Melanoma stem-like cells and melanoma cell lines: main molecular pathways and possible microRNA involvement

Abstract

Cancer Stem Cells (CSCs) have been identified in several malignancies and indirect evidences support the existence of melanoma stem-like cells. Since microRNAs (miRs), an abundant class of small non-coding RNAs playing important roles in regulating gene expression, are involved in melanoma development and progression, we investigated their possible involvement also in melanoma stem-like cells. We identified a subset of melanoma cells, isolated from lung metastatic melanomas that, cultured in appropriate serum free conditions, were able to propagate as non adherent spheres (SC), whereas in the presence of serum they adhere to the plastic and acquire the typical morphologic features of differentiated cells (PC). These melanoma spheres are also capable of self-renewing and tumorigenesis in vivo. Here we analyzed a group of selected miRs by qReal Time PCR and among them miR-221/-222 had the most consistent and significant increased expression in PC cells. In these cells we confirmed the inverse correlation between microRNA-221/-222 and p27Kip1 and c-KIT receptor, already reported in melanoma development and progression. Moreover, looking for new miR-221/-222-dependent target genes, we selected the proto-oncogene ETS-1 because its functional role in melanoma development was not completely elucidated. In fact ETS-1 has been reported either as a valuable diagnostic/prognostic marker or as molecule without a clear association with clinical outcome. Here we demonstrated that in melanoma stem cells as well as in melanoma cell lines ETS-1 is a direct target of miR-222. Furthermore we showed the existence of an ETS1-mediated repression of miR-222 in melanoma cell lines, thus we suggest that in putative melanoma stem cells proliferating as spheres (Mel SC) in comparison to their adherent counterpart (Mel PC) may be active an ETS-1 miR-222 circuitry whose deregulation could be relevant for melanoma progression. Further studies are in progress to understand the function of miRNAs in the context of gene networks controlling cell differentiation and tumorigenicity in melanoma stem-like cells.