Exposure of hematopoietic stem cells to benzene or 1,4-benzoquinone induces gender-specific gene expression.

Chronic exposure to benzene results in progressive decline of hematopoietic function and may lead to the onset of various disorders, including aplastic anemia, myelodysplastic syndrome, and leukemia. Damage to macromolecules resulting from benzene metabolites and misrepair of DNA lesions may lead to changes in hematopoietic stem cells (HSCs) that give rise to leukemic clones. We have shown previously that male mice exposed to benzene by inhalation were significantly more susceptible to benzene-induced toxicities than females. Because HSCs are targets for benzene-induced cytotoxicity and genotoxicity, we investigated DNA damage responses in HSC from both genders of 129/SvJ mice after exposure to 1,4-benzoquinone (BQ) in vitro or benzene in vivo. 1,4-BQ is a highly reactive metabolite of benzene that can cause cellular damage by forming protein and DNA adducts and producing reactive oxygen species. HSCs cultured in the presence of 1,4-BQ for 24 hours showed a gender-independent, dose-dependent cytotoxic response. RNA isolated from 1,4-BQ-treated HSCs and HSCs from mice exposed to 100 ppm benzene by inhalation showed altered expression of apoptosis, DNA repair, cell cycle, and growth control genes compared with unexposed HSCs. Rad51, xpc, and mdm-2 transcript levels were increased in male but not female HSCs exposed to 1,4-BQ. Males exposed to benzene exhibited higher mRNA levels for xpc, ku80, ccng, and wig1. These gene expression differences may partially explain the gender disparity in benzene susceptibility. HSC culture systems such as the one used here will be useful for testing the hematotoxicity of various substances, including other benzene metabolites.

Human CD34+ hematopoietic progenitor cells are sensitive targets for toxicity induced by 1,4-benzoquinone.

Chronic human exposure to benzene has been linked to several hematopoietic disorders, including leukemia and lymphomas. Certain benzene metabolites, including benzoquinone (BQ), are genotoxic and mutagenic. Bone marrow stem cells are targets for benzene-induced cytotoxicity and DNA damage that could result in changes to the genome of these progenitor cells, thereby leading to hematopoietic disorders and cancers. Human bone marrow CD34(+) hematopoietic progenitor cells (HPC) were exposed in vitro to 1,4-BQ to assess cytotoxicity, genotoxicity, and DNA damage responses and the molecular mechanisms associated with these events. CD34(+) HPC from 10 men and 10 women were exposed to 0, 1, 5, 10, 15, or 20 microM of 1,4-BQ and analyzed 72 h later. Apoptosis and cytotoxicity were dose-dependent, with exposure to 10 microM 1,4-BQ resulting in approximately 60% cytotoxicity relative to untreated controls. A significant increase in the percentage of micronucleated CD34(+) cells was detected in cultures treated with 1,4-BQ. In addition, the p21 mRNA level was elevated in 1,4-BQ-treated cells, suggesting that human CD34(+) cells utilize the p53 pathway in response to 1,4-BQ-induced DNA damage. However, there were no significant changes in mRNA levels of the DNA repair genes ku80, rad51, xpa, xpc, and ape1 as well as p53 following treatment with 1,4-BQ. Although interindividual variations were evident in the cellular response to 1,4-BQ, there was no gender difference in the response overall. These results show that human CD34(+) cells are sensitive targets for 1,4-BQ toxicity that use the p53 DNA damage response pathway in response to genotoxic stress. Human CD34(+) HPC will be useful for testing the toxicity of other benzene metabolites and various hematotoxic chemicals.

Variations in Prkdc and susceptibility to benzene-induced toxicity in mice.

Benzene, a carcinogen that induces chromosomal breaks, is strongly associated with leukemias in humans. Possible genetic determinants of benzene susceptibility include proteins involved in repair of benzene-induced DNA damage. The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), encoded by Prkdc, is one such protein. DNA-PKcs is involved in the nonhomologous end-joining (NHEJ) pathway of DNA double-strand break (DSB) repair. Here we compared the toxic effects of benzene on mice (C57BL/6 and 129/Sv) homozygous for the wild-type Prkdc allele and mice (129/SvJ) homozygous for a Prkdc functional polymorphism that leads to diminished DNA-PK activity and enhanced apoptosis in response to radiation-induced damage. Male and female mice were exposed to 0, 10, 50, or 100 ppm benzene for 6 h/d, 5 d/week for 2 weeks. Male mice were more susceptible to benzene toxicity compared with females. Hematotoxicity was evident in all male mice but was not seen in female mice. We observed similar, large increases in both micronucleated erythrocyte populations in all male mice. Female mice had smaller but significant increases in micronucleated cells. The p53-dependent response was induced in all strains and genders of mice following benzene exposure, as indicated by an increase in p21 mRNA levels in bone marrow that frequently corresponded with cell cycle arrest in G2/M. Prkdc does not appear to be a significant genetic susceptibility factor for acute benzene toxicity. Moreover, the role of NHEJ, mediated by DNA-PK, in restoring genomic integrity following benzene-induced DSB remains equivocal.

Male mice deficient in microsomal epoxide hydrolase are not susceptible to benzene-induced toxicity.

Enzymes involved in benzene metabolism are likely genetic determinants of benzene-induced toxicity. Polymorphisms in human microsomal epoxide hydrolase (mEH) are associated with an increased risk of developing leukemia, specifically those associated with benzene. This study was designed to investigate the importance of mEH in benzene-induced toxicity. Male and female mEH-deficient (mEH-/-) mice and background mice (129/Sv) were exposed to inhaled benzene (0, 10, 50, or 100 ppm) 5 days/week, 6 h/day, for a two-week duration. Total white blood cell counts and bone marrow cell counts were used to assess hematotoxicity and myelotoxicity. Micronucleated peripheral blood cells were counted to assess genotoxicity, and the p21 mRNA level in bone marrow cells was used as a determinant of the p53-regulated DNA damage response. Male mEH-/- mice did not have any significant hematotoxicity or myelotoxicity at the highest benzene exposure compared to the male 129/Sv mice. Significant hematotoxicity or myelotoxicity did not occur in the female mEH-/- or 129/Sv mice. Male mEH-/- mice were also unresponsive to benzene-induced genotoxicity compared to a significant induction in the male 129/Sv mice. The female mEH-/- and 129/Sv mice were virtually unresponsive to benzene-induced genotoxicity. While p21 mRNA expression was highly induced in male 129/Sv mice after exposure to 100-ppm benzene, no significant alteration was observed in male mEH-/- mice. Likewise, p21 mRNA expression in female mEH-/- mice was not significantly induced upon benzene exposure whereas a significant induction was observed in female 129/Sv mice. Thus mEH appears to be critical in benzene-induced toxicity in male, but not female, mice.

Genetic susceptibility to benzene-induced toxicity: role of NADPH: quinone oxidoreductase-1.

Enzymes that activate and detoxify benzene are likely genetic determinants of benzene-induced toxicity.NAD(P)H: quinone oxidoreductase-1 (NQO1) detoxifies benzoquinones, proposed toxic metabolites of benzene. NQO1 deficiency in humans is associated with an increased risk of leukemia, specifically acute myelogenous leukemia, and benzene poisoning. We examined the importance of NQO1 in benzene-induced toxicity by hypothesizing that NQO1-deficient (NQO1-/-) mice are more sensitive to benzene than mice with wild-type NQO1 (NQO1+/+; 129/Sv background strain). Male and female NQO1-/- and NQO1+/+ mice were exposed to inhaled benzene (0, 10, 50, or 100 ppm) for 2 weeks, 6 h/day, 5 days/week. Micronucleated peripheral blood cells were counted to assess genotoxicity. Peripheral blood counts and bone marrow histology were used to assess hematotoxicity and myelotoxicity. p21 mRNA levels in bone marrow cells were used as determinants of DNA damage response. Female NQO1-/- mice were more sensitive (6-fold) to benzene-induced genotoxicity than the female NQO1+/+ mice. Female NQO1-/- mice had a 9-fold increase (100 versus 0 ppm) in micronucleated reticulocytes compared with a 3-fold increase in the female NQO1+/+ mice. However, the induced genotoxic response in male mice was similar between the two genotypes (> or = 10-fold increase at 100 ppm versus 0 ppm). Male and female NQO1-/- mice exhibited greater hematotoxicity than NQO1+/+ mice. p21 mRNA levels were induced significantly in male mice (>10-fold) from both strains and female NQO1-/- mice (> 8-fold), which indicates an activated DNA damage response. These results indicate that NQO1 deficiency results in substantially greater benzene-induced toxicity. However, the specific patterns of toxicity differed between the male and female mice.