Metabolism of benzene and phenol in macrophages in vitro and the inhibition of RNA synthesis by benzene metabolites

False positives and false negatives in benzene biological monitoring

Benzene is a commonly used industrial chemical that is a significant environmental pollutant. Occupational health specialists and industrial toxicologists are concerned with determining the exact amount of exposure to chemicals in the workplace. There are two main approaches to assess chemical exposure; air monitoring and biological monitoring. Air monitoring has limitations, which biological monitoring overcomes and could be used as a supplement to it. However, there are several factors that influence biological monitoring results. It would be possible to assess exposure more accurately if these factors were taken into account. This study aimed to review published papers for recognizing and discussing parameters that could affect benzene biological monitoring. Two types of effects can be distinguished: positive and negative effects. Factors causing positive effects will increase the metabolite concentration in urine more than expected. Furthermore, the parameters that decrease the urinary metabolite level were referred to as false negatives. From the papers, sixteen influential factors were extracted that might affect benzene biological monitoring results. Identified factors were clarified in terms of their nature and mechanism of action. It is also important to note that some factors influence the quantity and quality of the influence of other factors. As a result of this study, a decision-making protocol was developed for interpreting the final results of benzene biological monitoring.

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.

Bone marrow progenitor cells from chemically exposed workers display an intrinsic ability for autonomous proliferation

In this study, the autonomous proliferation of bone marrow progenitor cells (CFU-C), a pathological phenomenon observed in many hematological abnormalities, was investigated in 31 individuals who had been diagnosed as having neutropenia. Of these subjects, 18 had been chronically exposed (range of exposure 5-30 years) to a variety of petroleum distillates. We observed that the group of exposed individuals presented higher numbers of autonomous CFU-C when compared with those unexposed subjects. In addition, follow-up data demonstrated that 20% of the exposed population (4 of the 18) developed malignant hematological diseases. The autonomous CFU-C obtained from all individuals studied was composed predominantly of macrophages. This suggests an involvement of these cells in the development of hematological abnormalities, probably as a result of increased production of chemical myelotoxic metabolites.

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.

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.

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.

Immunological and neurobiochemical alterations induced by repeated oral exposure of phenol in mice

Phenol, a major metabolite of benzene, is a potentially immunotoxic and neurotoxic substance of environmental significance. Male CD-1 mice were continuously exposed to 0, 4.7, 19.5, and 95.2 mg phenol/l in drinking water for 4 weeks. Various immune functions were evaluated and levels of selected neurotransmitters and metabolites measured in discrete brain regions. The doses of phenol did not produce any overt clinical signs of toxicity; peripheral red blood cell counts and hematocrits decreased. A dose of 95.2 mg/l suppressed the stimulation of cultured splenic lymphocytes by lipopolysaccharide, pokeweed mitogen, and phytohemagglutinin and the response in mixed lymphocyte cultures. The two high doses suppressed antibody production response to the T cell-dependent antigen (sheep erythrocytes), as determined by plaque-forming cells, and serum antibody levels. Mice treated with phenol had lower levels of neurotransmitters in several brain regions. In the hypothalamus, a major norepinephrine-containing compartment, the concentrations of norepinephrine significantly decreased by 29 and 40% in groups dosed with 19.5 and 95.2 mg/l, while dopamine concentrations decreased in the corpus striatum by 21, 26, and 35% at 4.7, 19.5 and 95.2 mg/l, respectively. Phenol also decreased 5-hydroxytryptamine in the hypothalamus, medulla oblongata, midbrain and corpus striatum. Levels of monoamine metabolites decreased in the hypothalamus (5-hydroxyindoleacetic acid), midbrain (vanillylmandelic acid), corpus striatum (vanillylmandelic acid and dihydroxyphenylacetic acid), cortex (vanillylmandelic acid), and cerebellum (dihydroxyphenylacetic acid).

The toxicity of benzene and its metabolism and molecular pathology in human risk assessment

Benzene, a common industrial chemical and a component of gasoline, is radiomimetic and exposure may lead progressively to aplastic anaemia, leukaemia, and multiple myeloma. Although benzene has been shown to cause many types of genetic damage, it has consistently been classified as a non-mutagen in the Ames test, possibly because of the inadequacy of the S9 microsomal activation system. The metabolism of benzene is complex, yielding glucuronide and sulphate conjugates of phenol, quinol, and catechol, L-phenylmercapturic acid, and muconaldehyde and trans, trans-muconic acid by ring scission. Quinol is oxidised to p-benzoquinone, which binds to vital cellular components or undergoes redox cycling to generate oxygen radicals; muconaldehyde, like p-benzoquinone, is toxic through depletion of intracellular glutathione. Exposure to benzene may also induce the microsomal mixed function oxidase, cytochrome P450 IIE1, which is probably responsible for the oxygenation of benzene, but also has a propensity to generate oxygen radicals. The radiomimetic nature of benzene and its ability to induce different sites of neoplasia indicate that formation of oxygen radicals is a major cause of benzene toxicity, which involves multiple mechanisms including synergism between arylating and glutathione-depleting reactive metabolites and oxygen radicals. The occupational exposure limit in the United Kingdom (MEL) and the United States (PEL) was 10 ppm based on the association of benzene exposure with aplastic anaemia, but recently was lowered to 5 ppm and 1 ppm respectively, reflecting a concern for the risk of neoplasia. The American Conference of Governmental Industrial Hygienists (ACGIH) has even more recently recommended that, as benzene is considered an A1 carcinogen, the threshold limit value (TLV) should be decreased to 0.1 ppm. Only one study in man, based on nine cases of benzene associated fatal neoplasia, has been considered suitable for risk assessment. Recent re-evaluation of these data indicated that past assessments may have overestimated the risk, and different authors have considered that lifetime exposure to benzene at 1 ppm would result in an excess of leukaemia deaths of 9.5 to 1.0 per 1000. Although in this study, deaths at low levels of benzene exposure were associated with multiple myeloma and a long latency period, instead of leukaemia, which might justify further lowering of the exposure limit, the risk assessment model has been found to be non-significant for response at low levels of exposure. The paucity of data for man, the complexity of the metabolic activation of benzene, the interactive and synergistic mechanisms of benzene toxicity and carcinogenicity, the different disease endpoints (aplastic anaemia, leukaemia, and multiple myeloma), and different individual susceptibilities, all indicate that in such a complex scenario, regulators should proceed with caution before making further changes to the exposure limit for this chemical.

The changes of BPA level in 31 cases of children with aplastic anaemia and its clinical significance

Burst-promoting activity (BPA) was measured in the sera from 31 children with aplastic anaemia (AA). BPA levels were elevated in most of the children with AA (65.2%), the mean value (137.7 +/- 18.4%) being significantly higher than that in normal children (69.6 +/- 9.4%), in children in the recovery period and in children with non-aplastic anaemia. There was a negative relationship between the BPA level in children with AA and the peripheral haemoglobin concentration. The BPA level was higher in those whose duration of illness was shorter than 1 year. In three cases of AA caused by chloramphenicol and benzene hexachloride and one case of congenital pure red cell AA, the BPA level was not elevated. Eleven patients received fetal liver cell suspensions intravenously (FLI). After FLI the BPA level in their sera was significantly reduced. According to these results, it appears that the elevation of BPA level is a special phenomenon of AA. The measurement of BPA in serum is helpful for differentiation between AA and other kinds of anaemia. The elevation of the BPA level in serum is a biological compensation for the haematopoietic disorder, and the measurement of BPA in the serum of patients with AA may be helpful in evaluating the haematopoietic condition.

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

Administration of benzene to mice causes bone marrow toxicity and elevations in prostaglandin E2 (PGE2), a negative regulator of myelopoiesis. In these experiments, benzene (400 mg/kg; 2 x/day for 2 days) administered to DBA/2 or C57Bl/6 mice decreased bone marrow cellularity and myeloid progenitor cell development (measured as colony-forming units per femur) by 40%. When inhibitors of the cyclooxygenase component of prostaglandin H synthase (PHS) (either indomethacin, 2 mg/kg; aspirin, 50 mg/kg; meclofenamate, 4 mg/kg) were coadministered with benzene, myelotoxicity and the elevation in bone marrow PGE level were prevented. Additionally, when indomethacin (1 microM) was added to cultures of bone marrow cells from benzene-treated mice, myeloid progenitor cell development was the same as the controls. The doses of indomethacin used had no affect on the hepatic conversion of benzene to its major metabolite, phenol. Using purified PHS, indomethacin (10 microM) inhibited the arachidonic acid-dependent oxidation of hydroquinone to p-benzoquinone, a putative reactive metabolite of benzene. Indomethacin (10 microM) had no effect on the H2O2-driven oxidation of hydroquinone catalysed by either PHS-peroxidase or myeloperoxidase. Coadministration of the benzene metabolites, phenol and hydroquinone, has been reported previously to reproduce the myelotoxicity of benzene. In our studies, phenol and hydroquinone (50 mg/kg each; 2 x/day for 2 days) decreased bone marrow cellularity by 40%; however, coadministration of indomethacin (2 mg/kg) or meclofenamate (4 mg/kg) with these metabolites did not prevent the decrease in bone marrow cell number. Our results implicate marrow PHS in mediating the short-term myelotoxicity of benzene.

Effect of exposure concentration, exposure rate, and route of administration on metabolism of benzene by F344 rats and B6C3F1 mice

To determine the effect of exposure concentration and the route of administration on benzene metabolism, male F344/N rats and B6C3F1 mice were orally exposed to 1, 10, and 200 mg benzene/kg, and by inhalation for 6 hr to 5, 50, and 600 ppm benzene vapor. The effect of different exposure rates on the metabolism of benzene was determined by exposing rodents over different time intervals to the same total amount of benzene [constant concentration X time factor (C X T) = 300 ppm.hr]. Water-soluble metabolites constituted greater than 90% of the metabolite dose to the tissues and were used as a measure of the metabolism of benzene via different pathways. Water-soluble metabolites were measured in the blood, urine, liver, lung, and bone marrow from animals killed following oral exposures and during and following inhalation exposures. The total "dose" to the tissue of individual metabolites was determined by the area under the curve (AUC). The results indicated a shift in metabolism from putative toxification pathways to detoxification pathways as the exposure concentration or oral dose increased. In mice, hydroquinone glucuronide and muconic acid (markers of toxification metabolic pathways) represented a greater percentage of the administered dose at low doses than at high doses. At high doses, phenylglucuronide and prephenylmercapturic acid (detoxification products) increased as a percentage of the administered dose. This same metabolic shift was observed in rats, except that hydroquinone glucuronide was a minor metabolite of benzene at all concentrations. The AUC of phenylsulfate (detoxification pathway) was proportional to the exposure concentration in both species. Within the range of C X T factors studied, the rate of the inhalation exposure to benzene did not affect the AUC of metabolites in tissues of rats; however, a high dose rate (600 ppm 0.5 hr) in mice caused a shift in metabolism to phenyl conjugates. The comparison of oral and 6-hr inhalation exposures indicated that, in terms of metabolite dose to tissues, there is no simple relationship between these two routes of administration. An oral dose and an inhalation exposure concentration which produce an equal dose of one metabolite produce very different doses of another metabolite. These studies demonstrated a species difference in benzene metabolism, as well as a metabolic shift in benzene metabolic pathways as the exposure concentration was increased.(ABSTRACT TRUNCATED AT 400 WORDS)

An in vivo study of benzene metabolite DNA adduct formation in liver of male New Zealand rabbits

Rabbits were treated with benzene (586 mg/kg/b.i.d./4 days) after which DNA was isolated from liver and analyzed for adduct formation using the [32P] post-labeling method of Randerath and coworkers (Randerath et al. 1981; Reddy et al. 1984, 1986, 1987). Liver 500 g and 9000 g fractions were analyzed for adducts. There appeared to be several adducts in both the 500 g and 9000 g fractions observed on radioautographs of cellulose-TLC plates. Several adducts were also observed when the 9000 g fraction was studied using HPLC.

Macrophage regulation of myelopoiesis is altered by exposure to the benzene metabolite hydroquinone

Hydroquinone, a myelotoxic metabolite of benzene, decreases the ability of murine bone marrow stromal cells to support myelopoiesis in vitro. Bone marrow stroma consists of macrophages and fibroblastoid stromal cells that participate coordinately in regulating myelopoiesis. The goal of this study was to determine if macrophage or fibroblastoid cell function is more sensitive to the myelotoxic actions of hydroquinone. To address this question, we developed purified populations of macrophages and fibroblastoid stromal cells and treated each population with hydroquinone. These cells were reconstituted together with nontreated cells of the opposite type and assayed for their ability to support the formation of granulocyte and macrophage colonies in an agar overlay. Reconstituted cultures containing hydroquinone-treated macrophages supported fewer colonies than did corresponding cultures containing untreated macrophages. Reconstituted cultures containing hydroquinone-treated fibroblastoid stromal cells were not affected. Moreover, hydroquinone reduced detectable interleukin-1 activity in purified macrophage cultures stimulated with lipopolysaccharide. These results indicate that hydroquinone selectively interferes with macrophage function possibly, in part, via alteration of macrophage interleukin-1 secretion.

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.

Recent advances in the metabolism and toxicity of benzene

Benzene is a heavily used industrial chemical, a petroleum byproduct, an additive in unleaded gas, and a ubiquitous environmental pollutant. Benzene is also a genotoxin, hematotoxin, and carcinogen. Chronic exposure causes aplastic anemia in humans and animals and is associated with increased incidence of leukemia in humans and lymphomas and certain solid tumors in rodents. Bioactivation of benzene is required for toxicity. In the liver, the major site of benzene metabolism, benzene is converted by a cytochrome P-450-mediated pathway to phenol, the major metabolite, and the secondary metabolites, hydroquinone and catechol. The target organ of benzene toxicity, the hematopoietically active bone marrow, metabolizes benzene to a very limited extent. Phenol is metabolized in the marrow cells by a peroxidase-mediated pathway to hydroquinone and catechol, and ultimately to quinones, the putative toxic metabolites. Benzene and its metabolites appear to be nonmutagenic, but they cause myeloclastogenic effects such as micronuclei, chromosome aberrations, and sister chromatid exchange. It is unknown whether these genomic changes, or the ability of the quinone metabolites to form adducts with DNA, are involved in benzene carcinogenicity. Benzene, through its active metabolites, appears to exert its hematological effects on the bone marrow stromal microenvironment by preventing stromal cells from supporting hemopoiesis of the various progenitor cells. Recent advances in our understanding of the mechanisms by which benzene exerts its genotoxic, hematotoxic, and carcinogenic effects are detailed in this review.