Résumé
Abstract only CAR (Constitutive Androstane Receptor, NR1I3) and PXR (Pregnane X Receptor, NR1I2) are nuclear receptors that control both endogenous and exogenous toxic compounds metabolism and elimination. In contrasts to PXR, CAR is expressed only in adult liver and not in fetal liver. Factors regulating human CAR (hCAR) gene expression remain largely unexplored. We focused our attention on the effect of Hepatocyte Nuclear Factor (HNF)‐4a isoforms onto hCAR gene promoter. HNF‐4a gene possesses two promoters, proximal P1 and distal P2, whose use results in HNF‐4a1 and HNF4a7 transcripts, respectively. While HNF‐4a7 is mainly expressed in the embryonic liver, HNF‐4a1 is almost exclusively in the adult liver. Transient transfections showed that HNF‐4a1, but not HNF‐4a7, enhances hCAR promoter activity. 5′‐Deletion and mutation analysis of hCAR promoter identified a proximal DR1 regulatory element, termed CAR HNF‐4a‐RE (−114ccAGGCCTtTGCCCTga). HNF‐4a1 increased the activity of a heterologous promoter driven by two copies of the CAR HNF‐4a‐RE. We demonstrate that in vitro synthetized HNF‐4a binds to this element in gel shift assays, while endogenous HNF4‐a interacts with hCAR promoter in chromatin immunoprecipitation studies performed with human hepatocytes. In addition, we observed that glucocorticoid receptor and HNF‐4a1 (but not HNF4‐a7) cooperate to maximally transactivate CAR gene expression, while HNF4‐a7 repressed this cooperation. Finally, we observed a strong correlation between CAR and HNF‐4a1 mRNA expression level in human liver tissu samples, and an inverse correlation between CAR and HNF‐4a7 in human hepatoma suggesting that, while HNF‐4a1 positively regulates CAR expression in adult, the fetal HNF‐4a7 isoform represses CAR gene expression.