Growth regulation in normal and malignant melanocytes

Induction of constitutive melanogenesis in amelanotic mouse melanoma cells by transfection of the human melanocortin-1 receptor gene

The human melanocortin-1 (MC1) receptor was stably expressed in the amelanotic mouse melanoma cell clone B16-G4F which does not express its own (mouse) MC1 receptor and hence is unresponsive to alpha melanocyte stimulating hormone (alpha MSH). From several stable transfectant cell lines expressing the human MC1 receptor in relatively high numbers, three melanin producing clones (G4F-12, 14, and 15) and one amelanotic clone (G4F-7) were further analyzed in competition binding experiments and in cAMP and melanin assays. The dissociation constants (KD) for [Nle4, D-Phe7]-alpha MSH in all four clones ranged from 0.187 to 0.705 nmol/l, thus corresponding to the KD observed with the different human melanoma cell lines so far studied. Intracellular cAMP content was 3- to 5-fold higher than that of control cells, and alpha MSH induced an additional 1.5- to 1.7-fold increase. G4F-15 cells secreted melanin into the medium whereas the other clones did not secrete melanin. The extent of melanin secretion was similar to that of fully alpha MSH-stimulated B16-F1 mouse melanoma cells but the onset of secretion was delayed. alpha MSH induced an additional dose-related increase (up to 1.3-fold) in melanin production which could be suppressed by the addition of specific alpha MSH antibodies without altering the constitutive part of melanogenesis. Human and mouse agouti proteins, which inhibit basal and alpha MSH-induced melanogenesis in B16-F1 cells, both reduced alpha MSH-induced melanin production in G4F-15 cells but did not affect the constitutive melanogenesis. These results indicate that human MC1 receptor expressed in mouse B16-G4F cells induces constitutive activation of the signalling pathway controlling melanogenesis, most likely by tightly coupling to Gs alpha, in a similar manner to that reported for constitutively active receptor mutants in other systems.

Effects of AGM-1470 and pentosan polysulphate on tumorigenicity and metastasis of FGF-transfected MCF-7 cells

Previously, we described FGF-1- or FGF-4-transfected MCF-7 breast carcinoma cells which are tumorigenic and metastatic in untreated or tamoxifen-treated ovariectomised nude mice. In this study, we have assessed the effects of AGM-1470, an antiangiogenic agent, and pentosan polysulphate (PPS), an agent that abrogates the effects of FGFs, on tumour growth and metastasis produced by these FGF-transfected MCF-7 cells. Untreated or tamoxifen-treated ovariectomised mice were injected with FGF-transfected cells, treated with AGM-1470 or PPS, and tumour growth and metastasis analysed. The sensitivity of FGF-transfected and parental MCF-7 cells to AGM-1470 or PPS was also determined in vitro. Both AGM-1470 and PPS inhibited tumour growth in otherwise untreated or tamoxifen-treated mice injected with either FGF- or FGF-4-transfected MCF-7 cells. This effect was more reliably seen in tamoxifen-treated animals. AGM-1470 was about 10(5) times less potent in inhibiting the anchorage-dependent growth of parental MCF-7 or FGF-transfected MCF-7 cells than in inhibiting the growth of human umbilical vein endothelial cells. PPS did not affect the in vitro growth of the transfectants or parental cells. Thus, the growth-inhibitory effect on tumours was in excess of the effect of either drug on the same cells in tissue culture, implying that stromal elements are important determinants of the effects of these drugs. There was a positive correlation between tumour size and the extent of proximal lymph node metastasis. However, neither drug had a significant effect on the extent of metastasis to proximal or distal lymph nodes or lungs. AGM-1470 or PPS may be helpful in cases of breast carcinoma in which angiogenesis is due to expression of FGFs by the tumour cells and may be more effective when combined with tamoxifen.

Genetics of Cutaneous Malignant Melanoma

An increase in the clinical significance of cutaneous malignant melanoma has paralleled a dramatic increase in the rate of death from this disease. The critical genetic changes associated with the genesis and progression of this disease are only beginning to be identified. This review highlights genetic changes in cutaneous melanoma and discusses the genetics of predisposition, cytogenetics, changes in proto-oncogenes and oncogenes, and evidence for the role of tumor suppressor genes in this malignancy. The viewpoint of this article is that malignant melanoma is a genetic disease.

Melanocortins and their implication in melanoma

The melanocortins (MCs), that is, the melanocyte-stimulating hormones (MSHs) and ACTH, are a group of related peptides containing the typical melanotropin core sequence, His-Phe-Arg-Trp, and are derived from a common precursor, pro-opiomelanocortin. They are pleiotropic molecules that occur in the pituitary, some brain regions, and also in several peripheral tissues, and they exert a variety of physiologic functions. Their effect on melanogenesis in the skin is well established, but their role in melanocyte and melanoma cell proliferation and metastasis is less clear. The recent cloning of five types of MC receptors (MC1-5), new studies on the regulation of these receptors, the discovery of a naturally occurring MSH antagonist, the agouti protein, and the finding that melanocytes and melanoma cells exclusively express MC1 receptors have laid the basis for the future development of specific MC ligands, which may become useful for melanoma diagnosis and eventually therapy.