Gliomas induce and exploit microglial MT1-MMP expression for tumor expansion

Abstract

for highly malignant gliomas (World Health Organization grade III and IV) there is no successful treatment; patients have an average survival time of approximately 1 y after diagnosis. Glioma cells are highly invasive and infiltrate normal brain tissue, and as a result, surgical resection is always incomplete. Degradation of ECM by membrane-bound and secreted metalloproteases facilitates glioma invasion. In particular, the membrane-bound metalloproteases are pivotal for tumor invasion as they very efficiently digest extracellular matrix proteins and also activate secreted metalloproteases (1) like matrix metalloproteinase-2 (MMP-2, also known as gelatinase A), which is one of the major proteases involved in glioma invasion in mouse models (2) and probably also in humans (3). Hence, membrane-inserted metalloproteases like membrane type 1 matrix metalloproteinase (MT1-MMP) can enable gliomas to invade the brain parenchyma as single cells (4).Microglia are the intrinsic immune cells of the brain; they control the innate and the adaptive immune response in the CNS and are activated by inflammatory or other pathological stimuli (5). Activation of microglial toll-like receptors (TLRs) triggers the innate immune response and can initiate host-defense and tissue repair mechanisms, but also CNS inflammation, neurodegeneration, and trauma (5, 6). As microglial cells are attracted toward glioma in large numbers-glioma tissue consists of as much as 30% microglial cells-and because microglia density in gliomas positively correlates with malignancy, invasiveness, and grading of the tumors (7-9), we investigated if microglia may actively contribute to glioma expansion. Here, we show that soluble factors released from glioma stimulate microglial TLRs, resulting in microglial MT1-MMP expression via the TLR downstream signaling molecules MyD88 and p38 MAPK. In turn, MT1-MMP expression and activity in these immune cells promotes glioma cell invasion and tumor expansion. ResultsGlioma Associated Microglia Over-Express MT1-MMP. We analyzed the expression pattern of the matrix protease MT1-MMP in mouse and human gliomas and found the enzyme to be expressed predominantly in microglial cells closely associated with the tumors. Whereas tumor-free human brain samples showed virtually no MT1-MMP expression, we detected intense MT1-MMP labeling, especially in higher-grade gliomas. Importantly, in human samples, immunolabeling for the microglial marker Iba1 and for MT1-MMP largely overlapped [supporting information (SI) Fig. S1 A-D and Table S1]. Likewise, after injection of a human glioma cell line (U373 cells) into immunodeficient mice, we detected that microglia represent the predominant cell type contributing intratumoral MT1-MMP expression (see Fig. S1E).In our in vivo mouse model, the glioma cells were identified by stable expression of EGFP and microglial cells by immunolabeling for Iba1. In sections obtained from mice 2 weeks after intracerebral injection with isogenic glioma cells (GL261 cells), we found an increased density of mic...

The brain tumor microenvironment

“…The up-regulation of MT1-MMP was mediated by toll-like receptor signaling since it was abolished in a mouse line with deletion of a molecule essential for most toll-like receptor signaling. Glioma growth was also strongly attenuated in the MyD88-deficient mouse model (Markovic et al, 2009). The importance of microglia for glioma growth was further substantiated by studying animals in which microglia were depleted.…”

“…To restrict the effect of ganciclovir on the intrinsic microglial population, the mice were irradiated before bone marrow transplanted with wild-type monocytes. The ganciclovir treatment led to a considerable depletion of microglia and to an 80% reduction in glioma volume (Markovic et al, 2009). Using the similar mouse model, Galarneau et al (2007) injected ganciclovir i.p.…”

Standard of care therapy for malignant glioma and its effect on tumor and stromal cells

“…Results from in vitro studies support the idea that microglia can promote tumor invasion and growth (Bettinger et al, 2002;Markovic et al, 2005). Perhaps the most convincing evidence of the importance of microglia to glioma formation is from in vivo studies that have shown that tumor cell growth is inhibited in microglia-deficient glioma mouse models (Daginakatte and Gutmann, 2007;Markovic et al, 2009). These findings suggest that therapies that target glioma-associated microglia may be promising antitumor agents.…”

“…In addition, brain tumor stem-like cells can act on microglia and induce the secretion of cytokines (for example, TGFb1, MIC-1, sCSF-1) that promote an immunosuppressive phenotype (Wu et al, 2010). In addition, it has been shown that soluble factors secreted by tumor cells can cause the upregulation of matrix metalloproteinase-14 (MMP-14) expression in microglia (Markovic et al, 2009). MMP-14 can activate inactive matrix metalloprotease-2 secreted by glioma cells and lead to the break down of the extracellular matrix.…”

Antidepressant use and risk of central nervous system metastasis

“…It is postulated that glia contribute to this inflammatory process by producing cytokines including IL-1β and TNF-α [14]. Glial cells may also affect metastatic growth within the brain [311,312] and be used by tumor cells to support its progression and invasion [313,314].…”