EVIDENCE FOR AN IMPORTANT ROLE OF DNA PYRIDYLOXOBUTYLATION IN RAT LUNG CARCINOGENESIS BY 4-(METHYLNITROSAMINO)-1-(3-PYRIDYL)-1-BUTANONE - EFFECTS OF DOSE AND PHENETHYL ISOTHIOCYANATE

The tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)- 1-butanone (NNK), selectively induces lung tumors in F344 rats. NNK is metabolically activated to intermediates that methylate and pyridylox obutylate DNA. To explore the importance of pyridyloxobutyl DNA adduct s in NNK-induced rat lung tumorigenesis, the first study in this repor t examined levels of these adducts in whole lung and pulmonary cells o f F344 rats treated with different doses of NNK (0.3, 1.0, 10.0, and 5 0 mg/kg; 3 x weekly for 2 weeks). Pyridyloxobutyl DNA adducts were hig hest in Clara cells compared to alveolar Type II cells, alveolar macro phages, and small cells, suggesting that enzymes involved in the forma tion of the pyridyloxobutylating species are concentrated in Clara cel ls. When we compared lung tumor incidence at the different doses of NN K (S. A. Belinsky et at, Cancer Res., 50: 3772-3780, 1990) versus pyri dyloxobutyl DNA adducts In Type II cells, we observed a significant co rrelation. Because NNK-induced lung tumors arise from the Type II cell s, this suggests an important role for pyridyloxobutyl DNA adducts. In the second study presented in this report, we examined the effect of dietary phenethyl isothiocyanate (PEITC), an inhibitor of lung tumor i nduction in F344 rats by NNK, on O-6-methyldeoxyguanosine (O-6-mG) and pyridyloxobutyl DNA adducts in whole lung and lung cells of F344 rats treated with NNK. F344 rats were fed control or PEITC-containing diet s (3 mu mol/g diet) before and throughout NNK treatment (1.76 mg/kg, t hree times weekly for 4, 8, 12, 16, or 20 weeks). PEITC inhibited form ation of pyridyloxobutyl DNA adducts in whole lung and all lung cells except macrophages. There was also inhibition of O-6-mG, but it varied with cell type and length of NNK treatment. Overall, PEITC treatment decreased pyridyloxobutyl DNA adducts by 57% in Clara cells, 51% in Ty pe II cells, 40% in small cells, and 44% in whole lung. PEITC treatmen t decreased O-6-mG levels by 52% in Clara cells, 19% in Type II cells and small cells, and 36% in whole lung. These results support the hypo thesis that PEITC inhibition of NNK-induced lung tumors is a result of decreased metabolic activation and DNA binding of NNK. The 50% reduct ion of pyridyloxobutyl DNA adducts in Type II cells agreed well with t he 50% reduction of NNK-induced lung tumors by PEITC, Because NNK-indu ced tumors arise from Type II cells, these results suggest an importan t role for pyridyloxobutyl DNA adducts in NNK-induced rat lung tumorig enesis.