What is the significance of increases in background levels of carcinogen-derived protein and DNA adducts? Some considerations for incremental risk assessment

Mode of action-based risk assessment of genotoxic carcinogens

The risk assessment of chemical carcinogens is one major task in toxicology. Even though exposure has been mitigated effectively during the last decades, low levels of carcinogenic substances in food and at the workplace are still present and often not completely avoidable. The distinction between genotoxic and non-genotoxic carcinogens has traditionally been regarded as particularly relevant for risk assessment, with the assumption of the existence of no-effect concentrations (threshold levels) in case of the latter group. In contrast, genotoxic carcinogens, their metabolic precursors and DNA reactive metabolites are considered to represent risk factors at all concentrations since even one or a few DNA lesions may in principle result in mutations and, thus, increase tumour risk. Within the current document, an updated risk evaluation for genotoxic carcinogens is proposed, based on mechanistic knowledge regarding the substance (group) under investigation, and taking into account recent improvements in analytical techniques used to quantify DNA lesions and mutations as well as "omics" approaches. Furthermore, wherever possible and appropriate, special attention is given to the integration of background levels of the same or comparable DNA lesions. Within part A, fundamental considerations highlight the terms hazard and risk with respect to DNA reactivity of genotoxic agents, as compared to non-genotoxic agents. Also, current methodologies used in genetic toxicology as well as in dosimetry of exposure are described. Special focus is given on the elucidation of modes of action (MOA) and on the relation between DNA damage and cancer risk. Part B addresses specific examples of genotoxic carcinogens, including those humans are exposed to exogenously and endogenously, such as formaldehyde, acetaldehyde and the corresponding alcohols as well as some alkylating agents, ethylene oxide, and acrylamide, but also examples resulting from exogenous sources like aflatoxin B1, allylalkoxybenzenes, 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx), benzo[a]pyrene and pyrrolizidine alkaloids. Additionally, special attention is given to some carcinogenic metal compounds, which are considered indirect genotoxins, by accelerating mutagenicity via interactions with the cellular response to DNA damage even at low exposure conditions. Part C finally encompasses conclusions and perspectives, suggesting a refined strategy for the assessment of the carcinogenic risk associated with an exposure to genotoxic compounds and addressing research needs.

Drinking water as a proportion of total human exposure to volatile N-nitrosamines

Some volatile N-nitrosamines, primarily N-nitrosodimethylamine (NDMA), are recognized as products of drinking water treatment at ng/L levels and as known carcinogens. The U.S. EPA has identified the N-nitrosamines as contaminants being considered for regulation as a group under the Safe Drinking Water Act. Nitrosamines are common dietary components, and a major database (over 18,000 drinking water samples) has recently been created under the Unregulated Contaminant Monitoring Rule. A Monte Carlo modeling analysis in 2007 found that drinking water contributed less than 2.8% of ingested NDMA and less than 0.02% of total NDMA exposure when estimated endogenous formation was considered. Our analysis, based upon human blood concentrations, indicates that endogenous NDMA production is larger than expected. The blood-based estimates are within the range that would be calculated from estimates based on daily urinary NDMA excretion and an estimate based on methylated guanine in DNA of lymphocytes from human volunteers. Our analysis of ingested NDMA from food and water based on Monte Carlo modeling with more complete data input shows that drinking water contributes a mean proportion of the lifetime average daily NDMA dose ranging from between 0.0002% and 0.001% for surface water systems using free chlorine or between 0.001% and 0.01% for surface water systems using chloramines. The proportions of average daily dose are higher for infants (zero to six months) than other age cohorts, with the highest mean up to 0.09% (upper 95th percentile of 0.3%).

Quantitative comparison between in vivo DNA adduct formation from exposure to selected DNA-reactive carcinogens, natural background levels of DNA adduct formation and tumour incidence in rodent bioassays

This study aimed at quantitatively comparing the occurrence/formation of DNA adducts with the carcinogenicity induced by a selection of DNA-reactive genotoxic carcinogens. Contrary to previous efforts, we used a very uniform set of data, limited to in vivo rat liver studies in order to investigate whether a correlation can be obtained, using a benchmark dose (BMD) approach. Dose-response data on both carcinogenicity and in vivo DNA adduct formation were available for six compounds, i.e. 2-acetylaminofluorene, aflatoxin B1, methyleugenol, safrole, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline and tamoxifen. BMD(10) values for liver carcinogenicity were calculated using the US Environmental Protection Agency BMD software. DNA adduct levels at this dose were extrapolated assuming linearity of the DNA adduct dose response. In addition, the levels of DNA adducts at the BMD(10) were compared to available data on endogenous background DNA damage in the target organ. Although for an individual carcinogen the tumour response increases when adduct levels increase, our results demonstrate that when comparing different carcinogens, no quantitative correlation exists between the level of DNA adduct formation and carcinogenicity. These data confirm that the quantity of DNA adducts formed by a DNA-reactive compound is not a carcinogenicity predictor but that other factors such as type of adduct and mutagenic potential may be equally relevant. Moreover, comparison to background DNA damage supports the notion that the mere occurrence of DNA adducts above or below the level of endogenous DNA damage is neither correlated to development of cancer. These data strongly emphasise the need to apply the mode of action framework to understand the contribution of other biological effect markers playing a role in carcinogenicity.

Biomarkers in natural fish populations indicate adverse biological effects of offshore oil production

Background

Despite the growing awareness of the necessity of a sustainable development, the global economy continues to depend largely on the consumption of non-renewable energy resources. One such energy resource is fossil oil extracted from the seabed at offshore oil platforms. This type of oil production causes continuous environmental pollution from drilling waste, discharge of large amounts of produced water, and accidental spills.

Methods and principal findings

Samples from natural populations of haddock (Melanogrammus aeglefinus) and Atlantic cod (Gadus morhua) in two North Sea areas with extensive oil production were investigated. Exposure to and uptake of polycyclic aromatic hydrocarbons (PAHs) were demonstrated, and biomarker analyses revealed adverse biological effects, including induction of biotransformation enzymes, oxidative stress, altered fatty acid composition, and genotoxicity. Genotoxicity was reflected by a hepatic DNA adduct pattern typical for exposure to a mixture of PAHs. Control material was collected from a North Sea area without oil production and from remote Icelandic waters. The difference between the two control areas indicates significant background pollution in the North Sea.

Conclusion

It is most remarkable to obtain biomarker responses in natural fish populations in the open sea that are similar to the biomarker responses in fish from highly polluted areas close to a point source. Risk assessment of various threats to the marine fish populations in the North Sea, such as overfishing, global warming, and eutrophication, should also take into account the ecologically relevant impact of offshore oil production.

DNA adduct levels in fish from pristine areas are not detectable or low when analysed using the nuclease P1 version of the32P-postlabelling technique

In order to understand and apply DNA adduct formation in fish liver as a biomarker for aquatic pollution, information concerning the natural background levels in noncontaminated organisms, caused by endogenous compounds, is of fundamental importance. In this study, DNA adducts were analysed in liver of 11 fish species from arctic and sub-arctic areas in the northern Atlantic using the nuclease P1 version of the 32P-postlabelling technique. The collected fish were assumed not to have been influenced by anthropogenic pollution apart from possible long-range transported pollutants. As polycyclic aromatic hydrocarbons (PAHs) are thought to be fundamental in forming the type of DNA adducts detected by the method used, biliary PAH metabolite levels were measured in a selection of the investigated species. In all investigated individuals, the levels of PAH metabolites were undetectable. Controlled on-site exposure experiments with benzo[a]pyrene (polar cod) and laboratory experiments with crude oil (polar cod and Atlantic cod) were conducted. DNA adducts were formed in both these species. The field-sampled fish showed undetectable levels of DNA adducts or levels just above the detection limit. The present study supports the assumption that when DNA adducts are detected by the nuclease P1 version of the 32P-postlabelling method in fish liver, it can be interpreted as DNA damage caused by pollutants.

Effects of environmental air pollution on endogenous oxidative DNA damage in humans

Epidemiological studies conducted in metropolitan areas have demonstrated that exposure to environmental air pollution is associated with increases in mortality. Carcinogenic polycyclic aromatic hydrocarbons (c-PAHs) are the major source of genotoxic activities of organic mixtures associated with respirable particulate matter, which is a constituent of environmental air pollution. In this study,we wanted to evaluate the relationship between exposure to these genotoxic compounds present in the air and endogenous oxidative DNA damage in three different human populations exposed to varying levels of environmental air pollution. As measures of oxidative DNA damage we have determined 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and cyclic pyrimidopurinone N-1,N(2) malondialdehyde-2'-deoxyguanosine (M(1)dG) by the immunoslot blot assay from lymphocyte DNA of participating individuals. The level of endogenous oxidative DNA damage was significantly increased in individuals exposed to environmental air pollution compared to unexposed individuals from Kosice (8-oxodG adducts) and Sofia (M(1)dG adducts). However, there was no significant difference in the level of endogenous oxidative DNA and exposure to environmental air pollution in individuals from Prague (8-oxodG and M(1)dG adducts) and Kosice (M(1)dG adducts). The average level of M(1)dG adducts was significantly lower in unexposed and exposed individuals from Kosice compared to those from Prague and Sofia. The average level of 8-oxodG adducts was significantly higher in unexposed and exposed individuals from Kosice compared to those from Prague. A significant increasing trend according to the interaction of c-PAHs exposure and smoking status was observed in levels of 8-oxodG adducts in individuals from Kosice. However, no other relationship was observed for M(1)dG and 8-oxodG adduct levels with regard to the smoking status and c-PAH exposure status of the individuals. The conclusion that can be made from this study is that environmental air pollution may alter the endogenous oxidative DNA damage levels in humans but the effect appears to be related to the country where the individuals reside. Genetic polymorphisms of the genes involved in metabolism and detoxification and also differences in the DNA repair capacity and antioxidant status of the individuals could be possible explanations for the variation observed in the level of endogenous oxidative DNA damage for the different populations.

Biological significance of DNA adducts investigated by simultaneous analysis of different endpoints of genotoxicity in L5178Y mouse lymphoma cells treated with methyl methanesulfonate

The biological significance of DNA adducts is under continuous discussion because analytical developments allow determination of adducts at ever lower levels. Central questions refer to the biological consequences of adducts and to the relationship between background DNA damage and exposure-related increments. These questions were addressed by measuring the two DNA adducts 7-methylguanine (7-mG) and O(6)-methyl-2'-deoxyguanosine (O(6)-mdGuo) by LC-MS/MS in parallel to two biological endpoints of genotoxicity (comet assay and in vitro micronucleus test), using large batches of L5178Y mouse lymphoma cells treated with methyl methanesulfonate (MMS). The background level of 7-mG was 1440 adducts per 10(9) nucleotides while O(6)-mdGuo was almost 50-fold lower (32 adducts per 10(9) nucleotides). In the comet assay and the micronucleus test, background was in the usual range seen with smaller batches of cells (2.1% Tail DNA and 12 micronuclei-containing cells per 1000 binucleated cells, respectively). For the comparison of the four endpoints for dose-related increments above background in the low-response region we assumed linearity at low dose and used the concept of the "doubling dose", i.e., we estimated the concentration of MMS necessary to double the background measures. Doubling doses of 4.3 and 8.7microM MMS were deduced for 7-mG and O(6)-mdGuo, respectively. For doubling the background measures in the comet assay and the micronucleus test, 5 to 15-fold higher concentrations of MMS were necessary (45 and 66microM, respectively). This means that the contribution of an increase in DNA methylation to biological endpoints of genotoxicity is overestimated. For xenobiotics that generate adducts without background, the difference is even more pronounced because the dose-response curve starts at zero and the limit of detection of an increase is not affected by background variation. Consequences for the question of thresholds in dose-response relationships and for the setting of tolerable exposure levels are discussed.

Chemical Reactivity as a Tool to Study Carcinogenicity: Reaction between Chloroethylene Oxide and Guanine

Chloroethylene oxide, an ultimate carcinogen of vinyl chloride, reacts with DNA giving rise to 7-(2-oxyethyl)guanine adduct in a nearly quantitative yield. This reaction represents an initial step of carcinogenesis associated with vinyl chloride. From experimental data for this reaction we calculated the second-order rate constant of 0.049 s(-1) M(-1), which corresponds to the activation free energy of 19.5 kcal/mol. We also performed a series of medium high ab initio and density functional theory simulations. Effects of hydration were considered in the framework of the Langevine dipoles solvation model and the solvent reaction field method of Tomasi and co-workers. In silico calculated activation free energies are in a good agreement with the experimental value. This fact presents strong evidence in favor of the validity of the proposed reaction mechanism and points to the applicability of quantum-chemical methods to studies of other reactions associated with carcinogenesis. Insignificant stereoselectivity of the studied reaction was also predicted.

Guanine Alkylation by Ethylene Oxide: Calculation of Chemical Reactivity

In this paper we report on calculations of the activation free energy for a chemical reaction between ethylene oxide and guanine. Ethylene oxide is biologically relevant per se and is also a model compound for numerous ultimate carcinogens. Calculations were performed on the medium-high ab initio, DFT, and semiempirical MO levels. Effects of solvation were considered using the Langevine dipole method and solvent reaction field method of Tomasi and co-workers. The calculated activation free energies are in reasonable agreement with the experimental value.

The mode of action of organic carcinogens on cellular structures

Most genotoxic organic carcinogens require metabolic activation to exert their detrimental effects. The present review summarizes the mechanisms of how organic carcinogens are bioactivated into DNA-reactive descendants. Beginning with the history of discovery of some important human organic carcinogens, the text guides through the development of the knowledge on their molecular mode of action that has grown over the past decades. Some of the most important molecular mechanisms in chemical carcinogenesis, the role of the enzymes involved in bioactivation, the target gene structures of some ultimate carcinogenic metabolites, and implications for human cancer risk assessment are discussed.

An approach to investigating the importance of high potency polycyclic aromatic hydrocarbons (PAHs) in the induction of lung cancer by air pollution

Evidence suggests that people living in urban areas have an increased risk of lung cancer due to higher levels of air pollution in these areas. Benzo[a]pyrene (B[a]P) is currently used as the main indicator of carcinogenic polycyclic aromatic hydrocarbons (PAHs) in air pollution, but there is concern that B[a]P may not be the ideal surrogate of choice for PAH mixtures since higher potency PAHs have recently been identified which could potentially contribute more and variably to the overall carcinogenicity. Dibenzo[a,h]anthracene (DBA) and dibenzo[a,l]pyrene (DB[a,l]P) are estimated to have carcinogenic potencies 10 or more times greater than B[a]P but data on their presence and formation in the environment are limited. Several occupational and environmental PAH biomonitoring studies are reviewed here, with particular focus on the specific exposure groups, study design, sample tissue, in particular the use of nasal tissues, and biomarkers used in each study. Consideration of these data is then used to propose a novel biomonitoring approach to evaluate exposure, uptake and the role of high potency PAHs in air pollution-related lung cancer. This is based upon an occupational study examining specific DNA adducts for DBA and DB[a,l]P in nasal cells to evaluate the extent to which these high potency PAHs might contribute to the increased risk of developing lung cancer from air pollution.

Review of the genotoxicity of styrene in humans

Styrene (CAS No. 100-42-5) is an important industrial chemical for which positive results have been reported in in vitro and in vivo genotoxicity assays. Styrene-exposed workers have been studied extensively over two decades for the induction of various types of genotoxic effects. The outcomes of these studies have been conflicting, and where positive responses have been reported, it has proved difficult to demonstrate clear relationships between levels of damage reported and exposure levels. In this review, we have assessed studies addressing mutagenicity (chromosome aberrations, micronuclei and gene mutations) and other endpoints (sister chromatid exchanges, DNA breaks and DNA adducts) using criteria derived from the IPCS guidelines for the conduct of human biomonitoring studies. Based on the re-evaluated outcomes, the data are not convincing that styrene induces gene mutations. The evidence for induction of clastogenicity in occupationally exposed workers is less clear, with a predominant lack of induction of micronuclei in different studies, but conflicting responses in chromosome aberration assays. The results of numerous studies on sister chromatid exchanges do not provide evidence of a clear positive response, despite these being induced in animals exposed to styrene at high concentrations. However, there is evidence that both DNA adducts and DNA single strand breaks are induced in styrene workers. These types of damage are considered indicative of exposure of the target cells and interaction with cellular DNA but do not necessarily result in heritable changes. There is evidence that the metabolism of styrene in humans is affected by genetic polymorphisms of metabolizing genes and that these polymorphisms affect the outcome of in vitro mutagenicity studies on styrene. Therefore, studies that have addressed the potential of this factor to affect in vivo responses were considered. To date, there are no consistent relationships between genetic polymorphisms and induction of genotoxicity by styrene in humans, but further work is warranted on larger samples. The analyses of individual studies, together with a consideration of dose-response relationships and the lack of a common profile of positive responses for the various endpoints in different studies, provide no clear evidence that styrene exposure in workers results in detectable levels of mutagenic damage. However, evidence of exposure to genotoxic metabolites is demonstrated by the formation of DNA adducts and strand breaks.

Nature and nurture - lessons from chemical carcinogenesis

The roles of genetic constitution versus environmental factors in cancer development have been a matter of debate even long before the discovery of 'oncogenes'. Evidence from epidemiological, occupational and migration studies has consistently pointed to environmental factors as the major contributing factors to cancer, so it seems reasonable to discuss the importance of chemical carcinogenesis in the present 'age of cancer genetics'.

Kinetics and mechanism of the general-acid-catalyzed ring-closure of the malondialdehyde-DNA adduct, N2-(3-oxo-1-propenyl)deoxyguanosine (N2OPdG-), to 3-(2'-Deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1,2-alpha]purin- 10(3H)-one (M1dG)

3-(2'-Deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1,2-alpha]purin-10(3H)-one (M1dG) is the major product of the reaction of deoxyguanosine with malondialdehyde (MDA). M1dG blocks replication by DNA polymerases in vitro and is mutagenic in vivo. M1dG reacts with hydroxide to form the N2-(3-oxo-1-propenyl)deoxyguanosine anion (N2OPdG-). This reaction is pH-dependent and reverses under neutral and acidic conditions to form M1dG. Here we describe the kinetics and mechanism of the ring-closure reaction in both the nucleoside and oligonucleotides. Kinetic analysis of absorbance and fluorescence changes demonstrates that ring-closure is biphasic, leading to the rapid formation of an intermediate that slowly converts to M1dG in a general-acid-catalyzed reaction. The dependence of the rate of the rapid phase on pH reveals the pKa for protonated N2OPdG is 6.9. One-dimensional 1H NMR and DQF-COSY experiments identified two distinct intermediates, N2OPdG-H and 8-hydroxy-6,7-propenodeoxyguanosine (HO-Prene-dG), that are formed upon acidification of N2OPdG-. Characterization of ring-closure in single-stranded and in melted duplex oligonucleotides shows M1dG formation is also acid-catalyzed in single-stranded oligonucleotides and that the denaturation of an oligonucleotide duplex enhances ring-closure. This work details the complexity of ring-closure in the nucleoside and oligonucleotides and provides new insight into the role of duplex DNA in catalyzing ring-opening and ring-closing of M1dG and N2OPdG.

Kinetic and thermodynamic analysis of the hydrolytic ring-opening of the malondialdehyde-deoxyguanosine adduct, 3-(2'-deoxy-beta-D-erythro-pentofuranosyl)- pyrimido[1,2-alpha]purin-10(3H)-one

3-(2'-Deoxy-beta-D-erythro-pentofuranosyl)pyrimido[1,2-alpha]purin-10(3H)-one (M1dG) is the major reaction product of deoxyguanosine with malondialdehyde or base propenals. M1dG undergoes hydrolytic ring-opening to N2-oxopropenyl-deoxyguanosine (N2OPdG) under basic conditions. We report that ring-opening of M1dG as a nucleoside or in oligonucleotides is a reversible second-order reaction with hydroxide ion. NMR and UV analysis revealed N2OPdG(-) to be the only product of M1dG ring-opening in basic solution. The rate constant for reaction of M1dG with hydroxide is 3.8 M(-1) s(-1), and the equilibrium constant is calculated to be 2.1 +/- 0.3 x 10(4) M(-1) at 25 degrees C. Equilibrium constants determined by spectroscopic analysis of the reaction end-point or by thermodynamic analysis of rate constants determined over a range of temperatures yielded a value 2.5 +/- 0.2 x 10(4) M(-1). Kinetic analysis of ring-opening of M1dG in oligonucleotides indicated the rate constant for ring-opening is decreased 10-fold compared to that in the nucleoside. Flanking purines or pyrimidines did not significantly alter the rate constants for ring-opening, but purines flanking M1dG enhanced the rate constant for the reverse reaction. A mechanism is proposed for ring-opening of M1dG under basic conditions and a role is proposed for duplex DNA in accelerating the rate of ring-opening of M1dG at neutral pH.

Mass spectrometric quantitation of deoxyguanosine and leukotriene A4-deoxyguanosine adducts of DNA

An assay was developed using electrospray ionization negative ion tandem mass spectrometry (MS) to identify and quantitate the major product in the reaction of leukotriene A(4) (LTA(4)) with deoxyguanosine (dGuo). A second quantitative assay was established using the same separation and detection techniques to determine the amount of dGuo isolated from enzymatically processed DNA. The amount of LTA(4)-dGuo adduct could then be analytically determined in DNA samples and normalized to the amount of dGuo that had been simultaneously derived from the DNA sample. Stable isotope-labeled internal standards used for these quantitative assays were readily synthesized from isotopically labeled [(15)N(5)(13)C(10)]deoxyguanosine triphosphate and analyzed for isotopic purity using MS. A comparison of fragment ions formed from stable isotope analogs of dGuo revealed the loss of deoxyribose and secondarily the loss of a series of stable neutral small molecules in a fashion similar to patterns described previously for the collisional fragmentation of protonated guanine determined by positive ion fast atom bombardment/MS/MS. The combined quantitative assays were used for the determination of the amount of endogenously formed LTA(4)-dGuo adducts observed in DNA when isolated human neutrophils that had been incubated with arachidonic acid were stimulated with calcium ionophore to initiate leukotriene biosynthesis.

Exocyclic malondialdehyde and aromatic DNA adducts in larynx tissues

Cigarette smoking and alcohol consumption, known to cause free radical generation and lipid peroxidation, are established risk factors for larynx cancer. Malondialdehyde (MDA) is a naturally occurring product of lipid peroxidation, capable of interacting with DNA to form exocyclic MDA-DNA adducts. In the present study, we investigated if the production of MDA-DNA adducts was increased in larynx cancer patients with respect to controls using (32)P-DNA postlabeling techniques. Moreover, we examined the potential effects of cigarette smoking and alcohol consumption on endogenous DNA adducts. We then analyzed the same set of larynx tissues for the presence of (32)P-postlabeled aromatic DNA adducts to determine more about the levels and types of adducts formed in the larynx. We observed that cancer patients tended to have increased levels of MDA and aromatic DNA adducts with respect to controls. In addition, smoking and alcohol were found to influence the formation of endogenous adducts in the larynx tissues. Finally, the amounts of endogenous adducts were found to be comparable to those observed for aromatic DNA adducts in the same set of larynx tissues. These findings imply that endogenous lesions, if not repaired, may contribute to larynx cancer development.

Formation of N-(2-hydroxyethyl)valine in human hemoglobin—effect of lifestyle factors

The formation of N-(2-hydroxyethyl)valine (HEV) in hemoglobin has been considered as a biomarker to assess exogenous and endogenous exposures to ethylene oxide (EO) and/or ethylene (ET). Factors associated with daily exposures to such compounds might significantly affect the formation of HEV. Tobacco smoke containing EO elicited a significant increase in the levels of HEV amongst smokers, although other factors related to lifestyles may warrant further studies. The objective of this study was to specifically analyze HEV using a modified Edman degradation technique in order to study the association between lifestyle related factors (smoking, second-hand smoke exposure, tea and alcohol consumption) and HEV formation in vivo. Total of 148 Taiwanese volunteers with no history of occupational exposure to either EO or ET were recruited in this study. The HEV levels for smokers (204 +/- 151 pmol HEV/g globin, n = 70 ) were greater than those for non-smokers (57 +/- 46 pmol HEV/g globin, n = 78), HEV level increasing with the number of cigarettes smoked by subjects per day with a rate of 8.8 pmol HEV/g globin per cigarettes per day. Further analysis revealed that the rate of HEV formation in our study subjects was significantly associated with the number of daily cigarettes smoked (P < 0.001), but was not associated with tea or alcohol consumption, second-hand smoke exposure, subject age, or subject gender. These results suggest that the significantly higher levels of HEV for smokers than for non-smokers were mainly due to subject exposure to EO contained in cigarette smoke.

Food mutagens

Several lines of evidence indicate that diet and dietary behaviors can contribute to human cancer risk. One way that this occurs is through the ingestion of food mutagens. Sporadic cancers result from a gene-environment interactions where the environment includes endogenous and exogenous exposures. In this article, we define environment as dietary exposures in the context of gene-environment interactions. Food mutagens cause different types of DNA damage: nucleotide alterations and gross chromosomal aberrations. Most mutagens begin their action at the DNA level by forming carcinogen-DNA adducts, which result from the covalent binding of a carcinogen or part of a carcinogen to a nucleotide. However the effect of food mutagens in carcinogenesis can be modified by heritable traits, namely, low-penetrant genes that affect mutagen exposure of DNA through metabolic activation and detoxification or cellular responses to DNA damage through DNA repair mechanisms or cell death. There are some clearly identified (e.g., aflatoxin) and suspected (e.g., N-nitrosamines, polycyclic aromatic hydrocarbons or heterocyclic amines) food mutagens. The target organs for these agents are numerous, but there is target-organ specificity for each. Mutagenesis however is not the only pathway that links dietary exposures and cancers. There is growing evidence that epigenetic factors, including changes in the DNA methylation pattern, are causing cancer and can be modified by dietary components. Also DNA damage may be indirect by triggering oxidative DNA damage. When considering the human diet, it should be recognized that foods contain both mutagens and components that decrease cancer risk such as antioxidants. Thus nutritionally related cancers ultimately develop from an imbalance of carcinogenesis and anticarcinogenesis. The best way to assess nutritional risks is through biomarkers, but there is no single biomarker that has been sufficiently validated. Although panels of biomarkers would be the most appropriate, their use as a reflection of target-organ risk remains to be determined. Also even when new biomarkers are developed, their application in target organs is problematic because tissues are not readily available. For now most biomarkers are used in surrogate tissues (e.g., blood, urine, oral cavity cells) that presumably reflect biological effects in target organs. This article reviews the role of food mutagens in mutagenesis and carcinogenesis and how their effects are modified by heritable traits and discusses how to identify and evaluate the effects of food mutagens.

Hydroxyl radical in living systems and its separation methods

It has recently been shown that hydroxyl radicals are generated under physiological and pathological conditions and that they seem to be closely linked to various models of pathology putatively implying oxidative stress. It is now recognized that the hydroxyl radical is well-regulated to help maintain homeostasis on the cellular level in normal, healthy tissues. Conversely, it is also known that virtually every disease state involves free radicals, particularly the most reactive hydroxyl radical. However, when hydroxyl radicals are generated in excess or the cellular antioxidant defense is deficient, they can stimulate free radical chain reactions by interacting with proteins, lipids, and nucleic acids causing cellular damage and even diseases. Therefore, a confident analytical approach is needed to ascertain the importance of hydroxyl radicals in biological systems. In this paper, we provide information on hydroxyl radical trapping and detection methods, including liquid chromatography with electrochemical detection and mass spectrometry, gas chromatography with mass spectrometry, capillary electrophoresis, electron spin resonance and chemiluminescence. In addition, the relationships between diseases and the hydroxyl radical in living systems, as well as novel separation methods for the hydroxyl radical are discussed in this paper.

Applications of mass spectrometry for quantitation of DNA adducts

DNA adducts are formed when electrophilic molecules or free radicals attack DNA. 32P-postlabeling has been the most commonly used assay for quantitation of DNA adducts due mainly to its excellent sensitivity that allows quantitation at concentrations as low as approximately 1 adduct per 10(9) normal bases. Such methods, however, do not have the specificity desired for accurate and reliable quantitation, and are prone to produce false positives and artifacts. In the last decade, mass spectrometry in combination with liquid and gas chromatography has presented itself as a good alternative to these techniques since it can satisfy the need for specificity and reliability through the use of stable isotope-labeled internal standards and highly specific detection modes such as selected reaction monitoring and high-resolution mass spectrometry. In this article, the contribution of mass spectrometry to the quantitation of DNA adducts is reviewed with special emphasis on unique applications of mass spectrometry in the area of DNA adduct quantitation and recent applications with improvements in sensitivity.

Methodologies for bulky DNA adduct analysis and biomonitoring of environmental and occupational exposures

It is undisputed that DNA adduct formation is one of the key processes in early carcinogenesis. Therefore, analysis of DNA adduct levels may be one of the best tools available to characterize exposure to complex mixtures of genotoxic chemicals as occurring in different environmental and occupational exposure settings. However, from an analytical point of view the detection and quantification of DNA adducts is a challenging enterprise as extremely high sensitivity and selectivity are required. The entire spectrum of chromatographic techniques, including thin-layer chromatography (TLC), gas and liquid chromatography as well as capillary electrophoresis has been used in combination with different detection systems, all with their own specific characteristics. Among the various combinations of techniques, the TLC-(32)P-postlabeling combination appears to meet best with criteria of sensitivity and requirements of minimal amounts of material. Recent developments in the application of capillary electrophoresis in combination with either immunochemical or mass spectrometric detection techniques may offer new and promising approaches, with higher selectivity as compared to TLC-(32)P postlabeling. The applicability of these new techniques in biomonitoring studies aiming at the exposure and risk assessment of low and chronic exposures remains to be determined. In this paper we compare and discuss the advantages and limitations of different techniques used in DNA adduct analysis, with specific emphasis on those adducts formed by the polycyclic aromatic hydrocarbons and heterocyclic aromatic amines.

Tobacco smoking, harm reduction, and biomarkers

The only known way to reduce cancer risk in smokers is complete cessation, but many smokers are unable or unwilling to quit. Consequently, tobacco companies are now marketing products that purport to reduce carcinogen exposure, with the implication that such products provide a safer way to smoke. Moreover, researchers are exploring ways to reduce the amount of cigarette smoke carcinogens to which the smokers are exposed. Although these methods are, in theory beneficial, it is possible that the perceived availability of "safe" ways to smoke will cause some former smokers to resume smoking and some current smokers to delay quitting. Thus, the extent of exposure reduction and the impact on public health of these methods need to be considered carefully. However, risk reduction and its relation to exposure are not simple to estimate. The way people smoke and the way they respond to carcinogen exposure are both highly variable, as evidenced by the previous history of smokers who switched to light, or low-tar cigarettes. This can actually increase risk in some smokers. The evaluation of exposure reduction will therefore need to be multidisciplinary and include in vitro cell culture studies, animal studies, human clinical studies, and epidemiologic studies. Biomarkers will be critical for rapidly evaluating the effects of new strategies or products to reduce exposure to tobacco smoke carcinogens. No single biomarker will likely satisfy our assessment needs, and so a panel of biomarkers should be used that includes biomarkers of exposure, biologically effective dose, and potential harm. In addition, usefulness of new products will need to be tested in people of different susceptibilities (i.e., who vary in behavior, sex, age, genetics, and prior tobacco use). Even if the new products are shown to be effective at reducing lung carcinogens, they should not be used alone but rather be incorporated into a comprehensive tobacco control program.

Methods for measuring DNA adducts and abasic sites I: isolation, purification, and analysis of DNA adducts in intact DNA

The major event involved in the formation of mutations and the initiation and progression of cancer is the induction of DNA damage by reactive intermediates arising from exposure to endogenous and exogenous chemicals. Many electrophilic metabolites of chemicals covalently bind to the bases of DNA causing specific DNA adducts. This unit includes protocols for preparing samples of intact DNA and adduct analysis to quantify the number of adducts that can potentially cause mutagenic or carcinogenic damage.

The contribution of epidemiology

Epidemiologic studies directly contribute data on risk (or benefit) in humans as the investigated species, and in the full food intake range normally encountered by humans. This paper starts with introducing the epidemiologic approach, followed by a discussion of perceived differences between toxicological and epidemiologic risk assessment. Areas of contribution of epidemiology to the risk assessment process are identified, and ideas for tailoring epidemiologic studies to the risk assessment procedures are suggested, dealing with data collection, analyses and reporting of both existing and new epidemiologic studies. The dietary habits and subsequent disease occurrence of over three million people are currently under observation worldwide in cohort studies, offering great potential for use in risk assessment. The use of biomarkers and data on genetic susceptibility are discussed. The paper describes a scheme to classify epidemiologic studies for use in risk assessment, and deals with combining evidence from multiple studies. Using a matrix approach, the potential contribution to each of the steps in the risk assessment process is evaluated for categories of food substances. The contribution to risk assessment of specific food substances depends on the quality of the exposure information. Strengths and weaknesses are summarized. It is concluded that epidemiology can contribute significantly to hazard identification, hazard characterisation and exposure assessment.

The gastrointestinal tract: a major site of antioxidant action?

Diets rich in fruits and vegetables delay the onset of many age-related diseases, and contain a complex mixture of antioxidants (including ascorbate, carotenoids, vitamin E and other phenolics such as the flavonoids). However, diet also contains pro-oxidants, including iron, copper, H2O2, haem, lipid peroxides and aldehydes. Nitrite is frequently present in diet, leading to generation of reactive nitrogen species in the stomach. In considering the biological importance of dietary antioxidants, attention has usually focussed on those that are absorbed through the gastrointestinal tract into the rest of the body. In the present paper we develop the argument that the high levels of antioxidants present in certain foods (fruits, vegetables, grains) and beverages (e.g. green tea) play an important role in protecting the gastrointestinal tract itself from oxidative damage, and in delaying the development of stomach, colon and rectal cancer. Indeed, carotenoids and flavonoids do not seem to be as well absorbed as vitamins C and E. Hence their concentrations can be much higher in the lumen of the GI tract than are ever achieved in plasma or other body tissues, making an antioxidant action in the GI tract more likely. Additional protective mechanisms of these dietary constituents (e.g. effects on intercellular communication, apoptosis, cyclooxygenases and telomerase) may also be important.

Biomarkers of exposure and effect as indicators of potential carcinogenic risk arising from in vivo metabolism of ethylene to ethylene oxide

The purposes of the present study were: (i) to investigate the potential use of several biomarkers as quantitative indicators of the in vivo conversion of ethylene (ET) to ethylene oxide (EO); (ii) to produce molecular dosimetry data that might improve assessment of human risk from exogenous ET exposures. Groups (n = 7/group) of male F344 rats and B6C3F1 mice were exposed by inhalation to 0 and 3000 p. p.m. ET for 1, 2 or 4 weeks (6 h/day, 5 days/week) or to 0, 40, 1000 and 3000 p.p.m. ET for 4 weeks. N:-(2-hydroxyethyl)valine (HEV), N:7-(2-hydroxyethyl) guanine (N7-HEG) and HPRT: mutant frequencies were assessed as potential biomarkers for determining the molecular dose of EO resulting from exogenous ET exposures of rats and mice, compared with background biomarker values. N7-HEG was quantified by gas chromatography coupled with high resolution mass spectrometry (GC-HRMS), HEV was determined by Edman degradation and GC-HRMS and HPRT: mutant frequencies were measured by the T cell cloning assay. N7-HEG accumulated in DNA with repeated exposure of rodents to 3000 p.p.m. ET, reaching steady-state concentrations around 1 week of exposure in most tissues evaluated (brain, liver, lung and spleen). The dose-response curves for N7-HEG and HEV were supralinear in exposed rats and mice, indicating that metabolic activation of ET was saturated at exposures >/=1000 p.p.m. ET. Exposures of mice and rats to 200 p.p.m. EO for 4 weeks (as positive treatment controls) led to significant increases in HPRT: mutant frequencies over background in splenic T cells from exposed rats and mice, however, no significant mutagenic response was observed in the HPRT: gene of ET-exposed animals. Comparisons between the biomarker data for both unexposed and ET-exposed animals, the dose-response curves for the same biomarkers in EO-exposed rats and mice and the results of the rodent carcinogenicity studies of ET and EO suggest that too little EO arises from exogenous ET exposure to produce a significant mutagenic response or a carcinogenic response under standard bioassay conditions.

Studies using specific biomarkers for human exposure assessment to exogenous and endogenous chemical agents

The extent of formation of carcinogen adducts with DNA and protein may be used to assess the biologically effective dose of these carcinogens in the tissue under study. In normal human tissues, such carcinogen adducts arise in part from exposures to exogenous genotoxic compounds, although it has been shown that endogenously formed carcinogens also make a significant contribution to the observed DNA and protein damage. Although, highly sensitive analytical methods, such as immunoassay, 32P-postlabelling and mass spectrometry have been developed and successfully applied to measure carcinogen adducts, further methodological advances are making these methods more amenable to molecular epidemiological studies. Thus, the use of immunoslot blot assays allows a higher sample throughput for adduct quantification. Liquid chromatographic separations of adducts, either for their radiochemical detection following 32P-postlabelling or for their determination by mass spectrometry, improves the specificity and applicability of these techniques. In this review, the sensitivities and specificities of the analytical methods used for adduct detection are compared and the limitations of these methods described.