Extracellular miRNAs: Origin, Function and Biomarkers in Hepatic Diseases

MicroRNAs (miRNAs), a class of 19–24 nucleotides non-coding RNAs, regulate gene expression by inhibiting both translation and stability of specific mRNAs at the post-transcriptional level. The existence of miRNAs in a series of mammalian body fluids as extracellular nuclease-resistant entities, together with the aberrant expression of miRNAs during the occurrence and development of a wide range of diseases, triggers the possibility that miRNAs as promising noninvasive diagnostic biomarkers are applied to predict the pathological status of the body. However, the origin and biological function of extracellular miRNAs have not been systematically elucidated. In this review, we summarize the current literature on the biogenesis and post-transcriptional regulation of miRNAs, discuss available evidence regarding the possible origin and release of extracellular miRNAs, and collect novel views on their potentials as key mediators in cell–cell communication processes. Finally, we shed light on the accumulating knowledge about their utilities as diagnostic biomarkers in hepatic diseases.