Photoprotection versus photodamage: updating an old but still unsolved controversy about melanin

Abstract

Melanin is the main pigment in animal skin. It is a heterogeneous polyphenolic molecule with a generalized absorption in the UV−visible range synthesized in melanocytes, a kind of specialized epidermal cells. There are two types of melanin, eumelanin and pheomelanin, more abundant in dark or fair skin respectively. Melanin plays several roles, but in mammalian skin the main one is related to photoprotection against sunlight. The absorption energy from UV light implies that elements of the melanin structure are photoexcited, and the subsequent transference of energy can provoke several effects on surrounding molecules. These processes raise the possibility that melanin becomes a photosensitizer instead of a photoprotector. Thus, melanins should be considered double-faced molecules. This was first evident for pheomelanin, but very recent new data on eumelanin indicate that the balance is subtler than thought so far, and photodamage can overcome photoprotection for both types. In this updated review, the consequences of the eumelanin versus pheomelanin photochemical properties are discussed in relation to DNA lesions. Melanin is considered photoprotective, but it can also produce DNA photodamage and this would be related to the high incidence of skin cancer, mostly in Caucasian people.

Eumelanin for nature‐inspired UV‐absorption enhancement of plastics

“…The long‐term effects on our commercial EVA of the UV‐absorption enhancement were then evaluated by means of a UV‐aging test. Findings from both medical doctors and physicists point to a dichotomous role, either photoprotective or photodamaging, conferred on eumelanin by its broad UV absorption . When eumelanin converts the energy of photons into heat (photothermal effect), it can act as a photoprotector .…”

“…Findings from both medical doctors and physicists point to a dichotomous role, either photoprotective or photodamaging, conferred on eumelanin by its broad UV absorption. 40 When eumelanin converts the energy of photons into heat (photothermal effect), it can act as a photoprotector. 65,66 This is the effect by which UV absorbers protect polymers against photo-oxidation.…”

“…The assessment included the evaluation of possible synergistic or antagonistic effects between eumelanin and the thermal stabilizer (butylated hydroxytoluene) already present in the commercial‐grade EVA . Finally, eumelanin's UV‐absorption properties in nature can have a dichotomous role, either photoprotective or photodamaging . The latter effect was suggested to prevail after prolonged exposure of the biopigment to UVA radiation (320–400 nm) .…”

“…[37][38][39] Finally, eumelanin's UV-absorption properties in nature can have a dichotomous role, either photoprotective or photodamaging. 40 The latter effect was suggested to prevail after prolonged exposure of the biopigment to UVA radiation (320-400 nm). 41 Consequently, we also assessed whether, in the polymeric matrix, the photoprotective or the photodamaging role of the biopigment prevails upon prolonged UV irradiation.…”

Photoprotection and Skin Pigmentation: Melanin-Related Molecules and Some Other New Agents Obtained from Natural Sources

“…The first one is produced through direct DNA absorption of mutagenic radiations, thereby increasing lesions called the UV "signature" (pyrimidine dimers or 6-4 photoproducts). The second mechanism is indirect, through interaction with other biochromes generating reactive oxygen species (ROS) which produce harmful cellular effects and can reach the nucleus causing oxidative DNA modifications and strand breaks [4][5][6]. In both cases, accumulative damage of DNA can finally induce apoptosis or lead to cancer appearance [5,7].…”