The sirtuin activator represses glucokinase expression via the combined action of FoxO1 and HNF-4

Abstract

Glucokinase (GK) plays a key role in promoting glucose utilization in the liver. Recently, it has been reported that the polyphenol resveratrol regulates several cellular processes among them fat and glucose metabolism. Although it was proposed that resveratrol may exert its effect via its target Sirt1 through deacetylation of FoxO1, the molecular mechanisms by which resveratrol affects glucose metabolism remain largely unknown. Therefore, it was the aim of the present study to better understand the role of resveratrol in the regulation of GK expression in the liver. Treatment of hepatocytes with resveratrol dose-dependently inhibited GK mRNA and enzyme activity while siRNA against FoxO1 reversed the resveratrol-dependent inhibition. Resveratrol was then shown to mediate acetylation of FoxO1 and transfection of hepatocytes with expression vectors for a deacetylation resistant FoxO1 mutant reduced GK promoter activity stronger than wild-type FoxO1. In addition, mutation of a FoxO1 binding element (FBEb) disrupted resveratrol and FoxO1-mediated repression of the GK promoter. Further, reporter gene assays and co-immunoprecipitation studies showed that interaction of FoxO1 with HNF-4 contributed to the repressive effects of resveratrol and FoxO1 on GK promoter activity. To our knowledge, this report provides the first evidence that resveratrol represses the activity of the liver-specific GK promoter via modulation of FoxO1 acetylation, and that interactions between FoxO1 target sites and HNF-4 contribute to the regulation of GK expression in the liver.