Chapter 33 - Photobiology and Noncalcemic Actions of Vitamin D

Abstract

Publisher Summary This chapter focuses on the photobiological and noncalcemic actions of Vitamin D. When human skin is exposed to sunlight, a photochemical process occurs that is essential for the maintenance of calcium homeostasis and a healthy skeleton. During exposure to sunlight, the ultraviolet B portion of the solar spectrum photolyzes 7-dehydrocholesterol (the precursor cholesterol; provitamin D3) to previtamin D3. Once formed, previtamin D3 undergoes an internal isomerization of its three double bonds to form a more thermodynamically stable, 5,6-cis-triene and is transformed in vitamin D3. 7-Dehydrocholesterol is found principally in the cell membrane and within the membrane, the hydrophobic side chain of 7-dehydrocholesterol is aligned with the hydrophobic chains of the fatty acids and cholesterol, thereby restraining the confirmation of previtamin D3 when it is formed. Thus, when 7-dehydrocholesterol is exposed to sunlight, 7- dehydrocholesterol is photolyzed to the s-cis, s-cis conformer of previtamin D3. 1,25(OH)2D3 was found to decrease cellular proliferative activity, reduce c-myc-mRNA, and induce the expression of monocyte-specific cell surface antigen 63D3. This suggests the possibility of using 1,25(OH)2D3 or one of its analogs as an antiproliferative agent for the treatment of some leukemias and other malignant disorders. Most cells and tissues in the human body possess VDR and are therefore potential target tissues for 1,25(OH)2D3. Although there is very strong evidence in vitro and in vivo that 1,25(OH)2D3 can have a wide range of noncalcemic activities that affect the function of the immune system, skin, gonads, prostate gland, brain, skeletal and smooth muscle, and pancreas, the true physiologic function of 1,25(OH)2D3 is not well understood.