Synergistic Effect of Thrombin on Collagen-Induced Platelet Procoagulant Activity Is Mediated Through Protease-Activated Receptor-1

Important regulatory role of activated platelet-derived procoagulant activity in the propagation of thrombi formed under arterial blood flow conditions

BACKGROUND

The functional links between the activation of platelets and the coagulation system have not been clarified.

METHODS AND RESULTS

Immobilized collagen fibrils were perfused with human blood containing fluoresceinated platelets in the presence of various concentrations of thrombin inhibitor. Coagulant activity around platelet thrombi was detected using a FITC-conjugated antibody against the fibrin monomer complex (F-405). Intra-cytosolic calcium ion concentrations ([Ca(2+)](i)) in individual platelets and the volume of thrombi were detected with an ultrafast confocal laser microscope equipped with a piezo-motor control unit. The volume of platelet thrombi formed after 8 min of blood perfusion in the presence of 10, 25, 50, and 100 micromol/L argatroban was 7.69+/-0.46 microm(3), 6.61+/-1.96 microm(3), 3.63+/-1.54 microm(3), and 1.67+/-0.75 microm(3), respectively. There was a positive correlation between the volume of platelet thrombi and the amount of fibrin monomer complex produced around them. The [Ca(2+)](i) of the platelets forming the thrombi oscillated between a minimum of 92.0+/-57.4 nmol/L, 120.1+/-68.1 nmol/L, and a maximum of 217.6+/-131.5 nmol/L, 367.6+/-189.1 nmol/L, respectively, in the presence of 100 and 10 mumol/L argatroban.

CONCLUSIONS

The results suggest a crucial role of coagulant activity in both the generation of fibrin and the growth of platelet thrombi.

The in-vitro effect of tirofiban, glycoprotein IIb/IIIa antagonist, on various responses of porcine blood platelets

The present study systematically evaluates the in-vitro effect of tirofiban, glycoprotein IIb/IIIa (integrins alphaIIbbetaIII) antagonist, on porcine blood platelets. It was found that tirofiban at concentrations up to 5,000 ng/ml did not affect the calcium signal produced by thrombin. Tirofiban, in a concentration-dependent manner reduced platelet aggregation evoked by ADP (IC50 approximately 70 ng/ml), collagen (IC50 approximately 200 ng/ml), and thrombin (IC50 approximately 5,000 ng/ml). Substantial thrombin-evoked platelet aggregation still occurred at high (5,000 ng/ml) tirofiban concentrations. The concentrations of tirofiban completely blocking the optical aggregation evoked by ADP or collagen failed to eliminate microaggregate formation totally. Tirofiban strongly inhibited the dense-granule and lysosome secretion induced by ADP (IC50 approximately 70-170 ng/ml), moderately inhibited that induced by collagen (IC50 approximately 420-500 ng/ml) and very poorly inhibited that elicited by thrombin (IC50 approximately 1,500-5,000 ng/ml). The extent of the inhibition of aggregation and secretion rose as concentrations of the stimulus lowered. Tirofiban was a moderate inhibitor (IC50 approximately 200 ng/ml) of adhesion and a poor inhibitor of platelet procoagulant response induced by collagen. Thromboelastography measurements indicate that, in whole blood, tirofiban, up to concentrations of 2,000 ng/ml, did not affect the kinetics of tissue factor induced clot formation. The obtained results reveal that in porcine platelets, the maximal concentrations of tirofiban used in human medicine (250 ng/ml), effectively block platelet responses triggered by ADP, partly block those induced by collagen and very poorly block those evoked by thrombin. The reason for this phenomenon seems to be the inability of tirofiban to reduce platelet secretion completely.

Lack of early augmentation of platelet reactivity after coronary intervention in patients treated with bivalirudin

To determine whether a regimen of aspirin pretreatment, bivalirudin during the procedure and clopidogrel (600 mg) immediately after percutaneous coronary intervention (PCI) was associated with platelet activation during and shortly after (1 and 2 h) PCI, we characterized platelet function in 10 patients with the use of flow cytometry in the absence of agonist and in response to thrombin (10 nM), ADP (1 microM), the collagen-mimetic convulxin (5 ng/ml), and platelet activating factor (10 nM). Activation of platelets in peripheral blood was rare (<0.5% of platelets) before, during and after PCI. Platelet reactivity in response to each of the agonists was lower after the PCI compared with that in blood taken before the PCI. Accordingly, platelet activation and platelet reactivity were not increased after elective PCI when patients were treated during the procedure with aspirin and bivalirudin and immediately after the procedure with a 600 mg loading dose of clopidogrel.

Collagen/endogenous thrombin-induced platelet aggregation in whole blood samples

The aim of the study was to investigate the individual and combined effects of collagen (3.5 microg/ml) and endogenously generated thrombin (due to addition of 0.35 pmol/l tissue factor) on platelet aggregation in the physiological environment of whole blood by means of the impedance method. Lag times were significantly shorter when a combination of collagen and endogenous thrombin was used to provoke platelet aggregation (41.9 +/- 16.3 s) compared with collagen (173.8 +/- 52.1 s, P < 0.0001) or endogenous thrombin (94.3 +/- 43.6 s, P < 0.001). Amplitudes and slopes were the lowest in collagen-induced experiments (2.83 +/- 1.59 Omega and 1.79 +/- 0.45 Omega/min, respectively), whereas they were approximately the same in endogenous thrombin-induced experiments whether collagen was present or not (13.7 +/- 3.1 versus 11.2 +/- 4.0 Omega and 6.3 +/- 2.8 versus 5.6 +/- 2.3 Omega/min, respectively). No synergistic effect of collagen and endogenous thrombin on the clot formation process was observed by means of thrombelastometry. Moreover, thrombin potentials in tissue factor-activated plasma samples were approximately the same whether collagen was present or not (834 +/- 67 versus 809 +/- 63 nmol/l.min). In conclusion, endogenously generated thrombin is a potent platelet agonist in whole blood, and a combination of collagen and endogenous thrombin synergistically shortens the lag time until the onset of platelet aggregation.

Microparticles adhere to collagen type I, fibrinogen, von Willebrand factor and surface immobilised platelets at physiological shear rates

Microparticles (MPs), shed during the storage of platelets, support blood coagulation and could be helpful in restoring the haemostatic system in thrombocytopenic patients. The mechanisms by which MPs support haemostasis under flow conditions were investigated. Fluorescent-labelled MPs were perfused at shear rates of 100 and 1000/s over surfaces coated with collagen, fibrinogen, von Willebrand factor (VWF) or surface-adherent platelets. Adhesion was monitored in real-time by fluorescence microscopy. In addition, thrombin-antithrombin (TAT) complex formation was measured in flowing thrombocytopenic blood. MPs attained the capacity to firmly adhere to collagen, VWF, fibrinogen and surface-adherent platelets at high and low shear rate. Antibodies against glycoprotein Ibalpha and alpha(IIb)beta(3) were used to demonstrate the specificities of these interactions. The addition of MPs to thrombocytopenic blood did not affect platelet adhesion. TAT complex formation was increased in the presence of MPs in capillaries coated with fibrinogen, but not on collagen fibres. We confirmed that MPs adhere to a damaged vascular bed in vivo after infusion in denuded arteries in a mouse model. MPs have platelet-like adhering properties and accelerate thrombin generation. These properties strongly support the notion that MPs can be beneficial in maintaining normal haemostasis when platelet function is impaired or reduced, as in thrombocytopenic patients.

Persistence of procoagulant surface expression on activated human platelets: involvement of apoptosis and aminophospholipid translocase activity

BACKGROUND

Activated platelets express a procoagulant surface when the asymmetric distribution of membrane phospholipids is scrambled, leading to phosphatidylserine (PS) exposure. PS expression, associated with apoptosis in nucleated cells, would be expected to be reversed by aminophospholipid translocase (APLT) activity.

OBJECTIVE

To determine whether the procoagulant surface of activated platelets persists after it forms; to examine whether PS expression on platelets is associated with loss of mitochondrial inner membrane potential (DeltaPsi(m)), a hallmark of apoptosis; and to investigate the role of APLT in persistence of PS expression.

METHODS

Platelets were stimulated with thrombin, collagen, a combination of both, or the Ca(2+)-ionophore A23187. Up to 4 h after activation, procoagulant surface expression was measured by annexin A5 binding by flow cytometry and by a prothrombinase assay. Flow cytometry was also used to measure PS expression concurrently with DeltaPsi(m) collapse, using CMXRos. APLT activity in annexin A5-negative and -positive platelets was measured flow cytometrically as the percent of 1-palmitoyl-2-[6-[(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]caproyl]-sn-glycero-3-phosphatidylserine (NBD-PS) translocated from the outer to the inner membrane leaflet.

RESULTS AND CONCLUSIONS

Procoagulant surface expression on activated platelets persisted in vitro for at least 4 h; if such persistence occurs in vivo, there are important implications for the propagation of thrombosis. With the physiological stimuli, only 10-20% of the activated platelets expressed PS on their surface, and of these, only a portion exhibited DeltaPsi(m) collapse, indicating that PS expression can be associated with platelet apoptosis, but can also occur independently. APLT activity was very low in the PS-expressing platelet subpopulation for up to 4 h after activation, indicating that the persistence of a procoagulant surface may be attributed, at least in part, to this reduced APLT activity.

Anticoagulant and antithrombotic properties of intracellular protease-activated receptor antagonists

BACKGROUND

Blockade of the thrombin receptors protease-activated receptor (PAR)1 and PAR4 with pepducins, cell-penetrating lipopeptides based on the third intracellular loop of PAR1 and PAR4, effectively inhibits platelet aggregation. We have previously shown that PAR1 pepducin also exerts an anticoagulant activity by partial inhibition of the thrombin plus collagen-induced externalization of phosphatidylserine (PS) at the platelet plasma membrane.

OBJECTIVE

In the present study we examined the effects of PAR1 and PAR4 pepducins on tissue factor (TF)-initiated thrombin generation in platelet-rich plasma (PRP) and the interaction between PAR4 pepducin-loaded mouse platelets and a growing thrombus to confirm the relevance of the in vitro data.

RESULTS

Localization of pepducins at the inner leaflet of the plasma membrane was confirmed with a fluorescence resonance energy transfer assay. Both the PAR1 pepducin, P1pal12, and the PAR4 pepducin, P4pal10, inhibited TF-initiated thrombin generation in PRP. Concentrations of P1pal12 and P4pal10, which blocked the thrombin-induced influx of extracellular calcium ions and inhibited platelet aggregation, reduced the rate of thrombin generation during the propagation phase by 38% and 36%, respectively. Whether this anticoagulant effect is relevant in inhibiting in vivo arterial thrombin growth is uncertain because P4pal10 prevented the incorporation of platelets in a growing thrombus.

CONCLUSIONS

Our findings suggest that in spite of their potential anticoagulant activities the in vivo antithrombotic effect of intracellular PAR pepducins is mainly based on inhibiting platelet-platelet interactions.

GPVI potentiation of platelet activation by thrombin and adhesion molecules independent of Src kinases and Syk

OBJECTIVE

The present study investigates the role of Src and Syk tyrosine kinases in signaling by G-protein coupled and platelet adhesion receptors.

METHODS AND RESULTS

Using Syk-/- platelets or the Src kinase inhibitor PP2, we demonstrate a critical role for Src and Syk kinases in mediating lamellipodia formation on VWF, collagen, CRP, fibrinogen, and fibronectin. In all cases, the spreading defect was overcome by addition of thrombin. Conversely, platelet aggregation and alphaIIb beta3 activation induced by thrombin was similar to controls, arguing against a functional role for Src and Syk in alphaIIb beta3 activation. Unexpectedly, CRP potentiated integrin alphaIIb beta3 activation and platelet aggregation induced by subthreshold concentrations of thrombin in Syk-/- platelets or in the presence of the Src kinase inhibitor PP2. Potentiation in the presence of PP2 was lost in the absence of FcRgamma-chain or GPVI confirming that it was mediated through the immunoglobulin receptor. Further delineation of this PP2-resistant synergy revealed that PAR4 could trigger the enhanced response in combination with CRP.

CONCLUSIONS

We show that Syk is critical for lamellipodia formation on a range of immobilized proteins but that this can be overcome by addition of thrombin. Further, we reveal a novel role for GPVI in supporting thrombin-induced activation, independent of Syk and Src kinases.

Differential involvement of thrombin receptors in Ca2+ release from two different intracellular stores in human platelets

Physiological agonists increase cytosolic free Ca2+ concentration to regulate a number of cellular processes. The platelet thrombin receptors, PAR (protease-activated receptor) 1 PAR-4 and GPIb-IX-V (glycoprotein Ib-IX-V) have been described as potential contributors of thrombin-induced platelet aggregation. Platelets present two separate Ca2+ stores, the DTS (dense tubular system) and acidic organelles, differentiated by the distinct sensitivity of their respective SERCAs (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPases) to TG (thapsigargin) and TBHQ [2,5-di-(tert-butyl)-1,4-hydroquinone]. However, the involvement of the thrombin receptors in Ca2+ release from each Ca2+ store remains unknown. Depletion of the DTS using ADP, which releases Ca2+ solely from the DTS, in combination with 10 nM TG, to selectively inhibit SERCA2 located on the DTS reduced Ca2+ release evoked by the PAR-1 agonist, SFLLRN, and the PAR-4 agonist, AYPGKF, by 80 and 50% respectively. Desensitization of PAR-1 and PAR-4 or pre-treatment with the PAR-1 and PAR-4 antagonists SCH 79797 and tcY-NH2 reduced Ca2+ mobilization induced by thrombin, and depletion of the DTS after desensitization or blockade of PAR-1 and PAR-4 had no significant effect on Ca2+ release stimulated by thrombin through the GPIb-IX-V receptor. Converse experiments showed that depletion of the acidic stores using TBHQ reduced Ca2+ release evoked by SFLLRN or AYPGKF, by 20 and 50% respectively, and abolished thrombin-stimulated Ca2+ release through the GPIb-IX-V receptor when PAR-1 and PAR-4 had been desensitized or blocked. Our results indicate that thrombin-induced activation of PAR-1 and PAR-4 evokes Ca2+ release from both Ca2+ stores, while activation of GPIb-IX-V by thrombin releases Ca2+ solely from the acidic compartments in human platelets.

Thrombin-triggered platelet apoptosis

BACKGROUND

Thrombin is primarily known as a coagulation factor and as an inducer of platelet activation and aggregation. It has been reported that thrombin modulates apoptosis of nucleated cells.

OBJECTIVES

The current study investigated whether thrombin can affect apoptosis in anucleated human platelets.

METHODS

Using flow cytometry, we studied platelet apoptosis at the single-cell level, analyzing markers of mitochondrial and cytoplasmic apoptosis. Western blotting was also employed, in addition to flow cytometry, for determining the expression of Bcl-2 family proteins.

RESULTS

We found that human alpha-thrombin induced four key manifestations of apoptosis in human platelets: (i) mitochondrial inner transmembrane potential (DeltaPsi m) depolarization; (ii) strong expression of pro-apoptotic Bax and Bak proteins but only weak expression of anti-apoptotic Bcl-2 protein; (iii) caspase-3 activation; and (iv) phosphatidylserine (PS) exposure.

CONCLUSIONS

This study demonstrates that, aside from its 'classical' function as an inducer of platelet activation, thrombin can trigger platelet apoptosis, where it acts as a death ligand. These data indicate that thrombin triggers platelet apoptosis by impacting on several intracellular apoptotic targets, including shifting the balance between Bcl-2 regulatory proteins in a pro-apoptotic direction, depolarizing the inner mitochondrial membrane, activating the executioner caspase-3, and stimulating aberrant exposure of PS on the platelet surface.

Active tissue factor pathway inhibitor is expressed on the surface of coated platelets

The incorporation of blood-borne forms of tissue factor (TF) into a growing blood clot is necessary for normal fibrin generation and stabilization of the blood clot. Tissue factor pathway inhibitor (TFPI) is the primary physiologic inhibitor of tissue factor and is present within platelets. Expression of TFPI on the platelet surface may be the optimal location for it to abrogate blood-borne TF activity that incorporates within the blood clot, balancing the need for adequate hemostasis while preventing development of occlusive thrombosis. TFPI is produced by megakaryocytes but is not expressed on the platelet surface. Activation of platelets with thrombin receptor activation peptide does not cause release or surface expression of TFPI, demonstrating that TFPI is not stored within platelet alpha granules. TFPI is expressed on the platelet surface following dual-agonist activation with convulxin plus thrombin to produce coated platelets. In association with its expression on the surface of coated platelets TFPI is also released in microvesicles or as a soluble protein.

Protease-activated receptors in cardiovascular diseases

Thrombosis associated with the pathophysiological activation of platelets and vascular cells has brought thrombin and its receptors to the forefront of cardiovascular medicine. Thrombin signaling through the protease-activated receptors (PARs) has been shown to influence a wide range of physiological responses including platelet activation, intimal hyperplasia, inflammation, and maintenance of vascular tone and barrier function. The thrombin receptors PAR1 and PAR4 can be effectively targeted in animals in which acute or prolonged exposure to thrombin leads to thrombosis and/or restenosis. In the present study, we describe the molecular and pharmacological basis of small-molecule inhibitors that target PAR1. In addition, we discuss a new class of cell-penetrating inhibitors, termed pepducins, that provide insight into previously unidentified roles of PAR1 and PAR4 in protease signaling.

Reversible inhibition of the platelet procoagulant response through manipulation of the Gardos channel

The platelet procoagulant response requires a sustained elevation of the intracellular Ca2+ concentration, [Ca2+]i, causing exposure of phosphatidylserine (PS) at the outer surface of the plasma membrane. An increased [Ca2+]i also activates Ca2+-dependent K+ channels. Here, we investigated the contribution of the efflux of K+ ions on the platelet procoagulant response in collagen-thrombin–activated platelets using selective K+ channel blockers. The Gardos channel blockers clotrimazol, charybdotoxin, and quinine caused a similar decrease in prothrombinase activity as well as in the number of PS-exposing platelets detected by fluorescence-conjugated annexin A5. Apamin and iberiotoxin, inhibitors of other K+ channels, were without effect. Only clotrimazol showed a significant inhibition of the collagen-plus-thrombin–induced intracellular calcium response. Clotrimazol and charybdotoxin did not inhibit aggregation and release under the conditions used. Inhibition by Gardos channel blockers was reversed by valinomycin, a selective K+ ionophore. The impaired procoagulant response of platelets from a patient with Scott syndrome was partially restored by pretreatment with valinomycin, suggesting a possible defect of the Gardos channel in this syndrome. Collectively, these results provide evidence for the involvement of efflux of K+ ions through Ca2+-activated K+ channels in the procoagulant response of platelets, opening potential strategies for therapeutic interventions.

Rolling and adhesion of apoptotic monocytes is impaired by loss of functional cell surface-expressed P-selectin glycoprotein ligand-1

BACKGROUND

Apoptosis induces cellular membrane changes that are thought to be linked to thrombotic processes, for example, surface exposure of procoagulant phosphatidylserine (PtdSer), upregulation of tissue factor (TF), and microvesicle formation. The latter, though, could downregulate this cellular response by shedding prothrombotic membrane elements, for example, integrins and TF. To test this hypothesis, etoposide-treated, apoptotic, monocytic cells (human monocytic leukemia cell line [THP-1]) were examined for rolling and adhesion on adherent platelets and for TF expression.

METHODS AND RESULTS

Etoposide treatment did not result in a significant change in TF antigen expression. However, TF activity, measured in a continuous factor Xa generation assay, was increased fivefold concomitantly with increased exposure of PtdSer. Laminar flow adhesion assays specific for interaction between P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) revealed that in contrast to non-treated cells, apoptotic cells did not roll or firmly attach on adherent platelets. Lack of apoptotic THP-1 platelet interaction could be attributed to both a loss of cell surface-expressed PSGL-1 and loss of functional PSGL-1 as a result of disruption of the binding of PSGL-1 with the cytoskeleton.

CONCLUSION

Etoposide-induced apoptosis in THP-1 cells evokes a procoagulant response by increasing TF activity associated with an increased PtdSer exposure. However, in contrast to TF, PSGL-1 shedding and loss of function, makes that apoptotic monocytes are unlikely involved in a thrombotic action because of their inability to adhere to an injured vessel wall or developing thrombus.