Search

Aim The importance of endothelial cell (EC) autophagy to vascular homeostasis in the context of health and disease is evolving. Earlier, we reported that intact EC autophagy is requisite to maintain shear-stress-induced nitric oxide (NO) generation via glycolysis-dependent purinergic signalling to endothelial NO synthase (eNOS). Here, we illustrate the translational and functional significance of these findings. Methods and results First, we assessed translational relevance using older male humans and mice that exhibit blunted EC autophagy and impaired arterial function vs. adult controls. Active hyperaemia evoked by rhythmic handgrip exercise-elevated radial artery shear-rate similarly from baseline in adult and older subjects for 60 min. Compared with baseline, indexes of autophagy initiation, p-eNOSS1177 activation, and NO generation, occurred in radial artery ECs obtained from adult but not older volunteers. Regarding mice, indexes of autophagy and p-eNOSS1177 activation were robust in ECs from adult but not older animals that completed 60-min treadmill-running. Furthermore, 20 dyne • cm2 laminar shear stress × 45-min increased autophagic flux, glycolysis, ATP production, and p-eNOSS1177 in primary arterial ECs obtained from adult but not older mice. Concerning functional relevance, we next questioned whether the inability to initiate EC autophagy, glycolysis, and p-eNOSS1177in vitro precipitates arterial dysfunction ex vivo. Compromised intraluminal flow-mediated vasodilation displayed by arteries from older vs. adult mice was recapitulated in vessels from adult mice by (i) NO synthase inhibition; (ii) acute autophagy impairment using 3-methyladenine (3-MA); (iii) EC Atg3 depletion (iecAtg3KO mice); (iv) purinergic 2Y1-receptor (P2Y1-R) blockade; and (v) germline depletion of P2Y1-Rs. Importantly, P2Y1-R activation using 2-methylthio-ADP (2-Me-ADP) improved vasodilatory capacity in arteries from (i) adult mice treated with 3-MA; (ii) adult iecAtg3KO mice; and (iii) older animals with repressed EC autophagy. Conclusions Arterial dysfunction concurrent with pharmacological, genetic, and age-associated EC autophagy compromise is improved by activating P2Y1-Rs.

Although several members of protein disulfide isomerase (PDI) family support thrombosis, other PDI family members with the CXYC motif remain uninvestigated. ERp46 has 3 CGHC redox-active sites and a radically different molecular architecture than other PDIs. Expression of ERp46 on the platelet surface increased with thrombin stimulation. An anti-ERp46 antibody inhibited platelet aggregation, adenosine triphosphate (ATP) release, and αIIbβ3 activation. ERp46 protein potentiated αIIbβ3 activation, platelet aggregation, and ATP release, whereas inactive ERp46 inhibited these processes. ERp46 knockout mice had prolonged tail-bleeding times and decreased platelet accumulation in thrombosis models that was rescued by infusion of ERp46. ERp46-deficient platelets had decreased αIIbβ3 activation, platelet aggregation, ATP release, and P-selectin expression. The defects were reversed by wild-type ERp46 and partially reversed by ERp46 containing any of the 3 active sites. Platelet aggregation stimulated by an αIIbβ3-activating peptide was inhibited by the anti-ERp46 antibody and was decreased in ERp46-deficient platelets. ERp46 bound tightly to αIIbβ3 by surface plasmon resonance but poorly to platelets lacking αIIbβ3 and physically associated with αIIbβ3 upon platelet activation. ERp46 mediated clot retraction and platelet spreading. ERp46 more strongly reduced disulfide bonds in the β3 subunit than other PDIs and in contrast to PDI, generated thiols in β3 independently of fibrinogen. ERp46 cleaved the Cys473-Cys503 disulfide bond in β3, implicating a target for ERp46. Finally, ERp46-deficient platelets have decreased thiols in β3, implying that ERp46 cleaves disulfide bonds in platelets. In conclusion, ERp46 is critical for platelet function and thrombosis and facilitates αIIbβ3 activation by targeting disulfide bonds.

PI-3 Kinase plays an important role in platelet activation mainly through regulation of RASA3. Akt phosphorylation is an indicator for the activity of PI3 kinase. The aim of this study is to characterize the pathways leading to Akt phosphorylation in platelets. We performed concentration response curves of LY294002, a pan-PI3 kinase inhibitor, on platelet aggregation and Akt phosphorylation, in washed human and mouse platelets. At concentrations as low as 3.12 μM, LY294002 abolished Akt phosphorylation induced by 2MeSADP and SFLLRN, but not by AYPGKF. It required much higher concentrations of LY294002 (12.5-25 μM) to abolish AYPGKF-induced Akt phosphorylation, both in wild type and P2Y12 null mouse platelets. We propose that 3.12 μM LY294002 is sufficient to inhibit PI3 kinase isoforms in platelets and higher concentrations might inhibit other pathways regulating Akt phosphorylation by AYPGKF. We conclude that Protease-activated receptor 4 (PAR4) might cause Akt phosphorylation through pathways distinctly different from those of Protease-activated receptor 1 (PAR1).

We performed a database mining on 102 transcriptomic datasets for the expressions of 29 m

Objective: The importance of endothelial cell (EC) autophagy to vascular homeostasis is evolving. Earlier we reported that purinergic 2Y 1 receptor (P2Y 1 -R) activation rejuvenates shear-stress induced nitric oxide (NO) generation in bovine aortic endothelial cells that is otherwise compromised after pharmacological and genetic autophagy repression. Here we determined the translational and functional relevance of these findings. Approach and Results: First we assessed translational relevance using older humans and mice that exhibit blunted EC autophagy at rest together with impaired arterial function vs. appropriate controls. Rhythmic handgrip exercise elevated radial artery shear rate similarly in adult and older males for 60-min. Compared to baseline, autophagy initiation, p-eNOS S1177 activation, and NO generation, occurred in radial artery ECs from adult but not older subjects.Regarding mice, indexes of autophagy and p-eNOS S1177 activation were robust in ECs from adult but not older mice in response to 60-min treadmill-running. Next we questioned whether an inability to initiate EC autophagy precipitates arterial dysfunction. Age-associated reductions in intraluminal flow-mediated vasodilation observed in older vs. adult mice were recapitulated in arteries from adult mice by : (i) NO synthase inhibition; (ii) autophagy impairment using 3-methyladenine (3-MA); (iii) EC Atg3 depletion (iecAtg3KO mice); (iv) P2Y 1 -R blockade; and (v) germline depletion of P2Y 1 -Rs. Importantly, P2Y 1 -R activation using 2-methylthio-ADP (2-Me-ADP) improved vasodilatory capacity in arteries from : (i) adult mice treated with 3-MA; (ii) adult iecAtg3KO mice; and (iii) older animals with repressed EC autophagy. Conclusions: Arterial dysfunction concurrent with pharmacological, genetic, and age-associated EC autophagy compromise is improved by activating P2Y 1 -Rs.

Immune cells express receptors bearing an immune tyrosine activation motif (ITAM) containing two YXXL motifs or hemITAMs containing only one YXXL motif. Phosphorylation of the ITAM/hemITAM is mediated by Src family kinases allowing for the binding and activation of spleen tyrosine kinase (Syk). It is believed that Syk must be phosphorylated on tyrosine residues for activation, and Tyr342, а conserved tyrosine in the interdomain B region, has been shown to be critical for regulating Syk in FcεR1-activated mast cells. Syk is a key mediator of signaling pathways downstream of several platelet pathways including the ITAM bearing glycoprotein VI (GPVI)/Fc receptor gamma chain collagen receptor and the hemITAM containing C-type lectin-like receptor-2 (CLEC-2). Since platelet activation is a crucial step in both hemostasis and thrombosis, we evaluated the importance of Syk Y342 in these processes by producing an Syk Y342F knock-in mouse. When using a CLEC-2 antibody as an agonist, reduced aggregation and secretion were observed in Syk Y342F mouse platelets when compared with control mouse platelets. Platelet reactivity was also reduced in response to the GPVI agonist collagen-related peptide. Signaling initiated by either GPVI or CLEC-2 was also greatly inhibited, including Syk Y519/520 phosphorylation. Hemostasis, as measured by tail bleeding time, was not altered in Syk Y342F mice, but thrombus formation in response to FeCl

ObjectiveThe importance of endothelial cell (EC) autophagy to vascular homeostasis is evolving. Earlier we reported that purinergic 2Y1 receptor (P2Y1-R) activation rejuvenates shearstress induced nitric oxide (NO) generation in bovine aortic endothelial cells that is otherwise compromised after pharmacological and genetic autophagy repression. Here we determined the translational and functional relevance of these findings.Approach and ResultsFirst we assessed translational relevance using older humans and mice that exhibit blunted EC autophagy at rest together with impaired arterial function vs. appropriate controls. Rhythmic handgrip exercise elevated radial artery shear rate similarly in adult and older males for 60-min. Compared to baseline, autophagy initiation, p-eNOSS1177 activation, and NO generation, occurred in radial artery ECs from adult but not older subjects. Regarding mice, indexes of autophagy and p-eNOSS1177 activation were robust in ECs from adult but not older mice in response to 60-min treadmill-running. Next we questioned whether an inability to initiate EC autophagy precipitates arterial dysfunction. Age-associated reductions in intraluminal flow-mediated vasodilation observed in older vs. adult mice were recapitulated in arteries from adult mice by : (i) NO synthase inhibition; (ii) autophagy impairment using 3-methyladenine (3-MA); (iii) EC Atg3 depletion (iecAtg3KO mice); (iv) P2Y1-R blockade; and (v) germline depletion of P2Y1-Rs. Importantly, P2Y1-R activation using 2-methylthio-ADP (2-Me-ADP) improved vasodilatory capacity in arteries from : (i) adult mice treated with 3-MA; (ii) adult iecAtg3KO mice; and (iii) older animals with repressed EC autophagy.ConclusionsArterial dysfunction concurrent with pharmacological, genetic, and age-associated EC autophagy compromise is improved by activating P2Y1-Rs.

Background and Objective CD45 is a receptor protein tyrosine phosphatase present on the surface of all hematopoietic cells except for erythrocytes and platelets. Proteomics studies, however, have demonstrated the presence of a CD45 c-terminal catalytic peptide in platelets. Therefore, we investigated the functional role of this truncated isoform of CD45 in platelets, which contains the c-terminal catalytic domain but lacks the extracellular region. Methods and Results We used an antibody specific to the c-terminus of CD45 to confirm the presence of a truncated CD45 isoform in platelets. We also examined ex vivo and in vivo platelet function using CD45 knockout (KO) mice. Aggregation and secretion mediated by the glycoprotein VI (GPVI) receptor was impaired in CD45 KO platelets. Consequently, CD45 KO mice had impaired hemostasis indicated by increased tail bleeding times. Also, using a model of pulmonary embolism we showed that CD45 KO mice had defective in vivo thrombus formation. Next, we investigated whether or not the truncated isoform of CD45 had a role in GPVI signaling. The full-length isoform of CD45 is known to regulate Src family kinase (SFK) activation in lymphocytes. We find a similar role for the truncated isoform of CD45 in platelets. SFK activation was impaired downstream of the GPVI receptor in the CD45 KO murine platelets. Consequently, Syk, PLCγ2, and pleckstrin phosphorylations were also impaired in CD45 KO murine platelets. Conclusion We conclude that the truncated CD45 isoform regulates GPVI-mediated signaling and platelet functional responses by regulating SFK activation.

Background Neutrophil dysfunction plays an important role in inflammation-induced tissue injury. Previously, we identified protein kinase C-δ (PKCδ) as a critical controller of neutrophil activation and trafficking but how PKCδ is regulated in inflammation has not been delineated. PKCδ activity is regulated by tyrosine phosphorylation on multiple sites. Tyrosine155 is a key regulator of apoptosis and gene expression, but its role in proinflammatory signaling is not known. Methods In-vitro studies - superoxide anion (O2) and neutrophil extracellular traps (NETs) were measured in bone marrow neutrophils (BMN) isolated from wild type (WT) and PKCδY155F knock-in mice (PKCδ tyrosine 155 → phenylalanine). Our novel 3D biomimetic microfluidic assay (bMFA) was used to delineate PKCδ-mediated regulation of individual steps in neutrophil adhesion and migration using WT and PKCδY155F BMN and mouse lung microvascular endothelial cells (MLMVEC). In-vivo studies - WT and PKCδY155F knock-in mice underwent sham or cecal ligation and puncture surgery and the lungs harvested 24 h post-surgery. Results In vitro - PKCδY155F BMN had significantly reduced O2 and NETs release compared with WT. WT BMN, but not PKCδY155F BMN, demonstrated significant adhesion and migration across tumor necrosis factor-activated MLMVEC in bMFA. PKCδ inhibition significantly reduced WT BMN adhesion and migration under low shear and near bifurcations, but had no effect on PKCδY155F BMN. In vivo - mutation of PKCδ tyrosine 155 significantly decreased neutrophil migration into the lungs of septic mice. Conclusions PKCδ tyrosine 155 is a key phosphorylation site controlling proinflammatory signaling and neutrophil-endothelial cell interactions. These studies provide mechanistic insights into PKCδ regulation during inflammation.

Background: Cardiac rupture is a major lethal complication of acute myocardial infarction (MI). Despite significant advances in reperfusion strategies, mortality from cardiac rupture remains high. Studies suggest that cardiac rupture can be accelerated by thrombolytic therapy, but the relevance of this risk factor remains controversial. Methods: We analyzed protease-activated receptor 4 (Par4) expression in mouse hearts with MI and investigated the effects of Par4 deletion on cardiac remodeling and function after MI by echocardiography, quantitative immunohistochemistry, and flow cytometry. Results: Par4 mRNA and protein levels were increased in mouse hearts after MI and in isolated cardiomyocytes in response to hypertrophic and inflammatory stimuli. Par4-deficient mice showed less myocyte apoptosis, reduced infarct size, and improved functional recovery after acute MI relative to wild-type (WT). Conversely, Par4 −/− mice showed impaired cardiac function, greater rates of myocardial rupture, and increased mortality after chronic MI relative to WT. Pathological evaluation of hearts from Par4 −/− mice demonstrated a greater infarct expansion, increased cardiac hemorrhage, and delayed neutrophil accumulation, which resulted in impaired post-MI healing compared with WT. Par4 deficiency also attenuated neutrophil apoptosis in vitro and after MI in vivo and impaired inflammation resolution in infarcted myocardium. Transfer of Par4 −/− neutrophils, but not of Par4 −/− platelets, in WT recipient mice delayed inflammation resolution, increased cardiac hemorrhage, and enhanced cardiac dysfunction. In parallel, adoptive transfer of WT neutrophils into Par4 −/− mice restored inflammation resolution, reduced cardiac rupture incidence, and improved cardiac function after MI. Conclusions: These findings reveal essential roles of Par4 in neutrophil apoptosis and inflammation resolution during myocardial healing and point to Par4 inhibition as a potential therapy that should be limited to the acute phases of ischemic insult and avoided for long-term treatment after MI.

Essentials RAS proteins are expressed in platelets but their functions are largely uncharacterized. TC21/RRas2 is required for glycoprotein VI-induced platelet responses and for thrombus stability in vivo. TC21 regulates platelet aggregation by control of αIIb β3 integrin activation, via crosstalk with Rap1b. This is the first indication of functional importance of a proto-oncogenic RAS protein in platelets. Summary Background Many RAS family small GTPases are expressed in platelets, including RAC, RHOA, RAP, and HRAS/NRAS/RRAS1, but most of their signaling and cellular functions remain poorly understood. Like RRAS1, TC21/RRAS2 reverses HRAS-induced suppression of integrin activation in CHO cells. However, a role for TC21 in platelets has not been explored. Objectives To determine TC21 expression in platelets, TC21 activation in response to platelet agonists, and roles of TC21 in platelet function in in vitro and in vivo thrombosis. Results We demonstrate that TC21 is expressed in human and murine platelets, and is activated in response to agonists for the glycoprotein (GP) VI-FcRγ immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor, in an Src-dependent manner. GPVI-induced platelet aggregation, integrin αIIb β3 activation, and α-granule and dense granule secretion, as well as phosphorylation of Syk, phospholipase Cγ2, AKT, and extracellular signal-regulated kinase, were inhibited in TC21-deficient platelets ex vivo. In contrast, these responses were normal in TC21-deficient platelets following stimulation with P2Y, protease-activated receptor 4 and C-type lectin receptor 2 receptor agonists, indicating that the function of TC21 in platelets is GPVI-FcRγ-ITAM-specific. TC21 was required for GPVI-induced activation of Rap1b. TC21-deficient mice did not show a significant delay in injury-induced thrombosis as compared with wild-type controls; however, thrombi were unstable. Hemostatic responses showed similar effects. Conclusions TC21 is essential for GPVI-FcRγ-mediated platelet activation and for thrombus stability in vivo via control of Rap1b and integrins.

We investigated the effect of platelets on ovarian cancer and the role of adenosine diphosphate (ADP) receptors (P2Y12 and P2Y1) on platelets in the growth of primary ovarian cancer tumors. We showed that in murine models of ovarian cancer, a P2Y12 inhibitor (ticagrelor) reduced tumor growth by 60% compared with aspirin and by 75% compared with placebo. In P2Y12-/- mice, the growth of syngeneic ovarian cancer tumors was reduced by >85% compared with wild-type (WT) mice. In contrast, there was no difference in tumor growth between P2Y1-/- and WT mice. Reconstitution of hematopoiesis in irradiated P2Y12-/- mice by hematopoietic progenitor cells from WT mice (WT→P2Y12-/-) restored tumor growth in P2Y12-/- mice. Finally, knockdown of ecto-apyrase (CD39) on ovarian cancer cells increased tumor growth in tumor-bearing mice. Although in the absence of platelets, ADP, the P2Y12 inhibitor, recombinant apyrase, or knockdown of CD39 did not affect cancer cell proliferation, in the presence of platelets, the P2Y12 inhibitor and recombinant apyrase reduced and knockdown of CD39 increased platelet-enhanced cancer cell proliferation. These results suggest that P2Y12 on platelets and ADP concentration at the interface between cancer cells and platelets affect the growth of primary ovarian cancer tumors in mice. If additional studies in mice and in pilot human trials confirm our results, inhibition of P2Y12 might be a new therapeutic option that can be used in adjuvant to the traditional surgery and chemotherapy in patients with ovarian cancer.

Platelets play a key role in the physiological hemostasis or pathological process of thrombosis. Rhodocytin, an agonist of the C-type lectin-like receptor-2 (CLEC-2), elicits powerful platelet activation signals in conjunction with Src family kinases (SFKs), spleen tyrosine kinase (Syk), and phospholipase γ2 (PLCγ2). Previous reports have shown that rhodocytin-induced platelet aggregation depends on secondary mediators such as thromboxane A2 (TxA2) and ADP, which are agonists for G-protein-coupled receptors (GPCRs) on platelets. How the secondary mediators regulate CLEC-2-mediated platelet activation in terms of signaling is not clearly defined. In this study, we report that CLEC-2-induced Syk and PLCγ2 phosphorylation is potentiated by TxA2 and that TxA2 plays a critical role in the most proximal event of CLEC-2 signaling, i.e. the CLEC-2 receptor tyrosine phosphorylation. We show that the activation of other GPCRs, such as the ADP receptors and protease-activated receptors, can also potentiate CLEC-2 signaling. By using the specific Gq inhibitor, UBO-QIC, or Gq knock-out murine platelets, we demonstrate that Gq signaling, but not other G-proteins, is essential for GPCR-induced potentiation of Syk phosphorylation downstream of CLEC-2. We further elucidated the signaling downstream of Gq and identified an important role for the PLCβ-PKCα pathway, possibly regulating activation of SFKs, which are crucial for initiation of CLEC-2 signaling. Together, these results provide evidence for novel Gq-PLCβ-PKCα-mediated regulation of proximal CLEC-2 signaling by Gq-coupled receptors.

Background: Platelets from patients with diabetes mellitus are hyperactive. Hyperactivated platelets may contribute to cardiovascular complications and inadequate responses to antiplatelet agents in the setting of diabetes mellitus. However, the underlying mechanism of hyperactivated platelets is not completely understood. Methods: We measured P2Y 12 expression on platelets from patients with type 2 diabetes mellitus and on platelets from rats with diabetes mellitus. We also assayed platelet P2Y 12 activation by measuring cAMP and VASP phosphorylation. The antiplatelet and antithrombotic effects of AR-C78511 and cangrelor were compared in rats. Finally, we explored the role of the nuclear factor-κB pathway in regulating P2Y 12 receptor expression in megakaryocytes. Results Platelet P2Y 12 levels are 4-fold higher in patients with type 2 diabetes mellitus compared with healthy subjects. P2Y 12 expression correlates with ADP-induced platelet aggregation (r=0.89, P 12 in platelets from patients with diabetes mellitus is constitutively activated. Although both AR-C78511, a potent P2Y 12 inverse agonist, and cangrelor have similar antiplatelet efficacy on platelets from healthy subjects, AR-C78511 exhibits more powerful antiplatelet effects on diabetic platelets than cangrelor (aggregation ratio 36±3% versus 49±5%, respectively, P 3 -injury mesenteric arteriole thrombosis model in rats and an arteriovenous shunt thrombosis model in rats, we found that the inverse agonist AR-C78511 has greater antithrombotic effects on GK rats with diabetes mellitus than cangrelor (thrombus weight 4.9±0.3 mg versus 8.3±0.4 mg, respectively, P 12 receptor expression in diabetes mellitus. Conclusions Platelet P2Y 12 receptor expression is significantly increased and the receptor is constitutively activated in patients with type 2 diabetes mellitus, which contributes to platelet hyperactivity and limits antiplatelet drug efficacy in type 2 diabetes mellitus.

Protein tyrosine phosphatase nonreceptor type 7 (PTPN7), also called hematopoietic protein tyrosine phosphatase, controls extracellular signal-regulated protein kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase in T lymphocytes. Because ERK1/2 plays an important role in regulating thromboxane A(2) (TXA(2)) generation in platelets, we investigated the function of PTPN7 in these cells. Using immunoblot analysis, we detected PTPN7 in both human and mouse platelets but not in PTPN7-null mice. PTPN7 KO mouse platelets exhibited increased platelet functional responses, including aggregation, dense granule secretion, and TXA(2) generation, compared with platelets from WT littermates, upon stimulation with both G protein–coupled receptor (GPCR) and glycoprotein VI (GPVI) agonists. Using the GPCR agonist AYPGKF in the presence of the COX inhibitor indomethacin, we found that PTPN7 KO mouse platelets aggregated and secreted to the same extent as WT platelets, suggesting that elevated TXA(2) is responsible for the potentiation of platelet functional responses in PTPN7-KO platelets. Phosphorylation of ERK1/2 was also elevated in PTPN7 KO platelets. Stimulation of platelets with the GPVI agonist collagen-related peptide along with the COX inhibitor indomethacin did not result in phosphorylation of ERK1/2, indicating that GPVI-mediated ERK phosphorylation occurs through TXA(2). Although bleeding times did not significantly differ between PTPN7-null and WT mice, time to death was significantly faster in PTPN7-null mice than in WT mice in a pulmonary thromboembolism model. We conclude that PTPN7 regulates platelet functional responses downstream of GPCR agonists, but not GPVI agonists, through inhibition of ERK activation and thromboxane generation.

Protein-tyrosine phosphatase TULA-2 has been shown to regulate receptor signaling in several cell types, including platelets. Platelets are critical for maintaining vascular integrity; this function is mediated by platelet aggregation in response to recognition of the exposed basement membrane collagen by the GPVI receptor, which is non-covalently associated with the signal-transducing FcRγ polypeptide chain. Our previous studies suggested that TULA-2 plays an important role in negatively regulating signaling through GPVI-FcRγ and indicated that the tyrosine-protein kinase Syk is a key target of the regulatory action of TULA-2 in platelets. However, the molecular basis of the down-regulatory effect of TULA-2 on Syk activation via FcRγ remained unclear. In this study, we demonstrate that suppression of Syk activation by TULA-2 is mediated, to a substantial degree, by dephosphorylation of Tyr(P)346, a regulatory site of Syk, which becomes phosphorylated soon after receptor ligation and plays a critical role in initiating the process that yields fully activated Syk. TULA-2 is capable of dephosphorylating Tyr(P)346 with high efficiency, thus controlling the overall activation of Syk, but is less efficient in dephosphorylating other regulatory sites of this kinase. Therefore, dephosphorylation of Tyr(P)346 may be considered an important “checkpoint” in the regulation of Syk activation process. Putative biological functions of TULA-2-mediated dephosphorylation of Tyr(P)346 may include deactivation of receptor-activated Syk or suppression of Syk activation by suboptimal stimulation.

SummaryPhospholipase C (PLC)-β2 (gene PLCB2) is a critical regulator of platelet responses upon activation. Mechanisms regulating of PLC-β2 expression in platelets/MKs are unknown. Our studies in a patient with platelet PLC-β2 deficiency revealed the PLCB2 coding sequence to be normal and decreased platelet PLC-β2 mRNA, suggesting a defect in transcriptional regulation. PLCB2 5’- upstream region of the patient revealed a heterozygous 13 bp deletion (-1645/-1633 bp) encompassing a consensus sequence for nuclear factor-κB (NF-κB). This was subsequently detected in three of 50 healthy subjects. To understand the mechanisms regulating PLC-β2, we studied the effect of this variation in the PLCB2. Gel-shift studies using nuclear extracts from human erythroleukaemia (HEL) cells or recombinant p65 showed NF-κB binding to oligonucleotide with NF-κB site; in luciferase reporter studies its deletion reduced PLCB2 promoter activity. PLCB2 expression was decreased by siRNA knockdown of NF-κB p65 subunit and increased by p65 overexpression. By immunoblotting platelet PLC-β2 in 17 healthy subjects correlated with p65 (r=0.76, p=0.0005). These studies provide the first evidence that NF-kB regulates MK/platelet PLC-β2 expression. This interaction is important because of the major role of PLC-β2 in platelet activation and of NF-κB in processes, including inflammation and atherosclerosis, where both are intimately involved.Supplementary Material to this article is available online at www.thrombosis-online.com.

Interleukin-17 (IL-17)-secreting T helper 17 cells were recently identified as a CD4(+) T helper subset and implicated in various inflammatory and autoimmune diseases. The issues of whether and by what mechanism hyperlipidemic stress induces IL-17A to activate aortic endothelial cells (ECs) and enhance monocyte adhesion remained largely unknown. Using biochemical, immunological, microarray, experimental data mining analysis, and pathological approaches focused on primary human and mouse aortic ECs (HAECs and MAECs) and our newly generated apolipoprotein E (ApoE)(-/-)/IL-17A(-/-) mice, we report the following new findings. 1) The hyperlipidemia stimulus oxidized low density lipoprotein up-regulated IL-17 receptor(s) in HAECs and MAECs. 2) IL-17A activated HAECs and increased human monocyte adhesion in vitro. 3) A deficiency of IL-17A reduced leukocyte adhesion to endothelium in vivo. 3) IL-17A activated HAECs and MAECs via up-regulation of proinflammatory cytokines IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), chemokine CXC motif ligand 1 (CXCL1), and CXCL2. 4) IL-17A activated ECs specifically via the p38 mitogen-activated protein kinases (MAPK) pathway; the inhibition of p38 MAPK in ECs attenuated IL-17A-mediated activation by ameliorating the expression of the aforementioned proinflammatory cytokines, chemokines, and EC adhesion molecules including intercellular adhesion molecule 1. Taken together, our results demonstrate for the first time that IL-17A activates aortic ECs specifically via p38 MAPK pathway.

Protease-activated receptor (PAR)4 is a low affinity thrombin receptor with less understood function relative to PAR1. PAR4 is involved in platelet activation and hemostasis, but its specific actions on myocyte growth and cardiac function remain unknown. This study examined the role of PAR4 deficiency on cardioprotection after myocardial ischemia-reperfusion (IR) injury in mice. When challenged by in vivo or ex vivo IR, PAR4 knockout (KO) mice exhibited increased tolerance to injury, which was manifest as reduced infarct size and a more robust functional recovery compared to wild-type mice. PAR4 KO mice also showed reduced cardiomyocyte apoptosis and putative signaling shifts in survival pathways in response to IR. Inhibition of PAR4 expression in isolated cardiomyocytes by shRNA offered protection against thrombin and PAR4-agonist peptide-induced apoptosis, while overexpression of wild-type PAR4 significantly enhanced the susceptibility of cardiomyocytes to apoptosis, even under low thrombin concentrations. Further studies implicate Src- and epidermal growth factor receptor-dependent activation of JNK on the proapoptotic effect of PAR4 in cardiomyocytes. These findings reveal a pivotal role for PAR4 as a regulator of cardiomyocyte survival and point to PAR4 inhibition as a therapeutic target offering cardioprotection after acute IR injury.

Essentials Phosphoinositide 3-kinase and MAPK pathways crosstalk via PDK1. PDK1 is required for adenosine diphosphate-induced platelet activation and thromboxane generation. PDK1 regulates RAF proto-oncogene Ser/Thr kinase (Raf1) activation in the MAPK pathway. Genetic ablation of PDK1 protects against platelet-dependent thrombosis in vivo. Summary Background Platelets are dynamic effector cells with functions that span hemostatic, thrombotic and inflammatory continua. Phosphoinositide-dependent protein kinase 1 (PDK1) regulates protease-activated receptor 4-induced platelet activation and thrombus formation through glycogen synthase kinase3β. However, whether PDK1 also signals through the ADP receptor and its functional importance in vivo remain unknown. Objective To establish the mechanism of PDK1 in ADP-induced platelet activation and thrombosis. Methods We assessed the role of PDK1 on 2MeSADP-induced platelet activation by measuring aggregation, thromboxane generation and phosphorylation events in the presence of BX-795, which inhibits PDK1, or by using platelet-specific PDK1 knockout mice and performing western blot analysis. PDK1 function in thrombus formation was assessed with an in vivo pulmonary embolism model. Results PDK1 inhibition with BX-795 reduced 2-methylthio-ADP (2MeSADP)-induced aggregation of human and murine platelets by abolishing thromboxane generation. Similar results were observed in pdk1-/- mice. PDK1 was also necessary for the phosphorylation of mitogen-activated protein kinase kinase 1/2 (MEK1/2), extracellular signal-regulated kinase 1/2, and cytosolic phospholipase A2, indicating that PDK1 regulates an upstream kinase in the mitogen-activated protein kinase (MAPK) pathway. We next determined that this upstream kinase is Raf-1, a serine/threonine kinase that is necessary for the phosphorylation of MEK1/2, as pharmacological inhibition and genetic ablation of PDK1 were sufficient to prevent Raf1 phosphorylation. Furthermore, in vivo inhibition or genetic ablation of PDK1 protected mice from collagen/epinephrine-induced pulmonary embolism. Conclusion PDK1 governs thromboxane generation and thrombosis in platelets that are stimulated with 2MeSADP by regulating activation of the MAPK pathway.

Sepsis is characterized by an intense systemic inflammatory response activating a cascade of proinflammatory events resulting in leukocyte dysregulation and host tissue damage. The lung is particularly susceptible to systemic inflammation, leading to acute lung injury. Key to inflammation-induced lung damage is the excessive migration of neutrophils across the vascular endothelium. The mechanisms which regulate neutrophil activation and migration in sepsis are not well defined but there is growing evidence that platelets are actively involved and play a key role in microvascular permeability and neutrophil-mediated organ damage. We previously identified PKC-delta (PKCδ) as a critical regulator of the inflammatory response in sepsis and demonstrated PKCδ inhibition was lung protective. However, the role of PKCδ in sepsis-induced platelet activation and platelet-leukocyte interactions is not known. In this study, rats underwent sham surgery or cecal ligation and puncture (CLP) to induce sepsis. Following surgeries, a PKCδ inhibitor (200μg/kg) or vehicle (PBS) was administered intra-tracheally. At 24 hours post-surgeries, lung tissue, BAL fluid, and blood samples were collected. While sepsis caused thrombocytopenia, the remaining circulating platelets were activated as demonstrated by increased p-selectin expression, elevated plasma PF4, and enhanced platelet-leukocyte aggregate formation compared to Sham animals. Platelet activation was associated with increased platelet PKCδ activity. Inhibition of PKCδ attenuated sepsis-induced platelet activation, secretion and aggregate formation. Sepsis-induced thrombocytopenia was also significantly reduced and circulating platelet numbers were similar to sham animals. In the lung, sepsis induced significant influx of platelets and neutrophils and the development of lung injury. Administration of the PKCδ inhibitor decreased platelet and neutrophil influx, and was lung protective. Thus, PKCδ inhibition modulated platelet activity both locally and systemically, decreased neutrophil influx into the lung, and was lung protective. We demonstrate for the first time that PKCδ plays an important role in platelet activation and platelet-neutrophil interaction during sepsis.

Phosphatidylinositol 3-kinase is an important signaling molecule that, once activated, leads to the generation of phosphatidylinositol (3,4,5)-trisphosphate (PIP3). We performed a proteomic screen to identify PIP3-interacting proteins in human platelets. Among these proteins, we found engulfment and cell motility 1 (ELMO1), a scaffold protein with no catalytic activity. ELMO1 is expressed in platelets and interacts with active RhoG. However, the function of ELMO1 in platelets is not known. The focus of this study was to determine the function of ELMO1 in platelets utilizing ELMO1−/− mice. Platelet aggregation, granule secretion, integrin αIIbβ3 activation, and thromboxane generation were enhanced in ELMO1−/− platelets in response to glycoprotein VI (GPVI) agonists but unaltered when a protease-activated receptor 4 agonist was used. The kinetics of spreading on immobilized fibrinogen was enhanced in ELMO1−/− platelets compared with wild-type (WT) littermate controls. This suggests that ELMO1 plays a role downstream of the GPVI and integrin αIIbβ3 pathway. Furthermore, whole blood from ELMO1−/− mice perfused over collagen exhibited enhanced thrombus formation compared with WT littermate controls. ELMO1−/− mice showed reduced survival compared with control following pulmonary embolism. ELMO1−/− mice also exhibited a shorter time to occlusion using the ferric-chloride injury model and reduced bleeding times compared with WT littermate controls. These results indicate that ELMO1 plays an important role in hemostasis and thrombosis in vivo. RhoG activity was enhanced in ELMO1−/− murine platelets compared with WT littermate controls in response to GPVI agonist. Together, these data suggest that ELMO1 negatively regulates GPVI-mediated thrombus formation via RhoG.

In blood platelets, stimulation of G protein-coupled receptors (GPCRs) by thrombin triggers the activation of Src family kinases (SFKs), resulting in the tyrosine-phosphorylation of multiple substrates, but the mechanism underlying this process is still poorly understood. In the present study, we show that the time-dependent protein-tyrosine phosphorylation triggered by thrombin in human or murine platelets was totally suppressed only upon concomitant chelation of intracellular Ca2+ and inhibition of SFKs. Thrombin-induced activation of SFKs was regulated by intracellular Ca2+ and accordingly the Ca2+ ionophore A23187 was sufficient to stimulate SFKs. A23187 also triggered the phosphorylation and activation of the Ca2+-dependent focal adhesion kinase Pyk2 and Pyk2 activation by thrombin was Ca2+-dependent. Stimulation of SFKs by thrombin or A23187 was strongly reduced in platelets from Pyk2 knockout (KO) mice, as was the overall pattern of protein-tyrosine phosphorylation. By immunoprecipitation experiments, we demonstrate that Lyn and Fyn, but not Src, were activated by Pyk2. Inhibition of SFKs by PP2 also reduced the phosphorylation of Pyk2 in thrombin or A23187-stimulated platelets. Analysis of KO mice demonstrated that Fyn, but not Lyn, was required for complete Pyk2 phosphorylation by thrombin. Finally, PP2 reduced aggregation of murine platelets to a level comparable to that of Pyk2-deficient platelets, but did not have further effects in the absence of Pyk2. These results indicate that in thrombin-stimulated platelets, stimulation of Pyk2 by intracellular Ca2+ initiates SFK activation, establishing a positive loop that reinforces the Pyk2/SFK axis and allows the subsequent massive tyrosine phosphorylation of multiple substrates required for platelet aggregation.

Platelets are activated by a range of stimuli that share little or no resemblance in structure to each other or to recognized ligands, including diesel exhaust particles (DEP), small peptides [4N1-1, Champs (computed helical anti-membrane proteins), LSARLAF (Leu-Ser-Ala-Arg-Leu-Ala-Phe)], proteins (histones) and large polysaccharides (fucoidan, dextran sulfate). This miscellaneous group stimulate aggregation of human and mouse platelets through the glycoprotein VI (GPVI)–FcR γ-chain complex and/or C-type lectin-like receptor-2 (CLEC-2) as shown using platelets from mice deficient in either or both of these receptors. In addition, all of these ligands stimulate tyrosine phosphorylation in GPVI/CLEC-2-double-deficient platelets, indicating that they bind to additional surface receptors, although only in the case of dextran sulfate does this lead to activation. DEP, fucoidan and dextran sulfate, but not the other agonists, activate GPVI and CLEC-2 in transfected cell lines as shown using a sensitive reporter assay confirming a direct interaction with the two receptors. We conclude that this miscellaneous group of ligands bind to multiple proteins on the cell surface including GPVI and/or CLEC-2, inducing activation. These results have pathophysiological significance in a variety of conditions that involve exposure to activating charged/hydrophobic agents.

Tyrosine kinase pathways are known to play an important role in the activation of platelets. In particular, the GPVI and CLEC-2 receptors are known to activate Syk upon tyrosine phosphorylation of an immune tyrosine activation motif (ITAM) and hemITAM, respectively. However, unlike GPVI, the CLEC-2 receptor contains only one tyrosine motif in the intracellular domain. The mechanisms by which this receptor activates Syk are not completely understood. In this study, we identified a novel signaling mechanism in CLEC-2-mediated Syk activation. CLEC-2-mediated, but not GPVI-mediated, platelet activation and Syk phosphorylation were abolished by inhibition of PI3K, which demonstrates that PI3K regulates Syk downstream of CLEC-2. Ibrutinib, a Tec family kinase inhibitor, also completely abolished CLEC-2-mediated aggregation and Syk phosphorylation in human and murine platelets. Furthermore, embryos lacking both Btk and Tec exhibited cutaneous edema associated with blood-filled vessels in a typical lymphatic pattern similar to CLEC-2 or Syk-deficient embryos. Thus, our data show, for the first time, that PI3K and Tec family kinases play a crucial role in the regulation of platelet activation and Syk phosphorylation downstream of the CLEC-2 receptor.

Akt is an important signaling molecule regulating platelet aggregation. Akt is phosphorylated after translocation to the membrane through Gi signaling pathways by a phosphatidylinositol-3,4,5-trisphosphate (PIP3)-dependent mechanism. However, Akt is more robustly phosphorylated by thrombin compared with adenosine 5'-diphosphate in platelets. This study investigated the mechanisms of Akt translocation as a possible explanation for this difference. Stimulation of washed human platelets with protease-activated receptor agonists caused translocation of Akt to the membrane rapidly, whereas phosphorylation occurred later. The translocation of Akt was abolished in the presence of a Gq-selective inhibitor or in Gq-deficient murine platelets, indicating that Akt translocation is regulated downstream of Gq pathways. Interestingly, phosphatidylinositol 3-kinase (PI3K) inhibitors or P2Y12 antagonist abolished Akt phosphorylation without affecting Akt translocation to the membrane, suggesting that Akt translocation occurs through a PI3K/PIP3/Gi-independent mechanism. An Akt scaffolding protein, p21-activated kinase (PAK), translocates to the membrane after stimulation with protease-activated receptor agonists in a Gq-dependent manner, with the kinetics of translocation similar to that of Akt. Coimmunoprecipitation studies showed constitutive association of PAK and Akt, suggesting a possible role of PAK in Akt translocation. These results show, for the first time, an important role of the Gq pathway in mediating Akt translocation to the membrane in a novel Gi/PI3K/PIP3-independent mechanism.

Objective— We previously determined that protein kinase C δ (PKCδ) regulates platelet function. However, the function of PKCδ in megakaryopoiesis is unknown. Approach and Results— Using PKCδ -/- and wild-type littermate mice, we found that deficiency of PKCδ caused an increase in white blood cells and platelet counts, as well as in bone marrow and splenic megakaryocytes ( P -/- mouse bone marrow after culturing with exogenous thrombopoietin compared with wild-type ( P -/- megakaryocytes compared with wild-type. Finally, PKCδ -/- mice recovered faster and had a heightened rebound thrombocytosis after thrombocytopenic challenge. Conclusions— These data suggest that PKCδ is an important megakaryopoietic protein, which regulates signaling induced by thrombopoietin and represents a potential therapeutic target.

Background It has long been postulated that Protein Kinase C (PKC) is an important regulator of megakaryopoiesis. Recent contributions to the literature have outlined the functions of several individual PKC isoforms with regard to megakaryocyte differentiation and platelet production. However, the exact role of PKCe remains elusive. Objective To delineate the role of PKCe in megakaryopoiesis. Approach and results We used a PKCe knockout mouse model to examine the effect of PKCe deficiency on platelet mass, megakaryocyte mass, and bone marrow progenitor cell distribution. We also investigated platelet recovery in PKCe null mice and TPO-mediated signaling in PKCe null megakaryocytes. PKCe null mice have higher platelet counts due to increased platelet production compared to WT littermate controls (p

Phosphoinositide-dependent protein kinase 1 (PDK1) is known to regulate PAR4 induced platelet activation and thrombus formation through GSK3β. However, whether PDK1 signaling also involves the ADP receptor and, if so, downstream functional consequences are unknown. We employed both pharmacologic (e.g. the selective PDK1 inhibitor, BX795) and genetic (platelet specific deletion of PDK1) approaches to dissect the role of PDK1 in ADP-induced platelet activation and protein synthesis. Inhibition of PDK1 with BX795 reduced 2MeSADP-induced platelet aggregation by abolishing thromboxane generation. Similar results were observed in PDK1 -/- mice (Fig A). Inhibition of PDK1 protected mice from collagen and epinephrine-induced pulmonary embolism (Fig B). PDK1 was also necessary for the phosphorylation of MEK1/2, Erk1/2 and cPLA2, indicating that PDK1 regulates an upstream kinase in MAPK pathway. We next identified that this upstream kinase is Raf1 (necessary for the phosphorylation of MEK1/2), as pharmacologic inhibition and genetic ablation of PDK1 was sufficient to prevent Raf1 phosphorylation (Fig C). Pharmacologic inhibition and genetic ablation of PDK1 blocked MAPK- and mTORC1-dependent protein synthesis in platelets through a mechanism requiring the phosphorylation of eIF4E and S6K. Concordantly, PDK1 is necessary for signal-dependent synthesis of the protein bcl3, which is under mTORC1-dependent control (Fig C). Taken together, our findings show for the first time that PDK1, a master kinase in the PI3K pathway, directly governs thromboxane generation, thrombosis, and protein synthesis in platelets through regulating MAPK and mTORC1 pathways.