Use of genetically modified mouse models to assess pathways of benzene-induced bone marrow cytotoxicity and genotoxicity

Ameliorative Effects of Bovine Lactoferrin on Benzene-Induced Hematotoxicity in Albino Rats

Benzene (Bz) is one of the major products of the petrochemical industry globally, which induces aplastic anemia and leukemia in humans and animals. This study aimed to investigate the modulatory effects of bovine lactoferrin (bLf) on Bz-induced hematotoxicity in albino rats. Eighty male rats were randomly divided into eight groups: corn oil group [2 mL/kg body weight (BW)], bLf groups (100, 200, and 300 mg/kg BW), Bz group (Bz 2 mL/kg BW; corn oil 2 mL/kg BW), and Bz + bLf groups (Bz 2 mL/kg BW; corn oil 2 mL/kg BW; bLf 100, 200, and 300 mg/kg BW). Hematobiochemical results exhibited marked pancytopenia, a significant decrease in total protein, albumin, α2- and γ-globulin, ferritin, serum iron, and total iron-binding capacity (TIBC), and an increase in serum bioactivities of aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase (ALP), lactate dehydrogenase (LDH), and erythropoietin hormone levels in Bz-treated rats. Histopathological examination revealed a marked reduction in all hematopoietic cell lines in the bone marrow (BM), necrosis in the white pulp of the spleen and cytosolic hydrops, and apoptosis of hepatocytes in the Bz-treated group. Rats treated with bLf (300 mg/kg BW) revealed marked increases in total protein, albumin, α2- and γ-globulin, ferritin, serum iron, and TIBC levels and decreases both in ALP and LDH bioactivities and erythropoietin hormone levels compared with the Bz-treated group. Histopathological results were concomitant with hematobiochemical parameters in rats treated with bLf (300 mg/kg BW), almost showing restoration of the normal cellularity of BM, the architecture of red and white pulps of the spleen, and even the normal hypertrophy of hepatocytes compared with the control groups. To conclude, bLf (300 mg/kg BW) can be recommended to treat Bz-induced hematotoxicity.

Gene-Environment Interactions Between Environmental Response Genes Polymorphisms and Mitochondrial DNA Copy Numbers Among Benzene Workers

OBJECTIVE

To determine the effect of mitochondrial DNA copy number (mtDNAcn) as a biomarker of benzene exposure.

METHODS

A total of 294 benzene-exposed workers and 102 controls were recruited. Biomarkers of mtDNAcn, cytokinesis-block micronucleus (MN) frequency, and peripheral blood white blood cells (WBC) were detected. Eighteen polymorphism sites in DNA damage repair and metabolic genes were analyzed.

RESULTS

Benzene exposure increased mtDNAcn and indicated a dose-response relationship (P < 0.001). mtDNAcn was negatively correlated with WBC count and DNA methylation and positively correlated with MN frequency. The AG type in rs1695 interacted with benzene exposure to aggravate mtDNAcn (β = 0.006, 95% CI: 0, 0.012, P = 0.050). rs13181, rs1695, rs1800975, and GSTM1 null were associated with benzene-induced mtDNAcn. Rs1695 interacted with benzene to increase mitochondrial damage.

CONCLUSIONS

Benzene exposure increases mtDNAcn levels in benzene-exposed workers.

Interaction effects of environmental response gene polymorphisms and benzene exposure on telomere length in shoe-making workers

Benzene is a globally occurring environmental and occupational pollutant that causes leukemia. To better understand telomere length (TL) as a function of benzene toxicity, we recruited 294 shoe-making workers and 102 controls from Wenzhou, China in 2011. Biomarkers of TL, cytokinesis-block micronucleus (MN) frequency, and white blood cells (WBC) were measured. In total, 18 polymorphic sites in environmental response genes, including metabolic and DNA repair genes, were analyzed. Results indicate that benzene exposure led to a longer TL at a threshold of 32 mg/m³-year of cumulative exposure dose (CED). Furthermore, the TL was longer in members of the damaged group, when evaluated for MN frequency (P < 0.001) and reduced WBC (P < 0.001), than in those of the normal group. Workers carrying genotype TT (β = 0.32, P = 0.042) in rs3212986 of ERCC1 and genotype TC (β = 0.24, P = 0.082) in rs1051740 of mEH exon3 were associated with a longer TL as compared to the wild-type group. TA (β = -0.53, P < 0.001) in rs6413432 of CYP2E1 was associated with a shorter TL. Benzene exposure interacted with the TA type in rs6413432 (β = 0.003, 95% CI: 0, 0.006, P = 0.042) and the CC type in rs1051740 (β = 0.007, 95% CI: 0.001, 0.013, P = 0.015) after adjusting for confounding factors. Our results indicate that benzene induces an increase in TL at a threshold of CED ≥32mg/m³-year. Rs1051740, rs3212986, and rs6413432 were found to be involved in benzene-induced telomere growth; in particular, rs1051740 and rs6413432 interacted with the benzene exposure, resulting in an extended TL.

Epimedium polysaccharides attenuates hematotoxicity by reducing oxidative stress and enhancing immune function in mice model of benzene-induced bone marrow failure

Chronic benzene (BZ) exposure is associated with multiple adverse health effects and leads to progressive bone marrow failure (BMF). BZ-induced BMF is an acquired aplastic anemia characterized by severe anemia, neutropenia and thrombocytopenia, which is likely caused by immunotoxicity and oxidative stress. Previous studies showed that Epimedium polysaccharides (EPS), a natural and major herbal compound derived from Epimedium, has immunomodulatory and antioxidant potential. The purpose of this study was to evaluate the potential efficacy of EPS against BZ-induced BMF. BMF mouse model was established by subcutaneous injection of 2 ml/kg BZ in CD1 mice. Mice received daily oral treatment with 100 mg/kg high-dose EPS and 20 mg/kg low-dose EPS for four weeks. Our data showed that EPS treatment alleviated BZ-associated weight loss and increased the number of whole blood cells in peripheral blood and nucleated cells in bone marrow. Furthermore, EPS treatment decreased apoptotic rate and reactive oxygen species production, S-phase arrest in bone marrow cells. Finally, EPS treatment improved T cell-mediated immune suppression by increasing CD3+, CD4 + T-cell counts, and CD4+/CD8+ ratio. and modulated hematopoietic cytokines including EPO, IL-11, and IL-2 in peripheral blood. Our study suggests that EPS is a potential therapeutic target to attenuate hematotoxicity induced by BZ.

Genetic Polymorphisms in XRCC1, CD3EAP, PPP1R13L, XPB, XPC, and XPF and the Risk of Chronic Benzene Poisoning in a Chinese Occupational Population

Objectives

Individual variations in the capacity of DNA repair machinery to relieve benzene-induced DNA damage may be the key to developing chronic benzene poisoning (CBP), an increasingly prevalent occupational disease in China. ERCC1 (Excision repair cross complementation group 1) is located on chromosome 19q13.2–3 and participates in the crucial steps of Nucleotide Excision Repair (NER); moreover, we determined that one of its polymorphisms, ERCC1 rs11615, is a biomarker for CBP susceptibility in our previous report. Our aim is to further explore the deeper association between some genetic variations related to ERCC1 polymorphisms and CBP risk.

Methods

Nine single nucleotide polymorphisms (SNPs) of XRCC1 (X-ray repair cross-complementing 1), CD3EAP (CD3e molecule, epsilon associated protein), PPP1R13L (protein phosphatase 1, regulatory subunit 13 like), XPB (Xeroderma pigmentosum group B), XPC (Xeroderma pigmentosum group C) and XPF (Xeroderma pigmentosum group F) were genotyped by the Snapshot and TaqMan-MGB^® probe techniques, in a study involving 102 CBP patients and 204 controls. The potential interactions between these SNPs and lifestyle factors, such as smoking and drinking, were assessed using a stratified analysis.

Results

An XRCC1 allele, rs25487, was related to a higher risk of CBP (P<0.001) even after stratifying for potential confounders. Carriers of the TT genotype of XRCC1 rs1799782 who were alcohol drinkers (OR = 8.000; 95% CI: 1.316–48.645; P = 0.022), male (OR = 9.333; 95% CI: 1.593–54.672; P = 0.019), and had an exposure of ≤12 years (OR = 2.612; 95% CI: 1.048–6.510; P = 0.035) had an increased risk of CBP. However, the T allele in PPP1R13L rs1005165 (P<0.05) and the GA allele in CD3EAP rs967591 (OR = 0.162; 95% CI: 0039~0.666; P = 0.037) decreased the risk of CBP in men. The haplotype analysis of XRCC1 indicated that XRCC1 rs25487^A, rs25489^G and rs1799782^T (OR = 15.469; 95% CI: 5.536–43.225; P<0.001) were associated with a high risk of CBP.

Conclusions

The findings showed that the rs25487 and rs1799782 polymorphisms of XRCC1 may contribute to an individual’s susceptibility to CBP and may be used as valid biomarkers. Overall, the genes on chromosome 19q13.2–3 may have a special significance in the development of CBP in occupationally exposed Chinese populations.

Are polymorphisms in metabolism protective or a risk for reduced white blood cell counts in a Chinese population with low occupational benzene exposures?

BACKGROUND

Genetic variations in metabolic enzyme genes may enhance hematotoxicity in benzene-exposed populations.

OBJECTIVE

To investigate the association between polymorphisms of metabolism genes and white blood cells (WBCs).

METHODS

Three hundred and eighty-five benzene-exposed workers and 220 unexposed indoor workers were recruited in China. We explored the relationship between metabolic enzymes polymorphisms [glutathione S-transferase T1/M1 (GSTT1/M1) null, glutathione S-transferase P1 (GSTP1)rs1695, Cytochrome P450 2E1 (CYP2E1) rs3813867, rs2031920, rs6413432, microsomal epoxide hydrolase (mEH) rs1051740, rs2234922] by polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) analysis and WBC.

RESULTS

The exposed group had lower WBC counts (P<0·001) than the unexposed group. Increased susceptibility to hematotoxicity, as evidenced by lower WBC counts, was found in workers with null-GSTT1 (P = 0·045), null-GSTM1 (P = 0·030), rs2031920 (P = 0·020), and rs3813867 (P = 0·014) genotypes. White blood cell counts were also lower in workers with null-GSTT1 and null-GSTM after adjusting for age, gender, smoking, and alcohol consumption.

CONCLUSION

Null-GSTT1 and null-GSTM1 genotypes and Cytochrome P4502E1 (CYP2E1: rs2031920, rs3813867) may support the hematotoxicity of benzene-exposed workers in China, and we can make use of it to select susceptible population.

Effect of polymorphic metabolizing genes on micronucleus frequencies among benzene-exposed shoe workers in China

It is well-known that metabolism of benzene is required for the induction of toxicity and consequent health problems. Therefore, genetic variation in benzene (BZ) metabolism genes can influence health outcomes. However, large population studies are needed to provide more evidence for such relationship. We have conducted a large population investigation (385 BZ-exposed shoe workers and 197 matched healthy controls) on the association between inheritance of certain BZ metabolizing genes and the expression of micronuclei (MN). The latter was based on the cytokinesis-blocked MN assay. We analyzed the polymorphisms of GSTM1, GSTT1, GSTP1 (rs1695), CYP2E1 (rs3813867), CYP2E1 (rs2031920), CYP2E1 (rs6413432), mEH exon 3 (rs1051740), mEH exon 4 (rs2234922). Univariate Poisson regression analysis demonstrated that the BZ-exposed workers had significantly increased MN frequency compared with the controls (FR=1.84, 95% CI: 1.56-2.18; P<0.001), and showed a cumulative exposure dose-response relationship. The CYP2E1 rs3813867 mutant allele (CC+GC) (FR 1.15, 95% CI 1.02-1.29; P=0.020) and rs2031920 variant allele (CT+TT) (FR=1.23, 95% CI: 1.09-1.37, P<0.01) was associated with higher MN frequency significantly compared with the wild genotype separately. Furthermore, the MN frequency in rs2031920 variant allele (CT+TT) (FR=1.17, 95% CI: 1.04-1.31, P<0.01) was also higher than the wild genotype when the age, gender and cumulative exposure dose was adjusted in Poisson regression. In addition, the CYP2E1, however, GSTM1null, GSTT1null, GSTP1 rs1695, rs6413432, rs1051740 and rs2234922 polymorphisms showed no association with MN frequency. Our results indicate that two promoter polymorphisms in the CYP2E1 gene, especially the rs2031920 variant allele, were involved with the BZ-induction of MN and may contribute to risk of cancer among exposed workers.

Association of the genetic polymorphism of EPHX1 and EPHX2 with the susceptibility to chronic benzene poisoning

The aim of this study was to explore the association of the genetic polymorphism of EPHX1 and EPHX2 with the susceptibility to chronic benzene poisoning (CBP). A case-control study of 268 patients with CBP and 268 healthy workers matched by age and sex, all of whom were occupationally exposed to benzene, was conducted. The single nucleotide polymorphisms (SNPs, rs2854451, rs3738047, rs2234922 and rs1051741) of EPHX1 gene and the SNP (rs751141) of EPHX2 gene were tested by the TaqMan PCR method. In the subjects carrying the genotype of EPHX1 rs3738047 GG, the risk of CBP was decreased in the individuals simultaneously carrying EPHX1 rs2234922 G (P = 0.02). Alternatively, in the subjects carrying the genotype of EPHX1 rs2234922 AA, the risk of CBP was increased in the individuals simultaneously carrying the allele of EPHX2 rs751141A (P = 0.03). It was also found that there were potential interactions between alcohol consumption and the polymorphism of EPHX1 rs1051741 (χH (2) = 5.28, P = 0.02) or rs2234922 (χH (2) = 6.71, P = 0.01). Compared to individuals with EPHX1 rs1051741 CC or rs2234922 AA genotype in the drinkers, the risk of CBP in those carrying genotypes of EPHX1 rs1051741 CT+TT or rs2234922 AG+GG was decreased, respectively (P = 0.04, P < 0.01). Haplotype analysis of polymorphisms in EPHX1 showed that the risk of CBP was increased in the subjects with haplotype 2 (rs2854451-A, rs3738047-G, rs2234922-A, rs1051741-C) or haplotype 4 (rs2854451-G, rs3738047-A, rs2234922-G, rs1051741-T), but decreased in those with haplotype 6 (rs2854451-G, rs3738047-G, rs2234922-G, rs1051741-T) or haplotype 10 (rs2854451-A, rs3738047-A, rs2234922-G, rs1051741-T), respectively. Logistic regression analysis revealed that smoking might play a role in modifying the risk of CBP (OR = 0.313, 95% CI: 0.123-0.794, P = 0.015). The genetic polymorphism in EPHX1 may be associated with the risk of CBP in the Chinese occupational population and further research is needed for the association between the genetic polymorphism in EPHX2 and the susceptibility to CBP.

Association of genetic polymorphisms in ERCC1 and ERCC2/XPD with risk of chronic benzene poisoning in a Chinese occupational population

DNA damage induced by benzene and its metabolites is thought of as an important mechanism underlying benzene genotoxicity in chronic benzene poisoning (CBP). Therefore, genetic variation in DNA repair genes may contribute to susceptibility to CBP in the exposed population. Since benzene-induced DNA damages include DNA adducts, we hypothesized that the polymorphisms of ERCC1 (Excision repair cross complementation group 1) and ERCC2/XPD (Excision repair cross complementation group 2/xeroderma pigmentosum group D) are associated with the risk of CBP. A case-control study involving 102 benzene-poisoned patients and 204 none-benzene-poisoned controls occupationally exposed to benzene was carried out in the Northeast region of China. The polymorphisms of codon 118 (rs11615) and C8092A (rs3212986) of ERCC1, codon 751 (rs13181), 312 (rs1799793) and 156 (rs238406) of ERCC2/XPD were genotyped by TaqMan(®) Real-time PCR. The results showed that individuals carrying the ERCC1 codon 118 TT genotype had an increased risk of CBP (OR(adj)=3.390; 95%CI: 1.393-8.253; P=0.007) comparing with its CC genotype. After stratified by smoking, gender and exposure duration we found that the increased risk of CBP associated with the ERCC1 codon 118 TT genotype confined to nonsmokers (OR=3.214; 95% CI: 1.359-7.601; P=0.006), female (OR=3.049; 95% CI: 1.235-7.529; P=0.013) and exposure duration> 12 years (OR=3.750; 95% CI: 1.041-13.513; P=0.035). Since ERCC1 and ERCC2/XPD are both located on chromosome 19q13.3, haplotype analysis of all 5 SNPs was also conducted. However no correlations between the risks of CBP and other genotypes or haplotypes were found. Therefore, our findings suggest an important role of ERCC1 codon 118 polymorphisms for a biomarker to CBP in the Chinese occupational population.

Study on the cytogenetic changes induced by benzene and hydroquinone in human lymphocytes

Benzene (BN) is a prototypical hematotoxicant, genotoxic carcinogen, and ubiquitous environmental pollutant. Although the molecular mechanisms of BN-induced cytotoxicity and genotoxic damage are poorly understood in humans, previous studies suggested that bioactivated BN metabolites are capable of oxidative stress, cell cycle arrest, apoptosis, and DNA damage. The objective of the current study was to investigate the BN-induced cytogenetic changes and underlying mechanisms based on these hypotheses. Peripheral blood lymphocytes (PBLs) might be the targets for BN-induced cytotoxicity and genotoxicity, and therefore DNA damage responses of PBLs after exposure to different concentrations of BN (0.25, 3.5, 50 μmol/L) or BN metabolite, hydroquinone (HQ; 50, 150, 450 μmol/L) were studied in vitro. Microculture tetrazolium assay, flow cytometry, 2′,7′-dichlorodihydrofluorescein-diacetate assay, comet assay, micronuclei assay, and attenuated total reflectance microspectroscope were chosen for this study. Based on the results, we reached the conclusion that different concentrations of BN or HQ significantly inhibited cell growth, induced the arrest of S phase and G2/M phase, and increased late apoptosis in a concentration-dependent manner. Furthermore, evidence was also provided to support the conclusion that BN and HQ induced DNA strand breaks and chromosomal mutations in PBL, which indicated the genotoxicity of BN and HQ. Current evidence has indicated that multiple mechanisms including dysfunction of cell cycle, programmed cell death, oxidative stress, and DNA lesions are likely to contribute to BN-induced cytogenetic changes.

Metabolism and toxicity of styrene in microsomal epoxide hydrolase-deficient mice

Styrene, which is widely used in manufacturing, is both acutely and chronically toxic to mice. Styrene is metabolized by cytochromes P-450 to the toxic metabolite styrene oxide, which is detoxified via hydrolysis with microsomal epoxide hydrolase (mEH) playing a major role. The purpose of these studies was to characterize the importance of this pathway by determining the hepatotoxicity and pneumotoxicity of styrene in wild-type and mEH-deficient (mEH(-/-)) mice. While the mEH(-/-) mice metabolized styrene to styrene oxide at the same rate as the wild-type mice, as expected there was minimal metabolism of styrene oxide to glycol. mEH(-/-) mice were more susceptible to the lethal effects of styrene. Twenty-four hours following the administration of 200 mg/kg ip styrene, mice demonstrated a greater hepatotoxic response due to styrene, as measured by increased serum sorbitol dehydrogenase activity and greater pneumotoxicity as shown by increased protein levels, cell numbers, and lactate dehydrogenase activity in bronchioalveolar lavage fluid. mEH(-/-) mice were also more susceptible to styrene-induced oxidative stress, as indicated by greater decreases in hepatic glutathione levels 3 h after styrene. Styrene oxide at a dose of 150 mg/kg did not produce hepatotoxicity in either wild-type or mEH(-/-) mice. However, styrene oxide produced pneumotoxicity that was similar in the two strains. Thus, mEH plays an important role in the detoxification of styrene but not for exogenously administered styrene oxide.

Relationships between metabolic and non-metabolic susceptibility factors in benzene toxicity

Reactive metabolites formed from benzene include benzene oxide, trans,trans muconaldehyde, quinones, thiol adducts, phenolic metabolites and oxygen radicals. Susceptibility to the toxic effects of benzene has been suggested to occur partly because of polymorphisms in enzymes involved in benzene metabolism which include cytochrome P450 2E1, epoxide hydrolases, myeloperoxidase, glutathione-S-transferases and quinone reductases. However, susceptibility factors not directly linked to benzene metabolism have also been associated with its toxicity and include p53, proteins involved in DNA repair, genomic stability and expression of cytokines and/or cell adhesion molecules. In this work, we examine potential relationships between metabolic and non-metabolic susceptibility factors using the enzyme NAD(P)H:quinone oxidoreductase (NQO1) as an example. NQO1 may also impact pathways in addition to metabolism of quinones due to protein-protein interactions or other mechanisms related to NQO1 activity. NQO1 has been implicated in stabilizing p53 and in maintaining microtubule integrity. Inhibition or knockdown of NQO1 in bone marrow endothelial cells has been found to lead to deficiencies of E-selectin, ICAM-1 and VCAM-1 adhesion molecule expression after TNFalpha stimulation. These examples illustrate how the metabolic susceptibility factor NQO1 may influence non-metabolic susceptibility pathways for benzene toxicity.

The aryl hydrocarbon receptor has an important role in the regulation of hematopoiesis: implications for benzene-induced hematopoietic toxicity

The aryl hydrocarbon receptor (AhR) belongs to the basic helix-loop-helix (bHLH) Per-Arnt-Sim (PAS) family of transcription factors. Many of these proteins are involved in regulating responses to signals in the tissue environment such as hypoxia, oxidation-reduction status, and circadian rhythms. Although the AhR is well studied as a mediator of the toxicity of certain xenobiotics, the normal physiological function remains unknown. However, accumulating data support a hypothesis that the AhR has an important function in the regulation of hematopoietic stem cells (HSCs). Persistent AhR activation by dioxin, a potent xenobiotic AhR agonist, results in altered numbers and function of HSCs in mouse bone marrow. Analysis of HSCs from AhR null-allele mice also indicates that lack of AhR expression results in altered characteristics and function of these cells. HSCs from these animals are hyperproliferative and have altered cell cycle. In addition, aging AhR-KO mice show characteristics consistent with premature bone marrow senescence and are prone to hematopoietic disease. Finally, some data suggest that the expression of the Ahr gene is regulated under conditions that control HSC proliferation. The presence of a normal functioning AhR may provide an important advantage to organisms by regulating the balance between quiescence and proliferation and preventing the premature exhaustion of HSCs and sensitivity to genetic alterations. This function assists in the preservation of HSC function and long-term multi-lineage generation over the lifespan of the organism. This also implicates a role for the AhR in the aging process. Furthermore, these functions may affect the sensitivity of HSCs to certain xenobiotics, including benzene. Defining the molecular mechanisms by which these events occur may lead to the identification of previously undefined roles of this transcription factor in human diseases, particularly those caused or affected by xenobiotics.

Mammalian epoxide hydrolases in xenobiotic metabolism and signalling

Epoxide hydrolases catalyse the hydrolysis of electrophilic--and therefore potentially genotoxic--epoxides to the corresponding less reactive vicinal diols, which explains the classification of epoxide hydrolases as typical detoxifying enzymes. The best example is mammalian microsomal epoxide hydrolase (mEH)-an enzyme prone to detoxification-due to a high expression level in the liver, a broad substrate selectivity, as well as inducibility by foreign compounds. The mEH is capable of inactivating a large number of structurally different, highly reactive epoxides and hence is an important part of the enzymatic defence of our organism against adverse effects of foreign compounds. Furthermore, evidence is accumulating that mammalian epoxide hydrolases play physiological roles other than detoxification, particularly through involvement in signalling processes. This certainly holds true for soluble epoxide hydrolase (sEH) whose main function seems to be the turnover of lipid derived epoxides, which are signalling lipids with diverse functions in regulatory processes, such as control of blood pressure, inflammatory processes, cell proliferation and nociception. In recent years, the sEH has attracted attention as a promising target for pharmacological inhibition to treat hypertension and possibly other diseases. Recently, new hitherto uncharacterised epoxide hydrolases could be identified in mammals by genome analysis. The expression pattern and substrate selectivity of these new epoxide hydrolases suggests their participation in signalling processes rather than a role in detoxification. Taken together, epoxide hydrolases (1) play a central role in the detoxification of genotoxic epoxides and (2) have an important function in the regulation of physiological processes by the control of signalling molecules with an epoxide structure.

Polymorphisms in phase I and phase II metabolism genes and risk of chronic benzene poisoning in a Chinese occupational population

It is widely accepted that the cytotoxicity and genotoxicity of benzene results from the action of reactive metabolites. Therefore, genetic variation in metabolic enzyme genes may contribute to susceptibility to chronic benzene poisoning (CBP) in the exposed population. Using a case-control study that included 268 benzene-poisoned patients and 268 workers occupationally exposed to benzene in South China, we aimed to investigate the association between single-nucleotide polymorphisms in genes with phase I and II of metabolism and risk of CBP. The TaqMan technique was used to detect polymorphisms of CYP1A1, CYP1A2, CYP1B1, ADH1B, EPHX1, EPHX2, NQO1, MPO, GSTP1 and UGT1A6 genes. We also explored potential interactions of these polymorphisms with lifestyle factors such as cigarette smoking and alcohol consumption. A weak positive association was found between glutathione S-transferase pi-1 (GSTP1) rs1695 polymorphism and the risk of CBP (P = 0.046), but this association was not statistically significant (P = 0.117) after adjustment for potential confounders. Further analysis showed that the risk of CBP increased in the subjects with EPHX1 GGAC/GAGT diplotype (P = 0.00057) or AGAC/GAGT diplotype (P = 0.00086). In addition, we found that alcohol drinkers with the EPHX1 rs3738047 GA + AA genotypes and non-alcohol drinkers with the GSTP1 rs1695 AA genotype tended to be more susceptible to benzene toxicity. Our results suggest that genetic polymorphisms in EPHX1 may contribute to risk of CBP in a Chinese occupational population.

Genotoxicity of intermittent co-exposure to benzene and toluene in male CD-1 mice

Benzene is an important industrial chemical. At certain levels, benzene has been found to produce aplastic anemia, pancytopenia, myeloblastic anemia and genotoxic effects in humans. Metabolism by cytochrome P450 monooxygenases and myeloperoxidase to hydroquinone, phenol, and other metabolites contributes to benzene toxicity. Other xenobiotic substrates for cytochrome P450 can alter benzene metabolism. At high concentrations, toluene has been shown to inhibit benzene metabolism and benzene-induced toxicities. The present study investigated the genotoxicity of exposure to benzene and toluene at lower and intermittent co-exposures. Mice were exposed via whole-body inhalation for 6h/day for 8 days (over a 15-day time period) to air, 50 ppm benzene, 100 ppm toluene, 50 ppm benzene and 50 ppm toluene, or 50 ppm benzene and 100 ppm toluene. Mice exposed to 50 ppm benzene exhibited an increased frequency (2.4-fold) of micronucleated polychromatic erythrocytes (PCE) and increased levels of urinary metabolites (t,t-muconic acid, hydroquinone, and s-phenylmercapturic acid) vs. air-exposed controls. Benzene co-exposure with 100 ppm toluene resulted in similar urinary metabolite levels but a 3.7-fold increase in frequency of micronucleated PCE. Benzene co-exposure with 50 ppm toluene resulted in a similar elevation of micronuclei frequency as with 100 ppm toluene which did not differ significantly from 50 ppm benzene exposure alone. Both co-exposures - 50 ppm benzene with 50 or 100 ppm toluene - resulted in significantly elevated CYP2E1 activities that did not occur following benzene or toluene exposure alone. Whole blood glutathione (GSH) levels were similarly decreased following exposure to 50 ppm benzene and/or 100 ppm toluene, while co-exposure to 50 ppm benzene and 100 ppm toluene significantly decreased GSSG levels and increased the GSH/GSSG ratio. The higher frequency of micronucleated PCE following benzene and toluene co-exposure when compared with mice exposed to benzene or toluene alone suggests that, at the doses used in this study, toluene can enhance benzene-induced clastogenic or aneugenic bone marrow injury. These findings exemplify the importance of studying the effects of binary chemical interactions in animals exposed to lower exposure concentrations of benzene and toluene on benzene metabolism and clastogenicity. The relevance of these data on interactions for humans exposed at low benzene concentrations can be best assessed only when the mechanism of interaction is understood at a quantitative level and incorporated within a biologically based modeling framework.

Investigating the role of the aryl hydrocarbon receptor in benzene-initiated toxicity in vitro

Chronic occupational exposure to benzene has been correlated with aplastic aneamia and acute myelogenous leukemia, however mechanisms behind benzene toxicity remain unknown. Interestingly, benzene-initiated hematotoxicity is absent in mice lacking the aryl hydrocarbon receptor (AhR) suggesting an imperative role for this receptor in benzene toxicities. This study investigated two potential roles for the AhR in benzene toxicity using hepa 1c1c7 wild type and AhR deficient cells. Considering that many toxic effects of AhR ligands are dependent on AhR activation, our first objective was to determine if benzene, hydroquinone (HQ) or benzoquinone (BQ) could activate the AhR. Secondly, because the AhR regulates a number of enzymes involved in oxidative stress pathways, we sought to determine if the AhR had a role in HQ and BQ induced production of reactive oxygen species (ROS). Dual luciferase assays measuring dioxin response element (DRE) activation showed no significant change in DRE activity after exposure to benzene, HQ or BQ for 24h. Immunofluorescence staining showed cytosolic localization of the AhR after 2h incubations with benzene, HQ or BQ. Western blot analysis of cells exposed to benzene, HQ or BQ for 1, 12 and 24h did not demonstrate induction of CYP1A1 protein expression. Dichlorodihydrofluorescein staining of cells exposed to benzene, HQ or BQ revealed that the presence of the AhR did not affect BQ and HQ induced ROS production. These results indicate that the involvement of the AhR in benzene toxicity does not seem to be through classical activation of this receptor or through interference of oxidative stress pathways.