Exosome mediated communication in cancer: melanoma and sarcoma models

Exosomes (EXOs) are nanovesicles of diameter ranging between 50 to 140 nm, distinguished from other cell-derived vesicles by their origin, size, morphology and composition. Their stimulatory or inhibitory signaling activities are mediated by their content (mRNAs, microRNAs and proteins) that can be transferred from the cells of origin to recipient cells, influencing the surrounding microenvironment besides cell behavior. In this study we investigated EXO-mediated communications in two cancer models, melanoma and Ewing’s sarcoma. In view of our previous results demonstrating miR-221&222 as key factors for melanoma development and dissemination, we demonstrated that the EXO-mediated horizontal transfer of miR-222 was competent to deliver miR-222-associated properties increasing tumor malignancy. Melanoma-purified vesicles were characterized and investigated for the functionality of miR-222 in EXO-mediated tumorigenesis. Our data showed that EXOs secreted by miR-222-overexpressing cells induced a protumorigenic program in target cells, mainly through the upmodulation of the PI3K/AKT pathway. The reverse effects were obtained with EXOs recovered after inhibition of endogenous miR-221 and miR-222 by antagomir transfections. The possible differential significance of PI3K/AKT blockade in miR-222-transduced vs control cells was assessed by using BKM120, a pan inhibitor of PI3K. Results showed the capability of miR-222 overexpression to overcome BKM120-dependent effects. We then demonstrated the role of Ewing’s sarcoma-derived EXOs as mediators of signals involved in cancer growth, metastases and differentiation. Ewing’s sarcoma (EWS) is an aggressive childhood bone tumor characterized in the majority of cases by the presence of the fusion oncoprotein EWS-FLI1 and by high expression of the membrane glycoprotein CD99. These features, which are the necessary conditions for the pathogenesis of EWS, mediate tumor progression and maintain the cells in a dedifferentiated state. We evaluated the ability of EXOs, expressing or not CD99, to modulate the phenotype of EWS cells. We observed that the delivery of EXOs devoid of CD99 was sufficient to induce neural differentiation in EWS recipient cells through the inhibition of Notch-NF-kB signaling mediated by miR-34a overexpression. All together these observations would provide a significant step toward new biomarker discovery and innovative therapeutic options. These data on one side support miR-222 responsibility in the exosome-associated melanoma properties, on the other the role of CD99-shRNA/miR-34a-derived EXOs to induce differentiation in EWS, thus further indicating microRNAs as potential diagnostic, prognostic and eventually therapeutic biomarkers.