Sister chromatid exchange in murine alveolar macrophages, bone marrow, and regenerating liver cells induced by styrene inhalation

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.

Perspectives on the genotoxic risk of styrene

Styrene is a highly reactive monomer widely used in the plastics industry. The potential for styrene to produce genotoxic effects has been studied extensively in experimental systems. Styrene can induce sister chromatid exchanges (SCE) and chromosome aberrations (CA) in vitro under test conditions that enhance metabolism of styrene to styrene 7,8-oxide (SO)or reduce detoxification of 50 by epoxide hydrolase. The in vivo animal data indicate that styrene is not clastogenic at concentrations (doses) likely encountered by humans under ambient or occupational exposure conditions. DNA binding studies with styrene in rats and mice demonstrated no increased adducts in mice compared to rats or in mouse lung compared to liver. As a result, DNA adducts in the lungs are unlikely to be the sole explanation of the development of lung tumors in mice exposed to styrene for 2 yr. Some epidemiological studies reported that DNA and/or protein adducts and DNA strand breaks result from occupational exposure to styrene and/or 50. Results of some of these studies, how-ever, are difficult to interpret, given that the statistical significance of reported effects (SCE, CA, and micronucleus formation) was often near or at p values of .05; dose and/or temporal response relationships often were missing; confounding variables could not be excluded; and, concomitant exposures to other industrial chemicals that are potentially genotoxic may also have occurred. These studies suggest that styrene, through metabolism to SO, could be clastogenic in humans at workplace levels in excess of 125 mg/m3. However, results from controlled animal studies involving in vivo exposure to styrene alone do not show clastogenic effects at exposures of up to 1500 mg/m3/d. In any event, these studies show that there is an apparent threshold for styrene-mediated effects.

Review of the in vivo genotoxicity tests performed with styrene

Results from new genotoxicity tests in laboratory animals have necessitated a comprehensive re-evaluation of the mutagenic potential of styrene in vivo. Available data suggest that styrene, after being metabolized to styrene oxide, is weakly positive in indicator tests detecting DNA adducts, DNA strand breaks and sister chromatid exchanges (SCEs). There is no convincing evidence of styrene clastogenicity in experimental animals when the quality of the studies and the plausibility of the test results are considered. Equivocal results were obtained after exposure to high doses causing lethality. A recently published in vivo micronucleus test (MNT) in bone marrow cells of mice conforming to the current OECD guideline was clearly negative. Consequently, our evaluation of the published genotoxicity data comes to the conclusion that styrene at high doses can induce genotoxic effects in indicator tests. These DNA effects depend upon the exposure levels of the target cells, the metabolic activation to styrene oxide and the efficiency of detoxification. Mutagenic effects of styrene can only be expected under extreme exposure conditions if styrene oxide is not efficiently detoxified and primary DNA lesions are not completely repaired. However, there is no clear evidence that styrene induces mutagenic/clastogenic effects in vivo when tested under appropriate test conditions.

Micronucleus assay in pulmonary alveolar macrophages, a simple model to detect genotoxicity of environmental agents entering through the inhalation route

A simple and short-term micronucleus (MN) test in pulmonary alveolar macrophages (PAMs) of rats has been developed to assess potential genotoxic effects of gaseous environmental agents. The protocol has been tested in model experiments with indoor air pollutants like mosquito coil smoke (MCS) and mosquito mat vapour (MMV). Smears of pulmonary lavage fluid collected in hypotonic (0.56%) KCl solution were fixed in absolute methanol and stained in Giemsa (10%). Characteristically the large size of the PAMs facilitates easy scoring of MN. An interval of 32 h post exposure seems to be suitable for MN preparation. A comparison of the concentration-response data on CAs (at 24 h post exposure) and MN (at 32 h post exposure) clearly reveals the validity of the MN assay in PAMs.

A re-evaluation of the cytogenetic effects of styrene

Results from new chromosome studies in laboratory animals, comparative investigations of styrene metabolism and pharmacokinetics in humans and animals, and several recent cytogenetic surveys of styrene-exposed workers have necessitated a comprehensive re-evaluation of the chromosome-damaging effects of this chemical. Both styrene and its genotoxic metabolite, styrene oxide, can induce chromosome aberrations (CA) and sister chromatid exchanges (SCE) in vitro, but the chromosome-damaging ability of styrene is only manifested if test conditions favour its metabolic activation over inactivation. There is no convincing evidence of styrene clastogenicity in experimental animals. Styrene oxide is clastogenic only at lethal concentrations via i.p. injection in Chinese hamsters (but not via inhalation) or after oral treatment of mice, a route considered inappropriate for investigating the chromosome-damaging potential of inhaled styrene in man. Styrene and styrene oxide can induce SCE in animals at very high concentrations. Eighteen of 52 cytogenetic studies (CA, micronuclei, SCE) on peripheral blood lymphocytes of styrene workers have reported increases in chromosome damage. The positive findings are not compatible with the conclusion that styrene is responsible for the cytogenetic effects for the following reasons. (a) The positive or negative outcome of the various investigations bears no relationship to the degree of exposure of the workers. (b) There is no convincing evidence of a positive dose response relationship. (c) The relative induction of CA and SCE in worker studies are the opposite of observations of styrene effects in cultured lymphocytes and in laboratory animals. (d) The reports of chromosome-type exchanges in some studies of styrene workers is inconsistent with observations of styrene clastogenicity in cultured lymphocytes. (e) Reports of SCE induction in workers exposed to low concentrations of styrene are not compatible with results of animal inhalation studies, particularly in view of the differences in styrene metabolism and pharmacokinetics between humans and rodents. The increases in cytogenetic effects reported in some studies on styrene workers are probably attributable to the presence of other chromosome-damaging agents in the workplace and/or to inadequate investigations.

Induction of chromosome aberrations and micronuclei in pulmonary alveolar macrophages of rats following inhalation of mosquito coil smoke

The genotoxic potential of inhalation of mosquito coil (MC) smoke was evaluated by using metaphase chromosome aberration and micronucleus assays in pulmonary alveolar macrophages (PAMs) of rats following short-term as well as long-term whole body intermittent exposure. For short-term exposure, the animals were exposed for 15 min/h, 8 h/day to smoke collected for 1, 5 or 10 min, and they were killed 16 or 24 h after the final exposure. For long-term exposure, they were exposed for 15 min/h, 8 h/day, 7 days/week to smoke collected for 10 min and then they were killed 24 h after the final exposure. Each time before exposure, fresh smoke was collected by burning a mosquito coil. Pulmonary lavage was collected, and conventional flame-drying preparation was done for metaphase chromosome analysis and micronuclei (MN) were analyzed from smear preparations. Significantly higher frequencies of chromosome aberrations, including as well as excluding gaps, and micronucleated PAMs in smoke-exposed animals, compared to controls, indicated genotoxic capacity of MC smoke. The increases significantly correlated with the "concentration" of the gas. Mitotic indices also showed a significant and concentration-dependent increase. The frequencies of chromosome aberrations and MN following 7-day exposure were very similar to those for 1-day exposure. This was probably due to the transient nature of PAMs. A post-exposure gap of 24 h, compared to the 16-h gap, yielded a higher incidence of both mitoses and chromosome aberrations.

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

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

Dose-dependent genotoxic effects of styrene on human blood lymphocytes and the relationship to its oxidative and metabolic effects

Although the genotoxic potential of styrene is known, very limited information is available regarding its dose-dependent genotoxic response to human blood lymphocytes and how such response correlates with different metabolic events in whole blood lymphocytes. The present study was therefore carried out to study such a relationship using in vitro human blood lymphocytes from healthy volunteers. To study genotoxic response to styrene, sister chromatid exchanges (SCEs), cell cycle, and cell survival were analyzed. Lymphocytes were cultured for 72 hr in the presence of different concentrations of styrene (0-1,000 microM). Twenty-four hr before harvest, BrdU (5 micrograms/ml) was added to assess the increase in SCEs and cell cycle delay. Both the SCE frequency and the cell cycle length were increased linearly with increasing concentrations of styrene up to 200 microM, without addition of any exogenous metabolizing system. Above 200 microM, no further increase in genotoxic response occurred. The range of concentrations (10-200 microM) at which increase of cell cycle length due to styrene was observed did not impair the viability of the cells, suggesting that such cell cycle delay is a genotoxic-related event and not caused by cytotoxicity. In vitro metabolic transformation of styrene in whole-blood lymphocyte cultures without the presence of any exogenous metabolic activation system showed the formation of a reactive intermediate, styrene 7,8-oxide, to be capacity-limited, as verified from a nonlinear increase in the formation of styrene glycol. The value of such metabolic parameter reached a plateau above 200 microM styrene. The same phenomenon of saturation has also been observed with regard to other metabolic effects due to styrene in whole blood lymphocytes in culture, such as dose-dependent increase in lipid peroxidation and depletion of blood lymphocyte glutathione. Based on the relationship between the formation of different metabolic events and the genotoxicity of styrene, it may be possible that the genotoxic properties of styrene in human blood lymphocytes may be mediated initially not only by the formation of the presumably reactive styrene 7,8-oxide, but also by that of a reactive oxygen species as well. However, the present data are not sufficient enough to definitely identify the role of reactive oxygen species in such toxicity and therefore it warrants further study.

Cytogenetic studies of mice exposed to styrene by inhalation

The data for the in vivo genotoxicity of styrene (STY) are equivocal. To evaluate the clastogenicity and sister-chromatid exchange (SCE)-inducing potential of STY in vivo under carefully controlled conditions, B6C3F1 female mice were exposed by inhalation for 6 h/day for 14 consecutive days to either 0, 125, 250 or 500 ppm STY. One day after the final exposure, peripheral blood, spleen, and lungs were removed and cells were cultured for the analysis of micronucleus (MN) induction using the cytochalasin B-block method, chromosome breakage, and SCE induction. Peripheral blood smears were also made for scoring MN in erythrocytes. There was a significant concentration-related elevation of SCE frequency in lymphocytes from the spleen and the peripheral blood as well as in cells from the lung. However, no statistically significant concentration-related increases were found in the frequency of chromosome aberrations in the cultured splenocytes or lung cells, and no significant increases in MN frequencies were observed in binucleated splenocytes or normochromatic erythrocytes in peripheral blood smears.

Haemopoietic stem cells in mice chronically exposed to styrene vapour

Female C57BL/6X DBA/2 hybrid mice were exposed to two concentrations of styrene in inhalation chambers for 6 h/day, 5 days per week. The concentrations were 220 ppm for a period of up to 8 weeks and 440 ppm for 2 days, followed by 330 ppm for up to 8 weeks due to early deaths after 440 ppm. Parallel groups were also treated with 5% ethanol in the drinking water during the days of styrene exposure. In the peripheral blood only a lymphocytopenia was found. The numbers of pluripotent haemopoietic stem cells, CFU-S, were unaffected, as were granuloid committed stem cells, CFU-C. Early and late erythroid stem cell numbers, BFU-E and CFU-E, showed high variability. At 220 ppm about half of the animals had CFU-E numbers below 80% of controls, at 440/330 ppm about two-thirds of the animals had BFU-E and CFU-E numbers below this level. An additional effect of ethanol was not found.

Cytogenetic effects of cigarette smoke on pulmonary alveolar macrophages of the rat

To determine accurately the potential genetic damage induced by toxic inhaled agents, the cells that receive a high concentration of such agents should be analyzed. Pulmonary alveolar macrophages (PAMs) represent such cells. We compared the cytogenetic effects of cigarette smoke on PAMs of rats exposed repeatedly by different methods. This study was part of a larger investigation of the health effects resulting from different methods of exposing rats to cigarette smoke. Fischer 344/N male rats (4/group) were randomly selected from five different exposure groups: 1) nose-only sham-exposed (air) control, 2) whole-body sham-exposed control, 3) nose-only intermittent, 4) nose-only continuous, and 5) whole-body continuous. The rats were exposed 6 hr/day, 5 days/week for 22-24 days. All smoke-exposed rats received the same daily concentration x time product (600 mg.hr.m-3 for the first week, 1200 mg.hr.m-3 thereafter) of cigarette smoke. Rats were injected intraperitoneally with colchicine at the end of exposure. PAMs were obtained by lung lavage and chromosomal damage was measured. Highly significant smoke-induced differences in both structural and numerical aberrations were observed in continuously exposed rats vs. sham controls, regardless route of exposure. The structural aberrations observed were chromatid-type deletions. Both hypoploid and hyperploid cells were detected. Our data suggest that cigarette smoke is clastogenic and may disrupt spindle-fiber formation. These activities may play a role in the induction of human carcinogenesis caused by cigarette smoke exposure.

Sister chromatid exchange and chromosome aberration analyses in mice after in vivo exposure to acrylonitrile, styrene, or butadiene monoxide

The use of polymers in plastic and rubber products has generated concern that monomers potentially active in biological systems may be eluted from these substances. We have evaluated two such monomers, acrylonitrile and styrene, for the induction of chromosome damage in mice. Butadiene monoxide, a presumed metabolite of a third important monomer, 1,3-butadiene, was also tested. These chemicals were administered as a single intraperitoneal injection; sister chromatid exchanges and chromosome aberrations were analyzed in bone marrow cells. Acrylonitrile and styrene were largely negative for these endpoints when tested at doses ranging to 60 mg/kg and 1,000 mg/kg, respectively. Butadiene monoxide, which previously has not been tested in a mammalian system, was determined to be a very effective inducer of sister chromatid exchanges and chromosome aberrations. Both endpoints showed a clear dose response and a greater than ten-fold increase over control levels at high doses. These studies represent an initial step in our efforts to evaluate genetic risk associated with exposure to common polymeric chemicals.

Dose-responsive increase in sister-chromatid exchanges in bone-marrow cells of mice exposed nose-only to whole cigarette smoke

The effect of whole cigarette smoke exposure on bone-marrow sister-chromatid exchanges (SCEs) was studied in B6C3F1 mice. Animals were exposed nose-only to 10% (v/v) cigarette smoke 5 days/week for 2 weeks. Four dose levels of cigarette smoke (1, 4, 9 and 18 exposures/day) were studied using 2 cigarette types, Kentucky reference 3A1 (3A1) and American Blend (AB). A single exposure represented approximately 1 cigarette. A dose-dependent increase in SCEs was observed for both the 3A1 and AB cigarettes at dose levels which had no effect on bone-marrow cell-replication kinetics. These findings represent the first demonstration of a dose-responsive increase in cigarette smoke-induced SCEs in a rodent model system.

Effect of blood on styrene oxidation in perfused rat liver

The oxidation of styrene to styrene oxide was studied in the isolated perfused rat liver in the presence and absence of blood at styrene concentrations of 2.5 and 50 mM. Erythrocytes contained in whole blood increased the levels of styrene glycol about 5 times after a short perfusion time with both concentrations. This increase was observed up to 1 h with 2.5 mM styrene. At both styrene concentrations styrene oxide was not detectable, either in the presence or absence of blood indicating that the liver was able completely to detoxify the styrene oxide produced by the mixed-function oxidases (MFO) and the oxyhemoglobin in the erythrocytes.

Increased sister-chromatid exchange in bone-marrow cells of mice exposed to whole cigarette smoke

Using defined cigarette smoke exposure conditions, BC3F1/Cum mice were exposed nose-only to two different types of whole cigarette smoke on a daily basis for 1 week and up to 46 weeks. The number of sister-chromatid exchanges (SCEs) per metaphase was determined in bone-marrow cells. Studies were scheduled so that all cytogenetic observations were made 2-3 days after the last smoke exposure. Exposure to either type of smoke on a daily basis for 1 week or up to 46 weeks resulted in a 2-fold increase in SCEs over sham-exposed control mice. In animals exposed either chronically or for 1 week to either type of smoke, the increase in SCEs persisted for at least 1 week after cessation of smoke exposure. This is the first demonstration of the induction of SCEs in laboratory animals that have been exposed to cigarette smoke in vivo.

Bone marrow cell chromosomal aberrations and styrene biotransformation in mice given styrene on a repeated oral schedule

Styrene's capacity to induce chromosomal aberrations was studied in bone marrow cells of CD1 male mice. No mutagenic effect could be detected after either a 4-day treatment course with daily oral doses of 500 mg/kg or a 70-day course with daily oral doses of 200 mg/kg. Urinary elimination of styrene metabolites related to styrene-7,8-oxide formation (i.e. phenylethylene glycol, mandelic acid, benzoic acid, phenylglyoxylic acid and total mercapturic acids) was quantitatively evaluated in the group of mice given the 200 mg/kg dose. In parallel, kinetic studies were made on styrene and styrene-7,8-oxide blood concentrations in the same group of animals. These determinations were carried out on days 1 and 70 of treatment by spectrophotometric, gas chromatographic and mass fragmentographic procedures. Not even nanograms of styrene-7,8-oxide were found in the blood of styrene-treated mice. This suggests that the metabolite does not migrate from the cellular compartment where it is formed being immediately metabolized or irreversibly bound to cellular structures. This observation could well explain the lack of mutagenic effects observed.

Styrene oxidation to styrene oxide by hydroxyl radicals produced during reaction of xanthine with xanthine oxidase in the presence of Fe3+

Styrene was oxidized to styrene oxide during reaction of xanthine (X) with xanthine oxidase (XO) in the presence of Fe3+. This reaction showed a dose-dependent requirement of iron and was inhibited by superoxide dismutase (SOD) and catalase, indicating that both the superoxide anion and H2O2 were essential. Styrene oxide production was inhibited by hydroxyl radical scavengers indicating that this reactive oxygen intermediate could be the proximal oxidant involved in styrene oxidation to styrene oxide.

Chromosome aberrations and sister chromatid exchanges in styrene-exposed workers with reference to their smoking habits

The incidences of chromosome aberrations and the frequencies of sister chromatid exchanges (SCE) were investigated in cultured lymphocytes of 18 styrene-exposed workers in comparison with six controls. There was a marginal increase in the incidence of structural chromosomal aberrations in first-division metaphases in the styrene-exposed workers, as compared with the nonexposed controls. However, there was no difference in SCE frequencies. When each group was divided into smokers and nonsmokers, styrene-exposed smokers tended to have higher SCE frequencies than styrene-exposed nonsmokers. Furthermore, cell proliferation was inhibited in styrene-exposed workers (both smokers and nonsmokers) and control smokers.

Ferrodoxin reductase catalyzes styrene oxidation to styrene oxide

The flavoprotein ferredoxin reductase catalyzed the oxidation of styrene to styrene oxide in the presence of NADPH. This reaction was inhibited by the addition of catalase and superoxide dismutase. The addition of the nonheme iron protein ferredoxin partially inhibited styrene oxidation. H2O2 was also able to catalyze this reaction when added to the enzyme in the absence of NADPH.

Sister chromatid exchange (SCE) and structural chromosome aberration in mutagenicity testing

Data from previous studies published on the induction by mutagens of sister chromatid exchanges (SCEs) and structural chromosome damage were compared qualitatively and quantitatively. Although a good correlation between the incidence of both cytogenetic phenomena has been pointed out in many previous publications, about 30% of the agents for which comparable data were available yielded non-corresponding qualitative results concerning both indicator effects. However, even in groups with good qualitative agreement distinct quantitative differences indicated different molecular mechanisms of the formation of SCEs and breaks. Additional information supporting the importance of these differences for the validity of both indicator systems has been derived from the results obtained using strong clastogens exhibiting a low or no SCE-inducing activity and vice versa, from special observations on chromosomal breakage syndromes, and from studies on the action of known co- and anti-clastogens on SCE-induction by chemical mutagens. As a result, it has been suggested that the SCE-technique should be considered as a valuable additional method for cytogenetic mutagenicity testing, which, however, is not adequate to replace the classical methods of analysis of structural chromosome damage.

Styrene and vinyltoluene induce micronuclei in mouse bone marrow

Male C57BL/6 mice were treated i.p. with a single dose of styrene (250, 500, 1000 or 1500 mg/kg b.wt; 4 animals/group), vinyltoluene (100, 200, 300 or 500 mg/kg b.wt; 4-5 animals/group) or olive oil (controls, 13 animals). A significant increase in micronucleated polychromatic erythrocytes was observed at 250 and 1000 mg/kg b.wt of styrene and at 200, 300 and 500 mg/kg b.wt of vinyltoluene. 1000 and 1500 mg/kg b.wt of styrene and 100, 200 and 300 mg/kg b.wt of vinyltoluene significantly decreased the ratio of polychromatic to normochromatic erythrocytes. There was no increase in normochromatic cells with micronuclei.

Multicellular in vivo sister-chromatid exchanges induced by urethane

Following urethane inhalation exposure, clear dose--response relationships were apparent in all cell types examined in hepatectomized and intact mice. At concentrations of 0.1 mg/l and higher, induced SCE frequencies were linearly related to log urethane concentrations. No significant differences in SCE response between like cell types of hepatectomized and intact mice were apparent. In hepatectomized mice, there was no significant difference in the SCE response of regenerating liver and alveolar macrophage cells. However, bone-marrow response was significantly lower (p = 0.01). Likewise, in intact mice bone marrow response was significantly lower than in alveolar macrophages (p = 0.01). Inhalation and intravenous infusions of the same total dose of urethane (193 mg/kg) administered over a 4-h period produced comparable SCE responses in all cell types. However, a single intraperitoneal injection of 193 mg/kg just prior to BrdU infusion produced significantly higher SCE frequencies in bone marrow (alpha = 0.01), and alveolar macrophages (alpha = 0.05) of intact mice than did the equivalent inhalation dose. Intraperitoneal injections produced similar results in 2-month-old mice as in 4-month-old mice. However, regardless of the route of administration SCE frequencies in regenerating liver and/or alveolar macrophages were significantly higher than in bone marrow.