Pathway List

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Pathway List
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Name Species Description View
Cell Cycle G2/M Transition 1182 Cell cycle checkpoints help ensure the accuracy of DNA replication and division. These checkpoints allow progression through the cell-cycle or arrest in response to DNA damage to allow time for DNA repair. DNA damage may cause cell cycle arrests both in G1 and in late G2 phase.<br><br>The G2 checkpoint controls cell-cycle progression from G2 to M phase. Active Cdk1(cdc2) complexed to cyclin B1 is required for progression from G2 to M phase. Regulation of the cdc2-B1 complex involves inhibitory phosphorylation at a pair of amino acids in the roof of the active site of cdc2 by Wee1. Dephosphorylation of these sites by the phosphatase Cdc25C increases Cdk activity. DNA damage activates ATM/ATR kinase, which then activate Chk1/Chk2, then inactivate Cdc25C through phosphorylation of cdc25C, resulting in the phosphorylation and inactivity of cdc2-B1 and G2-M arrest. Cdc2 can also be maintained in an inactive state by the kinases Wee1 and Mt1. DNA damage can also activate p53, which can turn on expression of several effectors to cause G2/M arrest, including: 14-3-3 which can bind to phosphorylated cdc2/cyclin B and export it from the nucleus, Gadd45 which can bind to and dissociated cdc2/cyclinB complex, and p21 which can inhibit cdc2 activity. View
DNA Damage Response Pathway 1182 DNA damage arises from both endogenous and environmental origins such as irradiation, reactive oxygen species, mutagens and drugs etc. This requires constant excision and repair of the damaged DNA by the DNA repair pathways. The inability to repair damaged DNA properly leads to various disorders and expedite tumor development. Organisms respond to chromosomal insult by activating a complex DNA damage response pathway which regulates genes involved in cell-cycle arrest, apoptosis as well as DNA repair. <br><br>The DNA damage response pathway is a signal transduction pathway that consists of sensors, transducers and effectors. ATM, ATR DNA-PK are protein kinases that act as sensors to DNA damage. They transmit signals through phosphorylation and activation of their downstream targets such as chk1, chk2. ATM, DNAPK can also activate p53 (see &quot;p53 Signaling&quot;) which in turn activate various genes to mediate diverse cellular processes. Bax and Fas can be activated by p53 to induce apoptosis; p21 can be activated to mediate cell cycle arrest; Gadd45, Brca1, Nbs1 can be activated to mediate DNA repair. Mutation or abnormal expression of these key intermediates in DNA response pathways often leads to genomic instability and carcinogenesis. Circadian rhythm gene, PER1, has also been shown to be important for the cellular response to IR and possibly function as a tumor suppressor gene. Overexpression of PER1 was reported to induce high expression levels of p53 and Myc. Therefore, cells that express PER1 are less able to arrest in G2/M and have higher rates of apoptosis. PER1 can interact with ATM and may facilitate the activation of ATM or may act as an adaptor protein to recruit other proteins that are required for ATM-mediated response to DNA damage. View
Glucocorticoids & Inflammation 1182 The effect of glucocorticoids on the immune system is species dependent as well as cell-type, dose, and duration dependent. Diametrically opposite effects of glucocorticoids on the immune system have been reported. Thus, glucocorticoids increase neutrophils by releasing bone-marrow polymorphonuclear cells and by inhibiting neutrophil migration and apoptosis, while on the other hand glucocorticoids promote apoptosis of eosinophils and basophils and suppress the synthesis of inflammatory mediators (such as cytokines, prostaglandins).<br><br>Glucocorticoid-effects occurring on a time scale of seconds to minutes have been shown to be mediated via membrane located and G-protein coupled receptors. Dexamethasone was found to induce rapid Ser-phosphorylation and membrane translocation of annexin-1 in a human folliculo stellate cell line via PKC, PI3K and MAPK. Annexin together with p11 act to inhibit the function of activated cPLA2 in mediating arachidonic acid release (see &quot;Arachidonic Acid Release&quot;). On a longer time scale, ligand-bound GR associate with IL-6-activated STAT3 and mediate (via either IL-6-responsive or glucocorticoid-responsive elements) the transcription of pro-inflammatory proteins. View
Beta-Oxidation of Fatty Acid 1182 The process of fatty acid oxidation is termed beta-oxidation since it occurs through the sequential removal of 2-carbon units by oxidation at the beta-carbon position of a fatty acyl-CoA molecule. In the mitochondria, each round of beta-oxidation produces 1 molecule each of NADH, FADH2, and acetyl-CoA. The acetyl-CoA enters the TCA cycle and is further oxidized to CO2 while generating 3 molecules of NADH, and 1 molecule each of FADH2 and ATP. The NADH and FADH2 generated during the beta oxidation and TCA cycles enter the respiratory chain to produce ATP. <br><br>In animal cells, beta-oxidation occurs in mitochondria and peroxisomes. Mitochondria is responsible for the oxidation of the major portion of the short-(<C8), medium- (C8-C12), and long- (C14-C20) chain fatty acids, while peroxisome is responsible for the metabolism of very long straight-chain (>C20), 2-methyl-branched chain (e.g. pristanic & phytanic) fatty acids, prostanoids, dicarboxylic acids, and the C27 bile acid intermediates. The peroxisomal beta-oxidation is carried out by two different sets of enzymes depending on whether the substrate is straight chain (AOX, AOX2/3, L-PBE/MFP1, ACAA1) or branched-chain (AOX2/3, D-PBE/MFP2, SCPx). The straight chain, but not the branched-chain, beta-oxidation enzymes (both mitochondrial and peroxisomal) are inducible by PPARalpha.<br><br>Mitochondrial beta-oxidation requires the carnitine palmitoyltransferases (CPTs) to shuttle acyl-CoA as acyl-carnitine across the mitochondrial inner membrane. Excess malonyl-CoA (endogenous inhibitor of beta-oxidation) in the cytoplasm inhibits CPT-I (residing in the mitochondrial outer membrane). View
Apoptosis 1182 Apoptosis, also known as programmed cell death, is an active cell-suicide program that rapidly removes unwanted, damaged or potentially dangerous cells. Inappropriate apoptotic responses have profound implications in various diseases, including cancer, Alzheimer's and autoimmune diseases.<br><br>Apoptosis can be triggered by a wide variety of stimuli, including DNA damage, growth factor withdrawal, certain peptide hormones (e.g. Fas ligand, TNF, & Trail), drugs, and toxins. Signals initiated by these stimuli are transduced via a series of protein-protein interactions which converge to activate the caspase-mediated proteolytic cascade that cleaves multiple nuclear and cytoskeletal proteins, ultimately leading to cell death.<br><br>In the death receptor pathway, receptors such as Fas, TNFR1, & DR-3,4 bind to and interact with their cognate ligands leading to the recruitment of adaptor protein FADD. FADD recruits and activates initiator caspase-8. This caspase then directly cleaves and activates effector caspases, such as caspase-3, which are predominantly responsible for substrate cleavage and cell dismantling. Alternatively, DNA damage leads to activation of p53, which in turn up-regulate transcription of pro-apoptotic Bax. Bax promotes the release of cytochrome C from mitochondria, which binds Apaf-1 and activates another initiator caspase, caspase-9. Once activated, caspase-9 can activate effector caspase-3,6,7. <br><br>Apoptosis is highly regulated. In addition to the caspase family, another class of regulator is the Bcl-2 family of proteins (see &quot;Bcr-Abl Signaling (Glevec Action)&quot;) that control mitochondria permeability and regulate the release of cytochrome C and other proteins (see &quot;Ca++ Signaling, Cytochrome c Release, & Apoptosis&quot;). Anti-apoptotic family members such as Bcl-2 and Bcl-xL inhibit the release of cytochrome C, whereas the pro-apoptotic proteins Bax, BID, Bad and Bim reside in the cytosol but translocate to mitochondria to promote cytochrome C release from the mitochondria. Pro- and anti-apoptotic regulators interact with one another and modulate each others' activities. Additional regulators are the IAPs (Inhibitors of Apoptotic Protein), which can bind and inhibit caspases-3, 6, and 9. View
MAP Kinase Signaling 1182 Mitogen-activated protein kinases (MAPK) are a family of serine/threonine protein kinases that directly phosphorylate transcription factors and other proteins that in turn modulate gene expression. They are highly conserved among eukaryotes and are involved in many cellular processes including cell proliferation, differentiation, apoptosis, stress response and, cytokine production. <br><br>MAPK activity is controlled through three-tiered cascades that are composed of MAPK, MAPK kinase (MAPKK, MKK or MEK) and MAPKK kinase (MAKKK, MEKK). There are at least 4 distinctly regulated groups of MAPKs in mammals: 1) extracellular signal-regulated kinases (ERK1/2), 2) Jun amino-terminal kinases (JNK 1/2/3), 3) p38 protein (p38 a/b/g/d), and 4) ERK5. These MAPKs are activated by their specific MAPKKs, which are activated by their specific MAPKKKs. MAPKKKs are in turn activated by interaction with a family of small GTPases and/or other protein kinases, connecting the MAPK module to the cell surface receptors or extracellular stimuli. View
Cholesterol Biosynthesis 1182 Cholesterol biosynthesis starts in the cytosol with the condensation of 2 acetyl-CoA units to form acetoacetyl-CoA which is then condensed with a third unit of acetyl-CoA to produce HMG-CoA (5-hydroxy-3-methyl-glutaryl-CoA). The conversion of HMG-CoA to mevalonate by HMG-CoA reductase occurs in the endoplasmic reticulum (ER). The conversion of mevalonate via 6 enzymatic steps to farnesyl pyrophosphate occurs in the peroxisome. The head to tail dimerization of farnesyl-PP to squalene as well as all the subsequent enzymatic steps that complete the synthesis of cholesterol take place in the ER.<br><br>Cholesterol biosynthesis is regulated at the transcription level by cellular sterol (cholesterol and side chain hydroxylated cholesterol) level via the transcription factor, SREBP-2 (see &quot;SREBP, Regulation of Fatty Acids & Cholesterol Biosynthesis&quot;). HMG-CoA reductase, in addition to being regulated by SREBP-2, is regulated at the protein level via binding to the ER protein Insig (see &quot;Regulation of SREBP&quot;). High cellular level of cholesterol promotes the binding of HMG-CoA reductase to Insig and leads degradation of the HMG-CoA reductase via the ubiquitin-proteasome pathway.<br><br>Products of the peroxisomal steps, isopentenyl-PP, geranyl-PP, and farnesyl-PP, in addition to being building blocks of cholesterol, are siphoned off for the synthesis of isopentenyl-tRNAs and the post-translational modification of some signaling proteins.<br><br>The mammalian cells contain two genes each of acetoacetyl-CoA thiolase and HMG-CoA synthase. For both enzymes, the mitochondrial and the cytosolic/peroxisomal isoforms are coded by two different genes while the location between the cytosol and peroxisome appears to be regulated by alternative splicing. The mitochondrial enzymes are involved in ketogenesis and are not regulated by SREBP-2. View
Bcr-Abl Signaling (Glevec Action) 1182 The Bcr-Abl oncogene is a constitutively active tyrosine kinase found in most of the human chronic myeloid leukemia (CML) cells. It's the product of genetic translocation between chromosomes 9 and 22 that occurred in transformed bone marrow cells, and consists of BCR sequence at the amino-terminus and c-Abl sequence at the carboxyl terminus. Bcr-Abl has transforming potentials and has been shown to inhibit apoptosis and cause the unregulated cell proliferation that is characteristic of CML and other malignancies.<br><br>Many signaling proteins can interact with BCR-ABL through its many functional domains and become phosphorylated in BCR-ABL-expressing cells. These proteins in turn activate a range of signaling pathways that activate proteins in the Ras, PI3K, AKT, JNK, & SRC families of kinases and protein and lipid phosphatases, as well as downstream targets of these proteins including transcription factors such as the STATs and Myc. Constitutive activation of Ras may lead to transformation and proliferation, while activation of the PI-3K/Akt and JAK/STAT pathways is most likely responsible for the observed anti-apoptotic potential. Activation of focal adhesion molecules (FAK/paxillin) contributes to the adhesive and migratory abnormalities of leukemia cells.<br><br>Glivec (imatinib, STI-571), is the first drug that specifically inhibits the tyrosine kinase activity of BCR-ABL to help reverse uncontrolled cell growth. It is one of the new classes of anti-cancer drugs and has revolutionized the treatment of CML and GIST (gastrointestinal stromal tumors). In addition to inhibiting the abl kinase, Glivec inhibits the PDGF tyrosine kinase and the c-kit tyrosine kinase. Glivec was designed to seek out and selectively bind to the Bcr/Abl ATP-binding pocket. By binding to this active site, Glivec turns off downstream signaling, halting uncontrolled proliferation, and apoptosis occurs. Treating patients with this drug dramatically improves patient survival, but people with a blast crisis form of the cancer begin to develop resistance following an initial response. View
Akt Signaling 1182 The serine/threonine protein kinase Akt/PKB is the cellular homologue of the viral oncogene v-Akt and is a key player downstream of PI3K in mediating growth and apoptosis. Activation of Akt accelerates tumorigenesis and allows tumors to become resistant to chemotherapeutic agents. Akt can be activated by PDK1, and repressed by tumor suppressor PTEN. <br><br>Activated Akt promotes cell survival through blocking apoptosis at multiple levels as shown in the pathway and described below. Akt inhibits apoptosis by phosphorylating the pro-apoptotic Bad component of the Bad/Bcl-XL complex. Phosphorylated Bad binds to 14-3-3 causing dissociation of the Bad/Bcl-XL complex and allows cell survival. Akt activates IKK and leads to NF-kB activation and cell survival. Akt phosphorylates and inactivates caspase-9 (see &quot;Apoptosis&quot; pathway). Akt phosphorylates forkhead transcription factors and blocks the induction of apoptosis through down-regulation of FasL. Akt inhibits YAP and MDM2 which are important for p53/73 mediated induction of pro-apoptotic genes such as Bax. Akt inhibits tumor suppressor TSC proteins that inhibit Rheb which activates mTOR. Activation of mTOR causes inactivation of 4E-BP1 which, in the active state, binds to translation initiation factor eIF4e. eIF-4E is rate limiting for the translation of many mRNA into proteins. Recently eIF-4E was discovered to have apoptosis suppression function through inhibiting the release of cytochrome c from mitochondria. View
Prostaglandin Action 1182 Prostaglandins (PG) are lipid mediators synthesized by most cells in our bodies and function as autocrine (signal at site of synthesis) and paracrine (signal at sites immediately adjacent to sites of synthesis). They are not stored but are synthesized from membrane-released arachidonic acids when cells are stimulated by mechanical trauma or biochemical signaling molecules from either external (e.g. allergens) or internal (e.g., cytokines, growth factors) sources. The activities of the prostaglandins via signaling through the different receptors are diverse although in general, they elicit an inflammatory response. Key receptor specific responses are depicted in the pathway diagram and summarized below.<br><br>PGD2 is the predominant PG produced by allergen-activated mast cells and has been implicated in the pathogenesis of allergic asthma and atopic dermatitis. In epithelial, smooth muscle, and fibroblast cells, PGD2 signal through the DP receptor to activate NF-kB and leading to induction of COX-2 gene expression. In Th2 lymphocytes, PtgD2 signal through the CRTH2 receptor to increase [Ca++]i and induce chemotaxis. PGD2 was found to be the most potent activator of basophils (responsible for late-phase allergic reactions).<br><br>PtgF2 is secreted by human endometrium throughout the menstrual cycle and is present in menstrual fluid. The PGF2 receptor (FP) in the corpus luteum is essential for maintaining normal reproductive cycle. FP expression in human endometrium was found to be the highest in the mid to late proliferative phase. The FP mRNA is extremely low in steroidogenic follicular cells but is expressed at high levels in the corpus luteum. Expression of FP was up-regulated in all endometrial adenocarcinomas. Activation of FP at the end of the cycle leads to a decrease in progesterone and luteolysis. <br><br>The Ptg EP1 receptor was found to be responsible for the neurotoxicity mediated by COX-2 and is associated with neuropathic pain. <br><br>In the kidney, the maintenance of normal renal blood flow and function is dependent on PGE2 which modulates renal hemodynamics and salt/water excretion via the function of all 4 EP receptors that are situated strategically in different segments of the nephron.<br><br>The EP2 receptor was implicated in murine osteoclastogenesis based on studies on Ep2 knockout mice. PtgE2 was found to enhance NF-kB induced osteoclastic differentiation of bone marrow-derived macrophages through EP2 and EP4 signaling. However, during differentiation, EP2 and Ep4 are down-regulated in the osteoclast precursor cells. PGE2 has been shown to directly inhibit bone-resorbing activity of mature osteoclasts.<br><br> View
Transportation of Lipids 1182 Dietary triglycerides emulsify with pancreatic juice and bile salts in the intestine and are hydrolyzed primarily by pancreatic lipase (PNLIP) into fatty acids and monoacylglycerols which are then absorbed together with cholesterol and bile salts by the enterocytes through an endocytosis process involving SR-B1 and CD36. Triacylglycerols are resynthesized from the absorbed fatty acids and monoacylglycerols in the enterocytes and are recruited by ApoB48 and assembled into chylomicrons which contain lipid droplets surrounded by the more polar lipids and a layer of apolipoproteins. Chylomicrons are then released into the blood via the lymph system and transport dietary triglyceride to peripheral tissues and cholesterol to the liver (see also &quot;LXR, FXR & Cholesterol Homeostasis&quot;).<br><br>Lipoprotein lipase (insulin sensitive, see &quot;Lipolysis: Mobilization of Triacylglycerols&quot;) on cell surface of peripheral tissues, using ApoC-II residing on chylomicron as co-factor, hydrolyzes chylomicron triglycerides and transport free fatty acids to muscle and adipose tissues. As a result, new particles called chylomicron remnants, enriched in cholesteryl ester and fat-soluble vitamins, are formed. Chylomicron and chylomicron remnants acquire ApoE from HDL and vLDL particles in circulation and is removed from the circulation by liver through endocytosis, once they drop to certain size, through receptors that recognize ApoE. The lipid pool in the liver are recruited by ApoB100, assembled into vLDL and released into the circulation. VLDL particles are secreted in an incomplete nascent state and acquire ApoE and ApoC-II from circulating HDL particles. Similar to the chylomicrons, vLDL particles supply peripheral tissues with fatty acid and continue to drop in size and become IDL particles. About = of the IDL particles are taken up by the liver via interaction between ApoB100 and the LDL receptor on hepatocytes, while the triglyceride on the other = of the IDL particles are taken up by the liver (via hepatic lipase) to produce LDL particles.<br><br> About 75% of the LDL particles are taken up by the liver, while 24% the LDL particles are delivered to other tissues. Approximately 1% of the LDL particles persist in circulation to become oxidized-LDL particles which are recognized by the endothelial scavenger receptor (see &quot;Monocytes, Macrophages & Atheroscleorosis&quot;). Persistence of LDL particles in the circulation may result from the excessive vLDL production associated with a high dietary intake of fats rich in saturated fatty acids. View
Prostaglandin Biosynthesis 1182 Prostaglandin endoperoxide synthase (PGHS, COX) is a heme protein with two distinct activities, both of which are required for the conversion of arachidonic acid (AA) to prostaglandin H2. The cyclooxygenase activity catalyzes the incorporation of 2 oxygen molecules into AA to form the hydroperoxide, prostaglandin G2 (PGG2), and the peroxidase activity reduces PGG2 to the corresponding alcohol, prostaglandin H2 (PGH2). Insertion of the first molecular oxygen is initiated by extraction of the C-13 hydrogen from a bound AA by a tyrosyl radical at the active site of the cyclooxygenase. This radical then reacts with one molecular-oxygen and rearranges itself to form PGG2 after the addition of the second molecular oxygen. <br><br>Depending on the size and shape of the NSAIDs, they are either rapidly reversible competitive inhibitors (ibuprofen) or slow tight-binding inhibitors (alclofenac & indomethacin) at the cyclooxygenase catalytic site. Among all the NSAIDs in medical use, aspirin is the only covalent modifier (acetylates) of the cyclooxygenase catalytic site (ser 530 of COX-1). COX-1 and COX-2 are 60% identical at the amino acid level. The most pronounced differences between the two enzymes are their signal peptides and their membrane binding domains, however, the significance of this is controversial.<br><br>The expression of COX-1 gene is generally constitutive and ubiquitous and relatively little is known about the transcription elements that control its transcription. COX-1 expression is known to be induced in quiescent endothelial cells and in differentiating mast cells. The promoter region of COX-2 is well characterized, thus kinases (ERK, c-Jun) in the MAPK pathway induce COX-2 transcription via the cAMP response element, while induction by cytokines and growth factors are through the binding of transcription factors to the CREB/NF-IL-6, and NF-kB elements. COX-2 can be induced (2-6 h) in fibroblasts, endothelial cells, monocytes, and ovarian follicles in response to growth factors, tumor promoters, hormones, LPS, and cytokines.<br><br>PGH2 is rapidly converted to one of the active prostaglandins, PGE2, PGD2, PGF2, PGI2 (prostacyclin) and thromboxane (TXA), by the corresponding synthases (isomerases) depending on cell type and stimuli. In mast cells, the early (30 min) and late (>6 hr) phase of PGD2 synthesis are coupled, respectively, to COX-1 and COX-2 (see &quot;Arachidonic Acid Release&quot;). In human embryonic kidney epithelial cell (HEK293), PGE2, PGI2 and thromboxane synthesis are preferentially coupled to COX-2. Stimulating quiescent rat peritoneal macrophage cells with Ca++ ionophore (A23187) produced thromboxane in preference to PGE2 in 30-60 min that is mediated by cPLA2 and COX-1; stimulating the same cells with LPS produced predominantly PGE2 in 3-24 hr and is mediated by cPLA2, sPLA2, and COX-2. The active prostaglandins are released from the cells immediately after synthesis through prostaglandin transporter (PGT) and function as paracrines locally (see &quot;Prostaglandin Action&quot;). PGI2 and TXA are chemically unstable (1/2 life < a few min), while PGD2, PGE2, and PGF2 are chemically stable but are inactivated by metabolism. View
Hemes from Protoporphyrin IX 1182 The rate-limiting step of heme biosynthesis in red blood cells is Fe acquisition from transferrin while that in nonerythroid cells is the synthesis of delta- aminolevulinic acid. How Fe sequestered by the Tf-TfR1 pathway (See &quot;Iron Homeostasis&quot;) is directed to the mitochondria for the final step of heme synthesis, insertion of Fe++ into protoporphyrin IX, is unknown. Once Fe++ is in the mitochondria, in addition to heme synthesis, it is incorporated into [2Fe-2S] and [4fe-4S] clusters and inserted into [Fe-S] containing proteins both in the mitochondria and in the cytosol. Excess Fe++ is stored in mitochondrial ferritin. <br><br>Heme b is incorporated into hemoglobin, myoglobin, and the heme containing oxidoreductases including the P450 enzymes. During hematopoiesis, in erythroid precursor cells, heme b induces the expression of beta-globin by disrupting the binding of Bach1 to the tandem MARE sequences on the beta-globin promoter.<br><br>In mammalian and yeast cells, heme b is covalently attached to apocytochrome c by the action of cytochrome c heme-lyase to give cytochrome c which is part of the mitochondrial respiration chain. Heme b is also converted to heme a, the heme in cytochrome c oxidase. View
Corticosteroid Biosynthesis 1182 The synthesis of corticosteroids from cholesterol at the adrenal cortex is catalyzed by many of the same enzymes used in the synthesis of testosterone and 17beta-estradiol in the gonads; however, the same transformations are catalyzed by different enzyme subtypes which are products of different genes. The synthesis of pregnenolone from cholesterol is regulated at the level of cholesterol delivery to CYP11A as described for &quot;Steroid Hormone Biosynthesis&quot;. 3beta-HSD2 is the predominant 3HSD isoform that leads to the synthesis of hydrocortisone and aldosterone. CYP21 which catalyzes the introduction of a hydroxyl group on C21 of progesterone and 17alpha-hydroxyprogesterone is present only in the adrenal cortex. There are two genes of this enzyme, CYP21A and B, in both human and mouse, however, only CYP21B encodes an active enzyme in human and only CYP21A encodes the active enzyme in mouse. <br><br>CYP11B1, located on the inner mitochondrial membrane, catalyzes the introduction of a hydroxyl group on C11 of cortexone and 17alpha hydroxylcortexone, as well as the on C18 of corticosterone but can not catalyze the oxidation of C18-hydroxyl to form aldosterone. CYP11B2 (aldosterone synthase), also located on the inner mitochondrial membrane, catalyzes the introduction of C11-hydroxyl on cortexone, C18-hydroxyl on corticosterone followed by the oxidation of the C18-hydroxyl to yield the final product aldosterone.<br><br>Hydrocortisone, the potent hormone, and cortisone (1/100 as active) are inter-convertible by the two isoforms of 11HSD as described in &quot;Regulation of Hydrocortisone Biosynthesis&quot; & &quot;Glucocorticoids & Inflammation&quot;. View
Regulation of Renin Release 1182 Renin is the first enzyme of the reaction cascade that converts circulating angiotensinogen to the major effecter hormone, angiotensin II (Ang-II) of the rennin-angiotensin system (RAS). Renin is synthesized as a pro-protein (pro-renin) and stored in the juxtaglomerular cells (JG cells) of the kidney. At the level of the whole kidney, renin secretion from the JG cells is stimulated by renal nerve activities via beta1-adrenoceptors and dopamine receptors, as well as by lowered perfusion pressure (low Na+ intake) in the renal arterial tree. In terms of RAS, renin secretion is subject to negative feedback regulation by angiotensin II. At the local level, prostaglandin E2, prostacyclin and nitric oxide stimulate renin secretion. At the intracellular level, elevation of [cAMP] rapidly (within a few minutes) stimulates renin secretion by the JG cells, while elevation of [Ca++] has the opposite effect. View
Regulation of Aldosterone Biosynthesis 1182 The biosynthesis of corticosteroid in steroidogenic tissues is mediated by the rate of cholesterol delivery to the first enzyme, CYP11A, of the biosynthesis cascade. Transportation of cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane where CYP11A resides is mediated by the steroidogenic acute regulatory protein (StAR). Upon stimulation by trophic hormone, the StAR precursor protein is rapidly synthesized in the cytoplasm and escorted by a chaperone protein (unidentified) to mitochondria, become associated with the mitochondrial outer membrane, and is imported into the mitochondrial matrix. During the importing process, cholesterol molecules are moved from the outer membrane to the inner membrane and immediately converted to pregnenolone (and progesterone in some tissues, see &quot;Corticosteroid Biosynthesis&quot; & &quot;Steroid Hormone Biosynthesis&quot;). <br><br>Synthesis of the StAR protein in steroidogenic tissues is regulated by the abundance of transcription factor, steroidogenic factor (SF-1). SF-1 is a global regulator of endocrine development and function; is expressed in adult adrenocortical cells, testicular Leydig cells, and ovarian theca and granulose cells. Its transcriptional activity is activated by adrenocorticotropic hormone and type-A atrial natriuretic peptide signaling and is inhibited by DAX-1, a transcriptional repressor. DAX-1 inhibits the transcriptional activity by 1) biding to the hairpin loops on the promoters of SF-1 regulated genes, 2) binding to transcriptional co-activator LRH-1, and 3) binding to SF-1 itself. SF-1 binding site is present on the DAX-1 promoter, thus preventing excessive production of the steroidal hormones. Expression of the enzymes in the aldosterone biosynthesis cascade is regulated in the same manner as StAR. View
Regulation of Hydrocortisone Biosynthesis 1182 The biosynthesis of corticosteroid in steroidogenic tissues is mediated by the rate of cholesterol delivery to the first enzyme, CYP11A, of the biosynthesis cascade. Transportation of cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane where CYP11A resides is mediated by the steroidogenic acute regulatory protein (StAR). Upon stimulation by trophic hormones, the StAR precursor protein is rapidly synthesized in the cytoplasm and escorted by a chaperone protein (unidentified) to mitochondria, become associated with the mitochondrial outer membrane, and is imported into the mitochondrial matrix. During the importing process, cholesterol molecules are moved from the outer membrane to the inner membrane and immediately converted to pregnenolone (and progesterone in some tissues). <br><br>Physical or psychological stress activates the production Crh in the hypothalamus and its release into circulation. Acting via the portal circulation, Crh, in conjunction with vasopressin, induces the production and release of ATCH from the pituitary into to circulation. ATCH then up-regulates the expression of StAR, as well as the down stream enzymes, to produce and release glucocorticoids (hydrocortisone in human and other primates, corticosterone in mice and rats). Both hydrocortisone and aldosterone have high affinity to the mineralocorticoid receptor (MR) and much lower affinity for the glucocorticoid receptor (MR). It is believed that MR plays a primary role in mediating the effects of the glucocorticoids under basal conditions. As glucocorticoid levels rise, either in response to stress or as a function of the circadian cycle, MR saturate and GR become the primary mediator of glucocorticoid activity including the feedback inhibition of Crh synthesis and release.<br><br>Two 11beta-hydroxysteroid dehydrogenases, 11bHSD1 and 11bHSD2, encoded by two distinct genes catalyze the interconversion of the hormonally active hydrocortisone and the inactive cortisone. In human 11bHSD1 is widely distributed but most abundantly in liver and adipose tissue while 11bHSD2 is found predominantly in mineralocorticoid target tissues, such as kidney, colon, and salivary gland, where it serves to protect MR from glucocorticoid excess. MR has the same affinity for hydrocortisone and aldosterone in vitro. Aldosterone is not metabolized by 11bHSD2. Mutations in the 11bHSD2 gene account for an inherited form of hypertension with the syndrome of apparent mineralocorticoid excess. Polymorphic variability in the 11bHSD2 gene in part determines salt sensitivity, a forerunner for adult onset hypertension. View
Renin-Angiotensin System 1182 The rennin-angiotensin system (RAS) plays a pivotal role in blood pressure homeostasis and electrolyte balance by regulating renal salt and water metabolism. The major effecter molecule of the system is Angiotensin II (Ang-II), an octa-peptide derived from circulating angiotensinogen (glycoprotein, 452 amino acid residues) produced in the liver. Ang-II is produced by sequential peptidase cleavage of angiotensinogen by rennin (an aspartyl protease synthesized by the juxtaglomerular cells located in the walls of the afferent arteriole of the kidney) to produce an inactive deca-peptide (angiotensin I) and a subsequent cleavage of the deca-peptide by angiotensin-converting enzyme (ACE, a membrane associated endopeptidase, cleaved and secreted into circulation). Transcription of the rennin gene is under complex developmental and tissue-specific regulation, while the expression of angiotensinogen is stimulated by glucose and down-regulated by insulin.<br><br>Although Ang-II binds to both AT1 and AT2, the AT1 receptor mediates most of the cardiovascular effects of Ang-II including vasoconstriction, sympathetic activation, cell proliferation, aldosterone release and renal sodium resorption. The AT2 receptor functions as a modulator of the effect of AT1 receptor including down-regulating rennin release. View
Glycolysis 1182 Glucose is the most ready energy source for most tissues and the only energy source for some tissues such as the brain, testes, erythrocytes and kidney medulla. Aerobic glycolysis as depicted in this pathway drawing generates 2 each of ATP, NADH, and pyruvate from each glucose molecule. Pyruvate enters the TCA cycle and generates additional 4 NADH and 1 FADH2 from each 2 pyruvate molecules. NADH and FADH2 are converted to mitochondrial membrane potential by the mitochondrial oxidative phosphorylation enzyme complexes and that, in turn, produces 3 ATP from each NADH and 2 ATP from each FADH2. Thus, under aerobic conditions, oxidation of one glucose molecule ultimately produces ~20 ATP molecules.<br><br>The rate of glycolysis in a cell is regulated by 1) the amount of glucose entering the cell via the glucose transporters (GLUT), 2) the cellular activity of the kinases that activate glucose to glucose-6-phosphate (G6P) for entry into glycolysis, and 3) the cellular [ATP]/[ADP] ratio (see &quot;Gluconeogenesis&quot; pathway description). In hepatocytes and muscle cells, based on cellular [ATP]/[ADP] ratio, excess G6P is converted to glycogen and excess pyruvate is converted to fatty acids and triglycerides for energy storage. View
Insulin Receptor Signaling 1182 Insulin is a key hormone controlling the storage and synthesis of lipid, proteins and carbohydrates. In addition to regulating the level of blood glucose, insulin suppresses hepatic gluconeogenesis and promotes glycogen synthesis and storage in liver and muscle, triglyceride synthesis in liver, triglyceride storage in adipose tissue, and amino acid storage in muscle. Insulin increases energy storage through enriching the concentration of glucose transporter Glut4 at the plasma membrane. The effects of insulin-receptor signaling on Glut4 and glycogen synthesis reduce serum glucose level immediately. <br><br>Upon binding of insulin to insulin receptor (INSR), the intrinsic tyrosine kinase associated with INSR is activated and phosphorylate intracellular substrates that including the insulin receptor substrates (IRS1/2/3) which then recruit PI3K and initiates a signaling cascade leading to the activation of Akt and atypical protein kinase C. Akt as well as other components the INSR signaling cascade stimulate Glut4 translocation to the plasma membrane and leads to increased glucose uptake into muscle cells and adipocytes. Akt signaling also increases glycogen synthesis via GSK3 phosphorylation (see &quot;Glycogen Synthesis & Glycogenolysis&quot;). PKC has also been demonstrated to participate in the termination of insulin signaling through phosphorylation of IRS1.<br><br>In parallel to the PI3K mediated signaling, activated insulin receptor phosphorylates the adaptor protein APS and Cbl. Cbl interacts with CAP which recruits them to the lipid raft. CAP expression correlates well with insulin sensitivity. Phosphorylated Cbl also interacts with CrkI which then binds to exchanges factor C3G. C3G then catalyze the exchange of GDP and GTP and activate the lipid-raft associated protein TC10. Upon its activation, TC10 is able to interact with a number of effector molecules including CIP4, Exo70, to stimulate Glut4 translocation and glucose uptake.<br><br>Other signaling transduction proteins that interact with IRS molecules include Grb2 and SHP2. SHP-2 has been shown to be required for activation of the MAPK pathway by insulin and for PKC-dependent phosphorylation of IRS-1. The mitogenic effects of insulin are mediated primarily through Ras/MAPK pathway and the Akt cascade.<br><br>Insulin regulates the expression of many genes. Insulin-stimulated phosphorylation of FKHR promotes its exclusion from the nucleus thus confer transcriptional suppression of many genes, including PEPCK, insulin-like growth factor-binding protein 1 (IGFBP-1), tyrosine aminotransferase, and the glucose-6-phosphatase (G6Pase). Insulin increases the expression of genes such as c-Fos. View
Cell Cycle G1/S Transition 1182 Eukaryotic cell cycle can be divided into four different phases where Gap 1 (G1) is the interval between mitosis and DNA replication. The transition that occurs at the restriction point (R) in G1 commits the cell to the proliferative cycle. If the conditions that signal this transition are not present, the cell exits the cell cycle and enters G0, a nonproliferative phase during which growth, differentiation or apoptosis may occur. Replication of DNA occurs during the synthesis (S) phase, which is followed by a second gap phase (G2) during which growth and preparation for cell division occurs. Mitosis and the production of two daughter cells occur in M phase.<br><br>Cell cycle is regulated by a complex network of regulatory proteins including a family of cyclins that act as regulatory subunits of cyclin-dependent kinases (cdks). The G1/S cell cycle checkpoint controls the passage of eukaryotic cells from the first 'gap' phase (G1) into the DNA synthesis phase (S). Two cell cycle kinases, CDK4/6-cyclin D and CDK2-cyclin E, and the transcription complex that includes Rb and E2F are pivotal to the control of this checkpoint. During G1 phase, the Rb-HDAC (histone deacetylase) repressor complex binds to the E2F transcription factor, inhibiting the downstream transcription mediated by E2F. Phosphorylation of Rb by CDK4/6 and CDK2 dissociates the Rb-repressor complex, allowing the transcription of S-phase genes encoding for proteins that amplify the G1 to S phase switch and that are required for DNA replication. Various stimuli exert G1/S-checkpoint control including TGFbeta, DNA damage, growth factor withdrawal, contact inhibition and replicative senescence. Many act by inducing members of the INK4 or Kip/Cip families of cell cycle kinase inhibitors e.g., P21, p27, p15, p16 etc. TGFbeta inhibits the transcription of Cdc25A, a phosphatase that activates the cell cycle kinases. Growth factor withdrawal activates GSK3beta, which phosphorylates cyclin D, leading to its rapid ubiquitination and proteasomal degradation. Cell cycle checkpoint ensures integrity of the genome. Cell does not enter mitosis until DNA replication is complete and DNA damage is repaired. View
Gluconeogenesis 1182 Gluconeogenesis is the biosynthesis of new glucose from lactate (produced during anaerobic glycolysis in skeletal muscles), alanine (produced form transamination of pyruvate in muscles and other peripheral tissues and transported in blood to the liver), glycerol (left over lipid backbone from beta-oxidation), and other amino acids (converted to oxaloacetate via the TCA cycle). Synthesis of glucose from three and four carbon precursors is essentially a reversal of glycolysis using the same enzymes, with the exception of the three highly regulated enzymes, glucokinase (or hexokinase), 6-phosphofructose-1-kinase (PFK-1), and pyruvate kinase. These three enzymes were substituted with three other highly regulated enzymes, glucose-6-phosphatase (G6Pase), fructose-1,6-bisphosphatase, and phosphoenolpyruvate carboxykinase (PEPCK), respectively.<br><br>The flux of gluconeogenesis or glycolysis is controlled at the interconversion between fructose-1,6-diphosphate (F1,6DP) and fructose-6-phosphate (F6P) by PFK-1 toward glycolysis and by fructose-1,6-bisphosphatase toward gluconeogenesis. The activity of these two enzymes are regulated allosterically in the opposite direction by the level of fructose-2,6-diphosphate (F2,6DP) and cellular [ATP]/[ADP] ratio, while the level of F2,6DP is regulated by the bifunctional enzyme, phosphofructokinase-2/fructose-2,6-bisphosphatase (PFK-2/F2,6Pase). Whether PFK-2/F2,6Pase functions as a kinase (favor glycolysis) or as a phosphatase (favor gluconeogenesis) is controlled by blood glucose level and glucagon.<br><br>Due to the high energy difference between pyruvate and phosphoenolpyruvate (PEP), pyruvate is first converted to oxaloacetate (also derived from the TCA cycle) which is then converted to PEP by PEPCK for entry into gluconeogenesis. Gluconeogenesis (as well as glycogenolysis) results in the formation of glucose-6-phosphate which is converted by G6Pase to glucose for release into circulation. Cellular levels of both PEPCK and G6Pase are regulated at the transcription level. View
Regulation of Phosphoenolpyruvate Carboxykinase (PEPCK) 1182 Phosphoenolpyruvate carboxykinase (PEPCK) is the key enzyme for gluconeogenesis (liver, kidney), glyceroneogenesis (adipose tissue), and cataplerosis (removal of citric acid cycle anions formed from entry of amino acid derived 4-carbon units into the cycle, all tissues). Gluconeogenesis is the major cataplerotic pathway in the liver. Transcriptional regulation of the cytosolic forms of PEPCK (cPEPCK) is among the best studied and is a well-defined model for metabolic regulation of gene expression. The sequence of the promoter region of cPEPCK is well conserved (>95% sequence identity) among mouse, rat, and human. cPEPCK mRNA is induced by glucocorticoids, thyroid hormone, or glucagon, and is repressed by insulin. The transcriptional effects of the hormones are further modulated by transcriptional co-activators or co-repressors in response cues arising from diverse signaling pathways. The transcriptional regulations shown in this pathway are examples of key regulatory elements and factors that are known to mediate the expression of cPEPCK. It is not meant to show a complete picture, rather to demonstrate how complex the transcriptional regulation of a key metabolic enzyme can be. View
Phosphoinositide-3-Kinase (PI3K) Signaling 1182 Phosphoinositide 3-kinases (PI3Ks) are a subfamily of lipid kinases that catalyze the addition of phosphate to the 3-position of the inositol-ring of phosphoinositides. The PI3K signaling pathway regulates a variety of biological processes including survival, proliferation, cell growth, cell motility and glycogen metabolism. Dysregulation of PI3K signaling contributes to the pathogenesis of several human diseases including diabetes and cancer.<br><br>The prototypical class IA PI3K consists of a catalytic subunit (P110a, b) and a regulatory subunit (P85). The only identified member of class IB is p110g (PI3Kg). PI3K can be activated by many growth and survival factors as well as G-proteins to convert one lipid signaling molecule (PIP2) into another (PIP3). Signaling proteins with pleckstrin-homology (PH) domains, such as the serine/threonine protein kinases, including protein kinase B (PKB, also known as Akt), phosphoinositide-dependent kinase-1 (PDK-1) and PDK-2, accumulate at sites of PI3K activation. Association with PIP3 at the membrane brings these proteins into proximity and facilitates phosphorylation of Akt by PDK-1 and PDK-2. The activated Akt phosphorylates a host of target proteins to affect cell growth and survival (see &quot;Akt Signaling&quot;). PI3K can also activate Rac and ARF to influence cell motility and vesicle budding, as well as GSK3b and mTOR to influence glycogen and protein synthesis. View
NF-kappa B Signaling 1182 Nuclear factor kB (NF-kB) consists of a family of inducible nuclear transcription factors that function as important regulators of host immune and inflammatory responses. It is also involved in protecting cells from undergoing apoptosis due to DNA damage or cytokine treatments. NF-kB can be activated by a variety of stimuli that include growth factors, cytokines, lymphokines, UV, pharmacological agents, and stress. The inactive NF-kB is normally sequestered in the cytoplasm, bound by members of the IkB family of inhibitor proteins. The activating signals activate the Ikb kinases (Ikka and Ikkb), which then phosphorylate the inhibitory protein IkB and lead to their ubiquitination and degradation via the proteasomal pathway. The freed NF-kB then translocates to the nucleus where it binds target genes and stimulate transcription. <br><br>NF-kB stimulates the expression of many anti-apoptotic proteins, which contribute to its pro-survival property. NF-kB also up-regulates cytokines and their modulators, and proteins involved in proliferation, cell adhesion, transcription factors, acute phase proteins and immunoreceptors etc. <br><br>Pharmacological inhibition of NF-kB pathway has been widely used in the treatment of inflammation and cancer. They act at several steps to interfere with NF-kB activation. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin and sodium salicylate has been shown to inhibit ATP-binding to IKKb, resulting in greatly reduced IKKb-dependent phosphorylation of IkBa and its degradation by the proteasome, thus suppressing activation of NF-kB pathway. Alternatively, PS-341 can directly inhibit proteasome function to prevent NF-kB activation. NF-KB pathway can also be inhibited by glucocorticoids, such as dexamethasone and prednisone, through inhibition of DNA binding, as well as increased expression of IkBa thus enhancing the cytosolic retention of NF-kB. View
Hepatic Stellate Cell Activation & Fibrosis 1182 Chronic liver injuries, such as viral hepatitis, alcohol abuse, drugs, iron or copper overload, leads to fibrosis (reversible) and eventually cirrhosis (the end stage to liver fibrosis). As the liver becomes fibrotic, both the collagens & non-collagenous components of the hepatic extra-cellular matrix (ECM) increase, and shift from sub-endothelial space (low density matrix) to interstitial space with fibril-forming collagens. <br><br>Hepatic stellate cells comprise 15% of the total number of liver cells and are the principal storage site for retinoids in normal liver. Liver injuries activate the stellate cells and cause them to transition from quiescent cells into proliferative, fibrogenic, & contractile myofigroblasts which is responsible for the ECM remodeling and the collagenous matrix observed in hepatic fibrosis. During the recovery from acute liver injury, the number of activated stellate cells decrease (by what signal?) as tissue integrity is restored. Stimuli that activate the stellate cells come from injured hepatocytes, neighboring Kupffer cells & endothelial cells. Hepatocytes are potent source of ROS derived from imbalanced lipid & glucose metabolism and from the various CYP up-regulated for xenobiotic metabolism. Injured sinusoidal endothelial cells, through the activation of plasmin, convert latent TGF-beta1 released from the Kupffer cells to TGF-beta1 which then participates in stellate cell activation. Endothelial cell injury also leads to platelet activation and the release of PDGF. Activation of the TGF-beta and the PDGF receptor signaling cascades are the key events of stellate cell activation, the former is responsible for the synthesis of ECM proteins and wound healing, while the later is responsible for the production of more stellate cells for more wound healing. The proliferative effect mediated by PDGF-R is tempered by the anti-proliferative effect mediated by endothelin B (ETB) receptor which is up-regulated by PDGF-BB & thrombin.<br><br>Recent studies have demonstrated that expression of IGF-1 by activated hepatic stellate cells reduces fibrogenesis & enhances regeneration after liver injury. View
Ion Channel & Cardiac Action Potential 1182 The rhythm of the heart is generated and regulated by pacemaker cells within the sinoatrial (SA) node located within the wall of the right atrium. The action-potentials generated by the pacemaker cells spread, by cell-to-cell conduction, to cardiomyocytes throughout different sections of heart in a well defined sequence and route, thereby maintaining the pumping function of the heart. The spontaneous pacemaker activity of the pacemaker cells is 100-110 beats/min. At rest, this intrinsic rhythm is primarily influenced by the parasympathetic cholinergic nerve (vagus) which release acetylcholine that binds to the M2 receptor and brings the heart rate down to 60-80 beats/min. Both the rate of pacing and cell-to-cell conduction process are also strongly influenced by sympathetic adrenergic nerves which release norepinephrine (NE) that bind preferentially to adrenoceptor beta-1 in the heart resulting in increased heart rate (by increasing intracellular [Ca++], thereby conduction velocity). <br><br>The course of a cardiac action potential triggered by cell-to-cell conduction (non-pacemaker action potential, as depicted in the pathway) is a function of the resting (equilibrium) potential of K+ (-96 mV), Ca++ (134 mV), and Na+ (50 mV) across the cardiomyocyte cell membrane and the gating voltage of the following voltage gated channels, NaV1.5, CaV1.2/1.3, & CaV3.1/3.2. The resting potential is primarily established by the Na+/K+-ATPase (SUR2A) and the plasma membrane Ca++-ATPase which work in concert to keep the K+ concentration high inside the cells and the Na+ & Ca++ concentration high outside the cells. <br><br>When a resting cardiac myocytes (-90 mV) are rapidly depolarized to above the gating voltage of NaV1.5 (~-70 mV), the channel opens up (Phase 0) and Na+ rushes in (while K+ goes out via the voltage gated Kv1.4, Kv4.2/4.3) raising the membrane potential to ~20 mV. Opening of KvLQT and ERG initiates (Phase 1) the repolarization phase which soon plateaus (Phase 2) due to the large but slow influx of Ca++ from the L-type calcium channels (dihydropyridine receptor ) that opened up when the membrane potential depolarized to ~-40 mV. Repolarization (Phase 3) occurs when the K+ conductance increases and Ca++ conductance decreases through the pumping activities of the respective ATPases. The length of Phase 2 (refractory period) limits the frequency of action potentials (contractions) that can be generated by the heart. The delayed slow rectifier K+ channels provide a slow leak of K+ in order to achieve the cells resting potential. The inward rectifier K+ channel is open at near resting potential presumably to maintain the resting potential prior the next action potential.<br><br>In non-pacemaker cardiac myocytes, the opposing effects of acetylcholine and norepinephrine on heart rate is transmitted via the corresponding G-proteins to the regulatory subunits of SERCA and RyR2 on sarcoplasmic reticulum (SR). During phase 2 of an action potential, the influx of Ca++ from the L-type Ca++ channels triggers Ca++ release from SR via RyR2 (physically interact with the L-type calcium channel). The combination of Ca++ influx and Ca++ release raises the free intracellular Ca++ concentration [Ca++]i, allowing Ca++ to bind to the troponin molecules on the actin filament to initiate contraction. [Ca++]i decline (for relaxation) is affected by Ca++ export via PMCA and NCX, as well as, Ca++ re-uptake into the SR via SERCA. Binding of Angiotensin-II to AT-I receptor (see &quot;Cardiac Hypertrophy & Cell Death&quot;) triggers additional Ca++ release from intracellular stores (see &quot;Calcium Signaling in Nonexcitable Cells&quot;), thus increase the strength of contraction. View
Glucocorticoids & Glucose Metabolism 1182 Glucocorticoids increase hepatic glucose production and decrease glucose uptake of insulin sensitive tissues in man. Glucocorticoids are counter-regulatory hormones that increase blood glucose concentration by inducing systemic insulin resistance, gluconeogenesis and glycogenolysis in liver. This effect is thought to allow increased glucose delivery to insulin independent cells such as inflammatory cells and neurons.<br><br>Glucocorticoid response element (GRE) is present in PEPCK and G6Pase, the two rate limiting enzymes of gluconeogenesis. Glucocorticoids also increase the expression of G6Pase in pancreatic islet leading to increased glucose cycling (simultaneous phosphorylation of glucose to G6P and Dephosphorylation of G6P to glucose) and insulin resistance. The cytosolic form of PEPCK is selectively expressed in liver, kidney and adipose tissue. The transcription of PEPCK is induced by glucocorticoids in the liver and kidney but repressed in the white adipose tissue. In NIH3T3 adipocytes, ligand-activated GR was found to interfere with binding of adipocyte-derived nuclear proteins (C/EBPalpha and beta) to C/EBP recognition sites on the PEPCK promoter. View
Bile Acid Synthesis 1182 About 500 mg of cholesterol is converted into bile acid each day in the adult human liver and ~95% of the bile acids secreted by the liver is reabsorbed in the intestine during each cycle of enterohepatic circulation and reused. The 5% lost is replaced by new synthesis in the liver. This 500 mg/day usage accounts for ~90% of the cholesterol that is actively metabolized in the body, while steroid hormone biosynthesis accounts for the remaining 10%.<br><br>This pathway depicts the classical route of bile acid synthesis which starts with the hydroxylation (Cyp7a) of C-7 of cholesterol. The non-classical route (see &quot;Bile Acid Biosynthesis via Oxysterols&quot;) starts with hydroxylation of the cholesterol side chain to produce a number of different oxysterols which are then hydroxylated (Cyp7b) at C-7. The product of C-7 hydroxylation from both the classical and the non-classical route can either under go side chain oxidation and cleavage to give chenodeoxycholic acid (CDCA) or hydroxylation (Cyp8b1) at C-12 prior to side chain oxidations to give cholic acid (CA). Expression of the CYP7A1 gene is up-regulated by LXR (mouse only) and down regulated by FXR (see &quot;Hepatic Cholestasis&quot;), while the expression of CYP8B1 is up-regulated by SREBP-1 which, in turn, is kept in the inactive form by oxysterols (see &quot;SREBP, Regulation of Fatty Acids & Cholesterol Biosynthesis&quot;). CDCA is more hydrophobic than cholic acid and is 10X more effective in activating FXR than cholic acid. Thus, when circulating cholesterol is low, SREBP-1 is activated and CYP8B1 is induced leading to increased ratio of [CA]/[CDCA], increased hydrophilicity of the bile, and increased intestine cholesterol absorption.<br><br>During de novo bile acid synthesis, cholic acid and chenodeoxycholic acids are formed as their respective CoA derivative and conjugated directly with taurine or glycine via BAAT to give, respectively, the taurocholates and the glycocholates. These conjugated bile acids are then excreted via BSEP (Hepatic Cholestasis) into the bile duct. View
Vasopressin System 1182 The primary function of vasopressin (arginine vasopressin, AVP) is the regulation of body's water content and blood pressure via regulating the rate of water excretion by the kidney and is essential for cardiovascular homeostasis. Vasopressin is a nona-peptide with a disulfide bridge between two cysteine residues. Vasopressin is synthesized by the magnocellular neurons of the hypothalamus as precursor consisting of a signal sequence and three moieties: the nona-peptide hormone, neurophysin II (a vasopressin-associated carrier protein) and a copeptide (a glycopeptide). The precursor is targeted to ER and processed in the Golgi apparatus so that the mature nona-peptide hormone is targeted to and stored in secretory granules at the distal end of the axons. Vasopressin is released into circulation in response to low blood volume (low blood pressure, high serum osmolality) and hyponatremia as sensed by baroreceptors located in the heart and osmoreceptors located in the hypothalamus.<br><br>The actions of vasopressin are mediated by five tissue-specific G-protein-coupled receptors, V1R (vascular), V2R (renal), V3R (pituitary) and OTR (oxytocin receptor) and P2R (purinergic receptors). V1Rs, coupled to G-protein Gi, are found in high density on vascular smooth muscle and cause vasoconstriction upon stimulation. They are also present on platelets and facilitate thrombosis upon stimulation. V2R, coupled to G-protein Gs, is found only in the collecting-duct of the kidney and mediates the well known antidiuretic effect of vasopressin by regulating the shuttling of aquaporin-2 to cell surface and by increasing the transcription of aquaporin-2 to increase water re-absorption. V3R is found only in the pituitary and mediates vasopressin induced ATCH secretion. OTR, coupled to G-protein Gq, has equal affinity for vasopressin and oxytocin, whereas V1R has a 30-fold higher affinity for vasopressin than for oxytocin. OTRs are present in a variety of reproductive and nonreproductive tissues. Activation of OTR in heart stimulates the release of atrial natriuretic peptide which has diuretic and natriuretic properties and is associated with cardiac hypertrophy. View
Mechanism of Muscle Contraction 1182 Muscle contraction occurs when myosin heads of the thick filament attach to and exert force on the actin molecules in the thin filament. This occurs when an action potential activate voltage gated Ca++ channels causing an increase in cellular Ca++ concentration.<br><br>In the absence of Ca++, troponin attaches to two actins (via the TnI subunit) and tropomyosin (via the TnT subunit) while the tropomyosin blocks the myosin binding site on actin. Ca++ binding (via the TnC subunit) causes troponin to detach from the actin molecules and allowing tropomyosin (Tm) to slide into its place between the two actin molecules. This movement of Tm exposes the myosin head binding sites and the ADP-Pi-bound myosin heads become attached to actin. In the presence of Ca++, Tm does not occupy a fixed position, rather &quot;slips & slides&quot; back & forth over the actin surface.<br><br>Strong binding of the ADP-Pi-bound myosin heads to actin triggers a conformation change in myosin so that Pi is released and simultaneously causing the thick and the thin filament to slide past each other, and the replacement of ADP by ATP. Binding of ATP causes the ATP-bound myosin heads to detach from actin; and a ATP to ADP-Pi equilibrium establishes on the molecular complex keeping this ADP-Pi-myosin in a energized state ready for next contraction. View
T Cell Receptor Signaling 1182 T-cells are a type of white blood cells that are essential for cell-mediated immunity. They function to destroy virally infected cells and tumor cells and are central players for immune response and immune tolerance. T-cell receptor (TCR) plays a pivotal role in the generation, maturation, activation and survival of the T-cells. T-cell activation requires not only the interaction between TCR molecules and specific MHC/antigen complexes on antigen presenting cells, but also the co-stimulatory signals provided by CD28 and B7 interaction.<br><br>TCR consists of several molecules including the alpha/beta antigen receptor that binds the MHC (major histocompatibility complex) and the CD3 complex that populate signals into the cell. Upon antigen engagement, one of the first events is activation of Lck, a src family tyrosine kinase. Lck can be positively regulated by CD45 and negatively regulated by Csk. Lck activation leads to phosphorylation and activation of key substrates such as CD3. ZAP-70 is subsequently, recruited and propagates signal transduction through the phosphorylation of downstream targets including the adapter molecules LAT and SLP-76. These adapters, in turn, lead to activation of phospholipase C-gamma 1 (PLC-gamma1) and generation of second messengers such as IP3 and DAG. IP3 triggers calcium mobilization, which leads to activation of nuclear factor of activated T cells (NF-AT). DAG activates protein kinase C-theta which in turn leads to activation of the nuclear factor kappa-B (NF-kB). TCR can also lead to activation of other signaling cascade such as PI3K, Ras/MAPK, and JNK pathways. TCR signaling plays an important role in immune response, cytoskeleton reorganization, cell proliferation, survival and activate target genes such as IL-2, 3, 4 etc. View
Calcium Signaling in Nonexcitable Cells 1182 Calcium signaling in non-excitable cells plays a central role in regulating many physiological processes such as proliferation, differentiation, hormone release, and apoptosis. The signaling system depicted in this pathway is also present in muscle cells, but is not the Ca++ signaling system that regulates muscle contraction.<br><br>Agonist binding to cell surface G-protein coupled receptors (Gq & Gi) or tyrosine kinase receptors activates phospholipase C beta (GPCR) or gamma (tyrosine kinase receptors) which hydrolyzes membrane phosphatidyl-inositol 4,5-bisphosphate (PIP2) and generates two second messengers, inositol 1,4,5-triphosphate (IP3) and 1,2-diacylglycerol (DAG). Binding of IP3 to IP3R in the endoplasmic reticulum (ER, intracellular Ca++ store) releases Ca++ bound to junctate associated with IP3R through the IP3R ion channel to the cytoplasm and generate the initial wave of Ca++ signal. Since free [Ca++] in the ER lumen is estimated to be 500 - 600 uM while junctate binds Ca++ with an affinity of 217 um, thus junctate is fully loaded with Ca++ at rest. Calsequestrin is a local sensor of ER luminal [Ca++] and inhibits the IP3R channel at low ER [Ca++]. Depletion of store Ca++ triggers the opening of store operated cation channels (SOCC, capacitive Ca++ entry) on the plasma membrane resulting in rapid increase of intracellular Ca++. The store is replenished by the pumping action of SERCA. Within the ER lumen, Ca++ is buffered by binding to calreticulin. DAG interacts directly with receptor operated cation channels (ROCC) to affect Ca++ influx across the plasma membrane. Phosphorylation of PIP2 by activated PI3K produces PI3P which also interacts (at a different site) directly with ROCC to induce Ca++ influx. Ca++ influx from the TRPC (SOCC & ROCC, transient receptor potential channels) are slow thus maintaining an increased level of [Ca++]i for signal propagation through Ca++-regulated proteins.<br><br>The effect of cytosolic [Ca++] on mitochondrial function and apoptosis is detailed in &quot;Ca++ Signaling, Cytochrome c Release, & Apoptosis&quot; View
SREBP, Regulation of Fatty Acids & Cholesterol Biosynthesis 1182 The SREBPs (sterol regulatory element binding proteins) are synthesized as inactive pre-proteins which immediately after leaving the nucleus are captured by SCAP (SREBP cleavage active protein) and escorted to the endoplasmic reticulum (ER). The SCAP-SREBP complexes are held in the smooth ER, in the presence of high cellular sterol concentration, through binding between SCAP and the Insigs (insulin signaling proteins). When the cellular sterol is depleted or when Insig protein concentration drops, SCAP-SREBP migrate to rough ER and is processed by two sequential protease cleavages to give the activated SREBP which then migrate into the nucleus and participate in the regulation of genes containing sterol response element (SRE), including themselves. Thus the transcriptional effects of activated SREBPs are amplified until cellular conditions are changed so that they are held in the inactive form.<br><br>Cholesterol biosynthesis appears to be exclusively controlled by SREBP-2 which induces all 12 enzymes of the cholesterol biosynthesis pathway. SREBP-2 also induces the synthesis of key fatty acid biosynthesis genes, ApoA-II and the LDL-receptor. SREBP-1c primarily regulates fatty acid biosynthesis and appears to mediate all effects of insulin on lipogenesis. The promoter region of SREBP-1c contains response elements for insulin, glucagon, and LXR, while the induction of SREBP-1c expression by LXR is tempered by ligand activated PPARalpha (see &quot;Regulation of SREBP&quot; pathway). Fatty acid biosynthesis is coordinately regulated with lipogenesis, triacylglycerol and phospholipid synthesis, and is not regulated by SREBP-1c along. The expression of microsomal triglyceride transfer protein (MTP, facilitates the translocation & folding of Apo-B, and the lipid loading & secretion of Apo-B containing lipoproteins) is repressed by both SREBP-2 and SREBP-1c. View
Ubiquitin-Proteasome Pathway & Protein Degradation 1182 The Ubiquitin-proteasome pathway is the principle cellular proteolytic mechanism for recognizing and destroying misfolded, oxidized, or damaged proteins. It also plays a key role in controlling the levels of many regulatory proteins that are involved in cell cycle, transcriptional activation, apoptosis and cell signaling. Degradation of a protein by the ubiquitin-proteasome pathway involves 2 successive steps, 1) covalent attachment of multiple ubiquitin molecules to the target protein, and 2) degradation of the protein by the 26S proteasome. Ubiquitin conjugation is further divided into a three-step mechanism. First, ubiquitin is activated at C-terminal by ubiquitin-activating enzyme, E1. Secondly, the ubiquitin-carrier protein, E2, transfers ubiquitin from E1 to ubiquitin-protein ligase E3. E3 catalyzes the final step of the conjugation process by attaching more ubiquitin to the substrate protein to generate a substrate-anchored poly-ubiquitin chain which is the recognition signal for the 26S proteasome complex. <br><br>Proteasome is a large multi catalytic protease that degrades polyubiquitinated proteins into small peptides. It is composed of a 20S barrel-shaped core particle that has the catalytic activity and two19S regulatory complexes that cap each ends of the catalytic barrel and facilitate recognition of ubiquitinated proteins. Following protein degradation, ubiquitin molecules are recycled by the ubiquitin recycling enzymes and reused. <br><br>Rapid and irreversible protein degradation is critical to the activation or repression of a wide variety of cellular processes, including cell cycle progression, apoptosis, and lipid homeostasis. Aberrations in the highly regulated ubiquitin-proteasome system have been implicated in the pathogenesis of several important human diseases. Several proteasome inhibitors have been studied and have been shown to sensitize cells to apoptosis. Bortezomib (Velcade, PS-341), indicated for multiple myeloma, is the first FDA approved proteasome inhibitor. View
Insulin & Lipid Homeostasis 1182 Hepatic GLUT2 is a high Km glucose transporter which equilibrates the glucose concentration between the portal vein and the hepatocytes. Glucose taken into the hepatocyte is rapidly converted to glucose-6-phosphate (G6P) via glucokinase (GCK) which is not inhibited by G6P but is reciprocally regulated by the cellular concentration of fructose-1-phosphate and fructose-6-phosphate (see &quot;Regulation of Glucose Utilization&quot;). Since glucose in hepatocyte is rapidly converted to G6P and metabolized (glycolysis), portal vein glucose is taken up into the hepatocytes rapidly. Glucose flux into the pancreatic beta-cells induces insulin release into the circulation (see &quot;Pancreatic beta-Cell Function & Dysfunction&quot;). <br><br>Insulin signaling in hepatocyte activates and induces SREBP-1c which induces lipogenic enzymes to hasten conversion of pyruvate derived from glycolysis to triglyceride for energy storage. SREBP-1c also induces GCK and GLUT2 to increase the capacity of glucose handling. Concurrent to insulin release, glucagon is released from pancreatic alpha-cells and functions as a repressor of hepatic gluconeogenesis and inducer of glycogen synthesis.<br><br>GLUT4 is the major glucose uptake transporter in muscle and adipose tissue. Insulin stimulates glucose uptake in these tissues by increasing the GLUT4 exocytosis rate, effectively recruiting up to 50% of the transporter to cell surfaces. Hexokinase II (HKII, inhibited by G6P) is the major hexokinases that convert glucose to G6P in muscle and adipose tissue. G6K is converted to glycogen (stimulated by insulin) or triglyceride (stimulated by SREBP-1c) and released as energy source when needed. Insulin receptor-signaling both activates and induces HKII. Fatty acid is the sustaining energy source for both cardiac and skeletal muscles. Muscle cells up-take fatty acids from circulating serum via lipoprotein lipase (LPL). Excess fatty acids and triglyceride stored in skeletal muscle inhibits insulin signaling.<br><br>In adipose tissue, insulin receptor signaling, in addition to increase glucose uptake, glycolysis, and lipogenesis, directly stimulates adipocyte differentiation (see &quot;LXR & Adipocyte Differentiation&quot;). SREBP-1c signaling in adipocytes also induces the synthesis of cell surface transporters (LDLR & SR-BI) to take up circulating lipoproteins. View
ChREBP & Regulation of Carbohydrate Metabolism 1182 ChREBP (carbohydrate response element binding protein) is activated in response to high glucose concentration in liver independent of insulin. When there is excess glucose-6-phosphate (G6P) or when cellular NADP+ increases (under conditions of oxidative stress), the pentose pathway is activated. Xylulose-5-phosphate (Xu5P) is a key intermediate of the pentose pathway and acts as an insulin independent glucose sensor. Xu5P signals by activating protein phosphatase 2A (PP2A), which then activate ChREBP and a number of enzymes; resulting in increased rate of glycolysis and fatty acid synthesis. Exactly how Xu5P activate PP2A at the molecular level is not well understood.<br><br>The expression of ChREBP is ubiquitous, but most highly expressed in liver, adipose, kidney, muscle, and small intestine. In the absence of Xu5P, ChREBP resides in the cytosol. Upon dephosphorylation (Ser-196) by activated PP2A, ChREBP translocates to the nucleus where it is dephosphorylated (Thr-666) by another Xu5P activated PP2 and associates with its heterodimer partner MIx (Mas-like protein X) to induce gene transcription through the carbohydrate-response element (ChORE). ChORE was found on the promoter of LPK, AAC, FAS, S14 & ACL (ATP citrate lyase). ChREBP also up-regulate malic enzyme, stearoyl-CoA desaturase-1 (SCD-1), long-chain fatty acyl elongase (LCE), & Glut2. View
Pancreatic beta-Cell Function & Dysfunction 1182 Glucose influx into the pancreatic beta-cells after ingestion of carbohydrate is mediated by GLUT2 (high-Km glucose transporter, Km = 17) and glucokinase (GCK, high-Km hexokinase). The ATP produced directly from the glycolysis of glucose-6-phosphate, and those produced from oxidative phosphorylation of NADH derived from both glycolysis (shuttled into the mitochondria) and metabolism of pyruvate in the TCA cycle, work in concert to increase the beta-cell [ATP]/[ADT] ratio. The pancreatic K(ATP) channel is an octameric ATP-sensitive K+ channel formed from 4 inwardly rectifying K+ channel (Kir6.0) and 4 sulfonylurea receptor (SUR). The tetrameric pore formed from Kir6.0 contains the site for ATP inhibition while the SUR subunits contain sites for Mg-ADP activation and modulation by pharmacologically active agents such as the sulfonylureas (inhibition) and the diazoxide (activation). Under normal physiological conditions, increased [ATP]i closes the K(ATP) channel leading to depolarization of the plasma membrane which triggers opening of the voltage gated Ca++ channel leading to increased [Ca++]i which then stimulate insulin release by exocytosis. Exocytosis of insulin granules further increases glucokinase activity in the beta-cells leading to further increase in [ATP]/[ADP] ratio and further insulin release.<br><br>Insulin receptor signaling in beta-cells, similar to that observed in hepatocytes, induces and activates SREBP-1c. Over-expression of SREBP-1c in rat pancreatic islets induces lipogenesis genes (FAS, ACC, & PPARgamma), impairs glucose stimulated insulin secretion (GLUT2 & GCK repression, UCP-2 induction), impairs beta-cell growth and differentiation (cdk4 & IRS-1 suppression, p21 induction), and promote beta-cell apoptosis (APO-1, BAD & BAX induction). Beta-cells appear to adapt to insulin resistance by repressing the activity of PDH and increasing the activity of pyruvate carboxylase thus increase the flux of pyruvate through the TCA cycle to produce more ATP and further increase the [ATP]/[ADP] ratio to promote insulin release.<br><br>LDL and vLDL were found to promote beta-cell apoptosis and suppress preproinsulin mRNA and beta-cell proliferation. View
Oxidative Stress 1182 Oxidative stress arises from an imbalance between the generation and the elimination of reactive oxygen species (ROS). ROS has the capacity to oxidize and damage a variety of cellular components including lipids, DNA and proteins, and often lead to cell death. There is strong evidence that oxidant stress plays a major role in the pathogenesis of many human diseases, such as cancer, Alzheimer's disease, Parkinson's disease, and aging.<br><br>ROS are either free radicals, reactive anions that contain oxygen atoms or molecules that contain oxygen atoms that can either produce free radicals or activated by them. Examples of ROS include superoxide anion (O2-), hydrogen peroxide (H2O2) and hydroxyl radicals (OH.). ROS can be generated in vivo by aerobic respiration or produced by peroxisomal fatty acids oxidation, xenobiotic metabolism, inflammatory cytokines, ER stress, radiation and aging. Under normal conditions, ROS are cleared from the cell by the action of antioxidant defense system. Antioxidant enzymes, SOD, glutathione peroxidase (GPx), and catalase, directly interact with ROS for its elimination. Glutathione reductase is a support enzyme that maintains cellular levels of reduced glutathione and NADPH, which are necessary cofactors for the function of SOD and GPx.<br><br>When excessive production of ROS overwhelms the antioxidant defense system or when there is a significant decrease or lack of antioxidant defenses, oxidative stress occurs. Oxidative stress influences a variety of cellular process through modulating gene transcription. ROS can activate gene transcription via 1) transcription factors (such as NF-kB, STAT3, AP-1 and ARE-binding proteins, or 2) activation of the MAP kinase cascades. Both of which activate transcription factors that trigger transcriptional up-regulation of genes involved in inflammation and/or fibrogenesis, including cytokines (IL-1, TNF-a, IL6) chemokines (IL-8), adhesion molecules (VCAM-1, ICAM-1) and growth factors (GM-CSF). View
LXR, FXR & Cholesterol Homeostasis 1182 HDL promotes reverse cholesterol transport by facilitating transfer of cholesterol from peripheral tissues to the liver for disposal. HDL induces cholesterol efflux from peripheral cells via ABCA1 by two different mechanisms: 1) efflux of free cholesterol via ABCA1 to cell surface and &quot;picked up&quot; directly by Lipid poor HDL particles (or ApoA-I) that can interact efficiently with the cell surface, and 2) desorption of effluxed free cholesterol on cell surface by the lipid rich HDL particles and esterification and internalization of free cholesterol into HDL by LCAT (lecithin-cholesterol acyltransferase). ABCA1 is widely expressed, but most abundant in macrophages.<br><br>SR-BI (scavenger receptor class B type I) binds HDL with high affinity and is structurally dissimilar to ABCA1. SR-BI mediates both the selective uptake of cholesteryl esters from lipid-rich HDL by cells, and the efflux of free cholesterol from cells to HDL. SR-BI is expressed at high levels in liver and steroidogenic tissues. SR-BI on the cell surface of hepatocytes takes up free cholesterol and metabolizes it to bile acid for excretion.<br><br>HDL and VLDL particles are assembled in the liver (hepatocyte) under the regulation of LXR and FXR respectively. Once released into the circulation, HDL particles pick up cholesterol from peripheral tissues and deliver cholesterol to steroidogenic tissues and return the excess cholesterol load back to liver. VLDL is very rich in phospholipids and delivers fatty acids to peripheral tissues. In circulation, HDL transfer cholesterol esters to VLDL and accept phospholipids and lipoproteins from VLDL so that eventually there is a LDL population.<br><br>See description of &quot;Transportation of Lipids&quot; pathway for the synthesis of chylomicrons in the intestine from dietary lipids, and more details on the conversion of VLDL to LDL. View
Phospholipid Biosynthesis & Remodeling 1182 Phosphatidic acid (PtdOH, diacylglycerol-3-phosphate) is an intermediate of the biosynthesis of both the triacylglycerols (lipid droplets in adipocytes and some other cells) and glycerolphospholipids (building blocks of cell membrane). It can also be synthesized from existing phospholipids and triacylglycerol through action of the various lipases as depicted in the pathway drawing. PtdOH can enter phospholipids biosynthesis through CTP-dependent activation catalyzed by CDP-diacylglycerol synthase (CDS). CDP-diacylglycerol is a direct precursor of phosphatidylinositol (PtdIns), phosphatidylglycerol phosphate, and cardiolipin (&quot;Cardiolipin & Mitochondrial Function&quot;).<br><br>PtdOH can also be de-phosphorylated to give diacylglycerol which serves as precursor for the synthesis of phosphatidyl-ethanolamine (PtdEtn) and phosphatidyl-choline (PtdCho) through the CDP-ethanolamine and the CDP-choline routes. PtdCho can also be synthesized from PtdEtn by sequential addition of three methyl groups via the action of PtdEtn N-methyltransferase (PEMT). In mammalian cells, phosphatidylserine is synthesized from PtdEtn from PtdCho by head-group exchange. View
O-Linked GlcNAc & Insulin Resistance 1182 Once glucose enters a cell, it is rapidly converted to glucose-6-phosphate (G6P) which can be converted to glucose-1-phosphate for glycogen synthesis, or converted to fructose-6-phosphate (F6P) for glycolysis. A small fraction of F6P is converted to glucosamine via the action of glutamine fructose-6-phosphate-aminotransferase (GFAT). The activity of GFAT is modulated by glucagon signaling (phosphorylation by PKA) in a manner similar to that of the activity of glycogen synthase (see &quot;Glycogen Synthesis & Glycogenolysis&quot;). Glucosamine is rapidly converted to uridin-diphospho N-acetylglucosamine (UDP-GlcNAc) which serves as a high energy donor sugar nucleotide for O-glycosylation of cellular proteins. N-acetylglucosamine is a nutrient sensor and mediator of insulin resistance.<br><br>Glycoproteins bearing O-GlcNAc on Ser/Thr residues are abundant in mammalian cell nucleus and cytosol, including components of the transcription machinery. Protein O-GlcNAc glycosylation was found to be dynamic and reversible, and the Ser/Thr residues that undergo O-GlcNAc glycosylation are the same ones that undergo O-phosphorylation. There appears to be only one (and essential) O-GlcNAc transferase (OGT) in mammalian systems; it catalyses the formation of beta O-GlcNAc-glycosides and its activity is very sensitive and responsive to a light shift in UDP-GlcNAc concentration. Removal of the beta O-GlcNAc unit is accomplished by O-Glc_NAcase (previously known as hyaluronidase). It has been proposed that OGT is regulated by a multitude of binding partners similar to phosphatases and has been co-purified with Ser/Thr phosphatases.<br><br>Glucosamine has been used to induce insulin resistance in cell cultures, tissues, experimental animals and humans. Elevated levels of UDP-GlcNAc have been associated, in cell cultures, animal models, and type-2 diabetic patient with hyperglycemia and hyperinsulinemia. O-GlcNAc-glycosylation of insulin receptor substrates (IRS), GSK-3, Akt, eNOS, etc, work in concert to produce the clinically observed insulin resistance, including, hyperglycemia, declined pancreatic beta-cell function (insulin signaling transcriptional induces insulin production), atherosclerosis (NO plays an important role in preventing vascular diseases), hypertension, etc.<br><br> View
Hepatic Cholestasis 1182 Cholestasis is an arrest of the normal bile flow resulting in elevated bilirubin in the blood stream (jaundice). Cholestasis could be due to blockage of the bile ducts or intra-hepatic signaling events that lead to reduced amount of functional bile salt transport pumps. In general, there is a common pattern of responses that serves to partially protect the hepatocytes from toxic bile salts by reducing the amount of bile salt uptake (down regulation of NTCP & OATP1, and up-regulation of Mrp1) while maintaining bile salt export (BSEP). In prolonged/progressive cholestasis, the canalicular Mrp2 pump is down regulated (IL-1 mediated) and a compensatory up-regulation of Mrp3 pump occurs to facilitate efflux of substances into the sinusoidal blood that normally would have been extruded into the bile. MDR1 pumps which efflux xenobiotics & their metabolites, but do not extrude bile salts, are up-regulated in drug induced cholestasis. <br><br>Excess bile acids (in particular chenodeoxycholic acid) in the hepatocyte activate FXR signaling which leads to the repression of NTCP (via SHP & RASR), and induction of BSEP gene expression. Activation of FXR also results in decreased LXR signaling (via SHP & LRH-1) which, in turn, reduce the amount of hepatic bile acid synthesis from cholesterol. Repression of NTCP can also occur through endotoxin and cytokine signaling via the TLR4 (see &quot;LPS & IL-1 Mediated Inhibition of RXR Function&quot;) signaling cascade. Lithocholic acid (synthesized by gut bacteria and recirculated back to the liver) and many drugs & xenobiotics activate PXR signaling resulting in increased expression of MRD1 & OATP leading to increased bile flow to facilitate extruding the offending molecules from hepatocytes. Expression of the majority of the transporters mediating bile flow, are regulated by nuclear receptors that partner with RXRalpha and are subject to JNK mediated degradation of the RXRalpha protein. This JNK mediated effect is prominent during inflammatory and acute phase responses (see &quot;LPS & IL-1 Mediated Inhibition of RXR Function&quot;). View
Integrin Signaling 1182 Integrins are transmembrane glycoproteins consists of non-covalently linked alpha and beta subunits. They are the main receptors for the extracellular matrix (ECM) proteins (e.g. fibronectin, collagens, laminin, etc.) and are thus essential for cell adhesion, migration, growth, and survival. Upon ligand binding, integrins become clustered at cell membrane and transduce signals leading to the activation of intracellular signaling pathways. They promote assembly of actin filaments and stress fiber formation, leading to cytoskeleton remodeling, and exert a stringent control on cell survival and cell proliferation. Integrin signaling regulates diverse cellular processes and has been implicated in many pathological processes including angiogenesis, cancer and arthritis.<br><br>Integrin's cytoplasmic tail can bind to -actinin, talin, and filamin, which then recruit actin-binding proteins such as vinculin to connect adhesion complexes with actin-cytoskeleton. Integrin also regulate Rho family of GTPases, which control the actin dynamics and stress fiber formation. Activation of Rho and Rac are important for cell contractibility and migration through regulation of myosin light-chain kinase (MLCK) which phosphorylates myosin light chains (MLC). Similar to receptor tyrosine kinases (RTK), integrin can activate Ras GTPase, which activate MAPK signaling pathways and promote cell cycle progression. Finally, Integrins contribute to the governing of cell survival through activation of the PI3K-Akt pathway. View
Cardiac Hypertrophy & Cell Death 1182 The rhythm of the heart is generated and regulated by pacemaker cells within the sinoatrial (SA) node located within the wall of the right atrium. The action-potentials generated by the pacemaker cells spread, by cell-to-cell conduction, to myocytes throughout different sections of the heart in a well defined sequence and route, thereby maintaining the pumping function of the heart. The cell-to-cell conduction process is strongly influenced by sympathetic adrenergic nerves which release norepinephrine (NE) that bind preferentially to adrenoceptor beta-1 in the heart resulting in increased heart rate (by increasing intracellular [Ca++], thereby conduction velocity). Circulating epinephrine released by the adrenal medulla binds preferentially to adrenoceptor-alpha-1 in the blood vessel wall and cause vasoconstriction. Physiological conditions that leads to increased sympathetic nerve output (NE), increased rennin release (leads to increased circulating angiotensin II), and increased vassopresin release, and will collectively increase conduction (increased heart rate) and blood volume (increased pressure). <br><br>Persistent increase in conduction and blood volume, as in hypertension, results in hypertrophic adaptation of the ventricular chamber wall in order to handle the increased load. This response is initially mediated by the increased intra-cellular Ca++ concentration (via NF-AT, & c-jun mediated transcription) and, in the long run, by angiotensin-II signaling via the AT-I receptor on cardiac fibroblasts (see &quot;Angiotensin II & Cardiac Hypertrophy&quot;). Persistent activation of the adrenoceptor beta of cultured rat cardiomyocytes leads to apoptosis (likely due to persistent elevation of intracellular [Ca++], see &quot;Ca++ Signaling, Cytochrome c Release, & Apoptosis&quot;). NAB1 (a transcriptional repressor), highly expressed in mammalian cardiac myocytes, is an endogenous regulator of cardiac growth through the repression of Egr1 (early growth response 1). Egr1 is broadly expressed in many tissues and is up-regulated in heart under conditions of cardiac stress and mediate the expression of many genes known to be up-regulated in cardiac hypertrophy. NAB1 is also induced under conditions of cardiac hypertrophy and was proposed to represent an endogenous feedback loop to limit the effect of Egr1 activation.<br><br>5HT2B signaling (G-protein Gq) was demonstrated to protect against cardiomyocyte apoptosis in cultured cardiomyocytes and 5-HT2B receptor knockout mice (via Nf-kB signaling mediated repression of ANT-1, a key component of the mitochondrial permeability transition pore). Thus, persistent elevation of NE, on the one hand, leads to persistently high cardiomyocyte cellular [Ca++] (through G-protein Gs signaling) that can lead to an adaptive cardiac hypertrophy, and on the other hand leads to increased angiotensin-II release and G-protein Gq signaling and protect cardiomyocyte from apoptosis.<br><br> View
Arachidonic Acid Release 1182 The first step in the biosynthesis of the prostaglandins and leukotrienes is the release of arachidonic acid (AA) from cell membrane phospholipids by the action of phospholipase A2. Extracellular stimuli such as cytokines, hormones, neurotransmitters, mitogens, and endotoxins triggers the release of AA residues attached to sn-2 positions of membrane phospholipids. Three modes of AA release have been described as detailed below.<br><br>Immediately upon exposure to external stimuli that trigger increased [Ca++]i, cytosolic PLA2alpha (cPLA2alpha, Type IV PLA2alpha) translocates to the nuclear envlope and the perinuclear membranes to hydrolyze and release AA into the cytoplasm. AA released via this mode is associated with COX-1 mediated prostaglandin synthesis. The lysophosphatidylcholine formed during the AA release process, under certain conditions, is converted to platelet activating factor (PAF). PAF can activate previously activated (primed) cPLA2alpha to release AA which is then converted by co-localized COX-2 to the prostaglandins. cPLA2alpha is highly selective for glycerolphospholipids containing AA at the sn-2 position<br><br>Sustained AA release in response to pro-inflammatory stimuli is associated with increased expression of secreted phospholipases A2 (sPLA2-V, sPLA2-IIA) that can release AA both prior and after secretion. Activation of cPLA2 is required for the up-regulation of sPLA2 by a yet to be defined mechanism (appear to involve chemokines such as MIP-2). AA released by sPLA2 is converted to the prostaglandins or leukotrienes by co-localized COX-2 or 5-LO, respectively. View
Thyroid Hormone Synthesis, Regulation & Release 1182 Thyroid hormones, triiodothyronine (T3) and thyroxin (T4), are key regulators of metabolism and development. T3 and T4 are synthesized from thyroglobulin (Tg), a large glycoprotein containing 115 tyrosine residues that is synthesized in the thyroid follicle cells, and iodinated via the action of thyroperoxidase, and secreted into the follicle lumen. Approximately 20% of the tyrosine residues on Tg are iodinated at the 3 or both the 3 and 5 positions, and 2 iodinated tyrosine units are coupled while remaining on the Tg peptide chain. The iodinated Tg forms a large colloidal store of thyroid hormone in the follicle lumen of the thyroid gland with little turnover of this hormone precursor in the thyroid tissue. Iodinated Tg is taken up in vesicles by the follicle cells and fuses with lysosomes where lysosomal proteases degrade Tg releasing amino acids, T3 and T4. T3 and T4 are secreted by the thyrocytes into the circulation, bound to thyroxin-binding globulin (Tbg, synthesized in the liver) and transported throughout the body.<br><br>T4 is the main product of thyroid secretion while the active hormone T3 is produced locally in peripheral tissues from T4 by the action of thyroxin deiodinase (Dio). The Action of T3 is initiated by the thyroid hormone receptors (TR) which can form homodimers or heterodimerize with RXR to regulate transcription of cognate genes. Besides their ligand-dependent properties, TRs function as ligand-independent silencers on positive TREs (TR response elements) and as ligand-independent activators on negative TREs. Ligand free TRs achieve transcriptional repression by recruiting co-repressors (NCoR and SMART) and histone deacetylases to positive TREs, and achieve transcriptional activation by recruiting NCoR and GAGA-binding factor (GAF) to negative TREs. <br><br>The production of Tg is controlled by serum thyrotropin (TSH, thyroid stimulating hormone) synthesized and released by the anterior pituitary gland in response to TSH-releasing hormone (TRH) which is secreted by the hypothalamus. T3 signaling through the beta-2 receptor exerts a negative feedback on the synthesis and release of TSH and TRH.<br><br>The sodium/iodide symporter (NIS) is an integral plasma membrane glycoprotein located in the basolateral membrane of thyroid follicle cells and mediates transport of iodide ions from the blood stream to the Tg colloid in the follicle lumen. NIS also mediates active iodide transport in salivary glands, gastric mucosa and lactating mammary gland. View
Matrix Metalloproteinases (MMP) & Their Regulation 1182 MMPs are a family of zinc-dependent endopeptidases capable of degrading essentially all components of extracellular matrix (ECM) and many other extracellular proteins. Their substrates include other proteinases, proteinase inhibitors, collagens, clotting factors, chemotactic molecules, cell surfaces receptors and adhesion molecules. According to their substrate specificity and domain structure, members of the MMP family can be classified into four main subgroups, collagenases, stromelysins, gelatinases, and membrane-type MMPs. MMPs play important roles in a number of physiological and pathological processes, such as cancer invasion and metastasis, development of arthritis, angiogenesis, atherosclerosis and aneurysms.<br><br>MMP transcripts are normally maintained at low levels, however, their levels increase dramatically when tissues undergo remodeling, such as wound healing, inflammation and cancer. MMP expression can also be induced by a variety of growth factors, hormones, inflammatory cytokines, and oncogenes.<br><br>MMP activity is tightly controlled at least at three levels, 1) gene transcription, 2) proteolytic activation of the zymogen form, 3) and inhibition of the active enzyme by general protease inhibitors such as alpha-2-macroglobulin, and tissue inhibitors of metalloproteinase (TIMPs). TIMPs are specialized MMP inhibitors that act by binding to active site of MMP at 1:1 molar ratio to prevent MMP enzymatic activity. Currently 4 TIMPs have been identified (TIMP-1, -2, -3, and -4). RECK, a membrane-anchored MMP inhibitor, inhibits MMP-2,-9 and MT1-MMP. Most of the MMPs are synthesized as latent precursors (zymogens) that are secreted and proteolytically activated in the extracellular space. The exceptions are MMP-11 and MT1-MMP, which are activated by Golgi-associated proteases prior to secretion. Due to the important roles of MMP in cancer invasion and metastasis and other pathological conditions, extensive research efforts were devoted to developing small molecule MMP inhibitors (MPI) as therapeutic strategy. These inhibitors include Marimastat, Batimastat, Metastat, AG-3340 etc. Many are in various stages of clinical trials. <br><br>Of the growth factors that promote ECM production, TGF-beta1 is the most important. This is primarily achieved through modulating plasmin, a protease that activates MMPs from their latent precursors, through the plasminogen-plasmin pathway. It has been shown that TGF-beta can down-regulate urokinase-type plasminogen activator (uPA) activity while up-regulating PA inhibitor (PAI)-1 expression, which in turn inhibit plasmin activity and is a key mechanism for the observed inhibition of tumor growth by TGF-beta in vivo. View
Arachidonic Acid Biosynthesis & Phospholipid Remodeling 1182 Arachidonic acid (AA) is the common precursor of the eicosanoids (prostaglandins, leukotrienes, 5-oxo-eicosatetraenoic acid, and lipoxins). AA is synthesized from lenoleic acid (18:2n-6) via three consecutive steps: desaturation via delta-6 fatty acid desaturase, 2-carbon chain-elongation at C1, and desaturation via delta-5 fatty acid desaturase. Once synthesized, AA is efficiently converted to arachidonyl-CoA and inserted into the sn-2 position of lysophosphatidylcholine. <br><br>AA is introduced into existing phosphatidylethanolamine (PtdEtn, see &quot;Phospholipid Biosynthesis & Remodeling&quot;) via a remodeling process where the sn-2 fatty acid on existing PtdEtn is removed by the action of Ca++-independent phospholipase A2 (iPLA2) and is re-acylated with AA from arachidonyl-phosphatidylcholine via the action of a Co-A independent transferase (some literature evidence [PMID: 15944408] suggest this is cPLAgamma functioning both as lipase and trans-acylase). AA is introduced into phosphatidylserine and phosphatidylinositol via a similar remodeling process, but using the AA from arachidonyl-PtdEtn. View
Bile Acid & Cholesterol Metabolism 1182 Bile acids are physiological detergents that facilitate absorption and transport of lipids & lipid soluble vitamins, and excretion of lipids and cholesterol. About 500 mg of cholesterol is converted into bile acid each day in the adult human liver and ~95% of the bile acids secreted by the liver is reabsorbed in the intestine during each cycle of enterohepatic circulation and reused. The 5% lost is replaced by new synthesis in the liver. This 500 mg/day usage accounts for ~90% of the cholesterol that is actively metabolized in the body, while steroid hormone biosynthesis accounts for the remaining 10%<br><br>Bile acids are reabsorbed from the intestine via the ileal apical sodium-dependent bile salt transporter (ISBT, IBAT, ASBT, Slc10a2) and the sodium independent bile salt transporter OATP3. ISBT and OATP3 have transport characteristic similar to NTCP and OATP1/2, respectively, located on the basolateral membrane of hepatocytes. Once transported across the apical membrane, bile acid is transported across the cell to the basolateral membrane via I-BABP and effluxed into enterohepatic circulation via MRP3. Bile salts are then taken up by the hepatocytes via NTCP and OATP1/2.<br><br>Excess cholesterol in peripheral tissues is effluxed via ABCA1 and picked up by circulating HDL particles and transported back to the liver (&quot;LXR, FXR & Cholesterol Homeostasis&quot;). SR-BI is primarily expressed in the liver and steroidogenic tissues, and mediates HDL & Cholesterol ester (CE) uptake. Inside the hepatocyte, CE is converted by CEH to free cholesterol and become part of the cholesterol pool that participates in the regulation of LXR, and SREBP as well as been converted to bile acid and become part of the bile acid pool. Expression of CYP8B is up-regulated (by SREBP-1c) when hepatic cholesterol is low so that more of the less polar bile acid, chenodeoxycholic acid (CDCA), is synthesized, resulting in higher bile acid reabsorption from the intestine. Higher level of CDCA, the most effective activator of FXR among all of the bile acids, also leads to FXR activation and resulting in reduced bile acid synthesis. Intestinal bacteria convert CDCA to lithocholic acid, which is toxic and is a strong activator of PXR, resulting in increased expression of PXR cognate genes and increased excretion of the less polar bile acids and xenobiotics. View
Regulation of Fatty Acid Biosynthesis 1182 The central enzyme in lipogenesis is the fatty acid (FA) synthase (FAS) which catalyzes all of the steps in the conversion of malonyl-CoA to palmitic acid (See &quot;Fatty Acid Biosynthesis & its Regulation&quot;). FAS activity is regulated primarily at the expression level by SREBP-1c, SREBP-2, and ChREBP. SREBP-1c is activated by insulin and induces its own transcription once activated. ChREBP is activated when the pentose pathway is activated under conditions of oxidative stress and hyperglycemia (see &quot;Pentose Pathway & Hyperglycemia&quot;). Thus both high portal glucose and oxidative stressed mediated by Nrf2 (such as oxidative stress mediated by induction of drug metabolism enzymes) increase lipogenesis and triglyceride accumulation in hepatocytes. <br><br>SREBP-1c is the primary regulator of FA synthesis and mediates the transcription of a large number of enzymes involved in the production and availability of FAS substrates acetyl-CoA and malonyl-CoA, as well as enzymes required for further processing of FAS product, palmitic acid, to triglycerides (see &quot;SREBP, Regulation of Fatty Acids & Cholesterol Biosynthesis&quot;). ChREBP is primarily concerned with intracellular glucose level and its effect on the transcription of lipogenic enzymes appears to be secondary and additive to that of SREBP-1c.<br><br>Exercise increases [AMP]/[ATP] ratio and activates AMPK leading to both reduced FA synthesis and FA beta-oxidation while switching the metabolic system to extract energy primarily form glycolysis (see &quot;AMP Kinase: A Metabolic Master Switch&quot;). View
Glycogen Synthesis & Glycogenolysis 1182 Glycogen molecules are branched chain polymers of glucose produced in the liver and skeletal muscle when there are excess of glucose, and released on demand as a ready source of energy. The major site of daily glucose consumption is the brain (75%), followed by erythrocytes, skeletal muscle, and heart muscle. Glycogens stored in muscles are for local use (muscle has no glucose-6-phosphatase which converts glucose-6-phosphate to free glucose) while glycogens stored in the liver is released as free glucose into the circulation and serves as the main buffer of blood glucose.<br><br> Alpha D-glucose-6-phosphate (alpha-G6P) is the form of glucose that exists in cells and tissues. Alpha-G6P can be converted to beta-D-fructose-6-phosphate to enter glycolysis or it can be converted to alpha-D-glucose-1-phosphate (alpha-G1P) and be stored as glycogen (see &quot;Regulation of Glucose Utilization&quot;). Alpha-G1P is also the form of glucose released from the glycogen stores upon glycogenolysis. For glycogen synthesis, alpha-G1P is activated by UGP2 to UDP-glucose and then attached to the non-reducing end of existing glycogen by the action of glycogen synthase (GYS). Glycogen synthesis is primarily controlled through the regulation of activity of GYS by phosphorylation and dephosphorylation. Insulin promotes glycogen synthesis through inhibiting GSK-3 and activating PPP1 via insulin receptor signaling. Adrenaline and acetylcholine promote glycogenolysis through G-protein receptor mediated Ca++ signaling. Glycogenin is a glycosyltransferase that serves as autocatalytic initiator for glycogen synthesis. During glycogenolysis, glucose units are sequentially removed and released as alpha-G1P by glycogen phosphorylase and the debranching enzyme (4 glucose units or less from the branching point). View
Platelet Activation 1182 Platelets are tightly regulated adhesion machineries that normally circulate in a quiescent state prevented from inappropriate activation by the presence of the endothelial cell monolayer, and the inhibitory effect of prostacyclin (PGI2) and nitric-oxide (NO). Upon vessel injury, von Willebrand factor (vWF) in circulating plasma becomes associated with exposed collagen and exposes its binding site for platelet membrane GPIb-IX-V. Once the platelet-vWF-collagen complex is formed, it activates 1) platelet integrin (GPIIb-IIa) to form a temporary clot, 2) the vitamin-K dependent coagulation cascade (see &quot;Vitamin-K & Coagulation&quot;), and 3) signals through the FcgammaRIIa receptor on the platelet surface to initiate both Ca++ and ADP mediated plate activation. Vessel injury also expose tissue factor on the sub-endothelial cells to circulating factor VII and initiate thrombin mediated coagulation. Thrombin also signal to initiate both Ca++ and ADP mediated plate activation. Activated platelets also release thromboxane A2 to further enhance the coagulation cascade. <br><br>ADP molecules are released from activated platelets and signal through P2Y12 and P2Y1 receptors to activate circulating platelets to eventually form an irreversible plug at the injured vessel wall. ADP signaling through P2Y12 receptor plays a central role in platelet activation, in the recruitment of other plates to the site of injury and the subsequent formation of a permanent plug. P2Y12 signals through G-protein-Gi resulting in the inhibition of adenylyl cyclase as well as activation of PI3K, & Akt leading to activation of fibrinogen receptor and platelet aggregation. P2Y12 receptor is also responsible for shear-induced platelet activation. View
Pentose Pathway & Hyperglycemia 1182 The pentose pathway is an anaerobic, non-energy-producing pathway. The pathway is turned on under conditions of oxidative stress or when there is excess of glucose. For each molecule of glucose going into the pathway, 2 molecules of NADPH and one molecule of xylulose-5-phosphate are produced. <br><br>NADPH is required for converting oxidized glutathione to its reduced form and is required for Xenobiotic detoxification, fatty acid & steroid biosynthesis. The [NADPH]/[NADP+] ratio of most cells is close to 10:1. The enzyme activity of Glucose 6-phosphate dehydrogenase (G6PD) is regulated by the availability of NADP+. During oxidative stress, NADPH is consumed and G6PD is activated immediately to take G6P into the pentose pathway and recycles 2 NADP+ to 2 NADPH for each committed G6P. G6PD deleted mouse embryonic stem cells were found to have [NADPH]/[NADP+] ratio ~1/2 that of wild type under normal culturing conditions and 15% that of the wild type when the cells were subjected to oxidative stress. Prolonged hypoxia increased the expression of G6PD (induced by ROS, see &quot;Oxidative Stress Response Mediated by Nrf2&quot;) although the time course of the induction is much slower than that of phosphoglycerate kinase 1 (PGK1, induced by HIF, see &quot;Hypoxia and HIF Signaling&quot;). PGK1 is the enzyme down stream of glyceraldehydes-3-phosphate dehydrogenase which is the only NADPH generating enzyme in aerobic glycolysis (see &quot;Glycolysis&quot; pathway).<br><br>Xylulose-5-phosphate, an intermediate of the pentose pathway down stream of NADPH production, is the glucose sensor that regulates glucose metabolism (via modulating the transcriptional activity of ChREBP) independent of insulin. The pentose pathway facilitates the removal of glucose from circulation by further promoting glycolysis & utilization of the glycolysis product (converting acetyl-CoA to fatty acid, see &quot;ChREBP & Regulation of Carbohydrate Metabolism&quot;) in addition to that promoted by insulin. When excess glyceraldehydes-3-phosphate can not be utilized, methylglyoxal is formed chemically leading to glycation of cellular proteins and diabetic complications. Excess fructose-6-phosphate reduce cellular glucose uptake (see &quot;Regulation of Glucose Utilization&quot;) and promote the formation of UDP-NAc-glucosamine (see &quot;O-Linked GlcNAc & Insulin Resistance&quot;) leading to O-GlcNAcylation of IRS (insulin receptor substrate) and insulin resistance. View
PPARalpha & Fatty Acid Metabolism 1182 PPARs are metabolic sensors of fatty acids and eicosanoids and regulate most of the pathways associated with lipid metabolism. PPARalpha regulates fatty acid oxidation in the liver, heart, muscle, and other tissues, while PPARgamma regulates fatty acid storage in adipose tissue. Food deprivation increases the hepatic expression and PPARalpha which in turn stimulate beta-oxidation of the fatty acids released from adipose tissue in order to maintain energy homeostasis.<br><br>Ligand activated PPARalpha induces the expression of proteins involved in the cellular uptake of fatty acids, the transport of cellular fatty acids to peroxisomes and mitochondrias, as well as, enzymes involved in both peroxisomal and mitochondrial beta-oxidation. Beta-oxidation of very long chain (> C20) fatty acids and fatty acid with beta-methyl groups from the diet, as well as, the deactivation of eicosanoids by beta-oxidation, occur only in the peroxisome. Mitochondrial beta-oxidation serves to provide most of the energy requirement during fasting. The inhibitory effect of ligand activated PPARalpha on HNF-4alpha results in reduced hepatic cellular uptake of fatty acid, as well as, reduced release of VLDL particles by the liver. View
Acute Phase Response 1182 Acute phase response is the physiological and metabolic changes that occur immediately after an infection or tissue injury in order to repair the damage, containing the offending organisms, or promote wound healing. Characteristic features of APR include fever, neutophilia, changes in lipid metabolism (suppression of pathways for cholesterol, fatty acid and phospholipid synthesis), hypoferremia, increased gluconeogenesis, increased protein catabolism, hormonal changes and induction of acute phase proteins. Tissue macrophages are most commonly used to initiate the acute phase responses through direct stimulation and secretion of various inflammatory cytokines such as TNFalpha, IL-1, IL-6, INFgamma etc. A number of proteins, called the acute phase proteins (APPs) are induced in the liver. These include positive APPs (upregulated) and negative APPs (downregulated) in response to infection or injury. The magnitude of the changes in APP varies from about 50% to more than 1000 folds. C-reactive protein (CRP) and serum amyloid A (SAA) are the major positive APPs that increase dramatically upon infection, while fibrinogen, complement components, PAI-1 etc., increase to a lesser extent. View
Steroid Hormone Biosynthesis 1182 While the steroid hormones have diverse physiological functions, their biosynthesis all begins with the conversion of cholesterol to pregnenolone. Cellular cholesterol resides in lipid droplets, outer mitochondrial membrane, and the plasma membranes of steroidogenic cells. For the first reaction to occur, cholesterol needs to be made available to CYP11A located on the matrix side of the inner mitochondrial membrane. This substrate delivery process is the rate limiting step of steroid hormone biosynthesis and is regulated by trophic hormones via the hormone regulated synthesis of the steroidogenic acute regulatory protein StAR (see &quot;Regulation of Aldosterone Biosynthesis&quot; & &quot;Regulation of Hydrocortisone Biosynthesis&quot;) that mediates the transport of cholesterol from outer mitochondrial membrane to inner mitochondrial membrane.<br><br>CYP11A sequentially catalyzes the hydroxylation of C22, C20, and the cleavage between C20 and C22 to yield pregnenolone and isocaproaldehyde (which is then oxidized to isocaproic acid). Pregnenolone is converted to progesterone by 3beta-HSD associated with CYP11A on the mitochondrial membrane and by 3beta-HSD in the endoplasmic reticulum. 3beta-HSD1 appears to be the isoform predominantly responsible for this conversion step which ultimately leads to the synthesis of testosterone and 17beta-estradiol. CYP17 catalyzes the next two reactions, the hydroxylation of progesterone at C17 followed by the cleavage of the C17-20 bond to produce the C19 steroid, 4-androstene-3,17-dione (androstenedione, dehydroepiandrosterone, DHEA) that is the immediate precursor to testosterone. <br><br>Androstenedione, a weak androgen, is converted to testosterone by 17beta-HSD3 which is expressed exclusively in the testes. Testosterone is further reduced to 5alpha-dihydro-testosterone (DHT) by the steroid 5alpha-reductase. Testosterone and DHT are the major circulating androgens in human and other vertebrates. DHT is crucial for the normal development of the male genitalia and prostate, while testosterone appears to be essential for promoting spermatogenesis and gonadotropin regulation. The two isoforms of steroid 5alpha-reductase, SRD5A1 and SRD5A2, are products of two independently regulated genes. SRD5A1 is expressed predominantly in liver, while SRD5A2 is predominantly expressed in tissues of the genital tract. <br><br>Androstenedione and testosterone are converted to estrone and 17beta-estradiol, respectively, by the widely distributed CYP19, and estrone is converted to 17beta-estradiol by 17beta-HSD1 or 3. The 17beta-HSDs convert the weakly active 17-ketosteroids to the potent 17beta-hydroxysteroids and are not involved in the biosynthesis of corticosteroids. While the human 17HSD1 predominantly catalyzes the conversion of estrone to estradiol, the mouse and rat enzyme also efficiently convert androstenedione to testosterone. View
Wnt / Ca++ Signaling in Development 1182 Different Wnt proteins can regulate distinct biological processes by activating distinct downstream pathways. The most common and well studied one is the canonical Wnt/beta-catenin pathway. Wnt can also signal through Wnt/Ca++ pathway to increase intracellular concentration of Ca++ and decrease the concentration of cyclic guanosine monophosphate (cGMP). Wnt-Ca++ pathway plays important roles in determining ventral cell fate, in regulating gastrulation movements and heart development.<br><br>In the Wnt-Ca++ pathway, Wnt/Frizzled functions through heterotrimeric guanine nucleotide-binding proteins (G proteins) to activate phospholipase C (PLC) and phosphodiesterase (PDE). The binding of Wnt5 activates the rat Frizzled-2 (Rfz2), resulting in activation of G proteins (G-alpha and G-beta subunits). G-alpha then activates PDE 6, causing decline in intracellular cGMP concentration. The G-beta subunits activate PLC-beta. PLC-beta can hydrolyze phosphatidylinositol 4,5-bisphosphate to give inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). DAG activates PKC directly while IP3 catalyzes the release of stored intracellular Ca++. Increased [Ca++]i in turn activates CaMKII and the Ca++-sensitive protein phosphatase calcineurin (Calcn). Both PKC and Calcn signal through various transcription factors such as NF-kb and NFAT to modulate gene expression. <br><br>The Wnt/Ca++ pathway also interferes with the canonical Wnt/beta-catenin pathway. Activation of CaMKII by the Wnt/Ca++ pathway has been shown to stimulate the TAK1-NLK MAP kinase cascade, which can antagonize the Wnt/beta catenin induced transcriptional activation. View
Chemokine Receptor Signaling 1182 Chemokines are a large family of proinflammatory polypeptide cytokines that regulate inflammation, leukocyte trafficking, angiogenesis and immune cell differentiation. Dysregulation of the system results in excess recruitment of leukocytes to the inflammatory sites. Intercepting the chemokine- receptor interaction and subsequent signaling events may provide therapeutic opportunities in areas of anti-inflammatory and anti-HIV.<br><br>Chemokines activate cells through their binding to specific seven-transmembrane (STM) domain receptors that are coupled to heterotrimeric G-proteins. Several chemokine receptors are found to be fusion co-factors for human HIV-1. Upon ligand binding, G-proteins dissociate into G-alpha and G-beta-gamma subunits. G-beta-gamma activates PLC, which cleaves PIP2 to form the second messengers IP3 and DAG. IP3 mobilizes calcium and activate CaMKII and PKC, while DAG activates PKC and NF-kB signaling. Simultaneously, G-alpha directly activates receptor tyrosine kinases such as Src, which in turn activates PI3K and Rac signaling. Ras/MAPK pathway has also been shown to be activated by chemokine signaling. Through activating multiple signal transduction pathways, chemokines and their receptors mediate cell survival and proliferation by inducing gene expression and enabling cell migration.<br><br> View
Lipogenesis: Biosynthesis of Triacylglycerols 1182 Fatty acids are stored for future use as triacylglycerols (TAG) in all cells, but primarily in adipocytes of the white adipose tissue. Triacylglycerols are molecules with 2 saturated fatty acids and one unsaturated fatty acid esterified on to one glycerol backbone. Triacylglycerols are called neutral fats since the carboxyl groups in the fatty acids are tied up as esters and no longer ionizable. The composition of fatty acids on TAG molecules is such that TAG is essentially insoluble in water and form oily droplets in cells at body temperature. Triacylglycerols with 3 saturated fatty acids would be waxy, while TAG with more than one unsaturated fatty acids would be too fluid for storage purpose.<br><br>The backbone for TAG synthesis, glycerol-3-phosphate is primarily derived from glycerol in tissues other than adipose tissues, while that in adipose tissue is derived from dihydroxy-acetone phosphate (an intermediate of glycolysis) since adipocytes do not have glycerol kinase. Fatty acids (from de novo synthesis or from the diet) are activated to acyl-CoAs through fatty-acid-CoA ligases (ACS1/4 in liver, ACS3/6 in brain, ACS2 for phospholipid synthesis) prior condensation with the glycerol backbone. The first acylation step, conversion of glycerol-3-phosphate to lysophosphatidic acid by glycerol-3P-O-acyltransferase, is rate limiting, and lysophophatidates do not accumulate in cells while phosphatidic acid may accumulate. Fatty acids from the diet are carried as TAG incorporated in VLDLs and chylomicrons, or as fatty acids bound to albumin. Lipoprotein lipase (LPL, regulated by insulin level) on adipocyte cell surface facilitate fatty acid uptake for the production of TAG for energy storage. LPL is expressed in many tissues, and, in general, mediates the hydrolysis and uptake of TAG from chylomicrons and VLDL into cells as fatty acids for energy supply and other metabolic needs. LPL is not expressed in adult liver except in response to TNF, PPAR (alpha & gamma) and LXR agonists. Liver LPL targets circulating TAG to the liver and increase ketone body production, but can cause fatty livers. View
Wnt / b-Catenin Signaling in Cancer Development 1182 The Wnt family of signaling proteins are secreted glycoproteins that participate in multiple development events and physiological processes. Activation of Wnt signal transduction pathways upon ligand binding can regulate diverse processes including cell proliferation, adhesion, migration, polarity, differentiation as well as axon outgrowth. Deregulation of Wnt signaling has been implicated in cancer, neurodegenerative diseases, bone density regulation, osteoarthritis, and regulation of the survival and proliferation of stem cells.<br><br>Wnt proteins signal across the plasma membrane by binding to Frizzled (Fz) family proteins and members of the low-density-lipoprotein-related protein (LRP) family at the cell surface. A major effector of the canonical Wnt signaling pathway is the transcription factor beta-catenin which is stabilized upon Wnt binding, and enters the nucleus to regulate Wnt pathway target genes. Wnt-binding also acts through beta-catenin-independent, noncanonical pathways, such as the planar cell polarity (PCP) pathway and a pathway involving Ca2+ signaling (see &quot;Wnt / Ca++ Signaling in Development&quot;). At steady state in the absence of Wnt signaling, cytoplasmic beta-catenin levels are kept low through interaction with APC and Axin scaffold proteins and phosphorylation by CKI and GSK3beta. Phosphorylated beta-catenin complexed with GSK-3beta/APC/Axin is ubiquitinated and degraded via proteasome pathway (see &quot;Ubiquitin-Proteasome Pathway & Protein Degradation&quot;). When Wnt binds Fz/LRP, formation of the GSK-3beta/APC/Axin/CK1 destruction complex is compromised resulting in the stabilization of beta-catenin and its accumulation in the cytoplasm and nucleus. Nuclear beta-catenin interacts with transcription factors such as LEF/TCF to modulation target gene expression. A large number of Wnt regulated genes contribute to the development of colon cancer and have been well characterized, including many genes in proliferation, adhesion, extracellular matrix formation and apoptosis, and contributes to the development of colon cancer. Constitutive activation of the Wnt/ beta-catenin pathway has been observed in transformed cells due to inactivating mutations in APC and Axin (thus reduce beta-catenin degradation), and mutations in beta-catenin where GSK-3 phosphorylate and render it stable. View
Ca++ Signaling, Cytochrome c Release, & Apoptosis 1182 Formally the mitochondrial permeability transition pore (PTP) is composed of hexokinase in the cytosol, VDAC (Voltage-dependent anion channel) in the outer membrane, ANT-1 in the inner membrane, cyclophilin D in the matrix, and the peripheral benzodiazepine receptor (Bzrp) spanning the inner and the outer membranes. VDAC comprises 20% of the outer membrane protein. It is ordinarily open and considered to be the main &quot;pore&quot; for solute (<1kD) diffusion between the cytosol and the inter-membrane space. Transient opening of the PTP allows for the transfer of certain solutes (ATP, ADP, cholesterol, & phosphatidylserine) in and out of the mitochondrial matrix. PTP opens up and let out inter membrane protein including cytochrome c when there is an excess of pro-apoptotic membrane proteins or when the mitochondrial [Ca++] exceeds a certain limit.<br><br>Both isoforms of adenine nucleotide translocase (ANT-1 & ANT-2) are found in the mitochondrial inner membrane. ANT-1 is found only in the peripheral inner membrane, while ANT-2 is found primarily in the crista-forming part of the inner membrane. ANT-2 catalyzes the exchange of ATP for ADP across the mitochondrial inner membrane. ANT-2 expression is up-regulated when growth- arrested cells are activated to enter G1 phase, and is maintained throughout the cell cycle. ANT-2 expression is down regulated by direct binding of NF1 family of transcription factors to its promoter when cells enter into growth arrest (at confluence). ANT-1 over expression in cells resulted in apoptosis while ANT-2 over expression did not. Co-expression of cyclophilin D with ANT-1 suppressed the ANT-1 induced apoptosis. <br><br>BH-3 only members of the Bcl-2 proteins, acting at the interface between stress signaling pathways and the core apoptotic machinery, are essential initiators of Bcl-2 regulated (death receptor independent) pathway for programmed cell death & stress induced apoptosis. Bcl-2 heterodimerizes with pro-apoptotic proteins (Bad, Bax, & Bak) via the BH3 domain and prevents the pro-apoptotic proteins from accumulating in the mitochondrial outer membrane. The ratio rather than the amount of the antiapoptotic proteins (Bcl-2, Bcl-XL) or the pro-apoptotic proteins (Bax, Bak, Bad, Bin, Blk) determines if apoptosis will proceed. Stress increase the concentration of BH-3 only proteins such as Bad, and Bad competes with pro-apoptotic proteins such as Bax for binding with Bcl-2. Freed Bax, Bad, & Bak relocate to mitochondrial outer membrane and complex with the ANT/VDAC complex to open up the PTP. Bad and hexokinase compete for the same binding site on VDAC.<br><br>Cells treated with Bcl-2 inhibitor produced a Ca++ spike within seconds, redistribution of Bax to mitochondria in 15 min, cytochrome c release in 30 minutes, and ROS increase starting at 30 min. Under normal physiological conditions, increased [Ca++]m increases mitochondrial metabolism, however persistent increase in [Ca++]m leads to apoptosis. Cytochrome c, which transfers electrons from oxidative phosphorylation complex III to complex IV, resides on the inter-membrane side of the mitochondrial inner membrane and is retained by binding to cardiolipin (negatively charged) of the inner membrane. High matrix [Ca++] causes cardiolipin to migrate to the matrix side of the membrane and cytochrome c is released into the inter-membrane space and is released into the cytosolic space when the PTP is opened up. View
Lipolysis: Mobilization of Triacylglycerols 1182 In response to energy demand, fatty acids stored in the form of tracylglycerol (TAG) in adipose tissue are mobilized for use by peripheral tissues. The mobilization process is regulated by hormone sensitive lipase (HSL) that is under the regulation of a number of G-protein Gs signaling cascades that result in the activation PKA which then activate HSL by phosphorylation. Activation of the Gs signaling cascade in adipocytes can be initiated by binding of glucagon, epinephrine or beta-corticotropin to their cognate receptors. Insulin antagonizes HSL activation primarily by lowering [cAMP] through the activation of PDE3B. PKA also phosphorylates perilipin which is expressed abundantly in adipocyte and is located on the surface of lipid droplets. Perilipin appear to be a suppressor for basal lipolysis as well as a necessary component for full lipolytic stimulation. Phosphorylation of perilipin A results in dynamic restructuring of the lipid droplet to allow access of the activated HSL, while TNFalpha mediated lipolysis is associated with down-regulation of perilipin expression.<br><br>Hormone sensitive lipase, also known as cholesterol ester hydrolase (CEH), exists in 3 isoforms, derived from the same gene through alternative splicing. HSL hydrolyze a broad range of substrates, including TAG, DAG, cholesterol esters, retinyl esters, steroid esters, and p-nitrophenyl esters. However it lacks phospholipase activity. At the biochemical level, HSL hydrolyses cholesterol esters at a much faster rate than the CEH that is activated by ligand-bound FXR. The adipocyte fatty acid binding protein (A-FABP) forms a complex with HSL and facilitates removal of fatty acid from HSL. View
Thromboxane & Prostacycline Action 1182 Thromboxane is synthesized from cell membrane derived arachidonic acid via two sequential enzymatic steps, involving COX-1 and thromboxane synthase (see &quot;Prostaglandin Biosynthesis&quot;). The synthesis is initiated by stimulus induced arachidonic acid release (see &quot;Arachidonic Acid Release&quot;) from a number of cell types including platelets, macrophages and vascular smooth muscle cells (VSMCs). Thromboxane signal through thromboxane receptor-beta (TPbeta) to increase [Ca++]i resulting in vasoconstriction, and signal through TPalpha to increase extra cellular [ADP] leading to increased platelet aggregation.<br><br>Prostacyclin is synthesized from arachidonic acid primarily via COX-2 and prostacyclin synthase, and is produced mainly by the vascular endothelial cells. It is a potent inhibitor of platelet aggregation, a vasodilator, and a key player in the local control of vascular homeostasis. The ligand bound prostacyclin receptor (IP) signals through G-protein Gs initially to increase [cAMP] which reduce platelet aggregation as well as activate PKA. PKA phosphorylation of Ser357 of IP enable IP to signal through G-proteins Gi and Gq thus modulate the initial signaling through Gs. PKA activated by IP signaling also selectively phosphorylates TPalpha and desensitize TPalpha from agonist induced signaling. Isoprenylation of IP is not required for ligand binding but is required for signaling through Gs.<br><br>The cholesterol-lowering unrelated cardiovascular benefits of the statins is in part due to reduced cellular level of isoprenoids which is required for post translational modification of both Rho (known to control the expression of vascular proteins) and the prostacyclin receptor IP. View
Protoporphyrin IX Biosynthesis 1182 Protoporphyrin IX, the organic molecule that chelates an iron atom to form heme, is a tetrapyrrole that is synthesized from 4 units of porphobilinogen via three lyases in the cytosol and two oxidoreductases in the mitochondria. The uroporphyrinogen III synthase gene has two promoters generating a housekeeping enzyme that is present in all tissues and an erythroid-specific enzyme. Mutations in the erythroid promoter leading to insufficient uroporphyrinogen III synthase were associated with congenital erythropoietic porphyria.<br><br>Coproporphyrinogen oxidase (Cpox) is located in the inter-membrane space of mitochondria while protoporphyrinogen oxidase (Ppox) is localized to the inter-membrane side of the inner membrane. It is not known how the substrates for these two oxidases are transported from the cytosol through the two layers of mitochondrial membranes. Partial deficiency of Cpox leads to hereditary coproporphyria.<br><br>Heme synthesis is regulated primarily at the rate of 5-amino-levulinate (precursor of porphobilinogen) synthesis in nonerythroid cells but at the rate of iron acquisition for red blood cells. ALAS1 and ALAS2 are encoded by separate genes, while ALAS1 is ubiquitously expressed and provides heme for cytochromes and other hemoproteins; ALAS2 is expressed exclusively in erythroid cells and synthesizes heme specifically for hemoglobin. The protein stability of ALAS2 is regulated by cellular oxygen tension via binding to the von Hippel-Lindau protein in a manner similar to regulation of the hypoxia-inducible factor (see &quot;HIF & Cellular Oxygen Sensing&quot;). The translation of ALAS2 mRNA is regulated by cellular iron level via the IRE/IRP interaction (see &quot;Iron Homeostasis&quot;). ALAS1 is not regulated by hypoxia or iron. Hepatic ALAS1 is regulated by the energy state and by PXR. <br><br>ALADH, an octameric enzyme, catalyzes the condensation of 2 molecules of 5-aminolevulinate to form the pyrrole, porphobilinogen, which is the precursor of protoporphyrin IX as well as the cobalamins. View
Vitamin-K & Coagulation 1182 In addition to typical posttranslational processing steps required of secreted proteins, proteins in the blood coagulation cascade undergo a unique modification involving the addition of gamma-carboxyl groups to specific N-terminal glutamic acid residues. This property is shared with a number of other proteins involved in bone morphogenesis and growth control. The enzyme, gamma-glutamyl carboxylase, which carries out the gamma-carboxylation requires vitamin-K as a cofactor. Mammals obtain vitamin-K from ingesting green leafy vegetables. The functionally active cofactor is Vitamin-K hydroquinone (KH2) which is converted to vitamin-K epoxide at the end of each gamma-carboxylation cycle. Conversion of vitamin-K epoxide back to vitamin K is carried out by the warfarin sensitive vitamin-K epoxide reductase.<br><br>The N-terminal gamma-carboxyglutamic acid-rich domain in these proteins is a membrane binding motif that, in the presence of calcium ions, folds into a stable structure. This stable structure interact with the head group of phosphatidylserines and dock the protein on cell membrane. Tissue factor (TF) is a transmembrane cellular receptor for factor-VII/VIIa (FVII/FVIIa). It is constitutively expressed in fibroblasts and blood vessel walls, but is not expressed in blood cells or the endothelial cells that line blood vessels. TF exposed after endothelial injury binds FVIIa to initiate the cascade for thrombin generation and blood clot. View
Leukotriene Biosynthesis & Action 1182 Leukotrienes (LTs) are synthesized predominantly by inflammatory cells such as eosinophils, mast cells, monocytes and macrophages, and are intimately associated with the pathogenesis of asthma. LTs are products of the 5-lipoxygenase pathway of arachidonic acid metabolism through the corporative action of 5-lipoxygenase (5-LO) and 5-lipoxygenase-activating protein (FLAP) and downstream hydrolases and synthases. FLAP is a small resident nuclear envelope protein that is critical to cellular 5-LO activity and to its membrane interaction but not to 5-LO translocation from the cytosol to the nuclear envelope. FLAP is thought to &quot;dock&quot; 5-LO to its membrane target and to present arachidonic acid to 5-LO to be transformed to LTA4.<br><br>LTA4 is converted, in polymorphonuclear leukocytes, into LTB4 which, when signaling through BLT1, is a potent chemoattractant that triggers adherence and aggregation of leukocytes to endothelium. LTB4 is also a chemoattractant for T-cells and is a mediator of both acute and chronic inflammatory diseases such as nephritis, arthritis, dermatitis, vascular inflammation, and atherosclerosis. LTA4 is conjugated with glutathione, in smooth muscle cells, to form LTC4 which is converted to LTD4 and LTE4 once it is exported outside the cells. LTC4, LTD4 and LTE4 are collectively known as the cysteinyl leukotrienes (cysLTs). In airway smooth muscle cells, cysLTs signal through cysLT1 and promote bronchoconstriction.<br><br>The synthesis of LTB4 and LTC4 are compartmentalized in a cell type dependent manner. LTA4 hydrolase was found to co-localize with 5-LO in macrophages and basophiles while LTC4 synthase and FLAP are co-localized in bone marrow-derived cells. Both sPLA2 and cPLA2 (see &quot;Arachidonic Acid Release&quot;) which are responsible for stimulus induced arachidonic acid release, were found to induce cysLT synthesis in human eosinophils, suggesting that these two PLA2s under certain conditions co-localize with 5-LO, FLAP and LTC4 synthase. View
Toll-like Receptor Signaling 1182 Toll-like receptors are the key players in host defense response. They recognize conserved motifs predominantly found in microorganisms but not in the vertebrates (eg. LPS, flagellin, and viral DNA/RNAs), and initiate signaling pathways that culminate in increased expression of immune and inflammatory genes. TLRs also recognize members of the proinflammatory IL-1 family. TLRs on endosome recognize nucleic acids signals while cell-curface TLRs sense lipids and proteins. <br><br>Two major pathways activated by TLRs have been documented. The core pathway activated by most TLRs leads to activation of transcription factor NF-kB and Map kinases ERKs, p38 and JNK, which leads to induction of many pro-inflammatory genes. The second pathways is activated by TLR3 and TLR4 and leads to activation of both NF-kB and another transcription factor IRF3, thus allowing additional set of genes to be induced, such the inteferons (antiviral) which then activate the expression of a set of INF-dependent genes. The extracellular region of TLRs is quite diverse for the cells to recognize a variety of pathogens. Whereas that the cytoplasmic region of TLR have a very conserve domain called TIR domain. The IL-1R and TLR family signal through shared downstream signaling molecules including adaptor molecule Myd88, IL-1R associated protein kinase (IRAKs), TGFb activated kinase (TAK1), TAK1 binding protein Tab1 and Tab2, and the TNF receptor associated factor 6 (Traf6). View
TNF Induced Inflammatory Response & Apoptosis Signaling 1182 Tumor necrosis factor (TNF) is an important cytokine that mediates immune and inflammatory responses. Aberrant TNF signaling has been associated with a diverse array of disorders including diabetes, cancer, osteoporosis, allograft rejection, and autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, and inflammatory bowel diseases.<br><br>TNF signals through binding to two distinct cell surface receptors, TNFR1 and TNFR2 to mediate a variety of cellular responses including induction of apoptosis, anti-viral response and activation of NF-kB and MAPK signaling cascades. TNF triggers both apoptotic and survival signals. First, TNFR1 can recruit TRADD, which subsequently recruit FADD and caspase 8 which then activate the downstream effector caspases leading to apoptosis. In addition, TRADD binds the serine-threonine kinase RIP and TRAF thereby coupling stimulation of TNFR1 to the activation of NF-kB and protect against TNF-induced apoptosis via induction of many molecules (e.g., gadd45b, x-IAP) that block caspase activation. The balance between the strength of apoptotic vs. survival signals determines the final cell fate. Recent studies have also demonstrated that cells in which the NF-kB signaling pathway is blocked are more likely to undergo apoptosis in response to TNF, suggesting that NF-kB plays a critical role in the ability of TNF to function as an apoptosis-inducer and anti-tumor agent. View
Cadherin Mediated Cell Adhesion 1182 Cadherins are a family of adhesion molecules that mediate Ca2+-dependent cell-cell adhesion in solid tissues and modulate a wide variety of processes including cell migration, polarization and tissue morphogenesis. E-cadherin (epithelial cadherin), is the most commonly studied cadherins. It forms adherens junctions in epithelial cells and functions in profound remodeling of the actin cytoskeleton. E-cadherin function or expression is frequently downregulated in carcinomas, leading to invasiveness of the tumor cells. E-cadherin has been used as a prognostic marker in prostatic cancer, based on the correlation of its expression and tumour grade, stage, metastasis and survival, as well as recurrence after radical prostatectomy.<br><br>Cadherin interacts with a number of proteins to modulate cell adhesion and/or activate downstream signaling pathways. The core of the multiprotein complex at the junctions consists of cadherin in association with p120ctn and beta-catenin, which in turn interacts with alpha-catenin, thus connecting cadherin to the actin cytoskeleton. There are multiple ways for cadherin to induce actin polymerization. First, cadherin can recruit and inhibit Arp2/3 actin nucleator complex, creating special sites for assembly of the F-actin. Second, alpha-catenin can bind to formin-1 and/or bind directly to actin for actin-cable formation. Cadherin also interacts with p120-catenin, which has been shown to regulate the Rho family GTPase activity and influence actin organization. <br><br>In addition to the role in regulating actin cytoskeleton, cadherin also acts as signaling nodes by binding to EGFR, phosphatase and adaptor proteins, and activating PI-3 kinases. Beta-catenin, which functions in the Wnt signaling pathway to activate the transcription of genes in cellular proliferation and differentiation, also interacts with E-cadherin. Free intracellular beta-catenin can be sequestered by cadherin to the adheren junction resulting inhibition of its transcriptional activity. View
Vascular Tissue Angiotensin System 1182 Vascular tissues contain angiotensin I-converting enzyme (ACE) independent system for converting angiotensin I (Ang-I) to angiotensin-II (Ang-II). Cultured endothelial cells have been shown to contain renin and angiotensinogen and are capable for synthesizing and secreting angiotensins. In vivo, it has been demonstrated that ACE is chiefly in the luminal compartment of the heart and blood vessels, whereas the chymase mediated conversion of Ang-I to Ang-II takes place primarily in the interstitial compartment. The two pathways, work in concert, to regulate blood pressure and cardiac remodeling. In normal homeostatic state, ACE in the endothelium plays the major role, while chymase (contained in mast cells) assumes the bigger role in grafted veins or injured cardiac tissue (such as after a myocardial infarction). Studies using rat vascular smooth muscle cells (VSMCs) demonstrated that upon exposure to fibronectin (a glycoprotein found in blood plasma & extra-cellular matrixes), differentiated VSMC (contractile phenotype) dedifferentiate and develop into a proliferative phenotype that produce angiotensinogen & cathepsin D, is able to convert Ang-I to Ang-II, and secretes the growth factors induced by Ang-II. Chymase inhibitor, NK-3201, inhibited intimal hyperplasia in a canine balloon injury model where AT-I receptor antagonist was effective while ACE inhibitor was not. View
Rho Signaling to Actin Cytoskeleton 1182 Rho family proteins (RhoA, B, C, Rac1,2, Cdc42) are small GTPases in the Ras superfamily. Rho regulates cell morphology via actin cytoskeleton reorganization in response to extracellular signals such as serum and LPA. In addition to controlling cytoskeletal changes, Rho GTPase increases the stability of actin-based structures such as stress fibers and focal adhesions, key regulators of cell motility, cell polarity and cell-cycle progression.<br><br>Rho proteins are molecular switches which cycle between and inactive GDP-bound and an active GTP-bound state. The switch is activated by guanine nucleotide exchange factors (GEFs), and repressed by GTPase-activating proteins (GAPs). In the active state, Rho interacts with multiple effector-proteins including ROCK, mDia, MEKK1and PI4P5K. Rock1 provides a direct link from RhoA to cell morphology through phosphorylation of the myosin light chain. Rock1 also phosphorylates and activates LIM kinase which phosphorylates and inactivates cofilin. Cofilin stimulates actin depolymerization and changes in cell structure, phosphorylation of cofilin by LIM kinase inhibits actin depolymerization and stabilizes F-actin and enhances actin-myosin contractility. Rho signaling through mDia, recruits profiling, and promotes actin polymerization. View
Complement Activation 1182 The complement system is an intricate part of immune response. Activation of the complement pathways lyses invading bacterial cells, forms chemotactic peptides (C3a and C5a) and increases phagocytic clearance of infecting cells. The complement system is a complex system of serum proteins circulating in blood plasma. Most of these exist in plasma as inactive precursors and activate each other in a proteolytic cascade in response to 3 different mechanisms, the classical pathway, the lectin pathway and the alternative pathway as shown in the pathway diagram.<br><br>The three different pathways differ by their way of initiation. The classical pathway is triggered by binding of antibody to its antigen, while the alternative pathway is initiated when a previously activated complement component binds to the surface of a pathogen. The lectin pathway is initiated by binding of mannose binding lectin (MBL) to microbial pathogens. Each of these pathways converges in the cleavage of C3 by specific multi-protein convertase enzymes (C4b2a and C3b Bb) and the subsequence generation of the membrane attack complex (MAC). View
Rac Signaling & Cell Motility 1182 Rac-1 is a small G-protein in the Rho family that regulates cell motility in response to extracellular signals. Rac-1 is activated by GEF factors (guanine nucleotide exchange factors), and repressed by GAPs (GTPase-activating proteins). Rac stimulates formation of the actin-based structures such as filopodia and lamellopodia, while GAPs such as chimerin oppose the formation of these Rac dependent structures. <br><br>Rac signals to cytoskeleton through activating its multiple effectors. PAK1 (p21-activated kinase 1) is an effector of Rac. PAK1 phosphorylates MLCK, resulting in decreased MLCK activity and MLC phosphorylation. PAK1 also signals to LIMK which then inhibits actin depolymerization and induces membrane ruffling. POR1 is another effector that interacts with Rac, and deletion or POR1 inhibits Rac induced membrane ruffles. WAVE is a member of the WASP family of proteins that regulates actin organization and is involved in Rac signaling to cause membrane ruffling. Interaction of MEKK1 with Rac-1 integrates Rac signaling with pathways signaling through map kinases. View
Mitochondrial Oxidative Phosphorylation 1182 The mitochondrial electron transport system (NADH dehydrogenase, succinate-ubiquinone oxidoreductase, ubiquinol-cytochrome c reductase, & cytochrome c oxidase) captures the free energy (in the form of NADH & FADH2) derived from substrate oxidation (TCA cycle, beta-oxidation of fatty acids, and glycolysis via the malate/ aspartate & the glycerol phosphate shuttles) and converts it to proton electrochemical gradient across the mitochondrial inner membrane. The flow of protons back across the inner membrane in the energetically favorable direction is coupled to the phosphorylation of ADP to produce ATP via the ATP synthase. The electrochemical gradient also drives the transhydrogenase in the forward direction to provide NADPH for biosynthesis, detoxification (reduction of glutathione & recycling of catalase), and fine tuning the flux through the TCA-cycle in high energy demand tissues (heart, muscle). <br><br>ATP synthesis by the ATP synthase is tightly coupled to the uptake of phosphate anion (mitochondrial phosphate carrier) and the exchange of ATP for ADP across the inner mitochondrial membrane (ANT-2). Expression of ANT-2 is induced when quiescent cells are activated to enter the G1 phase and expression is maintained throughout the cell cycle. Expression of ANT-2 is down-regulated when cells become growth-arrested.<br><br>Each of the proteins in the electron transport system, the ATP synthesis machinery, and the NADP-transhydrogenase, is tightly associated with the membrane lipid, cardiolipin (see &quot;Cardiolipin & Mitochondrial Function&quot;), which is considered a proton trap for the electron transport system. Cytochrome c is held on the outer surface of the inner membrane by cardiolipin which is rich in unsaturated fatty acids (90% linoleic acid) and prone to oxidative damage by hydroxyl radicals. Oxidative damage to cardiolipin is associated with cytochrome c release from mitochondria during reactive oxygen species mediated apoptosis. Conditions that cause the collapse of the electrochemical potential across the inner membrane, such as high Ca++ in the mitochondrial matrix or shortage of NADH, will also lead to cytochrome c release & apoptosis. View
IL-6 Signaling 1182 Interleukin-6 (IL-6) is a cytokine that provokes a broad range of cellular and physiological responses. It is a potent cytokine that induce many genes involved in inflammatory and acute phase response. IL-6 signals through a receptor composed of two different subunits, an alpha subunit that endows ligand specificity and a signal transducer, gp130 that is shared with a number of other cytokines in the IL-6 family.<br><br>Binding of IL-6 to its receptor initiates cellular events including activation of JAK kinases and activation of ras-mediated signaling. Activated JAK kinases phosphorylate and activate STAT transcription factors, particularly STAT3, that move into the nucleus to activate transcription of genes containing STAT3 response elements. The ras-mediated pathway, acting through Shc, Grb-2 and Sos-1 upstream and activating Map kinases downstream, activates transcription factors such as ELK-1 and NF-IL-6 (C/EBP-beta) that can act through their own cognate response elements in the genome. These factors and other transcription factors like AP-1 and SRF (serum response factor) that respond to many different signaling pathways come together to regulate a variety of complex promoters and enhancers that respond to IL-6 and other signaling molecules. View
Monocytes, Macrophages & Atherosclerosis 1182 Atherosclerosis is a multifactorial disease characterized, in part, by the accumulation of lipids and inflammatory molecules within the arterial wall. Excess LDL particles in the blood stream can accumulate in the intima (endothelial cells that line vessel walls) where they undergo oxidation & glycation and become recognizable by the endothelial cell scavenger receptor (SREC). Treatment of human aortic smooth muscle cells with oxidized LDL up-regulated the expression of SREC-II. Development of atherosclerosis is accelerated by LPS (a major component of Gram-negative bacteria), while activation of NF-kB is associated with increased adhesiveness (increased P- & E-selectin on endothelial cell surface, increased vascular cell and intracellular adhesion molecules) and permeability (increased monocyte chemo-attractant protein) of the endothelium. Monocytes accumulated in the intima mature into active macrophages, together with the T-cells they produce many inflammatory cytokines and growth factors. The macrophages take up oxidized LDL particles and long chain fatty acids via the scavenger receptors and become filled with fatty droplets. Collections of foam cells become encased in fibrous caps (plaque) effectively increase the thickness of the intima and narrowing the artery. Injury of the fibrous cap caused by shear stress or other inflammatory stimuli can rupture the plaque, release clotting factors, and cause a thrombus. View
Hypoxia and HIF Signaling 1182 Sensing and responding to fluxes in oxygen tension is one of the most important variables in physiology, and animal tissues have developed a number of essential mechanisms to cope with the stress of low physiological oxygen levels, or hypoxia. Hypoxia is a central aspect of many disease processes including cancer, cardiovascular and respiratory diseases. At the organism level, hypoxia response involves an increase in red blood cell production. Within tissues, HIF activation increases the blood supply and blood vessel growth. At the individual cell level, hypoxia is manifested as an increase in anaerobic metabolism in order to sustain basic cellular functions.<br><br>The hypoxic response is primarily mediated by hypoxia-inducible factors (HIF) while HIF activity is regulated by cellular oxygen tension and some of the major growth factor induced signal transduction pathways. When oxygen is not limiting (normoxia), the HIF-1 protein is transcriptionally inactive and its protein level is kept low by rapidly degradation via the ubiquitin/proteasome pathway. Under hypoxic or low oxygen conditions, HIF-1alpha is stabilized, translocates into the nucleus, and heterodimerizes with HIF-1beta (also known as ARNT, the AhR binding partner). This transcriptionally active complex then associates with hypoxia response elements (HREs) in the regulatory regions of target genes, binds transcriptional coactivators (p300) and induces target gene expression.<br><br>Stimulation of different receptor tyrosine kinases has been shown to increase the levels of HIF-1alpha (increased translation and protein level) and induces the transcription of oxygen-dependent genes. This effect could be inhibited by antagonist of PI3K and FRAP (mTOR). PTEN dephosphorylates PI-2, the product of PI3K reaction, thus counter regulates the PI3k/frap cascade. Loss of PTEN in tumor cells leads to a drastic increase of HIF-1alpha protein level that is associated with neoangiogenesis and tumor progression. <br><br>HIF regulates both short term responses to hypoxia, such as erythropoiesis and glycolysis, and long term responses such as angiogenesis (VEGF). HIF regulates a diverse range of processes including adipogenesis, apoptosis, and beta-cell development. HIF induced glucose uptake and glycolysis presumably is to provide more NADPH (2 NADPH are produced from each glucose in either aerobic or anaerobic glycolysis). View
EIF2 Kinase Mediated Stress Response 1182 Eukaryotic cells respond to oxidants, amino acid starvation, and unfolded proteins in their endoplasmic reticulum (ER) by phosphorylating (ser-51) the alpha-subunit of the translation initiation factor 2 (eIF2alpha). This is a highly conserved adaptive response that simultaneously reduces the rate of general protein synthesis and promotes the transcription and translation of the transcription factor ATF4 leading to the up-regulation of its down-stream genes involved in stress response. The kinases responsible for eIF2alpha phosphorylation, PEK & GCN2 are normally sequestered in the ER membrane by binding to ER chaperones (heat shock proteins) such as GRP94 and GRP78, and are released under conditions of ER stress or elevated levels of uncharged tRNAs. Hypoxia induced down regulation of protein synthesis is also mediated through the activation of PEK (by hyper-phosphorylation). The heme regulated HRI (eIF2alpha kinase-1, coordinates globin synthesis with heme & iron availability, activated by heme deficiency) in reticulocytes and the double-stranded RNA dependent, interferon inducible PKR (eIF2alpha kinase-2, activated by viral infection) are two other kinases that phosphorylate eIF2alpha at the same site.<br><br>GTP-bound EIF2alpha participates in the initiation of protein synthesis and is released from the ribosome as EIF2alpha-GDP. Recycling of eIF2alpha-GDP to eIF2alpha-GTP requires eIF2B (a guanine nucleotide exchange factor, rate limiting, present at ~20% of the level of eIF2alpha). Phosphorylation of eIF2alpha-GDP by its cognate kinases converts eIF2alpha-GDP from a substrate to an inhibitor of eIF2B. Phosphorylated-eIF2alpha-GDP binds to eIF2B much more tightly than eIF2alpha-GDP. The resulting decrease in EIF2alpha-GTP level reduces general translation and gives the cell time to correct the problems.<br><br>Phosphorylated-eIF2alpha-GDP initiates the translation of ATF4 which then induce the transcription of ATF3 and the down stream genes including Gadd34. Gadd34 on the one hand activates a cascade leading to G2/M cell cycle arrest, while on the other hand forms a complex with PP1 and modulates the phosphorylation of eIF2alpha-GDP, thus mediating translational recovery. It was proposed that Gadd34 controls a programmed shift from translational repression to stress-induced gene expression. In response to amino acid starvation, ATF3 induces CHOP (Gadd153) which can lead to apoptosis. ATF3 can function as a homodimer or as a heterodimer with CHOP for transcriptional repression. The activation of NF-kB in response to ER-stress or amino acid starvation is also mediated by phosphorylated-eIF2alpha-GDP. View
Benzo[a]pyrene Induced Toxicity 1182 Benzo[a]pyrene is a prototypical member of a collection of polyaromatic hydrocarbons formed as byproducts of combustion and found in diesel exhaust particles, cigarette smoke and grilled food. Benzo[a]pyrene is a micro molar range activator of the AhR and is a pro-carcinogen. Benzo[a]pyrene itself is not carcinogenic, however its metabolite trans-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDHE), via the action of CYP1 and Ephx (epoxide hydrolase), reacts directly with DNA to form adducts. This benzo[a]pyrene adduct 1) is known to cause mutation in K-ras & p53 leading to lung cancer, 2) activate DNA-base excision & repair associated signaling cascade via the activation of p53. In addition of forming DNA adducts, BPDHE induces oxidative stress responses that are cell type dependent. This pathway is a good example of the multifaceted effects a chemical can produce through multiple pathways. View
Adiponectin & Energy Metabolism 1182 Adiponectin (ADIPOQ), also referred as 30-kDa adipocyte complement-related protein (Acrp30) and gelatin-binding protein-28 (Gbp28), is expressed exclusively in adipose tissue. Adiponectin is a collagen-like protein abundantly synthesized and secreted by the adipose tissue. It is present normally in plasma at concentrations 3-orders of magnitude higher than other hormones. Serum levels of adiponectin correlate with systemic insulin sensitivity and inversely correlate with percent body fat.<br><br>Adiponectin exist in the circulation as a full length protein (fAd) as well as proteolytically cleaved globular C-terminal domain (gAd). The adiponectin receptors, AdipoR1 and AdipoR2 have 7 trans-membrane domains but are topologically and functionally distinct from GPCRs. AdipoR1 is expressed abundantly in skeletal muscle while AdipoR2 is predominantly expressed in liver. gAD has high affinity for AdipoR1, both fAd and gAd have moderate affinity for AdipoR2, while fAd has weak affinity for AdipoR1. This difference in affinity of fAd and gAd for the two receptors and the tissue distribution of the two receptors correlates with the observation that fAd has a greater effect on liver while gAd has a greater effect on skeletal muscles. Adiponectin exert its metabolic effect through AMPK (also see &quot;AMP Kinase: A Metabolic Master Switch&quot;).<br><br>Adiponectin is an anti-inflammatory cytokine with 3-dimensional structure closely resembles that of TNFalpha (a pro-inflammatory cytokine). Both in vivo and in vitro experiments have demonstrated that adiponectin and TNFalpha suppress each other's production and antagonize each other's action in their target tissues (See &quot;Adiponectin & Atheroscleorosis&quot;). View
Sphingosine-1-Phosphate (S1P) Signaling 1182 Sphingosine-1-phosphate (S1P, present in high concentrations in serum & platelets) signals through five subtypes of the endothelial differentiation gene (EDG) family G-protein coupled receptors, S1P specific receptors, S1P1(Edg1), S1P2 (edg5, AGR16), S1P3(Edg3), S1P4(Edg6), & S1P5(Edg8). The S1P receptors are expressed ubiquitously and are coupled to different G-proteins, thus the biological processes regulated by S1P (angiogenesis, vascular maturation, heart development, & immunity) is dependent on the tissue distribution pattern of the S1P receptors. S1P also functions intracellularly as a second messenger for the regulation of Ca++ homeostasis & apoptosis. S1P1 (coupled to Gi), S1P2 (coupled to G12/13) & S1P3 (coupled to G12/13) are widely expressed, while S1P4 (coupled to Gi) is restricted to lymphoid tissue & S1P5 (coupled to Gi & Gq) is expressed in brain & spleen. <br><br>Cellular levels of S1P in most tissues are low and regulated by the balance between sphingosine kinase and S1P lyase. S1P reduce the concentration of cellular ceramide by inhibiting both ceramide synthase and serine-palmitoyltransferase. S1P increase cytosolic Ca++ via the SIP receptors while sphingosine blocks the store-operated Ca++ influx.<br><br>PDGFR, EGFR, & VEGFR mediate proliferation & migration via the activation of ERK1/2 and play an important role in the growth of many cell types and cancers. Sphingosine is converted to S1P via VEGF treatment and S1P (intracellular) opposes the activation of RasGAP by sphingosine, resulting in the activation of the Ras-Raf-MEK-ERK pathway. S1P, in the absence of TGFbeta, can activate smad2/3 and causing it to dimerize with smad4 and initiate the TGFbeta-smad signaling cascade.<br><br>Platelets do not have SIP lyase and accumulate high concentrations of S1P. S1P is released in large amounts from activated platelets. S1P, signaling through the different receptors, mediate the different stages of wound healing including cell migration, & differentiation. S1P-induced transcription of PDGF is directly associated with neointimal tissue proliferation & atherosclerosis. View
Adiponectin & Atherosclerosis 1182 Adiponectin is a relatively abundant plasma protein secreted specifically from the adipose tissue and circulates as multimeric complexes (see &quot;Adiponectin & Energy Metabolism&quot;). It is an anti-inflammatory cytokine with 3-dimensional structure closely resembles that of TNFalpha, a pro-inflammatory cytokine. Both in vivo and in vitro experiments have demonstrated that adiponectin and TNFalpha suppress each other's production and antagonize each other's action in their target tissues. <br><br>Adiponectin levels correlate negatively with fasting plasma insulin level and percent body fat, and are significantly lower in patients with coronary artery diseases. Adiponectin knock-out mice showed severe neointimal thickening and increased proliferation of vascular smooth muscle cells in mechanically injured arteries. Treatment with adenovirus-mediated adiponectin suppress the development of atherosclerosis in susceptible (adiponectin E-deficient) mice where adiponectin was found to associate with foam cells in the fatty streak lesions, and suppress the expression of VCAM-1. Adiponectin binds to subendothelial collagens through its collagenous domain and abundantly accumulates into subendothelial space of acute injured lesions of human artery. In human monocyte-derived macrophages, adiponectin was found to suppress the expression of class-A MSR (responsible for foam cell formation) without affecting that of CD36.<br><br>In human macrophage, activation of either PPARalpha or PPARgamma was found to induce the expression of both adiponectin receptors (adipoR). Adiponectin treatment of vascular smooth muscle cells in culture attenuated proliferation induced by growth factors. View
Renal Function: Regulation of Protein Endocytosis 1182 The renal glomerular membrane retains cells and large proteins within the blood. Small proteins (< 70 kD, such as albumin, myoglobin, hemoglobin, polypeptide hormones, apolipoproteins, lactoferrin, vitamin carriers, metallothionein, and polybasic drugs) pass into the primary urine and are reabsorbed by receptor mediated endocytosis at the brush-border membrane of the proximal tubules. Conditions that compromised the glomerular barrier causes saturation of the endocytic capacity and results in progressive nephropathy (protein-urea, sodium retention and up-regulation of vasoactive & inflammatory genes).<br><br>The endocytosis process is clathrin-dependent and requires megalin and/or cubilin, and V-type H+-ATPase, NHE3 (Na+-H+ exchanger) and Clcn5 (chloride channel) as a protein complex within the endosome. Disabled-2 (Dab2) is a cytoplasmic adaptor protein that is abundantly expressed and co-localizes with megalin in renal proximal tubules, and is required for megalin function. ARH (autosomal recessive hypercholesterolemia) is an adaptor protein that enhances megalin mediated endocytosis. Receptor-associated protein (RAP) is a chaperone that assists the processing of LDLR-related proteins (LRP) such as vLDLR, LDLR, apoER2, megalin, LRP5. RAP is able to inhibit binding and/or uptake of all LRP ligands presumably by binding to and cause a conformation change in megalin. The endocytic vesicles are progressively acidified (via the action of NHE3, Clcn5, and the Na+/K+ ATPase) leading to the dissociation of small proteins from megalin and the subsequent degradation of albumin in lysosomes. <br><br>Clcn5 colocalizes with the V-type H+-ATPase in the renal proximal tubules and is necessary for endocytosis mediated via megalin and cubilin. The C-terminal tail of Clcn5 binds to cofilin, a ubiquitously expressed actin-associated protein that binds to both filamentous (F-actin) and monomeric (G-actin) to stimulate de-polymerization of the actin microfilaments. Cofilin is a terminal effector of signaling cascades that evoke cytoskeletal rearrangement essential for endocytosis. Phosphorylation of cofilin was found to be accompanied by pronounced inhibition of albumin uptake.<br><br>NHE3 mediates the majority of NaCl and NaCO3 absorption in the proximal tubule. Elevation of [Ca++]i inhibits NaCl absorption and brush border Na+/H+ exchange through the oligomerization and endocytosis of NHE3 via enhanced interaction between Actinin-4 and E3KARP. Parathyroid hormone (PTH) is a potent inhibitor of NaHCO3 absorption that results in increased concentration of NaHCO3 in the distal nephron and acts as an important stimulant to Ca++ absorption by distal tubules. Calcium sensing receptors (CaSR) are found on the lumen epithelial-cells throughout the renal tubules. CaSR is activated by proinflammatory cytokines, high extracellular (urine) [Ca++], and 1,25-dihydroxyvitamin D3. Activation of CaSR decreases Ca++ (and other divalent cations) reabsorption and decrease circulating PTH. View
Sphingolipids: Synthesis & Physiological Function 1182 Ceramide is synthesized de novo from palmitoyl-CoA and serine and incorporated into cell membrane as sphingomyelin. Sphingomyelin is ubiquitous in animal cell membrane and is nearly as abundant as phosphatidylcholine (usually the most abundant phospholipids in animal cells, accounting for up to 50% of the cellular phospholipids). Ceramide is glycosylated to form galactosylceramide (cerebroside), the principal glycosphingolipid in the brain (12% dry weight of white matter), and glucosylceramide which is further converted to oligoglycosylcerimides & gangliosides, as well as, sulfated glycosylsphingolipids.<br><br>The major source of ceramide that serves as a second messenger involved in apoptosis is generated from sphingomyelin hydrolysis by sphingomyelinases. Ceramide is the source of other bioactive sphingolipids which can work in concert with ceramide or against the effect of ceramide. TNF-alpha, Fas-ligand, interferon-alpha, & interleukin-1 activate caspase-3, which, in turn, activates neutral sphingomyelinase and deactivates sphingomyelin synthase resulting in increased cytosolic ceramide. Ceramide can initiate a number of cascades of cellular responses including both, pro- and anti- growth arrest/apoptosis, as well as, inflammation depending on the balance of many other factors. View
Xenobiotic Metabolism 1182 Xenobiotics are foreign small molecules which are introduced into the body from the environment (e.g., inhalation of air pollutants), through the ingestion of food (e.g., fruits, vegetables, food preservatives), and the use of medication. Xenobiotics are eventually excreted via either the biliary or the renal route but may require extensive metabolism in order to be converted to a form that the excretion system can handle. All major routes of exposure (skin, nasal mucosa, lung, liver, kidney, & gastrointestinal tract) have substantial xenobiotic metabolism capabilities. <br><br>Highly water soluble molecules often enter Phase III metabolism directly and are excreted unchanged in the urine. Xenobiotics that are less water soluble but with hydroxyl, amino, carboxyl, or sulfhydryl functional groups can enter Phase II where they are acylated, sulfated, conjugated with amino acids, or glucuronidated in order to be excreted via either the biliary or the renal route. The majority of the marketed drugs is highly lipophilic and requires &quot;activation&quot; prior Phase II metabolism. This &quot;Activation&quot; process is Phase-I xenobiotic metabolism and involves oxidation, reduction, and/or hydrolysis of the a drug molecule in order to introduce hydroxyl and carboxyl groups into the molecule for Phase II metabolism.<br><br>Many of the enzymes and transporters involved in xenobiotic metabolism are inducible through drug interaction with AhR, PXR, or CAR. The flavin-containing monooxygenases (FMOs) are constitutively expressed and their expression levels appear to be regulated by endogenous steroids. View
Iron Homeostasis 1182 Iron is an essential cofactor for the survival of all cells but is highly toxic in excess. Mammalian iron homeostasis is regulated at the level of intestinal absorption by the body's demand for iron, since there is no iron excretion route. Plasma iron level is maintained by both regulation of intestinal iron absorption and by release from iron stored in liver, spleen, and tissue macrophages. Dietary heme is absorbed via the heme carrier protein 1 (HCP1) localized on the brush-border membrane of duodenal enterocytes. The luminal surface of enterocyte lacks transferrin receptors and dietary ferric iron (Fe+++) is first reduced by duodenal cytochrome b (Dcytb, a brush-border membrane ferric reductase) to ferrous iron (Fe++) and then transported into the epithelial cells by the divalent metal ion transporter 1 (DMT1, Nramp2, DCT1) on the apical membrane of duodenal enterocytes. Fe++ is exported from enterocytes via ferroportin (Fpn) on the basolateral membrane and oxidized to F+++ by membrane-bound hephaestin (Heph) and circulating ceruloplasmin (Cp). F+++ is captured by circulating transferring (Tf) and delivered to the liver and other cells. DMT1 is a proton-dependent Fe++ ion importer that is also present in hepatocytes and the transferring-cycle endosomes in erythroid precursors. The DMT1 gene produces two splice variants, one with iron response element (DMT1A) and one without (DMT1B). DMT1A is predominantly expressed by epithelial cell lines, while DMT1B is expressed by the blood cell lines.<br><br>Hepciding is a peptide produced by the hepatocytes and released into circulation in response to inflammation and iron overload, and its expression is suppressed by anemia and hypoxia. It resembles peptides involved in innate immunity and has antimicrobial properties. Circulating hepcidin binds to Fpn on cell surface and induces its internalization and degradation, leading to decreased iron export and increased cellular iron retention, thus inhibits iron absorption in the duodenum and inhibits iron release from macrophages.<br><br>Fe+++ loaded Tf are taken up by other cells via receptor mediated endocytosis. Fe+++ are released in the acidic environment of the endosome and reduced to Fe++ by an unidentified ferrireductase while Apo-Tf and the receptor are recycled to the cell surface for the next cycle. Fe++ is transported across the endosomal membrane via DMT-1 into the cytoplasm. Expression of the transferring receptor TfR1 is under the control of IRE/IRP, while that of TfR2 is not. TfR2 binds Tf with 30-fold lower affinity than TfR1. Mutations in TfR2 are associated with hemochromatosis.<br><br>Excess cellular Fe++ are stored in ferritin which also function as a ferroxidase (associated with the heavy chain) and converts Fe++ to Fe+++ prior internalization. Ferritin is a spherical shell composed of 24 light and heavy chains that are joined together to form a hollow sphere. Each ferritin molecule can sequester up to 4500 atoms of Fe+++ in the form of [FeO(OH)]8[FeO(H2PO4)]. Ferritin is degraded for the stored Fe+++ to become available.<br><br>Trans-acting iron regulated proteins IRP1 and IRP2, regulate the translation of proteins involved in iron uptake, storage, utilization and export (H-ferritin, L-ferritin, eALAS, mitochondrial. aconitase, ferroportin, DMT1) by binding to IREs (conserved hairpin structures) located close to coding region of the mRNA. Formation of the IRP/IRE complex inhibits early steps of the translation process. When iron levels are high, a cubane [4Fe-4S] cluster assembles in IRP1, inhibiting IRE-binding activity and converting IRP1 to aconitase. When iron level is low, IRP1 binds to IRE as an apo-protein. IRP2 accumulates in iron-deficient cells and is targeted for degradation when iron levels are high by binding to heme. Multiple IREs are present in TfR1 far away from the coding region, and binding of IRPs to these IREs serves to stabilize thee TfR1. View
Oxidative Stress Response Mediated by Nrf2 1182 This pathway is one of the cascades down-stream of that depicted in the &quot;Oxidative Stress&quot; pathway. Nrf2 is the key transcription factor that regulates the constitutive and the inducible expression of a battery of enzymes, scavenger receptors, chaperon proteins, and proteasome components that mediate the amplification of mammalian defense system against environmental insults such as xenobiotics (including pharmaceuticals), air pollutants (such as diesel exhaust, airborne pesticides and other carcinogens) and acute inflammatory response.<br><br>Under normal physiological conditions, Nrf2 is sequestered in the cytoplasm by association with KEAP1. Association with KEAP1, not only keeps Nrf2 in the cytoplasm, but also increases its rate of proteasomal degradation. KEAP1 is a cysteine-rich protein and, in vitro, the KEAP1-Nrf2 complex is formed only under strong reducing condition. Antioxidant response, or agents that reacts with sulfhydryl groups (-SH), will lower cellular GSH (reduced form of glutathione) level, and cause KEAP1 to dissociate from Nrf2, allowing it to translocate into the nucleus and bind to the antioxidant/electrophile response element (ARE/EpRE), thereby activates Nrf2-responsive genes. One of the small Maf proteins is an obligatory heterodimeric partner for Nrf2 mediated transcription activation. In addition to KEAP1, phosphorylation of Nrf2 by protein kinases, including PKC, ERK, p38, JNK, PERK, & PI3K, promote its dissociation from KEAP1. Phosphorylation of Nrf2 by PERK (eIF2 kinase-3, phosphorylates eIF2alpha during the ER unfolded protein response), specifically, was found to be sufficient for its dissociation from KEAP1 and migration into the nucleus for the transcription of its cognate genes. View
p38 MAPK Signaling 1182 p38 is a member of mitogen-activated protein kinase (MAPK) family and is involved in a wide variety of cellular processes such as inflammation, apoptosis, proliferation, transcription regulation and differentiation. P38 signaling is regulated by environmental stress and cytokines, and represents a point of convergence for multiple signaling cascades. There are four p38 isoforms, alpha, beta, gamma, and delta that differ in their tissue expression, and affinity for upstream activators and downstream effectors. The p38 alpha and beta genes are ubiquitously expressed, p38gamma is predominantly expressed in skeletal muscle, while p38delta is enriched in lung, testis, pancreas, and the small intestine.<br><br>The p38 protein can be activated by a wide variety of stimuli including pathogens (LPS), inflammatory cytokines (TNFalpha. IL-1, IL-2, and IL-18, etc.), growth factors (TGFbeta, GM-CSF, CSF1, EPO, FGF, IGF, NGF, PDGF, VEGF, bFGF, etc.), and stress from the environment such as heat shock, UV, and osmotic shock. In the un-phosphorylated state p38 is inactive, it is rapidly activated by phosphorylation. Similar to the ERK and JNK cascades, the p38 signaling cascade consists of a linked series of protein kinases that sequentially phosphorylate and activate the next kinase in the cascade. A number of downstream targets of the p38 MAPK have been identified, including other kinases, cytosolic proteins and transcription factors. These effector molecules are essential for cellular responses such as cytokine production and transcription regulation. The p38 alpha MAPK has been implicated particularly in the regulation of inflammatory mediators, such as cytokines, PGE2, and iNOS.<br><br>Several p38 inhibitors have been shown to block the production of TNFalpha, IL-1 and other cytokines, suggesting that such compounds may be effective as therapeutic agents against inflammatory disorders. Several compounds have reached phase II and III clinical trials. However, due to possible cross-reactivities against other kinases or other cellular signaling molecules, these drugs may result in a complex phenotype that will require careful safety evaluation and monitoring. View
Aryl Hydrocarbon Receptor Signaling 1182 The Aryl-hydrocarbon receptor (AhR) is activated by polycyclic or halogenated aromatic hydrocarbons such as dioxin (TCDD), PCDD, PCB etc. In the absence of ligands, AhR is retained in the cytosol through complexing with Hsp90, co-chaperone p23, and immunophilin-like protein XAP2. Ligand binding to AhR triggers its nuclear translocations and binding with a different partner, ARNT. AhR/ARNT complex then binds to multiple DREs (Dioxin response element) in the promoter region of the target genes and activate their transcription. <br><br>The genes activated by AhR include genes of some Phase 1 and phase 2 drug metabolism enzymes, as well as, some of the genes involved in cell proliferation and apoptosis. AhR also induces the expression of one of its own repressors, AHRR (AhR repressor) which heterodimerize with ANRT. AHRR/ANRT dimer also binds to the DREs, but function as a repressor. Therefore, AhRR function as a feedback repressor of AhR signaling by competing with AhR for binding with ARNT, and with AhR/ARNT in occupying DRE.<br><br>AhR activation also down-regulates the expression of PEPCK2 and estrogen receptors. PEPCK2 was reported to be down-regulated by AhR ligands at both mRNA and protein activity level, which explained the wasting syndrome found in rodent following dioxin treatment. Estrogen receptors crosstalk with AhR through competitive binding to ARNT. Expression of the estrogen receptors is down-regulated by AhR activation. View
AMP Kinase: A Metabolic Master Switch 1182 Historically, the AMPK pathway was considered primarily as a sensor and regulator of cellular energy balance. It is now clear that in mammals, AMPK is also involved in the fundamental regulation of whole body energy balance by responding to hormonal and nutrient signals. AMPK is a heterotrimer composing of a catalytic subunit (alpha) and two regulatory subunits (beta and gamma). In mammalian cells the AMPK complex is allosterically (via the gamma subunit) activated by AMP, thus exposing a critical threonine residue (Thr172) on the alpha subunit for phosphorylation by upstream kinases. LKB1 and CAMKKbeta have been shown to be AMPK upstream kinases responding, respectively, to energy ([AMP]/[ATP] ratio) and oxidative/hyperosmotic ([Ca++]i) stresses.<br><br>Activation of AMPK switches on catabolic pathways that generate ATP such as glycolysis and the TCA cycle while switching off ATP-consuming processes such as gluconeogenesis and fatty acid synthesis. It achieves these effects by both direct phosphorylating metabolic enzymes and through inducing gene expression changes via phosphorylating transcription factors. Activation of AMPK in muscle increases glucose uptake, mitochondrial biogenesis, fatty acid uptake and oxidation. Activation of AMPK in liver suppresses fatty acid synthesis, cholesterol synthesis, and gluconeogenesis. Activation of AMPK in adipose tissue suppresses fatty acid synthesis and lipolysis. View
Interleukin-1 (IL-1) Signaling 1182 IL-1 (interleukin-1) is a pivotal pro-inflammatory cytokine centrally involved in local and systemic responses in the immune system, and can lead to various effects of inflammation. Dysregulated prolonged-synthesis and release of IL-1 in chronic inflammatory situations contributes diseases such as rheumatoid arthritis.<br><br>In IL-1 signaling, IL-1 receptor Type I (IL1-RI) forms a heterodimer with a second molecule, the IL-1 receptor accessory protein (IL-1RAcP). Binding of IL-1 to this receptor complex leads to activation of the transcription factor NF-kB through different signaling molecules. In addition, IL-1 signaling also activates MAP kinases which then activate transcription factor c-Jun, c-Fos and Elk-1 and leads to the expression of many genes involved in inflammation and acute phase response.<br><br> View
Jak-Stat Signaling 1182 The JAK/STAT pathway is one of the principle signaling mechanisms for a wide variety of cytokines, lymphokines, interferons and growth factors. JAK activation can stimulate cell proliferation, differentiation, migration and apoptosis. These cellular events are pivotal to hematopoiesis, immune development, adipogenesis, viral defense and other processes. Mutations that cause constitutive activation or de-regulation of JAK/STAT pathway may lead to inflammatory disease, erythrocytocis, gigantism and leukemias, etc. <br><br>The JAK-STAT signaling is initiated upon ligand binding to its receptor. Subsequent receptor multimerization causes trans-phosphorylation of JAKs in close proximity. The activated JAKs then phosphorylate additional targets such as the STATs. STATs are transcriptional factors that remain dormant in the cytoplasm until phosphorylated by JAKs. Phosphorylation by JAKs induces dimerization of the STATs which then enters the nucleus and binds to specific regulatory sequences to activate the transcription of target genes. The JAK/STAT pathway can be modulated by other signaling molecules. Adaptor molecule such as STAM, SHP-2 and StIP can bind and be phosphorylated by JAKs and facilitate the transcriptional activation of certain target genes. Negative regulators of JAK/STAT pathway include SOCSs, PIASs and protein tyrosine phosphatases such as SHP-1. SOCSs form a negative feedback loop in the JAK/STAT signaling since activated STATs stimulate the transcription of SOCS while the SOCS protein can bind phosphorylated JAKs to shut down their signaling. PIASs inhibit the JAK signaling via binding to STAT dimers and prevent them from binding to DNA. SHP-1 functions by binding to JAKs and dephosphorylates these activated signaling molecules. JAK/STAT pathway has also been demonstrated to cross-talk with other signaling pathways such as the MAPK and PI-3K pathways, providing another level of complexity toward understanding the biological consequences of JAK/STAT signaling. View
LPS & IL-1 Mediated Inhibiton of RXR Function 1182 Lipopolysaccharide (LPS, endotoxin) is an abundant component of the outer membrane of Gram-negative bacteria. The mammalian guts are colonized by Gram-negative bacterial, and the liver, by way of the portal vain, serves as the first line of defense against these bacteria. LPS, after entering into the bloodstream, are captured by the LPS binding protein (LBP) and activates the Toll-like receptor 4 (TLR4). LPS-binding protein is an acute phase protein, produced constitutively by the hepatocytes and its serum level is dramatically increased upon exposure to LPS<br><br>TLR4 signals through components of the signaling cascades of IL-1beta, TNFalpha, and TGFbeta, thus the effect of LPS in the bloodstream encompass a subset of the combined effects of these three receptors. One component of the TLR4 signaling cascades is the activation JNK (also activated by other stress signals, including proinflammatory cytokines, UV-light, DNA damage, heat shock) which phosphorylate a number of tyrosine residues on RXRalpha and induces a rapid and profound reduction of hepatic nuclear RXRalpha protein. Experimental evidence suggests that the Phosphorylated RXRalpha protein is exported out of the nucleus and targeted for degradation.<br><br>RXRalpha is the heterodimer partner of LXR, PPAR, FXR, PXR, CAR, TR, & RAR for activating the transcription of genes regulated by these nuclear receptors. LPS treatment or other stress conditions that leads to the activation of JNK resultss in decreased nuclear RXRalpha protein level. Since less RXR is available for heterodimerization with the RXRalpha associated nuclear hormone receptors, the transcription of PPAR-RXR, LXR-RXR, FXR-RXR, PXR-RXR, CAR-RXR, TR-RXR & RAR-RXR regulated genes are all down regulated leading to impaired xenobiotic metabolism, cholestasis, as well as, increased serum lipid and cholesterol (also see &quot;LPS & IL-1 Induced Changes in Lipid Metabolism&quot;). View
Angiotensin II Regulated Cardiac Hypertrophy & Myocyte Apoptosis 1182 Cardiac hypertrophy and remodeling is a compensatory response of myocardial tissue upon increased mechanical load. Multiple lines of experimental and clinical evidence indicate that angiotensin-II induces not only hypertension but also directly contribute to cardiac hypertrophy & remodeling and heart failure. In vitro experiments demonstrated that stretched cardiomycyte activate p53 mediated cellular rennin-angiotensin system, resulting in local synthesis of angiotensin-II (see &quot;Vascular Tissue Angiotensin System&quot;) which then initiate hypertrophic growth as well as reinforce the p53 signal. Animal experiments have demonstrated that the pathological changes associated with myocardial remodeling (left ventricular hypertrophy) are associated with angiotensin-II and are characterized by myocyte growth, fibrosis, & and death. Cardiomyocytes occupy approximately 75% of the total tissue space in heart, but account for only 30-40% of total cell number. Major cell types, within the cardiac non-muscle cell population, are cardiac fibroblasts, endothelial cells, and vascular smooth muscle cells (VSMCs). Myocardial remodeling is coordinately regulated by mediators released from all cardiac cell types upon stimulation by angiotensin-II. <br><br>Angiotensin-II signaling through AT-I receptor on cardiac fibroblast increases the production and release of TGFbeta, bEGF, HB-EGF, and Et-1, which collectively promote cardiomyocyte hypertrophy and cardiac fibrosis. Angiotensin-II signaling also activate the NADPH oxidase in fibroblasts, in endothelial cells and VSMCs by 1) hyper-phosphorylating p47(phox) leading to the translocation of p47(phox), and P67(phox) from cytosol as a complex to the membrane and associate with P22(phox) and NOX on the membrane to form the active NADPH oxidase complex, and 2) inducing the transcription of p22 and p67. NADPH oxidase functions to increase cellular superoxide anion which serves to increase vascular tone (vasoconstriction). However, excess superoxide anion is converted to H2O2 (by superoxide dismutase) and activates the apoptosis signal-regulating kinase (ASK-1) which then activates pro-hypertrophy and pro-apoptosis signaling cascades. View
Hepatomegaly and hepatocarcinogenesis 1182 None View
Cardiolipin & Mitochondrial Function 1182 Mitochondrial oxidative phosphorylation produces most of the ATP that powers a cell. The release of cytochrome c (shuttles electrons from complex III to complex IV of oxidative phosphorylation) into the cytoplasm is one of the first events that trigger cell death. Cytochrome c as well as all other proteins participating in oxidative phosphorylation is closely associated with cardiolipin which is synthesized in the inner membrane and accounts for up to 20% of the entire phospholipid mass of a cell. Cardiolipin was co-isolated with each of the proteins involved in oxidative phosphorylation. <br><br>After de novo synthesis, the lipids on cardiolipin are remodeled to incorporate unsaturated fatty acids so that mature cardiolipins are the most unsaturated lipids in a cell. This unsaturation is required for cardiolipin association with inner membrane proteins, and oxidation of the lipids by hydroxyl radical derived from reactive oxygen species causes dissociation of the membrane proteins from cardiolipin and interruption of oxidative phosphorylation. Approximately 10% of the mature cardiolipin is shuttled from the inner membrane to the outer membrane by phospholipid scramblase 3 (PLS3). During apoptosis, PKC-delta translocates to mitochondria and enhanced the activity of PLS3.<br><br>Bid, a Pro-apoptotic Bcl-2 family member (see &quot;Ca++ Signaling, Cytochrome c Release, & Apoptosis&quot;), is required for Fas-mediated apoptosis in liver and was found to associates preferentially with MLCL (monolysocardiolipin). MLCL is formed from cardiolipin via hydrolysis by PLA2 during cardiolipin remodel. As apoptosis progresses, amount of Bid associated with mitochondria increases simultaneously with a increase in the amount of MLCL and a decrease in the amount of cardiolipin. Caspase-8 cleaved Bid was found to bound MLCL tighter than Bid.<br><br>Thyroxin treatment increase the amount of cardiolipin synthase and the activity associated with acyl-CoA:lysocardiolipin acyltransferase 1 (ALCAT1). The amount of linoleate units on cardiolipin and the functional efficiency of mitochondria are dependent on the availability of linoleate from diet. Increased peroxisomal beta-oxidation (PPARalpha treatment) results in excess H2O2 which can overwhelm available catalase and leads to the production of hydroxyl radicals. Hydroxyl radicals peroxidize unsaturated membrane lipid, including cardiolipin, causing deactivation of mitochondrial enzymes and reduced mitochondrial function. Over expression of mitochondrial phospholipid hydroperoxide glutathione peroxidase (PHGPx) suppresses the peroxidation of cardiolipin. Oxidative stress caused by massive induction of P450 enzymes can overwhelm the capacity of available PHGPx and compromise mitochondrial function, presumably resulting in liver steatosis. View
Hepatic Steatosis 1182 Hepatic steatosis (fatty liver) can be due to chronic alcohol consumption, obesity, hyperinsulinemia (Type II diabetes), and drug or virus induced inflammation. Persistent hepatic steatosis eventually progress to fibrosis and cirrhosis. Macrovesicular steatosis, where accumulated triglyceride forms small number of large droplets occupying most of the space in the hepatocytes, is more common; while microvesicular steatosis, where the excess triglyceride forms numerous small lipid droplets in the hepatocytes, is less common but more serious, and is associated with mitochondrial dysfunction. <br><br>Ethanol induced steatosis is mediated by excess cellular acetaldehyde (from the oxidation of ethanol by alcohol dehydrogenase) which forms adducts with PPARalpha, reduce PPARalpha binding to PPRE and essentially reduce PPARalpha signaling in hepatocyte resulting in 1) reduced beta-oxidation, and 2) increased LXR signaling leading to increased SREBP-1c and increased fatty acid (FA) synthesis. Increased cellular malonyl-CoA (an intermediate of FA synthesis, synthesized via acetyl-CoA carboxylase) further inhibits beta-oxidation by inhibiting (palmitoyl-CoA transferase-1, CPT-1) mitochondrial uptake of fatty acids. A combination of increased fatty acid synthesis and reduced beta-oxidation leads to hepatic accumulation of triglyceride and macrovesicular steatosis. <br><br> CYP2E1 which also oxidizes ethanol is induced by ethanol, hormones, and various chemicals and produces reactive oxygen species (ROS) at a higher rate than other P450 enzymes due to a higher rate of oxidase activity. ROS readily reacts with polyunsaturated fatty acids (PUFA) and produce more stable aldehyde-containing reactive species that further propagate the initial ROS effects. Oxidative damage to cardiolipin (a PUFA rich phospholipids that anchors the respiration chain enzymes, see &quot;Cardiolipin & Mitochondrial Function&quot;) of Inner mitochondrial membrane results in impaired mitochondrial function leading to microvesicular steatosis, more ROS, and mitochondria mediated cell death. ROS initiated oxidative stress response can lead to the production of inflammatory cytokines and hepatic stellate cell activation and the state of steatohepatitis.<br><br>Physiological conditions and drugs that induce fatty acid biosynthesis, repress beta-oxidation, induce hepatic oxidative stress, or induce the production of inflammatory cytokines can lead to nonalcoholic steatohepatitis (NASH). View
Pyruvate Dehydrogenase Kinase & Fuel Switching 1182 The activity of the pyruvate dehydrogenase complex (PDC) determines (or reflects) cellular fuel preference. The enzymatic activity of PDC is increased by high concentrations of pyruvate, and is decreased by phosphorylation and under conditions of high [acetyl-CoA]/[CoA] and [NADH]/[NAD+] ratio. Activation of PDC promotes both glucose disposal and fatty acid synthesis, whereas the suppression of PDC activity is associated with glucose conservation. <br><br>Metabolic hormones and other signals regulates the activity of PDC via the transcriptional induction and suppression of pyruvate dehydrogenase kinases (PDK) which phosphorylate PDC and kinases (e.g., PKCdelta) which activate pyruvate dehydrogenase phosphatase (PDP) to de-phosphorylate PDC. PDK2 and PDK4 are expressed ubiquitously in the fed state, while PDK1 is expressed at high level primarily in the heart. In heart and skeletal muscle, suppression of PDC activity (such as starvation or treatment with PPAR agonists) increases diversion of pyruvate to oxaloacetate and facilitates entry of acetyl-CoA derived from fatty acid beta-oxidation into the TCA cycle. In liver and kidney, inactivation of PDC serves to conserve glucose as well as to divert available pyruvate to gluconeogenesis for maintaining circulating glucose level. PPARalpha exerts a direct role in the regulation of gluconeogenesis via PDK4, guiding the utilization of pyruvate for gluconeogenesis rather than for fatty acid synthesis. View
HIF & Cellular Oxygen Sensing 1182 The hypoxia inducing factor, HIF-1alpha is constitutively expressed in most tissues. Its protein level is regulated by ubiquitin mediated degradation. In order for the protein degradation to occur, two prolin residues (pro-402 & pro-564) in HIF-1alpha need to be hydroxylated so that binding with the von Hippel-Lindau (VHL) protein can occur. HIF-1alpha binding to VHL is a prerequisite for its degradation via the ubiquitin pathway. Hydroxylation of the prolin residue is affected by three HIF-prolyl hydroxylases (HIF-PHD). HIF-PHD is a member of the non-heme Fe++, alpha-ketoglutarate dioxygenases and requires ascorbate, alpha-ketoglutarate, Fe++, and O2 to perform its enzymatic function. When the partial pressure of O2 is low (as in hypoxia), the enzymatic activity is low, allowing HIF-1alpha to accumulate, bind to HIF-1beta, migrate to nucleus and initiate transcription of genes regulated by hypoxia response element (HRE, see &quot;Hypoxia and HIF Signaling&quot;). <br><br>First row group-8B transition metals, Co++ & Ni++, can displace Fe++ at the active site of the alpha-ketoglutarate dioxygenases but can not replace the function of Fe++. Co++ (CoCl2) has been clearly documented to cause hypoxia by preventing interaction between HIF-1alpha and VHL. Acute exposure to nickel has been documented to cause accumulation of HIF-1alpha and was thought to be the reason behind the carcinogenic property of nickel. Sodium orthovanadate was reported to induce the gene expression of HIF-1alpha via a PI3K and mTOR dependent pathway involving the generation of hydrogen peroxide from the reduction of vanadium V to vanadium IV. View
Reabsorption in Kidney - Late Proximal Tubule 1182 The renal glomerular membrane retains cells and large proteins within the blood. Small proteins (< 70 kD, such as albumin, myoglobin, hemoglobin, and polypeptide hormones), glucose, amino acids, nitrogenous waste products, drugs and their metabolites, salts, and water pass into the glomerular filtrate. Small proteins are reabsorbed by receptor mediated endocytosis at the brush-border membrane of the proximal tubules (see &quot;Renal Function: Regulation of Protein Endocytosis&quot;). The proximal tubules, residing in the cortex, is the &quot;bulk&quot; phase of the reabsorption and up to 100% of filtered Glucose, amino acids, di- and tripeptides, and bicarbonate are reabsorbed here. Up to 80% of filtered phosphate, sulfate, Na+, K+, Ca++, Mg++ and water are reabsorbed in this area of the nephron. Fine tuning of the electrolyte balance takes place in the distal tubules and the collecting ducts after the urine has been concentrated in the loop of Henle.<br><br>The majority of the NaCl filtered is reabsorbed via an active transporter complex on the apical brush-border membrane that is composed of a chloride-formate exchanger (CFEX, Dlc26a6) functioning in concert with NHE-3 (Na+-H+ exchange) and H+-formate exchange, and exit the cell through voltage-gated chloride-channels (ClC-5, ClC-K1 and ClC-Kb) on the basolateral membrane. CFEX is also capable of transporting sulfate, formate and oxalate. The bulk of the sulfate in lumen fluid is reabsorbed via a Na+-sulfate cotransporter (NaSi-1) on the brush-border membrane and exit through a Na+-independent - oxalate-dependent anion exchanger, sulfate anion transporter (SAT-1, Slc26a1) on the basolateral membrane. The driving force of the ion transport is maintained by the Na+/K+ ATPase. View
Angiotensin II & Cardiac Hypertrophy 1182 Cardiac hypertrophy and remodeling is a compensatory response of myocardial tissue upon increased mechanical load. Multiple lines of experimental and clinical evidence indicate that angiotensin-II induces not only hypertension but also directly contribute to cardiac hypertrophy & remodeling and heart failure. Cardiomyocytes occupy approximately 75% of the total tissue space in heart, but account for only 30-40% of total cell number. Major cell types, within the cardiac non-muscle cell population, are cardiac fibroblasts, endothelial cells, and vascular smooth muscle cells (VSMCs). Angiotensin-II induced cardiac remodeling is coordinately regulated by mediators released from all cardiac cell types. <br><br>Angiotensin-II signaling (G-protein Gq) in cardiomyocyte through AT-I receptor via c-Src releases bFGF and HB-EGF which reinforce the c-Src signal. c-Sar also enhances the release of Ca++ from intracellular stores initiated by Gq signaling and leads to the activation of NF-AT3. Angiotensin-II also activate ASK1 (a member of MAPKKK) in cardiomyocyte (see &quot;Angiotensin-II Regulated Cardiac Hypertrophy & Myocyte Apoptosis&quot;) which activate atrial natriuretic peptide (BNP). Angiotensin-II signaling in cardiac fibroblast increases the production and release of a host of local hormones, growth factors and other cytokines which signal via their cognate receptors on cardiomyocyte to STAT-3, ACTH, and BNP. Signaling through TGFbeta, ANT, ATCH, and the activation of NF-AT3 collectively, promotes cardiomyocyte hypertrophy and cardiac fibrosis. View
LPS & IL-1 Induced Changes in Lipid Metabolism 1182 Lipopolysaccharide (LPS, endotoxin) is an abundant component of the outer membrane of Gram-negative bacteria. The mammalian guts are colonized by Gram-negative bacterial, and the liver, by way of the portal vain, serves as the first line of defense against these bacteria. LPS, after entering into the bloodstream, are captured by the LPS binding protein (LBP) and activates the Toll-like receptor 4 (TLR4). LPS-binding protein is an acute phase protein, produced constitutively by the hepatocytes and its serum level is dramatically increased upon exposure to LPS<br><br>TLR4 signals through components of the signaling cascades of IL-1beta, TNFalpha, and TGFbeta, thus the effect of LPS in the bloodstream encompass a subset of the combined effects of these three receptors. One component of the TLR4 signaling cascades is the activation JNK (also activated by other stress signals, including proinflammatory cytokines, UV-light, DNA damage, heat shock) which phosphorylate a number of tyrosine residues on RXRalpha and induces a rapid and profound reduction of hepatic nuclear RXRalpha protein. Experimental evidence suggests that the Phosphorylated RXRalpha protein is exported out of the nucleus and targeted for degradation.<br><br>RXRalpha is the heterodimer partener of LXR, PPAR, FXR, PXR, CAR, TR, & RAR for activating the transcription of genes regulated by these nuclear receptors. LPS treatment or other stress conditions that leads to the activation of JNK results in decreased nuclear RXRalpha protein level. Since less RXR is available for heterodimerization with the RXRalpha associated nuclear hormone receptors, the transcription of PPAR-RXR, LXR-RXR, FXR-RXR, PXR-RXR, CAR-RXR, TR-RXR & RAR-RXR regulated genes are all down regulated leading to cholestasis, as well as, increased serum lipid and cholesterol (also see &quot;LPS & IL-1 Mediated Inhibiton of RXR Function&quot;). View
Urea & Aspartate Cycle 1182 In mammals excess nitrogen arising from metabolic break down of amino acids are converted to urea in the liver and excreted in the urine. As depicted in the pathway drawing, ornithine, citrulline, arginosuccinate, and arginine make up the urea cycle. In addition to converting ammonia to the less toxic urea, the urea cycle formally provides arginine for the synthesis of creatine. In mammals, a complete urea cycle operates only in the liver, Arg biosynthesis for other body tissues take place primarily in the kidney from citrulline synthesized in the liver. <br><br>One of the nitrogen in urea comes from cellular ammonia by way of incorporation into citrulline as carbamoyl phosphate; the other nitrogen comes from aspartate while aspartate, arginosuccinate, fumarate, malate, and oxaloacetate make up the aspartate cycle. The aspartate cycle operates in the cytosol while the high energy consumption portion of the urea cycle resides in the mitochondria. Carbamoyl phosphate synthase-1 (CPS1), which catalyzes the condensation of ammonia and bicarbonate to form carbamoyl phosphate, is allosterically activated by N-acetylglutamate. High rates of amino acid breakdown result in high cellular concentration of [glutamate] and synthesis of N-acetylglutamate. Activity of other enzymes of the urea cycle is [substrate] dependent. View
LPS Induced Inflammatory Responses 1182 Lipopolysaccharide (LPS, endotoxin) is an abundant component of the outer membrane of Gram-negative bacteria. The mammalian guts are colonized by Gram-negative bacterial, and the liver, by way of the portal vain, serves as the first line of defense against these bacteria. LPS, after entering into the bloodstream, are captured by the LPS binding protein (LBP) and escorted to membrane bound CD14 on the surface of immune cells (such as the macrophages and Kupffer cells in liver) and presented to the Toll-like receptor 4 (TLR4). TLR4, CD14, and MD-2 make up the LPS receptor complex. LPS-binding protein is an acute phase protein, produced constitutively by the hepatocytes and its serum level is dramatically increased upon exposure to LPS.<br><br>TLR4 signals through components of the signaling cascades of IL-1beta, and TNFalpha, and TGFbeta, thus the effect of LPS in the bloodstream encompass a subset of the combined effects of these three receptors. TLR4 signaling leads to the production of TNFalpha, IL-1beta, and IL-6 (see &quot;Toll-like Receptor Signaling&quot;) which then leads to the observed hepatotoxicity associated with LPS. <br><br>TRAF6 (TNF receptor associated factor 6), once activated (by IRAK, IL-1 receptor associated kinase), become ubiquitinated by the associated ubiquitin-conjugating enzymes, Ubc13 & Uev1A. IRAK also activates the adaptor proteins TAB2 & TAB3 which, after relocating to cytoplasm, binds to ubiquitinated TRAF6 and TAK1 (TGFbeta activated kinase) and causing TAK1 to be activated. In the TNFalpha signaling cascade, binding of RIP to TRAF2 cause RIP to be ubiquitinated followed by association with TAB2 and TAK1 activation. View
LXR & Adipocyte Differentiation 1182 Adipose tissue, in addition to being a major site of energy storage and expenditure, is an endocrine that secrete adipokines, such as adiponectin, resistin, leptin, TNF-alpha, & IL-6, which regulate glucose and lipid metabolism. Increase in adipocyte number is initiated by the recruitment of preadipose cells, and increase in the galectin-12 protein which arrests the preadipocytes at the G1-phase and enable them to response to stimulus signal from activated insulin receptor. In 3T3-L1 cell culture, IGF-1 receptor signaling leads to the activation of CREB and ATF1 (both constitutively expressed by 3T3-L1 preadipocytes) which then cooperatively induce the transcription of C/EBP-beta/delta. Although C/EBP-beta/delta are induced within 2 - 4 h, it is unable to bind DNA until after 10 - 12 h when the cells have reentered the cell cycle and have passed the G1-S check-point. Upon acquiring DNA-binding activity, C/EBP-beta/delta activates the transcription of the C/EBP-alpha and PPAR-gamma genes which then induced the transcription of cascades of down-stream genes that give rise to the adipocyte phenotype.<br><br>Activated LXR-alpha, a down-stream gene of PPAR-gamma, up-regulates the expression of both PPAR-gamma and SREBP-1c. Thus, the endogenous RXR ligands, the oxysterols, can enhance both adipocyte differentiation and lipogenesis.<br><br>Adiponectin is exclusively expressed in differentiated adipocytes and plays important roles in energy metabolism and inhibits the development of atherosclerosis. View
Reabsorption in Kidney - Distal Tubule and Collecting Duct 1182 The filtration and reabsorption process that take place in the nephron in order to eliminate metabolic waste without unduly removal of reusable metabolites is regionally specialized with respect to kidney anatomy. In the cortex, the pressure inside the afferent arterioles provides the driving forces that promote glomerular filtration, while the peritubular capillary (arising from efferent arterioles) surrounding the proximal and distal convoluted tubules accommodates the enormous reabsorption of the glomerular filtrate. In the medulla the hairpin band of the loop of Henle (renal tubule between the proximal and the distal tubule) and the surrounding vasa recta (capillaries that descend from the cortex to the medulla, forming a hairpin band in the medulla and return to the cortex) coordinately form a counter-current mechanism to concentrate the urine. The tubular fluid, after passing through the loop of Henle, is hypotonic as it enters the distal convoluted tubule residing in the cortex. <br><br>The tight junction in this portion of the nephron is impermeable to both water and ions, and the absorption of either is regulated independently by hormones including aldosterone, vasopressin, angiotensin II, and the parathyroid hormone. The distal tubule and the collecting duct are the main nephron segments where Na+ reabsorption is adjusted to maintain excretion at a level appropriate for the dietary intake. The activity of the apical ENaC (positively correlated with plasma aldosterone level, negatively correlated with dietary NaCl intake) largely determines the rate of Na+ reabsorption in this part of the nephron. Low NaCl diet and high plasma aldosterone markedly increase the abundance of ENaC, TSC, NKCC2, and ROMK on the apical membrane. View
Regulation of SREBP 1182 Immediately after synthesis, SREBPs form a complex with the ER protein SCAP (SREBP cleavage-activating protein). In the presence of sterols, this complex is retained in the ER by sterol-induced binding of SCAP to Insigs (insulin induced genes 1 & 2) which are resident ER membrane proteins. SCAP directly binds to cholesterol and undergoes a conformational change that allows it to bind Insigs. Oxysterols do not interact with SCAP directly but appears to bind to another protein which then induces SCAP binding to Insigs. Oxysterols are >50-fold more potent than cholesterol in inhibiting the activation of SREBP-2. High cellular lanosterol, the first steroidal intermediate in the cholesterol biosynthesis pathway, and oxysterols promotes HMG-CoA reductase (HMGCR) binding to Insigs at the same binding site used by SCAP. Binding of HMGCR to Insigs leads to ubiquitination & degradation of HMGCR. Cholesterol itself has no effect on the HMGCR protein level.<br><br>Although Insig-1 and Insig-2 have identical biochemical function in terms of regulating SREBP activation, these two proteins are regulated differently. Insig-1 has a high turn-over rate, is up-regulated by insulin, SREBP, PPAR-gamma, and down-regulated by ER-stress (via reduced protein synthesis). Thus, agents/drugs or conditions that cause ER-stress can activate the SREBPs in liver and lead to fatty liver. Insig-2 has slower turn-over rate, is down-regulated by insulin, but is not affected by ER-stress. Insig-1 mRNA increase progressively with a high-fat diet (mice) and declined on a restricted diet. Studies in 3Tc-L1 preadipocytes suggested that insig-1 over-expression restricts lipogenesis in mature adipocytes and blocks differentiation of pre-adipocytes.<br><br>Oxysterols, in addition to inhibiting SREBP-activation, is the natural ligand of LXR. Ligand-activated LXR competes with RXR for binding with PPAR-alpha leading to down regulation of genes required for fatty acid beta-oxidation.<br><br>The transcription of SREBP-1c can be regulated positively by nuclear SREBP (together with NF-Y or Sp1, the SRE on SREBP-1c promoter is flanked NF-Y and Sp1 binding sites) and by ligand activated LXR. Insulin up-regulation of SREBP-1c transcription appears to work through LXRE in conjunction with NF-Y site and SRE. It is possible that the well documented positive effect of insulin on the transcription of SREBP-1c is entirely mediated at the level of Insig-2, since decreased amount of Insig-2 will increase the amount of nuclear SBEBP-1c which auto-regulate its own transcription as well as the fatty acid synthesis genes it regulates. View
G-protein Gs Signaling 1182 G-proteins are heterotrimeric guanine nucleotide-binding proteins that communicate and route extracellular signals to distinct intracellular pathways. G-protein signaling are initiated by extracellular ligand (neurotransmitters, hormones, chemokines, and autocrine/paracrines) binding to their cognate G-protein-coupled receptors (GPCRs). The signaling pathways of the different G-proteins interact with each other to form a network that regulate the activity of proteins involved in cellular metabolism and a broad range of other cellular. G-proteins signal as if they were dimers since the signals are communicated either by the G-alpha subunit of by the G-beta-gamma complex. G-proteins are divided into four families, Gs, Gq/11, Gi/o, and G121/13, corresponding to their sequence similarity and effector coupling.<br><br>The Gs family are sensitive to cholera toxin and signal to activate adenylyl cyclase resulting in increased [cAMP]i which can activates two signaling cascades mediated respectively by PKA and Rap. Both direct signaling from PKA and Rap signaling via the MAP kinase cascade can increase the transcription of cAMP regulated genes via activation of the transcription factor CREB. <br><br>PKA activates phosphorylase b kinase by phosphorylating the alpha and the beta regulatory subunits leading to the phosphorylation and activation of glycogen phosphorylase and glycogen release from glycogen stores in the liver and muscle. PKA also phosphorylate L-type Ca++ channel in skeletal and cardiac muscles resulting increased [Ca++]i and enhanced contraction. View
Sphingosine Signaling 1182 Three sphingosine dependent protein kinases, SDK1, SDK2, & SDK3 have been characterized. SDK1 phosphorylate the 14-3-3 proteins (ER chaperones), SDK2 phosphorylate protein disulfide isomerase (PDI, ER chaperone essential for maintaining disulfide-dependent secondary protein structure) & calreticulin (binds Ca++, regulates intracellular Ca++ level, essential for the proper folding of glycoproteins), while SDK3 phosphorylate heat-shock proteins (HSP) and glucose-regulated proteins (GRP). Thus, the common function of SDK is the modulation of molecular chaperones for maintaining homeostasis & responding to environmental stress. SDK1 was found to be the kinase domain of PKC-delta produced by capase-3 cleavage of PKC-delta. 14-3-3 proteins are chaperones which, in the non-phosphorylated state, form dimmers and associate with Raf1, Bad, Bax, Cdc25 and KSR, etc., to sequester them in the inactive form. Phosphorylation of 14-3-3 by SDK1 dissociates the dimmer and releases the bound proteins. 14-3-3 binding to target protein is also dependent on specific phosphoserine and phosphothreonine motifs on the target protein. Thus the effect of 14-3-3 is modulated by protein kinases and protein phosphatases.<br><br>Exogenous C2-ceramide and sphingosine were demonstrated to up-regulate the expression of the transcription factor, c-jun, in HL-60 cells; the effect of C2-cerimide was inhibited by PKC inhibitors, while the effect of sphingosine was enhanced by PKA inhibitor. C-2 ceramide was also shown to enhance the binding of AP-1 (hetero-dimmer of c-jun and c-fos) to its consensus DNA sequence. The effect of C2-ceramide was antagonized by curcumin which is known to associate with AP-1 and inhibits its binding to DNA.<br><br>In mammalian cells, the phospholipid recycling pathway recovers sphingosine from late endosomes & lysosomes, and converts it to S1P (more hydrophilic) for transportation to ER where it is dephosphorylated by SPP to sphingosine. Depending on the balance of sphingosine, ceramide, S1P and the cell's metabolic state, this sphingosine can participate in signaling or be converted to ceramide and eventually sphingomyelin.<br><br>Agonist activated Gq/ Gi coupled G-proteins receptors induce two waves of diacylglycerol; the first wave is short and coincides with the release Ca++ and IP3 (from phosphatidyl-inositol-diphosphate) while the second wave is larger, prolonged, and results from the transcriptional up-regulation of PLD by PKC. The source of the second wave of diacylglycerol is from the hydrolysis of phosphatidylcholine by PLD (to give phosphatidic acid) followed by the action of phosphatidic acid phosphatase (PAP). By simultaneously activating PLD and inhibiting PAP, sphingosine shuts down the PKC/DAG pathway and cause accumulation of phosphatidic acid leading to the activation of sphingosine kinase and the production of sphingosine-1-phosphate (S1P, see &quot;Sphingosine-1-Phosphate (S1P) Signaling&quot;). Sphingosine and S1P produce opposing effects on growth arrest & apoptosis. View
Estrogen Receptor Signaling 1182 Estrogens are hormones important for development of reproductive systems, the regulation of bone metabolism, and confer cardioprotective effects. The biological action of estrogens are mediated by binding to the two specific estrogen receptors (ERs), ERalpha and ERbeta, which are ligand-regulated transcription factors that belong to nuclear receptor family. The ligand-free ERs mainly reside in the nucleus. In the classic model, binding of estrogen to ER releases the ER from heat shock proteins and immunophilins, and induces receptor dimerization and binding to specific ERE elements of target genes. The DNA bound ERs modulate target gene expression by interacting with a number of transcription co-activators, such as p160SRC and CBP.<br><br>In addition to the classical model, ER also control target gene transcription through a range of other protein-DNA, and protein-protein interactions. ER is known to function cooperatively with SP1 in the absence of ERE. In the absence of DNA binding, ERs may modulate transcription activities of other transcription factors; for example, ER represses the transcriptional activity of AP-1 and NF-kB. <br><br>ERalpha regulates MAP kinase activity by interacting with SH2 domain of Src, an effect associated with induction of cell proliferation. ERalpha was also shown to interact with p85a regulatory subunit of PI3K, increasing the activity of PI3K in vascular endothelial cells and leading to the activation of Akt and endothelial NOS. This may contribute to the cardioprotective effects of estrogens, and appears to be ERalpha specific. Akt and MAPK can also phosphorylate ER, thereby enhancing the transcriptional activation by the ER. View
G-Protein Gq Signaling 1182 G-proteins are heterotrimeric guanine nucleotide-binding proteins that communicate and route extracellular signals to distinct intracellular pathways. G-protein signaling are initiated by extracellular ligand (neurotransmitters, hormones, chemokines, and autocrine/paracrines) binding to their cognate G-protein-coupled receptors (GPCRs). The signaling pathways of the different G-proteins interact with each other to form a network that regulate the activity of proteins involved in cellular metabolism and a broad range of other cellular. G-proteins signal as if they were dimers since the signals are communicated either by the G-alpha subunit of by the G-beta-gamma complex. G-proteins are divided into four families, Gs, Gq/11, Gi/o, and G121/13, corresponding to their sequence similarity and effector coupling.<br><br>The Gq family are insensitive to either the cholera or pertussis toxin and signal to activate phospholipase C-beta for the production of intracellular messengers inositol triphosphate (IP3) and diacylglycerol (DAG) from membrane phospholipids. IP3 and DAG signal coordinately to release the intracellular Ca++ stores and activate the store-operated Ca++ channel on the cell plasma-membrane to increase [Ca++]i which then initiate down-stream signaling cascades (see &quot;Calcium Signaling in Nonexcitable Cells&quot;). Gq can also signal to activate PYK2 leading to PI3K activation and down-stream activation of the transcription factor NF-kB (see &quot;NF-kappa B Signaling&quot;).<br><br> View
Ceramide Signaling & Apoptosis 1182 A number of direct targets of ceramide have been identified: 1) ceramide activated protein phosphatases (PP1, PP2A), 2) ceramide activated protein kinases (KSR [kinase suppressor of ras], PKC-zeta, PKC-delta), 3) cathepsin D, <br><br>Anti-TNF-alpha antibodies reverse inflammatory bowel disease while TNF-alpha over expression induces the disease in animal models. In intestinal epithelial cells (YAMC cells), addition of TNF-alpha, C8-ceramide, or sphingomyelinase stimulate ERK1/2 activation via KSR-autophosphorylation. Both Raf and KSR are normally sequestered in the cytosol by specific phosphorylation dependent binding to the 14-3-3 proteins. Ceramide activated PP2A disrupt this binding and allow the KSR mediated MAPK signaling cascade to progress. In EGF stimulated ERK1/2 activation, de-phosphorylated form of KSR serves as a scaffold for Raf & MEK of the ras signaling pathway (the kinase activity is not required).<br><br>PKC-zeta is phosphorylated seconds after TNF-alpha stimulation (U937 cells) and ceramide was found to bind and regulate the kinase activity of PKC-zeta in a biphasic manner (early high affinity, later lower affinity). Arachidonic acid was found to compete for ceramide binding and inhibit PKC-zeta activity.<br><br>Ceramide binds to PKC-delta and escorting it to the Gogi complex to be activated by phosphrylation which then phosphorylate and increase the activity of phospholipid scramblases leading to increased amount of cardiolipin in mitochondrial outer membrane (see &quot;Cardiolipin & Mitochondrial Function&quot;). View
Renal Function: Calcium Homeostasis 1182 Renal tubular calcium reabsorption is a critical determinant of extracellular fluid Ca++ concentration, and for this reason the renal tubular Ca++ handling is tightly controlled. Parathyroid hormone (PTH), 25-OH-vitamin D3, calcitonin, and extra-tubular [Ca++] are involved in controlling the renal tubular Ca++ reabsorption. <br><br>Vitamin D3 is synthesized from 7-dehydrocholesterol (a precursor of cholesterol) in the skin by exposure to UV light, or can be derived from dietary source. Vitamin D3 is converted to 25-OH-vitamin D3 (25D) in the liver by the mitochondrial enzyme, CYP27A1 (same enzyme involved in oxysterol biosynthesis), and a number of microsomal enzymes (CYP2D25, CYP2R1, CYP2C11, and CYP3A4). 25D bound to vitamin D binding protein (DBP or GC a plasma transport protein) is stable and is the major circulating vitamin D metabolite in blood. The metabolic fate of 25D is dependent on Ca++ requirement of the animal.<br><br>Extracellular [ca++] regulates renal Ca++ reabsorption via the calcium sensing receptor (CaSR) which is expressed in the apical membrane of proximal tubules and the inner medullary collecting duct, and in the basolateral membrane of the thick ascending limb (TAL) and the distal convoluted tubule (DCT). CaSR activation results in inhibition of PTH release, decreased urinary Ca++ reabsorption, and increased calcitonin secretion. Loss-of-function CaSR mutations give rise to hypocalciuric hypercalcemia, while gain-of function CaSR mutations cause hypoparathyroidism. Activated CaSR also increase intracellular free [Ca++] and decrease hormone-dependent cAMP accumulation.<br><br>Reduced amount of calcitriol (the active form of vitamin D), in turn, leads to less activation of the vitamin D receptor (VDR) and reduced transcription of its cognate genes, and reduced tubular Ca++ and Mg++ reabsorption via the tight junction (involving paracellin-1). Calbindins (Calb) are vitamin D-dependent Ca++ binding proteins involved in intracellular Ca++ transport in kidney. Calcium regulates Calb-D9k expression by modulating the circulating level of calcitriol (via the expression level of CYP24).<br><br> View
p53 Signaling 1182 P53 is a tumor suppressor protein at the nexus of multiple stress response pathways. It is the most commonly mutated genes in human cancer. In the absence of stress, p53 protein is maintained at low levels by ongoing protein degradation mediated by mdm2 (a p53-specific E2 ubiquitin ligase). However, in response to stress, p53 becomes activated and acts as a transcription factor. By activating different subset of it target genes, p53 mediates a wide range of biological processes including cell-cycle arrest, DNA repair, angiogenesis, differentiation and apoptosis.<br><br>DNA damage induced p53 activation involves ATM/ATR protein kinases, which activate chk2 that, in turn, phosphorylates p53. Phosphorylation of p53 blocks the ability of Mdm2 to target p53 for degradation. Mdm 2 is negatively regulated by ARF (a cyclin dependent kinase inhibitor) which is induced by E2f, thus, linking the Rb/E2f system with the p53 pathway. Phosphorylation of Mdm2 by Akt is required for Mdm2 to translocate from the cytoplasm to the nucleus, where it targets p53 for degradation. Thus, elevated Akt level will lead to attenuation of p53 function. Activation of p53 by hyperactive oncogenes such as beta-catenin and Ras is considered to be a protective mechanism aimed at alerting p53 for the tumorigenic threat. View
TGF-beta Signaling 1182 TGF-beta regulates a wide variety of cellular processes, including embryonic development, cell growth, differentiation and apoptosis. TGF-beta can cause growth inhibition of many nontransformed cell types, while stimulating proliferation of others. Defects in TGF-beta signaling, including mutation in SMADs, have been implicated in various malignancies. TGF-beta can cause cell cycle arrest by activating SMAD, which then induces the transcription of growth inhibitory regulators such as p21 & p15, and suppresses the transcription of growth stimulatory proteins such as c-Myc.<br><br>The TGF-beta receptor is composed of type-1 and type-2 subunits that are serine-threonine kinases which signal through the SMAD family of transcriptional regulators. Prior to activation, receptor regulated SMADs are anchored to the cell membrane by SARA (SMAD Anchor for Receptor Activation) that brings the SMADs into the proximity of TGF receptor kinases. Once a ligand binds to the receptor, the type-2 receptor phosphorylates the type-1 receptor. The activated type-I receptor then transmits the signal to the SMADs. TGF-beta R1 phosphorylates SMAD2 and SMAD3, which then bind to the mediator SMAD4 to move into the nucleus and form complexes that regulate transcription. The inhibitory SMAD-7 represses signaling by other SMADs to down-regulate the system. SnoN regulates TGF-beta signaling, by binding to SMADs to block transcriptional activation. The MAP kinase-ERK cascade is activated by TGF-beta signaling and functions as a negative feedback mechanism, possibly through JNK, to modulate SMAD activation. SMADs regulate transcription in several ways, including binding to DNA to induce transcription of target genes, interacting with other transcription factors, and interacting with transcription co-repressors and co-activators such as p300 and CBP.<br><br>SMAD2,3,1,5,8 are receptor activated SMADs, SMAD4 is a cooperating SMAD. SMAD6 and SMAD7 are inhibitory SMADs that interfere with the phosphorylation of receptor-activated SMADs and their association with SMAD4. SMAD6 is activated by BMPs and de-activated by TGFbeta while SMAD7 is activated by TGFbeta. The regulation of transcription by the TGFbeta family signaling systems is a dynamic process. Competition for SMAD interaction between co-activators and co-repressors determines the out come of signaling events. The balance is determined by the relative level of the proteins involved as well as signaling input that affect their activity. Many of the SMADs, their co-activators, and co-repressors also participate in other pathways. View
Renal Injury: TGFbeta Induced Hypertrophy & Fibrosis 1182 Ischemic acute renal failure is characterized by a reduction in glomerular filtration rate, reduced renal blood flow, & tubular (primarily proximal) injury. Recovery occurs by the restoration of renal blood flow and regeneration of damaged tubular epithelium. If the tubule injury is severe, the recovery progress in multiple phases starting with tubular cell proliferation followed by basement membrane remodeling, cellular hypertrophy, differentiation, and apoptosis of hyperplastic epithelial cells. This process ultimately returns the proximal tubule to normal morphology. <br><br>The immediate post-ischemic environment is dominated by pro-mitogenic growth factors such as HB-EGF, HGF, FGF, and IGF-I in the first 1-2 days, thereafter more greatly influenced by the anti-proliferative effects of TGFbeta. TGFbeta inhibits proliferation of renal proximal tubule cells in vitro; stimulate ECM synthesis, cell clustering, differentiation, & apoptosis. Expression of TGFbeta is elevated up to 14 days after tubular damage. TGFbeta signaling participates in the regulation of both ECM synthesis and dissolution, the balance of which determines the amount of matrix accumulation.<br><br>Renal injury can result in proliferation, hypertrophy, or apoptosis. Proliferation requires normal progression through the cell cycle; hypertrophy occurs when cells are engaged in the cell cycle but could not progress through the G1/S checkpoint; apoptosis is associated with exit from cell cycle. Entry of quiescent cells (G0) into G1 requires cyclin-D which is transcriptional regulated by extracellular growth factors and is repressed by TGFbeta through the CDK-inhibitor p27. The level of p27 is critical in renal cell differentiation, apoptosis, proliferation, and hypertrophy; proliferating glomerular & tubular cells have almost undetectable level of p27.<br><br>Renal fibrosis is the terminal fate of many chronic renal diseases and is the main cause of kidney failure requiring dialysis or kidney transplant. Smad3 appears to be the dominant fibrosis mediator for TGFbeta. The non-smad signals that are also induced by TGFbeta (p38, ERK, JNK activation) appear to function primarily as smad2/3-signal modifier and their effects are condition-dependent. Bone morphogenetic protein-7 (BMP7) is a member of the TGFbeta superfamily of cytokines and plays pivotal roles during embryogenic renal and eye development. In adults, BMP7 expression is only retained in kidney and its level declines progressively in early experimental diabetic or obstructive nephropathy, predating histological onset of glomerular & tubular/interstitial fibrosis. In experimental models, treatment with BMP7 reduces fibrosis and preserves renal function. The transcriptional activation of plasminogen activator inhibitor (PAI-1, lowers matrix degradation) by TFbeta is an important contributor to glomerular fibrogenesis. BMP7 signals through smad5 to antagonize the TGFbeta induced fibrogenesis by modulating the induction of PAI by TGFbeta. Smad6 is induced by BMP7 downstream of smad7 and functions the same way as smad6 in repressing TGFbeta induced fibrogenesis. View
G-Protein G12 Signaling 1182 G-proteins are heterotrimeric guanine nucleotide-binding proteins that communicate and route extracellular signals to distinct intracellular pathways. G-protein signaling are initiated by extracellular ligand (neurotransmitters, hormones, chemokines, and autocrine/paracrines) binding to their cognate G-protein-coupled receptors (GPCRs). The signaling pathways of the different G-proteins interact with each other to form a network that regulate the activity of proteins involved in cellular metabolism and a broad range of other cellular. G-proteins signal as if they were dimers since the signals are communicated either by the G-alpha subunit of by the G-beta-gamma complex. G-proteins are divided into four families, Gs, Gq/11, Gi/o, and G121/13, corresponding to their sequence similarity and effector coupling.<br><br>The G12/13 families were discovered trough sequence similarity to known G-alpha proteins. In contrast to the Gs, Gi, and the Gq families, It is not clear which GPCRs are coupled to them and the biological end-points of their activation are also not well characterized. The major downstream effector of G12/G13 is Rho (a family of small GTPase, see &quot;Rho Signaling to Actin Cytoskeleton&quot;). Rho is activated (GTP-bound form) by RhoGEF (Rho guanine nucleotide exchange factors) and deactivated (GDP-bound form) by GAPs (GTPase activating proteins). N-terminal short sequences of G12 alpha-subunit interact with RhoGEFs through a motif similar to that found in RGS (regulators of G-protein signaling, function as GAPs [GTPase activating protein]). Thus, through it N-terminal sequence, G12/13 can signal to activate Rho via RhoGEF and signal to deactivate other small GTPases such as Ras via GAP. <br><br>G12/13 can also activate PLD to hydrolyze membrane phosphatidylcholine and release phosphatidic acid which acts in concert with diacylglycerol to increase intracellular [Ca++].<br><br> View
P450 Family 1182 Cytochrome P450s constitute a superfamily of microsomal heme-thiolate proteins that catalyze the oxidative, peroxidative and reductive metabolism of a wide variety of endogenous and exogenous compounds. This superfamily is divided into families and subfamilies based on nucleic acid sequence similarities. Most biotransformation of drugs and xenobiotics is carried out by enzymes from the family of CYP1, CYP2 and CYP3. Other families are mainly involved in metabolizing endogenous metabolites and building blocks, such as fatty acid, bile acid, prostaglandins and steroid hormones. The unique properties of P450s play profound roles in drug or carcinogen activation and detoxification, as well as drug-drug interactions.<br><br>The CYP gene expressions are selectively regulated by drugs through action of ligand-activated nuclear receptors (see &quot;Xenobiotic Metabolism&quot; pathway). Polyaromatic hydrocarbons such as TCDD induce CYP1a and CYP1b genes through binding to the aryl-hydrocarbon receptor (AhR). Activation of constitutive androstane receptor (CAR) by phenobarbital-like compounds correlates with induction of endogenous CYP2b mRNA, while activation of pregnane X receptor (PXR) correlates with CYP3A induction. The binding and activation of CYP4A promoter by PPARalpha is also well documented. Cross-talk among different nuclear receptors creates tightly controlled network for P450 expression. View
Bile Acid Biosynthesis via Oxysterols 1182 About 500 mg of cholesterol is converted into bile acid each day in the adult human liver and ~95% of the bile acids secreted by the liver is reabsorbed in the intestine during each cycle of enterohepatic circulation. The 5% lost is replaced by new synthesis in the liver. This 500 mg/day usage accounts for ~90% of the cholesterol that is actively metabolized in the body, while steroid hormone biosynthesis accounts for the remaining 10%.<br><br>The classical route of bile acid synthesis starts with hydroxylation of C-7 of cholesterol as shown in the &quot;Bile Acid Synthesis&quot; pathway. Bile acid biosynthesis can alternatively (non-classical route as shown in this pathway) start with hydroxylation of the cholesterol side chain at C-24, C-25, & C-27 to produce the respective oxysterols. Hydroxylation of the oxysterols at C-7 is carried out by a different enzyme from that used in the classical route.<br><br>7-hydroxylated C-25 & C-27 oxysterols are highly effective FXR agonist with potency similar to that of chenodeoxycholic acid (CDCA). The physiological concentrations of these oxysterols are similar to that of CDCA while cholic acid (1/10 as potent as FXR inducer) is present at 300X the concentration. Thus precursors of bile acid biosynthesis from the non-classical route can regulated the rate of bile acid synthesis.<br><br>The C-22, C-24, C-25, & C-27 oxysterols are potent activators of LXR at their physiological relevant concentrations. View
Reabsorption in Kidney - Early Proximal Tubule 1182 The renal glomerular membrane retains cells and large proteins within the blood. Small proteins (< 70 kD, such as albumin, myoglobin, hemoglobin, and polypeptide hormones), glucose, amino acids, nitrogenous waste products, drugs and their metabolites, salts, and water pass into the glomerular filtrate. Small proteins are reabsorbed by receptor mediated endocytosis at the brush-border membrane of the proximal tubules (see &quot;Renal Function: Regulation of Protein Endocytosis&quot;). The proximal tubules, residing in the cortex, is the &quot;bulk&quot; phase of the reabsorption and up to 100% of filtered Glucose, amino acids, di- and tripeptides, and bicarbonate are reabsorbed here. Up to 80% of filtered phosphate, sulfate, Na+, K+, Ca++, Mg++ and water are reabsorbed in this area of the nephron. Fine tuning of the electrolyte balance takes place in the distal tubules and the collecting ducts after the urine has been concentrated in the loop of Henle.<br><br>Reabsorption of organic solutes and inorganic anions such as phosphate and sulfate are generally through the Na+-coupled co-transporters while the [Na+]i is maintained low by the Na+/K+ ATPase. Bicarbonate is reabsorbed from the lumen fluid as CO2 in a process involving the membrane-bond Na+/H+ exchanger (NHE-3) and carbonic anhydrase on both side of the apical membrane. The bicarbonate is then effluxed out of the cell from the basolateral Na+/bicarbonate cotransporter (NBC-1). Reabsorption of bicarbonate in the early proximal tubule helps maintaining the lumen fluid close to neutral pH. View
Retinoid Acid Synthesis & Signaling 1182 Retinoids (vitamin A derivatives) serve two unrelated functions. The 11-cis-retinal is the universal chromophore of photoreceptor pigments of the eye, while all-trans- and 9-cis-retinoic acids are endogenous ligands of nuclear receptors RAR and RXR, respectively. Dietary source of retinoids can be retinyl esters, carotenoids, or vitamin A supplement. Vitamin A is converted to retinyl ester in the intestine before it is released into the circulation and made available to other tissues as retinol (vitamin A). Carotenoids, such as beta-carotene, are converted to all-trans-retinal in the intestine by dioxygenases, reduced to retinol and transported as retinyl esters. Target cells take up all-trans-retinol either from retinol-binding protein or from chylomicrons (transport retinyl esters).<br><br>Retinols are stored in the hepatic stellate cells as retinyl esters. In the visual cycle, both cellular retinol binding protein (CRBP) and lecithin retinol acyltransferase (LRAT) are required for the all trans-retinal to 9-cis-retinal conversion. The substrate for the isomerization step appears to be the retinyl ester. In the liver, 9-cis- isomerization also takes place prior the first oxidation step. Two classes of retinol dehydrogenases, the classical cytosolic alcohol dehydrogenases (ADHs, medium chain dehydrogenase family), and some members of the microsomal short-chain dehydrogenase/reductase (SDR) family have been characterized in vitro to oxidize retinol to retinal (or the reduction of retinal to retinol). However, evidence for ADHs' involvement in vivo is lacking. On the other hand, genetic evidences have provided strong support for some members of the SDR family, including RDH5, RDH10, RDH11, RDH12, and RDH14, to be responsible for converting retinol to retinal. These RDHs appear to oxidize only CRBP1-bound retinol but not free retinol. Retinals are converted to retinoic acids by aldehyde dehydrogenase Raldh1. In mouse hepatoma cells (heap-1), the expression of Raldh1 was down-regulated by retinoic acid signaling via the RAR/RXR heterodimer. View
Control of Gastric Emptying 1182 None View
Hepatic Glutathione Synthesis and Regulation 1182 None View
Metabolic Effects of CAR Activation 1182 None View
Renal Injury: Heme & Heme Oxygenase 1182 Heme serves as the prosthetic moiety for heme proteins, such as the globins, cytochromes, COX, NOS, catalase, peroxidases, etc. Heme is lipophilic, readily permeates cellular membranes and become distributed throughout the cell. Free heme damages lipid bilayers of mitochondria and nuclei and interferes with the function of proteins anchored in the lipid bilayers. Free heme binds to Bach1 (transcriptional repressor of ARE/EpRE) and de-represses ARE/EpRE, thus facilitates transcription of Nrf2 mediated oxidative stress response genes (see &quot;Oxidative Stress Response Mediated by Nrf2&quot;) including heme oxigenase-1 (HO-1).<br><br>Two isoforms of heme oxygenase, coded by two different genes, have been characterized; HO-1 is an inducible enzyme while HO-2 is constitutively expressed. Induction of HO-1 is an adaptive & beneficial response to acute renal injury secondary to ischemia-reperfusion injury, nephrotoxins, glomerulonephritis, and rhabdomyolysis.<br><br>The heme oxygenase system catalyzes the rate limiting step in heme degradation, producing biliverdin, iron, and carbon monoxide (CO). Biliverdin is subsequently converted to a potent endogenous antioxidant, bilirubin, that is capable of scavenging peroxy radicals and inhibits lipid peroxidation. Induction of HO-1 essentially causes a shift of cellular redox from a pro-oxidant environment toward a more antioxidant state. HO-1 induction is associated with iron efflux and ferritin induction, allowing iron liberated from heme degradation to be safely sequestered for future use. In a rat proximal tubular epithelial cell culture system, Fe+++ and CO released from HO action on heme was found to induce p21 (independent of p53) resulting in apoptosis under serum-deprived conditions and cell cycle arrest under serum-replete conditions.<br><br>Cadmium, after absorption through the lung or the GI tract, is bound to metallothionein and the Cd-protein complex is filtered through the glomeruli into the urinary space where it is endocytosed by the proximal tubule cells and degraded by the lysosomes. The free Cd++ accumulates in the kidney and liver (with a half-life of 15 years) and generates reactive oxygen species that leads to lipid peroxidation, protein crosslinking, and DNA damage. View
G-Protein Gi Signaling 1182 G-proteins are heterotrimeric guanine nucleotide-binding proteins that communicate and route extracellular signals to distinct intracellular pathways. G-protein signaling are initiated by extracellular ligand (neurotransmitters, hormones, chemokines, and autocrine/paracrines) binding to their cognate G-protein-coupled receptors (GPCRs). The signaling pathways of the different G-proteins interact with each other to form a network that regulate the activity of proteins involved in cellular metabolism and a broad range of other cellular. G-proteins signal as if they were dimers since the signals are communicated either by the G-alpha subunit of by the G-beta-gamma complex. G-proteins are divided into four families, Gs, Gq/11, Gi/o, and G121/13, corresponding to their sequence similarity and effector coupling.<br><br>The Gi family are sensitive to pertussis toxin and signal to inhibit adenylyl cyclase. In NIH-3T3 cells and Neuro-2A cells, Go was found to signal through c-Src to activate STAT3 transcriptional activity. Signaling through RapGAPII, Gi can control Rap1 induced cell adhesion mediated by integrins. The Gi family can also signal through the G-beta-gamma complex to activate PI3K and phospholipase C-beta and the corresponding signaling cascades. View
Fatty Acid Biosynthesis & its Regulation 1182 Fatty acid synthesis occurs in the cytoplasm and uses NADPH as cofactors while beta-oxidation occurs in the mitochondria (or the peroxisome) and uses FAD and NAD as cofactors. In animal, the de novo synthesis of long-chain Fatty acids from malonyl-CoA is catalyzed by a single protein, the fatty acid synthase (FAS). FAS consists of two large identical multifunctional polypeptides, each (272 kD protein) containing six catalytic domains (beta-ketoacyl synthase, malonyl/acetyl transferase, dehydrase, enoyl-reductase, beta-ketoacyl reductase, and thioesterase) and an acyl carrier protein (ACP). FAS in yeast is composed of two nonidentical units. These multifunctional FAS complexes are referred to as type I enzymes while the discrete mono-functional enzymes found in prokaryotes and plants are referred to as type II enzymes.<br><br>Acetyl-CoA is generated in the mitochondria from either the pyruvate dehydrogenase reaction (see &quot;Citric Acid Cycle (Tricarboxylic Acid Cycle)&quot;) or fatty acid beta-oxidation. During times of caloric excess, citrate is siphoned off the TCA cycle and transported via a dedicated transporter (mitochondrial tricarboxylate transporter) to the cytosol where ATP citrate lyase converts it to acetyl-CoA (and oxaloacetate). Acetyl-CoA is then converted by acetyl-CoA carboxylase alpha to malonyl-CoA. Fatty acid synthesis on the FAS complex starts with one acetyl-CoA and one malonyl-CoA unit, and at each elongation step a new malonyl-CoA unit is added. After completing 7 cycles of elongation and reduction, the final product is a saturated C16 fatty acid linked to the ACP. The palmitoyl moiety is released from ACP as free palmitic acid by the action of the resident thioesterase. The free palmitic acid is escorted from the cytoplasm to the endoplasmic reticulum (ER) by L-FABP where it is converted to palmitoyl-CoA for the synthesis of stearic & oleic acids for incorporation into triglycerides, cell membrane, and the synthesis of sphingolipids. Dietary fatty acids are also elongated and modified at the ER. View
Apoptosis Regulation mediated by death receptors 1182 Apoptosis can be initiated via two major mechanisms, the death-receptor mediated apoptosis and the mitochondria mediated apoptosis. Death-receptor mediated apoptosis (also called extrinsic pathway) involves engagement of particular 'death' receptors (members of the tumor necrosis factor receptor (TNF-R) superfamily) and through the formation of the death-inducing-signaling-complex (DISC), leading to a cascade of activated caspases, which in turn induce apoptosis. Mitochondria mediated apoptosis is triggered by intracellular stress (DNA damage, ER stress, etc.) that lead to the perturbation of mitochondria, resulting in the leakage of cytochrome C and the activation of apoptosis (see &quot;Ca++ Signaling, Cytochrome c Release, & Apoptosis&quot;). The two pathways are not mutually exclusive, death receptor signaling may also mediate apoptosis through the mitochondria pathway via modulation of Bcl-2 family members, and both pathways converge and activate the same cascade of caspases.<br><br>Death receptors are cell surface receptors belonging to the TNF receptor superfamily, including Fas (CD95), TNFR, DR3, DR4, and DR5. Upon activation by their natural ligands, death-receptors oligomerize and initiate signaling via recruitment of adaptor proteins (e.g., FADD and TRADD) and initiator caspases (caspase 8/10), resulting in the activation of initiator caspases. These initiator caspases cleave and activate effector caspases (caspase 3/6/7), which then cleave a number of target substrates leading to phenotypic and biochemical characteristics of apoptosis. <br><br>FasL binds to Fas and triggers receptor trimerization, and subsequent recruitment of adaptor molecule FADD, which then recruit and activate caspase 8. Activated caspase 8 stimulates apoptosis by directly cleaves and activates effector caspases, or by cleaving Bid. The truncated Bid (tBid) translocates to mitochondria and lead to cytochrome C release, which then activates caspase 9 and 3. TNF signaling is more complex and involves both pro-and anti-apoptosis signals. TNFR1 can recruit TRADD, which subsequently recruit FADD and caspase 8. In addition, TRADD also binds the serine-threonine kinase RIP and TRAF thereby coupling stimulation of TNFR1 to the activation of NF-kB. Activation of NF-kB protects against TNF-induced apoptosis via induction of many molecules (eg. FLIP, c-IAP, Bcl2a1 and surviving) that block caspase activation. The balance between the strength of apoptogenic and the survival signals determines the final cell fate. View
G-Protein G13 Signaling 1182 G-proteins are heterotrimeric guanine nucleotide-binding proteins that communicate and route extracellular signals to distinct intracellular pathways. G-protein signaling are initiated by extracellular ligand (neurotransmitters, hormones, chemokines, and autocrine/paracrines) binding to their cognate G-protein-coupled receptors (GPCRs). The signaling pathways of the different G-proteins interact with each other to form a network that regulate the activity of proteins involved in cellular metabolism and a broad range of other cellular. G-proteins signal as if they were dimers since the signals are communicated either by the G-alpha subunit of by the G-beta-gamma complex. G-proteins are divided into four families, Gs, Gq/11, Gi/o, and G121/13, corresponding to their sequence similarity and effector coupling.<br><br>The G12/13 families were discovered trough sequence similarity to known G-alpha proteins. In contrast to the Gs, Gi, and the Gq families, It is not clear which GPCRs are coupled to them and the biological end-points of their activation are also not well characterized. The major downstream effector of G12/G13 is Rho (a family of small GTPase, see &quot;Rho Signaling to Actin Cytoskeleton&quot;). Rho is activated (GTP-bound form) by RhoGEF (Rho guanine nucleotide exchange factors) and deactivated (GDP-bound form) by GAPs (GTPase activating proteins). N-terminal short sequences of G12 alpha-subunit interact with RhoGEFs through a motif similar to that found in RGS (regulators of G-protein signaling, function as GAPs [GTPase activating protein]). Thus, through it N-terminal sequence, G12/13 can signal to activate Rho via RhoGEF and signal to deactivate other small GTPases such as Ras via GAP. <br><br>G12/13 can also activate PLD to hydrolyze membrane phosphatidylcholine and release phosphatidic acid which acts in concert with diacylglycerol to increase intracellular [Ca++].<br><br>The major function of Rho Signaling is to affect focal adhesion, actin fiber formation, and cytoskeleton reorganization associated to regulate changes in cell morphology, migration, phagocytosis, cell cycle progression, and cytokinesis. Some of the immediate event upon G12/13 signaling to RhoA is depicted in this pathway. View
B-Cell Receptor Signaling 1182 B-cell antigen receptor (BCR) plays a central role in the generation, maturation, activation and survival of B-cells. It is composed of a membrane immunoglobulin molecule (mIg) and a signaling complex, Ig-alpha/beta heterodimers. The mIG subunits bind antigens and cause receptor aggregation, while the Ig-alpha/beta complex transduces signals to the cell interior. Receptor aggregation induces phosphorylation of the ITAMs motif on Ig-alpha/beta by the Src family kinases, including Lyn, Btk and Fyn. This recruits another kinase, Syk, and initiates the downstream signaling cascades. BCR signaling leads to diverse cellular response including proliferation, differentiation, apoptosis, survival or immune tolerance. The precise outcome of BCR signaling depends on the maturation state of the cell, the affinity of antigen/antibody interaction, the cellular environment and the nature of the antigen. Defective BCR signaling can lead to not only impaired B-cell development and immunodeficiency, but also a predisposition to autoimmunity.<br><br>Upon binding to the antigen, BCR activates numerous second messengers via membrane recruitment and activation of PI3K and PLC-gamma2. The production of PIP3 by PI3K and subsequent activation of Akt is critical to promote B-cell survival. The production of IP3 and DAG by PLC-gamma2 will then activate PKC or calcium signaling respectively. Small GTPase such as Ras and Rac are also activated by BCR and these ultimately lead to the activation of MAP kinases including ERK, JNK and p38. The activation and crosstalk of multiple signaling pathways by BCR results in concerted regulation of multiple transcription factors including NF-kB, NFAT, Jun and Fos that control target gene transcription in B-cells. View
Citric Acid Cycle (Tricarboxylic Acid Cycle) 1182 None View
PGC-1 & Metabolic Regulation 1182 None View
Control of Gastric Acid Secretion 1182 None View
Regulation of Inducible Nitric Oxide Synthesis 1182 Nitric oxide (NO), synthesized locally by nitric oxide synthase (NOS) from L-arginine, is a second messenger involved in numerous biological processes, including neurotransmission, vasodilatation, immune modulation, and regulation of apoptosis. Three isoforms of NOS, NOS1 (neuronal), NOS2 (iNOS, inducible), and NOS3 (eNOS, endothelial) has been characterized. NOS1 and eNOS are expressed constitutively, activated by Ca++ signaling, and produce nanomolar concentrations of NO of short duration to effect neurotransmission, vasodilatation, and smooth muscle contraction. The expression of iNOS is inducible in many cell types by inflammatory cytokines and hypoxia and produces micromolar concentrations of NO for extended period.<br><br>The enzymatic activity of NOS requires 6(R)-tetrahydrobiopterin (BH4) as cofactor, and in the absence of BH4, NADPH oxidation by eNOS and NOS1 leads to the formation of superoxide anion and H2O2. Cytokines induce the synthesis of both BH4 and NO, and intracellular BH4 level can modulate the activity of both the inducible form (iNOS) and the constitutive form (eNOS) of NOS. BH4 is synthesized from GTP via two relatively unstable intermediates, dihydroneopterin triphosphate (H2NPt-P3) and 6(R)-pyruvoyl-tetrahydropterin (PH4Pt). Conversion of GTP to H2NPt-P3 is rate limiting and the enzyme responsible for this step; GTP-cyclohydrolase-1 (GTPCH-1) is regulated at the transcriptional level as well at the protein level. BH4 is also the cofactor for aromatic amino acid (phenylalanine, tyrosine, and tryptophan) hydroxylases, and the activity of GTPCH-1 is modulated by phenylalanine and BH4 in opposite directions via interaction with GFRP (GTPCH-1 feedback regulatory protein). <br><br>In hepatocytes, BH4 is regenerated from the phenylalanine hydroxylase (PAH) system but not from the NOS system. Phenylalanine (Phe) and BH4 are the major regulator of PAH; while Phe is a positive allosteric activator, BH4 association with PAH reduce the enzyme activity. Interconversion between the active and the inactive form of PAH is dependent on [Phe]. View
Reabsorption in Kidney - Thick Ascending Limb 1182 The filtration and reabsorption process that take place in the nephron in order to eliminate metabolic waste without unduly removal of reusable metabolites is regionally specialized with respect to kidney anatomy. In the cortex, the pressure inside the afferent arterioles provides the driving forces that promote glomerular filtration, while the peritubular capillary (arising from efferent arterioles) surrounding the proximal and distal convoluted tubules accommodates the enormous reabsorption of the glomerular filtrate. In the medulla the hairpin band of the loop of Henle (renal tubule between the proximal and the distal tubule) and the surrounding vasa recta (capillaries that descend from the cortex to the medulla, forming a hairpin band in the medulla and return to the cortex) coordinately form a counter-current mechanism to concentrate the urine.<br><br>As urine passing through the thin descending limb (highly permeable to water) of the Henle's loop, water is reabsorbed from the lumen by osmotic diffusion so that the urine at the tip of the loop has high osmolarity and some NaCl is reabsorbed here by diffusion. The thick ascending limb of the loop of Henle has very low permeability to water and active reabsorption of NaCl from the lumen fluid takes place via the Na+-K+-2Cl- cotransporter (NKCC2) in apical membrane, a process driven by the Na+/K+ ATPase in the basolateral membrane. Chloride ions are effluxed out of the cells from the chloride channel ClC-5 and the chloride-K+ cotransporter on the basolateral membrane. View
Regulation of Glucose Utilization 1182 After ingestion of carbohydrates, the hepatic portal vein glucose concentration increases to 10-15 mM. The glucose influx into hepatocytes is mediated by GLUT2 (Km = 17 mM) and results in glucose concentration in the hypatocytes proportional to that in the portal vein. GLUT2 is constitutively expressed in liver, kidney, intestine, & pancreas (where glucose flux is high). In contrast to the other hexokinases, GK (glucokinase, hexokinase IV, liver & pancreatic beta cell specific) is not inhibited by its reaction product (glucose-6-phosphate, G6P), instead it is regulated by glucokinase regulatory protein (GKRP). The Km for glucose in the GK/GKRP system is in the 15-20 mM range. The kinetic characteristic above implies that the rate of G6P formation is proportional to the plasma concentration of glucose. G6P can enter 3 major pathways: glycogen synthesis (see&quot; Glycogen Synthesis & Glycogenolysis&quot;), pentose pathway (see &quot;Pentose Pathway & Hyperglycemia&quot;), and glycolysis (see &quot;Glycolysis&quot; pathway). The major function of glycolysis in liver is to provide pyruvate for fatty acid synthesis.<br><br>GKPR resides in the nucleus while GK translocates between the nucleus & the cytoplasm. GKRP appears to 1) sequester GK in the nucleus in the fasting state, 2) protecting GK from proteolytic degradation, and 3) maintaining a nuclear reserve of GK that can be quickly released. GKRP over expression in mice protects against the development of diet-induced diabetes. View
Cell Cycle: The Spindle Checkpoint 1182 None View