Role of glutathione S-transferase mu (GSTM1) in styrene-7,8-oxide toxicity and mutagenicity

In vivo mutagenicity assessment of styrene in MutaMouse liver and lung

BACKGROUND

Styrene (CAS 100-42-5) is widely used as polystyrene and acrylonitrile-butadiene-styrene resin such as plastic, rubber, and paint. One of the primary uses of styrene is food utensils and containers, but a small amount of styrene transferred into food can be ingested by eating. Styrene is metabolized into styrene 7,8-oxide (SO). SO is mutagenic in bacteria and mouse lymphoma assays. It is clastogenic in cultured mammalian cells. However, styrene and SO are not clastogenic/aneugenic in rodents, and no rodent in vivo gene mutation studies were identified.

METHODS

To investigate the mutagenicity of orally administered styrene, we used the transgenic rodent gene mutation assay to perform an in vivo mutagenicity test (OECD TG488). The transgenic MutaMouse was given styrene orally at doses of 0 (corn oil; negative control), 75, 150, and 300 mg/kg/day for 28 days, and mutant frequencies (MFs) were determined using the lacZ assay in the liver and lung (five male mice/group).

RESULTS

There were no significant differences in the MFs of the liver and lung up to 300 mg/kg/day (close to maximum tolerable dose (MTD)), when one animal with extremely high MFs that were attributed to an incidental clonal mutation was omitted. Positive and negative controls produced the expected results.

CONCLUSIONS

These findings show that styrene is not mutagenic in the liver and lung of MutaMouse under this experimental condition.

Critical review of styrene genotoxicity focused on the mutagenicity/clastogenicity literature and using current organization of economic cooperation and development guidance

Styrene is an important high production volume chemical used to manufacture polymeric products. In 2018, International Agency for Research on Cancer classified styrene as probably carcinogenic to humans; National Toxicology Program lists styrene as reasonably anticipated to be a human carcinogen. The genotoxicity literature for styrene and its primary metabolite, styrene 7,8-oxide (SO), begins in the 1970s. Organization of Economic Cooperation and Development (OECD) recently updated most genotoxicity test guidelines, making substantial new recommendations for assay conduct and data evaluation for the standard mutagenicity/clastogenicity assays. Thus, a critical review of the in vitro and in vivo rodent mutagenicity/clastogenicity studies for styrene and SO, based on the latest OECD recommendations, is timely. This critical review considered whether a study was optimally designed, conducted, and interpreted and provides a critical assessment of the evidence for the mutagenicity/clastogenicity of styrene/SO. Information on the ability of styrene/SO to induce other types of genotoxicity endpoints is summarized but not critically reviewed. We conclude that when styrene is metabolized to SO, it can form DNA adducts, and positive in vitro mutagenicity/clastogenicity results can be obtained. SO is mutagenic in bacteria and the in vitro mouse lymphoma gene mutation assay. No rodent in vivo mutation studies were identified. SO is clastogenic in cultured mammalian cells. Although the in vitro assays gave positive responses, styrene/SO is not clastogenic/aneugenic in vivo in rodents. In addition to providing updated information for styrene, this review demonstrates the application of the new OECD guidelines for chemicals with large genetic toxicology databases where published results may or may not be reliable. Environ. Mol. Mutagen. 2019. © 2019 Wiley Periodicals, Inc.

Glycogen debranching enzyme 6 (AGL), enolase 1 (ENOSF1), ectonucleotide pyrophosphatase 2 (ENPP2_1), glutathione S-transferase 3 (GSTM3_3) and mannosidase (MAN2B2) metabolism computational network analysis between chimpanzee and human left cerebrum

We identified significantly higher expression of the genes glycogen debranching enzyme 6 (AGL), enolase 1 (ENOSF1), ectonucleotide pyrophosphatase 2 (ENPP2_1), glutathione S-transferase 3 (GSTM3_3) and mannosidase (MAN2B2) from human left cerebrums versus chimpanzees. Yet the distinct low- and high-expression AGL, ENOSF1, ENPP2_1, GSTM3_3 and MAN2B2 metabolism networks between chimpanzee and human left cerebrum remain to be elucidated. Here, we constructed low- and high-expression activated and inhibited upstream and downstream AGL, ENOSF1, ENPP2_1, GSTM3_3 and MAN2B2 metabolism network between chimpanzee and human left cerebrum in GEO data set by gene regulatory network inference method based on linear programming and decomposition procedure, under covering AGL, ENOSF1, ENPP2_1, GSTM3_3 and MAN2B2 pathway and matching metabolism enrichment analysis by CapitalBio MAS 3.0 integration of public databases, including Gene Ontology, KEGG, BioCarta, GenMapp, Intact, UniGene, OMIM, etc. Our results show that the AGL, ENOSF1, ENPP2_1, GSTM3_3 and MAN2B2 metabolism network has more activated and less inhibited molecules in chimpanzee, but less activated and more inhibited in the human left cerebrum. We inferred stronger carbohydrate, glutathione and proteoglycan metabolism, ATPase activity, but weaker base excision repair, arachidonic acid and drug metabolism as a result of inducing cell growth in low-expression AGL, ENOSF1, ENPP2_1, GSTM3_3 and MAN2B2 metabolism network of chimpanzee left cerebrum; whereas stronger lipid metabolism, amino acid catabolism, DNA repair but weaker inflammatory response, cell proliferation, glutathione and carbohydrate metabolism as a result of inducing cell differentiation in high-expression AGL, ENOSF1, ENPP2_1, GSTM3_3 and MAN2B2 metabolism network of human left cerebrum. Our inferences are consistent with recent reports and computational activation and inhibition gene number patterns, respectively.

Association of GSTM1 and GSTT1 polymorphisms with DNA damage in coal-tar workers

DNA damage was evaluated by alkaline comet assay in peripheral blood lymphocytes of 115 coal-tar workers occupationally exposed to polycyclic aromatic hydrocarbons (PAHs) and 105 control subjects. The effect of polymorphisms of glutathione S-transferase (GST) genotypes on the DNA damage was assessed. The mean tail moment (TM) value in the coal-tar workers was significantly higher as compared to the control subjects (12.06 ± 0.55 versus 0.44 ± 0.31; P<0.05). No significant association (P>0.05) between the GSTT1 and GSTM1 genotypes and the TM values was found, however highest mean rank TM value was reported in GSTM1 null and GSTT1 null genotypes in both control and exposed subjects. Our results suggest that there is increased DNA damage in coal-tar workers due to PAHs exposure. Polymorphisms in GSTM1 and GSTT1 genes do not show significant effect (P>0.05) on DNA damage.

Genomic damages in peripheral blood lymphocytes and association with polymorphisms of three glutathione S-transferases in workers exposed to formaldehyde

DNA and chromosome damages in peripheral blood lymphocytes were evaluated in 151 workers occupationally exposed to formaldehyde (FA) and 112 non-FA exposed controls. The effects of polymorphisms in three glutathione-S-transferase (GSTs) genes on the DNA and chromosome damages were assessed as well. Alkaline comet assay and cytokinesis-block micronucleus (CBMN) assay were used to determine DNA and chromosome damages, respectively. The genotypes of GSTP1 (Ile105Val), GSTT1, and GSTM1 were assayed. The mean 8-h time-weighted average (TWA) concentrations of FA in two plywood factories were 0.83ppm (range: 0.08-6.30ppm). FA-exposed workers had higher olive tail moment (TM) and CBMN frequency compared with controls (Olive TM, 3.54, 95%CI=3.19-3.93 vs. 0.93, 95%CI=0.78-1.10, P<0.01; CBMN frequency, 5.51+/-3.37 vs. 2.67+/-1.32, P<0.01). Olive TM and the CBMN frequency also had a dose-dependent relation with the personal FA exposure. Significant association between FA exposure history and olive TM and CBMN frequency were also identified. The level of olive TM was slightly higher in FA-exposed workers with GSTM1 null genotype than those with non-null genotype (3.86, 95%CI=3.31-4.50 vs. 3.27, 95%CI=2.83-3.78, P=0.07) with adjustment of covariates. We also found that FA-exposed workers carrying GSTP1 Val allele had a slightly higher CBMN frequency compared with workers carrying only the wild-type allele (6.32+/-3.78 vs. 5.01+/-2.98, P=0.05). Our results suggest that the FA exposure in this occupational population increased DNA and chromosome damages and polymorphisms in GSTs genes may modulate the genotoxic effects of FA exposure.

Styrene Metabolism, Genotoxicity, and Potential Carcinogenicity

This report reviews styrene biotransformation, including minor metabolic routes, and relates metabolism to the genotoxic effects and possible styrene-related carcinogenicity. Styrene is shown to require metabolic activation in order to become notably genotoxic and styrene 7,8-oxide is shown to contribute quantitatively by far the most (in humans more than 95%) to the genotoxicity of styrene, while minor ring oxidation products are also shown to contribute to local toxicities, especially in the respiratory system. Individual susceptibility depending on metabolism polymorphisms and individual DNA repair capacity as well as the dependence of the nonlinearity of the dose-response relationships in the species in question and the consequences for risk evaluation are analyzd.

The Effects of GSTM1 and GSTT1 Polymorphisms on Micronucleus Frequencies in Human Lymphocytes In vivo

The influence of genetic polymorphisms in GSTM1 and GSTT1 genes on micronucleus frequencies in human peripheral blood lymphocytes was assessed through a pooled analysis of data from seven laboratories that did biomonitoring studies using the in vivo cytokinesis-block micronucleus assay. A total of 301 nonoccupationally exposed individuals (207 males and 94 females) and 343 workers (237 males and 106 females) occupationally exposed to known or suspected genotoxic substances were analyzed by Poisson regression. The results of the pooled analysis indicate that the GSTT1 null subjects had lower micronucleus frequencies than their positive counterparts in the total population (frequency ratio, 0.55; 95% confidence interval, 0.33-0.89). The protective effect of this genotype is reversed with increasing age, with a frequency ratio of 1.33 (95% confidence interval, 1.06-1.68) in subjects aged 60 years. A significant overall increase in micronucleus frequency with age and gender (P < 0.001 and P = 0.024, respectively) was observed, females having higher micronucleus frequencies than males, when occupationally exposed (P = 0.002). Nonoccupationally exposed smokers had lower micronucleus frequencies than nonsmokers (P = 0.001), whereas no significant difference in micronucleus level was observed between smokers and nonsmokers in the occupationally exposed group (P = 0.79). This study confirms that pooled analyses, by increasing the statistical power, are adequate for assessing the involvement of genetic variants on genome stability and for resolving discrepancies among individual studies.

Cytogenetic biomarkers, urinary metabolites and metabolic gene polymorphisms in workers exposed to styrene

The present study comprised a biomonitoring study in 95 workers occupationally exposed to styrene and 98 unexposed controls, employing an integrated approach involving biomarkers of exposure, effect, and susceptibility. Airborne styrene was evaluated at workplace, and urinary styrene metabolites, mandelic acid (MA), phenylglyoxylic acid (PGA), vinylphenols (VPTs) and phenylhydroxyethylmercapturic acids (PHEMAs), were measured as biomarkers of internal dose. Cytogenetic alterations were evaluated by analysing the frequency of chromosomal aberrations (CAs) and micronucleated binucleated cells (MNBN) in peripheral blood lymphocytes. The micronucleus assay was coupled with centromeric fluorescence in situ hybridization to distinguish micronuclei (MN) arising from chromosomal breakage (C- MN) from those harboring whole chromosomes (C+ MN). The possible influence of genetic polymorphisms of xenobiotic-metabolizing enzymes involved in styrene biotransformation (EPHX1, GSTT1, GSTM1, GSTP1) and NAT2 on the cytogenetic endpoints was investigated. The exposed workers showed a significantly higher frequency of MNBN (13.8+/-0.5% versus 9.2+/-0.4%; P<0.001) compared to control subjects. The effect appeared to concern both C- and C+ MN. A positive correlation was seen between the frequency of C+ MN and urinary level of MA+PGA (P<0.05) and VPTs (P<0.001). Chromosome-type CAs positively correlated with airborne styrene level and VPTs (P<0.05), whereas chromatid-type CAs correlated with PHEMAs (P<0.05). Workers bearing GSTM1 null genotype showed lowered levels of PHEMAs (P<0.001). The GSTT1 null genotype was associated with increased MNBN frequencies in the exposed workers (P<0.05) and the fast activity EPHX genotype with a moderate decrease in both MNBN and CAs in the controls. Our results suggest that occupational exposure to styrene has genotoxic effects that are potentiated by the GSTT1 gene deletion. These observations may have relevance considering the risk of lymphatic and haematopoietic malignancies tentatively associated with styrene exposure.

Review of the genotoxicity of styrene in humans

Styrene (CAS No. 100-42-5) is an important industrial chemical for which positive results have been reported in in vitro and in vivo genotoxicity assays. Styrene-exposed workers have been studied extensively over two decades for the induction of various types of genotoxic effects. The outcomes of these studies have been conflicting, and where positive responses have been reported, it has proved difficult to demonstrate clear relationships between levels of damage reported and exposure levels. In this review, we have assessed studies addressing mutagenicity (chromosome aberrations, micronuclei and gene mutations) and other endpoints (sister chromatid exchanges, DNA breaks and DNA adducts) using criteria derived from the IPCS guidelines for the conduct of human biomonitoring studies. Based on the re-evaluated outcomes, the data are not convincing that styrene induces gene mutations. The evidence for induction of clastogenicity in occupationally exposed workers is less clear, with a predominant lack of induction of micronuclei in different studies, but conflicting responses in chromosome aberration assays. The results of numerous studies on sister chromatid exchanges do not provide evidence of a clear positive response, despite these being induced in animals exposed to styrene at high concentrations. However, there is evidence that both DNA adducts and DNA single strand breaks are induced in styrene workers. These types of damage are considered indicative of exposure of the target cells and interaction with cellular DNA but do not necessarily result in heritable changes. There is evidence that the metabolism of styrene in humans is affected by genetic polymorphisms of metabolizing genes and that these polymorphisms affect the outcome of in vitro mutagenicity studies on styrene. Therefore, studies that have addressed the potential of this factor to affect in vivo responses were considered. To date, there are no consistent relationships between genetic polymorphisms and induction of genotoxicity by styrene in humans, but further work is warranted on larger samples. The analyses of individual studies, together with a consideration of dose-response relationships and the lack of a common profile of positive responses for the various endpoints in different studies, provide no clear evidence that styrene exposure in workers results in detectable levels of mutagenic damage. However, evidence of exposure to genotoxic metabolites is demonstrated by the formation of DNA adducts and strand breaks.

Occupational exposure to styrene: modulation of cytogenetic damage and levels of urinary metabolites of styrene by polymorphisms in genes CYP2E1, EPHX1, GSTM1, GSTT1 and GSTP1

Styrene is widely used in the production of various plastics, synthetic rubber and resins. The aim of this study was to evaluate if individual polymorphisms in xenobiotic metabolizing enzymes, related with the metabolic fate of styrene, could modify individual susceptibility to the possible genotoxic effects of the styrene exposure. Twenty-eight reinforced plastic workers and 28 control subjects were studied. In the selected population the urinary styrene metabolites mandelic (MA) and phenylglyoxylic (PGA) acids were quantified, sister chromatid exchanges (SCE) and micronuclei (MN) were assessed in peripheral lymphocytes and all the subjects were genotyped for GSTM1, GSTT1 (gene deletions), GSTP1 (codon 105 ile==>val), EPHX1 (codons 113 tyr==>his and 139 his==>arg) and CYP2E1 (DraI polymorphism in intron 6). The results obtained showed a significant difference between the levels of SCE, but not in MN levels, in exposed workers as compared with the control group. The GSTP1 and CYP2E1 individual genotypes modulate the baseline levels of SCE that are lower in non-wild type individuals for both polymorphisms. The GSTM1 null individuals with low levels of exposure have significantly higher urinary levels of MA+PGA. The present data seem to suggest that apart from the methodology usually used for monitoring populations occupationally exposed to styrene (urinary metabolites and biomarkers of early biological effects) the analysis of individual genotypes associated with the metabolic fate of styrene should also be carried out in order to evaluate the individual genetic susceptibility of exposed populations.

A recombinant model for assessing the role of GSTM1 in styrene-7,8-oxide toxicity and mutagenicity

Styrene-7,8-oxide (SO) is a highly reactive epoxide able to undergo reactions with endogenous nucleophiles, such as DNA. SO is inactivated by glutathione-S-transferase M1 (GSTM1). This detoxification enzyme is absent in approximately one-half of Caucasian (49%) populations. A GSTM1 recombinant human lymphoblastoid cell line (FB7) was generated from a GSTM1 negative parental cell line (WIL2NS). GSTM1 status was determined using RT-PCR and immunochemistry. Cells were challenged with a range of SO doses and subsequent toxicity (population growth in flasks) and genotoxicity (mutations at the HPRT locus) were monitored. FB7 (GSTM1 positive) exhibited greater cell survival after SO exposure relative to the GSTM1 negative parental line. The IC50 following a 1 h exposure to SO was 0.5 mM for WIL2NS, compared to greater than 2.5 mM for FB7. The extrapolated IC50 for FB7 was 5.5 mM. Significantly fewer mutant cells were induced by SO for FB7 than for WIL2NS at equivalent doses of SO. These findings suggest that the sensitivity of cells to styrene-7,8-oxide is influenced by GSTM1 status and that a recombinant GSTM1 positive cell line can efficiently detoxify styrene-7,8-oxide.

GSTM1, GSTT1, and GSTP1 genotypes and the genotoxicity of hydroquinone in human lymphocytes

Hydroquinone is a myelotoxin that is found in many foods and is also formed through the metabolism of benzene. Human exposure to benzene is associated with the development of myelodysplastic syndrome and acute myelogenous leukemia. Hydroquinone is genotoxic in several in vitro and in vivo test systems, inducing micronuclei (MN), sister-chromatid exchange (SCE), and chromosomal aberrations. Glutathione S-transferases (GSTs) are a superfamily of polymorphic enzymes involved in the conjugation of reactive chemical intermediates to soluble forms. These enzymes play a key role in the detoxification of endogenous and exogenous compounds, and the polymorphic genes GSTM1, GSTT1, and GSTP1 have been associated with the differential metabolism of several genotoxicants. In the present study, we have evaluated the effect of GSTM1, GSTT1, and GSTP1 polymorphisms on the frequency of MN and SCE induced by hydroquinone in human lymphocytes. Lymphocytes were obtained from 15 healthy non-smoking donors, and their GSTM1, GSTT1, and GSTP1 genotypes determined. Treatment of cultures of the lymphocytes with hydroquinone significantly increased the overall frequencies of MN and SCE (P<0.0001). Individuals with the GSTM1 null genotype had a significantly higher frequency of MN compared with GSTM1-present individuals (P=0.013); in contrast, the GSTM1 genotype had no effect on hydroquinone-induced SCE frequency. The other polymorphisms did not significantly affect the frequencies of MN or SCE. These results suggest that GSTM1 is involved in the metabolic fate of hydroquinone and that polymorphisms in GSTM1 could be related to inter-individual differences in DNA damage arising from the exposure to this compound.

Influence of genetic polymorphisms on biomarkers of exposure and genotoxic effects in styrene-exposed workers

A study on 44 workers exposed to styrene and 44 matched referents was performed in order to examine the influence of genetic polymorphisms in biotransformation and DNA repair enzymes on the levels of N-terminal hemoglobin adducts and genotoxicity biomarkers. Urinary mandelic acid concentration averaged 201.57 mg/g creatinine +/-148.32 in exposed workers, corresponding to a calculated average airborne styrene exposure of 9.5 ppm +/-9.6. Individuals with a high level of N-terminal valine adducts had higher levels of DNA damage, as evaluated by the Comet assay (r = 0.29, P = 0.008). Frequencies of micronucleated mononucleated lymphocytes (MNMC) (0.71 +/- 0.88 vs 0.11 +/- 0.20, P<0.0001), micronucleated binucleated lymphocytes (MNBC) (3.93 +/- 2.75 vs 2.65 +/- 1.94, p = 0.02) and micronucleated nasal epithelial cells (0.52 +/- 0.49 vs 0.23 +/- 0.31, p = 0.04) differed significantly between the exposed and referent groups. In the whole group of 88 individuals, higher frequencies of MNMC were found in individuals possessing the XRCC3 Met(241) allele and those individuals with the XRCC1 Gln( (399) ) allele showed higher frequencies of MNMC and MNCB. In vitro DNA repair capacity, as measured by residual DNA strand breaks in peripheral blood leukocytes after a styrene oxide challenge, was also influenced by styrene exposure, with an apparent induction of early repair mechanisms associated with the intensity of recent exposure and a reduction of late (24 h) repair capacity that was associated with the duration of employment. After 1 h of repair, lower levels of residual DNA damage were found in individuals possessing GSTT1 (P = 0.043). After 24 h of repair, lower residual DNA damage was found in individuals homozygous for XRCC1 Arg(194) (P = 0.013). Multivariate regression analysis indicated that the duration of exposure, smoking habits and polymorphisms of XRCC1 at codon 399 were important variables affecting the genotoxic responses. Our data suggest that DNA damage is formed in workers exposed to low concentrations of styrene, and that genotypes of metabolising and DNA-repair genes are important for the assessment of individual genotoxic risk to styrene. The in vitro DNA repair phenotype assay might be a valuable method to estimate the susceptibility of workers.

Genetic susceptibility, biomarker respones, and cancer

A large number of studies have reported associations between polymorphisms of xenobiotic-metabolizing enzymes (XMEs) and various cancers. However, the carcinogenic exposures behind such findings have usually been unclear. Information on susceptibility to specific carcinogens could better be obtained by examining situations where the exposure and the endpoint studied are nearer in time, i.e., by studying biomarkers of carcinogen exposure and early (genotoxic) effect in exposed humans. For example, analyses of DNA adducts and cytogenetic endpoints have indicated an increased susceptibility of glutathione S-transferase M1 (GSTM1) null genotype to genotoxicity of tobacco smoking, supporting the view that the associations of the GSTM1 null genotype with bladder and lung cancer are partly related to smoking. In vitro genotoxicity studies with human cells offer an experimental tool that can be used, within the limits of the cell systems, to predict individual sensitivity and genotype-carcinogen interactions. In vitro sensitivity to the genotoxicity of 1,2:3,4-diepoxybutane, an epoxide metabolite of 1,3-butadiene has clearly been shown to depend on GSTT1 genotype, which has also been implicated to modify, along with GSTM1 genotype, the in vitro genotoxicity of 1,2-epoxy-3-butene, another epoxide metabolite of 1,3-butadiene. These genotypes appear to modulate the excretion of 1,3-butadiene-specific mercapturic acids, and influence genotoxicity biomarker levels in 1,3-butadiene-exposed workers. The excretion of specific mercapturic acids (PHEMA) in workers exposed to styrene has clearly been shown to depend on GSTM1 genotype, and GSTT1 genotype seems to modulate the excretion of one PHEMA diastereoisomer. These genotypes have also been implicated to modulate the in vitro genotoxicity of styrene. In general, the genetic polymorphisms potentially important for biomarker response largely depend on the exposing agent, biological material examined, and ethnicity of the population under study. Individual exposure level may vary a lot, and a reliable estimate of the exposure is essential for correct interpretation of genotype-exposure interaction. Besides XME polymorphisms, any polymorphisms that affect cellular response to DNA damage could, in principle, modify individual sensitivity to genotoxins. For instance, those concerning DNA repair proteins are presently being studied by many laboratories.

Evaluation of bioanalytical assays for toxicity assessment and mode of toxic action classification of reactive chemicals

The toxicity of electrqphiles, including reactive organochlorines, epoxides, and compounds with an activated double bond was investigated. A set of different bioanalytical assays based on genetically modified Escherichia coli strains was set up to quantify cytotoxicity and specific reactivity toward the important biological nucleophiles DNA and glutathione (GSH). The significance of GSH for detoxification was assessed by cellular GSH depletion as well as by growth inhibition of a GSH-deficient strain. Tests for DNA damage comprised the measurement of induction of DNA repair systems, DNA fragmentation, and growth inhibition of a strain deficient in major DNA repair mechanisms. The most suitable assays for detection of mechanisms that underlie the observable cytotoxicity of the tested electrophiles were two sets of strains either lacking GSH or DNA repair in combination with their corresponding parent strains. Comparison of toxicity observed in those strains suggests three clearly distinguishable modes of toxic action for electrophilic chemicals: "DNA damage", "GSH depletion-related toxicity", and "unspecific reactivity". The class of chemicals causing DNA damage includes the epoxides 1,2-epoxybutane, (2,3-epoxypropyl)benzene, and styrene oxide. The class of chemicals with GSH depletion-related toxicity includes compounds with an activated double bond, like acrylates and acrolein. All reactive organochlorines and some epoxides were classified as unspecifically reactive because their toxicity is initiated by reactions with both biological nucleophiles. The work presented here is a contribution for an alternative hazard and effect assessment of organic pollutants based on mode of toxic action classification.

Applicability and limitation of OSARs for the toxicity of electrophilic chemicals

The appropriate selection and application of quantitative structure-activity relationships (QSARs) for the prediction of toxicity is based on the prior assignment of a chemical to its mode of toxic action. This classification is often derived from structural characteristics with the underlying assumption that chemically similar compounds have similar mechanisms of action, which is often but not necessarily the case. Instead of using structural characteristics for classification toward a mode of toxic action, we used Escherichia coli based bioanalytical assays to classify electrophilic chemicals. Analyzing a series of reactive organochlorines, epoxides, and compounds with an activated double bond, three subclasses of reactive toxicity were distinguished: "glutathione depletion-related toxicity", "DNA damage", and "unspecific reactivity". For both subsets of specifically reacting compounds a direct correlation between effects and chemical reactivity was found. Reaction rate constants with either glutathione or 2'-deoxyguanosine, which was used as a model for complex DNA, served well to set up preliminary QSARs for either glutathione depletion-related toxicity or toxicity based on DNA damage in the model organism E. coli. The applicability of QSARs for electrophilic chemicals based on mechanistically relevant reaction rate constants is a priori limited to a small subset of compounds with strictly identical mechanism of toxic action and similar metabolic rates. In contrast, the proposed bioanalytical assays not only allowed the experimental identification of molecular mechanisms underlying the observable toxicity but also their toxicity values are applicable to quantitatively predict toxic effects in higher organisms by linear correlation models, independent of the assigned mode of toxic action.

Markers of genetic susceptibility in human environmental hygiene and toxicology: the role of selected CYP, NAT and GST genes

Inherited genetic traits co-determine the susceptibility of an individual to a toxic chemical. Special emphasis has been put on individual responses to environmental and industrial carcinogens, but other chronic diseases are of increasing interest. Polymorphisms of relevant xenobiotic metabolising enzymes may be used as toxicological susceptibility markers. A growing number of genes encoding enzymes involved in biotransformation of toxicants and in cellular defence against toxicant-induced damage to the cells has been identified and cloned, leading to increased knowledge of allelic variants of genes and genetic defects that may result in a differential susceptibility toward environmental toxicants. "Low penetrating" polymorphisms in metabolism genes tend to be much more common in the population than allelic variants of "high penetrating" cancer genes, and are therefore of considerable importance from a public health point of view. Positive associations between cancer and CYP1A1 alleles, in particular the *2C I462V allele, were found for tissues following the aerodigestive tract. Again, in most cases, the effect of the variant CYP1A1 allele becomes apparent or clearer in connection with the GSTM1 null allele. The CYP1B1 codon 432 polymorphism (CYP1B1*3) has been identified as a susceptibility factor in smoking-related head-and-neck squameous cell cancer. The impact of this polymorphic variant of CYP1B1 on cancer risk was also reflected by an association with the frequency of somatic mutations of the p53 gene. Combined genotype analysis of CYP1B1 and the glutathione transferases GSTM1 or GSTT1 has also pointed to interactive effects. Of particular interest for the industrial and environmental field is the isozyme CYP2E1. Several genotypes of this isozyme have been characterised which seem to be associated with different levels of expression of enzyme activity. The acetylator status for NAT2 can be determined by genotyping or by phenotyping. In the pathogenesis of human bladder cancer due to occupational exposure to "classical" aromatic amines (benzidine, 4-aminodiphenyl, 1-naphthylamine) acetylation by NAT2 is regarded as a detoxication step. Interestingly, the underlying European findings of a higher susceptibility of slow acetylators towards aromatic amines are in contrast to findings in Chinese workers occupationally exposed to aromatic amines which points to different mechanisms of susceptibility between European and Chinese populations. Regarding human bladder cancer, the hypothesis has been put forward that genetic polymorphism of GSTM1 might be linked with the occurrence of this tumour type. This supports the hypothesis that exposure to PAH might causally be involved in urothelial cancers. The human polymorphic GST catalysing conjugation of halomethanes, dihalomethanes, ethylene oxide and a number of other industrial compounds could be characterised as a class theta enzyme (GSTT1) by means of molecular biology. "Conjugator" and "non-conjugator" phenotypes are coincident with the presence and absence of the GSTT1 gene. There are wide variations in the frequencies of GSTT1 deletion (GSTT1*0/0) among different ethnicities. Human phenotyping is facilitated by the GST activity towards methyl bromide or ethylene oxide in erythrocytes which is representative of the metabolic GSTT1 competence of the entire organism. Inter-individual variations in xenobiotic metabolism capacities may be due to polymorphisms of the genes coding for the enzymes themselves or of the genes coding for the receptors or transcription factors which regulate the expression of the enzymes. Also, polymorphisms in several regions of genes may cause altered ligand affinity, transactivation activity or expression levels of the receptor subsequently influencing the expression of the downstream target genes. Studies of individual susceptibility to toxicants and gene-environment interaction are now emerging as an important component of molecular epidemiology.

Effect of epoxide hydrolase and glutathione S-tranferase genotypes on the induction of micronuclei and DNA damage by styrene-7,8-oxide in vitro

Styrene is one of the most important organic chemicals used worldwide. Its main metabolite, styrene-7,8-oxide (SO), is considered responsible for the genotoxic effects associated with exposure to styrene. SO is detoxified by hydrolysis catalyzed by epoxide hydrolase (EH), or, to a minor extent, by conjugation mediated by glutathione S-transferases (GSTs). The purpose of the present study was to investigate whether EH (exons 3 and 4), GSTP1 (exons 5 and 6), GSTM1 and GSTT1 polymorphisms have any influence on the genotoxicity of SO in human leukocytes. Peripheral leukocytes from 30 healthy donors were exposed to SO (50 and 200 micro M) and genotoxicity was evaluated by means of the micronucleus (MN) test and alkaline comet assay, using 1% DMSO as solvent control. When EH genotypes were classified in low, medium, and high with respect to the expected EH activity, an increase in induced comet tail length was observed with decreasing EH activity in SO-exposed cells. An increase was seen in induced MN frequency in EH low-activity donors. These findings are consistent with the detoxifying activity of this enzyme. In addition, increases in MN frequencies for GSTP1 *A/*B and *A/*C genotypes with regard to the wild-type homozygous *A/*A genotype were detected. This may be due to a low detoxifying activity as a consequence of altered SO affinity of the variant protein, but must be confirmed using homozygote variant individuals, not included in this study. No clear results were obtained for GSTM1 or GSTT1 genotypes, even when performing the analysis after grouping individuals with the same expected EH activity, probably due to the minor role that glutathione conjugation plays in styrene metabolism. The present in vitro findings using human leukocytes suggest that polymorphisms in EH, and, to a lesser extent, in GSTP1, may influence induction of cytogenetic and DNA damage by SO.

Genetic polymorphism of drug-metabolizing enzymes and styrene-induced DNA damage

A cross-sectional study was carried out on 48 workers exposed to styrene and 14 unexposed healthy controls in order to investigate the genotoxic potential of styrene exposure. DNA damage was assessed in peripheral blood leukocytes (WBCs) by the comet assay. Polymorphisms in glutathione S-transferase genes (GSTM1, GSTT1, GSTP1) and the gene encoding microsomal epoxide hydrolase (EPHX) were characterized to assess their possible modifying role in styrene metabolism and subsequent DNA damage. Exposed workers showed significantly higher levels of DNA damage compared to controls. Among workers, the GSTM1 and GSTT1 polymorphisms significantly affected comet parameters. Subjects bearing a GSTM1pos genotype showed a significantly higher proportion of damaged nuclei compared to people lacking GSTM1-1 expression (GSTM1null), whereas GSTT1pos workers showed significantly lower DNA damage than GSTT1null individuals. Styrene-7,8-oxide (SO)-induced DNA damage was assessed in vitro in WBCs isolated from the healthy controls. A clear dose-response relationship at micromolar doses of SO was found for the whole group. WBCs collected from subjects bearing the homozygous wildtype GSTP1 genotype showed a significant protection compared to cells from subjects bearing at least one GSTP1 variant allele. The field survey confirms that styrene exposure is associated with increased DNA damage and indicates a modulating role for GSTM1 and GSTT1 genotypes. In vitro experiments suggest that the extent of SO-induced DNA strand breaks depends, at least in part, on interindividual differences in GSH-conjugation capabilities.