MicroRNAs (miRNAs) play an important role in tumorigenesis, but their role in tumor-induced immune suppression is largely unknown. STAT3 signaling, a key pathway mediating immune suppression in the tumor microenvironment, is responsible for the transcription of several important miRNAs. In this study, we observed that miR-146a, a known important regulator of immune responses, was downregulated by blocking activated STAT3 in hepatocellular carcinoma (HCC) cells. Furthermore, miR-146a inhibition in HCC cells not only altered the STAT3 activation-associated cytokine profile but also reversed HCC-induced NK cell dysfunction in vitro and improved the anti-tumor effect of lymphocytes in vivo. Importantly, ChIP and luciferase reporter assays confirmed that STAT3 directly bound to the miR-146a promoter and induced miR-146a expression. These findings indicated that miR-146a expression was regulated by aberrantly activated STAT3 in HCC cells and exerted negative effects on anti-tumor immune response, which resulted in the upregulation of cytokines such as TGF-β, IL-17, VEGF and downregulation of type I IFN to create an immunosuppressive microenvironment. This further insight into understanding the mechanism responsible for tumor-induced immune suppression highlights the potential application of miR-146a as a novel immunotherapeutic target for HCC.
Keywords: HCC; STAT3; anti-tumor immune suppression; miR-146a; miRNA.