LACK OF CORRELATION BETWEEN DNA METHYLATION AND HEPATOCARCINOGENESIS IN RATS AND HAMSTERS TREATED WITH 4-(METHYLNITROSAMINO)-1-(3-PYRIDYL)-1-BUTANONE

Previous studies have demonstrated that the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-l-butanone (NNK) induced liver tumors in F344 rats but not in Syrian golden hamsters. The aim of this study was to determine whether there was a correlation between the persistence of O6-methylguanine (O6-mGua) adducts and the rate of recovery of O6-methylguanine-DNA methyltransferase (O6-mGuaT) after depletion in the liver and susceptibility to NNK in F344 rat and Syrian golden hamster injected s.c. with NNK (80 mg/kg). The levels of both 7-methylguanine and O6-mGua reached a maximum 24 h after NNK treatment. O6-mGua in NNK-treated rat liver was undetectable after 48 h. In the rat, the depletion of O6-mGuaT activity occurred within 4 h following NNK treatment. A subsequent rapid recovery of enzyme activity was observed 36 h after NNK exposure. In contrast, high levels of O6-mGua persisted in hamster liver DNA and no O6-mGuaT activity was detected up to 336 h after NNK injection. Thus, the persistence of O6-mGua in hamster liver is most likely related to a lack of recovery of the O6-mGuaT. These results suggested that factors other than O6-mGua may be determining NNK-induced hepatocarcinogenesis in rats. An aldehyde generated by alpha-hydroxylation of NNK, 4-oxo-4-(3-pyridyl)butanal, inhibited O6-mGuaT activity in rat hepatocytes, suggesting that this aldehyde contributes to the carcinogenicity of NNK by inhibiting this repair enzyme.