HuRdling Senescence: HuR Breaks BRAF-Induced Senescence in Melanocytes and Supports Melanoma Growth

n addition to genetic changes, post-transcriptional events strongly contribute to the progression of malignant tumors. The RNA-binding protein HuR (ELAVL1) is able to bind and stabilize a large group of target mRNAs, which contain AU-rich elements (ARE) in their 3′-untranslated region. We found HuR to be upregulated in malignant melanoma in vitro and in vivo, significantly correlating with progression in vivo. Additionally, we could show that miR-194-5p can regulate HuR expression level. HuR knockdown in melanoma cells led to the suppression of proliferation and the induction of cellular senescence. Interestingly, HuR overexpression was sufficient to inhibit senescence in BRAFV600E-expressing melanocytes and to force their growth. Here, MITF (Microphthalmia-associated transcription factor), a key player in suppressing senescence and an ARE containing transcript, is positively regulated by HuR. Our results show for the first time that the overexpression of HuR is an important part of the regulatory pathway in the development of malignant melanoma and functions as a switch to overcome oncogene-induced senescence and to support melanoma formation. These newly defined alterations may provide possibilities for innovative therapeutic approaches

Complex Formation with Monomeric α-Tubulin and Importin 13 Fosters c-Jun Protein Stability and Is Required for c-Jun’s Nuclear Translocation and Activity

Microtubules are highly dynamic structures, which consist of α- and β-tubulin heterodimers. They are essential for a number of cellular processes, including intracellular trafficking and mitosis. Tubulin-binding chemotherapeutics are used to treat different types of tumors, including malignant melanoma. The transcription factor c-Jun is a central driver of melanoma development and progression. Here, we identify the microtubule network as a main regulator of c-Jun activity. Monomeric α-tubulin fosters c-Jun protein stability by protein–protein interaction. In addition, this complex formation is necessary for c-Jun’s nuclear localization sequence binding to importin 13, and consequent nuclear import and activity of c-Jun. A reduction in monomeric α-tubulin levels by treatment with the chemotherapeutic paclitaxel resulted in a decline in the nuclear accumulation of c-Jun in melanoma cells in an experimental murine model and in patients’ tissues. These findings add important knowledge to the mechanism of the action of microtubule-targeting drugs and indicate the newly discovered regulation of c-Jun by the microtubule cytoskeleton as a novel therapeutic target for melanoma and potentially also other types of cancer

Down-regulation of CYLD expression by Snail promotes tumor progression in malignant melanoma

High malignancy and early metastasis are hallmarks of melanoma. Here, we report that the transcription factor Snail1 inhibits expression of the tumor suppressor CYLD in melanoma. As a direct consequence of CYLD repression, the protooncogene BCL-3 translocates into the nucleus and activates Cyclin D1 and N-cadherin promoters, resulting in proliferation and invasion of melanoma cells. Rescue of CYLD expression in melanoma cells reduced proliferation and invasion in vitro and tumor growth and metastasis in vivo. Analysis of a tissue microarray with primary melanomas from patients revealed an inverse correlation of Snail1 induction and loss of CYLD expression. Importantly, tumor thickness and progression-free and overall survival inversely correlated with CYLD expression. Our data suggest that Snail1-mediated suppression of CYLD plays a key role in melanoma malignancy

Learning from Embryogenesis—A Comparative Expression Analysis in Melanoblast Differentiation and Tumorigenesis Reveals miRNAs Driving Melanoma Development

Malignant melanoma is one of the most dangerous tumor types due to its high metastasis rates and a steadily increasing incidence. During tumorigenesis, the molecular processes of embryonic development, exemplified by epithelial–mesenchymal transition (EMT), are often reactivated. For melanoma development, the exact molecular differences between melanoblasts, melanocytes, and melanoma cells are not completely understood. In this study, we aimed to identify microRNAs (miRNAs) that promote melanoma tumorigenesis and progression, based on an in vitro model of normal human epidermal melanocyte (NHEM) de-differentiation into melanoblast-like cells (MBrCs). Using miRNA-sequencing and differential expression analysis, we demonstrated in this study that a majority of miRNAs have an almost equal expression level in NHEMs and MBrCs but are significantly differentially regulated in primary tumor- and metastasis-derived melanoma cell lines. Further, a target gene analysis of strongly regulated but functionally unknown miRNAs yielded the implication of those miRNAs in many important cellular pathways driving malignancy. We hypothesize that many of the miRNAs discovered in our study are key drivers of melanoma development as they account for the tumorigenic potential that differentiates melanoma cells from proliferating or migrating embryonic cells

Multipler Einfluß des Zell-Zell-Adhäsionsmoleküls E-Cadherin auf Invasion und Metastasierung des malignen Melanoms

Der Kontakt zwischen Melanozyten und Keratinozyten ist von enormer Bedeutung für die Aufrechterhaltung des geordneten Gewebeverbands der menschlichen Epidermis. Keratinozyten regulieren durch das Zell-Zell-Adhäsionsmolekül E-cadherin die Morphologie und die Proliferation von Melanozyten. Ein Verlust der E-Cadherin Expression und Neuexpression von N-Cadherin in entarteten Melanozyten konnte bereits seit langem für das große Metastasierungspotential des malignen Melanoms verantwortlich gemacht werden. Durch den Verlust des E-Cadherin Moleküls entziehen sich die veränderten Melanozyten der Adhäsions- und Wachstumskontrolle der sie umgebenden Keratinozyten. Durch Expression von N-Cadherin erlangen die transformierten Melanozyten neue Adhäsionseigenschaften zu Fibroblasten der Dermis, die sogar MMPs für transformierte Melanozyten produzieren. Melanomzellen, die kein endogenes E-Cadherin mehr produzieren, zeigten in Immunfluoreszenz und Western Blot Daten eine erhöhte beta-Catenin Ansammlung im Zytoplasma. Im Gegensatz zum Kolon- bzw. Mammakarzinom bleibt das beta-Catenin ungebunden im Zytoplasma lokalisiert und translokiert nicht in den Zellkern. Durch das zytoplasmatische beta-Catenin kommt es zu einer Aktivitätssteigerung der MAP Kinase p38. Im weiteren wurde, durch bisher noch ungeklärte Signalweiterleitungen, eine Aktivierung von NFkappaB festgestellt. Wir konnten auch zeigen, dass N-Cadherin ein neu entdecktes Zielmolekül von NFkappaB ist

Cell–cell contacts in melanoma and the tumor microenvironment

Cell-contacts are essential for intercellular communication and are involved in proliferation, differentiation, and homeostasis. Melanocytes establish multiple contacts with keratinocytes, which in turn control melanocyte growth and expression of cell surface receptors. Most melanoma arise within the epidermis (melanoma in situ) and then invade across the basement membrane. These melanoma cells escape from control by keratinocytes through five major mechanisms: (1) downregulation of receptors important for communication with keratinocytes such as E-cadherin, P-cadherin, desmoglein, and connexins; (2) upregulation of receptors and signaling molecules important for interactions between melanoma cells and other melanoma cells, fibroblasts, or endothelial cells, such as N-cadherin, Mel-CAM, and zonula occludens protein-1 (ZO-1); (3) deregulation of morphogens such as Notch receptors and their ligands; (4) loss of anchorage to the basement membrane due to altered expression of cell-matrix adhesion molecules; (5) increased expression of metalloproteinases. Melanoma depends on, interacts with and reacts to its stroma, including extracellular matrix, growth factors, cytokines, fibroblasts, endothelial cells, and immune cells. In turn, melanoma is known to produce factors that influence its environment, and may force it to alter cell-cell communication. In this chapter, we describe the alterations in cell-cell contacts in melanoma and the tumor microenvironment associated with melanoma development and progression

Amphiregulin Regulates Melanocytic Senescence

Oncogene-induced senescence (OIS) is a decisive process to suppress tumor development, but the molecular details of OIS are still under investigation. Using an established OIS model of primary melanocytes transduced with BRAF V600E and compared to control cells, amphiregulin (AREG) was shown to be induced. In addition, AREG expression was observed in nevi, which by definition, are senescent cell clusters, compared to melanocytes. Interestingly, treatment of melanocytes with recombinant AREG did induce senescence. This led to the assumption that extracellular AREG has an important function in this process. Inhibition of the epidermal growth factor receptor (EGFR) using Gefitinib identified AREG as one of EGFR ligands responsible for senescence. Furthermore, depletion of AREG expression in senescent BRAF V600E melanocytes resulted in a significant reduction of senescent melanocytes. This study reveals AREG as an essential molecular component of signaling pathways leading to senescence in melanocytes

Amphiregulin Regulates Melanocytic Senescence

Oncogene-induced senescence (OIS) is a decisive process to suppress tumor development, but the molecular details of OIS are still under investigation. Using an established OIS model of primary melanocytes transduced with BRAF V600E and compared to control cells, amphiregulin (AREG) was shown to be induced. In addition, AREG expression was observed in nevi, which by definition, are senescent cell clusters, compared to melanocytes. Interestingly, treatment of melanocytes with recombinant AREG did induce senescence. This led to the assumption that extracellular AREG has an important function in this process. Inhibition of the epidermal growth factor receptor (EGFR) using Gefitinib identified AREG as one of EGFR ligands responsible for senescence. Furthermore, depletion of AREG expression in senescent BRAF V600E melanocytes resulted in a significant reduction of senescent melanocytes. This study reveals AREG as an essential molecular component of signaling pathways leading to senescence in melanocytes

Functional Implication of Netrin Expression in Malignant Melanoma

Background: Malignant melanoma cells are known to have altered expression of genes supporting proliferation and invasion, however, the expression of molecules of the Netrin family of repellent factors has not been analyzed in melanomas until now