Mechanism of stimulation of osteoclastic bone resorption through Gas6/Tyro3, a receptor tyrosine kinase signaling, in mouse osteoclasts

The signaling through receptor tyrosine kinases expressed on mature osteocl asts has recently been suggested to be involved in osteoclastic bone resorp tion. This study investigated the mechanism and the possible physiological relevance of Gas6/Tyro 3, a receptor tyrosine kinase signaling pathway in o steoclasts 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 osteocl ast function to form resorbed pits on a dentine slice. The expression of it s receptor, Tyro 3, was seen only in mature osteoclasts among bone cells. G as6 up-regulated the phosphorylation of cellular proteins including p42/p44 mitogen-activated protein kinase (MAPK), but not p38 or c-Jun N-terminal k inase MAPK, and increased the kinase activity:of immunoprecipitated Tyro 3 in isolated osteoclasts. The ability of Gas6 to stimulate pit formation res orbed 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 stron gly suggest that Gas6 acts directly on mature osteoclasts through activatio n of Tyro 3 and p42/p44 MAPK, possibly contributing to the bone loss by est rogen deficiency.