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

Directing Role of Organic Anion Transporters in the Excretion of Mercapturic Acids of Alkylated Polycyclic Aromatic Hydrocarbons

Excretion of mercapturic acids of a xenobiotic is a good indicator for the formation of electrophilic intermediates. However, the route of excretion, urine or feces, is important for usage of a given mercapturic acid as a biomarker in humans. In the present study we investigated the excretion routes of 1-methylpyrenyl mercapturic acid (MPMA) and 1,8-dimethylpyrenyl mercapturic acid (DMPMA) formed from the corresponding benzylic alcohols in rats. Whereas MPMA was primarily excreted in urine (72% of the total urinary and fecal level), DMPMA clearly preferred the fecal route (88%). We then examined interactions of these mercapturic acids with renal basolateral organic anion transporters (OATs) using HEK293 cells stably expressing human OAT1 and OAT3. The uptake rates of MPMA by OAT1- and OAT3-expressing cells were 2.8- and 1.7-fold, respectively, higher than that by control cells. MPMA was a competitive inhibitor of p-aminohippurate uptake by OAT1 and estrone sulfate uptake by OAT3 with K(i) values of 14.5 microM and 1.5 microM, respectively. In contrast, DMPMA was not transported by OAT1 and only modestly transported by OAT3 (1.25-fold over control). Thus, we suspect that the substrate specificities, alone or together with other factors, played a directing role in the excretion of MPMA and DMPMA. Although the mechanistic link requires verification, our results clearly show that a minute structural difference (the presence or absence of an additional methyl group in an alkylated four-ring polycyclic hydrocarbon) can strongly affect the interaction with transporter proteins and direct the excretion route of mercapturic acids.

Glutathione conjugation and conversion to mercapturic acids can occur as an intrahepatic process

By catalyzing the reaction of electrophilic compounds with the sulfhydryl group of glutathione, the glutathione S-transferases play physiologically important roles in the detoxication of potential alkylating agents. The glutathione S-conjugates thus formed are transported out of cells for further metabolism by gamma-glutamyltransferase and dipeptidases, ectoproteins that catalyze the sequential removal of the glutamyl and glycyl moieties, respectively. These ectoproteins are not found in all cells, but are localized predominantly to the apical surface of epithelial tissues. The resulting cysteine S-conjugates can be reabsorbed by specific cell types, and acetylated on the amino group of the cysteinyl residue by intracellular N-acetyl-transferases, to form the corresponding mercapturic acids (N-acetylcysteine S-conjugates). Mercapturic acids are then released into the circulation and delivered to the kidney for excretion in urine, or they may undergo further metabolism. Mercapturic acid biosynthesis is generally considered to be an interorgan process, with the liver serving as the major site of glutathione conjugation, and the kidney as the primary site for conversion of glutathione conjugates to cysteine conjugates. Cysteine conjugates formed in the kidney appear to be transported back to the liver for acetylation. This interorgan model of mercapturic acid synthesis is based largely on the interorgan distribution of the enzymes involved in their formation, and in particular of the enzyme gamma-glutamyltransferase. Rats have relatively low hepatic and high renal activities of gamma-glutamyltransferase, the only protein known to initiate the breakdown of glutathione S-conjugates. The low gamma-glutamyltransferase activity in rat liver limits the hepatic degradation of glutathione S-conjugates, particularly after large doses of xenobiotic. In contrast, hepatic gamma-glutamyltransferase is significantly higher in species such as rabbit, guinea pig, and dog, and as a consequence, nearly all of the glutathione and glutathione S-conjugates released by liver cells of these species is degraded within the liver. Recent studies demonstrate that glutathione S-conjugates synthesized within hepatocytes are secreted preferentially across the canalicular membrane into bile, and are broken down within biliary spaces to form cysteine S-conjugates. The latter are then reabsorbed by the liver, N-acetylated to form mercapturic acids, and reexcreted into bile, completing an intrahepatic pathway for mercapturic acid biosynthesis. The contribution of this intrahepatic pathway to overall mercapturate formation is dependent on dose of the electrophile, route of exposure, and the physicochemical properties of the glutathione S-conjugate formed, as well as the tissue distribution and activity of gamma-glutamyltransferase.(ABSTRACT TRUNCATED AT 400 WORDS)

The nephrotoxicity, cytotoxicity and renal handling of a cisplatin-methionine complex in male Wistar rats

When cisplatin is administered in the form of a cisplatin-methionine substitution complex, high doses of cisplatin can be tolerated with no obvious signs of renal toxicity. We have demonstrated that male Wistar rats receiving a single i.p. dose of cisplatin-methionine at a 1:5 ratio (by weight) did not exhibit cisplatin-induced nephrotoxicity, while cisplatin administered alone at an identical concentration (6 mg/kg) resulted in marked renal toxicity in all animals treated. Using renal cortical slices prepared from untreated rats, we demonstrated that cisplatin, but not cisplatin-methionine, inhibited the accumulation of 14C-tetraethylammonium (TEA). This observation suggests that cisplatin, unlike cisplatin-methionine, is a substrate for the organic base transport system. In addition, cisplatin alone was more cytotoxic to C6 glioma cells in vitro than the cisplatin-methionine complex. Exposure of C6 glioma cells to cisplatin-methionine, however, resulted in a 50%-60% reduction in 3H-thymidine incorporation at cisplatin:methionine ratios up to 1:10. These results indicate that cisplatin-methionine is significantly cytotoxic yet lacks cisplatin-associated renal toxicity and may, therefore, have a role in the treatment of human malignancies.

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.

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.

Role of gamma-glutamyltranspeptidase and beta-lyase in the nephrotoxicity of hexachloro-1,3-butadiene and methyl mercury in mice

Male Swiss OF1 mice received a single oral dose of either 80 mg/kg hexachloro-1,3-butadiene (HCBD) or 80 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. Pretreatment with the gamma-glutamyltranspeptidase (gamma-GT) inactivator AT-125 (Acivin, 50 mg/kg i.p., plus 50 mg/kg p.o., reduced the number of damaged tubules by 59 and 58% in mice treated with HCBD and MeHg, respectively. Pretreatment with the two beta-lyase inhibitors, amino-oxyacetic acid (AOAA, 3 x 100 mg/kg p.o.) and DL-propargylglycine (PPG, 300 mg/kg i.p. plus 300 mg/kg p.o.), reduced HCBD nephrotoxicity by 46 and 59%, respectively, but did not protect against MeHg nephrotoxicity. The results support a role for gamma-GT and beta-lyase in the mouse renal toxicity of HCBD and implicate gamma-GT but not beta-lyase in MeHg-induced nephrotoxicity in mice.

Cytotoxicity of S-(1,2,3,4,4-pentachlorobutadienyl)-L cysteine after apical and basolateral exposure of LLC-PK monolayers. Involvement of an amino acid transport system

Glutathione conjugation and subsequent formation of cysteine conjugates are key steps in the nephrotoxicity of halogenated alkenes. In this metabolic activation several organs are involved. However little is known about the transporters responsible for the uptake of cysteine conjugates. Recent evidence suggest that amino acid transporters play a role in this uptake. Monolayers of LLC-PK1 cells, a kidney cell line, were exposed to S-(1,2,3,4,4-pentachlorobutadienyl)-L-cysteine (PCBD-CYS). Cytotoxicity was used as a parameter for PCBD-CYS uptake. Basolateral exposure (1 h: 400 microM and 16 h: 25 microM) to PCBD-CYS resulted in a much higher aminooxyacetic acid inhibitable cytotoxicity than apical exposure, suggesting a preferential basolateral uptake of PCBD-CYS. Exposure to PCBD-CYS in the absence of sodium did not result in a decrease of the cytotoxicity, suggesting a sodium independency of the PCBD-CYS uptake. Amino acids and amino acid analogues were used as diagnostic compounds in the further identification of the PCBD-CYS transporter. In cis-inhibition experiments monolayers were co-incubated with PCBD-CYS and these diagnostic compounds during one hour. System L substrates such as 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid (BCH) and cycloleucine did not inhibit cytotoxicity. D-Tryptophan, a model inhibitor of System T, caused a strong inhibition. System L has, in contrast to System T, a high sensitivity to trans-stimulation. Pre-loading the monolayers with the diagnostic compounds should cause an increase in cytotoxicity when System L is involved. Neither System L substrates such as BCH and cycloleucine nor D-tryptophan increased cytotoxicity. These results suggest a preferential basolateral uptake of PCBD-CYS in LLC-PK1 monolayers and involvement of an amino acid transporter with characteristics of System T.

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.

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.

N-acetyl S-(1,2-dichlorovinyl)-L-cysteine produces a similar toxicity to S-(1,2-dichlorovinyl)-L-cysteine in rabbit renal slices: differential transport and metabolism

Renal cortical slices were used to determine the toxicity of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (N-acetyl-DCVC) as well as to investigate the transport and metabolism of S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and the N-acetyl derivative. N-Acetyl-DCVC produced dose- and time-dependent decreases in intracellular K+ content and lactate dehydrogenase activity. Histopathology demonstrated an initial S3 lesion followed by a lesion inclusive of all proximal tubules. N-Acetyl-DCVC was shown to be transported via the organic anion system by its ability to inhibit PAH transport by the cells and the ability of probenecid to decrease uptake (80%) and toxicity of N-acetyl-DCVC. DCVC, in contrast, was not transported by the organic anion system, but may be transported by one or more amino acid systems. N-Acetyl-DCVC must be deacetylated before undergoing metabolism by beta-lyase. This process must occur since covalent binding of a 35S-labeled reactive product from N-acetyl [35S]DCVC is observed within 1 hr. Both the uptake inhibitor, probenecid, and aminooxyacetic acid (AOAA), a beta-lyase inhibitor, decreased the covalent binding from N-acetyl [35S]DCVC (80 and 50%, respectively), but only AOAA inhibited the covalent binding of DCVC. AOAA also partially inhibited the toxicity of DCVC and N-acetyl-DCVC as determined by intracellular K+ content, lactate dehydrogenase activity, and histopathology. Despite the fact that a separate transport system and an additional enzymatic step (deacetylation) are required, N-acetyl-DCVC produces a lesion with similar intratubular specificity to that seen with DCVC. Therefore, the S3 specificity seen in vivo could be produced by either compound.

Toxicity of the cysteine-S-conjugates and mercapturic acids of four structurally related difluoroethylenes in isolated proximal tubular cells from rat kidney. Uptake of the conjugates and activation to toxic metabolites

Isolated proximal tubular cells from rat kidney were incubated with the cysteine-S-conjugates and corresponding mercapturates of the potent nephrotoxicants tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), 1,1-dichloro-2,2-difluoroethylene (DCDFE) and 1,1-dibromo-2,2-difluoroethylene (DBDFE). Toxicity of these S-conjugates was determined by their ability to inhibit alpha-methylglucose uptake by the cells. The cytotoxicity of the cysteine-S-conjugates and mercapturates of TFE and CTFE was similar, but the cysteine-S-conjugates of DCDFE and DBDFE were more toxic than their mercapturates. The cytotoxicity of the conjugates decreased in the following order TFE approximately CTFE greater than DCDFE greater than DBDFE, which is the same as observed in vivo. Inhibition of renal cysteine-S-conjugate beta-lyase by aminooxyacetic acid alleviated the cytotoxicity of both the cysteine-S-conjugates and the mercapturic acids of the four haloethylenes. The cytotoxicity of the mercapturates, but not of the cysteine-S-conjugates, could be reduced by probenecid, suggesting that the cysteine-S-conjugates are transported by a different carrier system than the mercapturates. The deacetylation of the mercapturates of TFE and CTFE in the cells was much higher than that of the mercapturates of DCDFE and DBDFE. The cysteine-S-conjugates of DCDFE and DBDFE were N-acetylated by the cells whereas the other cysteine-S-conjugates were not (TFE) or only marginally (CTFE) N-acetylated. The observed differences in cytotoxicity may be explained by differences in (1) the balance between acetylation/deacetylation by the cells, (2) the conversion rate of the S-conjugates to toxic metabolites by renal beta-lyase and (3) the transport into the proximal tubular cells.

Structure-nephrotoxicity relationships of glutathione pathway intermediates derived from organic solvents

The nephrotoxicity of glutathione (GSH) pathway metabolites derived from toluene (TOL), styrene (STYR), bromobenzene (BB), acrylonitrile (ACLN) and 2-chloroacrylonitrile (CACLN) were compared with that of dichlorovinylcysteine (DCVC), using renal brush border and basal-lateral uptake parameters as indices. Cysteine conjugates and mercapturates of ACLN did not alter p-aminohippurate (PAH) uptake by renal tubule suspensions in contrast to its chlorinated homologue. O-, m- and p-conjugates of BB inhibited PAH uptake by 43-82%, the mercapturates showing more potency than corresponding cysteine conjugates. The TOL derivatives N-acetylbenzylcysteine curtailed PAH uptake but benzylcysteine was more effective. The GSH conjugate and mercapturate synthesized from STYR oxide were also active inhibitors but not its cysteine conjugate. Among all GSH pathway metabolites studied, only DCVC and phenylhydroxyethylglutathione, derived from STYR oxide, impeded the renal basal-lateral uptake of [14C]tetraethylammonium (TEA) while DCVC was the sole inhibitor of brush border transport events such as the uptakes of [3H]glutamate and [14C]alpha-methyl-D-glucoside. These data indicate that GSH conjugation represents a non-nephrotoxic detoxication pathway for ACLN. In contrast, GSH conjugation with 2-chloroacrylonitrile and with aromatic solvents like TOL, STYR, BB gives rise to nephrotoxic mercapturates which may be less potent but show more specificity for the organic anion transport system than DCVC.

Cysteine conjugate beta-lyase of rat kidney cytosol: characterization, immunocytochemical localization, and correlation with hexachlorobutadiene nephrotoxicity

Cysteine conjugate beta-lyase (beta-lyase) was purified to electrophoretic homogeneity from the kidney cytosol of male Wistar rats. The highly purified enzyme exhibited a monomeric molecular weight of 50,000 Da and was active in the alpha-beta elimination of cysteine conjugates including S-(1,2-dichlorovinyl)-L-cysteine (DCVC), S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), and S-(2-benzothiazolyl)-L-cysteine, particularly toward DCVC and TFEC. The purified enzyme also exhibited glutamine transaminase K activity with phenylalanine and alpha-keto-gamma-methiolbutyrate as substrates. An antibody was raised to the purified rat protein in sheep and the crude immune serum affinity purified, yielding a specific antibody that recognized only the beta-lyase protein in whole kidney homogenates. Immunocytochemical studies on rat kidney sections stained with the purified antibody revealed that the cytosolic beta-lyase enzyme was mainly localized in the pars recta of the proximal tubule in untreated rats. This localization is coincident with the site-specific kidney necrosis produced by hexachloro-1,3-butadiene (HCBD). These results indicate that the tissue localization of beta-lyase in the proximal tubule plays an important role in determining the specific nephrotoxicity produced by halogenated alkenes such as HCBD.

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.

Deacetylation and further metabolism of the mercapturic acid of hexachloro-1,3-butadiene by rat kidney cytosol in vitro

Hexachloro-1,3-butadiene (HCBD) is more nephrotoxic to female than male rats. Metabolism of HCBD involves conjugation with glutathione followed by formation of the cysteine conjugate S-(pentachloro-1,3-butadienyl) cysteine (PCBD-CYS) and then the mercapturic acid N-acetyl-S-pentachloro-1,3-butadienyl-cysteine (PCBD-NAC). PCBD-NAC is also more nephrotoxic to female rats than male rats. The deacetylation of [14C]-PCBD-NAC to PCBD-CYS and the binding of radiolabelled metabolites to protein has been studied using renal cytosol preparations from male and female rats in vitro, since a sex-related difference in these reactions could explain the difference in nephrotoxicity found in vivo. PCBD-NAC was rapidly metabolised by renal cytosol. The rate of metabolism was similar with either male or female renal cytosol, and the major metabolite identified was PCBD-CYS. N-Acetylation of PCBD-CYS to PCBD-NAC was not detected in the presence of either male or female renal cytosol. Covalent binding of radioactivity from [14C]-PCBD-NAC to cytosolic protein could be detected after 5 min incubation, and although the extent of binding was similar for both male and female cytosol at early time periods, after 60 min incubation more binding was found in the presence of male cytosol. Covalent binding was largely prevented by aminooxyacetic acid, an inhibitor of cysteine conjugate beta-lyase, suggesting a role for this enzyme in the activation of HCBD. These results indicate that the sex differences in the nephrotoxicity of HCBD and PCBD-NAC in the rat are not attributable to differences in the rate of deacetylation of PCBD-NAC to give the proximate nephrotoxin PCBD-CYS.

Nephrotoxicity: a rational approach to target cell injury in vitro in the kidney

1. The kidney is a complex organ in which there is cellular heterogeneity. Many nephrotoxic chemicals target preferentially for discrete cell types, but adjacent, morphologically different cells are unaffected. This selectivity has made the assessment of nephrotoxicity in vivo (and the study of underlying mechanisms) difficult. Discrete renal injury can, however, be exploited in vitro, to study the interactions between the toxic compound and the target cell. 2. Several in vitro models have been used to study the potential interaction between the target cells and chemicals, including perfusion of the isolated kidney, renal slices, freshly isolated fragments, primary cultures and continuous cell lines. Where appropriate, isolated organelles and purified enzymes can also be used. 3. The target cell toxicity in vivo of adriamycin, 2-bromoethanamine and hexachlorobutadiene N-acetyl cysteine conjugate is selectively maintained towards glomerular epithelial, medullary interstitial and proximal tubular cells, respectively, in vitro, showing that the "in vivo-in vitro gap" can be bridged. Characteristics unique to each of these renal cell types, such as the selective uptake of a toxin, enzyme systems for generating biologically reactive intermediates, and the presence of lipid droplets (rich in polyunsaturated fatty acid) and peroxidase activity have been identified, and one or more of these may explain the mechanisms of selective injury in discrete regions of the kidney.

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.

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.