Hepatic triacylglycerol accumulation and insulin resistance
2008; Elsevier BV; Volume: 50; Linguagem: Inglês
10.1194/jlr.r800053-jlr200
ISSN1539-7262
AutoresCynthia A. Nagle, Eric L. Klett, Rosalind Coleman,
Tópico(s)Lipid metabolism and biosynthesis
ResumoThe association of hepatic steatosis with hepatic insulin resistance and type 2 diabetes has prompted investigators to elucidate the underlying mechanism. In this review we focus on pathways of lipid metabolism, and we review recent data, primarily from mouse models, that link lipid intermediates with insulin resistance. Most of the studies that implicate acyl-CoA, lysophosphatidic acid, phosphatidic acid, diacylglycerol, or ceramide rely on indirect associations. Convincing data to support the hypothesis that specific lipid intermediates initiate pathways that alter insulin signaling will require studies in which the concentration of each purported signaling molecule can be manipulated independently. The association of hepatic steatosis with hepatic insulin resistance and type 2 diabetes has prompted investigators to elucidate the underlying mechanism. In this review we focus on pathways of lipid metabolism, and we review recent data, primarily from mouse models, that link lipid intermediates with insulin resistance. Most of the studies that implicate acyl-CoA, lysophosphatidic acid, phosphatidic acid, diacylglycerol, or ceramide rely on indirect associations. Convincing data to support the hypothesis that specific lipid intermediates initiate pathways that alter insulin signaling will require studies in which the concentration of each purported signaling molecule can be manipulated independently. Nonalcoholic fatty liver disease (NAFLD) is characterized by excessive lipid accumulation in the liver in the absence of 'significant' alcohol consumption and may progress to nonalcoholic steatohepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma. In the United States, the prevalence of NAFLD is estimated to be 20% (1Ruhl C.E. Everhart J.E. Epidemiology of nonalcoholic fatty liver.Clin. Liver Dis. 2004; 8: 501-519Abstract Full Text Full Text PDF PubMed Scopus (152) Google Scholar); however, the true prevalence of NAFLD is unknown because many individuals in earlier stages of the disease are asymptomatic and remain undiagnosed. Although NAFLD was previously thought to be a benign condition, its development parallels the development of the insulin resistance (IR) syndrome and is associated with type 2 diabetes mellitus. Patients with the IR syndrome have a 4- to 11-fold increased risk of developing NAFLD. With overnutrition and lack of exercise, liver and other tissues store excess energy as triacylglycerol (TAG). Shunting carbon-energy into a storage form is likely protective against cytotoxic FA accumulation. Hepatic IR is associated with the accumulation of TAG and FA metabolites (fatty acyl-CoA, diacylglycerol (DAG), ceramide, and glycosphingolipid). The short-term protection achieved from sequestering FA may lead to long-term morbidity with the development of further IR leading to type 2 diabetes mellitus and the deterioration of hepatic function. Although fatty liver correlates with hepatic IR, it remains unclear whether IR or excess TAG stores develop first and whether fatty liver invariably leads to IR. Peripheral IR may cause fatty liver by elevating plasma FA, glucose, and insulin, which stimulate hepatic lipid synthesis and impair hepatic β-oxidation. However, high-fat feeding may cause hepatic IR before systemic IR develops. We will focus on the metabolic pathways involved in the development of NAFLD, the role of altered hepatic TAG metabolism, and the role of lipid metabolites in the development of IR. Hepatic lipid accumulation can be caused by four different metabolic perturbations: increased FA delivery to hepatocytes from lipolyzed adipose TAG, dietary lipids, or hepatic de novo lipogenesis (DNL); increased TAG synthesis; decreased hepatic FA oxidation; and inadequate TAG secretion in VLDL (Fig. 1). After consuming a meal containing fat, chylomicron-TAG is delivered to the liver and is lipolyzed in lysosomes with the release of FA. FA can also arise from DNL in response to a high carbohydrate meal, as excess glucose is metabolized to acetyl-CoA, the major substrate for FA synthesis. Malonyl-CoA, the product of acetyl-CoA carboxylase (ACC), is not only a substrate for FAS , but also inhibits carnitine-palmitoyltransferase-1 (CPT-1), which regulates long-chain FA entry into mitochondria for β-oxidation. FAS sequentially adds two carbons from malonyl-CoA to synthesize 16 and 18 carbon FA. These saturated FAs must be activated by long-chain acyl-CoA synthetase (ACSL) before they can be elongated by an elongase (ELOVL6), desaturated by stearoyl-CoA desaturase (SCD), or used for the synthesis of glycerolipids or cholesterol esters. Hepatic FAs are derived from entering chylomicron remnants, TAG lipolysis in lipid droplets, or DNL. After activation, the acyl-CoAs enter the glycerolipid synthetic pathway. In the liver, TAG may either be stored in cytoplasmic lipid droplets or incorporated into VLDL particles and secreted into the blood. The four-step synthesis of TAG, first identified by Eugene Kennedy and his colleagues (2Coleman R.A. Lee D.P. Enzymes of triacylglycerol synthesis and their regulation.Prog. Lipid Res. 2004; 43: 134-176Crossref PubMed Scopus (723) Google Scholar), occurs on the cytosolic surface of the mitochondrial outer membrane and the endoplasmic reticulum. Four glycerol-3-phosphate acyltransferases (GPAT), potentially six 1-acylglycerol-3-phosphate acyltransferases (AGPAT), 1AGPAT8 and AGPAT6 are correctly termed GPAT3 and GPAT4, respectively., 2CGI-58, and endophilin both have AGPAT activity. three phosphatidic acid phosphatases (PAP), and two DAG acyltransferases (DGAT) have been cloned 3Several additional enzymes exhibit some DGAT activity, including MGAT2.. During fasting, insulin levels fall, adipose TAG is hydrolyzed, and FAs are released and travel to the liver. It is not clear whether exogenously derived FAs must first be reesterified and stored in lipid droplets before they can be oxidized, or whether acyl-CoAs can be directly converted to acyl-carnitines and enter the oxidative pathway. Within the mitochondrial matrix, acyl-carnitines are converted to acyl-CoAs by CPT-2. Very long chain acyl-CoA dehydrogenase (VLCAD) acts on C12-C24 carbon acyl-CoAs, long chain acyl-CoA dehydrogenase (LCAD) acts on C8-C20 acyl-CoAs, medium chain acyl-CoA dehydrogenase (MCAD) acts on C4-C12 acyl-CoAs, and short-chain acyl-CoA dehydrogenase (SCAD) acts on C4-C6 acyl-CoAs. When the tricarboxylic acid (TCA) cycle cannot metabolize all the acetyl-CoA generated from β-oxidation, excess acetyl-CoA is converted to ketone bodies. The origin of VLDL-TAG remains poorly understood. A precursor VLDL particle that contains apoB and a small amount of TAG is formed in the endoplasmic reticulum (ER). This small VLDL precursor may fuse within the ER with a larger droplet of TAG to form a TAG-rich particle. The TAG that fuses is thought to arise from cytosolic TAG derived primarily from plasma FA uptake and less from DNL (7Uyeda K. Repa J.J. Carbohydrate response element binding protein, ChREBP, a transcription factor coupling hepatic glucose utilization and lipid synthesis.Cell Metab. 2006; 4: 107-110Abstract Full Text Full Text PDF PubMed Scopus (402) Google Scholar). Incorporation of cytosolic TAG into VLDL may require lipolysis and reesterification steps at the ER (3Gibbons G.F. Wiggins D. Brown A.M. Hebbachi A.M. Synthesis and function of hepatic very-low-density lipoprotein.Biochem. Soc. Trans. 2004; 32: 59-64Crossref PubMed Scopus (194) Google Scholar). An excess of glucose, FA, and insulin ultimately lead to hepatic steatosis and worsening hepatic IR via a network of transcription factors (Fig. 2). Activation of hepatic liver-X-receptor (LXR) by endogenous oxysterol ligands results in up-regulation of genes involved in cholesterol, lipid, and bile acid metabolism. In mice, oral LXR agonists result in enhanced hepatic FA synthesis, hepatic steatosis, and hypertriglyceridemia, all mediated by increased expression of SREBP-1c (4Grefhorst A. Elzinga B.M. Voshol P.J. Kok T.Plo¨sch, T. Bloks V.W. Sluijs F.H. van der Havekes L.M. Romijn J.A. Verkade H.J. al et Stimulation of lipogenesis by pharmacological activation of the liver X receptor leads to production of large, triglyceride-rich very low density lipoprotein particles.J. Biol. Chem. 2002; 277: 34182-34190Abstract Full Text Full Text PDF PubMed Scopus (408) Google Scholar). Transgenic hepatic over-expression of SREBP-1c produces a fatty liver and a 4-fold increase in the rate of hepatic FA synthesis, with increases in lipogenic genes like FAS, ACC and SCD-1 (5Shimano H. Horton J.D. Shimomura I. Hammer R.E. Brown M.S. Goldstein J.L. Isoform 1c of sterol regulatory element binding protein is less active than isoform 1a in livers of transgenic mice and in cultured cells.J. Clin. Invest. 1997; 99: 846-854Crossref PubMed Scopus (688) Google Scholar), and GPAT1 (5Shimano H. Horton J.D. Shimomura I. Hammer R.E. Brown M.S. Goldstein J.L. Isoform 1c of sterol regulatory element binding protein is less active than isoform 1a in livers of transgenic mice and in cultured cells.J. Clin. Invest. 1997; 99: 846-854Crossref PubMed Scopus (688) Google Scholar). Insulin increases LXR's ability to activate the SREBP-1c promoter and increase hepatic lipogenesis (6Chen G. Liang G. Ou J. Goldstein J.L. Brown M.S. Central role for liver X receptor in insulin-mediated activation of Srebp-1c transcription and stimulation of fatty acid synthesis in liver.Proc. Natl. Acad. Sci. USA. 2004; 101: 11245-11250Crossref PubMed Scopus (439) Google Scholar). Thus, hyperinsulinemia can activate LXR and SREBP-1c and result in increased lipogenesis, liver TAG accumulation, and worsening hepatic IR. ChREBP regulates hepatic lipid synthesis through transcriptional control of the lipogenic genes ACC and FAS in response to glucose (7Uyeda K. Repa J.J. Carbohydrate response element binding protein, ChREBP, a transcription factor coupling hepatic glucose utilization and lipid synthesis.Cell Metab. 2006; 4: 107-110Abstract Full Text Full Text PDF PubMed Scopus (402) Google Scholar). Liver-specific ChREBP inhibition results in decreased hepatic lipogenesis and ameliorated hepatic steatosis in ob/ob mice (7Uyeda K. Repa J.J. Carbohydrate response element binding protein, ChREBP, a transcription factor coupling hepatic glucose utilization and lipid synthesis.Cell Metab. 2006; 4: 107-110Abstract Full Text Full Text PDF PubMed Scopus (402) Google Scholar). Although IR correlates highly with TAG content in skeletal muscle and liver, TAG itself is believed to be a surrogate marker for the true disruptor of the insulin signal, variously hypothesized to be a) a cytokine released from macrophages or adipocytes, b) activation of the NFκB pathway (8Cai D. Yuan M. Frantz D.F. Melendez P.A. Hansen L. Lee J. Shoelson S.E. Local and systemic insulin resistance resulting from hepatic activation of IKK-and NF-B.Nat. Med. 2005; 11: 183-190Crossref PubMed Scopus (1819) Google Scholar), or c) a signaling pathway initiated by a FA-derived lipid. Lysophosphatidic acid (LPA), phosphatidic acid (PA), and DAG are well-established initiators of signaling pathways but have been studied primarily after their hydrolysis from membrane phospholipids. In liver, however, the major production of LPA, PA, and DAG occurs via de novo glycerolipid synthesis; lipid intermediates derived from this pathway and related lipids [acyl-CoAs (9Chen M.T. Kaufman L.N. Spennetta T. Shrago E. Effects of high fat-feeding to rats on the interrelationship of body weight, plasma insulin, and fatty acyl-coenzyme A esters in liver and skeletal muscle.Metabolism. 1992; 41: 564-569Abstract Full Text PDF PubMed Scopus (60) Google Scholar, 10Samuel V.T. Liu Z-X. Qu X. Elder B.D. Bliz S. Befroy D. Romanelli A.J. Shulman G.I. Mechanism of hepatic insulin resistance in non-alcoholic fatty liver disease.J. Biol. Chem. 2004; 279: 32345-32353Abstract Full Text Full Text PDF PubMed Scopus (1044) Google Scholar), ceramide (11Holland W.L. Brozinick J.T. Wang L.P. Hawkins E.D. Sargent K.M. Liu Y. Narra K. Hoehn K.L. Knotts T.A. Siesky A. al et Inhibition of ceramide synthesis ameliorates glucocorticoid-, saturated-fat-, and obesity-induced insulin resistance.Cell Metab. 2007; 5: 167-179Abstract Full Text Full Text PDF PubMed Scopus (926) Google Scholar) and acyl-carnitines] may also act as signaling molecules. Because incubating cells with FA or infusing FA into mice causes IR within a few hours, it appears that a direct FA metabolite can promote IR. With hepatic IR, insulin cannot effectively suppress hepatic glucose output in a hyperinsulinemic-euglycemic clamp study. IR can also be inferred by impaired tyrosine phosphorylation of the insulin receptor and aberrant phosphorylation of serine residues of insulin receptor substrate-1 (IRS-1) and downstream members of the insulin signaling pathway. Our studies implicate GPAT1-produced DAG as a cause of IR (12Neschen S. Morino K. Hammond L.E. Zhang D. Liu Z.X. Romanelli A.J. Cline G.W. Pongratz R.L. Zhang X.M. Choi C.S. al et Prevention of hepatic steatosis and hepatic insulin resistance in mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 knock out mice.Cell Metab. 2005; 2: 55-65Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 13Nagle C.A. An J. Shiota M. Torres T.P. Cline G.W. Liu Z-X. Wang S. Catlin R.L. Shulman G.I. Newgard C.B. al et Hepatic overexpression of glycerol-sn-3-phosphate acyltransferase 1 in rats causes insulin resistance.J. Biol. Chem. 2007; 282: 14807-14815Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar); when excess DAG is formed, protein kinase C (PKC)ε is activated, and IRS-1 is phosphorylated on Ser307, which suppresses IRS-1 tyrosine phosphorylation and diminishes phosphatidylinositol 3-kinase activation. These data suggest that the DAG formed during glycerolipid synthesis can interact with and activate PKC at distant membranes and show that GPAT1 modulates the cell content of lipid intermediates. This is not entirely surprising because lipogenic diets normally increase SREBP-1c-mediated GPAT1 mRNA expression (2Coleman R.A. Lee D.P. Enzymes of triacylglycerol synthesis and their regulation.Prog. Lipid Res. 2004; 43: 134-176Crossref PubMed Scopus (723) Google Scholar). In addition to DAG, the glycerolipid intermediates LPA and PA can also initiate signaling pathways. LPA is a ligand for peroxisome proliferator-activated receptor (PPAR) γ (14McIntyre T.M. Pontsler A.V. Silva A.R. Hilaire A.St. Xu Y. Hinshaw J.C. Zimmerman G.A. Hama K. Aoki J. Arai H. al et Identification of an intracellular receptor for lysophosphatidic acid (LPA): LPA is a transcellular PPAR agonist.Proc. Natl. Acad. Sci. USA. 2003; 100: 131-136Crossref PubMed Scopus (466) Google Scholar) and might be responsible for the up-regulation of PPARγ target genes observed in NAFLD, whereas intracellular PA activates mammalian target of rapamycin (15Foster D.A. Regulation of mTOR by phosphatidic acid?.Cancer Res. 2007; 67: 1-4Crossref PubMed Scopus (139) Google Scholar), which down-regulates the insulin signal by promoting serine phosphorylation of IRS when nutrients are present in excess. Although fatty liver is associated with IR, several rodent models with steatosis remain insulin sensitive (16Monetti M. Levin M.C. Watt M.J. Sajan M.P. Marmor S. Hubbard B.K. Stevens R.D. Bain J.R. Newgard C.B. Farese R.V. al Sr., et Dissociation of hepatic steatosis and insulin resistance in mice overexpressing DGAT in the liver.Cell Metab. 2007; 6: 69-78Abstract Full Text Full Text PDF PubMed Scopus (422) Google Scholar, 17Minehira K. Young S.G. Villanueva C.J. Yetukuri L. Oresic M. Hellerstein M.K. Farese Jr., R.V. Horton J.D. Preitner F. Thorens B. al et Blocking VLDL secretion causes hepatic steatosis but does not affect peripheral lipid stores or insulin sensitivity in mice.J. Lipid Res. 2008; 49: 2038-2044Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar, 18Grefhorst A. Hoekstra J. Derks T.G. Ouwens D.M. Baller J.F. Havinga R. Havekes L.M. Romijn J.A. Kuipers F. 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Lipid Res. 2005; 46: 582-588Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). Factors that may explain the variable relationship between fatty liver and IR include the specific pathways that contribute to hepatic fat accumulation, the type of lipid metabolites that accumulate, the cellular location and FA composition of those metabolites, the presence or absence of hepatic or systemic inflammation, and the type of rodent diet used. In humans with NAFLD, the estimated contribution of adipose lipolysis, dietary FA, and hepatic DNL to hepatic TAG and VLDL-TAG are 60%, 15%, and 24%, respectively (22Donnelly K.L. Smith C.I. Schwarzenberg S.J. Jessurun J. Boldt M.D. Parks E.J. Sources of fatty acids stored in liver and secreted via lipoproteins in patients with nonalcoholic fatty liver disease.J. Clin. Invest. 2005; 115: 1343-1351Crossref PubMed Scopus (2470) Google Scholar). It is not known, however, whether the lipid intermediates derived from these pathways have similar effects on the development of hepatic IR, or whether one pathway predominates. When over-expression of hepatic LPL increases FA flux into the liver from lipoproteins, hepatic IR is associated with an increased hepatic content of TAG and acyl-CoA (23Kim J.K. Fillmore J.J. Chen Y. Yu C. Moore I.K. Pypaert M. Lutz E.P. Kako Y. Goldberg W.Velez-Carrasco, I.J. al et Tissue-specific overexpression of lipoprotein lipase causes tissue-specific insulin resistance.Proc. Natl. Acad. Sci. USA. 2001; 98: 7522-7527Crossref PubMed Scopus (591) Google Scholar). Augmented DNL also appears to be important, because ob/ob mice deficient in either SREBP-1c or ChREBP are protected from hepatic steatosis and IR (24Dentin R. Benhamed F. Hainault I. Fauveau V. Foufelle F. Dyck J.R. Girard J. Postic C. 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Despite the fatty liver that results from decreased β-oxidation, FAS null mice have impaired gluconeogenesis and are hypoglycemic and insulin sensitive. Increased SREBP-1c activity in ob/ob mouse liver increases GPAT1 mRNA expression and hepatic de novo glycerolipid synthesis (29Lindén D. Rhedin L.William-Olsson, M. Clapham A-K. Asztély, J.C. Schreyer S. Overexpression of mitochondrial glycerol-3-phosphate acyltransferase in rat hepatocytes leads to decreased fatty acid oxidation and increased glycerolipid biosynthesis.J. Lipid Res. 2004; 45: 1279-1288Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar), whereas hepatic GPAT1 knockdown in ob/ob mice lowers hepatic TAG and DAG content and plasma glucose concentrations (30Xu H. Wilcox D. Nguyen P. Voorbach M. Suhar T. Morgan S.J. An W.F. Ge L. Green J. Wu Z. al et Hepatic knockdown of mitochondrial GPAT1 in ob/ob mice improves metabolic profile.Biochem. Biophys. Res. Commun. 2006; 349: 439-448Crossref PubMed Scopus (53) Google Scholar). Studies in GPAT1 knock out (KO) mice and in rats with hepatic over-expression of GPAT1 suggest that the lipid accumulation from de novo glycerolipid synthesis contributes to the development of hepatic IR in the absence of obesity or a high fat diet (12Neschen S. Morino K. Hammond L.E. Zhang D. Liu Z.X. Romanelli A.J. Cline G.W. Pongratz R.L. Zhang X.M. Choi C.S. al et Prevention of hepatic steatosis and hepatic insulin resistance in mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 knock out mice.Cell Metab. 2005; 2: 55-65Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar, 13Nagle C.A. An J. Shiota M. Torres T.P. Cline G.W. Liu Z-X. Wang S. Catlin R.L. Shulman G.I. Newgard C.B. al et Hepatic overexpression of glycerol-sn-3-phosphate acyltransferase 1 in rats causes insulin resistance.J. Biol. Chem. 2007; 282: 14807-14815Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar). Unlike lipid accumulation from DNL, hepatic steatosis caused by a block in VLDL-TAG secretion does not cause hepatic IR, perhaps because lipid intermediates do not accumulate, suggesting that lipids destined for VLDL secretion are in a separate cellular pool that cannot affect inhibitors of insulin signaling (17Minehira K. Young S.G. Villanueva C.J. Yetukuri L. Oresic M. Hellerstein M.K. Farese Jr., R.V. Horton J.D. Preitner F. Thorens B. al et Blocking VLDL secretion causes hepatic steatosis but does not affect peripheral lipid stores or insulin sensitivity in mice.J. Lipid Res. 2008; 49: 2038-2044Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar). Obese mouse models largely demonstrate that activating β-oxidation decreases hepatic lipid accumulation and improves insulin sensitivity. For example, activating PPARα with Wy-14,643 increases β-oxidation, reduces hepatic TAG accumulation, and improves insulin signaling in ob/ob mice and in lipoatrophic A-ZIP/F-1 mice (31Ide T. Tsunoda M. Mochizuki T. Murakami K. Enhancement of insulin signaling through inhibition of tissue lipid accumulation by activation of peroxisome proliferators-activated receptor (PPAR) alpha in obese mice.Med. Sci. Monit. 2004; 10: BR388-BR395PubMed Google Scholar, 32Chou C.J. Haluzik M. Gregory C. Dietz K.R. Vinson C. Garvrilova O. Reitman M.L. WY14,643, a peroxisome proliferator-activated receptor alpha (PPARalpha) agonist, improves hepatic and muscle steatosis and reverses insulin resistance in lipoatrophic A-ZIP/F-1 mice.J. Biol. Chem. 2002; 277: 24484-24489Abstract Full Text Full Text PDF PubMed Scopus (167) Google Scholar). However, the role of suppressed β-oxidation in the development of IR is less clear. Both prolonged fasting and short-term pharmacological inhibition of CPT-1 cause a fatty liver without IR (18Grefhorst A. Hoekstra J. Derks T.G. Ouwens D.M. Baller J.F. Havinga R. Havekes L.M. Romijn J.A. Kuipers F. Acute hepatic steatosis in mice by blocking beta-oxidation does not reduce insulin sensitivity of very-low-density lipoprotein production.Am. J. Physiol. Gastrointest. Liver Physiol. 2005; 289: G592-G598Crossref PubMed Scopus (53) Google Scholar, 21Heijboer A.C. Donga E. Voshol P.J. Dang Z.C. Havekes L.M. Romijn J.A. Corssmit E.P. Sixteen hours of fasting differentially affects hepatic and muscle insulin sensitivity in mice.J. Lipid Res. 2005; 46: 582-588Abstract Full Text Full Text PDF PubMed Scopus (81) Google Scholar). Unfortunately, DAG and other lipid intermediates were not measured in this study. When hepatic TAG accumulates in lean fasted animals, it does not cause hepatic IR. Perhaps signaling lipids are sequestered in pools that cannot interfere with insulin signaling. Similar to pharmacological inhibition of β-oxidation, genetic deficiency of PPARα, VLCAD, or MCAD causes a fatty liver and mild to severe hypoglycemia under fasting conditions (33Leone T.C. Winheimer C.J. Kelly D.P. A critical role for the peroxisome proliferator-activated receptor alpha in the cellular fasting response: the PPARalpha-null mouse as a model of fatty acid oxidation disorders.Proc. Natl. Acad. Sci. USA. 1999; 96: 7473-7478Crossref PubMed Scopus (824) Google Scholar, 34Cox K.B. Hamm D.A. Millington D.S. Matern D. Vockley J. Rinaldo P. Pinkert C.A. Rhead W.J. Lindsey J.R. Wood P.A. Gestational, pathologic and biochemical differences between very long-chain acyl-CoA dehydrogenase deficiency and long-chain acyl-CoA dehydrogenase deficiency in the mouse.Hum. Mol. Genet. 2001; 10: 2069-2077Crossref PubMed Scopus (101) Google Scholar, 35Tolwani R. Hamm D. Tian L. Sharer J. Vockley J. Rinaldo P. Matern D. Schoeb T. Wood P. Medium-chain acyl-CoA dehydrogenase deficiency in gene-targeted mice.PLOS Genet. 2005; 1: e23Crossref PubMed Scopus (87) Google Scholar). PPARα KO mice are protected from hepatic IR when fed a high fat (HF)-coconut oil diet (20Guerre-Millo M. Rouault C. Poulain P. Poitout J.André, V. Peters J.M. Gonzalez F.J. Fruchart J.C. Reach G. Staels B. PPAR-alpha-null mice are protected from high-fat diet-induced insulin resistance.Diabetes. 2001; 50: 2809-2814Crossref PubMed Scopus (215) Google Scholar), but not when fed HF-palm oil or HF-lard diets (36Patsouris D. Reddy J.K. Kersten M.Mu¨ller, and S. Peroxisome proliferator-activated receptor alpha mediates the effects of high-fat diet on hepatic gene expression.Endocrinology. 2006; 147: 1508-1516Crossref PubMed Scopus (248) Google Scholar). The insulin sensitivity of VLCAD and MCAD mice has not been studied, but LCAD KO mice develop hepatic steatosis and reduced FA oxidation without hypoglycemia (37Zhang D. Liu Z.X. Choi C.S. Tian L. Kibbey R. Dong J. Cline G.W. Wood P.A. Shulman G.I. Mitochondrial dysfunction due to long-chain Acyl-CoA dehydrogenase deficiency causes hepatic steatosis and hepatic insulin resistance.Proc. Natl. Acad. Sci. USA. 2007; 104: 17075-17080Crossref PubMed Scopus (220) Google Scholar). Although fasting hepatic DAG and ceramide content do not differ between wild-type (WT) and LCAD KO mice, the hepatic DAG content increases 3-fold in the LCAD KO livers after insulin stimulation during a hyperinsulinemic-euglycemic clamp, and is associated with hepatic IR (43Buettner R. Parhofer K.G. Woenckhaus M. Wrede C.E. Bollheimer L.A. Kunz-Schughart, J.Scho¨lmerich, and L.C. Defining high-fat-diet rat models: metabolic and molecular effects of different fat types.J. Mol. Endocrinol. 2006; 36: 485-501Crossref PubMed Scopus (538) Google Scholar). Thus, inhibiting β-oxidation in fasted lean mice causes hepatic steatosis without IR, whereas inhibiting β-oxidation in previously high-fat-fed animals interferes with insulin signaling. Perhaps impaired β-oxidation is most detrimental to insulin sensitivity when DNL is activated in response to feeding, thereby resulting in increased synthesis of FA and glycerolipid intermediates. It is not clear what makes one kind of lipid accumulation differ from another, but differences in cellular location or in lipid species may determine whether insulin signaling is impaired. Additionally, none of these models describes a liver-specific inhibition of β-oxidation. Inhibiting β-oxidation in adipose tissue or muscle may alter plasma cytokine levels that secondarily improve or inhibit hepatic insulin signaling. Both DAG and ceramide can diminish hepatic insulin sensitivity, and feeding rats a HF safflower oil diet (HF-SD) for three days increases hepatic DAG and PKCε activity and hepatic IR (10Samuel V.T. Liu Z-X. Qu X. Elder B.D. Bliz S. Befroy D. Romanelli A.J. Shulman G.I. Mechanism of hepatic insulin resistance in non-alcoholic fatty liver disease.J. Biol. Chem. 2004; 279: 32345-32353Abstract Full Text Full Text PDF PubMed Scopus (1044) Google Scholar). Conversely, a liver-specific knock-down of PKCε prevents HF-SD-induced hepatic IR, strengthening the hypothesis that PKCε and DAG mediate hepatic IR in this model (38Samuel V.T. Liu Z.X. Wang A. Beddow S.A. Geisler J.G. Kahn M. Zhang X.M. Monia B.P. Bhanot S. Shulman G.I. Inhibition of protein kinase Cepsilon prevents hepatic insulin resistance in nonalcoholic fatty liver disease.J. Clin. Invest. 2007; 117: 739-745Crossref PubMed Scopus (394) Google Scholar). Other mouse models that support this hypothesis include hepatic over-expression of GPAT1 (13Nagle C.A. An J. Shiota M. Torres T.P. Cline G.W. Liu Z-X. Wang S. Catlin R.L. Shulman G.I. Newgard C.B. al et Hepatic overexpression of glycerol-sn-3-phosphate acyltransferase 1 in rats causes insulin resistance.J. Biol. Chem. 2007; 282: 14807-14815Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar), LCAD KO mice (37Zhang D. Liu Z.X. Choi C.S. Tian L. Kibbey R. Dong J. Cline G.W. Wood P.A. Shulman G.I. Mitochondrial dysfunction due to long-chain Acyl-CoA dehydrogenase deficiency causes hepatic steatosis and hepatic insulin resistance.Proc. Natl. Acad. Sci. USA. 2007; 104: 17075-17080Crossref PubMed Scopus (220) Google Scholar), Elovl6 KO mice fed a high fat-high sucrose diet (19Matsuzaka T. Shimano H. Yahagi N. Kato T. Atsumi A. Yamamoto T. Inoue N. Ishikawa M. Okada S. Ishigaki N. al et Crucial role of a long-chain fatty acid elongase, Elovl6, in obesity-induced insulin resistance.Nat. Med. 2007; 13: 1193-1202Crossref PubMed Scopus (414) Google Scholar), and mouse models fed with the HF-SD including: ACC2 KO (19Matsuzaka T. Shimano H. Yahagi N. Kato T. Atsumi A. Yamamoto T. Inoue N. Ishikawa M. Okada S. Ishigaki N. al et Crucial role of a long-chain fatty acid elongase, Elovl6, in obesity-induced insulin resistance.Nat. Med. 2007; 13: 1193-1202Crossref PubMed Scopus (414) Google Scholar), ACC1 and 2 knockdown (26Savage D.B. Choi C.S. Samuel V.T. Liu Z.X. Zhang D. Wang A. Zhang X.M. Cline G.W. Yu X.X. Geisler J.G. al et Reversal of diet-induced hepatic steatosis and insulin resistance by antisense oligonucleotide inhibitors of acetyl-CoA carboxylases 1 and 2.J. Clin. Invest. 2006; 116: 817-824Crossref PubMed Scopus (362) Google Scholar), GPAT1 KO (12Neschen S. Morino K. Hammond L.E. Zhang D. Liu Z.X. Romanelli A.J. Cline G.W. Pongratz R.L. Zhang X.M. Choi C.S. al et Prevention of hepatic steatosis and hepatic insulin resistance in mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase 1 knock out mice.Cell Metab. 2005; 2: 55-65Abstract Full Text Full Text PDF PubMed Scopus (218) Google Scholar), and DGAT2 knockdown mice (16Monetti M. Levin M.C. Watt M.J. Sajan M.P. Marmor S. Hubbard B.K. Stevens R.D. Bain J.R. Newgard C.B. Farese R.V. al Sr., et Dissociation of hepatic steatosis and insulin resistance in mice overexpressing DGAT in the liver.Cell Metab. 2007; 6: 69-78Abstract Full Text Full Text PDF PubMed Scopus (422) Google Scholar). Although these studies provide strong evidence that DAG and PKCε are important mediators of hepatic IR in HF-SD fed animals, several studies question the importance of DAG. For example, GPAT1 KO mice are protected from hepatic IR when fed the HF-SD, but not when fed a diet high in saturated fat (39Hammond L.E. Neschen S. Romanelli A.J. Cline G.W. Ilkayeva O.R. Shulman G.I. Muoio D.M. Coleman R.A. Mitochondrial glycerol-3-phosphate acyltransferase-1 is essential in liver for the metabolism of excess acyl-CoAs.J. Biol. Chem. 2005; 280: 25629-25636Abstract Full Text Full Text PDF PubMed Scopus (92) Google Scholar, 40Yazdi M. Ahnmark A. Snaith L.William-Olsson, M. Turner N. Osla F. Wedin M. Elmgren A.K. Asztély, A. Schreyer M.Bohlooly-Y., S. Lindén D. The role of mitochondrial glycerol-3-phosphate acyltransferase-1 in regulating lipid and glucose homeostasis in high-fat diet fed mice.Biochem. Biophys. Res. Commun. 2008; 369: 1065-1070Crossref PubMed Scopus (18) Google Scholar). Similarly, mice treated with DGAT2 ASO are protected from HF-SD IR, but not from IR induced by a HF saturated diet (26Savage D.B. Choi C.S. Samuel V.T. Liu Z.X. Zhang D. Wang A. Zhang X.M. Cline G.W. Yu X.X. Geisler J.G. al et Reversal of diet-induced hepatic steatosis and insulin resistance by antisense oligonucleotide inhibitors of acetyl-CoA carboxylases 1 and 2.J. Clin. Invest. 2006; 116: 817-824Crossref PubMed Scopus (362) Google Scholar, 41Yu X.X. Murray S.F. Pandey S.K. Booten S.L. Bao D. Song X.Z. Kelly S. Chen S. McKay R. Monia B.P. al et Antisense oligonucleotide reduction of DGAT2 expression improves hepatic steatosis and hyperlipidemia in obese mice.Hepatology. 2005; 42: 362-371Crossref PubMed Scopus (241) Google Scholar). These discrepancies suggest that different fatty acid species affect mechanisms of hepatic IR differently. For example, a HF-lard diet causes a ceramide-associated hepatic IR that is reversed by inhibiting sphingolipid synthesis, whereas a HF-soy oil diet causes a DAG-associated IR that is not affected by sphingolipid inhibition (11Holland W.L. Brozinick J.T. Wang L.P. Hawkins E.D. Sargent K.M. Liu Y. Narra K. Hoehn K.L. Knotts T.A. Siesky A. al et Inhibition of ceramide synthesis ameliorates glucocorticoid-, saturated-fat-, and obesity-induced insulin resistance.Cell Metab. 2007; 5: 167-179Abstract Full Text Full Text PDF PubMed Scopus (926) Google Scholar). Perhaps high-saturated, but not high-unsaturated fat diets induce a low-grade systemic inflammation. Increased macrophage production of TNFα activates NFκB in hepatocytes and increases ceramide and hepatic IR (8Cai D. Yuan M. Frantz D.F. Melendez P.A. Hansen L. Lee J. Shoelson S.E. Local and systemic insulin resistance resulting from hepatic activation of IKK-and NF-B.Nat. Med. 2005; 11: 183-190Crossref PubMed Scopus (1819) Google Scholar, 42Nieto-Vazquez I. Lorenzo S.Fernández-Veledo, D.K. Kra¨mer, R.Vila-Bedmar, L.Garcia-Guerra, and M. Insulin resistance associated to obesity: the link TNF-alpha.Arch. Physiol. Biochem. 2008; 114: 183-194Crossref PubMed Scopus (329) Google Scholar). However, none of the studies using a HF-SD to induce IR reported increases in inflammation, so this diet may not be as proinflammatory as diets high in saturated fat. Human diets consist of both saturated and unsaturated fats, and DAG and ceramide may both be relevant lipid mediators of IR in human NAFLD. Mice can have elevated hepatic TAG, DAG, and ceramide without hepatic IR (16Monetti M. Levin M.C. Watt M.J. Sajan M.P. Marmor S. Hubbard B.K. Stevens R.D. Bain J.R. Newgard C.B. Farese R.V. al Sr., et Dissociation of hepatic steatosis and insulin resistance in mice overexpressing DGAT in the liver.Cell Metab. 2007; 6: 69-78Abstract Full Text Full Text PDF PubMed Scopus (422) Google Scholar, 17Minehira K. Young S.G. Villanueva C.J. Yetukuri L. Oresic M. Hellerstein M.K. Farese Jr., R.V. Horton J.D. Preitner F. Thorens B. al et Blocking VLDL secretion causes hepatic steatosis but does not affect peripheral lipid stores or insulin sensitivity in mice.J. Lipid Res. 2008; 49: 2038-2044Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar). This discrepancy might be explained if different pools of DAG and ceramide exist within cells, with only certain pools able to regulate inhibitors of insulin signaling. Thus, measuring hepatic DAG and ceramide total content instead of the content in specific membrane fractions may obscure the relevant mechanism. Additionally, some DAG and sphingolipid species may be poor inhibitors of insulin signaling. The fact that diets high in different FA species have remarkably different effects on hepatic insulin sensitivity and gene expression (11Holland W.L. Brozinick J.T. Wang L.P. Hawkins E.D. Sargent K.M. Liu Y. Narra K. Hoehn K.L. Knotts T.A. Siesky A. al et Inhibition of ceramide synthesis ameliorates glucocorticoid-, saturated-fat-, and obesity-induced insulin resistance.Cell Metab. 2007; 5: 167-179Abstract Full Text Full Text PDF PubMed Scopus (926) Google Scholar, 43Buettner R. Parhofer K.G. Woenckhaus M. Wrede C.E. Bollheimer L.A. Kunz-Schughart, J.Scho¨lmerich, and L.C. Defining high-fat-diet rat models: metabolic and molecular effects of different fat types.J. Mol. Endocrinol. 2006; 36: 485-501Crossref PubMed Scopus (538) Google Scholar) suggests that DAG FA composition could be relevant. Further, ceramide-induced IR may be caused by ganglioside metabolites of ceramide (44Aerts J.M. Ottenhoff R. Powlson A.S. Grefhorst A. Eijk M.van Dubbelhuis P.F. Aten J. Kuipers F. Serlie M.J. Wennekes T. al et Pharmacological inhibition of glucosylceramide synthase enhances insulin sensitivity.Diabetes. 2007; 56: 1341-1349Crossref PubMed Scopus (257) Google Scholar). Future researchers should measure not only gross lipid content, but also the intracellular locations and FA composition of lipids, as well as inflammatory markers that are associated with IR. A major difficulty in assessing the importance or the significance of any lipid intermediate to impaired insulin signaling is the lack of direct experiments. Most of the studies that implicate acyl-CoA, LPA, PA, DAG, or ceramide rely on indirect associations. The effects of different diets are not well documented or explained mechanistically, and the inter-relationships between different putative signaling pathways have been inadequately delineated. Convincing data to support the hypothesis that specific lipid intermediates initiate signaling pathways that alter insulin signaling may require additional studies in cultured hepatocytes in which the concentration of each purported signaling molecule can be manipulated independently.
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