Objective: Alpha6 integrin subunit (alpha6) expression is increased by proangiogenic growth factors such as vascular endothelial growth factor (VEGF) and fibroblast growth factor. This increase correlates with enhanced in vitro tube formation by endothelial cells and their progenitors called endothelial colony-forming cells (ECFCs). We thus studied the role of alpha6 in vasculogenesis induced by human ECFCs, in a mouse model of hindlimb ischemia.
Methods and results: We used small interfering RNA (siRNA) to inhibit alpha6 expression on the surface of ECFCs. For in vivo studies, human ECFCs were injected intravenously into a nude mouse model of unilateral hind limb ischemia. Transfection with siRNA alpha6 abrogated neovessel formation and reperfusion of the ischemic hind limb induced by ECFCs (P<0.01 and P<0.001, respectively). It also inhibited ECFC incorporation into the vasculature of the ischemic muscle (P<0.001). In vitro, siRNA alpha6 inhibited ECFC adhesion (P<0.01), pseudotube formation on Matrigel, migration, and AKT phosphorylation (P<0.0001), with no effect on cell proliferation or apoptosis.
Conclusions: alpha6 Expression is required for ECFC migration, adhesion, recruitment at the site of ischemia, and the promotion of the postischemic vascular repair. Thus, we have demonstrated a major role of alpha6 in the proangiogenic properties of ECFCs.