Phenobarbital response elements of cytochrome P450 genes and nuclear receptors

Phenobarbital (PB) response elements are composed of various nuclear recept or (NR)-binding sites. A 51-bp distal element PB-responsive enhancer module (PBREM) conserved in the PB-inducible CYP2B genes contains two NR-binding direct repeat (DR)-4 motifs. Responding to PB exposure in liver, the NR con stitutive active receptor (CAR) translocates to the nucleus, forms a dimer with the retinoid X receptor (RXR), and activates PBREM via binding to DR-4 motifs. For CYP3A genes, a common NR site [DR-3 or everted repeat (ER)-6] is present in proximal promoter regions. In addition, the distal element ca lled the xenobiotic responsive module (XREM) is found in human CYP3A4 genes , which contain both DR-3 and ER-6 motifs. Pregnane X receptor (PXR) could bind to all of these sites and, upon PB induction, a PXR:RXR heterodimer co uld transactivate XREM. These response elements and NRs are functionally ve rsatile, and capable of responding to distinct but overlapping groups of xe nochemicals.