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

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.

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

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

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.

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

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

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.

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.

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.

Role of aldehyde dehydrogenases in endogenous and xenobiotic metabolism

Aldehydes are highly reactive molecules that are intermediates or products involved in a broad spectrum of physiologic, biologic and pharmacologic processes. Aldehydes are generated from chemically diverse endogenous and exogenous precursors and aldehyde-mediated effects vary from homeostatic and therapeutic to cytotoxic, and genotoxic. One of the most important pathways for aldehyde metabolism is their oxidation to carboxylic acids by aldehyde dehydrogenases (ALDHs). Oxidation of the carbonyl functional group is considered a general detoxification process in that polymorphisms of several human ALDHs are associated a disease phenotypes or pathophysiologies. However, a number of ALDH-mediated oxidation form products that are known to possess significant biologic, therapeutic and/or toxic activities. These include the retinoic acid, an important element for vertebrate development, gamma-aminobutyric acid (GABA), an important neurotransmitter, and trichloroacetic acid, a potential toxicant. This review summarizes the ALDHs with an emphasis on catalytic properties and xenobiotic substrates of these enzymes.

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.

The extracellular signal-regulated kinase pathway contributes to mitogenic and antiapoptotic effects of peroxisome proliferators in vitro

Peroxisome proliferators are a class of nongenotoxic rodent hepatocarcinogens thought to induce tumors by altering the balance between mitosis and apoptosis. Previous studies suggest mitogenic growth factors that act through the extracellular signal-regulated kinase (ERK) pathway, including insulin and epidermal growth factor (EGF), modulate peroxisome proliferator-activated receptor alpha activation as well as the mitogenic activity of peroxisome proliferators. We have investigated whether the ERK pathway plays a role in regulating the growth and survival altering properties of peroxisome proliferators in primary mouse hepatocytes. Exposure of hepatocytes to Wy-14,643 and trichloroacetate resulted in a dose-dependent phosphorylation and activation of ERK. Peroxisome proliferator-induced ERK phosphorylation was blocked when cells were pretreated with the MEK (ERK kinase) inhibitor, PD098059, or the phosphatidyl-inositol 3-kinase (PI3K) inhibitors, LY294002 and apigenin, suggesting that both MEK and PI3K are involved in the initial response. The pathway leading to peroxisome proliferator-induced ERK activation is different than that induced by phorbol ester or EGF, since the PI3K inhibitors had no effect on ERK phosphorylation induced by these agents. Under defined culture conditions, Wy-14,643 increased the level of BrdU incorporation in primary hepatocytes and suppressed the incidence of apoptosis induced by transforming growth factor beta 1. In contrast, concentrations of PD098059 that block Wy-14,643-induced ERK phosphorylation also blocked the stimulation of DNA replicative synthesis and suppression of apoptosis by Wy-14,643. These studies indicate that activation of the ERK pathway through a PI3K-dependent mechanism may play a significant role in the tumor-promoting properties of peroxisome proliferators.