The mechanism of action of alpha-naphthoflavone as an inhibitor of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced CYP1A1 gene expression.

Treatment of rat hepatoma H-4-II E cells with alpha-naphthoflavone (alpha NF) (10(-8), 10(-7), 10(-6)M) resulted in only minimum induction of ethoxyresorufin O-deethylase (EROD) activity and cytochrome P4501A1 mRNA levels only at 10(-6)M. In contrast, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) caused maximum or near maximum induction responses at 10(-8) and 10(-9)M. In a time-course study with TCDD (10(-9)M), and TCDD plus alpha NF (cotreated), alpha NF significantly inhibited the induction of EROD activity and cytochrome P4501A1 mRNA levels by TCDD for 6-24 h after initial exposure of the cells to the chemicals. In addition, treatment of the cells with 10(-9)M TCDD in the presence or absence of 10(-8), 10(-7), and 10(-9)M alpha NF showed that the latter compound inhibited the induction effects by TCDD in a concentration-dependent manner and these inhibitory effects could be overcome, in part, by a higher concentration of TCDD (10(-8)M). Treatment of the rat hepatoma H-4-II E cells with [3H]TCDD showed that within 60 min, there was an initial rapid increase in nuclear [3H]TCDD receptor complex levels (38 fmol/mg protein) which decreased to less than 10 fmol/mg protein within 4 h and remained relatively constant for up to 24 h. However, in cells treated with [3H]TCDD (10(-9)M) plus alpha NF (10(-6)M) the levels of the nuclear [3H]TCDD receptor complex were less than 5 fmol/mg protein throughout the 24-h time course. These data, coupled with the results which indicate that the alpha NF competitively inhibits the binding of [3H]-TCDD to the cytosolic aryl hydrocarbon (Ah) receptor, suggest that alpha NF inhibits the TCDD-mediated induction of CYP1A1 gene transcription and translation by direct competition for cytosolic Ah receptor binding sites.