The effects of benzene and other leukaemogenic agents on haematopoietic stem and progenitor cell differentiation

A characteristic shared by a diverse group of myelotoxic compounds and leukaemogens is the ability to act synergistically with granulocyte-macrophage colony stimulating factor (GM-CSF) in increasing clonogenic response. Pretreatment of murine or human bone marrow cells with the benzene metabolite, hydroquinone, but not phenol, catechol or trans, trans-muconaldehyde, results in a selective enhancement of GM-CSF but not an interleukin-3 (IL-3)-mediated clonogenic response. Clonal enhancement is preserved and magnified in enriched populations of CD34+ cells (> 95% purity), suggesting an intrinsic effect on haematopoietic progenitor cell (HPC) recruitment rather than a secondary effect involving accessory cytokines. Clonogenic enhancement of murine HPCs is not accompanied by alterations in GM-CSF receptor expression or ligand affinity and appears to be mediated via a p53-independent mechanism. These observations suggest that hydroquinone treatment alters recruitment and differentiation in a primitive subpopulation of CD34+ cells and are consistent with a role for altered stem cell differentiation in the development of chemically induced myelodysplasias.

Comparative toxicity of dithiocarbamates and butadiene metabolites in human lymphoid and bone marrow cells

Apparent differences in the pattern of leukemia risk have been observed between workers employed in 1,3-butadiene (BD) monomer production and those working in styrene-butadiene rubber production (SBR). There are a number of possible explanations for these discrepancies, including differences in disease classification and diagnosis as well as possible quantitative and qualitative differences in occupational exposure between these two industries. This led us to evaluate the possibility that the pattern of disease observed in SBR might be influenced by the presence of an important class of biologically reactive chemicals, dithiocarbamates (DTC), that were present in SBR but not BD monomer production. Therefore, we compared the immunotoxic and hematotoxic activities of DTC and BD metabolites in human immune and hematopoietic cells. Relative to the mouse, human CD34+ bone marrow cells are relatively resistant to the direct effects of BD metabolites, with only the bis-oxide producing any evidence of suppression of clonogenic response at concentrations between 1 and 10 microM. Similarly, treatment of human CD4+ lymphocytes with known (2,3-epoxybutene) and putative BD metabolites (D,L-butane-bis-oxide, (2S,3R)-3-epoxybutane-1,2-diol) does not result in appreciable T-cell toxicity at concentrations likely to be encountered in vivo. In contrast, treatment of human cells with DTC at concentrations as low as 100 nM results in significant suppression of hematopoietic clonogenic response and T-lymphocyte function. Additional studies in our laboratory and others suggest a role for copper in DTC toxicity in both human lymphocytes and bone marrow cells, although the pattern of altered transcriptional regulation observed is markedly different in these two cell populations. These results are consistent with the pattern of DTC toxicity previously observed in clinical and molecular studies.

Hydroquinone inhibits PMA-induced activation of NFkappaB in primary human CD19+ B lymphocytes

Hydroquinone (HQ), a reactive metabolite of benzene, is known to inhibit mitogen-stimulated activation of both T and B lymphocytes. Despite extensive study, the underlying mechanism for the immunotoxicity of the HQ is not clear. We have previously demonstrated that 1 micromol/L HQ inhibits TNF-induced activation of NFkappaB in CD4+ T cells, resulting in decreased IL-2 production. NFkappaB, known to be important in T lymphocytes, also plays a critical role in normal B cell development and activation. We therefore hypothesized that alterations in NFkappaB might be involved in HQ-induced B cell immunosuppression as well. In this study, we demonstrate that 1-10 micromol/L HQ inhibits PMA/ionomycin-induced activation of NFkappaB in primary human CD19+ B cells. Inhibition of NFkappaB is accompanied by a dose-dependent decrease in PMA-stimulated production of TNF with no corresponding loss in viability or increased apoptosis. HQ also does not appear to alter NFkappaB directly, as preincubation of B cell nuclear extracts with HQ does not diminish DNA binding activity of this protein. In contrast to T cells, inhibition of NFkappaB by HQ in B cells is not reversible after 72 h in culture, suggesting a long-term functional suppression. These data support our original findings in T cells and indicate that NFkappaB is particularly susceptible to inhibition by HQ. We further hypothesize that inhibition of NFkappaB in lymphocytes, and perhaps other cell types as well, may play a significant role in the observed toxicity of HQ.

Comparative toxicity of known and putative metabolites of 1, 3-butadiene in human CD34(+) bone marrow cells

Species-specific susceptibility to the hematotoxic effects of 1, 3-butadiene (BD) is well known. Previous studies have revealed that murine bone marrow is uniquely susceptible to toxicity following exposure to the parent compound in vivo or exposure of bone marrow cells to the monoepoxide metabolite, 3,4-epoxybutane, in vitro. Studies described herein compare the relative ability of putative and known BD metabolites to produce concentration dependent suppression of colony formation and cytotoxicity in human CD34(+) bone marrow cells. Compounds evaluated included 3,4-epoxybutane, D, L-butane-bis-oxide, meso-butane-bis-oxide and (2S, 3R)-3-epoxybutane-1,2-diol. In contrast to results previously observed in mice, only the bis-oxides produced significant suppression of colony formation at potentially relevant concentrations (10(-8) to 10(-3) M). No enantiospecific differences were observed between the meso- and D,L-bis-oxides and no significant lineage-specific differences in susceptibility to inhibition of clonogenic response were observed among early multi-potential myeloid and erythroid hematopoietic progenitor cells. The relative potencies of the bis-oxides were found to be comparable to that of the prototype hematotoxic compound, hydroquinone. These results confirm previous studies that reveal marked species-specific differences in the susceptibility of bone marrow cells to 3,4-epoxybutane. Moreover, these results suggest that the bis-oxides of BD are capable of suppressing the clonogenic function of human hematopoietic progenitor cells, if, in fact, they are produced in human bone marrow in significant concentration. Further interpretation of these findings requires a better understanding of the metabolism of BD in humans.

Dithiocarbamates inhibit hematopoiesis via a copper-dependent mechanism

Dithiocarbamates (DTC), an important class of therapeutic and industrial chemicals, have alternatively been reported to be either beneficial or toxic to the hematopoietic and immune systems. In the present study, we investigated the potential of dimethyl- and diethyl-dithiocarbamate to alter clonogenic response of primary human CD34(+) bone marrow cells in vitro. Our results demonstrate that both compounds are potent inhibitors of clonogenic response in human CD34(+) bone marrow cells, suppressing cytokine-induced colony formation at concentrations between 100 and 500 nM. Pretreatment of bone marrow cells for 1 h with very high doses of DTC (30 microM) had no effect on colony formation. DTCs are known inhibitors of nuclear factor-kappa B (NF-kappa B); however, data presented herein demonstrate that DTC do not inhibit cytokine activation of NF-kappa B in CD34(+) bone marrow cells. Additional experiments demonstrate that DTCs induce a dose-related increase in apoptosis, potentially acting via a cytotoxic mechanism. We further demonstrate that the addition of copper sulfate greatly potentiates the hematotoxicity of DTC and that the addition of a copper-specific chelator completely abrogates DTC clonogenic suppression. These data support a role for copper in DTC-induced hematotoxicity.

The benzene metabolite, hydroquinone, selectively induces 5q31- and -7 in human CD34+CD19- bone marrow cells

OBJECTIVE

Chronic exposure to high concentrations of benzene is associated with an increased incidence of myelodysplastic syndrome and acute myelogenous leukemia. Acute myelogenous leukemia developing in patients treated with alkylating agents for other cancers or occupationally exposed to benzene exhibit a pattern of cytogenetic aberrations predominantly involving loss of all or part of chromosomes 5 and/or 7. In contrast, trisomy 8 is observed equally in both de novo and secondary acute myelogenous leukemia. Studies using peripheral lymphocytes or lymphoblastoid cell lines have observed dose-dependent loss of chromosomes 5, 7, and 8 following treatment with the benzene metabolite, hydroquinone. The purpose of this study was to determine the dose response and specificity of hydroquinone-induced aberrations on chromosomes 5, 7, and 8 using human CD34+CD19 bone marrow cells.

MATERIALS AND METHODS

Fluorescence in situ hybridization analysis was performed on CD34+CD19- bone marrow cells using the locus-specific probes, 5q31, 5p15.2, and centromeric probes specific for human chromosomes 7 and 8 following hydroquinone exposure.

RESULTS

Hydroquinone exposure results in -7, selective deletion of 5q31 but not chromosome 5 and no loss or gain of chromosome 8 in human CD34+CD19- cells.

CONCLUSION

CD34+ bone marrow cells are more susceptible and show a different pattern of cytogenetic aberrations as a result of hydroquinone exposure compared to lymphocytes. CD34+ bone marrow cells exhibit unique susceptibility to the development of specific chromosome aberrations that have been identified as the earliest structural changes occurring in the development of secondary myelodysplastic syndrome and acute myelogenous leukemia.

The benzene metabolites hydroquinone and catechol act in synergy to induce dose-dependent hypoploidy and -5q31 in a human cell line

Chronic exposure to high concentrations of benzene is associated with an increased incidence of myelodysplastic syndrome (MDS) and acute myelogenous leukemia (AML). Studies of patients occupationally exposed to benzene show a pattern of cytogenetic aberrations involving loss of all or part of chromosomes 5 and/or 7 as well as trisomy 8 and we have previously reported that hydroquinone (HQ) induces deletions of 5, 7 and 8. Benzene metabolism is a requirement for bone marrow toxicity and the phenolic metabolites, HQ and catechol (CAT), have been implicated in benzene hematotoxicity. A research project was designed to determine whether CAT by itself and in conjunction with HQ could directly induce loss of chromosome 5 and/or 7 and gain of chromosome 8. Using fluorescence in situ hybridization with chromosome-specific 5, 7, and 8 probes we demonstrate that 5 to 150 uM CAT does not produce chromosomal aberrations, however CAT and 25 uM HQ can act in synergy to induce dose dependent loss of these chromosomes. In addition HQ/CAT selectively induces -5q which is not observed for HQ only. These results demonstrate for the first time that CAT/HQ act in synergy to induce specific chromosome loss found in secondary MDS/AML.

An essential role for NF-kappaB in human CD34(+) bone marrow cell survival

The transcription factor, NF-kappaB, is important for T-cell activation, B-cell maturation, and human immunodeficiency virus transcription and plays a role in alternatively mediating and protecting against apoptosis in a variety of cell types. However, a role for NF-kappaB in human CD34(+) bone marrow cells has not been described. We provide evidence here that virtually all human CD34(+) bone marrow cells express NF-kappaB that can be activated by exposure to phorbol 12-myristate 13-acetate and a variety of cytokines, eg, tumor necrosis factor alpha, interleukin-3, and granulocyte-macrophage colony-stimulating factor. In addition, we demonstrate that NF-kappaB may be required for human CD34(+) bone marrow cell clonogenic function and survival. These results offer insight into a new role for NF-kappaB in maintaining survival and function in hematopoietic stem and progenitor cells and suggest that proposed strategies involving inhibition of NF-kappaB activation as an adjunct to cancer chemotherapy should be approached with caution.

Dimethyldithiocarbamate inhibits in vitro activation of primary human CD4+ T lymphocytes

Dithiocarbamates (DTC), a diverse group of industrial and therapeutic chemicals, have been reported to inhibit, enhance or have no effect on the immune system. These apparent inconsistencies reflect the complexity of the DTCs biological activities and are probably due in part to differences in dose, route of exposure, animal species used and/or specific compound tested. The studies described herein were undertaken to investigate the immunotoxicity of one member of this family, dimethyldithiocarbamate (DMDTC). We demonstrate that 0.1-0.5 microM DMDTC inhibits TNF-alpha-induced activation of NF-kappaB in primary human CD4+ T cells. This inhibition is not accompanied by a loss in viability, and DMDTC-treated T cells retain other active signaling pathways throughout the exposure duration. The inhibition of NF-kappaB is apparently permanent as DMDTC-treated T cells did not regain normal TNF-alpha activation, even after 72 h in culture. DMDTC does not appear to alter NF-kappaB directly as pre-incubation of nuclear extracts with DMDTC does not diminish binding activity of this protein. We further demonstrate that 0.1-0.5 microM DMDTC inhibits intracellular IL-2 production and decreases surface expression of CD25 (the alpha subunit of the IL-2 receptor) in T cells stimulated with phorbol ester. These data demonstrate that DMDTC is a potent immunosuppressive compound in vitro.

Hydroquinone, a reactive metabolite of benzene, inhibits NF-kappa B in primary human CD4+ T lymphocytes

Hydroquinone (HQ), a reactive metabolite of benzene, is present in cigarette smoke and is known to inhibit mitogen-stimulated activation of both T and B lymphocytes. Despite extensive study, the underlying mechanism for HQ's immunotoxicity is not clear. NF-kappa B is a transcription factor known to regulate the expression of a number of genes critical for normal T cell activation. We therefore hypothesized that NF-kappa B might be involved in HQ-induced immunosuppression. In this study, we demonstrate that 1 microM HQ inhibits tumor necrosis factor alpha induced activation of NF-kappa B in primary human CD4+ T cells. This inhibition is not accompanied by a loss in viability, and HQ-treated T cells maintain other active signaling pathways throughout the exposure duration. Additionally, the inhibition of NF-kappa B is reversible as HQ-treated T cells regain normal functioning after 72 h in culture. HQ does not appear to alter NF-kappa B directly as preincubation of nuclear extracts with HQ does not diminish activity of this protein. We further demonstrate that 1 microM HQ inhibits intracellular IL-2 production in T cells stimulated with phorbol ester but does not alter surface expression of CD25 (the alpha-subunit of the IL-2 receptor). These data suggest that NF-kappa B may be an important molecular mediator of HQ's (and benzene's) immunotoxicity.

Characterization and phenotypic analysis of differentiating CD34+ human bone marrow cells in liquid culture

Our current understanding of human haematopoietic stem cell biology is based in part on the characterization of human CD34+ bone marrow cell differentiation in vitro. CD34 is highly expressed on early stem cells and haematopoietic progenitor cells with clonogenic potential and is gradually lost during differentiation and commitment. However, CD71 (transferrin receptor) is expressed at low levels on early stem cells and generally increases during haematopoietic progenitor cell proliferation. We reasoned that the combination of these surface markers would provide a useful framework for the simultaneous analysis of multiple lineage differentiation of CD34+ haematopoietic progenitor cells in liquid culture. In this report, we identify the phenotype of distinct subpopulations of myeloid, erythroid and lymphoid cells in liquid suspension culture using differential expression of CD34 vs. CD71 in combination with specific lineage markers. Freshly isolated human CD34+ bone marrow cells were introduced into suspension culture and monitored over a 6-d period using 3-colour flow cytometry. This is the first demonstration that differential expression of CD34 vs. CD71 can be used to simultaneously monitor differentiation of multiple haematopoietic cell lineages in liquid suspension culture, facilitating the study of cytokine-, drug- or chemical-induced alterations in haematopoietic progenitor cell differentiation in vitro.

The benzene metabolite, hydroquinone, induces dose-dependent hypoploidy in a human cell line

Chronic exposure to high concentrations of benzene can result in the development of myelodysplastic syndrome (MDS) and acute myelogenous leukemia (AML). Studies of patients occupationally exposed to benzene show a pattern of cytogenetic aberrations involving high frequency of loss of all or part of chromosomes 5 and/or 7 as well as trisomy 8. The pattern of reoccurring chromosome abnormalities associated with the development of leukemia can be used as a guide in understanding the etiology and pathogenesis of these diseases. Therefore, a research project was designed to determine whether a metabolite of benzene, hydroquinone (HQ), could directly induce loss of chromosome 5 and/or 7 and gain of chromosome 8. Using fluorescence in situ hybridization with chromosome-specific 5, 7 and 8 probes we demonstrate that 42, 49 and 26 microM HQ induces monosomy 5, 7 and 8, respectively, in the human lymphoblast cell line GM09948. These results demonstrate for the first time that HQ induces a specific chromosome loss found in secondary MDS/AML. The pattern of chromosome 5 and/or 7 loss in benzene-induced MDS/AML is probably due to selective cell survival after HQ exposure rather than specific targeting of HQ for chromosomes 5 or 7.

Impact of benzene metabolites on differentiation of bone marrow progenitor cells

Interleukin-3 (IL-3) and granulocyte/macrophage-colony-stimulating factor (GM-CSF) are responsible for maintaining survival and stimulating growth of early dormant hematopoietic progenitor cells (HPC). These cytokines exhibit extensive overlap, with GM-CSF supporting growth and differentiation of myeloid HPC. A characteristic shared by a diverse group of leukemogens is the ability to act synergistically with GM-CSF to increase clonogenic response. Previous studies have revealed that pretreatment of murine HPC with hydroquinone (HQ) but not phenol, catechol, or trans-trans-muconaldehyde results in a selective enhancement of GM-CSF but not IL-3-mediated clonogenic response. Pretreatment of murine bone marrow cells with these agents or their metabolites in vitro results in increased numbers of HPC dividing and forming colonies in response to GM-CSF but not IL-3. The present studies explored the molecular mechanisms associated with altered cytokine response in early HPC in murine bone marrow and extended our initial observations in murine bone marrow to human bone marrow cells. HQ pretreatment of murine HPC did not induce either an up- or a down-regulation of GM-CSF receptors or any change in receptor affinity. CD34+ cells, which represent between 1 and 5% of human bone marrow, contain virtually all clonogenic stem and HPC. Pretreatment of CD34+ cells (approximately 95% purity) with HQ also results in enhanced clonogenic response with GM-CSF but not IL-3. These findings suggest that an early step in chemical leukemogenesis may involve transient alterations in the regulation of cytokine response to GM-CSF.

An epidemiologic study of early biologic effects of benzene in Chinese workers

Benzene is a recognized hematotoxin and leukemogen, but its mechanisms of action in humans are still uncertain. To provide insight into these processes, we carried out a cross-sectional study of 44 healthy workers currently exposed to benzene (median 8-hr time-weighted average; 31 ppm), and unexposed controls in Shanghai, China. Here we provide an overview of the study results on peripheral blood cells levels and somatic cell mutation frequency measured by the glycophorin A (GPA) gene loss assay and report on peripheral cytokine levels. All peripheral blood cells levels (i.e., total white blood cells, absolute lymphocyte count, platelets, red blood cells, and hemoglobin) were decreased among exposed workers compared to controls, with the exception of the red blood cell mean corpuscular volume, which was higher among exposed subjects. In contrast, peripheral cytokine levels (interleukin-3, interleukin-6, erythropoietin, granulocyte colony-stimulating factor, tissue necrosis factor-alpha) in a subset of the most highly exposed workers (n = 11) were similar to values in controls (n = 11), suggesting that benzene does not affect these growth factor levels in peripheral blood. The GPA assay measures stem cell or precursor erythroid cell mutations expressed in peripheral red blood cells of MN heterozygous subjects, identifying NN variants, which result from loss of the GPA M allele and duplication of the N allele, and N phi variants, which arise from gene inactivation. The NN (but not N phi) GPA variant cell frequency was elevated in the exposed workers compared with controls (mean +/- SD, 13.9 +/- 8.4 mutants per million cells versus 7.4 +/- 5.2 per million cells, (respectively; p = 0.0002), suggesting that benzene produces gene-duplicating but not gene-inactivating mutations at the GPA locus in bone marrow cells of exposed humans. These findings, combined with ongoing analyses of benzene macromolecular adducts and chromosomal aberrations, will provide an opportunity to comprehensively evaluate a wide range of early biologic effects associated with benzene exposure in humans.

The process of leukemogenesis

Leukemias are monoclonal diseases that arise from cells in the hematopoietic stem and progenitor cell compartment. Consistent with emerging models of carcinogenesis, leukemogenesis is an evolutionary process that involves multiple independent genetic and epigenetic events. Over the last half-century a predominant paradigm has emerged to describe leukemia developing secondary to alkylating drug therapy or exposure to benzene in which progressive dysplastic changes, accompanied by a distinct pattern of clonal cytogenetic abnormalities, give rise to acute myelogenous leukemia. Characterization of these clonal chromosomal aberrations, together with observed alterations in other growth-promoting genes, provides a useful framework for studying chemical leukemogenesis and for use in understanding the origins and development of leukemia in general.

Murine thymic lymphoma is associated with a species-specific hematopoietic progenitor cell subpopulation

Many strains of laboratory mouse are uniquely susceptible to the development of T cell lymphoma/leukemia, either spontaneously or as a result of chemical or radiation exposure. In contrast, T cell leukemias or lymphomas which are relatively uncommon in human populations, are not easily induced by radiation, and are not generally associated with chemotherapy or chemical exposure. Evidence is presented to suggest that differences in the susceptibility to the development of these malignancies is related to subtle but important variations in the regulation of hematopoietic stem cell differentiation between these two species.

Inorganic lead activates NF-kappa B in primary human CD4+ T lymphocytes

Inorganic lead (Pb) is a ubiquitous environmental contaminant that produces a variety of effects on humoral and cell mediated immune responses. The underlying molecular mechanism for Pb's complex effects on the immune system remain obscure. Many of Pb's effects on the immune system could be explained through activation of the transcription factor, NF-kappa B. NF-kappa B is critical for T lymphocyte function and is a strong inducer of HIV-LTR activation. We demonstrate that Pb at physiologically relevant concentrations activates NF-kappa B in primary human CD4+ T lymphocytes. Pb-induced activation of NF-kappa B is blocked by antibodies for p65 and p50 subunits but not cRel, indicating that the p65:p50 heterodimer (NF-kappa B) is involved. Functional activation of gene expression by Pb was confirmed using primary CD4+ T cells transfected with an NF-kappa B dependent reporter gene construct. Pb did not activate NF-kappa B in 4 different T cell lines, suggesting that lymphoid cell lines may not be reliable surrogates for the study of transcriptional activation in human T cells. These data suggest that NF-kappa B may be an important molecular mediator of Pb-induced immunotoxicity.

Reactive oxygen species mediate stem cell factor synergy with granulocyte/macrophage colony-stimulating factor in a subpopulation of primitive murine hematopoietic progenitor cells

Reactive oxygen species (ROS) have been shown to stimulate proliferation and growth responses in a variety of mammalian cell types and to act as important mediators in many cellular processes, including hematolymphopoiesis. We examined the effect on primitive murine hematopoietic progenitor cells (HPC) of ROS generated by xanthine plus xanthine oxidase (xanthine/XO) and various antioxidants. Pretreatment of murine HPC (C57BL/6) with xanthine/XO produced a dose-dependent enhancement of clonogenic response to granulocyte/macrophage colony-stimulating factor (GM-CSF) but not to interleukin-3 or granulocyte colony-stimulating factor. Stem cell factor (SCF), a potent comitogen for many hematopoietic growth factors, also synergized with GM-CSF. However, the synergistic enhancement of GM-CSF with xanthine/XO and SCF was not additive, indicating that xanthine/XO and SCF may target the same subpopulation of HPC. Support for this conclusion came from experiments demonstrating that 1) mutant mice strains constitutively lacking a SCF-responsive population of HPC [White spotted (W/WV) and Steel (SI/SId)] are unresponsive to xanthine/XO- and SCF-induced enhancement of GM-CSF and 2) 3,4-epoxybutene, which selectively abrogates SCF synergy with GM-CSF, inhibits xanthine/XO-induced enhancement. As xanthine/XO can mimic SCF in this population of HPC, the possibility exists that ROS also play a role in normal SCF-mediated proliferation of these cells. To test this hypothesis, we used the antioxidants N-tert-butyl-alpha-phenylnitrone, exogenous superoxide dismutase, and catalase. Both N-tert-butyl-alpha-phenylnitrone and superoxide dismutase effectively inhibited SCF and xanthine/XO synergism with GM-CSF, whereas catalase had no effect, indicating that the superoxide anion may be involved. Also, none of these compounds affected SCF synergism with other hematopoietic growth factors, such as interleukin-3 or granulocyte colony-stimulating factor, suggesting a population-specific phenomenon. These findings indicate that xanthine/XO mimics SCF in stimulating a subpopulation of murine HPC to proliferate and that SCF synergy with GM-CSF in this population is sensitive to antioxidant inhibition.

2'3'-Dideoxycytidine-induced thymic lymphoma correlates with species-specific suppression of a subpopulation of primitive hematopoietic progenitor cells in mouse but not rat or human bone marrow

The nucleoside analogue, 2',3'-dideoxycytidine (ddC), is a potent inhibitor of HIV replication, and AIDS patients receiving ddC experience clinical improvement without significant hematologic toxicity. Repeated ddC administration (1,000 mg/kg per day) for 13 wk produces an increased incidence of thymic lymphoma in B6C3F1 mice. Previous studies reveal a common link between chemically induced and genetically associated models of mouse thymic lymphoma that involves a defect in a subpopulation of primitive hematopoietic progenitor cells. This defect is characterized by suppression of a subpopulation of IL-3-responsive cells and ablation of stem cell factor synergy with GM-CSF. The present study was undertaken to ascertain whether ddC produces the same pattern of bone marrow toxicity in mice, and whether this effect is observed in rat and human bone marrow. ddC exposure in vivo and in vitro produced a select suppression of murine CFU identical to that previously described for other models of mouse thymic lymphoma. In contrast, this selective CFU suppression was not observed in rat and human bone marrow or in CD34+ cells. These studies suggest that the mouse may not be a good predictive model for ddC hematotoxicity in humans and that susceptibility to the development of thymic lymphoma may be unique to the mouse.

Peroxidase activity in murine and human hematopoietic progenitor cells: potential relevance to benzene-induced toxicity

Peroxidases may be important in the mechanism of toxicity of a number of compounds including benzene, a chemical that has been associated with bone marrow toxicity and leukemia after chronic exposure. The major peroxidase in bone marrow is myeloperoxidase (MPO), which has been previously thought to be expressed at the promyelocytic stage of differentiation. Hematopoietic progenitor cells are important potential cellular targets of bone marrow toxins and leukemogens. We therefore examined peroxidase activity in both murine and human progenitor cells. Murine progenitor populations were purified as lineage-negative cells (> 99% enriched) and human progenitor populations were purified as CD34+ cells (> 95% enriched). Using conventional biochemical assays for peroxidase activity, murine and human progenitor cells were found to have 30% and 11% of the peroxidase activity of murine and human unpurified marrow, respectively. Peroxidase activity was confirmed in purified murine and human progenitor populations by flow cytometry using a 2,7-dichlorofluorescein assay, adapted to measure peroxidase activity. In addition, two-color flow cytometry of murine whole marrow using phycoerythrin-conjugated antibodies to lineage markers confirmed the peroxidase activity of the murine progenitor cell population. A reverse transcription-polymerase chain reaction assay was developed for MPO mRNA, which was detected in murine progenitor cells. These data show that MPO mRNA is expressed in murine progenitor cells and that both murine and human progenitor cells have marked peroxidase activity. These data may have relevance for studies of hematopoietic cell differentiation and for the examination of mechanisms underlying cell-specific toxicity in bone marrow.

Cell proliferation and differentiation in chemical leukemogenesis

In tissues such as bone marrow with normally high rates of cell division, proliferation is tightly coordinated with cell differentiation. Survival, proliferation and differentiation of early hematopoietic progenitor cells depend on the growth factors, interleukin 3 (IL-3) and/or granulocyte-macrophage colony stimulating factor (GM-CSF) and their synergism with other cytokines. We provide evidence that a characteristic shared by a diverse group of compounds with demonstrated leukemogenic potential is the ability to act synergistically with GM-CSF. This results in an increase in recruitment of a resting population of hematopoietic progenitor cells normally unresponsive to the cytokine and a twofold increase in the size of the proliferating cell population normally regarded to be at risk of transformation in leukemogenesis. These findings support the possibility that transient alterations in hematopoietic progenitor cell differentiation may be an important factor in the early stages of development of leukemia secondary to chemical or drug exposure.

Chemical suppression of a subpopulation of primitive hematopoietic progenitor cells: 1,3-butadiene produces a hematopoietic defect similar to steel or white spotted mutations in mice

Chronic exposure of mice to 1,3-butadiene produces a macrocytic-megaloblastic anemia, thymic hypoplasia, and an increased incidence of T-cell lymphoma/leukemia. This is reminiscent of pathologies observed in mice bearing mutations at the W and Sl loci, which are deficient in c-kit and c-kit ligand (CKL), respectively. The influence of 3,4-epoxybutene (EB), the primary metabolite of 1,3-butadiene, on the colony-forming response of hematopoietic progenitor cells (HPCs) from C57BL/6, Sl, and W mice was investigated in order to elucidate the role of altered HPC regulation in the pathogenesis of 1,3-butadiene toxicity. EB pretreatment suppressed interleukin 3 colony formation and abrogated CKL synergism of the granulocyte-macrophage/colony-stimulating factor (GM-CSF) response in C57BL/6 cells, had no effect on colony formation induced by GM-CSF or granulocyte/colony-stimulating factor (G-CSF) alone, and failed to suppress CKL-induced synergism of the G-CSF response. Experiments conducted with cells from Sl and W mice revealed that they lack the same primitive HPC targeted by EB. EB pretreatment in vitro and butadiene exposure in vivo mimic hematopoietic defects seen in W and Sl mice, suggesting that the pleotypic pathologies encountered in these murine models may be largely due to a common defect in primitive HPCs. Susceptibility to EB appears to define a functional subpopulation of primitive HPCs and illustrates that differences observed in the susceptibility of specific cytokine responses to chemical/drug exposure may provide a valuable tool for characterizing functional subpopulations of HPCs.

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.

Susceptibility to 1,3-butadiene-induced leukemogenesis correlates with endogenous ecotropic retroviral background in the mouse

Previous studies have revealed marked differences in the pattern of carcinogenesis between rats and mice exposed to 1,3-butadiene (BD) that do not appear to be readily explained on the basis of pharmacokinetics or metabolism. Chronic exposure of B6C3F1 mice to BD produces a high incidence of thymic lymphoma (TL) that is not observed in rats. The potential of the endogenous ecotropic retroviral background to influence susceptibility to BD leukemogenesis was examined by comparing the incidence of TL between B6C3F1 and NIH swiss mice. Proviral ecotropic sequences are truncated in the NIH Swiss mouse, and the virus is not expressed. Chronic exposure to BD (1250 ppm) for up to 1 year resulted in a fourfold difference in the incidence of TL between B6C3F1 (57%) and NIH Swiss (14%) mice. These results provide presumptive evidence for retrovirus involvement since NIH Swiss mice lack ecotropic viruses and appear to be relatively resistant to induction of lymphoma by BD.

Selective activation of endogenous ecotropic retrovirus in hematopoietic tissues of B6C3F1 mice during the preleukemic phase of 1,3-butadiene exposure

1,3-Butadiene (BD), a comonomer used in the production of synthetic rubber, is a rodent carcinogen. We have observed a marked increase in the incidence of thymic lymphoma in male B6C3F1 relative to NIH Swiss mice chronically exposed to BD in the absence of demonstrable differences in bone marrow (target organ) toxicity. Increased expression of murine leukemia virus (MuLV) antigens was also observed on lymphomas from BD-exposed B6C3F1 mice. Because NIH Swiss mice do not usually express endogenous retroviruses and their ecotropic proviral sequences are not intact, these findings provide presumptive evidence of a role for endogenous retrovirus sequences in BD-induced lymphoma in the B6C3F1 mouse. The present study was conducted to examine the expression and behavior of endogenous retroviruses in these strains during the preleukemic phase of BD exposure. Chronic exposure to BD (1250 ppm) 6 hr/day, 5 days/wk for 3 to 21 weeks increased markedly the quantity of ecotropic retrovirus recoverable from bone marrow, thymus, and spleen of B6C3F1 mice. However, expression of other endogenous retroviruses (xenotropic, MCF-ERV) was not enhanced. No viruses of any type were found in similarly treated NIH Swiss mice. The mechanism of this increase in ecotropic retrovirus in B6C3F1 mice is believed to be de novo activation in greater numbers of cells because changes in the Fv-1 tropism of the replicating viruses or changes in Fv-1 host restriction were not found. Endogenous retroviruses are thus implicated in BD-induced leukemogenesis in B6C3F1 mice. Further studies will examine the role of retrovirus in BD-induced leukemogenesis and the mechanisms of activation of ecotropic proviral sequences in murine cells.

Effect of short-term inhalation exposure to 1,3-butadiene on murine immune functions

Interest in 1,3-butadiene (BD) as a potential immunomodulator was prompted by reports of an increased incidence of neoplasia in humans exposed to BD during the manufacture of styrene-butadiene synthetic rubber, and by a recent study which demonstrated a high incidence of thymic lymphomas in B6C3F1 mice. B6C3F1 mice were exposed to 1250 ppm BD by inhalation 6 hr per day, 5 days per week, for 6 or 12 weeks. Immune function assays were selected to evaluate specific humoral and cell-mediated immunity and spontaneous cytotoxicity; lymphoid organ histopathology was also evaluated. A slight decrease in antibody plaque-forming cells (PFC) per spleen was observed in exposed mice, although PFC per 10(6) splenic lymphocytes was normal. Significant extramedullary hematopoiesis and erythroid hyperplasia was observed in spleens from exposed mice, and correlated with a twofold increase in thymidine incorporation in spontaneously proliferating splenocytes. No differences in proliferation to alloantigens were demonstrable between control and BD-exposed splenocytes. Mitogenesis by phytohemagglutinin, Concanavalin A, and lipo polysaccharide was suppressed in splenocytes from exposed mice, but may have been due to the cellular dilution effect of hematopoietic activity. Cytotoxic T-lymphocyte generation was suppressed after a 6-week exposure to BD, but was comparable to controls after 12 weeks of exposure. No differences in spontaneous cytotoxicity were observed between control and exposed mice. Overall, no persistent immunological defects were detectable after inhalation exposure to this tumorigenic agent.

Macrocytic-megaloblastic anemia in male NIH Swiss mice following repeated exposure to 1,3-butadiene

Thymic lymphoma/leukemia is the major cause of death in B6C3F1 mice chronically exposed to 1,3-butadiene (BD). Similar to radiation-induced murine thymic lymphoma, the bone marrow is also a major target organ. Because of the association of murine thymic lymphoma with endogenous type-C murine leukemia retroviruses (MuLV) present in the germ line of most strains of laboratory mice, including B6C3F1 and its parent strains, we examined the effects of BD exposure on NIH Swiss mice which do not possess intact endogenous ecotropic MuLV. Male NIH Swiss mice exhibited a macrocytic-megaloblastic anemia following inhalation of 1250 ppm BD for 6 weeks. Treatment-related changes included decreases in circulating erythrocytes, total hemoglobin, and hematocrit and an increase in mean corpuscular volume. An eightfold increase in circulating micronuclei was also observed. The anemia was not accompanied by a significant alteration in mean corpuscular hemoglobin concentration, an increase in circulating reticulocytes, or an increase in circulating nucleated erythrocytes. These findings are consistent with a treatment-related macrocytic-megaloblastic anemia and indicate that the bone marrow is an important target for BD toxicity in mice independent of MuLV background and expression.

Macrocytic-megaloblastic anemia in male B6C3F1 mice following chronic exposure to 1,3-butadiene

In the present study exposure to 1,3-butadiene (BD) resulted in a macrocytic-megaloblastic anemia in male B6C3F1 mice following chronic inhalation of 1250 ppm for 6 to 24 weeks. Treatment-related changes evident after 6 weeks of exposure included a decrease in circulating erythrocytes, total hemoglobin, and hematocrit and an increase in mean corpuscular volume. A leukopenia, due primarily to a decrease in segmented neutrophils, and a five- to sixfold increase in circulating micronuclei were observed after 6 and 24 weeks of exposure. These changes were not accompanied by a significant alteration in mean corpuscular hemoglobin concentration, an increase in circulating reticulocytes, or circulating nucleated erythrocytes. A consistent treatment-related alteration in bone marrow cellularity was not found. However, flow cytofluorometric analysis of bone marrow DNA cell cycle kinetics revealed a 44% increase in proliferative index relative to controls, due primarily to an increase in the proportion of cells in S phase. These findings are consistent with a treatment-related macrocytic-megaloblastic anemia and indicate the bone marrow to be an important target organ for BD toxicity.

Altered hematopoietic stem cell development in male B6C3F1 mice following exposure to 1,3-butadiene

The effects of the murine lymphomagen, 1,3-butadiene (BD), on the proliferation and differentiation of hematopoietic stem cells were examined in male B6C3F1 mice. Exposure to 1250 ppm BD for 6 weeks resulted in no demonstrable alteration in the frequency of spleen colony-forming units (CFU-S); however, colonies derived from treated animals were smaller than those from controls. The absence of any difference in the frequency of CFU-GM after 6 weeks exposure suggests that BD produces an alteration in the relative proportion of immature to mature pluripotent stem cells in BD-exposed animals. This was confirmed by the examination of the effects of BD on stem cell development in long-term bone marrow culture. After 14 days, the number of CFU-GM derived from cultures of animals exposed for 6 weeks was reduced compared to controls. However, at 28 days an increase relative to controls was observed. This shift in the course of differentiation of the granulocyte/macrophage precursor cell, as assessed by the CFU-GM, provides further evidence that there is an increase in the relative frequency of primitive or immature stem cells in BD-treated mice. After a 30-31 week exposure to BD, a decrease in the numbers of both CFU-S and CFU-GM was observed. These findings indicate that BD causes alterations in stem cell development and suggest that alterations in bone marrow stem cells may play an essential role in the pathogenesis of BD-induced thymic lymphoma.

Studies of immune function and host resistance in B6C3F1 mice exposed to formaldehyde

A series of immune function and host resistance parameters were examined in female B6C3F1 mice following a 21-day (6 hr/day) inhalation exposure to 15 ppm of formaldehyde (HCHO). Immune parameters examined included delayed hypersensitivity to keyhole limpet hemocyanin, antibody plaque-forming cell response to sheep erythrocytes (T-lymphocyte-dependent antigen) and TNP-Ficoll (T-lymphocyte-independent antigen), lymphoid organ weights and histopathology, routine hematology, bone marrow cellularity and CFU progenitor cell enumeration, lymphocyte subpopulation quantitation by cell surface markers, mitogen-induced lymphocyte blastogenesis, macrophage function parameters, and host resistance to challenge with the bacterium Listeria monocytogenes and transplantable tumor cells. Lymphoid organ weight, bone marrow cellularity, and hematology parameters were unchanged in HCHO exposed mice. Similarly, the percentage of T and B lymphocytes and their proliferative responses to mitogens were not significantly altered. Antibody (IgM) plaque-forming cell response following antigen challenge was unchanged. Macrophage function was normal although some evidence of enhanced H2O2 production associated with elevated bactericidal activity was observed in resident macrophages. Resistance to challenge with the bacteria Listeria monocytogenes was significantly enhanced, while resistance to tumor challenge remained unchanged. No evidence of immunosuppression following short-term exposure to HCHO was observed.

Phytohemagglutinin-induced acquisition of T-cell surface markers by rat bone marrow precursor cells in the absence of the thymic microenvironment

Two monoclonal antibodies specific for different rat T-cell subpopulations, the anti-helper-T-cell antibody, W3/25, and the OX8 suppressor cell antibody were used to investigate lectin-stimulated T-lymphocyte differentiation of F-344 rat bone marrow cells in culture. Cytofluorometric analysis of freshly isolated lymphocytes from thymus and spleen revealed that these tissues contained both W3/25- and OX8-positive populations but differed with respect to the number of cells and receptor density distribution. By contrast, bone marrow-derived lymphocytes exhibited negligible W3/25- or OX8-associated fluorescence. However, several days after stimulation of bone marrow lymphocytes with phytohemagglutinin (PHA), cells appeared bearing these markers. Two-parameter histogram analysis of light scatter measurements with cell surface immunofluorescence indicated that this phenomenon represented the appearance of a new population of cells, presumably mature T cells, bearing an increased density of marker. These findings suggest an induction of differentiation of bone marrow T precursor cells by nonthymic factors (PHA) since lymphocytes lacking mature T-cell marker expression developed this characteristic after several days in culture.

Raji cell assay for immune complexes. Evidence for detection of Raji-directed immunoglobulin G antibody in sera from patients with systemic lupus erythematosus

We asked whether binding of human immunoglobulin (Ig)G antibody reacting with Raji cells could be distinguished from binding of IgG immune complexes. Using a standard Raji assay employing 125I-IgG goat anti-human Fc gamma, we found that digestion of Raji cells with pronase reduced by 95% their ability to bind complement-fixed aggregated human gamma globulin and complement-fixed tetanus toxoid-antitetanus toxin complexes. However, binding at 37 degrees C of IgG from the sera of 16 patients with systemic lupus erythematosus (SLE) to pronase-digested Raji cells was reduced much less consistently and extensively (9-100% reduction; mean reduction of 51%). In more detailed studies of two SLE sera, sucrose density gradient centrifugation showed that greater than 50% of the IgG binding to undigested Raji cells sedimented in the 7S region. Pepsin digestion of immunoglobulin fractions from four SLE sera caused a reduction in SLE IgG binding to undigested Raji cells when detected with 125I anti-Fc gamma, but an increase when binding was detected with 125I-anti-Fab, suggesting that substantial SLE IgG can bind through F(ab')2 regions. Binding of IgG from SLE sera was not directed at neoantigenic sites induced by pronase digestion because binding activity was adsorbed with undigested cells as readily as with digested cells. Moreover, sera from 10 SLE patients that had negative Raji assays contained no IgG that bound to pronase-digested Raji cells. We conclude that much of the IgG bound at 37 degrees C to Raji cells from the sera of many patients with SLE does not represent immune complexes but is probably antibody directed toward sites on the Raji cell.