PO-210 Anticancer drug-induced senescence in glioblastoma cells is associated with changes in DNA repair capacity

Abstract

IntroductionDespite extensive research, malignant glioma remains the most aggressive and fatal type of brain tumour. Following resection, therapy is based on radiation concomitant with the methylating agent temozolomide (TMZ), followed by adjuvant high dose TMZ treatment. The success of glioma therapy depends largely on the DNA repair capacity of the tumour cells, notable repair by MGMT, which confers resistance to TMZ. Beside DNA repair, other mechanisms involved in tumour protection may also play a role. These include the induction of a transient cell cycle arrest that provides time for repair and the induction of an irreversible cell cycle arrest in the form of senescence.Material and methodsMZ-induced senescence and repression of DNA repair was analysed in LN229, U87, LN308 and U138 glioblastoma cells using different techniques, including immunodetection, RT-qPCR, ChIP, Co-immunoprecipitation, ELISA, ß-Gal staining, C12FDG staining, AnexinV/PI staining, flow cytometry, pharmacological inhibition and si-RNA mediated knockdown.Results and discussionsWe show that following single and chronic TMZ exposure most glioma cells evade apoptosis and enter a senescent state. TMZ-induced senescence occurs in the G2/M phase of the cell cycle and is initiated by the ATR/CHK1-mediated degradation of CDC25c. It is further sustained by activation of p21 and NF-κB. Analysing the transcriptional regulation of DNA repair factors upon TMZ exposure, we found a strong repression of the mismatch repair (MMR) proteins MSH2, MSH6 and EXO1 as well as of Rad51, the central component of the homologous recombination pathway. The repression of these genes was regulated by the disruption of the E2F1/DP1 complex and is a specific trait of TMZ-induced senescent cells.ConclusionRepression of DNA repair in senescent cells may result in acquired drug resistance to TMZ and could render these cells susceptible to the accumulation of additional genomic alterations induced by anticancer therapy. In case of an escape from senescence, these genetically altered cells could contribute to increased aggressiveness and therapy resistance in a recurring tumour.