Distinct pigmentary and melanocortin 1 receptor-dependent components of cutaneous defense against ultraviolet radiation

Genetic variation at the melanocortin 1 receptor (MC1R) is an important risk factor for developing ultraviolet (UV) radiation-induced skin cancer, the most common form of cancer in humans. The underlying mechanisms by which the MC1R defends against UV- induced skin cancer are not known. We used neonatal mouse skin (which, like human skin, contains a mixture of melanocytes and keratinocytes) to study how pigment cells and Mc1r genotype affect the genome-level response to UV radiation. Animals without viable melanocytes (Kit(W-v)/Kit(W-v)) or animals lacking a functional Mc1r (Mc1r(e)/Mc1r(e)) were exposed to sunburn-level doses of UVB radiation, and the patterns of large-scale gene expression in the basal epidermis were compared to each other and to nonmutant animals. Our analysis revealed discrete Kit- and Mc1r-dependent UVB transcriptional responses in the basal epidermis. The Kit- dependent UVB response was characterized largely by an enrichment of oxidative and endoplasmic reticulum stress genes, highlighting a distinctive role for pigmented melanocytes in mediating antioxidant defenses against genotoxic stresses within the basal epidermal environment. By contrast, the Mc1r-dependent UVB response contained an abundance of genes associated with regulating the cell cycle and oncogenesis. To test the clinical relevance of these observations, we analyzed publicly available data sets for primary melanoma and melanoma metastases and found that the set of genes specific for the Mc1r-dependent UVB response was able to differentiate between different clinical subtypes. Our analysis also revealed that the classes of genes induced by UVB differ from those repressed by UVB with regard to their biological functions, their overall number, and their size. The findings described here offer new insights into the transcriptional nature of the UV response in the skin and provide a molecular framework for the underlying mechanisms by which melanocytes and the Mc1r independently mediate and afford protection against UV radiation.