Transcriptional regulation of murine cytochrome P450 2a5 by glucocorticoids: Implications for nicotine metabolism
Cytochrome P450 2a5 (Cyp2a5), the mouse orthologue of human CYP2A6, catalyzes the metabolism of nicotine to cotinine. Several xenobiotics induce ' Cyp2a5' expression; however, the effect of glucocorticoids on Cyp2a5 expression and nicotine metabolism has not been investigated. Treatment of primary mouse hepatocytes with corticosterone and dexamethasone (DEX) induced ' Cyp2a5' and increased cotinine formation. RU486 did not abrogate ' Cyp2a5' induction by corticosterone; however, supramicromolar concentrations of RU alone and the pregnane X receptor (PXR) agonist pregnenolone-16[alpha]-carbonitrile induced 'Cyp2a5' mRNA expression, suggesting involvement of PXR. Reporter activity using a p2a5-3033+10luc reporter construct decreased in DEX-treated Hepa 1-6 cells and hepatocytes. Co-transfection with a PXR expression plasmid recovered some luciferase activity. Chromatin immunoprecipitation revealed that DEX may increase PXR binding at the proximal HNF4[alpha] response element. In summary, glucocorticoids appear to up-regulate Cyp2a5 via a mechanism involving PXR. Glucocorticoid-mediated induction of Cyp2a5 enhances nicotine metabolism, and may influence smoking cessation.