miR-21 regulates immunosuppression mediated by myeloid-derived suppressor cells by impairing RUNX1-YAP interaction in lung cancer

BACKGROUND: Myeloid-derived suppressor cells (MDSCs) are known suppressors of antitumor immunity and contribute to immunosuppressive microenvironment during tumor development including lung cancer. Accumulating evidence shows microRNAs (miRNAs) affect tumor-expanded MDSC accumulation and function in tumor microenvironment and favor solid tumor growth. Herein, we aim to characterize the role of miR-21 in regulating the accumulation and activity of MDSCs in lung cancer. METHODS: The proportions of MDSCs, T helper cells (Th), and cytotoxic T lymphocytes (CTL) were evaluated by flow cytometric analyses of peripheral blood and tumor tissues collected from Lewis lung-cancer-bearing mice. T cell proliferation assay was performed in CD4+ or CD8+ T cells cocultured with MDSCs. MDSC apoptosis was examined by flow cytometric analysis. The levels of IL-10, TGF-β, and GM-CSF in mouse serum were determined by ELISA. miR-21 targeting RUNX1 and RUNX1 interaction with YAP were evaluated by RIP, dual-luciferase reporter gene, and ChIP assays. RESULTS: MiR-21 inhibition by its antagomir reduced the proportion of MDSCs, increased the proportion of Th and CTL in peripheral blood and tumor tissues of Lewis lung-cancer-bearing mice, protected Th and CTL from the suppression of MDSCs, increased apoptosis of MDSCs, but reduced IL-10, TGF-β and GM-CSF levels in mouse serum. RUNX1 could transcriptionally inhibit the YAP expression, whereas miR-21 targeting RUNX1 led to elevated YAP expression levels. Mechanistic investigation showed that miR-21 maintained MDSC accumulation in tumor microenvironment and promoted immunosuppressive ability of MDSCs in Lewis lung-cancer-bearing mice by down-regulating RUNX1and up-regulating YAP. CONCLUSIONS: Taken together, the study provides evidence that targeting miR-21 in MDSCs may be developed as an immunotherapeutic approach to combat lung cancer development.