Despite the general focus on an invasive and de-differentiated phenotype as main driver of cancer metastasis, in melanoma patients many metastatic lesions display a high degree of pigmentation, indicative for a differentiated phenotype.,Indeed, studies in mice and fish show that melanoma cells switch to a differentiated phenotype at secondary sites, possibly because in melanoma differentiation is closely linked to proliferation through the lineage-specific transcriptional master regulator MITF.,Importantly, while a lot of effort has gone into identifying factors that induce the de-differentiated/invasive phenotype, it is not well understood how the switch to the differentiated/proliferative phenotype is controlled.,We identify collagen as a contributor to this switch.,We demonstrate that collagen stiffness induces melanoma differentiation through a YAP/PAX3/MITF axis and show that in melanoma patients increased collagen abundance correlates with nuclear YAP localization.,However, the interrogation of large patient datasets revealed that in the context of the tumour microenvironment, YAP function is more complex.,In the absence of fibroblasts, YAP/PAX3-mediated transcription prevails, but in the presence of fibroblasts tumour growth factor-β suppresses YAP/PAX3-mediated MITF expression and induces YAP/TEAD/SMAD-driven transcription and a de-differentiated phenotype.,Intriguingly, while high collagen expression is correlated with poorer patient survival, the worst prognosis is seen in patients with high collagen expression, who also express MITF target genes such as the differentiation markers TRPM1, TYR and TYRP1, as well as CDK4.,In summary, we reveal a distinct lineage-specific route of YAP signalling that contributes to the regulation of melanoma pigmentation and uncovers a set of potential biomarkers predictive for poor survival.
Tenascin-C (TNC) is highly expressed in melanoma; however, little is known about its functions.,Recent studies indicate that TNC plays a role within the stem cell niche.,We hypothesized that TNC creates a specific environment for melanoma cells to exhibit a stem cell-like phenotype, driving tumor growth and evading conventional therapies.,TNC expression was strongly up-regulated in melanoma cells grown as 3D spheres (enriched for stem-like cells) when compared to adherent cells.,Down-modulation 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 IL2Rγnull mice compared to control cells.,Melanoma spheres contain and increased number of side population (SP) cells, which exhibited stem cell characteristics and 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 due to the down-regulation of multiple ABC transporters, including ABCB5.,These data suggest that TNC plays a critical role in melanoma progression by mediating protective signals in the therapy-resistant population of melanoma.