The pregnane x receptor: A promiscuous xenobiotic receptor that has diverged during evolution

Transcription of genes encoding cytochrome P450 3A (CYP3A) monooxygenases i s induced by a variety of xenobiotics and natural steroids. There are marke d differences in the compounds that induce CYP3A gene expression between sp ecies. Recently, the mouse and human pregnane X receptor (PXR) were shown t o be activated by compounds that induce CYP3A expression. However, most stu dies of CYP3A regulation have been performed using rabbit and rat hepatocyt es. Here, we report the cloning and characterization of PXR from these two species. PXR is remarkably divergent between species, with the rabbit, rat, and human receptors sharing only approximately 80% amino acid identity in their ligand-binding domains. This sequence divergence is reflected by mark ed pharmacological differences in PXR activation profiles. For example, the macrolide antibiotic rifampicin, the antidiabetic drug troglitazone, and t he hypocholesterolemic drug SR12813 are efficacious activators of the human and rabbit PXR but have little activity on the rat and mouse PXR. Converse ly, pregnane 16 alpha-carbonitrile is a more potent activator of the rat an d mouse PXR than the human and rabbit receptor. The activities of xenobioti cs in PXR activation assays correlate well with their ability to induce CYP 3A expression in primary hepatocytes. Through the use of a novel scintillat ion proximity binding assay, we demonstrate that many of the compounds that induce CYP3A expression bind directly to human PXR. These data establish P XR as a promiscuous xenobiotic receptor that has diverged during evolution.