Retinoids are a class of chemical compounds that are similar in structure to or derived from vitamin A. Retinoids serve many important and diverse functions throughout the body, including roles in vision,1 regulation of cell proliferation and differentiation,2 growth of bone tissue,3, 4 immune functions,5 and as activators of tumor suppressor genes.6 These compounds are also being investigated as preventive agents for skin cancer.7
The basic structure of a retinoid compound consists of four isoprenoid units joined head-to-tail with a cyclic group on one end, a polyene side chain, and a functional group on the other end.8 The conjugation of the polyene side chain—i.e. alternating single and double bonds—is responsible for the color of retinoids, which are typically yellow, orange or red, as well as their ability to act as chromophores. Variations in the side chains and polar end groups of these compounds lead to different classes of retinoids. Retinoids are classified into three generations (see Table 1),9 the first of which contains naturally occurring retinoids, though all are also produced synthetically. Both first- and second-generation retinoids are ligands for (bind to) several retinoid-binding proteins, called receptors, found in serum, cytoplasm and nuclei. Third- generation retinoids are less flexible than first- and second-generation retinoids and interact with fewer retinoid receptors. The body stores the majority of its retinoid reserves in the liver, mostly as retinyl esters of palmitic, stearic, oleic and linoleic acids.10
Once retinol, or vitamin A, has been taken up by a cell, it can be oxidized to retinal, which is further oxidized to retinoic acid.11 Retinoic acid acts as a ligand for both the RAR and retinoid X receptors (RXR). Retinol appears to function in maintaining skin health and is being used in an increasing number of consumer skin care products for treating aged skin and wrinkles, presumably by stimulating new collagen production.