Induction and persistence of SCE-inducing damage in human lymphocytes exposed to vinyl acetate and acetaldehyde in vitro

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.

Investigations on the new mechanism of action for acetaldehyde-induced clastogenic effects in human lung fibroblasts

Acetaldehyde (AA) has been classified as a probable human carcinogen by the International Agency for Research on Cancer (IARC, WHO) and by the US Environmental Protection Agency due to its ability to cause tumours following inhalation or alcohol consumption in animals. Humans are constantly exposed to AA through inhalation from the environment through cigarette smoke, vehicle fumes and industrial emissions as well as by persistent alcohol ingestion. Individuals with deficiencies in the enzymes that are involved in the metabolism of AA are more susceptible to its toxicity and constitute a vulnerable human population. Studies have shown that AA induces DNA damage and cytogenetic abnormalities. A study was undertaken to elucidate the clastogenic effects induced by AA and any preceding DNA damage that occurs in normal human lung fibroblasts as this will further validate the detrimental effects of inhalation exposure to AA. AA exposure induced DNA damage, involving DNA double strand breaks, which could possibly occur at the telomeric regions as well, resulting in a clastogenic effect and subsequent genomic instability, which contributed to the cell cycle arrest. The clastogenic effect induced by AA in human lung fibroblasts was evidenced by micronuclei induction and chromosomal aberrations, including those at the telomeric regions. Co-localisation between the DNA double strand breaks and telomeric regions was observed, suggesting possible induction of DNA double strand breaks due to AA exposure at the telomeric regions as a new mechanism beyond the clastogenic effect of AA. From the cell cycle profile following AA exposure, a G2/M phase arrest and a decrease in cell viability were also detected. Therefore, these effects due to AA exposure via inhalation may have implications in the development of carcinogenesis in humans.

The Japan Flavour and Fragrance Materials Association's (JFFMA) safety assessment of acetal food flavouring substances uniquely used in Japan

Using the procedure devised by the Joint FAO/WHO Expert Committee on Food Additives (JECFA), we performed safety evaluations on five acetal flavouring substances uniquely used in Japan: acetaldehyde 2,3-butanediol acetal, acetoin dimethyl acetal, hexanal dibutyl acetal, hexanal glyceryl acetal and 4-methyl-2-pentanone propyleneglycol acetal. As no genotoxicity study data were available in the literature, all five substances had no chemical structural alerts predicting genotoxicity. Using Cramer's classification, acetoin dimethyl acetal and hexanal dibutyl acetal were categorised as class I, and acetaldehyde 2,3-butanediol acetal, hexanal glyceryl acetal and 4-methyl-2-pentanone propyleneglycol acetal as class III. The estimated daily intakes for all five substances were within the range of 1.45-6.53 µg/person/day using the method of maximised survey-derived intake based on the annual production data in Japan from 2001, 2005, 2008 and 2010, and 156-720 µg/person/day using the single-portion exposure technique (SPET), based on the average use levels in standard portion sizes of flavoured foods. The daily intakes of the two class I substances were below the threshold of toxicological concern (TTC) - 1800 μg/person/day. The daily intakes of the three class III substances exceeded the TTC (90 μg/person/day). Two of these, acetaldehyde 2,3-butanediol acetal and hexanal glyceryl acetal, were expected to be metabolised into endogenous products after ingestion. For 4-methyl-2-pentanone propyleneglycol acetal, one of its metabolites was not expected to be metabolised into endogenous products. However, its daily intake level, based on the estimated intake calculated by the SPET method, was about 1/15 000th of the no observed effect level. It was thus concluded that all five substances raised no safety concerns when used for flavouring foods at the currently estimated intake levels. While no information on in vitro and in vivo toxicity for all five substances was available, their metabolites were judged as raising no safety concerns at the current levels of intake.

The in vitro genotoxicity of ethanol and acetaldehyde

Ability of ethanol to produce chromosomal changes has been controversial in past many years; nevertheless many recent studies have shown that ethanol itself produces genotoxic effects like acetaldehyde. This study was carried out to evaluate the ability of ethanol and its metabolite acetaldehyde to induce chromosomal changes using in vitro CBMN assay (Cytokinesis Blocked Micronucleus assay) in conjunction with immunofluorescent labeling of kinetochores. Kinetochore staining was used with a view to differentiate, between the genotoxic effects of both chemicals, and ascertain the mechanisms of genotoxicity induction by ethanol and acetaldehyde. Both ethanol and acetaldehyde produced statistically significant (P<0.05) dose dependent increase in MN induction as compared with the controls over the dose range tested. Kinetochore analysis proved that the MN induced in ethanol were originated by an aneugenic mechanism, whereas in the case of acetaldehyde most of the MN had originated by a clastogenic mechanism. This not only confirms the ability of ethanol to produce DNA damage in vitro but it also establishes the efficacy of CBMN assay to detect and differentiate between the genotoxic effects of different genotoxins. Here we report that ethanol is itself genotoxic, at least in vitro, and produces genotoxic effects mainly through an aneugenic mechanism whereas its metabolite acetaldehyde is a clastogen.

Association of ALDH2 polymorphism with sensitivity to acetaldehyde-induced micronuclei and facial flushing after alcohol intake

To investigate whether sensitivity to the induction of micronuclei by acetaldehyde is associated with genetic polymorphisms of the aldehyde dehydrogenase-2 (ALDH2) gene, cytokinesis-block micronucleus (CBMN) assays were performed on peripheral lymphocytes from 47 healthy human subjects exposed to acetaldehyde in vitro. Facial flushing following alcohol intake was analyzed to determine if it was correlated with ALDH2 gene polymorphisms. The frequencies of the ALDH2 genotypes ALDH2(1)/ALDH2(1), ALDH2(1)/ALDH2(2), and ALDH2(2)/ALDH2(2) were 66.0, 27.7, and 6.4%, respectively, in the 47 subjects. Therefore, 34% of the studied subjects carried the mutant allele ALDH2(2), which is associated with the lack of enzyme activity. The frequency of micronuclei induced by acetaldehyde increased in a dose-dependent manner with the largest increase seen in subjects that were homozygous for the ALDH2(2) allele. A significant association was observed between the ALDH2 genotype and alcohol-induced facial flushing. Average alcohol consumption of the study subjects was also associated with the ALDH2 genotype. The frequency of heavy drinking was significantly higher among subjects with the ALDH2(1)/ALDH2(1) genotype than among subjects with the ALDH2(2) allele (ALDH2(1)/ALDH2(2) and ALDH2(2)/ALDH2(2) genotypes). Alcohol-induced facial flushing was also associated with an increased frequency of micronuclei in lymphocytes treated with acetaldehyde. The results suggest that the ALDH2 genotype is significantly associated with acetaldehyde-induced micronuclei and alcohol-induced facial flushing.

Differentiating between local cytotoxicity, mitogenesis, and genotoxicity in carcinogen risk assessments: the case of vinyl acetate

Understanding the mode of action of carcinogens is critical to scientifically assessing exposure-related risk. Regulatory hazard classification schemes and dose-response assessment paradigms generally require basic knowledge of genotoxic potential to guide decisions on which scheme or paradigm is most appropriate. Although convention suggests that classification and dose-response assessment of genotoxic chemicals should be assessed using conservative assumptions of no threshold, several examples, such as vinyl acetate, exist that challenge this assumption. Vinyl acetate is carcinogenic at portals of entry (nasal cavity and upper gastrointestinal tract). Local metabolism of vinyl acetate produces DNA-reactive acetaldehyde but also produces acetic acid and protons, which contribute to intracellular acidification, cytotoxicity and cell proliferation. This paper reviews their relative contributions to the overall mode of action. Elevated cellular proliferation, well understood to be a risk factor for carcinogenesis, is observed at concentrations associated with tumor formation. Cytotoxicity and compensatory tissue regeneration is one pathway for stimulating cellular proliferation while intracellular acidification is a mitogenic stimulus. Both of these pathways may be operative in nasal tissues while mitogenic proliferation alone appears to be induced in the upper gastrointestinal tract. Using a physiologically-based pharmacokinetic model, quantitative relationships between critical tissue dosimeters and tissue responses are developed to assess the relative importance of genotoxicity and cell proliferation in the overall mode of action of vinyl acetate. This approach supports the concept that intracellular acidification is the sentinel response that precedes cytotoxicity and cellular proliferation. Secondarily, the carcinogenic potential of vinyl acetate is expressed only when tissue exposure to acetaldehyde is high and when cellular proliferation is simultaneously elevated. This mode of action suggests that exposure levels that do not increase intracellular acidification beyond homeostatic bounds will be adequately protective of adverse downstream responses including cancer. These mechanistic insights provide the scientific basis for a cancer classification that incorporates thresholds for cytotoxic and/or mitogenic cell proliferation secondary to intracellular acidification.

Population distribution of aldehyde dehydrogenase-2 genetic polymorphism: implications for risk assessment

The role of genetic polymorphisms in modulating xenobiotic metabolism and susceptibility to cancer and other health effects has been suggested in numerous studies. However, risk assessments have generally not used this information to characterize population variability or adjust risks for susceptible subgroups. This paper focuses upon the aldehyde dehydrogenase-2 (ALDH2) system because it exemplifies the pivotal role genetic polymorphisms can play in determining enzyme function and susceptibility. Allelic variants in ALDH2 cause decreased ability to clear acetaldehyde and other aldehyde substrates, with homozygous variants (ALDH2*2/2) having no activity and heterozygotes (ALDH2*1/2) having intermediate activity relative to the predominant wild type (ALDH2*1/1). These polymorphisms are associated with increased buildup of acetaldehyde following ethanol ingestion and increased immediate symptoms (flushing syndrome) and long-term cancer risks. We have used Monte Carlo simulation to characterize the population distribution of ALDH2 allelic variants and inter-individual variability in aldehyde internal dose. The nonfunctional allele is rare in most populations, but is common in Asians such that 40% are heterozygotes and 5% are homozygote variants. The ratio of the 95th or 99th percentiles of the Asian population compared to the median of the U.S. population is 14- to 26-fold, a variability factor that is larger than the default pharmacokinetic uncertainty factor (3.2-fold) commonly used in risk assessment. Approaches are described for using ALDH2 population distributions in physiologically based pharmacokinetic-Monte Carlo refinements of risk assessments for xenobiotics which are metabolized to aldehyde intermediates (e.g., ethanol, toluene, ethylene glycol monomethyl ether).

Cell cycle specificity of cytogenetic damage induced by 3,4-epoxy-1- butene

3,4-epoxy-1-butene (EB), a primary metabolite of butadiene, is a direct-acting "S-dependent" genotoxicant that can induce sister chromatid exchanges (SCEs) and chromosome aberrations (CAs) in cycling cells in vitro. However, EB is almost inactive when splenic or peripheral blood lymphocytes are exposed at the G(0) stage of the cell cycle. To investigate whether repair of DNA lesions is responsible for the lack of cytogenetic responses seen after G(0) treatments, we used cytosine arabinoside (ara-C) to inhibit DNA polymerization during DNA repair. If enough repairable lesions are present, double-strand breaks should accumulate and form chromosome-type ("S-independent") deletions and exchanges. This is exactly what occurred. EB induced chromosome deletions and dicentrics at the first division following treatment, when the EB exposure was followed by ara-C. Without ara-C treatment, there was no induction of CAs. These experiments indicate that the relatively low levels of damage induced by EB in G(0) lymphocytes are removed by DNA repair prior to DNA synthesis and thus, before the production of SCEs or chromatid-type aberrations.

Cytogenetic effects of ethylene oxide, with an emphasis on population monitoring

Cytogenetic assays are an integral component of the battery of short-term assays that are used for the hazard identification component of a cancer risk assessment. The protocol for the conduct of such assays for maximal sensitivity for detecting clastogenicity has to be attendant to the mechanism of induction of the endpoint being assessed and the fact that several aberration types are cell lethal necessitates that analysis be for cells at their first posttreatment metaphase. Cytogenetic assays for human populating monitoring have been used for predicting potential for carcinogenicity in humans. However, the assays as typically conducted are not appropriate for chronic exposures because nontransmissible alterations are assessed. The use of fluorescent in situ hybridization (FISH) techniques for the assessment of transmissible changes such as reciprocal translocations are required to make population monitoring studies interpretable, and for removing some of the concern over the influence of confounders on outcome. The database for the cytogenetic effects of ethylene oxide in vitro and in vivo, with an emphasis on human population monitoring, has been critically reviewed. Based on the endpoints studied, the size of the study groups, the information on exposure, the nature of any exposure response data, and the possible influence of confounders (i.e., control matching), it is concluded that acute, high exposures to ethylene oxide with sampling shortly (a few days) after exposure can be detected by increases in chromosome aberrations or SCE in peripheral lymphocytes. Such increases are indicators of exposure to a genotoxic chemical and not predictors of subsequent adverse health effects to individuals. The effect of chronic and/or low level (less than about 25 ppm) exposures cannot be reliably evaluated using current methods. The use of FISH, for example, for assessing reciprocal translocation frequencies (as a measure of transmissible events) will greatly improve the ability to detect chronic exposures to clastogenic chemicals.

In vivo repair during G1 of DNA lesions eliciting sister chromatid exchanges induced by methylnitrosourea or ethylnitrosourea in BrdU substituted or unsubstituted DNA in murine salivary gland cells

The difference in efficiency of methylnitrosourea (MNU) and ethylnitrosourea (ENU) to induce SCE in early or late G1 was determined in synchronized murine salivary gland cells in vivo, as a measure of the capacity of this tissue to repair the lesions involved in SCE formation during G1. The repair during G1 was determined by treating the cells in early or late G1. Treatment was in the first cycle (G1 before incorporation of 5-bromodeoxyuridine (BrdU)) or in G1 of the second cycle (after a single round of BrdU incorporation). It was observed that 50% of the lesions induced by MNU that elicit SCE are repaired during G1. BrdU incorporation into DNA increases the sensitivity of the cell to SCE induction by MNU nearly 40%; however under this circumstance a slightly lower SCE frequency was observed in the cells exposed to MNU at early G1, indicating that during G1 only few lesions are repaired. The ENU-induced DNA-lesions involved in SCE production are nearly 100% persistent along G1; besides, a slight but significantly higher SCE frequency was observed in cells exposed at early G1, suggesting the formation of SCE-inducing lesions during G1. BrdU incorporation to DNA sensitizes the cell to SCE induction by ENU, increasing the SCE frequency to nearly to a 40%, although these additional lesions involved in SCE induction seem to be susceptible to repair during G1.

A comprehensive approach for integration of toxicity and cancer risk assessments

Experimental observations and theoretical considerations indicate a dose threshold for most chemically induced noncancer toxic effects below which the increased risk of toxicity is zero. Thus, the historical approach for minimizing risk from toxic chemicals has been to experimentally determine a no-observed-adverse-effect-level (NOAEL) and then to apply safety or uncertainty factors to estimate a dose not expected to produce that toxic effect in humans. In contrast, for radiation and chemically induced cancer, it has been assumed that all agents operate by a genotoxic mode of action and that some risk can be assigned to even vanishingly small doses. Accordingly, risk assessments for carcinogens have commonly been based on the assumption that the tumor dose-response curve at low doses is linear and passes through the origin. Mode of action is defined as a fundamental obligatory step in the induction of toxicity or cancer. It is now clear that tumor induction can arise in a variety of ways including not only a DNA-reactive genotoxic mode of action, but also non-DNA-reactive nongenotoxic-cytotoxic and nongenotoxic-mitogenic modes of action. Initial risk assessment approaches that recognized this distinction identified a chemical carcinogen as either genotoxic or nongenotoxic, with no middle ground. The realization that there is a continuum whereby different chemicals can act by a combination of modes of action and the recent explosion of research into molecular mechanisms of carcinogenesis indicate that all relevant information should be integrated into the risk assessment process on a case by case basis. A comprehensive approach to risk assessment demands that default assumptions be replaced with an integrated understanding of the rate-limiting steps in the induction of toxicity or cancer along with quantitative measures of the shapes of those dose-response curves. The examples of more contemporary risk assessments are presented for chloroform and vinyl acetate.

Specific tandem GG to TT base substitutions induced by acetaldehyde are due to intra-strand crosslinks between adjacent guanine bases

Acetaldehyde is present in tobacco smoke and automotive exhaust gases, is produced by the oxidation of ethanol, and causes respiratory organ cancers in animals. We show both the types and spectra of acetaldehyde-induced mutations in supF genes in double- and single-stranded shuttle vector plasmids replicated in human cells. Of the 101 mutants obtained from the double-stranded plasmids, 63% had tandem base substitutions, of which the predominant type is GG to TT transversions. Of the 44 mutants obtained from the single-stranded plasmids, 39% had tandem mutations that are of a different type than the double-stranded ones. The GG to TT tandem substitutions could arise from intra-strand crosslinks. Our data indicate that acetaldehyde forms intra- as well as inter-strand crosslinks between adjacent two-guanine bases. Based upon the following observations: XP-A protein binds to acetaldehyde-treated DNA, DNA excision repair-deficient xeroderma pigmentosum (XP) cells were more sensitive to acetaldehyde than the repair-proficient normal cells, and a higher frequency of acetaldehyde-induced mutations of the shuttle vectors was found in XP cells than in normal cells, we propose that the DNA damage caused by acetaldehyde is removed by the nucleotide excision repair pathway. Since treatment with acetaldehyde yields very specific GG to TT tandem base substitutions in DNA, such changes can be used as a probe to identify acetaldehyde as the causal agent in human tumors.

Acetaldehyde: genotoxicity and cytotoxicity in human lymphocytes

We evaluated the DNA damaging effects of ethanol and its major metabolite, acetaldehyde. Freshly isolated human lymphocytes from two healthy donors were incubated with 0, 1.56, 6.25, 25, and 100 mM of ethanol or acetaldehyde in complete medium for 1 h. We used a newly developed, sensitive, alkaline microgel electrophoresis technique to quantitate single- and double-strand DNA breaks or alkali-labile sites in individual cells. Ethanol did not induce DNA strand breaks. However, acetaldehyde induced both single-strand (even at the lowest concentration of 1.56 mM) and double-strand breaks (only at the highest concentration of 100 mM). Following exposure to acetaldehyde, cells were incubated in complete medium for 30, 60, and 120 min. During this incubation period, most cells were unable to repair DNA single- and double-strand breaks caused by acetaldehyde. We also observed a significant cell loss after exposure to acetaldehyde. To our knowledge, this is the first study demonstrating DNA single- and double-strand breaks by acetaldehyde.

Persistence during G1 of gamma ray- or mitomycin C-induced lesions eliciting SCE in murine salivary gland cells in vivo

The efficiency of mitomycin C or gamma rays to induce SCE in early or late G1 was determined in synchronized murine salivary gland cells in vivo, as a measure of the capacity of this tissue to repair the lesions involved in SCE formation before S. The SCE frequencies induced by MMC in the first division (before BrdU incorporation) were significantly lower in the early G1 compared to the late G1, indicating some repair of SCE-inducing lesions. In the second division (after BrdU incorporation), there was no difference between SCE induced in early and late G1, indicating that MMC-induced lesions in such conditions are very persistent and not repairable during G1. The radio induced SCE frequency at early G1 was significantly lower than that observed in late G1, in cells irradiated after BrdU incorporation, suggesting that half of gamma ray-induced DNA lesions that elicit SCEs were repaired.

Formation and stability of acetaldehyde-induced crosslinks between poly-lysine and poly-deoxyguanosine

The amino acid residue and nucleoside specificity of acetaldehyde-induced DNA-protein crosslinks (DPXLs) were studied using a modified filter binding assay. A 40% inhibition of acetaldehyde-induced pUC13 plasmid DNA-calf thymus histone crosslink formation was achieved by addition of 50 mM lysine (free base), while arginine was unable to affect crosslink formation at concentrations to 150 mM. Polymers (5-mers) of lysine (poly-lys5) were able to substitute for histones in acetaldehyde-induced plasmid crosslink formation, being equally effective at equimolar concentrations. Homopolymers (6-mers) of deoxyguanosine (poly-dG6) (but not deoxyadenosine, deoxycytidine or thymidine) served as an efficient substrate for acetaldehyde-induced DPXL formation, using either calf thymus histones or poly-lys5 as the protein source. Acetaldehyde-induced crosslinks between poly-dG6 and poly-lys5 were formed rapidly, but were unstable at 37 degrees C (a half-life or 1.5-2 h). Stability of these crosslinks was unaffected by pH at a range of 5.5-9.0 at 37 degrees C for 2 h. Results presented here suggest that unstable complexes of deoxyguanosine and lysine constitute a major portion of the DPXLs formed by acetaldehyde in vitro.

Sister-chromatid exchange: second report of the Gene-Tox Program

This paper reviews the ability of a number of chemicals to induce sister-chromatid exchanges (SCEs). The SCE data for animal cells in vivo and in vitro, and human cells in vitro are presented in 6 tables according to their relative effectiveness. A seventh table summarizes what is known about the effects of specific chemicals on SCEs for humans exposed in vivo. The data support the concept that SCEs provide a useful indication of exposure, although the mechanism and biological significance of SCE formation still remain to be elucidated.

Sister-chromatid exchanges induced by vinyl esters and respective carboxylic acids in cultured human lymphocytes

Vinyl acetate--an efficient inducer of sister-chromatid exchanges (SCEs)--is known to be hydrolyzed in mammalian cells into acetic acid and acetaldehyde, the latter being the likely metabolite responsible for the SCE induction. As similar hydrolysis to acetaldehyde and to a carboxylic acid is also expected for other vinyl esters, five such compounds--vinyl formate, vinyl chloroformate, vinyl propionate, vinyl crotonate and vinyl-2-ethylhexanoate--and five carboxylic acids--formic acid, acetic acid, propionic acid, crotonic acid and 2-ethylhexanoic acid--were tested for their ability to induce SCEs in cultured (72 h) human lymphocytes with a 48-h treatment, starting at 24 h after culture initiation. Vinyl formate, vinyl propionate and vinyl crotonate induced a clear dose-dependent increase in the number of SCEs/cell at concentrations of 0.125-0.5 mM and vinyl chloroformate at 0.063-1 mM, i.e., at roughly the same concentration range as vinyl acetate and acetaldehyde. Vinyl-2-ethylhexanoate required slightly higher concentrations (0.25-4 mM) for SCE induction. All of the carboxylic acids tested also elevated SCEs, but only slightly. Formic acid and crotonic acid produced some SCE increase at a concentration of 10 mM, acetic acid at 5 and 10 mM and propionic acid at 2.5 mM. 2-Ethylhexanoic acid induced SCEs at a lower concentration range (0.63-2.5 mM) than the other acids. The positive concentrations of the first three carboxylic acids lowered the pH of the culture medium immediately after the treatment by 0.5-1.0 pH unit (lowest observed pH 6.53). The pH differences from the control cultures became smaller in measurements done 24 h and 48 h after the beginning of treatment. Propionic acid and 2-ethylhexanoic acid affected medium pH only slightly (maximum drop 0.2 pH units) at the concentrations that induced SCEs. The results lend support to the idea that the efficient SCE induction observed with the vinyl esters results from the formation of acetaldehyde, with carboxylic acids--with the possible exception of 2-ethylhexanoic acid--playing no significant role. The slight SCE induction obtained with the carboxylic acids cannot be explained by lowered pH alone.

Increased frequency of acetaldehyde-induced sister-chromatid exchanges in human lymphocytes treated with an aldehyde dehydrogenase inhibitor

Acetaldehyde, the first metabolite of ethanol oxidation, in concentrations ranging from 100 microM to 400 microM caused a dose-dependent linear increase in the frequency of sister-chromatid exchanges (SCE) in cultured human peripheral lymphocytes. The SCE frequency was on an average 2-fold higher when the cells were exposed to the acetaldehyde after 24 h incubation instead of at the time of mitogen stimulation (0 h). When acetaldehyde was added together with the potent aldehyde dehydrogenase inhibitor 1-aminocyclopropanol (0.1 mM), the SCE response was significantly (p less than 0.05) increased. The present results indicate that acetaldehyde is metabolized within human lymphocytes, and, moreover, that alcohol consumption during treatment with drugs that inactivate aldehyde dehydrogenase may cause a further increased incidence of acetaldehyde-induced SCE and concomitant lesions.

Aldehydes: occurrence, carcinogenic potential, mechanism of action and risk assessment

Aldehydes constitute a group of relatively reactive organic compounds. They occur as natural (flavoring) constituents in a wide variety of foods and food components, often in relatively small, but occasionally in very large concentrations, and are also widely used as food additives. Evidence of carcinogenic potential in experimental animals is convincing for formaldehyde and acetaldehyde, limited for crotonaldehyde, furfural and glycidaldehyde, doubtful for malondialdehyde, very weak for acrolein and absent for vanillin. Formaldehyde carcinogenesis is a high-dose phenomenon in which the cytotoxicity plays a crucial role. Cytotoxicity may also be of major importance in acetaldehyde carcinogenesis but further studies are needed to prove or disprove this assumption. For a large number of aldehydes (relevant) data on neither carcinogenicity nor genotoxicity are available. From epidemiological studies there is no convincing evidence of aldehyde exposure being related to cancer in humans. Overall assessment of the cancer risk of aldehydes in the diet leads to the conclusion that formaldehyde, acrolein, citral and vanillin are no dietary risk factors, and that the opposite may be true for acetaldehyde, crotonaldehyde and furfural. Malondialdehyde, glycidaldehyde, benzaldehyde, cinnamaldehyde and anisaldehyde cannot be evaluated on the basis of the available data. A series of aldehydes should be subjected to at least mutagenicity, cytogenicity and cytotoxicity tests. Priority setting for testing should be based on expected mechanism of action and degree of human exposure.

Fate of DNA lesions that elicit sister-chromatid exchanges

Using 3-way differential staining (TWD) of sister chromatids, the fate of DNA lesions involved in sister-chromatid exchange (SCE) formation was determined in murine bone marrow cells in vivo, after treatment with either mitomycin C (MMC) or cyclophosphamide (CP). Both MMC (2.6 mg/kg b.w.) and CP (7 mg/kg b.w.) induced an SCE frequency near the expected in the 2 subsequent cell divisions, but the frequency of SCE occurring at the same locus in successive cell divisions was substantially lower than expected. The results are compared with previous data obtained after exposure to gamma-rays. A model of SCE induction is proposed.

Effects of vinyl acetate and acetaldehyde on sperm morphology and meiotic micronuclei in mice

The testicular genotoxic effects of vinylacetate (VA) and its hydrolysis product, acetaldehyde (AA), were studied in mice by analyzing the induction of morphologically abnormal sperm and meiotic micronuclei. VA significantly increased the frequency of sperm abnormalities at 500 mg/kg/day while lower doses were ineffective. AA did not induce abnormal sperm. Neither of the compounds influenced the frequency of meiotic micronuclei. VA, but not AA, caused a dose-dependent decrease in sperm production and a reduction of testicular weight at 500 and 125 mg/kg/day.

A mutagenicity assessment of acetaldehyde

Acetaldehyde has been shown in studies by several different laboratories to be a clastogen (chromosome-breaking) and inducer of sister-chromatid exchanges in cultured mammalian cells (Chinese hamster cells and human lymphocytes). Although there have been very few studies in intact mammals, the available evidence suggests that acetaldehyde produces similar cytogenetic effects in vivo. The production of cytogenetic abnormalities may be related to the ability of acetaldehyde to form DNA-DNA and/or DNA-protein cross-links. Acetaldehyde apparently has not been evaluated for its ability to cause gene mutations in cultured mammalian cells, but it has been shown to produce sex-linked recessive lethals in Drosophila. In general, bacteria tests have been negative. Although acetaldehyde is a genotoxic cross-linking agent, it does not appear to cause DNA strand breaks. There were no studies available regarding the potential of acetaldehyde to produce genetic damage in mammalian germ cells in vivo. Most mutagenicity testing on acetaldehyde has been motivated by attempts to define the proximate mutagen in ethanol metabolism.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 15/1. Genetic effects of ethanol

Alcoholics have a higher frequency of chromosomal aberrations and sister-chromatid exchanges (SCEs) in their peripheral lymphocytes. In human and mammalian cells in vitro, ethanol generally does not induce genetic damage, but it induces SCEs in the presence of an exogenous metabolic system. In human lymphocytes in vitro, ethanol induces SCEs in the presence of alcohol dehydrogenase. In animals in vivo, ethanol induces a variety of genetic effects, including SCEs, micronuclei, dominant lethal mutations and aneuploidy in mouse eggs. There is some indication that ethanol may lead to genetic damage in sperm. In bacteria, ethanol is at best marginally active. Ethanol leads to anomalous chromosome segregation in Aspergillus, to mutations in yeast, to chromosomal aberrations and SCEs in plant root tips and to disturbances of meiosis and micronuclei in tetrads in Zea and Tradescantia respectively. The first metabolite of ethanol, acetaldehyde is mutagenic in a variety of test systems. The mutagenic activity of acetaldehyde in bacteria is questionable, but there is no doubt of its mutagenic activity in a variety of eukaryotic test systems in vitro as well as in vivo.

Induction of micronuclei by vinyl acetate in mouse bone marrow cells and cultured human lymphocytes

A dose-dependent increase in micronucleated polychromatic erythrocytes was observed in the bone marrow of male C57B1/6 mice 30 h after a single intraperitoneal injection of vinyl acetate (250, 500, 1000 or 2000 mg/kg b.wt.; (9-14 animals per group). The effect was statistically significant at 1000 mg/kg (1.33 +/- 0.29% vs. 0.6 +/- 0.10% in olive oil-treated controls) and at 2000 mg/kg (1.57 +/- 0.19%) of vinyl acetate. These doses were fatal to 6 (1000 mg/kg) and 8 (2000 mg/kg) out of 14 animals in both groups. The ratio of polychromatic to normochromatic cells decreased as a function of vinyl acetate dose. Cyclophosphamide (20 mg/kg), used as a positive control chemical, induced a clear increase in micronucleated polychromatic erythrocytes (2.07 +/- 0.20%). None of the treatments affected the number of micronuclei in normochromatic erythrocytes. In human whole-blood lymphocyte cultures, micronucleus induction by a 48-h treatment with vinyl acetate (0.125, 0.25, 0.5, 1 and 2 mM; 24 h after culture initiation) was studied in lymphocytes with preserved cytoplasm from smear slides prepared by a method involving the removal of erythrocytes at harvest by sodium cyanide treatment to improve preparation quality. The frequency of micronucleated lymphocytes reached a peak at 0.5 mM (3.2 +/- 1.0% vs. 0.9 +/- 0.1% in control cultures) and 1 mM (3.1 +/- 0.7%), with a decline at 2 mM probably because of a toxic effect resulting in mitotic inhibition.

Effects of UV-irradiation on the SCE frequency in human lymphocyte cultures

UV-irradiation (254 nm) was found to induce a smaller increase of SCE in human lymphocytes than in human fibroblasts and CHO cells. The UV-induced SCE frequency in human lymphocytes was not influenced by the duration between irradiation and the subsequent S-phase. UV-irradiated lymphocytes showed a slightly more than additive response to the SCE-inducing effect of HN2 and acetaldehyde in comparison with non-irradiated cells. The UV-induced SCE frequency was similar in lymphocyte cultures containing 20 and 100 microM of BrdUrd. The results suggest that human lymphocytes are relatively insensitive to the SCE-inducing effect of UV-irradiation, and that SCE-inducing damage caused by UV is not removed during the G1 phase in these cells.

DNA cross-links in human leucocytes treated with vinyl acetate and acetaldehyde in vitro

Human leucocytes were incubated in the presence of vinyl acetate or acetaldehyde (10-20 mM) for 4 h at 37 degrees C in vitro. DNA damage was analysed by alkaline elution. None of the compounds induced a detectable increase in the frequency of DNA strand breaks. Cells exposed to 5 Gy of X-ray immediately after treatment and before alkaline elution showed a clear, dose-dependent retardation of the elution rate in comparison with X-irradiated control cells. These results demonstrate that both vinyl acetate and acetaldehyde induce DNA cross-links in human cells.