Autophagy in endothelial progenitor cells is cytoprotective in hypoxic conditions

Endothelial progenitor cells (EPCs) may be incorporated into local vessels to enhance angiogenesis within ischemic tissue. Recently, EPC transplantation has become a potential therapy for improving tissue function in cardiovascular disease. However, the mechanisms of proliferation, differentiation, and survival of EPCs in a hypoxic microenvironment remain unclear. In this study, CD34(+)VEGFR-2(+) EPCs were isolated from mononuclear cells of human umbilical cord blood, and differentiation to endothelial cells was induced with VEGF. When EPC autophagy was inhibited with 3-methyladenine (3-MA) under normoxic conditions, proliferation and viability of the cells were decreased, and the cells failed to differentiate into endothelial cells. Under hypoxic conditions (1% O(2)), Beclin-1 expression of the cells was upregulated and both MDC-labeled and LC3-positive puncta and autophagic ultrastructures in the cells increased significantly. The number of lysosomes also increased in hypoxia-exposed cells. When autophagy was inhibited with 3-MA under hypoxic conditions, the number of apoptotic cells increased, and the number and size of lysosomes decreased. Conversely, apoptosis of the hypoxic EPCs was reduced when autophagy was induced by pretreatment with rapamycin. These results demonstrate that autophagy is involved in proliferation and differentiation of EPCs. Furthermore, hypoxia activates autophagy, promoting EPC survival by inhibiting apoptosis. Enhancing autophagy with hypoxic preconditioning may be beneficial for survival of the transplanted EPCs in a local hypoxic environment.