Early Endothelial Outgrowth Cells significantly protect mice from acute ischemic renal failure. This has meanwhile been proven in a number of different experimental studies [
5,
23‐
26]. In attempting to establish the cells as future therapeutic tool in ischemic AKI, different strategies have been identified, helpful to increase renoprotective effects of eEOCs. In 2010, 8-O-cAMP was identified as eEOC agonist, inducing redistribution of integrin molecules towards the plasma membrane of the cells thereby enhancing postischemic cell homing [
24]. The hormone melatonin improved antiischemic actions of eEOCs in AKI as well [
23]. Recently, the protein BMP-5 was identified as another eEOC agonist in this situation [
27]. Thus, Early Endothelial Outgrowth Cells increasingly become a therapeutic option in ischemic AKI.
Aim of the current study was to analyze consequences of Angiopoietin-1 (Ang-1) treatment of eEOCs in the context of eEOC-mediated postischemic kidney repair. Pretreatment of the cells resulted in significant lower concentrations of serum creatinine in cell-injected mice, which was accompanied by higher resistance of cultured eEOCs against TGF-β induced apoptosis/necrosis and by increased cell migration. In addition, supernatant from Ang-1 incubated eEOCs markedly enhanced migratory activity of cultured mature endothelial cells. Postischemic cell homing or production/secretion of proangiogenic mediators remained unaffected by Ang-1. The latter findings are somehow surprising, regarding the fact that eEOCs predominantely act through secreted signals. The cells home into the interstitial space of postischemic tissues where they release proangiogenic mediators in a paracrinic manner [
28]. Miscellaneous substances have been identified in the past, including VEGF, HGF, IGF-1 [
28,
29], matrix metalloproteinase 9, IL-8, and others [
30,
31]. Although VEGF plays a fundamental role in orchestrating protective effects of the cells [
23,
28], we did not find increased cellular secretion of VEGF (and HGF). An alternative mechanism of EPC(eEOC)-mediated vasoprotection has lately been introduced by the group of Goligorsky [
32]. Yasuda and colleagues showed direct interaction between EPCs and mature endothelial cells by the generation of so-called nanotubes, cell membrane-build structures which allow direct transfer of lysosome constituents and other molecules from one cell to the other [
32]. This study indicated the potential relevance of direct cell-cell communication, in addition to indirect paracrinic EPC effects. However, our investigation did [
5,
23,
24] not reveal any relevant cell incorporation into the postischemic endothelium or any accumulation of eEOCs in close proxomity to endothelial cells. This is by the way in line with the currently favored concept of eEOC-mediated vasorepair [
26,
30]. The virtual absence of eEOCs in the postischemic kidney at 48 hours has been reported in previous eEOC/AKI-related studies [
23,
33] and comparable data has been documented for mesenchymal stem cells as well [
34]. So far, significant and increased postischemic eEOC homing has only been observed in one of our studies. The substance 8-O-cAMP induced redistribution of β1-integrin molecules towards the plasma membrane of eEOCs resulting in increased transvascular cell migration into the interstitial space [
24]. Nevertheless, our analyses of postischemic kidneys were performed 48 hours after ischemia. Severe ischemia-related endothelial dysfunction typically occurs within the first 24 hours and the absence of injecetd cells at the end of day two only supports the concept of (transient) paracrinic cell actions of eEOCs [
35]. Angiopoietin-1 plays a crucial role in maintaining vascular homeostasis under physiological conditions and this study undoubtedly proves an eEOC-agonistic role of the protein in iAKi although it still remains to be elucidated which excact mechanisms promote eEOC renoprotection in vivo. At this point, several endogenous [
25] and exogenous agonists of eEOCs have been identified (8-O-cAMP, Melatonin, BMP-5, Ang-1 [
23,
24,
27]). A first study on therapeutic administration of eEOCs in human AKI is being planned at the moment. Thus, exogenous measures, helpful to improve renoprotective competence of the cells in ischemic AKI are needed. Another important question with regard to an EPC-based therapy of ischemic AKI is related to the mid- to long-term outcome of cell treated animals/patients. It has to be analyzed whether EPCs diminish postischemic vascular rarefication/interstitial fibrosis, two events which substantially worsen prognosis of patients in the long-term.
In summary, Angiopoietin-1 has been identified as potent agonist of syngeneic murine eEOCs in ischemic AKI. The protein increases renoprotective effects of the cells in vivo and activates cultured cells on different levels. Angiopoietin-1 is therefore a promising tool for stimulating eEOC-mediated renoprotection in ischemic AKI.