SUPPRESSION OF CYTOCHROME-P450 (CYPLA-1) INDUCTION IN MOUSE HEPATOMA HEPA-1C1C7 CELLS BY METHOXSALEN

Cultured mouse hepatoma cell line Hepa-1c1c7 cells were treated with methoxsalen to assess the role of methoxsalen in the process of Cyp1a-1 induction. Treatment of Hepa-1c1c7 cultures with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced Cyp1a-1, as indicated by analysis of 7-ethoxyresorufin O-deethylation (EROD) activity and P4501A1 protein. When methoxsalen and TCDD were both added to cultures, TCDD-inducible EROD activity was greatly suppressed by methoxsalen in a dose-dependent manner. We find that treatment of Hepa-1c1c7 cells with methoxsalen inhibited CYP1A1 mRNA induction by TCDD as well as the concomitant increase P4501A1 protein. Formation of DNA-protein complexes between the dioxin receptor and its DRE target was inhibited by methoxsalen, as determined by gel mobility shift assays using oligonucleotides corresponding to DRE 3 of the Cyp1a-1 gene. These results suggest that the inhibitory action of methoxsalen on TCDD induction of the Cyp1a-1 gene expression in Hepa-1c1c7 cells might be antagonism of the DNA binding potential of nuclear dioxin receptor. (C) 1995 Academic Press, Inc.