Melanocortin-1 Receptor Signaling Markedly Induces the Expression of the NR4A Nuclear Receptor Subgroup in Melanocytic Cells

The melanocortin-1 receptor (MCIR) is a G-protein-coupled receptor expressed primarily in melanocytes and is known to play a pivotal role in the regulation of pigmentation in mammals. In humans MC1R has been found to be highly polymorphic with several functional variants associated with the phenotype of red hair color and fair skin, cutaneous UV sensitivity, and increased risk of developing melanoma and non-melanoma skin cancer. Recent evidence suggests that MC1R plays a photo-protective role in melanocytes in response to UV irradiation. Relatively few genetic targets of MC1R signaling have been identified independent of the pigmentation pathway. Here we show that MC1R signaling in B16 mouse melanoma cells and primary human melanocytes rapidly, and transiently, induces the transcription of the NR4A subfamily of orphan nuclear receptors. Furthermore, primary human melanocytes harboring homozygous RHC variant MC1R alleles exhibited an impaired induction of NR4A genes in response to the potent MC1R agonist (Nle4,D-Phe7)-?-melanocyte-stimulating hormone. Using small interference RNA-mediated attenuation of NR4A1 and NR4A2 expression in melanocytes, the ability to remove cyclobutane pyrimidine dimers following UV irradiation appeared to be impaired in the context of MC1R signaling. These data identify the NR4A receptor family as potential mediators of an MC1R-coordinated DNA damage response to UV exposure in melanocytic cells.

It has long been recognized that individuals with fair skin and poor tanning ability have an increased incidence of melanoma and non-melanoma skin cancer. Cutaneous pigmentation in humans is the result of melanin synthesis by epidermal melanocytes within specialized organelles termed melanosomes, which are transferred to surrounding keratinocytes.

MC1R is a seven-transmembrane G-protein-coupled receptor that has been found to be highly polymorphic in Caucasian individuals, with a number of functional variants strongly associating with a phenotype of red hair, fair skin, and poor tanning ability often referred to as the Red Hair Color (RHC) phenotype. MC1R preferentially binds the pro-opiomelanocortin-derived peptides ?-melanocyte-stimulating hormone (?-MSH) and adrenocorticotropic hormone, which leads to an elevation in intracellular cAMP levels. RHC variant MC1Rs have been demonstrated to elicit weak cAMP responses as a result of reduced receptor coupling or abnormal receptor localization. Molecular analysis has revealed that MC1R signaling modulates the expression and function of the microphthalmia-associated transcription factor, a key regulator of pigmentation genes, suggesting that this pathway underlies MC1R coordination of melanogenesis. Accumulating evidence suggests the photo-protective effect of MC1R signaling goes beyond quantitative and qualitative changes in melanin synthesis and extends to protection against UV-induced genomic damage via the induction of nucleotide excision repair mechanisms. Although variant MC1R is anticipated to have a significant effect on melanocyte gene expression, few downstream transcriptional targets of MC1R signaling have been identified to date and the differential transcriptional response between wild-type and RHC signaling remains poorly understood.