CALCITONIN-INDUCED CHANGES IN THE CYTOSKELETON ARE MEDIATED BY A SIGNAL PATHWAY ASSOCIATED WITH PROTEIN-KINASE-A IN OSTEOCLASTS

Calcitonin is known to inhibit osteoclastic bone resorption through it s receptor, which is abundantly expressed on the plasma membrane of os teoclasts. Recently, it was reported that calcitonin receptors were co upled to both cAMP-dependent protein kinase (PKA) and protein kinase C (PKC). To examine how the PKA and PKC pathways are involved in the ef fects of calcitonin, we focused on changes in the cytoskeleton of muri ne osteoclast-like multinucleated cells (OCLs) formed in vitro. When O CLs were cultured on dentine slices, they formed resorption pits and r inged structures of F-actin dots (actin rings). Elcatonin, a synthetic analogue of eel calcitonin, disrupted actin rings and inhibited pit f ormation in a dose-dependent manner. Forskolin and dibutyryl cAMP, bot h of which have the ability to activate PKA, mimicked the effects of e lcatonin. Phorbol myristate acetate and phorbol 12,13-dibutyrate, both of which have the ability to activate PKC, also inhibited pit-forming activity, but little affected actin rings of OCLs. The inhibitory eff ects of elcatonin on the pit formation and actin ring formation were p artially restored by the treatment with Rp-cAMPs, a cAMP antagonist. E lcatonin induced a rapid increase in PKA activity within a few minutes , and its activation by elcatonin occurred in a dose-dependent manner. The time- and dose-dependent profiles of elcatonin for the activation of PKA were similar to those for the disruption of actin rings. Moreo ver, microinjection of activated PKA into OCLs disrupted actin rings w ithin 10 min on culture dishes. Actin rings were little affected by th e microinjection of the PKA preincubated with a cAMP-dependent protein kinase inhibitor (IP-20) into OCLs. These results suggest that PRA ac tivation, rather than PKC activation, is involved in mediating the eff ects of calcitonin, through the disruption of actin organization.