Protein kinase C and cyclic AMP regulate reversible exposure of binding sites for fibrinogen on the glycoprotein IIB-IIIA complex of human platelets

Difluorinated thromboxane A2 reveals crosstalk between platelet activatory and inhibitory pathways by targeting both the TP and IP receptors

BACKGROUND AND PURPOSE

Thromboxane A2 (TXA2) is a prostanoid produced during platelet activaton, important in enhancing platelet reactivity by activation of TP receptors. However, due to the short half-life, studying TXA2 signalling is challenging. To enhance our understanding of TP receptor-mediated platelet biology, we therefore synthesised mono and difluorinated TXA2 analogues and explored their pharmacology on heterologous and endogenously expressed TP receptor function.

EXPERIMENTAL APPROACH

Platelet functional and signalling responses were studied using aggregometry, Ca2+ mobilisation experiments and immunoblotting and compared with an analogue of the TXA2 precursor prostaglandin H2, U46619. Gαq/Gαs receptor signalling was determined using a bioluminescence resonance energy transfer (BRET) assay in a cell line overexpression system.

KEY RESULTS

BRET studies revealed that F-TXA2 and F2-TXA2 promoted receptor-stimulated TP receptor G-protein activation similarly to U46619. Unexpectedly, F2-TXA2 caused reversible aggregation in platelets, whereas F-TXA2 and U46619 induced sustained aggregation. Blocking the IP receptor switched F2-TXA2-mediated reversible aggregation into sustained aggregation. Further BRET studies confirmed F2-TXA2-mediated IP receptor activation. F2-TXA2 rapidly and potently stimulated platelet TP receptor-mediated protein kinase C/P-pleckstrin, whereas IP-mediated protein kinase A/P-vasodilator-stimulated phosphoprotein was more delayed.

CONCLUSION AND IMPLICATIONS

F-TXA2 is a close analogue to TXA2 used as a selective tool for TP receptor platelet activation. In contrast, F2-TXA2 acts on both TP and IP receptors differently over time, resulting in an initial wave of TP receptor-mediated platelet aggregation followed by IP receptor-induced reversibility of aggregation. This study reveals the potential difference in the temporal aspects of stimulatory and inhibitory pathways involved in platelet activation.

Platelet Activation Pathways Controlling Reversible Integrin αIIbβ3 Activation

Background  Agonist-induced platelet activation, with the integrin αIIbβ3 conformational change, is required for fibrinogen binding. This is considered reversible under specific conditions, allowing a second phase of platelet aggregation. The signaling pathways that differentiate between a permanent or transient activation state of platelets are poorly elucidated. Objective  To explore platelet signaling mechanisms induced by the collagen receptor glycoprotein VI (GPVI) or by protease-activated receptors (PAR) for thrombin that regulate time-dependent αIIbβ3 activation. Methods  Platelets were activated with collagen-related peptide (CRP, stimulating GPVI), thrombin receptor-activating peptides, or thrombin (stimulating PAR1 and/or 4). Integrin αIIbβ3 activation and P-selectin expression was assessed by two-color flow cytometry. Signaling pathway inhibitors were applied before or after agonist addition. Reversibility of platelet spreading was studied by microscopy. Results  Platelet pretreatment with pharmacological inhibitors decreased GPVI- and PAR-induced integrin αIIbβ3 activation and P-selectin expression in the target order of protein kinase C (PKC) > glycogen synthase kinase 3 > β-arrestin > phosphatidylinositol-3-kinase. Posttreatment revealed secondary αIIbβ3 inactivation (not P-selectin expression), in the same order, but this reversibility was confined to CRP and PAR1 agonist. Combined inhibition of conventional and novel PKC isoforms was most effective for integrin closure. Pre- and posttreatment with ticagrelor, blocking the P2Y 12 adenosine diphosphate (ADP) receptor, enhanced αIIbβ3 inactivation. Spreading assays showed that PKC or P2Y 12 inhibition provoked a partial conversion from filopodia to a more discoid platelet shape. Conclusion  PKC and autocrine ADP signaling contribute to persistent integrin αIIbβ3 activation in the order of PAR1/GPVI > PAR4 stimulation and hence to stabilized platelet aggregation. These findings are relevant for optimization of effective antiplatelet treatment.

Alterations in platelet proteome signature and impaired platelet integrin αIIbβ3 activation in patients with COVID-19

BACKGROUND

Patients with COVID-19 are at increased risk of thrombosis, which is associated with altered platelet function and coagulopathy, contributing to excess mortality.

OBJECTIVES

To characterize the mechanism of altered platelet function in COVID-19 patients.

METHODS

The platelet proteome, platelet functional responses, and platelet-neutrophil aggregates were compared between patients hospitalized with COVID-19 and healthy control subjects using tandem mass tag proteomic analysis, Western blotting, and flow cytometry.

RESULTS

COVID-19 patients showed a different profile of platelet protein expression (858 altered of the 5773 quantified). Levels of COVID-19 plasma markers were enhanced in the platelets of COVID-19 patients. Gene ontology pathway analysis demonstrated that the levels of granule secretory proteins were raised, whereas those of platelet activation proteins, such as the thrombopoietin receptor and protein kinase Cα, were lowered. Basally, platelets of COVID-19 patients showed enhanced phosphatidylserine exposure, with unaltered integrin α_IIbβ₃ activation and P-selectin expression. Agonist-stimulated integrin α_IIbβ₃ activation and phosphatidylserine exposure, but not P-selectin expression, were decreased in COVID-19 patients. COVID-19 patients had high levels of platelet-neutrophil aggregates, even under basal conditions, compared to controls. This association was disrupted by blocking P-selectin, demonstrating that platelet P-selectin is critical for the interaction.

CONCLUSIONS

Overall, our data suggest the presence of 2 platelet populations in patients with COVID-19: one of circulating platelets with an altered proteome and reduced functional responses and another of P-selectin-expressing neutrophil-associated platelets. Platelet-driven thromboinflammation may therefore be one of the key factors enhancing the risk of thrombosis in COVID-19 patients.

The contribution of zinc to platelet behaviour during haemostasis and thrombosis

Platelets are the primary cellular determinants of haemostasis and pathological thrombus formation leading to myocardial infarction and stroke. Following vascular injury or atherosclerotic plaque rupture, platelets are recruited to sites of damage and undergo activation induced by a variety of soluble and/or insoluble agonists. Platelet activation is a multi-step process culminating in the formation of thrombi, which contribute to the haemostatic process. Zinc (Zn(2+)) is acknowledged as an important signalling molecule in a diverse range of cellular systems, however there is limited understanding of the influence of Zn(2+) on platelet behaviour during thrombus formation. This review evaluates the contributions of exogenous and intracellular Zn(2+) to platelet function and assesses the potential pathophysiological implications of Zn(2+) signalling. We also provide a speculative assessment of the mechanisms by which platelets could respond to changes in extracellular and intracellular Zn(2+) concentration.

Vasodilator-stimulated phosphoprotein-phosphorylation by ginsenoside Ro inhibits fibrinogen binding to αIIb/β3 in thrombin-induced human platelets

BACKGROUND

Glycoprotein IIb/IIIa (αIIb/β3) is involved in platelet adhesion, and triggers a series of intracellular signaling cascades, leading to platelet shape change, granule secretion, and clot retraction. In this study, we evaluated the effect of ginsenoside Ro (G-Ro) on the binding of fibrinogen to αIIb/β3.

METHODS

We investigated the effect of G-Ro on regulation of signaling molecules affecting the binding of fibrinogen to αIIb/β3, and its final reaction, clot retraction.

RESULTS

We found that G-Ro dose-dependently inhibited thrombin-induced platelet aggregation and attenuated the binding of fibrinogen to αIIb/β3 by phosphorylating cyclic adenosine monophosphate (cAMP)-dependently vasodilator-stimulated phosphoprotein (VASP; Ser157). In addition, G-Ro strongly abrogated the clot retraction reflecting the intensification of thrombus.

CONCLUSION

We demonstrate that G-Ro is a beneficial novel compound inhibiting αIIb/β3-mediated fibrinogen binding, and may prevent platelet aggregation-mediated thrombotic disease.

Total saponin from Korean Red Ginseng inhibits binding of adhesive proteins to glycoprotein IIb/IIIa via phosphorylation of VASP (Ser(157)) and dephosphorylation of PI3K and Akt

Background

Binding of adhesive proteins (i.e., fibrinogen, fibronectin, vitronectin) to platelet integrin glycoprotein IIb/IIIa (αIIb/β₃) by various agonists (thrombin, collagen, adenosine diphosphate) involve in strength of thrombus. This study was carried out to evaluate the antiplatelet effect of total saponin from Korean Red Ginseng (KRG-TS) by investigating whether KRG-TS inhibits thrombin-induced binding of fibrinogen and fibronectin to αIIb/β₃.

Methods

We investigated the effect of KRG-TS on phosphorylation of vasodilator-stimulated phosphoprotein (VASP) and dephosphorylation of phosphatidylinositol 3-kinase (PI3K) and Akt, affecting binding of fibrinogen and fibronectin to αIIb/β₃, and clot retraction.

Results

KRG-TS had an antiplatelet effect by inhibiting the binding of fibrinogen and fibronectin to αIIb/β₃ via phosphorylation of VASP (Ser¹⁵⁷), and dephosphorylation of PI3K and Akt on thrombin-induced platelet aggregation. Moreover, A-kinase inhibitor Rp-8-Br-cyclic adenosine monophosphates (cAMPs) reduced KRG-TS-increased VASP (Ser¹⁵⁷) phosphorylation, and increased KRG-TS-inhibited fibrinogen-, and fibronectin-binding to αIIb/β₃. These findings indicate that KRG-TS interferes with the binding of fibrinogen and fibronectin to αIIb/β₃ via cAMP-dependent phosphorylation of VASP (Ser¹⁵⁷). In addition, KRG-TS decreased the rate of clot retraction, reflecting inhibition of αIIb/β₃ activation. In this study, we clarified ginsenoside Ro (G-Ro) in KRG-TS inhibited thrombin-induced platelet aggregation via both inhibition of [Ca²⁺]_i mobilization and increase of cAMP production.

Conclusion

These results strongly indicate that KRG-TS is a beneficial herbal substance inhibiting fibrinogen-, and fibronectin-binding to αIIb/β₃, and clot retraction, and may prevent platelet αIIb/β₃-mediated thrombotic disease. In addition, we demonstrate that G-Ro is a novel compound with antiplatelet characteristics of KRG-TS.

Effect of Cordycepin-Enriched WIB801C from Cordyceps militaris Suppressing Fibrinogen Binding to Glycoprotein IIb/IIIa

In this study, we investigated the effects of cordycepin-enriched (CE)-WIB801C, a n-butanol extract of Cordyceps militaris-hypha on collagen-stimulated platelet aggregation. CE-WIB801C dose dependently inhibited collagen-induced platelet aggregation, and had a synergistic effect together with cordycepin (W-cordycepin) from CE-WIB801C on the inhibition of collagen-induced platelet aggregation. CE-WIB801C and cordycepin stimulated the phosphorylation of VASP (Ser(157)) and the dephosphorylation of PI3K and Akt, and inhibited the binding of fibrinogen to glycoprotein IIb/IIIa (αIIb/β3) and the release of ATP and serotonin in collagen-induced platelet aggregation. A-kinase inhibitor Rp-8-Br-cAMPS reduced CE-WIB801C-, and cordycepin-increased VASP (Ser(157)) phosphorylation, and increased CE-WIB801C-, and cordycepin-inhibited the fibrinogen binding to αIIb/β3. Therefore, we demonstrate that CE-WIB801C-, and cordycepin-inhibited fibrinogen binding to αIIb/β3 are due to stimulation of cAMP-dependent phosphorylation of VASP (Ser(157)), and inhibition of PI3K/Akt phosphorylation. These results strongly indicate that CE-WIB801C and cordycepin may have preventive or therapeutic potential for platelet aggregation-mediated diseases, such as thrombosis, myocardial infarction, atherosclerosis, and ischemic cerebrovascular disease.

Cordycepin-enriched WIB801C from Cordyceps militaris inhibits ADP-induced [Ca(2+)] i mobilization and fibrinogen binding via phosphorylation of IP 3R and VASP

In this study, we investigated the effect of cordycepin-enriched (CE)-WIB801C from Cordyceps militaris on ADP (20 µM)-stimulated platelet aggregation. CE-WIB801C dose-dependently inhibited ADP-induced platelet aggregation, and its IC50 value was 18.5 μg/mL. CE-WIB801C decreased TXA2 production, but did not inhibit the activities of COX-1 and thromboxane synthase (TXAS) in ADP-activated platelets, which suggests that the inhibition of TXA2 production by CE-WIB801C is not resulted from the direct inhibition of COX-1 and TXAS. CE-WIB801C inhibited ATP release and [Ca(2+)]i mobilization, and increased cAMP level and IP3RI (Ser(1756)) phosphorylation in ADP-activated platelets. cAMP-dependent protein kinase (A-kinase) inhibitor Rp-8-Br-cAMPS increased CE-WIB801C-inhibited [Ca(2+)]i mobilization, and strongly inhibited CE-WIB801C-increased IP3RI (Ser(1756)) phosphorylation. CE-WIB801C elevated the phosphorylation of VASP (Ser(157)), an A-kinase substrate, but inhibited fibrinogen binding to αIIb/β3. These results suggest that CE-WIB801C-elevated cAMP involved in IP3RI (Ser(1756)) phosphorylation to inhibit [Ca(2+)]i mobilization and, VASP (Ser(157)) phosphorylation to inhibit αIIb/β3 activation. Therefore, in this study, we demonstrate that CE-WIB801C may have a preventive or therapeutic potential for platelet aggregation-mediated diseases, such as thrombosis, myocardial infarction, atherosclerosis, and ischemic cerebrovascular disease.

Protein kinase C ε expression in platelets from patients with acute myocardial infarction

Objective

Platelets play crucial roles in the pathophysiology of thrombosis and myocardial infarction. Protein kinase C ε (PKCε) is virtually absent in human platelets and its expression is precisely regulated during human megakaryocytic differentiation. On the basis of what is known on the role of platelet PKCε in other species, we hypothesized that platelets from myocardial infarction patients might ectopically express PKCε with a pathophysiological role in the disease.

Methods and Results

We therefore studied platelet PKCε expression from 24 patients with myocardial infarction, 24 patients with stable coronary artery disease and 24 healthy subjects. Indeed, platelets from myocardial infarction patients expressed PKCε with a significant frequency as compared to both stable coronary artery disease and healthy subjects. PKCε returned negative during patient follow-up. The forced expression of PKCε in normal donor platelets significantly increased their response to adenosine diphosphate-induced activation and adhesion to subendothelial collagen.

Conclusions

Our data suggest that platelet generations produced before the acute event retain PKCε-mRNA that is not down-regulated during terminal megakaryocyte differentiation. Results are discussed in the perspective of peri-infarctual megakaryocytopoiesis as a critical component of myocardial infarction pathophysiology.

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.

Treprostinil inhibits the adhesion and differentiation of fibrocytes via the cyclic adenosine monophosphate-dependent and Ras-proximate protein-dependent inactivation of extracellular regulated kinase

Fibrocytes comprise a recently described cell type of blood-derived, fibroblast-like cells that are recruited from the circulation to sites of wound repair, vascular remodeling, or fibrotic tissue remodeling. We recently showed that the stable prostacyclin analogue treprostinil, a clinically approved drug for pulmonary arterial hypertension (PAH), significantly reduced the recruitment of fibrocytes to sites of vascular remodeling in experimental hypoxic pulmonary hypertension. Here we report on the molecular mechanism underlying the inhibitory action of treprostinil on the adhesion and differentiation of human fibrocytes. Human fibrocytes expressed the prostanoid receptors, prostaglandin I (IP) receptors and prostaglandin E subtype receptors (EP2 and EP4). The generation of intracellular cyclic adenosine monophosphate (cAMP) by treprostinil reduced the expression of the integrins CD49 and CD29 when freshly isolated human peripheral blood mononuclear cells were treated with treprostinil. Cell-matrix adhesion was significantly impaired by treatment with treprostinil. We present evidence for a treprostinil/cAMP-induced downstream suppression of extracellular regulated kinase (ERK) that is transmitted via a protein kinase A-independent pathway through Rap proteins, which sequester Ras. The resulting dephosphorylated state of c-Raf limits the activity of ERK. The cell-matrix adhesion assay with the ERK inhibitor further confirmed that the adhesion of fibrocytes was impaired. Thus our data suggest that treprostinil inhibits the adhesion and differentiation of fibrocytes by limiting the activity of ERK via the cAMP-Rap axis.

Platelet protein kinase C-theta deficiency with human RUNX1 mutation: PRKCQ is a transcriptional target of RUNX1

OBJECTIVE

Mutations in the hematopoietic transcription factor RUNX1 cause thrombocytopenia and impaired platelet function. In a patient with a heterozygous mutation in RUNX1, we have described decreased platelet pleckstrin phosphorylation and protein kinase C- (PKC-, gene PRKCQ) associated with thrombocytopenia, impaired platelet aggregation, and dense granule secretion. Little is known regarding regulation of PKC- in megakaryocytes and platelets. We have addressed the hypothesis that PRKCQ is a direct transcriptional target of RUNX1.

METHODS AND RESULTS

In a chromatin immunoprecipitation assay using megakaryocytic cells, there was RUNX1 binding in vivo to PRKCQ promoter region -1225 to -1056 bp containing a RUNX1 consensus site ACCGCA at -1088 to -1069 bp; an electrophoretic mobility shift assay showed RUNX1 binding to the specific site. In RUNX1 overexpression studies, PKC- protein expression and promoter activity were enhanced; mutation of RUNX1 site showed decreased activity even with RUNX1 overexpression. Lastly, PRKCQ promoter activity and PKC- protein were decreased by short interfering RNA knockdown of RUNX1.

CONCLUSIONS

Our results provide the first evidence that PRKCQ is regulated at the transcriptional level by RUNX1 in megakaryocytic cells and a mechanism for PKC- deficiency associated with RUNX1 haplodeficiency.

Sphingolipid therapy in myocardial ischemia-reperfusion injury

Sphingolipids are known to play a significant physiological role in cell growth, cell differentiation, and critical signal transduction pathways. Recent studies have demonstrated a significant role of sphingolipids and their metabolites in the pathogenesis of myocardial ischemia-reperfusion injury. Our laboratory has investigated the cytoprotective effects of N,N,N-trimethylsphingosine chloride (TMS), a stable N-methylated synthetic sphingolipid analogue on myocardial and hepatic ischemia-reperfusion injury in clinically relevant in vivo murine models of ischemia-reperfusion injury. TMS administered intravenously at the onset of ischemia reduced myocardial infarct size in the wild-type and obese (ob/ob) mice. Following myocardial I/R, there was an improvement in cardiac function in the wild-type mice. Additionally, TMS also decreased serum liver enzymes following hepatic I/R in wild-type mice. The cytoprotective effects did not extend to the ob/ob mice following hepatic I/R or to the db/db mice following both myocardial and hepatic I/R. Our data suggest that although TMS is cytoprotective following I/R in normal animals, the cytoprotective actions of TMS are largely attenuated in obese and diabetic animals which may be due to altered signaling mechanisms in these animal models. Here we review the therapeutic role of TMS and other sphingolipids in the pathogenesis of myocardial ischemia-reperfusion injury and their possible mechanisms of cardioprotection.

Cytoprotective effects of N,N,N-trimethylsphingosine during ischemia- reperfusion injury are lost in the setting of obesity and diabetes

N,N,N-trimethylsphingosine chloride (TMS), a stable N-methylated synthetic sphingolipid analog, has been shown to modulate protein kinase C (PKC) activity and exert a number of important biological effects, including inhibition of tumor cell growth and metastasis, inhibition of leukocyte migration and respiratory burst, and inhibition of platelet aggregation. We hypothesized that TMS would be cytoprotective in clinically relevant in vivo murine models of myocardial and hepatic ischemia-reperfusion (I/R) injury. Wild-type, obese (ob/ob), and diabetic (db/db) mice were subjected to 30 min of left coronary artery occlusion followed by 24 h of reperfusion in the myocardial I/R model. In additional studies, mice were subjected to 45 min of hepatic artery occlusion followed by 5 h of reperfusion. TMS was administered intravenously at the onset of ischemia. Myocardial infarct size, cardiac function, and serum liver enzymes were measured to assess the extent of tissue injury. TMS attenuated myocardial infarct size by 66% in the wild type and by 36% in the ob/ob mice. Furthermore, TMS reduced serum alanine transaminase levels by 43% in wild-type mice. These benefits did not extend to the ob/ob mice following hepatic I/R or to the db/db mice following both myocardial and hepatic I/R. A likely mechanism is the failure of TMS to inhibit PKC-delta translocation in the diseased heart. These data suggest that although TMS is cytoprotective following I/R in normal animals, the cytoprotective actions of TMS are largely attenuated in obese and diabetic animals.

Dual Role of Platelet Protein Kinase C in Thrombus Formation

Protein kinase C (PKC) isoforms regulate many platelet responses in a still incompletely understood manner. Here we investigated the roles of PKC in the platelet reactions implicated in thrombus formation as follows: secretion aggregate formation and coagulation-stimulating activity, using inhibitors with proven activity in plasma. In human and mouse platelets, PKC regulated aggregation by mediating secretion and contributing to alphaIIbbeta3 activation. Strikingly, PKC suppressed Ca(2+) signal generation and Ca(2+)-dependent exposure of procoagulant phosphatidylserine. Furthermore, under coagulant conditions, PKC suppressed the thrombin-generating capacity of platelets. In flowing human and mouse blood, PKC contributed to platelet adhesion and controlled secretion-dependent thrombus formation, whereas it down-regulated Ca(2+) signaling and procoagulant activity. In murine platelets lacking G(q)alpha, where secretion reactions were reduced in comparison with wild type mice, PKC still positively regulated platelet aggregation and down-regulated procoagulant activity. We conclude that platelet PKC isoforms have a dual controlling role in thrombus formation as follows: (i) by mediating secretion and integrin activation required for platelet aggregation under flow, and (ii) by suppressing Ca(2+)-dependent phosphatidylserine exposure, and consequently thrombin generation and coagulation. This platelet signaling protein is the first one identified to balance the pro-aggregatory and procoagulant functions of thrombi.

Reduced thromboxane biosynthesis in carriers of toll-like receptor 4 polymorphisms in vivo

The recent demonstration that platelets express a functional toll-like receptor 4 (TLR4) prompted us to explore the influence of TLR4 polymorphisms (Asp299Gly alone or in combination with Thr399Ile) on thromboxane A2 (TXA2) biosynthesis in vivo. In 17 subjects with TLR4 polymorphisms versus 17 wild type (untreated with aspirin, matched for age, sex, and cardiovascular risk factors), intima-media thickness in the common carotid arteries was significantly lower. Average urinary excretion of 11-dehydro-TXB2, an index of systemic biosynthesis of TX, was significantly reduced by 65%. The urinary excretion of 2,3-dinor-6-keto-prostaglandin F1α, an index of systemic biosynthesis of prostacyclin, was marginally depressed but the prostacyclin/TXA2 biosynthesis ratio was significantly higher than in wild type. Selective inhibition of cyclooxygenase 2-dependent prostacyclin (by rofecoxib or etoricoxib) was associated with increased urinary excretion of 11-dehydro-TXB2 in carriers of TLR4 polymorphisms, but not in wild-type, suggesting a restrainable effect of prostacyclin on platelet function in vivo in this setting. Reduced TXA2 biosynthesis may contribute to the protective cardiovascular phenotype of TLR4 polymorphisms.

Endothelial cell integrins and COX-2: mediators and therapeutic targets of tumor angiogenesis

Vascular integrins are essential regulators and mediators of physiological and pathological angiogenesis, including tumor angiogenesis. Integrins provide the physical interaction with the extracellular matrix (ECM) necessary for cell adhesion, migration and positioning, and induce signaling events essential for cell survival, proliferation and differentiation. Integrins preferentially expressed on neovascular endothelial cells, such as alphaVbeta3 and alpha5beta1, are considered as relevant targets for anti-angiogenic therapies. Anti-integrin antibodies and small molecular integrin inhibitors suppress angiogenesis and tumor progression in many animal models, and are currently tested in clinical trials as anti-angiogenic agents. Cyclooxygense-2 (COX-2), a key enzyme in the synthesis of prostaglandins and thromboxans, is highly up-regulated in tumor cells, stromal cells and angiogenic endothelial cells during tumor progression. Recent experiments have demonstrated that COX-2 promotes tumor angiogenesis. Chronic intake of nonsteroidal anti-inflammatory drugs and COX-2 inhibitors significantly reduces the risk of cancer development, and this effect may be due, at least in part, to the inhibition of tumor angiogenesis. Endothelial cell COX-2 promotes integrin alphaVbeta3-mediated endothelial cell adhesion, spreading, migration and angiogenesis through the prostaglandin-cAMP-PKA-dependent activation of the small GTPase Rac. In this article, we review the role of integrins and COX-2 in angiogenesis, their cross talk, and discuss implications relevant to their targeting to suppress tumor angiogenesis.

Involvement of the beta3 E749ATSTFTN756 region in stabilizing integrin alphaIIbbeta3-ligand interaction

Platelet integrin alphaIIbbeta3 must be activated via intracellular mechanisms before it binds soluble ligands, and it is thought to be activated at its extracellular site by surface-bound ligands. Integrin activation is associated with rearrangement of the cytoskeleton and phosphorylation of proteins that become localized in focal contacts. In these processes, the cytoplasmic tail of the beta-subunit plays a central role. We introduced peptides homologous to the E749ATSTFTN756 domain (E-N peptide) and the T755NITYRGT762 domain (T-T peptide) of beta3 in streptolysin O-permeabilized platelets and analyzed the initial interaction with soluble fibronectin, fibrinogen and PAC-1 after stimulation with thrombin. E-N peptide left the initial binding of fibronectin intact but interfered with stable receptor occupancy. E-N peptide also inhibited fibrinogen binding, thereby reducing the formation of large aggregates. Strikingly, E-N peptide did not disturb the binding of PAC-1, which is known to reflect activation of the integrin. E-N peptide also inhibited tyrosine phosphorylation of focal adhesion kinase, a response known to be dependent on alphaIIbbeta3. T-T peptide did not affect these processes. In a model for outside-in integrin activation, E-N peptide disrupted the binding of CHO cells expressing alphaIIbbeta3 to surface-bound ligand. Again, T-T peptide had no effect. We conclude that the E749ATSTFTN756 region of the beta3-tail stabilizes the binding of soluble and surface-bound ligand to integrin alphaIIbbeta3 via a mechanism that involves the phosphorylation of FAK.

Cytoplasmic regions of the beta3 subunit of integrin alphaIIbbeta3 involved in platelet adhesion on fibrinogen under flow conditions

Platelet adhesion to surface-bound fibrinogen depends on integrin alphaIIbbeta3. In the present study, we investigated the role of the regions 749EATSTFT756N and 755TNITYRG762T of the beta3 cytoplasmic tail in the regulation of platelet adhesion under flow conditions, by introducing peptide mimetics in platelets. Introduction of peptide EATSTFTN (E-N) increased surface coverage by 35%, an effect caused by 25% more adhesion. In contrast, peptide TNITYRGT (T-T) decreased surface coverage by 16%, as a result of 25% less adhesion. An S-->P substitution in the E-N peptide, thereby mimicking a mutation in Glanzmann's thrombasthenia, abolished the effect of E-N. A suboptimal concentration of cytochalasin D is known to enhance ligand binding to alphaIIbbeta3 in platelet suspensions. Under flow, cytochalasin D (1 micro mol L-1) induced 50% more platelet adhesion, with a strong reduction in platelet spreading. Both peptides opposed the increase in adhesion by cytochalasin D and partly (E-N) and completely (T-T) restored platelet spreading. Thus, the 749EATSTFT756N and 755TNITYRG762T regions of beta3 contribute to the regulation of alphaIIbbeta3 anchorage to the cytoskeleton and platelet spreading to an adhesive surface.

Protein kinase C- and calcium-regulated pathways independently synergize with Gi pathways in agonist-induced fibrinogen receptor activation

Platelet fibrinogen receptor activation is a critical step in platelet plug formation. The fibrinogen receptor (integrin alphaIIbbeta3) is activated by agonist-mediated G(q) stimulation and resultant phospholipase C activation. We investigated the role of downstream signalling events from phospholipase C, namely the activation of protein kinase C (PKC) and rise in intracellular calcium, in agonist-induced fibrinogen receptor activation using Ro 31-8220 (a PKC inhibitor) or dimethyl BAPTA [5,5'-dimethyl-bis-(o-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid], a high-affinity calcium chelator. All the experiments were performed with human platelets treated with aspirin, to avoid positive feedback from thromboxane A2. In the presence of Ro 31-8220, platelet aggregation caused by U46619 was completely inhibited while no effect or partial inhibition was seen with ADP and the thrombin-receptor-activating peptide SFLLRN, respectively. In the presence of intracellular dimethyl BAPTA, ADP- and U46619-induced aggregation and anti-alphaIIbbeta3 antibody PAC-1 binding were completely abolished. However, similar to the effects of Ro 31-8220, dimethyl BAPTA only partially inhibited SFLLRN-induced aggregation, and was accompanied by diminished dense-granule secretion. When either PKC activation or intracellular calcium release was abrogated, aggregation and fibrinogen receptor activation with U46619 or SFLLRN was partially restored by additional selective activation of the G(i) signalling pathway. In contrast, when both PKC activity and intracellular calcium increase were simultaneously inhibited, the complete inhibition of aggregation that occurred in response to either U46619 or SFLLRN could not be restored with concomitant G(i) signalling. We conclude that, while the PKC- and calcium-regulated signalling pathways are capable of inducing activating fibrinogen receptor independently and that each can synergize with G(i) signalling to cause irreversible fibrinogen receptor activation, both pathways act synergistically to effect irreversible fibrinogen receptor activation.

The integrin alpha IIb/beta 3 in human platelet signal transduction

Platelets are critical for the maintenance of the integrity of the vascular system and are the first line of defence against haemorrhage. When they encounter a subendothelial matrix exposed by injury to a vessel, platelets adhere, are activated, and become adhesive for other platelets so that they aggregate. alpha IIb/beta 3, a platelet-specific integrin, is largely prominent amongst the adhesion receptors and is essential for platelet aggregation. The ligands for alpha IIb/beta 3 are the multivalent adhesive proteins fibrinogen and von Willebrand factor. In resting platelets, alpha IIb/beta 3 is normally in a low activation state, unable to interact with soluble fibrinogen. Stimulation of platelets with various agonists will induce a conformational change in alpha IIb/beta 3 (inside-out signalling), which is then able to bind soluble fibrinogen resulting in the onset of platelet aggregation. However, fibrinogen binding to its membrane receptor is not simply a passive event allowing the formation of intercellular bridges between platelets. Indeed, a complex signalling pathway triggered by integrin ligation and clustering (outside-in signalling) will regulate the extent of irreversible platelet aggregation and clot retraction. Amongst the signalling enzymes activated downstream of alpha IIb/beta 3 engagement, phosphoinositide 3-kinase plays an important role in the control of the irreversible phase of aggregation.

Activated platelets in patients with severe hypertriglyceridemia: effects of triglyceride-lowering therapy

Hypertriglyceridemia, a risk factor for cardiovascular disease, has been associated with hypercoagulability, but whether platelet activation is implicated is unknown. This study was designed to compare the in vivo platelet activation status between patients with severe hypertriglyceridemia and age- and sex-matched control subjects, and to evaluate the effects of triglyceride-lowering therapy. Sixteen patients with primary hypertriglyceridemia were included in a double-blind, placebo-controlled cross-over trial with 400 mg bezafibrate once daily. Platelet activation was analysed by double label flow cytometry, using monoclonal antibodies against GP53, P-selectin, and platelet-bound fibrinogen. Surface expression of the lysosomal membrane protein GP53 was significantly higher in the hypertriglyceridemic patients at baseline as compared to the group of age- and sex-matched controls (16.3+/-4.8% vs. 8.9+/-3.4%, respectively, P<0.001). No differences in the expression of P-selectin and fibrinogen binding were observed. In response to bezafibrate therapy, the expression of GP53 in the patient group decreased from 16.3+/-4.8% to 13.1+/-4.1% (P=0.018). The expression of P-selectin and fibrinogen binding was not affected by bezafibrate therapy. In conclusion, patients with hypertriglyceridemia have an increased in vivo platelet activation status, which can be improved by bezafibrate therapy.

Inhibition of platelet integrin alpha(IIb)beta(3) by peptides that interfere with protein kinases and the beta(3) tail

alpha-Thrombin stimulation of human platelets initiates inside-out signaling to integrin alpha(IIb)beta(3) (glycoprotein IIb/IIIa), resulting in the exposure of ligand binding sites. In the present study, the regulation of alpha(IIb)beta(3) via protein kinases was investigated in platelets permeabilized with streptolysin O by introducing peptides that interfere with these enzymes and with possible regulatory domains in the cytosolic tail of the beta(3) subunit. Compared with intact platelets, the permeabilized platelets preserved >80% of the aggregation, secretion, and alpha(IIb)beta(3) ligand binding capacity. The peptide YIYGSFK, a substrate for Src kinases, inhibited alpha-thrombin-induced ligand binding to alpha(IIb)beta(3), but a reversed peptide with Y-->F substitutions (KFSGFIF) had no effect. Ligand binding to alpha(IIb)beta(3) was also inhibited by the peptide RKRCLRRL, which binds irreversibly to the catalytic domain of protein kinase C. Peptides corresponding to parts of the protein C inhibitor and beta(2)-glycoprotein I were used as negative controls and failed to interfere with ligand binding. Possible target domains for protein kinases are present in the cytoplasmic tail of the beta(3) subunit. The LLITIHDR peptide, matching the membrane-proximal domain of beta(3) (residues 717 to 724), had no effect, but NNPLYKEA (residues 743 to 750), EATSTFTN (residues 749 to 756), and TNITYRGT (residues 755 to 762), which mimicked overlapping domains of the carboxy-terminal part of beta(3), reduced alpha-thrombin-induced ligand binding by 60+/-4%, 97+/-1%, and 97+/-2% (n=3) at 500 micromol/L peptide, respectively. These observations indicate that Src kinases and protein kinase C take part in inside-out signaling to integrin alpha(IIb)beta(3) and identify target domains in beta(3) that contribute to the regulation of this integrin.

Probing chemical and conformational differences in the resting and active conformers of platelet integrin alpha(IIb)beta(3)

Integrin alpha(IIb)beta(3) is the fibrinogen receptor that mediates platelet adhesion and aggregation. The ligand binding function of alpha(IIb)beta(3) is "activated" on the platelet surface by physiologic stimuli. Two forms of alpha(IIb)beta(3) can be purified from platelet lysates. These forms are facsimiles of the resting (Activation State-1 or AS-1) and the active (Activation State-2 or AS-2) conformations of the integrin found on the platelet surface. Here, the differences between purified AS-1 and AS-2 were examined to gain insight into the mechanism of activation. Four major findings are put forth. 1) The association rate (k(1)) between fibrinogen and the integrin is a key difference between AS-1 and AS-2. 2) Although the divalent ion Mn(2+) enhances the ligand binding function of AS-1, this ion is unable to convert AS-1 to AS-2. Therefore, its effect on integrin is unrelated to activation. 3) Peptide mass fingerprints indicate that the chemical structure of AS-1 and AS-2 are virtually identical, calling into question the idea that post-translational modifications are necessary for activation. 4) The two forms of alpha(IIb)beta(3) have significant conformational differences at three positions. These include the junction of the heavy and light chain of alpha(IIb), the divalent ion binding sites on alpha(IIb), and at a disulfide-bonded knot linking the amino terminus of beta(3) to the cysteine-rich domain. These observations indicate that integrin is activated by a series of specific conformational rearrangements in the ectodomain that increase the rate of ligand association.

Endothelins activate Ca(2+)-gated K(+) channels via endothelin B receptors in CD-1 mouse erythrocytes

Cell dehydration mediated by Ca(2+)-activated K(+) channels plays an important role in the pathogenesis of sickle cell disease. CD-1 mouse erythrocytes possess a Ca(2+)-activated K(+) channel (Gardos channel) with maximal velocity (V(max)) of 0.154 +/- 0.02 mmol. l cells(-1). min(-1) and an affinity constant (K(0.5)) for Ca(2+) of 286 +/- 83 nM in the presence of A-23187. Cells pretreated with 500 nM endothelin-1 (ET-1) increased their V(max) by 88 +/- 9% (n = 8) and decreased their K(0.5) for Ca(2+) to 139 +/- 63 nM (P < 0.05; n = 4). Activation of the Gardos channel resulted in an EC(50) of 75 +/- 20 nM for ET-1 and 374 +/- 97 nM for ET-3. Analysis of the affinity of unlabeled ET-1 for its receptor showed two classes of binding sites with apparent dissociation constants of 167 +/- 51 and 785 +/- 143 nM and with capacity of binding sites of 298 +/- 38 and 1,568 +/- 211 sites/cell, respectively. The Gardos channel was activated by the endothelin B (ET(B)) receptor agonist IRL 1620 and inhibited by BQ-788, demonstrating the involvement of ET(B) receptors. Calphostin C inhibited 73% of ET-1-induced Gardos activation and 84% of the ET-1-induced membrane protein kinase C activity. Thus endothelins regulate erythrocyte Gardos channels via ET(B) receptors and a calphostin-sensitive mechanism.

Reversible exposure of human platelet fibrinogen receptors by antiplatelet tetraspanin monoclonal antibodies via induction of a conformational change in membrane glycoprotein IIb/IIIa complex

Antihuman platelet tetraspanin (CD9 antigen) monoclonal antibodies, HI117 and SJ9A4, can induce human platelet aggregation and secretion. As platelet aggregation is mediated by fibrinogen binding to its receptors exposed on platelet glycoprotein IIb/IIIa complex, we, therefore, investigated the induction of platelet fibrinogen receptors by HI117 and SJ9A4. It was found that HI117 and SJ9A4 induced specific fibrinogen binding to human platelets, suggesting that the two monoclonal antibodies evoked obvious exposure of fibrinogen receptors on human platelets. But in the absence of fibrinogen, the monoclonal antibody-exposed fibrinogen receptors gradually lost their capacity to bind fibrinogen and closed. Our results also showed that HI117 and SJ9A4, when activating platelets, caused a conformational change in glycoprotein IIb/IIIa complex, which must contribute to the exposure of functional fibrinogen receptors on this integrin. The effect of HI117 and SJ9A4 on glycoprotein IIb/IIIa complex seems, however, to be indirect, because the HI1117 and SJ9A4-induced fibrinogen binding was reduced by pretreatment of platelets with sphingosine, aspirin, apyrase, and/or PGI2. Taken together, we conclude that the antihuman platelet tetraspanin monoclonal antibodies, HI117 and SJ9A4, reversibly expose platelet fibrinogen receptors via inducing a conformational change in glycoprotein IIb/IlIa complex. Three signaling pathways, that is, thromboxane, secreted ADP, and cAMP pathways may be involved in this process, while protein kinase C activation seems to be the final common step of the three pathways.

Differential involvement of tyrosine and serine/threonine kinases in platelet integrin alphaIIbbeta3 exposure

The relative contributions of protein tyrosine kinases (PTKs) and protein kinase C isoenzymes (PKCs), a family of serine/threonine kinases, in integrin alpha(IIb)beta3 (glycoprotein IIb/IIIa) exposure are the subject of much controversy. In the present study we measured the effect of the PTK inhibitor herbimycin A and the PKC inhibitor bisindolylmaleimide I on 125I-fibrinogen binding to alpha(IIb)beta3 and on aggregation/secretion induced by different agonists. Dose-response studies showed complete inhibition of alpha(IIb)beta3 exposure by 30 micromol/L (ADP stimulation) and 35 to 40 micromol/L (alpha-thrombin stimulation) herbimycin A. In contrast, inhibition of exposure by bisindolylmaleimide I varied from none (for ADP and epinephrine), to 30% (for platelet-activating factor), and to approximately 80% (for alpha-thrombin). Studies with a submaximal dose of herbimycin A (approximately 50% inhibition of the ADP-response) and a maximal dose of bisindolylmaleimide I showed that optical aggregation had a similar sensitivity to the inhibitors as alpha(IIb)beta3 exposure with minimal interference by secreted ADP. Thus, the relative contributions of tyrosine and serine/threonine kinases in alpha(IIb)beta3 exposure and aggregation differ among the different agonists, with an exclusive role for PTKs in ADP- and epinephrine-induced responses and a role for both PTKs and PKCs in responses induced by platelet-activating factor and alpha-thrombin.

Platelets from eclampsia patients have reduced membrane microviscosity and lower activities of the signalling enzymes

It has been shown that platelets from patients suffering from eclampsia are hyporesponsive to stimulation by agonists like thrombin and ADP. Although platelet hyporeactivity contributes to the pathogenesis of the disease process, the cause for this is still not known. Platelet aggregation and secretion are membrane-based phenomena initiated by the processes of cell signalling. Hence, to understand the mechanisms underlying platelet hyporeactivity in eclampsia, membrane microviscosity and activities of the signalling enzymes were measured in human platelets stimulated with thrombin. Membrane fluidity was determined from the steady-state fluorescence anisotropy of diphenylhexatriene incorporated in cell membranes. Activities of phospholipase C and protein kinase C in stimulated platelets were assessed from the extents of phosphatidic acid generation and pleckstrin phosphorylation, respectively. Platelet membrane microviscosity in eclampsia (2.3 +/- 0.2 SEM, n = 5) was significantly lower (P < 0.05) than that in the matched gravid control subjects (3.1 +/- 0.2, n = 4). In eclampsia, generation of phosphatidic acid and phosphorylation of pleckstrin were decreased by 25% (P < 0.05, n = 3) and 35% (P < 0.05, n = 3), respectively, after 60 sec of platelet stimulation. It was concluded that the hyporeactive platelets obtained from eclampsia have more fluid membranes and diminished activities of phospholipase C and protein kinase C. In summary, this study shows that alterations in membrane fluidity and activities of the signalling enzymes (phospholipase C and protein kinase C) may contribute to the diminished platelet responsiveness observed in the eclamptic condition.

Allostery and proteolysis: two novel modes of regulating integrin function

Integrins are involved in transmitting signals between the cytoplasm and the extracellular matrix. Importantly, the transfer of information is bi-directional: signals flow inside-out and outside-in. Here, I discuss two potential modes by which integrin function is likely to be regulated. It is hypothesized that the integrin cytoplasmic tails are proteolytic substrates, and that cleavage of the cytoplasmic domain regulates the ligand binding affinity of integrins. It is also hypothesized that the ligand binding site is allosterically regulated by separate divalent ion binding sites that independently control ligand association and dissociation rate. Both hypotheses are suggested by reports in the literature and can be tested experimentally.

The activation of human platelets mediated by two monoclonal antibodies raised against CD9

The platelet activation induced by two anti-human platelet P24/CD9 McAbs was investigated. The results showed that: the pathway of platelet aggregation induced by the two McAbs (HI117 and SJ9A4) is not the same; HI117 and SJ9A4 induced the phosphorylation of platelet proteins (40KD and 20KD) when platelets were activated; but HI117 didn't cause a rise in intracellular Ca2+ concentration in activated platelets compared with SJ9A4; the epitope recognized by HI117 and SJ9A4 is different and this is probably the real reason why the two CD9 McAbs play different roles in platelet activation. Additionally McAbs HI117 and SJ9A4 could not promote associates of other proteins (e.g.: GPIIb/IIIa) with P24/CD9 on activated human platelets. All these results indicate that the mechanism of platelet activation induced by HI117 or SJ9A4 is different form each other. It suggests that CD9 antigen play an important and complex role in platelet activation.

Affinity modulation of platelet integrin alphaIIbbeta3 by beta3-endonexin, a selective binding partner of the beta3 integrin cytoplasmic tail

Platelet agonists increase the affinity state of integrin alphaIIbbeta3, a prerequisite for fibrinogen binding and platelet aggregation. This process may be triggered by a regulatory molecule(s) that binds to the integrin cytoplasmic tails, causing a structural change in the receptor. beta3-Endonexin is a novel 111-amino acid protein that binds selectively to the beta3 tail. Since beta3-endonexin is present in platelets, we asked whether it can affect alphaIIbbeta3 function. When beta3-endonexin was fused to green fluorescent protein (GFP) and transfected into CHO cells, it was found in both the cytoplasm and the nucleus and could be detected on Western blots of cell lysates. PAC1, a fibrinogen-mimetic mAb, was used to monitor alphaIIbbeta3 affinity state in transfected cells by flow cytometry. Cells transfected with GFP and alphaIIbbeta3 bound little or no PAC1. However, those transfected with GFP/beta3-endonexin and alphaIIbbeta3 bound PAC1 specifically in an energy-dependent fashion, and they underwent fibrinogen-dependent aggregation. GFP/beta3-endonexin did not affect levels of surface expression of alphaIIbbeta3 nor did it modulate the affinity of an alphaIIbbeta3 mutant that is defective in binding to beta3-endonexin. Affinity modulation of alphaIIbbeta3 by GFP/beta3-endonexin was inhibited by coexpression of either a monomeric beta3 cytoplasmic tail chimera or an activated form of H-Ras. These results demonstrate that beta3-endonexin can modulate the affinity state of alphaIIbbeta3 in a manner that is structurally specific and subject to metabolic regulation. By analogy, the adhesive function of platelets may be regulated by such protein-protein interactions at the level of the cytoplasmic tails of alphaIIbbeta3.

Exposure of ligand-binding sites on platelet integrin alpha IIB/beta 3 by phosphorylation of the beta 3 subunit

The exposure of ligand-binding sites for adhesive proteins on platelet integrin alpha IIB/beta 3 (glycoprotein IIB/IIIA) by platelet-activating (PAF) is transient, whereas sites exposed by alpha-thrombin remain accessible. The same difference is seen in the phosphorylation of the beta 3 subunit. Inhibition of protein kinases (1 microM staurosporine) and protein kinase C (10 microM bisindolylmaleimide) closes binding sites exposed by both agonists and induces dephosphorylation of beta 3. Inhibition of Tyr-kinases (20 microM Herbimycin A) has only a slight effect. Inhibition of Ser/Thr-phosphatases (1 microM okadaic acid, 30 s preincubation) changes the transient exposure and beta phosphorylation by PAF into the 'permanent' patterns induced by alpha-thrombin. Inhibition of Tyr-phosphatases (100 microM vanadate) has little effect. Preincubation with okadaic acid makes exposed binding sites and phosphorylated beta 3 insensitive to staurosporine, resulting in exposed alpha IIB/beta 3 independent of concurrent phosphorylation/dephosphorylation. The stoichiometry of beta 3 phosphorylation by alpha-thrombin is 0.80+/-0.10. Thus, one of the mechanisms that regulates exposure and closure of ligand-binding sites on the alpha IIb/beta 3 is phosphorylation/dephosphorylation of a Ser/Thr-residue in the beta 3 subunit.

Tolerance to nitrates and simultaneous upregulation of platelet activity prevented by enhancing antioxidant state

We analysed the induction of tolerance to nitrates both in the vasculature (in vivo) and platelets (ex vivo). Simultaneously, we tested mechanisms underlying the induction of tolerance and interventions to prevent or overcome this phenomenon. For this purpose nitroglycerin (GTN 1.5 micrograms/kg per min i.v.), alone or in combination with ascorbate (55 micrograms/kg per min i.v.) as antioxidant, was infused continuously for a period of 5 days into chronically instrumented dogs. Along with haemodynamic parameters, ex vivo platelet function was continuously monitored. Following the start of GTN infusions there was a maximal coronary dilator response (245 +/- 15 microm) and, as an index of venodilation, a fall of left ventricular end-diastolic pressure (by 2.3 +/- 0.4 mmHg). Both responses declined progressively and disappeared during the infusion period. However, in combination with ascorbate as antioxidant the dilator responses were maintained fully throughout the infusion period. With GTN alone there was a progressive, unexpected upregulation of platelet activity demonstrated by enhanced thrombin-stimulated intracellular Ca2+ levels and increases in the microviscosity of platelet membranes (indicating enhanced receptor expression) associated with a progressive impairment in basal, unstimulated cGMP levels. These changes could also be prevented completely by i.v. co-administration of ascorbate. From these results it is concluded that vascular tolerance is closely reflected by simultaneous changes in platelet function and further, that both can be prevented completely by appropriate antioxidants such as ascorbate.

Regulation of platelet glycoprotein IIb/IIIa (integrin alpha IIB beta 3) function via the thrombin receptor

Binding sites on glycoprotein (GP) IIb/IIIa exposed by 0.5 unit/ml alpha-thrombin are insensitive to prostaglandin I2 (PGI2), in contrast with sites exposed by ADP or platelet-activating factor. Here we show that the thrombin receptor agonist peptide (TRAP) (SFLLRN; 15 microM) opens almost the same number of GPIIb/IIIa molecules as 0.5 unit/ml alpha-thrombin (64840 +/- 8920 compared with 81050 +/- 6030 molecules of fibronectin bound/platelet), but these sites rapidly close on addition of PGI2. To investigate whether alpha-thrombin and TRAP initiate different signalling pathways, we measured phospholipase C (PLC)-mediated control of GPIIb/IIIa and its sensitivity to cyclic AMP. Optimal concentrations of alpha-thrombin and TRAP activated PLC maximally, but TRAP induced only about 50% protein kinase C PKC) activation after 10 min stimulation compared with alpha-thrombin. These concentrations also suppressed PGI2-induced cyclic AMP accumulation, with alpha-thrombin inducing complete inhibition and TRAP about 10% less. Direct activation of PKC by phorbol 12-myristate 13-acetate confirmed earlier observations that PGI2-induced cyclic AMP accumulation is partly inhibited via PKC. Applying different concentration of alpha-thrombin, TRAP or a combination of alpha-thrombin and the thrombin receptor inhibitory peptide (TRIP) (Mpr-F-Cha-Cha-RKPNDK-NH2; 800 microM) (Mpr, 3-mercaptopropionic acid; Cha, cyclohexylalanine), we show that the different means of stimulating the thrombin receptor all suppressed PGI2-induced cyclic AMP accumulation via (i) activation of PKC and (ii) activation of the heterotrimeric G-protein, Gi. We conclude that complete inhibition of cyclic AMP accumulation requires activation of both PKC and Gi, as observed with 0.5 unit/ml alpha-thrombin. Although TRAP almost fully exposes GPIIb/IIIa, its activation of PKC is incomplete, enabling PGI2 to raise cyclic AMP concentration from 1.4 +/- 0.7 to 4.1 +/- 1.3 nmol/10(11) platelets (P < 0.005) which is sufficient to close exposed GPIIb/IIIa molecules.

Characterization of a potent platelet aggregation inducer from Cerastes cerastes (Egyptian sand viper) venom

A potent, proteinaceous inducer of platelet aggregation designated as IVa, has been purified to homogeneity from Cerastes cerastes venom by molecular sieve and ion exchange chromatography. It is composed of 2 subunits with total M(r) of 62,000 as shown by native gel chromatography and chemical cross-linking with disuccinimidyl suberate. It is not clear at the present time whether both subunits are identical gene products, however, both have identical N-terminal sequences for the first 15 amino acids. The protein has a pI above 9.6. IVa (0.1 micrograms/ml) could aggregate platelets up to 80% and was inhibited by p-APMSF, leupeptin, iodoacetamide, protein kinase C inhibitor, phosphatase inhibitor, ATP and PGE1, while it was insensitive to acetylsalicylic acid, ADP scavenger system, protein kinase A inhibitor and hirudin. Protein IVa is a serine proteinase with thrombin-like activity as it hydrolysed thrombin chromogenic substrate CBS 34.47, its aggregatory activity was partially inhibited by monoclonal antibodies against GPIb and the thrombin receptor, as was the thrombin, and its ability to induce intracellular Ca2+ release was blocked by pretreating platelets with thrombin. Unlike thrombin, the IVa protein showed very weak coagulant activity as indicated by plasma recalcification time and fibrinogen clotting time although it could hydrolyse fibrinogen alpha-chains.

Phosphoinositide 3-kinase inhibition spares actin assembly in activating platelets but reverses platelet aggregation

Platelet stimulation by thrombin leads to the activation of phosphoinositide 3-kinase (PI 3K) and to the production of the D3 phosphoinositides, phosphatidylinositol 3,4-bisphosphate (PdtIns-3,4P2) and 3,4,5-trisphosphate (PdtIns-3,4,5-P3). Because changes in the levels of these phosphoinositides correlate with the kinetics of actin assembly, they have been proposed to mediate actin assembly, causing cell shape changes. Wortmannin and LY294002, two unrelated inhibitors of PI 3-K, were used to investigate the role of PI 3-K in platelet actin assembly and aggregation. Both PI 3-K inhibitors abrogated the production of PdtIns-3,4-P2 and PdtIns-3,4,5-P3 in thrombin receptor-activating peptide (TRAP)-stimulated cells. However, neither wortmannin nor LY294002 altered the kinetics of actin assembly or the exposure of nucleation sites in TRAP-stimulated cells. In contrast, PI 3-K inhibitors showed a specific inhibitory pattern of cell aggregation, characterized by a primary phase of aggregation followed by progressive disaggregation. Flow cytometry analysis with the PAC1 monoclonal antibody or with FITC-labeled fibrinogen indicated that wortmannin inhibited the maintenance of the platelet integrin GPIIb-IIIa in its active state. Wortmannin also inhibited, in a dose-dependent manner, platelet aggregation induced by the binding of the monoclonal antibodies P256 and LIBS-6 to GPIIb-IIIa. LIBS Fab-induced aggregation also led to the production of PdtIns-3,4-P2. Platelet secretion, as evidenced by the release of preloaded 14C-5-hydroxy-tryptamine secretion or P-selectin up-regulation, was not affected by PI 3-K inhibition. These results demonstrate that the generation of D3 phosphoinositides is not required for actin assembly in TRAP-activated platelets. However, PI 3-K stimulation is necessary for prolonged GPIIb-IIIa activation and irreversible platelet aggregation. PI 3-K stimulation downstream of GPIIb-IIIa engagement may provide positive feedback required to sustain active GPIIb-IIIa.

Inhibition of TPA and 12(S)-HETE-stimulated tumor cell adhesion by prostacyclin and its stable analogs: rationale for their antimetastatic effects

We have investigated the regulatory role of PGI2 and its stable analogs, i.e., iloprost and cicaprost, on 12(S)-HETE- and TPA-enhanced tumor cell integrin expression and adhesion. Walker 256 carcinosarcoma cells express alpha IIb beta 3 integrin receptors, which mediate their adhesion to endothelium, subendothelial matrix and fibronectin. Adhesion is enhanced by treatment with exogenous 12(S)-HETE but not 12(R)-HETE or other lipoxygenase-derived hydroxy fatty acids, as well as by TPA. Both 12(S)-HETE and TPA enhanced alpha IIb beta 3 expression on W256 cells. PGI2 iloprost and cicaprost inhibited both 12(S)-HETE- and TPA-enhanced adhesion to endothelium and subendothelial matrix as well as alpha IIb beta 3 expression on W256 cells. The mechanism responsible for the effect of PGI2 was explored. Prostacyclin treatment of W256 cells resulted in an enhanced production of cAMP in a time- and dose-dependent manner. Pre-treatment of tumor cells with increasing concentrations of adenosine resulted in a dose-dependent decrease in the PGI2 effect on TPA or 12(S)-HETE-enhanced adhesion, suggesting that the PGI2 effect is mediated through PKA. Dibutyryl cAMP also blocked the 12(S)-HETE- or TPA-enhanced adhesion, and adenosine pre-treatment did not result in an inhibition of the dibutyryl cAMP effect. Collectively, our results suggest that the cyclooxygenase metabolite PGI2 can antagonize the lipoxygenase metabolite 12(S)-HETE- and TPA-enhanced alpha IIb beta 3 expression and tumor cell adhesion via activation of adenylate cyclase and elevation of intracellular levels of cAMP.

Phosphorylation of focal adhesion vasodilator-stimulated phosphoprotein at Ser157 in intact human platelets correlates with fibrinogen receptor inhibition

Integrins and other adhesion receptors are essential components for outside-in and inside-out signaling through the cell membrane. The platelet glycoprotein IIb-IIIa (also known as fibrinogen receptor or integrin alpha IIb beta 3) is activated by platelet agonists, inhibited by cyclic-nucleotide-elevating agents, and is involved in the activation of protein tyrosine kinases including the 125-kDa focal adhesion kinase (pp125FAK). However, the molecular details of glycoprotein IIb-IIIa regulation are not well understood. Here we report that in ADP-activated human platelets cAMP- and cGMP-dependent protein-kinase-mediated phosphorylation of the focal adhesion vasodilator-stimulated phosphoprotein (VASP) at Ser157 correlates well with glycoprotein IIb-IIIa inhibition. Human platelets contain similar concentrations of glycoprotein IIb-IIIa complexes (fibrinogen binding sites) and VASP. Using gel-filtered platelets, cAMP-elevating agents [e.g. prostaglandin E1 and the forskolin analog 6-(3-dimethylaminopropionyl)forskolin (NKH 477)] caused VASP Ser157 phosphorylation and inhibited glycoprotein IIb-IIIa activation up to 70-100%. NO-generating, cGMP-elevating agents [e.g. 3-morpholinosydnonimine hydrochloride (SIN1) and sodium nitroprusside] stimulated VASP Ser157 phosphorylation and inhibited glycoprotein IIb-IIIa activation up to a maximal extent of 30-50%. The effects of cAMP- and cGMP-elevating agents on VASP phosphorylation and fibrinogen binding were reversible and could be mimicked by membrane-permeant selective activators of platelet cAMP- or cGMP-dependent protein kinase, respectively. Using threshold concentrations, the nitrovasodilator SIN 1 potentiated the effects of the forskolin analog NKH 477 with respect to inhibition of platelet aggregation, VASP phosphorylation and glycoprotein IIb-IIIa inhibition. It is proposed that the inhibition of glycoprotein IIb-IIIa induced by cyclic nucleotide involves cAMP-and cGMP-dependent protein-kinase-mediated VASP phosphorylation at Ser157.

Absence of ligands bound to glycoprotein IIB-IIIA on the exposed surface of a thrombus may limit thrombus growth in flowing blood

We examined the distribution of glycoprotein IIb-IIIa (GPIIb-IIIa) and its ligands fibrinogen and von Willebrand factor (vWf) on platelets which had adhered under flow conditions. Immunoelectron microscopy was performed on whole mounts and frozen thin sections of adhering platelets. GPIIb-IIIa was homogeneously distributed on dendritic platelets and on interplatelet membranes of formed thrombi. Fibrinogen and vWf were predominantly associated with interplatelet membranes and membranes facing the substrate. On whole mounts, vWf appeared in clumps and linear arrays, representing the tangled or extended forms of the multimeric molecule. From semiquantitative analysis, it appeared that fibrinogen and vWf were, respectively, nine- and fourfold higher on interplatelet membranes than on surface membranes facing the blood stream, while GPIIb-IIIa was evenly distributed over all platelet plasma membranes. Ligand-induced binding sites (LIBS) of GPIIb-IIIa, as measured with conformation specific monoclonal antibodies RUU 2.41 and LIBS-1, were present on the surface of adhered platelets and thrombi. A redistribution of LIBS-positive forms of GPIIb-IIIa towards interplatelet membranes was not observed. Our data support the hypothesis that, under flow conditions, ligands have first bound to activated GPIIb-IIIa but this binding is reversed on the upper surface of adhering platelets. This relative absence of ligands on the exposed surface of thrombi may play a role in limiting their size.