The novel roles of vitamin D were discovered and valued in this century. In addition to the maintenance of calcium and phosphorus balance, vitamin D regulates the function of the kidneys, heart, and immune system. Moreover, its anti-inflammatory, antiapoptotic, and antifibrotic roles have gained considerable attention. Vitamin D is also important for the maintenance of homeostasis by regulation of hormone secretion, cell proliferation, and differentiation. This paper will review these pleiotropic functions of vitamin D. 1. Introduction Since the beginning of the 20th century, scientists have been exploring the functions of vitamin D. The roles of this vitamin in endocrine system and metabolic bone diseases were already well studied by 1970. In this century, the discovery of vitamin D receptor has provided more insight on its additional functions [1]. Vitamin D receptors are present on many organs, such as the pancreas, large and small intestines, muscles, and nervous system [2]. Vitamin D was found to regulate the cell cycle and subsequently influence organ functions by binding to its receptor on the cells of the immune, nervous, and cardiovascular systems [3]. In the kidneys, vitamin D exerts protective effects by inhibiting renal fibrosis, inflammation, and progression of proteinuria. Vitamin D deficiency is strongly associated with various cardiovascular and metabolic diseases such as hypertension, type 1 diabetes, myocardial infarction, and stroke. Moreover, vitamin D deficiency is related to several autoimmune diseases such as rheumatoid arthritis, systemic sclerosis, and systemic lupus erythematosus. Studies also have shown a negative correlation between serum vitamin D concentration and incidence of colorectal cancer and breast cancer [4]. These phenomena suggest that vitamin D plays protective roles in many diseases. As the importance of vitamin D for endocrine function has gained attention, the pursuit of paracrine and autocrine functions of vitamin D will continue in this century [5]. 2. Metabolism of Vitamin D Vitamin D is a fat-soluble vitamin produced by exposure of the skin to sufficient ultraviolet B radiation and absorption from the gastrointestinal tract. After vitamin D3 is synthesized, it is transported to the liver where 25-hydroxyvitamin D3 is formed via hydroxylation by 25-hydroxylase. 25-Hydroxyvitamin D3 is further converted into the physiologically active vitamin D3 (1,25-dihydroxyvitamin D3) in the mitochondria of the proximal convoluted tubules. The active vitamin D3 and vitamin D-binding protein are then transported to
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