High-grade gliomas, glioblastomas (GB), are refractory to conventional treatment combining surgery, chemotherapy, mainly temozolomide, and radiotherapy. This highlights an urgent need to develop novel therapies and increase the efficacy of radio/chemotherapy for these very aggressive and malignant brain tumors. Recently, tumor metabolism became an interesting potential therapeutic target in various cancers. Accordingly, combining drugs targeting cell metabolism with appropriate chemotherapeutic agents or radiotherapy has become attractive. In light of these perspectives, we were particularly interested in the anti-cancer properties of a biguanide molecule used for type 2 diabetes treatment, metformin. In our present work, we demonstrate that metformin decreases mitochondrial-dependent ATP production and oxygen consumption and increases lactate and glycolytic ATP production. We show that metformin induces decreased proliferation, cell cycle arrest, autophagy, apoptosis and cell death in vitro with a concomitant activation of AMPK, Redd1 and inhibition of the mTOR pathway. Cell sensitivity to metformin also depends on the genetic and mutational backgrounds of the different GB cells used in this study, particularly their PTEN status. Interestingly, knockdown of AMPK and Redd1 with siRNA partially, but incompletely, abrogates the induction of apoptosis by metformin suggesting both AMPK/Redd1-dependent and –independent effects. However, the primary determinant of the effect of metformin on cell growth is the genetic and mutational backgrounds of the glioma cells. We further demonstrate that metformin treatment in combination with temozolomide and/or irradiation induces a synergistic anti-tumoral response in glioma cell lines. Xenografts performed in nude mice demonstrate in vivo that metformin delays tumor growth. As current treatments for GB commonly fail to cure, the need for more effective therapeutic options is overwhelming. Based on these results, metformin could represent a potential enhancer of the cytotoxic effects of temozolomide and/or radiotherapy.

Glioblastomas (GBM) are very aggressive and malignant brain tumors, with frequent relapses despite an appropriate treatment combining surgery, chemotherapy and radiotherapy. In GBM, hypoxia is a characteristic feature and activation of Hypoxia Inducible Factors (HIF-1α and HIF-2α) has been associated with resistance to anti-cancer therapeutics. Int6, also named eIF3e, is the “e” subunit of the translation initiation factor eIF3, and was identified as novel regulator of HIF-2α. Eukaryotic initiation factors (eIFs) are key factors regulating total protein synthesis, which controls cell growth, size and proliferation. The functional significance of Int6 and the effect of Int6/EIF3E gene silencing on human brain GBM has not yet been described and its role on the HIFs is unknown in glioma cells. In the present study, we show that Int6/eIF3e suppression affects cell proliferation, cell cycle and apoptosis of various GBM cells. We highlight that Int6 inhibition induces a diminution of proliferation through cell cycle arrest and increased apoptosis. Surprisingly, these phenotypes are independent of global cell translation inhibition and are accompanied by decreased HIF expression when Int6 is silenced. In conclusion, we demonstrate here that Int6/eIF3e is essential for proliferation and survival of GBM cells, presumably through modulation of the HIFs.

Recent research on translation and protein synthesis in several pathologies, including cancer, peripheral artery disease, and wound healing, demonstrates the key role played by translational factors in tumorigenic and angiogenic processes. This review will focus on one specific translational factor, eIF3e also called INT6, the “e” subunit of the translation initiation factor eIF3. INT6/eIF3e has recently been described as a multifunction protein playing a role in translation, protein degradation, DNA repair, nonsense-mediated mRNA decay, cell cycle and control of cell response to low oxygen (hypoxia or ischemia) through modulation of the Hypoxia Inducible Factors (HIFs). Interestingly, INT6/eIF3e is a double-edged swordthat has both oncogenic and tumor suppressive abilities. In addition to its role in tumorigenesis, its silencing has recently been suggested as a potential therapeutic strategy to improve cell survival and function after ischemic injuries. Although a deeper understanding of the molecular mechanisms involved in these pathophysiological functions is essential, particularly to transform the in vitro /in vivo findings into clinical applications, INT6/eIF3e modulation could provide therapeutic benefit for a variety of human diseases such as cancer or vascular diseases.

PURPOSE: Resistance of glioblastoma to radiotherapy is mainly due to tumor cell radioresistance, which is partially controlled by growth factors such as fibroblast growth factor (FGF). Because we have previously shown the role of FGF-2 through RhoB in tumor cell radioresistance, here we investigated whether FGF receptor (FGFR) inhibition may represent a new strategy to optimize the efficiency of radiotherapy in glioblastomas. METHODS: We first treated prior to irradiation four different human glioblastoma cell lines in vitro with the specific FGFR inhibitor, SSR12819E, and studied the radiosensitizer effect of this compound. We then investigated the effect of SSR12819E on the downstream pathway. We generated orthotopic xenografts in nude mice to study the in vivo radiosensitizer effect of SSR128129E. We studied the survival of the animals without any neurological signs. RESULTS: Treating glioma cells with SSR12819E increases the radiosensitivity of radioresistant glioma cell lines by activating radiation-induced mitotic cell death. This in vitro radiosensitizer effect of SSR128129E is a consequence of an SSR12819E-induced FGFR-1 internalization and ubiquitination and of an inhibition of radiation-induced RhoB activation. Treating hypoxic U87 cells with SSR128129E 1 µM led to the activation of the proteasome-dependant degradation of the hypoxia-inducible factor, HIF1alpha, suggesting a role of SSR128129E and thus of FGFR-1 inhibition in the regulation of hypoxia. Moreover, treating orthotopically U87 xenografted mice with SSR128129 before submitting the animals to two subsequent 2.5 Gy fractions significantly increased their survival compared with the other treatment groups. This in vivo radiosensitizer effect was not associated in our experimental conditions with the regulation of angiogenesis, demonstrating that controlling the intrinsic tumor cells' radioresistance by inhibiting FGFR-1 pathways is sufficient to radiosensitize these tumors. CONCLUSION: Inhibiting FGFR-1 pathways with SSR128129E, the first-in-class compound able to inhibit FGF-2 binding to FGFR-1, may represent an interesting strategy to conceive new radiosensitizer agents.