Glioma-associated microglial MMP9 expression is upregulated by TLR2 signaling and sensitive to minocycline

The invasiveness of malignant gliomas is one of the major obstacles in glioma therapy and the reason for the poor survival of patients. Glioma cells infiltrate into the brain parenchyma and thereby escape surgical resection. Glioma associated microglia/macrophages support glioma infiltration into the brain parenchyma by increased expression and activation of extracellular matrix degrading proteases such as matrix-metalloprotease 2, matrix-metalloprotease 9 and membrane-type 1 matrix metalloprotease. In this work we demonstrate that, matrix-metalloprotease 9 is predominantly expressed by glioma associated microglia/macrophages in mouse and human glioma tissue but not by the glioma cells. Supernatant from glioma cells induced the expression of matrix-metalloprotease 9 in cultured microglial cells. Using mice deficient for different Toll-like receptors we identified Toll-like receptor 2/6 as the signalling pathway for the glioma induced upregulation of microglial matrix-metalloprotease 9. Also in an experimental mouse glioma model, Toll-like receptor 2 deficiency attenuated the upregulation of microglial matrix-metalloprotease 9. Moreover, glioma supernatant triggered an upregulation of Toll-like receptor 2 expression in microglia. Both, the upregulation of matrix-metalloprotease 9 and Toll-like receptor 2 were attenuated by the antibiotic minocycline and a p38 mitogen activated protein kinase antagonist in vitro. Minocycline also extended the survival rate of glioma bearing mice when given to the drinking water. Thus glioma cells change the phenotype of glioma associated microglia/macrophages in a complex fashion using Toll-like receptor 2 as an important signalling pathway and minocycline further proved to be a potential candidate for adjuvant glioma therapy.