The vinyl chloride DNA derivative N2,3-ethenoguanine produces G----A transitions in Escherichia coli

Detection of DNA Adduct Formation in Rat Lungs by a Micro-HPLC/MS/MS Approach

Aldehydes are abundantly present in tobacco smoke and in urban air pollution and are endogenously generated as products of the lipid peroxidation process. These molecules can react with DNA bases forming mutagenic exocyclic adducts, which have been used as biomarkers of aldehyde exposure and as potential tools for the study of inflammation, metal storage diseases, neurodegenerative disorders, and cancer. High-performance liquid chromatography-tandem mass spectrometry (HPLC/MS/MS) provides a highly precise, specific and ultrasensitive method for the detection of exocyclic DNA adducts. Here we present and describe a validated micro-HPLC-Electro Spray Ionization (ESI)-MS/MS method for the quantification of 1,N²-propanodGuo, an adduct produced following the reaction between 2'-deoxyguanosine and acetaldehyde or crotonaldehyde.

DNA adducts: Formation, biological effects, and new biospecimens for mass spectrometric measurements in humans

Hazardous chemicals in the environment and diet or their electrophilic metabolites can form adducts with genomic DNA, which can lead to mutations and the initiation of cancer. In addition, reactive intermediates can be generated in the body through oxidative stress and damage the genome. The identification and measurement of DNA adducts are required for understanding exposure and the causal role of a genotoxic chemical in cancer risk. Over the past three decades, 32 P-postlabeling, immunoassays, gas chromatography/mass spectrometry, and liquid chromatography/mass spectrometry (LC/MS) methods have been established to assess exposures to chemicals through measurements of DNA adducts. It is now possible to measure some DNA adducts in human biopsy samples, by LC/MS, with as little as several milligrams of tissue. In this review article, we highlight the formation and biological effects of DNA adducts, and highlight our advances in human biomonitoring by mass spectrometric analysis of formalin-fixed paraffin-embedded tissues, untapped biospecimens for carcinogen DNA adduct biomarker research.

DNA damage, repair and the improvement of cancer therapy - A tribute to the life and research of Barbara Tudek

Professor Barbara Tudek received the Frits Sobels Award in 2019 from the European Environmental Mutagenesis and Genomics Society (EEMGS). This article presents her outstanding character and most important lines of research. The focus of her studies covered alkylative and oxidative damage to DNA bases, in particular mutagenic and carcinogenic properties of purines with an open imidazole ring and 8-oxo-7,8-dihydroguanine (8-oxoGua). They also included analysis of mutagenic properties and pathways for the repair of DNA adducts of lipid peroxidation (LPO) products arising in large quantities during inflammation. Professor Tudek did all of this in the hope of deciphering the mechanisms of DNA damage removal, in particular by the base excision repair (BER) pathway. Some lines of research aimed at discovering factors that can modulate the activity of DNA damage repair in hope to enhance existing anti-cancer therapies. The group's ongoing research aims at deciphering the resistance mechanisms of cancer cell lines acquired following prolonged exposure to photodynamic therapy (PDT) and the possibility of re-sensitizing cells to PDT in order to increase the application of this minimally invasive therapeutic method.

Chronic Ethanol Consumption and Generation of Etheno-DNA Adducts in Cancer-Prone Tissues

Chronic ethanol consumption is a risk factor for several human cancers. A variety of mechanisms may contribute to this carcinogenic effect of alcohol including oxidative stress with the generation of reactive oxygen species (ROS), formed via inflammatory pathways or as byproducts of ethanol oxidation through cytochrome P4502E1 (CYP2E1). ROS may lead to lipidperoxidation (LPO) resulting in LPO-products such as 4-hydoxynonenal (4-HNE) or malondialdehyde. These compounds can react with DNA bases forming mutagenic and carcinogenic etheno-DNA adducts. Etheno-DNA adducts are generated in the liver (HepG2) cells over-expressing CYP2E1 when incubated with ethanol;and are inhibited by chlormethiazole. In liver biopsies etheno-DNA adducts correlated significantly with CYP2E1. Such a correlation was also found in the esophageal- and colorectal mucosa of alcoholics. Etheno-DNA adducts also increased in liver biopsies from patients with non alcoholic steatohepatitis (NASH). In various animal models with fatty liver either induced by high fat diets or genetically modified such as in the obese Zucker rat, CYP2E1 is induced and paralleled by high levels of etheno DNA-adducts which may be modified by additional alcohol administration. As elevation of adduct levels in NASH children were already detected at a young age, these lesions may contribute to hepatocellular cancer development later in life. Together these data strongly implicate CYP2E1 as an important mediator for etheno-DNA adduct formation, and this detrimental DNA damage may act as a driving force for malignant disease progression.

Carcinogenic Etheno DNA Adducts in Alcoholic Liver Disease: Correlation with Cytochrome P-4502E1 and Fibrosis

BACKGROUND

One mechanism by which alcoholic liver disease (ALD) progresses is oxidative stress and the generation of reactive oxygen species, among others due to the induction of cytochrome P-4502E1 (CYP2E1). Experimental data underline the key role of CYP2E1 because ALD could be partially prevented in rats by the administration of the specific CYP2E1 inhibitor chlormethiazole. As CYP2E1 is linked to the formation of carcinogenic etheno DNA adducts in ALD patients, a causal role of alcohol-induced CYP2E1 in hepatocarcinogenesis is implicated. The purpose of this study was to investigate CYP2E1 induction in ALD, and its correlation with oxidative DNA lesions and with hepatic histology.

METHODS

Hepatic biopsies from 97 patients diagnosed with ALD were histologically scored for steatosis, inflammation, and fibrosis. CYP2E1 and the exocyclic etheno DNA adduct 1,N⁶ -etheno-2'deoxyadenosine (εdA) were determined immunohistochemically. In addition, in 42 patients, 8-hydroxydeoxyguanosine (8-OHdG) was also evaluated using immunohistochemistry.

RESULTS

A significant positive correlation was found between CYP2E1 and εdA (p < 0.0001) as well as between CYP2E1 and 8-OHdG (p = 0.039). Both CYP2E1 (p = 0.0094) and ɛdA (p < 0.0001) also correlated significantly with the stage of hepatic fibrosis. Furthermore, a significant correlation between the fibrosis stage and the grade of lobular inflammation (p < 0.0001) was observed. However, the amount of alcohol consumed did not correlate with any of the parameters determined.

CONCLUSIONS

These data suggest an important role of CYP2E1 in the generation of εdA, in the fibrotic progression of ALD, and thus in alcohol-mediated hepatocarcinogenesis. CYP2E1 may be a target in the treatment of ALD and a potential prognostic marker for disease progression.

A novel role for transcription-coupled nucleotide excision repair for the in vivo repair of 3,N4-ethenocytosine

Etheno (ε) DNA base adducts are highly mutagenic lesions produced endogenously via reactions with lipid peroxidation (LPO) products. Cancer-promoting conditions, such as inflammation, can induce persistent oxidative stress and increased LPO, resulting in the accumulation of ε-adducts in different tissues. Using a recently described fluorescence multiplexed host cell reactivation assay, we show that a plasmid reporter bearing a site-specific 3,N⁴-ethenocytosine (εC) causes transcriptional blockage. Notably, this blockage is exacerbated in Cockayne Syndrome and xeroderma pigmentosum patient-derived lymphoblastoid and fibroblast cells. Parallel RNA-Seq expression analysis of the plasmid reporter identifies novel transcriptional mutagenesis properties of εC. Our studies reveal that beyond the known pathways, such as base excision repair, the process of transcription-coupled nucleotide excision repair plays a role in the removal of εC from the genome, and thus in the protection of cells and tissues from collateral damage induced by inflammatory responses.

Lipid peroxidation in face of DNA damage, DNA repair and other cellular processes

Exocyclic adducts to DNA bases are formed as a consequence of exposure to certain environmental carcinogens as well as inflammation and lipid peroxidation (LPO). Complex family of LPO products gives rise to a variety of DNA adducts, which can be grouped in two classes: (i) small etheno-type adducts of strong mutagenic potential, and (ii) bulky, propano-type adducts, which block replication and transcription, and are lethal lesions. Etheno-DNA adducts are removed from the DNA by base excision repair (BER), AlkB and nucleotide incision repair enzymes (NIR), while substituted propano-type lesions by nucleotide excision repair (NER) and homologous recombination (HR). Changes of the level and activity of several enzymes removing exocyclic adducts from the DNA was reported during carcinogenesis. Also several beyond repair functions of these enzymes, which participate in regulation of cell proliferation and growth, as well as RNA processing was recently described. In addition, adducts of LPO products to proteins was reported during aging and age-related diseases. The paper summarizes pathways for exocyclic adducts removal and describes how proteins involved in repair of these adducts can modify pathological states of the organism.

Occurrence, Biological Consequences, and Human Health Relevance of Oxidative Stress-Induced DNA Damage

A variety of endogenous and exogenous agents can induce DNA damage and lead to genomic instability. Reactive oxygen species (ROS), an important class of DNA damaging agents, are constantly generated in cells as a consequence of endogenous metabolism, infection/inflammation, and/or exposure to environmental toxicants. A wide array of DNA lesions can be induced by ROS directly, including single-nucleobase lesions, tandem lesions, and hypochlorous acid (HOCl)/hypobromous acid (HOBr)-derived DNA adducts. ROS can also lead to lipid peroxidation, whose byproducts can also react with DNA to produce exocyclic DNA lesions. A combination of bioanalytical chemistry, synthetic organic chemistry, and molecular biology approaches have provided significant insights into the occurrence, repair, and biological consequences of oxidatively induced DNA lesions. The involvement of these lesions in the etiology of human diseases and aging was also investigated in the past several decades, suggesting that the oxidatively induced DNA adducts, especially bulky DNA lesions, may serve as biomarkers for exploring the role of oxidative stress in human diseases. The continuing development and improvement of LC-MS/MS coupled with the stable isotope-dilution method for DNA adduct quantification will further promote research about the clinical implications and diagnostic applications of oxidatively induced DNA adducts.

Epigenetic alterations induced by genotoxic occupational and environmental human chemical carcinogens: A systematic literature review

Accumulating evidence suggests that epigenetic alterations play an important role in chemically-induced carcinogenesis. Although the epigenome and genome may be equally important in carcinogenicity, the genotoxicity of chemical agents and exposure-related transcriptomic responses have been more thoroughly studied and characterized. To better understand the evidence for epigenetic alterations of human carcinogens, and the potential association with genotoxic endpoints, we conducted a systematic review of published studies of genotoxic carcinogens that reported epigenetic endpoints. Specifically, we searched for publications reporting epigenetic effects for the 28 agents and occupations included in Monograph Volume 100F of the International Agency for the Research on Cancer (IARC) that were classified as "carcinogenic to humans" (Group 1) with strong evidence of genotoxic mechanisms of carcinogenesis. We identified a total of 158 studies that evaluated epigenetic alterations for 12 of these 28 carcinogenic agents and occupations (1,3-butadiene, 4-aminobiphenyl, aflatoxins, benzene, benzidine, benzo[a]pyrene, coke production, formaldehyde, occupational exposure as a painter, sulfur mustard, and vinyl chloride). Aberrant DNA methylation was most commonly studied, followed by altered expression of non-coding RNAs and histone changes (totaling 85, 59 and 25 studies, respectively). For 3 carcinogens (aflatoxins, benzene and benzo[a]pyrene), 10 or more studies reported epigenetic effects. However, epigenetic studies were sparse for the remaining 9 carcinogens; for 4 agents, only 1 or 2 published reports were identified. While further research is needed to better identify carcinogenesis-associated epigenetic perturbations for many potential carcinogens, published reports on specific epigenetic endpoints can be systematically identified and increasingly incorporated in cancer hazard assessments.

Next-generation sequencing reveals the biological significance of the N(2),3-ethenoguanine lesion in vivo

Etheno DNA adducts are a prevalent type of DNA damage caused by vinyl chloride (VC) exposure and oxidative stress. Etheno adducts are mutagenic and may contribute to the initiation of several pathologies; thus, elucidating the pathways by which they induce cellular transformation is critical. Although N(2),3-ethenoguanine (N(2),3-εG) is the most abundant etheno adduct, its biological consequences have not been well characterized in cells due to its labile glycosidic bond. Here, a stabilized 2'-fluoro-2'-deoxyribose analog of N(2),3-εG was used to quantify directly its genotoxicity and mutagenicity. A multiplex method involving next-generation sequencing enabled a large-scale in vivo analysis, in which both N(2),3-εG and its isomer 1,N(2)-ethenoguanine (1,N(2)-εG) were evaluated in various repair and replication backgrounds. We found that N(2),3-εG potently induces G to A transitions, the same mutation previously observed in VC-associated tumors. By contrast, 1,N(2)-εG induces various substitutions and frameshifts. We also found that N(2),3-εG is the only etheno lesion that cannot be repaired by AlkB, which partially explains its persistence. Both εG lesions are strong replication blocks and DinB, a translesion polymerase, facilitates the mutagenic bypass of both lesions. Collectively, our results indicate that N(2),3-εG is a biologically important lesion and may have a functional role in VC-induced or inflammation-driven carcinogenesis.

Covalent modification of DNA by α, β-unsaturated aldehydes derived from lipid peroxidation: Recent progress and challenges

Oxidative stress-induced lipid peroxidation (LPO) has been associated with human physiology and pathophysiology. LPO generates an array of oxidation products and among them reactive lipid aldehydes have received intensive research attentions due to their roles in modulating functions of biomolecules through covalent modification. Thus, covalent modification of DNA by these reactive lipid electrophiles has been postulated to be partially responsible for the biological roles of LPO. In this review, we summarized recent progress and challenges in studying the roles of covalent modification of DNA including nuclear and mitochondrial DNA by reactive lipid metabolites from LPO. We focused on the novel mechanistic insights into generation of lipid aldehydes from cellular membranes especially mitochondria through LPO. Recent advances in the technological front using mass spectrometry have also been highlighted in the settings of studying DNA damage caused by LPO and its biological relevance.

The generation of carcinogenic etheno-DNA adducts in the liver of patients with nonalcoholic fatty liver disease

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD), in particular its more aggressive form nonalcoholic steatohepatitis (NASH) is increasingly observed as a cause of end stage liver disease and hepatocellular carcinoma (HCC). Reactive oxygen species (ROS) are an important factor in the pathogenesis of HCC. ROS can react with polyunsaturated fatty acids derived from membrane phospholipids resulting in the production of reactive aldehydes as lipid oxidation (LPO) byproducts, such as 4-hydroxynonenal (4 HNE). 4 HNE can react with DNA to form mutagenic exocyclic etheno-DNA adducts. ROS is induced by inflammatory processes, but also by induction of cytochrome P450 2E1 (CYP2E1), as seen with chronic alcohol consumption.

METHODS

Immunohistochemical detection of CYP2E1, 4 HNE and hepatic exocyclic etheno-DNA adducts was performed on liver sections from 39 patients with NFLD. Spearman rank correlation was calculated to examine possible correlations.

RESULTS

Exocyclic etheno-DNA adducts were detected and correlated significantly with 4 HNE, but not with CYP2E1.

CONCLUSIONS

This is the first description of highly carcinogenic exocyclic etheno-DNA adducts in NAFLD patients. We could show that exocyclic etheno-DNA adducts significantly correlated with lipid peroxidation product 4 HNE, but not with CYP2E1, implying that in NAFLD ROS generation with consecutive DNA damage is rather inflammation driven through various cytokines than by induction of CYP2E1.

The role of reactive oxygen species (ROS) and cytochrome P-450 2E1 in the generation of carcinogenic etheno-DNA adducts

Exocyclic etheno-DNA adducts are mutagenic and carcinogenic and are formed by the reaction of lipidperoxidation (LPO) products such as 4-hydoxynonenal or malondialdehyde with DNA bases. LPO products are generated either via inflammation driven oxidative stress or via the induction of cytochrome P-450 2E1 (CYP2E1). In the liver CYP2E1 is induced by various compounds including free fatty acids, acetone and ethanol. Increased levels of CYP2E1 and thus, oxidative stress are observed in the liver of patients with non-alcoholic steatohepatitis (NASH) as well as in the chronic alcoholic. In addition, chronic ethanol ingestion also increases CYP2E1 in the mucosa of the oesophagus and colon. In all these tissues CYP2E1 correlates significantly with the levels of carcinogenic etheno-DNA adducts. In contrast, in patients with non-alcoholic steatohepatitis (NASH) hepatic etheno-DNA adducts do not correlate with CYP2E1 indicating that in NASH etheno-DNA adducts formation is predominately driven by inflammation rather than by CYP2E1 induction. Since etheno-DNA adducts are strong mutagens producing various types of base pair substitution mutations as well as other types of genetic damage, it is strongly believed that they are involved in ethanol mediated carcinogenesis primarily driven by the induction of CYP2E1.

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Cytochrome P-450 2E1 is induced following chronic ethanol ingestion.

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CYP2E1 correlates with carcinogenic etheno-DNA formation.

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CYP2E1 and oxidative stress are important mechanisms in alcohol mediated carcinogenesis in the liver, undefined and colon.

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In NASH hepatic etheno-DNA adducts occur but possible due to inflammation.

The role of reactive oxygen species (ROS) and cytochrome P-450 2E1 in the generation of carcinogenic etheno-DNA adducts

Exocyclic etheno-DNA adducts are mutagenic and carcinogenic and are formed by the reaction of lipidperoxidation (LPO) products such as 4-hydoxynonenal or malondialdehyde with DNA bases. LPO products are generated either via inflammation driven oxidative stress or via the induction of cytochrome P-450 2E1 (CYP2E1). In the liver CYP2E1 is induced by various compounds including free fatty acids, acetone and ethanol. Increased levels of CYP2E1 and thus, oxidative stress are observed in the liver of patients with non-alcoholic steatohepatitis (NASH) as well as in the chronic alcoholic. In addition, chronic ethanol ingestion also increases CYP2E1 in the mucosa of the oesophagus and colon. In all these tissues CYP2E1 correlates significantly with the levels of carcinogenic etheno-DNA adducts. In contrast, in patients with non-alcoholic steatohepatitis (NASH) hepatic etheno-DNA adducts do not correlate with CYP2E1 indicating that in NASH etheno-DNA adducts formation is predominately driven by inflammation rather than by CYP2E1 induction. Since etheno-DNA adducts are strong mutagens producing various types of base pair substitution mutations as well as other types of genetic damage, it is strongly believed that they are involved in ethanol mediated carcinogenesis primarily driven by the induction of CYP2E1.

The endogenous exposome

The concept of the Exposome is a compilation of diseases and one's lifetime exposure to chemicals, whether the exposure comes from environmental, dietary, or occupational exposures; or endogenous chemicals that are formed from normal metabolism, inflammation, oxidative stress, lipid peroxidation, infections, and other natural metabolic processes such as alteration of the gut microbiome. In this review, we have focused on the endogenous exposome, the DNA damage that arises from the production of endogenous electrophilic molecules in our cells. It provides quantitative data on endogenous DNA damage and its relationship to mutagenesis, with emphasis on when exogenous chemical exposures that produce identical DNA adducts to those arising from normal metabolism cause significant increases in total identical DNA adducts. We have utilized stable isotope labeled chemical exposures of animals and cells, so that accurate relationships between endogenous and exogenous exposures can be determined. Advances in mass spectrometry have vastly increased both the sensitivity and accuracy of such studies. Furthermore, we have clear evidence of which sources of exposure drive low dose biology that results in mutations and disease. These data provide much needed information to impact quantitative risk assessments, in the hope of moving towards the use of science, rather than default assumptions.

1,2-Alkylarylation of activated alkenes with dual C–H bonds of arenes and alkyl halides toward polyhalo-substituted oxindoles

Alkylarylation of N-arylacrylamides with alkyl halides through selective scission of the C(sp³)–H bond adjacent to halide atoms is presented.

Oxidative stress-induced 1, N6-ethenodeoxyadenosine adduct formation contributes to hepatocarcinogenesis

Numerous studies have found that oxidative stress-derived 1, N⁶-ethenodeoxyadenosine (ɛ-dA) can act as a driving force towards hepatocellular carcinoma (HCC) in cancer-prone liver diseases. The aim of the present study was to determine the oxidative stress status and the occurrence of ɛ-dA in HCC and adjacent non-tumor liver tissue, and to clarify whether the occurrence of ɛ-dA is related to liver inflammatory activity, fibrosis and mutant p53 expression. Oxidative stress-related parameters were examined in tumor and (or) non-tumor liver tissues of 32 patients with HCC. ɛ-dA, mutant p53 and proliferating cell nuclear antigen (PCNA) were immunohistochemically investigated in control, HCC and non-tumor liver tissues. The total antioxidant capacity and total superoxide dismutase activity of HCC tissues were lower compared to those of non-tumor tissues (P<0.05 vs. P<0.001). The prevalence of ɛ-dA in HCC was significantly higher compared to control (P<0.0001) and non-tumor liver tissues (P<0.001). A significant correlation between the positive rate of ɛ-dA and mutant p53 was observed (r=0.5162, P<0.01). The positive rate of PCNA in HCC was significantly higher compared to control (P<0.0001) and non-tumor liver tissues (P<0.0001). There was a possible link between the formation of ɛ-dA and chronic inflammation and fibrosis. Therefore, ɛ-dA lesions may gradually accumulate in chronic liver diseases, and partially contribute to mutant p53 overexpression and excessive cell proliferation, making it a potential mechanism in oxidative stress-mediated hepatocarcinogenesis.

Basis of miscoding of the DNA adduct N2,3-ethenoguanine by human Y-family DNA polymerases

N(2),3-Ethenoguanine (N(2),3-εG) is one of the exocyclic DNA adducts produced by endogenous processes (e.g. lipid peroxidation) and exposure to bioactivated vinyl monomers such as vinyl chloride, which is a known human carcinogen. Existing studies exploring the miscoding potential of this lesion are quite indirect because of the lability of the glycosidic bond. We utilized a 2'-fluoro isostere approach to stabilize this lesion and synthesized oligonucleotides containing 2'-fluoro-N(2),3-ε-2'-deoxyarabinoguanosine to investigate the miscoding potential of N(2),3-εG by Y-family human DNA polymerases (pols). In primer extension assays, pol η and pol κ replicated through N(2),3-εG, whereas pol ι and REV1 yielded only 1-base incorporation. Steady-state kinetics revealed that dCTP incorporation is preferred opposite N(2),3-εG with relative efficiencies in the order of pol κ > REV1 > pol η ≈ pol ι, and dTTP misincorporation is the major miscoding event by all four Y-family human DNA pols. Pol ι had the highest dTTP misincorporation frequency (0.71) followed by pol η (0.63). REV1 misincorporated dTTP and dGTP with much lower frequencies. Crystal structures of pol ι with N(2),3-εG paired to dCTP and dTTP revealed Hoogsteen-like base pairing mechanisms. Two hydrogen bonds were observed in the N(2),3-εG:dCTP base pair, whereas only one appears to be present in the case of the N(2),3-εG:dTTP pair. Base pairing mechanisms derived from the crystal structures explain the slightly favored dCTP insertion for pol ι in steady-state kinetic analysis. Taken together, these results provide a basis for the mutagenic potential of N(2),3-εG.

Facile Synthesis of Mutagen X (MX): 3-Chloro-4-(dichloromethyl)-5-hydroxy-5H-furan-2-one

3-Chloro-4-(dichloromethyl)-5-hydroxy-5H-furan-2-one (Mutagen X, MX) was synthesized in six steps from commercially-available and inexpensive starting materials (27% overall yield). This synthesis enables the preparation of MX analogs and does not require the use of chlorine gas, as do previously reported methods.

Damage of DNA and proteins by major lipid peroxidation products in genome stability

Oxidative stress and lipid peroxidation (LPO) accompanying infections and chronic inflammation may induce several human cancers. LPO products are characterized by carbohydrate chains of different length, reactive aldehyde groups and double bonds, which make these molecules reactive to nucleic acids, proteins and cellular thiols. LPO-derived adducts to DNA bases form etheno-type and propano-type exocyclic rings, which have profound mutagenic potential, and are elevated in several cancer-prone diseases. Adducts of long chain LPO products to DNA bases inhibit transcription. Elimination from DNA of LPO-induced lesions is executed by several repair systems: base excision repair (BER), direct reversal by AlkB family proteins, nucleotide excision repair (NER) and recombination. Modifications of proteins with LPO products may regulate cellular processes like apoptosis, cell signalling and senescence. This review summarizes consequences of LPO products' presence in cell, particularly 4-hydroxy-2-nonenal, in terms of genomic stability.

A new LC-MS/MS method for the quantification of endogenous and vinyl chloride-induced 7-(2-Oxoethyl)guanine in sprague-dawley rats

Vinyl chloride (VC) is an industrial chemical that is known to be carcinogenic to animals and humans. VC primarily induces hepatic angiosarcomas following high exposures (≥50 ppm). VC is also found in Superfund sites at ppb concentrations as a result of microbial metabolism of trichloroethylene and perchloroethylene. Here, we report a new sensitive LC-MS/MS method to analyze the major DNA adduct formed by VC, 7-(2-oxoethylguanine) (7-OEG). We used this method to analyze tissue DNA from both adult and weanling rats exposed to 1100 ppm [(13)C(2)]-VC for 5 days. After neutral thermal hydrolysis, 7-OEG was derivatized with O-t-butyl hydroxylamine to an oxime adduct, followed by LC-MS/MS analysis. The limit of detection was 1 fmol, and the limit of quantitation was 1.5 fmol on the column. The use of stable isotope VC allowed us to demonstrate for the first time that endogenous 7-OEG was present in tissue DNA. We hypothesized that endogenous 7-OEG was formed from lipid peroxidation and demonstrated the formation of [(13)C(2)]-7-OEG from the reaction of calf thymus DNA with [(13)C(18)]-ethyl linoleate (EtLa) under peroxidizing conditions. The concentrations of endogenous 7-OEG in liver, lung, kidney, spleen, testis, and brain DNA from adult and weanling rats typically ranged from 1.0 to 10.0 adducts per 10(6) guanine. The exogenous 7-OEG in liver DNA from adult rats exposed to 1100 ppm [(13)C(2)]-VC for 5 days was 104.0 ± 23.0 adducts per 10(6) guanine (n = 4), while concentrations in other tissues ranged from 1.0 to 39.0 adducts per 10(6) guanine (n = 4). Although endogenous concentrations of 7-OEG in tissues in weanling rats were similar to those of adult rats, exogenous [(13)C(2)]-7-OEG concentrations were higher in weanlings, averaging 300 adducts per 10(6) guanine in liver. Studies on the persistence of [(13)C(2)]-7-OEG in adult rats sacrificed 2, 4, and 8 weeks postexposure to [(13)C(2)]-VC demonstrated a half-life of 7-OEG of 4 days in both liver and lung.

Model calculations for the misincorporation of nucleotides opposite five-membered exocyclic DNA adduct: N(2),3-ethenoguanine

Five-membered exocyclic DNA adducts are biologically very significant because of their potential to block DNA replication and transcription. N(2),3-Ethenoguanine (N(2,3)-εG) has been identified in the liver DNA of vinyl chloride-exposed rats as a five-membered DNA adduct. Singer et al. ( Carcinogenesis 1987 , 8 , 745 - 747 ) reported that the misincorporation of thymine (T), with two hydrogen bonds to N(2,3)-εG, represents the mutagenic event. Although the base-pairing specificity and mode of misincorporation have been studied experimentally for the N(2),3-ethenoguanine adduct, molecular-level information is not yet clear. In this study, we have considered all four different DNA nucleotides paired with the N(2),3-ethenoguanine adduct for model calculations toward the determination of base-pairing specificity. To provide insight into the mutagenic process of DNA damage based on geometric characteristics and electronic properties, the B3LYP and M06 methods were employed for these model calculations. Single-point energy calculations at the MP2/6-311++G** level on the corresponding optimized geometries were also carried out to better estimate the hydrogen-bonding strengths. The polarizable conductor calculation model (CPCM), which accounts for the overall polarizability of the solvent, was also employed. The computed reaction enthalpy values lie in the order εG-G(2) (10.3 kcal/mol) > εG-G(4) (9.6 kcal/mol) > εG-T(4) (9.2 kcal/mol) > εG-G(1) (9.1 kcal/mol) > εG-A(5) (8.2 kcal/mol) > εG-C(2) (7.9 kcal/mol) at the M06 level, which indicates that guanine and thymine are most favorable for mispairing with the N(2),3-ethenoguanine adduct.

Endogenous versus exogenous DNA adducts: their role in carcinogenesis, epidemiology, and risk assessment

There is a strong need for science-based risk assessment that utilizes known data from diverse sources to arrive at accurate assessments of human health risk. Such assessments will protect the public health without mandating unreasonable regulation. This paper utilizes 30 years of research on three "known human carcinogens": formaldehyde, vinyl chloride (VC), and ethylene oxide (EO), each of which forms DNA adducts identical to endogenous DNA adducts in all individuals. It outlines quantitative data on endogenous adducts, mutagenicity, and relationships between endogenous and exogenous adducts. Formaldehyde has the richest data set, with quantitative data on endogenous and exogenous DNA adducts from the same samples. The review elaborates on how such data can be used to inform the current risk assessment on formaldehyde, including both the biological plausibility and accuracy of projected risks. Finally, it extends the thought process to VC, EO, and additional areas of potential research, pointing out needs, nuances, and potential paths forward to improved understanding that will lead to strong science-based risk assessment.

Exocyclic DNA adducts as biomarkers of lipid oxidation and predictors of disease. Challenges in developing sensitive and specific methods for clinical studies

Exocyclic DNA adducts are emerging as potential new tools for the study of oxidative stress-related diseases as well as the determination of cancer etiology and cancer risk. It is important to determine whether levels of exocyclic DNA adducts reflect redox stress in vivo and what role these adducts play in human diseases. To answer these important questions, interindividual differences, tissue distribution, background levels, and repair have to be assessed. This review focuses on recent developments in the use of these adducts as possible biomarkers for disease risk related to oxidative stress and on the challenges in developing sensitive and specific methods for clinical studies.

Chloroacetaldehyde-induced mutagenesis in Escherichia coli: the role of AlkB protein in repair of 3,N(4)-ethenocytosine and 3,N(4)-alpha-hydroxyethanocytosine

Etheno (epsilon) adducts are formed in reaction of DNA bases with various environmental carcinogens and endogenously created products of lipid peroxidation. Chloroacetaldehyde (CAA), a metabolite of carcinogen vinyl chloride, is routinely used to generate epsilon-adducts. We studied the role of AlkB, along with AlkA and Mug proteins, all engaged in repair of epsilon-adducts, in CAA-induced mutagenesis. The test system used involved pIF102 and pIF104 plasmids bearing the lactose operon of CC102 or CC104 origin (Cupples and Miller (1989) [17]) which allowed to monitor Lac(+) revertants, the latter arose by GC-->AT or GC-->TA substitutions, respectively, as a result of modification of guanine and cytosine. The plasmids were CAA-damaged in vitro and replicated in Escherichia coli of various genetic backgrounds. To modify the levels of AlkA and AlkB proteins, mutagenesis was studied in E. coli cells induced or not in adaptive response. Formation of varepsilonC proceeds via a relatively stable intermediate, 3,N(4)-alpha-hydroxyethanocytosine (HEC), which allowed to compare repair of both adducts. The results indicate that all three genes, alkA, alkB and microg, are engaged in alleviation of CAA-induced mutagenesis. The frequency of mutation was higher in AlkA-, AlkB- and Mug-deficient strains in comparison to alkA(+), alkB(+), and microg(+) controls. Considering the levels of CAA-induced Lac(+) revertants in strains harboring the pIF plasmids and induced or not in adaptive response, we conclude that AlkB protein is engaged in the repair of epsilonC and HEC in vivo. Using the modified TTCTT 5-mers as substrates, we confirmed in vitro that AlkB protein repairs epsilonC and HEC although far less efficiently than the reference adduct 3-methylcytosine. The pH optimum for repair of HEC and epsilonC is significantly different from that for 3-methylcytosine. We propose that the protonated form of adduct interact in active site of AlkB protein.

Chemical biology of mutagenesis and DNA repair: cellular responses to DNA alkylation

The reaction of DNA-damaging agents with the genome results in a plethora of lesions, commonly referred to as adducts. Adducts may cause DNA to mutate, they may represent the chemical precursors of lethal events and they can disrupt expression of genes. Determination of which adduct is responsible for each of these biological endpoints is difficult, but this task has been accomplished for some carcinogenic DNA-damaging agents. Here, we describe the respective contributions of specific DNA lesions to the biological effects of low molecular weight alkylating agents.

Ethanol-induced cytochrome P4502E1 causes carcinogenic etheno-DNA lesions in alcoholic liver disease

Oxidative stress is thought to play a major role in the pathogenesis of hepatocellular cancer (HCC), a frequent complication of alcoholic liver disease (ALD). However, the underlying mechanisms are poorly understood. In hepatocytes of ALD patients, we recently detected by immunohistochemistry significantly increased levels of carcinogenic etheno-DNA adducts that are formed by the reaction of the major lipid peroxidation product, 4-hydroxynonenal (4-HNE) with nucleobases. In the current study, we show that protein-bound 4-HNE and etheno-DNA adducts both strongly correlate with cytochrome P450 2E1 (CYP2E1) expression in patients with ALD (r = 0.9, P < 0.01). Increased levels of etheno-DNA adducts were also detected in the liver of alcohol-fed lean (Fa/?) and obese (fa/fa) Zucker rats. The number of nuclei in hepatocytes stained positively for etheno-DNA adducts correlated significantly with CYP2E1 expression (r = 0.6, P = 0.03). To further assess the role of CYP2E1 in the formation of etheno-DNA adducts, HepG2 cells stably transfected with human CYP2E1 were exposed to ethanol with or without chlormethiazole (CMZ), a specific CYP2E1 inhibitor. Ethanol increased etheno-DNA adducts in the nuclei of CYP2E1-transfected HepG2 cells in a concentration-dependent and time-dependent manner, but not in vector mock-transfected control cells. CMZ blocked the generation of etheno-DNA adducts by 70%-90% (P < 0.01).

CONCLUSION

Our data support the assumption that ethanol-mediated induction of hepatic CYP2E1 leading inter alia to highly miscoding lipid peroxidation-derived DNA lesions may play a central role in hepatocarcinogenesis in patients with ALD.

Vinyl Chloride—A Classical Industrial Toxicant of New Interest

The carcinogenicity of vinyl chloride in humans was recognized in 1974 based on observations of hepatic angiosarcomas in highly exposed workers. A multiplicity of endpoints has been demonstrated. The primary target organ, the liver, displays differential susceptibilities of hepatocytes and sinusoidal cells, which are modified by factors of age and dose. There is consistency in organotropism between experimental animals and humans. Vinyl chloride is a pluripotent carcinogen, predominantly directed toward hepatic endothelial (sinusoidal) cells, and second toward the parenchymal cells of the liver. The similarity of results between experimental animals and humans is a solid basis of an amalgamation of experimental and epidemiological risk estimates. Vinyl chloride requires metabolic activation for carcinogenicity and mutagenicity, and toxicokinetics are a key to interpret the dose response. Practically the entire initial metabolism of vinyl chloride is oxidative. At higher exposure concentrations this is nonlinear, and metabolic saturation of metabolism in rats is reached at about 250 ppm. This is consistent with the plateau of hepatic angiosarcoma incidence in rat bioassays. Physiologically based pharmacokinetic/toxicokinetic (PBPK) models have been developed and successfully applied within the frame of human cancer risk assessments. The major DNA adduct induced by vinyl chloride (approximately 98% of total adducts in rats), 7-(2-oxoethyl)guanine, is almost devoid of promutagenic activity. The clearly promutagenic "etheno" adducts N2,3-ethenoguanine and 3,N4-ethenocytosine each represent approximately 1% of the vinyl chloride DNA adducts in rats, and 1,N6-ethenoadenine is found at even lower concentrations. Etheno adducts appear to have a long persistence and are repaired by glycosylases. Vinyl chloride represents a human carcinogen for which a series of mechanistic events connects exposure with the carcinogenic outcome. These include (1) metabolic activation (to form chloroethylene oxide), (2) DNA binding of the reactive metabolite (to exocyclic etheno adducts), (3) promutagenicity of these adducts, and (4) effects of such mutations on protooncogenes/tumor suppressor genes at the gene and gene product levels. In rat hepatocytes, a further event is a biomarker response. Cancer prestages (enzyme-altered foci), as quantitative biomarkers, provide a tool to study dose response even within low dose ranges where a carcinogenic risk cannot be seen in cancer bioassays directly. Such biomarker responses support a linear nonthreshold extrapolation for low-dose assessment of carcinogenic risks due to vinyl chloride. Published risk estimates based on different sets of data (animal experiments, epidemiological studies) appear basically consistent, and on this basis an angiosarcoma risk of approximately 3 x 10(-4) has been deduced by extrapolation, for exposure to 1 ppm vinyl chloride over an entire human working lifetime. An important point that should be considered in regulatory standard settings is the presence of a physiological background of those etheno DNA adducts, which are also produced by vinyl chloride. Likely reasons for this background are oxidative stress and lipid peroxidation. In essence, fundamentals of the hepatocarcinogenicity of vinyl chloride appear now well established, providing a solid scientific basis for regulatory activities.

Biological properties of single chemical-DNA adducts: a twenty year perspective

The genome and its nucleotide precursor pool are under sustained attack by radiation, reactive oxygen and nitrogen species, chemical carcinogens, hydrolytic reactions, and certain drugs. As a result, a large and heterogeneous population of damaged nucleotides forms in all cells. Some of the lesions are repaired, but for those that remain, there can be serious biological consequences. For example, lesions that form in DNA can lead to altered gene expression, mutation, and death. This perspective examines systems developed over the past 20 years to study the biological properties of single DNA lesions.

AlkB influences the chloroacetaldehyde-induced mutation spectra and toxicity in the pSP189 supF shuttle vector

2-Chloroacetaldehyde (CAA), a metabolite of the carcinogen vinyl chloride, reacts with DNA to form cyclic etheno ()-lesions. AlkB, an iron-/alpha-ketoglutarate-dependent dioxygenase, repairs 1, N (6)-ethenodeoxyadenosine (A) and 3, N (4)-ethenodeoxycytidine (C) in site-specifically modified single-stranded viral genomes in vivo and also protects the E. coli genome from the toxic effects of CAA. We examined the role of AlkB as a cellular defense against CAA by characterizing the frequencies, types, and distributions of mutations induced in the double-stranded supF gene of pSP189 damaged in vitro and replicated in AlkB-proficient (AlkB (+)) and AlkB-deficient (AlkB (-)) E. coli. AlkB reduced mutagenic potency and increased the survival of CAA-damaged plasmids. Toxicity and mutagenesis data were benchmarked to levels of -adducts and DNA strand breaks measured by LC-MS/MS and a plasmid nicking assay. CAA treatment caused dose-dependent increases in A, C, and 1, N (2)-ethenodeoxyguanosine (1, N (2)-G) and small increases in strand breaks and abasic sites. Mutation frequency increased in plasmids replicated in both AlkB (+) and AlkB (-) cells; however, at the maximum CAA dose, the mutation frequency was 5-fold lower in AlkB (+) than in AlkB (-) cells, indicating that AlkB protected the genome from CAA lesions. Most induced mutations in AlkB (-) cells were G:C to A:T transitions, with lesser numbers of G:C to T:A transversions and A:T to G:C transitions. G:C to A:T and A:T to G:C transitions were lower in AlkB (+) cells than in AlkB (-) cells. Mutational hotspots at G122, G123, and G160 were common to both cell types. Three additional hotspots were found in AlkB (-) cells (C133, T134, and G159), with a decrease in mutation frequency and change in mutational signature in AlkB (+) cells. These results suggest that the AlkB protein contributes to the elimination of exocyclic DNA base adducts, suppressing the toxic and mutagenic consequences induced by this damage and contributing to genetic stability.

Free radicals, metals and antioxidants in oxidative stress-induced cancer

Oxygen-free radicals, more generally known as reactive oxygen species (ROS) along with reactive nitrogen species (RNS) are well recognised for playing a dual role as both deleterious and beneficial species. The "two-faced" character of ROS is substantiated by growing body of evidence that ROS within cells act as secondary messengers in intracellular signalling cascades, which induce and maintain the oncogenic phenotype of cancer cells, however, ROS can also induce cellular senescence and apoptosis and can therefore function as anti-tumourigenic species. The cumulative production of ROS/RNS through either endogenous or exogenous insults is termed oxidative stress and is common for many types of cancer cell that are linked with altered redox regulation of cellular signalling pathways. Oxidative stress induces a cellular redox imbalance which has been found to be present in various cancer cells compared with normal cells; the redox imbalance thus may be related to oncogenic stimulation. DNA mutation is a critical step in carcinogenesis and elevated levels of oxidative DNA lesions (8-OH-G) have been noted in various tumours, strongly implicating such damage in the etiology of cancer. It appears that the DNA damage is predominantly linked with the initiation process. This review examines the evidence for involvement of the oxidative stress in the carcinogenesis process. Attention is focused on structural, chemical and biochemical aspects of free radicals, the endogenous and exogenous sources of their generation, the metal (iron, copper, chromium, cobalt, vanadium, cadmium, arsenic, nickel)-mediated formation of free radicals (e.g. Fenton chemistry), the DNA damage (both mitochondrial and nuclear), the damage to lipids and proteins by free radicals, the phenomenon of oxidative stress, cancer and the redox environment of a cell, the mechanisms of carcinogenesis and the role of signalling cascades by ROS; in particular, ROS activation of AP-1 (activator protein) and NF-kappaB (nuclear factor kappa B) signal transduction pathways, which in turn lead to the transcription of genes involved in cell growth regulatory pathways. The role of enzymatic (superoxide dismutase (Cu, Zn-SOD, Mn-SOD), catalase, glutathione peroxidase) and non-enzymatic antioxidants (Vitamin C, Vitamin E, carotenoids, thiol antioxidants (glutathione, thioredoxin and lipoic acid), flavonoids, selenium and others) in the process of carcinogenesis as well as the antioxidant interactions with various regulatory factors, including Ref-1, NF-kappaB, AP-1 are also reviewed.

Site specific synthesis and polymerase bypass of oligonucleotides containing a 6-hydroxy-3,5,6,7-tetrahydro-9H-imidazo[1,2-a]purin-9-one base, an intermediate in the formation of 1,N2-etheno-2'-deoxyguanosine

The reaction of DNA with certain bis-electrophiles such as chlorooxirane and chloroacetaldehyde produces etheno adducts. These lesions are highly miscoding, and some of the chemical agents that produce them have been shown to be carcinogenic in laboratory animals and in humans. An intermediate in the formation of 1,N2-ethenoguanine is 6-hydroxy-3,5,6,7-tetrahydro-9H-imidazo[1,2-a]purin-9-one (6-hydroxyethanoguanine), which undergoes conversion to the etheno adduct. The chemical properties and miscoding potential of the hydroxyethano adduct have not been previously studied. A synthesis of the hydroxyethano-adducted nucleoside was developed, and it was site specifically incorporated into oligonucleotides. This adduct had a half-life of between 24 and 48 h at neutral pH and 25 degrees C at the nucleoside and oligonucleotide levels. The miscoding potential of the hydroxyethano adduct was examined by primer extension reactions with the DNA polymerases Dpo4 and pol T7-, and the results were compared to the corresponding etheno-adducted oligonucleotide. Dpo4 preferentially incorporated dATP opposite the hydroxyethano adduct and dGTP opposite the etheno adduct; pol T7- preferentially incorporated dATP opposite the etheno adduct while dGTP and dATP were incorporated opposite the hydroxyethano adduct with nearly equal catalytic efficiencies. Collectively, these results indicate that the hydroxyethano adduct has a sufficient lifetime and miscoding properties to contribute to the mutagenic spectrum of chlorooxirane and related genotoxic species.

Evaluation in vinyl chloride monomer (VCM)-exposed workers and the relationship between liver lesions and gene polymorphisms of metabolic enzymes

AIM: The permissible exposure limit (PEL) of vinyl chloride monomer (VCM) in developed country was 1 p/m (2.79 mg/m3); and threshold limit value-short term exposure limit (TLV-STEL) in China was 11 times higher [11 p/m (30 mg/m3)] than it, till 2002. The mechanism of vinyl chloride monomer (VCM)-related carcinogenesis remains unclear. We aimed to analyze occupational health hazards exposure to doses lower than the Chinese occupational health standard in a selected VC polymerization plant in China, and also to elucidate the relationship between genetic polymorphisms and genetic susceptibility on liver lesions of workers exposed to VCM.

METHODS: In order to explore the mechanism of VCM-related health effects, we used a case-control design to investigate the association between the genetic polymorphisms of metabolic enzymes and liver lesions in workers occupationally exposed to VCM. Genotypes of CYP2E1, GSTT1, GSTM1, ALDH2 and ADH2 were identified using PCR and PCR-RFLP.

RESULTS: Even when the concentration of VCM was lower than the current Chinese occupational health standard, neurasthenia, pharyngeal irritation, liver ultrasonography abnormalities and hemoglobin disorders were significantly higher in exposure subjects compared to non-exposure subjects, and the relative risks (RR and 95% CI) were 1.74 (1.06-2.85), 1.97 (1.56-2.48), 10.69 (4.38-26.12), and 2.07 (1.20-3.57). CYP2E1 c1c2/c2c2 genotype was significantly associated with liver damages (OR 3.29, 95% CI 1.51-7.20, P < 0.01).

CONCLUSION: The incidences of neurasthenia and liver ultrasonography abnormalities significantly increase when the cumulative exposure dose increases. The genotypes of metabolic enzymes (CYP2E1 c1c2/c2c2, null GSTT1 and ADH2 1-1) play important roles in VCM metabolism. Polymorphisms of CYP 2E1, GSTT1 and ADH2 may be a major reason of genetic susceptibility in VCM-induced hepatic damage.

Age-dependent increase of etheno-DNA-adducts in liver and brain of ROS overproducing OXYS rats

Reactive oxygen species (ROS) and lipid peroxidation (LPO) play a role in aging and degenerative diseases. To correlate oxidative stress and LPO-derived DNA damage, we determined etheno-DNA-adducts in liver and brain from ROS overproducing OXYS rats in comparison with age-matched Wistar rats. Liver DNA samples from 3- and 15-month-old OXYS and Wistar rats were analyzed for 1,N6-ethenodeoxyadenosine (epsilondA) and 3,N4-ethenodeoxycytidine (epsilondC) by immunoaffinity/32P-postlabelling. While epsilondA and epsilondC levels were not different in young rats, adduct levels were significantly higher in old OXYS rats when compared to old Wistar or young OXYS rats. Frozen rat brain sections were analyzed for epsilondA by immunostaining of nuclei. Brains from old OXYS rats accumulated epsilondA more frequently than age-matched Wistar rats. Our results demonstrate increased LPO-induced DNA damage in organs of OXYS rats which correlates with their known shorter life-span and elevated frequency of chronic degenerative diseases.

DNA damage and mutations produced by chloroacetaldehyde in a CpG-methylated target gene

Chloroacetaldehyde (CAA) is a metabolite of the human carcinogen vinyl chloride. CAA produces several types of DNA adducts including the exocyclic base adducts 3,N(4)-ethenocytosine, 1,N(6)-ethenoadenine, N(2),3-ethenoguanine, and 1,N(2)-ethenoguanine. Adducts of CAA with 5-methylcytosine have not yet been characterized. Here we have analyzed the mutational spectra produced by CAA in the supF gene of the pSP189 shuttle vector when present in either an unmethylated or CpG-methylated state. The vectors were replicated in human nucleotide excision repair-deficient XP-A fibroblasts. The mutational spectra obtained with the unmethylated and methylated supF target genes were generally similar with a preponderance of C/G to T/A transitions and C/G to A/T transversions. CAA-induced DNA adducts were mapped along the supF gene by using thermostable thymine DNA glycosylase (TDG) in conjunction with ligation-mediated PCR or by a Taq polymerase stop assay. Prominent CAA-induced TDG-sensitive sites were seen at several CpG positions but were independent of methylation. Methylated CpG sites were sites of CAA-induced mutations but were not the major mutational hotspots. Taq polymerase arrest sites were observed at numerous sequence positions in the supF gene and reflected the rather broad distributions of mutations along the sequence. We conclude that methylated CpG sites are not preferential targets for chloroacetaldehyde-induced mutagenesis.

Apoptosis and age-dependant induction of nuclear and mitochondrial etheno-DNA adducts in Long-Evans Cinnamon (LEC) rats: enhanced DNA damage by dietary curcumin upon copper accumulation

Long-Evans Cinnamon (LEC) rats, a model for human Wilson's disease, develop chronic hepatitis and liver tumors owing to accumulation of copper and induced oxidative stress. Lipid peroxidation (LPO)-induced etheno-DNA adducts in nuclear- and mitochondrial-DNA along with apoptosis was measured in LEC rat liver. Levels of etheno-DNA adducts (1,N6-ethenodeoxyadenosine and 3,N4-ethenodeoxycytidine) increased with age reaching a peak at 8 and 12 weeks in nuclear and mitochondrial DNA, respectively. This is the first demonstration that etheno-DNA adducts are also formed in mitochondrial DNA. Apoptosis was assessed by TUNEL+ cells in liver sections. CD95L RNA expression was also measured by in situ hybridization in the same sections. The highest nuclear DNA adduct levels coincided with a reduced apoptotic rate at 8 weeks. Mitochondrial-DNA adducts peaked at 12 weeks that coincided with the highest apoptotic rate, suggesting a link of etheno-DNA adducts in mitochondrial DNA to apoptosis. The DNA damage in liver was further enhanced and sustained by 0.5% curcumin in the diet. Treatment for 2 weeks elevated etheno-DNA adducts 9- to 25-fold in nuclear DNA and 3- to 4-fold in mitochondrial-DNA, providing a plausible explanation as to why in our earlier study [Frank et al. (2003) Mutat. Res., 523-524, 127-135], curcumin failed to prevent liver tumors in LEC rats. Our results also confirm the reported in vitro DNA damaging potential of curcumin in the presence of copper ions by reactive oxygen species. LPO-induced adduct formation in nuclear and mitochondrial DNA appear as early lesions in LEC rat liver carcinogenesis and are discussed in relation to apoptotic events in the progression of malignant disease.

Role of mismatch-specific uracil-DNA glycosylase in repair of 3,N4-ethenocytosine in vivo

The 3,N(4)-ethenocytosine (epsilon C) residue might have biological role in vivo since it is recognized and efficiently excised in vitro by the E. coli mismatch-specific uracil-DNA glycosylase (MUG) and the human thymine-DNA glycosylase (hTDG). In the present work we have generated mug defective mutant of E. coli by insertion of a kanamycin cassette to assess the role of MUG in vivo. We show that human TDG complements the enzymatic activity of MUG when expressed in a mug mutant. The epsilon C-DNA glycosylase defective strain did not exhibit spontaneous mutator phenotype and did not show unusual sensitivity to any of the following DNA damaging treatments: methylmethanesulfonate, N-methyl-N'-nitro-N-nitrosoguanidine, ultraviolet light, H(2)O(2), paraquat. However, plasmid DNA damaged by 2-chloroacetaldehyde treatment in vitro was inactivated at a greater rate in a mug mutant than in wild-type host, suggesting that MUG is required for the in vivo processing of the ethenobases. In addition, 2-chloroacetaldehyde treatment induces preferentially G.C --> C.G and A.T --> T.A transversions in mug mutant. Comparison of the mutation frequencies induced by the site-specifically incorporated epsilon C residue in E. coli wild-type versus mug indicates that MUG repairs more than 80% of epsilon C residues in vivo. Furthermore, the results show that nucleotide excision repair and recombination are not involved in the processing of epsilon C in E. coli. Based on the mutagenesis data we suggest that epsilon C may be less toxic and less mutagenic than expected. The increased spontaneous mutation rate for G.C --> A.T transition in the ung mug double mutant as compared to the single ung mutant suggest that MUG may be a back-up repair enzyme to the classic uracil-DNA glycosylase.

2'-deoxyribonolactone lesion produces G->A transitions in Escherichia coli

2'-deoxyribonolactone (dL) is a C1'-oxidized abasic site damage generated by a radical attack on DNA. Numerous genotoxic agents have been shown to produce dL including UV and gamma-irradiation, ene-dye antibiotics etc. At present the biological consequences of dL present in DNA have been poorly documented, mainly due to the lack of method for introducing the lesion in oligonucleotides. We have recently designed a synthesis of dL which allowed investigation of the mutagenicity of dL in Escherichia coli by using a genetic reversion assay. The lesion was site-specifically incorporated in a double-stranded bacteriophage vector M13G*1, which detects single-base-pair substitutions at position 141 of the lacZalpha gene by a change in plaque color. In E.coli JM105 the dL-induced reversion frequency was 4.7 x 10(-5), similar to that of the classic abasic site 2'-deoxyribose (dR). Here we report that a dL residue in a duplex DNA codes mainly for thymidine. The processing of dL in vivo was investigated by measuring lesion-induced mutation frequencies in DNA repair deficient E.coli strains. We showed a 32-fold increase in dL-induced reversion rate in AP endonuclease deficient (xth nfo) mutant compared with wild-type strain, indicating that the Xth and Nfo AP endonucleases participate in dL repair in vivo.

Immunohistochemical detection of 1,N6-ethenodeoxyadenosine in nuclei of human liver affected by diseases predisposing to hepato-carcinogenesis

Increased oxidative stress and lipid peroxidation (LPO) are implicated in multistage carcinogenesis. Recent studies have shown that LPO-derived reactive hydroxyalkenals can form promutagenic exocyclic etheno-DNA adducts in vivo. Such DNA damage was found to be increased in the liver of patients with metal storage diseases and in colon adenomas of familial adenomatous polyposis patients. We now have investigated the levels of 1,N(6)-ethenodeoxyadenosine (epsilon dA) in human liver samples obtained from a group of patients diagnosed with hepatitis, fatty liver, fibrosis and cirrhosis, primary hemochromatosis and Wilson's disease. Using an immunohistochemical method, the relative mean pixel intensity of randomly selected nuclei was measured by imaging software; positively stained cell nuclei (arbitrary mean pixel intensity > or =0.5) were counted. Prevalence of epsilon dA (%) was calculated from the ratio of a number of positively stained cell nuclei over a total number of cells counted. When compared with normal livers (3.1%), the percent prevalence (means) was significantly higher in specimens of alcoholic fatty liver (15%) and fibrosis patients (50%) but not in samples with hepatitis (induced by various factors) (6.2%). The percent prevalence in alcohol fibrosis was as high as in the liver from Wilson's disease (50.7%) and hemochromatosis (33%) patients. This is the first demonstration of increased epsilon dA in human liver diseases due to alcohol abuse. We conclude that excessive hepatic DNA damage, as assessed by miscoding etheno-DNA adduct in the nuclei of liver biopsies, is probably caused by alcohol-induced oxidative stress and LPO. In cancer-prone liver diseases (fatty liver, cirrhosis/fibrosis) such damage may act as a driving force towards malignancy.

Molecular epidemiology of plasma oncoproteins in vinyl chloride monomer workers in Taiwan

AIMS

To determine the presence of Asp13-p21-ki-ras oncoprotein and p53 oncoprotein in the plasma of vinyl chloride monomer (VCM)-workers in Taiwan.

METHODS

We used enhanced chemiluminescence (ECL) western blotting to detect Asp13-p21-ki-ras and ELISA to detect mutant p53 protein (p53-Ag) and anti-p53 antibodies (p53-Ab) in the plasma of VCM-exposed workers.

RESULTS

Twenty-five out of 251 (10%) VCM-workers were positive for Asp13-p21-ki-ras in plasma, but 0 out of 36 controls were positive. There were 15 out of 95 (15.8%) plasma-positives among the more highly exposed (> 480 ppm-month) workers and 10 out of 156 (6.4%) plasma-positives among the lesser exposed (< or = 480 ppm-month). Compared to the unexposed controls, age and drinking adjusted odds ratios (95% CI) were 1.2 (0.1, 9.8) in the lower exposed workers, and 4.8 (0.8, 28) in the higher exposed workers, and there was a significant linear trend between exposure and plasma positivity (P=0.001). Thirty-three out of 251 (13.2%) VCM-workers were positive for the p53 over-expression (10% with positive p53-Ag and 2.8% with positive p53-Ab). There was a significant association between cumulative VCM exposure concentration and positive p53 expression (P=0.032) among VCM-workers after adjusting for age, hepatitis, drinking and smoking status.

CONCLUSIONS

Asp13-p21-ki-ras oncoprotein and p53 over-expression (p53-Ag or p53-Ab) can be found in the plasma of VCM-workers in Taiwan, and a significant dose-response relationship exists between plasma oncoproteins expression and VCM exposure.

Methods for measuring DNA adducts and abasic sites II: methods for measurement of DNA adducts

This unit contains protocols for analyzing DNA adducts separated from the DNA backbone. HPLC is used to quantify total guanine or ribo- or deoxynucleotides as well as methods for analyzing specific adducts. These methods include HPLC with electrochemical detection, immunoaffininty chromatography to enrich for specific adducts, and gas and liquid chromatography in combination with HPLC and mass spectrometry.

Comparison of cancer risk estimates for vinyl chloride using animal and human data with a PBPK model

Vinyl chloride (VC) is a trans-species carcinogen, producing tumors in a variety of tissues, from both inhalation and oral exposures, across a number of species. In particular, exposure to VC has been associated with a rare tumor, liver angiosarcoma, in a large number of studies in mice, rats, and humans. The mode of action for the carcinogenicity of VC appears to be a relatively straightforward example of DNA adduct formation by a reactive metabolite, leading to mutation, mistranscription, and neoplasia. The objective of the present analysis was to investigate the comparative potency of a classic genotoxic carcinogen across species, by performing a quantitative comparison of the carcinogenic potency of VC using data from inhalation and oral rodent bioassays as well as from human epidemiological studies. A physiologically-based pharmacokinetic (PBPK) model for VC was developed to support the target tissue dosimetry for the cancer risk assessment. Unlike previous models, the initial metabolism of VC was described as occurring via two saturable pathways, one representing low capacity-high affinity oxidation by CYP2E1 and the other (in the rodent) representing higher capacity-lower affinity oxidation by other isozymes of P450, producing in both cases chloroethylene oxide (CEO) and chloroacetaldehyde (CAA) as intermediate reactive products. Depletion of glutathione by reaction with CEO and CAA was also described. Animal-based risk estimates for human inhalation exposure to VC using total metabolism estimates from the PBPK model were consistent with risk estimates based on human epidemiological data, and were lower than those currently used in environmental decision-making by a factor of 80.

Responses to the major acrolein-derived deoxyguanosine adduct in Escherichia coli

Acrolein, a reactive alpha,beta-unsaturated aldehyde found ubiquitously in the environment and formed endogenously in mammalian cells, reacts with DNA to form an exocyclic DNA adduct, 3H-8-hydroxy-3-(beta-D-2'-deoxyribofuranosyl)-5,6,7,8-tetrahydropyrido[3,2-a]purine-9-one (gamma-OH-PdG). The cellular processing and mutagenic potential of gamma-OH-PdG have been examined, using a site-specific approach in which a single adduct is embedded in double-strand plasmid DNA. Analysis of progeny plasmid reveals that this adduct is excised by nucleotide excision repair. The apparent level of inhibition of DNA synthesis is approximately 70% in Escherichia coli DeltarecA, uvrA. The block to DNA synthesis can be overcome partially by recA-dependent recombination repair. Targeted G --> T transversions were observed at a frequency of 7 x 10(-4)/translesion synthesis. Inactivation of polB, dinB, and umuD,C genes coding for "SOS" DNA polymerases did not affect significantly the efficiency or fidelity of translesion synthesis. In vitro primer extension experiments revealed that the Klenow fragment of polymerase I catalyzes error-prone synthesis, preferentially incorporating dAMP and dGMP opposite gamma-OH-PdG. We conclude from this study that DNA polymerase III catalyzes translesion synthesis across gamma-OH-PdG in an error-free manner. Nucleotide excision repair, recombination repair, and highly accurate translesion synthesis combine to protect E. coli from the potential genotoxicity of this DNA adduct.