[Cell differentiation and vitamin D]

After the discovery of the effect of vitamin D on cell differentiation by Abe, Suda and their colleagues, it has become clear that many of the vitamin D actions are mediated by their effect on terminal differentiation of various cells. Similarly important are the findings that the vitamin D receptors are almost ubiquitously distributed in various tissues, and the active metabolite of vitamin D exerts its effects in almost all types of cells. Thus, the actions of vitamin D are not confined to their classical target tissues that regulate calcium metabolism, but also are extended to the regulation of the proliferation and differentiation of various cells such as immunoregulatory cells, epidermal cells and malignant tumor cells. These discoveries have created a new era in the field of vitamin D endocrinology, and led to the development of vitamin D analogues that exert their effects mostly in the nonclassical target tissues with little effects on calcium metabolism. Efforts are currently under way to apply these analogues to the treatment of various disturbances such as immunological disorders, skin lesions and malignant tumors. This review briefly summarizes the current understandings on the effect of vitamin D on cell differentiation and proliferation, and discusses the future prospects of the application of these effects on the treatment of various disorders.