Inhibition of agonist-induced p42 and p38 mitogen-activated protein kinase phosphorylation and CD40 ligand/P-selectin expression by cyclic nucleotide-regulated pathways in human platelets

Arterial Baroreflex Dysfunction Promotes Neuroinflammation by Activating the Platelet CD40L/Nuclear Factor Kappa B Signaling Pathway in Microglia and Astrocytes

Arterial baroreflex (ABR) dysfunction has previously been associated with neuroinflammation, the most common pathological feature of neurological disorders. However, the mechanisms mediating ABR dysfunction-induced neuroinflammation are not fully understood. In the present study, we investigated the role of platelet CD40 ligand (CD40L) in neuroinflammation in an in vivo model of ABR dysfunction, and microglia and astrocyte activation in vitro. ABR dysfunction was induced in Sprague‒Dawley rats by sinoaortic denervation (SAD). We used ELSA and immunofluorescence to assess the effect of platelet CD40L on glial cell polarization and the secretion of inflammatory factors. By flow cytometry, we found that rats subjected to SAD showed a high level of platelet microaggregation and upregulation of CD40L on the platelet surface. The promotion of platelet invasion and accumulation was also observed in the brain tissues of rats subjected to SAD. In the animal model and cultured N9 microglia/C6 astrocytoma cells, platelet CD40L overexpression promoted neuroinflammation and activated M1 microglia, A1 astrocytes, and the nuclear factor kappa B (NFκB) signaling pathway. These effects were partially blocked by inhibiting platelet activity with clopidogrel or inhibiting CD40L-mediated signaling. Our results suggest that during ABR dysfunction, CD40L signaling in platelets converts microglia to the M1 phenotype and astrocytes to the A1 phenotype, activating NFκB and resulting in neuroinflammation. Thus, our study provides a novel understanding of the pathogenesis of ABR dysfunction-induced neuroinflammation and indicates that targeting platelet CD40L is beneficial for treating central nervous system (CNS) disorders associated with ABR dysfunction.

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

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

The involvement of toll-like receptors 2 and 4 in human platelet signalling pathways

In addition to haemostasis, platelets play an essential role in mechanisms of inflammation and in immunological reactions. Platelets express various toll-like receptors (TLR) on their surface, among them TLR2 and TLR4, which are important for the recognition of bacterial patterns. This study compared TLR2- and TLR4-dependent platelet signalling and their effect on platelet function. Platelet-rich-plasma and washed platelets were prepared from peripheral blood samples of healthy donors. Pam3CSK4 or LPS (lipopolysaccharides from Escherichia coli) were used for stimulation of TLR2 and TLR4. Intracellular signalling pathways were investigated by Western blot. TLR2- and TLR4-mediated specific transcription factor DNA binding activity was measured by the nuclear factor kappa B (NFκB) transcription factor assay kit. Platelet adhesion and glycoprotein Ib function were assessed by immunofluorescence staining and analysis of ristocetin-induced agglutination. Both, Pam3CSK4 and LPS were able to induce NFκB-mediated and classical activating platelet signalling with a higher stimulatory capacity of TLR2. In addition, TLR2 and TLR4 activation led to a similar activation of inhibitory pathways. In contrast to TLR2, stimulation of TLR4 resulted in decreased Akt/protein kinase B phosphorylation conditioned by enhanced protein phosphatase 2A activity. TLR4-mediated signalling induced platelet adhesion and facilitated ristocetin-induced platelet agglutination. In conclusion, Pam3CSK4 directly induces aggregation via classical activation cascades, whereas LPS enhances platelet adhesion and glycoprotein receptor Ib-dependent platelet agglutination.

The Immune Nature of Platelets Revisited

Platelets are the primary cellular mediators of hemostasis and this function firmly acquaints them with a variety of inflammatory processes. For example, platelets can act as circulating sentinels by expressing Toll-like receptors (TLR) that bind pathogens and this allows platelets to effectively kill them or present them to cells of the immune system. Furthermore, activated platelets secrete and express many pro- and anti-inflammatory molecules that attract and capture circulating leukocytes and direct them to inflamed tissues. In addition, platelets can directly influence adaptive immune responses via secretion of, for example, CD40 and CD40L molecules. Platelets are also the source of most of the microvesicles in the circulation and these miniscule elements further enhance the platelet’s ability to communicate with the immune system. More recently, it has been demonstrated that platelets and their parent cells, the megakaryocytes (MK), can also uptake, process and present both foreign and self-antigens to CD8+ T-cells conferring on them the ability to directly alter adaptive immune responses. This review will highlight several of the non-hemostatic attributes of platelets that clearly and rightfully place them as integral players in immune reactions.

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Platelets can act as circulating sentinels by expressing pathogen-associated molecular pattern receptors that bind pathogens and induce their killing and elimination.

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Activated platelets secrete and express a multitude of pro- and anti-inflammatory molecules that attract and capture circulating leukocytes and direct them to inflamed tissues.

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Platelets express and secrete many critical immunoregulatory molecules that significantly affect both innate and adaptive immune responses.

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Platelets are the primary source of microparticles in the circulation and these augment the platelet’s ability to communicate with the immune system.

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Platelets and megakaryocytes can act as antigen presenting cells and present both foreign- and self-peptides to T-cells.

Platelet CD40 ligand and bleeding during P2Y12 inhibitor treatment in acute coronary syndrome

Antiplatelet therapy through inhibition of the adenosine diphosphate (ADP)/P2Y12 pathway is commonly used in the treatment of acute coronary syndrome (ACS). Although efficient in preventing platelet activation and thrombus formation, it increases the risk of bleeding complications. In patients with ACS receiving platelet aggregation inhibitors, that is, P2Y12 blockers (n = 923), we investigated the relationship between plasma and platelet-associated CD40L levels and bleeding events (n = 71). Treatment with P2Y12 inhibitors in patients with ACS did not affect plasma-soluble CD40L levels, but decreased platelet CD40L surface expression (pCD40L) and platelet-released CD40L (rCD40L) levels in response to stimulation as compared to healthy controls. In vitro inhibition of the ADP pathway in healthy control platelets reduced both pCD40L and rCD40L levels. In a multivariable analysis, the reduced pCD40L level observed in ACS patients was significantly associated with the risk of bleeding occurrence (adjusted odds ratio = 0.15; 95% confidence interval = 0.034-0.67). P2Y12 inhibitor-treated (ticagrelor) mice exhibited a 2.5-fold increase in tail bleeding duration compared with controls. A significant reduction in bleeding duration was observed on CD40L+/+ but not CD40L-/- platelet infusion. In addition, CD40L blockade in P2Y12 inhibitor-treated blood samples from a healthy human reduced thrombus growth over immobilized collagen under arterial flow. In conclusion, measurement of pCD40L may offer a novel approach to assessing bleeding risk in patients with ACS who are being treated with P2Y12 inhibitors.

The role of proteasome activity for activating and inhibitory signalling in human platelets

Platelets express key proteins of the proteasome system, but its functional role in the regulation of platelet integrity, however, is not fully understood yet. Therefore, this study evaluated activating and inhibitory platelet signalling pathways using the potent and selective proteasome inhibitor bortezomib. In washed platelets, the effect of bortezomib on viability and on aggregation was assessed. In addition, fibrinogen binding and CD62P expression were determined. The influence on activating and inhibitory signalling was detected by phosphorylation levels of essential messenger molecules. Platelet viability was maintained after incubation with 0.01 μM to 1 μM bortezomib, but tampered with 100 μM bortezomib. Agonist-induced aggregation was only reduced under 100 μM bortezomib and with weak induction by 10 μM adenosine diphosphate. Similarly, phosphorylated kinase levels of the activating signalling pathways were not affected by 0.01 μM to 1 μM bortezomib. In contrast, proteasome inhibition resulted in the reduction of inhibitor-induced vasodilator-stimulated phosphoprotein phosphorylation, accompanied with the partial decrease of induced inhibition of fibrinogen binding and CD62P expression. In conclusion, platelet activation and aggregation are not dependent on proteasome activity. Instead, inhibitory signalling is partially attenuated under proteasome inhibition. Supramaximal inhibitory concentrations of bortezomib (above 1 μM) lead to heterogeneous effects on activating or inhibitory systems, probably caused by decreasing platelet viability.

Cyclic nucleotide-dependent inhibitory signaling interweaves with activating pathways to determine platelet responses

Platelets are regulated by extracellular cues that impact on intracellular signaling. The endothelium releases prostacyclin and nitric oxide which stimulate the synthesis of cyclic nucleotides cAMP and cGMP leading to platelet inhibition. Other inhibitory mechanisms involve immunoreceptor tyrosine-based inhibition motif-containing receptors, intracellular receptors and receptor desensitization. Inhibitory cyclic nucleotide pathways are traditionally thought to represent a passive background system keeping platelets in a quiescent state. In contrast, cyclic nucleotides are increasingly seen to be dynamically involved in most aspects of platelet regulation. This review focuses on crosstalk between activating and cyclic nucleotide-mediated inhibitory pathways highlighting emerging new hub structures and signaling mechanisms. In particular, interactions of plasma membrane receptors like P2Y12 and GPIb/IX/V with the cyclic nucleotide system are described. Furthermore, differential regulation of the RGS18 complex, second messengers, protein kinases, and phosphatases are presented, and control over small G-proteins by guanine-nucleotide exchange factors and GTPase-activating proteins are outlined. Possible clinical implications of signaling crosstalk are discussed.

The role of agonist-induced activation and inhibition for the regulation of purinergic receptor expression in human platelets

INTRODUCTION

Adenosine diphosphate (ADP) as physiological activator of human platelets mediates its effects via three purinergic receptors: P2Y1, P2Y12 and P2X1. The inhibition of P2Y12 is used pharmacologically to suppress aggregation underlining the physiological significance of this receptor. Since the regulation of purinergic receptor expression has not thoroughly been investigated yet, this study analyzed the content of purinergic receptors on the platelet surface membrane upon activation and inhibition.

MATERIALS AND METHODS

The surface expression of purinergic receptors was measured by flow cytometry using two different polyclonal antibodies as basal values and after incubation with thrombin receptor activating peptide (TRAP-6) or with inhibitors DEA/NO, MAHMA/NO or Prostaglandin E1 (PGE1). Western blot analysis was used to confirm inhibitory effects.

RESULTS

Both investigated antibodies revealed a significant increase of purinergic receptor expression upon TRAP-6 stimulation. The NO donors, DEA/NO and MAHMA/NO, did not influence basal or TRAP-6 stimulated values. PGE1 did not affect basal receptor expression, but diminished TRAP-6 stimulated purinergic receptor expression in a dose-dependent manner.

CONCLUSIONS

In summary, TRAP-6 induced platelet activation leads to an elevation of purinergic receptor expression. In contrast to other surface ligands, this effect is not suppressed by cGMP-mediated inhibition, but almost completely abrogated by enhanced cAMP-mediated signaling as induced by PGE1.

Cd40 Activation as Potential Tool in Malignant Neoplasms

Background

CD40, a cell surface molecule, is expressed on B-cell malignancies and many different solid tumors. It is capable of mediating diverse biological phenomena such as the induction of apoptosis in tumors and stimulation of the immune response. It has thus been studied as a possible target for antitumor therapy. The general aim of this review is to focus the attention of clinical oncologists on the involvement of CD40 in tumors and the rationale of CD40-activation-based therapies in new, biologically oriented antitumor protocols.

Methods

A Medline review of published papers about the role of CD40 activation in cancer therapy.

Results

Many authors have shown that CD40 activation promotes apoptotic death of tumor cells and that the presence of the molecule on the surface of carcinoma lines is an important factor in the generation of tumor-specific T-cell responses that contribute to tumor cell elimination. The CD40 ligand (CD40L) is the natural ligand for CD40; it is expressed primarily on the surface of activated T lymphocytes. Preclinical studies suggest that CD40-CD40L interaction could be useful for cytotoxicity against CD40-expressing tumors and for immune stimulation. Tumor inhibition was observed when tumor cells were treated with agonistic anti-CD40 monoclonal antibodies or with the soluble form of CD40L. The results of the first phase I clinical trial to treat cancer patients with subcutaneous injection of recombinant human CD40L have been recently reported. Immunohistochemical studies have revealed that detection of CD40 in primary cutaneous malignant melanoma and lung cancer may have a negative prognostic value. Interestingly, up-regulation of CD40 was observed in the tumor vessels of renal carcinomas and Kaposi's sarcoma, suggesting possible involvement of CD40 in tumor angiogenesis. Recently, it has also been shown that CD40 engagement on endothelial cells induces in vitro tubule formation and expression of matrix metalloproteinases, two processes involved in the neovascularization and progression of tumors.

Conclusions

CD40 activation represents an exciting target for hematological malignancies and solid tumors expressing the molecule, but its functional role in cancer development still remains unclear and controversial.

Fractalkine – a local inflammatory marker aggravating platelet activation at the vulnerable plaque

Chemokines play an important role in inducing chemotaxis of cells, piloting white blood cells in immune surveillance and are crucial parts in the development and progression of atherosclerosis. Platelets are mandatory players in the initiation of atherosclerotic lesion formation and are susceptible targets for and producers of chemokines. Several chemokine receptors on platelets have been described previously, amongst them CX3CR1, the receptor for fractalkine. The unique chemokine fractalkine (CX3CL1, FKN) exists as a soluble as well as a membrane-anchored glycoprotein. Its essential role in the formation of atherosclerotic lesions and atherosclerosis progression has been impressively described in mouse models. Moreover, fractalkine induces platelet activation and adhesion via a functional fractalkine receptor (CX3CR1) expressed on the platelet surface. Platelet activation via the FKN/CX3CR1-axis triggers leukocyte adhesion to activated endothelium, and fractalkine-induced platelet P-selectin is mandatory for leukocyte recruitment under arterial flow conditions. This review summarises the role of fractalkine as a potential local inflammatory mediator which influences platelet activation in the setting of atherosclerosis. Beyond that, aspects of a potential interaction between fractalkine and platelet responsiveness to antiplatelet drugs are described. Furthermore, the possible impact of high-density lipoprotein cholesterol (HDL-C) on atherosclerosis progression, platelet activation and fractalkine signalling are discussed.

Effect of diabetic duration on hemorheological properties and platelet aggregation in streptozotocin-induced diabetic rats

Diabetes mellitus with abnormal glucose concentration is associated with changes in hemorheological properties, endothelial function, and platelets hyperactivity. Disturbances may significantly be responsible for diabetes-related vascular complications. In this study, hemorheological and hemodynamic properties were measured according to diabetic duration after streptozotocin treatment in rats. For ex vivo measurements, an extracorporeal model was adopted. Flow rate and blood viscosity were measured using a microfluidic device. Erythrocyte aggregation and morphological parameters of erythrocytes were measured by modified erythrocyte sedimentation rate and the phase-contrast holography under in vitro conditions. The platelet aggregation and mean pressure in the femoral artery were estimated under ex vivo conditions. Hemorheological properties including blood viscosity, erythrocyte aggregation and shape parameters for the control group are significantly different with those for diabetic groups. The changes with respect to diabetic duration were relatively unnoticeable. However, the platelet aggregation is strongly dependent on the diabetic duration. Based on these results, hyperglycemia exposure may induce hemorheological variations in early stages of diabetes mellitus. High platelet aggregation may become more pronounced according to the diabetic duration caused by variations in hemorheological properties resulting in endothelial dysfunction. This study would be helpful in understanding the effects of diabetic duration on biophysical properties.

Extracellular ATP metabolism on vascular endothelial cells: A pathway with pro-thrombotic and anti-thrombotic molecules

Vascular endothelial contributes to the metabolism and interconversion of extracellular adenine nucleotides via ecto-ATPase/ADPase (CD39) and ecto-5'nucleotidase (CD73) activities. These enzymes collectively dephosphorylate ATP, ADP, and AMP with the production of additional adenosine. In the vascular system, adenine nucleotides (ATP and ADP) and nucleoside adenosine represent an important class of extracellular molecules involved in modulating the processes linked to vascular thrombosis exerting various effects in platelets. Yet, the mechanisms by which the extracellular ATP metabolism in the local environment trigger pro-thrombotic and anti-thrombotic states are yet to be fully elucidated. In this article, the relative contribution of extracellular ATP metabolism in platelet regulation is explored.

Mechanisms of Nifedipine-Downregulated CD40L/sCD40L Signaling in Collagen Stimulated Human Platelets

The platelet-derived soluble CD40L (sCD40L) release plays a critical role in the development of atherosclerosis. Nifedipine, a dihydropyridine-based L-type calcium channel blocker (CCB), has been reported to have an anti-atherosclerotic effect beyond its blood pressure-lowering effect, but the molecular mechanisms remain unclear. The present study was designed to investigate whether nifedipine affects sCD40L release from collagen-stimulated human platelets and to determine the potential role of peroxisome proliferator-activated receptor-β/-γ (PPAR-β/-γ). We found that treatment with nifedipine significantly inhibited the platelet surface CD40L expression and sCD40L release in response to collagen, while the inhibition was markedly reversed by blocking PPAR-β/-γ activity with specific antagonist such as GSK0660 and GW9662. Meanwhile, nifedipine also enhanced nitric oxide (NO) and cyclic GMP formation in a PPAR-β/-γ-dependent manner. When the NO/cyclic GMP pathway was suppressed, nifedipine-mediated inhibition of sCD40L release was abolished significantly. Collagen-induced phosphorylation of p38MAPK, ERK1/2 and HSP27, matrix metalloproteinase-2 (MMP-2) expression/activity and reactive oxygen species (ROS) formation were significantly inhibited by nifedipine, whereas these alterations were all attenuated by co-treatment with PPAR-β/-γ antagonists. Collectively, these results demonstrate that PPAR-β/-γ-dependent pathways contribute to nifedipine-mediated downregulation of CD40L/sCD40L signaling in activated platelets through regulation of NO/ p38MAPK/ERK1/2/HSP27/MMP-2 signalings and provide a novel mechanism regarding the anti-atherosclerotic effect of nifedipine.

Regulatory mechanisms of cAMP levels as a multiple target for antiplatelet activity and less bleeding risk

Platelet activation is a critical component of atherothrombosis. The multiple pathways of platelet activation limit the effect of specific receptor/pathway inhibitors, resulting in limited clinical efficacy. Recent research has confirmed that combination therapy results in enhanced antithrombotic efficacy without increasing bleeding risk. In this way, the best-known inhibitor and turn off signaling in platelet activation is cAMP. In this article we discuss the mechanisms of regulation of intraplatelet cAMP levels, a) platelet-dependent pathway: Gi/Gs protein-coupled receptors, phosphodiesterase inhibition and activation of PPARs and b) platelet-independent pathway: inhibition of adenosine uptake by erythrocytes. With respect to the association between intraplatelet cAMP levels and bleeding risk it is possible to establish that compounds/drugs with pleitropic effect for increased intraplatelet cAMP level could have an antithrombotic activity with less risk of bleeding.

Relationship between Platelet PPARs, cAMP Levels, and P-Selectin Expression: Antiplatelet Activity of Natural Products

Platelets are no longer considered simply as cells participating in thrombosis. In atherosclerosis, platelets are regulators of multiple processes, with the recruitment of inflammatory cells towards the lesion sites, inflammatory mediators release, and regulation of endothelial function. The antiplatelet therapy has been used for a long time in an effort to prevent and treat cardiovascular diseases. However, limited efficacy in some patients, drug resistance, and side effects are limitations of current antiplatelet therapy. In this context, a large number of natural products (polyphenols, terpenoids, alkaloids, and fatty acids) have been reported with antiplatelet activity. In this sense, the present paper describes mechanisms of antiplatelet action of natural products on platelet P-selectin expression through cAMP levels and its role as peroxisome proliferator-activated receptors agonists.

Cyclic nucleotides inhibit MAP kinase activity in low-dose collagen-stimulated platelets

Collagen-induced platelet activation is a complex process involving multiple signaling pathways. The role(s) of MAP kinases (ERKs and p38(MAPK)) are unclear, although at high, but not low, collagen concentrations p38(MAPK) is involved in cPLA(2)-mediated arachidonic acid release, prior to thromboxane generation. Cyclic nucleotides are conventionally regarded as mediators of platelet inhibition. However recent studies suggested a role for cGMP early in a MAP kinase pathway in platelet activation. In the current study the roles and relationships of MAP kinases, cyclic nucleotides and cPLA(2) in platelet activation by low-dose collagen and a thromboxane analogue (U46619) have been evaluated. Stimulants of neither adenylate cyclase (PGI(2)) nor guanylate cyclase (NaNP) alone had any effect on the basal phosphorylation of either MAP kinase. PGI(2) inhibited ERK/p38(MAPK) phosphorylation in response to both agonists which was unaffected by a cPLA(2) inhibitor (AACOCF(3)). NaNP inhibited collagen-induced ERK/p38(MAPK) phosphorylation, which was enhanced by AACOCF(3) and reversed by a guanylate cyclase inhibitor (ODQ). However NaNP had no effect on U46619-induced p38(MAPK) phosphorylation. Thus adenylate cyclase activation inhibits low-dose collagen-induced MAP kinase phosphorylation both prior, and distal, to thromboxane release. The study also supports an inhibitory, rather than stimulatory, role for guanylate cyclase in platelet signaling.

cGMP and cGMP-dependent protein kinase in platelets and blood cells

Platelets are specialized adhesive cells that play a key role in normal and pathological hemostasis through their ability to rapidly adhere to subendothelial matrix proteins (platelet adhesion) and to other activated platelets (platelet aggregation). NO plays a crucial role in preventing platelet adhesion and aggregation. In platelets, cGMP synthesis is catalyzed by sGC, whereas PDE2, PDE3 and PDE5 are responsible for cGMP degradation. Stimulation of cGK by cGMP leads to phosphorylation of multiple target substrates. These substrates inhibit elevation of intracellular calcium, integrin activation, cytoskeletal reorganization, and platelet granule secretion, events normally associated with platelet activation. The NO/cGMP pathway also plays a significant role in many other blood cell types in addition to platelets. In leukocytes, depending on the specific cell type, cGMP signaling regulates gene expression, differentiation, migration, cytokine production, and apoptosis.

Resveratrol, at concentrations attainable with moderate wine consumption, stimulates human platelet nitric oxide production

The mechanisms through which moderate wine consumption reduces ischemic cardiovascular events are not yet fully unraveled. Grape extracts or a mixture of the polyphenols contained in wine were previously shown to increase nitric oxide (NO); however, little information is available on the effect of resveratrol, one of the main polyphenols of wine, on platelet NO production. We assessed the effects of resveratrol, at the concentrations attainable after moderate wine intake, on platelet NO production and the mechanism of this activity. Twenty healthy volunteers were studied before and after 15 d of controlled white or red wine intake (300 mL/d). After wine intake, plasma resveratrol and the release of NO by stimulated platelets increased significantly. Resveratrol, at the concentrations detected in plasma after wine intake, was incubated in vitro with washed platelets and several variables related to NO production and to signal transduction were measured. Resveratrol in vitro enhanced significantly the production of NO by stimulated platelets, the activity of platelet NO synthase (NOS), phosphorylation of protein kinase B, an activator of the endothelial NOS (eNOS), and phosphorylation of vasodilator-activated protein (VASP), an expression of the biologic activity of NO in platelets. Simultaneously, we observed decreased phosphorylation of P38 mitogen-activated protein kinase (p38MAPK), a proinflammatory pathway in human platelets, a reduction of the activity of NADPH oxidase, a major source of reactive oxygen species (ROS) and of the generation of O(2)(-) radicals, as detected by cytochrome C reduction. In conclusion, resveratrol, at concentrations attainable after moderate wine intake, activates platelet eNOS and in this way blunts the proinflammatory pathway linked to p38MAPK, thus inhibiting ROS production and ultimately platelet function. This activity may contribute to the beneficial effects of moderate wine intake on ischemic cardiovascular disease.

Improved endothelial function and reduced platelet activation by chronic HMG-CoA-reductase inhibition with rosuvastatin in rats with streptozotocin-induced diabetes mellitus

Diabetes is associated with endothelial dysfunction and platelet activation, both of which may contribute to increased cardiovascular risk. We investigated whether the hydroxy-3-methyl-glutaryl CoA reductase inhibitor rosuvastatin improves endothelial function and reduces platelet activation in diabetic rats. Therefore, male Wistar rats were injected with streptozotocin (STZ, 50mg/kg i.v.) to induce insulin-deficient diabetes. Treatment with rosuvastatin (20mg/[kg day]) or vehicle was initiated 2 weeks after injection of STZ and continued for 2 weeks. Thereafter, platelet activation was assessed in fresh whole blood and vascular function was characterized in isolated aortic segments in organ bath chambers. Endothelium-dependent relaxation induced by acetylcholine was significantly attenuated in diabetic rats and improved by treatment with rosuvastatin (maximum relaxation, % of precontraction-control: 99.8+/-0.2, STZ-vehicle: 80.7+/-2.9, STZ-rosuvastatin: 98.9+/-0.7; p<0.01). Similarly, treatment with rosuvastatin significantly reduced fibrinogen-binding to activated GPIIb/IIIa (mean fluorescence-control: 161.0+/-6.9, STZ-vehicle: 207.8+/-15.9, rosuvastatin: 173.6+/-5.3; p<0.05) and P-Selectin surface expression on platelets (mean fluorescence-control: 76.5+/-7.3, STZ-vehicle: 92.1+/-5.5, rosuvastatin: 75.2+/-6.5; p<0.05), while both markers of platelet activation were increased in diabetic rats. Therefore, rosuvastatin treatment normalizes endothelial function and reduces platelet activation in diabetic rats. These effects may contribute to the reduction of cardiovascular events by statins in diabetic patients.

Soluble guanylyl cyclase activation with HMR1766 attenuates platelet activation in diabetic rats

OBJECTIVE

Platelet activation significantly contributes to cardiovascular morbidity and mortality in diabetes. An association between impaired NO-mediated platelet inhibition and platelet activation has recently been demonstrated in experimental diabetes. Guanylyl cyclase activation enhances the reduced signaling via the NO/cGMP pathway. We investigated whether chronic guanylyl cyclase activation would beneficially modulate platelet activation in experimental diabetes mellitus.

METHODS AND RESULTS

Diabetes was induced by streptozotocin-injection in male Wistar rats. After 2 weeks, treatment with either placebo or the guanylyl cyclase activator HMR1766 (10 mg/kg twice daily by gavage) was initiated. Two weeks later, in vivo platelet activation and in vitro platelet reactivity were assessed. Chronic treatment with HMR1766 enhanced NO/cGMP-mediated signaling in platelets from diabetic rats determined by in vivo phosphorylation of platelet vasodilator-stimulated phosphoprotein (VASP) at Ser157 and Ser239. In parallel, platelet-binding of fibrinogen, surface-expression of P-selectin, appearance of platelet-derived microparticles, and platelet-aggregates with other blood cells were significantly reduced by chronic treatment with HMR1766.

CONCLUSIONS

Chronic activation of soluble guanylyl cyclase in diabetic rats improved markers of platelet activation and is a rationale approach for prevention of adverse cardiovascular events in diabetes.

Thrombin stimulation of p38 MAP kinase in human platelets is mediated by ADP and thromboxane A2 and inhibited by cGMP/cGMP-dependent protein kinase

p38 MAP kinase in human platelets is activated by platelet agonists including thrombin, thromboxane A2 (TxA2), ADP, and others. However, both upstream mechanisms of p38 MAP kinase activation, and their downstream sequelae, are presently controversial and essentially unclear. Certain studies report sequential activation of cGMP-dependent protein kinase (PKG) and p38/ERK pathways by platelet agonists, leading to integrin activation and secretion, whereas others establish an essential role of Src/ERK-mediated TxA2 generation for fibrinogen receptor activation in human platelets. Here, we show that ADP secreted from platelet-dense granules, and subsequent activation of P2Y12 receptors, as well as TxA2 release are important upstream mediators of p38 MAP kinase activation by thrombin. However, p38 MAP kinase activation did not significantly contribute to calcium mobilization, P-selectin expression, alphaIIbbeta3 integrin activation, and aggregation of human platelets in response to thrombin. Finally, PKG activation did not stimulate, but rather inhibited, p38 MAP kinase in human platelets.

Activation of p38 MAPKinase/cPLA2 pathway in homocysteine-treated platelets

Hyperhomocysteinemia is considered a risk factor in arterial and venous thrombosis. The mechanism by which homocysteine (HCy) supports atherothrombosis is still unknown and may be multifactorial. Earlier in vitro studies demonstrated that HCy induced arachidonic acid release and increased thromboxane B2 (TXB2) formation. In this work, we found that HCy stimulated the rapid and sustained phosphorylation of platelet p38 mitogen-activated protein kinase (p38 MAPK). The effect was time- and dose-dependent. The HCy effect on p38 MAPK phosphorylation was prevented by N-acetyl-L-cysteine and iloprost and was partially inhibited by nordihydroguaiaretic acid. Moreover, the incubation of platelets with HCy led to the phosphorylation of cytosolic phospholipase A2 (cPLA2). In addition HCy promoted cPLA2 activation, assessed as arachidonic acid release. The cPLA2 phosphorylation and activation were both impaired by the inhibition of p38 MAPK through SB203580. This effect was not complete, reaching at the most the 50% of the total. In FURA 2-loaded platelets, HCy induced a dose-dependent intracellular calcium rise suggesting that the calcium elevation promoted by HCy could participate in the cPLA2 activation, leading to arachidonic acid release and TXB2 formation. In conclusion, our data provide insight into the mechanisms of platelet activation induced by HCy, suggesting that the p38 MAPK/cPLA2 pathway could play a relevant role in platelet hyperactivity described in hyperhomocysteinemia.

Inhibition of platelet aggregation by anthrax edema toxin

Edema toxin is a key virulence determinant in anthrax pathogenesis that causes augmentation of cAMP inside host cells. This exotoxin has been implicated in facilitating bacterial invasion by impairing host defenses. Here, we report for the first time that edema toxin plays an important role in suppression of platelet aggregation; an effect that could be of vital significance in anthrax afflicted subjects. It was found that edema toxin induces a dose dependent and time dependent increase in cAMP inside rabbit platelets. Elevation of cAMP led to suppression of platelet aggregation as demonstrated by in vitro aggregation assays. A 95% suppression of platelet aggregation in response to thrombin and a complete suppression in response to ADP, at toxin concentrations of 7 and 2.2 nM, respectively, were observed. Antibody neutralized wild type edema factor and non-toxic mutants of this binary toxin failed to show any alteration in the normal aggregation pattern. Edema toxin caused the activation of cAMP dependent protein kinase A inside platelets, a phenomenon that could be speculated to initiate the cascade of events responsible for suppressing platelet aggregation. Furthermore, in vivo bleeding time registered a sharp increase in response to edema toxin. These findings can explicate the systemic occurrence of hemorrhage, which is a prominent symptom of anthrax. This study exemplifies how Bacillus anthracis has evolved the ability to use host's physiological processes by mimicking the eukaryotic signal transduction machinery, thus inflicting persistent infection.

Reduced vascular NO bioavailability in diabetes increases platelet activation in vivo

OBJECTIVE

Platelet activation is a feature of cardiovascular disease that is also characterized by endothelial dysfunction. The direct relationship between impaired endothelium-derived NO bioavailability and platelet activation remains unclear. We investigated whether acute inhibition of NO production modulates platelet activation in mice and whether specific rescue of endothelial function in diabetes modifies platelet activation.

METHODS AND RESULTS

Intravenous injection of the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester in wild-type (WT) mice significantly reduced platelet vasodilator-stimulated phosphoprotein (VASP) phosphorylation and increased platelet surface expression of P-selectin, CD40 ligand, and fibrinogen platelet binding, demonstrating that NO production exerts tonic inhibition of platelet activation in mice. Diabetes was induced by streptozotocin injection in WT or endothelial-targeted guanosine 5'-triphosphate cyclohydrolase I (GCH)-transgenic (GCH-Tg) mice protected from endothelial dysfunction in diabetes by sustained levels of tetrahydrobiopterin in vascular endothelium. Platelet VASP phosphorylation was significantly reduced in diabetic WT but not in diabetic GCH-Tg mice. P-selectin, CD40 ligand expression, and fibrinogen binding were increased in diabetic WT mice but remained unchanged compared with controls in endothelial-targeted GCH-Tg mice.

CONCLUSIONS

Platelet activation results from acute and chronic reduction in NO bioactivity. Rescue of platelet activation in diabetes by endothelial-specific restoration of NO production demonstrates that platelet function in vivo is principally regulated by endothelium-derived NO. Endothelial dysfunction caused by uncoupling of endothelial NO synthase is well described in diabetes mellitus and may lead to platelet activation. Acute loss of systemic NO bioavailability causes platelet activation. eNOS uncoupling prevention in diabetes preserved systemic NO bioavailability and maintained a physiological platelet state without activation in vivo.

Rapid Regulation of Platelet Activation In Vivo by Nitric Oxide

Background— Platelet activation is a feature of many cardiovascular diseases characterized by endothelial dysfunction. The mechanistic relationship between impaired systemic nitric oxide (NO) bioavailability and platelet activation in vivo remains unclear. We investigated whether acute inhibition of NO production in humans modulates platelet activation in vivo and whether exogenous NO would counteract such an effect.

Methods and Results— Intravenous injection of the NO synthase inhibitor N G -monomethyl- l -arginine in healthy volunteers resulted in NO synthase inhibition as detected by increased blood pressure and by significantly reduced phosphorylation of platelet vasodilator-stimulated phosphoprotein, an indicator of NO signaling. NO synthase inhibition increased platelet activation as determined by enhanced platelet binding of fibrinogen and surface expression of P-selectin, glycoprotein 53, and CD40 ligand, demonstrating tonic inhibition of platelet activation by NO production in vivo. Sublingual administration of the NO donor glyceryl trinitrate normalized platelet VASP phosphorylation and restored markers of platelet activation to baseline levels.

Conclusions— Acute inhibition of endogenous NO production in humans causes rapid platelet activation in vivo, which is reversed by exogenous NO, demonstrating that platelet function in vivo is rapidly regulated by NO bioavailability.

Novel role of the membrane-bound chemokine fractalkine in platelet activation and adhesion

Chemokines released by the endothelium have proaggregatory properties on platelets. Fractalkine, a recently discovered membrane-bound chemokine with a transmembrane domain, is expressed in vascular injury; however, the effects of fractalkine on platelets have not yet been investigated. Blood was taken from healthy Wistar-Kyoto rats and the expression of the fractalkine receptor on platelets was demonstrated. The modulation of surface expression of P-selectin was assessed by flow cytometry. P-selectin expression was significantly enhanced by in vitro stimulation with recombinant rat fractalkine compared with baseline levels. Selectively inhibiting the function of recombinant fractalkine by an antagonizing antibody or the disruption of the G-protein-coupled intracellular signaling cascade of the fractalkine receptor by pertussis toxin (PTX) completely prevented fractalkine-mediated platelet activation. Preincubation with apyrase significantly attenuated the fractalkine-induced degranulation. In a flow chamber model of platelet adhesion, stimulation with fractalkine significantly enhanced platelet adhesion to collagen and fibrinogen. Similar to P-selectin expression, enhanced adhesion could be prevented by the antagonizing antibody or preincubation of platelets with PTX. Fractalkine, which is overexpressed in atherosclerosis and vascular injury, contributes to platelet activation and adhesion and hence is likely to play a pathophysiologically important role for increased thrombogenesis in vascular diseases.

Abnormal germinal center reactions in systemic lupus erythematosus demonstrated by blockade of CD154-CD40 interactions

To determine the role of CD154-CD40 interactions in the B cell overactivity exhibited by patients with active systemic lupus erythematosus (SLE), CD19+ peripheral B cells were examined before and after treatment with humanized anti-CD154 mAb (BG9588, 5c8). Before treatment, SLE patients manifested activated B cells that expressed CD154, CD69, CD38, CD5, and CD27. Cells expressing CD38, CD5, or CD27 disappeared from the periphery during treatment with anti-CD154 mAb, and cells expressing CD69 and CD154 disappeared from the periphery during the post-treatment period. Before treatment, active-SLE patients had circulating CD38 (bright) Ig-secreting cells that were not found in normal individuals. Disappearance of this plasma cell subset during treatment was associated with decreases in anti-double-stranded DNA (anti-dsDNA) Ab levels, proteinuria, and SLE disease activity index. Consistent with this finding, peripheral B cells cultured in vitro spontaneously proliferated and secreted Ig in a manner that was inhibited by anti-CD154 mAb. Finally, the CD38(+/++)IgD(+), CD38(+++), and CD38(+)IgD(-) B cell subsets present in the peripheral blood also disappeared following treatment with humanized anti-CD154. Together, these results indicate that patients with active lupus nephritis exhibit abnormalities in the peripheral B cell compartment that are consistent with intensive germinal center activity, are driven via CD154-CD40 interactions, and may reflect or contribute to the propensity of these patients to produce autoantibodies.

The C-terminal peptide of thrombospondin-1 stimulates distinct signaling pathways but induces an activation-independent agglutination of platelets and other cells

A peptide from the C-terminal domain of thrombospondin-1 (4N1-1) has been proposed to stimulate platelet aggregation by a novel mechanism involving both an activation-independent agglutination and an activation-dependent, glycoprotein (GP) IIb/IIIa-mediated aggregation which involves GPVI signaling but does not involve CD47. The present study demonstrates that 4N1-1 stimulated a different pattern of signal transduction pathways than the GPVI agonist convulxin. Furthermore, 4N1-1-induced platelet aggregation was activation-independent and not dependent on GPVI or GPIIb/IIIa. Interestingly, 4N1-1 also stimulated activation-independent agglutination of different megakaryocytic and non-megakaryocytic cells. 4N1-1-induced cell agglutination but not platelet signaling was inhibited by anti-CD47 antibodies.

Atrial natriuretic peptide type C induces a cell-cycle switch from proliferation to differentiation in brain-derived neurotrophic factor- or nerve growth factor-primed olfactory receptor neurons

With the discovery of postnatal stem cells within the brain, it has become important to understand how extracellular factors might affect the maturation of neuronal precursors in the postnatal brain. Neurotrophic factors are known to play a role in neuronal development but display pleiotrophic effects, in part because of their physiological interactions with other factors. One factor positioned to interact with neurotrophins in the brains of postnatal animals is atrial C-type natriuretic peptide (CNP). In this study, we used olfactory receptor neurons (ORNs) as a model, because their precursors demonstrate the most robust and functional postnatal neurogenesis of those systems thus far described. We examined the effects of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) and the interactions of these neurotrophins and CNP in postnatal olfactory neuronal precursors. Results obtained using mice with targeted deletion of the gene for BDNF indicated that BDNF is a neuroproliferation-inducing and survival factor for ORN precursors. These roles were confirmed in vitro using primary cultures of ORNs. NGF was found to be a proliferation-inducing factor but not a survival factor. The addition of CNP to either BDNF- or NGF-treated neuronal precursors resulted in an inhibition of proliferation and the promotion of maturation. These effects were accompanied by changes in cell-cycle proteins that suggest possible mechanisms for these effects. Thus, CNP may function in the postnatal brain to regulate the exit from the cell cycle in neuronal precursor cells.

Upregulation of CD40 and CD40 ligand (CD154) in patients with moderate hypercholesterolemia

BACKGROUND

Hypercholesterolemia, a risk factor for cardiovascular disease, is associated with inflammation and hypercoagulability. Both can be mediated by the CD40 system. This study investigated whether the CD40 system is upregulated in patients with moderate hypercholesterolemia and whether it is influenced by therapy with a hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase inhibitor.

METHODS AND RESULTS

Fifteen patients with moderate hypercholesterolemia and 15 healthy control subjects were investigated. CD154 and P-selectin were analyzed on platelets and CD40 was analyzed on monocytes before and under therapy with the statin cerivastatin by double-label flow cytometry. Blood concentrations of soluble CD154 and monocyte chemoattractant protein-1 (MCP-1) were evaluated. Our main findings were as follows. Patients with moderate hypercholesterolemia showed a significant increase of CD154 and P-selectin on platelets and CD40 on monocytes compared with healthy subjects. Soluble CD154 showed a nonsignificant trend for higher plasma levels in patients. A positive correlation was found for total or LDL cholesterol and CD154, but not for CD40 on monocytes. The latter was upregulated in vitro by C-reactive protein, which was found to be significantly elevated in patients with moderate hypercholesterolemia. CD154 on platelets proved to be biologically active because it enhanced the release of MCP-1, which was markedly elevated in an in vitro platelet-endothelial cell coculture model and in the serum of patients. Short-term therapy with a HMG-CoA reductase inhibitor significantly downregulated CD40 on monocytes and serum levels of MCP-1.

CONCLUSION

Patients with moderate hypercholesterolemia show upregulation of the CD40 system, which may contribute to the known proinflammatory, proatherogenic, and prothrombotic milieu found in these patients.

Taming platelets with cyclic nucleotides

Cardiovascular diseases are often accompanied and aggravated by pathologic platelet activation. Tight regulation of platelet function is an essential prerequisite for intact vessel physiology or effective cardiovascular therapy. Physiological platelet antagonists as well as various pharmacological vasodilators inhibit platelet function by activating adenylyl and guanylyl cyclases and increasing intracellular cyclic AMP (cAMP) and cyclic GMP (cGMP) levels, respectively. Elevation of platelet cyclic nucleotides interferes with basically all known platelet activatory signaling pathways, and effectively blocks complex intracellular signaling networks, cytoskeletal rearrangements, fibrinogen receptor activation, degranulation, and expression of pro-inflammatory signaling molecules. The major target molecules of cyclic nucleotides in platelets are cyclic nucleotide-dependent protein kinases that mediate their effects through phosphorylation of specific substrates. They directly affect receptor/G-protein activation and interfere with a variety of signal transduction pathways, including the phospholipase C, protein kinase C, and mitogen-activated protein kinase pathways. Regulation of these pathways blocks several steps of cytosolic Ca(2+) elevation and controls a multitude of cytoskeleton-associated proteins that are directly involved in organization of the platelet cytoskeleton. Due to their multiple sites of action and strong inhibitory potencies, cyclic nucleotides and their regulatory pathways are of particular interest for developing new approaches for the treatment of thrombotic and cardiovascular disorders.