Platelets, atherosclerosis and the endothelium: new therapeutic targets?

Platelet Microparticles: A Tool to Predict Infarction Area in Rats

Background: Whether there is a quantitative correlation between platelet microparticles (PMPs)/calpain and infarction area is still unclear. Whether present antiplatelet agents can improve myocardial infarction by influencing PMPs need to be revealed. The object of our study was to answer those questions. Methods: Male Wistar rats were used for all studies. All rats were randomly divided into five groups: sham-operated group, myocardial infarction group (blank control group), aspirin intervention group, aspirin combined with clopidogrel intervention group, and aspirin combined with ticagrelor intervention group. Venous blood and hearts were collected at day 7 following MI. ELISA was applied to detect PMPs level. Infarction size was determined by TTC staining method. The comparisons of multiple means were tested with analysis of variance. And the two-two comparisons among the means were done by Student-Newman-Keuls and LSD method. Results: PMPs level and infarction area did not differ between aspirin combined with clopidogrel intervention group and aspirin combined with ticagrelor intervention group. However, significant differences were detected between any two other groups. PMPs were decreased more in dual antiplatelet intervention group. Pearson correlation analysis showed a strong correlation between PMPs and infarction area (r = 0.90) as well as calpain 10 and infarction area (r = 0.84). We created a regression model: y = 4.61 + 0.28*x (y: infarction area, x: PMPs) to assess myocardial infarction area by PMPs level. Conclusions: Antiplatelet agents may decrease infarction areas by modifying PMPs. There was a strong correlation between PMPs and infarction area. Therefore, PMPs could be used as a tool to assess infarction area.

Higher Plasma Concentrations of Platelet Microparticles in Patients With Acute Coronary Syndrome: A Systematic Review and Meta-analysis

BACKGROUND

Platelet microparticles (PMP), shedding on platelet activation, have been proposed as key components in the procoagulant and proinflammatory process. The aim of this study was to clarify the correlation between plasma PMP concentration and the presence of acute coronary syndrome (ACS).

METHODS

We searched for potential relevant studies in PubMed, EMBASE, the Cochrane Library, and Web of Science databases before December 2015. After screening for eligibility, 11 observational studies that tested the plasma concentration of PMP in patients with ACS were retrieved for comprehensive review, quality assessment, and data extraction.

RESULTS

Seven studies (64%) provided explicit information between healthy controls and patients with ACS. Five studies (45%) addressed the plasma levels of PMP between patients with ACS and patients with stable angina. Moreover, 5 studies (45%) compared changes in PMP concentration before and after percutaneous coronary intervention (PCI) in patients with ACS. The results showed a significant difference in plasma PMP levels between the patients with ACS and healthy controls, with the pooled standardized mean difference of 1.95 (95% confidence intervals, 0.87-3.02; P < 0.0001). And the plasma concentration of PMP in patients with ACS was higher before PCI than after PCI (standardized mean difference, -0.97; 95% confidence interval, -1.91 to -0.03; P = 0.043). Four of the five studies described that patients with ACS had higher plasma PMP concentration than patients with stable angina, but there was no significant difference between these 2 patient cohorts.

CONCLUSIONS

PMP is a promising biomarker for the development of ACS. Moreover, PCI, the most common treatment for ACS, could effectively decrease the plasma concentration of PMP, indicating PMP as a prognostic factor.

Platelet microparticles: detection and assessment of their paradoxical functional roles in disease and regenerative medicine

There is increasing research on and clinical interest in the physiological role played by platelet microparticles (PMPs). PMPs are 0.1-1-μm fragments shed from plasma membranes of platelets that are undergoing activation, stress, or apoptosis. They have a phospholipid-based structure and express functional receptors from platelet membranes. As they are the most abundant microparticles in the blood and they express the procoagulant phosphatidylserine, PMPs likely complement, if not amplify, the functions of platelets in hemostasis, thrombosis, cancer, and inflammation, but also act as promoters of tissue regeneration. Their size and structure make them instrumental in platelet-cell communications as a delivery tool of platelet-borne bioactive molecules including growth factors, other signaling molecules and micro (mi)RNA. PMPs can therefore be a pathophysiological threat or benefit to the cellular environment when interacting with the blood vasculature. There is also increasing evidence that PMP generation is triggered during blood collection, separation into components, and storage, a phenomenon potentially leading to thrombotic and inflammatory side effects in transfused patients. Evaluating PMPs requires strict pre-analytical and analytical procedures to avoid artifactual generation and ensure accurate assessment of the number, size repartitioning, and functional properties. This review describes the physical and functional methods developed for analyzing and quantifying PMPs. It then presents the functional roles of PMPs as markers or triggers of diseases like thrombosis, atherosclerosis, and cancer, and discusses the possible detrimental immunological impact of their generation in blood components. Finally we review the potential function of PMPs in tissue regeneration and the prospects for their use in therapeutic strategies for human health.

The role of platelets in the pathogenesis of cerebral malaria

Malaria is a major cause of morbidity and mortality in the developing world and cerebral malaria is responsible for the majority of malaria-associated deaths. There is a strong association between thrombocytopenia and outcome in malaria, suggesting a role for platelets in the pathogenesis of malaria. This thrombocytopenia is likely due to platelet activation possibly through an interaction between PfEMP1 on plasmodium and CD36 on platelets. Platelet activation by plasmodium has two potential consequences. It can lead to the formation of micro-aggregates of infected red blood cells and platelets which can occlude blood vessels and it also leads to binding to and activation of the endothelium.

Imaging of the vulnerable plaque: noninvasive and invasive techniques

In a large proportion of previously asymptomatic individuals, sudden coronary death or acute myocardial infarction occurs as the first manifestation of coronary atherosclerosis. Imaging of coronary atheromatous plaques has traditionally centered on assessing the degree of luminal stenosis. The angiographic techniques that are routinely used to identify stenotic atherosclerotic lesions are unable to identify high-risk plaques; plaques prone to rupture and cause a cardiovascular event. This is partly due to the fact that the majority of culprit lesions that produce acute cardiovascular syndromes are not severely stenotic, possibly due to significant positive remodeling and reduced protective collateral circulation as well as because the risk of plaque rupture is more closely related to plaque content than plaque size. Recently, the focus of new imaging techniques is to identify the high risk plaques; the "vulnerable plaques." In this review, we will refer to the noninvasive and invasive techniques that can detect the vulnerable plaque.

Involvement of Ca2+ in the inhibition by crocetin of platelet activity and thrombosis formation

Crocetin, a unique carotenoid with potent antioxidative and anti-inflammatory activities, is a major ingredient of saffron used as an important spice and food colorant in various parts of the world. In the present study, the effects of crocetin on platelet activity and thrombosis formation were systematically investigated. Crocetin showed a dose-dependent inhibition of platelet aggregation induced by ADP, collagen, but not by arachidonic acid (AA). Crocetin significantly attenuated dense granule release, while neither platelets adhesion to collagen nor cyclic AMP level was altered by crocetin. Pretreatment with crocetin was confirmed to partially inhibit Ca (2+) mobilization via reducing both intracellular Ca (2+) release and extracellular Ca (2+) influx. Besides that, crocetin prolonged the occlusive time in electrical stimulation-induced carotid arterial thrombosis. These findings suggest that the favorable impacts of crocetin on platelet activity and thrombosis formation may be related to the inhibition of Ca (2+) elevation in stimulated platelets.

Clinically apparent atherosclerotic disease in diabetes is associated with an increase in platelet microparticle levels

BACKGROUND

The commonest cause of mortality in patients with Type 2 diabetes is atherothrombosis, which can be related to abnormalities in the coagulation and fibrinolytic pathways, as well as in platelet function. Platelet microparticles (PMPs) may contribute to the prothrombotic state and may promote the progression of atherosclerosis. We hypothesized that PMPs are elevated in Type 2 diabetes and that patients with Type 2 diabetes and clinically apparent atherosclerosis would have the highest levels. Similarly, we hypothesized that soluble plasma P-selectin (sPsel) and CD40L (both molecules which are released by activated platelets), as well as %CD62P (P-selectin) and %CD63 positivity on platelets quantified by flow cytometry, would be highest in patients with Type 2 diabetes and clinically apparent atherosclerotic disease, and might be correlated to PMP levels.

METHODS

Venous blood was obtained from 21 Type 2 diabetic patients without atherosclerotic complications, 18 diabetic patients with clinically apparent atherosclerotic disease and 21 non-diabetic control subjects. PMPs, as well as %CD62P and %CD63 positivity on platelets, were quantified by flow cytometry. sPsel and CD40L were measured using ELISA.

RESULTS

Patients with Type 2 diabetes and clinically apparent atherosclerotic disease had the highest PMP (P=0.045) and sPsel (P=0.046) levels, compared with patients without complications (who had intermediate PMP levels) and control subjects. Control subjects had the lowest CD40L levels (P<0.001) when compared with patients with Type 2 diabetes, with no difference in sCD40L levels between the two diabetic subgroups. %CD62P and %CD63 positivity did not differ between the groups. PMP levels correlated with %CD62P positivity (P=0.026) but not to %CD63 positivity (P=0.089), sCD40L (P=0.407) or sP-sel (P=0.163); sCD40L levels did not correlate with any other marker of platelet activation.

CONCLUSION

PMPs are elevated in Type 2 diabetes. In addition, patients with clinically apparent atherosclerosis had the highest levels of PMPs and sPsel. Thus, PMPs may be a marker of symptomatic atherosclerotic vascular disease in Type 2 diabetes, and may both represent a useful risk stratification tool as well as a novel therapeutic target for anti-thrombotic drugs.

Estrogenic regulation of tissue factor and tissue factor pathway inhibitor in platelets

Oral estrogen treatment increases thrombotic risk. Tissue factor (TF), tissue factor pathway inhibitor (TFPI), and platelet interaction with leukocytes are important determinants of thrombogenesis. Therefore, the present study was designed to define and compare platelet TF and TFPI mRNA and adhesion protein expression in platelets derived from animals treated with different types of oral estrogens. Ovariectomized pigs were treated with 17beta-estradiol (2 mg/day), conjugated equine estrogen (CEE; 0.625 mg/day), or raloxifene (60 mg/day) for 4 wk. Compared with intact animals, ovariectomy and treatment differentially affected populations of leukocytes: neutrophils decreased whereas lymphocytes increased significantly 4 wk after ovariectomy and with 17beta-estradiol and CEE treatments; eosinophils increased only with 17beta-estradiol treatment. Content of TF protein increased in platelets from 17beta-estradiol- and raloxifene-treated pigs, whereas TF mRNA was detected only in platelets from 17beta-estradiol- and CEE treated pigs. TFPI mRNA increased in platelets after ovariectomy and estrogen treatment. Only a trace of TFPI protein was detected, but a higher-molecular-mass protein was observed in all treatment groups. Expression of CD40 and CD40 ligand increased with ovariectomy and decreased with 17beta-estradiol and CEE treatments more than with raloxifene. The ratio of activated to basal P-selectin expression decreased with ovariectomy and increased with raloxifene treatments. These results suggest that estrogenic formulations may affect individual thrombotic risk by different mechanisms that regulate TF and platelet-leukocytic interactions. These studies provide the rationale for evaluation of interactions among platelets and TF and TFPI expression on thrombin generation during estrogen treatment in humans.

CD40-ligand-dependent induction of COX-2 gene expression in endothelial cells by activated platelets: inhibitory effects of atorvastatin

Increasing evidence shows the importance of platelet-endothelial cell interactions in the progression of atherosclerosis. Platelets contribute to coronary events both as major components of thrombi and as a triggering factor in inflammation that leads to plaque vulnerability. Recent data suggest that statins, besides their lipid-lowering properties, exert pleiotropic effects that may be beneficial in atherosclerosis. Whether activated platelets influence cyclooxygenase-2 (COX-2) expression in human umbilical vein endothelial cells (HUVEC), the effect of atorvastatin, and possible mechanisms were investigated. COX-2 gene expression in HUVEC was studied using real-time polymerase chain reaction. CD40 ligand surface expression of platelets was tested by fluorescence-activated cell sorting analyses. Activated platelets significantly up-regulated COX-2 gene expression in HUVEC. Co-incubation of platelets with atorvastatin was shown to reverse this up-regulation via reduction of CD40 ligand surface expression on platelets. Data suggest that atorvastatin influences CD40-CD40-ligand-dependent platelet-endothelial interaction and that this influence affects platelet-induced COX-2 expression in HUVEC.