TYRO-3 RECEPTOR TYROSINE KINASE AND ITS LIGAND, GAS6, STIMULATE THE FUNCTION OF OSTEOCLASTS

Bone is continuously being formed and resorbed, This process is accomp lished by the precise coordination of two cell types: osteoblasts and osteoclasts, Osteoclasts are large, multinucleated cells that are deri ved from the same hematopoietic precursors as macrophages, However, th ese bone-resorbing cells are difficult to study directly because of th eir relative inaccessibility. The purification of primary osteoclasts from rabbit bones by their adherent nature provides an opportunity for investigating the molecules in osteoclasts, We have examined the expr ession of receptor tyrosine kinase by polymerase chain reaction (PCR) and found that Tyro 3 was frequently identified from primary osteoclas ts in PCR cloning. Immunohistochemistry revealed that Tyro 3 was expre ssed on the multinucleated osteoclasts which were positive for tartrat e-resistant acid phosphatase (TRAP), but not on mononuclear TRAP-posit ive cells, The Tyro 3 Ligand, Gas6, induced the phosphorylation of Tyr o 3 receptors in osteoclasts in two to five min. Gas6 and protein S di rectly enhanced the bone resorbing activity of mature osteoclasts, Thi s effect of Gas6 was inhibited by the addition of a tyrosine kinase in hibitor, herbimycin A, However, Gas6 did not affect the differentiatio n of osteoclasts from bone marrow cells. Gas6 and protein S are depend ent on vitamin K, a cofactor for the enzyme responsible for carboxylat ion of glutamic acid residues. The findings in this study are the firs t to indicate a new biological activity of Gas6 and protein S as a dir ect regulator of osteoclastic function; they give an insight into the role of these vitamin K-dependent ligands in bone resorption in vivo.