Melanoma-initiating cells: a compass needed

Endosomal signalling via exosome surface TGFβ-1

Extracellular vesicles such as exosomes convey biological messages between cells, either by surface-to-surface interaction or by shuttling of bioactive molecules to a recipient cell's cytoplasm. Here we show that exosomes released by mast cells harbour both active and latent transforming growth factor β-1 (TGFβ-1) on their surfaces. The latent form of TGFβ-1 is associated with the exosomes via heparinase-II and pH-sensitive elements. These vesicles traffic to the endocytic compartment of recipient human mesenchymal stem cells (MSCs) within 60 min of exposure. Further, the exosomes-associated TGFβ-1 is retained within the endosomal compartments at the time of signalling, which results in prolonged cellular signalling compared to free-TGFβ-1. These exosomes induce a migratory phenotype in primary MSCs involving SMAD-dependent pathways. Our results show that mast cell-derived exosomes are decorated with latent TGFβ-1 and are retained in recipient MSC endosomes, influencing recipient cell migratory phenotype. We conclude that exosomes can convey signalling within endosomes by delivering bioactive surface ligands to this intracellular compartment.

Intracellular Notch1 Signaling in Cancer-Associated Fibroblasts Dictates the Plasticity and Stemness of Melanoma Stem/Initiating Cells

Cancer stem cells (CSCs) play critical roles in cancer initiation, metastasis, recurrence, and drug resistance. Recent studies have revealed involvement of cancer‐associated fibroblasts (CAFs) in regulating CSCs. However, the intracellular molecular mechanisms that determine the regulatory role of CAFs in modulating the plasticity of CSCs remain unknown. Here, we uncovered that intracellular Notch1 signaling in CAFs serves as a molecular switch, which modulates tumor heterogeneity and aggressiveness by inversely controlling stromal regulation of the plasticity and stemness of CSCs. Using mesenchymal stem cell‐derived fibroblasts (MSC‐DF) harboring reciprocal loss‐of‐function and gain‐of‐function Notch1 signaling, we found that MSC‐DF^{Notch1−/−} prompted cocultured melanoma cells to form more spheroids and acquire the phenotype (CD271⁺ and Nestin⁺) of melanoma stem/initiating cells (MICs), whereas MSC‐DF^N1IC+/+ suppressed melanoma cell sphere formation and mitigated properties of MICs. MSC‐DF^{Notch1−/−} increased stemness of CD271⁺ MIC, which resultantly exhibited stronger aggressiveness in vitro and in vivo, by upregulating Sox2/Oct4/Nanog expression. Consistently, when cografted with melanoma cells into NOD scid gamma (NSG) mice, MSC‐DF^{Notch1−/−} increased, but MSC‐DF^N1IC+/+ decreased, the amounts of CD271⁺ MIC in melanoma tissue. The amounts of CD271⁺ MIC regulated by MSC‐DF carrying high or low Notch1 pathway activity is well correlated with capability of melanoma metastasis, supporting that melanoma metastasis is MIC‐mediated. Our data demonstrate that intracellular Notch1 signaling in CAFs is a molecular switch dictating the plasticity and stemness of MICs, thereby regulating melanoma aggressiveness, and therefore that targeting the intracellular Notch1 signaling pathway in CAFs may present a new therapeutic strategy for melanoma. stem cells2019;37:865–875

Characterization of a B16-F10 melanoma model locally implanted into the ear pinnae of C57BL/6 mice

The common experimental use of B16-F10 melanoma cells focuses on exploring their metastatic potential following intravenous injection into mice. In this study, B16-F10 cells are used to develop a primary tumor model by implanting them directly into the ears of C57BL/6J mice. The model represents a reproducible and easily traceable tool for local tumor growth and for making additional in vivo observations, due to the localization of the tumors. This model is relatively simple and involves (i) surgical opening of the ear skin, (ii) removal of a square-piece of cartilage followed by (iii) the implantation of tumor cells with fibrin gel. The remodeling of the fibrin gel within the cartilage chamber, accompanying tumor proliferation, results in the formation of blood vessels, lymphatics and tissue matrix that can be readily distinguished from the pre-existing skin structures. Moreover, this method avoids the injection-enforced artificial spread of cells into the pre-existing lymphatic vessels. The tumors have a highly reproducible exponential growth pattern with a tumor doubling time of around 1.8 days, reaching an average volume of 85mm³ 16 days after implantation. The melanomas are densely cellular with proliferative indices of between 60 and 80%. The induced angiogenesis and lymphangiogenesis resulted in the development of well-vascularized tumors. Different populations of immunologically active cells were also present in the tumor; the population of macrophages decreases with time while the population of T cells remained quasi constant. The B16-F10 tumors in the ear frequently metastasized to the cervical lymph nodes, reaching an incidence of 75% by day 16. This newly introduced B16-F10 melanoma model in the ear is a powerful tool that provides a new opportunity to study the local tumor growth and metastasis, the associated angiogenesis, lymphangiogenesis and tumor immune responses. It could potentially be used to test different treatment strategies.

Mek inhibition results in marked antitumor activity against metastatic melanoma patient-derived melanospheres and in melanosphere-generated xenografts

One of the key oncogenic pathways involved in melanoma aggressiveness, development and progression is the RAS/BRAF/MEK pathway, whose alterations are found in most patients. These molecular anomalies are promising targets for more effective anti-cancer therapies. Some Mek inhibitors showed promising antitumor activity, although schedules and doses associated with low systemic toxicity need to be defined. In addition, it is now accepted that cancers can arise from and be maintained by the cancer stem cells (CSC) or tumor-initiating cells (TIC), commonly expanded in vitro as tumorspheres from several solid tumors, including melanoma (melanospheres). Here, we investigated the potential targeting of MEK pathway by exploiting highly reliable in vitro and in vivo pre-clinical models of melanomas based on melanospheres, as melanoma initiating cells (MIC) surrogates. MEK inhibition, through PD0325901, provided a successful strategy to affect survival of mutated-BRAF melanospheres and growth of wild type-BRAF melanospheres. A marked citotoxicity was observed in differentated melanoma cells regardless BRAF mutational status. PD0325901 treatment, dramatically inhibited growth of melanosphere-generated xenografts and determined impaired tumor vascularization of both mutated- and wild type-BRAF tumors, in the absence of mice toxicity. These results suggest that MEK inhibition might represent a valid treatment option for patients with both mutated- or wild type-BRAF melanomas, affecting tumor growth through multiple targets.

Recent advances in sunlight-induced carcinogenesis using the Xiphophorus melanoma model

Unlike breast and prostate cancers, the nature and sequence of critical genetic and epigenetic events involved in the initiation and progression of melanoma are not well understood. A contributing factor to this dilemma, especially given our current understanding of the importance of UV light in melanoma etiology, is the lack of quality UV-inducible melanoma animal models. In this study we elaborate on the capability of UV light to induce cutaneous malignant melanomas (CMM) in Xiphophorus fishes, which were previously found to develop melanomas after acute neonatal UVB irradiation. In two separate tumorigenesis experiments, we exposed adult Xiphophorus hybrids to either acute UVB irradiations (5 consecutive daily treatments) or chronic solar irradiations (continuous UVA/UVB treatment for 9 months). Acute adult UVB irradiation resulted in the significant induction of melanomas, and moreover, this induction rate is equivalent to that of animals exposed to acute neonatal UVB irradiation. This study represents the first evidence that acute adult UVB irradiation, in the absence of any early life exposures, induces CMM. Similar to the findings conducted on other divergent melanoma models, including HGF/SF transgenic mice and Monodelphis domestica, prolonged chronic solar UV was not a factor in melanomagenesis.

The role of stem and circulating cells in cancer metastasis

While many solid tumors have been reported to contain stem cell-like cells termed cancer stem cells, the case for a melanoma stem cell has been debated over the last few years. Herein, we summarize current knowledge of melanoma-initiating cells and provide an update on recently gained knowledge regarding cancer stem cells and melanoma.

Melanoma spheroids grown under neural crest cell conditions are highly plastic migratory/invasive tumor cells endowed with immunomodulator function

Background

The aggressiveness of melanoma tumors is likely to rely on their well-recognized heterogeneity and plasticity. Melanoma comprises multi-subpopulations of cancer cells some of which may possess stem cell-like properties. Although useful, the sphere-formation assay to identify stem cell-like or tumor initiating cell subpopulations in melanoma has been challenged, and it is unclear if this model can predict a functional phenotype associated with aggressive tumor cells.

Methodology/Principal Findings

We analyzed the molecular and functional phenotypes of melanoma spheroids formed in neural crest cell medium. Whether from metastatic or advanced primary tumors, spheroid cells expressed melanoma-associated markers. They displayed higher capacity to differentiate along mesenchymal lineages and enhanced expression of SOX2, NANOG, KLF4, and/or OCT4 transcription factors, but not enhanced self-renewal or tumorigenicity when compared to their adherent counterparts. Gene expression profiling attributed a neural crest cell signature to these spheroids and indicated that a migratory/invasive and immune-function modulating program could be associated with these cells. In vitro assays confirmed that spheroids display enhanced migratory/invasive capacities. In immune activation assays, spheroid cells elicited a poorer allogenic response from immune cells and inhibited mitogen-dependent T cells activation and proliferation more efficiently than their adherent counterparts. Our findings reveal a novel immune-modulator function of melanoma spheroids and suggest specific roles for spheroids in invasion and in evasion of antitumor immunity.

Conclusion/Significance

The association of a more plastic, invasive and evasive, thus a more aggressive tumor phenotype with melanoma spheroids reveals a previously unrecognized aspect of tumor cells expanded as spheroid cultures. While of limited efficiency for melanoma initiating cell identification, our melanoma spheroid model predicted aggressive phenotype and suggested that aggressiveness and heterogeneity of melanoma tumors can be supported by subpopulations other than cancer stem cells. Therefore, it could be constructive to investigate melanoma aggressiveness, relevant to patients and clinical transferability.

ABCB1 identifies a subpopulation of uveal melanoma cells with high metastatic propensity

Metastasis of tumor cells to distant organs is the leading cause of death in melanoma. Yet, the mechanisms of metastasis remain poorly understood. One key question is whether all cells in a primary tumor are equally likely to metastasize or whether subpopulations of cells preferentially give rise to metastases. Here, we identified a subpopulation of uveal melanoma cells expressing the multidrug resistance transporter ABCB1 that are highly metastatic compared to ABCB1(-) bulk tumor cells. ABCB1(+) cells also exhibited enhanced clonogenicity, anchorage-independent growth, tumorigenicity and mitochondrial activity compared to ABCB1(-) cells. A375 cutaneous melanoma cells contained a similar subpopulation of highly metastatic ABCB1(+) cells. These findings suggest that some uveal melanoma cells have greater potential for metastasis than others and that a better understanding of such cells may be necessary for more successful therapies for metastatic melanoma.

RANK is expressed in metastatic melanoma and highly upregulated on melanoma-initiating cells

Melanoma accounts for ∼ 79% of skin cancer-related deaths, and the receptor activator of NF-κB (RANK)-receptor activator of NF-κB ligand (RANKL) pathway has been shown to be involved in the migration and metastasis of epithelial tumor cells. In this study, we demonstrate that RANK was significantly increased in peripheral circulating melanoma cells, primary melanomas, and metastases from stage IV melanoma patients compared with tumor cells from stage I melanoma patients. However, upregulated RANK expression was not found in stage IV melanoma patients with bone metastases compared with stage IV melanoma patients without bone metastases, providing a possible explanation for the clinical observation that melanoma cells do not preferentially metastasize to bone tissue. Strikingly, RANK-expressing melanoma cells from peripheral blood, primary tumors, or metastases of stage IV patients coexpressed ATP-binding cassette (ABC) B5 and CD133, both markers characteristic of melanoma-initiating cells, suggesting a tumor stem cell-like phenotype. In support of this hypothesis, RANK-expressing melanoma cells showed a reduced Ki67 proliferation index compared with RANK(-) melanoma cells from the same patient and are able to induce tumor growth in immunodeficient mice. Together, our data demonstrate that RANK expression is increased in metastatic melanoma and highly upregulated on melanoma-initiating cells, suggesting that RANK might be involved in the development and maintenance of melanoma-initiating cells and possibly in metastatic spreading.

Tenascin-C promotes melanoma progression by maintaining the ABCB5-positive side population

Tenascin-C (TNC) is highly expressed in melanoma; however, little is known about its functions. Recent studies indicate that TNC has a role within the stem cell niche. We hypothesized that TNC creates a specific environment for melanoma cells to show a stem cell-like phenotype, promoting tumor growth and evading conventional therapies. TNC expression was strongly upregulated in melanoma cells grown as 3D spheres (enriched for stem-like cells) when compared to adherent cells. Downmodulation of TNC by shRNA lentiviruses significantly decreased the growth of melanoma spheres. The incidence of pulmonary metastases after intravenous injection of TNC knockdown cells was significantly lower in NOD/SCID IL2Rγ(null) mice compared with control cells. Melanoma spheres contain an increased number of side population (SP) cells, which show stem cell characteristics, and have the potential for drug resistance due to their high efflux capacity. Knockdown of TNC dramatically decreased the SP fraction in melanoma spheres and lowered their resistance to doxorubicin treatment, likely because of the downregulation of multiple ATP-binding cassette (ABC) transporters, including ABCB5. These data suggest that TNC is critical in melanoma progression as it mediates protective signals in the therapy-resistant population of melanoma.

Cancer stem cells-clinical relevance

Therapeutic advances over the past three decades now allow most cancer patients to achieve major clinical responses. Although clinical responses can clearly decrease side effects and improve quality of life, most cancer patients still eventually relapse and die of their disease. Many cancers have now been shown to harbor cells that are phenotypically and biologically similar to normal cells with self-renewal capacity; these so-called cancer stem cells (CSC) typically constitute only a small fraction of the total tumor burden, but theoretically harbor all the self-renewal capacity. Moreover, the CSC appears to be relatively resistant to standard anticancer therapies by co-opting normal stem cells' intrinsic defense mechanisms, such as quiescence, efflux pumps, and detoxifying enzymes. However, the clinical importance of CSC, if any, remains unproven. Nevertheless, emerging evidence suggests that initial responses in cancer represent therapeutic effectiveness against the bulk cancer cells, while the rare CSC is responsible for relapse. Better understanding of the biology of CSC, as well as reexamining both our preclinical and clinical drug development paradigms to include the CSC concept, has the potential to revolutionize the treatment of many cancers.