Signaling Mechanisms Controlled by Melanocortins in Melanoma, Lacrimal, and Brain Astroglial Cells (original) (raw)

1993, Annals of the New York Academy of Sciences

SALOMON et af.: SIGNALING MECHANISMS 365 12-16 amino acids, respectively, and arise in various mammalian tissues along with other bioactive peptides by posttranscriptional processing from the 3 1-36-kDa POMC precursor. The melanocortin peptides contain a consensus 7 amino acid sequence flanked by peptide-specific sequences of considerable homology on both the Nand C-terminal ends.'O Other common features of these peptides relate to (1) their unique dependence on extraceflular calcium required for interaction with their respective receptor molecules2' and (2) their ability to regulate target cell function via G-p r~t e i n s~~*~~ and adenylate cyclase (AC),20724 as well as other signaling pathways as further discussed below. A requirement of extracellular Ca2+ ions for the activity of ACTH was first noted in the early 50s by Birmingham and associates,2s studying ACTH-induced steroidogenesis in rat adrenal cortical tissue. ACTH stimulation of lipolysis was also shown to be Ca2+ dependent.26 Understanding of this phenomenon increased when Birnbaumer and Rodbell demonstrated that AC in fat cells serves as the common effector enzyme for several lipolytic hormones (epinephrine, glucagon, ACTH).*' Among these hormones, only ACTH appeared to require Ca2+ ions and its stimulatory effect was reduced to basal levels by EGTA. The difficulty of obtaining biologically active radiolabeled ACTH deferred further progress in the analysis of this phenomenon until, in 1985, Cheitlin and his associates first synthesized [1251][Tyr23 Phe2 Nle41ACTH-(1-38)-a derivative that maintained full biological activity.28 Binding of this peptide to adrenal glomerulosa cells required physiological calcium concentration, Furthermore, transfer of the cells to Ca2+-free medium induced the release of the receptor-bound hormone. Studies performed in lower vertebrates on the stimulation of skin melanophores, in which MSH induces melanosome dispersion, also revealed that the response to this hormone required extracellular calcium.20~29*30 Pigment dispersion could, however, be induced in the absence of calcium by exogenous cyclic AMP (CAMP), dibutyryl cAMP or other agents, such as theophylline, that nonspecifically raise intracellular CAMP."-^^ These observations, therefore, suggested that the elevation of cAMP and, hence, the cellular response depend on extracellular calcium only when stimulated by MSH. In experiments performed with photoreactive a-MSH, De Graan et al. noted that covalent cross-linking of MSH to the MSH receptor (MSH-R), in the presence but not in the absence of calcium, resulted in persistent activation of amphibian m e l a n o p h~r e s .~~-~~ However, removal of calcium reversed the stimulation, presumably while maintaining the active covalently cross-linked receptor-MSH complex. This was inferred since replenishment of calcium in MSH-free medium restored the cellular response. These authors, therefore, suggested a dual role for Ca2+ ions in the amphibian melanocyte, at the receptor level and at sites in the MSH-responsive pathways that are distal to the MSH-R itself. In this short review, we will discuss our recent work on MSH receptors in melanoma, lacrimal, and rat brain astrocytes, tissues in which MSH or ACTH appears to control rather different cellular responses. Furthermore, the various cell-specific responses seem to be mediated by different signaling mechanisms. M2R MOUSE MELANOMA The Calcium Dependence of MSH Receptor Function in Mouse Melanoma To further understand the role of Ca2+ in MSH-R function, particularly in mammalian systems, the mouse melanoma M2R cell line was chosen as a model.