The constitutive active/androstane receptor regulates phenytoin induction of Cyp2c29.

Many cytochrome P450 isoforms are known to be drug-inducible. The anticonvulsant phenytoin has been reported to be an inducer of human CYP2B6, CYP3A4, and murine CYP2C29. However, the molecular mechanism mediating phenytoin induction remains unclear. Herein, we used in vivo and in vitro gene reporter assays of the Cyp2c29 promoter to delineate the phenytoin-response activity to a phenytoin-responsive module located at -1371 kb upstream of the Cyp2c29 translation start site. The phenytoin-responsive module, consisting of two motifs of two imperfect direct repeat hexamers spaced by four nucleotides and a putative CCAAT/enhancer-binding protein-binding site, mediated luciferase reporter induction by phenytoin in mouse livers in vivo and was activated by CAR in HepG2 cells. Hepatic CYP2C29 mRNA was induced by phenytoin in wild-type but not in CAR-null mice, indicating that constitutive active or androstane receptor (CAR) regulates phenytoin-induced transcription of the Cyp2c29 gene. Furthermore, the constitutive levels of CYP2C29 mRNA were reduced approximately 77-fold in CAR-null mice compared with those in the wild-type mice, suggesting that CAR may also regulate the constitutive expression of the Cyp2c29 gene either directly or indirectly.