PHYSIOLOGICAL LEVELS OF CALCITONIN REGULATE THE MOUSE OSTEOCLAST CALCITONIN RECEPTOR BY A PROTEIN-KINASE A-MEDIATED MECHANISM

We have reported that calcitonin (CT) treatment induced downregulation of the CT receptor (CTR) in mouse osteoclast-like cells (OCLs). Here, we studied the features of homologous down-regulation of the CTR in m ature mouse OCLs. Treatment with salmon CT (sCT) and human CT (hCT) re duced [I-125]sCT specific binding. The decreased binding after 24 h of CT treatment was associated with a decrease in the cell surface recep tor concentration. The extent of CT-induced down-regulation in 24 h wa s dose dependent, and the ED(50) value was 3.6 +/- 4.1 (mean +/- SD; n = 3) x 10(-13) M for sCT and 4.9 +/- 3.3 x 10(-11) M for hCT. These v alues were very similar to those for the CT inhibition of the bone-res orbing activity of OCLs. The data suggest that these two distinct acti ons of CT may be mediated by a common intracellular pathway. Treatment of OCLs with activators of protein kinase A (PKA) mimicked the effect of CT on CTR down-regulation, whereas neither activation of protein k inase C nor elevation of intracellular Ca2+ did so. Attenuation of CT- induced CTR down-regulation by the competitive cAMP antagonist, RpcAMP , and high concentrations of H-7, but not by protein kinase C-specific inhibitors (sphingosine, staurosporine, and a lower concentration of H-7), suggested that the PKA pathway is primarily involved in homologo us regulation of the CTR. The changes in CTR messenger RNA confirm the findings in binding studies and demonstrate that CT treatment of OCLs results in decreased CTR synthesis through the PKA pathway. The low c oncentrations of hCT that result in CTR regulation are very close to t he physiological range, providing new insights into a dynamic relation ship between circulating levels of CT and CTR expression in osteoclast s.