RPL9 acts as an oncogene by shuttling miRNAs through exosomes in human hepatocellular carcinoma cells

The exosomal pathway is an essential mechanism that regulates the abnormal content of microRNAs (miRNAs) in hepatocellular carcinoma (HCC). The directional transport of miRNAs requires the assistance of RNA-binding proteins (RBPs). The present study found that RBPs participate in the regulation of miRNA content through the exosomal pathway in HCC cells. First, differential protein expression profiles in the serum exosomes of patients with HCC and benign liver disease were detected using mass spectrometry. The results revealed that ribosomal protein L9 (RPL9) was highly expressed in serum exosomes of patients with HCC. In addition, the downregulation of RPL9 markedly suppressed the proliferation, migration and invasion of HCC cells and reduced the biological activity of HCC-derived exosomes. In addition, using miRNA microarrays, the changes in exosomal miRNA profiles in HCC cells caused by RPL9 knockdown were examined. miR-24-3p and miR-185-5p were most differentially expressed, as verified by reverse transcription-quantitative PCR. Additionally, using RNA immunoprecipitation, it was found that RPL9 was directly bound to the two miRNAs and immunofluorescence assays confirmed that RPL9 was able to carry miRNAs into recipient cells via exosomes. Overexpression of miR-24-3p in cells increased the accumulation of miR-24-3p in exosomes and simultaneously upregulated RPL9. Excessive expression of miR-24-3p in exosomes also increased their bioactivity. Exosome-mediated miRNA regulation and transfer require the involvement of RBPs. RPL9 functions as an oncogene, can directly bind to specific miRNAs and can be co-transported to receptor cells through exosomes, thereby exerting its biological functions. These findings provide a novel approach for modulating miRNA profiles in HCC.