Activation of SIRT1 by Resveratrol Represses Transcription of the Gene for the Cytosolic Form of Phosphoenolpyruvate Carboxykinase (GTP) by Deacetylating Hepatic Nuclear Factor 4α

The SIRT1 activators isonicotinamide (IsoNAM), resveratrol, fisetin, and butein repressed transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C). An evolutionarily conserved binding site for hepatic nuclear factor (HNF) 4 alpha (-272/-252) was identified, which was required for transcriptional repression of the PEPCK-C gene promoter caused by these compounds. This site contains an overlapping AP-1 binding site and is adjacent to the C/EBP binding element (-248/-234); the latter is necessary for hepatic transcription of PEPCK-C. AP-1 competed with HNF4 alpha for binding to this site and also decreased HNF4 alpha stimulation of transcription from the PEPCK-C gene promoter. Chromatin immunoprecipitation experiments demonstrated that HNF4 alpha and AP-1, but not C/EBP beta, reciprocally bound to this site prior to and after treating HepG2 cells with IsoNAM. IsoNAM treatment resulted in deacetylation of HNF4 alpha, which decreased its binding affinity to the PEPCK-C gene promoter. In HNF4 alpha-null Chinese hamster ovary cells, IsoNAM and resveratrol failed to repress transcription from the PEPCK-C gene promoter; overexpression of HNF4 alpha in Chinese hamster ovary cells re-established transcriptional inhibition. Exogenous SIRT1 expression repressed transcription, whereas knockdown of SIRT1 by RNA interference reversed this effect. IsoNAM decreased the level of mRNA for PEPCK-C but had no effect on mRNA for glucose-6-phosphatase in AML12 mouse hepatocytes. We conclude that SIRT1 activation inhibited transcription of the gene for PEPCK-C in part by deacetylation of HNF4 alpha. However, SIRT1 deacetylation of other key regulatory proteins that control PEPCK-C gene transcription also likely contributed to the inhibitory effect.