Study on mechanism of the coupling between bone resorption and bone formation

研究目的：RANKL的发现阐明了破骨细胞分化的机制以及成骨细胞对其功能的调控。骨保护素(OPG)是RANKL的一种可溶性诱骗受体，对破骨细胞的分化和功能均有抑制作用。OPG缺陷(OPG-/-)小鼠表现出严重的骨质疏松，其原因是破骨细胞性骨吸收增强。人类OPG缺乏被证明是导致青少年Paget病的原因。以往的形态学研究表明，OPG-/-小鼠的成骨细胞骨形成被激活。这些结果表明，破骨细胞性骨吸收巧合地通过一种未知的因素(称为偶联因素)诱导成骨细胞骨形成。在目前的研究中，我们利用脂多糖处理的成熟破骨细胞，检测了成熟破骨细胞产生影响骨代谢的细胞因子的可能性。利用OPG-/-小鼠，我们还探讨了这种偶联因子是否存在于OPG-/-小鼠的骨骼中。结果：(1)p38MAP激酶为Essentia…更多地参与了骨髓巨噬细胞(破骨细胞前体)向破骨细胞的分化。在成熟的破骨细胞中，p38MAP激酶信号通路全部死亡。(2)成熟破骨细胞产生成骨细胞活化因子，但不产生IL-1、IL-6等骨吸收细胞因子。(3)OPG-/-小鼠血中骨钙素和碱性磷酸酶水平显著升高。骨组织形态计量学研究表明，OPG-/-小鼠的骨吸收和骨形成均被激活。(4)OPG-/-小鼠血清可溶性RANKL水平也明显升高。(5)给OPG-/-和野生型小鼠注射1，25(OH)_2D_3后，OPG-/-小鼠血清RANKL浓度显著升高，而野生型小鼠血清RANKL浓度无明显变化。(6)OPO-/-成骨细胞在培养上清液中释放大量RANKL，而加入OPG(7)可完全抑制这一作用。(8)利塞膦酸钠可降低OPG-/-小鼠血清骨钙素和碱性磷酸酶水平，但不降低血清RANKL水平。(9)骨形态发生蛋白诱导的异位骨形成在OPG-/-小鼠体内没有加速。结论：(1)成熟破骨细胞产生偶联因子，而不产生骨吸收因子；(2)骨吸收与骨形成在骨吸收发生部位紧密耦合。较少

Aim of the study :The discovery of RANKL elucidates the mechanism of osteoclast differentiation and function regulated by osteoblasts. Osteoprotegerin (OPG), a soluble decoy receptor of RANKL, inhibits both differentiation and function of osteoclasts. OPG-deficient (OPG-/-) mice exhibited severe osteoporosis caused by enhanced osteoclastic bone resorption. Deficiency of OPG in human has been shown to result in juvenile Paget's disease. The previous morphological study showed that osteoblastic bone formation was activated in OPG-/-mice. These results suggest that osteoclastic bone resorption coincidentally induces osteoblastic bone formation by an unknown factor (called coupling factor). In the present study, we examined a possibility that mature osteoclasts produce cytokines which influence bone metabolism using mature osteoclasts treated with LPS. Using OPG -/-mice, we also explored whether such a coupling factor is present in bone in OPG -/-mice.Results :(1)p38MAP kinase was essentia … More lly involved in the differentiation of bone marrow macrophages (osteoclast precursors) into osteoclasts. The p38MAP kinase-signaling pathway was all dead in mature osteoclasts. (2)Mature osteoclasts were shown to produce an osteoblast-activating factor but not bone-resorbing cytokines such as IL-1 and IL-6 in response to LPS. (3)Blood levels of osteocalcin and ALP of OPG-/-mice were markedly increased in OPG-/-mice. Bone histomorphometric studies showed that both bone resorption and bone formation were activated in OPG-/-mice. (4)Serum levels of soluble RANKL were also elevated in OPG-/-mice. (5)When 1,25(OH)_2D_3 was administered into OPG -/-and wild-type mice, serum RANKL concentrations were markedly increased in OPG-/-mice but not in wild-type mice. (6)OPO -/-osteoblasts released a large amount of RANKL in the culture medium, which was completely inhibited by adding OPG (7)When bisphosphonate (risedronate) was injected into OPG -/-mice, bone formation -related parameters as well as bone resorption-related parameters were sharply decreased in the treated mice. (8)Treatment of OPG -/-mice with risedronate decreased serum osteocalcin and ALP levels but not the serum RANKL level. (9)Ectopic bone formation induced by BMP is not accelerated in OPG -/-mice.Conclusion :These results suggest (1)that mature osteoclast produce a coupling factor but not bone resorbing factors, and (2)that bone resorption is tightly coupled with bone formation at the sites where borne resorption takes place. Less