cAMP-mediated regulation of HNF-4alpha depends on the level of coactivator PGC-1alpha

Abstract

Hepatocyte nuclear factor-4 alpha (HNF-4alpha) is a member of the nuclear receptor superfamily with important roles in hepatic metabolism. Fasting induces the cAMP/protein kinase A (PKA)-signaling pathway. The mechanisms whereby cAMP regulates HNF-4alpha transcriptional activity are incompletely understood. We have therefore investigated the role of cAMP/PKA in regulation of HNF-4alpha in COS-1 cells and the hepatoma HepG2 cell line. cAMP/PKA inhibited the transcriptional activity of HNF-4alpha in COS-1 cells, whereas a stimulatory effect was observed in HepG2 cells. The cAMP-induced inhibition of HNF-4alpha in COS-1 cells was counteracted by overexpression of the nuclear receptor coactivator PGC-1alpha, and cAMP/PKA-dependent induction of the PGC1A gene in HepG2 cells seems to explain the cell specific differences. This was further supported by knock-down of PGC-1alpha in HepG2 cells, which abolished the stimulatory effect of PKA on HNF-4alpha transcriptional activity. Similar to the cAMP/PKA-mediated regulation of HNF-4alpha, overexpression of the cAMP-response element binding protein (CREB) inhibited the transcriptional activity of HNF-4alpha in COS-1 cells, regardless of cAMP/PKA activation and CREB phosphorylation. Moreover, activation of CREB by cAMP/PKA further stimulated HNF-4alpha transactivation in HepG2 cells. cAMP induced the expression of the HNF-4alpha target genes PCK1 and G6Pase in these cells. In conclusion, our results suggest that the level of PGC-1alpha determines whether the cAMP/PKA-pathway overall stimulates or inhibits HNF-4alpha transcriptional activation.