The effect of cotinine or cigarette smoke co-administration on the formation of O6-methylguanine adducts in the lung and liver of A/J mice treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

Abstract

4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific nitrosamine, induces lung adenomas in A/J mice, following a single intraperitoneal (i.p.) injection. However, inhalation of tobacco smoke has not induced or promoted tumors in these mice. NNK-induced lung tumorigenesis is thought to involve O6-methylguanine (O6MeG) formation, leading to GC-->AT transitional mispairing and an activation of the K-ras proto-oncogene in the A/J mouse. NNK can be metabolized by several different cytochromes P450, resulting in a number of metabolites. Formation of the promutagenic DNA adduct O6MeG is believed to require metabolic activation of NNK by cytochrome P450-mediated alpha-hydroxylation of the methylene group adjacent to the N-nitroso nitrogen to yield the unstable intermediate, methanediazohydroxide. Nicotine, cotinine (the major metabolite of nicotine), and aqueous cigarette tar extract (ACTE) have all been shown to effectively inhibit metabolic activation of NNK to its mutagenic form, most likely due to competitive inhibition of the cytochrome P450 enzymes involved in alpha-hydroxylation of NNK. The objective of the current study was to monitor the effects of cotinine and cigarette smoke (CS) on the formation of O6MeG in target tissues of mice during the acute phase of NNK treatment. To test the effect of cotinine, mature female A/J mice received a single intraperitoneal injection of NNK (0, 2.5, 5, 7.5, or 10 mumole/mouse) with cotinine administered at a total dose of 50 mumole/mouse in 3 separate i.p. injections, administered 30 min before, immediately after, and 30 min after NNK treatment. To test the effect of whole smoke exposure on NNK-related O6MeG formation, mice were exposed to smoke generated from Kentucky 1R4F reference cigarettes at 0, 0.4, 0.6, or 0.8 mg wet total particulate matter/liter (WTPM/L) for 2 h, with a single i.p. injection of NNK (0, 3.75, or 7.5 mumole/mouse) midway through the exposure. Cigarette smoke alone failed to yield detectable levels of O6MeG. The number of O6MeG adducts following i.p. injection of NNK was significantly (p < 0.05) reduced in both lung and liver by cotinine and by cigarette smoke exposure. Our results demonstrate that NNK-induced O6MeG DNA adducts in A/J mice are significantly reduced when NNK is administered together with either cotinine, the major metabolite of nicotine, or the parental complex mixture, cigarette smoke.