Hydroquinone, a bioreactive metabolite of benzene, inhibits apoptosis in myeloblasts

MiR-146a affects the alteration in myeloid differentiation induced by hydroquinone in human CD34+ hematopoietic progenitor cells and HL-60 cells

The MiR-146a/TRAF6/NF-κB axis is important for the regulation of hematopoiesis and the immune system. To identify the key axis that regulates benzene-induced hematotoxicity or even leukemia, we investigated miR-146a expression in human CD34+ hematopoietic progenitor cells (HPCs) and human acute promyelocytic leukemia cells (HL-60) during the differentiation process. By performing a colony formation assay and flow cytometry on cells in the differentiation process after hydroquinone treatment, we found that hydroquinone induced a marked reduction of differentiation toward myeloid cells and immune cells in CD34+ cells (5 days exposure) as well as in HL-60 cells (3 h exposure). Further study using real-time PCR and western blot showed that the impaired myeloid differentiation was accompanied by the up-regulation of miR-146a and the down-regulation of TRAF6 and NF-κB. Using the miR-146a-5p inhibitor to suppress miR-146a expression could relieve the inhibitory effect on myeloid differentiation induced by hydroquinone to a certain extent. At the same time, the level of TRAF6 protein, as well as the phosphorylated IκBα protein which indicates NF-κB transcriptional activity was restored to the same levels as the control group. These results suggested that hydroquinone induced a dysregulation of miR-146a and its downstream NF-κB transcriptional factor pathway, which might be an early event in the generation of benzene-induced differentiation disturbance and subsequent leukemogenesis.

Small Molecule Active Site Directed Tools for Studying Human Caspases

Caspases are proteases of clan CD and were described for the first time more than two decades ago. They play critical roles in the control of regulated cell death pathways including apoptosis and inflammation. Due to their involvement in the development of various diseases like cancer, neurodegenerative diseases, or autoimmune disorders, caspases have been intensively investigated as potential drug targets, both in academic and industrial laboratories. This review presents a thorough, deep, and systematic assessment of all technologies developed over the years for the investigation of caspase activity and specificity using substrates and inhibitors, as well as activity based probes, which in recent years have attracted considerable interest due to their usefulness in the investigation of biological functions of this family of enzymes.

Influence of acetylsalicylic acid on hematotoxicity of benzene

OBJECTIVES

The aim of the study was to evaluate the influence of acetylsalicylic acid (ASA) on benzene hematotoxicity in rats.

MATERIALS AND METHODS

The study was carried out on rats exposed for 2, 4 and 8 weeks to benzene vapour at a concentration of 1.5 or 4.5 mmol/m(3) of air (5 days per week, 6 hours per day) alone or together with ASA at the doses of 5, 150 or 300 mg/kg body weight (per os).

RESULTS

Benzene at a concentration of 4.5 mmol/m(3) caused a slight lymphopenia, granulocytosis and reticulocytosis in blood. In bone marrow traits of megaloblastic renewal, presence of undifferentiated cells and giant forms of granulocytes as well as an increase in myeloperoxidase and decrease in chloroacetate esterase activity and lipids content were noted. ASA (150 and 300 mg/kg b.w.) influenced some of hematological parameters, altered by benzene intoxication. ASA limited the solvent-induced alteration in blood reticulocyte count and in the case of bone marrow in the erythroblasts count. Traits of megaloblastic renewal in bone marrow were less pronounced. Besides, higher activity of myeloperoxidase and the decrease in the level of lipids in granulocytes were noted.

CONCLUSION

Our results suggest that ASA limited the benzene-induced hematotoxicity.

Differential gene expression profiles of human leukemia cell lines exposed to benzene and its metabolites

Benzene is a well-known environmental pollutant that can induce hematotoxicity, aplastic anemia, acute myelogenous leukemia, and lymphoma. Benzene toxicity is likely mediated through metabolites induced by means of multiple pathways. Although benzene metabolites are known to induce oxidative stress and disrupt the cell cycle, the mechanism underlying leukemogenesis is not fully understood. The aim of this study was to analyze the genome-wide expression profiles of human promyelocytic leukemia HL-60 cells that had been exposed to benzene and its metabolites. This was carried out using whole human genome oligonucleotide microarrays to ascertain potential biomarkers. Genes that were differentially expressed (>1.5-fold and p-values <0.05) after exposure to benzene (BZ), hydroquinone (HQ), and 1,4-benzoquinone (BQ) were then classified with GO, KEGG and GSEA pathway annotation. All genes that were identified were then functionally categorized as being involved in the cell cycle, the p53 signaling pathway, apoptosis, the MAPK signaling pathway, or the T cell receptor signaling pathway. Functionally important genes were further validated by means of real-time RT-PCR. The results showed that EGR1, PMAIP1, AR, CCL2, CD69, HSPA8, SLC7A11, HERPUD1, ELK1, and MKI57 genes altered their expression profiles. Similar expression profiles were also found in human erythromyeloblastoid leukemia K562 cells and in human leukemic monocyte lymphoma U937 cells. In conclusion, gene expression profiles along with GO, KEGG and GSEA pathway annotation analysis have provided an insight into the leukemogenesis as well as highlighted potential gene-based biomarkers of human leukemia cell lines when they are exposed to benzene and its metabolites.

Benzene activates caspase-4 and -12 at the transcription level, without an association with apoptosis, in mouse bone marrow cells lacking the p53 gene

Benzene is a well-known environmental pollutant that can induce hematotoxicity, aplastic anemia, acute myelogenous leukemia, and lymphoma. However, although benzene metabolites are known to induce oxidative stress and disrupt the cell cycle, the mechanism underlying lympho/leukemogenicity is not fully understood. Caspase-4 (alias caspase-11) and -12 are inflammatory caspases implicated in inflammation and endoplasmic reticulum stress-induced apoptosis. The objectives of this study were to investigate the altered expression of caspase-4 and -12 in mouse bone marrow after benzene exposure and to determine whether their alterations are associated with benzene-induced bone marrow toxicity, especially cellular apoptosis. In addition, we evaluated whether the p53 gene is involved in regulating the mechanism, using both wild-type (WT) mice and mice lacking the p53 gene. For this study, 8-week-old C57BL/6 mice [WT and p53 knockout (KO)] were administered a benzene solution (150 mg/kg diluted in corn oil) via oral gavage once daily, 5 days/week, for 1 or 2 weeks. Blood and bone marrow cells were collected and cell counts were measured using a Coulter counter. Total mRNA and protein extracts were prepared from the harvested bone marrow cells. Then qRT-PCR and Western blotting were performed to detect changes in the caspases at the mRNA and protein level, respectively. A DNA fragmentation assay and Annexin-V staining were carried out on the bone marrow cells to detect apoptosis. Results indicated that when compared to the control, leukocyte number and bone marrow cellularity decreased significantly in WT mice. The expression of caspase-4 and -12 mRNA increased significantly after 12 days of benzene treatment in the bone marrow cells of benzene-exposed p53KO mice. However, apoptosis detection assays indicated no evidence of apoptosis in p53KO or WT mice. In addition, no changes of other apoptosis-related caspases, such as caspase-3 and -9, were found in WT or p53KO mice at the level of mRNA and proteins. These results indicated that upregulation of caspase-4 and -12 in mice lacking the p53 gene is not associated with cellular apoptosis. In conclusion, caspase-4 and -12 can be activated by benzene treatment without inducing cell apoptosis in mouse bone marrow, which are partly under the regulation of the p53 gene.

A Class of Benzenoid Chemicals Suppresses Apoptosis in C. elegans

Benzene is a human carcinogen that might act through both genotoxic and nongenotoxic mechanisms to promote tumorigenesis. The genotoxic effects of benzene are well established, however, its potential nongenotoxic roles in carcinogenesis are poorly understood. We find that benzene suppresses somatic apoptosis in C. elegans; this suggests a potential nongenotoxic mechanism by which this chemical might promote tumorigenesis. We find that two other benzenoid chemicals, biphenyl and toluene, also inhibit apoptosis in C. elegans. Notably, these chemicals are suspected carcinogens in mammals; this suggests that a subclass of benzenoid chemicals might promote tumorigenesis by suppressing apoptosis. A benzene metabolite, 1,4-benzoquinone, can directly inhibit the activity of caspase-3; this suggests a general molecular mechanism by which benzenoid chemicals might suppress apoptosis. These findings suggest that C. elegans is an excellent alternative animal model for studying the antiapoptotic activity of tumor-promoting chemicals and for identifying in vivo targets of these chemicals.

Case report: Hydroquinone and/or glutaraldehyde induced acute myeloid leukaemia?

BACKGROUND

Exposures to high doses of irradiation, to chemotherapy, benzene, petroleum products, paints, embalming fluids, ethylene oxide, herbicides, pesticides, and smoking have been associated with an increased risk of acute myelogenous leukemia (AML). Although there in no epidemiological evidence of relation between X-ray developer, fixer and replenisher liquids and AML, these included glutaraldehyde which has weakly associated with lymphocytic leukemia in rats and hydroquinone has been increasingly implicated in producing leukemia, causing DNA and chromosomal damage, inhibits topo-isomerase II, alter hematopoiesis and inhibit apoptosis of neoplastic cells.

CASE PRESENTATION

Two white females (A and B) hired in 1985 as medical radiation technologists in a primary care center, in Greece. In July 2001, woman A, 38-years-old, was diagnosed as having acute monocytic leukaemia (FAB M5). The patient did not respond to therapy and died three weeks later. In August 2001, woman B, 35-year-old, was diagnosed with acute promyelocytic leukaemia (FAB M3). Since discharge, she is in continuous complete remission. Both women were non smokers without any medical history. Shortly after these incidents official inspectors and experts inspected workplace, examined equipment, archives of repairs, notes, interviewed and monitored employees. They concluded that shielding was inadequate for balcony's door but personal monitoring did not show any exceeding of TLV of 20 mSv yearly and cytogenetics analysis did not reveal findings considered to be characteristics of ionizing exposure. Equipment for developing photos had a long list of repairs, mainly leakages of liquids and increases of temperature. On several occasions the floor has been flooded especially during 1987-1993 and 1997-2001. Inspection confirmed a complete lack of ventilation and many spoiled medical x-ray films. Employees reported that an "osmic" level was continuously evident and frequently developed symptoms of respiratory irritation and dizziness.

CONCLUSION

The findings support the hypothesis that the specific AML cases might have originated from exposure to chemicals, especially hydroquinone and/or glutaraldehyde. The report also emphasises the crucial role of inspection of facilities and enforcement of compliance with regulations in order to prevent similar incidents.

Effect of in vivo phenol or hydroquinone exposure on events related to neutrophil delivery during an inflammatory response

Phenol (PHE) and hydroquinone (HQ) are metabolites of benzene that affect leukocytes after solvent intoxication. Hence, we investigated the effects of PHE or HQ exposure on neutrophil mobilization during an inflammatory response. Male Wistar rats received intraperitoneal injections of PHE, HQ or vehicle only and assays were performed 24 h after the last dose. Quantifications of bone marrow or circulating leukocytes showed that only HQ exposure induced neutrophilia, probably due to the accelerated mobilization from the bone marrow compartment, since reduced numbers of segmented cells in the last phase of maturation were detected there. Intravital microscopy showed that circulating leukocytes of HQ-exposed rats increased their rolling behavior and adherence to the mesenteric postcapillary venule wall in vivo. The enhanced leukocyte-endothelium interaction was not dependent on microvascular reactivity or perivascular mast cell degranulation. Instead, it was the result of neutrophil activation, demonstrated by a decrease in L-selectin and an increase in beta2 integrin expression on neutrophil membranes. This pattern of neutrophil activation may have contributed to the higher number of neutrophils in the subcutaneous inflammatory response of HQ-exposed rats after oyster glycogen injection. Taken together, our results indicate that HQ exposure alters neutrophil mobilization, which results in an exacerbated response after an injury. Although PHE is endogenously metabolized to HQ, PHE exposure only induced an increment in rolling behavior, which was not sufficient to alter the inflammatory response.

Hydroquinone and its analogues in dermatology - a potential health risk

Hydroquinone has been used for decades as a skin lightening agent. Since January 1, 2001, its use in cosmetics has been banned. This ban is as a result of mid-term effects such as leukoderma-en-confetti/occupational vitiligo and exogenous ochronosis. However, a recent literature search on hydroquinone as a skin lightening agent suggests that possible long-term effects such as carcinogenesis may be expected as well. Metabolites of hydroquinone formed in the liver, e.g., p-benzoquinone and glutathione conjugates of hydroquinone, are mainly responsible for this. In the bone marrow, hydroquinone is oxidized into p-benzoquinone because of the high myeloperoxidase activity. Topically applied hydroquinone-containing creams may give rise to accumulation of these compounds, which can cause DNA damage and mutations. They also have the capability to disrupt protective mechanisms, whereby they facilitate further development of cancer. In the bone marrow, long-term effects such as aplastic anemia and acute myeloid leukemias may occur. Most of the evidence stems from research on benzene toxicity, which appears to arise via its metabolite hydroquinone. There is no report yet demonstrating carcinogenesis resulting from the application of hydroquinone-containing creams. However doctors should be aware of these potential health risks which were up until now disregarded.

Benzoquinone activates the ERK/MAPK signaling pathway via ROS production in HL-60 cells

Benzene (BZ) is a class I carcinogen and its oxidation to reactive intermediates is a prerequisite of hematoxicity and myelotoxicity. The generated metabolites include hydroquinone, which is further oxidized to the highly reactive 1,4-benzoquinone (BQ) in bone marrow. Therefore, we explored the mechanisms underlying BQ-induced HL-60 cell proliferation by studying the role of BQ-induced reactive oxygen species (ROS) in the activation of the ERK-MAPK signaling pathway. BQ treatment (0.01-30 microM) showed that doses below 10 microM did not significantly reduce viability. ROS production after 3 microM BQ treatment increased threefold; however, catalase addition reduced ROS generation to basal levels. FACS analysis showed that BQ induced a fivefold increase in the proportion of cells in S-phase. We also observed a high proportion of Bromodeoxyuridine (BrdU) stained cells, indicating a higher DNA synthesis rate. BQ also produced rapid and prolonged phosphorylation of ERK1/2 proteins. Simultaneous treatment with catalase or PD98059, a potent MEK protein inhibitor, reduced cell recruitment into the S-phase and also abolished the ERK1/2 protein phosphorylation induced by BQ, suggesting that MEK/ERK is an important pathway involved in BQ-induced ROS mediated proliferation. The prolonged activation of ERK1/2 contributes to explain the increased S-phase cell recruitment and to understand the leukemogenic processes associated with exposure to benzene metabolites. Thus, the possible mechanism by which BQ induce HL-60 cells to enter the cell cycle and proliferate is linked to ROS production and its growth promoting effects by specific activation of regulating genes known to be activated by redox mechanisms.

Xenobiotic Metabolism and the Mechanism(s) of Benzene Toxicity

The investigation of the mechanism(s) of benzene toxicity/leukemogenesis over the past 50 years has been contemporaneous with developments in the study of xenobiotic metabolism. Research on the cytochrome P450 (CYP) enzyme system, and related systems in vivo and in vitro, which culminated in the isolation and reconstitution of the many CYPs, established pathways for the study of xenobiotic metabolism and its relationship to the biological activity of many chemicals. The essential role for metabolism of benzene as a precursor to the demonstration of benzene toxicity led to extensive studies of benzene metabolism, many of which will be reviewed here. Benzene toxicity/leukemogenesis, however, is a function of the bone marrow, a site remote from the liver where most benzene metabolism occurs. Studies of benzene metabolism have delineated the array of metabolites which appear to play a role in bone marrow damage, but further studies, both in vivo and in vitro, using appropriate animal models, will be needed to fully understand the impact of benzene and its metabolites on bone marrow function.

Exposure of Hematopoietic Stem Cells to Benzene or 1,4-Benzoquinone Induces Gender-Specific Gene Expression

Chronic exposure to benzene results in progressive decline of hematopoietic function and may lead to the onset of various disorders, including aplastic anemia, myelodysplastic syndrome, and leukemia. Damage to macromolecules resulting from benzene metabolites and misrepair of DNA lesions may lead to changes in hematopoietic stem cells (HSCs) that give rise to leukemic clones. We have shown previously that male mice exposed to benzene by inhalation were significantly more susceptible to benzene-induced toxicities than females. Because HSCs are targets for benzene-induced cytotoxicity and genotoxicity, we investigated DNA damage responses in HSC from both genders of 129/SvJ mice after exposure to 1,4-benzoquinone (BQ) in vitro or benzene in vivo. 1,4-BQ is a highly reactive metabolite of benzene that can cause cellular damage by forming protein and DNA adducts and producing reactive oxygen species. HSCs cultured in the presence of 1,4-BQ for 24 hours showed a gender-independent, dose-dependent cytotoxic response. RNA isolated from 1,4-BQ-treated HSCs and HSCs from mice exposed to 100 ppm benzene by inhalation showed altered expression of apoptosis, DNA repair, cell cycle, and growth control genes compared with unexposed HSCs. Rad51, xpc, and mdm-2 transcript levels were increased in male but not female HSCs exposed to 1,4-BQ. Males exposed to benzene exhibited higher mRNA levels for xpc, ku80, ccng, and wig1. These gene expression differences may partially explain the gender disparity in benzene susceptibility. HSC culture systems such as the one used here will be useful for testing the hematotoxicity of various substances, including other benzene metabolites.

Dysregulation of apoptosis by benzene metabolites and their relationships with carcinogenesis

Benzene is a widely recognized human carcinogen, the effect of which is attributed to the production of reactive oxygen species (ROS) from its metabolites. Although there have been many reports on the relationship between DNA damage induced by benzene metabolites and carcinogenesis, only a report approached the subject by examining the benzene-induced dysregulation of apoptosis. Inhibition of apoptosis, aberrantly prolonging cell survival, may contribute to cancer by facilitating the insurgence of mutations and by creating a permissive environment for genetic instability. In this study, we examined the mechanism of antiapoptotic effects by benzene metabolites, p-benzoquinone (BQ) and hydroquinone (HQ), and their relationships with carcinogenesis. BQ and HQ inhibited the apoptotic death of NIH3T3 cells induced by both serum starvation and lack of an extracellular matrix (ECM). An antioxidant agent, N-acetylcysteine, significantly inhibited the antiapoptotic effects induced by benzene metabolites, indicating that the effects were mainly due to the production of ROS. Furthermore, BQ and HQ inhibited the in vitro caspase-3 activation, suggesting that the inhibition of caspase-3 activation due to ROS produced by BQ- and HQ-treatment was related to the suppression of apoptosis. The cells that escaped apoptosis could survive with the addition of serum and attachment to the ECM. Levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine were higher in the cells which survived after BQ- and HQ-treatment than in the normal cells. Furthermore, the cells treated with BQ and HQ showed greater proliferation than normal cells under low-serum conditions and anchorage-independent growth in soft agar. These findings suggested that benzene metabolites induced dysregulation of apoptosis due to caspase-3 inhibition, which contributes to carcinogenesis.

Gene expression profile in bone marrow and hematopoietic stem cells in mice exposed to inhaled benzene

Acute myeloid leukemia and chronic lymphocytic leukemia are associated with benzene exposure. In mice, benzene induces chromosomal breaks as a primary mode of genotoxicity in the bone marrow (BM). Benzene-induced DNA lesions can lead to changes in hematopoietic stem cells (HSC) that give rise to leukemic clones. To gain insight into the mechanism of benzene-induced leukemia, we investigated the DNA damage repair and response pathways in total bone marrow and bone marrow fractions enriched for HSC from male 129/SvJ mice exposed to benzene by inhalation. Mice exposed to 100 ppm benzene for 6h per day, 5 days per week for 2 week showed significant hematotoxicity and genotoxicity compared to air-exposed control mice. Benzene exposure did not alter the level of apoptosis in BM or the percentage of HSC in BM. RNA isolated from total BM cells and the enriched HSC fractions from benzene-exposed and air-exposed mice was used for microarray analysis and quantitative real-time RT-PCR. Interestingly, mRNA levels of DNA repair genes representing distinct repair pathways were largely unaffected by benzene exposure, whereas altered mRNA expression of various apoptosis, cell cycle, and growth control genes was observed in samples from benzene-exposed mice. Differences in gene expression profiles were observed between total BM and HSC. Notably, p21 mRNA was highly induced in BM but was not altered in HSC following benzene exposure. The gene expression pattern suggests that HSC isolated immediately following a 2 weeks exposure to 100 ppm benzene were not actively proliferating. Understanding the toxicogenomic profile of the specific target cell population involved in the development of benzene-associated diseases may lead to a better understanding of the mechanism of benzene-induced leukemia and may identify important interindividual and tissue susceptibility factors.

Autonomous growth of committed hematopoietic progenitors from peripheral blood of workers exposed to low levels of benzene

We investigated whether exposure to low levels of benzene (geometric mean of exposures = 0.28-0.41 parts per million) results in perturbations of the hemopoietic system found in patients with myeloproliferative diseases. Erythroid (BFU-E) and granulocyte-monocyte (CFU-GM) colonies from peripheral blood were grown in methylcellulose with and without the addition of cytokines (erythropoietin and granulocyte-stimulating factor). Colony growth from 17 workers exposed to low levels of benzene was compared with 20 healthy control subjects. Exposed workers had significantly increased growth of autonomous BFU-E and unstimulated CFU-GM when compared with the control subjects. Unexposed smokers had increased colony growth without the addition of cytokines. Colony growth was not significantly different between the groups after the addition of cytokines. Workers exposed to low concentrations of benzene and unexposed smokers have increased cytokine-independent hematopoiesis.

Caspase inhibitors as anti-inflammatory and antiapoptotic agents

The striking efficacy of Z-VAD-fmk in the various animal models presented above may reflect its ability to inhibit multiple enzymes including caspases. In accord with this, more selective, reversible inhibitors usually show low efficacy in multifactorial models such as ischaemia, but may offer some protection against NMDA-induced excitotoxicity and hepatitis. Importantly, caspase inhibitors may exhibit significant activity in vivo even when they are applied post insult. As far as the CNS is concerned, the first systemically active inhibitors have emerged. Functional recovery could be achieved in some ischaemia models, but long-term protection by caspase inhibitors is still being questioned. Recent developments in drug design enabled the first caspase inhibitors to enter the clinic. Although initially directed towards peripheral indications such as rheumatoid arthritis, caspase inhibitors will no doubt eventually be used to target CNS disorders. For this purpose the peptidic character of current inhibitors will have to be further reduced. Small molecule, nonpeptidic caspase inhibitors, which have appeared recently, indicate that this goal can be accomplished. Unfortunately, many fundamental questions still remain to be addressed. In particular, the necessary spectrum of inhibitory activity required to achieve the desired effect needs to be determined. There is also a safety aspect associated with prolonged administration. Therefore, the next therapeutic areas for broader-range caspase inhibitors are likely to involve acute treatment. Recent results with synergistic effects between MK-801 and caspase inhibitors in ischaemia suggest that caspase inhibitors may need to be used in conjunction with other drugs. It can be expected that, in the near future, research on caspases and their inhibitors will remain a rapidly developing area of biology and medicinal chemistry. More time, however, may be needed for the first caspase inhibitors to appear on the market.

Differential involvement of caspases in hydroquinone-induced apoptosis in human leukemic hl-60 and jurkat cells

The benzene metabolite hydroquinone (HQ) is postulated to exert its myelotoxicity by bioactivation to reactive quinone derivatives in myeloperoxidase (MPO)-containing cells. In this study, the role of caspases in hydroquinone-induced apoptosis in MPO-rich HL-60 promyelocytic leukemia and MPO-deficient Jurkat T-lymphoblastic leukemia cells was investigated. HQ-induced apoptosis in both cell types was accompanied by phosphatidylserine (PS) exposure, caspases-3/-7 activation, PARP cleavage, DNA fragmentation, and ultrastructural changes as assessed by electron microscopy. In HL-60 cells, the general caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone (Z-VAD.FMK) blocked activation of caspases-3/-7, cleavage of PARP, and DNA, but PS externalization and cytoplasmic changes were not significantly affected. In marked contrast, all features of apoptosis were completely inhibited by Z-VAD.FMK in HQ-treated Jurkat cells. These data provide evidence for Z-VAD.FMK-insensitive and caspases-3/-7-independent pathway(s) in the externalization of PS and cytoplasmic changes during HQ-induced apoptosis in HL-60 cells. In contrast, in Jurkat cells, all of these changes required caspase activation. The ability of HQ to induce equivalent apoptosis in both MPO-deficient Jurkat cells and MPO-rich HL-60 cells demonstrates that MPO-catalyzed bioactivation of HQ is not a prerequisite for toxicity. The differential mechanisms of apoptosis in HL-60 and Jurkat T cells may reflect the MPO activity of these cells and, as a result, the amount of reactive BQ and other metabolites that are generated.

DNA-Protein Crosslink and DNA Strand Break Formation in HL-60 Cells Treated with Trans,trans-Muconaldehyde, Hydroquinone and Their Mixtures

The toxicity of benzene, a human leukemogen and ubiquitous environmental pollutant, is mediated in part by ring-hydroxylated metabolites including hydroquinone (HQ) and ring-opened metabolites including trans,trans-muconaldehyde (muconaldehyde, MUC), and their interactions. DNA-protein crosslinks (DNAPC) and DNA strand breaks (DNASB) are toxic lesions associated with the mechanism(s) of toxicity of carcinogenic compounds. In the present studies, we examined the hypothesis that individual and interactive effects of MUC and HQ are involved in the formation of DNAPC and DNASB. We extended our previous studies on DNAPC induction by MUC in HL-60 cells to HQ and mixtures of MUC and HQ, and determined DNASB levels, including 3'OH DNASB. Treatment of HL-60 cells with 25 to 100 μM HQ followed by incubation for 4 hours resulted in 1.3- to 2.8-fold increases in DNAPC levels compared with control, as determined by a K+/sodium dodecyl sulfate (SDS) precipitation assay. At 25 and 100 μM, MUC was 1.8 and 4.9 fold more effective at inducing DNAPC than HQ. Treatment with equimolar mixtures of 25 or 50 μM MUC and HQ resulted in higher DNAPC formation relative to the DNAPC levels expected if the effects were only additive. 3'OH DNASB levels as determined by the TUNEL assay showed a significant concentration-dependent increase 1 hour after treatment with 5 to 25 μM MUC, whereas HQ treatment had no effect. Cotreatment with 25 and 50 μM MUC/HQ mixtures resulted in significant decreases in TUNEL labeling relative to treatment with MUC alone. HL-60 cells treated with 1 to 50 μM MUC or HQ exhibited concentration- and time-dependent increases in DNASB as determined by the FADU assay, which measures a variety of single- and double-strand breaks and alkali labile sites. Exposure to 10 μM MUC gave Qdnasb values (1 Qdnasb ≍100 DNASB/cell) of 7.5 ± 1.2 and 15.4 ± 1.4 at the 1- and 2-hour time points respectively, compared with 0.1 ± 3.8 and 0.0 ± 1.5 for the corresponding time controls. The Qdnasb values after treatment with 10 μM HQ were 4.4 ± 0.7 and 17.7 ± 2.1 at the 1- and 2-hour time points, respectively, compared with 0.0 ± 0.5 and 0.0 ± 1.3 for the corresponding time controls. Induction of DNASB was additive 1 hour after treatment with equimolar MUC/HQ mixtures of 5 to 50 μM. These in vitro findings are significant in that DNAPC and DNASB lesions induced by MUC and HQ as well as their interactions could contribute to benzene-induced hematotoxicity and leukemogenesis.

Modulation of pro- and anti-apoptotic genes in lymphocytes exposed to bone cements

The ability of bone cements to modify the apoptotic program in activated immune cells and the mechanisms by which they act were evaluated. Mononuclear cells were collected from healthy individuals, cultured for 4 and 24 h with phytohemoagglutinina-P and cement extracts and then tested to assess: (a) cell viability; (b) early apoptotic events, by Annexin V/propidium iodide staining; and (c) the expression of pro- (p53, c-myc, ICE) and anti-apoptotic (bcl-2) genes. After 4 h three cements were able to increase significantly the percentage of apoptotic cells, while after 24 h no differences were found. The proportion of dead cells was not significantly changed at either culture time. The simultaneous expression of both pro-apoptotic (ICE, c-myc, p53) and antiapoptotic genes (bcl-2) was investigated only with regard to the materials which induced significant changes in apoptosis: two cements induced the p53 expression, while the third down-regulated bcl-2. As apoptosis regulates the balance of immune response, the authors recommend that the interaction between materials and immune cells should be assessed, so that the use of pro-apoptotic materials may be avoided in patients with immune defects.

Benzene and non-Hodgkin's lymphoma

Incidence rates for non-Hodgkin's lymphoma (NHL) have been rising throughout the world for several decades, and no convincing explanation exists for the majority of this increase. The commonest subtypes of NHL have no well-defined aetiological factors but lymphoma development has been linked with exposure to a variety of chemicals, including nitrates, pesticides, herbicides, and solvents. Benzene, a solvent and important constituent of petrochemical products, is a potent lymphomagen in experimental animals and high-dose exposure in humans is associated with both acute myeloid leukaemia and NHL. Much current interest centres on the possibility that environmental benzene exposure in the general public may underlie a proportion of the increase in NHL. Seventy per cent of benzene exposure in the environment is derived from vehicle exhaust emissions, whose increase has closely paralleled the rise in frequency of the disease. Mathematical modelling has been used to calculate an acceptable concentration of benzene in air based on risk estimates derived from industrial exposure, but the recommended target concentration in the U.K. of 1 ppb is regularly exceeded in urban locations. Detailed investigation of the health effects of low-level benzene exposure awaits an accurate assay for quantifying long-term human exposure. The (32)P post-labelling technique for the detection of toxin-specific DNA adducts is extremely sensitive and has been applied in the biomonitoring of exposure to a number of carcinogens, but benzene-DNA adducts have to date proved elusive of detection.

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.

Enhancement of myeloid cell growth by benzene metabolites via the production of active oxygen species

In low concentrations, benzene and its metabolite hydroquinone are known to have diverse biological effects on cells, including the synergistic stimulation with GM-CSF of hematopoietic colony formation in vitro, stimulation of granulocytic differentiation in vitro and in vivo, and general suppression of hematopoiesis in vivo. These chemicals are also known to be active in the induction of active oxygen species. We used several assays to determine the effects of benzene metabolites (hydroquinone, benzenetriol, benzoquinone) and active oxygen species (xanthine/xanthine oxidase) on cell growth and cell cycle kinetics of the human myeloid cell line HL-60. HL-60 cells treated with these chemicals for 2 h in PBS showed increased growth over untreated controls in a subsequent 18h growth period in complete media. Incorporation of 3H-thymidine was also increased proportionately by these treatments. Catalase treatment abrogated the increased cell growth of all chemicals, suggesting an oxidative mechanism for the effect of all treatments alike. Cell cycle kinetics assays showed that the growth increase was caused by an increased recruitment of cells from G0/G1 to S-phase for both hydroquinone and active oxygen, rather than a decrease in the length of the cell cycle. Benzene metabolite's enhancement of growth of myeloid cells through an active oxygen mechanism may be involved in a number of aspects of benzene toxicity, including enhanced granulocytic growth and differentiation, stimulation of GM-CSF-induced colony formation, apoptosis inhibition, and stimulation of progenitor cell mitogenesis in the bone marrow. These effects in sum may be involved in the benzene-induced "promotion" of a clonal cell population to the fully leukemic state.

Novel cytokine release inhibitors. Part I: Triterpenes

Tripterine and closely related triterpenoid derivatives as IL-1 beta release inhibitors are discussed.

Scientific update on benzene

The mechanism of benzene toxicity has been extremely difficult to fully characterize. Much progress has been made in assessing the relative potency of benzene metabolites but specific pathways to leukemia remain to be determined. Metabolite and mechanistic studies will have to focus on aplastic anemia and MDS and separate endpoints. This may serve to clarify the array of metabolite effects and consequent disparate effects. Biomarker research can contribute to the understanding of the toxicity process. The significance of understanding benzene toxicity will also lead to better clinical treatment of aplastic anemia and therapy-related MDS and AML, detection of populations particularly susceptible to benzene toxicity, screening of populations with suspected or unknown exposures, and determination of meaningful values for occupational and individual health risk while effectively monitoring ongoing exposures for early signs of toxicity.