Epithelial tissue, cell membrane vitamin

The mechanism of action of 13-cis retinoic acid in normal and diseased human skin has not been studied. Many biological effects of vitamin A, especially its ability to labilize biological membranes and release lysosomal enzymes, have been described in detail. Vitamin A and its derivatives control cell growth and differentiation in epithelial tissues. Retinoic acid, in particular, is a potent stimulator of mitosis in the epidermis. Excess vitamin A and its derivatives also have been demonstrated to inhibit keratinization and induce a mucous metaplasia with enhanced glycoprotein synthesis in several laboratory models, such as embryonic chick skin in organ culture . The mode of action of topical retinoic acid in acne is thought to be the increased production of nonadherent surface scales, which prevents follicular occlusion and comedo formation . None of the known effects, however, can fully explain the apparent disease and individual specificity of the clinical response to 13-cis retinoic acid nor do they explain its ability to reduce the erythema in these diseases. 

Induction of Calbindin-D In response to calcitriol administration, there is an increase in mRNA synthesis and then in the synthesis of calbindin-D in intestinal mucosal cells, which is correlated with the later and more sustained increase in calcium absorption. In vitamin D-deficient animals, there is no detectable calbindin in the intestinal mucosa, whereas in animals adequately provided with vitamin D, it may account for 1 % to 3% of soluble protein in the cytosol of the colunmar epithelial ceils. Although the rapid response to calcitriol is an increase in the permeability of the brush border membrane to calcium, the induction of calbindin permits intracellular accumulation and transport of calcium. The rapid increase in net calcium transport in tissue from vitamin D-replete animals is presumably dependent on the calbindin that is already present in deficient animals, there can be no increase in calcium transport until sufficient calbindin has accumulated to permit intracellular accumulation, despite the increased permeability of the brush border. 

---