A Chemokine Receptor, CXCR4, Which Is Regulated by Hypoxia-Inducible Factor 2α, Is Crucial for Functional Endothelial Progenitor Cells Migration to Ischemic Tissue and Wound Repair

Endothelial progenitor cells (EPCs) have the ability to form new blood vessels and protect ischemic tissues from damage. We previously reported that EPCs with low activity of aldehyde dehydrogenase (Alde-Low EPCs) possess the greater ability to treat ischemic tissues compared with Alde-High EPCs. The expression level of the hypoxia-inducible factors (HIFs), HIF-1α and HIF-2α, was found to be greater in Alde-Low EPCs than in Alde-High EPCs. However, the precise role of the HIF factors in the regulation of EPC activity remains obscure. In this study, we demonstrate a critical role of HIF-2α and its target gene CXCR4 for controlling the migratory activity of EPC to ischemic tissue. We found that coculture of Alde-High EPCs with microvesicles derived from Alde-Low EPCs improved their ability to repair an ischemic skin flap, and the expression of CXCR4 and its ligand SDF1 was significantly increased following the coculture. In Alde-Low EPCs, the expression of CXCR4 was suppressed by short hairpin RNA (shRNA)-mediated HIF-2α, but not HIF-1α downregulation. Chromatin immunoprecipitation assays showed that HIF-2α, but not HIF-1α, binds to the promoter region of CXCR4 gene. The CXCR4 shRNA treatment in Alde-Low EPCs almost completely abrogated their migratory activity to ischemic tissues, whereas the reduction of vascular endothelial growth factor (VEGF) showed much less effect. The CXCR4 overexpression in Alde-High EPCs resulted in a partial, but significant improvement in their repairing ability in an ischemic skin flap. Collectively, these findings indicate that the CXCR4/SDF-1 axis, which is specifically regulated by HIF-2α, plays a crucial role in the regulation of EPC migration to ischemic tissues.