Xenobiotic induction of cytochrome P450 2B1 (CYP2B1) is mediated by the orphan nuclear receptor constitutive androstane receptor (CAR) and requires steroid co-activator 1 (SRC-1) and the transcription factor Sp1.
The constitutive androstane receptor (CAR) activates the expression of a reporter gene attached to the phenobarbital-response element (PBRE) of the cytochrome P450 2B1 (CYP2B1) gene in response to the barbiturate phenobarbital and the plant product picrotoxin. The xenobiotic-mediated increase in transactivation occurs in transfected primary hepatocytes and in liver transfected by biolistic-particle-mediated DNA transfer, but not in the transformed cell lines HepG2, CV-1 and HeLa, which support only constitutive activation of gene expression by CAR. Steroid co-activator 1 (SRC-1) enhances both constitutive and xenobiotic-induced CAR-mediated transactivation via the CYP2B1 PBRE in transfected primary hepatocytes. The nuclear receptor 1 (NR1) site of the PBRE is sufficient for CAR-mediated transactivation, but additional sequences within the PBRE, and hence the proteins that bind to them, are required for the interaction of CAR with SRC-1. The NR2 site of the PBRE binds proteins other than CAR, including an unidentified nuclear receptor heterodimerized with retinoid X receptor alpha. By binding to the proximal promoter of CYP2B1, the transcription factor Sp1 increases both basal transcription and xenobiotic-induced expression via the PBRE. Thus induction of CYP2B1 expression by xenobiotics is mediated by the nuclear receptor CAR and, for optimal expression, requires SRC-1 and Sp1.
Muangmoonchai
R
R
Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K.
Smirlis
D
D
Wong
S C
SC
Edwards
M
M
Phillips
I R
IR
Shephard
E A
EA
D000818:Animals; D001483:Base Sequence; D002461:Cell Line, Transformed; D002478:Cells, Cultured; D000090702:Constitutive Androstane Receptor; D019362:Cytochrome P-450 CYP2B1 / Q000096:biosynthesis* / Q000235:genetics; D017931:DNA Primers; D004790:Enzyme Induction / Q000187:drug effects / Q000502:physiology; D022781:Hepatocytes / Q000187:drug effects / Q000201:enzymology; D051548:Histone Acetyltransferases; D056920:Nuclear Receptor Coactivator 1; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000502:physiology*; D016329:Sp1 Transcription Factor / Q000502:physiology*; D014157:Transcription Factors / Q000502:physiology*; D015533:Transcriptional Activation / Q000187:drug effects; D015262:Xenobiotics / Q000494:pharmacology*
D000090702:Constitutive Androstane Receptor; D017931:DNA Primers; D018160:Receptors, Cytoplasmic and Nuclear; D016329:Sp1 Transcription Factor; D014157:Transcription Factors; D015262:Xenobiotics; D019362:Cytochrome P-450 CYP2B1; D051548:Histone Acetyltransferases; D056920:Nuclear Receptor Coactivator 1