Metabolism of 1,3-butadiene to butadiene monoxide in mouse and human bone marrow cells

Connecting volatile organic compounds exposure to osteoporosis risk via oxidative stress based on adverse outcome pathway methodology

Existing evidence has demonstrated the association between exposure to volatile organic compounds (VOCs) and osteoporosis (OP) risk, but the underlying mechanistic framework remains unclear. This study aimed to explore potential pathways using adverse outcome pathway (AOP) analysis, and evidence this association in middle-to-old-aged American adults using the updated National Health and Nutrition Examination Survey data. Multivariable-adjusted general linear and weighted quantile sum models were employed to analyze associations of VOC metabolites (VOCMs), representing internal VOCs exposure levels, with OP-related phenotypes. An AOP framework based on network analysis was developed by extracting target genes and phenotypes. Among 3555 American adults aged ≥ 40 years (539 OP participants), we found that increasing urinary 3- and 4-methylhippuric acid, N-acetyl-S-(n-propyl)-l-cysteine (BPMA), and N-acetyl-S-(3-hydroxypropyl)-l-cysteine were associated with elevated OP odds with odds ratios (ORs) (95 % confidence intervals, 95 % CIs) being 1.254 (1.016 to 1.548), 1.182 (1.014 to 1.377), and 1.244 (1.029 to 1.505), respectively, per standard deviation. Urinary BPMA and N-acetyl-S-(2-cyanoethyl)-l-cysteine were inversely associated with lumbar spine bone mineral density (BMD), while urinary N-acetyl-S-(2-hydroxypropyl)-l-cysteine was positively associated with hip BMD. Additionally, OP odds increased by 46.0 % (95 % CI: 3.9 % to 105.1 %) per quartile increment in the VOC mixture. AOP analysis identified 53 target genes and 9 target phenotypes, and 5 of 9 target phenotypes were oxidative stress (OS)-related. Literature and the "AOP 482″ framework implied the core role of OS in the VOC exposure and prevalent OP association, with the interleukin-6 as the molecular initiating event. Our findings provided a theoretical basis for further investigation.

DNA repair decline during mouse spermiogenesis results in the accumulation of heritable DNA damage

The postmeiotic phase of mouse spermatogenesis (spermiogenesis) is very sensitive to the genomic effects of environmental mutagens because as male germ cells form mature sperm they progressively lose the ability to repair DNA damage. We hypothesized that repeated exposures to mutagens during this repair-deficient phase result in the accumulation of heritable genomic damage in mouse sperm that leads to chromosomal aberrations in zygotes after fertilization. We used a combination of single or fractionated exposures to diepoxybutane (DEB), a component of tobacco smoke, to investigate how differential DNA repair efficiencies during the 3 weeks of spermiogenesis affected the accumulation of DEB-induced heritable damage in early spermatids (21-15 days before fertilization (dbf)), late spermatids (14-8dbf) and sperm (7-1dbf). Analysis of chromosomal aberrations in zygotic metaphases using PAINT/DAPI showed that late spermatids and sperm are unable to repair DEB-induced DNA damage as demonstrated by significant increases (P<0.001) in the frequencies of zygotes with chromosomal aberrations. Comparisons between single and fractionated exposures suggested that the DNA repair-deficient window during late spermiogenesis may be less than 2 weeks in the mouse and that during this repair-deficient window there is accumulation of DNA damage in sperm. Finally, the dose-response study in sperm indicated a linear response for both single and repeated exposures. These findings show that the differential DNA repair capacity of postmeiotic male germ cells has a major impact on the risk of paternally transmitted heritable damage and suggest that chronic exposures that may occur in the weeks prior to fertilization because of occupational or lifestyle factors (i.e., smoking) can lead to an accumulation of genetic damage in sperm and result in heritable chromosomal aberrations of paternal origin.

1,3-Butadiene: exposure estimation, hazard characterization, and exposure-response analysis

1,3-Butadiene has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to 1,3-butadiene primarily through ambient air. Inhaled 1,3-butadiene is carcinogenic in both mice and rats, inducing tumors at multiple sites at all concentrations tested in all identified studies. In addition, 1,3-butadiene is genotoxic in both somatic and germ cells of rodents. It also induces adverse effects in the reproductive organs of female mice at relatively low concentrations. The greater sensitivity in mice than in rats to induction of these effects by 1,3-butadiene is likely related to species differences in metabolism to active epoxide metabolites. Exposure to 1,3-butadiene in the occupational environment has been associated with the induction of leukemia; there is also some limited evidence that 1,3-butadiene is genotoxic in exposed workers. Therefore, in view of the weight of evidence of available epidemiological and toxicological data, 1,3-butadiene is considered highly likely to be carcinogenic, and likely to be genotoxic, in humans. Estimates of the potency of butadiene to induce cancer have been derived on the basis of both epidemiological investigation and bioassays in mice and rats. Potencies to induce ovarian effects have been estimated on the basis of studies in mice. Uncertainties have been delineated, and, while there are clear species differences in metabolism, estimates of potency to induce effects are considered justifiably conservative in view of the likely variability in metabolism across the population related to genetic polymorphism for enzymes for the critical metabolic pathway.

Glutathione S-transferase M1 genotype influences sister chromatid exchange induction but not adaptive response in human lymphocytes treated with 1,2-epoxy-3-butene

Induction of sister chromatid exchanges (SCEs) by 1,2-epoxy-3-butene (monoepoxybutene, MEB), an epoxide metabolite of 1,3-butadiene, in human whole-blood lymphocyte cultures has previously been observed to depend on the glutathione S-transferase M1 (GSTM1) and T1 (GSTT1) genotype of the blood donor. Pretreatment of lymphocyte cultures with a low dose of MEB has been shown to reduce the SCE response obtained by later treatment with a higher concentration of MEB. To investigate whether this adaptive response depends on the GSTM1 genotype of the donor, SCE induction by MEB (25 and 250 microM at 48 h for 24 h) was studied from whole-blood lymphocyte cultures of young non-smoking male and female subjects representing GSTM1 positive (n=7) and null (n=7) genotypes, with or without a MEB pretreatment (12.5 microM at 24 h). A higher mean number of induced SCEs per cell at 250 microM MEB was observed in lymphocytes of the GSTM1 null than positive donors, a statistically significant difference being obtained in the presence of the adaptive treatment (9.44 vs. 6.56; results from ethanol-treated controls subtracted). The pretreatment resulted in a statistically significant reduction in the response of the GSTM1 null group at both concentrations of MEB and in the GSTM1 positive group at 250 microM. However, there were no statistically significant differences in the adaptive response of the two genotypes. In conclusion, the present study further supported earlier findings on an increased sensitivity of GSTM1 null donors to SCE induction by MEB, suggesting that GSTM1 is involved in the detoxification of MEB in human lymphocyte cultures. As an adaptive response was observed in both GSTM1 positive and null donors, the phenomenon cannot be explained by GSTM1 induction. It may represent induction of other enzymes operating in MEB detoxification, or activation of DNA repair.

Butadiene: species comparison for metabolism and genetic toxicology

The synthetic monomer 1,3-butadiene and its metabolites have been reviewed in various in vitro and in vivo metabolic studies and in genetic toxicology assays. The species differences have been compared.

Oxidation of 1,3-butadiene to (R)- and (S)-butadiene monoxide by purified recombinant cytochrome P450 2E1 from rabbit, rat and human

1,3-Butadiene (BD) is a gas used widely in the rubber and plastics industry as an intermediate in production processes and has been detected in automobile exhaust and cigarette smoke. BD requires metabolic activation to exert toxicity and has been shown to be carcinogenic in rodents. IARC has classified BD as a group 2A (probably carcinogenic to humans) carcinogen. The initial oxidation of BD to butadiene monoxide (BMO) occurs primarily via cytochrome P450 2E1 and two stereoisomers of BMO (R and S) can be formed. (R) and (S)-BMO are metabolized differently and demonstrate markedly different toxicities in isolated rat hepatocytes. This work examined the generation of (R) and (S)-BMO from BD by cytochrome P450 2E1 from rabbit, rat and human. BMO level was measured by GC-MS analysis and enantiomeric composition was determined by GC-FID. The greatest rate of formation of BMO from BD was obtained with rabbit cytochrome P4502E1 followed by human and then by rat. Enantiomeric distribution of R and S-BMO produced by the three species demonstrated no significant differences.