Mechanism of S-(1,2-dichlorovinyl)glutathione-induced nephrotoxicity

Extracellular Regucalcin: A Potent Suppressor in the Cancer Cell Microenvironment

The regucalcin gene is located on the X chromosome, comprising seven exons and six introns. This gene and protein are expressed in various tissues and cells and is predominantly expressed in human liver, kidney, and adrenal tissues. Regucalcin gene expression is enhanced via a mechanism mediated by several signaling molecules and transcription factors. Regucalcin plays a multifunctional role in cellular regulation in maintaining cell homeostasis. In addition, regucalcin has been implicated in several metabolic disorders and diseases. In particular, regucalcin plays a role as a novel suppressor in several types of cancer patients. Increased expression of regucalcin suppresses the growth of human cancer cells, suggesting its pivotal role in suppressing tumor development. The survival time of cancer patients is prolonged with increased expression of regucalcin in the tumor tissues. The adhesion, migration, invasion, and bone metastatic activity of cancer cells are blocked by the overexpression of regucalcin, promoting dormancy in cancer patients. Interestingly, regucalcin is also found in human serum, suggesting its character as a novel biomarker in various diseases. This extracellular regucalcin has been shown to suppress human cancer cells' growth and bone metastatic activity. Thus, extracellular regucalcin may play a vital role as a suppressor of human cancer activity. Alteration of the serum regucalcin levels in physiological and pathophysiological conditions may influence the activity of cancer cells in the microenvironment. This review will discuss the potential role of extracellular regucalcin in cancer cell activity as a critical suppressor in the cancer microenvironment.

A Novel Assay Method to Determine the β-Elimination of Se-Methylselenocysteine to Monomethylselenol by Kynurenine Aminotransferase 1

Kynurenine aminotransferase 1 (KYAT1 or CCBL1) plays a major role in Se-methylselenocysteine (MSC) metabolism. It is a bi-functional enzyme that catalyzes transamination and beta-elimination activity with a single substrate. KYAT1 produces methylselenol (CH₃SeH) via β-elimination activities with MSC as a substrate. This methylated selenium compound is a major cytotoxic selenium metabolite, causing apoptosis in a wide variety of cancer cells. Methylselenol is volatile and possesses extraordinary nucleophilic properties. We herein describe a simple spectrophotometric assay by combining KYAT1 and thioredoxin reductase (TrxR) to detect CH₃SeH in a coupled activity assay. The metabolite methylselenol and its oxidized form from MSC metabolism is utilized as a substrate for TrxR1 and this can be monitored spectroscopically at 340 nm. Our results show the feasibility of monitoring the β-elimination of KYAT1 by our assay and the results were compared to the previously described β-elimination assays measuring pyruvate. By using known inhibitors of KYAT1 and TrxR1, we further validated the respective reaction. Our data provide a simple but accurate method to determine the β-elimination activity of KYAT1, which is of importance for mechanistic studies of a highly interesting selenium compound.

Toxicity mechanism-based prodrugs: glutathione-dependent bioactivation as a strategy for anticancer prodrug design

INTRODUCTION

6-Mercaptopurine (6-MP) and 6-thioguanine (6-TG), two anticancer drugs, have high systemic toxicity due to a lack of target specificity. Therefore, increasing target selectivity should improve drug safety. Areas covered: The authors examined the hypothesis that new prodrug designs based upon mechanisms of kidney-selective toxicity of trichloroethylene would reduce systemic toxicity and improve selectivity to kidney and tumor cells. Two approaches specifically were investigated. The first approach was based upon bioactivation of trichloroethylene-cysteine S-conjugate by renal cysteine S-conjugate β-lyases. The prodrugs obtained were kidney-selective but exhibited low turnover rates. The second approach was based on the toxic mechanism of trichloroethylene-cysteine S-conjugate sulfoxide, a Michael acceptor that undergoes rapid addition-elimination reactions with biological thiols. Expert opinion: Glutathione-dependent Michael addition-elimination reactions appear to be an excellent strategy to design highly efficient anticancer drugs. Targeting glutathione could be a promising approach for the development of anticancer prodrugs because cancer cells usually upregulate glutathione biosynthesis and/or glutathione S-transferases expression.

Metabolism of Glutathione S-Conjugates: Multiple Pathways

Many potentially toxic electrophilic xenobiotics and some endogenous compounds are detoxified by conversion to the corresponding glutathione S-conjugate, which is metabolized to the N-acetylcysteine S-conjugate (mercapturate) and excreted. Some mercapturate pathway components, however, are toxic. Bioactivation (toxification) may occur when the glutathione S-conjugate (or mercapturate) is converted to a cysteine S-conjugate that undergoes a β-lyase reaction. If the sulfhydryl-containing fragment produced in this reaction is reactive, toxicity may ensue. Some drugs and halogenated workplace/environmental contaminants are bioactivated by this mechanism. On the other hand, cysteine S-conjugate β-lyases occur in nature as a means of generating some biologically useful sulfhydryl-containing compounds.

Multigenerational study of chemically induced cytotoxicity and proliferation in cultures of human proximal tubular cells

Primary cultures of human proximal tubular (hPT) cells are a useful experimental model to study transport, metabolism, cytotoxicity, and effects on gene expression of a diverse array of drugs and environmental chemicals because they are derived directly from the in vivo human kidney. To extend the model to investigate longer-term processes, primary cultures (P0) were passaged for up to four generations (P1-P4). hPT cells retained epithelial morphology and stained positively for cytokeratins through P4, although cell growth and proliferation successively slowed with each passage. Necrotic cell death due to the model oxidants tert-butyl hydroperoxide (tBH) and methyl vinyl ketone (MVK) increased with increasing passage number, whereas that due to the selective nephrotoxicant S-(1,2-dichlorovinyl)-l-cysteine (DCVC) was modest and did not change with passage number. Mitochondrial activity was lower in P2-P4 cells than in either P0 or P1 cells. P1 and P2 cells were most sensitive to DCVC-induced apoptosis. DCVC also increased cell proliferation most prominently in P1 and P2 cells. Modest differences with respect to passage number and response to DCVC exposure were observed in expression of three key proteins (Hsp27, GADD153, p53) involved in stress response. Hence, although there are some modest differences in function with passage, these results support the use of multiple generations of hPT cells as an experimental model.

Glutathione S-conjugates as prodrugs to target drug-resistant tumors

Living organisms are continuously exposed to xenobiotics. The major phase of enzymatic detoxification in many species is the conjugation of activated xenobiotics to reduced glutathione (GSH) catalyzed by the glutathione-S-transferase (GST). It has been reported that some compounds, once transformed into glutathione S-conjugates, enter the mercapturic acid pathway whose end products are highly reactive and toxic for the cell responsible for their production. The cytotoxicity of these GSH conjugates depends essentially on GST and gamma-glutamyl transferases (γGT), the enzymes which initiate the mercapturic acid synthesis pathway. Numerous studies support the view that the expression of GST and γGT in cancer cells represents an important factor in the appearance of a more aggressive and resistant phenotype. High levels of tumor GST and γGT expression were employed to selectively target tumor with GST- or γGT-activated drugs. This strategy, explored over the last two decades, has recently been successful using GST-activated nitrogen mustard (TLK286) and γGT-activated arsenic-based (GSAO and Darinaparsin) prodrugs confirming the potential of GSH-conjugates as anticancer drugs.

Mutagenicity of the cysteine S-conjugate sulfoxides of trichloroethylene and tetrachloroethylene in the Ames test

The nephrotoxicity and nephrocarcinogenicity of trichloroethylene (TCE) and tetrachloroethylene (PCE) are believed to be mediated primarily through the cysteine S-conjugate β-lyase-dependent bioactivation of the corresponding cysteine S-conjugate metabolites S-(1,2-dichlorovinyl)-l-cysteine (DCVC) and S-(1,2,2-trichlorovinyl)-l-cysteine (TCVC), respectively. DCVC and TCVC have previously been demonstrated to be mutagenic by the Ames Salmonella mutagenicity assay, and reduction in mutagenicity was observed upon treatment with the β-lyase inhibitor aminooxyacetic acid (AOAA). Because DCVC and TCVC can also be bioactivated through sulfoxidation to yield the potent nephrotoxicants S-(1,2-dichlorovinyl)-l-cysteine sulfoxide (DCVCS) and S-(1,2,2-trichlorovinyl)-l-cysteine sulfoxide (TCVCS), respectively, the mutagenic potential of these two sulfoxides was investigated using the Ames Salmonella typhimurium TA100 mutagenicity assay. The results show both DCVCS and TCVCS were mutagenic, and TCVCS exhibited 3-fold higher mutagenicity than DCVCS. However, DCVCS and TCVCS mutagenic activity was approximately 700-fold and 30-fold lower than DCVC and TCVC, respectively. DCVC and DCVCS appeared to induce toxicity in TA100, as evidenced by increased microcolony formation and decreased mutant frequency above threshold concentrations. TCVC and TCVCS were not toxic in TA100. The toxic effects of DCVC limited the sensitivity of TA100 to DCVC mutagenic effects and rendered it difficult to investigate the effects of AOAA on DCVC mutagenic activity. Collectively, these results suggest that DCVCS and TCVCS exerted a definite but weak mutagenicity in the TA100 strain. Therefore, despite their potent nephrotoxicity, DCVCS and TCVCS are not likely to play a major role in DCVC or TCVC mutagenicity in this strain.

Characterization of the chemical reactivity and nephrotoxicity of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine sulfoxide, a potential reactive metabolite of trichloroethylene

N-Acetyl-S-(1,2-dichlorovinyl)-L-cysteine (NA-DCVC) has been detected in the urine of humans exposed to trichloroethylene and its related sulfoxide, N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (NA-DCVCS), has been detected as hemoglobin adducts in blood of rats dosed with S-(1,2-dichlorovinyl)-L-cysteine (DCVC) or S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS). Because the in vivo nephrotoxicity of NA-DCVCS was unknown, in this study, male Sprague-Dawley rats were dosed (i.p.) with 230 μmol/kg b.w. NA-DCVCS or its potential precursors, DCVCS or NA-DCVC. At 24 h post treatment, rats given NA-DCVC or NA-DCVCS exhibited kidney lesions and effects on renal function distinct from those caused by DCVCS. NA-DCVC and NA-DCVCS primarily affected the cortico-medullary proximal tubules (S(2)-S(3) segments) while DCVCS primarily affected the outer cortical proximal tubules (S(1)-S(2) segments). When NA-DCVCS or DCVCS was incubated with GSH in phosphate buffer pH 7.4 at 37°C, the corresponding glutathione conjugates were detected, but NA-DCVC was not reactive with GSH. Because NA-DCVCS exhibited a longer half-life than DCVCS and addition of rat liver cytosol enhanced GSH conjugate formation, catalysis of GSH conjugate formation by the liver could explain the lower toxicity of NA-DCVCS in comparison with DCVCS. Collectively, these results provide clear evidence that NA-DCVCS formation could play a significant role in DCVC, NA-DCVC, and trichloroethylene nephrotoxicity. They also suggest a role for hepatic metabolism in the mechanism of NA-DCVC nephrotoxicity.

Alcohol dehydrogenase- and rat liver cytosol-dependent bioactivation of 1-chloro-2-hydroxy-3-butene to 1-chloro-3-buten-2-one, a bifunctional alkylating agent

1,3-Butadiene (BD) is an air pollutant whose toxicity and carcinogenicity have been considered primarily mediated by its reactive metabolites, 3,4-epoxy-1-butene and 1,2,3,4-diepoxybutane, formed in liver and extrahepatic tissues by cytochromes P450s. A possible alternative metabolic pathway in bone marrow and immune cells is the conversion of BD to the chlorinated allylic alcohol 1-chloro-2-hydroxy-3-butene (CHB) by myeloperoxidase in the presence of hydrogen peroxide and chloride ion. In the present study, we investigated the in vitro bioactivation of CHB by alcohol dehydrogenases (ADH) under in vitro physiological conditions (pH 7.4, 37 °C). The results provide clear evidence for CHB being converted to 1-chloro-3-buten-2-one (CBO) by purified horse liver ADH and rat liver cytosol. CBO readily reacted with glutathione (GSH) under assay conditions to form three products: two CBO-mono-GSH conjugates [1-chloro-4-(S-glutathionyl)butan-2-one (3) and 1-(S-glutathionyl)-3-buten-2-one (4)] and one CBO-di-GSH conjugate [1,4-bis(S-glutathionyl)butan-2-one (5)]. CHB bioactivation and the ratios of the three GSH conjugates formed were dependent upon incubation time, GSH and CHB concentrations, and the presence of ADH or rat liver cytosol. The ADH enzymatic reaction followed Michaelis-Menten kinetics with a K(m) at 3.5 mM and a k(cat) at 0.033 s(-1). After CBO was incubated with freshly isolated mouse erythrocytes, globin dimers were detected using SDS-PAGE and silver staining, providing evidence that CBO can act as a protein cross-linking agent. Collectively, the results provide clear evidence for CHB bioactivation by ADH and rat liver cytosol to yield CBO. The bifunctional alkylating ability of CBO suggests that it may play a role in BD toxicity and/or carcinogenicity.

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

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

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.

Role of reactive metabolites in the circulation in extrahepatic toxicity

INTRODUCTION

Reactive metabolite-mediated toxicity is frequently limited to the organ where the electrophilic metabolites are generated. Some reactive metabolites, however, might have the ability to translocate from their site of formation. This suggests that for these reactive metabolites, investigations into the role of organs other than the one directly affected could be relevant to understanding the mechanism of toxicity.

AREAS COVERED

The authors discuss the physiological and biochemical factors that can enable reactive metabolites to cause toxicity in an organ distal from the site of generation. Furthermore, the authors present a case study which describes studies that demonstrate that S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS) and N-acetyl-S-(1,2-dichlorovinyl-L-cysteine sulfoxide (N-AcDCVCS), reactive metabolites of the known trichloroethylene metabolites S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (N-AcDCVC), are generated in the liver and translocate through the circulation to the kidney to cause nephrotoxicity.

EXPERT OPINION

The ability of reactive metabolites to translocate could be important to consider when investigating mechanisms of toxicity. A mechanistic approach, similar to the one described for DCVCS and N-AcDCVCS, could be useful in determining the role of circulating reactive metabolites in extrahepatic toxicity of drugs and other chemicals. If this is the case, intervention strategies that would not otherwise be feasible might be effective for reducing extrahepatic toxicity.

The transcriptional regulation of regucalcin gene expression

Regucalcin, which is discovered as a calcium-binding protein in 1978, has been shown to play a multifunctional role in many tissues and cell types; regucalcin has been proposed to play a pivotal role in keeping cell homeostasis and function for cell response. Regucalcin and its gene are identified in over 15 species consisting of regucalcin family. Comparison of the nucleotide sequences of regucalcin from vertebrate species is highly conserved in their coding region with throughout evolution. The regucalcin gene is localized on the chromosome X in rat and human. The organization of rat regucalcin gene consists of seven exons and six introns and several consensus regulatory elements exist upstream of the 5'-flanking region. AP-1, NF1-A1, RGPR-p117, β-catenin, and other factors have been found to be a transcription factor in the enhancement of regucalcin gene promoter activity. The transcription activity of regucalcin gene is enhanced through intracellular signaling factors that are mediated through the phosphorylation and dephosphorylation of nuclear protein in vitro. Regucalcin mRNA and its protein are markedly expressed in the liver and kidney cortex of rats. The expression of regucalcin mRNA in the liver and kidney cortex has been shown to stimulate by hormonal factors (including calcium, calcitonin, parathyroid hormone, insulin, estrogen, and dexamethasone) in vivo. Regucalcin mRNA expression is enhanced in the regenerating liver after partial hepatectomy of rats in vivo. The expression of regucalcin mRNA in the liver and kidney with pathophysiological state has been shown to suppress, suggesting an involvement of regucalcin in disease. Liver regucalcin expression is down-regulated in tumor cells, suggesting a suppressive role in the development of carcinogenesis. Liver regucalcin is markedly released into the serum of rats with chemically induced liver injury in vivo. Serum regucalcin has a potential sensitivity as a specific biochemical marker of chronic liver injury with hepatitis. Regucalcin has been proposed to be a key molecule in cellular regulation and metabolic disease.

Enzymes Involved in Processing Glutathione Conjugates

Many potentially toxic electrophiles react with glutathione to form glutathione S-conjugates in reactions catalyzed or enhanced by glutathione S-transferases. The glutathione S-conjugate is sequentially converted to the cysteinylglycine-, cysteine- and N-acetyl-cysteine S-conjugate (mercapturate). The mercapturate is generally more polar and water soluble than the parent electrophile and is readily excreted. Excretion of the mercapturate represents a detoxication mechanism. Some endogenous compounds, such as leukotrienes, prostaglandin (PG) A2, 15-deoxy-Δ^12,14-PGJ₂, and hydroxynonenal can also be metabolized to mercapturates and excreted. On occasion, however, formation of glutathione S- and cysteine S-conjugates are bioactivation events as the metabolites are mutagenic and/or cytotoxic. When the cysteine S-conjugate contains a strong electron-withdrawing group attached at the sulfur, it may be converted by cysteine S-conjugate β-lyases to pyruvate, ammonium and the original electrophile modified to contain an –SH group. If this modified electrophile is highly reactive then the enzymes of the mercapturate pathway together with the cysteine S-conjugate β-lyases constitute a bioactivation pathway. Some endogenous halogenated environmental contaminants and drugs are bioactivated by this mechanism. Recent studies suggest that coupling of enzymes of the mercapturate pathway to cysteine S-conjugate β-lyases may be more common in nature and more widespread in the metabolism of electrophilic xenobiotics than previously realized.

Cysteine conjugate beta-lyase activity of rat erythrocytes and formation of beta-lyase-derived globin monoadducts and cross-links after in vitro exposure of erythrocytes to S-(1,2-dichlorovinyl)-L-cysteine

S-(1,2-Dichlorovinyl)-L-cysteine (DCVC), a mutagenic and nephrotoxic metabolite of trichloroethylene, can be bioactivated to reactive metabolites, S-(1,2-dichlorovinyl)-L-cysteine sulfoxide (DCVCS) or chlorothioketene and/or 2-chlorothionoacetyl chloride, by cysteine conjugate S-oxidase (S-oxidase) and cysteine conjugate beta-lyase (beta-lyase), respectively. Previously, we characterized the reactivity of DCVCS with Hb upon incubation of erythrocytes with DCVCS and provided evidence for the formation of distinct DCVCS-Hb monoadducts and cross-links in both isolated erythrocytes and rats given DCVCS. In the present study, we investigated DCVC bioactivation and Hb adduct formation in isolated rat erythrocytes incubated with DCVC (9 and 450 microM) at 37 degrees C and pH 7.4. The results suggested that no DCVCS monoadducts or cross-links were formed; however, LC/electrospray ionization/MS and matrix-assisted laser desorption/ionization/MS of trypsin-digested globin peptides revealed the presence of beta-lyase-derived globin monoadducts and cross-links. Adducts and cross-links in which the sulfur atom of the reactive sulfur intermediates were replaced by oxygen have also been detected. Use of SDS-PAGE provided additional evidence for globin cross-link formation in the presence of DCVC. Interestingly, the MS results suggest that the observed peptide selectivity of the beta-lyase-derived reactive sulfur/oxygen-containing species was different than that previously observed with DCVCS. While these results suggested that erythrocytes have beta-lyase but not S-oxidase activity, further support for this hypothesis was obtained using S-(2-benzothiazolyl)-L-cysteine, an alternative substrate for beta-lyases. Collectively, the results demonstrate the utility of Hb adducts and cross-links to characterize the metabolic pathway responsible for DCVC bioactivation in erythrocytes and to provide distinct biomarkers for each reactive metabolite.

Mouse aminoacylase 3: a metalloenzyme activated by cobalt and nickel

Aminoacylase 3 (AA3) deacetylates N-acetyl-aromatic amino acids and mercapturic acids including N-acetyl-1,2-dichlorovinyl-L-cysteine (Ac-DCVC), a metabolite of a xenobiotic trichloroethylene. Previous studies did not demonstrate metal-dependence of AA3 despite a high homology with a Zn(2+)-metalloenzyme aminoacylase 2 (AA2). A 3D model of mouse AA3 was created based on homology with AA2. The model showed a putative metal binding site formed by His21, Glu24 and His116, and Arg63, Asp68, Asn70, Arg71, Glu177 and Tyr287 potentially involved in catalysis/substrate binding. The mutation of each of these residues to alanine inactivated AA3 except Asn70 and Arg71, therefore the corrected 3D model of mouse AA3 was created. Wild type (wt) mouse AA3 expressed in E. coli contained approximately 0.35 zinc atoms per monomer. Incubation with Co(2+) and Ni(2+) activated wt-AA3. In the cobalt-activated AA3 zinc was replaced with cobalt. Metal removal completely inactivated wt-AA3, whereas addition of Zn(2+), Mn(2+) or Fe(2+) restored initial activity. Co(2+) and to a lesser extent Ni(2+) increased activity several times in comparison with intact wt-AA3. Co(2+) drastically increased the rate of deacetylation of Ac-DCVC and significantly increased the toxicity of Ac-DCVC in the HEK293T cells expressing wt-AA3. The results indicate that AA3 is a metalloenzyme significantly activated by Co(2+) and Ni(2+).

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.

Role of mitochondrial dysfunction in cellular responses to S-(1,2-dichlorovinyl)-L-cysteine in primary cultures of human proximal tubular cells

The nephrotoxic metabolite of the environmental contaminant trichloroethylene, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), is known to elicit cytotoxicity in rat and human proximal tubular (rPT and hPT, respectively) cells that involves inhibition of mitochondrial function. DCVC produces a range of cytotoxic and compensatory responses in hPT cells, depending on dose and exposure time, including necrosis, apoptosis, repair, and enhanced cell proliferation. The present study tested the hypothesis that induction of mitochondrial dysfunction is an obligatory step in the cytotoxicity caused by DCVC in primary cultures of hPT cells. DCVC-induced necrosis was primarily a high concentration (> or =50 microM) and late (> or =24h) response whereas apoptosis and increased proliferation occurred at relatively low concentrations (<50 microM) and early time points (< or =24h). Decreases in cellular DNA content, indicative of cell loss, were observed at DCVC concentrations as low as 1 microM. Involvement of mitochondrial dysfunction in DCVC-induced cytotoxicity was supported by showing that DCVC caused modest depletion of cellular ATP, inhibition of respiration, and activation of caspase-3/7. Cyclosporin A protected cells against DCVC-induced apoptosis and both cyclosporin A and ruthenium red protected cells against DCVC-induced loss of mitochondrial membrane potential. DCVC caused little or no activation of caspase-8 and did not significantly induce expression of Fas receptor, consistent with apoptosis occurring only by the mitochondrial pathway. These results support the conclusion that mitochondrial dysfunction is an early and obligatory step in DCVC-induced cytotoxicity in hPT cells.

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.

Modulation of hepatic and renal metabolism and toxicity of trichloroethylene and perchloroethylene by alterations in status of cytochrome P450 and glutathione

The relative importance of metabolism of trichloroethylene (Tri) and perchloroethylene (Perc) by the cytochrome P450 (P450) and glutathione (GSH) conjugation pathways in their acute renal and hepatic toxicity was studied in isolated cells and microsomes from rat kidney and liver after various treatments to modulate P450 activity/expression or GSH status. Inhibitors of P450 stimulated GSH conjugation of Tri and, to a lesser extent, Perc, in both kidney cells and hepatocytes. Perc was a more potent, acute cytotoxic agent in isolated kidney cells than Tri but Perc-induced toxicity was less responsive than Tri-induced toxicity to modulation of P450 status. These observations are consistent with P450-dependent bioactivation being more important for Tri than for Perc. Incubation of isolated rat hepatocytes with Tri produced no acute cytotoxicity in isolated hepatocytes while Perc produced comparable cytotoxicity as in kidney cells. Modulation of P450 status in hepatocytes produced larger changes in Tri- and Perc-induced cytotoxicity than in kidney cells, with non-selective P450 inhibitors increasing toxicity. Induction of CYP2E1 with pyridine also markedly increased sensitivity of hepatocytes to Tri but had little effect on Perc-induced cytotoxicity. Increases in cellular GSH concentrations increased Tri- and Perc-induced cytotoxicity in kidney cells but not in hepatocytes, consistent with the role of GSH conjugation in Tri- and Perc-induced nephrotoxicity. In contrast, depletion of cellular GSH concentrations moderately decreased Tri- and Perc-induced cytotoxicity in kidney cells but increased cytotoxicity in hepatocytes, again pointing to the importance of different bioactivation pathways and modes of action in kidney and liver.

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.

Cisplatin-induced toxicity is associated with platinum deposition in mouse kidney mitochondria in vivo and with selective inactivation of the alpha-ketoglutarate dehydrogenase complex in LLC-PK1 cells

The anticancer drug cisplatin is nephrotoxic and neurotoxic. Previous data support the hypothesis that cisplatin is bioactivated to a nephrotoxicant. The final step in the proposed bioactivation is the formation of a platinum-cysteine S-conjugate followed by a pyridoxal 5'-phosphate (PLP)-dependent cysteine S-conjugate beta-lyase reaction. This reaction would generate pyruvate, ammonium, and a highly reactive platinum (Pt)-thiol compound in vivo that would bind to proteins. In this work, the cellular location and identity of the PLP-dependent cysteine S-conjugate beta-lyase were investigated. Pt was shown to bind to proteins in kidneys of cisplatin-treated mice. The concentration of Pt-bound proteins was higher in the mitochondrial fraction than in the cytosolic fraction. Treatment of the mice with aminooxyacetic acid (AOAA, a PLP enzyme inhibitor), which had previously been shown to block the nephrotoxicity of cisplatin, decreased the binding of Pt to mitochondrial proteins but had no effect on the amount of Pt bound to proteins in the cytosolic fraction. These data indicate that a mitochondrial enzyme catalyzes the PLP-dependent cysteine S-conjugate beta-lyase reaction. PLP-dependent mitochondrial aspartate aminotransferase (mitAspAT) is a mitochondrial enzyme that catalyzes beta-elimination reactions with cysteine S-conjugates of halogenated alkenes. We reasoned that the enzyme might also catalyze a beta-lyase reaction with the cisplatin-cysteine S-conjugate. In this study, mitAspAT was stably overexpressed in LLC-PK(1) cells. Cisplatin was significantly more toxic in confluent monolayers of LLC-PK(1) cells that overexpressed mitAspAT than in control cells containing vector alone. AOAA completely blocked the cisplatin toxicity in confluent mitAspAT-transfected cells. The Pt-thiol compound could rapidly bind proteins and inactivate enzymes in close proximity of the PLP-dependent cysteine S-conjugate beta-lyase. Treatment with 50 or 100 microM cisplatin for 3 h, followed by removal of cisplatin from the medium for 24 h, resulted in a pronounced loss of alpha-ketoglutarate dehydrogenase complex (KGDHC) activity in both mitAspAT-transfected cells and control cells. Exposure to 100 microM cisplatin resulted in a significantly greater loss of KGDHC activity in the cells overexpressing mitAspAT than in control cells. Aconitase activity was diminished in both cell types, but only at the higher level of exposure to cisplatin. AspAT activity was also significantly decreased by cisplatin treatment. By contrast, several other enzymes (both cytosolic and mitochondrial) involved in energy/amino acid metabolism were not significantly affected by cisplatin treatment in the LLC-PK(1) cells, whether or not mitAspAT was overexpressed. The susceptibility of KGDHC and aconitase to inactivation in kidney cells exposed to cisplatin metabolites may be due to the proximity of mitAspAT to KGDHC and aconitase in mitochondria. These findings support the hypothesis that a mitochondrial cysteine S-conjugate beta-lyase converts the cisplatin-cysteine S-conjugate to a toxicant, and the data are consistent with the hypothesis that mitAspAT plays a role in the bioactivation of cisplatin.

Protein kinase B/Akt modulates nephrotoxicant-induced necrosis in renal cells

Protein kinase B (Akt) activation is well known for its protective effects against apoptosis. However, the role of Akt in regulation of necrosis is unknown. This study was designed to test whether Akt activation protects against nephrotoxicant-induced injury and death in renal proximal tubular cells (RPTC). Exposure of primary cultures of RPTC to the nephrotoxic cysteine conjugate, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), resulted in 9% apoptosis and 30% necrosis at 24 h following the exposure. Akt was activated during 8 h but not at 24 h following toxicant exposure. No RPTC necrosis was observed during Akt activation. Blocking Akt activation using a phosphatidylinositol 3-kinase inhibitor, LY294002 (20 muM), or expressing dominant negative (inactive) Akt increased DCVC-induced RPTC necrosis to 42%. In contrast, Akt activation by expression of constitutively active Akt diminished necrosis to 15%. Modulation of Akt activity had no effect on DCVC-induced apoptosis. DCVC-induced RPTC injury was accompanied by decreases in respiration (51% of controls) and ATP levels (57% of controls). Akt inhibition exacerbated decreases in RPTC respiration and intracellular ATP content (both to 30% of controls). In contrast, Akt activation reduced DCVC-induced decreases in respiration (80% of controls) and prevented decline in ATP content. These data show that in RPTC, Akt activation reduces 1) toxicant-induced mitochondrial dysfunction, 2) decreases in ATP levels, and 3) necrosis. We conclude that Akt activation plays a protective role against necrosis caused by nephrotoxic insult in RPTC. Furthermore, we identified mitochondria as a subcellular target of protective actions of Akt against necrosis.

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

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

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

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

Modulation of expression of rat mitochondrial 2-oxoglutarate carrier in NRK-52E cells alters mitochondrial transport and accumulation of glutathione and susceptibility to chemically induced apoptosis

We previously showed that two anion carriers of the mitochondrial inner membrane, the dicarboxylate carrier (DIC; Slc25a10) and oxoglutarate carrier (OGC; Slc25a11), transport glutathione (GSH) from cytoplasm into mitochondrial matrix. In the previous study, NRK-52E cells, derived from normal rat kidney proximal tubules, were transfected with the wild-type cDNA for the DIC expressed in rat kidney; DIC transfectants exhibited increased mitochondrial uptake and accumulation of GSH and were markedly protected from chemically induced apoptosis. In the present study, cDNAs for both wild-type (WT) and a double-cysteine mutant of rat OGC (rOGC and rOGC-C221,224S, respectively) were expressed in Escherichia coli, purified, and reconstituted into proteoliposomes to assess their function. Although both WT rOGC and rOGC-C221,224S exhibited transport properties for GSH and 2-oxoglutarate that were similar to those found in mitochondria of rat kidney proximal tubules, rates of transport and mitochondrial accumulation of substrates were reduced by >75% in rOGC-C221,224S compared with the WT carrier. NRK-52E cells were stably transfected with the cDNA for WT-rOGC and exhibited 10- to 20-fold higher GSH transport activity than nontransfected cells and were markedly protected from apoptosis induced by tert-butyl hydroperoxide (tBH) or S-(1,2-dichlorovinyl)-L-cysteine (DCVC). In contrast, cells stably transfected with the cDNA for rOGC-C221,224S were not protected from tBH- or DCVC-induced apoptosis. These results provide further evidence that genetic manipulation of mitochondrial GSH transporter expression alters mitochondrial and cellular GSH status, resulting in markedly altered susceptibility to chemically induced apoptosis.

Pulmonary bronchiolar cytotoxicity and formation of dichloroacetyl lysine protein adducts in mice treated with trichloroethylene

This study was undertaken to test the hypothesis that bronchiolar damage induced by trichloroethylene (TCE) is associated with bioactivation within the Clara cells with the involvement of CYP2E1 and CYP2F2. Histopathology confirmed dose-dependent Clara cell injury and disintegration of the bronchiolar epithelium in CD-1 mice treated with TCE doses of 500 to 1000 mg/kg i.p. Immunohistochemical studies, using an antibody that recognizes dichloroacetyl lysine adducts, revealed dose-dependent formation of adducts in the bronchiolar epithelium. Localization of dichloroacetyl adducts in the Clara cells coincided with damage to this cell type in TCE-treated mice. Pretreatment of CD-1 mice with diallyl sulfone, an inhibitor of CYP2E1 and CYP2F2, abrogated the formation of the dichloroacetyl adducts and protected against TCE-induced bronchiolar cytotoxicity. Treatment of wild-type and CYP2E1-null mice with TCE (750 mg/kg i.p.) also elicited bronchiolar damage that correlated with the formation of adducts in the Clara cells. Immunoblotting, using lung microsomes from TCE-treated CD-1 mice, showed dose-dependent production of dichloroacetyl adducts that comigrated with CYP2E1 and CYP2F2. However, TCE treatment resulted in a loss of immunoreactive CYP2E1 and CYP2F2 proteins and p-nitrophenol hydroxylation, a catalytic activity associated with both cytochrome P450 enzymes. The TCE metabolite, chloral hydrate, was formed in incubations of TCE with lung microsomes from CD-1, wild-type, and CYP2E1-null mice. The levels were higher in CD-1 than in either wild-type or CYP2E1-null mice, although levels were higher in CYP2E1-null than in wild-type mice. These findings supported the contention that TCE bioactivation within the Clara cells, predominantly involving CYP2F2, correlated with bronchiolar cytotoxicity in mice.

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.

Molecular markers of trichloroethylene-induced toxicity in human kidney cells

Difficulties in evaluation of trichloroethylene (TRI)-induced toxicity in humans and extrapolation of data from laboratory animals to humans are due to the existence of multiple target organs, multiple metabolic pathways, sex-, species-, and strain-dependent differences in both metabolism and susceptibility to toxicity, and the lack or minimal amount of human data for many target organs. The use of human tissue for mechanistic studies is thus distinctly advantageous. The kidneys are one target organ for TRI and metabolism by the glutathione (GSH) conjugation pathway is responsible for nephrotoxicity. The GSH conjugate is processed further to produce the cysteine conjugate, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), which is the penultimate nephrotoxic species. Confluent, primary cultures of human proximal tubular (hPT) cells were used as the model system. Although cells in log-phase growth, which are undergoing more rapid DNA synthesis, would give lower LD(50) values, confluent cells more closely mimic the in vivo proximal tubule. DCVC caused cellular necrosis only at relatively high doses (>100 muM) and long incubation times (>24 h). In contrast, both apoptosis and enhanced cellular proliferation occurred at relatively low doses (10-100 muM) and early incubation times (2-8 h). These responses were associated with prominent changes in expression of several proteins that regulate apoptosis (Bcl-2, Bax, Apaf-1, Caspase-9 cleavage, PARP cleavage) and cellular growth, differentiation and stress response (p53, Hsp27, NF-kappaB). Effects on p53 and Hsp27 implicate function of protein kinase C, the mitogen activated protein kinase pathway, and the cytoskeleton. The precise pattern of expression of these and other proteins can thus serve as molecular markers for TRI exposure and effect in human kidney.

Formation and toxicity of anesthetic degradation products

Toxic degradation products are formed from a range of old and modern anesthetic agents. The common element in the formation of degradation products is the reaction of the anesthetic agent with the bases in the carbon dioxide absorbents in the anesthesia circuit. This reaction results in the conversion of trichloroethylene to dichloroacetylene, halothane to 2-bromo-2-chloro-1,1-difluoroethylene, sevoflurane to 2-(fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (Compound A), and desflurane, isoflurane, and enflurane to carbon monoxide. Dichloroacetylene, 2-bromo-2-chloro-1,1-difluoroethylene, and Compound A form glutathione S-conjugates that undergo hydrolysis to cysteine S-conjugates and bioactivation of the cysteine S-conjugates by renal cysteine conjugate beta-lyase to give nephrotoxic metabolites. The elucidation of the mechanisms of formation and bioactivation of degradation products has allowed for the safe use of anesthetics that may undergo degradation in the anesthesia circuit.

NMR-based metabonomic approaches for evaluating physiological influences on biofluid composition

Strategies such as genomics, proteomics and metabonomics are being applied with increasing frequency in the pharmaceutical industry. For each of these approaches, toxicological response can be measured by terms of deviation from control or baseline status. However, in order to accurately define drug-induced response, it is necessary to characterize the normal degree of physiological variation in the absence of stimuli. Here, 1H NMR spectroscopic-based analyses of the metabolic composition of urine in experimental animals under various normal physiological conditions are reviewed. In particular, the effects of inter-animal and diurnal variation, gender, age, diet, species, strain, hormonal status and stress on the biochemical composition of urine are explored. Pattern recognition methods facilitate the comparison of urine NMR spectra over a given time-course, enabling the establishment of changes in profile and highlighting the dynamic metabolic status of an organism. Thus metabonomic approaches based on information-rich spectroscopic data sets can be used to evaluate normal physiological variation and for investigation of drug safety issues.

Glutathione‐dependent Bioactivation of Haloalkanes and Haloalkenes

Haloalkanes and haloalkenes constitute an important group of widely used chemicals that have the potential to induce toxicity and cancer. The toxicity of haloalkanes and haloalkenes may be associated with cytochromes P450- or glutathione transferase-dependent bioactivation. This review is concerned with the glutathione- and glutathione transferase-dependent bioactivation of dihalomethanes, 1,2-dihaloalkanes, and haloalkenes. Dihalomethanes, e.g., dichloromethane, and 1,2-dihaloethanes, e.g., 1,2-dichloroethane and 1,2-dibromoethane, undergo glutathione transferase-catalyzed bioactivation to give S-(halomethyl)glutathione or glutathione episulfonium ions, respectively, as reactive intermediates. Haloalkenes, e.g., trichloroethene, hexachlorobutadiene, chlorotrifluoroethene, and tetrafluoroethene, undergo cysteine conjugate beta-lyase-dependent bioactivation to thioacylating intermediates, including thioacyl halides, thioketenes, and 2,2,3-trihalothiiranes. With all of these compounds, the formation of reactive intermediates is associated with their observed toxicity.

Acivicin-induced alterations in renal and hepatic glutathione concentrations and in γ-glutamyltransferase activities

gamma-Glutamyltransferase (gamma-GT) catalyzes the hydrolysis of glutathione, glutathione S-conjugates, and gamma-substituted l-glutamate derivatives. Acivicin is an irreversible inhibitor of gamma-GT that has been used to study the role of gamma-GT in glutathione homeostasis and glutathione-dependent bioactivation reactions. The present studies were undertaken because of reported conflicting effects of acivicin on the nephrotoxicity of some haloalkenes that undergo glutathione-dependent bioactivation. The objective of this study was to test the hypothesis that acivicin may alter renal glutathione concentrations; acivicin-induced changes in renal glutathione concentrations may alter the susceptibility of the kidney to the nephrotoxic effects of haloalkenes. Hence, diurnal and acivicin-induced changes in renal and hepatic glutathione concentrations along with renal and hepatic gamma-GT activities were investigated. The previously observed diurnal variations in hepatic glutathione concentrations in fed rats were confirmed, but no diurnal variations were observed in renal glutathione concentrations or in renal or hepatic gamma-GT activities. Renal and hepatic glutathione concentrations and gamma-GT activities were measured in tissue homogenates from rats given 0, 0.1, or 0.2 mmol acivicin/kg (i.p.) and killed 0, 2, 4, 8, 12, or 24 hr later. Renal glutathione concentrations were increased above control values in acivicin-treated rats, whereas acivicin had no effect on hepatic glutathione concentrations. Renal gamma-GT activities decreased within 2 hr after giving acivicin and remained decreased for 24 hr. Acivicin had no effect on hepatic gamma-GT activities, except at 24 hr after treatment when values in acivicin-treated rats were elevated compared with controls. Although the present studies do not afford an explanation of the mechanism whereby acivicin increases the nephrotoxicity of some haloalkenes, they do indicate that acivicin is not a reliable probe to investigate the role of gamma-GT in haloalkene-induced nephrotoxicity.

Protein kinase C-alpha inhibits the repair of oxidative phosphorylation after S-(1,2-dichlorovinyl)-L-cysteine injury in renal cells

Previously, we showed that physiological functions of renal proximal tubular cells (RPTC) do not recover following S-(1,2-dichlorovinyl)-l-cysteine (DCVC)-induced injury. This study investigated the role of protein kinase C-alpha (PKC-alpha) in the lack of repair of mitochondrial function in DCVC-injured RPTC. After DCVC exposure, basal oxygen consumption (Qo(2)), uncoupled Qo(2), oligomycin-sensitive Qo(2), F(1)F(0)-ATPase activity, and ATP production decreased, respectively, to 59, 27, 27, 57, and 68% of controls. None of these functions recovered. Mitochondrial transmembrane potential decreased 53% after DCVC injury but recovered on day 4. PKC-alpha was activated 4.3- and 2.5-fold on days 2 and 4, respectively, of the recovery period. Inhibition of PKC-alpha activation (10 nM Go6976) did not block DCVC-induced decreases in mitochondrial functions but promoted the recovery of uncoupled Qo(2), oligomycin-sensitive Qo(2), F(1)F(0)-ATPase activity, and ATP production. Protein levels of the catalytic beta-subunit of F(1)F(0)-ATPase were not changed by DCVC or during the recovery period. Amino acid sequence analysis revealed that alpha-, beta-, and epsilon-subunits of F(1)F(0)-ATPase have PKC consensus motifs. Recombinant PKC-alpha phosphorylated the beta-subunit and decreased F(1)F(0)-ATPase activity in vitro. Serine but not threonine phosphorylation of the beta-subunit was increased during late recovery following DCVC injury, and inhibition of PKC-alpha activation decreased this phosphorylation. We conclude that during RPTC recovery following DCVC injury, 1). PKC-alpha activation decreases F(0)F(1)-ATPase activity, oxidative phosphorylation, and ATP production; 2). PKC-alpha phosphorylates the beta-subunit of F(1)F(0)-ATPase on serine residue; and 3). PKC-alpha does not mediate depolarization of RPTC mitochondria. This is the first report showing that PKC-alpha phosphorylates the catalytic subunit of F(1)F(0)-ATPase and that PKC-alpha plays an important role in regulating repair of mitochondrial function.

Role of cysteine S-conjugate beta-lyase in the metabolism of cisplatin

Cisplatin is nephrotoxic, but the mechanism by which cisplatin kills renal proximal tubule cells is not well defined. Inhibition of gamma-glutamyl transpeptidase or pyridoxal 5'-phosphate (PLP)-dependent enzymes blocks the nephrotoxicity. Our hypothesis is that cisplatin is metabolized to a renal toxin through a platinum-glutathione conjugate to a reactive sulfur-containing compound. The final step in this bioactivation is the conversion of a platinum-cysteine S-conjugate to a reactive thiol by a PLP-dependent cysteine S-conjugate beta-lyase. LLC-PK1 cells, a proximal tubule cell line with low cysteine S-conjugate beta-lyase activity, are used to study cisplatin nephrotoxicity. We proposed that the beta-elimination reaction catalyzed by cysteine S-conjugate beta-lyase is the rate-limiting step in the metabolism of cisplatin to a toxin in these cells. In this study, LLC-PK1 cells were transfected with human glutamine transaminase K, which catalyzes the beta-elimination reaction. Cisplatin was significantly more toxic in confluent monolayers of cells with increased cysteine S-conjugate beta-lyase activity. In contrast, carboplatin, a non-nephrotoxic derivative of cisplatin, was 20-fold less toxic than cisplatin in confluent cells, and its toxicity was not altered by overexpression of cysteine S-conjugate beta-lyase. We propose that carboplatin is not nephrotoxic because it is not metabolized through this pathway. Dividing cells were more sensitive to both cisplatin and carboplatin toxicity. Overexpression of cysteine S-conjugate beta-lyase activity had no effect on the toxicity of either drug. These data demonstrate that cisplatin kills quiescent renal cells by a mechanism that is distinct from the mechanism by which it kills dividing cells and that the renal toxicity of cisplatin is dependent on cysteine S-conjugate beta-lyase activity.

Evidence for trichloroethylene bioactivation and adduct formation in the rat epididymis and efferent ducts

Recent studies indicate that trichloroethylene (TCE) may be a male reproductive toxicant. It is metabolized by conjugation with glutathione and cytochrome p450-dependent oxidation. Reactive metabolites produced along both pathways are capable of forming protein adducts and are thought to be involved in TCE-induced liver and kidney damage. Similarly, in situ bioactivation of TCE and subsequent binding of metabolites may be one mechanism by which TCE acts as a reproductive toxicant. Cysteine-conjugate beta-lyase (beta-lyase) bioactivates the TCE metabolite dichlorovinyl cysteine (DCVC) to a reactive intermediate that is capable of binding cellular macromolecules. In the present study, Western blot analysis indicated that the soluble form of beta-lyase, but not the mitochondrial form, was present in the epididymis and efferent ducts. Both forms of beta-lyase were detected in the kidney. When rats were dosed with DCVC, no protein adducts were detected in the epididymis or efferent ducts, although adducts were present in the proximal tubule of the kidney. Trichloroethylene can also be metabolized and form protein adducts through a cytochrome p450-mediated pathway. Western blot analysis detected the presence of cytochrome p450 2E1 (CYP2E1) in the efferent ducts. Immunoreactive proteins were localized to efferent duct and corpus epididymis epithelia. Metabolism of TCE was demonstrated in vitro using microsomes prepared from untreated rats. Metabolism was inhibited 77% when efferent duct microsomes were preincubated with an antibody to CYP2E1. Dichloroacetyl adducts were detected in epididymal and efferent duct microsomes exposed in vitro to TCE. Results from the present study indicate that the cytochrome p450-dependent formation of reactive intermediates and the subsequent covalent binding of cellular proteins may be involved in the male reproductive toxicity of TCE.

Protein kinase C mediates repair of mitochondrial and transport functions after toxicant-induced injury in renal cells

Previously, we have shown that renal proximal tubular cells (RPTCs) recover physiological functions after injury induced by the oxidant tert-butylhydroperoxide (TBHP), but not by the nephrotoxic cysteine conjugate dichlorovinyl-l-cysteine (DCVC). This study examined the role of protein kinase C (PKC) in the repair of RPTC functions after sublethal injury produced by these toxicants. Total PKC activity decreased 65 and 86% after TBHP and DCVC exposures, respectively, and recovered in TBHP-injured but not in DCVC-injured RPTCs. Mitochondrial function, active Na+ transport, and Na+-dependent glucose uptake decreased after toxicant exposure and recovered in TBHP- but not in DCVC-injured RPTCs. PKC inhibition decreased the repair of RPTC functions after TBHP injury. PKC activation promoted recovery of mitochondrial function and active Na+ transport in TBHP- and DCVC-injured RPTCs but had no effect on recovery of Na+-dependent glucose uptake. We conclude that in RPTCs, 1) total PKC activity decreases after TBHP and DCVC injury and recovers after TBHP but not after DCVC exposure, 2) recovery of PKC activity precedes the return of physiological functions after oxidant injury, 3) PKC inhibition decreases recovery of physiological functions, and 4) PKC activation promotes recovery of mitochondrial function and active Na+ transport but not Na+-dependent glucose uptake. These results suggest that the repair of renal functions is mediated through PKC-dependent mechanisms and that cysteine conjugates may inhibit renal repair, in part, through inhibition of PKC signaling.

Roles of necrosis, Apoptosis, and mitochondrial dysfunction in S-(1,2-dichlorovinyl)-L-cysteine sulfoxide-induced cytotoxicity in primary cultures of human renal proximal tubular cells

S-(1,2-Dichlorovinyl)-L-cysteine (DCVC) is the penultimate nephrotoxic metabolite of the environmental contaminant trichloroethylene. Although metabolism of DCVC by the cysteine conjugate beta-lyase is the most studied bioactivation pathway, DCVC may also be metabolized by the flavin-containing monooxygenase (FMO) to yield DCVC sulfoxide (DCVCS). Renal cellular injury induced by DCVCS was investigated in primary cultures of human proximal tubular (hPT) cells by assessment of time- and concentration-dependent effects on cellular morphology, acute cellular necrosis, apoptosis, mitochondrial function, and cellular glutathione (GSH) status. Confluent hPT cells incubated with as little as 10 microM DCVCS for 24 h exhibited morphological changes, although at least 100 microM DCVCS was required to produce marked changes. Acute cellular necrosis did not occur until 48 h with at least 200 microM DCVCS, indicating that this is a high-dose, late response. The extent of necrosis was similar to that with DCVC. In contrast, apoptosis occurred as early as 1 h with as little as 10 microM DCVCS and the extent of apoptosis was much less than that with DCVC. Mitochondrial function was maintained with DCVCS concentrations up to 100 microM, consistent with hPT cells only being competent to undergo apoptosis at early time points and relatively low concentrations. Marked depletion (>50%) of cellular GSH content was only observed with 500 microM DCVCS. These results, combined with previous studies showing protection from DCVC-induced necrosis and apoptosis by the FMO inhibitor methimazole, suggest that formation of DCVCS plays a significant role in trichloroethylene-induced renal cellular injury in hPT cells.

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

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

Metabolism of Cisplatin to a nephrotoxin in proximal tubule cells

Cisplatin, a commonly used chemotherapeutic agent, is nephrotoxic. The mechanism by which cisplatin selectively kills the proximal tubule cells was heretofore unknown. Recent studies in mice and rats have shown that the nephrotoxicity of cisplatin can be blocked by acivicin or (aminooxy)acetic acid, the same enzyme inhibitors that block the metabolic activation of a series of nephrotoxic halogenated alkenes. In this study, it was hypothesized that cisplatin is activated in the kidney to a toxic metabolite through the same pathway that has been shown to activate the halogenated alkenes. This activation begins with the formation of a glutathione-conjugate that is metabolized to a cysteinyl-glycine-conjugate, to a cysteine-conjugate, and finally to a reactive thiol. In this study, a protocol was developed in which confluent monolayers of LLC-PK(1) cells were exposed to clinically relevant concentrations of cisplatin or cisplatin-conjugate for 3 h. Cell viability was assayed at 72 h. The role of gamma-glutamyl transpeptidase (GGT) and cysteine-S-conjugate beta-lyase in the metabolism of each of the cisplatin-conjugates was investigated. Pre-incubation of cisplatin with glutathione, cysteinyl-glycine, or N-acetyl-cysteine to allow for the spontaneous formation of cisplatin-conjugates increased the toxicity of cisplatin toward LLC-PK(1) cells. Inhibition of GGT activity showed that GGT was necessary only for the toxicity of the cisplatin-glutathione-conjugate. Inhibition of cysteine-S-conjugate beta-lyase reduced the toxicity of each of the cisplatin-conjugates. These data demonstrate that metabolism of cisplatin in proximal tubule cells is required for its nephrotoxicity. The elucidation of this pathway provides new targets for the inhibition of cisplatin nephrotoxicity.

Cellular energetics and glutathione status in NRK-52E cells: toxicological implications

Cellular energetics and redox status were evaluated in NRK-52E cells, a stable cell line derived from rat proximal tubules. To assess toxicological implications of these properties, susceptibility to apoptosis induced by S-(1,2-dichlorovinyl)-L-cysteine (DCVC), a well-known mitochondrial and renal cytotoxicant, was studied. Cells exhibited high activities of several glutathione (GSH)-dependent enzymes, including gamma-glutamylcysteine synthetase, GSH peroxidase, glutathione disulfide reductase, and GSH S-transferase, but very low activities of gamma-glutamyltransferase and alkaline phosphatase, consistent with a low content of brush-border microvilli. Uptake and total cellular accumulation of [14C]alpha-methylglucose was significantly higher when cells were exposed at the basolateral as compared to the brush-border membrane. Similarly, uptake of GSH was nearly 2-fold higher across the basolateral than the brush-border membrane. High activities of (Na(+)+K(+))-ATPase and malic dehydrogenase, but low activities of other mitochondrial enzymes, respiration, and transport of GSH and dicarboxylates into mitochondria were observed. Examination of mitochondrial density by confocal microscopy, using a fluorescent marker (MitoTracker Orange), indicated that NRK-52E cells contain a much lower content of mitochondria than rat renal proximal tubules in vivo. Incubation of cells with DCVC caused time- and concentration-dependent ATP depletion that was largely dependent on transport and bioactivation, as observed in the rat, on induction of apoptosis, and on morphological damage. Comparison with primary cultures of rat and human proximal tubular cells suggests that the NRK-52E cells are modestly less sensitive to DCVC. In most respects, however, NRK-52E cells exhibited functions similar to those of the rat renal proximal tubule in vivo.

Sulfoxides as urinary metabolites of S-allyl-L-cysteine in rats: evidence for the involvement of flavin-containing monooxygenases

S-Allyl-L-cysteine (SAC), a component of garlic and a metabolite of allyl halides, is a known substrate for multiple flavin-containing monooxygenases (FMOs). In the current study, we characterize the in vivo SAC metabolism by investigating the presence of SAC, N-acetyl-S-allyl-L-cysteine (NASAC), and their corresponding sulfoxides in the urine of rats given SAC (200 or 400 mg/kg i.p.). In some experiments, rats were given aminooxyacetic acid (AOAA), an inhibitor of cysteine conjugate beta-lyase, or methimazole, an alternative FMO substrate, 30 min prior to treatment with 200 mg/kg SAC. Nearly 40 to 50% of the dose was recovered in the 24-h collection period. In all treatment groups, the majority of the metabolites were excreted within 8 h. The major metabolites detected were NASAC and NASAC sulfoxide (NASACS; nearly 30-40% and 5-10% of the dose, respectively). Only small amounts of the dose (approximately 1.5%) were recovered as SAC and SAC sulfoxide (SACS). Methimazole pretreatment significantly reduced amounts of both SACS and NASACS detected in the urine when compared with rats given SAC only, whereas AOAA pretreatment had no effect. In vitro assays using rat liver microsomes were also carried out to compare the sulfoxidation rates of SAC and NASAC. The results showed that SAC was much more readily oxidized than NASAC. Collectively, the results provide evidence for the involvement of FMOs in the in vivo metabolism of SAC and that SAC is a much better substrate for FMOs than its corresponding mercapturic acid.

Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate beta-lyases to pyruvate, ammonia, and an alpha-chloroenethiolate (with DCVC) or an alpha-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with epsilon-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5'-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate beta-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCAT(m)). Most of the cysteine S-conjugate beta-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the alpha-ketoglutarate- and branched-chain alpha-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCAT(m) may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

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.

Renal toxicity of perchloroethylene and S-(1,2,2-trichlorovinyl)glutathione in rats and mice: sex- and species-dependent differences

Suspensions of renal cells from rats and renal mitochondria from rats and mice were used to assess the sex and species dependence of acute toxicity due to perchloroethylene (Perc) and its glutathione conjugate S-(1,2,2-trichlorovinyl)glutathione (TCVG). A marked sex dependence in the acute cytotoxicity of both Perc and TCVG was observed: Perc caused significant release of lactate dehydrogenase (LDH) in isolated kidney cells from male but not female rats, and TCVG caused much more LDH release from male than female rat kidney cells. Assessment of toxicity in suspensions of isolated mitochondria from kidneys of male and female rats revealed a generally similar pattern of sensitivity, with mitochondria from males exhibiting significantly more inhibition of State 3 respiration and decrease of respiratory control ratio than mitochondria from females. Respiratory function in mitochondria from male and female mice, however, was also significantly inhibited by Perc or TCVG but exhibited little sex dependence in the degree of inhibition. Comparison with results from similar studies using the congener trichloroethylene and its glutathione conjugate suggested that Perc and TCVG are more potent nephrotoxicants. Neither Perc nor TCVG produced any significant effects on cytotoxicity or mitochondrial function in isolated hepatocytes from rats or in isolated liver mitochondria from rats or mice, suggesting that the liver is not a major acute target for Perc or its glutathione conjugate. Thus, many of the species-, sex-, and tissue-dependent differences in toxicity of Perc and TCVG that are observed in vivo are also observed in these in vitro models.

Inhibition of gamma-glutamyl transpeptidase or cysteine S-conjugate beta-lyase activity blocks the nephrotoxicity of cisplatin in mice

Cisplatin is nephrotoxic. The mechanism underlying this organ-specific toxicity is unknown. We hypothesize that cisplatin is metabolized via a gamma-glutamyl transpeptidase (GGT) and cysteine S-conjugate beta-lyase-dependent pathway that has been shown to activate several haloalkenes to nephrotoxins. To test this hypothesis, we inhibited GGT and cysteine S-conjugate beta-lyase in C57BL/6 mice and analyzed the effect of the inhibitors on the nephrotoxicity of cisplatin. GGT was inhibited by pretreating the mice with acivicin. Cysteine S-conjugate beta-lyase was inhibited by aminooxyacetic acid (AOAA). Male C57BL/6 mice were treated with 15 mg/kg cisplatin (i.p.) and sacrificed on day 5. Half the mice treated with cisplatin alone died before sacrifice. The cisplatin-treated mice sacrificed at 5 days had significantly elevated levels of blood urea nitrogen (BUN). Histologic analysis revealed severe damage to the renal proximal tubules. Pretreatment with acivicin or AOAA protected the mice from the nephrotoxicity of cisplatin. None of the pretreated animals died before sacrifice. BUN levels and quantitative histologic analysis of the kidneys confirmed the protective effect of acivicin and AOAA. Platinum levels in the kidneys were not altered by acivicin or AOAA, indicating that neither affected the uptake of cisplatin into the kidney. Likewise, cisplatin-induced weight loss was not altered by acivicin or AOAA, suggesting that weight loss and nephrotoxicity are via distinct mechanisms. These data support the hypothesis that the nephrotoxicity of cisplatin is due to the metabolism of a platinum-glutathione conjugate by GGT and cysteine S-conjugate beta-lyase to a potent nephrotoxin.

Apoptosis, necrosis, and cell proliferation induced by S-(1,2-dichlorovinyl)-L-cysteine in primary cultures of human proximal tubular cells

Apoptosis, necrosis, and cell proliferation induced by S-(1,2-dichlorovinyl)-L-cysteine (DCVC), the cysteine conjugate of the environmental and occupational contaminant trichloroethylene, were studied in primary cultures of human proximal tubular (hPT) cells. Cells from male and female donors were incubated with a range of concentrations of DCVC (10 to 1000 microM) for up to 48 h, and assessments of cellular morphology (phase-contrast microscopy), necrosis (lactate dehydrogenase (LDH) release), apoptosis(cell cycle analysis, annexin V staining, and caspase activation), and proliferation (cell cycle analysis and DNA synthesis) were made. Time- and concentration-dependent changes in cellular morphology, including elongation of cell shape, formation of intracellular vesicles, and formation of apoptotic bodies, were observed. Significant increases in LDH release occurred in hPT cells incubated with < or =100 microM DCVC for at least 24 h. hPT cells from males were modestly more sensitive to DCVC than those from females, with maximal LDH release of 78 and 65% in cells from males and females, respectively. Flow cytometry analysis of propidium iodide-stained and DCVC-treated hPT cells showed that apoptosis occurred at markedly lower concentrations (10 microM) and at much earlier incubation times (2 h) than necrosis. A small increase was also noted in the percentage of cells in S-phase after a 4-h treatment with as little as 10 microM DCVC, suggesting that cell proliferation was stimulated. This was supported further by increased DNA synthesis. These results show that DCVC causes apoptosis and enhances cell proliferation in hPT cells at environmentally relevant doses and at earlier time points and lower concentrations than necrosis.