Quantitation of pyridyloxobutyl DNA adducts of tobacco-specific nitrosamines in rat tissue DNA by high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry

Formation and distribution of NNK metabolites in an isolated perfused rat lung

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a lung-specific tobacco carcinogen. Metabolism is critical to its elimination given its lipophilic nature. Although NNK can be metabolized through detoxification pathways that safely eliminate it from the body, it can also be bioactivated, resulting in the formation of potentially carcinogenic DNA adducts. The isolated perfused rat lung (IPRL) system was used to determine the effect of NNK perfusate concentration (0.1 and 1.2 microM) on the formation and distribution of metabolites, the level of individual DNA adducts, and total covalent binding in the lung. Coadministration of the chemopreventive agent phenethyl isothiocyanate (PEITC; 20 microM) was also examined to determine its effect on NNK metabolism. NNK was readily metabolized in the IPRL system. In the 0.1 muM perfusions approximately 55% of metabolites formed were through detoxification pathways, whereas roughly 30% were the result of bioactivation pathways. An increase in NNK concentration increased the percentage of unmetabolized NNK and decreased the apparent metabolic clearance in the lung, but the metabolite profiles remained similar between concentrations. The addition of PEITC reduced the formation of oxidative metabolites and increased 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) formation and the percentage of unmetabolized NNK. PEITC also significantly decreased the formation of DNA adducts in the lung tissue. The level of O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine (O(2)-POB-dThd) and O(6)-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O(6)-POB-dGuo) decreased by 70 to 75%, and that of O(6)-methylguanine (O(6)-methyl-Gua) and 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua) decreased by 40 to 45%. Pyridylhydroxybutyl-DNA adducts were not detected in any of the treatment groups. Thus, the IPRL system is useful in determining pulmonary metabolism and DNA adduct formation separate from other metabolizing organs.

Stable-isotope dilution LC–MS for quantitative biomarker analysis

The ability to conduct validated analyses of biomarkers is critically important in order to establish the sensitivity and selectivity of the biomarker in identifying a particular disease. The use of stable-isotope dilution (SID) methodology in combination with LC–MS/MS provides the highest possible analytical specificity for quantitative determinations. This methodology is now widely used in the discovery and validation of putative exposure and disease biomarkers. This review will describe the application of SID LC–MS methodology for the analysis of small-molecule and protein biomarkers. It will also discuss potential future directions for the use of this methodology for rigorous biomarker analysis.

Eukaryotic initiation factor 4E binding protein family of proteins: sentinels at a translational control checkpoint in lung tumor defense

The usurping of translational control by sustained activation of translation initiation factors is oncogenic. Here, we show that the primary negative regulators of these oncogenic initiation factors--the 4E-BP protein family--operate as guardians of a translational control checkpoint in lung tumor defense. When challenged with the tobacco carcinogen 4-(methylnitrosamino)-I-(3-pyridyl)-1-butanone (NNK), 4ebp1(-/-)/4ebp2(-/-) mice showed increased sensitivity to tumorigenesis compared with their wild-type counterparts. The 4E-BP-deficient state per se creates pro-oncogenic, genome-wide skewing of the molecular landscape, with translational activation of genes governing angiogenesis, growth, and proliferation, and translational activation of the precise cytochrome p450 enzyme isoform (CYP2A5) that bioactivates NNK into mutagenic metabolites. Our study provides in vivo proof for a translational control checkpoint in lung tumor defense.

Investigating the biochemical impact of DNA damage with structure-based probes: abasic sites, photodimers, alkylation adducts, and oxidative lesions

DNA sustains a wide variety of damage, such as the formation of abasic sites, pyrimidine dimers, alkylation adducts, or oxidative lesions, upon exposure to UV radiation, alkylating agents, or oxidative conditions. Since these forms of damage may be acutely toxic or mutagenic and potentially carcinogenic, it is of interest to gain insight into how their structures impact biochemical processing of DNA, such as synthesis, transcription, and repair. Lesion-specific molecular probes have been used to study polymerase-mediated translesion DNA synthesis of abasic sites and TT dimers, while other probes have been developed for specifically investigating the alkylation adduct O(6)-Bn-G and the oxidative lesion 8-oxo-G. In this review, recent examples of lesion-specific molecular probes are surveyed; their specificities of incorporation opposite target lesions compared to unmodified nucleotides are discussed, and limitations of their applications under physiologically relevant conditions are assessed.

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.

Quantitation of pyridyloxobutyl DNA adducts in nasal and oral mucosa of rats treated chronically with enantiomers of N'-nitrosonornicotine

N'-Nitrosonornicotine (NNN) is one of the most important strong carcinogens in tobacco products and is believed to play a significant role in the induction of esophageal cancer in smokers and oral cavity cancer in snuff dippers. NNN is metabolically activated through cytochrome P450-catalyzed alpha-hydroxylation. 2'-Hydroxylation produces a reactive intermediate 4-(3-pyridyl)-4-oxobutanediazohydroxide (7), which alkylates DNA to form pyridyloxobutyl (POB)-DNA adducts. DNA pyridyloxobutylation from NNN treatment, as measured by released 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB, 8), has been observed in vitro and in vivo. In the present study, we have used liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) to analyze specific POB-DNA adducts in the nasal olfactory, nasal respiratory, and oral mucosa of F344 rats treated chronically with (R)-NNN or (S)-NNN in the drinking water (10 ppm, 1-20 weeks). Adduct levels in the nasal respiratory mucosa exceeded those in the nasal olfactory and oral mucosa. (R)-NNN treatment generated 2-4 times more adducts in the nasal olfactory and respiratory mucosa than did (S)-NNN at most time points. O(2)-[4-(3-Pyridyl)-4-oxobut-1-yl]thymidine (O(2)-POB-dThd, 11) predominated in the nasal olfactory and respiratory mucosa, followed by 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua, 14). Levels of O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]cytosine (O(2)-POB-Cyt, 13) and O(6)-[4-(3-pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O(6)-POB-dGuo, 12) were significantly lower. In the oral mucosa, the opposite stereoselectivity was observed, with (S)-NNN treatment producing 3-5 times more POB-DNA adducts than did (R)-NNN. O(2)-POB-dThd and 7-POB-dGuo were the two major adducts, and their levels were similar. Overall, POB-DNA adduct formation in the nasal olfactory and nasal respiratory mucosa was similar to that previously observed in the lung, whereas in the oral mucosa, the trend resembled that in the esophagus. These results indicate that different mechanisms are involved in NNN metabolism and tumorigenesis in rat nasal and oral tissues. NNN enters the nasal mucosa through the circulation, and tissue-specific metabolism is important, while in the oral mucosa, direct exposure and local activation both play significant roles. Our results also support the potential importance of NNN as an oral carcinogen in people who use smokeless tobacco products.

Analysis of pyridyloxobutyl and pyridylhydroxybutyl DNA adducts in extrahepatic tissues of F344 rats treated chronically with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and enantiomers of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and its metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) are potent pulmonary carcinogens in rats. NNK and NNAL require metabolic activation to express their carcinogenicity. Cytochrome P450-catalyzed alpha-hydroxylation at the methyl position of NNK or NNAL generates reactive intermediates, which alkylate DNA to form pyridyloxobutyl (POB)-DNA adducts or pyridylhydroxybutyl (PHB)-DNA adducts. NNK is metabolized to NNAL in a reversible and stereoselective manner, and the tissue-specific retention of (S)-NNAL is believed to be important to the carcinogenicity of NNK. In the present study, we investigated the formation of POB- and PHB-DNA adducts in extrahepatic tissues of F344 rats treated chronically with NNK and (R)- and (S)-NNAL (10 ppm in the drinking water, 1-20 weeks). POB- and PHB-DNA adducts were quantified in nasal olfactory mucosa, nasal respiratory mucosa, oral mucosa, and pancreas of treated rats. Adduct formation in the nasal respiratory mucosa exceeded that in the other tissues. O(2)-[4-(3-Pyridyl)-4-oxobut-1-yl]thymidine (O(2)-POB-dThd) or O(2)-[4-(3-pyridyl)-4-hydroxybut-1-yl]thymidine (O(2)-PHB-dThd) was the major adduct, followed by 7-[4-(3-pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua) or 7-[4-(3-pyridyl)-4-hydroxybut-1-yl]guanine (7-PHB-Gua). There was a remarkable similarity in adduct formation between the NNK and the (S)-NNAL groups, both of which were distinctively different from that in the (R)-NNAL group. For example, in the nasal olfactory mucosa, POB-DNA adduct levels in the NNK and (S)-NNAL groups were not significantly different from each other, while (R)-NNAL treatment generated 6-33 times lower amounts of POB-DNA adducts than did NNK treatment. In contrast, (R)-NNAL treatment produced significantly higher levels of PHB-DNA adducts than did NNK or (S)-NNAL treatment. Similar trends were observed in the nasal respiratory mucosa, oral mucosa, and pancreas. These results suggest extensive retention of (S)-NNAL in various tissues of NNK-treated rats and support a mechanism in which the preferential metabolism of NNK to (S)-NNAL, followed by sequestration of (S)-NNAL in the target tissues and reoxidation to NNK, is important to NNK tumorigenesis.

In vivo treatment with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) induces organ-specific alterations in in vitro repair of DNA pyridyloxobutylation

To investigate the mechanisms responsible for inter-organ differences in susceptibility to 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK)-induced carcinogenesis, the objectives were to compare DNA repair activities of extracts from mouse lung and liver towards NNK-induced pyridyloxobutyl (POB) damage to plasmid DNA, and to determine if and the mechanism by which in vivo NNK treatment of mice alters DNA repair. Repair activity of POB adducts was three times greater in mouse liver than in mouse lung (P<0.05). Repair activities of lung extracts from mice 4 or 24 h post-NNK treatment were 30-45% those of control (P<0.05). Conversely, POB adduct repair was 2-3 times higher in liver extracts from NNK treated mice than in controls (4 h, 24 h, P<0.05). NNK treatment also decreased incision of POB adducts by 92% (4 h, P<0.05) in lung and increased incision by 169% (24 h, P<0.05) in liver. NNK decreased immunoreactive levels of the incision protein RPA in lung (P<0.05) 4 h post-treatment but increased immunoreactive lung RPA and XPB after 24 h (P<0.05). In liver, levels of immunoreactive proteins, XPA, XPB and ERCC1 were increased after NNK treatment (24 h, P<0.05). Binding of XPA and XPB from liver extracts to POB adducts increased following NNK treatment, while binding of XPA and XPB from lung decreased (4 h, 24 h). These results suggest that lower incision activity of nucleotide excision repair and NNK-mediated alterations in levels and activities of key incision proteins contribute to the relative susceptibility of mouse lung to NNK-induced carcinogenesis.

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.

Screening for DNA adducts by data-dependent constant neutral loss-triple stage mass spectrometry with a linear quadrupole ion trap mass spectrometer

A two-dimensional linear quadrupole ion trap mass spectrometer (LIT/MS) was employed to simultaneously screen for DNA adducts of environmental, dietary, and endogenous genotoxicants, by data-dependent constant neutral loss scanning followed by triple-stage mass spectrometry (CNL-MS3). The loss of the deoxyribose (dR) from the protonated DNA adducts ([M + H - 116]+) in the MS/MS scan mode triggered the acquisition of MS3 product ion spectra of the aglycone adducts [BH2]+. Five DNA adducts of the tobacco carcinogen 4-aminobiphenyl (4-ABP) were detected in human hepatocytes treated with 4-ABP, and three DNA adducts of the cooked-meat carcinogen 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) were identified in the livers of rats exposed to MeIQx, by the CNL-MS3 scan mode. Buccal cell DNA from tobacco smokers was screened for DNA adducts of various classes of carcinogens in tobacco smoke including 4-ABP, 2-amino-9H-pyrido[2,3-b]indole (AalphaC), and benzo[a]pyrene (BaP); the cooked-meat carcinogens MeIQx, AalphaC, and 2-amino-1-methyl-6-phenylmidazo[4,5-b]pyridine (PhIP); and the lipid peroxidation products acrolein (AC) and trans-4-hydroxynonenal (HNE). The CNL-MS3 scanning technique can be used to simultaneously screen for multiple DNA adducts derived from different classes of carcinogens, at levels of adduct modification approaching 1 adduct per 108 unmodified DNA bases, when 10 microg of DNA is employed for the assay.

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.

Human aldo-keto reductases: Function, gene regulation, and single nucleotide polymorphisms

Aldo-keto reductases (AKRs) are a superfamily of NAD(P)H linked oxidoreductases that are generally monomeric 34-37kDa proteins present in all phyla. The superfamily consists of 15 families, which contains 151 members (www.med.upenn.edu/akr). Thirteen human AKRs exist that use endogenous substrates (sugar and lipid aldehydes, prostaglandins, retinals and steroid hormones), and in many instances they regulate nuclear receptor signaling. Exogenous substrates include metabolites implicated in chemical carcinogenesis: NNK (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone), polycyclic aromatic hydrocarbon trans-dihydrodiols, and aflatoxin dialdehyde. Promoter analysis of the human genes identifies common elements involved in their regulation which include osmotic response elements, anti-oxidant response elements, xenobiotic response elements, AP-1 sites and steroid response elements. The human AKRs are highly polymorphic, and in some instances single nucleotide polymorphisms (SNPs) of high penetrance exist. This suggests that there will be inter-individual variation in endogenous and xenobiotic metabolism which in turn affect susceptibility to nuclear receptor signaling and chemical carcinogenesis.

Development of liquid chromatography electrospray ionization tandem mass spectrometry methods for analysis of DNA adducts of formaldehyde and their application to rats treated with N-nitrosodimethylamine or 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

Reaction of formaldehyde with DNA in vitro produces a variety of adducts among which are observed the cross-link di-(N(6)-deoxyadenosyl)methane (dAdo-CH 2-dAdo, 1) and the hydroxymethyl adduct N(6)-hydroxymethyl-dAdo (N(6)-HOCH 2-dAdo, 2). While the structures of these adducts have been known for decades, there have been no reports of their formation in vivo. Formaldehyde is released during intracellular metabolism of carcinogenic N-nitrosomethyl compounds such as N-nitrosodimethylamine (NDMA) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), but DNA adducts formed by this pathway have not been previously characterized. In this study, we addressed these questions by developing highly sensitive liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring methods for quantitation of adducts 1 and 2, the latter as N(6)-methyl-dAdo ( 3). Considerable effort was devoted to the problem of artifactual formation of 1, which can occur during storage of DNA samples by reaction of dAdo with 2. This problem was solved by the addition of adenosine deaminase during the DNA hydrolysis step, thus removing dAdo as a reactant. The instability of adduct 2 was another potential block to analysis, and this was solved by converting it to 3 with NaBH 3CN. Separate aliquots of DNA were analyzed for adducts 1 and 2, using the [(15)N]-labeled adducts as internal standards. The application of these methods to rat hepatic DNA to which adducts 1 and 3 were added demonstrated accuracy and precision. The detection limits for adducts 1 and 3 were 1-4 adducts per 10 (9) nucleotides using 100-150 microg of DNA. The method was applied to analyze hepatic and pulmonary DNA from rats treated with NDMA and NNK. The results clearly demonstrated the dose-dependent presence of N(6)-HOCH 2-dAdo ( 2) in all DNA samples. The cross-link adduct dAdo-CH 2-dAdo ( 1) was observed in hepatic DNA of NNK-treated rats, with lower amounts in pulmonary DNA. Levels of these adducts were generally less than those of DNA adducts formed by the classical diazohydroxide pathway of nitrosamine metabolism. The results of this study demonstrate for the first time the presence of formaldehyde DNA adducts in tissues of rats treated with carcinogenic nitrosamines and suggest that these adducts may play a role in cancer induction by these compounds.

DNA adduct profiles: chemical approaches to addressing the biological impact of DNA damage from small molecules

Diverse small molecules alkylate DNA and form covalently linked adducts that can influence crucial biological processes, contributing to toxicity and mutation. Understanding the chemical reactivity dictating DNA alkylation and interactions of adducts with biological pathways can impact disease prevention and treatment. The ambident reactivity of DNA-alkylating small molecules, and of DNA itself, often results in formation of multiple adducts. Determining which structures impart biological responses is important for understanding the underlying relationships between small-molecule structure and biology. With application of sensitive and structure-specific experimental and analytical methodology, such as heteronuclear NMR spectroscopy and mass spectrometry, there are increasing numbers of studies that evaluate DNA alkylation from the perspective of resulting adduct profiles. DNA adduct profiles have been examined for both exogenous and endogenous reactive small molecules. Examples of recent findings are in the areas of tobacco-specific carcinogens, lipid peroxidation products, environmental and dietary chlorophenols, and natural-product-derived antitumor therapies. As more profile data are obtained, correlations with biological impact are being observed that would not be identified by a simplified single agent/single adduct approach.

Analysis of pyridyloxobutyl DNA adducts in F344 rats chronically treated with (R)- and (S)-N'-nitrosonornicotine

NNN (1) is an esophageal carcinogen in rats. 2'-Hydroxylation of NNN is believed to be the major bioactivation pathway for NNN tumorigenicity. (S)-NNN is preferentially metabolized by 2'-hydroxylation in cultured rat esophagus, whereas there is no preference for 2'-hydroxylation versus 5'-hydroxylation in the metabolism of (R)-NNN. 2'-Hydroxylation of NNN generates the reactive intermediate 4-oxo-4-(3-pyridyl)butanediazohydroxide (8), resulting in the formation of pyridyloxobutyl (POB)-DNA adducts. On the basis of these observations, we hypothesized that (S)-NNN treatment would produce higher levels of POB-DNA adducts than that by (R)-NNN in the rat esophagus. We tested this hypothesis by treating male F344 rats with 10 ppm of (R)-NNN or (S)-NNN in drinking water. After 1, 2, 5, 10, 16, or 20 weeks of treatment, POB-DNA adducts in esophageal, liver, and lung DNA were quantified by HPLC-ESI-MS/MS. In the rat esophagus, (S)-NNN treatment generated levels of POB-DNA adducts 3-5 times higher than (R)-NNN treatment, which supports our hypothesis. 7-[4-(3-Pyridyl)-4-oxobut-1-yl]guanine (7-POB-Gua, 14) was the major adduct detected, followed by O2-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine (O2-POB-dThd, 11) and O2-[4-(3-pyridyl)-4-oxobut-1-yl]cytosine (POB-Cyt, 15). O6-[4-(3-Pyridyl)-4-oxobut-1-yl]-2'-deoxyguanosine (O6-POB-dGuo, 10) was not detected. The total POB-DNA adduct levels in the esophagus were 3-11 times higher than those in the liver for (R)-NNN and 2-6 times higher than those for (S)-NNN. In contrast to the esophagus and liver, (R)-NNN treatment produced more POB-DNA adducts than (S)-NNN treatment in the rat lung, which suggested an important role for cytochrome P450 2A3 in NNN metabolism in the rat lung. In both the liver and lung, O2-POB-dThd was the predominant adduct and accumulated during the experiment. The results of this study demonstrate that individual POB-DNA adducts form and persist in the esophagi, livers, and lungs of rats chronically treated with NNN enantiomers and demonstrate that (S)-NNN produces higher levels of POB-DNA adducts in the esophagus than (R)-NNN, suggesting that (S)-NNN is more tumorigenic than (R)-NNN to the rat esophagus.

Novel LC-ESI/MS/MS(n) method for the characterization and quantification of 2'-deoxyguanosine adducts of the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by 2-D linear quadrupole ion trap mass spectrometry

An accurate and sensitive liquid chromatography-electrospray ionization/multi-stage mass spectrometry (LC-ESI/MS/MS(n)) technique has been developed for the characterization and quantification of 2'-deoxyguanosine (dG) adducts of the dietary mutagen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). PhIP is an animal and potential human carcinogen that occurs in grilled meats. Following enzymatic digestion and adduct enrichment by solid-phase extraction (SPE), PhIP-DNA adducts were analyzed by MS/MS and MS(n) scan modes on a 2-D linear quadrupole ion trap mass spectrometer (QIT/MS). The major DNA adduct, N-(deoxyguanosin-8-yl)-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (dG-C8-PhIP), was detected in calf thymus (CT) DNA modified in vitro with a bioactivated form of PhIP and in the colon and liver of rats given PhIP as part of the diet. The lower limit of detection (LOD) was 1 adduct per 10(8) DNA bases, and the limit of quantification (LOQ) was 3 adducts per 10(8) DNA bases in both MS/MS and MS(3) scan modes, using 27 microg of DNA for analysis. Measurements were based on isotope dilution with the internal standard, N-(deoxyguanosin-8-yl)-2-amino-1-(trideutero)methyl-6-phenylimidazo[4,5-b]pyridine (dG-C8-[2H3C]-PhIP). The selected reaction monitoring (SRM) scan mode in MS/MS was employed to monitor the loss of deoxyribose (dR) from the protonated molecules of the adducts ([M + H - 116]+). The consecutive reaction monitoring (CRM) scan modes in MS(3) and MS(4) were used to measure and further characterize product ions of the aglycone ion (BH2+) (Guanyl-PhIP). The MS(3) scan mode was effective in eliminating isobaric interferences observed in the MS/MS scan mode and resulted in an improved signal-to-noise (S/N) ratio. Moreover, the product ion spectra obtained by the MS(n) scan modes provided rich structural information about the adduct and were used to corroborate the identity of dG-C8-PhIP. In addition, an isomeric dG-PhIP adduct was detected in vivo. This LC-ESI/MS/MS(n) method is the first reported application on the use of the MS(3) scan mode for the analysis of DNA adducts in vivo.

Formation and accumulation of pyridyloxobutyl DNA adducts in F344 rats chronically treated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and enantiomers of its metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, 1) and its metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL, 2) are both potent pulmonary carcinogens in rats. The metabolism of NNK to NNAL is stereoselective and reversible, with (S)-NNAL being the major enantiomer formed from NNK. In rats, (R)-NNAL undergoes facile glucuronidation and is rapidly excreted in urine, whereas (S)-NNAL is preferentially retained in tissues and converted to NNK. We hypothesized that the lung carcinogenicity of NNK in the rat is due in part to the preferential retention of (S)-NNAL in the lung, the reconversion to NNK, and then the metabolic activation of NNK to pyridyloxobutyl (POB)-DNA adducts. We tested this hypothesis by treating male F344 rats with 10 ppm of NNK, (R)-NNAL, or (S)-NNAL in drinking water. After 1, 2, 5, 10, 16, or 20 weeks of treatment, POB-DNA adducts in liver and lung DNA were quantified by HPLC-ESI-MS/MS. At each time point, total adduct levels were higher in the lung than in the liver. O2-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine (O2-POB-dThd, 13) was the major adduct detected. Total adduct levels in the rats treated with (S)-NNAL were 0.6-1.3 times as great as those in the NNK group in the lung and 0.7-1.4 times in the liver, and 6-14 times higher than those in the (R)-NNAL group in the lung and 11-17 times in the liver. These results suggest that (S)-NNAL is stereoselectively retained in tissues. This study demonstrates for the first time the accumulation and persistence of specific POB-DNA adducts in the rat lung and liver during chronic treatment with NNK, (R)-NNAL, and (S)-NNAL and supports the hypothesis that the preferential retention of (S)-NNAL in the lung, followed by reconversion to NNK and then the metabolic activation of NNK is critical for lung carcinogenesis by NNK and NNAL.

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

An improved analytical method was developed for the analysis of 4-hydroxy-1-(3-pyridyl)-1-butanone (HPB)-releasing DNA adducts in lung samples of patients undergoing surgery for lung cancer. HPB-releasing adducts can be formed by metabolic activation of the tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and N'-nitrosonornicotine, and have been reported to play an important role in tobacco carcinogenesis. [2,2,3,3-D(4)]HPB (D(4)-HPB) was used as an internal standard, and HPB released by acid hydrolysis of DNA was determined by gas chromatography/mass spectrometry in the negative ion chemical ionisation mode. The method is sensitive with a limit of detection of 5.9 fmol HPB and a limit of quantification of 15.2 fmol HBP/mg DNA. The recovery of HPB was 82+/-17% and the background response was 10.1+/-1.8 fmol HPB/sample. The concentration of HPB-releasing lung DNA adducts was significantly higher (p<0.0001) in 21 self-reported smokers compared to in 11 self-reported nonsmokers (404+/-258 fmol versus 59+/-56 fmol HPB/mg DNA, respectively). HPB-releasing hemoglobin adduct concentrations were only marginally higher in a subset of 12 smokers compared to in 7 nonsmokers (63+/-53 fmol versus 42+/-34 fmol HPB/g hemoglobin; p=0.36). No correlation was found between HPB-releasing adducts in DNA and hemoglobin (p=0.074).

Combined genotoxic effects of radiation and a tobacco-specific nitrosamine in the lung of gpt delta transgenic mice

It is important to evaluate the health effects of low-dose-rate or low-dose radiation in combination with chemicals as humans are exposed to a variety of chemical agents. Here, we examined combined genotoxic effects of low-dose-rate radiation and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), the most carcinogenic tobacco-specific nitrosamine, in the lung of gpt delta transgenic mice. In this mouse model, base substitutions and deletions can be separately analyzed by gpt and Spi- selections, respectively. Female gpt delta mice were either treated with gamma-irradiation alone at a dose rate of 0.5, 1.0 or 1.5 mGy/h for 22 h/day for 31 days or combined with NNK treatments at a dose of 2 mg/mouse/day, i.p. for four consecutive days in the middle course of irradiation. In the gpt selection, the NNK treatments enhanced the mutation frequencies (MFs) significantly, but no obvious combined effects of gamma-irradiation were observable at any given radiation dose. In contrast, NNK treatments appeared to suppress the Spi- large deletions. In the Spi- selection, the MFs of deletions more than 1 kb in size increased in a dose-dependent manner. When NNK treatments were combined, the dose-response curve became bell-shaped where the MF at the highest radiation dose decreased substantially. These results suggest that NNK treatments may elicit an adaptive response that eliminates cells bearing radiation-induced double-strand breaks in DNA. Possible mechanisms underlying the combined genotoxicity of radiation and NNK are discussed, and the importance of evaluation of combined genotoxicity of more than one agent is emphasized.