Vitamin A, like vitamin D, has a nuclear receptor. Vitamin A technically has two nuclear receptors, the retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Vitamin A, like polyunsaturated fatty acids, can be found in trans and cis forms, depending on the conformation of its double bonds. The ligand for RARs is all-trans-retinoic acid, and the ligand for RXRs is 9-cis retinoic acid.
As shown in the figure below, all-trans retinol is brought to the cell by RBP and TTR. All-trans retinol is converted to all-trans-retinal, and then to all-trans-retinoic acid. RAR and RXR are paired, or dimerized, on the retinoic acid response element (RARE) in the promoter region of target genes. The binding of all-trans retinoic acid causes the transcription and ultimately the translation of target proteins. This is why all-trans-retinoic acid is the active form of vitamin A because it is the ligand for RARs, leading to many of the biologic effects attributed to vitamin A.
Figure 12.631 All-trans retinoic acid and retinoid X receptors