Exosomal microRNAs in cancer metastasis: A bridge between tumor micro and macroenvironment

Malignant tumors are complicated structures of cancer cells that are constantly in communication with their local and distant environment. Exosomes are released by tumor cells and can facilitate the cell-cell interaction within the local microenvironment and the primary tumor. In fact, exosomes are secreted by both tumor and non-tumor cells, to provide a mutual communication network between cells and their micro- and/or macro-environments. Exososmes can contain a variety of biological cargos mostly based on their originated cells. Uptake of these exosomes by their recipient cells results in the alterations that their cargo can exert. MicroRNAs are identified as one of the most critical exosomal components, considering their pivotal regulatory roles in distinct biological process, including metastasis. Release and absorbance of exosomal microRNAs is possible by various cells within the host, and can have distinct biological consequences. Therefore, in this review we will discuss the role of exosomal microRNAs derived from tumor cells and untransformed cells within their micro- and macroenvironment in cancer progression and metastasis.