Summary
The process of neovascularization greatly depends on the induction of the angiogenic
phenotype of endothelial cells that is strictly controlled by humoral factors as well
as by cellular communications in the vascular system. Although blood platelets contain
several secretable pro-and antiangiogenic components, their overall role in angiogenesis
remains poorly understood. In a mouse model of hypoxia-induced retinal angiogenesis,
the situation of thrombocytopenia as well as inhibition of platelet aggregation by
a highly specific αIIbß3-integrin antagonist or acetyl salicylic acid (Aspirin™) administration,
respectively, resulted in about 35-50% reduction of retinal neovascularization, compatible
with a significant contribution of blood platelets in angiogenesis. Platelet remnants
and microvesicles were found at sites of angiogenic sprouts. In vitro isolated platelets incorporated in a fibrin gel induced capillary sprouting of microvascular
endothelial cells. Similarly, platelet releasate elevated the permeability of confluent
endothelial cell monolayers to the same extent as hypoxia did. Platelet-derivedVEGF
as well as butanol-extractable lipid mediators were identified as predominant activators
of angiogenesis, particularly of microvascular endothelial cell proliferation and
migration. In addition, a synergistic effect between platelet-derived VEGF and bFGF
in capillary sprouting and endothelial cell proliferation was found. Based on this
proangiogenic role of platelets in neovascularization, anti-platelet substances can
be considered as potent inhibitors of angiogenesis.
Keywords
Vascular endothelial growth factor - retinal neovascularization - lysophospholipids
- thrombocytopenia - anti-platelet drugs