Effects of toluene on the metabolism,disposition and hemopoietic toxicity of [3H]benzene

The administration of [³H]benzene to mice resulted in the decreased incorporation of ⁵⁹Fe into red cells and the accumulation of benzene and its metabolites in bone marrow and other tissues. Toluene protected against the benzene-induced depression of red cell ⁵⁹Fe uptake and reduced the levels of benzene metabolites in bone marrow without affecting the level of benzene in this tissue. The results of this study suggest that toluene exerted its protective effect by inhibiting benzene metabolism and that a metabolite of benzene probably mediates the observed hemopietic toxicity of benzene.     

Detection of nitrated benzene metabolites in bone marrow of B6C3F1 mice treated with benzene

Benzene, a constituent of cigarette smoke, is a human leukemogen and induces bone marrow toxicity. The mechanism of benzene-induced toxicity is not well-established. We hypothesized that relatively high levels of nitric oxide formed in bone marrow can react with oxygen and/or superoxide anion that is generated during redox cycling of ring-hydroxylated benzene metabolites to yield peroxynitrite as well as other NO-derived intermediates. Peroxynitrite can either directly damage cellular macromolecules or form nitrated toxic metabolites. Toward this end, we investigated whether nitro derivatives of benzene are formed in bone marrow of mice treated with benzene. First, we have characterized products formed during activation of benzene in Fenton's system in the absence or presence of NO-releasing compound in vitro by GC/MS. The result of above experiment prompted us to determine whether similar products can be formed in vivo. Groups of B6C3F1 male mice, eight weeks of age, were given a single intraperitoneal dose of [14C]benzene (400 mg/kg body wt, 9.7 mCi/mmol) or an equal dose of unlabeled benzene in corn oil, and the mice were killed 0.5 or 1 h posttreatment. The control group received only vehicle injections. Organic solvent extractable metabolites from bone marrow, liver, lungs, and blood of mice treated with [14C]benzene were identified by comparison of their respective retention times under two different HPLC conditions with authentic standard samples. These metabolites were further characterized by comparison of their GC/MS properties to those of reference standards. Nitro metabolites, namely, nitrobenzene, nitrobiphenyl, and nitrophenol isomers, were detected in the bone marrow of the mice 1 h after benzene treatment. Formation of nitro derivatives in other tissues was either not observed or was significantly less than that formed in bone marrow. This study clearly demonstrates that nitric oxide is a contributor to benzene metabolism and can form nitrated derivatives that may, in part, account for bone marrow toxicity.     

Partial hepatectomy reduces both metabolism and toxicity of benzene

Removal of 70--80% of the liver reduced both the metabolism and the toxicity of benzene in rats. Metabolism was evaluated by measuring the levels of urinary metabolites in both sham-operated and partially hepatectomized rats given 2200 mg/kg [3H]benzene sc. Toxicity was evaluated by measuring the incorporation of 59Fe into circulating erythrocytes according to the method of Lee et al. The observation that partial hepatectomy decreases benzene metabolism and protects against benzene toxicity indicates that the liver may play a primary role in the development of benzene-induced bone marrow toxicity. The fact that benzene administration also reduces the ability of the liver to regenerate after partial hepatectomy suggests that the regenerating liver may serve as a model system in lieu of the bone marrow for studying the mechanism by which benzene inhibits cell proliferation.     

Stem cell and benzene-induced malignancy and hematotoxicity

The biological effect of benzene on the hematopoietic system has been known for over a century. The rapid advancement in understanding the biology of hematopoietic stem cells (HSCs) and cancer stem cells (CSCs) in recent years has renewed interest in investigating the role of stem cells in benzene-induced malignancy and bone marrow depression. The interplay between benzene and stem cells is complex involving the stem cell, progenitor, and HSC niche compartments of the bone marrow. In this prospect, benzene metabolites formed through metabolism in the liver and bone marrow cause damage in hematopoietic cells via multiple mechanisms that, in addition to traditionally recognized chromosomal aberration and covalent binding, incorporate oxidative stress, alteration of gene expression, apoptosis, error-prone DNA repair, epigenetic regulation, and disruption of tumor surveillance. However, benzene-exposed individuals exhibit variable susceptibility to benzene effect that arises, in part, from genetic variations in benzene metabolism, DNA repair, genomic stability, and immune function. These new studies of benzene leukemogenesis and hematotoxicity are expected to provide insights into how environmental and occupational chemicals affect stem cells to cause cancer and toxicity, which impact the risk assessment, permissible level, and therapy of benzene exposure.     

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.     

Metabonomics biomarkers for subacute toxicity screening for benzene exposure in mice

Benzene is known to produce hematotoxicity in occupational exposure workers. This study examined the utility of metabonomic biomarkers to ascertain subacute toxicity produced by benzene in male C3H/He mice. A 30-d intermittent collection of urine was obtained from mice in this experiment. The relative organ weights, blood parameters, and bone marrow smears were examined to identify specific changes of benzene-induced toxicity. In addition, an integrated analytical approach based on liquid chromatography coupled with mass spectrometry (LC-MS) was developed to map metabolic responses in urine. Five endogenous metabolites, hypoxanthine, spermidine, 4-aminohippuric acid, indolelactic acid, and glutamylphenylalanine, were identified as potential biomarkers of benzene-induced toxicity, indicating that pathways of purine, spermidine, fatty acid, tryptophan, and peptides metabolism might be disturbed in benzene-exposed mice. Our findings showed that the use of urine metabonomics was a more sensitive tool to detect benzene-induced toxicity compared to body weight or blood parameter changes.     

A Perspective on Benzene Leukemogenesis

Abstract

Although benzene is best known as a compound that causes bone marrow depression leading to aplastic anemia in animals and humans, it also induces acute myelogenous leukemia in humans. The epidemiological evidence for leukemogenesis in humans is contrasted with the results of animal bioassays. This review focuses on several of the problems that face those investigators attempting to unravel the mechanism of benzene-induced leukemogenesis. Benzene metabolism is reviewed with the aim of suggesting metabolites that may play a role in the etiology of the disease. The data relating to the formation of DNA adducts and their potential significance are analyzed. The clastogenic activity of benzene is discussed both in terms of biomarkers of exposure and as a potential indication of leukemogenesis. In addition to chromosome aberrations, sister chromatid exchange, and micronucleus formation, the significance of chromosomal translocations is discussed. The mutagenic activity of benzene metabolites is reviewed and benzene is placed in perspective as a leukemogen with other carcinogens and the lack of leukemogenic activity by compounds of related structure is noted. Finally, a pathway from exposure to benzene to eventual leukemia is discussed in terms of biochemical mechanisms, the role of cytokines and related factors, latency, and expression of leukemia.     

In vitro effects of benzene metabolites on mouse bone marrow stromal cells

Benzene exposure can result in bone marrow myelotoxicity. We examined the effects of benzene metabolites on bone marrow stromal cells of the hemopoietic microenvironment. Male B6C3F1 mouse bone marrow adherent stromal cells were plated at 4 X 10(6) cells per 2 ml of DMEM medium in 35-mm tissue culture dishes. The growing stromal cell cultures were exposed to log 2 doses of five benzene metabolites: hydroquinone, benzoquinone, phenol, catechol, or benzenetriol for 7 days. The dose which caused a 50% decrease in colony formation (TD50) was 2.5 X 10(-6) M for hydroquinone, 17.8 X 10(-6) M for benzoquinone, 60 X 10(-6) M for benzenetriol, 125 X 10(-6) M for catechol, and 190 X 10(-6) M for phenol. We next examined the effect of benzene metabolites on the ability of stromal cells to influence granulocyte/monocyte colony growth (G/M-CFU-C) in a coculture system. Adherent stromal cells were plated and incubated for 14 days and then exposed to a benzene metabolite. After 3 days the medium and metabolite were removed and an agar:RPMI layer containing 10(6) fresh bone marrow cells was placed over the stromal layer. After incubation for 7 days the cultures were scored for G/M colony formation. Hydroquinone and benzoquinone were most toxic, while catechol and benzenetriol inhibited colony growth only at high doses. These results indicate that injured bone marrow stromal cells may be a significant factor in benzene-induced hemotoxicity.     

Toxicity-Related Changes in Benzene Metabolism in vivo

1. The influence of microsomal enzyme inducers and inhibitors on the metabolism of [U-¹⁴C]benzene in the rat was examined.

2. Pre-treatment of animals with the inhibitors piperonyl butoxide and cobaltous chloride tended to reduce the urinary excretion of metabolites in 24?h but increase the overall urinary excretion. Piperonyl butoxide tended to increase expired benzene.

3. Pre-treatment of animals with the inducer phenobarbital tended to increase urinary excretion of metabolites but decrease expired benzene, as did pretreatment with benzene itself.

4. All pre-treatments appeared to increase the excretion of phenolic glucu-ronides, particularly benzene pre-treatment. Phenobarbital and benzene pre-treatment tended to increase phenylmercapturic acid excretion and benzene pre-treatment markedly increased excretion of quinol.

5. The results are discussed in relation to the effect of microsomal enzyme inhibitors and inducers on benzene-induced bone marrow toxicity.     

Xenobiotic metabolism and the mechanism(s) of benzene toxicity

The investigation of the mechanism(s) of benzene toxicity/leukemogenesis over the past 50 years has been contemporaneous with developments in the study of xenobiotic metabolism. Research on the cytochrome P450 (CYP) enzyme system, and related systems in vivo and in vitro, which culminated in the isolation and reconstitution of the many CYPs, established pathways for the study of xenobiotic metabolism and its relationship to the biological activity of many chemicals. The essential role for metabolism of benzene as a precursor to the demonstration of benzene toxicity led to extensive studies of benzene metabolism, many of which will be reviewed here. Benzene toxicity/leukemogenesis, however, is a function of the bone marrow, a site remote from the liver where most benzene metabolism occurs. Studies of benzene metabolism have delineated the array of metabolites which appear to play a role in bone marrow damage, but further studies, both in vivo and in vitro, using appropriate animal models, will be needed to fully understand the impact of benzene and its metabolites on bone marrow function.     

Cell-specific metabolism in mouse bone marrow stroma: studies of activation and detoxification of benzene metabolites.

Two of the major cell types in bone marrow stroma, macrophages and fibroblasts, have been shown to be important regulators of both myelopoiesis and lymphopoiesis. The enzymology relating to cell-specific metabolism of phenolic metabolites of benzene in isolated mouse bone marrow stromal cells was examined. Fibroblastoid stromal cells had elevated glutathione-S-transferase (4.5-fold) and DT-diaphorase (4-fold) activity relative to macrophages, whereas macrophages demonstrated increased UDP-glucuronosyltransferase (UDP-GT, 7.5-fold) and peroxidase activity relative to stromal fibroblasts. UDP-GT and glutathione-S-transferase activities in macrophages and fibroblasts, respectively, were significantly greater than those in unpurified white marrow. Aryl sulfotransferase activity could not be detected in either bone marrow-derived macrophages or fibroblasts, and there were no significant differences in GSH content between the two cell types. Because UDP-GT activity is high in macrophages, these data suggest that DT-diaphorase levels would be rate limiting in the detoxification of benzene-derived quinones in bone marrow macrophages. The peroxidase responsible for bioactivation of benzene-derived phenolic metabolites in bone marrow macrophages is unknown but has been suggested to be prostaglandin H synthase (PGS). Hydrogen peroxide, but not arachidonic acid, supported metabolism of hydroquinone to reactive species in bone marrow-derived macrophage lysates. These data do not support a major role for PGS in peroxidase-mediated bioactivation of hydroquinone in bone marrow-derived macrophages, although PGS mRNA could be detected in these cells. Similarly, hydrogen peroxide, but not arachidonic acid, supported metabolism of hydroquinone in a human bone marrow homogenate. Peroxidase-mediated interactions between phenolic metabolites of benzene occurred in bone marrow-derived macrophages. Bioactivation of hydroquinone to species that would bind to acid-insoluble cellular macromolecules was increased by phenol and was markedly stimulated by catechol. Bioactivation of catechol was also stimulated by phenol but was inhibited by hydroquinone. These data define the enzymology and the cell-specific metabolism of benzene metabolites in bone marrow stroma and demonstrate that interactions between phenolic metabolites may contribute to the toxicity of benzene in this critical bone marrow compartment.     

Hydroquinone inhibits bone marrow pre-B cell maturation in vitro

Environmental exposure to benzene results in both myelotoxicity and immunotoxicity. Although benzene-induced immunotoxicity has been well documented, no studies to date have addressed the possibility that benzene toxicity is due in part to altered differentiation of marrow lymphoid cells. We investigated the effect of acute exposure to the benzene metabolite, hydroquinone, on murine bone marrow B-lymphopoiesis. Bone marrow cell suspensions from B6C3F1 (C57BL/6J x C3H/HeJ) mice were depleted of mature surface IgM+ (sIgM+) B cells and cultured for 0, 24, 48, or 72 hr and production of newly formed B cells was assayed both by sIgM expression and colony formation in soft agar cultures. One hr exposure of bone marrow cells to hydroquinone before culture reduced the number of sIgM+ cells generated in liquid cultures. Small pre-B cells (cytoplasmic mu heavy chain+, sIgM-) were numerically elevated as compared with control cultures. Hydroquinone exposure also decreased the number of adherent cells found in cultures of bone marrow cells. These results suggest that short-term exposure to hydroquinone, an oxidative metabolite of benzene, may in some way block the final maturation stages of B cell differentiation. This apparent differentiation block resulted in reduced numbers of B cells generated in culture and a corresponding accumulation of pre-B cells. Reduction of adherent cells in treated cultures may also suggest that toxicity to regulatory cells for the B lineage may be in part responsible for this aspect of hydroquinone myelotoxicity.     

Benzene activates caspase-4 and -12 at the transcription level,without an association with apoptosis,in mouse bone marrow cells lacking the p53 gene

Benzene is a well-known environmental pollutant that can induce hematotoxicity, aplastic anemia, acute myelogenous leukemia, and lymphoma. However, although benzene metabolites are known to induce oxidative stress and disrupt the cell cycle, the mechanism underlying lympho/leukemogenicity is not fully understood. Caspase-4 (alias caspase-11) and -12 are inflammatory caspases implicated in inflammation and endoplasmic reticulum stress-induced apoptosis. The objectives of this study were to investigate the altered expression of caspase-4 and -12 in mouse bone marrow after benzene exposure and to determine whether their alterations are associated with benzene-induced bone marrow toxicity, especially cellular apoptosis. In addition, we evaluated whether the p53 gene is involved in regulating the mechanism, using both wild-type (WT) mice and mice lacking the p53 gene. For this study, 8-week-old C57BL/6 mice [WT and p53 knockout (KO)] were administered a benzene solution (150 mg/kg diluted in corn oil) via oral gavage once daily, 5 days/week, for 1 or 2 weeks. Blood and bone marrow cells were collected and cell counts were measured using a Coulter counter. Total mRNA and protein extracts were prepared from the harvested bone marrow cells. Then qRT-PCR and Western blotting were performed to detect changes in the caspases at the mRNA and protein level, respectively. A DNA fragmentation assay and Annexin-V staining were carried out on the bone marrow cells to detect apoptosis. Results indicated that when compared to the control, leukocyte number and bone marrow cellularity decreased significantly in WT mice. The expression of caspase-4 and -12 mRNA increased significantly after 12 days of benzene treatment in the bone marrow cells of benzene-exposed p53KO mice. However, apoptosis detection assays indicated no evidence of apoptosis in p53KO or WT mice. In addition, no changes of other apoptosis-related caspases, such as caspase-3 and -9, were found in WT or p53KO mice at the level of mRNA and proteins. These results indicated that upregulation of caspase-4 and -12 in mice lacking the p53 gene is not associated with cellular apoptosis. In conclusion, caspase-4 and -12 can be activated by benzene treatment without inducing cell apoptosis in mouse bone marrow, which are partly under the regulation of the p53 gene. J.-Y. Yi and Y. Hirabayashi contributed equally for this study.     

Evidence for strain-specific differences in benzene toxicity as a function of host target cell susceptibility

It has long been recognized that benzene exposure produces disparate toxic responses among different species or even among different strains within the same species. There is ample evidence that species- or strain-dependent differences in metabolic activity correlate with the disparate responses to benzene. However, bone marrow cells (the putative targets of benzene toxicity) may also exhibit species- or strain-dependent differences in susceptibility to the toxic effects of benzene. To investigate this hypothesis, two sets of companion experiments were performed. First, two strains of mice, Swiss Webster (SW) and C57B1/6J (C57), were exposed to 300 ppm benzene via inhalation and the effects of the exposures were determined on bone marrow cellularity and the development of bone marrow CFU-e (Colony Forming Unit-erythroid, an early red cell progenitor). Second, bone marrow cells from the same strains were exposed in vitro to five known benzene metabolites (1,4 benzoquinone, catechol, hydroquinone, muconic acid, and phenol) individually and in binary combinations. Benzene exposure, in vivo, reduced bone marrow cellularity and the development of CFU-e in both strains; however, reductions in both these endpoints were more severe in the SW strain. When bone marrow cells from the two strains were exposed in vitro to the five benzene metabolites individually, benzoquinone, hydroquinone, and catechol reduced the numbers of CFU-e in both strains in dose-dependent responses, phenol weakly reduced the numbers of the C57 CFU-e only and in a non-dose-dependent manner, and muconic acid was without effect on cells from either strain. Only benzoquinone and hydroquinone exhibited differential responses to CFU-e from the two strains and both of these metabolites were more toxic to SW cells than to C57 cells. Six of the ten possible binary mixtures of metabolites were differentially toxic to the CFU-e from the two strains and five of these mixtures were more toxic to SW cells than to C57 cells. Thus, SW mice were more susceptible to the toxic effects of inhaled benzene and their bone marrow cells were more severely affected by in vitro exposure to benzene metabolites. The binary combinations containing phenol produced little or no enhancement of the toxic effects of the non-phenol metabolites. The weak toxic response induced by phenol, whether delivered alone or in binary mixtures, suggests that little metabolism occurred during the 48 h of the in vitro exposures since benzoquinone and hydroquinone, which were clearly toxic when added to the CFU-e culture system, are formed by further metabolic oxidation of phenol. Thus, strain-dependent differential metabolism appeared to play a minimal role in the disparate toxicity observed in the in vitro studies, implying that the diverse responses were due to inherent differences in the susceptibilities of the CFU-e to the toxic action of the benzene metabolites.     

Differences in the metabolism and disposition of inhaled [3H]benzene by F344/N rats and B6C3F1 mice

Benzene is a potent hematotoxin and has been shown to cause leukemia in man. Chronic toxicity studies indicate that B6C3F1 mice are more susceptible than F334/N rats to benzene toxicity. The purpose of the studies presented in this paper was to determine if there were metabolic differences between F344/N rats and B6C3F1 mice which might be responsible for this increased susceptibility. Metabolites of benzene in blood, liver, lung, and bone marrow were measured during and following a 6-hr 50 ppm exposure to benzene vapor. Hydroquinone glucuronide, hydroquinone, and muconic acid, which reflect pathways leading to potential toxic metabolites of benzene, were present in much greater concentrations in the mouse than in rat tissues. Phenylsulfate, a detoxified metabolite, and an unknown water-soluble metabolite were present in approximately equal concentrations in these two species. These results indicate that the proportion of benzene metabolized via pathways leading to the formation of potentially toxic metabolites as opposed to detoxification pathways was much higher in B6C3F1 mice than in F344 rats, which may explain the higher susceptibility of mice to benzene-induced hematotoxicity and carcinogenicity.     

Benzene-Induced Hematotoxicity and Bone Marrow Compensation in B6C3F1 Mice

Long-term inhalation exposure of benzene has been shown to causehematotoxicity and an increased incidence of acute myelogenousleukemia in humans. The progression of benzene-induced hematotoxicityand the features of the toxicity that may play a major rolein the leukemogenesis are not known. We report the hematologicalconsequences of benzene inhalation in B6C3F1 mice exposed to1, 5, 10, 100, and 200 ppm benzene for 6 hr/day, 5 days/weekfor 1, 2, 4, or 8 weeks and a recovery group. There were nosignificant effects on hematopoietic parameters from exposureto 10 ppm benzene or less. Exposure of mice to 100 and 200 ppmbenzene reduced the number of total bone marrow cells, progenitorcells, differentiating hematopoietic cells, and most blood parameters.Replication of primitive progenitor cells in the bone marrowwas increased during the exposure period as a compensation forthe cytotoxicity induced by 100 and 200 ppm benzene. In miceexposed to 200 ppm benzene, the primitive progeni or cells maintainedan increased percentage of cells in S-phase through 25 daysof recovery compared with controls. The increased replicationof primitive progenitor cells in concert with the reported genotoxicityinduced by benzene provides the components necessary for producingan increased incidence of lymphoma in mice. Furthermore, wepropose this mode of action as a biologically plausible mechanismfor benzene-induced leukemia in humans exposed to high concentrationsof benzene.     

Amifostine protects bone marrow from benzene-induced hematotoxicity in mice

Benzene is one of the most widely used industrial chemical agents. Long-term benzene exposure causes bone marrow aplasia and leads to a wide range of hematopoietic disorders including aplastic anaemia (AA). There are currently no effective approaches to protect people from benzene-induced hematotoxicity and AA. In addition, current treatments for AA have limitations with short- and long-term risks. Protective agents and new therapeutic approaches, therefore, are needed to prevent and treat the disease. Amifostine is a well-known cytoprotective agent and has been widely used in clinical for protecting normal tissues from the toxic effects of chemotherapy and radiotherapy. The authors utilized an established mouse model to determine the protective effect of amifostine on benzene-induced bone marrow hematotoxicity. Whole-blood cell count, morphological and histopathological alterations in the bone marrow and spleen, as well as the production of inducible toxic oxidative species were examined and compared among the mouse groups. Amifostine treatment in benzene-exposed mice significantly improved blood cell counts, and morphological and histopathological signs of hematotoxicity in the bone marrow as well as in the spleen. Moreover, amifostine prevented benzene-induced bone marrow and spleen cell apoptosis and rescinded the inhibition of cell proliferation induced by benzene exposure. Finally, amifostine significantly inhibited the levels of reactive oxidative species and lipid peroxidation induced by benzene exposure. These data suggest that amifostine appears to have substantial protective effect on benzene-induced bone marrow hematotoxicity.     

Benzene's toxicity: a consolidated short review of human and animal studies

A large population of humans is exposed to benzene from various occupational and environmental sources. Benzene is an established human and animal carcinogen. Exposure to benzene has been associated with leukaemia in humans and several types of malignancies in animals. The exact mechanism of benzene-induced toxicity is poorly understood. It is believed that benzene exerts its adverse effects by metabolic activation to toxic metabolites. Certain benzene metabolites are genotoxic and mutagenic. This consolidated short-review is composed of human and animal studies to summarize the adverse effects of benzene with special reference to molecular mechanisms involved in benzene-induced toxicity.     

Suppression of bone marrow stromal cell function by benzene and hydroquinone is ameliorated by indomethacin

Administration of benzene to mice will inhibit bone marrow stromal cell-supported hemopoiesis in culture. Hydroquinone, a major metabolite of benzene, will cause a similar inhibition of stromal cell function in vitro. Stromal cells produce both an inducer (colony-stimulating factor) and an inhibitor (prostaglandin E2; PGE2) of hemopoiesis. This research was conducted to determine if prostaglandin synthesis is involved in the suppression of stromal cell function by benzene and hydroquinone. Male B6C3F1 mice were administered benzene (100 mg/kg), indomethacin (1 mg/kg), or benzene plus indomethacin twice a day for 4 consecutive days. On Day 5 bone marrow cells were removed to determine the effect of treatment. In a second series of experiments mouse bone marrow stromal cells in culture were treated with hydroquinone (10(-7) to 10(-4) M), indomethacin (10(-6) M), or a combination of hydroquinone plus indomethacin. Stromal cell function was based on the ability of the treated stromal cells to support granulocyte/monocyte colony development in coculture. The results demonstrated that preadministration of indomethacin in vivo ameliorated benzene-induced inhibition of bone marrow stromal cell function. In vitro, indomethacin ameliorated hydroquinone toxicity to stromal cell function. Benzene administration in vivo induced elevated PGE2 in bone marrow samples which were prevented by preadministration of indomethacin. However, hydroquinone in vitro did not induce a consistent increase in PGE2 levels. These results suggested that toxicity to stromal cells was not due solely to increased prostaglandin synthetase activity.     

Shanghai Health Study (2001–2009): What was learned about benzene health effects?

The Shanghai Health Study (SHS) was a large epidemiology study conducted as a joint effort between the University of Colorado and Fudan University in Shanghai, China. The study was funded by members of the American Petroleum Institute between 2001 and 2009 and was designed to evaluate the human health effects associated with benzene exposure. Two arms of the SHS included: an occupational-based molecular epidemiology study and several hospital-based case control studies. Consistent with historical literature, following sufficient exposure to relatively high airborne concentrations and years of exposure, the SHS concluded that exposure to benzene resulted in an increased risk of various blood and bone marrow abnormalities such as benzene poisoning, aplastic anemia (AA), myelodysplastic syndrome (MDS), and acute myeloid leukemia (AML). Non-Hodgkin lymphoma (NHL) was not significantly increased for the exposures examined in this study. Perhaps the most important contribution of the SHS was furthering our understanding of the mechanism of benzene-induced bone marrow toxicity and the importance of identifying the proper subset of MDS relevant to benzene. Investigators found that benzene-exposed workers exhibited bone marrow morphology consistent with an immune-mediated inflammatory response. Contrary to historic reports, no consistent pattern of cytogenetic abnormalities was identified in these workers. Taken together, findings from SHS provided evidence that the mechanism for benzene-induced bone marrow damage was not initiated by chromosome abnormalities. Instead, chronic inflammation, followed by an immune-mediated response, is likely to play a more significant role in benzene-induced disease initiation and progression than previously thought.