Inhibition of lectin-stimulated lymphocyte agglutination and mitogenesis by hydroquinone: reactivity with intracellular sulfhydryl groups

Stump classification was correlated with retear in the suture-bridge and double-row repair techniques for arthroscopic rotator cuff repair

PURPOSE

The Stump classification is significantly correlated with a retear after arthroscopic rotator cuff repair. However, no study has evaluated whether or not the stump classification is correlated with retear in the suture-bridge or double-row repair techniques. The aim of this study was to evaluate the relationship between a retear and the stump classification in the suture-bridge and double-row repair techniques.

METHODS

Among 389 patients who underwent arthroscopic repairs of full-thickness rotator cuff tears using suture-bridge or double-row repair techniques, 326 patients (median age 67.0 years; range 25-85) were included. There were 51 small, 172 medium, 83 large, and 20 massive tears. Two hundred forty patients were treated with the suture-bridge technique, and 86 patients were treated with the double-row technique. The following variables were analyzed: age, sex, the Cofield classification, anteroposterior and mediolateral tear size on preoperative MRI, global fatty degeneration index, and the stump classification. Cuff integrity was evaluated on magnetic resonance imaging at 6 months after surgery. The patients were divided into the intact and retear groups and the relationship between the variables and retear was evaluated by multivariate logistic regression analysis.

RESULTS

The overall retear rate was 10.1%. In the multivariate logistic regression analysis, the independent predictors of a retear were the stump classification type 3 (Odds ratio: 4.71, p = 0.0246), global fatty degeneration index (Odds ratio: 3.87, p = 0.0030), and anteroposterior tear size (Odds ratio: 1.07, p = 0.0077) in the suture bridge technique. In the double-row technique, the independent predictors of retear were stump classification type 3 (Odds ratio: 7.82, p = 0.0348), and age (Odds ratio: 1.22, p = 0.0163).

CONCLUSION

The stump classification was significantly correlated with retear in the suture-bridge and double-row repair technique. Stump classification type 3 was indicated to be an important risk factor for predicting retear.

LEVEL OF EVIDENCE

III.

Identification of human cell responses to benzene and benzene metabolites

Benzene is a common air pollutant and confirmed carcinogen, especially in reference to the hematopoietic system. In the present study we analyzed cytokine/chemokine production by, and gene expression induction in, human peripheral blood mononuclear cells upon their exposure to the benzene metabolites catechol, hydroquinone, 1,2,4-benzenetriol, and p-benzoquinone. Protein profiling showed that benzene metabolites can stimulate the production of chemokines, the proinflammatory cytokines TNF-alpha and IL-6, and the Th2 cytokines IL-4 and IL-5. Activated cells showed concurrent suppression of anti-inflammatory cytokine IL-10 expression. We also identified changes in global gene expression patterns in response to benzene metabolite challenges by using high-density oligonucleotide microarrays. Treatment with 1,2,4-benzenetriol resulted in the suppression of genes related to the regulation of protein expression and a concomitant activation of genes that encode heat shock proteins and cytochrome P450 family members. Protein and gene expression profiling identified unique human cellular responses upon exposure to benzene and benzene metabolites.

Monitoring of the benzene and toluene contents in human milk

Twenty-three samples of human milk collected from the milk bank of a children's hospital were analysed with a view to monitoring the possible presence of some of the most common aromatic hydrocarbons (benzene and toluene) and to quantify their concentrations. The analysis was carried out by the "purge and trap" technique combined with gas chromatography and with the use of the mass spectrometer as detector. The hydrocarbons themselves were used in a deuterated form as internal standards. The analysis of the data showed the presence of both hydrocarbons, even though their quantity was much lower than that detected in other foods.

The toxicology of hydroquinone--relevance to occupational and environmental exposure

Hydroquinone (HQ) is a high-volume commodity chemical used as a reducing agent, antioxidant, polymerization inhibitor, and chemical intermediate. It is also used in over-the-counter (OTC) drugs as an ingredient in skin lighteners and is a natural ingredient in many plant-derived products, including vegetables, fruits, grains, coffee, tea, beer, and wine. While there are few reports of adverse health effects associated with the production and use of HQ, a great deal of research has been conducted with HQ because it is a metabolite of benzene. Physicochemical differences between HQ and benzene play a significant role in altering the pharmacokinetics of directly administered when compared with benzene-derived HQ. HQ is only weakly positive in in vivo chromosomal assays when expected human exposure routes are used. Chromosomal effects are increased significantly when parenteral or in vitro assays are used. In cancer bioassays, HQ has reproducibly produced renal adenomas in male F344 rats. The mechanism of tumorigenesis is unclear but probably involves a species-, strain-, and sex-specific interaction between renal tubule toxicity and an interaction with the chronic progressive nephropathy that is characteristic of aged male rats. Mouse liver tumors (adenomas) and mononuclear cell leukemia (female F344 rat) have also been reported following HQ exposure, but their significance is uncertain. Various tumor initiation/promotion assays with HQ have shown generally negative results. Epidemiological studies with HQ have demonstrated lower death rates and reduced cancer rates in production workers when compared with both general and employed referent populations. Parenteral administration of HQ is associated with changes in several hematopoietic and immunologic endpoints. This toxicity is more severe when combined with parenteral administration of phenol. It is likely that oxidation of HQ within the bone marrow compartment to the semiquinone or p-benzoquinone (BQ), followed by covalent macromolecular binding, is critical to these effects. Bone marrow and hematologic effects are generally not characteristic of HQ exposures in animal studies employing routes of exposure other than parenteral. Myelotoxicity is also not associated with human exposure to HQ. These differences are likely due to significant route-dependent toxicokinetic factors. Fetotoxicity (growth retardation) accompanies repeated administration of HQ at maternally toxic dose levels in animal studies. HQ exposure has not been associated with other reproductive and developmental effects using current USEPA test guidelines. The skin pigment lightening properties of HQ appear to be due to inhibition of melanocyte tyrosinase. Adverse effects associated with OTC use of HQ in FDA-regulated products have been limited to a small number of cases of exogenous ochronosis, although higher incidences of this syndrome have been reported with inappropriate use of unregulated OTC products containing higher HQ concentrations. The most serious human health effect related to HQ is pigmentation of the eye and, in a small number of cases, permanent corneal damage. This effect has been observed in HQ production workers, but the relative contributions of HQ and BQ to this process have not been delineated. Corneal pigmentation and damage has not been reported at current exposure levels of <2 mg/m3. Current work with HQ is being focused on tissue-specific HQ-glutathione metabolites. These metabolites appear to play a critical role in the renal effects observed in F344 rats following HQ exposure and may also be responsible for bone marrow toxicity seen after parenteral exposure to HQ or benzene-derived HQ.

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.

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.

Benzene metabolite, 1,2,4-benzenetriol, induces micronuclei and oxidative DNA damage in human lymphocytes and HL60 cells

The triphenolic metabolite of benzene, 1,2,4-benzenetriol (BT), is readily oxidized to its corresponding quinone via a semiquinone radical. During this process, active oxygen species are formed that may damage DNA and other cellular macromolecules. The ability of BT to induce micronuclei (MN) and oxidative DNA damage has been investigated in both human lymphocytes and HL60 cells. An antikinetochore antibody based micronucleus assay was used to distinguish MN containing kinetochores and potentially entire chromosomes (kinetochore-positive, K+) from those containing acentric chromosome fragments (kinetochore-negative, K-). BT increased the frequency of MN formation twofold in lymphocytes and eightfold in HL60 cells with the MN being 62% and 82% K+, respectively. A linear dose-related increase in total MN, mainly in K(+)-MN, was observed in both HL60 cells and lymphocytes. Addition of copper ions (Cu2+) potentiated the effect of BT on MN induction threefold in HL60 cells and altered the pattern of MN formation from predominantly K+ to K-. BT also increased the level of 8-hydroxy-2'-deoxyguanosine (8-OH-dG), a marker of active oxygen-induced DNA damage. Cu2+ again enhanced this effect. Thus, BT has the potential to cause both numerical and structural chromosomal changes in human cells. Further, it may cause point mutations indirectly by generating oxygen radicals. BT may therefore play an important role in benzene-induced leukemia.

Clastogenic effects of hydroquinone: induction of chromosomal aberrations in mouse germ cells

The clastogenic activity of hydroquinone (HQ) in germ cells of male mice was evaluated by analysis of chromosomal aberrations in primary spermatocytes and differentiating spermatogonia. In the first experiment with treated spermatocytes the most sensitive stage of meiotic prophase to aberration induction by HQ was determined. Testicular material was sampled for microscopic analysis of cells in diakinesis-metaphase I at 1, 5, 9, 11, and 12 days after treatment with 80 mg/kg of HQ, corresponding to treated diplotene, pachytene, zygotene, leptotene and preleptotene. The frequencies of cells with structural chromosome aberrations peaked at 12 days after treatment (p less than 0.01). This indicates that the preleptotene when DNA synthesis occurred was the most sensitive stage of meiotic prophase. In the second experiment the dose response was determined 12 days post treatment by applying 2 additional doses of 40 mg/kg and 120 mg/kg. The clastogenic effects induced by 40 and 80 mg/kg were significantly different from the controls (p less than or equal to 0.01) and higher than the results obtained with 120 mg/kg of HQ. A humped dose-effect relationship was observed. In a third experiment the same doses were used to analyse chromosomal aberrations in dividing spermatogonia of mice 24 h after treatment with HQ. All the administered doses gave results statistically different from the control values (p less than or equal to 0.01) and the data were fitted to a linear equation. HQ was found to be clastogenic in male mouse germ cells. It is concluded that the clastogenic effect in male germ cells is of the same order of magnitude as in mouse bone marrow cells.

Two benzene metabolites, catechol and hydroquinone, produce a synergistic induction of micronuclei and toxicity in cultured human lymphocytes

A mixture of two benzene metabolites, hydroquinone and catechol, produces a striking synergistic genotoxic response in cultured human lymphocytes. This was demonstrated using an anti-kinetochore antibody modification of the micronucleus assay. Treatment with hydroquinone alone or in combination with phenol produced a 3-fold increase in micronucleated cells over background. Treatment with catechol or phenol alone and in combination produced only minor increases in the number of micronucleated cells. In contrast, simultaneous treatment with equimolar (75 microM) concentrations of hydroquinone and catechol resulted in a greater than 16-fold induction of micronucleated cells. Given an additivity model, 20 additional micronucleated cells would be expected (after correcting for background frequencies), yet 140 were observed. Further analysis revealed that over 90% of the micronucleated cells stained positively for kinetochores, indicating a high probability that these micronuclei contain entire chromosomes. This synergistic response appears to occur only at equimolar levels of hydroquinone and catechol. These results suggest that these metabolites are acting together to disrupt the mitotic spindle and interfere with chromosome segregation. These data provide further support for the hypothesis that multiple metabolites acting in concert are involved in the benzene-induced genotoxicity and leukemia in humans.

Subclinical effects of groundwater contaminants. III. Effects of repeated oral exposure to combinations of benzene and toluene on immunologic responses in mice

Toxicity of environmental pollutants may be expressed as combined effects of a chemicals. Benzene, a proven hematotoxic agent, frequently occurs with toluene in cocontaminated groundwater. Groups of CD-1 male mice were exposed continuously for 4 weeks to benzene (166 mg/l), toluene (80 and 325 mg/l), and combinations of benzene (166 mg/l) + toluene (80 mg/l or 325 mg/l) in drinking water. Benzene-induced anemia was alleviated by simultaneous toluene treatment. Leukopenia and lymphopenia were observed in the case of benzene only and benzene + toluene (80 mg/l)-treated mice. The cytopenia, however, was less severe in the benzene + toluene (325 mg/l)-treated group. Immunotoxicity induced by benzene treatment alone was characterized by involution of thymic mass and suppressions of both B- and T-cell mitogeneses, mixed lymphocyte culture response to alloantigens, the tumor lytic ability of cytotoxic T-lymphocytes as determined by 51Cr-release assay, and antibody production response to T-dependent antigen (sheep red blood cells). IL-2 secretion by Con A-stimulated mouse T-cells was decreased in the benzene-treated group. Toluene (325 mg/l) completely inhibited these adverse effects when it was coadministered with benzene, while the low dose of toluene (80 mg/l) did not protect against benzene-induced depressions of immune functions. Toluene administered alone at levels up to 325 mg/l showed no obvious immunotoxic effects. Results of this study demonstrated that toluene, in sufficient amounts, has an antagonistic effect on benzene immunotoxicity.

Oxidatively stressed lymphocytes remain in G0/G1a on mitogenic stimulation

Thiol modifiers and oxidants inhibit lymphocyte activation. To investigate which of the many cell functions sensitive to oxidation are critical in this inhibition, mouse splenic lymphocytes were treated with oxidants prior to exposure to mitogen, and progression into the cell cycle was assayed. Different treatments were used to chemically dissect different potential targets within the cell: copper phenanthroline (CuP), to oxidize surface sulfhydryls; N-ethyl maleimide (NEM), to alkylate extra- and intracellular thiols; and hydrogen peroxide, which generates the highly reactive hydroxyl radical within the cell. Progression into the cell cycle was assayed with acridine orange (AO) and assays of interleukin-2 (IL-2) production and IL-2 receptor (IL-2R) expression. The contribution of ADP-ribosylation to inhibition of mitogenesis was assessed using 3-aminobenzamide (3AB) to inhibit adenosine 5'-diphosphate (ADP)-ribose transferases. The results indicate that the CuP and NEM treatments both produce two independent inhibitory effects, that is, a failure in the production of and response to IL-2. Cells treated with these compounds were able to progress only through G1a upon mitogenic stimulation. H2O2 had more complex effects. Both ADP-ribosylation and modulations of cytosolic Ca2+ were involved in the inhibitory effects. With lower inhibitory doses of H2O2, lymphocytes were completely unresponsive to mitogen and failed to exit Go upon mitogenic stimulation. If intra- and extracellular Ca2+ were buffered before treatment with H2O2, higher concentrations were required, and under these conditions cells were able to enter G1a but could not progress into G1b. Under neither of these conditions could cells produce IL-2 or express IL-2R.

Modulation of benzene toxicity by polyinosinic-polycytidilic acid, an interferon inducer

Repeated intraperitoneal administration of benzene (1.0 ml/kg body wt.) for 3 days produced leucopenia, lymphocytopenia and significantly decreased body wt. (P less than 0.001) and organ weights of thymus (P less than 0.001) and spleen (P less than 0.001) in female albino rats. Total iron content, lipid peroxidation and superoxide dismutase activity of the liver and bone marrow were significantly increased as a result of benzene exposure. Low molecular weight (LMW) bleomycin detectable iron content was accumulated in bone marrow, whereas hepatic LMW iron was not detectable after benzene intoxication to rats. Prior administration of single dose (250 micrograms/100 g body wt.) of Poly IC, an interferon inducer with immunomodulating potential was found to be ameliorate some of the adverse effects of benzene as well as restoration of hepatic architecture histologically. Superoxide dismutase activity, lipid peroxidation, total iron content and LMW iron content (bone marrow) were normalised. Pretreatment of animals with Poly IC was able to enhance the SRBC antibody titre in benzene-treated animals. This study suggests that the beneficial effects of Poly IC in the amelioration of the acute toxicity of benzene has clinical significance.

Activation of bone marrow phagocytes following benzene treatment of mice

Techniques in flow cytometry/cell sorting were used to characterize the effects of benzene and its metabolites on subpopulations of bone marrow cells. Treatment of male Balb/c mice with benzene (880 mg/kg) or a combination of its metabolites, hydroquinone and phenol (50 mg/kg), resulted in a 30 to 40% decrease in bone marrow cellularity. Flow cytometric analysis revealed two subpopulations of bone marrow cells that could be distinguished by their size and density or granularity. The larger, more dense subpopulation was found to consist predominantly of macrophages and granulocytes as determined by monoclonal antibody binding and by cell sorting. Benzene treatment had no selective cytotoxic effects on subpopulations of bone marrow cells. To determine if benzene treatment activated bone marrow phagocytes, we quantified production of hydrogen peroxide by these cells using the fluorescent indicator dye, 2',7'-dichlorofluorescein diacetate. We found that macrophages and granulocytes from bone marrow of treated mice produced 50% more hydrogen peroxide in response to the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate than did cells from control animals. It is hypothesized that phagocyte activation and production of cytotoxic reactive oxygen intermediates may contribute to hematotoxicity induced by benzene.

The inhibitory effects of N-ethylmaleimide, colchicine and cytochalasins on human T-cell functions

The thiol-alkylating agent, N-ethylmaleimide (NEM), was found to inhibit the response of human peripheral blood mononuclear cells to the T-cell mitogen, concanavalin A (Con A). NEM (10 microM) blocked Con A-induced agglutination, production of interleukin-2 (IL-2) and expression of the IL-2 receptors (Tac) without toxicity. In order to determine whether the effects of NEM on lymphokine production were related to inhibition of agglutination, we compared the immunosuppressive effects of NEM with those of cytochalasin A, cytochalasin B and colchicine. NEM did not inhibit E-rosette (ER) formation, suggesting that it does not interfere with actin filaments. Low concentrations of NEM (4 microM) inhibited IL-2 production and Tac expression without inhibiting agglutination, while 6-10 microM NEM blocked agglutination and DNA synthesis as well. In contrast, 5-10 microM cytochalasin B (CB) inhibited ER formation, agglutination, Tac expression and DNA synthesis, but augmented IL-2 production by three- to ten-fold. Colchicine (0.1-10 microM) had no effect on ER formation or agglutination and augmented IL-2 production by as much as 18-fold. However, colchicine blocked Tac expression by greater than 40% and DNA synthesis by greater than 80%. Cytochalasin A (CA), which has the thiol-reactive properties of NEM, the actin filament-disrupting properties of CB, and the microtubule-disrupting properties of colchicine, exhibited the immunosuppressive effects of all three compounds. These studies suggest that the inhibitory effects of NEM on IL-2 production do not appear to be due to reactivity with the cytoskeleton, but are probably due to effects on signal transduction pathways leading to IL-2 production and expression of IL-2 receptors.

Anthralin, a non-phorbol tumor promoter, fails to inhibit metabolic cooperation in mutant human fibroblasts, but inhibits phytohemagglutinin-induced lymphocyte blastogenesis in vitro

Two (among many) of the hypotheses put forward to explain mechanisms of action of tumor promoters are: (1) immunosuppression of the host; and (2) inhibition of intercellular junctional communication. Murine spleen cells were exposed for 30 min to various concentrations of anthralin (1,8-dihydroxy-9-anthrone), a polyphenolic non-phorbol promoter, and 1,8-dihydroxyanthraquinone (1,8-DHAQ), an inactive congener. Phytohemagglutinin (PHA)-induced T cell blastogenesis, an indicator of lymphocyte function, was then assessed in vitro. Exposure to anthralin resulted in a concentration-dependent suppression of lymphocyte proliferation with complete suppression occurring at 1 microM. The inactive congener, 1,8-DHAQ, failed to suppress lectin-induced blastogenesis at concentrations up to 10 microM. Dithiothreitol (DTT), a sulfhydryl (SH) compound, failed to protect against the suppression of lymphocyte function by anthralin. In addition, anthralin failed to inhibit in vitro microtubule assembly, a SH-dependent process, in a crude rat brain extract. Finally, unlike 12-O-tetradecanoylphorbol-13-acetate (TPA), the most potent skin tumor promoter known, anthralin failed to inhibit metabolic cooperation between mutant human fibroblasts as assayed by [14C]citrulline incorporation. In summary, the data suggest that anthralin may act as a tumor promoter by suppressing immune parameters, a property which is shared by the phorbol esters.

Iodoacetic acid and related sulfhydryl reagents fail to inhibit cell-cell communication: mechanisms of immunotoxicity in vitro

The present study was undertaken to examine the effects of iodoacetic acid, a non-phorbol tumor promoter, on metabolic cooperation between mutant human fibroblasts as measured by [14C]citrulline incorporation. Other thiol-reactive polyphenolic compounds such as hydroquinone and 2-hydroxyestrone were also examined. 12-O-Tetradecanoyl phorbol-13-acetate (TPA), a potent skin tumor promoter, inhibited the cell-cell communication by more than 60% at 20 ng/ml. However, iodoacetic acid, hydroquinone, and 2-hydroxyestrone, had no effect on the process even at cytotoxic concentrations. Induction of intercellular contact (agglutination) among lymphocytes during the course of phytohemagglutinin (PHA)-induced blastogenesis was monitored turbidometrically at 620 nm. Hydroquinone and 2-hydroxyestrone suppressed the PHA-induced lymphocyte agglutination at 1-2 microM in vitro concentrations while iodoacetic acid was devoid of any effects at concentrations up to 100 microM. Hydroquinone and 2-hydroxyestrone concomitantly suppressed PHA-induced lymphocyte blastogenesis at 1-2 microM in vitro concentrations while the suppression by iodoacetic acid was significant at 10 microM. All 3 compounds failed to disrupt microtubule assembly, a sulfhydryl-dependent process, in a rat brain crude extract. However, p-benzoquinone, an oxidation product of hydroquinone, did inhibit the process at 1 mM. In summary, these studies suggest that, unlike TPA, thiol-reactive non-phorbol tumor promoters and polyphenolic compounds do not inhibit cell-cell communication between mutant human fibroblasts. Although the compounds demonstrate diverse molecular mechanisms of action, they all inhibit in vitro immune functions suggesting that immunosuppression may play a role in tumor promotion.

Inhaled benzene reduces aspects of cell-mediated tumor surveillance in mice

Benzene is a potent bone marrow toxicant with particular activity against lymphocytes. Despite the recognized effects of benzene on lymphocyte populations, few data exist concerning the effects of benzene on in vivo immune responses. We have been conducting a series of studies concerning the effects of inhaled benzene on murine cell-mediated immune responses. The studies in this report involve the interaction of inhaled benzene with some of those cell-mediated immune responses associated with tumor surveillance. Exposures to 100 ppm benzene (5 days/week X 20 weeks) induced lethal tumor growth in 9/10 C57Bl/6 mice inoculated with 10(4) viable PYB6 tumor cells. Lethal tumor incidences in air controls and mice exposed to lower benzene concentrations were 3/10 or less. Exposures to 100 ppm benzene (5 days/week X 4 weeks) also reduced the tumor lytic abilities of cytotoxic T lymphocytes as determined by 51Cr-release assays. In addition, splenocytes taken from mice exposed to 10 or 100 ppm benzene (5 days/week X 4 weeks) exhibited delays in peak mixed leukocyte responses. Coculture experiments demonstrated that these delays were not due to an induction of suppressor cell activity by benzene. There were no alterations in the relative percentages of B cells, T cells, or T-cell subsets among splenocytes from animals exposed to any concentration of benzene tested. These results demonstrate that inhaled benzene can inhibit some of the processes associated with tumor surveillance, and that this inhibition is due, at least in part, to impairments of the functional abilities of some of the cells responsible for tumor surveillance.

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.

Modulation of lectin-stimulated lymphocyte agglutination and mitogenesis by estrogen metabolites: effects on early events of lymphocyte activation

Pharmacological doses of estrogens such as 17-beta estradiol (17-beta E) and diethylstilbestrol (DES) suppress cell-mediated immunity in vivo. In this report, we investigated the direct in vitro effects of 17-beta E and its major metabolites on lymphocyte proliferation in response to the T cell lectin phytohemagglutinin (PHA). PHA-induced lymphocyte agglutination, an early event indicative of active, cytoskeletal-dependent membrane alterations, was monitored in conjunction with blastogenesis. Without exception, the effects of individual estrogen metabolites on the PHA-induced agglutination occurring within minutes were accompanied, at every concentration of compound, by equivalent effects on the blastogenic response of activated cells measured after several days. This observation suggested a role for estrogens in modulating lymphocyte activation at the cell surface rather than through cytosolic receptor-mediated events. As suggested by previous studies with quinone metabolites of benzene, the catechol estrogen metabolite 2-OH estrone (2-OH E) was significantly more potent than the parent compound at suppressing lymphocyte proliferation in vitro and in vivo.

Natural cysteine proteinase inhibitors reduce lectin induced lymphocyte stimulation

Lymphocyte stimulation by lectins can be inhibited by several synthetic inhibitors of proteolytic enzymes, notably those of cysteine proteinases. The effects of naturally occurring enzyme inhibitors are less well known. The effect of the neutral low-molecular weight cysteine proteinase inhibitor (NCPI) recently purified from lymph nodes and spleen was therefore investigated. Cultured peripheral blood lymphocytes were stimulated by PHA or ConA in the presence or absence of NCPI and the incorporation of 3H-thymidine was measured. NCPI was found to inhibit these lymphocyte responses in these circumstances.

Induction of cytogenetic damage in rodents after short-term inhalation of benzene

Experiments were designed to investigate both the induction of sister chromatid exchanges (SCEs) in peripheral blood lymphocytes (PBLs) and micronuclei (MN) in bone marrow polychromatic erythrocytes (PCEs) of mice and rats after inhalation of benzene (BZ). Male DBA/2 mice (17-19 weeks old) were exposed to target concentrations of either 0, 10, 100, or 1,000 ppm BZ for 6 hr. Male Sprague-Dawley rats (11-14 weeks old) were exposed to target concentrations of either 0, 0.1, 0.3, 1, 3, 10, or 30 ppm BZ for 6 hr. Blood was obtained by cardiac puncture 18 hr after exposure, and PBLs were cultured in the presence of lipopolysaccharide (mouse B cells, 60 micrograms/ml) or concanavalin A (rat T cells, 30 micrograms/ml) to stimulate blastogenesis for SCE analysis. Femoral bone marrow smears from both species were analyzed for MN in PCEs 18 hr after BZ exposure. Mouse PBLs revealed a significant concentration-related increase in the SCE frequency over controls at 10, 100, or 1,000 ppm BZ. Mouse bone marrow showed a significant concentration-dependent increase in MN over controls after exposure to 10, 100, or 1,000 ppm BZ. Rat PBLs showed a significant increase in the SCE frequency after exposure to 3, 10, or 30 ppm BZ. The statistical significance of the 1 ppm BZ result was borderline and dependent on the statistical test chosen. Rat cells revealed a significant concentration-related increase in MN after inhalation of either 1, 3, 10, or 30 ppm BZ. PBLs from treated mice showed significant concentration-dependent decreases in mitotic indices; however, cell cycle kinetics and leucocyte counts remained unaffected. Rat PBLs showed significant decreases in mitotic activity only after exposure to 3 and 30 ppm BZ, whereas cell cycle kinetics and leucocyte counts were unaffected. These results show that BZ can induce statistically significant cytogenetic effects in PBLs and PCEs of both mice and rats after a 6-hr inhalation of BZ at low concentrations.

Inhibition of RNA synthesis and interleukin-2 production in lymphocytes in vitro by benzene and its metabolites, hydroquinone and p-benzoquinone

The effects of benzene and its metabolites, hydroquinone and p-benzoquinone (PBQ) on RNA synthesis in mouse spleen lymphocytes in vitro were studied. Benzene and the quinones were shown to inhibit RNA synthesis in a dose-dependent manner at concentrations which had no significant effect on lymphocyte viability. Furthermore, 5 microM PBQ, the putative toxic metabolite of benzene, was shown to inhibit the formation of the T-cell growth factor IL-2. These results suggest that inhibition of RNA synthesis in lymphocytes by benzene may prevent the production of factors required for hemopoiesis and thus contribute to the aplastic anemia caused by benzene.

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.

Modulation of nonspecific cell-mediated growth inhibition by estrogen metabolites

Chronic exposure of mice to estrogens such as 17-beta estradiol and diethylstilbestrol inhibits natural killer cell-mediated cytotoxicity in vivo. In this report, we investigated the direct in vitro effects of 17-beta estradiol and its major metabolites on nonspecific effector cell function measured as the ability of naive lymphocytes to inhibit the growth of the YAC-1 lymphoma, a classical natural killer-sensitive target cell. Without exception, the effects of individual estrogen metabolites on the growth inhibitory properties of these cells were accompanied, at every concentration of compound, by identical effects on the blastogenic response of lymphocytes to the T cell lectin phytohemagglutinin. These observations suggested membrane-mediated immunomodulation of lymphocyte function by estrogen metabolites. As suggested by previous studies with quinone metabolites of benzene, the catechol estrogen metabolite 2-OH estrone was significantly more potent than the parent compound at suppressing lymphocyte functions in vitro; however, dosing regimens of 2-OH estrone that suppressed blastogenic response in vivo failed to inhibit nonspecific cell-mediated growth inhibition.

The inhibition of mitochondrial DNA replication in vitro by the metabolites of benzene, hydroquinone and p-benzoquinone

Rat liver mitochondria incubated with the metabolites of benzene, p-benzoquinone or 1,2,4-benzenetriol, showed a dose-dependent inhibition of [3H]dTTP incorporation into mtDNA with median inhibitory concentrations of 1 mM for each compound. Benzene and the metabolites phenol, catechol and hydroquinone did not inhibit at concentrations up to 10 mM. Similarly, incubation of p-benzoquinone or hydroquinone with rabbit bone marrow mitochondria showed a dose-dependent inhibition of mtDNA synthesis with 50% inhibition at 1 mM and 10 mM, respectively. That these metabolites inhibit mitochondrial replication was evidenced by the fact that [3H]dTTP incorporation into characteristic 38S, 27S and 7S mitochondrial replication intermediates was decreased by the quinones, as analyzed on 5-20% neutral sucrose velocity gradients. p-Benzoquinone, hydroquinone and 1,2,4-benzenetriol inhibited the activity of partially purified rat liver mtDNA polymerase gamma using either activated calf thymus DNA or poly(rA) X p(dT)12-18 as primer/template, with 50% inhibitory concentrations of 25 microM, 25 microM and 180 microM, respectively. Preincubation of the metabolites with polymerase gamma or primer/template, followed by removal of the unreacted metabolite by gel filtration, indicated that inhibition resulted from interaction of the metabolites with the enzyme, rather than with the template. Binding appeared to involve a sulfhydryl residue on the enzyme since the binding of [14C]hydroquinone was prevented by N-ethylmaleimide. The ability of hydroquinone or p-benzoquinone to inhibit binding of [14C]hydroquinone to the enzyme suggests that the compounds bind to a common site or are converted to a common intermediate. Inhibition of, or changes in, replication in mitochondria of bone marrow cells by hydroquinone and p-benzoquinone may explain the changes in the mitochondrial genome observed in marrow stem cells in acute myelogenous leukemia and may suggest a mechanism for benzene leukemogenesis.

Modulation of lymphokine-induced macrophage activation by estrogen metabolites

Pharmacological doses of estrogens such as 17-beta estradiol (17- beta E) and diethylstilbestrol (DES) activate macrophages in a thymic-dependent manner in vivo. In this report, we investigated the direct in vitro effects of 17- beta E and its major metabolites on macrophage activation in response to lectin-stimulated lymphocyte supernatants containing macrophage-activating factor (MAF), a T cell lymphokine (LK). Activation was measured in terms of macrophage cytostasis against cultured tumor cells. As suggested by previous studies with quinone metabolites of benzene, the catechol estrogen metabolite 2-OH estrone (2-OH E) was the most potent metabolite at suppressing LK-induced macrophage activation. However, if macrophages were first LK-induced, and then exposed to estrogens before addition of tumor cells, then all the estrogens, including 2-OH E, enhanced cytostasis. These observations suggested membrane-mediated immunomodulation of macrophage function by estrogen metabolites and, indirectly, a role for the thymus in these effects via the maintenance of a mature, LK-producing T cell population necessary for macrophage activation.

Quinones as toxic metabolites of benzene

Occupational exposure to benzene has long been associated with toxicity to the blood and bone marrow, including lymphocytopenia, pancytopenia, aplastic anemia, acute myelogenous leukemia, and possibly lymphoma. A variety of studies have established that benzene itself is not the toxic species but requires metabolism to reactive intermediates. The bioactivation of benzene is complex. Both primary and secondary oxidation of benzene and its metabolites are mediated via cytochrome P-450 in the liver, although the role of secondary metabolism in the bone marrow is not clear. Toxicity is associated with the dihydroxy metabolites, hydroquinone and catechol, which concentrate in bone marrow. Hydroquinone and its terminal oxidation product, p-benzoquinone, have been demonstrated to be potent suppressors of cell growth in culture. Suppression of lymphocyte blastogenesis by these compounds is a sulfhydryl-dependent process and occurs at concentrations that do not result in cell death, or in detectable alterations in energy metabolism, intracellular glutathione concentration, or protein synthesis. Recent studies suggest that these compounds and other membrane-penetrating sulfhydryl alkylating agents, such as N-ethylmaleimide and cytochalasin A, and endogenous regulatory molecules, such as soluble immune response suppressor (SIRS), interfere with microtubule assembly in vitro and selectively interfere with microtubule-dependent cell functions at identical concentrations. These agents appear to react with nucleophilic sulfhydryl groups essential for guanosine triphosphate binding to tubulin that are particularly sensitive to sulfhydryl-alkylating agents.

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

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

Mechanisms of dialkyltin induced immunopathology

In this report the immunotoxic effects of dialkyltin compounds are reviewed. In rats they induce lymphocyte depletion in the thymus and the thymus dependent areas of the peripheral lymphoid organs, without signs of a generalized toxicity. As a consequence of their selective lymphocytotoxic action they cause immunosuppression, especially of the cell mediated immunity. Various mechanisms are discussed that may be involved in the selective effect on the thymus, indirectly through changes in endocrine control or the reticular epithelial cells, and directly on the intrathymic rapidly dividing cells. It is suggested that the immunotoxicity of dialkyltin compounds is related to their affinity for dithiol groups, resulting in a disturbance of cell energetics as well as in cell proliferation.

Inhibition of mRNA synthesis in rabbit bone marrow nuclei in vitro by quinone metabolites of benzene

mRNA synthesis by rabbit bone marrow nuclei has been shown to be inhibited by the quinone metabolites of benzene, hydroquinone and p-benzoquinone, in a concentration-dependent manner with 50% inhibitory concentration (IC50[M]) for both compounds of 6 X 10(-6) M. Catechol and 1,2,4-benzenetriol also showed a concentration-dependent inhibition of synthesis, however, 50% inhibition was not reached by 10(-4) M. Phenol did not inhibit mRNA synthesis even at 10(-3) M. It is possible that myelotoxicity from benzene might result from such an inhibition of mRNA synthesis by quinone metabolites in pluripotent and/or committed bone marrow stem cells.

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.

Alteration of lymphocyte function by quinones through a sulfhydryl-dependent disruption of microtubule assembly

The cytoskeleton plays a pivotal role in lectin-induced lymphocyte blastogenesis. Microtubule disrupting agents, many of which are sulfhydryl (SH) reagents, interfere with cytoskeletal-dependent cell functions including lymphocyte blastogenesis and agglutination. For example, hydroquinone (HQ) and N-ethylmaleimide (NEM) inhibit lectin-stimulated lymphocyte blastogenesis and agglutination at concentrations (10(-5)M) that do not reduce cell viability or ATP production. Indicative of the SH-specificity of these effects, only L-cysteine protects against HQ or NEM inhibition of blastogenesis and agglutination. Other compounds, including L-serine, DL-lysine and imidazole, have no protective effect. These and other findings previously reported suggest a selective interaction of HQ, or its oxidation product, p-benzoquinone (p-BQ) with SH groups critical to early G1 events associated with lymphocyte activation. These compounds show similar SH specificity in inhibiting microtubule assembly in vitro. The subcellular target specificity (cytoskeleton) exhibited by these compounds was compared to that of Adriamycin (ADR), a complex polycyclic quinone with known immunotoxic activity. ADR inhibited microtubule assembly in vitro and inhibited lymphocyte blastogenesis, however, these effects were not correlated with a loss of agglutination nor was toxicity protected against by the addition of SH compounds. The combination of cell culture methods together with application of techniques to measure microtubule assembly in vitro provides an effective means to discriminate between agents that selectively interfere with cytoskeletal-dependent function and those producing non-specific effects associated with cell death, such as decreased energy production or increased membrane permeability.

Hydroquinone and catechol reduce the frequency of progenitor B lymphocytes in mouse spleen and bone marrow

Hydroquinone and catechol are two metabolites of benzene that are potential inducers of hematotoxicity. We investigated the in vivo toxicity of these metabolites toward the development of polyclonal, plaque-forming cells (PC-PFC) from progenitor B lymphocytes. Dextran sulfate (DxS), lipopolysaccharide (LPS), or the two mitogens combined (DxS + LPS) were used to induce proliferation and maturation of these progenitors to PC-PFC. Groups of 4 C57BL/6 mice were exposed to 2 daily doses, either intravenously or intraperitoneally, of hydroquinone (100 mg/kg) or catechol (75 mg/kg) for 3 consecutive days. Spleen and marrow cells were harvested for culture 1 day later. The results demonstrated that both metabolites were cytotoxic to spleen cells. Hydroquinone (100 mg/kg) also reduced marrow cellularity, whereas catechol (75 mg/kg) did not significantly affect marrow cellularity. Each compound reduced the frequency of PC-PFC developed from the spleens and marrows of treated mice, but only catechol selectively inhibited the maturation of LPS-activated marrow progenitors into end-stage PC-PFC. These experiments demonstrate the immunotoxic potential of hydroquinone and catechol in vivo through the reduction of progenitor B lymphocytes and suggest that inhibition of precursor cell maturation may play a significant role in the hematotoxicity observed after chronic exposure to benzene.