Nucleotide receptor signaling in platelets

Metastasis and angiogenesis in cervical cancer: key aspects of purinergic signaling in platelets and possible therapeutic targets

Cervical cancer ranks as the fourth most common and fatal cancer among women worldwide. Studies have demonstrated a strong association between purinergic platelet signaling and tumor progression in this type of cancer. The literature shows that neoplastic cells, when in the bloodstream, secrete adenosine triphosphate (ATP) and adenosine nucleotide diphosphate (ADP) that act on their corresponding platelet P2Y and P2X receptors. The interaction of these nucleotides with their receptors results in platelet activation and degranulation, ensuing several consequences, such as vascular endothelial growth factor (VEGF), platelet-derived growth factor, matrix metalloproteinases, ADP, and ATP. These molecules play essential roles in angiogenesis and tumor metastasis in cervical cancer. Several purinergic receptors are found in endothelial cells. Their activation, especially P2Y2, by the nucleotides released by platelets can induce relaxation of the endothelial barrier and consequent extravasation of tumor cells, promoting the development of metastases. Cancer cells that enter the bloodstream during the metastatic process are also subject to high shear stress and immune surveillance. In this context, activated platelets bind to circulating tumor cells and protect them against shear stress and the host's immune system, especially against natural killer cells, facilitating their spread throughout the body. Furthermore, activation of the P2Y12 receptor present on the platelet surface promotes the release of VEGF, the main inducer of angiogenesis in cervical cancer, in addition to increasing the concentration of several other pro-angiogenic molecules. Therefore, this review will address the role of platelet purinergic signaling in tumor progression of cervical cancer and propose possible therapeutic targets.

Distinct Role of GRK3 in Platelet Activation by Desensitization of G Protein-Coupled Receptors

BACKGROUND

 Many platelet agonists mediate their cellular effects through G protein-coupled receptors (GPCRs) to induce platelet activation, and GPCR kinases (GRKs) have been demonstrated to have crucial roles in most GPCR functions in other cell types. Here, we investigated the functional role of GRK3 and the molecular basis for the regulation of GPCR desensitization by GRK3 in platelets.

METHODS

 We used mice lacking GRK3 as well as β-arrestin2, which has been shown to be important in GPCR function in platelets.

RESULTS

 Platelet aggregation and dense granule secretion induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in both GRK3 -/- and β-arrestin2 -/- platelets compared with wild-type (WT) platelets, whereas non-GPCR agonist collagen-induced platelet aggregation and secretion were not affected. We have previously shown that GRK6 is not involved in the regulation of Gq-coupled 5HT2A and Gz-coupled α2A adrenergic receptors. Interestingly, in contrast to GRK6, platelet aggregation induced by costimulation of serotonin and epinephrine, which activate 5-HT2A and α2A adrenergic receptors, respectively, was significantly potentiated in GRK3 -/- platelets, suggesting that GRK3 is involved in general GPCR regulation. In addition, platelet aggregation in response to the second challenge of adenosine diphosphate was restored in GRK3 -/- platelets, whereas restimulation of the agonist failed to induce aggregation in WT platelets, confirming that GRK3 contributes to general GPCR desensitization. Furthermore, 2-MeSADP- and AYPGKF-induced AKT and ERK phosphorylation were significantly potentiated in GRK3 -/- platelets. Finally, GRK3 -/- mice showed shorter tail bleeding times compared with WT, indicating that GRK3 -/- mice is more susceptible to hemostasis.

CONCLUSION

 GRK3 plays a crucial role in the regulation of platelet activation through general GPCR desensitization in platelets.

The Signaling Pathway of the ADP Receptor P2Y12 in the Immune System: Recent Discoveries and New Challenges

P2Y12 is a G-protein-coupled receptor that is activated upon ADP binding. Considering its well-established role in platelet activation, blocking P2Y12 has been used as a therapeutic strategy for antiplatelet aggregation in cardiovascular disease patients. However, receptor studies have shown that P2Y12 is functionally expressed not only in platelets and the microglia but also in other cells of the immune system, such as in monocytes, dendritic cells, and T lymphocytes. As a result, studies were carried out investigating whether therapies targeting P2Y12 could also ameliorate inflammatory conditions, such as sepsis, rheumatoid arthritis, neuroinflammation, cancer, COVID-19, atherosclerosis, and diabetes-associated inflammation in animal models and human subjects. This review reports what is known about the expression of P2Y12 in the cells of the immune system and the effect of P2Y12 activation and/or inhibition in inflammatory conditions. Lastly, we will discuss the major problems and challenges in studying this receptor and provide insights on how they can be overcome.

Expression of microRNA Predicts Cardiovascular Events in Patients with Stable Coronary Artery Disease

BACKGROUND

 New biomarkers are warranted to identify patients with coronary artery disease (CAD) at high risk of recurrent cardiovascular events. It has been reported that the expression of microRNAs (miRs) may influence the development of CAD.

OBJECTIVES

 We aimed to investigate whether the expression of selected candidate miRs is a predictor of cardiovascular events in a cohort of stable CAD patients.

METHODS

 We performed a single-center prospective study of 749 stable CAD patients with a median follow-up of 2.8 years. We investigated the expression of nine candidate miRs and their relation to cardiovascular events in this cohort. The primary endpoint was the composite of nonfatal myocardial infarction (MI), stent thrombosis (ST), ischemic stroke, and cardiovascular death. The composite of nonfatal MI and ST was analyzed as a secondary endpoint. Furthermore, nonfatal MI, ST, ischemic stroke, and all-cause mortality were analyzed as individual endpoints.

RESULTS

 Employing receiver operating characteristic curves, it was shown that compared with traditional cardiovascular risk factors alone, combining the expression of miR-223-3p with existing traditional cardiovascular risk factors increased the predictive value of ST (area under the curve: 0.88 vs. 0.77, p = 0.04), the primary composite endpoint (0.65 vs. 0.61, p = 0.049), and the secondary endpoint of the composite of nonfatal MI and ST (0.68 vs. 0.62, p = 0.04).

CONCLUSION

 Among patients with CAD, adding miR-223-3p expression to traditional cardiovascular risk factors may improve prediction of cardiovascular events, particularly ST. Clinical trials confirming these findings are warranted.

Sex-related differences in the response of anti-platelet drug therapies targeting purinergic signaling pathways in sepsis

Sepsis, a complex clinical syndrome resulting from a serious infection, is a major healthcare problem associated with high mortality. Sex-related differences in the immune response to sepsis have been proposed but the mechanism is still unknown. Purinergic signaling is a sex-specific regulatory mechanism in immune cell physiology. Our studies have shown that blocking the ADP-receptor P2Y12 but not P2Y1 receptor was protective in male mice during sepsis, but not female. We now hypothesize that there are sex-related differences in modulating P2Y12 or P2Y1 signaling pathways during sepsis. Male and female wild-type (WT), P2Y12 knock-out (KO), and P2Y1 KO mice underwent sham surgery or cecal ligation and puncture (CLP) to induce sepsis. The P2Y12 antagonist ticagrelor or the P2Y1 antagonist MRS2279 were administered intra-peritoneally after surgery to septic male and female mice. Blood, lungs and kidneys were collected 24 hours post-surgery. Sepsis-induced changes in platelet activation, secretion and platelet interaction with immune cells were measured by flow cytometry. Neutrophil infiltration in the lung and kidney was determined by a myeloperoxidase (MPO) colorimetric assay kit. Sepsis-induced platelet activation, secretion and aggregate formation were reduced in male CLP P2Y12 KO and in female CLP P2Y1 KO mice compared with their CLP WT counterpart. Sepsis-induced MPO activity was reduced in male CLP P2Y12 KO and CLP P2Y1 KO female mice. CLP males treated with ticagrelor or MRS2279 showed a decrease in sepsis-induced MPO levels in lung and kidneys, aggregate formation, and platelet activation as compared to untreated male CLP mice. There were no differences in platelet activation, aggregate formation, and neutrophil infiltration in lung and kidney between female CLP mice and female CLP mice treated with ticagrelor or MRS2279. In human T lymphocytes, blocking P2Y1 or P2Y12 alters cell growth and secretion in vitro in a sex-dependent manner, supporting the data obtained in mice. In conclusion, targeting purinergic signaling represents a promising therapy for sepsis but drug targeting purinergic signaling is sex-specific and needs to be investigated to determine sex-related targeted therapies in sepsis.

Indications that the Antimycotic Drug Amphotericin B Enhances the Impact of Platelets on Aspergillus

Platelets are currently thought to harbor antimicrobial functions and might therefore play a crucial role in infections, e.g., those caused by Aspergillus or mucormycetes. The incidence of invasive fungal infections is increasing, particularly during the coronavirus disease 2019 (COVID-19) pandemic, and such infections continue to be life-threatening in immunocompromised patients. For this reason, the interaction of antimycotics with platelets is a key issue to evaluate modern therapeutic regimens. Amphotericin B (AmB) is widely used for the therapy of invasive fungal infections either as deoxycholate (AmB-D) or as a liposomal formulation (L-AmB). We showed that AmB strongly activates platelets within a few minutes. AmB concentrations commonly measured in the blood of patients were sufficient to stimulate platelets, indicating that this effect is highly relevant in vivo. The stimulating effect was corroborated by a broad spectrum of platelet activation parameters, including degranulation, aggregation, budding of microparticles, morphological changes, and enhanced adherence to fungal hyphae. Comparison between the deoxycholate and the liposomal formulation excluded the possibility that the liposomal part of L-Amb is responsible for these effects, as no difference was visible. The induction of platelet activation and alteration by L-AmB resulted in the activation of other parts of innate immunity, such as stimulation of the complement cascade and interaction with granulocytes. These mechanisms might substantially fuel the antifungal immune reaction in invasive mycoses. On the other hand, thrombosis and excessive inflammatory processes might occur via these mechanisms. Furthermore, the viability of L-AmB-activated platelets was consequently decreased, a process that might contribute to thrombocytopenia in patients.

Upregulation of P2Y12 inhibits chondrocyte apoptosis in lumbar osteoarthritis through the PI3K/AKT signaling pathway

Lumbar facet osteoarthritis (FJOA) is a major cause of severe lower back pain and disability worldwide. However, the mechanism underlying cartilage degeneration in FJOA remains unclear. The purpose of this study was to investigate the regulation and mechanism of P2Y12 on chondrocyte apoptosis in FJOA. The experimental rats were randomly divided into non-operation (n = 20) and operation groups (n = 20). In the operation group, Sodium iodoacetate (MIA, Sigma, 200 mg/mL) was injected into the right L4/5 facet process using a blunt nanoneedle 26 (WPI, Sarasota, FL, USA) under the control of an injection pump. The final injection volume was 5µL and the injection rate was 2µL/min. The facet joint was removed four weeks after surgery. After the operation, samples were stored at -80 °C until further use, whereby the right facet joints in each group were tested. Hematoxylin and eosin (HE) and iron-red solid green staining were used to observe the degeneration of articular chondrocytes in rats. Immunohistochemistry and western blotting were used to observe the expressions of P2Y12, Matrix metalloproteinase 13 (MMP13), Collagen II (COL2), and other cartilage degeneration and apoptosis-related genes. Co-localization of P2Y12-cleaved caspase-3 in the apoptosis model was detected by dual-standard immunofluorescence staining. Apoptosis was also detected by flow cytometry and TUNEL assay.P2Y12 is highly expressed in OA cartilage tissue, and inhibits IL-1β -induced chondrocyte apoptosis through PI3K/AKT signaling pathway, thus playing a certain protective role on cartilage.

PI3K Isoform Signalling in Platelets

Platelets are unique anucleated blood cells that constantly patrol the vasculature to seal and prevent injuries in a process termed haemostasis. Thereby they rapidly adhere to the subendothelial matrix and recruit further platelets, resulting in platelet aggregates. Apart from their central role in haemostasis, they also kept some of their features inherited by their evolutionary ancestor-the haemocyte, which was also involved in immune defences. Together with leukocytes, platelets fight pathogenic invaders and guide many immune processes. In addition, they rely on several signalling pathways which are also relevant to immune cells. Among these, one of the central signalling hubs is the PI3K pathway. Signalling processes in platelets are unique as they lack a nucleus and therefore transcriptional regulation is absent. As a result, PI3K subclasses fulfil distinct roles in platelets compared to other cells. In contrast to leukocytes, the central PI3K subclass in platelet signalling is PI3K class Iβ, which underlines the uniqueness of this cell type and opens new ways for potential platelet-specific pharmacologic inhibition. An overview of platelet function and signalling with emphasis on PI3K subclasses and their respective inhibitors is given in this chapter.

ATP transporters in the joints

Extracellular adenosine triphosphate (ATP) plays a central role in a wide variety of joint diseases. ATP is generated intracellularly, and the concentration of the extracellular ATP pool is determined by the regulation of its transport out of the cell. A variety of ATP transporters have been described, with connexins and pannexins the most commonly cited. Both form intercellular channels, known as gap junctions, that facilitate the transport of various small molecules between cells and mediate cell-cell communication. Connexins and pannexins also form pores, or hemichannels, that are permeable to certain molecules, including ATP. All joint tissues express one or more connexins and pannexins, and their expression is altered in some pathological conditions, such as osteoarthritis (OA) and rheumatoid arthritis (RA), indicating that they may be involved in the onset and progression of these pathologies. The aging of the global population, along with increases in the prevalence of obesity and metabolic dysfunction, is associated with a rising frequency of joint diseases along with the increased costs and burden of related illness. The modulation of connexins and pannexins represents an attractive therapeutic target in joint disease, but their complex regulation, their combination of gap-junction-dependent and -independent functions, and their interplay between gap junction and hemichannel formation are not yet fully elucidated. In this review, we try to shed light on the regulation of these proteins and their roles in ATP transport to the extracellular space in the context of joint disease, and specifically OA and RA.

The Predominant Role of Arrestin3 in General GPCR Desensitization in Platelets

Arrestins in concert with GPCR kinases (GRKs) function in G protein-coupled receptor (GPCR) desensitization in various cells. Therefore, we characterized the functional differences of arrestin3 versus arrestin2 in the regulation of GPCR signaling and its desensitization in platelets using mice lacking arrestin3 and arrestin2. In contrast to arrestin2, platelet aggregation and dense granule secretion induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in arrestin3-deficient platelets compared to wild-type (WT) platelets, while non-GPCR agonist CRP-induced platelet aggregation and secretion were not affected. Surprisingly, in contrast to GRK6, platelet aggregation induced by the co-stimulation of serotonin and epinephrine was significantly potentiated in arrestin3-deficient platelets, suggesting the central role of arrestin3 in general GPCR desensitization in platelets. In addition, the second challenge of ADP and AYPGKF restored platelet aggregation in arrestin3-deficient platelets but failed to do so in WT and arrestin2-deficient platelets, confirming that arrestin3 contributes to GPCR desensitization. Furthermore, ADP- and AYPGKF-induced Akt and ERK phosphorylation were significantly increased in arrestin3-deficient platelets. Finally, we found that arrestin3 is critical for thrombus formation in vivo. In conclusion, arrestin3, not arrestin2, plays a central role in the regulation of platelet functional responses and thrombus formation through general GPCR desensitization in platelets.

Platelet infiltration predicts survival in postsurgical colorectal cancer patients

Platelets promote tumor growth and metastasis in several tumor types. Recent research has found platelets can extravasate and infiltrate into the tumor stroma and interact with the tumor microenvironment. The prognostic role of platelet infiltration in colorectal cancer (CRC) remains controversial. A pan-cancer survival analysis was performed to find the potential prognostic value of platelet infiltration in patients with cancer. A survival analysis and a nomogram prognostic model were established to further confirm the results with data from our center. The correlations between patient outcomes and tumor-infiltrating platelets (TIPs) were identified by immunohistochemical staining for CD42b. The prognostic accuracy and discriminative ability of the nomogram were determined by the concordance index (C-index) and a calibration curve. The pan-cancer survival analysis showed platelet infiltration can lead to a poor prognosis in patients with several types of cancers, including CRC. Platelet infiltration was associated with overall survival (OS) and disease-free survival (DFS) in both primary and validation cohorts. The C-index values of the nomogram for predicting OS and DFS were 0.774 and 0.769, respectively, which were higher than that of the TNM staging system alone. Our study found platelet infiltration has a potential prognostic value regarding postsurgical survival in CRC patients. The proposed nomogram resulted in a more accurate prognostic prediction for postsurgical CRC patients.

Cangrelor ameliorates CLP-induced pulmonary injury in sepsis by inhibiting GPR17

BACKGROUND

Sepsis is a common complication of severe wound injury and infection, with a very high mortality rate. The P2Y12 receptor inhibitor, cangrelor, is an antagonist anti-platelet drug.

METHODS

In our study, we investigated the protective mechanisms of cangrelor in CLP-induced pulmonary injury in sepsis, using C57BL/6 mouse models.

RESULTS

TdT-mediated dUTP Nick-End Labeling (TUNEL) and Masson staining showed that apoptosis and fibrosis in lungs were alleviated by cangrelor treatment. Cangrelor significantly promoted surface expression of CD40L on platelets and inhibited CLP-induced neutrophils in Bronchoalveolar lavage fluid (BALF) (p < 0.001). We also found that cangrelor decreased the inflammatory response in the CLP mouse model and inhibited the expression of inflammatory cytokines, IL-1β (p  < 0.01), IL-6 (p < 0.05), and TNF-α (p < 0.001). Western blotting and RT-PCR showed that cangrelor inhibited the increased levels of G-protein-coupled receptor 17 (GPR17) induced by CLP (p < 0.001).

CONCLUSION

Our study indicated that cangrelor repressed the levels of GPR17, followed by a decrease in the inflammatory response and a rise of neutrophils in BALF, potentially reversing CLP-mediated pulmonary injury during sepsis.

MicroRNA as Biomarkers for Platelet Function and Maturity in Patients with Cardiovascular Disease

Patients with cardiovascular disease (CVD) are at increased risk of suffering myocardial infarction. Platelets are key players in thrombus formation and, therefore, antiplatelet therapy is crucial in the treatment and prevention of CVD. MicroRNAs (miRs) may hold the potential as biomarkers for platelet function and maturity. This systematic review was conducted using the guidelines of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). To identify studies investigating the association between miRs and platelet function and maturity in patients with CVD, PubMed and Embase were searched on October 13 and December 13, 2020 without time boundaries. Risk of bias was evaluated using a standardized quality assessment tool. Of the 16 included studies, 6 studies were rated "good" and 10 studies were rated "fair." In total, 45 miRs correlated significantly with platelet function or maturity (rho ranging from -0.68 to 0.38, all p < 0.05) or differed significantly between patients with high platelet reactivity and patients with low platelet reactivity (p-values ranging from 0.0001 to 0.05). Only four miRs were investigated in more than two studies, namely miR-223, miR-126, miR-21 and miR-150. Only one study reported on the association between miRs and platelet maturity. In conclusion, a total of 45 miRs were associated with platelet function or maturity in patients with CVD, with miR-223 and miR-126 being the most frequently investigated. However, the majority of the miRs were only investigated in one study. More data are needed on the potential use of miRs as biomarkers for platelet function and maturity in CVD patients.

P2Y12 antagonism results in altered interactions between platelets and regulatory T cells during sepsis

Sepsis is a complex clinical condition resulting from a serious bloodstream infection. With mortality rates as high as 50%, improved treatments are needed. Regulatory T cells (Tregs), a subset of T lymphocytes, promote the resolution of inflammation. Septic patients have elevated levels of circulating Tregs. Platelets influence the proliferation and activation of Tregs in vitro. However, modulating platelet-Tregs interaction during sepsis may restraing Treg proliferation, leading to the restoration of immunologic homeostasis. P2Y₁₂ is a purinergic receptor present on platelets and T lymphocytes. Blocking P2Y₁₂ improves the outcome of sepsis. We investigated whether blocking P2Y₁₂ alters platelet-Treg interaction in vivo. We used the murine model of sepsis, cecal ligation, and puncture (CLP) and we blocked P2Y₁₂ using the P2Y₁₂ antagonist, clopidogrel. Twenty-four hours after surgery, we measured Treg population sizes in the spleens of the Sham, CLP, and CLP + clopidogrel groups. We investigated the effect of blocking P2Y₁₂ in vitro using cocultures of human platelets and T cells with or without anti-CD3/CD28. P2Y₁₂ was blocked using AR-C69931MX. Treg population sizes were reduced in the septic mice treated with clopidogrel compared with untreated septic mice. Aggregation of platelets and CD4⁺ T cells was reduced in treated CLP mice compared with untreated CLP mice. P2Y₁₂ antagonism changes how platelets influence T cells in vitro, depending on T-cell activation. In conclusion, blockade of the P2Y₁₂ signaling pathway restrains Treg proliferation in vivo and in vitro. Targeting platelets to control Treg proliferation and activity may be a promising strategy for treating sepsis.

Rocuronium Has a Suppressive Effect on Platelet Function via the P2Y12 Receptor Pathway In Vitro That Is Not Reversed by Sugammadex

Rocuronium is an aminosteroid nondepolarizing neuromuscular blocker that is widely used for anesthesia and intensive care. In this study, we investigated the effect of rocuronium on human platelet functions in vitro. The effects of rocuronium on platelet aggregation, P-selectin expression, and cyclic adenosine monophosphate (cAMP) levels in platelets were measured using an aggregometer, an enzyme immunoassay, and flow cytometry, respectively. Rocuronium inhibited ADP-induced platelet aggregation, P-selectin expression and suppression of cAMP production. These effects were not antagonized by equimolar sugammadex, a synthetic γ-cyclodextrin derivative that antagonizes rocuronium-induced muscle relaxation by encapsulating the rocuronium molecule. Morpholine, which constitutes a part of the rocuronium molecule but is not encapsulated by sugammadex, inhibited ADP-induced platelet aggregation. Vecuronium, which has a molecular structure similar to that of rocuronium but does not possess a morpholine ring, had no significant effect on ADP-induced platelet aggregation. These results indicate that rocuronium has a suppressive effect on platelet functions in vitro that is not reversed by sugammadex and suggest that this effect is mediated by blockade of the P2Y12 receptor signaling pathway via the morpholine ring of rocuronium.

Proteomic Mapping of Multifunctional Complexes Within Triatomine Saliva

Triatomines are hematophagous insects that transmit Trypanosoma cruzi, the etiological agent of Chagas disease. This neglected tropical disease represents a global health issue as it is spreading worldwide. The saliva of Triatominae contains miscellaneous proteins crucial for blood feeding acquisition, counteracting host's hemostasis while performing vasodilatory, anti-platelet and anti-coagulant activities, besides modulating inflammation and immune responses. Since a set of biological processes are mediated by protein complexes, here, the sialocomplexomes (salivary protein complexes) of five species of Triatominae were studied to explore the protein-protein interaction networks. Salivary multiprotein complexes from Triatoma infestans, Triatoma dimidiata, Dipetalogaster maxima, Rhodnius prolixus, and Rhodnius neglectus were investigated by Blue-Native- polyacrylamide gel electrophoresis coupled with liquid chromatography tandem mass spectrometry. More than 70 protein groups, uncovering the landscape of the Triatominae salivary interactome, were revealed. Triabin, actin, thioredoxin peroxidase and an uncharacterized protein were identified in sialocomplexes of the five species, while hexamerin, heat shock protein and histone were identified in sialocomplexes of four species. Salivary proteins related to triatomine immunity as well as those required during blood feeding process such as apyrases, antigen 5, procalins, and nitrophorins compose different complexes. Furthermore, unique proteins for each triatomine species were revealed. This study represents the first Triatominae sialocomplexome reference to date and shows that the approach used is a reliable tool for the analysis of Triatominae salivary proteins assembled into complexes.

Rhizoma Bletillae polysaccharide elicits hemostatic effects in platelet-rich plasma by activating adenosine diphosphate receptor signaling pathway

Rhizoma Bletillae, the tubes of Bletilla striata, has been traditionally used in China as a hemostatic agent. In preliminary studies, the major active fraction responsible for its hemostatic effect have been confirmed to be Rhizoma Bletillae polysaccharide (RBp), but the hemostatic mechanism of action of RBp is still unknown.The main aim of this study was to clarify its mechanism of hemostatic effect. RBp was prepared by 80 % ethanol precipitation of the water extract of Rhizoma Bletillae followed by the Sevag method to remove proteins. The average molecular weight (Mw) of the crude RBp maintained at a range of 30.06-200 KDa. The hemostatic effects of RBp were evaluated by testing its effect on the platelet aggregation of rat platelet-rich plasma (PRP). PRP was dealt with different concentrations of RBp and platelet aggregation was measured by the turbidimetric method. The hemostatic mechanism of RBp was investigated by examining its effect on platelet shape, platelet secretion, and activation of related receptors (P2Y₁, P2Y₁₂ and TXA2) by electron microscopy and the turbidimetric method. RBp significantly enhanced the platelet aggregations at concentrations of 50-200 mg/L in a concentration-dependent manner. The inhibitory rate of platelet aggregation was significantly increased by apyrase and Ro31-8220 in a concentration-dependent manner, while RBp-induced platelet aggregation was completely inhibited by P2Y₁, P2Y₁₂ and the PKC receptor antagonists. However, the aggregation was not sensitive to TXA2. RBp, the active ingredients of Rhizoma Bletillae responsible for its hemostatic effect, could significantly accelerate the platelet aggregation and shape change. The hemostatic mechanism may involve activation of the P2Y₁, P2Y₁₂, and PKC receptors in the adenosine diphosphate (ADP) receptor signaling pathway.

Role of GRK6 in the Regulation of Platelet Activation through Selective G Protein-Coupled Receptor (GPCR) Desensitization

Platelet G protein-coupled receptors (GPCRs) regulate platelet function by mediating the response to various agonists, including adenosine diphosphate (ADP), thromboxane A₂, and thrombin. Although GPCR kinases (GRKs) are considered to have the crucial roles in most GPCR functions, little is known regarding the regulation of GPCR signaling and mechanisms of GPCR desensitization by GRKs in platelets. In this study, we investigated the functional role of GRK6 and the molecular basis for regulation of specific GPCR desensitization by GRK6 in platelets. We used GRK6 knockout mice to evaluate the functional role of GRK6 in platelet activation. Platelet aggregation, dense- and α-granule secretion, and fibrinogen receptor activation induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in GRK6^−/− platelets compared to the wild-type (WT) platelets. However, collagen-related peptide (CRP)-induced platelet aggregation and secretion were not affected in GRK6^−/− platelets. Interestingly, platelet aggregation induced by co-stimulation of serotonin and epinephrine which activate G_q-coupled 5HT_2A and G_z-coupled α_2A adrenergic receptors, respectively, was not affected in GRK6^−/− platelets, suggesting that GRK6 was involved in specific GPCR regulation. In addition, platelet aggregation in response to the second challenge of ADP and AYPGKF was restored in GRK6^−/− platelets whereas re-stimulation of the agonist failed to induce aggregation in WT platelets, indicating that GRK6 contributed to P2Y₁, P2Y₁₂, and PAR4 receptor desensitization. Furthermore, 2-MeSADP-induced Akt phosphorylation and AYPGKF-induced Akt, extracellular signal-related kinase (ERK), and protein kinase Cδ (PKCδ) phosphorylation were significantly potentiated in GRK6^−/− platelets. Finally, GRK6^−/− mice exhibited an enhanced and stable thrombus formation after FeCl₃ injury to the carotid artery and shorter tail bleeding times, indicating that GRK6^−/− mice were more susceptible to thrombosis and hemostasis. We conclude that GRK6 plays an important role in regulating platelet functional responses and thrombus formation through selective GPCR desensitization.

Opposite Modulatory Effects of Crataegus aronia Aqueous Extract on Platelet Aggregation in Rats

OBJECTIVES

To reveal the mechanisms behind the dual effects of Crataegus aronia (C. aronia) aqueous extract on platelet aggregation by focusing on function, regulation, expression, and signaling of platelets P₂Y₁₂ receptors.

METHODS

Adult male Wistar rats (120 ± 10 g) were classified as control received the vehicle, C. aronia (200 mg/kg), and C. aronia (2,000 mg/kg)-treated rats. After treatments for consecutive 7 days, hematological and molecular experiments were conducted to detect alterations in platelet aggregation, thromboxane B2 (THXB2) and intracellular reactive oxygen species (ROS) content; protein levels of P₂Y₁₂, p-Akt, cyclic adenosine monophosphate (cAMP), phosphorylated vasodilator-stimulated-phosphoprotein (p-VASP), nuclear factor κB (NF-κB), P-selectin, and etc. in platelets were determined by Western blot; mRNA expressions of P₂Y₁₂ and some inflammatory markers were determined by real-time polymerase chain reaction.

RESULTS

At a concentration of 200 mg/kg, C. aronia inhibited platelet aggregation through multiple interconnected mechanisms including downregulation P₂Y₁₂ synthesis and expression, stimulating intracellular cAMP levels and protein levels of p-VASP, inhibiting platelets THXB2 release and protein levels of P-selectin. Also, it inhibited platelets level of ROS and of NF-κB, a major signaling pathway that stimulates the expression of P₂Y₁₂ and THXA2 synthesis. Opposite findings were seen in platelets of rats received C. aronia at a concentration of 2,000 mg/kg. Interestingly, co-administration of N-acetylcysteine prevented all hematological and molecular alterations exerted by the high dose of the extract and inhibited platelet aggregation.

CONCLUSION

Oral administration of C. aronia at low dose inhibits platelet aggregation by reducing THXB2 release, expression of P-selectin and activating cAMP and Akt signaling through two major mechanisms including downregulation of P₂Y₁₂ and inhibition of ROS-induced activation of NF-κB, an effect that is observed to be in the opposite direction with its high dose.

Clopidogrel or prasugrel reduces mortality and lessens cardiovascular damage from acute myocardial infarction in hypercholesterolemic male rats

AIMS

Hypercholesterolemia is a hazard for increasing susceptibility of the heart to myocardial infarction (MI) by inducing platelet hyperaggregability. Clopidogrel and prasugrel have documented cardioprotective effects in clinical studies. Herein, we investigated whether clopidogrel and prasugrel could protect against isoproterenol-induced acute MI (A-MI) under hypercholesterolemic conditions in rats.

MAIN METHODS

Dietary hypercholesterolemic rats were subjected to acute doses of isoproterenol. Serum lipids, inflammatory markers, aortic endothelin1 and endothelial nitric oxide synthase (eNOS) mRNAs expression and immunexpression of BCL2 were determined.

KEY FINDINGS

Hypercholesterolemic rats showed infiltration of inflammatory cells and reduction in aortic wall thickness, deposition of fibrous tissue between cardiac muscle fibers. Protective doses of prasugrel or clopidogrel for 28 days before A-MI increased survival, amended the ECG parameters -including ST segment elevation- and improved the histopathological picture in hypercholesterolemic rats. This was coupled with reductions in platelet aggregation, creatine kinase-MB activity, endothelin 1, systemic inflammation and cardiac lipid peroxidation and increment in aortic eNOS expression. Clopidogrel and prasugrel groups showed enhanced BCL2 expression in cardiac fibers and aortic wall.

SIGNIFICANCE

Prasugrel and clopidogrel protected against A-MI via anti-aggregatory and anti-inflammatory effects. These results add to the value of these drugs in correcting cardiovascular dysfunction in patients vulnerable to A-MI after confirmation by appropriate human studies.

Dye D, Falasca M, Metharom P. Antiplatelet Drug Ticagrelor Enhances Chemotherapeutic Efficacy by Targeting the Novel P2Y12-AKT Pathway in Pancreatic Cancer Cells

Background: Extensive research has reported that extracellular ADP in the tumour microenvironment can stimulate platelets through interaction with the platelet receptor P2Y12. In turn, activated platelets release biological factors supporting cancer progression. Experimental data suggest that the tumour microenvironment components, of which platelets are integral, can promote chemotherapy resistance in pancreatic ductal adenocarcinoma (PDAC). Thus, overcoming chemoresistance requires combining multiple inhibitors that simultaneously target intrinsic pathways in cancer cells and extrinsic factors related to the tumour microenvironment. We aimed to determine whether ticagrelor, an inhibitor of the ADP–P2Y12 axis and a well-known antiplatelet drug, could be a therapeutic option for PDAC. Methods: We investigated a functional P2Y12 receptor and its downstream signalling in a panel of PDAC cell lines and non-cancer pancreatic cells termed hTERT-HPNE. We tested the synergistic effect of ticagrelor, a P2Y12 inhibitor, in combination with chemotherapeutic drugs (gemcitabine, paclitaxel and cisplatin), in vitro and in vivo. Results: Knockdown studies revealed that P2Y12 contributed to epidermal growth factor receptor (EGFR) activation and the expression of SLUG and ZEB1, which are transcriptional factors implicated in metastasis and chemoresistance. Studies using genetic and pharmacological inhibitors showed that the P2Y12–EGFR crosstalk enhanced cancer cell proliferation. Inhibition of P2Y12 signalling significantly reduced EGF-dependent AKT activation and promoted the anticancer activity of anti-EGFR treatment. Importantly, ticagrelor significantly decreased the proliferative capacity of cancer but not normal pancreatic cells. In vitro, synergism was observed when ticagrelor was combined with several chemodrugs. In vivo, a combination of ticagrelor with gemcitabine significantly reduced tumour growth, whereas gemcitabine or ticagrelor alone had a minimal effect. Conclusions: These findings uncover a novel effect and mechanism of action of the antiplatelet drug ticagrelor in PDAC cells and suggest a multi-functional role for ADP-P2Y12 signalling in the tumour microenvironment.

Pleiotropic Roles of P2X7 in the Central Nervous System

The purinergic receptor P2X7 is expressed in neural and immune cells known to be involved in neurological diseases. Its ligand, ATP, is a signaling molecule that can act as a neurotransmitter in physiological conditions or as a danger signal when released in high amount by damaged/dying cells or activated glial cells. Thus, ATP is a danger-associated molecular pattern. Binding of ATP by P2X7 leads to the activation of different biochemical pathways, depending on the physiological or pathological environment. The aim of this review is to discuss various functions of P2X7 in the immune and central nervous systems. We present evidence that P2X7 may have a detrimental or beneficial role in the nervous system, in the context of neurological pathologies: epilepsy, Alzheimer’s disease, multiple sclerosis, amyotrophic lateral sclerosis, age-related macular degeneration and cerebral artery occlusion.

Characterization of the distinct mechanism of agonist-induced canine platelet activation

Platelet activation has a major role in hemostasis and thrombosis. Various agonists including adenosine diphosphate (ADP) and thrombin interact with G protein-coupled receptors (GPCRs) which transduce signals through various G proteins. Recent studies have elucidated the role of GPCRs and their corresponding G proteins in the regulation of events involved in platelet activation. However, agonist-induced platelet activation in companion animals has not been elucidated. This study was designed to characterize the platelet response to various agonists in dog platelets. We found that 2-methylthio-ADP-induced dog platelet aggregation was blocked in the presence of either P2Y₁ receptor antagonist MRS2179 or P2Y₁₂ receptor antagonist AR-C69931MX, suggesting that co-activation of both the P2Y₁ and P2Y₁₂ receptors is required for ADP-induced platelet aggregation. Thrombin-induced dog platelet aggregation was inhibited in the presence of either AR-C69931MX or the PKC inhibitor GF109203X, suggesting that thrombin requires secreted ADP to induce platelet aggregation in dog platelets. In addition, thrombin-mediated Akt phosphorylation was inhibited in the presence of GF109203X or AR-C69931MX, indicating that thrombin causes Gi stimulation through the P2Y₁₂ receptor by secreted ADP in dog platelets. Unlike human and murine platelets, protease-activated receptor 4 (PAR4)-activating peptide AYPGKF failed to cause dog platelet aggregation. Moreover, PAR1-activating peptide SFLLRN or co-stimulation of SFLLRN and AYPGKF failed to induce dog platelet aggregation. We conclude that ADP induces platelet aggregation through the P2Y₁ and P2Y₁₂ receptors in dogs. Unlike human and murine platelets, selective activation of the PAR4 receptor may be insufficient to cause platelet aggregation in dog platelets.

Dual Anticoagulant/Antiplatelet Activity of Polyphenolic Grape Seeds Extract

Because of the side-effects of commonly used anti-platelet and anticoagulant drugs, investigations into plant substances with similar activities are very common. Based on our own studies in recent years, we estimate that it is possible to use natural compounds to both inhibit coagulation pathway enzymes and to reduce blood platelets’ activation. As such, in our current study we wanted to verify the anti-platelet and anticoagulant properties of grape seed extract (GSE) using in vitro models. During our analysis, the following parameters were analyzed: Coagulation times, thromboelastometry assays (coagulation time, clot formation time and maximum clot firmness), aggregation of platelets and phosphorylation of vasodilator-stimulated phosphoprotein (VASP). Adenosine diphosphate (ADP)-induced aggregation was lower in GSE 7.5 µg/mL as well as in GSE 15.0 µg/mL. A similar dependence was observed in VASP assays for GSE 7.5 µg/mL and GSE 15 µg/mL. The effect on plasma coagulation tests was distinct only with GSE 15 µg/mL. All of the thromboelastometry variables were statistically significant with 15.0 µg/mL GSE concentration. Our results show, for the first time, the multi-potential effect of grape seed extract on coagulation systems, and clearly suggest that grape seed extract could be considered a promising nutraceutical in the prevention of cardiovascular thrombotic events caused by different mechanisms.

Islet-Derived eATP Fuels Autoreactive CD8+ T Cells and Facilitates the Onset of Type 1 Diabetes

Extracellular ATP (eATP) activates T cells by engaging the P2X7R receptor. We identified two loss-of-function P2X7R mutations that are protective against type 1 diabetes (T1D) and thus hypothesized that eATP/P2X7R signaling may represent an early step in T1D onset. Specifically, we observed that in patients with newly diagnosed T1D, P2X7R is upregulated on CD8⁺ effector T cells in comparison with healthy control subjects. eATP is released at high levels by human/murine islets in vitro in high-glucose/inflammatory conditions, thus upregulating P2X7R on CD8⁺ T cells in vitro. P2X7R blockade with oxidized ATP reduces the CD8⁺ T cell-mediated autoimmune response in vitro and delays diabetes onset in NOD mice. Autoreactive CD8⁺ T-cell activation is highly dependent upon eATP/P2X7R-mediated priming, while a novel sP2X7R recombinant protein abrogates changes in metabolism and the autoimmune response associated with CD8⁺ T cells. eATP/P2X7R signaling facilitates the onset of autoimmune T1D by fueling autoreactive CD8⁺ cells and therefore represents a novel targeted therapeutic for the disorder.

The Platelet Lifeline to Cancer: Challenges and Opportunities

Besides their function in limiting blood loss and promoting wound healing, experimental evidence has highlighted platelets as active players in all steps of tumorigenesis including tumor growth, tumor cell extravasation, and metastasis. Additionally, thrombocytosis in cancer patients is associated with adverse patient survival. Due to the secretion of large amounts of microparticles and exosomes, platelets are well positioned to coordinate both local and distant tumor-host crosstalk. Here, we present a review of recent discoveries in the field of platelet biology and the role of platelets in cancer progression as well as challenges in targeting platelets for cancer treatment.

Inhibitory Effect of Flavonolignans on the P2Y12 Pathway in Blood Platelets

Adenosine diphosphate (ADP) is the major platelet agonist, which is important in the shape changes, stability, and growth of the thrombus. Platelet activation by ADP is associated with the G protein-coupled receptors P2Y1 and P2Y12. The pharmacologic blockade of the P2Y12 receptor significantly reduces the risk of peripheral artery disease, myocardial infarction, ischemic stroke, and vascular death. Recent studies demonstrated the inhibition of ADP-induced blood platelet activation by three major compounds of the flavonolignans group: silybin, silychristin, and silydianin. For this reason, the aim of the current work was to verify the effects of silybin, silychristin, and silydianin on ADP-induced physiological platelets responses, as well as mechanisms of P2Y12-dependent intracellular signal transduction. We evaluated the effect of tested flavonolignans on ADP-induced blood platelets’ aggregation in platelet-rich plasma (PRP) (using light transmission aggregometry), adhesion to fibrinogen (using the static method), and the secretion of PF-4 (using the ELISA method). Additionally, using the double labeled flow cytometry method, we estimated platelet vasodilator-stimulated phosphoprotein (VASP) phosphorylation. We demonstrated a dose-dependent reduction of blood platelets’ ability to perform ADP-induced aggregation, adhere to fibrinogen, and secrete PF-4 in samples treated with flavonolignans. Additionally, we observed that all of the tested flavonolignans were able to increase VASP phosphorylation in blood platelets samples, which is correlated with P2Y12 receptor inhibition. All of these analyses show that silychristin and silybin have the strongest inhibitory effect on blood platelet activation by ADP, while silydianin also inhibits the ADP pathway, but to a lesser extent. The results obtained in this study clearly demonstrate that silybin, silychristin, and silydianin have inhibitory properties against the P2Y12 receptor and block ADP-induced blood platelet activation.

Platelets, inflammation and tissue regeneration

Blood platelets have long been recognised to bring about primary haemostasis with deficiencies in platelet production and function manifesting in bleeding while upregulated function favourises arterial thrombosis. Yet increasing evidence indicates that platelets fulfil a much wider role in health and disease. First, they store and release a wide range of biologically active substances including the panoply of growth factors, chemokines and cytokines released from α-granules. Membrane budding gives rise to microparticles (MPs), another active participant within the blood stream. Platelets are essential for the innate immune response and combat infection (viruses, bacteria, micro-organisms). They help maintain and modulate inflammation and are a major source of pro-inflammatory molecules (e.g. P-selectin, tissue factor, CD40L, metalloproteinases). As well as promoting coagulation, they are active in fibrinolysis; wound healing, angiogenesis and bone formation as well as in maternal tissue and foetal vascular remodelling. Activated platelets and MPs intervene in the propagation of major diseases. They are major players in atherosclerosis and related diseases, pathologies of the central nervous system (Alzheimers disease, multiple sclerosis), cancer and tumour growth. They participate in other tissue-related acquired pathologies such as skin diseases and allergy, rheumatoid arthritis, liver disease; while, paradoxically, autologous platelet-rich plasma and platelet releasate are being used as an aid to promote tissue repair and cellular growth. The above mentioned roles of platelets are now discussed.

Role of P2Y12 Receptor in Thrombosis

P2Y₁₂ receptor is a 342 amino acid Gi-coupled receptor predominantly expressed on platelets. P2Y₁₂ receptor is physiologically activated by ADP and inhibits adenyl cyclase (AC) to decrease cyclic AMP (cAMP) level, resulting in platelet aggregation. It also activates PI3 kinase (PI3K) pathway leading to fibrinogen receptor activation, and may protect platelets from apoptosis. Abnormalities of P2Y₁₂ receptor include congenital deficiencies or high activity in diseases like diabetes mellitus (DM) and chronic kidney disease (CKD), exposing such patients to a prothrombotic condition. A series of clinical antiplatelet drugs, such as clopidogrel and ticagrelor, are designed as indirect or direct antagonists of P2Y₁₂ receptor to reduce incidence of thrombosis mainly for patients of acute coronary syndrome (ACS) who are at high risk of thrombotic events. Studies on novel dual-/multi-target antiplatelet agents consider P2Y₁₂ receptor as a promising part in combined targets. However, the clinical practical phenomena, such as "clopidogrel resistance" due to gene variations of cytochrome P450 or P2Y₁₂ receptor constitutive activation, call for better antiplatelet agents. Researches also showed inverse agonist of P2Y₁₂ receptor could play a better role over neutral antagonists. Personalized antiplatelet therapy is the most ideal destination for antiplatelet therapy in ACS patients with or without other underlying diseases like DM or CKD, however, there is still a long way to go.

Sepsis induced by cecal ligation and perforation (CLP) alters nucleotidase activities in platelets of rats

Sepsis is a potentially lethal condition, and it is associated with platelet alterations. The present study sought to investigate the activity of ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase), E-5'-nucleotidase, and ecto-adenosine deaminase (E-ADA) in the platelets of rats that were induced with sepsis. Male Wistar rats were divided into three groups of ten animals each: a negative control group (normal; NC); a group that underwent surgical procedures (sham); and a group that underwent cecal ligation and perforation (CLP). The induction of sepsis was confirmed by bacteremia, and the causative pathogen identified was Escherichia coli. Hematological parameters showed leukocytosis and thrombocytopenia in animals in the septic group. The results also revealed that there were significant (p < 0.05) increases in adenosine triphosphate (ATP) and adenosine monophosphate (AMP) hydrolyses, and in the deamination of adenosine in the CLP group compared to the sham and control groups. Conversely, ADP hydrolysis was significantly decreased (p < 0.05) in the CLP group compared to the sham and control groups. Purine levels were analyzed by high-performance liquid chromatography (HPLC) in serum samples from control, sham, and CLP groups. Increased concentrations of ATP, adenosine, and inosine were found in the CLP group compared to the sham and control groups. Conversely, the concentrations of ADP and AMP in the CPL group were not significantly altered. We suggest that alterations in hematological parameters, nucleotide hydrolysis in platelets, and nucleotide concentrations in serum samples of rats with induced sepsis may be related to thromboembolic events.

Platelet Aggregometry Testing: Molecular Mechanisms, Techniques and Clinical Implications

Platelets play a fundamental role in normal hemostasis, while their inherited or acquired dysfunctions are involved in a variety of bleeding disorders or thrombotic events. Several laboratory methodologies or point-of-care testing methods are currently available for clinical and experimental settings. These methods describe different aspects of platelet function based on platelet aggregation, platelet adhesion, the viscoelastic properties during clot formation, the evaluation of thromboxane metabolism or certain flow cytometry techniques. Platelet aggregometry is applied in different clinical settings as monitoring response to antiplatelet therapies, the assessment of perioperative bleeding risk, the diagnosis of inherited bleeding disorders or in transfusion medicine. The rationale for platelet function-driven antiplatelet therapy was based on the result of several studies on patients undergoing percutaneous coronary intervention (PCI), where an association between high platelet reactivity despite P2Y12 inhibition and ischemic events as stent thrombosis or cardiovascular death was found. However, recent large scale randomized, controlled trials have consistently failed to demonstrate a benefit of personalised antiplatelet therapy based on platelet function testing.

Uridine Triphosphate Thio Analogues Inhibit Platelet P2Y12 Receptor and Aggregation

Platelet P2Y₁₂ is an important adenosine diphosphate (ADP) receptor that is involved in agonist-induced platelet aggregation and is a valuable target for the development of anti-platelet drugs. Here we characterise the effects of thio analogues of uridine triphosphate (UTP) on ADP-induced platelet aggregation. Using human platelet-rich plasma, we demonstrate that UTP inhibits P2Y₁₂ but not P2Y₁ receptors and antagonises 10 µM ADP-induced platelet aggregation in a concentration-dependent manner with an IC₅₀ value of ~250 µM. An eight-fold higher platelet inhibitory activity was observed with a 2-thio analogue of UTP (2S-UTP), with an IC₅₀ of 30 µM. The 4-thio analogue (4S-UTP) with an IC₅₀ of 7.5 µM was 33-fold more effective. A three-fold decrease in inhibitory activity, however, was observed by introducing an isobutyl group at the 4S- position. A complete loss of inhibition was observed with thio-modification of the γ phosphate of the sugar moiety, which yields an enzymatically stable analogue. The interaction of UTP analogues with P2Y₁₂ receptor was verified by P2Y₁₂ receptor binding and cyclic AMP (cAMP) assays. These novel data demonstrate for the first time that 2- and 4-thio analogues of UTP are potent P2Y₁₂ receptor antagonists that may be useful for therapeutic intervention.

Electrical stimulation drives chondrogenesis of mesenchymal stem cells in the absence of exogenous growth factors

Electrical stimulation (ES) is known to guide the development and regeneration of many tissues. However, although preclinical and clinical studies have demonstrated superior effects of ES on cartilage repair, the effects of ES on chondrogenesis remain elusive. Since mesenchyme stem cells (MSCs) have high therapeutic potential for cartilage regeneration, we investigated the actions of ES during chondrogenesis of MSCs. Herein, we demonstrate for the first time that ES enhances expression levels of chondrogenic markers, such as type II collagen, aggrecan, and Sox9, and decreases type I collagen levels, thereby inducing differentiation of MSCs into hyaline chondrogenic cells without the addition of exogenous growth factors. ES also induced MSC condensation and subsequent chondrogenesis by driving Ca2+/ATP oscillations, which are known to be essential for prechondrogenic condensation. In subsequent experiments, the effects of ES on ATP oscillations and chondrogenesis were dependent on extracellular ATP signaling via P2X4 receptors, and ES induced significant increases in TGF-β1 and BMP2 expression. However, the inhibition of TGF-β signaling blocked ES-driven condensation, whereas the inhibition of BMP signaling did not, indicating that TGF-β signaling but not BMP signaling mediates ES-driven condensation. These findings may contribute to the development of electrotherapeutic strategies for cartilage repair using MSCs.

Flavonolignans inhibit ADP induced blood platelets activation and aggregation in whole blood

Flavonolignans are a group of active chemical compounds presented in the silymarin - a standardized extract obtained from fruits and seeds of Milk thistle (Silybum marianum L. Gaernt.). Since the 70s of the last century, flavonolignans have been regarded to the official medicine as a substances having hepatoprotective properties. However many researches performed in recent years have demonstrated that flavonolignans posses many other healthy properties including modulation of variety cell-signaling pathways. The aim of our study was to examine the effects of three major flavonolignans (silybin, silychristin and silydianin) on ADP-induced blood platelet activation using the flow cytometry analysis as well as determine the mechanism of this interaction by bioinformatic ligand docking method. We observed that all tested flavonolignans in dose-dependent manner inhibit formation of blood platelet aggregates and microparticles as well as decrease expression of P-selectin and activation of integrin α_IIbβ₃. Our computer-generated models confirm the flow cytometry analysis. We observed that all tested flavonolignans have conformations which are able to bind to the extracellular domain of P2Y12 receptor and probably block interaction with ADP. Our studies may help in the development of a new potential anti-platelet agent, which might be an alternative to the current using drugs.

Effect of dietary supplementation of Padauk (Pterocarpus soyauxii) leaf on high fat diet/streptozotocin induced diabetes in rats' brain and platelets

BACKGROUND

This study investigated the effects of Padauk leaf on brain malondialdehyde (MDA) content, acetylcholinesterase (AChE) activities, ectonucleotidases and adenosine deaminase (ADA) activities in the platelet of high fat diet and streptozotocin (STZ)-induced diabetic rats.

METHODS

The animals were divided into six groups (n=7): normal control rats; diabetic rats+high fat diet (HFD); diabetic rats+HFD+Metformin; diabetic rats+HFD+acarbose; diabetic rats+HFD+10% Padauk leaf; normal rats+basal diet+10% Padauk leaf. After 30days of experiment comprising of acclimatization, dietary manipulation, pre-treatment with STZ and supplementation with Padauk leaf, the animals were sacrificed and the rats' brain and blood were collected for subsequent analysis.

RESULTS

The results demonstrated that the elevated MDA content and AChE activity in the diabetic rats were significantly reduced when compared with the control rats. Furthermore, the increased NTPDases, 5'-nucleotidase and ADA activities in the diabetic rats were significantly reduced when compared with the control rats.

CONCLUSION

This study demonstrated that Padauk leaf exhibited modulatory effects on purinergic and cholinergic enzymes involved in the prevention of platelet abnormality and consequent vascular complications in diabetic state.

Inverse agonism at the P2Y12 receptor and ENT1 transporter blockade contribute to platelet inhibition by ticagrelor

Ticagrelor is a potent antagonist of the P2Y₁₂ receptor (P2Y₁₂R) and consequently an inhibitor of platelet activity effective in the treatment of atherothrombosis. Here, we sought to further characterize its molecular mechanism of action. Initial studies showed that ticagrelor promoted a greater inhibition of adenosine 5'-diphosphate (ADP)-induced Ca²⁺ release in washed platelets vs other P2Y₁₂R antagonists. This additional effect of ticagrelor beyond P2Y₁₂R antagonism was in part as a consequence of ticagrelor inhibiting the equilibrative nucleoside transporter 1 (ENT1) on platelets, leading to accumulation of extracellular adenosine and activation of G_s-coupled adenosine A_2A receptors. This contributed to an increase in basal cyclic adenosine monophosphate (cAMP) and vasodilator-stimulated phosphoprotein phosphorylation (VASP-P). In addition, ticagrelor increased platelet cAMP and VASP-P in the absence of ADP in an adenosine receptor-independent manner. We hypothesized that this increase originated from a direct effect on basal agonist-independent P2Y₁₂R signaling, and this was validated in 1321N1 cells stably transfected with human P2Y₁₂R. In these cells, ticagrelor blocked the constitutive agonist-independent activity of the P2Y₁₂R, limiting basal G_i-coupled signaling and thereby increasing cAMP levels. These data suggest that ticagrelor has the pharmacological profile of an inverse agonist. Based on our results showing insurmountable inhibition of ADP-induced Ca²⁺ release and forskolin-induced cAMP, the mode of antagonism of ticagrelor also appears noncompetitive, at least functionally. In summary, our studies describe 2 novel modes of action of ticagrelor, inhibition of platelet ENT1 and inverse agonism at the P2Y₁₂R that contribute to its effective inhibition of platelet activation.

P2Y12 Receptor Modulates Sepsis-Induced Inflammation

OBJECTIVE

Platelets modulate hemostasis and immune responses via interactions with immune cells through secretion of immunemodulators and cell-cell interactions. The P2Y12 receptor mediates ADP-induced aggregation and secretion in platelets.

APPROACH AND RESULTS

Using a mouse model of intra-abdominal sepsis and acute lung injury, we investigated the role of the P2Y12 receptor in neutrophil migration and lung inflammation in P2Y12 null mice and in mice pretreated with the P2Y12 antagonist clopidogrel. Our data show a decrease in circulating white blood cells and a decrease in platelet activation and platelet-leukocyte interactions in treated mice compared with untreated mice. Additionally, lung injury and platelet sequestration were diminished in clopidogrel-treated mice compared with their untreated septic littermates. Similar results were observed in P2Y12 null mice: platelet activation and platelet-leukocyte aggregates were decreased in septic P2Y12 null mice compared with wild-type mice. P2Y12 null mice were refractory to lung injury compared with wild-type mice. Finally, to evaluate P2Y12-independent effects of clopidogrel, we pretreated P2Y12 null mice. Interestingly, the number of circulating neutrophils was reduced in treated septic P2Y12 null mice, suggesting neutrophils as a target for clopidogrel pleiotropic effects. No difference was observed in P2Y1 null mice during sepsis, indicating that the P2Y12 receptor is responsible for the effects.

CONCLUSIONS

P2Y12 null mice are refractory to sepsis-induced lung injury, suggesting a key role for activated platelets and the P2Y12 receptor during sepsis.

Effect of antiretroviral therapy in thromboregulation through the hydrolysis of adenine nucleotides in platelets of HIV patients

The human immunodeficiency virus (HIV) infection results in biochemical and vascular dysfunctions. The highly active antiretroviral therapy (HAART) markedly reduces mortality and opportunistic diseases associated with acquired immunodeficiency syndrome (AIDS). This increased survival time predisposes the development of cardiovascular diseases. Platelets present purinergic system ectoenzymes such as E-NTPDase, E-5'-nucleotidase and E-ADA on its surface. In view of this, the aim of this study was to evaluate the activity of these ectoenzymes in platelets as well as the platelet aggregation and lipid profile of patients with HIV infection and also patients receiving HAART. The results showed an increase in the E-NTPDase activity for ATP hydrolysis in the HIV group compared with the control group and the HIV/HAART group. When assessing the activity E-NTPDase hydrolysis to ADP, the results revealed an increase in activity in the HIV group when compared to the control group, and a decrease in activity when in the HIV/HAART group when compared to the control and HIV groups. The activity of E-5'-nucleotidase revealed an increase in AMP hydrolysis in the HIV group, as the results from control and HIV/HAART groups showed no statistical difference. Regarding the E-ADA activity, the HIV and HIV/HAART groups revealed a decreased deamination of adenosine when compared with the control group. Furthermore, we observed an increased platelet aggregation of HIV/HAART group compared with the control group. Thus, our results suggest that antiretroviral treatment against HIV has a significant effect on the activity of purinergic system ectoenzymes demonstrating that thromboregulation is involved in the process.

Calcium Signalling through Ligand-Gated Ion Channels such as P2X1 Receptors in the Platelet and other Non-Excitable Cells

Ligand-gated ion channels on the cell surface are directly activated by the binding of an agonist to their extracellular domain and often referred to as ionotropic receptors. P2X receptors are ligand-gated non-selective cation channels with significant permeability to Ca(2+) whose principal physiological agonist is ATP. This chapter focuses on the mechanisms by which P2X1 receptors, a ubiquitously expressed member of the family of ATP-gated channels, can contribute to cellular responses in non-excitable cells. Much of the detailed information on the contribution of P2X1 to Ca(2+) signalling and downstream functional events has been derived from the platelet. The underlying primary P2X1-generated signalling event in non-excitable cells is principally due to Ca(2+) influx, although Na(+) entry will also occur along with membrane depolarization. P2X1 receptor stimulation can lead to additional Ca(2+) mobilization via a range of routes such as amplification of G-protein-coupled receptor-dependent Ca(2+) responses. This chapter also considers the mechanism by which cells generate extracellular ATP for autocrine or paracrine activation of P2X1 receptors. For example cytosolic ATP efflux can result from opening of pannexin anion-permeable channels or following damage to the cell membrane. Alternatively, ATP stored in specialised secretory vesicles can undergo quantal release via the process of exocytosis. Examples of physiological or pathophysiological roles of P2X1-dependent signalling in non-excitable cells are also discussed, such as thrombosis and immune responses.

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.

The sGC stimulator riociguat inhibits platelet function in washed platelets but not in whole blood

BACKGROUND AND PURPOSE

Stimulation of soluble guanylyl cyclase (sGC) is a valuable therapeutic strategy for the treatment of several cardiovascular diseases. The sGC stimulator riociguat has been approved for the treatment of two forms of pulmonary hypertension. Platelets contain large amounts of sGC and play a key role in the regulation of haemostasis. Therefore, we investigated the effects of riociguat on platelet function.

EXPERIMENTAL APPROACH

The effect of riociguat treatment on human platelet activation and aggregation was investigated. The sGC-specific effects of riociguat were determined by comparing wild-type and platelet-specific sGC-knockout mice.

KEY RESULTS

Riociguat induced cGMP synthesis and subsequent PKG activation in human platelets, suggesting that the inhibitory effects are mediated by cGMP signalling. This finding was confirmed when sGC-knockout platelets were not inhibited by riociguat. In washed human platelets, 100 nM riociguat reduced ADP-induced GPIIb/IIIa activation, while a 10-fold higher concentration was required to reduce convulxin-stimulated GPIIb/IIIa activation. Riociguat inhibited ADP-induced platelet shape change and aggregation, while ATP-induced shape change remained unaffected. However, in PRP and whole blood, 50-100 μM riociguat was required to inhibit platelet activation and aggregation. Riociguat in combination with iloprost significantly inhibited platelet aggregation, even in whole blood.

CONCLUSIONS AND IMPLICATIONS

Riociguat inhibits platelet activation in whole blood only at concentrations above 50 μM, while the plasma concentrations in riociguat-treated patients are 150 to 500 nM. This finding indicates that riociguat treatment does not affect platelet function in patients. Nevertheless, the possibility that riociguat acts synergistically with iloprost to inhibit platelet activation should be considered.

A complementary role for tetraspanin superfamily member CD151 and ADP purinergic P2Y12 receptor in platelets

P2Y12 receptor is required for sustained activation of integrin αIIbβ3, irreversible platelet aggregation and thrombus stabilisation. Tetraspanin superfamily member CD151 associates with integrin αIIbβ3 and plays critical roles in regulation of thrombus growth and stability in vivo. The possible functional relationship between P2Y12 and CD151 in a molecular cluster in platelets may affect thrombus formation. Hence our aim was to investigate the physical and functional requirements for this association in platelets. Our investigations reveal a specific and constitutive association between CD151 and P2Y12 receptor in human platelets shown by immunoprecipitation/western blot studies and by flow cytometry. Specifically, the prominent association involves CD151 with P2Y12 oligomers, and to a lesser extent P2Y12 monomers. This association is not altered by platelet aggregation induced by different agonists. There is also a distinct complex of tetraspanin CD151 with ADP purinergic receptor P2Y12 but not P2Y1. P2Y12 oligomer interaction with CD151 is selective as compared to other tetraspanins. To investigate the functional relationship between these receptors in platelets we used wild-type or CD151 knockout (KO) mice treated with either PBS or 50 mg/kg clopidogrel. CD151 KO mice treated with clopidogrel exhibited synergy in delayed kinetics of clot retraction, in PAR-4 and collagen-mediated platelet aggregation, platelet spreading on fibrinogen and without restricting cAMP inhibition. Clopidogrel treated CD151 KO arterioles showed smaller and less stable thrombi with increased tendency to embolise ex vivo and in vivo. These studies demonstrate a complementary role between CD151 and P2Y12 receptor in platelets in regulating thrombus growth and stability.

A review on the chemical synthesis of pyrophosphate bonds in bioactive nucleoside diphosphate analogs

Currently, there is an ongoing interest in the synthesis of nucleoside diphosphate analogs as important regulators in catabolism/anabolism, and their potential applications as mechanistic probes and chemical tools for bioassays. However, the pyrophosphate bond formation step remains as the bottleneck. In this Digest, the chemical synthesis of the pyrophosphate bonds of representative bioactive nucleoside diphosphate analogs, i.e. phosphorus-modified analogs, nucleoside cyclic diphosphates, and nucleoside diphosphate conjugates, will be described.

Rho-kinase regulates human platelet activation induced by thromboxane A2 independently of p38 MAP kinase

We have previously demonstrated that ristocetin, an activator of GPIb/IX/V, induces the release of soluble CD40 ligand (sCD40L) via thromboxane A2 production in human platelets. It has been shown that thromboxane A2 induces the activation of Rho-kinase, a downstream effector of Rho, in human platelets. In the present study, we investigated the exact roles of Rho-kinase in thromboxane A2-induced platelet activation. We found that U46619, a thromboxane receptor (TP) agonist, induced the phosphorylation of cofilin, a target of Rho-kinase signaling, and that the cofilin phosphorylation by U46619 was suppressed by Y27632 or fasudil, specific inhibitors of Rho-kinase. Y27632 and fasudil markedly decreased large platelet aggregate formation by U46619. The release of sCD40L and secretion of platelet-derived growth factor (PDGF)-AB stimulated by U46619 were inhibited by Y27632 and fasudil. SB203580, a specific inhibitor of p38 mitogen-activated protein (MAP) kinase, reduced the sCD40L release and PDGF-AB secretion. Y27632 and fasudil failed to affect the phosphorylation of p38 MAP kinase whereas SB203580 had little effect on the phosphorylation of cofilin induced by U46619. In conclusion, our results strongly suggest that Rho-kinase regulates thromboxane A2-induced human platelet activation independently of p38 MAP kinase.