A morphological analysis of the short-term effects of benzene on the development of the hematological cells in the bone marrow of mice and the effects of interleukin-1 alpha on the process

The fate of benzene-oxide

Metabolism is a prerequisite for the development of benzene-mediated myelotoxicity. Benzene is initially metabolized via cytochromes P450 (primarily CYP2E1 in liver) to benzene-oxide, which subsequently gives rise to a number of secondary products. Benzene-oxide equilibrates spontaneously with the corresponding oxepine valence tautomer, which can ring open to yield a reactive alpha,beta-unsaturated aldehyde, trans-trans-muconaldehyde (MCA). Further reduction or oxidation of MCA gives rise to either 6-hydroxy-trans-trans-2,4-hexadienal or 6-hydroxy-trans-trans-2,4-hexadienoic acid. Both MCA and the hexadienal metabolite are myelotoxic in animal models. Alternatively, benzene-oxide can undergo conjugation with glutathione (GSH), resulting in the eventual formation and urinary excretion of S-phenylmercapturic acid. Benzene-oxide is also a substrate for epoxide hydrolase, which catalyzes the formation of benzene dihydrodiol, itself a substrate for dihydrodiol dehydrogenase, producing catechol. Finally, benzene-oxide spontaneously rearranges to phenol, which subsequently undergoes either conjugation (glucuronic acid or sulfate) or oxidation. The latter reaction, catalyzed by cytochromes P450, gives rise to hydroquinone (HQ) and 1,2,4-benzene triol. Co-administration of phenol and HQ reproduces the myelotoxic effects of benzene in animal models. The two diphenolic metabolites of benzene, catechol and HQ undergo further oxidation to the corresponding ortho-(1,2-), or para-(1,4-)benzoquinones (BQ), respectively. Trapping of 1,4-BQ with GSH gives rise to a variety of HQ-GSH conjugates, several of which are hematotoxic when administered to rats. Thus, benzene-oxide gives rise to a cascade of metabolites that exhibit biological reactivity, and that provide a plausible metabolic basis for benzene-mediated myelotoxicity. Benzene-oxide itself is remarkably stable, and certainly capable of translocating from its primary site of formation in the liver to the bone marrow. However, therein lies the challenge, for although there exists a plethora of information on the metabolism of benzene, and the fate of benzene-oxide, there is a paucity of data on the presence, concentration, and persistence of benzene metabolites in bone marrow. The major metabolites in bone marrow of mice exposed to 50 ppm [(3)H]benzene are muconic acid, and glucuronide and/or sulfate conjugates of phenol, HQ, and catechol. Studies with [(14)C/(13)C]benzene revealed the presence in bone marrow of protein adducts of benzene-oxide, 1,4-BQ, and 1,4-BQ, the relative abundance of which was both dose and species dependent. In particular, histones are bone marrow targets of [(14)C]benzene, although the identity of the reactive metabolite(s) giving rise to these adducts remain unknown. Finally, hematotoxic HQ-GSH conjugates are present in the bone marrow of rats receiving the HQ/phenol combination. In summary, although the fate of benzene-oxide is known in remarkable detail, coupling this information to the site, and mechanism of action, remains to be established.

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.

Formaldehyde as a potential human leukemogen: an assessment of biological plausibility

The International Agency for Research on Cancer (IARC, 2004) recently reevaluated the epidemiological data on formaldehyde and concluded that there was "strong but not sufficient evidence for a causal association between leukaemia and occupational exposure to formaldehyde." This conclusion was tempered since a mechanism for leukemia induction could not be identified. Chemically induced leukemia is a well-studied phenomenon with benzene and a number of cancer chemotherapeutic drugs recognized as capable of causing this effect. Abundant in vitro and in vivo data in animals and humans demonstrate that exposure to sufficient doses of these recognized leukemogens can initiate a cascade of events leading to hematopoietic toxicity and the subsequent development of leukemia. This review addresses the biological plausibility that formaldehyde might be capable of causing any type of leukemia by providing a broad overview of the scientific data that must be considered in order to support or refute a conclusion that a particular substance might be leukemogenic. Data on benzene and selected chemotherapeutic cancer drugs are used as examples and are briefly summarized to demonstrate the similar biological events thought to result in leukemogenesis. These data are compared and contrasted with the available data on formaldehyde in order to judge whether they fulfill the criteria of biological plausibility that formaldehyde would be capable of inducing leukemia as suggested by the epidemiological data. Based on the epidemiological data, it is reasonable to expect that if formaldehyde was capable of inducing leukemia, in vivo and in vitro data would offer supporting evidence for biological plausibility. In particular, there is (1) no evidence to suggest that formaldehyde reaches any target organ beyond the site of administration including the bone marrow, (2) no indication that formaldehyde is toxic to the bone marrow/hematopoietic system in in vivo or in vitro studies, and (3) no credible evidence that formaldehyde induces leukemia in experimental animals. As discussed in this review, based on the key biological events that occur in the process of chemically induced leukemia, there is inadequate biological evidence currently available to corroborate existing weak epidemiological associations. This provides an insufficient database to conclude that there is a causal relationship for formaldehyde and leukemia risk.

Animal models for acquired bone marrow failure syndromes

Bone marrow failure is a disease characterized by a drastic decline in the marrow's functional ability to produce mature blood cells. Aplastic anemia, a disease in which patients have essentially empty bone marrow accompanied by severe anemia, neutropenia, and thrombocytopenia, presents a paradigm for bone marrow failure. Damage to the marrow may first result from exposure to toxic chemicals, drug overdose, radiation, and viral infection; however, it is the extended immune-mediated reaction that causes massive destruction of hematopoietic cells and leads to marrow hypoplasia and peripheral pancytopenia. In recent years, animal models of acquired bone marrow failure syndromes have helped to strengthen our understanding of the mechanisms causing bone marrow failure. In this review, animal models for bone marrow failure are summarized by two groups: 1) bone marrow failure induced by toxic chemicals and drugs such as benzene, busulfan, and chloramphenicol, and radiation, and 2) models developed by an immune-related mechanism such as viral infection or foreign lymphocyte infusion.

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.

Drugs toxic to the bone marrow that target the stromal cells

Drugs that cause toxicity to the bone marrow are a heterogeneous group of compounds that act by various mechanisms. The etiology of this pathology is poorly understood but the highly proliferative nature of the hematopoietic cells is assumed to make the bone marrow more sensitive to toxicity. Recent evidence suggests that drugs can also affect specific aspects of stromal cells and the extracellular matrix that they establish. The data support the view that characteristics other than a high proliferation rate could confer susceptibility of the bone marrow to the toxic effects of drugs. This article discusses those drugs that have been shown to have direct effects on the bone marrow stromal cells.

The pharmacology and toxicology of polyphenolic-glutathione conjugates

Polyphenolic-glutathione (GSH) conjugates and their metabolites retain the electrophilic and redox properties of the parent polyphenol. Indeed, the reactivity of the thioether metabolites frequently exceeds that of the parent polyphenol. Although the active transport of polyphenolic-GSH conjugates out of the cell in which they are formed will limit their potential toxicity to those cells, once within the circulation they can be transported to tissues that are capable of accumulating these metabolites. There are interesting physiological similarities between the organs that are known to be susceptible to polyphenolic-GSH conjugate-mediated toxicity. In addition, the frequent localization of gamma-glutamyl transpeptidase to cells separating the circulation from a second fluid-filled compartment coincides with tissues that are susceptible either to polyphenolic-GSH conjugate-induced toxicity or to quinone and reactive oxygen species-induced toxicity. Polyphenolic-GSH conjugates therefore contribute to the nephrotoxicity, nephrocarcinogenicity, and neurotoxicity of a variety of polyphenols.

Benzene--a review of the literature from a health effects perspective

A literature review of the impact on human health of exposure to benzene was conducted. Special emphasis in this report is given to the health effects reported in excess of national norms by participants in the Benzene Subregistry of the National Exposure Registry--people having documented exposure to benzene through the use of benzene-contaminated water for domestic purposes. The health effects reported in excess (p < or = .01) by some or all of the sex and age groups studied were diabetes, kidney disease, respiratory allergies, skin rashes, and urinary tract disorders; anemia was also increased for females, but not significantly so.

Relationships between smoking habits, smoking-associated hematological changes, and urinary benzene metabolites

It has been suggested that benzene metabolites might be good indicators of smoking. Moreover, benzene could stimulate the neutrophil lineage while depressing the lymphocytic and erythroid lineages, possibly by an interference with cytokines. The effect on the neutrophil lineage could explain the smokers' leukocytosis, the mechanism of which is presently unknown. Therefore, the usefulness of benzene metabolites as indicators of smoking was compared to that of cotinine and thiocyanate, and the relationships between benzene metabolites, the hematological parameters of smokers, and interleukin 1 alpha production were examined. The results show that benzene metabolites are not better indicators of smoking status than cotinine or thiocyanate. Furthermore, it seems unlikely that the smokers' leukocytosis is benzene induced.

Inhibition of the conversion of pre-interleukins-1 alpha and 1 beta to mature cytokines by p-benzoquinone, a metabolite of benzene

Chronic exposure of humans to benzene causes severe bone marrow cell depression leading to aplastic anemia. Marrow stromal macrophage dysfunction and deficient interleukin-1 production has been reported for patients with severe aplastic anemia. The stromal macrophage, a target of benzene toxicity, is involved in hematopoietic regulation through the synthesis of several cytokines including interleukin-1, which is required for production by stromal fibroblasts of a number of cytokines required for the survival of hematopoietic progenitor cells. We have previously demonstrated that hydroquinone, a major toxic metabolite of benzene in marrow, prevents the proteolytic conversion of 31 kDa pre-interleukin-1 alpha to the 17 kDa cytokine by calpain in purified murine stromal macrophages. Furthermore, stromal macrophages from benzene-treated mice produce the 31 kDa pre-interleukin-1 alpha when stimulated in culture with endotoxin, but cannot convert the precursor to interleukin-1 alpha. In this report, we show that 1,4-benzoquinone, the oxidation product of hydroquinone in the cell, causes a concentration-dependent inhibition of highly purified human platelet calpain with an IC50 of 3 microM. Hydroquinone also inhibits the processing of pre-interleukin-1 beta by interleukin-1 beta convertase. The addition of 2 microM hydroquinone to B1 cells that undergo autocrine stimulation by interleukin-1 beta resulted in the cessation of autocrine cell growth and interleukin-1 beta secretion into the culture medium, as determined by Western immunoblots of the culture supernatants. Purified converting enzyme treated with 3 microM benzoquinone was incapable of converting 31 kDa recombinant pre-interleukin-1 beta to the 17 kDa mature cytokine as analyzed by polyacrylamide gel electrophoresis and Western immunoblotting. These findings support our observations in a mouse model that benzene-induced bone marrow cell depression results from a lack of interleukin-1 alpha subsequent to an inhibition by benzoquinone of calpain, the protease required for converting pre-interleukin-1 alpha to active cytokine. The results may provide a basis for studying benzene-induced aplastic anemia in a mouse model.

Benzene and its metabolite, hydroquinone, induce granulocytic differentiation in myeloblasts by interacting with cellular signaling pathways activated by granulocyte colony-stimulating factor

Chronic exposure of humans to benzene (BZ) causes acute myelogenous leukemia. These studies determined whether BZ, or its reactive metabolite, hydroquinone (HQ), affect differentiation of myeloblasts. BZ or HQ administered to C57BL/6J mice specifically induced terminal granulocytic differentiation of myeloblasts. The ability of the compounds to induce differentiation of the myeloblast was tested directly using the murine interleukin 3 (IL-3)-dependent myeloblastic cell line, 32D.3 (G) and the human HL-60 promyelocytic leukemic cell line. Treatment of HL-60 myeloblasts with BZ activated protein kinase C and upregulated the 5-lipoxygenase (LPO) pathway for the production of leukotriene D4 (LTD4), an essential effector of granulocytic differentiation. Differentiation was prevented by sphinganine, a kinase C inhibitor, as well as by LPO inhibitors and LTD4 receptor antagonists. BZ and HQ also induced differentiation in 32D.3 (G) myeloblasts. Both compounds interact with cellular signaling pathways activated by granulocyte colony-stimulating factor (G-CSF) and thus replace the requirement for G-CSF. IL-3 induces a growth response, whereas G-CSF provides both growth and differentiation signals. BZ does not induce growth in the absence of IL-3, but provides a differentiation signal. Both HQ and LTD4 induce differentiation and synergize with IL-3 for growth, however, neither support growth in the absence of IL-3. BZ-induced 32D cells showed a gradual progression of progenitor differentiation to granulocytes similar to that seen with G-CSF or LTD4. HQ blocks differentiation at the myelocyte stage; only a small percentage of progenitors proceed to granulocytes. BZ, like G-CSF, upregulates LTD4 production, whereas HQ obviates the requirement for LTD4 by activating the LTD4 receptor.