Study on signal transduction in osteoclastogenesis for the development of anti-osteoporosis drugs.

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Inhibition of signal transduction for osteoclastogenesis is thought to provide important information for development of anti-osteoporosis drugs. We have succeeded in molecular cloning of an essential factor for osteoclastogenesis, RANKL, which is expressed as a membrane associated factor in osteoblasts in response to many bone-resorbing factors. Using culture systems for osteoclast formation, we have studied signal transduction for osteoclastogenesis including RANK-RANKL signaling. The findings obtained from the study are as follows. (1) TNFα stimulated osteoclastogenesis in bone marrow-derived macrophage cultures in the presence of M-CSF.(2) SaOS-4/3, a subclone of the human osteosarcoma cell line SaOS-2, established by transfecting the human PTH/PTHrP receptor cDNA, supported osteoclast formation in response to PTH in co-culture with mouse bone marrow cells. Expression of mRNAs for RANKL and M-CSF by SaOS-4/3 cells was up-regulated in response to PTH.Both factors were expressed as membrane-associated factors. (3) Ionomycin and A23187 stimulated osteoclast formation in mouse co-cultures of bone marrow cells and osteoblasts. We also found that PMA, an activator of protein kinase C, stimulated osteoclast formation in the co-culture though up-regulation of RANKL expression in osteoblasts. (4) BMP-2 markedly enhanced osteoclast differentiation induced by RANKL and M-CSF.Addition of a soluble form of BMP receptor type-IA to the culture inhibited not only osteoclast formation induced by RANKL and BMP-2, but also the basal osteoclast formation supported by RANKL alone. Both bone marrow macrophages and mature osteoclasts expressed BMP-2 and BMP receptor type IA mRNAs. These findings provide important information for the development of anti-osteoporosis drugs.