Melanin, lipofuscin and the effects of visible light in the skin

Photoexcitation of endogenous photosensitizers (ePS) in the human skin by sunlight culminates with the formation of electronic excited states, such as triplet excited states, singlet oxygen, and a variety of reactive oxygen and nitrogen species, either free radicals or two-electron oxidants. Considering the endogenous skin photosensitizers, melanin is the most abundant and is involved in photoprotection mechanisms, while lipofuscin is an unintentionally pigment accumulated during cell aging or oxidative stress. Both pigments severely increase the phototoxicity of visible light (VL) to skin cells. In the presence of these pigments, VL induces significant oxidative damage in nucleic acids, lipids and proteins, triggering regulated and unregulated cell death mechanisms. Besides, there is accumulation of premutagenic DNA lesions, indicated by the formamidopyrimidine glycosylase (Fpg)- and endonuclease III (Endo III)-sensitive sites in the nuclear DNA. As consequence of the photoinduced oxidative damage, skin cells, under the stimulus of VL release pro-inflammatory cytokines and metalloproteinases, prompting further cell death and skin aging. In this review we describe the photochemical properties of both melanin and lipofuscin pigments and discuss the consequences of their accumulation in terms of the phototoxicity of VL to the human skin.