Effect of troglitazone on CYP1A1 induction

Several peroxisome proliferators enhance CYP1A1 activity, but the mechanisms involved in this enhancement remain unknown. In this study, we examined the effect of troglitazone, a peroxisome proliferator-activated receptor-γ (PPAR-γ) agonist, on CYP1A1 gene expression and explored the mechanisms involved in these effects. Troglitazone increased gene expression of CYP1A1 mRNA and also increased CYP1A1-specific 7-ethoxyresorufin-O-deethylase (EROD) activity in a dose-dependent manner. Moreover, concomitant treatment with troglitazone and GW9662, a PPAR antagonist, markedly reduced the troglitazone-inducible EROD activity. Luciferase reporter assays using Hepa-1c1c7 cells showed a significant transactivation by troglitazone with a reporter plasmid containing a region from −1395 to +7 of the CYP1A1 gene. We found that a putative peroxisome proliferator-response element (PPRE) between −521 and −500 is located in the CYP1A1 gene promoter. Their inactivation by deletion mutagenesis suppressed the inductive effect of troglitazone on CYP1A1 promoter activation. Electrophoretic mobility shift assay revealed that troglitazone induced the activation of the PPAR-γ to a form capable of binding specifically to the PPRE sequence of the CYP1A1 gene promoter. Furthermore, troglitazone increased the formation of the benzo[a]pyrene (BaP)–DNA adduct. Overall, our results suggest that troglitazone induces CYP1A1 enzyme activity and gene expression through PPAR-γ activation, and may be involved in carcinogenesis.