Bisphenol-A, an environmental estrogen, activates the human orphan nuclear receptor, steroid and xenobiotic receptor-mediated transcription

Background: There is increasing concern about endocrine-disrupting chemicals (EDCs) which may produce adverse health effects in humans and other species. One such chemical, bisphenol-A (BPA), a monomer of polycarbonate plastics, is widely used in consumer products; it has been reported to contain estrogenic activity through binding to estrogen receptors. Cytochrome P450 monooxygenase 3A4 (CYP3A4) is one of the key enzymes for the metabolism of endogenous steroids and foreign chemicals in liver. The orphan nuclear receptor. steroid and xenobiotic receptor (SXR/PXR). has recently been isolated. A variety of known inducers of CYP3A4 bind to SXR/PXR, and stimulate transcription on xenobiotic-response elements (XREs) located in the promoter region of the CYP3A4 gene. Recent study has shown that EDCs, diethylhexylphthalate (DEHP) and nonylphenol, but not BPA, induce mouse SXR/PXR-mediated transcription. However, it is known that species differences in SXR alter CYP3A inducibility. Objective: To test whether BPA stimulates human SXR/PXR-mediated transcription using reporter gene assays. Methods: Transfection assays were performed with human SXR/PXR expression plasmid and a reporter plasmid containing the XREs in the CYP3A4 gene promoter in HepG2 cells. BPA-induced interaction of human SXR/PXR with steroid receptor coactivator-1 (SRC-1) was analyzed by mammalian two-hybrid assays. Results: BPA, as well as DEHP, activated human SXR-mediated transcription on the XREs. In mammalian two-hybrid assays, BPA recruited SRC-1 to the ligand-binding domain of human SXR/PXR. Conclusions: Our observations have indicated that BPA may be a human-specific inducer of the CYP3A4 gene, and may influence the metabolism of endogenous steroids, drugs, and other xenobiotics.