The genotoxicity of enantiomeric aliphatic epoxides

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.

The Induction of Gene Mutations and Micronuclei by Oxiranes and Siloranes in Mammalian Cells in vitro

Oxiranes and siloranes are candidate molecules for the development of composite materials with low shrinkage. Since some of these molecules are highly reactive, they could lead to adverse biological effects from underlying genetic mechanisms. Therefore, we analyzed the formation of micronuclei (chromosomal aberrations) and the induction of gene mutations (HPRT assay) in mammalian cells. The numbers of micronuclei induced by the oxirane di(cyclohexene-epoxidemethyl)ether (Eth-Ep) at low concentrations (10 μM) were about five-fold higher than controls. The related compound epoxy cyclohexyl methyl-epoxy cyclo-hexane carboxylate (Est-Ep) was less effective. The activity of diglycidylether of bisphenol A (BADGE) was even lower but similar to the most reactive silorane, di-3,4-epoxy cyclohexylmethyl-dimethyl-silane (DiMe-Sil). No induction of micronuclei was detected in the presence of a rat liver homogenate (S9). Est-Ep and Eth-Ep also induced gene mutations. Our analyses indicated low mutagenic potentials of siloranes; however, some oxiranes induced strong effects at two genetic endpoints.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Depletion by styrene of glutathione in plasma and bronchioalveolar lavage fluid of non-Swiss albino (NSA) mice

Styrene is a widely used chemical, but it is known to produce lung and liver damage in mice. This may be related to oxidative stress associated with the decrease in the levels of reduced glutathione (GSH) in the target tissues. The purpose of this study was to investigate the effect of styrene and its primary metabolites R-styrene oxide (R-SO) and S-styrene oxide (S-SO) on GSH levels in the lung lumen, as determined by amounts of GSH in bronchioalveolar lavage fluid (BALF) and in plasma. When non-Swiss albino (NSA) mice were administered styrene (600 mg/kg, ip), there was a significant fall in GSH levels in both BALF and plasma within 3 h. These returned to control levels by 12 h. The active metabolite R-SO (300 mg/kg, ip) also produced significant decreases in GSH in both BALF and plasma, but S-SO was without marked effect. Since GSH is a principal antioxidant in the lung epithelial lining fluid, this fall due to styrene may exert a significant influence on the ability of the lung to buffer oxidative damage.

Indicators of oxidative stress and apoptosis in mouse whole lung and Clara cells following exposure to styrene and its metabolites

In mice, styrene is hepatotoxic, pneumotoxic, and causes lung tumors. One explanation for the mechanism of toxicity is oxidative stress/damage. Previous studies have shown decreased glutathione levels, linked to increased apoptosis, in lung homogenates and isolated Clara cells 3 h following styrene or styrene oxide (SO) administration or in vitro exposure. The objective of the current studies was to determine what effects styrene and its active metabolites, primarily styrene oxide, had on indicators of oxidative stress and attendant apoptosis in order to understand better the mechanism of styrene-induced toxicity. Three hours following in vitro exposure of Clara cells to styrene or SO there were increases in reactive oxygen species (ROS). Following administration of styrene or styrene oxide ip, increases in ROS, superoxide dismutase (SOD), and 8-hydroxydeoxyguanosine (8-OHdG) formation were observed. Since increases in ROS have been linked to increases in apoptosis ratios of bax/bcl-2, mRNA and protein expression were determined 3-240 h following the administration of styrene and R-styrene oxide (RSO). The bax/bcl-2 mRNA ratio increased 12 and 24 h following R-SO and 120 h following styrene administration. However, the bax/bcl-2 protein ratio was not increased until 240 h following R-SO, and 24 and 240 h following styrene administration. However, only a slight increase in caspase 3 was observed. These results indicated that oxidative stress occurred 3h following styrene or styrene oxide as evidenced by increased ROS and SOD. This increased ROS may be responsible for the increased 8-OHdG formation. Our findings of limited apoptosis in Clara cells following acute exposure to styrene or SO are in agreement with others and may reflect the minimal extent to which apoptosis plays a role in acute styrene toxicity. It is clear, however, that oxidative stress and oxidative effects on DNA are increased following exposure to styrene or styrene oxide, and these may play a role in the lung tumorigenesis in mice.

Oxidative stress due to (R)-styrene oxide exposure and the role of antioxidants in non-Swiss albino (NSA) mice

Styrene produces lung and liver damage that may be related to oxidative stress. The purpose of this study was to investigate the toxicity of (R)-styrene oxide (R-SO), the more active enantiomeric metabolite of styrene, and the protective properties of the antioxidants glutathione (GSH), N-acetylcysteine (NAC), and 4-methoxy-L-tyrosinyl-gamma-L-glutamyl-L-cysteinyl-glycine (UPF1) against R-SO-induced toxicity in non-Swiss Albino (NSA) mice. UPF1 is a synthetic GSH analog that was shown to have 60 times the ability to scavenge reactive oxygen species (ROS) in comparison to GSH. R-SO toxicity to the lung was measured by elevations in the activity of lactate dehydrogenase (LDH), protein concentration, and number of cells in bronchoalveolar lavage fluid (BALF). Toxicity to the liver was measured by increases in serum sorbitol dehydrogenase (SDH) activity. Antioxidants were not able to decrease the adverse effects of R-SO on lung. However, NAC (200 mg/kg) ip and GSH (600 mg/kg), administered orally prior to R-SO (300 mg/kg) ip, showed significant protection against liver toxicity as measured by SDH activity. Unexpectedly, a synthetic GSH analog, UPF1 (0.8 mg/kg), administered intravenously (iv) prior to R-SO, produced a synergistic effect with regard to liver and lung toxicity. Treatment with UPF1 (0.8 mg/kg) iv every other day for 1 wk for preconditioning prior to R-SO ip did not result in any protection against liver and lung toxicity, but rather enhanced the toxicity when administered prior R-SO. The results of the present study demonstrated protection against R-SO toxicity in liver but not lung by the administration of the antioxidants NAC and GSH.

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.

Comparison of styrene and its metabolites styrene oxide and 4-vinylphenol on cytotoxicity and glutathione depletion in Clara cells of mice and rats

Styrene is a widely used compound in the manufacturing industry. In mice and rats, it is both hepatotoxic and pneumotoxic. It causes lung tumors in mice, but not in rats. The Clara cell is the main target for the toxicity of styrene and its metabolites, and it also has the greatest activity for styrene metabolism. Therefore, Clara cells isolated from CD-1 mice and Sprague-Dawley rats were used to compare the cytotoxicities induced by styrene and its metabolites. The cytotoxicity of styrene was greater in vitro than that of its metabolites styrene oxide (racemic, R- and S-) and 4-vinylphenol in contrast with what has been observed in vivo in previous studies on hepatotoxicity and pneumotoxicity. Susceptibility of rats to styrene and its metabolites are 4-fold less than that observed with mice. Glutathione levels were also measured in mice following addition of the chemicals in vitro and treatment of the CD-1 mice in vivo. Decreases in glutathione concentrations were seen even at doses which did not cause the death of mouse Clara cells. Significant decreases in glutathione were observed 3h after treatment with racemic SO and R-SO. At 12h, rebound effects were seen for all compounds, with all but R-SO rebounding above controls. These studies suggest that in vitro cytotoxicity of styrene and its metabolites does not strictly follow in vivo effects and that decreases in mouse glutathione levels may be related to oxidative stress.

Genotoxic effects of ethylene oxide, propylene oxide and epichlorohydrin in humans: update review (1990-2001)

Ethylene oxide (EtO), propylene oxide (PO) and epichlorohydrin (ECH) are important industrial chemicals widely used as intermediates for various synthetic products. EtO and PO are also environmental pollutants. In this review we summarize data published during the period 1990-2001 concerning both the genotoxic and carcinogenic effects of these epoxides in humans. The use of DNA and hemoglobin adducts as biomarkers of exposure and the role of polymorphism, as well as confounding factors, are discussed. We have also included recent in vitro data comprising genotoxic effects induced by EtO, PO and ECH in mammalian cells. The uncertainties regarding cancer risk estimation still persist, in spite of the large database collected.

Styrene respiratory tract toxicity and mouse lung tumors are mediated by CYP2F-generated metabolites

Mice are particularly sensitive to respiratory tract toxicity following styrene exposure. Inhalation of styrene by mice results in cytotoxicity in terminal bronchioles, followed by increased incidence of bronchioloalveolar tumors, as well as degeneration and atrophy of nasal olfactory epithelium. In rats, no effects on terminal bronchioles are seen, but effects in the nasal olfactory epithelium do occur, although to a lesser degree and from higher exposure concentrations. In addition, cytotoxicity and tumor formation are not related to blood levels of styrene or styrene oxide (SO) as measured in chronic studies. Whole-body metabolism studies have indicated major differences in styrene metabolism between rats and mice. The major differences are 4- to 10-fold more ring-oxidation and phenylacetaldehyde pathways in mice compared to rats. The data indicate that local metabolism of styrene is responsible for cytotoxicity in the respiratory tract. Cytotoxicity is seen in tissues that are high in CYP2F P450 isoforms. These tissues have been demonstrated to produce a high ratio of R-SO compared to S-SO (at least 2.4 : 1). In other rat tissues the ratio is less than 1, while in mouse liver the ratio is about 1.1. Inhibition of CYP2F with 5-phenyl-1-pentyne prevents the styrene-induced cytotoxicity in mouse terminal bronchioles and nasal olfactory epithelium. R-SO has been shown to be more toxic to mouse terminal bronchioles than S-SO. In addition, 4-vinylphenol (ring oxidation of styrene) has been shown to be highly toxic to mouse terminal bronchioles and is also metabolized by CYP2F. In human nasal and lung tissues, styrene metabolism to SO is below the limit of detection in nearly all samples, and the most active sample of lung was approximately 100-fold less active than mouse lung tissue. We conclude that styrene respiratory tract toxicity in mice and rats, including mouse lung tumors, are mediated by CYP2F-generated metabolites. The PBPK model predicts that humans do not generate sufficient levels of these metabolites in the terminal bronchioles to reach a toxic level. Therefore, the postulated mode of action for these effects indicates that respiratory tract effects in rodents are not relevant for human risk assessment.

Quantum mechanical quantitative structure activity relationships to avoid mutagenicity in dental monomers

The objective of this study was to identify through quantum mechanical quantitative structure activity relationships (Q-QSARs) chemical structures in dental monomers that influence their mutagenicity. AMPAC, a semiempirical computer program that provides quantum mechanical information for chemical structures, was applied to three series of reference chemicals: a set of methacrylates, a set of aromatic and a set of aliphatic epoxy compounds. QSAR models were developed using this chemical information together with mutagenicity data (Salmonella TA 100, Ames Test). CODESSA, a QSAR program that calculates quantum chemical descriptors from information generated by AMPAC and statistically matches these descriptors with observed biological properties was used. QSARs were developed which had r2 values exceeding 0.90 for each study series. These QSARs were used to accurately predict the mutagenicity of BISGMA. a monomer commonly used in dentistry, and two epoxy monomers with developing use in dentistry, GY-281 and UVR-6105. The Q-QSAR quantum mechanical descriptors correctly predicted the level of mutagenicity for all three compounds. The descriptors in the correlation equation pointed to components of structure that may contribute to mutagenesis. The QSARs also provided 'dose windows' for testing mutagenicity, circumventing the need for extensive dose exploration in the laboratory. The Q-QSAR method promises an approach for biomaterials scientists to predict and avoid mutagenicity from the chemicals used in new biomaterial designs.

Stereochemistry of styrene biotransformation

Metabolism of styrene, an important industrial monomer, is reviewed. Attention is focused on the stereoselectivity of its oxidation to 7,8-styrene oxide as well as on further stereoselective biotransformation by hydrolytic and mercapturic acid pathway. Toxic effects such as mutagenicity, genotoxicity, hepatotoxicity, and pneumotoxicity may be related to the ratio of styrene oxide enantiomers at the target site. In rats formation of the less mutagenic (S)-styrene oxide and a faster detoxication of the (R)-enantiomer is favored. In mice metabolic activation of styrene favors the formation of (R)-styrene oxide but this more toxic enantiomer is detoxified faster, so that a nearly racemic styrene oxide results. Stereochemistry of biotransformation can contribute to the species differences in toxicity but can hardly be interpreted as a crucial factor. Due to lack of relevant data the stereochemistry of human metabolism cannot be interpreted in relation to the toxic effects.

Genetic toxicology of epichlorohydrin: a review

Epichlorohydrin (ECH) is one of the more commercially important aliphatic epoxides used extensively as an industrial intermediate, a laboratory reagent, and as an insecticide. It is a volatile, colourless liquid with an ethereal odour. It behaves as an alkylating agent. Reports have shown it to cause the respiratory and dermal toxicity in animals and humans. It has also been reported to be carcinogenic in experimental models. Thus, the wide-spread use of this aliphatic epoxide is of great concern in human health problem. The purpose of this paper is to critically review and update the mutagenic and clastogenic effects of ECH based on available literature.

Propylene oxide and epichlorohydrin induce DNA strand breaks in human diploid fibroblasts

The induction of DNA strand breaks in human diploid fibroblasts (VH-10) was demonstrated after in vitro exposure with two carcinogenic epoxides, propylene oxide (PO) and epichlorohydrin (ECH). Alkaline DNA unwinding (ADU), pulsed field gel electropharosis (PFGE), and the comet assay were used to measure DNA single. (SSBs) and double-strand breaks (DSBs). A dose-dependent increase of DNA strand breaks, measured by ADU, was observed in the dose range 2.5-20 mMh of PO and 0.25-2 mMh of ECH. The dose-response of ECH was about five times higher compared with that of PO (211 vs. 41 SSBs. 100 Mbp-1.mMh-1). The induction rates of DSBs, measured by PFGE, were found to be 18 times higher for ECH compared to PO (4.8 and 0.27 DSBs.100 Mbp-1.mMh-1 for ECH and PO, respectively). Using these two methods, the SSBs/ DSBs ratio was estimated to be 148 for PO and 44 for ECH. The data obtained by the comet assay also demonstrated a dose-dependent ability of PO and ECH to induce DNA damage. It was found that ECH was about six times more effective as an inducer of DNA strand breaks compared to PO (200 and 32x100 Mbp-1.mMh-1 for ECH and PO, respectively). The SSBs/DSBs ratios calculated using comet assay and PFGE data were 125 for ECH and 41 for PO. In addition, ECH is about 10 times more toxic than PO with respect to survival. These properties of ECH can at least in part be explained by its higher chemical reactivity connected with a higher rate of DNA alkylation.

Pneumotoxicity and hepatotoxicity of styrene and styrene oxide

The purpose of this study was to investigate the toxicity of styrene and styrene oxide in the lung in comparison to the toxicity in the liver. Pneumotoxicity caused by styrene or styrene oxide was measured by elevations in the release of gamma-glutamyltranspeptidase (GGT) and lactate dehydrogenase (LDH) into bronchoalveolar lavage fluid (BALF), while hepatotoxicity was measured by increases in serum sorbitol dehydrogenase (SDH) in non-Swiss Albino (Hsd:NSA) mice. Intraperitoneal administration of styrene at doses of 500-1000 mg/kg caused consistent dose-dependent increases in both sets of biomarkers with the hepatic effect appearing earlier than the pulmonary effect. Pyridine, phenobarbital, and beta-naphthoflavone, inducers of CYP2E1, CYP2B, and CYP1A, respectively, increased the toxicity of styrene. Pyridine and phenobarbital treatments increased mortality due to styrene. Styrene oxide exists in two enantiomeric forms: (R)- and (S)-styrene oxide, and the differential toxicities of the two enantiomers and racemic styrene oxide were compared. In all studies, (R)-styrene oxide caused greater toxicity than the (S) enantiomer, especially in the liver. Trichloropropene oxide, an epoxide hydrolase inhibitor, was used to inhibit styrene oxide detoxification and increased its hepatotoxicity, while buthionine sulfoxamine, a glutathione depletor, did not. These results demonstrated the greater role of epoxide hydrolase in styrene oxide detoxification.

Reaction of epichlorohydrin with 2'-deoxynucleosides: characterization of adducts

Epichlorohydrin (ECH) is a simple 3-carbon epoxide of industrial importance and thus has the potential for human exposure in the workplace. It has been shown to be genotoxic in several systems and is a compound capable of reacting with biological nucleophiles. This study details the products formed from the reaction of ECH with 2'-deoxynucleosides at pH 7 and 37 degrees C for 6 h. Reaction with 2'-deoxyguanosine yielded 7-(3-chloro-2-hydroxypropyl) guanine (7-CHP-Gua) resulting from alkylation at N-7 of 2'-deoxyguanosine followed by depurination. Two unusual adducts were also partially characterized which resulted from further reaction of 7-CHP-Gua with another molecule of ECH to yield 1,7-bis(3-chloro-2-hydroxypropyl)guanine (1,7-bis-CHP-Gua) which could then cyclize with the exocyclic amino group to yield 1,N2-(2-hydroxypropano)-7-(3-chloro-2-hydroxypropyl) guanine (1,N2-HP-7-CHP-Gua). Reaction with 2'-deoxyadenosine gave only one product, namely 1,N6-(2-hydroxypropano)-2'-deoxyadenosine (1,N6-HP-dAdo). The reaction of 2'-deoxythymidine with ECH also yielded one product which was identified as 3-(3-chloro-2-hydroxypropyl)-2'-deoxythymidine (3-CHP-dThd). A 3-(3-chloro-2-hydroxypropyl)-2'-deoxyuridine (3-CHP-dUrd) product was isolated from the reaction of ECH with 2'-deoxycytidine. This product most likely resulted from the deamination of an initially formed 3-(3-chloro-2-hydroxypropyl) -2'-deoxycytidine (3-CHP-dCyd), a phenomenon which we have previously reported to occur during the reaction of 2'-deoxycytidine with other aliphatic epoxides. Evidence is also presented that 3-CHP-dUrd is converted to 3-(2,3-dihydroxypropyl)-2'deoxyuridine (3-DHP-dUrd) under physiological conditions, with a half-life of 213 h. Reaction of ECH with calf thymus DNA (pH 7.0, 37 degrees C, 3 h) resulted in the formation of 7-CHP-Gua (200 nmol/mg DNA.