Genoprotection by UDP-glucuronosyltransferases in peroxidase-dependent, reactive oxygen species-mediated micronucleus initiation by the carcinogens 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and benzo[a]pyrene.

UDP-glucuronosyltransferases (UGTs) catalyze the glucuronidation and elimination of putative tobacco carcinogens such as benzo[a]pyrene (B[a]P) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), which may reduce competing bioactivation and toxicity. B[a]P-initiated cytotoxicity and micronucleus formation, believed to reflect carcinogenic initiation, are enhanced in UGT-deficient rat fibroblasts, and UGTs may provide similar genoprotection against NNK. Using skin fibroblasts from wild-type UGT-normal (+/+) and congenic heterozygous (+/j) and homozygous (j/j) UGT-deficient rats, this study evaluated NNK in relation to B[a]P with respect to the mechanism of genotoxicity, evidenced by micronucleus formation, and genoprotection++ by UGTs. Molecular mechanisms were determined by changes in B[a]P- and NNK-initiated micronucleus formation when cells were incubated with the antioxidative enzyme superoxide dismutase (1680 IU/ml), inhibitors of cytochrome P450 (1 mM 1-aminobenzotriazole) and peroxidases (1-aminobenzotriazole; 40 microM eicosatetraynoic acid), and inducers of CYP1A1/2(10 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin) and peroxidases [2,3,7,8-tetrachlorodibezo-p-dioxin; 0.625 ng/ml (0.0367 nM) interleukin 1alpha; 1 microM 12-0-tetradecanoylphorbol-13-acetate]. In +/+ fibroblasts, NNK and B[a]P initiated concentration-dependent, respective maximum 2.7-fold and 1.7-fold increases over DMSO controls in micronucleus formation (P < 0.05), with 10 microM NNK being 2.4-fold more genotoxic than B[a]P (P < 0.05). In both +/j and j/j UGT-deficient cells, micronuclei initiated by NNK and B[a]P each were over 2-fold higher than that in +/+ UGT normal cells (P < 0.05). Both NNK- and B[a]P-initiated micronuclei were decreased by superoxide dismutase and cytochrome P450/peroxidase inhibitors, while only that initiated by B[a]P was enhanced, up to 2.4-fold, by inducers, of which only interleukin 1alpha was effective in all UGT phenotypes (P < 0.05). These results provide the first evidence that: (a) UGTs may be genoprotective for NNK, with even heterozygous UGT deficiencies being toxicologically critical; and (b) peroxidase-catalyzed bioactivation, reactive oxygen species, and molecular target oxidation may contribute differentially to the genotoxicity of both NNK and B[a]P.