NCOA6 differentially regulates the expression of the CYP2C9 and CYP3A4 genes

CYP2Cs and CYP3A4 sub families of enzymes of the Cytochrome P450 super family metabolize clinically prescribed therapeutics. Constitutive and induced expressions of these enzymes are under the control of HNF4α and rifampicin activated PXR. In the present study, we show a mechanism for ligand dependent synergistic cross talk between PXR and HNF4α. Two-hybrid screening identified NCOA6 as a HNF4α interacting protein. NCOA6 was also found to interact with PXR through the first LXXLL motif in GST pull down and mammalian two hybrid assays. NCOA6 enhances the synergistic activation of CYP2C9 and CYP3A4 promoter activity by PXR and HNF4α in the presence of rifampicin. However silencing NCOA6 abrogated the synergistic activation and induction of CYP2C9 by PXR–HNF4α but not of CYP3A4. ChIP analysis revealed that NCOA6 could bridge HNF4α and PXR binding sites of the CYP2C9 promoter. Our results indicate that NCOA6 is responsible for the synergistic activation of CYP2C9 by HNF4α and PXR and NCOA6 differentially regulates CYP2C9 and CYP3A4 gene expression though both the genes are regulated by the same nuclear receptors.

Graphical abstractHypothetical model for the synergistic up regulation of CYP2C9: (A) Simplified cartoon depicting ligand bound PXR/RXR heterodimer binding to the PXR responsive element on the CYP2C9 promoter. Chromatin remodeling complex (Brg1 (Brahma-related gene 1) and helicases), histone acyl transferases (HATs, SRC family and CBP), histone methyl transferases (HMTs, CARM1 and PRMT1)) are recruited on the PXR/RXR hetero dimer in a sequential manner for acetylation and methylation followed by the mediator complex anchored by PBP and finally the basal transcription machinery containing RNA Pol II sub complex which initiates the transcription process of CYP2C9. (B) A similar cartoon depicting HNF4α binding to all the possible sites (three binding sites at −150, 180 and −211 bp) of the promoter of CYP2C9 gene with the above sub complexes binding sequentially to initiate the transcription process. (C) A model showing NCOA6 bridging PXR and HNF4α binding sites, responsible for the synergistic activation/induction followed by the sequential recruitment of sub complexes to initiate the transcription of CYP2C9 gene expression.Figure optionsDownload full-size imageDownload as PowerPoint slide