Prominent Stereoselectivity of NNAL Glucuronidation in Upper Aerodigestive Tract Tissues

From cultivation to cancer: formation of N-nitrosamines and other carcinogens in smokeless tobacco and their mutagenic implications

Tobacco use is a major cause of preventable morbidity and mortality globally. Tobacco products, including smokeless tobacco (ST), generally contain tobacco-specific N-nitrosamines (TSNAs), such as N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-butanone (NNK), which are potent carcinogens that cause mutations in critical genes in human DNA. This review covers the series of biochemical and chemical transformations, related to TSNAs, leading from tobacco cultivation to cancer initiation. A key aim of this review is to provide a greater understanding of TSNAs: their precursors, the microbial and chemical mechanisms that contribute to their formation in ST, their mutagenicity leading to cancer due to ST use, and potential means of lowering TSNA levels in tobacco products. TSNAs are not present in harvested tobacco but can form due to nitrosating agents reacting with tobacco alkaloids present in tobacco during certain types of curing. TSNAs can also form during or following ST production when certain microorganisms perform nitrate metabolism, with dissimilatory nitrate reductases converting nitrate to nitrite that is then released into tobacco and reacts chemically with tobacco alkaloids. When ST usage occurs, TSNAs are absorbed and metabolized to reactive compounds that form DNA adducts leading to mutations in critical target genes, including the RAS oncogenes and the p53 tumor suppressor gene. DNA repair mechanisms remove most adducts induced by carcinogens, thus preventing many but not all mutations. Lastly, because TSNAs and other agents cause cancer, previously documented strategies for lowering their levels in ST products are discussed, including using tobacco with lower nornicotine levels, pasteurization and other means of eliminating microorganisms, omitting fermentation and fire-curing, refrigerating ST products, and including nitrite scavenging chemicals as ST ingredients.

Alkylating and oxidative stresses in smoking and non-smoking patients with COPD: Implications for lung carcinogenesis

Chronic obstructive pulmonary disease (COPD) is a disease characterized by chronic inflammation and irreversible airway obstruction. Cigarette smoking is the predominant risk factor for developing COPD. It is well-known that the COPD is also strongly associated with an increased risk of developing lung cancer. Cigarette smoke contains elevated concentrations of oxidants and various carcinogens (e.g., tobacco-derived nitrosamines) that can cause oxidative and alkylating stresses, which can also arise from inflammation. However, it is surprising that, except for oxidative stress, little information is available on the burden of alkylating stress and the detoxification efficiency of tobacco-derived carcinogens in COPD patients. In this study, we used LC-MS/MS to measure the archetypical tobacco-specific carcinogenic 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), its major metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), three biomarkers of oxidative stress (8-oxo-7,8-dihydroguanine, 8-oxoGua; 8-oxo-7,8-dihydro-2'-deoxyguanosine, 8-oxodGuo; 8-oxo-7,8-dihydroguanosine, 8-oxoGuo) and two biomarkers of alkylating stress (N7-methylguanine, N7-MeGua and N3-methyladenine, N3-MeAde), in the urine of smoking and non-smoking COPD patients and healthy controls. Our results showed that not only was oxidative stress significantly elevated in the COPD patients compared to the controls, but also alkylating stress. Significantly, levels of alkylating stress (i.e., N7-MeGua) were highly correlated with the COPD severity and not affected by age and smoking status. Furthermore, COPD smokers had significantly higher ratios of free NNAL to the total NNAL than control smokers, implying a lower detoxification efficiency of NNK in COPD smokers. This ratio was even higher in COPD smokers with stages 3-4 than in COPD smokers with stages 1-2. Taken together, our results demonstrated that the detoxification efficiency of tobacco-derived carcinogens (e.g., NNK) was associated with the pathogenesis and possibly the progression of COPD. In addition to oxidative stress, alkylating stress derived from chronic inflammation appears to be also dominant in COPD patients.

Role of l- and d-Menthol in the Glucuronidation and Detoxification of the Major Lung Carcinogen, NNAL

Menthol, which creates mint flavor and scent, is often added to tobacco in both menthol and nonmenthol cigarettes. A potent tobacco carcinogen, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), is extensively metabolized to its equally carcinogenic metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) as (R)- or (S)-NNAL enantiomers. NNAL is detoxified by UDP-glucuronosyltransferase (UGT) enzymes, with glucuronidation occurring on either NNAL's pyridine ring nitrogen (NNAL-N-Gluc) or the chiral alcohol [(R)- or (S)-NNAL-O-Gluc]. To characterize a potential effect by menthol on NNAL glucuronidation, in vitro menthol glucuronidation assays and menthol inhibition of NNAL-Gluc formation assays were performed. Additionally, NNAL and menthol glucuronides (MG) were measured in the urine of smokers (n = 100) from the Southern Community Cohort Study. UGTs 1A9, 1A10, 2A1, 2A2, 2A3, 2B4, 2B7, and 2B17 all exhibited glucuronidating activity against both l- and d-menthol. In human liver microsomes, both l- and d-menthol inhibited the formation of each NNAL-Gluc, with a stereospecific difference observed between the formation of (R)-NNAL-O-Gluc and (S)-NNAL-O-Gluc in the presence of d-menthol but not l-menthol. With the exception of three nonmenthol cigarette smokers, urinary MG was detected in all menthol and nonmenthol smokers, with l-MG comprising >98% of total urinary MG. Levels of urinary NNAL-N-Gluc were significantly (P < 0.05) lower among subjects with high levels of total urinary MG; no significant changes in free NNAL were observed. These data suggest that the presence of menthol could lead to increases in alternative, activating metabolic pathways of NNAL in tobacco target tissues, increasing the opportunity for NNAL to damage DNA and lead to the development of tobacco-related cancers. SIGNIFICANCE STATEMENT: High levels of the major menthol metabolite, menthol-glucuronide, was observed in the urine of smokers of either menthol or nonmenthol cigarettes. The fact that a significant inverse correlation was observed between the levels of urinary menthol-glucuronide and NNAL-N-glucuronide, a major detoxification metabolite of the tobacco carcinogen, NNK, suggests that menthol may inhibit clearance of this important tobacco carcinogen.