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Background: Brain-tumor-initiating cells (BTICs) of proneural and mesenchymal origin contribute to the highly malignant phenotype of glioblastoma (GB) and resistance to current therapies. Methods: BTICs of different subtypes representing tumor heterogeneity were challenged with OXPHOS (oxidative phosphorylation) inhibition to assess the differential effects of metabolic intervention on key resistance features. Results: Whereas mesenchymal BTICs were more invasive, more glycolytic and less responsive to OXPHOS-inhibition, proneural BTICs were less invasive, catabolized glucose more via the pentose phosphate pathways, and responded better to OXPHOS inhibition. Conclusion: Targeting glycolysis may be a promising approach to inhibit highly invasive tumor cells of mesenchymal origin, whereas proneural cells are more responsive to OXPHOS inhibition. Future clinical trials exploring metabolic interventions should account for metabolic heterogeneity of brain tumors to overcome resistance to current treatments.

Glioblastoma (GBM) is a particularly devastating tumor with a median survival of about 16 months. Recent research has revealed novel insights into the outstanding heterogeneity of this type of brain cancer. However, all GBM subtypes share the hallmark feature of aggressive invasion into the surrounding tissue. Invasive glioblastoma cells escape surgery and focal therapies and thus represent a major obstacle for curative therapy. This review aims to provide a comprehensive understanding of glioma invasion mechanisms with respect to tumor-cell-intrinsic properties as well as cues provided by the microenvironment. We discuss genetic programs that may influence the dissemination and plasticity of GBM cells as well as their different invasion patterns. We also review how tumor cells shape their microenvironment and how, vice versa, components of the extracellular matrix and factors from non-neoplastic cells influence tumor cell motility. We further discuss different research platforms for modeling invasion. Finally, we highlight the importance of accounting for the complex interplay between tumor cell invasion and treatment resistance in glioblastoma when considering new therapeutic approaches.

// Verena Leidgens 1 , Judith Proske 1, * , Lisa Rauer 1, * , Sylvia Moeckel 1 , Kathrin Renner 2 , Ulrich Bogdahn 1 , Markus J. Riemenschneider 3 , Martin Proescholdt 4 , Arabel Vollmann-Zwerenz 1 , Peter Hau 1 , Corinna Seliger 1 1 Department of Neurology and Wilhelm Sander-NeuroOncology Unit, University Hospital Regensburg, Regensburg, Germany 2 Department of Internal Medicine III, University Hospital Regensburg, Regensburg, Germany 3 Department of Neuropathology, Regensburg University Hospital, Regensburg, Germany 4 Department of Neurosurgery, University Hospital Regensburg, Regensburg, Germany * These authors have contributed equally to this work Correspondence to: Corinna Seliger, email: corinna.seliger@klinik.uni-regensburg.de Keywords: glioma, BTIC, STAT3, Stattic, metformin Received: August 20, 2016 Accepted: November 21, 2016 Published: December 24, 2016 ABSTRACT Glioblastoma (GBM) is the most common and malignant type of primary brain tumor and associated with a devastating prognosis. Signal transducer and activator of transcription number 3 (STAT3) is an important pathogenic factor in GBM and can be specifically inhibited with Stattic. Metformin inhibits GBM cell proliferation and migration. Evidence from other tumor models suggests that metformin inhibits STAT3, but there is no specific data on brain tumor initiating cells (BTICs). We explored proliferation and migration of 7 BTICs and their differentiated counterparts (TCs) after treatment with Stattic, metformin or the combination thereof. Invasion was measured in situ on organotypic brain slice cultures. Protein expression of phosphorylated and total STAT3, as well as AMPK and mTOR signaling were explored using Western blot. To determine functional relevance of STAT3 inhibition by Stattic and metformin, we performed a stable knock-in of STAT3 in selected BTICs. Inhibition of STAT3 with Stattic reduced proliferation in all BTICs, but only in 4 out of 7 TCs. Migration and invasion were equally inhibited in BTICs and TCs. Treatment with metformin reduced STAT3-phosphorylation in all investigated BTICs and TCs. Combined treatment with Stattic and metformin led to significant additive effects on BTIC proliferation, but not migration or invasion. No additive effects on TCs could be detected. Stable STAT3 knock-in partly attenuated the effects of Stattic and metformin on BTICs. In conclusion, metformin was found to inhibit STAT3-phosphorylation in BTICs and TCs. Combined specific and unspecific inhibition of STAT3 might represent a promising new strategy in the treatment of glioblastoma.

To this day, glioblastoma (GBM) remains an incurable brain tumor. Previous research has shown that metformin, an oral anti-diabetic drug, may decrease GBM cell proliferation and migration especially in brain tumor initiating cells (BTICs). As transforming growth factor β 2 (TGF-β2) has been reported to promote high-grade glioma and is inhibited by metformin in other tumors, we explored whether metformin directly interferes with TGF-β2-signaling. Functional investigation of proliferation and migration of primary BTICs after treatment with metformin+/−TGF-β2 revealed that metformin doses as low as 0.01 mM metformin thrice a day were able to inhibit proliferation of susceptible cell lines, whereas migration was impacted only at higher doses. Known cellular mechanisms of metformin, such as increased lactate secretion, reduced oxygen consumption and activated AMPK-signaling, could be confirmed. However, TGF-β2 and metformin did not act as functional antagonists, but both rather inhibited proliferation and/or migration, if significant effects were present. We did not observe a relevant influence of metformin on TGF-β2 mRNA expression (qRT-PCR), TGF-β2 protein expression (ELISA) or SMAD-signaling (Western blot). Therefore, it seems that metformin does not exert its inhibitory effects on GBM BTIC proliferation and migration by altering TGF-β2-signaling. Nonetheless, as low doses of metformin are able to reduce proliferation of certain GBM cells, further exploration of predictors of BTICs' susceptibility to metformin appears justified.

Versican is a large chondroitin sulphate proteoglycan produced by several tumor cell types, including high-grade gliomas. Increased expression of distinct versican isoforms in the extracellular matrix plays a role in tumor cell growth, adhesion and migration. We have recently shown that transforming growth factor (TGF-beta)2, an important modulator of glioma invasion, interacts with versican isoforms V0/V1 during malignant progression of glioma in vitro. However, the distinct subtype of versican that modulates these effects could not be specified. Here, we show that transient down-regulation of V1 by siRNA leads to a significant reduction of proliferation and migration in glioblastoma cell lines and glioblastoma progenitor cells, whereas tumor cell attachment stays unaffected. We conclude that V1 plays a predominant role in modulating central pathophysiological mechanisms as proliferation and migration in glioblastoma. Considering that TGF-beta is a master regulator of glioma pathophysiology, and that V0/1 is induced by TGF-beta2, therapeutic regulation of V1 may induce meaningful effects on glioma cell migration not only in vitro, but also in vivo.