The alternatively spliced murine pregnane X receptor isoform, mPXRΔ171–211 exhibits a repressive action

The orphan nuclear receptor pregnane X receptor regulates enzymes and transport proteins involved in the detoxification and clearance of numerous endobiotic and xenobiotic compounds, including pharmaceutical agents. Multiple alternatively spliced pregnane X receptor isoforms have been identified which are significantly expressed in humans and mice (up to 30% of the total pregnane X receptor transcript), however, little is known about their biological action. We explored functional differences between the major mouse pregnane X receptor isoforms mPXR431 and mPXRΔ171–211 that lacks 41 amino acids adjacent to the ligand-binding pocket. Transient transfection assays showed that mPXRΔ171–211 reduced the basal transcription of cytochrome P450 3A4 and the drug transporter P-glycoprotein/Multi Drug Resistance Protein 1 and directly repressed the regulatory effects of mPXR431 on these genes. Replacement of the mPXRΔ171–211 DNA-binding domain with that of GAL4 showed mPXRΔ171–211 retained its repressive role independent of binding to PXR responsive elements located within the cytochrome P450 3A4 and Multi Drug Resistance Protein 1 regulatory regions. Use of the histone deacetylase inhibitor, trichostatin A, demonstrated that the repressive function of mPXRΔ171–211 acts independently of histone acetylation state. Protein interaction assays revealed mPXRΔ171–211 and mPXR431 differentially bind the obligatory heterodimer partner retinoid X receptor. Furthermore, mPXR431 and mPXRΔ171–211 proteins could heterodimerize. These studies demonstrate that the variant mouse PXR isoform, mPXRΔ171–211, has a distinct repressive function from mPXR431 in regulating genes encoding important drug metabolizing enzymes and transport proteins.