Minocycline reduces glioma expansion and invasion by attenuating microglial MT1-MMP expression.

Glioma cells release soluble factors, which induce the expression of membrane type 1 matrix metalloprotease (MT1-MMP) in tumor associated microglia and then exploit MT1-MMP mediated matrix degradation for invasion. Here, we show that minocycline blocked the increase in MT1-MMP expression and activity in cultivated microglia stimulated with glioma conditioned medium. Glioma growth within an organotypic brain slice preparation was reduced by minocycline and this reduction depended on the presence of microglia. Glioma growth in an experimental mouse model was strongly reduced by the addition of minocycline to drinking water, compared to untreated controls. Coherently, we observed in our orthotopic glioma implantation model, that MT1-MMP was abundantly expressed in glioma associated microglia in controls, but was strongly attenuated in tumors of minocycline treated animals. Overall, our study indicates that the clinically approved antibiotic minocycline is a promising new candidate for adjuvant therapy against malignant gliomas.

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

Diffuse infiltration of glioma cells into normal brain tissue is considered to be a main reason for the unfavorable outcomes of patients with malignant gliomas. Invasion of glioma cells into the brain parenchyma is facilitated by metalloprotease-mediated degradation of the extracellular matrix. Metalloproteases are released as inactive pro-forms and get activated upon cleavage by membrane bound metalloproteases. Here, we show that membrane type 1 metalloprotease (MT1-MMP) is up-regulated in glioma-associated microglia, but not in the glioma cells. Overexpression of MT1-MMP is even lethal for glioma cells. Glioma-released factors trigger the expression and activity of MT1-MMP via microglial toll-like receptors and the p38 MAPK pathway, as deletion of the toll-like receptor adapter protein MyD88 or p38 inhibition prevented MT1-MMP expression and activity in cultured microglial cells. Microglial MT1-MMP in turn activates glioma-derived pro-MMP-2 and promotes glioma expansion, as shown in an ex vivo model using MT1-MMP-deficient brain tissue and a microglia depletion paradigm. Finally, MyD88 deficiency or microglia depletion largely attenuated glioma expansion in 2 independent in vivo models.

Transferrin-receptor-mediated iron accumulation controls proliferation and glutamate release in glioma cells.

Transferrin receptors (TfR) are overexpressed in brain tumors, but the pathological relevance has not been fully explored. Here, we show that TfR is an important downstream effector of ets transcription factors that promotes glioma proliferation and increases glioma-evoked neuronal death. TfR mediates iron accumulation and reactive oxygen formation and thereby enhanced proliferation in clonal human glioma lines, as shown by the following experiments: (1) downregulating TfR expression reduced proliferation in vitro and in vivo; (2) forced TfR expression in low-grade glioma accelerated proliferation to the level of high-grade glioma; (3) iron and oxidant chelators attenuated tumor proliferation in vitro and tumor size in vivo. TfR-induced oxidant accumulation modified cellular signaling by inactivating a protein tyrosine phosphatase (low-molecular-weight protein tyrosine phosphatase), activating mitogen-activated protein kinase and Akt and by inactivating p21/cdkn1a and pRB. Inactivation of these cell cycle regulators facilitated S-phase entry. Besides its effect on proliferation, TfR also boosted glutamate release, which caused N-methyl-D-aspartate-receptor-mediated reduction of neuron cell mass. Our results indicate that TfR promotes glioma progression by two mechanisms, an increase in proliferation rate and glutamate production, the latter mechanism providing space for the progressing tumor mass.