Cytochrome P450 (P450) enzymes play important roles in the metabolism of endogenous and xenobiotic substrates in humans. CYP2C8 is an important member of the CYP2C subfamily, which metabolizes both endogenous compounds (i.e., arachidonic acids and retinoic acid) and xenobiotics (e.g., paclitaxel). Induction of P450 enzymes by drugs can result in tolerance as well as drug-drug interactions. CYP2C8 is the most strongly inducible member of the CYP2C subfamily in human hepatocytes, but the mechanism of induction by xenobiotics has not been delineated. To determine the mechanisms controlling the regulation of this important P450, we cloned the 5'-flanking region of CYP2C8 and investigated its transcriptional regulation by nuclear factors such as the pregnane X receptor (PXR), constitutive androstane receptor (CAR), glucocorticoid receptor (GR), and hepatic nuclear factor 4 (HNF4alpha) that are known to be involved in the induction of other P450 enzymes using both cell lines and primary hepatocyte models. We initially identified a distal PXR/CAR-binding site in the CYP2C8 promoter that confers inducibility of CYP2C8 via the PXR agonist/ligand rifampicin and the CAR agonist/ligand CITCO [6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime]. A glucocorticoid-responsive element was identified that mediates dexamethasone induction via the GR. We finally identified an HNF4alpha-binding site within the CYP2C8 basal promoter region that is cis-activated by cotransfected HNF4alpha. In summary, the present studies show that CAR, PXR, GR, and HNF4alpha can regulate CYP2C8 expression and identify specific cis-elements within the promoter that control these regulatory pathways.
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