The role of alpha-melanocyte-stimulating hormone in cutaneous biology

alpha-Melanocyte stimulating hormone is a neuroimmunomodulating peptide that was recently detected in many non-pituitary tissues including the skin. Accordingly, epidermal cells such as keratinocytes and melanocytes (as well as dermal cells such as fibroblasts and endothelial cells), after stimulation with pro-inflammatory cytokines or UV light, synthesize, and release alpha MSH. The effects of these peptides are mediated through specific melanocortin (MC) receptors that can be detected on immunocompetent and inflammatory cells as well as on keratinocytes, melanocytes, fibroblasts, and endothelial cells. In addition to its well known pigment-inducing capacity, alpha MSH is able to modulate keratinocyte proliferation and differentiation. Endothelial cell and fibroblast cytokine production and fibroblast collagenase production are also regulated by alpha MSH. The immunosuppressive capacity of alpha MSH is mediated mainly through its effects on monocyte and macrophage functions. Accordingly, alpha MSH downregulates the production of pro-inflammatory cytokines and accessory molecules on antigen-presenting cells. The production of suppressor factors such as IL-10, however, is upregulated by alpha MSH. The in vivo relevance of these data is documented by the finding that systemic application of alpha MSH inhibits the induction and the elicitation of murine contact hyper-sensitivity and induces hapten-specific tolerance. These findings indicate that alpha MSH is part of the mediator network that regulates cutaneous inflammation and hyper-proliferative skin diseases.