Grundt Alexander, Grafe Ingo Alexander, Liegibel Ute, Sommer Ulrike, Nawroth Peter, Kasperk Christian
Department of Medicine I and Clinical Chemistry, Division of Osteology, University of Heidelberg, Heidelberg, Germany.
Biochem Biophys Res Commun. 2009 Nov 20;389(3):550-5. doi: 10.1016/j.bbrc.2009.09.026. Epub 2009 Sep 11.
Osteoprotegerin (OPG) affects bone metabolism by intercepting the RANK-RANKL interaction which prevents osteoclastic differentiation and consequently reduces bone resorption. Different bone phenotypes of mice overexpressing OPG and of mice with knockdown of receptor activator of NF-kappaB (RANK) or RANK-ligand (RANKL) suggest that the mechanism of action of the OPG-RANKL-RANK system in regulating bone remodeling is not completely understood. Furthermore, OPG increases bone mass and density independently from reduced osteoclastogenesis which is consistent with the possibility that OPG may directly affect bone metabolism beyond its known role as decoy receptor for RANKL.
We treated primary human osteoblastic cells with OPG and inhibitory anti-RANKL antibodies and measured cellular ALP activity, in vitro mineralization, vitronectin receptor protein expression and ERK phosphorylation. We also analyzed the mRNA co-expression of ALP and OPG ex vivo in bone biopsies from acute and old stable vertebral fractures.
OPG directly increased ALP activity and in vitro mineralization of HOC, enhanced expression of the vitronectin receptor thereby increasing adherence of HOC to vitronectin and stimulated ERK phosphorylation. All OPG-mediated effects could be prevented by RANKL antibodies or RANKL-siRNA transfection and MAPK inhibitor PD98059 reduced the stimulatory effect of OPG on integrin alphav expression. In acutely fractured vertebrae OPG and ALP mRNA expression was significantly increased compared to stable vertebral fractures. In conclusion, OPG exerts direct osteoanabolic effects on HOC metabolism via RANKL in addition to its well described role as decoy receptor for RANKL.
骨保护素（OPG）通过阻断RANK-RANKL相互作用来影响骨代谢，该相互作用可防止破骨细胞分化，从而减少骨吸收。过表达OPG的小鼠以及核因子κB受体激活剂（RANK）或RANK配体（RANKL）基因敲低的小鼠具有不同的骨表型，这表明OPG-RANKL-RANK系统在调节骨重塑中的作用机制尚未完全明确。此外，OPG增加骨量和骨密度，这一作用独立于破骨细胞生成减少，这与OPG可能直接影响骨代谢的可能性一致，而不仅仅是作为RANKL的诱饵受体发挥已知作用。
我们用OPG和抗RANKL抑制性抗体处理原代人成骨细胞，并测量细胞碱性磷酸酶（ALP）活性、体外矿化、玻连蛋白受体蛋白表达和细胞外信号调节激酶（ERK）磷酸化。我们还在急性和陈旧性稳定椎体骨折的骨活检标本中对ALP和OPG的mRNA共表达进行了离体分析。
OPG直接增加人成骨细胞（HOC）的ALP活性和体外矿化，增强玻连蛋白受体的表达，从而增加HOC与玻连蛋白的黏附，并刺激ERK磷酸化。所有OPG介导的效应均可被RANKL抗体或RANKL小干扰RNA（siRNA）转染所阻断，丝裂原活化蛋白激酶（MAPK）抑制剂PD98059可降低OPG对整合素αv表达的刺激作用。与稳定椎体骨折相比，急性骨折椎体中OPG和ALP mRNA表达显著增加。总之，除了作为RANKL的诱饵受体这一广为人知的作用外，OPG还通过RANKL对HOC代谢发挥直接的促骨合成作用。
