Platelet angiogenic activities : a concert of multiple players

Abstract

Angiogenesis, the sprouting of new capillaries from pre-existing blood vessels, is crucial for many physiological and pathological processes. Platelets are an important player in angiogenesis, whilst understanding of platelet engagement remains obscure. The present thesis aims to further elucidate the mechanisms of platelet angiogenic activities, including those by platelet releasate, platelet membrane, and miRNA-regulated platelet angiogenic factor synthesis.

Our earlier studies have demonstrated that thrombin protease activated receptor 1 (PAR1) and PAR4 stimuli induce selective platelet release of proangiogenic and antiangiogenic regulators, respectively. The present thesis work shows that both PAR1-stimulated platelet releasate (PAR1-PR) and PAR4-PR enhanced tube formation and migration of endothelial progenitor cells (EPCs, also termed as endothelial colony forming cells, ECFCs, in paper II and IV) in vitro and angiogenesis in vivo, while the enhancements by PAR1-PR were more potent than those of PAR4-PR. Our findings suggest that the selective release of platelet angiogenic regulators may mainly concern the different release levels of platelet angiogenic regulators, and the final outcome of angiogenic regulating effects will depend on the negotiation of all factors in the platelet releasates.

We further demonstrated that platelet-promoted angiogenesis depends on not only platelet-released mediators but also platelet membrane components. Thus, we found that platelet-enhanced EPC angiogenic responses were more pronounced than those by platelet releasates. We showed that platelet membrane glycoproteins had a major role in promoting EPC tube formation, and have identified that platelet tetraspanin CD151 and the integrin α6β1 expressed on both platelets and EPCs contributing to platelet-promoted tube formation. Albeit as anucleated cells, platelets retain certain capacities of protein synthesis. Our work gives further evidence that thrombin stimulation induced de novo synthesis of thrombospondin 1 (TSP1). More importantly, we showed that thrombin stimulation altered platelet miroRNA profile, and that thrombin-reduced platelet miR-27b expression enhanced TSP1 synthesis. The latter was supported by our data showing that miR-27b mimic transfection inhibited TSP1 synthesis in the platelet precursor megakaryocytes.

We have also studied if type 2 diabetes mellitus (T2DM) would impair platelet angiogenic activities. We found that platelets from mild T2DM subjects were hyperreactive, but had similar angiogenic activities (angiogenic regulator release and platelet-regulated EPC tube formation), as compared to those of age/gender-matched controls. Our data suggest that a good glucose control may be beneficial for maintaining platelet angiogenic function.

Together, this thesis work shows the platelets can regulate angiogenesis via different elements, and that platelet angiogenic activities are a concert of multiple players, namely released angiogenic factors, membrane components, as well de novo angiogenic factor synthesis and its regulation by miRNAs.