Influence of beta-naphthoflavone on 7,12-dimethylbenz(a)anthracene metabolism, DNA adduction, and tumorigenicity in rainbow trout

Metabolism, DNA adduction, and tumor induction by 7,12-dimethylbenz(a)anthr acene (DMBA) were examined in cultured trout liver cells and in vivo in tro ut. Modulating CYP1A1 activity indicated this enzyme plays a significant ro le in metabolizing DMBA to water-soluble compounds in isolated trout liver cells. The major DMBA metabolites identified in trout liver cells were 10-, 11-, 8,9-, and 5,6-DMBA dihydrodiols, and DMBA, 2- or 3- or 4-phenol; 7-OH -methyl-12-methyl-benz(a)anthracene and 12-OH-methyl-7-methyl-benz(a)anthra cene were minor metabolites. A very small amount of DMBA-3,4-dihydrodiol wa s detected, and polar metabolites, which did not migrate with any DMBA meta bolite standards, were observed. Incubating trout hepatocytes with DMBA-3,4 -dihydrodiol produced three prominent, nonpolar adducts indistinguishable f rom those in mouse embryo cells. However, DMBA-DNA adducts, formed in trout in vivo or in trout liver cells exposed to DMBA, were predominantly more p olar than those formed in mouse embryo fibroblasts, and levels of DMBA-DNA adducts formed in trout liver cells were not significantly altered by modul ating CYP1A1 activity. No significant repair of DMBA-DNA adducts was detect ed in cultured trout liver cells over a 48-h period, supporting previous st udies indicating that fish are less efficient than mammals in repairing pol yaromatic hydrocarbon DNA adducts, Compared to animals receiving DMBA alone , beta-naphthoflavone pretreatment in vivo did not affect hepatic CYP1A1, D MBA-DNA adducts, nor hepatic tumor response; but did significantly reduce t umor response in two other target organs. These results collectively indica te that DMBA bioactivation to DNA-binding metabolites in trout liver cells and mouse embryo cells predominantly involve different metabolic pathways t o form the DNA-binding intermediates.