The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) at 10-9M stimulated the formation of osteoclast-like multinucleated cells (MNCs) in the presence of 1 alpha, 25-dihydroxyvitamin D3in rat bone marrow cultures. However, at 10-7M, it clearly inhibited 1 α, 25-dihydroxyvitamin D3-dependent osteoclast-like MNC formation at 6 days of culture. In cultures treated with 10-7M TPA, numerous MNCs that lack the marker enzyme tartrate-resistant acid phosphatase (TRAP) were formed. These TRAP-negative MNCs had neither receptors for calcitonin nor dentine-resorbing activity. The reactivity of the cells against antirat macrophage antibodies was completely different from that of authentic osteoclasts. These data suggest that TRAP-negative MNCs formed in the presence of 10-7M TPA are macrophage polykaryons. Time-course studies showed that 10-7M TPA stimulated osteoclast-like Mnc formation at 4 days of culture, but these osteoclast-like MNCs were converted to TRAP-negative MNCs. Furthermore, 1-(5-isoquinolinyl-sulfonyl)2-methylpiperazine (H-7), a protein kinase-C inhibitor, inhibited osteoclast-like Mnc formation in a dose-dependent fashion. These results suggest that activation of protein kinase-C may play a role in osteoclast differentiation.
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