The xenobiotic compound 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene is an agonist ligand for the nuclear receptor CAR

A wide range of xenobiotic compounds are metabolized by cytochrome P450 (CY P) enzymes, and the genes that encode these enzymes are often induced in th e presence of such compounds. Here, we show that the nuclear receptor CAR c an recognize response elements present in the promoters of xenobiotic-respo nsive CYP genes, as well as other novel sites. CAR has previously been show n to be an apparently constitutive transactivator, and this constitutive ac tivity is inhibited by androstanes acting as inverse agonists. As expected, the ability of CAR to transactivate the CYP promoter elements is blocked b y the inhibitory inverse agonists. However, CAR transactivation is increase d in the presence of 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), the most potent known member of the phenobarbital-like class of CYP-inducin g agents. Three independent lines of evidence demonstrate that TCPOBOP is a n agonist ligand for CAR. The first is that TCPOBOP acts in a dose-dependen t manner as a direct agonist to compete with the inhibitory effect of the i nverse agonists. The second is that TCPOBOP acts directly to stimulate coac tivator interaction with the CAR ligand binding domain, both in vitro and i n vivo. The third is that mutations designed to block ligand binding block not only the inhibitory effect of the androstanes but also the stimulatory effect of TCPOBOP. Importantly, these mutations do not block the apparently constitutive transactivation by CAR, suggesting that this activity is trul y ligand independent. Both its ability to target CYP genes and its activati on by TCPOBOP demonstrate that CAR is a novel xenobiotic receptor that may contribute to the metabolic response to such compounds.