Comparison of cytogenetic effects of 3,4-epoxy-1-butene and 1,2:3, 4-diepoxybutane in mouse, rat and human lymphocytes following in vitro G0 exposures

Effects of GSTT1 Genotype on the Detoxification of 1,3-Butadiene Derived Diepoxide and Formation of Promutagenic DNA-DNA Cross-Links in Human Hapmap Cell Lines

Smoking is a leading cause of lung cancer, accounting for 81% of lung cancer cases. Tobacco smoke contains over 5000 compounds, of which more than 70 have been classified as human carcinogens. Of the many tobacco smoke constituents, 1,3-butadiene (BD) has a high cancer risk index due to its tumorigenic potency and its abundance in cigarette smoke. The carcinogenicity of BD has been attributed to the formation of several epoxide metabolites, of which 1,2,3,4-diepoxybutane (DEB) is the most toxic and mutagenic. DEB is formed by two oxidation reactions carried out by cytochrome P450 monooxygenases, mainly CYP2E1. Glutathione-S-transferase theta 1 (GSTT1) facilitates the conjugation of DEB to glutathione as the first step of its detoxification and subsequent elimination via the mercapturic acid pathway. Human biomonitoring studies have revealed a strong association between GSTT1 copy number and urinary concentrations of BD-mercapturic acids, suggesting that it plays an important role in the metabolism of BD. To determine the extent that GSTT1 genotype affects the susceptibility of individuals to the toxic and genotoxic properties of DEB, GSTT1 negative and GSTT1 positive HapMap lymphoblastoid cell lines were treated with DEB, and the extent of apoptosis and micronuclei (MN) formation was assessed. These toxicological end points were compared to the formation of DEB-GSH conjugates and 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) DNA-DNA cross-links. GSTT1 negative cell lines were more sensitive to DEB-induced apoptosis as compared to GSTT1 positive cell lines. Consistent with the protective effect of GSH conjugation against DEB-derived apoptosis, GSTT1 positive cell lines formed significantly more DEB-GSH conjugate than GSTT1 negative cell lines. However, GSTT1 genotype did not affect formation of MN or bis-N7G-BD cross-links. These results indicate that GSTT1 genotype significantly influences BD metabolism and acute toxicity.

Applying Tobacco, Environmental, and Dietary-Related Biomarkers to Understand Cancer Etiology and Evaluate Prevention Strategies

Many human cancers are caused by environmental and lifestyle factors. Biomarkers of exposure and risk developed by our team have provided critical data on internal exposure to toxic and genotoxic chemicals and their connection to cancer in humans. This review highlights our research using biomarkers to identify key factors influencing cancer risk as well as their application to assess the effectiveness of exposure intervention and chemoprevention protocols. The use of these biomarkers to understand individual susceptibility to the harmful effects of tobacco products is a powerful example of the value of this type of research and has provided key data confirming the link between tobacco smoke exposure and cancer risk. Furthermore, this information has led to policy changes that have reduced tobacco use and consequently, the tobacco-related cancer burden. Recent technological advances in mass spectrometry led to the ability to detect DNA damage in human tissues as well as the development of adductomic approaches. These new methods allowed for the detection of DNA adducts in tissues from patients with cancer, providing key evidence that exposure to carcinogens leads to DNA damage in the target tissue. These advances will provide valuable insights into the etiologic causes of cancer that are not tobacco-related.See all articles in this CEBP Focus section, "Environmental Carcinogenesis: Pathways to Prevention."

1,3-Butadiene-Induced Adenine DNA Adducts Are Genotoxic but Only Weakly Mutagenic When Replicated in Escherichia coli of Various Repair and Replication Backgrounds

The adverse effects of the human carcinogen 1,3-butadiene (BD) are believed to be mediated by its DNA-reactive metabolites such as 3,4-epoxybut-1-ene (EB) and 1,2,3,4-diepoxybutane (DEB). The specific DNA adducts responsible for toxic and mutagenic effects of BD, however, have yet to be identified. Recent in vitro polymerase bypass studies of BD-induced adenine (BD-dA) adducts show that DEB-induced N6,N6-DHB-dA (DHB = 2,3-dihydroxybutan-1,4-diyl) and 1,N6-γ-HMHP-dA (HMHP = 2-hydroxy-3-hydroxymethylpropan-1,3-diyl) adducts block replicative DNA polymerases but are bypassed by human polymerases η and κ, leading to point mutations and deletions. In contrast, EB-induced N6-HB-dA (HB = 2-hydroxy-3-buten-1-yl) does not block DNA synthesis and is nonmutagenic. In the present study, we employed a newly established in vivo lesion-induced mutagenesis/genotoxicity assay via next-generation sequencing to evaluate the in vivo biological consequences of S-N6-HB-dA, R,R-N6,N6-DHB-dA, S,S-N6,N6-DHB-dA, and R,S-1,N6-γ-HMHP-dA. In addition, the effects of AlkB-mediated direct reversal repair, MutM and MutY catalyzed base excision repair, and DinB translesion synthesis on the BD-dA adducts in bacterial cells were investigated. BD-dA adducts showed the expected inhibition of DNA replication in vivo but were not substantively mutagenic in any of the genetic environments investigated. This result is in contrast with previous in vitro observations and opens the possibility that E. coli repair and bypass systems other than the ones studied here are able to minimize the mutagenic properties of BD-dA adducts.

Bis-butanediol-mercapturic acid (bis-BDMA) as a urinary biomarker of metabolic activation of butadiene to its ultimate carcinogenic species

Human carcinogen 1,3-butadiene (BD) undergoes metabolic activation to 3,4-epoxy-1-butene (EB), hydroxymethylvinyl ketone (HMVK), 3,4-epoxy-1,2-butanediol (EBD) and 1,2,3,4-diepoxybutane (DEB). Among these, DEB is by far the most genotoxic metabolite and is considered the ultimate carcinogenic species of BD. We have shown previously that BD-exposed laboratory mice form 8- to 10-fold more DEB-DNA adducts than rats exposed at the same conditions, which may be responsible for the enhanced sensitivity of mice to BD-mediated cancer. In the present study, we have identified 1,4-bis-(N-acetyl-L-cystein-S-yl)butane-2,3-diol (bis-BDMA) as a novel DEB-specific urinary biomarker. Isotope dilution high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry was employed to quantify bis-BDMA and three other BD-mercapturic acids, 2-(N-acetyl-L-cystein-S-yl)-1-hydroxybut-3-ene/1-(N-acetyl-L-cystein-S-yl)-2-hydroxy-but-3-ene (MHBMA, from EB), 4-(N-acetyl-L-cystein-S-yl)-1,2-dihydroxybutane (DHBMA, from HMVK) and 4-(N-acetyl-L-cystein-S-yl)-1,2,3-trihydroxybutane (THBMA, from EBD), in urine of confirmed smokers, occupationally exposed workers and BD-exposed laboratory rats. Bis-BDMA was formed in a dose-dependent manner in urine of rats exposed to 0-200 p.p.m. BD by inhalation, although it was a minor metabolite (1%) as compared with DHBMA (47%) and THBMA (37%). In humans, DHBMA was the most abundant BD-mercapturic acid excreted (93%), followed by THBMA (5%) and MHBMA (2%), whereas no bis-BDMA was detected. These results reveal significant differences in metabolism of BD between rats and humans.

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

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

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.

DNA oligomers containing site-specific and stereospecific exocyclic deoxyadenosine adducts of 1,2,3,4-diepoxybutane: synthesis, characterization, and effects on DNA structure

1,2,3,4-Diepoxybutane (DEB) is a carcinogenic metabolite of 1,3-butadiene (BD), an important industrial and environmental chemical present in urban air and in cigarette smoke. DEB is considered the ultimate carcinogenic species of BD because of its potent genotoxicity and mutagenicity attributed to its ability to form DNA-DNA cross-links and exocyclic nucleoside adducts. Mutagenesis studies suggest that DEB adducts formed at adenine bases may be critically important, as it induces large numbers of A → T transversions. We have recently identified three types of exocyclic DEB-dA lesions: N⁶,N⁶-(2,3-dihydroxybutan-1,4-diyl)-2'-deoxyadenosine (N⁶,N⁶-DHB-dA), 1,N⁶-(2-hydroxy-3-hydroxymethylpropan-1,3-diyl)-2'-deoxyadenosine (1,N⁶-γ-HMHP-dA), and 1,N⁶-(1-hydroxymethyl-2-hydroxypropan-1,3-diyl)-2'-deoxyadenosine (1,N⁶-α-HMHP-dA) [Seneviratne, U., et al. (2010) Chem. Res. Toxicol. 23, 118-133]. In the work presented here, a postsynthetic methodology for preparing DNA oligomers containing stereospecific and site-specific N⁶,N⁶-DHB-dA and 1,N⁶-γ-HMHP-dA adducts was developed. DNA oligomers containing site-specific 6-chloropurine were coupled with optically pure 1-amino-2-hydroxy-3,4-epoxybutanes to generate oligomers containing N⁶-(2-hydroxy-3,4-epoxybut-1-yl)-2'-deoxyadenosine adducts, followed by their spontaneous cyclization to 1,N⁶-γ-HMHP-dA lesions. N⁶,N⁶-DHB-dA containing strands were prepared analogously by coupling 6-chloropurine containing DNA with (3S,4S)- or (3R,4R)-pyrrolidine-3,4-diols. Oligodeoxynucleotide structures were confirmed by ESI-MS, exonuclease ladder sequencing, and HPLC-MS/MS of enzymatic digests. UV melting and CD spectroscopy studies of DNA duplexes containing N⁶,N⁶-DHB-dA and 1,N⁶-γ-HMHP-dA revealed that both lesions lower the thermodynamic stability of DNA. Interestingly, structurally modified DNA duplexes were more thermodynamically stable when an adenine residue was placed opposite 1,N⁶-γ-HMHP-dA instead of thymine, suggesting that these adducts may preferentially pair with dA.

Molecular dosimetry of 1,2,3,4-diepoxybutane-induced DNA-DNA cross-links in B6C3F1 mice and F344 rats exposed to 1,3-butadiene by inhalation

1,3-Butadiene (BD) is an important industrial and environmental chemical classified as a human carcinogen based on epidemiologic studies in occupationally exposed workers and animal studies in laboratory rats and mice. BD is metabolically activated to three epoxides that can react with nucleophilic sites in biomolecules. Among these, 1,2,3,4-diepoxybutane (DEB) is considered the ultimate carcinogen due to its high genotoxicity and mutagenicity attributed to its ability to form DNA-DNA cross-links. Our laboratory has developed quantitative high-performance liquid chromatography-muESI(+)-tandem mass spectrometry methods for two DEB-specific DNA-DNA cross-links, 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) and 1-(guan-7-yl)-4-(aden-1-yl)-2,3-butanediol (N7G-N1A-BD). This report describes molecular dosimetry analysis of these adducts in tissues of B6C3F1 mice and F344 rats exposed to a range of BD concentrations (0-625 ppm). Much higher (4- to 10-fold) levels of DEB-DNA cross-links were observed in mice compared with rats exposed to the same BD concentrations. In both species, bis-N7G-BD levels were 1.5- to 4-fold higher in the liver than in other tissues examined. Interestingly, tissues of female animals exposed to BD contained higher concentrations of bis-N7G-BD adducts than tissues of male animals, which is in accord with previously reported differences in tumor incidence. The molecular dosimetry data presented herein suggest that species and gender differences observed in BD-induced cancer are directly related to differences in the extent of BD metabolism to DEB. Furthermore, a rat model of sensitivity to BD may be more appropriate than a mouse model for assessing human risk associated with BD exposure, because rats and humans seem to be similar with respect to DEB formation.

Quantitative high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry analysis of the adenine-guanine cross-links of 1,2,3,4-diepoxybutane in tissues of butadiene-exposed B6C3F1 mice

1,3-Butadiene (BD) is an important industrial chemical used in the manufacture of rubber and plastics as well as an environmental pollutant present in automobile exhaust and cigarette smoke. It is classified as a known human carcinogen based on the epidemiological evidence in occupationally exposed workers and its ability to induce tumors in laboratory animals. BD is metabolically activated to several reactive species, including 1,2,3,4-diepoxybutane (DEB), which is hypothesized to be the ultimate carcinogenic species due to its bifunctional electrophilic nature and its ability to form DNA-DNA and DNA-protein cross-links. While 1,4- bis-(guan-7-yl)-2,3,-butanediol ( bis-N7G-BD) is the only type of DEB-specific DNA adduct previously quantified in vivo, four regioisomeric guanine-adenine (G-A) cross-links have been observed in vitro: 1-(guan-7-yl)-4-(aden-1-yl)-2,3-butanediol (N7G-N1A-BD), 1-(guan-7-yl)-4-(aden-3-yl)-2,3-butanediol (N7G-N3A-BD), 1-(guan-7-yl)-4-(aden-7-yl)-2,3-butanediol (N7G-N7A-BD), and 1-(guan-7-yl)-4-(aden-6-yl)-2,3-butanediol (N7G-N (6)A-BD) ( Park ( 2004) Chem. Res. Toxicol. 17, 1638- 1651 ). The goal of the present work was to develop an isotope dilution HPLC-positive mode electrospray ionization-tandem mass spectrometry (HPLC-ESI (+)-MS/MS) method for the quantitative analysis of G-A DEB cross-links in DNA extracted from BD-exposed laboratory animals. In our approach, G-A butanediol conjugates are released from the DNA backbone by thermal or mild acid hydrolysis. Following solid-phase extraction, samples are subjected to capillary HPLC-ESI (+)-MS/MS analysis with (15)N 3, (13)C 1-labeled internal standards. The detection limit of our current method is 0.6-1.5 adducts per 10 (8) normal nucleotides. The new method was validated by spiking G-A cross-link standards (10 fmol each) into control mouse DNA (0.1 mg), followed by sample processing and HPLC-ESI (+)-MS/MS analysis. The accuracy and precision were calculated as 105 +/- 17% for N7G-N3A-BD, 102 +/- 25% for N7G-N7A-BD, and 79 +/- 11% for N7G-N (6)A-BD. The regioisomeric G-A DEB adducts were formed in a concentration-dependent manner in DEB-treated calf thymus DNA, with N7G-N1A-BD found in the highest amounts. Under physiological conditions, N7G-N1A-BD underwent Dimroth rearrangement to N7G-N (6)A-BD ( t 1/2 = 114 h), while hydrolytic deamination of N7G-N1A-BD to the corresponding hypoxanthine lesion was insignificant. We found that for in vivo samples, a greater sensitivity could be achieved if N7G-N1A-BD adducts were converted to the corresponding N7G-N (6)A-BD lesions by forced Dimroth rearrangement. Liver DNA extracted from female B6C3F1 mice that underwent inhalation exposure to 625 ppm BD for 2 weeks contained 3.1 +/- 0.6 N7G-N1A-BD adducts per 10 (8) nucleotides ( n = 5) (quantified as N7G-N (6)A-BD following base-induced Dimroth rearrangement), while the amounts of N7G-N3A-BD and N7G-N7A-BD were below the detection limit of our method. None of the G-A cross-links was present in control animals. The formation of N7G-N1A-BD cross-links may contribute to the induction of AT base pair mutations following exposure to BD. Quantitative methods presented here may be used not only for studies of biological significance in animal models but potentially to predict risk associated with human exposure to BD.

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.

Toxicology of 1,3-butadiene, chloroprene, and isoprene

The diene monomers, 1,3-butadiene, chloroprene, and isoprene, respectively, differ only in substitution of a hydrogen, a chlorine, or a methyl group at the second of the four unsaturated carbon atoms in these linear molecules. Literature reviewed in the preceding sections indicates that these chemicals have important uses in synthesis of polymers, which offer significant benefits within modern society. Additionally, studies document that these monomers can increase the tumor formation rate in various organs of rats and mice during chronic cancer bioassays. The extent of tumor formation versus animal exposure to these monomers varies significantly across species, as well among strains within species. These studies approach, but do not resolve, important questions of human risk from inhalation exposure. Each of these diene monomers can be activated to electrophilic epoxide metabolites through microsomal oxidation reactions in mammals. These epoxide metabolites are genotoxic through reactions with nucleic acids. Some of these reactions cause mutations and subsequent cancers, as noted in animal experiments. Significant differences exist among the compounds, particularly in the extent of formation of highly mutagenic diepoxide metabolites, when animals are exposed. These metabolites are detoxified through hydrolysis by epoxide hydrolase enzymes and through conjugation with glutathione with the aid of glutathione S-transferase. Different strains and species perform these reactions with varying efficacy. Mice produce these electrophilic epoxides more rapidly and appear to have less adequate detoxification mechanisms than rats or humans. The weight of evidence from many studies suggests that the balance of activation versus detoxification offers explanation of differing sensitivities of animals to these carcinogenic actions. Other aspects, including molecular biology of the many processes that lead through specific mutations to cancer, are yet to be understood. Melnick and Sills (2001) compared the carcinogenic potentials of these three dienes, along with that of ethylene oxide, which also acts through an epoxide intermediate. From the number of tissue sites where experimental animal tumors were detected, butadiene offers greatest potential for carcinogenicity of these dienes. Chloroprene and then isoprene appear to follow in this order. Comparisons among these chemicals based on responses to external exposures are complicated by differences among studies and of species and tissue susceptibilities. Physiologically based pharmacokinetic models offer promise to overcome these impediments to interpretation. Mechanistic studies at the molecular level offer promise for understanding the relationships among electrophilic metabolites and vital genetic components. Significant improvements in minimization of industrial worker exposures to carcinogenic chemicals have been accomplished after realization that vinyl chloride caused hepatic angiosarcoma in polymer production workers (Creech and Johnson 1974; Falk et al. 1974). Efforts continue to minimize disease, particularly cancer, from exposures to chemicals such as these dienes. Industry has responded to significant challenges that affect the health of workers through efforts that minimize plant exposures and by sponsorship of research, including animal and epidemiological studies. Governmental agencies provide oversight and have developed facilities that accomplish studies of continuing scientific excellence. These entities grapple with differences in perspective, objectives, and interpretation as synthesis of knowledge develops through mutual work. A major challenge remains, however, in assessment of significance of environmental human exposures to these dienes. Such exposure levels are orders of magnitude less than exposures studied in experimental or epidemiological settings, but exposures may persist much longer and may involve unknown but potentially significant sensitivities in the general population. New paradigms likely will be needed for toxicological evaluation of these human exposures, which are ongoing but as yet are not interpreted.

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.

Acute exposure of human lung cells to 1,3-butadiene diepoxide results in G1 and G2 cell cycle arrest

1,3-Butadiene (BD) causes genetic damage, including adduct formation, sister chomatid exchange, and point mutations. Previous studies have focused on the types of genetic damage and tumors found after long-term exposure of rodents to butadiene. This study examined the effect of the most active BD metabolite, butadiene diepoxide (BDO2), on cell cycle entry and progression in human lung fibroblasts (LU cells) with a normal diploid karyotype. Serum-arrested (G0) LU cells were exposed to BDO2 for 1 hr and stimulated to divide with medium containing 10% fetal bovine serum. The BDO2-treated LU cells were evaluated for cell cycle progression, nuclear localization of arrest mediators, mitotic index, and cellular proliferation. The BDO2-treated cells demonstrated a substantial inhibition of cell proliferation when treated with 100 microM BDO2 for 1 hr. No appreciable levels of apoptosis or mitotic figures were observed in the BDO2-treated cells through 96 hr posttreatment. Flow cytometric analysis revealed that the lack of proliferation in BDO2-treated LU cells was related to G1 arrest in about half of the cells and a delayed progression through S and G2 arrest in nearly all of the remaining cells. Both G1 and G2 arrest were prolonged and only a very small percentage of BDO2-treated cells were eventually able to replicate. Increased nuclear localization of both p53 and p21(cip1) was observed in BDO2-treated cells, suggesting that the cell cycle arrest was p21(cip1)-mediated. These results demonstrate that BDO2 induces cell cycle perturbation and arrest even with short-term exposure that does not produce other pathologic cellular effects.

Genetic susceptibility, biomarker respones, and cancer

A large number of studies have reported associations between polymorphisms of xenobiotic-metabolizing enzymes (XMEs) and various cancers. However, the carcinogenic exposures behind such findings have usually been unclear. Information on susceptibility to specific carcinogens could better be obtained by examining situations where the exposure and the endpoint studied are nearer in time, i.e., by studying biomarkers of carcinogen exposure and early (genotoxic) effect in exposed humans. For example, analyses of DNA adducts and cytogenetic endpoints have indicated an increased susceptibility of glutathione S-transferase M1 (GSTM1) null genotype to genotoxicity of tobacco smoking, supporting the view that the associations of the GSTM1 null genotype with bladder and lung cancer are partly related to smoking. In vitro genotoxicity studies with human cells offer an experimental tool that can be used, within the limits of the cell systems, to predict individual sensitivity and genotype-carcinogen interactions. In vitro sensitivity to the genotoxicity of 1,2:3,4-diepoxybutane, an epoxide metabolite of 1,3-butadiene has clearly been shown to depend on GSTT1 genotype, which has also been implicated to modify, along with GSTM1 genotype, the in vitro genotoxicity of 1,2-epoxy-3-butene, another epoxide metabolite of 1,3-butadiene. These genotypes appear to modulate the excretion of 1,3-butadiene-specific mercapturic acids, and influence genotoxicity biomarker levels in 1,3-butadiene-exposed workers. The excretion of specific mercapturic acids (PHEMA) in workers exposed to styrene has clearly been shown to depend on GSTM1 genotype, and GSTT1 genotype seems to modulate the excretion of one PHEMA diastereoisomer. These genotypes have also been implicated to modulate the in vitro genotoxicity of styrene. In general, the genetic polymorphisms potentially important for biomarker response largely depend on the exposing agent, biological material examined, and ethnicity of the population under study. Individual exposure level may vary a lot, and a reliable estimate of the exposure is essential for correct interpretation of genotype-exposure interaction. Besides XME polymorphisms, any polymorphisms that affect cellular response to DNA damage could, in principle, modify individual sensitivity to genotoxins. For instance, those concerning DNA repair proteins are presently being studied by many laboratories.

Role of haemoglobin in the protection of cultured lymphocytes against diepoxybutane (DEB), assessed by in vitro induced chromosome breakage

Diepoxybutane (DEB) is an alkylating agent that can be used to assess chromosome instability in repair-deficient subjects. Previous authors investigated the role of red blood cells (RBC) in determining individual susceptibility to DEB in normal healthy donors, and demonstrated that a polymorphic enzyme in RBC, Glutathione S-transferase T1 (GSTT1), is involved in DEB detoxification. In the present work we studied the influence of individual GSTM1 and GSTT1 genotypes and the presence of RBC on the frequency of DEB-induced chromosome breakage in lymphocyte cultures from normal individuals and, in particular, the influence of isolated components of RBC: RBC membranes, RBC lysate, and haemoglobin. Our results confirm that individual GSTT1 genotypes modulate the level of genetic lesions induced by DEB; however, this effect was not sufficient to explain the highly significant variation in chromosome breakage between whole blood and RBC-depleted cultures. We showed that RBC can protect cultured lymphocytes against chromosome breakage induced by DEB and we demonstrated the particular role of haemoglobin in the protective effect.

Diepoxybutane and mitomycin C toxicity is associated with the induction of oxidative DNA damage in sea urchin embryos

Diepoxybutane (DEB)- and mitomycin C (MMC)-associated toxicity was investigated in embryos from the sea urchin (SU) species Sphaerechinus granularis. DEB- and MMC-induced toxicity resulted in S. granularis embryos and larvae at concentrations ranging 10(-5) to 10(-4) M DEB, and 3 x 10(-6) to 3 x 10(-5) M MMC, in terms of larval malformations, developmental arrest and mortality. The formation of DNA oxidative damage, 8-hydroxy-2'-deoxyguanosine (8-OHdG) was measured in DEB- and in MMC-exposed embryos (at gastrula stage). A dose-dependent increase in 8-OHdG levels was observed that was significantly correlated with DEB- and MMC-induced developmental defects. The results lend further support to the body of evidence associating both DEB and MMC toxicity with oxidative stress, including DNA oxidative damage.

Genetic polymorphisms and chromosome damage

Genetic polymorphisms that affect xenobiotic metabolism or cellular response to DNA damage can modulate individual sensitivity to genotoxins. Information on the effects of such polymorphisms on the level of chromosome damage may facilitate the identification of risk groups and increase the sensitivity of cytogenetic endpoints as biomarkers of genotoxic exposure and effect. Glutathione S-transferase M1 (GSTM1) is an important detoxification enzyme which, due to a homozygous gene deletion (null genotype), is lacking from about 50% of Caucasians. A higher level of DNA adducts and chromosome damage has been detected in lymphocytes of tobacco smokers and bus drivers who lack the GSTM1 gene. Other polymorphic glutathione S-transferases include GSTM3, GSTP1, and GSTT1. The GSTT1 null genotype (10-20% of Caucasians) has been associated with an increased "baseline" level of sister chromatid exchanges (SCEs) in lymphocytes. N-acetyltransferase 2 (NAT2), metabolizing xenobiotics with primary aromatic amine and hydrazine structures, is another important polymorphic phase II enzyme. Subjects having the NAT2 slow acetylator genotype appear to show an increased baseline frequency of lymphocyte CAs in the absence of identified environmental exposure. Besides human biomonitoring studies, genetic polymorphisms may be important in explaining individual variation in genotoxic response observed in genetic toxicology tests with human cells. Several studies have suggested that blood cultures from GSTT1 null and GSTM1 null individuals have increased in vitro sensitivity to various genotoxins. The best-known example is probably the diepoxybutane sensitivity of GSTT1 null donors. Recently discovered polymorphisms affecting DNA repair may be expected to be of special importance in modulating genotoxic effects; the first available studies have suggested that the exon 10 Arg399Gln polymorphism of XRCC1 gene (X-ray repair cross-complementing group 1) could affect individual genotoxic response. In conclusion, the genetic polymorphism of GSTM1 influences the frequency of chromosome damage in exposed humans, while that of GSTT1 and NAT2 affect the "baseline" level of such damage. Both GSTM1 and GSTT1 genotypes may shape the in vitro genotoxic response of human lymphocytes. The significance of DNA repair polymorphisms is presently unclear.

Genetic and reproductive toxicity of butadiene and isoprene

Butadiene (BD) and its 2-methyl analogue, isoprene, have been extensively studied in animals and BD in population studies. Both chemicals are metabolised by liver cytochrome P450 dependent monogenases to monoepoxide and diepoxide intermediates. The diepoxide intermediates of both compounds were mutagenic in Salmonella typhimurium. However, unlike the monoepoxide of BD, the monoepoxides of isoprene were not mutagenic. It appears that they have no alkylating capacity. BD did not induce somatic cell mutation and recombination or sex-linked recessive lethal mutation in Drosophila melanogaster and isoprene produced no increase in chromosomal aberrations in CHO cells in vitro. Comparative concentrations of haemoglobin adducts in the blood of mice and rats after exposure to BD indicated that reaction with blood may decrease the levels of reactive intermediates available to tissues in rats, but not in mice contributing to greater potency of BD in the mouse. For isoprene, the adducts reach approximately the same concentrations in both species. DNA adducts have also been detected in testicular and lung cells of mice after BD exposure. The level of epoxybutene haemoglobin adducts was significantly elevated in BD-exposed workers, but lower than in rats and mice. In conjunction with the toxicology and carcinogenesis studies for BD and isoprene, additional mice were included for the evaluation of cytogenetic effects. Both chemicals produced increases in sister chromatid exchanges in bone marrow cells and in the frequency of micronuclei in normochromatic and polychromatic erythrocytes, but only BD produced an increase in the percent of bone marrow cells with chromosomal aberrations. At similar doses, the effects with BD were 2-3 times larger than with isoprene. There were also increased hprt mutation frequencies in rats and mice after BD exposure. Biomonitoring studies with hprt mutations in lymphocytes showed conflicting results, with both positive and negative findings. BD has been shown to be positive in one human cytogenetic biomonitoring study and not in three others, but chromosomal aberrations were increased in BD-exposed workers after challenge with gamma rays. Re-analysis of GSTTI null individuals showed positive results. There was an increase in spermatid micronuclei in mice by BD and its metabolites and in rats only by its metabolites. The cytotoxic response of germ cells in mice is greater than in rats. Dominant lethal mutations have been induced by BD and diepoxybutane, but not by epoxybutene. There was some evidence of congenital malformations in mice after BD exposure and there was a linear concentration-related induction of heritable translocations in mice. There was no induction of dominant lethal mutations or congenital malformations in rats. Using the heritable translocation data in mice, it has been determined that if a worker is continually exposed over 5 or 6 weeks to 20-25 ppm of BD, the risk of producing a child with a balanced reciprocal translocation is twice as high as the background risk. Since genetic damage cannot be measured directly in human germ cells, risk to such cells can also be estimated from germ cells and somatic cells of the mouse and human somatic cells using the parallelogram approach. Using doubling doses, the fourth corner of the parallelogram was calculated as a doubling dose for human germ cells of 4390 ppm/h. However, it is still questioned if man is more like rat than mouse in terms of sensitivity to exposure. Similar germ cell data do not exist for isoprene. In conventional developmental studies, where rats and mice were exposed to BD, maternal toxicity was shown in rats but there was no evidence of developmental toxicity or teratogenic effects and there was a small effect on sperm morphology. After exposure to isoprene, there was no adverse effect on rat dams or other reproductive indices. In mice, there was reduced foetal body weight and decreased maternal weight gain and isoprene also affected ovarian follicles. There was a reduction in testicular function parameters such as testicular weight and sperm motility.

A review of the genetic and related effects of 1,3-butadiene in rodents and humans

In this paper, the metabolism and genetic toxicity of 1,3-butadiene (BD) and its oxidative metabolites in humans and rodents is reviewed with attention to newer data that have been published since the latest evaluation of BD by the International Agency for Research on Cancer (IARC). The oxidative metabolism of BD in mice, rats and humans is compared with emphasis on the major pathways leading to the reactive intermediates 1,2-epoxy-3-butene (EB), 1,2:3, 4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBdiol). Results from recent studies of DNA and hemoglobin adducts indicate that EBdiol may play a more significant role in the toxicity of BD than previously thought. All three metabolites are capable of reacting with macromolecules, such as DNA and hemoglobin, and have been shown to induce a variety of genotoxic effects in mice and rats as well as in human cells in vitro. DEB is clearly the most potent of these genotoxins followed by EB, which in turn is more potent than EBdiol. Studies of mutations in lacI and lacZ mice and of the Hprt mutational spectrum in rodents and humans show that mutations at G:C base pairs are critical events in the mutagenicity of BD. In-depth analyses of the mutational spectra induced by BD and/or its oxidative metabolites should help to clarify which metabolite(s) are associated with specific mutations in each animal species and which mutational events contribute to BD-induced carcinogenicity. While the quantitative relationship between exposure to BD, its genotoxicity, and the induction of cancer in occupationally exposed humans remains to be fully established, there is sufficient data currently available to demonstrate that 1,3-butadiene is a probable human carcinogen.

Mammalian class theta GST and differential susceptibility to carcinogens: a review

Glutathione S-transferases (GSTs) are an important part of the cellular detoxification system and, perhaps, evolved to protect cells against reactive oxygen metabolites. Theta is considered the most ancient among the GSTs and theta-like GSTs are found in mammals, fish, insects, plants, unicellular algae, and bacteria. It is thought that an ancestral theta-gene underwent an early duplication before the divergence of fungi and animals and further duplications generated the variety of the other classes of GSTs (alpha, mu, phi, etc.). The comparison of the aminoacidic homologies among mammals suggests that a duplication of an ancient GST theta occurred before the speciation of mammals and resulted in the subunits GSTT1 and GSTT2. The ancestral GST theta has a dehalogenase activity towards several halogenated compounds, such as the dichloromethane. In fact, some aerobic and anaerobic methylotrophic bacteria can use these molecules as the sole carbon and energy source. The mammalian GST theta cannot sustain the growth of bacteria but still retains the dehalogenating activity. Therefore, although mammalian GST theta behaves as a scavenger towards electrophiles, such as epoxides, it acts also as metabolic activator for halogenated compounds, producing a variety of intermediates potentially dangerous for DNA and cells. For example, mice exposed to dichloromethane show a dose-dependent incidence of cancer via the GSTT1-1 pathway. Because GSTT1-1 is polymorphic in humans, with about 20% of Caucasians and 80% of Asians lacking the enzyme, the relationship between the phenotype and the incidence of cancer has been investigated extensively in order to detect GSTT1-1-associated differential susceptibility towards endogenous or exogenous carcinogens. The lack of the enzyme is related to a slightly increased risk of cancer of the bladder, gastro-intestinal tract, and for tobacco-related tumors (lung or oral cavity). More pronounced risks were found in males with the GSTT1-null genotype for brain diseases and skin basal cell carcinomas not related to sunlight exposures. Moreover, there was an increased risk of kidney and liver tumors in humans with the GSTT1-1 positive genotype following exposures to halogenated solvents. Interestingly, the liver and kidney are two organs that express the highest level of GST theta in the human body. Thus, the GSTT1-1 genotype is suspected to confer decreased or increased risk of cancer in relation to the source of exposure; in vitro studies, mostly conducted on metabolites of butadiene, confirm the protective action of GSTT1-1, whereas, thus far, experimental studies prove that the increasing risk is limited.

Butadiene-induced intrastrand DNA cross-links: a possible role in deletion mutagenesis

To initiate studies designed to identify the mutagenic spectrum associated with butadiene diepoxide-induced N(2)-N(2) guanine intrastrand cross-links, site specifically adducted oligodeoxynucleotides were synthesized in which the adducted bases were centrally located within the context of the human ras 12 codon. The two stereospecifically modified DNAs and the corresponding unmodified DNA were ligated into a single-stranded M13mp7L2 vector and transfected into Escherichia coli. Both stereoisomeric forms (R, R and S,S) of the DNA cross-links resulted in very severely decreased plaque-forming ability, along with an increased mutagenic frequency for both single base substitutions and deletions compared with unadducted DNAs, with the S,S stereoisomer being the most mutagenic. Consistent with decreased plaque formation, in vitro replication of DNA templates containing the cross-links by the three major E. coli polymerases revealed replication blockage by both stereoisomeric forms of the cross-links. The same DNAs that were used for replication studies were also assembled into duplex DNAs and tested as substrates for the initiation of nucleotide excision repair by the E. coli UvrABC complex. UvrABC incised linear substrates containing these intrastrand cross-links with low efficiency, suggesting that these lesions may be inefficiently repaired by the nucleotide excision repair system.

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

3,4-epoxy-1-butene (EB), a primary metabolite of butadiene, is a direct-acting "S-dependent" genotoxicant that can induce sister chromatid exchanges (SCEs) and chromosome aberrations (CAs) in cycling cells in vitro. However, EB is almost inactive when splenic or peripheral blood lymphocytes are exposed at the G(0) stage of the cell cycle. To investigate whether repair of DNA lesions is responsible for the lack of cytogenetic responses seen after G(0) treatments, we used cytosine arabinoside (ara-C) to inhibit DNA polymerization during DNA repair. If enough repairable lesions are present, double-strand breaks should accumulate and form chromosome-type ("S-independent") deletions and exchanges. This is exactly what occurred. EB induced chromosome deletions and dicentrics at the first division following treatment, when the EB exposure was followed by ara-C. Without ara-C treatment, there was no induction of CAs. These experiments indicate that the relatively low levels of damage induced by EB in G(0) lymphocytes are removed by DNA repair prior to DNA synthesis and thus, before the production of SCEs or chromatid-type aberrations.