Formation and repair of pyridyloxobutyl DNA adducts and their relationship to tumor yield in A/J mice.

The nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a known human carcinogen. It generates methyl and pyridyloxobutyl DNA adducts. The role of the methyl DNA adducts has been well-established in the tumorigenic properties of NNK However, the role of the pyridyloxobutyl DNA adducts is unclear. Four pyridyloxobutyl DNA adducts have been characterized: 7-[4-3-(pyridyl)-4oxobut-l-Aguanine (7-pobG), O-2-[4-3-(pyridyl)-4-oxobut-1-yl]-cytodine (O-2-pobC), O-2-[4-3-(pyridyl)-4-oxobut-1-yl]thymidine (O-2-pobdT), and O-6[4-3-(pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O-6-pobdG). Mutagenic O-6-pobdG is thought to contribute to the tumorigenic properties of the pyridyloxobutylation pathway. It is repaired by O-6-alkylguanine-DNA alkyltransferase (AGT). To explore the role of O-6-pobdG formation and repair in the tumorigenic properties of NNK A/J mice were given single or multiple doses of the model pyridyloxobutylating agent 4-(acetoxymethyl-nitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc) in the presence or absence of the AGT depletor, O-6-benzylguanine. Levels of the four pyridyloxobutyl DNA adducts were measured in the lung at 8, 48, or 96 h following treatment and compared to the lung tumorigenic activity of these treatments. AGT depletion had only a modest effect on the levels of O-6-pobdG and did not increase tumor formation. Three pyridyloxobutyl DNA adducts, 7-pobG, O-2-pobdT, and O-6-pobdG, persisted in lung DNA at significant levels for up to 96 h post-treatment, suggesting that all three adducts may contribute to the tumorigenic properties of NNK.