Intracellular calcium homeostasis during hydrogen peroxide injury to cultured P388D1 cells

A cellular model of oxidant-mediated neuronal injury

Oxidants derived from the partial reduction of oxygen are thought to play a significant role in neuronal injury. We present here a cellular model of neuronal injury mediated by hydrogen peroxide (H2O2) using the PC 12 rat pheochromocytoma cell line. The organization of microtubules and microfilaments within neurites of PC 12 cells differentiated by exposure to nerve growth factor was examined after H2O2 injury using fluorescence microscopy. Concentrations of H2O2 as low as 100 microM produced an initial periodic pattern of microtubule depolymerization over 3-4 h which later progressed to complete depolymerization. Neuritic microspikes containing actin filaments were relatively more resistant to injury by H2O2 than microtubules. Blebbing of PC 12 cell bodies and neurites also was seen after H2O2 injury and the blebs appeared to contain microtubules. The destructive changes affecting neuritic structure preceded but were not essential for PC 12 cell lysis. Exposure of the cells to the Ca2+ ionophore, ionomycin (25 microM) also produced the same pattern of microtubule depolymerization in PC 12 neurites as was seen after H2O2 injury suggesting that H2O2 may mediate its destructive effect on the neurites via elevation of intracellular Ca2+.

Cytotoxic effects of biphenyl and hydroxybiphenyls on isolated rat hepatocytes

The cytotoxic effects of biphenyl (BP) and its hydroxylated derivatives, o-phenylphenol (OPP), m-phenylphenol (MPP), p-phenylphenol (PPP), 2-biphenylyl glycidyl ether (OPP-epoxide), phenyl-hydroquinone (PHQ), o,o'-biphenol (o,o'-BPol) and p,p'-biphenol (p,p'-BPol), were investigated in freshly isolated rat hepatocytes. OPP, MPP and PPP, at concentration of 0.75 mM, resulted in the loss of intracellular ATP, glutathione (GSH) and protein thiols, causing cell death. OPP-epoxide and BP were less toxic than the OPP isomers. MPP or PPP compared with OPP caused serious impairments in oxidative phosphorylation in mitochondria isolated from rat liver. PHQ (0.75 mM) caused a rapid loss of intracellular ATP which preceded the onset of cell death. PHQ was more toxic than o,o'-BPol or p,p'-BPol. PHQ dissolved in Krebs-Henseleit buffer without hepatocytes was rapidly converted to its corresponding quinone, phenyl-benzoquinone. The cytotoxicity produced by PHQ depends on the rate of formation of reactive intermediates. These results indicate that the addition of a hydroxyl group to the aromatic ring of BP enhances BP-induced cytotoxicity and that the mitochondria are a common target of the OPP isomers and other BP derivatives. In addition, the para- or meta-hydroxyl groups rather than the ortho-hydroxyl group increase the toxicity. The cytotoxicity produced by PHQ depends on the rate of formation of reactive intermediate(s) such as phenyl-benzoquinone.

Effects of hydrogen peroxide on mitochondrial enzyme function studied in situ in rat heart myocytes

Our previous work indicated that energy transduction, as measured by myocyte respiration, was inhibited by hydrogen peroxide, but the mitochondrial membrane potential was relatively unaffected. Therefore, we determined in the present study the critical steps in mitochondrial energy transduction by measuring the sensitivity to hydrogen peroxide of NADH-CoQ reductase, ATP synthase, and adenine nucleotide translocase in situ in myocytes. Adult rat heart cells were isolated using collagenase and incubated in the presence of 0.1-10 mM hydrogen peroxide for 30 min. Activities of NADH-CoQ reductase and oligomycin-sensitive ATP synthase were assayed enzymatically with sonicated myocytes, and adenine nucleotide translocase activities were determined by atractyloside-inhibitable [14C]ADP uptake of myocytes, permeabilized by saponin. The NADH-CoQ reductase and ATP synthase activities were inhibited to 77% and 67% of control, respectively, following an exposure to 10 mM hydrogen peroxide for 30 min. The adenine nucleotide translocase activities were inhibited in a concentration- and time-dependent manner and by 10 mM hydrogen peroxide to 44% of control. The dose-response relationship indicated that the translocase was the most susceptible to hydrogen peroxide among the three enzymes studied. Combined treatment of myocytes with 3-amino-1,2,4-triazole, 1,3-bis(2-chloroethyl)-1-nitrosourea and diethyl maleate (to inactivate catalase, to inhibit glutathione reductase activity, and to deplete glutathione, respectively) enhanced the sensitivity of translocase to hydrogen peroxide, supporting the view that the cellular defense mechanism is a significant factor in determining the toxicity of hydrogen peroxide. The results indicate that hydrogen peroxide can cause dysfunction in mitochondrial energy transduction, principally as the result of inhibition of adenine nucleotide translocase.

Mechanisms of hydroxyl free radical-induced cellular injury and calcium overloading in alveolar macrophages

Excessive production of reactive oxygen radicals by alveolar macrophages is proposed to play an important role in oxidative lung injury. A major product oxygen radical formation is the highly reactive hydroxyl radical (.OH) generated via a biologic Fenton reaction. In addition to its known ability to induce lipid peroxidation, recent studies have suggested that the .OH may exert its cytotoxic effect through the alteration of [Ca2+]i homeostasis. To test this potential mechanism as well as to investigate the relationship between .OH and Ca2+ overloading in cytotoxic injury, isolated rat alveolar macrophages were exposed to externally generated radical system, H2O2 (0.01 to 1 mM) and Fe2+ (1 mM) and their [Ca2+]i levels and cell injury were monitored using quantitative fluorescence microscopy with the aid of the specific Ca2+ indicator, Fura-2, and membrane integrity indicator, propidium iodide. Electron spin resonance measurements using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) confirmed the production of the .OH radical by this system. Upon the addition of the radicals, the macrophages displayed a rapid initial rise in [Ca2+]i which was followed by a slower but more pronounced [Ca2+]i elevation that reached a level 3 to 5 times higher than the basal level. This process preceded cell death as evident by nuclear propidium iodide fluorescence. Depletion of extracellular Ca2+ inhibited both the [Ca2+]i response and cell injury. Preincubation of the cells with the Ca2+ channel blocker verapamil or .OH radical scavenger mannitol similarly inhibited the [Ca2+]i rise and loss of viability. Firefly luciferase assay of cellular ATP content demonstrated that the alterations in [Ca2+]i following .OH treatment preceded the depletion of ATP.(ABSTRACT TRUNCATED AT 250 WORDS)

H2O2-mediated damage to lysosomal membranes of J-774 cells

The effects of hydrogen peroxide on cell viability and, in particular, on lysosomal integrity were investigated in a model system of cultured, established, macrophage-like J-774 cells. The cells were found to rapidly degrade added hydrogen peroxide, withstanding concentrations < or = 250 microM without cell death; however, all tested concentrations (100-500 microM) substantially decreased cellular ATP to approximately the same degree. Concentrations of hydrogen peroxide > or = 500 microM resulted in a pronounced and rapid decrease in cell viability preceded by the loss of lysosomal integrity, as judged by the relocalization of acridine orange, a lysosomotropic weak base, in pre-labelled cells. Hydrogen peroxide-induced relocalization of acridine orange and cell death were either enhanced or much prevented, according to if the cells were initially allowed to endocytose ferric iron or the specific iron-chelator deferoxamine, respectively. Depletion of ATP, however, was not associated with the loss of lysosomal integrity and viability regardless of iron or deferoxamine pretreatment. Pre-exposure to E-64, an inhibitor of lysosomal thiol proteases, resulted in the reduction of both lysosomal membrane damage and cell death. The results are interpreted as indicating (i) generation of hydroxyl radicals within the secondary lysosomal compartment due to the occurrence of reactive ferrous iron, leading to (ii) peroxidative alterations of the lysosomal membrane resulting in (iii) loss of lysosomal membrane integrity with dissipation of the proton gradient and leakage of lysosomal contents, including hydrolytic enzymes, into the cell sap.

Cyclosporin A protects hepatocytes against prooxidant-induced cell killing. A study on the role of mitochondrial Ca2+ cycling in cytotoxicity

Cyclosporin A (CsA) is a potent inhibitor of the prooxidant-induced release of Ca2+ from isolated mitochondria. In this investigation, pretreatment of hepatocytes with CsA before exposure to the prooxidants tert-butyl hydroperoxide (tBH), cumene hydroperoxide or 3,5-dimethyl-N-acetyl-p-benzoquinone imine (3,5-Me2-NAPQI) prevented the loss of cell viability. HPLC analysis of adenine and pyridine nucleotide concentrations in hepatocytes treated with 3,5-Me2-NAPQI showed a rapid depletion of ATP prior to the loss of cell viability versus the maintenance of near control levels of ATP in hepatocytes treated with CsA before 3,5-Me2-NAPQI. In 3,5-Me2-NAPQI-exposed hepatocytes there was also a rapid loss of cellular NAD+ which could be accounted for initially by a transient increase in NADP+. Measurement of the intracellular Ca2+ pools showed an early depletion of the mitochondrial Ca2+ pool in hepatocytes exposed to 3,5-Me2-NAPQI, tBH or cumene hydroperoxide; this loss was prevented by CsA. In conclusion, these results show that CsA protected hepatocytes from prooxidant injury by preventing mitochondrial Ca2+ cycling and subsequent mitochondrial dysfunction. This suggests that in prooxidant injury, excessive Ca2+ cycling is an early and important event leading to mitochondrial damage and subsequently to cell death.

Inhibition of organic anion transport in endothelial cells by hydrogen peroxide

ATP loss is a prominent feature of cellular injury induced by oxidants or ischemia. How reduction of cellular ATP levels contributes to lethal injury is still poorly understood. In this study we examined the ability of H2O2 to inhibit in a dose-dependent manner the extrusion of fluorescent organic anions from bovine pulmonary artery endothelial cells. Extrusion of fluorescent organic anions was inhibited by probenecid, suggesting an organic anion transporter was involved. In experiments in which ATP levels in endothelial cells were varied by treatment with different degrees of metabolic inhibition, it was determined that organic anion transport was ATP-dependent. H2O2-induced inhibition of organic anion transport correlated well with the oxidant's effect on cellular ATP levels. Thus H2O2-mediated inhibition of organic anion transport appears to be via depletion of ATP, a required substrate for the transport reaction. Inhibition of organic anion transport directly by probenecid or indirectly by metabolic inhibition with reduction of cellular ATP levels was correlated with similar reductions of short term viability. This supports the hypothesis that inhibition of organic anion transport after oxidant exposure or during ischemia results from depletion of ATP and may significantly contribute to cytotoxicity.

Hydrogen peroxide increases the availability of arachidonic acid for oxidative metabolism by inhibiting acylation into phospholipids in the alveolar macrophage

Reactive oxygen species stimulate metabolism of arachidonic acid (AA) to eicosanoids in a variety of cells and tissues, yet the pathway(s) by which oxidants increase the availability of AA for oxidative metabolism are not known. Thus, we explored the effects of hydrogen peroxide (H2O2) on deacylation and reacylation of AA to determine the enzymatic mechanism(s) by which this oxidant increases levels of free, unesterified AA, and thereby its oxidative metabolism to eicosanoids, in the rat alveolar macrophage (AM). Over the range from 0.1 to 0.5 mM, H2O2 caused marked time- and dose-dependent inhibition of incorporation of [3H]AA into macrophage phospholipids, whereas calcium ionophore A23187 and zymosan particles did not cause such inhibition. Within this concentration range, there was an almost exact reciprocal correlation between inhibition of [3H]AA acylation and H2O2-stimulated accumulation of free [3H]AA in prelabeled AM cultures. Thimerosal, which blocks AA reacylation but spares deacylation via phospholipase A2 (PLA2), did not affect accumulation of free [3H]AA in prelabeled cells stimulated with H2O2, while markedly augmenting [3H]AA release in response to A23187 and to zymosan. Despite its ability to block AA acylation almost completely, H2O2 did not directly inhibit arachidonoyl CoA synthetase or arachidonoyl CoA:lysophosphatide acyltransferase, which catalyze AA incorporation into phospholipids. However, H2O2 (0.1 to 0.5 mM) markedly depleted AMs of ATP, required for synthesis of the acylation intermediate arachidonoyl CoA, suggesting that this was the means by which H2O2 inhibited acylation. Notably, H2O2 (0.03 to 3 mM) failed to stimulate macrophage PLA2 activity. We conclude that H2O2, in contrast to A23187 and zymosan, inhibits incorporation of AA into phospholipids, and that this represents the major mechanism by which the oxidant increases the availability of free AA for oxidative metabolism in the AM. This may be an important basis for release of eicosanoids in oxidant-induced inflammation and injury of the lung.

Reactive oxygen molecules, oxidant injury and renal disease

Oxidant injury has been implicated in the pathogenesis of inflammatory, metabolic and toxic insults, in ischemic-reperfusion injury, and in carcinogenesis, aging and atherosclerosis. Oxidant injury is initiated by free radicals and reactive oxygen molecules which are generated by activated neutrophils, monocytes, and mesangial cells, during normal and abnormal metabolic processes, and from the metabolism of exogenous drugs and toxins. When cells and organs are exposed to oxidant stress, several different antioxidant defense mechanisms operate to prevent or limit oxidant injury. When antioxidant defense mechanisms are decreased, or when the generation of reactive oxygen molecules is increased, oxidant injury results from the shift in the oxidant/antioxidant balance. Oxidant-induced alterations of proteins, membranes, DNA, and basement membranes leads to cell and organ dysfunction. Several renal diseases including glomerulonephritis, vasculitis, toxic nephropathies, pyelonephritis, acute renal failure, and others are likely to be mediated at least in part by oxidant injury. In the future, mechanisms to decrease the generation of reactive oxygen molecules and/or antioxidant therapy may develop into new avenues of therapeutic intervention.

Cytosolic free Ca2+ and proteolysis in lethal oxidative injury in endothelial cells

Oxygen free radicals (OFR) are thought to mediate ischemia-reperfusion injury to endothelium of heart, lung, brain, liver, and kidney and contribute to development of atherosclerosis, pulmonary O2 toxicity, and adult respiratory distress syndrome. Increased cytosolic free Ca2+ (Cai2+) has been proposed as a mechanism of injury from oxidative stress, yet the pathways by which an increase in Cai2+ may cause OFR-mediated endothelial cell injury remain unknown. Using multiparameter digitized video microscopy and the fluorescent probes, fura-2 acetoxymethyl ester and propidium iodide, we measured Cai2+ and cell viability in human umbilical endothelial cells during oxidative stress with xanthine (50 microM) plus xanthine oxidase (40 mU/ml). Oxidative stress caused a sustained increase in Cai2+ from a resting level of 90-100 nM to near 500 nM, which was preceded by formation of plasma membrane blebs. The increase in Cai2+ was prevented by removal of extracellular Ca2+ (Cao2+). Prevention of the increase in Cai2+ was associated with prolonged cell viability. Readdition of Cao2+ resulted in an immediate large increase in Cai2+ and rapid onset of cell death. The protease inhibitors, leupeptin and pepstatin, delayed the increase in Cai2+ and prolonged cell viability. The results are consistent with the hypothesis that endothelial cell injury due to oxidative stress may be the result of Cai2+ influx and resultant activation of Ca(2+)-dependent proteases.

Enhanced proteolysis and changes in membrane-associated calpain following phenylhydrazine insult to human red cells

Phenylhydrazine-mediated protein damage in human red cells has been assessed using HPLC, one- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and immunoblot analysis of major membrane proteins. The association of the Ca(2+)-activated neutral protease, calpain, with membrane proteins following hydrazine insult was also examined using immunoblot analysis. HPLC amino acid analysis of red cell suspensions was employed to quantify proteolysis. Phenylhydrazine (4 mM) increased the rate of leucine, lysine, and histidine release by approximately 12-, 7-, and 5-fold, respectively. N-acetylcysteine (20 mM), dithiothreitol (50 mM), and dimethylthiourea (50 mM) decreased the rate of phenylhydrazine-stimulated amino acid release by approximately 30-50%; in contrast, the free radical scavengers and antioxidants dimethylfuran (50 mM) and dimethyl sulfoxide (50 mM) were without significant effect. The calcium chelator, EGTA (10 mM) inhibited phenylhydrazine-stimulated proteolysis by approximately 30%. Phenylhydrazine (4 mM) caused attenuation of the major membrane protein bands present in the SDS-PAGE pattern and extensive smearing of a band in the region of approximately 28 kDa. Free radical scavengers and antioxidants failed to ameliorate significantly membrane protein damage in phenylhydrazine-treated cells as judged by SDS-PAGE. Immunoblot analysis of spectrin confirmed these results. Two-dimensional SDS-PAGE of membrane proteins following phenylhydrazine treatment, however, revealed the appearance of new protein spots and a loss of existing protein spots as compared to control. Western blot analysis of membrane-associated calpain (79 kDa (proenzyme), 77- and 75-kDa forms) was also performed. Phenylhydrazine-treated red blood cells exhibited concentration- and time-dependent changes in the level of membrane-associated procalpain relative to control. The inhibitors N-acetylcysteine, dithiothreitol, dimethylthiourea, and dimethyl sulfoxide in the presence of phenylhydrazine appeared to preserve the level of procalpain in association with the membrane proteins, but only N-acetylcysteine and dithiothreitol protected the 77- and 75-kDa forms. In contrast, dimethylfuran in the presence of phenylhydrazine caused a substantial decrease in all three forms of membrane-associated calpain. In phenylhydrazine-treated hemolysate, the level of the 77- and 75-kDa forms of membrane-associated calpain was decreased relative to control. These forms were absent when EGTA (10 mM) was included in the incubation and the level of proenzyme was decreased. These data suggest that calpain is recruited to the membrane following hydrazine insult, undergoes a Ca(2+)-dependent conversion to the active forms, and may be involved in the degradation of damaged cytosolic and membrane protein(s).

Metabolic inhibition potentiates oxidant injury

Toxic oxygen species have been implicated as important mediators of injury after reperfusion of an ischemic organ. The aim of this study was to determine if prior metabolic inhibition, such as that which occurs during ischemia, potentiates oxidant injury in vitro. Bovine pulmonary artery endothelial cells were metabolically inhibited for various periods of time with or without the mitochondrial inhibitor oligomycin (650 nM). The cells were rescued from metabolic inhibition by a wash step and subsequent addition of 5.5 mM glucose. At the same time that metabolic inhibition was relieved the cells were subjected to doses of H2O2 ranging from 0 to 100 microM. ATP levels were monitored over a 2-hr time course after rescue from metabolic inhibition by the luciferin-luciferase assay. Cell viability at 2 hr after relief of metabolic inhibition was assessed by trypan blue exclusion. Intracellular pH during metabolic inhibition was determined with the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5(and-6) carboxyfluorescein tetraacetomethoxymethyl ester. H2O2 consumption, a measure of H2O2 scavenging capability, was determined by a fluorescent assay. The viability and ATP levels of cells not subjected to metabolic inhibition were unaffected by these low concentrations of H2O2. Cells metabolically inhibited with glucose depletion and oligomycin were exquisitely sensitive to H2O2. Cells that were only deprived of glucose demonstrated no potentiation of injury, while cells subjected to mitochondrial inhibition with oligomycin alone also showed significant potentiation of oxidant injury. H2O2 consumption was not affected by metabolic inhibition. Conditions associated with mitochondrial inhibition consistently resulted in a decrease in intracellular pH. These experiments suggest that a synergism exists between metabolic inhibition and subsequent oxidant exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydrogen peroxide stimulates phospholipase A2-mediated arachidonic acid release in cultured intestinal epithelial cells (INT 407)

The mechanisms by which hydrogen peroxide and, for comparison, 4-beta-phorbol-12-myristate-13-acetate (PMA) stimulate release of radiolabeled arachidonic acid (14C-AA) in cultured intestinal epithelial cells (INT 407) were investigated. Both hydrogen peroxide and PMA caused a rapid (3 min) and dose-related intracellular release of free 14C-AA, followed by a dose- and time-dependent release of 14C-AA into the extracellular medium, but hydrogen peroxide was about 50,000 times less effective than PMA in releasing 14C-AA. No 14C-AA was released on stimulation with 4-alpha-phorbol-12,13-di-decanoate (PDD), a phorbol ester that does not activate protein kinase C. The 14C-AA release was reduced by the phospholipase A2 inhibitors nordihydroguaiaretic acid and 4-bromophenacyl bromide and by the calmodulin/protein kinase C inhibitor trifluoperazine and the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7). However, H-7 was less effective than the other inhibitors in reducing the hydrogen peroxide-stimulated 14C-AA release. The hydrogen peroxide-stimulated, but not the PMA-stimulated, rapid (3 min) 14C-AA release was associated with an increased influx of extracellular calcium. Stimulation of the cells with PMA resulted in phosphorylation of a cellular protein of about 32 kDa, whereas no phosphorylation of this protein was detected after stimulation with hydrogen peroxide. Taken together, these findings indicate that (i) both PMA and hydrogen peroxide may stimulate phospholipase A2-mediated AA release from human intestinal epithelial cells; (ii) this stimulation is brought about via protein kinase C and calmodulin-mediated events; (iii) PMA-stimulated 14C-AA release is associated with phosphorylation of a 32-kDa protein, possibly lipocortin, whereas the hydrogen peroxide-stimulated release is not; and (iv) calmodulin is more important for the hydrogen peroxide-stimulated 14C-AA release than is protein kinase C. The possibility that hydrogen peroxide-evoked AA release may contribute to the mucosal abnormality in Crohn's disease is discussed.

Free radicals accelerate the decay of long-term potentiation in field CA1 of guinea-pig hippocampus

Free radicals have been implicated in a number of pathological conditions. To evaluate the neurophysiological consequences of free radical exposure, slices of hippocampus isolated from guinea-pigs were exposed to hydrogen peroxide which reacts with tissue iron to generate hydroxyl free radicals. Long-term potentiation, a sustained increase in synaptic responses, was elicited in field CA1 by high frequency stimulation of an afferent pathway. We found that 0.002% peroxide did not directly affect the responses evoked by stimulation of the afferent pathway but did prevent maintenance of long-term potentiation. Short-term potentiation and paired-pulse facilitation were not affected by peroxide treatment. Peroxide was less effective if removed following high frequency stimulation and was ineffective if applied only after high frequency stimulation. Input/output analysis showed that the increase in synaptic efficacy was reduced with peroxide treatment. Changes in the enhanced ability of the synaptic potential to generate a spike were less apparent. These data show that the interference of free radicals with long-term potentiation may contribute to pathological deficits. It is possible that intracellular calcium regulation is disrupted by peroxide treatment. A number of second messenger systems involved with long-term potentiation are potential targets for free radical attack.

Hydrogen peroxide-induced alterations in prostaglandin secretion in the rat colon in vitro

Although the specific cause(s) of inflammatory bowel diseases (IBD) has not been identified, one theory suggests ischemia as the early event that occurs in IBD and reperfusion causes sustained release of oxyradicals, leading to inflammation and ulceration. In this study, we have confirmed that H2O2 in the concentration seen during ischemia/reperfusion is primarily responsible for cellular membrane damage in the rat colonic fragments in vitro. Hydrogen peroxide caused a time and dose-dependent increase in 6-keto-PGF1 alpha and TXB2 release. Hydrogen peroxide-stimulated 6-keto-PGF1 alpha release was blocked (50%) by phospholipase A2 (PLA2) inhibitors quinacrine and dimethyleicosadienoic acid at 5 min. Hydrogen peroxide-stimulated 6-keto-PGF1 alpha release was completely blocked by indomethacin, significantly blocked (69%) by nordihydroguiaretic acid, and completely blocked by catalase. Superoxide dismutase and uric acid failed to inhibit H2O2-stimulated 6-keto-PGF1 alpha release. Endogenous catalase inhibitors 3-aminotriazole and sodium azide further enhanced the release of 6-keto-PGF1 alpha stimulated by H2O2 by 29% and 73%, respectively. Xanthine-xanthine oxidase also increased 6-keto-PGF1 alpha release from the fragments by 110%. This release was not inhibited by superoxide dismutase and uric acid, but was completely inhibited by catalase. These studies suggest a direct effect of H2O2 on colonic fragments leading to submicroscopic cellular membrane damage and excess prostanoid production utilizing a PLA2/cyclooxygenase and catalase-sensitive pathway without the formation of toxic hydroxyl ions. The quick release of 6-keto-PGF1 alpha also suggests an early manifestation of H2O2-induced damage in rat colonic fragments.

Reactive oxygen molecule-mediated injury in endothelial and renal tubular epithelial cells in vitro

To investigate renal tubular epithelial cell injury mediated by reactive oxygen molecules and to explore the relative susceptibility of epithelial cells and endothelial cells to oxidant injury, we determined cell injury in human umbilical vein endothelial cells and in four renal tubular epithelial cell lines including LLC-PK1, MDCK, OK and normal human kidney cortical epithelial cells (NHK-C). Cells were exposed to reactive oxygen molecules including superoxide anion, hydrogen peroxide and hydroxyl radical generated by xanthine oxidase and hypoxanthine. We determined early sublethal injury with efflux of 3H-adenine metabolites and a decline in ATP levels, while late lytic injury and cell detachment were determined by release of 51chromium. When the cells were exposed to 25, 50, and 100 mU/ml xanthine oxidase with 5.0 mM hypoxanthine, ATP levels were significantly lower (P less than 0.001) in LLC-PK1, NHK-C and OK cells compared to MDCK cells while ATP levels were significantly lower (P less than 0.01) in endothelial cells compared to all tubular cell lines. A similar pattern of injury was seen with efflux of 3H-adenine metabolites. When the cells were exposed to 50 mU/ml xanthine oxidase with 5.0 mM hypoxanthine for five hours, total 51chromium release was significantly (P less than 0.001) greater in LLC-PK1, NHK-C and OK cells compared to MDCK cells, while total 51chromium release was significantly (P less than 0.001) greater in endothelial cells compared to all tubular cells. However, lytic injury was the greatest in LLC-PK1 cells and NHK-C cells while cell detachment was the greatest in endothelial cells. MDCK cells were remarkably resistant to oxidant-mediated cell detachment and cell lysis. In addition, we determined ATP levels, 3H-adenine release and 51chromium release in LLC-PK1, NHK-C and endothelial cells in the presence of superoxide dismutase to dismute superoxide anion, catalase to metabolize hydrogen peroxide, DMPO to trap hydroxyl radical and DMTU to scavenge hydrogen peroxide and hydroxyl radical. We found that catalase and DMTU (scavengers of hydrogen peroxide) provided significant protection from ATP depletion, prevented efflux of 3H-adenine metabolites and cell detachment while DMPO (scavenger of hydroxyl radical) prevented lytic injury. In addition, we found that the membrane-permeable iron chelator, phenanthroline, and preincubation with deferoxamine prevented cell detachment and cell lysis, confirming the role of hydroxyl radical in cell injury.(ABSTRACT TRUNCATED AT 400 WORDS)

Protective effect of glutamine on endothelial cell ATP in oxidant injury

Endothelial cell dysfunction following exposure to H2O2 is associated with rapid inhibition of glucose-dependent pathways of ATP synthesis. The role other substrates for ATP synthesis (e.g., amino acids) may play in the metabolism of H2O2-injured cells is unclear. The effect of glutamine, a precursor of the Kreb's cycle intermediate alpha-ketoglutarate on ATP levels in bovine pulmonary artery endothelial cells exposed to H2O2 was examined. The presence of glutamine during H2O2 injury significantly enhanced ATP levels in the injured cells. Concentrations of glutamine as low as 50 microM produced significant improvement of ATP levels in endothelial cells exposed to 5 mM H2O2. The 2 mM concentration of glutamine produced the greatest benefit, while greater concentrations of glutamine (5-20 mM) were actually associated with progressive decrements of the maximal benefit seen with the 2 mM concentration. The 2 mM concentration of glutamine produced similar enhancement of ATP with 1 and 10 mM H2O2 injury as well. Short-term viability following 5 mM H2O2 injury was significantly improved by the presence of 2 mM glutamine. The most effective concentration of glutamine (2 mM) did not scavenge H2O2 in a fluorometric assay. These observations suggest that mitochondrial substrates, such as glutamine, that bypass glucose-dependent pathways of ATP synthesis may be useful therapeutic agents for maintenance of ATP levels in oxidant-injured cells.

Prevention of thromboxane B2-induced hepatocyte plasma membrane bleb formation by certain prostaglandins and a proteinase inhibitor

Isolated hepatocytes incubated in the presence of thromboxane B2 developed many plasma membrane blebs which are a characteristic feature of toxic or ischaemic cell injury. When hepatocytes were incubated in the presence of both thromboxane B2 and the non-lysosomal proteinase inhibitor, leupeptin, were also well protected from the formation of blebs. This implies that thromboxane B2 is able to activate non-lysosomal proteinases which appear to attack certain cytoskeletal proteins. The data presented are consistent with thromboxane B2 acting as an intermediary in a proposed mechanism of cell injury and death in which elevated cytosolic free Ca2+ levels activate phospholipase A2 and the arachidonic acid cascade.

Mechanisms of hypochlorite injury of target cells

HOCl, which is produced by the action of myeloperoxidase during the respiratory burst of stimulated neutrophils, was used as a cytotoxic reagent in P388D1 cells. Low concentrations of HOCl (10-20 microM) caused oxidation of plasma membrane sulfhydryls determined as decreased binding of iodoacetylated phycoerythrin. These same low concentrations of HOCl caused disturbance of various plasma membrane functions: they inactivated glucose and aminoisobutyric acid uptake, caused loss of cellular K+, and an increase in cell volume. It is likely that these changes were the consequence of plasma membrane SH-oxidation, since similar effects were observed with para-chloromercuriphenylsulfonate (pCMBS), a sulfhydryl reagent acting at the cell surface. Given in combination pCMBS and HOCl showed an additive effect. Higher doses of HOCl (greater than 50 microM) led to general oxidation of -SH, methionine and tryptophan residues, and formation of protein carbonyls. HOCl-induced loss of ATP and undegraded NAD was closely followed by cell lysis. In contrast, NAD degradation and ATP depletion caused by H2O2 preceded cell death by several hours. Formation of DNA strand breaks, a major factor of H2O2-induced injury, was not observed with HOCl. Thus targets of HOCl were distinct from those of H2O2 with the exception of glyceraldehyde-3-phosphate dehydrogenase, which was inactivated by both oxidants.

Inhibition by linoleic acid hydroperoxide of alveolar macrophage superoxide production: effects upon mitochondrial and plasma membrane potentials

Linoleic acid hydroperoxide (LOOH) is a naturally occurring product of lipid peroxidation. Incubation of rat alveolar macrophages with LOOH produced alterations of membrane properties and function at concentrations of LOOH as low as 0.1 microM. These included phorbol myristate acetate (PMA)-stimulated superoxide production, mitochondrial membrane potential, and plasma membrane potentials. These effects were clearly separated from gross loss of structural integrity as measured by lactate dehydrogenase release, in terms of both time of incubation and concentration of LOOH. PMA-stimulated superoxide production measured 15 min after addition of 10 microM LOOH was inhibited approximately 50%; however, addition of this concentration of the hydroperoxide after PMA stimulation was without effect. Superoxide production was also measured in a cell-free system produced by incubation of alveolar macrophages with sodium dodecyl sulfate. Prior incubation of alveolar macrophages with LOOH, H2O2, or t-butyl hydroperoxide, under conditions that significantly inhibited superoxide production by the intact cells, did not produce inhibition of the NADPH-dependent superoxide generating system in the cell-free preparation. These results suggest that the effect of LOOH was upon signal transduction involved in the stimulation of superoxide production rather than on the NADPH oxidase itself. Measurements of membrane potential changes were made using the lipophilic ions, 3,3'-dipentyloxacarbocyanine (DiOC5(3] and bis(3-phenyl-5-oxoisoxazol-4-yl)pentamethineoxonol (oxonol V). On the basis of their charge, DiOC5(3) fluorescence primarily reports mitochondrial potential and oxonol V absorbance reports plasma membrane potential. With 10 microM LOOH, depolarization of the plasma and mitochondrial membranes appeared to occur within seconds. As prior depolarization depresses superoxide production, these hydroperoxide-induced changes in membrane potential may be responsible for decreased PMA-stimulated superoxide production.

Differential lymphocyte growth-modifying effects of oxidants: changes in cytosolic Ca+2

An increase in the concentration of cytosolic Ca+2 ([Ca-2]i) is among the earliest changes seen in mitogen-stimulated lymphocytes and is a consequence of signal transduction which usually results in the initiation of cell cycle progression. However, increased [Ca+2]i has also been correlated with cytotoxicity. We have determined whether modulations of [Ca+2]i are involved in the functional inactivation of cells observed with sublethal concentrations of oxidants. Specifically, [Ca+2]i was measured in mouse splenic lymphocytes that were treated with different oxidants in order to determine if oxidative stress interferes with mitogen-stimulated increases in [Ca+2]i, if oxidants themselves modulated [Ca+2]i, and, if so, whether such Ca+2 modulations by oxidants had stimulatory or inhibitory effects on the response of lymphocytes to mitogens. The oxidants employed were copper phenanthroline (CuP; surface thiol oxidizer), N-ethyl maleimide (NEM; permeant thiol alkylator), hydrogen peroxide (H2O2; generates hydroxyl radical within the cell), and radiation (Cs137; generates hydroxyl radical by radiolysis). Growth of all treated cells was equally inhibited upon stimulation with Con A or PMA/A23187, suggesting that all the oxidants inhibited cell functions required distal in activation to the transduction pathway utilized by Con A but bypassed by PMA/A23187. Doses of CuP, NEM, and radiation which fully inhibited Con A-stimulated proliferation had little effect on resting or mitogen-stimulated changes of [Ca+2]i, but H2O2 doses which fully inhibited proliferation increased [Ca+2]i in unstimulated cells and prevented the increase normally caused by Con A. Both intra- and extracellular Ca+2 contributed to the increased [Ca+2]i seen in unstimulated cells. An elevated [Ca+2]i was sufficient to reduce responsiveness, since pharmacologically increasing the [Ca+2]i with the ionophore A23187 rendered lymphocytes less responsive to Con A. Unlike A23187, H2O2 was unable to synergize with PMA, suggesting that the H2O2-induced increase of [Ca+2]i delivered predominantly negative signals to the cell. The results also suggest that [Ca+2]i utilization by Con A versus PMA-activated lymphocytes must be different. When cells were treated with H2O2 under conditions where intracellular and extracellular Ca+2 were chelated with BAPTA and EGTA, respectively, the response to Con A was restored. Under these conditions, higher concentrations of H2O2 were required to inhibit the response to Con A. Our results indicate that signal transduction may be compromised in cells treated with H2O2, but not in cells treated with CuP, NEM, or radiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Pathophysiological mechanisms of active oxygen

Besides being toxic, oxidants can induce pathophysiological effects in mammalian cells. For example they can stimulate rather than inhibit cell growth. Since oxidants are ubiquitous they may represent 'natural' tumour promoters. Our work with xanthine/xanthine-oxidase as an extracellular source of active oxygen (AO) and promotable (clone 41) and non-promotable (clone 30) mouse epidermal cells JB6 allows insights into the mechanism of action of oxidant promoters. We found that AO stimulated the growth only of promotable clone 41 after an initial period of moderate inhibition while it was strongly cytostatic for non-promotable clone 30. Active oxygen induced larger amounts of DNA-strand breaks and poly ADP-ribosylation of chromosomal proteins in non-promotable cells. In addition, AO was capable of inducing the growth- and differentiation-related proto-oncogenes c-fos and c-myc in promotable and non-promotable JB6 cells. We speculate that these genes can exert their functions only in the promotable clone 41 because the general cytostatic effects of AO are moderate. A possible explanation for the differences between these 2 clones was discovered when we compared the constitutive activities, protein concentrations and mRNA levels for the antioxidant enzymes catalase (CAT), Cu,Zn-superoxide dismutase (SOD) and glutathione-peroxidase (GPx). We found that CAT and SOD (but not GPx) levels were 2-3-fold higher in the promotable clone 41. We propose that promotable cells possess a superior antioxidant defence which protects them from excessive cytostatic effects of AO.

Alterations in energy status by menadione metabolism in hepatocytes isolated from fasted and fed rats

The biochemical mechanism of cytotoxicity, induced by the quinoid compound 2-methyl 1,4-naphthoquinone (menadione), was investigated in hepatocytes freshly isolated from fasted and fed rats. Hepatocytes from fasted rats were significantly more vulnerable to the toxicity of menadione than hepatocytes from fed rats. Menadione (150 microM) induced a 50% loss of viability of cells (LT50) from fasted rats after 55 min of incubation, whereas a LT50 of 80 min was observed after exposure of hepatocytes from fed rats to menadione. Glutathione and NADPH levels were rapidly depleted by menadione metabolism. This depletion was sustained during the incubation period. No significant differences were found in the time course and extent of the menadione-induced glutathione and NADPH depletion in hepatocytes of both nutritional states. Menadione also affected the energy status of the hepatocytes. The ATP content of cells from fasted rats decreased to 50% (AT50) within 18 min of exposure to menadione, whereas a 50% loss of ATP content of hepatocytes from fed rats was reached at 65 min. In contrast to depletion of glutathione and NADPH, the time course and extent of menadione-induced ATP depletion correlated well with the time of onset and rate of cell killing. Our results suggest that menadione metabolism may interfere with both mitochondrial and glycolytic ATP production. Depletion of ATP might be a critical step in menadione-induced cytotoxicity.

Mechanism of endothelial cell shape change in oxidant injury

Changes in endothelial cell morphology induced by neutrophil-generated hydrogen peroxide (H2O2) may account for the capillary leak of the adult respiratory distress syndrome (ARDS). The relationship of H2O2 effects on the concentration of intracellular Ca2+ [( Ca2+]i) and ATP to changes in microfilaments and microtubules, important determinants of cell shape, was examined. Bovine pulmonary artery endothelial cells were injured over a 2-hr time course with a range of H2O2 doses (0-20 mM). The higher concentrations of H2O2 consistently produced contraction and rounding of greater than 50-75% of cells by 1-2 hr. The range of 1-20 mM H2O2 produced rapid, significant reductions in endothelial ATP levels over the time course of injury. Although there were significant increases in mean endothelial [Ca2+]i in response to 5, 10, and 20 mM H2O2, 1 mM H2O2 did not affect the [Ca2+]i. Fluorescence microscopy revealed that microfilament disruption occurred as ATP levels fell and preceded depolymerization of microtubules which developed after [Ca2+]i approached 1 X 10(-6) M. H2O2 at 1 mM injury caused microfilament disruption but did not depolymerize microtubules. Microfilament disruption occurred without oxidant exposure, when ATP levels were reduced by glucose depletion and mitochondrial inhibition with oligomycin (650 nM). If a Ca2+ ionophore, ionomycin (5 microM), was then added, [Ca2+]i rose to greater than 1 X 10(-6) M, microtubules fragmented and depolymerized, and cell contraction and rounding very similar to that induced by H2O2 occurred. These results suggest that endothelial cell dysfunction and capillary leak in ARDS may be due to H2O2-mediated changes in cellular ATP and [Ca2+]i.

Morphologic and functional correlates of plasma membrane injury during oxidant exposure

Plasma membrane injury by exposure to hydrogen peroxide was examined in a renal epithelial cell line (LLC-PK1). Morphologic and functional parameters of plasma membrane integrity were studied in an attempt to eludicate the sequence of membrane alterations during the evolution of hydrogen peroxide-mediated injury. These parameters included plasma membrane potential and permeability, plasma membrane bleb formation, cellular size, and plating efficiency. Plasma membrane potential was the earliest parameter affected by hydrogen peroxide exposure. Half maximal depolarization occurred within 15-30 min of exposure to 1 mM, after 10-15 min exposure to 100 mM and after over 150 min exposure to 10 microM hydrogen peroxide. After exposure to 1 mM hydrogen peroxide, the following sequence of events was seen; increased plasma membrane blebbing (30 min), cell swelling (90-125 min) and increased plasma membrane permeability (150-240 min). After a 30 min exposure to 1 mM hydrogen peroxide, cellular plating efficiency, measured at 24 h, was reduced by 50% (P less than .001). These changes were accelerated, although their order of appearance was unchanged, at higher concentrations of hydrogen peroxide. We conclude that functional and morphologic expressions of cellular injury in this model occur in a defined sequence with plasma membrane depolarization representing the earliest marker of membrane injury during hydrogen peroxide exposure.

Oxidant-induced DNA damage of target cells

In this study we examined the leukocytic oxidant species that induce oxidant damage of DNA in whole cells. H2O2 added extracellularly in micromolar concentrations (10-100 microM) induced DNA strand breaks in various target cells. The sensitivity of a specific target cell was inversely correlated to its catalase content and the rate of removal of H2O2 by the target cell. Oxidant species produced by xanthine oxidase/purine or phorbol myristate acetate-stimulated monocytes induced DNA breakage of target cells in proportion to the amount of H2O2 generated. These DNA strand breaks were prevented by extracellular catalase, but not by superoxide dismutase. Cytotoxic doses of HOCl, added to target cells, did not induce DNA strand breakage, and myeloperoxidase added extracellularly in the presence of an H2O2-generating system, prevented the formation of DNA strand breaks in proportion to its H2O2 degrading capacity. The studies also indicated that H2O2 formed hydroxyl radical (.OH) intracellularly, which appeared to be the most likely free radical responsible for DNA damage: .OH was detected in cells exposed to H2O2; the DNA base, deoxyguanosine, was hydroxylated in cells exposed to H2O2; and intracellular iron was essential for induction of DNA strand breaks.

Incubation of endothelial cells in a superoxide-generating system: impaired low-density lipoprotein receptor-mediated endocytosis

Endothelial cells (EC) of blood vessels are submitted to oxidative stress under various circumstances. These conditions may modify EC functions; therefore, in the present work we have studied the receptor-mediated endocytosis of low-density lipoproteins (LDL) and malondialdehyde-modified LDL by the LDL receptor and the "scavenger" receptor, respectively, in cultured human umbilical vein EC after short (0-120 minutes) incubations in a superoxide anion (O2-) generating system. In both receptor-mediated processes, the oxidative stress produces a significant decrease at four different LDL concentrations (5-50 micrograms/ml) after 120 minutes of oxidation. On the other hand, the fluid-phase endocytosis of sucrose by EC seems to be stimulated by these conditions. Furthermore, incorporation of antioxidant enzymes in the O2- -producing system shows that H2O2 is an obligatory intermediate in order to produce the effect on the receptor-mediated processes. Hypotheses concerning the mechanisms involved in the modifications of endocytotic processes and their implications in vivo are discussed.

Cellular Ca2+ homeostasis and Ca2+-mediated cell processes as critical targets for toxicant action: conceptual and methodological pitfalls

Because of the central role of the calcium messenger system in diverse functions of tissues, organs, and cells, Ca2+ homeostasis and function may prove to be critical cellular and molecular targets for a diverse range of toxicants. Experimental proof of these targets as a specific site of toxicant action is challenging and technically difficult as a result of the complexity of Ca2+ homeostatic and Ca2+-mediated processes. However, as the investigation of normal physiological control of Ca2+ and function will continue to be an active and productive area of basic research for several years to come, it is anticipated that these insights will be increasingly applied to the understanding of the mechanisms of action of toxic agents.

Role of selenium-dependent glutathione peroxidase in antioxidant defenses in rat alveolar macrophages

Glutathione peroxidase is a crucial component of cellular antioxidant defenses. Using tertiary butyl hydroperoxide (tBOOH) as a model for oxidant stress in alveolar macrophages, we determined the effectiveness of glutathione peroxidase in preventing both cell "death" (lactate dehydrogenase release) and more subtle alterations in cell function. The KM of glutathione peroxidase for tBOOH was 54 microM, and the Vmax was 26 nmol/min/10(6) cells in alveolar macrophages. Concentrations of tBOOH greater than 100 microM caused lactate dehydrogenase release; however, a lag greater than 30 min was observed when with 10 mM tBOOH. With 200 microM tBOOH, the rate of decrease in membrane potential, measured by 3,3'-dipentyloxacarbocyanine iodide fluorescence, inversely correlated with glutathione peroxidase. Computer-enhanced microscopy showed that this fluorescence predominately was in mitochondria. NADPH fluorescence was altered in selenium-deficient alveolar macrophages; the tBOOH-dependent rate of NADPH oxidation was slowed, and higher concentrations of tBOOH were required to disturb the steady state NADPH/NADP+ ratio. Although alteration in NADPH or glutathione oxidation can reflect oxidant stress and can adversely affect cell function, such a change does not dictate irreversible injury. Nevertheless, irreversible injury by oxidants appears to involve an overwhelming of the glutathione-NADPH antioxidant system.