Pyridyloxobutyl DNA adducts inhibit the repair of O6-methylguanine.
The carcinogenic tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), both methylates and pyridyloxobutylates DNA. O6-Methylguanine (O6-mG) persistence has been correlated to NNK-induced lung tumor formation in A/J mice. The pyridyloxobutylation pathway enhances the tumorigenicity of the methylation pathway. In this paper we test the hypothesis that DNA pyridyloxobutylation increases O6-mG persistence by inhibiting the repair protein O6-alkylguanine-DNA alkyltransferase (AGT). Pyridyloxobutylated DNA was generated by reacting calf thymus DNA with the model pyridyloxobutylating agent 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc) in the presence of esterase. The alkylated DNA inhibited the ability of partially purified rat liver AGT to repair O6-mG when it was incubated with AGT prior to the addition of 3H-methylated DNA. The extent of inhibition was dependent on the amount of NNKOAc reacted with DNA. The ability of NNKOAc-treated DNA to inhibit AGT was destroyed when the DNA was subjected to neutral thermal hydrolysis. Approximately 1 pmol of AGT was inhibited for every 25 to 50 pmol of 4-hydroxy-1-(3-pyridyl)-1-butanone- releasing adducts present in NNKOAc-treated DNA. The inhibitory activity of this alkylated DNA was relatively stable under physiological conditions (pH 7.4, 37 degrees C). Only 13% of the AGT reactive activity was lost after 7 days. When pyridyloxobutylated DNA was incubated simultaneously with 3H-methylated DNA and AGT, a significant reduction in [3H]methyl transfer to AGT was observed. The levels of reduction were similar to those observed when unlabeled methylated DNA containing comparable levels of O6-mG was substituted for NNKOAc-treated DNA. Based on these results, a cocarcinogenic role for pyridyloxobutylation in NNK-induced lung tumorigenesis is proposed in which pyridyloxobutyl DNA adduct(s) compete with O6-mG for reaction with AGT resulting in sustained levels of O6-mG. These enhanced levels then increase the probability of tumor induction by NNK.
Division of Chemical Carcinogenesis, American Health Foundation, Valhalla, New York 10595.
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LA
Peterson
X K
XK
Liu
S S
SS
Hecht
D000478:Alkylation; D000818:Animals; D002417:Cattle; D004247:DNA / Q000187:drug effects / Q000378:metabolism*; D004260:DNA Repair / Q000187:drug effects*; D006147:Guanine / Q000031:analogs & derivatives* / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008745:Methylation / Q000187:drug effects; D008780:Methyltransferases / Q000378:metabolism*; D009602:Nitrosamines / Q000378:metabolism* / Q000494:pharmacology; D019853:O(6)-Methylguanine-DNA Methyltransferase; D011725:Pyridines / Q000378:metabolism* / Q000494:pharmacology; D051381:Rats
D009602:Nitrosamines; D011725:Pyridines; C081510:4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone; D006147:Guanine; C016583:4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone; D004247:DNA; C008449:O-(6)-methylguanine; D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase