The Reproductive Effects Assessment Group's report on the mutagenicity of 1,3-butadiene and its reactive metabolites

Molecular and genetic toxicology of 1,3-butadiene

During the last 9 years, there have been many studies published concerning the mutagenic potential of butadiene in mammalian systems, including alterations at the molecular level. Butadiene has tested positive in several mouse in vivo and in vitro assays, but has generally tested negative in rat studies. Most of these studies are cytogenetic and include positive data in mice for chromosomal aberrations, micronucleus formation, and sister chromatid exchanges. Butadiene also induces mutations in lung, spleen, and bone marrow of transgenic mice. The positive bone marrow cytogenetic and transgenic data may be significant in view of the increased lymphohematopoietic malignancies observed in mice and probably in humans. In addition, butadiene causes mutations in the K-ras protooncogene and in the p53 tumor suppressor gene in mouse studies. Mutations in these genes are associated with oncogenesis in humans as well as in rodents. Also, positive mutagenicity data have been obtained in a pilot study of workers exposed to butadiene. Positive dominant lethal studies in rodents suggest that exposure to butadiene can result in germ cell mutation and heritable risk. These mutagenicity and molecular data suggest that butadiene is both a somatic and germ cell mutagen in mammals, possibly including humans.

Clastogenic effects of 1,3-butadiene and its metabolites 1,2-epoxybutene and 1,2,3,4-diepoxybutane in splenocytes and germ cells of rats and mice in vivo

Clastogenicity of 1,3-butadiene (BD), 1,2-epoxybutene (EB), and 1,2,3,4-diepoxybutane (DEB) was studied in splenocytes and germ cells of rats and mice by means of micronucleus assays (cytokinesis-block method for splenocytes, suspension method for germ cells). Inhalation exposure of mice to 200, 500, or 1,300 ppm BD (6 h/d; 5 days) induced significant chromosome damage in spermatocytes at the preleptotene stage. EB and DEB induced significant amounts of clastogenic damage in splenocytes and spermatocytes of rats and mice. The lowest tested effective doses for mice and rats were, respectively, 40 and 80 mg/kg for EB, and 15 and 30 mg/kg for DEB. In splenocytes, 80 mg EB/kg induced 3.6 times more MN in mice than in rats, whereas 30 mg DEB/kg induced the same amount of damage in both species. Damage in germ cells of mice was induced in early spermatocytes treated with 40 and 80 mg EB/kg, and in late spermatocytes exposed to 30 mg DEB/kg. In rats, 40 mg EB/kg induced damage in early spermatocytes, whereas 80 mg EB/kg induced chromosomal damage in early and late spermatocytes. In rats treated with DEB, clastogenic damage was induced in spermatocytes at preleptotene, zygotene, diplotene, and diakinesis stages. When the clastogenic potential of EB and DEB in splenocytes and germ cells of mice and rats was compared, DEB always showed a stronger effect than EB. Body weight, testis weight, ratio of testis weight to body weight, and ratio of Golgi to Golgi + cap phase spermatids were used as parameters for toxicity. Exposures to 500 and 1,300 ppm BD were somewhat toxic to mice. Doses of 80 mg EB/kg and 30 mg DEB/kg exhibited toxic effects in mice and rats.

Sister-chromatid exchanges and cell-cycle kinetics in the lymphocytes of workers occupationally exposed to a chemical mixture in the tyre industry

Cytogenetic studies of clinically healthy workers employed in the rubber industry showed an increase in chromosome aberrations (CAs), sister-chromatid exchanges (SCEs) and a decrease in proliferation indices (PIs). The aim of the present study was to establish, using the SCE and PI tests, genotoxic effects of hazardous chemicals in the rubber industry. An increase in mean SCEs in the lymphocytes of vulcanizers as compared to controls was observed. Since the PI in the exposed group was insignificantly decreased as compared to the controls, it could be concluded that the SCE test is the most sensitive cytogenetic test for the detection of a genotoxic effect of chemicals in the rubber industry. There was no evidence in the present study that the genotoxic effect of chemicals in the rubber industry was enhanced by cigarette smoking.

Cytogenetic studies of workers from the rubber industry

Lymphocytes from 21 vulcanizers from a tire factory were analyzed for the presence of chromosome aberrations (CAs), sister-chromatid exchanges (SCEs) and for proliferation indices (PIs). An increase in the frequencies of CAs and SCEs and a decrease in PIs were observed. Cytogenetic parameters were correlated with age, smoking habits, and duration of occupational exposure. From the present study it can be concluded that cigarette smoking enhances the genotoxic effects of mixtures of chemicals to which workers in the rubber industry are exposed.

1,3-Butadiene and its epoxides induce sister-chromatid exchanges in human lymphocytes in vitro

Sister-chromatid exchanges (SCEs) were induced in human lymphocytes by 1,3-butadiene and its epoxides 3,4-epoxy-1-butene and 1,2:3,4-diepoxybutane. After a pulse treatment of 2 h, 1,3-butadiene produced a weak but reproducible increase in SCEs both with and without S9 mix. The response was similar in cultures of whole blood and of isolated lymphocytes. The 2 epoxide metabolites of butadiene, studied in whole-blood lymphocyte cultures without exogenous metabolic activation, were highly active SCE inducers. The lowest effective concentrations of butadiene, monoepoxybutene, and diepoxybutane were 2000 microM, 25 microM and 0.5 microM, respectively. A slight but dose-dependent increase in SCEs was also observed without an exogenous metabolic system after a 48-h treatment with 1,3-butadiene. Already the lowest concentration tested (500 microM) was effective. Again, the response was similar in cultures of whole blood and isolated lymphocytes, suggesting that the lymphocytes are capable of metabolically activating 1,3-butadiene.

Sister-chromatid exchanges induced by 1.3-butadiene and its epoxides in CHO cells

Sister-chromatid exchanges (SCEs) were analyzed in CHO cells after pulse treatment with 1,3-butadiene, 3,4-epoxy-1-butene (monoepoxybutene) and 1,2:3,4-diepoxybutane (diepoxybutane). A weak dose effect was observed after exposure to 1,3-butadiene but only in the presence of S9 mix. Monoepoxybutene and diepoxybutane were highly effective in inducing SCEs at concentrations of 0.1-1 microM both in the presence and in the absence of S9 mix. At higher concentrations the response was more pronounced without S9 mix.

Chromosomal abnormalities and sister-chromatid exchange in bone marrow cells of mice and Chinese hamsters after inhalation and intraperitoneal administration: I. Diepoxybutane

Diepoxybutane (DEB), a direct-acting animal carcinogen, was found to increase the frequency of structural chromosomal abnormalities (CA) and sister-chromatid exchange (SCE) in bone marrow cells of mice and Chinese hamsters, when inhaled from an aerosol during a 2-h head-only exposure or administered as a single intraperitoneal injection. For the purpose of comparing the genotoxicity in the 2 species, both after inhalation and intraperitoneal administration, the systemic DEB dose obtained by inhalation was determined on the basis of blood concentrations and inhalation duration after the investigation of the blood kinetics. The bone marrow cells of male and female NMRI mice were found to be more sensitive than those of Chinese hamsters to the genotoxic activity of DEB.

Comparative cytogenetic analysis of bone marrow damage induced in male B6C3F1 mice by multiple exposures to gaseous 1,3-butadiene

Groups of male B6C3F1 mice (N = 12) were exposed to ambient air or to gaseous 1,3-butadiene (BD) at 6.25, 62.5, and 625 ppm for 10 exposure days (6 hr + T90/day). Exposure to BD induced in bone marrow: 1) a significant increase in the frequency of chromosomal aberrations (CA); 2) a significant elevation in the frequency of sister chromatid exchanges (SCE); 3) a significant lengthening of the average generation time (AGT); 4) a significant depression in the mitotic index (MI); and, as measured in the peripheral blood, 5) a significant increase in the proportion of circulating polychromatic erythrocytes (%PCE), and 6) a significant increase in the level of micronucleated PCE (MN-PCE) and micronucleated normochromatic erythrocytes (MN-NCE). The most sensitive indicator of genotoxic damage was the frequency of SCE (significant at 6.25 ppm), followed by MN-PCE levels (significant at 62.5 ppm), and then by CA and MN-NCE frequencies (significant at 625 ppm). The most sensitive measure of cytotoxic damage was AGT (significant at 62.5 ppm), followed by %PCE (significant at 625 ppm), and then by MI (significant by trend test only). Because each cytogenetic endpoint was evaluated in every animal, a correlation analysis was conducted to evaluate the degree of concordance among the various indicators of genotoxic and cytotoxic damage. The extent of concordance ranged from a very good correlation between the induction of MN-PCE and the induction of SCE (correlation coefficient r = 0.9562) to the lack of a significant correlation between the depression in the MI and any other endpoint (r less than 0.37).

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.