A mechanism of inhibition of aflatoxin B(1) hepatocarcinogenesis by β-naphthoflavone pretreatment of rats

Abstract

Pretreatment with β-naphthoflavone (BNF) inhibits aflatoxin B1 (AFB1) hepatocarcinogenesis in the rat by stimulation of aflatoxin M1 (AFM1) hydroxylation. We have now examined the effect of BNF pretreatment of rats on AFB1-hydroxylations, AFB1- DNA binding and AFB1-glutathione (AFB1-SG) conjugation in liver in vitro, in vivo and in isolated hepatocytes. Male Fischer rats were injected i.p. with either corn oil or BNF (25 mg/kg body weight) 24 h before for in vitro and in isolated hepatocyte studies. Cytochrome P-450 contents in BNF-treated rat liver microsomes and hepatocytes were 170% and 200% of controls, respectively. At two AFB1- concentrations (2 µM and 100 µM), AFB1- epoxidation measured as AFB1-DNA binding and AFM1 hydroxylation were 60% and 300% of controls, respectively. At low AFBl level (33 nM), AFB1-binding and AFM1 formation in BNF-treated hepatocytes were 40% and 225% of controls, respectively. Formation of AFB1-SG, aflatoxin Q1 and aflatoxin P1 were not affected significantly. Hepatic nuclear AFB1-DNA binding 2 h after i.p. injection of 0.4 mg [3H]AFB1/kg body wt. in BNF-treated rats was 60% of controls. The overall data indicate that lower AFB1-DNA binding in addition to higher inactivation via AFM1 hydroxylation are responsible for the inhibition of AFB1 hepato- carcinogenesis by BNF pretreatment of rats. It is suggested that this inhibition of AFB1-DNA binding may be a direct result of lower levels of constitutive cytochromes P-450 responsible for AFB1-epoxidation.