O⁶-Pyridyloxobutylguanine Adducts Contribute to the Mutagenic Properties of Pyridyloxobutylating Agents

Abstract

The tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) are potent carcinogens in animal models and likely human carcinogens. Both NNK and NNN can be activated to a pyridyloxobutylating agent. This alkylating agent contributes to the carcinogenic effects of NNK and NNN via the formation of miscoding DNA adducts. One of these adducts, O6-[4-oxo-4-(3-pyridyl)butyl]guanine (O6-pobG) has been characterized as a mutagenic adduct which is a substrate for the repair protein O6-alkylguanine-DNA alkyltransferase (AGT). Repair of O6-alkylguanine adducts by AGT protects cells from the mutagenic and carcinogenic effects of alkylating agents and is likely to play a similar role in shielding cells from the adverse effects of pyridyloxobutylating agents. Therefore, we examined the mutagenicity of the model pyridyloxobutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc), in Salmonella typhimurium YG7108 expressing hAGT. Expression of hAGT protected cells from NNKOAc-induced mutagenicity. Interestingly, hAGT did not shield cells from the toxicity of this agent. To confirm that the repair of O6-pobG was increased in the bacteria expressing hAGT, we measured levels of this adduct in NNKOAc-treated cultures. The levels of O6-pobG were lower in DNA from bacteria expressing hAGT. This work establishes an important role for O6-pobG in mediating the mutagenic, and possibly carcinogenic, effects of pyridyloxobutylating compounds.

Quantitation of Pyridyloxobutyl DNA Adducts of Tobacco-Specific Nitrosamines in Rat Tissue DNA by High-Performance Liquid Chromatography−Electrospray Ionization−Tandem Mass Spectrometry

“…These results indicate that there is efficient cellular repair of O 6 -POB-dGuo in vivo, especially in the liver. O 6 -Alkylguanine-DNA-alkyltransferase (AGT) repairs O 6 -POB-dGuo efficiently both in vitro and in vivo (5,(26)(27)(28). AGT is widely distributed in rat tissues such as liver, lung, esophagus, colon, and kidney (29)(30)(31).…”

4-Hydroxy-1-(3-pyridyl)-1-butanone-releasing DNA adducts in lung, lower esophagus and cardia of sudden death victims

“…First of all, our method for determining total HPB-releasing DNA adducts by acid hydrolysis of DNA cannot distinguish between individual adducts formed by NNK and NNN (Wang et al, 1997(Wang et al, , 2003Haglund et al, 2002;Hecht et al, 2004). Individual HPB-releasing DNA adducts differ in their mutagenic and carcinogenic potential, and stability to repair; and one specific HPB-releasing adduct, O 6 -[4-(3-pyridyl)-4-oxobut-1-yl]-2 -deoxyguanosine, which is thought to play a significant role in lung carcinogenesis by NNK (Pauly et al, 2002;Mijal et al, 2005), contributes only 0.3-2.1% to total POB-DNA adducts in the lung of rats given high doses of either NNK or (S)-NNAL (Lao et al, 2007b). Secondly, the formation of DNA adducts and their role in the development of pulmonary tumors in rodents following exposure to NNK is highly cell-specific (Belinsky et al, 1991).…”

DNA Sequence Context Affects Repair of the Tobacco-Specific Adduct O6-[4-Oxo-4-(3-pyridyl)butyl]guanine by Human O6-Alkylguanine-DNA Alkyltransferases

“…The mutagenicity of methylating agents is directly related to levels of O 6 -methylguanine formed (7). Recently, our group established that O 6 -[4-oxo-4-(3-pyridyl)butyl]guanine (O 6 -pobG) plays a major role in the mutagenic activity of pyridyloxobutylating compounds (10).…”

“…Thus, DNA repair by AGT is stoichiometric and can be depleted. Repair of O 6 -methylguanine and O 6 -pobG by AGT shields cells from the mutagenic effects of methylating (14,15) and pyridyloxobutylating agents (10), respectively. In addition, AGT protects against carcinogenesis in animals exposed to alkylating agents (11,15).…”