BACKGROUND & 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. Insulin sensitivity of the rats was altered by adding a high-fat (HF) diet or the peroxisomal-proliferator activated receptor-gamma agonist pioglitazone to the MCD diet. RESULTS: The MCD diet-induced steatohepatitis was accelerated in OLETF rats after 8 weeks. Steatosis preceded inflammation, which led to fibrosis and the development of steatohepatitis. The hepatic gene expression for transforming growth factor-beta, alpha1 procollagen and plasminogen activator inhibitor-1 was up-regulated in OLETF rats compared with LETO rats. The MCD + HF diet further enhanced insulin resistance and led to rapid development of pre-cirrhosis in OLETF rats by increasing the triglyceride pool, activating stellate cells, and up-regulating gene expression for sterol regulatory element-binding protein-1c and fatty acid synthase in the liver. In contrast, pioglitazone attenuated the MCD diet-induced steatohepatitis in OLETF rats but not in LETO rats by reversing the underlying pathogenesis involved in this model through improvement of insulin resistance. These results confirm a link between insulin resistance and the development/progression of steatohepatitis, at least partly via up-regulation of genes for lipogenesis, inflammation, and fibrogenesis, in animal models. CONCLUSIONS: Insulin resistance and/or diabetes may accelerate the entire pathologic spectrum of NASH.
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