Brain tumor and brain endothelial cells' response to ionizing radiation and phytochemical treatments

Glioblastoma multiform (GBM) represents the most aggressive and vascularised primary cerebral neoplasm in adults. Median length of survival without further therapy is usually less than one year from the time of diagnosis. Unfortunately, 90% of patients receiving radiotherapy following GBM resection develop a tumor recurrence. More recently, treatment of GBM with combined radiotherapy and temozolomide, an agent recognized for its antiangiogenic activity, increased the median survival to 14,6 months.  Efforts have been oriented towards identifying naturally occurring substances capable of inhibiting, delaying or reversing the multi-stage carcinogenesis process.  Epigallocatechin-3-gallate (EGCG), a green tea polyphenol, has been recognized for its anticancerous and antiangiogenic property.  EGCG may represent a potential agent capable of sensitizing brain tumor cells and their derived endothelial cells (ECs) to conventional treatments.  In chapter II, the first part of this doctorate project aimed at determining if EGCG, in synergy with radiotherapy, can sensitize GBM’s response to radiation and whether specific molecular markers are involved.  We documented that U-87 cells were relatively radioresistant and that Survivin, an inhibitor of apoptosis protein, may be involved in GBM’s radioresistance.  We also found that pre-treatment of U-87 cells with EGCG could overcome the cytoprotective effect of Survivin overexpression and potentiate the cytoreductive effect of irradiation (IR).  In chapter III, we characterized the impact of IR on human brain microvascular endothelial cell (HBMEC) survival and determined whether EGCG, could optimize this effect.  We found that although EGCG treatment and IR individually decreased HBMEC survival, the combined treatment synergistically reduced survival.  We documented that the combined treatment increased cell death, more specifically necrosis. In chapter IV, we investigated the impact of IR exposure on the angiogenic functions i.e. cell proliferation, cell migration in response to brain tumor-derived growth factors, and capacity for tubulogenesis of surviving human brain tumor-derived ECs.  The Rho signalling pathway was also investigated in relation to the functional properties of radioresistant HBMEC.  Our data suggests that IR significantly alters radioresistant HBMEC migration response to tumor-secreted growth factors and tubulogenesis.  Response to growth factors important for tumor expansion and angiogenesis is significantly attenuated in these cells. In conclusion, this doctorate project confirmed IR’s cytoreductive properties on malignant gliomas.  We proposed a novel mechanism to explain GBMs’ radioresistance.  This project documented for the first time IR’s cytotoxic effect in HBMEC.  It also described the existence of radioresistant HBMEC and characterized their altered angiogenic functions.  The combination of natural anticancerous and antiangiogenic molecules such as EGCG with radiotherapy could improve IR’s effect on human malignant glioma cells and microvascular ECs, especially through increased necrosis of HBMEC.  The thesis supports integrating nutrients bearing anticancerous and antiangiogenic properties, such as EGCG, in the management of gliomas to sensitize tumor and tumor-associated ECs to conventional therapies.

Keyphrases: Astrocytoma, Angiogenesis, Epigallocathechin-3-gallate, Irradiation, Radiotherapy, Rho signaling pathway, Survivin, Astrocytome, Irradiation, Survivin