In cancer, deregulated expression levels of miRNAs are associated with tumorigenesis, tumor progression, metastasis and drug resistance. Apart from their release by apoptotic and necrotic cells, miRNAs can also be actively secreted into the blood circulation by exosomes. Exosomal miRNAs are thought to play an important role in cell-to-cell communication. Based on their biological functions and the possibility of quantifying miRNAs in patient blood in real time, these small non-coding RNA molecules may be a new promising class of potential non-invasive biomarkers. Screening of these liquid biopsies may provide information on target molecules of miRNAs and aberrant signaling pathways that can be blocked by a chosen targeted therapy. Consequently, therapy-associated modulations may facilitate treatment decisions. The present article discusses the potential clinical use of exosomal miRNAs as diagnostic and prognostic tumor markers. The emerging role of exosomes as a mediator of cell-to-cell signaling, to transfer miRNAs between cells, is also discussed.
