Development of a 32P-postlabelling method for the analysis of adducts arising through the reaction of acetaldehyde with 2'-deoxyguanosine-3'-monophosphate and DNA

A translation proofreader of archaeal origin imparts multi-aldehyde stress tolerance to land plants

Aldehydes, being an integral part of carbon metabolism, energy generation, and signalling pathways, are ingrained in plant physiology. Land plants have developed intricate metabolic pathways which involve production of reactive aldehydes and its detoxification to survive harsh terrestrial environments. Here, we show that physiologically produced aldehydes, i.e., formaldehyde and methylglyoxal in addition to acetaldehyde, generate adducts with aminoacyl-tRNAs, a substrate for protein synthesis. Plants are unique in possessing two distinct chiral proofreading systems, D-aminoacyl-tRNA deacylase1 (DTD1) and DTD2, of bacterial and archaeal origins, respectively. Extensive biochemical analysis revealed that only archaeal DTD2 can remove the stable D-aminoacyl adducts on tRNA thereby shielding archaea and plants from these system-generated aldehydes. Using Arabidopsis as a model system, we have shown that the loss of DTD2 gene renders plants susceptible to these toxic aldehydes as they generate stable alkyl modification on D-aminoacyl-tRNAs, which are recycled only by DTD2. Bioinformatic analysis identifies the expansion of aldehyde metabolising repertoire in land plant ancestors which strongly correlates with the recruitment of archaeal DTD2. Finally, we demonstrate that the overexpression of DTD2 offers better protection against aldehydes than in wild type Arabidopsis highlighting its role as a multi-aldehyde detoxifier that can be explored as a transgenic crop development strategy.

Acetaldehyde as a Food Flavoring Substance: Aspects of Risk Assessment

The Senate Commission on Food Safety (SKLM) of the German Research Foundation (DFG) has reviewed the currently available data in order to assess the health risks associated with the use of acetaldehyde as a flavoring substance in foods. Acetaldehyde is genotoxic in vitro. Following oral intake of ethanol or inhalation exposure to acetaldehyde, systemic genotoxic effects of acetaldehyde in vivo cannot be ruled out (induction of DNA adducts and micronuclei). At present, the key question of whether acetaldehyde is genotoxic and mutagenic in vivo after oral exposure cannot be answered conclusively. There is also insufficient data on human exposure. Consequently, it is currently not possible to reliably assess the health risk associated with the use of acetaldehyde as a flavoring substance. However, considering the genotoxic potential of acetaldehyde as well as numerous data gaps that need to be filled to allow a comprehensive risk assessment, the SKLM considers that the use of acetaldehyde as a flavoring may pose a safety concern. For reasons of precautionary consumer protection, the SKLM recommends that the scientific base for approval of the intentional addition of acetaldehyde to foods as a flavoring substance should be reassessed.

Stellungnahme zu Acetaldehyd als Aromastoff: Aspekte der Risikobewertung

Opinion on acetaldehyde as a flavouring substance: considerations for risk assessment

Acetaldehyde occurs naturally in many foods and is also used as a flavouring due to its fruity aroma. The International Agency for Research on Cancer (IARC) classified acetaldehyde as possibly carcinogenic to humans and, in combination with oral intake via alcoholic beverages, as carcinogenic to humans. Therefore, the question arises whether the use of acetaldehyde as a flavouring agent is still justifiable. The Senate Commission on Food Safety (SKLM) of the German Research Foundation (DFG) reviewed the scientific basis for health risk assessment of the use of acetaldehyde as a flavouring substance and adopted an opinion. Based on the available data, it is at present not possible to conclude if acetaldehyde is genotoxic and mutagenic in vivo after oral exposure. There is also uncertainty regarding the contribution of acetaldehyde as a flavouring substance to the overall exposure to acetaldehyde. Therefore, a science-based assessment on health risk related to the use of acetaldehyde as a flavouring is not possible at present. Considering the genotoxic potential as well as numerous data gaps that need to be closed for a full risk assessment, the SKLM is concerned about the safety of acetaldehyde as a flavouring substance. For reasons of precautionary consumer protection, the SKLM considers that the use of acetaldehyde as a food additive should be re-evaluated.

Recruitment of archaeal DTD is a key event toward the emergence of land plants

Streptophyte algae emerged as a land plant with adaptations that eventually led to terrestrialization. Land plants encounter a range of biotic and abiotic stresses that elicit anaerobic stress responses. Here, we show that acetaldehyde, a toxic metabolite of anaerobic stress, targets and generates ethyl adducts on aminoacyl-tRNA, a central component of the translation machinery. However, elongation factor thermo unstable (EF-Tu) safeguards l-aminoacyl-tRNA, but not d-aminoacyl-tRNA, from being modified by acetaldehyde. We identified a unique activity of archaeal-derived chiral proofreading module, d-aminoacyl-tRNA deacylase 2 (DTD2), that removes N-ethyl adducts formed on d-aminoacyl-tRNAs (NEDATs). Thus, the study provides the molecular basis of ethanol and acetaldehyde hypersensitivity in DTD2 knockout plants. We uncovered an important gene transfer event from methanogenic archaea to the ancestor of land plants. While missing in other algal lineages, DTD2 is conserved from streptophyte algae to land plants, suggesting its role toward the emergence and evolution of land plants.

Protective effects of Alda-1, an ALDH2 activator, on alcohol-derived DNA damage in the esophagus of human ALDH2*2 (Glu504Lys) knock-in mice

Alcohol consumption is the key risk factor for the development of esophageal squamous cell carcinoma (ESCC), and acetaldehyde, a metabolite of alcohol, is an alcohol-derived major carcinogen that causes DNA damage. Aldehyde dehydrogenase2 (ALDH2) is an enzyme that detoxifies acetaldehyde, and its activity is reduced by ALDH2 gene polymorphism. Reduction in ALDH2 activity increases blood, salivary and breath acetaldehyde levels after alcohol intake, and it is deeply associated with the development of ESCC. Heavy alcohol consumption in individuals with ALDH2 gene polymorphism significantly elevates the risk of ESCC; however, effective prevention has not been established yet. In this study, we investigated the protective effects of Alda-1, a small molecule ALDH2 activator, on alcohol-mediated esophageal DNA damage. Here, we generated novel genetically engineered knock-in mice that express the human ALDH2*1 (wild-type allele) or ALDH2*2 gene (mutant allele). Those mice were crossed, and human ALDH2*1/*1, ALDH2*1/*2 and ALDH2*2/*2 knock-in mice were established. They were given 10% ethanol for 7 days in the presence or absence of Alda-1, and we measured the levels of esophageal DNA damage, represented by DNA adduct (N²-ethylidene-2′-deoxyguanosine). Alda-1 significantly increased hepatic ALDH2 activity both in human ALDH2*1/*2 and/or ALDH2*2/*2 knock-in mice and reduced esophageal DNA damage levels after alcohol drinking. Conversely, cyanamide, an ALDH2-inhibitor, significantly exacerbated esophageal DNA adduct level in C57BL/6N mice induced by alcohol drinking. These results indicate the protective effects of ALDH2 activation by Alda-1 on esophageal DNA damage levels in individuals with ALDH2 gene polymorphism, providing a new insight into acetaldehyde-mediated esophageal carcinogenesis and prevention.

Preparation of N2-Ethyl-2'-deoxyguanosine-d4 as an Internal Standard for the Electrospray Ionization-Tandem Mass Spectrometric Determination of DNA Damage by Acetaldehyde

The deuteration of N²-ethyl-2'-deoxyguanosine (Et-dG), which is a DNA adduct generated from acetaldehyde, was studied by the addition reaction of acetaldehyde-d₄ to 2'-deoxyguanosine (dG) in deuterium oxide (D₂O), with the aim to obtain an isotope internal standard for the liquid chromatography/tandem mass spectrometry (LC/MS/MS) quantitation of Et-dG. The replacement of the dG C-8 hydrogen atom by a deuteron atom took place at 50°C in D₂O and afforded a mixture of Et-dG-d₄ and Et-dG-d₅. Et-dG-d₄, which was stable in aqueous solutions, was prepared by incubating the mixture in H₂O at 60°C for 48 h. The calibration curve was obtained by multiple reaction monitoring (MRM) measurements using a hydrophilic interaction chromatography-electrospray ionization-tandem mass spectrometric (HILIC/ESI-MS/MS) system between the Et-dG concentration, ranging from 1.0 × 10^-10 to 4.0 × 10^-9 M in the sample solutions, and the relative peak areas of Et-dG (m/z: 296.1 → 180.1) to the value of Et-dG-d₄ (m/z: 300.2 → 184.2), with an internal standard showing good linearity (R² = 0.995, n = 5).

Methylglyoxal-induced miR-223 suppresses rat vascular KATP channel activity by downregulating Kir6.1 mRNA in carbonyl stress

The vascular ATP-sensitive K⁺ (K_ATP) channel composed of Kir6.1 and SUR2B subunits regulates cellular activity by coupling intermediary metabolism to membrane excitability. Our previous studies have shown that both Kir6.1 and SUB2B are post-transcriptionally downregulated by methylglyoxal (MGO) which is a reactive carbonyl specie and can cause disruption of vascular tone regulation under diabetic conditions. We have shown that the SUB2B downregulation is mediated by the microRNA (miR) miR-9a, while the mechanism underlying Kir6.1 inhibition is still unclear. Studying the microRNA databases, we found that miR-223 has sequence similarities to the 3' untranslated sequence (3'UTR) of Kir6.1 mRNA suggesting their potential interactions. Therefore, we explored the role of miR-233 in K_ATP channel regulation by up/down-regulation of miR-223 in smooth muscle cells (SMCs) and mesenteric arterials. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) analysis showed augmentation of miR-223 expression in the cultured SMCs after 300 μM MGO exposure by 5-6 folds. miR-223 overexpression down-regulated Kir6.1 mRNA levels by ~2.6 times while miR-223 knockdown diminished the effect of 300 μM MGO by ~50% in the SMCs. Luciferase assay and mutagenesis studies showed that the effect of miR-223 was abolished when the potential interaction site in the 3' UTR was mutated. Studies with Western blot, patch clamp, and perfused mesenteric arterial rings showed that transfection of miR-223 downregulated K_ATP protein expression, inhibited K_ATP channel activity and enhanced vasoconstriction. These results therefore suggest that miR-223 is induced by MGO exposure, which subsequently downregulates the Kir6.1 mRNA, suppresses K_ATP channel function, and impairs functional regulation of vascular tones.

BACKGROUND

Methylglyoxal causes transcriptional inhibition of the vascular K_ATP channel.

RESULTS

Exogenous miR-223 down-regulated Kir6.1. miR-223 knockdown alleviated the effect of MGO.

CONCLUSION

Vascular K_ATP channel is important for miR-223 targeting.

SIGNIFICANCE

Regulation of the miR-223 level may be a novel strategy for clinical treatment of diabetes.

Construction of a risk model for the development of metachronous squamous cell carcinoma after endoscopic resection of esopahageal squamous cell carcinoma

BACKGROUND

Previously, we identified that rs1229984 in ADH1B, rs671 in ALDH2, and smoking status were independently associated with the risk of developing metachronous squamous cell carcinoma (SCC) after endoscopic resection (ER) for esophageal SCC (ESCC). However, this analysis included cases with short-term follow-up. In the present study, we investigated the environmental and genetic factors associated with developing metachronous SCC using long-term follow-up observation after ER for ESCC.

METHODS

One hundred and thirty ESCC patients who underwent treatment with ER were followed up using endoscopy for ≥ 30 months. We investigated the incidence of, and genetic/environmental factors associated with, metachronous SCC development after ER for ESCC. We also analyzed the potential risk factors for multiple metachronous SCC development using Cox's proportional hazards model. Moreover, we constructed a risk model for the development of metachronous SCC after ER for ESCC.

RESULTS

Male, rs1229984, rs671, alcohol consumption (> 20 g/day), smoking, and multiple Lugol-voiding lesions (LVLs) significantly affected the incidence of multiple metachronous SCCs. Multiple Cox proportional analysis revealed that rs1229984, rs671, alcohol consumption, smoking, and multiple LVLs were independently associated with the risk of developing metachronous SCC. Patients who had ≤ 2 risk factors did not develop metachronous SCC, and the risk of developing metachronous SCC in patients with ≥ 3 risk factors was significantly higher than in patients with ≤ 2 risk factors.

CONCLUSION

The risk model using these 5 genetic and environmental factors is useful as an indication for multiple metachronous development in ESCC patients.

Replication and repair of a reduced 2΄-deoxyguanosine-abasic site interstrand cross-link in human cells

Apurinic/apyrimidinic (AP) sites, or abasic sites, which are a common type of endogenous DNA damage, can forge interstrand DNA–DNA cross-links via reaction with the exocyclic amino group on a nearby 2΄-deoxyguanosine or 2΄-deoxyadenosine in the opposite strand. Here, we utilized a shuttle vector method to examine the efficiency and fidelity with which a reduced dG–AP cross-link-containing plasmid was replicated in cultured human cells. Our results showed that the cross-link constituted strong impediments to DNA replication in HEK293T cells, with the bypass efficiencies for the dG- and AP-containing strands being 40% and 20%, respectively. While depletion of polymerase (Pol) η did not perturb the bypass efficiency of the lesion, the bypass efficiency was markedly reduced (to 1–10%) in the isogenic cells deficient in Pol κ, Pol ι or Pol ζ, suggesting the mutual involvement of multiple translesion synthesis polymerases in bypassing the lesion. Additionally, replication of the cross-linked AP residue in HEK293T cells was moderately error-prone, inducing a total of ∼26% single-nucleobase substitutions at the lesion site, whereas replication past the cross-linked dG component occurred at a mutation frequency of ∼8%. Together, our results provided important insights into the effects of an AP-derived interstrand cross-link on the efficiency and accuracy of DNA replication in human cells.

Alcohol and Hepatocellular Carcinoma: Adding Fuel to the Flame

Primary tumors of the liver represent the fifth most common type of cancer in the world and the third leading cause of cancer-related death. Case-control studies from different countries report that chronic ethanol consumption is associated with an approximately 2-fold increased odds ratio for hepatocellular carcinoma (HCC). Despite the substantial epidemiologic data in humans demonstrating that chronic alcohol consumption is a major risk factor for HCC development, the pathways causing alcohol-induced liver cancer are poorly understood. In this overview, we summarize the epidemiological evidence for the association between alcohol and liver cancer, review the genetic, oncogenic, and epigenetic factors that drive HCC development synergistically with ethanol intake and discuss the essential molecular and metabolic pathways involved in alcohol-induced liver tumorigenesis.

Molecular Mechanisms of Acetaldehyde-Mediated Carcinogenesis in Squamous Epithelium

Acetaldehyde is a highly reactive compound that causes various forms of damage to DNA, including DNA adducts, single- and/or double-strand breaks (DSBs), point mutations, sister chromatid exchanges (SCEs), and DNA-DNA cross-links. Among these, DNA adducts such as N²-ethylidene-2'-deoxyguanosine, N²-ethyl-2'-deoxyguanosine, N²-propano-2'-deoxyguanosine, and N²-etheno-2'-deoxyguanosine are central to acetaldehyde-mediated DNA damage because they are associated with the induction of DNA mutations, DNA-DNA cross-links, DSBs, and SCEs. Acetaldehyde is produced endogenously by alcohol metabolism and is catalyzed by aldehyde dehydrogenase 2 (ALDH2). Alcohol consumption increases blood and salivary acetaldehyde levels, especially in individuals with ALDH2 polymorphisms, which are highly associated with the risk of squamous cell carcinomas in the upper aerodigestive tract. Based on extensive epidemiological evidence, the International Agency for Research on Cancer defined acetaldehyde associated with the consumption of alcoholic beverages as a "group 1 carcinogen" (definite carcinogen) for the esophagus and/or head and neck. In this article, we review recent advances from studies of acetaldehyde-mediated carcinogenesis in the squamous epithelium, focusing especially on acetaldehyde-mediated DNA adducts. We also give attention to research on acetaldehyde-mediated DNA repair pathways such as the Fanconi anemia pathway and refer to our studies on the prevention of acetaldehyde-mediated DNA damage.

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.

The SUR2B subunit of rat vascular KATP channel is targeted by miR-9a-3p induced by prolonged exposure to methylglyoxal

ATP-sensitive K(+) (K(ATP)) channels regulate plasma membrane excitability. The Kir6.1/SUR2B isoform of K(ATP) channels is expressed in vascular smooth muscles and plays an important role in vascular tone regulation. This K(ATP) channel is targeted by several reactive species. One of them is methylglyoxal (MGO), which is overly produced with persistent hyperglycemia and contributes to diabetic vascular complications. We have previously found that MGO causes posttranscriptional inhibition of the K(ATP) channel, aggravating vascular tone regulation. Here we show evidence for the underlying molecular mechanisms. We screened microRNA databases and found several candidates. Of them, miR-9a-3p, increased its expression level by ∼240% when the cultured smooth muscle cell line was exposed to micromolar concentrations of MGO. Treatments with exogenous miR-9a-3p downregulated the SUR2B but not Kir6.1 mRNA. Antisense nucleotides of miR-9a-3p alleviated the effects of MGO. Quantitative PCR showed that the targeting sites of the miR-9a-3p were likely to be in the coding region of SUR2B. The effects of miR-9a-3p were mostly eliminated when the potential targeting site in SUR2B was site-specifically mutated. Our functional assays showed that K(ATP) currents were impaired by miR-9a-3p induced with MGO treatment. These results suggest that MGO exposure raises the expression of miR-9a-3p, which subsequently downregulates the SUR2B mRNA, compromising K(ATP) channel function in vascular smooth muscle.

Involvement of DNA damage response pathways in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) has been known as one of the most lethal human malignancies, due to the difficulty of early detection, chemoresistance, and radioresistance, and is characterized by active angiogenesis and metastasis, which account for rapid recurrence and poor survival. Its development has been closely associated with multiple risk factors, including hepatitis B and C virus infection, alcohol consumption, obesity, and diet contamination. Genetic alterations and genomic instability, probably resulted from unrepaired DNA lesions, are increasingly recognized as a common feature of human HCC. Dysregulation of DNA damage repair and signaling to cell cycle checkpoints, known as the DNA damage response (DDR), is associated with a predisposition to cancer and affects responses to DNA-damaging anticancer therapy. It has been demonstrated that various HCC-associated risk factors are able to promote DNA damages, formation of DNA adducts, and chromosomal aberrations. Hence, alterations in the DDR pathways may accumulate these lesions to trigger hepatocarcinogenesis and also to facilitate advanced HCC progression. This review collects some of the most known information about the link between HCC-associated risk factors and DDR pathways in HCC. Hopefully, the review will remind the researchers and clinicians of further characterizing and validating the roles of these DDR pathways in HCC.

Trapping of a cross-link formed by a major purine adduct of a metabolite of the carcinogen N-nitrosomorpholine by inorganic and biological reductants

3-Hydroperoxy-N-nitrosomorpholine in buffered aqueous media in the presence of calf thymus DNA was treated with a phosphine reductant to generate the transient α-hydroxynitrosamine and subsequent diazonium ion that alkylated the DNA, as previously reported. Subsequent addition of hydride donors, for 30 min, followed by acid hydrolysis of the mixture allowed detection and quantification of 6-(2-{2-[(9H-purin-6-yl)amino]ethoxy}ethoxy)-9H-purin-2-amine, the reduced cross-link formed from deposition, via the diazonium ion, of a 3-oxapentanal fragment on O(6)-Gua, and condensation with N(6)-Ade, presumably in the vicinity. Decreasing the temperature of the reaction mixtures and decreasing the pH modestly increased the yields of the trapped cross-link. Among three borohydride reductants, NaNCBH3 is superior, being ∼4 times more effective on a molar basis, as opposed to a hydride equivalent basis, than NaBH4 or Na(AcO)3BH. For trapping with NaNCBH3, it is deduced that the reaction likely occurs with the iminium ion that is in protonic equilibrium with its conjugate base imine. In an experiment in which the hydroperoxide was decomposed and NaNCBH3 was introduced after various periods of time, the amount of cross-link was observed to increase, nearly linearly, by ∼4-fold over 1 week. These data indicate that there are a minimum of two populations of cross-links, one that forms rapidly, in minutes, and another that grows in with time, over days. Reduced nicotinamide cofactors and ascorbate are observed to effect reduction (over 3 days) of the cross-links, confirming the possibility that otherwise reversible cross-links might be immortalized under biological conditions.

Biomarkers of exposure and effect in human lymphoblastoid TK6 cells following [13C2]-acetaldehyde exposure

The dose-response relationship for biomarkers of exposure (N(2)-ethylidene-dG adducts) and effect (cell survival and micronucleus formation) was determined across 4.5 orders of magnitude (50nM-2mM) using [(13)C2]-acetaldehyde exposures to human lymphoblastoid TK6 cells for 12h. There was a clear increase in exogenous N (2)-ethylidene-dG formation at exposure concentrations ≥ 1µM, whereas the endogenous adducts remained nearly constant across all exposure concentrations, with an average of 3.0 adducts/10(7) dG. Exogenous adducts were lower than endogenous adducts at concentrations ≤ 10µM and were greater than endogenous adducts at concentrations ≥ 250µM. When the endogenous and exogenous adducts were summed together, statistically significant increases in total adduct formation over the endogenous background occurred at 50µM. Cell survival and micronucleus formation were monitored across the exposure range and statistically significant decreases in cell survival and increases in micronucleus formation occurred at ≥ 1000µM. This research supports the hypothesis that endogenously produced reactive species, including acetaldehyde, are always present and constitute the majority of the observed biological effects following very low exposures to exogenous acetaldehyde. These data can replace default assumptions of linear extrapolation to very low doses of exogenous acetaldehyde for risk prediction.

Alcohol-induced one-carbon metabolism impairment promotes dysfunction of DNA base excision repair in adult brain

The brain is one of the major targets of chronic alcohol abuse. Yet the fundamental mechanisms underlying alcohol-mediated brain damage remain unclear. The products of alcohol metabolism cause DNA damage, which in conditions of DNA repair dysfunction leads to genomic instability and neural death. We propose that one-carbon metabolism (OCM) impairment associated with long term chronic ethanol intake is a key factor in ethanol-induced neurotoxicity, because OCM provides cells with DNA precursors for DNA repair and methyl groups for DNA methylation, both critical for genomic stability. Using histological (immunohistochemistry and stereological counting) and biochemical assays, we show that 3-week chronic exposure of adult mice to 5% ethanol (Lieber-Decarli diet) results in increased DNA damage, reduced DNA repair, and neuronal death in the brain. These were concomitant with compromised OCM, as evidenced by elevated homocysteine, a marker of OCM dysfunction. We conclude that OCM dysfunction plays a causal role in alcohol-induced genomic instability in the brain because OCM status determines the alcohol effect on DNA damage/repair and genomic stability. Short ethanol exposure, which did not disturb OCM, also did not affect the response to DNA damage, whereas additional OCM disturbance induced by deficiency in a key OCM enzyme, methylenetetrahydrofolate reductase (MTHFR) in Mthfr(+/-) mice, exaggerated the ethanol effect on DNA repair. Thus, the impact of long term ethanol exposure on DNA repair and genomic stability in the brain results from OCM dysfunction, and MTHFR mutations such as Mthfr 677C→T, common in human population, may exaggerate the adverse effects of ethanol on the brain.

Detection of acetaldehyde derived N(2)-ethyl-2'-deoxyguanosine in human leukocyte DNA following alcohol consumption

Epidemiological studies have shown an association between alcohol (ethanol) consumption and increased cancer risk. The effect of alcohol consumption on the levels and persistence of N(2)-ethylidene-2'-deoxyguanosine (N(2)-ethylidene-dG) formed by acetaldehyde, the oxidative metabolite of ethanol, in human leukocyte DNA was investigated. DNA was isolated from venous blood samples obtained from 30 male non-smoking individuals before consumption of alcohol (0h) and subsequently at 3-5h following the consumption of 150mL of vodka (containing 42% pure ethanol). Additional samples were collected 24h and 48h post-alcohol consumption. The levels of N(2)-ethyl-2'-deoxyguanosine (N(2)-ethyl-dG) in the DNA were determined following reduction of N(2)-ethylidene-dG with sodium cyanoborohydride using a liquid chromatography-tandem mass spectrometry selected reaction monitoring method. A slight time-dependent trend showing an increase and decrease in the levels of N(2)-ethyl-dG was observed following consumption of alcohol compared to time 0h, however, the differences were not statistically significant. The average levels of N(2)-ethyl-dG observed at 0h, 3-5h, 24h and 48h time points following ingestion of alcohol were 34.6±21.9, 35.1±21.0, 36.8±20.7 and 35.6±21.1 per 10(8) 2'-deoxynucleosides, respectively. In conclusion, alcohol consumption that could be encountered under social drinking conditions, does not significantly alter the levels of the acetaldehyde derived DNA adduct, N(2)-ethyl-dG in human leukocyte DNA from healthy individuals.

Combination of ADH1B*2/ALDH2*2 polymorphisms alters acetaldehyde-derived DNA damage in the blood of Japanese alcoholics

The acetaldehyde associated with alcoholic beverages is an evident carcinogen for the esophagus. Genetic polymorphisms of the alcohol dehydrogenase 1B (ADH1B) and aldehyde dehydrogenase 2 (ALDH2) genes are associated with the risk of esophageal cancer. However, the exact mechanism via which these genetic polymorphisms affect esophageal carcinogenesis has not been elucidated. ADH1B*2 is involved in overproduction of acetaldehyde due to increased ethanol metabolism into acetaldehyde, and ALDH2*2 is involved in accumulation of acetaldehyde due to the deficiency of acetaldehyde metabolism. Acetaldehyde can interact with DNA and form DNA adducts, resulting in DNA damage. N(2)-ethylidene-2'-deoxyguanosine (N(2)-ethylidene-dG) is the most abundant DNA adduct derived from acetaldehyde. Therefore, we quantified N(2)-ethylidene-dG levels in blood samples from 66 Japanese alcoholic patients using liquid chromatography/electrospray tandem mass spectrometry, and investigated the relationship between N(2)-ethylidene-dG levels and ADH1B and ALDH2 genotypes. The median N(2)-ethylidene-dG levels (25th percentile, 75th percentile) in patients with ADH1B*1/*1 plus ALDH2*1/*1, ADH1B*2 carrier plus ALDH2*1/*1, ADH1B*1/*1 plus ALDH2*1/*2, and ADH1B*2 carrier plus ALDH2*1/*2 were 2.14 (0.97, 2.37)/10(7) bases, 2.38 (1.18, 2.98)/10(7) bases, 5.38 (3.19, 6.52)/10(7) bases, and 21.04 (12.75, 34.80)/10(7) bases, respectively. In the ALDH2*1/*2 group, N(2)-ethylidene-dG levels were significantly higher in ADH1B*2 carriers than in the ADH1B*1/*1 group (P < 0.01). N(2)-ethylidene-dG levels were significantly higher in the ALDH2*1/*2 group than in the ALDH2*1/*1 group, regardless of ADH1B genotype (ADH1B*1/*1, P < 0.05; ADH1B*2 carriers, P < 0.01) N(2)-ethylidene-dG levels in blood DNA of the alcoholics was remarkably higher in individuals with a combination of the ADH1B*2 and ALDH2*2 alleles. These results provide a new perspective on the carcinogenicity of the acetaldehyde associated with alcoholic beverages, from the aspect of DNA damage.

DNA damage and neurotoxicity of chronic alcohol abuse

Chronic alcohol abuse results in a variety of pathological effects including damage to the brain. The causes of alcohol-induced brain pathology are presently unclear. Several mechanisms of pathogenicity of chronic alcoholism have been proposed, including accumulation of DNA damage in the absence of repair, resulting in genomic instability and death of neurons. Genomic instability is a unified genetic mechanism leading to a variety of neurodegenerative disorders. Ethanol also likely interacts with various metabolic pathways, including one-carbon metabolism (OCM). OCM is critical for the synthesis of DNA precursors, essential for DNA repair, and as a methyl donor for various methylation events, including DNA methylation. Both DNA repair and DNA methylation are critical for maintaining genomic stability. In this review, we outline the role of DNA damage and DNA repair dysfunction in chronic alcohol-induced neurodegeneration.

Time course of DNA adduct formation in peripheral blood granulocytes and lymphocytes after drinking alcohol

Alcohol consumption is an established risk factor for cancers of the head and neck, colorectum, liver and female breast. Acetaldehyde, the primary metabolite of ethanol, is suspected to play a major role in alcohol-related carcinogenesis. Acetaldehyde binds to DNA resulting in formation of adducts. DNA adducts are involved in mutagenesis and carcinogenesis. N (2)-Ethylidenedeoxyguanosine (N (2)-ethylidene-dGuo) is the major adduct formed in this reaction. Studies have shown an association between alcohol drinking and levels of this DNA adduct, suggesting its potential use as a biomarker for studying alcohol-related carcinogenesis. However, there are no reports on the kinetics of formation and repair of N (2)-ethylidene-dGuo after alcohol consumption. Therefore, we investigated levels of N (2)-ethylidene-dGuo in DNA from human peripheral blood cells at several time points after consumption of increasing doses of alcohol. Ten healthy non-smokers were recruited and asked to abstain from alcohol consumption except for the study doses. The subjects were given measured doses of alcohol once a week for 3 weeks, targeting increasing blood alcohol levels. Blood was collected at several time points before and after each dose, DNA was isolated from granulocytes and lymphocytes and N (2)-ethylidene-dGuo was quantified as its NaBH(3)CN reduction product N ( 2 )-ethyldeoxyguanosine by liquid chromatography-electrospray ionisation-tandem mass spectrometry. Significant increases in N (2)-ethylidene-dGuo were observed after all doses and in both cell types. However, there was substantial intraindividual variability, indicating that there are other important sources of this adduct in peripheral blood DNA. Further studies are needed to better understand the origins of N (2)-ethylidene-dGuo in blood cells, the exposures it reflects, and thus its potential use as a marker of alcohol's genotoxic effects.

Kinetics of DNA adduct formation in the oral cavity after drinking alcohol

BACKGROUND

Alcohol consumption is one of the top 10 risks for the worldwide burden of disease and an established cause of head and neck cancer, as well as cancer at other sites. Acetaldehyde, the major metabolite of ethanol, reacts with DNA to produce adducts, which are critical in the carcinogenic process and can serve as biomarkers of exposure and, possibly, of disease risk. Acetaldehyde associated with alcohol consumption is considered "carcinogenic to humans." We have previously developed the technology to quantify acetaldehyde-DNA adducts in human tissues, but there are no studies in the literature defining the formation and removal of acetaldehyde-DNA adducts in people who consumed alcohol.

METHODS

We investigated levels of N(2)-ethylidene-dGuo, the major DNA adduct of acetaldehyde, in DNA from human oral cells at several time points after consumption of increasing alcohol doses. Ten healthy nonsmokers were dosed once a week for three weeks. Mouthwash samples were collected before and at several time points after the dose. N(2)-Ethylidene-dGuo was measured as its NaBH(3)CN reduction product N(2)-ethyl-dGuo by liquid chromatography-electrospray-tandem mass spectrometry.

RESULTS

N(2)-ethylidene-dGuo levels increased as much as 100-fold from baseline within 4 hours after each dose for all subjects and in a dose-responsive manner (P = 0.001).

CONCLUSION

These results show an effect of alcohol on oral cell DNA adduct formation, strongly supporting the key role of acetaldehyde in head and neck cancer caused by alcohol drinking.

IMPACT

Our results provide some of the first conclusive evidence linking exposure to a lifestyle carcinogen and kinetics of DNA adduct formation in humans.

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

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

DNA damage induced by endogenous aldehydes: current state of knowledge

DNA damage plays a major role in various pathophysiological conditions including carcinogenesis, aging, inflammation, diabetes and neurodegenerative diseases. Oxidative stress and cell processes such as lipid peroxidation and glycation induce the formation of highly reactive endogenous aldehydes that react directly with DNA, form aldehyde-derived DNA adducts and lead to DNA damage. In occasion of persistent conditions that influence the formation and accumulation of aldehyde-derived DNA adducts the resulting unrepaired DNA damage causes deregulation of cell homeostasis and thus significantly contributes to disease phenotype. Some of the most highly reactive aldehydes produced endogenously are 4-hydroxy-2-nonenal, malondialdehyde, acrolein, crotonaldehyde and methylglyoxal. The mutagenic and carcinogenic effects associated with the elevated levels of these reactive aldehydes, especially, under conditions of stress, are attributed to their capability of causing directly modification of DNA bases or yielding promutagenic exocyclic adducts. In this review, we discuss the current knowledge on DNA damage induced by endogenously produced reactive aldehydes in relation to the pathophysiology of human diseases.

Bypass of N²-ethylguanine by human DNA polymerase κ

The efficiency and fidelity of nucleotide incorporation and next-base extension by DNA polymerase (pol) κ past N(2)-ethyl-Gua were measured using steady-state and rapid kinetic analyses. DNA pol κ incorporated nucleotides and extended 3' termini opposite N(2)-ethyl-Gua with measured efficiencies and fidelities similar to that opposite Gua indicating a role for DNA pol κ at the insertion and extension steps of N(2)-ethyl-Gua bypass. The DNA pol κ was maximally activated to similar levels by a twenty-fold lower concentration of Mn(2+) compared to Mg(2+). In addition, the steady state analysis indicated that high fidelity DNA pol κ-catalyzed N(2)-ethyl-Gua bypass is Mg(2+)-dependent. Strikingly, Mn(2+) activation of DNA pol κ resulted in a dramatically lower efficiency of correct nucleotide incorporation opposite both N(2)-ethyl-Gua and Gua compared to that detected upon Mg(2+) activation. This effect is largely governed by diminished correct nucleotide binding as indicated by the high K(m) values for dCTP insertion opposite N(2)-ethyl-Gua and Gua with Mn(2+) activation. A rapid kinetic analysis showed diminished burst amplitudes in the presence of Mn(2+) compared to Mg(2+) indicating that DNA pol κ preferentially utilizes Mg(2+) activation. These kinetic data support a DNA pol κ wobble base pairing mechanism for dCTP incorporation opposite N(2)-ethyl-Gua. Furthermore, the dramatically different polymerization efficiencies of the Y-family DNA pols κ and ι in the presence of Mn(2+) suggest a metal ion-dependent regulation in coordinating the activities of these DNA pols during translesion synthesis.

Formation of acetaldehyde-derived DNA adducts due to alcohol exposure

Epidemiological studies have identified chronic alcohol consumption as a significant risk factor for cancers of the upper aerodigestive tract, including the oral cavity, pharynx, larynx and esophagus, and for cancer of the liver. Ingested ethanol is mainly oxidized by the enzymes alcohol dehydrogenase (ADH), cytochrome P-450 2E1 (CYP2E1), and catalase to form acetaldehyde, which is subsequently oxidized by aldehyde dehydrogenase 2 (ALDH2) to produce acetate. Polymorphisms of the genes which encode enzymes for ethanol metabolism affect the ethanol/acetaldehyde oxidizing capacity. ADH1B*2 allele (ADH1B, one of the enzyme in ADH family) is commonly observed in Asian population, has much higher enzymatic activity than ADH1B*1 allele. Otherwise, approximately 40% of Japanese have single nucleotide polymorphisms (SNPs) of the ALDH2 gene. The ALDH2 *2 allele encodes a protein with an amino acid change from glutamate to lysine (derived from the ALDH2*1 allele) and devoid of enzymatic activity. Neither the homozygote (ALDH2*2/*2) nor heterozygote (ALDH2*1/*2) is able to metabolize acetaldehyde promptly. Acetaldehyde is a genotoxic compound that reacts with DNA to form primarily a Schiff base N(2)-ethylidene-2'-deoxyguanosine (N(2)-ethylidene-dG) adduct, which may be converted by reducing agents to N(2)-ethyl-2'-deoxyguanosine (N(2)-ethyl-dG) in vivo, and strongly blocked translesion DNA synthesis. Several studies have demonstrated a relationship between ALDH2 genotypes and the development of certain types of cancer. On the other hand, the drinking of alcohol induces the expression of CYP2E1, resulting in an increase in reactive oxygen species (ROS) and oxidative DNA damage. This review covers the combined effects of alcohol and ALDH2 polymorphisms on cancer risk. Studies show that ALDH2*1/*2 heterozygotes who habitually consume alcohol have higher rates of cancer than ALDH2*1/*1 homozygotes. Moreover, they support that chronic alcohol consumption contributes to formation of various DNA adducts. Although some DNA adducts formation is demonstrated to be an initiation step of carcinogenesis, it is still unclear that whether these alcohol-related DNA adducts are true factors or initiators of cancer. Future studies are needed to better characterize and to validate the roles of these DNA adducts in human study.

Characteristics of aldehyde dehydrogenase 2 (Aldh2) knockout mice

Acetaldehyde is an intermediate of ethanol oxidation. It covalently binds to DNA, and is known as a carcinogen. Aldehyde dehydrogenase 2 (ALDH2) is an important enzyme that oxidizes acetaldehyde. Approximately 45% of Chinese and Japanese individuals have the inactive ALDH2 genotypes (ALDH2*2/*2 and ALDH2*1/*2), and Aldh2 knockout mice appear to be a valid animal model for humans with inactive ALDH2. This review gives an overview of published studies on Aldh2 knockout mice, which were treated with ethanol or acetaldehyde. According to these studies, it was found that Aldh2 -/- mice (Aldh2 knockout mice) are more susceptible to ethanol and acetaldehyde-induced toxicity than Aldh2 +/+ mice (wild type mice). When mice were fed with ethanol, the mortality was increased. When they were exposed to atmospheres containing acetaldehyde, the Aldh2 -/- mice showed more severe toxic symptoms, like weight loss and higher blood acetaldehyde levels, as compared with the Aldh2 +/+ mice. Thus, ethanol and acetaldehyde treatment affects Aldh2 knockout mice more than wild type mice. Based on these findings, it is suggested that ethanol consumption and acetaldehyde inhalation are inferred to pose a higher risk to ALDH2-inactive humans. These results also support that ALDH2-deficient humans who habitually consume alcohol have a higher rate of cancer than humans with functional ALDH2.

Association of ALDH2 genotypes with periodontitis progression

The progression of periodontitis may be affected by ALDH2 genotypes with respect to the oxidation of acetaldehyde to acetate, which leads to the accumulation of acetaldehyde in plasma and potential toxic effects. We examined the prospective association of ALDH2 genotypes in terms of alcohol sensitivity between alcohol consumption and periodontal disease progression. In 2003, 224 of 256 (87.5%) individuals examined at baseline (1999) completed probing pocket depth measurements for the evaluation of periodontitis progression. Missing data on self-reported questionnaires and blood samples were excluded; therefore, 183 samples were analyzed. Individuals who consumed > or = 33.0 g/day of alcohol exhibited high periodontal disease progression risk (OR = 3.54). ALDH2 *1/*2 individuals who consumed > or = 33 g/day of alcohol displayed a significant odds ratio (OR = 4.28) of periodontitis progression risk, in contrast to ALDH2 *1/*1 individuals. These results suggested that alcohol consumption as well as alcohol sensitivity may be a risk factor for periodontitis progression.

Acetaldehyde as an underestimated risk factor for cancer development: role of genetics in ethanol metabolism

Chronic ethanol consumption is a strong risk factor for the development of certain types of cancer including those of the upper aerodigestive tract, the liver, the large intestine and the female breast. Multiple mechanisms are involved in alcohol-mediated carcinogenesis. Among those the action of acetaldehyde (AA), the first metabolite of ethanol oxidation is of particular interest. AA is toxic, mutagenic and carcinogenic in animal experiments. AA binds to DNA and forms carcinogenic adducts. Direct evidence of the role of AA in alcohol-associated carcinogenesis derived from genetic linkage studies in alcoholics. Polymorphisms or mutations of genes coding for AA generation or detoxifying enzymes resulting in elevated AA concentrations are associated with increased cancer risk. Approximately 40% of Japanese, Koreans or Chinese carry the AA dehydrogenase 2*2 (ALDH2*2) allele in its heterozygous form. This allele codes for an ALDH2 enzyme with little activity leading to high AA concentrations after the consumption of even small amounts of alcohol. When individuals with this allele consume ethanol chronically, a significant increased risk for upper alimentary tract and colorectal cancer is noted. In Caucasians, alcohol dehydrogenase 1C*1 (ADH1C*1) allele encodes for an ADH isoenzyme which produces 2.5 times more AA than the corresponding allele ADH1C*2. In studies with moderate to high alcohol intake, ADH1C*1 allele frequency and rate of homozygosity was found to be significantly associated with an increased risk for cancer of the upper aerodigestive tract, the liver, the colon and the female breast. These studies underline the important role of acetaldehyde in ethanol-mediated carcinogenesis.

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

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

Chemistry and biology of DNA containing 1,N(2)-deoxyguanosine adducts of the alpha,beta-unsaturated aldehydes acrolein, crotonaldehyde, and 4-hydroxynonenal

The α,β-unsaturated aldehydes (enals) acrolein, crotonaldehyde, and trans-4-hydroxynonenal (4-HNE) are products of endogenous lipid peroxidation, arising as a consequence of oxidative stress. The addition of enals to dG involves Michael addition of the N²-amine to give N²-(3-oxopropyl)-dG adducts, followed by reversible cyclization of N1 with the aldehyde, yielding 1,N²-dG exocyclic products. The 1,N²-dG exocyclic adducts from acrolein, crotonaldehyde, and 4-HNE exist in human and rodent DNA. The enal-induced 1,N²-dG lesions are repaired by the nucleotide excision repair pathway in both Escherichia coli and mammalian cells. Oligodeoxynucleotides containing structurally defined 1,N²-dG adducts of acrolein, crotonaldehyde, and 4-HNE were synthesized via a postsynthetic modification strategy. Site-specific mutagenesis of enal adducts has been carried out in E. coli and various mammalian cells. In all cases, the predominant mutations observed are G→T transversions, but these adducts are not strongly miscoding. When placed into duplex DNA opposite dC, the 1,N²-dG exocyclic lesions undergo ring opening to the corresponding N²-(3-oxopropyl)-dG derivatives. Significantly, this places a reactive aldehyde in the minor groove of DNA, and the adducted base possesses a modestly perturbed Watson−Crick face. Replication bypass studies in vitro indicate that DNA synthesis past the ring-opened lesions can be catalyzed by pol η, pol ι, and pol κ. It also can be accomplished by a combination of Rev1 and pol ζ acting sequentially. However, efficient nucleotide insertion opposite the 1,N²-dG ring-closed adducts can be carried out only by pol ι and Rev1, two DNA polymerases that do not rely on the Watson−Crick pairing to recognize the template base. The N²-(3-oxopropyl)-dG adducts can undergo further chemistry, forming interstrand DNA cross-links in the 5′-CpG-3′ sequence, intrastrand DNA cross-links, or DNA−protein conjugates. NMR and mass spectrometric analyses indicate that the DNA interstand cross-links contain a mixture of carbinolamine and Schiff base, with the carbinolamine forms of the linkages predominating in duplex DNA. The reduced derivatives of the enal-mediated N²-dG:N²-dG interstrand cross-links can be processed in mammalian cells by a mechanism not requiring homologous recombination. Mutations are rarely generated during processing of these cross-links. In contrast, the reduced acrolein-mediated N²-dG peptide conjugates can be more mutagenic than the corresponding monoadduct. DNA polymerases of the DinB family, pol IV in E. coli and pol κ in human, are implicated in error-free bypass of model acrolein-mediated N²-dG secondary adducts, the interstrand cross-links, and the peptide conjugates.

Lesion bypass of N2-ethylguanine by human DNA polymerase iota

Nucleotide incorporation and extension opposite N2-ethyl-Gua by DNA polymerase iota was measured and structures of the DNA polymerase iota-N2-ethyl-Gua complex with incoming nucleotides were solved. Efficiency and fidelity of DNA polymerase iota opposite N2-ethyl-Gua was determined by steady state kinetic analysis with Mg2+ or Mn2+ as the activating metal. DNA polymerase iota incorporates dCMP opposite N2-ethyl-Gua and unadducted Gua with similar efficiencies in the presence of Mg2+ and with greater efficiencies in the presence of Mn2+. However, the fidelity of nucleotide incorporation by DNA polymerase iota opposite N2-ethyl-Gua and Gua using Mn2+ is lower relative to that using Mg2+ indicating a metal-dependent effect. DNA polymerase iota extends from the N2-ethyl-Gua:Cyt 3' terminus more efficiently than from the Gua:Cyt base pair. Together these kinetic data indicate that the DNA polymerase iota catalyzed reaction is well suited for N(2)-ethyl-Gua bypass. The structure of DNA polymerase iota with N2-ethyl-Gua at the active site reveals the adducted base in the syn configuration when the correct incoming nucleotide is present. Positioning of the ethyl adduct into the major groove removes potential steric overlap between the adducted template base and the incoming dCTP. Comparing structures of DNA polymerase iota complexed with N2-ethyl-Gua and Gua at the active site suggests movements in the DNA polymerase iota polymerase-associated domain to accommodate the adduct providing direct evidence that DNA polymerase iota efficiently replicates past a minor groove DNA adduct by positioning the adducted base in the syn configuration.

Liquid chromatography-electrospray ionization tandem mass spectrometry analysis of 7-ethylguanine in human liver DNA

Considerable evidence in the literature demonstrates the exposure of humans to an unknown ethylating agent. Previous studies have demonstrated the presence of 7-ethyl-Gua and 3-ethyl-Ade in urine, 7-ethyl-dGuo and O4-ethyl-dThd in human lung, and ethylvaline in hemoglobin. Some studies also report higher levels of ethyl adducts in smokers than in nonsmokers, and there is convincing evidence for an uncharacterized ethylating agent in cigarette smoke. To further investigate this question, we have developed a liquid chromatography-electrospray ionization tandem mass spectrometry-selected reaction monitoring method for analysis of 7-ethyl-Gua in human liver DNA. To our knowledge, there are no previous reports of MS analyses of 7-ethyl-Gua in human tissues. [15N 5]7-Ethyl-Gua was synthesized and used as the internal standard. Human liver DNA was heated to release 7-ethyl-Gua. After partial purification by solid-phase extraction, analysis was carried out using the transition m/z 180 [M+H]+-->m/z 152 [Gua+H]+ for 7-ethyl-Gua and m/z 185-->m/z 157 for the internal standard. The method was accurate and precise. The detection limit was approximately 8-9 fmol/micromol Gua, starting with 1-2 mg of DNA. Clear coeluting peaks for 7-ethyl-Gua and the internal standard were observed in the human liver DNA samples. Twenty-six human liver DNA samples (0.77+/-0.40 mg) were analyzed, and 25 were positive for 7-ethyl-Gua. The mean level of 7-ethyl-Gua was 42.2+/-43.0 fmol/micromol Gua (8.4+/-8.6 adducts per 10(9) nucleotides). These results demonstrate that 7-ethyl-Gua is a common DNA adduct in human liver with likely endogenous sources that require further investigation.

Molecular mechanisms of alcohol-mediated carcinogenesis

Approximately 3.6% of cancers worldwide derive from chronic alcohol drinking, including those of the upper aerodigestive tract, the liver, the colorectum and the breast. Although the mechanisms for alcohol-associated carcinogenesis are not completely understood, most recent research has focused on acetaldehyde, the first and most toxic ethanol metabolite, as a cancer-causing agent. Ethanol may also stimulate carcinogenesis by inhibiting DNA methylation and by interacting with retinoid metabolism. Alcohol-related carcinogenesis may interact with other factors such as smoking, diet and comorbidities, and depends on genetic susceptibility.

The role of acetaldehyde in upper digestive tract cancer in alcoholics

Chronic excessive alcohol consumption is the strongest risk factor for upper aerodigestive tract (UADT) cancer. Multiple mechanisms are involved in alcohol-associated cancer development of the UADT, including acetaldehyde (AA) effects. AA is toxic, mutagenic, and carcinogenic. Evidence of the role of AA in alcohol-associated carcinogenesis derived from genetic linkage studies in alcoholics. Polymorphism or mutation in genes coding for AA generation or detoxification enzymes are associated with increased cancer risk. It has been clearly shown in Asians that individuals carrying the acetaldehyde dehydrogenase 2*2 (ALDH2*2) allele have a significantly increased cancer risk when they consume alcohol. In Caucasians, alcohol dehydrogenase 1*1 (ADH1C*1) allele encodes for an alcohol dehydrogenase (ADH) isoenzyme, which produces 2.5 times more AA than the corresponding allele ADH1C*2. The authors found that the ADH1C*1 allele frequency and rate of homozygosity was significantly associated with an increased risk for alcohol-related cancer. AA seems to be an important factor in alcohol-associated carcinogenesis of the UADT.

Analyses of representative biomarkers of exposure and effect by chromatographic, mass spectrometric, and nuclear magnetic resonance techniques: method development and application in life sciences

Biomarkers are essential tools in monitoring studies, which include environmental monitoring, biological monitoring, biological effect monitoring, and health surveillance, as well as drug development processes. Their discovery, validation, and analysis require highly sensitive and selective analytical technologies. In this regard, gas and liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy have facilitated great achievements in all these areas. In addition and closely related to biomarkers, the ongoing developments in these techniques promise a better understanding of the nature and mechanisms of toxic effects originating from various chemical, biological, or physical sources. This Review compiles studies performed on selected biomarkers with respect to both method development and application. Section 1 summarizes the concept of biomarkers; their application in various industrial/occupational, agricultural, drug developmental, and medical/clinical platforms. This section also focuses on biotransformation studies in close relation to biomarker discovery and validation, and on major techniques utilized in this area. In Section 2, biotransformation of volatile anesthetics in humans with a focus on mercapturic acid derivatives as potential biomarkers of effect is reviewed. The use of GC-ECD, GC/MS, and 19F-NMR in these studies is described. Section 3 focuses on the analysis of aldehydic lipid peroxidation degradation products by GC-ECD in mammalian cells in which oxidative stress induced chemically, and in humans after various challenges; anesthetic exposure, ischemia-reperfusion, and controlled endurance exercise. In Section 4, method development for protein and DNA oxidation products by LC-tandem MS and its application in mammalian cells and in humans are summarized. Possibilities, limitations, and future perspectives are discussed in Section 5.

Alcohol dehydrogenase 1C*1 allele is a genetic marker for alcohol-associated cancer in heavy drinkers

Chronic alcohol consumption is associated with an increased risk for upper aerodigestive tract cancer and hepatocellular carcinoma. Increased acetaldehyde production via alcohol dehydrogenase (ADH) has been implicated in the pathogenesis. The allele ADH1C*1 of ADH1C encodes for an enzyme with a high capacity to generate acetaldehyde. So far, the association between the ADH1C*1 allele and alcohol-related cancers among heavy drinkers is controversial. ADH1C genotypes were determined by polymerase chain reaction and restriction fragment length polymorphism in a total of 818 patients with alcohol-associated esophageal (n=123), head and neck (n=84) and hepatocellular cancer (n=86) as well as in patients with alcoholic pancreatitis (n=117), alcoholic liver cirrhosis (n=217), combined liver cirrhosis and pancreatitis (n=17) and in alcoholics without gastrointestinal organ damage (n=174). The ADH1C*1 allele and genotype ADH1C*1/1 were significantly more frequent in patients with alcohol-related cancers than that in individuals with nonmalignant alcohol-related organ damage. Using multivariate analysis, ADH1C*1 allele frequency and rate of homozygosity were significantly associated with an increased risk for alcohol-related cancers (p<0.001 in all instances). The odds ratio for genotype ADH1C*1/1 regarding the development of esophageal, hepatocellular and head and neck cancer were 2.93 (CI, 1.84-4.67), 3.56 (CI, 1.33-9.53) and 2.2 (CI, 1.11-4.36), respectively. The data identify genotype ADH1C*1/1 as an independent risk factor for the development of alcohol-associated tumors among heavy drinkers, indicating a genetic predisposition of individuals carrying this genotype.

Mutagenesis by exocyclic alkylamino purine adducts in Escherichia coli

Exocyclic alkylamino purine adducts, including N(2)-ethyldeoxyguanosine, N(2)-isopropyldeoxyguanosine, and N(6)-isopropyldeoxyadenosine, occur as a consequence of reactions of DNA with toxins such as the ethanol metabolite acetaldehyde, diisopropylnitrosamine, and diisopropyltriazene. However, there are few data addressing the biological consequences of these adducts when present in DNA. Therefore, we assessed the mutagenicities of these single, chemically synthesized exocyclic amino adducts when placed site-specifically in the supF gene in the reporter plasmid pLSX and replicated in Escherichia coli, comparing the mutagenic potential of these exocyclic amino adducts to that of O(6)-ethyldeoxyguanosine. Inclusion of deoxyuridines on the strand complementary to the adducts at 5' and 3' flanking positions resulted in mutant fractions of N(2)-ethyldeoxyguanosine and N(2)-isopropyldeoxyguanosine-containing plasmid of 1.4+/-0.5% and 5.7+/-2.5%, respectively, both of which were significantly greater than control plasmid containing deoxyuridines but no adduct (p=0.04 and 0.003, respectively). The mutagenicities of the three exocyclic alkylamino purine adducts tested were of smaller magnitude than O(6)-ethyldeoxyguanosine (mutant fraction=21.2+/-1.2%, p=0.00001) with the N(6)-isopropyldeoxyadenosine being the least mutagenic (mutant fraction=1.2+/-0.5%, p=0.13). The mutation spectrum generated by the N(2)-ethyl and -isopropyldeoxyguanosine adducts included adduct site-targeted G:C-->T:A transversions, adduct site single base deletions, and single base deletions three bases downstream from the adduct, which contrasted sharply with the mutation spectrum generated by the O(6)-ethyldeoxyguanosine lesion of 95% adduct site-targeted transitions. We conclude that N(2)-ethyl and -isopropyldeoxyguanosine are mutagenic adducts in E. coli whose mutation spectra differ markedly from that of O(6)-ethyldeoxyguanosine.

Identification of an acetaldehyde adduct in human liver DNA and quantitation as N2-ethyldeoxyguanosine

Acetaldehyde, an ubiquitous mutagen and carcinogen, could be involved in human cancer etiology. Because DNA adducts are important in carcinogenesis, we have used liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) to explore the presence in human liver DNA of the major acetaldehyde DNA adduct, N2-ethylidenedeoxyguanosine (1). DNA was isolated and enzymatically hydrolyzed in the presence of NaBH3CN, which quantitatively converts adduct 1 to N2-ethyldeoxyguanosine (N2-ethyl-dGuo, 2). [15N5]N2-Ethyl-dGuo was synthesized and used as an internal standard. Adduct 2 was enriched from the hydrolysate by solid phase extraction and analyzed by LC-ESI-MS/MS. Clear peaks were observed for adduct 2 in analyses of human liver DNA, calf thymus DNA, and rat liver DNA. These peaks were not observed, or were much smaller, when the NaBH3CN step was omitted. When the DNA was subjected to neutral thermal hydrolysis prior to NaBH3CN treatment, adduct 2 was not observed. Control experiments using [13C2]acetaldehyde demonstrated that adducts 1 and 2 were not formed as artifacts during DNA isolation and analysis. These results strongly indicate that adduct 1 is present in human liver DNA and demonstrate that it can be quantified as adduct 2. Levels of adduct 2 measured in 12 human liver samples were 534 +/- 245 fmol/micromol dGuo (mean +/- SD). The results of this study establish the presence of an acetaldehyde adduct in human liver DNA and suggest that it is a commonly occurring endogenous DNA adduct.

Polyamines stimulate the formation of mutagenic 1,N2-propanodeoxyguanosine adducts from acetaldehyde

Alcoholic beverage consumption is associated with an increased risk of upper gastrointestinal cancer. Acetaldehyde (AA), the first metabolite of ethanol, is a suspected human carcinogen, but the molecular mechanisms underlying AA carcinogenicity are unclear. In this work, we tested the hypothesis that polyamines could facilitate the formation of mutagenic α-methyl-γ-hydroxy-1,N²-propano-2′-deoxyguanosine (Cr-PdG) adducts from biologically relevant AA concentrations. We found that Cr-PdG adducts could be formed by reacting deoxyguanosine with μM concentrations of AA in the presence of spermidine, but not with either AA or spermidine alone. The identities of the Cr-PdG adducts were confirmed by both liquid and gas chromatography-mass spectrometry. Using a novel isotope-dilution liquid chromatography-mass spectrometry assay, we found that in the presence of 5 mM spermidine, AA concentrations of 100 μM and above resulted in the formation of Cr-PdG in genomic DNA. These AA levels are within the range that occurs in human saliva after alcoholic beverage consumption. We also showed that spermidine directly reacts with AA to generate crotonaldehyde (CrA), most likely via an enamine aldol condensation mechanism. We propose that AA derived from ethanol metabolism is converted to CrA by polyamines in dividing cells, forming Cr-PdG adducts, which may be responsible for the carcinogenicity of alcoholic beverage consumption.

DNA adducts from acetaldehyde: implications for alcohol-related carcinogenesis

Alcoholic beverage consumption is classified as a known human carcinogen, causally related to an increased risk of cancer of the upper gastrointestinal tract. The formation of acetaldehyde from ethanol metabolism seems to be the major mechanism underlying this effect. Acetaldehyde is carcinogenic in rodents and causes sister chromatid exchanges and chromosomal aberrations in human cells. The best-studied DNA adduct from acetaldehyde is N(2)-ethyl-2'-deoxyguanosine, which is increased in liver DNA obtained from ethanol-treated rodents and in white blood cells obtained from human alcohol abusers. However, the carcinogenic relevance of this adduct is unclear in view of the lack of evidence that it is mutagenic in mammalian cells. A different DNA adduct, 1,N(2)-propano-2'-deoxyguanosine (PdG), can also be formed from acetaldehyde in the presence of histones and other basic molecules. PdG has been shown to be responsible for the genotoxic and mutagenic effects of crotonaldehyde. The PdG adduct can exist in either of two forms: a ring-closed form or a ring-opened aldehyde form. Whereas the ring-closed form is mutagenic, the aldehyde form can participate in the formation of secondary lesions, including DNA-protein cross-links and DNA interstrand cross-links. The formation of these types of complex secondary DNA lesions resulting from PdG may explain many of the observed genotoxic effects of acetaldehyde described above. Repair of PdG and its associated adducts is complex, involving multiple pathways. Inherited variation in the genes encoding the proteins involved in the repair of PdG and its secondary adducts may contribute to susceptibility to alcoholic beverage-related carcinogenesis.

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.

Enhancement of camptothecin-induced topoisomerase I cleavage complexes by the acetaldehyde adduct N2-ethyl-2'-deoxyguanosine

The activity of DNA topoisomerase I (Top1), an enzyme that regulates DNA topology, is impacted by DNA structure alterations and by the anticancer alkaloid camptothecin (CPT). Here, we evaluated the effect of the acetaldehyde-derived DNA adduct, N2-ethyl-2'-deoxyguanosine (N2-ethyl-dG), on human Top1 nicking and closing activities. Using purified recombinant Top1, we show that Top1 nicking-closing activity remains unaffected in N2-ethyl-dG adducted oligonucleotides. However, the N2-ethyl-dG adduct enhanced CPT-induced Top1-DNA cleavage complexes depending on the relative position of the N2-ethyl-dG adduct with respect to the Top1 cleavage site. The Top1-mediated DNA religation (closing) was selectively inhibited when the N2-ethyl-dG adduct was present immediately 3' from the Top1 site (position +1). In addition, when the N2-ethyl-dG adduct was located at the -5 position, CPT enhanced cleavage at an alternate Top1 cleavage site immediately adjacent to the adduct, which was then at position +1 relative to this new alternate Top1 site. Modeling studies suggest that the ethyl group on the N2-ethyl-dG adduct located at the 5' end of a Top1 site (position +1) sterically blocks the dissociation of CPT from the Top1-DNA complex, thereby inhibiting further the religation (closing) reaction.

Association of ALDH(2) genotypes and alcohol consumption with periodontitis

There is little information regarding the association between alcohol consumption and periodontitis risk. We assessed whether alcohol consumption and ALDH(2) genotypes were associated with periodontitis. Subjects' lifestyle was examined by a self-administered questionnaire, and the percentage of pocket depths > or = 3.5 mm was used as a periodontal parameter. ALDH(2) genotypes were determined with the use of a PCR/RFLP method. Multiple logistic analyses showed that alcohol consumption was significantly associated with periodontitis, and its odds ratio was 1.98. There was no significant relationship between periodontal status and ALDH(2) genotypes. However, ALDH(2)*1/*2 subjects who consumed > or = 33 g/day of alcohol had a significantly greater percentage of pocket depths > or = 3.5 mm than those whose daily consumption was lower, while there was no significant difference in periodontal status associated with alcohol consumption in ALDH(2)*1/*1 subjects. Our results suggest that alcohol consumption may be a risk indicator for periodontitis in ALDH(2)*1/*2 subjects who consume larger amounts of alcohol.