Cyclic Nucleotides and Phosphodiesterases in Platelets

Anti-thrombotic effects of ginsenoside Rk3 by regulating cAMP and PI3K/MAPK pathway on human platelets

Background and objective

The ability to inhibit aggregation has been demonstrated with synthetically derived ginsenoside compounds G-Rp (1, 3, and 4) and ginsenosides naturally found in Panax ginseng 20(S)-Rg3, Rg6, F4, and Ro. Among these compounds, Rk3 (G-Rk3) from Panax ginseng needs to be further explored in order to reveal the mechanisms of action during inhibition.

Methodology

Our study focused to investigate the action of G-Rk3 on agonist-stimulated human platelet aggregation, inhibition of platelet signaling molecules such as fibrinogen binding with integrin αIIbβ3 using flow cytometry, intracellular calcium mobilization, dense granule secretion, and thromboxane B2 secretion. In addition, we checked the regulation of phosphorylation on PI3K/MAPK pathway, and thrombin-induced clot retraction was also observed in platelets rich plasma.

Key Results

G-Rk3 significantly increased amounts of cyclic adenosine monophosphate (cAMP) and led to significant phosphorylation of cAMP-dependent kinase substrates vasodilator-stimulated phosphoprotein (VASP) and inositol 1,4,5-trisphosphate receptor (IP3R). In the presence of G-Rk3, dense tubular system Ca2+ was inhibited, and platelet activity was lowered by inactivating the integrin αIIb/β3 and reducing the binding of fibrinogen. Furthermore, the effect of G-Rk3 extended to the inhibition of MAPK and PI3K/Akt phosphorylation resulting in the reduced secretion of intracellular granules and reduced production of TXA2. Lastly, G-Rk3 inhibited platelet aggregation and thrombus formation via fibrin clot.

Conclusions and implications

These results suggest that when dealing with cardiovascular diseases brought upon by faulty aggregation among platelets or through the formation of a thrombus, the G-Rk3 compound can play a role as an effective prophylactic or therapeutic agent.

The Inhibitory Effects of Protaetia brevitarsis seulensis Larvae Extract on Human Platelet Aggregation and Glycoprotein IIb/IIIa Expression

The white-spotted flower chafer, Protaetia brevitarsis seulensis, is used as a traditional remedy against liver cirrhosis, hepatitis, and hepatic cancer. In this study, we investigated if P. brevitarsis extract (PBE) inhibited platelet aggregation via integrin αIIb/β3 regulation. We observed that PBE inhibited αIIb/β3 activation by regulating the cyclic nucleotides, cyclic adenosine monophosphate and cyclic guanosine monophosphate. Additionally, PBE affected phosphatidylinositol-3 kinase, Akt, SYK, glycogen synthase kinase-3α/β, cytosolic phospholipase A2, and p38 expression, which are signal transduction molecules expressed by platelets, and consequently suppressed αIIbβ3 activity and thromboxane A2 generation. Taken together, PBE showed strong antiplatelet effects and may be used to block thrombosis- and platelet-mediated cardiovascular diseases.

Antiplatelet Effects of Selected Xanthine-Based Adenosine A2A and A2B Receptor Antagonists Determined in Rat Blood

The platelet aggregation inhibitory activity of selected xanthine-based adenosine A2A and A2B receptor antagonists was investigated, and attempts were made to explain the observed effects. The selective A2B receptor antagonist PSB-603 and the A2A receptor antagonist TB-42 inhibited platelet aggregation induced by collagen or ADP. In addition to adenosine receptor blockade, the compounds were found to act as moderately potent non-selective inhibitors of phosphodiesterases (PDEs). TB-42 showed the highest inhibitory activity against PDE3A along with moderate activity against PDE2A and PDE5A. The antiplatelet activity of PSB-603 and TB-42 may be due to inhibition of PDEs, which induces an increase in cAMP and/or cGMP concentrations in platelets. The xanthine-based adenosine receptor antagonists were found to be non-cytotoxic for platelets. Some of the compounds showed anti-oxidative properties reducing lipid peroxidation. These results may provide a basis for the future development of multi-target xanthine derivatives for the treatment of inflammation and atherosclerosis and the prevention of heart infarction and stroke.

Pharmacological Actions of 5-Hydroxyindolin-2 on Modulation of Platelet Functions and Thrombus Formation via Thromboxane A2 Inhibition and cAMP Production

Platelets play a very significant role in hemostasis while simultaneously posing a risk for the development of various cardiovascular diseases. Platelet-mediated issues can occur in blood vessels and trigger various medical problems. Therefore, controlling platelet function is important in the prevention of thrombosis. In this regard, we need to find compounds that provide potent antiplatelet activity with minimum side effects. Therefore, we examined the effect of 5-hydroxyindolin-2-one isolated from Protaetia brevitarsis larvae having antiplatelet properties and investigated different pathways that mediate the antiplatelet activity. We examined the effect of 5-hydroxyindolin-2-one (5-HI) on the regulation of phosphoproteins, thromboxane A₂ generation, and integrin αIIbβ3 action. Our data showed that human platelet aggregation was inhibited by 5-HI (75, 100, 150, 200 μM) without cytotoxicity, and it suppressed intracellular Ca²⁺ concentration through the regulation of inositol 1, 4, 5-triphosphate receptor I (Ser¹⁷⁵⁶) and extracellular signal-regulated kinase (ERK). Moreover, collagen-elevated thromboxane A₂ production and αIIbβ3 action were inhibited by 5-HI through the regulation of cytosolic phospholipase A₂ (cPLA₂), mitogen-activated protein kinase p38 (p38^MAPK), vasodilator-stimulated phosphoprotein (VASP), phosphoinositide 3-kinase (PI3K), and Akt (protein kinase B). Therefore, we suggested that 5-HI could be a potential substance for the prevention of thrombosis-mediated thrombosis.

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 NO/cGMP/PKG pathway in platelets: The therapeutic potential of PDE5 inhibitors in platelet disorders

Platelets are the "guardians" of the blood circulatory system. At sites of vessel injury, they ensure hemostasis and promote immunity and vessel repair. However, their uncontrolled activation is one of the main drivers of thrombosis. To keep circulating platelets in a quiescent state, the endothelium releases platelet antagonists including nitric oxide (NO) that acts by stimulating the intracellular receptor guanylyl cyclase (GC). The latter produces the second messenger cyclic guanosine-3',5'-monophosphate (cGMP) that inhibits platelet activation by stimulating protein kinase G, which phosphorylates hundreds of intracellular targets. Intracellular cGMP pools are tightly regulated by a fine balance between GC and phosphodiesterases (PDEs) that are responsible for the hydrolysis of cyclic nucleotides. Phosphodiesterase type 5 (PDE5) is a cGMP-specific PDE, broadly expressed in most tissues in humans and rodents. In clinical practice, PDE5 inhibitors (PDE5i) are used as first-line therapy for erectile dysfunction, pulmonary artery hypertension, and lower urinary tract symptoms. However, several studies have shown that PDE5i may ameliorate the outcome of various other conditions, like heart failure and stroke. Interestingly, NO donors and cGMP analogs increase the capacity of anti-platelet drugs targeting the purinergic receptor type Y, subtype 12 (P2Y12) receptor to block platelet aggregation, and preclinical studies have shown that PDE5i inhibits platelet functions. This review summarizes the molecular mechanisms underlying the effect of PDE5i on platelet activation and aggregation focusing on the therapeutic potential of PDE5i in platelet disorders, and the outcomes of a combined therapy with PDE5i and NO donors to inhibit platelet activation.

Reversible Platelet Integrin αIIbβ3 Activation and Thrombus Instability

Integrin αIIbβ3 activation is essential for platelet aggregation and, accordingly, for hemostasis and arterial thrombosis. The αIIbβ3 integrin is highly expressed on platelets and requires an activation step for binding to fibrinogen, fibrin or von Willebrand factor (VWF). A current model assumes that the process of integrin activation relies on actomyosin force-dependent molecular changes from a bent-closed and extended-closed to an extended-open conformation. In this paper we review the pathways that point to a functional reversibility of platelet αIIbβ3 activation and transient aggregation. Furthermore, we refer to mouse models indicating that genetic defects that lead to reversible platelet aggregation can also cause instable thrombus formation. We discuss the platelet agonists and signaling pathways that lead to a transient binding of ligands to integrin αIIbβ3. Our analysis points to the (autocrine) ADP P2Y₁ and P2Y₁₂ receptor signaling via phosphoinositide 3-kinases and Akt as principal pathways linked to reversible integrin activation. Downstream signaling events by protein kinase C, CalDAG-GEFI and Rap1b have not been linked to transient integrin activation. Insight into the functional reversibility of integrin activation pathways will help to better understand the effects of antiplatelet agents.

Prolonged oral sildenafil use-induced Mondor disease: a case report

Penile Mondor disease (MD) is a palpable, painful, subcutaneous induration caused by superficial dorsal penile vein thrombosis. We report a case of penile MD that was suspected to be related to prolonged oral sildenafil use. A 46-year-old man visited our emergency department with sustained penile pain and swelling that began 7 hours after sexual intercourse. He had used oral sildenafil intermittently for 11 years and engaged in sexual intercourse the previous night after taking sildenafil. Examination revealed no evidence of intercourse-related trauma to the genital area or an increase in penile skin temperature. However, penile swelling and tenderness over the protruding dorsal penile vein were noted. A color Doppler ultrasound examination was performed immediately, which showed hyperechoic thrombosis in the right superficial dorsal penile vein that was dilated, with soft tissue swelling and no detectable flow signal in the thrombotic lesion. The patient was diagnosed as having penile MD. The patient was treated conservatively. Some reports have indicated the involvement of sildenafil in thrombogenesis. Physicians should be aware that prolonged oral sildenafil use may be associated with penile MD.

Anti-Thrombotic Effects of Artesunate through Regulation of cAMP and PI3K/MAPK Pathway on Human Platelets

Normal activation of platelets and their aggregation are crucial for proper hemostasis. It appears that excessive or abnormal aggregation of platelets may bring about cardiovascular diseases such as stroke, atherosclerosis, and thrombosis. For this reason, finding a substance that can regulate platelet aggregation or suppress aggregation will aid in the prevention and treatment of cardiovascular diseases. Artesunate is a compound extracted from the plant roots of Artemisia or Scopolia, and its effects have shown to be promising in areas of anticancer and Alzheimer’s disease. However, the role and mechanisms by which artesunate affects the aggregation of platelets and the formation of a thrombus are currently not understood. This study examines the ways artesunate affects the aggregation of platelets and the formation of a thrombus on platelets induced by U46619. As a result, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) production were increased significantly by artesunate relative to the doses, as well as phosphorylated vasodilator-stimulated phosphoprotein (VASP) and inositol 1,4,5-trisphosphate receptor (IP₃R), substrates to cAMP-dependent kinase and cGMP-dependent kinase, in a significant manner. The Ca²⁺, normally mobilized from the dense tubular system, was inhibited due to IP₃R phosphorylation from artesunate, and phosphorylated VASP aided in inhibiting platelet activity via αIIb/β₃ platelet membrane inactivation and inhibiting fibrinogen binding. In addition, MAPK and PI3K/Akt phosphorylation was inhibited via artesunate in a significant manner, causing the production of TXA₂ and intracellular granular secretion (serotonin and ATP release) to be reduced. Therefore, we suggest that artesunate has value as a substance that inhibits platelet aggregation and thrombus formation through an antiplatelet mechanism.

Coenzyme Q10 Upregulates Platelet cAMP/PKA Pathway and Attenuates Integrin αIIbβ3 Signaling and Thrombus Growth

SCOPE

Platelet integrin αIIbβ3 is the key mediator of atherothrombosis. Supplementation of coenzyme Q10 (CoQ10), a fat-soluble molecule that exists in various foods, exerts protective cardiovascular effects. This study aims to investigate whether and how CoQ10 acts on αIIbβ3 signaling and thrombosis, the major cause of cardiovascular diseases.

METHODS AND RESULTS

Using a series of platelet functional assays in vitro, it is demonstrated that CoQ10 reduces human platelet aggregation, granule secretion, platelet spreading, and clot retraction. It is further demonstrated that CoQ10 inhibits platelet integrin αIIbβ3 outside-in signaling. These inhibitory effects are mainly mediated by upregulating cAMP/PKA pathway, where CoQ10 stimulates the A_2A adenosine receptor and decreases phosphodiesterase 3A phosphorylation. Moreover, CoQ10 attenuates murine thrombus growth and vessel occlusion in a ferric chloride (FeCl₃ )-induced thrombosis model in vivo. Importantly, the randomized, double-blind, placebo-controlled clinical trial in dyslipidemic patients demonstrates that 24 weeks of CoQ10 supplementation increases platelet CoQ10 concentrations, enhances the cAMP/PKA pathway, and attenuates αIIbβ3 outside-in signaling, leading to decreased platelet aggregation and granule release.

CONCLUSION

Through upregulating the platelet cAMP/PKA pathway, and attenuating αIIbβ3 signaling and thrombus growth, CoQ10 supplementation may play an important protective role in patients with risks of cardiovascular diseases.

Citrullinated Histone H3: Early Biomarker of Neutrophil Extracellular Traps in Septic Liver Damage

BACKGROUND

Neutrophil extracellular traps (NETs) play a crucial role in host defense, but excess and prolonged interaction of NETs with platelets can cause severe inflammation and host organ damage. Modification of histone H3 by citrullination is involved in in vitro NET formation. The phosphodiesterase III inhibitor, cilostazol (Ciz), which has a protective effect on liver sinusoidal endothelial cells and inhibits platelet aggregation, may prevent organ damage caused by excess NETosis. In this study, we investigated whether citrullinated histone H3 (H3Cit) could serve as a biomarker for the detection of critical liver damage in sepsis and the efficacy of phosphodiesterase-III inhibition for preventing the liver dysfunction induced by NETosis.

MATERIALS AND METHODS

Mice injected with lipopolysaccharide (LPS; 1 mg/kg) were used as a sepsis model with or without treatment with Ciz (200 mg/kg). H3Cit, myeloperoxidase, and neutrophil elastase levels were measured by immunohistochemistry. We evaluated H3Cit-positive neutrophils in the peripheral blood by flow cytometry.

RESULTS

Immunohistochemistry revealed that H3Cit-, neutrophil elastase-, and myeloperoxidase-positive cell numbers in the livers peaked at 12 h after LPS administration. However, flow cytometry showed a significant increase in H3Cit-positive neutrophils in the peripheral blood only 4 h after LPS injection. Treatment with Ciz significantly ameliorated all parameters.

CONCLUSIONS

H3Cit is a useful biomarker for early detection of NETosis or liver dysfunction, and Ciz may be an effective treatment for septic liver damage.

Phosphodiesterase III inhibitor attenuates rat sinusoidal obstruction syndrome through inhibition of platelet aggregation in Disse's space

BACKGROUND AND AIM

Sinusoidal obstruction syndrome (SOS) is a serious drug-induced liver injury. However, the pathophysiology of the disease remains unclear. This study investigated the effects of cilostazol (CZ), a phosphodiesterase III inhibitor, in a monocrotaline (MCT)-induced rat model of SOS.

METHODS

Male Wistar rats were administrated MCT to induce SOS. Rats were divided into control, MCT, and MCT + CZ groups. In the MCT + CZ group, CZ was administered at 48 h, 24 h, and 30 min prior to and 8 h and 24 h after MCT administration. The MCT group was treated with water instead of CZ. At 48 h after MCT administration, blood and liver samples were collected to assess biochemistry and liver histology. Expression of rat endothelial cell antigen, CD34, CD41, P-selectin, and caspase-3 in the liver were analyzed. Plasminogen activator inhibitor-1 (PAI-1) in hepatocytes was analyzed using western blotting and polymerase chain reaction.

RESULTS

In the MCT group, macroscopic findings showed a dark-red liver surface. Histological findings showed sinusoidal dilatation, coagulative necrosis of hepatocytes, and endothelial damage of the central vein. These changes were attenuated in the MCT + CZ group. Elevated serum transaminase and decreased platelet counts were observed in the MCT + CZ group compared with those in the MCT group. Treatment with CZ reduced MCT-induced damage to the liver sinusoidal endothelial cells, inhibited extravasated platelet aggregation, and suppressed hepatocyte apoptosis around the central vein. CZ attenuated hepatic PAI-1 protein and mRNA levels.

CONCLUSIONS

Cilostazol attenuated MCT-induced SOS by preventing damage to liver sinusoidal endothelial cells and extravasated platelet aggregation. Hepatic PAI-1 levels were suppressed with CZ treatment.

Effects of the NO/soluble guanylate cyclase/cGMP system on the functions of human platelets

Platelets are circulating sentinels of vascular integrity and are activated, inhibited, or modulated by multiple hormones, vasoactive substances or drugs. Endothelium- or drug-derived NO strongly inhibits platelet activation via activation of the soluble guanylate cyclase (sGC) and cGMP elevation, often in synergy with cAMP-elevation by prostacyclin. However, the molecular mechanisms and diversity of cGMP effects in platelets are poorly understood and sometimes controversial. Recently, we established the quantitative human platelet proteome, the iloprost/prostacyclin/cAMP/protein kinase A (PKA)-regulated phosphoproteome, and the interactions of the ADP- and iloprost/prostacyclin-affected phosphoproteome. We also showed that the sGC stimulator riociguat is in vitro a highly specific inhibitor, via cGMP, of various functions of human platelets. Here, we review the regulatory role of the cGMP/protein kinase G (PKG) system in human platelet function, and our current approaches to establish and analyze the phosphoproteome after selective stimulation of the sGC/cGMP pathway by NO donors and riociguat. Present data indicate an extensive and diverse NO/riociguat/cGMP phosphoproteome, which has to be compared with the cAMP phosphoproteome. In particular, sGC/cGMP-regulated phosphorylation of many membrane proteins, G-proteins and their regulators, signaling molecules, protein kinases, and proteins involved in Ca²⁺ regulation, suggests that the sGC/cGMP system targets multiple signaling networks rather than a limited number of PKG substrate proteins.

Mathematical Modelling of Nitric Oxide/Cyclic GMP/Cyclic AMP Signalling in Platelets

Platelet activation contributes to normal haemostasis but also to pathologic conditions like stroke and cardiac infarction. Signalling by cGMP and cAMP inhibit platelet activation and are therefore attractive targets for thrombosis prevention. However, extensive cross-talk between the cGMP and cAMP signalling pathways in multiple tissues complicates the selective targeting of their activities. We have used mathematical modelling based on experimental data from the literature to quantify the steady state behaviour of nitric oxide (NO)/cGMP/cAMP signalling in platelets. The analysis provides an assessment of NO-induced cGMP synthesis and PKG activation as well as cGMP-mediated cAMP and PKA activation though modulation of phosphodiesterase (PDE2 and 3) activities. Both one- and two-compartment models of platelet cyclic nucleotide signalling are presented. The models provide new insight for understanding how NO signalling to cGMP and indirectly cAMP, can inhibit platelet shape-change, the initial step of platelet activation. Only the two-compartment models could account for the experimental observation that NO-mediated PKA activation can occur when the bulk platelet cAMP level is unchanged. The models revealed also a potential for hierarchical interplay between the different platelet phosphodiesterases. Specifically, the models predict, unexpectedly, a strong effect of pharmacological inhibitors of cGMP-specific PDE5 on the cGMP/cAMP cross-talk. This may explain the successful use of weak PDE5-inhibitors, such as dipyridamole, in anti-platelet therapy. In conclusion, increased NO signalling or PDE5 inhibition are attractive ways of increasing cGMP-cAMP cross-talk selectively in platelets.

New Concepts and Mechanisms of Platelet Activation Signaling

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

cGMP signaling inhibits platelet shape change through regulation of the RhoA-Rho Kinase-MLC phosphatase signaling pathway

Essentials Platelet shape change requires cytoskeletal rearrangement via myosin-mediated actin contraction. We investigated whether nitric oxide (NO) affected thrombin-induced platelet shape change. NO inhibits shape change, RhoA/ROCK signalling and myosin light chain (MLC) phosphorylation. NO promotes MLC phosphatase activity, thus prevents MLC phosphorylation and shape change.

SUMMARY

Background Platelet shape change, spreading and thrombus stability require activation of the actin cytoskeleton contractile machinery. The mechanisms controlling actin assembly to prevent unwanted platelet activation are unclear. Objectives We examined the effects of nitric oxide on the signaling pathways regulating platelet actin-myosin activation. Results S-nitrosoglutathione (GSNO) inhibited thrombin-induced platelet shape change and myosin phosphorylation of the myosin light chain (MLC). Because thrombin stimulates phospho-MLC through the RhoA/ ROCK dependent inhibition of MLC phosphatase (MLCP) we examined the effects of NO on this pathway. Thrombin caused the GTP loading and activation of RhoA, leading to the ROCK-mediated phosphorylation of MLCP on threonine 853 (thr853 ), which is known to inhibit phosphatase activity. Treatment of platelets with GSNO blocked ROCK-mediated increases in phosphoMLCP-thr853 induced by thrombin. This effect was mimicked by the direct activator of protein kinase G, 8-pCPT-PET-cGMP, and blocked by the inhibition of guanylyl cyclase, but not inhibitors of protein kinase A. Further exploration of the mechanism demonstrated that GSNO stimulated the association of RhoA with protein kinase G (PKG) and the inhibitory phosphorylation (serine188) of RhoA in a cGMP-dependent manner. Consistent with these observations, in vitro experiments revealed that recombinant PKG caused direct phosphorylation of RhoA. The inhibition of RhoA by GSNO prevented ROCK-mediated phosphorylation and inhibition of MLCP activity. Conclusions These data suggest novel crosstalk between the NO-cGMP-PKG and RhoA/ROCK signaling pathways to control platelet actin remodeling.

The expansive role of oxylipins on platelet biology

In mammals, three major oxygenases, cyclooxygenases (COXs), lipoxygenases (LOXs), and cytochrome P450 (CYP450), generate an assortment of unique lipid mediators (oxylipins) from polyunsaturated fatty acids (PUFAs) which exhibit pro- or anti-thrombotic activity. Over the years, novel oxylipins generated from the interplay of theoxygenase activity in various cells, such as the specialized pro-resolving mediators (SPMs), have been identified and investigated in inflammatory disease models. Although platelets have been implicated in inflammation, the role and mechanism of these SPMs produced from immune cells on platelet function are still unclear. This review highlights the burgeoning classes of oxylipins that have been found to regulate platelet function; however, their mechanism of action still remains to be elucidated.

Aged Garlic Extract Inhibits Human Platelet Aggregation by Altering Intracellular Signaling and Platelet Shape Change

BACKGROUND

Increased platelet aggregation plays a pivotal role in the etiology of cardiovascular disease. Upon platelet aggregation, an increase in free cytoplasmic Ca(2+) results in the inhibition of soluble guanylyl cyclase (sGC) and adenylyl cyclase (AC), leading to a decrease in cyclic guaninosine-5'-monophosphate (cGMP) and cAMP, respectively. This leads to the activation of the glycoprotein IIb/IIIa (GPIIb/IIIa) fibrinogen receptor, resulting in platelet shape change. Aged garlic extract (AGE) decreases platelet aggregation; however, the mechanisms involved are not clearly defined.

OBJECTIVE

Our objective was to investigate the effects of AGE on intraplatelet cell signaling and platelet shape change.

METHODS

Platelets from 14 participants were studied. Platelet aggregation was induced by ADP in the presence of AGE up to a concentration of 6.25% (vol:vol) alone or in combination with 3-morpholinosydnonimine (Sin-1), a nitric oxide donor. The experiments with AGE were repeated in the presence of 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor. In a series of separate experiments, platelet aggregation was induced in the presence of either 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an sGC inhibitor, or 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ22536), an AC inhibitor, or a combination of both in the presence of IBMX and AGE. Intraplatelet cGMP and cAMP were measured. The platelets were also subjected to scanning electron microscopic analysis, and their binding to fibrinogen was determined.

RESULTS

AGE decreased platelet aggregation at all concentrations tested; this decrease was more marked in the presence of Sin-1 and ranged between 15% and 67%.The presence of IBMX also led to a decrease (17-35%) in platelet aggregation at all AGE concentrations and a significant decrease in the amounts of cGMP (24-41%) and cAMP (19-70%), respectively, in the presence of ODQ and SQ22536. The presence of AGE significantly inhibited the binding of activated platelets to fibrinogen, preventing changes in platelet shape.

CONCLUSION

These results indicate that AGE inhibits platelet aggregation by increasing cyclic nucleotides and inhibiting fibrinogen binding and platelet shape change.

Regulation of cAMP Intracellular Levels in Human Platelets Stimulated by 2-Arachidonoylglycerol

We demonstrated that in human platelets the endocannabinoid 2-arachidonoylglycerol (2-AG) decreased dose- and time-dependently cAMP intracellular levels. No effect on cAMP decrease induced by 2-AG was observed in the presence of the adenylate cyclase inhibitor SQ22536 as well in platelets pretreated with the thromboxane A2 receptor antagonist, SQ29548 or with aspirin, inhibitor of arachidonic acid metabolism through the cyclooxygenase pathway. An almost complete recovering of cAMP level was measured in platelets pretreated with the specific inhibitor of phosphodiesterase (PDE) 3A, milrinone. In platelets pretreated with LY294002 or MK2206, inhibitors of PI3K/AKT pathway, and with U73122, inhibitor of phospholipase C pathway, only a partial prevention was shown. cAMP intracellular level depends on synthesis by adenylate cyclase and hydrolysis by PDEs. In 2-AG-stimulated platelets adenylate cyclase activity seems to be unchanged. In contrast PDEs appear to be involved. In particular PDE3A was specifically activated, as milrinone reversed cAMP reduction by 2-AG. 2-AG enhanced PDE3A activity through its phosphorylation. The PI3K/AKT pathway and PKC participate to this PDE3A phosphorylation/activation mechanism as it was greatly inhibited by platelet pretreatment with LY294002, MK2206, U73122, or the PKC specific inhibitor GF109203X. Taken together these data suggest that 2-AG potentiates its power of platelet agonist reducing cAMP intracellular level.

Tadalafil for lower urinary tract symptoms secondary to benign prostatic hyperplasia: a review of clinical data in Asian men and an update on the mechanism of action

Tadalafil, a phosphodiesterase type 5 (PDE5) inhibitor, is approved worldwide for the treatment of lower urinary tract symptoms secondary to benign prostatic hyperplasia (BPH-LUTS). The purpose of this narrative review is to summarize the clinical data on tadalafil 5 mg once-daily, primarily focusing on Asian men with BPH-LUTS, and to update the current understanding of the mechanism of action underlying PDE5 inhibition. Findings from studies have demonstrated that PDE5 is highly expressed in the lower urinary tract and supporting vasculature, and that PDE5 inhibition potentially decreases smooth muscle cell proliferation in the prostate, relaxes smooth muscle in the prostate, bladder neck and supporting vasculature, increases blood perfusion to the lower urinary tract, and modulates bladder afferent nerve activity. A total of 11 larger, 12-week, double-blind, randomized, placebo-controlled studies of tadalafil, including four Asian studies, have been conducted globally, enrolling >3000 men with BPH-LUTS. In addition, two long-term (42- and 52-week) studies enrolled 394 Japanese and 428 North American men, respectively, with BPH-LUTS. Overall, tadalafil 5 mg once-daily resulted in significant improvements in the change from baseline to endpoint in total International Prostate Symptom Scores (IPSS), IPSS storage and voiding subscores, and IPSS quality of life index compared with placebo. Tadalafil was well tolerated and had a favorable safety profile. These findings support tadalafil 5 mg once-daily for treating men, including Asian men, with BPH-LUTS.

The control of blood platelets by cAMP signalling

Blood platelet activation must be tightly regulated to ensure a balance between haemostasis and thrombosis. The cAMP signalling pathway is the most powerful endogenous regulator of blood platelet activation. PKA (protein kinase A), the foremost effector of cAMP signalling in platelets, phosphorylates a number of proteins that are thought to modulate multiple aspects of platelet activation. In the present mini-review, we outline our current understanding of cAMP-mediated platelet inhibition and discuss some of the issues that require clarification.

A review and discussion of platelet nitric oxide and nitric oxide synthase: do blood platelets produce nitric oxide from L-arginine or nitrite?

The NO/sGC/cGMP/PKG system is one of the most powerful mechanisms responsible for platelet inhibition. In numerous publications, expression of functional NO synthase (NOS) in human and mouse platelets has been reported. Constitutive and inducible NOS isoforms convert L-arginine to NO and L-citrulline. The importance of this pathway in platelets and in endothelial cells for the regulation of platelet function is discussed since decades. However, there are serious doubts in the literature concerning both expression and functionality of NOS in platelets. In this review, we aim to present and critically evaluate recent data concerning NOS expression and function in platelets, and to especially emphasise potential pitfalls of detection of NOS proteins and measurement of NOS activity. Prevailing analytical problems are probably the main sources of contradictory data on occurrence, activity and function of NOS in platelets. In this review we also address issues of how these problems can be resolved. NO donors including organic nitrites (RONO) and organic nitrate (RONO2) are inhibitors of platelet activation. Endogenous inorganic nitrite (NO2 (-)), the product of NO autoxidation, and exogenous inorganic nitrite are increasingly investigated as NO donors in the circulation. The role of platelets in the generation of NO from nitrite is also discussed.

Discovering anti-platelet drug combinations with an integrated model of activator-inhibitor relationships, activator-activator synergies and inhibitor-inhibitor synergies

Identifying effective therapeutic drug combinations that modulate complex signaling pathways in platelets is central to the advancement of effective anti-thrombotic therapies. However, there is no systems model of the platelet that predicts responses to different inhibitor combinations. We developed an approach which goes beyond current inhibitor-inhibitor combination screening to efficiently consider other signaling aspects that may give insights into the behaviour of the platelet as a system. We investigated combinations of platelet inhibitors and activators. We evaluated three distinct strands of information, namely: activator-inhibitor combination screens (testing a panel of inhibitors against a panel of activators); inhibitor-inhibitor synergy screens; and activator-activator synergy screens. We demonstrated how these analyses may be efficiently performed, both experimentally and computationally, to identify particular combinations of most interest. Robust tests of activator-activator synergy and of inhibitor-inhibitor synergy required combinations to show significant excesses over the double doses of each component. Modeling identified multiple effects of an inhibitor of the P2Y12 ADP receptor, and complementarity between inhibitor-inhibitor synergy effects and activator-inhibitor combination effects. This approach accelerates the mapping of combination effects of compounds to develop combinations that may be therapeutically beneficial. We integrated the three information sources into a unified model that predicted the benefits of a triple drug combination targeting ADP, thromboxane and thrombin signaling.

Drugs are often used in combinations, but establishing the best combinations is a considerable challenge for basic and clinical research. Anti-platelet therapies reduce thrombosis and heart attacks by lowering the activation of platelet cells. We wanted to find good drug combinations, but a full systems model of the platelet is absent, so we had no good predictions of how particular combinations might behave. Instead, we put together three sources of knowledge. The first concerned what inhibitors act on what activators; the second concerned what pairs of activators synergise together (having a bigger effect than expected); and the third concerned what pairs of inhibitors synergise together. We implemented an efficient experimental approach to collect this information from experiments on platelets. We developed a statistical model that brought these separate results together. This gave us insights into how platelet inhibitors act. For example, an inhibitor of an ADP receptor showed multiple effects. We also worked out from the model what further (triple) combinations of drugs may be most efficient. We predicted, and then tested experimentally, the effects of a triple drug combination. This simultaneously inhibited the platelet’s responses to three stimulants that it encounters during coronary thrombosis, namely ADP, thromboxane and thrombin.

Post receptor determinants of acute platelet response to clopidogrel in patients with symptomatic myocardial ischemia

BACKGROUND

Clopidogrel resistance is more common in patients with loss-of-function CYP2C19 genotypes. Since adenylate cyclase (AC) and soluble guanylate cyclase (sGC) pathways are variably impaired in patients with ischaemic heart disease, we tested the relevance of these determinants in patients undergoing acute loading with clopidogrel (600 mg) prior to non-emergent coronary stenting.

METHODS

Inhibitory effects of prostaglandin E1 (PGE1, an AC activator) and sodium nitroprusside (NP, a sGC activator) on platelet aggregation were determined at baseline and compared with platelet responses to clopidogrel (4 h after administration) assessed as ∆ADP, and Platelet Reactivity Index (∆PRI). Data were analysed according to CYP2C19 genotype.

RESULTS

In patients without loss of function mutations (n=18), ∆ADP but not ∆PRI, was directly correlated with baseline PGE1 responsiveness (rs=0.62, p=0.005)). NP responsiveness did not predict ∆ADP. However there was no relationship between clopidogrel responses and either PGE1 or NP responsiveness in patients with loss of function mutations. Multivariate correlates of clopidogrel response were both the genotype status (β=-0.609, p<0.001) and the baseline response to PGE1 (β=0.303, p=0.03).

CONCLUSIONS

While genetically impaired bio-activation markedly limits acute (4 h) clopidogrel response, impaired AC signalling provides an additional cause for clopidogrel resistance.

Multiplexed phosphospecific flow cytometry enables large-scale signaling profiling and drug screening in blood platelets

BACKGROUND

Dissecting the signaling events that contribute to platelet activation will increase our understanding of platelet function and aid in the development of new antiplatelet agents. However, high-throughput methodology for the quantitative analysis of platelet signaling events is still lacking.

OBJECTIVE

To develop a high-throughput assay for the analysis of platelet signaling events in whole blood.

METHODS AND RESULTS

We developed a fluorescent barcoding protocol to facilitate multiplexing and enable large-scale signaling profiling in platelets in whole blood. The methodology allowed simultaneous staining and acquisition of 24-96 samples in a single analysis tube with a standard flow cytometer. This approach significantly reduced experimental numbers, data acquisition time, and antibody consumption, while providing automated statistically rich quantitative data on signaling events. Using vasodilator-stimulated phosphoprotein (VASP), an established marker of platelet inhibition and antiplatelet drug therapy, we demonstrated that the assay could detect subtle changes in phosphoVASP-Ser157/239 in response to cAMP-elevating agents of varying potency and known modulators of the cAMP signaling cascade. The assay could be used with washed platelets or whole blood, analyzed immediately or frozen, without any significant change in assay performance. To demonstrate the usefulness of the assay as a drug discovery platform, we examined a prostaglandin screening library. Our screen of 70 prostaglandin derivatives revealed three previously uncharacterized lipids that stimulated phosphorylation of VASP-Ser157. Follow-up analyses demonstrated that these agents elevated intraplatelet cAMP and inhibited collagen-induced platelet aggregation.

CONCLUSIONS

This novel method enables rapid, large-scale quantitative signaling profiling and compound screening in human platelets present in whole blood.

Time-dependent inhibitory effects of cGMP-analogues on thrombin-induced platelet-derived microparticles formation, platelet aggregation, and P-selectin expression

In platelets, nitric oxide (NO) activates cGMP/PKG signalling, whereas prostaglandins and adenosine signal through cAMP/PKA. Cyclic nucleotide signalling has been considered to play an inhibitory role in platelets. However, an early stimulatory effect of NO and cGMP-PKG signalling in low dose agonist-induced platelet activation have recently been suggested. Here, we investigated whether different experimental conditions could explain some of the discrepancy reported for platelet cGMP-PKG-signalling. We treated gel-filtered human platelets with cGMP and cAMP analogues, and used flow cytometric assays to detect low dose thrombin-induced formation of small platelet aggregates, single platelet disappearance (SPD), platelet-derived microparticles (PMP) and thrombin receptor agonist peptide (TRAP)-induced P-selectin expression. All four agonist-induced platelet activation phases were blocked when platelets were costimulated with the PKG activators 8-Br-PET-cGMP or 8-pCPT-cGMP and low-doses of thrombin or TRAP. However, extended incubation with 8-Br-PET-cGMP decreased its inhibition of TRAP-induced P-selectin expression in a time-dependent manner. This effect did not involve desensitisation of PKG or PKA activity, measured as site-specific VASP phosphorylation. Moreover, PKG activators in combination with the PKA activator Sp-5,6-DCL-cBIMPS revealed additive inhibitory effect on TRAP-induced P-selectin expression. Taken together, we found no evidence for a stimulatory role of cGMP/PKG in platelets activation and conclude rather that cGMP/PKG signalling has an important inhibitory function in human platelet activation.

Decreasing phosphodiesterase 5A activity contributes to platelet cGMP accumulation during storage of apheresis-derived platelet concentrates

BACKGROUND

Platelet storage lesion (PSL) considerably decreases the quality of platelets (PLTs) in concentrates characterized by a loss of signaling responses to agonists and impaired PLT activation, secretion, and aggregation. To understand the role of inhibitory signaling pathways in the mechanism of PSL, the basal state of the cyclic nucleotide (CN)-dependent signaling systems in stored PLTs was investigated.

STUDY DESIGN AND METHODS

Whole blood samples (WB) and apheresis-derived PLT concentrates (APCs) were obtained from healthy volunteers. Washed PLTs were prepared from WB on Day 0 and from APCs on Days 0, 2, and 5. The basal phosphorylation of the vasodilator-stimulated phosphoprotein (VASP) and phosphodiesterase 5A (PDE5A) levels were quantified by Western blot. CN and PDE5A activity were measured by enzyme-linked immunoassay kits. Fibrinogen binding and aggregation were measured in PLT-rich plasma of WB or APC samples. Unpaired t test was used for statistical analysis.

RESULTS

Basal VASP phosphorylation levels were comparable in WB and APCs on Day 0. VASP phosphorylation increased significantly during storage of APCs, more pronounced at Ser(239) than at Ser(157) . Similarly, intracellular cGMP, but not cAMP, concentration continuously increased in stored PLTs, whereas PDE5A levels and activity significantly decreased accompanied by diminished thrombin receptor activator peptide 6-induced fibrinogen binding and aggregation.

CONCLUSION

Storage of APCs leads to intracellular cGMP accumulation that could be caused by degradation of PDE5A. Enhanced cGMP level supports subsequent cGMP-dependent protein kinase-mediated increase of VASP phosphorylation resulting in reduced fibrinogen binding and aggregation.

ATP antagonizes thrombin-induced signal transduction through 12(S)-HETE and cAMP

In this study we have investigated the role of extracellular ATP on thrombin induced-platelet aggregation (TIPA) in washed human platelets. ATP inhibited TIPA in a dose-dependent manner and this inhibition was abolished by apyrase but not by adenosine deaminase (ADA) and it was reversed by extracellular magnesium. Antagonists of P2Y1 and P2Y12 receptors had no effect on this inhibition suggesting that a P2X receptor controlled ATP-mediated TIPA inhibition. ATP also blocked inositol phosphates (IP1, IP2, IP3) generation and [Ca(2+)]i mobilization induced by thrombin. Thrombin reduced cAMP levels which were restored in the presence of ATP. SQ-22536, an adenylate cyclase (AC) inhibitor, partially reduced the inhibition exerted by ATP on TIPA. 12-lipoxygenase (12-LO) inhibitors, nordihidroguaretic acid (NDGA) and 15(S)-hydroxy-5,8,11,13-eicosatetraenoic acid (15(S)-HETE), strongly prevented ATP-mediated TIPA inhibition. Additionally, ATP inhibited the increase of 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12(S)-HETE) induced by thrombin. Pretreatment with both SQ-22536 and NDGA almost completely abolished ATP-mediated TIPA inhibition. Our results describe for the first time that ATP implicates both AC and 12-LO pathways in the inhibition of human platelets aggregation in response to agonists.

Harnessing the platelet signaling network to produce an optimal hemostatic response

Once released into the circulation by megakaryocytes, circulating platelets can undergo rapid activation at sites of vascular injury and resist unwarranted activation, which can lead to heart attacks and strokes. Historically, the signaling mechanisms underlying the regulation of platelet activation have been approached as a collection of individual pathways unique to agonist. This review takes a different approach, casting platelet activation as the product of a signaling network, in which activating and restraining mechanisms interact in a flexible network that regulates platelet adhesiveness, cohesion between platelets, granule secretion, and the formation of a stable hemostatic thrombus.

Influence of Nitric Oxide and cGMP on Agonist-Induced Platelet Adhesion – An In Vitro Study in Platelets Isolated from Patients with Liver Cirrhosis / Uticaj Azot-Oksida I cGMP Na Adheziju Trombocita Izazvanu Agonistima – In Vitro Studija Na Izolovanim Trombocitima Pacijenata Sa Cirozom Jetre

Variceal bleeding, one of the major complications of liver cirrhosis, is primarily due to platelet activation defect and secondarily due to coagulation defects. Platelet adhesion is the key event in hemostasis. Since nitric oxide (NO) related stress is known to influence platelet functions in liver cirrhosis, we undertook the present study to evaluate the possible mechanism involved in the inhibition of platelet adhesion by NO.

Methods:Agonist-induced platelet adhesion in vitro was measured in platelets isolated from normal subjects and cirrhosis patients. The time-dependent changes in nitric oxide synthase (NOS), NO, 3',5'-cyclic guanosine monophosphate (cGMP) and cytosolic calcium (Ca2+) levels were monitored during adhesion. The percentage of platelet adhesion was also monitored in the presence of an eNOS inhibitor and a cGMP inhibitor.

Results:The percentage of adhesion was significantly lower in cirrhosis platelets. Time-dependent changes in the cGMP NO and NOS level in platelets stimulated with collagen were significantly high, with a significantly low level of elevation of cytosolic Ca2+in cirrhosis as adhesion proceeded. The results showed improved platelet adhesion with inhibitors of NOS and cGMP with concomitant elevation in Ca2+level.

Conclusions:It is inferred that elevation in the formation of cGMP due to stimulation of NOS activity inhibits Ca2+mobilization from the internal store, an essential process to trigger platelet activation. The abnormal alterations were significantly lower in cirrhosis patients without bleeding complications. So, it could be stated that the bleeding abnormality in liver cirrhosis might be due to defective platelet adhesion influenced by the NO-cGMP pathway.

Endogenous cGMP-dependent protein kinase reverses EGF-induced MAPK/ERK signal transduction through phosphorylation of VASP at Ser239

In our previous study, we demonstrated that type II cGMP-dependent protein kinase (PKG II) was expressed at lower levels in different human cancer cell lines and that exogenous PKG II inhibited epidermal growth factor (EGF)-induced MAPK/ERK signaling. In order to investigate its functions further in this signaling pathway, it is necessary to elucidate whether endogenous PKG has the same effect or not. This study aimed to investigate the possible inhibitory effect of endogenous PKG activity on EGF-induced MAPK/ERK signal transduction in human lung cancer cells and its mechanism. Human small cell lung carcinoma cells (SCLCs) were treated with the PKG-selective cGMP analog 8-pCPT-cGMP to activate endogenous PKG, EGF and cGMP followed by EGF, respectively. The results showed that increased endogenous PKG activity inhibited the EGF-induced phosphorylation of the epidermal growth factor receptor (EGFR) and the binding between Sos1 and Grb2. In addition, EGF-triggered Ras activation was reversed by increased endogenous PKG activity. While the EGF-induced phosphorylation of MEK and ERK were inhibited by increased endogenous PKG activity, there was a significant increase of phosphorylated vasodilator-stimulated phosphoprotein (p-VASP) at Ser239. Furthermore, we investigated whether endogenous PKG exerted its effects on EGF-induced MAPK/ERK signaling through phosphorylation of VASP at Ser239. Downregulation of the levels of p-VASP Ser239 by point mutation blocked the effects of endogenous PKG on EGF-induced MAPK/ERK signal transduction. The data shown here suggest that endogenous PKG reverses the EGF-induced MAPK/ERK signaling pathway by phosphorylating VASP at Ser239.

Roles of purinergic receptor P2Y, G protein-coupled 12 in the development of atherosclerosis in apolipoprotein E-deficient mice

OBJECTIVE

The aim of the study was to evaluate the role of purinergic receptor P2Y, G protein-coupled 12 (P2Y12), an ADP receptor, in the development of atherosclerotic lesions.

METHODS AND RESULTS

Apolipoprotein E-null mice were crossed with P2y12(-/-) mice to generate double knockout mice. The double knockout mice and the control apolipoprotein E-null mice were fed a high-fat diet for 20 weeks. Assessment of the atherosclerotic lesions in the control and double knockout mice demonstrated that P2Y12 deficiency caused a diminished lesion area, an increased fibrous content at the plaque site, and decreased monocyte/macrophage infiltration of the lesions. Polymerase chain reaction studies revealed that white blood cells do not express significant levels of P2Y12. Bone marrow transplantation experiments confirmed that P2Y12 expressed on platelets is a key factor responsible for atherosclerosis, but do not exclude a role of smooth muscle cell P2Y12. Supernatant fluid from activated P2y12(+/+) but not P2y12(-/-) platelets was capable of causing monocyte migration. In vitro studies showed that platelet P2Y12 deficiency suppressed platelet factor 4 secretion and P-selectin expression. Further work demonstrated that platelet P2Y12, through inhibition of the cAMP/protein kinase A pathway, critically regulates the release of platelet factor 4, and thereby affects monocyte recruitment and infiltration.

CONCLUSIONS

These results demonstrate that P2Y12 modulates atherogenesis, at least in part by augmenting inflammatory cell recruitment via regulation of platelet α-granule release.

PGE(2) reverses G(s)-mediated inhibition of platelet aggregation by interaction with EP3 receptors, but adds to non-G(s)-mediated inhibition of platelet aggregation by interaction with EP4 receptors

Prostaglandin E(2) (PGE(2)) has intriguing effects on platelet function in the presence of agents that raise cyclic adenosine 3'5'-monophosphate (cAMP). PGE(2) reverses inhibition of platelet aggregation by agents that stimulate cAMP production via a G(s)-linked receptor, but adds to the inhibition of platelet function brought about by agents that raise cAMP through other mechanisms. Here, we used the EP receptor antagonists DG-041 (which acts at the EP3 receptor) and ONO-AE3-208 (which acts at the EP4 receptor) to investigate the role of these receptors in mediating these effects of PGE(2). Platelet aggregation was measured in platelet-rich plasma obtained from healthy volunteers in response to adenosine diphosphate (ADP) using single platelet counting. The effects of a range of concentrations of PGE(2) were determined in the presence of (1) the prostacyclin mimetic iloprost, which operates through G(s)-linked IP receptors, (2) the cAMP PDE inhibitor DN9693 and (3) the direct-acting adenylate cyclase stimulator forskolin. Vasodilator-stimulated phosphoprotein (VASP) phosphorylation was also determined as a measure of cAMP. PGE(2) reversed the inhibition of aggregation brought about by iloprost; this was prevented in the presence of the EP3 antagonist DG-041, indicating that this effect of PGE(2) is mediated via the EP3 receptor. In contrast, PGE(2) added to the inhibition of aggregation brought about by DN9693 or forskolin; this was reversed by the EP4 antagonist ONO-AE3-208, indicating that this effect of PGE(2) is mediated via the EP4 receptor. Effects on aggregation were accompanied by corresponding changes in VASP phosphorylation. The dominant role of EP3 receptors circumstances where cAMP is increased through a Gs-linked mechanism may be relevant to the situation in vivo where platelets are maintained in an inactive state through constant exposure to prostacyclin, and thus the main effect of PGE(2) may be prothrombotic. If so, the results described here further support the potential use of an EP3 receptor antagonist in the control of atherothrombosis.

Novel roles of cAMP/cGMP-dependent signaling in platelets

Endothelial prostacyclin and nitric oxide potently inhibit platelet functions. Prostacyclin and nitric oxide actions are mediated by platelet adenylyl and guanylyl cyclases, which synthesize cyclic AMP (cAMP) and cyclic GMP (cGMP), respectively. Cyclic nucleotides stimulate cAMP-dependent protein kinase (protein kinase A [PKA]I and PKAII) and cGMP-dependent protein kinase (protein kinase G [PKG]I) to phosphorylate a broad panel of substrate proteins. Substrate phosphorylation results in the inactivation of small G-proteins of the Ras and Rho families, inhibition of the release of Ca(2+) from intracellular stores, and modulation of actin cytoskeleton dynamics. Thus, PKA/PKG substrates translate prostacyclin and nitric oxide signals into a block of platelet adhesion, granule release, and aggregation. cAMP and cGMP are degraded by phosphodiesterases, which might restrict signaling to specific subcellular compartments. An emerging principle of cyclic nucleotide signaling in platelets is the high degree of interconnection between activating and cAMP/cGMP-dependent inhibitory signaling pathways at all levels, including cAMP/cGMP synthesis and breakdown, and PKA/PKG-mediated substrate phosphorylation. Furthermore, defects in cAMP/cGMP pathways might contribute to platelet hyperreactivity in cardiovascular disease. This article focuses on recent insights into the regulation of the cAMP/cGMP signaling network and on new targets of PKA and PKG in platelets.

Anti-platelet therapy: phosphodiesterase inhibitors

Inhibition of platelet aggregation can be achieved either by the blockade of membrane receptors or by interaction with intracellular signalling pathways. Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are two critical intracellular second messengers provided with strong inhibitory activity on fundamental platelet functions. Phosphodiesterases (PDEs), by catalysing the hydrolysis of cAMP and cGMP, limit the intracellular levels of cyclic nucleotides, thus regulating platelet function. The inhibition of PDEs may therefore exert a strong platelet inhibitory effect. Platelets possess three PDE isoforms (PDE2, PDE3 and PDE5), with different selectivity for cAMP and cGMP. Several nonselective or isoenzyme-selective PDE inhibitors have been developed, and some of them have entered clinical use as antiplatelet agents. This review focuses on the effect of PDE2, PDE3 and PDE5 inhibitors on platelet function and on the evidence for an antithrombotic action of some of them, and in particular of dipyridamole and cilostazol.

Targeting phosphodiesterases in anti-platelet therapy

There are two primary modes of platelet inhibition: blockade of membrane receptors or neutralization of intracellular pathways. Both means of inhibition have proven benefits in the prevention and resolution of atherothrombotic events. With regard to intracellular inhibition, phosphodiesterases (PDEs) are fundamental for platelet function. Platelets possess several PDEs (PDE2, PDE3 and PDE5) that catalyze the hydrolysis of cyclic adenosine 3'-5'-monophosphate (cAMP) and cyclic guanosine 3'-5'-monophosphate (cGMP), thereby limiting the levels of intracellular nucleotides. PDE inhibitors, such as cilostazol and dipyridamole, dampen platelet function by increasing cAMP and cGMP levels. This review focuses on the roles of PDE inhibitors in modulating platelet function, with particular attention paid to drugs that have anti-platelet clinical indications.

Streptococcus sanguinis-induced cytokine release from platelets

BACKGROUND

There is increasing evidence that both chronic and acute infections play a role in the development and progression of atherothrombotic disorders. One potential mechanism is the direct activation of platelets by bacteria. A wide range of bacterial species activate platelets through heterogeneous mechanisms. The oral micro-organism S. sanguinis stimulates platelet aggregation in vitro in a strain-dependent manner, although there are no reports of associated cytokine production.

OBJECTIVE

The aim of the present study was to determine whether platelet activation by S. sanguinis involved the release of pro-inflammatory and immune modulating factors, and whether activation was enhanced by epinephrine.

METHODS AND RESULTS

Four strains of S. sanguinis and one of S. gordonii stimulated the release of RANTES, PF4, sCD40L and PDGF-AB, whereas only one S. sanguinis strain caused the release of sCD62p. Epinephrine enhanced S. sanguinis-induced platelet aggregation and phosphorylation of phospholipase Cγ2 and Erk, but inhibited RANTES, PF4, sCD40L and PDGF-AB release. Wortmannin inhibited S. sanguinis-induced aggregation and release; however, only aggregation was partially reversed by epinephrine.

CONCLUSIONS

The present study demonstrates that platelets respond to S. sanguinis with both prothrombotic and pro-inflammatory/immune-modulating responses. Epinephrine, potentially released in response to infection and/or stress, can significantly enhance the prothrombotic response, thereby providing a putative link between bacteraemia and acute coronary events during stress. In contrast, epinephrine inhibited the pro-inflammatory/immune-modulating response by an undetermined mechanism.

Differentiation of cGMP-dependent and -independent nitric oxide effects on platelet apoptosis and reactive oxygen species production using platelets lacking soluble guanylyl cyclase

Platelet activation is an irreversible process resulting in platelet apoptosis and necrosis, and circulating platelets contain many components of the apoptotic machinery. Cyclic guanosine monophosphate (cGMP) generated by nitric oxide (NO) activated soluble guanylyl cyclase (sGC) plays a crucial role in preventing platelet activation. However, in addition to activation of sGC, cGMP-independent NO effects in platelets have been described. To differentiate between cGMP-dependent and -independent NO effects on platelet apoptosis and reactive oxygen species (ROS) production, we generated platelet-specific sGC-deficient mice (PS-GCKO). Platelet apoptosis was induced by a combination of thrombin/convulxin (Thr/Cvx) and assessed by phosphatidylserine (PS) surface exposure, and loss of the mitochondrial membrane potential. NO-induced inhibition of PS externalisation was mediated only by cGMP-dependent mechanisms. Inhibition of the mitochondrial membrane potential decrease at low NO concentration was also cGMP-dependent but became cGMP-independent at high NO concentrations. In contrast, inhibition of ROS formation at any NO concentration was mediated by cGMP-independent mechanisms, very likely due to direct radical scavenging. NO inhibits platelet apoptosis by cGMP-dependent mechanisms and ROS production by cGMP-independent mechanisms. The PS-GCKO mouse model is an important tool for the differentiation of cGMP-dependent and -independent NO effects on platelets.

Nitric oxide at a glance

In healthy blood vessels excessive platelet activation is counterbalanced by negative signalling cascades that modulate activation. This is achieved primarily through endothelial-derived nitric oxide (NO) and prostacyclin (PGI2). The biological effects of NO are mediated through stimulation soluble guanylyl cyclase (sGC) activation of cyclic AMP and GMP-mediated signalling pathways. In the present review examine our current understanding of NO-mediated regulation of platelets and highlight key issues that remain unresolved.

Characterization of the L-arginine-NO-cGMP pathway in spontaneously hypertensive rat platelets: the effects of pregnancy

Nitric oxide (NO) is a short-lived intercellular messenger that provides an efficient vascular regulatory mechanism to support homeostasis and prevent thrombosis. Endothelial dysfunction and reduced NO bioavailability have a central role in hypertension associated with pregnancy. The purpose of this study was to investigate the impact of pregnancy on the L-arginine-NO-cGMP pathway in platelets and its correlation to platelet function and blood pressure in normotensive rats and spontaneously hypertensive rats (SHRs). Platelets were obtained from blood on the 20th day of pregnancy from female SHRs (SHR-P) and normotensive controls (P) or age-matched nonpregnant rats (SHR-NP and NP). Intraplatelet NO synthase (NOS) activity was reduced in P compared to NP, despite unchanged L-arginine influx. The expression levels of endothelial NOS (eNOS) and inducible NOS (iNOS) were diminished during pregnancy in normotensive rats. Paradoxically, cyclic guanosine monophosphate (cGMP) levels were similar between NP and P, as were phosphodiesterase type 5 (PDE5) expression and platelet aggregation induced by adenosine diphosphate. In SHRs, L-arginine influx was reduced in SHR-P compared to SHR-NP. SHR-P exhibited impaired NOS activity and reduced iNOS expression compared with SHR-NP. Soluble guanylyl cyclase and PDE5 expression in platelets were lower in SHR-P than in SHR-NP, whereas no differences were noted between groups with respect to cGMP levels. However, increased levels of cGMP were observed in SHR-P compared to normotensive groups and platelet aggregability remained unaltered. In conclusion, these observations prompted the hypothesis that normal platelet aggregation in pregnant SHRs may be related to a reduction in PDE5 expression and consequently the maintenance of cGMP levels, independently of reduced platelet NO bioavailability.

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.

Thrombin and collagen induce a feedback inhibitory signaling pathway in platelets involving dissociation of the catalytic subunit of protein kinase A from an NFkappaB-IkappaB complex

Protein kinase A (PKA) activation by cAMP phosphorylates multiple target proteins in numerous platelet inhibitory pathways that have a very important role in maintaining circulating platelets in a resting state. Here we show that in thrombin- and collagen-stimulated platelets, PKA is activated by cAMP-independent mechanisms involving dissociation of the catalytic subunit of PKA (PKAc) from an NFkappaB-IkappaBalpha-PKAc complex. We demonstrate mRNA and protein expression for most of the NFkappaB family members in platelets. From resting platelets, PKAc was co-immunoprecipitated with IkappaBalpha, and conversely, IkappaBalpha was also co-immunoprecipitated with PKAc. This interaction was significantly reduced in thrombin- and collagen-stimulated platelets. Stimulation of platelets with thrombin- or collagen-activated IKK, at least partly by PI3 kinase-dependent pathways, leading to phosphorylation of IkappaBalpha, disruption of an IkappaBalpha-PKAc complex, and release of free, active PKAc, which phosphorylated VASP and other PKA substrates. IKK inhibitor inhibited thrombin-stimulated IkBalpha phosphorylation, PKA-IkBalpha dissociation, and VASP phosphorylation, and potentiated integrin alphaIIbbeta3 activation and the early phase of platelet aggregation. We conclude that thrombin and collagen not only cause platelet activation but also appear to fine-tune this response by initiating downstream NFkappaB-dependent PKAc activation, as a novel feedback inhibitory signaling mechanism for preventing undesired platelet activation.

Protein kinase C-mediated phosphorylation and activation of PDE3A regulate cAMP levels in human platelets

The elevation of [cAMP](i) is an important mechanism of platelet inhibition and is regulated by the opposing activity of adenylyl cyclase and phosphodiesterase (PDE). In this study, we demonstrate that a variety of platelet agonists, including thrombin, significantly enhance the activity of PDE3A in a phosphorylation-dependent manner. Stimulation of platelets with the PAR-1 agonist SFLLRN resulted in rapid and transient phosphorylation of PDE3A on Ser(312), Ser(428), Ser(438), Ser(465), and Ser(492), in parallel with the PKC (protein kinase C) substrate, pleckstrin. Furthermore, phosphorylation and activation of PDE3A required the activation of PKC, but not of PI3K/PKB, mTOR/p70S6K, or ERK/RSK. Activation of PKC by phorbol esters also resulted in phosphorylation of the same PDE3A sites in a PKC-dependent, PKB-independent manner. This was further supported by the finding that IGF-1, which strongly activates PI3K/PKB, but not PKC, did not regulate PDE3A. Platelet activation also led to a PKC-dependent association between PDE3A and 14-3-3 proteins. In contrast, cAMP-elevating agents such as PGE(1) and forskolin-induced phosphorylation of Ser(312) and increased PDE3A activity, but did not stimulate 14-3-3 binding. Finally, complete antagonism of PGE(1)-evoked cAMP accumulation by thrombin required both G(i) and PKC activation. Together, these results demonstrate that platelet activation stimulates PKC-dependent phosphorylation of PDE3A on Ser(312), Ser(428), Ser(438), Ser(465), and Ser(492) leading to a subsequent increase in cAMP hydrolysis and 14-3-3 binding.

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.