Inhibition of platelet aggregation by unsaturated fatty acids through interference with a thromboxane-mediated process

Large-scale lipidomics profiling reveals characteristic lipid signatures associated with an increased cardiovascular risk

BACKGROUND AND AIMS

Patients with cardiovascular disease (CVD) are at high risk to develop adverse events. The distinct risk of developing adverse cardiovascular (CV) events is not solely explained by traditional risk factors. Platelets are essentially involved in progression of CVD including coronary artery disease (CAD) and platelet hyperreactivity leads to development of adverse CV events. Alterations in the platelet lipidome lead to platelet hyperresponsiveness and thus might alter the individual risk profile. In this study, we investigate the platelet lipidome of CAD patients by untargeted lipidomics and elucidate alterations in the lipid composition of patients with adverse CV events.

METHODS

We characterized the platelet lipidome in a large consecutive CAD cohort (n = 1057) by an untargeted lipidomics approach using liquid chromatography coupled to mass spectrometry.

RESULTS

The platelet lipidome in this study identified 767 lipids and characteristic changes occurred in patients with adverse CV events. The most prominent upregulated lipids in patients with cardiovascular events primarily belong to the class of phospholipids and fatty acyls. Further, upregulated platelet lipids are associated with an increased cardiovascular or bleeding risk and independently associated with adverse events. In addition, alterations of the platelet lipidome are associated with modulation of in vitro platelet functions.

CONCLUSIONS

Our results reveal that the composition of the platelet lipidome is altered in CVD patients with an increased cardiovascular risk and distinct platelet lipids may indicate adverse events. Results of this study may contribute to improved risk discrimination and classification for cardiovascular events in patients with CVD. Main findings of this study and hypothetical impact of altered platelet lipid signatures in patients with adverse cardiovascular events on platelet function and clinical outcome. LPE lysophosphatidylethanolamines, CAR acylcarnitines, FA fatty acids.

Protective Effect of Olive Oil Microconstituents in Atherosclerosis: Emphasis on PAF Implicated Atherosclerosis Theory

Atherosclerosis is a progressive vascular multifactorial process. The mechanisms underlining the initiating event of atheromatous plaque formation are inflammation and oxidation. Among the modifiable risk factors for cardiovascular diseases, diet and especially the Mediterranean diet (MedDiet), has been widely recognized as one of the healthiest dietary patterns. Olive oil (OO), the main source of the fatty components of the MedDiet is superior to the other "Mono-unsaturated fatty acids containing oils" due to the existence of specific microconstituents. In this review, the effects of OO microconstituents in atherosclerosis, based on data from in vitro and in vivo studies with special attention on their inhibitory activity against PAF (Platelet-Activating Factor) actions, are presented and critically discussed. In conclusion, we propose that the anti-atherogenic effect of OO is attributed to the synergistic action of its microconstituents, mainly polar lipids that act as PAF inhibitors, specific polyphenols and α-tocopherol that also exert anti-PAF activity. This beneficial effect, also mediated through anti-PAF action, can occur from microconstituents extracted from olive pomace, a toxic by-product of the OO production process that constitutes a significant ecological problem. Daily intake of moderate amounts of OO consumed in the context of a balanced diet is significant for healthy adults.

Inhibition of platelet-activating factor (PAF)-induced platelet aggregation by fatty acids from human saliva

Experiments were undertaken to identify the nature of a previously identified inhibitor of PAF-induced platelet aggregation (PA) in human saliva. Human saliva fractionated by preparative thin layer chromatography (TLC) yielded a fraction that co-migrated with fatty acids (FAs) and inhibited PAF-induced aggregation of platelets. Synthetic FAs tested for their capacities to inhibit 0.1 nM PAF-induced PA showed that only the cis-unsaturated compounds were inhibitory with activities of some of the polyunsaturated FAs (PUFA) reaching almost 100% at 20 μM. Eicosapentanoic acid (EPA) and 8,11,14-eicosatrienoic acid also deaggregated the PAF-induced aggregates. With the exception of oleic acid (OLA), cis-monounsaturated FAs, and elaidic acid, the trans isomer of OLA, were poor inhibitors. In a direct comparison with other platelet agonists, ADP, thrombin, and ionophore A23187, the active saliva fraction and selected individual (FA) inhibited, to greater or lesser extent, PA induced by each of the agonists. EPA, OLA, linoleic acid (LNA), and the active saliva fraction were potent inhibitors of ADP-induced PA, EPA completely inhibited thrombin-induced PA and the saliva fraction showed only weak - moderate inhibitory activity to both thrombin- and ionophore A23187-induced PA. Other reports of endogenous PAF inhibitors in mammalian tissues are compared to the present results. PAF can trigger and amplify inflammatory cascades suggesting a possible modulation role for cis-unsaturated FAs in some diseases.

Trans-fatty acid promotes thrombus formation in mice by aggravating antithrombogenic endothelial functions via Toll-like receptors

SCOPE

Since excessive intake of trans-fatty acid (TFA) increases the risk of myocardial infarction, we investigated the effects of TFA on thrombus formation using animal and cell culture experiments.

METHODS AND RESULTS

C57BL/6 mice were fed a diet containing TFA or cis-fatty acid (5% each of total calories) or a chow diet for 4 weeks, and thrombus formation was induced in the carotid artery by He-Ne laser irradiation. The high-TFA diet significantly promoted thrombus formation in the carotid artery compared to the chow or cis-fatty acid diet. TFA activated the inflammatory signaling pathway in cultured endothelial cells and in mice; aortic gene expression levels of antithrombogenic molecules, including thrombomodulin and tissue factor pathway inhibitor, were decreased, and the expression levels of prothrombogenic molecules were increased in TFA-treated mice. TFA markedly upregulated the prothrombogenic molecules and downregulated the antithrombogenic molecules in endothelial cells. In addition, TFA induced phosphorylation of c-Jun N-terminal kinase, extracellular signal-regulated kinase, and nuclear factor-κB. The TFA-activated signal pathways and prothrombogenic phenotypic changes of endothelial cells were inhibited by genetic or pharmacological inactivation of Toll-like receptors 2 and 4.

CONCLUSION

TFA aggravates the antithrombogenic phenotypes of vascular endothelial cells via Toll-like receptors and promotes thrombus formation in mice.

Antiplatelet effects of conjugated linoleic acid isomers

Conjugated diene isomers of linoleic acid (CLA) are normal constituents of certain foods and exhibit anticarcinogenic and antiatherogenic properties. In the present study, the effects of several CLA isomers on human platelet aggregation and arachidonic acid metabolism were examined. It was found that 9c,11t-CLA, 10t, 12c-CLA and 13-hydroxy-9c,11t-octadecadienoic acid (13-HODE) inhibited arachidonic acid- and collagen-induced platelet aggregation with I50s in the 5-7 microM range. The nonconjugated 9c, 12c-LA was about 300% and 50%, respectively, less potent an inhibitor with these aggregating agents. Using either thrombin or the calcium ionophore A23187 as aggregating agents, a CLA isomer mix was also found to be more inhibitory than 9c,12c-LA. The 9c,11t- and 10t,12c-CLA isomers as well as the CLA isomer mix inhibited formation of the proaggregatory cyclooxygenase-catalyzed product TXA2, as measured by decreased production of its inactive metabolite [14C]TXB2 from exogenously added [14C]arachidonic acid (I50s=9-16 microM). None of the CLA isomers tested inhibited production of the platelet lipoxygenase metabolite [14C]12-HETE. The additional presence of a hydroxyl group gave opposite results: 13-HODE (I50=3 microM) was about 4-fold more potent a cyclooxygenase inhibitor than the 9c,11t-CLA isomer but 9-HODE was 2- to 3-fold less effective an inhibitor (I50=34 microM) of [14C]TXB2 formation than the corresponding 10t,12c-CLA. In both the aggregation and arachidonic acid metabolism experiments, the inhibitory effects of CLA on platelets were reversible and dependent on the time of addition of either the aggregating agent or the [14C]arachidonic acid substrate. These studies suggest that CLA isomers may also possess antithrombotic properties.

Thromboxane A2 fails to induce proliferation of smooth muscle cells enriched with eicosapentaenoic acid and docosahexaenoic acid

Thromboxane A2 (TXA2) released from aggregating platelets and injured vessel wall stimulates smooth muscle cell proliferation, which may contribute to the development of vascular lesion formation after percutaneous transluminal coronary angioplasty. Polyunsaturated fatty acids (n-3) present in the fish oils have been shown to have anti-atherosclerotic effects. In view of this, we examined the effect of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the active ingredients of fish oils on TXA2 induced smooth muscle cell proliferation. To find out the specificity of these fatty acids we used gamma-linolenic acid (n-6) and oleic acid (n-9) as controls. It was found that TXA2 failed to stimulate proliferation of smooth muscle cells preloaded with EPA or DHA but not with gamma-linolenic acid or oleic acid. Further, when smooth muscle cells were preloaded with both EPA and DHA, they acted together in preventing the TXA2 induced smooth muscle cell proliferation. These results demonstrate that one of the mechanisms by which fish oils may prevent neointima formation is by making smooth muscle cells less responsive to TXA2 induced proliferation of smooth muscle cells.

Hydrogenated fat high in trans monoenes with an adequate level of linoleic acid has no effect on prostaglandin synthesis in rats

Our study was designed to determine whether hydrogenated fat high in trans monoenes concentration affected prostaglandin synthesis. Corn oil (CO), butter (B), hydrogenated vegetable oil (HF) and coating fat (CF) were used in this study. These fats were fed to rats for 10 wk at 10 g/100 g diet. The phospholipid (PL) fatty acid content of platelets, aorta and heart was determined by gas liquid chromatography, and the in vitro aorta production of prostacyclin (PGI2) from exogenous or endogenous arachidonic acid (AA) was measured using the radioimmuno-assay (RIA) method. Serum thromboxane B2 (TXB2) released by platelets as thromboxane A2 (TXA2) during incubation of whole blood was also measured by this method. In the group fed CF, AA was significantly lower in the PL of aorta, platelet and heart, and the ratio 20:3(n-9)/20:4(n-6) was greater than in the groups fed CO, B or HF, indicating that the group fed CF was essential fatty acid (EFA) deficient. Although AA was significantly lower in the aorta and platelet PL of the group fed HF compared to the group fed CO, that difference did not affect the amounts of PGI2 or TXB2 produced in these groups. The group fed CF had significantly less PGI2 and TXB2 released by aorta and platelets than the other groups. This was the result of the reduced level of AA and the presence of higher amounts of 20:3(n-9) acid in the PL, which might act as a competitive inhibitor for cyclooxygenase. The aortic production of PGI2 from exogenous AA did not differ among the groups indicating that prostaglandin synthetase was not affected by the dietary fat. We conclude that the consumption of hydrogenated fats high in trans 18:1 acids with adequate amount of linoleic acid had no effect on the amount of thromboxane or prostacyclin produced by platelet or aorta in vitro.

Effects of isomeric cis and trans eighteen carbon monounsaturated fatty acids on porcine platelet function

The effects of different positional cis and trans isomers (9, 11, 12 and 13) of C(18) monoenoic fatty acids on the response of porcine platelets to collagen and thrombin stimulation were determined. Cis isomers inhibited aggregation in response to 2 microgram/ml or 5 microgram/ml collagen and 0.1 U/ml of thrombin with almost equal effectiveness, with the exception of the 9- and 11-isomers which gave reduced inhibition of aggregation with thrombin and collagen stimulation respectively. All cis isomers inhibited TXA(2) formation (determined as TXB(2)) elicited by collagen with almost equal effectiveness. Trans isomers were less effective than cis in inhibiting collagen induced aggregation particularly at the higher collagen concentrations. Inhibition of TXB(2) formation was less marked with trans isomers but a clear dissociation between the extent of inhibition of aggregation and TXB(2) formation was evident. Cis isomers also inhibited aggregation in response to 0.1 U/ml thrombin whereas trans isomers augmented aggregation but still reduced TXB(2) formation. Isomers did not elicit their effects through incorporation into platelet membrane phospholipids. Aggregation studies were carried out using the impedance method which is more sensitive than the optical method hitherto described, particularly for lipid studies.

Arachidonic acid liberation induced by phosphatidic acid endogenously generated from membrane phospholipids in rabbit platelets

The action of phosphatidic acid generated from membrane phospholipids on phospholipase A2 activation in rabbit platelets was investigated. When [3H]arachidonic acid-labelled platelets were treated with phorbol 12-myristate 13-acetate (PMA) and the membranes isolated from the cells incubated at 37 degrees C with 50 microM CaCl2 and 50 microM guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), both phosphatidic acid production and arachidonic acid liberation increased in PMA- and GTP gamma S-concentration-dependent manners. Ethanol dose-dependently inhibited these responses, accompanied by the formation of phosphatidylethanol. Since propranolol, an inhibitor of phosphatidic acid phosphohydrolase, had no influence on the production of phosphatidic acid, the arachidonic acid liberated does not appear to be derived from diacylglycerol which may be produced from phosphatidic acid through the action of this enzyme. In another approach, treatment of [3H]arachidonic acid-labelled membranes with phospholipase D from Streptomyces chromofuscus induced arachidonic acid liberation as well as phosphatidic acid formation in time- and dose-dependent manners. The former response was suppressed by p-bromophenacyl bromide, a phospholipase A2 inhibitor. These results suggest that phosphatidic acid derived from membrane phospholipids potentiates phospholipase A2 activation and contributes to the amplification of platelet activation.

Inhibitory mechanism of cis-polyunsaturated fatty acids on platelet aggregation: the relation with their effects on Ca2+ mobilization, cyclic AMP levels and membrane fluidity

The in vitro inhibitory effects of cis-polyunsaturated fatty acids, linolenic (18:2 delta 9,12), alpha-linoleic (18:3 delta 9,12,15) and eicosatrienoic (20:3 delta 11,14,17) acid, on bovine platelet aggregation and their inhibitory mechanism were investigated. These fatty acids inhibited platelet aggregation induced by ADP and thrombin to similar extent. Fluorescence analyses with fura-2-loaded platelets showed that, in the concentration ranges that inhibited aggregation, they also inhibited agonist-induced increase in cytoplasmic Ca2+. According to radioimmunoassay study, addition of these fatty acids increased cyclic AMP contents in the presence of theophylline corresponded with their inhibitory effects on aggregation. These fatty acids induced a 1.6-1.8-fold increase over basal concentration of cyclic AMP in the concentration ranges that fully inhibited aggregation. On the other hand, saturated fatty acid, stearic acid, affected neither aggregation nor cyclic AMP levels. As reported previously [1985) Biochim. Biophys. Acta 818, 391), these unsaturated fatty acids induced increase in membrane fluidity in the same concentration range. These results suggest that inhibition of platelet aggregation by cis-polyunsaturated fatty acids is due to the increase in cyclic AMP levels. This increase seems to be due to stimulation of adenylate cyclase which is mediated by membrane perturbation.

Dose responses in platelet fatty acid composition, aggregation and prostanoid metabolism during moderate freshwater fish diet

The dose responses in platelet fatty acid composition, aggregation and thromboxane production and in plasma prostacyclin level during moderate freshwater fish diet was studied in healthy male students (n = 100). There were four fish diet groups eating 0.9, 1.5, 2.3 or 3.8 fish-containing meals per week for 12 weeks. The meals provided about 0.25, 0.5, 0.6 or 1.1 g n-3 fatty acids per day, respectively. The increase of n-3 at the expense of n-6 fatty acids in total platelet lipids took place already with 1.5 weekly fish meals (0.5 g n-3 fatty acids/d). Most of the observed changes in platelet fatty acids were seen already after 5 weeks. ADP-and collagen-induced platelet aggregation was measured from controls and 1.5 and 3.8 fish meals per week groups. The maximum platelet aggregation values of the group with the highest fish intake were significantly lower than in the controls at the end of dietary period. A tendency towards reduced platelet aggregability was observed also in the group eating 1.5 fish meals per weeks. Thromboxane B2 and 6-keto-PGF1 alpha were determined from controls and two groups with highest fish intake. The highest intake was needed to decrease the thromboxane B2 production of clotted blood and the plasma 6-keto-PGF1 alpha concentration. A positive correlation between ADP-induced aggregation and thromboxane B2 production was found. These results show that platelet characteristics can already be modified with a very moderate freshwater fish intake.

Platelet desensitization by arachidonic acid is associated with the suppression of endoperoxide/thromboxane A2 binding to the membrane receptor

The inhibitory mechanism of high levels of exogenously added arachidonic acid on activation of washed human platelets was investigated. While low levels of arachidonic acid (5-10 microM) induced aggregation, ATP secretion and increase in cytoplasmic free Ca2+ concentration (first phase of activation), these platelet responses did not occur significantly at high concentrations (30-50 microM). However, much higher concentrations than 80 microM again elicited these responses (second phase). The first phase of platelet activation was inhibited by cyclooxygenase inhibitor, indomethacin, whereas the second one was independent of such treatment. Thromboxane B2 was produced dose-dependently until reaching a plateau at arachidonic acid concentrations higher than 20 microM, irrespective of the lack of aggregation and secretion at high concentrations. After that the amount of free arachidonic acid which remained unmetabolized in platelets gradually increased. High concentrations of arachidonic acid as well as other polyunsaturated fatty acids caused desensitization of platelets in response to U46619, and also depressed the specific [3H]U46619-binding to the receptor as well as other polyunsaturated fatty acids. The amount free arachidonic acid needed in platelets to suppress [3H]U46619 binding corresponded to that needed to inhibit platelet aggregation. Furthermore, arachidonic acid dose-dependently induced fluidization of lipid phase of platelet membranes as detected by 1,6-diphenyl-1,3,5-hexatriene. These results suggest that the inhibition of platelet response by high levels of arachidonic acid can be attributed to interference with endoperoxide/thromboxane A2 binding to the receptor, probably due to perturbation of the membrane lipid phase due to excess amounts of free arachidonic acid remaining in the membranes.

Influence of dietary fatty acids on membrane fluidity and activation of rat platelets

The apparent steady-state fluorescence anisotropy of DPH- or TMA-DPH-labeled washed rat platelets is strongly affected by factors that also influence the turbidity by these platelet suspensions. Sonicated preparations from platelet lipids have a low turbidity and give anisotropy values which are hardly affected by the experimental conditions. We studied the effect of four high-fat diets on membrane fluidity, lipid composition and activation tendency of washed platelets. The diets contained 50 energy% of oils with different levels of saturated and (poly)unsaturated fatty acids. Only small diet-induced differences in DPH fluorescence anisotropy were found, which were comparable for intact platelets and platelet lipids. These differences were unrelated to the degree of saturation of the dietary fatty acids. Platelets from rats fed mainly saturated fatty acids differed significantly from other diet groups in a higher unsaturation degree of phospholipids and a lower cholesterol/phospholipid ratio, but this was not detected by DPH in terms of decreased anisotropy. These platelets aggregated less than other platelets in response to thrombin or collagen. The lower response to collagen persisted in indomethacin-treated platelets activated with the thromboxane A2 mimetic U46619, indicating a different sensitivity of these platelets for thromboxane A2. We conclude that in rat platelets: (a) the overall membrane fluidity and phospholipid unsaturation degree are subject to strong homeostatic control; (b) steady-state anisotropy with DPH or TMA-DPH label is inadequate to reveal subtile changes in lipid profile; (c) changes in platelet responsiveness to thrombin and thromboxane A2, rather than (plasma) membrane fluidity, determine the effect of dietary fatty acids on platelet aggregation.

Eicosapentaenoic acid and docosahexaenoic acid are antagonists at the thromboxane A2/prostaglandin H2 receptor in human platelets

The present study investigated the mechanism by which eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) inhibit platelet activation induced by thromboxane A2. DHA was found to be more potent than EPA in blocking platelet aggregation induced by the stable thromboxane A2 mimetic, U46619. Furthermore, this inhibition by DHA or EPA was competitive. Binding studies using 3H-U46619 demonstrated that both EPA and DHA interact with the platelet thromboxane receptor. The potency of the inhibition of binding corresponded with that seen for the inhibition of aggregation. These results suggest that thromboxane receptor antagonism may be an important mechanism by which EPA and DHA modulate platelet reactivity in vivo.