Contribution of dichloroacetate and trichloroacetate to liver tumor induction in mice by trichloroethylene

Persistent gene expression and DNA methylation alterations linked to carcinogenic effects of dichloroacetic acid

Background

Mechanistic understanding of transient exposures that lead to adverse health outcomes will enhance our ability to recognize biological signatures of disease. Here, we measured the transcriptomic and epigenomic alterations due to exposure to the metabolic reprogramming agent, dichloroacetic acid (DCA). Previously, we showed that exposure to DCA increased liver tumor incidence in B6C3F1 mice after continuous or early life exposures significantly over background level.

Methods

Using archived formalin-fixed liver samples, we utilized modern methodologies to measure gene expression and DNA methylation levels to link to previously generated phenotypic measures. Gene expression was measured by targeted RNA sequencing (TempO-seq 1500+ toxicity panel: 2754 total genes) in liver samples collected from 10-, 32-, 57-, and 78-week old mice exposed to deionized water (controls), 3.5 g/L DCA continuously in drinking water ("Direct" group), or DCA for 10-, 32-, or 57-weeks followed by deionized water until sample collection ("Stop" groups). Genome-scaled alterations in DNA methylation were measured by Reduced Representation Bisulfite Sequencing (RRBS) in 78-week liver samples for control, Direct, 10-week Stop DCA exposed mice.

Results

Transcriptomic changes were most robust with concurrent or adjacent timepoints after exposure was withdrawn. We observed a similar pattern with DNA methylation alterations where we noted attenuated differentially methylated regions (DMRs) in the 10-week Stop DCA exposure groups compared to the Direct group at 78-weeks. Gene pathway analysis indicated cellular effects linked to increased oxidative metabolism, a primary mechanism of action for DCA, closer to exposure windows especially early in life. Conversely, many gene signatures and pathways reversed patterns later in life and reflected more pro-tumorigenic patterns for both current and prior DCA exposures. DNA methylation patterns correlated to early gene pathway perturbations, such as cellular signaling, regulation and metabolism, suggesting persistence in the epigenome and possible regulatory effects.

Conclusion

Liver metabolic reprogramming effects of DCA interacted with normal age mechanisms, increasing tumor burden with both continuous and prior DCA exposure in the male B6C3F1 rodent model.

Assessment of the mode of action of perchloroethylene-induced mouse liver tumors

Perchloroethylene (PCE) is used as a solvent and chemical intermediate. Following chronic inhalation exposure, PCE selectively induced liver tumors in mice. Understanding the mode of action (MOA) for PCE carcinogenesis in mice is important in defining its possible human cancer risk. The proposed MOA is based on the extensive examination of the peer-reviewed studies that have assessed the mouse liver effects of PCE and its major oxidative metabolite trichloroacetic acid (TCA). Similar to PCE, TCA has also been demonstrated to liver tumors selectively in mice following chronic exposure. The Key Events (KE) of the proposed PCE MOA involve oxidative metabolism of PCE to TCA [KE 1]; activation of the peroxisome proliferator-activated receptor alpha (PPARα) [KE 2]; alteration in hepatic gene expression including cell growth pathways [KE 3]; increase in cell proliferation [KE 4]; selective clonal expansion of hepatic preneoplastic foci [KE 5]; and formation of hepatic neoplasms [KE 6]. The scientific evidence supporting the PPARα MOA for PCE is strong and satisfies the requirements for a MOA analysis. The PPARα liver tumor MOA in rodents has been demonstrated not to occur in humans; thus, human liver cancer risk to PCE is not likely.

Toxicity assessments of selected trichloroethylene and perchloroethylene metabolites in three in vitro human placental models

Residential and occupational exposures to the industrial solvents perchloroethylene (PERC) and trichloroethylene (TCE) present public health concerns. In humans, maternal PERC and TCE exposures can be associated with adverse birth outcomes. Because PERC and TCE are biotransformed to toxic metabolites and placental dysfunction can contribute to adverse birth outcomes, the present study compared the toxicity of key PERC and TCE metabolites in three in vitro human placenta models. We measured cell viability and caspase 3 + 7 activity in the HTR-8/SVneo and BeWo cell lines, and caspase 3 + 7 activity in first trimester villous explant cultures. Cultures were exposed for 24 h to 5-100 µM S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC), or 5-200 µM trichloroacetate (TCA) and dichloroacetate (DCA). DCVC significantly reduced cell viability and increased caspase 3 + 7 activity in HTR-8/SVneo cells at a lower concentration (20 µM) compared with concentrations toxic to BeWo cells and villous explants. Similarly, TCVC reduced cell viability and increased caspase 3 + 7 activity in HTR-8/SVneo cells but not in BeWo cells. TCA and DCA had only negligible effects on HTR-8/SVneo or BeWo cells. This study advances understanding of potential risks of PERC and TCE exposure during pregnancy by identifying metabolites toxic in placental cells and tissues.

Targeted Metabolomic Assessment of the Sub-Lethal Toxicity of Halogenated Acetic Acids (HAAs) to Daphnia magna

Halogenated acetic acids (HAAs) are amongst the most frequently detected disinfection by-products in aquatic environments. Despite this, little is known about their toxicity, especially at the molecular level. The model organism Daphnia magna, which is an indicator species for freshwater ecosystems, was exposed to sub-lethal concentrations of dichloroacetic acid (DCAA), trichloroacetic acid (TCAA) and dibromoacetic acid (DBAA) for 48 h. Polar metabolites extracted from Daphnia were analyzed using liquid chromatography hyphened to a triple quadrupole mass spectrometer (LC-MS/MS). Multivariate analyses identified shifts in the metabolic profile with exposure and pathway analysis was used to identify which metabolites and associated pathways were disrupted. Exposure to all three HAAs led to significant downregulation in the nucleosides: adenosine, guanosine and inosine. Pathway analyses identified perturbations in the citric acid cycle and the purine metabolism pathways. Interestingly, chlorinated and brominated acetic acids demonstrated similar modes of action after sub-lethal acute exposure, suggesting that HAAs cause a contaminant class-based response which is independent of the type or number of halogens. As such, the identified metabolites that responded to acute HAA exposure may serve as suitable bioindicators for freshwater monitoring programs.

The Importance of Gender-Related Anticancer Research on Mitochondrial Regulator Sodium Dichloroacetate in Preclinical Studies In Vivo

Sodium dichloroacetate (DCA) is an investigational medicinal product which has a potential anticancer preparation as a metabolic regulator in cancer cells’ mitochondria. Inhibition of pyruvate dehydrogenase kinases by DCA keeps the pyruvate dehydrogenase complex in the active form, resulting in decreased lactic acid in the tumor microenvironment. This literature review displays the preclinical research data on DCA’s effects on the cell pyruvate dehydrogenase deficiency, pyruvate mitochondrial oxidative phosphorylation, reactive oxygen species generation, and the Na⁺–K⁺–2Cl⁻ cotransporter expression regulation in relation to gender. It presents DCA pharmacokinetics and the hepatocarcinogenic effect, and the safety data covers the DCA monotherapy efficacy for various human cancer xenografts in vivo in male and female animals. Preclinical cancer researchers report the synergistic effects of DCA combined with different drugs on cancer by reversing resistance to chemotherapy and promoting cell apoptosis. Researchers note that female and male animals differ in the mechanisms of cancerogenesis but often ignore studying DCA’s effects in relation to gender. Preclinical gender-related differences in DCA pharmacology, pharmacological mechanisms, and the elucidation of treatment efficacy in gonad hormone dependency could be relevant for individualized therapy approaches so that gender-related differences in treatment response and safety can be proposed.

GSTZ1-1 Deficiency Activates NRF2/IGF1R Axis in HCC via Accumulation of Oncometabolite Succinylacetone

The IGF1R signaling is important in the malignant progression of cancer. However, overexpression of IGF1R has not been properly assessed in HCC. Here, we revealed that GSTZ1‐1, the enzyme in phenylalanine/tyrosine catabolism, is downregulated in HCC, and its expression was negatively correlated with IGF1R. Mechanistically, GSTZ1‐1 deficiency led to succinylacetone accumulation, alkylation modification of KEAP1, and NRF2 activation, thus promoting IGF1R transcription by recruiting SP1 to its promoter. Moreover, inhibition of IGF1R or NRF2 significantly inhibited tumor‐promoting effects of GSTZ1 knockout in vivo. These findings establish succinylacetone as an oncometabolite, and GSTZ1‐1 as an important tumor suppressor by inhibiting NRF2/IGF1R axis in HCC. Targeting NRF2 or IGF1R may be a promising treatment approach for this subset HCC.

Metabolism of Glutathione S-Conjugates: Multiple Pathways

Many potentially toxic electrophilic xenobiotics and some endogenous compounds are detoxified by conversion to the corresponding glutathione S-conjugate, which is metabolized to the N-acetylcysteine S-conjugate (mercapturate) and excreted. Some mercapturate pathway components, however, are toxic. Bioactivation (toxification) may occur when the glutathione S-conjugate (or mercapturate) is converted to a cysteine S-conjugate that undergoes a β-lyase reaction. If the sulfhydryl-containing fragment produced in this reaction is reactive, toxicity may ensue. Some drugs and halogenated workplace/environmental contaminants are bioactivated by this mechanism. On the other hand, cysteine S-conjugate β-lyases occur in nature as a means of generating some biologically useful sulfhydryl-containing compounds.

The PPARα-dependent rodent liver tumor response is not relevant to humans: addressing misconceptions

A number of industrial chemicals and therapeutic agents cause liver tumors in rats and mice by activating the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). The molecular and cellular events by which PPARα activators induce rodent hepatocarcinogenesis have been extensively studied elucidating a number of consistent mechanistic changes linked to the increased incidence of liver neoplasms. The weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis is summarized here. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators. The key events (KE) identified in the MOA are PPARα activation (KE1), alteration in cell growth pathways (KE2), perturbation of hepatocyte growth and survival (KE3), and selective clonal expansion of preneoplastic foci cells (KE4), which leads to the apical event-increases in hepatocellular adenomas and carcinomas (KE5). In addition, a number of concurrent molecular and cellular events have been classified as modulating factors, because they potentially alter the ability of PPARα activators to increase rodent liver cancer while not being key events themselves. These modulating factors include increases in oxidative stress and activation of NF-kB. PPARα activators are unlikely to induce liver tumors in humans due to biological differences in the response of KEs downstream of PPARα activation. This conclusion is based on minimal or no effects observed on cell growth pathways and hepatocellular proliferation in human primary hepatocytes and absence of alteration in growth pathways, hepatocyte proliferation, and tumors in the livers of species (hamsters, guinea pigs and cynomolgus monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Despite this overwhelming body of evidence and almost universal acceptance of the PPARα MOA and lack of human relevance, several reviews have selectively focused on specific studies that, as discussed, contradict the consensus opinion and suggest uncertainty. In the present review, we systematically address these most germane suggested weaknesses of the PPARα MOA.

SLC transporters as a novel class of tumour suppressors: identity, function and molecular mechanisms

The role of plasma membrane transporters in cancer is receiving increasing attention in recent years. Several transporters for essential nutrients are up-regulated in cancer and serve as tumour promoters. Transporters could also function as tumour suppressors. To date, four transporters belonging to the SLC gene family have been identified as tumour suppressors. SLC5A8 is a Na(+)-coupled transporter for monocarboxylates. Among its substrates are the bacterial fermentation products butyrate and propionate and the ubiquitous metabolite pyruvate. The tumour-suppressive function of this transporter relates to the ability of butyrate, propionate and pyruvate to inhibit histone deacetylases (HDAC). SLC5A8 functions as a tumour suppressor in most tissues studied thus far, and provides a molecular link to Warburg effect, a characteristic feature in most cancers. It also links colonic bacteria and dietary fibre to the host. SLC26A3 as a tumour suppressor is restricted to colon; it is a Cl(-)/HCO(-) 3 exchanger, facilitating the efflux of HCO(-) 3 The likely mechanism for the tumour-suppressive function of SLC26A3 is related to intracellular pH regulation. SLC39A1 is a Zn(2+) transporter and its role in tumour suppression has been shown in prostate. Zn(2+) is present at high concentrations in normal prostate where it elicits its tumour-suppressive function. SLC22A18 is possibly an organic cation transporter, but the identity of its physiological substrates is unknown. As such, there is no information on molecular pathways responsible for the tumour-suppressive function of this transporter. It is likely that additional SLC transporters will be discovered as tumour suppressors in the future.

Therapeutic applications of dichloroacetate and the role of glutathione transferase zeta-1

Dichloroacetate (DCA) has several therapeutic applications based on its pharmacological property of inhibiting pyruvate dehydrogenase kinase. DCA has been used to treat inherited mitochondrial disorders that result in lactic acidosis, as well as pulmonary hypertension and several different solid tumors, the latter through its ability to reverse the Warburg effect in cancer cells and restore aerobic glycolysis. The main clinically limiting toxicity is reversible peripheral neuropathy. Although administration of high doses to rodents can result in liver cancer, there is no evidence that DCA is a human carcinogen. In all studied species, including humans, DCA has the interesting property of inhibiting its own metabolism upon repeat dosing, resulting in alteration of its pharmacokinetics. The first step in DCA metabolism is conversion to glyoxylate catalyzed by glutathione transferase zeta 1 (GSTZ1), for which DCA is a mechanism-based inactivator. The rate of GSTZ1 inactivation by DCA is influenced by age, GSTZ1 haplotype and cellular concentrations of chloride. The effect of DCA on its own metabolism complicates the selection of an effective dose with minimal side effects.

Fatal Liver and Bone Marrow Toxicity by Combination Treatment of Dichloroacetate and Artesunate in a Glioblastoma Multiforme Patient: Case Report and Review of the Literature

A 52-year-old male patient was treated with standard radiochemotherapy with temozolomide for glioblastoma multiforme (GBM). After worsening of his clinical condition, further tumor-specific treatment was unlikely to be successful, and the patient seeked help from an alternative practitioner, who administered a combination of dichloroacetate (DCA) and artesunate (ART). A few days later, the patient showed clinical and laboratory signs of liver damage and bone marrow toxicity (leukopenia, thrombocytopenia). Despite successful restoration of laboratory parameters upon symptomatic treatment, the patient died 10 days after the infusion. DCA bears a well-documented hepatotoxic risk, while ART can be considered as safe concerning hepatotoxicity. Bone marrow toxicity can appear upon ART application as reduced reticulocyte counts and disturbed erythropoiesis. It can be assumed that the simultaneous use of both drugs caused liver injury and bone marrow toxicity. The compassionate use of DCA/ART combination therapy outside of clinical trials cannot be recommended for GBM treatment.

Dichloroacetate Decreases Cell Health and Activates Oxidative Stress Defense Pathways in Rat Alveolar Type II Pneumocytes

Dichloroacetate (DCA) is a water purification byproduct that is known to be hepatotoxic and hepatocarcinogenic and to induce peripheral neuropathy and damage macrophages. This study characterizes the effects of the haloacetate on lung cells by exposing rat alveolar type II (L2) cells to 0–24 mM DCA for 6–24 hours. Increasing DCA concentration and the combination of increasing DCA concentration plus longer exposures decrease measures of cellular health. Length of exposure has no effect on oxidative stress biomarkers, glutathione, SOD, or CAT. Increasing DCA concentration alone does not affect total glutathione or its redox ratio but does increase activity in the SOD/CAT oxidative stress defense pathway. These data suggest that alveolar type II cells rely on SOD and CAT more than glutathione to combat DCA-induced stress.

The Contribution of Peroxisome Proliferator-Activated Receptor Alpha to the Relationship Between Toxicokinetics and Toxicodynamics of Trichloroethylene

Exposure to the ubiquitous environmental contaminant trichloroethylene (TCE) is associated with cancer and non-cancer toxicity in both humans and rodents. Peroxisome proliferator-activated receptor-alpha (PPARα) is thought to be playing a role in liver toxicity in rodents through activation of the receptor by the TCE metabolite trichloroacetic acid (TCA). However, most studies using genetically altered mice have not assessed the potential for PPARα to alter TCE toxicokinetics, which may lead to differences in TCA internal doses and hence confound inferences as to the role of PPARα in TCE toxicity. To address this gap, male and female wild type (129S1/SvImJ), Pparα-null, and humanized PPARα (hPPARα) mice were exposed intragastrically to 400 mg/kg TCE in single-dose (2, 5 and 12 h) and repeat-dose (5 days/week, 4 weeks) studies. Interestingly, following either a single- or repeat-dose exposure to TCE, levels of TCA in liver and kidney were lower in Pparα-null and hPPARα mice as compared with those in wild type mice. Levels of trichloroethanol (TCOH) were similar in all strains. TCE-exposed male mice consistently had higher levels of TCA and TCOH in all tissues compared with females. Additionally, in both single- and repeat-dose studies, a similar degree of induction of PPARα-responsive genes was observed in liver and kidney of hPPARα and wild type mice, despite the difference in hepatic and renal TCA levels. Additional sex- and strain-dependent effects were observed in the liver, including hepatocyte proliferation and oxidative stress, which were not dependent on TCA or TCOH levels. These data demonstrate that PPARα status affects the levels of the putative PPARα agonist TCA following TCE exposure. Therefore, interpretations of studies using Pparα-null and hPPARα mice need to consider the potential contribution of genotype-dependent toxicokinetics to observed differences in toxicity, rather than attributing such differences only to receptor-mediated toxicodynamic effects.

Occupational health hazards of trichloroethylene among workers in relation to altered mRNA expression of cell cycle regulating genes (p53, p21, bax and bcl-2) and PPARA

Trichloroethylene (TCE) is widely used as a metal degreaser in industrial processes. The present study reports on the effects of TCE exposure on workers employed in the lock industries. To ensure exposure of the workers to TCE, its toxic metabolites, trichloroacetic acid (TCA), dichloroacetic acid (DCA) and trichloroethanol (TCEOH) were detected in the plasma of the subjects through solid phase microextraction-gas chromatography-electron capture detection. TCA, DCA and TCEOH were detected in the range of 0.004–2.494 μg/mL, 0.01–3.612 μg/mL and 0.002–0.617 μg/mL, respectively. Quantitative reverse transcription polymerase chain reaction analysis revealed up-regulated expression of p53 (2.4-fold; p < 0.05), p21 (2-fold; p < 0.01), bax (2.9-fold; p < 0.01) mRNAs and down-regulated expression of bcl-2 (67%; p < 0.05) mRNAs, indicating DNA damaging potential of these metabolites. No effects were observed on the levels of p16 and c-myc mRNAs. Further, as TCA and DCA, the ligand of peroxisome proliferator activated receptor alpha (PPARA), are involved in the process of hepatocarcinogenesis in rodents, we examined expression of PPARA mRNA and let-7c miRNA in the workers. No statistically significant differences in expression of PPARA mRNA and let-7c miRNA in patients were observed as compared to values in controls. Dehydroepiandosterone sulfate (DHEAS) is a reported endogenous ligand of PPARA so its competitive role was also studied. We observed decreased levels of DHEAS hormone in the subjects. Hence, its involvement in mediation of the observed changes in the levels of various mRNAs analyzed in this study appears unlikely.

Trichloroethylene biotransformation and its role in mutagenicity, carcinogenicity and target organ toxicity

Metabolism is critical for the mutagenicity, carcinogenicity, and other adverse health effects of trichloroethylene (TCE). Despite the relatively small size and simple chemical structure of TCE, its metabolism is quite complex, yielding multiple intermediates and end-products. Experimental animal and human data indicate that TCE metabolism occurs through two major pathways: cytochrome P450 (CYP)-dependent oxidation and glutathione (GSH) conjugation catalyzed by GSH S-transferases (GSTs). Herein we review recent data characterizing TCE processing and flux through these pathways. We describe the catalytic enzymes, their regulation and tissue localization, as well as the evidence for transport and inter-organ processing of metabolites. We address the chemical reactivity of TCE metabolites, highlighting data on mutagenicity of these end-products. Identification in urine of key metabolites, particularly trichloroacetate (TCA), dichloroacetate (DCA), trichloroethanol and its glucuronide (TCOH and TCOG), and N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (NAcDCVC), in exposed humans and other species (mostly rats and mice) demonstrates function of the two metabolic pathways in vivo. The CYP pathway primarily yields chemically stable end-products. However, the GST pathway conjugate S-(1,2-dichlorovinyl)glutathione (DCVG) is further processed to multiple highly reactive species that are known to be mutagenic, especially in kidney where in situ metabolism occurs. TCE metabolism is highly variable across sexes, species, tissues and individuals. Genetic polymorphisms in several of the key enzymes metabolizing TCE and its intermediates contribute to variability in metabolic profiles and rates. In all, the evidence characterizing the complex metabolism of TCE can inform predictions of adverse responses including mutagenesis, carcinogenesis, and acute and chronic organ-specific toxicity.

Human health effects of tetrachloroethylene: key findings and scientific issues

BACKGROUND

The U.S. Environmental Protection Agency (EPA) completed a toxicological review of tetrachloroethylene (perchloroethylene, PCE) in February 2012 in support of the Integrated Risk Information System (IRIS).

OBJECTIVES

We reviewed key findings and scientific issues regarding the human health effects of PCE described in the U.S. EPA's Toxicological Review of Tetrachloroethylene (Perchloroethylene).

METHODS

The updated assessment of PCE synthesized and characterized a substantial database of epidemiological, experimental animal, and mechanistic studies. Key scientific issues were addressed through modeling of PCE toxicokinetics, synthesis of evidence from neurological studies, and analyses of toxicokinetic, mechanistic, and other factors (tumor latency, severity, and background rate) in interpreting experimental animal cancer findings. Considerations in evaluating epidemiological studies included the quality (e.g., specificity) of the exposure assessment methods and other essential design features, and the potential for alternative explanations for observed associations (e.g., bias or confounding).

DISCUSSION

Toxicokinetic modeling aided in characterizing the complex metabolism and multiple metabolites that contribute to PCE toxicity. The exposure assessment approach-a key evaluation factor for epidemiological studies of bladder cancer, non-Hodgkin lymphoma, and multiple myeloma-provided suggestive evidence of carcinogenicity. Bioassay data provided conclusive evidence of carcinogenicity in experimental animals. Neurotoxicity was identified as a sensitive noncancer health effect, occurring at low exposures: a conclusion supported by multiple studies. Evidence was integrated from human, experimental animal, and mechanistic data sets in assessing adverse health effects of PCE.

CONCLUSIONS

PCE is likely to be carcinogenic to humans. Neurotoxicity is a sensitive adverse health effect of PCE.

Do Antioxidant Enzymes and Glutathione Play Roles in the Induction of Hepatic Oxidative Stress in Mice upon Subchronic Exposure to Mixtures of Dichloroacetate and Trichloroacetate?

Dichloroacetate (DCA) and trichloroacetate (TCA) are water chlorination byproducts, and their mixtures were previously found to induce additive to greater than additive effects on hepatic oxidative stress (OS) induction in mice after subchronic exposure. To investigate the roles of antioxidant enzymes and glutathione (GSH) in those effects, livers of B6C3F1 mice treated by gavage with 7.5, 15, or 30 mg DCA/kg/day, 12.5, 25, or 50 mg TCA/kg/day, and mixtures (Mix I, Mix II and Mix III) at DCA:TCA ratios corresponding to 7.5:12.5, 15:25 and 25:50 mg/kg/day, respectively, for 13 weeks. Livers were assayed for superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), as well as for GSH levels. In general, DCA suppressed SOD and GSH-Px activities and GSH levels but caused no changes in CAT activity; TCA increased SOD and CAT activities, suppressed GSH-Px activity, but did not change GSH levels; mixtures of DCA and TCA increased SOD and CAT activities and suppressed GSH-Px activity and GSH levels. In conclusion, antioxidant enzymes contribute to DCA-, TCA- and mixtures-induced OS, but not to changes from additive to greater than additive effects produced by different mixture compositions of the compounds. GSH on the hand may contribute to these changes.

The effects of mixtures of dichloroacetate and trichloroacetate on induction of oxidative stress in livers of mice after subchronic exposure

Dichloroacetate (DCA) and trichloroacetate (TCA) are drinking-water chlorination by-products previously found to induce oxidative stress (OS) in hepatic tissues of B6C3F1 male mice. To assess the effects of mixtures of the compounds on OS, groups of male B6C3F1 mice were treated daily by gavage with DCA at doses of 7.5, 15, or 30 mg/kg/d, TCA at doses of 12.5, 25, or 50 mg/kg/d, and 3 mixtures of DCA and TCA (Mix I, Mix II, and Mix III), for 13 wk. The concentrations of the compounds in Mix I, Mix II, and Mix III corresponded to those producing approximately 15, 25, and 35%, respectively, of maximal induction of OS by individual compounds. Livers were assayed for production of superoxide anion (SA), lipid peroxidation (LP), and DNA single-strand breaks (SSB). DCA, TCA, and the mixtures produced dose-dependent increases in the three tested biomarkers. Mix I and II effects on the three biomarkers, and Mix III effect on SA production were found to be additive, while Mix III effects on LP and DNA-SSB were shown to be greater than additive. Induction of OS in livers of B6C3F1 mice after subchronic exposure to DCA and TCA was previously suggested as an important mechanism in chronic hepatotoxicity/hepatocarcinogenicity induced by these compounds. Hence, there may be rise in exposure risk to these compounds as these agents coexist in drinking water.

Micronucleus induction by oxidative metabolites of trichloroethylene in cultured human peripheral blood lymphocytes: a comparative genotoxicity study

The genotoxic effects of oxidative metabolites of trichloroethylene (TCE), namely chloral hydrate, trichloroacetic acid (TCA), dichloroacetic acid (DCA), and trichloroethanol (TCEOH) were examined in human peripheral blood lymphocytes. In this context, lymphocytes were exposed in vitro to 25, 50, and 100 μg/ml concentrations of these metabolites separately for a period of 48 h and examined for micronucleus (MN) induction through flow cytometer. At 50 μg/ml TCE metabolites, TCA (6.33 ± 0.56 %), DCA (5.06 ± 0.55), and TCEOH (4.70 ± 1.73) induced highly significant (p<0.001) frequency of MN in comparison to control (1.03 ± 0.40) suggestive of their genotoxic potential. However, exposure of 100 μg/ml of all the metabolites consistently declined the frequencies of MN which in some cases was equable to that of observed at 25 μg/ml. Further, cytotoxicity and cell cycle disturbances were also measured to find out the association of these endpoints with the MN induction. DNA content analysis revealed 3-4-fold elevation of S-phase at all the concentrations tested. Particularly, at 100 μg/ml, treatment elevation of S-phase was significantly (p<0.0001) higher as compared to the control. Present findings together with earlier reports indicate that TCE induces genotoxicity through its metabolites. Interaction of these metabolites with DNA, as evident by elevated S-phase, seems to be the major cause of MN induction. However, involvement of spindle disruption cannot be ruled out. This comparative study also suggests that after TCE exposure, the metabolic efficiency of human to generate oxidative metabolites determines the extent of genotoxicity.

Mode of action framework analysis for receptor-mediated toxicity: The peroxisome proliferator-activated receptor alpha (PPARα) as a case study

Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is "not relevant to humans" with the remaining members concluding that the MOA is "unlikely to be relevant to humans". The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Trichloroethylene: Mechanistic, epidemiologic and other supporting evidence of carcinogenic hazard

The chlorinated solvent trichloroethylene (TCE) is a ubiquitous environmental pollutant. The carcinogenic hazard of TCE was the subject of a 2012 evaluation by a Working Group of the International Agency for Research on Cancer (IARC). Information on exposures, relevant data from epidemiologic studies, bioassays in experimental animals, and toxicity and mechanism of action studies was used to conclude that TCE is carcinogenic to humans (Group 1). This article summarizes the key evidence forming the scientific bases for the IARC classification. Exposure to TCE from environmental sources (including hazardous waste sites and contaminated water) is common throughout the world. While workplace use of TCE has been declining, occupational exposures remain of concern, especially in developing countries. The strongest human evidence is from studies of occupational TCE exposure and kidney cancer. Positive, although less consistent, associations were reported for liver cancer and non-Hodgkin lymphoma. TCE is carcinogenic at multiple sites in multiple species and strains of experimental animals. The mechanistic evidence includes extensive data on the toxicokinetics and genotoxicity of TCE and its metabolites. Together, available evidence provided a cohesive database supporting the human cancer hazard of TCE, particularly in the kidney. For other target sites of carcinogenicity, mechanistic and other data were found to be more limited. Important sources of susceptibility to TCE toxicity and carcinogenicity were also reviewed by the Working Group. In all, consideration of the multiple evidence streams presented herein informed the IARC conclusions regarding the carcinogenicity of TCE.

Metabolomics reveals trichloroacetate as a major contributor to trichloroethylene-induced metabolic alterations in mouse urine and serum

Trichloroethylene (TCE)-induced liver toxicity and carcinogenesis is believed to be mediated in part by activation of the peroxisome proliferator-activated receptor α (PPARα). However, the contribution of the two TCE metabolites, dichloroacetate (DCA) and trichloroacetate (TCA) to the toxicity of TCE, remains unclear. The aim of the present study was to determine the metabolite profiles in serum and urine upon exposure of mice to TCE, to aid in determining the metabolic response to TCE exposure and the contribution of DCA and TCA to TCE toxicity. C57BL/6 mice were administered TCE, TCA, or DCA, and urine and serum subjected to ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC-ESI-QTOFMS)-based global metabolomics analysis. The ions were identified through searching metabolomics databases and by comparison with authentic standards, and quantitated using multiple reactions monitoring. Quantitative polymerase chain reaction of mRNA, biochemical analysis, and liver histology were also performed. TCE exposure resulted in a decrease in urine of metabolites involved in fatty acid metabolism, resulting from altered expression of PPARα target genes. TCE treatment also induced altered phospholipid homeostasis in serum, as revealed by increased serum lysophosphatidylcholine 18:0 and 18:1, and phosphatidylcholine metabolites. TCA administration revealed similar metabolite profiles in urine and serum upon TCE exposure, which correlated with a more robust induction of PPARα target gene expression associated with TCA than DCA treatment. These data show the metabolic response to TCE exposure and demonstrate that TCA is the major contributor to TCE-induced metabolite alterations observed in urine and serum.

Diosmin protects against trichloroethylene-induced renal injury in Wistar rats: plausible role of p53, Bax and caspases

Diosmin (DM) is a naturally occurring flavone and has been found to possess numerous therapeutic properties. In this study, we used DM as a protective agent against the nephrotoxic effects of the environmental toxicant trichloroethylene (TCE). Male Wistar rats were divided into five groups (I-V, n 6). Groups II, III and IV received an oral administration of TCE at a dose of 1000 mg/kg body weight for twenty consecutive days. The animals in groups II and III received an oral treatment of DM at doses of 20 and 40 mg/kg body weight, respectively, for twenty consecutive days, while groups I and V were given maize oil (5 ml/kg body weight and DM 40 mg/kg body weight, respectively) for 20 d. The protective effects of DM on TCE-induced oxidative stress and caspase-dependent apoptosis were investigated by assaying oxidative stress biomarkers, lipid peroxidation (LPO), serum toxicity markers, alkaline unwinding assay, caspase-3, -7 and -9, Bax and p53 expression. Oral administration of TCE in rats enhanced renal LPO, depleted glutathione content and antioxidant enzymes, induced DNA strand breaks (P<0·001), modulated the expression of Bax and p53 protein and induced the expression of caspase-3, -7 and -9. Co-treatment with DM prevented oxidative stress by restoring the levels of antioxidant enzymes; furthermore, a significant dose-dependent decrease in DNA disintegration and kidney toxicity markers such as blood urea N, creatinine, lactate dehydrogenase and kidney injury molecule-1 was observed. DM also effectively decreased the TCE-induced up-regulation of Bax and p53. Data from the present study establish the protective role of DM against TCE-induced renal damage.

Interstrain differences in the liver effects of trichloroethylene in a multistrain panel of inbred mice

Trichloroethylene (TCE) is a widely used industrial chemical and a common environmental contaminant. It is a well-known carcinogen in rodents and a probable carcinogen in humans. Studies utilizing panels of mouse inbred strains afford a unique opportunity to understand both metabolic and genetic basis for differences in responses to TCE. We tested the hypothesis that strain- and liver-specific toxic effects of TCE are genetically controlled and that the mechanisms of toxicity and susceptibility can be uncovered by exploring responses to TCE using a diverse panel of inbred mouse strains. TCE (2100 mg/kg) or corn oil vehicle was administered by gavage to 6- to 8-week-old male mice of 15 mouse strains. Serum and liver were collected at 2, 8, and 24 h postdosing and were analyzed for TCE metabolites, hepatocellular injury, and gene expression of liver. TCE metabolism, as evident from the levels of individual oxidative and conjugative metabolites, varied considerably between strains. TCE treatment-specific effect on the liver transcriptome was strongly dependent on genetic background. Peroxisome proliferator-activated receptor-mediated molecular networks, consisting of the metabolism genes known to be induced by TCE, represent some of the most pronounced molecular effects of TCE treatment in mouse liver that are dependent on genetic background. Conversely, cell death, liver necrosis, and immune-mediated response pathways, which are altered by TCE treatment in liver, are largely genetic background independent. These studies provide better understanding of the mechanisms of TCE-induced toxicity anchored on metabolism and genotype-phenotype correlations that may define susceptibility or resistance.

Trichloroethylene risk assessment: a review and commentary

Trichloroethylene (TCE) is a widespread environmental contaminant that is carcinogenic when given in high, chronic doses to certain strains of mice and rats. The capacity of TCE to cause cancer in humans is less clear. The current maximum contaminant level (MCL) of 5 ppb (microg/L) is based on an US Environment Protection Agency (USEPA) policy decision rather than the underlying science. In view of major advances in understanding the etiology and mechanisms of chemically induced cancer, USEPA began in the late 1990s to revise its guidelines for cancer risk assessment. TCE was chosen as the pilot chemical. The USEPA (2005) final guidelines emphasized a "weight-of-evidence" approach with consideration of dose-response relationships, modes of action, and metabolic/toxicokinetic processes. Where adequate data are available to support reversible binding of the carcinogenic moiety to biological receptors as the initiating event (i.e., a threshold exists), a nonlinear approach is to be used. Otherwise, the default assumption of a linear (i.e., nonthreshold) dose-response is utilized. When validated physiologically based pharmacokinetic (PBPK) models are available, they are to be used to predict internal dosimetry as the basis for species and dose extrapolations. The present article reviews pertinent literature and discusses areas where research may resolve some outstanding issues and facilitate the reassessment process. Key research needs are proposed, including role of dichloroacetic acid (DCA) in TCE-induced liver tumorigenesis in humans; extension of current PBPK models to predict target organ deposition of trichloroacetic acid (TCA) and DCA in humans ingesting TCE in drinking water; use of human hepatocytes to ascertain metabolic rate constants for use in PBPK models that incorporate variability in metabolism of TCE by potentially sensitive subpopulations; measurement of the efficiency of first-pass elimination of trace levels of TCE in drinking water; and assessment of exogenous factors' (e.g., alcohol, drugs) ability to alter metabolic activation and risks at such low-level exposure.

Enzymes Involved in Processing Glutathione Conjugates

Many potentially toxic electrophiles react with glutathione to form glutathione S-conjugates in reactions catalyzed or enhanced by glutathione S-transferases. The glutathione S-conjugate is sequentially converted to the cysteinylglycine-, cysteine- and N-acetyl-cysteine S-conjugate (mercapturate). The mercapturate is generally more polar and water soluble than the parent electrophile and is readily excreted. Excretion of the mercapturate represents a detoxication mechanism. Some endogenous compounds, such as leukotrienes, prostaglandin (PG) A2, 15-deoxy-Δ^12,14-PGJ₂, and hydroxynonenal can also be metabolized to mercapturates and excreted. On occasion, however, formation of glutathione S- and cysteine S-conjugates are bioactivation events as the metabolites are mutagenic and/or cytotoxic. When the cysteine S-conjugate contains a strong electron-withdrawing group attached at the sulfur, it may be converted by cysteine S-conjugate β-lyases to pyruvate, ammonium and the original electrophile modified to contain an –SH group. If this modified electrophile is highly reactive then the enzymes of the mercapturate pathway together with the cysteine S-conjugate β-lyases constitute a bioactivation pathway. Some endogenous halogenated environmental contaminants and drugs are bioactivated by this mechanism. Recent studies suggest that coupling of enzymes of the mercapturate pathway to cysteine S-conjugate β-lyases may be more common in nature and more widespread in the metabolism of electrophilic xenobiotics than previously realized.

Immuno- and hepato-toxicity of dichloroacetic acid in MRL(+/+) and B(6)C(3)F(1) mice

Dichloroacetic acid (DCA) is a by-product of chlorination that occurs in drinking water disinfected with chlorine. Metabolism of trichloroethene (TCE) also generates DCA. TCE exposure is associated with the development of autoimmune diseases, which may be induced by TCE metabolites, such as DCA. Thus, it is important to understand immunotoxic responses to DCA. We chose 2 murine models, autoimmune-prone MRL(+/+) and normal B(6)C(3)F(1) mice. Both strains of mice were exposed to DCA for 12 weeks. Following DCA treatment, liver weights and liver-to-body weight ratios were significantly increased in both strains of mice when compared to their respective controls. The serum activity of alanine and aspartate aminotransferases was not significantly altered in either strain. In MRL(+/+) mice, the serum concentrations of IgG and IgM were significantly increased, whereas in B(6)C(3)F(1) mice, only serum IgG(3) was increased. DCA treatment did not change the levels of inflammatory cytokines in the serum. However, independent of treatment, the concentrations of G-CSF in the serum were lower in MRL(+/+) mice than in B(6)C(3)F(1) mice, whereas IL-12 serum levels were higher in MRL(+/+) mice. DCA treatment decreased IL-10 and KC chemokine concentrations in the livers of MRL(+/+) mice, whereas T-helper cell cytokines (IL-4, IL-5, IL-10, IFNgamma, and GM-CSF), pro-inflammatory cytokines (IL-6, IL-12, and G-CSF), and KC chemokine were increased in the livers of DCA-treated B(6)C(3)F(1) mice. Stimulation of splenic T-lymphocytes with antibodies against CD3 and CD28 resulted in a marked difference in the secreted cytokines between the two strains of mice. T-lymphocytes from MRL(+/+) mice secreted more IL-2, IL-4 and IL-10, but less IFNgamma and GM-CSF, than did T-lymphocytes from B(6)C(3)F(1) mice. Thus, the cytokine levels in serum and liver, and the cytokine secretion patterns from stimulated splenic T-lymphocytes suggested a higher propensity of inflammatory responses in B(6)C(3)F(1) than in MRL(+/+) mice. Treatment with DCA also affected lipid accumulation in the liver more severely in B(6)C(3)F(1) than in MRL(+/+) mice. Thus, these results indicate that DCA induced stronger inflammatory responses leading to more severe hepatotoxicity in B(6)C(3)F(1) mice than in MRL(+/+) mice, and more pronounced immune responses in the latter.

Toxicity, biomarkers, genotoxicity, and carcinogenicity of trichloroethylene and its metabolites: a review

Trichloroethylene (TCE) is a prevalent occupational and environmental contaminant that has been reported to cause a variety of toxic effects. This article reviews toxicity, mutagenicity, and carcinogenicity caused by the exposure of TCE and its metabolites in the living system as well as on their (TCE and its metabolites) toxicity biomarkers.

Trichloroethylene liver toxicity in mouse and rat: microarray analysis reveals species differences in gene expression

Trichloroethylene (TCE), an industrial organic solvent found in the environment, is a known carcinogen in laboratory animals and is believed to be carcinogenic in humans. Its carcinogenicity is subject to species-specific differences in biological activity, causing hepatocellular carcinoma in mouse and renal-cell carcinoma in rat. We have sought to better understand TCE's mode of action (MOA) by studying the alterations in gene expression profiles of liver in mice and rats that were administrated TCE by oral gavage either once or daily for 14 days. Microarray analysis revealed distinct transcriptional profiles and differences in biological pathways not only species-specific, but also pulse-dose effects within each species. For example, inhibition of the TGF-beta pathway and activation of MAPK signaling were specific to mice repeatedly exposed to TCE. A better understanding of the MOA in mice and rats will lead to better hypotheses of TCE's affect on humans.

Evaluation of the role of peroxisome proliferator-activated receptor alpha (PPARalpha) in mouse liver tumor induction by trichloroethylene and metabolites

Trichloroethylene (TCE) is an industrial solvent and a widespread environmental contaminant. Induction of liver cancer in mice by TCE is thought to be mediated by two metabolites, dichloroacetate (DCA) and trichloroacetate (TCA), both of which are themselves mouse liver carcinogens. TCE, TCA, and DCA are relatively weak peroxisome proliferators (PP), a group of rodent hepatocarcinogens that activate a nuclear receptor, PP-activated receptor alpha (PPARalpha. The objective of this review is to assess the weight of evidence (WOE) that PPARalpha is or is not mechanistically involved in mouse liver tumor induction by TCE and metabolites. Based on similarities of TCE and TCA to typical PP, including dose-response characteristics showing PPARalpha-dependent responses coincident with liver tumor induction and abolishment of TCE and TCA effects in PPARalpha-null mice, the WOE supports the hypothesis that PPARalpha plays a dominant role in TCE- and TCA-induced hepatocarcinogenesis. Data indicates that the MOA for DCA tumor induction is PPARalpha-independent. Uncertainties remain regarding the genesis of the TCE-induced tumors. In contrast to the TCA-induced tumors, which have molecular features similar to those induced by typical PP, there is evidence, albeit weak, that TCE tumors arise by a mode of action (MOA) different from that of TCA tumors, based largely on dissimilarities in molecular markers found in TCE versus TCA-induced tumors. In summary, the WOE indicates that TCA-induced liver tumors arise by a PPARalpha-dependent MOA. Although the TCE MOA is likely dominated by a PPARalpha-dependent contribution from TCA, the contribution of a PPARalpha-independent MOA from DCA cannot be ruled out.

Evaluation of dichloroacetic acid for carcinogenicity in genetically modified Tg.AC hemizygous and p53 haploinsufficient mice

There has been considerable interest in the use of genetically modified mice for detecting potential environmental carcinogens. For this reason, the National Toxicology Program has been evaluating Tg.AC hemizygous and p53 haploinsufficient mice as models to detect potential carcinogens. It was reasoned that these mouse models might also prove more effective than standard rodent models in evaluating the numerous disinfection byproducts that are found in low concentrations in drinking water. Dichloroacetic acid (DCA) is one of the most frequently found disinfection byproducts and DCA has been consistently shown to cause hepatocellular tumors in rats and mice in standard rodent studies. Tg.AC hemizygous and p53 haploinsufficient mice were exposed in the drinking water to DCA for up to 41 weeks. In a second study Tg.AC mice were subjected to dermal DCA exposure for up to 39 weeks. Increased incidences and severity of cytoplasmic vacuolization of hepatocytes were seen in the p53 mice, but there was no evidence of carcinogenic activity at exposures of up to 2000 mg/l in the drinking water. Increased incidences and severity of cytoplasmic vacuolization of hepatocytes were seen in the drinking water study with Tg.AC mice and a modest non-dose-related increase in pulmonary adenomas was observed in males exposed to 1000 mg/l in the drinking water. Dermal exposure up to 500 mg/kg for 39 weeks resulted in increased dermal papillomas at the site of application in Tg.AC mice. No significant increase in papillomas under the same study conditions was seen in the 26-week study. For DCA under these study conditions, the p53 and Tg.AC mice appear less sensitive to hepatocarcinogenesis than standard rodent models. These results suggest caution for the use of Tg.AC and p53 mice to screen unknown chemicals in drinking water for potential carcinogenicity.

Genetic signature for human risk assessment: lessons from trichloroethylene

Trichloroethylene (TCE), an organic solvent commonly used for metal degreasing and as a chemical additive, is a significant environmental contaminant that poses health concerns in humans. The US Environmental Protection Agency (EPA) is currently revising the 2001 TCE human risk assessment draft. The next draft is expected to be ready in 2008. TCE metabolites are detectable in humans and carry varying potencies for induction of cancers in animals. Genomic mechanisms have been explored in animals and humans to link TCE to carcinogenesis. DNA analysis provides an opportunity for detection of unique genetic alterations representing a signature of TCE exposure. These alterations can arise from genotoxic and nongenotoxic pathways at multiple points throughout tumorigenesis. Although fixation of alterations may require several stages of selection and modification, the spectra can be specific to TCE. Only a fraction of these alterations eventually lead to tumor formation and some contribute to tumor progression. Genetic events in two major TCE target organs are reviewed, including the VHL gene in kidney, and the Ras gene and genome-wide hypomethylation in liver. Attempts to identify a genetic signature of TCE exposure are challenged by inconsistent findings, lack of evidence of promutagenic lesions, biological relevance of specific genomic changes, and likelihood of coexposures. For human risk assessment, genome-wide screening is useful and is possible with the development of new DNA-sequencing technologies. Genetic screening for preneoplastic and tumor tissues from high-risk population is proposed to exclude the noise of passenger mutations and genetic polymorphisms.

Chemical measures of similarity among disinfection by-product mixtures

There are few measures that can be used to distinguish among mixtures of disinfection by-products (DBPs) produced in the chlorination or chloramination of drinking water. Objective measures of similarities among DBP mixtures would greatly simplify judgments about the risk that may be associated with exposure to DBPs in a given water supply. Major by-products of chlorination/chloramination include the trihalomethanes (THMs) and haloacetic acids (HAAs), which are routinely measured for compliance to regulations. A key question is whether measurement of similar amounts of these DBPs is indicative of the myriad other DBPs that are known to be produced. This article utilized data from a survey of 35 utilities in the United States that included several additional parameters, including members of the haloacetonitrile, trihaloacetaldehyde, and halopropanone classes. Based upon the distribution of bromine in the THM class, the concentrations of unmeasured brominated and bromochlorinated compounds could be determined. This allowed determination of whether measures of the THM and/or HAA classes reflected the amounts of these less abundant classes. Variations in relative yields among DBP classes were observed with water source type and with whether chlorine or chloramine was used as the disinfectant. However, most of the variability was attributable to geographic location. The relative abundance of brominated by-products also varied among water sources. Recent documentation that potent by-products, such as nitrosamines, are selectively produced in particular water systems and preferentially with chloramination indicates that more measures of individual DBP are needed to evaluate similarity among DBPs mixtures.

Determinants of whether or not mixtures of disinfection by-products are similar

Reactive chemicals have been used to disinfect drinking waters for over a century. In the 1970s, it was first observed that the reaction of these chemicals with the natural organic matter (NOM) in source waters results in the production of variable, complex mixtures of disinfection by-products (DBP). Because limited toxicological and epidemiological data are available to assess potential human health risks from complex DBP mixture exposures, methods are needed to determine when health effects data on a specific DBP mixture may be used as a surrogate for evaluating another environmental DBP mixture of interest. Before risk assessors attempt such efforts, a set of criteria needs to be in place to determine whether two or more DBP mixtures are similar in composition and toxicological potential. This study broadly characterizes the chemical and toxicological measures that may be used to evaluate similarities among DBP mixtures. Variables are discussed that affect qualitative and quantitative shifts in the types of DBP that are formed, including disinfectants used, their reactions with NOM and with bromide/iodide, pH, temperature, time, and changes in the water distribution system. The known toxicological activities of DBP mixtures and important single DBPs are also presented in light of their potential for producing similar toxicity. While DBP exposures are associated with a number of health effects, this study focuses on (1) mutagenic activity of DBP mixtures, (2) DBP cancer epidemiology, and (3) toxicology studies to evaluate similarity among DBP mixtures. Data suggest that further chemical characterization of DBP mixtures and more systematic study of DBP toxicology will improve the quality and usefulness of similarity criteria.

Applying Mode-of-Action and Pharmacokinetic Considerations in Contemporary Cancer Risk Assessments: An Example with Trichloroethylene

The guidelines for carcinogen risk assessment recently proposed by the U.S. Environmental Protection Agency (U.S. EPA) provide an increased opportunity for the consideration of pharmacokinetic and mechanistic data in the risk assessment process. However, the greater flexibility of the new guidelines can also make their actual implementation for a particular chemical highly problematic. To illuminate the process of performing a cancer risk assessment under the new guidelines, the rationale for a state-of-the-science risk assessment for trichloroethylene (TCE) is presented. For TCE, there is evidence of increased cell proliferation due to receptor interaction or cytotoxicity in every instance in which tumors are observed, and most tumors represent an increase in the incidence of a commonly observed, species-specific lesion. A physiologically based pharmacokinetic (PBPK) model was applied to estimate target tissue doses for the three principal animal tumors associated with TCE exposure: liver, lung, and kidney. The lowest points of departure (lower bound estimates of the exposure associated with 10% tumor incidence) for lifetime human exposure to TCE were obtained for mouse liver tumors, assuming a mode of action primarily involving the mitogenicity of the metabolite trichloroacetic acid (TCA). The associated linear unit risk estimates for mouse liver tumors are 1.5 x 10(-6) for lifetime exposure to 1 microg TCE per cubic meter in air and 0.4 x 10(-6) for lifetime exposure to 1 microg TCE per liter in drinking water. However, these risk estimates ignore the evidence that the human is likely to be much less responsive than the mouse to the carcinogenic effects of TCA in the liver and that the carcinogenic effects of TCE are unlikely to occur at low environmental exposures. Based on consideration of the most plausible carcinogenic modes of action of TCE, a margin-of-exposure (MOE) approach would appear to be more appropriate. Applying an MOE of 1000, environmental exposures below 66 microg TCE per cubic meter in air and 265 microg TCE per liter in drinking water are considered unlikely to present a carcinogenic hazard to human health.

Comments on Article “Applying Mode-of-Action and Pharmacokinetic Considerations in Contemporary Cancer Risk Assessments: An Example with Trichloroethylene” by Clewell and Andersen

In their 2004 article, Clewell and Andersen provide their perspective on the application of mode-of-action (MOA) and pharmacokinetic considerations in contemporary cancer risk assessment using trichloroethylene (TCE) as a case example. TCE is a complex chemical toxicologically, with multiple metabolites, multiple sites of observed toxicity, and multiple potential MOAs. As scientists who are responsible for revising the U.S. Environmental Protection Agency's draft risk assessment of TCE, we welcome input of the quality to which the Agency is held accountable. However, in our view, Clewell and Andersen do not present a sufficiently current, complete, accurate, and transparent review of the pertinent scientific literature. In particular, their article would need to incorporate substantial recently published scientific information, better support its conclusions about MOA and choice of linear or nonlinear dose-response extrapolation, and increase its transparency as to quantitative analyses in order to make a significant contribution to the scientific discussion of TCE health risks.

Use of mechanistic data in IARC evaluations

Consideration of mechanistic data has the potential to improve the analysis of both epidemiologic studies and cancer bioassays. IARC has a classification system in which mechanistic data can play a pivotal role. Since 1991, IARC has allowed an agent to be classified as carcinogenic to humans (Group 1) when there is less than sufficient evidence in humans but there is sufficient evidence in experimental animals and "strong evidence in exposed humans that the agent acts through a relevant mechanism of carcinogenicity." Mechanistic evidence can also substitute for conventional cancer bioassays when there is less than sufficient evidence in experimental animals, just as mechanistic evidence can substitute for conventional epidemiologic studies when there is less than sufficient evidence in humans. The IARC Monographs have used mechanistic data to raise or lower a classification that would be otherwise based on epidemiologic studies and cancer bioassays only. Recently, the IARC Monographs have evaluated several agents where mechanistic data were pivotal to the overall evaluation: benzo[a]pyrene, carbon black and other poorly soluble particles, ingested nitrates and nitrites, and microcystin-LR. In evaluating mechanistic data, it is important to consider alternative mechanistic hypotheses, because an agent may induce tumors through multiple mechanisms.

Difficulty of mode of action determination for trichloroethylene: An example of complex interactions of metabolites and other chemical exposures

The mode(s) of action (MOA) of a pollutant for adverse health effects may be dependent on the mixture of metabolites resulting from exposure to a single agent and may also be affected by coexposure to pollutants that have similar targets or affected pathways. Trichloroethylene (TCE) can be an useful example for illustration of the complexity coexposure can present to elucidation of the MOA of an agent. TCE exposure has been associated with increased risk of liver and kidney cancer in both laboratory animal and epidemiologic studies. There are a number of TCE metabolites that could play a role in the induction of these effects. Coexposures of other chemicals with TCE typically occurs as a result of environmental cocontamination that include its own metabolites, such as trichloroacetic acid, dichloroacetic acid, and other pollutants with similar metabolites such as perchloroethylene. Behaviors such as alcohol consumption can also potentially modify TCE toxicity through similar MOAs. The U.S. Environmental Protection Agency (EPA)'s 2001 draft TCE risk assessment, Trichloroethylene (TCE) Health Risk Assessment: Synthesis and Characterization, concluded that it was difficult to determine which of the metabolites of TCE may be responsible for these effects, what key events in their hypothesized MOAs are involved, and the relevance of some of the hypothesized MOAs to humans. Since the publication of U.S. EPA's draft TCE assessment, several studies have been conducted to understand the effects of coexposures to TCE. They cover both pharmacodynamic and pharmacokinetic considerations. This article highlights some of the recently published scientific literature on toxicological interactions between TCE, its metabolites, and other coexposures, including solvents, haloacetates, and ethanol. These studies give insight into both the potential MOAs of TCE exposure itself and putative modulators of TCE toxicity, and illustrate the difficulties encountered in determining the MOAs and modulators of toxicity for pollutants with such complex metabolism and coexposures.

Kinetics of chloral hydrate and its metabolites in male human volunteers

Chloral hydrate (CH) is a short-lived intermediate in the metabolism of trichloroethylene (TRI). TRI, CH, and two common metabolites, trichloroacetic acid (TCA) and dichloroacetic acid (DCA) have been shown to be hepatocarcinogenic in mice. To better understand the pharmacokinetics of these metabolites of TRI in humans, eight male volunteers, aged 24-39, were administered single doses of 500 or 1,500 mg or a series of three doses of 500 mg given at 48 h intervals, in three separate experiments. Blood and urine were collected over a 7-day period and CH, DCA, TCA, free trichloroethanol (f-TCE), and total trichloroethanol (T-TCE=trichloroethanol and trichloroethanol-glucuronide [TCE-G]) were measured. DCA was detected in blood and urine only in trace quantities (<2 microM). TCA, on the other hand, had the highest plasma concentration and the largest AUC of any metabolite. The TCA elimination curve displayed an unusual concentration-time profile that contained three distinct compartments within the 7-day follow-up period. Previous work in rats has shown that the complex elimination curve for TCA results largely from the enterohepatic circulation of TCE-G and its subsequent conversion to TCA. As a result TCA had a very long residence time and this, in turn, led to a substantial enhancement of peak concentrations following the third dose in the multiple dose experiment. Approximately 59% of the AUC of plasma TCA following CH administration is produced via the enterohepatic circulation of TCE-G. The AUC for f-TCE was found to be positively correlated with serum bilirubin concentrations. This effect was greatest in one subject that was found to have serum bilirubin concentrations at the upper limit of the normal range in all three experiments. The AUC of f-TCE in the plasma of this individual was consistently about twice that of the other seven subjects. The kinetics of the other metabolites of CH was not significantly modified in this individual. These data indicate that individuals with a more impaired capacity for glucuronidation may be very sensitive to the central nervous system depressant effects of high doses of CH, which are commonly attributed to plasma levels of f-TCE.

Key issues in the modes of action and effects of trichloroethylene metabolites for liver and kidney tumorigenesis

Trichloroethylene (TCE) exposure has been associated with increased risk of liver and kidney cancer in both laboratory animal and epidemiologic studies. The U.S. Environmental Protection Agency 2001 draft TCE risk assessment concluded that it is difficult to determine which TCE metabolites may be responsible for these effects, the key events involved in their modes of action (MOAs) , and the relevance of these MOAs to humans. In this article, which is part of a mini-monograph on key issues in the health risk assessment of TCE, we present a review of recently published scientific literature examining the effects of TCE metabolites in the context of the preceding questions. Studies of the TCE metabolites dichloroacetic acid (DCA) , trichloroacetic acid (TCA) , and chloral hydrate suggest that both DCA and TCA are involved in TCE-induced liver tumorigenesis and that many DCA effects are consistent with conditions that increase the risk of liver cancer in humans. Studies of S-(1,2-dichlorovinyl) -l-cysteine have revealed a number of different possible cell signaling effects that may be related to kidney tumorigenesis at lower concentrations than those leading to cytotoxicity. Recent studies of trichloroethanol exploring an alternative hypothesis for kidney tumorigenesis have failed to establish the formation of formate as a key event for TCE-induced kidney tumors. Overall, although MOAs and key events for TCE-induced liver and kidney tumors have yet to be definitively established, these results support the likelihood that toxicity is due to multiple metabolites through several MOAs, none of which appear to be irrelevant to humans.

Application of cryopreserved human hepatocytes in trichloroethylene risk assessment: relative disposition of chloral hydrate to trichloroacetate and trichloroethanol

BACKGROUND

Trichloroethylene (TCE) is a suspected human carcinogen and a common groundwater contaminant. Chloral hydrate (CH) is the major metabolite of TCE formed in the liver by cytochrome P450 2E1. CH is metabolized to the hepatocarcinogen trichloroacetate (TCA) by aldehyde dehydrogenase (ALDH) and to the noncarcinogenic metabolite trichloroethanol (TCOH) by alcohol dehydrogenase (ADH). ALDH and ADH are polymorphic in humans, and these polymorphisms are known to affect the elimination of ethanol. It is therefore possible that polymorphisms in CH metabolism will yield subpopulations with greater than expected TCA formation with associated enhanced risk of liver tumors after TCE exposure.

METHODS

The present studies were undertaken to determine the feasibility of using commercially available, cryogenically preserved human hepatocytes to determine simultaneously the kinetics of CH metabolism and ALDH/ADH genotype. Thirteen human hepatocyte samples were examined. Linear reciprocal plots were obtained for 11 ADH and 12 ALDH determinations.

RESULTS

There was large interindividual variation in the Vmax values for both TCOH and TCA formation. Within this limited sample size, no correlation with ADH/ALDH genotype was apparent. Despite the large variation in Vmax values among individuals, disposition of CH into the two competing pathways was relatively constant.

CONCLUSIONS

These data support the use of cryopreserved human hepatocytes as an experimental system to generate metabolic and genomic information for incorporation into TCE cancer risk assessment models. The data are discussed with regard to cellular factors, other than genotype, that may contribute to the observed variability in metabolism of CH in human liver.

Cysteine S-conjugate beta-lyases

Cysteine S-conjugate beta-lyases are pyridoxal 5'-phosphate-containing enzymes that catalyze beta-elimination reactions with cysteine S-conjugates that possess an electron-withdrawing group attached at the sulfur. The end products of the beta-lyase reaction are pyruvate, ammonium and a sulfur-containing fragment. If the sulfur-containing fragment is reactive, the parent cysteine S-conjugate may be toxic, particularly to kidney mitochondria. Halogenated alkenes are examples of electrophiles that are bioactivated (toxified) by conversion to cysteine S-conjugates. These conjugates are converted by cysteine S-conjugate beta-lyases to thioacylating fragments. Several cysteine S-conjugates found in allium foods (garlic and onion) are beta-lyase substrates. This finding may account in part for the chemopreventive activity of allium products. This review (1) identifies enzymes that catalyze cysteine S-conjugate beta-lyase reactions, (2) suggests that toxicant channeling may contribute to halogenated cysteine S-conjugate-induced toxicity to mitochondria, and (3) proposes mechanisms that may contribute to the antiproliferative effects of sulfur-containing fragments eliminated from allium-derived cysteine S-conjugates.

Neuroembryopathic effect of trichloroacetic acid in rats exposed during organogenesis

BACKGROUND

Halogenated hydrocarbons such as trichloroacetic acid (TCA) are among the most common water supply contaminants in the world. This study examines the effect of TCA on the developing brain of the Charles Foster rat.

METHODS

Adult pregnant rats were placed in the test group and exposed to various concentration of TCA (i.e., 1000, 1200, 1400, 1600, and 1800 mg/kg body weight [b.w.]) by oral gavage throughout the period of organogenesis from Gestation Day (GD) 6-15 of gestation. Trichloroacetic acid was administered in the form of trichloroacetate, which is reduced to TCA in the body. The control mother rats were administered an equal volume of distilled water. Fetal brains were examined for their external and histological malformation.

RESULTS

On GD 19, TCA administration led to an initial increase of brain weight at 1000 mg/kg b.w. and then a weight reduction after TCA doses of 1200 mg/kg b.w. and over. The brain of the formalin-fixed fetuses at 1000 and 1200 mg/kg b.w. showed hydrocephalus with breech of the ependymal lining, altered choroids plexus architecture, and increased apoptosis. At doses of 1400 mg/kg b.w. and above, the brain showed not only enhanced apoptosis of the neuronal cells, but extravasation of erythrocytes within the cortical parenchyma, vacuolation of the neuropil, and multiple cavity formation.

CONCLUSION

With an increase in dose of TCA i.e., 1200 mg/kg b.w. and above, there is enhanced apoptosis, leading to increased neuronal death, which consequently led to the reduction in the brain weight as compared to controls. The fetal central nervous system is susceptible to the toxic effect of TCA.

A review of analytical methods for the determination of trichloroethylene and its major metabolites chloral hydrate, trichloroacetic acid and dichloroacetic acid

Trichloroethylene (TCE) and some of its metabolites are potentially carcinogenic compounds that the general population is commonly exposed to in drinking water. Concentrations of TCE, dichloroacetic acid (DCA) and trichloroacetic acid (TCA) given to laboratory animals in cancer bioassays are high, whereas drinking water levels of the compounds are very low. It is not clear whether the trace amounts of TCE, DCA and TCA in drinking water pose a cancer risk to humans. The accuracy of pharmacokinetic studies relies on the analytical method from which blood and tissue concentration data are obtained. Models that extrapolate cancer risks of TCE and its metabolites from laboratory animals to humans, in turn, rely on the results of pharmacokinetic studies. Therefore, it is essential to have reliable analytical methods for the analysis of TCE and its metabolites. This paper reviews the methods currently in the literature for the analysis of TCE, DCA, TCA and, to a lesser extent, chloral hydrate (CH). Additional aspects of analytical methods such as method validation, species preservation and future directions in the analysis of TCE and its metabolites are also discussed.

Pulmonary bronchiolar cytotoxicity and formation of dichloroacetyl lysine protein adducts in mice treated with trichloroethylene

This study was undertaken to test the hypothesis that bronchiolar damage induced by trichloroethylene (TCE) is associated with bioactivation within the Clara cells with the involvement of CYP2E1 and CYP2F2. Histopathology confirmed dose-dependent Clara cell injury and disintegration of the bronchiolar epithelium in CD-1 mice treated with TCE doses of 500 to 1000 mg/kg i.p. Immunohistochemical studies, using an antibody that recognizes dichloroacetyl lysine adducts, revealed dose-dependent formation of adducts in the bronchiolar epithelium. Localization of dichloroacetyl adducts in the Clara cells coincided with damage to this cell type in TCE-treated mice. Pretreatment of CD-1 mice with diallyl sulfone, an inhibitor of CYP2E1 and CYP2F2, abrogated the formation of the dichloroacetyl adducts and protected against TCE-induced bronchiolar cytotoxicity. Treatment of wild-type and CYP2E1-null mice with TCE (750 mg/kg i.p.) also elicited bronchiolar damage that correlated with the formation of adducts in the Clara cells. Immunoblotting, using lung microsomes from TCE-treated CD-1 mice, showed dose-dependent production of dichloroacetyl adducts that comigrated with CYP2E1 and CYP2F2. However, TCE treatment resulted in a loss of immunoreactive CYP2E1 and CYP2F2 proteins and p-nitrophenol hydroxylation, a catalytic activity associated with both cytochrome P450 enzymes. The TCE metabolite, chloral hydrate, was formed in incubations of TCE with lung microsomes from CD-1, wild-type, and CYP2E1-null mice. The levels were higher in CD-1 than in either wild-type or CYP2E1-null mice, although levels were higher in CYP2E1-null than in wild-type mice. These findings supported the contention that TCE bioactivation within the Clara cells, predominantly involving CYP2F2, correlated with bronchiolar cytotoxicity in mice.

Lack of direct mitogenic activity of dichloroacetate and trichloroacetate in cultured rat hepatocytes

Dichloroacetate (DCA) and trichloroacetate (TCA) are hepatocarcinogenic metabolites of the common groundwater contaminant, 1,1,2-trichloroethylene. DCA and TCA have been shown to induce hepatocyte proliferation in vivo, but it is not known if this response is the result of direct mitogenic activity or whether cell replication occurs indirectly in response to tissue injury or inflammation. In this study we used primary cultures of rat hepatocytes, a species susceptible to DCA- but not TCA-induced hepatocarcinogenesis, to determine whether DCA and TCA are direct hepatocyte mitogens. Rat hepatocytes, cultured in growth factor-free medium, were treated with 0.01-1.0 mM DCA or TCA for 10-40 h; cell replication was then assessed by measuring incorporation of 3H-thymidine into DNA and by cell counts. DCA or TCA treatment did not alter 3H-thymidine incorporation in the cultured hepatocytes. Although an increase in cell number was not observed, DCA treatment significantly abrogated the normal background cell loss, suggesting an ability to inhibit apoptotic cell death in primary hepatocyte cultures. Furthermore, treatment with DCA synergistically enhanced the mitogenic response to epidermal growth factor. The data indicate that DCA and TCA are not direct mitogens in hepatocyte cultures, which is of interest in view of their ability to stimulate hepatocyte replication in vivo. Nevertheless, the synergistic enhancement of epidermal growth factor-induced hepatocyte replication by DCA is of particular interest and warrants further study.