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 e limination of putative tobacco carcinogens such as benzo[a]pyrene (B[a ]P) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), which ma y reduce competing bioactivation and toxicity. B[a]P-initiated cytotox icity and micronucleus formation, believed to reflect carcinogenic ini tiation, are enhanced in UGT-deficient rat fibroblasts, and UGTs may p rovide similar genoprotection against NNK. Using skin fibroblasts from wild-type UGT-normal (+/+) and congenic heterozygous (+/j) and homozy gous (j/j) UGT-deficient rats, this study evaluated NNK in relation to B[a]P with respect to the mechanism of genotoxicity, evidenced by mic ronucleus formation, and genoprotection by UGTs. Molecular mechanisms were determined by changes in B[a]P- and NNK-initiated micronucleus fo rmation when cells were incubated with the antioxidative enzyme supero xide dismutase (1680 IU/ml), inhibitors of cytochrome P450 (1 mM 1-ami nobenzotriazole) and peroxidases (1-aminobenzotriazole; 40 mu M eicosa tetraynoic acid), and inducers of CYP1A1/2(10 nM 2,3,7,8-tetrachlorodi benzo-p-dioxin) and peroxidases [2,3,7,8-tetrachlorodibezo-p-dioxin; 0 .625 ng/ml (0.0367 nM) interleukin 1 alpha; 1 mu M 12-O-tetradecanoylp horbol-13-acetate]. In +/+ fibroblasts, NNK and B[a]P initiated concen tration-dependent, respective maximum 2.7-fold and 1.7-fold increases over DMSO controls in micronucleus formation (P < 0.05), with 10 mu M NNK being 2.4-fold more genotoxic than B[a]P (P < 0.05). In both +/j a nd j/j UGT-deficient cells, micronuclei initiated by NNK and B[a]P eac h 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 superoxi de dismutase and cytochrome P450/peroxidase inhibitors, while only tha t initiated by B[a]P was enhanced, up to 2.4-fold, by inducers, of whi ch only interleukin Icu was effective in all UGT phenotypes (P < 0.05) . These results provide the first evidence that: (a) UGTs may be genop rotective for NNK, with even heterozygous UGT deficiencies being toxic ologically critical; and (b) peroxidase-catalyzed bioactivation, react ive oxygen species, and molecular target oxidation may contribute diff erentially to the genotoxicity of both NNK and B[a]P.