MicroRNA-99b-5p downregulates protein synthesis in human primary myotubes

Abstract

MicroRNAs (miRNAs) are important regulators of cellular homeostasis and exert their effect by directly controlling protein expression. We have previously reported an age-dependent, negative association between microRNA-99b (miR-99b-5p) expression and muscle protein synthesis in human muscle in vivo. Here we investigated the role of miR-99b-5p as a potential negative regulator of protein synthesis via inhibition of MTOR signalling in human primary myocytes. Overexpressing miR-99b-5p in human primary myotubes from young and old subjects significantly decreased protein synthesis with no effect of donor age. A binding interaction between miR-99b-5p and its putative binding site within the MTOR 3ʹ UTR was confirmed in C₂C₁₂ myoblasts. The observed decline in protein synthesis was however not associated with a suppression of the MTOR protein, but of its regulatory associated protein RPTOR. These results demonstrate that modulating the expression levels of a miRNA can regulate protein synthesis in human muscle cells and provide a potential mechanism for muscle wasting in vivo.