Promotion by dichloroacetic acid and trichloroacetic acid of N-methyl-N-nitrosourea-initiated cancer in the liver of female B6C3F1 mice

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

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

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

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

N-Methyl-N-nitrosourea as a mammary carcinogenic agent

The administration of chemical carcinogens is one of the most commonly used methods to induce tumors in several organs in laboratory animals in order to study oncologic diseases of humans. The carcinogen agent N-methyl-N-nitrosourea (MNU) is the oldest member of the nitroso compounds that has the ability to alkylate DNA. MNU is classified as a complete, potent, and direct alkylating compound. Depending on the animals' species and strain, dose, route, and age at the administration, MNU may induce tumors' development in several organs. The aim of this manuscript was to review MNU as a carcinogenic agent, taking into account that this carcinogen agent has been frequently used in experimental protocols to study the carcinogenesis in several tissues, namely breast, ovary, uterus, prostate, liver, spleen, kidney, stomach, small intestine, colon, hematopoietic system, lung, skin, retina, and urinary bladder. In this paper, we also reviewed the experimental conditions to the chemical induction of tumors in different organs with this carcinogen agent, with a special emphasis in the mammary carcinogenesis.

Application of talcum powder, trichloroacetic acid and silver nitrate in female rats for non-surgical sterilization: evaluation of the apoptotic pathway mRNA and miRNA genes

There are several methods used for non-surgical sterilization in birth control including quinacrine, trichloroacetic acid (TCA), erythromycin, tetracycline, silver nitrate and talcum powder. Among these, talcum powder, TCA and silver nitrate are the most commonly used. However, the toxic and carcinogenic activities of these chemicals in ovarian tissue have been poorly elucidated. This study demonstrates the expression levels of antioxidant, apoptotic and anti-apoptotic genes after administration of talc powder, TCA and silver nitrate for non-surgical sterilization in female rat models. The expression changes of some microRNAs (miR-15b, miR-21, miR-34a and miR-98) that play key roles in the apoptosis pathway were also included. All expression analyses were evaluated with real-time PCR. The expression levels of all genes appeared to be upregulated in the talcum powder group, but the results were not statistically significant. Increased expression of Gsr and Sod1 genes was statistically significant in the talcum powder group. In TCA and silver nitrate group, expression of all genes was appeared to be elevated but only the Gsr expression was statistically significant in the TCA-administrated group; there were no statistically significant changes in the silver nitrate group. miRNA expression levels were increased in talcum powder and TCA-administrated groups, but these results were not significant. Expression levels of miR-15b, miR-21 and miR-98 in the silver nitrate group were significantly increased. Consequently, these chemicals appear to be non-carcinogenic agents for rat ovarian tissue which do not induce apoptosis. However, talcum powder and TCA can be considered as agents that are toxic to ovarian tissue.

Trichloroethylene risk assessment: a review and commentary

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

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

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

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

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

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

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

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.

Experimental cancer studies of chlorinated by-products

Chlorinated drinking water contains a number of different by-products formed during the chlorination process from organic matter. The carcinogenicity of only a fraction of them have been evaluated in experimental animals. The focus has been on compounds and groups of compounds that are most abundant in chlorinated drinking water or the in vitro toxicity data have suggested genotoxic potential. From trihalomethanes, chloroform causes liver tumors in mice and female rats and renal tumors in male mice and rats. Tumor formation by chloroform is strongly associated with cytotoxicity and regenerative cell proliferation in tissues and that has been considered to be one determinant of its carcinogenicity. From halogenic acetic acids, dichloroacetic acid (DCA) and trichlotoacetic acid (TCA) are hepatocarcinogenic in mice and DCA in male rats. Their genotoxicity is equivocal and nongenotoxic mechanisms, such as peroxisome proliferation and hypomethylation of DNA in the liver, likely contribute to tumor development. From chlorinated furanones (CHFs), 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is a multisite carcinogen in rats (e.g. in thyroid glands and liver) and it has caused DNA damage in vivo. MX may be a complete carcinogen because it also has promoter properties in vitro. Chlorinated drinking water may also contain brominated by-products providing the raw water contains bromide. At least some of them (bromodichloromethane, bromoform) have been shown to be carcinogenic in laboratory animals. Altogether, although several by-products have been shown to have carcinogenic potential in laboratory animals, it not yet possible to state which compounds or groups of by-products cause the cancer risk in chlorinated drinking water. The cellular mechanisms of their effects and these effects at low concentrations are still poorly understood. The few studies with mixtures of these by-products suggest that the mixture effects may be complex and unpredictable (inhibitory, additive, synergistic).

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.

Pharmacologic or genetic ablation of maleylacetoacetate isomerase increases levels of toxic tyrosine catabolites in rodents

Dichloroacetate (DCA) is both an environmental contaminant and an investigational drug for diseases involving perturbed mitochondrial energetics. DCA is biotransformed to glyoxylate by maleylacetoacetate isomerase (MAAI). Previous studies have shown that DCA decreases MAAI activity in rat liver in a time- and dose-dependent manner and may target the protein for degradation in vivo. We now report that the MAAI protein is depleted in a time- and dose-dependent manner in the livers of Sprague-Dawley rats exposed to DCA. This decrease in protein expression is not mirrored by a decrease in the steady-state levels of MAAI mRNA, indicating that the depletion is exclusively a post-transcriptional event. We also investigated the pharmacokinetics of DCA in the recently developed MAAI knockout (MAAI-KO) mouse. MAAI-KO mice maintain high plasma and urine drug concentrations and do not biotransform DCA to monochloroacetate to a significant extent. Therefore, no alternative pathways for DCA clearance appear to exist in mice other than by MAAI-mediated biotransformation. DCA-nai;ve MAAI-KO mice accumulate very high levels of the tyrosine catabolites maleylacetone and succinylacetone, and DCA exposure did not significantly increase the levels of these compounds. MAAI-KO mice also have high levels of fumarylacetone and normal levels of fumarate. These results demonstrate that pharmacologic or genetic ablation of MAAI cause potentially toxic concentrations of tyrosine intermediates to accumulate in mice and perhaps in other species.

The effect of dichloroacetic acid and trichloroacetic acid on DNA methylation and cell proliferation in B6C3F1 mice

The chlorine disinfection by-products, dichloroacetic acid (DCA) and trichloroacetic acid (TCA), are carcinogenic in mouse liver. We have previously reported that DCA and TCA induced DNA hypomethylation in mouse liver. In the present study, we determined the temporal association for DNA hypomethylation and cell proliferation. Female B6C3F1 mice were administered daily doses of 500 mg/kg DCA or TCA by gavage and sacrificed at 24, 36, 48, 72, and 96 hours after the first dose. The proliferating cell nuclear antigen-labeling index in the liver was increased at 72 and 96 hours by both DCA and TCA, that is, at 72 hours the index was 1.00 +/- 0.21, 0.51 +/- 0.11, and 0.095 +/- 0.016 for DCA, TCA, and the vehicle control, respectively. The mitotic index was also significantly increased at 96 hours. The promoter region for the c-myc gene was hypomethylated only at 72 and 96 hours and not at the earlier sacrifices. Similarly, the methylation of the c-myc gene in the kidney and urinary bladder was decreased only at 72 and 96 hours. In summary, enhancement of cell proliferation and decreased methylation of the c-myc gene were first observed simultaneously at 72 hours after the start of exposure. Thus, the results support the hypothesis that DCA and TCA induce DNA hypomethylation by inducing DNA replication and preventing the methylation of the newly synthesized strands of DNA.

Differential effects of dihalogenated and trihalogenated acetates in the liver of B6C3F1 mice

Haloacetates are produced in the chlorination of drinking water in the range 10--100 microg l(-1). As bromide concentrations increase, brominated haloacetates such as bromodichloroacetate (BDCA), bromochloroacetate (BCA) and dibromoacetate (DBA) appear at higher concentrations than the chlorinated haloacetates: dichloroacetate (DCA) or trichloroacetate (TCA). Both DCA and TCA differ in their hepatic effects; TCA produces peroxisome proliferation as measured by increases in cyanide-insensitive acyl CoA oxidase activity, whereas DCA increases glycogen concentrations. In order to determine whether the brominated haloacetates DBA, BCA and BDCA resemble DCA or TCA more closely, mice were administered DBA, BCA and BDCA in the drinking water at concentrations of 0.2--3 g l(-1). Both BCA and DBA caused liver glycogen accumulation to a similar degree as DCA (12 weeks). The accumulation of glycogen occurred in cells scattered throughout the acinus in a pattern very similar to that observed in control mice. In contrast, TCA and low concentrations of BDCA (0.3 g l(-1)) reduced liver glycogen content, especially in the central lobular region. The high concentration of BDCA (3 g l(-1)) produced a pattern of glycogen distribution similar to that in DCA-treated and control mice. This effect with a high concentration of BDCA may be attributable to the metabolism of BDCA to DCA. All dihaloacetates reduced serum insulin levels. Conversely, trihaloacetates had no significant effects on serum insulin levels. Dibromoacetate was the only brominated haloacetate that consistently increased acyl-CoA oxidase activity and rates of cell replication in the liver. These results further distinguish the effects of the dihaloacetates from those of peroxisome proliferators like TCA.

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.

Dichloroacetate and trichloroacetate promote clonal expansion of anchorage-independent hepatocytes in vivo and in vitro

Dichloroacetate (DCA) and trichloroacetate (TCA) are hepatocarcinogenic by-products of water chlorination and metabolites of several industrial solvents. To determine whether DCA and TCA promote the clonal expansion of anchorage-independent liver cells in vitro, a modification of the soft agar assay (over agar assay) was utilized to quantitate growth and analyze phenotype of anchorage-independent hepatocellular colonies. Hepatocytes from naïve male B6C3F1 mice were isolated and cultured with 0-2.0 mM DCA or TCA over agar for 10 days, at which time colonies of eight cells or more were scored. Both DCA and TCA promoted the formation of anchorage-independent colonies in a dose-dependent manner. Immunocytochemical analysis using a c-Jun antibody demonstrated that colonies promoted by DCA were primarily c-Jun+, whereas TCA-promoted colonies were primarily c-Jun-. This corresponds to the differences in c-Jun immunoreactivity reported in tumors induced by DCA and TCA. Neither DCA nor TCA induced c-Jun expression in hepatocyte monolayers, indicating that these haloacetates selectively affect subpopulations of anchorage-independent hepatocyts. The latency of colony formation was decreased by the concentration of DCA, although the same number of colonies appeared after 25 days in culture at all DCA concentrations used. The plating density of hepatocytes also affected colony formation. At lower cell densities, promotion of colony formation by DCA was significantly reduced. Pretreatment of male B6C3F1 mice with 0.5 g/liter DCA in drinking water resulted in a fourfold increase in in vitro colony formation above hepatocytes isolated from naïve mice, suggesting that DCA is promoting the clonal expansion of anchorage-independent hepatocytes in vivo. Results from this study indicate that DCA and TCA promote the survival and growth of initiated cells. Furthermore, results from over agar assays reflect observations made in vivo, indicating this assay provides a valid means to investigate the mechanism by which chemicals promote clonal expansion of initiated hepatocytes.

Dissimilar characteristics of N-methyl-N-nitrosourea-initiated foci and tumors promoted by dichloroacetic acid or trichloroacetic acid in the liver of female B6C3F1 mice

Dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are metabolites of the industrial solvent and environmental contaminant trichloroethylene (TCE), as well as contaminants of chlorinated drinking water. Human exposure to these chemicals is of concern as all three have been shown to increase liver tumor incidence in mice. Differences in dose-response curves, progression to cancer, and postexposure regression of lesions suggest that TCA and DCA work through different mechanisms. The purpose of this study was to further characterize the proliferative hepatocellular lesions promoted by TCA and DCA using biomarkers of cell growth, differentiation, and metabolism in liver sections to better delineate the distinctions in the mechanism of the two chloroacetates. Fifteen-day-old female mice were initiated with 25 mg/kg N-methyl-N-nitrosourea. The initiated mice were administered DCA or TCA (20.0 mmol/L) in drinking water from age 49 days until euthanasia at age 413 days. The pathologic assessment showed that the foci of altered hepatocytes and tumors occurring in the animals promoted with DCA were eosinophilic and positive immunohistochemically for TGF-alpha, c-jun, c-myc, CYP 2E1, CYP 4A1, and glutathione S-transferase-pi (GST-pi). The DCA lesions also were essentially negative for c-fos and TGF-beta, but nontumor hepatocytes were consistently TGF-beta-positive. In contrast, tumors promoted by TCA were predominantly basophilic, lacked GST-pi, and stained variably; usually, more than 50% of the tumor hepatocytes were essentially negative for the other biomarkers. This study demonstrates some striking differences in certain molecular biomarkers of cell growth, differentiation, and metabolism between DCA and TCA. The results also suggest some potential growth signal transduction pathways that may contribute to the DCA promotion of tumors, further support the premise that these two chloroacetates promote hepatocarcinogenesis in different ways, and provide a rational basis for a similar comparison with TCE. Such a comparison should give some insight as to whether DCA, TCA, or both are playing a significant role in the murine liver carcinogenesis of the parent compound, TCE.

Differences in phenotype and cell replicative behavior of hepatic tumors induced by dichloroacetate (DCA) and trichloroacetate (TCA)

Dichloroacetate (DCA) and trichloroacetate (TCA) are two hepatocarcinogenic by-products of water chlorination. To compare the effects of DCA and TCA on cell replication in the nodules and tumors they induce, male B6C3F1 mice were administered 2.0 g/L DCA or TCA in their drinking water for 38 or 50 weeks, respectively. The pretreated mice were then given water containing 0, 0.02, 0.5, 1.0, or 2.0 g/L DCA or TCA for two additional weeks to determine whether cell proliferation in the normal liver or tumors that had been induced by DCA or TCA was dependent on continued treatment. Prior to sacrifice the mice were subcutaneously implanted with mini-osmotic pumps to label DNA in dividing cells with 5-bromo-2'-deoxyuridine (BrdU). Serial sections of nodules/tumors and normal liver were stained immunohistochemically for BrdU, the oncoproteins c-Jun and c-Fos, and hematoxylin and eosin (H & E); or with Periodic acid-Schiff (PAS) stain, BrdU, and H & E, respectively. DCA and TCA transiently stimulated the division of normal hepatocytes relative to rates observed in the livers of control mice. However, at 40 and 52 weeks of treatment, replication of normal hepatocytes was substantially inhibited by DCA and TCA, respectively. Cell division within DCA-induced lesions that were identified macroscopically was significantly higher with increasing dose of DCA administered in the last 2 weeks of the experiment. DCA-induced lesions were found to display immunoreactivity to anti-c-Jun and anti-c-Fos antibodies, were predominantly basophilic, and contained very little glycogen relative to surrounding hepatocytes. In contrast, rates of cell division within TCA-induced altered hepatic foci and tumors were very high and appeared to be independent of continued treatment. TCA-induced lesions did not display immunoreactivity to either c-Jun or c-Fos antibodies. Results from this study suggest that the mechanisms by which DCA and TCA induce hepatocarcinogenesis in the male B6C3F1 mouse differ.

Promotion by mixtures of dichloroacetic acid and trichloroacetic acid of N-methyl-N-nitrosourea-initiated cancer in the liver of female B6C3F1 mice

Hepatic tumor promoting activity was determined for mixtures of dichloroacetic acid (DCA) and trichloroacetic acid (TCA) in female B6C3F1 mice initiated on day 15 of age with 25 mg/kg N-methyl-N-nitrosourea. The mice received in their drinking water from 6 to 50 weeks of age either DCA (7.8, 15.6, or 25 mmol/l) with/without 6.0 mmol/l TCA or TCA (6.0 or 25 mmol/l) with/without 15.6 mmol/l DCA. Proliferative lesions (foci of altered hepatocytes and hepatocellular adenomas) promoted by TCA increased linearly with its concentration and were predominantly basophilic and negative for glutathione S-transferase-pi (GST-pi), while those promoted by DCA increased exponentially with its concentration and were eosinophilic and positive for GST-pi. The promoting activity of DCA and TCA in mixtures was at least additive. The proliferative lesions resulting from exposure to the mixtures were predominately similar to those promoted by DCA, i.e. contained eosinophilic and GST-pi-positive hepatocytes.

Trichloroethylene cancer risk: simplified calculation of PBPK-based MCLs for cytotoxic end points

Cancer risk assessments for trichloroethylene (TCE) based on linear extrapolation from bioassay results are questionable in light of new data on TCE's likely mechanism of action involving induced cytotoxicity, for which a threshold-type dose-response model may be more appropriate. Previous studies have shown that if a genotoxic mechanism for TCE is assumed, algebraic methods can considerably simplify the use of physiologically based pharmacokinetic (PBPK) models to estimate virtually safe environmental concentrations for humans based on rodent cancer-bioassay data. We show here how such methods can be extended to the case in which TCE is assumed to induce cancer via cytotoxicity, to estimate environmentally safe concentrations based on rodent toxicity data. These methods can be substituted for the numerical methods typically used to calculate PBPK-effective doses when these are defined as peak concentrations. We selected liver and kidney as plausible target tissues, based on an analysis of rodent TCE-bioassay data and on a review of related data bearing on mechanism. Tumor patterns in rodent bioassays are shown to be consistent with our estimates of PBPK-based, effective cytotoxic doses to mice and rats used in these studies. When used with a margin of exposure of 1000, our method yielded maximum concentration levels for TCE of 16 ppb (87 micrograms/m3) for TCE in air respired 24 hr/day, 700 ppb (3.8 mg/m3) for TCE in air respired for relatively brief daily periods (e.g., 0.5 hr while showering/bathing), and 210 micrograms/liter for TCE in drinking water assuming a daily 2-liter ingestion. Cytotoxic effective doses were also estimated for occupational respiratory exposures. These estimates indicate that the current OSHA permissible exposure limit for TCE would produce metabolite concentrations that exceed an acute no observed adverse effect level for hepatotoxicity in mice. On this basis, the OSHA TCE limit is not expected to be protective.

Loss of heterozygosity on chromosome 6 in dichloroacetic acid and trichloroacetic acid-induced liver tumors in female B6C3F1 mice

Dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are major metabolites of tetrachloroethylene (PCE) and trichloroethylene (TCE) and are found in chlorinated drinking water. All four chlorinated compounds are liver carcinogens in B6C3F1 mice. It has previously been reported that approximately 20% of hepatic tumors induced by PCE exhibited loss of heterozygosity (LOH) on chromosome 6, suggesting the presence of a tumor suppressor gene. In the current investigation, we determined whether TCA or DCA also induced LOH on chromosome 6. Liver tumors were initiated in 15 day old female B6C3F1 mice with N-methyl-N-nitrosourea (MNU) and promoted with 20 mmol/l DCA or TCA in their drinking water. Twenty-four and thirty-seven liver tumors promoted by DCA and TCA, respectively, were examined for LOH using 4 polymorphic loci on chromosome 6. Ten of 37 (27%) tumors (7 of 27 carcinomas and 3 of 10 adenomas) promoted by TCA exhibited LOH at least for two loci on chromosome 6. All 10 tumors that exhibited LOH, lost the C57BL/6J allele at both the D6mit9 loci, while two also lost at least one of the C3H/HeJ alleles. No LOH on chromosome 6 was observed in the 24 liver tumors promoted by DCA. The LOH on chromosome 6 in TCA but not in DCA-promoted tumors supports it as an active metabolite of PCE and demonstrates different pathogenesis at least for some of the DCA and TCA-promoted liver cancer.