Endothelial progenitor cells (EPCs) trigger angiogenesis and vasculogenesis, and thus have been used in the treatment of diseases related to ischemia. EPCs have been isolated from peripheral blood [15], bone marrow [68], and umbilical cord blood [912]. Several studies have demonstrated successful preclinical application of EPCs in various disease models, such as rat myocardial infarction [1315], murine hindlimb ischemia [16], rat ischemic myocardium [17], dermal wound healing [18], and mouse ovarian grafts [19]. A recent study showed that EPC transplantation could increase neovascularization in porcine models [20]. Clinically, EPC transplantation can improve cardiovascular outcomes. In a phase I/II clinical trial of 24 patients [21] followed by a phase IIb trial with 167 patients [22], almost all patients subjected to EPC transplantation showed improved angina frequency and exercise tolerance. In another phase III trial of 444 patients, CD34 cell transplantation could improve the functional capacity in patients with refractory angina [23]. In addition to coronary artery diseases, other pilot studies have also used autologous CD34 to treat critical limb ischemia and showed reduced amputation rates in critical limb ischemic patients [24]. Several studies have also genetically modified EPCs by transfecting cells with genes to increase the treatment efficacy, such as hTERT-transfected EPCs for ischemic myocardium of rats [25], vascular endothelial growth factor (VEGF)-transfected EPCs for myocardial infarction [26], and VEGF and heme oxygenase-1-transfected EPCs for the hindlimb ischemia mice model [27, 28]. Human cord blood endothelial progenitors could also promote postischemic angiogenesis in an immunocompetent mouse model [29].

Successful application of EPC treatments in the clinic, however, has encountered several obstacles, such as limited EPC sources, low numbers of cells, and low proliferation. The number of EPCs in peripheral blood and bone marrow is extremely low [5, 30]. Therefore, deriving endothelial cells as well as EPCs from pluripotent stem cells was studied [3133]. Although EPCs derived from embryonic stem cells were suggested to be a more promising therapy compared with umbilical cord EPCs [34], this application has not been translated to the clinic owing to the risk of pluripotent stem cells stimulating tumor formation [35, 36]. Current research has thus focused on human fibroblasts (HFs) as a cell source for reprogramming to EPCs.

The first effort towards direct reprogramming of fibroblasts to EPCs was performed by Margariti et al. [37]. The authors developed a method to generate partial-induced pluripotent stem cells by transferring four reprogramming factors (OCT4, SOX2, KLF4, and c-MYC) to HFs for 4 days. These partial-induced pluripotent stem cells were capable of differentiating into endothelial cells in response to defined media and culture conditions [37]. However, although the endothelial cells obtained could not form tumors, these cells carried oncogenes such as Sox-2 [37]. Li et al. and Han et al. successfully removed the Sox-2 gene in a revised version of the procedure and instead only used two genes (Oct4 and Klf4) in combination with soluble factors [38] or Foxo1, Er71, Klf2, Tal1, and Lmo2 [39]. These studies used a mixture of factors to induce fibroblasts to EPCs and involved complex procedures with low efficacy.

Recently, ETV2 was reported as a single factor that could induce direct reprogramming of fibroblasts into EPCs [40, 41] and of amniotic cells into EPCs [42]. In fact, ETV2 is a master gene that regulates various signaling pathways and functions as an essential regulator for vasculogenesis and hematopoiesis. ETV2 and GATA2 bind to the promoter of SPI1 and regulate its expression during embryogenesis [43]. ETV2 regulates cardiac development [44], and vascular regeneration [45]. However, the direct reprogramming of ETV2 transduction was low (about 1 %) [41]. Several studies have reported that hypoxia could improve reprogramming of cells. Foja et al. [46] showed that hypoxia improved the reprogramming of MSCs into induced pluripotent stem cells (iPSCs). Adipose stem cells were also stimulated for reprogramming to iPSCs by hypoxia [47]. Hypoxia also enhanced the reprogramming of fibroblasts into iPSCs [48] and dental pulp cells into iPSCs [49]. This study therefore examined the potential enhancement of direct reprogramming efficacy to EPCs by single-factor ETV2 under hypoxia treatment.