Mechanism of stimulation of osteoclastic bone resorption through Gas6/Tyro 3, a receptor tyrosine kinase signaling, in mouse osteoclasts

The signaling through receptor tyrosine kinases expressed on mature osteoclasts has recently been suggested to be involved in osteoclastic bone resorption. This study investigated the mechanism and the possible physiological relevance of Gas6/Tyro 3, a receptor tyrosine kinase signaling pathway in osteoclasts in stimulating osteoclastic bone resorption using several mouse culture systems. Gas6, expressed ubiquitously in bone cells, did not affect the differentiation or the survival of osteoclasts, but stimulated osteoclast function to form resorbed pits on a dentine slice. The expression of its receptor, Tyro 3, was seen only in mature osteoclasts among bone cells. Gas6 up-regulated the phosphorylation of cellular proteins including p42/p44 mitogen-activated protein kinase (MAPK), but not p38 or c-Jun N-terminal kinase MAPK, and increased the kinase activity of immunoprecipitated Tyro 3 in isolated osteoclasts. The ability of Gas6 to stimulate pit formation resorbed by osteoclasts was abrogated by PD98059, a specific inhibitor of p42/p44 MAPK. In addition, the Gas6 mRNA level in bone marrow was up-regulated by ovariectomy and was reduced by estrogen replacement. These results strongly suggest that Gas6 acts directly on mature osteoclasts through activation of Tyro 3 and p42/p44 MAPK, possibly contributing to the bone loss by estrogen deficiency.