Polysome arrest restricts miRNA turnover by preventing exosomal export of miRNA in growth-retarded mammalian cells.

MicroRNAs (miRNAs) are tiny posttranscriptional regulators of gene expression in metazoan cells, where activity and abundance of miRNAs are tightly controlled. Regulated turnover of these regulatory RNAs is important to optimize cellular response to external stimuli. We report that the stability of mature miRNAs increases inversely with cell proliferation, and the increased number of microribonucleoproteins (miRNPs) in growth-restricted mammalian cells are in turn associated with polysomes. This heightened association of miRNA with polysomes also elicits reduced degradation of target mRNAs and impaired extracellular export of miRNA via exosomes. Overall polysome sequestration contributes to an increase of cellular miRNA levels but without an increase in miRNA activity. Therefore miRNA activity and turnover can be controlled by subcellular distribution of miRNPs that may get differentially regulated as a function of cell growth in mammalian cells.