MSX1-Induced Neural Crest-Like Reprogramming Promotes Melanoma Progression

An MITF- and mTOR-dependent FLCN pathway suppresses TFE3-driven metastasis in melanoma

Cancer cells have remarkable plasticity allowing them to acquire many biological states. Melanoma cells have the ability to switch from a proliferative melanocytic state to an invasive mesenchymal state and back again resulting in intratumoral heterogeneity. While microphthalmia-associated transcription factor (MITF) promotes the melanocytic phenotype, it is unclear what transcription factors drive the mesenchymal phenotype, and what mechanisms regulate the switch from the proliferative state to the mesenchymal state. We show that nuclear localization of the MITF paralog TFE3 correlates positively with metastatic potential in melanoma cell lines and tumors, and that deletion of TFE3 in MITF-low melanoma cell lines eliminates migration and metastatic ability. Further, we find that MITF suppresses the mesenchymal phenotype by activating expression of FNIP2, which encodes a component of an mTORC1-stimulated pathway promoting cytoplasmic retention and lysosomal degradation of TFE3. These findings point to the mTOR pathway and TFE3 as key regulators of melanoma plasticity.

Single-cell profiling of MC1R-inhibited melanocytes

The human red hair color (RHC) trait is caused by increased pheomelanin (red-yellow) and reduced eumelanin (black-brown) pigment in skin and hair due to diminished melanocortin 1 receptor (MC1R) function. In addition, individuals harboring the RHC trait are predisposed to melanoma development. While MC1R variants have been established as causative of RHC and are a well-defined risk factor for melanoma, it remains unclear mechanistically why decreased MC1R signaling alters pigmentation and increases melanoma susceptibility. Here, we use single-cell RNA sequencing (scRNA-seq) of melanocytes isolated from RHC mouse models to define a MC1R-inhibited Gene Signature (MiGS) comprising a large set of previously unidentified genes which may be implicated in melanogenesis and oncogenic transformation. We show that one of the candidate MiGS genes, TBX3, a well-known anti-senescence transcription factor implicated in melanoma progression, binds both E-box and T-box elements to regulate genes associated with melanogenesis and senescence bypass. Our results provide key insights into further mechanisms by which melanocytes with reduced MC1R signaling may regulate pigmentation and offer new candidates of study toward understanding how individuals with the RHC phenotype are predisposed to melanoma.

Immune escape and metastasis mechanisms in melanoma: breaking down the dichotomy

Melanoma is one of the most lethal neoplasms of the skin. Despite the revolutionary introduction of immune checkpoint inhibitors, metastatic spread, and recurrence remain critical problems in resistant cases. Melanoma employs a multitude of mechanisms to subvert the immune system and successfully metastasize to distant organs. Concerningly, recent research also shows that tumor cells can disseminate early during melanoma progression and enter dormant states, eventually leading to metastases at a future time. Immune escape and metastasis have previously been viewed as separate phenomena; however, accumulating evidence is breaking down this dichotomy. Recent research into the progressive mechanisms of melanoma provides evidence that dedifferentiation similar to classical epithelial to mesenchymal transition (EMT), genes involved in neural crest stem cell maintenance, and hypoxia/acidosis, are important factors simultaneously involved in immune escape and metastasis. The likeness between EMT and early dissemination, and differences, also become apparent in these contexts. Detailed knowledge of the mechanisms behind "dual drivers" simultaneously promoting metastatically inclined and immunosuppressive environments can yield novel strategies effective in disabling multiple facets of melanoma progression. Furthermore, understanding progression through these drivers may provide insight towards novel treatments capable of preventing recurrence arising from dormant dissemination or improving immunotherapy outcomes.

Identifying biomarkers and novel therapeutic targets in uveal melanoma

Uveal melanoma (UM) is an orphan cancer despite being the most common eye tumor in adults. Patients often present to skin cancer centers for treatment of metastatic disease although there are significant genetic, biological, and clinical differences from cutaneous melanoma. The treatments most commonly used for metastatic UM are tebentafusp and combined immune checkpoint blockade, both of which yield low response rates and may be accompanied by high treatment costs and significant immune-related toxicities. Thus, it is of paramount importance to identify biomarkers and clinical profiles predictive of treatment response and to find novel therapeutic targets. The use of immune checkpoint blockade showed more favorable outcomes in patients with extrahepatic disease and normal levels of serum lactate dehydrogenase in a panel of retrospective studies, making its use more reasonable in this subgroup. To identify novel drug targets, we will analyze the expression and relevance of neural crest transcription factors in patient bio-specimens using next-generation nanopore sequencing. Computer algorithms and network-based analysis will facilitate the identification of druggable targets which will subsequently be validated in patient-derived short-term cell cultures. This approach will help to find novel and personalized treatments for UM.

Identifizierung von Biomarkern und neuen therapeutischen Zielen beim Aderhautmelanom: Identifying biomarkers and novel therapeutic targets in uveal melanoma

Das Uveamelanom (UM) ist eine seltene Krebserkrankung, obwohl es der häufigste Tumor des Auges bei Erwachsenen ist. Die Patienten werden meist in Hautkrebszentren zur Behandlung der metastasierten Erkrankung vorgestellt. Es gibt jedoch erhebliche genetische, biologische und klinische Unterschiede zum kutanen Melanom. Die bei metastasiertem UM am häufigsten eingesetzten Therapien sind Tebentafusp und die kombinierte Immuncheckpoint‐Blockade, die beide niedrige Ansprechraten aufweisen und mit hohen Behandlungskosten und erheblichen immunbedingten Toxizitäten verbunden sein können. Daher ist es von größter Bedeutung, einerseits Biomarker und klinische Profile zu identifizieren, die das Ansprechen auf die Behandlung vorhersagen können und andererseits neue therapeutische Ziele zu finden. Der Einsatz der Immuncheckpoint‐Blockade zeigte in einer Reihe retrospektiver Studien günstigere Ergebnisse bei Patienten mit extrahepatischer Metastasierung und normalen Laktatdehydrogenase‐Werten im Serum, so dass ihr Einsatz in dieser Subgruppe sinnvoller scheint. Um neue Zielmoleküle für Medikamente zu identifizieren, werden wir die Expression und Relevanz von Transkriptionsfaktoren der Neuralleiste in Bioproben von Patienten mit Next‐Generation‐Sequenzierung der dritten Generation analysieren. Computeralgorithmen und netzwerkbasierte Analysen werden die Identifizierung von Zielstrukturen für Medikamente erleichtern, die anschließend in Kurzzeit‐Zellkulturen von Patienten validiert werden. Dieser Ansatz wird dazu beitragen, neue und personalisierte Behandlungen für das UM zu finden.

Hepatic passaging of NRAS-mutant melanoma influences adhesive properties and metastatic pattern

BACKGROUND

Liver metastasis is a poor prognostic factor for treatment of advanced cutaneous melanoma with either immunotherapy or targeted therapies. In this study we focused on NRAS mutated melanoma, a cohort with high unmet clinical need.

METHODS

WT31 melanoma was repeatedly passaged over the liver after intravenous injections five times generating the subline WT31_P5IV. The colonization of target organs, morphology, vascularization and the gene expression profiles of metastases were analyzed.

RESULTS

After intravenous injection lung metastasis was significantly decreased and a trend towards increased liver metastasis was detected for WT31_P5IV as compared to parental WT31. Besides, the ratio of lung to liver metastases was significantly smaller. Histology of lung metastases revealed reduced proliferation of WT31_P5IV in relation to WT31 while both size and necrotic areas were unaltered. Liver metastases of both sublines showed no differences in vascularization, proliferation or necrosis. To identify tumor-intrinsic factors that altered the metastatic pattern of WT31_P5IV RNA sequencing was performed and revealed a differential regulation of pathways involved in cell adhesion. Ex vivo fluorescence imaging confirmed that initial tumor cell retention in the lungs was significantly reduced in WT31_P5IV in comparison to WT31.

CONCLUSION

This study demonstrates that tumor-intrinsic properties influencing the metastatic pattern of NRAS mutated melanoma are strongly affected by hepatic passaging and the hematogenous route tumor cells take. It has implications for the clinical setting as such effects might also occur during metastatic spread or disease progression in melanoma patients.

Loss of MC1R signaling implicates TBX3 in pheomelanogenesis and melanoma predisposition

The human Red Hair Color (RHC) trait is caused by increased pheomelanin (red-yellow) and reduced eumelanin (black-brown) pigment in skin and hair due to diminished melanocortin 1 receptor (MC1R) function. In addition, individuals harboring the RHC trait are predisposed to melanoma development. While MC1R variants have been established as causative of RHC and are a well-defined risk factor for melanoma, it remains unclear mechanistically why decreased MC1R signaling alters pigmentation and increases melanoma susceptibility. Here, we use single-cell RNA-sequencing (scRNA-seq) of melanocytes isolated from RHC mouse models to reveal a Pheomelanin Gene Signature (PGS) comprising genes implicated in melanogenesis and oncogenic transformation. We show that TBX3, a well-known anti-senescence transcription factor implicated in melanoma progression, is part of the PGS and binds both E-box and T-box elements to regulate genes associated with melanogenesis and senescence bypass. Our results provide key insights into mechanisms by which MC1R signaling regulates pigmentation and how individuals with the RHC phenotype are predisposed to melanoma.

Biology of cancer; from cellular and molecular mechanisms to developmental processes and adaptation

Cancer research has been largely focused on the cellular and molecular levels of investigation. Recent data show that not only the cell but also the extracellular matrix plays a major role in the progression of malignancy. In this way, the cells and the extracellular matrix create a specific local microenvironment that supports malignant development. At the same time, cancer implies a systemic evolution which is closely related to developmental processes and adaptation. Consequently, there is currently a real gap between the local investigation of cancer at the microenvironmental level, and the pathophysiological approach to cancer as a systemic disease. In fact, the cells and the matrix are not only complementary structures but also interdependent components that act synergistically. Such relationships lead to cell-matrix integration, a supracellular form of biological organization that supports tissue development. The emergence of this supracellular level of organization, as a structure, leads to the emergence of the supracellular control of proliferation, as a supracellular function. In humans, proliferation is generally involved in developmental processes and adaptation. These processes suppose a specific configuration at the systemic level, which generates high-order guidance for local supracellular control of proliferation. In conclusion, the supracellular control of proliferation act as an interface between the downstream level of cell division and differentiation, and upstream level of developmental processes and adaptation. Understanding these processes and their disorders is useful not only to complete the big picture of malignancy as a systemic disease, but also to open new treatment perspectives in the form of etiopathogenic (supracellular or informational) therapies.

LINC01021 maintains tumorigenicity by enhancing N6-methyladenosine reader IMP2 dependent stabilization of MSX1 and JARID2: implication in colorectal cancer

Insulin-like growth factor-2 mRNA-binding protein 2 (IGF2BP2, also known as IMP2), a novel class III N6-methyladenosine (m⁶A) reader, has recently gained attention due to its critical functions in recognizing and stabilizing m⁶A modified oncogenic transcripts. However, whether and how long non-coding RNAs (lncRNAs) facilitate IMP2's role as m⁶A "reader" remains elusive, particularly in colorectal cancer (CRC). Here, we demonstrated that oncogenic LINC021 specifically bound with the m⁶A "reader" IMP2 protein and enhanced the mRNA stability of MSX1 and JARID2 in an m⁶A regulatory manner during CRC tumorigenesis and pathogenesis. Specifically, a remarkable upregulation of LINC021 was confirmed in CRC cell lines and clinical tissues (n = 130). High level of LINC021acted as an independent prognostic predictor for CRC clinical outcomes. Functional assays demonstrated that LINC021 exerted its functions as an oncogene to aggravate CRC malignant phenotypes including enhanced cell proliferation, colony formation, migration capabilities, and reduced cell apoptosis. Mechanistically, LINC021 directly recognized IMP2 protein, the latter enhanced the mRNA stability of transcripts such as MSX1 and JARID2 by recognizing their m⁶A-modified element RGGAC. Thus, these findings uncovered an essential LINC021/IMP2/MSX1 and JARID2 signaling axis in CRC tumorigenesis, which provided profound insights into our understanding of m⁶A modification regulated by lncRNA in CRC initiation and progression and shed light on the targeting of this axis for CRC treatment.

Angiogenic and molecular diversity determine hepatic melanoma metastasis and response to anti-angiogenic treatment

BACKGROUND

Cutaneous melanoma exhibits heterogeneous metastatic patterns and prognosis. In this regard, liver metastasis, which is detected in ~ 10-20% of stage 4 patients, came to the fore of melanoma research, as it recently evolved as decisive indicator of treatment resistance to immune checkpoint inhibition.

METHODS

Hepatic metastases were induced by intrasplenic injection of five different murine melanoma cell lines. The efficiencies of hepatic colonization, morphologic patterns, gene expression profiles and degree of vascularization were analyzed and Sorafenib was applied as anti-angiogenic treatment.

RESULTS

WT31 melanoma showed the highest efficiency of hepatic colonization, while intermediate efficiencies were observed for B16F10 and RET, and low efficiencies for D4M and HCmel12. RNAseq-based gene expression profiles of high and intermediate metastatic melanomas in comparison to low metastatic melanomas indicated that this efficiency predominantly associates with gene clusters involved in cell migration and angiogenesis. Indeed, heterogeneous vascularization patterns were found in the five models. Although the degree of vascularization of WT31 and B16F10 metastases differed, both showed a strong response to Sorafenib with a successful abrogation of the vascularization.

CONCLUSION

Our data indicate that molecular heterogeneity of melanomas can be associated with phenotypic and prognostic features of hepatic metastasis paving the way for organ-specific anti-angiogenic therapeutic approaches.

Developmental chromatin programs determine oncogenic competence in melanoma

[Figure: see text].

Progression of melanoma is suppressed by targeting all transforming growth factor‑β isoforms with an Fc chimeric receptor

Melanoma is an aggressive type of cancer originating from the skin that arises from neoplastic changes in melanocytes. Transforming growth factor-β (TGF-β) is a pleiotropic cytokine and is known to contribute to melanoma progression by inducing the epithelial-mesenchymal transition (EMT) program and creating an environment that favors tumor progression. There are three TGF-β isoforms, TGF-β1, TGF-β2 and TGF-β3, all of which engage in pro-tumorigenic activities by activating SMAD signaling pathways. All TGF-β isoforms activate signaling pathways by binding to their TGF-β type I (TβRI) and type II (TβRII) receptors. Thus, effective targeting of all TGF-β isoforms is of great importance. In the present study, chimeric proteins comprising the extracellular domains of TβRI and/or TβRII fused with the Fc portion of human immunoglobulin (IgG) were validated in the melanoma context. The Fc chimeric receptor comprising both TβRI and TβRII (TβRI-TβRII-Fc) effectively trapped all TGF-β isoforms. Conversely, TβRII-Fc chimeric receptor, that comprises TβRII only, was able to interact with TGF-β1 and TGF-β3 isoforms, but not with TGF-β2, which is a poor prognostic factor for melanoma patients. Accordingly, it was revealed that TβRI-TβRII-Fc chimeric receptor suppressed the EMT program in melanoma cells in vitro induced by any of the three TGF-β isoforms, as revealed by decreased expression of mesenchymal markers. Conversely, TβRII-Fc chimeric receptor inhibited the EMT program induced by TGF-β1 and TGF-β3. In addition, it was established that tumor growth in subcutaneous mouse melanoma was inhibited by TβRI-TβRII-Fc chimeric receptor indicating that Fc chimeric receptor could be applied to modify the tumor microenvironment (TME) of melanoma. Therefore, designing of Fc chimeric receptors targeting TGF-β signals that affect various components of the TME may result in the development of effective anti-melanoma agents.

How Neural Crest Transcription Factors Contribute to Melanoma Heterogeneity, Cellular Plasticity, and Treatment Resistance

Cutaneous melanoma represents one of the deadliest types of skin cancer. The prognosis strongly depends on the disease stage, thus early detection is crucial. New therapies, including BRAF and MEK inhibitors and immunotherapies, have significantly improved the survival of patients in the last decade. However, intrinsic and acquired resistance is still a challenge. In this review, we discuss two major aspects that contribute to the aggressiveness of melanoma, namely, the embryonic origin of melanocytes and melanoma cells and cellular plasticity. First, we summarize the physiological function of epidermal melanocytes and their development from precursor cells that originate from the neural crest (NC). Next, we discuss the concepts of intratumoral heterogeneity, cellular plasticity, and phenotype switching that enable melanoma to adapt to changes in the tumor microenvironment and promote disease progression and drug resistance. Finally, we further dissect the connection of these two aspects by focusing on the transcriptional regulators MSX1, MITF, SOX10, PAX3, and FOXD3. These factors play a key role in NC initiation, NC cell migration, and melanocyte formation, and we discuss how they contribute to cellular plasticity and drug resistance in melanoma.

A narrative review of the roles of muscle segment homeobox transcription factor family in cancer

Deregulation of many homeobox genes has been observed in various cancers and has caused functional implications in the tumor progression. In this review, we will focus on the roles of the human muscle segment homeobox (MSX) transcription factor family in the process of tumorigenesis. The MSX transcription factors, through complex downstream regulation mechanisms, are promoters or inhibitors of diverse cancers by participating in cell proliferation, cell invasion, cell metastasis, cell apoptosis, cell differentiation, drug resistance of tumors, maintenance of tumor stemness, and tumor angiogenesis. Moreover, their upstream regulatory mechanisms in cancers may include: gene mutation and chromosome aberration; DNA methylation and chromatin modification; regulation by non-coding RNAs; regulation by other transcription factors and post-translational modification. These mechanisms may provide a better understanding of why MSX transcription factors are abnormally expressed in tumors. Notably, intermolecular interactions and post-translational modification can regulate the transcriptional activity of MSX transcription factors. It is also crucial to know what affects the transcriptional activity of MSX transcription factors in tumors for possible interventions in them in the future. This systematic summary of the regulatory patterns of the MSX transcription factor family may help to further understand the mechanisms involved in transcriptional regulation and also provide new therapeutic approaches for tumor progression.

Cell Adhesion Molecules in Plasticity and Metastasis

Prior to metastasis, modern therapeutics and surgical intervention can provide a favorable long-term survival for patients diagnosed with many types of cancers. However, prognosis is poor for patients with metastasized disease. Melanoma is the deadliest form of skin cancer, yet in situ and localized, thin melanomas can be biopsied with little to no postsurgical follow-up. However, patients with metastatic melanoma require significant clinical involvement and have a 5-year survival of only 34% to 52%, largely dependent on the site of colonization. Melanoma metastasis is a multi-step process requiring dynamic changes in cell surface proteins regulating adhesiveness to the extracellular matrix (ECM), stroma, and other cancer cells in varied tumor microenvironments. Here we will highlight recent literature to underscore how cell adhesion molecules (CAM) contribute to melanoma disease progression and metastasis.

A Novel Apoptosis-Related Gene Signature Predicts Biochemical Recurrence of Localized Prostate Cancer After Radical Prostatectomy

Background: Nowadays, predictions of biochemical recurrence (BCR) in localized prostate cancer (PCa) patients after radical prostatectomy (RP) are mainly based on clinical parameters with a low predictive accuracy. Given the critical role of apoptosis in PCa occurrence and progression, we aimed to establish a novel predictive model based on apoptosis-related gene signature and clinicopathological parameters that can improve risk stratification for BCR and assist in clinical decision-making. Methods: Expression data and corresponding clinical information were obtained from four public cohorts, one from The Cancer Genome Atlas (TCGA) dataset and three from the Gene Expression Omnibus (GEO) dataset. Weighted gene co-expression network analysis (WGCNA) was performed to identify candidate modules closely correlated to BCR, and univariate and multivariate Cox regression analyses were utilized to build the gene signature. Time-dependent receiver operating curve (ROC) and Kaplan-Meier (KM) survival analysis were used to assess the prognostic value. Finally, we analyzed the expression of genes in the signature and validated the results using quantitative real-time PCR (qRT-PCR). Results: The novel gene signature we established exhibited a high prognostic value and was able to act as an independent risk factor for BCR [Training set: P < 0.001, hazard ratio (HR) = 7.826; Validation set I: P = 0.006, HR = 2.655; Validation set II: P = 0.003, HR = 4.175; Validation set III: P < 0.001, HR = 3.008]. Nomogram based on the gene signature and clinical parameters was capable of distinguishing high-risk BCR patients. Additionally, functional enrichment analysis showed several enriched pathways and biological processes, which might help reveal the underlying mechanism. The expression results of qRT-PCR were consistent with TCGA results. Conclusion: The apoptosis-related gene signature could serve as a powerful predictor and risk factor for BCR in localized PCa patients after RP.

Immune Checkpoint Blockade in Advanced Cutaneous Squamous Cell Carcinoma: What Do We Currently Know in 2020?

Cutaneous squamous cell carcinoma (cSCC) is the second most common skin cancer that predominantly arises in chronically sun-damaged skin. Immunosuppression, genetic disorders such as xeroderma pigmentosum (XP), exposure to certain drugs and environmental noxae have been identified as major risk factors. Surgical removal of cSCC is the therapy of choice and mostly curative in early stages. However, a minority of patients develop locally advanced tumors or distant metastases that are still challenging to treat. Immune checkpoint blockade (ICB) targeting CTLA-4, PD-L1 and PD-1 has tremendously changed the field of oncological therapy and especially the treatment of skin cancers as tumors with a high mutational burden. In this review, we focus on the differences between cSCC and cutaneous melanoma (CM) and their implications on therapy, summarize the current evidence on ICB for the treatment of advanced cSCC and discuss the chances and pitfalls of this therapy option for this cancer entity. Furthermore, we focus on special subgroups of interest such as organ transplant recipients, patients with hematologic malignancies, XP and field cancerization.

Reemergence of neural crest stem cell-like states in melanoma during disease progression and treatment

Melanoma is the deadliest of all skin cancers due to its high metastatic potential. In recent years, advances in targeted therapy and immunotherapy have contributed to a remarkable progress in the treatment of metastatic disease. However, intrinsic or acquired resistance to such therapies remains a major obstacle in melanoma treatment. Melanoma disease progression, beginning from tumor initiation and growth to acquisition of invasive phenotypes and metastatic spread and acquisition of treatment resistance, has been associated with cellular dedifferentiation and the hijacking of gene regulatory networks reminiscent of the neural crest (NC)—the developmental structure which gives rise to melanocytes and hence melanoma. This review summarizes the experimental evidence for the involvement of NC stem cell (NCSC)‐like cell states during melanoma progression and addresses novel approaches to combat the emergence of stemness characteristics that have shown to be linked with aggressive disease outcome and drug resistance.

Single-Cell RNA Sequencing Unravels Heterogeneity of the Stromal Niche in Cutaneous Melanoma Heterogeneous Spheroids

Heterogeneous spheroids have recently acquired a prominent position in melanoma research because they incorporate microenvironmental cues relevant for melanoma. In this study, we focused on the analysis of microenvironmental factors introduced in melanoma heterogeneous spheroids by different dermal fibroblasts. We aimed to map the fibroblast diversity resulting from previously acquired damage caused by exposure to extrinsic and intrinsic stimuli. To construct heterogeneous melanoma spheroids, we used normal dermal fibroblasts from the sun-protected skin of a juvenile donor. We compared them to the fibroblasts from the sun-exposed photodamaged skin of an adult donor. Further, we analysed the spheroids by single-cell RNA sequencing. To validate transcriptional data, we also compared the immunohistochemical analysis of heterogeneous spheroids to melanoma biopsies. We have distinguished three functional clusters in primary human fibroblasts from melanoma spheroids. These clusters differed in the expression of (a) extracellular matrix-related genes, (b) pro-inflammatory factors, and (c) TGFβ signalling superfamily. We observed a broader deregulation of gene transcription in previously photodamaged cells. We have confirmed that pro-inflammatory cytokine IL-6 significantly enhances melanoma invasion to the extracellular matrix in our model. This supports the opinion that the aspects of ageing are essential for reliable melanoma 3D modelling in vitro.

Intrinsic Balance between ZEB Family Members Is Important for Melanocyte Homeostasis and Melanoma Progression

It has become clear that cellular plasticity is a main driver of cancer therapy resistance. Consequently, there is a need to mechanistically identify the factors driving this process. The transcription factors of the zinc-finger E-box-binding homeobox family, consisting of ZEB1 and ZEB2, are notorious for their roles in epithelial-to-mesenchymal transition (EMT). However, in melanoma, an intrinsic balance between ZEB1 and ZEB2 seems to determine the cellular state by modulating the expression of the master regulator of melanocyte homeostasis, microphthalmia-associated transcription factor (MITF). ZEB2 drives MITF expression and is associated with a differentiated/proliferative melanoma cell state. On the other hand, ZEB1 is correlated with low MITF expression and a more invasive, stem cell-like and therapy-resistant cell state. This intrinsic balance between ZEB1 and ZEB2 could prove to be a promising therapeutic target for melanoma patients. In this review, we will summarise what is known on the functional mechanisms of these transcription factors. Moreover, we will look specifically at their roles during melanocyte-lineage development and homeostasis. Finally, we will overview the current literature on ZEB1 and ZEB2 in the melanoma context and link this to the 'phenotype-switching' model of melanoma cellular plasticity.

Malignant Melanoma: Autoimmunity and Supracellular Messaging as New Therapeutic Approaches

OPINION STATEMENT

Melanoma is one of the most aggressive forms of cancer, with a high mortality rate in the absence of a safe and curable therapy. As a consequence, several procedures have been tested over time, with the most recent (immunological and targeted) therapies proving to be effective in some patients. Unfortunately, these new treatment options continue to generate debate related to the therapeutic strategy (intended to maximize the long-term results of patients with melanoma), not only about the monotherapy configuration but also regarding association/succession between distinct therapeutic procedures. As an example, targeted therapy with BRAF inhibitors proved to be effective in advanced BRAF-mutant melanoma. However, such treatments with BRAF inhibitors lead to therapy resistance in half of patients after approximately 6 months. Even if most benign nevi incorporate oncogenic BRAF mutations, they rarely become melanoma; therefore, targeted therapy with BRAF inhibitors should be viewed as an incomplete or perfectible therapy. Another example is related to the administration of immune checkpoint inhibitors/ICIs (anti-CTLA-4 antibodies, anti-PD-1/PD-L1 antibodies), which are successfully used in metastatic melanoma. It is currently believed that CTLA-4 and PD-1 blockade would favor a strong immune response against cancer cells. The main side effects of ICIs are represented by the development of immune-related adverse events, which in some cases can be lethal. These ICI side effects would thus be not only therapeutically counterproductive but also potentially dangerous. Surprisingly, a subset of immune-related adverse events (especially autoimmune toxicity) seems to be clearly correlated with better therapeutic results, perhaps due to an additional therapeutic effect (currently insufficiently studied/exploited). Contrary to the classical approach of cancer (considered until now an uncontrolled division of cells), a very recent and comprehensive theory describes malignancy as a supracellular disease. Cancerous disease would therefore be a disturbed supracellular process (embryogenesis, growth, development, regeneration, etc.), which imposes/coordinates an increased rhythm of cell division, angiogenesis, immunosuppression, etc. Melanoma is presented from such a supracellular perspective to be able to explain the beneficial role of autoimmunity in cancer (autoimmune abortion/rejection of the melanoma-embryo phenotype) and to create premises to better optimize the newly emerging therapeutic options. Finally, it is suggested that the supracellular evolution of malignancy implies complex supracellular messaging (between the cells and host organism), which would be interfaced especially by the extracellular matrix and noncoding RNA. Therefore, understanding and manipulating supracellular messaging in cancer could open new treatment perspectives in the form of digitized (supracellular) therapy.

Get with the Program! Stemness and Reprogramming in Melanoma Metastasis

Cancer cells are highly plastic and adopt multiple phenotypic states that contribute to tumor progression. Heppt et al. demonstrate that the homeodomain transcription factor Msh homeobox 1 reprograms melanoma cells to a precursor state associated with melanoma progression and increased liver metastasis. Identification of this new role for Msh homeobox 1 may facilitate the development of new therapies that limit melanoma dissemination.

Expression of Neural Crest Markers GLDC and ERRFI1 is Correlated with Melanoma Prognosis

Regulation of particular genes during the formation of neural crest (NC) cells is also described during progression of malignant melanoma. In this context, it is of paramount importance to develop neural crest models allowing the identification of candidate genes, which could be used as biomarkers for melanoma prognosis. Here, we used a human induced Pluripotent Stem Cells (iPSC)-based approach to present novel NC-associated genes, expression of which was upregulated in melanoma. A list of 8 candidate genes, based on highest upregulation, was tested for prognostic value in a tissue microarray analysis containing samples from advanced melanoma (good versus bad prognosis) as well as from high-risk primary melanomas (early metastasizing versus non or late-metastasizing). CD271, GLDC, and ERRFI1 showed significantly higher expression in metastatic patients who died early than the ones who survived at least 30 months. In addition, GLDC and TWIST showed a significantly higher immunohistochemistry (IHC) score in primary melanomas from patients who developed metastases within 12 months versus those who did not develop metastases in 30 months. In conclusion, our iPSC-based study reveals a significant association of NC marker GLDC protein expression with melanoma prognosis.

Hepatic Endothelial Notch Activation Protects against Liver Metastasis by Regulating Endothelial-Tumor Cell Adhesion Independent of Angiocrine Signaling

The interaction of tumor cells with organ-specific endothelial cells (EC) is an important step during metastatic progression. Notch signaling in organ-specific niches has been implicated in mediating opposing effects on organotropic metastasis to the lungs and the liver, respectively. In this study, we scrutinized the role of endothelial Notch activation during liver metastasis. To target hepatic EC (HEC), a novel EC subtype-specific Cre driver mouse was generated. Clec4g-Cre^tg/wt mice were crossed to Rosa26^{N1ICD-IRES-GFP} to enhance Notch signaling in HEC (NICD^OE-HEC). In NICD^OE-HEC mice, hepatic metastasis of malignant melanoma and colorectal carcinoma was significantly reduced. These mice revealed reduced liver growth and impaired metabolic zonation due to suppression of hepatic angiocrine Wnt signaling. Hepatic metastasis, however, was not controlled by angiocrine Wnt signaling, as deficiency of the Wnt cargo receptor Wls in HEC of Wls^HEC-KO mice did not affect hepatic metastasis. In contrast, the hepatic microvasculature in NICD^OE-HEC mice revealed a special form of sinusoidal capillarization, with effacement of endothelial zonation functionally paralleled by reduced tumor cell adhesion in vivo. Notably, expression of endothelial adhesion molecule ICAM1 by HEC was significantly reduced. Treatment with an anti-ICAM1 antibody significantly inhibited tumor cell adhesion to HEC in wild-type mice confirming that Notch controls hepatic metastasis via modulation of HEC adhesion molecules. As endothelial Notch activation in the lung has been shown to promote lung metastasis, tumor therapy will require approaches that target Notch in an organ-, cell type-, and context-specific manner. SIGNIFICANCE: Manipulation of Notch signaling in the endothelium has opposing, organ-specific effects on metastasis to the lung and the liver, demonstrating that this pathway should be targeted in a cell- and context-specific fashion.

Targeted Therapy-Resistant Melanoma Cells Acquire Transcriptomic Similarities with Human Melanoblasts

The mechanisms of adaptive and acquired drug resistance in tumors are not completely understood. So far, gene amplifications or mutations, leading to the reactivation of the MAPK or PI3K pathways have been described. In this study, we used two different methods to generate human melanoblasts: (1) via differentiation from induced pluripotent stem cells (iPSCs) and (2) via dedifferentiation from melanocytes. The melanoblast transcriptomes were then compared to the transcriptome of MAPK inhibitor-resistant melanoma cells. We observed that the expression of genes associated with cell cycle control, DNA damage control, metabolism, and cancer was altered in both melanoblast populations and in both adaptive and acquired resistant melanoma samples, compared to drug-sensitive samples. However, genes involved in antigen presentation and cellular movement were only regulated in the melanoblast populations and in the acquired resistant melanoma samples, compared to the drug-sensitive samples. Moreover, melanocyte-derived melanoblasts and adaptive resistant melanoma samples were characterized by different expression levels of certain transcription factors or genes involved in the CDK5 pathway. In conclusion, we show here that in vitro models of human melanoblasts are very important tools to comprehend the expression profiles of drug-resistant melanoma.

Loss of neural crest-associated gene FOXD1 impairs melanoma invasion and migration via RAC1B downregulation

Recent studies suggest that malignant melanoma heterogeneity includes subpopulations of cells with features of multipotent neural crest (NC) cells. Zebrafish and mouse models have shown that reactivation of neural crest-specific pathways during transformation determines the invasiveness of melanoma cells. In our study, we show that the neural crest-associated transcription factor FOXD1 plays a key role in the invasion and the migration capacities of metastatic melanomas both in vivo and in vitro. Gene expression profiling analysis identified both an upregulation of FOXD1 in NC and melanoma cells, as well as a downregulation of several genes related to cell invasion in FOXD1 knockdown cells, including MMP9 and RAC1B. Furthermore, we demonstrate that knockdown of RAC1B a tumor-specific isoform of RAC1, significantly impaired melanoma cell migration and invasion and could abrogate enhanced invasiveness induced by FOXD1 overexpression. We conclude that FOXD1 may influence invasion and migration via indirect regulation of MMP9 and RAC1B alternative splicing in melanoma cells.