The mechanisms of endothelial progenitor cell homing and differentiaion

Endothelial progenitor cells (EPC) are found circulating in adults and preferentially incorporate into growing vessels at sites of active angiogenesis. The mechanisms for their homing to angiogenic vessels, factors that regulate their differentiation into an endothelial-like phenotype and extent to which these cells become endothelial are not known. Understanding these mechanisms would be valuable for therapeutically directing vascularisation, especially where normal compensatory angiogenic response is lacking or impaired. In this study, we have examined the possibility that ex vivo expanded EPC migration and adhesion could be regulated by Angiopoietic-1 (Ang1) and -2 (Ang2), a soluble ligand expressed by endothelial cells at sites of vessel remodelling and angiogenesis and Vascular Endothelial Growth Factor (VEGF). We show VEGF and Ang1 stimulate EPC migration and show for the first time the same effect is seen with Ang2. This was specific for EPC as the ligand failed to affect endothelial cell migration. Ang2 stimulated EPC migration was inhibited by soluble Tie2 ectodomain. Furthermore, these ligands were found to stimulate adhesion between EPC and endothelial monolayers using cell:cell adhesion assays. The Angiopoietin stimulated increase EPC:endothelial cell interaction was again inhibited by the soluble Tie2 ectodomain. The roles of these soluble mediators and adhesion in signalling differentiation were defined by examining EPC marker expression. These results showed a phenotypic overlap between EPC and monocytes and allowed definition of a moncyte sub-population with endothelial characteristics.