Mass spectrometric analysis of 4-hydroxy-1-(3-pyridyl)-1-butanone-releasing DNA adducts in human lung

Human cancer genomes harbor the mutational signature of tobacco-specific nitrosamines NNN and NNK

Tobacco usage is linked to multiple cancer types and accounts for a quarter of all cancer-related deaths. Tobacco smoke contains various carcinogenic compounds, including polycyclic aromatic hydrocarbons (PAH), though the mutagenic potential of many tobacco-related chemicals remains largely unexplored. In particular, the highly carcinogenic tobacco-specific nitrosamines NNN and NNK form pre-mutagenic pyridyloxobutyl (POB) DNA adducts. In the study presented here, we identified genome-scale POB-induced mutational signatures in cell lines and rat tumors, while also investigating their role in human cancer. These signatures are characterized by T>N and C>T mutations forming from specific POB adducts damaging dT and dC residues. Analysis of 2,780 cancer genomes uncovered POB signatures in ∼180 tumors; from cancer types distinct from the ones linked to smoking-related signatures SBS4 and SBS92. This suggests that, unlike PAH compounds, the POB pathway may contribute uniquely to the mutational landscapes of certain hematological malignancies and cancers of the kidney, breast, prostate and pancreas.

Metabolism and DNA Adduct Formation of Tobacco-Specific N-Nitrosamines

The tobacco-specific N-nitrosamines 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) always occur together and exclusively in tobacco products or in environments contaminated by tobacco smoke. They have been classified as "carcinogenic to humans" by the International Agency for Research on Cancer. In 1998, we published a review of the biochemistry, biology and carcinogenicity of tobacco-specific nitrosamines. Over the past 20 years, considerable progress has been made in our understanding of the mechanisms of metabolism and DNA adduct formation by these two important carcinogens, along with progress on their carcinogenicity and mutagenicity. In this review, we aim to provide an update on the carcinogenicity and mechanisms of the metabolism and DNA interactions of NNK and NNN.

Mass Spectrometric Quantitation of Apurinic/Apyrimidinic Sites in Tissue DNA of Rats Exposed to Tobacco-Specific Nitrosamines and in Lung and Leukocyte DNA of Cigarette Smokers and Nonsmokers

Metabolic activation of the carcinogenic tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) results in formation of reactive electrophiles that modify DNA to produce a variety of products including methyl, 4-(3-pyridyl)-4-oxobutyl (POB)-, and 4-(3-pyridyl)-4-hydroxybutyl adducts. Among these are adducts such as 7-POB-deoxyguanosine (N7POBdG) which can lead to apurinic/apyrimidinic (AP) sites by facile hydrolysis of the base-deoxyribonucleoside bond. In this study, we used a recently developed highly sensitive mass spectrometric method to quantitate AP sites by derivatization with O-(pyridin-3-yl-methyl)hydroxylamine (PMOA) (detection limit, 2 AP sites per 108 nucleotides). AP sites were quantified in DNA isolated from tissues of rats treated with NNN and NNK and from human lung tissue and leukocytes of cigarette smokers and nonsmokers. Rats treated with 5 or 21 mg/kg bw NNK for 4 days by s.c. injection had 2-6 and 2-17 times more AP sites than controls in liver and lung DNA (p < 0.05). Increases in AP sites were also found in liver DNA of rats exposed for 10 and 30 weeks (p < 0.05) but not for 50 and 70 weeks to 5 ppm of NNK in their drinking water. Levels of N7POBG were significantly correlated with AP sites in rats treated with NNK. In rats treated with 14 ppm (S)-NNN in their drinking water for 10 weeks, increased AP site formation compared to controls was observed in oral and nasal respiratory mucosa DNA (p < 0.05). No significant increase in AP sites was found in human lung and leukocyte DNA of cigarette smokers compared to nonsmokers, although AP sites in leukocyte DNA were significantly correlated with urinary levels of the NNK metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). This is the first study to use mass spectrometry based methods to examine AP site formation by carcinogenic tobacco-specific nitrosamines in laboratory animals and to evaluate AP sites in DNA of smokers and nonsmokers.

Synergistic Effect between Human Papillomavirus 18 and 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone on Malignant Transformation of Immortalized SHEE Cells

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is an important tobacco-specific nitrosamine (TSNA) that induces malignant tumors in rodents. High-risk human papillomavirus (hr-HPV) infection is an important cause of several human cancers. Epidemiological evidence has shown that HPV cooperatively induces carcinogenesis with tobacco smoke. In the present study, the synergistic carcinogenesis of NNK and HPV18 was investigated. Immortalized human esophageal epithelial SHEE cells containing the HPV18 E6E7 gene were constructed by lentiviral transfection. SHEE-E6E7 cells were exposed to NNK along with SHEE-V cells without HPV18 E6E7 as a negative control. The cooperation of NNK and HPV was examined by wound-healing, transwell, and colony-forming assays. The results showed that NNK exposure promoted the migration, invasion, and proliferation abilities of both SHEE-E6E7 and SHEE-V cells; however, the changes in these phenotypic features were remarkably stronger in SHEE-E6E7 cells than those in SHEE-V cells. Our findings indicate that NNK promotes malignant transformation of human esophageal epithelial cells and suggest a synergistic carcinogenesis with the HPV18 E6E7 oncogene. As reported previously, the formation of pyridyloxybutylated DNA adducts is a crucial step in NNK-mediated carcinogenesis. In order to clarify the influence of HPV on the formation of NNK-induced DNA adducts, the amounts of 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB)-releasing DNA adducts were determined using high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry. We observed that the levels of HPB-releasing adducts in SHEE-E6E7 cells were significantly higher (p < 0.01) than those of SHEE-V cells, which was in line with results of the phenotypic assays. In conclusion, this study provides direct evidence that NNK and HPV18 exhibit a synergistic effect on formation of DNA adducts, resulting in malignant transformation of esophageal epithelial cells. Such knowledge on the interaction between infection and smoking habits in the development of cancers informs cancer-prevention strategies. Further studies to delineate the molecular mechanism and to identify specific intervention targets are worthwhile.

Recent Studies on DNA Adducts Resulting from Human Exposure to Tobacco Smoke

DNA adducts are believed to play a central role in the induction of cancer in cigarette smokers and are proposed as being potential biomarkers of cancer risk. We have summarized research conducted since 2012 on DNA adduct formation in smokers. A variety of DNA adducts derived from various classes of carcinogens, including aromatic amines, polycyclic aromatic hydrocarbons, tobacco-specific nitrosamines, alkylating agents, aldehydes, volatile carcinogens, as well as oxidative damage have been reported. The results are discussed with particular attention to the analytical methods used in those studies. Mass spectrometry-based methods that have higher selectivity and specificity compared to 32P-postlabeling or immunochemical approaches are preferred. Multiple DNA adducts specific to tobacco constituents have also been characterized for the first time in vitro or detected in vivo since 2012, and descriptions of those adducts are included. We also discuss common issues related to measuring DNA adducts in humans, including the development and validation of analytical methods and prevention of artifact formation.

Liquid Chromatography-Tandem Mass Spectrometry for the Quantification of Tobacco-Specific Nitrosamine-Induced DNA Adducts in Mammalian Cells

Quantification of DNA lesions constitutes one of the main tasks in toxicology and in assessing health risks accompanied by exposure to carcinogens. Tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) can undergo metabolic transformation to give a reactive intermediate that pyridyloxobutylates nucleobases and phosphate backbone of DNA. Here, we reported a highly sensitive method, relying on the use of nanoflow liquid chromatography-nanoelectrospray ionization-tandem mass spectrometry (nLC-nESI-MS/MS), for the simultaneous quantifications of O6-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O6-POBdG) as well as O2- and O4-[4-(3-pyridyl)-4-oxobut-1-yl]-thymidine (O2-POBdT and O4-POBdT). By using this method, we measured the levels of the three DNA adducts with the use of 10 μg of DNA isolated from cultured mammalian cells exposed to a model pyridyloxobutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc). Our results demonstrated, for the first time, the formation of O4-POBdT in naked DNA and in genomic DNA of cultured mammalian cells exposed with NNKOAc. We also revealed that the levels of the three lesions increased with the dose of NNKOAc and that O2-POBdT and O4-POBdT could be subjected to repair by the nucleotide excision repair (NER) pathway. The method reported here will be useful for investigations about the involvement of other DNA repair pathways in the removal of these lesions and for human toxicological studies in the future.

High Level of Tobacco Carcinogen-Derived DNA Damage in Oral Cells Is an Independent Predictor of Oral/Head and Neck Cancer Risk in Smokers

Exposure to tobacco-specific nitrosamines (TSNA) and polycyclic aromatic hydrocarbons (PAH) is recognized to play an important role in the development of oral/head and neck squamous cell cancer (HNSCC). We recently reported higher levels of TSNA-associated DNA adducts in the oral cells of smokers with HNSCC as compared with cancer-free smokers. In this study, we further investigated the tobacco constituent exposures in the same smokers to better understand the potential causes for the elevated oral DNA damage in smokers with HNSCC. Subjects included cigarette smokers with HNSCC (cases, n = 30) and cancer-free smokers (controls, n = 35). At recruitment, tobacco/alcohol use questionnaires were completed, and urine and oral cell samples were obtained. Analysis of urinary 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and N'-Nitrosonornicotine (NNN; TSNA biomarkers), 1-hydroxypyrene (1-HOP, a PAH), cotinine, 3'-hydroxycotinine, and the nicotine metabolite ratio (NMR) were performed. Cases and controls differed in mean age, male preponderance, and frequency of alcohol consumption (but not total alcoholic drinks). Univariate analysis revealed similar levels of NNN, 1-HOP, and cotinine between groups but, as reported previously, significantly higher DNA adduct formation in the cases. Multiple regression adjusting for potential confounders showed persistent significant difference in DNA adduct levels between cases and controls [ratio of geometric means, 20.0; 95% CI, 2.7-148.6). Our cohort of smokers with HNSCC demonstrates higher levels of TSNA-derived oral DNA damage in the setting of similar exposure to nicotine and tobacco carcinogens. Among smokers, DNA adduct formation may act as a predictor of eventual development of HNSCC that is independent of carcinogen exposure indicators. Cancer Prev Res; 10(9); 507-13. ©2017 AACRSee related editorial by Johnson and Bauman, p. 489.

Use of rodent data for cancer risk assessment of smokeless tobacco in the regulatory context

To support risk management decisions, information from different fields has been integrated in this presentation to provide a realistic quantitative cancer risk assessment of smokeless tobacco. Smoking among Swedish men is currently below 10%, while about 20% use a special smokeless tobacco (snus) as a substitute for cigarettes. Epidemiological data and molecular biomarkers demonstrate that rodent bioassays with tobacco specific nitrosamines (TSNA) overestimate cancer risk from snus by more than one order of magnitude. The underlying reasons are discussed. DNA damage constitutes a necessary, although not sufficient prerequisite for cancer initiation. Individuals who have not used tobacco exhibit DNA lesions identical with those induced by TSNA. No increase above this adduct background can be shown from snus, and extensive epidemiological studies in Sweden have failed to demonstrate elevated cancer risks even in long term users. A "bench mark" for acceptable risk of 1/10(6) derived from rodent data has been suggested when regulating snus. By relating similarly derived estimates for some food contaminants, the implementation even of a limit of 1/10(4) may be unrealistic. The management of smokeless tobacco products has rarely been based on a scientifically sound risk assessment, where attention is given to the outstandingly higher hazards associated with smoking.

Context Matters: Contribution of Specific DNA Adducts to the Genotoxic Properties of the Tobacco-Specific Nitrosamine NNK

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent pulmonary carcinogen in laboratory animals. It is classified as a Group 1 human carcinogen by the International Agency for Cancer Research. NNK is bioactivated upon cytochrome P450 catalyzed hydroxylation of the carbon atoms adjacent to the nitrosamino group to both methylating and pyridyloxobutylating agents. Both pathways generate a spectrum of DNA damage that contributes to the overall mutagenic and toxic properties of this compound. NNK is also reduced to form 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), which is also carcinogenic. Like NNK, NNAL requires metabolic activation to DNA alkylating agents. Methyl hydroxylation of NNAL generates pyridylhydroxybutyl DNA adducts, and methylene hydroxylation leads to DNA methyl adducts. The consequence of this complex metabolism is that NNK generates a vast spectrum of DNA damage, any form of which can contribute to the overall carcinogenic properties of this potent pulmonary carcinogen. This Perspective reviews the chemistry and genotoxic properties of the collection of DNA adducts formed from NNK. In addition, it provides evidence that multiple adducts contribute to the overall carcinogenic properties of this chemical. The adduct that contributes to the genotoxic effects of NNK depends on the context, such as the relative amounts of each DNA alkylating pathway occurring in the model system, the levels and genetic variants of key repair enzymes, and the gene targeted for mutation.

Optimized Liquid Chromatography Nanoelectrospray-High-Resolution Tandem Mass Spectrometry Method for the Analysis of 4-Hydroxy-1-(3-pyridyl)-1-butanone-Releasing DNA Adducts in Human Oral Cells

Metabolic activation of the carcinogenic tobacco-specific N-nitrosamines leads to the formation of 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB)-releasing DNA adducts. We recently developed a liquid chromatography (LC)-tandem mass spectrometry (MS/MS) method for the analysis of HPB-releasing DNA adducts in human oral cells. However, given the limited amounts of DNA that can be extracted from oral cells, higher sensitivity and selectivity are required for the reliable analysis of these adducts in future studies. We have developed a new sensitive LC-nanoelectrospray ionization-high-resolution MS/MS method for the analysis of HPB-releasing DNA adducts in oral cells. A new procedure was also developed for guanine analysis by LC-MS/MS. The detection limit of the developed assay is 5 amol, and the limit of quantitation is 0.35 fmol HPB on-column, starting with 50 pg of DNA. The method was tested by analyzing oral samples from 65 smokers, including 30 head and neck squamous cell carcinoma (HNSCC) patients and 35 cancer-free controls. In all smokers, the levels of HPB-releasing DNA adducts averaged 6.22 ± 16.18 pmol/mg DNA, with significant interindividual variation being consistent with previous reports. The median HPB-releasing DNA adduct level was 6.6 times greater for those with HNSCC than for smokers without HNSCC (p = 0.002). The developed highly sensitive and selective method is a valuable tool for future measurement of HPB-releasing DNA adducts in tobacco users, which can potentially provide critical insights for the identification of individuals at risk for cancer.

Exposure and Metabolic Activation Biomarkers of Carcinogenic Tobacco-Specific Nitrosamines

Lung cancer is the leading cause of cancer death in the world, and cigarette smoking is its main cause. Oral cavity cancer is another debilitating and often fatal cancer closely linked to tobacco product use. While great strides have been made in decreasing tobacco use in the United States and some other countries, there are still an estimated 1 billion men and 250 million women in the world who are cigarette smokers and there are hundreds of millions of smokeless tobacco users, all at risk for cancer. Worldwide, lung cancer kills about three people per minute. This Account focuses on metabolites and biomarkers of two powerful tobacco-specific nitrosamine carcinogens, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN), considered to be among the main causes of lung cancer and oral cavity cancer in people who use tobacco products. Three properties of NNK and NNN are critical for successful biomarker studies: they are present in all tobacco products, they are tobacco-specific and are not found in any other product, and they are strong carcinogens. NNK and NNN are converted in humans to urinary metabolites that can be quantified by mass spectrometry as biomarkers of exposure to these carcinogens. They are also metabolized to diazonium ions and related electrophiles that react with DNA to form addition products that can be detected and quantified by mass spectrometry. These urinary metabolites and DNA addition products can serve as biomarkers of exposure and metabolic activation, respectively. The biomarkers of exposure, in particular the urinary NNK metabolites 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronides, have been extensively applied to document tobacco-specific lung carcinogen uptake in smokers and nonsmokers exposed to secondhand tobacco smoke. Highly sensitive mass spectrometric methods have been developed for quantitative analysis of these NNK metabolites as well as metabolites of NNN in human urine, blood, and toenails. Urinary and serum NNAL have been related to lung cancer risk, and urinary NNN has been related to esophageal cancer risk in prospective epidemiology studies. These results are consistent with carcinogenicity studies of NNK, NNAL, and NNN in rats, which show that NNK and NNAL induce mainly lung tumors, while NNN causes tumors of the esophagus and oral cavity. Biomarkers of metabolic activation of NNK and NNN applied in human studies include the metabolism of deuterium labeled substrates to distinguish NNK and NNN metabolism from that of nicotine and the determination of DNA and hemoglobin adducts in tissues, blood, and oral cells from people exposed to tobacco products. As these methods are continually improved in parallel with the ever increasing sensitivity and selectivity of mass spectrometers, development of a comprehensive biomarker panel for identifying tobacco users at high risk for cancer appears to be a realistic goal. Targeting high risk individuals for smoking cessation and cancer surveillance can potentially decrease the risk of developing fatal cancers.

Targeted omics analyses, and metabolic enzyme activity assays demonstrate maintenance of key mucociliary characteristics in long term cultures of reconstituted human airway epithelia

3D reconstituted respiratory epithelia have emerged as better in vitro models for toxicological testing compared to cell lines due to the conservation of key morphological features and functions. MucilAir™ is a commercially available human airway epithelia system that can potentially maintain functional attributes for up to a year, however, detailed mucociliary characteristics and xenobiotic metabolism relevant to inhaled pro-toxicant bioactivation is lacking. Here, we assessed in MucilAir™ some key biomarkers that are characteristic of the respiratory epithelia including morphology, function and xenobiotics metabolism. The end points that were measured included targeted proteomics using a panel of 243 airway surface liquid (ASL) proteins, cilia beat frequency (CBF), a qRT-PCR screen of xenobiotic metabolizing enzymes, and CYP2A6/13, CYP1A1/1B1 activity. Comparison of ASL proteomics with human sputum identified key proteins common to both matrices, but present at different levels. Xenobiotic metabolism gene profiling demonstrated strong similarities with the normal human lung and did not reveal any consistent changes when assessed over a 6 month period. Inducibility and activity of CYP1A1/1B1 and activity of CYP2A6/2A13 were present at one month in culture and maintained in one tested MucilAir™ donor for several months. In conclusion, MucilAir™ presented important morphological and metabolic characteristics of a mucociliary epithelium in short and long term culture. MucilAir™ is therefore a potentially useful model to test repeated sub-cytotoxic doses of toxicants.

Liquid chromatography-electrospray ionization-tandem mass spectrometry quantitation of urinary [pyridine-D4]4-hydroxy-4-(3-pyridyl)butanoic acid, a biomarker of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone metabolic activation in smokers

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, 1) is a potent tobacco-specific lung carcinogen believed to play a key role in the development of lung cancer in smokers. Metabolic activation of NNK to DNA damaging reactive intermediates proceeds via α-hydroxylation pathways. The end products of these pathways are excreted in the urine of smokers as 4-oxo-4-(3-pyridyl)butanoic acid (keto acid, 3) and 4-hydroxy-4-(3-pyridyl)butanoic acid (hydroxy acid, 4). The sum of these biomarkers (after NaBH4 treatment), referred to as total hydroxy acid, could potentially be used to measure the extent of NNK metabolic activation in smokers. However, the same metabolites are formed from nicotine; therefore, there is a need to distinguish the NNK- and nicotine-derived keto and hydroxy acid in smokers' urine. We previously developed a unique methodology based on the use of [pyridine-D4]NNK ([D4]1), which metabolizes to the correspondingly labeled biomarkers. In this study, we developed a sensitive and reproducible assay for the detection and quantitation of total [pyridine-D4]hydroxy acid ([D4]4) in human urine. A two-step derivatization approach was used to convert [D4]4 to [pyridine-D4]methyl 4-hexanoyl-4-(3-pyridyl)butanoate ([D4]6), and an LC-ESI-MS/MS method was developed for the analysis of this derivative with excellent sensitivity, accuracy, and precision. The robustness and reproducibility of the assay was further confirmed by its application for the analysis of urine samples from 87 smokers who smoked [D4]1-containing cigarettes for 1 week. The measured level averaged 130 fmol/mL urine. The developed assay can be used in future studies that may require evaluation of the relative efficiency of NNK metabolic activation in humans.

Kinetics of O(6)-pyridyloxobutyl-2'-deoxyguanosine repair by human O(6)-alkylguanine DNA alkyltransferase

Tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosonicotine (NNN) are potent carcinogens believed to contribute to the development of lung tumors in smokers. NNK and NNN are metabolized to DNA-reactive species that form a range of nucleobase adducts, including bulky O(6)-[4-oxo-4-(3-pyridyl)but-1-yl]deoxyguanosine (O(6)-POB-dG) lesions. If not repaired, O(6)-POB-dG adducts induce large numbers of G → A and G → T mutations. Previous studies have shown that O(6)-POB-dG can be directly repaired by O(6)-alkylguanine-DNA alkyltransferase (AGT), which transfers the pyridyloxobutyl group from O(6)-alkylguanines in DNA to an active site cysteine residue within the protein. In the present study, we investigated the influence of DNA sequence context and endogenous cytosine methylation on the kinetics of AGT-dependent repair of O(6)-POB-dG in duplex DNA. Synthetic oligodeoxynucleotide duplexes containing site-specific O(6)-POB-dG adducts within K-ras and p53 gene-derived DNA sequences were incubated with recombinant human AGT protein, and the kinetics of POB group transfer was monitored by isotope dilution HPLC-ESI(+)-MS/MS analysis of O(6)-POB-dG remaining in DNA over time. We found that the second-order rates of AGT-mediated repair were influenced by DNA sequence context (10-fold differences) but were only weakly affected by the methylation status of neighboring cytosines. Overall, AGT-mediated repair of O(6)-POB-dG was 2-7 times slower than that of O(6)-Me-dG adducts. To evaluate the contribution of AGT to O(6)-POB-dG repair in human lung, normal human bronchial epithelial cells (HBEC) were treated with model pyridyloxobutylating agent, and O(6)-POB-dG adduct repair over time was monitored by HPLC-ESI(+)-MS/MS. We found that HBEC cells were capable of removing O(6)-POB-dG lesions, and the repair rates were significantly reduced in the presence of an AGT inhibitor (O(6)-benzylguanine). Taken together, our results suggest that AGT plays an important role in protecting human lung against tobacco nitrosamine-mediated DNA damage and that inefficient AGT repair of O(6)-POB-dG at a specific sequences contributes to mutational spectra observed in smoking-induced lung cancer.

Analysis of 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB)-releasing DNA adducts in human exfoliated oral mucosa cells by liquid chromatography-electrospray ionization-tandem mass spectrometry

Quantitation of DNA adducts could provide critical information on the relationship between exposure to tobacco smoke and cancer risk in smokers. In this study, we developed a robust and sensitive liquid chromatography-tandem mass spectrometry method for the analysis of 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB)-releasing DNA adducts in human oral cells, a noninvasive source of DNA for biomarker studies. Isolated DNA undergoes acid hydrolysis, after which samples are purified by solid-phase extraction and analyzed by LC-ESI-MS/MS. The developed method was applied to the analysis of samples obtained via collection with a commercial mouthwash from 30 smokers and 15 nonsmokers. In smokers, the levels of HPB-releasing DNA adducts averaged 12.0 pmol HPB/mg DNA (detected in 20 out of 28 samples with quantifiable DNA yield), and in nonsmokers, the levels of adducts averaged 0.23 pmol/mg DNA (detected in 3 out of 15 samples). For the 30 smoking subjects, matching buccal brushings were also analyzed, and HPB-releasing DNA adducts were detected in 24 out of 27 samples with quantifiable DNA yield, averaging 44.7 pmol HPB/mg DNA. The levels of adducts in buccal brushings correlated with those in mouthwash samples of smokers (R = 0.73, p < 0.0001). Potentially, the method can be applied in studies of individual susceptibility to tobacco-induced cancers in humans.

DNA and protein adducts in human tissues resulting from exposure to tobacco smoke

Tobacco smoke contains a variety of genotoxic carcinogens that form adducts with DNA and protein in the tissues of smokers. Not only are these biochemical events relevant to the carcinogenic process, but the detection of adducts provides a means of monitoring exposure to tobacco smoke. Characterization of smoking-related adducts has shed light on the mechanisms of smoking-related diseases and many different types of smoking-derived DNA and protein adducts have been identified. Such approaches also reveal the potential harm of environmental tobacco smoke (ETS) to nonsmokers, infants and children. Because the majority of tobacco-smoke carcinogens are not exclusive to this source of exposure, studies comparing smokers and nonsmokers may be confounded by other environmental sources. Nevertheless, certain DNA and protein adducts have been validated as biomarkers of exposure to tobacco smoke, with continuing applications in the study of ETS exposures, cancer prevention and tobacco product legislation. Our article is a review of the literature on smoking-related adducts in human tissues published since 2002.

Metabolic effects of CYP2A6 and CYP2A13 on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced gene mutation--a mammalian cell-based mutagenesis approach

Both cytochrome P450 2A6 (CYP2A6) and cytochrome P450 2A13 (CYP2A13) are involved in metabolic activation of tobacco-specific nitrosamines and may play important roles in cigarette smoking-induced lung cancer. Unlike CYP2A6, effects of CYP2A13 on the tobacco-specific nitrosamine-induced mutagenesis in lung cells remain unclear. This study uses a supF mutagenesis assay to examine the relative effects of CYP2A6 and CYP2A13 on metabolic activation of a tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), and its resulting mutagenesis in human lung cells. A recombinant adenovirus-mediated CYP2A6/CYP2A13 expression system was established to specifically address the relative effects of these two CYPs. Mutagenesis results revealed that both CYP2A6 and CYP2A13 significantly enhanced the NNK-induced supF mutation and that the mutagenic effect of CYP2A13 was markedly higher than that of CYP2A6. Analysis of NNK metabolism indicated that ≥70% of NNK was detoxified to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), either with or without CYP2A6/CYP2A13 expression. Both CYP2A6 and CYP2A13 significantly enhanced the α-hydroxylation of NNK; and the α-hydroxylation activity of CYP2A13 was significantly higher than that of CYP2A6. Analysis of the NNK-related DNA adduct formation indicated that, in the presence of CYP2A13, NNK treatments caused marked increases in O(6)-methylguanine (O(6)-MeG). The present results provide the first direct in vitro evidence demonstrating the predominant roles of CYP2A13 in NNK-induced mutagenesis, possibly via metabolic activation of NNK α-hydroxylation.

DNA adducts of ortho-toluidine in human bladder

BACKGROUND

4-Aminobiphenyl (4-ABP) and o-toluidine are known human bladder carcinogens, but only 4-ABP-releasing DNA adducts are known.

METHODS

Determination of 4-ABP and o-toluidine-releasing DNA adducts in epithelial and submucosal bladder tissues of sudden death victims (SDV: n=46), and bladder tumours (n=12) by gas chromatography/mass spectrometry.

RESULTS

Above background, 4 and 11 of 12 tumour samples contained adducts of 4-ABP (0.057 ± 0.125 fmol/µg DNA) and o-toluidine (8.72 ± 4.49 fmol/µg DNA), respectively. Lower adduct levels were present in both epithelial and submucosal bladder tissues of SDV (4-ABP: 0.011 ± 0.022 and 0.019 ± 0.047 fmol/µg DNA; o-toluidine: 0.24 ± 0.63 and 0.27 ± 0.70 fmol/µg DNA).

CONCLUSION

Detection of o-toluidine-releasing DNA adducts support the carcinogenicity of o-toluidine in the human bladder.

Formation, repair, and genotoxic properties of bulky DNA adducts formed from tobacco-specific nitrosamines

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N′-nitrosonornicotine (NNN) are tobacco-specific nitrosamines present in tobacco products and smoke. Both compounds are carcinogenic in laboratory animals, generating tumors at sites comparable to those observed in smokers. These Group 1 human carcinogens are metabolized to reactive intermediates that alkylate DNA. This paper focuses on the DNA pyridyloxobutylation pathway which is common to both compounds. This DNA route generates 7-[4-(3-pyridyl)-4-oxobut-1-yl]-2′-deoxyguanosine, O²-[4-(3-pyridyl)-4-oxobut-1-yl]-2′-deoxycytosine, O²-[4-(3-pyridyl)-4-oxobut-1-yl]-2′-deoxythymidine, and O⁶-[4-(3-pyridyl)-4-oxobut-1-yl]-2′-deoxyguanosine as well as unstable adducts which dealkylate to release 4-hydroxy-1-{3-pyridyl)-1-butanone or depyriminidate/depurinate to generate abasic sites. There are multiple repair pathways responsible for protecting against the genotoxic effects of these adducts, including adduct reversal as well as base and nucleotide excision repair pathways. Data indicate that several DNA adducts contribute to the overall mutagenic properties of pyridyloxobutylating agents. Which adducts contribute to the carcinogenic properties of this pathway are likely to depend on the biochemistry of the target tissue.

Analysis of myosmine, cotinine and nicotine in human toenail, plasma and saliva

Myosmine is a minor tobacco alkaloid with widespread occurrence in the human diet. Myosmine is genotoxic in human cells and is readily nitrosated and peroxidated yielding reactive intermediates with carcinogenic potential. For biomonitoring of short-term and long-term exposure, analytical methods were established for determination of myosmine together with nicotine and cotinine in plasma, saliva and toenail by gas chromatography-mass spectrometry (GC/MS). Validation of the method with samples of 14 smokers and 10 non-smokers showed smoking-dependent differences of myosmine in toenails (66 +/- 56 vs 21 +/- 15 ng g(-1), p <0.01) as well as saliva (2.54 +/- 2.68 vs 0.73 +/- 0.65 ng ml(-1), p <0.01). However, these differences were much smaller than those with nicotine (1971 +/- 818 vs 132 +/- 82 ng g(-1), p <0.0001) and cotinine (1237 +/- 818 vs <35 ng g(-1)) in toenail and those of cotinine (97.43 +/- 84.54 vs 1.85 +/- 4.50 ng ml(-1), p <0.0001) in saliva. These results were confirmed in plasma samples from 84 patients undergoing gastro-oesophageal endoscopy. Differences between 25 smokers and 59 non-smokers are again much lower for myosmine (0.30 +/- 0.35 vs 0.16 +/- 0.18 ng ml(-1), p <0.05) than for cotinine (54.67 +/- 29.63 vs 0.61 +/- 1.82 ng ml(-1), p <0.0001). In conclusion, sources other than tobacco contribute considerably to the human body burden of myosmine.

The influence of repair pathways on the cytotoxicity and mutagenicity induced by the pyridyloxobutylation pathway of tobacco-specific nitrosamines

Tobacco-specific nitrosamines, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and N'-nitrosonornicotine, are considered to be human carcinogens. Both compounds are metabolized to pyridyloxobutylating intermediates that react with DNA to form adducts such as 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine, O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]cytosine, O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxythymidine (O(2)-pobdT), O(6)-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O(6)-pobdG), and 4-hydroxy-1-(3-pyridyl)-1-butanone-releasing adducts. The role of specific DNA adducts in the overall genotoxic activity of the pyridyloxobutylation pathway is not known. One adduct, O(6)-pobdG, is mutagenic. To characterize the mutagenic and cytotoxic properties of pyridyloxobutyl DNA adducts, the impact of DNA repair pathways on the cytotoxic and mutagenic properties of the model pyridyloxobutylating agent, 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc), was investigated in Chinese hamster ovary cell lines proficient or deficient in O(6)-alkylguanine DNA alkyltransferase (AGT), deficient in both AGT and base excision repair (BER), or deficient in both AGT and nucleotide excision repair (NER). The repair of the four pyridyloxobutyl DNA adducts was determined in the same cell lines via sensitive LC-MS/MS methods. NNKOAc was more cytotoxic in the cell lines lacking AGT, BER, and NER repair pathways. It also induced more mutations in the hprt gene in the BER- and NER-deficient cell lines. However, AGT expression did not influence NNKOAc's mutagenicity despite efficient repair of O(6)-pobdG. Analysis of the hprt mutational spectra indicated that NNKOAc primarily caused point mutations at AT base pairs. GC to AT transition mutations were a minor contributor to the overall mutation spectrum, providing a rationale for the observation that AGT does not protect against the overall mutagenic properties of NNKOAc in this model system. The only adduct affected by the absence of effective NER was O(2)-pobdT. Slower repair of O(2)-pobdT in NER-deficient cells was associated with increased AT to TA transversion mutations, supporting the hypothesis that these mutations are caused by O(2)-pobdT. Together, these data support a hypothesis that the pyridyloxobutylation pathway generates multiple mutagenic and toxic adducts.

Mitochondrial DNA adducts in the lung and liver of F344 rats chronically treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and (S)-4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

Two recent studies conducted in our laboratory have demonstrated formation and accumulation of pyridyloxobutyl (POB) and pyridylhydroxybutyl (PHB) adducts in lung and liver total DNA of F344 rats chronically treated with the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and (R)- and (S)-enantiomers of its metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). In this study, we measured POB and PHB adducts in lung and liver mitochondrial DNA (mtDNA), as previous studies suggest a potentially important role of mtDNA in carcinogenesis. Rats were sacrificed after 1, 2, 5, 10, 16, and 20 weeks of treatment with 10 ppm of NNK or (S)-NNAL in drinking water, and mtDNA and nuclear DNA (nDNA) adduct levels in the lung and liver were determined by LC-ESI-MS/MS-SRM. The mean levels of individual POB adducts in mtDNA at all time points were slightly higher than those in nDNA for both NNK and (S)-NNAL-treated rats in the lung (P < 0.001 for both treatments) but not in the liver (P > 0.05). Lung mtDNA of both NNK- and (S)-NNAL-treated rats contained higher concentrations of the sum of three POB adducts (P < 0.001 for both treatments) than nDNA, while the levels of mtDNA and nDNA total POB adducts in the liver were not significantly different in either NNK- or (S)-NNAL-treated rats. Analysis of PHB adducts in mtDNA and nDNA produced results similar to those obtained for POB adducts. The steady accumulation of the lung and liver mtDNA adducts over the course of the study indicates inefficient repair of these adducts in mtDNA. This is the first study to examine the formation of NNK- and (S)-NNAL-derived adducts in rat mtDNA. The results support the hypothesis that preferential binding of tobacco carcinogens to mtDNA of the lung might be functionally important in the development of smoking-induced lung cancer.

Identification of adducts formed in the reactions of 5'-acetoxy-N'-nitrosonornicotine with deoxyadenosine, thymidine, and DNA

N'-Nitrosonornicotine (NNN) is the most prevalent of the carcinogenic tobacco-specific nitrosamines found in all tobacco products. Previous studies have demonstrated that cytochrome P450-mediated 5'-hydroxylation of NNN is a major metabolic pathway leading to mutagenic products, but to date, DNA adducts formed by this pathway have been only partially characterized, and there have been no studies reported on adducts formed with bases other than dGuo. Because adducts with dAdo and dThd have been identified in the DNA of the livers of rats treated with the structurally related carcinogen N-nitrosopyrrolidine, we investigated dAdo and dThd adduct formation from 5'-acetoxyNNN (3), a stable precursor to 5'-hydroxyNNN (2). Reaction of 3 with dAdo gave diastereomeric products, which were identified by their spectral properties and LC-ESI-MS/MS-SRM analysis as N(6)-[5-(3-pyridyl)tetrahydrofuran-2-yl]dAdo (9). This adduct was further characterized by NaBH(3)CN reduction to N(6)-[4-hydroxy-4-(3-pyridyl)but-1-yl]dAdo (17). A second dAdo adduct was identified, after NaBH(3)CN treatment, as 6-[2-(3-pyridyl)pyrrolidin-1-yl]purine-2'-deoxyriboside (18). Reaction of 3 with dThd, followed by NaBH(3)CN reduction, gave O(2)-[4-(3-pyridyl)-4-hydroxybut-1-yl]thymidine (11). Adducts 9, 11, 17, and 18 were all identified by LC-ESI-MS/MS-SRM comparison to synthetic standards. The reaction of 3 with calf thymus DNA was then investigated. The DNA was enzymatically hydrolyzed to deoxyribonucleosides, and the resulting mixture was treated with NaBH(3)CN and analyzed by LC-ESI-MS/MS-SRM. Adducts 11, 17, and 18, as well as the previously identified dGuo adducts, were identified. The results of this study provide a more comprehensive picture of DNA adduct formation by the quantitatively important 5'-hydroxylation pathway of NNN and will facilitate investigation of the presence of these adducts in laboratory animals treated with NNN or in people who use tobacco products.

Quantitation of pyridylhydroxybutyl-DNA adducts in liver and lung of F-344 rats treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and enantiomers of its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a potent pulmonary carcinogen in rats and is believed to be one cause of lung cancer in smokers. NNK is metabolized to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), which is also a strong lung carcinogen in rats and has a chiral center at its 1-carbon. Previous studies have demonstrated that cytochrome P450-catalyzed alpha-hydroxylation of NNK in the lung leading to the formation of methyl and pyridyloxobutyl (POB)-DNA adducts is critical for its carcinogenicity. alpha-Hydroxylation of NNAL would similarly produce pyridylhydroxybutyl (PHB)-DNA adducts, but these have not been previously investigated in vivo. POB- and PHB-DNA adduct levels can indicate the amounts of pyridyloxobutylating and pyridylhydroxybutylating agents present in tissues of NNK- or NNAL-treated rats at any given point. Therefore, in this study, we developed a sensitive and quantitative liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring method to determine levels of the PHB-DNA adducts O(6)-[4-(3-pyridyl)-4-hydroxybut-1-yl]-2'-deoxyguanosine (O(6)-PHB-dGuo, 10b), O(2)-[4-(3-pyridyl)-4-hydroxybut-1-yl]thymidine (O(2)-PHB-dThd, 11b), and 7-[4-(3-pyridyl)-4-hydroxybut-1-yl]-2'-deoxyguanosine (7-PHB-dGuo, 12b), the latter as the corresponding base 7-[4-(3-pyridyl)-4-hydroxybut-1-yl]-Gua (7-PHB-Gua, 14b) in DNA isolated from liver and lung of rats treated with 10 ppm NNK, (S)-NNAL, or (R)-NNAL in the drinking water for 20 weeks and sacrificed at 1, 2, 5, 10, 16, and 20 weeks. PHB-DNA adduct levels were higher in lung than in liver at each time point, consistent with previous studies of POB-DNA adducts in rats treated with NNK and NNAL in the drinking water. The results showed that NNK and (S)-NNAL behaved in a similar fashion, while (R)-NNAL was strikingly different. In the rats treated with NNK or (S)-NNAL, levels of each adduct at each time point were remarkably similar in lung, and levels of O(2)-PHB-dThd were generally greater than 7-PHB-Gua > O(6)-PHB-dGuo. The highest PHB-DNA adduct levels were found in lung and liver of rats treated with (R)-NNAL, suggesting that there are cytochrome P450s that efficiently catalyze the alpha-methyl hydroxylation of this compound. The results of this study provide further support for our hypothesis that (S)-NNAL is rapidly formed from NNK, sequestered at an unknown site in the lung, and then released and reoxidized to NNK with consequent DNA adduct formation resulting in lung carcinogenicity.

Progress and challenges in selected areas of tobacco carcinogenesis

Tobacco use continues to be a major cause of cancer in the developed world, and despite significant progress in this country in tobacco control, which is driving a decrease in cancer mortality, there are still over 1 billion smokers in the world. This perspective discusses some selected issues in tobacco carcinogenesis focusing on progress during the 20 years of publication of Chemical Research in Toxicology. The topics covered include metabolism and DNA modification by tobacco-specific nitrosamines, tobacco carcinogen biomarkers, an unidentified DNA ethylating agent in cigarette smoke, mutations in the K-RAS and p53 gene in tobacco-induced lung cancer and their possible relationship to specific carcinogens, secondhand smoke and lung cancer, emerging issues in smokeless tobacco use, and a conceptual model for understanding tobacco carcinogenesis. It is hoped that a better understanding of mechanisms of tobacco-induced cancer will lead to new and useful approaches for the prevention of lung cancer and other cancers caused by tobacco use.