Homologous regulation of the calcitonin receptor in mouse osteoclast-like cells and human breast cancer T47D cells.

Calcitonin (CT) down-regulates its receptor in several cancer cell lines, including T47D human breast cancer cells. Removal of CT results in the recovery of CT receptor (CTR) binding. However, homologous regulation of the CTR in osteoclasts is not well understood. To elucidate these phenomena in cells of the osteoclast lineage, mouse osteoblasts and bone marrow cells were cocultured on type 1 collagen gels. For the experiments, osteoclast-like cell (OCL)-enriched populations were subcultured from the collagen gels into multiwell dishes on days 7-8, and CT regulation of the CTR was determined and compared with that in T47D cells. When cells of either type were treated with CT for 1 h and then washed, binding capacity for [125I] salmon CT ([125I]sCT) was decreased dependent upon the preincubating concentration of CT. After removal of CT, the binding capacity in OCLs recovered toward the control level over 12 h. However, in contrast to that in T47D cells, recovery was transient, so that 24 h after removal of CT, the binding capacity in preincubated cells was strikingly reduced compared with that in control cells. This occurred even when the preincubating concentration of CT was too low to cause down-regulation of binding in 1 h. Scatchard analysis showed a decrease in receptor number in CT-treated compared with control OCLs 24 h after CT removal, with unchanged receptor affinity. By autoradiography, decreased CTR density on multinuclear OCLs was indicated. Preexposure of either OCLs or T47D cells to CT caused elevation of intracellular cAMP, which persisted for 6-12 h after removal of CT. In addition, there was desensitization to a rechallenge with CT, which, in T47D cells, recovered by 24-36 h. In contrast, OCLs showed incomplete recovery of desensitization. These data correlated with the results of semiquantitative reverse transcription-polymerase chain reaction studies; the CTR messenger RNA level was increased to about 150% of the control level in sCT-treated T47D cells 18-36 h after sCT removal; the level was markedly decreased to about 20% of the control value in sCT-treated OCLs 12-48 h after sCT removal and remained suppressed. This study suggests that CT-induced homologous down-regulation is a potential cause of the "escape" phenomenon, by producing a population of CT-resistant osteoclasts.