retinoic acid
Meaning of retinoic acid: The working form of vitamin A, a gene switch powerful enough that an excess of it deforms a developing embryo.
Definition of retinoic acid
Retinoic acid is the active form of vitamin A produced inside the body, and it works by entering the nucleus of a cell and switching genes on and off. It governs how cells mature and specialize, and it is essential to building organs in the embryo. Manufactured versions treat acne when applied to the skin and one form of leukemia when taken by mouth.
Is it a vitamin?
No, and the distinction is worth keeping straight. Vitamin A is the nutrient that has to arrive in food, either as retinol from animal sources or as pigments in vegetables that the body converts. Retinoic acid is what the body then makes from that supply, much as a hormone is built from a raw material. The conversion runs one way only. Retinoic acid cannot be turned back into the form the eye requires, so it supports none of vitamin A's role in vision while carrying out almost everything else vitamin A is credited with. Describing it as a nutrient needed in small amounts describes the vitamin rather than this molecule.
How does it act on a cell?
By binding receptors that sit inside the nucleus, already attached to the genetic material. Most signaling molecules dock on the outside of a cell and pass a message inward. This one crosses the membrane, joins its receptor at the DNA, and the pair switches particular genes on or off directly. Hundreds of genes answer to it, so the effect is less a nudge than a change of program, pushing an immature cell to grow up into its final form. Skin cells respond by altering how the surface layer is shed and how pores behave, which is the basis of its use against acne.
What forms are used as drugs?
| Form | How it is given | Main use |
|---|---|---|
| All-trans retinoic acid | By mouth or on the skin | Acute promyelocytic leukemia, and acne |
| 13-cis retinoic acid | By mouth | Severe acne, and neuroblastoma in children |
| 9-cis retinoic acid | On the skin | Skin lesions of Kaposi sarcoma |
The leukemia use is the striking one. In acute promyelocytic leukemia the cells are stuck at an immature stage because a genetic rearrangement has jammed the very receptor this molecule works through. Giving all-trans retinoic acid in large amounts forces that receptor to function again, so the trapped cells mature and die on schedule, turning one of the most lethal forms of leukemia into one of the most curable. The approach is called differentiation therapy and it differs from chemotherapy, which kills dividing cells outright. Every form, including 13-cis retinoic acid, causes severe malformations in a developing embryo, which is why the oral preparations are supplied under strict pregnancy prevention arrangements.
Also known as
Used in a sentence
Retinoic acid was started as differentiation therapy once the diagnosis was confirmed.
Good to know
Retinoic acid and its relatives cause severe birth defects, and the molecule cannot be converted back into the forms of vitamin A the eye uses, so it does not substitute for the vitamin in supporting vision.
Word origin
Where “retinoic acid” comes from
Retinoic acid is named from the retina, from the Latin rete, meaning net, because vitamin A was first tied to the workings of the eye.