Molecular definition of the pro‐tumorigenic phenotype of glioma‐activated microglia

Microglia are myeloid cells residing in the central nervous system that participate in inflammatory responses and could promote injury and repair. Gliomas attract microglia and polarize them into tumor‐supporting cells that participate in matrix remodeling, invasion, angiogenesis, and suppression of adaptive immunity. Although signaling pathways and critical regulators underlying classical inflammation are well established, signal transduction and transcriptional circuits underlying the alternative activation of microglia are poorly known. Using primary rat microglial cultures exposed to glioma conditioned medium or lipopolysaccharide (LPS), we demonstrate that microglia adapt different fates and polarize into pro‐inflammatory or alternatively activated cells. Glioma‐derived factors increased cell motility, phagocytosis, and sustained proliferation of microglial cells that was mediated by enhanced focal adhesion kinase and PI‐3K/Akt signaling. The signals from glioma cells induced ERK and p38 MAPK but not JNK signaling and failed to activate pro‐inflammatory Stat1 and NFκB signaling in microglial cells. Transcriptome analysis of microglial cultures at 6 h after exposure to glioma‐conditioned medium or LPS revealed different patterns of gene expression. Glioma‐induced activation was associated with induction of genes coding for ID (inhibitor of DNA binding) 1/3 and c‐Myc, markers of the alternative phenotype Arg1, MT1‐MMP, CXCL14, and numerous cytokines/chemokines implicated in immune cell trafficking. Many classical inflammation‐related genes and signaling pathways failed to be induced. Our study indicates for the first time molecular pathways that direct microglia toward the pro‐invasive, immunosuppressive phenotype.