Nonalcoholic steatohepatitis: Association of insulin resistance and mitochondrial abnormalities

Background & Aims: The pathogenesis of nonalcoholic steatohepatitis (NASH) is unknown. We tested the hypothesis that NASH is associated with 2 defects : (1) peripheral insulin resistance, which increases lipolysis, delivery of free fatty acids (FFA) to the liver, and hepatic fatty acid beta oxidation , thereby creating oxidative stress; and (2) an abnormality within the hepa tocytes that might render them more susceptible to injury from oxidative st ress. Methods: The hypothesis was tested by evaluation of (1) insulin resis tance by a 2-step hyperinsulinemic (10 and 40 mU . m(-2) min(-1)) euglycemi c clamp; (2) insulin effects on lipolysis by enrichment of [U-C-13]glycerol ; (3) frequency and severity of structural defects in hepatocyte mitochondr ia in vivo; (4) fatty acid beta oxidation from serum IF-OH butyrate], relea se of water-soluble radioactivity from H-3-palmitate by cultured fibroblast s and urinary dicarboxylic acid excretion; and (5) hepatic lipid peroxidati on by immunohistochemical staining for 9-nitrotyrosine (3-NT). Subjects wit h NASH (n = 6-10 for different studies) were compared with those with fatty liver (n = 6) or normal controls (n = 6), Results: HASH and fatty liver we re both associated with insulin resistance, with mean glucose infusion rate s (normal/fatty liver/NASH) of step 1, 4.5/1,6/0.9; step 2, 9.5/7.7/4.5 (P < 0.03 for both steps). Although baseline rates of glycerol appearance were higher in those with NASH than in those with fatty liver (means, 14.6 vs. 21.6 <mu>mol . kg(-l) . min(-1); P < 0.05), neither group significantly sup pressed glycerol appearance at insulin infusion rates of 10 mU m-2 min-l. N ASH was associated with loss of mitochondrial cristae and paracrystalline i nclusions in 9 of 10 subjects, compared with 0 of 6 subjects with fatty liv er. However, no evidence of a generalized defect in fatty acid P oxidation was noted in any group. Also, mean [<beta>-OH butyrate] was highest in thos e with NASH (means, 90 vs. 110 vs. 160 mu mol/L; P < 0,04). Increased stain ing for 3-NT was present in fatty liver, and even greater staining was seen in NASH. Conclusions: These data indicate that peripheral insulin resistan ce, increased fatty acid P oxidation, and hepatic oxidative stress are pres ent in both fatty liver and NASH, but HASH alone is associated with mitocho ndrial structural defects.