Different metabolic behavior of long-chain n-3 polyunsaturated fatty acids in human platelets

In vitro and in vivo bimodal effects of docosahexaenoic acid supplements on redox status and platelet function

Docosahexaenoic acid (DHA) is a prominent nutrient of marine lipids. Together with eicosapentaenoic acid, it is recognized as a protective molecule against atherosclerosis and thrombosis through the regulation of blood cell functions, especially platelets. Its high unsaturation index may however make it prone to peroxidation, which is usually considered as deleterious. This short review takes into consideration this possibility related to DHA concentrations both in vitro and in vivo. It is suggested that protective effects of DHA on platelet activation depend on the reduction of oxidative stress, and appear bimodal with the abolishment of such a protection when DHA is used at relatively high concentrations.

In vitro and in vivo bimodal effects of docosahexaenoic acid supplements on redox status and platelet function

Docosahexaenoic acid (DHA) is a prominent nutrient of marine lipids. Together with eicosapentaenoic acid, it is recognized as a protective molecule against atherosclerosis and thrombosis through the regulation of blood cell functions, especially platelets. Its high unsaturation index may however make it prone to peroxidation, which is usually considered as deleterious. This short review takes into consideration this possibility related to DHA concentrations both in vitro and in vivo. It is suggested that protective effects of DHA on platelet activation depend on the reduction of oxidative stress, and appear bimodal with the abolishment of such a protection when DHA is used at relatively high concentrations.

Dietary manipulation of platelet function

Activated platelets contribute to plaque formation within blood vessels in the early and late stages of atherogenesis, and therefore they have been proposed as risk factor for cardiovascular disease. Anti-platelet drugs, such as aspirin, are now the most prescribed pharmacological treatment in Europe. Certain dietary bioactives also beneficially affect platelet function, and with less side effects, albeit that effects are generally more subtle. Therefore, consumption of dietary bioactives could play a role in the prevention of atherothrombotic vascular disease. Here we review the efficacy of dietary treatment strategies, especially those involving certain dietary fatty acids and polyphenols, to modulate platelet function in healthy subjects or in patients with cardiovascular disease. Variation in study populations, small study sizes and lack of comparability between methods to assess platelet function currently limit robust evidence on the efficacy of dietary bioactives in healthy subjects or specific patient groups. Also, limited knowledge of the metabolism of dietary bioactives, and therefore of the bioavailability of bioactive ingredients, restricts our ability to identify the most effective dietary regimes to improve platelet function. Implementation of uniform point-of-care tests to assess platelet function, and enhanced knowledge of the efficacy by which specific dietary compounds and their metabolites affect platelet function, may enable the identification of functional anti-platelet ingredients that are eligible for a health claim, or combined treatment strategies, including both pharmacological anti-platelet treatment as well as dietary intervention, to tackle atherothrombotic vascular disease.

Dose-effect and metabolism of docosahexaenoic acid: pathophysiological relevance in blood platelets

Docosahexaenoic acid (DHA) is known as a major nutrient from marine origin. Considering its beneficial effect in vascular risk prevention, the effect of DHA on blood components, especially platelets, will be reviewed here. Investigating the dose-effect of DHA in humans shows that daily intake lower than one gram/day brings several benefits, such as inhibition of platelet aggregation, resistance of monocytes against apoptosis, and reinforced antioxidant status in platelets and low-density lipoproteins. However, higher daily intake may be less efficient on those parameters, especially by losing the antioxidant effect. On the other hand, a focus on the inhibition of platelet aggregation by lipoxygenase end-products of DHA is made. The easy conversion of DHA by lipoxygenases and the formation of a double lipoxygenation product named protectin DX, reveal an original way for DHA to contribute in platelet inhibition through both the cyclooxygenase inhibition and the antagonism of thromboxane A₂ action.

Inhibition of platelet aggregation by omega-3 polyunsaturated fatty acids is gender specific-Redefining platelet response to fish oils

Existence of gender differences in cardiovascular disease (CVD) following long-chain omega-3 polyunsaturated fatty acid (LCn-3 PUFA) supplementation have suggested that sex hormones play a role in cardio-protection. The objective of this study was to determine gender specific responses in the efficacy of LCn-3 PUFA to inhibit platelet aggregation in vitro. Blood was analyzed for collagen-induced platelet aggregation following pre-incubation with LCn-3 PUFA in healthy adults (n=42). Eicosapentaenoic acid (EPA) was significantly more effective in reducing platelet aggregation compared with docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA). When grouped by gender, this differential pattern was followed in males only. In females, DHA, DPA and EPA were all equally effective. Between group analyses (LCn-3 PUFA vs. gender) showed that both DHA and DPA were significantly less effective in males compared with females. EPA was equally effective in reducing platelet aggregation in both groups. These findings show that significant gender differences exist in platelet aggregation in response to various LCn-3 PUFA treatments.

EPA, but not DHA, decreases mean platelet volume in normal subjects

The first indication of platelet activation is an increase in mean platelet volume (MPV). n-3 FA are known to inhibit platelet function and to reduce the risk for coronary heart disease. The purpose of this study was to determine the effects of EPA and DHA on MPV. Healthy subjects received olive oil placebo for 4 wk and then were randomly assigned to receive 4 g of ethyl esters of either safflower oil (n = 11), EPA (n = 10), or DHA (n = 12) for 4 wk. At the end of placebo run-in and treatment periods, MPV (fL; mean +/- SEM) and platelet count (PLT-CT; 10(3)/microL blood) were measured in the basal state and after ex vivo stimulation with collagen (10 microg/mL), cold (4 degrees C), and heat (37 degrees C). Unlike DHA, EPA lowered MPV as compared with safflower oil (7.2 +/- 0.1 vs. 7.5 +/- 0.1 fL; P < 0.05) and raised PLT-CT (211 +/- 18 vs.192 +/- 18 10(3)/microL; P < 0.05) in the fasting state. Collagen and cold significantly increased MPV whereas heat lowered MPV regardless of treatment. All stimuli decreased PLT-CT. EPA significantly increased platelet EPA (0.2 +/- 0.1 vs. 3.3 +/- 0.4%) and docosapentaenoic acid (DPA; 2.2 +/- 0.3 vs. 2.9 +/- 0.3%) concentrations, but not DHA. DHA treatment significantly increased DHA (1.4 +/- 0.2 vs. 4.1 +/- 0.5%) and DPA (2.0 +/- 0.4 vs. 3.0 +/- 0.4%) concentrations, but not EPA. In conclusion, EPA, but not DHA, reduces platelet activation, an early step in platelet aggregation.

Inhibition of prostaglandin H synthase and activation of 12-lipoxygenase by 8,11,14,17-eicosatetraenoic acid in human endothelial cells and platelets

The effects of the marine fatty acid 20:4n-3, an isomer of arachidonic acid (20:4n-6), have been compared to that of 20:5n-3 on 20:4n-6 oxygenation in human platelets and endothelial cells. In platelets, 20:4n-3 added along with 20:4n-6 was as potent as 20:5n-3 in inhibiting prostaglandin H synthase (PGH synthase) activity. From 2.5- to 10 microM of 20:4n-6, the synthesis of thromboxane B2 and 12-hydroxy-5,8,10-heptadecatrienoic acid, reflecting the PGH/thromboxane synthase activity, was lowered by 5 and 10 microM of both fatty acids. In contrast, 20:4n-3, but not 20:5n-3, strongly stimulated the lipoxygenase activity at each concentration of 20:4n-6 used whatever the amount of 20:4n-3 added. The effects of both n-3 polyunsaturated fatty acids on endothelial cell PGH/prostacyclin synthases were compared after 2- and 24-hr incubation with the cells, leading to moderate (2 hr) and high (24 hr) concentrations of these fatty acids in membrane phospholipids. The incorporation of 20:4n-3 and 20:5n-3 occurred mostly in phosphatidylcholine and phosphatidylethanolamine and did not alter the 20:4n-6 level of phospholipid classes after 2-hr supplementation, whereas it was drastically decreased after 24 hr. The synthesis of prostacyclin obtained after cell stimulation by 0.1 U/mL thrombin was unaffected by the fatty acid modifications induced after 2-hr supplementation, whereas it was strongly depressed after 24 hr. It was concluded that 20:4n-3 is not an agonist for platelet activation, despite its close structural analogy with 20:4n-6, and is as potent as 20:5n-3 in inhibiting PGH synthase activities, showing that the double bond at C5 is not necessary for inhibition. In contrast, the oxygenation of 20:4n-6 by 12-lipoxygenase was stimulated by 20:4n-3 but not by 20:5n-3, which might be related to the efficient oxygenation of 20:4n-3 by this enzyme compared with 20:5n-3.

Effect of prostanoids and their precursors on the aggregation of rainbow trout thrombocytes

The role of prostanoids and their precursor fatty acids in the aggregatory response of thrombocytes (platelet equivalents of fish) from the rainbow trout, Oncorhynchus mykiss, was studied. Aggregation of these cells was induced by the thromboxane mimetic U-46619 or arachidonic acid (AA) in the presence of human or trout fibrinogen. The production of TXB2/3 by thrombocytes in response to stimulation with AA was inhibited by aspirin, ibuprofen, and indomethacin. However, thrombocyte aggregation in response to AA stimulation was not significantly altered by these agents at the concentrations tested (10-100 microM), with the exception of indomethacin at 20 and 40 microM. Effects on cytosolic calcium concentration have been suggested as an alternative mechanism for the inhibitory action of indomethacin on human platelet aggregation. The present study, however, failed to identify this as a mechanism for the inhibition of U-46619-induced trout thrombocyte aggregation by indomethacin. The polyunsaturated fatty acids docosahexaenoic acid and eicosapentaenoic acid both exhibited an inhibitory effect on U-46619-induced thrombocyte aggregation similar to that observed with mammalian platelets. Unlike the case in mammalian hemostasis, prostacyclin inhibited thrombocyte aggregation only at high concentrations (>5 microM). Prostaglandin E2, however, inhibited thrombocyte aggregation at much lower concentrations (>0.01 microM), suggesting that it may be the major inhibitory eicosanoid in trout.

Involvement of lipid peroxidation in platelet signalling

A well-known signalling pathway in blood platelets consists in the release of arachidonic acid (AA) from membrane phospholipids and its specific oxygenation into bioactive derivatives. In particular, cyclic prostaglandin endoperoxides and thromboxane A2 are potent inducers of platelet functions and are produced in greater amounts when the level of lipid hydroperoxides is higher than normal, as 'physiological concentrations' of such peroxides activate the cyclooxygenation of AA. In this context, a lower activity of platelet glutathione peroxidase (GPx), the key-enzyme for the degradation of lipid hydroperoxides, has been reported in aging, which will ensure a longer life span to those peroxides. Accordingly, the biosynthesis of pro-aggregatory prostanoids is elevated in platelets from the elderly. On the other hand, fatty acids from marine origin have been recognized as inhibitors of platelet functions, and they may alter the redox status of cells. They may for instance increase the platelet GPx activity, an effect that can be prevented by antioxidants. Overall, these data point out the relevance of the redox status in platelet functions.

Fish oil fatty acids and human platelets: dose-dependent decrease in dienoic and increase in trienoic thromboxane generation

Dietary enrichment of membrane phospholipids with n-3 (fish-oil-derived) fatty acids has attracted attention as a putative therapeutic regimen for suppression of inflammatory and coagulatory events. Use of n-3 fatty-acid-enriched lipid infusions for parenteral nutrition results in micromolar concentrations of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DCHA) in the plasma-free fatty acid fraction. We investigated the influence of free EPA and DCHA on platelet thromboxane (Tx) A2 and A3 formation by using a recently developed high performance liquid chromatography-ELISA technique for separate quantification of the stable hydrolysis products TxB2 and TxB3. Washed human thrombocytes were incubated with free arachidonic acid (AA; 1 microM), A23187 (0.1 microM) or thrombin (5 U/mL) for stimulation; all regimens provoked large quantities of TxA2 in the absence of TxA3. Simultaneous admixture of free EPA or free DCHA to the incubation medium (concentration range, 0.01-50 microM) largely suppressed platelet TxA2 generation in response to all stimuli used in a dose-dependent manner. The effective concentration with 50% influence of arachidonic acid was 4.2 microM, whereas the inhibitory concentration with 50% effect of EPA and DCHA were both in the same order of magnitude but differed with the nature of the agonist (0.2-7 microM). Platelet (co-)incubation with EPA, but not DCHA, provoked dose-dependent synthesis of n-3-lipid-derived thromboxane: kinetics of formation and absolute quantities of TxA3 approximated 20% of the respective TxA2 data upon stimulation with AA. Both EPA and DCHA dose-dependently suppressed U46619-provoked platelet aggregation. We conclude that EPA and DCHA are potent competitive inhibitors of TxA2 generation by intact platelets, with EPA acting as poor substrate and DCHA being no substrate for the cyclooxygenase/thromboxane synthase complex. Enrichment of the plasma-free fatty acid fraction with n-3 lipids may offer a therapeutic regimen to suppress the synthesis of the potent proaggregatory and vasoconstrictory agent TxA2.

Docosapentaenoic acid (22:5,n-3): metabolism and effect on prostacyclin production in endothelial cells

Eicosapentaenoic acid (EPA, 20:5,n-3) and docosahexaenoic acid (DHA, 22:6, n-3), the two main fatty acids of fish oil, have been shown to inhibit prostacyclin production and to be actively interconverted, leading to the accumulation of docosapentaenoic acid (DPA, 22:5,n-3) in endothelial cell phospholipids. We have investigated the effect of supplementing endothelial cells with DPA on their capacity to produce prostacyclin. We found that endothelial cells incubated for 22 h with 25 microM DPA bound to albumin (fatty acid/albumin ratio of 1.3) produced two-fold less prostacyclin compared to control cells when stimulated with endogenous arachidonic acid-mobilizing agents such as bradykinin and calcium ionophore A23187. Since the formation of prostacyclin from 0.1-15 microM exogenous arachidonic acid was also reduced, it is suggested that prostacyclin inhibition observed in DPA-treated cells might not proceed from a reduction of arachidonic acid availability only. Such an inhibition was already observed after 1 h incubation of the cells with DPA, and with 2-20 times lower DPA concentrations. The inhibition might depend on EPA which was formed by retroconversion of DPA.

Differential effects of n-6 and n-3 polyunsaturated fatty acids on cell growth and early gene expression in Swiss 3T3 fibroblasts

Dietary n-3 polyunsaturated fatty acids (PUFAs) have been found to reduce accelerated cell growth. To study the underlying molecular mechanisms, we evaluated the effects of the n-3 PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) compared with the n-6 PUFA arachidonic acid (AA) on cell growth and early gene mRNA accumulation in Swiss 3T3 fibroblasts. AA significantly increased cell numbers and incorporation of [3H]-thymidine compared with cells treated with EPA and DHA, which did not stimulate cell growth. In contrast to AA and parallel to its effect on cell growth, EPA and DHA did not lead to a pronounced increase in Egr-1 and c-fosmRNA levels. When they were incubated together with AA, both DHA and EPA reduced AA-induced Egr-1 and c-fosmRNA accumulation and incorporation of [3H]-thymidine. We have recently shown that AA strongly increases Egr-1 and c-fosmRNA accumulation 3T3 fibroblasts through its metabolism to prostaglandin E2 (PGE2) and its subsequent activation of protein kinase C (Danesch et al., 1994, J. Biol. Chem., 269:27258-27263). Consistent with the notion that increased PGE2 formation is required for the AA-induced early gene mRNA accumulation, EPA and DHA reduced PGE2 formation from exogenous [14C]-AA by more than 60%, but they did not decrease mRNA levels following stimulation with PGE2. We suggest that, in 3T3 fibroblasts, EPA and DHA antagonize AA-induced early gene mRNA accumulation and cell growth by reducing PGE2 formation.

Influence of polyunsaturated fatty acids on lipid metabolism in human blood mononuclear cells and early biochemical events associated with lymphocyte activation

n-3 and n-6 polyunsaturated fatty acids are involved in the regulation of the immune response. Although different hypotheses related to modifications of arachidonic acid metabolism or alterations at the level of the cell membrane have been put forward to explain their suppressive effect on the lymphocyte growth, their mechanism of action remains largely unknown. Cyclic nucleotide phosphodiesterase (PDE) has been shown to be an important target involved in the control of lymphocyte proliferation. The present study aimed to determine whether in vitro addition of a physiological concentration (5 microM) of n-6 (20:3n-6) or n-3 (18:4n-3, 20:5n-3, 22:6n-3) fatty acids to human peripheral blood mononuclear cells (PBMC) was able to alter the PDE activity of these cells, and especially the PDE increase in response to Con A stimulation. Pretreatment of human PBMC for a short period of time (90 min) with 5 microM of either 20:3n-6, 20:5n-3 or 22:6n-3 was sufficient to induce a significant enrichment of cellular phospholipids in the corresponding fatty acid, whereas 18:4n-3 was poorly incorporated. Either fatty acid significantly increased both cAMP- and cGMP-PDE activities in the cytosolic compartment, the particulate PDE activities being less sensitive to their stimulatory effect. In contrast, they significantly lowered the PDE increase to Con A stimulation. Except 20:5 n-3, the three other fatty acids did not alter significantly the basal or Con A-induced oxygenated metabolism of arachidonic acid (AA), appreciated by the measurement of radioactive eicosanoids formed in [3H]AA-labelled cells. Furthermore, only 20:5n-3 significantly inhibited the lymphoproliferative response to Con A, whereas 16:0, 18:0, 18:1n-9, 20:3n-6 and 20:4n-6 were inactive. The inhibitory effect was not prevented by antioxidant vitamins C and E. The present results suggest that the lymphocyte growth suppressive effect of 20:5n-3 20:5n-3 is very likely to be independent on both the cAMP system and eicosanoid synthesis, and does not seem to involve their conversion to peroxidised products.

Interactions between arachidonic and eicosapentaenoic acids during their dioxygenase-dependent peroxidation

Eicosapentaenoic acid (EPA), a major polyunsaturated fatty acid of fish has been widely proposed as a potential nutrient for decreasing platelet-endothelial cell interactions and the subsequent atherogenesis and thrombogenesis. This is mainly based upon the decrease of arachidonic acid (AA) oxygenation into bioactive molecules like thromboxane A2. In addition, EPA may be oxygenated into its own active derivatives via cell dioxygenases. We report evidence for the requirement of specific peroxides, adequately provided by AA, to allow EPA to be oxygenated into its bioactive products like prostaglandin I3, a prostacyclin mimetic. On the other hand, we present some data that argue for a decreased basal AA dioxygenation (specific peroxidation) by small concentrations of EPA. The interactions between AA and EPA are then dual, EPA being able to counteract AA oxygenation whereas EPA requires AA to be efficiently oxygenated.

Diethyldithiocarbamate (ditiocarb sodium) effect on arachidonic acid metabolism in human mononuclear cells. Glutathione peroxidase-like activity

Diethyldithiocarbamate (DTC), a thiol delivery agent, has been shown to significantly reduce the frequency of primary opportunistic infections in HIV-infected patients. This therapeutic effect has been related to the capacity of DTC to reverse the deleterious effects of the oxidative stress occurring in HIV infection. The influence of DTC on the oxygenated metabolism of arachidonic acid (AA) was investigated in mitogen-stimulated human peripheral blood mononuclear cells (PBMC). Upon incubation with PBMC previously labelled with [3H]AA, Concanavalin A (Con A) markedly increased cyclooxygenase and lipoxygenase activities, within 30 min, as judged by thromboxane B2 (TxB2) and hydroxyeicosatetraenoic acid (HETE) production. Con A activation of [3H]AA platelets also increased 12-HETE production but did not induce any TxB2 synthesis. Micromolar concentrations of DTC, added simultaneously with the mitogen, significantly enhanced the synthesis of HETEs above the Con A-induced level while TxB2-induced synthesis was inhibited but only at DTC concentrations higher than 50 microM. In the presence of nordihydroguaiaretic acid, a lipoxygenase inhibitor, which inhibited the Con A-induced synthesis of HETEs by 78%, DTC no longer stimulated HETE production above the Con A-induced level. Reverse phase HPLC analysis showed that Con A increased the PBMC production of 5-, 12- and 15-HETEs. In the presence of 5 microM DTC, 5-HETE production was entirely suppressed whereas the 15-HETE level was markedly enhanced, 12-HETE production by the contaminating platelets remained unchanged. In vitro experiments indicated that DTC alone did not significantly influence 15-hydroperoxyeicosatetraenoic (15-HPETE) production by the soybean 15-lipoxygenase but, in the presence of added reduced glutathione, DTC markedly reduced 15-HPETE into 15-HETE. In addition, DTC was able to substitute for cellular extract in the glutathione peroxidase (GPx) assay system. Taken together, these results indicate that DTC, through its "GPx-like" activity is able to modify the lipoxygenase cascade. Its ability to selectively reduce 15-HPETE known to stimulate immunosuppressive T-cells might help to explain its positive regulatory effect upon the immune system.

Incorporation and turnover of eicosapentaenoic and docosahexaenoic acids in human blood platelets in vitro

Mass changes in the incorporation of linoleic (C18:2), eicosapentaenoic (C20:5) and docosahexaenoic (C22:6) acids in human blood platelet phospholipids were induced by incubating the cells and these fatty acids complexed to albumin. The remodelling of [14C]C18:2, [14C]C20:5 and [14C]C22:6 in classes, subclasses and molecular species of platelet phospholipids was studied in resting and thrombin-stimulated cells. More than 85% of the incorporation was located in phospholipids, representing 5-fold and 2.5-fold increases in the phospholipid C20:5 and C22:6 endogenous content respectively. Thrombin stimulation induced a 30% degradation of 1-acyl-2-C20:5-glycerophosphocholine (GPC) and 1-acyl-2-C22:6-GPC, but did not induce significant release of C18:2 from 1-acyl-2-C18:2-GPC. There was no change in the [14C]fatty acid composition of 1-alkyl-2-acyl-GPC. Thrombin-dependent increases in 1-alkenyl-2-C20:5-glycerophosphoethanolamine (GPE) and 1-alkenyl-2-C22:6-GPE of 2.1-fold and 2.5-fold respectively accounted for the rise in GPE radioactivity and partly compensated for the loss of these fatty acids from 1,2-diacyl-GPC: transfer to 1-alkenyl-2-acyl-GPE was 0.4 and 1.5 nmol/10(9) platelets for C20:5 and C22:6 respectively. [14C]C20:5 and [14C]C22:6 were incorporated into six different species of 1,2-diacyl-GPC, with acylation in the major endogenous forms (C18:1 +C16:0 and C18:0 species) representing 76% and 66% respectively of the total radioactivity present in 1,2-diacyl-GPC. Stimulation by thrombin induced significant release of these fatty acids from the main molecular species of 1,2-diacyl-GPC, but significantly stimulated the synthesis of alkenyl forms of GPE containing C18:1/C22:6 +C16:0/C22:6, C18:0/C22:6 and C18:0/C20:5. C18:0/C18:2, the major endogenous C18:2 molecular species, represented only 10.5% of the incorporation; none of the [14C]C18:2 molecular species was a substrate for transfer towards 1-alkenyl-2-acyl-GPE. It is concluded that when C20:5 and C22:6, but not C18:2, are acylated in 1,2-diacyl-GPC, they participate in thrombin-dependent phospholipid remodelling, and might compete with the turnover and release of arachidonic acid from platelet phospholipids and the subsequent activation of the cells.

A unique pool of free arachidonate serves as substrate for both cyclooxygenase and lipoxygenase in platelets

Stimulation of platelets induces a rapid release of arachidonate from specific phospholipids and subsequent remodeling of arachidonate-containing phospholipids. This process is accompanied by transformation of released arachidonate by cyclooxygenase and lipoxygenase enzymes. We addressed the question of whether the cyclooxygenase and the lipoxygenase products originated from the same arachidonate-containing phospholipids. [14C]Arachidonate prelabeled platelets were stimulated by thrombin or by ionophore A 23187. We monitored the cyclooxygenase pathway by following 12-hydroxy-5,8,10-heptadecatrienoic acid [12(S)-HHT] formation and the lipoxygenase pathway by following 12-hydroxy-5,8,10,14-eicosatetraenoic acid [12(S)-HETE] formation and compared specific activities. The data showed that the same pool of released arachidonate can be utilized by either cyclooxygenase or by lipoxygenase. Indeed, the specific activity of both products was identical when both enzymes were acting. Since cyclooxygenase was rapidly deactivated while lipoxygenase continued to be active, the specific activity of 12(S)-HETE became lower than the specific activity of 12(S)-HHT when large amounts of 12(S)-HETE were synthesized. Based on comparison of specific activity between phospholipids and oxygenated products, the pools of arachidonate-containing phospholipids involved in the synthesis of oxygenated products are dependent on the amount of arachidonate released.

Influence of dietary fatty acids on phospholipid composition and prostaglandin E synthesis in rat kidneys

The effects of oils with different amounts of n6 and n3 fatty acid precursors and derivatives were evaluated on phospholipid composition and PGE2 synthesis of rat kidneys. Dietary lipids were: olive oil, an olive-blackcurrant-fish oil mixture and a blackcurrant-fish oil mixture. We observed in the kidneys of rats fed the blackcurrant-fish oil mixture a significant decrease in PGE2 synthesis, while arachidonate values did not show significant variations. A decrease of PGE2 synthesis could be due to competitive and inhibitory effects of fatty acids other than arachidonate, observed in the kidney phospholipid composition in our dietary conditions.

[In vitro studies of the effect of different mixture proportions of omega-3 and omega-6 fatty acids on thrombocyte aggregation and thromboxane synthesis in human thrombocytes]

In order to estimate the influence of the tested fatty acids on platelet aggregation, synthesis of prostaglandin E and thromboxane B in vitro, platelet rich plasma (PRP) was incubated with the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), with linoleic acid as representative of the omega-6 fatty acids, as well as with mixtures of EPA and DHA and all fatty acids, resp., with and without addition of alpha-tocopherol. For the determinations, platelets were prepared from blood of young adult male volunteers (age 26.6 +/- 8 years). Platelet aggregation and synthesis of thromboxane were measured after 30 and 60 min of incubation. Smoking habits were not regarded. The incubation of platelets with DHA and EPA itself, as well as the mixture of fatty acids dominated by omega-3 fatty acids (omega-3/omega-6 = 15/1) caused a significant decrease (p less than 0.05) of collagen-induced platelet aggregation. Tocopherol, linoleic acid, and the linoleic-acid-rich mixtures (omega-3/omega-6 = 1/4) caused only a slight inhibition of platelet aggregation. No uniform influence of omega-3 fatty acids could be observed that showed their influence on synthesis of thromboxane to be of importance for the promotion of platelet aggregation. EPA and the mixture of EPA and DHA did decrease thromboxane synthesis significantly (p less than 0.05). On the other hand, single incubation with DHA as well as with linoleic acid rich mixtures caused a statistically not significant increase of rate of the synthesis, which did not increase the aggregation. This observation indicates the formation of less effective TXA3. An influence of tocopherol could also not be observed.

Modulation of prostanoid formation by various polyunsaturated fatty acids during platelet-endothelial cell interactions

Previous studies have reported that polyunsaturated fatty acids (PUFAs) of nutritional interest may influence arachidonic acid (20:4n-6) metabolism in both platelets and endothelium, when tested separately. In the present study, platelets (PL) and cultured endothelial cells (EC) were first pre-enriched with eight different PUFAs for a two hour incubation in the presence of free fatty acid albumin pre-coated with each acid. EC, PL or both cell populations in combination, were then stimulated by thrombin (0.1 U/ml) for five minutes. Prostanoids were extracted, purified by thin-layer chromatography, and TxB2, 6-keto-PGF1 alpha and PGE2 were quantitated by radioimmunoassays. Prostanoids or dihomoprostanoids formed from cyclooxygenase substrates other than 20:4n-6 were measured by gas chromatography-negative chemical ionisation mass-spectrometry (GC-MS). When co-incubated with EC, PL produced less TxB2 (-15 and -85% in the absence and presence of thrombin, respectively). In contrast, 6-keto-PGF1 alpha increased by 189 (basal conditions) and 358% (thrombin stimulation) when PL were added to EC, in agreement with PGH2 transfers from PL to EC. PGE2, produced by both cell populations, reached amounts which roughly represent the sum of those measured in PL and EC alone, except when cells were pre-enriched with linoleic (18:2n-6) and the n-3 family fatty acids (18:3-, 20:5- and 22:6n-3). 6-keto-PGF1 alpha was markedly inhibited by adrenic acid (22:4n-6), while this acid was converted into dihomo-6-keto-PGF1 alpha, the stable metabolite of dihomoprostacyclin. 22:4n-6 also inhibited TxB2 formation and was converted into dihomo-TxA2.(ABSTRACT TRUNCATED AT 250 WORDS)

Albumin-bound docosahexaenoic acid and collagen-induced human platelet reactivity

An in vitro system designed to mimic the effect of various plasma nonesterified (polyunsaturated) fatty acids on platelet function and metabolism was employed. Human platelet aggregation induced by submaximal (1.8 micrograms/ml) collagen stimulation was significantly inhibited by 2 min preincubation with 20 microM albumin-bound docosahexaenoic acid (22:6n-3) (DHA), but not by the other fatty acids tested. [3H]Phosphatidic acid (PA) formation, an indicator of phospholipase C activation following platelet stimulation, was moderately inhibited by eicosapentaenoic acid (20:5n-3), 11,14,17-eicosatrienoic acid (20:3n-3), dihomo-gamma-linolenic acid (20:3n-6), as well as DHA, but not by arachidonic acid (20:4n-6); this inhibition of phospholipase C activation could not explain the differential effect of DHA on platelet aggregation. The decreased production of thromboxane A2 (TxA2), as assessed by [3H]12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) formation, may account for the inhibition of collagen-induced aggregation by 20 microM DHA. Surprisingly, preincubation with 40 microM albumin-bound DHA, even though resulting in greater inhibition of collagen-induced aggregation, had less impact on HHT formation. A small but significant increase in [3H]prostaglandin D2 (PGD2) levels following 3-min collagen stimulation may have contributed to the greater antiaggregatory effect of 40 muM DHA. It is concluded that increased plasma nonesterified DHA may contribute to the dampened platelet activation and altered metabolism following fish oil supplementation of the diet.

Functions and tocopherol content of blood platelets from elderly people after low intake of purified eicosapentaenoic acid

Elderly people present an increased incidence of atherosclerosis and vascular cerebral damages, associated with blood platelet hyperactivity and a stimulation of arachidonic acid metabolism in vivo. The effects of a low intake of purified eicosapentaenoic acid (EPA) on platelet hyperactivity in old human subjects has been investigated. In a randomized, double blind study, 8 people took during 2 months a daily intake of 100 mg of eicosapentaenoic acid (EPA) given as a triglyceride (1,3-dioctanoyl,2-eicosapentaenoyl-glycerol), and 8 other subjects ingested a placebo. A slight, but significant reduction of platelet-rich plasma aggregation in response to epinephrine and arachidonic acid occurred after EPA intake, as well as a decreased aggregation of washed platelets induced by thrombin, although collagen- and U-46619-induced aggregations were not significantly modified. EPA intake failed to affect arachidonic acid metabolism in thrombin-stimulated platelets or in clotted venous blood. The urinary excretion of thromboxane, 6-keto-PGF1 alpha and their 2,3-dinor-metabolites was also not modified. Similarly, no change in the plasma and platelet lipid fatty acid compositions could be observed. Platelet, but not plasma, alpha- and gamma-tocopherol were enhanced by EPA intake. An increase of platelet vitamin E has been associated with a decrease of aggregation, especially in vitamin E-deficient subjects, like elderly people. Therefore, low intake of EPA might have contributed to inhibit platelet aggregation by increasing cellular vitamin E.

Inhibitory effect of stearidonic acid (18:4 n-3) on platelet aggregation and arachidonate oxygenation

The effect of stearidonic acid (18:4 n-3) present in fish and some plant oils, such as black currant seed oil, was studied on human platelets. When added to platelets simultaneously with collagen, arachidonic acid or endoperoxide mimetic U46619, 18:4 n-3 appeared as a weak inhibitor of platelet aggregation. In addition, 18:4 n-3 did not alter the metabolism of exogenous arachidonic acid. In contrast, when preincubated with platelets after precoating onto albumin, 18:4 n-3 inhibited platelet aggregation induced by thrombin, collagen, arachidonic acid or U46619, and was as potent as eicosapentaenoic acid (20:5 n-3) tested under similar conditions. Stearidonic acid also altered the endogenous arachidonate oxygenation stimulated by low doses of thrombin, but to a significantly lesser extent than did 20:5 n-3. It seems therefore that, in addition to competing with endogenous arachidonate metabolism, 18:4n-3 may affect platelet aggregation by another mechanism.

The effect of a fish oil diet on the fatty acid composition of individual phospholipids and eicosanoid production by rat platelets

When rats were fed a diet containing chow or fish oil for six weeks, the platelet phospholipid content and percent distribution were similar. In the fish oil fed animals there was a 54, 40, 41, and 24% reduction, respectively, in the levels of 20:4(n-6) in the choline-, ethanolamine-, inositol- and serine-containing glycerophospholipids. Dietary fish oil increased the total (n-3) polyunsaturated fatty acid content in all lipids. This effect was most pronounced in the ethanolamine glycerophospholipids which now contained 26, 11, and 4 nmols of 20:5(n-3), 22:5(n-3), and 22:6(n-3) in 10(9) cells. Ionophore A23187 stimulation of platelets from the chow fed rats resulted in the synthesis of 7, 64, and 3.5 nmols of 12-hydroxy-5,8,10-heptadecatrienoic acid, 12-hydroxy-5,8,10,14-eicosatetraenoic acid and 12-hydroxy-5,8,10,14,17-eicosapentaenoic acid, respectively, from 1 X 10(9) cells. The values from animals fed fish oil were 4, 18, and 27 nmol/10(9) platelets. It was not possible to detect any lipoxygenase products from 22:5(n-3) or 22:6(n-3), even though both acids are readily metabolized by lipoxygenase when added directly to platelets. These findings suggest that 22-carbon (n-3) fatty acids are not liberated when phospholipases are activated by calcium mobilization.