Insulin Resistance: Cause or Consequence of Nonalcoholic Steatohepatitis?

Abstract

See “Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis” by Ota T, Takamura T, Kurita S, Matsuzawa N, Kita Y, Uno M, Akahori H, Misu H, Sakurai M, Zen Y, Nakanuma Y, and Kaneko S, on page 282. See “Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis” by Ota T, Takamura T, Kurita S, Matsuzawa N, Kita Y, Uno M, Akahori H, Misu H, Sakurai M, Zen Y, Nakanuma Y, and Kaneko S, on page 282. Nonalcoholic fatty liver disease (NAFLD) refers to a range of disorders associated with fatty liver that occur in the absence of infection or significant consumption of alcohol.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar Most patients with NAFLD have increased liver fat content alone (simple steatosis), but others develop features of inflammation, hepatocellular injury, and fibrosis, known as nonalcoholic steatohepatitis (NASH) (Figure 1).1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 2Fassio E. Alvarez E. Dominguez N. Landeira G. Longo C. Natural history of nonalcoholic steatohepatitis: a longitudinal study of repeat liver biopsies.Hepatology. 2004; 40: 820-826Crossref PubMed Scopus (425) Google Scholar Up to 20% of patients with NASH may eventually progress to advanced liver disease in the form of cirrhosis or liver failure.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 2Fassio E. Alvarez E. Dominguez N. Landeira G. Longo C. Natural history of nonalcoholic steatohepatitis: a longitudinal study of repeat liver biopsies.Hepatology. 2004; 40: 820-826Crossref PubMed Scopus (425) Google Scholar In addition, the risk of development of hepatocellular carcinoma in NASH-related cirrhosis is comparable to hepatitis C infection.3Hui J.M. Kench J.G. Chitturi S. Sud A. Farrell G.C. Byth K. Hall P. Khan M. George J. Long-term outcomes of cirrhosis in nonalcoholic steatohepatitis compared with hepatitis C.Hepatology. 2003; 38: 420-427Crossref PubMed Scopus (397) Google Scholar, 4Bugianesi E. Leone N. Vanni E. Marchesini G. Brunello F. Carucci P. Musso A. De Paolis P. Capussotti L. Salizzoni M. Rizzetto M. Expanding the natural history of nonalcoholic steatohepatitis: from cryptogenic cirrhosis to hepatocellular carcinoma.Gastroenterology. 2002; 123: 134-140Abstract Full Text Full Text PDF PubMed Scopus (1264) Google Scholar NAFLD and NASH are closely associated with the “metabolic syndrome,” a cluster of disorders including central obesity, insulin resistance with or without type 2 diabetes, dyslipidemia, and hypertension.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar Compared with normal subjects, patients with NAFLD manifest severe reductions in their insulin’s ability to suppress endogenous glucose production and increase glucose uptake by muscle and fat, indicative of insulin resistance.5Utzschneider K.M. Kahn S.E. The role of insulin resistance in non-alcoholic fatty liver disease.J Clin Endocrinol Metab. 2006; 91: 4753-4761Crossref PubMed Scopus (611) Google Scholar NAFLD also blunts insulin-mediated suppression of free fatty acid release from adipocytes, and reduces fatty acid oxidation.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar The “first hit” of NAFLD is steatosis characterized by an increase in lipid deposition, mostly in the form of triglycerides in hepatocytes (Figure 1).1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 6Day C.P. Pathogenesis of steatohepatitis.Best Pract Res Clin Gastroenterol. 2002; 16: 663-678Abstract Full Text PDF PubMed Scopus (378) Google Scholar This has been attributed to excessive fatty acid influx into hepatocytes derived from higher rates of lipolysis in obesity, and changes in adipocyte hormones, for example, hyperleptinemia and hypoadiponectinemia, which promote triglyceride synthesis and reduce fatty acid oxidation (Figure 1).1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar Hyperinsulinemia in obese patients increases hepatic steatosis by stimulating de novo lipogenesis and inactivating pathways involved in β-oxidation of fatty acids.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 5Utzschneider K.M. Kahn S.E. The role of insulin resistance in non-alcoholic fatty liver disease.J Clin Endocrinol Metab. 2006; 91: 4753-4761Crossref PubMed Scopus (611) Google Scholar The “second hit” of NAFLD includes mechanisms that promote inflammation and fibrosis.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 6Day C.P. Pathogenesis of steatohepatitis.Best Pract Res Clin Gastroenterol. 2002; 16: 663-678Abstract Full Text PDF PubMed Scopus (378) Google Scholar The rise in fatty acid levels generates reactive oxygen species, enhances lipid peroxidation, and promotes the generation of cytokines, resulting in hepatocellular injury, infiltration of inflammatory cells, activation of stellate cells, and fibrogenic reaction.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar Although an association between NASH and insulin resistance is well known, questions remain as to whether insulin resistance per se is a cause of NASH.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 5Utzschneider K.M. Kahn S.E. The role of insulin resistance in non-alcoholic fatty liver disease.J Clin Endocrinol Metab. 2006; 91: 4753-4761Crossref PubMed Scopus (611) Google Scholar The prevalence of the metabolic syndrome is far greater than NASH, and some patients with NASH do not manifest insulin resistance or other features of the metabolic syndrome.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar Furthermore, weight loss from diet and exercise improves insulin sensitivity, but only modestly decreases fatty liver and plasma transaminase levels, and does not reverse NASH.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 7American Gastroenterological Association medical position statement: nonalcoholic fatty liver disease.Gastroenterology. 2002; 123: 1702-1704Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar Nonetheless, insulin sensitizers (eg, metformin and thiazolidinediones) attenuate fatty liver, inflammation, and fibrosis in NASH, suggesting a causal role for insulin resistance.8Marchesini G. Brizi M. Bianchi G. Tomassetti S. Zoli M. Melchionda N. Metformin in non-alcoholic steatohepatitis.Lancet. 2001; 358: 893-894Abstract Full Text Full Text PDF PubMed Scopus (637) Google Scholar, 9Bugianesi E. Gentilcore E. Manini R. Natale S. Vanni E. Villanova N. David E. Rizzetto M. Marchesini G. A randomized controlled trial of metformin versus vitamin E or prescriptive diet in nonalcoholic fatty liver disease.Am J Gastroenterol. 2005; 100: 1082-1090Crossref PubMed Scopus (595) Google Scholar, 10Neuschwander-Tetri B.A. Brunt E.M. Wehmeier K.R. Oliver D. Bacon B.R. Improved nonalcoholic steatohepatitis after 48 weeks of treatment with the PPAR-gamma ligand rosiglitazone.Hepatology. 2003; 38: 1008-1017Crossref PubMed Scopus (720) Google Scholar, 11Promrat K. Lutchman G. Uwaifo G.I. Freedman R.J. Soza A. Heller T. Doo E. Ghany M. Premkumar A. Park Y. Liang T.J. Yanovski J.A. Kleiner D.E. Hoofnagle J.H. A pilot study of pioglitazone treatment for nonalcoholic steatohepatitis.Hepatology. 2004; 39: 188-196Crossref PubMed Scopus (671) Google Scholar In this issue of Gastroenterology, Ota et al12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar explore the connection between insulin resistance and NASH by feeding rats a methionine-choline deficient (MCD) diet.13Kirsch R. Clarkson V. Shephard E.G. Marais D.A. Jaffer M.A. Woodburne V.E. Kirsch R.E. Hall Pde L. Rodent nutritional model of non-alcoholic steatohepatitis: species, strain and sex difference studies.J Gastroenterol Hepatol. 2003; 18: 1272-1282Crossref PubMed Scopus (203) Google Scholar, 14Sahai A. Malladi P. Pan X. Paul R. Melin-Aldana H. Green R.M. Whitington P.F. Obese and diabetic db/db mice develop marked liver fibrosis in a model of nonalcoholic steatohepatitis: role of short-form leptin receptors and osteopontin.Am J Physiol Gastrointest Liver Physiol. 2004; 287: G1035-G1043Crossref PubMed Scopus (244) Google Scholar An MCD diet causes steatosis, steatohepatitis, and fibrosis, similar to human NASH.13Kirsch R. Clarkson V. Shephard E.G. Marais D.A. Jaffer M.A. Woodburne V.E. Kirsch R.E. Hall Pde L. Rodent nutritional model of non-alcoholic steatohepatitis: species, strain and sex difference studies.J Gastroenterol Hepatol. 2003; 18: 1272-1282Crossref PubMed Scopus (203) Google Scholar, 14Sahai A. Malladi P. Pan X. Paul R. Melin-Aldana H. Green R.M. Whitington P.F. Obese and diabetic db/db mice develop marked liver fibrosis in a model of nonalcoholic steatohepatitis: role of short-form leptin receptors and osteopontin.Am J Physiol Gastrointest Liver Physiol. 2004; 287: G1035-G1043Crossref PubMed Scopus (244) Google Scholar The precise mechanism is unclear; however, supplementation of S-adenosylmethionine prevents the changes induced by MCD diet, indicating a crucial role for methionine in the pathogenesis of steatohepatitis. However, the use of MCD diet in mice is confounded by weight loss and discordant effects on insulin sensitivity.14Sahai A. Malladi P. Pan X. Paul R. Melin-Aldana H. Green R.M. Whitington P.F. Obese and diabetic db/db mice develop marked liver fibrosis in a model of nonalcoholic steatohepatitis: role of short-form leptin receptors and osteopontin.Am J Physiol Gastrointest Liver Physiol. 2004; 287: G1035-G1043Crossref PubMed Scopus (244) Google Scholar, 15Rinella M.E. Green R.M. The methionine-choline deficient dietary model of steatohepatitis does not exhibit insulin resistance.J Hepatol. 2004; 40: 47-51Abstract Full Text Full Text PDF PubMed Scopus (336) Google Scholar By contrast, Ota et al12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar show that MCD induces NASH in rats without substantial effects on weight. They tested the hypothesis that obesity, insulin resistance, and steatosis precede the onset of NASH, by studying 2 strains of rats, Otsuka Long-Evans Tokushima Fatty (OLETF) and Long-Evans Tokushima Otsuka (LETO), bred for divergent metabolic features.16Shima K. Zhu M. Mizuno A. Pathoetiology and prevention of NIDDM lessons from the OLETF rat.J Med Invest. 1999; 46: 121-129PubMed Google Scholar, 17Man Z.W. Hirashima T. Mori S. Kawano K. Decrease in triglyceride accumulation in tissues by restricted diet and improvement of diabetes in Otsuka Long-Evans Tokushima fatty rats, a non-insulin-dependent diabetes model.Metabolism. 2000; 49: 108-114Abstract Full Text PDF PubMed Scopus (39) Google Scholar OLETF rats are prone to obesity, insulin resistance, hyperinsulinemia, diabetes, and steatosis, similar to patients with the metabolic syndrome.16Shima K. Zhu M. Mizuno A. Pathoetiology and prevention of NIDDM lessons from the OLETF rat.J Med Invest. 1999; 46: 121-129PubMed Google Scholar, 17Man Z.W. Hirashima T. Mori S. Kawano K. Decrease in triglyceride accumulation in tissues by restricted diet and improvement of diabetes in Otsuka Long-Evans Tokushima fatty rats, a non-insulin-dependent diabetes model.Metabolism. 2000; 49: 108-114Abstract Full Text PDF PubMed Scopus (39) Google Scholar However, LETO rats are lean, insulin sensitive, and less susceptible to steatosis.16Shima K. Zhu M. Mizuno A. Pathoetiology and prevention of NIDDM lessons from the OLETF rat.J Med Invest. 1999; 46: 121-129PubMed Google Scholar, 17Man Z.W. Hirashima T. Mori S. Kawano K. Decrease in triglyceride accumulation in tissues by restricted diet and improvement of diabetes in Otsuka Long-Evans Tokushima fatty rats, a non-insulin-dependent diabetes model.Metabolism. 2000; 49: 108-114Abstract Full Text PDF PubMed Scopus (39) Google Scholar OLETF rats developed hepatic steatosis after 2 weeks on MCD diet, followed by inflammation by 4 weeks and fibrosis by 8 weeks. A high-fat (Western) diet enhanced insulin resistance and accelerated NASH in MCD-OLETF rats. MCD-induced NASH led to rapid increase in hepatic triglycerides associated with expression of lipogenic genes (SREBP-1c, fatty acid synthase and steroyl CoA desaturase), increase in plasma interleukin-6 and hepatic tumor necrosis factor-α and transforming growth factor-β, stellate cell activation, and fibrogenesis.12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar By contrast, LETO rats exhibited greater insulin sensitivity and milder response to MCD and high-fat diets.12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar The link between insulin response and MCD-induced NASH was explored further by measuring adiponectin levels and testing pioglitazone treatment.12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar Adiponectin is secreted by adipocytes and regulates glucose and lipid metabolism, by suppressing gluconeogenesis and stimulating glucose utilization and fatty acid oxidation.18Trujillo M.E. Scherer P.E. Adiponectin—journey from an adipocyte secretory protein to biomarker of the metabolic syndrome.J Intern Med. 2005; 257: 167-175Crossref PubMed Scopus (650) Google Scholar, 19Yamauchi T. Kamon J. Minokoshi Y. Ito Y. Waki H. Uchida S. Yamashita S. Noda M. Kita S. Ueki K. Eto K. Akanuma Y. Froguel P. Foufelle F. Ferre P. Carling D. Kimura S. Nagai R. Kahn B.B. Kadowaki T. Adiponectin stimulates glucose utilization and fatty-acid oxidation by activating AMP-activated protein kinase.Nat Med. 2002; 8: 1288-1295Crossref PubMed Scopus (3460) Google Scholar Interestingly, the insulin-sensitizing action of thiazolidinediones is mediated via adiponectin.18Trujillo M.E. Scherer P.E. Adiponectin—journey from an adipocyte secretory protein to biomarker of the metabolic syndrome.J Intern Med. 2005; 257: 167-175Crossref PubMed Scopus (650) Google Scholar Ota et al12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar observed a decrease in adiponectin in MCD-OLETF rats. Pioglitazone treatment increased adiponectin concurrently with its insulin-sensitizing action, and reduction in NASH in MCD-OLETF rats. By contrast, pioglitazone did not affect adiponectin or NASH in MCD-LETO rats.12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar This lack of effect of insulin sensitization was interpreted as indicating a causal link between insulin resistance and NASH.12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar The study by Ota et al12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar provides new information on the natural history of NASH in an animal model, and supports an association between obesity, insulin resistance, and NASH. However, the use of glucose and insulin tolerance tests does not allow the site of insulin resistance to be ascertained. Insulin clamp and tracer kinetic studies are better suited for delineating how MCD and high-fat diets affect glucose production by the liver and glucose utilization by key insulin-sensitive targets, such as skeletal muscle and adipose tissue.20Samuel V.T. Liu Z.X. Qu X. Elder B.D. Bilz 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: 32345-32353Crossref PubMed Scopus (1033) Google Scholar, 21Ye J.M. Dzamko N. Cleasby M.E. Hegarty B.D. Furler S.M. Cooney G.J. Kraegen E.W. Direct demonstration of lipid sequestration as a mechanism by which rosiglitazone prevents fatty-acid-induced insulin resistance in the rat: comparison with metformin.Diabetologia. 2004; 47: 1306-1313Crossref PubMed Scopus (105) Google Scholar The observation that the MCD diet induced histologic and biochemical features of NASH in LETO rats, albeit to lesser degrees than OLETF rats, suggests that NASH per se does not induce insulin resistance. Although the rise in adiponectin in pioglitazone-treated MCD-OLETF rats suggests a role in ameliorating insulin resistance and NASH, this idea requires further study to determine the underlying mechanisms. Most likely, other adipokines and humoral factors are involved in the pathogenesis of NASH. Another shortcoming of the study by Ota et al12Ota T. Takamura T. Kurita S. Matsuzawa N. Kita Y. Uno M. Akahori H. Misu H. Sakurai M. Zen Y. Nakanuma Y. Kaneko S. Insulin resistance accelerates a dietary rat model of nonalcoholic steatohepatitis.Gastroenterology. 2007; 132: 282-293Abstract Full Text Full Text PDF PubMed Scopus (202) Google Scholar is that the MCD diet did not affect plasma lipid levels in OLETF and LETO rats, in contrast to humans where NAFLD frequently occurs with dyslipidemia.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar Moreover, the contents of diacylglycerol and various lipid metabolites (eg, ceramide), which disrupt insulin signaling in the liver and other tissues, were not measured.1Angulo P. Nonalcoholic fatty liver disease.N Engl J Med. 2002; 346: 1221-1231Crossref PubMed Scopus (4171) Google Scholar, 20Samuel V.T. Liu Z.X. Qu X. Elder B.D. Bilz 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: 32345-32353Crossref PubMed Scopus (1033) Google Scholar The MCD model used by Ota et al, although imperfect, is relevant to human NASH and should assist in the investigation and treatment of this condition. The study clearly demonstrates that rats genetically predisposed to the metabolic syndrome are more susceptible to NASH. Importantly, the study shows that improvement in insulin sensitivity via thiazolidinedione treatment attenuates NASH in susceptible rats. This model may prove useful for delineating the molecular mechanisms by which insulin resistance results in key features of NASH, including steatosis, inflammation, and fibrosis (Figure 1). Insulin Resistance Accelerates a Dietary Rat Model of Nonalcoholic SteatohepatitisGastroenterologyVol. 132Issue 1PreviewBackground & Aims: The increasing prevalence of nonalcoholic steatohepatitis (NASH) is due to the epidemic of obesity and type 2 diabetes, both of which are associated with insulin resistance. Methods: To clarify the causal relationship between insulin resistance and the development of NASH, steatohepatitis was induced in obese diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) and nondiabetic control Long-Evans Tokushima Otsuka (LETO) rats by feeding them a methionine and choline-deficient (MCD) diet. Full-Text PDF