Functions of osteocytes in bone metabolism

为了研究骨细胞在骨吸收中的作用，用小鼠未分离的骨细胞进行凹坑形成试验，对纯化的鸡头盖骨细胞匀浆和条件培养液进行了检测。骨细胞匀浆明显抑制凹坑的形成，而细胞的条件培养液对凹坑的形成无影响。抑制活性不是由于匀浆的细胞毒性所致。在这项研究中，从胶原酶消化的富含骨细胞的鸡头骨碎片中纯化出一种骨吸收抑制蛋白。该抑制蛋白大小为18.5 kDa，对小鼠、兔未分离的骨细胞和人巨细胞瘤细胞的凹坑形成有明显的抑制作用，且呈剂量依赖性。在没有其他效应细胞的情况下，该蛋白还在凹坑形成试验中抑制纯化的破骨细胞的骨吸收活性。将该蛋白质显微注射到破骨细胞中会导致细胞中足体的破坏。该蛋白的N-末端25个氨基酸序列与部分Rho-GDI有68%的同源性。骨细胞可能参与了破骨细胞对骨吸收的调控机制。2.维生素D_3调节钙受体(CaSR)mRNA的含量维生素D_3降低枕骨CaSR mRNA的含量，而激素增加长骨和肾脏CaSR mRNA的含量。这些结果表明维生素D_3对CASR具有组织特异性调节作用。

I.In order to investigate roles of osteocytes in bone resorption, the homogenate and conditioned medium of purified chick calvarial osteocytes were examined in a pit formation assay using unfractionated bone cells from mice. The osteocyte homogenate markedly inhibited the pit formation, whereas the conditioned medium of the cells had no effect on the pit formatin. The inhibitory activity was not a result of cytotoxicity of the homogenate. In this study, a bone-resorption inhibitory protein was purified from collagenase-digested chick calvarial fragments enriched in osteocytes. The inhibitory protein, 18.5kDa, showed a significant dose-dependent inhibition of the pit formation by unfractionated bone cells from mice and rabbits, and by human giant cell tumor cells. This protein also inhibited the bone-resorbing activity of purified osteoclasts in the pit formation assay in the absence of other effector cells. Microinjection of the protein into osteoclasts caused disruption of the podosomes in the cells. The N-terminal 25-amino acid sequence of the protein was 68% homologous to a part of Rho-GDI.Osteocytes may be involved in the regulatory mechanism of bone resorption by osteoclasts.II.Vitamin D_3 regulates the amount of calcium receptor (CaSR) mRNA Vitamin D_3 decreases the amount of CaSR mRNA in occipital bone, but the hormone increases that of CaSR mRNA in long bone and kidney. These results indicate the presence of tissue specific regulation in CaSR by Vitamin D_3.