Department of Orthopedic Surgery, Mayo Clinic, Rochester, MN, USA; Developmental Biology and Cancer, UCL GOS Institute of Child Health, London, UK; Comparative Bioveterinary Sciences, Royal Veterinary College, London, UK.
Mayo Clinic Graduate School of Biomedical Sciences, Mayo Clinic, Rochester, MN, USA; Center for Regenerative Medicine, Mayo Clinic, Rochester, MN, USA.
Gene. 2020 Dec;763S:100027. doi: 10.1016/j.gene.2020.100027. Epub 2020 Jan 16.
Mechanical loading-related strains trigger bone formation by osteoblasts while suppressing resorption by osteoclasts, uncoupling the processes of formation and resorption. Osteocytes may orchestrate this process in part by secreting sclerostin (SOST), which inhibits osteoblasts, and expressing receptor activator of nuclear factor-κB ligand (RANKL/TNFSF11) which recruits osteoclasts. Both SOST and RANKL are targets of the master osteoblastic transcription factor RUNX2. Subjecting human osteoblastic Saos-2 cells to strain by four point bending down-regulates their expression of SOST and RANKL without altering RUNX2 expression. RUNX2 knockdown increases basal SOST expression, but does not alter SOST down-regulation following strain. Conversely, RUNX2 knockdown does not alter basal RANKL expression, but prevents its down-regulation by strain. Chromatin immunoprecipitation revealed RUNX2 occupies a region of the RANKL promoter containing a consensus RUNX2 binding site and its occupancy of this site decreases following strain. The expression of epigenetic acetyl and methyl writers and readers was quantified by RT-qPCR to investigate potential epigenetic bases for this change. Strain and RUNX2 knockdown both down-regulate expression of the bromodomain acetyl reader BRD2. BRD2 and RUNX2 co-immunoprecipitate, suggesting interaction within regulatory complexes, and BRD2 was confirmed to interact with the RUNX2 promoter. BRD2 also occupies the RANKL promoter and its occupancy was reduced following exposure to strain. Thus, RUNX2 may contribute to bone remodeling by suppressing basal SOST expression, while facilitating the acute strain-induced down-regulation of RANKL through a mechanosensitive epigenetic loop involving BRD2.
机械负荷相关的应变通过成骨细胞触发骨形成，同时抑制破骨细胞的吸收，从而使形成和吸收过程脱偶联。骨细胞可能通过分泌抑制成骨细胞的硬化素 (SOST) 和表达招募破骨细胞的核因子-κB 配体受体激活剂 (RANKL/TNFSF11) 来部分协调这一过程。SOST 和 RANKL 都是主成骨转录因子 RUNX2 的靶点。通过四点弯曲使人类成骨细胞 Saos-2 细胞承受应变会下调其 SOST 和 RANKL 的表达，而不改变 RUNX2 的表达。RUNX2 敲低会增加基础 SOST 的表达，但不会改变应变后 SOST 的下调。相反，RUNX2 敲低不会改变基础 RANKL 的表达，但会阻止其应变后的下调。染色质免疫沉淀显示 RUNX2 占据 RANKL 启动子的一个区域，该区域包含一个共识 RUNX2 结合位点，其结合位点在应变后减少。通过 RT-qPCR 定量了表观遗传乙酰和甲基写入器和读取器的表达，以研究这种变化的潜在表观遗传基础。应变和 RUNX2 敲低均下调溴结构域乙酰读取器 BRD2 的表达。BRD2 和 RUNX2 共免疫沉淀，表明在调节复合物内相互作用，并且 BRD2 被证实与 RUNX2 启动子相互作用。BRD2 还占据 RANKL 启动子，其结合在暴露于应变后减少。因此，RUNX2 可能通过抑制基础 SOST 的表达来促进骨重塑，同时通过涉及 BRD2 的机械敏感表观遗传环促进急性应变诱导的 RANKL 下调。
