Proopiomelanocortin production by epidermal cells: evidence for an immune neuroendocrine network in the epidermis.

Proopiomelanocortin (POMC) is known to be synthesized in the pituitary gland and is subsequently cleaved by specific prohormone convertases into biologically active peptide hormones such as melanocyte stimulating hormones (MSH), adrenocorticotropin (ACTH) and endorphins (EP). Guanine nucleotide-binding protein (G-protein)-coupled receptors, which have only recently been discovered, are involved in the transmission of their message. There is also evidence indicating that POMC is not only produced by pituitary cells but is an ubiquitous molecule, that is cleaved cell- and tissue-specific. It has also been shown that the epidermis keratinocytes as well as melanocytes express POMC upon stimulation and release alpha MSH and ACTH. In addition to their function as hormones, POMC peptides have been shown to exert a variety of immunoregulatory effects by modulating the function of immunocompetent cells as well as cytokines. These findings provide further evidence for the immunoneuroendocrine network playing a crucial role during the pathogenesis of immune and inflammatory skin disease.