Topical retinoic acid augments ultraviolet light-induced melanogenesis

Melanin, the natural pigment found in human skin, absorbs and protects against the ultraviolet (UV) components of sunlight. Melanin production (melanogenesis) is increased by exposure to sunlight, causing a darker skin colour which is regarded as aesthetically pleasing by many humans, who therefore expose themselves to large amounts of potentially damaging sunlight. We have found that topically applied all-trans retinoic acid, a metabolic derivative of vitamin A, greatly enhances UV light-induced melanogenesis: the same preparation on its own had no effect on skin pigmentation. An orally administered retinoid, temarotene, did not have this effect. These observations were made using a lightly pigmented mouse strain, HRA: Skh-2, and confirmed in 2 human volunteers. This is the first time that metabolic derivatives of vitamin A have been shown to augment UV light-induced melanogenesis, suggesting a role for vitamin A in this process.