Nephrotoxicity and hepatotoxicity of 1,1-dichloro-2,2-difluoroethylene in the rat. Indications for differential mechanisms of bioactivation

"Commandeuring" Xenobiotic Metabolism: Advances in Understanding Xenobiotic Metabolism

The understanding of how exogenous chemicals (xenobiotics) are metabolized, distributed, and eliminated is critical to determine the impact of the chemical and its metabolites to the (human) organism. This is part of the research and educational discipline ADMET (absorption, distribution, metabolism, elimination, and toxicity). Here, we review the work of Jan Commandeur and colleagues who have not only made a significant impact in understanding of phase I and phase II metabolism of several important compounds but also contributed greatly to the development of experimental techniques for the study of xenobiotic metabolism. Jan Commandeur’s work has covered a broad area of research, such as the development of online screening methodologies, the use of a combination of enzyme mutagenesis and molecular modeling for structure–activity relationship (SAR) studies, and the development of novel probe substrates. This work is the bedrock of current activities and brings the field closer to personalized (cohort-based) pharmacology, toxicology, and hazard/risk assessment.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Evidence for the Formation of Difluoroacetic Acid in Chlorofluorocarbon-Contaminated Ground Water

The concentrations of difluoroacetic acid (DFA) and trifluoroacetic acid (TFA) in rainwater and surface water from Berlin, Germany resembled those reported for similar urban areas, and the TFA/DFA ratio in rainwater of 10:1 was in accordance with the literature. In contrast, nearby ground water historically contaminated with 1,1,2-trichloro-1,2,2-trifluoroethane (R113) displayed a TFA/DFA ratio of 1:3. This observation is discussed versus the inventory of microbial degradation products present in this ground water along with the parent R113 itself. A microbial transformation of chlorotrifluoroethylene (R1113) to DFA so far has not been reported for environmental media, and is suggested based on well-established mammalian metabolic pathways.

Chemical Toxicology of Reactive Intermediates Formed by the Glutathione-Dependent Bioactivation of Halogen-Containing Compounds

The concept that reactive intermediate formation during the biotransformation of drugs and chemicals is an important bioactivation mechanism was proposed in the 1970s and is now accepted as a major mechanism for xenobiotic-induced toxicity. The enzymology of reactive intermediate formation as well as the characterization of the formation and fate of reactive intermediates are now well-established. The mechanism by which reactive intermediates cause cell damage and death is, however, still poorly understood. Although most xenobiotic-metabolizing enzymes catalyze the bioactivation of chemicals, glutathione-dependent biotransformation has been largely associated with detoxication processes, particularly mercapturic acid formation. Abundant evidence now shows that glutathione-dependent biotransformation constitutes an important bioactivation mechanism for halogen-containing drugs and chemicals and has for many compounds been implicated in their organ-selective toxicity and in their mutagenic and carcinogenic potential. The glutathione-dependent biotransformation of haloalkenes is the first step in the cysteine S-conjugate beta-lyase pathway for the bioactivation of nephrotoxic haloalkenes. This pathway has been a rich source of reactive intermediates, including thioacyl halides, alpha-chloroalkenethiolates, 3-halo-alpha-thiolactones, 2,2,3-trihalothiiranes, halothioketenes, and vinylic sulfoxides. Glutathione-dependent bioactivation of gem-dihalomethanes and 1,2-, 1,3-, and 1,4-dihaloalkanes leads to the formation of alpha-chlorosulfides, thiiranium ions, sulfenate esters, and tetrahydrothiophenium ions, respectively, and these reactions lead to reactive intermediate formation.

Effect of beta-naphthoflavone and phenobarbital on the nephrotoxicity of chlorotrifluoroethylene and 1,1-dichloro-2,2-difluoroethylene in the rat

The role of cytochrome P450 activity in the nephrotoxicity of chlorotrifluoroethylene (CTFE) and 1,1-dichloro-2,2-difluoroethylene (DCDFE) was investigated in the male rat. Hepatic cytochrome P450 1A1 and principally P450 2B1/2 were induced by beta-naphthoflavone and phenobarbital, respectively. Nephrotoxicity was evaluated by investigating urine biochemical parameters, kidney histochemistry and histopathological modifications. Both CTFE and DCDFE induce severe nephrotoxicity in rats after 4 h of exposure to 200 and 100 ppm, respectively. Compared with controls, activity levels of gamma-glutamyltranspeptidase (gamma GT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) and N-acetyl-beta-D-glucosaminidase (NAG) in 24-h urine were increased similarly, but urinary excretion of glucose, proteins and beta2-microglobulin (beta2-m) and serum urea and creatinine levels were increased. Histopathological and histochemical examinations of kidney sections of CTFE- and DCDFE-exposed rats revealed cellular necrosis and tubular lesions 24 h after exposure. Beta-naphthoflavone-pretreated rats were afforded some protection against the nephrotoxicity of CTFE and DCDFE. Phenobarbital did not modify DCDFE nephrotoxicity but afforded some protection against CTFE nephrotoxicity. In conclusion, CTFE and DCDFE are strong nephrotoxins. Cytochrome P450 1A1 is implicated in CTFE and DCDFE metabolism and one or several cytochromes induced by phenobarbital are implicated in CTFE metabolism. The P450 cytochromes involved in CTFE and DCDFE metabolism probably constitute detoxication metabolic pathways. The nephrotoxicity of CTFE and DCDFE is therefore subordinated to the cytochrome P450 activity involved in their metabolism.

Chemical-induced nephrotoxicity mediated by glutathione S-conjugate formation

Glutathione conjugation has been identified as an important detoxication reaction. However, several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions is discussed in this article. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl) glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed and interact with nucleic acids. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cystein conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is likely to be responsible for the observed nephrotoxicity of the parent compounds. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they cause alkylation or redox cycling reactions, or both, that cause organ-selective damage.

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.

In vitro metabolism of 5-fluoro-2-glutathionyl-nitrobenzene by kidney proximal tubular cells studied by 19F-NMR

Proximal tubular biotransformation of the glutathionyl (GSH) conjugate derived from 2,5-difluoronitrobenzene (5-fluoro-2-glutathionyl-nitrobenzene) was studied by means of 19F-NMR. This method allows a direct and specific detection of the fluorinated metabolites formed, at a detection limit of 1 microM for an overnight NMR run. Incubation of a monolayer of LLCPK1 cells with 100 microM 5-fluoro-2-glutathionyl-nitrobenzene for 24 h showed that these cells metabolize this GSH conjugate into the corresponding cysteinylglycyl and cysteine conjugate. The expected N-acetylcysteine conjugate however was not formed as an endproduct. Additional experiments demonstrated the absence of N-acetyltransferase activity in LLCPK1 cell lysates incubated with FCysNB and also the rapid loss of this activity in isolated renal proximal tubular cells (RPT): freshly isolated RPT cells do convert FCysNB to FNAcNB as major metabolite but, upon cultivation, quickly lose this capacity. Since uptake of FCysNB might also be a limiting factor, we investigated transport of FCysNB from the apical to the basolateral side of the culture RPT cells. No indication for such transport was obtained. Thus, the absence of mercapturic acid formation in LLCPK1 cells and cultured RPT cells is the results of a decline in N-acetyltransferase activity and perhaps a deficient cellular uptake of the cysteine conjugate.

Metabolism of 1,1-dichloro-2,2,2-trifluoroethane in rats

1. Chlorofluorohydrocarbons are presently being developed as alternatives for ozone-depleting chlorofluorocarbons. 1,1-Dichloro-2,2,2-trifluoro-[2-14C]-ethane (HCFC-123) is a chlorofluorohydrocarbon with potential widespread use and associated human exposure. As a part of the toxicological evaluation of HCFC-123, its metabolism was studied in rodents in a closed recirculating exposure system. 2. Two male rats were individually exposed for 6 h. Excretion of radioactivity was monitored for 48 h after the start of the exposure. Of the radioactivity introduced into the chamber, 14% was recovered in urine within the period of observation. Excretion of metabolites in the urine was very slow. 3. Trifluoroacetic acid was the major metabolite of HCFC-123 and N-trifluoroacetyl-2-aminoethanol and N-acetyl-S-(2,2-dichloro-1,1-difluoroethyl)-L-cysteine were identified as minor urinary metabolites of HCFC-123. 4. Forty-eight hours after the start of the exposure, covalent binding of radioactive metabolites to protein was highest in liver followed by kidney and lung. Covalent binding above background levels was not observed in pancreas and testis, the target organs of HCFC-123 tumourigenicity. 5. These results suggest that the biotransformation of HCFC-123 in rodents follows a pathway identical to those of the extensively studied structural analogue halothane.

Glutathione-dependent bioactivation of xenobiotics

Glutathione conjugation has been identified as an important detoxication reaction. However, in recent years several glutathione-dependent bioactivation reactions have been identified. Current knowledge on the mechanisms and the possible biological importance of these reactions are discussed. 1. Dichloromethane is metabolized by glutathione conjugation to formaldehyde via S-(chloromethyl)glutathione. Both compounds are reactive intermediates and may be responsible for the dichloromethane-induced tumorigenesis in sensitive species. 2. Vicinal dihaloalkanes are transformed by glutathione S-transferase-catalyzed reactions to mutagenic and nephrotoxic S-(2-haloethyl)glutathione S-conjugates. Electrophilic episulphonium ions are the ultimate reactive intermediates formed. 3. Several polychlorinated alkenes are bioactivated in a complex, glutathione-dependent pathway. The first step is hepatic glutathione S-conjugate formation followed by cleavage to the corresponding cysteine S-conjugates, and, after translocation to the kidney, metabolism by renal cysteine conjugate beta-lyase. Beta-Lyase-dependent metabolism of halovinyl cysteine S-conjugates yields electrophilic thioketenes, whose covalent binding to cellular macromolecules is responsible for the observed toxicity of the parent compounds. 4. Finally, hepatic glutathione conjugate formation with hydroquinones and aminophenols yields conjugates that are directed to gamma-glutamyltransferase-rich tissues, such as the kidney, where they undergo alkylation or redox cycling reactions, or both, that cause organ-selective damage.

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.

Mutagenicity and cytotoxicity of two regioisomeric mercapturic acids and cysteine S-conjugates of trichloroethylene

The mutagenicity, cytotoxicity and metabolism of two regioisomic L-cysteine- and N-acetyl-L-cysteine-S-conjugates of trichloroethylene were studied. The 1,2-dichlorovinyl(1,2-DCV) isomers of both the cysteine conjugate and the mercapturate were much stronger mutagens in the Ames test with Salmonella typhimurium TA2638 when compared to the corresponding 2,2-dichlorovinyl (2,2-DCV) isomers. Similarly, the 1,2-DCV isomers were more cytotoxic towards isolated rat kidney proximal tubular cells, as assessed by inhibition of alpha-methylglucose uptake, than the 2,2-DCV isomers. The 3-4-fold higher rate of beta-lyase-dependent activation of S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCV-Cys) when compared to S-(1,2-dichlorovinyl)-L-cysteine (2,2-DCV-Cys) as well as the different nature of the reactive intermediates formed is probably responsible for these structure-dependent effects. The cytotoxicity of N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCV-NAc) toward isolated kidney cells showed a delayed time course as compared to that of 1,2-DCV-Cys, probably due to the relatively low rate of deacetylation of 1,2-DCV-NAc. The time course of cytotoxicity of N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine (2,2-DCV-NAc), however, parallelled that of 2,2-DCV-Cys. Due to the relatively high rate of N-acetylation and low rate of beta-lyase activation, for 2,2-DCV-Nac the beta-lyase activation step may be rate limiting. Different rates of cellular uptake also may play a role in time course of toxicity of the cysteine conjugates and the mercapturic acids in the renal cells.

Metabolism of L-cysteine S-conjugates and N-(trideuteroacetyl)-L-cysteine S-conjugates of four fluoroethylenes in the rat. Role of balance of deacetylation and acetylation in relation to the nephrotoxicity of mercapturic acids

The relationship between the relative nephrotoxicity of the mercapturic acids (NAc) of the fluorinated ethylenes tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), 1,1-dichloro-2,2-difluoroethylene (DCDFE) and 1,1-dibromo-2,2-difluoroethylene (DBDFE), and the biotransformation by activating (N-deacetylase and beta-lyase) and inactivating (N-acetyltransferase) enzymes was studied in the rat. After intraperitoneal (i.p.) administration of 50 mumol/kg of N-(trideuteroacetyl)-labeled mercapturic acids of DCDFE and DBDFE to rats, significant amounts of the dose were excreted unchanged: with DCDFE-NAc, 17% of the dose, and DBDFE-NAc, 31% of the dose. In contrast, the corresponding deuterium-labeled mercapturic acids of TFE and CTFE were excreted unchanged at less than 1% of the dose. With DCDFE-NAc and DBDFE-NAc, also high amounts of unlabeled mercapturic acids were excreted, respectively 48% and 28% of the dose, indicating extensive N-deacetylation followed by reacetylation in vivo. Only small amounts (less than 2%) of unlabeled mercapturic acids were excreted with TFE-NAc and CTFE-NAc. After administration of the cysteine S-conjugates DCDFE-Cys and DBDFE-Cys to rats, high amounts of the corresponding mercapturic acids were detected in urine, respectively 57% and 45% of the dose. After administration of TFE-Cys and CTFE-Cys, however, only small amounts were excreted as the corresponding mercapturic acid, approximately 4% of the dose. The strongly different amounts of mercapturic acids in urine may be attributed to the strong differences in N-deacetylation activities which were found in rat renal fractions. The threshold dose of the mercapturic acids to cause nephrotoxicity in male Wistar rats increased in the order: CTFE-NAc (25 mumol/kg) less than TFE-NAc (50 mumol/kg) less than DCDFE-NAc (75 mumol/kg) less than DBDFE-NAc (100 mumol/kg). A higher ratio of N-deacetylation and N-acetylation activities, resulting in a higher availability of cysteine S-conjugate, in addition to a higher specific activity of cysteine S-conjugate beta-lyase, probably explains the higher nephrotoxicity of TFE-NAc and CTFE-NAc when compared to DCDFE-NAc and DBDFE-NAc. The much lower activities of N-deacetylation and beta-lyase which are observed in hepatic fractions may explain the lack of hepatotoxicity of the mercapturic acids studied.

The role of metallothionein in the reduction of cisplatin-induced nephrotoxicity by Bi3(+)-pretreatment in the rat in vivo and in vitro. Are antioxidant properties of metallothionein more relevant than platinum binding?

Nephrotoxicity induced by cisplatin (CDDP) was reported to be reduced by Bi3(+)-pretreatment, which selectively induces renal metallothionein (MT). In the present study renal MT had increased to 250% of control in rats that received bismuth subnitrate (50 mumol/kg/day, orally) for 8 days. In vitro experiments demonstrated that the reduction of CDDP-induced toxicity is a renal effect: in proximal tubular cells (PTC) isolated from Bi3(+)-treated rats the toxicity of CDDP, and also of HgCl2, CdCl2 and p-aminophenol, was reduced as compared to PTC from untreated rats. In contrast to the reduction in CDDP, Hg2+ and Cd2+ toxicity, the reduction in p-aminophenol toxicity cannot be explained by the metal-binding properties of MT. MT was reported to act as a free radical scavenger, which may explain our observation since p-aminophenol toxicity is thought to be a consequence of the generation of oxygen radicals. In vivo experiments showed that the overall renal Pt-content as well as the Pt bound to renal MT is lower in Bi3(+)-pretreated rats than in untreated rats, 24 hr after administration of CDDP (12 mg/kg), suggesting that the reduction in nephrotoxicity is not due to increased binding of Pt2+ to renal MT. Renal superoxide dismutase (SOD) activity was increased in rats that had only received CDDP. Such a rise in SOD may result from peroxidative damage caused by exposure to CDDP. The fact that SOD was not elevated in rats that received Bi3+ prior to CDDP suggests that (i) peroxidation contributes to CDDP-induced nephrotoxicity and (ii) the anti-oxidant properties of MT are responsible for the reduction of this toxicity.

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.

Primary culture of proximal tubular cells from normal rat kidney as an in vitro model to study mechanisms of nephrotoxicity. Toxicity of nephrotoxicants at low concentrations during prolonged exposure

The aim of this study was to set up an in vitro system to study nephrotoxicity of xenobiotics which allows exposure at low concentrations for long periods (1-5 days). A very pure preparation of isolated proximal tubular cells (PTC) from rat kidney (Boogaard et al., Toxicol Appl Pharmacol 101: 135-143, 1989) was brought into primary culture. Cells grew to confluence in 3 days and could be maintained up to 8 days in a modification of Dulbecco's modified Eagle's medium Ham F12 nutrient mixture supplemented with fetal calf serum. Fibroblast growth was completely suppressed by replacement of L-valine by D-valine and of L-arginine by L-ornithine. Polarity was retained: in cells grown on filters organic anions were transported at the basolateral membrane while D-glucose transport was located at the apical membrane. Inhibition of the latter was used to assess the functional integrity of the cells after exposure to nephrotoxins. The newly grown cells expressed gamma-glutamyltranspeptidase activity since incubation with the glutathione-conjugate of 1,1-dichloro-2,2-difluoroethylene (DCDFE) induced cytotoxicity. Both beta-lyase and acylase activities were expressed because the cysteine-S-conjugate and the corresponding mercapturate of DCDFE showed cytotoxicity. Cultured cells showed toxicity on prolonged exposure to very low concentrations of gentamicin, cephaloridine, cisplatin and the cysteine-S-conjugate of chlorotrifluoroethylene. The lowest concentrations at which toxicity can be observed are 1-3 orders of magnitude lower in primary cultures than in freshly isolated PTC in suspension. This indicates that this cell model is suitable to investigate mechanisms of nephrotoxicity in vitro, at prolonged exposure to the low concentrations that are relevant in vivo levels.

Metabolism of trichloroethene--in vivo and in vitro evidence for activation by glutathione conjugation

The metabolism of trichloroethene by glutathione conjugation was investigated in rat liver subcellular fractions and in male rats in vivo. In the presence of glutathione, rat liver microsomes transformed [14C]trichloroethene to S-(1,2-dichlorovinyl)glutathione (DCVG) identified by gas chromatography mass spectrometry after hydrolysis to the corresponding cysteine S-conjugate and chemical derivatisation. In bile of rats given 2.2 g/kg trichloroethene. DCVG was present in concentrations of 5 nmol (7 ml bile collected over 9 h) and identified by thermospray mass spectrometry after HPLC-purification. E- and Z-N-acetyl-dichlorovinyl-L-cysteine (3.1 nmol present in the pooled 24-h urine) were identified by GC/MS after methylation and butylation as urinary metabolites of trichloroethene (2.2 g/kg, orally). The presented results demonstrate that glutathione-dependent metabolism of trichloroethene is a minor route in the biotransformation of this haloalkene in rats. Formation of S-(1,2-dichlorovinyl)-glutathione, processing to S-(1,2-dichlorovinyl)-L-cysteine and metabolism of this S-conjugate by cysteine beta-lyase in the kidney to reactive and genotoxic intermediates may account for the nephrocarcinogenicity observed after long time administration of trichloroethene in male rats.

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.

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.

Isolated proximal tubular cells from rat kidney as an in vitro model for studies on nephrotoxicity. II. Alpha-methylglucose uptake as a sensitive parameter for mechanistic studies of acute toxicity by xenobiotics

Many nephrotoxic agents exert their effect primarily on the cells of the proximal tubules. We isolated these cells and investigated whether the uptake of alpha-methylglucose (alpha-MG) could serve as a parameter to assess effects of nephrotoxins on the functional integrity of the cells. Agents that are acutely nephrotoxic in vivo, CD2+, Hg2+, UO22+, p-aminophenol, and bis-2,3-dibromopropylphosphate, inhibited alpha-MG uptake at low concentrations. Most agents that exert their effect in vivo with delay or only when used chronically (gentamicin, cephaloridine, phenacetin, and acetaminophen) inhibited alpha-MG uptake only at much higher concentrations; cisplatin, however, inhibited alpha-MG uptake at a low concentration. S-(1,1-Difluoro-2,2-dichloroethyl)-L-cysteine and its N-acetyl derivate, two metabolites of the nephrotoxic agent 1,1-dichloro-2,2-difluoroethylene, inhibit alpha-MG uptake. Aminooxyacetic acid, which prevents the formation of the ultimate toxic metabolite by inhibition of beta-lyase, abolished almost completely the toxicity of both compounds. The nephrotoxic conjugate of hexachlorobutadiene, S-(1,2,3,4,4-pentachlorobutadienyl)-glutathione, also inhibited alpha-MG uptake. The selective inhibitor of gamma-glutamyltranspeptidase, anthglutin, completely prevented this inhibition. These results indicate that the uptake of alpha-methylglucose by isolated proximal tubular cells from rat kidney is a useful parameter for the study of nephrotoxicity, since in vitro results reflect acute nephrotoxicity as observed in vivo.

N-acetyl-S-(2-hydroxyethyl)-L-cysteine as a potential tool in biological monitoring studies? A critical evaluation of possibilities and limitations

In mammalian species, including man, N-acetyl-S-(2-hydroxyethyl)-L-cysteine (2-HEMA) is a common urinary metabolite of a large number of structurally different xenobiotic chemicals. It is a common urinary end product of glutathione pathway metabolism of a variety of chemicals possessing electrophilic properties and, in most cases, also a genotoxic potential. Five different chemically reactive intermediates, with different electrophilic properties, may be involved in the formation of 2-HEMA. An inventory of chemicals known to lead to the formation of 2-HEMA, or based on their chemical structure expected to do so, is presented. Furthermore, an attempt is made to evaluate the possibilities and limitations in terms of the potential use of urinary 2-HEMA as a tool in biomonitoring studies. Two other related, sulfur-containing urinary metabolites, i.e. N-acetyl-(S-carboxymethyl)-L-cysteine and thio-diacetic acid, are proposed as possible alternatives to urinary 2-HEMA. It is suggested that 2-HEMA might be seen as a potentially useful and sensitive signal parameter for the assessment of exposure of animals and man to a variety of electrophilic and therefore potentially toxic xenobiotic chemicals.

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.

Nephrotoxicity of mercapturic acids of three structurally related 2,2-difluoroethylenes in the rat. Indications for different bioactivation mechanisms

The biotransformation and the hepato- and nephrotoxicity of the mercapturic acids (N-acetyl-1-cysteine S-conjugates) of three structurally related 2,2-difluoroethylenes were investigated in vivo in the rat. All mercapturic acids appeared to cause nephrotoxicity, without any measureable effect on the liver. The mercapturic acid of tetrafluoroethylene (TFE-NAC) appeared to be the most potent nephrotoxin, causing toxicity upon an i.p. dose of 50 mumol/kg. The mercapturic acids of 1,1-dichloro-2,2-difluoroethylene (DCDFE-NAC) and 1,1-dibromo-2,2-difluoroethylene (DBDFE-NAC) were nephrotoxic at slightly higher doses, i.e. at 75 and 100 mumol/kg, respectively. In the urine of TFE-NAC-treated rats significant amounts of difluoroacetic acid (DFAA) could be detected. With increasing doses, the relative amount of DFAA in urine increased progressively (5-18% of dose). In urine of rats treated with DCDFE-NAC and DBDFE-NAC, however, the corresponding dihaloacetic acids, dichloroacetic acid and dibromoacetic acid, could not be detected. Formation of DFAA and pyruvate could also be observed during in vitro metabolism of the cysteine conjugate of tetrafluoroethylene (TFE-CYS) by rat renal cytosol. Inhibition by aminooxyacetic acid (AOA) pointed to a beta-lyase dependency for the DFAA-formation. Next to DFAA and pyruvate, also formation of hydrogen sulfide and thiosulfate could be detected. These results suggest that TFE-CYS is bioactivated to a significant extent to difluorothionacyl fluoride, which most likely is subsequently hydrolysed to difluorothio(no)acetic acid and difluoroacetic acid. According to formation of pyruvate, the cysteine conjugates derived from DCDFE-NAC and DBDFE-NAC also were efficiently metabolized by rat renal beta-lyase. However, the formation of corresponding dihaloacetic acids, dichloroacetic acid and dibromoacetic acid, could not be detected in vitro at all. Only very small amounts of hydrogen sulfide and thiosulfate were detected. These results suggest that bioactivation of the latter two conjugates to a dichloro- or dibromothionoacyl fluoride represents only a minor route. Because of better leaving group abilities of chloride and bromide compared to fluoride, rearrangement of the initially formed ethanethiol to a thiirane might be favoured. Based on the present in vivo and in vitro data, it is concluded that the nephrotoxicity of the structurally related mercapturic acids of 2,2-difluoroethylenes is dependent on halogen substitution and presumably the result of at least two different mechanisms of bioactivation.

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.