Differences in xenobiotic detoxifying activities between bone marrow stromal cells from mice and rats: implications for benzene-induced hematotoxicity

Antioxidants and Phase 2 Enzymes in Cardiomyocytes: Chemical Inducibility and Chemoprotection Against Oxidant and Simulated Ischemia-Reperfusion Injury

The increasing recognition of the role for oxidative stress in cardiac disorders has led to extensive investigation on the protection by exogenous antioxidants against oxidative cardiac injury. On the other hand, another strategy for protecting against oxidative cardiac injury may be through upregulation of the endogenous antioxidants and phase 2 enzymes in the myocardium by chemical inducers. However, our current understanding of the chemical inducibility of cardiac cellular antioxidants and phase 2 enzymes is very limited. In this study, using rat cardiac H9c2 cells we have characterized the concentration- and time-dependent induction of cellular antioxidants and phase 2 enzymes by 3H-1,2-dithiole-3-thione (D3T), and the resultant chemoprotective effects on oxidative cardiac cell injury. Incubation of H9c2 cells with D3T resulted in a marked concentration- and time-dependent induction of a number of cellular antioxidants and phase 2 enzymes, including catalase, reduced glutathione (GSH), GSH peroxidase, glutathione reductase (GR), GSH S-transferase (GST), and NAD(P)H:quinone oxidoreductase-1 (NQO1). D3T treatment of H9c2 cells also caused an increase in mRNA expression of catalase, γ-glutamylcysteine ligase catalytic subunit, GR, GSTA1, M1 and P1, and NQO1. Moreover, both mRNA and protein expression of Nrf2 were induced in D3T-treated cells. D3T pretreatment led to a marked protection against H9c2 cell injury elicited by various oxidants and simulated ischemia-reperfusion. D3T pretreatment also resulted in decreased intracellular accumulation of reactive oxygen in H9c2 cells after exposure to the oxidants as well as simulated ischemia-reperfusion. This study demonstrates that a series of endogenous antioxidants and phase 2 enzymes in H9c2 cells can be induced by D3T in a concentration- and time-dependent fashion, and that the D3T-upregulated cellular defenses are accompanied by a markedly increased resistance to oxidative cardiac cell injury.

Association of GSTM1 and GSTT1 deletion polymorphisms with Pakistani aplastic anemia patients and controls and meta-analysis

Interaction of environmental and genetic elements plays a vital role in the pathogenesis of aplastic anemia (AA). Glutathione S-transferase (GST) is a key detoxifying enzyme. Absence or low levels of this enzyme may genetically predispose individuals to AA. GST genes GSTM1 and GSTT1 are polymorphic. The aim of this study was to screen Pakistani AA patients and controls for GSTM1 deletion GSTM0 and GSTT1 deletion GSTT0 and perform meta-analysis using our data and other published data regarding these polymorphisms. DNA samples from 137 patients and 220 controls were screened using multiplex polymerase chain reaction. GSTM0 emerged as susceptible genotype for AA in Pakistan with a percentage frequency of 49.6 % as compared to 30 % in controls with odds ratio (OR) of 2.25, 95 % confidence interval (CI) of 1.4-3.5 and corrected p = 0.006. The meta-analysis showed a significant association between the null genotype GSTT0 and AA in overall analysis with OR of 1.47, 95 % CI of 1.01-2.13 and p value of 0.04 in random effects model. Studies like these could play a role in understanding the underlying path in AA pathogenesis and therefore can help in designing means for prevention, diagnose and treatment.

Aplastic anemia: what have we learned from animal models and from the clinic

Aplastic anemia (AA) is currently perceived as an immune-mediated disease in which aberrant effector cells recognize and destroy primitive marrow elements, resulting in pancytopenia. The immune hypothesis is based on clinical observations of responsiveness of AA to immunomodulatory agents such as anti-thymocyte globulin (ATG) and the requirement of cyclosporine to maintain response; evidence of an immune system in disarray provided by abnormal regulatory, TH1, TH17, and expanded CD8(+) T-cell populations, and animal models of immune-mediated marrow destruction, where many of the observed clinical and in vitro alterations can be confirmed and expanded. Murine models mimicking AA have used exposure to agents that result in marrow destruction through a direct toxic effect, but models that explore antigenic disparities between strains have resulted in immune-mediated destruction of the marrow, more closely modeling human AA. Many experiments in mice have helped confirm and elucidate specific mechanisms of marrow destruction. However, clinical development of regimens in AA has not relied on establishing their success in murine model. Instead, drugs and their combinations investigated in AA were those shown clinically to be active in AA, in other hematologic diseases, or in other specialties such as in rheumatology, and solid and bone marrow transplantation. In this review, the evolution of murine models and their clinical relevance in AA are discussed.

Comparative proteomic analysis of cellular response of human airway epithelial cells (A549) to benzo(a)pyrene

This work aimed to investigate the cellular response of human airway epithelial cells (A549) to oxidative stress induced by benzo(a)pyrene [B(a)P]. Levels of intracellular reactive oxygen species (ROS) and lipid peroxidation were investigated in A549 cells treated with varying concentrations of B(a)P. A comparative proteomic analysis of total proteins was performed in cells treated with 1 µM B(a)P [B(a)P-1] and untreated cells. The expression of Mn superoxide dismutase (Mn SOD), one of the identified down-regulated proteins in B(a)P-1 cells, was then analyzed by Western blotting. The total antioxidant activity, total superoxide dismutase activity, catalase (CAT) activity, and glutathione reductase (GR) activity were all analyzed after B(a)P treatment. Our results demonstrated that 1 µM B(a)P could induce ROS generation and lead to lipid peroxidation in A549 cells, and 23 differentially expressed proteins were identified. The expression levels of Mn SOD and the total SOD were induced at 0.1 µM and suppressed at 1 µM and 10 µM. Up-regulation of CAT and GR activity resulted in an increase in total antioxidant activity in A549 after exposure to B(a)P. These findings provide a basis for understanding the mechanisms of mitochondrial dysfunction and perturbation of antioxidant status induced by B(a)P on airway epithelial cells.

NAD(P)H:quinone acceptor oxidoreductase 1 (NQO1), a multifunctional antioxidant enzyme and exceptionally versatile cytoprotector

NAD(P)H:quinone acceptor oxidoreductase 1 (NQO1) is a widely-distributed FAD-dependent flavoprotein that promotes obligatory 2-electron reductions of quinones, quinoneimines, nitroaromatics, and azo dyes, at rates that are comparable with NADH or NADPH. These reductions depress quinone levels and thereby minimize opportunities for generation of reactive oxygen intermediates by redox cycling, and for depletion of intracellular thiol pools. NQO1 is a highly-inducible enzyme that is regulated by the Keap1/Nrf2/ARE pathway. Evidence for the importance of the antioxidant functions of NQO1 in combating oxidative stress is provided by demonstrations that induction of NQO1 levels or their depletion (knockout, or knockdown) are associated with decreased and increased susceptibilities to oxidative stress, respectively. Furthermore, benzene genotoxicity is markedly enhanced when NQO1 activity is compromised. Not surprisingly, human polymorphisms that suppress NQO1 activities are associated with increased predisposition to disease. Recent studies have uncovered protective roles for NQO1 that apparently are unrelated to its enzymatic activities. NQO1 binds to and thereby stabilizes the important tumor suppressor p53 against proteasomal degradation. Indeed, NQO1 appears to regulate the degradative fate of other proteins. These findings suggest that NQO1 may exercise a selective "gatekeeping" role in regulating the proteasomal degradation of specific proteins, thereby broadening the cytoprotective role of NQO1 far beyond its highly effective antioxidant functions.

Relationships between metabolic and non-metabolic susceptibility factors in benzene toxicity

Reactive metabolites formed from benzene include benzene oxide, trans,trans muconaldehyde, quinones, thiol adducts, phenolic metabolites and oxygen radicals. Susceptibility to the toxic effects of benzene has been suggested to occur partly because of polymorphisms in enzymes involved in benzene metabolism which include cytochrome P450 2E1, epoxide hydrolases, myeloperoxidase, glutathione-S-transferases and quinone reductases. However, susceptibility factors not directly linked to benzene metabolism have also been associated with its toxicity and include p53, proteins involved in DNA repair, genomic stability and expression of cytokines and/or cell adhesion molecules. In this work, we examine potential relationships between metabolic and non-metabolic susceptibility factors using the enzyme NAD(P)H:quinone oxidoreductase (NQO1) as an example. NQO1 may also impact pathways in addition to metabolism of quinones due to protein-protein interactions or other mechanisms related to NQO1 activity. NQO1 has been implicated in stabilizing p53 and in maintaining microtubule integrity. Inhibition or knockdown of NQO1 in bone marrow endothelial cells has been found to lead to deficiencies of E-selectin, ICAM-1 and VCAM-1 adhesion molecule expression after TNFalpha stimulation. These examples illustrate how the metabolic susceptibility factor NQO1 may influence non-metabolic susceptibility pathways for benzene toxicity.

Upregulation of endogenous glutathione system by 3H-1,2-dithiole-3-thione in pancreatic RINm5F beta-cells as a novel strategy for protecting against oxidative beta-cell injury

This study was undertaken to investigate the inducibility of glutathione (GSH), glutathione reductase (GR) and glutathione peroxidase (GPx) by 3H-1,2-dithiole-3-thione (D3T) in beta-cells, and the resultant cytoprotection against oxidant injury. Incubation of the insulin-secreting RINm5F cells with D3T led to significant induction of GSH, GR and GPx. D3T-mediated induction of GSH was abolished by buthionine sulfoximine (BSO), suggesting a critical involvement of gamma-glutamylcysteine ligase (gammaGCL). Consistently, incubation of RINm5F cells with D3T resulted in increased expression of gammaGCL protein and mRNA. Pretreatment of RINm5F cells with D3T provided remarkable protection against oxidant-elicited cytotoxicity. On the other hand, depletion of cellular GSH by BSO sensitized RINm5F cells to oxidant injury. Furthermore, cotreatment of RINm5F cells with BSO to reverse D3T-mediated GSH induction abolished the cytoprotective effects of D3T on oxidant injury. Taken together, this study demonstrates that upregulation of glutathione system by D3T is effective for protecting against oxidative beta-cell injury.

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

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

Resveratrol protects primary rat hepatocytes against oxidative stress damage: activation of the Nrf2 transcription factor and augmented activities of antioxidant enzymes

Oxidative stress is recognized as an important factor in the development of liver pathologies. The reactive oxygen species endogenously generated or as a consequence of xenobiotic metabolism are eliminated by enzymatic and nonenzymatic cellular systems. Besides endogen defences, the antioxidant consumption in the diet has an important role in the protection against the development of diseases product of oxidative damage. Resveratrol is a naturally occurring compound which is part of the human diet. This molecule has been shown to have many biological properties, including antioxidant activity. We decided to test if resveratrol could protect primary hepatocytes in culture from oxidative stress damage and if so, to determine if this compound affects the cellular detoxifying systems and their regulation through the Nrf2 transcription factor that regulates the expression of antioxidant and phase II detoxifying enzymes. Cell death by necrosis was detected by measuring the activity of lactate dehydrogenase liberated to the medium. The activities of antioxidant and phase II enzymes were measured using previously described methods. Activation of the Nrf2 transcription factor was studied by confocal microscopy and the Nrf2 and its coding mRNA levels were determined by western blot and quantitative PCR respectively. Resveratrol pre-treatment effectively protected hepatocytes in culture exposed to oxidative stress, increasing the activities of catalase, superoxide dismutase, glutathione peroxidase, NADPH quinone oxidoreductase and glutathione-S-transferase. Resveratrol increases the level of Nrf2 and induces its translocation to the nucleus. Also, it increases the concentration of the coding mRNA for Nrf2. In this work we show that resveratrol could be a useful drug for the protection of liver cells from oxidative stress induced damage.

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

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

Glutathione and glutathione-linked enzymes in normal human aortic smooth muscle cells: chemical inducibility and protection against reactive oxygen and nitrogen species-induced injury

Substantial evidence suggests a crucial role for glutathione (GSH) and GSH-linked enzymes in protecting against oxidative vascular disorders. However, studies on the chemical inducibility of these antioxidant defenses and their protective effects on oxidant injury in normal human vascular cells are currently lacking. Accordingly, this study was undertaken to investigate the inducibility of GSH, glutathione reductase (GR), glutathione peroxidase (GPx), and glutathione S-transferase (GST) by the chemoprotective agent, 3H-1,2-dithiole-3-thione (D3T) in cultured normal human aortic smooth muscle cells (HASMCs). HASMCs expressed measurable levels/activities of GSH, GR, GPx, and GST. Incubation of HASMCs with low micromolar concentrations of D3T resulted in a marked elevation in total cellular GSH content and GR activity. The protein and mRNA expression of gamma-glutamylcysteine ligase (GCL) and GR were also upregulated by D3T. In addition, D3T caused significant increases in mitochondrial GSH content and GR activity. In contrast, neither cellular GPx nor GST activity was altered after D3T treatment. Pretreatment of HASMCs with D3T afforded remarkable protection against reactive oxygen and nitrogen species (ROS/RNS)-mediated cell injury. Depletion of cellular GSH by pretreatment with buthionine sulfoximine (BSO), an inhibitor of GSH biosynthesis led to marked potentiation of the ROS/RNS-induced cell injury. Moreover, co-treatment of HASMCs with BSO was found to completely abolish the D3T-mediated GSH elevation, and remarkably reverse D3T cytoprotection against the ROS/RNS-elicited injury. Taken together, this study demonstrates that both GSH/GCL and GR in normal HASMCs are inducible by D3T, and that upregulation of GSH biosynthesis appears to be the predominant mechanism underlying D3T-mediated cytoprotection against ROS/RNS-elicited injury to human vascular smooth muscle cells.

Toward refinement of the colony-forming unit-granulocyte/macrophage clonogenic assay: Inclusion of a metabolic system

This work represents a first attempt to refine the colony-forming unit-granulocyte/macrophage (CFU-GM) clonogenic assay by incorporating liver microsomes and co-factors as a metabolic system into the in vitro test system in response to an ECVAM recommendation. From the comparison of results obtained with the CFU-GM clonogenic assay currently used and with the new experimental protocol, different toxicity on granulocyte/macrophage precursors was demonstrated, when drugs with a known metabolism in vivo were tested.

Role of Nrf2 signaling in regulation of antioxidants and phase 2 enzymes in cardiac fibroblasts: protection against reactive oxygen and nitrogen species-induced cell injury

Understanding the molecular pathway(s) of antioxidant gene regulation is of crucial importance for developing antioxidant-inducing agents for the intervention of oxidative cardiac disorders. Accordingly, this study was undertaken to determine the role of Nrf2 signaling in the basal expression as well as the chemical inducibility of endogenous antioxidants and phase 2 enzymes in cardiac fibroblasts. The basal expression of a scope of key cellular antioxidants and phase 2 enzymes was significantly lower in cardiac fibroblasts derived from Nrf2-/- mice than those from wild type control. These include catalase, reduced glutathione (GSH), glutathione reductase (GR), GSH S-transferase (GST), and NAD(P)H:quinone oxidoreductase-1 (NQO1). Incubation of Nrf2+/+ cardiac fibroblasts with 3H-1,2-dithiole-3-thione (D3T) led to a significant induction of superoxide dismutase (SOD), catalase, GSH, GR, glutathione peroxidase (GPx), GST, and NQO1. The inducibility of SOD, catalase, GSH, GR, GST, and NQO1, but not GPx by D3T was completely abolished in Nrf2-/- cells. The Nrf2-/- cardiac fibroblasts were much more sensitive to reactive oxygen and nitrogen species-mediated cytotoxicity. Upregulation of antioxidants and phase 2 enzymes by D3T in Nrf2+/+ cardiac fibroblasts resulted in a dramatically increased resistance to the above species-induced cytotoxicity. In contrast, D3T-treatment of the Nrf2-/- cells only provided a slight cytoprotection. Taken together, this study demonstrates for the first time that Nrf2 is critically involved in the regulation of the basal expression and chemical induction of a number of antioxidants and phase 2 enzymes in cardiac fibroblasts, and is an important factor in controlling cardiac cellular susceptibility to reactive oxygen and nitrogen species-induced cytotoxicity.

Differential roles of 3H-1,2-dithiole-3-thione-induced glutathione, glutathione S-transferase and aldose reductase in protecting against 4-hydroxy-2-nonenal toxicity in cultured cardiomyocytes

4-hydroxy-2-nonenal (HNE) plays an important role in the pathogenesis of cardiac disorders. While conjugation with glutathione (GSH) catalyzed by GSH S-transferase (GST) has been suggested to be a major detoxification mechanism for HNE in target cells, whether chemically upregulated cellular GSH and GST afford protection against HNE toxicity in cardiac cells has not been investigated. In addition, the differential roles of chemically induced GSH and GST as well as other cellular factors in detoxifying HNE in cardiomyocytes are unclear. In this study, we have characterized the induction of GSH and GST by 3H-1,2-dithiole-3-thione (D3T) and the protective effects of the D3T-elevated cellular defenses on HNE-mediated toxicity in rat H9C2 cardiomyocytes. Treatment of cardiomyocytes with D3T resulted in a significant induction of both GSH and GST as well as the mRNA expression of gamma-glutamylcysteine ligase catalytic subunit and GSTA. Both GSH and GST remained elevated for at least 72 h after removal of D3T from the culture media. Treatment of cells with HNE led to a significant decrease in cell viability and an increased formation of HNE-protein adducts. Pretreatment of cells with D3T dramatically protected against HNE-mediated cytotoxicity and protein-adduct formation. HNE treatment caused a significant decrease in cellular GSH level, which preceded the loss of cell viability. Either depletion of cellular GSH by buthionine sulfoximine (BSO) or inhibition of GST by sulfasalazine markedly sensitized the cells to HNE toxicity. Co-treatment of cardiomyocytes with BSO was found to completely block the D3T-mediated GSH elevation, which however failed to reverse the cytoprotective effects of D3T, suggesting that other cellular factor(s) might be involved in D3T cytotprotection. In this regard, D3T was shown to induce cellular aldose reductase (AR). Surprisingly, inhibition of AR by sorbinil failed to potentiate HNE toxicity in cardiomyocytes. In contrast, sorbinil dramatically augmented HNE cytotoxicity in cells with GSH depletion induced by BSO. Similarly, in BSO-treated cells, D3T cytoprotection was also largely reversed by sorbinil, indicating that AR played a significant role in detoxifying HNE only under the condition of GSH depletion in cardiomyocytes. Taken together, this study demonstrates that D3T can induce GSH, GST, and AR in cardiomyocytes, and that the above cellular factors appear to play differential roles in detoxification of HNE in cardiomyocytes.

Aplastic Anemia in a Professional Musician Exposed to Instrument Polish

Chemicals are known to cause toxin-induced aplastic anemia. However, some chemicals documented in only a few cases to possess only a possibility of toxic potential may also be responsible for the development of aplastic anemia. This report presents a case of a string musician with bone marrow failure. The patient used a certain type of polish (komalak) to shine his musical instrument and did this frequently. He presented with nasal bleeding, and a workup on admission revealed pancytopenia. Aplastic anemia was diagnosed on the basis of bone marrow histology results. An analysis for gene polymorphism related to the metabolic detoxification enzymes glutathione S-transferase and N-acetyltransferase 2 indicated that the patient was genetically susceptible to developing toxicity. This case suggests that frequent use of this polish may cause a toxic effect that leads to bone marrow failure. Musicians should be made aware of the risks associated with these types of chemicals.

Animal models for acquired bone marrow failure syndromes

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

Modulation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine-induced mutation in the cecum and colon of big blue rats by conjugated linoleic acid and 1,2-dithiole-3-thione

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a potent mutagen and suspected human carcinogen present in cooked protein-rich food. It preferentially induced colon tumors in male rats and mammary tumors in female rats. In the present study, the in vivo antimutagenic efficacy of two dietary compounds, conjugated linoleic acid (CLA) and 1,2-dithiole-3-thione (DTT), against PhIP was explored using 1acI transgenic Big Blue rats. Five- or six-week-old male Big Blue rats were fed a diet containing CLA (0.5%, wt/wt) or DTT (0.005%, wt/wt) starting one week before exposure to 200 ppm PhIP for 61 days. PhIP treatment induced a approximately 8- to 16-fold increase in the mutation frequency (MF) in the colon. The induced MF was significantly lower in the cecum than in the proximal and distal colon (approximately 52 x 10(-5) vs. 100 x 10(-5), p < 0.008). CLA and DTT significantly reduced the PhIP-induced MF in the distal colon (p < 0.05) by 14% and 24%, respectively. The frequency of -1 frameshift mutations was lower in the distal colon of CLA- or DTT-treated rats. This protective effect was not observed in the cecum or in the proximal colon. In contrast, the PhIP-induced MF in the cecum (specifically, the frequency of -1 frameshifts and GC-->TA transversions) was elevated by 43% after treatment with CLA. In conclusion, CLA and DTT modulate PhIP-induced mutagenesis in a tissue-specific manner, and different modulation pathways are employed by CLA and DTT.

Increased frequencies of glutathione S-transferase (GSTM1 and GSTT1) gene deletions in Korean patients with acquired aplastic anemia

Patients with reduced ability to metabolize environmental carcinogens or toxins may be at risk of developing aplastic anemia. Glutathione S-transferase (GST) has been implicated in detoxifying mutagenic electrophilic compounds. This study asked whether the homozygous gene deletions of GSTM1 and GSTT1 affect the likelihood of developing aplastic anemia. The incidence of GSTM1 and GSTT1 gene deletions was significantly higher for aplastic anemia patients (odds ratio [OR]: 3.1, P =.01 and OR: 3.1, P =.004, respectively) than for healthy controls. Among the aplastic anemia patients, 17.5% (10:57) had chromosomal abnormalities at the time of diagnosis, and all aplastic anemia patients with chromosomal abnormalities showed GSTT1 gene deletions (P =.048). Individuals with GSTM1 and GSTT1 gene deletions may have greater susceptibility to aplastic anemia. It is possible that genetic instability or chromosomal damage due to abnormal detoxification of environmental toxins might have worked as an important pathophysiologic mechanism of aplastic anemia for patients with GSTT1 gene deletions.

Effect of hydroquinone on meiotic segregation in Drosophila melanogaster females

The induction of sex chromosomes meiotic nondisjunction (ND) by hydroquinone (HQ) given orally was investigated in Drosophila melanogaster 2-7, 8-22, 24, 48, 72 and 96 h-old females. ND was assessed by a system where exceptional females (XXY) and only 1/4 of the expected regular progeny are viable. Oocytes were treated at different stages of development. 4% HQ tested only in 72 h-old females induced ND in oocytes sampled in brood I (mostly mature oocytes at metaphase I). 6% HQ increased ND in brood I of 8-22 h-old females, while other broods, (including cells treated at early prophase) were also affected in older flies, the highest significance being attained in the 48 h-old series. Newly hatched females (2-7 h-old) were refractory to the treatment, though oocytes sampled in the first three subcultures are comparable to cells showing enhancement of ND in series run with older females. Toxicity of 2, 4 and 6% HQ increased with concentration and females' age: (a) 2% was not toxic; (b) 4% was toxic only to 72 h-old females; (c) 6% was increasingly toxic to females 24, 48 and 72 h-old. The results indicate that age plays a significant role on both chromosomal segregation and toxicity and suggest that in Drosophila HQ is metabolized to its reactive species. The lack of toxic and aneugenic effect in very young females could reflect a more efficient detoxification due to the known high specific activity of glutathione-S-transferase (GST) after eclosion. The decline in GST activity around day 2 of adult life coincides with the high effect of HQ in 48 h-old females.

Bioreactivity of glutathionyl hydroquinone with implications to benzene toxicity

Glutathionyl hydroquinone (GHQ), a highly reactive metabolite of benzene, has been implicated as a causative intermediate of benzene toxicity. To substantiate, the bioreactivity of GHQ was investigated under in vitro and in vivo conditions using end points, characteristic of benzene toxicity. Under in vitro conditions, the presence of GHQ: (a) linearly increased the release of aldehydic products from L-glutamate or deoxyuridine at GHQ concentrations of 5-25 microM and from rat liver homogenates at GHQ concentrations of 50-250 microM; (b) cleaved plasmid pUC 18 supercoiled DNA through a single strand nick to yield open circular relaxed DNA, and through a double strand cut to give out linear DNA at GHQ concentrations of 25-200 microM, with evidence of protection by catalase and superoxide dismutase; and (c) induced cross-linking and polymerization of lymphocyte nuclear DNA through in situ generation of GHQ, which was protected by pretreatment of lymphocytes with N-ethylmaleimide. In vivo exposure of Swiss albino mice to GHQ (100 mg/kg, intraperitoneally once daily for 30 days) resulted in significant increase of liver weight and inhibition of mitotic index in the bone marrow. The other test parameters, namely spleen weight, hematological indices, hepatic sulphahydryl content and nonenzymatic lipid peroxidation, and chromosomal aberrations in the bone marrow were, however, unaffected by GHQ treatment. The observations indicate pro-oxidant and cytotoxic potential of GHQ, mediated by the reactive oxygen species generated during the course of its auto-oxidation. Bioreactivity of GHQ with cellular macromolecules in vitro and inhibition of mitotic index of bone marrow on in vivo exposure have relevance to benzene toxicity, although in situ generation of GHQ at the site of action appears critical in bringing about hematological and chromosomal effects that were probably spared due to rapid metabolic disposition and, consequently, poor bioavailability of intraperitoneally administered GHQ.

Persuasive evidence that quinone reductase type 1 (DT diaphorase) protects cells against the toxicity of electrophiles and reactive forms of oxygen

An extensive body of evidence supports the conclusion that by catalyzing obligatory two-electron reductions of quinones to hydroquinones, NAD(P)H:quinone reductase (QR1) protects cells against the deleterious effects of redox cycling of quinones, their ability to deplete glutathione, and to produce neoplasia. The effects of elevation of QR1 levels by various enzyme inducers, inhibition of the enzyme by dicumarol, and genetic deletion of the enzyme (knockout mouse) are all consistent with the proposed protective functions. Measurement of QR1 activity in murine hepatoma cells grown in 96-well microtiter plates has provided a rapid and quantitative method for detecting inducer activity and determining inducer potency. This constitutes a strategy for the identification of potential chemoprotectors against cancer. Epidemiological studies show that humans who are genetically deficient in QR1 are more susceptible to the hematological toxicity and carcinogenicity of benzene exposure, and may be more susceptible to the development of a number of malignant tumors.

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

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

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.

The National Exposure Registry: analyses of health outcomes from the benzene subregistry

The purpose of the National Exposure Registry is to assess the long-term health consequences to a general population from long-term, low-level exposures to specific substances in the environment. This study investigates the health outcomes of 1,143 persons (1,127 living, 16 deceased) living in south central Texas who had documented environmental exposure to benzene (up to 66ppb) in tap water. As with all subregistries, face-to-face interviews were used to collect self-reported information for 25 general health status questions. Using computer-assisted telephone interviewing, the same health questions were asked 1 year (Followup 1, F1) and 2 years later (Followup 2, F2). The health outcome rates for Baseline and Followup 1 and 2 data collections for the Benzene Subregistry were compared with national norms, that is, the National Health Interview Survey (NHIS) rates. For at least one of the three reporting periods, specific age and sex groups of the Benzene Subregistry population reported more adverse health outcomes when compared with the NHIS population, including anemia and other blood disorders, ulcers, gall bladder trouble, and stomach or intestinal problems, stroke, urinary tract disorders, skin rashes, diabetes, kidney disease, and respiratory allergies. Statistically significant deficits for the Benzene Subregistry population overall were found for asthma, emphysema, or chronic bronchitis; arthritis, rheumatism, or other joint disorders; hearing impairment; and speech impairment. No statistically significant differences between the two populations were seen for the outcomes hypertension; liver disease; mental retardation; or cancer. These results do not identify a causal relationship between benzene exposure and adverse health effects; however, they do reinforce the need for continued followup of registrants.

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.