In vivo and in vitro nephrotoxicity of the cysteine conjugate of hexachlorobutadiene

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).

Role of glutathione S-transferase Pi in cisplatin-induced nephrotoxicity

One of the dose-limiting toxicities of cisplatin is nephrotoxicity. Renal toxicity is localized to quiescent proximal tubule cells, where the formation of DNA-adducts cannot account for the dose-limiting toxicity. Our earlier results have shown that a glutathione conjugate of cisplatin is metabolized to a nephrotoxicant via gamma-glutamyl transpeptidase (GGT) and a cysteine S-conjugate beta-lyase. The present study was designed to evaluate the potential role of glutathione S-transferase Pi (GSTP) in the initial steps of the bioactivation of cisplatin. Wild-type mice and mice deficient in both murine GSTP genes (GstP1/P2) were treated with cisplatin. Toxicity in both male and female mice was evaluated 5 days after treatment and renal damage was most severe in wild-type male mice. Wild-type males have approximately 10-fold higher levels of GSTP expression in the liver than females, suggesting that hepatic GSTP in the wild-type males contributed to the formation of the nephrotoxic platinum-glutathione conjugate. In GstP1/P2 null mice the gender difference in toxicity was eliminated. Our data show that GSTP expression is a determinant in cisplatin-induced nephrotoxicity and its levels contribute to sex-dependent differences.

Cysteine S-conjugate beta-lyases

Cysteine S-conjugate beta-lyases are pyridoxal 5'-phosphate-containing enzymes that catalyze beta-elimination reactions with cysteine S-conjugates that possess an electron-withdrawing group attached at the sulfur. The end products of the beta-lyase reaction are pyruvate, ammonium and a sulfur-containing fragment. If the sulfur-containing fragment is reactive, the parent cysteine S-conjugate may be toxic, particularly to kidney mitochondria. Halogenated alkenes are examples of electrophiles that are bioactivated (toxified) by conversion to cysteine S-conjugates. These conjugates are converted by cysteine S-conjugate beta-lyases to thioacylating fragments. Several cysteine S-conjugates found in allium foods (garlic and onion) are beta-lyase substrates. This finding may account in part for the chemopreventive activity of allium products. This review (1) identifies enzymes that catalyze cysteine S-conjugate beta-lyase reactions, (2) suggests that toxicant channeling may contribute to halogenated cysteine S-conjugate-induced toxicity to mitochondria, and (3) proposes mechanisms that may contribute to the antiproliferative effects of sulfur-containing fragments eliminated from allium-derived cysteine S-conjugates.

An evaluation of the occupational health risks to workers in a hazardous waste incinerator

A study was conducted to evaluate the health impact of airborne pollutants on incinerator workers at IZAYDAS Incinerator, Turkey. Ambient air samples were taken from two sampling points in the incinerator area and analyzed for particulate matter, heavy metals, volatile and semi-volatile organic compounds (VOCs and SVOCs) and dioxins. The places where the maximum exposure was expected to occur were selected in determining the sampling points. The first point was placed in the front area of the rotary kiln, between the areas of barrel feeding, aqueous and liquid waste storage and solid waste feeding, and the second one was near the fly ash transfer line from the ash silo. Results were evaluated based on the regulations related to occupational health. Benzene, dibromochloropropane (DBCP) and hexachlorobutadiene (HCBD) concentrations in the ambient air of the plant were measured at levels higher than the occupational exposure limits. Dioxin concentrations were measured as 0.050 and 0.075 pg TEQ.m(-3), corresponding to a daily intake between 0.007 and 0.01 pg TEQ. kg body weight(-1).day (-1). An assessment of dioxin congener and homologue profiles suggested that gaseous fractions of dioxin congeners are higher in front of the rotary kiln, while most of them are in particle-bound phases near the ash conveyor. Finally, the necessity of further studies including occupational health and medical surveillance assessments on the health effects of the pollutants for the workers and the general population in such an industrialized area was emphasized.

High pressure liquid chromatography and mass spectrometry characterization of the nephrotoxic biotransformation products of Cisplatin

Previous studies have shown that cisplatin requires metabolic activation to become nephrotoxic. The activation is proposed to be via the metabolism of a glutathione-platinum conjugate to a cysteinyl-glycine-platinum conjugate, which is further processed to a cysteine conjugate. Preincubating cisplatin with glutathione (GSH), cysteinyl-glycine, or N-acetylcysteine (NAC) results in a transient increase in the toxicity of cisplatin toward renal proximal tubular cells. In this study, the preincubation solutions were analyzed by high pressure liquid chromatography (HPLC), atomic absorption spectrometry, and mass spectrometry (MS) to characterize the formation and structure of the platinum conjugates. HPLC analysis of the cisplatin-GSH, cisplatin-cysteinyl-glycine, and cisplatin-NAC preincubation solutions revealed two new platinum-containing peaks in each of the solutions. MS-MS analysis of the peaks revealed a diplatinum- and a monoplatinum conjugate in each of the solutions. Analysis of the composition and toxicity of the solutions with time showed that the transient increase in toxicity correlated with the formation of the monoplatinum conjugate whereas prolonged preincubation decreased toxicity and correlated with the formation of the diplatinum conjugate. The monoplatinum-monoglutathione conjugate is a substrate for gamma-glutamyl transpeptidase, an enzyme that is essential for the nephrotoxicity of cisplatin. The monoplatinum-mono-NAC conjugate can be deacetylated to a cysteine conjugate, which is a substrate for pyroxidol phosphate (PLP)-dependent cysteine S-conjugate beta-lyase. This PLP-dependent enzyme is proposed to catalyze the final step in the metabolic activation of cisplatin. Identification of the structure and toxicity of these conjugates further elucidates the metabolism of cisplatin to a nephrotoxin.

Assessing the health risks following environmental exposure to hexachlorobutadiene

Hexachloro-1,3-butadiene (HCBD) has been reported to be toxic to the rat kidney in a 2 year study at doses higher than 0.2 mg/kg/day. The toxicity is known to be a consequence of the metabolism of HCBD by glutathione conjugation and the renal beta-lyase pathway. Neither toxicity data, nor data on the metabolism of HCBD, are available in humans. In the current work, the potential of HCBD to cause kidney damage in humans environmentally exposed to this chemical has been assessed quantitatively by comparing the key metabolic steps in rats and humans. To that end, the hepatic conjugation of HCBD with glutathione, the metabolism of the cysteine conjugate by renal beta-lyases and N-acetyltransferases, and the metabolism of the N-acetylcysteine conjugate by renal acylases has been compared in vitro in rat and human tissues. Rates for each metabolic step were lower in humans than in rats; 5-fold for glutathione conjugation, 3-fold for beta-lyase and 3.5-fold for N-acetyltransferase. Acylase activity could not be detected in human kidney cytosol. Use of these data in a physiologically based toxicokinetic model to quantify metabolism by the beta-lyase pathway demonstrated that metabolism in humans was an order of magnitude lower than that in rats. At the no effect level for kidney toxicity in the rat the concentration of beta-lyase metabolites was calculated by the model to be 137.7 mg/l. In humans the same concentration would be achieved following exposure to 1.41 ppm HCBD. This is in contrast to the figure of 0.6 ppb which is obtained when it is assumed that the risk is associated with the internal dose of HCBD itself rather than beta-lyase metabolites.

Metabolism of Cisplatin to a nephrotoxin in proximal tubule cells

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

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.

Bioactivation of xenobiotics by formation of toxic glutathione conjugates

Evidence has been accumulating that several classes of compounds are converted by glutathione conjugate formation to toxic metabolites. The aim of this review is to summarize the current knowledge on the biosynthesis and toxicity of glutathione S-conjugates derived from halogenated alkanes, halogenated alkenes, and hydroquinones and quinones. Different types of toxic glutathione conjugates have been identified and will be discussed in detail: (i) conjugates which are transformed to electrophilic sulfur mustards, (ii) conjugates which are converted to toxic metabolites in an enzyme-catalyzed multistep mechanism, (iii) conjugates which serve as a transport form for toxic quinones and (iv) reversible glutathione conjugate formation and release of the toxic agent in cell types with lower glutathione concentrations. The kidney is the main, with some compounds the exclusive, target organ for compounds metabolized by pathways (i) to (iii). Selective toxicity to the kidney is easily explained due to the capability of the kidney to accumulate intermediates formed by processing of S-conjugates and to bioactivate these intermediates to toxic metabolites. The influences of other factors participating in the renal susceptibility are discussed.

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.

Contraluminal para-aminohippurate (PAH) transport in the proximal tubule of the rat kidney. VI. Specificity: amino acids, their N-methyl-, N-acetyl- and N-benzoylderivatives; glutathione- and cysteine conjugates, di- and oligopeptides

In order to evaluate the specificity of the renal contraluminal PAH transport system for amino acids, oligopeptides and their conjugates, the inhibitory potency of these substances against contraluminal [3H] PAH influx has been determined. For this, inhibition of 3H-PAH flux from the interstitium into cortical tubular cells of the rat kidney in situ has been measured. Apparent Ki values were evaluated by a computer program assuming competitive inhibition. Unconjugated amino acids (glycine, cysteine, alanine, leucine, phenylalanine, tyrosine, aspartate, glutamate, arginine, ornithine and lysine) do not inhibit [3H] PAH influx. The very hydrophobic tryptophan, however, does. N-alpha-methylation does not change this behaviour. N-alpha-acetylation does not evoke interaction with the PAH transporter when it occurs with glycine, cysteine (to yield mercapturic acid), arginine, ornithine and lysine. However, it renders alanine, leucine, phenylalanine, tryptophan, L-aspartate moderately, and L-glutamate strongly, inhibitory. The acetylated D-isomers of alanine, leucine and phenylalanine exert a higher inhibitory potency compared with the respective L-isomers. N-alpha-benzoylation of L-lysine is ineffective. N-alpha-benzoylation, however, evokes interaction with the PAH transporter, when it occurs with ornithine less than arginine less than histidine less than glycine = leucine less than alanine = phenylalanine = aspartate = glutamate. Dipeptides interact with the PAH transporter according to their hydrophobicity (Nozaki scale down to 0.9, Fauchère scale up to 1.0). N-acetylation does not change this behaviour. Hydrophobicity also renders oligopeptides, as angiotensin II, inhibitory against PAH transport. Similarly the anionic angiotensin I converting enzyme inhibitors Captopril, Enalapril and Ramipril inhibit contraluminal PAH influx.

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.

Effects of AT-125 on the nephrotoxicity of hexachloro-1,3-butadiene in rats

The role of gamma-glutamyl transpeptidase (gamma-GTP) in the nephrotoxicity of hexachloro-1,3-butadiene (HCBD) was studied using male Sprague-Dawley rats pretreated with AT-125 (Acivicin; L-(alpha S, 5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid). Inhibition of gamma-GTP by more than 95% did not affect urine output, glomerular filtration rate, or tubular reabsorption of filtrate, sodium, or glucose. Nephrotoxicity observed during the first 24 hr after HCBD was not decreased by inhibition of gamma-GTP and beyond 24 hr nephrotoxicity was increased, rather than decreased, in the AT-125-pretreated group. HCBD impairs glucose reabsorption and this was greatly increased in the AT-125-pretreated group, indicating that function of the initial segment of the nephron is impaired by HCBD. Since inhibition of gamma-GTP did not protect against HCBD nephrotoxicity, it is concluded that gamma-GTP inhibition does not limit the formation of metabolites(s) which cause HCBD nephrotoxicity. Therefore, distribution of gamma-glutamyltranspeptidase does not account for the selective nephrotoxicity of hexachloro-1,3-butadiene.

Differential toxicity as a result of apical and basolateral treatment of LLC-PK1 monolayers with S-(1,2,3,4,4-pentachlorobutadienyl)glutathione and N-acetyl-S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine

Monolayers of LLC-PK1 cells, a cell line with features typical of proximal tubular epithelial cells, were treated at the apical and basolateral side with S-(1,2,3,4,4-pentachlorobutadienyl)glutathione (PCBD-GSH) and N-acetyl-S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine (PCBD-NAC). Apical treatment with PCBD-GSH (greater than 20 microM) resulted in cytotoxicity, which could be inhibited by acivicin and aminooxyacetic acid (AOAA), inhibitors of gamma-glutamyltranspeptidase (gamma GT) and beta-lyase respectively. In contrast apical treatment with PCBD-NAC was only toxic at high concentrations (greater than 850 microM), and this effect could hardly be inhibited by AOAA. Basolateral treatment of confluent LLC-PK1 monolayers, grown on porous membranes, with PCBD-GSH gave a much smaller response than apical treatment, consistent with the fact that gamma GT is predominantly present at the apical side. Basolateral treatment even with high concentrations of PCBD-NAC (1.1 mM) did not show an increase in cytotoxicity when compared to the effect after apical treatment. These results suggest the absence of an organic anion transporter, by which these conjugates in vivo are transported into the cells from the basolateral side. This supposition was substantiated in a study of transcellular transport of the model ions tetraethyl ammonium (TEA) and para-aminohippurate (PAH), in LLC-PK1 monolayers, grown as indicated above. No active PAH transport could be demonstrated, whereas an active TEA transport was present. The absence of an organic anion transporter limits the usefulness of LLC-PK1 cells for the study of nephrotoxicity of compounds, like PCBD-NAc, needing this transport to enter the cells. However, the finding of an active basolateral organic cation transporter, together with the presence of gamma GT, dipeptidase and beta-lyase, makes this system especially interesting for testing all compounds that use this transporter or these enzymes in order to elicit toxicity.

Probenecid-induced protection against acute hexachloro-1,3-butadiene and methyl mercury toxicity to the mouse kidney

Male Swiss OF1 mice received a single oral dose of either 80 mg/kg hexachloro-1,3-butadiene (HCBD) or 40 mg/kg methyl mercury (MeHg). Examination of cryostat kidney sections stained for alkaline phosphatase (APP) revealed damage to about 50% of the proximal tubules after 8 h. Treatment with the organic anion transport inhibitor probenecid (i.p., 3 x 0.75 mmol/kg) did not have any renal effect in normal mice but reduced the number of damaged tubules by 80 and 90% in mice treated with HCBD and MeHg respectively. The results support the conclusion that the toxicity of HCBD and MeHg to the mouse kidney is related to a probenecid-sensitive transport process. It cannot be stated from the present investigation whether the inhibition nephrotoxicity data are related to classic organic anion secretion by the kidney.

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.

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

S-(1,2,3,4,4-Pentachloro-1,3-butadienyl)-L-cysteine (PCBC) has been identified as the penultimate compound responsible for hexachlorobutadiene-induced nephrotoxicity. The primary goal of these studies was to determine the mechanism of PCBC-induced toxicity in rabbit renal proximal tubules by examining the early changes in tubular physiology. PCBC (20-500 microM) induced a specific sequence of toxic events. Following 15 min of exposure, 200 microM PCBC increased basal (25%) and ouabain-insensitive (78%) respiration. This was followed by a decrease in basal (46%), nystatin-stimulated (54%), and ouabain-insensitive (21%) respiration and a decrease in glutathione content (79%). Finally, there was a decrease in cell viability as measured by a decrease in LDH retention at 60 min. Direct probing of mitochondrial function revealed that the initial increase in respiration resulted from the uncoupling of oxidative phosphorylation, while the late changes in respiration appeared to result from gross mitochondrial damage characterized by inhibited state 3 respiration, inhibited cytochrome c-cytochrome oxidase, and inhibited electron transport. Studies utilizing tubules with decreased glutathione content revealed that glutathione plays little if any role in the early events of PCBC-induced toxicity. These results suggest that PCBC-induced mitochondrial dysfunction may initiate the renal proximal tubule injury.

Enzymatic transformation of mercapturic acids derived from halogenated alkenes to reactive and mutagenic intermediates

The metabolism of the mercapturic acids S-pentachlorobutadienyl-N-acetylcysteine (N-Ac-PCBC), S-trichlorovinyl-N-acetylcysteine (N-Ac-TCVC) and S-dichlorovinyl-N-acetylcysteine (N-Ac-DCVC) by subcellular fractions from male rat liver and kidney homogenates was studied. As a model compound, N-Ac-PCBC, 14C labelled, was synthesised. It was intensively metabolised by cytosolic but not by microsomal enzymes from rat liver and kidney. The major metabolite identified by GC/MS was pentachlorobutadienylcysteine, the amount produced being highest in kidney cytosol. Metabolic conversion of 14C-N-Ac-PCBC by kidney and liver cytosol resulted in covalent binding of radioactivity to protein, binding was strongly inhibited by the beta-lyase inhibitor aminooxyacetic acid (AOAA). N-Ac-TCVC and N-Ac-DCVC were also transformed by cytosolic enzymes to the corresponding cysteine conjugates (trichlorovinylcysteine and dichlorovinylcysteine). The three mercapturic acids tested were strong mutagens in the Ames-test after addition of rat kidney cytosol. In the absence of cytosol, N-Ac-TCVC and N-Ac-DCVC were weakly but definitely mutagenic, whereas N-Ac-PCBC was not. In contrast to N-Ac-PCBC, the "direct" mutagens N-Ac-TCVC and N-Ac-DCVC were both transformed to pyruvate by bacterial (S. typhimurium TA100) homogenate 100,000 g supernatants. It is concluded that mercapturic acids are deacetylated to the corresponding cysteine conjugates by cytosolic (N-Ac-PCBC, N-Ac-TCVC and N-Ac-DCVC) and bacterial enzymes (N-Ac-TCVC and N-Ac-DCVC) and further cleaved to reactive and mutagenic intermediates by mammalian and/or bacterial beta-lyase. The observed activation mechanisms for the mercapturic acids, whose formation from hexachlorobutadiene, tetrachloroethylene and trichloroethylene has been proven, might contribute to the nephrotoxicity and nephrocarcinogenicity of the parent alkenes.

The mechanism of pentachlorobutadienyl-glutathione nephrotoxicity studied with isolated rat renal epithelial cells

Isolated renal epithelial cells were used to study the mechanism of toxicity of pentachlorobutadienyl-glutathione (PCBG), a nephrotoxic glutathione conjugate of hexachlorobutadiene. The cytotoxicity of PCBG displayed a very steep dose-response relationship; at 10 microM PCBG no toxicity was observed whereas 25, 50, and 100 microM PCBG all resulted in a similar degree of toxicity. In all cases, loss of cell viability was observed only after a 30-min lag period and reached a plateau of 50 to 60% nonviable cells between 90 and 100 min. Toxic doses of PCBG also resulted in the depletion of cellular thiols. Blocking PCBG metabolism by inhibition of gamma-glutamyl transpeptidase [1-gamma-L-glutamyl-2-(2-carboxyphenyl)hydrazine (anthglutin), 2 mM] or renal cysteine conjugate beta-lyase (aminooxyacetic acid, 0.5 mM) resulted in complete protection against PCBG-induced cell damage. Exposure of isolated renal epithelial cells to 100 microM PCBG resulted in the rapid formation of plasma membrane blebs which appeared to be associated with a loss of Ca2+ from the mitochondrial compartment and an elevation of cytosolic Ca2+ concentration as measured by Quin-2. PCBG treatment also resulted in the inhibition of cell respiration and a marked depletion of cellular ATP content, indicating additional mitochondrial effects of the toxin. Our results support a role for renal cysteine conjugate beta-lyase in the metabolic activation of PCBG and suggest that PCBG-induced renal cell injury may be the result of selective effects on mitochondrial function.

In vitro studies of mechanisms of cytotoxicity

A variety of model systems has been used to study mechanisms of cytotoxicity in vitro. These include purified enzymes, subcellular fractions, freshly isolated cells and perfused organs. Freshly isolated cells provide a number of distinct advantages and have become a popular experimental model in many laboratories. This paper discusses some of the advantages and disadvantages of isolated cell preparations for studies of cytotoxicity and summarizes the results of recent work in which isolated hepatocytes and renal epithelial cells have been used to examine the metabolism and toxicity of menadione and hexachloro-1,3-butadiene.

Identification of S-1,2,2-trichlorovinyl-N-acetylcysteine as a urinary metabolite of tetrachloroethylene: bioactivation through glutathione conjugation as a possible explanation of its nephrocarcinogenicity

The elimination and metabolism of [14-C]-tetrachloroethylene (Tetra) was studied in female rats and mice after the oral administration of 800 mg/kg [14-C]-Tetra. Elimination of unchanged Tetra was the main pathway of elimination in both species and amounted to 91.2% of the dose in rats and 85.1% in mice. [14-C]-Carbon dioxide (CO2) was found to be a trace metabolite of [14-C]-Tetra. Only a small part of the applied dose was transformed to urinary (rats = 2.3%, mice = 7.1%) and fecal (rats = 2.0%, mice = 0.5%) metabolites. The urinary metabolites were separated and quantified by high performance liquid chromatography (HPLC) and identified by gas liquid chromatography/mass spectrometry (GC/MS). The following metabolites could be identified: oxalic acid (8.0% of urinary radioactivity in rats, 2.9% in mice), dichloroacetic acid (5.1%, 4.4%), trichloroacetic acid (54.0%, 57.8%), N-trichloroacetyl-aminoethanol (5.4%, 5.7%), trichloroethanol, free and conjugated (8.7%, 8.0%), S-1,2,2-trichlorovinyl-N-acetylcysteine (N-acetyl TCVC) (1.6%, 0.5%), and another conjugate of trichloroacetic acid (1.8%, 1.3%). The structures of the identified metabolites indicate two different pathways operative in Tetra biotransformation: cytochrome P-450-mediated epoxidation forming reactive metabolites in the liver and conjugation of Tetra with glutathione (GSH) catalyzed by glutathione transferase(s). The formation of reactive intermediates by renal processing of the glutathione conjugates may provide a molecular mechanism for the nephrotoxicity and nephrocarcinogenicity of Tetra in male rats.

Transport of N-acetyl-S-pentachloro-1,3-butadienylcysteine by rat renal cortex

N-acetyl-S-pentachloro-1,3-butadienyl-L-cysteine (PCBD-NAC) is a postulated metabolite derived from glutathione conjugation of hexachloro-1,3-butadiene and is nephrotoxic in the rat. Because PCBD-NAC causes selective necrosis to the pars recta of the proximal tubule, and is an organic anion it might be expected to be transported by the renal organic anion transport system. Rat renal cortical slices were used to characterise the transport. 14C-PCBD-NAC uptake was temperature dependent and reduced by the metabolic inhibitors cyanide and iodoacetate. Probenecid and sulphinpyrazone, specific competitive inhibitors of the anion transport system, and dinitrophenol, a metabolic inhibitor as well as a competitive inhibitor of anion transport, reduced PCBD-NAC transport. Organic cations or uric acid transport inhibitors did not alter PCBD-NAC accumulation by the slices. These data are consistent with the transport of PCBD-NAC by the renal organic anion secretory system.

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

S-(1,2-Dichlorovinyl)glutathione and S-(1,2-dichlorovinyl)-DL-cysteine are potent nephrotoxins. Agents that inhibit gamma-glutamyl transpeptidase, cysteine conjugate beta-lyase, and renal organic anion transport systems, namely L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125), aminooxyacetic acid, and probenecid, respectively, protected against S-conjugate-induced nephrotoxicity. Furthermore, S-(1,2-dichlorovinyl)-DL-alpha-methylcysteine, which cannot be cleaved by cysteine conjugate beta-lyase, was not nephrotoxic. These results strongly support a role for renal gamma-glutamyl transpeptidase, cysteine conjugate beta-lyase, and organic anion transport systems in S-(1,2-dichlorovinyl)glutathione- and S-(1,2-dichlorovinyl)cysteine-induced nephrotoxicity.

Cellular defense mechanisms against toxic substances

Recent studies of cellular defense mechanisms against toxic substances are reviewed with particular emphasis on the critical functions of reduced glutathione. Studies of the metabolism of paracetamol and of the redox active quinone menadione in isolated rat hepatocytes, are summarized in order to illustrate how multiple defense mechanisms are involved in the protection of the cell against the toxicity of these agents. Cytotoxicity with both agents occurs only after the cellular defense mechanisms have become exhausted.

The formation of both a mono- and a bis-substituted glutathione conjugate of hexachlorobutadiene by isolated hepatocytes and following in vivo administration to the rat

The nephrotoxicity of hexachloro-1,3-butadiene (HCBD) appears to depend on the initial formation of a glutathione (GSH) conjugate in the liver. In the present study we have examined the hepatic metabolism of HCBD using isolated hepatocytes and following in vivo administration. Exposure of isolated hepatocytes to HCBD resulted in a dose-dependent depletion of GSH. HPLC analysis of the incubation medium demonstrated the formation of two products. When isolated hepatocytes containing [3H]GSH were exposed to [14C]HCBD, coincident elution of 3H and 14C corresponding to the previously recognized HPLC peaks was observed. Both products were sensitive to treatment with gamma-glutamyl transpeptidase (gamma-GT), providing additional support for their identification as GSH conjugates. The ratio of 3H to 14C in the two peaks indicated the formation of both a mono- and a bis-substituted GSH conjugate of HCBD. The identification of the mono- and bis-GSH conjugates was further confirmed by the preparation of synthetic standards which displayed retention times by HPLC identical to the biological products. The production of the total and individual GSH conjugates displayed both dose and time dependence. The production of the total as well as the ratio of mono- to bis-conjugate was found to depend on the availability of GSH. At low HCBD exposure levels the bis-substituted conjugate accounted for more than 20% of the total conjugate produced by isolated hepatocytes. This value decreased at higher HCBD concentrations. Analysis of bile collected from rats following intraportal administration of [14C]HCBD revealed the presence of both the mono- and bis-substituted GSH conjugates of HCBD as well as additional 14C-containing metabolites. The results of the present study clearly demonstrate the production of both a mono- and a bis-substituted GSH conjugate of HCBD. The potential importance of this finding in terms of the nephrotoxicity of HCBD is discussed.

Role of intrarenal biotransformation in chloroform-induced nephrotoxicity in rats

Various ketonic agents potentiate the hepatic and renal toxicity of halogenated solvents in mice and rats. Characteristics of CHCl3 nephrotoxicity and of 2-hexanone potentiation were evaluated in adult male Fischer 344 rats pretreated with vehicle (oil, 10 ml/kg, po) or 2-hexanone (10 mmol/kg, po) 18 hr prior to CHCl3 exposure. In contrast to the liver, little metabolism of 14CHCl3 by renal cortical microsomes from vehicle- or 2-hexanone-pretreated rats was detected. However, CHCl3 produced a concentration-related dysfunction when added to renal cortical slices from Fischer 344 or Sprague-Dawley rats. The degree of CHCl3 toxicity in vitro was not altered when renal cortical slices were preincubated with CHCl3 (8.5 microliter) under an atmosphere of carbon monoxide. In renal cortical slices, deuterated-CHCl3 was less toxic than CHCl3. Although 2-hexanone pretreatment increased renal slice metabolism of 14CHCl3 twofold, this increase was not associated with an increase in nephrotoxicity after direct exposure of slices to CHCl3 (0 to 10 microliter) in vitro. CHCl3 (0.5 ml/kg, ip) did not alter renal cortical glutathione concentrations in vehicle or 2-hexanone pretreated rats. The association of 14CHCl3-derived radiolabel was increased over control by 2-hexanone pretreatment in protein, lipid, and acid soluble fractions from the renal cortex by approximately two-, two-, and fivefold, respectively. In conclusion, renal cytochrome P-450 did not appear to mediate CHCl3 metabolism and nephrotoxicity in the rat to the extent observed previously in mice. 2-Hexanone appeared to potentiate nephrotoxicity by a mechanism different than that observed in rat liver.

Structure/activity studies of the nephrotoxic and mutagenic action of cysteine conjugates of chloro- and fluoroalkenes

The cysteine conjugates of the nephrotoxins hexachlorobutadiene (HCBD), tetrafluoroethylene (TFE) and hexafluoropropene (HFP), together with those of trichloroethylene and perchloroethylene, have been chemically synthesized and a relationship determined between their structures and their nephrotoxicity and mutagenicity in vitro. All of the conjugates had a marked effect on the uptake of both the organic anion p-aminohippuric acid (PAH) and the cation tetraethylammonium bromide (TEA) into rat kidney slices, suggesting activation of the conjugates in the slices to a toxic species which interferes with ion transport. This observation is consistent with the known nephrotoxicity of HCBD, TFE and HFP in vivo. Each of the conjugates was found to be metabolised by rat kidney slices and by semi-purified rat kidney beta-lyase to pyruvate, ammonia and an unidentified reactive metabolite. When semi-purified beta-lyase was used stoichiometric amounts of pyruvate and ammonia were produced. Although all of the conjugates were activated by beta-lyase and had a similar effect on ion transport their mutagenicity differed markedly. The conjugates of HCBD, trichloroethylene and perchloroethylene were mutagenic in the Ames bacterial mutation assay when activated by rat kidney S9. Metabolic cofactors were not required suggesting that activation was due to the enzyme beta-lyase. In the same assay the conjugates of TFE and HFP were not mutagenic either in the presence or absence of rat kidney S9 and cofactors. With a limited number of cysteine conjugates a clear distinction has been identified between the conjugates of chloroalkenes which were were similarly nephrotoxic but were not mutagenic. The mutagenicity of the cysteine conjugate of HCBD is consistent with the known renal carcinogenicity of this chemical.

Hexachloro-1,3-butadiene-induced hydropic change in mouse liver

Hexachloro-1,3-butadiene (HCBD) produced a time- and dose-related increase in hepatic water content following i.p. administration to male Alderley Park (Alpk/AP) mice. The increase in liver water was maximal 1-2 days after a single dose of 50 mg kg-1 and had returned to normal by day 5. Associated with the increased water, there was a parallel increase in Na+ and K+ ions, with no overall change in intracellular cation concentration. Liver non-protein sulphydryl content showed no consistent time-related decrease after 50 mg kg-1 HCBD. Histopathological examination of the liver showed fine cytoplasmic vacuolation of periportal hepatocytes which was more marked following 100 or 200 mg kg-1 than 50 mg kg-1 HCBD. In one animal, following 200 mg kg-1 HCBD, the liver showed ballooning and degeneration of periportal hepatocytes. Ultrastructural changes were evident 4 h after 50 mg kg-1 and consisted of mitochondrial swelling in periportal hepatocytes, whilst pericentral hepatocytes appeared normal. By 16 h, marked mitochondrial swelling and some proliferation of smooth endoplasmic reticulum were evident in periportal hepatocytes. Male mice of the C57BL/10J and C3H strains appeared to be more sensitive to HCBD-induced hepatic hydropic change than did the male and female Alpk/AP strain and male Balbc and DBA/2J strains. It appears that HCBD or a metabolite causes disruption of mitochondria in periportal hepatocytes which results in an influx of water and ions into the cell without compromising the Na+ pump.

Excretion pattern and metabolism of hexachlorobutadiene in rats. Evidence for metabolic activation by conjugation reactions

Excretion, covalent binding and metabolism of hexachloro-1,3-butadiene (HCBD), a nephrotoxic and nephrocarcinogenic compound, have been studied in female rats. Seventy-two hours after administration of a single oral dose of 1 mg/kg [14C]HCBD, 5.3% of the dose were exhaled as unchanged HCBD and 76.3% were metabolized and excreted in urine and feces or exhaled as 14CO2. After a 50 mg/kg dose of [14C]HCBD, the amount of exhaled parent compound was nearly unchanged at 5.4%. At the higher dose the gastro-intestinal absorption of HCBD appeared to be saturated with the result that unchanged HCBD constituted the major portion of the 69% radioactivity eliminated. Covalent binding to proteins in kidney and liver agreed well with the organ-specific toxicity of HCBD: binding was higher in the kidney, independent of the dose. It increased significantly when the rats were pretreated with phenobarbital, an inducer of monooxygenases; it decreased when the inhibitor piperonyl butoxide was given. Urinary radioactivity in 24 hr urine was separated by column chromatography into four fractions. High performance liquid chromatography, radio gas chromatography and gas chromatography/mass spectrometry were used for further separation and identification. Two major metabolites were identified as pentachlorobutadiene methylthio ether and pentachlorobutadiene carboxymethylthio ether. Their formation is plausibly explained via glutathione conjugation, which appears to be the first step in HCBD metabolism. The mechanism of the conjugation at the olefinic double bond of HCBD is explained by an addition-elimination reaction. This pathway, which appears to lead to a destabilization of the HCBD molecule, could explain the distinct nephrotoxic effects of HCBD.

The formation and biotransformation of cysteine conjugates of halogenated ethylenes by rabbit renal tubules

The nephrotoxicity of chlorotrifluoroethylene ( CTFE ) was examined using isolated rabbit renal tubules suspensions. Exposure of the tubules to CTFE resulted in consumption of CTFE , formation of a glutathione conjugate and inhibition of active organic acid transport. Synthetic cysteine, N-acetylcysteine or glutathione conjugates of CTFE inhibited transport indicating S-conjugation as a possible toxic pathway. 1,2-dichlorovinyl glutathione ( DCVG ), a model synthetic glutathione conjugate, was used to examine the degradation and toxicity of these conjugates. DCVG inhibited rabbit renal tubule transport in vivo and in vitro. The DCVG was found to be degraded with the evolution of glutamine and glycine to produce the ultimate nephro-toxicant, dichlorovinyl cysteine. Dichlorovinyl cysteine is then bioactivated with the release of ammonia. This sequential degradation explains the latency of DCVG -induced renal transport inhibition relative to dichlorovinyl cysteine. It is now evident that certain halogenated ethylenes are capable of being biotransformed to glutathione conjugates in the kidney with their subsequent hydrolysis to nephrotoxic cysteine conjugates.

Nephrotoxicity of hexachloro-1,3-butadiene in the mouse: the effect of age, sex, strain, monooxygenase modifiers, and the role of glutathione

Administration of a single ip dose of hexachloro-1,3-butadiene (HCBD) to adult male or female Alderley Park mice at 96 mumol/kg or above produced renal tubular necrosis of the pars convoluta and pars recta of the proximal tubule by 24 hr. Renal damage was also observed biochemically by the reduced accumulation of the organic anion (p-aminohippurate) and organic cation (tetraethylammonium) by renal slices and by an elevated plasma urea. The toxicity of HCBD to adult Alderley Park mice was similar for males and females. Young male mice (21 and 28 days old) were slightly more susceptible to HCBD-induced nephrotoxicity than adult (7-week-old) males. A strain difference in susceptibility to HCBD-induced renal damage was observed, the BALB/c strain being slightly more sensitive than C57BL/10J, C3H, DBA/2J, and Alderley Park strains. This strain difference was reflected in the lethality of HCBD to mice. Prior administration of the monooxygenase inducers, phenobarbitone or beta-naphthoflavone, or of the monooxygenase inhibitor, piperonyl butoxide, did not alter the extent of renal damage produced by HCBD in male Alderley Park mice. However, HCBD produced a marked decrease in kidney but not liver nonprotein sulfydryl content in this strain of mouse, suggesting the formation of glutathione conjugates in the kidney. Administration of the glutathione or N-acetylcysteine conjugates of HCBD produced a marked renal tubular necrosis similar to that seen with HCBD. It is suggested that the mechanism of HCBD-induced nephrotoxicity in the mouse involves nonoxidative metabolism of HCBD in the kidney and that the cysteine conjugates formed are further metabolized to a nephrotoxic agent.