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 hepatocytes that might render them more susceptible to injury from oxidative stress. Methods: The hypothesis was tested by evaluation of (1) insulin resistance by a 2-step hyperinsulinemic (10 and 40 mU . m(-2) min(-1)) euglycemic clamp; (2) insulin effects on lipolysis by enrichment of [U-C-13]glycerol; (3) frequency and severity of structural defects in hepatocyte mitochondria in vivo; (4) fatty acid beta oxidation from serum IF-OH butyrate], release of water-soluble radioactivity from H-3-palmitate by cultured fibroblasts and urinary dicarboxylic acid excretion; and (5) hepatic lipid peroxidation by immunohistochemical staining for 9-nitrotyrosine (3-NT). Subjects with 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 were both associated with insulin resistance, with mean glucose infusion rates (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 suppressed glycerol appearance at insulin infusion rates of 10 mU m-2 min-l. NASH was associated with loss of mitochondrial cristae and paracrystalline inclusions in 9 of 10 subjects, compared with 0 of 6 subjects with fatty liver. 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 those with NASH (means, 90 vs. 110 vs. 160 mu mol/L; P < 0,04). Increased staining for 3-NT was present in fatty liver, and even greater staining was seen in NASH. Conclusions: These data indicate that peripheral insulin resistance, increased fatty acid P oxidation, and hepatic oxidative stress are present in both fatty liver and NASH, but HASH alone is associated with mitochondrial structural defects.