MSH receptors and the response of human A375 melanoma cells to interleukin-1 beta

Synthesis and characterization of a melanoma-targeted fluorescence imaging probe by conjugation of a melanocortin 1 receptor (MC1R) specific ligand

The incidence of malignant melanoma is rising more rapidly than that of any other cancer in the United States. The melanocortin 1 receptor (MC1R) is overexpressed in most human melanoma metastases, thus making it a promising target for imaging and therapy of melanomas. We have previously reported the development of a peptidomimetic ligand with high specificity and affinity for MC1R. Here, we have conjugated near-infrared fluorescent dyes to the C-terminus of this ligand via lysine-mercaptopropionic acid linkers to generate MC1R specific optical probes (MC1RL-800, 0.4 nM K(i); and MC1RL-Cy5, 0.3 nM K(i)). Internalization of the imaging probe was studied in vitro by fluorescence microscopy using engineered A375/MC1R cells and B16F10 cells with endogenous MC1R expression. The in vivo tumor targeting of MC1RL-800 was evaluated by intravenous injection of probe into nude mice bearing bilateral subcutaneous A375 xenograft tumors with low MC1R expression and engineered A375/MC1R tumors with high receptor expression. Melanotic B16F10 xenografts were also studied. Fluorescence imaging showed that the agent has higher uptake values in tumors with high expression compared to low (p < 0.05), demonstrating the effect of expression levels on image contrast-to-noise. In addition, tumor uptake was significantly blocked by coinjection of excess NDP-α-MSH peptide (p < 0.05). In conclusion, the MC1R-specific imaging probe developed in this study displays excellent potential for the intraoperative detection of regional node involvement and for margin detection during melanoma metastasis resection.

Anti-inflammatory and anti-invasive effects of alpha-melanocyte-stimulating hormone in human melanoma cells

α-Melanocyte stimulating hormone (α-MSH) is known to have pleiotrophic functions including pigmentary, anti-inflammatory, antipyretic and immunoregulatory roles in the mammalian body. It is also reported to influence melanoma invasion with levels of α-, β- and γ-MSH correlated clinically with malignant melanoma development, but other studies suggest α-MSH acts to retard invasion. In the present study, we investigated the action of α-MSH on three human melanoma cell lines (HBL, A375-SM and C8161) differing in metastatic potential. α-melanocyte-simulating hormone reduced invasion through fibronectin and also through a human reconstructed skin composite model for the HBL line, and inhibited proinflammatory cytokine-stimulated activation of the NF-κB transcription factor. However, A375-SM and C8161 cells did not respond to α-MSH. Immunofluorescent microscopy and Western blotting identified melanocortin-1 receptor (MC-1R) expression for all three lines and MC-2R on HBL and A375-SM lines. Receptor binding identified a similar affinity for α-MSH for all three lines with the highest number of binding sites on HBL cells. Only the HBL melanoma line demonstrated a detectable cyclic adenosine monophosphate (cAMP) response to α-MSH, although all three lines responded to acute α-MSH addition (+(−)-N⁶-(2-phenylisopropyl)-adenosine (PIA)) with an elevation in intracellular calcium. The nonresponsive lines displayed MC-1R polymorphisms (C8161, Arg (wt) 151/Cys 151; A375-SM, homozygous Cys 151), whereas the HBL line was wild type. Stable transfection of the C8161 line with wild-type MC-1R produced cells whose invasion was significantly inhibited by α-MSH. From this data, we conclude that α-MSH can reduce melanoma cell invasion and protect cells against proinflammatory cytokine attack in cells with the wild-type receptor (HBL).

Evidence for expression of melanocortin-1 receptor in human sebocytes in vitro and in situ

Many lines of evidence indicate that the activity of sebaceous glands can be modulated by neuropeptides. Direct evidence in man, however, is still missing. We show that SZ95 sebocytes, an immortalized human sebaceous gland cell line, express receptors for alpha-melanocyte-stimulating hormone. Reverse transcription polymerase chain reaction with primers against the five melanocortin receptors and immunofluorescence studies using an antibody directed against a peptide corresponding to the amino acids 2-18 of the human melanocortin-1 receptor disclosed specific transcripts and immunoreactivity for melanocortin-1 receptor in these cells. Melanocortin-1 receptor expression was confirmed in sebocytes of normal human skin by immunohistochemistry. In contrast, no immunostaining for the melanocortin-5 receptor could be detected in sebocytes in situ, in accordance with the lack of specific transcripts for this melanocortin receptor in SZ95 sebocytes. As cytokines play an important role in the recruitment of inflammatory cells in acne and related disorders and alpha-melanocyte-stimulating hormone exerts immunomodulatory effects in many other cell types, we investigated the effect of alpha-melanocyte-stimulating hormone on interleukin-8 secretion by SZ95 sebocytes. Treatment with interleukin-1beta resulted in a marked increase in interleukin-8 release that was partially blocked by coincubation with alpha-melanocyte-stimulating hormone in a dose-dependent manner. Taken together, we show here that the melanocortin-1 receptor is expressed in vitro and in situ in human sebocytes. By modulating interleukin-8 secretion, alpha-melanocyte-stimulating hormone may act as a modulator of inflammatory responses in the pilosebaceous unit.

Alpha-melanocyte-stimulating hormone inhibits NF-kappaB activation in human melanocytes and melanoma cells

Alpha-melanocyte-stimulating hormone is produced by several different cell types including neural cells, endothelial cells, monocytes, and keratinocytes. A biologic role in melanocyte pigmentation is widely recognized, but more recent studies describe a part in modulating inflammatory and immune responses. The aim of the this study was to investigate the mechanism by which alpha-melanocyte-stimulating hormone antagonizes proinflammatory cytokine action. We report that alpha-melanocyte-stimulating hormone (10-9 M) was effective in opposing a tumor necrosis factor-alpha stimulated increase in NF-kappaB DNA binding activity in: (i) normal ocular melanocytes; (ii) cells cultured from ocular melanoma tumors; and (iii) two cutaneous melanoma cell lines. NF-kappaB is activated by many inflammatory mediators and controls transcription of genes required for immune and inflammatory responses. The transcription factor complex was positively identified as the p50/p65 heterodimer, recognized to have transcriptional activating potential. Maximum reduction of NF-kappaB DNA binding activity with alpha-melanocyte-stimulating hormone was detected 2 h after cellular stimulation and varied from between 53% and 18% depending on cell type. Whereas the acute inhibitory effects could be mimicked by elevating cyclic adenosine monophosphate, alpha-melanocyte-stimulating hormone was not found to have any effect on the relative level of IkappaBalpha protein expression over 24 h. These data show that alpha-melanocyte-stimulating hormone has a pronounced effect on NF-kappaB activity in melanocytes and melanoma cells, identifying a specific dimeric complex, and suggest this to be a key pathway by which immunomodulation/anti-inflammation may operate. The results may also be considered in the broader context of general inflammatory pathologies concerning cells which express alpha-melanocyte-stimulating hormone receptors and utilize the NF-kappaB signaling pathway.