SPECIFIC DOWN-REGULATION OF PARATHYROID-HORMONE (PTH) RECEPTORS AND RESPONSES TO PTH BY TUMOR-NECROSIS-FACTOR-ALPHA AND RETINOIC ACID IN UMR 106-06 OSTEOBLAST-LIKE OSTEOSARCOMA CELLS

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) act via PTH receptors in bone to stimulate bone resorption. Bone resorption is also stimulated by certain cytokines, which are produced in bone and bone marrow. The effects of such cytokines on the PTH-receptor system were studied in the osteoblast-like osteosarcoma cell line UMR 106-06. I-125-labelled PTHrP-(1-84)-peptide bound specifically to the cells, and PTHrP-(1-34) and -(1-84) competed with equimolar affinity for binding to UMR 106-06 cells. The specific binding of I-125-PTHrP-(1-84) could be completely blocked by PTH. Therefore I-125-PTHrP-(1-84) bound to a classical PTH receptor in UMR 106-06 cells. Preincubation for 3 days with either tumour necrosis factor-alpha (TNF-alpha) or retinoic acid (RA) both decreased the specific binding of I-125-PTHrP-(1-84) to about 40% of control levels. These effects were specific for PTH binding, since there was little effect on I-125-salmon-calcitonin binding. Both TNF-alpha and RA required 24 h exposure to cells to produce a measurable effect. The decrease in I-125-PTHrP-(1-84) binding was due to a reduced number of binding sites, with little apparent change in affinity. Half-maximal effects were seen with 1 ng of TNF-alpha/ml, whereas 1-mu-M-RA was needed to observe the loss of PTH receptors. Combinations of RA and TNF-alpha produced a greater effect than that of either agonist alone. The loss of PTH receptors was accompanied by a specific loss of PTH-stimulated cyclic AMP production. Preincubation with TNF-alpha increased the basal plasminogen activator (PA) activity in the cells and decreased the amplitude of the response of PA activity to PTH compared with control cells. Furthermore TNF-alpha decreased sensitivity to PTH (50% stimulation of PA activity with 0.1 nM-PTH in control cells versus 50 % stimulation with 0. 3 nM-PTH in TNF-alpha-treated cells). In contrast, TNF-alpha pretreatment increased the amplitude of the response of PA activity to calcitonin, whereas sensitivity to calcitonin was not altered. These data are consistent with a specific down-regulation of PTH receptors in osteoblast-like UMR 106-06 cells after exposure to TNF-alpha or RA. The loss of PTH receptors is accompanied by a decreased responsiveness to PTH, as measured with the PA system in these cells. A loss of PTH receptors could modulate PTH responses in osteoblasts, either in the local control of bone formation and resorption, or in pathological conditions such as humoral hypercalcaemia of malignancy.