Tyrosine phosphorylation in platelets

Platelet Oxidative Stress and its Relationship with Cardiovascular Diseases in Type 2 Diabetes Mellitus Patients

Enhanced platelet activation and thrombosis are linked to various cardiovascular diseases (CVD). Among other mechanisms, oxidative stress seems to play a pivotal role in platelet hyperactivity. Indeed, upon stimulation by physiological agonists, human platelets generate and release several types of reactive oxygen species (ROS) such as O2 -, H2O2 or OH-, further amplifying the platelet activation response via various signalling pathways, including, formation of isoprostanes, Ca2+ mobilization and NO inactivation. Furthermore, excessive platelet ROS generation, incorporation of free radicals from environment and/or depletion of antioxidants induce pro-oxidant, pro-inflammatory and platelet hyperaggregability effects, leading to the incidence of cardiovascular events. Here, we review the current knowledge regarding the effect of oxidative stress on platelet signaling pathways and its implication in CVD such as type 2 diabetes mellitus. We also summarize the role of natural antioxidants included in vegetables, fruits and medicinal herbs in reducing platelet function via an oxidative stress-mediated mechanism.

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Species differences in platelet responses to thrombin and SFLLRN. receptor-mediated calcium mobilization and aggregation, and regulation by protein kinases

The thrombin receptor on human platelets is activated by thrombin to stimulate platelet aggregation through the tethered ligand SFLLRN. This study examined the effects of thrombin and SFLLRN on aggregation and calcium mobilization ([Ca2+]i) in rat, guinea pig, rabbit, dog, monkey, and human platelets, and the role of protein kinases in regulating these functions. Thrombin induced platelet aggregation and [Ca2+]i in all species studied; however, only guinea pig, monkey and human platelets were responsive to SFLLRN. Similar species specific effects were obtained with [Ca2+]i studies. The kinetic profile for [Ca2+]i differed among species, suggesting that regulatory mechanisms for calcium differed between agonists and among species. Staurosporine, a non-selective inhibitor of protein kinases, inhibited platelet aggregation induced by thrombin or SFLLRN in all species. Staurosporine inhibited thrombin-induced [Ca2+]i in guinea pigs, had no effect in rat, and increased [Ca2+]i in all other species. Staurosporine inhibited SFLLRN-induced [Ca2+]i in guinea pig, yet had no effect in monkey or human. Tyrphostin 23, a specific inhibitor of tyrosine protein kinases, inhibited thrombin-induced aggregation of rabbit, monkey, dog and human platelets. SFLLRN-induced aggregation was also inhibited by tyrphostin 23. Tyrphostin 23 inhibited [Ca2+]i induced by either thrombin or SFLLRN in all species. Based on the differential response to agonist stimulation, we propose that thrombin can activate platelets via SFLLRN-dependent and independent mechanisms, which could involve yet unrecognized subtypes of the thrombin receptor or distinct cellular activating mechanisms. Furthermore, differential regulation of calcium mobilization and aggregation was observed in those platelets responding to either thrombin or SFLLRN.

Modulation of platelet aggregation by areca nut and betel leaf ingredients: roles of reactive oxygen species and cyclooxygenase

There are 2 to 6 billion betel quid (BQ) chewers in the world. Areca nut (AN), a BQ component, modulates arachidonic acid (AA) metabolism, which is crucial for platelet function. AN extract (1 and 2 mg/ml) stimulated rabbit platelet aggregation, with induction of thromboxane B2 (TXB2) production. Contrastingly, Piper betle leaf (PBL) extract inhibited AA-, collagen-, and U46619-induced platelet aggregation, and TXB2 and prostaglandin-D2 (PGD2) production. PBL extract also inhibited platelet TXB2 and PGD2 production triggered by thrombin, platelet activating factor (PAF), and adenosine diphosphate (ADP), whereas little effect on platelet aggregation was noted. Moreover, PBL is a scavenger of O2(*-) and *OH, and inhibits xanthine oxidase activity and the (*)OH-induced PUC18 DNA breaks. Deferoxamine, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) and neomycin prevented AN-induced platelet aggregation and TXB2 production. Indomethacin, genistein, and PBL extract inhibited only TXB2 production, but not platelet aggregation. Catalase, superoxide dismutase, and dimethylthiourea (DMT) showed little effect on AN-induced platelet aggregation, whereas catalase and DMT inhibited the AN-induced TXB2 production. These results suggest that AN-induced platelet aggregation is associated with iron-mediated reactive oxygen species production, calcium mobilization, phospholipase C activation, and TXB2 production. PBL inhibited platelet aggregation via both its antioxidative effects and effects on TXB2 and PGD2 production. Effects of AN and PBL on platelet aggregation and AA metabolism is crucial for platelet activation in the oral mucosa and cardiovascular system in BQ chewers.

Adherence of platelets under flow conditions results in specific phosphorylation of proteins at tyrosine residues

Collagen is a powerful platelet activating agent that promotes adhesion and aggregation of platelets. To differentiate the signals generated in these processes we have analyzed the tyrosine phosphorylation occurring in platelets after activation with collagen in suspension or under flow conditions. For the suspension studies, washed platelets were activated with different concentrations of purified type I collagen (ColI). Studies under flow conditions were performed using two different adhesive substrata: ColI and endothelial cells extracellular matrix (ECM). Coverslips coated with ColI or ECM were perfused through a parallel-plate perfusion chamber at 800 s(-1) for 5 min. After activation of platelets either in suspension or by adhesion, samples were solubilized and proteins were resolved by electrophoresis. Tyrosine-phosphorylated proteins were detected in immunoblots by specific antibodies. Activation of platelet suspensions with collagen induced tyrosine phosphorylation before aggregation could be detected. Profiles showing tyrosine-phosphorylated proteins from platelets adhered on ColI or on ECM were almost identical and lacked proteins p95, p80, p66, and p64, which were present in profiles from platelets activated in suspension. The intensity of phosphorylation was quantitatively weaker in those profiles from platelets adhered on ECM. Results from the present work indicate that activation of platelets in suspension or by adhesion induces differential tyrosine phosphorylation patterns. Phosphorylation of proteins p90 and p76 may be related to early activation events occurring during initial contact and spreading of platelets. Considering that adhesion is the first step of platelet activation, studies on signal transduction mechanisms under flow conditions may provide new insights to understand the signaling processes taking place at earliest stages of platelet activation.

Relocation of Syk protein-tyrosine kinase to the actin filament network and subsequent association with Fak

Previous studies demonstrated that Syk protein-tyrosine kinase (Syk) is activated by thrombin in platelets. To elucidate the function of Syk in platelets, we have biochemically examined the intracellular location of Syk and the molecules associated with Syk, following platelet activation. In human platelets, thrombin induces the relocation of Syk to the cytoskeletal fraction presumably via Syk tyrosine phosphorylation. Relocated Syk is associated with the actin filament network, and the early phase (10-90 s) of this association can be partially inhibited by the pretreatment of platelets with cytochalasin D, an inhibitor of actin polymerization. Upon thrombin stimulation, Syk becomes associated with Fak as demonstrated by co-immunoprecipitation. The association of both kinases can be inhibited by pretreatment of platelets with cytochalasin D. Interestingly, reconstitution experiments, using COS cells transfected with various porcine Syk mutants, revealed that the kinase domain, but not the kinase activity, of Syk is required for the association of Syk with the actin filament network. These findings suggest that thrombin-induced association of Syk with Fak correlates with the state of actin polymerization, and may play an important role in platelet activation.

Inhibition by ajoene of protein tyrosine phosphatase activity in human platelets

The effects of ajoene (a potent antithrombotic agent obtained from garlic) on the tyrosine phosphorylation status of human platelet proteins were investigated by immunoblotting-based experiments using an anti-phosphotyrosine antibody. Incubation of platelets with ajoene enhanced the phosphorylation of at least four proteins (estimated MWs 76, 80, 84 and 120 kDa), both in resting platelets and in platelets subsequently stimulated with thrombin (0.1 U/ml). This effect was both dose- and incubation-time-dependent. High concentrations of ajoene (50 microM) or long periods of incubation (10 min) led to nonselective 'hyperphosphorylation' of numerous proteins. The effects of ajoene on protein tyrosine phosphatase (PTP) activity in platelet lysates were also investigated, PTP activity was inhibited when platelets were incubated with ajoene before lysis, but not when ajoene was added to lysates of platelets which had not been pre-exposed to ajoene.

Oxygen free radicals and platelet activation

This article reviews our current understanding of the role of oxygen free radicals in platelet activation. Several studies have indicated that platelets, in analogy to other circulating blood cells, are able to produce oxygen free radicals, which are likely to play an important role in the mechanism of platelet activation and aggregation. Platelet activation has been obtained with very low, physiologically relevant concentrations of radicals generated chemically, by leukocytes, and by hemoglobin derived from membrane leakage of erythrocytes. Knowledge of the role of reactive species in platelet physiology is relevant because platelets are brought into close contact with other cells capable of producing free radicals, such as neutrophils, macrophages, and endothelial cells, during the formation of thrombus. The physiopatological importance of these findings is high because it is now emerging that free radicals may have a role in the mechanism of atherosclerosis and its thrombotic complications, where the causative role of platelets is well documented. This background suggests therapeutic interventions with antioxidants as antiplatelet agents to improve the pharmacological effect of classical antiplatelet drug such as aspirin.

Total inhibition of phospholipase C and phosphatidylinositol 3-kinase by okadaic acid in thrombin-stimulated platelets

The strong inhibition of thrombin-induced platelet functions induced by okadaic acid is not correlated with the partial modification of pleckstrin phosphorylation, which remains still phosphorylated two min after stimulation, indicating that protein kinase C is not affected by okadaic acid. We then investigated the effect of okadaic acid on platelet lipid metabolism. Our data indicate that inhibition indeed strongly affects phosphatidic acid as well as phosphatidylinositol 3,4-bisphosphate synthesis at low concentrations of okadaic acid, and phosphatidylinositol 4,5-bisphosphate at higher concentrations. Since thrombin-induced tyrosine phosphorylations were completely inhibited in the presence of okadaic acid, as a consequence, phosphatidylinositol 3-kinase was no longer detected in antiphosphotyrosine immunoprecipitates, thus explaining the absence of phosphatidylinositol, 3,4-bisphosphate synthesis. Finally, okadaic acid inhibited thrombin-induced fibrinogen binding, indicating that serine/threonine phosphatases may affect the inside-out signalling which regulates the alpha 11bb3 integrin, downstream protein kinase C activation.

Inhibitory effect of EGTA on serotonin transport into rabbit blood platelets: possible involvement of the glycoprotein IIb/IIIa complex

We reported previously that serotonin (5-HT) transport was attenuated by treatment of platelets with EGTA, and that this inhibitory effect of EGTA was restored by CaCl2. In the present study, the inhibitory effect of EGTA was found to be uncompetitive, and no inhibitory effect was observed when EGTA was added at 20 degrees C. Genistein and thyrphostin A47, both protein tyrosine kinase inhibitors, inhibited Ca(2+)-induced restoration of 5-HT transport. In contrast, the protein tyrosine phosphatase inhibitor phenylarsine oxide significantly augmented Ca(2+)-induced restoration of 5-HT transport. These results might support the hypothesis that the glycoprotein (GP) IIb/IIIa complex, a platelet membrane integrin protein, might regulate 5-HT transport into blood platelets. It is conceivable that Ca2+ chelation by EGTA might cause temperature-dependent dissociation of the GP IIb/IIIa complex, which results in the reduction of 5-HT transport. Rearrangement of the GP IIb/IIIa complex by replenishment of the Ca2+ binding sites might restore the EGTA-induced reduction of 5-HT transport.

Thrombopoietin modulates platelet activation in vitro through protein-tyrosine phosphorylation

To determine the roles of thrombopoietin (TPO) in platelet function in vitro, we examined the effects of TPO on platelet aggregation. Although several proteins in platelets were tyrosine-phosphorylated by TPO treatment, TPO alone was unable to induce platelet aggregation. However, the secondary wave of platelet aggregation induced by adenosine diphosphate (ADP) was enhanced by TPO in a dose-dependent manner. TPO in conjunction with ADP augmented tyrosine phosphorylation of platelet proteins, including tyrosine-phosphorylated proteins induced by TPO alone. Genistein inhibited protein-tyrosine phosphorylation in platelets induced by TPO with ADP and suppressed TPO-enhanced platelet aggregation. Moreover, tyrosine phosphorylation of MAP-kinases induced by TPO alone and TPO with ADP was consistent with TPO-enhanced platelet aggregation. These findings in the present study suggest that signal transduction involved in TPO-enhanced platelet aggregation is mediated in part by tyrosine-phosphorylated proteins, including MAP-kinases, in platelets through TPO-stimulated c-Mpl, TPO receptor.

Receptor-mediated regulation of leukotriene C4 synthase activity in human platelets

Human platelets possess a specific membrane-bound leukotriene (LT) C4 synthase, which catalyzes the conversion of LTA4 to LTC4. Stimulation of the receptors for thrombin, collagen or thromboxane A2 provoked inhibition of this enzyme, as judged by suppressed transformation of exogenous LTA4 to LTC4. Similarly, direct activation of protein kinase (PK) C with nanomolar concentrations of 4 beta-phorbol 12-myristate 13-acetate (PMA) inhibited the production of LTC4. Kinetic studies demonstrated that the inhibition induced by thrombin and PMA was non-competitive. Elevation of intracellular cAMP levels with carbacyclin did not affect basal LTC4 formation, but abolished the attenuation of platelet LTC4 synthase activity induced by the thromboxane receptor agonist U-46619. The unselective protein kinase inhibitor staurosporine prevented both receptor-mediated and PMA-induced suppression of LTC4 formation. In contrast, two selective PKC inhibitors, Ro 31-8220 and GF 109203X, reversed the inhibitory effect provoked by PMA, but failed to prevent thrombin-induced inhibition. Furthermore, the protein tyrosine phosphatase inhibitor, sodium orthovanadate, induced dose-dependent inhibition of LTC4 production in platelet sonicates. In conclusion, receptor-mediated activation of human platelets leads to decreased LTC4 synthase activity via phosphoregulation. Although the present results demonstrate that platelet LTC4 synthase can be regulated via PKC-dependent events, alternative mechanisms appears to be involved in the physiological regulation of this enzyme. The findings suggest the possible importance of protein tyrosine phosphorylations in this process.

Protein tyrosine phosphorylation in equine platelets: the effect of stimulation by thrombin and platelet-activating factor (PAF)

Protein tyrosine phosphorylation (PTP) in thrombin- and platelet-activating factor (PAF)-stimulated equine platelet activation was investigated in the absence and presence of 2 protein tyrosine kinase inhibitors (PTKIs), methyl 2,5-dihydroxycinnamate (MDHC) and genistein. Washed equine platelets aggregated irreversibly in response to thrombin or PAF in an agonist concentration dependent fashion. MDHC produced an MDHC concentration and time dependent inhibitory effect on rate and extent of thrombin- and PAF-induced aggregations, whereas the effect of genistein on the same parameters was only genistein concentration dependent. Western blotting demonstrated tyrosine phosphorylated proteins in resting platelets. Changes in the PTP pattern occurred both when platelets were stimulated with varying concentrations of thrombin or PAF for a standard time (3 min) or with a standard agonist concentration (0.17 u/ml thrombin or 10(-10) mol/l PAF) for varying times. Different patterns of PTP were produced by thrombin and PAF. 500 mumol/l MDHC and 300 mumol/l genistein each affected the PTP patterns produced in response to thrombin or PAF, but in different ways. PTP results with thrombin and PAF in the presence of 500 mumol/l MDHC were similar, as were those in the presence of 300 mumol/l genistein. However, there were many differences in the PTP results between thrombin (or PAF) in the presence of MDHC and between thrombin (or PAF) in the presence of genistein. Therefore, although both inhibitors are PTKIs, they have different effects on the PTP induced by either agonist. Our work has produced the first evidence of PTP in equine platelets. It is probable that the changes in PTP are related to events in the signal transduction pathway.

Regulation of protein tyrosine kinases in platelets

Platelet activation is accompanied by a dramatic increase in tyrosine phosphorylation of many cellular proteins. Phosphorylation of these proteins occurs in successive waves during the activation process, suggesting that several distinct mechanisms, occurring in a temporal order, regulate protein tyrosine kinases and/or phosphatases in activated platelets. Several tyrosine kinases, including Src family kinases, Syk and FAK, have been implicated in these phosphorylation events. These kinases are regulated by distinct receptor-mediated events involving activation of their catalytic activity and alterations in their cellular localization.