Review of the genotoxicity of 2-butoxyethanol

Compensatory erythropoiesis has no impact on the outcome of the in vivo Pig-a mutation assay in rats following treatment with the haemolytic agent 2-butoxyethanol

The Pig-a assay has rapidly gained international interest as a useful tool for assessing the mutagenic potential of compounds in vivo. Although a large number of compounds, including both mutagens and non-mutagens, have been tested in the rat Pig-a assay in haematopoietic cells, there is limited understanding of how perturbations in haematopoiesis affect assay performance. Of particular concern is the possibility that regenerative haematopoiesis alone, without exposure to a genotoxic agent, could result in elevated Pig-a mutant cell frequencies. To address this concern, Wistar-Han rats were dosed by oral gavage with a non-genotoxic haemolytic agent, 2-butoxyethanol (2-BE). Dose levels ranging from 0 to 450 mg/kg were tested using both single administration and 28-day treatment regimens. Haematology parameters were assessed at minimum within the first 24h of treatment and 8 days after the final administration. Pig-a mutant frequencies were assessed on Days 15 and ~30 for both treatment protocols and also on Days 43 and 57 for the 28-day protocol. Even at doses of 2-BE that induced marked intravascular lysis and strong compensatory erythropoiesis, the average Pig-a mutant phenotype red blood cell and reticulocyte frequencies were within the historical vehicle control distribution. 2-BE therefore showed no evidence of in vivo mutagenicity in these studies. The data suggest that perturbations in haematopoiesis alone do not lead to an observation of increased mutant frequency in the Pig-a assay.

Chemical dispersants used in the Gulf of Mexico oil crisis are cytotoxic and genotoxic to sperm whale skin cells

The 2010 Deepwater Horizon oil rig explosion in the Gulf of Mexico drew attention to the need for toxicological studies of chemical dispersants. We are still learning the effects these spills had on wildlife. Little is known about the toxicity of these substances in marine mammals. The objective of this study was to determine the toxicity of the two dispersants (Corexit 9500 and 9527). Corexit 9500 and 9527 were both cytotoxic to sperm whale skin fibroblasts. Corexit 9527 was less cytotoxic than 9500. S9 mediated metabolism did not alter cytotoxicity of either dispersant. Both dispersants were genotoxic to sperm whale skin fibroblasts; S9 mediated metabolism increased Corexit 9527 genotoxicity.

The role of hypoxia in 2-butoxyethanol-induced hemangiosarcoma

To understand the molecular mechanisms underlying compound-induced hemangiosarcomas in mice, and therefore, their human relevance, a systems biology approach was undertaken using transcriptomics and Causal Network Modeling from mice treated with 2-butoxyethanol (2-BE). 2-BE is a hemolytic agent that induces hemangiosarcomas in mice. We hypothesized that the hemolysis induced by 2-BE would result in local tissue hypoxia, a well-documented trigger for endothelial cell proliferation leading to hemangiosarcoma. Gene expression data from bone marrow (BM), liver, and spleen of mice exposed to a single dose (4 h) or seven daily doses of 2-BE were used to develop a mechanistic model of hemangiosarcoma. The resulting mechanistic model confirms previous work proposing that 2-BE induces macrophage activation and inflammation in the liver. In addition, the model supports local tissue hypoxia in the liver and spleen, coupled with increased erythropoeitin signaling and erythropoiesis in the spleen and BM, and suppression of mechanisms that contribute to genomic stability, events that could be contributing factors to hemangiosarcoma formation. Finally, an immunohistochemistry method (Hypoxyprobe) demonstrated that tissue hypoxia was present in the spleen and BM. Together, the results of this study identify molecular mechanisms that initiate hemangiosarcoma, a key step in understanding safety concerns that can impact drug decision processes, and identified hypoxia as a possible contributing factor for 2-BE-induced hemangiosarcoma in mice.

In vivometabolism and kinetics of ethylene glycol monobutyl ether and its metabolites, 2-butoxyacetaldehyde and 2-butoxyacetic acid, as measured in blood, liver and forestomach of mice

1. Ethylene glycol monobutyl ether (EGBE) causes forestomach hyperplasia and neoplasia in mice when administered chronically by inhalation. 2. The study was initiated to test the physiologically based pharmacokinetic (PBPK) model prediction that 2-butoxyacetaldehyde (BAL), a transient, labile intermediate in the oxidation of EGBE to butoxyacetic acid (BAA), is unlikely to achieve concentrations sufficient to cause DNA damage in target tissues. 3. Male and female B6C3F1 mice were administered a high oral dose of EGBE (600mgkg(-1)), and tissues were collected at 5, 15, 45 and 90min following the dose. The tissues were processed for determination of EGBE, BAL and BAA by gas chromatography-mass spectrometry. 4. BAL was detected at low concentrations in all tissues sampled and at all time points following EGBE administration (about 0.3-33 microM). BAL concentrations were highest in the initial samples (5 min) in all tissues and declined from that point. 5. BAL concentrations in liver and forestomach tissues corresponded to the peak concentrations predicted by an already published PBPK model, and are higher than BAL concentrations that could be achieved by inhalation exposure to EGBE. 6. Mouse inhalation exposure to EGBE is therefore unlikely to generate BAL concentrations in tissues sufficient to initiate a carcinogenic response.

Mechanisms of 2-Butoxyethanol–Induced Hemangiosarcomas

Chronic exposure to 2-butoxyethanol increased liver hemangiosarcomas in male mice. The mechanism for the selective induction of hemangiosarcomas by 2-butoxyethanol is unknown but has been suggested to occur through non-DNA-reactive mechanisms. The occurrence of liver hemangiosarcomas in male mice has been linked to oxidative damage subsequent to RBC hemolysis and iron deposition and activation of macrophages (Kupffer cells) in the liver, events that exhibit a threshold in both animals and humans. 2-Butoxyethanol is metabolized to 2-butoxyacetaldehyde and 2-butoxyacetic acid, and although the aldehyde metabolite is short lived, the potential exists for this metabolite to cause DNA damage. The present study examined whether 2-butoxyethanol and its metabolites, 2-butoxyacetaldehyde and 2-butoxyacetic acid, damaged mouse endothelial cell DNA using the comet assay. No increase in DNA damage was observed following 2-butoxyethanol (1-10mM), 2-butoxyacetaldehyde (0.1-1.0mM), or 2-butoxyacetic acid (1-10mM) in endothelial cells after 2, 4, or 24 h of exposure. Additional studies examined the involvement of hemolysis and macrophage activation in 2-butoxyethanol carcinogenesis. DNA damage was produced by hemolyzed RBCs (10 x 10(6), 4 h), ferrous sulfate (0.1-1.0 microM; 2-24 h), and hydrogen peroxide (50-100 microM; 1-4 h) in endothelial cells. Hemolyzed RBCs also activated macrophages, as evidenced by increased tumor necrosis factor (TNF) alpha, while neither 2-butoxyethanol nor butoxyacetic acid increased TNF-alpha from macrophages. The effect of activated macrophages on endothelial cell DNA damage and DNA synthesis was also studied. Coculture of endothelial cells with activated macrophages increased endothelial cell DNA damage after 4 or 24 h and increased endothelial cell DNA synthesis after 24 h. These data demonstrate that 2-butoxyethanol and related metabolites do not directly cause DNA damage. Supportive evidence also demonstrated that damaged RBCs, iron, and/or products from macrophage activation (possibly reactive oxygen species) produce DNA damage in endothelial cells and that activated macrophages stimulate endothelial cell proliferation. These events coupled together provide the events necessary for the induction of hemangiosarcomas by 2-butoxyethanol.

U.S. EPA's IRIS assessment of 2-butoxyethanol: the relationship of noncancer to cancer effects

U.S. EPA's integrated risk information system (IRIS) assessment of 2-butoxyethanol (EGBE) indicates that the human carcinogenic potential of EGBE cannot be determined at this time, but that "suggestive evidence" for cancer exists from laboratory animal studies (hemangiosarcoma of the liver in male mice and forestomach squamous cell papilloma or carcinoma in female mice [National Toxicology Program (NTP), 2000a. Toxicology and carcinogenesis studies of 2-butoxyethanol (CAS no. 111-76-2) in F344/N rats and B6C3F1 mice (inhalation studies). National Toxicology Program Technical Report Series No. 484. U.S. Department of Health and Human Services, National Institutes of Health, Washington, DC]). Since the last EGBE IRIS assessment, a number of studies have provided evidence that the carcinogenic effects observed in mice are nonlinear in their mode of action and may be dependent on threshold events such as EGBE-induced hemolytic effects. EPA is in the process of considering several questions relating to this issue. First, can a plausible mode of action be determined for the two types of tumors observed in mice? Second, are the mechanisms involved applicable to humans? If so, should the mode of action be considered to result in a linear or nonlinear dose-response? These questions will be addressed within the context of the agency's new cancer guidelines and with regard to how the answers might affect a revised IRIS assessment for EGBE.

INRS activities on risk assessment of glycol ethers

The occupational exposure assessment uses data from published sources, from Industry (most often from the producers), and from dedicated occupational exposure data bases, as well as evaluations using the EASE model (Estimation and Assessment of Substance Exposure). Atmospheric concentrations and characteristics of skin contacts are evaluated in different scenarios (such as manufacturing, formulating, main and most polluting uses) and sub-scenarios (e.g. warm water dilution). Air concentrations of EGBE are low during production (most often <0.5 mg/m(3)), incidental excursions being <50 mg/m(3); the "worst-case" mean concentration is proposed as 9 mg/m(3). Skin contact, according to EASE, may be in the range of 0-0.1 mg/cm(2)(day), and should be mitigated by the use of suitable gloves. For formulations of products containing EGBE, air concentrations are evaluated as 10 mg/m(3) and skin contact as 0.19 mg/cm(2)(day). The "reasonable worst case" air concentrations (8-Hr TWA) are assessed at around 11 mg/m(3) (coating industry), from 5 to 20 mg/m(3) in printing activities (depending on the task), and in the 20-70 mg/m(3) range (upper limit 40 mg/m(3) in better controlled situations) for cleaning activities. Skin contact would be around twice the preceding level, i.e., 0.4 mg/cm(2)(day) for coating as well as cleaning activities. EGBE and its major metabolites, 2-butoxyacetaldehyde (2-BAL) and 2-butoxyacetic acid (2-BAA) have been subjected to tests for genetic toxicity tests both in vitro and in vivo. While some positive responses have been obtained, the balance of the evidence indicates that EGBE does not express significant genotoxic activity. There are no epidemiological data investigating a relationship between exposure to EGBE and human cancer. Two carcinogenicity inhalation bioassays have been conducted in rodents, one in rats and one in mice. Significant increases were found in forestomach tumours in female mice and haemangiosarcomas in male mice. No increases in tumour incidences were found in either male or female rats. Mechanistic studies have suggested the crucial involvement in the pathogenesis of haemangiosarcomas of a chain of events consisting of (1) haemolysis due to BAA, followed by (2) hepatic haemosiderin deposition and (3) the subsequent generation of reactive oxygen species within the endothelial cells from which haemangiosarcomas arise. Since human erythrocytes are particularly resistant to the haemolytic effects of BAA, it is extremely unlikely, according to this model, that the haemangiosarcomas observed in male mice will have human significance. Similarly, mechanistic studies on the female mouse forestomach tumours have suggested that these also are not important as an indication of human risk. In vivo, EGBE tested in a continuous breeding study and in repeated dose toxicity tests, did not produced specific effects on reproductive organs or fertility parameters. For developmental toxicity, rats, mice and rabbits were dosed via oral and/or inhalation routes. Foeto- and embryo-toxicity was observed in presence or maternal toxicity (haemolytic anaemia). The data available give plausible support to the hypothesis that this developmental toxicity is a direct consequence of maternal toxicity. There are no epidemiological data investigating a relationship between exposure to EGBE alone and human reproductive effects.

Mode of action of butoxyethanol-induced mouse liver hemangiosarcomas and hepatocellular carcinomas

Chronic exposure to 2-butoxyethanol resulted in an increase in liver hemangiosarcomas and hepatic carcinomas in male mouse liver. No increase in liver neoplasia was observed in similarly exposed male and female rats or female mice. We have proposed that the production of liver neoplasia in the male mouse is the result of oxidative damage secondary to the hemolytic deposition of iron in the liver. Our working hypothesis is that the mode of action of butoxyethanol-induced mouse liver hemangiosarcomas and hepatic neoplasia involves the metabolism of 2-butoxyethanol to butoxyacetic acid which results in the induction of RBC hemolysis. This hemolytic response is translated into the accumulation of iron in both liver hepatocytes and Kupffer cells. The Kupffer cell response to this insult is two-fold: (1) the production of oxidative species-through both Kupffer cell activation and through the Fenton reaction involving iron and (2) the production of cytokines (for example TNF alpha). The induction of reactive oxygen species can, if not scavenged, produce oxidative DNA damage (the formation of OH8dG), as well as increase cell growth through modulation of gene expression. While the reactive oxygen species generation would occur in the both rats and mice, the ability of the rat to detoxify the reactive oxygen species would preclude the remaining steps from occurring. In contrast, in the mouse, the reactive oxygen species would override antioxidant defense mechanisms and allow the proposed mode of action to move forward. Our results to date in male B6C3F1 mice and male F344 rats treated with 2-butoxyethanol (via daily gavage; five times per week) at doses of 0, 225, 450, and 900 mg/kg/day (mice) and 0, 225, 450 mg/kg/day (rats), respectively, showed: an increase in hemolysis in 2-butoxyethanol treated rats and mice in a dose-dependent manner, in addition, an increase in the percent of iron stained Kupffer cells in the liver was observed following treatment with 450 and 900 mg/kg of 2-butoxyethanol in mice and 225 and 450 mg/kg of 2-butoxyethanol in rat. With the iron deposition, a biphasic increase in oxidative damage (OH8dG and malondialdehyde) was seen in mouse liver after treatment with 2-butoxyethanol. In contrast, no increase in oxidative damage was observed in the rat liver at any of the doses examined. Concomitant with the increase in oxidative damage, Vitamin E levels were similarly reduced by 2-butoxyethanol in both mice and rat liver. However, the basal level of Vitamin E in rat liver was 2.5-fold greater than in mouse liver. A biphasic induction of DNA synthesis was seen following 2-butoxyethanol in the mouse. In mouse liver, increased DNA synthesis was observed in hepatocytes at 90 days and in endothelial cells at 7 and 14 days at all doses. No change in DNA synthesis was seen in 2-butoxyethanol treated rat liver. No apparent differences in apoptosis and mitosis in the liver were observed in mouse and rat liver between 2-butoxyethanol treatment groups and untreated controls. These results suggest that the induction of DNA synthesis, possibly from oxidative stress and/or Kupffer cell activation, occurs selectively in the mouse liver, in endothelial cells and in hepatocytes following exposure to 2-butoxyethanol, and support the hypothesis proposed above.

Review of studies concerning the tumorigenicity of 2-butoxyethanol in B6C3F1 mice and its relevance for human risk assessment

The U.S. National Toxicology Program (NTP) has completed 2-yr inhalation exposures in rats and mice with 2-butoxyethanol (BE). This review concerns the most significant findings from those studies and describes recent research into the mechanistic aspects of BE-mediated tumorigenesis in the mouse and the relevance of such effects to humans. Two tumor types were increased in B6C3F1 mice leading to the classification of "some evidence" of carcinogenicity: liver hemangiosarcomas in male mice and forestomach tumors in female mice (primarily benign papillomas). The results of research collected to date indicate that the tumorigenesis noted for BE was produced by indirect mechanisms. In particular, the occurrence of liver hemangiosarcomas in male mice has been linked to oxidative damage subsequent to red blood cell hemolysis and iron deposition in this organ. Oral administration of BE in mice up to 600 mg/kg/d for up to 90 d produces a dose-related increase in iron (Perl's staining) in Kupffer cells and hepatocytes, increased DNA synthesis in endothelial cells, and enhanced oxidative damage. Further, iron alone, and not BE or BAA, is responsible for producing oxidative damage in cultured hepatocytes from rats or mice. Forestomach neoplasms in female mice were most likely a result of prolonged exposure-induced irritation with compensatory hyperplasia and subsequent tumor promotion. This mechanism is supported by studies indicating elevated levels of BE and BAA in the mouse forestomach tissues and stomach contents following multiple routes of exposure, forestomach epithelial cell cytotoxicity and cell proliferation following administration of BE and BAA, and the increased capacity of forestomach tissues from female mice to metabolize BE to the more irritating metabolite, BAA. The current article summarizes the results of a number of in vivo and in vitro studies designed to elucidate the underlying mechanisms of tumorigenesis by BE in the mouse and discusses the relevance of these for human risk.

The development of forestomach tumours in the mouse following exposure to 2-butoxyethanol by inhalation: studies on the mode of action and relevance to humans

2-Butoxyethanol, a forestomach carcinogen in mice exposed by inhalation, has been shown to enter the forestomach as a result of grooming and ingestion of material condensed on the skin and fur during exposure. The material entering the stomach concentrates in the forestomach region and persists for at least 48 h post-exposure. Mice given single oral doses of either 2-butoxyethanol or 2-butoxyacetic acid, daily for 10 days, developed a marked hyperkeratosis in the forestomach. 2-Butoxyacetic acid was more potent than 2-butoxyethanol, the NOEL for the former being 50 mg/kg and for the latter, 150 mg/kg. Although a dose dependent increase in cell replication was also seen with both chemicals, the results were confounded by a high labelling rate in the controls. There was no evidence of significant binding of radiolabelled 2-butoxyethanol to proteins in stomach tissues. 2-Butoxyethanol was metabolised in vitro in both mouse and rat forestomach and glandular stomach fractions by alcohol dehydrogenases forming 2-butoxyacetaldehyde which was rapidly converted by aldehyde dehydrogenases to 2-butoxyacetic acid. There was a marked species difference in alcohol dehydrogenase activity between rats and mice with the maximum rates up to one order of magnitude greater in mouse than rat. The alcohol and aldehyde dehydrogenases were heavily concentrated in the stratified squamous epithelium of the forestomach of both rats and mice whereas in the glandular stomach the distribution was more diffuse. In human stomach both enzymes were evenly distributed throughout the epithelial cells of the mucosa. It is concluded that 2-butoxyethanol is ingested following inhalation exposure and concentrates in the forestomach where it is metabolised to 2-butoxyacetic acid which causes cellular damage, increased cell replication and hyperkeratosis. These changes are believed to lead to the tumours seen in mice exposed to 2-butoxyethanol for a lifetime. Differences in structure and enzyme distribution between the rodent and human stomach suggest that the responses seen in the mouse are unlikely to occur in humans.

Effects of 2-butoxyethanol on hepatic oxidative damage

2-Butoxyethanol has been reported to induce an increase in liver tumors in male B6C3F1 mice following chronic inhalation while rats, similarly treated, showed no increase in liver tumors. The mechanism for the selective induction of cancer in mouse liver is unknown, however, 2-butoxyethanol has been shown to induce hemolysis in mice, resulting in an accumulation of hemosiderin (iron) in the liver. Previous studies by our group and others have shown that mouse liver compared to other rodent species has a lower antioxidant capacity and appears to be more susceptible to chemically-induced oxidative damage. Since iron is known to produce hydroxyl radicals (through the Fenton reaction), we have proposed that the 2-butoxyethanol-induced iron overload (through hemolysis) may contribute to the induction of liver neoplasia in the mouse. In the present studies, 2-butoxyethanol induced oxidative stress in the liver of mice following 7-day treatment by gavage. These studies also examined whether 2-butoxyethanol, 2-butoxy acetic acid (a major metabolite of 2-butoxyethanol) or iron (FeSO(4)) produced oxidative stress in mouse and rat hepatocytes. Oxidative stress was examined by measuring oxidative DNA damage (OH8dG), lipid peroxidation (MDA formation) and cellular vitamin E concentrations. Neither 2-butoxyethanol or 2-butoxyacetic acid induced changes in the oxidative stress parameters examined in either rat or mouse hepatocytes. In contrast, FeSO(4) produced a dose-related increase in OH8dG and MDA and a decrease in vitamin E levels following 24 h treatment. Mouse hepatocytes were more sensitive than rat hepatocytes to the oxidative damage induced by the FeSO(4). FeSO(4)-induced oxidative stress was not increased by co-treatment of FeSO(4) with either 2-butoxyethanol or 2-butoxy acetic acid. These results support the proposal that the induction of hepatic oxidative stress by 2-butoxyethanol in vivo occurs secondary to induction of hemolysis and iron deposition in the liver rather than as a direct action of 2-butoxyethanol or its main metabolite, 2-butoxy acetic acid.

Cytotoxic effects of 2-butoxyethanol in vitro are related to butoxyacetaldehyde, an intermediate oxidation product

Ethylene glycol ethers belong to a group of solvents with a wide spectrum of applications, particularly because of their compatibility to both hydrophilic and lipophilic systems. Especially ethylene glycol monobutyl ether (2-butoxyethanol, BE) is widely used as a key ingredient in many industrial and consumer cleaning products. Therefore, the risk of human exposure and toxicity by BE as well as its potential for environmental contamination have to be carefully evaluated. By using an established kidney epithelial cell line from the proximal tubule (opossum kidney cells), we investigated the effects of BE on viability, proliferative activity, volume and the organization of the intracellular cytoskeleton of the cells. The experiments were performed with freshly used BE and BE that had been stored at room temperature in the original packing for 3 months after use. After this period of storage the latter BE contained-besides butyraldehyde and n-butanol-0.5 vol% butoxyacetaldehyde (BAL) as measured by capillary gas chromatography and mass spectrometry. Freshly used BE did not cause a toxic effect in the in vitro assays at all concentrations tested (up to 1 mg/ml). In contrast, stored BE which contained BAL reduced cell viability and mitotic activity in a dose-dependent manner. The effective concentration of stored BE causing a 50% loss in cell viability (EC(50/24h)) was calculated to be 1 mg/ml. The toxic effect of stored BE also resulted in alterations of cell morphology and a depolymerization of actin-containing stress fibers. Moreover, administration of stored BE also caused a dose-dependent cell volume increase by the uptake of water, pointing to a necrotic process. In addition, synthesized BAL with a purity of 73.5% (gas chromatography) was also tested and caused an EC(50/24h) of 15 μg/ml, which is a 70-fold lower concentration when compared with stored BE. The present study provides evidence that BE possesses only a low cytotoxic potential in vitro, whereas the corresponding BAL, an intermediate in the oxidation process of BE to butoxyacetic acid, has marked toxic effects. The occurrence of the aldehyde might explain the predominant hematological effects of BE observed in vivo.