Fatty acid supplementation protects pulmonary artery endothelial cells from oxidant injury

Intracranial Aneurysms and Lipid Metabolism Disorders: From Molecular Mechanisms to Clinical Implications

Many vascular diseases are linked to lipid metabolism disorders, which cause lipid accumulation and peroxidation in the vascular wall. These processes lead to degenerative changes in the vessel, such as phenotypic transformation of smooth muscle cells and dysfunction and apoptosis of endothelial cells. In intracranial aneurysms, the coexistence of lipid plaques is often observed, indicating localized lipid metabolism disorders. These disorders may impair the function of the vascular wall or result from it. We summarize the literature on the relationship between lipid metabolism disorders and intracranial aneurysms below.

Acute hyperoxia increases lipid peroxidation and induces plasma membrane blebbing in human U87 glioblastoma cells

Atomic force microscopy (AFM), malondialdehyde (MDA) assays, and amperometric measurements of extracellular hydrogen peroxide (H(2)O(2)) were used to test the hypothesis that graded hyperoxia induces measurable nanoscopic changes in membrane ultrastructure and membrane lipid peroxidation (MLP) in cultured U87 human glioma cells. U87 cells were exposed to 0.20 atmospheres absolute (ATA) O(2), normobaric hyperoxia (0.95 ATA O(2)) or hyperbaric hyperoxia (HBO(2), 3.25 ATA O(2)) for 60 min. H(2)O(2) (0.2 or 2 mM; 60 min) was used as a positive control for MLP. Cells were fixed with 2% glutaraldehyde immediately after treatment and scanned with AFM in air or fluid. Surface topography revealed ultrastructural changes such as membrane blebbing in cells treated with hyperoxia and H(2)O(2). Average membrane roughness (R(a)) of individual cells from each group (n=35 to 45 cells/group) was quantified to assess ultrastructural changes from oxidative stress. The R(a) of the plasma membrane was 34+/-3, 57+/-3 and 63+/-5 nm in 0.20 ATA O(2), 0.95 ATA O(2) and HBO(2), respectively. R(a) was 56+/-7 and 138+/-14 nm in 0.2 and 2 mM H(2)O(2). Similarly, levels of MDA were significantly elevated in cultures treated with hyperoxia and H(2)O(2) and correlated with O(2)-induced membrane blebbing (r(2)=0.93). Coapplication of antioxidant, Trolox-C (150 microM), significantly reduced membrane R(a) and MDA levels during hyperoxia. Hyperoxia-induced H(2)O(2) production increased 189%+/-5% (0.95 ATA O(2)) and 236%+/-5% (4 ATA O(2)) above control (0.20 ATA O(2)). We conclude that MLP and membrane blebbing increase with increasing O(2) concentration. We hypothesize that membrane blebbing is an ultrastructural correlate of MLP resulting from hyperoxia. Furthermore, AFM is a powerful technique for resolving nanoscopic changes in the plasma membrane that result from oxidative damage.

Methods of human islet culture for transplantation

The ability to maintain isolated human islet preparations in tissue culture has recently been adopted by most islet transplant centers, and improves the safety as well as the practicality of islet transplantation. Maintaining islet viability and recovery, however, remains challenging in a clinical setting, due to stringent conditions required for culture. Islet culture is further complicated by the fact that islets do not form a monolayer. This review aims to clarify media, supplementation, and conditions that have been shown to be relevant to human islets, as well as to offer avenues of future research. Factors examined that may influence islet survival include base medium, glucose concentration, vitamin, inorganic ion, lipid, hormone, growth factor, amino acid, and binding protein composition and concentration, as well as culture temperature and seeding density. In addition, this article reviews novel techniques, such as coculture and matrices, that have been employed in an attempt to improve islet survival and functional viability.

Fatty Acids Differentially Modulate Insulin-Stimulated Endothelial Nitric Oxide Production by an Akt-lndependent Pathway

Background

Insulin increases endothelial nitric oxide (NO) production by activating endothelial nitric oxide synthase (eNOS) through protein kinase B (Akt)-mediated phosphorylation of serine residue 1179 (p-eNOS serine 1179). Because fatty acids modulate insulin-stimulated Akt signaling cascades in smooth muscle cells, we hypothesized that fatty acids would differentially regulate endothelial Akt signaling, eNOS phosphorylation, and NO production.

Methods

Porcine pulmonary artery endothelial cells (PAECs) were treated for 3 hours with 100 μM oleic (18:1) or eicosapentaenoic (20:5) acids or with an equivalent volume of ethanol vehicle (0.1%). PAECs were then treated with graded concentrations (109–10-5 M) of insulin or incubated overnight (24 hours) in culture medium without fatty acids before insulin treatment. Activation and phosphorylation of Akt and eNOS were determined by immunoblotting. NO production was measured with a chemiluminescence NO analyzer or with a NO-selective carbon fiber microelectrode.

Results

Insulin-stimulated Akt phosphorylation, eNOS phosphorylation, and NO production. The phosphatidylinositol-3 kinase inhibitor wortmannin attenuated insulin-stimulated Akt activation and NO production. Treatment with the co-3 fatty acid 20:5, but not 18:1, enhanced insulin-stimulated NO production but failed to alter insulin-stimulated Akt activation or eNOS serine 1179 phosphorylation.

Conclusion

Individual fatty acyl species have distinct effects on insulin-stimulated endothelial NO production. Although fatty acids alter Akt signaling in muscle cells, the current results indicate that fatty acids do not modulate endothelial NO production through alterations in insulin-stimulated, Akt-mediated eNOS phosphorylation.

Peroxisome proliferator-activated receptor gamma ligands increase release of nitric oxide from endothelial cells

OBJECTIVE

Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands reduce lesion formation in animal models of atherosclerosis by mechanisms that have not been defined completely. We hypothesized that PPARgamma ligands stimulate endothelial-derived nitric oxide release (*NO) to protect the vascular wall.

METHODS AND RESULTS

The PPARgamma ligands, 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2) or ciglitazone, stimulated a PPAR response element-luciferase reporter construct in transfected porcine pulmonary artery endothelial cells (PAECs), demonstrating that PPARgamma was transcriptionally functional. Treatment with 15d-PGJ2 or ciglitazone significantly increased release of *NO from PAECs or human aortic endothelial cells and augmented calcium ionophore-induced *NO release from human umbilical vein endothelial cells measured by chemiluminescence analysis of culture media. Increases in *NO release caused by treatment with 15d-PGJ2 occurred at 24 hours, but not after 1 to 16 hours, and were abrogated by treatment with the transcriptional inhibitor alpha-amanitin. Overexpression of PPARgamma or treatment with 9-cis retinoic acid also enhanced PAEC *NO release. Neither 15d-PGJ2 nor ciglitazone altered eNOS mRNA, whereas 15d-PGJ2, but not ciglitazone, decreased eNOS protein.

CONCLUSIONS

Taken together, these findings demonstrate that PPARgamma ligands stimulate *NO release from endothelial cells derived from multiple vascular sites, through a transcriptional mechanism unrelated to eNOS expression.

Attenuation of oxidant-mediated endothelial cell injury with docosahexaenoic acid: the role of intracellular iron

Previous studies have demonstrated that altering the fatty acid composition of porcine pulmonary artery endothelial cells (PAEC) significantly modulates their susceptibility to oxidative stimuli, e.g. H2O2. Based on observations that fatty acids also function to transport iron, an important catalyst for H2O2-mediated hydroxyl radical generation, we hypothesized that fatty acid-induced alterations in PAEC iron metabolism contribute to modulation of PAEC oxidant susceptibility. To test this hypothesis, PAEC were treated with culture medium supplemented with 0.1 mM oleic (18:1), linolenic (18:3) or docosahexaenoic (22:6) acids or with an equivalent volume of ethanol vehicle for 3 h. After thorough washing and incubation in unsupplemented culture medium for 24 h, PAEC monolayers were subjected to additional studies. Supplementation with 22:6 attenuated lactate dehydrogenase (LDH) release from PAEC 2 h following treatment with 100 microM H2O2 for 30 min (% LDH release: ETOH-control = 7.9 +/- 1.6, 22:6-control = 5.9 +/- 0.9, ETOH-H2O2 = 26.4 +/- 4.2, 22:6-H2O2* = 16.2 +/- 2.9; *P < 0.05 vs ETOH-H2O2). In a non-cellular system, 18:1 and 18:3 were more effective than their methyl ester derivatives or 22:6 at translocating iron from aqueous to hydrophobic environments. In contrast, only supplementation with 22:6 significantly increased PAEC uptake of 57Fe and human umbilical vein endothelial cell (HUVEC) ferritin content, whereas none of the supplementation conditions altered PAEC catalytic iron measured with bleomycin. These novel observations indicate that specific fatty acids are capable of altering PAEC iron uptake and ferritin content thereby contributing to the understanding of the mechanisms by which fatty acids modulate the oxidant susceptibility of vascular endothelial cells.

H2O2 causes endothelial barrier dysfunction without disrupting the arginine-nitric oxide pathway

We have previously demonstrated that nitric oxide (.NO) donors attenuate and that inhibition of endogenous nitric oxide synthase (NOS) enhances hydrogen peroxide (H2O2)-mediated porcine pulmonary artery endothelial cell (PAEC) injury. The current study investigates the hypothesis that oxidant-mediated inhibition of NOS contributes to PAEC injury. PAEC barrier function, measured as the transmonolayer clearance of albumin, was significantly impaired by H2O2 (10-100 microM) in the absence of cytotoxicity. Treatment with H2O2 did not alter NOS activity, measured as the conversion of [3H]arginine to [3H]citrulline in PAEC lysates, either immediately after treatment with 0-250 microM H2O2 for 30 min or for up to 120 min after treatment with 100 microM H2O2. H2O2 had little effect on NOS activity in intact PAECs, measured as 1) the formation of [3H]citrulline in [3H]arginine-loaded PAECs, 2) PAEC guanosine 3',5'-cyclic monophosphate content, and 3) PAEC.NO release to the culture media. These results indicate that the arginine-.NO pathway remains intact after exposure to oxidant conditions sufficient to promote functional derangements of vascular endothelial cells.

Role of 8-epi PGF2alpha, 8-isoprostane, in H2O2-induced derangements of pulmonary artery endothelial cell barrier function

The non-enzymatic peroxidation product of arachidonic acid, 8-epi-PGF2alpha or 8-isoprostane (8-IP) was measured in H2O2-exposed cultured pulmonary artery endothelial cell (PAEC) monolayers using a commercially-available enzyme immunoassay kit. H2O2 (50 microM for 1-30 min) significantly increased 8-IP production in a time-dependent fashion. Treatment with higher H2O2 concentrations (100 or 250 microM) failed to further increase 8-IP generation. Determinations of thiobarbituric acid reactive substances (TBARS) and lipid hydroperoxides (LOOH) were not sufficiently sensitive to detect lipid peroxidation in PAEC exposed to 50 microM H2O2 for 15 min. 8-IP (100 pM-500 nM for 2 h) caused PAEC monolayer barrier dysfunction measured as the transmonolayer clearance of albumin without causing significant PAEC cytotoxicity (measured as intracellular lactate dehydrogenase release). This is the first report to provide evidence that 8-IP generated in H2O2-exposed PAEC contributes to oxidant-mediated alterations in monolayer barrier function at non-cytotoxic concentrations.

Oleic acid reduces oxidant stress in cultured pulmonary artery endothelial cells

Altering the fatty acid composition of cultured porcine pulmonary artery endothelial cells (PAEC) modulates their susceptibility to oxidant stress. This study demonstrates that supplementing PAEC with oleic acid (18:1 omega 9), but not gamma-linolenic acid (18:3 omega 6), provided dose-dependent protection from hydrogen peroxide (H2O2)-induced cytotoxicity. It was hypothesized that 18:1 reduced PAEC susceptibility to oxidant stress by altering H2O2 metabolism. To test this hypothesis, confluent PAEC monolayers were treated with 100-200 microM H2O2 or control conditions 24 h after supplementation with 0.1 mM 18:1, 18:3, or vehicle for 3 h. Intracellular [H2O2] in control cells (14.4-29.0 pM), estimated from the rate of aminotriazole-mediated inactivation of endogenous catalase activity, increased following treatment with 200 microM H2O2 (19.0-37.3 pM). Supplementation with 18:1 attenuated increases in intracellular [H2O2] only in oxidant-exposed cells, whereas supplementation with 18:3 attenuated intracellular [H2O2] only in control cells. Supplementation with 18:1 or 18:3 tended to reduce or enhance PAEC lipid hydroperoxide content following H2O2 exposure, respectively, but did not alter PAEC reduced glutathione content, the activities of glutathione peroxidase or catalase, or H2O2 uptake and release. Alteration of H2O2 metabolism in cultured PAEC may contribute to the ability of fatty acids to modulate cellular oxidant susceptibility.

Endothelial cell monolayer dysfunction caused by oxidized low density lipoprotein: attenuation by oleic acid

Oleic acid (18:1) may exert beneficial effects on the pathogenesis of vascular disease by a variety of mechanisms. To determine if 18:1 exerts direct protective effects on vascular endothelial cells, porcine pulmonary artery endothelial cells (PAEC) were supplemented with 0.1 mM 18:1, gamma-linolenic acid (18:3), or ethanol vehicle (ETOH) prior to treatment with low density lipoprotein (LDL), or CU(2+)-oxidized LDL (OXLDL). Treatment with neither LDL nor OXLDL (100 micrograms protein/ml) for 24-48 h caused PAEC cytotoxicity, whereas OXLDL, but not LDL, caused derangements in PAEC actin microfilament architecture and monolayer barrier dysfunction. Supplementation with 18:1, but not 18:3, attenuated derangements caused by OXLDL and lysophosphatidylcholine, a component of OXLDL. These results demonstrate that monounsaturated fatty acids directly alter the response of vascular endothelial cells to OXLDL and may retard the atherosclerotic process by decreasing the efflux of macromolecules (e.g. LDL) into the vessel wall.

Exogenous fatty acids modulate the functional and cytotoxic responses of cultured pulmonary artery endothelial cells to oxidant stress

We previously reported that supplementation with exogenous fatty acids modulated the susceptibility of cultured pulmonary artery endothelial cells (PAEC) to oxidant-mediated cytotoxicity. The current study investigates the effects of fatty acids with increasing degrees of unsaturation on oxidant-mediated dysfunction and cytotoxicity in cultured porcine pulmonary artery and aortic endothelial cells (AEC). Monolayers supplemented with 0.1 mmol/L oleic (18:1), linoleic (18:2), or gamma-linolenic (18:3) acids were exposed to oxidant stress (100 mumol/L hydrogen peroxide (H2O2)) or to control conditions for 30 minutes. Gas chromatographic analysis of the PAEC fatty acids confirmed incorporation of supplemental fatty acids into PAEC lipids. Cytotoxicity, measured as the release of intracellular lactate dehydrogenase (LDH), and PAEC monolayer barrier function, assessed by measuring the monolayer clearance of Evans blue dye bound to albumin, were determined for 1 to 3 hours after oxidant stress. The PAEC and AEC demonstrated comparable responses to H2O2. Hydrogen peroxide caused increases in monolayer permeability and detachment of cells from the monolayer that were most attenuated by supplementation with 18:2 or 18:3, and to a lesser degree with 18:1. In contrast, H2O2-mediated LDH release was attenuated by supplementation with 18:1, whereas 18:2 and 18:3 potentiated cytotoxicity after exposure to H2O2. These results indicate that the relationship between PAEC lipid composition and oxidant susceptibility is complex and that the extent of fatty acid unsaturation does not predict the functional or cytotoxic responses of PAEC to oxidant stress. Furthermore, these results suggest that functional derangements may not correlate with traditional assays of cytotoxicity induced by oxidant injury in cultured endothelium.

Nitric oxide regulates vascular cell adhesion molecule 1 gene expression and redox-sensitive transcriptional events in human vascular endothelial cells

Decreased nitric oxide (NO) activity, the formation of reactive oxygen species, and increased endothelial expression of the redox-sensitive vascular cell adhesion molecule 1 (VCAM-1) gene in the vessel wall are early and characteristic features of atherosclerosis. To explore whether these phenomena are functionally interrelated, we tested the hypothesis that redox-sensitive VCAM-1 gene expression is regulated by a NO-sensitive mechanism. In early passaged human umbilical vein endothelial cells and human dermal microvascular endothelial cells, the NO donor diethylamine-NO (DETA-NO, 100 microM) reduced VCAM-1 gene expression induced by the cytokine tumor necrosis factor alpha (TNF-alpha, 100 units/ml) at the cell surface level by 65% and intracellular adhesion molecule 1 (ICAM-1) gene expression by 35%. E-selectin gene expression was not affected. No effect on expression of cell adhesion molecules was observed with DETA alone. Moreover, DETA-NO suppressed TNF-alpha-induced mRNA accumulation of VCAM-1 and TNF-alpha-mediated transcriptional activation of the human VCAM-1 promoter. Conversely, treatment with NG-monomethyl-L-arginine (L-NMMA, 1 mM), an inhibitor of NO synthesis, augmented cytokine induction of VCAM-1 and ICAM-1 mRNA accumulation. By gel mobility shift analysis, DETA-NO inhibited TNF-alpha activation of DNA binding protein activity to the VCAM-1 NF-kappa B like binding sites. Peroxy-fatty acids such as 13-hydroperoxydodecanoeic acid (linoleyl hydroperoxide) may serve as an intracellular signal for NF-kappa B activation. Using thin layer chromatography, DETA-NO (100 microM) suppressed formation of this metabolite, suggesting that DETA-NO modifies the reactivity of oxygen intermediates in the vascular endothelium. Through this mechanism, NO may function as an immunomodulator of the vessel wall and thus mediate inflammatory events involved in the pathogenesis of atherosclerosis.

Differential effects of endothelial cell fatty acid modification on the sensitivity of their membrane phospholipids to peroxidation

In order to study the relationship between the fatty acid (FA) composition of human umbilical vein endothelial cells (HUVEC) and their susceptibility to oxidative stress, we modified their FA composition by long-term culturing in media supplemented with a saturated, monounsaturated, or polyunsaturated FA. Sensitivity of the cellular phospholipids to peroxidation was monitored by measuring conjugated diene formation and decrease of polyunsaturated FAs induced by CuSO4 and H2O2 in liposomes prepared from the respective phospholipid extracts. The extent of phospholipid peroxidation was found to increase with increasing content of polyunsaturated FAs. In addition, the sensitivity of individual polyunsaturated FAs to peroxidation was directly proportional to the number of double bonds present. However, no unequivocal relationship was observed between conjugated diene formation and the phospholipid unsaturation index (an indicator for the combined effect of number of double bonds and polyunsaturated FA contents of the membrane phospholipids). The results suggest that long-term FA modification of endothelial cells differentially alters the sensitivity of their membrane phospholipids to peroxidation: long-term modification with oleic acid may protect against lipid peroxidation, whereas linoleic acid may increase sensitivity to peroxidation. In contrast to what might be expected, long-term modification of endothelial cells with eicosapentaenoic or docosahexaenoic acid does not increase the sensitivity of phospholipids to peroxidation.

Oleic acid supplementation reduces oxidant-mediated dysfunction of cultured porcine pulmonary artery endothelial cells

We have previously shown that supplementing cultured porcine pulmonary artery endothelial cells (PAEC) with exogenous oleic acid (18:1 omega 9) alters the fatty acid composition of the cells and reduces oxidant-mediated cytotoxicity. Because the mechanisms by which lipid alterations modulate oxidant susceptibility have not been defined, the ability of 18:1 to reduce hydrogen peroxide (H2O2)-mediated PAEC dysfunction was evaluated. PAEC monolayers on polycarbonate filters were incubated for 3 h in maintenance medium supplemented with either 0.1 mM 18.1 in ethanol vehicle (ETOH) or with an equivalent volume of vehicle alone. Twenty-four hours later monolayers were treated for 30 min with 50 or 100 microM H2O2 in Hanks' balanced salt solution (HBSS) or with HBSS alone (nonoxidant control). As a functional index of PAEC monolayer integrity, the permeability of monolayers to albumin was then measured for 3 h. Treatment with 100 microM H2O2 caused cytotoxicity and progressive increases in PAEC monolayer permeability that were attenuated by 18:1 supplementation, whereas 50 microM H2O2 caused only a transient increase in permeability without cytotoxicity. Supplementation with 18:1 also attenuated H2O2-induced reductions in PAEC adenosine triphosphate (ATP) content and disruption of PAEC microfilament architecture. The ATP content of PAEC monolayers was reversibly reduced in the absence of oxidant stress by incubation with glucose-depleted medium containing deoxyglucose and antimycin A. Metabolic inhibitor-induced ATP depletion increased monolayer permeability and altered cytoskeletal architecture, alterations that resolved during recovery of PAEC ATP content. These results demonstrate that ATP depletion plays a critical role in barrier dysfunction and suggests that the ability of 18:1 to reduce oxidant-mediated PAEC dysfunction and injury may relate directly to its ability to preserve PAEC ATP content.

Quantitative fatty acid analyses in cultured porcine pulmonary artery endothelial cells: the combined effects of fatty acid supplementation and oxidant exposure

Supplemental fatty acids can modify the oxidant susceptibility of pulmonary artery endothelial cells (PAEC) in monolayer culture. In addition, in vivo dietary modifications have altered tissue and animal susceptibility to a variety of forms of oxidant stress. These modifications of oxidant injury have been attributed to changes in the numbers of fatty acid double bonds in cell lipids. We tested this hypothesis by incubating porcine PAEC in culture medium supplemented with either 0.1 mM oleic acid (18:1 omega 9) or with an equivalent volume of ethanol vehicle alone (ETOH-0.1%) for 3 h. After supplementation, PAEC were exposed to either oxidant stress, 100 microM hydrogen peroxide (H2O2) in Hanks' balanced salt solution (HBSS), or to control condition, HBSS alone, for 30 min. Supplemental PAEC were exposed to HBSS or H2O2 either immediately or 24, 48, or 72 h after supplementation. Supplementation with 18:1 protected PAEC from H2O2-induced injury at all time points. The fatty acid composition of PAEC phospholipid (PL), triglyceride (TG), and free fatty acid (FFA) subclasses was determined using thin layer and gas chromatography. The PL fraction contained the majority of PAEC fatty acids, and H2O2 reduced the polyunsaturates in this fraction regardless of supplementation. Supplementation with 18:1 increased the 18:1 content of PAEC PL, TG, and FFA at all time points, modified other fatty acids to a lesser extent, but failed to alter the overall number of fatty acid double bonds at all time points. These results indicate that modification of double bond number does not fully explain the mechanisms by which changes in lipid composition can modulate oxidant injury.

Mechanism of hydrogen peroxide-induced inhibition of sheep airway cilia

To study the effect of the inflammatory mediator hydrogen peroxide (H2O2) on airway ciliary activity, we measured ciliary beat frequency (CBF) in cultured tracheal explants from sheep. Addition of H2O2 (10(-8) to 10(-4) M) produced a concentration-dependent mean (+/- SEM) decrease in CBF between 11.1 +/- 0.4% (P less than 0.01) and 100 +/- 0% (P less than 0.001); at each concentration, the maximal effect was reached by 20 to 25 min. Between 10(-8) and 10(-6) M H2O2, the decrease in CBF was reversible, lactate dehydrogenase (LDH) release was not significantly increased, and major morphologic lesions were not seen. At higher concentrations of H2O2, incomplete recovery of CBF (10(-5) M) or irreversible ciliostasis (10(-4) M) developed, and a significant increase in LDH and morphologic lesions were present. Catalase (2,000 U/ml) and H-7 (10(-5) M), a protein kinase inhibitor, abolished cilioinhibition produced by H2O2 at 10(-6) M and lower concentrations but not at 10(-5) M and higher concentrations. Phorbol 12-myristate 13-acetate (PMA), a protein kinase C activator, caused a dose-dependent (10(-11) to 10(-5) M), reversible decrease in CBF; this effect was abolished by H-7. We suggest that at nonlethal concentrations, H2O2 inhibits the beat frequency of airway epithelial cilia reversibly, through the activation of second messengers, including protein kinase C. This mechanism might contribute to the previously demonstrated impairment of mucociliary clearance in airway inflammation.

Hypoxia increases the susceptibility of pulmonary artery endothelial cells to hydrogen peroxide injury

The effect of hypoxia on subsequent susceptibility of porcine pulmonary artery endothelial cells (PAEC) to hydrogen peroxide (H2O2) injury was studied. Preexposure of PAEC to hypoxia for 3 or more h significantly increased susceptibility to subsequent H2O2 challenge. Analysis of the activities of antioxidant enzymes and xanthine oxidase/dehydrogenase suggested that changes in these enzymes in hypoxic PAEC were not responsible for the increased susceptibility. However, hypoxia resulted in significant time-dependent decreases in total glutathione at 12 h or more. The rate of glutathione regeneration in diethylmaleate-treated PAEC and the rate of uptake of cystine and glycine were significantly lower during hypoxia. Hypoxia also caused depletion of ATP and NADPH levels in PAEC, but these did not occur until well after hypoxia-enhanced susceptibility to H2O2 injury was demonstrable. Alterations in glutathione levels and enhanced susceptibility were reversible when hypoxic PAEC were returned to normoxia. These results indicate that hypoxia increased the susceptibility to H2O2 injury by decreasing the ability of PAEC to maintain and regenerate cellular glutathione content in response to H2O2 challenge.