Role of pregnane X receptor in the upregulation of human aldehyde oxidase gene expression

The present study was designed to test the hypothesis that pregnane X receptor (PXR) regulates human aldehyde oxidase (AOX1) gene expression. Treatment of LS180 human colon adenocarcinoma cells, a commonly used human PXR (hPXR) experimental model, with a hPXR agonist (rifampicin, T0901317, SR12813, ritonavir, or paclitaxel) increased AOX1 mRNA expression by 5.2-, 10.2-, 4.7-, 2.2-, and 6.5-fold, respectively, and increased a prototypic hPXR target gene (CYP3A4 mRNA) expression by 9.5-, 13.5-, 7.2-, 3.4-, and 18.0-fold, respectively. A correlation coefficient of 0.83 was obtained between the increase in AOX1 and CYP3A4 mRNA levels amongst our panel of hPXR agonists. The increase in AOX1 mRNA expression by rifampicin was accompanied by an increase in AOX1 protein expression. It was abolished by actinomycin D, indicating the mRNA increase occurred by a transcriptional mechanism. The increase in AOX1 mRNA level by rifampicin was attenuated by pharmacological antagonism (SPA70) of hPXR, gene knockdown (siRNA) of hPXR, and pharmacological antagonism (HX531) of the heterodimer partner (retinoid X receptor) of hPXR. Rifampicin induced AOX1 (mRNA and catalytic activities) and other hPXR target genes (CYP3A5, UGT1A1, UGT1A3, and ABCB1) in primary cultures of human hepatocytes. The in vivo administration of a mouse PXR agonist (pregnenolone 16α-carbonitrile, PCN) to C57BL/6 mice increased hepatic cytosolic Aox1 mRNA expression and Aox-mediated enzyme activities (carbazeran 4-oxidation and O 6 -benzylguanine 8-oxidation). Control analysis indicated that PCN increased mouse PXR-regulated Cyp3a11 mRNA and Cyp3a-mediated enzyme activity. Overall, our novel data provide evidence for a role of PXR in AOX1 gene expression.