Abstract
Since several aspects of the effects of vitamin A and 1 alpha,25-dihydroxyvitamin D3 (1,25-(OH)2D3) on bone metabolism are quite similar, we examined the possibility that vitamin A effects on bone were mediated through the regulation of cytosolic 1,25-(OH)2D3 receptors. A clonal osteoblast-like cell line derived from rat osteosarcoma (ROS 17/2) was used as a model system. Vitamin A acid (retinoic acid) in concentrations ranging from 10(-8) to 10(-5) M was found to elicit a dose-dependent increase in 1,25-(OH)2D3 binding in these cells. This effect was maximal after 24 h, was independent of cell density, and was inhibited by actinomycin D (0.05-0.5 microgram/ml). The 1,25-(OH)2D3 binding macromolecule in cytosol preparations from both vehicle- and retinoic acid-treated cells had a sedimentation coefficient of 3.2 S and binding specificities for vitamin D3 metabolites in the order: 1,25-(OH)2D3 greater than 25-(OH)-D3 greater than 24,25-(OH)2D3. Sucrose density gradient analysis, vitamin D3 metabolite displacement studies, and saturation and Scatchard analyses all indicated that the specific increase in 1,25-(OH)2[3H]D3 binding in these cells was the result of a selective increase in the number of specific 1,25-(OH)2D3 receptors.
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