Genetic evidence for a predominant role of PI3Kβ catalytic activity in ITAM- and integrin-mediated signaling in platelets

Mechanism of Bile Acid in Regulating Platelet Function and Thrombotic Diseases

Platelets play a key role in physiological hemostasis and pathological thrombosis. Based on the limitations of current antiplatelet drugs, it's important to elucidate the mechanisms of regulating platelet activation. In addition to dissolving lipid nutrients, bile acids (BAs) can regulate platelet function. However, the specific mechanisms underlying BAs-mediated effects on platelet activation and thrombotic diseases remain unknown. Therefore, the effects of BAs on platelets and intracellular regulatory mechanisms are explored. It is showed that the inhibitory effect of secondary BAs is more significant than that of primary BAs; lithocholic acid (LCA) shows the highest inhibitory effect. In the process of platelet activation, BAs suppress platelet activation via the spleen tyrosine kinase (SYK), protein kinase B (Akt), and extracellular signal-regulated kinase1/2 (Erk1/2) pathways. Nck adaptor proteins (NCK1) deficiency significantly suppress the activity of platelets and arterial thrombosis. Phosphorylated proteomics reveal that LCA inhibited phosphorylation of syntaxin-11 at S80/81 in platelets. Additional LCA supplementation attenuated atherosclerotic plaque development and reduced the inflammation in mice. In conclusion, BAs play key roles in platelet activation via Syk, Akt, ERK1/2, and syntaxin-11 pathways, which are associated with NCK1. The anti-platelet effects of BAs provide a theoretical basis for the prevention and therapy of thrombotic diseases.

Study on the Mechanism of the Adrenaline-Evoked Procoagulant Response in Human Platelets

Adrenaline has recently been found to trigger phosphatidylserine (PS) exposure on blood platelets, resulting in amplification of the coagulation process, but the mechanism is only fragmentarily established. Using a panel of platelet receptors' antagonists and modulators of signaling pathways, we evaluated the importance of these in adrenaline-evoked PS exposure by flow cytometry. Calcium and sodium ion influx into platelet cytosol, after adrenaline treatment, was examined by fluorimetric measurements. We found a strong reduction in PS exposure after blocking of sodium and calcium ion influx via Na+/H+ exchanger (NHE) and Na+/Ca2+ exchanger (NCX), respectively. ADP receptor antagonists produced a moderate inhibitory effect. Substantial limitation of PS exposure was observed in the presence of GPIIb/IIIa antagonist, phosphoinositide-3 kinase (PI3-K) inhibitors, or prostaglandin E1, a cyclic adenosine monophosphate (cAMP)-elevating agent. We demonstrated that adrenaline may develop a procoagulant response in human platelets with the substantial role of ion exchangers (NHE and NCX), secreted ADP, GPIIb/IIIa-dependent outside-in signaling, and PI3-K. Inhibition of the above mechanisms and increasing cytosolic cAMP seem to be the most efficient procedures to control adrenaline-evoked PS exposure in human platelets.

Inverse relationship between platelet Akt activity and hippocampal atrophy: A pilot case-control study in patients with diabetes mellitus

BACKGROUND

Akt plays diverse roles in humans. It is involved in the pathogenesis of type 2 diabetes mellitus (T2DM), which is caused by insulin resistance. Akt also plays a vital role in human platelet activation. Furthermore, the hippocampus is closely associated with memory and learning, and a decrease in hippocampal volume is reportedly associated with an insulin-resistant phenotype in T2DM patients without dementia.

AIM

To investigate the relationship between Akt phosphorylation in unstimulated platelets and the hippocampal volume in T2DM patients.

METHODS

Platelet-rich plasma (PRP) was prepared from the venous blood of patients with T2DM or age-matched controls. The pellet lysate of the centrifuged PRP was subjected to western blotting to analyse the phosphorylation of Akt, p38 mitogen-activated protein (MAP) kinase and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Phosphorylation levels were quantified by densitometric analysis. Hippocampal volume was analysed using a voxel-based specific regional analysis system for Alzheimer's disease on magnetic resonance imaging, which proposes the Z-score as a parameter that reflects hippocampal volume.

RESULTS

The levels of phosphorylated Akt corrected with phosphorylated p38 MAP kinase were inversely correlated with the Z-scores in the T2DM subjects, whereas the levels of phosphorylated Akt corrected with GAPDH were not. However, this relationship was not observed in the control patients.

CONCLUSION

These results suggest that an inverse relationship may exist between platelet Akt activation and hippocampal atrophy in T2DM patients. Our findings provide insight into the molecular mechanisms underlying T2DM hippocampal atrophy.

Development of small-molecule inhibitors that target PI3Kβ

Phosphatidylinositol-3 kinase (PI3K) β, a subtype of class I PI3Ks, has an essential role in PTEN-deficient tumors and links to thrombosis, male fertility, and Fragile X syndrome. PI3Kβ-specific targeting therapy could be an efficacious treatment for diseases highly dependent on PI3Kβ, while mitigating the severe toxicity of pan-PI3K inhibitors. Achieving selectivity can be accomplished through three primary strategies, namely, binding to the induced lipophilic pocket, targeting the unique amino acid residue of PI3Kβ, or using atropisomerism to lock conformation. In this review, we focus on advances in the development of these β-isoform-selective PI3K inhibitors, providing potential guidance for the further development of novel clinical candidates.

Phosphoinositides take a central stage in regulating blood platelet production and function

Blood platelets are produced by megakaryocytes through a complex program of differentiation and play a critical role in hemostasis and thrombosis. These anucleate cells are the target of antithrombotic drugs that prevent them from clumping in cardiovascular disease conditions. Platelets also significantly contribute to various aspects of physiopathology, including interorgan communications, healing, inflammation, and thromboinflammation. Their production and activation are strictly regulated by highly elaborated mechanisms. Among them, those involving inositol lipids have drawn the attention of researchers. Phosphoinositides represent the seven combinatorially phosphorylated forms of the inositol head group of inositol lipids. They play a crucial role in regulating intracellular mechanisms, such as signal transduction, actin cytoskeleton rearrangements, and membrane trafficking, either by generating second messengers or by directly binding to specific domains of effector proteins. In this review, we will explore how phosphoinositides are implicated in controlling platelet production by megakaryocytes and in platelet activation processes. We will also discuss the diversity of phosphoinositides in platelets, their role in granule biogenesis and maintenance, as well as in integrin signaling. Finally, we will address the discovery of a novel pool of phosphatidylinositol 3-monophosphate in the outerleaflet of the plasma membrane of human and mouse platelets.

Novel strategies in antithrombotic therapy: targeting thrombosis while preserving hemostasis

Antithrombotic therapy is a delicate balance between the benefits of preventing a thrombotic event and the risks of inducing a major bleed. Traditional approaches have included antiplatelet and anticoagulant medications, require careful dosing and monitoring, and all carry some risk of bleeding. In recent years, several new targets have been identified, both in the platelet and coagulation systems, which may mitigate this bleeding risk. In this review, we briefly describe the current state of antithrombotic therapy, and then present a detailed discussion of the new generation of drugs that are being developed to target more safely existing or newly identified pathways, alongside the strategies to reverse direct oral anticoagulants, showcasing the breadth of approaches. Combined, these exciting advances in antithrombotic therapy bring us closer than we have ever been to the "holy grail" of the field, a treatment that separates the hemostatic and thrombotic systems, preventing clots without any concurrent bleeding risk.

Current concepts and novel targets for antiplatelet therapy

Platelets have a crucial role in haemostasis and atherothrombosis. Pharmacological control of platelet hyper-reactivity has become a cornerstone in the prevention of thrombo-ischaemic complications in atherosclerotic diseases. Current antiplatelet therapies substantially improve clinical outcomes in patients with coronary artery disease, but at the cost of increased risk of bleeding. Beyond their role in thrombosis, platelets are known to regulate inflammatory (thrombo-inflammatory) and microcirculatory pathways. Therefore, controlling platelet hyper-reactivity might have implications for both tissue inflammation (myocardial ischaemia) and vascular inflammation (vulnerable plaque formation) to prevent atherosclerosis. In this Review, we summarize the pathophysiological role of platelets in acute myocardial ischaemia, vascular inflammation and atherosclerotic progression. Furthermore, we highlight current clinical concepts of antiplatelet therapy that have contributed to improving patient care and have facilitated more individualized therapy. Finally, we discuss novel therapeutic targets and compounds for antiplatelet therapy that are currently in preclinical development, some of which have a more favourable safety profile than currently approved drugs with regard to bleeding risk. These novel antiplatelet targets might offer new strategies to treat cardiovascular disease.

Blocking of PI3-kinase beta protects against cerebral ischemia/reperfusion injury by reducing platelet activation and downstream microvascular thrombosis in rats

Phosphoinositide 3-kinase beta (PI3Kβ) plays an important role in platelet activation and thrombosis, but its role in stroke pathology remains unknown. In this study, we investigated whether inhibition of PI3Kβ protects against cerebral ischemia/reperfusion (I/R) injury by preventing circulating platelet activation and downstream microvascular thrombosis. We used a rat intraluminal filament model of transient middle cerebral artery occlusion (tMCAO) because the rapid restoration of cerebral blood flow to the ischemic area in both tMCAO and endovascular thrombectomy provides clinical relevance for this model. The results showed that TGX221, a selective PI3Kβ inhibitor, treatment immediately before the onset of reperfusion dose-dependently reduced infarct volume and improved neurological function. The protective effects were associated with blocking platelet activation and thrombotic response, thereby reducing downstream microvascular thrombosis, and maintaining reperfusion efficiency. These results suggest that PI3Kβ might be a promising target for treating downstream microvascular thrombosis induced by cerebral I/R injury and offer a novel adjunctive treatment to improve reperfusion therapy for acute ischemic stroke.

Platelet p110β mediates platelet-leukocyte interaction and curtails bacterial dissemination in pneumococcal pneumonia

Phosphatidylinositol 3-kinase catalytic subunit p110β is involved in tumorigenesis and hemostasis. However, it remains unclear if p110β also regulates platelet-mediated immune responses, which could have important consequences for immune modulation during anti-cancer treatment with p110β inhibitors. Thus, we investigate how platelet p110β affects inflammation and infection. Using a mouse model of Streptococcus pneumoniae-induced pneumonia, we find that both platelet-specific p110β deficiency and pharmacologic inhibition of p110β with TGX-221 exacerbate disease pathogenesis by preventing platelet-monocyte and neutrophil interactions, diminishing their infiltration and enhancing bacterial dissemination. Platelet p110β mediates neutrophil phagocytosis of S. pneumoniae in vitro and curtails bacteremia in vivo. Genetic deficiency or inhibition of platelet p110β also impairs macrophage recruitment in an independent model of sterile peritonitis. Our results demonstrate that platelet p110β dysfunction exacerbates pulmonary infection by impeding leukocyte functions. Thereby, our findings provide important insights into the immunomodulatory potential of PI3K inhibitors in bacterial infection.

The role of PI3Kγ in the immune system: new insights and translational implications

Over the past two decades, new insights have positioned phosphoinositide 3-kinase-γ (PI3Kγ) as a context-dependent modulator of immunity and inflammation. Recent advances in protein structure determination and drug development have allowed for generation of highly specific PI3Kγ inhibitors, with the first now in clinical trials for several oncology indications. Recently, a monogenic immune disorder caused by PI3Kγ deficiency was discovered in humans and modelled in mice. Human inactivated PI3Kγ syndrome confirms the immunomodulatory roles of PI3Kγ and strengthens newly defined roles of this molecule in modulating inflammatory cytokine release in macrophages. Here, we review the functions of PI3Kγ in the immune system and discuss how our understanding of its potential as a therapeutic target has evolved.

PI3K Isoform Signalling in Platelets

Platelets are unique anucleated blood cells that constantly patrol the vasculature to seal and prevent injuries in a process termed haemostasis. Thereby they rapidly adhere to the subendothelial matrix and recruit further platelets, resulting in platelet aggregates. Apart from their central role in haemostasis, they also kept some of their features inherited by their evolutionary ancestor-the haemocyte, which was also involved in immune defences. Together with leukocytes, platelets fight pathogenic invaders and guide many immune processes. In addition, they rely on several signalling pathways which are also relevant to immune cells. Among these, one of the central signalling hubs is the PI3K pathway. Signalling processes in platelets are unique as they lack a nucleus and therefore transcriptional regulation is absent. As a result, PI3K subclasses fulfil distinct roles in platelets compared to other cells. In contrast to leukocytes, the central PI3K subclass in platelet signalling is PI3K class Iβ, which underlines the uniqueness of this cell type and opens new ways for potential platelet-specific pharmacologic inhibition. An overview of platelet function and signalling with emphasis on PI3K subclasses and their respective inhibitors is given in this chapter.

Bioactive constituents and the molecular mechanism of Curcumae Rhizoma in the treatment of primary dysmenorrhea based on network pharmacology and molecular docking

BACKGROUND

Curcumae Rhizoma (CR) has a clinical efficacy in activating blood circulation to dissipate blood stasis and has been used for the clinical treatment of qi stagnation and blood stasis (QSBS) primary dysmenorrhea for many years. However, its molecular mechanism is unknown.

OBJECTIVE

The present study aimed to demonstrate the multicomponent, multitarget and multipathway regulatory molecular mechanisms of CR in the treatment of QSBS primary dysmenorrhea.

METHODS

Observations of pathological changes in uterine tissues and biochemical assays were used to confirm that a rat model was successfully established and that CR was effective in the treatment of QSBS primary dysmenorrhea. The main active components of CR in rat plasma were identified and screened by ultra-performance liquid chromatography-quadrupole/time-of-flight mass spectrometry (UPLC-Q/TOF-MS). The component-target-disease network and protein-protein interaction (PPI) network of CR were constructed by a network pharmacology approach. Then, we performed Gene Ontology (GO) functional enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Molecular docking was adopted to verify the interactions between the core components and targets of CR to confirm the accuracy of the network pharmacology prediction results. Furthermore, we evaluated the bioactive constituents of CR and molecular mechanism of by which CR promote blood circulation and remove blood stasis via platelet tests in vivo and in vitro and Western blot analysis.

RESULTS

The results of HE staining and biochemical assays of PGF2α, TXB2 and Ca2+ showed that CR was effective in the treatment of QSBS primary dysmenorrhea. A total of 36 active components were identified in CR, and 329 common targets were obtained and used to construct the networks. Of these, 14 core components and 10 core targets of CR in the treatment of primary dysmenorrhea were identified. The GO and KEGG enrichment analyses revealed that the common targets were involved in multiple signaling pathways, including the calcium, cAMP, MAPK, and PI3K-Akt signaling pathways, as well as platelet activation, which is closely related to platelet aggregation. The molecular docking results showed that the 14 core components and 10 core targets could bind spontaneously. Two core targets (MAPK1 and CCR5) and 7 core components (Isoprocurcumenol, Curcumadione, Epiprocurcumenol, (+)-Curdione, Neocurdione, Procurcumenol, and 13-Hydroxygermacrone) were closely related to CR in the treatment of primary dysmenorrhea. Furthermore, the in vivo platelet test showed that CR clearly inhibited platelet aggregation. Five core components ((+)-Curdione, Neocurdione, Isoprocurcumenol, Curcumadione and Procurcumenol) obviously inhibited platelet aggregation in vitro. In addition, based on the relationships among the signaling pathways, we confirmed that CR can effectively inhibit the expression of MAPK and PI3K-Akt signaling pathway-related proteins and decrease the protein expression levels of ERK, JNK, MAPK, PI3K, AKT and CCR5, thereby inhibiting platelet aggregation.

CONCLUSION

This study demonstrated the bioactive constituents and mechanisms of CR in promoting blood circulation and removing blood stasis and its multicomponent, multitarget and multipathway treatment characteristics in primary dysmenorrhea. The results provide theoretical evidence for the development and utilization of CR.

Exogenous Integrin αIIbβ3 Inhibitors Revisited: Past, Present and Future Applications

The integrin αIIbβ3 is the most abundant integrin on platelets. Upon platelet activation, the integrin changes its conformation (inside-out signalling) and outside-in signalling takes place leading to platelet spreading, platelet aggregation and thrombus formation. Bloodsucking parasites such as mosquitoes, leeches and ticks express anticoagulant and antiplatelet proteins, which represent major sources of lead compounds for the development of useful therapeutic agents for the treatment of haemostatic disorders or cardiovascular diseases. In addition to hematophagous parasites, snakes also possess anticoagulant and antiplatelet proteins in their salivary glands. Two snake venom proteins have been developed into two antiplatelet drugs that are currently used in the clinic. The group of proteins discussed in this review are disintegrins, low molecular weight integrin-binding cysteine-rich proteins, found in snakes, ticks, leeches, worms and horseflies. Finally, we highlight various oral antagonists, which have been tested in clinical trials but were discontinued due to an increase in mortality. No new αIIbβ3 inhibitors are developed since the approval of current platelet antagonists, and structure-function analysis of exogenous disintegrins could help find platelet antagonists with fewer adverse side effects.

The PI3Kδ Inhibitor Idelalisib Diminishes Platelet Function and Shows Antithrombotic Potential

Background: Clinical management of ischemic events and prevention of vascular disease is based on antiplatelet drugs. Given the relevance of phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) as a candidate target in thrombosis, the main goal of the present study was to identify novel antiplatelet agents within the existing inhibitors blocking PI3K isoforms. Methods: We performed a biological evaluation of the pharmacological activity of PI3K inhibitors in platelets. The effect of the inhibitors was evaluated in intracellular calcium release and platelet functional assays, the latter including aggregation, adhesion, and viability assays. The in vivo drug antithrombotic potential was assessed in mice undergoing chemically induced arterial occlusion, and the associated hemorrhagic risk evaluated by measuring the tail bleeding time. Results: We show that PI3K Class IA inhibitors potently block calcium mobilization in human platelets. The PI3K p110δ inhibitor Idelalisib inhibits platelet aggregation mediated by ITAM receptors GPVI and CLEC-2, preferentially by the former. Moreover, Idelalisib also inhibits platelet adhesion and aggregation under shear and adhesion to collagen. Interestingly, an antithrombotic effect was observed in mice treated with Idelalisib, with mild bleeding effects at high doses of the drug. Conclusion: Idelalisib may have antiplatelet effects with minor bleeding effects, which provides a rationale to evaluate its antithrombotic efficacy in humans.

Organismal roles for the PI3Kα and β isoforms: their specificity, redundancy or cooperation is context-dependent

PI3Ks are important lipid kinases that produce phosphoinositides phosphorylated in position 3 of the inositol ring. There are three classes of PI3Ks: class I PI3Ks produce PIP3 at plasma membrane level. Although D. melanogaster and C. elegans have only one form of class I PI3K, vertebrates have four class I PI3Ks called isoforms despite being encoded by four different genes. Hence, duplication of these genes coincides with the acquisition of coordinated multi-organ development. Of the class I PI3Ks, PI3Kα and PI3Kβ, encoded by PIK3CA and PIK3CB, are ubiquitously expressed. They present similar putative protein domains and share PI(4,5)P2 lipid substrate specificity. Fifteen years after publication of their first isoform-selective pharmacological inhibitors and genetically engineered mouse models (GEMMs) that mimic their complete and specific pharmacological inhibition, we review the knowledge gathered in relation to the redundant and selective roles of PI3Kα and PI3Kβ. Recent data suggest that, further to their redundancy, they cooperate for the integration of organ-specific and context-specific signal cues, to orchestrate organ development, physiology, and disease. This knowledge reinforces the importance of isoform-selective inhibitors in clinical settings.

Naringenin Inhibits Platelet Activation and Arterial Thrombosis Through Inhibition of Phosphoinositide 3-Kinase and Cyclic Nucleotide Signaling

Citrus flavanoids intake can reduce the risk of cardiovascular diseases. Naringenin, a natural predominant flavonoid abundant in citrus fruits, possesses protective effects against atherothrombotic diseases. As platelet activation plays central roles in atherothrombogenesis, we studied the effects of naringenin on platelet activation, signaling, thrombosis and hemostasis. Naringenin dose-dependently inhibited agonist-induced platelet aggregation in vitro, and exhibited more-potent efficacy on ADP-induced platelet aggregation. It also suppressed platelet aggregation stimulated by ADP ex vivo. Naringenin inhibited ADP-induced platelet α-granule secretion, fibrinogen binding, intracellular calcium mobilization and platelet adhesion on collagen-coated surface. Naringenin also inhibited platelet spreading on fibrinogen and clot retraction, processes mediated by outside-in integrin signaling. Mechanism studies indicated that naringenin suppressed PI3K-mediated signaling and phosphodiesterase activity in platelets, in addition to increasing cGMP levels and VASP phosphorylation at Ser239. Furthermore, naringenin-induced VASP phosphorylation and inhibition of platelet aggregation were reversed by a PKA inhibitor treatment. Interestingly, naringenin inhibited thrombus formation in the (FeCl₃)-induced rat carotid arterial thrombus model, but not cause a prolonged bleeding time in mice. This study suggests that naringenin may represent a potential antiplatelet agent targeting PI3K and cyclic nucleotide signaling, with a low bleeding risk.

Prevention of platelet aggregation and arterial thrombosis using a modified Shenzhu Guanxin Formula

OBJECTIVE

Modified Shenzhu Guanxin Formula (mSGF) has beneficial effects in coronary artery disease. Previously, we found that mSGF inhibited platelet aggregation in rats. In the present study we further investigated the antiplatelet and antithrombotic activities of mSGF in rats.

METHODS

Rats were orally administered mSGF (4.2, 8.4, or 16.8 g crude drug/kg), the adenosine 5'-diphosphate (ADP) receptor antagonist clopidogrel (7.875 mg/kg), or saline once a day for 7 days. The effects of mSGF on platelet aggregation were measured. Levels of cyclic adenosine monophosphate (cAMP) and phosphoinositide 3-kinase (PI3K) signaling were analyzed by ELISA and western blotting, respectively. The antithrombotic effect of mSGF was investigated using a FeCl3-induced carotid arterial thrombosis model and effects on bleeding time were assessed in a rat tail transection model.

RESULTS

mSGF significantly inhibited ADP-induced platelet aggregation in a dose-dependent manner, elevated cAMP levels and inhibited phosphorylation of extracellular signal-regulated kinase (ERK) and PI3K/protein kinase B (Akt). Moreover, mSGF dose-dependently inhibited thrombosis in a FeCl3-induced carotid arterial thrombus model and had a significantly smaller effect on bleeding time compared with clopidogrel.

CONCLUSIONS

mSGF represents a potent and safe antithrombotic agent whose antiplatelet activity is probably mediated through blockade of PI3K/Akt signaling and increased cAMP generation.

PI3K inhibitors in thrombosis and cardiovascular disease

Phosphoinositide 3-kinases (PI3Ks) are lipid kinases that regulate important intracellular signalling and vesicle trafficking events via the generation of 3-phosphoinositides. Comprising eight core isoforms across three classes, the PI3K family displays broad expression and function throughout mammalian tissues, and the (patho)physiological roles of these enzymes in the cardiovascular system present the PI3Ks as potential therapeutic targets in settings such as thrombosis, atherosclerosis and heart failure. This review will discuss the PI3K enzymes and their roles in cardiovascular physiology and disease, with a particular focus on platelet function and thrombosis. The current progress and future potential of targeting the PI3K enzymes for therapeutic benefit in cardiovascular disease will be considered, while the challenges of developing drugs against these master cellular regulators will be discussed.

Stimulation of mTORC2 by integrin αIIbβ3 is required for PI3Kβ-dependent activation of Akt but is dispensable for platelet spreading on fibrinogen

Phosphatidylinositol 3 kinase (PI3K) is a major player in platelet activation and regulates thrombus formation and stabilization. The β isoform of PI3K is implicated in integrin αIIbβ3 outside-in signaling, is required for the phosphorylation of Akt, and controls efficient platelet spreading upon adhesion to fibrinogen. In this study we found that during integrin αIIbβ3 outside-in signaling PI3Kβ-dependent phosphorylation of Akt on Serine473 is mediated by the mammalian target of rapamycin complex 2 (mTORC2). The activity of mTORC2 is stimulated upon platelet adhesion to fibrinogen, as documented by increased autophosphorylation. However, mTORC2 activation downstream of integrin αIIbβ3 is PI3Kβ-independent. Inhibition of mTORC2, but not mTORC1, also prevents Akt phosphorylation of Threonine308 and affects Akt activity, resulting in the inhibition of GSK3α/β phosphorylation. Nevertheless, mTORC2 or Akt inhibition does not alter PI3Kβ-dependent platelet spreading on fibrinogen. The activation of the small GTPase Rap1b downstream of integrin αIIbβ3 is regulated by PI3Kβ but is not affected upon inhibition of either mTORC2 or Akt. Altogether, these results demonstrate for the first time the activation of mTORC2 and its involvement in Akt phosphorylation and stimulation during integrin αIIbβ3 outside-in signaling. Moreover, the results demonstrate that the mTORC2/Akt pathway is dispensable for PI3Kβ-regulated platelet spreading on fibrinogen.

Focal Adhesion Kinases in Platelet Function and Thrombosis

The focal adhesion kinase family includes 2 homolog members, FAK and Pyk2 (proline-rich tyrosine kinase 2), primarily known for their roles in nucleated cells as regulators of cytoskeletal dynamics and cell adhesion. FAK and Pyk2 are also expressed in megakaryocytes and platelets and are activated by soluble agonists and on adhesion to the subendothelial matrix. Despite high sequence homology and similar molecular organization, FAK and Pyk2 play different roles in platelet function. Whereas FAK serves mostly as a traditional focal adhesion kinase activated downstream of integrins, Pyk2 coordinates multiple signals from different receptors. FAK, but not Pyk2, is involved in megakaryocyte maturation and platelet production. In circulating platelets, FAK is recruited by integrin αIIbβ3 to regulate hemostasis, whereas it plays minimal roles in thrombosis. By contrast, Pyk2 is implicated in platelet activation and is an important regulator of thrombosis. The direct activation of Pyk2 by calcium ions provides a connection between GPCRs (G-protein coupled receptors) and Src family kinases. In this review, we provide the comprehensive overview of >20 years of investigations on the role and regulation of focal adhesion kinases in blood platelets, highlighting common and distinctive features of FAK and Pyk2 in hemostasis and thrombosis.

Low expression of PIK3C2A gene: A potential biomarker to predict the risk of acute myocardial infarction

AIMS

Phosphoinositide 3-kinases (PI3Ks) are a family of enzymes that phosphorylate the 3'-OH of inositol ring of phosphatidylinositol (PI) and regulate a broad range of signaling pathways. PIK3C2A is structurally distinct from the other members of this class and is expressed in endothelial cells, vascular endothelium, and smooth muscle. In ischemic cardiovascular diseases, such as coronary artery disease, pathology is associated with endothelial damage and inflammation, downregulation of the EPC cell population and function, and impaired angiogenesis. This study aims to make an assessment on whether expression of PIK3C2A gene can be used as a biomarker for predicting the risk of acute myocardial infarction (AMI).

METHODS

We collected peripheral blood from 84 subjects with non-coronary heart disease and 70 patients with AMI. The real-time quantitative PCR test was applied to measure levels of PIK3C2A gene expression at mRNA level in peripheral blood.

RESULTS

Our results indicated that the level of PIK3C2A gene expression in peripheral blood of AMI patients was significantly lower than one in the non-coronary heart disease subjects. Binary logistic regression analysis showed that low expression of PIK3C2A gene was an independent risk factor of AMI and increased the risk of AMI by 2.231 folds. Moreover, it was found that low expression of PIK3C2A gene was not associated with level of fasting blood glucose, platelet count, Gensini score of coronary artery, and quantity of cardiac troponin.

CONCLUSION

The level of PIK3C2A gene expression in patients with AMI is significantly lower than that of healthy people. Low expression of PIK3C2A gene is an independent risk factor of AMI. Low expression of PIK3C2A could serve as a potential biomarker to predict risk of AMI.

PI3Kβ-A Versatile Transducer for GPCR, RTK, and Small GTPase Signaling

The phosphoinositide 3-kinase (PI3K) family includes eight distinct catalytic subunits and seven regulatory subunits. Only two PI3Ks are directly regulated downstream from G protein-coupled receptors (GPCRs): the class I enzymes PI3Kβ and PI3Kγ. Both enzymes produce phosphatidylinositol 3,4,5-trisposphate in vivo and are regulated by both heterotrimeric G proteins and small GTPases from the Ras or Rho families. However, PI3Kβ is also regulated by direct interactions with receptor tyrosine kinases (RTKs) and their tyrosine phosphorylated substrates, and similar to the class II and III PI3Ks, it binds activated Rab5. The unusually complex regulation of PI3Kβ by small and trimeric G proteins and RTKs leads to a rich landscape of signaling responses at the cellular and organismic levels. This review focuses first on the regulation of PI3Kβ activity in vitro and in cells, and then summarizes the biology of PI3Kβ signaling in distinct tissues and in human disease.

Critical roles for the phosphatidylinositide 3-kinase isoforms p110β and p110γ in thrombopoietin-mediated priming of platelet function

Thrombopoietin (TPO) enhances platelet activation through activation of the tyrosine kinase; JAK2 and the lipid kinase phosphatidylinositide 3-kinase (PI3K). The aim of our study was to identify the PI3K isoforms involved in mediating the effect of TPO on platelet function and elucidate the underlying mechanism. We found that p110β plays an essential role in TPO-mediated (i) priming of protease-activated receptor (PAR)-mediated integrin α_IIbβ₃ activation and α-granule secretion, (ii) synergistic enhancement of PAR-mediated activation of the small GTPase RAP1, a regulator of integrin activation and (iii) phosphorylation of the PI3K effector Akt. More importantly, the synergistic effect of TPO on phosphorylation of extracellular-regulated kinase (ERK1/2) and thromboxane (TxA₂) synthesis was dependent on both p110β and p110γ. p110β inhibition/deletion, or inhibition of p110γ, resulted in a partial reduction, whereas inhibiting both p110β and p110γ completely prevented the synergistic effect of TPO on ERK1/2 phosphorylation and TxA₂ synthesis. The latter was ablated by inhibition of MEK, but not p38, confirming a role for ERK1/2 in regulating TPO-mediated increases in TxA₂ synthesis. In conclusion, the synergistic effect of TPO on RAP1 and integrin activation is largely mediated by p110β, whereas p110β and p110γ contribute to the effect of TPO on ERK1/2 phosphorylation and TxA₂ formation.

The role of P2Y12 receptor in ischemic stroke of atherosclerotic origin

Atherosclerosis is a chronic and progressive disease of the arterial walls and a leading cause of non-cardioembolic ischemic stroke. P2Y12 is a well-recognized receptor that is expressed on platelets and is a target of thienopyridine-type antiplatelet drugs. In the last few decades, P2Y12 receptor inhibitors, such as clopidogrel, have been applied for the secondary prevention of non-cardioembolic ischemic stroke. Recent clinical studies have suggested that these P2Y12 receptor inhibitors may be more effective than other antiplatelet drugs in patients with ischemic stroke/transient ischemic attack of atherosclerotic origin. Moreover, animal studies have also shown that the P2Y12 receptor may participate in atherogenesis by promoting the proliferation and migration of vascular smooth muscle cells (VSMCs) and endothelial dysfunction, and affecting inflammatory cell activities in addition to amplifying and maintaining ADP-induced platelet activation and platelet aggregation. P2Y12 receptor inhibitors may also exert neuroprotective effects after ischemic stroke. Thus, P2Y12 receptor inhibitors may be a better choice for secondary prevention in patients with atherosclerotic ischemic stroke subtypes because of their triple functions (i.e., their anti-atherosclerotic, anti-platelet aggregation, and neuroprotective activities), and the P2Y12 receptor may also serve as a noval therapeutic target for atherosclerosis. In this review, we summarize the current knowledge on the P2Y12 receptor and its key roles in atherosclerosis and ischemic stroke of atherosclerotic origin.

Ligustrazine inhibits platelet activation via suppression of the Akt pathway

Aberrant activation of platelets has a critical role in thrombotic vascular events, including atherosclerosis, arterial thrombosis and myocardial infarction. The process of platelet activation is associated with multiple intracellular signaling pathways, including the phosphoinositide 3‑kinase/AKT serine/threonine kinase (Akt) pathway. The well‑known medicinal herb Rhizoma Ligusticum Wallichii (RLW) has long been used in China to clinically treat various cardiovascular disorders. As the most pharmacologically active component of RLW, ligustrazine has been demonstrated to possess a potent antiplatelet activity. However, the precise mechanisms mediating the bioactivities of ligustrazine have not been thoroughly elucidated. The present study evaluated the effects of ligustrazine hydrochloride (LH; the clinical‑grade form of ligustrazine) on platelet activation and investigated the underlying molecular mechanisms. In vitro and ex vivo platelet activation models were used, established by stimulating rat platelet‑rich plasma either with the platelet activator adenosine diphosphate (ADP) or with the specific Akt pathway activator insulin‑like growth factor‑1 (IGF‑1). The results demonstrated that treatment with LH significantly and dose‑dependently inhibited ADP‑induced platelet aggregation, in addition to thromboxane A2 (TXA2) secretion and intracellular Ca2+ mobilization in platelets, in vitro and ex vivo. In addition, LH markedly suppressed ADP‑induced Akt phosphorylation in vitro and ex vivo. Furthermore, LH markedly inhibited IGF‑1‑induced Akt phosphorylation, platelet aggregation, TXA2 formation and Ca2+ mobilization in vitro. Finally, LH was able to reverse adrenaline‑induced shortening of bleeding time. Taken together, these results suggested that ligustrazine possesses a broad range of antiplatelet activities without apparent hemorrhagic side-effects, and suppression of Akt signaling may be one of the mechanisms by which ligustrazine exerts its antiplatelet activities.

Impact of PI3Kα (Phosphoinositide 3-Kinase Alpha) Inhibition on Hemostasis and Thrombosis

Objective- PI3Kα (phosphoinositide 3-kinase alpha) is a therapeutic target in oncology, but its role in platelets and thrombosis remains ill characterized. In this study, we have analyzed the role of PI3Kα in vitro, ex vivo, and in vivo in 2 models of arterial thrombosis. Approach and Results- Using mice selectively deficient in p110α in the megakaryocyte lineage and isoform-selective inhibitors, we confirm that PI3Kα is not mandatory but participates to thrombus growth over a collagen matrix at arterial shear rate. Our data uncover a role for PI3Kα in low-level activation of the GP (glycoprotein) VI-collagen receptor by contributing to ADP secretion and in turn full activation of PI3Kβ and Akt/PKB (protein kinase B). This effect was no longer observed at high level of GP VI agonist concentration. Our study also reveals that over a vWF (von Willebrand factor) matrix, PI3Kα regulates platelet stationary adhesion contacts under arterial flow through its involvement in the outside-in signaling of vWF-engaged αIIbβ3 integrin. In vivo, absence or inhibition of PI3Kα resulted in a modest but significant decrease in thrombus size after superficial injuries of mouse mesenteric arteries and an increased time to arterial occlusion after carotid lesion, without modification in the tail bleeding time. Considering the more discrete and nonredundant role of PI3Kα compared with PI3Kβ, selective PI3Kα inhibitors are unlikely to increase the bleeding risk at least in the absence of combination with antiplatelet drugs or thrombopenia. Conclusions- This study provides mechanistic insight into the role of PI3Kα in platelet activation and arterial thrombosis.

Integrating platelet and coagulation activation in fibrin clot formation

Platelets interact with the coagulation system in a multitude of ways, not only during the phases of thrombus formation, but also in specific areas within a formed thrombus. This review discusses current concepts of platelet control of thrombin generation, fibrin formation and structure, and anticoagulation. Indicated are how combined signalling via the platelet receptors for collagen (glycoprotein VI) and thrombin induces the secretion of (anti)coagulation factors, as well as surface exposure of phosphatidylserine, thereby catalysing thrombin generation. This procoagulant platelet response is also facilitated by the adhesive complexes glycoprotein Ib-V-IX and integrin αIIbβ3. In the buildup of a platelet-fibrin thrombus, the extrinsic, tissue factor-driven coagulation pathway is predominant in early stages, while the intrinsic, factor XII pathway seems to promote at later time points. Already early generation of thrombin enforces platelet responses and stimulates intra-thrombus heterogeneity with patches of loosely aggregated, contracted, and phosphatidylserine-exposing platelets. Fibrin actively formed on the surface of activated platelets supports thrombus growth, but also captures thrombin. The fibrin distribution in a thrombus appears to rely on the local procoagulant trigger and the blood flow rate. Clinical studies support the importance of the platelet-coagulation interplay, by showing beneficial effects of combination therapy in the secondary prevention of cardiovascular disease.

Profiling of phosphoinositide molecular species in human and mouse platelets identifies new species increasing following stimulation

Phosphoinositides are bioactive lipids essential in the regulation of cell signaling as well as cytoskeleton and membrane dynamics. Their metabolism is highly active in blood platelets where they play a critical role during activation, at least through two well identified pathways involving phospholipase C and phosphoinositide 3-kinases (PI3K). Here, using a sensitive high-performance liquid chromatography-mass spectrometry method recently developed, we monitored for the first time the profiling of phosphatidylinositol (PI), PIP, PIP₂ and PIP₃ molecular species (fatty-acyl profiles) in human and mouse platelets during the course of stimulation by thrombin and collagen-related peptide. Furthermore, using class IA PI3K p110α or p110β deficient mouse platelets and a pharmacological inhibitor, we show the crucial role of p110β and the more subtle role of p110α in the production of PIP₃ molecular species following stimulation. This comprehensive platelet phosphoinositides profiling provides important resources for future studies and reveals new information on phosphoinositides biology, similarities and differences in mouse and human platelets and unexpected dramatic increase in low-abundance molecular species of PIP₂ during stimulation, opening new perspectives in phosphoinositide signaling in platelets.

Phosphoinositide 3-kinase p110α negatively regulates thrombopoietin-mediated platelet activation and thrombus formation

Phosphoinositide 3-kinase (PI3K) plays an important role in platelet function and contributes to platelet hyperreactivity induced by elevated levels of circulating peptide hormones, including thrombopoietin (TPO). Previous work established an important role for the PI3K isoform; p110β in platelet function, however the role of p110α is still largely unexplored. Here we sought to investigate the role of p110α in TPO-mediated hyperactivity by using a conditional p110α knockout (KO) murine model in conjunction with platelet functional assays. We found that TPO-mediated enhancement of collagen-related peptide (CRP-XL)-induced platelet aggregation and adenosine triphosphate (ATP) secretion were significantly increased in p110α KO platelets. Furthermore, TPO-mediated enhancement of thrombus formation by p110α KO platelets was elevated over wild-type (WT) platelets, suggesting that p110α negatively regulates TPO-mediated priming of platelet function. The enhancements were not due to increased flow through the PI3K pathway as phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P₃) formation and phosphorylation of Akt and glycogen synthase kinase 3 (GSK3) were comparable between WT and p110α KO platelets. In contrast, extracellular responsive kinase (ERK) phosphorylation and thromboxane (TxA₂) formation were significantly enhanced in p110α KO platelets, both of which were blocked by the MEK inhibitor PD184352, whereas the p38 MAPK inhibitor VX-702 and p110α inhibitor PIK-75 had no effect. Acetylsalicylic acid (ASA) blocked the enhancement of thrombus formation by TPO in both WT and p110α KO mice. Together, these results demonstrate that p110α negatively regulates TPO-mediated enhancement of platelet function by restricting ERK phosphorylation and TxA₂ synthesis in a manner independent of its kinase activity.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

Integrin αIIbβ3 outside-in signaling

Integrin α_IIbβ₃ is a highly abundant heterodimeric platelet receptor that can transmit information bidirectionally across the plasma membrane, and plays a critical role in hemostasis and thrombosis. Upon platelet activation, inside-out signaling pathways increase the affinity of α_IIbβ₃ for fibrinogen and other ligands. Ligand binding and integrin clustering subsequently stimulate outside-in signaling, which initiates and amplifies a range of cellular events driving essential platelet processes such as spreading, thrombus consolidation, and clot retraction. Integrin α_IIbβ₃ has served as an excellent model for the study of integrin biology, and it has become clear that integrin outside-in signaling is highly complex and involves a vast array of enzymes, signaling adaptors, and cytoskeletal components. In this review, we provide a concise but comprehensive overview of α_IIbβ₃ outside-in signaling, focusing on the key players involved, and how they cooperate to orchestrate this critical aspect of platelet biology. We also discuss gaps in the current understanding of α_IIbβ₃ outside-in signaling and highlight avenues for future investigation.

In-depth PtdIns(3,4,5)P3 signalosome analysis identifies DAPP1 as a negative regulator of GPVI-driven platelet function

The class I phosphoinositide 3-kinase (PI3K) isoforms play important roles in platelet priming, activation, and stable thrombus formation. Class I PI3Ks predominantly regulate cell function through their catalytic product, the signaling phospholipid phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P₃], which coordinates the localization and/or activity of a diverse range of binding proteins. Notably, the complete repertoire of these class I PI3K effectors in platelets remains unknown, limiting mechanistic understanding of class I PI3K-mediated control of platelet function. We measured robust agonist-driven PtdIns (3,4,5)P₃ generation in human platelets by lipidomic mass spectrometry (MS), and then used affinity-capture coupled to high-resolution proteomic MS to identify the targets of PtdIns (3,4,5)P₃ in these cells. We reveal for the first time a diverse platelet PtdIns(3,4,5)P₃ interactome, including kinases, signaling adaptors, and regulators of small GTPases, many of which are previously uncharacterized in this cell type. Of these, we show dual adaptor for phosphotyrosine and 3-phosphoinositides (DAPP1) to be regulated by Src-family kinases and PI3K, while platelets from DAPP1-deficient mice display enhanced thrombus formation on collagen in vitro. This was associated with enhanced platelet α/δ granule secretion and α_IIbβ₃ integrin activation downstream of the collagen receptor glycoprotein VI. Thus, we present the first comprehensive analysis of the PtdIns(3,4,5)P₃ signalosome of human platelets and identify DAPP1 as a novel negative regulator of platelet function. This work provides important new insights into how class I PI3Ks shape platelet function.

The Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) Binder Rasa3 Regulates Phosphoinositide 3-kinase (PI3K)-dependent Integrin αIIbβ3 Outside-in Signaling

The class I PI3K family of lipid kinases plays an important role in integrin α_IIbβ₃ function, thereby supporting thrombus growth and consolidation. Here, we identify Ras/Rap1GAP Rasa3 (GAP1^IP4BP) as a major phosphatidylinositol 3,4,5-trisphosphate-binding protein in human platelets and a key regulator of integrin α_IIbβ₃ outside-in signaling. We demonstrate that cytosolic Rasa3 translocates to the plasma membrane in a PI3K-dependent manner upon activation of human platelets. Expression of wild-type Rasa3 in integrin α_IIbβ₃-expressing CHO cells blocked Rap1 activity and integrin α_IIbβ₃-mediated spreading on fibrinogen. In contrast, Rap1GAP-deficient (P489V) and Ras/Rap1GAP-deficient (R371Q) Rasa3 had no effect. We furthermore show that two Rasa3 mutants (H794L and G125V), which are expressed in different mouse models of thrombocytopenia, lack both Ras and Rap1GAP activity and do not affect integrin α_IIbβ₃-mediated spreading of CHO cells on fibrinogen. Platelets from thrombocytopenic mice expressing GAP-deficient Rasa3 (H794L) show increased spreading on fibrinogen, which in contrast to wild-type platelets is insensitive to PI3K inhibitors. Together, these results support an important role for Rasa3 in PI3K-dependent integrin α_IIbβ₃-mediated outside-in signaling and cell spreading.

Downstream Regulatory Element Antagonist Modulator (DREAM), a target for anti-thrombotic agents

Circulating platelets participate in the process of numerous diseases including thrombosis, inflammation, and cancer. Thus, it is of great importance to understand the underlying mechanisms mediating platelet activation under disease conditions. Emerging evidence indicates that despite the lack of a nucleus, platelets possess molecules that are involved in gene transcription in nucleated cells. This review will summarize downstream regulatory element antagonist modulator (DREAM), a transcriptional repressor, and highlight recent findings suggesting its novel non-transcriptional role in hemostasis and thrombosis.

DREAM plays an important role in platelet activation and thrombogenesis

Downstream regulatory element antagonist modulator (DREAM), a transcriptional repressor, is known to modulate pain responses. However, it is unknown whether DREAM is expressed in anucleate platelets and plays a role in thrombogenesis. By using intravital microscopy with DREAM-null mice and their bone marrow chimeras, we demonstrated that both hematopoietic and nonhematopoietic cell DREAMs are required for platelet thrombus formation following laser-induced arteriolar injury. In a FeCl₃-induced thrombosis model, we found that compared with wild-type (WT) control and nonhematopoietic DREAM knockout (KO) mice, DREAM KO control and hematopoietic DREAM KO mice showed a significant delay in time to occlusion. Tail bleeding time was prolonged in DREAM KO control mice, but not in WT or DREAM bone marrow chimeric mice. In vivo adoptive transfer experiments further indicated the importance of platelet DREAM in thrombogenesis. We found that DREAM deletion does not alter the ultrastructural features of platelets but significantly impairs platelet aggregation and adenosine triphosphate secretion induced by numerous agonists (collagen-related peptide, adenosine 5'-diphosphate, A23187, thrombin, or U46619). Biochemical studies revealed that platelet DREAM positively regulates phosphoinositide 3-kinase (PI3K) activity during platelet activation. Using DREAM-null platelets and PI3K isoform-specific inhibitors, we observed that platelet DREAM is important for α-granule secretion, Ca²⁺ mobilization, and aggregation through PI3K class Iβ (PI3K-Iβ). Genetic and pharmacological studies in human megakaryoblastic MEG-01 cells showed that DREAM is important for A23187-induced Ca²⁺ mobilization and its regulatory function requires Ca²⁺ binding and PI3K-Iβ activation. These results suggest that platelet DREAM regulates PI3K-Iβ activity and plays an important role during thrombus formation.

PDK1 Determines Collagen-Dependent Platelet Ca 2+ Signaling and Is Critical to Development of Ischemic Stroke In Vivo

Objective—

Activation of platelets by subendothelial collagen results in an increase of cytosolic Ca 2+ concentration ([Ca 2+ ] i ) and is followed by platelet activation and thrombus formation that may lead to vascular occlusion. The present study determined the role of phosphoinositide-dependent protein kinase 1 (PDK1) in collagen-dependent platelet Ca 2+ signaling and ischemic stroke in vivo.

Approach and Results—

Platelet activation with collagen receptor glycoprotein VI agonists collagen-related peptide or convulxin resulted in a significant increase in PDK1 activity independent of second-wave signaling. PDK1 deficiency was associated with reduced platelet phospholipase Cγ2–dependent inositol-1,4,5-trisphosphate production and intracellular [Ca 2+ ] i in response to stimulation with collagen-related peptide or convulxin. The defective increase of [Ca 2+ ] i resulted in a substantial defect in activation-dependent platelet secretion and aggregation on collagen-related peptide stimulation. Furthermore, Rac1 activation and spreading, adhesion to collagen, and thrombus formation under high arterial shear rates were significantly diminished in PDK1-deficient platelets. Mice with PDK1-deficient platelets were protected against arterial thrombotic occlusion after FeCl 3 -induced mesenteric arterioles injury and ischemic stroke in vivo. These mice had significantly reduced brain infarct volumes, with a significantly increased survival of 7 days after transient middle cerebral artery occlusion without increase of intracerebral hemorrhage. Tail bleeding time was prolonged in pdk1 −/− mice, reflecting an important role of PDK1 in primary hemostasis.

Conclusions—

PDK1 is required for Ca 2+ -dependent platelet activation on stimulation of collagen receptor glycoprotein VI, arterial thrombotic occlusion, and ischemic stroke in vivo.

Discovery and antiplatelet activity of a selective PI3Kβ inhibitor (MIPS-9922)

A series of amino-substituted triazines were developed and examined for PI3Kβ inhibition and anti-platelet function. Structural adaptations of a morpholine ring of the prototype pan-PI3K inhibitor ZSTK474 yielded PI3Kβ selective compounds, where the selectivity largely derives from an interaction with the non-conserved Asp862 residue, as shown by site directed mutagenesis. The most PI3Kβ selective inhibitor from the series was studied in detail through a series of in vitro and in vivo functional studies. MIPS-9922, 10 potently inhibited ADP-induced washed platelet aggregation. It also inhibited integrin αIIbβ3 activation and αIIbβ3 dependent platelet adhesion to immobilized vWF under high shear. It prevented arterial thrombus formation in the in vivo electrolytic mouse model of thrombosis without inducing prolonged bleeding or excess blood loss.

Suppressive effect of exogenous carbon monoxide on endotoxin-stimulated platelet over-activation via the glycoprotein-mediated PI3K-Akt-GSK3β pathway

Platelet activation is an important event involved in the pathophysiological processes of the coagulation system. Clinical evidence has shown that platelets undergo distinctive pathological processes during sepsis. Unfortunately, how platelets physiologically respond to inflammation or sepsis is not well understood. In this study, we used a lipopolysaccharide (LPS)-stimulated platelet model to systemically investigate alterations in membrane glycoprotein expression, molecular signaling, morphology and critical functions of platelets. We found that platelet adhesion, aggregation, secretion, and spreading on immobilized fibrinogen and the expression of platelet membrane glycoproteins were significantly increased by LPS stimulation, and these changes were accompanied by a significant decrease in cGMP levels and an abnormal distribution of platelet α-granules. Exogenous CO reversed these alterations. Profound morphological changes in LPS-stimulated platelets were observed using atomic force microscopy and phase microscopy. Furthermore, the elevated activities of PI3Ks, AKt and GSK-3β were effectively suppressed by exogenous CO, leading to the improvement of platelet function. Together, these results provide evidence that platelet over-activation persists under LPS-stimulation and that exogenous CO plays an important role in suppressing platelet activation via the glycoprotein-mediated PI3K-Akt-GSK3β pathway.

The inhibitory activity of ginsenoside Rp4 in adenosine diphosphate-induced platelet aggregation

BACKGROUND

Korean ginseng, Panax ginseng Meyer, has been used as a traditional oriental medicine to treat illness and promote health for several thousand years. Ginsenosides are the main constituents for the pharmacological effects of P. ginseng. Since several ginsenosides, including ginsenoside (G)-Rg3 and G-Rp1, have reported antiplatelet activity, here we investigate the ability of G-Rp4 to modulate adenosine diphosphate (ADP)-induced platelet aggregation. The ginsenoside Rp4, a similar chemical structure of G-Rp1, was prepared from G-Rg1 by chemical modification.

METHODS

To examine the effects of G-Rp4 on platelet activation, we performed several experiments, including antiplatelet ability, the modulation of intracellular calcium concentration, and P-selectin expression. In addition, we examined the activation of integrin αIIbβ₃ and the phosphorylation of signaling molecules using fibrinogen binding assay and immunoblotting in rat washed platelets.

RESULTS

G-Rp4 inhibited ADP-induced platelet aggregation in a dose-dependent manner. We found that G-Rp4 decreased calcium mobilization and P-selectin expression in ADP-activated platelets. Moreover, fibrinogen binding to integrin αIIbβ₃ by ADP was attenuated in G-Rp4-treated platelets. G-Rp4 significantly attenuated phosphorylation of extracellular signal-regulated protein kinases 1 and 2, p38, and c-Jun N-terminal kinase, as well as protein kinase B, phosphatidylinositol 3-kinase, and phospholipase C-γ phosphorylations.

CONCLUSION

G-Rp4 significantly inhibited ADP-induced platelet aggregation and this is mediated via modulating the intracellular signaling molecules. These results indicate that G-Rp4 could be a potential candidate as a therapeutic agent against platelet-related cardiovascular diseases.

The role of class I, II and III PI 3-kinases in platelet production and activation and their implication in thrombosis

Blood platelets play a pivotal role in haemostasis and are strongly involved in arterial thrombosis, a leading cause of death worldwide. Besides their critical role in pathophysiology, platelets represent a valuable model to investigate, both in vitro and in vivo, the biological roles of different branches of the phosphoinositide metabolism, which is highly active in platelets. While the phospholipase C (PLC) pathway has a crucial role in platelet activation, it is now well established that at least one class I phosphoinositide 3-kinase (PI3K) is also mandatory for proper platelet functions. Except class II PI3Kγ, all other isoforms of PI3Ks (class I α, β, γ, δ; class II α, β and class III) are expressed in platelets. Class I PI3Ks have been extensively studied in different models over the past few decades and several isoforms are promising drug targets to treat cancer and immune diseases. In platelet activation, it has been shown that while class I PI3Kδ plays a minor role, class I PI3Kβ has an important function particularly in thrombus growth and stability under high shear stress conditions found in stenotic arteries. This class I PI3K is a potentially interesting target for antithrombotic strategies. The role of class I PI3Kα remains ill defined in platelets. Herein, we will discuss our recent data showing the potential impact of inhibitors of this kinase on thrombus formation. The role of class II PI3Kα and β as well as class III PI3K (Vps34) in platelet production and function is just emerging. Based on our data and those very recently published in the literature, we will discuss the impact of these three PI3K isoforms in platelet production and functions and in thrombosis.

[Phosphoinositides: the lipids coordinating cell dynamics]

Within the glycerophospholipid family, phosphoinositides, which are minor components of eukaryotic cell membranes, play a critical role as spatiotemporal organizers of cell dynamics. By specifically interacting with proteins, they coordinate the formation and the organization of multiprotein complexes involved in cell signaling, intracellular trafficking and cytoskeleton rearrangement. The highly precise spatiotemporal dynamics of phosphoinositides-regulated mechanisms is ensured by kinases and phosphatases that specifically produce, hydrolyze and control the interconversion of these lipids. The direct implication of these enzymes in human pathologies such as genetic diseases, cancer or infectious pathologies, and the recent arrival of inhibitors targeting some phosphoinositide kinases in clinic, illustrate the mandatory functions of these fascinating lipids.

Deubiquitinases Modulate Platelet Proteome Ubiquitination, Aggregation, and Thrombosis

OBJECTIVE

Platelets express a functional ubiquitin-proteasome system. Mass spectrometry shows that platelets contain several deubiquitinases, but whether these are functional, modulate the proteome, or affect platelet reactivity are unknown.

APPROACH AND RESULTS

Platelet lysates contained ubiquitin-protein deubiquitinase activity hydrolyzing both Lys48 and Lys63 polyubiquitin conjugates that was suppressed by the chemically unrelated deubiquitinase inhibitors PYR41 and PR619. These inhibitors acutely and markedly increased monoubiquitination and polyubiquitination of the proteome of resting platelets. PYR41 (intravenous, 15 minutes) significantly impaired occlusive thrombosis in FeCl3-damaged carotid arteries, and deubiquitinase inhibition reduced platelet adhesion and retention during high shear flow of whole blood through microfluidic chambers coated with collagen. Total internal reflection microscopy showed that adhesion and spreading in the absence of flow were strongly curtailed by these inhibitors with failure of stable process extension and reduced the retraction of formed clots. Deubiquitinase inhibition also sharply reduced homotypic platelet aggregation in response to not only the incomplete agonists ADP and collagen acting through glycoprotein VI but also to the complete agonist thrombin. Suppressed aggregation was accompanied by curtailed procaspase activating compound-1 binding to activated IIb/IIIa and inhibition of P-selectin translocation to the platelet surface. Deubiquitinase inhibition abolished the agonist-induced spike in intracellular calcium, suppressed Akt phosphorylation, and reduced agonist-stimulated phosphatase and tensin homolog phosphatase phosphorylation. Platelets express the proteasome-associated deubiquitinases USP14 and UCHL5, and selective inhibition of these enzymes by b-AP15 reproduced the inhibitory effect of the general deubiquitinase inhibitors on ex vivo platelet function.

CONCLUSIONS

Remodeling of the ubiquitinated platelet proteome by deubiquitinases promotes agonist-stimulated intracellular signal transduction and platelet responsiveness.

Phosphoinositides: Important lipids in the coordination of cell dynamics

By interacting specifically with proteins, phosphoinositides organize the spatiotemporal formation of protein complexes involved in the control of intracellular signaling, vesicular trafficking and cytoskeleton dynamics. A set of specific kinases and phosphatases ensures the production, degradation and inter-conversion of phosphoinositides to achieve a high level of precision in the regulation of cellular dynamics coordinated by these lipids. The direct involvement of these enzymes in cancer, genetic or infectious diseases, and the recent arrival of inhibitors targeting specific phosphoinositide kinases in clinic, emphasize the importance of these lipids and their metabolism in the biomedical field.

PI3K signaling in arterial diseases: Non redundant functions of the PI3K isoforms

Cardiovascular diseases are the most common cause of death around the world. This includes atherosclerosis and the adverse effects of its treatment, such as restenosis and thrombotic complications. The development of these arterial pathologies requires a series of highly-intertwined interactions between immune and arterial cells, leading to specific inflammatory and fibroproliferative cellular responses. In the last few years, the study of phosphoinositide 3-kinase (PI3K) functions has become an attractive area of investigation in the field of arterial diseases, especially since inhibitors of specific PI3K isoforms have been developed. The PI3K family includes 8 members divided into classes I, II or III depending on their substrate specificity. Although some of the different isoforms are responsible for the production of the same 3-phosphoinositides, they each have specific, non-redundant functions as a result of differences in expression levels in different cell types, activation mechanisms and specific subcellular locations. This review will focus on the functions of the different PI3K isoforms that are suspected as having protective or deleterious effects in both the various immune cells and types of cell found in the arterial wall. It will also discuss our current understanding in the context of which PI3K isoform(s) should be targeted for future therapeutic interventions to prevent or treat arterial diseases.