WORTMANNIN, A SPECIFIC INHIBITOR OF PHOSPHATIDYLINOSITOL-3 KINASE, BLOCKS OSTEOCLASTIC BONE-RESORPTION

The biological role of phosphatidylinositol (PI)-3 kinase was examined in osteoclast-like multinucleated cells (OCLs) formed in co-cultures of mouse osteoblastic cells and bone marrow cells. The expression of P I-3 kinase in OCLs was confirmed by Western blot analysis. Wortmannin (WT), a specific inhibitor of PI-3 kinase, inhibited PI-3 kinase activ ity in OCLs both in vitro and in vivo. WT also inhibited pit-forming a ctivity on dentine slices and disrupted a ringed structure of F-actin- containing dots (an actin ring) in OCLs in a dose-dependent manner. Th e inhibitory profiles of WT for pit and actin ring formation were simi lar to that for PI-3 kinase activity in OCLs. Electron microscopic ana lysis revealed that OCLs treated with WT did not form ruffled borders. Instead, numerous electron lucent vacuoles of differing sizes were fo und throughout the cytoplasm. These results suggest that PI-3 kinase i s important in osteoclastic bone resorption.