Mutagenicity of formaldehyde in Chinese hamster lung fibroblasts: synergy with ionizing radiation and N-nitroso-N-methylurea

The multiplex interactions and molecular mechanism on genotoxicity induced by formaldehyde and acrolein mixtures on human bronchial epithelial BEAS-2B cells

Aldehydes are common air pollutants with carcinogenicity. Genotoxicity of single aldehyde has been studied well, but the combined genotoxicity is rarely known. Here, we evaluated the combined genotoxicity of formaldehyde and acrolein on BEAS-2B cells in terms of DNA strands breakage, chromosome damage and gene mutation below subcytotoxic concentrations covering smoking-related concentrations. Meanwhile, the molecular mechanism was investigated further based on oxidative stress, DNA-protein crosslinks (DPCs), cell cycle and DNA damage-repair pathway. Co-exposure to formaldehyde and acrolein mixtures showed significantly synergistic interaction on DNA strands breakage and chromosome damage in a concentration/time-dependent manner, while antagonism was shown on the late genotoxic endpoints (e.g. cytoplasmic block micronucleus (CBMN) and HPRT gene mutation). Moreover, formaldehyde synergistically potentiated acrolein-induced S-phase arrest, inhibition of DNA repair and up-regulation of genes related to cell stress, which conversely strengtherned mixture-induced DNA/chromosome damage and finally resulted in antagonism on late genotoxic events. Additionally, formaldehyde-induced DNA damage mainly resulted from the direct covalent bonding (e.g. DPCs), while acrolein-induced DNA damage mainly generated from oxidative damage (e.g. oxidative stress), which dominated the synergistic DNA strand breakage induced by mixtures. Summarily, aldehyde mixtures (formaldehyde and acrolein) induced multiplex combined genotoxicity on BEAS-2B cells even at smoking-related concentrations, which was dependent on genotoxic endpoints and closely related to that formaldehyde potentiated acrolein-induced cell stress, S-phase arrest and inhibition of DNA repair. So prolonged exposure to aldehyde mixtures may have a more serious risk to respiratory system in animal and human than the expectation based on the toxicity of single aldehyde even at environmentally relevant concentrations.

The Antibiotic Trimethoprim Displays Strong Mutagenic Synergy with 2-Aminopurine

We show that trimethoprim (TMP), an antibiotic in current use, displays a strong synergistic effect on mutagenesis in Escherichia coli when paired with the base analog 2-aminopurine (2AP), resulting in a 35-fold increase in mutation frequencies in the rpoB-Rif^r system. Combination therapies are often employed both as antibiotic treatments and in cancer chemotherapy. However, mutagenic effects of these combinations are rarely examined. An analysis of the mutational spectra of TMP, 2AP, and their combination indicates that together they trigger a response via an alteration in deoxynucleoside triphosphate (dNTP) ratios that neither compound alone can trigger. A similar, although less strong, response is seen with the frameshift mutagen ICR191 and 2AP. These results underscore the need for testing the effects on mutagenesis of combinations of antibiotics and chemotherapeutics.

Mutagenesis: Interactions with a parallel universe

Unexpected observations in mutagenesis research have led to a new perspective in this personal reflection based on years of studying mutagenesis. Many mutagens have been thought to operate via a single principal mechanism, with secondary effects usually resulting in only minor changes in the observed mutation frequencies and spectra. For example, we conceive of base analogs as resulting in direct mispairing as their main mechanism of mutagenesis. Recent studies now show that in fact even these simple mutagens can cause very large and unanticipated effects both in mutation frequencies and in the mutational spectra when used in certain pair-wise combinations. Here we characterize this leap in mutation frequencies as a transport to an alternate universe of mutagenesis.

Investigations on potential co-mutagenic effects of formaldehyde

The genotoxicity and mutagenicity of formaldehyde (FA) has been well-characterized during the last years. Besides its known direct DNA-damaging and mutagenic activity in sufficiently exposed cells, FA at low concentrations might also enhance the mutagenic and carcinogenic effects of other environmental mutagens by interfering with the repair of DNA lesions induced by these mutagens. To further assess potential co-mutagenic effects of FA, we exposed A549 human lung cells to FA in combination with various mutagens and measured the induction and removal of DNA damage by the comet assay and the production of chromosomal mutations by the cytokinesis-block micronucleus assay (CBMN assay). The mutagens tested were ionizing radiation (IR), (±)-anti-B[a]P-7,8-dihydrodiol-9,10-epoxide (BPDE), N-nitroso-N-methylurea (methyl nitrosourea; MNU) and methyl methanesulfonate (MMS). FA (10-75μM) did not enhance the genotoxic and mutagenic activity of these mutagens under the test conditions applied. FA alone and in combination with MNU or MMS did not affect the expression (mRNA level) of the gene of the O(6)-methylguanine-DNA methyltransferase (MGMT) in A549 cells. The results of these experiments do not support the assumption that low FA concentrations might interfere with the repair of DNA damage induced by other mutagens.

Evaluation of mutagenic effects of formocresol: detection of DNA-protein cross-links and micronucleus in mouse bone marrow

OBJECTIVE

The genotoxic potential of formocresol was assessed by comet assay on human peripheral blood lymphocytes and in vivo micronucleus in mice.

STUDY DESIGN

Peripheral blood lymphocytes, obtained from healthy donors, were exposed directly with different dilutions of formocresol for 45 minutes at 37 degrees C. To verify the possibility of formocresol to induce DNA-protein cross-links, treated lymphocytes were incubated with proteinase K. Micronucleus test was performed on male Swiss mice treated with several dilutions of formocresol by single intraperitoneal injection. After treatment, bone marrow was sampled 24 and 48 hours after formocresol administration.

RESULTS

Formocresol did not produce detectable DNA damage as evaluated by comet assay. However, after proteinase K exposure, a dose-dependent increase of DNA migration was observed. Formocresol induced a significant increase in micronucleus frequencies at the highest dilution only at 24 hours after administration.

CONCLUSION

Formocresol induced DNA-protein cross-links and an increased frequency of micronucleus.

DNA-protein crosslinks and p53 protein expression in relation to occupational exposure to formaldehyde

BACKGROUND

Formaldehyde (FA) is classified as a probable human carcinogen.

AIMS

To examine DNA protein crosslinks (DPC) and p53, which are generally known to be involved in carcinogenesis, in peripheral blood lymphocytes of workers exposed to FA.

METHODS

DPC and p53 ("wild type" and mutant) were examined in peripheral blood lymphocytes of 186 workers exposed to FA (mean years of exposure = 16) and 213 unexposed workers. Every worker completed a questionnaire on demographic data, occupational and medical history, smoking, and hygiene.

RESULTS

The adjusted mean level of DPC in the exposed and the unexposed workers differed significantly. Adjustment was made for age, sex, years of education, smoking, and origin. Exposure to FA increased the risk of having a higher level of pantropic p53 above 150 pg/ml (OR 1.6, 95% CI 0.8 to 3.1). A significant positive correlation was found between the increase of pantropic p53 protein and mutant p53 protein, as well as between pantropic p53 >150 pg/ml and mutant p53 protein. In the exposed group a significantly higher proportion of p53 >150 pg/ml was found among workers with DPC >0.187 (55.7%) (0.187 = median level of DPC) than among workers with DPC < or =0.187 (33.3%). The risk of having pantropic p53 protein >150 pg/ml was determined mainly by levels of DPC. Workers with DPC above the median level had a significantly higher risk of having pantropic p53 >150 pg/ml (adjusted OR 2.5, 95% CI 1.2 to 5.4).

CONCLUSIONS

Results suggest that DPC and mutation in p53 may represent steps in FA carcinogenesis and a possible causal relation between DPC and mutation in p53. These biomarkers can be applied in the assessment of the development of cancer due to FA exposure.

A study on the effect of inhalative formaldehyde exposure on water labyrinth test performance in rats

We investigated the effects of repeated inhalative exposure to several formaldehyde concentrations (2 hours/day, 10 consecutive days) on the behavior of adult male and female LEW.1K rats during a period in which they learned to perform a water labyrinth task. We also examined the effects on the histology of some organ tissues. While the controls needed increasingly shorter swimming periods to complete the water labyrinth test and made fewer errors with advancing trial duration, such progress could not be observed in the learning behavior of the exposed animals. They took significantly longer swimming periods to reach the finish and made significantly more errors in comparison to the controls. The statistical comparison between the collected data in the formaldehyde exposed male and female rats reveals that females in general reached the end point of the swimming labyrinth in significantly less time. On some trial days, however, they made more mistakes than males. In the highest concentration group, no gender differences were evident in the frequency of errors. The histological examination revealed no pathological changes attributable to formaldehyde exposure. Since the water labyrinth test is used to investigate changes in memory and learning behavior in animals, we conclude that under investigational conditions, formaldehyde affects the learning behavior and the memory of male and female rats.

Functional polymorphism in the alcohol dehydrogenase 3 (ADH3) promoter

The ADH3 gene encodes alcohol dehydrogenase 3 (ADH3)/glutathione-dependent formaldehyde dehydrogenase, the ancestral and most conserved form of alcohol dehydrogenase. ADH3 is expressed in all tissues examined and the enzyme is essential for formaldehyde scavenging. We have screened the promoter region including exon 1 and exons 5, 6 and 7 of the ADH3 gene for allelic variants. Using 80 samples of genomic DNA from Swedes as template, the various parts of the gene were PCR amplified and subsequently analyzed on single strand conformation polymorphism (SSCP) gels. No abnormal migration patterns could be detected by SSCP analysis of exons 5, 6 and 7 while for the promoter region, a large number of the samples displayed differences in SSCP gel migration patterns. Cloning and sequence analysis revealed four possible base pair exchanges in the promoter region. Two transitions were found at position -197 and -196, GG --> AA, one at position -79, G --> A and finally, close to the transcription start site, a fourth transition was found at position +9, C --> T. An allele specific PCR method was developed and allele frequencies were determined in three populations: Chinese, Spanish and Swedish. GG-197,-196 and AA-197,-196 alleles were common in all three populations, G-79 and A-79 were common in Swedes and Spaniards but only A-79 was found among Chinese. T+9 was the most rare allele with an allele frequency of 1.5% in Swedes. Finally, promoter activity assessments and electrophoretic mobility shift assays demonstrated that the C+9 --> T+9 exchange resulted in a significant transcriptional decrease in HeLa cells and a decreased binding of nuclear proteins. These base pair exchanges may have an effect on the expression of the enzyme and thereby influence the capacity of certain individuals to metabolize formaldehyde.

Identification of fluorescent 2'-deoxyadenosine adducts formed in reactions of conjugates of malonaldehyde and acetaldehyde, and of malonaldehyde and formaldehyde

2'-Deoxyadenosine was reacted with malonaldehyde in the presence of formaldehyde or acetaldehyde. The reactions were carried out at 37 degrees C in aqueous solution at acidic conditions. The reaction mixtures were analyzed by HPLC. In both reactions, two major products were formed. The reaction products were isolated and purified by C18 chromatography, and their structures were characterized by UV absorbance, fluorescence emission, (1)H and (13)C NMR spectroscopy, and mass spectrometry. The reaction products isolated from the mixture containing formaldehyde, malonaldehyde, and deoxyadenosine were identified as 3-(2'-deoxy-beta-D-ribofuranosyl)-7H-8-formyl[2,1-i]pyrimidopurine (M(1)FA-dA) and 9-(2'-deoxy-beta-D-ribofuranosyl)-6-(3,5-diformyl-1, 4-dihydro-1-pyridyl)purine (M(2)FA-dA). In the reaction mixture consisting of acetaldehyde, malonaldehyde, and deoxyadenosine, the identities of the products were determined to be 3-(2'-deoxy-beta-D-ribofuranosyl)-7-methyl-8-formyl[2, 1-i]pyrimidopurine (M(1)AA-dA) and 9-(2'-deoxy-beta-D-ribofuranosyl)-6-(3,5-diformyl-4-methyl-1, 4-dihydro-1-pyridyl)purine (M(2)AA-dA). The yields of the compounds were 1.8 and 0.7% for M(1)FA-dA and M(2)FA-dA, respectively, and 6.8 and 10% for M(1)AA-dA and M(2)AA-dA, respectively. All compounds exhibited marked fluorescent properties. These findings show that in addition to direct reaction of a specific aldehyde with 2'-deoxyadenosine, aldehyde conjugates also may react with the base. Although three of the adducts (M(1)FA-dA, M(2)FA-dA, and M(1)AA-dA) could not be detected in reactions carried out under neutral conditions, the possibility that trace amounts of the adducts may be formed under physiological conditions cannot be ruled out. Therefore, conjugate adducts must be considered in work that aims at clarifying the mechanism of aldehyde genotoxicity.

Expression of alcohol dehydrogenase 3 in tissue and cultured cells from human oral mucosa

Because formaldehyde exposure has been shown to induce pathological changes in human oral mucosa, eg, micronuclei, the potential enzymatic defense by alcohol dehydrogenase 3 (ADH3)/glutathione-dependent formaldehyde dehydrogenase was characterized in oral tissue specimens and cell lines using RNA hybridization and immunological methods as well as enzyme activity measurements. ADH3 mRNA was expressed in basal and parabasal cell layers of oral epithelium, whereas the protein was detected throughout the cell layers. ADH3 mRNA and protein were further detected in homogenates of oral tissue and various oral cell cultures, including, normal, SV40T antigen-immortalized, and tumor keratinocyte lines. Inhibition of the growth of normal keratinocytes by maintenance at confluency significantly decreased the amount of ADH3 mRNA, a transcript with a determined half-life of 7 hours. In contrast, decay of ADH3 protein was not observed throughout a 4-day period in normal keratinocytes. In samples from both tissue and cells, the ADH3 protein content correlated to oxidizing activity for the ADH3-specific substrate S:-hydroxymethylglutathione. The composite analyses associates ADH3 mRNA primarily to proliferative keratinocytes where it exhibits a comparatively short half-life. In contrast, the ADH3 protein is extremely stable, and consequently is retained during the keratinocyte life span in oral mucosa. Finally, substantial capacity for formaldehyde detoxification is shown from quantitative assessments of alcohol- and aldehyde-oxidizing activities including K:(m) determinations, indicating that ADH3 is the major enzyme involved in formaldehyde oxidation in oral mucosa.

Significance of formaldehyde-induced DNA-protein crosslinks for mutagenesis

Formaldehyde (FA) is a genotoxic substance, induces tumors in the nasal epithelium of rats, and is suspected to be a human carcinogen. As a primary DNA lesion, FA induces DNA-protein crosslinks (DPC) and the formation of DPC has been used as a measure of exposure for risk estimation. However, the significance of DPC for mutagenesis and carcinogenesis is at present poorly understood. We therefore performed comparative investigations on the induction of DPC and other genetic endpoints by FA in V79 Chinese hamster cells. The amount of DPC was comparatively determined with the K-SDS assay and the comet assay. Both tests gave similar results but the comet assay was much foster and easier to perform. Our results show that FA significantly induces DPC, sister-chromatid exchanges, and micronuclei in the same range of concentrations, parallel to the induction of cytotoxicity (relative cloning efficiency). In contrast, treatment of V79 cells with FA did not induce gene mutations in the HPRT test even after variations of the treatment protocol. Our results indicate that FA-induced DPC seem to be related to cytotoxicity and clastogenicity but do not lead to the formation of gene mutations in mammalian cells. It is suggested that FA-induced DPC do not cause gene mutations that are involved in FA-induced carcinogenesis.