* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Role of Liver In Triglyceride Homeostasis
Survey
Document related concepts
Point mutation wikipedia , lookup
Nucleic acid analogue wikipedia , lookup
Genetic code wikipedia , lookup
Peptide synthesis wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Lipid signaling wikipedia , lookup
Basal metabolic rate wikipedia , lookup
Proteolysis wikipedia , lookup
Citric acid cycle wikipedia , lookup
Specialized pro-resolving mediators wikipedia , lookup
Butyric acid wikipedia , lookup
Amino acid synthesis wikipedia , lookup
Biochemistry wikipedia , lookup
Biosynthesis wikipedia , lookup
Glyceroneogenesis wikipedia , lookup
Transcript
Role of Liver In Triglyceride Homeostasis Larry L. Swift, Ph.D. Department of Pathology Vanderbilt University School of Medicine Metabolic Cycle of Fatty Acids and Triglycerides VLDLTG Fatty Acids • Aliphatic carboxylic acids usually having an even number of carbon atoms • Chains may be saturated or unsaturated • Position of double bonds described in relation to carboxy-terminus ( ) or methyl carbon ( or n) Common Fatty Acids Fatty Acid C:DB, position Stearic 18:0 Oleic 18:1 ω9 Linoleic 18:2 ω6 α-Linolenic 18:3 ω3 γ-Linolenic 18:3 ω6 Arachidonic 20:4 ω6 Eicosapentaenoic 20:5 ω3 Structure Fatty Acid Biosynthesis • Takes place in the cytosol • Acetyl CoA is substrate; palmitate (C16:0) is product • Fatty acids are synthesized by a multi-enzyme complex, fatty acid synthase (FAS) • Elongation beyond C16:0 and desaturation are catalyzed by enzymes associated with the endoplasmic reticulum Amino Acids Glucose Fatty acids Pyruvate Pyruvate Denhydrgenase Cytosol Acetyl CoA Fatty Acids Mitochondria Fatty Acid Biosynthesis •First committed step of fatty acid synthesis pathway •Acetyl CoA Carboxylase (ACC) is major regulatory site of fatty acid synthesis Fatty Acid Biosynthesis Final product is palmitic acid, C16:0 Fatty Acid Synthase Fatty Acid Elongation / Desaturation Palmitate (16:0) elongation Mammals and plants Stearate (18:0) desaturation Oleate 18:1 (∆9 or 9) desaturation * Plants only * Other PUFAs Longer saturated fatty acids * Linoleate 18:2 (∆9,12 or 6) -linoleate 18:3 (∆9,12,15 or 3) Palmitoleate 16:1 (∆9 or 7) -linoleate 18:3 (∆6,9,12 or 6) elongation Eicosatrienoate 20:3 (∆8,11,14 or 6) desaturation Arachidonate 20:4 (∆5,8,11,14 or 6) Synthesis of Polyunsaturated Fatty Acids Regulation of Fatty Acid Biosynthesis Acetyl CoA Carboxylase • Allosteric regulation – citrate activates and palmitoyl CoA, stearoyl CoA and arachidyl CoA inhibit ACC • Hormonal regulation – glucagon inactivates and insulin activates ACC via phosphorylation/dephosphorylation of the enzyme Regulation of ACC by Phosphorylation • Inactivation (starved state) – glucagon increases cAMP – activates protein kinase A – inactivates ACC • Activation (fed state) – insulin induces protein phosphatase and activates ACC Regulation of Lipid Biosynthesis via SREBP Horton et al., J. Clin. Invest. 109: 1125, 2002. SREBP1c • Enhances transcription of genes required for FA synthesis • Responsive genes include: – – – – – – – ATP citrate lyase Acetyl-CoA carboxylase Fatty acid synthase Fatty acid elongase Stearoyl-CoA desaturase Glycerol-3-phosphate acyltransferase Genes required to generate NADPH Insulin • Stimulates hepatic FA synthesis during carbohydrate excess • Increases SREBP-1c mRNA in liver • Induction of target genes blocked if dominant negative form of SREBP-1c is expressed • Incubating primary hepatocytes with glucagon decreases mRNAs for SREBP-1c and its associated target genes What happens to these newly synthesized fatty acids? • Depends on the cell and metabolic state • Can be stored (but not as fatty acids) or utilized for energy • Synthesis of other lipid classes Triglyceride • Glycerol backbone with 3 fatty acids (FA) • Highly concentrated source of energy, easily stored in cytosolic droplets • Before use as energy source, it must undergo lipolysis to release FA Triglyceride Synthesis Synthesis of Glycerol Phosphate in Liver Glucose Glycolysis CH2OH Glycerol CH2OH CHOH DHAP C=O O CH2-O-P-OO ATP ADP NADH Glycerol 3-phosphate dehydrogenase CH2OH NAD+ CH2OH HO-C-H O CH2-O-P-OO L-Glycerol 3-Phosphate Glycerol Kinase Biosynthesis of Triglyceride What Happens to TG Synthesized by the Liver? • Energy • Export as VLDL • Storage Lipoprotein Model Very Low Density Lipoproteins (VLDL) • Spherical TG-rich particles 40-60 nm in diameter • Synthesized by the liver and transport triglyceride to periphery for storage and/or utilization • Major structural protein is apo B Apolipoprotein B • Structural apoprotein for triglyceride-rich lipoproteins • Expressed in two forms: apoB100 & apoB48 • B100 is found on VLDL, IDL, and LDL • B48 is found on chylomicrons • ApoB100 and B48 synthesis are essential for VLDL and chylomicron assembly Apolipoprotein B VLDL Assembly • Initiated in endoplasmic reticulum (ER) • Assembly and secretion are dependent on the ability of the cell to synthesize apoB and MTP • Constitutive process modulated by a variety of factors Assembly of Triglyceride-Rich Lipoproteins G. Shelness, Wake Forest School of Medicine Microsomal Triglyceride Transfer Protein (MTP) • A heterodimeric protein complex consisting of 97 kDa subunit and protein disulfide isomerase (PDI) • Transfers lipid from the ER membrane to the forming lipoprotein in the ER lumen • Absolute requirement for assembly of VLDL by the liver • Inhibition of MTP leads to reduction in plasma triglycerides and cholesterol Regulation of VLDL Production • Availability of functional MTP • Apo B degradation • Substrate availability Functional MTP • Inhibition of MTP leads to decreased VLDL secretion and a reduction in plasma TGs • Over-expression of MTP leads to increased VLDL secretion • Absence of MTP (abetalipoproteinemia) leads to absence of TG-rich lipoproteins in plasma ApoB Degradation • Proteasome inhibitors block degradation of apoB and ubiquitinated apoB accumulates within the cell • ALLN, a calpain inhibitor, inhibits degradation of apoB, but does not increase apoB secretion • Insulin regulates degradation of apoB through PI3-kinase pathway • Insulin resistance leads to decreased apoB degradation and increased secretion Assembly of Triglyceride-Rich Lipoproteins Davidson and Shelness, Ann. Rev. Nutr. 20: 169, 2000 Targets for Regulation of VLDL Assembly G.F. Gibbons, et al., Biochem Soc Trans 32: 59-64, 2004 Hepatic Lipid Mobilization Gibbons et al., Biochim. Biophys. Acta 1483: 37, 2000 What is the fate of VLDL secreted from the liver? VLDL Metabolic Pathway Chylomicron Metabolic Pathway Adipocyte Lipid Metabolism Sethi J.K. , Vidal-Puig A.J., J. Lipid Res. 48:1253-1262, 2007 Sources of Fatty Acids for Liver and VLDL Triglycerides Adiels, M. et al. Arterioscler Thromb Vasc Biol 28:1225-1236, 2008. Why Does Liver Store TG? • TG accumulates when plasma NEFA exceeds hepatic disposal via secretion and oxidation • Neutralizes potential toxicity of fatty acids released from adipose tissue • Hepatic TG secretion in post-absorptive state exceeds hepatic NEFA esterification, suggesting utilization of hepatic TG stores Insulin – A Key Regulator of Hepatic Lipid Metabolism • • • • • Fatty acid flux to the liver Hepatic de novo lipogenesis MTP synthesis ApoB degradation Stimulates production of lipoprotein lipase • Fatty acid oxidation De Novo Lipogenesis • Insulin stimulates SREBP-1c expression and increases cleavage of SREBP-1c to its mature form which activates production of lipogenic enzymes • Hepatic IR exists in the pathway regulating gluconeogenesis, but this does not affect insulin’s ability to stimulate lipogenesis • ChREBP is also up-regulated by insulin and works in synergy with SREBP-1c to promote lipogenesis MTP Synthesis • Mttp gene expression is negatively regulated by insulin in part by MAPK pathway • Suppression of MTP by insulin also occurs via Forkhead box 01 (Fox01) • Insulin induces phosphorylation of Fox01 leading to its exclusion from the nucleus, resulting in inhibition of MTP expression • Fox01 gain of function is associated with enhanced MTP expression • Insulin resistance leads to increased MTP expression Insulin Signaling through Fox01 Kamagate et al. J. Clin. Invest. 118: 234702364, 2008 Degradation of ApoB • Insulin targets apoB for degradation via a PI3kinase pathway • Acute increases in insulin decrease hepatic VLDL and apoB secretion via a post-ER, non-proteasomal process • Insulin resistance leads to decreased apoB degradation and may contribute to overproduction of VLDL Hepatic VLDL Assembly and Secretion Sparks and Sparks, J. Clin. Invest. 118: 2012-2015, 2008 Substrate Sources Driving Assembly of ApoB-Lipoproteins in IR Ginsberg et al., Arch. Med. Res. 36: 232, 2005 Insulin, VLDL Secretion, and Fatty Liver Ginsberg, et al., Endocrin. Metab. Clinics 35: 491, 2006 Summary • De novo lipogenesis and the regulation of fatty acid synthesis • Sources of fatty acids for liver TG biosynthesis • Secretion of hepatic TG with VLDL and the fate of TG-rich particles • Insulin resistance and its impact on hepatic TG homeostasis Effects of Insulin Resistance on Hepatic TG Homeostasis • Increased fatty acid flux from adipose tissue to the liver • Increased hepatic uptake of VLDL, IDL, and chylomicron remnants • Increased de novo lipogenesis • Decreased degradation of apoB • Increased activity of MTP • Overproduction of VLDL • Hepatic steatosis Questions? Increased Fatty Acid Flux from Adipose Tissue to the Liver • Insulin stimulates adipocyte fatty acid uptake and inhibits fatty acid release (HSL) leading to decreased plasma NEFA • Insulin resistance leads to increased mobilization of fatty acids from adipose tissue and increased plasma NEFA De Novo Lipogenesis • Hepatic insulin resistance leads to upregulation of sterol regulatory element-binding protein-1 (SREBP-1c) and activates lipogenic enzymes • Carbohydrate response element-binding protein (ChREBP) regulates expression of key glucoseresponsive genes of lipogenesis • Synergistic action of SREBP-1c and ChREBP directs conversion of excess glucose to fatty acids and enhances esterification Substrate Sources Driving Assembly of ApoB-Lipoproteins in IR Ginsberg et al., Arch. Med. Res. 36: 232, 2005 Endocrine Reviews 20(5): 649-88 Changes in Lipoprotein Metabolism in the Metabolic Syndrome Adiels, M. et al. Arterioscler Thromb Vasc Biol 2008;28:1225-1236 Metabolic Syndrome • Abdominal obesity • Elevated blood pressure • Insulin resistance • Prothrombotic state • Proinflammatory state • Atherogenic dyslipidemia Metabolic Syndrome Dyslipidemia • Elevated plasma triglycerides • Decreased HDL cholesterol • Normal levels of LDL cholesterol carried in small dense particles • Lipoprotein alterations contribute to increased risk for CHD Lecture Outline • Lipoproteins, structure, apoproteins, characteristics • Source of triglyceride for lipoproteins • Assembly of triglyceride-rich lipoproteins • Triglyceride-rich lipoprotein catabolism • Effects of insulin resistance on triglyceride-rich lipoprotein production • VLDL secretion and fatty liver Hormonal control of adipocyte lipolysis Biochem. Soc. Trans. (2003) 31, 1120-1124 Apolipoproteins Lipoprotein Class Tissue Source MW liver and intestine 30K 17K 45K intestine liver 240K 512K apoAI apoAII apoA-IV HDL HDL HDL apoB48 apoB100 chylomicrons VLDL, LDL apoCI apoCII apoCIII HDL HDL and VLDL HDL and VLDL liver 6.6K 8.9K 8.8K apoE HDL, LDL, VLDL liver 34K Apolipoprotein B • Structural apoprotein for triglyceride-rich lipoproteins • Expressed in two forms: apoB100 & apoB48 • B100 is found on VLDL, IDL, and LDL • B48 is found on chylomicrons • ApoB100 and B48 synthesis are essential for VLDL and chylomicron assembly Increased MTP Activity • Mttp gene has insulin response element in the promoter • In human liver cells Mttp gene expression is negatively regulated by insulin through the MAPK cascade • Increased MTP mRNA is associated with enhanced synthesis of VLDL in wild-type animals and in animal models of insulin resistance Increased MTP Activity • Forkhead box 01 (Fox01) has been shown to mediate inhibitory action of insulin on target gene expression • Fox01 stimulates hepatic MTP expression; the effect is counteracted by insulin • Fox01 gain-of-function is associated with enhanced MTP expression, augmented hepatic VLDL production, and elevated plasma TG levels in Fox01 transgenic mice Essential Fatty Acids (EFA) • Fatty acids that cannot be synthesized and must be obtained from the diet • C18:2 -6 - linoleic acid • C18:3 -3 - -linolenic acid • These fatty acids are starting point for synthesizing longer and more highly unsaturated fatty acids Arachidonic Acid • C20:4 ω6 COOH CH3 • A precursor for prostaglandins, leukotrienes, and thromboxanes Common Fatty Acids • C14:0 - myristic acid • C16:0 - palmitic acid • C18:0 - stearic acid • C18:1 9 - oleic acid • C18:2 6 - linoleic acid* • C18:3 3 - -linolenic acid*; 6 - -linolenic acid • C20:4 6 - arachidonic acid • C20:5 3 - eicosapentaenoic acid • C22:5 3 - docosapentaenoic acid • C22:6 3 - docosahexaenoic acid Source of Acetyl CoA • Derived primarily from pyruvate via pyruvate dehydrogenase in mitochondria • Transported into cytosol as citrate, then regenerated by ATP citrate lyase • Available for malonyl CoA formation and fatty acid synthesis O R1-C-S-CoA CH2OH HO-C-H O CH2-O-P-OO Acyl Transferase O R2-C-S-CoA Acyl Transferase L-Glycerol 3-Phosphate CH2-OOCR1 R2COO-C-H O CH2-O-P-OO Phosphatidic Acid Addition of Fatty Acids to Form Triglycerides CH2-OOCR1 R2COO-C-H O R3-C-S-CoA DGAT Phosphatidate phosphohydrolase CH2-OOCR1 R2COO-C-H CH2OH CH2OOCR3 Triacylglycerol Diacylglycerol Plasma Lipoprotein Classes Chylomicrons • TG-rich particles 75 to 200 nm in diameter • Assembled by enterocytes to transport dietary fat to periphery for storage or utilization • Present in postprandial plasma • ApoB-48 is the sole B apoprotein (humans and rodents) Negative Stain of Lipoproteins Chylomicrons LDL VLDL HDL Hepatic TG Storage • Human liver can store 5-45 mol/g under normal conditions (6.7-60.3 g/1.5 kg liver) • Hepatic TG in house musk shrew can exceed 100 mol/g during starvation • Livers of mice that overexpress SREBP1a contain >250 mol/g • Liver accomodates TG in cytosolic droplets Adipocyte Lipid Metabolism Sethi J.K. , Vidal-Puig A.J., J. Lipid Res. 48:1253-1262, 2007