Keratinocytes and fibroblasts in a human skin equivalent model enhance melanocyte survival and melanin synthesis after ultraviolet irradiation

To investigate paracrine effects of fibroblasts and keratinocytes on melanocyte behavior after ultraviolet (UV) irradiation, we compared an in vitro skin equivalent model with melanocyte cultures. Human melanocytes were maintained alone in monolayer cultures or on dermal equivalents with or without keratinocytes and were irradiated daily with solar-simulated light. After seven daily UV irradiations, monolayer melanocytes displayed dose-dependent increases in cellular damage. In contrast, melanocytes on dermal equivalents survived strikingly better. Moreover, UV-irradiated skin equivalent melanocytes became highly dendritic as compared with sham-irradiated cells, closely mimicking their morphology in UV-irradiated skin. In addition, in skin equivalents melanocytes migrated from the center to the periphery of the keratinocyte layer after UV irradiation. Melanin production per culture, as measured by 14C-dihydroxyphenylalanine incorporation, was consistently higher in skin equivalent melanocytes than in monolayer melanocytes from the same donor, and it was highest in melanocytes from skin equivalents containing both keratinocytes and fibroblasts. Our data strongly suggest that fibroblasts and keratinocytes modulate melanocyte function in skin. The skin equivalent is a valuable model for investigating paracrine effects on melanocytes after UV irradiation.