Arachidonate metabolism in cultured human vascular endothelial cells. Evidence for two prostaglandin synthetic pathways sensitive to acetylsalicylic acid

Effects of various doses of aspirin on platelet activity and endothelial function

Although aspirin has become an established medicine for cardiac and cerebrovascular diseases, the optimal dose remains unknown. We evaluated the optimal dose of aspirin on platelet activity and endothelial function by administering 11 healthy male volunteers (32 ± 6 years of age) doses of aspirin that were increased in a stepwise manner (0, 81, 162, 330 and 660 mg/day) every 3 days. Platelet activity was assessed as surface P-selectin expression (%) measured by flow cytometry and the platelet aggregation ratio. Endothelial function in the brachial artery was assessed by measuring flow-mediated dilation (FMD) before and after reactive hyperemia. Platelet aggregation and P-selectin expression were significantly and dose-dependently suppressed (81-660 mg), and the FMD ratio tended to increase from 0 to 162 mg, but decreased significantly at 660 mg. In conclusion, although aspirin suppressed platelet activity and even surface P-selectin expression, higher doses worsened endothelial-mediated arterial dilation.

Lipopolysaccharide induces prostaglandin H synthase-2 protein and mRNA in human alveolar macrophages and blood monocytes

We and others have previously demonstrated that human alveolar macrophages produce more PGE2 in response to lipopolysaccharide (LPS) than do blood monocytes. We hypothesized that this observation was due to a greater increase in prostaglandin H synthase-2 (PGHS-2) enzyme mass in the macrophage compared to the monocyte. To evaluate this hypothesis, alveolar macrophages and blood monocytes were obtained from healthy nonsmoking volunteers. The cells were cultured in the presence of 0 to 10 micrograms/ml LPS. LPS induced the synthesis of large amounts of a new 75-kD protein in human alveolar macrophages, and a lesser amount in monocytes. Synthesis of this protein required more than 6 h and peaked in 24 to 48 h; the protein reacted with an anti-PGHS-2 antibody prepared against mouse PGHS-2. Associated with synthesis of the protein was a marked increase in LPS-stimulated and arachidonic acid-stimulated synthesis of PGE2 by alveolar macrophages compared to monocytes. Cells not exposed to LPS contained only PGHS-1 and synthesized very little PGE2 during culture or in response to exogenous arachidonic acid. An LPS-induced mRNA, which hybridized to a human cDNA probe for PGHS-2 mRNA, was produced in parallel with production of this new protein and was produced in much greater amounts by alveolar macrophages compared to blood monocytes. This mRNA was not detectable in cells not exposed to LPS. In contrast, both types of cells contain mRNA, which hybridizes to a cDNA probe for PGHS-1. This mRNA did not increase in response to LPS. LPS also had no effect on PGHS-1 protein. These data demonstrate that PGE2 synthesis in human alveolar macrophages and blood monocytes correlates to the mass of PGHS-2 in the cell. We conclude that the greater ability of the macrophage to synthesize PGE2 in response to LPS is due to greater synthesis of PGHS-2 by the macrophage.

Enhancement of the endothelial production of prostacyclin by inhibitors of protein synthesis

1. Pretreatment of bovine aortic endothelial cells with cycloheximide enhanced their capacity to release prostacyclin in response to adenosine 5'-triphosphate (ATP) and bradykinin. 2. The action of cycloheximide was time-dependent; it became detectable after a 1 h exposure to the cells and was maximal after 3 h. 3. Puromycin mimicked the effect of cycloheximide. For these two agents, the enhancement of prostacyclin release was obtained at concentrations producing a partial inhibition of protein synthesis. 4. Cycloheximide increased the mobilization of free arachidonic acid induced by ATP in bovine aortic endothelial cells. 5. In conclusion, the synthesis of new proteins is not involved in the stimulatory action of ATP and bradykinin on prostacyclin production by bovine aortic endothelial cells. Despite the short half-life of prostaglandin H synthase in endothelial cells, cycloheximide and puromycin enhanced the release of prostacyclin induced by agonists. Our data suggest that this release might be under the control of rapidly turning-over phospholipase inhibitory proteins.

Differential localization of von Willebrand factor, fibronectin and 13-HODE in human endothelial cell cultures

Von Willebrand factor (vWF), fibronectin (FN) and 13-hydroxy-octadecadienoic acid (13-HODE) are known to influence the regulation of the adhesive properties of vascular surfaces. In the present study vWF, FN and 13-HODE were comparatively localized in endothelial cells (EC) and in the extracellular matrix (ECM) produced by EC. An indirect immunofluorescent technique was applied to coverslips containing human EC cultures previously fixed and permeabilized following different procedures: A. Alcohol/acetone; B. Paraformaldehyde alone and C. Paraformaldehyde followed by Triton X-100. vWF was observed inside EC (A), on the ECM produced by EC (B) or in EC and ECM (C) depending on the fixation procedures used. FN was mainly localized in the ECM despite the fixation procedures employed. FN was only seen in relation to cell bodies after strong permeabilization (A). Under our experimental conditions 13-HODE was never found in ECM. This latter antigen was observed randomly dispersed in those preparations fixed with alcohol/acetone, indicating that it is probably extracted by this fixative. 13-HODE was detected in granular shaped structures in EC after permeabilization with detergent (C). These results suggest that the cellular localization of vWF and FN is compatible with an adhesive role related to the abluminal side of ECs. 13-HODE was readily observed after mild permeabilization. This finding would be morphologically consistent with its contribution to the regulation of the vessel wall thromboresistance.

Oxidized low density lipoprotein stimulates prostacyclin production by adult human vascular endothelial cells

Interactions between vascular endothelium and low density lipoprotein (LDL) have been implicated in the development of atherosclerosis. The effect of normal and oxidized LDL (Ox-LDL) on prostaglandin release by cultured adult human saphenous vein endothelial cells was investigated. Ox-LDL induced a rapid release of prostacyclin (PGI2) to levels which were several-fold higher than those observed with control LDL. PGI2 release was concentration-dependent and was biphasic, with a first peak occurring within 30 minutes (followed by a decrease), and a second peak occurring after several hours of incubation. PGI2 production was inhibited by lipoprotein-depleted serum and by indomethacin, an antagonist of cyclooxygenase activity. These cells produced mainly PGF2 alpha, with some PGE2 and PGI2 when stimulated by the ionophore A23187 at confluency. However, among these prostanoids, mainly PGI2 was produced in response to Ox-LDL. The data indicate that Ox-LDL induces the production of PGI2 by human vascular endothelial cells. Since Ox-LDL is cytotoxic, this phenomenon may be a manifestation of an early response to injury.

Role of prostaglandins in blood-induced vasoconstriction of canine cerebral arteries

We tested the hypothesis that the vasoconstriction produced by the application of blood to the adventitial surfaces of the vessels of an isolated perfused canine circle of Willis preparation was mediated by products of prostaglandin metabolism. In this preparation (perfused at constant flow and outflow pressure), topical application of blood produced an average 16.6 +/- 1.8 (SE) mm Hg increase in inflow pressure. This response could be prevented with four structurally dissimilar cyclooxygenase inhibitors (aspirin, indomethacin, ibuprofen, and meclofenamate), suggesting that the blood-induced increase in vascular resistance was mediated by prostaglandins. Imidazole, an inhibitor of thromboxane synthetase, had no effect on the blood response. Further support for the involvement of prostaglandins in this response was provided by additional experiments in which either arachidonic acid, prostaglandin E2 (PGE2), or PGF2 alpha were administered. All three treatments produced vasoconstriction. These results suggest that the vessels of this preparation are capable of synthesizing vasoconstrictor prostaglandins and indicate that they are reactive to known vasoactive prostaglandins.

Aspirin in cardiovascular disease

Although other mechanisms may be contributory, the antithrombotic properties of aspirin derive predominantly from its platelet-inhibitory effects. These are mediated via irreversible acetylation of platelet cyclo-oxygenase with subsequent blockade of platelet thromboxane synthesis. Long term administration of doses of aspirin as low as 20mg daily depresses platelet thromboxane formation by more than 90%; however, higher doses appear to be necessary to prevent thromboxane-dependent platelet activation in vivo. While there is evidence of biochemical selectivity with low doses of aspirin, significant reduction of the platelet-inhibitory eicosenoid, prostacyclin, occurs even at dosages ranging from 20 to 40mg daily. The ability of aspirin to prevent the occurrence or recurrence of vaso-occlusion has been extensively investigated. In the secondary prevention of myocardial infarction 7 placebo-controlled trials involving more than 15,000 patients have been completed. The dose of aspirin varied from 300 to 1500mg daily. Although none of the individual trials produced statistically significant reductions in total or coronary mortality, taken together the results are highly suggestive of a beneficial effect of aspirin. Similarly, 2 recent studies in patients with unstable angina demonstrated a protective effect of aspirin against acute myocardial infarction and death. While each study employed widely different doses of aspirin (324mg and 1250mg daily) similar reductions in mortality were reported. The effects of aspirin on the prevention of coronary artery bypass graft occlusion have been evaluated in 9 trials. Aspirin in doses of 100 to 975mg daily was shown to be of benefit in preventing early (less than 6 months) graft occlusion, particularly when therapy was started within 24 hours of operation. In patients with prosthetic vascular grafts of the lower limbs, aspirin has been shown to reduce platelet deposition, however further controlled trials will be required to establish the patient population most likely to benefit and, as in all these studies, the optimum dose of aspirin to employ. In patients with prosthetic heart valves it is clear that aspirin alone is insufficient to prevent thromboembolic complications and when administered as an adjunct to anticoagulant therapy it is associated with a high incidence of bleeding. In contrast, there is convincing evidence from several studies for the efficacy of aspirin in doses of 990 to 1300mg daily in the prevention of stroke and death in patients with transient ischaemic attacks.(ABSTRACT TRUNCATED AT 400 WORDS)

Dietary biotin effects on polyunsaturated fatty acids in chick tissue lipids and prostaglandin E2 levels in freeze-clamped hearts

Chicks were fed a purified diet with 0, 100, 200, 300, 400, or 500 micrograms/kg diet of added biotin to determine the effects of biotin deficiency on polyunsaturated fatty acids in tissue lipids. Body weight was reduced by biotin deficiency and liver and heart biotin levels varied with the biotin in the diet. Fatty acids in liver and lung from biotin-deficient chicks at 15 days contained elevated (P less than .03) 18:3 omega 3 and 18:2 omega 6 but prostaglandin precursors 20:3 omega 6 and 20:4 omega 6 were reduced (P less than .03) in liver lipids. Heart tissues from 15-day-old chicks fed the biotin-deficient diet were low (P less than .03) in 20:3 omega 6. Feeding acetylsalicylic acid in diets containing added biotin (0, 100, 400, and 500 micrograms/kg) did not significantly alter fatty acid levels in chick tissue lipids but significantly reduced plasma prostaglandin E2 (PGE2). Biotin deficiency reduced heart PGE2 levels in 22-day-old chicks. An 8-h fast reduced (P less than .04) 20:4 omega 6 in chick heart total fatty acids.

Interleukin-1 inhibits the synthesis of von Willebrand factor in endothelial cells, which results in a decreased reactivity of their matrix toward platelets

We have studied the influence of recombinant human and murine interleukin-1 (IL-1) on the synthesis and secretion of von Willebrand factor by human endothelial cells. Treatment of endothelial cells with IL-1 caused a decline in the steady-state level of von Willebrand factor mRNA in endothelial cells. This decline resulted in a decreased secretion to the culture medium, a decreased storage of von Willebrand factor in the Weibel-Palade bodies, and a decreased incorporation into the extracellular matrix. As a consequence of the decreased amount of von Willebrand factor in the extracellular matrix we have found a strongly impaired platelet adhesion to these matrices. When the matrices of IL-1-treated cells were incubated with purified von Willebrand factor, their ability to support platelet adhesion was restored. These results suggest that perturbation of endothelial cells by inflammatory mediators like IL-1 results in a decreased adhesion of platelets to the subendothelium owing to a diminished synthesis of von Willebrand factor.

Alteration of uptake and distribution of eicosanoid precursor fatty acids by aspirin

Aspirin is an important drug in the treatment of numerous disorders, especially rheumatic diseases. Its several mechanisms of action include inhibition of prostaglandin production by acetylation of prostaglandin synthetase. To explore further the modulatory effect of aspirin on eicosanoid production, we examined its effect on uptake and incorporation of fatty acids into phospholipids of human peripheral blood monocytes. Aspirin ingestion by normal volunteers inhibited uptake of arachidonic acid and linoleic acid in monocytes cultured for 3 days. Similar inhibition was observed when cultured normal human peritoneal macrophages were treated with aspirin for 3 days. In contrast, monocytes cultured for 12 days from both normal volunteers who had ingested aspirin and normal cells treated with aspirin in vitro for the first 3 days of a 12-day culture period expressed an increased uptake of both arachidonic and linoleic acids. Similarly, incorporation of fatty acid into phosphatidylcholine was depressed in 3-day cultured cells but was increased in 12-day cultured cells. Thus, aspirin, whether administered in vivo or added in vitro, modulates cellular uptake and incorporation of eicosanoid precursor fatty acids and their insertion into membrane phospholipids in cultured human monocytes and macrophages.

Ozone inhibits endothelial cell cyclooxygenase activity through formation of hydrogen peroxide

We have previously demonstrated that a 2H exposure of cultured pulmonary endothelial cells to ozone (0.0-1.0 ppm) in-vitro resulted in a concentration-dependent reduction of endothelial prostacyclin production (90% decrease at the 1.0 ppm level). Ozone-exposed endothelial cells, incubated with 20 uM arachidonate, also demonstrated a significant inhibition of prostacyclin synthesis. To further examine the mechanisms of the inhibition of prostacyclin synthesis, bovine pulmonary endothelial cells were exposed to 1.0 ppm ozone for 2H. A significant decrease in prostacyclin synthesis was found within 5 min of exposure (77 +/- 36% of air-exposed control values, p less than 0.05). Endothelial prostacyclin synthesis returned to baseline levels by 12H after ozone exposure, a time point which was similar to the recovery time of unexposed endothelium treated with 0.5 uM acetylsalicylic acid. Incubation of endothelial cells, previously exposed to 1.0 ppm ozone for 2 hours, with 4 uM PGH2 resulted in restoration of essentially normal prostacyclin synthesis. When endothelial cells were co-incubated with catalase (5 U/ml) during ozone exposure, no inhibition of prostacyclin synthesis was observed. Co-incubation with either heat-inactivated catalase or superoxide dismutase (10 U/ml) did not affect the ozone-induced inhibition of prostacyclin synthesis. These data suggest that H2O2 is a major toxic species produced in endothelial cells during ozone exposure and responsible for the inhibition of endothelial cyclooxygenase activity.

14C-labeled arachidonic acid bioconversion in guinea pig placenta during the last third of gestation

In the present study we investigated the arachidonic acid metabolism in guinea pig placenta during the last third of gestation. Homogenates were incubated with 14C-labeled substrate, and eicosanoid formation was determined using rp HPLC. Arachidonic acid was substantially converted to cyclooxygenase products i.e 6-keto-PGF1 alpha, TxB2, PGF2 alpha, PGE2, PGD2 and 12-HHT. Lipoxygenase activity was also found but of a much lower degree and represented by the mono-hydroxy acids 12-HETE and 15-HETE. The total conversion of arachidonic acid exhibited a progressive rise from day 50 to term, due principally to the increasing part of TxB2, PGE2 and 12-HHT throughout this gestational period and in addition, near term, of 6-keto-PGF1 alpha and PGF2 alpha. These results suggest that there is an increasing concentration and/or activity of cyclooxygenase system enzymes with placental development in guinea pig, which may contribute to the augmented intrauterine availability of prostanoids near parturition. Additional experiments were performed to compare the metabolism of exogenously added 14C-arachidonic acid and endogenously present 12C-arachidonic acid during placental homogenate incubation by means of isotope dilution GC-MS. Although the 14C- and 12C-prostanoid patterns were comparable, the 14C/12C ratios of the prostanoids formed during incubation were significantly different. These data indicate that exogenous arachidonic acid and endogenous arachidonic acid in placental homogenate do not follow up exactly the same metabolic pathway so that the assumption of biochemical identity between exogenous radio-tracer and studied endogenous substrate is not quite true.

Macrophage eicosanoid formation is stimulated by platelet arachidonic acid and prostaglandin endoperoxide transfer

The present study was designed to determine whether platelets transfer arachidonic acid or prostaglandin endoperoxide intermediates to macrophages which may be further metabolized into cyclooxygenase products. Adherent peritoneal macrophages were prepared from rats fed either a control diet or an essential fatty acid-deficient diet, and incubated with a suspension of washed rat platelets. Macrophage cyclooxygenase metabolism was inhibited by aspirin. In the presence of a thromboxane synthetase inhibitor, 7-(1-imidazolyl)heptanoic acid, immunoreactive 6-ketoprostaglandin F1 alpha formation was significantly increased 3-fold. Since this increase was greater (P less than 0.01) than that seen with either 7-(1-imidazolyl)heptanoic acid-treated platelets or aspirin-treated macrophages alone, these results indicate that shunting of endoperoxide from platelets to macrophages may have occurred. In further experiments, macrophages from essential fatty acid-deficient rats were substituted for normal macrophages. Essential fatty acid-deficient macrophages, depleted of arachidonic acid, produced only 2% of the amount of eicosanoids compared to macrophages from control rats. When platelets were exposed to aspirin, stimulated with thrombin, and added to essential fatty acid-deficient macrophages, significantly more immunoreactive 6-ketoprostaglandin F1 alpha was formed than in the absence of platelets. This increased macrophage immunoreactive 6-ketoprostaglandin F1 alpha synthesis, therefore, must have occurred from platelet-derived arachidonic acid. These data indicate that in vitro, in the presence of an inhibition of thromboxane synthetase, prostaglandin endoperoxides, as well as arachidonic acid, may be transferred between these two cell types.

Effect of preoperative antiplatelet drugs on vascular prostacyclin synthesis

Patients undergoing aortocoronary bypass using autogenous saphenous veins were randomly divided into three comparable groups. Group 1 (n = 10) acted as a control, Group 2 (n = 14) received 80 mg of aspirin at midnight before the operation, and Group 3 (n = 12) received 80 mg of aspirin and 75 mg of dipyridamole at midnight and an additional 75-mg dose of dipyridamole at 6 AM. The purpose was to determine which regimen would maximally inhibit platelet function without depressing vascular prostacyclin synthesis. Serum thromboxane A2, saphenous vein wall and aortic wall prostacyclin, platelet aggregation, and bleeding time were measured in all patients. None was restarted on a regimen of aspirin or dipyridamole postoperatively. Aspirin alone and in combination with dipyridamole significantly inhibited thromboxane A2 and platelet aggregation in all treated patients but spared venous prostacyclin synthesis. Aortic prostacyclin synthesis was partially inhibited in both treated groups. Chest tube drainage was comparable in all three groups. These results indicate that the combination of aspirin and dipyridamole offers no measurable advantage over aspirin alone in the perioperative period.

Effect of hypoxia on prostacyclin production in cultured pulmonary artery endothelium

Exposure of cultured bovine pulmonary artery endothelial cells to varying levels of hypoxia (10% or 0% O2) for 4 hours resulted in a significant dose-dependent inhibition in endothelial prostacyclin synthesis (51% and 98%, at the 10% and 0% O2 levels respectively, p less than 0.05, compared to 21% O2 exposure values). Release of 3H-arachidonic acid from cellular pools was not altered by hypoxia. Some of the cells were incubated with arachidonic acid (20 microM for 5 min) or PGH2 (4 microM for 2 min) immediately after exposure. Endothelium exposed to 0% O2, but not to 10% O2, produced significantly less prostacyclin after addition of either arachidonic acid (25 +/- 5% of 21% O2 exposure values, n = 6, p less than 0.01) or PGH2 (31 +/- 3% of 21% O2 exposure values, n = 6, p less than 0.05). These results suggest that hypoxia inhibits cyclooxygenase at the 10% O2 level and both cyclooxygenase and prostacyclin synthetase enzymes at the 0% O2 exposure levels. Exposure of aortic endothelial cells resulted in a 44% inhibition of prostacyclin at the 0% exposure level. No significant alteration in prostacyclin production was found in pulmonary vascular smooth muscle cells exposed to hypoxia. These data suggest that the increased prostacyclin production reported in lungs exposed to hypoxia is not due to a direct effect of hypoxia on the main prostacyclin producing cells of the pulmonary circulation.

Possible inhibitory function of endogenous 15-hydroperoxyeicosatetraenoic acid on prostacyclin formation in bovine aortic endothelial cells

Arachidonic acid is metabolized via the cyclooxygenase pathway to several potent compounds that regulate important physiological functions in the cardiovascular system. The proaggregatory and vasoconstrictive thromboxane A2 produced by platelets is opposed in vivo by the antiaggregatory and vasodilating activity of prostacyclin (prostaglandin I2) synthesized by blood vessels. Furthermore, arachidonic acid is metabolized by lipoxygenase enzymes to different isomeric hydroxyeicosatetraenoic acids (HETE's). This metabolic pathway of arachidonic acid was studied in detail in endothelial cells obtained from bovine aortae. It was found that this tissue produced 6-ketoprostaglandin F1 alpha as a major cyclooxygenase metabolite of arachidonic acid, whereas prostaglandins F2 alpha and E2 were synthesized only in small amounts. The monohydroxy fatty acids formed were identified as 15-HETE, 5-HETE, 11-HETE and 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT). The latter two compounds were produced by cyclooxygenase activity. Nordihydroguaiaretic acid (NDGA), a rather selective lipoxygenase inhibitor and antioxidant blocked the synthesis of 15- and 5-HETE. It also strongly stimulated the cyclooxygenase pathway, and particularly the formation of prostacyclin. This could indicate that NDGA might exert its effect on prostacyclin levels by preventing the synthesis of 15-hydroperoxyeicosatetraenoic acid (15-HPETE), a potent inhibitor of prostacyclin synthetase. 15-HPETE could therefore act as an endogenous inhibitor of prostacyclin production in the vessel wall.

Inhibition by aspirin of human arterial and venous prostacyclin synthesis

Prostacyclin (PGI2) synthesis by human mesenteric arteries and veins was measured ex vivo in 62 patients who received either no medication or a single oral dose of aspirin 40 mg, 75 mg or 300 mg approximately 24 hrs pre-operatively. Each dose of aspirin caused a significant reduction in both arterial and venous PGI2 synthesis compared with the untreated group. Arterial PGI2 synthesis did not differ significantly from venous PGI2 synthesis whether assessed by sample weight or sample area.

The role of thrombin in the regulation of the endothelial prostaglandin production

Prostaglandin synthesis in endothelial cells may be initiated by the addition of exogenous substrate (arachidonic acid) or by addition of thrombin or the CA2+-ionophore A23187, which leads to prostacyclin formation from endogenous substrates. We noticed that endothelial cells produce more than twice the amount of prostacyclin when incubated with thrombin and arachidonic acid together than with arachidonic acid alone. In addition, it was found that the thrombin-induced conversion of endogenous substrates was inhibited by exogenous arachidonic acid. This means that the conversion of exogenous added arachidonic acid to prostacyclin was stimulated by thrombin. This activation of the enzymes involved in prostacyclin synthesis lasted about 5 min and could be inhibited by phospholipase inhibitors such as mepacrine and p-bromophenyl-acylbromide but not by the cAMP analogue dibutyryl cAMP, an inhibitor of arachidonic acid release from cellular phospholipids. These data demonstrate that, in addition to causing release of endogenous substrate, thrombin and the Ca2+-ionophore also activate the enzyme system involved in the further transformation of arachidonic acid.

Prostaglandins, other eicosanoids and endothelial cells

Endothelial cells are an important source of eicosanoid formation in the cardiovascular systems. All major pathways of eicosanoid production have been demonstrated in endothelial cells, yielding significant amounts of prostacyclin (PGI2), PGE2, PGF2 alpha, thromboxane A2, leukotrienes and a number of hydroxy fatty acids. The regulation of eicosanoid formation by endothelial cells is poorly understood. There is evidence that precursors, such as arachidonic acid or prostaglandin endoperoxides, may also be provided by other cell types. Endothelial cell-derived eicosanoids are involved in the regulation of local vessel tone, intravascular platelet activation, cell locomotion and, eventually, cell proliferation. Most of the available information considers PGI2. This compound is the quantitatively dominating eicosanoid in endothelial cells. Major actions of PGI2 include inhibition of platelet activation and aggregation, relaxation of arterial vessels and inhibition of growth-factor release. There is probably a tight interaction with other biologically active mediators which needs further evaluation. This also applies to the clinical significance of eicosanoid-related pathways for the mechanism of action of cardiovascular drugs, such as organic nitrates or acetylsalicylic acid. The unique property of the eicosanoid system to become activated only in response to stimulation, the local nature of this reaction, the multiplicity of products formed and the short half-time of most of them are currently the most significant obstacles to define the role of endothelial cell-derived eicosanoids in clinical practice.

Prostaglandin and hydroxyeicosatetraenoic acid synthesis by human mesenchymal tumors

The metabolism of arachidonic acid was investigated by radioimmunoassay and chromatographic techniques in 5 sarcomas and one embryonal carcinoma of human origin maintained as transplantable tumors in nude mice. The results obtained indicate that: the absolute quantities of arachidonic acid metabolites produced by a given tumor varied between experiments but the overall distribution pattern of these products, in general, remained constant from passage to passage; each tumor showed a different arachidonic acid metabolite profile in quality and quantity; 2 sarcomas of the same histological type could be clearly distinguished by their arachidonic acid metabolites; the predominant product in all tumors was 12-HETE or 15-HETE, whereas thromboxane A2 was synthesized in low quantities by all tumors; PGF2 alpha was synthesized at the highest rate by an alveolar rhabdomyosarcoma; PGE2 synthesis was highest in a malignant fibrous histiocytoma; and total prostaglandin synthesis was low in the chondrosarcoma and synovial-cell sarcomas. All results reported in this study are for the complete tumor which includes both neoplastic and stromal cells. The role that these products play in the biological behavior of mesenchymal tumor cells and normal tissues of the host remains to be determined.

Effect of hydrogen peroxide on prostaglandin production and cellular integrity in cultured porcine aortic endothelial cells

Oxidative damage to the vascular endothelium may play an important role in the pathogenesis of atherosclerosis and aging, and may account in part for reduced vascular prostacyclin (PGI2) synthesis associated with both conditions. Using H2O2 to induce injury, we investigated the effects of oxidative damage on PGI2 synthesis in cultured endothelial cells (EC). Preincubation of EC with H2O2 produced a dose-dependent inhibition (inhibitory concentration [IC50] = 35 microM) of PGI2 formation from arachidonate. The maximum dose-related effect occurred within 1 min after exposure although appreciable H2O2 remained after 30 min (30% of original). In addition, H2O2 produced both a time- and dose-dependent injury leading to cell disruption, lactate dehydrogenase release, and 51Cr release from prelabeled cells. However, in dramatic contrast to H2O2 effects on PGI2 synthesis, loss of cellular integrity required doses in excess of 0.5 mM and incubation times in excess of 1 h. The superoxide-generating system, xanthine plus xanthine oxidase, produced a similar inhibition of PGI2 formation. Such inhibition was dependent on the generation of H2O2 but not superoxide in that catalase was completely protective whereas superoxide dismutase was not. H2O2 (50 microM) also effectively inhibited basal and ionophore A23187 (0.5 microM)-stimulated PGI2 formation. However, H2O2 had no effect on phospholipase A2 activity, because ionophore A23187-induced arachidonate release was unimpaired. To determine the effects on cyclooxygenase and PGI2 synthase, prostaglandin products from cells prelabeled with [3H]arachidonate and stimulated with ionophore A23187, or products formed from exogenous arachidonate were examined. Inhibition of cyclooxygenase but not PGI2 synthase was observed. Incubation of H2O2-treated cells with prostaglandin cyclic endoperoxide indicated no inhibition of PGI2 synthase. Thus, in EC low doses of H2O2 potently inhibit cyclooxygenase after brief exposure whereas larger doses and prolonged exposure are required for classical cytolytic effects. Surprisingly, PGI2 synthase, which is known to be extremely sensitive to a variety of lipid peroxides, is not inhibited by H2O2. Lipid solubility, enzyme location within the EC membrane, or the local availability of reducing factors may explain these results, and may be important determinants of the response of EC to oxidative stress.

Cumulative inhibitory effect of low-dose aspirin on vascular prostacyclin and platelet thromboxane production in patients with atherosclerosis

The relationship between the antithrombotic and antiplatelet effects of aspirin is complex, since aspirin influences other systems that protect against thrombosis as well as inhibiting platelet function. We investigated possible cumulative effects of low-dose aspirin on vascular production of prostacyclin in patients with documented atherosclerotic cardiovascular disease. Candidates for coronary artery vein graft bypass ingested 20 mg of aspirin daily during the week before surgery, and platelet aggregation, platelet formation of thromboxane A2 (TXA2), aortic and saphenous vein production of prostacyclin (PGI2), and hemostatic status were measured at the time of the bypass surgery. Low-dose aspirin markedly inhibited platelet aggregation responses and reduced TXA2 generation by greater than 90%, effects similar to those observed with much higher doses of aspirin. Both aortic and saphenous vein production of PGI2 were inhibited by 50% compared with PGI2 produced by vascular tissues of control subjects who received no aspirin preoperatively (51 +/- 10 pg 6-keto-PGF1 alpha/mg aortic wet weight [mean +/- SEM] in aspirin-treated subjects vs 130 +/- 16 pg/mg in control subjects, and 71 +/- 8 pg/mg saphenous vein wet weight vs 131 +/- 17 pg/mg). Blood loss at surgery was not significantly increased by preoperative low-dose aspirin as measured by chest tube drainage (754 +/- 229 ml in aspirin-treated subjects vs 645 +/- 271 ml in control subjects), hematocrit nadir (31.2 +/- 1.9% vs 31.8 +/- 1.7%), or transfusions (2.2 +/- 1.3 units of red blood cells vs 2.2 +/- 1.7 units).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of inhibitors of eicosanoid synthesis on insulin release by neonatal pancreatic islets

In the pancreatic islet, eicosanoids may arise from both cyclooxygenase- and lipoxygenase-dependent metabolism of arachidonic acid. The inclusion of inhibitors of selective steps in these pathways indicated that in cultured neonatal rat islets, arachidonic acid may be metabolised through both pathways, concurrent with insulin release stimulated by D-glucose, D-glyceraldehyde and 2-ketoisocaproate. The effects of the inhibitors suggested that the products of the lipoxygenase pathway were necessary for the stimulatory effects of nutrients to be observed. In contrast to glucose, where insulin release was stimulated in the presence of inhibitors of cyclooxygenase, the stimulatory action of D-glyceraldehyde, 2-ketoisocaproate and melittin was only minimally affected by these inhibitors, although it was inhibited by lipoxygenase inhibition. These findings support a major stimulatory role for products of the lipoxygenase pathway of arachidonic acid metabolism in nutrient-induced secretion, and a negative or modulatory role of cyclooxygenase pathway products on glucose-stimulated insulin release in the neonatal islet.

Unidirectional transfer of prostaglandin endoperoxides between platelets and endothelial cells

An important determinant of platelet-vessel wall interactions is the local balance of production of endothelial prostacyclin (PGI2) and platelet thromboxane (TX) A2, labile eicosanoids with opposing effects on hemostasis. Disputed evidence suggests that platelet-derived prostaglandin endoperoxide intermediates may be utilized as substrates for vascular PGI2 synthesis. Using several different approaches, we have found that platelets can transfer endoperoxides to cultured endothelial cells for efficient conversion to PGI2, but a reciprocal transfer of endothelial endoperoxides for utilization by platelet thromboxane synthetase does not occur under the same experimental conditions. However, platelets can utilize arachidonic acid released by endothelial cells for lipoxygenase metabolism. We have directly demonstrated the production of [3H]6-keto-PGF1 alpha (the breakdown product of [3H]PGI2) by aspirin-treated endothelial cells in the presence of platelets stimulated with [3H]arachidonic acid. In coincubation experiments using either arachidonate or ionophore A23187 as a stimulus, radioimmunoassay of the net production of arachidonic acid metabolites showed that 6-keto-PGF1 alpha generation by aspirin-treated endothelial cells in the presence of platelets may actually exceed its generation by uninhibited endothelial cells alone. In functional assays, platelet aggregation was inhibited in the presence of aspirin-treated endothelial cells after stimulation with either arachidonate or ionophore A23187. In contrast, the inverse experiments, using aspirin-treated platelets and uninhibited endothelial cells, failed to demonstrate platelet utilization of endothelial endoperoxides for TXA2 production by any of the above methods. These studies thus provide evidence that efficient unidirectional transfer and utilization of platelet-derived endoperoxides for endothelial PGI2 production can occur. This process may serve to amplify PGI2 generation adjacent to areas of vascular injury and permit tight localization of platelet plug formation at these sites.

Specific effect of dietary saturated fat on rat aortic 6-keto-PGF1 alpha synthesis

The effect of dietary lard and corn oil-supplementation on rat aortic 6-keto-PGF1 alpha synthesis from both exogenous and endogenous arachidonic acid was investigated. Only lard-supplementation resulted in any change when compared to rats fed normal chow. Lard-supplementation resulted in both a significant increase in the conversion of exogenous arachidonic acid to 6-keto-PGF1 alpha and a significant decrease in the ratio of linoleate to arachidonate in aortic phospholipids. Basal aortic synthesis of 6-keto-PGF1 alpha from endogenous arachidonic acid was not different for any group. We conclude that at least for the aortic conversion of exogenous arachidonic acid to 6-keto-PGF1 alpha the level of inhibitor linoleic acid in tissue phospholipids may be important.

The effect of calcium on the secretion of factor VIII-related antigen by cultured human endothelial cells

Cultured human endothelial cells derived from the umbilical cord vein are able to release factor VIII-related antigen into the culture medium. The experiments described in this paper show the presence of two pathways for the secretion of factor VIII-related antigen from endothelial cells. There is a basal release of this antigen, independent of the presence of extracellular calcium ions. This release can be inhibited by cycloheximide and is therefore directly related to de novo protein synthesis. Besides this basal release, there is an extra release of factor VIII-related antigen that can be stimulated by thrombin, the Ca2+-ionophore A23187 or phorbol myristate acetate. As demonstrated by immunofluorescence, the stimulus-inducible release originates from storage granules in the cells. This stimulus-inducible release is dependent on extracellular Ca2+ but independent of intracellular cAMP.