1 alpha,25(OH)2 vitamin D3: a steroid hormone capable of producing pleiotropic receptor-mediated biological responses by both genomic and nongenomic mechanisms.

Vitamin D3 is now considered a pro-hormone and its biological activities are in a great majority due to its hormonally active derivative 1 alpha,25-dihydroxyvitamin D3. The actions of vitamin D3 have been extensively studied both in vivo and in vitro using different animal models as well as a diversity of cell types including intestinal epithelial cells, kidney cells, lymphocytes, macrophages and a number of malignant cell lines. Results thus obtained have considerably extended the number of target tissues for this vitamin and hormone far beyond its classical target organs, parathyroid gland, kidney, intestine and bone. Studies performed at the cellular, subcellular and molecular levels have led to the establishment of new concepts on the mode of action of vitamin D3 which is not believed to act through receptor-mediated genomic and nongenomic events leading to the expression of the full biological response.