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

Abstract 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 [ 3 H]TCDD showed that within 60 min, there was an initial rapid increase in nuclear [ 3 H]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 [ 3 H]TCDD (10 −9 m ) plus αNF (10 −6 m ) the levels of the nuclear [ 3 H]TCDD receptor complex were 3 H]-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.