Abstract
Vitamin D is a principal factor required for the development and maintenance of bone as well as for maintaining normal calcium and phosphorus homeostasis. In addition, evidence has indicated the involvement of vitamin D in a number of diverse cellular processes, including effects on differentiation and cell proliferation, on hormone secretion, and on the immune system. Vitamin D, which is taken in the diet or is synthesized in the skin from 7-dehydrocholesterol in a reaction catalyzed by ultraviolet irradiation, is transported in the blood by the vitamin D-binding protein to the liver. In the liver, vitamin D is hydroxylated at C-25, resulting in the formation of 25-hydroxyvitamin D3 [25(OH)D3]. This chapter describes the vitamin D gene regulation. It begins with an explanation of the role of 1,25(OH)2D3 in classical target tissues, including bone, intestine, kidney, and parathyroid glands. Following this, it considers the nonclassical actions of 1,25(OH)2D. These include the effects of 1,25(OH)2D3 on differentiation and proliferation, on hormone secretion, and on the immune system. Subsequently, it provides an understanding of the transcriptional regulation by 1,25(OH)2D3, elucidating the factors involved in vitamin D mediated transcriptional regulation.