Vitamin A : Retinoic acid mechanism of action

Retinol is oxidized to retinoic acid. Retinoic acid binds with high affinity to specific receptor proteins (retinoic acid receptors [RAR]) present in the nucleus of target tissues such as epithelial cells (Fig. 1).

The activated retinoic acid–RAR complex binds to response elements on DNA and recruits activators or repressors to regulate retinoid-specific RNA synthesis, resulting in control of the production of specific proteins that mediate several physiologic functions. For example, retinoids control the expression of the gene for keratin in most epithelial tissues of the body. [Note: The RAR proteins are part of the superfamily of transcriptional regulators that includes the nuclear receptors for steroid and thyroid hormones and vitamin D, all of which function in a similar way .]

Figure 1:  Action of the retinoids. [Note: Retinoic acid–receptor complex forms a dimer, but is shown as monomer for simplicity.] TTR = transthyretin; RBP = retinol-binding protein; mRNA = messenger RNA.