Sterol regulatory element-binding protein-1c mimics the negative effect ofinsulin on phosphoenolpyruvate carboxykinase (GTP) gene transcription

We have assessed the potential role of sterol regulatory element-binding pr otein-1c (SREBP-1c) on the transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK-C). SREBP- 1c introduced into primary hepatocytes with an adenovirus vector caused a t otal loss of PEPCK-C mRNA and a marked induction of fatty acid synthase mRN A that directly coincided with the appearance of SREBP-1c in the hepatocyte s. It also blocked the induction of PEPCK-C mRNA by cAMP and dexamethasone in these cells. In contrast, a dominant negative form of SREBP-1c (dnSREBP- 1c) stimulated the accumulation of PEPCK-C mRNA in these cells. SREBP-1c co mpletely blocked the induction of PEPCK-C gene transcription by the catalyt ic subunit of protein kinase A (PKA), and increasing concentrations of dnSR EBP-1c reversed the negative effect of insulin on transcription from the PE PCK-C gene promoter in WT-IR cells. The more than 10-fold induction of PKA- stimulated PEPCK-C gene transcription caused by the co-activator CBP, was a lso blocked by SREBP-1c. In addition, dnSREBP-1c reversed the strong negati ve effect of EIA and NF1 on PKA-stimulated transcription from the PEPCK-C g ene promoter. An analysis of the possible site of action of SREBP-1c using stepwise truncations of the PEPCK-C gene promoter indicated that the negati ve effect of SREBP-1c on transcription is exerted at a site between -355 an d -277. We conclude that SREBP-1c is an intermediate in the action of insul in on PEPCK-C gene transcription in the liver and acts by blocking the stim ulatory effect cAMP that is mediated via an interaction with cAMP-binding p rotein.