Ligand-dependent regulation of the 1,25-dihydroxyvitamin D3 receptor in rat osteosarcoma cells

The specific binding of radiolabeled 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) to intact rat osteosarcoma (ROS 17/2) cells was followed for 24 h. In the presence of 0.5-1.5 nM 1,25(OH)2D3, hormone binding increased over a period of 12 h, from 1.1 X 10(4) to 1.3 X 10(5) receptors/cell. The elevated level of hormone binding persisted through 24 h provided that the initial concentration of hormone was maintained. The concentration dependence of this increase in receptor level was centered between 10 and 30 pM 1,25(OH)2D3, and the binding at 12 h exhibited the metabolite specificity expected for a 1,25(OH)2D3 receptor. The t 1/2 values for the disappearance of unoccupied and occupied receptors were roughly the same, approximately 2.7 h; therefore, the increase in hormone binding was not due to receptor stabilization. In comparison, hormone-receptor complexes appeared to dissociate with a t 1/2 of 1 h. alpha-Amanitin treatment reduced the magnitude of receptor accumulation by 50-60%, indicating that mRNA synthesis was required to achieve the maximal response. Ligand-dependent regulation of cellular receptor levels provides a mechanism for amplifying the primary hormonal signal and is predicted to influence the kinetics, magnitude, and dose dependence of cellular responses.