BIOCHEMICAL AND MORPHOLOGICAL CHARACTERIZATION OF PARATHYROID-HORMONE RECEPTOR-BINDING TO THE RAT OSTEOSARCOMA CELL-LINE UMR-106

We have used both biochemical and morphological techniques to characterize PTH receptors on the clonal osteosarcoma cell line UMR-106, a widely used model of the osteoblast phenotype.125I-Labeled rat (r) PTH-(l-34) bound to a single class of specific saturable receptors on both whole cells and membranes prepared from UMR-106 cells in a time-and temperature-dependent manner. A decrease in PTH receptor affinity seen in the presence of guanine nucleotides demonstrated that PTH receptors on the UMR-106 cells are coupled to guanyl nucleotide-binding proteins. Although PTH is a potent stimulator of adenylate cyclase in the UMR-106 cells, comparison of PTH-stimulated adenylate cyclase and PTH binding curves indicated the presence of receptors that are not linked to the adenylate cyclase system. Our studies also demonstrated that125I-labeled rPTH-(l-34) bound to UMR-106 cells is rapidly internalized at 22 C, whereas PTH bound at 4 C remains intact and on the cell surface. Internalization of125I-labeled rPTH-(l-34) was associated with degradation and release of the hormone at 22 C. Three morphologically distinct cell types were identified in subconfluent cultures of UMR-106 cells. Autoradiographic analysis of125I-labeled rPTH-(l-34) binding demonstrated differential PTH receptor expression in these cell types. The most abundant PTH binding was observed over a cell type with long cytoplasmic extensions. This cell was reminiscent of the predominant PTH target cell previously identified in the rat metaphysis in vivo, suggesting that the UMR-106 cell line may represent neoplastic transformation of the PTH target cell. © 1990 by The Endocrine Society.