Role of uncoupling protein 2 in the regulation of bioenergetics and metabolism in the liver.

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Fatty acid oxidation, ketone body production, redox state and ATP levels, and their relations with mitochondrial uncoupling protein (UCP) 2 were examined in the early stages of liver regeneration after partial hepatectomy, using a perfusion system of the rat liver. The rate of fatty acid oxidation was increased in the regenerating liver, accompanied with increased ratio of oxygen consumption to ketone body production, suggesting that acetyl-CoA produced by β-oxidation preferred to be oxidized rather than being converted to ketone bodies. Mitochondrial NADH/NAD^+ ratio (redox states) was sustained at low levels even when fatty acid was infused. Thus, supply of sufficient amounts of NAD^+ presumably explains the increased fatty acid oxidation. UCP2 was remarkably increased in the remnant liver, especially in hepatocytes, and also in the sham-operated control liver, suggesting that UCP2 is unlikely to control redox states and consequent increase of fatty acid oxidation. The redox states were also not related directly with ATP levels, an index of oxidative phosphorylation, and with conversion of acetoacetate to 3-hydroxybutyrate.Next, to determine the regulatory mechanisms of UCP2 expression in the hepatocytes, transcription activity of UCP2 gene was analyzed in two hepatoma cell lines, one of which lacks endogenous UCP2 expression. In both the cells, the minimal region exhibiting full promoter activity was located between the translation-initiation site of the mouse UCP2 gene and the100bp upstream region, and the sequence and the gene mutation analyzes suggest involvement of specificity protein 1 in the full activation. In addition, treatment of the cells lacking endogenous UCP2 with azacytidine, a DNA methylation inhibitor, caused the expression of UCP2 gene. These results suggest that epigenetic control of UCP2 gene, as well as cis-acting DNA elements, is important for its expression in hepatocytes.