Acute and chronic dose/response effects of inhaled benzene on multipotential hematopoietic stem (CFU-S) and granulocyte/macrophage progenitor (GM-CFU-C) cells in CD-1 mice

Pyridoxine deficiency modulates benzene inhalation-induced hematotoxicity associated with hepatic CYP2E1 activity in B6C3F1 mice

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Modifying effects of pyridoxine deficiency on benzene toxicity were assessed in B6C3F1 mice.

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The mice were fed either a pyridoxine-deficient diet or control diet.

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Pyridoxine deficiency was combined with nose-only benzene inhalation exposure (100 ppm).

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Pyridoxine deficiency and benzene inhalation led to lower body, thymus and spleen weights.

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The treatment combination also led to higher hepatic CYP2E1 protein expression and activity.

Pyridoxine is a co-factor in many enzymatic reactions and impacts of deficiency have been observed in affected populations. A possible modifying effect of pyridoxine deficiency on benzene toxicity was assessed in male B₆C₃F₁ mice fed either a pyridoxine-deficient diet or a control diet. This treatment was combined with benzene inhalation exposure (100 ppm) or no benzene treatment. Pyridoxine-deficient mice exposed to 100 ppm benzene had significantly lower body, thymus and spleen weights. While total white blood cell counts, percentage of lymphocytes, hematocrit and hemoglobin levels were lower, the percentage of neutrophils was significantly higher in deficient and benzene-exposed mice compared to non-exposed controls. Hepatic CYP2E1 protein expression and activity in the deficient and exposed mice were also significantly higher compared to the non-exposed controls. A significant correlation between CYP2E1 activity and several hematological parameters was observed. These results demonstrated that pyridoxine deficiency significantly impacted benzene-induced hematotoxicity. Moreover, the observed agonistic effect of pyridoxinedeficiency and benzene inhalation exposure on CYP2E1 would seem to indicate an involvement of metabolism, but this needs to be further assessed.

Use of toxicant sensitivity distributions (TSD) for development of exposure guidelines for risk to human health from benzene

This technique for setting guideline values differs from that currently used by regulatory agencies throughout the world. Data for benzene were evaluated from epidemiological studies on human populations (29 studies). Exposure durations were evaluated in terms of Long Term Exposure (LTE) and Lifetime Exposure. All data was reported as Lowest Observed Adverse Effect Levels (LOAEL) and converted into exposure doses using Average Daily Dose (ADD) and Lifetime Average Daily Dose (LADD). These values were plotted as a Toxicant Sensitivity Distribution (TSD) which was the cumulative probability of LOAEL-ADD and LOAEL-LADD. From the TSD plots, linear regression equations gave correlation coefficients (R²) ranging from 0.69 to 0.97 indicating normal distributions. Guideline Values (GVs) for LTE (8hr/day) and Lifetime (24hr/70yrs) exposure to benzene were calculated using data from human epidemiological studies as 5% level of cumulative probability (CP) of LOAEL-ADD and LOAEL-LADD from the cumulative probability distributions (CPD). The derived guideline values from the human epidemiological studies were 92 μg/kg/day for LTE and 3.4 μg/kg/day for lifetime exposure. GV for LTE is appropriate for occupational exposure and GV derived for lifetime exposure appropriate for the general population. The guideline value for occupational exposure limit was below all the guideline values developed by regulatory agencies. But the general population guideline is within the range of values formulated by European Union, ATSDR, EPAQS, USEPA and OEHHA for air quality for the general population. This is an alternative method which eliminates the application of safety factors and other sources of errors in deriving guideline values for benzene.

Quantitative measurement of oxidative damage in erythrocytes as indicator in benzene intoxications

The metabolism of aromatic hydrocarbons by the organism forms products that cause cell death depending on the type of exposure. Benzene exposure has been linked to oxidative stress, hepatic damage, aplastic anemia, and hematopoietic cancer as lymphoid and myeloid leukemia. However, there are not fast methods to evaluate chronic benzene exposure in human blood. The objective of this work was the evaluation of the correlation between oxidative damage with benzene exposure and the level of cellular plasma membrane stability (CPMS) in erythrocytes to use it as a future indicator to determine the grade of benzene intoxications. CPMS in vitro assays were used to evaluate damage for benzene, toluene, and xylene. Erythrocytes CPMS assays in vitro shows a progressive reduction with benzene, toluene, and xylene suggesting that aromatic hydrocarbons complexity favors CPMS damage. Eight groups of Wistar rats (n = 5) were used to study the level of damage on CPMS by acute and chronic benzene administration. Enzymatic, metabolic, histological, and oxidative damage tests were performed. Acute administration (100 μL/100 g/single dose) showed a decrease of 66.7% in CPMS, while 63.6% for chronic administration (5 μL/100 g/every 2 days/3 months) showing a correlation with liver damage principally (transaminases activity increase, glycogen level decrease, and high oxidative damage). Tissue damage was observed in bone marrow, kidney, spleen, and lungs. Benzene produces damage on CPMS depending on the exposure time and dose. The CPMS technique could be used as an important aromatic hydrocarbons intoxication indicator.

Altered Development of Immunocompetence Following Prenatal or Combined Prenatal-Postnatal Insult: A Timely Review

There is an unequal distribution of interest between assessment of immunocompetence at the adult level following the administration of various agents/protocols and evaluation of immunocompetence postnatally subsequent to prenatal insult. This paper reviews the effects of dietary manipulation, selected pharmaceuticals, and certain environmental agents on the functional status of the postnatal immune system after in utero exposure to these agents and protocols. These data are discussed in light of what has also been observed for adult exposure to the same or similar experimental design. A discussion of congenital disorders of immunocompetence is also provided. Alteration in the development of postnatal immunocompetence following prenatal insult may manifest itself in several ways, some of which may reflect a permanent defect while others may be of a transient nature. In either instance, however, it behooves scientists in all arenas to (1) determine the precise nature and magnitude of such developmentally related immunologic deficits and (2) focus on the selection and standardization of the most relevant procedures to be employed in the assessment of immunocompetence.

Hematological effect of benzene exposure with emphasis of muconic acid as a biomarker

Human exposure to benzene in work environment is a global occupational health problem. It is established that benzene requires to be metabolized to induce its effects. Benzene has been associated with various hematotoxins and carcinogens. The aim of this study was to investigate the effect of benzene on complete blood picture, with emphasis of trans, trans-muconic acid (t,t-MA) as a biomarker of benzene in urine, considering the influence of cigarette smoke. A total of 81 workers (61 males and 20 females) have been occupationally exposed to benzene. In addition, 83 workers (55males and 28 females) were also recruited as a control group. Complete blood picture was analyzed and urinary t,t-MA was determined by liquid chromatography. In addition, creatinine in the urine samples was determined. Levels of blood elements (white blood cells, red blood cells and platelets) were decreased among exposed workers compared with the controls. The urinary level of t,t-MA/creatinine of the exposed workers was elevated especially in the smoking group compared to the controls. This study recommends that complete blood picture and t,t-MA are helpful biomarker tests that should be done to detect the early effects of benzene exposure.

Physiologically based toxicokinetic modeling of secondary acute myelolytic leukemia

Benzene, designated as environmental and occupational carcinogen and hematotoxin, has been associated with secondary leukemia. To develop a toxicokinetic model of AML, benzene can be used as leukemogenic agent. The aim of the present study was to optimize the dose, period and time of cumulative benzene exposure of Swiss Albino mice and to analyze survival rate; alteration in cell cycle regulation and other clinical manifestations in mice exposed to benzene vapour at a dose 300 ppm × 6 h/day × 5 days/week for 2 weeks, i.e., 9000(a)ppm cumulative dose. Analyzing physiological parameters like plasma enzyme profile, complete hematology (Hb %, RBC indices and WBC differentials), hematopoietic cells morphology, expression of cell cycle regulatory proteins, tissue histology and analysis of DNA fragmentation, optimum conditions were established. Down regulation of p53 and p21 and up regulation of CDK2, CDK4, CDK6, cyclin D1 and E in this exposed group were marked as the optimum conditions of cellular deregulation for the development of secondary AML. Elevated level of Plasma AST/ALT with corresponding changes in liver histology showing extended sinusoids within the hepatocytic cell cords in optimally exposed animals also confirmed the toxicokinetic relation of benzene with leukemia. It can be concluded from the above observations that the 9000(a)ppm exposed animals can serve as the induced laboratory model of secondary acute myeloid leukemia.

The Toxicological Properties of Petroleum Gases

To characterize the toxicological hazards of petroleum gases, 90-day inhalation toxicity (Organization for Economic Cooperation and Development [OECD] 413) and developmental toxicity (OECD 414) tests were conducted with liquefied propane gas (LPG) at concentrations of 1000, 5000, or 10 000 ppm. A micronucleus test (OECD 474) of LPG was also conducted. No systemic or developmental effects were observed; the overall no observed adverse effect concentration (NOAEC) was 10 000 ppm. Further, there was no effect of LPG exposure at levels up to 10 000 ppm on micronucleus induction and no evidence of bone marrow toxicity. Other alkane gases (ethane, propane, n-butane, and isobutane) were then evaluated in combined repeated exposure studies with reproduction/development toxicity screening tests (OECD 422). There were no toxicologically important changes in parameters relating to systemic toxicity or neurotoxicity for any of these gases at concentrations ranging from 9000 to 16 000 ppm. There was no evidence of effects on developmental or reproductive toxicity in the studies of ethane, propane, or n-butane at the highest concentrations tested. However, there was a reduction in mating in the high-exposure group (9000 ppm) of the isobutane study, which although not significantly different was outside the range previously observed in the testing laboratory. Assuming the reduction in mating to have been toxicologically significant, the NOAEC for the isobutane reproductive toxicity screening test was 3000 ppm (7125 mg/m3). A method is proposed by which the toxicity of any of the 106 complex petroleum gas streams can be estimated from its composition.

Leukemia and benzene

Excessive exposure to benzene has been known for more than a century to damage the bone marrow resulting in decreases in the numbers of circulating blood cells, and ultimately, aplastic anemia. Of more recent vintage has been the appreciation that an alternative outcome of benzene exposure has been the development of one or more types of leukemia. While many investigators agree that the array of toxic metabolites, generated in the liver or in the bone marrow, can lead to traumatic bone marrow injury, the more subtle mechanisms leading to leukemia have yet to be critically dissected. This problem appears to have more general interest because of the recognition that so-called "second cancer" that results from prior treatment with alkylating agents to yield tumor remissions, often results in a type of leukemia reminiscent of benzene-induced leukemia. Furthermore, there is a growing literature attempting to characterize the fine structure of the marrow and the identification of so called "niches" that house a variety of stem cells and other types of cells. Some of these "niches" may harbor cells capable of initiating leukemias. The control of stem cell differentiation and proliferation via both inter- and intra-cellular signaling will ultimately determine the fate of these transformed stem cells. The ability of these cells to avoid checkpoints that would prevent them from contributing to the leukemogenic response is an additional area for study. Much of the study of benzene-induced bone marrow damage has concentrated on determining which of the benzene metabolites lead to leukemogenesis. The emphasis now should be directed to understanding how benzene metabolites alter bone marrow cell biology.

ATSDR evaluation of health effects of benzene and relevance to public health

As part of its mandate, the Agency for Toxic Substances and Disease Registry (ATSDR) prepares toxicological profiles on hazardous chemicals found at Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) National Priorities List (NPL) sites that have the greatest public health impact. These profiles comprehensively summarize toxicological and environmental information. This article constitutes the release of portions of the Toxicological Profile for Benzene. The primary purpose of this article is to provide public health officials, physicians, toxicologists, and other interested individuals and groups with an overall perspective on the toxicology of benzene. It contains descriptions and evaluations of toxicological studies and epidemiological investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health.

Immunotoxic effects of chemicals: A matrix for occupational and environmental epidemiological studies

BACKGROUND

Many biological and chemical agents have the capacity to alter the way the immune system functions in human and animals. This study evaluates the immunotoxicity of 20 substances used widely in work environments.

METHODS

A systematic literature search on the immunotoxicity of 20 chemicals was performed. The first step was to review literature on immunotoxicity testing and testing schemes adopted for establishing immunotoxicity in humans. The second step consisted of providing a documentation on immunotoxicity of substances that are widely used in work environment, by building tables for each chemical of interest (benzene, trichloroethylene, PAHs, crystalline silica, diesel exhausts, welding fumes, asbestos, styrene, formaldehyde, toluene, vinyl chloride monomer, tetrachloroethylene, chlorophenols, 1,3-butadiene, mineral oils, P-dichlorobenzene, dichloromethane, xylene, 1,1,1-trichloroethane, ethylene oxide). The third step was the classification of substances; an index (strong, intermediate, weak, nil) was assigned on the basis of the evidence of toxicity and type of immunotoxic effects (immunosuppression, autoimmunity, hypersensitivity) on the basis of the immune responses. Finally substances were assigned a score of immunotoxic power.

RESULTS

Tables have been produced that include information for the 20 substances of interest, based on 227 animal studies and 94 human studies. Each substance was assigned an index of immunotoxic evidence, a score of immunotoxic power and type of immunotoxic effect.

CONCLUSIONS

This matrix can represent a tool to identify chemicals with similar properties concerning the toxicity for the immune system, and to interpret epidemiological studies on immune-related diseases.

Exposure of Tg.AC transgenic mice to benzene suppresses hematopoietic progenitor cells and alters gene expression in critical signaling pathways

The effects of acute benzene (BZ) exposure on hematopoietic progenitor cells (HPCs) derived from bone marrow cells were studied using homozygous male v-Ha-ras Tg.AC mice at 8-10 weeks of age. The mice were given 0.02% BZ in their drinking water for 28 days with the dose rate estimated to be 34 mg benzene/kg BW/day. Analysis of cultured HPCs indicated that BZ suppressed the proliferation of the multilineage colony forming unit-granulocyte, erythrocyte, macrophage, megakaryocyte (CFU-GEMM); colony forming unit-granulocyte, macrophage (CFU-GM); and blast forming unit erythrocyte/colony forming unit erythrocyte (BFUE/CFUE). A gene expression profile was generated using nylon arrays spotted with 23 cDNAs involved in selected signal pathways involved in cell distress, inflammation, DNA damage, cell cycle arrest, and apoptosis. Of the 23 marker genes, 6 (bax, c-fos, E124, hsf1, ikBa, and p57) were significantly (Mann-Whitney U tests, P < 0.05) overexpressed in BZ-exposed mice. Two genes (c-myc and IL-2) approached significance (at P = 0.053). The pattern of gene expression was consistent with BZ toxicity and the suppression of HPCs.

Mechanism of action of benzene toxicity: cell cycle suppression in hemopoietic progenitor cells (CFU-GM)

The aim of this study was to clarify previously reported controversial data and hypotheses concerning the effect of benzene on the cell cycle of hemopoietic stem cells. In this study, the bromodeoxyuridine UV (BUUV) suicide assay was performed in normal C57BL/6 and p53 knockout (KO) C57BL/6 mice during and after exposure to 300 ppm of benzene for 2 weeks. Our kinetic studies revealed that the cell cycle of hemopoietic myeloid progenitor cells (colony-forming unit granulocyte-macrophage [CFU-GM]), rather than being stimulated, was suppressed by exposure to benzene. The fraction of CFU-GM in S phase was significantly depressed, from 37.1% in controls to 16.3% in normal mice. BrdUrd incorporation in both groups revealed significantly different slopes for untreated and benzene-exposed normal C57BL/6 mice. p53 appeared to induce suppression of both the number and the cycling fraction of hemopoietic progenitor cells, as demonstrated by the lack of benzene-induced suppression of these parameters in p53 KO mice. The likelihood that suppression of bone marrow cellularity and cell cycling is mediated by p53 was supported by the upregulation of p21, a cyclin-dependent kinase inhibitor. Our present study revealed the mechanism of action of benzene hematotoxicity. Benzene suppresses the cell cycle by p53-mediated overexpression of p21, a cyclin-dependent kinase inhibitor, resulting not simply in suppression of hemopoiesis but rather in a dynamic change of hemopoiesis during and after benzene exposure. Thus, the controversies raised by previously reported data are resolved by our present findings of hemopoietic stem cell kinetics.

A potential mechanism underlying the increased susceptibility of individuals with a polymorphism in NAD(P)H:quinone oxidoreductase 1 (NQO1) to benzene toxicity

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a two-electron reductase that detoxifies quinones derived from the oxidation of phenolic metabolites of benzene. A polymorphism in NQO1, a C609T substitution, has been identified, and individuals homozygous for this change (T/T) have no detectable NQO1. Exposed workers with a T/T genotype have an increased risk of benzene hematotoxicity. This finding suggests NQO1 is protective against benzene toxicity, which is difficult to reconcile with the lack of detectable NQO1 in human bone marrow. The human promyeloblastic cell line, KG-1a, was used to investigate the ability of the benzene metabolite hydroquinone (HQ) to induce NQO1. A concentration-dependent induction of NQO1 protein and activity was observed in KG-1a cells cultured with HQ. Multiple detoxification systems, including NQO1 and glutathione protect against benzene metabolite-induced toxicity. Indeed, exposure to a noncytotoxic concentration of HQ induced both NQO1 and soluble thiols and protected against HQ-induced apoptosis. NQO1 protein and activity increased in wild-type human bone marrow cells (C/C) exposed to HQ, whereas no NQO1 was induced by HQ in bone marrow cells with the T/T genotype. Intermediate induction of NQO1 by HQ was observed in heterozygous bone marrow cells (C/T). NQO1 also was induced by HQ in wild-type (C/C) human bone marrow CD34(+) progenitor cells. Our data suggest that failure to induce functional NQO1 may contribute to the increased risk of benzene poisoning in individuals homozygous for the NQO1 C609T substitution (T/T).

Influences of gender, development, pregnancy and ethanol consumption on the hematotoxicity of inhaled 10 ppm benzene

The hematotoxic effects of benzene in both humans and animals are well documented. Current estimates concerning the risks associated with benzene exposure are usually based on adult, male cohort studies; however, there are indications that females may respond differently than males to benzene and that fetuses may respond differently than adults. Another factor to be considered in risk estimates is the impact of personal habits. In experimental animals, ethanol consumption is known to increase the hematotoxicity of benzene; therefore, alcohol consumption may also alter the potential risk of individuals exposed to benzene. To address some of the factors that may confound risk estimates for benzene exposure, a series of experiments were performed. Age-matched male as well as pregnant and virgin female Swiss Webster mice were exposed to 10 ppm benzene for 6 h a day over 10 consecutive days (days 6 through 15 of gestation for the pregnant females). Half of the animals also received 5% ethanol in the drinking water during this period. On day 11, bone marrow cells from the adults and liver cells from the fetuses were assayed for the numbers of erythroid colony-forming units (CFU-e). CFU-e assays were also performed on bone marrow cells isolated from 6-week postpartum dams exposed during gestation and from in utero-exposed 6-week old males and females. Gender differences were clearly observed in the responses to the various exposure protocols. Depressions in CFU-e numbers were only seen in male mice while elevations in CFU-e numbers were only seen in female mice. Male mice exposed as adults for 10 days to benzene (B), ethanol (E) or benzene+ethanol (B+E) exhibited depressed CFU-e levels as did male fetal mice exposed to B in utero. In addition, adult male mice which had been exposed in utero to either B or to E individually displayed depressed CFU-e levels. In contrast, none of the groups of female mice exhibited any depressions in CFU-e numbers after any of the exposures. Elevations in CFU-e numbers were observed among pregnant females exposed to E and among adult females exposed to B+E in utero. In summary, a majority (6/9) of the exposure protocols produced depressions in the CFU-e numbers of male mice, whereas a majority (7/9) of the exposure protocols produced no changes in the CFU-e numbers of female mice. Those changes that were observed in females consisted of elevations of CFU-e numbers. These results suggest that the male erythron is more susceptible than the female erythron to the hematotoxicants benzene and ethanol, regardless of whether exposures occur in utero or during adulthood.

A perspective on benzene leukemogenesis

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.

Synergistic action of the benzene metabolite hydroquinone on myelopoietic stimulating activity of granulocyte/macrophage colony-stimulating factor in vitro

The effects of in vitro pretreatment with benzene metabolites on colony-forming response of murine bone marrow cells stimulated with recombinant granulocyte/macrophage colony-stimulating factor (rGM-CSF) were examined. Pretreatment with hydroquinone (HQ) at concentrations ranging from picomolar to micromolar for 30 min resulted in a 1.5- to 4.6-fold enhancement in colonies formed in response to rGM-CSF that was due to an increase in granulocyte/macrophage colonies. The synergism equaled or exceeded that reported for the effects of interleukin 1, interleukin 3, or interleukin 6 with GM-CSF. Optimal enhancement was obtained with 1 microM HQ and was largely independent of the concentration of rGM-CSF. Pretreatment with other authentic benzene metabolites, phenol and catechol, and the putative metabolite trans, trans-muconaldehyde did not enhance growth factor response. Coadministration of phenol and HQ did not enhance the maximal rGM-CSF response obtained with HQ alone but shifted the optimal concentration to 100 pM. Synergism between HQ and rGM-CSF was observed with nonadherent bone marrow cells and lineage-depleted bone marrow cells, suggesting an intrinsic effect on recruitment of myeloid progenitor cells not normally responsive to rGM-CSF. Alterations in differentiation in a myeloid progenitor cell population may be of relevance in the pathogenesis of acute myelogenous leukemia secondary to drug or chemical exposure.

Hematotoxic effects of benzene analyzed by mathematical modeling

The hematopoietic cell response to benzene intoxication in mice (during and after long-term inhalation) was analyzed by a mathematical model of murine hematopoiesis. Two complementary methods, Time-Curve and Steady-State Analysis, were developed to identify target cells for benzene toxicity and to quantify the extent of damage in different stages of development of these target cells. We found that (i) erythropoietic cells were the most sensitive; (ii) granulopoietic cells were about half as sensitive as erythropoietic and (iii) hematopoietic stem cells exhibited a sensitivity that ranged between that of erythropoietic and granulopoietic cells. A dose-response relationship between benzene levels and damage in target cells (valid from 1 to more than 900 ppm) was derived that was linear for doses up to 300 ppm and plateaued thereafter. This relationship indicated that benzene-induced hematotoxicity is subject to a saturable process. Recovery of hematopoiesis following chronic benzene intoxication was simulated for different doses and preceding exposure periods. The impaired recovery following exposure periods greater than 8 weeks could be explained by a severe reduction in the maximum self-maintenance of stem cells. This study indicates that the present mathematical model represents a useful approach to investigate alternate hypotheses for the action of hematotoxic agents.

Short term benzene exposure provides a growth advantage for granulopoietic progenitor cells over erythroid progenitor cells

Because chronic benzene exposure is associated with acute myeloblastic leukemia and other myeloproliferative disorders, we sought to determine whether short-term benzene exposure provides a growth advantage for granulopoietic elements over erythropoietic elements. Groups of male DBA/2J mice were exposed to 0, 10, 30, or 100 ppm benzene (6 h/day for 5 days). One day and 5 days after the benzene exposures, the numbers of the two most primitive erythroid progenitor cells (BFU-E and CFU-E) and the numbers of the most primitive granulocytic progenitor cells (GM-CFU-C) were assessed. Additional groups of mice were given hemolytic doses of phenylhydrazine (PHZ) during the 5 days of benzene exposure, while other groups of mice were given PHZ during the 5 days of recovery from benzene exposure. These experiments were designed to determine the effects of benzene exposure on progenitor cell numbers during periods of markedly heightened erythropoiesis. The results demonstrate that short-term benzene exposure does induce a growth advantage for granulocytic cells in both the bone marrow and spleen of exposed mice. Moreover, a benzene-induced shift toward granulopoiesis is observed even in those mice treated with a powerful erythropoietic stimulus. These effects disappear 5 days after cessation of benzene exposure in the bone marrow but persist in the spleen of mice treated with phenylhydrazine.

Pharmacokinetics of benzene

A physiologically based pharmacokinetic model was developed and used to describe the pharmacokinetics of benzene in three species: mice, rats, and humans. For each species, the body was divided into five anatomical compartments, consisting of liver, fat, bone marrow, and muscle, and organs such as brain, heart, kidney, and viscera, connected by the arterial and venous blood flow pathways. Metabolism of benzene followed Michaelis-Menten (nonlinear) kinetics in all species and occurred primarily in the liver compartment and, to a lesser extent, in the bone marrow. Comparison of model results with empirical data on inhalation, gavage, and intraperitoneal and subcutaneous injection in mice, rats, and humans, demonstrates the utility of a physiological pharmacokinetic model in describing the pharmacokinetics of benzene in three species across multiple routes of exposure.

Ferrokinetics and erythropoiesis in mice after long-term inhalation of benzene

Ferrokinetics and erythropoiesis were examined in mice exposed for 6 or 7 weeks to an airborne concentration of 300 ppm of benzene, for 6 h per day, and 5 days per week. Ferrokinetic indicators showed only a slightly enhanced production of haeme and erythrocytes in the spleen (133% +/- 18% and 122% +/- 17%, respectively). Production did not change in the femoral marrow; a decline of CFU-C, BFU-E and especially CFU-E (34% +/- 8%) took place there and a shift of cellularity into less mature developmental classes in the erythroblast compartment, without this compartment as a whole being damaged. The erythrocytes produced have an enhanced MCV (109% +/- 0%) and MCH (109% +/- 1%) with an unchanged MCHC; their concentration in blood sank to 87% +/- 1%. The absolute reticulocyte count rose to 160% +/- 16%. 59Fe incorporation into the liver declined far below the level attributable to decreased accessibility of the tracer (84% +/- 4%). A shortening of the life span of late erythroblasts and circulating erythrocytes was deduced from these findings and methodological problems related to some of the seemingly controversial findings are discussed.

The ability of the murine erythron to respond to hemolytic doses of phenylhydrazine is significantly impaired by exposures to 10 ppm benzene

Male C57Bl mice were given 50 exposures (6 h/d x 5 d/wk x 10 wk) to 10 ppm benzene. At regular intervals during the course of the exposures, the numbers of erythroid colony-forming cells (CFU-E) and the numbers of granulocytic colony-forming cells (GM-CFU-C) were assayed. At the end of the benzene exposures, additional groups of mice were given 4 daily injections of phenylhydrazine (PHZ) to induce anemia. During the course of the exposures, the numbers of colony-forming cells from benzene-exposed mice were, with infrequent exceptions, statistically indistinguishable from the numbers of these cells in air-exposed mice. However, in response to the PHZ-induced anemia, the numbers of late erythroid (CFU-E) and granulocytic (GM-CFU-C) progenitor cells were about 30% lower among benzene-exposed mice than among air-exposed mice. These results suggest that concentrations of benzene that induce little or no observable hematopoietic changes may, in fact, greatly alter the hematopoietic capacity of an exposed individual.

Prolonged red blood cell glycerol hemolysis in mice inhaling benzene

Inhalation of benzene produces a prolongation of mouse red blood cell glycerol hemolysis time. This was not observed in red blood cells directly incubated in benzene. Increased resistance to the hemolytic action of glycerol should be explored as a potentially useful biological monitoring procedure in the red blood cells of benzene-exposed humans.

Chemical of current interest--benzene

Benzene is one of the world's major commodity chemicals. It is derived from petroleum and coal and is used both as a solvent and as a starting material in chemical syntheses. The numerous industrial uses of benzene over the last century need not be recounted here, but the most recent addition to the list of uses of benzene is as a component in a mixture of aromatic compounds added to gasoline for the purpose of replacing lead compounds as anti-knock ingredients. The best known and longest recognized toxic effect of benzene is the depression of bone marrow function seen in occupationally exposed individuals. These people have been found to display anemia, leucopenia, and/or thrombocytopenia. When pancytopenia, i.e., the simultaneous depression of all three cell types, occurs and is accompanied by bone marrow necrosis, the syndrome is called aplastic anemia. In addition to observing this decrease in humans and relating it to benzene exposure, it has been possible to establish animal models which mimic the human disease. The result has been considerable scientific investigation into the mechanism of benzene toxicity. Although the association between benzene exposure and aplastic anemia has been recognized and accepted throughout most of this century, it is only recently that leukemia, particularly of the acute myelogenous type, has been related to benzene. The acceptance of benzene as an etiological agent in aplastic anemia in large measure derives from our ability to reproduce the disease in most animals treated with sufficiently high doses of benzene over the necessary time period. Unfortunately, despite extensive efforts in several laboratories, it has not been possible to establish a reproducible, reliable model for the study of benzene-induced leukemia. The recent demonstration that several animals exposed to benzene either by inhalation or in the drinking water during studies by Drs. B. Goldstein and C. Maltoni suggests that such a model may be forthcoming. Nevertheless, at this time it is not clear whether bone marrow damage of the type that leads to aplastic anemia is required for the development of leukemia. Most studies of benzene toxicity have involved dosing animals with benzene either by inhalation or by injection, using high doses to ensure a toxic response. Very few studies have concentrated on the oral route of administration and none have concentrated on administering benzene by mouth at the low doses occasionally detected in drinking water.(ABSTRACT TRUNCATED AT 400 WORDS)

Recent advances in the metabolism and toxicity of benzene

Benzene is a heavily used industrial chemical, a petroleum byproduct, an additive in unleaded gas, and a ubiquitous environmental pollutant. Benzene is also a genotoxin, hematotoxin, and carcinogen. Chronic exposure causes aplastic anemia in humans and animals and is associated with increased incidence of leukemia in humans and lymphomas and certain solid tumors in rodents. Bioactivation of benzene is required for toxicity. In the liver, the major site of benzene metabolism, benzene is converted by a cytochrome P-450-mediated pathway to phenol, the major metabolite, and the secondary metabolites, hydroquinone and catechol. The target organ of benzene toxicity, the hematopoietically active bone marrow, metabolizes benzene to a very limited extent. Phenol is metabolized in the marrow cells by a peroxidase-mediated pathway to hydroquinone and catechol, and ultimately to quinones, the putative toxic metabolites. Benzene and its metabolites appear to be nonmutagenic, but they cause myeloclastogenic effects such as micronuclei, chromosome aberrations, and sister chromatid exchange. It is unknown whether these genomic changes, or the ability of the quinone metabolites to form adducts with DNA, are involved in benzene carcinogenicity. Benzene, through its active metabolites, appears to exert its hematological effects on the bone marrow stromal microenvironment by preventing stromal cells from supporting hemopoiesis of the various progenitor cells. Recent advances in our understanding of the mechanisms by which benzene exerts its genotoxic, hematotoxic, and carcinogenic effects are detailed in this review.

Protracted exposure of C57BL/6 mice to 300 ppm benzene depresses B- and T-lymphocyte numbers and mitogen responses. Evidence for thymic and bone marrow proliferation in response to the exposures

Groups of C57BL/6J, male mice were exposed to 300 ppm benzene via inhalation for 115 exposures (6 h/day, 5 days/week), a regimen known to cause thymic lymphoma in these animals. The effects of these exposures on lymphoid parameters were determined by measuring the numbers of B- and T-lymphocytes and mitogen-induced proliferation of B- and T-lymphocytes in bone marrow, spleen, and thymus after 6, 30, and 115 exposures. The numbers of B-lymphocytes in bone marrow and spleen and the numbers of T-lymphocytes in thymus and spleen were found to be markedly reduced after all 3 periods. Mitogen-induced proliferation of bone marrow and splenic B-lymphocytes exhibited a progressive depression throughout the exposure period reaching a point of no observable response after 115 exposures. Splenic T-cell mitogen-induced proliferation was also markedly depressed throughout the exposures, but there was no evidence of a progressive decline in this response during the exposures. Bone marrow cellularity increased 3-fold and the numbers of thymic T-cells increased 15-fold in benzene-exposed mice between the 6th and 30th exposure. No corresponding increase in splenic cells was observed in benzene-exposed mice during this interval. The marked increases in the numbers of cells in bone marrow and thymus are interpreted as arising from compensatory proliferation of a subpopulation of cells in response to the exposures. The absence of increases in cell number in the spleen is interpreted as reflecting the lack of lymphoid restorative capacity in this organ. The marked increases of thymic and bone marrow cellularity are discussed relative to the known ability of this benzene exposure regimen to produce thymic lymphoma in these animals.

Short-term toxicity of trans,trans-muconaldehyde

Toxicological and biochemical effects of muconaldehyde, a six-carbon diene dialdehyde, were evaluated in 6- to 9-week-old CD-1 male mice. The LD50 of trans,trans-muconaldehyde was 6.7 and 7.1 mg/kg body wt when calculated by two different methods. Administration of trans,trans-muconaldehyde (2 mg/kg, ip) daily for 10 and 16 days resulted in a statistically significant decrease in red blood cell count, hematocrit, hemoglobin, bone marrow cellularity, and hepatic total and free sulfhydryl content. There was an increase in white blood cell count and spleen weight at 16 days. Similar effects, but of much lesser magnitude, were observed when the mice were given the same total dose of 2 mg/kg divided into three daily ip injections of 0.67 mg/kg trans,trans-muconaldehyde for 10 and 16 days. This alpha,beta-unsaturated aldehyde is a potent bone marrow toxin in mice.

Benzene hematotoxicity and leukemogenesis

Eight-to-twelve-week-old male and female C57B1/6 BNL mice were exposed to air or benzene vapor in air at a concentration of 10, 25, 100, 300, or 400 ppm. Benzene at concentrations of 100 ppm or higher for 10 exposures of 6 hours per day 5 days a week produced a reduction in bone marrow cellularity and the number of pluripotent stem cells in the bone marrow. The fraction of stem cells in DNA synthesis was also increased. Exposure to 300 ppm 6 hours a day 5 days a week for 2, 4, 8, and 16 weeks produced a diminution in the stem cell levels in bone marrow which returned to those of controls 2 weeks after benzene exposure for 2 and 4 weeks, 16 weeks after exposure for 8 weeks, and to 92% of controls 25 weeks after 16 weeks of exposure. There was a more rapid return of blood lymphocytes to the control level. Mice exposed to 300 ppm for 6 hours/day, 5 days per week for 16 weeks began dying at 330 days of age, whereas no deaths were observed in sham-exposed mice until 440 days of age. The benzene-exposed mice died in two waves: the first was from 330-390 days of age, with a second wave commencing at 570 days of age. The first wave of mortality was due primarily to thymic lymphomata. The second wave was due to a mixture of nonthymic lymphomata and solid tumors.

The temporal relationship between behavioral and hematological effects of inhaled benzene

The time-effect relationship of the behavioral and hematological changes caused by inhaled benzene was investigated in C57BL mice. Mice were exposed to air, or 100, 300, 1000, or 3000 ppm benzene in standard inhalation chambers employing dynamic vapor exposure techniques. Mice were exposed for 6 hr/day for the number of days necessary to achieve a minimum concentration X time (Ct) product of 3000 ppm-days. The intermittent exposure regimen of 6 hr/day was employed to simulate occupational exposure. The most sensitive behavioral index (milk-licking) was affected by the lowest concentration tested (100 ppm), while homecage food intake, hindlimb grip strength, and body weight were reduced only at 1000 and 3000 ppm. Some of these previously undocumented behavioral changes occurred as rapidly as hematological changes that have been considered hallmarks of benzene toxicity. A significant decrease in circulating lymphocytes occurred after exposure to all concentrations. Circulating red blood cells were variably affected by benzene, in that anemia resulted after 10 days exposure to 100 ppm and after 3 days exposure to 300 ppm but not after 3 days exposure to 1000 ppm or a single exposure of 3000 ppm. The data indicate a departure from Ct relationships, and suggest that exposure duration as well as daily dose may be an important factor in benzene toxicity.

Benzene inhalation produces leukemia in mice

Female C57Bl/6 mice were exposed to 300 ppm benzene 6 hr/day, 5 days/week for 16 weeks and then held for lifetime observation. Sixty-four weeks after commencement of the study, 10 of 90 exposed mice had died as opposed to only 1 of 88 controls. Of the 10 exposed mice that died, 6 had thymic lymphomas, 2 had unspecified lymphomas, 1 was killed when moribund and found leukemia-free, and 1 was undiagnosed due to autolysis and partial cannibalization. The single dead control animal did not have lymphoma or leukemia. These data provide proof of the leukemogenicity of benzene in female C57Bl/6 mice.

Repeated exposure of C57Bl mice to inhaled benzene at 10 ppm markedly depressed erythropoietic colony formation

Exposure of C57Bl mice to 10 ppm benzene (the current occupational exposure limit) for 6 h/day, 5 days/week causes a progressive depression in the in vitro colony forming ability of one of the erythroid progenitor cells, the colony-forming unit-erythroid (CFU-E). Colony growth of cells from exposed mice was only 5% of control colony growth after 178 days of exposure. Burst-forming-cell growth was depressed to 55% of control growth after 66 days but returned to control growth values at 178 days. In addition, benzene-exposed mice exhibited depressions in the numbers of splenic nucleated red cells and in the numbers of circulating red cells and lymphocytes. These results suggest that low-level exposure to benzene may be hematotoxic.

Depressions in B- and T-lymphocyte mitogen-induced blastogenesis in mice exposed to low concentrations of benzene

In a short-term (6 h/day X 6 days) benzene inhalation dose-response study, mitogen-induced blastogenesis of both B- and T-lymphocytes in male, C57Bl mice was observed to be significantly depressed at relatively low levels of benzene. Exposure to 10 ppm benzene resulted in a significant depression in femoral lipopolysaccharide (LPS)-induced B-colony-forming ability, while total numbers of B-lymphocytes at this concentration were not significantly depressed. Similarly, splenic phytohemagglutinin (PHA)-induced blastogenesis was significantly depressed at 31 ppm, without a concomitant significant depression in numbers of T-lymphocytes. These data indicate that concentrations of benzene at or near the current standard for occupational exposure (10 ppm) can affect certain immune-associated processes.

The effects of benzene inhalation on murine hematopoietic precursor cells (CFU-e, BFU-e and CFU-gm)

Male B6D2F1 mice were exposed by inhalation to 4000 ppm of benzene for 8 h/day for 1, 2, 3, 5, 7 or 14 days and then sacrificed at 18 h following the last exposure. The cellularities of both femur and spleen were depressed for the duration of benzene exposure. BFU-e/femur declined for 3 days, rebounded to control levels by day 5, and were again depressed by day 7. CFU-e/femur were initially depressed, but by day 7, the concentrations of CFU-e were so elevated that the total number had actually rebounded to slightly higher than control values. CFU-gm/femur remained depressed for the duration of the exposure periods. CFU-e, BFU-e and CFU-gm/spleen were depressed following all exposures. The toxic effects of benzene on hematopoiesis that are immediate are not ascribable to migration of stem cells between femur and spleen. The depressive effects of benzene inhalation on erythropoiesis may be compensated by the rapid proliferation of CFU-e.

Effect of in vivo exposure to benzene on the characteristics of bone marrow adherent cells

The effect of benzene on the adherent cell population, cultured from the bone marrow of exposed mice was investigated in the presence and absence of hydrocortisone. The adherent CFUs from exposed animals did not differ either in numbers or self-replicate ability to those derived from shown exposed animals. Adherent layers from mice exposed to 100 or 400 pp-benzene were devoid of fat cells regardless of the presence or absence of hydrocortisone. Hydrocortisone was shown to influence the proportion of acid phosphatase-positive cells derived from benzene-exposed animals. Those results suggest that benzene exposure may influence the bone marrow stromal cells.