Differential regulation of bone marrow-derived endothelial progenitor cells and endothelial outgrowth cells by the Notch signaling pathway.

Endothelial progenitor cells (EPCs) are heterogeneous populations of cells that participate in vasculogenesis and promote tissue regeneration. However the different roles of EPC populations in vasculogenesis and tissue regeneration, as well as their regulation and mechanisms remain elusive. In the present study, we cultured bone marrow (BM)-derived early EPCs (EEPCs) and endothelial outgrowth cells (EOCs), and investigated their roles in liver regeneration and their regulation by the Notch signaling pathway. We found that Notch signaling exhibited different effects on the proliferation and migration of EEPCs and EOCs. Our results also showed that while EEPCs failed to form vessel-like structures in a three dimensional sprouting model in vitro, EOCs could sprout and form endothelial cords, and this was regulated by the Notch signaling. We further showed that, by using a conditional knockout model of RBP-J (the critical transcription factor mediating Notch signaling), Notch signaling differentially regulates EEPCs and EOCs. In a partial hepatectomy (PHx) model, EEPCs Notch-dependently benefitted liver regeneration with respect to liver function and hepatocyte proliferation and apoptosis. In contrast, EOCs appeared not directly involved in the recovery of liver function and the increase of hepatocytes. These data suggested that the RBP-J-mediated Notch signaling differentially regulated the two types of EPCs, which showed different roles in liver regeneration.