Tetraspanin CD151 and integrin α6β1 mediate platelet-enhanced endothelial colony forming cell angiogenesis

Abstract

Background: Platelets promote angiogenesis of endothelial colony forming cells (ECFCs), with the underlying mechanisms not being fully understood. Objective: To investigate if platelets regulate the angiogenic property of ECFCs via mechanisms beyond platelet-released angiogenic regulators. Methods and Results: Endothelial colony forming cells were generated by ECFC-directed cell culture of peripheral blood mononuclear cells. Capillary-like tube formation of ECFCs was assessed using a Matrigel assay. Platelets promoted ECFC tube formation in both basic and complete ECFC medium. Importantly, the ECFC angiogenic responses induced by platelets were stronger than those induced by platelet releasates. Thus, the branching points of ECFC tube formation (30.5 ± 9.0/field, ECFC alone) were increased by platelet releasates (58.2 ± 8.3/field) and even more profoundly by platelets (95.5 ± 17.6/field), indicating that platelet membrane components also promoted ECFC tube formation. The latter was further supported by evidence that fixed platelets did enhance ECFC tube formation. Subsequent experiments revealed that the promotion was dependent on platelet-surface glycoproteins, as removal of sialic acid from platelet glycoproteins by neuraminidase abolished the enhancement. Furthermore, platelet-expressed, but not ECFC-expressed, CD151 was important for the enhancement, as pretreatment of platelets, but not ECFCs, with a CD151-blocking antibody attenuated the effect. Integrin α6β1 on both ECFCs and platelets also participated in platelet-promoted tube formation, as integrin α6 or β1 blockade of either cell type markedly or totally inhibited the phenomenon. Moreover, platelets exerted the enhancement via the Src-PI3K signaling pathway of ECFCs. Conclusion: Platelet-enhanced ECFC angiogenesis requires platelet tetraspanin CD151 and α6β1 integrin, as well as ECFC α6β1 integrin and Src-PI3K signaling.