In vitro inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced activity by α-naphthoflavone and 6-methyl-1,3,8-trichlorodibenzofuran using an aryl hydrocarbon (Ah)-responsive construct

Rat hepatoma H4IIE and mouse hepatoma Hepa 1c1c7 cells were transiently transfected with a plasmid construct that contained the bacterial chloramphenicol acetyltransferase (CAT) gene under the control of the mouse mammary tumor virus promoter and one copy of the dioxin responsive element. Treatment of transfected H4IIE and Hepa 1c1c7 cells with 10^-13 to 10^-6 M 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in a concentration-dependent increase in transient CAT activity. Maximum CAT activity was induced in both cell lines by exposure to 10^-9 M TCDD. The induction of CAT activity correlated well with the TCDD-induced, P4501A1-dependent ethoxyresorufin O-deethylase activity. Cotreatment of transfected cells with 10^-9 M TCDD and 10^-8 to 10^-6 M α-naphthoflavone (αNF) or 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) resulted in a concentration-dependent reduction of TCDD-induced CAT activity. Treatment of cells with 10^-6 M αNF or MCDF alone resulted in only minimal induction of CAT activity. Both antagonists inhibited the induction of genes under the control of the CYP1A1 and mouse mammary tumor virus promoters, which indicates that the αNF- and MCDF-mediated antagonism of TCDD-induced, aryl hydrocarbon receptor-dependent gene transcription does not depend on promoter context.