Progress and challenges in selected areas of tobacco carcinogenesis

Evolution of research on the DNA adduct chemistry of N-nitrosopyrrolidine and related aldehydes

This perspective reviews our work on the identification of DNA adducts of N-nitrosopyrrolidine and some related aldehydes. The research began as a focused project to investigate mechanisms of cyclic nitrosamine carcinogenesis but expanded into other areas, as aldehyde metabolites of NPYR were shown to have their own diverse DNA adduct chemistry. A total of 69 structurally distinct DNA adducts were identified, and some of these, found in human tissues, have provided intriguing leads for investigating carcinogenesis mechanisms in humans due to exposure to both endogenous and exogenous agents.

Steroid hormone levels associated with passive and active smoking

CONTEXT

Cigarette tobacco smoke is a potent environmental contaminant known to adversely affect health including fertility and pregnancy.

OBJECTIVE

To examine the associations between second-hand cigarette tobacco-smoke exposure, or active smoking and serum concentrations of steroid hormones using tandem mass spectrometry.

DESIGN

Healthy women (18-45 y) from the general community in the Metropolitan Washington, DC were recruited at the follicular stage of their menstrual cycle. Participants were assigned to one of three study groups: active smokers (N=107), passive smokers (N=86), or non-smokers (N=100). Classifications were based on a combination of self-reporting and serum cotinine concentrations.

METHODS

Serum androgens, estrogens, progestins, androstenedione, aldosterone, cortisol, corticosterone, dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulfate (DHEAS), 11-deoxycortisol and 25-hydroxy-vitamin D3 (25-OHVitD3) and cotinine were measured by isotope dilution tandem mass spectrometry (LC/MS/MS) (API-5000). Kruskal-Wallis tests were used to assess median differences among the three groups, with Dunn's multiple comparison test for post hoc analysis.

RESULTS

Serum estrone, estradiol, and estriol concentrations were lower in active and passive smokers than in non-smokers. The three study groups differed significantly in serum concentrations of 16-OHE1, aldosterone and 25-OHVitD3, as well as in the ratios of many of the steroids. Pair-wise comparison of the groups demonstrated significant differences in hormone concentrations between (i) smokers and non-smokers for aldosterone: (ii) passive smokers and non-smokers for aldosterone, progesterone and estriol. Moreover, for smokers and passive smokers, there were no significant differences in these hormone concentrations.

CONCLUSIONS

Smoke exposure was associated with lower than normal median steroid hormone concentrations. These processes may be instrumental in explaining some adverse effects of tobacco smoke on female health and fertility.

Cytochrome P450-mediated pulmonary metabolism of carcinogens: regulation and cross-talk in lung carcinogenesis

Lung cancer is strongly associated with exogenous risk factors, in particular tobacco smoking and asbestos exposure. New research data are accumulating about the regulation of the metabolism of tobacco carcinogens and the metabolic response to oxidative stress. These data provide mechanistic details about why well known risk factors cause lung cancer. The purpose of this review is to evaluate the present knowledge of the role of cytochrome P450 (CYP) enzymes in the metabolism of tobacco carcinogens and associations with tobacco and asbestos carcinogenesis. Major emphasis is placed on human data and regulatory pathways involved in CYP regulation and lung carcinogenesis. The most exciting new research findings concern cross-talk of the CYP-regulating aryl hydrocarbon receptor with other transcription factors, such as nuclear factor-erythroid 2-related factor 2, involved in the regulation of xenobiotic metabolism and antioxidant enzymes. This cross-talk between transcription factors may provide mechanistic evidence for clinically relevant issues, such as differences in lung cancers between men and women and the synergism between tobacco and asbestos as lung carcinogens.

Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a widely distributed, unique DNA repair protein that acts as a single agent to directly remove alkyl groups located on the O(6)-position of guanine from DNA restoring the DNA in one step. The protein acts only once, and its alkylated form is degraded rapidly. It is a major factor in counteracting the mutagenic, carcinogenic, and cytotoxic effects of agents that form such adducts including N-nitroso-compounds and a number of cancer chemotherapeutics. This review describes the structure, function, and mechanism of action of AGTs and of a family of related alkyltransferase-like proteins, which do not act alone to repair O(6)-alkylguanines in DNA but link repair to other pathways. The paradoxical ability of AGTs to stimulate the DNA-damaging ability of dihaloalkanes and other bis-electrophiles via the formation of AGT-DNA cross-links is also described. Other important properties of AGTs include the ability to provide resistance to cancer therapeutic alkylating agents, and the availability of AGT inhibitors such as O(6)-benzylguanine that might overcome this resistance is discussed. Finally, the properties of fusion proteins in which AGT sequences are linked to other proteins are outlined. Such proteins occur naturally, and synthetic variants engineered to react specifically with derivatives of O(6)-benzylguanine are the basis of a valuable research technique for tagging proteins with specific reagents.

Nicotine exposure and metabolizer phenotypes from analysis of urinary nicotine and its 15 metabolites by LC-MS

Smokers who inhale less deeply are exposed to lower amounts of the toxic substances present in tobacco smoke. In order to more rigorously assess tobacco smoke exposure, it is necessary to have an accurate method for quantifying nicotine and all of its known metabolites.

METHODS

A stable-isotope dilution LC-MRM/MS assay has been developed for quantification of urinary nicotine and the 15 possible metabolites that could arise from known metabolic pathways. Nicotine, cotinine, trans-3´-hydroxy-cotinine, nicotine-N-oxide, cotinine-N-oxide, nornicotine, norcotinine and 4-hydroxy-4-(3-pyridyl)butanoic acid were quantified by direct analysis. The corresponding glucuronide metabolites were quantified after urine hydrolysis with β-glucuronidase.

RESULTS

Nicotine and all 15 nicotine metabolites were quantified by LC-MRM/MS in most urine samples from 61 tobacco smokers. Urinary nicotine and metabolite concentrations ranged from 7.9 to 337.8 µM (mean 75.5 ± 67.8 µM). Three nicotine metabolizer phenotypes were established as reduced metabolizers (ratio < 8), normal metabolizers (ratio 8-30), and extensive metabolizers (ratio > 30). 4-hydroxy-4-(3-pyridyl)butanoic acid, which has not been quantified previously, was an abundant metabolite in all three phenotypes.

CONCLUSION

Using this assay it will now be possible to determine whether there are relationships between nicotine exposure and/or metabolizer phenotype with exposure to toxic substances that are present in tobacco smoke and/or to biological response biomarkers to tobacco smoking. This will help in identifying individuals at high risk for developing smoking-related diseases as well as those amenable to smoking cessation programs.

TP53 mutations in nonsmall cell lung cancer

The tumor suppressor gene TP53 is frequently mutated in human cancers. Abnormality of the TP53 gene is one of the most significant events in lung cancers and plays an important role in the tumorigenesis of lung epithelial cells. Human lung cancers are classified into two major types, small cell lung cancer (SCLC) and nonsmall cell lung cancer (NSCLC). The latter accounts for approximately 80% of all primary lung cancers, and the incidence of NSCLC is increasing yearly. Most clinical studies suggest that NSCLC with TP53 alterations carries a worse prognosis and may be relatively more resistant to chemotherapy and radiation. A deep understanding of the role of TP53 in lung carcinogenesis may lead to a more reasonably targeted clinical approach, which should be exploited to enhance the survival rates of patients with lung cancer. This paper will focus on the role of TP53 in the molecular pathogenesis, epidemiology, and therapeutic strategies of TP53 mutation in NSCLC.

A new liquid chromatography/mass spectrometry method for 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) in urine

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a carcinogenic nitrosamine produced upon curing tobacco. It is present in tobacco smoke and undergoes metabolism to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) in the lungs. NNAL undergoes further uridine diphosphate glucuronosyltransferase (UGT)-mediated metabolism to give N- and O-glucuronide metabolites, which together with free (non-conjugated) NNAL are then excreted in the urine. The ability to conduct validated analyses of free and conjugated NNAL in human urine is important in order to assess inter-individual differences in lung cancer risk from exposure to cigarette smoke. The use of stable isotope dilution (SID) methodology in combination with liquid chromatography/multiple reaction monitoring/mass spectrometry (LC/MRM-MS) provides the highest bioanalytical specificity possible for such analyses. We describe a novel derivatization procedure, which results in the formation of a pre-ionized N-propyl-NNAL derivative. The increased LC/MS sensitivity arising from this derivative then makes it possible to analyze free NNAL in only 0.25 mL urine. This substantial reduction in urine volume when compared with other methods that have been developed will help preserve the limited amounts of stored urine samples that are available from on-going longitudinal biomarker studies. The new high sensitivity SID LC/MRM-MS assay was employed to determine free and conjugated NNAL concentrations in urine samples from 60 individual disease-free smokers. Effects of inter-individual differences in urinary creatinine clearance on NNAL concentrations were then assessed and three metabolizer phenotypes were identified in the 60 subjects from the ratio of urinary NNAL glucuronides/free NNAL. Poor metabolizers (PMs, 14 subjects) with a ratio of NNAL glucuronides/free NNAL <2 (mean = 1.3), intermediate metabolizers (IMs, 36 subjects) with a ratio between 2 and 5 (mean = 3.4), and extensive metabolizers (EMs, 10 subjects) with a ratio >5 (mean = 11.1).

Measuring Tobacco Specific Lung Carcinogen among Nonsmoking Hospitality Workers in Richmond Virginia: A Preliminary Evaluation of Exposure before the Smoking Ban

The purpose of this pilot study is to measure the exposure to SHS among Richmond bar and restaurant workers and identify the prevalence of the tobacco-specific lung carcinogen-4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNAL) among nonsmoking bar and restaurants workers. Hair and urine samples were obtained from 40 nonsmoking bar and restaurant workers from Richmond establishments that do not have smoke free policies. Workers' exposure to SHS was estimated through measuring nicotine concentration in hair (NG/MG). Self-reported tobacco use was assessed in addition to performing urine analysis and listing sources and intensity of SHS exposure. Urine specimens were analyzed for total NNAL.

Results

Hair nicotine ranged from 0.05 to 42.15 ng/mg among workers with an average of 23.3 hours of self reported exposure in workplaces where smoking was permitted, indicating that workers in smoking restaurants and bars are exposed to SHS. Also, 60% of workers had a detectable level of NNAL that ranged from 0.019 to 1.9 PMol/ML. Increased levels of NNAL were mostly associated with the number of continuous hours of a single workplace exposure. In the city of Richmond, most bar and restaurant workers are continuously exposed to SHS in their workplace. To achieve complete protection for all workers and patrons in Richmond, Virginia, smoke free initiatives in all occupational settings are required. This information can be used to advocate for smoke free policies when discussing potential ways to strengthen the law.

Urinary biomarkers for secondhand smoke

The use of a biomarker is mandatory for quantitative analysis of exposure to secondhand smoke (SHS). This article summarizes urinary biomarkers of smoke exposure which can be now quantified. The most reliable urinary biomarkers to assess the exposure to SHS are NNAL 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol and NNAL-Glucuronides, which is metabolites of tobacco-specific nitrosamine. These substances were detected even in 50% of children who had undetectable level of cotinine (<0.5 ng/ml). Urinary cotinine, which is determined by a highly sensitive competing enzyme immunoassay, is also a useful biomarker. However, individual variability of CYP2A6 allele,in which nicotine is catalyzed to cotinine, affects the level of urinary cotinine. Approximately 20% of Japanese subjects have homozygotes or heterozygotes of the CYP2A6*4 allele, which has impaired nicotine metabolism and subsequently may underestimate the actual exposure to SHS. In assessing the exposure to SHS, therefore, individual variability of CYP2A6 gene polymorphism should be taken into consideration. The combination of urinary cotinine measurement and self-report of parents' smoking seems to be accurate to assess the exposure to SHS in mass screening.

Acetaldehyde adducts in alcoholic liver disease

Chronic alcohol abuse causes liver disease that progresses from simple steatosis through stages of steatohepatitis, fibrosis, cirrhosis, and eventually hepatic failure. In addition, chronic alcoholic liver disease (ALD), with or without cirrhosis, increases risk for hepatocellular carcinoma (HCC). Acetaldehyde, a major toxic metabolite, is one of the principal culprits mediating fibrogenic and mutagenic effects of alcohol in the liver. Mechanistically, acetaldehyde promotes adduct formation, leading to functional impairments of key proteins, including enzymes, as well as DNA damage, which promotes mutagenesis. Why certain individuals who heavily abuse alcohol, develop HCC (7.2-15%) versus cirrhosis (15-20%) is not known, but genetics and co-existing viral infection are considered pathogenic factors. Moreover, adverse effects of acetaldehyde on the cardiovascular system and hematologic systems leading to ischemia, heart failure, and coagulation disorders, can exacerbate hepatic injury and increase risk for liver failure. Herein, we review the role of acetaldehyde adducts in the pathogenesis of chronic ALD and HCC.

Formation and repair of tobacco carcinogen-derived bulky DNA adducts

DNA adducts play a central role in chemical carcinogenesis. The analysis of formation and repair of smoking-related DNA adducts remains particularly challenging as both smokers and nonsmokers exposed to smoke are repetitively under attack from complex mixtures of carcinogens such as polycyclic aromatic hydrocarbons and N-nitrosamines. The bulky DNA adducts, which usually have complex structure, are particularly important because of their biological relevance. Several known cellular DNA repair pathways have been known to operate in human cells on specific types of bulky DNA adducts, for example, nucleotide excision repair, base excision repair, and direct reversal involving O⁶-alkylguanine DNA alkyltransferase or AlkB homologs. Understanding the mechanisms of adduct formation and repair processes is critical for the assessment of cancer risk resulting from exposure to cigarette smoke, and ultimately for developing strategies of cancer prevention. This paper highlights the recent progress made in the areas concerning formation and repair of bulky DNA adducts in the context of tobacco carcinogen-associated genotoxic and carcinogenic effects.

Cigarette smoke extract induces disruption of structure and function of tubulin-microtubule in lung epithelium cells and in vitro

In the present study, we have investigated the effect of the aqueous extract of cigarette smoke (AECS) on tubulin-microtubule, a major cytoskeleton protein that maintains cellular morphology and participates in cell division. We found that treatment of AECS results in the loss of both structural and functional properties of tubulin-microtubule. Disruption of the microtubule network was observed in AECS-treated human lung epithelial (A549) cells and noncarcinoma human lung alveolar epithelium (L132) cells, in a dose and time-dependent manner. Tubulin-microtubule mediated important cellular properties, such as proliferation, migration, and maintenance of the cellular morphology, were affected by AECS in A549 cells. The aqueous extract of cigarette smoke (AECS) was also found to interfere the microtubule dynamics inside the cell and induce tubulin degradation. The structure of microtubules was also disrupted by AECS in the presence of protease inhibitors accompanied by a change of morphology of cells and loss of cell viability. In vitro, the functional properties of tubulin, such as the ability of polymerization, was inhibited by AECS in a dose and time-dependent manner, and it was accompanied by the loss of reactive cysteine residues, destabilization of the secondary structure, and quenching of intrinsic tryptophan fluorescence. Carbonyl content of tubulin was increased after treatment with AECS, indicating that one of the pathways of tubulin damage is protein oxidation. The damage of tubulin by AECS thus may be correlated with the pathogenesis of cigarette smoke induced disorders, which result in cellular apoptosis and tissue damage.

Oxygen as a friend and enemy: How to combat the mutational potential of 8-oxo-guanine

The maintenance of genetic stability is of crucial importance for any form of life. Prior to cell division in each mammalian cell, the process of DNA replication must faithfully duplicate the three billion bases with an absolute minimum of mistakes. Various environmental and endogenous agents, such as reactive oxygen species (ROS), can modify the structural properties of DNA bases and thus damage the DNA. Upon exposure of cells to oxidative stress, an often generated and highly mutagenic DNA damage is 7,8-dihydro-8-oxo-guanine (8-oxo-G). The estimated steady-state level of 8-oxo-G lesions is about 10(3) per cell/per day in normal tissues and up to 10(5) lesions per cell/per day in cancer tissues. The presence of 8-oxo-G on the replicating strand leads to frequent (10-75%) misincorporations of adenine opposite the lesion (formation of A:8-oxo-G mispairs), subsequently resulting in C:G to A:T transversion mutations. These mutations are among the most predominant somatic mutations in lung, breast, ovarian, gastric and colorectal cancers. Thus, in order to reduce the mutational burden of ROS, human cells have evolved base excision repair (BER) pathways ensuring (i) the correct and efficient repair of A:8-oxo-G mispairs and (ii) the removal of 8-oxo-G lesions from the genome. Very recently it was shown that MutY glycosylase homologue (MUTYH) and DNA polymerase lambda play a crucial role in the accurate repair of A:8-oxo-G mispairs. Here we review the importance of accurate BER of 8-oxo-G damage and its regulation in prevention of cancer.

An air transfer experiment confirms the role of volatile cues in communication between plants

Previous studies reported that sagebrush plants near experimentally clipped neighbors experienced less herbivory than did plants near unclipped neighbors. Blocking air flow with plastic bags made this effect undetectable. However, some scientists remained skeptical about the possibility of volatile communication between plants since the existence and identity of a cue that operates in nature have never been demonstrated. We conducted an air transfer experiment that collected air from the headspace of an experimentally clipped donor plant and delivered it to the headspace of an unclipped assay plant. We found that assay plants treated with air from clipped donors were less likely to be damaged by naturally occurring herbivores in a field experiment. This simple air transfer experiment fulfills the most critical of Koch's postulates and provides more definitive evidence for volatile communication between plants. It also provides an inexpensive experimental protocol that can be used to screen plants for interplant communication in the field.

UVA activation of N-dialkylnitrosamines releasing nitric oxide, producing strand breaks as well as oxidative damages in DNA, and inducing mutations in the Ames test

We investigated the photo-mutagenicity and photo-genotoxicity of N-dialkylnitrosamines and its mechanisms of UVA activation. With simultaneous irradiation of UVA, photo-mutagenicity of seven N-dialkylnitrosamines was observed in Ames bacteria (Salmonella typhimurium TA1535) in the absence of metabolic activation. Mutagenicity of pre-irradiated N-dialkylnitrosamines was also observed with S. typhimurium hisG46, TA100, TA102 and YG7108 in the absence of metabolic activation. UVA-mediated mutation with N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) decreased by adding either the NO or OH radical scavenger. When superhelical DNA was irradiated with N-dialkylnitrosamines, nicked circular DNA appeared. Ten N-dialkylnitrosamines examined produced strand breaks in the treated DNA in the presence of UVA. The level of single-strand breaks in phiX174 DNA mediated by N-nitrosomorpholine (NMOR) and UVA decreased by adding either a radical scavenger or superoxide dismutase. When calf thymus DNA was treated with N-dialkylnitrosamines (NDMA, NDEA, NMOR, N-nitrosopyrrolidine (NPYR) and N-nitrosopiperidine (NPIP)) and UVA, the ratio of 8-oxodG/dG in the DNA increased. Action spectra were obtained to determine if nitrosamine acts as a sensitizer of UVA. Both mutation frequency and NO formation were highest at the absorption maximum of nitrosamines, approximately 340 nm. The plots of NO formation and mutation frequency align with the absorption curve of NPYR, NMOR and NDMA. A significant linear correlation between the optical density of N-dialkynitrosamines at 340 nm and NO formation in each irradiated solution was revealed by ANOVA. We would like to propose the hypothesis that the N-nitroso moiety of N-dialkylnitrosamines absorbs UVA photons, UVA-photolysis of N-dialkylnitrosamines brings release of nitric oxide, and subsequent production of alkyl radical cations and active oxygen species follow as secondary events, which cause DNA strand breaks, oxidative and alkylative DNA damages and mutation.

The 1-carbamoyl-2-oxo-4,5-dihydroxyimidazolidine component of ROS-induced DNA damage in white blood cells

The 1-carbamoyl-2-oxo-4,5-dihydroxyimidazolidine modification of cytosine is a known base modification produced in vitro by oxidative stress. However, the presence of this modification in vivo has not been established. In this study the introduction of this base modification into dinucleoside monophosphates was accomplished using the Fenton reaction. Subsequently, the modification was produced in short isotopically labeled oligomers. Labeled tetramers bearing the lesion were used as internal standards for LC-MS/MS determinations of the base modification in the DNA of white blood cells from healthy donors. The background level of the 1-carbamoyl-2-oxo-4,5-dihydroxyimidazolidine modification of cytosine was found to be larger than the levels of the formamide and thymine glycol base modifications.

Lung cancer in Chinese women: evidence for an interaction between tobacco smoking and exposure to inhalants in the indoor environment

BACKGROUND

Epidemiologic data suggest that Chinese women have a high incidence of lung cancer in relation to their smoking prevalence. In addition to active tobacco smoke exposure, other sources of fumes and airborne particles in the indoor environment, such as cooking and burning of incense and mosquito coils, have been considered potential risk factors for lung cancer.

OBJECTIVES

We used a case-control study to explore effects of inhalants from combustion sources common in the domestic environment on lung cancer and their modification by active tobacco smoking.

METHODS

We analyzed 703 primary lung cancer cases and 1,578 controls. Data on demographic background and relevant exposures were obtained by face-to-face interviews in the hospital.

RESULTS

We observed a positive relationship with daily exposure to incense or mosquito coils and to cooking fumes only among smokers, and no association among lifetime nonsmokers. Interactions between smoking and frequency of cooking, or exposure to incense or mosquito coils were statistically significant and consistent with synergistic effects on lung cancer. The odds ratio (OR) comparing smokers without daily incense or mosquito coil exposure with nonsmokers without daily exposure was 2.80 [95% confidence interval (CI), 1.86-4.21], whereas the OR comparing smokers with daily exposure to the same referent group was 4.61 (95% CI, 3.41-6.24). In contrast, daily exposure to incense or mosquito coils was not associated with lung cancer among nonsmokers (OR = 0.91; 95% CI, 0.72-1.16). We observed the same pattern of associations for smokers without (OR = 2.31; 95% CI, 1.52-3.51) and with (OR = 4.50; 95% CI, 3.21-6.30) daily cooking exposure compared with nonsmokers, with no evidence of an association with daily cooking exposure among nonsmokers.

CONCLUSION

Our results suggest that active tobacco smoking not only is an important risk factor for development of lung cancer, but also may cause smokers to be more susceptible to the risk-enhancing effects of other inhalants.

The effect of cigarette smoking on cell proliferation in normal uterine cervix epithelium

BACKGROUND

The aim of this study was to evaluate the effect of smoking on cell proliferation in normal cervical epithelium.

METHODS

A total of 45 women undergoing hysterectomies due to uterine leiomyomata were investigated. Thirty were nonsmokers and 15 were current smokers. A segment of tissue was taken from cervix after removal of the uterus, and the identification of cell proliferation was performed by immunohistochemical analysis employing Ki-67 staining. The number of positive cells colored by Ki-67 staining was finally expressed as number of cells per square millimeter (mm²) of epithelium.

RESULTS

The comparison of the number of proliferating cells measured by ki-67 staining among smoking and nonsmoking women did not show significant difference. Among smoking women, there was no significant difference related to the number of cigarettes smoked per day or time of consumption and epithelial cell proliferation. However, the total amount of cigarettes smoked throughout presented significant association with Ki-67 staining (p < 0.001); the number of proliferating cells per mm² increased proportionally to the increase in consumption of cigarettes.

CONCLUSIONS

The results indicated that there was no significant difference between cell proliferation in smokers and nonsmokers; however, it was shown that in the smokers' group, amount of cigarettes smoked during throughout lifetime can increase cell proliferation in normal cervical epithelium.

Applying tobacco carcinogen and toxicant biomarkers in product regulation and cancer prevention

Tobacco carcinogen and toxicant biomarkers are metabolites or protein or DNA adducts of specific compounds in tobacco products. Highly reliable analytical methods, based mainly on mass spectrometry, have been developed and applied in large studies of many of these biomarkers. A panel of tobacco carcinogen and toxicant biomarkers is suggested here, and typical values for smokers and nonsmokers are summarized. This panel of biomarkers has potential applications in the new and challenging area of tobacco product regulation and in the development of rational approaches to cancer prevention by establishing carcinogen and toxicant uptake and excretion in people exposed to tobacco products.

Divergent ß-hairpins determine double-strand versus single-strand substrate recognition of human AlkB-homologues 2 and 3

Human AlkB homologues ABH2 and ABH3 repair 1-methyladenine and 3-methylcytosine in DNA/RNA by oxidative demethylation. The enzymes have similar overall folds and active sites, but are functionally divergent. ABH2 efficiently demethylates both single- and double-stranded (ds) DNA, whereas ABH3 has a strong preference for single-stranded DNA and RNA. We find that divergent F1 β-hairpins in proximity of the active sites of ABH2 and ABH3 are central for substrate specificities. Swapping F1 hairpins between the enzymes resulted in hybrid proteins resembling the donor proteins. Surprisingly, mutation of the intercalating residue F102 had little effect on activity, while the double mutant V101A/F102A was catalytically impaired. These residues form part of an important hydrophobic network only present in ABH2. In this functionally important network, F124 stacks with the flipped out base while L157 apparently functions as a buffer stop to position the lesion in the catalytic pocket for repair. F1 in ABH3 contains charged and polar residues preventing use of dsDNA substrate. Thus, E123 in ABH3 corresponds to F102 in ABH2 and the E123F-variant gained capacity to repair dsDNA with no loss in single strand repair capacity. In conclusion, divergent sequences outside of the active site determine substrate specificities of ABH2 and ABH3.

Induction of lacZ mutations in MutaMouse primary hepatocytes

We have developed an in vitro mutation assay using primary hepatocytes from the transgenic MutaMouse. Primary hepatocytes were isolated using a two-step perfusion method with purification by Percoll, cultured, and treated with benzo[a]pyrene (BaP), 2-amino-1-methyl-6-phenyl- imidazo[4,5-b]pyridine (PhIP), 3-nitrobenzoanthrone (3-NBA), and cigarette smoke condensate (CSC). The mean lacZ mutant frequency (MF) for the solvent control was approximately twofold greater than the spontaneous MF observed in liver tissue. A concentration-dependent increase in MF (up to 3.7-fold above control) was observed following exposure to BaP. Fourfold and twofold increases in mutant frequency were observed for 3-NBA and PhIP exposures, respectively, without the addition of any exogenous metabolic activation. A slight but statistically significant increase in lacZ MF was observed for CSC, but only at the lowest concentration. This is the first report demonstrating that mutations can be detected in cultured primary hepatocytes from MutaMouse. The preliminary results presented suggest that the MutaMouse primary hepatocyte mutagenicity assay can be used as a cost-effective tool for screening of environmental mutagens and therapeutic products.

Tobacco and cancer: an American Association for Cancer Research policy statement

The evidence against tobacco use is clear, incontrovertible, and convincing; so is the need for urgent and immediate action to stem the global tide of tobacco-related death and suffering and to improve public health. The American Association for Cancer Research makes an unequivocal call to all who are concerned about public health to take the following immediate steps:Increase the investment in tobacco-related research, commensurate with the enormous toll that tobacco use takes on human health, to provide the scientific evidence to drive the development of effective policies and treatments necessary to dramatically reduce tobacco use and attendant disease. Develop new evidence-based strategies to more effectively prevent the initiation of tobacco use, especially for youth and young adults. Promote the further development of evidence-based treatments for tobacco cessation, including individualized therapies, and ensure coverage of and access to evidence-based behavioral and pharmacological treatments. Develop evidence-based strategies for more effective public communication to prevent, reduce, and eliminate tobacco use and to guide health policies and clinical practice. Develop effective, evidence-based policies to reduce disparities across the tobacco continuum among social groups and developed and developing nations. Implement to the fullest extent existing evidence-based, systems-wide tobacco control programs to prevent initiation and foster cessation. Adapt and implement appropriate approaches to reduce the growing burden of tobacco use in the developing world. Enhance and coordinate surveillance efforts, both in the United States and globally, to monitor tobacco products, tobacco use, and tobacco-related disease, including tobacco use in oncology clinical trials. Establish a comprehensive, science-based regulatory framework to evaluate tobacco products and manufacturers' claims. Promote research that addresses the following: the potential harms of current and new tobacco products; the impact of altering the levels of addictive components in tobacco products; the identification of risk and risk-reduction measures for current and former tobacco users; enhanced early detection methods for tobacco-related cancers; and effective treatments against tobacco-related cancers tailored to the unique effects of tobacco on cancer. Pursue domestic and international economic policies that support tobacco control. Urge the United States to ratify the World Health Organization Framework Convention on Tobacco Control. Foster global scientific efforts to support the Framework. Work together with stakeholders worldwide, including federal agencies, to develop and implement effective tobacco control strategies and to deter counter-tobacco control efforts by the tobacco industry. Only such concerted global actions by scientists, policymakers, and advocates together can prevent the invidious impact of tobacco, the use of which is cutting wide swathes of death and disease around the world.

Clinical significance of phosphatidyl inositol synthase overexpression in oral cancer

BACKGROUND

We reported increased levels of phosphatidyl inositol synthase (PI synthase), (enzyme that catalyses phosphatidyl inositol (PI) synthesis-implicated in intracellular signaling and regulation of cell growth) in smokeless tobacco (ST) exposed oral cell cultures by differential display. This study determined the clinical significance of PI synthase overexpression in oral squamous cell carcinoma (OSCC) and premalignant lesions (leukoplakia), and identified the downstream signaling proteins in PI synthase pathway that are perturbed by smokeless tobacco (ST) exposure.

METHODS

Tissue microarray (TMA) Immunohistochemistry, Western blotting, Confocal laser scan microscopy, RT-PCR were performed to define the expression of PI synthase in clinical samples and in oral cell culture systems.

RESULTS

Significant increase in PI synthase immunoreactivity was observed in premalignant lesions and OSCCs as compared to oral normal tissues (p = 0.000). Further, PI synthase expression was significantly associated with de-differentiation of OSCCs, (p = 0.005) and tobacco consumption (p = 0.03, OR = 9.0). Exposure of oral cell systems to smokeless tobacco (ST) in vitro confirmed increase in PI synthase, Phosphatidylinositol 3-kinase (PI3K) and cyclin D1 levels.

CONCLUSION

Collectively, increased PI synthase expression was found to be an early event in oral cancer and a target for smokeless tobacco.

A review of the analysis of tobacco-specific nitrosamines in biological matrices

Tobacco use constitutes a leading cause of mortality and morbidity worldwide. Tobacco-specific nitrosamines (TSNAs) are an important class of biomarkers for tobacco carcinogen uptake. The current review focuses on the issues and developments in analysis of these compounds in human biological matrices. The two most widely used techniques for TSNA bioanalysis are gas chromatography coupled with thermal energy analysis and liquid chromatography coupled with mass spectrometry, employing various sample preparation techniques. The review provides an overview of the tools and techniques currently available for TSNA bioanalysis that will help towards the ultimate goal of understanding the mechanisms of cancer caused by the use of tobacco products. A contrast and comparison of the important aspects of bioanalysis such as sample preparation, compound detection, and throughput is discussed for the thermal energy analysis- and mass spectrometry-based techniques. Complex sample extraction procedures, throughput, and the ability to validate are important issues of concern for the gas chromatography-thermal energy analysis-based methods. On the other hand, addressing ion suppression matrix effects remains an important challenge for hyphenated mass spectrometry-based methods. The review also provides an extensive summary of analytical procedures for various studies measuring tobacco-specific nitrosamines in different biological matrices.

The Cys326 allele of the 8-oxoguanine DNA N-glycosylase 1 gene as a risk factor in smoking- and drinking-associated larynx cancer

Tobacco smoke-related products and ethanol would induce oxidative modifications to the DNA bases, thereby contributing to larynx cancer. Human 8-oxoguanine DNA N-glycosylase 1 (hOGG1) deals with oxidative DNA damage, and the base changes in the hOGG1 gene may alter the susceptibility of the human cells to tobacco smoke-related compounds and/or ethanol. In the present work, we investigated the association between smoking, drinking or the Ser326Cys polymorphism of the hOGG1 gene and the risk of larynx cancer in a Polish population. It has been reported that the Ser326 allele exhibits higher activity than the Cys326 variant. In this study, 253 age-matched controls and 253 patients with larynx cancer were enrolled. The polymorphism was determined with DNA from blood lymphocytes by polymerase chain reaction. The frequencies (%) of the genotypes were Ser/Ser 65.6, Ser/Cys 30.4, and Cys/Cys 4.0 in the controls and those in patients were 55.7, 36.0 and 8.3, respectively. Stratification of individuals according to their smoking and drinking habits indicated that these habits might be significant risk factors in larynx cancer. The Ser/Cys and Cys/Cys genotypes are significantly associated with the increased risk of larynx cancer. These genotypes increased the risk ratio of larynx cancer among heavy smokers, but did not change the risk in former smokers and moderate smokers. These genotypes also increased the risk of larynx cancer in moderate and heavy drinkers. Therefore, the Cys326 allele of the hOGG1 gene may increase the risk of larynx cancer associated with smoking or alcohol consumption.

A small-cell lung cancer genome with complex signatures of tobacco exposure

Cancer is driven by mutation. Worldwide, tobacco smoking is the principal lifestyle exposure that causes cancer, exerting carcinogenicity through >60 chemicals that bind and mutate DNA. Using massively parallel sequencing technology, we sequenced a small-cell lung cancer cell line, NCI-H209, to explore the mutational burden associated with tobacco smoking. A total of 22,910 somatic substitutions were identified, including 134 in coding exons. Multiple mutation signatures testify to the cocktail of carcinogens in tobacco smoke and their proclivities for particular bases and surrounding sequence context. Effects of transcription-coupled repair and a second, more general, expression-linked repair pathway were evident. We identified a tandem duplication that duplicates exons 3-8 of CHD7 in frame, and another two lines carrying PVT1-CHD7 fusion genes, indicating that CHD7 may be recurrently rearranged in this disease. These findings illustrate the potential for next-generation sequencing to provide unprecedented insights into mutational processes, cellular repair pathways and gene networks associated with cancer.

Acetaldehyde adducts in alcoholic liver disease

Chronic alcohol abuse causes liver disease that progresses from simple steatosis through stages of steatohepatitis, fibrosis, cirrhosis, and eventually hepatic failure. In addition, chronic alcoholic liver disease (ALD), with or without cirrhosis, increases risk for hepatocellular carcinoma (HCC). Acetaldehyde, a major toxic metabolite, is one of the principal culprits mediating fibrogenic and mutagenic effects of alcohol in the liver. Mechanistically, acetaldehyde promotes adduct formation, leading to functional impairments of key proteins, including enzymes, as well as DNA damage, which promotes mutagenesis. Why certain individuals who heavily abuse alcohol, develop HCC (7.2-15%) versus cirrhosis (15-20%) is not known, but genetics and co-existing viral infection are considered pathogenic factors. Moreover, adverse effects of acetaldehyde on the cardiovascular system and hematologic systems leading to ischemia, heart failure, and coagulation disorders, can exacerbate hepatic injury and increase risk for liver failure. Herein, we review the role of acetaldehyde adducts in the pathogenesis of chronic ALD and HCC.

Is it time to advance the chemoprevention of environmental carcinogenesis with microdosing trials?

This perspective on Jubert et al. (beginning on page [1015] in this issue of the journal) discusses the use of microdosing with environmental carcinogens to accelerate the evaluation and optimization of chemopreventive interventions. The need for chemoprevention of environmental carcinogenesis is considered, as are the structure of microdosing, or phase 0, trials, technologies required to conduct microdose studies in this context, and ethical concerns. We also reflect on what microdosing studies have taught us to date.

Exposure to the carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in smokers from 3 populations with different risks of lung cancer

Native Hawaiian smokers are at higher risk and Japanese-American smokers at lower risk of lung cancer (LC), compared with white smokers, even after accounting for smoking history. Because variation in carcinogen exposure/metabolism may occur separately of smoking amount, we compared urinary biomarkers of uptake and detoxification of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-a potent lung carcinogen-among 578 smokers in these ethnic/racial groups in Hawaii. We measured the NNK metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its glucuronide (NNAL-Gluc) and examined total NNAL (NNAL + NNAL-Gluc) and the NNAL detoxification ratio (NNAL-Gluc:NNAL). Native Hawaiians and Japanese-Americans had lower age- and sex-adjusted mean total NNAL, compared with whites. When further adjusting for urinary nicotine equivalents (the sum of nicotine, cotinine, trans-3'-hydroxycotinine and their respective glucuronides), only the difference between Japanese-Americans and whites was eliminated. Therefore, consistent with their lower LC risk, a lower cigarette smoke exposure explains the lower NNK dose of Japanese-Americans, but it does not explain that of Native Hawaiians. The mean detoxification ratio was also lower in Native Hawaiians and Japanese-Americans, compared with whites, even after adjusting for nicotine equivalents (p < 0.0001). Lower NNAL glucuronidation in Native Hawaiians might contribute to their increased LC risk; however, this is inconsistent with the low glucuronidation ratio similarly observed in the low-risk Japanese-American group and because Native Hawaiians had lower total NNAL levels. Thus, exposure and detoxification of NNK are unlikely to explain, by themselves, the differences in LC risk among the 3 populations studied.

DNA damage response induced by tobacco smoke in normal human bronchial epithelial and A549 pulmonary adenocarcinoma cells assessed by laser scanning cytometry

Cigarette smoke (CS) is a major cause of lung cancer and a contributor to the development of a wide range of other malignancies. There is an acute need to develop a methodology that can rapidly assess the potential carcinogenic properties of the genotoxic agents present in CS. We recently reported that exposure of normal human bronchial epithelial cells (NHBEs) or A549 pulmonary carcinoma cells to CS induces the activation of ATM through its phosphorylation on Ser1981 and phosphorylation of histone H2AX on Ser139 (gammaH2AX) most likely in response to the formation of potentially carcinogenic DNA double-strand breaks (DSBs). To obtain a more complete view of the DNA damage response (DDR) we explored the correlation between ATM activation, H2AX phosphorylation, activation of Chk2 through its phosphorylation on Thr68, and phosphorylation of p53 on Ser15 in NHBE and A549 cell exposed to CS. Multiparameter analysis by laser scanning cytometry made it possible to relate these DDR events, detected immunocytochemically, with cell cycle phase. The CS-dose-dependent induction and increase in the extent of phosphorylation of ATM, Chk2, H2AX, and p53 were seen in both cell types. ATM and Chk2 were phosphorylated approximately 1 h prior to phosphorylation of H2AX and p53. The dephosphorylation of ATM, Chk2, and H2AX was seen after 2 h following CS exposure. The dose-dependency and kinetics of DDR were essentially similar in both cell types, which provide justification for the use of A549 cells in the assessment of genotoxicity of CS in lieu of normal bronchial epithelial cells. The observation that DDR was more pronounced in S-phase cells is consistent with the mechanism of induction of DSBs occurring as a result of collision of replication forks with primary lesions such as DNA adducts that can be caused by CS-generated oxidants. The cytometric assessment of CS-induced DDR provides a means to estimate the genotoxicity of CS and to explore the mechanisms of the response as a function of cell cycle phase and cell type.

Chronic inflammation, chronic obstructive pulmonary disease, and lung cancer

PURPOSE OF REVIEW

Smoking is a major risk factor for lung cancer, which is the leading cause of cancer-related deaths both in the USA and worldwide. Chronic obstructive pulmonary disease and emphysema are comorbid conditions often found in lung cancer patients. The inflammatory pathways that link chronic obstructive pulmonary disease, emphysema, and lung cancer likely involve genetic and epigenetic modulations due to chronic tissue injury and abnormal tumor immunity in susceptible hosts.

RECENT FINDINGS

Chronic airway inflammation contributes to alterations in the bronchial epithelium and lung microenvironment, provoking a milieu conducive to pulmonary carcinogenesis. For example, inflammation-inducible cyclooxygenase-2 is upregulated in nonsmall cell lung cancer and also plays an important role in promoting epithelial-to-mesenchymal transition. Genetic changes in the airway epithelium of smokers may help predict or identify individuals at risk for lung cancer. Finally, radiographic findings of emphysema have been established as independent risk factors for lung cancer.

SUMMARY

The relationships between inflammation, airflow obstruction, and lung cancer are complex. Deregulated inflammation is complicit in the pathogenesis of chronic obstructive pulmonary disease and lung cancer, but the overlap of signaling events is not yet fully understood. Tobacco exposure is an important risk factor that confers long-term risk of lung disease. Diagnostic sensitivity of detecting lung cancer may improve with the utilization of genetic profiling in combination with pathologic evaluation of airway epithelium. Additional research is required to understand the role of epithelial-to-mesenchymal transition in chronic inflammatory lung diseases and lung carcinogenesis.

Evaluation of the DNA damaging potential of cannabis cigarette smoke by the determination of acetaldehyde derived N2-ethyl-2'-deoxyguanosine adducts

Acetaldehyde is an ubiquitous genotoxic compound that has been classified as a possible carcinogen to humans. It can react with DNA to form primarily a Schiff base N(2)-ethylidene-2'-deoxyguanosine (N(2)-ethylidene-dG) adduct. An online column-switching valve liquid chromatography tandem mass spectrometry (LC-MS/MS) selected reaction monitoring (SRM) method was developed for the determination of N(2)-ethylidene-dG adducts in DNA following reduction with sodium cyanoborohydride (NaBH(3)CN) to the chemically stable N(2)-ethyl-2'-deoxyguanosine (N(2)-ethyl-dG) adduct. Accurate quantitation of the adduct was obtained by the addition of the [(15)N(5)]N(2)-ethyl-dG stable isotope-labeled internal standard prior to enzymatic hydrolysis of the DNA samples to 2'-deoxynucleosides with the incorporation of NaBH(3)CN in the DNA hydrolysis buffer. The method required 50 microg of hydrolyzed DNA on column for the analysis, and the limit of detection for N(2)-ethyl-dG was 2.0 fmol. The analysis of calf thymus DNA treated in vitro with acetaldehyde (ranging from 0.5 to 100 mM) or with the smoke generated from 1, 5, and 10 cannabis cigarettes showed linear dose-dependent increases in the level of N(2)-ethyl-dG adducts (r = 0.954 and r = 0.999, respectively). Similar levels (332.8 +/- 21.9 vs 348.4 +/- 19.1 adducts per 10(8) 2'-deoxynucleosides) of N(2)-ethyl-dG adducts were detected following the exposure of calf thymus DNA to 10 tobacco or 10 cannabis cigarettes. No significant difference was found in the levels of N(2)-ethyl-dG adducts in human lung DNA obtained from nonsmokers (n = 4) and smokers (n = 4) with the average level observed as 13.3 +/- 0.7 adducts per 10(8) 2'-deoxynucleosides. No N(2)-ethyl-dG adducts were detected in any of the DNA samples following analysis with the omission of NaBH(3)CN from the DNA hydrolysis buffer. In conclusion, these results provide evidence for the DNA damaging potential of cannabis smoke, implying that the consumption of cannabis cigarettes may be detrimental to human health with the possibility to initiate cancer development.

Blood-based CHRNA3 single nucleotide polymorphism and outcome in advanced non-small-cell lung cancer patients

Nicotine acetylcholine receptors (nAChRs) are associated with resistance to gemcitabine, cisplatin and paclitaxel in non-small-cell lung cancer (NSCLC) cell lines. Three single nucleotide polymorphisms (SNPs) of CHRNA3, CHRNA5 and LOC123688 increase lung cancer risk. These SNPs may have influenced outcome in patients treated in our phase III trial. Stage IV NSCLC patients were treated with customized chemotherapy based on ERCC1 (excision repair cross-complementing 1) mRNA expression. Patients in the control arm received docetaxel/cisplatin; patients in the genotypic arm with low levels of ERCC1 received docetaxel/cisplatin; patients in the genotypic arm with high levels of ERCC1 received docetaxel/gemcitabine. DNA was extracted from lymphocytes, and CHRNA3 (rs1051730), CHRNA5 (rs16969968) and LOC123688 (rs8034191) SNPs were genotyped with the Taqman allele discrimination assay. A significant interaction was found for CHRNA3 and PS (P=0.02). In patients with PS 0, CT patients had a better response than both CC (P=0.01) and TT (P=0.02) patients, and patients in the low genotypic group also had a better response (P=0.01). When the CHRNA3 genotype was added in the multivariate analysis for progression-free survival, an improvement was observed in the low genotypic group in PS 0 patients (P=0.02). PS 0 patients in the low genotypic group with the CT genotype attained an 84% response rate, 12.1-month progression-free survival, and 19-month median survival. CHRNA3 (rs1051730) genotyping can improve customized chemotherapy based on tumor assessment of ERCC1 mRNA in stage IV NSCLC with PS 0.

Cigarette smoking and risk of colorectal cancer among Norwegian women

OBJECTIVE

The association between cigarette smoking and colorectal cancer (CRC) is still not established. In 2002, Norwegian women had the second highest incidence of CRC in the world. A large proportion of Norwegian women are ever smokers. We examined the association between cigarette smoking and CRC incidence among Norwegian women.

METHODS

We followed 68,160 women, aged 30-69 years, from the Norwegian Women and Cancer Study who completed a questionnaire in 1996 or 1998 by linkages to national registers through 31 December 2005. Rate ratios (RRs) and 95% confidence intervals (CIs) were estimated by fitting Cox proportional hazard models. Subsequently, we estimated the population attributable fraction.

RESULTS

Altogether, 425 incident cases of primary, invasive CRC were identified. Ever smokers had a 20% increased risk of CRC (RR = 1.2; 95% CI = 1.0-1.5), a 30% increased risk of colon (RR = 1.3; 95% CI = 1.0-1.7), and a 10% increased risk of rectal (RR = 1.1; 95% CI = 0.7-1.5) cancer compared to never smokers. The population attributable fraction was estimated to be 12% which indicated that approximately one in eight of the CRC cases could have been prevented at a population level.

CONCLUSION

Our results support the hypothesis that cigarette smoking is a preventable cause of CRC among women.

Thyroid hormone levels associated with active and passive cigarette smoking

BACKGROUND

Active and passive cigarette smoking are a risk factor among women of reproductive age-leading to reproductive health morbidity, including fetal and infant death and developmental problems with the newborn. However, the underlying physiological mechanisms for these ill-effects are not fully understood. Smoke exposure may affect various metabolic and biological processes, including hormone biosynthesis and secretion, interfere with thyroid hormone release, binding, transport, storage, and clearance, associated with adverse effects on the thyroid resulting in changes in circulating hormone concentrations. We measured and compared serum thyroid hormone and thyroid stimulating hormone (TSH) concentrations in active, passive, and nonsmokers and determined their association with cigarette tobacco smoke exposure. We use a comprehensive approach to assess the interrelationships between active and passive tobacco smoke exposure and thyroid hormone levels by employing innovative hormone analysis techniques.

METHODS

Serum was obtained from women (18-44 years of age). Thyroxine (T4), triiodothyronine (T3), and cotinine concentrations were quantified using isotope dilution high performance liquid chromatography tandem mass spectrometry, and TSH concentrations by chemiluminescence.

RESULTS

Serum concentrations of the various hormones of active smokers, passive smokers, and nonsmokers (nonexposed), respectively, were as follows. Median TSH concentrations were 1.02, 1.06, and 1.12 mIU/L (p < 0.001 for the comparison of each group with the other two groups), and mean TSH levels were 1.40 mIU/L, confidence interval (CI) 0.07-6.83 mIU/L; 1.30 mIU/L, CI 0.25-3.01 mIU/L; and 1.50 mIU/mL, CI 0.71-4.00 mIU/L. Median serum T4 concentrations were 7.6, 7.9, and 8.7 microg/dL, and median serum T3 concentrations were 92.0, 84.0, and 102.0 ng/dL (p < 0.0001). Mean T3 levels were 99.1 ng/dL, CI 52.1-204.3 ng/dL; 87.6 ng/dL, CI 40.1-160.2 ng/dL; and 106.6 ng/dL, CI 46.4-175.0 ng/dL. Pair-wise comparisons of the three study groups indicate statistically significant differences in serum T4 (p < 0.01) and T3 (p < 0.001) means for the comparison of each group with the other two groups.

CONCLUSIONS

This study is unique in examining the association of serum cotinine and thyroid hormone concentrations using liquid chromatography tandem mass spectrometry in women smokers, passive smokers, and nonsmokers. Active and passive exposure to cigarette tobacco smoke is associated with a mild inhibitory effect on the thyroid reflected in higher serum T4 and T3 in nonsmokers compared to smokers in this cohort of women.

Cancer prevention: from 1727 to milestones of the past 100 years

The rich, multidisciplinary history of cancer prevention recounted here begins with surgical and workplace recommendations of the 1700s and ends with 2009 results of the enormous (35,535 men) Selenium and Vitamin E [prostate] Cancer Prevention Trial (SELECT). This history comprises a fascinating array of chemopreventive, vaccine, surgical, and behavioral science research, both preclinical and clinical. Preclinical milestones of cancer prevention include the 1913 and 1916 mouse studies by Lathrop and Loeb of cancer development associated with pregnancy or cancer prevention through castration (oophorectomy), preventing chemically induced mouse carcinogenesis as early as 1929, energy restriction studies in the 1940s, the 1950s discoveries and later molecular characterizations of field cancerization and multistep carcinogenesis, and the effects of angiogenesis inhibition in genetically engineered mice reported in 2009. The extraordinary panoply of clinical research includes numerous large and smaller chemoprevention studies of nutritional supplements, other dietary approaches, a Bacillus Calmette-Guérin trial in 1976, molecular-targeted agents, and agents to prevent infection-related cancers such as hepatitis B virus vaccine to prevent liver cancer in 1984. Clinical surgical prevention includes removal of intraepithelial neoplasia detected by screening (including Pap testing developed in 1929 and culposcopy for cervical premalignancy and colonoscopy and polypectomy to prevent colorectal cancer begun in the 1960s) and prophylactic surgeries, such as in Lynch syndrome patients begun in 1977. Behavioral studies include smoking cessation and control beginning in the 1950s, obesity control rooted in studies of 1841, and genetic-counseling and cancer-survivorship studies. This history of pioneering events may help in better understanding who we are and what we want to achieve as cancer prevention researchers and practitioners.

Comparative levels of O6-methylguanine, pyridyloxobutyl-, and pyridylhydroxybutyl-DNA adducts in lung and liver of rats treated chronically with the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a lung carcinogen in rats and may be a cause of lung cancer in smokers. NNK is metabolized by cytochromes P450 to intermediates that react with DNA forming methyl, pyridyloxobutyl (POB), and pyridylhydroxybutyl (PHB) adducts, which are critical in carcinogenesis. The methyl adduct O(6)-methylguanine (O(6)-methyl-G) has miscoding properties, but there are no reports on levels of this adduct in rats treated chronically with NNK in the drinking water, nor has its levels been compared with those of POB- and PHB-DNA adducts. We used liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring to quantify O(6)-methyl-G in lung and liver DNA of rats treated with a carcinogenic dose of 10 ppm of NNK in the drinking water and sacrificed after 1, 2, 5, 10, 16, and 20 weeks. The maximal level of O(6)-methyl-G in lung DNA, 2550 +/- 263 fmol/mg DNA, was reached at 5 weeks and was significantly greater (P < 0.05) at that point than all other adducts (measured previously) except O(2)-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine. Overall levels of O(6)-methyl-G in lung were intermediate between those of total POB- and PHB-DNA adducts. In liver, the wave of O(6)-methyl-G peaked at 2 weeks while that of total POB-DNA adducts peaked at 10 weeks, and levels of total PHB-DNA adducts were low throughout. The results of this study demonstrate that substantial amounts of O(6)-methyl-G are formed at various time points in lung and liver DNA of rats treated chronically with NNK, supporting its role in carcinogenesis.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Effects of smoking cessation on eight urinary tobacco carcinogen and toxicant biomarkers

We determined the persistence at various times (3, 7, 14, 21, 28, 42, and 56 days) of eight tobacco smoke carcinogen and toxicant biomarkers in the urine of 17 smokers who stopped smoking. The biomarkers were 1-hydroxy-2-(N-acetylcysteinyl)-3-butene (1) and 1-(N-acetylcysteinyl)-2-hydroxy-3-butene (2) [collectively called MHBMA for monohydroxybutyl mercapturic acid] and 1,2-dihydroxy-4-(N-acetylcysteinyl)butane (3) [DHBMA for dihydroxybutyl mercapturic acid], metabolites of 1,3-butadiene; 1-(N-acetylcysteinyl)-propan-3-ol (4, HPMA for 3-hydroxypropyl mercapturic acid), a metabolite of acrolein; 2-(N-acetylcysteinyl)butan-4-ol (5, HBMA for 4-hydroxybut-2-yl mercapturic acid), a metabolite of crotonaldehyde; (N-acetylcysteinyl)benzene (6, SPMA for S-phenyl mercapturic acid), a metabolite of benzene; (N-acetylcysteinyl)ethanol (7, HEMA for 2-hydroxyethyl mercapturic acid), a metabolite of ethylene oxide; 1-hydroxypyrene (8) and its glucuronides (1-HOP), metabolites of pyrene; and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (9) and its glucuronides (total NNAL), a biomarker of exposure to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). These biomarkers represent some of the major carcinogens and toxicants in cigarette smoke: 1,3-butadiene, acrolein, crotonaldehyde, benzene, ethylene oxide, polycyclic aromatic hydrocarbons (PAH), and NNK. With the exception of DHBMA, levels of which did not change after cessation of smoking, all other biomarkers decreased significantly after 3 days of cessation (P < 0.001). The decreases in MHBMA, HPMA, HBMA, SPMA, and HEMA were rapid, nearly reaching their ultimate levels (81-91% reduction) after 3 days. The decrease in total NNAL was gradual, reaching 92% after 42 days, while reduction in 1-HOP was variable among subjects to about 50% of baseline. Since DHBMA did not change upon smoking cessation, there appear to be sources of this metabolite other than 1,3-butadiene. The results of this study demonstrate that the tobacco smoke carcinogen/toxicant biomarkers MHBMA, HPMA, HBMA, SPMA, HEMA, 1-HOP, and NNAL are related to smoking and are good indicators of the impact of smoking on human exposure to 1,3-butadiene, acrolein, crotonaldehyde, benzene, ethylene oxide, PAH, and NNK.

Biomarkers of induced active and passive smoking damage

In addition to the well-known link between smoking and lung cancer, large epidemiological studies have shown a relationship between smoking and cancers of the nose, oral cavity, oropharynx, larynx, esophagus, pancreas, bladder, kidney, stomach, liver, colon and cervix, as well as myeloid leukemia. Epidemiological evidence has reported a direct link between exposure of non-smokers to environmental tobacco smoke and disease, most notably, lung cancer. Much evidence demonstrates that carcinogenic-DNA adducts are useful markers of tobacco smoke exposure, providing an integrated measurement of carcinogen intake, metabolic activation, and delivery to the DNA in target tissues. Monitoring accessible surrogate tissues, such as white blood cells or bronchoalveolar lavage (BAL) cells, also provides a means of investigating passive and active tobacco exposure in healthy individuals and cancer patients. Levels of DNA adducts measured in many tissues of smokers are significantly higher than in non-smokers. While some studies have demonstrated an association between carcinogenic DNA adducts and cancer in current smokers, no association has been observed in ex or never smokers. The role of genetic susceptibility in the development of smoking related-cancer is essential. In order to establish whether smoking-related DNA adducts are biomarkers of tobacco smoke exposure and/or its carcinogenic activity we summarized all data that associated tobacco smoke exposure and smoking-related DNA adducts both in controls and/or in cancer cases and studies where the effect of genetic polymorphisms involved in the activation and deactivation of carcinogens were also evaluated. In the future we hope we will be able to screen for lung cancer susceptibility by using specific biomarkers and that subjects of compared groups can be stratified for multiple potential modulators of biomarkers, taking into account various confounding factors.

Signal transduction molecules as targets for cancer prevention

Environmental and life-style aspects are major contributors to human carcinogenesis and, therefore, many human cancers may be preventable. Cancer is the end result of defects in cellular signaling processes that play a key role in the control of cell growth, survival, division, and differentiation. Therefore, identifying molecular and cellular targets critical in cancer development and prevention is an area of intensive research, driving the development of highly specific small-molecule inhibitors. A major idea today is that cancer may be prevented or treated by targeting the products of specific cancer-related genes, frequently encoding signaling proteins or transcription factors. Participants in these joint conferences discussed their latest findings in the identification of promising molecular targets and the development of agents directed against these targets with the goal of effectively transitioning these into the clinical setting.

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.

Smokers with the CHRNA lung cancer-associated variants are exposed to higher levels of nicotine equivalents and a carcinogenic tobacco-specific nitrosamine

A locus at 15q24/15q25.1, which includes the nicotinic acetylcholine receptor A subunits 3 and 5 (CHRNA3 and CHRNA5) genes, has recently been associated with lung cancer risk, self-reported number of cigarettes smoked per day, and a nicotine dependence scale. It is not clear whether the association with lung cancer is direct or mediated through differences in smoking behavior. We used urinary biomarkers to test whether two linked lung cancer risk variants in CHRNA3 (rs1051730) and CHRNA5 (rs16969968) are associated with intensity of smoking and exposure to a tobacco-specific carcinogenic nitrosamine per cigarette dose. We studied 819 smokers and found that carriers of these variants extract a greater amount of nicotine (P = 0.003) and are exposed to a higher internal dose of 4-(methylnitrosamino)-I-(3-pyridyl)-1-butanone (P = 0.03) per cigarette than noncarriers. Thus, smokers who carry the CHRNA3 and CHRNA5 variants are expected to be at increased risk for lung cancer compared with smokers who do not carry these alleles even if they smoked the same number of cigarettes. Number of cigarettes per day, even if it could be accurately assessed, is not an adequate measure of smoking dose.