Platelet angiogenic activities and their regulation on endothelial progenitor cell functions

Abstract

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 α6β1, as well as EPC α6β1 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 pro- and anti-angiogenic factors, which are stored in separate α-granules 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.