Translational control mechanisms in cutaneous malignant melanoma: the role of eIF2α.

BACKGROUND: Melanoma cells develop adaptive responses in order to cope with particular conditions of tumor microenvironment, characterized by stress conditions and deregulated proliferation. Recently, the interplay between the stress response and the gene expression programs leading to metastatic spread has been reported. METHODS: We evaluated levels and localization of eIF2α/peIF2α in V600BRAF and wtBRAF metastatic melanoma cell lines by means of western blot and confocal microscopy analyses. Furthermore, we performed a sequence analyses and structure and dynamics studies of eIF2α protein to reveal the role of eIF2α and its correlations in different pathways involved in the invasive phase of melanoma. RESULTS: We found peIF2α both in cytoplasm and nucleus. Nuclear localization was more represented in V600BRAF melanoma cell lines. Our studies on eIF2α protein sequence indicated the presence of a predicted bipartite NLS as well as a nuclear export signal NES and an S1 domain, typical of RNA interacting proteins. Furthermore, we found high levels of transcription factor EB (TFEB), a component of the MiT/TFE family, and low ß-catenin levels in V600BRAF cells. CONCLUSIONS: Based on our results, we suggest that peIF2α nuclear localization can be crucial in ER stress response and in driving the metastatic spread of melanoma, through lysosomal signaling and Wnt/ß-catenin pathway. In conclusion, this is the first evidence of nuclear localization of peIF2α, representing a possible target for future therapeutic approaches for metastatic melanoma.

New insight into the role of metabolic reprogramming in melanoma cells harboring BRAF mutations.

This study explores the V600BRAF-MITF-PGC-1α axis and compares metabolic and functional changes occurring in primary and metastatic V600BRAF melanoma cell lines. V600BRAF mutations in homo/heterozygosis were found to be correlated to high levels of pERK, to downregulate PGC-1α/ß, MITF and tyrosinase activity, resulting in a reduced melanin synthesis as compared to BRAFwt melanoma cells. In this scenario, V600BRAF switches on a metabolic reprogramming in melanoma, leading to a decreased OXPHOS activity and increased glycolytic ATP, lactate, HIF-1α and MCT4 levels. Furthermore, the induction of autophagy and the presence of ER stress markers in V600BRAF metastatic melanoma cells suggest that metabolic adaptations of these cells occur as compensatory survival mechanisms. For the first time, we underline the role of peIF2α as an important marker of metastatic behaviour in melanoma. Our results suggest the hypothesis that inhibition of the glycolytic pathway, inactivation of peIF2α and a reduction of basal autophagy could be suitable targets for novel combination therapies in a specific subgroup of metastatic melanoma.

Melanogenesis in visceral tissues of Salmo salar. A link between immunity and pigment production?

Melanogenesis is mostly studied in melanocytes and melanoma cells, but much less is known about other pigment cell systems. Liver, spleen, kidney, and other organs of lower vertebrates harbour a visceral pigment cell system with an embryonic origin that differs from that of melanocytes. In teleosts, melanin-containing cells occur in the reticulo-endothelial system and are mainly in the kidney and spleen. The Atlantic salmon (Salmo salar L.) is an ichthyic breeding species of considerable economic importance. The accumulation of pigments in salmon visceral organs and musculature adversely affects the quality of fish products and is a problem for the aquaculture industry. With the aim to reveal novel functions and behaviour of the salmonid extracutaneous pigment system, we investigated aspects of the melanogenic systems in the tissues of Atlantic salmon, as well as in SHK-1 cells, which is a long-term cell line derived from macrophages of the Atlantic salmon head-kidney. We demonstrate that a melanogenic system is present in SHK-1 cells, head-kidney, and spleen tissues. As teleosts lack lymph nodes and Peyer's patches, the head-kidney and spleen are regarded as the most important secondary lymphoid organs. The detection of tyrosinase activity in lymphoid organs indicates that a link exists between the extracutaneous pigmentary system and the immune system in salmon.

Molecular and ultrastructural analysis of a multiphasic oral malignant melanoma.

Melanomas of the oral cavity are extremely rare. Their rarity and their independence on exposure to UV radiation make them particularly interesting. The authors analyzed an oral multiphasic melanoma composed by a nodular nonpigmented ulcerated central region, a nodular ulcerated pigmented area, a pigmented nonulcerated region, and an area similar to a dysplastic nevus. They determined the expression of some genes involved in the differentiation and cellular transformation in morphologically different regions of melanoma. All these areas were also analyzed by electron microscopy. The various regions composing the melanoma expressed genes involved in melanogenesis and melanoma progression in a different manner. Electron microscopy observation of ultrathin sections of each region evidenced ultrastructural differences, being the cellular architecture more compromised in the most aggressive parts of the neoplasm. This pilot study identified morphological, molecular, and ultrastructural differences that characterize each region of the multiphasic melanoma.

Trop-2 cleavage by ADAM10 is an activator switch for cancer growth and metastasis.

Trop-2 is a transmembrane signal transducer that can induce cancer growth. Using antibody targeting and N-terminal Edman degradation, we show here that Trop-2 undergoes cleavage in the first thyroglobulin domain loop of its extracellular region, between residues R87 and T88. Molecular modeling indicated that this cleavage induces a profound rearrangement of the Trop-2 structure, which suggested a deep impact on its biological function. No Trop-2 cleavage was detected in normal human tissues, whereas most tumors showed Trop-2 cleavage, including skin, ovary, colon, and breast cancers. Coimmunoprecipitation and mass spectrometry analysis revealed that ADAM10 physically interacts with Trop-2. Immunofluorescence/confocal time-lapse microscopy revealed that the two molecules broadly colocalize at the cell membrane. We show that ADAM10 inhibitors, siRNAs and shRNAs abolish the processing of Trop-2, which indicates that ADAM10 is an effector protease. Proteolysis of Trop-2 at R87-T88 triggered cancer cell growth both in vitro and in vivo. A corresponding role was shown for metastatic spreading of colon cancer, as the R87A-T88A Trop-2 mutant abolished xenotransplant metastatic dissemination. Activatory proteolysis of Trop-2 was recapitulated in primary human breast cancers. Together with the prognostic impact of Trop-2 and ADAM10 on cancers of the skin, ovary, colon, lung, and pancreas, these data indicate a driving role of this activatory cleavage of Trop-2 on malignant progression of tumors.

Mechanism of dimerization of the human melanocortin 1 receptor.

The melanocortin 1 receptor (MC1R) is a dimeric G protein-coupled receptor expressed in melanocytes, where it regulates the amount and type of melanins produced and determines the tanning response to ultraviolet radiation. We have studied the mechanisms of MC1R dimerization. Normal dimerization of a deleted mutant lacking the seventh transmembrane fragment and the C-terminal cytosolic extension excluded coiled-coil interactions as the basis of dimerization. Conversely, the electrophoretic pattern of wild type receptor and several Cys-->Ala mutants showed that four disulfide bonds are established between the monomers. Disruption of any of these bonds abolished MC1R function, but only the one involving Cys35 was essential for traffic to the plasma membrane. A quadruple Cys35-267-273-275Ala mutant migrating as a monomer in SDS-PAGE in the absence of reducing agents was able to dimerize with WT, suggesting that in addition to disulfide bond formation, dimerization involves non-covalent interactions, likely of domain swap type.

Role of G protein-coupled receptor kinases in the homologous desensitization of the human and mouse melanocortin 1 receptors.

The melanocortin 1 receptor, a G protein-coupled receptor positively coupled to adenylyl cyclase, is a key regulator of epidermal melanocyte proliferation and differentiation and a determinant of human skin phototype and skin cancer risk. Despite its potential importance for regulation of pigmentation, no information is available on homologous desensitization of this receptor. We found that the human melanocortin 1 receptor (MC1R) and its mouse ortholog (Mc1r) undergo homologous desensitization in melanoma cells. Desensitization is not dependent on protein kinase A, protein kinase C, calcium mobilization, or MAPKs, but is agonist dose-dependent. Both melanoma cells and normal melanocytes express two members of the G protein-coupled receptor kinase (GRK) family, GRK2 and GRK6. Cotransfection of the receptor and GRK2 or GRK6 genes in heterologous cells demonstrated that GRK2 and GRK6 impair agonist-dependent signaling by MC1R or Mc1r. However, GRK6, but not GRK2, was able to inhibit MC1R agonist-independent constitutive signaling. Expression of a dominant negative GRK2 mutant in melanoma cells increased their cAMP response to agonists. Agonist-stimulated cAMP production decreased in melanoma cells enriched with GRK6 after stable transfection. Therefore, GRK2 and GRK6 seem to be key regulators of melanocortin 1 receptor signaling and may be important determinants of skin pigmentation.

Novel ß-HPV49 Transgenic Mouse Model of Upper Digestive Tract Cancer.

The beta genus of human papillomaviruses (ß-HPV) includes approximately 50 different viral types that are subdivided into five species (ß-1 through ß-5). Nonmelanoma cancers may involve some ß-1 and ß-2 HPV types, but the biology of most ß-HPV types and their possible connections to human disease are still little characterized. In this study, we studied the effects of ß-3 type HPV49 in a novel transgenic (Tg) mouse model, using a cytokeratin K14 promoter to drive expression of the E6 and E7 genes from this virus in the basal skin epidermis and the mucosal epithelia of the digestive tract (K14 HPV49 E6/E7-Tg mice). Viral oncogene expression only marginally increased cellular proliferation in the epidermis of Tg animals, compared with wild-type littermates, and we observed no spontaneous tumor formation during their entire lifespan. However, we found that K14 HPV49 E6/E7-Tg mice were highly susceptible to upper digestive tract carcinogenesis upon initiation with 4-nitroquinoline 1-oxide (4NQO). This was a selective effect, as the same mice did not exhibit any skin lesions after chronic UV irradiation. Opposite results were observed in an analogous Tg model expressing the ß-2 HPV38 E6 and E7 oncogenes at the same anatomic sites. While these mice were highly susceptible to UV-induced skin carcinogenesis, as previously shown, they were little affected by 4NQO treatment. Overall, our findings highlight important differences in the biologic properties of certain ß-type HPV that affect their impact on carcinogenesis in an anatomic site-specific manner. Cancer Res; 76(14); 4216-25. ©2016 AACR.

Melanocortin 1 receptor mutations impact differentially on signalling to the cAMP and the ERK mitogen-activated protein kinase pathways.

Melanocortin 1 receptor (MC1R), a Gs protein-coupled receptor expressed in melanocytes, is a major determinant of skin pigmentation, phototype and cancer risk. MC1R activates cAMP and mitogen-activated protein kinase ERK1/ERK2 signalling. When expressed in rat pheochromocytoma cell line cells, the R151C, R160W and D294H MC1R variants associated with melanoma and impaired cAMP signalling mediated ERK activation and ERK-dependent, agonist-induced neurite outgrowth comparable with wild-type. Dose-response curves for ERK activation and cAMP production indicated higher sensitivity of the ERK response. Thus, the melanoma-associated MC1R mutations impact differently on cAMP and ERK signalling, suggesting that cAMP is not responsible for functional coupling of MC1R to the ERK cascade.

Molecular cloning and biochemical characterization of the skin tyrosinase from Rana esculenta L.

Amphibian tyrosinases display unique and poorly understood properties such as seasonal activity variations, different activities in dorsal and ventral skin and the occurrence as inactive forms requiring proteolytic activation. For the first time we have sequenced and characterized Rana esculenta L. tyrosinase by functional expression of the cloned cDNA, and compared it with frog skin extracts. R. esculenta tyrosinase ORF is well conserved compared with tyrosinases of various sources. The amino acid similarities between the tyrosinases from R. esculenta and other amphibia range from 85% to 98%. Homology remains high with mammalian tyrosinases (65% identity with Homo sapiens, and 63% with Mus musculus) and with bird orthologues (66% identity with Gallus gallus). Tyrosinase was expressed in HEK293T cells as an active enzyme. Activity staining on non reducing SDS-PAGE revealed two bands around 63 and 68 kDa. R. esculenta skin extracts were mildly active and reached maximal activity upon protease treatment, revealing a high molecular weight dopa-positive band in the 200 kDa range and one of higher MW, after nagarse treatment, in activity stainings. The different behaviour of recombinant tyrosinase compared to skin extracts suggests formation in vivo of a multimeric complex.

Trop-1 are conserved growth stimulatory molecules that mark early stages of tumor progression.

BACKGROUND: Trop-1 is a cell-cell adhesion regulatory molecule that is overexpressed by a large fraction of tumors in man. METHODS: To identify fundamental, conserved functional features of Trop-1 in transformed cells, a search was performed for evolutionarily conserved structure, expression patterns, and function by gene cloning, DNA array and serial analysis of gene expression (SAGE), Northern and Western blotting, flow cytometry, and immunohistochemistry of sequential stages of tumor progression in experimental systems and in man. RESULTS: TROP1 genes demonstrate conserved structure and promoter regions with parallel expression patterns (high expression in the small intestine and colon; lower expression in prostate, thyroid, salivary glands, breast, kidney, lung, liver, and spleen; very low levels in skin and stomach; no expression in heart, muscle, and brain). Progenitor cells of different tissues were shown to express Trop-1. Hence, the expression and functional role of Trop-1 were analyzed at successive stages of tumor progression in vitro and in vivo. The findings show that Trop-1 is expressed at early stages of tumor development, eg, in dysplastic lesions and immortalized cells, is sufficient to stimulate cell growth of expressing transformed cells, and is required for tumor growth in vivo. CONCLUSIONS: The findings identify Trop-1 as a novel determinant of cell growth at early stages of tumor development and as a marker of early stages of development in normal tissues and in cancer, making this molecule a candidate for novel diagnostic and therapeutic procedures.

Melanogenic response of the Kupffer cells of Rana esculenta L to melanocyte stimulating hormone.

We have previously shown that Kupffer cells (KCs) of Rana esculenta L. possess melanogenic ability. The melanogenic enzyme activities in these cells are different from those described in skin melanocytes, and very little is known about their regulation by extracellular signalling molecules. In order to study this regulation, we analysed the effects of NDP-MSH on the levels of expression of the tyrosinase gene and on dopa-oxidase activity, using primary cultures of KCs. Incubation of the cells with NDP-MSH increases tyrosinase gene transcription, within the first 24 h of stimulation. To gain insight into the signalling mechanism involved in the cell response to the hormone, KCs in culture were incubated with IBMX or forskolin. These agents mimic the effects of alpha-MSH on melanocytes by increasing the intracellular level of cAMP. The experimental results showed that while the hormonal treatment always activated the KC tyrosinase system, treatment with IBMX or forskolin never did. Therefore, in KCs the tyrosinase-stimulating action of NDP-MSH was not mimicked by cAMP elevating agents. Assays of cAMP levels in cells stimulated with NDP-MSH demonstrated that the hormone does not produce significant increases in intracellular cAMP. On the contrary, forskolin produced significant increases in cAMP starting from 30 min of incubation. These results suggest that tyrosinase induction by melanocortins in KCs is not mediated by the cAMP pathway, and highlight the existence of substantial differences in the hormone signal transduction mechanisms between amphibian KCs and melanocytes or melanoma cells.