title: Functional interaction of hepatic nuclear factor-4 and peroxisome proliferator-activated receptor-gamma coactivator 1alpha in CYP7A1 regulation is inhibited by a key lipogenic activator, sterol regulatory element-binding protein-1c.
abstract: Insulin inhibits transcription of cholesterol 7alpha-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 1alpha (PGC-1alpha), 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-1alpha 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-1alpha transactivation of Cyp7a1. SREBP-1c also inhibited PGC-1alpha-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-1alpha 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-1alpha. This mechanism may be relevant to known repression of many other HNF-4 target genes upon feeding.
authors.affiliation: Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
authors.forename: Bhaskar
authors.initials: B
authors.lastname: Ponugoti
authors.forename: Sungsoon
authors.initials: S
authors.lastname: Fang
authors.forename: Jongsook Kim
authors.initials: JK
authors.lastname: Kemper
mesh_terms: D000818:Animals; D019556:COS Cells; D002460:Cell Line; D002522:Chlorocebus aethiops; D002790:Cholesterol 7-alpha-Hydroxylase / Q000096:biosynthesis*; D004268:DNA-Binding Proteins / Q000378:metabolism; D005786:Gene Expression Regulation; D051557:Hepatocyte Nuclear Factor 4 / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D051379:Mice; D047495:PPAR gamma / Q000378:metabolism*; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D011960:Receptors, Estrogen / Q000378:metabolism; D051780:Sterol Regulatory Element Binding Protein 1 / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism; D015533:Transcriptional Activation; D000099150:Receptor, Farnesoid X-Activated
chemical_list: D002790:Cholesterol 7-alpha-Hydroxylase; D004268:DNA-Binding Proteins; D051557:Hepatocyte Nuclear Factor 4; D007328:Insulin; D047495:PPAR gamma; D018160:Receptors, Cytoplasmic and Nuclear; D011960:Receptors, Estrogen; D051780:Sterol Regulatory Element Binding Protein 1; D014157:Transcription Factors; D000099150:Receptor, Farnesoid X-Activated; C502772:Cyp7a1 protein, mouse