Fu Sheng-Long, Pang Hao, Xu Jian-Zhong, Wu Xue-Hui
National & Regional Engineering Laboratory of Tissue Engineering, Department of Orthopaedics, Southwest Hospital, Third Military Medical University, Chongqing, China.
PLoS One. 2014 Mar 11;9(3):e91217. doi: 10.1371/journal.pone.0091217. eCollection 2014.
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/EBPβ is known to regulate SDF-1α 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/EBPβ signaling. Using an inflammatory EPC model we showed that EPCs upregulate protein levels of both SDF-1α and C/EBPβ. A luciferase assay confirmed that C/EBPβ acts on the SDF-1α promoter in these cells, and that binding is increased under conditions of inflammation, while silencing of C/EBPβ reduces expression of SDF-1 α and C/EBPβ. 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-1β, an effect which could be abolished by silencing C/EBPβ 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/EBPβ mediates this response.
组织工程骨移植与宿主骨的整合对于临界尺寸骨缺损的愈合至关重要。这一过程的一个重要方面是骨吸收，它必须由宿主来源的破骨细胞来完成。然而，宿主破骨细胞前体募集到移植部位的潜在机制仍不清楚。内皮祖细胞（EPCs）从骨髓动员到循环中，并归巢到诸如组织重塑等血管生成部位。由于EPCs表达SDF-1，并且已知C/EBPβ调节SDF-1α的表达，我们推测EPCs可能将表达CXCR4的宿主破骨细胞前体募集到修复区域，并且这种募集可能通过C/EBPβ信号传导介导。使用炎症EPC模型，我们表明EPCs上调SDF-1α和C/EBPβ的蛋白水平。荧光素酶测定证实C/EBPβ作用于这些细胞中的SDF-1α启动子，并且在炎症条件下结合增加，而C/EBPβ的沉默降低SDF-1α和C/EBPβ的表达。使用RAW264.7细胞作为破骨细胞单核细胞前体的模型，我们研究了它们对EPC条件培养基中迁移因子的反应。我们表明RAW264.7细胞向用IL-1β处理的EPCs的条件培养基迁移，这种效应可以通过沉默EPCs中的C/EBPβ来消除，并且在RAW264.7细胞中沉默CXCR4几乎完全阻断这种效应。这些发现表明EPCs通过经由SDF-1向破骨细胞前体发出信号来响应炎症刺激，并且C/EBPβ介导这种反应。
