Identification of miRNAs in small extracellular vesicles and their role in melanoma phenotypic plasticity

Abstract

Cutaneous melanoma is characterized by the malignant transformation of melanocytes, cells responsible for the production of melanin pigment, which is located close to the basal layer, between the dermis and epidermis. Despite having a low incidence, it is considered the most aggressive skin cancer due to its high metastatic capacity and resistance to therapies. The change in phenotypes (phenotype switch) observed in melanomas contributes to tumor heterogeneity and difficulties in the effective elimination of the tumor by different therapies. Both genetic and epigenetic alterations play a role in the progression of this tumor, and the participation of epigenetic mechanisms in the regulation of the phenotype switch is crucial. Among the epigenetic alterations that contribute to the development of melanoma are those that involve non-coding RNAs, such as miRNAs. MicroRNAs (miRNAs) constitute a class of small non-coding RNAs (ncRNA), which are characterized by not producing proteins. The wide regulation of gene expression by miRNAs implies the translational repression or degradation of mRNAs, influencing physiological processes such as cell proliferation, differentiation, angiogenesis, and apoptosis. This control of different cellular pathways, by miRNAs, allows the loss or gain of their expression to be related to the development of several diseases, including melanoma. The knowledge about miRNAs gained a new perspective when it was discovered that these small RNAs are not restricted to the cytoplasm of cells, but can be released into the extracellular medium in small extracellular vesicles (sEVs), also known as exosomes. This intercellular communication is related to signaling in the tumor microenvironment and, as a consequence, to the enrichment of pro-metastatic and pro-tumor factors. Therefore, exosomal miRNAs are being studied regarding their potential use as biomarkers, their prognostic value, and their interference in pathways related to the progression of melanoma. Using the linear cell model of melanoma progression in our laboratory, 3 miRNAs described in EMT (epithelium-mesenchymal transition) and metastasis signatures were selected based on data from miRNA expression arrays of cell lines that correspond to steps distinct from melanoma progression: melanocytes (melan-a), premalignant melanocytes (4C), undifferentiated and poorly proliferating (4C11-) and highly proliferative and metastatic pigmented (4C11+) cells. This group of miRNAs (miR-214, miR-125b-5p, and miR-125a-5p) was selected considering their relevance in tumorigenesis, and the existence of few (or none) studies associating them with the content of sEVs in melanoma. This study aims precisely to identify and examine the properties of such miRNAs present in sEVs in regulating the change in melanoma phenotypes, from the modulation of their expression by gene silencing and overexpression. This knowledge can result in the identification of new prognostic markers for melanoma, the development of more effective therapeutic strategies, and in addition to providing information about its role in the aggressiveness of the disease.(AU)