The Role of Sirt1 Activation of AMP-Activated Protein Kinase in the Effect of Polyphenols on Lipid Metabolism in HepG2 Cells Exposed to High Glucose.

Hou, Xiuyun; Xu, Shanqin; Jiang, Bingbing; Ido, Yasuo; Lan, Fan; Wierzbicki, Michel; Verbeuren, Tony J.; Cohen, Richard A.; Zang, Mengwei

Sirt1 is one of seven mammalian homologs of Sir2, an NAD-dependent deacetylase that functions as a novel metabolic sensor connecting aging and metabolism. We recently discovered that polyphenols stimulate AMPK activity and lower hepatic lipids, contributing to their beneficial effects on accelerated hyperlipidemia and atherosclerosis in diabetes. Here we examined if Sirt1 mediates the effect of polyphenols on AMPK and its lipid metabolic effects. The in vitro Fluor de Lys deacetylation assay showed that Sirt1 catalytic activity was significantly increased by polyphenols including resveratrol and a synthetic polyphenol, S17834, and decreased by Sirt1 inhibitors, nicotinamide and suramin, respectively. The two polyphenolic Sirt1 activators substantially stimulated phosphorylation of AMPK and its downstream effector, acetyl CoA carboxylase, in cultured human HepG2 hepatocytes and HEK293 cells, but not in HeLa cells deficient in LKB1. These responses were largely blocked by inhibition of Sirt1 deacetylation with nicotinamide or splitomicin. Importantly, downregulation of Sirt1 using lentivirus-mediated Sirt1 shRNA completely abolished the basal and polyphenol-induced AMPK activation. Furthermore, studies with a dominant negative Sirt1 (Sirt1H355A) and Sirt1 inhibitors revealed that the effects of polyphenols on hepatocellular AMPK signaling and lipids depend on Sirt1 activity in a HepG2 cell model of high glucose-induced lipid accumulation. Overexpression of wild type Sirt1 also stimulated AMPK, which was inhibited by high glucose, as well as reduced triglyceride levels that were enhanced by high glucose. The effects of wild type Sirtl were completely blocked by a dominant negative AMPK (AMP-Ka1phaK45R). These results implicate Sirt1 stimulation of AMPK as a key regulator of lipid metabolism in bepatocytes. Because of the associated improvements seen in hyperlipidemia and atherosclerosis caused by polyphenols in type 1 and type 2 mouse models, these data suggest that Sirt1 and AMPK may be attractive therapeutic targets for diabetes.

PROTEIN kinases; POLYPHENOLS; LIPID metabolism; HYPERLIPIDEMIA; ATHEROSCLEROSIS; DIABETES; NICOTINAMIDE; PHOSPHORYLATION

Diabetes, 2007, Vol 56, pA98

0012-1797

Academic Journal