High molecular weight kininogen inhibits thrombin-induced platelet aggregation and cleavage of aggregin by inhibiting binding of thrombin to platelets.

In this study we show that high molecular weight kininogen (HK) inhibited alpha-thrombin-induced aggregation of human platelets in a dose-dependent manner with complete inhibition occurring at plasma concentration (0.67 mumol/L) of HK. HK (0.67 mumol/L) also completely inhibited thrombin-induced cleavage of aggregin (Mr = 100 Kd), a surface membrane protein that mediates adenosine diphosphate (ADP)-induced shape change, aggregation, and fibrinogen binding. The inhibition of HK was specific for alpha- and gamma-thrombin-induced platelet aggregation, because HK did not inhibit platelet aggregation induced by ADP, collagen, calcium ionophore (A23187), phorbol myristate acetate (PMA), PMA + A23187, or 9,11-methano derivative of prostaglandin H2 (U46619). These effects were explained by the ability of HK, at physiologic concentration, to completely inhibit binding of 125I-alpha-thrombin to washed platelets. As a result of this action of HK, this plasma protein also completely inhibited thrombin-induced secretion of adenosine triphosphate, blocked intracellular rise in Ca2+ in platelets exposed to alpha- and gamma-thrombin, inhibited thrombin-induced platelet shape change, and blocked the ability of thrombin to antagonize the increase in intracellular cyclic adenosine monophosphate (cAMP) levels induced by iloprost. Because elevation of cAMP is known to inhibit binding of thrombin to platelets, we established that HK did not increase the intracellular concentration of platelet cAMP. Finally, HK did not inhibit enzymatic activity of thrombin. To study the role of HK in the plasma environment, we used gamma-thrombin to avoid fibrin formation by alpha-thrombin. Platelet aggregation induced by gamma-thrombin was also inhibited by HK in a dose-dependent manner. The EC50 (concentration to produce 50% of the maximum rate of aggregation) of gamma-thrombin for washed platelets was 7 nmol/L and increased to 102 nmol/L when platelets were suspended in normal human plasma. The EC50 for platelet aggregation induced by alpha-thrombin in plasma deficient in total kininogen was 40 nmol/L. When supplemented with HK at plasma concentration (0.67 mumol/L), the EC50 increased to 90 nmol/L, a value similar to that for normal human plasma. These results indicate that (1) HK inhibits thrombin-induced platelet aggregation and cleavage of aggregin by inhibiting binding of thrombin to platelets; (2) HK is a specific inhibitor of platelet aggregation induced by alpha- and gamma-thrombin; and (3) HK plays a role in modulating platelet aggregation stimulated by alpha-thrombin in plasma.     

Glycoprotein Ib (GPIb)-dependent and GPIb-independent pathways of thrombin-induced platelet activation

In this study, the question of whether glycoprotein Ib (GPIb) mediates both high and moderate affinity pathways of alpha-thrombin-induced platelet activation was examined. Flow cytometric studies, using a panel of monoclonal antibodies (MoAbs), showed that Serratia marcescens protease treatment removed greater than 97% of the glycocalicin portion of GPIb but did not affect the changes in the expression of GPIX or GMP-140 that were induced by high concentrations of alpha-thrombin (10 nmol/L). However, Serratia treatment almost completely abolished the increase in platelet surface GMP-140 induced by low concentrations of alpha-thrombin (0.5 nmol/L) and diminished the downregulation of platelet surface GPIX by 60.9% +/- 5.6% (mean +/- SEM, n = 3). When present in 20-fold molar excess, an MoAb directed against the alpha-thrombin/von Willebrand factor (vWf) binding domains of GPIb completely blocked the ristocetin-dependent binding of vWf to platelets but inhibited only to about 50% the binding of alpha-thrombin and the activation-dependent binding of vWf. In platelets treated with Serratia marcescens protease to remove GPIb, a concentration of this MoAb 16,000-fold in excess of the maximum possible remaining copies of GPIb failed to inhibit platelet activation by alpha-thrombin. These studies demonstrate that activation of intact platelets by alpha-thrombin proceeds by both GPIb-dependent and GPIb-independent mechanisms.     

Prostaglandin E2 potentiates platelet aggregation by priming protein kinase C

Prostaglandin E2 (PGE2) is produced by activated platelets and by several other cells, including capillary endothelial cells. PGE2 exerts a dual effect on platelet aggregation: inhibitory, at high, supraphysiologic concentrations, and potentiating, at low concentrations. No information exists on the biochemical mechanisms through which PGE2 exerts its proaggregatory effect on human platelets. We have evaluated the activity of PGE2 on human platelets and have analyzed the second messenger pathways involved. PGE2 (5 to 500 nmol/L) significantly enhanced aggregation induced by subthreshold concentrations of U46619, thrombin, adenosine diphosphate (ADP), and phorbol 12-myristate 13-acetate (PMA) without simultaneously increasing calcium transients. At a high concentration (50 mumol/L), PGE2 inhibited both aggregation and calcium movements. PGE2 (5 to 500 nmol/L) significantly enhanced secretion of beta-thromboglobulin (beta TG) and adenosine triphosphate from U46619- and ADP-stimulated platelets, but it did not affect platelet shape change. PGE2 also increased the binding of radiolabeled fibrinogen to the platelet surface and increased the phosphorylation of the 47-kD protein in 32P- labeled platelets stimulated with subthreshold doses of U46619. Finally, the amplification of U46619-induced aggregation by PGE2 (500 nmol/L) was abolished by four different protein kinase C (PKC) inhibitors (calphostin C, staurosporine, H7, and TMB8). Our results suggest that PGE2 exerts its facilitating activity on agonist-induced platelet activation by priming PKC to activation by other agonists. PGE2 potentiates platelet activation at concentrations produced by activated platelets and may thus be of pathophysiologic relevance.     

Protease-activated receptors 1 and 4 do not stimulate G(i) signaling pathways in the absence of secreted ADP and cause human platelet aggregation independently of G(i) signaling

Thrombin is an important agonist for platelet activation and plays a major role in hemostasis and thrombosis. Thrombin activates platelets mainly through protease-activated receptor 1 (PAR1), PAR4, and glycoprotein Ib. Because adenosine diphosphate and thromboxane A(2) have been shown to cause platelet aggregation by concomitant signaling through G(q) and G(i) pathways, we investigated whether coactivation of G(q) and G(i) signaling pathways is the general mechanism by which PAR1 and PAR4 agonists also activate platelet fibrinogen receptor (alphaIIbbeta3). A PAR1-activating peptide, SFLLRN, and PAR4-activating peptides GYPGKF and AYPGKF, caused inhibition of stimulated adenylyl cyclase in human platelets but not in the presence of either Ro 31-8220, a protein kinase C selective inhibitor that abolishes secretion, or AR-C66096, a P2Y12 receptor-selective antagonist; alpha-thrombin-induced inhibition of adenylyl cyclase was also blocked by Ro 31-8220 or AR-C66096. In platelets from a P2Y12 receptor-defective patient, alpha-thrombin, SFLLRN, and GYPGKF also failed to inhibit adenylyl cyclase. In platelets from mice lacking the P2Y12 receptor, neither alpha-thrombin nor AYPGKF caused inhibition of adenylyl cyclase. Furthermore, AR-C66096 caused a rightward shift of human platelet aggregation induced by the lower concentrations of alpha-thrombin and AYPGKF but had no effect at higher concentrations. Similar results were obtained with platelets from mice deficient in the P2Y12. We conclude that (1) thrombin- and thrombin receptor-activating peptide-induced inhibition of adenylyl cyclase in platelets depends exclusively on secreted adenosine diphosphate that stimulates G(i) signaling pathways and (2) thrombin and thrombin receptor-activating peptides cause platelet aggregation independently of G(i) signaling.     

Enhancement of cathepsin G-induced platelet activation by leukocyte elastase: consequence for the neutrophil-mediated platelet activation

We have focused our interest on the platelet-activating properties of two polymorphonuclear neutrophil (PMN)-derived proteinases, namely elastase (HLE) and cathepsin G (Cat.G). First of all, we observed that whereas HLE was unable to trigger platelet activation by itself, it enhanced platelet activation induced by Cat.G when both proteinases were added simultaneously. It has been recently described that, upon stimulation, PMN released Cat.G, which in turn activated surrounding platelets. Thus, we looked for a combined effect of Cat.G and HLE during this cell-to-cell interaction. When PMN (5 x 10(6)/mL) were stimulated by 0.5 mumol/L N-formyl-Met-Leu-Phe, they released 237.9 +/- 49.1 nmol/L Cat.G and 381.7 +/- 28.0 nmol/L HLE. Such a concentration of purified Cat.G (240 nmol/L) induced only a moderate platelet activation when added to a PMN-platelet mixture. However, when Cat.G (240 nmol/L) and HLE (380 nmol/L) were added together, the resulting platelet activation was strictly comparable to that corresponding to the addition of N-formyl-Met-Leu-Phe (P > .05) in terms of aggregation, dense and alpha granule secretion, and thromboxane B2 production. In fact, Elafin, a specific HLE inhibitor, when added to the PMN-platelet cooperation system triggered by N-formyl-Met-Leu-Phe, prevented platelet activation within the same range of concentrations as for inhibition of HLE activity. In conclusion, we now show that not only Cat.G, but also HLE is involved in the PMN-mediated platelet activation.     

Differential requirements for platelet aggregation and inhibition of adenylate cyclase by epinephrine. Studies of a familial platelet alpha 2-adrenergic receptor defect

We describe a family whose members have impaired platelet aggregation and secretion responses to epinephrine with normal responses to adenosine diphosphate and collagen. Platelet alpha 2-adrenergic receptors (measured using 3H methyl-yohimbine) were diminished in the propositus (78 sites per platelet), his two sisters (70 and 27 sites per platelet), and parents (37 and 63 sites per platelet), but not in two maternal aunts (12 normal subjects, 214 +/- 18 sites per platelet; mean +/- SE). However, the inhibition of cyclic adenosine monophosphate (cAMP) levels by epinephrine in platelets exposed to 400 nmol/L PGI2 was similar in the patients and five normal subjects (epinephrine concentration for 50% inhibition, 0.04 +/- 0.01 mumol/L v 0.03 +/- 0.01 mumol/L; P greater than .05). In normal platelets, the concentration of yohimbine (0.18 mumol/L) required for half maximal inhibition of aggregation induced by 2 mumol/L epinephrine was lower than that for inhibition of its effect on adenylate cyclase (1.6 mumol/L). In quin2 loaded platelets, thrombin (0.1 U/mL) stimulated rise in cytoplasmic Ca2+ concentration, [Ca2+]i, was normal in the two patients studied. The PGI2 analog ZK 36,374 completely inhibited thrombin-induced rise in [Ca2+]i; the reversal of this inhibition by epinephrine was normal in the two patients. Thus, despite the impaired aggregation response to epinephrine, platelets from these patients have normal ability to inhibit PGI2-stimulated cAMP levels. These patients with an inherited receptor defect provide evidence that fewer platelet alpha 2-adrenergic receptors are required for epinephrine-induced inhibition of adenylate cyclase than for aggregation.     

Thrombin stimulation of human endothelial cell phospholipase D activity. Regulation by phospholipase C, protein kinase C, and cyclic adenosine 3'5'-monophosphate.

The activation of membrane-bound phospholipase D (PLD) resulting in the generation of phosphatidic acid (PA) is increasingly recognized as an integral event in the initiation of a variety of cellular responses. We explored whether alpha-thrombin is a physiologic agonist for PLD activation in human umbilical vein endothelial cells (HUVEC). HUVEC monolayers were labeled with [32Pi] and PLD activity determined by formation of the PLD metabolite [32P] phosphatidylethanol (PEt) in the presence of 5 g/L ethanol by thin-layer chromatography. alpha-Thrombin rapidly (1 minute) increased PA and PEt formation in a dose-dependent manner (10(-6) to 10(-10)) with maximal PLD stimulation achieved with 10 nmol/L alpha-thrombin producing a threefold to fourfold increase in PA and a sixfold to eightfold increase in PEt over controls at 15 minutes. Esterolytically active zeta-thrombin (10 nmol/L) and gamma-thrombin (1 mumol/L), but not inactive DIP-alpha-thrombin (1 mumol/L) also increased PLD activity. The role of Ca2+ flux in human endothelial cell PLD activation was investigated and PEt formation was significantly enhanced by Ca2+ ionophores A23187 and ionomycin (1 mumol/L, three-fold to fourfold increase in PEt). Alpha-Thrombin-stimulated PEt formation was abolished (greater than 90% inhibition) with chelation of intracellular calcium (Ca2+i) by pretreatment with BAPTA-AM (25 mumol/L, 30 minutes) but only mildly attenuated (30% inhibition) by removal of extracellular calcium (Ca2+E) with EGTA (5 mmol/L). The protein kinase C (PKC) inhibitor staurosporine reduced alpha-thrombin-induced PEt formation in a dose-dependent manner (10 mumol/L, 78% inhibition) and PKC downregulation with chronic PMA treatment (18 hours) also resulted in marked inhibition of alpha-thrombin-induced PEt formation. Neither pertussis nor botulinum C bacterial toxins significantly altered alpha-thrombin-induced PLD responses. In contrast, similar pretreatment with cholera toxin (1 microgram/mL, 60 minutes) consistently augmented alpha-thrombin-stimulated PLD activity by 50% to 90%. Comparable results were observed with agents which increased cAMP such as forskolin, 8-bromo cAMP, or dibutyryl cAMP and cholera toxin augmentation was abolished by 2-dideoxyadenosine, a competitive inhibitor of adenylyl cyclase activity. These studies demonstrate that alpha-thrombin is a potent stimulus for human PLD-mediated PA formation and that cyclic adenosine nucleotides modulate agonist-induced cellular PLD activity. In this model of PLD activation, alpha-thrombin receptor occupancy leads to the breakdown of phosphatidylinositol 4,5-bisphosphate catalyzed by phospholipase C producing the Ca2+ secretagogue IP3 and DAG.(ABSTRACT TRUNCATED AT 400 WORDS)     

Circulating tumor gangliosides enhance platelet activation

Gangliosides enhance tumor formation in experimental animals, and high circulating concentrations of gangliosides shed by tumor cells are associated with rapid progression of human neuroblastoma. We studied these shed molecules for effects on platelet function, because platelet activation may play a role in the metastatic process. Preincubation of normal platelets in patient (tumor ganglioside-containing) serum resulted in their aggregation upon exposure to a subthreshold concentration (1 microgram/mL) of collagen (up to 34% v < 10% in normal serum) and ATP release (up to 1.1 nmol/2.5 x 10(7) platelets v < 0.2 in normal serum). Because circulating shed tumor gangliosides are lipoprotein-associated, we next assessed the effects of the serum lipoprotein fraction on platelet ATP release. The patient serum lipoprotein fraction (d > 1.210) enhanced ATP release (up to 3.1 nmol ATP), whereas the same fraction of normal serum, and both patient and control lipoprotein-depleted serum fractions (d > 1.210), were inactive (< 0.2 nmol ATP released). Finally, as little as 0.5 mumol/L patient serum gangliosides (purified from the lipoprotein fraction) caused significantly greater ATP release than did normal serum gangliosides (P < .01) and caused maximal release at 50 mumol/L (up to 3.0 nmol ATP released v < or = 0.3 nmol released by platelets exposed to normal serum gangliosides). Purified total human neuroblastoma tumor gangliosides, detected in the patient serum and isolated from LA-N5 cells, were highly active; preincubation of platelets with only 5 mumol/L of these gangliosides resulted in release of 2.5 +/- 0.1 nmol ATP. Thus, neuroblastoma patient serum, the lipoprotein fraction, and, specifically, the serum gangliosides enhance platelet activation. This activity appears to reside particularly in the tumor cell gangliosides, which are shed in vivo.     

Thrombin binding to platelets defines functional receptors: inhibition of thrombin-induced platelet activation by catalytically-inactivated thrombin

It has been widely questioned as to whether the observed binding of a-thrombin to intact platelets defines receptors coupled to signal transduction or merely thrombin binding sites. We have now shown that at α-thrombin concentrations sufficient to induce a full shape change response without aggregation (0.1 nM), PPACK-thrombin (that is, α-thrombin treated with the irreversible active site inhibitor D-phenylalanyl-L-prolyl-L-arginine chloromethylketone) dose-dependently inhibits platelet shape change (IC(50)～70 nM), the concomitant increases in [Ca(2+)Ii (IC(50)～75 nM) and ATP secretion (IC(50)～50 nM). Since PPACK-thrombin competes fully in the binding of a-thrombin to high, moderate and low affinity sites on intact platelets, these results show that this binding defines functional receptors coupled to platelet activation.     

Direct evidence for the interaction of the nucleotide affinity analog 5'-p-fluorosulfonylbenzoyl adenosine with a platelet ADP receptor

Previous reports have indicated that the nucleotide affinity analog 5'- p-fluorosulfonylbenzoyl adenosine (FSBA) at concentrations between 40 and 100 mumol/L and at times greater than 20 minutes covalently modifies a single protein component on the external platelet membrane surface and that adenosine diphosphate (ADP) protects against this reaction. That this protein is an ADP receptor linked to platelet activation is shown by FSBA inhibition of ADP-mediated platelet shape change, aggregation, and fibrinogen receptor exposure. In this report, further evidence for the interaction of FSBA with the ADP receptor on platelets is provided by the observation that FSBA at high concentrations (100 to 500 mumol/L) behaves as a weak agonist to produce platelet shape change within one minute as detected by spectroscopic assay and scanning electron microscopy with concomitant phosphorylation of the light chain of platelet myosin. The specificity of FSBA as an agonist is demonstrated by inhibition of FSBA-induced shape change by ATP and the covalent incorporation of SBA as well as the failure of 5'-fluorosulfonylbenozoyl guanosine (FSBG) to cause shape change. In contrast, incubation of platelets with low concentrations of [3H]-FSBA (40 mol/L) is not associated with stimulation of platelet shape change or myosin light chain phosphorylation.     

Impaired cytoplasmic ionized calcium mobilization in inherited platelet secretion defects

Defects in platelet cytoplasmic Ca++ mobilization have been postulated but not well demonstrated in patients with inherited platelet secretion defects. We describe studies in a 42-year-old white woman, referred for evaluation of easy bruising, and her 23-year-old son. In both subjects, aggregation and 14C-serotonin secretion responses in platelet-rich plasma (PRP) to adenosine diphosphate (ADP), epinephrine, platelet activating factor (PAF), arachidonic acid (AA), U46619, and ionophore A23187 were markedly impaired. Platelet ADP and adenosine triphosphate (ATP), contents and thromboxane synthesis induced by thrombin and AA were normal. In quin2-loaded platelets, the basal intracellular Ca++ concentration, [Ca++]i, was normal; however, peak [Ca++]i measured in the presence of 1 mmol/L external Ca++ was consistently diminished following activation with ADP (25 mumol/L), PAF (20 mumol/L), collagen (5 micrograms/mL), U46619 (1 mumol/L), and thrombin (0.05 to 0.5 U/mL). In aequorin-loaded platelets, the peak [Ca++]i studied following thrombin (0.05 and 0.5 U/mL) stimulation was diminished. Myosin light chain phosphorylation following thrombin (0.05 to 0.5 U/mL) stimulation was comparable with that in the normal controls, while with ADP (25 mumol/L) it was more strikingly impaired in the propositus. We provide direct evidence that at least in some patients with inherited platelet secretion defects, agonist-induced Ca++ mobilization is impaired. This may be related to defects in phospholipase C activation. These patients provide a unique opportunity to obtain new insights into Ca++ mobilization in platelets.     

Mechanism of human platelet activation by endotoxic glycolipid-bearing mutant Re595 of Salmonella minnesota

The mechanism through which human blood platelets interact with gram- negative bacteria with well-defined structural variations in endotoxic lipopolysaccharide was studied. Secretion of 14C-serotonin and aggregation of platelets separated from plasma proteins were observed on challenge with rough mutant Re595 of Salmonella minnesota possessing a glycolipid outer layer composed of Lipid A and 2-keto-3-deoxyoctonate (KDO) but lacking heptose phosphate in the core and O-polysaccharide in its outer portion. Both 14C-serotonin secretion and platelet aggregation were concentration-dependent, with a half-maximum response at the ratio of one bacterial colony-forming unit (CFU) to two platelets. The aggregation of human platelets induced by mutant Re595 was divalent cation-dependent and required secretion of ADP and fibrinogen from platelet storage granules because it was inhibited by chelators, by the ADP-splitting enzyme apyrase, and by monospecific antifibrinogen Fab fragments. The synthetic peptide analog of the platelet receptor recognition site on the gamma chain of fibrinogen, gamma 400-411, inhibited platelet aggregation induced by mutant Re595 (IC50 160 mumol/L), whereas serotonin secretion was unaffected. Tetrapeptide, RGDS, analogous to human fibrinogen alpha chain (alpha 572-575) and to the cell adhesion site of fibronectin, also inhibited aggregation induced by mutant Re595 (IC50 60 mumol/L). Secretion of 14C- serotonin was preceded by a very rapid phosphorylation of a platelet protein of mol wt 47,000, which is associated with protein kinase C activation. Myosin light chain (mol wt 20,000) was also phosphorylated. Both phosphoproteins were dephosphorylated while secretion was reaching maximum. Furthermore, release of 3H-arachidonic acid from platelet phospholipids and generation of thromboxane B2 via the cyclooxygenase pathway were observed. Inhibition of this pathway with acetylsalicylic acid (10(-4) mol/L) or indomethacin (5 X 10(-4) mol/L) reduced 14C- serotonin secretion and platelet aggregation. The role of Lipid A in the interaction of mutant Re595 with human platelets was deduced from the inhibitory effect of the Lipid A-binding protein present in Limulus amebocyte lysate. Likewise, polymyxin B, known to complex with Lipid A, was inhibitory. The reactivity of mutant Re595 toward platelets was attenuated by mild acid hydrolysis, during which KDO was dissociated from the glycolipid, and by alkaline hydrolysis, which breaks ester- linked fatty acids in Lipid A. In contrast to mutant Re595, strain S218 of S minnesota bearing "complete" endotoxic lipopolysaccharide did not induce secretion and aggregation of human platelets.(ABSTRACT TRUNCATED AT 400 WORDS)     

A role for intracellular histamine in collagen-induced platelet aggregation

We previously demonstrated that newly formed intracellular histamine mediates platelet aggregation in response to phorbol-12-myristate-13-acetate (PMA). We now report further investigations of the role of histamine during physiological activation of platelets by collagen. Platelets stirred with collagen produced histamine; the rise in histamine precedes the onset of aggregation. The dose response for collagen stimulation of histamine synthesis and platelet aggregation is similar. Inhibitors of histidine decarboxylase (HDC) block both aggregation and histamine synthesis in parallel. Histamine production is not dependent on aggregation; both the intracellular histamine receptor antagonist, N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine-HCl (DPPE), and the cyclooxygenase inhibitors, aspirin and indomethacin, inhibit collagen-induced aggregation but not histamine synthesis. DPPE also inhibits collagen-induced serotonin secretion and thromboxane production. The effects of DPPE and HDC inhibitors are significantly reversed by the addition of histamine (0.1 to 10 mumol/L) to saponin-permeabilized platelets, though histamine alone has no pro-aggregatory effects. The results suggest that newly synthesized intracellular histamine has a role in collagen-induced platelet activation and that it may act to promote the generation of thromboxane and the secretion responses of platelet granules.     

Inhibition of platelet aggregation by prostacyclin is attenuated after exercise in patients with angina pectoris.

We tested whether alteration of platelet sensitivity to prostacyclin (PGI2) is involved in the activation of platelets induced by exercise in patients with stable angina. Twenty patients and 20 control subjects underwent treadmill testing. Blood samples were obtained before and immediately after exercise for plasma thromboxane B2 (TXB2) and 6-keto-PGF1 alpha (6kP) assays and platelet aggregation studies. Dose-response curves for platelet aggregation to collagen were obtained in the presence and absence of 1 nmol/L PGI2 to quantify the antiaggregation effects of PGI2. At rest, platelet aggregation by collagen was enhanced in the patients. However, platelets were more sensitive to exogenous PGI2, apparently associated with lower plasma 6kP levels in the patients. After exercise, plasma TXB2 levels increased in the patients but not in the control subjects. Plasma 6kP levels remained unchanged and platelet sensitivity to PGI2 decreased in the patients whereas these values increased in the control subjects. The exercise-induced changes in platelet sensitivity to PGI2 correlated with those of platelet adenylate cyclase activity in response to 1 nmol/L PGI2 (r = 0.787, p less than 0.01). Thus impaired sensitivity of platelets to PGI2, in addition to the reduced response of prostanoid secretion, might be relevant to the platelet activation associated with exercise in patients with stable angina.     

Molecular requirements for the interaction of thrombospondin with thrombin-activated human platelets: modulation of platelet aggregation.

We have investigated the molecular requirements for thrombospondin (TSP) to bind to the platelet surface and to support the subsequent secretion-dependent platelet aggregation. For this, we used two distinct murine monoclonal antibodies (MoAbs), designated MAI and MAII, raised against human platelet TSP, and three polyclonal antibodies, designated R3, R6, and R5, directed against fusion proteins containing the type 1 (Gly 385-Ile 522), type 2 (Pro 559-Ile 669), and type 3 (Asp 784-Val 932) repeating sequences, respectively. Among them, R5 and R6, but not R3, inhibited thrombin-induced aggregation of washed platelets and the concomitant secretion of serotonin. These antibodies, however, did not inhibit the expression of TSP on thrombin-activated platelets, as measured by the binding of a radiolabeled MoAb to TSP, suggesting that they may inhibit platelet aggregation by interfering with a physiologic event subsequent to TSP binding. In contrast, MoAb MAII, which reacts with an epitope located within the heparin-binding domain of TSP, inhibited both TSP surface expression and platelet aggregation/secretion induced by thrombin. In addition, this MoAb inhibited in a dose-dependent manner (IC50 approximately 0.5 mumol/L) the interaction of 125I-TSP with immobilized fibrinogen and platelet glycoprotein IV, both potential physiologic receptors for TSP on thrombin-activated platelets. These results indicate that the interaction of TSP with the surface of activated platelets can be modulated at the level of a specific epitope located within the amino terminal heparin-binding domain of the molecule. Thus, selective inhibition of the platelet/TSP interaction may represent an alternative approach to the inhibition of platelet aggregation.     

Signaling via IRAG is essential for NO/cGMP-dependent inhibition of platelet activation

Platelet activation is strongly affected by nitric oxide/cyclic GMP (NO/cGMP) signaling involving cGMP-dependent protein kinase I (cGKI). Previously it was shown that interaction of the cGKI substrate IRAG with InsP(3)RI is essential for NO/cguanosine monophosphate (GMP)-dependent inhibition of platelet aggregation in vitro and in vivo. However, the role of Inositol-trisphosphate receptor associated cGMP kinase substrate (IRAG) for platelet adhesion or granule secretion was unknown. Here, we analysed the functional role of IRAG for platelet activation. Murine IRAG-deficient platelets displayed enhanced aggregability towards several agonists (collagen, thrombin and TxA2). NO- or cGMP-dependent inhibition of agonist induced ATP- or 5-HT secretion from dense granules, and P-selectin secretion from alpha granules was severely affected in IRAG-deficient platelets. Concomitantly, the effect of NO/cGMP on platelet aggregation was strongly reduced in IRAG-deficient platelets. Furthermore, GPIIb/IIIa-mediated adhesion of platelets to fibrinogen could only weakly be inhibited in IRAG-deficient mice contrary to wild-type (WT) mice. Our results suggest that signaling via IRAG is essential for NO/cGMP-dependent inhibition of platelet activation regarding granule secretion, aggregation and adhesion. This platelet disorder might cause that the bleeding time of IRAG-deficient mice was reduced.     

The common pathway for alpha- and gamma-thrombin-induced platelet activation is independent of GPIb: a study of Bernard-Soulier platelets

The responses to alpha- and gamma-thrombin were studied in normal and Bernard-Soulier platelets labelled with [32P]phosphate, to investigate the relationship between thrombin binding to the platelet membrane glycoprotein Ib (GPIb) and thrombin-induced platelet activation. For this purpose we conducted parallel studies of the kinetics of platelet aggregation, granule secretion, hydrolysis of polyphosphoinositides, formation of phosphatidic acid, phosphorylation of the myosin light chain (p20) and of the 43 kDa protein (p43), and thromboxane B2 formation. Like alpha-thrombin, gamma-thrombin activated control platelets via all the above metabolic responses, but only after a prolonged lag. In Bernard-Soulier platelets, alpha-thrombin induced polyphosphoinositide hydrolysis and phosphatidic acid formation, p20 and p43 phosphorylation, thromboxane B2 formation, secretion and to a lesser extent aggregation, but only after a prolonged lag. The metabolic responses of Bernard-Soulier platelets to gamma-thrombin were very similar to those of control platelets. We have previously showed that GPIb which is not present in Bernard-Soulier platelets binds alpha- but not gamma-thrombin. The present results indicate that thrombin binding to GPIb is not directly coupled either with the activation of phospholipase C specific to polyphosphoinositides, or with the activation of protein kinase C and phospholipase A2. However, thrombin binding to GPIb appears to promote an early mechanism which accelerates all the platelet responses.     

Effect of extracellular magnesium on platelet activation and intracellular calcium mobilization.

A dose-dependent effect of magnesium on the inhibition of platelet aggregation and release of ATP from dense granules was observed in human platelets (in whole blood, platelet-rich plasma, or washed platelets) against various aggregation agents (ADP, U46619, collagen, or thrombin). The synthesis and release of the proaggregatory cyclooxygenase (CO) and lipoxygenase (LO) products, thromboxane A2 (TXA2) and 12-hydroxyeicosatetraenoic acid (12-HETE), respectively, in platelets were also inhibited by Mg in a dose-dependent manner (IC50 4 to 6 mmol/L). These Mg-mediated activities were further enhanced when platelets were preincubated with insulin (100 microU/mL). The effect of extracellular Mg on the change of intracellular calcium concentration ([Ca2+]i) was assessed using Fura-2/AM loaded cells in the presence or absence of extracellular Ca. Thrombin-stimulated influx of Ca ions decreased from 194 +/- 30 nmol/L to 156 +/- 21 nmol/L in the presence of 5 mmol/L Mg and to 111 +/- 16 nmol/L in 10 mmol/L Mg. However, the intracellular Ca release (as determined in the presence of 5 mmol/L EGTA) was not affected by Mg. The intracellular Ca-dependent protein kinase C and myosin light chain kinase activities on the phosphorylation of endogenous p47 and p20 proteins studied after 2 min of thrombin addition decreased only 10 to 25% in the presence of 5 to 10 mmol/L Mg. Similar results were obtained when EGTA was added prior to the initiation of protein phosphorylation. We conclude that Mg can dose dependently inhibit a wide variety of agonists on platelet aggregation. Furthermore, insulin can potentiate the inhibitory effects of Mg on platelet activation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synergistic effects of butyrate on platelet responses to arachidonate, A23187, PGE1, and forskolin

With eukaryotic cells, butyrate is known to induce a series of morphological and biochemical changes that mimic cellular differentiation. With platelets, we have found that butyrate (10 mmol/L) caused an approximately threefold increase in sensitivity to calcium ionophore A23187 and arachidonate. Maximum aggregation was observed at agonist concentrations of 3 mumol/L and 170 mumol/L, respectively, as compared with required concentrations of 10 mumol/L and 400 mumol/L in the absence of butyrate. Similar effects were seen with isobutyric acid, and about one-half the effect was shown with valerate and caproate, but lower homologues showed no synergistic effect. No ultrastructural changes were observed in platelets incubated with butyrate, and the aggregation effects were reversible and returned to normal on removal of butyrate. Membrane fluidity was unchanged by butyrate as measured by changes in the fluorescence depolarization of diphenylhexatriene. Butyrate caused a 60% to 70% increase in the uptake of 3H-arachidonate. Butyrate also potentiated the inhibition of platelet function by prostaglandin E1 and forskolin and uptake of 3H- forskolin was increased approximately 20%. In contrast, platelet response to other agonists (ADP, epinephrine, collagen, thrombin, and platelet-activating factor) was essentially unaffected by butyrate. These results suggest that butyrate may increase the uptake of certain hydophobic agonists and antagonists by platelets. Similar mechanisms for uptake of endogenous effectors may explain the response of eukaryotic cells to butyrate in culture.     

Matching the evaluation of the clinical efficacy of clopidogrel to platelet function tests relevant to the biological properties of the drug

OBJECTIVES: This study aimed to explore platelet function tests relevant to the biological effects of clopidogrel that could help the clinical monitoring of drug efficacy. BACKGROUND: Clopidogrel selectively inhibits the P2Y12 receptor, the major role of which is stabilization of aggregation, whereas initiation of aggregation depends on activity of both P2Y1 and P2Y12 receptors. METHODS: Tests used were peak aggregation (Agg(max)) and late aggregation (Agg(6min)), and disaggregation, relating to P2Y1 and P2Y12 activity, respectively; and monoclonal antibody binding activated glycoprotein (GP) IIb/IIIa receptors (PAC-1) and P-selectin, measuring activation and secretion. A first study compared hirudin/PPACK (r-hirudin and D-phenylalanyl-prolyl-arginine chloromethyl ketone) with citrate as blood anticoagulant (16 patients), and a second control study compared the effects of clopidogrel, aspirin, or both (20 normal controls). RESULTS: Clopidogrel similarly inhibited adenosine 5'-diphosphate (ADP)-induced Agg(max) with either anticoagulant, but significantly more Agg(6min) (75% vs. 31%), P-selectin (72% vs. 53%), and PAC-1 (62% vs. 24%) in hirudin/PPACK. In the control study, it inhibited Agg(max) by 22%, and Agg(6min), P-selectin, and PAC-1, by 69%, 66%, and 55%, respectively (all p < 0.05). Disaggregation at six min reached 62% with clopidogrel, but was virtually absent with placebo and aspirin. Non-responsiveness as evaluated by inhibition of Agg(max) in citrate was diagnosed in 35% of patients; in half this rate by Agg(6min), P-selectin, and PAC-1; and in 6% to 12% with the latter tests performed in hirudin/PPACK. CONCLUSIONS: The evaluation of clopidogrel responsiveness by platelet function tests is largely influenced by the choice of blood preservative and functional tests. Measures of aggregation stabilization, and of consequent secretion and activation, identified most patients as responders, contrasting with measures of peak aggregation, by likely reflecting better the interactions clopidogrel and the P2Y12 receptor.