Vitamin D and bone.

Recent studies of the effects of vitamin D on bone include characterization of the receptors for 1,25-(OH)2D3 in bone and bone derived cells. The receptors show similar affinities in the different tissues. The receptor concentration is affected by the stage of the cell cycle. Glucocorticoid treatment affects the number of receptor sites. The most extensively studied effects of 1,25-(OH)2D3 on macromolecular synthesis in bone have been on collagen, where both anabolic and antianabolic effects are found. Differences in the response may reflect the state of differentiation of the cells. Significant effects are seen on osteocalcin synthesis, although the role of this protein in vitamin D action on bone is still unclear. 1,25-(OH)2D3 influences the activity of alkaline phosphatase and 25-OH-D3 24-hydroxylase in bone. Receptors for growth factors, and production of cytokines may be influenced by 1,25-(OH)2D3 treatment. Although some of these findings would be consistent with direct anabolic effects of vitamin D or its metabolites on bone, such a process has yet to be integrated into the complete picture of vitamin D action at physiological and pharmacological levels in vivo. Recent studies are consistent with earlier results that indicate that hypercalcemic effects of 1,25-(OH)2D3 are mediated by a direct effect on bone. Studies with analogs of 1,25-(OH)2D3 suggest that the stimulation of bone resorption can be dissociated from effects on differentiation of cells of the monocyte lineage.(ABSTRACT TRUNCATED AT 250 WORDS)