DOWN-REGULATION OF CALCITONIN RECEPTOR MESSENGER-RNA EXPRESSION BY CALCITONIN DURING HUMAN OSTEOCLAST-LIKE CELL-DIFFERENTIATION

Calcitonin inhibits both osteoclast formation and bone resorption, and is a primary treatment for patients with hypercalcemia and increased bone turnover. However, the clinical utility of calcitonin is limited because patients become refractory to calcitonin after several days (t he calcitonin ''escape phenomenon''). The molecular basis for calciton in ''escape'' is unclear. To determine the regulatory mechanisms contr olling calcitonin receptor (CTR) expression in osteoclasts and their p recursors, we treated immature mononuclear precursors for human osteoc last-like multinucleated cells (MNC) formed in vitro with 1,25-(OH)(2) D-3, to induce their differentiation to committed mononuclear precurso rs, and mature multinucleated osteoclasts, and used reverse transcript ase (RT)-PCR to assess expression of CTR mRNA in both committed mononu clear precursors and MNC. The PCR fragment produced was cloned and seq uenced to confirm that it was derived from CTR mRNA. CTR mRNA expressi on was detected in mononuclear MNC precursors after 7 d of 1,25-(OH)(2 )D-3 treatment. It was also present in osteoclast-like MNC and highly purified giant cells from osteoclastomas, but not in monocytes or macr ophage polykaryons formed in vitro. Calcitonin markedly decreased CTR but not actin mRNA expression in giant cells and MNC after 12 h, and r emoval of calcitonin restored CTR mRNA expression. Similarly, calciton in decreased calcitonin-induced adenylate cyclase activity. These data suggest: (a) downregulation of CTR gene expression by calcitonin may in part explain the calcitonin ''escape phenomenon'' and (b) expressio n of CTR mRNA occurs in mononuclear osteoclast precursors within 7 d a fter exposure to 1,25-(OH)(2)D-3.