Knockdown of milk‐fat globule EGF factor‐8 suppresses glioma progression in GL261 glioma cells by repressing microglial M2 polarization

Tumor‐associated microglial cells promote glioma growth, invasion, and chemoresistance by releasing inflammatory factors. Milk fat globule EGF factor 8 protein (MFG‐E8), a secreted glycoprotein, is closely related to tissue homeostasis and anti‐inflammation. In the present study, we investigated the role of MFG‐E8 in microglial polarization and glioma progression in vitro and in vivo. We found that glioma cells secrete comparable amounts of MFG‐E8 in culture media to astrocytes. Recombinant MFG‐E8 triggered microglia to express the M2 polarization markers, such as arginase‐1 (ARG‐1), macrophage galactose‐type C‐type lectin‐2 (MGL‐2), and macrophage mannose receptor (CD206). Forced expression of MFG‐E8 in BV‐2 microglia cells not only promoted IL‐4‐induced M2 polarization but also inhibited lipopolysaccharide (LPS)‐induced M1 microglial polarization. Mechanistic studies demonstrated that recombinant MFG‐E8 markedly induced signal transducer and activator of transcription 3 (STAT3) phosphorylation, and the STAT3 inhibitor stattic significantly blocked MFG‐E8‐induced ARG‐1 expression. Administration of antibody against MFG‐E8 and knockdown of its receptor, integrin β3, significantly attenuated MFG‐E8‐induced ARG‐1 expression. Similarly, knockdown of MFG‐E8 also markedly reduced IL‐4‐induced M2 marker expression and increased LPS‐induced M1 marker expression in microglia cells. Moreover, the knockdown of MFG‐E8 in GL261 glioma cells inhibited cell proliferation and enhanced chemosensitivity to 1,3‐bis(2‐chloroethyl)‐1‐nitrosourea (BCNU), which was likely associated with the downregulation of FAK/AKT activation and STAT3/cyclin D1 signaling. The murine GL261 glioma experimental model demonstrated that knockdown of MFG‐E8 significantly reduced tumor size and extended survival times. Additionally, attenuated CD11b+ cell infiltration and reduced CD206+ expression in CD11b+ cells were also observed in an MFG‐E8 knockdown GL261 murine glioma model. These results suggested that inhibition of MFG‐E8 might hamper the immunosuppressive microenvironment in gliomas and therefore ameliorate tumor progression. Knockdown of milk fat globule EGF factor 8 protein (MFG‐E8) repressed glioma progression in vitro and in vivo by directly affecting glioma growth and TAM M2 polarization.