Characterization of a novel PXR isoform with potential dominant-negative properties

Background & Aims The nuclear Pregnane X Receptor (PXR, NR1I2) plays a pivotal role in xenobiotic metabolism. Here, we sought to characterize a new PXR isoform (hereafter called small PXR or sPXR) stemming from alternative transcription starting sites downstream of a CpG Island located near exon 3 of the human PXR gene. Methods Quantitative RT-PCR, western blot, methylation-specific PCR, luciferase reporter assays, electro-mobility shift assays, and stable sPXR overexpression were used to examine sPXR expression and function in hepatocellular cell lines, healthy human liver (n = 99), hepatocellular adenomas (HCA, n = 91) and hepatocellular carcinoma samples (HCC, n = 213). Results Liver sPXR mRNA expression varied importantly among individuals and encodes a 37 kDa nuclear protein consisting of the ligand-binding domain of PXR that behaves as a dominant-negative of PXR transactivation properties. In vitro methylation of the sPXR upstream promoter abolished its activity, while the demethylation agent 5-aza-2-deoxycytidine increased sPXR mRNA expression in several cell lines. Finally, we observed that sPXR mRNA expression displayed significant differences related to HCA or HCC biology. Conclusions This novel PXR isoform, displaying a dominant-negative activity and regulated by DNA methylation, is associated with outcomes of patients with HCC treated by resection, suggesting that it represents a key modulator of PXR.