Eicosanoid/thromboxane A2-independent and -dependent generation of lysoplasmenylethanolamine via phospholipase A2 in collagen-stimulated human platelets

Agomelatine inhibits platelet aggregation through melatonin receptor-dependent and independent mechanisms

AIMS

Melatonin is known to inhibit platelet aggregation induced by arachidonic acid (AA). In the present study we investigated whether agomelatine (Ago), an antidepressant with agonist activity at melatonin receptor 1 (MT1) and MT2 could reduce platelets aggregation and adhesion.

MAIN METHODS

Human platelets from healthy donors were used to test the in vitro effects of Ago in the presence of different platelet activators. We performed aggregation and adhesion assays, thromboxane B₂ (TxB₂), cAMP and cGMP measurements, intra-platelet calcium registration and flow cytometry assays.

KEY FINDINGS

Our data revealed that different concentrations of Ago reduced AA- and collagen-induced human platelet aggregation in vitro. Ago also reduced AA-induced increase in thromboxane B₂ (TxB₂) production, intracellular calcium levels and P-selectin expression at plasma membrane. The effects of Ago in AA-activated platelets were likely dependent on MT1 as they were blocked by luzindole (a MT1/MT2 antagonist) and mimicked by the MT1 agonist UCM871 in a luzindole-sensitive manner. The MT2 agonist UCM924 was also able to inhibit platelet aggregation, but this response was not affected by luzindole. On the other hand, although UCM871 and UCM924 reduced collagen-induced platelet aggregation and adhesion, inhibition of collagen-induced platelet aggregation by Ago was not mediated by melatonin receptors because it was not affected by luzindole.

SIGNIFICANCE

The present data show that Ago suppresses human platelet aggregation and suggest that this antidepressant may have the potential to prevent atherothrombotic ischemic events by reducing thrombus formation and vessel occlusion.

AMPK-ACC signaling modulates platelet phospholipids and potentiates thrombus formation

AMP-activated protein kinase (AMPK) α1 is activated in platelets on thrombin or collagen stimulation, and as a consequence, phosphorylates and inhibits acetyl-CoA carboxylase (ACC). Because ACC is crucial for the synthesis of fatty acids, which are essential for platelet activation, we hypothesized that this enzyme plays a central regulatory role in platelet function. To investigate this, we used a double knock-in (DKI) mouse model in which the AMPK phosphorylation sites Ser79 on ACC1 and Ser212 on ACC2 were mutated to prevent AMPK signaling to ACC. Suppression of ACC phosphorylation promoted injury-induced arterial thrombosis in vivo and enhanced thrombus growth ex vivo on collagen-coated surfaces under flow. After collagen stimulation, loss of AMPK-ACC signaling was associated with amplified thromboxane generation and dense granule secretion. ACC DKI platelets had increased arachidonic acid-containing phosphatidylethanolamine plasmalogen lipids. In conclusion, AMPK-ACC signaling is coupled to the control of thrombosis by specifically modulating thromboxane and granule release in response to collagen. It appears to achieve this by increasing platelet phospholipid content required for the generation of arachidonic acid, a key mediator of platelet activation.

Inhibition of Collagen-Induced Platelet Aggregation by the Secobutanolide Secolincomolide A from Lindera obtusiloba Blume

Atherothrombosis is one of the main underlying cause of cardiovascular diseases. In addition to treating atherothrombosis with antithrombotic agents, there is growing interest in the role of natural food products and biologically active ingredients for the prevention and treatment of cardiovascular diseases. This study aimed to investigate the effect of secolincomolide A (3) isolated from Lindera obtusiloba Blume on platelet activity and identify possible signaling pathways. In our study, the antiplatelet activities of 3 were measured by collagen-induced platelet aggregation and serotonin secretion in freshly isolated rabbit platelets. Interestingly, 3 effectively inhibited the collagen-induced platelet aggregation and serotonin secretion via decreased production of diacylglycerol, arachidonic acid, and cyclooxygenase-mediated metabolites such as thromboxane B₂ (TXB₂), and prostaglandin D₂ (PGD₂). In accordance with the antiplatelet activities, 3 prolonged bleeding time and attenuated FeCl₃-induced thrombus formation in arterial thrombosis model. Notably, 3 abolished the phosphorylation of phospholipase Cγ2 (PLCγ2), spleen tyrosine kinase (Syk), p47, extracellular signal-regulated kinase 1/2 (ERK1/2), protein kinase B (Akt) by inhibiting the activation of the collagen receptor, glycoprotein VI (GPVI). Taken together, our results indicate the therapeutic potential of 3 in antiplatelet action through inhibition of the GPVI-mediated signaling pathway and the COX-1-mediated AA metabolic pathways.

Phospholipase A2-catalyzed hydrolysis of plasmalogen phospholipids in thrombin-stimulated human platelets

In the present study, phospholipase A(2) (PLA(2))-catalyzed hydrolysis of platelet membrane phospholipids was investigated by measuring PLA(2) activity, phospholipid hydrolysis, arachidonic acid release and choline lysophospholipid production in thrombin-stimulated human platelets. Thrombin-stimulated platelets demonstrated selective hydrolysis of arachidonylated plasmenylcholine and plasmenylethanolamine, with little change in diacyl phospholipids. Accelerated plasmalogen hydrolysis was accompanied by increased arachidonic acid and thromboxane B(2) release and increased lysoplasmenylcholine production. Thrombin stimulation caused an increase in PLA(2) activity measured in the cytosolic fraction with plasmenylcholine only; no increase in activity was measured with phosphatidylcholine. No change in membrane-associated PLA(2) activity was observed with either substrate tested. Pretreatment with the Ca(2+)-independent PLA(2)-selective inhibitor, bromoenol lactone, inhibited completely any thrombin-stimulated phospholipid hydrolysis. Thus, thrombin stimulation of human platelets activates a cytosolic PLA(2) that selectively hydrolyzes arachidonylated plasmalogen phospholipids.

Effects of lipid-esterified conjugated linoleic acid isomers on platelet function: evidence for stimulation of platelet phospholipase activity

The effects of four conjugated linoleic acid (CLA) isomers on in vitro collagen-induced human platelet aggregation and thromboxane (TXB(2), the inactive metabolite of the proaggregatory TXA(2)) production were examined. As the free fatty acid (FFA), 9t, 11t-CLA was the most effective inhibitor of these two processes (I(50)s of 2.2 and 4 microM, respectively) and the 9c, 11c-CLA was the least effective (I(50)s of 8.3 and 37 microM) of the isomers tested. When platelets were preesterified with either 25 microM 9t, 11t-CLA or 9c, 11c-CLA, CLA incorporation in total platelet lipids increased from 0.24% to 0.31% and 0.38%, and most of this increase was found to be in the phosphatidyl choline and phosphatidyl ethanolamine subclasses. The decrease in arachidonic acid (AA) content in total fatty acids or phospholipids was an order of magnitude greater. Furthermore, no significant differences between platelets prelabeled with either 9t, 11t- or 9c, 11c-CLA in the inhibition of collagen-induced aggregation and TXB(2) formation were observed. However, platelets prelabeled with 9c, 11c-CLA stimulated basal TXB(2) production (4-fold) which was not observed with platelets pretreated with either 9t, 11t-CLA, linoleic acid or stearic acid. This enhancement was associated with a 2.4-5-fold increase in the release of endogenous AA. Our results suggest that the presence of a conjugated cis, cis double bond appears to change the lipid environment sufficiently to stimulate the basal platelet phospholipase activity, which in turn increases the formation of TXB(2).

Hemoglobin A0 and alpha-crosslinked hemoglobin (alpha-DBBF) potentiate agonist-induced platelet aggregation through the platelet thromboxane receptor

Chemically modified hemoglobins are potential oxygen-carrying blood substitutes, but their in vivo administration has been associated with a variety of unexpected side events, including increased platelet reactivity. We studied the effects of hemoglobin A0 (HbA0) and alpha-crosslinked hemoglobin (alpha-DBBF) on platelets in vitro. Neither hemoglobin A0 nor alpha-DBBF activated platelets when added alone, but both proteins potentiated submaximal agonist-induced platelet aggregation without increasing other markers of platelet activation such as serotonin secretion. Only agonists that are known to cause release of platelet arachidonic acid (AA) were potentiated while aggregation induced by ADP, which does not release AA, was not potentiated. Blockade of the thromboxane receptor with SQ-29,548 prevented the HbA0-induced and the alpha-DBBF-induced potentiation suggesting that the AA/thromboxane signaling pathway mediates the interaction of platelets with hemoglobin.

Plasmalogens, phospholipases A2 and signal transduction

Several lines of evidence indicate that the breakdown of plasmalogens in neural membranes during neurodegenerative diseases is a receptor-mediated process catalyzed by a plasmalogen-selective phospholipase A2. This enzyme has recently been purified from bovine brain. It does not require Ca2+ and is localized in cytosol. It has a molecular mass of 39 kDa and is strongly inhibited by glycosaminoglycans, with the pattern of inhibition being heparan sulfate > hyaluronic acid > chondroitin sulfate > heparin. This plasmalogen-selective phospholipase A2 is also inhibited by gangliosides and sialoglycoproteins. Substrate specificity and the effects of metal ions, detergents and inhibitors suggest that this phospholipase A2 is different from the well-known 85 kDa Ca(2+)-dependent cytosolic phospholipase A2 that has recently been cloned and is not plasmalogen-selective. The plasmalogen-selective phospholipase A2 may be regulated by glycosaminoglycans and sialoglycoconjugates and may be involved in the regulation of K+ channels. This enzyme, which plays a major role in the release of fatty acids during ischemic injury and reperfusion, shows promise as a major target for drug therapy.

The cleavage of plasmenylethanolamine by phospholipase A2 appears to be mediated by the low affinity binding site of the TxA2/PGH2 receptor in U46619-stimulated human platelets

Two TxA2/PGH2 receptor binding sites linked to different effector systems have recently been identified. Since plasmenylethanolamine represents the major phospholipid reservoir of arachidonic acid (AA) in resting human platelets, we assessed the differential role of these binding sites on plasmenylethanolamine hydrolysis by phospholipase A2 activity upon platelet activation by determining the generation of the corresponding [3H]lysoplasmenylethanolamine. Ethanolamine-containing phospholipids in platelets were pre-labelled with [3H]ethanolamine prior to platelet stimulation with U46619 (1 microM), a TxA2 mimetic, in the presence or absence of S-145, an antagonist of the low affinity TxA2/PGH2 receptor. Labelled platelets were also treated with the TxA2/PGH2 receptor antagonist, GR32191B, prior to washing (which blocks the low affinity site of the receptor) and subsequent stimulation. The above conditions provided for blockage of platelet aggregation but not shape change with U46619. The rise in [3H]lysoplasmenylethanolamine accumulation (170% of unstimulated controls) with U46619 as the agonist was inhibited in platelets pre-treated with S-145 and in platelets washed from GR32191B. Similar findings were also obtained for [3H]lysophosphatidylethanolamine accumulation. The present results indicate that the TxA2-dependent activation of plasmenylethanolamine cleavage by phospholipase A2 in intact human platelets is predominantly linked to the low affinity site of the TxA2/PGH2 receptor and may be important for platelet aggregation but not shape change.