Retinoic acid

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Contents

All-trans-retinoic acid
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Retinoic acid 3D ball.png
Names
IUPAC name
Retinoic acid
Systematic IUPAC name
(2E,4E,6E,8E)-3,7-Dimethyl-9-(2,6,6-trimethylcyclohex-1-en-1-yl)nona-2,4,6,8-tetraenoic acid
Other names
vitamin A acid; RA
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
PubChem CID
UNII
  • InChI=1S/C20H28O2/c1-15(8-6-9-16(2)14-19(21)22)11-12-18-17(3)10-7-13-20(18,4)5/h6,8-9,11-12,14H,7,10,13H2,1-5H3,(H,21,22)/b9-6+,12-11+,15-8+,16-14+
    Key: SHGAZHPCJJPHSC-YCNIQYBTSA-N
  • CC1=C(C(CCC1)(C)C)/C=C/C(=C/C=C/C(=C/C(=O)O)/C)/C
Properties
C20H28O2
Molar mass 300.442 g·mol−1
AppearanceYellow to light orange crystalline powder with a characteristic of a floral scent [1]
Melting point 180 to 182 °C (356 to 360 °F; 453 to 455 K) Crystals from ethanol [1]
Nearly insoluble
Solubility in fatSoluble
Related compounds
Related compounds
 retinol; retinal; beta-carotene
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Retinoic acid (simplified nomenclature for all-trans-retinoic acid) is a metabolite of vitamin A 1 (all-trans-retinol) that is required for embryonic development, male fertility, regulation of bone growth and immune function. [2] 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. [3] It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages. [4] In adult tissues, the activity of endogenous retinoic acid appears limited to immune function [2] and male fertility. [5] Retinoic acid administered as a drug (see tretinoin and alitretinoin) causes significant toxicity that is distinct from normal retinoid biology. [6]

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. [7]

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. [8]

Mechanism of biological action

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. [9] 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. [10] Control of retinoic acid levels is maintained by a suite of proteins that control synthesis and degradation of retinoic acid. [3] [4] The concentration of retinoic acid is tightly controlled and governs activation of the retinoid nuclear receptor pathway. [11] In adults, retinoic acid is only detected at physiologically relevant levels in the testes, pancreas and immune tissues. [12]

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. [13]

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. [14] This pathway is used by cancer cells to suppress the immune system. [15] In the testes, retinoic acid is necessary for the process of spermatogenesis. [16] Experiments in healthy male subjects suggests that retinoic acid is only necessary for fertility in adult humans. [17]

Biosynthesis and metabolism

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) [18] that metabolize retinaldehyde to retinoic acid. [3] Enzymes that metabolize retinoic acid to turn off biological signaling include the cytochrome P450 members (CYP26). [19] Oxidized metabolites such as 4-oxoretinoic acid are eliminated by glucuronidation in the liver.

Function in embryonic development

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. [20]

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. [21] 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. [22] 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. [21]

Related Research Articles

<span class="mw-page-title-main">Vitamin A</span> Essential nutrient

Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most notably β-carotene (beta-carotene). Vitamin A has multiple functions: growth during embryo development, maintaining the immune system, and healthy vision. For aiding vision specifically, it combines with the protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light and color vision.

<span class="mw-page-title-main">Retinol</span> Chemical compound

Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family that is found in food and used as a dietary supplement. Retinol or other forms of vitamin A are needed for vision, cellular development, maintenance of skin and mucous membranes, immune function and reproductive development. Dietary sources include fish, dairy products, and meat. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xerophthalmia. It is taken by mouth or by injection into a muscle. As an ingredient in skin-care products, it is used to reduce wrinkles and other effects of skin aging.

<span class="mw-page-title-main">Retinal</span> Vitamin A aldehyde, a polyene chromophore

Retinal is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).

<span class="mw-page-title-main">Retinoid</span> Group of tetraterpenes

The retinoids are a class of chemical compounds that are natural derivatives of vitamin A or are chemically related to it. Synthetic retinoids are used in medicine where they regulate skin health, immunity and bone disorders.

<span class="mw-page-title-main">Morphogen</span> Biological substance that guides development by non-uniform distribution

A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the various specialized cell types within a tissue. More specifically, a morphogen is a signaling molecule that acts directly on cells to produce specific cellular responses depending on its local concentration.

The retinoic acid receptor (RAR) is a type of nuclear receptor which can also act as a ligand-activated transcription factor that is activated by both all-trans retinoic acid and 9-cis retinoic acid, retinoid active derivatives of Vitamin A. They are typically found within the nucleus. There are three retinoic acid receptors (RAR), RAR-alpha, RAR-beta, and RAR-gamma, encoded by the RARA, RARB, RARG genes, respectively. Within each RAR subtype there are various isoforms differing in their N-terminal region A. Multiple splice variants have been identified in human RARs: four for RARA, five for RARB, and two for RARG. As with other type II nuclear receptors, RAR heterodimerizes with RXR and in the absence of ligand, the RAR/RXR dimer binds to hormone response elements known as retinoic acid response elements (RAREs) complexed with corepressor protein. Binding of agonist ligands to RAR results in dissociation of corepressor and recruitment of coactivator protein that, in turn, promotes transcription of the downstream target gene into mRNA and eventually protein. In addition, the expression of RAR genes is under epigenetic regulation by promoter methylation. Both the length and magnitude of the retinoid response is dependent of the degradation of RARs and RXRs through the ubiquitin-proteasome. This degradation can lead to elongation of the DNA transcription through disruption of the initiation complex or to end the response to facilitate further transcriptional programs. RAR receptors are also known to exhibit many retinoid-independent effects as they bind to and regulate other nuclear receptor pathways, such as the estrogen receptor.

The retinoid X receptor (RXR) is a type of nuclear receptor that is activated by 9-cis retinoic acid, which is discussed controversially to be of endogenous relevance, and 9-cis-13,14-dihydroretinoic acid, which may be an endogenous mammalian RXR-selective agonist. Bexarotene is the only specific activator of the RXRs which does not activate the Retinoic Acid Receptors.

The RAR-related orphan receptors (RORs) are members of the nuclear receptor family of intracellular transcription factors. There are three forms of ROR, ROR-α, -β, and -γ and each is encoded by a separate gene, RORA, RORB, and RORC respectively. The RORs are somewhat unusual in that they appear to bind as monomers to hormone response elements as opposed to the majority of other nuclear receptors which bind as dimers. They bind to DNA elements called ROR response elements (RORE).

In enzymology, a retinol dehydrogenase (RDH) (EC 1.1.1.105) is an enzyme that catalyzes the chemical reaction

<span class="mw-page-title-main">Retinoid X receptor alpha</span> Protein-coding gene in the species Homo sapiens

Retinoid X receptor alpha (RXR-alpha), also known as NR2B1 is a nuclear receptor that in humans is encoded by the RXRA gene.

<span class="mw-page-title-main">Retinoic acid receptor alpha</span> Protein found in humans

Retinoic acid receptor alpha (RAR-α), also known as NR1B1, is a nuclear receptor that in humans is encoded by the RARA gene.

<span class="mw-page-title-main">Retinal dehydrogenase</span>

In enzymology, a retinal dehydrogenase, also known as retinaldehyde dehydrogenase (RALDH), catalyzes the chemical reaction converting retinal to retinoic acid. This enzyme belongs to the family of oxidoreductases, specifically the class acting on aldehyde or oxo- donor groups with NAD+ or NADP+ as acceptor groups, the systematic name being retinal:NAD+ oxidoreductase. This enzyme participates in retinol metabolism. The general scheme for the reaction catalyzed by this enzyme is:

<span class="mw-page-title-main">Retinoic acid receptor gamma</span> Protein-coding gene in the species Homo sapiens

Retinoic acid receptor gamma (RAR-γ), also known as NR1B3 is a nuclear receptor encoded by the RARG gene. Adapalene selectively targets retinoic acid receptor beta and retinoic acid receptor gamma and its agonism of the gamma subtype is largely responsible for adapalene's observed effects.

<span class="mw-page-title-main">CRABP2</span> Protein-coding gene in the species Homo sapiens

Cellular retinoic acid-binding protein 2 is a cytoplasmic binding protein that in humans is encoded by the CRABP2 gene.

<span class="mw-page-title-main">CRABP1</span> Protein-coding gene in the species Homo sapiens

Cellular retinoic acid-binding protein 1 is a protein that in humans is encoded by the CRABP1 gene.

<span class="mw-page-title-main">ALDH1A2</span> Protein-coding gene in the species Homo sapiens

Aldehyde dehydrogenase 1 family, member A2, also known as ALDH1A2 or retinaldehyde dehydrogenase 2 (RALDH2), is an enzyme that in humans is encoded by the ALDH1A2 gene.

<span class="mw-page-title-main">ALDH1A3</span> Protein-coding gene in the species Homo sapiens

Aldehyde dehydrogenase 1 family, member A3 (ALDH1a3), also known as retinaldehyde dehydrogenase 3 (RALDH3) or as ALDH6 in earlier published studies, is an enzyme that in humans is encoded by the ALDH1A3 gene.,

<span class="mw-page-title-main">RDH13</span> Protein-coding gene in humans

Retinol dehydrogenase 13 (all-trans/9-cis) is a protein that in humans is encoded by the RDH13 gene. This gene encodes a mitochondrial short-chain dehydrogenase/reductase, which catalyzes the reduction and oxidation of retinoids. The encoded enzyme may function in retinoic acid production and may also protect the mitochondria against oxidative stress. Alternatively spliced transcript variants have been described.

<span class="mw-page-title-main">Retinol-binding protein</span> Family of proteins that bind retinol

Retinol-binding proteins (RBP) are a family of proteins with diverse functions. They are carrier proteins that bind retinol. Assessment of retinol-binding protein is used to determine visceral protein mass in health-related nutritional studies.

The RARA gene, also known as NR1B1, is a protein coding gene located on chromosome 17 that provides the instructions required to make transcription factor Retinoic Acid Receptor Alpha.

References

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