760 Melanin may mediate visible light induced phototoxicity in darker skin types

Abstract

Visible light (VL), a major component of solar radiation, differentially induces pigmentation in skin of color, an effect likely related to inherent melanization. A chromophore, melanin absorbs light within the VL wavelength. We hypothesize that clinical and histologic responses to VL are determined by epidermal melanin content and distribution. Nuclear matrix metalloproteinase (MMP-1), a proapoptotic mediator of photodamage may play a central role in skin type specific responses to VL. Enrolled participants were grouped by skin type (light=6,dark=6). Photoprotected sites were exposed daily x4days to 480J/cm2 of VL at 200mW(FiberLite High Intensity Illuminator Series 180,Dolan Jenner Industries) for a total dose of 1920J/cm2as tolerated. Treated and control sites were biopsied 24h after the last exposure and stained with Masson’s Trichrome and Fontana-Masson (FM) to assess collagen and melanin deposition, respectively. MMP-1 deposition was detected by indirect immunofluorescence (IF). Dark skin types did not tolerate the full VL regimen with blistering occurring in all subjects at doses of 220-880J/cm2. We found epidermal MMP-1 expression and nuclear localization in keratinocytes in dark but not in light skin. Nuclear localization correlated with areas of highest melanin density per FM stain. While there was no significant difference in MMP expression of treated light skin from control, collagen stains showed decreased stain intensity and thickened, wavy collagen bundles in both light (mean fold change[FC]=1.46,SEM+0.11), and dark skin (FC=1.19,+0.05). Herein, we demonstrate the acute effects of high power VL exposure in light and dark skin. VL can cause degenerative changes in collagen similar to acute UV irradiation or thermal injury, occurring independent of skin melanization. While the role of melanin in photoprotection is well defined, given the pro-apoptotic function of nuclear MMPs, our findings suggest a potential mechanism by which melanin may mediate VL induced phototoxicity. These observations show a “light” and “dark” side to melanin that warrants further investigation.