Transcription of the human microsomal epoxide hydrolase gene (EPHX1) is regulated by an HNF-4α/CAR/RXR/PSF complex

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in the metabolism of numerous xenobiotics as well as mediating the sodium-dependent transport of bile acids into hepatocytes where they are involved in cholesterol excretion and metabolism, lipid digestion and regulating numerous signaling pathways. Previous studies have demonstrated the critical role of GATA-4 and a C/EBP alpha-NF/Y complex in the regulation of the mEH gene (EPHX1). In this study we show that HNF-4 alpha and CAR/RXR also bind to the proximal promoter region and regulate EPHX1 expression. Bile acids, which inhibit the expression of HNF-4 alpha also decrease the expression of EPHX1. Studies also established that the binding of HNF-4 alpha was essential for the activation of EPHX1 activity by CAR suggesting the formation of a complex between these adjacent factors. The nature of this regulatory complex was further explored using a biotinylated oligonucleotide of this region in conjunction with BioMag beads and mass spectrometric analysis which demonstrated the presence of an additional inhibitory factor (PSF), confirmed by co-immunoprecipitation and ChIP analyses, which interacted with DNA-bound CAR/RXR/HNF-4 alpha forming a 4-component regulatory complex. (C) 2013 Elsevier B.V. All rights reserved.