Abstract
1α, 25-Dihydroxyvitamin D_3 [1α,25 (OH)_2D_3], the most potent metabolite of vitamin D_3, is now considered to be a hormone that mediates calcium transport in target tissues such as intestine and bone. It is known that 1α, 25 (OH)_2D_3 enters the cell and associates with the specific nuclear receptor (VDR), which results in the de novo synthesis of several 1α, 25 (OH)_2D_3-dependent proteins. Recent studies have indicated that VDR is present not only in the classical target tissues of vitamin D (intestine, bone and kidney), but also in almost all tissues and cells examined. This raised a question whether 1α, 25 (OH)_2D_3 has a more subtle function in a wide variety of cells. A breakthrough in the study on 1α, 25 (OH)_2D_3 functions was made in 1981 by Abe et al., who clearly demonstrated that 1α, 25 (OH)_2D_3 induces differentiation of mouse myeloid leukemia cells (Ml) into cells of the monocyte-macrophage lineage. The differentiation-inducing activity of 1α, 25 (OH)_2D_3 has now been applied to the treatment of the skin disease, psoriasis vulgaris. In this article, the classical calcium-regulating activity of 1α, 25 (OH)_2D_3 is explained by the newly discovered differentiation-inducing activity of the vitamin, in particular focussing polyamine metabolism in intestine and osteoclast differentiation in bone.
