Journal of Photochemistry and Photobiology (Mar 2022)
Melanin photosensitization by green light reduces melanoma tumor size
Abstract
Upon excitation by visible light (VL), melanin engages in photosensitization reactions, generating singlet oxygen (1O2) and causing oxidative damage to biomolecules via both type I and type II mechanisms. Therefore, VL irradiation may be used to perturb the survival and growth of pigmented melanoma lesions. However, this hypothesis has not been investigated previously, and there is no information regarding the effectiveness of this wavelength of light on melanomas. Therefore, this hypothesis needs to be tested in vivo, as VL has also been shown to stimulate tumor growth via photo biomodulation. This study aimed to determine the effect of melanin photosensitization following exposure to VL in cultured B16-F10 cells and melanoma lesions induced in mice. Cell viability was evaluated by the MTT assay after irradiation with blue, green, and red lights (466 nm, 532 nm, and 630 nm, respectively). Singlet oxygen (1O2) was investigated in melanin extracted from M+B16-F10 cells. Melanin-loaded M+B16-F10 cells showed a significant reduction in viability (∼30%) and 1O2 generation only after irradiation with 532 nm light. Mice were divided into groups with carcinogenesis induction (C+) or without induction (control, C-), irradiated (I+) with green light (GL 36 J.cm−2) at 7-day intervals or without irradiation (I-). The average tumor size (TAS) and pigmentation were quantified using a caliper and ImageJ software, respectively. After seven weeks, skin samples were collected from all groups for histopathological analysis. Statistical analysis was performed using Origin 2021 and GraphPad Prism 9®. The C-/I+ group showed initial pigmentation but no histological changes following irradiation. C+/I- showed tumor progression up to three times the initial volume, whereas the C+/I+ group presented a significant ∼20% decrease in TAS. These results suggest that photosensitization of melanin by GL (532 nm) reduces the viability of M+B16-F10 cells, decreases TAS, and prevents the growth of melanoma in C57BL6 mice.
