Cytotoxicity of alkylating agents in isolated rat kidney proximal tubular and distal tubular cells

Renal Glutathione: Dual roles as antioxidant protector and bioactivation promoter

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

Methyl vinyl ketone and its analogs covalently modify PI3K and alter physiological functions by inhibiting PI3K signaling

Reactive carbonyl species (RCS), which are abundant in the environment and are produced in vivo under stress, covalently bind to nucleophilic residues such as Cys in proteins. Disruption of protein function by RCS exposure is predicted to play a role in the development of various diseases such as cancer and metabolic disorders, but most studies on RCS have been limited to simple cytotoxicity validation, leaving their target proteins and resulting physiological changes unknown. In this study, we focused on methyl vinyl ketone (MVK), which is one of the main RCS found in cigarette smoke and exhaust gas. We found that MVK suppressed PI3K-Akt signaling, which regulates processes involved in cellular homeostasis, including cell proliferation, autophagy, and glucose metabolism. Interestingly, MVK inhibits the interaction between the epidermal growth factor receptor and PI3K. Cys656 in the SH2 domain of the PI3K p85 subunit, which is the covalently binding site of MVK, is important for this interaction. Suppression of PI3K-Akt signaling by MVK reversed epidermal growth factor-induced negative regulation of autophagy and attenuated glucose uptake. Furthermore, we analyzed the effects of the 23 RCS compounds with structures similar to MVK and showed that their analogs also suppressed PI3K-Akt signaling in a manner that correlated with their similarities to MVK. Our study demonstrates the mechanism of MVK and its analogs in suppressing PI3K-Akt signaling and modulating physiological functions, providing a model for future studies analyzing environmental reactive species.

Detoxification Mechanism of α,β-Unsaturated Carbonyl Compounds in Cigarette Smoke Observed in Sheep Erythrocytes

Highly reactive α,β-unsaturated carbonyl compounds, such as acrolein (ACR), crotonaldehyde (CA) and methyl vinyl ketone (MVK), are environmental pollutants present in high concentrations in cigarette smoke. We have previously found that these carbonyl compounds in cigarette smoke extract (CSE) react with intracellular glutathione (GSH) to produce the corresponding GSH-ACR, GSH-CA and GSH-MVK adducts via Michael addition reaction. These adducts are then further reduced to the corresponding alcohol forms by intracellular aldo-keto reductases in highly metastatic mouse melanoma (B16-BL6) cells and then excreted into the extracellular fluid. This time, we conducted a similar study using sheep erythrocytes and found analogous changes in the sheep erythrocytes after exposure to CSE as those with B16-BL6 cells. This indicates similarity of the detoxification pathways of the α,β-unsaturated carbonyl compounds in sheep blood cells and B16-BL6 cells. Also, we found that the GSH-MVK adduct was reduced by aldose reductase in a cell-free solution to generate its alcohol form, and its reduction reaction was completely suppressed by pretreatment with epalrestat, an aldose reductase inhibitor, a member of the aldo-keto reductase family. In the presence of sheep blood cells, however, reduction of the GSH-MVK adduct was partially inhibited by epalrestat. This revealed that some member of the aldo-keto reductase superfamily other than aldose reductase is involved in reduction of the GSH-MVK adduct in sheep blood. These results suggest that blood cells, mainly erythrocytes are involved in reducing the inhalation toxicity of cigarette smoke via an aldo-keto reductase pathway other than that of aldose reductase.

Vanin-1: a potential biomarker for nephrotoxicant-induced renal injury

Because traditional markers for detecting renal injury are generally insensitive and nonspecific, we tried to identify some useful biomarkers. Microarray analyses and quantitative real-time PCR using human renal tubular cells showed that the mRNA expression of VNN-1 which encodes vanin-1, increased after the exposure of these cells to organic solvents (allyl alcohol, ethylene glycol, formaldehyde, chloroform, and phenol) for 24h. The mRNA levels of other inflammation-related molecules such as monocyte chemoattractant protein 1 (MCP-1) and kidney injury molecule-1 (KIM-1) also increased after the exposure to organic solvents, although their elevations were slower than that of vanin-1. In rats treated with ethylene glycol for 3 weeks, tubular injury was detected by histological examination, but not by traditional biomarkers including serum creatinine and urinary N-acetyl-β-glucosaminidase. The mRNA levels of vanin-1 and Kim-1, but not MCP-1, significantly elevated in the renal cortices of ethylene glycol-exposed rats. On immunofluorescence analyses, vanin-1 signal was detected specifically in the renal tubules with a remarkable expression in the ethylene glycol-treated rats. As a result, compared with control group, higher urinary and serum concentrations of vanin-1 were observed in the ethylene glycol-treated group. These results suggest that vanin-1 is a useful and rapid biomarker for renal tubular injury induced by organic solvents.

γ-Glutamylcysteine ameliorates oxidative injury in neurons and astrocytes in vitro and increases brain glutathione in vivo

γ-Glutamylcysteine (γ-GC) is an intermediate molecule of the glutathione (GSH) synthesis pathway. In the present study, we tested the hypothesis that γ-GC pretreatment in cultured astrocytes and neurons protects against hydrogen peroxide (H(2)O(2))-induced oxidative injury. We demonstrate that pretreatment with γ-GC increases the ratio of reduced:oxidized GSH levels in both neurons and astrocytes and increases total GSH levels in neurons. In addition, γ-GC pretreatment decreases isoprostane formation both in neurons and astrocytes, as well as nuclear factor erythroid 2-related factor 2 (Nrf2) nuclear translocation in astrocytes in response to H(2)O(2)-induced oxidative stress. Furthermore, GSH and isoprostane levels significantly correlate with increased neuron and astrocyte viability in cells pretreated with γ-GC. Finally, we demonstrate that administration of a single intravenous injection of γ-GC to mice significantly increases GSH levels in the brain, heart, lungs, liver, and in muscle tissues in vivo. These results support a potential therapeutic role for γ-GC in the reduction of oxidant stress-induced damage in tissues including the brain.

Methyl vinyl ketone induces apoptosis in murine GT1-7 hypothalamic neurons through glutathione depletion and the generation of reactive oxygen species

alpha,beta-Unsaturated carbonyl compounds have been implicated in a number of environmentally-related diseases. Often, the presence of alpha,beta-unsaturated carbonyl functionality as part of either an aliphatic or cyclic structure is considered a structural alert for cytotoxicity. We examined the cytotoxicity of methyl vinyl ketone (MVK), an aliphatic, straight-chain alpha,beta-unsaturated carbonyl compound, in murine GT1-7 hypothalamic neurons. In addition to its widespread environmental occurrence, MVK was selected due to its extensive use in the chemical industry. Also, MVK is a close structural analog of hydroxymethylvinyl ketone that, in part, mediates the cytotoxic effects of 1,3-butadiene in vivo. It was found that MVK at low micromolar concentrations induced extensive cell death that retained key features of apoptosis such as chromatin condensation and DNA fragmentation. The MVK-induced apoptosis was associated with depletion of glutathione, disruption of mitochondrial transmembrane potential, and increased generation of reactive oxygen species (ROS). Supplementation of neuronal cells with Trolox offered partial, but significant, protection against the MVK-induced cytotoxicity, presumably due to scavenging of ROS in situ. The suggested sequence of events in the MVK-induced apoptosis in neuronal cells involves the depletion of cellular glutathione followed by an increased generation of ROS and finally the loss of mitochondrial function.

Role of mitochondrial dysfunction in cellular responses to S-(1,2-dichlorovinyl)-L-cysteine in primary cultures of human proximal tubular cells

The nephrotoxic metabolite of the environmental contaminant trichloroethylene, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), is known to elicit cytotoxicity in rat and human proximal tubular (rPT and hPT, respectively) cells that involves inhibition of mitochondrial function. DCVC produces a range of cytotoxic and compensatory responses in hPT cells, depending on dose and exposure time, including necrosis, apoptosis, repair, and enhanced cell proliferation. The present study tested the hypothesis that induction of mitochondrial dysfunction is an obligatory step in the cytotoxicity caused by DCVC in primary cultures of hPT cells. DCVC-induced necrosis was primarily a high concentration (> or =50 microM) and late (> or =24h) response whereas apoptosis and increased proliferation occurred at relatively low concentrations (<50 microM) and early time points (< or =24h). Decreases in cellular DNA content, indicative of cell loss, were observed at DCVC concentrations as low as 1 microM. Involvement of mitochondrial dysfunction in DCVC-induced cytotoxicity was supported by showing that DCVC caused modest depletion of cellular ATP, inhibition of respiration, and activation of caspase-3/7. Cyclosporin A protected cells against DCVC-induced apoptosis and both cyclosporin A and ruthenium red protected cells against DCVC-induced loss of mitochondrial membrane potential. DCVC caused little or no activation of caspase-8 and did not significantly induce expression of Fas receptor, consistent with apoptosis occurring only by the mitochondrial pathway. These results support the conclusion that mitochondrial dysfunction is an early and obligatory step in DCVC-induced cytotoxicity in hPT cells.

Acrylamide-induced nerve terminal damage: relevance to neurotoxic and neurodegenerative mechanisms

Acrylamide (ACR) has demonstrable neurotoxic effects in animals and humans that stem from its chemical behavior as a soft electrophilic alpha,beta-unsaturated carbonyl compound. Evidence is presented that the nerve terminal is a primary site of ACR action and that inhibition of neurotransmission mediates the development of neurological deficits. At the mechanistic level, recent proteomic, neurochemical, and kinetic data are considered, which suggest that ACR inhibits neurotransmission by disrupting presynaptic nitric oxide (NO) signaling. Nerve-terminal damage likely mediates the neurological complications that accompany the occupational exposure of humans to ACR. In addition, the proposed molecular mechanism of synaptotoxicity has substantial implications for the pathogenesis of Alzheimer's disease and other neurodegenerative conditions that involve neuronal oxidative stress and the secondary endogenous generation of acrolein and other conjugated carbonyl chemicals.

Molecular mechanisms of the conjugated alpha,beta-unsaturated carbonyl derivatives: relevance to neurotoxicity and neurodegenerative diseases

Conjugated alpha,beta-unsaturated carbonyl derivatives such acrylamide, acrolein, and 4-hydroxy-2-nonenal (HNE) are members of a large class of chemicals known as the type-2 alkenes. Human exposure through diet, occupation, and pollution is pervasive and has been linked to toxicity in most major organs. Evidence suggests that these soft electrophiles produce toxicity by a common mechanism involving the formation of Michael-type adducts with nucleophilic sulfhydryl groups. In this commentary, the adduct chemistry of the alpha,beta-unsaturated carbonyls and possible protein targets will be reviewed. We also consider how differences in electrophilic reactivity among the type-2 alkenes impact corresponding toxicokinetics and toxicological expression. Whereas these concepts have mechanistic implications for the general toxicity of type-2 alkenes, this commentary will focus on the ability of these chemicals to produce presynaptic damage via protein adduct formation. Given the ubiquitous environmental presence of the conjugated alkenes, discussions of molecular mechanisms and possible neurotoxicological risks could be important. Understanding the neurotoxicodynamic of the type-2 alkenes might also provide mechanistic insight into neurodegenerative conditions where neuronal oxidative stress and presynaptic dysfunction are presumed initiating events. This is particularly germane to a recent proposal that lipid peroxidation and the subsequent liberation of acrolein and HNE in oxidatively stressed neurons mediate synaptotoxicity in brains of Alzheimer's disease patients. This endogenous neuropathogenic process could be accelerated by environmental type-2 alkene exposure because common nerve terminal proteins are targeted by alpha,beta-unsaturated carbonyl derivatives. Thus, the protein adduct chemistry of the conjugated type-2 alkenes offers a mechanistic explanation for the environmental toxicity induced by these chemicals and might provide insight into the pathogenesis of certain human neurodegenerative diseases.

Differences in lysine adduction by acrolein and methyl vinyl ketone: implications for cytotoxicity in cultured hepatocytes

Acrolein is a highly toxic environmental pollutant that readily alkylates the epsilon-amino group of lysine residues in proteins. In model systems, such chemistry involves sequential addition of two acrolein molecules to a given nitrogen, forming bis-Michael-adducted species that undergo aldol condensation and dehydration to form Nepsilon-(3-formyl-3,4-dehydropiperidino)lysine. Whether this ability to form cyclic adducts participates in the toxicity of acrolein is unknown. To address this issue, we compared the chemistry of protein adduction by acrolein to that of its close structural analogue methyl vinyl ketone, expecting that the alpha-methyl group would hinder the intramolecular cyclization of any bis-adducted species formed by methyl vinyl ketone. Both acrolein and methyl vinyl ketone displayed comparable protein carbonylating activity during in vitro studies with the model protein bovine serum albumin, confirming the alpha,beta,-unsaturated bond of both compounds is an efficient Michael acceptor for protein nucleophiles. However, differences in adduction chemistry became apparent during the use of electrospray ionization-MS to monitor reaction products in a lysine-containing peptide after modification by each compound. For example, although a Schiff base adduct was detected following reaction of the peptide with acrolein, an analogous species was not formed by methyl vinyl ketone. Furthermore, while ions corresponding to mono- and bis-Michael adducts were detected at the N-terminus and lysine residues following peptide modification by both carbonyls, only acrolein modification generated ions attributable to cyclic adducts. Despite these differences in adduction chemistry, in mouse hepatocytes, the two compounds exhibited very comparable abilities to induce rapid, concentration-dependent cell death as well as protein carbonylation. These findings suggest that the acute toxicity of short-chain alpha,beta-unsaturated carbonyl compounds involves their ability to form acyclic Michael addition adducts rather than Schiff conjugates or heterocyclic adducts.

Role of glutathione transport processes in kidney function

The kidneys are highly dependent on an adequate supply of glutathione (GSH) to maintain normal function. This is due, in part, to high rates of aerobic metabolism, particularly in the proximal tubules. Additionally, the kidneys are potentially exposed to high concentrations of oxidants and reactive electrophiles. Renal cellular concentrations of GSH are maintained by both intracellular synthesis and transport from outside the cell. Although function of specific carriers has not been definitively demonstrated, it is likely that multiple carriers are responsible for plasma membrane transport of GSH. Data suggest that the organic anion transporters OAT1 and OAT3 and the sodium-dicarboxylate 2 exchanger (SDCT2 or NaDC3) mediate uptake across the basolateral plasma membrane (BLM) and that the organic anion transporting polypeptide OATP1 and at least one of the multidrug resistance proteins mediate efflux across the brush-border plasma membrane (BBM). BLM transport may be used pharmacologically to provide renal proximal tubular cells with exogenous GSH to protect against oxidative stress whereas BBM transport functions physiologically in turnover of cellular GSH. The mitochondrial GSH pool is derived from cytoplasmic GSH by transport into the mitochondrial matrix and is mediated by the dicarboxylate and 2-oxoglutarate exchangers. Maintenance of the mitochondrial GSH pool is critical for cellular and mitochondrial redox homeostasis and is important in determining susceptibility to chemically induced apoptosis. Hence, membrane transport processes are critical to regulation of renal cellular and subcellular GSH pools and are determinants of susceptibility to cytotoxicity induced by oxidants and electrophiles.

The metabolic activation of abacavir by human liver cytosol and expressed human alcohol dehydrogenase isozymes

Abacavir (ZIAGEN) is a reverse transcriptase inhibitor marketed for the treatment of HIV-1 infection. A small percentage of patients experience a hypersensitivity reaction indicating immune system involvement and bioactivation. A major route of metabolism for abacavir is oxidation of a primary betagamma unsaturated alcohol to a carboxylic acid via an aldehyde intermediate. This process was shown to be mediated in vitro by human cytosol and NAD, and subsequently the alphaalpha and gamma2gamma2 human isoforms of alcohol dehydrogenase (ADH). The alphaalpha isoform effected two sequential oxidation steps to form the acid metabolite and two isomers, qualitatively reflective of in vitro cytosolic profiles. The gamma2gamma2 isozyme generated primarily an isomer of abacavir, which was minor in the alphaalpha profiles. The aldehyde intermediate could be trapped in incubations with both isozymes as an oxime derivative. These metabolites can be rationalized as arising via the aldehyde which undergoes isomerization and further oxidation by the alphaalpha enzyme or reduction by the gamma2gamma2 isozyme. Non-extractable abacavir protein residues were generated in cytosol, and with alphaalpha and gamma2gamma2 incubations in the presence of human serum albumin (HSA). Metabolism and residue formation were blocked by the ADH inhibitor 4-methyl pyrazole (4-MP). The residues generated by the alphaalpha and gamma2gamma2 incubations were analyzed by SDS-PAGE with immunochemical detection. The binding of rabbit anti-abacavir antibody to abacavir-HSA was shown to be dependent on metabolism (i.e. NAD-dependent and 4-MP sensitive). The mechanism of covalent binding remains to be established, but significantly less abacavir-protein residue was detected with an analog of abacavir in which the double bond was removed, suggestive of a double bond migration and 1,4 addition process.

Acrolein activates mitogen-activated protein kinase signal transduction pathways in rat vascular smooth muscle cells

Acrolein, a major component of cigarette smoke, an environmental pollutant and an endogenous lipid peroxidation product, has been implicated in the development of atherosclerosis. Although a link between vascular injury and acrolein has been indicated, the exact molecular mechanism of acrolein-induced toxicity to vasculature is unknown. In an effort to elucidate the molecular basis of acrolein-induced vascular toxicity, the possibility of the intracellular signaling system as one of the targets of acrolein-induced toxicity is investigated in the present study. Exposure of cultured rat vascular smooth muscle cells (VSMCs) to different doses of acrolein not only causes cytotoxicity but also alters cellular morphology in a concentration and time-dependent manner. VSMCs exhibit cytotoxicity to a narrow concentration range of 5-10 microg/ml and display no toxicity to 2 microg/ml acrolein even after 24 h of exposure. Furthermore, exposure to acrolein results in activation of members of the mitogen-activated protein kinase (MAPK) family and protein tyrosine kinases. The extracellular signal-regulated kinases 1 and 2 (ERK1/2), stress-activated protein kinases/c-jun NH2-terminal kinases (SAPK/JNK) and p38MAPK are effectively and transiently activated by acrolein in a concentration and time-dependent fashion. While all three MAPKs exhibit significant activation within 5 min of exposure to acrolein, maximum activation (ERK1/2 and p38MAPK) or close to maximum activation (SAPK/JNK) occurs on exposure to 5 microg/ml acrolein for 2 h. Acrolein-induced activation of MAPKs is further substantiated by the activation of transcription factors, c-jun and activator transcription factor-2 (ATF-2), by acrolein-activated SAPK/JNK and p38MAPK, respectively. Additionally several cellular proteins exhibit spectacular protein tyrosine phosphorylation, particularly in response to 2 and 5 microg/ml of acrolein. Interestingly, the acrolein-induced activation of MAPKs precedes acrolein-stimulated protein tyrosine phosphorylation, which occurs after 2 h of exposure to acrolein. However, the time course of maximum protein tyrosine phosphorylation profile corresponds to the peak activation profile of MAPKs. The activation of MAPKs and protein tyrosine phosphorylation by acrolein appears to be independent of acrolein-induced toxicity. VSMCs exposed to 2 microg/ml acrolein exhibit no toxicity but stimulates significant activation of MAPKs and protein tyrosine phosphorylation. Although acrolein-induced VSMC toxicity is not blocked by MAPK inhibitors, PD98059, an inhibitor of MAPK kinase and SB203580, an inhibitor of p38MAPK, eitheralone or in combination, each MAPK responds differently to the inhibitors. Most prominently, although SB203580, an inhibitor of both SAPK/JNK and p38MAPK, significantly inhibited acrolein-induced activation of p38MAPK, it also stimulated SAPK/JNK activation by acrolein alone and in combination with PD98059. These results provide the first evidence that the activation of both growth-regulated (ERK1/2) and stress-regulated (SAPK/JNK and p38MAPK) MAPKs as well as tyrosine kinases are involved in the mediation of acrolein-induced effects on VSMC, which may play a crucial role in vascular pathogenesis due to environmentally and endogenously produced acrolein.

Effects of manganese (Mn) on the developing rat brain: oxidative-stress related endpoints

We evaluated biochemical endpoints related to oxidative stress in brains of neonatal rats exposed to manganese (Mn). Oral Mn chloride (MnCl2) (0, 25, or 50 mg Mn chloride kg(-1) body weight per day) was given daily to neonatal rats throughout lactation (i.e. from postnatal day (PND) 1 to 21). As previously reported by [J. Appl. Toxicol. 20 (2000) 179], this treatment paradigm results in increased cerebral cortex (CTX) Mn concentrations in PND 21 rats from both Mn treatment groups. High dose Mn exposure also results in increased cerebellar Mn concentrations. This preliminary study determined whether this exposure paradigm also affects cerebrocortical or cerebellar metallothionein (MT) mRNA levels, glutamine synthetase (GS) activity, GS protein levels, as well as total glutathione (GSH) levels. High dose Mn exposure significantly increased (P < 0.05) total cerebrocortical GSH without accompanying changes in any of the other measured parameters. Therefore, it is unlikely that high dose Mn exposure is associated with oxidative stress in this experimental paradigm.

Functional and toxicological characteristics of isolated renal mitochondria: impact of compensatory renal growth

Mitochondria were isolated from renal cortical homogenates from control rats and rats that had undergone uninephrectomy and compensatory renal growth (NPX rats). Activities of selected mitochondrial processes, including key enzymes of intermediary metabolism, glutathione-dependent enzymes, and glutathione transport, were measured, and the effects of three mitochondrial toxicants were assessed to test the hypothesis that compensatory renal growth is accompanied by increases in mitochondrial metabolism and that this is associated with increased susceptibility to injury from oxidants or other mitochondrial toxicants. Activities of malic and succinic dehydrogenases were significantly higher in mitochondria from NPX rats than in mitochondria from control rats. Although the rates of state 3 respiration were significantly higher in mitochondria from NPX rats, the rates of state 4 respiration and respiratory control ratios were not different between mitochondria from control and NPX rats. Activities of glutathione redox cycle enzymes did not differ significantly between mitochondria from control and NPX rats. However, the rates of uptake of glutathione into mitochondria were approximately 2.5-fold higher in tissue from NPX rats than in tissue from control rats. Incubation of mitochondria from NPX rats with three mitochondrial toxicants [tert-butyl hydroperoxide, methyl vinyl ketone, and S-(1,2-dichlorovinyl)-L-cysteine] caused greater inhibition of state 3 respiration and larger increases in malondialdehyde formation than similar incubations of mitochondria from control rats. These results indicate that mitochondria from hypertrophied renal cells are more sensitive to oxidants or mitochondrial toxicants. Baseline levels of malondialdehyde were also significantly higher in mitochondria from NPX rats, suggesting that a basal oxidant stress exists in mitochondria from hypertrophied cells.

Renal cellular transport of exogenous glutathione: heterogeneity at physiological and pharmacological concentrations

Properties and kinetics of GSH transport into proximal tubular (PT) and distal tubular (DT) cells from rat kidney were determined to validate further the hypothesis that cellular differences in handling of GSH contribute to the greater susceptibility of DT cells to oxidant injury. PT and DT cells were incubated with a broad range of GSH concentrations, encompassing physiologically relevant (0.001 to 0.1 mM) to pharmacological (0.25 to 5 mM) levels of GSH. GSH uptake in PT cells was rapid, exhibiting an overshoot with a maximum at 1-min incubation. GSH uptake in DT cells reached maximal intracellular levels at 2- to 5-min incubations. GSH uptake in PT cells was resolved into two kinetically distinct processes, with Km values of 41.7 and 540 microM and Vmax values of 183 and 4885 pmol/min per 10(6) cells. In contrast, GSH uptake in DT cells was best described by one process, with Km and Vmax values of 1480 microM and 2094 pmol/min per 10(6) cells, respectively. Rates of GSH synthesis from 1 mM precursor amino acids were approximately 3-fold faster in PT cells, but rates of cysteine accumulation were 3.5-fold faster in DT cells. Accumulation of intracellular GSH in PT cells was 8-fold faster after incubation with 1 mM GSH than after incubation with 1 mM precursor amino acids. At both a physiological (10 microM) and a pharmacological (5 mM) GSH concentration, uptake exhibited marked Na+ and energy dependence, sensitivity to substrates for the organic anion and dicarboxylate carriers, and sensitivity to various gamma-glutamyl amino acids in PT cells only. Na+-dependent GSH uptake in PT cells was accounted for completely by activity of the organic anion and dicarboxylate carriers. These results indicate that DT cells possess limited capacity to transport GSH and suggest that exogenous GSH may not be effective in protecting other segments of the nephron besides the PT region from oxidants or other agents that alter GSH status.

Cellular and subcellular heterogeneity of glutathione metabolism and transport in rat kidney cells

Selective permeabilization of plasma membranes with digitonin produced separation of cytosolic and mitochondrial compartments of proximal tubular (PT) and distal tubular (DT) cells from a rat kidney. Subcellular distributions of several intracellular glutathione (GSH)-dependent enzymes were similar in the two cell types but specific activities were significantly higher in PT cells, indicating that DT cells, particularly in their mitochondrial fraction, have a diminished capacity to detoxify reactive oxygen species. To enable isolation of suspensions of mitochondria, renal cells were treated with digitonin followed by the bacterial protease nagarse and were filtered through polycarbonate membranes. Activity distributions of enzymatic markers for subcellular fractions were quantitated and uptake of GSH was studied in suspensions of PT and DT cell mitochondria. While PT cell mitochondria catalyzed rapid uptake of GSH that was inhibited by malate, indicating involvement of dicarboxylate carriers, DT cell mitochondria exhibited limited capacity for GSH uptake that was not inhibited by substrates for the two dicarboxylate carriers. This report provides the first description of methodology for the preparation of mitochondria from renal cells derived from specific nephron cell types and shows that mitochondria from DT cells have a significantly lower capacity to use GSH for detoxification and regulation of redox status.

In vitro methods of assessing renal damage

Freshly isolated and primary cultures of rat kidney cells derived from specific nephron segments can be useful in vitro models for studying processes such as drug metabolism, membrane transport, and biochemical mechanisms of chemically induced toxicity. Proximal tubular (PT) and distal tubular (DT) cells were isolated from rat renal cortex by collagenase perfusion and Percoll density-gradient centrifugation. Oxidants produced glutathione (GSH) oxidation and lipid peroxidation and were markedly more cytotoxic to DT cells than to PT cells. Similarly, alkylating agents that target soft nucleophiles such as GSH and protein sulfhydryls were more toxic to DT cells than to PT cells, whereas an alkylating agent that targets hard nucleophiles was equally cytotoxic in the 2 cell types. DT cells were also more sensitive to brief periods of oxygen deprivation and were markedly more susceptible to ATP depletion by treatment with iodoacetate and cyanide than were PT cells. Serum-free, hormonally defined conditions have been optimized for primary culture of rat renal PT and DT cells to maintain differentiated function for up to 9 days. Primary cultures exhibited similar susceptibilities as freshly isolated cells to acute injury from chemical toxicants and the cultures express several isoforms of cytochrome P-450. These studies show that freshly isolated and primary cultures of rat renal PT and DT cells can be used to study both short-term and long-term responses to toxic chemicals.

Pathways of glutathione metabolism and transport in isolated proximal tubular cells from rat kidney

Cellular uptake and metabolism of exogenous glutathione (GSH) in freshly isolated proximal tubular (PT) cells from rat kidney were examined in the absence and presence of inhibitors of GSH turnover [acivicin, L-buthionine-S,R-sulfoximine (BSO)] to quantify and assess the role of different pathways in the handling of GSH in this renal cell population. Incubation of PT cells with 2 or 5 mM GSH in the presence of acivicin/BSO produced 3- to 4-fold increases in intracellular GSH within 10-15 min. These significantly higher intracellular concentrations were maintained for up to 60 min. At lower concentrations of extracellular GSH, an initial increase in intracellular GSH concentrations was observed, but this was not maintained for the 60-min time course. In the absence of inhibitors, intracellular concentrations of GSH increased to levels that were 2- to 3-fold higher than initial values in the first 10-15 min, but these dropped below initial levels thereafter. In both the absence and presence of acivicin/BSO, PT cells catalyzed oxidation of GSH to glutathione disulfide (GSSG) and degradation of GSH to glutamate and cyst(e)ine. Exogenous tert-butyl hydroperoxide oxidized intracellular GSH to GSSG in a concentration-dependent manner and extracellular GSSG was transported into PT cells, but limited intracellular reduction of GSSG to GSH occurred. Furthermore, incubation of cells with precursor amino acids produced little intracellular synthesis of GSH, suggesting that PT cells have limited biosynthetic capacity for GSH under these conditions. Hence, direct uptake of GSH, rather than reduction of GSSG or resynthesis from precursors, may be the primary mechanism to maintain intracellular thiol redox status under toxicological conditions. Since PT cells are a primary target for toxicants, the ability of these cells to rapidly take up and metabolize GSH may serve as a defensive mechanism to protect against chemical injury.

Susceptibility of primary cultures of proximal tubular and distal tubular cells from rat kidney to chemically induced toxicity

Isolated proximal tubular (PT) and distal tubular (DT) cells from rat kidney were cultured for up to 9 days under serum-free, hormonally-defined conditions on 35-mm polystyrene culture dishes. Several hormonal and growth factor supplements were assessed for their ability to promote growth (increased protein and DNA content) and stability of differentiated phenotype (high activities of gamma-glutamyltransferase and alkaline phosphatase as brush-border membrane markers in PT cells; maintenance of high activities of glutamate dehydrogenase as a mitochondrial marker in both PT and DT cells; maintenance of low and high activities of lactate dehydrogenase in PT and DT cells, respectively; expression of cytokeratins). Basal supplemented media (DMEM/F12, 1:1 v/v) contained insulin, hydrocortisone, epidermal growth factor, sodium selenite and transferrin as supplements. Additionally, triiodothyronine selectively promoted growth and stability of differentiated phenotype in PT cells and thyrocalcitonin selectively promoted growth and stability of differentiated phenotype in DT cells. On Day 3 of primary culture, PT and DT cells were incubated for up to 8 h with either tert-butyl hydroperoxide (tBH; 0.5-10 mM), methyl vinyl ketone (MVK; 1-10 mM), or p-aminophenol (PAP; 1-10 mM) and cellular injury, as assessed by cellular release of lactate dehydrogenase, was determined. DT cells were significantly more susceptible to injury from both tBH and MVK, but the two cell populations were equally susceptible to injury from PAP, which is the same susceptibility pattern seen in freshly isolated cells. These results suggest that primary cultures of rat renal PT and DT cells reflect similar biochemical properties as freshly isolated cells and are, therefore, useful models for study of chemically induced injury.

Adenosine modulates methylmercuric chloride (MeHgCl)-induced D-aspartate release from neonatal rat primary astrocyte cultures

The effects of adenosine, and selective adenosine receptor agonists and antagonists on methylmercury (MeHg)-induced aspartate release were studied in neonatal rat primary astrocyte cultures. Whereas basal levels of D-[3H]aspartate release were unchanged upon treatment with adenosine or the selective A1 receptor agonists, N6-cyclopentyladenosine (CPA), cyclohexyladenosine (CHA), and R-phenylisopropyladenosine (R-PIA), all partially reversed the MeHg-induced release of D-aspartate. Treatment of astrocytes with the xanthine derivative, theophylline, an adenosine antagonist, reversed the inhibitory effect of adenosine on MeHg-induced D-[3H]aspartate release. Since the effect of MeHg on D-[3H]aspartate release is known to be associated with sulfhydryl (-SH) groups which are controlled by intracellular glutathione concentrations [GSH]i, we also evaluated the effects of adenosine, the A1 agonists CPA and CHP, and the adenosine antagonist, theophylline, on astrocytic [GSH]i. Attenuation of the stimulatory effect of MeHg on D-[3H]aspartate release by adenosine and its agonists occurred in the presence of reduced astrocytic [GSH]i, suggesting that other mechanisms must be invoked for this protective effect. Whilst the mechanism of MeHg-induced D-[3H]aspartate release is not known, the data suggest a role for adenosine in its regulation.

Fate and effects of acrolein

Acrolein is a highly toxic, reactive, and irritating aldehyde that occurs as a product of organic pyrolysis, as a metabolite of a number of compounds, and as a residue in water when used for the control of aquatic organisms. It is an intermediate in the production of acrylic acid, DL-methionine, and numerous other agents. Its major direct use is as a biocide for the control of aquatic flora and fauna. It is introduced to the environment from a variety of sources, including organic combustion such as automobile exhaust, cigarette smoke, and manufacturing and cooking emissions, as well as direct biocidal applications. Organic combustion from both fixed and mobile sources is the significant source of acrolein in the atmosphere; it represents up to 8% of the total aldehydes generated from vehicles and residential fireplaces and 13% of total atmospheric aldehydes. This reactive aldehyde also occurs in organisms as a metabolite of allyl alcohol, allylamine, spermine, spermidine, and the anticancer drug cyclophosphamide, and as a product of UV radiation of the skin lipid triolein. Furthermore, small amounts are found in foods; when animal or vegetable fats are overheated, however, large amounts are produced. Most human contact occurs during exposure to smoke from cigarettes, automobiles, industrial processes, and structural and vegetation fires. Besides cigarette smoke, occupational exposures are a common mode of human contact, particularly in industries that involve combustion of organic compounds. Firefighters, in particular, are exposed to extremely high levels during the extinguishment and overhaul phases of their work. Water may contain significant levels of the herbicide. It has been found in paper mill and municipal effluents at 20-200 micrograms/L, and at 30 micrograms/L as far as 64 km downstream from the point of application. The USEPA-recommended water quality criteria for freshwater are only 1.2 micrograms/L (24-hr avg) and 2.7 micrograms/L (maximum ceiling). Acrolein is highly reactive, and intercompartmental transport is limited. However, it is eliminated from aqueous environments by volatilization and hydration to beta-hydroxypropanal, after which biotransformation occurs, with a half-life of 7-10 d. The Koc for acrolein is 24, and it is not likely to be retained in soil; activated carbon adsorbs only 30% from solution. Thus, the aldehyde is either leached extensively in moist soil or volatilizes quickly from dry soil. It is eliminated from air by reaction with .OH (half-life, 0.5-1.2 d), NOx (half-life, 16 d), and O3 (half-life, 59 d), as well as by photolysis and wet deposition. As expected from its high water solubility, bioaccumulation is low. Acrolein is highly toxic by all routes of exposure. The respiratory system is the most common target: exposure causes localized irritation, respiratory distress, pulmonary edema, cellular necrosis, and increased susceptibility to microbial diseases. Additionally, acute inhalation studies verify that it is a severe respiratory irritant that affects respiratory rates. Respiratory rate depression may have a protective effect by minimizing vapor inhalation, thereby explaining the subadditive effect of acrolein when combined with the other toxic combustion by-products CO and HCHO. Liquid contact with the skin and eyes causes severe irritation, opaque or cloudy corneas, and localized epidermal necrosis, but no allergic contact dermatitis. The cardiovascular system is affected, resulting in increased blood pressure, platelet aggregation, and quick cessation of beating in perfused rat hearts. It may also inhibit mitochondrial oxidative phosphorylation in the myocardium. Acute LD50s and LC50s are low. Levels are 7-46 mg/kg and 18-750 mg/m3, respectively, in rats; aquatic organisms are affected above 11.4 micrograms/L.(ABSTRACT TRUNCATED)

Intracellular glutathione (GSH) levels modulate mercuric chloride (MC)- and methylmercuric chloride (MeHgCl)-induced amino acid release from neonatal rat primary astrocytes cultures

Mercuric chloride (MC) and methylmercury (MeHg) were found to increase amino acid release from astrocytes. This suggests interaction with sulfhydryl (-SH) groups which are controlled by glutathione [GSH] levels. In the present study, we evaluated the effects of alterations in intracellular glutathione concentrations [GSH]i on the outcome of MC and MeHg treatment. [GSH]i were increased in a time-dependent fashion by incubating the astrocytes with 1 mM L-2-oxothiazolidine-4-carboxylic acid (OTC), a cysteine precursor. OTC attenuated the release of [2,3-3H]D-aspartic acid from astrocytes exposed to MC- (5 microM) and MeHg-(10 microM). MeHg-induced [3H]D-taurine release was also reduced by pretreatment of astrocytes with OTC. Treatment with BSO (50 microM) decreased [GSH]i in astrocytes, and increased [2,3-3H]D-aspartate release from MC- and MeHg-treated astrocytes, and [3H]D-taurine release from MeHg-treated cells. Neither OTC nor BSO when added to cultures in the absence of MC or MeHg had an effect on amino acid release by astrocytes. The current study underscores both the sensitivity of astrocytes to mercurials in terms of amino acid release and the relationship of these effects of astrocytic [GSH]i.

Renal cell type specificity of cephalosporin-induced cytotoxicity in suspensions of isolated proximal tubular and distal tubular cells

We have developed an in vitro model for investigation of nephron heterogeneity and cell type-specific patterns of renal injury. To further validate our model and to study biochemical mechanisms of cephalosporin-induced injury, cytotoxicity of three cephalosporins was studied in freshly isolated proximal tubular (PT) and distal tubular (DT) cells from rat kidney. The three cephalosporins [cephaloridine (CPH), cephalexin (CXN), cephalothin (CTN)] were chosen because they exhibit varying degrees of nephrotoxicity in vivo and contain different functional groups. CPH produced greater amounts of lactate dehydrogenase release from PT cells than either CXN or CTN, indicating greater toxicity of CPH, which agrees with in vivo observations. DT cells were not affected by any of the cephalosporins. Thus, the cephem ring is sufficient to produce PT cell injury but the presence of other functional groups modifies toxicity. SKF-525A and alpha-tocopherol protected PT cells from both CPH and CTN, suggesting involvement of cytochrome P-450 metabolism and oxidative stress. Both PT and DT cells exhibited transport of CPH or CXN and transport of CPH into PT cells was inhibitable by probenecid, consistent with action of a specific carrier. Transport alone, therefore, cannot account for the cell type specificity pattern in vitro. Effects on intracellular glutathione status, malondaldehyde formation, and uncoupler-stimulated respiration were also investigated, and these generally correlated with cell type specificity patterns but not always with degree of cytotoxicity. These results validate further the isolated PT and DT cells as in vitro models to study cell type-specific renal injury and show a role for oxidative stress, cytochrome P-450 bioactivation, and mitochondrial dysfunction in cephalosporin-induced PT cell injury.

Hypoxia and oxygen dependence of cytotoxicity in renal proximal tubular and distal tubular cells

Ischemia and hypoxia are major causes of renal failure and altered oxygen supply may affect renal responses to toxic chemicals. In vitro experiments were designed to evaluate the susceptibility of isolated proximal tubular (PT) and distal tubular (DT) cells from rat kidney to brief periods of oxygen deprivation and to assess how variations in oxygen supply affect chemical-induced cytotoxicity. Isolated cells were incubated for 1 hr in either oxygen (95% O2/5% CO2), air (21% O2), or nitrogen (95% N2/5% CO2) atmosphere. PT cells exhibited no injury due to brief oxygen deprivation whereas DT cells exhibited moderate, but significant injury, indicating that DT cells are more susceptible than PT cells to hypoxic injury. The cytotoxicity of chemicals that alter cellular redox status [i.e. tert-butyl hydroperoxide (tBH), menadione, methyl vinyl ketone] and the cytotoxicity of "chemical hypoxia" [i.e. KCN + iodoacetic acid] were greatest in air, intermediate in oxygen, and lowest in nitrogen. In contrast, the cytotoxicity of the alkylating agent N-dimethylnitrosamine was independent of oxygen concentration and the cytotoxicity of p-aminophenol was related directly to oxygen concentration. The mechanism of the oxygen dependence of chemical injury was investigated further, employing tBH as a model toxicant. tBH metabolism was oxygen independent in both PT and DT cells. Depletion of cellular protein sulfhydryl groups by tBH increased with increasing oxygen concentration and lipid peroxidation due to tBH was inhibited in nitrogen but was not different in air as compared with oxygen. Although these processes may contribute to the much lower toxicity in nitrogen as compared with oxygen, it does not explain the higher toxicity in air as compared with that in oxygen. Other processes that predominate at lower oxygen concentrations but that only produce injury if enough oxygen is present are likely to be responsible for the enhanced susceptibility of both PT and DT cells to oxidants in air as compared with oxygen.

Uptake of glutathione by renal cortical mitochondria

Transport of GSH into renal cortical mitochondria was studied. Mitochondria were highly enriched with little contamination from other subcellular organelles (as assessed by marker enzymes), they exhibited coupled respiration (respiratory control ratio greater than 3.0), and they had initial GSH concentrations of 5.71 +/- 0.65 nmol/mg protein (n = 47). Incubation of mitochondria with GSH in a triethanolamine, pH 7.4, buffer containing sucrose, potassium phosphate, MgCl2, and KCl, produced time- and concentration-dependent increases in intramitochondrial GSH content. Uptake was linear versus time for at least 2 min and exhibited kinetics consistent with one low-affinity, high-capacity process (Km = 1.3 mM, Vmax = 5.59 nmol/min per mg protein), although the results cannot exclude the presence of other, less quantitatively significant pathways. The initial rate of uptake of 5 mM GSH was not significantly altered by uncouplers (0.1 mM 2,4-dinitrophenol and 25 microM carbonyl cyanide m-chlorophenylhydrazone) or by 1 mM ADP. In contrast, incubation with 1 mM ATP, 1 mM KCN, 0.1 mM or 1 mM CaCl2 inhibited uptake by 41, 39, 43, or 55%, respectively. GSH uptake was markedly inhibited by gamma-glutamylglutamate and by a series of S-alkyl GSH derivatives. Strong interactions (i.e., both cis and trans effects) were observed with other dicarboxylates (i.e., succinate, malate, glutamate) but not with monocarboxylates (i.e., lactate, pyruvate). Preincubation of mitochondria with GSH protected against tert-butyl hydroperoxide- or methyl vinyl ketone-induced inhibition of state 3 respiration. These results demonstrate uptake of GSH into renal cortical mitochondria that appears to involve electroneutral countertransport (exchange) with other dicarboxylates. Functionally, GSH uptake into mitochondria can protect these organelles from various forms of injury, such as oxidative stress.