Lyn, PKC-delta, SHIP-1 interactions regulate GPVI-mediated platelet-dense granule secretion

Icaritin Sensitizes Thrombin- and TxA2-Induced Platelet Activation and Promotes Hemostasis via Enhancing PLCγ2-PKC Signaling Pathways

BACKGROUND

 Vascular injury results in uncontrollable hemorrhage in hemorrhagic diseases and excessive antithrombotic therapy. Safe and efficient hemostatic agents which can be orally administered are urgently needed. Platelets play indispensable roles in hemostasis, but there is no drug exerting hemostatic effects through enhancing platelet function.

METHODS

 The regulatory effects of icaritin, a natural compound isolated from Herba Epimedii, on the dense granule release, thromboxane A2 (TxA2) synthesis, α-granule release, activation of integrin αIIbβ3, and aggregation of platelets induced by multiple agonists were investigated. The effects of icaritin on tail vein bleeding times of warfarin-treated mice were also evaluated. Furthermore, we investigated the underlying mechanisms by which icaritin exerted its pharmacological effects.

RESULTS

 Icaritin alone did not activate platelets, but significantly potentiated the dense granule release, α-granule release, activation of integrin αIIbβ3, and aggregation of platelets induced by thrombin and U46619. Icaritin also shortened tail vein bleeding times of mice treated with warfarin. In addition, phosphorylated proteome analysis, immunoblotting analysis, and pharmacological research revealed that icaritin sensitized the activation of phospholipase Cγ2 (PLCγ2)-protein kinase C (PKC) signaling pathways, which play important roles in platelet activation.

CONCLUSION

 Icaritin can sensitize platelet activation induced by thrombin and TxA2 through enhancing the activation of PLCγ2-PKC signaling pathways and promote hemostasis, and has potential to be developed into a novel orally deliverable therapeutic agent for hemorrhages.

Platelet Activation Pathways Controlling Reversible Integrin αIIbβ3 Activation

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

Dynamic phosphatase-recruitment controls B-cell selection and oncogenic signaling

Initiation of B-cell receptor (BCR) ¹ signaling, and subsequent antigen-encounter in germinal centers ^2,3 represent milestones of B-lymphocyte development that are both marked by sharp increases of CD25 surface-expression. Oncogenic signaling in B-cell leukemia (B-ALL) ⁴ and lymphoma ⁵ also induced CD25-surface expression. While CD25 is known as an IL2-receptor chain on T- and NK-cells ^6-9 , the significance of its expression on B-cells was unclear. Our experiments based on genetic mouse models and engineered patient-derived xenografts revealed that, rather than functioning as an IL2-receptor chain, CD25 expressed on B-cells assembled an inhibitory complex including PKCδ and SHIP1 and SHP1 phosphatases for feedback control of BCR-signaling or its oncogenic mimics. Recapitulating phenotypes of genetic ablation of PKCδ ¹⁰ - ¹² , SHIP1 ^13,14 and SHP1 ^{14, 15,16} , conditional CD25-deletion decimated early B-cell subsets but expanded mature B-cell populations and induced autoimmunity. In B-cell malignancies arising from early (B-ALL) and late (lymphoma) stages of B-cell development, CD25-loss induced cell death in the former and accelerated proliferation in the latter. Clinical outcome annotations mirrored opposite effects of CD25-deletion: high CD25 expression levels predicted poor clinical outcomes for patients with B-ALL, in contrast to favorable outcomes for lymphoma-patients. Biochemical and interactome studies revealed a critical role of CD25 in BCR-feedback regulation: BCR-signaling induced PKCδ-mediated phosphorylation of CD25 on its cytoplasmic tail (S ²⁶⁸ ). Genetic rescue experiments identified CD25-S ²⁶⁸ tail-phosphorylation as central structural requirement to recruit SHIP1 and SHP1 phosphatases to curb BCR-signaling. A single point mutation CD25 ^S268A abolished recruitment and activation of SHIP1 and SHP1 to limit duration and strength of BCR-signaling. Loss of phosphatase-function, autonomous BCR-signaling and Ca ²⁺ -oscillations induced anergy and negative selection during early B-cell development, as opposed to excessive proliferation and autoantibody production in mature B-cells. These findings highlight the previously unrecognized role of CD25 in assembling inhibitory phosphatases to control oncogenic signaling in B-cell malignancies and negative selection to prevent autoimmune disease.

TULA-Family Regulators of Platelet Activation

The two members of the UBASH3/TULA/STS-protein family have been shown to critically regulate cellular processes in multiple biological systems. The regulatory function of TULA-2 (also known as UBASH3B or STS-1) in platelets is one of the best examples of the involvement of UBASH3/TULA/STS proteins in cellular regulation. TULA-2 negatively regulates platelet signaling mediated by ITAM- and hemITAM-containing membrane receptors that are dependent on the protein tyrosine kinase Syk, which currently represents the best-known dephosphorylation target of TULA-2. The biological responses of platelets to collagen and other physiological agonists are significantly downregulated as a result. The protein structure, enzymatic activity and regulatory functions of UBASH3/TULA/STS proteins in the context of platelet responses and their regulation are discussed in this review.

Modulation of Glycoprotein VI and Its Downstream Signaling Pathways as an Antiplatelet Target

Antiplatelet therapy aims to reduce the risk of thrombotic events while maintaining hemostasis. A promising current approach is the inhibition of platelet glycoprotein GPVI-mediated adhesion pathways; pathways that do not involve coagulation. GPVI is a signaling receptor integral for collagen-induced platelet activation and participates in the thrombus consolidation process, being a suitable target for thrombosis prevention. Considering this, the blocking or antibody-mediated depletion of GPVI is a promising antiplatelet therapy for the effective and safe treatment of thrombotic diseases without a significant risk of bleeding and impaired hemostatic plug formation. This review describes the current knowledge concerning pharmaceutical approaches to platelet GPVI modulation and its downstream signaling pathways in this context.

Clustering extent-dependent differential signaling by CLEC-2 receptors in platelets

Background

C-type lectin receptor family members play a role in many cells including platelets, where they are crucial in the separation of lymphatic and blood vessels during development. The C-type lectin-like receptor 2 (CLEC-2) receptor contains the canonical intracellular hemITAM motif through which it signals to activate Syk.

Objectives

One proposed hypothesis for signaling cascade is that Syk bridges two receptors through phosphorylated hemITAM motifs. We demonstrated that the phosphorylated hemITAM stimulates PI3 kinase/Btk pathways to activate Syk. To address this controversy, we used a CLEC-2 selective agonist and studied the role of Btk in platelet activation.

Results and Conclusions

Platelet activation and downstream signaling were abolished in murine and human platelets in the presence of the Btk inhibitors ibrutinib or acalabrutinib when a low concentration of a CLEC-2 antibody was used to crosslink CLEC-2 receptors. This inhibition was overcome by increasing concentrations of the CLEC-2 antibody. Similar results were obtained in X-linked immunodeficient mouse platelets, with an inactivating mutation in Btk or in Lyn null platelets. We conclude that at low crosslinking conditions of CLEC-2, Btk plays an important role in the activation of Syk, but at higher crosslinking conditions their role becomes less important and other mechanisms take over to activate Syk.

PI3K Isoform Signalling in Platelets

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

mTOR regulates GPVI-mediated platelet activation

Background

Due to mTOR (mammalian/mechanistic target of rapamycin) gene-loss mice die during embryonic development, the role of mTOR in platelets has not been evaluated using gene knockout technology.

Methods

A mouse model with megakaryocyte/platelet-specific deletion of mTOR was established, and be used to evaluate the role of mTOR in platelet activation and thrombus formation.

Results

mTOR^−/− platelets were deficient in thrombus formation when grown on low-concentration collagen-coated surfaces; however, no deficiency in thrombus formation was observed when mTOR^−/− platelets were perfused on higher concentration collagen-coated surfaces. In FeCl₃-induced mouse mesenteric arteriole thrombosis models, wild-type (WT) and mTOR^−/− mice displayed significantly different responses to low-extent injury with respect to the ratio of occluded mice, especially within the first 40 min. Additionally, mTOR^−/− platelets displayed reduced aggregation and dense granule secretion (ATP release) in response to low doses of the glycoprotein VI (GPVI) agonist collagen related peptide (CRP) and the protease-activated receptor-4 (PAR4) agonist GYPGKF-NH₂; these deficiencies were overcame by stimulation with higher concentration agonists, suggesting dose dependence of the response. At low doses of GPVI or PAR agonist, the activation of α_IIbβ₃ in mTOR^−/− platelets was reduced. Moreover, stimulation of mTOR^−/− platelets with low-dose CRP attenuated the phosphorylation of S6K1, S6 and Akt Ser473, and increased the phosphorylation of PKCδ Thr505 and PKCε Ser729. Using isoform-specific inhibitors of PKCs (δ, ɛ, and α/β), we established that PKCδ/ɛ, and especially PKCδ but not PKCα/β or PKCθ, may be involved in low-dose GPVI-mediated/mTOR-dependent signaling.

Conclusion

These observations indicate that mTOR plays an important role in GPVI-dependent platelet activation and thrombus formation.

Phosphorylation of protein kinase Cδ Tyr311 positively regulates thromboxane generation in platelets

Platelets are key mediators of physiological hemostasis and pathological thrombosis, whose function must be carefully balanced by signaling downstream of receptors such as protease-activated receptor (PAR)4. Protein kinase C (PKC) is known to regulate various aspects of platelet function. For instance, PKCδ is known to regulate dense granule secretion, which is important for platelet activation. However, the mechanism by which PKCδ regulates this process as well as other facets of platelet activity is unknown. We speculated that the way PKCδ regulates platelet function may be because of the phosphorylation of tyrosine residues on PKCδ. We investigated phosphorylation of PKCδ following glycoprotein VI-mediated and PAR4-mediated platelet activation and found that Y311 is selectively phosphorylated when PAR4 is activated in human platelets. Therefore, we generated PKCδ Y311F knock-in mice, which are viable and have no gross abnormalities. However, PKCδY311F mice have significantly enhanced tail-bleeding times compared with WT littermate controls, which means hemostasis is interrupted. Furthermore, PKCδY311F mice exhibit longer time to carotid artery occlusion compared with WT control using a ferric chloride in vivo thrombosis model, indicating that the phosphorylation of PKCδ Y311 is prothrombotic. Washed platelets from PKCδY311F mice have reduced reactivity after stimulation with a PAR-4 agonist indicating its importance in platelet signaling. The phenotype observed in Y311F mouse platelets is because of reduced thromboxane generation, as an inhibitor of thromboxane generation equalizes the PKCδY311F platelet response to that of WT. Therefore, phosphorylation of PKCδ on Y311 is important for regulation of platelet function and specifically thromboxane generation, which reinforces platelet activation.

The role of phospho-tyrosine signaling in platelet biology and hemostasis

Platelets are small enucleated cell fragments specialized in the control of hemostasis, but also playing a role in angiogenesis, inflammation and immunity. This plasticity demands a broad range of physiological processes. Platelet functions are mediated through a variety of receptors, the concerted action of which must be tightly regulated, in order to allow specific and timely responses to different stimuli. Protein phosphorylation is one of the main key regulatory mechanisms by which extracellular signals are conveyed. Despite the importance of platelets in health and disease, the molecular pathways underlying the activation of these cells are still under investigation. Here, we review current literature on signaling platelet biology and in particular emphasize the newly emerging role of phosphatases in these processes.

Diacylglycerol kinase ζ is a negative regulator of GPVI-mediated platelet activation

Diacylglycerol kinases (DGKs) are a family of enzymes that convert diacylglycerol (DAG) into phosphatidic acid (PA). The ζ isoform of DGK (DGKζ) has been reported to inhibit T-cell responsiveness by downregulating intracellular levels of DAG. However, its role in platelet function remains undefined. In this study, we show that DGKζ was expressed at significant levels in both platelets and megakaryocytes and that DGKζ-knockout (DGKζ-KO) mouse platelets were hyperreactive to glycoprotein VI (GPVI) agonists, as assessed by aggregation, spreading, granule secretion, and activation of relevant signal transduction molecules. In contrast, they were less responsive to thrombin. Platelets from DGKζ-KO mice accumulated faster on collagen-coated microfluidic surfaces under conditions of arterial shear and stopped blood flow faster after ferric chloride-induced carotid artery injury. Other measures of hemostasis, as measured by tail bleeding time and rotational thromboelastometry analysis, were normal. Interestingly, DGKζ deficiency led to increased GPVI expression on the platelet and megakaryocyte surfaces without affecting the expression of other platelet surface receptors. These results implicate DGKζ as a novel negative regulator of GPVI-mediated platelet activation that plays an important role in regulating thrombus formation in vivo.

Regulation of platelet activation and thrombus formation by reactive oxygen species

Reactive oxygen species (ROS) are generated within activated platelets and play an important role in regulating platelet responses to collagen and collagen-mediated thrombus formation. As a major collagen receptor, platelet-specific glycoprotein (GP)VI is a member of the immunoglobulin (Ig) superfamily, with two extracellular Ig domains, a mucin domain, a transmembrane domain and a cytoplasmic tail. GPVI forms a functional complex with the Fc receptor γ-chain (FcRγ) that, following receptor dimerization, signals via an intracellular immunoreceptor tyrosine-based activation motif (ITAM), leading to rapid activation of Src family kinase signaling pathways. Our previous studies demonstrated that an unpaired thiol in the cytoplasmic tail of GPVI undergoes rapid oxidation to form GPVI homodimers in response to ligand binding, indicating an oxidative submembranous environment in platelets after GPVI stimulation. Using a redox-sensitive fluorescent dye (H2DCF-DA) in a flow cytometric assay to measure changes in intracellular ROS, we showed generation of ROS downstream of GPVI consists of two distinct phases: an initial Syk-independent burst followed by additional Syk-dependent generation. In this review, we will discuss recent findings on the regulation of platelet function by ROS, focusing on GPVI-dependent platelet activation and thrombus formation.

Cutting Edge: Imbalanced Cation Homeostasis in MAGT1-Deficient B Cells Dysregulates B Cell Development and Signaling in Mice

Cation homeostasis, in relation to various immune-suppressive diseases, is a novel field of investigation. Recently, patients with a loss-of-function mutation in magnesium transporter 1 (MAGT1) were reported to present a dysregulated Mg²⁺ homeostasis in T lymphocytes. Using Magt1-knockout mice (Magt1^-/y ), we show that Mg²⁺ homeostasis was impaired in Magt1^-/y B cells and Ca²⁺ influx was increased after BCR stimulation, whereas T and NK cell function was unaffected. Consequently, mutant B cells displayed an increased phosphorylation of BCR-related proteins differentially affecting protein kinase C activation. These in vitro findings translated into increased frequencies of CD19⁺ B cells and marginal zone B cells and decreased frequencies of plasma cells among CD45⁺ splenocytes in vivo. Altogether, our study demonstrates for the first time, to our knowledge, that abolished MAGT1 function causes imbalanced cation homeostasis and developmental responses in B cells. Therefore, this study might contribute to a further understanding of B cell-related pathologies.

Dextran sulphate induces fibrinogen receptor activation through a novel Syk-independent PI-3 kinase-mediated tyrosine kinase pathway in platelets

In our attempt to find a physiological agonist that activates PAR3 receptors, we screened several coagulation proteases using PAR4 null platelets. We observed that FXIIa and heat inactivated FXIIa, but not FXII, caused platelet aggregation. We have identified a contaminant activating factor in FXIIa preparation as dextran sulfate (DxS), which caused aggregation of both human and mouse platelets. DxS-induced platelet aggregation was unaffected by YM254890, a Gq inhibitor, but abolished by pan-Src family kinase (SFK) inhibitor PP2, suggesting a role for SFKs in this pathway. However, DxS-induced platelet aggregation was unaffected in FcRγ-chain null murine platelets, ruling out the possibility of glycoprotein VI-mediated events. More interesting, OXSI-2 and Go6976, two structurally unrelated inhibitors shown to affect Syk, had only a partial effect on DxS-induced PAC-1 binding. DxS-induced platelet aggregation and intracellular calcium increases were abolished by the pan PI-3 kinase inhibitor LY294002, or an isoform-specific PI-3 kinase β inhibitor TGX-221. Pretreatment of platelets with Syk inhibitors or ADP receptor antagonists had little effect on Akt phosphorylation following DxS stimulation. These results, for the first time, establish a novel tyrosine kinase pathway in platelets that causes fibrinogen receptor activation in a PI-3 kinase-dependent manner without a crucial role for Syk.

PKC-epsilon deficiency alters progenitor cell populations in favor of megakaryopoiesis

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 PKCε remains elusive.

OBJECTIVE

To delineate the role of PKCε in megakaryopoiesis.

APPROACH AND RESULTS

We used a PKCε knockout mouse model to examine the effect of PKCε deficiency on platelet mass, megakaryocyte mass, and bone marrow progenitor cell distribution. We also investigated platelet recovery in PKCε null mice and TPO-mediated signaling in PKCε null megakaryocytes. PKCε null mice have higher platelet counts due to increased platelet production compared to WT littermate controls (p<0.05, n = 8). Furthermore, PKCε null mice have more bone marrow megakaryocyte progenitor cells than WT littermate control mice. Additionally, thrombopoietin-mediated signaling is perturbed in PKCε null mice as Akt and ERK1/2 phosphorylation are enhanced in PKCε null megakaryocytes stimulated with thrombopoietin. Finally, in response to immune-induced thrombocytopenia, PKCε null mice recovered faster and had higher rebound thrombocytosis than WT littermate control mice.

CONCLUSIONS

Enhanced platelet recovery could be due to an increase in megakaryocyte progenitor cells found in PKCε null mice as well as enhanced thrombopoietin-mediated signaling observed in PKCε deficient megakaryocytes. These data suggest that PKCε is a negative regulator of megakaryopoiesis.

Distinctive roles of PKC delta isozyme in platelet function

Platelet activation is a complex balance of positive and negative signaling pathways. Several protein kinase C (PKC) isoforms are expressed in human platelets. They are a major regulator of platelet granule secretion, activation and aggregation activity. One of those isoforms is the PKCδ isozyme, it has a central yet complex role in platelets such as opposite signaling functions depending on the nature of the agonist, it concentration and pathway. In fact, it has been shown that PKCδ has an overall negative influence on platelet function in response to collagen, while, following PAR stimulation, PKCδ has a positive effect on platelet function. Understanding the crucial role of PKCδ in platelet functions is recently emerging in the literature, therefore, further investigations should shed light into its specific role in hemostasis. In this review, we focus on the different roles of PKCδ in platelet activation, aggregation and thrombus formation.

Novel phosphatidylethanolamine derivatives accumulate in circulation in hyperlipidemic ApoE-/- mice and activate platelets via TLR2

A prothrombotic state and increased platelet reactivity are common in dyslipidemia and oxidative stress. Lipid peroxidation, a major consequence of oxidative stress, generates highly reactive products, including hydroxy-ω-oxoalkenoic acids that modify autologous proteins generating biologically active derivatives. Phosphatidylethanolamine, the second most abundant eukaryotic phospholipid, can also be modified by hydroxy-ω-oxoalkenoic acids. However, the conditions leading to accumulation of such derivatives in circulation and their biological activities remain poorly understood. We now show that carboxyalkylpyrrole-phosphatidylethanolamine derivatives (CAP-PEs) are present in the plasma of hyperlipidemic ApoE(-/-) mice. CAP-PEs directly bind to TLR2 and induces platelet integrin αIIbβ3 activation and P-selectin expression in a Toll-like receptor 2 (TLR2)-dependent manner. Platelet activation by CAP-PEs includes assembly of TLR2/TLR1 receptor complex, induction of downstream signaling via MyD88/TIRAP, phosphorylation of IRAK4, and subsequent activation of tumor necrosis factor receptor-associated factor 6. This in turn activates the Src family kinases, spleen tyrosine kinase and PLCγ2, and platelet integrins. Murine intravital thrombosis studies demonstrated that CAP-PEs accelerate thrombosis in TLR2-dependent manner and that TLR2 contributes to accelerate thrombosis in mice in the settings of hyperlipidemia. Our study identified the novel end-products of lipid peroxidation, accumulating in circulation in hyperlipidemia and inducing platelet activation by promoting cross-talk between innate immunity and integrin activation signaling pathways.

Cross talk between serine/threonine and tyrosine kinases regulates ADP-induced thromboxane generation in platelets

ADP-induced thromboxane generation depends on Src family kinases (SFKs) and is enhanced with pan-protein kinase C (PKC) inhibitors, but it is not clear how these two events are linked. The aim of the current study is to investigate the role of Y311 phosphorylated PKCδ in regulating ADP-induced platelet activation. In the current study, we employed various inhibitors and murine platelets from mice deficient in specific molecules to evaluate the role of PKCδ in ADP-induced platelet responses. We show that, upon stimulation of platelets with 2MeSADP, Y311 on PKCδ is phosphorylated in a P2Y1/Gq and Lyn-dependent manner. By using PKCδ and Lyn knockout murine platelets, we also show that tyrosine phosphorylated PKCδ plays a functional role in mediating 2MeSADP-induced thromboxane generation. 2MeSADP-induced PKCδ Y311 phosphorylation and thromboxane generation were potentiated in human platelets pre-treated with either a pan-PKC inhibitor, GF109203X or a PKC α/β inhibitor and in PKC α or β knockout murine platelets compared to controls. Furthermore, we show that PKC α/β inhibition potentiates the activity of SFK, which further hyper-phosphorylates PKCδ and potentiates thromboxane generation. These results show for the first time that tyrosine phosphorylated PKCδ regulates ADP-induced thromboxane generation independent of its catalytic activity and that classical PKC isoforms α/β regulate the tyrosine phosphorylation on PKCδ and subsequent thromboxane generation through tyrosine kinase, Lyn, in platelets.

Peroxiredoxin II is an antioxidant enzyme that negatively regulates collagen-stimulated platelet function

Collagen-induced platelet signaling is mediated by binding to the primary receptor glycoprotein VI (GPVI). Reactive oxygen species produced in response to collagen have been found to be responsible for the propagation of GPVI signaling pathways in platelets. Therefore, it has been suggested that antioxidant enzymes could down-regulate GPVI-stimulated platelet activation. Although the antioxidant enzyme peroxiredoxin II (PrxII) has emerged as having a role in negatively regulating signaling through various receptors by eliminating H2O2 generated upon receptor stimulation, the function of PrxII in collagen-stimulated platelets is not known. We tested the hypothesis that PrxII negatively regulates collagen-stimulated platelet activation. We analyzed PrxII-deficient murine platelets. PrxII deficiency enhanced GPVI-mediated platelet activation through the defective elimination of H2O2 and the impaired protection of SH2 domain-containing tyrosine phosphatase 2 (SHP-2) against oxidative inactivation, which resulted in increased tyrosine phosphorylation of key components for the GPVI signaling cascade, including Syk, Btk, and phospholipase Cγ2. Interestingly, PrxII-mediated antioxidative protection of SHP-2 appeared to occur in the lipid rafts. PrxII-deficient platelets exhibited increased adhesion and aggregation upon collagen stimulation. Furthermore, in vivo experiments demonstrated that PrxII deficiency facilitated platelet-dependent thrombus formation in injured carotid arteries. This study reveals that PrxII functions as a protective antioxidant enzyme against collagen-stimulated platelet activation and platelet-dependent thrombosis.

The BCR-ABL inhibitor ponatinib inhibits platelet immunoreceptor tyrosine-based activation motif (ITAM) signaling, platelet activation and aggregate formation under shear

BACKGROUND

Treatment of chronic myelogenous leukemia (CML) with the BCR-ABL tyrosine kinase inhibitor (TKI) imatinib significantly improves patient outcomes. As some patients are unresponsive to imatinib, next generation BCR-ABL inhibitors such as nilotinib have been developed to treat patients with imatinib-resistant CML. The use of some BCR-ABL inhibitors has been associated with bleeding diathesis, and these inhibitors have been shown to inhibit platelet functions, which may explain the hemostasis impairment. Surprisingly, a new TKI, ponatinib, has been associated with a high incidence of severe acute ischemic cardiovascular events. The mechanism of this unexpected adverse effect remains undefined.

OBJECTIVE AND METHODS

This study used biochemical and functional assays to evaluate whether ponatinib was different from the other BCR-ABL inhibitors with respect to platelet activation, spreading, and aggregation.

RESULTS AND CONCLUSIONS

Our results show that ponatinib, similar to other TKIs, acts as a platelet antagonist. Ponatinib inhibited platelet activation, spreading, granule secretion, and aggregation, likely through broad spectrum inhibition of platelet tyrosine kinase signaling, and also inhibited platelet aggregate formation in whole blood under shear. As our results indicate that pobatinib inhibits platelet function, the adverse cardiovascular events observed in patients taking ponatinib may be the result of the effect of ponatinib on other organs or cell types, or disease-specific processes, such as BCR-ABL+cells undergoing apoptosis in response to chemotherapy, or drug-induced adverse effects on the integrity of the vascular endothelium in ponatinib-treated patients.

Protein kinase C δ deficiency enhances megakaryopoiesis and recovery from thrombocytopenia

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<0.05). Additionally, the megakaryocyte number and DNA content were enhanced in PKCδ(-/-) mouse bone marrow after culturing with exogenous thrombopoietin compared with wild-type (P<0.05). Importantly, thrombopoietin-induced signaling was also altered with PKCδ deletion because both extracellular signal-regulated kinase and Akt308 phosphorylation were heightened in PKCδ(-/-) 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.

Src family kinases: at the forefront of platelet activation

Src family kinases (SFKs) play a central role in mediating the rapid response of platelets to vascular injury. They transmit activation signals from a diverse repertoire of platelet surface receptors, including the integrin αIIbβ3, the immunoreceptor tyrosine-based activation motif-containing collagen receptor complex GPVI-FcR γ-chain, and the von Willebrand factor receptor complex GPIb-IX-V, which are essential for thrombus growth and stability. Ligand-mediated clustering of these receptors triggers an increase in SFK activity and downstream tyrosine phosphorylation of enzymes, adaptors, and cytoskeletal proteins that collectively propagate the signal and coordinate platelet activation. A growing body of evidence has established that SFKs also contribute to Gq- and Gi-coupled receptor signaling that synergizes with primary activation signals to maximally activate platelets and render them prothrombotic. Interestingly, SFKs concomitantly activate inhibitory pathways that limit platelet activation and thrombus size. In this review, we discuss past discoveries that laid the foundation for this fundamental area of platelet signal transduction, recent progress in our understanding of the distinct and overlapping functions of SFKs in platelets, and new avenues of research into mechanisms of SFK regulation. We also highlight the thrombotic and hemostatic consequences of targeting platelet SFKs.

Reactive oxygen species play a critical role in collagen-induced platelet activation via SHP-2 oxidation

AIMS

The collagen-stimulated generation of reactive oxygen species (ROS) regulates signal transduction in platelets, although the mechanism is unclear. The major targets of ROS include protein tyrosine phosphatases (PTPs). ROS-mediated oxidation of the active cysteine site in PTPs abrogates the PTP catalytic activity. The aim of this study was to elucidate whether collagen-induced ROS generation leads to PTP oxidation, which promotes platelet stimulation.

RESULTS

SH2 domain-containing PTP-2 (SHP-2) is oxidized in platelets by ROS produced upon collagen stimulation. The oxidative inactivation of SHP-2 leads to the enhanced tyrosine phosphorylation of spleen tyrosine kinase (Syk), Vav1, and Bruton's tyrosine kinase (Btk) in the linker for the activation of T cells signaling complex, which promotes the tyrosine phosphorylation-mediated activation of phospholipase Cγ2 (PLCγ2). Moreover, we found that, relative to wild-type platelets, platelets derived from glutathione peroxidase 1 (GPx1)/catalase double-deficient mice showed enhanced cellular ROS levels, oxidative inactivation of SHP-2, and tyrosine phosphorylation of Syk, Vav1, Btk, and PLCγ2 in response to collagen, which subsequently led to increased intracellular calcium levels, degranulation, and integrin αIIbβ3 activation. Consistent with these findings, GPx1/catalase double-deficiency accelerated the thrombotic response in FeCl3-injured carotid arteries.

INNOVATION

The present study is the first to demonstrate that SHP-2 is targeted by ROS produced in collagen-stimulated platelets and suggests that a novel mechanism for the regulation of platelet activation by ROS is due to oxidative inactivation of SHP-2.

CONCLUSION

We conclude that collagen-induced ROS production leads to SHP-2 oxidation, which promotes platelet activation by upregulating tyrosine phosphorylation-based signal transduction.

Inhibiting tyrosine phosphorylation of protein kinase Cδ (PKCδ) protects the salivary gland from radiation damage

Radiation therapy for head and neck cancer can result in extensive damage to normal adjacent tissues such as the salivary gland and oral mucosa. We have shown previously that tyrosine phosphorylation at Tyr-64 and Tyr-155 activates PKCδ in response to apoptotic stimuli by facilitating its nuclear import. Here we have identified the tyrosine kinases that mediate activation of PKCδ in apoptotic cells and have explored the use of tyrosine kinase inhibitors for suppression of irradiation-induced apoptosis. We identify the damage-inducible kinase, c-Abl, as the PKCδ Tyr-155 kinase and c-Src as the Tyr-64 kinase. Depletion of c-Abl or c-Src with shRNA decreased irradiation- and etoposide-induced apoptosis, suggesting that inhibitors of these kinases may be useful therapeutically. Pretreatment with dasatinib, a broad spectrum tyrosine kinase inhibitor, blocked phosphorylation of PKCδ at both Tyr-64 and Tyr-155. Expression of "gate-keeper" mutants of c-Abl or c-Src that are active in the presence of dasatinib restored phosphorylation of PKCδ at Tyr-155 and Tyr-64, respectively. Imatinib, a c-Abl-selective inhibitor, also specifically blocked PKCδ Tyr-155 phosphorylation. Dasatinib and imatinib both blocked binding of PKCδ to importin-α and nuclear import, demonstrating that tyrosine kinase inhibitors can inhibit nuclear accumulation of PKCδ. Likewise, pretreatment with dasatinib also suppressed etoposide and radiation induced apoptosis in vitro. In vivo, pre-treatment of mice with dasatinib blocked radiation-induced apoptosis in the salivary gland by >60%. These data suggest that tyrosine kinase inhibitors may be useful prophylactically for protection of nontumor tissues in patients undergoing radiotherapy of the head and neck.

CGX1037 is a novel PKC isoform delta selective inhibitor in platelets

Platelets upon activation change their shape, aggregate and secrete alpha and dense granule contents among which ADP acts as a feedback activator. Different Protein Kinase C (PKC) isoforms have specific non-redundant roles in mediating platelet responses including secretion and thrombus formation. Murine platelets lacking specific PKC isoforms have been used to evaluate the isoform specific functions. Novel PKC isoform δ has been shown to play an important role in some pathological processes. Lack of specific inhibitors for PKCδ has restricted analysis of its role in various cells. The current study was carried out to evaluate a novel small molecule PKCδ inhibitor, CGX1037 in platelets. Platelet aggregation, dense granule secretion and western blotting experiments were performed to evaluate CGX1037. In human platelets, CGX1037 inhibited PAR4-mediated phosphorylation on PKD2, a PKCδ-specific substrate. Pre-treatment of human or murine platelets with CGX1037 inhibited PAR4-mediated dense granule secretion whereas it potentiated GPVI-mediated dense granule secretion similar to the responses observed in murine platelets lacking PKCδ· Furthermore, pre-treatment of platelets from PKCδ(-/-) mice with CGX1037 had no significant additive effect on platelet responses suggesting the specificity of CGX1037. Hence, we show that CGX1037 is a selective small molecule inhibitor of PKCδ in platelets.

Hemostatic abnormalities in sickle cell disease

PURPOSE OF REVIEW

Although it has long been recognized that sickle cell disease (SCD) and other hemoglobinopathies are associated with a state of chronic hyperactivation of coagulation, the study of the epidemiology of venous thromboembolic (VTE) complications in SCD is only now beginning to evolve. In parallel, mechanistic studies of the hypercoagulable state in humans and mouse models implicate an increasingly important causative role of hemolysis.

RECENT FINDINGS

The case for SCD as a thrombophilic state has been strengthened by the recent literature. In an attempt to better understand the underlying mechanism(s), global assays of coagulation (thromboelastography and thrombin generation assays) have been utilized by several groups, but thus far, the results have been inconsistent, probably because of the technical differences. However, global assays continue to support the case for an important role of peripheral blood cells and their derived microparticles in promoting coagulation activation.

SUMMARY

VTE is an underappreciated and potentially morbid complication of SCD. The mechanisms underlying this hypercoagulable state are complex. A greater understanding of these pathways may lead to the rational selection of therapies that not only prevent thrombosis, but also impact on many of the other vaso-occlusive complications of SCD.

Tyrosine phosphorylation on spleen tyrosine kinase (Syk) is differentially regulated in human and murine platelets by protein kinase C isoforms

Protein kinase C (PKC) isoforms differentially regulate platelet functional responses downstream of glycoprotein VI (GPVI) signaling, but the role of PKCs regulating upstream effectors such as Syk is not known. We investigated the role of PKC on Syk tyrosine phosphorylation using the pan-PKC inhibitor GF109203X (GFX). GPVI-mediated phosphorylation on Syk Tyr-323, Tyr-352, and Tyr-525/526 was rapidly dephosphorylated, but GFX treatment inhibited this dephosphorylation on Tyr-525/526 in human platelets but not in wild type murine platelets. GFX treatment did not affect tyrosine phosphorylation on FcRγ chain or Src family kinases. Phosphorylation of Lat Tyr-191 and PLCγ2 Tyr-759 was also increased upon treatment with GFX. We evaluated whether secreted ADP is required for such dephosphorylation. Exogenous addition of ADP to GFX-treated platelets did not affect tyrosine phosphorylation on Syk. FcγRIIA- or CLEC-2-mediated Syk tyrosine phosphorylation was also potentiated with GFX in human platelets. Because potentiation of Syk phosphorylation is not observed in murine platelets, PKC-deficient mice cannot be used to identify the PKC isoform regulating Syk phosphorylation. We therefore used selective inhibitors of PKC isoforms. Only PKCβ inhibition resulted in Syk hyperphosphorylation similar to that in platelets treated with GFX. This result indicates that PKCβ is the isoform responsible for Syk negative regulation in human platelets. In conclusion, we have elucidated a novel pathway of Syk regulation by PKCβ in human platelets.

New prospects for drug development: the hedgehog pathway revealed. Focus on hematologic malignancies

The hedgehog (Hh) pathway is a critical regulator of vertebrate embryonic development and is involved in the function of processes such as stem cell maintenance and differentiation, tissue polarity and cell proliferation. Given how critical these functions are, it is not surprising that mutations in Hh pathway components are often implicated in the tumorigenesis of a variety of human cancers. Promotion of tumor growth has recently been shown by activated Hh signaling in the tumor itself, as well as by pathway activation within surrounding cells comprising the tumor microenvironment. Targeted disruption of various Hh pathway proteins has been successfully employed as an anticancer strategy with several synthetic Hh antagonists now available. Here, the molecular basis of Hh signaling, the therapeutic rationales for targeting this pathway and the current status of Hh pathway inhibitors in the clinic are reviewed.

Activation/Inhibition of mast cells by supra-optimal antigen concentrations

Mast cells (MCs) are tissue resident cells of hemopoietic origin and are critically involved in allergic diseases. MCs bind IgE by means of their high-affinity receptor for IgE (FcεRI). The FcεRI belongs to a family of multi-chain immune recognition receptors and is activated by cross-linking in response to multivalent antigens (Ags)/allergens. Activation of the FcεRI results in immediate release of preformed granular substances (e.g. histamine, heparin, and proteases), generation of arachidonic acid metabolites, and production of pro-inflammatory cytokines. The FcεRI shows a remarkable, bell-shaped dose-response behavior with weak induction of effector responses at both low and high (so-called supra-optimal) Ag concentrations. This is significantly different from many other receptors, which reach a plateau phase in response to high ligand concentrations. To explain this unusual dose-response behavior of the FcεRI, scientists in the past have drawn parallels to so-called precipitin curves resulting from titration of Ag against a fixed concentration of antibody (Ab) in solution (a.k.a. Heidelberger curves). Thus, for high, supra-optimal Ag concentrations one could assume that every IgE-bound FcεRI formed a monovalent complex with “its own Ag”, thus resulting in marginal induction of effector functions due to absence of receptor cross-linking. However, this was never proven to be the case. More recently, careful studies of FcεRI activation and signaling events in MCs in response to supra-optimal Ag concentrations have suggested a molecular explanation for the descending part of this bell-shaped curve. It is obvious now that extensive FcεRI/IgE/Ag clusters are formed and inhibitory molecules and signalosomes are engaged in response to supra-optimal cross-linking (amongst them the Src family kinase Lyn and the inositol-5′-phosphatase SHIP1) and they actively down-regulate MC effector responses. Thus, the analysis of MC signaling triggered by supra-optimal crosslinking holds great potential for identifying novel targets for pharmacologic therapeutic intervention to benefit patients with acute and chronic allergic diseases.

Polymorphisms of protein tyrosine phosphatase CD148 influence FcγRIIA-dependent platelet activation and the risk of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is due primarily to IgG antibodies specific to platelet factor 4/heparin complexes (PF4/Hs) that activate platelets via FcγRIIA. CD148 is a protein tyrosine phosphatase that regulates Src kinases and collagen-induced platelet activation. Three polymorphisms affecting CD148 (Q276P, R326Q, and D872E) were studied in HIT patients and 2 control groups, with or without antibodies to PF4/Hs. Heterozygote status for CD148 276P or 326Q alleles was less frequent in HIT patients, suggesting a protective effect of these polymorphisms. Aggregation tests performed with collagen, HIT plasma, and monoclonal antibodies cross-linking FcγRIIA showed consistent hyporesponsiveness of platelets expressing the 276P/326Q alleles. In addition, platelets expressing the 276P/326Q alleles exhibited a greater sensitivity to the Src family kinases inhibitor dasatinib in response to collagen or ALB6 cross-linking FcγRIIA receptors. Moreover, the activatory phosphorylation of Src family kinases was considerably delayed as well as the phosphorylation of Linker for activation of T cells and phospholipase Cγ2, 2 major signaling proteins downstream from FcγRIIA. In conclusion, this study shows that CD148 polymorphisms affect platelet activation and probably exert a protec-tive effect on the risk of HIT in patients with antibodies to PF4/Hs.

Pharmacological targeting of phosphoinositide lipid kinases and phosphatases in the immune system: success, disappointment, and new opportunities

The predominant expression of the γ and δ isoforms of PI3K in cells of hematopoietic lineage prompted speculation that inhibitors of these isoforms could offer opportunities for selective targeting of PI3K in the immune system in a range of immune-related pathologies. While there has been some success in developing PI3Kδ inhibitors, progress in developing selective inhibitors of PI3Kγ has been rather disappointing. This has prompted the search for alternative targets with which to modulate PI3K signaling specifically in the immune system. One such target is the SH2 domain-containing inositol-5-phosphatase-1 (SHIP-1) which de-phosphorylates PI(3,4,5)P₃ at the D5 position of the inositol ring to create PI(3,4)P₂. In this article, we first describe the current state of PI3K isoform-selective inhibitor development. We then focus on the structure of SHIP-1 and its function in the immune system. Finally, we consider the current state of development of small molecule compounds that potently and selectively modulate SHIP activity and which offer novel opportunities to manipulate PI3K mediated signaling in the immune system.

Distinct and overlapping functional roles of Src family kinases in mouse platelets

BACKGROUND AND OBJECTIVES

Src family kinases (SFKs) play a critical role in initiating and propagating signals in platelets. The aims of this study were to quantitate SFK members present in platelets and to analyze their contribution to platelet regulation using glycoprotein VI (GPVI) and intregrin αIIbβ3, and in vivo.

METHODS AND RESULTS

Mouse platelets express four SFKs, Fgr, Fyn, Lyn and Src, with Lyn expressed at a considerably higher level than the others. Using mutant mouse models, we demonstrate that platelet activation by collagen-related peptide (CRP) is delayed and then potentiated in the absence of Lyn, but only marginally reduced in the absence of Fyn or Fgr, and unaltered in the absence of Src. Compound deletions of Lyn/Src or Fyn/Lyn, but not of Fyn/Src or Fgr/Lyn, exhibit a greater delay in activation relative to Lyn-deficient platelets. Fibrinogen-adherent platelets show reduced spreading in the absence of Src, potentiation in the absence of Lyn, but no change in the absence of Fyn or Fgr. In mice double-deficient in Lyn/Src or Fgr/Lyn, the inhibitory role of Lyn on spreading on fibrinogen is lost. Lyn is the major SFK-mediating platelet aggregation on collagen at arterial shear and its absence leads to a reduction in thrombus size in a laser injury model.

CONCLUSION

These results demonstrate that SFKs share individual and overlapping roles in regulating platelet activation, with Lyn having a dual role in regulating GPVI signaling and an inhibitory role downstream of αIIbβ3, which requires prior signaling through Src.

CD2AP/SHIP1 complex positively regulates plasmacytoid dendritic cell receptor signaling by inhibiting the E3 ubiquitin ligase Cbl

The human plasmacytoid dendritic cell (pDC) receptor BDCA2 forms a complex with the adaptor FcεR1γ to activate an ITAM-signaling cascade. BDCA2 receptor signaling negatively regulates the TLR7/9-mediated type 1 IFN responses in pDCs, which may play a key role in controlling self-DNA/RNA-induced autoimmunity. We report in this article that CD2-associated adaptor protein (CD2AP), which is highly expressed in human pDCs, positively regulates BDCA2/FcεR1γ receptor signaling. By immunoprecipitation and mass spectrometry analyses, we found that CD2AP bound to SHIP1. Knockdown of CD2AP or SHIP1 reduced the BDCA2/FcεR1γ-mediated ITAM signaling and blocked its inhibition of TLR9-mediated type 1 IFN production. Knockdown of CD2AP or SHIP1 also enhanced the ubiquitination and degradation of Syk and FcεR1γ that was mediated by the E3 ubiquitin ligase Cbl. This led us to discover that, upon BDCA2 cross-linking, the CD2AP/SHIP1 complex associated with Cbl and inhibited its E3 ubiquitin ligase activity. In human primary pDCs, cross-linking of the BDCA2/FcεR1γ complex induced the recruitment of the CD2AP/SHIP1/Cbl complex to the plasma membrane of pDCs, where it colocalized with the BDCA2/FcεR1γ complex. Therefore, CD2AP positively regulates BDCA2/FcεR1γ signaling by forming a complex with SHIP1 to inhibit the E3 ubiquitin ligase Cbl.

Overexpression of the autoimmunity-associated phosphatase PTPN22 promotes survival of antigen-stimulated CLL cells by selectively activating AKT

A polymorphic variant of the phosphatase PTPN22 has been associated with increased risk for multiple autoimmune diseases. The risk allele is thought to function by diminishing antigen-receptor signals responsible for negative selection of autoreactive lymphocytes. We now show that PTPN22 is markedly overexpressed in chronic lymphocytic leukemia (CLL), a common malignancy of autoreactive B lymphocytes. We also show that overexpression of PTPN22 significantly inhibits antigen-induced apoptosis of primary CLL cells by blocking B-cell receptor (BCR) signaling pathways that negatively regulate lymphocyte survival. More importantly, we show that PTPN22 positively regulates the antiapoptotic AKT kinase, which provides a powerful survival signal to antigen-stimulated CLL cells. This selective uncoupling of AKT from other downstream BCR signaling pathways is a result of inhibition of a negative regulatory circuit involving LYN, CD22, and SHIP. Finally, we show that PTPN22 can be effectively down-regulated by the PKC inhibitors ruboxistaurin and sotrastaurin, resulting in enhanced killing of CLL cells exposed to proapoptotic BCR stimuli. Collectively, these data suggest that PTPN22 overexpression represents a protective mechanism that allows autoantigen-activated CLL cells to escape from negative selection and indicate that this mechanism could be exploited for therapeutic purposes by targeting PTPN22 with PKC inhibitors.

Intracellular signaling as a potential target for antiplatelet therapy

Three classes of inhibitors of platelet aggregation have demonstrated substantial clinical benfits. Aspirin acts by irreversibly inhibiting COX-1 and therefore blocking the synthesis of proaggregatory thromboxane A (2) (TxA(2)). The indirect acting (ticlopidine, clopidogrel, prasugrel) and the direct acting (ticagrelor) antagonists of P2Y(12) block the thrombus stabilizing activity of ADP. Parenteral GP IIb-IIIa inhibitors directly block platelet-platelet interactions. Despite well-established benefits, all antiplatelet agents have important limitations: increased bleeding and gastrointestinal toxicities (aspirin), high incidence of thrombotic thrombocytopenic purpura (ticlopidine), potentially nonresponders (clopidogrel), severe bleeding (prasugrel, GP IIb-IIIa antagonists) and "complicated" relationships with aspirin ticagrelor). In this chapter, we present the genetic and pharmacological evidence that supports the development and expectations associated with novel antiplatelet strategies directed at intrasignaling pathways.

Platelet activation in hypertension associated with hypercholesterolemia: effects of irbesartan

AIM

The aim of this study was to determine the effect of simultaneous hypertension and hypercholesterolemia on platelet activation, nitric oxide (NO) production and oxidative stress, and to evaluate the role of irbesartan, an angiotensin II type 1 receptor antagonist.

METHODS

Golden Syrian hamsters were divided into three groups: controls, C (fed a standard diet); hypertensive-hypercholesterolemic, HH (fed a diet enriched in 3% cholesterol, 15% butter and 8% NaCl, for 4 months); and hypertensive-hypercholesterolemic treated with irbesartan, HHI (fed as HH group, plus irbesartan 10 mg kg(-1) per day, for 4 months).

RESULTS

Compared with the C group, platelets isolated from the HH group showed: morphological modifications; increased integrin β3 exposure and protein expression of P-selectin, FAK, PI3K, Akt and Src; reduced eNOS protein expression and NO production; higher generation of ROS, mostly produced by NADPH-oxidase, cyclooxygenase-1 (COX-1) and 12-lipoxygenase; and enhanced NAD(P)H oxidase activity and protein expression of gp91phox and p22phox subunits, 12-lipoxygenase, COX-1, cPLA(2) and PKC. Compared with the HH group, the treatment with irbesartan (HHI group) significantly attenuates the changes in all the molecules tested, reduces platelet aggregation, and improves intraplatelet redox balance.

CONCLUSIONS

Experimental hypertension associated with hypercholesterolemia produces major changes in morphology, signaling mechanisms and oxidative stress in blood platelets. These changes were significantly diminished by irbesartan administration, which functions as an antioxidant on platelets.

Inhibitor and activator: dual functions for SHIP in immunity and cancer

SHIP1 is at the nexus of intracellular signaling pathways in immune cells that mediate bone marrow (BM) graft rejection, production of inflammatory and immunosuppressive cytokines, immunoregulatory cell formation, the BM niche that supports development of the immune system, and immune cancers. This review summarizes how SHIP participates in normal immune physiology or the pathologies that result when SHIP is mutated. This review also proposes that SHIP can have either inhibitory or activating roles in cell signaling that are determined by whether signaling pathways distal to PI3K are promoted by SHIP's substrate (PI(3,4,5)P(3) ) or its product (PI(3,4)P(2) ). This review also proposes the "two PIP hypothesis" that postulates that both SHIP's product and its substrate are necessary for a cancer cell to achieve and sustain a malignant state. Finally, due to the recent discovery of small molecule antagonists and agonists for SHIP, this review discusses potential therapeutic settings where chemical modulation of SHIP might be of benefit.

Phosphorylation mechanisms in intensive care medicine

INTRODUCTION

The phosphorylation states of proteins, lipids, carbohydrates, amino acids, and nucleotides control the mechanisms behind nearly all cellular functions. Therefore, not surprisingly, recent findings have shown that alterations in these phosphorylation pathways play a central role in the development and progression of many disease states. This review provides a brief summary of the function and activity of various phosphorylation mechanisms, outlines some of the major phosphorylation signaling cascades, and describes the role of these phosphorylation mechanisms in intensive care medicine.

METHODS

This article will comprise a comprehensive review of the literature in the context of intensive care medicine. Specifically, we will discuss the involvement of phosphorylation in the pathogenesis, diagnosis, and treatment of heart failure, myocardial infarction, stroke, respiratory failure, ventilation-induced lung injury, traumatic brain injury, acute organ failure, systemic sepsis, and shock.

CONCLUSION

Phosphorylation mechanisms clearly play an important role in many pathologies and treatment strategies of intensive care and therefore further understanding of these mechanisms may lead to the development of novel therapies and improved patient care.

Tyrosine phosphorylation of SHIP promotes its proteasomal degradation

OBJECTIVE

The activity of the SH2-containing-phosphatidylinositol-5'-phosphatase (SHIP, also known as SHIP1), a critical hematopoietic-restricted negative regulator of the PI3 kinase (PI3K) pathway, is regulated in large part via its protein levels. We sought to determine the mechanism(s) involved in its downregulation by BCR-ABL and by interleukin (IL)-4.

MATERIALS AND METHODS

We used Ba/F3(p210-tetOFF) cells to study the downregulation of SHIP by BCR-ABL and bone marrow-derived macrophages to study SHIP's downregulation by IL-4.

RESULTS

We show herein that BCR-ABL downregulates SHIP, but not SHIP2 or PTEN, and this can be blocked with the Src kinase inhibitor PP2, which inhibits the tyrosine phosphorylation of SHIP, or with the proteasomal inhibitor MG-132. We also show, using anti-SHIP immunoprecipitates, that c-Cbl and Cbl-b are associated with SHIP and that BCR-ABL induces SHIP's polyubiquitination. This ubiquitination can be blocked with PP2, consistent with the tyrosine phosphorylation of SHIP acting as a signal for its ubiquitination. In bone marrow-derived macrophages, IL-4 also leads to the proteasomal degradation of SHIP but, unlike in Ba/F3(p210-tetOFF) cells, SHIP2 is also proteasomally degraded and the degradation of both inositol phosphatases can be prevented with PP2 or MG-132.

CONCLUSION

Our results suggest that SHIP protein levels can be reduced via BCR-ABL and/or Src family member-induced tyrosine phosphorylation of SHIP because this triggers its polyubiquitination and degradation within the proteasome.

Diverse functions of protein kinase C isoforms in platelet activation and thrombus formation

Platelet activation is a complex balance of positive and negative signaling pathways. The protein kinase C (PKC) family is a major regulator of platelet granule secretion, integrin activation, aggregation, spreading and procoagulant activity. As broad-spectrum PKC inhibitors reduce secretion and aggregation, the PKC family is generally considered to be a positive regulator of platelet activation. However, the individual members of the PKC family that are expressed in platelets are regulated in different ways, and an increasing body of evidence indicates that they have distinct, and often opposing, roles. Many of the recent advances in understanding the contributions of individual PKC isoforms have come from mouse gene knockout studies. PKCalpha, a classic isoform, is an essential positive regulator of granule secretion and thrombus formation, both in vitro and in vivo. Mice lacking PKCalpha show much reduced thrombus formation in vivo but do not have a bleeding defect, suggesting that PKCalpha could be an attractive antithrombotic target. Important, apparently non-redundant, roles, both positive and negative, for the novel PKC isoforms delta, theta and epsilon in granule secretion have also been proposed, indicating highly complex regulation of this essential process. Similarly, PKCbeta, PKCdelta and PKCtheta have non-redundant roles in platelet spreading, as absence of either PKCbeta or PKCtheta reduces spreading, whereas PKCdelta negatively regulates filopodial formation. This negative signaling by PKCdelta may reduce platelet aggregation and so restrict thrombus formation. In this review, we discuss the current understanding of the regulation and functions of individual PKC isoforms in platelet activation and thrombus formation.

Integrin alphaMbeta2 clustering triggers phosphorylation and activation of protein kinase C delta that regulates transcription factor Foxp1 expression in monocytes

Integrins are type I membrane and heterodimeric (alphabeta) cell adhesion receptors. Intracellular signals triggered by ligand-bound integrins are important for cell growth, differentiation, and migration. Integrin alpha(M)beta(2) plays key roles in myeloid cell adhesion, phagocytosis, and degranulation. In this study, we show that protein kinase C (PKC) delta is involved in alpha(M)beta(2) signaling. In human monocytic U937 cells and peripheral blood monocytes, alpha(M)beta(2) clustering induced PKCdelta translocation to the plasma membrane, followed by Tyr(311) phosphorylation and activation of PKCdelta by the src family kinases Hck and Lyn. Interestingly, alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation was not mediated by the tyrosine kinase Syk, which is a well reported kinase in beta(2) integrin signaling. Analysis of the beta(2) cytoplasmic tail showed that the sequence Asn(727)-Ser(734) is important in alpha(M)beta(2)-induced PKCdelta Tyr(311) phosphorylation. It has been shown that alpha(M)beta(2) clustering regulates the expression the transcription factor Foxp1 that has a role in monocyte differentiation. We show that Foxp1 expression was reduced in monocytes that were allowed to adhere to human microvascular endothelial cells. However, the expression of Foxp1 was not affected in monocytes that were treated with PKCdelta-targeting small interfering RNA, suggesting that PKCdelta regulates Foxp1 expression. These results demonstrate a role of PKCdelta in alpha(M)beta(2)-mediated Foxp1 regulation in monocytes.

Lyn and PKCδ order SHIP1 embargo

In this issue of Blood, Chari and colleagues provide a novel mechanism for the unique negative regulatory role of PKCδ in platelet dense granule release downstream of collagen signaling.