RGS10 shapes the hemostatic response to injury through its differential effects on intracellular signaling by platelet agonists.

Platelets express ≥2 members of the regulators of G protein signaling (RGS) family. Here, we have focused on the most abundant, RGS10, examining its impact on the hemostatic response in vivo and the mechanisms involved. We have previously shown that the hemostatic thrombi formed in response to penetrating injuries consist of a core of fully activated densely packed platelets overlaid by a shell of less-activated platelets responding to adenosine 5'-diphosphate (ADP) and thromboxane A2 (TxA2). Hemostatic thrombi formed in RGS10-/- mice were larger than in controls, with the increase due to expansion of the shell but not the core. Clot retraction was slower, and average packing density was reduced. Deleting RGS10 had agonist-specific effects on signaling. There was a leftward shift in the dose/response curve for the thrombin receptor (PAR4) agonist peptide AYPGKF but no increase in the maximum response. This contrasted with ADP and TxA2, both of which evoked considerably greater maximum responses in RGS10-/- platelets with enhanced Gq- and Gi-mediated signaling. Shape change, which is G13-mediated, was unaffected. Finally, we found that free RGS10 levels in platelets are actively regulated. In resting platelets, RGS10 was bound to 2 scaffold proteins: spinophilin and 14-3-3γ. Platelet activation caused an increase in free RGS10, as did the endothelium-derived platelet antagonist prostacyclin. Collectively, these observations show that RGS10 serves as an actively regulated node on the platelet signaling network, helping to produce smaller and more densely packed hemostatic thrombi with a greater proportion of fully activated platelets.

Coordination of platelet agonist signaling during the hemostatic response in vivo.

The local microenvironment within an evolving hemostatic plug shapes the distribution of soluble platelet agonists, resulting in a gradient of platelet activation. We previously showed that thrombin activity at a site of vascular injury is spatially restricted, resulting in robust activation of a subpopulation of platelets in the hemostatic plug core. In contrast, adenosine 5'-diphosphate (ADP)/P2Y12 signaling contributes to the accumulation of partially activated, loosely packed platelets in a shell overlying the core. The contribution of the additional platelet agonists thromboxane A2 (TxA2) and epinephrine to this hierarchical organization was not previously shown. Using a combination of genetic and pharmacologic approaches coupled with real-time intravital imaging, we show that TxA2 signaling is critical and nonredundant with ADP/P2Y12 for platelet accumulation in the shell region but not required for full platelet activation in the hemostatic plug core, where thrombin activity is highest. In contrast, epinephrine signaling is dispensable even in the presence of a P2Y12 antagonist. Finally, dual P2Y12 and thrombin inhibition does not substantially inhibit hemostatic plug core formation any more than thrombin inhibition alone, providing further evidence that thrombin is the primary driver of platelet activation in this region. Taken together, these studies show for the first time how thrombin, P2Y12, and TxA2 signaling are coordinated during development of a hierarchical organization of hemostatic plugs in vivo and provide novel insights into the impact of dual antiplatelet therapy on hemostasis and thrombosis.

Mean platelet volume is not associated with platelet reactivity and the extent of coronary artery disease in diabetic patients.

Platelets play a central role in the pathogenesis of coronary artery disease (CAD). Mean platelet volume (MPV) is an indicator of platelet activation, and has been demonstrated to be correlated with platelet reactivity. Diabetic patients have been shown to have larger MPV, that may contribute to higher platelet reactivity and atherothrombotic complications observed in these patients. Therefore, the aim of the current study was to investigate whether MPV is associated with platelet reactivity and the extent of CAD among diabetic patients. We performed a cohort study including 1016 consecutive diabetic patients undergoing coronary angiography at the University Hospital 'Maggiore della Carita', Novara, Italy. CAD is defined as stenosis above 50% in at least one coronary vessel at coronary angiography. Platelet reactivity was evaluated in 50 diabetic patients without history of CAD and who were free (in the past month) from medications which may affect platelet aggregation. Platelet aggregation was evaluated by light transmission aggregometry after stimulation with 1 µg/ml collagen type I. We additionally evaluated platelet surface expression of P-selectin after stimulation with U46619 (a stable synthetic analogue of the prostaglandin PGH2) and plasma concentration of thromboxane B2 (TxB2). Patients were grouped according to tertile values of MPV (<10.6 fl, group 1; 10.6-11.3 fl, group 2; >11.4 fl, group 3). MPV was associated with age (P=0.011), baseline fasting glucose (P=0.044), glycosylated haemoglobin (P=0.005), creatinine (P=0.052) and haemoglobin (P=0.003), but inversely related to platelet count (P<0.001) and triglycerides (P=0.031). Larger MPV was associated with therapy with statins (P=0.012) and diuretics (P=0.021). CAD was observed in 826 patients (81.3%). MPV was not associated with the prevalence of CAD [odds ratio (OR), 0.85 (0.7-1.03), P=0.11]. The results were confirmed in terms of severe CAD [OR, 1.03 (0.88-1.21), P=0.7]. The absence of any significant relationship between MPV and CAD was confirmed after correction for baseline confounding factors [OR, 0.9 (0.75-1.08), P=0.19]. Finally, MPV was not related to platelet reactivity. This is the first study showing that in diabetic patients MPV is not related to platelet reactivity and the prevalence and extent of CAD. Therefore, MPV may not be considered a risk factor for CAD among diabetic patients.

The phytoestrogen 8-prenylnaringenin inhibits agonist-dependent activation of human platelets.

BACKGROUND: Phytoestrogens are plant-derived polyphenolic compounds that exert beneficial effects on human health, mostly related to their estrogen mimetic activity. In particular a strong correlation between phytoestrogens intake and a lower risk of cardiovascular diseases has been reported. The flavanone 8-prenylnaringenin, extracted from hop flowers, has been identified as a novel phytoestrogen, unique with respect to estrogen receptors specificity and potency. However, to date no investigations on the 8-prenylnaringenin role in modulating platelet function have been undertaken. METHODS: We evaluated the effect of 8-prenylnaringenin on platelet aggregation, intracellular calcium mobilization and protein phosphorylation triggered by thrombin and collagen, and platelet adhesion and dense granule secretion triggered by collagen. RESULTS: 8-Prenylnaringenin inhibited platelet aggregation induced by different agonists and platelet adhesion to collagen matrix. 8-Prenylnaringenin directly increased intracellular cAMP and cGMP levels and thus promoted VASP phosphorylation. However, these molecular events were not responsible for the inhibitory action of 8-prenylnaringenin on platelets. Moreover, 8-prenylnaringenin inhibited the phosphorylation of Pyk2, Akt, and ERK1/2. Finally, 8-prenylnaringenin suppressed the mobilization of calcium and the secretion of dense granules. All these effects were independent of estrogen receptors recruitment. CONCLUSIONS: 8-Prenylnaringenin exerted anti-aggregatory and anti-adhesive effects on human platelets, independently of estrogen receptors, acting as an inhibitor of multiple proteins essential for the morphological and biochemical transformations that occur during platelet activation and aggregation. GENERAL SIGNIFICANCE: 8-Prenylnaringenin may represent a useful tool in the therapy and prevention of vascular diseases associated with platelet aggregation, such as atherosclerosis, myocardial infarction, coronary artery disease, and thrombosis.

Dehydroepiandrosterone-sulfate inhibits thrombin-induced platelet aggregation.

Dehydroepiandrosterone (DHEA) and its sulfated form, DHEA-S, are the most abundant steroids circulating in human blood. DHEA stimulates endothelial cells to release high amounts of nitric oxide in the circulation. Nitric oxide activates guanylyl cyclase in platelets thus decreasing the responsiveness of these cells to physiological agonists. However, the impact of DHEA-S and DHEA on platelet function and their possible role in modulating the response of human platelets to physiological agonists were not yet investigated. Here, DHEA-S, but not DHEA, inhibited in vitro thrombin-dependent platelet aggregation in a dose-dependent manner. DHEA-S exerted this effect by decreasing thrombin-dependent dense granule secretion, and so impairing the positive feed-back loop provided by ADP. Furthermore, DHEA-S inhibited thrombin-dependent activation of Akt, ERK1/2, and p38 MAP kinase. Although both DHEA-S and DHEA directly activated in platelets the inhibitory cGMP/PGK/VASP pathway, these events were not responsible for the inhibitory action of DHEA-S in platelets. In addition DHEA-S acted in synergism with nitric oxide in inhibiting platelet aggregation. In conclusion DHEA-S inhibited platelet activation caused by a mild stimulus without completely hampering platelet functionality and thus DHEA-S may participate in the physiological mechanisms that maintain circulating platelets in a resting state. The role played by DHEA-S could be relevant mainly when the functionality of the vascular endothelium is compromised.

Diacylglycerol kinase α mediates 17-ß-estradiol-induced proliferation, motility, and anchorage-independent growth of Hec-1A endometrial cancer cell line through the G protein-coupled estrogen receptor GPR30.

Increased levels of endogenous and/or exogenous estrogens are one of the well known risk factors of endometrial cancer. Diacylglycerol kinases (DGKs) are a family of enzymes which phosphorylate diacylglycerol (DAG) to produce phosphatidic acid (PA), thus turning off and on DAG-mediated and PA-mediated signaling pathways, respectively. DGK α activity is stimulated by growth factors and oncogenes and is required for chemotactic, proliferative, and angiogenic signaling in vitro. Herein, using either specific siRNAs or the pharmacological inhibitor R59949, we demonstrate that DGK α activity is required for 17-ß-estradiol (E2)-induced proliferation, motility, and anchorage-independent growth of Hec-1A endometrial cancer cell line. Impairment of DGK α activity also influences basal cell proliferation and growth in soft agar of Hec-1A, while it has no effects on basal cell motility. Moreover, we show that DGK α activity induced by E2, as well as its observed effects, are mediated by the G protein-coupled estrogen receptor GPR30 (GPER). These findings suggest that DGK α may be a potential target in endometrial cancer therapy.