Antagonism of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) - induced cyp1A1 gene expression by 6-methyl-1,3,8-trichlorodibenzofuran (MCDF): Mechanistic studies

Abstract

Treatment of rat hepatoma H-4-II E cells with 10 −9 M 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) resulted in maximal induction of cytochrome P4501A1-dependent ethoxyresorufin O-deethylase (EROD) activity. In contrast, 10 −6 M 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) was relatively inactive as an inducer. Cotreatment with TCDD (10 −9 M) and MCDF (10 −8, 10 −7, 10 −6 M) resulted in a concentration-dependent antagonism of EROD induction. In addition, MCDF caused a concentration-dependent decrease in TCDD-induced CYP1A1 mRNA levels. In a parallel study, MCDF reduced the accumulation of transcriptionally active nuclear [ 3H]TCDD:Ah receptor complexes. Receptor competition studies with freshly prepared Sprague-Dawley rat cytosol revealed that MCDF competes with [ 3H]TCDD for Ah receptor binding sites (EC 50 ≈ 140 nM). These results suggest that MCDF antagonizes TCDD-mediated induction of CYP1A1 gene expression by competition for the Ah receptor and by decreasing the accumulation of transcriptionally active nuclear Ah receptor complexes.