IMMUNOFLUORESCENT EVIDENCE FOR THE ABUNDANCE OF FOCAL ADHESION KINASE IN THE HUMAN AND AVIAN OSTEOCLASTS AND ITS DOWN-REGULATION BY CALCITONIN

We show here that osteoclasts possess an abundant level of focal adhesion kinase, a novel cytosolic tyrosine kinase with unique structural features that may play an important role in the action of pp(60c-src), cell surface integrins, and hormonal peptides. The presence of focal adhesion kinase in the bone cell osteoclast was determined using monoclonal antibodies to the kinase by employing immunofluorescent staining. The expression of focal adhesion kinase in the osteoclast was markedly suppressed following exposure to calcitonin. However, calcitonin-induced down regulation of the kinase was apparent only following a prolonged exposure. Our hypothesis that focal adhesion kinase is maximally expressed in the osteoclasts was confirmed when the transfection of avian osteoclasts and fibroblasts, with v-src containing plasmid pATV-8, induced increased expression of the kinase in the fibroblasts but did not alter the expression level of FAK in the osteoclasts.