P17.82 * ITGA6 IS INVOLVED IN RADIORESISTANCE OF GLIOBLASTOMA STEM CELLS

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BACKGROUND: In the last decade, the cancer stem cell hypothesis has gained progressively agreement towards researchers. Cancer stem cells (CSCs) or tumour initiating cells (TICs) are considered to be responsible for tumor initiation, propagation and therapeutic resistance. In glioblastoma multiforme (GBM), the most frequent and malignant primary brain tumour in adults, TICs are related with chemo- and radioresistance. Integrins are cell surface receptors involved in the crosstalk of tumoral cells with their extracellular matrix (ECM). In particular integrin α6 (ITGA6) is known to be extremely important in embryonic, hematopoietic and neural stem cells. In brain it is involved in maintaining adhesion to the ventricular zone and ensures proper cell division. Besides, recently it has been proposed as a functional marker of TICs. Given the raising evidence of the ECM and perivascular niche role in the maintenance and guidance of TICs, we focused on ITGA6 to determine whether ITGA6 expression is associated with radiation-resistance of TICs in glioblastoma patients. METHODS: Four different GBM cultures obtained from post-surgical specimens were growth in two different growth conditions: a differentiating FBS-containing media and an FBS-free media supplemented with bFGF and EGF which is considered to be selective for TICs. Self-renewal capability and proliferative potential has been evaluated. Tumour initiating capability has been assessed through injection of low number of cells in immunocompromised mice. ITGA6 expression has been analyzed at RNA and protein levels. TICs have been sorted in order to get a population enriched for ITGA6high cells and dose-response to ionizing radiation (IR) has been evaluated and compared to ITGA6low cells by clonogenic assays. RESULTS: TICs have shown noticeable self-renewal capability, extensive proliferative potential, and a remarkable radioresistance compared to the same primary cultures maintained under differentiated conditions. In TICs, ITGA6 RNA and protein levels were statistically significant higher than in differentiated glioblastoma cells. In addition, this expression is decreasing consistently when TICs were cultured under differentiating conditions. Response to radiotherapy assessed by the clonogenic surviving fraction showed that ITGA6high cells are remarkably more radioresistant than ITGA6low cells (p < 0.001). In addition, ITGA6high cells showed a higher α/β ratio than ITGA6low cells (30 Gy vs. 6 Gy, respectively). In conclusion, these results suggest that ITGA6 is involved in the radioresistant phenotype of glioblastoma stem cells.