Unraveling the Mystery of Cancer by Secretory microRNA: Horizontal microRNA Transfer between Living Cells

microRNAs (miRNAs) have been identified as a fine-tuner in a wide array of biological processes, including development, organogenesis, metabolism, and homeostasis. Deregulation of miRNAs causes diseases, especially cancer. This occurs through a variety of mechanisms, such as genetic alterations, epigenetic regulation, or altered expression of transcription factors, which target miRNAs. Recently, it was discovered that extracellular miRNAs circulate in the blood of both healthy and diseased patients. Since RNase is abundant in the bloodstream, most of the secretory miRNAs are contained in apoptotic bodies, microvesicles, and exosomes or bound to the RNA-binding proteins. However, the secretory mechanism and biological function, as well as the significance of extracellular miRNAs, remain largely unclear. In this article, we summarize the latest and most significant discoveries in recent peer-reviewed research on secretory miRNA involvement in many aspects of physiological and pathological conditions, with a special focus on cancer. In addition, we discuss a new aspect of cancer research that is revealed by the emergence of “secretory miRNA.”