Gastric Cancer-Derived Exosomal MATN3 Favours Immunosuppressive Tumour Microenvironment by Activating Autophagy in an EGFR/ELK1/ATG12 Signalling Dependent Manner.

Gastric cancer (GC)-derived exosomes (Exos) have been identified to facilitate GC progression by inducing M2 macrophage polarization. This study investigated the biological function of exosomal matrilin-3 (MATN3) in M2 macrophage polarisation during GC development and its underlying mechanism. Exos were isolated from GC cells and then co-cultured with THP-1-derived macrophages. Macrophage polarisation was evaluated by measuring the levels of M1/M2 macrophage markers. Target molecule expression was evaluated by RT-qPCR, Western blotting and immunohistochemical staining. LC3II expression and the co-localisation of MATN3 and epidermal growth factor receptor (EGFR) were determined by immunofluorescent staining. In vivo growth of GC cells was assessed in a xenograft mouse model. Molecular mechanisms were analysed by Co-IP, ChIP, dual-luciferase reporter assay and ubiquitination assay. MATN3 was highly expressed in GC and its high expression was negatively associated with the overall survival and M1 macrophage marker expression of GC patients. The in vitro experiments validated that MATN3 was secreted by GC-Exos, which promoted M2 macrophage polarisation via autophagy activation. In addition, exosomal MATN3 contributed to in vivo growth of GC cells via promoting M2 macrophage infiltration. Mechanistically, MATN3 interacted with EGFR to enhance its protein stability, which activated Ets-like protein-1 (ELK1) and consequently promoted ATG12-mediated autophagy. Activation of the EGFR/ELK1 pathway abolished exosomal MATN3 silencing-mediated inhibitory effect on autophagy and M2 macrophage polarisation. GC-derived exosomal MATN3 exerted an oncogenic role by inducing M2 macrophage polarisation via activation of the EGFR/ELK1/ATG12 axis-mediated autophagy, which provides potential therapeutic targets for GC.