Development of in vitro Human Pigmented Skin Equivalents to Study Skin Pigmentation, Photodamage and Photoprotection

Abstract

Daily solar UV radiation has an important impact on skin health. Skin cancer and photoageing are some of the well-known long-term effects. Therefore, understanding the initial events of the UV-induced response is critical to prevent these conditions. However, studies in human volunteers have ethical, technical, and economic implications that make the skin equivalents a valuable platform to investigate mechanisms related to UV exposure to the skin. In vitro human skin equivalents can recreate the structure and function of in vivo human skin and represent a useful tool for academic and industrial applications. The successful development of pigmented skin equivalents by adding melanocytes to the epidermis of full-thickness skin equivalents has led to the study of skin pigmentation mechanisms, responses to UV exposure and the testing of photoprotective sunscreen.
We have developed pigmented skin equivalents that recreate the microanatomy of human skin. The pigmented full-thickness skin equivalents generated contain an Alvetex® Scaffold-based dermis, where human dermal fibroblasts secrete their own endogenous extracellular matrix proteins, and which supports a well-differentiated and stratified epidermis containing epidermal melanocytes. These skin equivalents have been highly characterised and have recapitulated different aspects of human skin pigmentation. In addition, different constitutive skin pigmentations have been recreated by including melanocytes from different donors.
Previous studies have utilised non-pigmented full-thickness skin equivalents or pigmented epidermal skin equivalents to investigate skin responses to UV exposure. However, these do not recapitulate the dermal-epidermal crosstalk and the melanocyte role in photoprotection that occur in vivo. In addition, the doses included in these studies generally do not represent a real UV condition. We have found that chronic irradiation of the pigmented full-thickness skin equivalents with a physiologically relevant dose can recreate UV-associated responses in vitro that have been found in irradiated human skin: morphological damage, tanning, alterations in proliferation, differentiation and apoptosis, DNA lesions, inflammatory response, and ECM-remodelling. Furthermore, we have found a differential response compared to non-pigmented full-thickness skin equivalents emphasising the contribution of the melanocytes to photoprotection.
A successful photoprotection from chronic topical sunscreen application to the skin equivalent has been achieved in vitro. We have demonstrated that the generated UV-induced skin equivalents represent a valuable platform for testing cosmetic and dermatological formulations designed to prevent UV-induced damage.