Possible involvement of focal adhesion kinase,p125FAK,in osteoclastic bone resorption

Involvement of tyrosine phosphorylation in osteoclastic bone resorption was examined using osteoclast-like multinucleated cells prepared from co-cultures of mouse osteoblastic cells and bone marrow cells in the presence of 1α,25-dihydroxyvitamin D₃. When osteoclast-like cells were plated on culture dishes in the presence of 10% fetal bovine serum, they were sharply stained in their peripheral region by anti-phosphotyrosine antibody. Western blot analysis revealed that 115-to 130-kD proteins were tyrosine-phosphorylated in osteoclast-like cells. Using immunoprecipitation and immunoblotting, one of the proteins with 115–130 kD was identified as focal adhesion kinase (p125^FAK), a tyrosine kinase, which is localized in focal adhesions. Immunostaining with anti-p 125^FAK antibody revealed that p125^FAK was mainly localized at the periphery of osteoclast-like cells. Herbimycin A, a tyrosine kinase inhibitor, not only suppressed tyrosine phosphorylation of p125^FAK but also changed the intracellular localization of p125^FAK and disrupted a ringed structure of F-actin-containing podosomes in osteoclast-like cells. Antisense oligodeoxynucleotides to p125^FAK inhibited dentine resorption by osteoclast-like cells, whereas sense oligodeoxynucleotides did not. These results suggest that p125^FAK is involved in osteoclastic bone resorption and that tyrosine phosphorylation of p125^FAK is critical for regulating osteoclast function.