Genotoxic activity of chlorinated butenoic acids in Salmonella typhimurium strains TA98, TA100 and TA104

Genotoxicity assay and potential byproduct identification during different UV-based water treatment processes

Formation of genotoxic byproducts during different ultraviolet (UV) -related water/wastewater treatment processes (including medium pressure (MP) UV oxidation, LP UV oxidation, chlorination, biological activated carbon (BAC) treatment, H₂O₂ oxidation, and two or more combined processes) was investigated by Ames fluctuation test using Salmonella strains TA98 and TA100 with and without rat liver enzyme extract S9. Byproducts responsible for genotoxicity were identified. The results showed that MP UV can induce mutagenicity and LP UV treatment does not induce mutagenicity. H₂O₂ oxidation could degrade part of genotoxic compounds. Compared with chlorination, BAC treatment is more effective in removing genotoxicity. Mutagenicity was found mostly in samples tested with TA100 instead of TA98, especially with TA100 without S9, indicating that guanosine and/or cytosine adducts contribute to mutation or toxicological effects in MP UV treated samples. Potential genotoxic byproducts were selected, most of which were nitrogenous organic compounds with more than 10 carbon atoms. Nitrosamines and histidine were excluded from potential genotoxic candidates. The results could contribute to evaluation of mutagenicity of various UV-based water treatment processes.

Mutagen structure and transcriptional response: induction of distinct transcriptional profiles in Salmonella TA100 by the drinking-water mutagen MX and its homologues

The relationship between chemical structure and biological activity has been examined for various compounds and endpoints for decades. To explore this question relative to global gene expression, we performed microarray analysis of Salmonella TA100 after treatment under conditions of mutagenesis by the drinking-water mutagen MX and two of its structural homologues, BA-1, and BA-4. Approximately 50% of the genes expressed differentially following MX treatment were unique to MX; the corresponding percentages for BA-1 and BA-4 were 91 and 80, respectively. Among these mutagens, there was no overlap of altered Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways or RegulonDB regulons. Among the 25 Comprehensive Microbial Resource functions altered by these mutagens, only four were altered by more than one mutagen. Thus, the three structural homologues produced distinctly different transcriptional profiles, with none having a single altered KEGG pathway in common. We tested whether structural similarity between a xenobiotic and endogenous metabolites could explain transcriptional changes. For the 830 intracellular metabolites in Salmonella that we examined, BA-1 had a high degree of structural similarity to 2-isopropylmaleate, which is the substrate for isopropylmalate isomerase. The transcription of the gene for this enzyme was suppressed twofold in BA-1-treated cells. Finally, the distinct transcriptional responses of the three structural homologues were not predicted by a set of phenotypic anchors, including mutagenic potency, cytotoxicity, mutation spectra, and physicochemical properties. Ultimately, explanations for varying transcriptional responses induced by compounds with similar structures await an improved understanding of the interactions between small molecules and the cellular machinery.

Electrophilic Intermediates Produced by Bioactivation of Furan

The industrial and environmental chemical, furan, is a liver toxicant and carcinogen in laboratory animals. It has been classified as a possible human carcinogen. The mechanism of tumor induction is unknown. However, toxicity is initiated by cytochrome P450 catalyzed oxidation of furan to an alpha,beta-unsaturated dialdehyde, cis-2-butene-1,4-dial. This metabolite reacts readily with protein and DNA nucleophiles and is a bacterial mutagen in Ames assay strain TA104. Metabolism studies indicate that this reactive metabolite is formed in vivo. It is also an intermediate leading to other metabolites whose role in furan-derived toxicities has yet to be explored.

Detection of DNA adducts derived from the reactive metabolite of furan, cis-2-butene-1,4-dial

Furan is a toxic and carcinogenic compound used in industry and commonly found in the environment. The mechanism of furan's carcinogenesis is not well-understood and may involve both genotoxic and nongenotoxic pathways. Furan undergoes oxidation by cytochrome P450 to cis-2-butene-1,4-dial, which is thought to mediate furan's toxic effects. Consistently, cis-2-butene-1,4-dial readily reacts with glutathione, amino acids, and nucleosides. To determine the importance of DNA alkylation in furan-induced carcinogenesis, we developed an assay for the detection of cis-2-butene-1,4-dial-derived DNA adducts. DNA samples were treated with O-benzyl-hydroxylamine, which reacts with the aldehyde functionality of the DNA adducts. Enzyme hydrolysates of these samples were then analyzed by capillary electrospray tandem mass spectrometry with selected reaction monitoring. The dCyd and dAdo adducts were detected in digests of DNA treated with nanomolar concentrations of cis-2-butene-1,4-dial. In addition, these adducts were present in DNA isolated from Ames assay strain TA104 treated with mutagenic concentrations of cis-2-butene-1,4-dial. These data support the hypothesis that cis-butene-1,4-dial is a genotoxic metabolite of furan. This method will allow us to explore the role of these adducts in furan-induced carcinogenesis.

Carcinogenicity of the chlorination disinfection by-product MX

3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone, better known by its historical name 'mutagen X' or MX, is a chlorination disinfection byproduct that forms from the reaction of chlorine and humic acids in raw water. MX has been measured in drinking water samples in several countries at levels that ranged from non-detectable to 310 ng/L. Although the concentration of MX in drinking water is typically 100- to 1000-fold lower than other common chlorinated by-products of concern (e.g., trihalomethanes), some have hypothesized that MX might play a role in the increased cancer risks that have been associated with the consumption of chlorinated water. This hypothesis is based on observations that MX, in some test systems, is extremely potent relative to trihalomethanes in inducing DNA damage and altering pathways involved in cell growth, and that in some epidemiological studies increased cancer rates are associated with the bacterial mutagenicity of disinfected water of which MX contributes a significant portion. MX also appears to be more potent than other chlorination by-products in causing cancer in animals. This article reviews the available evidence on the carcinogenicity of MX. MX induced cancer at multiple sites in male and female rats, acted as a tumor initiator and promoter, enhanced tumor yields in genetically modified rodents, induced a myriad of genotoxic effects in numerous in vitro and in vivo test systems, and was a potent inhibitor of gap junction intercellular communication. Although the precise mechanism of MX-induced DNA damage is not known, MX is able to cause DNA damage through an unusual mechanism of ionizing DNA bases due to its extremely high reductive potential. MX may also cause mutations through DNA adduction. This article develops a mean cancer potency estimate for MX of 2.3 (mg/kg-d)(-1) and an upper 95% percentile estimate of 4.5 (mg/kg-d)(-1), and examines the potential health risks posed by this chlorination contaminant in drinking water. A discussion of additional data that would be desirable to better characterize the risks posed by MX and other halogenated hydroxyfuranones follows.

Mutagen X and chlorinated tap water are recombinagenic in yeast

This study determines the effects of a water disinfection by-product, 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (also known as mutagen X or MX) and chlorinated tap water on genomic instability in the yeast Saccharomyces cerevisiae. Tap water samples collected from Cherepovets (Russia) and Boston (MA, USA), were extracted using XAD absorption and ethyl acetate elution. MX and these water extracts were then tested for their ability to induce intrachromosomal recombination (deletions or DEL events), interchromosomal recombination (ICR) and aneuploidy (ANEU) using the yeast DEL assay. MX strongly induced DEL, ICR and ANEU events with a positive dose response and no threshold. Cherepovets tap water induced DEL and ICR events while evidence of ANEU induction was weak. The DEL induction potencies were stronger at higher concentrations. The estimated contribution of MX to DEL induction varied from over 50% at low concentrations (which is comparable to a typical contribution of MX to Ames mutagenicity of tap water) to between 2 and 10% at highest concentrations. For Boston tap water, there was only weak evidence of DEL induction and no evidence of ICR and ANEU induction. This is consistent with the results of other studies, which reported much higher concentrations of MX and stronger Ames mutagenicity in Cherepovets tap water than in Boston tap water.

Potent inhibition of gap junctional intercellular communication by 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) in BALB/c 3T3 cells

The chlorohydroxyfuranones (CHFs) MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone], MCA [3,4-dichloro-5-hydroxy-2(5H)-furanone], CMCF [3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone], and MCF [3-chloro-4-methyl-5-hydroxy-2(5H)-furanone] are genotoxic disinfection by-products of drinking water chlorination. MX, MCA, and MCF also promote foci formation in the two-stage cell transformation assay. The cellular mechanisms underlying this apparent promotional effect are not known. In the present study, the effects of MX, MCA, CMCF, and MCF on gap junctional intercellular communication (GJIC) were measured in BALB/c 3T3 cells using the scrape loading dye technique. The effect of MX on apoptosis in the same cell line was explored by assaying caspase-3-like protease activity. All the four CHFs inhibited GJIC after 30 min exposure in a dose-dependent fashion but there was a marked difference in the ranges of their active concentrations. MX was almost as potent an inhibitor of GJIC (inhibition at nanomolar concentrations) as 12-O-tetradecanoylphorbol-13-acetate (TPA) (positive control), while MCA was 10 times weaker, CMCF 10,000 times weaker, and MCF 20,000 times weaker than MX. After prolonged exposure periods (up to 6 h), GJIC recovered somewhat upon MX and MCA exposures, the inhibition of GJIC by MCF remained constant but CMCF showed an irreversible increasing inhibitory effect. MX caused apoptosis as a "window" effect at concentrations 2000-4000-fold higher than those needed to inhibit GJIC. The results indicate that MX is a potent inhibitor of GJIC in BALB/c 3T3 cells and this inhibition might be one mechanism by which MX can promote malignant foci formation. MCA also has a specific potential to inhibit GJIC whereas MCF and CMCF affected GJIC at concentrations, similar to those evoking genotoxicity in vitro.

Bacterial and mammalian-cell genotoxicity of mixtures of chlorohydroxyfuranones, by-products of water chlorination

The genotoxic responses of mixtures of four chlorohydroxyfuranones (CHFs), 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), 3,4-dichloro-5-hydroxy-2(5H)-furanone (MCA), 3-chloro- 4-(chloromethyl)-5-hydroxy-2(5H)-furanone (CMCF) and 3-chloro-4-methyl-5-hydroxy-2(5H)-furanone (MCF), were compared with the genotoxicity of the individual compounds. Genotoxicity was evaluated in the Salmonella reversion assay (Ames test), the in vitro Chinese hamster ovary (CHO) cell Hprt mutation assay, and in the CHO chromosome aberration test. When tested individually, the concentrations of the chemicals that were chosen for the mixtures induced no or only a modest increase in the genotoxic effects, and caused little or no cytotoxicity. In the Ames test, the genotoxic responses caused by the mixtures of CHFs did not follow simple additivity. Synergism was observed with strains TA97 and TA98, and antagonism with strain TA100. In the CHO/Hprt mutation assay, the mutagenic response of the mixtures was inconsistent, with near additivity seen with a mixture of CHFs that resulted in 12% cell survival. In contrast, the four CHFs together consistently caused more structural chromosome damage (mainly chromatid-type breaks and exchanges) compared to the sum of net effects of the four CHFs tested alone. Also, a potentiating effect was consistently seen for the cytotoxicity of the CHF mixtures both in the CHO/Hprt mutation assay and the chromosome aberration test. The present results indicate that the genotoxic effects of CHF mixtures can be greater than additive. Such effects may be worth considering in the cancer risk assessment of chlorinated drinking water.

Comparable DNA and chromosome damage in Chinese hamster ovary cells by chlorohydroxyfuranones

Chlorinated drinking water contains several chlorohydroxyfuranone (CHF) by-products whose contribution to cancer risk is not presently known. 3,4-Dichloro-5-hydroxy-2(5H)-furanone (MCA), 3-chloro-4-(chloromethyl)-5-hydroxy-2(5H)-furanone (CMCF), and 3- chloro-4-methyl-5-hydroxy-2(5H)-furanone (MCF) were studied for the induction of DNA damage, using the alkaline single-cell gel (SCG)/comet assay, and for chromosome damage, using sister-chromatid exchange (SCE) and chromosome aberration (CA) tests, in Chinese hamster ovary (CHO) cells. 3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), the known genotoxic chlorination by-product and a rat carcinogen, was used as a reference chemical. The SCG analyses were done using concentrations that caused little or no cytotoxicity compared to that of the concurrent control cultures. In the cytogenetic tests, the CHFs were tested up to maximum cytotoxicity. MX and MCA were the most cytotoxic of the compounds in CHO cells followed by CMCF and MCF. All of the CHFs induced DNA damage, SCEs and CAs (mainly chromatid-type breaks and exchanges) in a concentration-related manner, with the exception that MCA was a weak inducer of SCEs. There were no significant differences between the lowest concentration of MX, MCA, and CMCF to cause DNA damage (SCG assay). Based on comparisons of the slopes of regression lines, MX was somewhat more potent than either MCA or CMCF, and MCF was clearly less potent than the other three compounds in the assay. The order of potency was MX > CMCF > MCA > MCF in inducing SCEs and MX > MCA > CMCF > MCF in inducing CAs. The data show that there are differences in the potency of genotoxicity among the CHFs tested. In many cases, however, the extent of maximum effect observed was comparable between the compounds. The results suggest that besides MX other CHFs should be considered in the evaluation of genotoxic risks associated with the consumption of chlorinated drinking water.

Mutation spectra in Salmonella of analogues of MX: implications of chemical structure for mutational mechanisms

We determined the mutation spectra in Salmonella of four chlorinated butenoic acid analogues (BA-1 through BA-4) of the drinking water mutagen 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) and compared the results with those generated previously by us for MX and a related compound, MCF. We then considered relationships between the properties of mutagenic potency and mutational specificity for these six chlorinated butenoic acid analogues. In TA98, the three most potent mutagens, BA-3, BA-4, MX, and the organic extract, all induced large percentages of complex frameshifts (33-67%), which distinguish these agents from any other class of compound studied previously. In TA100, which has only GC sites for mutation recovery, >71% of the mutations induced by all of the agents were GC-->TA transversions. The availability of both GC and TA sites for mutation in TA104 resulted in greater distinctions in mutational specificity than in TA100. MX targeted GC sites almost exclusively (98%); the structurally similar BA-4 and BA-2 produced mutations at similar frequencies at both GC and AT sites; and the structurally similar BA-3 and BA-1 induced most mutations at AT sites (69%). Thus, large variations in structural properties influencing relative mutagenic potency appeared to be distinct from the more localized similar structural features influencing mutagenic specificity in TA104. Among a set of physicochemical properties examined for the six butenoic acids, a significant correlation was found between pK(a) and mutagenic potency in TA100, even when the unionized fraction of the activity dose was considered. In addition, a correlation in CLOGP for BA-1 to BA-4 suggested a role for bioavailability in determining mutagenic potency. These results illustrate the potential value of structural analyses for exploring the relationship between chemical structure and mutational mechanisms. To our knowledge, this is the first study in which such analyses have been applied to structural analogues for which both mutagenic potency and mutation spectra date were available.

Influence of DNA repair on mutation spectra in Salmonella

This paper reviews the influence of DNA repair on spontaneous and mutagen-induced mutation spectra at the base-substitution (hisG46) and -1 frameshift (hisD3052) alleles present in strains of the Salmonella (Ames) mutagenicity assay. At the frameshift allele (mostly a CGCGCGCG target), DeltauvrB influences the frequency of spontaneous hotspot mutations (-CG), duplications, and deletions, and it also shifts the sites of deletions and duplications. Cells with pKM101+DeltauvrB spontaneously produce complex frameshifts (frameshifts with an adjacent base substitution). The spontaneous frequency of 1-base insertions or concerted (templated) mutations is unaffected by DNA repair, and neither mutation is inducible by mutagens. Glu-P-1, 1-nitropyrene (1NP), and 2-acetylaminofluorene (2AAF) induce only hotspot mutations and are unaffected by pKM101, whereas benzo(a)pyrene and 4-aminobiphenyl induce only hotspot in pKM101(-), and hotspot plus complex in pKM101(+). At the base-substitution allele (mostly a CC/GG target), the DeltauvrB allele increases spontaneous transitions in the absence of pKM101 and increases transversions in its presence. The frequency of suppressor mutations is decreased 4x by DeltauvrB, but increased 7. 5x by pKM101. Both repair factors cause a shift in the proportion of mutations to the second position of the CC/GG target. With UV light and gamma-rays, the DeltauvrB allele increases the proportion of transitions relative to transversions. pKM101 is required for mutagenesis by Glu-P-1 and 4-AB, and the types and positions of the substitutions are not altered by the addition of the DeltauvrB allele. Changes in DNA repair appear to cause more changes in spontaneous than in mutagen-induced mutation spectra at both alleles. There is a high correlation (r(2)=0.8) between a mutagen's ability to induce complex frameshifts and its relative base-substitution/frameshift mutagenic potency. A mutagen induces the same primary class of base substitution in TA100 (DeltauvrB, pKM101) as it does in Escherichia coli, mammalian cells, or rodents as well as in the p53 gene of human tumors associated with exposure to that mutagen. Thus, a mutagen induces the same primary class of base substitution in most organisms, reflecting the conserved nature of DNA replication and repair processes.