Functional interaction of hepatic nuclear factor-4 and peroxisome proliferator-activated receptor-γ coactivator 1α in CYP7A1 regulation is inhibited by a key lipogenic activator, sterol regulatory element-binding protein-1c

Insulin inhibits transcription of cholesterol 7 alpha-hydroxylase ( Cyp7a1), a key gene in bile acid synthesis, and the hepatic nuclear factor-4 ( HNF-4) site in the promoter was identified as a negative insulin response sequence. Using a fasting/feeding protocol in mice and insulin treatment in HepG2 cells, we explored the inhibition mechanisms. Expression of sterol regulatory element-binding protein-1c ( SREBP-1c), an insulin-induced lipogenic factor, inversely correlated with Cyp7a1 expression in mouse liver. Interaction of HNF-4 with its coactivator, peroxisome proliferator-activated receptor-gamma coactivator 1 alpha ( PGC-1 alpha), was observed in livers of fasted mice and was reduced after feeding. Conversely, HNF- 4 interaction with SREBP-1c was increased after feeding. In vitro studies suggested that SREBP-1c competed with PGC-1 alpha for direct interaction with the AF2 domain of HNF-4. Reporter assays showed that SREBP-1c, but not of a SREBP-1c mutant lacking the HNF- 4 interacting domain, inhibited HNF-4/PGC-1 alpha transactivation of Cyp7a1. SREBP-1c also inhibited PGC-1 alpha-coactivation of estrogen receptor, constitutive androstane receptor, pregnane X receptor, and farnesoid X receptor, implying inhibition of HNF- 4 by SREBP-1c could extend to other nuclear receptors. In chromatin immunoprecipitation studies, HNF- 4 binding to the promoter was not altered, but PGC-1 alpha was dissociated, SREBP-1c and histone deacetylase-2 ( HDAC2) were recruited, and acetylation of histone H3 was decreased upon feeding. Adenovirus-mediated expression of a SREBP-1c dominant-negative mutant, which blocks the interaction of SREBP-1c and HNF- 4, partially but significantly reversed the inhibition of Cyp7a1 after feeding. Our data show that SREBP-1c functions as a non-DNA-binding inhibitor and mediates, in part, suppression of Cyp7a1 by blocking functional interaction of HNF- 4 and PGC-1(alpha). This mechanism may be relevant to known repression of many other HNF-4 target genes upon feeding.