Vitamin D and the skin: Physiology and pathophysiology

The keratinocytes of the skin are unique in being not only the primary source of vitamin D for the body, but in possessing both the enzymatic machinery to metabolize the vitamin D produced to active metabolites (in particular 1,25(OH)(2)D) and the vitamin D receptor (VDR) that enables the keratinocytes to respond to the 1,25(OH)(2)D thus generated. Numerous functions of the skin are regulated by vitamin D and/or its receptor. These include inhibition of proliferation, stimulation of differentiation including formation of the permeability barrier, promotion of innate immunity, regulation of the hair follicle cycle, and suppression of tumor formation. Regulation of these actions is exerted by a number of different coregulator complexes including the coactivators vitamin D receptor interacting protein (DRIP) complex also known as Mediator and the steroid receptor coactivator (SRC) family (of which SRC 2 and 3 are found in keratincytes), the inhibitor hairless (Hr), and β-catenin whose impact on VDR function is complex. Different coregulators appear to be involved in different VDR regulated functions. This review will examine the various functions of vitamin D and its receptor in the skin, and explore the mechanisms by which these functions are regulated.