Clinical and molecular change induced by repeated low-dose visible light exposure in both light-skinned and dark-skinned individuals

Background: Visible light (VL) is known to induce pigmentation in dark-skinned individuals and immediate erythema in light-skinned individuals. However, the effects of accumulated low-dose VL exposure across skin types are not well established. Methods: Thirty-one healthy subjects with light (Fitzpatrick skin types [FST] I-II, n = 13) and dark (FST V-VI, n = 18) skin types were enrolled. Subjects' buttocks were exposed daily to VL, wavelength 400–700 nm, with a dose of 120 J/cm² at 50 mW/cm², for four consecutive days. Microarray using Affymetrix GeneChip (49,395 genes) was performed followed by qRT-PCR on skin samples. Results: Repeated low-dose VL irradiation induced immediate pigment darkening and delayed tanning in dark-skinned individuals while no discernable pigmentation and erythema were observed in light-skinned individuals. Top ten upregulated genes by repeated VL exposure in microarray included melanogenic genes such as tyrosinase (TYR), tyrosinase-related protein-1 (TYRP1), dopachrome tautomerase (DCT), premelanosome protein (PMEL), melan-A (MLANA), and solute carrier family 24, member 5 (SLC24A5) and genes involved in inflammation/matrix remodeling/cell signaling including chemokine (C-C motif) ligand 18 (CCL18), BCL2-related protein A1 (BCL2A1), and cartilage oligomeric matrix protein (COMP). In qRT-PCR CCL18 was upregulated in light skin with a greater extent (mean fold change ± SD; 4.03 ± 3.28, p =.04) than in dark-skinned individuals (1.91 ± 1.32, p =.07) while TYR was not significantly upregulated in both skin types. Conclusion: This study highlights the genes upregulated by cumulative VL exposure involved in pigmentation, immune response, oxidation/reduction, and matrix remodeling across skin types providing relevant information on daily solar exposure.