Annexin A2 and Microglia/Macrophages in Glioma Progression
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Gliomas are highly invasive brain tumors with the occurrence of numerous microglia/macrophages (MG/MP) in and around the tumor. Annexin A2 is overexpressed in many cancers and correlates with increased plasmin activity on the tumor cell surface. Plasmin mediates degradation of extracellular matrix and angiogenesis to facilitate tumor growth. In this study, we used a mouse glioma cell line, GL261-EGFP, and stable clones transfected with an annexin A2 knockdown (annA2KD) construct, GL261-EGFP-annA2KD. We found that the annA2KD decreased glioma cell migration in vitro and decreased membrane-bound plasmin activity. In vivo we injected GL261-EGFP cells into the mouse brain and the glioma progression was followed. Knockdown of annexin A2 in glioma cells decreased tumor size and slowed down tumor progression, characterized by decreased invasion, angiogenesis and proliferation, as well as increased apoptosis in tumor tissue of the annA2KD group. We also investigated the interaction between glioma and MG/MPs, and the contribution of MG/MPs to glioma progression. We used a glioma-microglia in culture system to establish the effects the tumor and microglial cells have on each other. We then assessed glioma progression in vivo after MG/MP ablation or in the setting of exaggerated MG/MP activation. We show that glioma cells activate microglia but inhibit their phagocytic activities. Local ablation of MG/MP in vivo significantly decreased tumor size and improved survival curves. Conversely, pharmacological activation of MG/MP increased glioma size through stimulating tumor proliferation and inhibiting apoptosis. In agreement with recent reports we found that expression of the chemokine CCL21 was enhanced after MG/MP activation and correlates with tumor growth. However the macrophage/microglial inhibitory factor (MIF) had the opposite effects on glioma progression. We expect that these experiments will provide a better insight into the role of annexin A2 and MG/MPs in glioma progression, and will hopefully allow us to design potential ways to interfere with the glioma-microglia interaction and affect tumor growth and survival.