Cell cycle specificity of cytogenetic damage induced by 3,4-epoxy-1- butene

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

1,3-Butadiene: III. Assessing carcinogenic modes of action

1,3-Butadiene (BD) is a multisite carcinogen in laboratory rodents following lifetime exposure, with greater potency in the mouse than the rat, and is associated with an increase in leukemia mortality in highly exposed workers. Species differences in the formation of reactive metabolites underlie observed species differences in sensitivity to the carcinogenic effects of BD. The modes of action (MOAs) for human leukemia and rodent tumors are both likely related to mutagenic potencies of one or more of these metabolites. However, differences in the nature of genotoxic lesions associated with human leukemia and rodent tumors, along with their implications for risk assessment, require that they be discussed separately. The MOAs for BD are assessed in this review using the modified Hill criteria and human relevance framework. Key events in MOAs for human and rodent cancers are identified, along with important species differences and sources of nonlinearity for each event that can affect extrapolations made from high- to low-dose exposures. Because occupational exposures to BD have also included co-exposures to styrene and dimethyldithiocarbamide (DMDTC), potential interactions with BD carcinogenicity are also discussed. The MOAs for BD carcinogenesis will be used to guide key decisions made in the quantitative cancer dose-response assessment.

Propylene oxide: genotoxicity profile of a rodent nasal carcinogen

Propylene oxide (PO) is a DNA-reactive genotoxic agent; that is, it reacts with DNA to produce lesions in the genetic material. PO also induces tumors in rodents, although only at high concentrations and at portals of entry. This review of PO's genotoxicity profile is organized according to endpoints measured, that is, nonmutational or mutational endpoints, and as to whether the results were from in vitro or in vivo studies. In addition to results of experimental studies, PO's genotoxicity for humans is assessed by reviewing results of published biomarker studies. The weight of evidence indicates that although it is genotoxic, PO's potency as a DNA-reactive mutagen is weak. Other aspects of PO's overall tissue toxicities are also reviewed, with attention to glutathione (GSH) depletion and its consequences, that is, cell proliferation, death, and necrosis. These toxic tissue responses occur in the same anatomical regions in rodents as do the PO-induced tumors. Furthermore, some of these tissue toxicities can produce effects that may either augment PO's DNA-reactive mutagenicity or be genotoxic in themselves, not dependent on PO's DNA reactivity. Although its DNA reactivity may be a necessary component of PO's overall genotoxicity and rodent carcinogenicity, it is likely not sufficient, and the associated tissue toxicities, which are rate-limiting, also seem to be required. This complex mode of action has implications for estimations of PO's cancer potential in humans, especially at low exposure concentrations.

Some insights into the mode of action of butadiene by examining the genotoxicity of its metabolites

1,3-Butadiene (BTD) is an important commodity chemical and air pollutant that has been shown to be a potent carcinogen in mice, and to a lesser extent, a carcinogen in rats. To better assess butadiene's carcinogenic risk to humans, it is important to understand its mode of action and how this relates to differences in responses among species. In a series of in vitro experiments, lymphocytes from rats, mice, and humans were exposed to 3,4-epoxy-1-butene (EB) or 1,2:3,4-diepoxybutane (DEB) for 1h at the G(0) stage of the cell cycle, stimulated to divide, and cultured to assess the ability of these metabolites to induce sister chromatid exchange (SCE) and chromosome aberrations (CAs). EB induced no increases in SCEs or CAs in the cells from the three species. DEB was a potent SCE- and CA-inducer, with the results being similar in each rodent species. The response for SCEs seen in the human cells was more complex, with genetic polymorphism for glutathione-S-transferases (GST) possibly modulating the response. The single cell gel electrophoresis assay was used on genetically engineered V79 cell lines to investigate a possible influence of GST status. Experiments were also conducted to investigate the reason for EB's failure to induce SCEs or CAs in G(0) cells. The results indicate that EB-induced DNA damage was repaired before DNA synthesis in unstimulated lymphocytes, but EB caused a large increase in SCEs if actively cycling cells were treated. Thus, the results indicate that DEB damage is persistent in G(0) cells, and DEB is a much more potent genotoxicant than EB. The carcinogenic effect of butadiene will most likely depend on the degree to which DEB is produced and reaches target tissues, and to a lesser extent on the ability of EB to reach actively dividing or repair deficient cells.