Photobiology and Noncalcemic Actions of Vitamin D

Abstract

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 D 3 ) to previtamin D 3 . Once formed, previtamin D 3 undergoes an internal isomerization of its three double bonds to form a more thermodynamically stable, 5,6-cis-triene and is transformed in vitamin D 3 . 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 D 3 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 D 3 . 1,25(OH) 2 D 3 was found to decrease cellular proliferative activity, reduce c-myc-mRNA, and induce the expression of monocyte-specific cell surface antigen 63D 3 . This suggests the possibility of using 1,25(OH) 2 D 3 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) 2 D 3 . Although there is very strong evidence in vitro and in vivo that 1,25(OH) 2 D 3 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) 2 D 3 is not well understood.