Comparative effect of dietary butylated hydroxyanisole and {beta}-naphthoflavone on aflatoxin B1 metabolism, DNA adduct formation, and carcinogenesis in rainbow trout

Butylated hydroxyanisole (BHA) and ß-naphthoflavone(BNF), both chemicals with anti-carcinogeneic properties insome experimental animals, were compared for effects on afiatoxinB1 (AFB1) metabolism, hepatic DNA adduct formation and carcinogenesisin the rainbow trout. Dietary BHA had no effect on the hepatictumor incidence when fed at 0.03 or 0.3% 4 weeks prior to andduring a 4 week dietary exposure of 10 p.p.b. AFB1. BNF, whenfed at 0.005 or 0.05% under similar conditions, significantlyreduced tumor response, which confirms previous results in trout(Nixon et al.9 Carcinogenesis, 5, 615–619, 1984). BHAfed at either 0.03 or 0.3% for 8 weeks had no post-initiationeffect on the 52 week hepatic tumor incidence of trout exposedto a 0.5 p.p.m. AFB1 solution as embryos. A similar post-initiationexposure to 0.05% BNF significantly enhanced AFB1 tumor response.The influence of dietary BHA and BNF on AFB1 metabolism andDNA adduct formation and persistence in trout were examined.A 3 week pre-treatment with 0.3% dietary BHA had no effect onin vivo hepatic nuclear AFB1-DNA adduct formation at 0.5, 1,2 and 7 days after AFB1 i.p. injection. By contrast 0.05% dietaryBNF reduced hepatic AFB1-DNA adducts to 33–60% of controllevels at 0.5, 1, 2 and 4 days after AFB1 exposure. This wasaccompanied by significantly lower blood and liver levels ofAFB1 during the first 24 h after i.p. injection. Livers of BNFtrout also contained 4-fold more of the less carcinogenic metabolite,aflatoxin M1, and 50% less aflatoxicol (AFL), a metabolite withsimilar carcinogenicity as AFB1. Bile AFL-glucuronide levelswere significantly decreased in BNF-fed trout, but total bileglucuronides were significantly increased due to a 15-fold increasein aflatoxicol-M1 glucuronide. Freshly isolated hepatocytesfrom BHA-fed fish, when incubated with AFB1 for 1 h, showedno difference in levels of AFB1-DNA adducts or ratios of AFB1metabolites when compared to hepatocytes isolated from fishfed a control diet only. By contrast, dietary BNF has been previouslyshown to greatly enhance AFM1 production, reduce AFL production,and significantly reduce AFB1-DNA adduct formation in isolatedtrout hepatocytes (Bailey et al., Natl. Cancer Inst. Monograph,65, 379–385, 1984). These results indicate that dietaryBHA up to 0.3% does not alter AFB1 metabolism or DNA adductionin trout, nor does it inhibit or promote AFB1 hepatocarcinogenesisin this species. This is in contrast to anti-oxidant enhancementof AFB1-glutathione conjugation, reduction of AFB1-DNA binding,and consequent reduction of tumor response in rats. The nullresults in trout thus support enhanced glutathione conjugationas the major mechanism for BHA inhibition of AFB1 cardnogenesisin mammalian models. By contrast, BNF dietary pre-treatmentappears to inhibit AFB1 carcinogenicity in trout by enchancingglucuronide formation and elimination of the carcinogen, leadingto reduced DNA adduct formation in target tissue.