Inhibition of Human UGT2B7 Gene Expression in Transgenic Mice by the Constitutive Androstane Receptor

The xenobiotic receptors, constitutive androstane receptor (CAR), and pregnane X receptor (PXR) regulate and alter the metabolism of xenobiotic substrates. Among the 19 functional UDP-glucuronosyltransferases (UGTs) in humans, UGT2B7 is involved in the metabolism of many structurally diverse xenobiotics and plays an important role in the clearance and detoxification of many therapeutic drugs. To examine whether this gene is regulated by CAR and PXR in vivo, transgenic mice expressing the entire UGT2B7 gene (TgUGT2B7) were created. Gene expression profiles revealed that UGT2B7 is differentially expressed in liver, kidney, adipocytes, brain, and estrogen-sensitive tissues, such as ovary and uterus. Liver UGT2B7 expression levels were decreased when TgUGT2B7 mice were treated with the CAR ligand 1,4-b-s-[2-(3,5,-dichloropyridyloxy)] (TCPOBOP) but not the PXR ligand pregnenolone 16 alpha-carbonitrile. Although TCPOBOP decreased the levels of UGT2B7 mRNA in TgUGT2B7 mice, it had no affect on Tg(UGT2B7)Car(-/-) mice, adding support for a CAR-dependent mechanism contributing toward UGT2B7 gene suppression. Expression of promoter constructs in HepG2 cells showed the CAR-dependent inhibition was linked to hepatocyte nuclear factor-4 alpha (HNF4 alpha)-mediated transactivation of the UGT2B7 promoter. The inhibitory effect of CAR on UGT2B7 gene expression was validated in chromatin immunoprecipitation assays in which TCPOBOP treatment blocked HNF4 alpha binding to the UGT2B7 promoter. These results suggest that HNF4 alpha plays an important role in the constitutive expression of hepatic UGT2B7, and CAR acts as a negative regulator by interfering with HNF4 alpha binding activity.