Prevention of benzene-induced myelotoxicity and prostaglandin synthesis in bone marrow of mice by inhibitors of prostaglandin H synthase

Metabolomic Study of Urine from Workers Exposed to Low Concentrations of Benzene by UHPLC-ESI-QToF-MS Reveals Potential Biomarkers Associated with Oxidative Stress and Genotoxicity

Benzene is a human carcinogen whose exposure to concentrations below 1 ppm (3.19 mg·m^-3) is associated with myelotoxic effects. The determination of biomarkers such as trans-trans muconic acid (AttM) and S-phenylmercapturic acid (SPMA) show exposure without reflecting the toxic effects of benzene. For this reason, in this study, the urinary metabolome of individuals exposed to low concentrations of benzene was investigated, with the aim of understanding the biological response to exposure to this xenobiotic and identifying metabolites correlated with the toxic effects induced by it. Ultra-efficient liquid chromatography coupled to a quadrupole-time-of-flight mass spectrometer (UHPLC-ESI-Q-ToF-MS) was used to identify metabolites in the urine of environmentally (n = 28) and occupationally exposed (n = 32) to benzene (mean of 22.1 μg·m^-3 and 31.8 μg·m^-3, respectively). Non-targeted metabolomics analysis by PLS-DA revealed nine urinary metabolites discriminating between groups and statistically correlated with oxidative damage (MDA, thiol) and genetic material (chromosomal aberrations) induced by the hydrocarbon. The analysis of metabolic pathways revealed important alterations in lipid metabolism. These results point to the involvement of alterations in lipid metabolism in the mechanisms of cytotoxic and genotoxic action of benzene. Furthermore, this study proves the potential of metabolomics to provide relevant information to understand the biological response to exposure to xenobiotics and identify early effect biomarkers.

Addendum to the Final Report on the Safety Assessment of Hydroquinone

An assessment of the safety of Hydroquinone was first published in 1986 (J Am Coll Toxicol 5:123-65). The ingredient was found to be safe for use at limited concentrations for certain formulations. This addendum reviews new data and presents a revised conclusion regarding safety. Hydroquinone is an aromatic compound used principally in hair dyes and colors, but it is also in lipsticks, skin fresheners, and other skin care preparations. Hydroquinone in an aqueous solution was shown to be absorbed through human skin at a rate of 0.55 ± 0.13 μg/cm2/h. Hydroquinone is rapidly absorbed and excreted in urine in rats following oral administration. Absorption from an alcohol vehicle is greater than from an aqueous solution. Hydroquinone was found to be cytotoxic to rat hepatoma cells in culture, and nephrotoxic in male rats dosed orally by gavage. Oral administration of Hydroquinone to rats resulted in dose-dependent mortality, lethargy, tremors, and increased liver and kidney weights. Oral administration did not produce embryotoxic, fetotoxic, or teratogenic effects in rats. In rats, dermal application produced slight to severe irritation. In a guinea pig maximization test, induction with 2% Hydroquinone injected intradermal, followed by challenge with 0.5% Hydroquinone, showed extreme sensitization. In 80 patients known to be sensitive to aromatic compounds, 0.5% Hydroquinone elicited no reactions. Hydroquinone can cause depigmentation of skin. Various genotoxicity assays show that Hydroquinone can induce sister chromatid exchanges, chromosomal aberrations and loss, and increased frequency of mitotic crossovers. It also induced DNA strand breaks and inhibited DNA and RNA synthesis in rabbit bone marrow mitochondria. Forward mutation assays with or without metabolic activation were positive, but the results with the Ames test, a mouse test for somatic mutations, and other tests were negative. Hydroquinone, given to rats orally by gavage five times per week for up to 103 weeks at doses of 25 or 50 mg/kg, resulted in a significant increase of renal adenomas in males given 50 mg/kg and of mononuclear cell leukemia in females with both doses. At doses of 50 or 100 mg/kg on the same schedule, there was a significant increase in hepatocellular adenomas in both male and female mice. Other studies of Hydroquinone showed no significant difference in tumors between control and exposed groups, and marginal to no activity as a tumor promoter. It is concluded that Hydroquinone is safe at concentrations of ≤1% for aqueous cosmetic formulations designed for discontinuous, brief use followed by rinsing from the skin and hair. Hydroquinone should not be used in leave-on, nondrug cosmetic products.

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.

Role of hydroquinone-thiol conjugates in benzene-mediated toxicity

Hydroquinone (HQ) is a metabolite of benzene, and in combination with phenol (PHE), reproduces benzene myelotoxicity. HQ readily oxidizes to 1,4-benzoquinone (1,4-BQ) followed by the reductive addition of glutathione (GSH). Subsequent cycles of oxidation and GSH addition give rise to a variety of mono-, and multi-GSH substituted conjugates. Following administration of PHE/HQ (1.1 mmol/kg/0.9 mmol/kg, ip) to male Sprague-Dawley (SD) rats, 2-(glutathion-S-yl)HQ [GS-HQ], 2,5-bis-(glutathion-S-yl)HQ [2,5-GS-HQ], 2,6-bis-(glutathion-S-yl)HQ [2,6-GS-HQ], and 2,3,5-tris-(glutathion-S-yl)HQ [2,3,5-GS-HQ] were all identified in bone marrow. 2-(Cystein-S-ylglycine)HQ [2-(CysGly)HQ], 2-(cystein-S-yl)HQ [2-(Cys)HQ], and 2-(N-acetylcystein-S-yl)HQ [2-(NACys)HQ] were also found in the bone marrow of PHE/HQ and benzene treated rats and mice, indicating the presence of an active mercapturic acid pathway within bone marrow. Moreover, 2,6-GS-HQ and 2,3,5-GS-HQ were hematotoxic when administered to rats. All of the HQ-GSH conjugates retain the ability to redox cycle and generate reactive oxygen species (ROS), and to arylate target proteins. Recent in vitro and in vivo studies in our laboratory revealed lysine and arginine residues as primary targets of 1,4-BQ, GS-HQ and 2-(NACys)HQ adduction. In contrast 1,4-BQ-adduction of cysteine residues may be a transient interaction, where physiological conditions dictate adduct stability. The generation of ROS and alkylation of proteins may both contribute to benzene-mediated myelotoxicity, and the two processes may be inter-dependent. However, the precise molecular mechanism by which benzene and HQ-GSH conjugates induce hematotoxicity remains to be determined. Within 18h of administration of PHE/HQ to SD rats a significant decrease in blood lymphocyte count was observed. At this early time point, erythrocyte counts and hemoglobin concentrations remained within the normal range. Concomitant with the decrease in lymphocyte count, western blot analysis of bone marrow lysate, using HQ-GSH and 4-hydroxy-2-nonenal (4HNE) specific antibodies, revealed the presence of HQ-GSH- and 4HNE-derived protein adducts. Identification of these adducts is required before the functional significance of such protein modifications can be determined.

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.

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

Chronic exposure of humans to benzene (BZ), a widely used industrial chemical and a ubiquitous environmental pollutant, causes aplastic anemia and acute myeloid leukemia. The purpose of the studies reported here was to determine whether the observed depression of bone marrow (BM) cellularity in mice administered benzene was reflected in a suppression of development of all of the hematopoietic lineages and to confirm the ability of interleukin-1 alpha (IL-1 alpha) to prevent BZ-induced BM cell depression. We report that BZ, administered twice per day for 2 days to C57B1/6J mice at a dose of 600 mg/kg body weight, caused a significant depression of the total number of nucleated BM cells per femur when measured on day 3. The observed depression reflects a complex situation that represents the net effect of a decrease in the total number of cells of the lymphocytic and erythroid lineages, along with an increase in the number of intermediate and terminally differentiated cells of the granulocytic lineage. An experiment to monitor the effects of BZ over a 7-day period showed a progressive depressive effect on the lymphocytes and an initial depression of the erythroid cells at day 3 that remained constant until day 7. Conversely, the numbers of intermediate and terminally differentiated granulocytes progressively increased over the 7 days. The BM appeared to recover from the depressive effects of BZ immediately upon cessation of exposure, as the number of nucleated BM cells began to rise by day 5 and was equal to that of the control group by day 7. The results expand our earlier finding (Renz and Kalf 1991) that the overall depression of BM cellularity occurs because of an inability of the stromal fibroblast to produce colony-stimulating factors essential for stem and progenitor cell survival. This results from inhibition by the BZ metabolite, hydroquinone (HQ), of the processing of pre-IL-1 alpha to the mature cytokine.(ABSTRACT TRUNCATED AT 250 WORDS)

A perspective on benzene leukemogenesis

Although benzene is best known as a compound that causes bone marrow depression leading to aplastic anemia in animals and humans, it also induces acute myelogenous leukemia in humans. The epidemiological evidence for leukemogenesis in humans is contrasted with the results of animal bioassays. This review focuses on several of the problems that face those investigators attempting to unravel the mechanism of benzene-induced leukemogenesis. Benzene metabolism is reviewed with the aim of suggesting metabolites that may play a role in the etiology of the disease. The data relating to the formation of DNA adducts and their potential significance are analyzed. The clastogenic activity of benzene is discussed both in terms of biomarkers of exposure and as a potential indication of leukemogenesis. In addition to chromosome aberrations, sister chromatid exchange, and micronucleus formation, the significance of chromosomal translocations is discussed. The mutagenic activity of benzene metabolites is reviewed and benzene is placed in perspective as a leukemogen with other carcinogens and the lack of leukemogenic activity by compounds of related structure is noted. Finally, a pathway from exposure to benzene to eventual leukemia is discussed in terms of biochemical mechanisms, the role of cytokines and related factors, latency, and expression of leukemia.

Reduction of prostaglandin H synthase compound II by phenol and hydroquinone, and the effect of indomethacin

The reduction of prostaglandin H synthase compound II to native enzyme by phenol and by hydroquinone, in the presence of diethyldithiocarbamate as a stabilizing agent, was studied by rapid scan spectrometry and transient state kinetics at 4.0 +/- 0.5 degrees C in 0.1 M phosphate buffer, pH 8.0. The plot of pseudo-first-order rate constants for the conversion of prostaglandin H synthase compound II to native enzyme versus phenol concentration was linear with a non-zero intercept. The second-order rate constant was determined from the slope to be (5.3 +/- 0.3) x 10(5) M-1 s-1. For the reduction by hydroquinone, the second-order rate constant was determined from pointwise measurements of the pseudo-first-order rate constant to be (2.1 +/- 0.4) x 10(6) M-1 s-1. Rapid scan spectrum results also showed the reduction of compound I to compound II by both phenol and hydroquinone. Thus reduction of both compound I and compound II is one electron process. Our results suggest that the tyrosyl radical, detected in the presence of oxidizing agents, is formed by intramolecular electron transfer from the tyrosyl residue to the porphyrin pi-cation radical, and this reaction tends to disappear in the presence of sufficient reducing substrate. These in vitro results support speculation that there is a role of the peroxidase component of prostaglandin H synthase in benzene-induced toxicity. In the present work, the effect of indomethacin on the reduction of prostaglandin H synthase compound II by diethyldithiocarbamate, phenol, and hydroquinone was also investigated. Results revealed, for the first time, that indomethacin is an inhibitor of the peroxidase activity of prostaglandin H synthase, although not as effectively as in its well-known inhibition of cyclooxygenase activity.

Metabolic activation of hydroquinone by macrophage peroxidase

Lysates from macrophages, cells involved in hematopoiesis and immunological responses, catalyzed the metabolic activation of the benzene metabolite, hydroquinone, to protein-binding compounds and to free 1,4-benzoquinone. This reaction is mediated by a peroxidase since activation was dependent upon H2O2 and was prevented by the inhibitors aminotriazole and azide. Activation of hydroquinone was independent of HO. radicals since protein binding occurred in the presence of the HO. scavengers mannitol and dimethyl sulfoxide. In reactions with macrophage lysates, phenol, another hepatic metabolite of benzene, stimulated the production of 1,4-benzoquinone as well as the amount of hydroquinone equivalents bound to protein in a dose-dependent manner. Addition of cysteine to incubations with macrophage lysates resulted in a dose-dependent decrease in hydroquinone equivalents bound to protein. At 100 microM cysteine, protein binding was inhibited by 63% and this decrease was recovered as the monocysteine-hydroquinone conjugate. Macrophages catalyzed the arachidonic acid-mediated activation of hydroquinone to metabolites which bound to cellular macromolecules. This activation was inhibited by indomethacin indicating the action of prostaglandin synthase in hydroquinone metabolism by macrophages. The results of these experiments demonstrate that macrophage peroxidase catalyzes the metabolic oxidation of hydroquinone to 1,4-benzoquinone and that 1,4-benzoquinone and/or its semiquinone intermediate are binding to protein and cysteine. Hydroquinone activation by macrophages and subsequent macromolecular binding may be associated with the immunologic and hematopoietic toxicity of benzene.