Relationship between internalization and calcitonin-induced receptor loss in T 47D cells

Exposure of T 47D human breast cancer cells to salmon calcitonin (sCT) resulted in a reduction of binding capacity for [125I]iodo-sCT in washed cells. The reduction was both time and concentration dependent. Recovery of binding capacity in CT-pretreated T 47D cells occurred in the absence of CT, but was prevented by inhibitors of protein synthesis. Studies were carried out to determine the mechanism of CT-induced reduction of binding capacity. When T 47D cells were treated with sCT at 37 C or 4 C and washed with buffer at neutral pH, subsequently measured binding capacity was lost as a function of time of pretreatment. Cells pretreated under the same conditions were washed with isotonic buffer at pH 2.5 to release cell-surface bound sCT and to allow assessment of cell surface receptor concentration. It was found that at 37 C sCT induced a time-dependent loss of cell surface receptors, so that initially the lost binding capacity was largely reclaimable by acid treatment, whereas after longer exposure to sCT, acid treatment was much less effective in regenerating binding capacity. The CT-induced reduction in binding capacity was not observed when cells were pretreated with sCT at 4 C or in the presence of inhibitors of cellular metabolic energy. These results are consistent with the view that initially CT-induced loss of CT receptors in T 47D cells is primarily due to occupancy of cell-surface receptors and later to a reduction in the concentration of cell-surface receptors mediated by an energy requiring internalization process involving the CT-receptor complex; reappearance of receptors requires new protein synthesis.