Application of percent labeled mitoses (PLM) analysis to the investigation of melanoma cell responsiveness to MSH stimulation throughout the cell cycle

Melanotropic peptide receptors: membrane markers of human melanoma cells

The objectives of this research were to determine whether melanotropin receptors are characteristic (constant) membrane markers of human melanoma cells. Methodologies were developed to visualize these receptors by fluorescence microscopy. Multiple copies (10-20) of both [Nle4,D-Phe7]alpha-MSH, a superpotent analog of alpha-melanocyte stimulating hormone (alpha-MSH), and a fluorophore, were conjugated to polyvinyl alcohol (PVA). Incubation in the presence of the multivalent macromolecular conjugate (FITC-PVA-MSH) resulted in binding of human epidermal melanocytes and keratinocytes and human melanoma cells (both melanotic and amelanotic) to the fluorescent conjugate. Binding of the conjugate to the cells exhibited a unique cluster pattern (capping) suggesting a receptor internalization related phenomenon. Most importantly, every cell of every melanoma cell line, melanotic or amelanotic, possessed receptors as visualized by fluorescence microscopy. Since the cells were not synchronized, some binding apparently took place during all phases of the cell cycle. Therefore, receptor expression appears not to be cell-cycle dependent. Specificity of binding of FITC-PVA-MSH was demonstrated by several studies. (i) Binding of the conjugate to melanoma cells could be blocked by prior incubation of the cells in the presence of the unconjugated hormone analog; [Nle4,D-Phe7]alpha-MSH. (ii) The macromolecular conjugate lacking bound ligand (FITC-PVA) did not bind to the melanoma cells. (iii) Another peptide, a substance-P analog, attached to the substrate (FITC-PVA-SP) failed to bind to the cells. (iv) With the exception of keratinocytes, other cells of nonmelanocyte origin (e.g., fibroblasts, spleen, liver, kidney cells, and mammary cancer cells, lung cancer cells) did not bind to the conjugate. Thus, cell-specific melanotropin receptors appear to be characteristic cell surface markers of epidermal melanocytes, keratinocytes, and melanoma cells. In several human melanoma cell lines these receptors appeared to be functional since [Nle4,D-Phe7]alpha-MSH stimulated tyrosinase activity. Fluorescent melanotropin conjugates might prove useful in determining whether all human melanoma (primary and metastatic) tumors possess such receptors. These receptors might then provide targets for melanotropic peptides for the identification, localization, and chemotherapy of melanoma.

Melanotropic peptide-conjugated beads for microscopic visualization and characterization of melanoma melanotropin receptors

We developed two solid-phase reagent systems for microscopic visualization and characterization of melanocyte-stimulating hormone (MSH) receptors of melanoma cells. Multiple copies of [Nle-4,D-Phe-7]-alpha-MSH, a potent analog of alpha-MSH, were conjugated to microspheres (latex beads) or macrospheres (polyamide beads) through a thioether or disulfide bond. Binding between the beads and mouse and human melanoma cells was examined by scanning electron microscopy and by light microscopy. Each mouse and human melanoma cell (of all cell lines) evinced binding to the beads. Binding of the melanotropin conjugates was not restricted to any one phase of the cell cycle. Specificity of binding was demonstrated by several studies. Negative controls included cell types of nonmelanocyte origin (e.g., mammary cancer cells) and beads that lacked the melanotropic ligand or had other attached ligands. Beads with a disulfide-linked melanotropin analog served as a direct control. Treatment of these beads with DTT during or before incubation of the beads with melanoma cells (resulting in release of the MSH analog from the beads) eliminated binding of the beads to melanoma cells. Binding interactions between melanoma cells and melanotropin-bound beads also could be abolished by prior incubation with unconjugated MSH analog. During these experiments, certain membrane receptor-hormone associated phenomena, such as capping (aggregation) of the receptor-ligand complex, also were observed. These results provide visual evidence that MSH receptors are a property common to melanoma cells. Normal human epidermal melanocytes and keratinocytes were also shown to express melanotropin receptors by the same criteria established for melanoma cells.

Melanocortin receptors: identification and characterization by melanotropic peptide agonists and antagonists

Hormones are chemical messengers released from cells to act on and control the activity of other cells. Hormonal ligands initiate their actions by interacting with receptive substances (Langley, 1906) of the target cells. These receptors are proteins that are either integral components of the cell membrane or are localized cytoplasmically within cells. Ligand-receptor interaction results in either the stimulation or inhibition of cellular activity. Since most hormones bind rather specifically to receptors possessed by their target cells, labeling of hormonal ligands can be utilized to identify and localize cells within an animal. In this report we discuss what is presently known about melanocortin receptors (MCRs) as studied by the use of labeled melanotropic peptide ligands.

Homologous regulation of the MSH receptor in melanoma cells

We have examined the mechanism of homologous regulation of MSH receptor binding and receptor-mediated adenylate cyclase activation in three human and two mouse melanoma cell lines. Pretreatment with alpha-MSH resulted in a time- and dose-dependent up-regulation of MSH receptors in human D10 and 205 melanoma cells whereas in human HBL and in mouse B16-F1 and Cloudman S91 cells alpha-MSH induced receptor down-regulation. Up-regulation of receptors was maximal after a 24-h incubation period and an alpha-MSH concentration of 100 nM (EC50 = 2.4 nM). The increase in alpha-MSH binding was independent of adenylate cyclase activation and protein synthesis and appeared to be caused by recruitment of spare receptors. The structural requirements of the peptide for triggering this process differed from those found in receptor-binding analyses. Receptor down-regulation was maximal after 12 h and hence more rapid than up-regulation. In B16-F1 cells, 10 nM alpha-MSH caused the disappearance of 85-90% of the MSH receptors, the EC50 of 0.23 nM lying exactly between that for alpha-MSH-induced melanogenesis (0.027 nM) and the dissociation constant of receptor binding (1.31 nM). Down-regulation in B16-F1 cells appears to be the consequence of receptor internalization following MSH binding and seems to be initiated during an early step in MSH signalling, preceding the activation of adenylate cyclase and the cAMP signal. Receptor up- and down-regulation were not accompanied by an alteration in affinity to alpha-MSH, as demonstrated by Scatchard analysis of the binding curves.

Expression of melanocyte stimulating hormone receptors correlates with mammalian pigmentation, and can be modulated by interferons

The relationship between melanogenesis and the expression of melanocyte stimulating hormone (MSH) receptors on the surface of melanocytes was examined using sublines generated from the melanotic JB/MS melanoma. JB/MS cells were propagated in long term culture to allow for phenotypic drift in their characteristics of differentiation, and then were cloned; the cloned cells ranged from well differentiated and pigmented to undifferentiated and amelanotic. Spontaneous and MSH-induced melanogenesis in these different lines was measured and correlated with the number of MSH receptors expressed. After 6 months of in vitro culture, the ability of the cells to respond to MSH was significantly reduced, as were the number of MSH receptors expressed; the cells had reduced pigmentation and were relatively undifferentiated histologically. Subsequently, clonally-derived pigmented cells were found to have numbers of surface MSH receptors (approximately 60,000 per cell) and levels of melanogenic activity similar to the original JB/MS cell line. However, an amelanotic clone had an even more dramatically reduced level of pigmentation which correlated with a further decrease in the expression of MSH receptors (less than 1,000 per cell) and the production of a potent melanogenic inhibitor. We also examined the responses of these various sublines to alpha, beta, and gamma-interferons and found significant heterogeneity in their abilities to respond to these cytokines. This study clearly shows that there is a direct correlation between melanogenesis and the expression of MSH receptors on the surface of melanocytes, and that melanogenic inhibitors may be critically involved in the regulation of mammalian pigmentation.

Quantitative cytochemical analysis by microdensitometry of spontaneous or alpha-MSH-stimulated melanogenesis in B16 melanoma cells cultivated in vitro

A spontaneous cyclic phenomenon characterized by successive waves of either high proliferative rate or intense melanogenesis is described in non-confluent B16 melanoma cells subcultivated during 2 months (or more). Dopaoxidase activity is quantified in individual cells after L-dopa reaction, by an original method of visible light absorption cytophotometry. A 24-hr treatment with alpha-MSH increases dopa-oxidase activity. This increase is also noted during the following 14 hr, in a fresh medium devoid of alpha-MSH, in which cell proliferation resumes after 24 hr. Phenylthiourea, cycloheximide or actinomycin D inhibit dopaoxidase activity, but also cell proliferation in alpha-MSH pre-stimulated cells. The effects of the two latter agents suggest that de novo synthesis of the enzyme takes place following alpha-MSH treatment.

Cell-cycle analysis of insulin binding and internalization on mouse melanoma cells

Binding of 125I-labeled insulin to the surface receptors of Cloudman S-91 mouse melanoma cells (CCL 53.1) was studied at various phases (M, G1, S, and G2) in the cell cycle. Insulin-binding activity was persistently present during the cell cycle but the highest activity was noted at the S-phase. The insulin once bound to the cell surface receptors at any phase of the cell cycle was internalized and degraded, presumably through a lysosomal pathway. Insulin-indexing activity of melanoma cells was not affected by melanocyte-stimulating hormone.