Phosphatidylinositol 3'-kinase and tyrosine-phosphatase activation positively modulate Convulxin-induced platelet activation. Comparison with collagen

In this report we have studied the role of phosphatidylinositol 3'-kinase (PI3-K) and tyrosine phosphatase activation on platelet activation by Convulxin (Cvx). Wortmannin, a specific PI3-K inhibitor, and phenylarsine oxide (PAO), a sulfhydryl reagent that inhibits tyrosine phosphatase (PTPase), block Cvx-induced platelet aggregation, granule secretion, inositol phosphate production, and increase in [Ca2+]i. However, PAO does not inhibit Cvx-induced tyrosine phosphorylation of platelet proteins, including Syk and PLCgamma2, but blocked collagen-induced platelet aggregation as well as tyrosine phosphorylation of PLCgamma2. In contrast, Cvx-induced PLCgamma2 tyrosyl phosphorylation was partially inhibited by wortmannin. We conclude that (i) although Cvx and collagen activate platelets by a similar mechanism, different regulatory processes are specific to each agonist; (ii) mechanisms other than tyrosine phosphorylation regulate PLCgamma2 activity; and (iii) besides protein tyrosine kinases, PI3-K (and PTPase) positively modulate platelet activation by both Cvx and collagen, and this enzyme is required for effective transmission of GPVI-Fc receptor gamma chain signal to result in full activation and tyrosine phosphorylation of PLCgamma2 in Cvx-stimulated platelets.     

Humanin analogue,HNG, inhibits platelet activation and thrombus formation by stabilizing platelet microtubules

HNG, a highly potent mutant of the anti‐Alzheimer peptide‐humanin, has been shown to protect against ischaemia‐reperfusion (I/R) injury. However, the underlying mechanism related to platelet activation remains unknown. We proposed that HNG has an effect on platelet function and thrombus formation. In this study, platelet aggregation, granule secretion, clot retraction, integrin activation and adhesion under flow conditions were evaluated. In mice receiving HNG or saline, cremaster arterial thrombus formation induced by laser injury, tail bleeding time and blood loss were recorded. Platelet microtubule depolymerization was evaluated using immunofluorescence staining. Results showed that HNG inhibited platelet aggregation, P‐selectin expression, ATP release, and α_IIbβ₃ activation and adhesion under flow conditions. Mice receiving HNG had attenuated cremaster arterial thrombus formation, although the bleeding time was not prolonged. Moreover, HNG significantly inhibited microtubule depolymerization, enhanced tubulin acetylation in platelets stimulated by fibrinogen or microtubule depolymerization reagent, nocodazole, and inhibited AKT and ERK phosphorylation downstream of HDAC6 by collagen stimulation. Therefore, our results identified a novel role of HNG in platelet function and thrombus formation potentially through stabilizing platelet microtubules via tubulin acetylation. These findings suggest a potential benefit of HNG in the management of cardiovascular diseases.     

P2 receptors and platelet function

Following vessel wall injury, platelets adhere to the exposed subendothelium, become activated and release mediators such as TXA₂ and nucleotides stored at very high concentration in the so-called dense granules. Released nucleotides and other soluble agents act in a positive feedback mechanism to cause further platelet activation and amplify platelet responses induced by agents such as thrombin or collagen. Adenine nucleotides act on platelets through three distinct P2 receptors: two are G protein-coupled ADP receptors, namely the P2Y₁ and P2Y₁₂ receptor subtypes, while the P2X₁ receptor ligand-gated cation channel is activated by ATP. The P2Y₁ receptor initiates platelet aggregation but is not sufficient for a full platelet aggregation in response to ADP, while the P2Y₁₂ receptor is responsible for completion of the aggregation to ADP. The latter receptor, the molecular target of the antithrombotic drugs clopidogrel, prasugrel and ticagrelor, is responsible for most of the potentiating effects of ADP when platelets are stimulated by agents such as thrombin, collagen or immune complexes. The P2X₁ receptor is involved in platelet shape change and in activation by collagen under shear conditions. Each of these receptors is coupled to specific signal transduction pathways in response to ADP or ATP and is differentially involved in all the sequential events involved in platelet function and haemostasis. As such, they represent potential targets for antithrombotic drugs.     

A new platelet receptor specific to type III collagen. Type III collagen-binding protein

Platelet interaction with type III collagen is mediated by several platelet receptors that recognize specific sequences in collagen. We previously described an octapeptide KP*GEP*GPK within the alpha(1)III-CB4 fragment that binds to platelets and specifically inhibits platelet aggregation induced by type III collagen. In this study, we demonstrated that the octapeptide prevented platelet contact and spreading on type III collagen and subendothelium under static and flow conditions. Platelets adhered to the immobilized octapeptide, and anti-bodies directed against other platelet collagen receptors (glycoprotein (GP) Ia/IIa, GP IV, p65, p47) did not impair this adhesion. The platelet octapeptide receptor was identified by ligand blotting as a protein doublet with molecular masses of 68 and 72 kDa and does not correspond to any other already known platelet collagen receptors (GP Ia, GP IV GP VI, and p65). Our results indicate that a specific type III collagen receptor, expressed on the platelet surface, is involved in the first stages of platelet type III collagen interaction.     

The platelet receptor for type III collagen (TIIICBP) is present in platelet membrane lipid microdomains (rafts)

Platelet interactions with collagen are orchestrated by the presence or the migration of platelet receptor(s) for collagen into lipid rafts, which are specialized lipid microdomains from the platelet plasma membrane enriched in signalling proteins. Electron microscopy shows that in resting platelets, TIIICBP, a receptor specific for type III collagen, is present on the platelet membrane and associated with the open canalicular system, and redistributes to the platelet membrane upon platelet activation. After platelet lysis by 1% Triton X-100 and the separation of lipid rafts on a discontinuous sucrose gradient, TIIICBP is recovered in lipid raft-containing fractions and Triton X-100 insoluble fractions enriched in cytoskeleton proteins. Platelet aggregation, induced by type III collagen, was inhibited after disruption of the lipid rafts by cholesterol depletion, whereas platelet adhesion under static conditions did not require lipid raft integrity. These results indicate that TIIICBP, a platelet receptor involved in platelet interaction with type III collagen, is localized within platelet lipid rafts where it could interact with other platelet receptors for collagen (GP VI and α2β1 integrin) for efficient platelet activation. Pascal Maurice and Ludovic Waeckel have contributed equally to this work.     

Interaction between ATP and catecholamines in stimulation of platelet aggregation

Platelets, on activation by endothelial damage, release ADP, ATP, serotonin, epinephrine, and norepinephrine. Although ATP is known to augment the action of norepinephrine in cardiovascular and endocrine systems, the possible interaction between ATP and catecholamines in regulation of platelet reactivity has not been reported. The addition of ATP (1-5 microM) to human platelet-rich plasma did not induce platelet aggregation; however, it selectively augmented the aggregatory response to norepinephrine and epinephrine, but not to serotonin. This potentiating action of ATP was dose dependent and was not due to contamination by, or hydrolysis to, ADP. The action of ATP was blocked by 10 microM of adenosine 3'-phosphate 5'-phosphosulfate, a selective P(2)Y(1) receptor antagonist. ATP alone did not cause release of intracellular Ca(2+), but produced a significant Ca(2+) response in the presence of norepinephrine. In contrast, the P(2)X(1) receptor agonists P(1),P(6)-diadenosine-5' hexophosphate and alpha,beta-methylene-ATP had no effect on norepinephrine-induced platelet aggregation even when added at 100 microM. This synergistic interaction between ATP and norepinephrine in stimulating platelet aggregation may have significant clinical implications and suggests a prothrombotic role for ATP in stress.     

Sex-specific changes in platelet aggregation and secretion with sexual maturity in pigs.

Cardiovascular disease may begin early in adolescence. Platelets release factors contributing to vascular disease. Experiments were designed to test the hypothesis that hormonal transitions associated with sexual maturity differentially affect platelet aggregation and secretion in males and females. Platelets were collected from juvenile (2-3 mo) and sexually mature (adult; 5-6 mo) male and female pigs (n=8/group). Maturation was evidenced by increased weight of reproductive tissue and changes in circulating levels of gonadal hormones. Aggregation to ADP (10 microM) and collagen (6 microg/ml) and ATP secretion to 50 nM thrombin were determined by turbidimetric analysis and bioluminescence, respectively. Total platelet counts, platelet turnover, and mean platelet volume did not change with maturity. Platelet aggregation and ATP secretion decreased in females but increased in males with maturity, whereas total ATP content remained unchanged in platelets from females but increased in platelets from males. Platelet fibrinogen receptor, P-selectin expression, and receptors for sex steroids did not change with sexual maturation. Plasma C-reactive protein and brain-type natriuretic peptide also did not change. Results indicate that changes in platelet aggregation and secretion change with sexual maturity differently in females and males. These observations provide evidence on which clinical studies could be designed to examine platelet characteristics in human children and young adults.     

A platelet secretion pathway mediated by cGMP-dependent protein kinase

Platelet secretion (exocytosis) is critical in amplifying platelet activation, in stabilizing thrombi, and in arteriosclerosis and vascular remodeling. The signaling mechanisms leading to secretion have not been well defined. We have shown previously that cGMP-dependent protein kinase (PKG) plays a stimulatory role in platelet activation via the glycoprotein Ib-IX pathway. Here we show that PKG also plays an important stimulatory role in mediating aggregation-dependent platelet secretion and secretion-dependent second wave platelet aggregation, particularly those induced via Gq-coupled agonist receptors, the thromboxane A2 (TXA2) receptor, and protease-activated receptors (PARs). PKG I knock-out mouse platelets and PKG inhibitor-treated human platelets showed diminished aggregation-dependent secretion and also showed a diminished secondary wave of platelet aggregation induced by a TXA2 analog and thrombin receptor-activating peptides that were rescued by the granule content ADP. Low dose collagen-induced platelet secretion and aggregation were also reduced by PKG inhibitors. Furthermore PKG I knockout and PKG inhibitors significantly attenuated activation of the Gi pathway that is mediated by secreted ADP. These data unveil a novel PKG-dependent platelet secretion pathway and a mechanism by which PKG promotes platelet activation.     

P2X1-mediated ERK2 activation amplifies the collagen-induced platelet secretion by enhancing myosin light chain kinase activation

The ATP-gated P2X1 ion channel is the only P2X subtype expressed in human platelets. Via transmission electron microscopy, we found that P2X1 mediates fast, reversible platelet shape change, secretory granule centralization, and pseudopodia formation. In washed human platelets, the stable P2X1 agonist alpha,beta-methylene ATP (alpha,beta-meATP) causes rapid, transient (2-5 s), and dose-dependent myosin light chain (MLC) phosphorylation, requiring extracellular Ca2+. Phosphorylation was inhibited by the calmodulin (CaM) inhibitor W-7, but not by the Rho kinase inhibitor HA-1077, i.e. it is exclusively regulated by Ca2+/CaM-dependent MLC kinase. Correspondingly, the P2X1-induced platelet shape change was inhibited by W-7 and by the MLC kinase inhibitor ML-7 but not by HA-1077. W-7, ML-7, the protein kinase C inhibitor GF109203-X, and the Src family kinase inhibitor PP1 inhibited the collagen and convulxin-induced early platelet degranulation, shape change, and subsequent aggregation, indicating a role for Ca2+/CaM and MLC kinase in these glycoprotein VI-related platelet responses. The secreted ATP-mediated P2X1-dependent ERK2 activation induced by low collagen concentrations contributes to MLC kinase activation since P2X1 desensitization or blockade of ERK2 phosphorylation by U0126 strongly attenuated MLC phosphorylation, degranulation, and aggregation. We therefore conclude that at low doses of collagen, glycoprotein VI activation leads to early protein kinase C- and MLC kinase-dependent degranulation. Rapidly released ATP triggers P2X1 -mediated Ca2+ influx, activating ERK2, in turn amplifying platelet secretion by reinforcing the early MLC kinase phosphorylation. Hence, the P2X1-ERK2-MLC axis contributes to collagen-induced platelet activation by enhancing platelet degranulation.     

Identification of glycoprotein IV (CD36) as a primary receptor for platelet-collagen adhesion

The role of glycoprotein IV (GPIV) in platelet activation processes has been examined by several different approaches: (i) Fab fragments of a monospecific polyclonal antibody to purified platelet GPIV (approximately 20 micrograms/ml) completely inhibited platelet shape change, aggregation, and secretion induced by collagen. Aggregation and secretion by ADP (but not shape change) and by epinephrine were also inhibited, but there was no effect on platelet activation induced by thrombin, arachidonate, or ionophore A23187. (ii) Purified GPIV was able to compete completely with membrane-bound GPIV to inhibit platelet activation induced by collagen, including shape change, but not in activation induced by any of the other platelet agonists. 50% inhibition of collagen-induced activation and secretion were obtained at GPIV concentrations of approximately 10 nM (1 micrograms/ml). (iii) Purified GPIV bound rapidly and reversibly to collagen Type I fibrils, and binding was not inhibited by adhesive proteins such as denatured collagen, fibronectin, fibrinogen, or von Willebrand factor. The direct binding of purified GPIV to collagen Type I fibrils fit best to a single site model with Kd 0.34 +/- 0.10 nM. (iv) Using a microtiter assay, platelet adhesion to collagen was shown to be inhibited by Fab fragments of monospecific polyclonal anti-GPIV antibodies, but adhesion to other adhesive proteins was unaffected. (v) When anti-GPIV was added at various times during adhesion the time dependence of inhibition was seen to be biphasic. Anti-GP antibody was able to reverse adhesion that occurred within the first 5-8 min and to inhibit adhesion occurring thereafter. These results demonstrate that GPIV mediates the early stages of platelet recognition by and attachment to collagen but that there may be a second GPIV-independent mechanism that mediates the subsequent anchorage of these adherent platelets.     

Analysis of the interaction of platelet collagen receptor glycoprotein VI (GPVI) with collagen. A dimeric form of GPVI,but not the monomeric form,shows affinity to fibrous collagen

Glycoprotein VI (GPVI) is a platelet-specific glycoprotein that has been indicated to react with collagen and activate platelets. Its structure was recently identified by cDNA cloning (Clemetson, J. M., Polgar, J., Magnenat, E., Wells, T. N., and Clemetson, K. J. (1999) J. Biol. Chem. 274, 29019-29024). However, the mechanism of the interaction between collagen and GPVI has not been analyzed in detail because both collagen and GPVI are insoluble molecules. In this study, we expressed the extracellular domain of GPVI as soluble forms as follows: the monomeric form (GPVIex) and the dimeric form of GPVI fused with the human immunoglobulin Fc domain (GPVI-Fc(2)). Purified GPVIex strongly inhibited convulxin (Cvx)-induced platelet aggregation but only weakly inhibited that induced by collagen-related peptide. However, only GPVI-Fc(2), and not GPVIex, inhibited collagen-induced platelet aggregation. The dimeric form of GPVI exhibits high affinity for collagen, as concluded from measurements of GPVI binding to immobilized collagen by both the enzyme-linked immunosorbent assay and surface plasmon resonance methods. GPVI-Fc(2) bound to the surface of immobilized collagen with a dissociation constant (K(D)) of 5.76 x 10(-7) m, but the binding of GPVIex was too weak to allow estimation of this parameter. Cvx did not inhibit the binding of dimeric GPVI to collagen, indicating that the binding site of GPVI to collagen was different from that to Cvx. Taken together, our data indicate that the high affinity binding site for collagen is composed from two chains of GPVI. Furthermore, they suggest that the binding sites for Cvx are different from the collagen-binding sites and do not need to be formed by two GPVI molecules. Because dimeric GPVI is the only form that shows high affinity to fibrous collagen, our results indicate that GPVI would be present as a dimeric form on the platelet. Moreover, surface plasmon resonance indicated that there is no detectable interaction between soluble collagen and GPVI, supporting our previous observation that GPVI only reacts with fibrous collagen.     

Dual role of the p38 MAPK/cPLA2 pathway in the regulation of platelet apoptosis induced by ABT-737 and strong platelet agonists

p38 Mitogen-activated protein (MAP) kinase is involved in the apoptosis of nucleated cells. Although platelets are anucleated cells, apoptotic proteins have been shown to regulate platelet lifespan. However, the involvement of p38 MAP kinase in platelet apoptosis is not yet clearly defined. Therefore, we investigated the role of p38 MAP kinase in apoptosis induced by a mimetic of BH3-only proteins, ABT-737, and in apoptosis-like events induced by such strong platelet agonists as thrombin in combination with convulxin (Thr/Cvx), both of which result in p38 MAP kinase phosphorylation and activation. A p38 inhibitor (SB202190) inhibited the apoptotic events induced by ABT-737 but did not influence those induced by Thr/Cvx. The inhibitor also reduced the phosphorylation of cytosolic phospholipase A₂ (cPLA2), an established p38 substrate, induced by ABT-737 or Thr/Cvx. ABT-737, but not Thr/Cvx, induced the caspase 3-dependent cleavage and inactivation of cPLA2. Thus, p38 MAPK promotes ABT-737-induced apoptosis by inhibiting the cPLA2/arachidonate pathway. We also show that arachidonic acid (AA) itself and in combination with Thr/Cvx or ABT-737 at low concentrations prevented apoptotic events, whereas at high concentrations it enhanced such events. Our data support the hypothesis that the p38 MAPK-triggered arachidonate pathway serves as a defense mechanism against apoptosis under physiological conditions.     

Modulation of platelet activation by native DNA.

Native DNA (dsDNA) was found to induce the aggregation of isolated human platelets and the release of platelet 5HT; this activation was inhibited by both theophylline and TYA, suggesting a role for cAMP and metabolic products formed from arachidonate. By contrast, nonaggregating amounts of dsDNA inhibited platelet activation induced by collagen or thrombin. This inhibition, which could be overcome by use of greater amounts of the stimulatory agents, was not associated with the loss of platelet viability. Activation of platelets by dsDNA was not observed in plasma or in isolated platelet systems to which small amounts of cell-free plasma were added. However, dsDNA maintained in plasma its ability to inhibit platelet aggregation induced by collagen and thrombin. RNA and single-stranded DNA failed to induce platelet aggregation or release of 5HT and to block the platelet activation stimulated by dsDNA. Further, dsDNA did not significantly inhibit platelet aggregation in platelet-rich plasma stimulated by ADP or epinephrine. These data implicate dsDNA as a selective and potentially important activator and modulator of platelet responsiveness.     

Spatially distinct production of reactive oxygen species regulates platelet activation

Platelets play a key role in hemostasis and changes in redox balance are known to alter platelet activation and aggregation. Interestingly, activation of platelets leads to production of reactive oxygen species (ROS), but the role(s) of these ROS remain unclear. Using flow cytometry and chemiluminescence, agonist-induced ROS generation was found to be spatially distinct with stimulation through the major collagen receptor GPVI inducing only intraplatelet ROS while thrombin induced production of extracellular ROS. Platelet activation by either the GPVI-selective agonist convulxin or thrombin was differentially regulated by ROS generation. Thus, surface expression of CD62P, CD40L, or activated integrin alphaIIbbeta3 was abrogated by pharmacologic antioxidants but externalization of phosphatidylserine was not inhibited. Furthermore, extracellular antioxidants SOD/catalase markedly inhibited thrombin-, but not convulxin-, induced CD62P expression and alphaIIbbeta3 activation. The data suggest that ROS selectively regulate biochemical steps in platelet activation and that distinct source(s) of ROS and discrete redox-sensitive pathway(s) may control platelet activation in response to GPVI or thrombin stimulation. Thus, targeting ROS with site-specific antioxidants may differentially regulate platelet activation via thrombin or collagen.     

Protease-activating receptor-4 induces full platelet spreading on a fibrinogen matrix: involvement of ERK2 and p38 and Ca2+ mobilization

Although the involvement of protease-activating receptor PAR1 and PAR4 is well established in platelet aggregation, their role in platelet adhesion and spreading has yet to be characterized. We investigated platelet adhesion and spreading on a fibrinogen matrix after PAR1 and PAR4 stimulation in correlation with the activation of two MAPKs, ERK2 and p38. Of the two PAR-activating peptides (PAR-APs), PAR1-AP and PAR4-AP, which both induce adhesion, only PAR4-AP induced full platelet spreading. Although both PAR1-AP and PAR4-AP induced ADP secretion, which is required for platelet spreading, only PAR4-AP induced sustained Ca(2+) mobilization. In these conditions of PAR4 induction, ERK2 and p38 activation were involved in platelet spreading but not in platelet adhesion. p38 phosphorylation was dependent on ADP signaling through P2Y12, its receptor. ERK2 phosphorylation was triggered through integrin alphaIIbbeta3 outside-in signaling and was dependent on the Rho pathway. ERK2 and p38 activation induced phosphorylation of the myosin light chain and actin polymerization, respectively, necessary for cytoskeleton reorganization. These findings provide the first evidence that thrombin requires PAR4 for the full spreading response. ERK2 and p38 and sustained Ca(2+) mobilization, involved in PAR4-induced platelet spreading, contribute to the stabilization of platelet thrombi at sites of high thrombin production.     

Inhibition of collagen-induced platelet reactivity by DGEA peptide

Direct interactions between collagen, the most thrombogenic component of the extracellular matrix, and platelet surface membrane receptors mediate platelet adhesion and induce platelet activation and aggregation. In this process two glycoproteins are crucial: integrin alpha2beta1, an adhesive receptor, and GPVI, which is especially responsible for signal transduction. Specific antagonists of the collagen receptors are useful tools for investigating the complexity of platelet-collagen interactions. In this work we assessed the usefulness of DGEA peptide (Asp-Gly-Glu-Ala), the shortest collagen type I-derived motif recognised by the collagen-binding integrin alpha2beta1, as a potential antagonist of collagen receptors. We examined platelet function using several methods including platelet adhesion under static conditions, platelet function analyser PFA-100TM, whole blood electric impedance aggregometry (WBEA) and flow cytometry. We found that DGEA significantly inhibited adhesion, aggregation and release reaction of collagen activated blood platelets. The inhibitory effect of DGEA on static platelet adhesion reached sub-maximal values at millimolar inhibitor concentrations, whereas the specific blocker of alpha2beta1 - monoclonal antibodies Gi9, when used at saturating concentrations, had only a moderate inhibitory effect on platelet adhesion. Considering that 25-30% of total collagen binding to alpha2beta1 is specific, we conclude that DGEA is a strong antagonist interfering with a variety of collagen-platelet interactions, and it can be recognised not only by the primary platelet adhesion receptor alpha2beta1 but also by other collagen receptors.     

The roles of LAT in platelet signaling induced by collagen, TxA2, or ADP

The work presented here demonstrates that platelets from mice lacking LAT (linker for the activation of T cells) show reversible aggregation in response to concentrations of collagen that cause TxA2/ADP-dependent irreversible aggregation of control platelets. The aggregation defect of the LAT-deficient platelets was shown to be the result of almost no TxA2 production and significantly diminished ADP secretion. In contrast, the LAT deficiency does not affect aggregation induced by high concentrations of collagen because that aggregation is not dependent on TxA2 and/or ADP. Even though ADP and TxA2 provide amplification signals for platelet activation in response to low concentrations of collagen, LAT-deficient platelets hyperaggregate to low levels of U46619, a TxA2 analog, or ADP. Though the mechanism(s) of costimulatory signals by collagen, ADP, and TxA2 remains unidentified, it is clear that LAT plays a positive role in collagen-induced, TxA2/ADP-dependent aggregation, and a negative role in TxA2 or ADP-induced platelet aggregation.     

Molecular mechanisms of platelet activation and aggregation induced by breast cancer cells

Tumor cell-induced platelet aggregation represents a critical process both for successful metastatic spread of the tumor and for the development of thrombotic complications in cancer patients. To get further insights into this process, we investigated and compared the molecular mechanisms of platelet aggregation induced by two different breast cancer cell lines (MDA-MB-231 and MCF7) and a colorectal cancer cell line (Caco-2). All the three types of cancer cells were able to induce comparable platelet aggregation, which, however, was observed exclusively in the presence of CaCl₂ and autologous plasma. Aggregation was supported both by fibrinogen binding to integrin αIIbβ3 as well as by fibrin formation, and was completely prevented by the serine protease inhibitor PPACK. Platelet aggregation was preceded by generation of low amounts of thrombin, possibly through tumor cells-expressed tissue factor, and was supported by platelet activation, as revealed by stimulation of phospholipase C, intracellular Ca²⁺ increase and activation of Rap1b GTPase. Pharmacological inhibition of phospholipase C, but not of phosphatidylinositol 3-kinase or Src family kinases prevented tumor cell-induced platelet aggregation. Tumor cells also induced dense granule secretion, and the stimulation of the P2Y12 receptor by released ADP was found to be necessary for complete platelet aggregation. By contrast, prevention of thromboxane A₂ synthesis by aspirin did not alter the ability of all the cancer cell lines analyzed to induce platelet aggregation. These results indicate that tumor cell-induced platelet aggregation is not related to the type of the cancer cells or to their metastatic potential, and is triggered by platelet activation and secretion driven by the generation of small amount of thrombin from plasma and supported by the positive feedback signaling through secreted ADP.     

Differential Involvement of ERK2 and p38 in platelet adhesion to collagen

We investigated the role of two MAP kinases, ERK2 and p38, in platelet adhesion and spreading over collagen matrix in static and blood flow conditions. P38 was involved in collagen-induced platelet adhesion and spreading in static adhesion conditions, whereas ERK2 was not. In blood flow conditions, with shear rates of 300 or 1500 s(-1), ERK2 and p38 displayed differential involvement in platelet adhesion, depending on the presence or absence of the von Willebrand factor (vWF). Low collagen coverage densities (0.04 microg/cm2) did not support vWF binding. During perfusions over this surface, platelet adhesion was not affected by the inhibition of ERK2 phosphorylation by PD 98059. However, abolishing p38 activation by SB 203580 treatment reduced platelet adhesion by 67 +/- 9% at 300 s(-1) and 56 +/- 2% at 1500 s(-1). In these conditions, the p38 activity required for platelet adhesion depends on the alpha2beta1 collagen receptor. At higher collagen coverage densities (0.8 microg/cm2) supporting vWF binding, the inhibition of ERK2 activity by PD 98059 decreased adhesion by 47 +/- 6% at 300 s(-1) and 72 +/- 3% at 1500 s(-1), whereas p38 inhibition had only a small effect. The ERK2 activity required for platelet adhesion was dependent on the interaction of vWF with GPIb. In conclusion, ERK2 and p38 have complementary effects in the control of platelet adhesion to collagen in a shear stress-dependent manner.     

Effect of the 807 C/T polymorphism in glycoprotein la on blood platelet reactivity

The 807 C/T dimorphism in the GPla gene has been shown to affect GPlalla receptor density, which can affect platelet adhesiveness to collagens. In this work, we studied platelet function mediated by GPlalla. The 807 T/T genotype was associated with increased platelet adhesion to monomeric collagen after activation with ADP, but not following activation with thrombin and U46619. Adhesion to fibrillar collagen and PFA-100 closure time were not different between carriers of the C/C and T/T genotypes. Also, to monitor the role of the 807 C/T polymorphism in the sensitivity to platelet antagonists, anti-GPlalla monoclonal antibodies (Gi9) were used. Irrespective of the 807 C/T genotype, Gi9 inhibited the ADP-induced platelet adhesion to the monomeric collagen-coated surface stronger than adhesion evoked by thrombin. Moreover, Gi9 significantly inhibited platelet adhesion to both monomeric and fibrillar collagen in 807 T/T carriers, whereas in 807 C/C subjects, Gi9 blocked only adhesion to monomeric collagen. Our results indicate that the 807 T/T genotype is related to increased platelet activation induced by ADP and higher platelet sensitivity to GPlalla antagonists.