Cytoprotective effect of reduced glutathione in hydrogen peroxide-induced endothelial cell injury

Vitamin C-induced loss of redox-dependent viability in lung microvascular endothelial cells

Recent clinical trials have shown that vitamin C, at pharmacological concentrations (milligram to approximately gram), upon infusion into circulation, modulates vasodilation and vascular tone in humans. This also results in the elevated concentrations of vitamin C in circulation in the millimolar range. Here, it was hypothesized that vitamin C at pharmacological concentrations (millimolar) would induce oxidative stress and cause loss of redox-dependent cell viability in vascular endothelial cells (ECs). To test the hypothesis, bovine lung microvascular ECs (BLMVECs) in monolayer cultures were exposed to vitamin C (0-10 mM) for different time periods (0-2 h). Electron paramagnetic resonance spectroscopy revealed the intracellular formation of ascorbate free radical in a dose- and time-dependent fashion. Vitamin C also induced formation of intracellular reactive oxygen species in a dose-dependent fashion. It was observed that vitamin C induced morphological alterations and loss of cell viability in a dose- and time-dependent fashion, as measured by light microscopy and Alamar Blue redox cell viability assay, respectively. Vitamin C analogues failed to induce such changes. Vitamin C depleted cellular GSH levels in a dose-dependent fashion, suggesting that vitamin C altered thiol-redox status in BLMVECs. Antioxidants, intracellular iron chelator, and catalase protected cells against vitamin C-induced loss of redox-dependent cell viability, confirming the role of hydrogen peroxide and iron during redox cycling of vitamin C. These results, for the first time in detail, established that vitamin C at pharmacological doses induced oxidative stress and loss of redox-dependent cell viability in microvascular ECs.

Redox regulation of endothelial barrier integrity

Intestinal ischemia-reperfusion is associated with the generation of reactive oxygen metabolites as well as remote, oxidant-mediated lung injury. Oxidants elicit endothelial redox imbalance and loss of vascular integrity by disorganizing several junctional proteins that contribute to the maintenance and regulation of the endothelial barrier. To determine the specific effect of redox imbalance on pulmonary vascular barrier integrity, microvascular permeability was determined in lungs of animals subjected to chemically induced redox imbalance. The effect of redox imbalance on microvascular permeability and endothelial junctional integrity in cultured lung microvascular cells was also determined. Whole lung and cultured pulmonary endothelial cell permeability both increased significantly in response to chemical redox imbalance. Thiol depletion also resulted in decreased endothelial cadherin content and disruption of the endothelial barrier. These deleterious effects of intracellular redox imbalance were blocked by pretreatment with exogenous glutathione. The results of this study suggest that redox imbalance contributes to pulmonary microvascular dysfunction by altering the content and/or spatial distribution of endothelial junctional proteins.

Endogenous glutathione protects cerebral endothelial cells from traumatic injury

Blood-brain barrier breakdown and edema, indicative of cerebrovascular injury, are characteristic pathophysiologic outcomes following head trauma. These injuries result from both primary mechanical damage and from secondary events initiated by the traumatic insult. Free radicals are recognized as mediators of secondary injury in a number of trauma models. In this study, we used a novel in vitro model of traumatic microvascular injury to test the hypothesis that endogenous glutathione protects cerebral endothelial cells from secondary autooxidative injury following mechanical trauma. Porcine brain cerebral endothelial cells were grown in tissue culture wells with Silastic membrane bottoms, and cellular injury was induced by displacing the membrane different distances with user-defined pressure pulses from a customized device. The resultant endothelial cell injury 2 h following stretch was determined by measuring lactate dehydrogenase in the culture media. Significant stretch-dependent increases in endothelial injury were elicited that depended in a nonlinear fashion on the degree of membrane displacement. Depletion of intracellular glutathione with buthionine sulfoximine (1 mM) increased the extent of traumatic endothelial cell injury by 17-56%, particularly at low to moderate levels of traumatic injury (30-40% of total endothelial cell LDH release). Conversely, traumatic injury was reduced by 22-45% when endothelial cell glutathione levels were augmented threefold (to 140+/-8 nmol/mg protein) by preincubating cells with 2 mM glutathione; the extent of protection was inversely proportional to the extent of the traumatic stretch. Traumatic endothelial cell injury was also significantly and dose-dependently attenuated (up to 40%) by treatment with the xanthine oxidase inhibitor oxypurinol (50 and 100 microM). These results demonstrate that cerebral endothelial cells are the targets of hydrogen peroxide-mediated injury secondary to trauma-induced superoxide radical formation via the xanthine oxidase pathway. The neutralization of peroxides by the endogenous glutathione redox cycle provides endothelial cells a finite capacity to reduce free radical-mediated traumatic injury; this cycle may be amenable to therapeutic manipulation to mitigate posttraumatic edema and other manifestations of vascular dysfunction.

Decreased endothelial cell glutathione and increased sensitivity to oxidative stress in an in vitro blood-brain barrier model system

Using a cell culture model of the blood-brain barrier (BBB) we have evaluated the role of endothelial cell glutathione in protecting barrier integrity against nitric oxide (NO)-induced oxidative stress. The co-culture of human umbilical vein endothelial cells (ECV304) with rat (C6) glioma cells, or incubation with glioma cell or primary astrocytic conditioned medium, resulted in a decline in endothelial cell glutathione. Exposure to a single addition of NO gas induced a rapid breakdown in model barrier integrity in endothelial/glioma co-cultures. Addition of NO gas or tumour necrosis factor-alpha (TNF-alpha) also resulted in a loss of membrane integrity, as measured by an enhanced release of lactate dehydrogenase, only from endothelial cells treated with glioma conditioned medium. Furthermore, assessment of viability in endothelial cells grown alone or treated with glioma conditioned medium, by propidium iodide labelled flow cytometry. demonstrated no difference in the number of positively stained cells after NO exposure. These results indicate that when enhanced endothelial monolayer barrier formation occurs via astrocytic-endothelial interactions, cellular glutathione levels are decreased. This renders the barrier cells, under these conditions, more susceptible to oxidative stress but does no necessarily lead to greater cell death.

Overexpression of superoxide dismutase and catalase in immortalized neural cells: toxic effects of hydrogen peroxide

Hydrogen peroxide (H2O2) is a known toxicant which causes its damage via the production of hydroxyl radicals. It has been reported to cause both necrotic and apoptotic cell death. The present study was undertaken to evaluate the mode of H2O2-induced cell death and to assess if overexpression of catalase could protect against its toxicity. H2O2 causes cell death of immortalized CSM 14.1 neural cells in a dose-dependent manner. H2O2-induced death was associated with DNA laddering as shown by agarose gel electrophoresis. Stable overexpression of catalase by transfection of a vector containing human cDNA into these cells markedly attenuated H2O2-induced toxic effects. Transfection of a vector containing a SOD cDNA afforded no protection. These results indicate that H2O2 can lead to the activation of endonuclease enzyme that breaks DNA into oligosomes. These cells which overexpress catalase or SOD will help to determine the specific role of H2O2 or O2- in the deleterious effects of a number of toxins.

The ratio of formation of prostacyclin/thromboxane A2 in HUVEC decreased in each subsequent passage

In this study, we analyzed the antioxidative potential (SOD-, GSH-Px-activity) and the basal, H2O2- and ATP-stimulated formation of PGI2 and TXA2 in human umbilical vein endothelial cells (HUVEC) of different passages. The subcultivation of cells partly represents the process of aging. Both subcultivation of the cells and the H2O2 incubation did not significantly influence the activity of SOD and GSH-Px. H2O2 (0.1 mM and 1.0 mM) stimulated the generation of PGI2 and TXA2 in the cell passages time dependently. The formation ratio of PGI/TXA2 changed from 640:1 (0.1 mM H2O2) or 430:1 (1.0 mM H2O2, 40 min incubation) at the 1st passage, to 13:1 and 17:1, respectively, at the 4th passage. This resulted from the reduction of the PGI2 synthesis connected with more pronounced TXA2 formation. The same behavior was found in the basal and ATP-stimulated eicosanoid formation. Based on this, the age-dependent activation of the oxygen radical formation could be responsible for the modified eicosanoid metabolism resulting in vascular complications in the elderly.

Gender-related response to a tert-butyl hydroperoxide-induced oxidation in human neonatal tissue

Reports of gender-related differences in the activity of enzymes involved in the metabolism of intracellular antioxidants, led us to verify whether the prostaglandin response to tert-butyl hydroperoxide (TBH) differed according to the sex of infants. Segments of human umbilical veins were perfused in the presence or absence of TBH (0.25 mmol/l, and 1.0 mmol/l). Because TBH is quenched in the cell by glutathione peroxidase, total glutathione concentrations and production of glutathione-dependent prostaglandins (PGE2 and PGF2 alpha) as well as membrane-derived eicosanoids (PGI2 and thromboxane) were measured in the eluate. In veins from boys, TBH induced a sustained response for glutathione only, which was increased (p < 0.05). In female-derived tissue, the hydroperoxide induced a different response according to the dose of TBH. At 0.25 mmol/l, a drop (p < 0.005) in PGF2 alpha was associated with a rise (p < 0.001) in thromboxane. At 1.0 mmol/l, TBH had an opposite effect--there was a rise (p < 0.01) in PGE2 and PGI2. The prostaglandin concentration were not proportional to the oxidative stimulus, suggesting a critical level of TBH at which the oxidative state differs in tissues derived from boys or girls.