Immunoreactive α‐Melanotropin as an Autocrine Effector in Human Melanoma Cells

Abstract

Melanotropin is a peptide having several functions, including the stimulation of melanogenesis and the modulation of proliferation of melanocytes and melanoma cells. It acts through binding to high-affinity receptors of the melanocortin-1 subtype, exclusively expressed in cells of the melanocytic lineage.Elevated levels of immunoreactive a-melanotropin were previously reported in melanoma cell lines, tumours and plasma from patients with melanoma. Here, we show that this high ectopic production of melanotropin is restricted to melanoma and non-pituitary tumours with the same neuroectodermic origin.The occurrence of a melanotropin-specific autocrine loop was further investigated in human melanoma cells. Immunoreactive a-melanotropin was spontaneously released from a melanoma cell line (HBL) expressing melanotropin receptors on the cell surface. This release was significantly increased in the presence of melanotropin-related peptides such as corticotropin-(4-10)-peptide and P-melanotropin, competing for binding to the melanotropin receptor and was directly correlated to the displacement potential of these peptides. Both spontaneous and induced releases of immunoreactive a-melanotropin could be blocked at low temperatures, suggesting the involvement of intracellular protein movement in the release mechanism.The release of immunoreactive a-melanotropin was not significant in melanoma cells expressing very low levels of melanotropin receptors (IGR3) or in non-melanoma cells (SCCI). However, upon expression of the melanocortin-I receptor cDNA into IGR3 cells, spontaneous and competition-induced releases of immunoreactive a-melanotropin were both increased and also blocked at low temperatures. This observation further underlines a role for the melanotropin receptor in the release of immunoreactive a-melanotropin. These experiments indicate that an autocrine loop between the melanocortin-1 receptor and immunoreactive a-melanotropin may be functional in human melanoma cells.

Agonist‐Independent, High Constitutive Activity of the Human Melanocortin 1 Receptor

“…The amplification product was cloned, and its sequence was found identical to the one reported by others (GenBank accession numbers , , , ). This shows that mouse melanoma cells do express the Pomc gene, consistent with the occurrence of immunoreactive and biologically active melanocortin‐like peptides in melanoma cells and normal melanocytes (38). In a second series of experiments, the POMC transcript was amplified by RT‐PCR using a panel of human melanocytes, giant congenital nevus cells, melanoma surgical specimens and melanoma cell lines.…”

Rate Limiting Factors in Melanocortin 1 Receptor Signalling Throughthe cAMP Pathway

“…Another interesting aspect is that there was a trend towards increased basal cAMP levels, in the absence of any stimulation by exogenous ligands, in those clones that showed an increased responsiveness. This suggests that MC1R might display constitutive activity, or that an autocrine stimulatory loop could be operating in these cells, as a result of the expression of MSH‐related peptides by melanoma cells (30). Obviously, both possibilities are not mutually exclusive.…”

“…Moreover, overexpression of MC1R in human melanoma cells when compared with normal melanocytes may lead to higher cAMP levels in malignant melanocytes. Together with the occurrence of an autocrine stimulatory loop (30), this could account, at least partially, for the well known lack of requirement of cAMP elevating agents for melanoma cell proliferation when compared with the stringent requirement observed in normal melanocytes (32). However, a preliminary analysis of phenotypic effects of MC1R overexpression, particularly on basal pigmentation or on the levels of the melanogenic enzymatic activities, did not yield significant results.…”

The Role of Melanocortins in Skin Homeostasis

“…Regarding pigment lesions immunoreactivity of α-MSH, ACTH and β-ED was more intense and diffuse in samples of nodular melanoma, vertically growing acral lentiginous melanoma, superficial spreading melanoma and metastatic melanomas compared to nevi. The amount of α-MSH as determined by RIA was found to be markedly elevated in samples of cutaneous melanomas and lymph node metastases as compared to normal human skin [71]. Increased production of α-MSH by melanomas may explain elevated α-MSH serum levels of α-MSH that correlate with the clinical stage of the disease and the tumor depth [72].…”