PLATELET ANGIOGENIC ACTIVITIES AND THEIR REGULATION ON ENDOTHELIAL PROGENITOR CELL FUNCTIONS

Platelets are essential for haemostasis and thrombosis, but also play a major role in angiogenesis. Activated platelets recruit progenitor cells and induce differentiation into endothelial progenitor cells (EPCs), which contribute importantly to vascular regeneration and endothelium reparation. However, the interactions between platelets and EPCs are poorly understood. Therefore, aims of the present thesis work are to investigate platelet angiogenic activities, and to study the influence and mechanisms of platelet-regulated EPC angiogenic properties. We have elucidated that platelets promoted EPC angiogenesis via the glycoproteins on the platelet surface, and identified that platelet tetraspanin CD151 and integrin α61, as well as EPC α61 were required for platelet-enhanced EPC angiogenesis. Moreover, it has been shown that platelets exerted the enhancement via Src-PI3K signalling pathway of EPCs. Platelets contain both proand anti-angiogenic factors, which are stored in separate αgranules and distinctly release upon different stimulation. In paper II, we characterized that pro-angiogenic and anti-angiogenic regulators were mostly stored in separate α-granules. Furthermore, we found that protease-activated receptor (PAR) 1, adenosine diphosphate (ADP), and GPVI stimulation induced platelet secretion of pro-angiogenic regulators, whereas PAR4 stimulation selectively induced platelet secretion of anti-angiogenic regulators. In paper III, we determined if PAR1-stimulated platelet releasate (PAR1-PR) and PAR4-PR differently regulate angiogenic properties of EPCs. To our surprise, both PAR1-PR and PAR4-PR enhanced EPC migration and tube formation. PAR1-PR enhanced vasculogenesis more potently than PAR4-PR, and the enhancements required a cooperation of multiple platelet-derived angiogenic regulators. Platelets retain mRNAs from the megakaryocytes, and use these mRNAs as templates for de novo protein synthesis upon stimulation. In paper IV, we observed that thrombin stimulation induced SDF-1α mRNA maturation, which led to de novo synthesis of SDF-1α after activation. The data suggest that platelets may enhance angiogenesis by de novo synthesis of angiogenic regulators after activation. Together, the thesis work demonstrates that platelets enhance EPC angiogenic properties via both secreted angiogenic regulactors and surface receptors, and that platelets may regulate angiogenesis via de novo synthesis of angiogenic regulators after activation. LIST OF SCIENTIFIC PAPERS I. Huang Z, Patarroyo M, Miao X, Nilsson G, Pernow J, Li N. Platelet membrane glycoprotein CD151 promotes endothelial progenitor cell angiogenesis. Submitted manuscript. II. Chatterjee M, Huang Z, Zhang W, Jiang L, Hultenby K, Zhu L, Hu H, Nilsson GP, Li N. Distinct platelet packaging, release, and surface expression of proangiogenic and antiangiogenic factors on different platelet stimuli. Blood. 2011 Apr 7;117(14):3907-11. III. Huang Z, Miao X, Luan Y, Zhu L, Kong F, Lu Q, Pernow J, Nilsson G, Li N. PAR1-stimulated platelet releasate promotes angiogenic activities of endothelial progenitor cells more potently than PAR4-stimulated platelet releasate. J Thromb Haemost. 2015 Mar;13(3):465-76. IV. Huang Z, Rahman MF, Jiang L, Xie H, Hu H, Lui WO, Li N. Thrombin induces de novo protein synthesis of stromal cell-derived factor-1α but not angiostatin in human platelets. J Thromb Haemost. 2012 Oct;10(10):2202-5. OTHER PUBLICATIONS NOT INCLUDED IN THESIS 1. Zhu L, Huang Z, Stalesen R, Hansson GK, Li N. Platelets provoke distinct dynamics of immune responses of different CD4+ T cell subsets via selective regulations of cell proliferation. J Thromb Haemost. 2014 Jul;12(7):1156-65. 2. Huang Z, Liu P, Zhu L, Li N, Hu H. P2X1-initiated p38 signalling enhances thromboxane A2-induced platelet secretion and aggregation. Thromb Haemost. 2014 Jul 3;112(1):142-50.