Consideration of the target organ toxicity of trichloroethylene in terms of metabolite toxicity and pharmacokinetics

Current Research Trends in the Application of In Vitro Three-Dimensional Models of Liver Cells

The liver produces and stores various nutrients that are necessary for the body and serves as a chemical plant, metabolizing carbohydrates, fats, hormones, vitamins, and minerals. It is also a vital organ for detoxifying drugs and exogenous harmful substances. Culturing liver cells in vitro under three-dimensional (3D) conditions is considered a primary mechanism for liver tissue engineering. The 3D cell culture system is designed to allow cells to interact in an artificially created environment and has the advantage of mimicking the physiological characteristics of cells in vivo. This system facilitates contact between the cells and the extracellular matrix. Several technically different approaches have been proposed, including bioreactors, chips, and plate-based systems in fluid or static media composed of chemically diverse materials. Compared to conventional two-dimensional monolayer culture in vitro models, the ability to predict the function of the tissues, including the drug metabolism and chemical toxicity, has been enhanced by developing three-dimensional liver culture models. This review discussed the methodology of 3D cell cultures and summarized the advantages of an in vitro liver platform using 3D culture technology.

Ameliorative effect of vitamin E on trichloroethylene-induced nephrotoxicity in rats

Background:

1,1,2-Trichloroethylene (TCE) is an important organic solvent which is widespread in the environment. Work place exposure to TCE has been associated adverse effects in many organs including kidney. Vitamin E is an antioxidant that can overcome oxidative stress.

Objectives:

The aim of the present study is to examine the role of vitamin E against destructive effects of TCE on rat kidney.

Materials and Methods:

A total of 35 male Wistar rats were randomly divided into seven groups of equal number in each. The rats in group I were the controls received vehicle only. Animals in groups III, V and VII received intraperitoneal injection (i.p) of corn oil. Rats in groups of II, IV, and VI were received vitamin E at a dose of 200 mg/kg; 30 minutes later, animals were received TCE (i.p) at doses of 1000 mg/kg (groups II and III), 1500 mg/kg (groups of IV and V), and 2000 mg/kg (groups of VI and VII) respectively. The experiment repeated for 7 consecutive days. Twenty-four hours after last administration, animals were killed with overdose of sodium pentobarbital. Blood samples were analyzed for blood urea nitrogen (BUN) and creatinine (Cr). One part of the kidney tissues were excised for measuring malondialdehyde (MDA) and glutathione (GSH) concentrations. Another part were excised for histopathological estimation.

Results:

TCE induced a dose-dependent elevation in BUN, Cr, MDA and markedly decreased GSH level when compared to those in control rats. TCE-induced dose-dependent injury in rat kidney tissue. Vitamin E significantly decreased BUN, Cr, MDA and increased GSH levels and protected kidney damage in TCE treated animals.

Conclusions:

The observations suggest that vitamin E may have a protective effect against TCE-induced oxidative stress in the rat kidney.

Toxicity of seven priority hazardous and noxious substances (HNSs) to marine organisms: Current status, knowledge gaps and recommendations for future research

Shipping industry and seaborne trade have rapidly increased over the last fifty years, mainly due to the continuous increasing demand for chemicals and fuels. Consequently, despite current regulations, the occurrence of accidental spills poses an important risk. Hazardous and noxious substances (HNSs) have been raising major concern among environmental managers and scientific community for their heterogeneity, hazardous potential towards aquatic organisms and associated social-economic impacts. A literature review on ecotoxicological hazards to aquatic organisms was conducted for seven HNSs: acrylonitrile, n-butyl acrylate, cyclohexylbenzene, hexane, isononanol, trichloroethylene and xylene. Information on the mechanisms of action of the selected HNS was also reviewed. The main purpose was to identify: i) knowledge gaps in need of being addressed in future research; and ii) a set of possible biomarkers suitable for ecotoxicological assessment and monitoring in both estuarine and marine systems. Main gaps found concern the scarcity of information available on ecotoxicological effects of HNS towards marine species and their poorly understood mode of action in wildlife. Differences were found between the sensitivity of freshwater and seawater organisms, so endpoints produced in the former may not be straightforwardly employed in evaluations for the marine environment. The relationship between sub-individual effects and higher level detrimental alterations (e.g. behavioural, morphological, reproductive effects and mortality) are not fully understood. In this context, a set of biomarkers associated to neurotoxicity, detoxification and anti-oxidant defences is suggested as potential indicators of toxic exposure/effects of HNS in marine organisms. Overall, to support the development of contingency plans and the establishment of environmental safety thresholds, it will be necessary to undertake targeted research on HNS ecotoxicity in the marine environment. Research should address these issues under more realistic exposure scenarios reflecting the prevailing spatial and temporal variability in ecological and environmental conditions.

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.

Sex-related differences in renal toxicodynamics in rodents

INTRODUCTION

An issue yet to be addressed, in the investigation of the xenobiotic toxicity, is a detailed characterization of the sex differences in toxicological responses. The 'sex issue' is particularly significant in nephrotoxicology as the kidney is a relevant target organ for xenobiotics and few studies have approached this subject in the past. There is a strong need to improve our understanding regarding the influence of sex in toxicology, given their increased requirement to establish the limits of exposure to chemicals in the environment and at work.

AREAS COVERED

In this review, the authors provide the reader with the current knowledge of sex differences in kidney toxicity for rats and mice. To make the review easier to consult, these studies have been organized according to the class of xenobiotic.

EXPERT OPINION

From the analysis of the present knowledge emerges a dramatic need for information on sex differences in xenobiotics toxicity. Although animals are reasonably good predictors of adverse renal effects in patients, there is need to identify alternative methods (e.g. in vitro/ex vivo) to better study sex differences in organ toxicity.

Impact of environmental exposures on ovarian function and role of xenobiotic metabolism during ovotoxicity

The mammalian ovary is a heterogeneous organ and contains oocyte-containing follicles at varying stages of development. The most immature follicular stage, the primordial follicle, comprises the ovarian reserve and is a finite number, defined at the time of birth. Depletion of all follicles within the ovary leads to reproductive senescence, known as menopause. A number of chemical classes can destroy follicles, thus hastening entry into the menopausal state. The ovarian response to chemical exposure can determine the extent of ovotoxicity that occurs. Enzymes capable of bioactivating as well as detoxifying xenobiotics are expressed in the ovary and their impact on ovotoxicity has been partially characterized for trichloroethylene, 7,12-dimethylbenz[a]anthracene, and 4-vinylcyclohexene. This review will discuss those studies, as well as illustrate where knowledge gaps remain for chemicals that have also been established as ovotoxicants.

Ability of trichloroethylene metabolite to promote immune pathology is strain-specific

Chronic low-level exposure to the environmental pollutant trichloroethylene has been shown to promote autoimmune disease in association with CD4(+) T-lymphocyte activation in lupus-prone MRL(+/+) mice. One of the primary metabolites of trichloroethylene, trichloroacetaldehyde hydrate (TCAH), was similarly shown to increase the percentage of IFNgamma-producing CD4(+) T-lymphocytes when added to the drinking water of MRL(+/+) mice. In addition, TCAH-treated MRL(+/+) mice developed skin inflammation and alopecia. In the present study TCAH was tested for its ability to accelerate the development of alopecia in C3H/HeJ mice which tend to develop the disorder spontaneously late in life. In contrast to MRL(+/+) mice, C3H/HeJ mice treated with TCAH did not develop alopecia at an increased rate. In addition, TCAH did not promote the expansion of activated IFNgamma-producing CD4(+) T-lymphocytes in C3H/HeJ mice. CD4(+) T-lymphocytes from TCAH-treated C3H/HeJ mice, unlike their MRL(+/+) counterparts, did not become resistant to activation-induced apoptosis following in vivo exposure to TCAH. Taken together, it appears that the ability of TCAH to promote immune-mediated pathology is strain-specific and may require an autoimmune-prone genetic background.

Effects of Kombucha on oxidative stress induced nephrotoxicity in rats

BACKGROUND

Trichloroethylene (TCE) may induce oxidative stress which generates free radicals and alters antioxidants or oxygen-free radical scavenging enzymes.

METHODS

Twenty male albino rats were divided into four groups: (1) the control group treated with vehicle, (2) Kombucha (KT)-treated group, (3) TCE-treated group and (4) KT/TCE-treated group. Kidney lipid peroxidation, glutathione content, nitric oxide (NO) and total blood free radical concentrations were evaluated. Serum urea, creatinine level, gamma-glutamyl transferase (GGT) and lactate dehydrogenase (LDH) activities were also measured.

RESULTS

TCE administration increased the malondiahyde (MDA) and NO contents in kidney, urea and creatinine concentrations in serum, total free radical level in blood and GGT and LDH activities in serum, whereas it decreased the glutathione (GSH) level in kidney homogenate. KT administration significantly improved lipid peroxidation and oxidative stress induced by TCE.

CONCLUSION

The present study indicates that Kombucha may repair damage caused by environmental pollutants such as TCE and may be beneficial to patient suffering from renal impairment.

Metabolism of pyridalyl in rats: excretion, distribution, and biotransformation of dichloropropenyl-labeled pyridalyl

Metabolism of pyridalyl [2,6-dichloro-4-(3,3-dichloroallyloxy)phenyl 3-[5-(trifluoromethyl)-2-pyridyloxy]propyl ether] labeled at position 2 of the dichloropropenyl group with 14C was investigated after single oral administration to male and female rats at 5 and 500 mg/kg. Absorbed 14C was excreted into feces (68-79%), urine (8-14%), and expired air (6-10%) in all of the groups. Regarding 14C-tissue residues on the seventh day after administration, fat showed the highest levels at 0.98-2.34 ppm and 219-221 ppm with the low and high doses, respectively. 14C-Residues in other tissues accounted for 0.03-0.32 ppm at the low dose and 3-70 ppm at the high dose. The percentages of the 14C-residue in fat were 1.50-3.16% of the dose, and those of muscle and hair and skin were also relatively high, accounting for 0.49-1.20%. Total 14C-residues in the whole body were 2.95-6.80% of the dose. Fecal metabolites in male rats treated with 500 mg/kg pyridalyl were purified by a combination of chromatographic techniques, and chemical structures of 8 metabolites were identified by NMR and MS spectrometry. The biotransformation reactions in rats were proposed to be as follows: (1) epoxidation of the double bond in the dichloropropenyl group followed by hydration, dehydrochlorination, decarboxylation, and/or mercapturic acid conjugation; (2) CO2 formation after O-dealkylation of pyridalyl and its metabolites; (3) hydroxylation of C3 in the pyridyl ring; (4) O-dealkylation of the pyridyloxy and the trimethylene groups; (5) dehydrochlorination and hydration in the dichloropropenyl group.

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

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

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

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

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

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

Ovarian gene expression is stable after exposure to trichloroethylene

Exposure of female rats to trichloroethylene (TCE), an environmental toxicant commonly found in ground and surface waters throughout the United States, reduces the fertilizability of oocytes produced by these females compared with oocytes from control females. Localization of cytochrome P450 2E1 and glutathione s-transferase alpha, TCE-metabolizing enzymes, in the ovary suggests TCE metabolism occurs in the ovary. The production of bioactive TCE metabolites in the ovary may alter female reproductive function by altering ovarian gene transcription and/or protein expression and function. The purpose of the present study was to examine ovarian gene transcription after exposure of female rats to 0.45% TCE (v/v) in 3% Tween. Control rats received 3% Tween. Microarray analysis after 1 and 5 days of exposure indicated ovarian gene transcription was maintained during TCE exposure with the possible exception of a very few genes. Although conclusions for these few genes were ambiguous from the microarray analysis due to the minimal but statistically significant reductions, quantitative real time RT-PCR (qRT-PCR) analysis indicated expression of these genes was unaltered after TCE exposure. Protein analysis confirmed qRT-PCR results. This study suggests TCE-induced reductions in oocyte fertilizability are independent of currently detectable alterations in ovarian gene expression.

Trichloroethylene metabolism in the rat ovary reduces oocyte fertilizability

Exposure to trichloroethylene (TCE, an environmental toxicant) reduced oocyte fertilizability in the rat. In vivo, TCE may be metabolized by cytochrome P450 dependent oxidation or glutathione conjugation in the liver or kidneys, respectively. Cytochrome P450 dependent oxidation is the higher affinity pathway. The primary isoform of cytochrome P450 to metabolize TCE in the liver, cytochrome P450 2E1, is present in the rodent ovary. Ovarian metabolism of TCE by the oxidative pathway and the production of reactive oxygen species (ROS) may occur given the presence of the metabolizing enzyme. The objectives of this study were to define the sensitive interval of oocyte growth to TCE exposure, and to determine if TCE exposure resulted in the formation of ovarian protein carbonyls, an indicator of oxidative damage. Rats were exposed to TCE in drinking water (0.45% TCE (v/v) in 3% Tween) or 3% Tween (vehicle control) during three 4-5 day intervals of oocyte development preceding ovulation. Oocytes from TCE-exposed females were less fertilizable compared with vehicle-control oocytes. Immunohistochemical labeling of ovaries and Western blotting of ovarian proteins demonstrated TCE treatment induced a greater incidence of protein carbonyls compared with vehicle controls. Protein carbonyl formation in the ovary is consistent with TCE metabolism by the cytochrome P450 pathway. Oxidative damage following ovarian TCE metabolism or the presence of TCE metabolites may contribute to reduced oocyte fertilizability. In summary, these results indicate maturing oocytes are susceptible to very short in vivo exposures to TCE.

Interactive toxicity of inorganic mercury and trichloroethylene in rat and human proximal tubules: effects on apoptosis, necrosis, and glutathione status

Simultaneous or prior exposure to one chemical may alter the concurrent or subsequent response to another chemical, often in unexpected ways. This is particularly true when the two chemicals share common mechanisms of action. The present study uses the paradigm of prior exposure to study the interactive toxicity between inorganic mercury (Hg(2+)) and trichloroethylene (TRI) or its metabolite S-(1,2-dichlorovinyl)-l-cysteine (DCVC) in rat and human proximal tubule. Pretreatment of rats with a subtoxic dose of Hg(2+) increased expression of glutathione S-transferase-alpha1 (GSTalpha1) but decreased expression of GSTalpha2, increased activities of several GSH-dependent enzymes, and increased GSH conjugation of TRI. Primary cultures of rat proximal tubular (rPT) cells exhibited both necrosis and apoptosis after incubation with Hg(2+). Pretreatment of human proximal tubular (hPT) cells with Hg(2+) caused little or no changes in GST expression or activities of GSH-dependent enzymes, decreased apoptosis induced by TRI or DCVC, but increased necrosis induced by DCVC. In contrast, pretreatment of hPT cells with TRI or DCVC protected from Hg(2+) by decreasing necrosis and increasing apoptosis. Thus, whereas pretreatment of hPT cells with Hg(2+) exacerbated cellular injury due to TRI or DCVC by shifting the response from apoptosis to necrosis, pretreatment of hPT cells with either TRI or DCVC protected from Hg(2+)-induced cytotoxicity by shifting the response from necrosis to apoptosis. These results demonstrate that by altering processes related to GSH status, susceptibilities of rPT and hPT cells to acute injury from Hg(2+), TRI, or DCVC are markedly altered by prior exposures.

Lack of formic acid production in rat hepatocytes and human renal proximal tubule cells exposed to chloral hydrate or trichloroacetic acid

The industrial solvent trichloroethylene (TCE) and its major metabolites have been shown to cause formic aciduria in male rats. We have examined whether chloral hydrate (CH) and trichloroacetic acid (TCA), known metabolites of TCE, produce an increase in formic acid in vitro in cultures of rat hepatocytes or human renal proximal tubule cells (HRPTC). The metabolism and cytotoxicity of CH was also examined to establish that the cells were metabolically active and not compromised by toxicity. Rat hepatocytes and HRPTC were cultured in serum-free medium and then treated with 0.3-3mM CH for 3 days or 0.03-3mM CH for 10 days, respectively and formic acid production, metabolism to trichloroethanol (TCE-OH) and TCA and cytotoxicity determined. No increase in formic acid production in rat hepatocytes or HRPTC exposed to CH was observed over and above that due to chemical degradation, neither was formic acid production observed in rat hepatocytes exposed to TCA. HRPTC metabolized CH to TCE-OH and TCA with a 12-fold greater capacity to form TCE-OH versus TCA. Rat hepatocytes exhibited a 1.6-fold and three-fold greater capacity than HRPTC to form TCE-OH and TCA, respectively. CH and TCA were not cytotoxic to rat hepatocytes at concentrations up to 3mM/day for 3 days. With HRPTC, one sample showed no cytotoxicity to CH at concentrations up to 3mM/day for 10 days, while in another cytotoxicity was seen at 1mM/day for 3 days. In summary, increased formic acid production was not observed in rat hepatocytes or HRPTC exposed to TCE metabolites, suggesting that the in vivo response cannot be modelled in vitro. CH was toxic to HRPTC at millimolar concentrations/day over 10 days, while glutathione derived metabolites of TCE were toxic at micromolar concentrations/day over 10 days [Lock, E.A., Reed, C.J., 2006. Trichloroethylene: mechanisms of renal toxicity and renal cancer and relevance to risk assessment. Toxicol. Sci. 19, 313-331] supporting the view that glutathione derived metabolites are likely to be responsible for nephrotoxicity.

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

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

Comparative metabolism and disposition of trichloroethylene in Cyp2e1-/-and wild-type mice

Trichloroethylene (TCE)1 is an important environmental contaminant, a well established rodent carcinogen, and a "probable human carcinogen". Metabolism of TCE occurs primarily via cytochrome P450 (P450)-dependent oxidation. In vitro studies suggested that CYP2E1 is the principal high-affinity enzyme responsible for TCE metabolism. The objective of the present work is to more directly assess the role of CYP2E1 in the metabolism and disposition of 1,2-14C-TCE administered at 250 or 1000 mg/kg (gavage) using Cyp2e1-/-[knockout (KO)] versus wild-type (WT) mice. After dosing, animals were individually placed in glass metabolism cages that allowed the collection of expired air, urine, and feces. Exhalation of TCE-derived 14CO2 increased in a dose-dependent manner in mice of both genotypes and was significantly higher in WT versus KO mice. A significantly greater percentage of the dose was exhaled in KO versus WT mice as organic volatiles (mainly as TCE). Urinary excretion was the major route of TCE metabolism in WT mice, and the percentage of dose eliminated in urine was significantly higher at the 250 versus 1000 mg/kg dose. Furthermore, urinary excretion and CO2 exhalation significantly decreased in KO versus WT mice. Pretreatment with 1-aminobenzotriazole clearly inhibited TCE metabolism as evident from increased exhalation of parent TCE, and decreased urinary excretion and CO2 exhalation in mice of both genotypes. In conclusion, these data showed that whereas CYP2E1 plays an important role in TCE metabolism and disposition, other P450s also play a significant role and may explain earlier results showing that TCE causes lung damage in KO and WT mice.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Exposure to a metabolite of the environmental toxicant, trichloroethylene, attenuates CD4+ T cell activation-induced cell death by metalloproteinase-dependent FasL shedding

Long-term exposure to the environmental contaminant trichloroethylene (TCE) in drinking water has been shown to promote autoimmune disease in association with the expansion of activated CD4+ T cells. The effects of TCE on CD4+ T cells were linked in the present study to the ability of TCE metabolite, trichloroacetaldehyde hydrate (TCAH), to inhibit activation-induced cell death (AICD) in CD4+ T cells. TCAH attenuated AICD in CD4+ T cells by decreasing FasL (CD178) expression but not by altering Fas (CD95) expression or by interfering with Fas-signaling events following direct engagement of the Fas receptor. The TCAH-induced decrease in FasL expression did not appear to be mediated at the transcriptional level but was instead due to increased shedding of FasL from the surface of the CD4+ T cells. The ability of TCAH to cleave FasL and thereby decrease AICD appeared to be mediated by metalloproteinases and correlated with a TCAH-induced increase in matrix metalloproteinase-7. Thus, this study presents the novel finding that the environmental contaminant TCE works via its metabolite TCAH to attenuate AICD by increasing metalloproteinase activity that cleaves FasL from CD4+ T cells. This represents a mechanism by which an environmental trigger inhibits AICD in CD4+ T cells and may thereby promote CD4+ T cell-mediated autoimmune disease.

Trichloroethylene: mechanisms of renal toxicity and renal cancer and relevance to risk assessment

1,1,2-Trichloroethylene (TCE) is an important solvent that is widespread in the environment. We have reviewed carcinogenicity data from seven bioassays with regard to renal injury and renal tumors. We report a consistent but low incidence of renal tubule carcinoma in male rats. Epidemiology studies on workers exposed to TCE (and other chlorinated solvents) indicate a weak association between high-level exposure and renal cancer. There appears to be a threshold below which no renal injury or carcinogenicity is expected to arise. TCE is not acutely nephrotoxic to rats or mice, but subchronic exposure to rats produces a small increase in urinary markers of renal injury. Following chronic exposure, pathological changes (toxic nephrosis and a high incidence of cytomegaly and karyomegaly) were observed. The basis for the chronic renal injury probably involves bioactivation of TCE. Based on the classification by E. A. Lock and G. C. Hard (2004, Crit. Rev. Toxicol. 34, 211-299) of chemicals that induce renal tubule tumors, we found no clear evidence to place TCE in category 1 or 2 (chemicals that directly or indirectly interact with renal DNA), category 4 (direct cytotoxicity and sustained tubule cell regeneration), category 5 (indirect cytotoxicity and sustained tubule cell regeneration associated with alpha2u-globulin accumulation), or category 6 (exacerbation of spontaneous chronic progressive nephropathy). TCE is best placed in category 3, chemicals that undergo conjugation with GSH and subsequent enzymatic activation to a reactive species. The implication for human risk assessment is that TCE should not automatically be judged by linear default methods; benchmark methodology could be used.

Pulmonary bioactivation of trichloroethylene to chloral hydrate: relative contributions of CYP2E1, CYP2F, and CYP2B1

Pulmonary cytotoxicity induced by trichloroethylene (TCE) is associated with cytochrome P450-dependent bioactivation to reactive metabolites. In this investigation, studies were undertaken to test the hypothesis that TCE metabolism to chloral hydrate (CH) is mediated by cytochrome P450 enzymes, including CYP2E1, CYP2F, and CYP2B1. Recombinant rat CYP2E1 catalyzed TCE metabolism to CH with greater affinity than did the recombinant P450 enzymes, rat CYP2F4, mouse CYP2F2, rat CYP2B1, and human CYP2E1. The catalytic efficiencies of recombinant rat CYP2E1 (V(max)/K(m) = 0.79) for generating CH was greater than those of recombinant CYP2F4 (V(max)/K(m) = 0.27), recombinant mouse CYP2F2 (V(max)/K(m) = 0.11), recombinant rat CYP2B1 (V(max)/K(m) = 0.07), or recombinant human CYP2E1 (V(max)/K(m) = 0.02). Decreases in lung microsomal immunoreactive CYP2E1, CYP2F2, and CYP2B1 were manifested at varying time points after TCE treatment. The loss of immunoreactive CYP2F2 occurred before the loss of immunoreactive CYP2E1 and CYP2B1. These protein decreases coincided with marked reduction of lung microsomal p-nitrophenol hydroxylation and pentoxyresorufin O-dealkylation. Rates of CH formation in the microsomal incubations were time-dependent and were incremental from 5 to 45 min. The production of CH was also determined in human lung microsomal incubations. The rates were low and were detected in only three of eight subjects. These results showed that, although CYP2E1, CYP2F, and CYP2B1 are all capable of generating CH, TCE metabolism is mediated with greater affinity by recombinant rat CYP2E1 than by recombinant CYP2F, CYP2B1, or human CYP2E1. Moreover, the rates of CH production were substantially higher in murine than in human lung.

Effects of trichloroethylene and its metabolite trichloroacetic acid on the expression of vimentin in the rat H9c2 cell line

Trichloroethylene (TCE) and its metabolite trichloroacetic acid (TCAA) are environmental contaminants with specific toxicity for the embryonic heart. In an effort to identify the cellular pathways disrupted by TCE and TCAA during heart development, we investigated their effects on expression of vimentin, a marker of cardiac differentiation. Previous studies had shown that the level of vimentin transcript was inhibited in rat embryonic heart after maternal exposure to TCE via drinking water. In the same study, maternal exposure to TCAA produced the opposite effect, inducing an increased level of vimentin mRNA. In this study, we selected an in vitro system, the rat cardiac myoblast cell line H9c2, to further characterize the molecular mechanisms used by TCE and TCAA to disrupt normal heart development. In particular, we investigated the effects of both toxicants on vimentin, at both the RNA and protein levels, using dose-response and time course curves. Our experimental findings indicate that vimentin expression is affected by TCE and TCAA in H9c2 cells similarly as in vivo. The work is significant because it provides a suitable in vitro model for studies looking at toxicant effects on myocardiac cells, and it suggests that vimentin is a good marker of TCE exposure in the embryonic heart.

Environmental contaminant and disinfection by-product trichloroacetaldehyde stimulates T cells in vitro

It had been shown previously that MRL+/+ mice exposed to occupationally relevant doses of the environmental contaminant trichloroethylene in their drinking water developed lupus-like symptoms and autoimmune hepatitis in association with activation of Interferon-gamma (IFN-gamma)-producing CD4+ T cells. Since trichloroethylene must be metabolized in order to promote the T-cell activation associated with autoimmunity, the present study was initiated to determine whether the immunoregulatory effects of trichloroethylene could be mimicked by one of its major metabolites, trichloroacetaldehyde (TCAA). At concentrations ranging from 0.04 to 1 mM TCAA co-stimulated proliferation of murine T-helper type 1 (Th1) cells treated with anti-CD3 antibody or antigen in vitro. TCAA at similar concentrations also induced phenotypic alterations commensurate with activation (upregulation of CD28 and downregulation of CD62L) in both cloned memory Th1 cells, as well as naïve CD4+ T cells from MRL+/+ mice. TCAA-induced Th1 cell activation was accompanied by phoshorylation of activating transcription factor 2 (ATF-2) and c-Jun, two components of the activator protein-1 (AP-1) transcription factor. TCAA at higher concentrations was also shown to form a Schiff base on T cells, and inhibition of Schiff base formation suppressed the ability of TCAA to phosphorylate ATF-2. Taken together, these results suggest that TCAA promotes T-cell activation via stimulation of the mitogen-activated protein (MAP) kinase pathway in association with Schiff base formation on T-cell surface proteins. By demonstrating that TCAA can stimulate T-cell function directly, these results may explain how the environmental toxicant trichloroethylene promotes T-cell activation and related autoimmunity in vivo.

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

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

Activation and attenuation of apoptosis of CD4+ T cells following in vivo exposure to two common environmental toxicants, trichloroacetaldehyde hydrate and trichloroacetic acid

Exposure to occupationally relevant concentrations of the environmental pollutant, trichloroethylene (TCE), in the drinking water of autoimmune-prone MRL+/+ mice has been shown to promote the generation of lupus and autoimmune hepatitis in association with the activation of Interferon-gamma (IFN-gamma)-producing CD4+ T cells. Since blocking TCE metabolism suppressed the TCE-induced alteration in immune function, the present study was initiated to determine whether the major metabolites of TCE, trichloroacetaldehyde hydrate (TCAH) and trichloroacetic acid (TCA) could also mediate these immunoregulatory affects in vivo. TCAH and TCA were administered to the drinking water of MRL+/+ mice for 4 weeks. CD4+ T cells from TCAH and TCA-treated MRL+/+ mice, unlike CD4+ T cells from control mice, demonstrated functional and phenotypic signs of activation, as evidenced by increased IFN-gamma production in association with the increased percentage of CD62L(lo) CD4+ T cells. Interestingly, it was also found that the CD4+ T cells from the TCAH and TCA-treated mice showed a decreased susceptibility to the activation-induced cell death (AICD) form of apoptosis following re-stimulation in vitro. By demonstrating that TCAH and TCA can activate CD4+ T cells and inhibit their apoptosis following in vivo exposure represents a mechanism by which environmental toxicants may induce or accelerate the development of autoimmune disease.

Identification of trichloroethylene and its metabolites in human seminal fluid of workers exposed to trichloroethylene

We have investigated the potential of the male reproductive tract to accumulate trichloroethylene (TCE) and its metabolites, including chloral, trichloroethanol (TCOH), trichloroacetic acid (TCA), and dichloroacetic acid (DCA). Human seminal fluid and urine samples from eight mechanics diagnosed with clinical infertility and exposed to TCE occupationally were analyzed. In in vivo experimental studies, TCE and its metabolites were determined in epididymis and testis of mice exposed to TCE (1000 ppm) by inhalation for 1 to 4 weeks. In other studies, incubations of monkey epididymal microsomes were performed in the presence of TCE and NADPH. Our results showed that seminal fluid from all eight subjects contained TCE, chloral, and TCOH. DCA was present in samples from two subjects, and only one contained TCA. TCA and/or TCOH were also identified in urine samples from only two subjects. TCE, chloral, and TCOH were detected in murine epididymis after inhalation exposure with TCE for 1 to 4 weeks. Levels of TCE and chloral were similar throughout the entire exposure period. TCOH levels were similar at 1 and 2 weeks but increased significantly after 4 weeks of TCE exposure. Chloral was identified in microsomal incubations with TCE in monkey epididymis. CYP2E1, a P450 that metabolizes TCE, was localized in human and monkey epididymal epithelium and testicular Leydig cells. These results indicated that TCE is metabolized in the reproductive tract of the mouse and monkey. Furthermore, TCE and its metabolites accumulated in seminal fluid, and suggested associations between production of TCE metabolites, reproductive toxicity, and impaired fertility.

Measurement of subclinical atherosclerosis: beyond risk factor assessment

PURPOSE OF REVIEW

Assessment of subclinical atherosclerosis using the current available noninvasive imaging modalities holds promise for individual cardiovascular risk management and monitoring efficacy of therapeutic interventions (i.e. surrogate end-points). The present review addresses benefits and limitations of flow-mediated dilatation, intima-media thickness, electron-beam computed tomography and magnetic resonance coronary angiography.

RECENT FINDINGS

Both carotid intima-media thickness and peripheral flow-mediated dilatation correlate inversely with cardiovascular risk factors and coronary artery disease. They have been shown to carry predictive value for future cardiovascular events, but clinical application of both intima-media thickness and flow-mediated dilatation demands further methodological maturation of these techniques. Intima thickening has been successfully targeted in numerous intervention trials, but determination of an explicit threshold value beyond which cardiovascular risk significantly increases will facilitate its utility as a routine clinical tool. Electron-beam computed tomography can accurately detect and quantify coronary artery calcification (an established marker of the total coronary plaque burden). However, lack of evidence of its additional predictive power for future coronary events warrants for further research. Finally, magnetic resonance coronary angiography appears to be a promising technique, integrating both functional and anatomical aspects of coronary artery disease. Properly designed studies are needed to determine its value in clinical practice.

SUMMARY

Various noninvasive imaging techniques have recently emerged that may find applications in clinical research. However, before widespread clinical utilization, further technical refinement of all of the cited imaging modalities is mandatory. It will be a challenge over the coming few years to clarify whether improvements in surrogate end-points can directly be translated into improved outcomes.

Industrial solvents and liver toxicity: risk assessment, risk factors and mechanisms

Organic solvents utilized in various industrial processes may be associated with hepatotoxicity. The hepatotoxicity of some of the solvents was recognized as early as 1887, 1889 and 1904. Factors contributing to the hepatotoxicity of solvents include 1) species differences, 2) liver blood flow, 3) protein binding, 4) point of binding intracellularly, 5) genetic factors, 6) different cellular enzymatic degradation, 7) age, 8) nutritional condition, 9) interaction with alcohol, and 10) interaction with medications of use and abuse. The hepatotoxicity of solvents in general and of carbon tetrachloride, trichloroethylene, tetrachloroethylene, toluene, and 1,1,1-trichloroethene are discussed. Experimental animal data, human data, and in vitro studies are explored. Suggested mechanisms of direct toxicity, indirect toxicity and autoimmune mechanisms are elaborated. The most important message from this review is that laboratory testing that is commonly used by clinicians to detect liver toxicity may not be sensitive enough to detect early liver hepatotoxicity from industrial solvents and new methodologies are being encouraged and utilized in the early recognition and diagnosis of hepatotoxicity for solvents. The final clinical assessment of hepatotoxicity and industrial solvents must take into account synergism with medications, drugs of use and abuse, alcohol, age, and nutrition. Early recognition and reporting will be helpful in further understanding the incidence, cofactors and possible mechanisms.

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.

Structure-activity relationship for the biotransformation of haloalkenes by rat liver microsomal glutathione transferase 1

Many haloalkenes are nephrotoxic in rodents, and experimental evidence supports a glutathione-dependent bioactivation pathway that leads to nephrotoxicity or nephrocarcinogenicity, or both. The reaction of glutathione with haloalkenes is catalyzed by cytosolic glutathione transferases (cGST) and microsomal glutathione transferase 1 (MGST1). The aim of this study was to develop a computational approach to predict the competency of cGST and MGST1 to catalyze the reaction of glutathione with a range of haloalkenes. The hypothesis tested was that the semiempirically computed energy of the lowest unoccupied molecular orbital (E(LUMO)) of a haloalkene may be used to predict the competency of cGST and MGST1 to catalyze its reaction with glutathione. The MGST1- and cGST-catalyzed reaction of glutathione with nine haloalkenes with E(LUMO) values ranging from -1.14 to 0.38 eV was determined experimentally. The data indicated that the E(LUMO) values for haloalkenes were inversely related to the specific activity of the MGST1-catalyzed reaction but not the cGST-catalyzed reaction. These data also demonstrated that MGST1 catalyzed the reaction of glutathione with haloalkenes with E(LUMO) values equal to or more negative than -0.73 eV and that cGST catalyzed the reaction of glutathione with haloalkenes with E(LUMO) values more negative than -0.06 eV.

The use of in vitro metabolic parameters and physiologically based pharmacokinetic (PBPK) modeling to explore the risk assessment of trichloroethylene

A physiologically based pharmacokinetic (PBPK) model has been developed for trichloroethylene (1,1,2-trichloroethene, TRI) for rat and humans, based on in vitro metabolic parameters. These were obtained using individual cytochrome P450 and glutathione S-transferase enzymes. The main enzymes involved both for rats and humans are CYP2E1 and the μ- and π-class glutathione S-transferases. Validation experiments were performed in order to test the predictive value of the enzyme kinetic parameters to describe 'whole-body' disposition. Male Wistar rats were dosed orally or intravenously with different doses of trichloroethylene. Obtained exhaled radioactivity, excreted radioactivity in urine, and obtained blood concentration-time curves of trichloroethylene for all dosing groups were compared to predictions from the PBPK model. Subsequently, using the scaling factor derived from the rat experiments predictions were made for the extreme cases to be expected in humans, based on interindividual variations of the key enzymes involved. On comparing these predictions with literature data a very close match was found. This illustrates the potential application of in vitro metabolic parameters in risk assessment, through the use of PBPK modeling as a tool to understand and predict in vivo data. From a hypothetical 8 h exposure scenario to 35 ppm trichloroethylene in rats and humans, and assuming that the glutathione S-transferase pathway is responsible for the toxicity of trichloroethylene, it was concluded that humans are less sensitive for trichloroethylene toxicity than rats.

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

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

Cytochrome p450-dependent metabolism of trichloroethylene in rat kidney

The metabolism of trichloroethylene (Tri) by cytochrome P450 (P450) was studied in microsomes from liver and kidney homogenates and from isolated renal proximal tubular (PT) and distal tubular (DT) cells from male Fischer 344 rats. Chloral hydrate (CH) was the only metabolite consistently detected and was used as a measurement of P450-dependent metabolism of Tri. Pretreatment of rats with pyridine increased CH formation in both liver and kidney microsomes, whereas pretreatment of rats with clofibrate increased CH formation only in kidney microsomes. Pyridine increased CYP2E1 expression in both liver and kidney microsomes, whereas clofibrate had no effect on hepatic but increased renal CYP2E1 and CYP2C11 protein levels. These results suggest a role for CYP2E1 in both the hepatic and renal metabolism of Tri and a role for CYP2C11 in the renal metabolism of Tri. Studies with the general P450 inhibitor SKF-525A and the CYP2E1 competitive substrate chlorzoxazone provided additional support for the role of CYP2E1 in both tissues. CH formation was higher in PT cells than in DT cells and was time and reduced nicotinamide adenine dinucleotide phosphate (NADPH) dependent. However, pretreatment of rats with either pyridine or clofibrate had no effect on CYP2E1 or CYP2C11 protein levels or on CH formation in isolated cells. These data show for the first time that Tri can be metabolized to at least one of its P450 metabolites in the kidneys and quantitate the effect of P450 induction on Tri metabolism in the rat kidney.

Cytotoxicity of trichloroethylene and S-(1, 2-dichlorovinyl)-L-cysteine in primary cultures of rat renal proximal tubular and distal tubular cells

Activities of several glutathione-dependent enzymes, expression of cytochrome P450 isoenzymes, and time- and concentration-dependent cytotoxicity of trichloroethylene (TRI) and S-(1, 2-dichlorovinyl)-L-cysteine (DCVC) were evaluated in primary cultures of proximal tubular (PT) and distal tubular (DT) cells from rat kidney. These cells exhibited cytokeratin staining and maintained activities of all glutathione-dependent enzymes measured. Of the cytochrome P450 isoenzymes studied, only CYP4A expression was detected. CYP4A mRNA and protein expression were higher in primary cultures of DT cells than in PT cells and were increased in DT cells by ciprofibrate treatment. Incubation of cells for 6 h with concentrations of TRI as high as 10 mM resulted in minimal cytotoxicity, as determined by release of lactate dehydrogenase (LDH). In contrast, marked cytotoxicity resulted from incubation of PT or DT cells with DCVC. Addition to cultures of TRI (2-10 mM) for 24 or 72 h resulted in modest, but significant time- and concentration-dependent increases in LDH release. Treatment of cells with DCVC (0.1-1 mM) for 24 h caused significant increases in LDH release and alterations in cellular protein and DNA content. Finally, exposure of primary cultures to TRI or DCVC for 72 h followed by 3 h of recovery caused a slight increase in the expression of vimentin, consistent with cellular regeneration. These studies demonstrate the utility of the primary renal cell cultures for the study of CYP4A expression and mechanisms of TRI-induced cellular injury.

Contribution of direct solvent injury to the dose-dependent kinetics of trichloroethylene: portal vein administration to rats

Presystemic elimination of trichloroethylene (TCE), a common contaminant of drinking water, has been shown by Lee et al. (Toxicol. Appl. Pharmacol. 139, 262-271, 1996) to be inversely related to dose. When relatively high doses were administered to rats via the portal vein (PV), first-pass hepatic extraction became negligible. This phenomenon could result not only from metabolic saturation, but from suicidal destruction of cytochromes P450 and hepatocellular injury as well. The objectives of the current investigation were to: (a) clarify the relative roles of P450 depletion and hepatocellular toxicity in the apparent cessation of hepatic elimination of TCE in animals given relatively high doses of TCE via the PV; and (b) investigate mechanism(s) of hepatocellular injury under such exposure conditions. TCE (16 and 64 mg/kg body weight (bw) was incorporated into a 5% aqueous Alkamuls emulsion and injected via an indwelling jugular vein (JV) or PV cannula into male Sprague-Dawley rats. Some animals received 73.5 micromol/kg of p-nitrophenol (PNP), a competitive metabolic inhibitor of TCE, through the PV cannula 3 min before TCE. Administration of TCE via the PV resulted in deposition of relatively high levels of TCE in the liver. PV dosing resulted in lower total hepatic P450 levels than did JV dosing. PV dosing produced marked elevations of cytoplasmic enzymes in serum, but JV dosing did not. Decreases in hepatic P450 were not selective for cytochrome P4502E1. Histological examination of the liver of PV-dosed rats revealed periportal rather than centrilobular necrosis. PNP pretreatment failed to prevent the increase in serum enzymes, decrease in hepatic P450 content, and hepatic necrosis following PV TCE. It is concluded that PV injection of bolus doses of TCE >/= 16 mg/kg causes liver injury within minutes in rats, primarily through direct solvent action on hepatocellular membranes rather than by P450-mediated effects. This liver damage likely plays a modest role in reducing the liver's capacity to metabolize high PV doses of TCE.

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.

trans-Activation of PPARalpha and PPARgamma by structurally diverse environmental chemicals

A large number of industrial chemicals and environmental pollutants, including trichloroethylene (TCE), di(2-ethylhexyl)phthalate (DEHP), perfluorooctanoic acid (PFOA), and various phenoxyacetic acid herbicides, are nongenotoxic rodent hepatocarcinogens whose human health risk is uncertain. Rodent model studies have identified the receptor involved in the hepatotoxic and hepatocarcinogenic actions of these chemicals as peroxisome proliferator-activated receptor alpha (PPARalpha), a nuclear receptor that is highly expressed in liver. Humans exhibit a weak response to these peroxisome proliferator chemicals, which in part results from the relatively low level of PPARalpha expression in human liver. Cell transfection studies were carried out to investigate the interactions of peroxisome proliferator chemicals with PPARalpha, cloned from human and mouse, and with PPARgamma, a PPAR isoform that is highly expressed in multiple human tissues and is an important regulator of physiological processes such as adipogenesis and hematopoiesis. With three environmental chemicals, TCE, perchloroethylene, and DEHP, PPARalpha was found to be activated by metabolites, but not by the parent chemical. A decreased sensitivity of human PPARalpha compared to mouse PPARalpha to trans-activation was observed with some (Wy-14, 643, PFOA), but not other, peroxisome proliferators (TCE metabolites, trichloroacetate and dichloroacetate; and DEHP metabolites, mono[2-ethylhexyl]phthalate and 2-ethylhexanoic acid). Investigation of human and mouse PPARgamma revealed the transcriptional activity of this receptor to be stimulated by mono(2-ethylhexyl)phthalate, a DEHP metabolite that induces developmental and reproductive organ toxicities in rodents. This finding suggests that PPARgamma, which is highly expressed in human adipose tissue, where many lipophilic foreign chemicals tend to accumulate, as well as in colon, heart, liver, testis, spleen, and hematopoietic cells, may be a heretofore unrecognized target in human cells for a subset of industrial and environmental chemicals of the peroxisome proliferator class.

Theoretical study of the SNV reaction of trichloroethylene (TCE) and CH3S- as a model for glutathione conjugation of TCE

Trichloroethylene (TCE), a major environmental pollutant, is activated to mutagenic and nephrotoxic intermediates through a glutathione (GSH) conjugation pathway. Three product isomers of GSH-TCE conjugation, having potentially different toxicities, are theoretically possible: cis- or trans-S-(1, 2-dichlorovinyl)glutathione (cis- or trans-1,2-DCVG, respectively) or 2,2-DCVG. This study involved application of ab initio molecular orbital theory to computing potential energy profiles (PEPs) and predicting product outcome of the reaction of CH3S- with TCE as a model for GSH-TCE conjugation in biological systems. A goal of this study was to determine the extent to which a body of chemical knowledge pertaining to nucleophilic vinylic substitution (SNV) reactions, of which the GSH-TCE conjugation is a representative example, is relevant to this biological conjugation problem. PEPs were computed for all studied species at the HF/6-31+G level of theory; electron correlation effects were estimated at the MP2/6-31+G and MP4/6-31+G levels, and the influence of solvation was estimated using the PS-GVB solvation model. Multiple proposed reaction pathways were considered, including conjugation at the C1 or C2 site on TCE, by in-plane (sigma) or out-of-plane (pi) approach of the nucleophile. Some aspects of the MP2 and HF PEPs were found to differ significantly. However, on the basis of comparison of activation barriers, calculations at all levels of theory predict preference for C2 conjugation over C1 conjugation and formation of the trans-1,2-DCVM product over the cis-1,2-DCVM product. These predictions are consistent with GSH-TCE conjugation results from in vivo experiments. In contrast, relative product energies appear to be a poor indicator of the product outcome for this system. Hence, theoretical consideration of the reaction chemistry in the vicinity of the site of nucleophilic addition appears to be necessary and sufficient to predict the outcome of the enzyme-mediated GSH-TCE conjugation.

Neurobehavioral and respiratory findings in jet engine repair workers: A comparison of exposed and unexposed volunteers

Workers repairing jet engines had respiratory, rheumatic, and neurobehavioral symptoms. They had welded and ground stainless steel parts using hard metal tools and cleaned metal with chlorinated and fluorinated organic solvents. We compared 154 workers and 112 unexposed subjects, all volunteers of similar ages and with similar educational levels, for abnormalities on chest radiographs, spirometric measurements, and questionnaires. Also appraised were performance of reaction time, balance, blink reflex latency, color discrimination, Culture Fair, vocabulary, slotted pegboard, trail making A and B, profile of mood states (POMS), and frequencies of 35 symptoms. Compared to unexposed subjects, workers had significantly more respiratory symptoms but no differences in pulmonary function. They had significantly prolonged simple and choice reaction time (P<0.0001), and abnormal balance with eyes open and eyes closed (P<0. 0001), and abnormal color discrimination. Blink reflex latency was abnormal in both exposed workers and in local unexposed compared to other reference groups. Focus of the inquiry on lung disease helped ensure that for neurobehavioral tests confounding factors were minimal and known biases were small. We tentatively attribute the neurobehavioral impairments and increased symptom frequencies to chlorinated solvent exposure. Excessive respiratory symptoms are attributed to welding stainless steel combined with cigarette smoking. Specifically, manganese exposure may have affected the respiratory and the central nervous systems.

Physiologically based pharmacokinetic modeling of inhaled trichloroethylene and its oxidative metabolites in B6C3F1 mice

A physiologically based pharmacokinetic (PBPK) model for inhaled trichloroethylene (TCE) was developed for B6C3F1 mice. Submodels described four P450-mediated metabolites of TCE, which included chloral hydrate (CH), free and glucuronide-bound trichloroethanol (TCOH-f and TCOH-b), trichloroacetic acid (TCA), and dichloroacetic acid (DCA). Inhalation time course studies were carried out for calibration of the model by exposing mice to TCE vapor concentrations of either 100 or 600 ppm for 4 h. At several time points, mice were euthanized and blood, liver, kidney, lung, and fat were collected and analyzed for TCE and its oxidative metabolites. Peak blood TCE concentrations were 0.86 and 7.32 microgram/mL, respectively, in mice exposed to 100 and 600 ppm TCE. The model overpredicted the mixed venous blood and tissue concentrations of TCE for mice of both exposure groups. Fractional absorption of inhaled TCE was proposed to explain the discrepancy between the model predictions and the TCE blood time course data. When fractional absorption (53%) of inhaled TCE was incorporated into the model, a comprehensive description of the uptake, distribution, and clearance of TCE in the blood was obtained. Fractional uptake of inhaled TCE was further verified by collecting TCE in exhaled breath following a 4-h constant concentration exposure to TCE and validation was provided by testing the model against TCE blood concentrations from an independent data set. The submodels adequately simulated the distribution and clearance kinetics of CH and TCOH-f in blood and the lungs, TCOH-b in the blood, and TCA and DCA, which were respectively detected for up to 43 and 14 h postexposure in blood and livers of mice exposed to 600 ppm TCE. This is the first extensive tissue time course study of the major metabolites of TCE following an inhalation exposure to TCE and the PBPK model predictions were in good general agreement with the observed kinetics of the oxidative metabolites formed in mice exposed to TCE concentrations of 100 and 600 ppm.

Antagonistic interactions of an arsenic-containing mixture in a multiple organ carcinogenicity bioassay

Inorganic arsenic (As), 1,2-dichloroethane (DCE), vinyl chloride (VC) and trichloroethylene (TCE) are frequently identified as groundwater contaminants near hazardous waste disposal sites. While the carcinogenicity of each of these chemicals has been extensively studied individually, little information exists regarding their carcinogenic potential in combination. Therefore, we investigated the carcinogenic promoting potential of chemical mixtures containing arsenic, DCE, VC and TCE following multiple initiator administration in a multiple organ carcinogenicity bioassay (N. Ito, T. Shirai, S. Fukushima, Medium-term bioassay for carcinogens using multiorgan models, in: N. Ito, H. Sugano (Eds.), Modification of Tumor Development in Rodents, Prog. Exp. Tumor Res., 33, 41-57, Basel, Karger, 1991). Our results reveal a dose-responsive antagonistic effect of this four-chemical mixture on the development of preneoplastic hepatic lesions (altered hepatocellular foci and glutathione S-transferase pi positive foci) as well as bronchioalveolar hyperplasia and adenoma formation.

A human physiologically based pharmacokinetic model for trichloroethylene and its metabolites, trichloroacetic acid and free trichloroethanol

Nine male and eight female healthy volunteers were exposed to 50 or 100 ppm trichloroethylene vapors for 4 h. Blood, urine, and exhaled breath samples were collected for development of a physiologically based pharmacokinetic (PBPK) model for trichloroethylene and its two major P450-mediated metabolites, trichloroacetic acid and free trichloroethanol. Blood and urine were analyzed for trichloroethylene, chloral hydrate, free trichloroethanol and trichloroethanol glucuronide, and trichloroacetic acid. Plasma was analyzed for dichloroacetic acid. Trichloroethylene was also measured in exhaled breath samples. Trichloroethylene, free trichloroethanol, and trichloroacetic acid were found in blood samples of all volunteers and only trace amounts of dichloroacetic acid (4-12 ppb) were found in plasma samples from a few volunteers. Trichloroethanol glucuronide and trichloroacetic acid were found in urine of all volunteers. No chloral hydrate was detected in the volunteers. Gender-specific PBPK models were developed with fitted urinary rate constant values for each individual trichloroethylene exposure to describe urinary excretion of trichloroethanol glucuronide and trichloroacetic acid. Individual urinary excretion rate constants were necessary to account for the variability in the measured cumulative amount of metabolites excreted in the urine. However, the average amount of trichloroacetic acid and trichloroethanol glucuronide excreted in urine for each gender was predicted using mean urinary excretion rate constant values for each sex. A four-compartment physiological flow model was used for the metabolites (lung, liver, kidney, and body) and a six-compartment physiological flow model was used for trichloroethylene (lung, liver, kidney, fat, and slowly and rapidly perfused tissues). Metabolic capacity (Vmaxc) for oxidation of trichloroethylene was estimated to be 4 mg/kg/h in males and 5 mg/kg/h in females. Metabolized trichloroethylene was assumed to be converted to either free trichloroethanol (90%) or trichloroacetic acid (10%). Free trichloroethanol was glucuronidated forming trichloroethanol glucuronide or converted to trichloroacetic acid via back conversion of trichloroethanol to chloral (trichloroacetaldehyde). Trichloroethanol glucuronide and trichloroacetic acid were then excreted in urine. Gender-related pharmacokinetic differences in the uptake and metabolism of trichloroethylene were minor, but apparent. In general, the PBPK models for the male and female volunteers provided adequate predictions of the uptake of trichloroethylene and distribution of trichloroethylene and its metabolites, trichloroacetic acid and free trichloroethanol. The PBPK models for males and females consistently overpredicted exhaled breath concentrations of trichloroethylene immediately following the TCE exposure for a 2- to 4-h period. Further research is needed to better understand the biological determinants responsible for the observed variability in urinary excretion of trichloroethanol glucuronide and trichloroacetic acid and the metabolic pathway resulting in formation of dichloroacetic acid.

Glutathione-dependent bioactivation of haloalkenes

Several halogenated alkenes are nephrotoxic in rodents. A mechanism for the organ-specific toxicity of these compounds to the kidney has been elucidated. The mechanism involves hepatic glutathione conjugation to dihaloalkenyl or 1,1-difluoroalkyl glutathione S-conjugates, which are cleaved by gamma-glutamyltransferase and dipeptidases to cysteine S-conjugates. Haloalkene-derived cysteine S-conjugates may have four fates in the organism: (a) They may be substrates for renal cysteine conjugate beta-lyases, which cleave them to form reactive intermediates identified as thioketenes (chloroalkene-derived S-conjugates), thionoacyl halides (fluoroalkene-derived S-conjugates not containing bromide), thiiranes, and thiolactones (fluoroalkene-derived S-conjugates containing bromine); (b) cysteine S-conjugates may be N-acetylated to excretable mercapturic acids; (c) they may undergo transamination or oxidation to the corresponding 3-mercaptopyruvic acid S-conjugate; (d) finally, oxidation of the sulfur atom in halovinyl cysteine S-conjugates and corresponding mercapturic acids forms Michael acceptors and may also represent a bioactivation reaction. The formation of reactive intermediates by cysteine conjugate beta-lyase may play a role in the target-organ toxicity and in the possible renal tumorigenicity of several chlorinated olefins widely used in many chemical processes.

Covalent binding of trichloroethylene to proteins in human and rat hepatocytes

The environmental contaminant and occupational solvent trichloroethylene is metabolized to a reactive intermediate that covalently binds to specific hepatic proteins in exposed mice and rats. In order to compare covalent binding between humans and rodents, primary hepatocyte cultures were exposed to vaporized trichloroethylene at 0-10,000 parts per million for up to 2 h. Immunochemical detection of three major dose- and time-dependent trichloroethylene protein adducts at 50, 52 and 100 kDa was demonstrated in the rat hepatocytes, while a single, distinctively different 47 kDa adduct was detected in human hepatocytes. The 50 kDa adduct in rat hepatocytes was found to comigrate on SDS-PAGE with cytochrome P450 2E1 (CYP2E1), while the adduct found in humans did not comigrate with CYP2E1. These data show that reactive metabolites of trichloroethylene can be formed in human and rat hepatocytes and bind covalently to discrete hepatic proteins, and suggests that in rats, but not humans, that one of the targets is CYP2E1.

A physiologically based pharmacokinetic model for trichloroethylene and its metabolites, chloral hydrate, trichloroacetate, dichloroacetate, trichloroethanol, and trichloroethanol glucuronide in B6C3F1 mice

A six-compartment physiologically based pharmacokinetic (PBPK) model for the B6C3F1 mouse was developed for trichloroethylene (TCE) and was linked with five metabolite submodels consisting of four compartments each. The PBPK model for TCE and the metabolite submodels described oral uptake and metabolism of TCE to chloral hydrate (CH). CH was further metabolized to trichloroethanol (TCOH) and trichloroacetic acid (TCA). TCA was excreted in urine and, to a lesser degree, metabolized to dichloroacetic acid (DCA). DCA was further metabolized. The majority of TCOH was glucuronidated (TCOG) and excreted in the urine and feces. TCOH was also excreted in urine or converted back to CH. Partition coefficient (PC) values for TCE were determined by vial equilibrium, and PC values for nonvolatile metabolites were determined by centrifugation. The largest PC values for TCE were the fat/blood (36.4) and the blood/air (15.9) values. Tissue/blood PC values for the water-soluble metabolites were low, with all PC values under 2.0. Mice were given bolus oral doses of 300, 600, 1200, and 2000 mg/kg TCE dissolved in corn oil. At various time points, mice were sacrificed, and blood, liver, lung, fat, and urine were collected and assayed for TCE and metabolites. The 1200 mg/kg dose group was used to calibrate the PBPK model for TCE and its metabolites. Urinary excretion rate constant values were 0. 06/hr/kg for CH, 1.14/hr/kg for TCOH, 32.8/hr/kg for TCOG, and 1. 55/hr/kg for TCA. A fecal excretion rate constant value for TCOG was 4.61/hr/kg. For oral bolus dosing of TCE with 300, 600, and 2000 mg/kg, model predictions of TCE and several metabolites were in general agreement with observations. This PBPK model for TCE and metabolites is the most comprehensive PBPK model constructed for P450-mediated metabolism of TCE in the B6C3F1 mouse.

Detection of trichloroethylene-protein adducts in rat liver and plasma

Trichloroethylene is an industrial chemical with widespread occupational exposure and is a major environmental contaminant. In a Western blot using antiserum that recognizes trichloroethylene covalently bound to protein, a single 50 kDa microsomal adduct was detected in the livers of trichloroethylene-treated Sprague-Dawley rats. To determine if trichloroethylene-protein adducts could be detected in blood, plasma proteins were immunoaffinity purified using an antidichloroacetyl column. A single 50 kDa protein was detected in the affinity-purified fraction in a Western blot using dichloroacetyl antiserum. This protein was also immunochemically reactive with anti-cytochrome P450 2E1 antibodies. The 50 kDa trichloroethylene-protein adduct may be formed in the liver and released into the blood following exposure to trichloroethylene. The significance of adduct formation with respect to trichloroethylene toxicity remains to be established; however, the data suggest that this approach may be useful in the investigation of trichloroethylene-protein adducts and adverse effects following exposure.