Retinoic acid (simplified nomenclature for all-trans-retinoic acid) is a metabolite of vitamin A1 (all-trans-retinol) that is required for embryonic development, male fertility, regulation of bone growth and immune function.[1] All-trans-retinoic acid is required for chordate animal development, 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.[2] It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages.[3] In adult tissues, the activity of endogenous retinoic acid appears limited to immune function.[1] and male fertility.[4] Retinoic acid administered as a drug (see tretinoin and alitretinoin) causes significant toxicity that is distinct from normal retinoid biology.[5]
All-trans-retinoic acid is the major occurring retinoic acid, while isomers like 13-cis- and 9-cis-retinoic acid are also present in much lower levels.[6]
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 acne or retinol used for skin disorders. High oral doses of preformed vitamin A (retinyl palmitate), and all-trans-retinoic acid itself, also have teratogenic potential by this same mechanism.[7]
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.[8] 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.[9] Control of retinoic acid levels is maintained by a suite of proteins that control synthesis and degradation of retinoic acid.[2] [3] The concentration of retinoic acid is tightly controlled and governs activation of the retinoid nuclear receptor pathway.[10] In adults, retinoic acid is only detected at physiologically relevant levels in the testes, pancreas and immune tissues.[11]
The molecular basis for the interaction between all-trans-retinoic acid and the Hox genes has been studied by using deletion analysis in transgenic mice carrying constructs of GFP reporter genes. Such studies have identified functional RAREs within flanking sequences of some of the most 3′ Hox genes (including HOXA1, HOXB1, HOXB4, HOXD4), suggesting a direct interaction between the genes and retinoic acid. These types of studies strongly support the normal roles of retinoids in patterning vertebrate embryogenesis through the Hox genes.[12]
In adults, retinoic acid has a key role in preventing autoimmunity in mucosal tissues. Retinoic acid produced by dendritic cells promotes regulatory T cell formation to promote tolerance within the colon.[13] This pathway is used by cancer cells to suppress the immune system.[14] In the testes, retinoic acid is necessary for the process of spermatogenesis.[15] Experiments in healthy male subjects suggests that retinoic acid is only necessary for fertility in adult humans.[16]
All-trans-retinoic acid can be produced in the body by two sequential oxidation steps that convert all-trans-retinol to retinaldehyde to all-trans-retinoic acid, but once produced it cannot be reduced again to all-trans-retinal. The enzymes that generate retinoic acid for regulation of gene expression include retinol dehydrogenase (Rdh10) that metabolizes retinol to retinaldehyde, and three types of retinaldehyde dehydrogenase, i.e. ALDH1A1 (RALDH1), ALDH1A2 (RALDH2), and ALDH1A3 (RALDH3)[17] that metabolize retinaldehyde to retinoic acid. Enzymes that metabolize retinoic acid to turn off biological signaling include the cytochrome P450 members (CYP26).[18] Oxidized metabolites such as 4-oxoretinoic acid are eliminated by glucuronidation in the liver.
All-trans-retinoic acid is a morphogen signaling molecule, which means it is concentration dependent; malformations can arise when the concentration of retinoic acid is in excess or deficient. Other signaling pathways that interact with the retinoic acid pathway are fibroblast growth factor 8, Cdx and Hox genes, all participating in the development of various structures within the embryo. For example, retinoic acid plays an important role in activating Hox genes required for hindbrain development. The hindbrain, which later differentiates into the brain stem, serves as a major signaling center defining the border of the head and trunk.[19]
A double-sided retinoic acid gradient that is high in the trunk and low at the junction with the head and tail represses fibroblast growth factor 8 in the developing trunk to allow normal somitogenesis, forelimb bud initiation, and formation of the atria in the heart.[20] During exposure to excess retinoic acid, the hindbrain becomes enlarged, hindering the growth of other parts of the brain; other developmental abnormalities that can occur during excess retinoic acid are missing or fused somites, and problems with the aorta and large vessels within the heart. With an accumulation of these malformations, an individual can be diagnosed with DiGeorge syndrome.[21] However, since retinoic acid acts in various developmental processes, abnormalities associated with loss of retinoic acid are not only limited to sites associated with DiGeorge syndrome. Genetic loss-of-function studies in mouse and zebrafish embryos that eliminate retinoic acid synthesis or retinoic acid receptors (RARs) have revealed abnormal development of the somites, forelimb buds, heart, hindbrain, spinal cord, eye, forebrain basal ganglia, kidney, foregut endoderm, etc.