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.