Transcriptional Regulation of Pregnane X Receptor by Protein Arginine Methyltransferase

Pregnane X receptor (PXR) is a ligand-dependent transcription factor that plays an important role in xenobiotic/drug metabolism. The ligand-receptor interaction transcriptionally activates phase I and phase II enzymes, and membrane-bound transporters in a coordinated manner and ultimately leads to detoxification and excretion of the ligands. One of the direct target genes is cytochrome P450 3A4 (CYP3A4) which is responsible for metabolism of over 50% of clinically used drugs. Understanding the regulation of PXR is important for treatment of disease and avoidance of untoward drug-drug interactions. In this research, we have used various biochemical and molecular approaches to investigate factors that regulate the transcriptional activity of PXR. We have stably transfected PXR into HepG2 human liver hepatoma cells. Using these PXR-HepG2 cells, we surveyed the histone methyltransferases that interact with PXR. Based on results from co-immunoprecipitation/methyltransferase, N-terminal peptide sequencing, GST-pulldown assays, we found that protein arginine methyltransferase 1 (PRMT1) is a predominant histone methyltransferase in HepG2 cells. Evidence from other laboratories suggests that histone methylation by PRMT1 sets the stage for subsequent histone modifications such as the acetylation of histone H4. These modifications are believed to be important for transcriptional and epigenetic regulation of gene expression. We hypothesize that PRMT1 plays a role in the epigenetic changes regulated by PXR. PRMT1-dependent histone methylation changes may be involved in epigenetic cell memory where prior exposure to certain agents may alter the chromatin (or priming the chromatin) with a "primed" state which alters the subsequent magnitude or duration of gene expression. In our study, we have found that pretreatment of PXR-HepG2 cells with DMSO greatly enhanced PXR-mediated activation of CYP3A4 upon rifampicin treatment. DMSO pretreatment altered histone modifications association with the promoter of the PXR-regulated gene (CYP3A4). Inhibition of histone methylation by PRMT1 either through RNAi or the methyltransferase inhibitor (Adox) abolished the priming effects. My research results strongly indicate that PRMT1 is involved in transcriptional regulation of PXR and may be involved in epigenetic memory of liver cells where prior exposure to agents changes the subsequent detoxification responses.