Synergistic action of the benzene metabolite hydroquinone on myelopoietic stimulating activity of granulocyte/macrophage colony-stimulating factor in vitro

Shanghai Health Study (2001-2009): What was learned about benzene health effects?

The Shanghai Health Study (SHS) was a large epidemiology study conducted as a joint effort between the University of Colorado and Fudan University in Shanghai, China. The study was funded by members of the American Petroleum Institute between 2001 and 2009 and was designed to evaluate the human health effects associated with benzene exposure. Two arms of the SHS included: an occupational-based molecular epidemiology study and several hospital-based case control studies. Consistent with historical literature, following sufficient exposure to relatively high airborne concentrations and years of exposure, the SHS concluded that exposure to benzene resulted in an increased risk of various blood and bone marrow abnormalities such as benzene poisoning, aplastic anemia (AA), myelodysplastic syndrome (MDS), and acute myeloid leukemia (AML). Non-Hodgkin lymphoma (NHL) was not significantly increased for the exposures examined in this study. Perhaps the most important contribution of the SHS was furthering our understanding of the mechanism of benzene-induced bone marrow toxicity and the importance of identifying the proper subset of MDS relevant to benzene. Investigators found that benzene-exposed workers exhibited bone marrow morphology consistent with an immune-mediated inflammatory response. Contrary to historic reports, no consistent pattern of cytogenetic abnormalities was identified in these workers. Taken together, findings from SHS provided evidence that the mechanism for benzene-induced bone marrow damage was not initiated by chromosome abnormalities. Instead, chronic inflammation, followed by an immune-mediated response, is likely to play a more significant role in benzene-induced disease initiation and progression than previously thought.

Amended Final Report On the Safety Assessment of Pyrocatechol

Pyrocatechol is a phenol used primarily in hair dyes but has one reported use in skin care preparations. When this ingredient was first reviewed it was concluded that Pyrocatechol was safe for use in formulations designed for discontinuous, brief use followed by rinsing from the skin and hair at concentrations of up to 1.0%. This amendment to that report was prepared in order to incorporate the results from several studies, including those reporting immunologic, carcinogenic, and cocarcinogenic effects of Pyrocatechol. In vitro, Pyrocatechol has been shown to affect several immunologic and other properties of murine bone marrow cells, both alone and when combined with hydroquinone. Pyrocatechol, when produced as an hepatic metabolite of benzene, has been reported to concentrate in the bone marrow and to adversely affect hematopoietic precursor cells. These effects are somewhat species specific. In vivo, dermal exposure of mice to Pyrocatechol affects the number and distribution of Langerhans cells at the site of exposure but is not associated with a change in contact sensitivity. Oral exposure of pregnant rats to Pyrocatechol did result in an increase in teratogenic effects. The results of a micromass teratogen test in vitro were also positive. Pyrocatechol was tested in a number of bacterial and mammalian mutagenesis systems. Both negative and positive results were observed. Carcinogenicity studies were conducted in rats and mice. Pyrocatechol was provided in the feed. While adenomatous hyperplasia was noted in both species, adenocarcinomas were seen in male and female rats only. In cocarcinogenesis studies there is a similar pattern of hyperplasia and adenomas of the glandular stomach in the groups exposed to Pyrocatechol alone. When administered with several other carcinogens, Pyrocatechol had a cocarcinogenic effects. Pyrocatechol was not a tumor promoter in dermal studies. Based on these data and the significant potential for skin absorption of Pyrocatechol in leave-on preparations, it was concluded that Pyrocatechol is unsafe for use in leave-on cosmetic formulations. Pyrocatechol used in a rinse-off application such as hair dyes may not present a similar concern if Pyrocatechol is completely and rapidly oxidized. Because no data were available, it was concluded that there are insufficient data to support the safety of this ingredient. The data needed to complete the safety assessment include the extent and rate of oxidation when used in hair dyeing. If not rapidly and completely oxidized, then further chemical characterization of cosmetic grade Pyrocatechol is needed, along with cutaneous absorption data in aqueous and alcohol vehicles and ultraviolet absorption data (if positive, photosensitization studies may be needed).

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.

The possible role of liver kinase B1 in hydroquinone-induced toxicity of murine fetal liver and bone marrow hematopoietic stem cells

Epidemiological studies suggest that the increasing incidence of childhood leukemia may be due to maternal exposure to benzene, which is a known human carcinogen; however, the mechanisms involved remain unknown. Liver Kinase B1 (LKB1) acts as a regulator of cellular energy metabolism and functions to regulate hematopoietic stem cell (HSC) homeostasis. We hypothesize that LKB1 contributes to the deregulation of fetal or bone hematopoiesis caused by the benzene metabolite hydroquinone (HQ). To evaluate this hypothesis, we compared the effects of HQ on murine fetal liver hematopoietic stem cells (FL-HSCs) and bone marrow hematopoietic stem cells (BM-HSCs). FL-HSCs and BM-HSCs were isolated and enriched by a magnetic cell sorting system and exposed to various concentrations of HQ (0, 1.25, 2.5, 5, 10, 20, and 40 μM) for 24 h. We found that the inhibition of differentiation and growth, as well as the apoptosis rate of FL-HSCs, induced by HQ were consistent with the changes in BM-HSCs. Furthermore, G1 cell cycle arrest was observed in BM-HSCs and FL-HSCs in response to HQ. Importantly, FL-HSCs were more sensitive than BM-HSCs after exposure to HQ. The highest induction of LKB1 and adenosine monophosphate-activated protein kinase (AMPK) was observed with a much lower concentration of HQ in FL-HSCs than in BM-HSCs. LKB1 may play a critical role in apoptosis and cell cycle arrest of HQ-treated HSCs. This research has developed innovative ideas concerning benzene-induced hematopoietic toxicity or embryotoxicity, which can provide a new experimental evidence for preventing childhood leukemia. © 2014 Wiley Periodicals, Inc. Environ Toxicol 31: 830-841, 2016.

Comet assay based DNA evaluation of fuel filling stations and automobile workshops workers from Khyber Pakhtunkhwa province, Pakistan

BACKGROUND

Fuel filling stations workers and automobile workshops mechanics are consistently exposed to gasoline hydrocarbons during their occupation, this may cause DNA damage. Objective of this study was to evaluate the level of DNA damage in subjects occupationally exposed to these hydrocarbons.

METHODS

Comet assay was performed on blood lymphocytes of exposed subjects to assess the probable DNA damage. 100 cells per individual were scored and graded by comet tail length. Exposed group consisted of 98 subjects (age 25.4 ± 7.2 years), of which 68 were CNG/Petrol filling men and 30 were automobile workshop workers, selected randomly from different service stations and automobile workshops of populated and adjacent cities of Peshawar, Mardan and Nowshera of Khyber Pakhtunkhwa province, while control group included 92 subjects (age 26.7 ± 11.8 years) were also from the same areas.

RESULTS

Significantly high level of DNA damage was found in the subjects exposed to gasoline hydrocarbons as compared to control subjects (173.2 ± 50.1 and 61.0 ± 25.0, P = 0.001, respectively). Period of exposure and use of tobacco also showed considerable effects (P < 0.05) on DNA damage, while effect of age and daily working hours on total comet score (TCS) were non-significant (P > 0.05).

CONCLUSIONS

The results of our study concluded that petroleum hydrocarbons have the potential to cause DNA damage in the exposed subjects. The study also suggested that protective strategies should be implemented by the concerned authorities to minimize exposure to fuel hydrocarbons.

Modulation of Ras signaling alters the toxicity of hydroquinone, a benzene metabolite and component of cigarette smoke

Background

Benzene is an established human leukemogen, with a ubiquitous environmental presence leading to significant population exposure. In a genome-wide functional screen in the yeast Saccharomyces cerevisiae, inactivation of IRA2, a yeast ortholog of the human tumor suppressor gene NF1 (Neurofibromin), enhanced sensitivity to hydroquinone, an important benzene metabolite. Increased Ras signaling is implicated as a causal factor in the increased pre-disposition to leukemia of individuals with mutations in NF1.

Methods

Growth inhibition of yeast by hydroquinone was assessed in mutant strains exhibiting varying levels of Ras activity. Subsequently, effects of hydroquinone on both genotoxicity (measured by micronucleus formation) and proliferation of WT and Nf1 null murine hematopoietic precursors were assessed.

Results

Here we show that the Ras status of both yeast and mammalian cells modulates hydroquinone toxicity, indicating potential synergy between Ras signaling and benzene toxicity. Specifically, enhanced Ras signaling increases both hydroquinone-mediated growth inhibition in yeast and genotoxicity in mammalian hematopoetic precursors as measured by an in vitro erythroid micronucleus assay. Hydroquinone also increases proliferation of CFU-GM progenitor cells in mice with Nf1 null bone marrow relative to WT, the same cell type associated with benzene-associated leukemia.

Conclusions

Together our findings show that hydroquinone toxicity is modulated by Ras signaling. Individuals with abnormal Ras signaling could be more vulnerable to developing myeloid diseases after exposure to benzene. We note that hydroquinone is used cosmetically as a skin-bleaching agent, including by individuals with cafe-au-lait spots (which may be present in individuals with neurofibromatosis who have a mutation in NF1), which could be unadvisable given our findings.

Electronic noses for monitoring benzene occupational exposure in biological samples of Egyptian workers

OBJECTIVES

Benzene is commonly emitted in several industries, leading to widespread environmental and occupational exposure hazards. While less toxic solvents have been substituted for benzene, it is still a component of petroleum products and is a trace impurity in industrial products resulting in continued higher occupational exposures in industrial settings in developing countries.

MATERIALS AND METHODS

We investigated the potential use of an electronic nose (e-nose) to monitor the headspace volatiles in biological samples from benzene-exposed Egyptian workers and non-exposed controls. The study population comprised 150 non-smoking male workers exposed to benzene and an equal number of matching non-exposed controls. We determined biomarkers of benzene used to estimate exposure and risk including: benzene in exhaled air and blood; and its urinary metabolites such as phenol and muconic acid using gas chromatography technique and a portable e-nose.

RESULTS

The average benzene concentration measured in the ambient air of the workplace of all studied industrial settings in Alexandria, Egypt; was 97.56 ± 88.12 μg/m(3) (range: 4.69-260.86 μg/m(3)). Levels of phenol and muconic acid were significantly (p < 0.001) higher in both blood and urine of benzene-exposed workers as compared to non-exposed controls.

CONCLUSIONS

The e-nose technology has successfully classified and distinguished benzene-exposed workers from non-exposed controls for all measured samples of blood, urine and the exhaled air with a very high degree of precision. Thus, it will be a very useful tool for the low-cost mass screening and early detection of health hazards associated with the exposure to benzene in the industry.

United States Voluntary Children's Chemical Evaluation Program (VCCEP) risk assessment for children exposed to benzene

As part of the Voluntary Children's Chemical Evaluation Program (VCCEP) program, a risk assessment was performed to evaluate the risks to children from environmental benzene exposures. This paper summarizes this risk assessment. Risk was characterized using two distinct methods: USEPA's default type of risk assessment, which used the Reference Dose (RfD) and Cancer Slope Factor (CSF) to characterize non-cancer and cancer risks, as well as a Margin of Safety (MOS) approach that utilized a point of departure (POD). The exposures for most scenarios evaluated in this VCCEP risk assessment are lower than both the cancer and non-cancer PODs by several orders of magnitude, indicating a large MOS and corresponding low potential for toxicity at these exposures. The highest benzene exposures likely experienced by children, associated with the lowest MOS, are from cigarette smoke. In addition, the potential for age-related differences in the sensitivity towards benzene-induced toxicity was investigated. In general, this risk assessment does not indicate that children are likely to be at a elevated risk of AML or hematopoietic toxicity associated with environmental exposures to benzene.

The fate of benzene-oxide

Metabolism is a prerequisite for the development of benzene-mediated myelotoxicity. Benzene is initially metabolized via cytochromes P450 (primarily CYP2E1 in liver) to benzene-oxide, which subsequently gives rise to a number of secondary products. Benzene-oxide equilibrates spontaneously with the corresponding oxepine valence tautomer, which can ring open to yield a reactive alpha,beta-unsaturated aldehyde, trans-trans-muconaldehyde (MCA). Further reduction or oxidation of MCA gives rise to either 6-hydroxy-trans-trans-2,4-hexadienal or 6-hydroxy-trans-trans-2,4-hexadienoic acid. Both MCA and the hexadienal metabolite are myelotoxic in animal models. Alternatively, benzene-oxide can undergo conjugation with glutathione (GSH), resulting in the eventual formation and urinary excretion of S-phenylmercapturic acid. Benzene-oxide is also a substrate for epoxide hydrolase, which catalyzes the formation of benzene dihydrodiol, itself a substrate for dihydrodiol dehydrogenase, producing catechol. Finally, benzene-oxide spontaneously rearranges to phenol, which subsequently undergoes either conjugation (glucuronic acid or sulfate) or oxidation. The latter reaction, catalyzed by cytochromes P450, gives rise to hydroquinone (HQ) and 1,2,4-benzene triol. Co-administration of phenol and HQ reproduces the myelotoxic effects of benzene in animal models. The two diphenolic metabolites of benzene, catechol and HQ undergo further oxidation to the corresponding ortho-(1,2-), or para-(1,4-)benzoquinones (BQ), respectively. Trapping of 1,4-BQ with GSH gives rise to a variety of HQ-GSH conjugates, several of which are hematotoxic when administered to rats. Thus, benzene-oxide gives rise to a cascade of metabolites that exhibit biological reactivity, and that provide a plausible metabolic basis for benzene-mediated myelotoxicity. Benzene-oxide itself is remarkably stable, and certainly capable of translocating from its primary site of formation in the liver to the bone marrow. However, therein lies the challenge, for although there exists a plethora of information on the metabolism of benzene, and the fate of benzene-oxide, there is a paucity of data on the presence, concentration, and persistence of benzene metabolites in bone marrow. The major metabolites in bone marrow of mice exposed to 50 ppm [(3)H]benzene are muconic acid, and glucuronide and/or sulfate conjugates of phenol, HQ, and catechol. Studies with [(14)C/(13)C]benzene revealed the presence in bone marrow of protein adducts of benzene-oxide, 1,4-BQ, and 1,4-BQ, the relative abundance of which was both dose and species dependent. In particular, histones are bone marrow targets of [(14)C]benzene, although the identity of the reactive metabolite(s) giving rise to these adducts remain unknown. Finally, hematotoxic HQ-GSH conjugates are present in the bone marrow of rats receiving the HQ/phenol combination. In summary, although the fate of benzene-oxide is known in remarkable detail, coupling this information to the site, and mechanism of action, remains to be established.

Advances in understanding benzene health effects and susceptibility

Benzene is a ubiquitous chemical in our environment that causes acute leukemia and probably other hematological cancers. Evidence for an association with childhood leukemia is growing. Exposure to benzene can lead to multiple alterations that contribute to the leukemogenic process, indicating a multimodal mechanism of action. Research is needed to elucidate the different roles of multiple metabolites in benzene toxicity and the pathways that lead to their formation. Studies to date have identified a number of polymorphisms in candidate genes that confer susceptibility to benzene hematotoxicity. However, a genome-wide study is needed to truly assess the role of genetic variation in susceptibility. Benzene affects the blood-forming system at low levels of occupational exposure, and there is no evidence of a threshold. There is probably no safe level of exposure to benzene, and all exposures constitute some risk in a linear, if not supralinear, and additive fashion.

Integrating WHO 2001-2008 criteria for the diagnosis of Myelodysplastic Syndrome (MDS): a case-case analysis of benzene exposure

We characterized the prevalence of hematopoietic and lymphoid disease for 2923 consecutive patients presenting at 29 hospitals from August 2003 to June 2007. Diagnoses were made in our laboratory using WHO criteria based on morphologic, immunophenotypic, cytogenetic, FISH and molecular data. A total of 611 subjects (322 males/289 females) were prospectively diagnosed with MDS using WHO (2001) criteria. Update and re-evaluation of cases using MDS (2008) criteria resulted in 649 MDS cases. Using WHO (2008) criteria, refractory cytopenia with multilineage dysplasia (RCMD) accounted for 68% of total cases, refractory anemia with excess blasts (RAEB), 16.3%; refractory anemia (RA), 6.5%; refractory cytopenia with unilineage dysplasia (RCUD), 4%; and MDS-unclassifiable (MDS-U), 4.5%. Subjects were administered questionnaires and information on previous disease, work histories and exposures to potential etiologic agents such as benzene (BZ) was obtained. A total of 80/649 (13.2%) were determined to have some BZ exposure. The frequency of clonal cytogenetic abnormalities in all MDS was 30%, the most common being +8>del(20)q>del(7q)>del(5q), while the analogous frequency in BZ-exposed cases was only 24%. To further investigate the characteristics of MDS associated with BZ, we identified a subset of cases with high BZ exposure. These BZ signal cases were each matched by age and gender to two cases with no known BZ exposure. When contrasting BZ signal cases vs matched cases with no BZ exposure, we found a high odds ratio (OR) for the WHO subtype MDS-U (OR=11.1), followed by RAEB and RCUD (OR=1), RA (OR=0.7) and RCMD (OR=0.6). Multilineage dysplasia with abnormal eosinophils (MDS-Eo) was strongly associated with BZ exposure, whereas the relative risk of clonal cytogenetic abnormalities was reduced for high BZ-exposed cases (OR=0.5). These findings are strongly indicative that MDS subtypes are influenced by BZ exposure, and taken together with previous studies, the features of MDS-Eo suggest that altered immune regulation plays a major role in the pathogenesis of MDS following chronic exposure to BZ.

Genetic and environmental factors influencing human diseases with telomere dysfunction

Both genetic and environmental factors have been implicated in the mechanism underlying the pathogenesis of serious and fatal forms of human blood disorder (acquired aplastic anemia, AA) and lung disease (idiopathic pulmonary fibrosis, IPF). We and other researchers have recently shown that naturally occurring mutations in genes encoding the telomere maintenance complex (telomerase) may predispose patients to the development of AA or IPF. Epidemiological data have shown that environmental factors can also cause and/or exacerbate the pathogenesis of these diseases. The exact mechanisms that these germ-line mutations in telomere maintenance genes coupled with environmental insults lead to ineffective hematopoiesis in AA and lung scarring in IPF are not well understood, however. In this article, we provide a summary of evidence for environmental and genetic factors influencing the diseases. These studies provide important insights into the interplay between environmental and genetic factors leading to human diseases with telomere dysfunction.

Overview of benzene-induced aplastic anaemia

The scientific literature is replete with reports of cases of benzene-induced toxicity to the haematopoietic system. These mainly involve aplastic anaemia, the first cases of which were reported in 1897. At high level of benzene exposure (air concentration > 100 p.p.m.), the incidence of aplastic anaemia is approximately 1/100 individuals exposed, but this drops precipitously at lower levels of exposure (10-20 p.p.m.) to around 1/10,000. Factors that affect susceptibility may include high liver cytochrome P450 2E1 activity and low folic acid intake. The mechanism of benzene-induced aplastic anaemia remains unclear, but is likely to involve: (a) metabolism of benzene in the liver; (b) transport of metabolites to the marrow and their secondary activation to toxic quinones and free radicals by peroxidase enzymes; (c) induction of apoptosis, DNA damage and altered differentiation in early progenitor cells; and (d) depletion of the stem cell pool.

The effects of benzene and other leukaemogenic agents on haematopoietic stem and progenitor cell differentiation

A characteristic shared by a diverse group of myelotoxic compounds and leukaemogens is the ability to act synergistically with granulocyte-macrophage colony stimulating factor (GM-CSF) in increasing clonogenic response. Pretreatment of murine or human bone marrow cells with the benzene metabolite, hydroquinone, but not phenol, catechol or trans, trans-muconaldehyde, results in a selective enhancement of GM-CSF but not an interleukin-3 (IL-3)-mediated clonogenic response. Clonal enhancement is preserved and magnified in enriched populations of CD34+ cells (> 95% purity), suggesting an intrinsic effect on haematopoietic progenitor cell (HPC) recruitment rather than a secondary effect involving accessory cytokines. Clonogenic enhancement of murine HPCs is not accompanied by alterations in GM-CSF receptor expression or ligand affinity and appears to be mediated via a p53-independent mechanism. These observations suggest that hydroquinone treatment alters recruitment and differentiation in a primitive subpopulation of CD34+ cells and are consistent with a role for altered stem cell differentiation in the development of chemically induced myelodysplasias.

Multipronged evaluation of genotoxicity in Indian petrol-pump workers

Petrol (gasoline) contains a number of toxicants. This study used human biomonitoring to evaluate the genotoxic effects of exposure to benzene in petrol fumes in 100 Indian petrol-pump workers (PPWs) and an equal number of controls. The study was corroborated with in silico assessments of the Comet assay results from the human biomonitoring study. An in vitro study in human lymphocytes was also conducted to understand the genotoxicity of benzene and its metabolites. In a subset of the population studied, higher blood benzene levels were detected in the PPWs (n = 39; P < 0.01) than the controls (n = 18), and 100-250 ppb benzene was also detected in air samples from the petrol pumps. PPWs had higher levels of DNA damage than the controls (P < 0.01). In addition, the micronucleus assay was performed on lymphocytes from a subset of the subjects, and the micronucleus frequency for PPWs was significantly higher (n = 39; 14.79 +/- 3.92 per thousand) than the controls (n = 18; 7.54 +/- 3.00 per thousand). Human lymphocytes were treated in vitro with benzene and several of its metabolites and assayed for DNA damage with the Comet assay. Benzene and its metabolites produced significant (P < 0.05) levels of DNA damage at and above concentrations of 10 microM. The metabolite, p-benzoquinone, produced the greatest amount of DNA damage, followed by hydroquinone > benzene > catechol > 1,2,4,-benzenetriol > muconic acid. This study demonstrates that, using sensitive techniques, it is possible to detect human health risks at an early stage when intervention is possible. possible.

Physiologically-based pharmacokinetic modeling of benzene in humans: a Bayesian approach

Benzene is myelotoxic and leukemogenic in humans exposed at high doses (>1 ppm, more definitely above 10 ppm) for extended periods. However, leukemia risks at lower exposures are uncertain. Benzene occurs widely in the work environment and also indoor air, but mostly below 1 ppm, so assessing the leukemia risks at these low concentrations is important. Here, we describe a human physiologically-based pharmacokinetic (PBPK) model that quantifies tissue doses of benzene and its key metabolites, benzene oxide, phenol, and hydroquinone after inhalation and oral exposures. The model was integrated into a statistical framework that acknowledges sources of variation due to inherent intra- and interindividual variation, measurement error, and other data collection issues. A primary contribution of this work is the estimation of population distributions of key PBPK model parameters. We hypothesized that observed interindividual variability in the dosimetry of benzene and its metabolites resulted primarily from known or estimated variability in key metabolic parameters and that a statistical PBPK model that explicitly included variability in only those metabolic parameters would sufficiently describe the observed variability. We then identified parameter distributions for the PBPK model to characterize observed variability through the use of Markov chain Monte Carlo analysis applied to two data sets. The identified parameter distributions described most of the observed variability, but variability in physiological parameters such as organ weights may also be helpful to faithfully predict the observed human-population variability in benzene dosimetry.

PU.1 phosphorylation correlates with hydroquinone-induced alterations in myeloid differentiation and cytokine-dependent clonogenic response in human CD34+ hematopoietic progenitor cells

The transcriptional regulatory factor PU.1 is important for the regulation of a diverse group of hematopoietic and myeloid genes. Posttranslational phosphorylation of PU.1 has been demonstrated in the regulation of a variety of promoters in normal cells. In leukemia cells, differing patterns of PU.1 phosphorylation have been described among acute myelogenous leukemia (AML) subtypes. Therefore, we hypothesized that modulation of PU.1-dependent gene expression might be a molecular mediator of alterations in myeloid cell growth and differentiation that have been demonstrated to be early events in benzene-induced leukemogenesis. We found that freshly isolated human CD34(+) hematopoietic progenitor cells (HPC) exhibit multiple PU.1-DNA binding species that represent PU.1 proteins in varying degrees of phosphorylation states as determined by phosphatase treatment in combination with electrophoretic mobility shift assay (EMSA). Maturation of granulocyte and monocyte lineages is also accompanied by distinct changes in PU.1-DNA binding patterns. Experiments reveal that increasing doses of the benzene metabolite, hydroquinone (HQ) induce a time-and dose-dependent alteration in the pattern of PU.1-DNA binding in cultured human CD34(+) cells, corresponding to hyperphosphorylation of the PU.1 protein. HQ-induced alterations in PU.1-DNA binding are concomitant with a sustained immature CD34(+) phenotype and cytokine-dependent enhanced clonogenic activity in cultured human HPC. These results suggest that HQ induces a dysregulation in the external signals modulating PU.1 protein phosphorylation and this dysregulation may be an early event in the generation of benzene-induced AML.

Chronic exposure to benzene results in a unique form of dysplasia

Hematotoxicity following chronic benzene exposure has been recognized for over a century, although the mechanism remains unknown. We describe a novel form of bone marrow dysplasia in 23 workers exposed to high concentrations of benzene. Distinguishing features of benzene-induced dysplasia include: marked dyserythropoiesis, eosinophilic dysplasia and abnormal cytoplasmic granulation of neutrophilic precursors. Hematophagocytosis, stromal degeneration and bone marrow hypoplasia are also seen. Severe bone marrow dysplasia is frequently accompanied by clonal T cell expansion and alterations in T lymphocyte subsets. No clonal cytogenetic abnormalities were observed. These results suggest that autoimmune-mediated bone marrow injury is an early or predisposing event in the pathogenesis of benzene-induced persistent hematopoietic disease.

Hydroquinone modulates the GM-CSF signaling pathway in TF-1 cells

Human leukemogens, including alkylating chemotherapeutic agents and benzene, enhance granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent proliferation of human CD34+ bone marrow (BM) cells. The extracellular signal-regulated kinase (ERK) pathway plays an important role in GM-CSF-dependent proliferation and also has been implicated in the pathogenesis of acute myelogenous leukemia. Therefore, we investigated the effects of the benzene metabolite, hydroquinone (HQ), on alterations in the GM-CSF signaling pathway in TF-1 erythroleukemia cells and human CD34+ BM cells. HQ treatment in TF-1 cells results in a strong proliferative response that is dependent on ERK activation and GM-CSF production. HQ also induces ERK-dependent AP-1 activation with concomitant increased transcriptional activity of AP-1 reporter gene. However, the kinetics of ERK activation are different between rhGM-CSF and HQ in TF-1 cells: rhGM-CSF results in immediate activation of ERK, whereas HQ activation of ERK is delayed. Further, HQ and rhGM-CSF together produce an immediate increase in ERK phosphorylation, which is sustained for over 48 h. HQ also stimulates colony formation, AP-1 DNA binding and GM-CSF production in human CD34+ BM cells. These results suggest that HQ stimulates proliferation via activation of ERK/AP-1 and is at least partially mediated via the production of GM-CSF.

The use of non-tumor data in cancer risk assessment: reflections on butadiene, vinyl chloride, and benzene

The estimation and characterization of a cancer risk is grounded in the observation of tumors in humans and/or experimental animals. Increasingly, however, other kinds of data (non-tumor data) are finding application in cancer risk assessment. Metabolism and kinetics, adduct formation, genetic damage, mode of action, and biomarkers of exposure, susceptibility, and effects are examples. While these and other parameters have been studied for many important chemicals over the past 30-40 years, their use in risk assessments is more recent, and new insights and opportunities are continuing to unfold. To provide some perspective on this field, the ILSI Risk Science Institute asked a select working group to characterize the pertinent non-tumor data available for 1,3-butadiene, benzene, and vinyl chloride and to comment on the utility of these data in characterizing cancer risks. This paper presents the findings of that working group and concludes with 15 simple principles for the use of non-tumor data in cancer risk assessment.

Proliferation and differentiation of murine haemopoietic progenitor cells in stroma-free culture in the presence of metabolites of chlorinated pesticides

We have studied the influence of metabolites of chlorinated pesticides (lindane, pentachlorophenol, hexachlorobenzene) on proliferation and differentiation in two stroma-free murine bone marrow culture models, a multipotent progenitor cell line (FDCP-mix) and primary lineage-depleted bone marrow cells. Tetrachlorohydroquinone (Cl(4)pHQ), tetrachloro-p-benzoquinone (Cl(4p)BQ), but not their positional isomers, tetrachlorocatechol (Cl(4)oHQ) and tetrachloro-o-benzoquinone (Cl(4)oBQ), nor 2,4,6-trichlorophenol (2,4,6-Cl(3)P), were much more toxic to FDCP-mix cells cultured under conditions which lead to self-renewal than under conditions which lead to granulocyte-macrophage differentiation. Under the latter conditions, Cl(4)pHQ and Cl(4p)BQ even stimulated growth at intermediate concentration levels. In the primary cell cultures, pronounced differences were observed in the sensitivity between individual developmental pathways and between the different compounds. The percent of cells differentiating into the granulocytic lineage was increased at high concentration levels of each test compound. However, stimulatory effects on the macrophage lineage were observed at intermediate concentration levels of Cl(4)pHQ, Cl(4p)BQ and 2,4,6-Cl(3)P, and differentiation into erythrocytes was stimulated at low concentrations of 2,4,6-Cl(3)P. It is concluded that chlorinated monocyclic pesticides, after biotransformation to quinoid metabolites, may interact directly with haemopoietic progenitor cells with differential effects on self-renewal and differentiation. These mechanisms could lead to myeloplastic disorders.

Comparative toxicity of known and putative metabolites of 1, 3-butadiene in human CD34(+) bone marrow cells

Species-specific susceptibility to the hematotoxic effects of 1, 3-butadiene (BD) is well known. Previous studies have revealed that murine bone marrow is uniquely susceptible to toxicity following exposure to the parent compound in vivo or exposure of bone marrow cells to the monoepoxide metabolite, 3,4-epoxybutane, in vitro. Studies described herein compare the relative ability of putative and known BD metabolites to produce concentration dependent suppression of colony formation and cytotoxicity in human CD34(+) bone marrow cells. Compounds evaluated included 3,4-epoxybutane, D, L-butane-bis-oxide, meso-butane-bis-oxide and (2S, 3R)-3-epoxybutane-1,2-diol. In contrast to results previously observed in mice, only the bis-oxides produced significant suppression of colony formation at potentially relevant concentrations (10(-8) to 10(-3) M). No enantiospecific differences were observed between the meso- and D,L-bis-oxides and no significant lineage-specific differences in susceptibility to inhibition of clonogenic response were observed among early multi-potential myeloid and erythroid hematopoietic progenitor cells. The relative potencies of the bis-oxides were found to be comparable to that of the prototype hematotoxic compound, hydroquinone. These results confirm previous studies that reveal marked species-specific differences in the susceptibility of bone marrow cells to 3,4-epoxybutane. Moreover, these results suggest that the bis-oxides of BD are capable of suppressing the clonogenic function of human hematopoietic progenitor cells, if, in fact, they are produced in human bone marrow in significant concentration. Further interpretation of these findings requires a better understanding of the metabolism of BD in humans.

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.

Urinary excretion of phenol, catechol, hydroquinone, and muconic acid by workers occupationally exposed to benzene

OBJECTIVES

Animal inhalation studies and theoretical models suggest that the pattern of formation of benzene metabolites changes as exposure to benzene increases. To determine if this occurs in humans, benzene metabolites in urine samples collected as part of a cross sectional study of occupationally exposed workers in Shanghai, China were measured.

METHODS

With organic vapour monitoring badges, 38 subjects were monitored during their full workshift for inhalation exposure to benzene. The benzene urinary metabolites phenol, catechol, hydroquinone, and muconic acid were measured with an isotope dilution gas chromatography mass spectroscopy assay and strongly correlated with concentrations of benzene air. For the subgroup of workers (n = 27) with urinary phenol > 50 ng/g creatinine (above which phenol is considered to be a specific indicator of exposure to benzene), concentrations of each of the four metabolites were calculated as a ratio of the sum of the concentrations of all four metabolites (total metabolites) and were compared in workers exposed to > 25 ppm v < or = 25 ppm.

RESULTS

The median, 8 hour time weighted average exposure to benzene was 25 ppm. Relative to the lower exposed workers, the ratio of phenol and catechol to total metabolites increased by 6.0% (p = 0.04) and 22.2% (p = 0.007), respectively, in the more highly exposed workers. By contrast, the ratio of hydroquinone and muconic acid to total metabolites decreased by 18.8% (p = 0.04) and 26.7% (p = 0.006), respectively. Similar patterns were found when metabolite ratios were analysed as a function of internal benzene dose (defined as total urinary benzene metabolites), although catechol showed a more complex, quadratic relation with increasing dose.

CONCLUSIONS

These results, which are consistent with previous animal studies, show that the relative production of benzene metabolites is a function of exposure level. If the toxic benzene metabolites are assumed to be derived from hydroquinone, ring opened products, or both, these results suggests that the risk for adverse health outcomes due to exposure to benzene may have a supralinear relation with external dose, and that linear extrapolation of the toxic effects of benzene in highly exposed workers to lower levels of exposure may underestimate risk.

The pharmacology and toxicology of polyphenolic-glutathione conjugates

Polyphenolic-glutathione (GSH) conjugates and their metabolites retain the electrophilic and redox properties of the parent polyphenol. Indeed, the reactivity of the thioether metabolites frequently exceeds that of the parent polyphenol. Although the active transport of polyphenolic-GSH conjugates out of the cell in which they are formed will limit their potential toxicity to those cells, once within the circulation they can be transported to tissues that are capable of accumulating these metabolites. There are interesting physiological similarities between the organs that are known to be susceptible to polyphenolic-GSH conjugate-mediated toxicity. In addition, the frequent localization of gamma-glutamyl transpeptidase to cells separating the circulation from a second fluid-filled compartment coincides with tissues that are susceptible either to polyphenolic-GSH conjugate-induced toxicity or to quinone and reactive oxygen species-induced toxicity. Polyphenolic-GSH conjugates therefore contribute to the nephrotoxicity, nephrocarcinogenicity, and neurotoxicity of a variety of polyphenols.

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.

Benzene and Its Principal Metabolites Modulate Proinflammatory Cytokines and Growth Factors in Human Epidermal Keratinocyte Cultures

Benzene is an established leukemogen and hematotoxin in humans. However, the finding that benzene is a multiple-site carcinogen in rodents raises the possibility that other tissues could be susceptible to benzene-induced carcinogenicity, especially since a significant excess of squamous cell carcinomas and papillomas arise from epidermal and oral keratinocytes in benzene-exposed rats. Since inflammation and sustained hyperplasia are two integral components in carcinogenesis, the elaboration of proinflammatory cytokines and growth factors by keratinocytes might provide a mechanistic link between tumor initiation and promotion in benzene-induced cancers. We observed that the principal benzene metabolites, represented by hydroquinone, 1,4-benzoquinone, phenol, 1,2,4-benzenetriol, and catechol, significantly alters the production of transforming growth factor of (TGF)-α and interleukin (IL)-8 in human epidermal keratinocyte cultures. These cytokines represent the primary growth promoting factor and neutrophil chemotactant in the skin, respectively. Cytokine secretion correlated with the known redox potential of individual benzene metabolites and antioxidants, including dimethyl sulfoxide, 1,1,3,3-tetramethylthiourea, and N-acetylcysteine, attenuated the response. Binary combinations of selected benzene metabolites synergized in the induction of IL-8, while benzene, by itself, induced about a three-fold increase in IL-8 production. Taken together, our studies suggest that benzene and many of its phase I metabolites induce inflammatory cytokines and growth factors and this occurs through direct covalent binding or the generation of reactive oxygen species by autooxidation and reduction. The elaboration of proinflammatory cytokines and growth factors by keratinocytes in response to benzene and its principal metabolites may participate in benzene-induced skin carcinogenesis.

Characterization and phenotypic analysis of differentiating CD34+ human bone marrow cells in liquid culture

Our current understanding of human haematopoietic stem cell biology is based in part on the characterization of human CD34+ bone marrow cell differentiation in vitro. CD34 is highly expressed on early stem cells and haematopoietic progenitor cells with clonogenic potential and is gradually lost during differentiation and commitment. However, CD71 (transferrin receptor) is expressed at low levels on early stem cells and generally increases during haematopoietic progenitor cell proliferation. We reasoned that the combination of these surface markers would provide a useful framework for the simultaneous analysis of multiple lineage differentiation of CD34+ haematopoietic progenitor cells in liquid culture. In this report, we identify the phenotype of distinct subpopulations of myeloid, erythroid and lymphoid cells in liquid suspension culture using differential expression of CD34 vs. CD71 in combination with specific lineage markers. Freshly isolated human CD34+ bone marrow cells were introduced into suspension culture and monitored over a 6-d period using 3-colour flow cytometry. This is the first demonstration that differential expression of CD34 vs. CD71 can be used to simultaneously monitor differentiation of multiple haematopoietic cell lineages in liquid suspension culture, facilitating the study of cytokine-, drug- or chemical-induced alterations in haematopoietic progenitor cell differentiation in vitro.

Impact of benzene metabolites on differentiation of bone marrow progenitor cells

Interleukin-3 (IL-3) and granulocyte/macrophage-colony-stimulating factor (GM-CSF) are responsible for maintaining survival and stimulating growth of early dormant hematopoietic progenitor cells (HPC). These cytokines exhibit extensive overlap, with GM-CSF supporting growth and differentiation of myeloid HPC. A characteristic shared by a diverse group of leukemogens is the ability to act synergistically with GM-CSF to increase clonogenic response. Previous studies have revealed that pretreatment of murine HPC with hydroquinone (HQ) but not phenol, catechol, or trans-trans-muconaldehyde results in a selective enhancement of GM-CSF but not IL-3-mediated clonogenic response. Pretreatment of murine bone marrow cells with these agents or their metabolites in vitro results in increased numbers of HPC dividing and forming colonies in response to GM-CSF but not IL-3. The present studies explored the molecular mechanisms associated with altered cytokine response in early HPC in murine bone marrow and extended our initial observations in murine bone marrow to human bone marrow cells. HQ pretreatment of murine HPC did not induce either an up- or a down-regulation of GM-CSF receptors or any change in receptor affinity. CD34+ cells, which represent between 1 and 5% of human bone marrow, contain virtually all clonogenic stem and HPC. Pretreatment of CD34+ cells (approximately 95% purity) with HQ also results in enhanced clonogenic response with GM-CSF but not IL-3. These findings suggest that an early step in chemical leukemogenesis may involve transient alterations in the regulation of cytokine response to GM-CSF.

An epidemiologic study of early biologic effects of benzene in Chinese workers

Benzene is a recognized hematotoxin and leukemogen, but its mechanisms of action in humans are still uncertain. To provide insight into these processes, we carried out a cross-sectional study of 44 healthy workers currently exposed to benzene (median 8-hr time-weighted average; 31 ppm), and unexposed controls in Shanghai, China. Here we provide an overview of the study results on peripheral blood cells levels and somatic cell mutation frequency measured by the glycophorin A (GPA) gene loss assay and report on peripheral cytokine levels. All peripheral blood cells levels (i.e., total white blood cells, absolute lymphocyte count, platelets, red blood cells, and hemoglobin) were decreased among exposed workers compared to controls, with the exception of the red blood cell mean corpuscular volume, which was higher among exposed subjects. In contrast, peripheral cytokine levels (interleukin-3, interleukin-6, erythropoietin, granulocyte colony-stimulating factor, tissue necrosis factor-alpha) in a subset of the most highly exposed workers (n = 11) were similar to values in controls (n = 11), suggesting that benzene does not affect these growth factor levels in peripheral blood. The GPA assay measures stem cell or precursor erythroid cell mutations expressed in peripheral red blood cells of MN heterozygous subjects, identifying NN variants, which result from loss of the GPA M allele and duplication of the N allele, and N phi variants, which arise from gene inactivation. The NN (but not N phi) GPA variant cell frequency was elevated in the exposed workers compared with controls (mean +/- SD, 13.9 +/- 8.4 mutants per million cells versus 7.4 +/- 5.2 per million cells, (respectively; p = 0.0002), suggesting that benzene produces gene-duplicating but not gene-inactivating mutations at the GPA locus in bone marrow cells of exposed humans. These findings, combined with ongoing analyses of benzene macromolecular adducts and chromosomal aberrations, will provide an opportunity to comprehensively evaluate a wide range of early biologic effects associated with benzene exposure in humans.

The mechanism of benzene-induced leukemia: a hypothesis and speculations on the causes of leukemia

An overall hypothesis for benzene-induced leukemia is proposed. Key components of the hypothesis include a) activation of benzene in the liver to phenolic metabolites; b) transport of these metabolites to the bone marrow and conversion to semiquinone radicals and quinones via peroxidase enzymes; c) generation of active oxygen species via redox cycling; d) damage to tubulin, histone proteins, topoisomerase II, other DNA associated proteins, and DNA itself; and e) consequent damage including DNA strand breakage, mitotic recombination, chromosome translocations, and aneuploidy. If these effects take place in stem or early progenitor cells a leukemic clone with selective advantage to grow may arise, as a result of protooncogene activation, gene fusion, and suppressor gene inactivation. Epigenetic effects of benzene metabolites on the bone marrow stroma, and perhaps the stem cell itself, may then foster development and survival of the leukemic clone. Evidence for this hypothesis is mounting with the recent demonstration that benzene induces gene-duplicating mutations in human bone marrow and chromosome-specific aneuploidy and translocations in peripheral blood cells. If this hypothesis is correct, it also potentially implicates phenolic and quinonoid compounds in the induction of "spontaneous" leukemia in man.

Induction of granulocytic differentiation in a mouse model by benzene and hydroquinone

Chronic exposure of humans to benzene causes acute myelogenous leukemia (AML). The studies presented here were undertaken to determine whether benzene, or its reactive metabolite, hydroquinone (HQ), affects differentiation of myeloblasts. Benzene or HQ administered to C57BL/6J mice specifically induced granulocytic differentiation of myeloblasts. The ability of these compounds to induce differentiation of the myeloblasts was tested directly using the murine interleukin 3 (IL-3)-dependent 32D.3 (G) myeloblastic cell line, and the human HL-60 promyelocytic leukemia cell line. We have previously shown that benzene treatment of HL-60 myeloblasts activates protein kinase C (PKC) and upregulates the 5-lipoxygenase (LPO) pathway for the production of leukotriene D4 (LTD4), an essential effector or granulocytic differentiation. Differentiation was prevented by sphinganine, a PKC inhibitor, and, as shown here, by LPO inhibitors and LTD4 receptor antagonists. Benzene or HQ also induces differentiation in 32D.3 (G) myeloblasts. Both compounds interact with cellular signaling pathways normally activated by granulocyte colony stimulating factor (G-CSF) and can replace the requirement for G-CSF. While IL-3 induces a growth response in 32D.3 (G) cells, G-CSF has been shown to provide both growth and differentiated signals. Both HQ and LTD4 induce differentiation and synergize with IL-3 for growth; however, neither supports growth in the absence of IL-3. Benzene, like HQ, also provides a differentiation signal for 32D cells; however, it has no effect on their growth. Unlike G-CSF, benzene, or LTD4, each of which stimulates terminal differentiation; HQ blocks differentiation at the myelocyte stage, allowing only a small percentage of progenitors to proceed to mature segmented granulocytes. Benzene- and G-CSF-induced differentiation were prevented by the additional of either LPO inhibitors or LTD4 receptor antagonists, indicating that benzene, like G-CSF, upregulates LTD4 production. Hydroquinone-induced differentiation was not affected by the LPO inhibitors, but only by the specific receptor antagonists. Thus HQ appears to obviate the requirement for LTD4 by activating the LTD4 receptor directly.

The process of leukemogenesis

Leukemias are monoclonal diseases that arise from cells in the hematopoietic stem and progenitor cell compartment. Consistent with emerging models of carcinogenesis, leukemogenesis is an evolutionary process that involves multiple independent genetic and epigenetic events. Over the last half-century a predominant paradigm has emerged to describe leukemia developing secondary to alkylating drug therapy or exposure to benzene in which progressive dysplastic changes, accompanied by a distinct pattern of clonal cytogenetic abnormalities, give rise to acute myelogenous leukemia. Characterization of these clonal chromosomal aberrations, together with observed alterations in other growth-promoting genes, provides a useful framework for studying chemical leukemogenesis and for use in understanding the origins and development of leukemia in general.

Hydroquinone stimulates granulocyte-macrophage progenitor cells in vitro and in vivo

To investigate whether hydroxylated metabolites of benzene may be responsible for the amplification of granulocyte-macrophage progenitor cells (GM-CFC) observed in mice that inhale benzene, groups of six C57BL6 mice were injected with hydroquinone (HQ) (75 mg/kg) or HQ (50 mg/kg) plus phenol (PHE) (50 mg/kg) twice daily for 11 days. Deviations in blood leukocyte and erythrocyte levels by up to one-third were noted in the treated groups; however, the peripheral blood differential counts were unchanged. Although no changes in bone marrow cellularity were observed in mice treated with HQ, cellularity was decreased by a factor of two in the mice that had received HQ plus PHE. The number of GM-CFC per femur was doubled in both treated groups. In vitro experiments using the murine multipotent hematopoietic progenitor cells FDCP mix also showed a duplication of GM-CFC formation in the presence of HQ at concentrations between 10(-6) M and 10(-10) M. When HQ and PHE were present at equimolar concentrations, significantly increased colony formation was still observed with 10(-12) M of metabolites. The effect was independent of the concentration of GM-colony-stimulating factor used. We suggest that HQ is a major mediator of the stimulatory effect of benzene on GM-CFC in mice. In addition, the in vitro data indicate that a direct effect of GM-CFC is involved.

Mechanistic considerations in benzene physiological model development

Benzene, an important industrial solvent, is also present in unleaded gasoline and cigarette smoke. The hematotoxic effects of benzene in humans are well documented and include aplastic anemia, pancytopenia, and acute myelogenous leukemia. However, the risks of leukemia at low exposure concentrations have not been established. A combination of metabolites (hydroquinone and phenol, for example) may be necessary to duplicate the hematotoxic effect of benzene, perhaps due in part to the synergistic effect of phenol on myeloperoxidase-mediated oxidation of hydroquinone to the reactive metabolite benzoquinone. Because benzene and its hydroxylated metabolites (phenol, hydroquinone, and catechol) are substrates for the same cytochrome P450 enzymes, competitive interactions among the metabolites are possible. In vivo data on metabolite formation by mice exposed to various benzene concentrations are consistent with competitive inhibition of phenol oxidation by benzene. In vitro studies of the metabolic oxidation of benzene, phenol, and hydroquinone are consistent with the mechanism of competitive interaction among the metabolites. The dosimetry of benzene and its metabolites in the target tissue, bone marrow, depends on the balance of activation processes such as enzymatic oxidation and deactivation processes such as conjugation and excretion. Phenol, the primary benzene metabolite, can undergo both oxidation and conjugation. Thus the potential exists for competition among various enzymes for phenol. Zonal localization of phase I and phase II enzymes in various regions of the liver acinus also impacts this competition. Biologically based dosimetry models that incorporate the important determinants of benzene flux, including interactions with other chemicals, will enable prediction of target tissue doses of benzene and metabolites at low exposure concentrations relevant for humans.

Biomarkers of exposure to low concentrations of benzene: a field assessment

OBJECTIVE

To carry out a comprehensive field investigation to evaluate various conventional and recently developed biomarkers for exposure to low concentrations of benzene.

METHODS

Analyses were carried out on environmental air, unmetabolised benzene in blood and urine, urinary trans, transmuconic acid, and three major phenolic metabolites of benzene: phenol, catechol, and hydroquinone. Validations of these biomarkers were performed on 131 never smokers occupationally exposed to the time weighed average benzene concentration of 0.25 ppm (range, 0.01 to 3.5 ppm).

RESULTS

Among the six biomarkers studied, unmetabolised benzene in urine correlated best with environmental benzene concentration (correlation coefficient, r = 0.76), followed by benzene in blood (r = 0.64). When urinary metabolites were compared with environmental benzene, trans, trans-muconic acid showed a close correlation (r = 0.53) followed by hydroquinone (r = 0.44), and to a lesser extent with urinary phenol (r = 0.38). No correlation was found between catechol and environmental benzene concentrations. Although unmetabolised benzene in urine correlates best with benzene exposure, owing to serious technical drawbacks, its use is limited. Among the metabolites, trans, trans-muconic acid seems to be more reliable than other phenolic compounds. Nevertheless, detailed analyses failed to show that it is specific for monitoring benzene exposures below 0.25 ppm.

CONCLUSION

The overall results suggest that most of the currently available biomarkers are unable to provide sufficient specificity for monitoring of low concentrations of benzene exposure. If a lower occupational exposure limit for benzene is to be considered, the reliability of the biomarker and the technical limitations of measurements have to be carefully validated.

Evaluation of biomarkers for occupational exposure to benzene

OBJECTIVE

To evaluate the relations between environmental benzene concentrations and various biomarkers of exposure to benzene.

METHODS

Analyses were carried out on environmental air, unmetabolised benzene in urine, trans, trans-muconic acid (ttMA), and three major phenolic metabolites of benzene; catechol, hydroquinone, and phenol, in two field studies on 64 workers exposed to benzene concentrations from 0.12 to 68 ppm, the time weighted average (TWA). Forty nonexposed subjects were also investigated.

RESULTS

Among the five urinary biomarkers studied, ttMA correlated best with environmental benzene concentration (correlation coefficient, r = 0.87). When urinary phenolic metabolites were compared with environmental benzene, hydroquinone correlated best with benzene in air. No correlation was found between unmetabolised benzene in urine and environmental benzene concentrations. The correlation coefficients for environmental benzene and end of shift catechol, hydroquinone, and phenol were 0.30, 0.70, and 0.66, respectively. Detailed analysis, however, suggests that urinary phenol was not a specific biomarker for exposure below 5 ppm. In contrast, ttMA and hydroquinone seemed to be specific and sensitive even at concentrations of below 1 ppm. Although unmetabolised benzene in urine showed good correlation with atmospheric benzene (r = 0.50, P < 0.05), data were insufficient to suggest that it is a useful biomarker for exposure to low concentrations of benzene. The results from the present study also showed that both ttMA and hydroquinone were able to differentiate the background level found in subjects not occupationally exposed and those exposed to less than 1 ppm of benzene. This suggests that these two biomarkers are useful indices for monitoring low concentrations of benzene. Furthermore, these two metabolites are known to be involved in bone marrow leukaemogenesis, their applications in biological monitoring could thus be important in risk assessment.

CONCLUSION

The good correlations between ttMA, hydroquinone, and atmospheric benzene, even at concentrations of less than 1 ppm, suggest that they are sensitive and specific biomarkers for benzene exposure.

Risk of acute myeloid leukaemia and multiple myeloma in workers exposed to benzene

OBJECTIVE

To determine the risk of developing acute myeloid leukaemia (AML) and multiple myeloma in a cohort of workers exposed to benzene. The results were used to show the importance of taking specificity of disease into consideration in causation analysis.

METHODS

Data were derived from a cohort of workers employed at two Goodyear plants in Ohio in the manufacture of Pliofilm. Based on data in the Pliofilm study, several papers that examined the relation between exposure to benzene and leukaemia (all cell types combined) have been published. In the current analyses based on updated data in the study, standardised mortality ratios (SMRs) and 95% confidence intervals (95% CIs) were calculated for AML and multiple myeloma by cumulative exposure to benzene. The results based on AML were compared with those for leukaemia (all cell types combined) published previously.

RESULTS

An exposure response relation was shown between cumulative exposure to benzene and AML. No increased risk of AML was detected for cumulative exposure to benzene below 200 ppm-years (SMR 0.91). Above 200 ppm-years, risk of AML rose drastically; reaching a significant SMR of 98.37 for > 400 ppm-years. For multiple myeloma, no relation with exposure to benzene was detected.

CONCLUSION

Analysis specific to AML shows the importance of taking specificity of disease into consideration in causation analysis. This investigation shows that previous analyses based on all leukaemia cell types combined have incorrectly set the estimated threshold too low, and have underestimated risk above the threshold. Current regulatory policies that rely on previous analyses based on all leukaemia cell types combined should be re-examined.

Benzene and its metabolite, hydroquinone, induce granulocytic differentiation in myeloblasts by interacting with cellular signaling pathways activated by granulocyte colony-stimulating factor

Chronic exposure of humans to benzene (BZ) causes acute myelogenous leukemia. These studies determined whether BZ, or its reactive metabolite, hydroquinone (HQ), affect differentiation of myeloblasts. BZ or HQ administered to C57BL/6J mice specifically induced terminal granulocytic differentiation of myeloblasts. The ability of the compounds to induce differentiation of the myeloblast was tested directly using the murine interleukin 3 (IL-3)-dependent myeloblastic cell line, 32D.3 (G) and the human HL-60 promyelocytic leukemic cell line. Treatment of HL-60 myeloblasts with BZ activated protein kinase C and upregulated the 5-lipoxygenase (LPO) pathway for the production of leukotriene D4 (LTD4), an essential effector of granulocytic differentiation. Differentiation was prevented by sphinganine, a kinase C inhibitor, as well as by LPO inhibitors and LTD4 receptor antagonists. BZ and HQ also induced differentiation in 32D.3 (G) myeloblasts. Both compounds interact with cellular signaling pathways activated by granulocyte colony-stimulating factor (G-CSF) and thus replace the requirement for G-CSF. IL-3 induces a growth response, whereas G-CSF provides both growth and differentiation signals. BZ does not induce growth in the absence of IL-3, but provides a differentiation signal. Both HQ and LTD4 induce differentiation and synergize with IL-3 for growth, however, neither support growth in the absence of IL-3. BZ-induced 32D cells showed a gradual progression of progenitor differentiation to granulocytes similar to that seen with G-CSF or LTD4. HQ blocks differentiation at the myelocyte stage; only a small percentage of progenitors proceed to granulocytes. BZ, like G-CSF, upregulates LTD4 production, whereas HQ obviates the requirement for LTD4 by activating the LTD4 receptor.

Elevated levels of benzene-related compounds in the urine of cigarette smokers

Benzene exposure causes leukemia and lymphomas. Recent epidemiological findings have also shown an association between cigarette smoking and an increased risk of leukemia. However, further evidence is required to document the biological plausibility of this association. In evaluating this link, it is important to note that cigarette smoke contains benzene and various pyrolytic compounds, among other carcinogens. This study aims to determine the uptake of benzene by measuring 3 benzene-related compounds in cigarette smokers and non-smokers. Urinary concentrations of catechol (CAT), hydroquinone (HQ), and trans,trans-muconic acid (tt-MA) were measured by high-performance liquid chromatography (HPLC) with fluorimetric and UV detection, respectively. The results showed that these compounds were present in all urine samples. However, the concentrations were significantly higher in smokers than in non-smokers. The mean level of urinary tt-MA was 0.19 +/- 0.09 mg/g creatinine for 46 male smokers and the corresponding value for 40 non-smokers was 0.14 +/- 0.07 mg/g creatinine. The mean concentrations of HQ and CAT were 0.81 +/- 0.4 and 3.51 +/- 2.6 mg/g creatinine for smokers, and 0.45 +/- 0.4 and 1.94 +/- 1.2 mg/g creatinine for non-smokers, respectively. These results suggest that cigarette smoking is associated with a significant additional exposure to benzene and its related compounds. Furthermore, significant correlations were observed between the concentrations of cotinine, the metabolite of nicotine, and the above compounds. These findings suggest that the exposure originated from cigarette smoking.

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.

DNA lesions, inducible DNA repair, and cell division: three key factors in mutagenesis and carcinogenesis

DNA lesions that escape repair have a certain probability of giving rise to mutations when the cell divides. Endogenous DNA damage is high: 10(6) oxidative lesions are present per rat cell. An exogenous mutagen produces an increment in lesions over the background rate of endogenous lesions. The effectiveness of a particular lesion depends on whether it is excised by a DNA repair system and the probability that it gives rise to a mutation when the cell divides. When the cell divides, an unrepaired DNA lesion has a certain probability of giving rise to a mutation. Thus, an important factor in the mutagenic effect of an exogenous agent whether it is genotoxic or non-genotoxic, is the increment it causes over the background cell division rate (mitogenesis) in cells that appear to matter most in cancer, the stem cells, which are not on their way to being discarded. Increasing their cell division rate increases mutation and therefore cancer. There is little cancer from nondividing cells. Endogenous cell division rates can be influenced by hormone levels, decreased by calorie restriction, or increased by high doses of chemicals. If both the rate of DNA lesions and cell division are increased, then there will be a multiplicative effect on mutagenesis (and carcinogenesis), for example, by high doses of a mutagen that also increases mitogenesis through cell killing. The defense system against reactive electrophilic mutagens, such as the glutathione transferases, are also almost all inducible and buffer cells against increments in active forms of chemicals that can cause DNA lesions. A variety of DNA repair defense systems, almost all inducible, buffer the cell against any increment in DNA lesions. Therefore, the effect of a particular chemical insult depends on the level of each defense, which in turn depends on the past history of exposure. Exogenous agents can influence the induction and effectiveness of these defenses. Defenses can be partially disabled by lack of particular micronutrients in the diet (e.g., antioxidants).