2-(4'-chlorophenyl)benzothiazole is a potent inducer of cytochrome P450IA1 in a human and a mouse cell line. Anomalous correlation between protein and mRNA induction

Abstract

Two benzothiazole derivatives, 2-(4'-chlorophenyl)benzothiazole (CPBT) and 2-(4'-formylphenyl)benzothiazole (FPBT) were studied for their ability to induce aryl hydrocarbon hydroxylase activity in a mouse and a human cell line. In both the mouse hepatoma cell line, Hepa-1, and the human choriocarcinoma cell line, JEG-3, a high aryl hydrocarbon hydroxylase activity was observed after treatment with CPBT. In contrast, FPBT had a very weak inducing capacity in both cell lines. The maximal induction by CPBT was several times (e.g. in Hepa-1 about fourfold on average) greater than that observed with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). A specific cDNA probe for mouse cytochrome P4501A1 gene was used to quantify mRNA levels in Hepa-1 cells. CPBT increased cytochrome P450IA1 mRNA to a level of 88% of that induced by TCDD. Immunoblot analysis with monoclonal antibody 1-7-1, directed against rat liver cytochrome P450IA1 and P450IA2, showed that the amount of P450IA1 is substantially increased in Hepa-1 cells after treatment with CPBT. The observation that CPBT competed TCDD off its specific cytosolic binding site suggests a receptor-mediated induction of cytochrome P450IA1 mRNA. An in vitro activation effect did not explain the exceptionally high hydroxylase activity. The results show that CPBT is a more efficient inducer of aryl hydrocarbon hydroxylase than TCDD in Hepa-1 and JEG-3 cells and that the induction is supported by P450IA1. The discordant effect of CPBT on mRNA and aryl hydrocarbon hydroxylase activity suggests that post-translational modifications of P450IA1 account for a major part of the increased enzyme activity.