Detection of 1,N6-ethenodeoxyadenosine and 3,N4-ethenodeoxycytidine by immunoaffinity/32P-postlabelling in liver and lung DNA of mice treated with ethyl carbamate (urethane) or its metabolites

The Excited State Dynamics of a Mutagenic Guanosine Etheno Adduct Investigated by Femtosecond Fluorescence Spectroscopy and Quantum Mechanical Calculations

Femtosecond fluorescence upconversion experiments were combined with CASPT2 and time dependent DFT calculations to characterize the excited state dynamics of the mutagenic etheno adduct 1,N2-etheno-2'-deoxyguanosine (ϵdG). This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The ϵdG fluorescence is strongly modified with respect to that of the canonical nucleoside dG, notably by an about 6-fold increase in fluorescence lifetime and quantum yield at neutral pH. In addition, femtosecond fluorescence upconversion experiments reveal the presence of two emission bands with maxima at 335 nm for the shorter-lived and 425 nm for the longer-lived. Quantum mechanical calculations rationalize these findings and provide absorption and fluorescence spectral shapes similar to the experimental ones. Two different bright minima are located on the potential energy surface of the lowest energy singlet excited state. One planar minimum, slightly more stable, is associated with the emission at 335 nm, whereas the other one, with a bent etheno ring, is associated with the red-shifted emission.

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

Non-canonical genomic driver mutations of urethane carcinogenesis

The carcinogen urethane induces pulmonary tumors in mice initiated by an incredibly specific Q61L/R oncogenic mutation in the proto-oncogene Kras. Previous Whole-Exome Sequencing of urethane-induced tumors revealed a bias towards A➙T/G and G➙A substitutions. Subsequent ultra-sensitive Maximum-Depth Sequencing of Kras shortly after urethane exposure suggest a further refinement to CA➙CT/G substitutions. As C182AA➙C182T/GA substitutions in Kras result in Q61L/R mutations, the extreme bias of urethane towards these genomic driver mutations can be ascribed to the specificity of the carcinogen for CA➙CT/G substitutions. However, we previously found that changing rare codons to common in the Kras gene to increase protein expression shifted mutations in urethane-induced tumors away from Kras, or when detected in Kras, to G12D mutations that are usually rarely detected in such tumors. Moreover, the loss of p53 partially reversed this effect, generating tumors with either Q61L/R or G12D oncogenic Kras mutations, or no Kras mutations, presumably due to other genomic driver mutations. Determining the origin of these G12D and other unknown non-canonical genomic driver mutations would provide critical insight into the extreme bias of carcinogens for specific genomic driver mutations. We thus compared the types of Single Nucleotide Variations detected by previously performed Maximum-Depth Sequencing immediately after urethane exposure to the mutation signatures derived from Whole Exome Sequencing of urethane-induced tumors. This identified two types of non-canonical mutations. First, a V637E oncogenic mutation in the proto-oncogene Braf that conforms to the mutation signature of urethane, suggesting that the mutational bias of the carcinogen may account for this non-canonical mutation, similar to that for canonical Q61L/R mutations in Kras. Second, G12D and Q61H mutations in Kras that did not fit this mutation signature, and instead shared similarity with Single Nucleotide Variations detected by Maximum-Depth Sequencing from normal cells, suggesting that perhaps these mutations were pre-existing. We thus posit that when canonical Kras mutations are selected against that the carcinogen may instead promote the expansion of pre-existing genomic driver mutations, although admittedly we cannot rule out other mechanisms. Interrogating the mutation signatures of human lung cancers similarly identified KRAS genomic driver mutations that failed to match the mutation signature of the tumor. Thus, we also speculate that the selection for non-canonical genomic driver mutations during urethane carcinogenesis may reflect the process by which discordance between genomic driver mutations and mutational signatures arises in human cancers.

The Excited State Dynamics of a Mutagenic Cytidine Etheno Adduct Investigated by Combining Time-Resolved Spectroscopy and Quantum Mechanical Calculations

Joint femtosecond fluorescence upconversion experiments and theoretical calculations provide a hitherto unattained degree of characterization and understanding of the mutagenic etheno adduct 3,N4-etheno-2'-deoxycytidine (εdC) excited state relaxation. This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The εdC fluorescence is modified with respect to that of the canonical base dC, with a 3-fold increased lifetime and quantum yield at neutral pH. This behavior is amplified upon protonation of the etheno ring (εdCH⁺). Quantum mechanical calculations show that the lowest energy state ππ*1 is responsible for the fluorescence and that the main nonradiative decay pathway to the ground state goes through an ethene-like conical intersection, involving the out-of-plane motion of the C5 and C6 substituents. This conical intersection is lower in energy than the ππ* state (ππ*1) minimum, but a sizable energy barrier explains the increase of εdC and εdCH⁺ fluorescence lifetimes with respect to that of dC.

Capturing the primordial Kras mutation initiating urethane carcinogenesis

The environmental carcinogen urethane exhibits a profound specificity for pulmonary tumors driven by an oncogenic Q₆₁L/R mutation in the gene Kras. Similarly, the frequency, isoform, position, and substitution of oncogenic RAS mutations are often unique to human cancers. To elucidate the principles underlying this RAS mutation tropism of urethane, we adapted an error-corrected, high-throughput sequencing approach to detect mutations in murine Ras genes at great sensitivity. This analysis not only captured the initiating Kras mutation days after urethane exposure, but revealed that the sequence specificity of urethane mutagenesis, coupled with transcription and isoform locus, to be major influences on the extreme tropism of this carcinogen.

Why the carcinogen urethane causes only lung tumours driven by a specific oncogenic mutation in just one Ras gene in mice is unclear. Here, the authors capture mutations immediately after urethane exposure and show that the sequence specificity of mutagenesis, transcriptional status, and Ras genetic loci may all contribute to this specificity.

Transcription factor EB (TFEB)-mediated autophagy protects against ethyl carbamate-induced cytotoxicity

Ethyl carbamate (EC) is thought to be a toxicant that widely exists in cigarette smoke and polluted air, as well as fermented food and alcoholic beverages. However, the mechanism and approach to treat hepatic damage after EC exposure remain unclear. Here, we first found that EC caused decreased cell viability, reactive oxygen species (ROS) overproduction and glutathione (GSH) depletion in normal human hepatocytes L02 cells. Excessive ROS generation was found to be one of the major reasons for cell cytotoxicity of EC treatment. Furthermore, increased ROS levels also promoted autophagy, a lysosomal degradation process, which was confirmed by detection of LC3-II expression and puncta in GFP-RFP-LC3 transfection assay. Autophagy inhibitor chloroquine (CQ) pretreatment led to decreased cell viability and higher ROS levels compared with EC group, suggesting that autophagy protected EC-treated cells against oxidative stress and cytotoxicity. Notably, we observed increased lysosomal biogenesis and activation of transcription factor EB (TFEB), a master regulator of lysosomal generation, in the process of autophagy. Taken together, we unveiled a novel mechanism of hepatotoxicity and endogenous potent protection of TFEB-mediated autophagy against decreased cell viability and redox disturbance under EC exposure in normal human hepatocytes.

Modulation of Urethane (Ethyl Carbamate) Carcinogenicity by Ethyl Alcohol: A Review

Urethane (ethyl carbamate) is a carcinogen that has been detected as a contaminant in certain foods and alcoholic beverages. The carcinogenicity of urethane appears to be mediated through a metabolic pathway involving sequential cytochrome P-450-catalyzed oxidation to vinyl carbamate and vinyl carbamate epoxide, the latter of which reacts with DNA to yield etheno-type DNA adducts. The interactions of ethanol on urethane metabolism and carcinogenicity are varied and complex. Urethane is oxidized by cytochrome P-450 IIE1, an isoform induced by ethanol, which suggests that chronic ethanol consumption could increase the oxidation of urethane to its epoxide derivative. On the other hand, ethanol coadministration is known to inhibit the elimination and hydrolysis of urethane. Ethanol has decreased the tumorigenicity of urethane in two bioassays, but did not have an effect in a third. While the results to date suggest that ethanol will decrease the metabolic activation of urethane, with a resultant decrease in tumorigenicity, additional studies are clearly necessary to elucidate the mechanisms by which ethanol influences the carcinogenicity of urethane.

From immunotoxicity to carcinogenicity: the effects of carbamate pesticides on the immune system

The immune system can be the target of many chemicals, with potentially severe adverse effects on the host's health. In the literature, carbamate (CM) pesticides have been implicated in the increasing prevalence of diseases associated with alterations of the immune response, such as hypersensitivity reactions, some autoimmune diseases and cancers. CMs may initiate, facilitate, or exacerbate pathological immune processes, resulting in immunotoxicity by induction of mutations in genes coding for immunoregulatory factors and modifying immune tolerance. In the present study, direct immunotoxicity, endocrine disruption and inhibition of esterases activities have been introduced as the main mechanisms of CMs-induced immune dysregulation. Moreover, the evidence on the relationship between CM pesticide exposure, dysregulation of the immune system and predisposition to different types of cancers, allergies, autoimmune and infectious diseases is criticized. In addition, in this review, we will discuss the relationship between immunotoxicity and cancer, and the advances made toward understanding the basis of cancer immune evasion.

Mass spectrometry for the assessment of the occurrence and biological consequences of DNA adducts

Exogenous and endogenous sources of chemical species can react, directly or after metabolic activation, with DNA to yield DNA adducts. If not repaired, DNA adducts may compromise cellular functions by blocking DNA replication and/or inducing mutations. Unambiguous identification of the structures and accurate measurements of the levels of DNA adducts in cellular and tissue DNA constitute the first and important step towards understanding the biological consequences of these adducts. The advances in mass spectrometry (MS) instrumentation in the past 2-3 decades have rendered MS an important tool for structure elucidation, quantification, and revelation of the biological consequences of DNA adducts. In this review, we summarized the development of MS techniques on these fronts for DNA adduct analysis. We placed our emphasis of discussion on sample preparation, the combination of MS with gas chromatography- or liquid chromatography (LC)-based separation techniques for the quantitative measurement of DNA adducts, and the use of LC-MS along with molecular biology tools for understanding the human health consequences of DNA adducts. The applications of mass spectrometry-based DNA adduct analysis for predicting the therapeutic outcome of anti-cancer agents, for monitoring the human exposure to endogenous and environmental genotoxic agents, and for DNA repair studies were also discussed.

Ultrasensitive UPLC-MS/MS method for analysis of etheno-DNA adducts in human white blood cells

Etheno-DNA adducts are generated by interaction of cellular DNA with exogenous environmental carcinogens and end products of lipid peroxidation. It has been determined that 1,N(6)-etheno-2'-deoxyadenosine (εdA) and 3,N(4)-etheno-2'-deoxycytidine (εdC) adducts formed in human white blood cells can be used to serve as biomarkers of genetic damage mediated by oxidative stress. In this study, we developed an ultrasensitive ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method used to detect and quantify εdA and dC adducts in human white blood cells. The percent recoveries of εdA and dC adducts were found to be 88.9% ± 2.8 and 95.7% ± 3.7, respectively. The detection limits were ∼ 1.45 fmol for εdA and ∼ 1.27 fmol for εdC in 20 μg of human white blood cell DNA samples, both εdA and εdC adducts could be detected using only ∼ 5 μg of DNA per sample. For validation of the method, 34 human blood cell DNA samples were assayed and the results revealed a significant difference (P < 0.01) between levels (fmol/μg DNA) of 0.82 ± 0.83 (standard deviation [SD]) (range: 0.15-3.11) for εdA, 3.28 ± 3.15 (SD) (range: 0.05-9.6) for εdC in benzene-exposed workers; and 0.04 ± 0.08 (SD) (range: 0.0-0.27) for εdA and 0.77 ± 1.02 (SD) (range: 0.10-4.11) for εdC in non-benzene-exposed workers. Our method shows a high sensitivity and specificity when applied to small amounts of human white blood cell DNA samples; background levels of εdA and εdC could be reproducibly detected. The ultrasensitive and simple detection method is thus suitable for applications in human biomonitoring and molecular epidemiology studies.

Ultrasensitive UPLC-MS-MS method for the quantitation of etheno-DNA adducts in human urine

Etheno-DNA adducts are generated from the metabolism of exogenous carcinogens and endogenous lipid peroxidation. We and others have previously reported that 1,N⁶-ethenodeoxyadenosine (εdA) and 3,N⁴-ethenodeoxycytidine (εdC) are present in human urine and can be utilized as biomarkers of oxidative stress. In this study, we report a new ultrasensitive UPLC-ESI-MS/MS method for the analysis of εdA and εdC in human urine, capable of detecting 0.5 fmol εdA and 0.3 fmol εdC in 1.0 mL of human urine, respectively. For validation of the method, 20 human urine samples were analyzed, and the results revealed that the mean levels of εdA and εdC (SD) fmol/µmol creatinine are 5.82 ± 2.11 (range 3.0–9.5) for εdA and 791.4 ± 328.8 (range 116.7–1264.9) for εdC in occupational benzene-exposed workers and 2.10 ± 1.32 (range 0.6–4.7) for εdA and 161.8 ± 200.9 (range 1.8–557.5) for εdC in non-benzene-exposed workers, respectively. The ultrasensitive detection method is thus suitable for applications in human biomonitoring and molecular epidemiology studies.

Increased levels of inosine in a mouse model of inflammation

One possible mechanism linking inflammation with cancer involves the generation of reactive oxygen, nitrogen, and halogen species by activated macrophages and neutrophils infiltrating sites of infection or tissue damage, with these chemical mediators causing damage that ultimately leads to cell death and mutation. To determine the most biologically deleterious chemistries of inflammation, we previously assessed products across the spectrum of DNA damage arising in inflamed tissues in the SJL mouse model nitric oxide overproduction ( Pang et al. ( 2007 ) Carcinogenesis 28 , 1807 - 1813 ). Among the anticipated DNA damage chemistries, we observed significant changes only in lipid peroxidation-derived etheno adducts. We have now developed an isotope-dilution, liquid chromatography-coupled, tandem quadrupole mass spectrometric method to quantify representative species across the spectrum of RNA damage products predicted to arise at sites of inflammation, including nucleobase deamination (xanthosine and inosine), oxidation (8-oxoguanosine), and alkylation (1,N(6)-ethenoadenosine). Application of the method to the liver, spleen, and kidney from the SJL mouse model revealed generally higher levels of oxidative background RNA damage than was observed in DNA in control mice. However, compared to control mice, RcsX treatment to induce nitric oxide overproduction resulted in significant increases only in inosine and only in the spleen. Further, the nitric oxide synthase inhibitor, N-methylarginine, did not significantly affect the levels of inosine in control and RcsX-treated mice. The differences between DNA and RNA damage in the same animal model of inflammation point to possible influences from DNA repair, RcsX-induced alterations in adenosine deaminase activity, and differential accessibility of DNA and RNA to reactive oxygen and nitrogen species as determinants of nucleic acid damage during inflammation.

Longitudinal assessment of lung cancer progression in the mouse using in vivo micro-CT imaging

PURPOSE

Small animal micro-CT imaging is being used increasingly in preclinical biomedical research to provide phenotypic descriptions of genomic models. Most of this imaging is coincident with animal death and is used to show the extent of disease as an end point. Longitudinal imaging overcomes the limitation of single time-point imaging because it enables tracking of the natural history of disease and provides qualitative and, where possible, quantitative assessments of the effects of an intervention. The pulmonary system is affected by many disease conditions, such as lung cancer, chronic obstructive pulmonary disease, asthma, and granulomatous disorders. Noninvasive imaging can accurately assess the lung phenotype within the living animal, evaluating not only global lung measures, but also regional pathology. However, imaging the lung in the living animal is complicated by rapid respiratory motion, which leads to image based artifacts. Furthermore, no standard mouse lung imaging protocols exist for longitudinal assessment, with each group needing to develop their own systematic approach.

METHODS

In this article, the authors present an outline for performing longitudinal breath-hold gated micro-CT imaging for the assessment of lung nodules in a mouse model of lung cancer. The authors describe modifications to the previously published intermittent isopressure breath-hold technique including a new animal preparation and anesthesia protocol, implementation of a ring artifact reduction, variable scanner geometry, and polynomial beam hardening correction. In addition, the authors describe a multitime-point data set registration and tumor labeling and tracking strategy.

RESULTS

In vivo micro-CT data sets were acquired at months 2, 3, and 4 posturethane administration in cancer mice (n = 5) and simultaneously in control mice (n = 3). 137 unique lung nodules were identified from the cancer mice while no nodules were detected in the control mice. A total of 411 nodules were segmented and labeled over the three time-points. Lung nodule metrics including RECIST, Ortho, WHO, and 3D volume were determined and extracted. A tumor incidence rate of 30.44 +/- 1.93 SEM for n = 5 was found with identification of nodules as small as 0.11 mm (RECIST) and as large as 1.66 mm (RECIST). In addition, the tumor growth and doubling rate between months 2-3 and 3-4 were calculated. Here, the growth rate was slightly higher in the second period based on the 3D volume data (0.12 +/- 0.13 to 0.13 +/- 0.17 microl) but significantly less based on the linear diameter metrics [RECIST (0.33 +/- 0.19 to 0.17 +/- 0.18 mm); Ortho (0.24 +/- 0.15 to 0.16 +/- 0.15 mm)], indicating the need to understand how each metric is obtained and how to correctly interpret change in tumor size.

CONCLUSIONS

In conclusion, micro-CT imaging provides a unique platform for in vivo longitudinal assessment of pulmonary lung cancer progression and potentially tracking of therapies at very high resolutions. The ability to evaluate the same subject over time provides for a sensitive assay that can be carried out on a smaller sample size. When integrated with image processing and analysis routines as detailed in this study, the data acquired from micro-CT imaging can now provide a very powerful assessment of pulmonary disease outcomes.

Translesion synthesis polymerases in the prevention and promotion of carcinogenesis

A critical step in the transformation of cells to the malignant state of cancer is the induction of mutations in the DNA of cells damaged by genotoxic agents. Translesion DNA synthesis (TLS) is the process by which cells copy DNA containing unrepaired damage that blocks progression of the replication fork. The DNA polymerases that catalyze TLS in mammals have been the topic of intense investigation over the last decade. DNA polymerase η (Pol η) is best understood and is active in error-free bypass of UV-induced DNA damage. The other TLS polymerases (Pol ι, Pol κ, REV1, and Pol ζ) have been studied extensively in vitro, but their in vivo role is only now being investigated using knockout mouse models of carcinogenesis. This paper will focus on the studies of mice and humans with altered expression of TLS polymerases and the effects on cancer induced by environmental agents.

Mechanisms of lung tumorigenesis by ethyl carbamate and vinyl carbamate

Vinyl carbamate (VC) and ethyl carbamate (EC) induce the formation of lung tumors. The mechanism involves a two-step oxidation of EC to VC and VC to an epoxide, both of which are mediated mainly by CYP2E1. Interaction of the epoxide with DNA leads to the formation of DNA adducts, including 1,N(6)ethenodeoxyadenosine and 1,N(4)-ethenodeoxycytidine. The production of DNA adducts correlated with capacities for the bioactivation of VC, which are higher in the lungs of A/J than in C57BL/6 mice. Importantly, CYP2E1 is higher in the lungs of A/J than in C57BL/6 mice. Studies using F(1) (Big Blue x A/J) transgenic mice revealed the formation of mutations in the lambda cII gene after treatment with VC. Mutations induced by VC were mainly A:T-->G:C transitions and A:T-->T:A transversions, while mutations induced by EC were mainly G:C-->A:T transitions. An EC dose that was 17-fold higher than that for VC was required to produce a similar level of mutant frequency in the lung. Pretreatment of mice with the CYP2E1 inhibitor, diallyl sulfone, significantly inhibited the mutant frequencies induced by VC. Mutations in the endogeneous Kras2 gene were found in codon 61 of exon 2 and were identified as A:T transversions and A-->G transitions in the second base and A-->T transversions in the third base. These mutations were reduced by treatment of mice with diallyl sulfone before VC and coincided with a reduction in the number of lung tumors with Kras2 mutations. These findings affirmed that the metabolism of EC and VC is a prerequisite for, or at least substantially contributes to, initiation of the cascade of events leading to lung tumor formation.

Transcriptome of normal lung distinguishes mouse lines with different susceptibility to inflammation and to lung tumorigenesis

AIRmax and AIRmin mouse lines show a differential lung inflammatory response and differential lung tumor susceptibility after urethane treatment. The transcript profile of approximately 24,000 known genes was analyzed in normal lung tissue of untreated and urethane-treated AIRmax and AIRmin mice. In lungs of untreated mice, inflammation-associated genes involved in pathways such as "leukocyte transendothelial migration", "cell adhesion" and "tight junctions" were differentially expressed. Moreover, gene expression levels differed significantly in urethane-treated mice; in AIRmin mice, modulation of expression of genes involved in pathways associated with inflammatory response paralleled the previously observed persistent infiltration of inflammatory cells in the lung of these mice.

Inhibition of vinyl carbamate-induced lung tumors and Kras2 mutations by the garlic derivative diallyl sulfone

Vinyl carbamate (VC) is derived from ethyl carbamate (EC), a chemical found in alcoholic beverages and fermented foods. The objectives of this study were to characterize the formation of lung tumors induced by VC in F1 (Big BluexA/J) mice, and to identify the mutations formed in the Kras2 gene. In addition, we have tested the hypothesis that pretreatment with diallyl sulfone (DASO2) inhibits the adverse effects of VC. Mice were treated with VC (60 mg/kg, i.p.) or DASO2 (50 mg/kg, p.o.) 2 h prior to VC (DASO2/VC). Lung tumor multiplicity was significantly lower (21%) in mice treated with DASO2/VC than with VC. Lung tumors induced by VC are manifested as solid or papillary tumors, with the latter being regarded as a more malignant phenotype as they demonstrate no growth restrictions. Solid (42%) and papillary tumors (58%) were found in similar proportions in VC-treated mice. The number of papillary tumors was significantly decreased (44.5%) in mice treated with DASO2/VC, while there was a proportional increase (44.5%) in the number of solid tumors. The number of tumors with mutations in the first and second exon of Kras2 was significantly lower after treatment with DASO2/VC (7%) than after treatment with VC (61%). The mutations were mainly found in codon 61, and were identified as A-->T transversions (31%) and A-->G transitions (25%) in the second base, and A-->T transversions (12%) in the third base. All of these mutations were significantly reduced by DASO2 pretreatment. The number of tumors containing Kras2 mutations was highest (38%) in the large papillary tumors. Hence, mice treated with DASO2/VC had decreased frequencies of Kras2 mutations and reduced numbers of small and large papillary tumors, suggesting that activation of the Kras2 gene may be implicated in lung tumor formation and progression.

Urethane and N-nitrosodiethylamine are mutagenic for the Syrian hamster fetus

Urethane and N-nitrosodiethylamine are soluble environmental carcinogens that initiate tumors transplacentally, but have a mixed history of effectiveness in mutagenesis assays in vitro or in vivo with adult rodents. To test for their transplacental mutagenicity, Syrian hamster fetuses at 12 days in gestation were exposed transplacentally to urethane or N-nitrosodiethylamine at 0.5 or 1.0 mM/kg. The fetal cells were isolated on day 13 of gestation and tested for diphtheria toxin resistance as a mutation marker. Both compounds were significantly mutagenic, at both doses, causing 6- to 20-fold increases in mutations compared with controls. Compared with N-nitrosodiethylamine, urethane was somewhat more effective as a mutagen with a more marked dose-response. These results are consistent with mutagenesis as part of the mechanism of transplacental carcinogenicity of urethane and N-nitrosodiethylamine.

Genetic control of resistance to hepatocarcinogenesis by the mouse Hpcr3 locus

The genome of the BALB/c mouse strain provides alleles that dominantly inhibit hepatocellular tumor development in F1 crosses with the highly hepatocarcinogenesis-susceptible C3H/He strain. Genome-wide linkage analysis using a 1536-single-nucleotide polymorphism array in a (C3H/He x BALB/c)F2 intercross population treated with urethane to induce hepatocellular tumor development revealed a locus with a major role in the resistance to hepatocarcinogenesis. This locus, designated hepatocarcinogen resistance 3 (Hpcr3) and mapping to central chromosome 15, showed a linkage at LOD score = 16.52 and accounted for 40% of the phenotypical variance. The BALB/c-derived allele at Hpcr3 reduced tumor-occupied area of the liver up to 25-fold, in a semidominant way. Additional minor loci were mapped to chromosomes 1, 10, and 18. A gene expression profile of normal adult mouse liver showed a significant association with susceptibility of BALB/c, C3H/He, and F1 mice to hepatocarcinogenesis and identified the genes expressed in the Hpcr3 locus region; moreover, this analysis implicated the E2F1 pathway in the modulation of the phenotype susceptibility to hepatocarcinogenesis.

CONCLUSION

These findings, indicating the complex genetics of dominant resistance to hepatocarcinogenesis, represent a step toward the identification of the genes underlying this phenotype.

Etheno-DNA adduct formation in rats gavaged with linoleic acid, oleic acid and coconut oil is organ- and gender specific

Intake of linoleic acid (LA) increased etheno-DNA adducts induced by lipid peroxidation (LPO) in white blood cells (WBC) of female but not of male volunteers [J. Nair, C.E. Vaca, I. Velic, M. Mutanen, L.M. Valsta, H. Bartsch, High dietary omega-6 polyunsaturated fatty acids drastically increase the formation of etheno-DNA adducts in white blood cells of female subjects, Cancer Epidemiol. Biomarkers Prev. 6 (1997) 597-601]. Etheno-adducts were measured in rats gavaged with LA, oleic acid (OA) and saturated fatty acid rich coconut oil for 30 days. DNA from organs and total WBC was analyzed for 1, N(6)-ethenodeoxyadenosine (varepsilondA) and 3, N(4)-ethenodeoxycytidine (varepsilondC) by immunoaffinity/(32)P-postlabeling. Colon was the most affected target with LA-treatment, where etheno-adducts were significantly elevated in both sexes. In WBC both adducts were elevated only in LA-treated females. Unexpectedly, OA treatment enhanced etheno-adduct levels in prostate 3-9 fold. Our results in rodents confirm the gender-specific increase of etheno-adducts in WBC-DNA, likely due to LPO induced by redox-cycling of 4-hydroxyestradiol. Colon was a target for LPO-derived DNA-adducts in both LA-treated male and female rats, supporting their role in omega-6 PUFA induced colon carcinogenesis.

Lipid peroxidation-induced DNA damage in cancer-prone inflammatory diseases: a review of published adduct types and levels in humans

Persistent oxidative stress and excess lipid peroxidation (LPO), induced by inflammatory processes, impaired metal storage, and/or dietary imbalance, cause accumulations and massive DNA damage. This massive DNA damage, along with deregulation of cell homeostasis, leads to malignant diseases. Reactive aldehydes produced by LPO, such as 4-hydroxy-2-nonenal, malondialdehyde, acrolein, and crotonaldehyde, react directly with DNA bases or generate bifunctional intermediates which form exocyclic DNA adducts. Modification of DNA bases by these electrophiles, yielding promutagenic exocyclic adducts, is thought to contribute to the mutagenic and carcinogenic effects associated with oxidative stress-induced LPO. Ultrasensitive detection methods have facilitated studies of the concentrations of promutagenic DNA adducts in human tissues, white blood cells, and urine, where they are excreted as modified nucleosides and bases. Thus, immunoaffinity-(32)P-postlabeling, high-performance liquid chromatography-electrochemical detection, gas chromatography-mass spectrometry, liquid chromatography-tandem mass spectrometry, immunoslotblot assay, and immunohistochemistry have made it possible to detect background concentrations of adducts arising from endogenous LPO products in vivo and studies of their role in carcinogenesis. These background adduct levels in asymptomatic human tissues occur in the order of 1 adduct/10(8) and in organs affected by cancer-prone inflammatory diseases these can be 1 or 2 orders of magnitude higher. In this review, we critically discuss the accuracy of the available methods and their validation and summarize studies in which measurement of exocyclic adducts suggested new mechanisms of cancer causation, providing potential biomarkers for cancer risk assessment in humans with cancer-prone diseases.

Inhibition of vinyl carbamate-induced mutagenicity and clastogenicity by the garlic constituent diallyl sulfone in F1 (Big Blue® × A/J) transgenic mice

Vinyl carbamate (VC) is a metabolite of ethyl carbamate (EC), a naturally occurring compound found in fermented foods and alcoholic beverages. CYP2E1 mediates the sequential oxidation of EC to VC and subsequently to the vinyl carbamate epoxide, which is believed to be the ultimate carcinogenic species. Here, we have tested the hypothesis that bioactivation of VC by CYP2E1 plays a central role in the development of its mutagenicity and clastogenicity, and further that inhibition of CYP2E1 by diallyl sulfone (DASO(2)) leads to diminution in their incidences. DASO(2) is a garlic constituent that is oxidized by CYP2E1, leading to inactivation of this P450. F(1) (Big Blue x A/J) transgenic mice harboring the lambda cII gene were used for in vivo identification and quantitation of mutations in the lung and small intestine. Mice were pre-treated with DASO(2) (12.5-200 mg/kg, p.o.), treated 2 h later with VC (60 mg/kg, i.p.) and were killed 4 weeks later. Our results showed that pre-treatment of mice with DASO(2) at doses of 50-200 mg/kg significantly decreased the VC-induced mutant frequencies (MFs) by 50-70%. In the small intestine, pre-treatment with 200 mg/kg of DASO(2) decreased the MF by approximately 40%. Clastogenicity, as assessed by the frequency of micronucleated reticulocytes, was significantly decreased (33-44%) by pre-treatment with DASO(2) (50-200 mg/kg). These results demonstrated that bioactivation of VC by CYP2E1 plays a valid role in the development of mutagenicity and clastogenicity, and further that inhibition of this pathway by DASO(2) produces a protective effect.

Oxidation of vinyl carbamate and formation of 1,N6-ethenodeoxyadenosine in murine lung

Vinyl carbamate (VC) is derived from ethyl carbamate, a carcinogen formed in fermentation of food and alcoholic products. We have undertaken studies to test the hypothesis that an epoxide generated from VC oxidation leads to formation of 1,N6-ethenodeoxyadenosine (epsilon dAS). We have developed approaches using liquid chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry for identification and quantitation of epsilon dAS. Scanning and fragment ion analyses confirmed the identity of epsilon dAS based on the molecular ion [M + H]+ m/z 276 and the specific fragment ion m/z 160. Chemical oxidation of VC in reactions containing 2'-deoxyadenosine produced epsilon dAS with 1H NMR, chromatographic, and mass spectral characteristics identical to those of the authentic epsilon dAS, suggesting DNA alkylation by the VC epoxide. Subsequent studies evaluated formation of epsilon dAS in incubations of murine lung microsomes or recombinant CYP2E1 with VC. The formation of epsilon dAS in incubations of lung microsomes or recombinant CYP2E1 with VC was dependent on protein concentrations, CYP2E1 enzyme levels, and incubation time. The rates of epsilon dAS formation were highly correlated with VC concentrations. Peak rates were produced by lung microsomes and recombinant CYP2E1 at 3.0 and 2.5 mM VC, respectively. In inhibitory studies, incubations of VC were performed using lung microsomes from mice treated with the CYP2E1 inhibitor diallyl sulfone (100 mg/kg, p.o.). Results from these studies showed significantly decreased epsilon dAS formation in microsomes incubated with VC, with an inhibition of 70% at 3.0 mM. These findings suggested that CYP2E1 is a major enzyme mediating VC oxidation, leading to the formation of a metabolite that alkylates DNA to form the epsilon dAS adduct.

Report of the IWGT working group on strategy/interpretation for regulatory in vivo tests II. Identification of in vivo-only positive compounds in the bone marrow micronucleus test

A survey conducted as part of an International Workshop on Genotoxicity Testing (IWGT) has identified a number of compounds that appear to be more readily detected in vivo than in vitro. The reasons for this property varies from compound to compound and includes metabolic differences; the influence of gut flora; higher exposures in vivo compared to in vitro; effects on pharmacology, in particular folate depletion or receptor kinase inhibition. It is possible that at least some of these compounds are detectable in vitro if a specific in vitro test is chosen as part of the test battery, but the 'correct' choice of test may not always be obvious when testing a compound of unknown genotoxicity. It is noted that many of the compounds identified in this study interfere with cell cycle kinetics and this can result in either aneugenicity or chromosome breakage. A decision tree is outlined as a guide for the evaluation of compounds that appear to be genotoxic agents in vivo but not in vitro. The regulatory implications of these findings are discussed.

In vivomutagenicity of vinyl carbamate and ethyl carbamate in lung and small intestine of F1(Big Blue® × A/J) transgenic mice

Vinyl carbamate (VC) is a metabolite of ethyl carbamate (EC), a chemical found in alcoholic beverages and fermented foods. We undertook this study to: (i) evaluate the ability of both EC and VC to induce gene mutations in lung and various extrapulmonary tissues, and (ii) identify the type of mutations induced by the two compounds in various tissues. F1 (Big Blue x A/J) transgenic mice harboring the lambda cII transgene were used for identification and quantitation of mutations in vivo. Time-course studies in lung showed a plateau in mutant frequency (MF) 4 weeks after VC treatment, at which time mutations were fixed and were about 4-fold higher than in controls. Dose-dependent increases in MF were detected in the lung and small intestine (SI) after treatment with 15-75 mg/kg, i.p., of VC. VC was mutagenic in the lung and SI at doses of 45, 60 and 75 mg/kg. Sequencing of the cII gene in lung and SI showed that VC induced mainly A:T-->G:C transitions and A:T-->T:A transversions. EC was also mutagenic in the lung at 500 and 1,000 mg/kg and elicited mainly G:C-->A:T transitions. A VC dose of 60 mg/kg elicited a similar level of MF as an EC dose of 1,000 mg/kg. At 4 weeks after treatment, neither VC nor EC elicited mutations in the colon, bone marrow or kidney. These results demonstrated that VC and EC are mutagenic in vivo and affirm that VC is a more potent mutagen than EC.

New ultrasensitive 32P-postlabelling method for the analysis of 3,N4-etheno-2'-deoxycytidine in human urine

Etheno-DNA adducts are generated from exogenous carcinogens such as vinyl chloride and urethane and also from endogenous lipid peroxidation products such as trans-4-hydroxy-2-nonenal (HNE). The present authors and others have established that 1,N6-ethenodeoxyadenosine (epsilondA) and 3,N4-ethenodeoxycytidine (epsilondC) are present in human urine and could be explored as biomarkers for monitoring whole-body oxidative stress. The present study reports on a new ultrasensitive 32P-postlabelling/thin-layer chromatography (TLC) method for the analysis of epsilondC as deoxynucleoside in human urine. The urine samples were purified and enriched on a solid-phase silica C-18 column followed by a semi-preparative reverse-phase high-performance liquid chromatography. The purified sample was labelled with a multisubstrate deoxyribonucleoside kinase from Drosophila melanogaster (Dm-dNK) in the presence of 5'-bromo-2'-deoxyuridine (BrdU) as internal standard. The absolute sensitivity of the method was 0.1 fmol epsilondC detectable in 500 microl of human urine. The analysis of human urine samples from 15 healthy volunteers revealed a mean epsilondC level of 2.49+/-1.76 (SD) fmol micromol-1 creatinine (range 0.66-6.42). By this non-invasive method, epsilondC in human urine could be explored as a biomarker for oxidative stress-related human diseases.

Quantification of urinary etheno-DNA adducts by column-switching LC/APCI-MS/MS

Lipid peroxidation induced etheno-DNA adducts are promutagenic and have been suggested to play a causal role in the development of human cancers. Therefore, human biomonitoring of etheno-DNA adducts in urine has been suggested as a potential marker for oxidative stress-related DNA damage. For quantitative determination, a column-switching LC/APCI-MS/MS method was developed for simultaneous analysis of epsilonAde, epsilondC, and epsilondA in human urine. Quantitative validation parameters (precision, within-day repeatability, and between-day reproducibility) yielded satisfactory results below 10%. Limit of quantification for epsilonAde, epsilondC, and epsilondA was 5.3 fmol, 7.5 fmol, and 1.3 fmol on column, respectively. Mean urinary excretion rates of a six healthy volunteers were 45.8 pmol epsilonAde/24 h, 96.8 pmol epsilondC/24 h, and 18.1 pmol epsilondA/24 h. The demonstrated levels of performance suggest a future applicability of this method to studies of cancer and other diseases related to oxidative stress in humans. To our knowledge, this is the first method described that allows simultaneous determination of epsilonAde, epsilondC, and epsilondA in human urine samples.

Increased bioaccumulation of urethane in CYP2E1-/- versus CYP2E1+/+ mice

Urethane is a fermentation by-product and a potent animal carcinogen. Human exposure to urethane occurs through consumption of alcoholic beverages and fermented foods. Recently, CYP2E1 was identified as the primary enzyme responsible for the metabolism of [(14)C]carbonyl-labeled urethane. Subsequently, attenuation of urethane-induced cell proliferation and genotoxicity in CYP2E1-/- mice was reported. The present work compares the metabolism of single versus multiple exposures of CYP2E1-/- and CYP2E1+/+ mice to (14)C-ethyl-labeled urethane. Urethane was administered as a single 10 or 100 mg/kg gavage dose or at 100 mg/kg/day for 5 consecutive days. CYP2E1+/+ mice administered single or multiple doses exhaled 78 to 88% of dose as (14)CO(2)/day. CYP2E1-/- mice eliminated 30 to 38% of a single dose as (14)CO(2) in 24 h and plateaued after day 3 at approximately 52% of dose/day. The concentrations of urethane-derived radioactivity in plasma and tissues were dose-dependent, increased as a function of the number of doses administered, and were significantly higher in CYP2E1-/- versus CYP2E1+/+ mice. Whereas urethane was the main chemical found in the plasma and tissues of CYP2E1-/- mice, it was not detectable in CYP2E1+/+ mice. In conclusion, multiple dosing led to considerable bioaccumulation of urethane in mice of both genotypes; however, greater retention occurred in CYP2E1-/- versus CYP2E1+/+ mice. Furthermore, greater bioaccumulation of (14)C-ethyl-labeled than [(14)C]carbonyl-labeled urethane was observed in mice. Comparison of the metabolism of ethyl-versus carbonyl-labeled urethane was necessary for tracing the source of CO(2) and led us to propose for the first time that C-hydroxylation is a likely pathway of urethane metabolism.

Effect of ethanol on the tumorigenicity of urethane (ethyl carbamate) in B6C3F1 mice

Urethane is a carcinogen to which there is widespread exposure through the consumption of fermented foods and alcoholic beverages. In this study, we have assessed the carcinogenicity of urethane in combination with ethanol. Male and female B6C3F(1) mice (48 mice per sex per group) were exposed to 0, 10, 30, or 90 ppm urethane in the presence of 0%, 2.5%, or 5% ethanol in drinking water ad libitum for two years, at which time the extent of tumorigenesis was assessed. Additional mice (four per sex per group) received the same doses for four weeks to assess serum levels of urethane and ethanol, DNA adduct formation, and the induction of microsomal cytochromes P450, cell proliferation, and apoptosis. Urethane decreased cell replication in the livers of female, but not male, mice, decreased cell replication in the lungs of both sexes, and induced cytochrome P450 2E1 in the livers of female mice. Hepatic levels of the DNA adduct 1,N(6)-ethenodeoxyadenosine were increased by exposure to urethane and decreased by treatment with ethanol. Animal weights and survival were not affected by ethanol; in contrast, urethane administration decreased body weights and survival. Urethane caused dose-dependent increases in liver, lung, and harderian gland adenoma or carcinoma and hemangiosarcoma of the liver and heart in both sexes, mammary gland and ovarian tumors in females, and squamous cell papilloma or carcinoma of the skin and forestomach in males. The increase in hepatocellular tumors occurred in a relatively linear manner and was attributed to the formation of 1,N(6)-ethenodeoxyadenosine in hepatic DNA coupled with an increase in cell replication. Hemangiosarcomas were observed only at the 90 ppm urethane dose and were probably a result of high-dose urethane-induced toxicity. Lung alveolar/bronchiolar and harderian gland adenoma or carcinoma increased in a relatively linear manner, suggestive of a genotoxic mechanism for tumor induction. Ethanol induced a dose-dependent trend in hepatocellular adenoma or carcinoma in male mice, with the incidence being marginally increased at the highest dose. In female mice administered 10 ppm and 90 ppm urethane, ethanol caused dose-related increases in alveolar/bronchiolar adenoma or carcinoma and hemangiosarcoma of the heart, respectively. This may be due to ethanol decreasing the first-pass clearance of urethane, thus, increasing systemic distribution. In male mice a different relationship was observed: ethanol caused a dose-related decrease in alveolar/bronchiolar and harderian gland adenoma or carcinoma in mice administered 30 ppm urethane.

Urinary excretion of 3,N4-etheno-2'-deoxycytidine in humans as a biomarker of oxidative stress: association with cigarette smoking

Smokers are known to have elevated levels of lipid peroxidation, a form of oxidative stress. Etheno DNA adduct formation can originate from endogenous lipid peroxidation or from exogenous exposure of carcinogens. Using a modified stable isotope dilution GC/negative ion chemical ionization/MS assay originally developed for urinary 3,N(4)-ethenocytosine (epsilonCyt), the nucleoside 3,N(4)-etheno-2'-deoxycytidine (epsilondCyd) was detected for the first time in human urine. The presence of epsilondCyd in human urine was confirmed by LC/electrospray ionization/tandem MS. Concentrations of epsilondCyd in the 24 h urine samples from healthy individuals not occupationally exposed to industrial chemicals were in the range between 0 and 0.80 nM. A statistically significant correlation was established between cigarette smoking and urinary excretion of epsilondCyd after being adjusted for creatinine (p = 0.004). Furthermore, the urinary total antioxidant capacity was found to correlate inversely with the epsilondCyd levels (r = -0.50, p = 0.02). The results indicate that urinary epsilondCyd may provide a valuable noninvasive biomarker for oxidative DNA damage.

Quantification of 1,N6-etheno-2'-deoxyadenosine in human urine by column-switching LC/APCI-MS/MS

1,N6-etheno-2'-deoxyadenosine (epsilondA) is one of several promutagenic DNA modifications arising from cellular oxidative metabolism. It is believed that these background DNA lesions may contribute to various diseases, such as cancer. Therefore, human biomonitoring of epsilondA in urine could be a potential marker for oxidative stress-related DNA damage. Existing methods for quantifying urinary epsilondA use 32P postlabeling. We have developed a nonradioactive, fast, and easier method based on column-switching liquid chromatography with atmospheric pressure chemical ionization tandem mass spectrometry (LC/APCI-MS/MS) in the positive mode. Differences in column temperatures were used to influence analyte retention and sample focusing. With multiple reaction monitoring (MRM) mode the afforded limit of detection was about 0.7 pM when starting with 3 ml of urine. The urinary excretion rates of epsilondA from 28 nonsmoking and 5 smoking men were 10.0-99.6 pmol/24 h, and did not correlate with body weight, age, or plasma vitamin C concentration. The 5 smokers excreted 30.5 +/-8.5 and the 28 nonsmokers excreted 38.6 +/- 2.4 pmol epsilondA per 24 h, p=.37 (mean +/- SEM). The demonstrated level of performance suggests the future applicability of this method to studies of cancer and other diseases related to oxidative stress in humans.

Prevalence of liver tumours in HIV-1 tat-transgenic mice treated with urethane

The human immunodeficiency virus type 1 (HIV-1) Tat protein stimulates cell proliferation, inhibits apoptosis, displays angiogenic functions and is believed to be involved in the pathogenesis of Kaposi's sarcoma (KS) and other tumours arising in AIDS patients. Tat-transgenic (TT) mice, which constitutively express Tat in all tissues and organs, may therefore be predisposed to tumorigenesis. To test this hypothesis, we treated TT mice with urethane, a general carcinogen inducing tumours of various organs. The results indicate that, after injection of urethane, the incidence of lung tumours and lymphomas is not significantly different in the TT and control (CC) mice, whereas liver preneoplastic lesions and tumours show a significantly greater incidence in TT than in CC mice. This remarkable carcinogenic effect of urethane for the liver may be due to a tat-induced predisposition, manifested as a liver cell dysplasia (LCD), spontaneously affecting most of the TT mice. LCD may exert a promoting effect by stimulating proliferation of cell clones initiated by the mutagenic effect of urethane. In addition, LCD, which is associated with aneuploidy and chromosome instability, may enhance the progression to malignancy of the preneoplastic lesions induced by urethane. Interestingly, a significantly greater incidence of vascular ectasias and haemangiomas was detected in the liver of urethane-treated TT mice, most likely due to the marked angiogenic properties of Tat. This study suggests a role for Tat in the promotion and progression of tumours initiated by exogenous and endogenous carcinogens in HIV-1-infected patients, thereby contributing to the tumorigenesis in the course of AIDS.

Monitoring of DNA Adducts in Humans and 32P-Postlabelling Methods. A Review

DNA adduct formation in humans is a promising biomarker for elucidating the molecular epidemiology of cancer. For detection of DNA adducts, the most widely used methods include mass spectroscopy, fluorescence spectroscopy, immunoassays and 32P-postlabelling. Among them, the 32P-postlabelling method appears to meet best the criteria of sensitivity and amount of DNA needed, and, therefore, is one of the most appropriate methods for biomonitoring of human DNA adducts. Most classes of carcinogens have been subjected to 32P-postlabelling analysis, ranging from bulky and/or aromatic compounds to small and/or aliphatic compounds; it has also been used, with modifications, to detect apurinic sites in DNA, oxidative damage to DNA, UV-induced photodimers and, to a lesser extent, DNA damage caused by cytotoxic drugs. It has been used in human biomonitoring studies to detect DNA damage from occupational exposure to carcinogens, and also from environmental (i.e. non-occupational) exposures. It has also led to the discovery of the presence of numerous modifications in DNA arising from endogenous processes. The principle of the method is the enzymatic digestion of DNA to nucleotides, 5'-labelling of these nucleotides with an isotopically labelled phosphate group, and the resolution, detection and quantitation of the labelled products. Since the development of the original procedure in the early 1980s, many methods have been developed to increase the sensitivity by enrichment of modified nucleotides prior to labelling. The review presents the individual 32P-postlabelling techniques (standard procedure, enrichment methods) and a critical evaluation of these assays, besides reviewing the applications of the method to different DNA modifications, and its utilization in human biomonitoring studies. A review with 179 references.

Detection and quantification of 1,N6-ethenoadenine in human urine by stable isotope dilution capillary gas chromatography/negative ion chemical ionization/mass spectrometry

1,N(6)-Ethenoadenine (epsilonAde) is a promutagenic lesion detected in tissue DNA; it has been shown that epsilonAde can be repaired by human DNA glycosylases, and it is expected to be excreted in urine. In this paper, we present for the first time detection and accurate quantification of epsilonAde in human urine samples by a highly sensitive and specific stable isotope dilution gas chromatography/negative ion chemical ionization/mass spectrometric assay (GC/NICI/MS). Analysis by GC/NICI/MS includes adduct enrichment by a solid phase extraction column, followed by electrophore labeling and postderivatization cleanup. Using selective ion monitoring mode, the assay allows quantification of 0.5 pg of epsilonAde in as little as 0.1 mL of the urine sample, which is equivalent to corresponding concentration quantification limit of 31 pM. Using this assay, concentrations of epsilonAde in the 24 h urine samples of 23 healthy individuals were determined, which ranged from 0 to 124 pg/mL. After we adjusted for creatinine, a statistically significant correlation was found between epsilonAde excretion and cigarette smoking in males (p = 0.03). Thus, this stable isotope dilution GC/NICI/MS assay offers a sensitive and accurate quantification of urinary epsilonAde as a potential biomarker for oxidative damage of DNA and repair.

Potential role of lipid peroxidation derived DNA damage in human colon carcinogenesis: studies on exocyclic base adducts as stable oxidative stress markers

Molecular pathways to colorectal cancer involve multiple genetic changes that may be caused by overproduction of reactive oxygen species in cancer-related genes. Our aim was to investigate, whether besides direct oxidative DNA damage, reactive oxygen and nitrogen species induce lipid peroxidation (LPO) that could yield etheno-DNA adducts via trans-4-hydroxy-2-nonenal, a major aldehyde generated by LPO, in colon tissue. We analyzed the etheno-DNA adducts by a highly specific, ultrasensitive method involving immunoaffinity chromatography coupled with 32P-postlabelling [Carcinogenesis 16 (1995) 613] in affected colon epithelium from ulcerative colitis, Crohn's disease and familial adenomatous polyposis (FAP) and compared them with asymptomatic colon tissue. In all these cancer prone colon tissues, the formation of markedly enhanced etheno adduct levels was demonstrated for the first time. Etheno-DNA adducts are promutagenic and cause genomic instability that could drive the inflamed colonic epithelia to malignancy. Etheno-DNA adducts appear promising biomarkers for (i) quantifying increased DNA damage in early stages of colon carcinogenesis and for (ii) verifying the efficacy of new antioxidants (e.g. [Lancet Oncol. 1 (2000) 107]) and chemopreventive agents in lowering oxidative stress and related cancer risk.

Exocyclic DNA adducts as oxidative stress markers in colon carcinogenesis: potential role of lipid peroxidation, dietary fat and antioxidants

Molecular pathways to colorectal cancer involve multiple genetic changes, whereby extensive oxyradical damage causes mutations in cancer-related genes and leads to a cycle of cell death and regeneration. Besides direct oxidative DNA-damage, reactive oxygen and nitrogen species can induce etheno (epsilon)-DNA adducts mainly via trans-4-hydroxy-2-nonenal, generated as the major aldehyde by lipid peroxidation (LPO) of omega-6 PUFAs. Patients with familial adenomatous polyposis (FAP) develop multiple colorectal adenomas. In affected tissues increased LPO could be triggered due to increased arachidonic acid metabolism as a result of elevated cyclooxygenases. Our studies demonstrated an increased epsilon-DNA adduct level in affected colon epithelia of FAP patients. Epsilon-DNA adducts are promutagenic and can cause genomic instability that drives colorectal adenoma to malignancy. We have further investigated the potential chemopreventive properties of olive oil and its polyphenolic components. 'Mediterranean diet', of which olive oil is a major fatty acid source, has protective effects against human breast and colorectal cancers. Olive oil extracts and the newly identified lignan fractions showed high antioxidant capacity in vitro. As epsilon-DNA adducts are biomarkers for oxidative stress and LPO induced DNA damage, they can verify the efficacy of newly identified antioxidants, e.g. from olive oil, as chemopreventive agents against colon carcinogenesis.

Moderate G6PD deficiency increases mutation rates in the brain of mice

Mice that harbored the x-ray-induced low efficiency allele of the major X-linked isozyme of glucose-6-phospate dehydrogenase (G6PD), Gpdx(a-m2Neu), and, in addition, harbored the transgenic shuttle vector for the determination of mutagenesis in vivo, pUR288, were employed to further our understanding of the interdependence of general metabolism, oxidative stress control, and somatic mutagenesis. The Gpdx(a-m2Neu) mutation conferred moderate G6PD deficiency in hemizygous males (Gpdx(a-m2Neu/y)) displaying residual enzyme activities of 27% in red blood cells and 13% in brain (compared to wild-type controls, Gpdx(a/y) males). In spite of this mild phenotype, the brains of G6PD-deficient males exhibited a significant distortion of redox control ( approximately 3-fold decrease in the ratio of reduced glutathione to oxidized glutathione), a considerable accumulation of promutagenic etheno DNA adducts ( approximately 13-fold increase in ethenodeoxyadenosine and approximately 5-fold increase in ethenodeoxycytidine), and a substantial elevation of somatic mutation rates ( approximately 3-fold increase in mutant frequencies in lacZ, the target and reporter gene of mutagenesis in the shuttle vector, pUR288). The mutation pattern in the brain was dominated by illegitimate genetic recombinations, a presumed hallmark of oxidative mutagenesis. These findings suggested a critical function for G6PD in limiting oxidative mutagenesis in the mouse brain.

Ethanol enhances the formation of endogenously and exogenously derived adducts in rat hepatic DNA

To investigate the role of ethanol in chemically-induced carcinogenesis, we exposed Wistar rats to ethanol, either as an acute dose or for prolonged periods in a liquid diet and looked for effects on endogenously and exogenously derived DNA adducts. Changes in the cytochrome P450 protein (CYP 2E1) and its catalytic demethylase activity were also followed in order to provide a sequence of relatively well understood changes that are associated with free radical production and, therefore, potentially capable of affecting DNA. The exocyclic DNA adducts, ethenodeoxyadenosine (varepsilondA) and ethenodeoxycytidine (varepsilondC), known to arise from oxidative stress and lipid peroxidation (LPO) sources, were detected in the liver DNA of Wistar rats at background concentrations of 4-6 (varepsilondA) and 25-35 (varepsilondC) adducts per 10(9) parent bases. When rats were given either an acute dose of ethanol (5g/kg, i.g.) or exposed for 1 week to ethanol in a liquid diet (5%, w/v), etheno adduct levels were increased approximately 2-fold and this was statistically significant for varepsilondC (P<0.05 and P<0.02, respectively) for the two separate treatments.In N-nitrosodimethylamine (NDMA)-treated rats, acute ethanol treatment significantly increased the level of O(6)-methylguanine (O(6)-MeG) in hepatic DNA and this was paralleled by a decrease in O(6)-alkylguanine DNA alkyltransferase (ATase) activity; immunohistochemistry confirmed this increase of O(6)-MeG in both hepatic and renal nuclei. When rats were given ethanol in the diet and treated with NDMA, O(6)-MeG levels in hepatic DNA increased at 1 week which coincided with the peak of CYP 2E1-dependent NDMA-demethylase activity. Single cell gel electrophoresis of liver cells showed that after 1 week of exposure to ethanol, there was a small but significant increase in the frequency of DNA strand breaks induced by NDMA (P<0.05); after 4 weeks the increase was 1.4-fold (P<0.01). Our results indicate that exposures to ethanol, which resulted in blood ethanol concentrations similar to those seen in chronic alcoholics and increased levels of expression of the CYP 2E1 protein can exacerbate the DNA damaging effects of endogenous and exogenous alkylating agents. These observations provide indications of possible mechanisms for the carcinogenic or co-carcinogenic action of ethanol.

Quantitative analysis of etheno-2'-deoxycytidine DNA adducts using on-line immunoaffinity chromatography coupled with LC/ES-MS/MS detection

Etheno DNA adducts, including 3,N4-etheno-2'-deoxycytidine (etheno-dC), are promutagenic lesions present in normal animal and human tissues. These DNA adducts are believed to be important in the etiology of cancer. Existing methods for quantifying etheno-dC use 32p. postlabeling. Although highly sensitive, postlabeling requires the use of an energetic radioisotope and considerable time and effort. The new methodology reported here permits automated quantification of trace levels of etheno-dC in crude DNA hydrolysates on the order of 5 adducts in 10(8) normal nucleotides from 100-microg samples of DNA. This was accomplished by using on-line immunoaffinity chromatography, a reverse-phase LC separation on graphitized carbon, tandem mass spectrometric detection, and an isotopically labeled internal standard. The automated procedures permitted analysis of 4 DNA hydrolysates/hr. The sensitivity using immunoaffinity cleanup was approximately 100-fold greater than that observed when using a silica-based trapping system. The validated method was applied to the analysis of etheno-dC in commercial calf thymus DNA, untreated mouse liver, and untreated rat liver DNA. The demonstrated level of performance suggests future applicability of this method in studies of cancer in humans and experimental animals.

Quantification of etheno-DNA adducts using liquid chromatography, on-line sample processing, and electrospray tandem mass spectrometry

Etheno-DNA adducts are promutagenic lesions present in normal animal and human tissues that are believed to be important in the etiology of cancer related to diet and lifestyle. A method has been developed for the quantification of trace levels of etheno-DNA adducts using on-line sample preparation coupled with liquid chromatography and electrospray tandem mass spectrometry. The use of automated solid-phase extraction and stable labeled internal standards permitted the robust determination of ethenodeoxyadenosine contained in crude DNA hydrolysates from untreated rodent and human tissues at levels on the order of one adduct in 10(8) normal nucleotides from 100 microg of DNA. Inherent analyte response and matrix interference made sensitivity for simultaneous determination of ethenodeoxycytidine approximately 5-fold lower. The method was applied to the analysis of liver DNA from untreated and urethane-treated B6C3F1 mice, untreated rat liver, human placenta, and several commercial DNA preparations. Some sources of potential artifactual formation of etheno-DNA adducts were investigated.

Ultrasensitive and specific detection methods for exocylic DNA adducts: markers for lipid peroxidation and oxidative stress

Among exocyclic DNA adducts, etheno (epsilon) bases (epsilond A, epsilond C, N(2),3-epsilond G) are generated by reactions of DNA bases with lipid peroxidation (LPO) products derived from endogenous sources and from the carcinogens vinyl chloride or urethane. The recent development of ultrasensitive methods has made it possible to detect these epsilon-adducts in vivo and to study their formation and role in experimental and human carcinogenesis. The promutagenic epsilon-DNA modifications can be detected by immunoaffinity/32P-postlabelling or by immunohistochemistry. When epsilon-adducts are excised from tissue DNA, the modified nucleosides can be quantified in urine by an immunoaffinity-HPLC-fluorescence method. Highly variable background levels of epsilon-adducts were detected in tissues from unexposed humans and rodents, suggesting an endogenous pathway of formation from reaction of trans-4-hydroxy-2-nonenal (via its 2,3-epoxide) with DNA bases. Several known cancer risk factors increased the level of these DNA lesions: Elevated epsilon-adducts were found in hepatic DNA from patients with excess metal storage (haemochromatosis, Wilson's disease), resulting in oxidative stress and high risk of liver cancer. Reactive O/N-intermediates generated during inflammatory processes, for example in patients with inflammatory bowel disease (IBD) and familial adenomatous polyposis (FAP) led to the formation of epsilon-adducts likely through peroxynitrite-mediated LPO and/or increased oxidative arachidonic acid metabolism. A high omega-6-polyunsaturated fatty acid (PUFA) diet increased epsilon-DNA adducts in white blood cells (WBC), particularly in female subjects (about 40-fold), while the level of adducted malondialdehyde in deoxyguanosine of WBC-DNA was only moderately elevated. In conclusion, there is now growing evidence that epsilon-adducts were elevated in cancer-prone patients and in rodents (liver, pancreas, colon, skin), suggesting that promutagenic epsilon-adducts, when formed as a consequence of persistent oxidative stress, can drive cells to malignancy. Therefore, biomonitoring of exocyclic DNA adducts offers useful tools: (i) to evaluate the etiological contributions of dietary fats, oxidative stress, and chronic inflammatory/infectious processes; (ii) to verify the efficacy of chemopreventive agents on endogenous DNA damage and cancer risk; and (iii) to gain mechanistic insights into the role of oxidative stress/LPO-derived lesions in the initiation and progression of human cancer.

New DNA-based biomarkers for oxidative stress and cancer chemoprevention studies

Etheno (epsilon) modified DNA bases (epsilond A, epsilond C, epsilond G) are generated from the carcinogens vinyl chloride and urethane, but also by reactions of DNA with products derived from lipid peroxidation (LPO) and oxidative stress via endogenous pathways. Recently developed ultrasensitive methods allowed the detection of these epsilon-adducts in vivo and their role to be studied in experimental and human carcinogenesis. Highly variable background levels of epsilon-adducts were detected in DNA from different organs of unexposed humans and rodents. Several known cancer risk factors increased the level of these DNA lesions in target organs: elevated epsilon-adducts were found in hepatic DNA from patients with metal storage diseases, after overproduction of nitric oxide (NO) by inducible NO synthase (iNOS) in a mouse model, and in colonic polyps of familial adenomatous polyposis patients. A high omega-6-polyunsaturated fatty acid diet increased epsilon-DNA adducts in white blood cells of female subjects. In conclusion, epsilon-adducts were found to be elevated in cancer-prone tissues, suggesting that these promutagenic lesions could drive cells to malignancy. Therefore, exocyclic DNA adducts offer new tools in cancer aetiological research and in verifying the efficacy of chemopreventive agents.

Thymine-DNA glycosylase and G to A transition mutations at CpG sites

About 23% of mutations in hereditary human diseases and 24% of mutations in p53 in human cancers are G to A transitions at sites of cytosine methylation suggesting that these sites are either foci for DNA damage, or foci for damage that is poorly repaired. Thymine produced at these sites by the hydrolytic deamination of 5-methylcytosine is removed by thymine-DNA glycosylase. Thymine-DNA glycosylase will also remove 3,N(4)-ethenocytosine and uracil from DNA. The action of this enzyme is limited by its very low k(cat) and by tight binding to the apurinic site produced when the thymine is removed. These properties of the enzyme suggest that the inefficiency of the base excision repair pathway that it initiates may be the underlying cause of the prevalence of these mutations.

Etheno-adduct-forming chemicals: from mutagenicity testing to tumor mutation spectra

During the past 25 years, ethenobases have emerged as a new class of DNA lesions with promutagenic potential. Ethenobases were first investigated as DNA reaction products of vinyl chloride, an occupational carcinogen causing angiosarcoma of the liver (ASL). They were subsequently shown to be formed by several carcinogenic agents, including urethane (ethyl carbamate), and more recently, to occur in various tissues of unexposed humans and rodents. The endogenous source of ethenobases in DNA is thought to be a lipid peroxidation (LPO) product. Initial studies on metabolic activation, mutagenicity and carcinogenicity moved to the analyses of the formation of ethenobases in vivo and to the determination of their promutagenic properties. Quantification of etheno adducts in vivo became possible with the development of ultrasensitive techniques of analysis. To study the miscoding properties of ethenobases, the initial assays on the fidelity of replication or of transcription were replaced by site-directed mutagenesis assays in vivo. Ethenobases generate mainly base pair substitution mutations. With the advent of new techniques of molecular biology, mutations were investigated in the ras and p53 genes of tumors induced by vinyl chloride and urethane. In liver tumors induced by vinyl chloride, specific mutational patterns were found in the Ki-ras gene in human ASL, in the Ha-ras gene in hepatocellular carcinoma (HCC) in rats, and in the p53 gene in human and rat ASL. In tumors induced by urethane in mice, codon 61 of the Ha-ras gene (liver, skin) and of the Ki-ras gene (lung) seems to be a characteristic target. These tumor mutation spectra are compatible with the promutagenic properties of etheno adducts and with their formation in target tissues, suggesting that ethenobases can be initiating lesions in carcinogenesis. Another recent focus has been given to the repair of etheno adducts, and DNA glycosylases able to excise these adducts in vitro have been identified. The last two decades have brought ethenobases to light as potentially important DNA lesions in carcinogenesis. More research is needed to better understand the environmental and genetic factors that affect the formation and persistence of ethenobases in vivo.

Detection and quantification of 1,N(6)-ethenoadenine in human placental DNA by mass spectrometry

Exocyclic DNA adducts have been reported to derive from various exogenous as well as endogenous sources, such as lipid peroxidation. Among them, 1,N(6)-ethenoadenine (epsilonAde) has previously been detected in tissue DNA of untreated rodents and humans by an immunoaffinity/(32)P-postlabeling method. This study reports detection and quantification of the endogenous epsilonAde adduct in the same human placental DNA by three independent assays, namely, GC/MS, LC/MS, and HPLC/fluorescence. Using a recently reported gas chromatography/negative ion chemical ionization/mass spectrometry (GC/NICI/MS) method [Chen, H.-J. C., et al. (1998) Chem. Res. Toxicol. 11, 1474], the level of epsilonAde in human placental DNA from a commercial source was found to be 2.3 adducts per 10(6) Ade bases. To confirm these findings, a liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI/MS/MS) method was developed for epsilondAdo. With this LC/MS assay, epsilondAdo was detected at the level of 2.5 adducts per 10(6) dAdo nucleosides in the same human placental DNA. The stable isotopes of epsilonAde and epsilondAdo were added as internal standards in both GC/MS and LC/ESI/MS/MS assays, respectively, and thus provided high specificity, reproducibility, and accurate quantification. The relatively high levels of epsilonAde in this human placental DNA detected by mass spectrometry were further verified by HPLC/fluorescence analysis. The GC/MS method was validated by the HPLC/fluorescence assay using calf thymus DNA treated with chloroacetaldehyde or by the LC/MS method with 2, 3-epoxy-4-hydroxynonanal-modified calf thymus DNA. The epsilonAde level in human placental DNA freshly isolated in the presence of an antioxidant was similar to that in DNA from the commercial source. Since epsilonAde is a potential mutagenic lesion, analysis of epsilonAde by the specific and sensitive GC/NICI/MS method may provide a useful biomarker in cancer risk assessment.

Etheno DNA-base adducts from endogenous reactive species

Promutagenic etheno (epsilon) adducts in DNA are generated through reactions of DNA bases with LPO products derived from endogenous sources or from exposure to several xenobiotics. The availability of sensitive methods has made it possible to detect three epsilon-adducts in vivo, namely epsilon dA, epsilon dC and N2,3-epsilon dG. One probable endogenous source for the formation of these adducts arises from LPO products such as trans-4-hydroxy-2-nonenal (HNE), resulting in highly variable background epsilon-adduct levels in tissues from unexposed humans and rodents. The range of background levels of epsilon dAx10-8dA detected inhuman tissues was <0.05 to 25 and in rodent tissues 0.02 to 10; the corresponding values for epsilon dCx10-8dC were 0.01 to 11 and 0.03 to 24, respectively. Part of this variability may be associated with different dietary intake of antioxidants and/or omega-6 PUFAs which oxidize readily to form 4-hydroxyalkenals, as epsilon dA and epsilon dC levels in WBC-DNA of female volunteers on a high omega-6 PUFA diet were drastically elevated. Increased levels of etheno adducts were also found in the liver of cancer-prone patients suffering from hereditary metal storage diseases, i.e., Wilson's disease (WD) and primary hemochromatosis (PH) as well as in Long-Evans Cinnamon rats, an animal model for WD. Increased metal-induced oxidative stress and LPO-derive epsilon-adducts, along with other oxidative damage, may trigger this hereditary liver cancer. Epsilon-Adducts could hence be explored as biomarkers (i) to ascertain the role of LPO mediated DNA damage in human cancers associated with oxidative stress imposed by certain lifestyle patterns, chronic infections and inflammations, and (ii) to verify the reduction of these epsilon-adducts by cancer chemopreventive agents. This article summarizes recent results on the formation, occurrence and possible role of epsilon-DNA adducts in carcinogenesis and mutagenesis.