Selective Inhibition of Parallel DNA Damage Response Pathways Optimizes Radiosensitization of Glioblastoma Stem-like Cells

Abstract

Glioblastoma is the most common form of primary brain tumor in adults and is essentially incurable. Despite aggressive treatment regimens centered on radiotherapy, tumor recurrence is inevitable and is thought to be driven by glioblastoma stem-like cells (GSC) that are highly radioresistant. DNA damage response pathways are key determinants of radiosensitivity but the extent to which these overlapping and parallel signaling components contribute to GSC radioresistance is unclear. Using a panel of primary patient-derived glioblastoma cell lines, we confirmed by clonogenic survival assays that GSCs were significantly more radioresistant than paired tumor bulk populations. DNA damage response targets ATM, ATR, CHK1, and PARP1 were upregulated in GSCs, and CHK1 was preferentially activated following irradiation. Consequently, GSCs exhibit rapid G 2 -M cell-cycle checkpoint activation and enhanced DNA repair. Inhibition of CHK1 or ATR successfully abrogated G 2 -M checkpoint function, leading to increased mitotic catastrophe and a modest increase in radiation sensitivity. Inhibition of ATM had dual effects on cell-cycle checkpoint regulation and DNA repair that were associated with greater radiosensitizing effects on GSCs than inhibition of CHK1, ATR, or PARP alone. Combined inhibition of PARP and ATR resulted in a profound radiosensitization of GSCs, which was of greater magnitude than in bulk populations and also exceeded the effect of ATM inhibition. These data demonstrate that multiple, parallel DNA damage signaling pathways contribute to GSC radioresistance and that combined inhibition of cell-cycle checkpoint and DNA repair targets provides the most effective means to overcome radioresistance of GSC. Cancer Res; 75(20); 4416-28. Ó2015 AACR.

PP2A Inhibitor PME-1 Drives Kinase Inhibitor Resistance in Glioma Cells

“…S2F). Moreover, we have previously shown that these GSCs display characteristic infiltrative growth pattern in intracranial in vivo model (29,30). Indeed, a significant inhibition of colony growth was observed in PME-1 depleted GSCs (E2 and R10 cell lines) treated with either STS or UCN-01 (Fig.…”

GBM radiosensitizers: dead in the water…or just the beginning?

“…Mechanistic cellular studies indicate that dual inhibition of cell cycle checkpoints and DNA repair mechanisms yields the most effective radiosensitization of GBM stem-like cells and there is exciting potential to combine small molecule inhibitors of ATR with the PARP inhibitor compounds described above (22). Systemic toxicity is likely to be an issue in this context, especially if temozolomide is included in the therapeutic cocktail, but intelligent scheduling of DNA damage response inhibitors, along with judicious combination with targeted radiotherapy, should enable a beneficial therapeutic index to be achieved.…”

DNA Repair

“…Accordingly, CSC have the potential to self-renew, differentiate and maintain tumour growth and repopulation following radio-or chemotherapy. Two models exist for the organisation of CSC: one that assumes a hierarchically organised system in which only a small proportion of tumour cells actually have any tumourigenic potential whilst most tumour cells are unable to induce tumours (Lapidot et al 1994;Al-Hajj et al 2003), and a clonal evolution model in which random changes enable subclones to emerge with new functions and treatment responses within the tumour. In fact, both models may have some merit and could coexist, assuming that clonal evolution may shape the make-up of the small proportion of stem-like cells (Maugeri-Sacca et al 2014).…”

“…However, as multiple DNA damage response pathways may be simultaneously upregulated in CSC, as shown in glioma stem cells, a multi-targeting approach involving combined inhibition of DNA repair and cell cycle checkpoint mechanisms may be a more promising strategy for overcoming CSC resistance (Signore et al 2014;Ahmed et al 2015).…”

“…ATR, CHK1 and PARP1 have been shown to be prime targets for radiosensitisation of S-phase cells (Ahmed et al 2015;Dungey et al 2008;Pires et al 2012;Dobbelstein and Sorensen 2015). Furthermore, irradiated cells undergoing replication form secondary DSBs (Groth et al 2012) and suffer from blocked replication elongation (Parplys et al 2012).…”