Secretory MicroRNAs by Exosomes as a Versatile Communication Tool

In the past several years, the importance of microRNA (miRNA) in cancer cells has been recognized. Proper control of miRNA expression is essential for maintaining a steady state of the cellular machinery. Dysregulation of miRNAs leads to the cancer development, meaning that expression profile of miRNAs can be used as cancer biomarker, and recovery of down-regulated miRNAs or inhibition of up-regulated miRNAs will be a novel approach for cancer therapy. Recently, it was discovered that extracellular miRNAs circulate in the blood of both healthy and diseased patients. Most of the circulating miRNAs are included in protein, lipid or lipoprotein complexes, such as RNA-binding proteins, apoptotic bodies, microvesicles, or exosomes, and are, therefore, highly stable. The existence of circulating miRNAs in the blood of cancer patients has raised the possibility that miRNAs may serve as a novel diagnostic marker. However, the secretory mechanism and biological function, as well as the meaning of the existence of extracellular miRNAs, remain largely unclear. Our recent study revealed the secretory mechanism of miRNAs and showed their cell-to-cell transfer. Here we summarize current approaches to modulate the intercellular and interindividual network via silencing signals exported by secretory miRNAs and discuss about the usage of circulating miRNAs as a novel communication tool