Dichloroacetic acid-induced dysfunction in rat hippocampus and the protective effect of curcumin

The present study was designed to evaluate the role of cAMP-PKA-CREB signaling in mediating the neuroprotective effects of curcumin against DCAA-induced oxidative stress, inflammation and impaired synaptic plasticity in rats. Sixty Sprague-Dawley rats were randomly divided into five groups, and we assessed the histomorphological, behavioral and biochemical characteristics to investigate the beneficial effects of different concentrations of curcumin against DCAA-induced neurotoxicity in rat hippocampus. The results indicated that animal weight gain and food consumption were not significantly affected by DCAA. However, behavioral tests, including morris water maze and shuttle box, showed varying degrees of alterations. Additionally, we found significant changes in hippocampal neurons by histomorphological observation. DCAA exposure could increase lipid peroxidation, reactive oxygen species (ROS), inflammation factors while reducing superoxide dismutase (SOD) activity and glutathione (GSH) level accompanied by DNA damage in the hippocampus. Furthermore, we found that DCAA exposure could cause a differential modulation of mRNA and proteins (cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), cAMP-response element-binding protein (CREB), p-CREB, brain-derived neurotrophic factor (BDNF), postsynaptic density-95 (PSD-95), synaptophysin (SYP)). Conversely, various doses of curcumin attenuated DCAA-induced oxidative stress, inflammation response and impaired synaptic plasticity, while elevating cAMP, PKA, p-CREB, BDNF, PSD-95, SYP levels. Thus, curcumin could activate the cAMP-PKA-CREB signaling pathway, conferring neuroprotection against DCAA-induced neurotoxicity.

Degradation of haloacetic acids with the Fenton-like and analysis by GC-MS: use of bioassays for monitoring of genotoxic, mutagenic and cytotoxic effects

In this study, a method was developed to evaluate the degradation of haloacetic acids (HAAs) in water by a heterogenous Fenton-like process catalyzed by cobalt-doped magnetite nanoparticles (Fe_{3 - x}Co_xO₄), extraction of the contaminants by liquid-liquid extraction (LLE), and analysis by gas chromatography-mass spectrometry (GC-MS). The developed method was efficient in the degradation of HAAs, with the following degradation values: 63%, 62%, 30%, 39%, 37%, 50%, 84%, 41%, and 79% for monochloroacetic acid, monobromoacetic acid, dichloroacetic acid, trichloroacetic acid, bromochloroacetic acid, dibromoacetic acid, bromodichloroacetic acid, dibromochloroacetic acid, and tribromoacetic acid compounds, respectively. Through the application of the Allium cepa test, the cytotoxicity, genotoxicity, and mutagenicity of HAAs were evaluated. The results confirm its genotoxic and mutagenic effects on Allium cepa meristematic cells. Through this study, it was possible to verify the effectiveness of the developed method and its potential as a proposal for environmental remediation.

Metabolic Disruption Early in Life is Associated With Latent Carcinogenic Activity of Dichloroacetic Acid in Mice

Early-life environmental factors can influence later-life susceptibility to cancer. Recent evidence suggests that metabolic pathways may mediate this type of latency effect. Previously, we reported that short-term exposure to dichloroacetic acid (DCA) increased liver cancer in mice 84 weeks after exposure was stopped. Here, we evaluated time course dynamics for key events related to this effect. This study followed a stop-exposure design in which 28-day-old male B6C3F1 mice were given the following treatments in drinking water for up to 93 weeks: deionized water (dH2O, control); 3.5 g/l DCA continuously; or 3.5 g/l DCA for 4-52 weeks followed by dH2O. Effects were evaluated at eight interim time points. A short-term biomarker study was used to evaluate DCA effects at 6, 15, and 30 days. Liver tumor incidence was higher in all DCA treatment groups, including carcinomas in 82% of mice previously treated with DCA for only 4 weeks. Direct effects of DCA in the short-term study included decreased liver cell proliferation and marked mRNA changes related to mitochondrial dysfunction and altered cell metabolism. However, all observed short-term effects of DCA were ultimately reversible, and prior DCA treatment did not affect liver cell proliferation, apoptosis, necrosis, or DNA sequence variants with age. Key intermediate events resulting from transient DCA exposure do not fit classical cytotoxic, mitogenic, or genotoxic modes of action for carcinogenesis, suggesting a distinct mechanism associated with early-life metabolic disruption.

Complex I inhibition augments dichloroacetate cytotoxicity through enhancing oxidative stress in VM-M3 glioblastoma cells

The robust glycolytic metabolism of glioblastoma multiforme (GBM) has proven them susceptible to increases in oxidative metabolism induced by the pyruvate mimetic dichloroacetate (DCA). Recent reports demonstrate that the anti-diabetic drug metformin enhances the damaging oxidative stress associated with DCA treatment in cancer cells. We sought to elucidate the role of metformin's reported activity as a mitochondrial complex I inhibitor in the enhancement of DCA cytotoxicity in VM-M3 GBM cells. Metformin potentiated DCA-induced superoxide production, which was required for enhanced cytotoxicity towards VM-M3 cells observed with the combination. Similarly, rotenone enhanced oxidative stress resultant from DCA treatment and this too was required for the noted augmentation of cytotoxicity. Adenosine monophosphate kinase (AMPK) activation was not observed with the concentration of metformin required to enhance DCA activity. Moreover, addition of an activator of AMPK did not enhance DCA cytotoxicity, whereas an inhibitor of AMPK heightened the cytotoxicity of the combination. Our data indicate that metformin enhancement of DCA cytotoxicity is dependent on complex I inhibition. Particularly, that complex I inhibition cooperates with DCA-induction of glucose oxidation to enhance cytotoxic oxidative stress in VM-M3 GBM cells.

[Dichloroacetate--a healing toxin or a toxic drug?]

Dichloroacetate (DCA) is a compound which activity is observed by experimental and clinical toxicologists. DCA is a by-product of chlorination of water, it is toxic to many organs, such as liver, kidneys or nervous system. In a view of its metabolism it is also demonstrated that this substance may be treated as a drug in various medical conditions, such as different types of tumors for example.

Cancer risk assessment on trihalomethanes and haloacetic acids in drinking water of China using disability-adjusted life years

The cancer risks from exposure to trihalomethanes (THMs) and haloacetic acids (HAAs) through multiple pathways were assessed based on the result of a water quality survey in 35 major cities of China. To express the risks in disability-adjusted life years (DALYs), the excess cancer incidence estimates were combined with a two-stage disease model for calculation. The median total cancer risk of THMs and HAAs was calculated as 7.34 × 10(-7) DALYs per person-year (ppy), lower than the reference level of risk (10(-6)DALYsppy) set by WHO. The risk from ingestion and inhalation exposures contributed 93.6% and 6.3% of the total risk respectively, while dermal contact made a negligible contribution. The median risk of trichloroacetic acid (TCAA) (2.12 × 10(-7)DALYsppy) was highest among the disinfection by-products (DBPs) considered. The risk ratio of total HAAs (THAA) to total THMs (TTHM) was 1.12. The risk was highest in northeast China while lowest in northwest China. As for the 35 cities, Tianjin had the highest risk while Yinchuan had the lowest. This study attempted to use DALYs for the risk assessment of DBPs, which will provide useful information for risk comparison and prioritization of hazards in drinking water.

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.

The occurrence of disinfection by-products in municipal drinking water in China's Pearl River Delta and a multipathway cancer risk assessment

Disinfection byproducts were measured in the finished drinking water from ten water treatment plants in three Chinese cities - Guangzhou, Foshan and Zhuhai. A total of 155 water samples were collected in 2011 and 2012. The median (range) of trihalomethane (THM) and haloacetic acid (HAA) levels were 17.7 (0.7-62.7) μg/L and 8.6 (0.3-81.3) μg/L, respectively. Chloroform, dichloroacetic acid and trichloroacetic acid were the dominant species observed in Guangzhou and Foshan water, while brominated THMs predominated in water from Zhuhai. Haloacetonitriles, haloketones, chloral hydrate and trichloronitromethane were usually detected at levels ranging from unquantifiable (<0.2μg/L) to 12.2μg/L (choral hydrate). THMs and HAAs showed clear seasonal variations with the total concentrations higher in winter than in summer. Correlations among DBP levels varied, with the strongest linear correlation observed between chloroform and chloral hydrate levels (R(2)=0.77). The risk of cancer from ingestion, inhalation and dermal contact exposure to THMs was estimated. CHCl2Br contributed the highest percentage of the cancer risk from ingestion pathway and CHCl3 contributed the highest of cancer risk from inhalation pathway.

Mammalian cell cytotoxicity and genotoxicity of the haloacetic acids, a major class of drinking water disinfection by-products

The haloacetic acids (HAAs) are disinfection by-products (DBPs) that are formed during the disinfection of drinking water, wastewaters and recreational pool waters. Currently, five HAAs [bromoacetic acid (BAA), dibromoacetic acid (DBAA), chloroacetic acid (CAA), dichloroacetic acid (DCAA), and trichloroacetic acid (TCAA); designated as HAA5] are regulated by the U.S. EPA, at a maximum contaminant level of 60 μg/L for the sum of BAA, DBAA, CAA, DCAA, and TCAA. We present a comparative systematic analysis of chronic cytotoxicity and acute genomic DNA damaging capacity of 12 individual HAAs in mammalian cells. In addition to the HAA5, we analyzed iodoacetic acid (IAA), diiodoacetic acid (DiAA), bromoiodoacetic acid (BIAA), tribromoacetic acid (TBAA), chlorodibromoacetic acid (CDBAA), bromodichloroacetic acid (BDCAA), and bromochloroacetic acid (BCAA). Their rank order of chronic cytotoxicity in Chinese hamster ovary cells was IAA > BAA > TBAA > CDBAA > DIAA > DBAA > BDCAA > BCAA > CAA > BIAA > TCAA > DCAA. The rank order for genotoxicity was IAA > BAA > CAA > DBAA > DIAA > TBAA > BCAA > BIAA > CDBAA. DCAA, TCAA, and BDCAA were not genotoxic. The trend for both cytotoxicity and genotoxicity is iodinated HAAs > brominated HAAs > chlorinated HAAs. The use of alternative disinfectants other than chlorine generates new DBPs and alters their distribution. Systematic, comparative, in vitro toxicological data provides the water supply community with information to consider when employing alternatives to chlorine disinfection. In addition, these data aid in prioritizing DBPs and their related compounds for future in vivo toxicological studies and risk assessment.

[Cytotoxicity of dichloroacetic acid in lymphocyte and expression of chemokine receptor CXCR2 and chemokine receptor CXCR3 mRNA]

OBJECTIVE

To explore the effects of trichloroethylene (TCE) and its by-products (trichloroacetic acid, TCA; dichloroacetic acid, DCA) on the normal human peripheral blood lymphocyte and the role of DCA in dermatitis medicamentosa- like induced by trichloroethylene (DMLT).

METHODS

Lymphocyte was isolated from peripheral venous blood, and cytotoxicity of human lymphocytes treated with different concentrations (0.02 approximately 30.00 mmol/L) of DCA was determined at indicated times (2 h and 4 h) based on the MTS assay. Action of DCA on cell viability, membrane integrity was assessed by neutral red uptake (NRU) assay and lactate dehydrogenase (LDH) release test and measurement of superoxide dismutase (SOD) activity. Fluorescence quantitative reverse transcription polymerase chain reaction (FQ-RT-PCR) was employed for detection and quantization of the chemokine receptor CXCR2 and chemokine receptor CXCR3 mRNA in peripheral blood lymphocyte treated with different concentrations of DCA.

RESULTS

DCA had a more vital effect on peripheral blood lymphocyte than TCE and TCA. A concentration-dependent release of LDH was observed at 4 h after cells were exposed to different doses of DCA (0.88, 1.75, 3.50 and 7.00 mmol/L) (P < 0.05), and DCA also caused an inhibition of SOD activity in a concentration-dependent manner (P < 0.05). The results of FQ- RT- PCR indicated that CXCR2 and CXCR3 mRNA were all over- expression. At 48 h after the DCA of 0.5 mmol/L and 10.00 mmol/L was used, CXCR2 and CXCR3 mRNA were 10.34, 5.66-fold and 19.43, 8.75-fold of those in the control group (P < 0.01).

CONCLUSION

DCA is of a great cytotoxicity and may be one of crucial evocators on DMLT.

Kinetics and effects of dichloroacetic acid in rainbow trout

Halogenated acetic acids (HAAs) produced by chlorine disinfection of municipal drinking water represent a potentially important class of environmental contaminants. Little is known, however, about their potential to adversely impact fish and other aquatic life. In this study we examined the kinetics and effects of dichloroacetic acid (DCA) in rainbow trout. Branchial uptake was measured in fish confined to respirometer-metabolism chambers. Branchial uptake efficiency was <5%, suggesting passive diffusion through aqueous channels in the gill epithelium. DCA concentrations in tissues following prolonged (72, 168, or 336 h) waterborne exposures were expressed as tissue:plasma concentration ratios. Concentration ratios for the kidney and muscle at 168 and 336 h were consistent with the suggestion that DCA distributes primarily to tissue water. Reduced concentration ratios for the liver, particularly at 72 h, indicated that DCA was highly metabolized by this tissue. Routes and rates of elimination were characterized by injecting chambered animals with a high (5.0mg/kg) or low (50 microg/kg) bolus dose. DCA was rapidly cleared by naïve animals resulting in elimination half-lives (t(1/2)) of less than 4h. Waterborne pre-treatment of fish with DCA increased the persistence of a subsequently injected dose. In high dose animals, pre-treatment caused a 4-fold decrease in whole-body clearance (CL(B)) and corresponding increases in the area under the plasma concentration-time curve (extrapolated to infinity; AUC(0-->infinity)) and t(1/2). Qualitatively similar results were obtained in low dose fish, although the magnitude of the pre-treatment effect ( approximately 2.5-fold) was reduced. Renal and branchial clearance contributed little (combined, <3% of CL(B)) to the elimination of DCA. Biliary elimination of DCA was also negligible. The steady-state volume of distribution (V(SS)) did not vary among treatment groups and was consistent with results of the tissue distribution study. DCA had no apparent effects on respiratory physiology or acid-base balance; however, the concentration of blood lactate declined progressively during continuous waterborne exposures. A transient effect on blood lactate was also observed in bolus injection experiments. The results of this study suggest that clearance of DCA is due almost entirely to metabolism. The pathway responsible for this activity exhibits characteristics in common with those of mammalian glutathione S-transferase zeta (GSTzeta), including non-linear kinetics and apparent suicide inactivation by DCA. Observed effects on blood lactate are probably due to the inhibition of pyruvate dehydrogenase kinase in aerobic tissues and may require the participation of a monocarboxylase transport protein to move DCA across cell membranes.

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.

Modulation by ellagic acid of DCA-induced developmental toxicity in the zebrafish (Danio rerio)

The ability of ellagic acid (EA) to modulate dichloroacetic acid (DCA)-induced developmental toxicity and oxidative damage was examined in zebrafish embryos. Embryos were exposed to 20 mM EA administered concomitantly with 32 mM DCA at 4 hours postfertilization (hpf) and 20 h later. Embryos were observed through 144 hpf for developmental malformations, and production of superoxide anion (SA) and nitric oxide (NO) was determined in embryonic homogenates. DCA was shown to produce developmental abnormalities and significant levels of SA and NO in zebrafish embryos. EA exposure alleviated the developmental malformations observed in treated embryos and decreased the levels of SA and NO in those same embryos. Less than 10% of DCA + EA exposed embryos showed developmental malformations compared to 100% of embryos treated with DCA alone. Animals in this group that developed malformations were shown to have fewer defects than those treated with DCA only. Taken together, the results confirm the involvement of oxidative stress in the developmental toxicity of DCA in zebrafish embryos, and suggest possible protection against those effects with the use of antioxidants.

NTP report on the toxicology studies of dichloroacetic acid (CAS No. 79-43-6) in genetically modified (FVB Tg.AC hemizygous) mice (dermal and drinking water studies) and carcinogenicity studies of dichloroacetic acid in genetically modified [B6.129-Trp53(tm1Brd) (N5) haploinsufficient] mice (drinking water studies)

Dichloroacetic acid was nominated for study by the United States Environmental Protection Agency (EPA) and by the National Institute of Environmental Health Sciences because of its widespread occurrence in drinking water as a by-product of water disinfection using chlorination. It was also nominated because dichloroacetic acid is the most studied representative of the class of haloacetic acids and has been shown to cause liver tumors in both rats and mice. Haloacetic acids are second only to trihalomethanes as a family of disinfection by-products found in many drinking water supplies. Dichloroacetic acid is one of several disinfection by-products being evaluated to determine whether genetically modified mouse models can serve as a more rapid and cost-effective means of evaluating and ranking potential hazards of disinfection by-products. The NTP has explored the use of genetically altered mouse models as adjuncts to 2-year rodent cancer assays. These models may prove to be more rapid, use fewer animals, and provide some mechanistic insights into neoplastic responses. As part of the evaluation of new mouse cancer screening models, dichloroacetic acid was tested for potential toxicity and carcinogenicity in two relatively well-studied models, the Tg.AC hemizygous strain and the p53 haploinsufficient strain. Male and female Tg.AC hemizygous and p53 haploinsufficient mice were exposed to dichloroacetic acid in the drinking water (greater than 98% pure) for 26 or 39 weeks. Genetic toxicology studies were conducted in Salmonella typhimurium strains TA98, TA100, and TA1535 and in mouse peripheral blood erythrocytes. 26- AND 39-WEEK DERMAL STUDIES IN Tg.AC HEMIZYGOUS MICE Groups of 15 male and 15 female Tg.AC hemizygous mice were administered 0, 31.25, 125, or 500 mg dichloroacetic acid/kg body weight 5 days per week for 26 weeks with additional groups of 10 males and 10 females continued on treatment for 39 weeks. Survival of dosed males and females was similar to that of the vehicle control groups for both studies. Mean body weights of dosed males and females in the 26-week study were similar to those of the vehicle control groups. Mean body weights of dosed males in the 39-week study were similar to those of the vehicle control groups. Mean body weights of the 500 mg/kg females were greater than those of the vehicle controls in the 39-week study. The absolute liver weights were increased by greater than 50% compared to the vehicle controls for the 500 mg/kg males and females in both studies. At the site of application, the incidences of squamous cell papilloma were significantly increased in 500 mg/kg males and females at 39 weeks. In addition, one 125 mg/kg male, two 500 mg/kg males, and two 500 mg/kg females had squamous cell papillomas at 26 weeks. The incidences of epidermal hyperplasia and hyperkeratosis were significantly increased at the site of application in the 125 and 500 mg/kg males and females at 26 weeks. At 39 weeks, the incidence of epidermal hyperkeratosis was increased in the 31.25 mg/kg males, but in females, increased epidermal hyperkeratosis and hyperplasia occurred only in the 500 mg/kg group. There was a modest increase in pulmonary adenomas at 39 weeks that may have been related to the dichloroacetic acid exposure in males and females exposed to 125 or 500 mg/kg. In both studies, there was a dose-related increase in the mean severity of hepatocyte cytoplasmic vacuolization in males and females, and the incidence of nephropathy was increased in 500 mg/kg males. 26- AND 41-WEEK DRINKING WATER STUDIES IN Tg.AC HEMIZYGOUS MICE: Groups of 15 male and 15 female Tg.AC hemizygous mice were exposed to drinking water containing 0, 500, 1,000, or 2,000 mg/L dichloroacetic acid for 26 weeks with additional groups of 10 males and 10 females exposed for 41 weeks. The equivalent average daily doses were approximately 75, 145, and 235 mg dichloroacetic acid/kg body weight to males and approximately 100, 185, and 285 mg/kg to females. Survival of exposed males was similar in both studies. In the females, survival was decreased in the 26-week but not the 41-week study. While there was some variability, the mean body weights of mice exposed to dichloroacetic acid tended to be similar to those of the control groups. In the 41-week study, mean body weights of exposed males and females tended to be less than those of the control groups. Water consumption by males and females exposed to 1,000 and 2,000 mg/L was less than that by the controls throughout both studies. The incidences and/or severity of hepatocyte cytoplasmic vacuolization were increased in males and females in both studies. The incidence of pulmonary adenoma was increased in the male mice exposed to 1,000 mg/L dichloroacetic acid for 41 weeks. Two pulmonary adenomas were found in the 2,000 mg/L females at 41 weeks. At 26 weeks, a pulmonary carcinoma was found in one 1,000 mg/L male, one 500 mg/L female, and one 2,000 mg/L female. 26- AND 41-WEEK DRINKING WATER STUDIES IN p53 HAPLOINSUFFICIENT MICE: Groups of 15 male and 15 female p53 haploinsufficient mice were exposed to drinking water containing 0, 500, 1,000, or 2,000 mg/L dichloroacetic acid for 26 weeks with additional groups of 10 males and 10 females exposed for 41 weeks. The equivalent average daily doses were approximately 45, 80, and 145 mg/kg to males and approximately 75, 145, and 220 mg/kg to females. Survival of all exposed groups was similar to that of the control groups in both studies. Mean body weights of 1,000 and 2,000 mg/L males and females were generally less than those of the control groups throughout most of both studies; mean body weights of 500 mg/L males and females were less than those of the controls for much of the 41-week study. Water consumption by 1,000 and 2,000 mg/L males and females was less than that by the control groups throughout both studies. The incidences and/or severities of hepatocyte cytoplasmic vacuolization were increased in males in the 26-week study and females in both studies.

GENETIC TOXICOLOGY

Dichloroacetic acid was mutagenic in Salmonella typhimurium strains TA100 and TA1535 in tests conducted in the absence of S9 liver activation enzymes; no increase in mutations was observed in either strain in the presence of rat or hamster liver S9. Dichloroacetic acid was not mutagenic in S. typhimurium strain TA98 with or without S9. Dichloroacetic acid was also tested for micronucleus induction in peripheral blood erythrocytes of male and female Tg.AC hemizygous and p53 haploinsufficient mice treated by drinking water or dermal application for 26 weeks. No induction of micronuclei was seen in Tg.AC hemizygous mice treated by either route or in the p53 haploinsufficient mice, which were exposed only by the drinking water route. In another study, analysis of peripheral blood samples for frequency of micronucleated erythrocytes in male and female B6C3F1 mice exposed to dichloroacetic acid in drinking water for 3 months revealed no alteration in micronucleus frequencies in male mice; a small increase seen in females was judged to be equivocal.

CONCLUSIONS

Under the conditions of these drinking water studies, there was no evidence of carcinogenic activity of dichloroacetic acid in male or female p53 haploinsufficient mice exposed to 0, 500, 1,000, or 2,000 mg/L for 26 or 41 weeks. The incidences and/or severities of cytoplasmic vacuolization of the hepatocyte were increased in males and females exposed to dichloroacetic acid for 26 or 41 weeks. Under the conditions of these dermal studies, there were increased incidences of squamous cell papillomas at the site of application in male and female Tg.AC hemizygous mice exposed to 500 mg/kg for 39 weeks. There were dose-related increased incidences of epidermal hyperkeratosis and hyperplasia at the site of application in both male and female mice exposed to dichloroacetic acid for 26 or 39 weeks. Under the conditions of these drinking water studies, there was an increase in the incidence of alveolar/bronchiolar adenoma in male Tg.AC hemizygous mice exposed to 1,000 mg/L for 41 weeks. There were a few bronchiolar/alveolar carcinomas in males and females exposed to dichloroacetic acid in the drinking water for 26 weeks and a few bronchiolar/alveolar adenomas in females exposed to dichloroacetic acid in the drinking water for 41 weeks. There were increased incidences and/or severities of cytoplasmic vacuolization of the hepatocyte in male and female Tg.AC hemizygous mice exposed to dichloroacetic acid in the drinking water study for 26 or 41 weeks. The marginally increased incidences of pulmonary adenomas and/or carcinomas compared to the unexposed groups found in both the dermal and drinking water studies at 39 or 41 weeks were considered to be related to dichloroacetic acid exposure.

Dichloroacetate causes reversible demyelination in vitro: potential mechanism for its neuropathic effect

Dichloroacetate (DCA) is an investigational drug for genetic mitochondrial diseases whose use has been mitigated by reversible peripheral neuropathy. We investigated the mechanism of DCA neurotoxicity using cultured rat Schwann cells (SCs) and dorsal root ganglia (DRG) neurons. Myelinating SC-DRG neuron co-cultures, isolated SCs and DRG neurons were exposed to 1-20 mm DCA for up to 12 days. In myelinating co-cultures, DCA caused a dose- and exposure-dependent decrease of myelination, as determined by immunolabeling and immunoblotting for myelin basic protein (MBP), protein zero (P0), myelin-associated glycoprotein (MAG) and peripheral myelin protein 22 (PMP22). Partial recovery of myelination occurred following a 10-day washout of DCA. DCA did not affect the steady-state levels of intermediate filament proteins, but promoted the formation of anti-neurofilament antibody reactive whirls. In isolated SC cultures, DCA decreased the expression of P0 and PMP22, while it increased the levels of p75(NTR) (neurotrophin receptor), as compared with non-DCA-treated samples. DCA had modest adverse effects on neuronal and glial cell vitality, as determined by the release of lactate dehydrogenase. These results demonstrate that DCA induces a reversible inhibition of myelin-related proteins that may account, at least in part, for its clinical peripheral neuropathic effects.

New insights into the mechanism of methoxyflurane nephrotoxicity and implications for anesthetic development (part 2): Identification of nephrotoxic metabolites

BACKGROUND

Methoxyflurane nephrotoxicity results from its metabolism, which occurs by both dechlorination (to methoxydifluoroacetic acid [MDFA]) and O-demethylation (to fluoride and dichloroacetic acid [DCAA]). Inorganic fluoride can be toxic, but it remains unknown why other anesthetics, commensurately increasing systemic fluoride concentrations, are not toxic. Fluoride is one of many methoxyflurane metabolites and may itself cause toxicity and/or reflect formation of other toxic metabolite(s). This investigation evaluated the disposition and renal effects of known methoxyflurane metabolites.

METHODS

Rats were given by intraperitoneal injection the methoxyflurane metabolites MDFA, DCAA, or sodium fluoride (0.22, 0.45, 0.9, or 1.8 mmol/kg followed by 0.11, 0.22, 0.45, or 0.9 mmol/kg on the next 3 days) at doses relevant to metabolite exposure after methoxyflurane anesthesia, or DCAA and fluoride in combination. Renal histology and function (blood urea nitrogen, urine volume, urine osmolality) and metabolite excretion in urine were assessed.

RESULTS

Methoxyflurane metabolite excretion in urine after injection approximated that after methoxyflurane anesthesia, confirming the appropriateness of metabolite doses. Neither MDFA nor DCAA alone had any effects on renal function parameters or necrosis. Fluoride at low doses (0.22, then 0.11 mmol/kg) decreased osmolality, whereas higher doses (0.45, then 0.22 mmol/kg) also caused diuresis but not significant necrosis. Fluoride and DCAA together caused significantly greater tubular cell necrosis than fluoride alone.

CONCLUSIONS

Methoxyflurane nephrotoxicity seems to result from O-demethylation, which forms both fluoride and DCAA. Because their co-formation is unique to methoxyflurane compared with other volatile anesthetics and they are more toxic than fluoride alone, this suggests a new hypothesis of methoxyflurane nephrotoxicity. This may explain why increased fluoride formation from methoxyflurane, but not other anesthetics, is associated with toxicity. These results may have implications for the interpretation of clinical anesthetic defluorination, use of volatile anesthetics, and the laboratory methods used to evaluate potential anesthetic toxicity.

[Study on rat's DNA damage of p53 induced by chlorinated acetic acids of drinking water disinfection by-products]

OBJECTIVE

To study on the DNA damage of p53 induced by dichloroacetic acid(DCA) and trichloroacetic acid( TCA), approve their genotoxicity and discuss molecular mechanism of their carcinogenic action.

METHODS

Administered SD rats with DCA or TCA by i.p. injection, extracted DNA from rat's liver, and then used RDPCR to detect DNA damage of exon 70f p53 gene.

RESULTS

Two hybridization bands were detected in treated group induced by DCA. It was indicated that DCA can result in DNA damage of exon 7 of p53 gene of rat's liver tissue, and there were two broken sites. It was not detected damage of exon 7 of p53 gene of rat induced by TCA.

CONCLUSION

There may be the relationship between DCA carcinogenic action and the damage of p53. The result that the damage of p53 of target tissue detected by RDPCR was consistent well with rat carcinogenic test.

Autoimmune response in MRL+/+ mice following treatment with dichloroacetyl chloride or dichloroacetic anhydride

Dichloroacetyl chloride (DCAC) is formed from trichloroethene (TCE), which is implicated in inducing/accelerating autoimmune response. Due to its potent acylating activity, DCAC may convert proteins to neo-antigens and thus could induce autoimmune responses. Dichloroacetic anhydride (DCAA), which is a similar acylating agent, might also induce autoimmune responses. To evaluate if chloroacylation plays a role in the induction of autoimmunity, we have measured the autoimmune responses following treatment with DCAC or DCAA in autoimmune-prone MRL+/+ mice. Five-week-old female mice were injected intraperitoneally (twice weekly) with 0.2 mmol/kg of DCAC or DCAA in corn oil for 6 weeks. Total serum IgG, IgG1, and IgE levels were significantly increased in DCAC-treated mice as compared to controls. These increases corresponded with increases in DCAC-specific IgG and IgG1 levels. Total serum IgM was decreased in both DCAC- and DCAA-treated mice. Antinuclear antibodies, measured as an indication of systemic autoimmune responses, were increased in both DCAC- and DCAA-treated mice. Of eight Th1/Th2 cytokines measured in the serum, only IL-5 was significantly decreased in both treatment groups. The cytokine secretion patterns of splenic lymphocytes after stimulation with antibodies against CD3 (T cell receptor-mediated signal) and CD28 (costimulatory signal) differed between treatment and control groups. Levels of IL-1, IL-3, IL-6, IFN-gamma, G-CSF, and KC were higher in cultures of stimulated splenocytes from either DCAC- or DCAA-treated mice than from controls. The level of IL-17 was only increased in cultures from DCAC-treated mice. Increased lymphocytic populations were found in the red pulp of spleens following treatment with either DCAC or DCAA. In addition, thickening of the alveolar septa in the lungs of DCAC- or DCAA-treated mice was observed. The lung histopathology in exposed mice was consistent with the symptomology observed in welders exposed to DCAC/phosgene. Thickening was more pronounced in DCAC-treated mice. Our data suggest that DCAC and DCAA elicit autoimmune responses in MRL+/+ mice that might be reflective of their chloroacylation potential in vivo.

Short-term exposures to dihaloacetic acids produce dysmorphogenesis in mouse conceptuses in vitro

The haloacetic acids (HAAs) are a family of xenobiotics found in tap water as a result of drinking water disinfection. Administration of HAAs to rats produces a variety of adverse effects, including developmental toxicity. The dysmorphogenic potencies of all nine bromo/chloro-acetic acids have been determined in rodent whole embryo culture using standard 26-h exposure. Since the half-lives of the HAAs in vivo are typically <8 h, the developmental effects of short-term exposures to dihaloacetates were evaluated. Gestation day 8 (3-6 somite pairs) CD-1 mouse conceptuses were exposed to 11,000 microM dichloroacetic acid (DCA), 300 microM dibromoacetic acid (DBA) or 300 microM bromochloroacetic acid (BCA) for culture periods of 1, 3, 6 or 26 h. Following 1, 3 or 6 h of exposure to HAAs, conceptuses were transferred to control medium to complete a 26-h culture period. The amounts of HAAs present in embryos after 1, 3 and 6h of exposure were determined. Increased incidences of dysmorphic embryos were produced by 6 or 26-h exposures to DCA; a 26-h exposure to DBA; or 3, 6 or 26-h exposures to BCA. The dysmorphology produced was dependent upon the length of exposure and chemical. The embryonic concentration of each HAA (104.5, 2.5 and 2.6 pmol/microg protein for DCA, DBA and BCA, respectively) was reached by 1h of exposure and did not change at the subsequent time points examined. The current studies demonstrate that BCA is more potent than DBA or DCA at disrupting embryogenesis since shorter exposures alter morphogenesis. Since the embryonic HAA concentrations were the same at the three time points measured, the time-dependence in dysmorphogenesis does not appear to be a simple function of increasing embryonic concentration of these chemicals. These studies demonstrate that for these dihaloacetic acids relatively high concentrations and long exposures are needed to alter rodent development in vitro.

Trichloroethylene, trichloroacetic acid, and dichloroacetic acid: do they affect eye development in the Sprague-Dawley rat?

Maternal exposure to high doses of trichloroethylene (TCE) and its oxidative metabolites, trichloroacetic acid (TCA) and dichloroacetic acid (DCA), has been implicated in eye malformations in fetal rats, primarily micro-/anophthalmia. Subsequent to a cardiac teratology study of these compounds (Fisher et al. 2001, Int. J. Toxicol. 20:257-267), their potential to induce ocular malformations was examined in a subset of the same experimental animals. Pregnant, Sprague-Dawley Crl:CDR BR rats were orally treated on gestation days (GDs) 6 to 15 with bolus doses of either TCE (500 mg/kg/day), TCA (300 mg/kg/day), DCA (300 mg/kg/day), or all-trans retinoic acid (RA; 15 mg/kg/day). The heads of GD 21 fetuses were not only examined grossly for external malformations, but were sectioned using a modified Wilson's technique and subjected to computerized morphometry that allowed for the quantification of lens area, globe area, medial canthus distance, and interocular distance. Gross ocular malformations were essentially absent in all treatment groups except for the RA group in which 26% of fetuses exhibited micro-/anophthalmia. Using the litter as the experimental unit of analysis, lens area, globe area, and interocular distance were statistically significantly reduced in the DCA treatment group. Statistically significant reductions in lens and globe areas also occurred in the RA treatment group, all four ocular measures were reduced in the TCA treatment group but none significantly so, and TCE was without effect. Because DCA, TCA, and RA treatments were associated with significant reductions in fetal body weight (bw), data were also statistically analyzed after bw adjustment. Doing so dramatically altered the results of treatment group comparisons, but the severity of bw reduction and the degree of change in ocular measures did not always correlate. This suggests that bw reduction may not be an adequate explanation for all the changes observed in ocular measures. Thus, it is unclear whether DCA specifically disrupted ocular development even under these provocative exposure conditions. Clearly, however, if TCE is capable of disrupting ocular development in the Sprague-Dawley rat, a higher dose than that employed in the present study is required.

Dichloroacetate-induced developmental toxicity and production of reactive oxygen species in zebrafish embryos

Dichloroacetate (DCA) is one of the toxic by products that are formed during the chlorine disinfection process of drinking water. In this study, the developmental toxicity of DCA has been determined in zebrafish (Danio rerio) embryos. Embryos were exposed to different concentrations (4, 8, 16, and 32 mM) of the compound at the 4 h postfertilization (hpf) stage of development, and were observed for different developmental toxic effects at 8, 24, 32, 55, 80, and 144 hpf. Exposure of embryos to 8-32 mM of DCA resulted in significant increases in the heart rate and blood flow of the 55 and 80 hpf embryos that turned into significant decreases at the 144 hpf time point. At 144 hpf, malformations of mouth structure, notochord bending, yolk sac edema and behavioral effects including perturbed swimming and feeding behaviors were also observed. DCA was also found to produce time- and concentration-dependent increases in embryonic levels of superoxide anion (O2*-) and nitric oxide (NO), at various stages of development. The results of the study suggest that DCA-induced developmental toxic effects in zebrafish embryos are associated with production of reactive oxygen species in those embryos.

Developmental toxicity of mixtures: the water disinfection by-products dichloro-, dibromo- and bromochloro acetic acid in rat embryo culture

The chlorination of drinking water results in production of numerous disinfection by-products (DBPs). One of the important classes of DBPs is the haloacetic acids. We have previously shown that the haloacetic acids (HAs), dichloro (DCA), dibromo (DBA) and bromochloro (BCA) acetic acid are developmentally toxic in mouse whole embryo culture. Human exposure to these contaminants in drinking water would involve simultaneous exposure to all three HAs. This study explores the question of developmental toxicity interactions between these compounds. Gestational day (GD) 9.5 rat embryos were exposed to various concentrations of the three HAs (singly or in combination) for 48 h and then evaluated for dysmorphology. The embryonic effects from exposure to the single compounds and mixtures were evaluated using developmental score (DEVSC) as the parameter of comparison. Concentrations of individual compounds and mixtures were chosen (based on a dose-additivity model) which were predicted to produce scores 10 or 20% lower than control levels. Evaluations were performed on all possible combinations of the three HAs. The HAs were dysmorphogenic and resulted in primarily rotation and heart defects and to a lesser extent prosencephalic, visceral arch, and eye defects. The percent anomalies in the rat were comparable to those previously published for the mouse at comparable toxicant concentration. There was a low incidence of neural tube defects in the rat following exposure to the HAs. The rat neural tube appeared less sensitive to the HAs than did the mouse resulting in a higher rate of neural tube dysmorphology in the mouse. Following exposures to BCA and DBA, alone and in combination, there was a significant incidence of delayed embryonic caudal development with apparent normal development anterior to the second visceral arch. The developmental scores for embryos exposed to combinations of the three compounds, when compared to scores for embryos exposed to the single compounds, indicated that the dose-additivity model adequately predicted the observed toxicity and that the developmental toxicity of these water disinfection by-products appears to be additive in whole embryo culture (WEC).

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.

Role of the peroxisome proliferator-activated receptor alpha (PPARalpha) in responses to trichloroethylene and metabolites, trichloroacetate and dichloroacetate in mouse liver

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 carcinogenic metabolites, dichloroacetate (DCA) and trichloroacetate (TCA). TCE is considered to be a relatively weak peroxisome proliferator (PP), a group of rodent hepatocarcinogens that cause adaptive responses in liver through the PP-activated receptor alpha (PPARalpha). The objectives of this study were to determine whether effects of TCE, TCA and DCA in the liver associated with carcinogenesis are mediated by PPARalpha. Male wild-type and PPARalpha-null mice were given TCE by gavage for 3 days or 3 weeks; TCA or DCA were given in the drinking water for 1 week. Increases in relative liver and kidney weights by TCE were dependent on PPARalpha whereas liver weight increases by DCA were PPARalpha-independent. Dose-dependent increases in hepatocyte proliferation observed in wild-type mice after TCE exposure as determined by BrdU-labeling of hepatocytes were PPARalpha-dependent. Transcript profiling using macroarrays containing approximately 1200 genes showed that 93% (40 out of 43) of all expression changes observed in wild-type mice upon TCE exposure were dependent on PPARalpha and included known targets of PP (Cyp4a12, epidermal growth factor receptor) and additional genes involved in cell growth. Increases in enzymes that catalyze beta- and omega-oxidation of fatty acids were dependent on PPARalpha after exposure to TCE, TCA or DCA. TCE altered a unique set of genes in the livers of PPARalpha-null mice compared to wild-type mice including those that respond to different forms of stress. These data support the hypothesis that PPARalpha plays a dominant role in mediating the effects associated with hepatocarcinogenesis upon TCE exposure.

Interactions in the tumor-promoting activity of carbon tetrachloride, trichloroacetate, and dichloroacetate in the liver of male B6C3F1 mice

Interactions between carcinogens in mixtures found in the environment have been a concern for several decades. In the present study, male B6C3F1 mice were used to study the responses to mixtures of dichloroacetate (DCA), trichloroacetate (TCA), and carbon tetrachloride (CT). TCA produces liver tumors in mice with the phenotypic characteristics common to peroxisome proliferators. DCA increases the growth of liver tumors with a phenotype that is distinct in several respects from those produced by TCA. These chemicals are effective as carcinogens at doses that do not produce cytotoxicity. Thus, they encourage clonal expansion of initiated cells through subtle, selective mechanisms. CT is well known for its ability to promote the growth of liver tumors through cytotoxicity that produces a generalized growth stimulus in the liver that is reflected in a reparative hyperplasia. Thus, CT is relatively non-specific in its promotion of initiated cells within the liver. The objective of this study was to determine how the differing modes of action of these chemicals might interact when given as mixed exposures. The hypothesis was that the effects of two selective promoters would not be more than additive. On the other hand, CT would be selective only to cells not sensitive to its effects as a cytotoxin. Thus, it was hypothesized that neither DCA nor TCA would add significantly to the effects produced by CT. Mice were initiated by vinyl carbamate (VC), and then promoted by DCA, TCA, CT, or the pair-wised combinations of the three compounds. The effect of each treatment or treatment combination on tumor number per animal and mean tumor volume was assessed in each animal. Dose-related increases in mean tumor volume were observed with 20 and 50mg/kg CT, but each produced equal numbers of tumors at 36 weeks. As the dose of CT was increased to >/=100mg/kg substantial increases in the number of tumors per animal were observed, but the mean tumor size decreased. This finding suggests that initiation occurs as doses of CT increase to >/=100mg/kg, perhaps as a result of the inflammatory response that is known to occur with high doses of CT. When administered alone in the drinking water at 0.1, 0.5 and 2g/l, DCA increased both tumor number and tumor size in a dose-related manner. With TCA treatment at 2g/l in drinking water a maximum tumor number was reached by 24 weeks and was maintained until 36 weeks of treatment. DCA treatment did not produce a plateau in tumor number within the experimental period, but the numbers observed at the end of the experimental period were similar to TCA and doses of 50mg/kg CT. The tumor numbers observed at the end of the experiment are consistent with the assumption that the administered dose of the tumor initiator, vinyl carbamate, was the major determinant of tumor number and that treatments with CT, DCA, and TCA primarily affected tumor size. The results with mixtures of these compounds were consistent with the basic hypotheses that the responses to tumor promoters with differing mechanisms are limited to additivity at low effective doses. More complex, mutually inhibitory activity was more often observed between the three compounds. At 24 weeks, DCA produced a decrease in tumor numbers promoted by TCA, but the numbers were not different from TCA alone at 36 weeks. The reason for this result became apparent at 36 weeks of treatment where a dose-related decrease in the size of tumors promoted by TCA resulted from DCA co-administration. On the other hand, the low dose of TCA (0.1g/l) decreased the number of tumors produced by a high dose of DCA (2g/l), but higher doses of TCA (2g/l) produced the same number as observed with DCA alone. DCA inhibited the growth rate of CT-induced tumors (CT dose = 50mg/kg). TCA substantially increased the numbers of tumors observed at early time points when combined with CT, but this was not observed at 36 weeks. The lack of an effect at 36 weeks was attributable to the fact that more than 90% of the livers consisted of tumors and the earlier effect was masked by coalescence of tumors. Thus, the ability of TCA to significantly increase tumor numbers in CT-treated mice was probably real and contrary to our original hypothesis that CT was non-specific in its effects on initiated cells. It is probable that the interaction between CT and TCA is explained through stimulation of the growth of cells with differing phenotypes. These data suggest that the outcome of interactions between the mechanisms of tumor promotion vary based on the characteristics of the initiated cells. The interactions may result in additive or inhibitory effects, but no significant evidence of synergy was observed.

Hypomethylation of DNA and the insulin-like growth factor-II gene in dichloroacetic and trichloroacetic acid-promoted mouse liver tumors

Dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are mouse liver carcinogens. DNA hypomethylation is a common molecular event in cancer that is induced by DCA and TCA. Hypomethylation of DNA and the insulin-like growth factor-II (IGF-II) gene was determined in DCA- and TCA-promoted liver tumors. Mouse liver tumors were initiated by N-methyl-N-nitrosourea and promoted by either DCA or TCA. By dot-blot analysis using an antibody for 5-methylcytosine, the DNA in DCA- and TCA-promoted tumors was demonstrated to be hypomethylated. The methylation status of 28 CpG sites in the differentially methylated region-2 (DMR-2) of mouse IGF-II gene was determined. In liver, 79.3 +/- 1.7% of the sites were methylated, while in DCA- and TCA-treated mice, only 46.4 +/- 2.1% and 58.0 +/- 1.7% of them were methylated and 8.7 +/- 2.6% and 10.7 +/- 7.4% were methylated in tumors. The decreased methylation found in liver from mice exposed to DCA or TCA occurred only in the upstream region of DMR-2, while in tumors it occurred throughout the probed region. mRNA expression of the IGF-II gene was increased in DCA- and TCA-promoted liver tumors but not in non-involved liver from DCA- and TCA-exposed mice. The results support the hypothesis that DNA hypomethylation is involved in the mechanism for the tumorigenicity of DCA and TCA.

Prevention by Methionine of Dichloroacetic Acid-Induced Liver Cancer and DNA Hypomethylation in Mice

Dichloroacetic acid (DCA) is a liver carcinogen that induces DNA hypomethylation in mouse liver. To test the involvement of DNA hypomethylation in the carcinogenic activity of DCA, we determined the effect of methionine on both activities. Female B6C3F1 mice were administered 3.2 g/l DCA in their drinking water and 0, 4.0, and 8.0 g/kg methionine in their diet. Mice were sacrificed after 8 and 44 weeks of exposure. After 8 weeks of exposure, DCA increased the liver/body weight ratio and caused DNA hypomethylation, glycogen accumulation, and peroxisome proliferation. Methionine prevented completely the DNA hypomethylation, reduced by only 25% the glycogen accumulation, and did not alter the increased liver/body weight ratio and the proliferation of peroxisomes induced by DCA. After 44 weeks of exposure, DCA induced foci of altered hepatocytes and hepatocellular adenomas. The multiplicity of foci of altered hepatocytes/mouse was increased from 2.41 +/- 0.38 to 3.40 +/- 0.46 by 4.0 g/kg methionine and decreased to 0.94 +/- 0.24 by 8.0 g/kg methionine, suggesting that methionine slowed the progression of foci to tumors. The low and high concentrations of methionine reduced the multiplicity of liver tumors/mouse from 1.28 +/- 0.31 to 0.167 +/- 0.093 and 0.028 +/- 0.028 (i.e., by 87 and 98%, respectively). Thus, the prevention of liver tumors by methionine was associated with its prevention of DNA hypomethylation, indicating that DNA hypomethylation was critical for the carcinogenic activity of DCA.

Field level evaluation and risk assessment of the toxicity of dichloroacetic acid to the aquatic macrophytes Lemna gibba, Myriophyllum spicatum, and Myriophyllum sibiricum

Dichloroacetic acid (DCA), a haloacetic acid, is a common contaminant of aquatic ecosystems. A study to investigate potential phytotoxic effects on rooted and floating macrophytes (Myriophyllum spicatum, M. sibiricum, and Lemna gibba) was conducted. Replicate 12,000 L outdoor microcosms (n = 3) were treated with 3, 10, 30, and 100 mg/L of DCA that had been neutralized to the sodium salt, plus controls. Plants were sampled regularly over 21 days and assessed for a variety of endpoints including plant growth, root growth, number of nodes, wet and dry mass, chlorophyll-a, chlorophyll-b, carotenoids, and citrate levels. EC10, EC25, and EC50 values were calculated for each endpoint that exhibited a concentration-response. Overall, M. sibiricum was slightly more sensitive than M. spicatum to DCA exposure. The most sensitive plant endpoints were wet mass and plant length. Pigments showed no response with exposure to DCA. The probability of current concentrations of DCA in Canadian lake water and Swiss river waters exceeding thresholds of toxicity derived from single species effect measure distributions (EC10s) is << 0.01%. The use of effect measure distributions holds promise as a new risk assessment technique for aquatic plants. Currently, environmental levels of DCA do not pose a risk to these plants.

A 2-year dose-response study of lesion sequences during hepatocellular carcinogenesis in the male B6C3F(1) mouse given the drinking water chemical dichloroacetic acid

Dichloroacetic acid (DCA) is carcinogenic to the B6C3F(1) mouse and the F344 rat. Given the carcinogenic potential of DCA in rodent liver and the known concentrations of this compound in drinking water, reliable biologically based models to reduce the uncertainty of risk assessment for human exposure to DCA are needed. Development of such models requires identification and quantification of premalignant hepatic lesions, identification of the doses at which these lesions occur, and determination of the likelihood that these lesions will progress to cancer. In this study we determined the dose response of histopathologic changes occurring in the livers of mice exposed to DCA (0.05-3.5 g/L) for 26-100 weeks. Lesions were classified as foci of cellular alteration smaller than one liver lobule (altered hepatic foci; AHF), foci of cellular alteration larger than one liver lobule (large foci of cellular alteration; LFCA), adenomas (ADs), or carcinomas (CAs). Histopathologic analysis of 598 premalignant lesions revealed that (a)) each lesion class had a predominant phenotype; (b)) AHF, LFCA, and AD demonstrated neoplastic progression with time; and (c)) independent of DCA dose and length of exposure effects, some toxic/adaptive changes in non-involved liver were related to this neoplastic progression. A lesion sequence for carcinogenesis in male B6C3F(1) mouse liver has been proposed that will enable development of a biologically based mathematical model for DCA. Because all classes of premalignant lesions and CAs were found at both lower and higher doses, these data are consistent with the conclusion that nongenotoxic mechanisms, such as negative selection, are relevant to DCA carcinogenesis at lower doses where DCA genotoxicity has not been observed.

Nephrotoxicity of chlorofluoroacetic acid in rats

Dichloroacetic acid (DCA), chlorofluoroacetic acid (CFA), and difluoroacetic acid (DFA) are inhibitors of pyruvate dehydrogenase kinase. DCA is used for the clinical management of congenital lactic acidosis. Glutathione transferase zeta (GSTZ1-1) catalyzes the biotransformation of DCA and CFA, and DCA is a mechanism-based inactivator of GSTZ1-1. In rodents, DCA causes multiorgan toxicities and is hepatocarcinogenic. The toxic effects of CFA, which is an excellent substrate but a poor inactivator of GSTZ1-1, have not been investigated. The objective of this study was to investigate the nephrotoxicity of CFA. Rats given a single dose of 1.5 mmol/kg CFA became anuric and died within 24 h. Urinalysis and light microscopic analysis showed that rats given 0.6-1.2 mmol/kg CFA developed polyuria, glycosuria, and renal proximal tubular damage. Electron microscopic analysis indicated a role for apoptosis in CFA-induced cell death. The nephrotoxicity of CFA was associated with a dose-dependent increase in inorganic fluoride excretion. Treatment of rats with DCA for 5 days to inactivate GSTZ1-1 failed to prevent metabolism of CFA to fluoride and did not block CFA-induced renal damage. A role for GSTZ1-1-catalyzed release of fluoride from CFA is proposed but a role for other enzymes cannot be excluded. DFA, which is not metabolized to fluoride by GSTZ1-1, was given to rats as a control and was also nephrotoxic: rats given 1.2 mmol DFA/kg/day for 5 days had normal urine volumes but showed proximal and distal tubular damage; fluoride excretion was not elevated. The mechanism of DFA-induced nephrotoxicity is not known but appears to differ from that of CFA.

Dichloroacetate stimulates glycogen accumulation in primary hepatocytes through an insulin-independent mechanism

Dichloroacetate (DCA), a by-product of water chlorination, causes liver cancer in B6C3F1 mice. A hallmark response observed in mice exposed to carcinogenic doses of DCA is an accumulation of hepatic glycogen content. To distinguish whether the in vivo glycogenic effect of DCA was dependent on insulin and insulin signaling proteins, experiments were conducted in isolated hepatocytes where insulin concentrations could be controlled. In hepatocytes isolated from male B6C3F1 mice, DCA increased glycogen levels in a dose-related manner, independently of insulin. The accumulation of hepatocellular glycogen induced by DCA was not the result of decreased glycogenolysis, since DCA had no effect on the rate of glucagon-stimulated glycogen breakdown. Glycogen accumulation caused by DCA treatment was not hindered by inhibitors of extracellular-regulated protein kinase kinase (Erk1/2 kinase or MEK) or p70 kDa S6 protein kinase (p70(S6K)), but was completely blocked by the phosphatidylinositol 3-kinase (PI3K) inhibitors, LY294002 and wortmannin. Similarly, insulin-stimulated glycogen deposition was not influenced by the Erk1/2 kinase inhibitor, PD098509, or the p70(S6K) inhibitor, rapamycin. Unlike DCA-stimulated glycogen deposition, PI3K-inhibition only partially blocked the glycogenic effect of insulin. DCA did not cause phosphorylation of the downstream PI3K target protein, protein kinase B (PKB/Akt). The phosphorylation of PKB/Akt did not correlate to insulin-stimulated glycogenesis either. Similar to insulin, DCA in the medium decreased IR expression in isolated hepatocytes. The results indicate DCA increases hepatocellular glycogen accumulation through a PI3K-dependent mechanism that does not involve PKB/Akt and is, at least in part, different from the classical insulin-stimulated glycogenesis pathway. Somewhat surprisingly, insulin-stimulated glycogenesis also appears not to involve PKB/Akt in isolated murine hepatocytes.

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

Determining the key events in the induction of liver cancer in mice by trichloroethylene (TRI) is important in the determination of how risks from this chemical should be treated at low doses. At least two metabolites can contribute to liver cancer in mice, dichloroacetate (DCA) and trichloroacetate (TCA). TCA is produced from metabolism of TRI at systemic concentrations that can clearly contribute to this response. As a peroxisome proliferator and a species-specific carcinogen, TCA may not be important in the induction of liver cancer in humans at the low doses of TRI encountered in the environment. Because DCA is metabolized much more rapidly than TCA, it has not been possible to directly determine whether it is produced at carcinogenic levels. Unlike TCA, DCA is active as a carcinogen in both mice and rats. Its low-dose effects are not associated with peroxisome proliferation. The present study examines whether biomarkers for DCA and TCA can be used to determine if the liver tumor response to TRI seen in mice is completely attributable to TCA or if other metabolites, such as DCA, are involved. Previous work had shown that DCA produces tumors in mice that display a diffuse immunoreactivity to a c-Jun antibody (Santa Cruz Biotechnology, SC-45), whereas TCA-induced tumors do not stain with this antibody. In the present study, we compared the c-Jun phenotype of tumors induced by DCA or TCA alone to those induced when they are given together in various combinations and to those induced by TRI given in an aqueous vehicle. When given in various combinations, DCA and TCA produced a few tumors that were c-Jun+, many that were c-Jun-, but a number with a mixed phenotype that increased with the relative dose of DCA. Sixteen TRI-induced tumors were c-Jun+, 13 were c-Jun-, and 9 had a mixed phenotype. Mutations of the H-ras protooncogene were also examined in DCA-, TCA-, and TRI-induced tumors. The mutation frequency detected in tumors induced by TCA was significantly different from that observed in TRI-induced tumors (0.44 vs 0.21, p < 0.05), whereas that observed in DCA-induced tumors (0.33) was intermediate between values obtained with TCA and TRI, but not significantly different from TRI. No significant differences were found in the mutation spectra of tumors produced by the three compounds. The presence of mutations in H-ras codon 61 appeared to be a late event, but ras-dependent signaling pathways were activated in all tumors. These data are not consistent with the hypothesis that all liver tumors induced by TRI were produced by TCA.

Immunohistochemical localization and activity of glutathione transferase zeta (GSTZ1-1) in rat tissues

Glutathione transferase zeta (GSTZ1-1) catalyzes the biotransformation of a range of alpha-haloacids, including dichloroacetic acid (DCA), and the penultimate step in the tyrosine degradation pathway. DCA is a rodent carcinogen and a common drinking water contaminant. DCA also causes multiorgan toxicity in rodents and dogs. The objective of this study was to determine the expression and activities of GSTZ1-1 in rat tissues with maleylacetone and chlorofluoroacetic acid as substrates. GSTZ1-1 protein was detected in most tissues by immunoblot analysis after immunoprecipitation of GSTZ1-1 and by immunohistochemical analysis; intense staining was observed in the liver, testis, and prostate; moderate staining was observed in the brain, heart, pancreatic islets, adrenal medulla, and the epithelial lining of the gastrointestinal tract, airways, and bladder; and sparse staining was observed in the renal juxtaglomerular regions, skeletal muscle, and peripheral nerve tissue. These patterns of expression corresponded to GSTZ1-1 activities in the different tissues with maleylacetone and chlorofluoroacetic acid as substrates. Specific activities ranged from 258 +/- 17 (liver) to 1.1 +/- 0.4 (muscle) nmol/min/mg of protein with maleylacetone as substrate and from 4.6 +/- 0.89 (liver) to 0.09 +/- 0.01 (kidney) nmol/min/mg of protein with chlorofluoroacetic acid as substrate. Rats given DCA had reduced amounts of immunoreactive GSTZ1-1 protein and activities of GSTZ1-1 in most tissues, especially in the liver. These findings indicate that the DCA-induced inactivation of GSTZ1-1 in different tissues may result in multiorgan disorders that may be associated with perturbed tyrosine metabolism.

Dichloroacetate toxicokinetics and disruption of tyrosine catabolism in B6C3F1 mice: dose-response relationships and age as a modifying factor

Dichloroacetate (DCA) is a rodent carcinogen commonly found in municipal drinking water supplies. Toxicokinetic studies have established that elimination of DCA is controlled by liver metabolism, which occurs by the cytosolic enzyme glutathione-S-transferase-zeta (GST-zeta). DCA is also a mechanism based inhibitor of GST-zeta, and a loss in GST-zeta enzyme activity occurs following repeated doses or prolonged drinking water exposures. GST-zeta is identical to an enzyme that is part of the tyrosine catabolism pathway known as maleylacetoacetate isomerase (MAAI). In this pathway, GST-zeta plays a critical role in catalyzing the isomerization of maleylacetoacetate to fumarylacetoacetate. Disruption of tyrosine catabolism has been linked to increased cancer risk in humans. We studied the elimination of i.v. doses of DCA to young (10 week) and aged (60 week) mice previously treated with DCA in their drinking water for 2 and 56 weeks, respectively. The diurnal change in blood concentrations of DCA was also monitored in mice exposed to three different drinking water concentrations of DCA (2.0, 0.5 and 0.05 g/l). Additional experiments measured the in vitro metabolism of DCA in liver homogenates prepared from treated mice given various recovery times following treatment. The MAAI activity was also measured in liver cytosol obtained from treated mice. Results indicated young mice were the most sensitive to changes in DCA elimination after drinking water treatment. The in vitro metabolism of DCA was decreased at all treatment rates. Partial restoration ( approximately 65% of controls) of DCA elimination capacity and hepatic GST-zeta activity occurred after 48 h recovery from 14 d 2.0 g/l DCA drinking water treatments. Recovery from treatments could be blocked by interruption of protein synthesis with actinomycin D. MAAI activity was reduced over 80% in liver cytosol from 10-week-old mice. However, MAAI was unaffected in 60-week-old mice. These results indicate that in young mice, inactivation and re-synthesis of GST-zeta is a highly dynamic process and that exogenous factors that deplete or reduce GST-zeta levels will decrease DCA elimination and may increase the carcinogenic potency of DCA. As mice age, the elimination capacity for DCA is less affected by reduced liver metabolism and mice appear to develop some toxicokinetic adaptation(s) to allow elimination of DCA at rates comparable to naive animals. Reduced MAAI activity alone is unlikely to be the carcinogenic mode of action for DCA and may in fact, only be important during the early stages of DCA exposure.

Trichloroethylene, trichloroacetic acid, and dichloroacetic acid: do they affect fetal rat heart development?

Trichloroethylene (TCE), trichloroacetic acid (TCA), and dichloroacetic acid (DCA) are commonly found as groundwater contaminants in many regions of the United States. Cardiac birth defects in children have been associated with TCE, and laboratory studies with rodents report an increased incidence of fetal cardiac malformations resulting from maternal exposures to TCE, TCA, and DCA. The objective of this study was to orally treat pregnant CDR(CD) Sprague-Dawley rats with large bolus doses of either TCE (500 mg/kg), TCA (300 mg/kg), or DCA (300 mg/kg) once per day on days 6 through 15 of gestation to determine the effectiveness of these materials to induce cardiac defects in the fetus. All-trans retinoic acid (RA) dissolved in soybean oil was used as a positive control. Soybean oil is commonly used as a dosing vehicle for RA teratology studies and was also used in this study as a dosing vehicle for TCE. Water was used as the dosing vehicle for TCA and DCA. Fetal hearts were examined on gestation day (GD) 21 by an initial in situ, cardiovascular stereomicroscope examination, and then followed by a microscopic dissection and examination of the formalin-fixed heart. The doses selected for TCA and DCA resulted in a modest decrease in maternal weight gain during gestation (3% to 8%). The fetal weights on GD 21 in the TCA and DCA treatment groups were decreased 8% and 9%, respectively, compared to the water control group and 21% in the RA treatment group compared to soybean oil control group. The heart malformation incidence for fetuses from the TCE-, TCA-, and DCA-treated dams did not differ from control values on a per fetus or per litter basis. The rate of heart malformations, on a per fetus basis, ranged from 3% to 5% for TCE, TCA, and DCA treatment groups compared to 6.5% and 2.9% for soybean oil and water control groups. The RA treatment group was significantly higher with 33% of the fetuses displaying heart defects. For TCE, TCA, and DCA treatment groups 42% to 60% of the litters contained at least one fetus with a heart malformation, compared to 52% and 37% of the litters in the soybean oil and water control groups. For the RA treatment group, 11 of 12 litters contained at least one fetus with a heart malformation. Further research is needed to quantify the spontaneous rates of heart defects for vehicle control rats and to explain the disparity between findings in the present study and other reported findings on the fetal cardiac teratogenicity of TCE, TCA, and DCA.

Effect of chloroform on dichloroacetic acid and trichloroacetic acid-induced hypomethylation and expression of the c-myc gene and on their promotion of liver and kidney tumors in mice

Chloroform, dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are mouse liver carcinogens that are chlorine disinfection by-products found in drinking water. The effect of chloroform on DCA and TCA-induced hypomethylation and expression of the c-myc gene and on their promotion of liver and kidney tumors was determined. B6C3F1 mice were administered 0, 400, 800 and 1600 mg/l chloroform in drinking water and 500 mg/kg DCA or TCA-administered daily by gavage. DCA, TCA and to a lesser extent chloroform decreased the methylation and increased the mRNA expression of the c-myc gene. Co-administering chloroform prevented only DCA and not TCA-induced hypomethylation and increased mRNA expression of the gene. The effect of chloroform on tumor promotion by DCA and TCA was determined in female and male B6C3F1 mice initiated on day 15 of age with N-methyl-N-nitrosourea. Starting at 5 weeks of age, the mice received in their drinking water DCA (3.2 g/l) or TCA (4.0 g/l) with 0, 800 or 1600 mg/l chloroform until they were killed at 36 weeks. Liver tumors promoted by DCA and TCA were predominantly basophilic except for DCA-treated female mice that were eosinophilic. Only DCA promoted foci of altered hepatocytes and they were eosinophilic in both sexes. Chloroform prevented DCA, but not TCA promotion of liver foci and tumors. In male mice, TCA promoted kidney tumors while DCA promoted kidney tumors only when co-administered with chloroform. Hence, chloroform prevented the hypomethylation and increased mRNA expression of the c-myc gene and the promotion of liver tumors by DCA, while enhancing DCA-promotion of kidney tumors. Thus, the concurrent exposure to two carcinogens, chloroform and DCA resulted in less than additive activity in one organ and synergism in another organ.

Effects of dichloroacetate (DCA) on serum insulin levels and insulin-controlled signaling proteins in livers of male B6C3F1 mice

DCA is hepatocarcinogenic in rodents. At carcinogenic doses, DCA causes a large accumulation of liver glycogen. Thus, we studied the effects of DCA treatment on insulin levels and expression of insulin-controlled signaling proteins in the liver. DCA treatment (0.2-2.0 g/l in drinking water for 2 weeks) reduced serum insulin levels. The decrease persisted for at least 8 weeks. In livers of mice treated with DCA for 2-, 10-, and 52-week periods, insulin receptor (IR) protein levels were significantly depressed. Additionally, protein kinase B (PKBalpha) expression decreased significantly with DCA treatment. In normal liver, glycogen levels were increased as early as at 1 week, and this effect preceded changes in insulin and IR and PKBalpha. In contrast to normal liver, IR protein was elevated in DCA-induced liver tumors relative to that in liver tissue of untreated animals and to an even greater extent when compared to adjacent normal liver in the treated animal. Mitogen-activated protein kinase (MAP kinase) phosphorylation was also increased in tumor tissue relative to normal liver tissue and tissue from untreated controls. These data suggest that normal hepatocytes down-regulate insulin-signaling proteins in response to the accumulation of liver glycogen caused by DCA. Furthermore, these results suggest that the initiated cell population, which does not accumulate glycogen and is promoted by DCA treatment, responds differently from normal hepatocytes to the insulin-like effects of this chemical. The differential sensitivity of the 2 cell populations may contribute to the tumorigenic effects of DCA in the liver.

The disinfection by-products dichloro-, dibromo-, and bromochloroacetic acid impact intestinal microflora and metabolism in Fischer 344 rats upon exposure in drinking water

Human consumption of chlorinated drinking water has been linked epidemiologically to bladder, kidney, and rectal cancers. The disinfection by-product (DBP) dichloroacetic acid is a hepatocarcinogen in Fischer 344 rats and B6C3F1 mice. The objective of this study is to determine the effect of the DBPs dichloro-, bromochloro-, and dibromoacetic acids (DCA, BCA, DBA) on intestinal microbial populations and their metabolism, with emphasis on enzymes involved in the bioactivation of procarcinogens and promutagens. One-month-old male Fischer 344 rats were provided water ad libitum containing 1 g/l DCA, BCA, or DBA for up to 5 weeks. At 1, 3, and 5 weeks of treatment, beta-glucuronidase (GLR), beta-galactosidase (GAL), beta-glucosidase (GLU), nitroreductase (NR), azoreductase (AR), and dechlorinase (DC) activities were determined in cecal and small and large intestinal homogenates. After 5 weeks of treatment, intestinal populations were enumerated on selective media. Cecal GAL (DCA, BCA, DBA) and GLR (DCA, DBA) activities were reduced after 1 and 3 weeks of treatment and GAL activity was elevated at 5 weeks (BCA). Large intestinal GAL (DCA, BCA) and GLU (DCA, BCA, DBA) activities were elevated after 5 weeks of treatment. Week 5 cecal AR (DCA, BCA, DBA), NR (DCA), and DC (DCA, DBA) activities were reduced. Even though some significant changes in intestinal populations were observed, use of selective media was not sensitive enough to explain fluctuations in enzyme activity. Haloacetic acids in the drinking water alter intestinal metabolism, which could influence bioactivation of promutagens and procarcinogens in the drinking water.

Depletion of lactate by dichloroacetate reduces cardiac efficiency after hemorrhagic shock

We have demonstrated previously that dichloroacetate (DCA) treatment in rodents ameliorates, via activation of the pyruvate dehydrogenase complex, the cardiovascular depression observed after hemorrhagic shock. To explore the mechanism of this effect, we administered DCA in a large animal model of hemorrhagic shock. Mongrel hounds were anesthetized with 1.5% isoflurane and were measured for hemodynamics, myocardial contractility, and myocardial substrate utilization. They were hemorrhaged to a mean arterial pressure of 35 mm Hg for 90 min or until arterial lactate levels reached 7.0 mM (1137 +/- 47 mL or 49 +/- 2% total blood volume). Animals were chosen at random to receive DCA dissolved in water or an equal volume of saline at the onset of resuscitation. Two-thirds of the shed blood volume was returned immediately after giving an equivalent volume of saline. Two hours after the onset of resuscitation, mean arterial pressure was not different between DCA and control groups (79 +/- 3 vs. 82 +/- 3 mm Hg, respectively). Arterial lactate levels were significantly reduced by DCA (0.5 +/- 0.06 vs. 2.0 +/- 0.2 mM). However, DCA treatment was associated with a decreased stroke volume index (0.56 +/- 0.06 vs. 0.82 +/- 0.08 mL/kg/beat) and a decreased myocardial efficiency (19 vs. 41 L x mm Hg/mL/100 g tissue). During resuscitation by DCA, myocardial lactate consumption was reduced (21.4 +/- 3.7 vs. 70.7 +/- 16.3 micromole/min/100 g tissue) despite a three-fold increase in myocardial pyruvate dehydrogenase activity, while free fatty acid levels actually began to rise. Although increased lactate oxidation should be beneficial during resuscitation, we propose that DCA treatment led to a deprivation of myocardial lactate supply, which reduced net myocardial lactate oxidation, thus compromising myocardial function during resuscitation from hemorrhagic shock.

In vivo MRI measurements of tumor growth induced by dichloroacetate: implications for mode of action

Dichloroacetate (DCA) is an important by-product of the chlorination of drinking water that produces liver cancer in rodents. Assessment of the risk that results from concentrations that occur in drinking water will be dependent upon the mode of action held responsible for these tumors. A study by Stauber and Bull [Stauber, A.J. and Bull, R. J (1997) Differences in phenotype and cell replicative behavior of hepatic tumors inducted by dichloroacetate (DCA) and trichloroacetate (TCA). Toxicol. Appl. Pharmacol. 144, 235-246] in mice treated with DCA demonstrated a lesion distribution that was skewed towards many small, altered foci of cells that are assumed to be precursor lesions [EPA, (1996). U.S. Environmental Protection Agency: Proposed Guidelines for carcinogen risk assessment; notice. Fed. Reg. 61, pp. 17960-10811]. The present study was designed to determine the extent to which the tumorigenic effects of DCA could be explained by its effect on tumor growth rates (i.e. tumor promoting activity). In vivo magnetic resonance imaging (MRI) allowed accurate determination of growth rates of individual lesions in mice that had been treated with DCA in drinking water at 2 g/l. Out of thirty treated mice, ten were found to have hepatic tumors detectable by MRI at 48 weeks of treatment. These tumor-bearing animals were assigned to two groups matched on the size of lesions observed by in vivo MR1. Treatment with DCA continued in one group of five mice and was stopped in the other. For both groups, tumor growth rates were determined by measuring changes in size of all lesions greater than 1 mm(3) in volume during a 14-day period. Removal of DCA treatment resulted in growth rates that could not be distinguished from zero across all lesion sizes represented in the sample. These data are in agreement with previous observations of DCAs effects on replication rates within tumors (Stauber and Bull, (1997)). Tumor growth rates observed in animals maintained on treatment decreased with lesion volume in a manner that is consistent with a stochastic Gompertz birth-death process proposed by Tan [Tan, W.Y. (1986) A stochastic Gompertz birth-death process. Stat. Prob. Lett. 4, 25-28]. Parameters of this model obtained by fitting measured growth rates were used to predict the lesion-size distribution expected after one year of DCA treatment. The shape of the predicted lesion-size distribution was similar to that observed by Stauber and Bull (Stauber and Bull, (1997)) in mice sacrificed after 40 weeks of DCA treatment. We conclude that the effects of DCA on the division and/or death rates of spontaneously initiated cells can account for the predominance of small lesions in DCA-treated animals.

Effect of trichloroethylene and its metabolites, dichloroacetic acid and trichloroacetic acid, on the methylation and expression of c-Jun and c-Myc protooncogenes in mouse liver: prevention by methionine

Trichloroethylene (TCE), dichloroacetic acid (DCA), and trichloroacetic acid (TCA) are environmental contaminants that are carcinogenic in mouse liver. 5-Methylcytosine (5-MeC) in DNA is a mechanism that controls the transcription of mRNA, including the protooncogenes, c-jun and c-myc. We have previously reported that TCE decreased methylation of the c-jun and c-myc genes and increased the level of their mRNAs. Decreased methylation of the protooncogenes could be a result of a deficiency in S-adenosylmethionine (SAM), so that methionine, by increasing the level of SAM, would prevent hypomethylation of the genes. For 5 days, female B6C3F1 mice were administered, daily by oral gavage, either 1000 mg/kg body weight of TCE or 500 mg/kg DCA or TCA. At 30 min after each dose of carcinogen, the mice received, by ip injection, 0-, 30-, 100-, 300-, or 450-mg/kg methionine. Mice were euthanized at 100 min after the last dose of DCA, TCA, or TCE. Decreased methylation in the promoter regions of the c-jun and c-myc genes and increased levels of their mRNA and proteins were found in livers of mice exposed to TCE, DCA, and TCA. Methionine prevented both the decreased methylation and the increased levels of the mRNA and proteins of the two pro-tooncogenes. The prevention by methionine of DCA- TCA-, and TCE-induced DNA hypomethylation supports the hypothesis that these carcinogens act by depleting the availability of SAM. Hence, methionine would prevent DNA hypomethylation by maintaining the level of SAM. Furthermore, the results suggest that the dose of DCA, TCA, or TCE must be sufficient to decrease the level of SAM in order for these carcinogens to be active.

Lessons learned in applying the U.S. EPA proposed cancer guidelines to specific compounds

An expert panel was convened to evaluate the U.S. Environmental Protection Agency's "Proposed Guidelines for Carcinogen Risk Assessment" through their application to data sets for chloroform (CHCl3) and dichloroacetic acid (DCA). The panel also commented on perceived strengths and limitations encountered in applying the guidelines to these specific compounds. This latter aspect of the panel's activities is the focus of this perspective. The panel was very enthusiastic about the evolution of these proposed guidelines, which represent a major step forward from earlier EPA guidance on cancer-risk assessment. These new guidelines provide the latitude to consider diverse scientific data and allow considerable flexibility in dose-response assessments, depending on the chemical's mode of action. They serve as a very useful template for incorporating state-of-the-art science into carcinogen risk assessments. In addition, the new guidelines promote harmonization of methodologies for cancer- and noncancer-risk assessments. While new guidance on the qualitative decisions ensuing from the determination of mode of action is relatively straightforward, the description of the quantitative implementation of various risk-assessment options requires additional development. Specific areas needing clarification include: (1) the decision criteria for judging the adequacy of the weight of evidence for any particular mode of action; (2) the role of mode of action in guiding development of toxicokinetic, biologically based or case-specific models; (3) the manner in which mode of action and other technical considerations provide guidance on margin-of-exposure calculations; (4) the relative roles of the risk manager versus the risk assessor in evaluating the margin of exposure; and (5 ) the influence of mode of action in harmonizing cancer and noncancer risk assessment methodologies. These points are elaborated as recommendations for improvements to any revisions. In general, the incorporation of examples of quantitative assessments for specific chemicals would strengthen the guidelines. Clearly, any revisions should retain the emphasis present in these draft guidelines on flexibility in the use of scientific information with individual compounds, while simultaneously improving the description of the processes by which these mode-of-action data are organized and interpreted.

Hepatocarcinogenicity in the male B6C3F1 mouse following a lifetime exposure to dichloroacetic acid in the drinking water: dose-response determination and modes of action

Male B6C3F, mice were exposed to dichloroacetic acid (DCA) in the drinking water in order to establish a dose response for the induction of hepatocellular cancer and to examine several modes of action for the carcinogenic process. Groups of animals were exposed to control, 0.05, 0.5, 1, 2, or 3.5 g/L DCA in the drinking water for 90-100 wk. Mean daily doses (MDD) of 8, 84, 168, 315, and 429 mg/kg/d of DCA were calculated. The prevalence (percent of animals) with hepatocellular carcinoma (HC) was significantly increased in the 1-g/L (71%), 2-g/L (95%), and 3.5-g/L (100%) treatment groups when compared to the control (26%). HC multiplicity (tumors/animal) was significantly increased by all DCA treatments-0.05 g/L (0.58), 0.5 g/L (0.68), 1 g/L (1.29), 2 g/L (2.47), and 3.5 g/L (2.90)-compared to the control group (0.28). Based upon HC multiplicity, a no-observed-effect level (NOEL) for hepatocarcinogenicity could not be determined. Hepatic peroxisome proliferation was significantly increased only for 3.5 g/L DCA treatment at 26 wk. and did not correlate with the liver tumor response. The severity of hepatotoxicity increased with DCA concentration. Below 1 g/L, hepatotoxicity was mild and transient as demonstrated by the severity indices and serum lactate dehydrogenase activity. An analysis of generalized hepatocyte proliferation reflected the mild hepatotoxicity and demonstrated no significant treatment effects on the labeling index of hepatocytes outside proliferative lesions. Consequently, the induction of liver cancer by DCA does not appear to be conditional upon peroxisome induction or chemically sustained cell proliferation. Hepatotoxicity, especially at the higher doses, may exert an important influence on the carcinogenic process.

Behavioral evaluation of the neurotoxicity produced by dichloroacetic acid in rats

Dichloroacetic acid (DCA) is commonly found in drinking water as a by-product of chlorination disinfection. It is a known neurotoxicant in rats, dogs, and humans. We have characterized DCA neurotoxicity in rats using a neurobehavioral screening battery under varying exposure durations (acute, subchronic, and chronic) and routes of administration (oral gavage and drinking water). Studies were conducted in both weanling and adult rats, and comparisons were made between Long-Evans and Fischer-344 rats. DCA produced neuromuscular toxicity comprised of limb weakness and deficits in gait and righting reflex; altered gait and decreased hindlimb grip strength were the earliest indicators of toxicity. Other effects included mild tremors, ocular abnormalities, and a unique chest-clasping response (seen in Fischer-344 rats only). Neurotoxicity was permanent (i.e., through 2 years) following a 6-month exposure to high dose levels, whereas the effects of intermediate dose levels with exposures of 3 months or less were slowly reversible. The severity, specificity, and recovery of neurological changes were route, duration, and strain dependent. Fischer-344 rats were more sensitive than Long-Evans rats, and weanling rats may be somewhat more sensitive than adults. Oral gavage produced significantly less toxicity compared to the same intake level received in drinking water. Neurotoxicity was progressive with continued exposure, and was observed at exposure levels as low as 16 mg/kg/day (lowest dose level tested) when administered via drinking water in subchronic studies. The data from these studies characterize the neurotoxicity produced by DCA, and show it to be more pronounced, persistent, and occurring at lower exposures than has been previously reported. Further research should take into account these marked route, age, and strain differences.

Phase III interlaboratory study of FETAX. Part 3. FETAX validation using 12 compounds with and without an exogenous metabolic activation system

FETAX (Frog Embryo Teratogenesis Assay-Xenopus) is a 96-h whole-embryo developmental toxicity screening assay that can be used in ecotoxicology and in detecting mammalian developmental toxicants when an in vitro metabolic activation system is employed. A standardized American Society for Testing and Materials (ASTM) guide for the conduct of FETAX has been published, along with a companion atlas that helps in embryo staging and in identifying malformations. As part of the ASTM process, an interlaboratory validation study was undertaken to evaluate the repeatability and reliability of FETAX and to evaluate the potential teratogenic hazard of 12 compounds. Three different laboratories participated in the study. All three participating laboratories had extensive experience with the assay. FETAX intralaboratory and interlaboratory variability, as judged by coefficients of variation, were very low. Potential teratogenic hazard was evaluated using two major criteria from FETAX experiments employing metabolic activation systems (MAS). These were the teratogenic index TI (TI = 96-h lc(50)/96-h ec(50) (malformation)) and the minimum concentration that inhibits growth (MCIG). A compound was considered teratogenic by this criterion when the MCIG was significantly different from controls at concentrations below the 30% level of the MAS 96-h lc(50). Based on the results of this and other studies, a decision table was constructed in order to evaluate additional studies. Severity of malformations caused, especially near the MAS 96-h ec(50) (malformation), were also evaluated. Four compounds were non-teratogenic but two compounds were clearly teratogenic. The remaining six compounds were ranked as equivocal teratogens. The results were discussed in light of the difficulty of producing an adequate decision table. FETAX proved to yield repeatable and reliable data as long as care was taken during range-finding and technicians were adequately trained. The MAS was essential in using FETAX to predict developmental hazard in mammals, and still requires further development.

Inhibition of glutathione S-transferase zeta and tyrosine metabolism by dichloroacetate: a potential unifying mechanism for its altered biotransformation and toxicity

Dichloroacetate (DCA) inhibits its own metabolism and is converted to glyoxylate by glutathione S-transferase zeta (GSTz). GSTz is identical to maleylacetoacetate isomerase, an enzyme of tyrosine catabolism that converts maleylacetoacetate (MAA) to fumarylacetoacetate and maleylacetone (MA) to fumarylacetone. MAA and MA are alkylating agents. Rats treated with DCA for up to five days had markedly decreased hepatic GSTz activity and increased urinary excretion of MA. When dialyzed cytosol obtained from human liver was incubated with DCA, GSTz activity was unaffected. In contrast, DCA incubation inhibited enzyme activity in dialyzed hepatic cytosol from rats. Incubation of either rat or human hepatic cytosol with MA led to a dose dependent inhibition of GSTz. These data indicate that humans or rodents exposed to DCA may accumulate MA and/or MAA which inhibit(s) GSTz and, consequently, DCA biotransformation. Moreover, DCA-induced inhibition of tyrosine catabolism may account for the toxicity of this xenobiotic in humans and other species.

Dichloroacetate (DCA) dosimetry: interpreting DCA-induced liver cancer dose response and the potential for DCA to contribute to trichloroethylene-induced liver cancer

Pharmacokinetic studies with dichloroacetate (DCA) provide insights into the likelihood that trichloroethylene-induced liver cancers arise from formation of DCA as a metabolite and the mode of action by which DCA induces liver cancer. A simple physiologically based pharmacokinetic model was developed to analyze DCA blood concentration data from mice unexposed to or pre-treated with DCA. The large first pass metabolism of DCA in the liver is significantly reduced by DCA pretreatment. Because DCA inhibits its own metabolism, large increases in area under the blood concentration curve occur at lower doses than would be predicted from single-dose pharmacokinetic studies with naive mice. The dose metrics associated with the incidence of liver tumors in contrast to the multiplicity of tumors per animal may be different, suggesting potentially different roles in the cancer process for DCA versus its metabolites. By linking a model for trichloroethylene (TCE) pharmacokinetics with the DCA model, maximum levels of DCA potentially produced from TCE were estimated to be at or below the analytical chemistry detection limits. In addition, the predicted levels of DCA would be too small to produce the observed liver cancers following corn oil gavage exposure of mice to TCE.

Drinking water disinfection byproducts: review and approach to toxicity evaluation

There is widespread potential for human exposure to disinfection byproducts (DBPs) in drinking water because everyone drinks, bathes, cooks, and cleans with water. The need for clean and safe water led the U.S. Congress to pass the Safe Drinking Water Act more than 20 years ago in 1974. In 1976, chloroform, a trihalomethane (THM) and a principal DBP, was shown to be carcinogenic in rodents. This prompted the U.S. Environmental Protection Agency (U.S. EPA) in 1979 to develop a drinking water rule that would provide guidance on the levels of THMs allowed in drinking water. Further concern was raised by epidemiology studies suggesting a weak association between the consumption of chlorinated drinking water and the occurrence of bladder, colon, and rectal cancer. In 1992 the U.S. EPA initiated a negotiated rulemaking to evaluate the need for additional controls for microbial pathogens and DBPs. The goal was to develop an approach that would reduce the level of exposure from disinfectants and DBPs without undermining the control of microbial pathogens. The product of these deliberations was a proposed stage 1 DBP rule. It was agreed that additional information was necessary on how to optimize the use of disinfectants while maintaining control of pathogens before further controls to reduce exposure beyond stage 1 were warranted. In response to this need, the U.S. EPA developed a 5-year research plan to support the development of the longer term rules to control microbial pathogens and DBPs. A considerable body of toxicologic data has been developed on DBPs that occur in the drinking water, but the main emphasis has been on THMs. Given the complexity of the problem and the need for additional data to support the drinking water DBP rules, the U.S. EPA, the National Institute of Environmental Health Sciences, and the U.S. Army are working together to develop a comprehensive biologic and mechanistic DBP database. Selected DBPs will be tested using 2-year toxicity and carcinogenicity studies in standard rodent models; transgenic mouse models and small fish models; in vitro mechanistic and toxicokinetic studies; and reproductive, immunotoxicity, and developmental studies. The goal is to create a toxicity database that reflects a wide range of DBPs resulting from different disinfection practices. This paper describes the approach developed by these agencies to provide the information needed to make scientifically based regulatory decisions.