Early Phase Of Platelet Activation Research Articles

SHIP1 Controls Internal Platelet Contraction and αIIbβ3 Integrin Dynamics in Early Platelet Activation.

The Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is known to dephosphorylate PtdIns(3,4,5)P3 into PtdIns(3,4)P2 and to interact with several signaling proteins though its docking functions. It has been shown to negatively regulate platelet adhesion and spreading on a fibrinogen surface and to positively regulate thrombus growth. In the present study, we have investigated its role during the early phase of platelet activation. Using confocal-based morphometric analysis, we found that SHIP1 is involved in the regulation of cytoskeletal organization and internal contractile activity in thrombin-activated platelets. The absence of SHIP1 has no significant impact on thrombin-induced Akt or Erk1/2 activation, but it selectively affects the RhoA/Rho-kinase pathway and myosin IIA relocalization to the cytoskeleton. SHIP1 interacts with the spectrin-based membrane skeleton, and its absence induces a loss of sustained association of integrins to this network together with a decrease in αIIbβ3 integrin clustering following thrombin stimulation. This αIIbβ3 integrin dynamics requires the contractile cytoskeleton under the control of SHIP1. RhoA activation, internal platelet contraction, and membrane skeleton integrin association were insensitive to the inhibition of PtdIns(3,4,5)P3 synthesis or SHIP1 phosphatase activity, indicating a role for the docking properties of SHIP1 in these processes. Altogether, our data reveal a lipid-independent function for SHIP1 in the regulation of the contractile cytoskeleton and integrin dynamics in platelets.

Dextrose Effects on Platelet Count and Volume: Implications for Regenerative Medicine.

Platelet-rich plasma (PRP) and hypertonic dextrose solutions are commonly used injectates in regenerative medicine, sometimes used simultaneously. The effects of hypertonic dextrose on platelet lysis and activation have not been previously reported. We tested the effects of escalating dextrose concentration on cell counts and cell volume of platelets and red cells in PRP and whole blood (WB). A prompt partial reduction in platelet count occurred with all dextrose admixtures with either PRP or whole blood, consistent with partial lysis. After the first minute, platelet counts remained stable, suggesting a rapid accommodation of residual platelets to extreme (>2000 mOsm) hypertonicity. A 25% or higher dextrose concentration caused a significant increase in mean platelet volume (MPV), which suggests an early phase of platelet activation. Further investigation is warranted to confirm if platelet lysis or activation has occurred and whether additive clinical benefit may result from hypertonic dextrose injection alone or in combination with PRP.

The Effect of Tirofiban on Fibrinogen/Agonist-Induced Platelet Shape Change and Aggregation

There is evidence linking raised plasma fibrinogen (fib) and platelet hyperactivity with vascular events. One way to inhibit platelets is to block the platelet membrane glycoprotein (GP) IIb/IIIa receptor, which binds circulating fib or von Willebrand factor and cross-links platelets at the final common pathway to platelet aggregation. Tirofiban is a potent and specific fib receptor antagonist, used in the treatment of unstable angina. The authors assessed the effect of tirofiban on spontaneous platelet aggregation (SPA), fib-induced, serotonin (5HT)-induced, and adenosine diphosphate (ADP)-induced aggregation in whole blood by calculating the percentage free platelet count. These various agonists were used alone and in combination. The authors also measured the effect of tirofiban on agonists-induced (ADP, 5HT) platelet shape change (PSC). The effect of fib on PSC was also evaluated in platelet-rich plasma using a high-resolution (0.07 fL) channelyzer. Tirofiban significantly inhibited SPA, fib (2, 4, 8 g/L), ADP, ADP + fib combination, and 5HT-induced aggregation. Tirofiban had no effect on agonist-induced PSC. There was no apparent change in platelet volume with fib. In conclusion, tirofiban does not appear to have an effect on PSC, an early phase of platelet activation. Tirofiban seems to be a nonspecific and an effective inhibitor of platelet aggregation (a later phase of platelet activation) in whole blood. The clinical significance of these findings remains to be established.

Both the ADP receptors P2Y 1 and P2Y 12 , play a role in controlling shape change in human platelets

Two types of ADP receptors, P2Y 1 and P2Y 12 are involved in platelet aggregation. The P2X 1 receptor is also present but its role, in terms of platelet function, is not yet defined. The aim of this study was to establish if the ADP receptors, P2Y 1, P2Y 12 and P2X 1 play a role in controlling platelet shape change (PSC) in human platelets. PSC is an early phase of platelet activation that precedes aggregation. Using a high-resolution channelyzer, PSC was assessed by measuring the median platelet volume (MPV). The P2Y 1 receptor antagonist MRS 2179 (1.06 m 10.25 µmol/l) blocked ADP-induced PSC (by 100%). The median IC 50 was 3.16 µmol/l. MRS 2179 also significantly ( P = 0.01) inhibited PSC induced by the combination of ADP + serotonin (5HT). The P2Y 12 receptor antagonist AR-C69931MX significantly inhibited (at 10s, P = 0.009; 15 s, P = 0.001 and 30 s, P = 0.015) ADP-induced PSC. The P2X 1 receptor antagonist TNP-ATP had no significant effect on ADP- or ADP + 5HT-induced PSC. We conclude that the IC 50 of a P2Y 1 -blocker can be derived because of the high-resolution and reproducibility of the channelyzer technique. In addition to the P2Y 1 purinoceptor, the P2Y 12 receptor appears to be involved in ADP-induced PSC since this process was significantly inhibited by AR-C69931MX. The channelyzer technique may be more reliable than optical aggregometry to assess PSC.

RAFTK/Pyk2 involvement in platelet activation is mediated by phosphoinositide 3-kinase.

Platelet activation by different agonists initiates a signalling cascade involving the phosphorylation of several protein kinases, which control key regulatory events. Previously, we demonstrated that the related adhesion focal tyrosine kinase (RAFTK, Pyk2) was involved in an early phase of platelet activation, independent of integrin and glycoprotein IIb-IIIa activation. In this study, we demonstrate that RAFTK is co-immunoprecipitated with phosphoinositide 3-kinase (PI3K) upon platelet activation, and that thrombin, ADP and collagen induced the phosphorylation of both PI3K and RAFTK. A low dose of thrombin (0.015 U/ml) induced RAFTK phosphorylation and platelet aggregation in a PI3K activity-dependent manner, whereas a high dose of thrombin (0.1 U/ml) induced these events in a PI3K activity-independent manner. ADP and collagen also induced RAFTK phosphorylation and platelet aggregation in a PI3K activity-dependent manner, similar to that of the low-dose thrombin. Furthermore, protein tyrosine phosphatase activity was associated with RAFTK in response to platelet activation, and was found to be that of protein tyrosine phosphatase-2 (SHP-2). The association of SHP-2 with RAFTK was PI3K-dependent and was increased upon RAFTK phosphorylation. Taken together, our results strongly suggest that the involvement of RAFTK in platelet activation is mediated via the PI3K pathway.

Doxazosin, an alpha1-adrenoceptor antagonist, inhibits serotonin-induced shape change in human platelets.

Patients with peripheral vascular disease or diabetes mellitus tend to have elevated circulating levels of naturally occurring platelet agonists like serotonin (5 hydroxytryptamine; 5-HT). This bioamine can induce platelet shape change (PSC) an early phase of platelet activation, which is essentially aspirin resistant. In addition, 5-HT exerts other harmful effects (eg stimulating vascular smooth muscle proliferation and inducing vasoconstriction in atheromatous coronary vessels). The aim of this study was to determine whether doxazosin inhibits 5-HT-induced PSC. Doxazosin is a long acting alpha(1)-adrenoceptor antagonist, used in the treatment of essential hypertension and/or benign prostatic hyperplasia (BPH). Platelet rich plasma (PRP) was prepared from healthy volunteers (n = 8; five males and three females with a median age of 32 years, range: 26-57). Agonists (5-HT, 0.06-0.5; ADP, 0.1-0.2 micromol/l or U46619, a TXA(2)analogue, 0.025-0.05 micromol/l) were added to PRP and aliquots were removed at specific time points for median platelet volume (MPV) measurement (using a high-resolution channelyser). The MPV was used as an indicator of PSC. PRP was also incubated with doxazosin (final concentration: 0.33 microM, a concentration similar to therapeutic plasma levels) prior to the addition of each of the above-mentioned agonists. Doxazosin significantly inhibited (P = 0.007 and P = 0.008, at 30 sec and 60 sec, respectively) the 5-HT-induced increase in MPV. Doxazosin did not significantly inhibit ADP- or U46619-induced PSC. The inhibitory effect of doxazosin seems to be specific to platelet 5-HT(2) receptors, since there was no effect on ADP- or U46619-induced PSC. This inhibition of platelet activation may be an additional, clinically relevant, advantage. Future in vivo studies should consider assessing the effect of doxazosin on 5-HT-induced platelet activation.

Angiotensin II can induce and potentiate shape change in human platelets: effect of losartan

Platelet shape change (PSC) is an early phase of platelet activation that precedes platelet aggregation. This phase of platelet activation is essentially aspirin resistant. PSC was monitored, by measuring the median platelet volume (MPV) using a high resolution channelyser. Angiotensin (Ang) II, added in vitro, caused a significant (P = 0.004) increase in MPV in platelet rich plasma prepared from healthy subjects (n = 14). This increase in MPV was marked (>0.40 fl) in 57% (n = 8) of these subjects and was significantly inhibited (P<0.008) by losartan (a selective Ang II antagonist) at concentrations similar to those achieved in the circulation during treatment. Ang II also significantly enhanced sub-maximal PSC induced by ADP and serotonin in all subjects tested. Losartan significantly (n = 9; P<0.001) inhibited U46619 (a thromboxane A2 analogue)-induced PSC. These findings suggest that losartan, in addition to its blood pressure lowering action, has antiplatelet activity. This property may be clinically relevant because of the increased risk of vascular events in hypertensive patients.

Effect of endothelin-1 on human platelet shape change: reversal of activation by naftidrofuryl

Naftidrofuryl (Praxilene; NAF) significantly improves claudication distance in patients with peripheral vascular disease (PVD). Endothelin-1 (ET-1) is a powerful endogenous vasoconstrictor and the circulating levels of ET-1 are elevated in patients with vascular disease. Platelet rich plasma (PRP) was prepared from healthy volunteers. NAF at concentrations similar to therapeutic levels (3.5-14 w mol/l), inhibited (P < 0.02) platelet activation (as indicated by a fall in median platelet volume, MPV) induced by ET-1 (0.4 w mol/l) alone. NAF also inhibited (P <0.0001) shape change (PSC; an early phase of platelet activation, characterised by an increase in MPV) induced by ET-1 (0.4 w mol/l) in combination with ADP (0.05-0.15 w mol/l) or serotonin (0.03-0.13 w mol/ l). We assessed the effect of ET A (BQ123, 50 nmol/l) or ET B (BQ788, 50 nmol/l) receptor antagonists on PSC induced by ET-1 alone. Both antagonists significantly inhibited PSC. We conclude that ET-1 activates human platelets. Both ET and ET B receptors probably contribute to this response by a complex mechanism that A requires further elucidation. NAF antagonises the action of ET-1 on human platelets. These actions may contribute to the beneficial effects of NAF in PVD.

Shape change in human platelets: measurement with a channelyzer and visualisation by electron microscopy

The increase in median platelet volume (MPV) that occurs following the addition of various agonists can be recorded by high-resolution channelyzers. However, this increase in MPV may not represent the platelet shape change (PSC), an early phase of platelet activation characterised by the formation of pseudopodia. We therefore compared the MPV after adding various agonists (serotonin, ADP or a thromboxane A 2 analogue) with scanning electron micrographs (EM) of the platelets in the same sample. Saline alone did not increase the MPV or alter platelet morphology. In contrast, the selected agonists altered both the MPV and EM. Adrenaline (5.0 mu mol/l) alone did not influence either of these indices of platelet activation. However, when a lower concentration of adrenaline (0.2 mu mol/l) was added in combination with a dose of serotonin (0.02 mu mol/l) that did not affect the MPV on its own, the measured MPV increased significantly and the EM showed pseudopodia formation. Because the channelyzer method is reproducible and suited to serial measurements of the MPV, it may become a useful tool for the investigation of platelet activation.

Erythrocyte Promotion of Platelet Reactivity Decreases the Effectiveness of Aspirin as an Antithrombotic Therapeutic Modality

To the Editor: Valles et al1 report persistent serotonin release from platelets prepared from patients with vascular disease who were taking aspirin (200 to 300 mg/d). These authors point to the potential clinical significance of their observation because serotonin stimulates vascular smooth muscle cell (VSMC) proliferation. Serotonin is also a vasoconstrictor and can induce or enhance platelet shape change (PSC) and aggregation.2 3 4 5 PSC is an early phase of platelet activation that is aspirin resistant and precedes aggregation.2 A link between serotonin and platelet activity was demonstrated in dyslipidemic patients in whom simvastatin (an effective lipid-lowering drug) reduced platelet hyperactivity and corrected the intraplatelet levels of serotonin.3 Furthermore, after PTCA, the addition of ticlopidine (a potent inhibitor of platelet activation) to aspirin and heparin normalized serotonin release from platelets.4 …

Phosphatidylinositol 3,4,5-Trisphosphate-dependent Stimulation of Phospholipase C-γ2 Is an Early Key Event in FcγRIIA-mediated Activation of Human Platelets

Platelets express a single class of Fcgamma receptor (FcgammaRIIA), which is involved in heparin-associated thrombocytopenia and possibly in inflammation. FcgammaRIIA cross-linking induces platelet secretion and aggregation, together with a number of cellular events such as tyrosine phosphorylation, activation of phospholipase C-gamma2 (PLC-gamma2), and calcium signaling. Here, we show that in response to FcgammaRIIA cross-linking, phosphatidylinositol (3,4, 5)-trisphosphate (PtdIns(3,4,5)P3) is rapidly produced, whereas phosphatidylinositol (3,4)-bisphosphate accumulates more slowly, demonstrating a marked activation of phosphoinositide 3-kinase (PI 3-kinase). Inhibition of PI 3-kinase by wortmannin or LY294002 abolished platelet secretion and aggregation, as well as phospholipase C (PLC) activation, indicating a role of this lipid kinase in the early phase of platelet activation. Inhibition of PLCgamma2 was not related to its tyrosine phosphorylation state, since wortmannin actually suppressed its dephosphorylation, which requires platelet aggregation and integrin alphaIIb/beta3 engagement. In contrast, the stable association of PLCgamma2 to the membrane/cytoskeleton interface observed at early stage of platelet activation was fully abolished upon inhibition of PI 3-kinase. In addition, PLCgamma2 was able to preferentially interact in vitro with PtdIns(3,4,5)P3. Finally, exogenous PtdIns(3,4,5)P3 restored PLC activation in permeabilized platelets treated with wortmannin. We propose that PI 3-kinase and its product PtdIns(3,4,5)P3 play a key role in the activation and adequate location of PLCgamma2 induced by FcgammaRIIA cross-linking.

Tyrosine Phosphorylation of the Novel Protein-tyrosine Kinase RAFTK during an Early Phase of Platelet Activation by an Integrin Glycoprotein IIb-IIIa-independent Mechanism

A key regulatory event controlling platelet activation is mediated through the phosphorylation of several cellular proteins by protein-tyrosine kinases. The related adhesion focal tyrosine kinase (RAFTK) is a novel cytoplasmic tyrosine kinase and a member of the focal adhesion kinase (FAK) gene family. FAK phosphorylation in platelets is integrin-dependent, occurs in a late stage of platelet activation, and is dependent on platelet aggregation. In this study, we have investigated the involvement of RAFTK phosphorylation during different stages of platelet activation. Treatment of platelets with thrombin induced, in as early as 10 s, a rapid tyrosine phosphorylation of RAFTK in a time- and concentration-dependent manner. Treatment of platelets with thrombin in the absence of stirring or pretreatment of platelets with RGDS peptide prevented platelet aggregation, but not RAFTK phosphorylation. Furthermore, phosphorylation of RAFTK did not require integrin engagement since platelets treated with the 7E3 inhibitory antibodies that block fibrinogen binding to glycoprotein IIb-IIIa did not inhibit RAFTK phosphorylation. Similarly, platelets treated with LIBS6 antibodies, which specifically activate glycoprotein IIb-IIIa, did not induce RAFTK phosphorylation. Stimulation of platelets by several agonists such as collagen, ADP, epinephrine, and calcium ionophore A23187 induced RAFTK phosphorylation. Tyrosine phosphorylation of RAFTK in platelets is regulated by calcium and is mediated through the protein kinase C pathway. Phosphorylation of RAFTK is dependent upon the formation of actin cytoskeleton as disruption of actin polymerization by cytochalasin D significantly inhibited this phosphorylation. The RAFTK protein appears to be proteolytically cleaved by calpain in an aggregation dependent manner upon thrombin stimulation. These results demonstrate that RAFTK is tyrosine-phosphorylated during an early phase of platelet activation by an integrin- independent mechanism and is not dependent on platelet aggregation, suggesting different mechanisms of regulation for FAK and RAFTK phosphorylation during platelet activation.

The Antiplatelet Effects of a New Nitroderivative of Acetylsalicylic Acid - An In Vitro Study of Inhibition on the Early Phase of Platelet Activation and on TXA2 Production

We studied in vitro the antiplatelet activity of a new nitroderivative chemically related to acetylsalicylic acid: 2 acetoxybenzoate 2-[1-nitroxy-methyl]-phenyl ester (NCX 4016), in order to identify any effects due to the release of nitric oxide and the blockade of cyclo-oxygenase. The effects of scalar doses of NCX 4016 on the early phase of platelet activation, platelet aggregation and thromboxane A2 production were investigated. We observed inhibitory effects of NCX 4016 on platelet adhesion (IC50 = 7.3 x 10(-5) M), platelet cytosolic calcium concentration, assayed by fluorescent probe Fura 2, and the expression of glycoprotein IIb/IIIa (CD41/alpha IIb beta 3) (IC50 = 3.4 x 10(-5) M) and P-selectin (CD62/GMP-140) (IC50 = 4.9 x 10(-5) M) measured by flow cytometry. NCX 4016 also prevented thrombin-induced platelet aggregation (IC50 = 3.9 x 10(-5) M). None of these parameters were affected by acetylsalicylic acid. These inhibitory activities of NCX 4016 were abolished by oxyhaemoglobin and methylene blue. Intracellular cyclic GMP observed during thrombin-induced aggregation was increased by incubation with NCX 4016. These results appear to be attributable to the release of nitric oxide, which activates soluble platelet guanylylcyclase and promotes intracellular cyclic GMP increase. NCX 4016 almost completely inhibited platelet thromboxane A2 production and arachidonic acid-induced platelet aggregation. This also occurred in the presence of oxyhaemoglobin and methylene blue, indicating that its antiplatelet activity can be attributed not only to nitric oxide release but also to cyclo-oxygenase inhibition.

Regulation of the protein tyrosine kinase pp72syk by platelet agonists and the integrin alpha IIb beta 3.

Agonist stimulation of platelets induces multiple waves of tyrosine phosphorylation, several of which are dependent on the integrin alpha IIb beta 3. At least two classes of protein tyrosine kinases are activated during various stages of platelet activation, 1) Src family tyrosine kinases are activated during an early phase of platelet activation by an integrin-independent mechanism and 2) pp125FAK is activated during a late stage of platelet activation, and it is dependent on platelet aggregation mediated by fibrinogen binding to alpha IIb beta 3. In this report, we examined the mechanism of agonist-induced phosphorylation and activation of the tyrosine kinase pp72syk, which is known to couple with immune response receptors in B cells and mast cells. pp72syk was found to be regulated by both agonist and integrin receptors in a pattern distinct from that of pp60src and pp125FAK. Specifically, thrombin induced the tyrosine phosphorylation and activation of pp72syk independent of platelet aggregation. However, full activation of pp72syk required integrin engagement since treatment with antibodies that block fibrinogen binding to alpha IIb beta 3 reduced pp72syk phosphorylation by 40%. Furthermore, fibrinogen binding to alpha IIb beta 3 stimulated directly with an anti-beta 3 antibody activated pp72syk 3-fold and stimulated its tyrosine phosphorylation. Thus, pp72syk is the only platelet tyrosine kinase identified to date that can be directly activated through integrin ligation. In addition, we found that the activation of pp72syk is dependent upon the state of actin polymerization and that pp72syk redistributes to actin-rich cytoskeletal complexes in an aggregation-dependent manner. These results suggest a role for pp72syk in both early, integrin-independent tyrosine phosphorylation events as well as those dependent upon subsequent integrin engagement.

Neutrophil proteinase cathepsin G is proteolytically active on the human platelet glycoprotein Ib-IX receptor: characterization of the cleavage sites within the glycoprotein Ib alpha subunit.

The proteolytic activity of the neutrophil serine-proteinase cathepsin G (CG) on platelet adherence receptors, the glycoprotein (GP) Ib-IX complex and the integrin alpha IIb beta 3, has been investigated. In the range 50 to 200 nmol/l, CG is a potent platelet agonist which induces shape change, granule exocytosis and aggregation. Investigation of the proteolysis of the receptors' subunits during the course of platelet activation by CG was performed by immunoblot analysis of platelet proteins using a panel of specific antibodies. Exposure of platelets for 3 min at 37 degrees C to CG at a concentration that induces full cell activation resulted in an extensive cleavage of the N-terminal region of the extracellular domain of GPIb alpha, the largest (relative molecular mass, M(r), 143,000) of the three subunits constituting the GPIb-IX complex. In contrast, no detectable proteolytic modification of the two other subunits, GPIb beta and GPIX, was detected. Similarly, we observed that neither of the two subunits of the alpha IIb beta 3 receptor were proteolytically modified by CG. Cleavage of GPIb alpha by CG leaves a remnant of the polypeptide chain with M(r) approx. 106,000 in the plasma membrane, while releasing into the extracellular milieu the N-terminal domain with M(r) in the range 40,000 to 46,000. N-terminal sequencing of the CG-derived fragments of GPIb alpha indicated that the Leu275-Tyr276 peptide bond was the primary cleavage site for this proteinase. Proteolysis of GPIb alpha was already detectable at concentrations of CG as low as 25 nmol/l, while with 200 nmol/l the cleavage was detected as soon as 10 s after exposure of platelets to the proteinase. Comparison of the kinetics and concentration dependency for the proteolysis of GPIb alpha and for the activation of platelets by CG showed that cleavage of the GPIb-IX receptor is an early event that accompanies exocytosis and aggregation. Quantitative evaluation of the conversion of GPIb alpha into its membrane fragment indicated that, under optimal conditions, a maximum of approx. 50% of the total GPIb alpha can be affected by proteolysis. However, this proteolysis was > 90% complete when platelets were in the presence of the potent antagonist prostacyclin, suggesting that cellular redistribution of the GPIb-IX receptor may also occur during activation by CG. These results thus indicate that the very early phase of platelet activation by CG is accompanied by extensive modifications in the structure and expression of the GPIb-IX receptor, an effect that might be of functional significance for the interaction of platelets with the vessel wall.