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

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.

Polyunsaturated fatty acid enrichment enhances endothelial cell-induced low-density-lipoprotein peroxidation

Oxidative modification of low-density lipoprotein (LDL) is an important feature in the initiation and progression of atherosclerosis. LDL modification by endothelial cells was studied after supplementation of the cells with oleic acid and polyunsaturated fatty acids (PUFA) of the n-6 and n-3 series. In terms of the lipid peroxidation product [thiobarbituric acid reactive substances (TBARS)] content and diene level of the LDL particle, oleic acid had no significant effect, and linoleic acid was poorly effective. Gamma linolenic acid (C18:3,n-6) and arachidonic acid (C20:4,n-6) increased by about 1.6-1.9-fold the cell-mediated LDL modification. PUFA from the n-3 series, alpha linolenic acid (C18:3,n-3), eicosapentaenoic acid (C20:5,n-3) and docosahexaenoic acid (C22:6,n-3), induced a less marked effect (1. 3-1.6-fold increase). The relative electrophoretic mobility of the LDL particle and its degradation by macrophages were enhanced in parallel. Concomitantly, PUFA stimulated superoxide anion secretion by endothelial cells. The intracellular TBARS content was also increased by PUFA. Comparison of PUFA from the two series indicates a good correlation between LDL oxidative modification, superoxide anion secretion and intracellular lipid peroxidation. The lipophilic antioxidant vitamin E decreased the basal as well as the PUFA-stimulated LDL peroxidation. These results indicate that PUFAs with a high degree of unsaturation of the n-6 and n-3 series could accelerate cell-mediated LDL peroxidation and thus aggravate the atherosclerotic process.

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.

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.

Role of fatty acids and eicosanoids in modulating proteoglycan metabolism in endothelial cells

Endothelial cell dysfunction is considered to be a critical event in the etiology of atherosclerosis. Thus, the preservation of endothelial structure and function are a prerequisite for normal control of vascular permeability properties, mediation of both inflammatory and immunologic responses and the general 'communication' between blood-borne cells and abluminal tissues. Many of these properties can be influenced by proteoglycans present in vascular tissues. There is evidence that selected lipids can be atherogenic by altering endothelial proteoglycan metabolism. Little is known about the role of fatty acids in modulating proteoglycan composition in endothelial cells. Data suggest, however, that linoleic acid in particular can adversely alter proteoglycan metabolism, which may be related to an imbalance in eicosanoid synthesis patterns. These events could be sufficient to disrupt normal endothelial barrier function, initiate smooth muscle migration and proliferation, and result in other metabolic dysfunctions associated with the etiology of vascular diseases such as atherosclerosis. Thus, the focus of this review is on fatty acids and eicosanoids as they may alter proteoglycan metabolism of vascular tissues and in particular of the endothelium.

Nutrition, endothelial cell metabolism, and atherosclerosis

The vascular endothelium that forms an interface between the blood and the surrounding tissues is continuously exposed to both physiologic and pathophysiologic stimuli. These stimuli are often mediated by nutrients that can contribute to the overall function of the endothelial cell in the regulation of vascular tone, coagulation and fibrinolysis, cellular growth and differentiation, and immune and inflammatory responses. Therefore, nutrient-mediated functional changes of the endothelium and the underlying tissues may be significantly involved in the atherosclerotic disease process. There is evidence that individual nutrients or nutrient derivatives may either provoke or prevent metabolic and physiologic perturbations of the vascular endothelium. Preservation of nutrients that exhibit antiatherogenic properties may, therefore, be a critical issue in the preparation and processing of foods. This review focuses on selected nutrients as they affect endothelial cell metabolism and their possible implications in atherosclerosis.

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.