Skin type specific photobiological response to visible light is mediated by constitutional melanin

Visible light (VL) induces varying photobiological responses between skin types, likely influenced by inherent melanization. Individual typology angle (ITA) objectively measures skin types. We hypothesize that epidermal melanin content and distribution determine VL response.

This study describes clinical and histologic responses to VL and examines the potential role of melanin in the underlying mechanistic pathways.

We grouped enrolled participants by ITA (Light = 5, Intermediate = 4, Dark = 7) per colorimetry (CR-400, Konica Minolta). Photoprotected sites were exposed daily to 480 J/cm² of VL (Fiber-Lite High Intensity Illuminator, Series 180, Dolan Jenner Industries Inc.) for 4 days (total = 1920 J/cm² ), as tolerated. Treated and control sites were biopsied 96 h after first exposure. We used hematoxylin and eosin and Fontana-Mason to assess histological changes and melanin deposition, respectively. p53 and Ki67 immunohistochemical stains were done to assess DNA damage and proliferation. Matrix metalloproteinase (MMP)-1 expression was detected by immunohistochemical staining and immunofluorescence microscopy.

Darker skin did not tolerate the full VL regimen with blistering occurring in most subjects at doses of 220-880 J/cm² . Intermediate and Dark skin showed tanning. Light skin developed erythema. p53 counts were highest in Intermediate, followed by Light skin, although this was not statistically significant. VL treatment led to MMP-1 expression and nuclear localization in keratinocytes in Dark and Intermediate but not in Light skin, however differences between groups were not statistically significant.

Skin types demonstrate unique biological responses to VL. The role of melanin in photoprotection is well-defined. However, given the pro-apoptotic function of nuclear MMPs, we suggest a potential mechanism by which melanin may mediate VL-induced phototoxicity.