A mechanism of S-(1,2,3,4,4-pentachloro-1,3-butadienyl)-L-cysteine toxicity to rabbit renal proximal tubules

Mapping Adverse Outcome Pathways for Kidney Injury as a Basis for the Development of Mechanism-Based Animal-Sparing Approaches to Assessment of Nephrotoxicity

In line with recent OECD activities on the use of AOPs in developing Integrated Approaches to Testing and Assessment (IATAs), it is expected that systematic mapping of AOPs leading to systemic toxicity may provide a mechanistic framework for the development and implementation of mechanism-based in vitro endpoints. These may form part of an integrated testing strategy to reduce the need for repeated dose toxicity studies. Focusing on kidney and in particular the proximal tubule epithelium as a key target site of chemical-induced injury, the overall aim of this work is to contribute to building a network of AOPs leading to nephrotoxicity. Current mechanistic understanding of kidney injury initiated by 1) inhibition of mitochondrial DNA polymerase γ (mtDNA Polγ), 2) receptor mediated endocytosis and lysosomal overload, and 3) covalent protein binding, which all present fairly well established, common mechanisms by which certain chemicals or drugs may cause nephrotoxicity, is presented and systematically captured in a formal description of AOPs in line with the OECD AOP development programme and in accordance with the harmonized terminology provided by the Collaborative Adverse Outcome Pathway Wiki. The relative level of confidence in the established AOPs is assessed based on evolved Bradford-Hill weight of evidence considerations of biological plausibility, essentiality and empirical support (temporal and dose-response concordance).

The cellular effects of a unique pesticide sulfluramid (N-ethylperfluorooctane sulphonamide) on rabbit renal proximal tubules

The cellular effects of sulfluramid (N-ethylperfluorooctane sulphonamide, NEPFOS) and its major metabolite perfluorooctane sulphonamide (PFOS) were examined using a suspension of rabbit renal proximal tubules as a model. NEPFOS and PFOS were potent stimulators of proximal tubule basal oxygen consumption (QO(2)), with initial effects exhibited at 5-10 mum and maximal effects at 50-200 mum. The increase in basal QO(2) was ouabain insensitive, which suggests that NEPFOS and PFOS may act by uncoupling oxidative phosphorylation. Exposure of tubule suspensions to NEPFOS or PFOS concentrations of 100 mum or higher for 60 min produced tubule death, indicated by an increase in the release of lactate dehydrogenase. The tubule death did not appear to result from alkylation or lipid peroxidation, since glutathione and malondialdehyde levels were unaffected. To determine the mechanism by which NEPFOS and PFOS increased tubule QO(2), the effects of NEPFOS and PFOS on isolated renal cortical mitochondria were examined. NEPFOS (10 mum) and PFOS (5 mum) increased state-4 respiration of mitochondria in the absence of a phosphate acceptor. These results suggest that NEPFOS and PFOS uncouple oxidative phosphorylation and may produce cytotoxicity through this mechanism.

Primary cultures of rabbit renal proximal tubule cells: II. Selected phase I and phase II metabolic capacities

Specific characteristics of cells vary as a function of time in culture. We have determined the stability of selected Phase I and Phase II biotransformation capacities in rabbit renal proximal tubule cells in primary culture. When grown in hormonally-defined medium, proximal tubule cells lost Phase I metabolic capacity. Cytochrome P-450 content and associated mixed-function oxidase activities present in kidney cortex microsomes were not detectable after 14 days in culture. Phase II glutathione-dependent metabolic functions were well retained in cultured cells compared with freshly isolated proximal tubules (FIPT). Cellular total glutathione content was 2.8 mug/mg protein in FIPT compared with approximately 10 mug/mg protein in stable confluent cultures. A higher total glutathione content of 20.6 mug/mg was noted in preconfluent cultures. The glutathione redox state was initially perturbed in FIPT with 37% of the total glutathione present found in its oxidized form. Tubule cells recovered to a normal ratio (6-13% of total glutathione in the oxidized form) while in culture. The glutathione S-transferase activity in 4-day-old cells in culture was reduced to 50% of the 4 U/mg protein level found in FIPT. No appreciable further decline in glutathione S-transferase activity was detected during 15 days in culture. The level of gamma-glutamyl-transpeptidase (a brush-border enzyme necessary for glutathione uptake into proximal tubule cells) declined from 1499 mU/mg protein in homogenates of FIPT to 636 mU/mg in homogenates of 8-day-old cultured cells. A further decline in activity occurred during the next 7 days in culture. In conclusion, although Phase I metabolic functions were diminished in primary cultured rabbit proximal tubule cells, Phase II metabolic functions were retained at levels comparable with FIPT and well above those found in several established kidney cell lines.

Proliferative responses observed following vancomycin treatment in renal proximal tubule epithelial cells

Vancomycin (VAN) is a glycopeptide antibiotic used to treat gram-positive infections. Nephrotoxicity is a common side effect observed with vancomycin therapy. However, the mechanism of vancomycin-induced nephrotoxicity has not been fully characterized. In this study we examined the effect of vancomycin on cellular proliferation in renal proximal tubule cells. A dose- and time-dependent increase in cell number and total cellular protein was observed following vancomycin exposure. Vancomycin exposure also caused an increase in BrdU incorporation followed by the accumulation of renal proximal tubule cells in G(2)/M phase of the cell cycle. These effects were inhibited by pretreatment with the mitogen-activated protein kinase inhibitor, PD098059, suggesting an association between the cell proliferative effect of VAN and the induction of the mitogen-activated protein kinase signaling pathway. Mitochondrial function in renal proximal tubule cells was assessed using oxygen consumption and ATP concentrations. We observed an increase in oxygen consumption and ATP concentrations following short-term exposure to vancomycin. Together, our data suggest that vancomycin treatment produces alterations in mitochondrial function that coincide with a cell proliferative response in renal proximal tubule epithelial cells.

Glutathione-dependent bioactivation of haloalkenes

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

Mitochondrial bioactivation of cysteine S-conjugates and 4-thiaalkanoates: implications for mitochondrial dysfunction and mitochondrial diseases

The toxicity of most drugs and chemicals is associated with their enzymatic conversion to toxic metabolites. Bioactivation reactions occur in a range of organs and organelles, including mitochondria. The toxicity of haloalkene-derived cysteine S-conjugates and related 4-thiaalkanoates is associated with their mitochondrial bioactivation. Toxic cysteine S-conjugates are formed by the glutathione S-transferase-catalyzed addition of glutathione to haloalkenes to give glutathione S-conjugates, which are hydrolyzed by gamma-glutamyltransferase and dipeptidases. Mitochondrial cysteine conjugate beta-lyase-catalyzed bioactivation of cysteine S-conjugates affords unstable alpha-halothiolates. Haloalkene-derived 4-thiaalkanoates, which are analogs of cysteine S-conjugates that lack an alpha-amino group, undergo bioactivation by the enzymes of fatty acid beta-oxidation to give 3-hydroxy-4-thiaalkanoates that eliminate alpha-halothiolates. alpha-Halothiolates yield alkylating and acylating agents that interact with cellular macromolecules and thereby cause cell damage. Mitochondrial dysfunction is the hallmark of cysteine S-conjugate-induced cytotoxicity: decreased respiration, decreased ATP and total adenine nucleotide concentrations, depletion of the mitochondrial glutathione content, perturbations in cellular Ca2+ homeostasis, and damage to the mitochondrial genome are seen with cysteine S-conjugates. Similar changes are observed with cytotoxic 4-thiaalkanoates, but inhibition of the medium-chain acyl-CoA dehydrogenase and hypoglycemia are also observed.

Decreasing glycolysis increases sensitivity to mitochondrial inhibition in primary cultures of renal proximal tubule cells

We have previously shown that shaking the culture plates (SHAKE) of rabbit renal proximal tubule cells (RPTC) to maintain adequate aeration increased aerobic metabolism and decreased the induction of glycolysis compared to RPTC cultured under standard conditions (STILL). However, glycolysis in SHAKE RPTC remained elevated compared to glycolysis in proximal tubules in vivo. In the present study the contribution of culture medium sugar composition and concentration to glycolytic metabolism was assessed in RPTC. SHAKE and STILL RPTC cultured in 5 mM glucose contained lactate levels equivalent to the respective SHAKE and STILL RPTC cultured in standard culture medium which contains 17.5 mM glucose. Similarly, the activity of lactate dehydrogenase was unchanged by lowering the medium glucose concentration. Substituting 5 mM galactose for 5 mM glucose in the culture medium significantly reduced the lactate content of both SHAKE and STILL RPTC but had no effect on lactate dehydrogenase activity. Cell growth was equivalent under all culture conditions. Sensitivity to mitochondrial inhibition was determined for each culture condition by measuring cell death after exposure to the respiratory inhibitor antimycin A. The results showed a hierarchy of sensitivity to antimycin A (5 mM galactose SHAKE > 5 mM glucose SHAKE > 17.5 mM glucose SHAKE = 17.5 mM glucose STILL), which was generally inversely correlated with the level of glycolysis as measured by lactate content (17.5 mM glucose STILL > 17.5 mM glucose SHAKE = 5 mM glucose SHAKE > 5 mM galactose SHAKE).

Studies on the mechanism of 4-aminophenol-induced toxicity to renal proximal tubules

4-Aminophenol (PAP) is known to cause nephrotoxicity in the rat where it produces selective necrosis to renal proximal tubules. The aim of this work was to investigate the toxicity of PAP and its known nephrotoxic metabolite 4-amino-3-S-glutathionylphenol using a well defined suspension of rabbit renal proximal tubules. PAP at a concentration of 0.5 mM and 1 mM caused proximal tubule cell death (measured by lactate dehydrogenase release) in a time-dependent manner over a 4-h exposure. In contrast, 4-amino-3-S-glutathionylphenol at 1 mM produced no proximal tubule cell death over a similar 4-h exposure. At 2 h, 1 mM PAP inhibited proximal tubule respiration by 30% and decreased cellular adenosine triphosphate (ATP) levels by 60%. These events preceded cell death. The addition of PAP to proximal tubules led to a rapid depletion of cellular glutathione, exposure to 0.5 mM causing a 50% depletion within 1 h. The cytochrome P-450 inhibitors SKF525A (1 mM) and metyrapone (1 mM), the iron chelator deferoxamine (1 mM) and the antioxidant N,N'-phenyl-1,4-phenylenediamine (2 microM) had no effect on PAP-induced cell death. However ascorbic acid (0.1 mM), afforded a marked protection against the depletion of cellular glutathione and completely protected against the cell death produced by 1 mM-PAP. These results indicate that oxidation of PAP to generate a metabolite that can react with glutathione is an important step in the toxicity, while mitochondria appear to be a critical target for the reactive intermediate formed.

Cysteine conjugate toxicity in a human cell line: correlation with C-S lyase activity in human hepatic tissue

C-S lyase enzymes catalyse the generation of mutagenic and/or cytotoxic thiols from cysteine conjugated xenobiotics. These cysteine conjugates are produced subsequent to glutathione conjugations as a metabolic step in the mercapturic acid pathway, traditionally thought of as a pathway solely associated with detoxification. Human Chang liver (HCL) cells were challenged with a range of cysteine conjugates demonstrated to be substrates for human hepatic C-S lyases. The cellular toxicity of these compounds was determined and it was observed that the rank order of substrate toxicity obtained for the HCL cells followed the rank order of C-S lyase activity of the substrates in a freshly isolated mitochondrial fraction of human tissue. The presence of C-S lyase activity was also established in this cell line.

Differential cellular effects in the toxicity of haloalkene and haloalkane cysteine conjugates to rabbit renal proximal tubules

The relationship between the covalent binding, uptake, and toxicity produced by S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC) was investigated in suspensions of rabbit renal proximal tubules (RPT). The DCVC and TFEC at concentrations of 25 microM produced a time-dependent (1-6 hours) loss of RPT viability. The TFEC was biotransformed rapidly by beta-lyase to a reactive metabolite which bound covalently to tubular protein. Approximately 63% of the TFEC-equivalents inside the cell were bound to protein. Covalent binding of TFEC-equivalents was associated with a 30% decrease in tubular basal and state 3 respiration, a sevenfold increase in lipid peroxidation, and, ultimately, cell death. The DCVC was biotransformed rapidly to a reactive metabolite which bound covalently to tubular protein. Approximately 90% of the DCVC-equivalents inside the cell were bound covalently to tubular protein. Following exposure to 25 microM DCVC, the binding of DCVC-equivalents was associated with a 17-fold increase in lipid peroxidation but, in contrast to TFEC, had no effect on tubular respiration. However, exposure of RPT to 100 microM DCVC resulted in a ninefold increase in the binding of DCVC-equivalents and a 30% decrease in tubular state 3 respiration. The beta-lyase inhibitor aminooxyacetic acid (AOAA) blocked the covalent binding, mitochondrial dysfunction, lipid peroxidation, and cell death produced by TFEC. The AOAA decreased the covalent binding and the lipid peroxidation produced by DCVC by approximately 60-70% but had no effect on cell death.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentachlorobutadienyl-L-cysteine (PCBC) toxicity: the importance of mitochondrial dysfunction

The relationship between the covalent binding, uptake, and toxicity produced by pentachlorobutadienyl-L-cysteine (PCBC) was examined in rabbit renal proximal tubules (RPT), renal basolateral membrane vesicles, and isolated renal cortical mitochondria. Renal proximal tubules rapidly metabolized PCBC to a reactive intermediate that bound to tubular protein. Approximately 70-90% of PCBC found in the cell at any given time was bound to protein. PCBC initially uncoupled oxidative phosphorylation, followed by a 45% reduction of state 3 respiration and a 90% decrease in cellular adenosine triphosphate (ATP) levels. These events preceded cell death. Isolated mitochondria also metabolized PCBC to a reactive intermediate that bound to mitochondrial protein and initiated mitochondrial toxicity. These results show that PCBC-induced mitochondrial dysfunction occurred as a result of mitochondrial bioactivation and that the mitochondrion is the critical subcellular target in PCBC toxicity. Aminooxyacetic acid (AOAA), an inhibitor of cysteine conjugate beta-lyase, reduced the covalent binding of PCBC-equivalents to tubular protein by approximately 90% and decreased but did not prevent the toxic effects produced by PCBC on RPT respiration and cellular ATP levels. AOAA delayed but had no effect on the overall extent of cell death produced by PCBC. The protective effect of AOAA was independent of any effects on PCBC uptake. These results show that AOAA decreased but did not prevent the metabolism of PCBC by cysteine conjugate beta-lyase. The partial inhibition of PCBC metabolism, and hence, PCBC-induced cell death by AOAA, may be related to limited concentrations of AOAA within the tubule cell or mitochondria.

Nephrotoxicity of halogenated alkenyl cysteine-S-conjugates

In 1916 a relationship was postulated between the occurrence of aplastic anaemia in cattle and the soy bean meal that they had been fed, which had been extracted with trichloroethylene. The toxic compound was later identified as S-(1,2-dichlorovinyl)-L-cysteine (DCV-Cys). In addition to effects on the hemopoietic system it also produced nephrotoxicity in calves. In rats only renal tubular necrosis was found. Further research demonstrated that other halogenated hydrocarbons produced similar nephrotoxicity. The haloalkenyl cysteine-S-conjugates (Cys-S-conjugates) have extensively been studied; this has provided new insight into the biochemical processes that lead to nephrotoxicity. It has been shown that a combination of transport processes and specific metabolic pathways, resulting in reactive intermediates that bind to cellular macromolecules, makes the kidney vulnerable to the noxious effects of the haloalkenyl Cys-S-conjugates. The first part of this review gives a brief overview of the bioactivation of the haloalkenes; in the second part the present knowledge of the underlying mechanisms of cytotoxicity will be outlined.

Renal mitochondrial glutathione transport

Freshly isolated tightly coupled rabbit renal cortical mitochondria rapidly accumulated glutathione (GSH) against an electrical and concentration gradient, and in the presence and absence of pyruvate/malate, succinate, antimycin A, or FCCP. Mitochondrial GSH uptake was dependent on medium GSH concentration, was not saturable, and reached equilibrium within 1 min of addition. Mitochondrial GSH uptake was partially inhibited by glycine, ophthalmic acid, and serine but not glutamate, cysteine, gamma-glutamyl-glutamate, or proline. These results show that 1) mitochondrial GSH uptake is by both a carrier-mediated process and by diffusion, and 2) the GSH carrier system has structural specificity with the glycine residue being a recognition site.

Differences in enzymatic and mechanical isolated rabbit renal proximal tubules: comparison in long-term incubation

Suspensions of renal proximal tubules (RPT) are the in vitro model for many biochemical and physiologic investigations. Inasmuch as there are numerous procedures for tubule isolation and the more commonly used enzymatic procedures may disrupt the basement membrane, there is a need for information comparing the influence of various isolation methods on RPT viability and function in long-term suspension. Rabbit RPT isolated a) enzymatically (ENZ) by in vitro collagenase digestion and Percoll size and density purification, and b) mechanically (MECH) by in vitro iron oxide perfusion and purification by sieving and magnetic removal of glomeruli were compared for viability, morphology, and functional stability during long-term suspension. RPT isolated by ENZ and MECH methods had excellent viability (less than 15% lactate dehydrogenase release), limited lipid peroxidation (less than 0.2 nmol MDA.mg protein-1), and stable nystatin-stimulated oxygen consumption (QO2) (38 and 36 nmol O2.mg protein-1.min-1) throughout 24 h of incubation. Basal QO2 was higher in ENZ than MECH tubules (27 and 19 nmol O2.mg protein-1.min-1, respectively), and was unchanged over 24 h in each preparation. The higher basal QO2 in ENZ tubules was ouabain-sensitive, suggesting an increased rate of Na+,K(+)-ATPase activity in these tubules. Total glutathione content (oxidized + reduced) in ENZ and MECH tubules increased over the 24-h incubation from 8 to 18 nmol.mg protein-1. gamma-Glutamyltranspeptidase (GGT) activity of the RPT homogenates was equivalent in both preparations and stable over time. The ratio of suspension GGT activity to homogenate GGT activity doubled (0.4 to 0.8) during the incubation period. MECH tubules retained their tubule structure during 24 h of incubation whereas the ENZ tubules had a striking loss of tubular morphology over time. These results show that ENZ- and MECH-isolated renal proximal tubule suspensions exhibit similar biochemical properties in long-term incubations but differ in ouabain-sensitive QO2 and the retention of tubular morphology. The loss of tubular morphology and the increase in the rate of Na+,K(+)-ATPase activity in ENZ tubules may be secondary to the disruption of the tubular basement membrane.

The effect of haloalkene cysteine conjugates on rat renal glutathione reductase and lipoyl dehydrogenase activities

An early event in the nephrotoxicity of haloalkene cysteine conjugates is their metabolism by cysteine conjugate beta-lyase to generate a reactive "thiol moiety" which binds to protein. This reactive metabolite(s) has been reported to cause mitochondrial dysfunction. We have examined the effect of three haloalkene cysteine conjugates on the activity of rat renal cortical cytosolic glutathione reductase and mitochondrial lipoyl dehydrogenase, two enzymes which have been reported to be inhibited by S-(1,2-dichlorovinyl)-L-cysteine (DCVC) in the liver. N-Acetyl-S-(1,2,3,4,4-pentachloro-1,3-butadienyl)-L- cysteine (N-acetyl PCBC) produced a time- and concentration-dependent inhibition of glutathione reductase and kinetic studies showed that the inhibition was noncompetitive with a Ki of 215 microM. The enzyme activity from male rat kidney was more sensitive to N-acetyl PCBC than that from female rat kidney. Aminooxyacetic acid, an inhibitor of cysteine conjugate beta-lyase, and bis-p-nitrophenyl phosphate, an amidase inhibitor, blocked the effect of N-acetyl PCBC on glutathione reductase, indicating that metabolism by the cytosol is required to produce enzyme inhibition. S-(1,1,2,2-Tetrafluoroethyl)-L-cysteine (TFEC) and DCVC are also noncompetitive inhibitors of glutathione reductase but are less active than N-acetyl PCBC with Ki's of 2.6 and 6.2 mM for DCVC and TFEC, respectively, DCVC produced a time- and concentration-dependent inhibition of lipoyl dehydrogenase and kinetic studies showed that the inhibition was noncompetitive with a Ki of 762 microM. TFEC and PCBC also inhibit lipoyl dehydrogenase. Aminooxyacetic acid blocked the effect of DCVC, TFEC, and PCBC on lipoyl dehydrogenase, indicating that metabolism by the mitochondrial fraction is required to produce enzyme inhibition. Glutathione reductase activity in the renal cortex of male rats treated with 200 mg/kg hexachloro-1,3-butadiene (HCBD) was inhibited as early as 1 hour after dosing, before signs of marked morphological damage. The activity of lipoyl dehydrogenase was also reduced but was only statistically significant 8 hr after dosing when there was marked renal dysfunction. These findings indicate that the reactive thiol moiety formed by cysteine conjugate beta-lyase cleavage of PCBC can inhibit both glutathione reductase and lipoyl dehydrogenase activities in vivo following HCBD administration. We suggest that such inhibition is a general phenomenon, occurring with diverse and as yet unidentified renal proteins. The critical nature of mitochondrial function and the generation of reactive metabolites within this compartment make this organelle a prime target.

Bioactivation of hexachlorobutadiene by glutathione conjugation

Glutathione (GSH) conjugation reactions in the metabolism of hexachlorobutadiene (HCBD), in rats and mice, initiate a series of metabolic events resulting in the formation of reactive intermediates in the proximal tubular cells of the kidney. The GSH S-conjugate 1-(glutathion-S-yl)-1,2,3,4,4-pentachlorobutadiene (GPCB), which is formed by conjugation of HCBD with GSH in the liver, is not reactive and is eliminated from the liver in the bile or plasma, or both. GPCB may be translocated intact to the kidney and processed there by gamma-glutamyl transpeptidase and dipeptidases to the corresponding cysteine S-conjugate. Alternatively, gamma-glutamyl transpeptidase and dipeptidases present in epithelial cells of the bile duct and small intestine may catalyse the conversion of GPCB to cysteine S-conjugates. The kidney concentrates both GSH and cysteine S-conjugates and processes GSH conjugates to cysteine S-conjugates. A substantial fraction of HCBD cysteine S-conjugate thus concentrated in the kidney is metabolized by renal cysteine conjugate beta-lyase to reactive intermediates. The selective formation of reactive intermediates in the kidney most likely accounts for the organ-specific effects of HCBD. Alternatively, cysteine S-conjugates may be acetylated to yield excretable mercapturic acids.

Intracellular glutathione in the protection from anoxic injury in renal proximal tubules

Previous results (Weinberg, J. M., J. A. David, M. Abarzua, and T. Rajan. 1987. J. Clin. Invest. 80:1446-1454) have shown that GSH and glycine (GLY) are cytoprotective during anoxia when added extracellularly. The present studies investigate the role that intracellular GSH plays in this cytoprotection. Proximal renal tubules in suspension prepared with either high (11 +/- 1 nmol/mg protein) or low (6 +/- 1 nmol/mg protein) GSH contents were subjected to 40 min of anoxia and 40 min of reoxygenation. Low GSH tubules were protected from plasma membrane damage during anoxia by exogenous addition of 1 mM GSH or GLY, reducing lactate dehydrogenase (LDH) release from 42 +/- 7 to 14 +/- 1 and 10 +/- 1%, respectively. High GSH tubules were equally protected from anoxic damage without exogenous additions. Since the high GSH content approximates the in vivo values, it may be concluded that GSH may be cytoprotective during anoxia in vivo. However, it is not the intracellular GSH itself that is cytoprotective; rather, this protection resides in the ability to produce GLY, which appears to be the cytoprotective agent. Alanine was also shown to have similar cytoprotective properties, although higher concentrations were required. Sulfhydryl reducing agents such as cysteine and dithiothreitol offered less, but significant protection from anoxic damage. Protection by GSH, GLY, or alanine was not associated with higher ATP levels during anoxia. Tubules that were protected from membrane damage during anoxia recovered oxygen consumption and K and ATP contents significantly better during reoxygenation than unprotected tubules.

The effects of haloalkene cysteine conjugates on cytosolic free calcium levels in suspensions of rat renal proximal tubules

Disturbances in intracellular calcium homeostasis may play a role in the injury induced by various haloalkene cysteine conjugates. The effects of S-(1,2,3,4,4-pentachloro-1,3-butadienyl)-L-cysteine (PCBC), S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC) on cytosolic free calcium levels were examined in suspensions of rat renal proximal tubules. Cytosolic free calcium levels, measured with fura 2, in control tubules, were 112 +/- 3 nM and increased more than 200% within 1 minute after exposure to the calcium ionophore ionomycin (0.005 mM). PCBC (0.1 mM) increased cytosolic free calcium levels 18% after 5 minutes, while tubular oxygen consumption was unaffected. DCVC (1 mM) did not alter tubular cytosolic free calcium levels or oxygen consumption under similar conditions. TFEC (1 mM) increased cytosolic free calcium levels 36%, had no effect on basal oxygen consumption, and decreased nystatin-stimulated oxygen consumption 30% after 5 minutes. TFEC increased cytosolic free calcium levels in tubules incubated in a nominally calcium-free buffer but not in a calcium containing buffer in the presence of EGTA. The data suggest that the TFEC-induced increase in cytosolic free calcium levels may result from an influx of extracellular calcium or from inhibition of calcium efflux. The increase in cytosolic free calcium levels preceded changes in basal oxygen consumption in tubules exposed to PCBC and TFEC. This study shows that an increase in cytosolic free calcium levels is an early event following PCBC and TFEC but not DCVC exposure.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.

Pentachlorobutadienyl-L-cysteine uncouples oxidative phosphorylation by dissipating the proton gradient

A very early event in the toxicity of pentachlorobutadienyl-L-cysteine (PCBC) to rabbit renal proximal tubules is uncoupling of oxidative phosphorylation (R.G. Schnellmann, E. A. Lock, and L. J. Mandel (1986), Toxicologist 6, 176; (1987), Toxicol. Appl. Pharmacol. 90, 521). The mechanism of PCBC uncoupling of mitochondrial oxidative phosphorylation has been investigated using isolated rabbit renal cortical mitochondria (RCM). PCBC increased state 4 respiration of RCM respiring on pyruvate/malate or succinate in a concentration (10-100 microM)- and time (1-5 min)-dependent manner. PCBC also increased state 4 respiration in the presence of oligomycin, an inhibitor of F0F1-ATPase. The effect of PCBC on mitochondrial proton permeability was determined by measuring passive mitochondrial swelling. After a 2-min exposure to PCBC, RCM swelled when placed in NH4Cl or NaCl, but not KCl or sucrose. The protonophore carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) (1 microM) produced similar effects. After 5 min, RCM swelled when placed in NH4Cl, NaCl, or KCl, but not in sucrose. Aminooxyacetic acid, an inhibitor of cysteine conjugate beta-lyase, blocked the effects of PCBC on respiration, indicating that PCBC can be metabolized by RCM to produce RCM toxicity. These results show that PCBC initially uncouples oxidative phosphorylation by dissipating the proton gradient. Subsequently, additional ion permeabilities occur. These results are in complete agreement with previous observations in rabbit renal proximal tubule suspensions.

2-Bromohydroquinone-induced toxicity to rabbit renal proximal tubules: the role of biotransformation, glutathione, and covalent binding

2-Bromohydroquinone (BHQ) is a model toxic hydroquinone and plays an important role in bromobenzene-induced nephrotoxicity. Proximal tubules isolated to contain decreased glutathione (GSH) levels were at least twice as sensitive to the GSH depleting effects of BHQ and BHQ-induced mitochondrial dysfunction as were tubules with "normal" (i.e., in vivo) GSH content. The decrease in tubular GSH content resulted from BHQ-GSH conjugate formation. A mono-GSH conjugate (2-bromo-3-(glutathion-S-yl)hydroquinone) and a di-GSH conjugate (2-bromo-3,5- or 6-(diglutathion-S-yl)hydroquinone) were identified. In addition, a glucuronide conjugate was identified (2-bromo-1- or 4-O-glucuronylhydroquinone). BHQ-GSH conjugates were not responsible for BHQ-induced toxicity since (1) tubules with normal levels of GSH were more resistant to BHQ-induced toxicity even though they formed more BHQ-GSH conjugates than tubules with decreased GSH levels and (2) inhibition of gamma-glutamyltranspeptidase did not prevent BHQ-induced toxicity. BHQ-equivalents bound covalently to tubular protein in a concentration-, time-, and temperature-dependent manner with the majority of the binding (61%) occurring during the first 15 min after exposure to 0.2 mM BHQ. Tubules pretreated with GSH underwent less BHQ-protein alkylation and mitochondrial dysfunction, and the amount of BHQ recovered and BHQ-di-GSH conjugate formed increased. These data suggest that BHQ is biotransformed to a reactive intermediate (2-bromoquinone and/or 2-bromosemiquinone) and that this intermediate can react with GSH to form BHQ-GSH conjugates and/or bind covalently to tubular protein which may result in mitochondrial dysfunction and tubular death.

2-Bromohydroquinone-induced toxicity to rabbit renal proximal tubules: evidence against oxidative stress

2-Bromohydroquinone (BHQ) plays an important role in bromobenzene-induced nephrotoxicity and is a model toxic hydroquinone. Since BHQ has a quinone nucleus and various quinones have been shown to produce cytotoxicity via oxidative stress, the goal of this study was to determine whether BHQ produced cytotoxicity in a suspension of rabbit renal proximal tubules via oxidative stress. t-Butyl hydroperoxide (TBHP), an agent known to produce cytotoxicity via oxidative stress in this preparation, was used as a positive control. BHQ decreased tubular glutathione disulfide content whether glutathione reductase was inhibited or not. Inhibition of glutathione reductase did not result in the potentiation of BHQ-induced mitochondrial dysfunction or cell death. In contrast, TBHP increased tubular glutathione disulfide content. TBHP-induced increases in glutathione disulfide content, mitochondrial dysfunction, and cell death were potentiated when glutathione reductase was inhibited. Unlike TBHP, BHQ did not initiate lipid peroxidation nor was the antioxidant butylated hydroxytoluene protective. However, BHQ and TBHP both increased sodium cyanide-insensitive oxygen consumption. These results suggest that BHQ may undergo "redox cycling," but BHQ-induced mitochondrial dysfunction and cell death are not due to oxidative stress.

Assessment of unscheduled DNA synthesis in a cultured line of renal epithelial cells exposed to cysteine S-conjugates of haloalkenes and haloalkanes

The ability of S-(1,2-dichlorovinyl)-L-cysteine (DCVC), S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC), S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine (PCBC), S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine (CTFEC) and S-(2-chloroethyl)-L-cysteine (CEC) to induce DNA repair was investigated in LLC-PK1, a cultured line of porcine kidney tubular epithelial cells. DNA repair due to exposure of the cells to the S-conjugates was determined as unscheduled DNA synthesis (UDS) after inhibition of replicative DNA synthesis in confluent LLC-PK1 monolayers. DCVC, TCVC and PCBC induced dose-dependent UDS in LLC-PK1 at concentrations which did not impair the viability of the cells compared to untreated controls; higher concentrations were cytotoxic, resulting in lactate dehydrogenase leakage into the medium. Cell death was also induced by CTFEC, which failed to exert genotoxicity. CEC induced the highest response among these cysteine conjugates without impairing cell viability. Inhibition of cysteine conjugate beta-lyase with aminooxyacetic acid abolished the effects of DCVC, TCVC, PCBC and CTFEC but did not influence the genotoxicity of CEC.

Influence of aging on chemically induced hepatotoxicity: role of age-related changes in metabolism

The effects on hepatotoxicity of age-associated changes in drug metabolism are not always straightforward. In the case of allyl alcohol hepatotoxicity in male rats, there is a good relationship between increased metabolic activation by liver alcohol dehydrogenase and enhanced hepatotoxicity in old age. With regard to two other hepatotoxicants, some tentative conclusions about the role of metabolism can be drawn, but they must be tempered with caution due to gaps in the available information. Acetaminophen-induced hepatotoxicity is reduced in old age, and decreased formation of the toxic intermediate may be the reason. There is a prominent effect of aging on acetaminophen conjugation, a shift from sulfation to glucuronidation, but this change does not affect total clearance. The situation with carbon tetrachloride is difficult to interpret because the final outcome is unaltered hepatotoxicity in old age. Nevertheless, the available data suggest that an age-associated decrease in activation of carbon tetrachloride is counterbalanced by a loss in resistance to lipid peroxidation. These conclusions are summarized in Table 5. Again, it must be emphasized that all of these age-dependent changes in toxicity could be related to effects on other systems that are not necessarily involved in the metabolism of hepatotoxicants. Future research is needed to identify pathways of metabolic activation and detoxification in which age-dependent changes occur that result in significant changes in hepatotoxicity. The entire sequence of events from changes at the molecular level to their sequelae at the level of the cell, tissue and intact animal should be investigated, and the results should be confirmed in more than one mammalian model of aging. The aim would be to identify basic mechanisms that result in increased hazard for the aged liver from exposure to toxic compounds.

Reactive intermediates and their toxicological significance

Many chemicals that cause toxicity do so via metabolism to biologically reactive metabolites. However, the nature of the interaction between such reactive metabolites and various cellular components, and the mechanism(s) by which these interactions eventually lead to cell death are poorly understood. The relative importance of macromolecular alkylation (covalent binding), lipid peroxidation, alterations in thiol, calcium and energy homeostasis are discussed with reference to specific toxicants. It is concluded that the cytotoxic effects of reactive metabolites are a consequence of simultaneous and/or sequential alterations in several cellular processes. Further studies are required to determine the relationship between these alterations and cell death.

Intracellular distribution and depletion of glutathione in rabbit renal proximal tubules

The intracellular compartmentation of glutathione (GSH) in rabbit renal proximal tubules under various conditions was examined using the digitonin fractionation technique. Tubules with GSH contents similar to those found in vivo (13.4 +/- 0.8 nmol . mg protein-1) and with decreasing amounts of GSH had an apparently constant mitochondrial GSH pool of 1.9 +/- 0.1 nmol . mg protein-1. This renal mitochondrial GSH pool is similar in size to that of hepatic mitochondria and represents 10 to 15 percent of the total cellular GSH. Using phorone and diethyl maleate to decrease tubular GSH concentrations, the cytosolic GSH pool could be depleted without affecting the mitochondrial GSH pool. Depletion of the cytosolic GSH pool and decreases in the mitochondrial pool of up to 42 percent were not associated with mitochondrial dysfunction nor loss of tubular viability.

Mechanisms of t-butyl hydroperoxide-induced toxicity to rabbit renal proximal tubules

This study examined the mechanisms of t-butyl hydroperoxide (TBHP)-induced oxidative injury to a suspension of rabbit renal proximal tubules. TBHP (0.25-1 mM) produced a specific sequence of intracellular events in the tubules. Initially, TBHP increased tubular glutathione disulfide content and lipid peroxidation. Subsequently, there was an increase in ouabain-sensitive oxygen consumption (indicative of an increase in intracellular sodium concentrations), mitochondrial dysfunction, and a decrease in glutathione content. Finally, cell death, as measured by a decrease in tubular retention of lactate dehydrogenase activity, began between 30 and 60 min. The toxicity was dependent on iron-mediated free radical formation, since the iron chelator, deferoxamine, and the antioxidants, promethazine, butylated hydroxytoluene, and dithiotreitol, prevented the lipid peroxidation, the mitochondrial dysfunction, and cell death. Further studies with the antioxidants provided evidence that lipid peroxidation plays an important role in TBHP toxicity in proximal tubules.

Thioacylating agents as ultimate intermediates in the beta-lyase catalysed metabolism of S-(pentachloro-butadienyl)-L-cysteine

The transformation of the hexachloro-1,3-butadiene metabolite S-(1,2,3,4,4-pentachlorobuta-1,3-dienyl)-L-cysteine (PCBC) by bacterial cysteine conjugate beta-lyase (beta-lyase) and by N-dodecylpyridoxal bromide (PLP-Br) was investigated using GC/MS to identify products formed. PCBC was transformed by both bacterial beta-lyase and PLP-Br to the major products 2,3,4,4-tetrachlorobutenoic acid and 2,3,4,4-tetrachlorothiobutenoic acid, and to the minor metabolites trichloroacetic acid and S-(1,2,3,4,4-pentachlorobuta-1,3-dienyl)-mercaptoacetic acid. In the presence of diethylamine as model nucleophile, PLP-Br transformed PCBC to yield 2,3,4,4-tetrachlorothiobutenoic acid diethylamide; attempts to trap 1,2,3,4,4-pentachlorobutadienyl thiol, the initial metabolite formed by beta-elimination from PCBC, were unsuccessful. The results obtained suggest that the formation of a thioacylating intermediate (a thioketene or a thiono acyl chloride) may be the decisive reaction during the beta-lyase dependent activation of PCBC.

Studies on the mechanism of nephrotoxicity and nephrocarcinogenicity of halogenated alkenes

There is now a considerable weight of evidence from studies in a number of different laboratories with different haloalkenes to suggest that these compounds undergo conjugation with glutathione followed by degradation of the S-conjugate (Figure 1) to produce cytotoxic, and in some cases mutagenic, metabolites. These effects are dependent upon the sequential metabolism by gamma-glutamyl transferase and dipeptidases to produce the cysteine conjugates, and the presence of renal transport systems which concentrate the chemical in renal cells. These conjugates then appear to undergo further metabolism to a reactive thiol by the renal enzyme cysteine-conjugate beta-lyase, a process which can be blocked by inhibiting the enzyme with AOAA. Renal beta-lyase is present in both the cytosol and mitochondrial fractions, but toxicity studies in isolated cells and mitochondria indicate that the primary mode of action of these compounds is the inhibition of mitochondrial respiration, suggesting that the mitochondrial beta-lyase may be more important than the cytosolic enzyme in cysteine S-conjugate bioactivation. In addition to the renal cell injury caused by the presumed reactive thiol metabolite, reaction with DNA also occurs as the chlorinated, but not fluorinated, analogs are mutagenic, and in the case of HCBD, carcinogenic. Thus the target organ, cellular and subcellular specificity of haloalkene-S-conjugates, is due to the presence of bioactivating enzymes and the susceptibility of certain biochemical processes. The precise relationship between (1) the mitochondrial effects and cytotoxicity and (2) the interaction of the chemical with DNA and its mutagenicity require more precise understanding in order to elucidate the mechanism of S-conjugate-induced cell death and carcinogenicity. The routes and rates of metabolism of some of these compounds, with respect to glutathione conjugation vs. oxidative metabolism, in both experimental animals and man are required to help assess the risk associated with this class of chemicals.