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Department of Oral Cell Biology, Umeå University, S-901 87 Umeå, Sweden

(Correspondence should be addressed to U H Lerner; Email: ulf.lerner{at}odont.umu.se)

The effects of calcitonin (CT) on osteoclast formation and gene expression have been studied in cultured mouse spleen cells and mouse bone marrow macrophages (BMMs). CT inhibited the formation of multinucleated osteoclasts and resorption pits in spleen cell cultures and BMM as well as in CD115⁺ CD3^– CD45R^–sorted BMM cultures, incubated in the presence of macrophage colony-stimulating factor and receptor activator of NF-B ligand (RANKL). No effect on apoptosis by CT was observed. CT did not affect the mRNA expressions of RANK and c-Fms, or the mRNA expressions of a wide variety of transcription factors and genes important for osteoclast differentiation and activity. CT induced inhibition of tartrate-resistant acid phosphatase (TRAP), positive multinucleated osteoclast formation was not associated with any decrease of total TRAP activity, resulting in a large number of TRAP⁺ mononucleated cells in CT-treated cultures. CT did not affect the mRNA expression of dendritic cell-specific transmembrane protein, d2 isoform of vacuolar (H⁺) ATPase v_o domain, a disintegrin and metalloproteinase domain 8 (ADAM8), ADAM12, DNAX-activating protein or Fc receptor common chain suggested to be involved in fusion of mononucleated osteoclast progenitor cells. The inhibitory effect by CT was mimicked not only by compounds activating cAMP and protein kinase A (PKA) but also by a cAMP analogue activating the exchange protein directly activated by cAMP (Epac) pathway. It is concluded that CT, through cAMP/PKA/Epac cascades, inhibits osteoclast formation and that this effect is not associated with decreased transcription of genes known to be important for osteoclast progenitor cell differentiation, fusion or function.