Modulating effects of beta-naphthoflavone on the induction of SCEs by model compounds with emphasis on benzo[a]pyrene

The inducing capability of the synthetic flavonol beta-naphthoflavone (beta-NF) on cytochrome P-450 content was studied in primary chick embryo hepatocytes. In addition, the modulating effects of pretreatment with beta-NF on the induction of sister-chromatid exchanges (SCEs) in V79 cells by mutagens from different chemical classes were investigated in a co-cultivation system consisting of primary chick embryo hepatocytes and V79 Chinese hamster cells. Finally, the effects of pretreatment on benzo[a]pyrene (B(a)P) metabolism were studied in more detail. Pretreatment of cultured primary chick embryo hepatocytes with beta-NF resulted in a large increase in cytochrome P-450 content (a 2.8-fold increase after 31 h). Pretreatment with beta-NF had no effect on the level of SCEs induced by N-nitroso-dimethylamine (NDMA) and 2-aminoanthracene (2AA). Pretreatment with beta-NF resulted in a decrease in B(a)P-induced SCEs. This inhibitory potential was positively related to the beta-NF dose. However, there was an inverse relationship between the inhibitory action of beta-NF and the dose of B(a)P, at higher doses less inhibition was observed. When beta-NF was applied simultaneously with B(a)P the percentage of decrease was about the same as for pretreatment. Pretreatment with beta-NF followed by simultaneous application of beta-NF and B(a)P did not result in larger effects. In addition, subcellular fractions were prepared from chick embryos pretreated with beta-NF in ovo. The use of the S9 fraction resulted in a large decrease (80%) in the induction of SCEs in V79 cells by B(a)P whereas the use of the microsomal fraction resulted in a 70% increase in SCE induction compared with non-pretreated microsomes. Pretreatment with beta-NF in ovo gave rise to a large increase in aryl hydrocarbon hydroxylase (AHH) activity in the hepatic microsomal fraction. Increases were observed in the formation of all B(a)P metabolites. In particular the formation of the proximate carcinogenic and mutagenic metabolite B(a)P-7,8 dihydrodiol was increased 7-fold. The data strongly suggest that the inhibitory effects of pretreatment of cultured primary chick embryo hepatocytes with beta-NF cannot be ascribed to its inducing capabilities but instead seem to be due to the formation of an intracellular pool of beta-NF which acts as a competitive inhibitor for B(a)P metabolism.