Bone Resorption Is Inhibited by an Osteocyte-Derived Protein
In the process of bone resorption, osteoclasts encounter a large number of osteocytes excavated from the bone tissue. For instance, osteocytes released from the collagen matrix have been shown to be engulfed into osteoclasts by phagocytosis. After the osteoclast-osteocyte encounter or incorporation of osteocytes into osteoclasts, it is possible that osteocytes transmit some signals to osteoclasts.
To investigate roles of osteocytes in bone resorption, we examined the homogenate and conditioned medium from purified chick calvarial osteocytes in a pit-formation assay. The osteocyte homogenate markedly inhibited pit formation by unfractionated bone cells, whereas the conditioned medium of the cells had no effect. A novel bone-resorption-inhibitory protein was purified from collagenase-digested chick calvarial fragments enriched in osteocytes. The inhibitory protein, of molecular mass 18.5 kDa, showed significant inhibition of pit formation by purified osteoclasts as well as by unfractionated bone cells. Microinjection of the protein into osteoclasts caused disruption of podosomes in the cells. Thus osteocytes may be involved in the regulation of bone resorption by osteoclasts.
Key wordsOsteocyte Osteoclast Bone resorption Pit formation
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