Summary
Retinoic acid (used simplified here for all-trans-retinoic acid) is a metabolite of vitamin A1 (all-trans-retinol) that mediates the functions of vitamin A1 required for growth and development. All-trans-retinoic acid is required in chordate animals, which includes all higher animals from fish to humans. During early embryonic development, all-trans-retinoic acid generated in a specific region of the embryo helps determine position along the embryonic anterior/posterior axis by serving as an intercellular signaling molecule that guides development of the posterior portion of the embryo. It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages. All-trans-retinoic acid (ATRA) is the major occurring retinoic acid, while isomers like 13-cis- and 9-cis-retinoic acid are also present in much lower levels. The key role of all-trans-retinoic acid in embryonic development mediates the high teratogenicity of retinoid pharmaceuticals, such as isotretinoin (13-cis-retinoic acid) used for treatment of cancer and acne. Oral megadoses of preformed vitamin A (retinyl palmitate), and all-trans-retinoic acid itself, also have teratogenic potential by this same mechanism. All-trans-retinoic acid acts by binding to the retinoic acid receptor (RAR), which is bound to DNA as a heterodimer with the retinoid X receptor (RXR) in regions called retinoic acid response elements (RAREs). Binding of the all-trans-retinoic acid ligand to RAR alters the conformation of the RAR, which affects the binding of other proteins that either induce or repress transcription of a nearby gene (including Hox genes and several other target genes). RARs mediate transcription of different sets of genes controlling differentiation of a variety of cell types, thus the target genes regulated depend upon the target cells. In some cells, one of the target genes is the gene for the retinoic acid receptor itself (RAR-beta in mammals), which amplifies the response.
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