Ultraviolet radiation is a well established epidemiologic risk factor for malignant melanoma. This observation has been linked to the relative resistance of normal melanocytes to ultraviolet B (UVB) radiation-induced apoptosis, which consequently leads to accumulation of UVB radiation-induced DNA lesions in melanocytes. Therefore, identification of physiologic factors regulating UVB radiation-induced apoptosis and DNA damage of melanocytes is of utmost biological importance. We show that the neuropeptide ␣-melanocyte-stimulating hormone (␣-MSH) blocks UVB radiation-induced apoptosis of normal human melanocytes in vitro. The antiapoptotic activity of ␣-MSH is not mediated by filtering or by induction of melanin synthesis in melanocytes. ␣-MSH neither leads to changes in the cell cycle distribution nor induces alterations in the expression of the apoptosis-related proteins Bcl 2 , Bcl x , Bax, p53, CD95 (Fas/APO-1), and CD95L (FasL). In contrast, ␣-MSH markedly reduces the formation of UVB radiation-induced DNA damage as demonstrated by reduced amounts of cyclobutane pyrimidine dimers, ultimately leading to reduced apoptosis. The reduction of UV radiation-induced DNA damage by ␣-MSH appears to be related to induction of nucleotide excision repair, because UV radiation-mediated apoptosis was not blocked by ␣-MSH in nucleotide excision repair-deficient fibroblasts. These data, for the first time, demonstrate regulation of UVB radiation-induced apoptosis of human melanocytes by a neuropeptide that is physiologically expressed within the epidermis. Apart from its ability to induce photoprotective melanin synthesis, ␣-MSH appears to exert the capacity to reduce UV radiation-induced DNA damage and, thus, may act as a potent protection factor against the harmful effects of UV radiation on the genomic stability of epidermal cells.Apoptosis of epidermal cells by ultraviolet B (UVB 1 ; 290 -320 nm) radiation is a well described phenomenon in vitro and in vivo and has been extensively studied in keratinocytes, the major target cells of solar UV radiation. It is considered a protective mechanism for minimizing the survival of cells with irreparable DNA damage (1), thereby preventing malignant transformation. The molecular pathways leading to UVB radiation-induced apoptosis include the formation of cyclobutane pyrimidine dimers (CPDs) and (6 -4) photoproducts (2, 3), the activation of death receptors including CD95 (Fas/APO-1) (2, 4), the release of death ligands, e.g. tumor necrosis factor-␣ (5, 6), and the formation of reactive oxygen species (7). These pathways are orchestrated by positive and negative factors that act within the epidermis in an autocrine and/or paracrine fashion. For example, hepatocyte growth factor/scatter factor produced by dermal fibroblasts inhibits UVB radiation-induced apoptosis of human keratinocytes via the phosphatidylinositol 3-kinase/AKT pathway (8). Insulin-like growth factor-1, which is expressed by melanocytes and fibroblasts (9), delays UVB radiation-induced apoptosis in human keratinocytes...

Studies in mice indicate that α-melanocyte-stimulating hormone (αMSH) is immunosuppressive, but it is not known whether αMSH suppresses human immune responses to exogenous Ags. Human PBMCs, including monocytes, express the melanocortin 1 receptor (MC1R), and it is thought that the ability of αMSH to alter monocyte-costimulatory molecule expression and IL-10 release is mediated by this receptor. However, the MC1R gene is polymorphic, and certain MC1R variants compromise receptor signaling via cAMP, resulting in red hair and fair skin. Here, we have investigated whether αMSH can suppress Ag-induced lymphocyte proliferation in humans and whether these effects are dependent on MC1R genotype. αMSH suppressed streptokinase-streptodornase-induced lymphocyte proliferation, with maximal inhibition at 10−13–10−11 M αMSH. Anti-IL-10 Abs failed to prevent suppression by αMSH, indicating that it was not due to MC1R-mediated IL-10 release by monocytes. Despite variability in the degree of suppression between subjects, similar degrees of αMSH-induced immunosuppression were seen in individuals with wild-type, heterozygous variant, and homozygous/compound heterozygous variant MC1R alleles. RT-PCR of streptokinase-streptodornase-stimulated PBMCs for all five melanocortin receptors demonstrated MC1R expression by monocytes/macrophages, MC1R and MC3R expression by B lymphocytes, but no melanocortin receptor expression by T lymphocytes. In addition, αMSH did not significantly inhibit anti-CD3 Ab-induced lymphocyte proliferation, whereas αMSH and related analogs (SHU9119 and MTII) inhibited Ag-induced lymphocyte proliferation in monocyte-depleted and B lymphocyte-depleted assays. These findings demonstrate that αMSH, acting probably via MC1R on monocytes and B lymphocytes, and possibly also via MC3R on B lymphocytes, has immunosuppressive effects in humans but that suppression of Ag-induced lymphocyte proliferation by αMSH is independent of MC1R gene status.

Pigmentation is a significant determinant of individual susceptibility to cutaneous melanoma, with fair skinned subjects at highest risk of developing this neoplasm. Melanocortin 1 receptor (MC1R) gene variants alter pigment synthesis in vivo, and are causally associated with red hair and fair skin in humans. MC1R variants are more frequent in subjects with melanoma, and increase the risk of developing this tumour in sporadic and familial cases. MC1R variants may predispose to melanoma as a result of alterations in skin pigmentation (which affords less protection against incident ultraviolet radiation). However, melanoma cells synthesize and release alpha-melanocyte stimulating hormone (aMSH, the ligand for MC1R), therefore MC1R variants could alter the autocrine effects of aMSH on melanoma cell behaviour, thereby affecting early melanoma development and progression via non-pigmentary mechanisms. B16G4F melanoma cells, which are functionally null at Mc1r, were stably transfected with wild type and variant (Arg151Cys, Arg160Trp, and Asp294His) human MC1R. At similar MC1 receptor numbers per cell, aMSH increased intracellular cAMP in wild type MC1R transfected melanoma cells, but the cAMP response was compromised in the variant MC1R transfected clones. In growth inhibition experiments, aMSH significantly reduced growth of wild type MC1R transfected cells, but had no effect on cells transfected with variant MC1R. In addition, binding to fibronectin was significantly reduced by aMSH in the wild type transfectants whereas this was not observed in the variant transfected clones; binding to laminin was not affected by aMSH in this cell line. These results provide evidence for differences in melanoma cell behaviour secondary to MC1R variants, and suggest an alternative non-pigmentary mechanism whereby MC1R variants could modify melanoma susceptibility or progression.