| Abstract |
Constitutive androstane receptor (CAR) is one of the principal regulators of hepatic cytochrome P450s (CYPs) 3A (CYP3A). cDNA-mediated expression of a mature rat CAR (rCAR) into rat hepatoma cells induced CYP3A1 and CYP2B mRNAs. Aberrant rCAR failed in these inductions. Three important human CYP3A4 regulatory elements (REs), proximal ER6 (proER6), xenobiotic responsive enhancer module (XREM) and constitutive liver enhancer module (CLEM), support constitutive and inducible expression of CYP3As mediated by CAR and pregnane X receptor (PXR). NHR-scan software predicted proER6, XREM and CLEM at -255 b, -8 kb and -11.5 kb, respectively of CYP3A4, but neither XREM nor CLEM was predicted in rat CYP3A. A luciferase reporter construct carrying a 5'-flanking sequence of CYP3A1 (-31,739 to -31,585 from its transcription initiation site) revealed important for the rCAR-dependent transactivation of CYP3A1. This region includes two putative binding motifs of nuclear receptors (DR4 and DR2), a putative hepatocyte nuclear factor-1 binding motif (HNF1), nuclear factor-kappa B binding motif (NFκB), activator protein 1 binding motif (AP-1), and ecotropic viral integration site 1 binding motif (Evi1). We hereby conclude DR4 and/or DR2 motifs being primarily responsible and HNF1 being synergistically functioning elements for the rCAR-mediated transcription of CYP3A1.
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