In search of cancer stem cells in cutaneous melanoma.

Cutaneous melanoma is the most aggressive human skin cancer, because of high therapy-resistance and metastatic potential. Mechanisms underlying these processes are largely unknown. Cancer stem cells (CSC) are defined as cells that withstand conventional therapy and hence can re-grow the tumor. Moreover, because of their proposed migratory capacity, CSC are plausible effectors of metastasis. Therefore, we pursued their identification and characterization using not one but various approaches, based on CSC-associated functional and phenotypical characteristics. In the first chapter, we searched for CSC in human melanoma using the side population (SP) technique. Our study identified a SP in human melanoma that holds functional and molecular characteristics of CSC, including tumorigenicity, invasive phenotype and resistance to chemotherapy and hypoxia. In the second chapter, we analyzed the acute effects on melanoma of (high doses of) chemotherapy in a human clinical 'model' (isolated limb perfusion, or ILP, with melphalan). This study discovered a 5-gene set acutely induced in melanoma after chemotherapy, plausibly linked to CSC. The use of the clinical model strengthens the relevance of our findings and may open up strategies for future (combination) therapies aimed at targeting these stress-survival factors. In the third chapter, we examined whether the epithelial-to-mesenchymal transition (EMT) may be going on in melanoma. In many other cancers EMT is known to induce or activate CSC, and in particular to drive their invasion and migration. This study discovered a novel melanoma cell type; detailed characterization suggests that it resides at the crossroad of EMT and CSC induction, and may represent migrating CSC. In conclusion, this thesis study provides new insights into melanoma biology by providing several arguments for the existence of CSC and their role in therapy-resistance and metastasis. The CSC candidates identified by the diverse range of experimental approaches used show certain degrees of overlap. Our study instigates further research that may open the way to new therapeutic targets for malignant melanoma in the context of CSC, EMT and chemoresistance.nrpages: 206status: publishe