Identification of S-1,2-dichlorovinyl-N-acetyl-cysteine as a urinary metabolite of trichloroethylene: a possible explanation for its nephrocarcinogenicity in male rats

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

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

Genotoxicity of haloalkene and haloalkane glutathione S-conjugates in porcine kidney cells

The genotoxicity of the glutathione S-conjugates S-(12-dichlorovinyl)glutathione (DCVG), S-(1,2,2-trichlorovinyl)glutathione (TCVG), S-(1,2,3,4,4-pentachlorobutadienyl)glutathione (PCBG) and S-(2-chloroethyl)glutathione (CEG) was investigated in LLC-PK1, a cultured line of porcine kidney cells that exhibits many properties of proximal tubular cells. DNA damage caused by treatment of the cells with the S-conjugates was estimated by determining the induction of unscheduled DNA synthesis (UDS) after inhibition of replicative DNA synthesis in confluent LLC-PK1 monolayers. DCVG-, TCVG- and PCBG-induced dose-dependent UDS at concentrations not causing cytotoxicity, as determined by the release of lactate dehydrogenase into the medium. Acivicin, which inhibits irreversibly gamma-glutamyl-transpeptidase (GGT) and aminooxyacetic acid, an inhibitor of cysteine conjugate beta-lyase, blocked DCVG-, TCVG- and PCBG-induced genotoxicity. CEG, however, was genotoxic in subconfluent cells and this was not dependent on GGT and beta-lyase activities. The DNA damaging effects in kidney cells of DCVG, TCVG and PCBG, which are metabolites of the nephrocarcinogens trichloroethylene, tetrachloroethylene and hexachlorobutadiene, respectively, suggest that the parent haloalkenes are potentially genotoxic in the rat kidney, the target organ for both acute toxicity and carcinogenicity.

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.

Metabolism and tissue distribution of orally administered trichloroethylene in male and female rats: identification of glutathione- and cytochrome P-450-derived metabolites in liver, kidney, blood, and urine

Male and female Fischer 344 rats were administered trichloroethylene (TRI) (2, 5, or 15 mmol/kg body weight) in corn oil by oral gavage, and TRI and its metabolites were measured at times up to 48 h in liver, kidneys, blood, and urine. Studies tested the hypothesis that gender-dependent differences in distribution and metabolism of TRI could help explain differences in toxicity. Higher levels of TRI were generally observed in tissues of males at lower doses. Complex patterns of TRI concentration, sometimes with multiple peaks, were observed in liver, kidneys, and blood of both males and females, consistent with enterohepatic recirculation. Higher concentrations of cytochrome P-450 (P450)-derived metabolites were observed in livers of males than in females, whereas the opposite pattern was observed in kidneys. Trichloroacetate was the primary P450-derived metabolite in blood and urine, although it generally appeared at later times than chloral hydrate. Trichloroethanol was also a significant metabolite in urine. S-(1,2-Dichlorovinyl)glutathione (DCVG) was recovered in liver and kidneys of female rats only and in blood of both males and females, with generally higher amounts found in females. S-(1,2-Dichlorovinyl)-L-cysteine (DCVC), the penultimate nephrotoxic metabolite, was recovered in male and female liver, female kidneys, male blood, and in urine of both males and females. The relationship between gender-dependent differences in distribution and metabolism of TRI and susceptibility to TRI-induced toxicity is discussed.

Carcinogenicity and chronic toxicity of 1,4-dichloro-2-nitrobenzene in rats and mice by two years feeding

Carcinogenicity and chronic toxicity of 1,4-dichloro-2-nitrobenzene (DCNB) were examined by feeding each group of 50 F344 rats and 50 BDF1 mice of both sexes a DCNB-containing diet at a concentration of 0 (control), 320, 800 or 2,000 ppm (w/w) for 2 yr. In rats, incidences of hepatocellular adenomas and carcinomas and their combined incidence were increased in the 2,000 ppm-fed males, together with increased incidence of basophilic cell foci in the 800 and 2,000 ppm-fed males. A dose-related increase in combined incidences of renal cell adenomas and carcinomas was noted. Incidence of Zymbal gland adenomas tended to increase in the 2,000 ppm-fed males. In mice, incidences of hepatocellular adenomas in the 800 and 2,000 ppm-fed females and hepatocellular carcinomas in the 2,000 ppm-fed males and in the 800 and 2,000 ppm-fed females were increased. Incidence of hepatoblastomas was increased in all DCNB-fed males and in the 2,000 ppm-fed females. Signs of chronic toxicity were characterized by centrilobular hypertrophy of hepatocytes with nuclear atypia in mice, increased relative liver weight in rats, a dose-related increase in incidences of chronic progressive nephropathy with advanced grades of severity in male rats, and decreased hemoglobin concentration and hematocrit accompanied by increased bone marrow hematopoiesis in female rats. Carcinogenic activity of DCNB was evaluated for the three different tumors, and sensitive signs of the chronic toxicity were dis-

Chemically induced renal tubule tumors in the laboratory rat and mouse: review of the NCI/NTP database and categorization of renal carcinogens based on mechanistic information

The incidence of renal tubule carcinogenesis in male and female rats or mice with 69 chemicals from the 513 bioassays conducted to date by the NCI/NTP has been collated, the chemicals categorized, and the relationship between carcinogenesis and renal tubule hyperplasia and exacerbation of the spontaneous, age-related rodent disease chronic progressive nephropathy (CPN) examined. Where information on mechanism or mode of action exists, the chemicals have been categorized based on their ability to directly or indirectly interact with renal DNA, or on their activity via epigenetic pathways involving either direct or indirect cytotoxicity with regenerative hyperplasia, or exacerbation of CPN. Nine chemicals were identified as directly interacting with DNA, with six of these producing renal tubule tumors at high incidence in rats of both sexes, and in some cases also in mice. Ochratoxin A was the most potent compound in this group, producing a high tumor incidence at very low doses, often with metastasis. Three chemicals were discussed in the context of indirect DNA damage mediated by an oxidative free radical mechanism, one of these being from the NTP database. A third category included four chemicals that had the potential to cause DNA damage following conjugation with glutathione and subsequent enzymatic activation to a reactive species, usually a thiol-containing entity. Two chemicals were allocated into the category involving a direct cytotoxic action on the renal tubule followed by sustained compensatory cell proliferation, while nine were included in a group where the cell loss and sustained increase in renal tubule cell turnover were dependent on lysosomal accumulation of the male rat-specific protein, alpha2mu-globulin. In a sixth category, morphologic evidence on two chemicals indicated that the renal tumors were a consequence of exacerbated CPN. For the remaining chemicals, there were no pertinent data enabling assignment to a mechanistic category. Accordingly, these chemicals, acting through an as yet unknown mechanism, were grouped as either being associated with an enhancement of CPN (category 7, 16 chemicals), or not associated with enhanced CPN (category 8, 4 chemicals). A ninth category dealt with 11 chemicals that were regarded as producing increases in renal tubule tumors that did not reach statistical significance. A 10th category discussed 6 chemicals that induced renal tumors in mice but not in rats, plus 8 chemicals that produced a low incidence of renal tubule tumors in mice that did not reach statistical significance. As more mechanistic data are generated, some chemicals will inevitably be placed in different groups, particularly those from categories 7 and 8. A large number of chemicals in the series exacerbated CPN, but those in category 7 especially may be candidates for inclusion in category 6 when further information is gleaned from the relevant NTP studies. Also, new data on specific chemicals will probably expand category 5 as cytotoxicity and cell regeneration are identified as obligatory steps in renal carcinogenesis in more cases. Additional confirmatory outcomes arising from this review are that metastases from renal tubule tumors, while encountered with chemicals causing DNA damage, are rare with those acting through an epigenetic pathway, with the exception being fumonisin B1; that male rats and mice are generally more susceptible than female rats and mice to chemical induction of renal tubule tumors; and that a background of atypical tubule hyperplasia is a useful indicator reflecting a chemically associated renal tubule tumor response. With respect to renal tubule tumors and human risk assessment, chemicals in categories 1 and 2, and possibly 3, would currently be judged by linear default methods; chemicals in category 4 (and probably some in category 3) as exhibiting a threshold of activity warranting the benchmark approach; and those in categories 5 and 6 as representing mechanisms that have no relevance for extrapolation to humans.

Effects of superoxide dismutase and polyclonal tumor necrosis factor-alpha antibodies on chloroacetate-induced cellular death and superoxide anion production by J774.A1 macrophages

Dichloroacetate (DCA) and trichloroacetate (TCA) are by-products that are formed during the process of water chlorination and have been previously shown to induce superoxide anion (SA) production and cellular death when added to J774.A1 macrophage cultures. In this study, the effects of superoxide dismutase (SOD) and polyclonal tumor necrosis factor-alpha (TNF-alpha) antibodies on DCA- and TCA-induced SA production and cellular death have been tested on the J774.A1 macrophage cultures. TCA and DCA were added to different cultures either alone, each at a concentration of 16 mM, or in combination with SOD (2-12 units/ml), or with TNF-alpha antibodies (10 and 25 units/ml). Cells were incubated for 48 h, after which cellular death/viability, lactate dehydrognase (LDH) leakage by the cells, and SA production by the cells were determined. While TCA and DCA caused significant cellular toxicity, indicated by reduction in cellular viability and increases in LDH leakage and SA production, SOD addition resulted in significant reduction of the effects induced by the compounds. On the other hand, addition of TNF-alpha antibodies to the DCA- and TCA-treated cultures resulted in significant reduction of DCA- but not TCA-induced cellular death and SA production by the cells. Although these results suggest a significant role for SA in DCA- and TCA-induced cellular death, they may also suggest two different mechanisms for the chloroacetate-induced SA production by the cells.

Rodent carcinogenicity of peroxisome proliferators and issues on human relevance

A variety of substances such as hypolipidemic drugs, phthalate ester plasticizers, pesticides, and industrial solvents have been shown to increase the size and number of peroxisomes in rats and mice. They are grouped under the generic term peroxisome proliferators (PP) because of their unique property of inducing peroxisome proliferation. There are marked species differences in response to PP. Rats and mice are most sensitive, and hamsters show an intermediate response while guinea pigs, monkeys, and humans appear to be relatively insensitive or non-responsive at dose levels that produce a marked response in rodents. Out of over 100 PP identified to date, about 30 have been adequately tested and shown to be carcinogenic, inducing tumors (primarily in the liver) upon chronic administration to rats and/or mice; hence, chemicals which induce the proliferations of peroxisomes have formed a unique class of chemical carcinogens. It is not well documented that activation of the "peroxisome proliferator-activated receptor alpha" (PPARalpha) is involved in PP-induced liver growth and carcinogenesis in rodents. PPARalpha is also present in human cells; however, the levels reported are about 10% of those found in the liver of rodents. The human relevance of rodent tumors induced by PP has been the subject of debate over the last decade. Review of the existing evidence on PPAR-alpha agonists by a recent International Life Science Institute (ILSI) workgroup following a human relevance mode of action (MOA) framework has concluded that despite the presence of similar pathways in humans, it is unlikely that the proposed MOA for rodent tumors is plausible in humans, taking into account kinetic and dynamic factors. The data, however, did not permit a definitive conclusion that the animal MOA is not plausible in humans. While these agents appear unlikely to be hepatocarcinogens in humans at expected levels of human exposure, it remains uncertain to some experts in the field whether there is no possibility of carcinogenic potential under any circumstances of PP exposure, and if the potential human carcinogenicity of these chemicals can be summarily ignored. A number of remaining issues on human relevance of rodent tumors induced by PP are discussed.

Dietary controlled carcinogenicity study of chloral hydrate in male B6C3F1 mice

Chloral hydrate, which is used as a sedative in pediatric medicine and is a by-product of water chlorination, is hepatocarcinogenic in B6C3F1 mice, a strain that can exhibit high rates of background liver tumor incidence, which are associated with increased body weight. In this study, dietary control was used to manipulate body growth in male B6C3F1 mice in a 2-year bioassay of chloral hydrate. Male B6C3F1 mice were treated with water or 25, 50, or 100 mg/kg chloral hydrate by gavage. The study compared ad libitum-fed mice with dietary controlled mice. The latter received variably restricted feed allocations to maintain their body weights on a predetermined "idealized" weight curve predictive of a terminal background liver tumor incidence of 15-20%. These mice exhibited less individual body weight variation than did their ad libitum-fed counterparts. This was associated with a decreased variation in liver to body weight ratios, which allowed the demonstration of a statistically significant dose response to chloral hydrate in the dietary controlled, but not the ad libitum-fed, test groups. Chloral hydrate increased terminally adjusted liver tumor incidence in both dietary controlled (23.4, 23.9, 29.7, and 38.6% for the four dose groups, respectively) and ad libitum-fed mice (33.4, 52.6, 50.6, and 46.2%), but a statistically significant dose response was observed only in the dietary controlled mice. This dose response positively correlated with markers of peroxisomal proliferation in the dietary controlled mice only. The study suggests that dietary control not only improves terminal survival and decreases interassay variation, but also can increase assay sensitivity by decreasing intra-assay variation.

The induction of oxidative stress and cellular death by the drinking water disinfection by-products, dichloroacetate and trichloroacetate in J774.A1 cells

The in vitro toxicity of the drinking water disinfection by products dichloroacetate (DCA) and trichloroacetate (TCA) were studied using the J774A.1 macrophage cell line. DCA and TCA were added to cell cultures at concentrations ranging between 8-32 mM and incubated for 24, 36 and 60 h. DCA and TCA effects on cellular viability, lactate dehydrogenase (LDH) release and superoxide anion (SA) production by the cells, as well as superoxide dismutase (SOD) activities of the cells were determined. DCA and TCA caused time- and concentration-dependent increases in cellular death, in LDH release and production of SA by the cells. The compounds also caused modulations in SOD activities of the cells, with increases observed at the lower concentrations and/or shorter periods of incubations and suppression with the higher concentrations and/or longer periods of incubation. The results of the study indicate that DCA and TCA induce macrophage activation and that the activation is associated with cellular toxicity. Also, DCA and TCA are found to be equitoxic to J774.A1 cells.

Renal injury and repair following S-1, 2 dichlorovinyl-L-cysteine administration to mice

S-(1,2-dichlorovinyl)-L-cysteine (DCVC), a metabolite of a common environmental contaminant, trichloroethylene, is a selective proximal tubular nephrotoxicant. The objective of our study was to examine the dose-response relationship of renal injury and repair following DCVC administration. Male Swiss-Webster mice were injected with DCVC [15, 30, or 75 mg/kg ip in distilled water (10 ml/kg)] and the extent of nephrotoxicity and tissue repair was assessed over a 14-day period. The renal injury due to the low and medium doses of DCVC peaked at 36 and 72 h after dosing, respectively, and then regressed over time due to a timely and adequate tissue repair response. At the highest dose tissue repair was inhibited, thereby causing progression of renal injury, which led to acute renal failure and death of the mice. The possibility that compromised tissue repair was a result of the extensive nephrotoxic injury attendant to the high dose of DCVC was investigated via an equinephrotoxicity study in which separate groups of mice received 40 (LD40) and 75 (LD90) mg DCVC/kg, respectively. Bioactivation-based renal proximal tubular injury measured in these two groups over a time course was identical but there was a marked difference in mortality due to an early and robust tissue repair in the first group relative to the second group. These results support the concept that quantitative evaluation of renal tissue repair in parallel with injury is useful in the assessment of the likely toxic outcome associated with exposure to nephrotoxic drugs and toxicants.

Mercapturic acids in the biological monitoring of occupational exposure to chemicals

This paper reviews several procedures for determination of mercapturic acids in urine. Special attention was paid to methods useful in relation to human exposure to industrial pollutants, without any description for less sensitive methods used in animal research. Gas chromatographic and liquid chromatographic procedures were considered together with the little information available about thin layer chromatography and immunochemical techniques. After a description of the main industrial pollutants which lead to synthesis of their specific mercapturic acids, the methods for analysing these products are synthetically reported. The comparison among difficulties in sample preparation, complexity of instrumentation and their cost/benefit ratio are discussed.

Possible involvement of glutathione and p53 in trichloroethylene- and perchloroethylene-induced lipid peroxidation and apoptosis in human lung cancer cells

Trichloroethylene (TCE) and perchloroethylene (PERC) are volatile organic compounds (VOCs) that are primarily inhaled through the respiratory system. The aim of this study was to elucidate the role of glutathione (GSH) and p53 in TCE- and PERC-induced lung toxicity. Human lung adenocarcinoma cells NCI-H460 (p53-wild-type) have constitutively lower levels of GSH than NCI-H1299 (p53-null) cells. The results showed that exposure to vapor TCE and PERC produced a dose-dependent and more pronounced accumulation of H(2)O(2) in p53-WT H460 than p53-null H1299 cells. The accumulation of H(2)O(2) was accompanied by severe cellular damage, as indicated by the significant increase of lipid peroxidation and apoptosis in p53-WT H460 cells, but not p53-null H1299 cells. Cotreatment of p53-WT H460 cells with free radical scavengers, such as D-mannitol, uric acid, and sodium selenite, significantly attenuated the TCE- or PERC-induced lipid peroxidation. In contrast, depletion of GSH in p53-null H1299 cells enhanced TCE- or PERC-induced lipid peroxidation. The levels of p53 and Bax proteins were elevated, while Bcl-2 protein was downregulated in TCE- or PERC-treated p53-WT H460 cells. Activity of caspase 3, the apoptotic executioner, was also significantly enhanced in TCE- or PERC-treated cells. These data suggest that, in human lung cancer cells, GSH plays a vital role in the protection of TCE- and PERC-induced oxidative stress and apoptosis, which may be mediated through a p53-dependent pathway.

Chemical-induced nephrotoxicity mediated by glutathione S-conjugate formation

Glutathione conjugation has been identified as an important detoxication reaction. However, several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions is discussed in this article. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl) glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed and interact with nucleic acids. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cystein conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is likely to be responsible for the observed nephrotoxicity of the parent compounds. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they cause alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Renal toxicity and carcinogenicity of trichloroethylene: key results, mechanisms, and controversies

The discussion on renal carcinogenicity of trichloroethylene addresses epidemiological, mechanistic, and metabolic aspects. After trichloroethylene exposure of rats, renal cell tumors were found increased in males, and an increased incidence of interstitial cell tumors of the testes was reported. Studies on the metabolism of trichloroethylene in rodents and in humans support the role of bioactivation reactions for the development of tumors following exposure to trichloroethylene. Epidemiological cohort studies addressing the carcinogenicity of trichloroethylene with respect to the renal or urothelial target sites have been conducted, and no clear evidence for an elevated renal or urinary tract cancer risk in trichloroethylene-exposed groups was visible in exposed populations. However, a cohort study of 169 male workers having been exposed to unusually high levels of trichloroethylene in Germany within the period between 1956 and 1975 supported a nephrocarcinogenic effect of trichloroethylene in humans. The results of this study were discussed in the literature with considerable reserve; criticism was based mainly on the choice of the study group, which had been recruited from personnel of a company in which a cluster of four renal tumors was observed previously. Hence, a further case-control study was conducted in the same region. This study confirmed the results of the previous cohort study, supporting the concept of involvement of prolonged and high-dose trichloroethylene exposures in the development of renal cell cancer. Further investigations on patients with renal cell carcinoma and with histories of high trichloroethylene exposures, on the basis of excretion of marker proteins in the urine, pointed to toxic damage to the proximal renal tubules by trichloroethylene. The hypothesis of implication of a glutathione transferase-dependent bioactivating pathway of trichloroethylene, established in experimental animals, seems at least also plausible for humans. Apparently, the occurrence of renal cell carcinomas in man follows high-dose exposures to trichloroethylene that are also accompanied by damage to tubular renal cells. Development of renal cell carcinomas has been related to mutations in the vonHippel-Lindau (VHL) tumor suppressor gene. Renal cell carcinoma tissues of persons with histories of prolonged high-dose exposure to trichloroethylene were investigated for the occurrence of mutations of the vonHippel-Lindau (VHL) tumor suppressor gene. VHL gene mutations were found in the majority of renal cell tumors associated with high-level exposure to trichloroethylene. A specific mutational hot spot at the VHL nucleotide 454 was addressed as a unique mutation pattern of the VHL tumor suppressor gene. A synopsis of all experimental, clinical, and epidemiological data suggests that reactive metabolites of trichloroethylene, with likely involvement of dichlorovinyl-cysteine (DCVC), exert a genotoxic effect on the proximal tubule of the human kidney. This constitutes a tumor-initiating process of genotoxic nature, the initial genotoxic effect apparently being linked with mutational changes in the VHL tumor suppressor gene. However, there is compelling evidence that the full development of a malignant tumor requires continued promotional stimuli. Repetitive episodes of high peak exposures to trichloroethylene over a prolonged period of time apparently led to nephrotoxicity, visualized by the excretion of tubular marker proteins in the urine. This critical process of development of tubular damage by trichloroethylene must follow a "conventional" dose-dependence, implying a practical threshold. This view is much corroborated by the fact that the occurrence of human renal cell cancer is obviously confined to cases of unusually high trichloroethylene exposures in the past, with special characteristics of very high and repetitive peak exposures. Current instruments of regulation should be adjusted to allow adequate consideration of su

Role of cytochrome P450 and glutathione S-transferase alpha in the metabolism and cytotoxicity of trichloroethylene in rat kidney

The toxicity and metabolism of trichloroethylene (TRI) were studied in renal proximal tubular (PT) and distal tubular (DT) cells from male Fischer 344 rats. TRI was slightly toxic to both PT and DT cells, and inhibition of cytochrome P450 (P450; substrate, reduced-flavoprotein:oxygen oxidoreductase [RH-hydroxylating or -epoxidizing]; EC 1.14.14.1) increased TRI toxicity only in DT cells. In untreated cells, glutathione (GSH) conjugation of TRI to form S-(1,2-dichlorovinyl)glutathione (DCVG) was detected only in PT cells. Inhibition of P450 transiently increased DCVG formation in PT cells and resulted in detection of DCVG formation in DT cells. Formation of DCVG in PT cells was described by a two-component model (apparent Vmax values of 0.65 and 0.47 nmol/min per mg protein and Km values of 2.91 and 0.46 mM). Cytosol isolated from rat renal cortical, PT, and DT cells expressed high levels of GSH S-transferase (GST; RX:glutathione R-transferase; EC 2.5.1.18) alpha (GSTalpha) but not GSTpi. Low levels of GSTmu were detected in cortical and DT cells. Purified rat GSTalpha2-2 exhibited markedly higher affinity for TRI than did GSTalpha1-1 or GSTalpha1-2, but each isoform exhibited similar VmaX values. Triethyltinbromide (TETB) (9 microM) inhibited DCVG formation by purified GSTalpha-1 and GSTalpha2-2, but not GSTalpha1-2. Bromosulfophthalein (BSP) (4 microM) only inhibited DCVG formation by GSTalpha2-2. TETB and BSP inhibited approximately 90% of DCVG formation in PT cytosol but had no effect in DT cytosol. This suggests that GSTalpha1-1 is the primary isoform in rat renal PT cells responsible for GSH conjugation of TRI. These data, for the first time, describe the metabolism of TRI by individual GST isoforms and suggest that DCVG feedback inhibits TRI metabolism by GSTs.

Stereoselective formation of glutathione S-conjugates from halovinylmercapturate sulphoxides

1. The glutathione S-transferase catalysed formation of glutathione S-conjugates from halovinylmercapturate sulphoxides was investigated in rat liver and kidney cytosol, with purified glutathione S-transferases and in rat in vivo. 2. The two diastereomers of the sulphoxides of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine, N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine and N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine show different reactivities with glutathione and glutathione S-transferases. Rat liver and kidney cytosol catalyses the formation of a 1:1 mixture of two diastereomers of (E)-N-acetyl-S-(2-glutathione-S-yl-2-chlorovinyl)-L-cysteine sulphoxide from N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine sulphoxide and of (E)-N-acetyl-S-(2-glutathione-S-yl-1,2-dichlorovinyl)-L-cysteine sulphoxide from N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine sulphoxide. In contrast, only one diastereomer of the Z-isomers was formed. 3. N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine sulphoxide reacted spontaneously with glutathione at high rates, a 1:1 mixture of both diastereomers of N-acetyl-S-(2-glutathione-S-yl-1-chlorovinyl)-L-cysteine sulphoxide was formed. 4. Metabolism of N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine sulfoxide and N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine sulfoxide under by alpha-class glutathione S-transferases yielded identical products as observed with the cytosolic enzymes. No reaction was observed in the presence of rat liver mu class glutathione S-transferases or human glutathione S-transferase M1. 5. Formation of these glutathione conjugates was also observed in the bile of rat after i.p. administration of the mercapturic acid sulphoxides. The results obtained show that stereochemical aspects may govern the regioselectivity and substrate specificity in glutathione S-transferase-catalysed reactions.

Role of SAM-dependent thiol methylation in the renal toxicity of several solvents in mice

The role of S-adenosylmethionine (SAM)-dependent thiol methylation in the nephrotoxicity of seven industrial solvents was studied in mice. The seven following solvents were utilized: bromobenzene (BB), styrene (STY), tetrachloroethylene (TTCE), trichloroethylene (TCE), 1,1-dichloroethylene (DCE), 1,2-dichloroethane (DCA) and hexachlorobutadiene (HCB). The experimental model comprised mice pretreated with periodate oxidized adenosine (ADOX) (100 micromol kg(-1) i.p.) 30 min before injection of solvents. In the first 4 h after ADOX treatment, the SAM levels were about fourfold higher than controls for the liver and kidney. The S-adenosylhomocysteine (SAH) levels were increased by factors of 11 and 14 and the SAM/SAH ratios were decreased by factors of 3 and 10 for the liver and kidney, respectively. These results show that ADOX treatment probably induces an inhibition of methyltransferase SAM-dependent in the liver and kidney and thus decreases the methylation capabilities. A single oral administration of BB (500 or 800 mg kg(-1)), TTCE (3500 or 4000 mg kg(-1)), TCE (3000 or 3500 mg kg(-1)) or STY (400 or 600 mg kg(-1)) did not induce renal toxicity, evaluated by the percentage of damaged tubules compared to controls. On the other hand, the three solvents DCE, HCB and DCA were nephrotoxic and the percentage of damaged tubules observed for each solvent was significantly different from the value of <1.8% for controls: 19% and 40% for DCE (130 and 200 mg kg(-1)), 50% and 46% for HCB (80 and 100 mg kg(-1)) and 5.1% and 7.6% for DCA (1000 and 1500 mg kg(-1)). The ADOX treatment in the mice did not modify the renal toxicity of the seven solvents. Thus, their renal toxicity, when it existed, was probably independent of the SAM-dependent thiolmethyltransferase activity in the mice. The results of this study are discussed from two viewpoints. The first concerns the general considerations on inhibition of thiol methyltransferase activities in mice and the second is related to the different solvents that are evoked individually.

Chlorothioketene, the ultimate reactive intermediate formed by cysteine conjugate beta-lyase-mediated cleavage of the trichloroethene metabolite S-(1,2-Dichlorovinyl)-L-cysteine, forms cytosine adducts in organic solvents, but not in aqueous solution

Chlorothioketene has been suggested as a reactive intermediate formed by the cysteine conjugate beta-lyase-mediated cleavage of S-(1,2-dichlorovinyl)-L-cysteine, a minor metabolite of trichloroethene. Halothioketenes are highly reactive, and their intermediate formation may be confirmed by reactions such as cycloadditions and thioacylations of nucleophiles. A precursor of chlorothioketene, S-(1,2-dichlorovinyl)thioacetate, is readly accessible by the reaction of dichloroethyne with thioacetic acid. In presence of base, S-(1,2-dichlorovinyl)thioacetate is cleaved to chlorothioketene. Chlorothioketene is not stable at room temperature and was characterized after transformation to stable products by reaction with compounds such as cyclopentadiene, N,N-diethylamine, and ethanol. In organic solvents, the cleavage of S-(1, 2-dichlorovinyl)thioacetate in the presence of cytosine results in N4-acetylcytosine, N4-(chlorothioacetyl)cytosine, and small amounts of 3-(N4-thioacetyl)cytosine. No reaction products were seen with guanosine, adenosine, and thymidine under identical conditions. When cytosine was reacted with S-(1,2-dichlorovinyl)thioacetate in aqueous solutions, only N4-acetylcytosine was formed. N4-(Chlorothioacetyl)cytosine and 3-(N4-thioacetyl)cytosine were not detected even when using a very sensitive method, derivatization with pentafluorobenzyl bromide and electron capture mass spectrometry with a detection limit of 50 fmol/microliter of injection volume. Aqueous solutions of DNA cleave S-(1, 2-dichlorovinyl)thioacetate to give N4-acetyldeoxycytidine in DNA, but chlorothioketene adducts of deoxynucleosides were also not detected in these experiments. These results confirm the electrophilic reactivity of chlorothioketene toward nucleophilic groups of DNA constituents in inert solvents but also demonstrate that the formation of DNA adducts under physiological conditions likely is not efficient. Therefore, DNA adducts may not represent useful biomarkers of exposure and biochemical effects for trichloroethene.

Kinetics of trichloroacetic acid and dichloroacetic acid in the isolated perfused rat liver

Trichloroacetic acid (TCA) and dichloroacetic acid (DCA) are environmental contaminants that are suspected human carcinogens. To obtain more detail on the role of the liver in the kinetics of TCA and DCA, experimental studies in the isolated perfused rat liver (IPRL) system were conducted. The IPRL system was dosed with either 5 or 50 micromol of either TCA or DCA (25 or 250 microM initial concentration, respectively). TCA and DCA concentrations were followed in perfusion medium and bile for 2 h. The chemical concentration in liver was determined at the end of exposure. Liver viability was monitored by measuring leakage of lactate dehydrogenase (LDH) into perfusion medium and the rate of bile production. Studies performed with TCA showed that the total TCA concentration in perfusion medium decreased slightly during the first 30 min of exposure and remained constant thereafter. Most TCA, greater than 90% of total, was bound to albumin in the perfusion medium. A low, linear excretion rate of TCA in bile was obtained. The calculated free TCA concentration in the liver intracellular water space was higher than the unbound TCA concentration in the perfusion medium. Parallel studies with DCA showed that the DCA concentration in perfusion medium decreased rapidly. Of the total DCA in the perfusion medium, 60% was bound to albumin. The concentration of DCA in bile decreased over time. There was no DCA detectable in the liver after 2 h of exposure at both DCA concentrations. Enzyme leakage and bile production did not change in the presence of TCA or DCA, indicating that these concentrations were not acutely cytotoxic to the liver.

Formic acid excretion in rats exposed to trichloroethylene: a possible explanation for renal toxicity in long-term studies

Rats exposed to trichloroethylene, either by gavage or by inhalation, excreted large amounts of formic acid in urine which was accompanied by a change in urinary pH, increased excretion of ammonia, and slight increases in the excretion of calcium. Following a single 6-h exposure to 500 ppm trichloroethylene, the excretion of formic acid was comparable to that seen after a 500 mg/kg dose of formic acid itself, yet the half-life was markedly different. Formate excretion in trichloroethylene treated rats reached a maximum on day 2 and had a half-life of 4-5 days, whereas urinary excretion was complete within 24 h following a single dose of formic acid itself. Formic acid was shown not to be a metabolite of trichloroethylene. When rats were exposed to 250 or 500 ppm trichloroethylene, 6 h/day, for 28 days, the only significant effects were increased formic acid and ammonia excretion, and a change in urinary pH. There was no evidence of morphological liver or kidney damage. Long-term exposure to formic acid is known to cause kidney damage suggesting that excretion of this acid may contribute to the kidney damage seen in the long-term studies with trichloroethylene.

The role of glutathione conjugation in the development of kidney tumours in rats exposed to trichloroethylene

Trichloroethylene is metabolised to a very minor extent (< 0.01% of the dose) by conjugation with glutathione, a metabolic pathway which leads to the formation of S-(1,2-dichlorovinyl)-L-cysteine (DCVC), a bacterial mutagen and nephrotoxin activated by the renal enzyme beta-lyase. The role of this metabolic pathway in the development of the nephrotoxicity and subsequent tumour formation seen in rats exposed to trichloroethylene has been evaluated. The pathway has been assessed quantitatively in vivo in rats, and in rats, mice and humans in vitro. Trichloroethylene was found to be a very weak nephrotoxin. There was no evidence of morphological change in the kidneys and only small increases in biochemical markers of kidney damage in rats dosed with 2000 mg/kg trichloroethylene by gavage for 42 days. N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine was detected in the urine of rats dosed with 500 and 2000 mg/kg trichloroethylene for up to 10 days at levels equivalent to 0.001-0.008% of the dose. In vitro, the rate of conjugation of trichloroethylene with glutathione in the liver was higher in the mouse, 2.5 pmol/min per mg protein, than the rat, 1.6 pmol/min per mg protein, and in human liver the rates were extremely low, 0.02-0.37 pmol/min per mg protein. Comparisons of the metabolism of DCVC by renal beta-lyase and N-acetyl transferase showed that metabolism by N-acetyl transferase was two orders of magnitude greater than that by beta-lyase and that beta-lyase activity in rat kidney was 11-fold greater than that in human kidney. When the nephrotoxicity of DCVC was compared in rats and mice, the mouse was found to be 5-10 fold more sensitive than the rat. The no effect level in the rat was 10 mg/kg, a dose which is three orders of magnitude higher than the amount of DCVC formed from trichloroethylene in vivo. The lack of correlation between metabolism by this pathway and the rat specific tumours, together with questions concerning the potency of DCVC at the levels formed from trichloroethylene, suggests that DCVC may not be involved in the renal toxicity and subsequent tumour development seen in rats and that further evaluation of the mechanism(s) involved in the nephrotoxic response is warranted.

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.

Toxicity of mixtures of nephrotoxicants with similar or dissimilar mode of action

The toxicity of mixtures of chemicals with the same target organ was examined in rats using nephrotoxicants with similar or dissimilar modes of action. In a 4-wk feeding study, lysinoalanine, mercuric chloride, hexachloro-1,3-butadiene and d-limonene, each affecting renal proximal tubular cells but through different modes of action, were administered simultaneously at their individual lowest-observed-nephrotoxic-effect level (LONEL), no-observed-nephrotoxic-effect level (NONEL) and NONEL/4. Combined exposure at the LONEL resulted in increased growth depression and increased renal toxicity in male but not in female rats. Co-exposure at the NONEL produced only weak signs of toxicity (slightly retarded growth and increased renal weight), and rats co-exposed at the NONEL/4 did not show any treatment-related changes. The absence of an obviously increased hazard on combined exposure at the NONEL suggested absence of synergism and probably also of additivity. In a subsequent study the additivity assumption (dose addition) was tested, using the similarly acting nephrotoxicants tetrachloroethylene, trichloroethylene, hexachloro-1,3-butadiene and 1,1,2-trichloro-3,3,3-trifluoropropene. The compounds were given to female rats by daily oral gavage for 32 days either alone, at the LONEL and NONEL (= LONEL/4), or in combinations of four (at the NONEL and LONEL/2) or three (at the LONEL/3). Relative kidney weight was increased on exposure to the individual compounds at their LONEL and, to about the same extent, on combined exposure at the NONEL or the LONEL/3. As assessed by this endpoint, the renal toxicity of the mixtures corresponded to the effect expected on the basis of the additivity assumption. The other endpoints were not (or hardly) affected on combined exposure.

Alterations of the renal function in the isolated perfused rat kidney system after in vivo and in vitro application of S-(1,2-dichlorovinyl)-L-cysteine and S-(2,2-dichlorovinyl)-L-cysteine

The nephrotoxic effects of the two isomers S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCVC) and S-(2,2-dichlorovinyl)-L-cysteine (2,2-DCVC) were investigated comparatively in the isolated perfused rat kidney with two different treatment regimens. In the first approach, the kidneys were exposed to the test compounds dissolved in the perfusion media after removal from the animal. In the second approach the test compounds were administered to rats in vivo and the nephrotoxicity was assessed in the isolated perfused kidney 6 h and 18 h post-treatment. The vicinal isomer 1,2-DCVC produced concentration- and time-dependent nephrotoxicity with both treatment regimens, as indicated by the impairment of glucose reabsorption, the increase of protein excretion and of gamma-glutamyltransferase and alkaline phosphatase activities in urine. In contrast to the marked toxicity observed after in vivo and in vitro administration of 1,2-DCVC, the geminal isomer, 2,2-DCVC, was not nephrotoxic at all concentrations (0.5 and 2.5 mM in vitro, 40 and 70 mg/kg in vivo) investigated.

Sulfoxidation of mercapturic acids derived from tri- and tetrachloroethene by cytochromes P450 3A: a bioactivation reaction in addition to deacetylation and cysteine conjugate beta-lyase mediated cleavage

In the present study we investigated the formation of sulfoxides from N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine (N-Ac-TCVC), N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (N-Ac-1,2-DCVC), and N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine (N-Ac-2,2-DCVC), which are formed in the glutathione dependent bioactivation of tri- and tetrachloroethene. The first aim was to elucidate the enzymes involved in these oxidation reactions. N-Ac-TCVC, N-Ac-1,2-DCVC, and N-Ac-2,2-DCVC are oxidized to the corresponding sulfoxides mainly, if not exclusively, by cytochrome P450 enzymes in liver microsomes of untreated male rats, since no role for the flavin-containing monooxygenase (FMO) could be demonstrated by heat inactivation experiments and by the use of n-octylamine. The sulfoxidation rates were increased when using liver microsomes of phenobarbital and dexamethasone pretreated male rats as well as liver microsomes of dexamethasone pretreated female rats, while no sulfoxide formation was observed in liver microsomes of untreated female rats, suggesting an involvement of cytochrome P450 3A. Also, troleandomycin, a specific chemical inhibitor for cytochrome P450 3A, drastically reduced sulfoxidation rates. The observed rates of sulfoxidation also correlated well with the rates of oxidation of testosterone at the 6-beta-position, a specific marker for P450 3A activity. The second aim of this study was to compare the cytotoxicity of the sulfoxides with the cytotoxicity of the corresponding mercapturic acids in isolated rat renal epithelial cells. Both mercapturic acids and the corresponding sulfoxides were cytotoxic. Cytotoxicity of the mercapturic acids could be blocked by (aminooxy)acetic acid (AOAA), an inhibitor of cysteine conjugate beta-lyase, while the cytotoxicity of the sulfoxides was not influenced by this treatment. Moreover, the sulfoxides were significantly more cytotoxic than the corresponding mercapturic acids at equimolar doses. The results show that mercapturic acids derived from TRI and PER are oxidized to sulfoxides by microsomal monooxygenases from rat liver. The cytotoxicity of the produced sulfoxides could not be reduced by AOAA, consistent with a role of the sulfoxides as direct acting electrophiles (i.e., Michael acceptor substrates).

Renal activation of trichloroethene and S-(1,2-dichlorovinyl)-L-cysteine and cell proliferative responses in the kidneys of F344 rats and B6C3F1 mice

Covalent binding of reactive intermediates formed by renal beta-lyase activation of S-(1,2-dichlorovinyl)-L-cysteine (DCVC) has been suggested to be responsible for the greater renal sensitivity of rats than mice to the carcinogenic effects of chronic treatment with trichloroethene (TRI). Previous work demonstrated that the activation of DCVC results in acid-labile adducts to protein that can be distinguished from adducts formed by other pathways of TRI metabolism. By analyzing acid-labile adduct formation, the relationship between DCVC formation and activation from TRI and increases in rates of cell division in the kidneys of male F344 rats and B6C3F1 mice could be investigated. The delivered dose of DCVC from an oral dose of 1000 mg/kg TRI was approximately six times greater in rats than mice. However, renal activation of DCVC in mice was approximately 12 times greater than in rats. Therefore, the overall activation of TRI was about two times greater in mice than rats. Induction of cell replication in liver and kidney following doses of 1, 5, or 25 mg/kg DCVC or 1000 mg/kg TRI was also measured through the use of miniosmotic pumps that delivered BrdU subcutaneously for 3 d. Acid-labile adduct formation from DCVC and TRI displayed a consistent relationship with increased cell replication in mice and between mice and rats. Both cell replication and acid-labile adduct formation in rats given 25 mg/kg DCVC were approximately equal to that observed in mice given 1 mg/kg. Increased cell replication was not observed in rats receiving 1 or 5 mg/kg DCVC or 1000 mg/kg TRI, nor were there histological signs of nephrotoxicity. Thus, net activation of TRI by the cysteine S-conjugate pathway was found to be greater in mice than rats and these findings appeared related to differences in cell proliferative responses of the kidneys of the two species. Based on these data, it would appear that other factors must contribute to the greater sensitivity of the rat to the induction of renal carcinogenesis by TRI.

Developmental toxicity of trichloroethylene, tetrachloroethylene and four of their metabolites in rat whole embryo culture

The embryotoxicity of trichloroethylene (TRI), tetrachloroethylene (PER), and of four of their oxidative metabolites i.e. trichloroacetic acid, dichloroacetic acid, chloral hydrate, and trichloroacetyl chloride, was studied in vitro, using the rat whole embryo culture system. Embryos from Sprague-Dawley rats were explanted on gestational day 10 (plug day = day 0) and cultured for 46 h in the presence of the test chemical. All of the tested chemicals produced concentration-dependent decreases in growth and differentiation and increases in the incidence of morphologically abnormal embryos. TRI and PER produced qualitatively similar patterns of abnormalities, while TRI and/or PER metabolites, each elicited clearly distinguishable dysmorphogenic profiles. The presence of hepatic microsomal fractions in the culture medium produced marked decreases in TRI- and PER-induced embryotoxic effects, including mortality, severity of malformations, and delayed growth and differentiation.

Acid-labile adducts to protein can be used as indicators of the cysteine S-conjugate pathway of trichloroethene metabolism

Covalent binding of radiolabel to tissue proteins following [14C]trichloroethene (TRI) exposure has been used as a measure of TRI activation. Gross binding of 14C label does not differentiate between alternate routes of metabolism and can be confounded when there is significant metabolic incorporation of radiolabel. We examined the covalent association of 14C label to hepatic and renal proteins in male F344 rats and B6C3F1 mice following oral treatment with [14C]TRI and three metabolites of TRI: [14C]trichloroacetate (TCA), [14C]dichloroacetate (DCA), and [14C]dichlorovinylcysteine (DCVC) in vivo. Association of radiolabel from [14C]TRI with hepatic proteins reached a maximum at 2 and 4 h in mouse and rat hepatic proteins, respectively. Association of radiolabel with renal proteins reached a maximum at 8 h in both species. An approach was developed based upon formation of protein adducts that release acetate and monochloroacetate (MCA) on acid hydrolysis. These adducts were found to be specifically associated with the activation of DCVC to reactive intermediates. Acetate and MCA were identified by using two different conditions of high-performance liquid chromatography (HPLC) separation with differing selectivity. Diethylmaleate and aminooxyacetic acid pretreatment inhibited the formation of these adducts from TRI, consistent with requirements for glutathione and beta-lyase. No evidence of these adducts was detected following [14C]TCA and [14C]DCA treatment. Renal acid-labile adduct formation from 25 mg/kg DCVC was approximately 12-fold greater in male B6C3F1 mice than in male F344 rats. They accounted for 7.8 and 4.6% of the total adducts to renal protein in rats and mice, respectively. Acid-labile adducts formed from 1000 mg/kg TRI were approximately two times greater in mice than rats. In this case, they accounted for 1.4 and 3.3% of the total adduct formed in renal proteins from TRI (corrected for metabolic incorporation), respectively. This greater dilution of adducts associated with DCVC in renal proteins of the rat suggests that covalent binding of TRI has less specificity for the DCVC pathway in rats than in mice.

Trichloroethylene--a review of the literature from a health effects perspective

This report reviews the literature on the impact of exposure to trichloroethylene (TCE) on human health. Special emphasis is given to the health effects reported in excess of national norms by participants in the TCE Subregistry of the Volatile Organic Compounds Registry of the National Exposure Registries--persons with documented exposure to TCE through drinking and use of contaminated water. The health effects reported in excess by some or all of the sex and age groups studied were speech and hearing impairments, effects of stroke, liver problems, anemia and other blood disorders, diabetes, kidney disease, urinary tract disorders, and skin rashes.

Increased incidence of renal cell tumors in a cohort of cardboard workers exposed to trichloroethene

A retrospective cohort study was carried out in a cardboard factory in Germany to investigate the association between exposure to trichloroethene (TRI) and renal cell cancer. The study group consisted of 169 men who had been exposed to TRI for at least 1 year between 1956 and 1975. The average observation period was 34 years. By the closing day of the study (December 31, 1992) 50 members of the cohort had died, 16 from malignant neoplasms. In 2 out of these 16 cases, kidney cancer was the cause of death, which leads to a standard mortality ratio of 3.28 compared with the local population. Five workers had been diagnosed with kidney cancer: four with renal cell cancers and one with a urothelial cancer of the renal pelvis. The standardized incidence ratio compared with the data of the Danish cancer registry was 7.97 (95% CI: 2.59-18.59). After the end of the observation period, two additional kidney tumors (one renal cell and one urothelial cancer) were diagnosed in the study group. The control group consisted of 190 unexposed workers in the same plant. By the closing day of the study 52 members of this cohort had died, 16 from malignant neoplasms, but none from kidney cancer. No case of kidney cancer was diagnosed in the control group. The direct comparison of the incidence on renal cell cancer shows a statistically significant increased risk in the cohort of exposed workers. Hence, in all types of analysis the incidence of kidney cancer is statistically elevated among workers exposed to TRI.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the comparative toxicity of S-(1,2-dichlorovinyl)-L-cysteine, S-(1,2-dichlorovinyl)-L-homocysteine and 1,1,2-trichloro-3,3,3-trifluoro-1-propene in the Fischer 344 rat

The renal tubular toxicity of various halogenated xenobiotics has been attributed to their enzymatic bioactivation to reactive intermediates by S-conjugation. A combination of high resolution proton nuclear magnetic resonance (1H NMR) spectroscopy of urine, renal histopathology and more routinely used clinical chemistry methods has been used to explore the acute toxic and biochemical effects of S-(1,2-dichlorovinyl)-L-cysteine (DCVC), S-(1,2-dichlorovinyl)-L-homocysteine (DCVHC) and 1,1,2-trichloro-3,3,3-trifluoro-1-propene (TCTFP) up to 48 h following their administration to male Fischer 344 (F344) rats. In the absence of gross renal pathology, 1H NMR urinalysis revealed increased excretion of the tricarboxylic acid cycle intermediates citrate and succinate following DCVC administration. In contrast, both DCVHC and TCTFP produced functional defects in the S2 and S3 segments of the proximal tubule that were confirmed histologically. In these cases, 1H NMR urinalysis revealed increased excretion of glucose, L-lactate, acetate and 3-D-hydroxybutyrate (HB) as well as selective amino aciduria (alanine, valine, glutamate and glutamine). The significance of the proximal nephropathies induced by DCVHC and TCTFP is discussed in relation to biochemical observations on other xenobiotics that are toxic by similar mechanisms.

Quantitation of the tetrachloroethylene metabolite N-acetyl-S-(trichlorovinyl)cysteine in rat urine via negative ion chemical ionization gas chromatography/tandem mass spectrometry

A sensitive and selective negative ion chemical ionization gas chromatographic/tandem mass spectrometric (NICI GC/MS/MS) method was developed for the determination of the tetrachlorethylene metabolite, N-acetyl-S-(trichlorovinyl)cysteine (TCVC), in rat urine. Urine samples were fortified with a 13C,D2-analog of TCVC, acidified and extracted with ethyl acetate. The extract were derivatized with methanolic HCl, and the resulting methyl esters analyzed via NICI GC/MS/MS. Detection of the TCVC analogs was performed by monitoring the Cl- product ion of M-Cl2C2HS-. The limit of detection for TCVC by this method was estimated to be 0.1 ng ml-1 urine (3 x noise). The quantitation limit was determined to be 0.3 ng TCVC per milliliter of urine. The method was found to be linear for TCVC concentrations from 0.3 to 104 ng ml-1 urine. Relative recovery of TCVC from urine ranged from 95.4% to 108.5%. Additional data are given for GC/MS and GC/MS/MS analysis of the pentafluoro-benzyl ester derivative of TCVC. Data are also presented for the isolation and analysis of this compound obtained from dosed rats.

Glutathione-dependent bioactivation of xenobiotics

Glutathione conjugation has been identified as an important detoxication reaction. However, in recent years several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions are discussed. 1. Dichloromethane is metabolized by glutathione conjugation to formaldehyde via S-(chloromethyl)glutathione. Both compounds are reactive intermediates and may be responsible for the dichloromethane-induced tumorigenesis in sensitive species. 2. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl)glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed. 3. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cysteine conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is responsible for the observed toxicity of the parent compounds. 4. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they undergo alkylation or redox cycling reactions, or both, that cause organ-selective damage.

Pharmacokinetic modeling of trichloroethylene and trichloroacetic acid in humans

The development and application of appropriate physiologically based pharmacokinetic (PBPK) models of chemical contaminants will provide a rational basis for risk assessment extrapolation. Trichloroethylene (TCE) is a widespread contaminant found in soil, groundwater, and the atmosphere. Exposures to TCE and its metabolites have been found to be carcinogenic in rodents. In this study, a PBPK model for TCE and its major metabolite, trichloroacetic acid (TCA), is developed for humans. The model parameters, estimated from the relevant published literature on human exposures to TCE and its metabolites, are described. Key parameters describing the metabolism of TCE and the kinetics of TCA were estimated by optimization. The optimization was accomplished by simultaneously matching model predictions to observations of TCE concentrations in blood and exhaled breath, TCA plasma concentrations, and urinary TCA excretion from five published studies. The optimized human PBPK model provides an excellent description of TCE and TCA kinetics. The predictions were especially good for TCA plasma concentrations following repeated TCE inhalation, an exposure scenario similar to that occurring in the workplace. The human PBPK model can be used to estimate dose metrics resulting from TCE exposures and is therefore useful when considering the estimation of human health risks associated with such exposures.

Brain uptake of S-(1,2-dichlorovinyl)glutathione and S-(1,2-dichlorovinyl)-L-cysteine, the glutathione and cysteine S-conjugates of the neurotoxin dichloroacetylene

Dichloroacetylene causes trigeminal neuropathy in humans and animals. Glutathione conjugation of dichloroacetylene affords S-(1,2-dichlorovinyl)glutathione (DCVG), which is hydrolyzed to S-(1,2-dichlorovinyl)-L-cysteine (DCVC). This study was undertaken to test the hypothesis that the neurotoxicity of dichloroacetylene may be associated with glutathione S-conjugate formation and brain uptake and bioactivation of the dichloroacetylene-derived S-conjugates. With the Oldendorf technique, the Brain Uptake Index for [35S]DCVC and [35S]DCVG was determined and compared with the uptake of [35S]methionine and [14C]sucrose. Brain uptake of DCVC exceeded uptake of methionine and DCVG uptake was comparable to methionine uptake. Both [35S]DCVC and [35S]DCVG were recovered intact in brain tissue. The uptake of the 35S-labeled S-conjugates was inhibited by unlabeled DCVC and DCVG in a concentration-dependent manner. The data indicated that DCVC, but not DCVG, was transported by the sodium-independent system-L transporter for neutral amino acids. In vitro studies revealed that DCVG can be hydrolyzed to DCVC by brain tissue in a concentration-dependent manner.

Metabolism and lipoperoxidative activity of trichloroacetate and dichloroacetate in rats and mice

Trichloroacetate (TCA) and dichloroacetate (DCA) have been shown to be hepatocarcinogenic in mice when administered in drinking water. However, DCA produces pathological effects in the liver that are much more severe than those observed following TCA treatment in both rats and mice. To identify potential mechanisms involved in the liver pathology, the biotransformation of TCA and DCA was investigated in male Fischer 344 rats and B6C3F1 mice. Rodents were administered 5, 20, or 100 mg/kg [14C]TCA or [14C]DCA as a single oral dose in water. Elimination was examined by counting radioactivity in urine, feces, exhaled air, and carcass. Blood concentration over time curves were constructed for both TCA and DCA at the 20 and 100 mg/kg doses. Analysis of the data reveals two significant differences in the systemic clearance of TCA relative to DCA. First, DCA was much more extensively metabolized than TCA. More than 50% of any single dose of TCA was excreted unchanged in the urine of both rats and mice. In contrast, less than 2% of any dose of DCA was recovered in the urine as the parent compound. Second, while the blood concentration over time curves for TCA were similar in rats and mice, the blood concentrations of DCA were markedly greater in rats compared to those in mice, both when DCA was administered and when DCA resulted from metabolism of TCA. DCA was detected in the urine of TCA-treated animals and chloroacetate was found in the urine of DCA-treated animals. These metabolic products would be expected to arise from a free radical-generating, reductive dechlorination pathway. To evaluate the ability of acute doses of TCA and DCA to elicit a lipoperoxidative response, additional groups of mice were administered 0, 100, 300, 1000, and 2000 mg/kg TCA or DCA and thiobarbituric acid-reactive substances (TBARS) measured in liver homogenates. Both TCA and DCA enhanced the formation of TBARS in a dose-dependent manner, thereby providing further evidence of a reductive metabolic pathway. DCA was found to be the more potent of the chlorinated acetates in increasing TBARS formation in the livers of both rats and mice. In view of these data, it appears that the more extensive metabolism and rapid rate of elimination of DCA relative to TCA and the more potent lipoperoxidative activity of DCA may be important factors in the pathological effects associated with DCA treatment.

Species differences in the nephrotoxic response to S-(1,2-dichlorovinyl)glutathione

The present study was carried out to investigate the species differences in the nephrotoxic response to S-(1,2-dichlorovinyl)glutathione (DCVG) using rats, hamsters and guinea-pigs. DCVG was given intraperitoneally in physiological saline to groups of 5 animals at doses 0, 165 and 330 mumol/kg. Urine was collected for 24 h and the animals were then sacrificed. Significantly increased levels of urinary glucose, N-acetyl-beta-D-glucosaminidase, gamma-glutamyl transpeptidase, proteins and blood urea nitrogen were observed in rats at both dose levels of DCVG. An increase, but not of similar magnitude, of these biochemical parameters was noted in hamsters only at the higher dose of DCVG. Guinea-pigs showed significant increases in these biochemical parameters at the lower dose, but not at the higher dose. Light-microscopic studies showed increasing proximal tubular necrosis (PTN) in rats with increasing dose of DCVG, but PTN involving straight tubules only was observed at the higher dose in hamsters. PTN was again observed in guinea-pigs at the lower dose, but not at the higher dose of DCVG.

Male rat specific nephrotoxicity resulting from subchronic administration of hexachlorobenzene

Male rats are more sensitive to the nephrocarcinogenic effect of hexachlorobenzene (HCB) than are female rats. The purpose of this study was to shed light on this phenomenon by investigating mechanisms of subchronic nephrotoxicity of HCB. Groups of rats were administered HCB in corn oil (po) at 100 mg/kg, 5 days per week for 15 days or at 50 mg/kg, 5 days per week for 50 days. Urine was collected on Days 1, 8, and 15 for the 15-day treatment and on Day 50 for the 50-day treatment. Glucosuria, proteinuria, and enzymuria (gamma-glutamyl transpeptidase) were measured to assess renal function. Twenty-four hours after the last HCB treatment, the animals were killed and kidneys were removed for histopathological evaluation. Urine analyses showed no indication of renal dysfunction in treated animals compared to controls during the 15-day treatment. However, histology of male rat kidneys revealed degenerative and regenerative cellular foci accompanied by an increased accumulation of protein droplets in epithelial cells of the proximal tubules. The same histological observations were also made in male rats after a 50-day HCB treatment but this time they were accompanied by renal function alterations. In female rats, no such renal functional or histological alterations were observed. The histopathological observations in male rats correspond well with the protein droplet nephropathy; the latter is characteristic of the accumulation in kidney cells of alpha 2u-globulin probably caused by the reversible binding of a chemical to alpha 2u that renders the protein indigestible to kidney proteases. alpha 2u-Globulin was measured in the cytosol of male rats and was found to be increased 11-fold compared to controls. Also, HCB was found to be bound reversibly to alpha 2u. These results suggest that HCB induces a male rat specific nephropathy that could explain the higher incidence of kidney tumors in male rats compared to female rats.

Mutagenicity and cytotoxicity of two regioisomeric mercapturic acids and cysteine S-conjugates of trichloroethylene

The mutagenicity, cytotoxicity and metabolism of two regioisomic L-cysteine- and N-acetyl-L-cysteine-S-conjugates of trichloroethylene were studied. The 1,2-dichlorovinyl(1,2-DCV) isomers of both the cysteine conjugate and the mercapturate were much stronger mutagens in the Ames test with Salmonella typhimurium TA2638 when compared to the corresponding 2,2-dichlorovinyl (2,2-DCV) isomers. Similarly, the 1,2-DCV isomers were more cytotoxic towards isolated rat kidney proximal tubular cells, as assessed by inhibition of alpha-methylglucose uptake, than the 2,2-DCV isomers. The 3-4-fold higher rate of beta-lyase-dependent activation of S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCV-Cys) when compared to S-(1,2-dichlorovinyl)-L-cysteine (2,2-DCV-Cys) as well as the different nature of the reactive intermediates formed is probably responsible for these structure-dependent effects. The cytotoxicity of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCV-NAc) toward isolated kidney cells showed a delayed time course as compared to that of 1,2-DCV-Cys, probably due to the relatively low rate of deacetylation of 1,2-DCV-NAc. The time course of cytotoxicity of N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine (2,2-DCV-NAc), however, parallelled that of 2,2-DCV-Cys. Due to the relatively high rate of N-acetylation and low rate of beta-lyase activation, for 2,2-DCV-Nac the beta-lyase activation step may be rate limiting. Different rates of cellular uptake also may play a role in time course of toxicity of the cysteine conjugates and the mercapturic acids in the renal cells.