THE MECHANISM OF ACTION OF ALPHA-NAPHTHOFLAVONE AS AN INHIBITOR OF 2,3,7,8-TETRACHLORODIBENZO-PARA-DIOXIN-INDUCED CYP1A1 GENE-EXPRESSION

Treatment of rat hepatoma H-4-II E cells with α-naphthoflavone (αNF) (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-tetrachlorodi-benzo-p-dioxin (TCDD) caused maximum or near maximum induction responses at 10-8 and 10-9 m. In a timecourse study with TCDD (10-9 m), and TCDD plus αNF (cotreated), αNF 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 αNF 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 (10-9m) plus αNF (10-6 m) the levels of the nuclear [3H]TCDD receptor complex were <5 fmol/mg protein throughout the 24-h time course. These data, coupled with the results which indicate that the αNF competitively inhibits the binding of [3H]-TCDD to the cytosolic aryl hydrocarbon (Ah) receptor, suggest that αNF inhibits the TCDD-mediated induction of CYP1A1 gene transcription and translation by direct competition for cytosolic Ah receptor binding sites. © 1990.