Osteoprotegerin (OPG) Promotes Recruitment of Endothelial Progenitor Cells (EPCs) via CXCR4 Signaling Pathway to Improve Bone Defect Repair.

BACKGROUND The aim of this study was to investigate the effect of using osteoprotegerin (OPG) to treat bone defects mediated by endothelial progenitor cell (EPC) recruitment and migration through the CXCR4 signaling pathway. MATERIAL AND METHODS The EPCs extracted from human peripheral blood were cultured in vitro and the expression of CXCR4 and its downstream p-AKT was monitored by the Western blot analysis after OPG treatment. Using the scratch wound healing test and Transwell assay, we assessed the variables influencing the effect of OPG on EPCs after pre-treatment with CXCR4 blocker (AMD3100) and PI3K blocker (Ly294002). After 4 weeks, the bone defect repair condition was estimated via micro-CT and staining with HE and Masson trichrome. Then, immunofluorescence staining was performed to assess angiogenesis in bone defects, while the expression of EPC marker and vascular endothelial growth factor receptor 2 (VEGFR2) was detected by immunohistochemical staining. RESULTS The EPCs treated with OPG had increased levels of CXCR4 and p-AKT. Moreover, the difference in EPC levels among groups in the scratch wound healing experiment and migration experiment indicated that the OPG treatment promoted cell migration and AMD3100 and LY294002 inhibited the function of OPG. In addition, OPG promoted angiogenesis and repair of bone defect in rats, and these effects were abolished by AMD3100 and LY294002 administration. CONCLUSIONS OPG enhanced the proliferation and migration of EPCs through the CXCR4 pathway and promoted angiogenesis and bone formation at bone defect sites.