INDUCTION OF CYP1A1 GENE BY BENZIMIDAZOLE DERIVATIVES DURING CACO-2 CELL-DIFFERENTIATION - EVIDENCE FOR AN ARYL-HYDROCARBON RECEPTOR-MEDIATED MECHANISM

The Caco-2 cell line, derived from a human colon adenocarcinoma, is un ique in its property of spontaneously differentiating into a mature en terocyte cell type during its growth in culture. In this work, we comp ared the response of the CYP1A1 gene with the benzimidazole derivative s omeprazole and lansoprazole, and with the classical inducer beta-nap hthoflavone in the Caco-2 cells at various culture stages. In addition , we characterized the Caco-2 aryl-hydrocarbon receptor. The protein-s ynthesis inhibitor cycloheximide led to a derepression of the CYP1A1 g ene transcription, and to a superinduction when combined with either b eta-naphthoflavone or benzimidazoles. Taking advantage of the spontane ous differentiation of Caco-2 cells in long-term cultures, we observed a difference in behavior between the classical inducer beta-naphthofl avone and the atypical inducer omeprazole. In the poorly differentiate d cells, both compounds elicited comparable dose/response and rate of induction of CYP1A1 gene expression. In the fully differentiated cells , in contrast, the induction by omeprazole was only transient, whereas the response to beta-naphthoflavone was long lasting. The Caco-2 aryl -hydrocarbon receptor exhibited binding characteristics similar to tho se determined for human liver and other tissues. The induction of CYP1 A1 transcription by benzimidazole derivatives in Caco-2 cells occurred with no direct binding of benzimidazole derivatives to the aryl-hydro carbon receptor, as in human hepatocytes. However, transient transfect ion experiments clearly showed that the xenobiotic-responsive element enhancer, with which the activated aryl-hydrocarbon receptor interacts , could drive the induction of a heterologous promoter in the presence of benzimidazoles. Finally the presence of the activated aryl-hydroca rbon receptor in the nuclei of the Caco-2 cells exposed to these molec ules was clearly demonstrated by gel-retardation experiments. These re sults question about the mechanism of ligand-independent activation of the aryl-hydrocarbon receptor and intracellular signaling, initiated by benzimidazole derivatives.