The genetic toxicology of styrene and styrene oxide

Microplastics in the Human Body: Exposure, Detection, and Risk of Carcinogenesis: A State-of-the-Art Review

Background: Humans cannot avoid plastic exposure due to its ubiquitous presence in the natural environment. The waste generated is poorly biodegradable and exists in the form of MPs, which can enter the human body primarily through the digestive tract, respiratory tract, or damaged skin and accumulate in various tissues by crossing biological membrane barriers. There is an increasing amount of research on the health effects of MPs. Most literature reports focus on the impact of plastics on the respiratory, digestive, reproductive, hormonal, nervous, and immune systems, as well as the metabolic effects of MPs accumulation leading to epidemics of obesity, diabetes, hypertension, and non-alcoholic fatty liver disease. MPs, as xenobiotics, undergo ADMET processes in the body, i.e., absorption, distribution, metabolism, and excretion, which are not fully understood. Of particular concern are the carcinogenic chemicals added to plastics during manufacturing or adsorbed from the environment, such as chlorinated paraffins, phthalates, phenols, and bisphenols, which can be released when absorbed by the body. The continuous increase in NMP exposure has accelerated during the SARS-CoV-2 pandemic when there was a need to use single-use plastic products in daily life. Therefore, there is an urgent need to diagnose problems related to the health effects of MP exposure and detection. Methods: We collected eligible publications mainly from PubMed published between 2017 and 2024. Results: In this review, we summarize the current knowledge on potential sources and routes of exposure, translocation pathways, identification methods, and carcinogenic potential confirmed by in vitro and in vivo studies. Additionally, we discuss the limitations of studies such as contamination during sample preparation and instrumental limitations constraints affecting imaging quality and MPs detection sensitivity. Conclusions: The assessment of MP content in samples should be performed according to the appropriate procedure and analytical technique to ensure Quality and Control (QA/QC). It was confirmed that MPs can be absorbed and accumulated in distant tissues, leading to an inflammatory response and initiation of signaling pathways responsible for malignant transformation.

The genotoxicity of an organic solvent mixture: A human biomonitoring study and translation of a real-scenario exposure to in vitro

This study aimed to evaluate occupational exposure to a styrene and xylene mixture through environmental exposure assessment and identify the potential genotoxic effects through biological monitoring. Secondly, we also exposed human peripheral blood cells in vitro to both xylene and styrene either alone or in mixture at concentrations found in occupational settings in order to understand their mechanism of action. The results obtained by air monitoring were below the occupational exposure limits for both substances. All biomarkers of effect, except for nucleoplasmic bridges, had higher mean values in workers (N = 17) compared to the corresponding controls (N = 17). There were statistically significant associations between exposed individuals and the presence of nuclear buds and oxidative damage. As for in vitro results, there was no significant influence on primary DNA damage in blood cells as evaluated by the comet assay. On the contrary, we did observe a significant increase of micronuclei and nuclear buds, but not nucleoplasmic bridges upon in vitro exposure. Taken together, both styrene and xylene have the potential to induce genomic instability either alone or in combination, showing higher effects when combined. The obtained data suggested that thresholds for individual chemicals might be insufficient for ensuring the protection of human health.

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.

Health risk assessment of occupational exposure to styrene in Neyshabur electronic industries

Styrene is one of the essential components in making thousands of everyday products. Occupational exposure to styrene causes pulmonary, neurological, genetic and ocular complications, and leukemia and affects reproduction. The aim of this study was to assess the health risks of exposure to styrene in the electronics industry of Neyshabur, Iran. This descriptive and analytical cross-sectional study was carried out in three electronics industries, in Neyshabur city, in 2017-2018. Occupational exposure to styrene was measured according to the NIOSH1501 method, using a low-flow rate sampling pump (0.2 L/min) and an active charcoal absorber tube. Health risk assessment was done according to the Singapore semi-quantitative method and the United States Environmental Protection Agency (OEHHA) method. The average occupational exposure to styrene in men employed in the compact plastic parts production halls was 79.61 mg m^-3 (range 28-208.33). 45.8% of exposed subjects (27 people) encountered exposure above the permitted limit. The average lifetime carcinogenic risk of styrene was 1.4 × 10^-3; therefore, 100% (59 people) had a definite risk of getting cancer. The highest lifetime risk of getting cancer was observed in plastic injection device users (1.9 × 10^-3) and then in shift managers (1.6 × 10^-3). The results of this study indicate a definite risk of getting cancer for all workers. Strategies to reduce workers exposure to styrene through engineering controls and routine measurements are necessary.

Subcellular localization of rat CYP2E1 impacts metabolic efficiency toward common substrates

Cytochrome P450 2E1 (CYP2E1) detoxifies or bioactivates many low molecular-weight compounds. Most knowledge about CYP2E1 activity relies on studies of the enzyme localized to endoplasmic reticulum (erCYP2E1); however, CYP2E1 undergoes transport to mitochondria (mtCYP2E1) and becomes metabolically active. We report the first comparison of in vitro steady-state kinetic profiles for erCYP2E1 and mtCYP2E1 oxidation of probe substrate 4-nitrophenol and pollutants styrene and aniline using subcellular fractions from rat liver. For all substrates, metabolic efficiency changed with substrate concentration for erCYP2E1 reflected in non-hyperbolic kinetic profiles but not for mtCYP2E1. Hyperbolic kinetic profiles for the mitochondrial enzyme were consistent with Michaelis-Menten mechanism in which metabolic efficiency was constant. By contrast, erCYP2E1 metabolism of 4-nitrophenol led to a loss of enzyme efficiency at high substrate concentrations when substrate inhibited the reaction. Similarly, aniline metabolism by erCYP2E1 demonstrated negative cooperativity as metabolic efficiency decreased with increasing substrate concentration. The opposite was observed for erCYP2E1 oxidation of styrene; the sigmoidal kinetic profile indicated increased efficiency at higher substrate concentrations. These mechanisms and CYP2E1 levels in mitochondria and endoplasmic reticulum were used to estimate the impact of CYP2E1 subcellular localization on metabolic flux of pollutants. Those models showed that erCYP2E1 mainly carries out aniline metabolism at all aniline concentrations. Conversely, mtCYP2E1 dominates styrene oxidation at low styrene concentrations and erCYP2E1 at higher concentrations. Taken together, subcellular localization of CYP2E1 results in distinctly different enzyme activities that could impact overall metabolic clearance and/or activation of substrates and thus impact the interpretation and prediction of toxicological outcomes.

Cooperativity in CYP2E1 metabolism of acetaminophen and styrene mixtures

Risk assessment for exposure to mixtures of drugs and pollutants relies heavily on in vitro characterization of their bioactivation and/or metabolism individually and extrapolation to mixtures assuming no interaction. Herein, we demonstrated that in vitro CYP2E1 metabolic activation of acetaminophen and styrene mixtures could not be explained through the Michaelis-Menten mechanism or any models relying on that premise. As a baseline for mixture studies with styrene, steady-state analysis of acetaminophen oxidation revealed a biphasic kinetic profile that was best described by negative cooperativity (Hill coefficient=0.72). The best-fit mechanism for this relationship involved two binding sites with differing affinities (Ks=830μM and Kss=32mM). Introduction of styrene inhibited that reaction less than predicted by simple competition and thus provided evidence for a cooperative mechanism within the mixture. Likewise, acetaminophen acted through a mixed-type inhibition mechanism to impact styrene epoxidation. In this case, acetaminophen competed with styrene for CYP2E1 (Ki=830μM and Ksi=180μM for catalytic and effector sites, respectively) and resulted in cooperative impacts on binding and catalysis. Based on modeling of in vivo clearance, cooperative interactions between acetaminophen and styrene resulted in profoundly increased styrene activation at low styrene exposure levels and therapeutic acetaminophen levels. Current Michaelis-Menten based toxicological models for mixtures such as styrene and acetaminophen would fail to detect this concentration-dependent relationship. Hence, future studies must assess the role of alternate CYP2E1 mechanisms in bioactivation of compounds to improve the accuracy of interpretations and predictions of toxicity.

Cooperative effects for CYP2E1 differ between styrene and its metabolites

Cooperative interactions are frequently observed in the metabolism of drugs and pollutants by cytochrome P450s; nevertheless, the molecular determinants for cooperativity remain elusive. Previously, we demonstrated that steady-state styrene metabolism by CYP2E1 exhibits positive cooperativity. We hypothesized that styrene metabolites have lower affinity than styrene toward CYP2E1 and limited ability to induce cooperative effects during metabolism. To test the hypothesis, we determined the potency and mechanism of inhibition for styrene and its metabolites toward oxidation of 4-nitrophenol using CYP2E1 Supersomes® and human liver microsomes. Styrene inhibited the reaction through a mixed cooperative mechanism with high affinity for the catalytic site (67 µM) and lower affinity for the cooperative site (1100 µM), while increasing substrate turnover at high concentrations. Styrene oxide and 4-vinylphenol possessed similar affinity for CYP2E1. Styrene oxide behaved cooperatively like styrene, but 4-vinylphenol decreased turnover at high concentrations. Styrene glycol was a very poor competitive inhibitor. Among all compounds, there was a positive correlation with binding and hydrophobicity. Taken together, these findings for CYP2E1 further validate contributions of cooperative mechanisms to metabolic processes, demonstrate the role of molecular structure on those mechanisms and underscore the potential for heterotropic cooperative effects between different compounds.

CYP2E1 metabolism of styrene involves allostery

We are the first to report allosterism during styrene oxidation by recombinant CYP2E1 and human liver microsomes. At low styrene concentrations, oxidation is inefficient because of weak binding to CYP2E1 (K(s) = 830 μM). A second styrene molecule then binds CYP2E1 with higher affinity (K(ss) = 110 μM) and significantly improves oxidation to achieve a k(cat) of 6.3 nmol · min(-1) · nmol CYP2E1(-1). The transition between these metabolic cycles coincides with reported styrene concentrations in blood from exposed workers; thus, this CYP2E1 mechanism may be relevant in vivo. Scaled modeling of the in vitro-positive allosteric mechanism for styrene metabolism to its in vivo clearance led to significant deviations from the traditional model based on Michaelis-Menten kinetics. Low styrene levels were notably much less toxic than generally assumed. We interrogated the allosteric mechanism using the CYP2E1-specific inhibitor and drug 4-methylpyrazole, which we have shown binds two CYP2E1 sites. From the current studies, styrene was a positive allosteric effector on 4-methylpyrazole binding, based on a 10-fold increase in 4-methylpyrazole binding affinity from K(i) 0.51 to K(si) 0.043 μM. The inhibitor was a negative allosteric effector on styrene oxidation, because k(cat) decreased 6-fold to 0.98 nmol · min(-1) · nmol CYP2E1(-1). Consequently, mixtures of styrene and other molecules can induce allosteric effects on binding and metabolism by CYP2E1 and thus mitigate the efficiency of their metabolism and corresponding effects on human health. Taken together, our elucidation of mechanisms for these allosteric reactions provides a powerful tool for further investigating the complexities of CYP2E1 metabolism of drugs and pollutants.

DNA damage and susceptibility assessment in industrial workers exposed to styrene

Styrene is a widely used chemical in the manufacture of synthetic rubber, resins, polyesters, and plastics. The highest levels of human exposure to styrene occur during the production of reinforced plastic products. The objective of this study was to examine occupational exposure to styrene in a multistage approach, in order to integrate the following endpoints: styrene in workplace air, mandelic and phenylglyoxylic acids (MA + PGA) in urine, sister chromatid exchanges (SCE), micronuclei (MN), DNA damage (comet assay), and genetic polymorphisms of metabolizing enzymes (CYP2E1, EPHX1, GSTM1, GSTT1, and GSTP1). Seventy-five workers from a fiberglass-reinforced plastics factory and 77 unexposed controls took part in the study. The mean air concentration of styrene in the breathing zone of workers (30.4 ppm) and the mean concentration of urinary metabolites (MA + PGA = 443 ± 44 mg/g creatinine) exceeded the threshold limit value (TLV) and the biological exposure index (BEI). Significantly higher SCE frequency rate and DNA damage were observed in exposed workers, but MN frequency was not markedly modified by exposure. With respect to the effect of genetic polymorphisms on different exposure and effect biomarkers studied, an increase in SCE levels with elevated microsomal epoxide hydrolase activity was noted in exposed workers, suggesting a possible exposure-genotype interaction.

Cytogenetic and DNA damage on workers exposed to styrene

Styrene is a commercially important chemical widely used in the manufacture of synthetic rubber, resins, polyesters and plastics. The highest levels of human exposure to styrene occur during the production of reinforced plastic products. The objective of this work was to evaluate both DNA and cytogenetic damage in styrene-exposed workers, analysing only non-smoker individuals. Environmental levels of styrene and urinary concentrations of mandelic and phenylglyoxylic acids were determined, and genetic damage was studied by means of micronucleus (MN) test, sister chromatid exchanges (SCEs) and comet assay. Fifty-two fibreglass-reinforced plastics workers and 54 controls took part in the study. The mean air concentration of styrene in the breathing zone of workers exceeded the threshold limit value, and 24 workers exceeded the biological exposure index. A strong and significant correlation was found between styrene environmental concentrations and urinary metabolites. Higher SCE rate (P<0.01) was observed in exposed workers than in controls. Besides, significant correlations were obtained for SCE rate with both environmental and internal exposure parameters (r=0.496, P<0.01 and r=0.511, P<0.01, respectively). Results from MN test and comet assay showed slight and non-significant increases related to the exposure. Our data seem to support previous studies reporting genotoxicity associated with occupational exposure to styrene, excluding the confounding influence of smoking, although caution must be taken in the interpretation of these results since the significance of an increase in SCE rate is still unclear.

Preliminary study on the monitoring of glutathione S-tranferase activity toward styrene oxide by electromigration methods

A new method has been developed for the monitoring of glutathione S-tranferase (GST) detoxification activity toward styrene oxide (SO). The enzymatic reaction was carried out directly in a thermostatted autosampler vial and the formation of conjugates between glutathione (GSH) and SO was monitored by sequential MEKC runs. The determinations were performed in a 50-microm fused silica capillary using 50 mM SDS in 20 mM phosphate 20 mM tetraborate buffer (pH 8.3) as a background electrolyte; separation voltage 28 kV (positive polarity), temperature of capillary 25 degrees C, and detection at 200 nm. The method is rapid, amenable to automation, and requires only small amounts of samples, which is especially important in the case of GST isoenzyme analyses.

Chiral separations of mandelic acid by HPLC using molecularly imprinted polymers

Styrene is used in a variety of chemical industries. Environmental and occupational exposures to styrene occur predominantly through inhalation. The major metabolite of styrene is present in two enantiomeric forms, chiral R- and S- hydroxy-1-phenyl-acetic acid (R-and S-mandelic acid, MA). Thus, the concentration of MA, particularly of its enantiomers, has been used in urine tests to determine whether workers have been exposed to styrene. This study describes a method of analyzing mandelic acid using molecular imprinting techniques and HPLC detection to perform the separation of diastereoisomers of mandelic acid. The molecularly imprinted polymer (MIP) was prepared by non-covalent molecular imprinting using (+) MA, (-) MA or (+) phenylalanine, (-) phenylalanine as templates. Methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) were copolymerized in the presence of the template molecules. The bulk polymerization was carried out at 4ºC under UV radiation. The resulting MIP was grounded into 25~44¼m particles, which were slurry packed into analytical columns. After the template molecules were removed, the MIP-packed columns were found to be effective for the chromatographic resolution of (±)-mandelic acid. This method is simpler and more convenient than other chromatographic methods.

Kinetics of nucleotide incorporation opposite DNA bulky guanine N2 adducts by processive bacteriophage T7 DNA polymerase (exonuclease-) and HIV-1 reverse transcriptase

Six oligonucleotides with carcinogen derivatives bound at the N2 atom of deoxyguanosine were prepared, including adducts derived from butadiene, acrolein, crotonaldehyde, and styrene, and examined for effects on the replicative enzymes bacteriophage DNA polymerase T7- (T7-) and HIV-1 reverse transcriptase for comparison with previous work on smaller DNA adducts. All of these adducts strongly blocked dCTP incorporation opposite the adducts. dATP was preferentially incorporated opposite the acrolein and crotonaldehyde adducts, and dTTP incorporation was preferred at the butadiene- and styrene-derived adducts. Steady-state kinetic analysis indicated that the reduced catalytic efficiency with adducted DNA involved both an increased Km and attenuated kcat. Fluorescence estimates of Kd and pre-steady-state kinetic measurements of koff showed no significantly decreased affinity of T7- with the adducted oligonucleotides or the dNTP. Pre-steady-state kinetics showed no burst phase kinetics for dNTP incorporation with any of the modified oligonucleotides. These results indicate that phosphodiester bond formation or a conformational change of the enzyme.DNA complex is rate-limiting instead of the step involving release of the oligonucleotide. Thio elemental effects for dNTP incorporation were generally relatively small but variable, indicating that the presence of adducts may sometimes make phosphodiester bond formation rate-limiting but not always.

Anticlastogenic effects of Aevitum intake in a group of chemical industry workers

The incidence of chromosome aberrations (CAs) was investigated in cultured lymphocytes of 109 styrene-, formaldehyde-, and phenol-exposed workers in comparison with 64 controls. There was a marked increase in the incidence of the structural chromosome aberrations in the first mitotic division metaphases of occupationally exposed workers (3.59 +/- 0.26 CAs/100 cells vs 1.47 +/- 0.14 in controls (P < 0.01). 22 occupationally-exposed workers were selected for the trial including 1-month administration of a drug Aevitum (100,000 U of retinol palmitate plus 0.1 g of alpha-tocopherol acetate dissolved in 0.2 ml of oil) at a daily dose of 1-2 capsules for 5 days a week. The frequency of chromosome aberrations before, after the administration of a cumulative Aevitum dose of 2.0, 4.0 and 8.0 ml, and 6 weeks after the cessation of vitamin intake was 5.68 +/- 0.63, 4.33 +/- 0.45, 2.67 +/- 0.34, 2.00 +/- 0.25, and 2.64 +/- 0.21 per 100 cells, respectively. Thus, Aevitum was found to cause a significant decrease in occupationally-induced chromosome damage in human lymphocytes.

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.

Spectrum of styrene-induced DNA adducts: the relationship to other biomarkers and prospects in human biomonitoring

Styrene is an important industrial chemical that has shown genotoxicity in many toxicology assays. This is believed to be related to the DNA-binding properties of styrene-7,8-oxide (SO), a major metabolite of styrene. In this review, we have summarized knowledge on various aspects of styrene genotoxicity, especially in order to understand the formation and removal of primary DNA lesions, and the usefulness of biomarkers for risk assessment. Biological significances of specific DNA adducts and their role in the cascade of genotoxic events are discussed. Links between markers of external and internal exposure are evaluated, as well as metabolic aspects leading to the formation of DNA adducts and influencing biomarkers of biological effect. Finally, we suggest a design of a population study, which may contribute to our understanding genotoxic events in the exposure either to single xenobiotic or complex mixture.

Effects of occupational exposure to styrene on expression of adhesion molecule on leukocytes

Styrene is an indispensable chemical extensively used in plastic and synthetic rubber industries. Styrene is known to produce various types of hepatotoxic, neurotoxic, and genotoxic effects. Styrene may be immunotoxic by both direct and indirect mechanisms. Measurement of adhesion molecules is a new tool for the investigation of immune system modulation. The aim of this study was to evaluate the association of the expression of the adhesion molecules CD11a, CD11b, CD18, CD54, CD49d, and CD62-L in white blood cells and levels of soluble adhesion molecules ICAM-1 and L-selectin in serum with occupational exposure to styrene. Analyses by flow cytometry revealed elevated levels of most of the assessed adhesion molecules on surfaces of lymphocytes, monocytes, and granulocytes. Expression of the adhesion receptor antigens CD11a on lymphocytes, CD11b on monocytes, and CD18 on granulocytes were unaffected. Workers exposed to styrene had decreased concentrations of sICAM-1 and no changes in concentrations of sL-selectin. Styrene exposure appears to increase activation of the immune system and alter leukocyte adherence. This interaction is a critical first step in immune stimulation and leukocyte-endothelial interaction.

Styrene 7,8-oxide induces caspase activation and regular DNA fragmentation in neuronal cells

Neurobehavioral changes have been described in workers occupationally exposed to styrene vapors. Alterations of neurotransmitters and loss of neurons have been observed in brains of styrene-exposed rats. However, the mechanisms of neuronal damage are not yet clearly understood. We have characterized the cellular alterations induced by the main reactive intermediate of styrene metabolism, styrene 7,8-oxide (SO) in the human neuroblastoma SK-N-MC cell line and primary culture of rat cerebellar granule cells (CGC). SK-N-MC cells exposed to SO (0.3-1 mM) displayed apoptotic morphology, together with chromatin condensation and DNA cleavage into high molecular weight fragments of regular size. These features were accompanied by the activation of class II caspases, as detected with the DEVD assay, by following the cleavage of the caspase-substrate poly (ADP-ribose) polymerase (PARP) and by detection of the active fragment of caspase-3. Pre-incubation of the cells with the caspase inhibitor z-VAD-fmk reduced the cellular damage induced by SO, suggesting that caspases play an important role in SO toxicity. Increased proteolysis by class II caspases was detected also in primary culture of CGC exposed to SO. In addition, the presence of the 150-kDa cleavage product of alpha-fodrin suggests a possible activation of calpains in SK-N-MC cells. Moreover, SO did not affect the level of expression of the p53 protein, even though it is known to cause DNA damage. The identified intracellular pathways affected by SO exposure provides end-points that can be used in future studies for the evaluation of the neurotoxic effect of styrene in vivo.

Evaluation of genotoxic effects in a group of workers exposed to low levels of styrene

Occupational exposure to styrene was studied in a group of workers engaged in the production of fiberglass-reinforced plastics. Sister-chromatid exchanges (SCE), micronuclei (MN), and DNA damage (evaluated by means of comet assay) were measured in peripheral blood cells from the exposed workers and from a control population. Mandelic acid concentration, an indicator of styrene exposure level, was measured in urine samples collected at the end of the work shift. Average estimated values for styrene exposure were slightly below the threshold limit value (TLV) of 20 ppm recommended by the American Conference of Governmental Industrial Hygienists. Significant increases (P< or =0.01) have been found for SCE and MN frequencies and comet tail length among exposed individuals, as well as significant decreases (P< or =0.01) in the proliferation indices, as compared with control population. High correlation has been obtained between endpoints evaluated and exposure length, and increased values of SCE and MN frequencies and comet tail length have been found among smokers only in the exposed population. The high correlation obtained among SCE and MN frequencies and comet tail length, and the increase of these parameters in the exposed group with regard to control group justify the use of these three biomarkers in the evaluation of genotoxic effects in human populations exposed to styrene.

DNA damage and repair in human leukocytes exposed to styrene-7,8-oxide measured by the comet assay

Styrene-7,8-oxide (SO) is produced by cytochrome p450 monooxygenases as the main mammalian metabolite of styrene, an important industrial chemical present at high concentrations in the ambient air of fiberglass-reinforced plastic plants. Previous studies have shown positive results for SO in the induction of several cytogenetic endpoints in vitro. In this work we have evaluated, by means of the comet assay, the potential of SO to act as a DNA damaging agent in human peripheral leukocytes and the ability of white blood cells to repair the DNA damage induced by this compound. Our results show that SO induces DNA damage at concentrations higher than 50 microM in a dose-dependent manner, and that the lesions produced by SO are efficiently removed within a few hours after the end of treatment.

Association between genetic polymorphisms and biomarkers in styrene-exposed workers

A comprehensive approach to evaluate genotoxic effects induced by styrene exposure was employed in 44 hand-lamination workers in comparison with 18 unexposed controls. The acquired data on single-strand breaks in DNA (SSBs), frequency of chromosomal aberrations and HPRT mutant frequency in peripheral blood lymphocytes were compared to the results on genotyping of some of the xenobiotic-metabolising enzymes (CYP1A1, CYP2E1, epoxide hydrolase and GSTM1, GSTP1 and GSTT1). Multifactorial regression analysis indicated that SSB in DNA were significantly associated with styrene exposure and with heterozygosity in CYP2E1 (5'-flanking region and intron 6; r(2)=0.614). The frequency of chromosomal aberrations (CA), as analysed by linear multiple regression analysis, significantly correlated with years of employment (P=0.004) and with combinations of epoxide hydrolase (EPHX) genotypes (exon 3, Tyr/His and exon 4, His/Arg), where individuals with low and medium activity EPHX genotypes exhibited higher frequencies of CA than those with high activity genotypes (P=0.044, r(2)=0.563). Moderately higher HPRT mutant frequencies were detected in styrene-exposed individuals (20.2 +/- 25.8 x 10(-6)) as compared to controls (13.3 +/- 6.3 x 10(-6)), but this difference was not significant. ANOVA (in the whole set of data) revealed that mutant frequencies at the HPRT gene were significantly associated with years of employment (F=6.9, P=0.0001), styrene in blood (F=10.1, P=0.0001), and heterozygosity in CYP2E1 (intron 6; F=13.5, P=0.0008) and GSTP1 (exon 5; F=3.6, P=0.038). In conclusion, our present data suggest that analysed biomarkers of DNA damage may be modulated by polymorphic CYP2E1, EPHX and GSTP1. In our study, styrene-specific DNA and haemoglobin adducts are under investigation. Completing these data with the results of genotyping of metabolising enzymes may provide a useful tool for individual genotoxic risk assessment.

An application of the challenge assay in boat builders exposed to low levels of styrene--a feasibility study of a possible biomarker for acquired susceptibility

Sensitivity to carcinogens and susceptibility for malignant diseases may be related to genetic predisposition, e.g. polymorphisms in toxicant-metabolizing enzymes or DNA repair deficiencies. The latter may also be acquired by exposure to substances that interfere with DNA repair processes. Application of the challenge assay to an exposed population may allow scientists to study the interference of DNA repair as an acquired susceptibility phenomenon. The assay was therefore used in a feasibility study to evaluate its application. A group of 14 workers exposed to low levels of styrene (mean < 100 mg/m3 styrene in air; 35 micrograms/l styrene in blood) and a reference of seven controls were investigated for structural chromosomal aberrations using FISH. The rate of exchange-type aberrations per 100 metaphases was 0.14 (95% CI, 0.05-0.31) in controls and 0.22 (95% CI, 0.13-0.36) in exposed workers. The difference is not statistically significant. Interaction with DNA repair was measured in the 14 workers and 2 historical controls using the challenge assay. Exchange-type aberrations per 100 metaphases after X-ray challenge of 1.66 Gy were 13.26 (10.53-16.50) and 16.19 (15.00-17.40) for the controls and exposed, respectively. The difference is statistically significant (p < 0.038). Among the exposed group, the challenge response was also significantly correlated with the cumulative lifetime exposure to styrene (R2 = 0.3996; p < 0.015) but not with the current exposure as measured in blood (R2 = 0.0226; p = 0.700). The challenge responses in the short-term and long-term exposed subgroups were 15.55 (14.23-16.96) and 17.90 (15.64-20.39), respectively, based on sample sizes of 5 and 9, respectively. The difference was not significant. Hence, data from our study are consistent with the hypothesis that long-term exposure to styrene can interfere with DNA repair activities. The lack of statistically significant differences in some of the data may be due to the small sample size and a possible confounding by age in our investigation. Additional data from our ongoing study should clarify this uncertainty.

Simultaneous high-performance liquid chromatographic determination of urinary mandelic and phenylglyoxylic acids as indirect evaluation of styrene exposure

Styrene is rapidly metabolised to mandelic acid (MA) and phenylglyoxylic acid (PGA), which are excreted in urine. In this work, we have developed a simple, sensitive and specific high-performance liquid chromatographic method with minor sample preparation procedures for the simultaneous determination of MA and PGA in urine of workers exposed to styrene. Moreover, urine samples from workers of two plastic factories were analysed, styrene exposure levels of the workers were estimated and data obtained from the two factories were compared.

Genotoxic effects of styrene-7,8-oxide in human white blood cells: comet assay in relation to the induction of sister-chromatid exchanges and micronuclei

Styrene is used in the production of plastics, resins and rubber. The highest human exposures to styrene take place by inhalation during the production of fiberglass reinforced plastics. Styrene is metabolized mainly in the liver to styrene-7,8-oxide (SO), its principal in vivo mutagenic metabolite. In this study, human peripheral white blood cells were exposed to several SO concentrations (10-200 microM) in order to evaluate its genotoxic properties by means of comet assay, sister-chromatid exchanges (SCE) and cytokinesis-blocked micronucleus (MN) test, in addition to determine its clastogenic or aneugenic properties by combining MN with fluorescence in situ hybridization (FISH) procedures. Our results show that SO induces DNA damage, SCE and MN in human leukocytes in vitro at concentrations above 50 microM, and that there is a strong relationship between DNA damage, as measured by the comet assay, and cytogenetic damage induced by SO at the doses employed. SO shows preferentially a clastogenic activity and produces a cytostatic effect at high doses, reflected by the significant decrease of the calculated proliferation indices. A good dose-effect relationship is obtained in the three tests performed at the concentration range assayed.

Uridine uptake inhibition as a cytotoxicity test for a human hepatoma cell line (HepG2 cells): comparison with the neutral red assay

This study describes a sensitive microassay for measuring cytotoxicity based on the degree of inhibition of RNA synthesis in HepG2 cells. RNA synthesis is measured by the kinetic uptake of radiolabeled uridine. A large number of compounds were tested in a wide range of concentrations. The concentration required to induce 50% inhibition of HepG2 uridine uptake rates (IC(50)) was determined for each compound and used to rank its potency. These IC(50)s were compared with IC(50)s measured with the neutral red assay. 2-acetylaminofluorene, benzo[a]pyrene and methylnitrosourea were not cytotoxic in the neutral red assay. Uridine uptake was always inhibited at lower concentrations than those required in the neutral red assay, suggesting that the uridine uptake assay is a more sensitive indicator of toxic action than the neutral red inclusion. Uridine uptake assay provides a rapid and quantitative method for assessing toxicity in a human cell line. Application of this method to bottled spring waters are described. Due to its high sensitivity and reproducibility, this method provides a suitable tool for screening a great number of samples and will be a helpful test for evaluating food safety and controlling the recycling process of wrapping materials.

Effects of prenatal styrene exposure on postnatal development and brain serotonin and catecholamine levels in rats

Maternal reproductive effects in Wistar rats exposed to 0, 50, or 300 ppm styrene for 6 h/day during gestational days 6 to 20 were evaluated. Their offspring were observed postnatally for neurochemical changes, growth, and physical landmarks of development. Mothers exposed to styrene were compared with pair-fed and ad-lib-fed controls in order to adjust nutrient conditions. Prolongation of the gestational period, food intake, and the number of neonatal deaths or stillbirths in 300-ppm-exposed dams showed evidence of styrene-related effects. Other reproductive parameters, such as litter size, birth weight, and sex ratio, were found to exhibit no effects within the variation range studied. A neurochemical effect was observed in that the 5-HT and HVA concentrations in cerebrum were significantly decreased. Incisor eruption (mandible), eye opening, and the air-righting reflex were delayed in rat pups born to dams receiving 300 ppm styrene exposure compared with the pair-fed and ad lib control groups. Pups born to dams exposed to 50 ppm styrene also had a significantly delayed air-righting reflex compared with ad lib controls. These results suggest that the offspring were susceptible to the effects of styrene on a few developmental landmarks even when nutritional effects were controlled.

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

In the human glutathione S-transferase (GST) mu gene family, homozygous deletion of GSTM1 is the null phenotype (frequency of approximately 50% in Caucasians). In the current study, GSTM1 status was determined in human cell lines using reverse transcriptase, polymerase chain reaction, and immunochemistry. Cell lines were challenged with a range of doses of styrene-7,8-oxide (SO) and then toxicity and genotoxicity were monitored. Toxicity was determined by growth in flasks and genotoxicity by cloning in microplates in the presence/absence of 6-thioguanine, to detect mutations at the hypoxanthine phosphoribosyltransferase (hprt) locus. A SO concentration-dependent decrease in survival was observed for all cell lines, with GSTM1-deficient lines being more sensitive. The IC(50)s of deficient and proficient cell lines were 0.45 and 0.55 mM SO, respectively. The difference between survival of GSTM1-deficient and -proficient cell lines approached statistical significance. The background mutation frequency of GSTM1-deficient cell lines was 2 x 10(-5), and that of GSTM1-proficient cell lines was 3 x 10(-6). GSTM1-deficient cell lines were significantly more sensitive than GSTM1-proficient cell lines to mutation induction for concentrations up to 2.5 mM SO (P < 0.001, regression analysis). These results suggest that cell lines containing metabolically competent GSTM1 are able to efficiently use GSTM1 to conjugate SO and reduce its hazard. This supports the epidemiological evidence that GSTM1 influences sensitivity to chemical carcinogenesis and subsequent risk of cancer induction.

Metabolism of styrene-7,8-oxide in human liver in vitro: interindividual variation and stereochemistry

Styrene is an industrial solvent which is mainly oxidized by cytochrome P450 to an electrophilic, chiral epoxide metabolite: styrene-7,8-oxide (SO). SO has cytotoxic and genotoxic properties; the (R)-enantiomer is more mutagenic to Salmonella typhimurium TA 100 in the Ames test than the (S)-enantiomer. Detoxication proceeds via microsomal epoxide hydrolase (mEH). Interindividual differences in mEH activity as well as differences in mEH enantioselectivity are important factors for toxic effects of SO. To study the extent of the interindividual variation, microsomal preparations of 20 human livers were incubated with (R)- and (S)-SO separately (1-2000 microM) and Michaelis-Menten kinetics were determined. In addition, samples were genotyped for two genetic polymorphisms of the mEH gene. V(max), K(m) and V(max)/K(m) values of both enantiomers differed three- to fivefold between the livers. No association of the enzyme constants with the genetic polymorphisms of the epoxide hydrolase gene was found. Hydrolysis of the styrene oxide enantiomers proceeded in an enantioselective manner, with the (S)-enantiomer having an approximately six times higher K(m) and five times higher V(max) than the (R)-enantiomer. In vivo, both SO enantiomers are formed; therefore, time course incubations with racemic SO were carried out in vitro to investigate possible interactions between the enantiomers. When racemic SO was used as a substrate, the (R)-enantiomer acted as an inhibitor on the hydrolysis of the (S)-enantiomer. These results indicate that mEH-mediated hydrolysis of SO is subject to appreciable interindividual variation and that hydrolysis of the more toxic enantiomer is favored.

An evaluation of styrene genotoxicity using several biomarkers in a 3-year follow-up study of hand-lamination workers

A study employing several biomarkers of styrene exposure and genotoxicity was carried out in a group of lamination (reinforced plastic) workers and controls, who had been repeatedly sampled during a 3-year period. Special attention will be paid to the last sampling (S.VI), reported here for the first time. Styrene concentration in the breathing zone, monitored by personal dosimeters, and urinary mandelic acid (MA) were measured as indicators of external exposure. Blood samples were assayed for styrene-specific O6-guanine adducts in DNA, N-terminal valine adducts of styrene in haemoglobin, DNA single-strand breaks (SSB), determined by use of the single cell gel electrophoresis (Comet) assay), and hypoxanthine guanine phosphoribosyl transferase (HPRT) mutant frequencies (MF) in T-lymphocytes. O6-styrene guanine adduct levels were significantly higher in the exposed group (5.9 +/- 4.9 adducts/10(8) dNp) as compared to laboratory controls (0.7 +/- 0.8 adducts/10(8) dNp; P = 0.001). DNA adduct levels significantly correlated with haemoglobin adducts, SSB parameters and years of employment. Styrene-induced N-terminal valine adducts were detected in the lamination workers (1.7 +/- 1.1 pmol/g globin), but not in the control group (detection limit 0.1 pmol/g globin). N-terminal valine adducts correlated strongly with external exposure indicators, DNA adducts and HPRT MF. No significant correlation was found with SSB parameters. A statistically significant difference in HPRT MF was observed between the laminators (22.3 +/- 10.6/10(6)) and laboratory controls (14.2 +/- 6.5/10(6), P = 0.039). HPRT MF in the laminators significantly correlated with styrene concentration in air, MA and haemoglobin adducts, as well as with years of employment and age of the employees. No significant difference (P = 0.450) in MF between the laminators and the factory controls was observed. Surprisingly, we detected differences in MF between sexes. When data from all measurements were combined, women showed higher MF (geometric mean 15.4 vs. 11.2 in men, P = 0.020). The styrene-exposed group exhibited significantly higher SSB parameters (tail moment (TM), tail length (TL) and the percentage of DNA in the tail (TP)) than the control group (P < 0.001). SSB parameters correlated with indicators of external exposure and with O6-styrene guanine adducts. No significant correlation was found between SSB parameters and haemoglobin adducts or HPRT MF. The data encompassing biomarkers from repeated measurements of the same population over a 3-year period are discussed with respect to the mechanisms of genotoxic effects of styrene and the interrelationship of individual biomarkers.

Biomonitoring of occupational exposure to styrene in a plastics lamination plant

A comprehensive approach to biological monitoring of 44 workers occupationally exposed to styrene in a hand lamination plant was performed by using several end-points: styrene in workplace air, styrene in exhaled air, styrene in blood, DNA strand breaks (SBs) and oxidised bases in mononuclear leukocytes, chromosomal aberrations in lymphocytes, immune parameters and genotyping of polymorphic genes of some xenobiotic-metabolizing enzymes (CYP 1A1, EPHX, GSTM1 and GSTP1). We found a significantly higher number of DNA SBs, measured by a modified comet assay, in mononuclear leukocytes of the styrene-exposed workers compared with results from 19 unexposed controls (P<0.001). A fairly strong correlation was observed between SBs and years of exposure (P<0.001, r=0.545). The styrene-exposed workers also showed a significantly increased frequency of chromosomal aberrations (P<0.0001 for highly exposed group, P<0.004 for medium-exposed group, and P=0.0001 for low-exposed group). The proliferative response of T-lymphocytes stimulated with concanavalin A was significantly suppressed in people exposed to styrene (P<0.05). We recorded a significant increase of the percentage of monocytes in differential white blood cell counts in the exposed group (P<0.05). Using flow cytometry, we found an increased expression of adhesion molecules CD62L, CD18, CD11a, CD11b, CD49d and CD54 in the exposed workers as compared with the control group (P<0.05).

Detection of styrene and styrene oxide-induced DNA damage in various organs of mice using the comet assay

Styrene (100-500 mg/kg b.wt.) and styrene oxide (50-200 mg/kg b.wt.) were given as a single intraperitoneal injection to female mice (C57BL/6) at various time intervals before sacrifice. Primary DNA damage in various organs was studied using alkaline single cell gel electrophoresis (comet) assay. Both substances induced significant DNA damage in lymphocytes, liver, bone marrow and kidney after 4 hr. The lymphocytes and liver cells were found to be the most sensitive cells to the DNA damaging effects of both agents. With the exception of bone marrow cells, the degree of DNA damage in all other cell types was decreased from 4 hr to 16 hr after the administration of both compounds. A strong sublinear dose-response relationship was observed in the lymphocytes, liver and bone marrow cells, possibly indicating a saturation of the detoxifying enzyme systems in these organs. The present work suggests that the comet assay can be used for detection of primary DNA damage induced by styrene and styrene oxide in vivo and for comparing the sensitivity of various target organs.

Comparison of styrene-7,8-oxide adducts formed via reaction with cysteine, N-terminal valine and carboxylic acid residues in human, mouse and rat hemoglobin

The reactive metabolite of styrene, styrene-7,8-oxide (SO), reacts with a variety of nucleophilic sites in hemoglobin (Hb) to form SO-Hb adducts. Following the in vitro incubation of SO with blood from humans, NMRI mice and Sprague-Dawley rats, the second-order reaction rate constants were determined for the reaction of SO with cysteine (through both the alpha- and beta-carbons of SO), N-terminal valine (through the beta-carbon of SO), and carboxylic acid (presumably through both the alpha- and beta-carbons of SO) residues in Hb. The rate constants for cysteine adducts vary dramatically between species [2.04, 10.7, 133 L (mol Hb)-1 h-1 (alpha binding) for humans, mice and rats, respectively] and [0.078, 2.16, 20.4 L (mol Hb)-1 h-1 (beta binding), respectively]. The considerably higher rate of reaction with cysteine in rat Hb probably reflects the presence of an additional cysteine residue at position beta 125. Although the rate constants for valine adducts (1.82, 0.80, 0.29 L (mol Hb)-1 h-1, respectively) and COOH adducts (3.55, 1.94, 2.37 L (mol Hb)-1 h-1, respectively) are much more consistent, the inter-species differences are statistically significant for the reaction of SO with the N-terminal valine of Hb. Following the i.p. administration of styrene to mice and styrene and SO to rats, the levels of adducts at each of these sites were used in conjunction with the calculated rate constants to predict the integrated blood doses of SO. While the SO doses predicted from cysteine and valine adducts were very similar, that based upon COOH-binding was significantly different, presumably due to the instability of SO-COOH adducts. This research affirms the use of both cysteine and valine adducts, but not carboxylic acid adducts, as biomarkers of exposure to styrene and SO.

Evaluation of genotoxic potential of styrene in furniture workers using unsaturated polyester resins

Styrene is a widely used chemical, mostly in making synthetic rubber, resins, polyesters, plastics and insulators. Increasing attention has been focused on this compound since experiments using cytogenetic end-points have implicated styrene as a potential carcinogen and mutagen. In order to perform biological monitoring of genotoxic exposure to styrene monomer, we evaluated the urinary thioether (UT) excretion, and sister chromatid exchanges (SCEs) and micronuclei (MN) in peripheral lymphocytes from 53 furniture workers employed in small workplaces where polyester resin lamination processings were done and from 41 matched control subjects. The mean air concentration of styrene in the breathing zone of workers was 30.3 ppm. As a metabolic marker for styrene exposure, mandelic acid + phenylglyoxylic acid was measured in the urine and the mean value was 207 mg/g creatinine. The mean +/- SD value of UT excretions of workers was 4.43 +/- 3.42 mmol SH-/mol creatinine and also mean UT for controls was found to be a 2.75 +/- 1.78 mmol SH-/mol creatinine. The mean +/- SD/cell values of SCE frequency in peripheral lymphocytes from the workers and controls were 6.20 +/- 1.56 and 5.23 +/- 1.23, respectively. The mean +/- SD frequencies (%o) of MN in the exposed and control groups were 1.98 +/- 0.50 and 2.09 +/- 0.35, respectively. Significant effects of work-related exposure were detected in the UT excretion and SCEs analyzed in peripheral blood lymphocytes (p < 0.05 and p < 0.01, respectively). The MN frequency in lymphocytes from the styrene-exposed group did not differ from that in the controls (p > 0.05). Effect of smoking, age and duration of exposure on the genotoxicity parameters analyzed were also evaluated. In conclusion, although our data do not demonstrate a dose-response relationship, they do suggest that styrene exposure was evident and that this styrene exposure may contribute to the observed genotoxic damage in furniture workers.

Recombinant expression of human microsomal epoxide hydrolase protects V79 Chinese hamster cells from styrene oxide- but not from ethylene oxide-induced DNA strand breaks

Styrene 7,8-oxide and ethylene oxide are widely used genotoxic bulk chemicals, which have been associated with potential carcinogenic hazard for occupationally exposed workers. Both epoxides alkylate DNA preferentially at the N-7 position of guanine and consequently produce single-strand breaks and alkali labile sites in the DNA of exposed cells. In order to study the role of human microsomal epoxide hydrolase (hmEH) in protecting cells against genotoxicity of styrene 7,8-oxide and ethylene oxide, we expressed the cDNA of hmEH in V79 Chinese hamster cells. We obtained a number of cell clones that expressed functionally active epoxide hydrolase. Among these, the clone 92hmEH-V79 revealed an especially high enzymatic mEH activity toward styrene 7,8-oxide (10 nmol converted per mg of protein per min, measured in the 9,000 x g supernatant of the cell homogenate), that was 100 times higher than that determined in mock-transfected cells and within the range of mEH activity in human liver. Styrene 7,8-oxide-induced DNA single-strand breaks/alkali labile sites (dose range 10 microM to 1 mM styrene 7,8-oxide) measured by the alkaline elution technique were significantly lower in the 92hmEH-V79 cells as compared to the mock-transfected cells. The protection against styrene 7,8-oxide genotoxicity in 92hmEH-V79 cells could be abolished by addition of valpromide, a selective inhibitor of microsomal epoxide hydrolase. These results clearly show that the metabolism of styrene 7,8-oxide by hmEH in 92hmEH-V79 cells was responsible for the protection against styrene 7,8-oxide genotoxicity. On the other hand, no protective effect of epoxide hydrolase expression could be observed on ethylene oxide-induced DNA damage with the recombinant cell line over a dose range of 0.5-2.5 mM ethylene oxide. This selectivity of the protective effect on epoxide genotoxicity thus appears to be an important factor that must be taken into account for the prediction of the genotoxic risk of epoxides themselves or compounds that can be metabolically activated to epoxides.

Genotoxicity and cytotoxicity in male B6C3F1 mice following exposure to mixtures of 1,3-butadiene and styrene

1,3-Butadiene and styrene are oxidized, in part, by cytochrome P450 2E1 and have been shown to metabolically interact in rodents exposed by inhalation to mixtures of both compounds. Because the reactive metabolites of butadiene and styrene are thought to be responsible for the toxicity of each compound, metabolic interactions may alter the response in animals exposed to mixtures of butadiene and styrene compared with the response in animals exposed to butadiene alone or styrene alone. The purpose of this study was to quantitate alterations in genotoxicity and cytotoxicity in male B6C3F1 mice exposed to mixtures of butadiene and styrene. Male B6C3F1 mice were exposed to 6.25, 62.5, 200, or 625 ppm butadiene alone, 50 ppm styrene alone, or mixtures of 6.25, 62.5, 200, or 625 ppm butadiene and 50 ppm styrene. Genotoxicity was assessed by quantitating the frequency of micronucleated polychromatic erythrocytes in bone marrow. Cytotoxicity was assessed by counting total spleen and thymus cells and by quantitating the frequency of polychromatic erythrocytes in the peripheral blood. Butadiene and mixtures of butadiene and styrene were genotoxic in mice, as shown by a significant increase in the frequency of micronucleated polychromatic erythrocytes. The increased frequency following exposure to mixtures of butadiene and styrene was not significantly different compared with the frequency following exposure to butadiene alone. Styrene and mixtures of butadiene and styrene were cytotoxic in mice, as shown by significantly decreased number of spleen cells. Exposure to mixtures of butadiene and styrene with butadiene concentrations of 62.5 or 625 ppm significantly reduced the number of thymus cells. Exposure to 200 ppm or 625 ppm butadiene alone, or to mixtures of 200 ppm or 625 ppm butadiene and 50 ppm styrene, significantly reduced the frequency of polychromatic erythrocytes in the peripheral blood. The results of the study demonstrate that exposure to mixture of butadiene and styrene does not reduce the respective genotoxicity of butadiene or cytotoxicity of styrene.

A comparative study of chemically induced DNA damage in isolated human and rat testicular cells

Testicular cells prepared from human organ transplant donors or from Wistar rats were used to compare 15 known reproductive toxicants with respect to their ability to induce DNA damage, measured as single-strand DNA breaks and alkali labile sites (ssDNA breaks) with alkaline filter elution. The compounds tested included various categories of chemicals (i.e., pesticides, industrial chemicals, cytostatics, and mycotoxins) most of which are directly acting genotoxicants (i.e., reacting with DNA either spontaneously or via metabolic activation). In addition, a few indirect genotoxic and nongenotoxic reproductive toxicants were included. Six of the chemicals induced no significant levels of ssDNA breaks in human and rat testicular cells; methoxychlor (10 to 100 microM, human and rat), benomyl (10 to 100 microM, human and rat), thiotepa (10 to 1000 microM, human and rat), cisplatin (30 to 1000 microM, human; 100 to 1000 microM, rat), Cd2+ (30 to 1000 microM, human; 100 to 1000 microM, rat), and acrylonitrile (30 to 1000 microM, human; 30 to 300 microM, rat). Four chemicals induced significant levels of ssDNA breaks in testicular cells from both species: styrene oxide (> or = 100 microM, rat and human), 1,2-dibromoethane (EDB) (> or = 100 microM, rat; 1000 microM human), thiram (> or = 30 microM, rat; > or = 100 microM, human), and chlordecone (300 microM, rat; > or = 300 microM, human). Finally, five chemicals induced ssDNA breaks in one of the two species. Four chemicals induced significant ssDNA breaks in rat testicular cells only: 1,2-dibromo-3-chloropropane (DBCP) (> or = 10 microM), 1,3-dinitrobenzene (1,3-DNB) (> or = 300 microM), Cr6+ (1000 microM), and aflatoxin B1 (> or = 100 microM), the last two of these produced only a minor positive response. One chemical, acrylamide, induced a marginal increase in ssDNA breaks in human at 1000 microM, but not in rat testicular cells. Although based on a limited number of donors, the data indicate a close correlation between the induction of DNA damage in human and rat testicular cells in vitro. For some chemicals, however, there appears to be differences in the susceptibility to chemically induced ssDNA breaks of isolated testicular cells from the two species. The data indicate that the parallel use of human and rat testicular cells provides a valuable tool in the assessment of human testicular toxicity.

Cytogenetic effects in mice of divinylbenzene-55 inhalation

Male B6C3F1 mice (8 weeks of age) were exposed by inhalation to divinylbenzene-55 (DVB-55), at target concentrations of 0, 25, 50 and 75 ppm for 6 h per day for 3 days. Following exposure the animals were killed blood smears were prepared for micronucleus (MN) analysis, and the spleens were removed and cultured for sister chromatid exchange (SCE) and chromosome aberration (CA) analyses. DVB-55 induced a dose dependent increase in SCE with the two highest doses reaching statistical significance. Similarly, there was a statistically significant although less pronounced increase in the frequency of CAs in splenocytes and MN in polychromatic erythrocytes. There was no indication of toxicity as measured by cell cycle kinetics in the splenocytes or the percentage of polychromatic erythrocytes in the peripheral blood smears. Thus, DVB-55 appears to be a weak genotoxicant in vivo.

Determination of low level exposure to volatile aromatic hydrocarbons and genotoxic effects in workers at a styrene plant

OBJECTIVES

Low exposures to volatile aromatic hydrocarbons and cytogenetic effects in peripheral white blood cells were determined in 25 healthy workers employed in different areas of a styrene production plant in the former German Democratic Republic. The results were compared with 25 healthy unexposed controls (matched for age and sex) employed in the same company.

METHODS

The concentrations of aromatic hydrocarbons determined from active air sampling in all areas of the factory (styrene: 73-3540 micrograms/m3 (< 0.01-0.83 ppm); ethylbenzene 365-2340 micrograms/m3 (0.08-0.53 ppm); benzene 73-3540 micrograms/m3 ( < 0.02-1.11 ppm); toluene 54-2960 micrograms/m3 (0.01-0.78 ppm); xylenes 12-94 micrograms/m3 ( < 0.01-0.02 ppm)) were considerably lower than in the pump house ( > 4000 micrograms/m3 styrene, ethylbenzene, benzene, and toluene; > 500 micrograms/m3 xylenes), which was only intermittently occupied for short periods. Passive personal monitoring, biomonitoring of exhaled air and metabolites (mandelic, phenylglyoxylic, trans, trans-muconic, hippuric, o-, m- and p-methylhippuric acids, and phenol) in urine samples collected before and after an eight hour working shift was used to assess individual exposure. Questionnaires and examination of company records showed that the historical exposure was far higher than that measured. Genotoxic monitoring was performed by nuclease P1-enhanced 32P-postlabelling of DNA adducts in peripheral blood monocytes, and DNA single strand breaks, sister chromatid exchange, and micronuclei in lymphocytes. The content of kinetochores in the micronuclei was determined by immunofluorescence with specific antibodies from the serum of CREST patients.

RESULTS

No genotoxic effects related to exposure were detected by DNA adducts or DNA single strand breaks and sister chromatid exchange. The only effect related to exposure was an increase in kinetochore positive micronuclei in peripheral lymphocytes; the frequency of total micronuclei in peripheral lymphocytes did not change. Smoking was confirmed by measurement of plasma cotinine, and no confounding effect was found on any of the cytogenetic variables.

CONCLUSIONS

Low occupational exposure to styrene, benzene, and ethylbenzene did not induce alterations of genotoxicological variables except kinetochore positive micronuclei. This is the first reported use of the CREST technique for an in vivo study in occupational toxicology, which thus could serve as a valuable and sensitive technique for toxicogenic monitoring.

Exposure to styrene and chronic health effects: mortality and incidence of solid cancers in the Danish reinforced plastics industry

OBJECTIVES

To study the occurrence of non-malignant diseases and solid cancers in workers exposed to styrene in the Danish reinforced plastics industry.

METHODS

All 36,610 workers of 386 reinforced plastics companies and 14,293 workers not exposed to styrene from similar industries were followed up from 1970 to 1990. This industry is characterised by simple exposure conditions, exposure to high concentrations of styrene, and a high proportion of small companies, and the exposure assessment was based on experts' classification on a company level. The mortality from non-malignant causes and the incidence of solid cancers were compared with the national rates. Poisson models were used for internal comparisons.

RESULTS

A total of 3031 deaths and 1134 newly diagnosed cases of solid cancer were reported in the workers in the reinforced plastics industry. In companies where 50% or more of the workers produced reinforced plastics an increased mortality rate ratio (MRR) for degenerative disorders of the nervous system (multiple sclerosis, parkinsonism, and motor neurone disease; MRR 1.8, 16 cases, 95% confidence interval (95% CI) 0.9-3.8) and an increased incidence rate ratio (IRR) for pancreatic cancer (IRR 2.2, 17 cases, 95% CI 1.1-4.5) was found. For both disease categories increased occurrence was also found among long term workers, workers of the 1960s (the period with the highest exposure to styrene), and workers with a latent period of more than 10 years after the start of employment. No other non-malignant diseases or solid cancers showed these patterns.

CONCLUSION

The findings have to be interpreted with caution, due to the company based exposure assessment, but the possible association between exposures in the reinforced plastics industry, mainly styrene, and degenerative disorders of the nervous system and pancreatic cancer, deserves attention.

Dosimetry of styrene 7,8-oxide in styrene- and styrene oxide-exposed mice and rats by quantification of haemoglobin adducts

Rats (Sprague Dawley) and mice (NMRI) were administered nonlabelled or labelled styrene and styrene oxide by i.p. injection. Blood samples were collected 6 and 24 h after treatment for studies of dose-response and 6 h to 32 days after treatment for studies of adduct stability. Haemoglobin (Hb) and plasma protein adduct levels were determined by radioactivity measurements or, in the case of adducts to N-terminal valine in Hb, by the so-called N-alkyl Edman procedure. Adducts to N-terminal valine were found to be chemically stable during the life-span of the erythrocytes, whereas adducts to carboxylic acid residues showed a reduced stability. The Hb-adduct levels found after styrene oxide treatment were compatible with a linear dose-response at low doses (< or = 0.4 mmol/kg body weight). At higher doses the detoxification of styrene oxide was overloaded resulting in a higher than proportional increase in adduct levels. Saturation of detoxification of styrene oxide could also explain the non-linear dose-response relationship observed in the mouse following treatment with styrene. Styrene oxide gave 4-7 times higher adduct levels than styrene when administered to the animals at equimolar low concentration. For both compounds, the levels of adducts to N-terminal valine were 2-3 times higher in the mouse than in the rat. A comparison of Hb-adduct levels in the styrene-exposed animals with adduct levels in styrene-exposed reinforced plastics workers (Christakopoulos et al., Scand. J. Work Environ. Health, 19(4) (1993) 255-263) suggests that styrene is less effective in humans than in mice and rats.

Assessment of the neurotoxicity of styrene, styrene oxide, and styrene glycol in primary cultures of motor and sensory neurons

The neurotoxicity of styrene and its major metabolites, styrene oxide and styrene glycol, was investigated in dissociated primary cultures of murine spinal cord-dorsal root ganglia (DRG)-skeletal muscle using morphological and electrophysiological endpoints. Styrene and styrene oxide (but not styrene glycol) were acutely cytotoxic to both neuronal and non-neuronal cells in the cultures; concentrations in excess of 2 and 0.2 mM, respectively, induced blebbing, vacuolation, detachment from the substratum and cell death in neuronal and non-neuronal cells within 4 days. No effects on neuronal morphology were observed in cultures treated with sublethal concentrations of styrene or styrene oxide for up to 3 weeks. The results suggest that oxidation of multiple cellular macromolecules that underlies the toxicity of styrene in other organ systems may also be responsible for damage to cells in the nervous system. No changes in action potential production indicative of a 'solvent effect' on membrane electrical properties was apparent in cultures treated with up to 8 mM styrene or 10 mM styrene glycol.

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.