Adenosine deaminases acting on RNA modulate the expression of the human pregnane X receptor

The pregnane X receptor (PXR) is one of the major transcription factors that regulate the expression of different drug-metabolizing enzymes and transporters. Adenosine-to-inosine RNA editing, the most frequent nucleotide conversion on RNA, which is catalyzed by adenosine deaminase acting on RNA (ADAR) enzymes, may modulate gene expression and function. Here, we investigated the potential regulation of human PXR expression by adenosine-to-inosine RNA editing. Knockdown of ADAR1 increased PXR mRNA level, and the knockdown of ADAR1 or ADAR2 significantly increased PXR protein level in HepaRG cells. In HepG2 cells, the knockdown of ADAR1 or ADAR2 significantly increased PXR mRNA and protein levels. The increase in the PXR protein by ADAR1 knockdown resulted in increased cytochrome P450 3A4 (CYP3A4) transactivity and CYP3A4 and UDP-glucuronosyltransferase 1A1 (UGT1A1) expression. A reporter assay revealed that the 3'-untranslated region (UTR) of PXR mRNA, especially from +3371 to +3440, is responsible for the ADAR-mediated post-transcriptional control of PXR expression, despite the lack of RNA edited sites in this region. Collectively, we found that PXR is negatively regulated by ADAR1 via an indirect mechanism, which facilitates the degradation of PXR mRNA. We could demonstrate that ADAR1 can cause interindividual variability in hepatic drug metabolism potencies.