A phosphoinositide 3-kinase-AKT-nitric oxide-cGMP signaling pathway in stimulating platelet secretion and aggregation

The Role of NO/sGC/cGMP/PKG Signaling Pathway in Regulation of Platelet Function

Circulating blood platelets are controlled by stimulatory and inhibitory factors, and a tightly regulated equilibrium between these two opposing processes is essential for normal platelet and vascular function. NO/cGMP/ Protein Kinase G (PKG) pathways play a highly significant role in platelet inhibition, which is supported by a large body of studies and data. This review focused on inconsistent and controversial data of NO/sGC/cGMP/PKG signaling in platelets including sources of NO that activate sGC in platelets, the role of sGC/PKG in platelet inhibition/activation, and the complexity of the regulation of platelet inhibitory mechanisms by cGMP/PKG pathways. In conclusion, we suggest that the recently developed quantitative phosphoproteomic method will be a powerful tool for the analysis of PKG-mediated effects. Analysis of phosphoproteins in PKG-activated platelets will reveal many new PKG substrates. A future detailed analysis of these substrates and their involvement in different platelet inhibitory pathways could be a basis for the development of new antiplatelet drugs that may target only specific aspects of platelet functions.

Age-restricted functional and developmental differences of neonatal platelets

BACKGROUND

Developmental ontogeny of neonatal thrombopoiesis retains characteristics that are distinct from adults although molecular mechanisms remain unestablished.

METHODS

We applied multiparameter quantitative platelet responses with integrated ribosome profiling/transcriptomic studies to better define gene/pathway perturbations regulating the neonatal-to-adult transition. A bioinformatics pipeline was developed to identify stable, neonatal-restricted platelet biomarkers for clinical application.

RESULTS

Cord blood (CB) platelets retained the capacity for linear agonist-receptor coupling linked to phosphatidylserine (PS) exposure and α-granule release, although a restricted block in cross-agonist activation pathways was evident. Functional immaturity of synergistic signaling pathways was due to younger ontogenetic age and singular underdevelopment of the protein secretory gene network, with reciprocal expansion of developmental pathways (E2F, G2M checkpoint, c-Myc) important for megakaryocytopoiesis. Genetic perturbations regulating vesicle transport and fusion (TOM1L1, VAMP3, SNAP23, and DNM1L) and PS exposure and procoagulant activity (CLCN3) were the most significant, providing a molecular explanation for globally attenuated responses. Integrated transcriptomic and ribosomal footprints identified highly abundant (ribosome-protected) DEFA3 (encoding human defensin neutrophil peptide 3) and HBG1 as stable biomarkers of neonatal thrombopoiesis. Studies comparing CB- or adult-derived megakaryocytopoiesis confirmed inducible and abundant DEFA3 antigenic expression in CB megakaryocytes, ~3.5-fold greater than in leukocytes (the most abundant source in humans). An initial feasibility cohort of at-risk pregnancies manifested by maternal/fetal hemorrhage (chimerism) were applied for detection and validation of platelet HBG1 and DEFA3 as neonatal thrombopoiesis markers, most consistent for HBG1, which displayed gestational age-dependent expression.

CONCLUSIONS

These studies establish an ontogenetically divergent stage of neonatal thrombopoiesis, and provide initial feasibility studies to track disordered fetal-to-adult megakaryocytopoiesis in vivo.

TCM Regulates PI3K/Akt Signal Pathway to Intervene Atherosclerotic Cardiovascular Disease

Vascular endothelial injury is the initial stage of atherosclerosis (AS). Stimulating and activating the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway can regulate the expression of vascular endothelial cytokines, thus affecting the occurrence and development of AS. In addition, the PI3K/Akt signaling pathway can regulate the polarization and survival of macrophages and the expression of inflammatory factors and platelet function, thus influencing the progression of AS. In recent years, traditional Chinese medicine (TCM) has been widely recognized for its advantages of fewer side effects, multiple pathways, and multiple targets. Also, the research of TCM regulation of AS via the PI3K/Akt signaling pathway has achieved certain results. This study aimed to analyze the characteristics of the PI3K/Akt signaling pathway and its role in the pathogenesis of AS, as well as the role of Chinese medicine in regulating the PI3K/Akt signaling pathway. The findings are expected to provide a theoretical basis for the clinical treatment and pathological mechanism research of AS.

α-Ketoglutarate Inhibits Thrombosis and Inflammation by Prolyl Hydroxylase-2 Mediated Inactivation of Phospho-Akt

Background

Phospho-Akt1 (pAkt1) undergoes prolyl hydroxylation at Pro125 and Pro313 by the prolyl hydroxylase-2 (PHD2) in a reaction decarboxylating α-ketoglutarate (αKG). We investigated whether the αKG supplementation could inhibit Akt-mediated activation of platelets and monocytes, in vitro as well as in vivo, by augmenting PHD2 activity.

Methods

We treated platelets or monocytes isolated from healthy individuals with αKG in presence of agonists in vitro and assessed the signalling molecules including pAkt1. We supplemented mice with dietary αKG and estimated the functional responses of platelets and monocytes ex vivo. Further, we investigated the impact of dietary αKG on inflammation and thrombosis in lungs of mice either treated with thrombosis-inducing agent carrageenan or infected with SARS-CoV-2.

Findings

Octyl αKG supplementation to platelets promoted PHD2 activity through elevated intracellular αKG to succinate ratio, and reduced aggregation in vitro by suppressing pAkt1(Thr308). Augmented PHD2 activity was confirmed by increased hydroxylated-proline and enhanced binding of PHD2 to pAkt in αKG-treated platelets. Contrastingly, inhibitors of PHD2 significantly increased pAkt1 in platelets. Octyl-αKG followed similar mechanism in monocytes to inhibit cytokine secretion in vitro. Our data also describe a suppressed pAkt1 and reduced activation of platelets and leukocytes ex vivo from mice supplemented with dietary αKG, unaccompanied by alteration in their number. Dietary αKG significantly reduced clot formation and leukocyte accumulation in various organs including lungs of mice treated with thrombosis-inducing agent carrageenan. Importantly, in SARS-CoV-2 infected hamsters, we observed a significant rescue effect of dietary αKG on inflamed lungs with significantly reduced leukocyte accumulation, clot formation and viral load alongside down-modulation of pAkt in the lung of the infected animals.

Interpretation

Our study suggests that dietary αKG supplementation prevents Akt-driven maladies such as thrombosis and inflammation and rescues pathology of COVID19-infected lungs.

Funding

Study was funded by the Department of Biotechnology (DBT), Govt. of India (grants: BT/PR22881 and BT/PR22985); and the Science and Engineering Research Board, Govt. of India (CRG/000092).

Caveolin-1 promotes tumor cell proliferation and vasculogenic mimicry formation in human glioma

Vasculogenic mimicry (VM) plays an important role in human glioma progression and resistance to antiangiogenic therapy as a compensatory neovascularization mechanism in malignant tumors. Caveolin-1 (Cav-1) has been found to contribute to VM formation. However, it remains largely unknown whether Cav-1 expression correlates with VM in glioma. In this study, we examined CAV-1 expression levels and VM in human glioma cell lines and in 94 human gliomas with different grades of malignancy, and present Cox proportional hazards regression. The molecular role of Cav-1 in glioma cells was investigated using quantitative polymerase chain reaction (qRT-PCR) assays, western blotting, CCK-8 assays, and tubule formation assays. Cav-1 expression and VM formation were positively correlated with each other and both were closely associated with glioma development and progression as evidenced by the presence of cystic tumor, shortened survival time, and advanced-stage glioma in glioma patients with Cav-1 overexpression/increased VM formation. Cav-1 promoted U251 glioma cell proliferation and VM formation in a Matrigel-based 3D culture model. VM-associated factors including hypoxia-inducible factor 1α (HIF-1α) and p-Akt was significantly elevated by Cav-1 overexpression but suppressed by siCav-1 in U251 cells. Collectively, our study identified Cav-1 as an important regulator of glioma cell proliferation and VM formation, contributing to glioma development and progression.

Flow Cytometric Assessment of AKT Signaling in Platelet Activation: An Alternative Diagnostic Tool for Small Volumes of Blood

INTRODUCTION

 The diagnosis of platelet function disorder in children is challenging. Light transmission aggregometry is the gold standard for platelet function disorders. However, large blood volumes are required. Currently, there are no existing tools for the diagnosis of platelet function disorders that use small blood volumes. AKT signaling plays a central role in platelet activation during hemostasis and might be visualized by flow cytometry.

METHODS

 Platelet-rich plasma obtained by centrifugation of citrated blood from healthy volunteers was activated with arachidonic acid, thrombin receptor activating peptide-6 (TRAP-6), collagen, adenosine diphosphate ADP, collagen-related peptide (CRP), and epinephrine. After platelet activation, the phosphorylation of AKT was assessed by flow cytometer using a Navios cytometer.

RESULTS

 Healthy volunteers showed a reproducible phosphorylation of AKT upon activation. In comparison to nonactivated platelets, we documented an increase in pAKT expression with all agonists. Especially TRAP-6 and CRP caused considerable increase in percentage of pAKT expression throughout all the tested healthy volunteers.

CONCLUSION

 An activation of the AKT-signal pathway by different agonists can clearly be detected on the flow cytometer, indicating that the visualization of signaling in platelets by flow cytometry might be an efficient alternative for light transmission aggregometry to test platelet function in children.

Significant reductions in apoptosis-related proteins (HSPA6, HSPA8, ITGB3, YWHAH, and PRDX6) are involved in immune thrombocytopenia

To investigate differences in the expression of plasma proteins in immune thrombocytopenia (ITP) and normal control groups, bone marrow samples were collected from 20 active ITP patients and 20 healthy controls. Quantitative proteomics analysis based on mass spectrometry was used to measure the protein levels and understand the protein networks. We found differentially expressed proteins in ITP patients and healthy controls. Parallel reaction monitoring (PRM), a targeted proteome quantification technique, was used to quantitatively confirm the identified target proteins and verify the proteomics data. In this study, a total of 829 proteins were identified, and the fold-change cut-off was set at 1.5 (patients vs controls); a total of 26 proteins were upregulated, and 69 proteins were downregulated. The bioinformatics analysis indicated that some differentially expressed proteins were associated with apoptosis. KEGG enrichment analysis showed that the apoptosis-related proteins were closely related to the PI3K-Akt signalling pathway. PRM demonstrated that apoptosis-related proteins were significantly decreased in ITP patients, which further confirmed the important effect of apoptosis on ITP pathogenesis. We hypothesised that apoptosis may be closely related to ITP pathogenesis through the PI3K-Akt signalling pathway.

Targeting the phosphoinositide-3-kinase/protein kinase B pathway in airway innate immunity

The airway innate immune system maintains the first line of defense against respiratory infections. The airway epithelium and associated immune cells protect the respiratory system from inhaled foreign organisms. These cells sense pathogens via activation of receptors like toll-like receptors and taste family 2 receptors (T2Rs) and respond by producing antimicrobials, inflammatory cytokines, and chemokines. Coordinated regulation of fluid secretion and ciliary beating facilitates clearance of pathogens via mucociliary transport. Airway cells also secrete antimicrobial peptides and radicals to directly kill microorganisms and inactivate viruses. The phosphoinositide-3-kinase/protein kinase B (Akt) kinase pathway regulates multiple cellular targets that modulate cell survival and proliferation. Akt also regulates proteins involved in innate immune pathways. Akt phosphorylates endothelial nitric oxide synthase (eNOS) enzymes expressed in airway epithelial cells. Activation of eNOS can have anti-inflammatory, anti-bacterial, and anti-viral roles. Moreover, Akt can increase the activity of the transcription factor nuclear factor erythroid 2 related factor-2 that protects cells from oxidative stress and may limit inflammation. In this review, we summarize the recent findings of non-cancerous functions of Akt signaling in airway innate host defense mechanisms, including an overview of several known downstream targets of Akt involved in innate immunity.

PAK Membrane Translocation and Phosphorylation Regulate Platelet Aggregation Downstream of Gi and G12/13 Pathways

Platelet activation plays a pivotal role in physiological hemostasis and pathological thrombosis causing heart attack and stroke. Previous studies conclude that simultaneous activation of Gi and G_12/13 signaling pathways is sufficient to cause platelet aggregation. However, using Gq knockout mice and Gq-specific inhibitors, we here demonstrated that platelet aggregation downstream of coactivation of Gi and G_12/13 depends on agonist concentrations; coactivation of Gi and G_12/13 pathways only induces platelet aggregation under higher agonist concentrations. We confirmed Gi and G_12/13 pathway activation by showing cAMP (cyclic adenosine monophosphate) decrease and RhoA activation in platelets stimulated at both low and high agonist concentrations. Interestingly, we found that though Akt and PAK (p21-activated kinase) translocate to the platelet membrane upon both low and high agonist stimulation, membrane-translocated Akt and PAK only phosphorylate at high agonist concentrations, correlating well with platelet aggregation downstream of concomitant Gi and G_12/13 pathway activation. PAK inhibitor abolishes Akt phosphorylation, inhibits platelet aggregation in vitro and arterial thrombus formation in vivo. We propose that the PAK-PI3K/Akt pathway mediates platelet aggregation downstream of Gi and G_12/13, and PAK may represent a potential antiplatelet and antithrombotic target.

FUNDC2 regulates platelet activation through AKT/GSK-3β/cGMP axis

AIMS

AKT kinase is vital for regulating signal transduction in platelet aggregation. We previously found that mitochondrial protein FUNDC2 mediates phosphoinositide 3-kinase (PI3K)/phosphatidylinositol-3,4,5-trisphosphate (PIP3)-dependent AKT phosphorylation and regulates platelet apoptosis. The aim of this study was to evaluate the role of FUNDC2 in platelet activation and aggregation.

METHODS AND RESULTS

We demonstrated that FUNDC2 deficiency diminished platelet aggregation in response to a variety of agonists, including adenosine 5'-diphosphate (ADP), collagen, ristocetin/VWF, and thrombin. Consistently, in vivo assays of tail bleeding and thrombus formation showed that FUNDC2-knockout mice displayed deficiency in haemostasis and thrombosis. Mechanistically, FUNDC2 deficiency impairs the phosphorylation of AKT and downstream GSK-3β in a PI3K-dependent manner. Moreover, cGMP also plays an important role in FUNDC2/AKT-mediated platelet activation. This FUNDC2/AKT/GSK-3β/cGMP axis also regulates clot retraction of platelet-rich plasma.

CONCLUSION

FUNDC2 positively regulates platelet functions via AKT/GSK-3β/cGMP signalling pathways, which provides new insight for platelet-related diseases.

The effects of ginsenosides on platelet aggregation and vascular intima in the treatment of cardiovascular diseases: From molecular mechanisms to clinical applications

Thrombosis initiated by abnormal platelet aggregation is a pivotal pathological event that precedes most cases of cardiovascular diseases (CVD). Recently, growing evidence indicates that platelet could be a potential target for CVD prevention. However, as the conventional antithrombotic management strategy, applications of current antiplatelet agents are somewhat limited by their various side effects, such as bleeding risk and drug resistance. Hence, efforts have been made to search for agents as complementary therapies. Ginsenoside, the principal active component extracted from Panax ginseng, has gained much attention for its regulations on multiple crucial events of platelet aggregation. From structural characteristics to clinical applications, this review anatomized the intrinsic structure-function relationship of antiplatelet potency of ginsenosides, and the involved signal pathways were specifically summarized. Additionally, the emphasis was placed on clinical studies that investigate the antithrombotic efficacy of ginsenosides in the treatment of CVD. Further, a broad overview of approaches for improving the bioavailability of ginsenosides was concluded. Limitations and prospects of current studies were also discussed. This study may provide some new insights into the systematic understanding of ginsenosides in CVD treatment and lay a foundation for future research.

Short-Term Exposure to Waterpipe/Hookah Smoke Triggers a Hyperactive Platelet Activation State and Increases the Risk of Thrombogenesis

OBJECTIVE

Cardiovascular disease is a major public health problem. Among cardiovascular disease's risk factors, tobacco smoking is considered the single most preventable cause of death, with thrombosis being the main mechanism of cardiovascular disease mortality in smokers. While tobacco smoking has been on the decline, the use of waterpipes/hookah has been rising, mainly due to the perception that they are less harmful than regular cigarettes. Strikingly, there are few studies on the negative effects of waterpipes on the cardiovascular system, and none regarding their direct contribution to thrombus formation. Approach and Results: We used a waterpipe whole-body exposure protocol that mimics real-life human exposure scenarios and investigated its effects, relative to clean air, on platelet function, hemostasis, and thrombogenesis. We found that waterpipe smoke (WPS)-exposed mice exhibited both shortened thrombus occlusion and bleeding times. Further, our results show that platelets from WPS-exposed mice are hyperactive, with enhanced agonist-induced aggregation, dense and α-granule secretion, αIIbβ3 integrin activation, phosphatidylserine expression, and platelet spreading, when compared with clean air-exposed platelets. Finally, at the molecular level, it was found that Akt (protein kinase B) and ERK (extracellular signal-regulated kinases) phosphorylation are enhanced in the WPS and in nicotine-treated platelets.

CONCLUSIONS

Our findings demonstrate that WPS exposure directly modulates hemostasis and increases the risk of thrombosis and that this is mediated, in part, via a state of platelet hyperactivity. The negative health impact of WPS/hookah, therefore, should not be underestimated. Moreover, this study should also help in raising public awareness of the toxic effects of waterpipe/hookah.

Exogenous carbon monoxide suppresses LPS-Induced platelet SNAREs complex assembly and α-granule exocytosis via integrin αIIbβ3-Mediated PKCθ/Munc18a pathway

Activation of coagulation occurs in sepsis and contributes to the development of thrombosis. Platelet α-granule exocytosis plays an important role in septic coagulation abnormalities. The present study aimed to investigate the effects and the underlying mechanisms of exogenous carbon monoxide, carbon monoxide-releasing molecules II (CORM-2)-liberated CO, on suppressing platelet α-granule exocytosis in sepsis. It was shown that CORM-2 weakened α-granule membrane fusion with platelet plasma membrane and attenuated α-granule contents exocytosis in LPS-Induced platelet. Further studies revealed that CORM-2 suppressed the expression of integrin α_IIbβ₃ in platelets stimulated by LPS. This was accompanied by a decrease in production and phosphorylation of PKCθ and Munc18a, SNARE complex assembly and subsequently platelet α-granule exocytosis. Taken together, we suggested that the potential mechanism of suppressive effect of CORM-2 on LPS-induced platelet SNAREs complex assembly and α-Granule Exocytosis might involve integrin α_IIbβ₃-mediated PKCθ/Munc18a pathway activation.

Anti‑platelet activity of mineral‑balanced deep sea water is mediated via the regulation of Akt and ERK pathway crosstalk

Mineral‑balanced deep sea water (MBDSW), an unlimited natural sea source, has been demonstrated to minimize the risk of developing cardiovascular diseases, such as obesity, hypertension, inflammation and hyperlipidemia. This study investigated the effects of MBDSW [magnesium (Mg):calcium (Ca) ratio, 3:1] on platelet activation. MBDSW significantly inhibited the collagen‑ and thrombin‑induced platelet aggregation of human platelets. In collagen‑induced platelets, MBDSW inhibited intracellular calcium mobilization, granule secretion [serotonin, adenosine triphosphate (ATP) and P‑selectin expression] and thromboxane A2 (TXA2) production. Moreover, MBDSW markedly inhibited Akt and extracellular signal‑regulated kinase (ERK) phosphorylation, but not that of c‑Jun N‑terminal kinase (JNK) and p38. Moreover, MBDSW phosphorylated inositol 1,4,5‑triphosphate receptor (IP3R) and vasodilator‑stimulated phosphoprotein (VASP), and it increased the cyclic adenosine monophosphate (cAMP) level in collagen‑induced human platelets. Dipyridamole, a phosphodiesterase (PDE) inhibitor, significantly increased the cAMP level and regulated the Akt, ERK and VASP (Ser157) levels in a manner similar to that of MBDSW. In addition, LY294002, an Akt inhibitor, inhibited the phosphorylation of ERK, and U0126, an ERK inhibitor, inhibited the phosphorylation of Akt. Taken together, the results of the present investigation suggest that the inhibitory effects of MBDSW on platelet aggregation may be associated with the cross‑inhibition of Akt and ERK phosphorylation. These results strongly indicate that MBDSW may have preventive or therapeutic potential for platelet aggregation‑mediated diseases, such as thrombosis, atherosclerosis and myocardial infarction.

Platelet Mechanotransduction

The vasculature is a dynamic environment in which blood platelets constantly survey the endothelium for sites of vessel damage. The formation of a mechanically coherent hemostatic plug to prevent blood loss relies on a coordinated series of ligand-receptor interactions governing the recruitment, activation, and aggregation of platelets. The physical biology of each step is distinct in that the recruitment of platelets depends on the mechanosensing of the platelet receptor glycoprotein Ib for the adhesive protein von Willebrand factor, whereas platelet activation and aggregation are responsive to the mechanical forces sensed at adhesive junctions between platelets and at the platelet-matrix interface. Herein we take a biophysical perspective to discuss the current understanding of platelet mechanotransduction as well as the measurement techniques used to quantify the physical biology of platelets in the context of thrombus formation under flow.

TMPRSS2-ERG activates NO-cGMP signaling in prostate cancer cells

The aberrant activation of the ERG oncogenic pathway due to the TMPRSS2-ERG gene fusion is the major event that contributes to prostate cancer (PCa) development. However, the critical downstream effectors that can be therapeutically targeted remain to be identified. In this study, we have found that the expression of the α1 and β1 subunits of soluble guanylyl cyclase (sGC) was directly and specifically regulated by ERG in vitro and in vivo and was significantly associated with TMPRSS2-ERG fusion in clinical PCa cohorts. sGC is the major mediator of nitric oxide (NO)-cGMP signaling in cells that, upon NO binding, catalyzes the synthesis of cGMP and subsequently activates protein kinase G (PKG). We showed that cGMP synthesis was significantly elevated by ERG in PCa cells, leading to increased PKG activity and cell proliferation. Importantly, we also demonstrated that sGC inhibitor treatment repressed tumor growth in TMPRSS2-ERG-positive PCa xenograft models and can act in synergy with a potent AR antagonist, enzalutamide. This study strongly suggests that targeting NO-cGMP signaling pathways may be a novel therapeutic strategy to treat PCa with TMPRSS2-ERG gene fusion.

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.

Short-Term E-Cigarette Exposure Increases the Risk of Thrombogenesis and Enhances Platelet Function in Mice

Background

Cardiovascular disease is the main cause of death in the United States, with smoking being the primary preventable cause of premature death, and thrombosis being the main mechanism of cardiovascular mortality in smokers. Due to the perception that electronic/e‐cigarettes are “safer/less harmful” than conventional cigarettes, their usage—among a variety of ages—has increased tremendously during the past decade. Notably, there are limited studies regarding the negative effects of e‐cigarettes on the cardiovascular system, which is also the subject of significant debate.

Methods and Results

We employed a passive e‐Vape^TM vapor inhalation system and developed an in vivo whole‐body e‐cigarette mouse exposure protocol that mimics real‐life human exposure scenarios/conditions and investigated the effects of e‐cigarettes and clean air on platelet function and thrombogenesis. Our results show that platelets from e‐cigarette–exposed mice are hyperactive, with enhanced aggregation, dense and α granule secretion, activation of the αIIbβ3 integrin, phosphatidylserine expression, and Akt and ERK activation, when compared with clean air–exposed platelets. E‐cigarette–exposed platelets were also found to be resistant to inhibition by prostacyclin, relative to clean air. Furthermore, the e‐cigarette–exposed mice exhibited a shortened thrombosis occlusion and bleeding times.

Conclusions

Taken together, our data demonstrate for the first time that e‐cigarettes alter physiological hemostasis and increase the risk of thrombogenic events. This is attributable, at least in part, to the hyperactive state of platelets. Thus, the negative health consequences of e‐cigarette exposure should not be underestimated and warrant further investigation.

14-3-3 proteins in platelet biology and glycoprotein Ib-IX signaling

Members of the 14-3-3 family of proteins function as adapters/modulators that recognize phosphoserine/phosphothreonine-based binding motifs in many intracellular proteins and play fundamental roles in signal transduction pathways of eukaryotic cells. In platelets, 14-3-3 plays a wide range of regulatory roles in phosphorylation-dependent signaling pathways, including G-protein signaling, cAMP signaling, agonist-induced phosphatidylserine exposure, and regulation of mitochondrial function. In particular, 14-3-3 interacts with several phosphoserine-dependent binding sites in the major platelet adhesion receptor, the glycoprotein Ib-IX complex (GPIb-IX), regulating its interaction with von Willebrand factor (VWF) and mediating VWF/GPIb-IX-dependent mechanosignal transduction, leading to platelet activation. The interaction of 14-3-3 with GPIb-IX also plays a critical role in enabling the platelet response to low concentrations of thrombin through cooperative signaling mediated by protease-activated receptors and GPIb-IX. The various functions of 14-3-3 in platelets suggest that it is a possible target for the treatment of thrombosis and inflammation.

Panax notoginseng saponins reduce high-risk factors for thrombosis through peroxisome proliferator-activated receptor -γ pathway

The classic Virchow theory suggests that blood stasis, hypercoagulability and endothelial dysfunction are three major factors that cause venous thrombosis (VT). It is a complicated biological process involved multi-factors. Platelet plays a central role and participates in multiple links of this process. Panax notoginseng saponins (PNS), the principal constituents derived from panax notoginseng, has been widely described for its anti-platelet activity. However, its potential mechanism against platelet aggregation has not been clarified. In this present study, we evaluated the anti-platelet effects of PNS on thrombin-induced platelet activation and its possible molecular mechanism of action, and further explored the therapeutic action of PNS on thrombin induced hypercoagulability in rat. Our results showed that PNS treatment inhibited platelet aggregation induced by thrombin, which was accompanied with over-expression of Peroxisome proliferator-activated receptor γ (PPAR-γ) protein, mRNA and upregulation of phosphatidylinositol 3 kinase (PI₃K)/ protein kinase B (Akt)/ endothelial nitric oxide synthase (eNOS) pathway in platelet, and this effect could be reversed by PPAR-γ inhibitor T0070907. In vivo, PNS significantly reversed thrombin-induced hypercoagulable state in rat which was accompanied by PPAR-γ protein and mRNA upregulation in rat lung. In conclusion, these data suggested that PNS could suppress thrombin-induced platelet aggregation in vitro and effectively improve hypercoagulable state in vivo and PNS-induced activation of PPAR-γ and its downstream PI₃K/Akt/eNOS pathway played the central role.

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.

Class III PI3K Positively Regulates Platelet Activation and Thrombosis via PI(3)P-Directed Function of NADPH Oxidase

OBJECTIVE

Class III phosphoinositide 3-kinase, also known as VPS34 (vacuolar protein sorting 34), is a highly conserved enzyme regulating important cellular functions such as NADPH oxidase (NOX) assembly, membrane trafficking, and autophagy. Although VPS34 is expressed in platelets, its involvement in platelet activation remains unclear. Herein, we investigated the role of VPS34 in platelet activation and thrombus formation using VPS34 knockout mice.

APPROACH AND RESULTS

Platelet-specific VPS34-deficient mice were generated and characterized. VPS34 deficiency in platelets did not influence tail bleeding time. In a ferric chloride-induced mesenteric arteriolar thrombosis model, VPS34^-/- mice exhibited a prolonged vessel occlusion time compared with wild-type mice (42.05±4.09 versus 18.30±2.47 minutes). In an in vitro microfluidic whole-blood perfusion assay, thrombus formation on collagen under arterial shear was significantly reduced for VPS34^-/- platelets. VPS34^-/- platelets displayed an impaired aggregation and dense granule secretion in response to low doses of collagen or thrombin. VPS34 deficiency delayed clot retraction but did not influence platelet spreading on fibrinogen. We also demonstrated that VPS34 deficiency altered the basal level of autophagy in resting platelets and hampered NOX assembly and mTOR (mammalian target of rapamycin) signaling during platelet activation. Importantly, we identified the NOX-dependent reactive oxygen species generation as the major downstream effector of VPS34, which in turn can mediate platelet activation. In addition, by using a specific inhibitor 3-methyladenine, VPS34 was found to operate through a similar NOX-dependent mechanism to promote human platelet activation.

CONCLUSIONS

Platelet VPS34 is critical for thrombosis but dispensable for hemostasis. VPS34 regulates platelet activation by influencing NOX assembly.

Syk Activity Is Dispensable for Platelet GP1b-IX-V Signaling

The binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein 1b-IX (GP1b-IX) leads to activation of platelets. GP1b was shown to signal via the FcRγ-ITAM (Fc Receptor γ-Immunoreceptor tyrosine-based activation motif) pathway, activating spleen tyrosine kinase (Syk) and other tyrosine kinases. However, there have been conflicting reports regarding the role of Syk in GP1b signaling. In this study, we sought to resolve these conflicting reports and clarify the role of Syk in VWF-induced platelet activation. The inhibition of Syk with the selective Syk inhibitors, OXSI-2 and PRT-060318, did not inhibit VWF-induced platelet adhesion, agglutination, aggregation, or secretion. In contrast, platelets stimulated with the Glycoprotein VI (GPVI) agonist, collagen-related peptide (CRP), failed to cause any aggregation or secretion in presence of the Syk inhibitors. Furthermore, GP1b-induced platelet signaling was unaffected in the presence of Syk inhibitors, but GPVI-induced signaling was abolished under similar conditions. Thus, we conclude that Syk kinase activity does not play any functional role downstream of GP1b-mediated platelet activation.

Resveratrol suppresses pulmonary tumor metastasis by inhibiting platelet-mediated angiogenic responses

BACKGROUND

To explore the impact of Resveratrol (RSV) on the angiogenic potential of activated platelets and to elucidate the underlying mechanism.

METHODS

Vascular endothelial growth factor concentrations were measured by enzyme-linked immunosorbent assay. Capillary tube formation assay was used to examine the impact of RSV on the angiogenic potential of activated platelets. The levels of cyclic adenosine monophosphate and cyclic guanosine monophosphate (cGMP) in the supernatant were evaluated using corresponding enzyme-linked immunosorbent assay kits. Immunoblotting assays were used to determine the expression of vasodilator-stimulated phosphoprotein and Akt phosphorylation. A pulmonary metastasis experiment with male nude mice model was performed to test the effect of RSV on pulmonary metastasis and angiogenesis in vivo.

RESULTS

RSV inhibited platelets-mediated angiogenic responses induced by adenosine diphosphate (ADP)ADP through increased cGMP generation and cGMP-mediated vasodilator-stimulated phosphoprotein phosphorylation along with reduced intracellular Ca²⁺ mobilization. In addition, RSV attenuated the platelet secretion and angiogenic responses induced by A549 cells in vitro and suppressed A549 lung cancer metastasis and angiogenesis in nude mice.

CONCLUSIONS

RSV is a potential therapeutic drug for the prevention of tumor metastasis by interrupting the platelet-tumor cell amplification loop.

Receptor-interacting protein kinase 3 promotes platelet activation and thrombosis

Previous studies have shown that receptor-interacting protein kinase 3 (RIP3) is involved in many important biological processes, including necroptosis, apoptosis, and inflammation. Here we show that RIP3 plays a critical role in regulating platelet functions and in vivo thrombosis and hemostasis. Tail bleeding times were significantly longer in RIP3-knockout (RIP3^-/-) mice compared with their wild-type (WT) littermates. In an in vivo model of arteriole thrombosis, mice lacking RIP3 exhibited prolonged occlusion times. WT mice repopulated with RIP3^-/- bone marrow-derived cells had longer occlusion times than RIP3^-/- mice repopulated with WT bone marrow-derived cells, suggesting a role for RIP3-deficient platelets in arterial thrombosis. Consistent with these findings, we observed that RIP3 was expressed in both human and mice platelets. Deletion of RIP3 in mouse platelets caused a marked defect in aggregation and attenuated dense granule secretion in response to low doses of thrombin or a thromboxane A₂ analog, U46619. Phosphorylation of Akt induced by U46619 or thrombin was diminished in RIP3^-/- platelets. Moreover, RIP3 interacted with Gα₁₃ Platelet spreading on fibrinogen and clot retraction were impaired in the absence of RIP3. RIP3 inhibitor dose-dependently inhibited platelet aggregation in vitro and prevented arterial thrombus formation in vivo. These data demonstrate a role for RIP3 in promoting in vivo thrombosis and hemostasis by amplifying platelet activation. RIP3 may represent a novel promising therapeutic target for thrombotic diseases.

Ambient fine particles (PM2.5 ) attenuate collagen-induced platelet activation through interference of the PLCγ2/Akt/GSK3β signaling pathway

AIMS

It has been proven that carbon nanoparticles or diesel exhaust particles stimulate platelet activation. However, the effect of fine particle matter (PM_2.5 ) on platelet activation remains unknown, which motivates this study.

METHODS

PM_2.5 samples were collected in an urban area of Zhengzhou, China. To study the morphological characteristics and the mass concentrations of trace elements of PM_2.5 samples, a filed-emission scanning electron microscope, the Image-J software, and an inductively coupled plasma mass spectrometry were used. Washed human platelets or platelet-rich-plasma were used to study the effect of PM_2.5 on platelet aggregation, P-selectin expression, or platelet signaling pathways. The cytotoxicity in platelets exposed to PM_2.5 was evaluated by a lactate dehydrogenase assay kit. In addition, platelet adhesion and spreading were studied on collagen-coated surfaces in stable conditions.

RESULTS

The filed-emission scanning electron microscope scanning showed that PM_2.5 samples varied in shape and size distributions. The mean equivalent spherical diameter of these particles was 1.97 ± 0.04 μm, of which 82.40% were particles with equivalent spherical diameters of less than 2.5 μm. The mass concentration of Ca was higher than that of other elements. The other elements followed the trend of Al>Fe>Zn>Mg>Pb>K>Mn>Cu>Ti>Ba>As>Sr>Sn>Sb>Cd>B>Se>Mo>Ag>Ni>TI>V>Co. Furthermore, pretreatment of PM_2.5 significantly inhibited rather than potentiated collagen-induced platelet aggregation and P-selectin expression, whereas it had no significant effect on ADP-induced platelet aggregation and P-selectin expression. The lactate dehydrogenase analysis showed trivial cytotoxic effect of PM_2.5 exposure on platelets. Pretreatment of PM_2.5 inhibited platelet adhesion on immobilized collagen-coated surfaces; however, it almost did not impact the platelet spreading. Immunoblotting analysis indicated that PM_2.5 reduced collagen-induced phosphorylation of phospholipase C gamma-2 (PLCγ2) at Tyr759, Akt at Ser473, and glycogen synthase kinase 3β (GSK3β) at Ser9.

CONCLUSIONS

PM_2.5 attenuated collagen-induced platelet aggregation, α-granule secretion and adhesion, with the potential mechanism of impairing PLCγ2, Akt, and GSK3β signaling. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 530-540, 2017.

Atractylodes lactone compounds inhibit platelet activation

Platelets play a crucial role in the development and progression of atherosclerosis-thrombosis and, therefore, antiplatelet drugs are widely used in the treatment of coronary artery disease. Moreover, advances in understanding the biological functions of natural plant products can provide new pharmacological strategies aimed at promoting cardiovascular health. Atractylenolide I (ATL-1), ATL-2, and ATL-3 are the major bioactive components of a Qi tonifying medicinal herb Rhizoma Atractylodis Macrocephalae (Atractylodes macrocephala), which is commonly used in traditional Chinese medicine (TCM). These components possess well-documented anti-inflammatory and anticancer activities, but their effects on platelet activation are still unknown. In this study, the effects of ATL on platelet function in vitro and in vivo were investigated, and the underlying mechanism was explored. We found that ATL-2 and ATL-3 but not ATL-1 diminished agonist-induced platelet aggregation and diminished adenosine triphosphate (ATP) release from dense granules. The levels of phospho-Akt (Ser473) and phospho-p38 MAPK were downregulated in the presence of ATL-2 and ATL-3. We also found that ATL-2 and ATL-3 have a similar inhibitory effect on platelet activation as acetylsalicylic acid in response to agonists. Furthermore, ATL-2 and ATL-3 diminished the spreading of human platelets on immobilized fibrinogen (Fg), delayed clot retraction in platelet-depleted plasma containing human platelets, extended first occlusion time in a mouse model of ferric chloride (FeCl₃)-induced carotid arterial thrombosis, and prolonged the bleeding time. These observations suggest that ATL-2 and ATL-3 are potential candidate therapeutic drugs for the prevention of thrombosis.

Leukaemia-associated Rho guanine nucleotide exchange factor (LARG) plays an agonist specific role in platelet function through RhoA activation

Leukemia-Associated RhoGEF (LARG) is highly expressed in platelets, which are essential for maintaining normal haemostasis. We studied the function of LARG in murine and human megakaryocytes and platelets with Larg knockout (KO), shRNA-mediated knockdown and small molecule-mediated inhibition. We found that LARG is important for human, but not murine, megakaryocyte maturation. Larg KO mice exhibit macrothrombocytopenia, internal bleeding in the ovaries and prolonged bleeding times. KO platelets have impaired aggregation, α-granule release and integrin α2bβ3 activation in response to thrombin and thromboxane, but not to ADP. The same agonist-specific reductions in platelet aggregation occur in human platelets treated with a LARG inhibitor. Larg KO platelets have reduced RhoA activation and myosin light chain phosphorylation, suggesting that Larg plays an agonist-specific role in platelet signal transduction. Using two different in vivo assays, Larg KO mice are protected from in vivo thrombus formation. Together, these results establish that LARG regulates human megakaryocyte maturation, and is critical for platelet function in both humans and mice.

Increased prostacyclin levels inhibit the aggregation and activation of platelets via the PI3K-AKT pathway in prolonged isolated thrombocytopenia after allogeneic hematopoietic stem cell transplantation

OBJECTIVES

The aim of this study was to investigate the role of prostacyclin (PGI2) in prolonged isolated thrombocytopenia (PT) following allogeneic hematopoietic stem cell transplantation (allo-HSCT) and the effect of PGI2 on the activation and aggregation of platelets in PT.

METHODS

We enrolled 37 patients with PT and 36 controls following allo-HSCT in this study. Platelet aggregation and activation and PGI2 levels were measured. Endothelial progenitor cells (EPCs) from either PT or control patients were cultured ex vivo with serum from either PT or control patients. PGI2 secretions were then measured. PGI2 was added to the platelets ex vivo, and platelet aggregation and activation and PI3K/Akt phosphorylation were analyzed.

RESULTS

A higher PGI2 level was observed in the PT patients. The activation and aggregation of platelets were significantly lower in the PT patients. EPCs from PT patients cultured in PT serum secreted higher levels of PGI2, and PGI2 inhibited platelet activation and aggregation in a concentration-dependent manner ex vivo. PI3K/Akt phosphorylation of platelets was regulated by PGI2 after allo-HSCT. Disease status, serum PGI2 level and platelet aggregation were independent risk factors in patients with PT after allo-HSCT.

CONCLUSIONS

Higher PGI2 levels and lower platelet activation and aggregation occurred simultaneously in PT patients. PGI2 inhibited platelet activation and aggregation, probably by regulating the phosphorylation of PI3K/Akt.

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.

Inhibitory effect of caffeic acid on ADP-induced thrombus formation and platelet activation involves mitogen-activated protein kinases

Caffeic acid (CA), one of the active constituents of Radix Salvia miltiorrhizae, exhibits antioxidant and anti-inflammatory activities. However, few studies have assessed the ability of CA to inhibit platelet mediated thrombus generation in vivo. In this study, we investigated the antithrombotic effect of CA in mouse cerebral arterioles and venules using intravital microscopy. The antiplatelet activity of CA in ADP stimulated mouse platelets in vitro was also examined in attempt to explore the underlying mechanism. Our results demonstrated that CA (1.25–5 mg/kg) significantly inhibited thrombus formation in vivo. In vitro, CA (25–100 μM) inhibited ADP-induced platelet aggregation, P-selectin expression, ATP release, Ca²⁺ mobilization, and integrin αIIbβ3 activation. Additionally, CA attenuated p38, ERK, and JNK activation, and enhanced cAMP levels. Taken together, these data provide evidence for the inhibition of CA on platelet-mediated thrombosis in vivo, which is, at least partly, mediated by interference in phosphorylation of ERK, p38, and JNK leading to elevation of cAMP and down-regulation of P-selectin expression and αIIbβ3 activation. These results suggest that CA may have potential for the treatment of aberrant platelet activation-related diseases.

A tryptophan derivative TD-26 attenuates thrombus formation by inhibiting both PI3K/Akt signaling and binding of fibrinogen to integrin αIIbβ3

The incidence and mortality of thrombotic disorders are rapidly increasing worldwide. The existing antithrombotic drugs, however, are associated with side effects, especially bleeding complications. Therefore, there remains a need for the development of more effective and safer antithrombotic agents. In this study, we discovered a new synthetic tryptophan derivative TD-26, producing potent inhibitory effect on platelet aggregation while without causing obvious bleeding risk. It has been shown that TD-26 inhibited platelet aggregation induced by ADP, thrombin, U46619 and collagen in vitro and suppressed the platelet aggregation induced by ADP ex vivo. Mechanism studies indicated that TD-26 inhibited platelet adhesion to fibrinogen-coated surfaces, blocked the binding of fibrinogen to integrin αIIbβ3 and reduced Akt(Ser473) phosphorylation in platelet phosphatidylinositol 3-kinase (PI3K) signaling. Furthermore, TD-26 exhibited potent antithrombotic activity in vivo. In animal models, it decreased death of mice with acute pulmonary thrombosis by 90% and attenuated thrombosis weight by 60.3%, both at a dose of 3 mg/kg. Additionally, TD-26 did not obviously prolong bleeding time in mice. Taken together, our results reveal that TD-26 is a novel antithrombotic compound exhibiting both integrin αIIbβ3 inhibition and PI3K signaling blockage, with a low bleeding risk.

Nucleotide-binding oligomerization domain 2 receptor is expressed in platelets and enhances platelet activation and thrombosis

BACKGROUND

Pattern recognition receptor nucleotide-binding oligomerization domain 2 (NOD2) is well investigated in immunity, but its expression and function in platelets has never been explored.

METHOD AND RESULTS

Using reverse transcription polymerase chain reaction and Western blot, we show that both human and mouse platelets express NOD2, and its agonist muramyl dipeptide induced NOD2 activation as evidenced by receptor dimerization. NOD2 activation potentiates platelet aggregation and secretion induced by low concentrations of thrombin or collagen, and clot retraction, as well. These potentiating effects of muramyl dipeptide were not seen in platelets from NOD2-deficient mice. Plasma from septic patients also potentiates platelet aggregation induced by thrombin or collagen NOD2 dependently. Using intravital microscopy, we found that muramyl dipeptide administration accelerated in vivo thrombosis in a FeCl3-injured mesenteric arteriole thrombosis mouse model. Platelet depletion and transfusion experiments confirmed that NOD2 from platelets contributes to the in vivo thrombosis in mice. NOD2 activation also accelerates platelet-dependent hemostasis. We further found that platelets express receptor-interacting protein 2, and provided evidence suggesting that mitogen activated-protein kinase and nitric oxide/soluble guanylyl cyclase/cGMP/protein kinase G pathways downstream of receptor-interacting protein mediate the role of NOD2 in platelets. Finally, muramyl dipeptide stimulates proinflammatory cytokine interleukin-1β maturation and accumulation in human and mouse platelets NOD2 dependently.

CONCLUSIONS

NOD2 is expressed in platelets and functions in platelet activation and arterial thrombosis, possibly during infection. To our knowledge, this is the first study on NOD-like receptors in platelets that link thrombotic events to inflammation.

Impact of the PI3-kinase/Akt pathway on ITAM and hemITAM receptors: haemostasis, platelet activation and antithrombotic therapy

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that are activated in response to various stimulants, and they regulate many processes including inflammation; the stress response; gene transcription; and cell proliferation, differentiation, and death. Increasing reports have shown that the PI3Ks and their downstream effector Akt are activated by several platelet receptors that regulate platelet activation and haemostasis. Platelets express two immunoreceptor tyrosine based activation motif (ITAM) receptors, collagen receptor glycoprotein VI (GPVI) and Fcγ receptor IIA (FcγRIIA), which are characterized by two YxxL sequences separated by 6-12 amino acids. Activation of an ITAM receptor initiates a reaction cascade via its YxxL sequence in which signaling molecules such as spleen tyrosine kinase (Syk), linker for activation of T cells (LAT) and phospholipase C γ2 (PLCγ2) become activated, leading to platelet activation. Platelets also express another receptor, C-type lectin 2 (CLEC-2), which has a single YxxL sequence, so it is appropriately called a hemITAM receptor. ITAM receptors and the hemITAM receptor share many signaling features. Here we will summarize our current knowledge about how the PI3K/Akt pathway regulates (hem)ITAM receptor-mediated platelet activation and haemostasis and discuss the possible benefits of targeting PI3K/Akt as an antithrombotic therapy.

Role of inorganic nitrate and nitrite in driving nitric oxide-cGMP-mediated inhibition of platelet aggregation in vitro and in vivo

BACKGROUND

Nitric oxide (NO) is a critical negative regulator of platelets that is implicated in the pathology of thrombotic diseases. Platelets generate NO, but the presence and functional significance of NO synthase (NOS) in platelets is unclear. Inorganic nitrate/nitrite is increasingly being recognized as a source of bioactive NO, although its role in modulating platelets during health and vascular dysfunction is incompletely understood.

METHODS

We investigated the functional significance and upstream sources of NO-cGMP signaling events in platelets by using established methods for assessing in vitro and in vivo platelet aggregation, and assessed the bioconversion of inorganic nitrate to nitrite during deficiency of endothelial NOS (eNOS).

RESULTS

The phosphodiesterase 5 (PDE5) inhibitor sildenafil inhibited human platelet aggregation in vitro. This inhibitory effect was abolished by a guanylyl cyclase inhibitor and NO scavengers, but unaffected by NOS inhibition. Inorganic nitrite drove cGMP-mediated inhibition of human platelet aggregation in vitro and nitrate inhibited platelet function in eNOS(-/-) mice in vivo in a model of thromboembolic radiolabeled platelet aggregation associated with an enhanced plasma nitrite concentration as compared with wild-type mice.

CONCLUSIONS

Platelets generate transient, endogenous cGMP signals downstream of NO that are primarily independent of NOS and may be enhanced by inhibition of PDE5. Furthermore, nitrite can generate transient NO-cGMP signals in platelets. The absence of eNOS leads to enhanced plasma nitrite levels following nitrate administration in vivo, which negatively impacts on platelet function. Our data suggest that inorganic nitrate exerts an antiplatelet effect during eNOS deficiency, and, potentially, that dietary nitrate may reduce platelet hyperactivity during endothelial dysfunction.

Of von Willebrand factor and platelets

Hemostasis and pathological thrombus formation are dynamic processes that require multiple adhesive receptor-ligand interactions, with blood platelets at the heart of such events. Many studies have contributed to shed light on the importance of von Willebrand factor (VWF) interaction with its platelet receptors, glycoprotein (GP) Ib-IX-V and αIIbβ3 integrin, in promoting primary platelet adhesion and aggregation following vessel injury. This review will recapitulate our current knowledge on the subject from the rheological aspect to the spatio-temporal development of thrombus formation. We will also discuss the signaling events generated by VWF/GPIb-IX-V interaction, leading to platelet activation. Additionally, we will review the growing body of evidence gathered from the recent development of pathological mouse models suggesting that VWF binding to GPIb-IX-V is a promising target in arterial and venous pathological thrombosis. Finally, the pathological aspects of VWF and its impact on platelets will be addressed.

Baicalein inhibits agonist- and tumor cell-induced platelet aggregation while suppressing pulmonary tumor metastasis via cAMP-mediated VASP phosphorylation along with impaired MAPKs and PI3K-Akt activation

Recently, the importance of platelet activation in cancer metastasis has become generally accepted. As a result, the development of new platelet inhibitors with minimal adverse effects is now a promising area of targeted cancer therapy. Baicalein is a functional ingredient derived from the root of Scutellaria baicalensis Georgi, a plant used intraditional medicine. The pharmacological effects of this compound including anti-oxidative and anti-inflammatory activities have already been demonstrated. However, its effects on platelet activation are unknown. We therefore investigated the effects of baicalein on ligand-induced platelet aggregation and pulmonary cancer metastasis. In the present study, baicalein inhibited agonist-induced platelet aggregation, granule secretion markers (P-selectin expression and ATP release), [Ca(2+)]i mobilization, and integrin αIIbβ3 expression. Additionally, baicalein attenuated ERK2, p38, and Akt activation, and enhanced VASP phosphorylation. Indeed, baicalein was shown to directly inhibit PI3K kinase activity. Moreover, baicalein attenuated the platelet aggregation induced by C6 rat glioma tumor cells in vitro and suppressed CT26 colon cancer metastasis in mice. These features indicate that baicalein is a potential therapeutic drug for the prevention of cancer metastasis.

Plant food anthocyanins inhibit platelet granule secretion in hypercholesterolaemia: Involving the signalling pathway of PI3K-Akt

Controlling platelet granule secretion has been considered an effective strategy to dampen thrombosis and prevent atherosclerosis. Anthocyanins are natural plant pigments and possess a wide range of biological activities, including cardiovascular protective activity. In the present study we explored the effects and the potential mechanisms of anthocyanins on platelet granule secretion in hypercholesterolemia. In a randomised, double-blind clinical trial, 150 hypercholesterolaemic individuals were treated with purified anthocyanins (320 mg/day) or placebo for 24 weeks. Anthocyanins consumption significantly reduced plasma levels of β-thromboglobulin (β-TG), soluble P-selectin, and of Regulated on Activation Normal T cell Expressed and Secreted (RANTES) as compared with the placebo. A minor reduction in platelet factor 4 (PF4) and transforming growth factor β1 (TGF-β1) levels were also observed. In in vitro experiments, we observed that puriӿed anthocyanin mixture, as well as its two main anthocyanin components, delphinidin-3-glucoside (Dp-3-g) and cyanidin-3-glucoside (Cy-3g) directly inhibited platelet á-granule, dense granule, and lysosome secretion evaluated by P-selectin, RANTES, β-TG, PF4, TGF-β1, serotonin, ATP, and CD63 release. Further, anthocyanins inhibited platelet PI3K/Akt activation and consequently attenuated eNOS phosphorylation and cGMP production, thus interrupting MAPK activation. LY294002, a PI3K inhibitor, did not cause additional inhibitory efficacy, indicating that anthocyanin-induced effects may be involved in inhibition of the PI3K/Akt signalling pathway. These results provide evidence that by inhibiting platelet granule secretion, anthocyanins may be a potent cardioprotective agent.

Agonist-induced platelet procoagulant activity requires shear and a Rac1-dependent signaling mechanism

Activated platelets facilitate blood coagulation by exposing phosphatidylserine (PS) and releasing microvesicles (MVs). However, the potent physiological agonists thrombin and collagen poorly induce PS exposure when a single agonist is used. To obtain a greater procoagulant response, thrombin is commonly used in combination with glycoprotein VI agonists. However, even under these conditions, only a percentage of platelets express procoagulant activity. To date, it remains unclear why platelets poorly expose PS even when stimulated with multiple agonists and what the signaling pathways are of soluble agonist-induced platelet procoagulant activity. Here we show that physiological levels of shear present in blood significantly enhance agonist-induced platelet PS exposure and MV release, enabling low doses of a single agonist to induce full-scale platelet procoagulant activity. PS exposed on the platelet surface was immediately released as MVs, revealing a tight coupling between the 2 processes under shear. Using platelet-specific Rac1(-/-) mice, we discovered that Rac1 plays a common role in mediating the low-dose agonist-induced procoagulant response independent of platelet aggregation, secretion, and the apoptosis pathway. Platelet-specific Rac1 function was not only important for coagulation in vitro but also for fibrin accumulation in vivo following laser-induced arteriolar injury.

Apolipoprotein B100 danger-associated signal 1 (ApoBDS-1) triggers platelet activation and boosts platelet-leukocyte proinflammatory responses

Low-density lipoproteins (LDL), occurring in vivo in both their native and oxidative form, modulate platelet function and thereby contribute to atherothrombosis. We recently identified and demonstrated that 'ApoB100 danger-associated signal 1' (ApoBDS-1), a native peptide derived from Apolipoprotein B-100 (ApoB100) of LDL, induces inflammatory responses in innate immune cells. Platelets are critically involved in the development as well as in the lethal consequences of atherothrombotic diseases, but whether ApoBDS-1 has also an impact on platelet function is unknown. In this study we examined the effect of ApoBDS-1 on human platelet function and platelet-leukocyte interactions in vitro. Stimulation with ApoBDS-1 induced platelet activation, degranulation, adhesion and release of proinflammatory cytokines. ApoBDS-1-stimulated platelets triggered innate immune responses by augmenting leukocyte activation, adhesion and transmigration to/through activated HUVEC monolayers, under flow conditions. These platelet-activating effects were sequence-specific, and stimulation of platelets with ApoBDS-1 activated intracellular signalling pathways, including Ca2+, PI3K/Akt, PLC, and p38- and ERK-MAPK. Moreover, our data indicates that ApoBDS-1-induced platelet activation is partially dependent of positive feedback from ADP on P2Y1 and P2Y12, and TxA2. In conclusion, we demonstrate that ApoBDS-1 is an effective platelet agonist, boosting platelet-leukocyte's proinflammatory responses, and potentially contributing to the multifaceted inflammatory-promoting effects of LDL in the pathogenesis of atherothrombosis.

Akt signaling in platelets and thrombosis

Akt is a Ser-Thr kinase with pleiotropic effects on cell survival, growth and metabolism. Recent evidence from gene-deletion studies in mice, and analysis of human platelets treated with Akt inhibitors, suggest that Akt regulates platelet activation, with potential consequences for thrombosis. Akt activation is regulated by the level of phosphoinositide 3-phosphates, and proteins that regulate concentrations of this lipid also regulate Akt activation and platelet function. Although the effectors through which Akt contributes to platelet activation are not definitively known, several candidates are discussed, including endothelial nitric oxide synthase, glycogen synthase kinase 3β, phosphodiesterase 3A and the integrin β(3) tail. Selective inhibitors of Akt isoforms or of proteins that contribute to its activation, such as individual PI3K isoforms, may make attractive targets for antithrombotic therapy. This review summarizes the current literature describing Akt activity and its regulation in platelets, including speculation regarding the future of Akt or its regulatory pathways as targets for the development of antithrombotic therapies.

Antiplatelet effects of dietary nitrate in healthy volunteers: involvement of cGMP and influence of sex

Ingestion of vegetables rich in inorganic nitrate has emerged as an effective method, via the formation of a nitrite intermediate, for acutely elevating vascular NO levels. As such a number of beneficial effects of dietary nitrate ingestion have been demonstrated including the suggestion that platelet reactivity is reduced. In this study we investigated whether inorganic nitrate supplementation might also reduce platelet reactivity in healthy volunteers and have determined the mechanisms involved in the effects seen. We conducted two randomised crossover studies each in 24 (12 of each sex) healthy subjects assessing the acute effects of dietary nitrate (250 ml beetroot juice) or potassium nitrate capsules (KNO3, 8 mmol) vs placebo control on platelet reactivity. Inorganic nitrate ingested either from a dietary source or via supplementation raised circulating nitrate and nitrite levels in both sexes and attenuated ex vivo platelet aggregation responses to ADP and, albeit to a lesser extent, collagen but not epinephrine in male but not female volunteers. These inhibitory effects were associated with a reduced platelet P-selectin expression and elevated platelet cGMP levels. In addition, we show that nitrite reduction to NO occurs at the level of the erythrocyte and not the platelet. In summary, our results demonstrate that inorganic nitrate ingestion, whether via the diet or through supplementation, causes a modest decrease in platelet reactivity in healthy males but not females. Our studies provide strong support for further clinical trials investigating the potential of dietary nitrate as an adjunct to current antiplatelet therapies to prevent atherothrombotic complications. Moreover, our observations highlight a previously unknown sexual dimorphism in platelet reactivity to NO and intimate a greater dependence of males on the NO-soluble guanylate cyclase pathway in limiting thrombotic potential.

2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (THSG) attenuates human platelet aggregation, secretion and spreading in vitro

INTRODUCTION

2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside(THSG) is a water-soluble component of the rhizome extract from the traditional Chinese herb Polygonum multiflorum. Recent studies have demonstrated that THSG has potent anti-oxidative and anti-inflammatory effects. In this study, we investigated the anti-platelet aggregation, secretion and spreading of THSG with different methods. The purpose was to explore the anti-platelet effect of THSG and the underlying mechanism.

MATERIALS AND METHODS

We investigated the anti-platelet activity of THSG on platelet aggregation induced by collagen (2 μg/mL), thrombin(0.04U/mL), U46619 (3 μM) and ADP (2 μM). ATP secretion induced by collagen (2 μg/mL) was also investigated. P-selectin expression and PAC-1 binding were measured by flow cytometry. In addition, human platelet spreading on immobilized fibrinogen and immunoblotting were also tested.

RESULTS

THSG dose-dependently inhibited platelet aggregation and ATP secretion induced by collagen. It inhibited platelet P-selectin expression and PAC-1 binding induced by thrombin(0.1U/mL). THSG also inhibited human platelet spreading on immobilized fibrinogen, a process mediated by platelet outside-in signaling. Western blot analysis showed that THSG could inhibit platelet Fc γ RIIa, Akt(Ser473)and GSK3β(Ser9) phosphorylation.

CONCLUSIONS

Our study indicates that THSG has potent anti-platelet activity to collagen induced aggregation. THSG is likely to exert protective effects in platelet-associated thromboembolic disorders by modulating human platelet.

Chlorin e6 Prevents ADP-Induced Platelet Aggregation by Decreasing PI3K-Akt Phosphorylation and Promoting cAMP Production

A number of reagents that prevent thrombosis have been developed but were found to have serious side effects. Therefore, we sought to identify complementary and alternative medicinal materials that are safe and have long-term efficacy. In the present studies, we have assessed the ability of chlorine e6 (CE6) to inhibit ADP-induced aggregation of rat platelets and elucidated the underlying mechanism. CE6 inhibited platelet aggregation induced by 10 µM ADP in a concentration-dependent manner and decreased intracellular calcium mobilization and granule secretion (i.e., ATP and serotonin release). Western blotting revealed that CE6 strongly inhibited the phosphorylations of PI3K, Akt, c-Jun N-terminal kinase (JNK), and different mitogen-activated protein kinases (MAPKs) including extracellular signal-regulated kinase 1/2 (ERK1/2) as well as p38-MAPK. Our study also demonstrated that CE6 significantly elevated intracellular cAMP levels and decreased thromboxane A₂ formation in a concentration-dependent manner. Furthermore, we determined that CE6 initiated the activation of PKA, an effector of cAMP. Taken together, our findings indicate that CE6 may inhibit ADP-induced platelet activation by elevating cAMP levels and suppressing PI3K/Akt activity. Finally, these results suggest that CE6 could be developed as therapeutic agent that helps prevent thrombosis and ischemia.

p-Terphenyl curtisian E inhibits in vitro platelet aggregation via cAMP elevation and VASP phosphorylation

Mushrooms possess untapped source of enormous natural compounds showing anti-inflammatory, antioxidant and anti-platelet activities. Paxillus curtisii, wild mushroom, is a rich source of curtisian E (CE) reported for neuroprotective effects; however, its anti-platelet effect was unknown. Here, therefore, we investigated the anti-platelet activity of CE in rat platelets. Curtisian E (12.5-200μM) attenuated collagen (2.5μg/ml), thrombin (0.1U/ml) and ADP (10μM) induced platelet aggregation in vitro. Likewise, CE diminished intracellular calcium and adenosine triphosphate (ATP) release in collagen activated platelets. Fibrinogen binding and fibronectin adhesion to platelets were also inhibited. While CE downregulated c-jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), p38, and Akt dose dependently in collagen stimulated platelets, it upregulated intraplatelet cyclic adenosine monophosphate (cAMP) and vasodilator-stimulated-phosphoprotein (VASP) phosphorylation. Protein kinase A inhibitor (H-89) markedly inhibited p-VASP(157) protein expression, suggesting that cAMP-PKA-VASP(157) pathway may mediate its anti-platelet effect and thus CE could be considered as a potential anti-thrombotic agent.

PDK1 regulates platelet activation and arterial thrombosis

The effects of phosphoinositide-dependent protein kinase 1 (PDK1), a master kinase in the phosphoinositide 3-kinase/Akt pathway, on platelet activation are unknown. Accordingly, platelet-specific PDK1-deficient mice were characterized to elucidate the platelet-related function(s) of PDK1. We found that PDK1 deficiency caused mild thrombocytopenia. The aggregation of PDK1(-/-) platelets was diminished in response to low levels of thrombin, U46619, and adenosine 5'-diphosphate. Further results demonstrated that PDK1 regulates thrombin-induced platelet activation by affecting αIIbβ3-mediated outside-in signaling. This result provided an explanation for the diminished spreading of PDK1(-/-) platelets on immobilized fibrinogen (Fg) and the decreased rate of clot retraction in platelet-rich plasma (PRP) containing PDK1(-/-) platelets. PDK1 deficiency diminished agonist-induced Akt Ser473 phosphorylation and thoroughly abolished Akt Thr308 and Gsk3β Ser9 phosphorylation in response to agonist treatment and platelet spreading, respectively. A Gsk3β inhibitor fully restored the aggregation of PDK1(-/-) platelets in response to low levels of thrombin, normal spreading of PDK1(-/-) platelets on Fg, and normal clot retraction in PRP containing PDK1(-/-) platelets. Those results indicated that Gsk3β is one of the major downstream effectors of PDK1 in thrombin-induced platelet activation and αIIbβ3-mediated outside-in signaling. In addition, in vivo data demonstrated that PDK1 is an important regulator in arterial thrombosis formation.

Dual regulation of glycogen synthase kinase 3 (GSK3)α/β by protein kinase C (PKC)α and Akt promotes thrombin-mediated integrin αIIbβ3 activation and granule secretion in platelets

Glycogen synthase kinase-3 is a Ser/Thr kinase, tonically active in resting cells but inhibited by phosphorylation of an N-terminal Ser residue (Ser²¹ in GSK3α and Ser⁹ in GSK3β) in response to varied external stimuli. Recent work suggests that GSK3 functions as a negative regulator of platelet function, but how GSK3 is regulated in platelets has not been examined in detail. Here, we show that early thrombin-mediated GSK3 phosphorylation (0–30 s) was blocked by PKC inhibitors and largely absent in platelets from PKCα knock-out mice. In contrast, late (2–5 min) GSK3 phosphorylation was dependent on the PI3K/Akt pathway. Similarly, early thrombin-mediated inhibition of GSK3 activity was blocked in PKCα knock-out platelets, whereas the Akt inhibitor MK2206 reduced late thrombin-mediated GSK3 inhibition and largely prevented GSK3 inhibition in PKCα knock-out platelets. More importantly, GSK3 phosphorylation contributes to platelet function as knock-in mice where GSK3α Ser²¹ and GSK3β Ser⁹ were mutated to Ala showed a significant reduction in PAR4-mediated platelet aggregation, fibrinogen binding, and P-selectin expression, whereas the GSK3 inhibitor CHIR99021 enhanced these responses. Together, these results demonstrate that PKCα and Akt modulate platelet function by phosphorylating and inhibiting GSK3α/β, thereby relieving the negative effect of GSK3α/β on thrombin-mediated platelet activation.