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Calcitonin Receptor Regulation and Responsiveness to Calcitonin in Human Osteoclast-Like Cells Prepared in Vitro using Receptor Activator of Nuclear Factor-B Ligand and Macrophage Colony-Stimulating Factor¹

The Fourth Department of Internal Medicine, Saitama Medical School, Saitama 350-0495 Japan

Address all correspondence and requests for reprints to: Seiki Wada, M.D., Ph.D., The Fourth Department of Internal Medicine, Saitama Medical School, 38 Morohongo, Moroyama-cho, Iruma-gun, Saitama 350-0495 Japan. E-mail: wadas{at}saitama-med.ac.jp

Using mouse osteoclast-like cells (OCs), we have shown that short exposure to calcitonin (CT) resulted in prolonged reduction of CT binding by inhibiting de novo CT receptor (CTR) synthesis. Additionally, CT-treated OCs demonstrated resistance to CT rechallenge on the inhibitory effect of CT in osteoclastic bone resorption. There is, however, scant information on CT effects on human osteoclasts. In this study, we examined the features of CTR down-regulation and its recovery after short exposure to CT of human OCs. OCs were prepared by treatment of peripheral blood mononuclear cells in vitro with osteoclast differentiation factor and macrophage colony-stimulating factor. Treatment of OCs with salmon CT (sCT) and human CT (hCT) resulted in a dose-dependent reduction in [¹²⁵I]sCT binding capacity. Continued receptor occupancy by ligand was excluded by using a glycine-acid washing procedure. Treatment with sCT reduced CTR messenger RNA expression, suggesting that CTR down-regulation is, at least partly, attributable to an inhibition of de novo CTR synthesis. To investigate the intracellular signal transduction pathways that mediate these effects, we examined the effects of activation of the protein kinase (PK)A, PKC, and Ca²⁺-calmodulin-dependent kinases. Treatment with PKC activators mimicked CT, whereas neither activation of PKA nor elevation of intracellular Ca²⁺ did so. We further investigated the intracellular signaling pathways responsible for the inhibitory effects of CT on bone resorption, which showed that treatment with PKC activators reproduced the effects of CT. These data suggest that the PKC pathway plays an important role in homologous CTR down-regulation, as well as inhibition of bone-resorbing activity by CT, in human OCs. Short exposure of OCs to CT (10^-9 M, 1 h) reduced [¹²⁵l]sCT-specific binding for a prolonged period, as we have shown previously in mouse OCs. The reduced specific binding, CTR messenger RNA levels, and CT-sensitive adenylate cyclase responsiveness returned to the control levels by 96 h after removal of CT. These results strongly support the notion that escape from CT inhibition of osteoclastic bone resorption in humans is attributable to the development of resistance by OCs to CT. This study also showed that even short exposure to CT induced prolonged desensitization to CT rechallenge, although the OCs eventually regained responsiveness to sCT rechallenge.

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