Thrombospondin-1 promotes hemostasis through modulation of cAMP signaling in blood platelets

Thrombospondin-1 (TSP-1) is released by platelets upon activation and can increase platelet activation, but its role in hemostasis in vivo is unclear. We show that TSP-1 is a critical mediator of hemostasis that promotes platelet activation by modulating inhibitory cyclic adenosine monophosphate (cAMP) signaling. Genetic deletion of TSP-1 did not affect platelet activation in vitro, but in vivo models of hemostasis and thrombosis showed that TSP-1-deficient mice had prolonged bleeding, defective thrombosis, and increased sensitivity to the prostacyclin mimetic iloprost. Adoptive transfer of wild-type (WT) but not TSP-1-/- platelets ameliorated the thrombotic phenotype, suggesting a key role for platelet-derived TSP-1. In functional assays, TSP-1-deficient platelets showed an increased sensitivity to cAMP signaling, inhibition of platelet aggregation, and arrest under flow by prostacyclin (PGI2). Plasma swap experiments showed that plasma TSP-1 did not correct PGI2 hypersensitivity in TSP-1-/- platelets. By contrast, incubation of TSP-1-/- platelets with releasates from WT platelets or purified TSP-1, but not releasates from TSP-1-/- platelets, reduced the inhibitory effects of PGI2. Activation of WT platelets resulted in diminished cAMP accumulation and downstream signaling, which was associated with increased activity of the cAMP hydrolyzing enzyme phosphodiesterase 3A (PDE3A). PDE3A activity and cAMP accumulation were unaffected in platelets from TSP-1-/- mice. Platelets deficient in CD36, a TSP-1 receptor, showed increased sensitivity to PGI2/cAMP signaling and diminished PDE3A activity, which was unaffected by platelet-derived or purified TSP-1. This scenario suggests that the release of TSP-1 regulates hemostasis in vivo through modulation of platelet cAMP signaling at sites of vascular injury.

Effect of induced hypoglycemia on inflammation and oxidative stress in type 2 diabetes and control subjects

Intensive diabetes control has been associated with increased mortality in type 2 diabetes (T2DM); this has been suggested to be due to increased hypoglycemia. We measured hypoglycemia-induced changes in endothelial parameters, oxidative stress markers and inflammation at baseline and after a 24-hour period in type 2 diabetic (T2DM) subjects versus age-matched controls. Case-control study: 10 T2DM and 8 control subjects. Blood glucose was reduced from 5 (90 mg/dl) to hypoglycemic levels of 2.8 mmol/L (50 mg/dl) for 1 hour by incremental hyperinsulinemic clamps using baseline and 24 hour samples. Measures of endothelial parameters, oxidative stress and inflammation at baseline and at 24-hours post hypoglycemia were performed: proteomic (Somalogic) analysis for inflammatory markers complemented by C-reactive protein (hsCRP) measurement, and proteomic markers and urinary isoprostanes for oxidative measures, together with endothelial function. Between baseline and 24 -hours after hypoglycemia, 15 of 140 inflammatory proteins differed in T2DM whilst only 1 of 140 differed in controls; all returned to baseline at 24-hours. However, elevated hsCRP levels were seen at 24-hours in T2DM (2.4 mg/L (1.2-5.4) vs. 3.9 mg/L (1.8-6.1), Baseline vs 24-hours, P < 0.05). In patients with T2DM, between baseline and 24-hour after hypoglycemia, only one of 15 oxidative stress proteins differed and this was not seen in controls. An increase (P = 0.016) from baseline (73.4 ng/mL) to 24 hours after hypoglycemia (91.7 ng/mL) was seen for urinary isoprostanes. Hypoglycemia resulted in inflammatory and oxidative stress markers being elevated in T2DM subjects but not controls 24-hours after the event.

Atherogenic lipid stress induces platelet hyperactivity through CD36-mediated hyposensitivity to prostacyclin: the role of phosphodiesterase 3A

Prostacyclin (PGI₂) controls platelet activation and thrombosis through a cyclic adenosine monophosphate (cAMP) signaling cascade. However, in patients with cardiovascular diseases this protective mechanism fails for reasons that are unclear. Using both pharmacological and genetic approaches we describe a mechanism by which oxidized low density lipoproteins (oxLDL) associated with dyslipidemia promote platelet activation through impaired PGI₂ sensitivity and diminished cAMP signaling. In functional assays using human platelets, oxLDL modulated the inhibitory effects of PGI₂, but not a phosphodiesterase (PDE)-insensitive cAMP analog, on platelet aggregation, granule secretion and in vitro thrombosis. Examination of the mechanism revealed that oxLDL promoted the hydrolysis of cAMP through the phosphorylation and activation of PDE3A, leading to diminished cAMP signaling. PDE3A activation by oxLDL required Src family kinases, Syk and protein kinase C. The effects of oxLDL on platelet function and cAMP signaling were blocked by pharmacological inhibition of CD36, mimicked by CD36-specific oxidized phospholipids and ablated in CD36^−/− murine platelets. The injection of oxLDL into wild-type mice strongly promoted FeCl3-induced carotid thrombosis in vivo, which was prevented by pharmacological inhibition of PDE3A. Furthermore, blood from dyslipidemic mice was associated with increased oxidative lipid stress, reduced platelet sensitivity to PGI₂ex vivo and diminished PKA signaling. In contrast, platelet sensitivity to a PDE-resistant cAMP analog remained normal. Genetic deletion of CD36 protected dyslipidemic animals from PGI₂ hyposensitivity and restored PKA signaling. These data suggest that CD36 can translate atherogenic lipid stress into platelet hyperactivity through modulation of inhibitory cAMP signaling.

Platelet releasate promotes skeletal myogenesis by increasing muscle stem cell commitment to differentiation and accelerates muscle regeneration following acute injury

AIM

The use of platelets as biomaterials has gained intense research interest. However, the mechanisms regarding platelet-mediated skeletal myogenesis remain to be established. The aim of this study was to determine the role of platelet releasate in skeletal myogenesis and muscle stem cell fate in vitro and ex vivo respectively.

METHODS

We analysed the effect of platelet releasate on proliferation and differentiation of C2C12 myoblasts by means of cell proliferation assays, immunohistochemistry, gene expression and cell bioenergetics. We expanded in vitro findings on single muscle fibres by determining the effect of platelet releasate on murine skeletal muscle stem cells using protein expression profiles for key myogenic regulatory factors.

RESULTS

TRAP6 and collagen used for releasate preparation had a more pronounced effect on myoblast proliferation vs thrombin and sonicated platelets (P < 0.05). In addition, platelet concentration positively correlated with myoblast proliferation. Platelet releasate increased myoblast and muscle stem cell proliferation in a dose-dependent manner, which was mitigated by VEGFR and PDGFR inhibition. Inhibition of VEGFR and PDGFR ablated MyoD expression on proliferating muscle stem cells, compromising their commitment to differentiation in muscle fibres (P < 0.001). Platelet releasate was detrimental to myoblast fusion and affected differentiation of myoblasts in a temporal manner. Most importantly, we show that platelet releasate promotes skeletal myogenesis through the PDGF/VEGF-Cyclin D1-MyoD-Scrib-Myogenin axis and accelerates skeletal muscle regeneration after acute injury.

CONCLUSION

This study provides novel mechanistic insights on the role of platelet releasate in skeletal myogenesis and set the physiological basis for exploiting platelets as biomaterials in regenerative medicine.

Dyslipidemia-associated atherogenic oxidized lipids induce platelet hyperactivity through phospholipase Cγ2-dependent reactive oxygen species generation

Oxidized low-density lipoprotein (oxLDL) and associated oxidized phosphocholine-headgroup phospholipids (oxPCs) activate blood platelets through ligation of the scavenger receptor CD36. Previously, we found that oxLDL stimulated phosphorylation of phospholipase Cγ2 (PLCγ2). However, the functional relevance of PLCγ2 phosphorylation in oxLDL-mediated platelet hyperactivity remained elusive. Here, we set out to explore the functional importance of PLCγ2 in oxLDL-mediated platelet activation using human and genetically modified murine platelets. The CD36-specific oxidized phospholipid (oxPC_CD36) triggered the generation of reactive oxygen species (ROS) in platelets under static and arterial flow conditions. The ROS generation in response to oxPC_CD36 was sustained for up to 3 h but ablated in CD36- and PLCγ2-deficient platelets. The functional importance of ROS generation in response to atherogenic lipid stress was examined through measurement of P-selectin expression. OxPC_CD36 induced P-selectin expression, but required up to 60 min incubation, consistent with the timeline for ROS generation. P-selectin expression was not observed in CD36- and PLCγ2-deficient mice. The ability of oxPC_CD36 and oxLDL to stimulate P-selectin expression was prevented by incubation of platelets with the ROS scavenger N-acetyl-cysteine (NAC) and the NOX-2 inhibitor gp91ds-tat, but not with the NOX-1 inhibitor ML171. In summary, we provide evidence that prolonged exposure to oxLDL-associated oxidized phospholipids induces platelet activation via NOX-2-mediated ROS production in a CD36- and PLCγ2-dependent manner.

Attenuation of oxidative stress-induced lesions in skeletal muscle in a mouse model of obesity-independent hyperlipidaemia and atherosclerosis through the inhibition of Nox2 activity

Obesity leading to hyperlipidaemia and atherosclerosis is recognised to induce morphological and metabolic changes in many tissues. However, hyperlipidaemia can occur in the absence of obesity. The impact of the latter scenario on skeletal muscle and liver is not understood sufficiently. In this regard, we used the Apolipoprotein E-deficient (ApoE^-/-) mouse model, an established model of hyperlipidaemia and atherosclerosis, that does not become obese when subjected to a high-fat diet, to determine the impact of Western-type diet (WD) and ApoE deficiency on skeletal muscle morphological, metabolic and biochemical properties. To establish the potential of therapeutic targets, we further examined the impact of Nox2 pharmacological inhibition on skeletal muscle redox biology. We found ectopic lipid accumulation in skeletal muscle and the liver, and altered skeletal muscle morphology and intramuscular triacylglycerol fatty acid composition. WD and ApoE deficiency had a detrimental impact in muscle metabolome, followed by perturbed gene expression for fatty acid uptake and oxidation. Importantly, there was enhanced oxidative stress in the skeletal muscle and development of liver steatosis, inflammation and oxidative protein modifications. Pharmacological inhibition of Nox2 decreased reactive oxygen species production and protein oxidative modifications in the muscle of ApoE^-/- mice subjected to a Western-type diet. This study provides key evidence to better understand the pathophysiology of skeletal muscle in the context of hyperlipidaemia and atherosclerosis and identifies Nox2 as a potential target for attenuating oxidative stress in skeletal muscle in a mouse model of obesity-independent hyperlipidaemia.

Reversal of stress fibre formation by Nitric Oxide mediated RhoA inhibition leads to reduction in the height of preformed thrombi

Evidence has emerged to suggest that thrombi are dynamic structures with distinct areas of differing platelet activation and inhibition. We hypothesised that Nitric oxide (NO), a platelet inhibitor, can modulate the actin cytoskeleton reversing platelet spreading, and therefore reduce the capability of thrombi to withstand a high shear environment. Our data demonstrates that GSNO, DEANONOate, and a PKG-activating cGMP analogue reversed stress fibre formation and increased actin nodule formation in adherent platelets. This effect is sGC dependent and independent of ADP and thromboxanes. Stress fibre formation is a RhoA dependent process and NO induced RhoA inhibition, however, it did not phosphorylate RhoA at ser188 in spread platelets. Interestingly NO and PGI₂ synergise to reverse stress fibre formation at physiologically relevant concentrations. Analysis of high shear conditions indicated that platelets activated on fibrinogen, induced stress fibre formation, which was reversed by GSNO treatment. Furthermore, preformed thrombi on collagen post perfused with GSNO had a 30% reduction in thrombus height in comparison to the control. This study demonstrates that NO can reverse key platelet functions after their initial activation and identifies a novel mechanism for controlling excessive thrombosis.

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 Spatiotemporal Regulation of cAMP Signaling in Blood Platelets-Old Friends and New Players

Atherothrombosis, the pathology underlying numerous cardiovascular diseases, is a major cause of death globally. Hyperactive blood platelets play a key role in the atherothrombotic process through the release of inflammatory mediators and formation of thrombi. In healthy blood vessels, excessive platelet activation is restricted by endothelial-derived prostacyclin (PGI₂) through cyclic adenosine-5′-monophosphate (cAMP) and protein kinase A (PKA)-dependent mechanisms. Elevation in intracellular cAMP is associated with the control of a number of distinct platelet functions including actin polymerisation, granule secretion, calcium mobilization and integrin activation. Unfortunately, in atherosclerotic disease the protective effects of cAMP are compromised, which may contribute to pathological thrombosis. The cAMP signaling network in platelets is highly complex with the presence of multiple isoforms of adenylyl cyclase (AC), PKA, and phosphodiesterases (PDEs). However, a precise understanding of the relationship between specific AC, PKA, and PDE isoforms, and how individual signaling substrates are targeted to control distinct platelet functions is still lacking. In other cells types, compartmentalisation of cAMP signaling has emerged as a key mechanism to allow precise control of specific cell functions. A-kinase anchoring proteins (AKAPs) play an important role in this spatiotemporal regulation of cAMP signaling networks. Evidence of AKAP-mediated compartmentalisation of cAMP signaling in blood platelets has begun to emerge and is providing new insights into the regulation of platelet function. Dissecting the mechanisms that allow cAMP to control excessive platelet activity without preventing effective haemostasis may unleash the possibility of therapeutic targeting of the pathway to control unwanted platelet activity.

Targeting of type I protein kinase A to lipid rafts is required for platelet inhibition by the 3',5'-cyclic adenosine monophosphate-signaling pathway

BACKGROUND

Platelet adhesion to von Willebrand factor (VWF) is modulated by 3',5'-cyclic adenosine monophosphate (cAMP) signaling through protein kinase A (PKA)-mediated phosphorylation of glycoprotein (GP)Ibβ. A-kinase anchoring proteins (AKAPs) are proposed to control the localization and substrate specificity of individual PKA isoforms. However, the role of PKA isoforms in regulating the phosphorylation of GPIbβ and platelet response to VWF is unknown.

OBJECTIVES

We wished to determine the role of PKA isoforms in the phosphorylation of GPIbβ and platelet activation by VWF as a model for exploring the selective partitioning of cAMP signaling in platelets.

RESULTS

The two isoforms of PKA in platelets, type I (PKA-I) and type II (PKA-II), were differentially localized, with a small pool of PKA-I found in lipid rafts. Using a combination of Far Western blotting, immunoprecipitation, proximity ligation assay and cAMP pull-down we identified moesin as an AKAP that potentially localizes PKA-I to rafts. Introduction of cell-permeable anchoring disruptor peptide, RI anchoring disruptor (RIAD-Arg11 ), to block PKA-I/AKAP interactions, uncoupled PKA-RI from moesin, displaced PKA-RI from rafts and reduced kinase activity in rafts. Examination of GPIbβ demonstrated that it was phosphorylated in response to low concentrations of PGI2 in a PKA-dependent manner and occurred primarily in lipid raft fractions. RIAD-Arg11 caused a significant reduction in raft-localized phosphoGPIbβ and diminished the ability of PGI2 to regulate VWF-mediated aggregation and thrombus formation in vitro.

CONCLUSION

We propose that PKA-I-specific AKAPs in platelets, including moesin, organize a selective localization of PKA-I required for phosphorylation of GPIbβ and contribute to inhibition of platelet VWF interactions.

The effects of treatment with liraglutide on atherothrombotic risk in obese young women with polycystic ovary syndrome and controls

BACKGROUND

Polycystic ovary syndrome (PCOS) is associated with obesity and increased cardiovascular (CV) risk markers. In this study our aim was to assess the effects of six months treatment with liraglutide 1.8 mg od on obesity, and CV risk markers, particularly platelet function, in young obese women with PCOS compared to controls of similar age and weight.

METHODS

Carotid intima-media wall thickness (cIMT) was measured by B-mode ultrasonography, platelet function by flow cytometry, clot structure/lysis by turbidimetric assays and endothelial function by ELISA and post-ischaemic reactive hyperemia (RHI). Data presented as mean change (6-month - baseline) ± standard deviation.

RESULTS

Nineteen obese women with PCOS and 17 controls, of similar age and weight, were recruited; baseline atherothrombotic risk markers did not differ between the two groups. Twenty five (69.4%) participants completed the study (13 PCOS, 12 controls). At six months, weight was significantly reduced by 3.0 ± 4.2 and 3.8 ± 3.4 kg in the PCOS and control groups, respectively; with no significant difference between the two groups, P = 0.56. Similarly, HOMA-IR, triglyceride, hsCRP, urinary isoprostanes, serum endothelial adhesion markers (sP-selectin, sICAM and sVCAM), and clot lysis area were equally significantly reduced in both groups compared to baseline. Basal platelet P-selectin expression was significantly reduced at six months in controls -0.17 ± 0.26 but not PCOS -0.12 ± 0.28; between groups difference, 95% confidence interval = -0.14 - 0.26, P = 0.41. No significant changes were noted in cIMT or RHI.

CONCLUSIONS

Six months treatment with liraglutide (1.8 mg od) equally affected young obese women with PCOS and controls. In both groups, liraglutide treatment was associated with 3-4% weight loss and significant reduction in atherothrombosis markers including inflammation, endothelial function and clotting. Our data support the use of liraglutide as weight loss medication in simple obesity and suggest a potential beneficial effect on platelet function and atherothrombotic risk at 6 months of treatment.

TRIAL REGISTRATION

Clinical trial reg. no. ISRCTN48560305. Date of registration 22/05/2012.

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.

Acute hypertriglyceridemia induces platelet hyperactivity that is not attenuated by insulin in polycystic ovary syndrome

Background

Atherothrombosis is associated with platelet hyperactivity. Hypertriglyceridemia and insulin resistance (IR) are features of polycystic ovary syndrome (PCOS). The effect of induced hypertriglyceridemia on IR and platelet function was examined in young women with PCOS.

Methods and Results

Following overnight fasting, 13 PCOS and 12 healthy women were infused with saline or 20% intralipid for 5 hours on separate days. Insulin sensitivity was measured using a hyperinsulinemic euglycaemic clamp in the final 2 hours of each infusion. Platelet responses to adenosine diphosphate (ADP) and prostacyclin (PGI₂) were measured by flow cytometric analysis of platelet fibrinogen binding and P‐selectin expression using whole blood taken during each infusion (at 2 hours) and at the end of each clamp. Lipid infusion increased triglycerides and reduced insulin sensitivity in both controls (median, interquartile range ) (5.25 [3.3, 6.48] versus 2.60 [0.88, 3.88] mg kg⁻¹ min⁻¹, P<0.001) and PCOS (3.15 [2.94, 3.85] versus 1.06 [0.72, 1.43] mg kg⁻¹ min⁻¹, P<0.001). Platelet activation by ADP was enhanced and ability to suppress platelet activation by PGI₂ diminished during lipid infusion in both groups when compared to saline. Importantly, insulin infusion decreased lipid‐induced platelet hyperactivity by decreasing their response to 1 μmol/L ADP (78.7% [67.9, 82.3] versus 62.8% [51.8, 73.3], P=0.02) and increasing sensitivity to 0.01 μmol/L PGI₂ (67.6% [39.5, 83.8] versus 40.9% [23.8, 60.9], P=0.01) in controls, but not in PCOS.

Conclusion

Acute hypertriglyceridemia induced IR, and increased platelet activation in both groups that was not reversed by insulin in PCOS subjects compared to controls. This suggests that platelet hyperactivity induced by acute hypertriglyceridemia and IR could contribute athero‐thrombotic risk.

Clinical Trial Registration

URL: www.isrctn.org. Unique Identifier: ISRCTN42448814.

Polycystic ovary syndrome has no independent effect on vascular, inflammatory or thrombotic markers when matched for obesity

INTRODUCTION

Previous studies investigating cardiovascular (CV) risk in obese women with polycystic ovary syndrome (PCOS) have been potentially confounded by not adequately accounting for body weight.

OBJECTIVE

To assess if PCOS increases CV risk independently in young obese women by examining carotid intima-media wall thickness (cIMT) and platelet function.

DESIGN

A case-control study comparing women with PCOS (n = 21) to age (32·8 ± 7·2 vs 33·5 ± 6·7 years), and weight (100·9 ± 16·7 vs 99·3 ± 14·7 kg)-matched controls (n = 19). Platelet function was examined by flow cytometry, clot structure and fibrinolysis by turbidimetric assays and endothelial function by ELISA and post ischaemic reactive hyperaemia.

RESULTS

The PCOS group had higher testosterone 1·2 ± 0·3 vs 0·9 ± 0·3 nmol/l (P = 0·01), HOMA-IR 2·5 ± 1·7 vs 1·7 ± 1·0 (P = 0·08), impaired glucose regulation 33·3% vs 5·3% (P = 0·02), and urinary isoprostane 16·0 ± 4·4 vs 11·8 ± 7·1 ng/ml (P = 0·04) compared to controls. Mean cIMT 0·5 ± 0·05 vs 0·48 ± 0·06 mm (P = 0·36), and basal platelet surface expression (percentage of positive cells) of P-selectin 0·52 ± 0·3 vs 0·43 ± 0·23 (P = 0·40) and fibrinogen binding 0·97 ± 0·4 vs 0·83 ± 0·3 (P = 0·48) did not significantly differ between the PCOS and control groups respectively. Furthermore, platelets sensitivity to stimulation with adenosine-5'-diphosphate or inhibition with prostacyclin, clot structure and fibrinolytic efficiency ex vivo, endothelial reactive hyperaemic index (RHI), inflammation (hsCRP) and adhesion markers (sE-selectin, sP-selectin, sVCAM-1 and sICAM-1) were not significantly different between the two groups.

CONCLUSIONS

PCOS appeared not to independently increase atherothrombotic risk when matched for obesity. It is likely that any excess CV risk in young obese women with PCOS can either be attributed to obesity or is not yet apparent at this early stage of the condition.

Nitric oxide inhibits von Willebrand factor-mediated platelet adhesion and spreading through regulation of integrin alpha(IIb)beta(3) and myosin light chain

BACKGROUND

von Willebrand factor (VWF)-mediated platelet adhesion and spreading at sites of vascular injury is a critical step in hemostasis. This process requires two individual receptors: glycoprotein Ib (GPIb)-V-IX and integrin alpha(IIb)beta(3). However, little is known about the negative regulation of these events.

OBJECTIVES

To examine if the endogenous platelet inhibitor nitric oxide (NO) has differential effects on adhesion, spreading and aggregation induced by immobilized VWF.

RESULTS

S-nitrosoglutathione (GSNO) inhibited platelet aggregation on immobilized VWF under static and flow conditions, but had no effect on platelet adhesion. Primary signaling events underpinning the actions of NO required cyclic GMP but not protein kinase A. Dissecting the roles of GPIb and integrin alpha(IIb)beta(3) demonstrated that NO targeted alpha(IIb)beta(3)-mediated aggregation and spreading, but did not significantly influence GPIb-mediated adhesion. To understand the relationship between the effects of NO on adhesion and subsequent aggregation, we evaluated the activation of alpha(IIb)beta(3) on adherent platelets. NO reduced the phosphorylation of extracellular stimuli-responsive kinase (ERK) and p38, required for integrin activation resulting in reduced binding of the activated alpha(IIb)beta(3)-specific antibody PAC-1 on adherent platelets. Detailed analysis of platelet spreading initiated by VWF demonstrated key roles for integrin alpha(IIb)beta(3) and myosin light chain (MLC) phosphorylation. NO targeted both of these pathways by directly modulating integrin affinity and activating MLC phosphatase.

CONCLUSION

These data demonstrate that initial activation-independent platelet adhesion to VWF via GPIb is resistant to NO, however, NO inhibits GPIb-mediated activation of alpha(IIb)beta(3) and MLC leading to reduced platelet spreading and aggregation.

Globular adiponectin increases cGMP formation in blood platelets independently of nitric oxide

BACKGROUND

Platelet-derived nitric oxide (NO) has been shown to play conflicting roles in platelet function, although it is accepted that NO mediates its actions through soluble guanylyl cyclase (sGC). This confusion concerning the roles of platelet NO may have arisen because of an uncharacterized mechanism for activation of sGC.

OBJECTIVES

To examine the ability of the novel platelet agonist globular adiponectin (gAd) to stimulate the NO-independent cGMP-protein kinase G (PKG) signaling cascade.

METHODS

We used three independent markers of NO signaling, [(3)H]l-citrulline production, cGMP accrual, and immunoblotting of vasodilator-stimulated phosphoprotein (VASP), to examine the NO signaling cascade in response to gAd.

RESULTS

gAd increased platelet cGMP formation, resulting in a dose- and time-dependent increase in phospho-VASP(157/239). Phosphorylation of VASP in response to gAd was mediated by both protein kinase A and PKG. Importantly, cGMP formation occurred in the absence of NO synthase (NOS) activation and in the presence of NOS inhibitors. Indeed, inhibition of the NOS signaling cascade had no influence on gAd-mediated platelet aggregation. Exploration of the mechanism demonstrated that NO-independent cGMP formation, phosphorylation of VASP and association of sGCalpha(1) with heat shock protein-90 induced by gAd were blocked under conditions that inhibited Src kinases, implying a tyrosine kinase-dependent mechanism. Indeed, sGCalpha1 was reversibly tyrosine phosphorylated in response to gAd, collagen, and collagen-related peptide, an effect that required Src kinases and downstream Ca(2+) mobilization.

CONCLUSIONS

These data demonstrate activation of the platelet cGMP signaling cascade by a novel tyrosine kinase-dependent mechanism in the absence of NO.