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.

Interindividual Differences in DNA Adduct Formation and Detoxification of 1,3-Butadiene-Derived Epoxide in Human HapMap Cell Lines.

Smoking-induced lung cancer is a major cause of cancer mortality in the US and worldwide. While 11-24% of smokers will develop lung cancer, risk varies among individuals and ethnic/racial groups. Specifically, African American and Native Hawaiian cigarette smokers are more likely to get lung cancer as compared to Caucasians, Japanese Americans, and Latinos. It is important to identify smokers who are at the greatest risk of developing lung cancer as they should be candidates for smoking cessation and chemopreventive intervention programs. Among 60+ tobacco smoke carcinogens, 1,3-butadiene (BD) is one of the most potent and abundant (20-75 µg per cigarette in mainstream smoke and 205-361 µg per cigarette in side stream smoke). BD is metabolically activated to 3,4-epoxy-1-butene (EB), which can be detoxified by glutathione S-transferase theta 1 (GSTT1)-mediated conjugation with glutathione, or can react with DNA to form 7-(1-hydroxy-3-buten-2-yl)guanine (EB-GII) adducts. In the present study, we employed EBV-transformed human lymphoblastoid cell lines (HapMap cells) with known GSTT1 genotypes to examine the influence of the GSTT1 gene on interindividual variability in butadiene metabolism, DNA adduct formation/repair, and biological outcomes (apoptosis). We found that GSTT1- HapMap cells treated with EB in culture produced lower levels of glutathione conjugates and were more susceptible to apoptosis but had similar numbers of EB-GII adducts as GSTT1+ cells. Our results suggest that GSTT1 can influence an individual's susceptibility to butadiene-derived epoxides.

Urinary N7-(1-hydroxy-3-buten-2-yl) guanine adducts in humans: temporal stability and association with smoking.

1,3-Butadiene (BD) is a known human carcinogen found in cigarette smoke, automobile exhaust, and urban air. Workers occupationally exposed to BD in the workplace have an increased incidence of leukemia and lymphoma. BD undergoes cytochrome P450-mediated metabolic activation to 3,4-epoxy-1-butene (EB), 1,2,3,4-diepoxybutane (DEB) and 1,2-dihydroxy-3,4-epoxybutane (EBD), which form covalent adducts with DNA. We have previously reported a quantitative nanoLC/ESI+-HRMS3 method for urinary N7-(1-hydroxy-3-buten-2-yl) guanine (EB-GII) adducts as a mechanism-based biomarker of BD exposure. In the present study, the method was updated to include high throughput 96-well solid phase extraction (SPE) and employed to establish urinary EB-GII biomarker stability and association with smoking. Urinary EB-GII levels were measured bimonthly for 1 year in 19 smokers to determine whether single adduct measurement provides reliable levels of EB-GII in an individual smoker. In addition, association of EB-GII with smoking was studied in 17 individuals participating in a smoking cessation program. EB-GII levels decreased 34% upon smoking cessation, indicating that it is associated with smoking status, but may also originate from sources other than exposure to cigarette smoke.

1,3-Butadiene metabolite 1,2,3,4 diepoxybutane induces DNA adducts and micronuclei but not t(9;22) translocations in human cells.

Epidemiological studies of 1,3-butadiene (BD) exposures have reported a possible association with chronic myelogenous leukemia (CML), which is defined by the presence of the t(9;22) translocation (Philadelphia chromosome) creating an oncogenic BCR-ABL fusion gene. Butadiene diepoxide (DEB), the most mutagenic of three epoxides resulting from BD, forms DNA-DNA crosslink adducts that can lead to DNA double-strand breaks (DSBs). Thus, a study was designed to determine if (±)-DEB exposure of HL60 cells, a promyelocytic leukemia cell line lacking the Philadelphia chromosome, can produce t(9;22) translocations. In HL60 cells exposed for 3 h to 0-10 µM DEB, overlapping dose-response curves suggested a direct relationship between 1,4-bis-(guan-7-yl)-2,3-butanediol crosslink adduct formation (R = 0.977, P = 0.03) and cytotoxicity (R = 0.961, P = 0.002). Experiments to define the relationships between cytotoxicity and the induction of micronuclei (MN), a dosimeter of DNA DSBs, showed that 24 h exposures of HL60 cells to 0-5.0 µM DEB caused significant positive correlations between the concentration and (i) the degree of cytotoxicity (R = 0.998, p = 0.002) and (ii) the frequency of MN (R = 0.984, p = 0.016) at 48 h post exposure. To determine the relative induction of MN and t(9;22) translocations following exposures to DEB, or x-rays as a positive control for formation of t(9;22) translocations, HL60 cells were exposed for 24 h to 0, 1, 2.5, or 5 µM DEB or to 0, 2.0, 3.5, or 5.0 Gy x-rays, or treatments demonstrated to yield 0, 20%, 50%, or 80% cytotoxicity. Treatments between 0 and 3.5 Gy x-rays caused significant dose-related increases in both MN (p < 0.001) and t(9;22) translocations (p = 0.01), whereas DEB exposures causing similar cytotoxicity levels did not increase translocations over background. These data indicate that, while DEB induces DNA DSBs required for formation of MN and translocations, acute DEB exposures of HL60 cells did not produce the Philadelphia chromosome obligatory for CML.

Discovery of Novel N-(4-Hydroxybenzyl)valine Hemoglobin Adducts in Human Blood.

Humans are exposed to a wide range of electrophilic compounds present in our diet and environment or formed endogenously as part of normal physiological processes. These electrophiles can modify nucleophilic sites of proteins and DNA to form covalent adducts. Recently, powerful untargeted adductomic approaches have been developed for systematic screening of these adducts in human blood. Our earlier untargeted adductomics study detected 19 unknown adducts to N-terminal valine in hemoglobin (Hb) in human blood. We now describe a full characterization of one of these adducts, which corresponds to the addition of a 4-hydroxybenzyl (4-OHBn) group to N-terminal valine in Hb to form N(4-hydroxybenzyl)valine (4-OHBn-Val). The adduct structure was determined by comparison of its accurate mass, HPLC retention time, and MS/MS fragmentation to that of authentic standards prepared by chemical synthesis. Average 4-OHBn-Val adduct concentrations in 12 human blood samples were estimated to 380 ± 160 pmol/g Hb. Two possible routes of 4-OHBnVal adduct formation are proposed using two different precursor electrophiles: 4-quinone methide (4-QM) and 4-hydroxybenzaldehyde (4-OHBA). We found that 4-QM reacts rapidly with valine to form the 4-OHBn-Val adduct; however, the quinone methide is unstable under physiological conditions due to hydrolysis. It was shown that 4-OHBA forms reversible Schiff base adducts with valine, which can be stabilized via reduction in blood generating the 4-OHBn-Val adduct. In addition, trace amounts of isomeric 2-hydroxybenzyl-valine (2-OHBn-Val) adducts were detected in 12 human blood samples (estimated mean adduct level, 5.0 ± 1.4 pmol/g Hb). Further studies are needed to quantify the contributions from identified possible precursor electrophiles to the observed hydroxybenzyl adducts in humans.

Cellular Repair of DNA-DNA Cross-Links Induced by 1,2,3,4-Diepoxybutane.

Xenobiotic-induced interstrand DNA-DNA cross-links (ICL) interfere with transcription and replication and can be converted to toxic DNA double strand breaks. In this work, we investigated cellular responses to 1,4-bis-(guan-7-yl)-2,3-butanediol (bis-N7G-BD) cross-links induced by 1,2,3,4-diepoxybutane (DEB). High pressure liquid chromatography electrospray ionization tandem mass spectrometry (HPLC-ESI⁺-MS/MS) assays were used to quantify the formation and repair of bis-N7G-BD cross-links in wild-type Chinese hamster lung fibroblasts (V79) and the corresponding isogenic clones V-H1 and V-H4, deficient in the XPD and FANCA genes, respectively. Both V-H1 and V-H4 cells exhibited enhanced sensitivity to DEB-induced cell death and elevated bis-N7G-BD cross-links. However, relatively modest increases of bis-N7G-BD adduct levels in V-H4 clones did not correlate with their hypersensitivity to DEB. Further, bis-N7G-BD levels were not elevated in DEB-treated human clones with defects in the XPA or FANCD2 genes. Comet assays and γ-H2AX focus analyses conducted with hamster cells revealed that ICL removal was associated with chromosomal double strand break formation, and that these breaks persisted in V-H4 cells as compared to control cells. Our findings suggest that ICL repair in cells with defects in the Fanconi anemia repair pathway is associated with aberrant re-joining of repair-induced double strand breaks, potentially resulting in lethal chromosome rearrangements.

Mass Spectrometry-Based Tools to Characterize DNA-Protein Cross-Linking by Bis-Electrophiles.

DNA-protein cross-links (DPCs) are unusually bulky DNA adducts that form in cells as a result of exposure to endogenous and exogenous agents including reactive oxygen species, ultraviolet light, ionizing radiation, environmental agents (e.g. transition metals, formaldehyde, 1,2-dibromoethane, 1,3-butadiene) and common chemotherapeutic agents. Covalent DPCs are cytotoxic and mutagenic due to their ability to interfere with faithful DNA replication and to prevent accurate gene expression. Key to our understanding of the biological significance of DPC formation is identifying the proteins most susceptible to forming these unusually bulky and complex lesions and quantifying the extent of DNA-protein cross-linking in cells and tissues. Recent advances in bottom-up mass spectrometry-based proteomics have allowed for an unbiased assessment of the whole protein DPC adductome after in vitro and in vivo exposures to cross-linking agents. This MiniReview summarizes current and emerging methods for DPC isolation and analysis by mass spectrometry-based proteomics. We also highlight several examples of successful applications of these novel methodologies to studies of DPC lesions induced by bis-electrophiles such as formaldehyde, 1,2,3,4-diepoxybutane, nitrogen mustards and cisplatin.