Epigenetic Regulation of Transcriptional Activity of Pregnane X Receptor by Protein Arginine Methyltransferase 1

Abstract

Pregnane X receptor (PXR) is a ligand-dependent transcription factor, regulating gene expression of enzymes and transporters involved in xenobiotic/drug metabolism. Here, we report that protein arginine methyltransferase 1 (PRMT1) is required for the transcriptional activity of PXR. PRMT1 regulates expression of numerous genes, including nuclear receptor-regulated transcription, through methylating histone and non-histone proteins. Co-immunoprecipitation and histone methyltransferase assays revealed that PRMT1 is a major histone methyltransferase associated with PXR. The PXR ligand-binding domain is responsible for PXR-PRMT1 interaction as determined by mammalian two-hybrid and glutathione S-transferase (GST) pull-down assays. The chromatin immunoprecipitation (ChIP) assay showed that PRMT1 was recruited to the regulatory region of the PXR target gene cytochrome P450 3A4 (CYP3A4), with a concomitant methylation of arginine 3 of histone H4, in response to the PXR agonist rifampicin. In mammalian cells, small interfering RNA (siRNA) knockdown and gene deletion of PRMT1 greatly diminished the transcriptional activity of PXR, suggesting an indispensable role of PRMT1 in PXR-regulated gene expression. Interestingly, PXR appears to have a reciprocal effect on the PRMT1 functions by regulating its cellular compartmentalization as well as its substrate specificity. Taken together, these results demonstrated mutual interactions and functional interplays between PXR and PRMT1, and this interaction may be important for the epigenetics of PXR-regulated gene expression. Pregnane X receptor (PXR)2 is an orphan nuclear receptor that regulates metabolism and disposition of various xenobiotics and endobiotics (1). These physiological functions of PXR are achieved through coordinating transcriptional regulation of Phase I and Phase II drug-metabolizing enzymes as well as the "Phase III" transporters (2). The structural flexibility in the ligand binding pocket enables PXR to function as a xenobiotic receptor through interacting with a wide range of structurally diverse compounds (3). Xeno-and endobiotics that activate PXR include a variety of prescription and nonprescription drugs, herbal medicines, environmental toxicants, and bile acids (4). The transcriptional activity of PXR is not only regulated by ligands, but also by various signal transduction pathways including NF-B-regulated inflammatory pathway (5). We have shown that the transcriptional activity of PXR is negatively regulated by NF-B (5), which may play a role in the pathogenesis of inflammatory bowl diseases and colon cancer (6).Post-translational modifications on the N termini of histones have been shown to play critical roles in gene regulation including the regulation of transcriptional activity by nuclear receptors. These modifications include phosphorylation, acetylation, methylation, and ubiquitination (7). It is believed that the combination of modifications of the chromatin-associated histone and non-histone proteins, and the interplays between these modificat...

Pregnane X receptor suppresses proliferation and tumourigenicity of colon cancer cells

“…In earlier studies, we observed that many cancer cells lines derived from GI tract, such as HepG2, HT29 and HCT116, showed loss of PXR expression (Xie et al, 2009; data not shown). Upon ectopic expression of PXR through transfection, the PXR function could be restored and cells showed significant reduced proliferation, suggesting its role in controlling tumour cell proliferation.…”

“…In an earlier study, we noticed that in many cell lines derived from GI tract, such as HepG2 and HT29 cells, expression of PXR is lost (Xie et al, 2009). To investigate the effects of PXR on tumour cell growth, we restored PXR expression through transfection of human PXR into the HT29 colon cancer cell line.…”

Pregnane X receptor‐ and CYP3A4‐humanized mouse models and their applications

“…PKA regulates NF-kB-dependent transcription with different expression levels of endogenous A-kinase interacting protein 1 (AKIP1) in multiple cell lines (Gao et al, 2010). Moreover, a recent study revealed that the histone methyltransferase, protein arginine methyltransferase 1 (PRMT1), plays a role in the transcriptional activity of PXR by controlling its cellular compartmentalization (Xie et al, 2009). A recent report also revealed that rifampicin-activated human PXR SUMOylation directly represses NF-kB in liver (Hu et al, 2010).…”

Genetic and Epigenetic Factors Affecting Cytochrome P450 Phenotype and Their Clinical Relevance

“…It is recruited to 5'-region of the CYP3A4 gene to methylate histone H4 as a response to the PXR agonist rifampicin (Xie et al, 2009). CYP3A4 is also regulated by constitutive androstane receptor (CAR) albeit it at a lower rate of expression.…”