cAMP-mediated regulation of HNF-4alpha depends on the level of coactivator PGC-1alpha.
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.
Dankel
Simon Nitter
SN
Institute of Medicine, University of Bergen and the Hormone Laboratory, Haukeland University Hospital, N-5021 Bergen, Norway.
Hoang
Tuyen
T
Flågeng
Marianne Hauglid
MH
Sagen
Jørn Vegard
JV
Mellgren
Gunnar
G
D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D000242:Cyclic AMP / Q000494:pharmacology*; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism / Q000502:physiology; D005786:Gene Expression Regulation / Q000187:drug effects; D006360:Heat-Shock Proteins / Q000378:metabolism*; D056945:Hep G2 Cells; D051557:Hepatocyte Nuclear Factor 4 / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D000071248:Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha; D015534:Trans-Activators / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism*; D015533:Transcriptional Activation; D014162:Transfection
D017362:Cyclic AMP Response Element-Binding Protein; C496592:HNF4A protein, human; D006360:Heat-Shock Proteins; D051557:Hepatocyte Nuclear Factor 4; C486497:PPARGC1A protein, human; D000071248:Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha; D015534:Trans-Activators; D014157:Transcription Factors; D000242:Cyclic AMP