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

Abstract

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.

The role of the PI3K/AKT/mTOR pathway in brain tumor metastasis

“…[125][126][127] In a recent study, PAM signaling was upregulated in microglial cells that were exposed to glioma derived factors, indicating that PAM signaling is needed to force microglial cells into a tumor supportive M2 state. [128] This result was supported by a report showing that mTOR inhibition with rapamycin polarizes microglia cells to express a tumor suppressive M1 phenotype. [129] To date, the exact molecular mechanism by which PI3K signaling contributes to M2-polarization of microglia is still unknown and should be the subject of further investigation.…”

Glioblastoma-infiltrated innate immune cells resemble M0 macrophage phenotype

“…Gene set enrichment analysis (GSEA) identified hallmarks of biological states or processes enriched in glioblastoma patient CD14 + blood cells relative to healthy donors, including transcriptional regulation by MYC and E2F, protein secretion, oxidative phosphorylation, and fatty acid metabolism (Figure 1D). MYC has previously been shown to have a role in tumor-supportive microglia (24), and E2F is involved in macrophage differentiation (25). GSEA enrichment plots and expression heat maps are shown in Supplemental Figure 2.…”

Proinflammatory-Activated Glioma Cells Induce a Switch in Microglial Polarization and Activation Status, From a Predominant M2b Phenotype to a Mixture of M1 and M2a/B Polarized Cells

“…Consistent with this hypothesis, Kaminska and co-workers reported that glioma-derived soluble factors induce the activation of focal adhesion kinase, PI3K (phosphoinositide 3-kinase)/Akt (protein kinase B), ERK (extracellular-signal-regulated kinase), and P38 MAPK (mitogen-activated protein kinase) signaling pathways, resulting in enhancement of cell motility, phagocytosis, sustained proliferation and a distinct genomic response. Such activated microglial phenotype appeared to be unique, being not associated with activation of NFκB (nuclear factor κB) and Stat1 and thus up-regulation of inflammatory genes (Ellert-Miklaszewska et al, 2013). …”

“…Several reports have shown that intra-tumoral microglia express an M2 phenotype in response to soluble factors released by glioma cells. For example, Kaminska and co-workers have recently characterized the M2 activation state of rat microglia cells exposed to a conditioned medium harvested by rat C6 glioma cell cultures as well as the molecular pathways that direct microglia toward a pro-invasive and immunosuppressive phenotype (Gabrusiewicz et al, 2011; Ellert-Miklaszewska et al, 2013). Using co-cultures of the human microglia cell line CHME-5 and the rat glioma C6 cells, Bouzier-Sore and co-workers showed that microglial activation after exposure to C6 cells is bi-phasic, with a prior transitory phagocytic activated phenotype (M2) followed by a reversal to a non-phagocytic status (M1) (Voisin et al, 2010).…”