A Novel Interaction of the Catalytic Subunit of Protein Phosphatase 2A with the Adaptor Protein CIN85 Suppresses Phosphatase Activity and Facilitates Platelet Outside-in αIIbβ3 Integrin Signaling

Human Placental Mesenchymal Stem/Stromal cells (pMSCs) inhibit agonist-induced platelet functions reducing atherosclerosis and thrombosis phenotypes

Mesenchymal stem/stromal cells isolated from human term placenta (pMSCs) have potential to treat clinically manifested inflammatory diseases. Atherosclerosis is a chronic inflammatory disease, and platelets play a contributory role towards its pathogenesis. During transplantation, MSCs interact with platelets and exert influence on their functional outcome. In this study, we investigated the consequences of interaction between pMSCs and platelets, and its impact on platelet-mediated atherosclerosis in vitro. Human platelets were treated with various types of pMSCs either directly or with their secretome, and their effect on agonist-mediated platelet activation and functional characteristics were evaluated. Human umbilical vein endothelial cells (HUVECs) were used as control. The impact of pMSCs treatment on platelets was evaluated by the expression of activation markers and by platelet functional analysis. A subset of pMSCs reduced agonist-induced activation of platelets, both via direct contact and with secretome treatments. Decrease in platelet activation translated into diminished spreading, limited adhesion and minimized aggregation. In addition, pMSCs decreased oxidized LDL (ox-LDL)-inducedCD36-mediated platelet activation, establishing their protective role in atherosclerosis. Gene expression and protein analysis show that pMSCs express pro- and anti-thrombotic proteins, which might be responsible for the modulation of agonist-induced platelet functions. These data suggest the therapeutic benefits of pMSCs in atherosclerosis.

Platelet integrin αIIbβ3: signal transduction, regulation, and its therapeutic targeting

Integrins are a family of transmembrane glycoprotein signaling receptors that can transmit bioinformation bidirectionally across the plasma membrane. Integrin αIIbβ3 is expressed at a high level in platelets and their progenitors, where it plays a central role in platelet functions, hemostasis, and arterial thrombosis. Integrin αIIbβ3 also participates in cancer progression, such as tumor cell proliferation and metastasis. In resting platelets, integrin αIIbβ3 adopts an inactive conformation. Upon agonist stimulation, the transduction of inside-out signals leads integrin αIIbβ3 to switch from a low- to high-affinity state for fibrinogen and other ligands. Ligand binding causes integrin clustering and subsequently promotes outside-in signaling, which initiates and amplifies a range of cellular events to drive essential platelet functions such as spreading, aggregation, clot retraction, and thrombus consolidation. Regulation of the bidirectional signaling of integrin αIIbβ3 requires the involvement of numerous interacting proteins, which associate with the cytoplasmic tails of αIIbβ3 in particular. Integrin αIIbβ3 and its signaling pathways are considered promising targets for antithrombotic therapy. This review describes the bidirectional signal transduction of integrin αIIbβ3 in platelets, as well as the proteins responsible for its regulation and therapeutic agents that target integrin αIIbβ3 and its signaling pathways.

Functional links between Disabled-2 Ser723 phosphorylation and thrombin signaling in human platelets

Essentials Disabled-2 (Dab2) phosphorylation status in thrombin signaling of human platelet was investigated. Ser723 was the major Dab2 phosphorylation site in human platelets stimulated by thrombin. Dab2 S723 phosphorylation (pS723) caused the dissociation of Dab2-CIN85 protein complex. Dab2-pS723 regulated ADP release and integrin αIIbβ3 activation in thrombin-treated platelets.

SUMMARY

Background Disabled-2 (Dab2) is a platelet protein that is functionally involved in thrombin signaling in mice. It is unknown whether or not Dab2 undergoes phosphorylation during human platelet activation. Objectives To investigate the phosphorylation status of Dab2 and its functional consequences in thrombin-stimulated human platelets. Methods Dab2 was immunoprecipitated from resting and thrombin-stimulated platelet lysates for differential isotopic labeling. After enrichment of the phosphopeptides, the phosphorylation sites were analyzed by mass spectrometry. The corresponding phospho-specific antibody was generated. The protein kinases responsible for and the functional significance of Dab2 phosphorylation were defined by the use of signaling pathway inhibitors/activators, protein kinase assays, and various molecular approaches. Results Dab2 was phosphorylated at Ser227, Ser394, Ser401 and Ser723 in thrombin-stimulated platelets, with Ser723 phosphorylation being the most significantly increased by thrombin. Dab2 was phosphorylated by protein kinase C at Ser723 in a Gαq -dependent manner. ADP released from the stimulated platelets further activated the Gβγ -dependent pathway to sustain Ser723 phosphorylation. The Cbl-interacting protein of 85 kDa (CIN85) bound to Dab2 at a motif adjacent to Ser723 in resting platelets. The consequence of Ser723 phosphorylation was the dissociation of CIN85 from the Dab2-CIN85 complex. These molecular events led to increases in fibrinogen binding and platelet aggregation in thrombin-stimulated platelets by regulating αIIb β3 activation and ADP release. Conclusions Dab2 Ser723 phosphorylation is a key molecular event in thrombin-stimulated inside-out signaling and platelet activation, contributing to a new function of Dab2 in thrombin signaling.