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Although calcitonin has been clinically utilized as a primary treatment for several metabolic bone diseases, its inhibitory effects against osteoclastic function diminish after several days owing to the calcitonin 'escape phenomenon'. We have previously found a unique cell-surface antigen (Kat1-antigen) expressed on rat osteoclasts. Here we show evidence that, in the presence of calcitonin, the Kat1-antigen is involved in osteoclastogenesis. Treatment of bone marrow cultures for forming osteoclast-like cells with anti-Kat1-antigen monoclonal antibody (mAb Kat1) provoked a marked stimulation of osteoclast-like cell formation only in the presence of calcitonin but not in its absence. Osteoclastogenesis stimulated by the receptor activator of nuclear factor kappa B (NF-kappaB) ligand/osteoclast differentiation factor was further augmented by mAb Kat1 in the presence of calcitonin. Furthermore, even in the presence of the osteoprotegerin/osteoclast inhibitory factor, mAb Kat1 induced osteoclast-like cell formation. Our current data suggest that the Kat1-antigen is a molecule that is distinct from receptor activator of NF-kappaB. The presence of the unique Kat1-antigen on cells in the osteoclast lineage appears to contribute to the fine regulation of osteoclastogenesis in vivo. Expression of this cell-surface molecule in cells in the osteoclast lineage may partly explain the mechanism responsible for the escape phenomenon.

Macrophage inflammatory protein-1alpha (MIP-1alpha) is a member of the CC chemokines. We have previously reported the use of a whole bone marrow culture system to show that MIP-1alpha stimulates the formation of osteoclast-like multinucleated cells. Here we use rat bone marrow cells deprived of stromal cells, and clones obtained from murine macrophage-like cell line RAW264 to show that MIP-1alpha acts directly on cells in osteoclast lineage. We obtained several types of RAW264 cell clones, one of these clones, designated as RAW264 cell D clone (D clone), showed an extremely high response to receptor activator of NFkappaB ligand (RANKL) and tumor necrosis factor-alpha (TNF-alpha), while the other clone, RAW264 cell N clone (N clone), demonstrated no response to RANKL or TNF-alpha. Although both clones expressed receptor activator NFkappaB (RANK) before being stimulated for differentiation, only the D clone expressed cathepsin K when cells were stimulated to differentiate to osteoclasts. MIP-1alpha stimulated the formation of mononuclear preosteoclast-like cells from rat bone marrow cells deprived of stromal cells. MIP-1alpha also stimulated formation of osteoclast-like multinucleated cells from the D clone, when these cells were stimulated with RANKL and TNF-alpha. These findings provide strong evidence to show that MIP-1alpha acts directly on cells in the osteoclast lineage to stimulate osteoclastogenesis. Furthermore, pretreatment of RAW264 cell D clone with MIP-1alpha significantly induced adhesion properties of these cells to primary osteoblasts, suggesting a crucial role for MIP-1alpha in the regulation of the interaction between osteoclast precursors and osteoblasts in osteoclastogenesis.