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.
Merchant
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M
Department of Veterinary Physiology and Pharmacology, Texas A&M University, College Station 77843.
Arellano
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L
Safe
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S
D000818:Animals; D001571:Benzoflavones / Q000494:pharmacology*; D019363:Cytochrome P-450 CYP1A1; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis*; D004147:Dioxins / Q000037:antagonists & inhibitors*; D004347:Drug Interactions; D004790:Enzyme Induction / Q000187:drug effects; D005419:Flavonoids / Q000494:pharmacology*; D015870:Gene Expression / Q000187:drug effects*; D006838:Hydrocarbons / Q000378:metabolism; D008114:Liver Neoplasms, Experimental / Q000378:metabolism*; D010088:Oxidoreductases / Q000096:biosynthesis*; D000072317:Polychlorinated Dibenzodioxins / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D051381:Rats; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism
D001571:Benzoflavones; D004147:Dioxins; D005419:Flavonoids; D006838:Hydrocarbons; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug; C011512:alpha-naphthoflavone; D003577:Cytochrome P-450 Enzyme System; D010088:Oxidoreductases; D019363:Cytochrome P-450 CYP1A1