1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) is the accepted hormonal form of vitamin D which facilitates calcium and phosphorus homeostasis by acting on intestinal mineral absorption and bone remodeling. Recent evidence also implicates 1,25(OH)2D3 in normal hematopoiesis and cell differentiation. The proposed study will focus on elucidating the biochemical mechanism of 1,25(OH)2D3 action in a variety of target cells. Using monoclonal antibody we plan to further characterize the molecular properties of the 1,25(OH)2D3 receptor protein and prove that it is an obligatory mediator of the hormone's action. We will also study receptor modulation during target cell differentiation and analyze its binding to putative promotor regions of vitamin D regulated genes. The functions of 1,25(OH)2D3 in bone will be examined at the level of differentiating chondrocytes, osteoblasts, and osteoclast precursor cells. Another biological action of 1,25(OH)2D3, the induction of its catabolism via C-24 oxidation, will be investigated in cultured kidney cells. Biochemical details of the hormone's functions which may involve intracellular calcium and protein kinase C as well as nuclear protein modification and altered gene expression will be delineated. Finally, we will probe the regulation of cell growth and differentiation in normal and tumor cells by 1,25(OH)2D3. These basic studies will be complemented with measurements of 1,25(OH)2D3 receptor levels in patients with crush fracture osteoporosis, idiopathic hypercalciuria, osteogenic sarcoma and myelogenous leukemia. Our findings should not only reveal some of the early molecular events in the action of vitamin D, but they may add to our understanding of the pathophysiology and treatment of metabolic and malignant bone diseases.