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

Trichloroethylene exposure, multi-organ injury, and potential mechanisms: A narrative review

Trichloroethylene (TCE) is a common environmental pollutant and industrial chemical that has been associated with adverse health effects, especially on organ systems. The purpose of this review is to summarize the current findings on organ system damage caused by TCE exposure and the underlying mechanisms involved. Numerous studies have shown that TCE exposure may cause damage to multiple organ systems, mainly the skin, liver, kidney, and circulatory system. The mechanisms leading to TCE-induced organ system damage are complex and diverse. TCE is metabolized in vivo to reactive intermediates, through which TCE can induce oxidative stress, interfere with cell signaling pathways, and promote inflammatory responses. In addition, studies have shown that TCE interferes with DNA repair mechanisms, leading to genotoxicity and potentially carcinogenic effects. This review highlights the importance of understanding the deleterious effects of TCE exposure on organ systems and provides insights into the underlying mechanisms involved. Further research is needed to elucidate the full range of organ system damage caused by TCE and to develop effective prevention and treatment strategies.

Renal Glutathione: Dual roles as antioxidant protector and bioactivation promoter

The tripeptide glutathione (GSH) possesses two key structural features, namely the nucleophilic sulfur and the γ-glutamyl isopeptide bond. The former allows GSH to serve as a critical antioxidant and anti-electrophile. The latter allows GSH to translocate throughout the systemic circulation without being degraded. The kidneys exhibit several unique processes for handling GSH. This includes the extraction of 80% of plasma GSH, in part by glomerular filtration but mostly by transport across the basolateral plasma membrane. Studies on the protective effect of exogenous GSH are summarized, showing the different inherent susceptibility of proximal tubular and distal tubular cells and the impact on pathological or disease states, including hypoxia, diabetic nephropathy, and compensatory renal growth associated with uninephrectomy. Studies on mitochondrial GSH transport show the coordination between the citric acid cycle and oxidative phosphorylation in generating driving forces for both plasma membrane and mitochondrial carriers. The strong protective effects of increasing expression and activity of these carriers against oxidants and mitochondrial toxicants are summarized. Although GSH plays a cytoprotective role in most situations, two distinct exceptions to this are presented. In contrast to expectations, overexpression of the mitochondrial 2-oxoglutarate carrier markedly increased cell death from exposure to the nephrotoxic chemotherapeutic drug cisplatin (CDDP). Another key example of GSH serving a bioactivation role in the kidneys, rather than a detoxification role, is the metabolism of halogenated alkenes such as trichloroethylene (TCE). Although considerable research has gone into this topic, unanswered questions and emerging topics remain and are discussed.

Transcriptional profiling of the response to the trichloroethylene metabolite S-(1,2-dichlorovinyl)-L-cysteine revealed activation of the eIF2α/ATF4 integrated stress response in two in vitro placental models

Trichloroethylene (TCE) is an industrial solvent and widespread environmental contaminant. Although TCE exposure is prevalent, epidemiological studies of TCE exposure associations with adverse birth outcomes are inconclusive. Prior studies show that the TCE metabolite S-(1,2-dichlorovinyl)-l-cysteine (DCVC) exhibits toxicity in a placental cell line. In the current study, genome-wide gene expression and gene set enrichment analyses were used to identify novel genes and pathway alterations in the HTR-8/SVneo human trophoblast cell line and human placental villous explants treated with DCVC at concentrations relevant to human exposures. In the cells, concentration- and time-dependent effects were observed, as evidenced by the magnitude of altered gene expression after treatment with 20 µM DCVC versus 10 µM, and 12-h versus 6-h of treatment. Comparing the two models for the transcriptional response to 12-h 20 µM DCVC treatment, no differentially expressed genes reached significance in villous explants, whereas 301 differentially expressed genes were detected in HTR-8/SVneo cells compared with non-treated controls (FDR < 0.05 + LogFC > 0.35 [FC > 1.3]). GSEA revealed five upregulated enriched pathways in common between explants and cells (FDR < 0.05). Moreover, all 12-h DCVC treatment groups from both models contained upregulated pathways enriched for genes regulated by the ATF4 transcription factor. The overrepresentation of ATF4 regulation of differentially expressed genes indicated activation of the integrated stress response (ISR), a condition triggered by multiple stress stimuli, including the unfolded protein response. DCVC-induced ISR activation was confirmed by elevated eIF2α phosphorylation, ATF4 protein concentrations, and decreased global protein synthesis in HTR-8/SVneo cells. This study identifies a mechanism of DCVC-induced cytotoxicity by revealing the involvement of a specific stress signaling pathway.

The trichloroethylene metabolite S-(1,2-dichlorovinyl)-L-cysteine induces progressive mitochondrial dysfunction in HTR-8/SVneo trophoblasts

Trichloroethylene is an industrial solvent and common environmental pollutant. Despite efforts to ban trichloroethylene, its availability and usage persist globally, constituting a hazard to human health. Recent studies reported associations between maternal trichloroethylene exposure and increased risk for low birth weight. Despite these associations, the toxicological mechanism underlying trichloroethylene adverse effects on pregnancy remains largely unknown. The trichloroethylene metabolite S-(1,2-dichlorovinyl)-L-cysteine (DCVC) induces mitochondrial-mediated apoptosis in a trophoblast cell line. To gain further understanding of mitochondrial-mediated DCVC placental toxicity, this study investigated the effects of DCVC exposure on mitochondrial function using non-cytolethal concentrations in placental cells. Human trophoblasts, HTR-8/SVneo, were exposed in vitro to a maximum of 20 μM DCVC for up to 12 h. Cell-based oxygen consumption and extracellular acidification assays were used to evaluate key aspects of mitochondrial function. Following 6 h of exposure to 20 μM DCVC, elevated oxygen consumption, mitochondrial proton leak and sustained energy coupling deficiency were observed. Similarly, 12 h of exposure to 20 μM DCVC decreased mitochondrial-dependent basal, ATP-linked and maximum oxygen consumption rates. Using the fluorochrome TMRE, dissipation of mitochondrial membrane potential was detected after a 12-h exposure to 20 μM DCVC, and (±)-α-tocopherol, a known suppressor of lipid peroxidation, attenuated DCVC-stimulated mitochondrial membrane depolarization but failed to rescue oxygen consumption perturbations. Together, these results suggest that DCVC caused progressive mitochondrial dysfunction, resulting in lipid peroxidation-associated mitochondrial membrane depolarization. Our findings contribute to the biological plausibility of DCVC-induced placental impairment and provide new insights into the role of the mitochondria in DCVC-induced toxicity.

Metabolism and Toxicity of Trichloroethylene and Tetrachloroethylene in Cytochrome P450 2E1 Knockout and Humanized Transgenic Mice

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are structurally similar olefins that can cause liver and kidney toxicity. Adverse effects of these chemicals are associated with metabolism to oxidative and glutathione conjugation moieties. It is thought that CYP2E1 is crucial to the oxidative metabolism of TCE and PCE, and may also play a role in formation of nephrotoxic metabolites; however, inter-species and inter-individual differences in contribution of CYP2E1 to metabolism and toxicity are not well understood. Therefore, the role of CYP2E1 in metabolism and toxic effects of TCE and PCE was investigated using male and female wild-type [129S1/SvlmJ], Cyp2e1(-/-), and humanized Cyp2e1 [hCYP2E1] mice. To fill in existing gaps in our knowledge, we conducted a toxicokinetic study of TCE (600 mg/kg, single dose, i.g.) and a subacute study of PCE (500 mg/kg/day, 5 days, i.g.) in 3 strains. Liver and kidney tissues were subject to profiling of oxidative and glutathione conjugation metabolites of TCE and PCE, as well as toxicity endpoints. The amounts of trichloroacetic acid formed in the liver was hCYP2E1≈ 129S1/SvlmJ > Cyp2e1(-/-) for both TCE and PCE; levels in males were about 2-fold higher than in females. Interestingly, 2- to 3-fold higher levels of conjugation metabolites were observed in TCE-treated Cyp2e1(-/-) mice. PCE induced lipid accumulation only in liver of 129S1/SvlmJ mice. In the kidney, PCE exposure resulted in acute proximal tubule injury in both sexes in all strains (hCYP2E1 ≈ 129S1/SvlmJ > Cyp2e1(-/-)). In conclusion, our results demonstrate that CYP2E1 is an important, but not exclusive actor in the oxidative metabolism and toxicity of TCE and PCE.

Comparative analysis of metabolism of trichloroethylene and tetrachloroethylene among mouse tissues and strains

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are structurally similar chemicals that are metabolized through oxidation and glutathione conjugation pathways. Both chemicals have been shown to elicit liver and kidney toxicity in rodents and humans; however, TCE has been studied much more extensively in terms of both metabolism and toxicity. Despite their qualitative similarities, quantitative comparison of tissue- and strain-specific metabolism of TCE and PCE has not been performed. To fill this gap, we conducted a comparative toxicokinetic study where equimolar single oral doses of TCE (800 mg/kg) or PCE (1000 mg/kg) were administered to male mice of C57BL/6J, B6C3F1/J, and NZW/LacJ strains. Samples of liver, kidney, serum, brain, and lung were obtained for up to 36 h after dosing. For each tissue, concentrations of parent compounds, as well as their oxidative and glutathione conjugation metabolites were measured and concentration-time profiles constructed. A multi-compartment toxicokinetic model was developed to quantitatively compare TCE and PCE metabolism. As expected, the flux through oxidation metabolism pathway predominated over that through conjugation across all mouse strains examined, it is 1,200-3,800 fold higher for TCE and 26-34 fold higher for PCE. However, the flux through glutathione conjugation, albeit a minor metabolic pathway, was 21-fold higher for PCE as compared to TCE. The degree of inter-strain variability was greatest for oxidative metabolites in TCE-treated and for glutathione conjugation metabolites in PCE-treated mice. This study provides critical data for quantitative comparisons of TCE and PCE metabolism, and may explain the differences in organ-specific toxicity between these structurally similar chemicals.

Trichloroethylene metabolite S-(1,2-dichlorovinyl)-l-cysteine induces lipid peroxidation-associated apoptosis via the intrinsic and extrinsic apoptosis pathways in a first-trimester placental cell line

Trichloroethylene (TCE), a prevalent environmental contaminant, is a potent renal and hepatic toxicant through metabolites such as S-(1, 2-dichlorovinyl)-l-cysteine (DCVC). However, effects of TCE on other target organs such as the placenta have been minimally explored. Because elevated apoptosis and lipid peroxidation in placenta have been observed in pregnancy morbidities involving poor placentation, we evaluated the effects of DCVC exposure on apoptosis and lipid peroxidation in a human extravillous trophoblast cell line, HTR-8/SVneo. We exposed the cells in vitro to 10-100μM DCVC for various time points up to 24h. Following exposure, we measured apoptosis using flow cytometry, caspase activity using luminescence assays, gene expression using qRT-PCR, and lipid peroxidation using a malondialdehyde quantification assay. DCVC significantly increased apoptosis in time- and concentration-dependent manners (p<0.05). DCVC also significantly stimulated caspase 3, 7, 8 and 9 activities after 12h (p<0.05), suggesting that DCVC stimulates the activation of both the intrinsic and extrinsic apoptotic signaling pathways simultaneously. Pre-treatment with the tBID inhibitor Bl-6C9 partially reduced DCVC-stimulated caspase 3 and 7 activity, signifying crosstalk between the two pathways. Additionally, DCVC treatment increased lipid peroxidation in a concentration-dependent manner. Co-treatment with the antioxidant peroxyl radical scavenger (±)-α-tocopherol attenuated caspase 3 and 7 activity, suggesting that lipid peroxidation mediates DCVC-induced apoptosis in extravillous trophoblasts. Our findings suggest that DCVC-induced apoptosis and lipid peroxidation in extravillous trophoblasts could contribute to poor placentation if similar effects occur in vivo in response to TCE exposure, indicating that further studies into this mechanism are warranted.

Characterization of inter-tissue and inter-strain variability of TCE glutathione conjugation metabolites DCVG, DCVC, and NAcDCVC in the mouse

Trichloroethylene (TCE) is a ubiquitous environmental toxicant that is a liver and kidney carcinogen. Conjugation of TCE with glutathione (GSH) leads to formation of nepthrotoxic and mutagenic metabolites postulated to be critical for kidney cancerdevelopment; however, relatively little is known regarding their tissue levels as previous analytical methods for their detection lacked sensitivity. Here, an LC-MS/MS-based method for simultaneous detection of S-(1,2-dichlorovinyl)-glutathione (DCVG), S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (NAcDCVC) in multiple mouse tissues was developed. This analytical method is rapid, sensitive (limits of detection (LOD) 3-30 fmol across metabolites and tissues), and robust to quantify all three metabolites in liver, kidneys, and serum. The method was used to characterize inter-tissue and inter-strain variability in formation of conjugative metabolites of TCE. Single oral dose of TCE (24, 240 or 800 mg/kg) was administered to male mice from 20 inbred strains of Collaborative Cross. Inter-strain variability in the levels of DCVG, DCVC, and NAcDCVC (GSD = 1.6-2.9) was observed. Whereas NAcDCVC was distributed equally among analyzed tissues, highest levels of DCVG were detected in liver and DCVC in kidneys. Evidence indicated that inter-strain variability in conjugative metabolite formation of TCE might affect susceptibility to adverse health effects and that this method might aid in filling data gaps in human health assessment of TCE.

Xenobiotic transporters and kidney injury

Renal proximal tubules are targets for toxicity due in part to the expression of transporters that mediate the secretion and reabsorption of xenobiotics. Alterations in transporter expression and/or function can enhance the accumulation of toxicants and sensitize the kidneys to injury. This can be observed when xenobiotic uptake by carrier proteins is increased or efflux of toxicants and their metabolites is reduced. Nephrotoxic chemicals include environmental contaminants (halogenated hydrocarbon solvents, the herbicide paraquat, the fungal toxin ochratoxin, and heavy metals) as well as pharmaceuticals (certain beta-lactam antibiotics, antiviral drugs, and chemotherapeutic drugs). This review explores the mechanisms by which transporters mediate the entry and exit of toxicants from renal tubule cells and influence the degree of kidney injury. Delineating how transport proteins regulate the renal accumulation of toxicants is critical for understanding the likelihood of nephrotoxicity resulting from competition for excretion or genetic polymorphisms that affect transporter function.

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.

Modulation of trichloroethylene in vitro metabolism by different drugs in human

Toxicological interactions with drugs have the potential to modulate the toxicity of trichloroethylene (TCE). Our objective is to identify metabolic interactions between TCE and 14 widely used drugs in human suspended hepatocytes and characterize the strongest using microsomal assays. Changes in concentrations of TCE and its metabolites were measured by headspace GC-MS. Results with hepatocytes show that amoxicillin, cimetidine, ibuprofen, mefenamic acid and ranitidine caused no significant interactions. Naproxen and salicylic acid showed to increase both TCE metabolites levels, whereas acetaminophen, carbamazepine and erythromycin rather decreased them. Finally, diclofenac, gliclazide, sulphasalazine and valproic acid had an impact on the levels of only one metabolite. Among the 14 tested drugs, 5 presented the most potent interactions and were selected for confirmation with microsomes, namely naproxen, salicylic acid, acetaminophen, carbamazepine and valproic acid. Characterization in human microsomes confirmed interaction with naproxen by competitively inhibiting trichloroethanol (TCOH) glucuronidation (Ki=2.329 mM). Inhibition of TCOH formation was also confirmed for carbamazepine (partial non-competitive with Ki=70 μM). Interactions with human microsomes were not observed with salicylic acid and acetaminophen, similar to prior results in rat material. For valproic acid, interactions with microsomes were observed in rat but not in human. Inhibition patterns were shown to be similar in human and rat hepatocytes, but some differences in mechanisms were noted in microsomal material between species. Next research efforts will focus on determining the adequacy between in vitro observations and the in vivo situation.

Glutathione-dependent bioactivation

The classical view of the glutathione (GSH) conjugation pathway involves GSH S-transferase (GST)-dependent formation of thioether conjugates between GSH and an electrophilic substrate, processing to yield the corresponding cysteine S-conjugate, which is then converted to an N-acetylcysteine conjugate (or mercapturate). Mercapturates of most GST substrates are rendered more polar and thus readily excreted in urine. In contrast, there is a growing number of GST substrates that, rather than being detoxified, are bioactivated. These substrates include several halogenated solvents, many of which are nephrotoxic because of the tissue distribution of GSH conjugation pathway enzymes and membrane transporters, and prodrugs of certain chemotherapeutic agents. Although the initiating steps are the same regardless of whether the substrate is detoxified or bioactivated, the cysteine conjugate functions as a branch point. Bioactivated cysteine S-conjugates are metabolized in the kidneys by either cysteine conjugate β-lyase or flavin-containing monooxygenase to produce a reactive intermediate.

Diosmin protects against trichloroethylene-induced renal injury in Wistar rats: plausible role of p53, Bax and caspases

Diosmin (DM) is a naturally occurring flavone and has been found to possess numerous therapeutic properties. In this study, we used DM as a protective agent against the nephrotoxic effects of the environmental toxicant trichloroethylene (TCE). Male Wistar rats were divided into five groups (I-V, n 6). Groups II, III and IV received an oral administration of TCE at a dose of 1000 mg/kg body weight for twenty consecutive days. The animals in groups II and III received an oral treatment of DM at doses of 20 and 40 mg/kg body weight, respectively, for twenty consecutive days, while groups I and V were given maize oil (5 ml/kg body weight and DM 40 mg/kg body weight, respectively) for 20 d. The protective effects of DM on TCE-induced oxidative stress and caspase-dependent apoptosis were investigated by assaying oxidative stress biomarkers, lipid peroxidation (LPO), serum toxicity markers, alkaline unwinding assay, caspase-3, -7 and -9, Bax and p53 expression. Oral administration of TCE in rats enhanced renal LPO, depleted glutathione content and antioxidant enzymes, induced DNA strand breaks (P<0·001), modulated the expression of Bax and p53 protein and induced the expression of caspase-3, -7 and -9. Co-treatment with DM prevented oxidative stress by restoring the levels of antioxidant enzymes; furthermore, a significant dose-dependent decrease in DNA disintegration and kidney toxicity markers such as blood urea N, creatinine, lactate dehydrogenase and kidney injury molecule-1 was observed. DM also effectively decreased the TCE-induced up-regulation of Bax and p53. Data from the present study establish the protective role of DM against TCE-induced renal damage.

Transport of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine, a metabolite of trichloroethylene, by mouse multidrug resistance associated protein 2 (Mrp2)

N-acetyl-S-(1,2-dichlorovinyl)-l-cysteine (Ac-DCVC) and S-(1,2-dichlorovinyl)-l-cysteine (DCVC) are the glutathione conjugation pathway metabolites of a common industrial contaminant and potent nephrotoxicant trichloroethylene (TCE). Ac-DCVC and DCVC are accumulated in the renal proximal tubule where they may be secreted into the urine by an unknown apical transporter(s). In this study, we explored the hypothesis that the apical transport of Ac-DCVC and/or DCVC may be mediated by the multidrug resistance associated protein 2 (Mrp2, ABCC2), which is known to mediate proximal tubular apical ATP-dependent transport of glutathione and numerous xenobiotics and endogenous substances conjugated with glutathione. Transport experiments using membrane vesicles prepared from mouse proximal tubule derived cells expressing mouse Mrp2 utilizing ATPase assay and direct measurements of Ac-DCVC/DCVC using liquid chromatography/tandem mass-spectrometry (LC/MS/MS) demonstrated that mouse Mrp2 mediates ATP-dependent transport of Ac-DCVC. Expression of mouse Mrp2 antisense mRNA significantly inhibited the vectorial basolateral to apical transport of Ac-DCVC but not DCVC in mouse proximal tubule derived cells endogenously expressing mouse Mrp2. The results suggest that Mrp2 may be involved in the renal secretion of Ac-DCVC.

Effects of genetic polymorphisms of N-Acetyltransferase on trichloroethylene-induced hypersensitivity dermatitis among exposed workers

The hypersensitivity dermatitis induced by trichloroethylene (TCE) exposure was influenced by individual genetic susceptibility factors. In this paper, a case-control study was conducted to investigate effects of various genotypes and phenotypes of N-Acetyltransferases (NATs) on individual susceptibility to the disease. The study consists of 111 patients with hypersensitivity dermatitis and 154 healthy TCE-exposed workers. Polymerase chain reaction-restriction fragment length polymorphism was used to detect the polymorphic sites of NAT1 at nt 1095 and 1088 and the sites of NAT2 at nt 481, 590, and 857. Logistic regression was used to calculate the odds ratios (ORs) and 95% confidence intervals (95%CI). The results reveal that subjects with intermediate or slow acetylators of NAT2 have a 2.01 fold (95%CI=1.14-3.54) higher risk for the disease than subjects with the fast acetylators. When non-fast NAT2 phenotype (intermediate and slow acetylators) and a slow NAT1 phenotype were combined, the risk for the disease was significantly increased (OR=2.71, 95%CI 1.29-5.70) to the level higher than that observed for NAT2 non-fast acetylators phenotype alone. These findings suggest that slow metabolic phenotype of NAT2 maybe one of risk factor for TCE-induced hypersensitivity dermatitis and combined slow acetylator phenotypes of NAT1 and NAT2 further increase such risk.

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

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

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+).

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.

Drug metabolism enzyme expression and activity in primary cultures of human proximal tubular cells

We previously catalogued expression and activity of organic anion and cation, amino acid, and peptide transporters in primary cultures of human proximal tubular (hPT) cells to establish them as a cellular model to study drug transport in the human kidney [Lash, L.H., Putt, D.A., Cai, H., 2006. Membrane transport function in primary cultures of human proximal tubular cells. Toxicology 228, 200-218]. Here, we extend our analysis to drug metabolism enzymes. Expression of 11 cytochrome P450 (CYP) enzymes was determined with specific antibodies. CYP1B1, CYP3A4, and CYP4A11 were the only CYP enzymes readily detected in total cell extracts. These same CYP enzymes, as well as CYP3A5 and possibly CYP2D6, were detected in microsomes from confluent hPT cells, although expression levels varied among kidney samples. In agreement with Western blot data, only activity of CYP3A4/5 was detected among the enzyme activities measured. Expression of all three glutathione S-transferases (GSTs) known to be found in hPT cells, GSTA, GSTP, and GSTT, was readily detected. Variable expression of three sulfotransferases (SULTs), SULT1A3, SULT1E, and SULT2A1, and three UDP-glucuronosyltransferases (UGTs), UGT1A1, UGT1A6, and UGT2B7, was also detected. When examined over the course of cell growth to confluence, expression of all enzymes was generally maintained at readily measurable levels, although they were often lower than in fresh tissue. These results indicate that primary cultures of hPT cells possess significant capacity to metabolize many classes of drugs, and can be used as an effective model to study drug metabolism.

Akt activation improves oxidative phosphorylation in renal proximal tubular cells following nephrotoxicant injury

Previously, we showed that protein kinase B (Akt) activation increases intracellular ATP levels and decreases necrosis in renal proximal tubular cells (RPTC) injured by the nephrotoxicant S-(1, 2-dichlorovinyl)-l-cysteine (DCVC) (Shaik ZP, Fifer EK, Nowak G. Am J Physiol Renal Physiol 292: F292-F303, 2007). This study examined the role of Akt in improving mitochondrial function in DCVC-injured RPTC. Our data show a novel observation that phosphorylated (active) Akt is localized in mitochondria of noninjured RPTC, both in mitoplasts and the mitochondrial outer membrane. Mitochondrial levels of active Akt decreased in nephrotoxicant-injured RPTC, and this decrease was associated with mitochondrial dysfunction. DCVC decreased basal, uncoupled, and state 3 respirations; ATP production; activities of complexes I, II, and III; the mitochondrial membrane potential (DeltaPsi(m)); and F(0)F(1)-ATPase activity. Expressing constitutively active Akt in DCVC-injured RPTC increased the levels of phosphorylated Akt in mitochondria, reduced the decreases in basal and uncoupled respirations, increased complex I-coupled state 3 respiration and ATP production, enhanced activities of complex I, complex III, and F(0)F(1)-ATPase, and improved DeltaPsi(m). In contrast, inhibiting Akt activation by expressing dominant negative (inactive) Akt or using 20 microM LY294002 exacerbated decreases in electron transport rate, state 3 respiration, ATP production, DeltaPsi(m), and activities of complex I, complex III, and F(0)F(1)-ATPase. In conclusion, our data show that Akt activation promotes mitochondrial respiration and ATP production in toxicant-injured RPTC by 1) improving integrity of the respiratory chain and maintaining activities of complex I and complex III, 2) reducing decreases in DeltaPsi(m), and 3) restoring F(0)F(1)-ATPase activity.

Methods for measuring cysteine S-conjugate β-lyase activity

The cysteine conjugate β-lyase represents activities in cytoplasm and mitochondria catalyzed by at least eleven pyridoxal 5'-phosphate (PLP)-dependent enzymes in various tissues. These enzymes mediate bioactivation of cysteine S-conjugates of several haloalkanes and haloalkenes. The reaction occurs through either a direct β-elimination or a transamination followed by a retro-Michael rearrangement, resulting in the cleavage of a C-S bond. The resultant product is a reactive thiolate that rearranges to form thioacylating species. This unit presents several protocols for the assay of β-lyase activity and includes measurements of product formation and substrate loss as well as fluorescent activity stains. Support protocols describe the synthesis and high-performance liquid chromatography analysis of selected cysteine S-conjugates. Because of the diversity of enzymes that can catalyze a β-lyase reaction, each of the assays presented here may indicate only a portion of the potential β-lyase activity in a given biological preparation.

Localization of broadly selective equilibrative and concentrative nucleoside transporters, hENT1 and hCNT3, in human kidney

Nucleoside transporters in kidney mediate renal reabsorption and secretion of nucleosides. Using RT-PCR, we demonstrated mRNAs encoding hENT1, hENT2, hCNT1, hCNT2, and hCNT3 in both cortex and medulla. Immunoblotting with crude membrane preparations revealed abundant hENT1 and hCNT3 in both cortex and medulla, and little, if any, hENT2, hCNT1, or hCNT2, indicating that the latter were either absent or below limits of detection of immunoassays. hENT1 immunostaining was observed on apical surfaces of proximal tubules and on both apical and basal surfaces of thick ascending loops of Henle and collecting ducts. Prominent hCNT3 immunostaining was observed on apical surfaces of proximal tubules and thick ascending loops of Henle in addition to some cytoplasmic staining. Equilibrium binding of [3H]nitrobenzylmercaptopurine ribonucleoside (NBMPR), a high-affinity inhibitor of hENT1, to brush-border membrane vesicles from cortex confirmed the presence of hENT1 on apical surfaces of proximal tubules. Uptake of [3H]uridine by polarized renal proximal tubule cells exhibited a sodium-dependent component that was inhibited by thymidine and inosine as well as a sodium-independent component that was partially inhibited by NBMPR and completely inhibited by dilazep, indicating high levels of hENT1 and hCNT3 and low levels of hENT2 activities. The presence of 1) transcripts for hENT1/2 and hCNT1/2/3 and the hENT1 and hCNT3 proteins in human kidneys and 2) hENT1, hENT2, and hCNT3 activities in cultured proximal tubule cells suggest involvement of hENT1, hCNT3, and possibly also hENT2 in renal handling of nucleosides and nucleoside drugs.

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.

Glutamine donor pretreatment in rat kidney transplants with severe preservation reperfusion injury

BACKGROUND

Glutamine (GLN) has been shown to confer cytoprotection by enhancing endogenous heat shock protein (HSP) expression. We hypothesized that GLN donor pretreatment protects rat renal grafts against severe preservation reperfusion injury (PRI).

MATERIALS AND METHODS

GLN (0.75 g/kg) or saline was administered i.p. to male donor rats 24 h and 6 h before donor nephrectomy. Kidneys (n = 6/group) were cold-stored in UW solution for 40 h and transplanted into bilaterally nephrectomized syngeneic recipients. Grafts were removed after 24 h. Renal HSP 70 expression was determined by Western blotting. Graft function was assessed by serum creatinine. Renal cross sections were microscopically examined for acute tubular necrosis, apoptosis, tubular proliferation, and macrophage infiltration.

RESULTS

GLN donor pretreatment significantly increased intragraft HSP 70 expression. Serum creatinine was not different between groups: 2.6 +/- 0.2 mg/dL (saline) versus 2.7 +/- 0.5 mg/dL (GLN). Both treatment groups showed severe tubular damage with significantly less papillary necrosis in the GLN group (P < 0.05). GLN significantly reduced the number of apoptotic tubular cells in the cortex, medulla, and papilla (P < 0.001 versus saline). Postinjury tubular proliferation, measured by PCNA antigen expression, and intragraft macrophage infiltration was not influenced by GLN.

CONCLUSIONS

In rat renal grafts suffering severe PRI pharmacological preconditioning with GLN attenuates early structural damage, especially tubular cell apoptosis. Stimulation of renal HSP 70 expression could be an important mechanism of GLN-induced cytoprotection. Our findings may have implications for the treatment of delayed graft function in recipients of marginal donor kidneys.

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

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

Key issues in the modes of action and effects of trichloroethylene metabolites for liver and kidney tumorigenesis

Trichloroethylene (TCE) exposure has been associated with increased risk of liver and kidney cancer in both laboratory animal and epidemiologic studies. The U.S. Environmental Protection Agency 2001 draft TCE risk assessment concluded that it is difficult to determine which TCE metabolites may be responsible for these effects, the key events involved in their modes of action (MOAs) , and the relevance of these MOAs to humans. In this article, which is part of a mini-monograph on key issues in the health risk assessment of TCE, we present a review of recently published scientific literature examining the effects of TCE metabolites in the context of the preceding questions. Studies of the TCE metabolites dichloroacetic acid (DCA) , trichloroacetic acid (TCA) , and chloral hydrate suggest that both DCA and TCA are involved in TCE-induced liver tumorigenesis and that many DCA effects are consistent with conditions that increase the risk of liver cancer in humans. Studies of S-(1,2-dichlorovinyl) -l-cysteine have revealed a number of different possible cell signaling effects that may be related to kidney tumorigenesis at lower concentrations than those leading to cytotoxicity. Recent studies of trichloroethanol exploring an alternative hypothesis for kidney tumorigenesis have failed to establish the formation of formate as a key event for TCE-induced kidney tumors. Overall, although MOAs and key events for TCE-induced liver and kidney tumors have yet to be definitively established, these results support the likelihood that toxicity is due to multiple metabolites through several MOAs, none of which appear to be irrelevant to humans.

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.

Photocytotoxicity of a new Rh2(II,II) complex: increase in cytotoxicity upon irradiation similar to that of PDT agent hematoporphyrin

A new type of heteroleptic dirhodium complex cis-[Rh(2)(mu-O2CCH3)2-(bpy)(dppz)]2+ (3) was synthesized and its potential as a photodynamic therapy (PDT) agent was investigated. Although 27% hypochromicity of the absorption of 3 in the near-UV and visible regions is observed in the presence of duplex DNA, relative viscosity measurements reveal that the complex does not intercalate between the DNA bases. The DNA photocleavage with visible light by 3 proceeds via both oxygen dependent and independent mechanisms, and it is more efficient than that of related complexes. The increase in the cytotoxicity of 3 towards human skin cells is similar to that of hematoporphyrin, a key ingredient in a PDT drug currently in use. This feature makes this complex a useful candidate for further PDT studies.

Porphyromonas gingivalis enhances FasL expression via up-regulation of NFkappaB-mediated gene transcription and induces apoptotic cell death in human gingival epithelial cells

The interaction between epithelial cells and micro-organisms is often a crucial initiating event in infectious diseases. Infection with Porphyromonas gingivalis, a Gram-negative anaerobe, is strongly associated with severe periodontal disease. This bacterium possesses an array of virulence factors, some of which can induce apoptosis. The tumour necrosis factor (TNF) receptor family is involved in the regulation of cellular homeostasis, cell surface molecules involved in phagocytosis, Fas ligand (L) expression and activation of the caspase cascade resulting in DNA fragmentation and cell blebbing. The current study examined the role of nuclear factor-kappaB (NFkappaB) in FasL-mediated apoptotic cell death in primary human gingival epithelial cells (HGEC) induced by heat-killed P. gingivalis, probably through TLR signalling pathways. A marked up-regulation of TLR2 and Fas-FasL was detected in HGEC stimulated with P. gingivalis. Activation of NFkappaB by P. gingivalis in HGEC was demonstrated by an NFkappaB promoter luciferase assay as well as by phosphorylation of p65 as detected by Western blotting. Activation of cleaved caspase-3 and caspase-8 resulted in apoptotic cell death of HGEC. The survival proteins c-IAP-1/c-IAP-2 were decreased in HGEC exposed to P. gingivalis. HGEC apoptosis induced by P. gingivalis was inhibited by an anti-human FasL monoclonal antibody. Blockade of NFkappaB by helenalin resulted in down-regulation of FasL whereas a caspase-8 inhibitor did not decrease FasL. Taken together, these studies show that P. gingivalis can induce epithelial cell apoptosis through Fas-FasL up-regulation and activation of caspase-3 and caspase-8.

Changes in gene expression in human renal proximal tubule cells exposed to low concentrations of S-(1,2-dichlorovinyl)-l-cysteine, a metabolite of trichloroethylene

Epidemiology studies suggest that there may be a weak association between high level exposure to trichloroethylene (TCE) and renal tubule cell carcinoma. Laboratory animal studies have shown an increased incidence of renal tubule carcinoma in male rats but not mice. TCE can undergo metabolism via glutathione (GSH) conjugation to form metabolites that are known to be nephrotoxic. The GSH conjugate, S-(1,2-dichlorovinyl)glutathione (DCVG), is processed further to the cysteine conjugate, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), which is the penultimate nephrotoxic species. We have cultured human renal tubule cells (HRPTC) in serum-free medium under a variety of different culture conditions and observed growth, respiratory control and glucose transport over a 20 day period in medium containing low glucose. Cell death was time- and concentration-dependent, with the EC(50) for DCVG being about 3 microM and for DCVC about 7.5 microM over 10 days. Exposure of HRPTC to sub-cytotoxic doses of DCVC (0.1 microM and 1 microM for 10 days) led to a small number of changes in gene expression, as determined by transcript profiling with Affymetrix human genome chips. Using the criterion of a mean 2-fold change over control for the four samples examined, 3 genes at 0.1 microM DCVC increased, namely, adenosine kinase, zinc finger protein X-linked and an enzyme with lyase activity. At 1 microM DCVC, two genes showed a >2-fold decrease, N-acetyltransferase 8 and complement factor H. At a lower stringency (1.5-fold change), a total of 63 probe sets were altered at 0.1 microM DCVC and 45 at 1 microM DCVC. Genes associated with stress, apoptosis, cell proliferation and repair and DCVC metabolism were altered, as were a small number of genes that did not appear to be associated with the known mode of action of DCVC. Some of these genes may serve as molecular markers of TCE exposure and effects in the human kidney.

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

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

Gene expression profiling of nephrotoxicity from the sevoflurane degradation product fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether ("compound A") in rats

The major degradation product of the volatile anesthetic sevoflurane, the haloalkene fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE or "compound A"), is nephrotoxic in rats. FDVE undergoes complex metabolism and bioactivation, which mediates the nephrotoxicity. Nevertheless, the molecular and cellular mechanisms of FDVE toxification are unknown. This investigation evaluated the gene expression profile of kidneys in rats administered a nephrotoxic dose of FDVE. Male Fischer 344 rats (five per group) received 0.25 mmol/kg intraperitoneal FDVE or corn oil (controls) and were sacrificed after 24 or 72 h. Urine output and kidney histological changes were quantified. Kidney RNA was extracted for microarray analysis using Affymetrix GeneChip Rat Expression Array 230A arrays. Quantitative real-time PCR confirmed the modulation of several genes. FDVE caused significant diuresis and necrosis at 24 h, with normal urine output and evidence of tubular regeneration at 72 h. There were 517 informative genes that were differentially expressed >1.5-fold (p < 0.05) versus control at 24 h, of which 283 and 234 were upregulated and downregulated, respectively. Major classes of upregulated genes included those involved in apoptosis, oxidative stress, and inflammatory response (mostly at 24 h), and regeneration and repair; downregulated genes were generally associated with transporters and intermediary metabolism. Among the quantitatively most upregulated genes were kidney injury molecule, osteopontin, clusterin, tissue inhibitor of metalloproteinase 1, and TNF receptor 12, which have been associated with other forms of nephrotoxicity, and angiopoietin-like protein 4, glycoprotein nmb, ubiquitin hydrolase, and HSP70. Microarray results were confirmed by quantitative real-time PCR. FDVE causes rapid and brisk changes in gene expression, providing potential insights into the mechanism of FDVE toxification, and potential biomarkers for FDVE nephrotoxicity which are more sensitive than conventional measures of renal function.

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.

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.

Apoptosis induction on HL-60 cells of a novel polysaccharide from the mucus of the loach, Misgurnus anguillicaudatus

This study was designed to determine the anti-proliferative, apoptotic properties of a novel polysaccharide from the loach, Misgurnus anguillicaudatus (MAP), using a human promyelocytic leukemia line (HL-60) as a model system. HL-60 cells were cultured in the presence of MAP at various concentrations (50-800 mg/l) for 5 days and the percentage of cell viability was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. The results showed that MAP inhibited the cells viability in time and concentration-dependent characteristics. We found that anti-proliferative effect of MAP was associated with apoptosis on HL-60 cells by determinations of morphological changes and oligonucleosomal DNA fragments. In addition, the content of nitric oxide (NO) and activity of lactate dehydrogenase (LDH) release increased when the cells incubated with MAP at various concentrations and times. These investigations suggest that the polysaccharide from loach has the function of anti-proliferation and induction of apoptosis in tumor cells in vitro.

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.

A flow cytometric assay for simultaneous assessment of drug efflux, proliferation, and apoptosis

BACKGROUND

Proliferative status and multidrug resistance play a key role in determining cell response to chemotherapy. There is a need to develop multiple labeling method that allows simultaneous assessment of multidrug resistant (MDR) phenotype, proliferative status, apoptosis related changes in mitochondrial potential, in chemosensitive and chemoresistant tumor cell populations.

METHODS

A three-color labeling was performed using Hoechst 33342 (DNA), JC1 (mitochondrial potential), and a far red fluorescent membrane intercalating dye: PTIR271 (proliferation).

RESULTS

Combined staining of DNA and mitochondrial potential allows identification of subpopulations expressing and MDR phenotype mediated by P-glycoprotein (Pgp), and, in Pgp negative subpopulations, identification of apoptotic cells and evaluation of cell cycle status in viable cells. Addition of a far red fluorescent membrane intercalating dye, PTIR271, allows simultaneous monitoring of cell division status by dye dilution in both drug sensitive and drug resistant populations.

CONCLUSION

This triple labeling is an interesting method to study the proliferation status of drug sensitivity and drug resistance in viable tumor cells.

L-alanine-glyoxylate aminotransferase II of rat kidney and liver mitochondria possesses cysteine S-conjugate beta-lyase activity: a contributing factor to the nephrotoxicity/hepatotoxicity of halogenated alkenes?

Several halogenated alkenes are metabolized in part to cysteine S-conjugates, which are mitochondrial toxicants of kidney and, to a lesser extent, other organs. Toxicity is due to cysteine S-conjugate beta-lyases, which convert the cysteine S-conjugate into pyruvate, ammonia and a reactive sulphur-containing fragment. A section of the human population is exposed to halogenated alkenes. To understand the health effects of such exposure, it is important to identify cysteine S-conjugate beta-lyases that contribute to mitochondrial damage. Mitochondrial aspartate aminotransferase [Cooper, Bruschi, Iriarte and Martinez-Carrion (2002) Biochem. J. 368, 253-261] and mitochondrial branched-chain aminotransferase [Cooper, Bruschi, Conway and Hutson (2003) Biochem. Pharmacol. 65, 181-192] exhibit beta-lyase activity toward S -(1,2-dichlorovinyl)-L-cysteine (the cysteine S-conjugate of trichloroethylene) and S -(1,1,2,2-tetrafluoroethyl)-L-cysteine (the cysteine S-conjugate of tetrafluoroethylene). Turnover leads to eventual inactivation of these enzymes. Here we report that mitochondrial L-alanine-glyoxylate aminotransferase II, which, in the rat, is most active in kidney, catalyses cysteine S-conjugate beta-lyase reactions with S -(1,1,2,2-tetrafluoroethyl)-L-cysteine, S -(1,2-dichlorovinyl)-L-cysteine and S -(benzothiazolyl-L-cysteine); turnover leads to inactivation. Previous workers showed that the reactive-sulphur-containing fragment released from S -(1,1,2,2-tetrafluoroethyl)-L-cysteine and S -(1,2-dichlorovinyl)-L-cysteine is toxic by acting as a thioacylating agent - particularly of lysine residues in nearby proteins. Toxicity, however, may also involve 'self-inactivation' of key enzymes. The present findings suggest that alanine-glyoxylate aminotransferase II may be an important factor in the well-established targeting of rat kidney mitochondria by toxic halogenated cysteine S-conjugates. Previous reports suggest that alanine-glyoxylate aminotransferase II is absent in some humans, but present in others. Alanine-glyoxylate aminotransferase II may contribute to the bioactivation (toxification) of halogenated cysteine S-conjugates in a subset of individuals exposed to halogenated alkenes.

Cytotoxicity of S-conjugates of the sevoflurane degradation product fluoromethyl-2,2-difluoro-1-(trifluoromethyl) vinyl ether (Compound A) in a human proximal tubular cell line

Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE) is a fluorinated alkene formed by degradation of the volatile anesthetic sevoflurane in anesthesia machines. FDVE is nephrotoxic in rats but not humans. Rat FDVE nephrotoxicity is attributed to FDVE glutathione conjugation and bioactivation of subsequent FDVE-cysteine S-conjugates, in part by renal beta-lyase. Although FDVE conjugation and metabolism occur in both rats and humans, the mechanism for selective toxicity in rats and lack of effect in humans is incompletely elucidated. This investigation measured FDVE S-conjugate cytotoxicity in cultured human proximal tubular HK-2 cells, and compared this with known cytotoxic S-conjugates. HK-2 cells were incubated with FDVE and its GSH, cysteine S-mercapturic acid, cysteine S-sulfoxide, and mercapturic acid sulfoxide conjugates (0.1-2.7 mM) for 24 h. Cytotoxicity was determined by lactate dehydrogenase (LDH) release, total LDH, and the ability of viable cells to reduce a tetrazolium-based compound (MTT). FDVE was cytotoxic only at concentrations >/=0.9 mM. No increase in LDH release was observed with either FDVE-GSH conjugate. The FDVE-cysteine conjugates S-(1,1-difluoro-2-fluoromethoxy-2-(trifluoromethyl) ethyl)-L-cysteine (DFEC) and (Z)-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)-L-cysteine ((Z)-FFVC) caused significant differences in LDH release and MTT reduction only at 2.7 mM; (Z)-FFVC was slightly more cytotoxic. Both S-(1,1-difluoro-2-fluoromethoxy-2-(trifluoromethyl) ethyl)-L-cysteine sulfoxide (DFEC-SO) and (Z)-N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)-L-cysteine sulfoxide ((Z)-N-Ac-FFVC-SO) caused slightly greater changes in LDH release or total LDH than the corresponding equimolar DFEC and (Z)-N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)-L-cysteine ((Z)-N-Ac-FFVC) conjugates. In contrast to FDVE S-conjugates, S-(1,2-dichlorovinyl)-L-cysteine was markedly cytotoxic, at concentrations as low as 0.1 mM. These results show that human proximal tubular cells are relatively resistant to FDVE and FDVE S-conjugate cytotoxicity. This may partially explain the lack of FDVE nephrotoxicity in humans.

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.

Human mitochondrial and cytosolic branched-chain aminotransferases are cysteine S-conjugate beta-lyases, but turnover leads to inactivation

The mitochondrial and cytosolic branched-chain aminotransferases (BCAT(m) and BCAT(c)) are homodimers in the fold type IV class of pyridoxal 5'-phosphate-containing enzymes that also contains D-amino acid aminotransferase and 4-amino-4-deoxychorismate lyase (a beta-lyase). Recombinant human BCAT(m) and BCAT(c) were shown to have beta-lyase activity toward three toxic cysteine S-conjugates [S-(1,1,2,2-tetrafluoroethyl)-L-cysteine, S-(1,2-dichlorovinyl)-L-cysteine, and S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine] and toward beta-chloro-L-alanine. Human BCAT(m) is a much more effective beta-chloro-L-alanine beta-lyase than two aminotransferases (cytosolic and mitochondrial isozymes of aspartate aminotransferase) previously shown to possess this activity. BCAT(m), but not BCAT(c), also exhibits measurable beta-lyase activity toward a relatively bulky cysteine S-conjugate [benzothiazolyl-L-cysteine]. Benzothiazolyl-L-cysteine, however, inhibits the L-leucine-alpha-ketoglutarate transamination reaction catalyzed by both enzymes. Inhibition was more pronounced with BCAT(m). In the presence of beta-lyase substrates and alpha-ketoisocaproate (the alpha-keto acid analogue of leucine), no transamination could be detected. Therefore, with an amino acid containing a good leaving group in the beta position, beta-elimination is greatly preferred over transamination. Both BCAT isozymes are rapidly inactivated by the beta-lyase substrates. The ratio of turnover to inactivation per monomer in the presence of toxic halogenated cysteine S-conjugates is approximately 170-280 for BCAT(m) and approximately 40-50 for BCAT(c). Mitochondrial enzymes of energy metabolism are especially vulnerable to thioacylation and inactivation by the reactive fragment released from toxic, halogenated cysteine S-conjugates such as S-(1,1,2,2-tetrafluoroethyl)-L-cysteine. The present results suggest that BCAT isozymes may contribute to the mitochondrial toxicity of these compounds by providing thioacylating fragments, but inactivation of the BCAT isozymes might also block essential metabolic pathways.

Human kidney flavin-containing monooxygenases and their potential roles in cysteine s-conjugate metabolism and nephrotoxicity

The potential roles of human hepatic and renal flavin-containing monooxygenases (FMOs) in the metabolism of the cysteine S-conjugates S-allyl cysteine (SAC) and S-(1,2-dichlorovinyl)-L-cysteine (DCVC) were investigated. Incubations of human cDNA-expressed FMO1, FMO3, FMO4, and FMO5 with SAC resulted in detection of SAC sulfoxide, with FMO3 exhibiting approximately 3-, 4-, and 10-fold higher activity than FMO1, FMO4, and FMO5, respectively. DCVC sulfoxide formation was only detected with FMO3 and was 59-fold lower than SAC sulfoxide formation. Incubations of human liver microsomes with SAC or DCVC resulted in detection of the corresponding sulfoxides and provided evidence for the involvement of FMO3. Incubations of SAC or DCVC with human kidney microsomes, however, led only to the detection of SAC sulfoxide. Immunoblots with monospecific antibodies to FMO1, FMO3, and FMO5 in kidney microsomes from 26 humans showed that the average expression levels for FMO1, FMO3, and FMO5 were 5.8 +/- 2.3, 0.5 +/- 0.4, and 2.4 +/- 1.4 pmol/mg (means +/- S.D.), respectively. Interestingly, African-American kidney samples (n = 8) exhibited significantly higher FMO1 levels than Caucasian samples (n = 17), whereas no difference in expression level between males and females was observed with any of the examined FMO isoforms. Collectively, the results provide evidence for the expression of three FMO isoforms in the human kidney and show that the contribution of renal FMOs in cysteine S-conjugate metabolism is likely to vary depending upon the cysteine S-conjugate and the relative expression levels of the active FMOs.

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.