Regulation of osteoprotegerin localization and expression in bone remodeling

骨保护素（OPG）是肿瘤坏死因子受体家族的可溶性成员，通过与核因子-kB配体受体激活因子（RANKL）结合作为诱饵，从而通过中断RANKL介导的信号传导抑制破骨细胞的发生。为了阐明OPG在骨重塑中的作用，我们利用一种特异性肽抗体研究了OPG在大鼠胫骨中的定位，并通过原位杂交研究了OPG在大鼠胫骨中的表达。Western blotting结果显示，该抗体与大鼠胫骨裂解液的90 kD条带发生反应。免疫组化观察显示，OPG反应主要见于破骨细胞作用下的骨表面。在骨基质中，OPG也定位于软骨/骨界面和水泥线。然而，在破骨细胞和基质细胞之间的接触区域几乎没有检测到标记。部分成骨细胞和骨细胞标记弱。这些发现表明，来自成骨细胞系细胞和/或血清的OPG可能集中在被吸收的骨表面，随后集中在骨水泥线上。OPG可能通过抑制骨重塑过程中破骨细胞的分化和活性，在防止骨过度吸收方面发挥重要作用。由于OPG对肝素具有高亲和力，我们还比较了透明质酸、硫酸肝素、硫酸角蛋白和硫酸软骨素等蛋白聚糖的定位。然而，它们的模式与OPG不同。OPG在骨基质中的定位机制有待进一步研究。

Osteoprotegerin (OPG), a soluble member of the tumor necrosis factor receptor family, works as a decoy by binding to receptor activator of nuclear factor (NF)-kB ligand (RANKL) and thus inhibits osteoclastogenesis by interrupting RANKL-mediated signaling. We investigated the localization of OPG in rat tibia using a specific peptide antibody and its expression by in situ hybridization in order to clarify the role of OPG in bone remodeling. Western blotting showed that the antibody reacted with a 90 kD band in lysate from rat tibiae. Immunohistochemical observation revealed that OPG reactivity was mainly seen on bone surfaces under osteoclasts. In bone matrices, OPG was also localized on cartilage/bone interfaces and cement lines. However, labeling was hardly detected in the region of contact between osteoclasts and stromal cells. Some osteoblasts and osteocytes showed weak labeling. These findings suggest that OPG derived from osteoblast-lineage cells and/or serum may be concentrated on resorbed bone surfaces, and subsequently on cement lines. OPG may play an important role in the prevention of excess bone resorption by inhibiting differentiation and activity of osteoclasts in bone remodeling. Since OPG has high affinity for heparin, we also compared the localization of proteoglycans, such as hyaluronate, heparan sulfate, keratan sulfate and chondroitin sulfate proteoglycans. However, their patterns are different from OPG. The mechanism of OPG localization in bone matrix should be addressed in future research.