Reduced lung tumorigenesis in human methylguanine DNA-methyltransferase transgenic mice achieved by expression of transgene within the target cell

Human methylguanine-DNA methyltransferase (MGMT) transgenic mice expressing high levels of O-6-alkylguanine-DNA alkyltransferase (AGT) in lung were cr ossbred to A/J mice that are susceptible to pulmonary adenoma to study the impact of O-6-methylguanine (O(6)mG)-DNA adduct repair on NNK-induced lung tumorigenesis. Expression of the chimeric human MGMT transgene in lung was identified by northern and western blot analysis, immunohistochemistry assa y and enzymatic assay. AGT activity was 17.6 +/- 3.2 versus 1.2 +/- 0.4 fmo l/mu g DNA in lung of MGMT transgenic mice compared with nontransgenic mice , Immunohistochemical staining with antihuman AGT antibody showed that huma n AGT was expressed throughout the lung, However, some epithelial cells of bronchi and alveoli did not stain for human AGT, suggesting that the human MGMT transgene expression was heterogeneous, After 100 mg/kg NNK i.p. injec tion in MGMT transgenic mice, lung AGT activity remained much higher and le vels of lung O(6)mG-DNA adducts in MGMT transgenic mice were lower than tho se of non-transgenic mice. In the tumorigenesis study, mice received 100 mg /kg NNK at 6 weeks of age and were killed 44 weeks later. Ten of 17 MGMT tr ansgenic mice compared with 16 of 17 non-transgenic mice had lung tumors, P < 0.05, MGMT transgenic mice had lower multiplicity and smaller sized lung tumors than non-transgenic mice. Moreover, a reduction in the frequency of K-ras mutations in lung tumors was found in MGMT transgenic mice (6.7 vers us 50% in non-transgenic mice). These results indicate that high levels of AGT expressed in mouse lung reduce lung tissue susceptibility to NNK-induce d tumorigenesis due to increased repair capacity for O(6)mG, subsequently, decreased mutational activation of K-ras oncogene, Heterogeneity in the lev el of AGT expressed in different lung cell populations or other forms of ca rcinogenic DNA damage caused by NNK may explain the residual incidence of l ung tumors in MGMT transgenic mice.