Expression of melanocortin‐1 receptor in normal, malformed and neoplastic skin glands and hair follicles

Abstract: Melanocortin receptors (MC‐Rs) are G‐protein coupled receptors that mediate pleiotropic actions of melanocyte‐stimulating hormones and adrenocorticotropin. There is increasing evidence that one of the five so far identified melanocortin receptors, i.e. melanocortin‐1 receptor (MC‐1R), has a more ubiquitous distribution in the skin than originally expected. In the present study, the expression of MC‐1R in normal skin glands and hair follicles, various malformations and neoplasms with adnexal differentiation is described. Using an anti‐MC‐1R antibody directed against the amino acids 2–18 of the human MC‐1R, specimens of normal healthy skin (n = 10) as well as hamartomas, cysts, hyperplasias, and benign or malignant neoplasms with eccrine, apocrine, sebaceous gland, and hair follicle differentiation (n = 98) were immunostained. MC‐1R expression was widely preserved in various adnexal malformations and neoplasms as compared with normal skin and did not show major differences with regard to maturation of the neoplasms. The majority of adnexal epithelia showed an intracytoplasmically granular staining and, to a lesser extent, an intercellular staining pattern. Immunoelectron microscopical investigations revealed expression of MC‐1R both along the cell surface and intracytoplasmically within tubular endosomes, the latter suggesting internalisation of the receptor. In conclusion, preserved MC‐1R expression in adnexal epithelia suggests a functional role of proopiomelanocortin (POMC) in various malformations and neoplasms of the skin.