Soluble factors released by endothelial progenitor cells promote migration of endothelial cells and cardiac resident progenitor cells.

Circulating endothelial progenitor cells (EPC) are incorporated into newly formed capillaries, enhance neovascularization after hind limb ischemia and improve cardiac function after ischemic injury. Incorporated progenitor cells may also promote neovascularization and cardiac regeneration by releasing factors, which act in a paracrine manner to support local angiogenesis and mobilize tissue residing progenitor cells. Therefore, we analyzed the expression profile of cytokines in human peripheral blood-derived EPC as opposed to human umbilical vein endothelial cells (HUVEC), human microvascular endothelial cells (HMVEC), and CD14(+) monocytes by microarray technology. A gene tree analysis revealed a distinct expression pattern of angiogenic growth factors in EPC, mature endothelial cells, and CD14(+) monocytes. VEGF-A, VEGF-B, SDF-1, and IGF-1 mRNA levels were higher in EPC as compared to HUVEC or HMVEC. The enhanced mRNA expression was paralleled by a significant release of VEGF, SDF-1, and IGF-1 protein into the cell culture supernatant of EPC. Moreover, immunohistological analysis of ischemic limbs from nude rats revealed that VEGF is also released from recruited human EPC in vivo. As a functional consequence, conditioned medium of EPC induced a strong migratory response of mature endothelial cells, which was significantly inhibited by VEGF and SDF-1 neutralizing antibodies. Finally, conditioned medium of EPC significantly stimulated the migration of cardiac resident c-kit(+) progenitor cells in vitro. Taken together, EPC exhibit a high expression of angiogenic growth factors, which enhanced migration of mature endothelial cells and tissue resident cardiac progenitor cells. In addition to the physical contribution of EPC to newly formed vessels, the enhanced expression of cytokines may be a supportive mechanism to improve blood vessel formation and cardiac regeneration after cell therapy.