Hepatotoxicity Of Trichloroethylene Research Articles

Trichloroethylene injures rat liver and elevates the level of peroxisomal bifunctional enzyme (Ehhadh)

Trichloroethylene (TCE) is a common industrial solvent and an occupational toxicant. TCE exposure can cause severe hepatotoxicity, but its mode of action is poorly understood. Many studies have shown TCE-induced liver damage in mice, while few have examined the effects of TCE in rats. To explore the effects of TCE in Sprague–Dawley (SD) rats and the potential mechanisms in TCE-induced hepatocytotoxicity. The liver index and activities of liver damage marker enzymes (ALT, AST and ALP) in rat serum were elevated along with the increase in TCE dose, while the levels of total proteins and albumin in serum were reduced. The results suggest that TCE is hepatotoxic in rats. 2D-DIGE electrophoresis showed that the levels of 66 liver proteins in TCE-treated rats were abnormally altered (39 up-regulated and 27 down-regulated). In these proteins, six enzymes were related to liver damage and carcinogenesis as indicated by bioinformatics analysis, and Western blot analysis confirmed the alterations of three of them, i.e., aldehyde dehydrogenase 2 (Aldh2), glutathione S-transferase Mu 1 (Gstm1) and peroxisomal bifunctional enzyme (PBE, also named as Ehhadh). PBE was the only protein elevated in a dose dependent manner. Whether PBE can be a biomarker of TCE hepatotoxicity needs to be further studied. These findings indicate that TCE induces liver injury in rats.

Differential expression profile of membrane proteins in L-02 cells exposed to trichloroethylene.

Trichloroethylene (TCE), a halogenated organic solvent widely used in industries, is known to cause severe hepatotoxicity. However, the mechanisms underlying TCE hepatotoxicity are still not well understood. It is predicted that membrane proteins are responsible for key biological functions, and recent studies have revealed that TCE exposure can induce abnormal levels of membrane proteins in body fluids and cultured cells. The aim of this study is to investigate the TCE-induced alterations of membrane proteins profiles in human hepatic L-02 liver cells. A comparative membrane proteomics analysis was performed in combination with two-dimensional fluorescence difference gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry. A total of 15 proteins were identified as differentially expressed (4 upregulated and 11 downregulated) between TCE-treated cells and normal controls. Among this, 14 of them are suggested as membrane-associated proteins by their transmembrane domain and/or subcellular location. Furthermore, the differential expression of β subunit of adenosine triphosphate synthase (ATP5B) and prolyl 4-hydroxylase, β polypeptide (P4HB) were verified by Western blot analysis in TCE-treated L-02 cells. Our work not only reveals the association between TCE exposure and altered expression of membrane proteins but also provides a novel strategy to discover membrane biomarkers and elucidate the potential mechanisms involving with membrane proteins response to chemical-induced toxic effect.

Phosphoproteomic analyses of L-02 liver cells exposed to trichloroethylene

Trichloroethylene (TCE) is an environmental and occupational toxicant that has been shown to cause serious hepatotoxicity. However, the mechanisms underlying the hepatotoxicity of TCE remain unclear. Previously, we identified several apoptosis-related proteins in TCE-induced hepatic cytotoxicity. This study is aimed to analyze the changes in phosphoproteins in L-02 liver cells exposed to TCE using iTRAQ labeling, IMAC enrichment and LC–MS/MS. We identified 1878 phosphorylation sites in 107 proteins and found that 20 sites in 16 phosphoproteins were differentially phosphorylated in L-02 cells after TCE treatment. Among these phosphoproteins, 20% were protein localization and formation processes-related proteins, 38% were metabolism-related proteins and 42% were cellular process-related proteins, including transcriptional regulation and biogenesis. Moreover, two phosphoproteins, 4E-BP1 (37T) and MCM2 (139S), were validated as TCE-induced alteration of phosphorylation at specific sites by Western-blot analysis. Taken together, our study demonstrated that TCE exposure changed the levels of multiple phosphoproteins in L-02 liver cells, and the functional analysis suggested that these differentially expressed phosphoproteins might be involved in TCE-induced hepatic cytotoxicity.

Effects of trichloroethylene on hepatotoxicity in cytochrome 2E1-silenced hepatocytes

To prepare cytochrome (CYP)2E1-silenced hepatocytes by lentivirus-mediated RNA interference technology and to investigate the hepatotoxicity of trichloroethylene (TCE) in CYP2E1-silenced hepatocytes. Short hairpin RNA fragments were designed and synthesized and were then ligated into the lentiviral vector; single colonies were screened; the plasmid was extracted after PCR and sequence identification and then transferred into L02 hepatocytes; the CYP2E1-silenced hepatocytes were selected; real-time quantitative PCR and Western blot were used to evaluate the interference effects. The obtained CYP2E1-silenced hepatocytes, as well as normal L02 hepatocytes, were treated with TCE (0, 0.25, 0.50, 1.00, 2.00, and 4.00 mmol/L). The cell viability and half maximal inhibitory concentration (IC50) of TCE were measured; the apoptotic rate of cells was measured by flow cytometry; the mRNA expression levels of apoptosis genes and oncogenes were measured by real-time quantitative PCR. The IC50s of TCE for L02 hepatocytes and CYP2E1-silenced hepatocytes were 15.1 mmol/L and 23.6 mmol/L, respectively. The apoptotic rate increased as the dose of TCE rose in the two types of cells; the CYP2E1-silenced hepatocytes hada significantly lower apoptotic rate than L02 hepatocytes when they were exposed to 2.0 and 4.0 mmol/L TCE (P < 0.05 or P < 0.01). The mRNA expression level of bcl-2 (anti-apoptosis gene) in CYP2E1-silenced hepatocytes was 15% ∼ 60% higher than that in L02 hepatocytes (P < 0.01), while the mRNA expression levels of caspase-3 and caspase-9 (apoptosis genes) in CYP2E1-silenced hepatocytes were 30% ∼ 60% lower than those in L02 hepatocytes (P < 0.01). The mRNA expression level of p53 (cancer suppressor gene) in CYP2E1-silenced hepatocytes was 81 - 278% higher than that in L02 hepatocytes (P < 0.01), while the mRNA expression levels of c-fos and k-ras (oncogenes) in CYP2E1-silenced hepatocytes were 20-68% lower than those in L02 hepatocytes (P < 0.01). CYP2E1-silenced cells can be successfully prepared by lentivirus-mediated RNA interference technology. Silencing CYP2E1 gene can reduce the hepatotoxicity of TCE and inhibit the expression of some apoptosis genes and oncogenes, suggesting that CYP2E1 gene plays an important role in TCE metabolism and is related to the hepatotoxicity of TCE.

Proteomic analysis of trichloroethylene-induced alterations in expression, distribution, and interactions of SET/TAF-Iα and two SET/TAF-Iα-binding proteins, eEF1A1 and eEF1A2, in hepatic L-02 cells

Emerging evidence indicates that trichloroethylene (TCE) exposure causes severe hepatotoxicity. However, the mechanisms of TCE hepatotoxicity remain unclear. Recently, we reported that TCE exposure up-regulated the expression of the oncoprotein SET/TAF-Iα and SET knockdown attenuated TCE-induced cytotoxicity in hepatic L-02 cells. To decipher the function of SET/TAF-Iα and its contributions to TCE-induced hepatotoxicity, we employed a proteomic analysis of SET/TAF-Iα with tandem affinity purification to identify SET/TAF-Iα-binding proteins. We identified 42 novel Gene Ontology co-annotated SET/TAF-Iα-binding proteins. The identifications of two of these proteins (eEF1A1, elongation factor 1-alpha 1; eEF1A2, elongation factor 1-alpha 2) were confirmed by Western blot analysis and co-immunoprecipitation (Co-IP). Furthermore, we analyzed the effects of TCE on the expression, distribution and interactions of eEF1A1, eEF1A2 and SET in L-02 cells. Western blot analysis reveals a significant up-regulation of eEF1A1, eEF1A2 and two isoforms of SET, and immunocytochemical analysis reveals that eEF1A1 and SET is redistributed by TCE. SET is redistributed from the nucleus to the cytoplasm, while eFE1A1 is translocated from the cytoplasm to the nucleus. Moreover, we find by Co-IP that TCE exposure significantly increases the interaction of SET with eEF1A2. Our data not only provide insights into the physiological functions of SET/TAF-Iα and complement the SET interaction networks, but also demonstrate that TCE exposure induces alterations in the expression, distribution and interactions of SET and its binding partners. These alterations may constitute the mechanisms of TCE cytotoxicity.

Lentivirus-mediated silencing of I2PP2A through RNA interference attenuates trichloroethylene-induced cytotoxicity in human hepatic L-02 cells

Trichloroethylene (TCE) is a common chemical pollutant that exists in air, soil, and drinking water. TCE exposure is known to cause severe hepatotoxicity; however, the mechanisms underlying TCE hepatotoxicity remain poorly understood. In a previous proteomics study, we found that TCE exposure up-regulated the expression of the inhibitor 2 of protein phosphatase 2A (I2PP2A), a potent and specific endogenous inhibitor of protein phosphatase (PP) 2A, in human hepatic L-02 cells. Here, we employed lentivirus-mediated RNA interference (RNAi) to knock down I2PP2A expression in L-02 cells and explored the potential role of I2PP2A in TCE-induced cytotoxicity. We found that TCE treatment of L-02 cells causes decreased cell viability, increased apoptosis and elevated I2PP2A mRNA and protein levels. TCE-treated L-02 cells were also found to have significantly reduced PP2A activity. Lentivirus-mediated I2PP2A knockdown partially prevented the decrease in viability and increased apoptosis induced by TCE treatment. Knockdown of I2PP2A in TCE-treated L-02 cells also suppressed the inhibition of PP2A activity and prevented caspase-3 activation. These data for the first time demonstrate that the up-regulation of I2PP2A could mediate, at least in part, TCE-induced liver cell toxicity through the inhibition of PP2A activity and caspase-3-mediated pathway, and suggest that I2PP2A may play a crucial role in mediating TCE hepatotoxicity.

Body weight and the risk of breast cancer in BRCA1/2 families – the GEO-HEBON study

Emerging evidence indicates that trichloroethylene (TCE) exposure causes severe hepatotoxicity. However, the mechanisms of TCE hepatotoxicity remain unclear. Recently, we reported that TCE exposure up-regulated the expression of the oncoprotein SET/TAF-Iα and SET knockdown attenuated TCE-induced cytotoxicity in hepatic L-02 cells. To decipher the function of SET/TAF-Iα and its contributions to TCE-induced hepatotoxicity, we employed a proteomic analysis of SET/TAF-Iα with tandem affinity purification to identify SET/TAF-Iα-binding proteins. We identified 42 novel Gene Ontology co-annotated SET/TAF-Iα-binding proteins. The identifications of two of these proteins (eEF1A1, elongation factor 1-alpha 1; eEF1A2, elongation factor 1-alpha 2) were confirmed by Western blot analysis and co-immunoprecipitation (Co-IP). Furthermore, we analyzed the effects of TCE on the expression, distribution and interactions of eEF1A1, eEF1A2 and SET in L-02 cells. Western blot analysis reveals a significant up-regulation of eEF1A1, eEF1A2 and two isoforms of SET, and immunocytochemical analysis reveals that eEF1A1 and SET is redistributed by TCE. SET is redistributed from the nucleus to the cytoplasm, while eFE1A1 is translocated from the cytoplasm to the nucleus. Moreover, we find by Co-IP that TCE exposure significantly increases the interaction of SET with eEF1A2. Our data not only provide insights into the physiological functions of SET/TAF-Iα and complement the SET interaction networks, but also demonstrate that TCE exposure induces alterations in the expression, distribution and interactions of SET and its binding partners. These alterations may constitute the mechanisms of TCE cytotoxicity.

Evaluating the Relationship Between Variance in Enzyme Expression and Toxicant Concentration in Health Risk Assessment

ABSTRACT The replacement of default uncertainty factors with those based on chemical-specific data is a topic of interest to a growing number of government-based organizations and those in affiliated professional societies. The division of the uncertainty factors for animal-to-human extrapolation and human interindividual variance (UFA and UFH, respectively) into their pharmacodynamic (PD) and pharmacokinetic (PK) components invites additional and specific considerations. Where data are available, or substantiated PK models have been developed, the animal-to-human chemical-specific differences have been quantified and utilized to replace the PK component of the uncertainty factor. The increasing degree to which the genome is being characterized has stimulated additional interest in describing the impact of genetic polymorphisms on susceptibility. Frequently, proteins for which the genes are being evaluated are the group of xenobiotic metabolizing enzymes. In-depth understanding of the genetic polymorphisms of genes coding for Aldehyde dehydrogenase, glucuronyl transferase and cytochrome P450 enzyme forms has been combined with information on the bioactivation or detoxication of environmental contaminants. The preliminary conclusion of some of these considerations is that alterations in enzyme content or enzyme activity result in a de facto alteration of risk. While this may be true of the “all-or-none” genetic alterations, the impact of more subtle changes in enzyme content and/or activity are more difficult to predict. The hepatotoxicity of trichloroethylene (TCE) is dependent upon an initial, cytochrome P450 2E1 (CYP2E1)-mediated oxidative step. Variance of CYP2E1 content of human liver has been characterized from a bank of tissues from human organ donors and combined with data describing the in vitro Michaelis-Menten kinetic parameters in order to extrapolate the metabolic capacity (and variance thereof) from in vitro to in vivo and assess its impact on PK through incorporation in a physiologically based pharmacokinetic (PBPK) model. This presentation summarizes that work, and demonstrates and discusses why extremes of CYP2E1-mediated metabolic capacity in adult humans has virtually no impact on the PK metric most closely related to hepatotoxic injury from TCE exposure.

The Impact of Cytochrome P450 2E1‐Dependent Metabolic Variance on a Risk‐Relevant Pharmacokinetic Outcome in Humans

Risk assessments include assumptions about sensitive subpopulations, such as the fraction of the general population that is sensitive and the extent that biochemical or physiological attributes influence sensitivity. Uncertainty factors (UF) account for both pharmacokinetic (PK) and pharmacodynamic (PD) components, allowing the inclusion of risk-relevant information to replace default assumptions about PK and PD variance (uncertainty). Large numbers of human organ donor samples and recent advances in methods to extrapolate in vitro enzyme expression and activity data to the intact human enable the investigation of the impact of PK variability on human susceptibility. The hepatotoxicity of trichloroethylene (TCE) is mediated by acid metabolites formed by cytochrome P450 2E1 (CYP2E1) oxidation, and differences in the CYP2E1 expression are hypothesized to affect susceptibility to TCE's liver injury. This study was designed specifically to examine the contribution of statistically quantified variance in enzyme content and activity on the risk of hepatotoxic injury among adult humans. We combined data sets describing (1) the microsomal protein content of human liver, (2) the CYP2E1 content of human liver microsomal protein, and (3) the in vitro Vmax for TCE oxidation by humans. The 5th and 95th percentiles of the resulting distribution (TCE oxidized per minute per gram liver) differed by approximately sixfold. These values were converted to mg TCE oxidized/h/kg body mass and incorporated in a human PBPK model. Simulations of 8-hour inhalation exposure to 50 ppm and oral exposure to 5 micro g TCE/L in 2 L drinking water showed that the amount of TCE oxidized in the liver differs by 2% or less under extreme values of CYP2E1 expression and activity (here, selected as the 5th and 95th percentiles of the resulting distribution). This indicates that differences in enzyme expression and TCE oxidation among the central 90% of the adult human population account for approximately 2% of the difference in production of the risk-relevant PK outcome for TCE-mediated liver injury. Integration of in vitro metabolism information into physiological models may reduce the uncertainties associated with risk contributions of differences in enzyme expression and the UF that represent PK variability.

The Effects of Diethyldithiocarbamate on the Metabolism and Hepatotoxicity of Trichloroethylene

Objectives: The purpose of this study was to evaluate the trichloroethylene (TCE) metabolism, acute toxicity, and the effects of diethyldithiocarbamate (DDTC) on the acute toxicity in TCE-intoxicated rats. Methods: TCE was administered orally at doses of 600, 1,200 and 2,400 mg/kg of body weight following pretreatment with either saline or 500 mg/kg of DDTC. 12 hours after administration of TCE, the concentrations of TCE, trichloroacetic acid (TCA) and trichloroethanol (TCEOH) in the blood and solid organs, and the histopathological changes in each organ were examined. Results: The level of CYP2E1 markedly decreased in the DDTC-pretreated groups. The CYP2E1 content in the TCE-treated rats increased in a dose-dependent manner. The concentrations of TCE and TCEOH were highest in the liver, and the level of TCA was highest in the blood. The DDTC-pretreated rats had a markedly increased level of TCE and decreased levels of TCA and TCEOH, than the rats pretreated with saline. These findings indicated that CYP2E1 was important in the metabolism of TCE. From the histopathological findings, centrilobular necrosis was observed in the livers of the TCE-treated rats, but no significant change was found in those rats pretreated with DDTC. Conclusions: DDTC is considered to be effective in protecting TCE-induced hepatic damage because it inhibits the TCE metabolism.

Tissue Repair Response as a Function of Dose during Trichloroethylene Hepatotoxicity

Trichloroethylene (TCE), a widely used organic solvent and degreasing agent, is regarded as a hepatotoxicant. The objective of the present studies was to investigate whether the extent and timeliness of tissue repair has a determining influence on the ultimate outcome of hepatotoxicity. Male Sprague-Dawley rats (200-250 g) were injected with a 10-fold dose range of TCE and hepatotoxicity and tissue repair were studied during a time course of 0 to 96 h. Light microscopic changes as evaluated by H&E-stained liver sections revealed a dose-dependent necrosis of hepatic cells. Maximum liver cell necrosis was observed at 48 h after the TCE administration. However, liver injury as assessed by plasma sorbitol dehydrogenase (SDH) showed a dose response over a 10-fold dose range only at 6 h, whereas alanine aminotransferase (ALT) did not show a dose response at any of the time points studied. A low dose of TCE (250 mg/kg) showed an increase in SDH at all time points up to 96 h without peak levels, whereas higher doses showed peak only at 6 h. At later time points SDH declined but remained above normal. In vitro addition of trichloroacetic acid, a metabolite of TCE to plasma, decreased the activities of SDH and ALT indicating that metabolites formed during TCE toxicity may interfere with plasma enzyme activities in vivo. This indicates that the lack of dose-related increase in SDH and ALT activities may be because of interference by the TCE metabolite. Tissue regeneration response as measured by [3H]thymidine incorporation into hepatocellular nuclear DNA was stimulated maximally at 24 h after 500 mg/kg TCE administration. A higher dose of TCE led to a delay and diminishment in [3H]thymidine incorporation. At a low dose of TCE (250 mg/kg) [3H]thymidine incorporation peaked at 48 h and this could be attributed to very low or minimal injury caused by this dose. With higher doses tissue repair was delayed and attenuated allowing for unrestrained progression of liver injury. These results support the concept that the toxicity and repair are opposing responses and that a dose-related increase in tissue repair represents a dynamic, quantifiable compensatory mechanism.

Tissue Repair Response as a Function of Dose during Trichloroethylene Hepatotoxicity

Tissue Repair Response as a Function of Dose during Trichloroethylene Hepatotoxicity

Toxicity and metabolism of trichloroethylene in rat hepatocytes.

Trichloroethylene (TCE) hepatotoxicity is controversial. The present study was designed to investigate the mechanism of TCE hepatotoxicity. The toxicity of equimolar concentrations (5.7 mM) of TCE and its two major metabolites, trichloroacetic acid (TCA) and trichloroethanol (TCL), was determined. TCE cell viability was dose- and time-dependent. Enzyme leakage correlated with decrease in cell viability; significantly increased leakage started at 1.9 mM treatment. 5.7 mM TCA or TCL was not toxic compared with the same dose of TCE. Hepatocytes isolated from phenobarbital pretreated rats exhibited no significant alteration in toxicity parameters when exposed to 1.9 and 5.7 mM concentrations of TCE. While the phenobarbital pretreatment increased the rate of metabolism of TCE. The present study suggests that TCE toxicity occurs before the formation of TCA and TCL.

Effect of cimetidine on hepatic biochemical changes, liver toxicity and major urinary metabolite excretion of trichloroethylene in rats.

The effects of cimetidine (CIM) (an inhibitor of the hepatic microsomal monooxygenase system) on the metabolism and hepatotoxicity of trichloroethylene (TRI) were studied in male Sprague-Dawley rats. Rats were given three doses of 120 mg/kg i.p. (low-dose regimen) of CIM at 0, 6 and 11 h for 1 day, or ten doses of 200 mg/kg (high-dose regimen) at 8, 11, 14 and 17 h for 2 days and 8 and 11 h on 3rd day. Trichloroethylene (0.5 or 0.65 ml/kg) was administered i.p. 1 h after 2nd dose (low-dose regimen) or 9th dose (high-dose regimen) of CIM. In the low-dose regimen study, the activity of hepatic microsomal aminopyrine N-demethylase was decreased 1 and 5 h after the second dose and 7 h after the third dose of CIM, but became normal 20 h after the last dose. The cytochrome P-450 content and the activities of aniline hydroxylase and epoxide hydratase remained unchanged. Trichloroethylene at both dose levels produced liver toxicity, as verified by increase in activities of SDH and SGPT as well as by liver histology. Cimetidine alone had no such effect. An apparent reduction in TRI toxicity by CIM (at both dose regimens) could be observed histologically. The biochemical tests (SDH and SGPT) corroborated the histological changes only when TRI was given at a dose of 0.5 ml/kg combined with a high-dose regimen of CIM. Cimetidine at both dose regimens had a tendency to decrease the in vivo metabolism of TRI.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of pretreatment with cimetidine or phenobarbital on lipoperoxidation in carbon tetrachloride- and trichloroethylene-dosed rats

Lipid peroxidation (LP) in vivo as reflected by the exhalation of ethane and n-pentane and by thiobarbituric acid reactive substances (TBARS) in liver microsomes was studied in rats injected with carbon tetrachloride (CCl 4) and trichloroethylene (TCE), each at 2 dose levels. Interactions between these chlorinated solvents and cimetidine (CM), an inhibitor of cytochrome P-450-dependent monooxygenases, or phenobarbital (PB) the well known inducer of microsomal enzyme activities were also assessed. A non-hepatotoxic dose of CCl 4 did not cause a significant increase in ethane production except in PB-induced rats but did enhance n-pentane elimination, whereas an hepatotoxic dose increased the emission of both hydrocarbons. No interactions between CM and CCl 4 could be shown but, as expected, PB potentiated the effect of CCl 4. TCE administration led to a moderate dose-dependent elevation of n-pentane production but did not affect that of ethane and the effect of TCE was smaller in PB-induced than in CM- or non-pretreated rats. There was no difference in microsomal TBARS content in rats injected with the chlorinated hydrocarbons. The use of butylated hydroxytoluence (BHT) and ethylene diaminetetraacetic acid (EDTA) revealed that direct measurements of TBARS gave inadequate results due to substantial chemical LP in vitro during the whole procedure. With the “ethane-pentane test” it was established that: (i) CM cannot prevent CCl 4-induced LP; and (ii) TCE hepatotoxicity does not involve increased LP of membrane lipids.

Delineation of the role of metabolism in the hepatotoxicity of trichloroethylene and perchloroethylene: A dose-effect study

The relationships among dose, metabolism and hepatotoxicity in mice which resulted from subchronic exposure to the chlorinated solvents trichloroethylene (TRI) and perchloroethylene (PER) were examined. Male Swiss-Cox mice received either TRI (0 to 3200 mg/kg/day) or PER (0 to 2000 mg/kg/day) in corn oil by gavage for 6 weeks. Urinary metabolites from individual mice were quantified to estimate the extent to which each compound was metabolized. Four parameters of hepatotoxicity were assessed: liver weight, triglycerides, glucose-6-phosphatase (G6P) activity, and SGPT activity, TRI significantly affected liver weight and G6P activity; PER affected all four parameters. The metabolism of TRI was linearly related to dose through 1600 mg/kg, but then became saturated. The metabolism of PER was saturable. The dose-effect curves of the affected hepatotoxicity parameters of both compounds were nonlinear and resembled the dose-metabolism graph of the corresponding solvent. Plots of the hepatotoxicity data of each compound against total urinary metabolites were linear in all cases, suggesting that the hepatotoxicity of both PER and TRI in mice is directly related to the extent of their metabolism. This pattern is consistent with formation of the toxic intermediate in the primary metabolic pathway of each compound.

Hepatotoxicity of Trichloroethylene-Carbon Tetrachloride Mixtures in Rats

Liver histology was normal 24 h after the administration of trichloroethylene (1 ml . kg-1) in rats. It was normal, or showed necrosis of a few hepatocytes, after the administration of carbon tetrachloride (64 microliters . kg-1). In rats receiving both solvents, there was extensive centrilobular necrosis. In vitro, trichloroethylene did not initiate lipid peroxidation but potentiated that initiated by carbon tetrachloride; a similar potentiating effect was observed for a wide range of trichloroethylene concentrations (0.19-12 mM). In vivo, a wide range of trichloroethylene doses (0.064-1 ml . kg-1) similarly potentiated the hepatotoxicity of carbon tetrachloride. Administration of trichloroethylene (1 ml . kg-1), 5 h earlier, increased carbon tetrachloride-induced lipid peroxidation in vitro, and increased the hepatotoxicity of a subsequent dose of carbon tetrachloride (64 microliters . kg-1). Previous administration of carbon tetrachloride failed to modify lipid peroxidation and to increase the hepatotoxicity of trichloroethylene. We conclude that trichloroethylene potentiates the hepatotoxicity of carbon tetrachloride, possibly by increasing carbon tetrachloride-induced lipid peroxidation.

Metabolic activation and hepatotoxicity of trichloroethylene.

When we began our studies about six years ago on the relationship between the metabolism and hepatotoxicity of trichloroethylene, there seemed to be a great deal more to know about its mechanism of metabolism than about its hepatotoxicity. Knowledge about its metabolism began with the detection by Powell in 1945 of an oxidized metabolite of trichloroethylene with three chlorines on one carbon in human urine. On this basis, she suggested that the metabolism of trichloroethylene proceeds via an unstable epoxide intermediate. Subsequently Daniel (1963) demonstrated that the oxidation of tri­chloroethylene to trichloroethanol and trichloroacetic acid proceeds without loss of labeled chlorine, which supported an epoxidation mechanism of metabolism followed by an intramolecular chlorine shift. Metabolic investigations by Leibman (1965) identified the liver mixed-function oxidases as the enzyme system primarily responsible for the initial oxidation of trichloroethylene. Other enzyme systems, including those in the adjacent cytoplasm, were found to participate in subsequent hydration, oxidation, reduction, and/or conjugation of secondary metabolites (Byington and Leibman, 1965; Muller et al., 1975). Figure 1 summarizes probable and potential pathways of trichloroethylene biotransformation.

Enhancement of the metabolism and hepatotoxicity of trichloroethylene and perchloroethylene

Trichloroethylene anesthesia (1% for 2 hr) caused acute hepatic injury in rats pretreated with five different inducers of the hepatic mixed function oxidase system, phenobarbital, Aroclor 1254, hexachlorobenzene, 3-methylcholanthrene and pregnenolone-16-α-carbonitrile. Injury did not occur after trichloroethylene in rats pretreated with spironalactone or controls given vehicle alone. Morphologic liver injury was most severe in the phenobarbital- and Aroclor 1254-pretreated animals and was accompanied by marked perturbations in liver electrolyte content and more than 20-fold elevations in serum transaminase. Extent of serum transaminase elevation appears to relate directly to prolongation of anesthesia recovery time and the enhanced urinary excretion of trichlorinated metabolites. Metabolism of perchloroethylene (7.5 m-moles/kg, p.o.) was increased 5- and 7-fold, respectively, in phenobarbital- and Aroclor 1254-pretreated animals, but liver injury after perchloroethylene appeared only in Aroclor 1254 animals. Differential induction of various components of the microsomal mixed function oxidase system was quantified in parallel experiments using animals similarly pretreated with isomolar doses of the six inducers and the vehicle control and sacrificed at times corresponding to onset of chloroethylene exposure. Magnitude of induction of cytochrome P-450 among these seven groups of animals correlates with the mean extent of trichloroethylene-induced liver injury as quantitated by serum transaminases level ( r = 0.95), with prolongation of anesthesia recovery time ( r = 0.95 Z) and with enhanced urinary excretion of trichlorinated metabolites ( r = 0.88).

1188. Low hepatotoxicity of trichloroethylene confirmed biochemically

1188. Low hepatotoxicity of trichloroethylene confirmed biochemically