The pregnane X receptor regulates gene expression in a ligand- and promoter-selective fashion.

Recent studies have revealed that pregnane X receptor (PXR) can function as a master regulator to control the expression of phase I and phase II drug-metabolizing enzymes, as well as members of the drug transporter family, including multiple drug resistance (MDR) 1, which has a major role in multidrug resistance. Previously, we have demonstrated that steroid/xenobiotics metabolism by tumor tissue through the PXR-cytochrome P-450 3A (CYP3A) pathway might play an important role in endometrial cancer. In this study, we examined which endocrine-disrupting chemicals (EDCs) and anticancer agents might be ligands for PXR and whether these chemicals enhanced PXR-mediated transcription through two different PXR-responsive elements (PXREs), CYP3A4 and MDR1, in endometrial cancer cell lines. Some steroids/EDCs strongly activated PXR-mediated transcription through the CYP3A4-responsive element compared with the MDR1-responsive element, whereas these steroids/EDCs also enhanced the CYP3A4 expression compared with the MDR1 expression. In contrast, the anticancer agents, cisplatin and paclitaxel, strongly activated PXR-mediated transcription through the MDR1-responsive element compared with the CYP3A4-responsive element, whereas these drugs also enhanced the MDR1 expression compared with the CYP3A4 expression. We also analyzed how these ligands regulated PXR-mediated transcription through two different PXREs. In the presence of PXR ligands, there was no difference in the DNA binding affinity of the PXR/retinoid X receptor heterodimer to each PXRE, but there were different interactions of the coactivator to each PXR/PXRE complex. These data suggested that PXR ligands enhanced PXR-mediated transcription in a ligand- and promoter-dependent fashion, which in turn differentially regulated the expression of individual PXR targets, especially CYP3A4 and MDR1.