The intracellular action of sphingosine 1-phosphate in GPVI-mediated Ca2+ mobilization in platelets

Ginkgolide B inhibits platelet release by blocking Syk and p38 MAPK phosphorylation in thrombin-stimulated platelets

INTRODUCTION

Atherosclerosis is a chronic vascular inflammatory disease. Platelets play a critic role in the initiation of vascular inflammation in atherosclerosis. In the present study, we investigated the effects of ginkgolide B on the inhibition of platelet release and the potential mechanisms.

METHODS

Experiments were performed in freshly human platelets. Platelet aggregation and ATP release were measured with a Lumi-aggregometer. Thrombin (0.5 U/ml) was used to induce platelet activation. Protein expression and phosphorylation was examined by Western blotting.

RESULTS

The results showed that ginkgolide B significantly suppressed ATP release by 50.8% in thrombin-activated platelets. Ginkgolide B completely abolished the expression of platelet factor 4 (PF4) and CD40 Ligand (CD40L). Moreover, ginkgolide B fully attenuated the phosphorylation of Syk and p38MAPK. Similarly, R788 (a syk inhibitor) and SB203580 (a p38 MAPK inhibitor) inhibited the expression PF4 and CD40L, respectively. Furthermore, the combination of low concentrations of ginkgolide B and R788 or SB203580 has synergistic inhibition on the expression of PF4 and CD40L. Ginkgolide B partially reduced calcium efflux by 52.7% in thrombin-stimulated platelets.

CONCLUSION

Ginkgolide B potently inhibited the expression of PF4 and CD40L in thrombin-activated platelets. Ginkgolide B partially decreased ATP release and Ca(2+) efflux. The mechanism might be associated with the inhibition of Syk and p38 MAPK phosphorylation. These results demonstrated that ginkgolide B might be a promising drug on inhibiting platelet function and reducing inflammation in atherosclerosis.

The functional role of platelets in the regulation of angiogenesis

Functionally, platelets are primarily recognized as key regulators of thrombosis and hemostasis. Upon vessel injury, the typically quiescent platelet interacts with subendothelial matrix to regulate platelet adhesion, activation and aggregation, with subsequent induction of the coagulation cascade forming a thrombus. Recently, however, newly described roles for platelets in the regulation of angiogenesis have emerged. Platelets possess an armory of pro- and anti-angiogenic proteins, which are actively sequestered and highly organized in α-granule populations. Platelet activation facilitates their release, eliciting potent angiogenic responses through mechanisms that appear to be tightly regulated. In conjunction, the release of platelet-derived phospholipids and microparticles has also earned merit as synergistic regulators of angiogenesis. Consequently, platelets have been functionally implicated in a range of angiogenesis-dependent processes, including physiological roles in wound healing, vascular development and blood/lymphatic vessel separation, whilst facilitating aberrant angiogenesis in a range of diseases including cancer, atherosclerosis and diabetic retinopathy. Whilst the underlying mechanisms are only starting to be elucidated, significant insights have been established, suggesting that platelets represent a promising therapeutic strategy in diseases requiring angiogenic modulation. Moreover, anti-platelet therapies targeting thrombotic complications also exert protective effects in disorders characterized by persistent angiogenesis.

External calcium facilitates signalling, contractile and secretory mechanisms induced after activation of platelets by collagen

Platelet activation leads to the initiation of intracellular signalling processes, many of which are triggered by Ca2+. We have studied the involvement of exogenous Ca2+ in platelet response to collagen activation. Platelet suspensions were prepared with and without adding external calcium in the suspension buffers. Activation with collagen (Col-I) was carried out, before and after incubation with cytochalasin B (Cyt-B) to block the actin assembly and the cytoskeletal reorganization. We evaluated changes in (i) tyrosine phosphorylation of proteins, in platelet lysates and associated with the cytoskeletal fraction, (ii) the association of contractile proteins to the cytoskeleton, (iii) expression of intraplatelet substances at the surface, and (iv) cytosolic Ca2+ levels ([Ca2+]i). Ultrastructural evaluation of platelets by electron microscopy was also performed. Platelet activation by Col-I in the absence of added Ca2+ was followed by mild association of actin and other contractile proteins, low phosphorylation of proteins at tyrosine residues, lack of expression of intraplatelet substances at the membrane, and absence of aggregation. In the presence of millimolar Ca2+, Col-I induced intense actin filament formation with association of contractile proteins with the cytoskeleton, resulting in profound morphological changes. Under these conditions, Col-I induced signalling through tyrosine phosphorylation, with increases in the [Ca2+]i, release of intragranule content and aggregation. Inhibiting actin polymerization with Cyt-B prevented all these events. Our data indicates that platelet activation by collagen requires external Ca2+. Studies with Cyt-B indicate that assembly of new actin and cytoskeleton-mediated contraction, both dependent on exogenous Ca2+, are key events for platelet activation by collagen. In addition, our results confirm that entrance of exogenous Ca2+ depends on a functional cytoskeleton.