Exosome-derived microRNAs in cancer progression: angel or devil?

Exosomes, a class of small extracellular vesicles (sEVs) having a diameter of 40–100 nm, are produced by all cells and found circulating freely in all body fluids, including sputum, plasm, urine and bronchoalveolar lavage (1-3). This biogenesis of exosomes originates in the endosomal compartment of a parent cell, and then mediate specific cell-to-cell communication by transferring information (e.g., proteins, RNAs, and DNAs) to target cells (4). The process is completed through three main mechanisms: endocytosis by phagocytosis, direct fusion with the plasma membrane and receptor-ligand interactions (5). Recently, studies have shown that exosomes play a role in tumorigenesis, invasion, metastasis, and drug resistance via specific cellcell communication involving the transfer a series of small molecules, including microRNAs (6,7). The weight of evidence supports that exosome-derived microRNAs is associated with the development and progression of various cancers, such as esophageal cancer (8), bladder cancer (9), lung cancer (10), nasopharyngeal carcinoma (11) and colorectal cancer (12), etc. Over the years, abnormally elevated levels of exosome-derived microRNAs in cancer tissues have fueled a large number of studies attempting to determine their potential as new diagnostic biomarkers and therapeutic targets for cancer.