C/Ebp Beta Mediates Osteoclast Recruitment By Regulating Endothelial Progenitor Cell Expression Of Sdf-1 Alpha

Integration of tissue-engineered bone grafts with the host bone is vital for the healing of critical-size bone defects. An important aspect of this process is bone resorption, which must be carried out by osteoclasts derived from the host. However, the mechanism underlying recruitment of host osteoclast precursors to graft sites remains unclear. Endothelial progenitor cells (EPCs) mobilize from the bone marrow into the circulation and home to sites of angiogenesis such as tissue remodeling. Since EPCs express SDF-1, and C/EBP beta is known to regulate SDF-1 alpha expression, we hypothesized that EPCs may recruit CXCR4-expressing host osteoclast precursors to the repair area and that this recruitment may be mediated through C/EBP beta signaling. Using an inflammatory EPC model we showed that EPCs upregulate protein levels of both SDF-1 alpha and C/EBP beta. A luciferase assay confirmed that C/EBP beta acts on the SDF-1 alpha promoter in these cells, and that binding is increased under conditions of inflammation, while silencing of C/EBP beta reduces expression of SDF-1 alpha and C/EBP beta. Using RAW264.7 cells as a model of osteoclastic monocyte precursors, we investigated their responses to migratory factors in EPC conditioned medium. We showed that RAW264.7 cells migrate towards conditioned medium from EPCs treated with IL-1 beta, an effect which could be abolished by silencing C/EBP beta in EPCs, and was almost completely blocked by silencing CXCR4 in RAW264.7 cells. These findings show that EPCs respond to inflammatory stimuli by signaling to osteoclast precursors via SDF-1, and that C/EBP beta mediates this response.