STABILIZATION OF THE VITAMIN-D RECEPTOR IN RAT OSTEOSARCOMA CELLS THROUGH THE ACTION OF 1,25-DIHYDROXYVITAMIN-D(3)

A regulatory mechanism for the vitamin D receptor (VDR) in rat osteosa rcoma cells (ROS 17/2.8) is stabilization of the receptor through bind ing of its ligand, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Increased transcription of the gene encoding VDR does not occur upon treatment o f these osteoblast-like cells with 1,25-(OH)2D3. When 1 0 nm 1,25-(OH) 2D3 was administered to confluent cultures of ROS 17/2.8 cells, no cha nge in receptor mRNA was detected, as measured by a ribonuclease prote ction assay. VDR abundance was measured using an immunoradiometric ass ay at varying time points within a 24-h period after 1,25-(OH)2D3 trea tment. Receptor protein levels increased rapidly and continued to rise over 24 h. By 2 h, the level of receptor increased 2.5-fold, achievin g a maximum level of 8-fold above the baseline at 18 h. The half-life of the receptor protein is 2 h in the absence of hormone, as determine d by blockage of translation in cycloheximide-treated cells. In the pr esence of hormone, however, receptor levels were unchanged for at leas t 6 h. The administration of 1,25-(OH)2D3 stabilizes the receptor, the reby resulting in its accumulation in ROS 17/2.8 cells.