Vitamin D and the skin: an ancient friend, revisited.

Most vertebrates need vitamin D to develop and maintain a healthy mineralized skeleton. However, 1,25-dihydroxyvitamin D3 [1,25(OH)(2)D(3)], the biologically active vitamin D metabolite, exerts a multitude of important physiological effects independent from the regulation of calcium and bone metabolism. We know today that the skin has a unique role in the human body's vitamin D endocrine system. It is the only site of vitamin D photosynthesis, and has therefore a central role in obtaining a sufficient vitamin D status. Additionally, the skin has the capacity to synthesize the biologically active vitamin D metabolite 1,25(OH)(2)D(3), and represents an important target tissue for 1,25(OH)(2)D(3). In keratinocytes and other cell types, 1,25(OH)(2)D(3) regulates growth and differentiation. Consequently, vitamin D analogues have been introduced for the treatment of the hyperproliferative skin disease psoriasis. Recently, sebocytes were identified as 1,25(OH)(2)D(3)-responsive target cells, indicating that vitamin D analogues may be effective in the treatment of acne. Other new functions of vitamin D analogues include profound effects on the immune system as well as in various tissues protection against cancer and other diseases, including autoimmune and infectious diseases. It can be speculated that the investigation of biological effects of vitamin D analogues will lead to new therapeutic applications that, besides cancer prevention, may include the prevention and treatment of infectious as well as of inflammatory skin diseases. Additionally, it can be assumed that dermatological recommendations on sun protection and health campaigns for skin cancer prevention will have to be re-evaluated to guarantee a sufficient vitamin D status.