Radiosensitization of Glioblastoma Cell Lines by the Dual PI3K and mTOR Inhibitor NVP-BEZ235 Depends on Drug-Irradiation Schedule

Abstract

Previous studies have shown that the dual phosphatidylinositide 3-kinase/mammalian target of rapamycin (PI3K/mTOR) inhibitor NVP-BEZ235 radiosensitizes tumor cells if added shortly before ionizing radiation (IR) and kept in culture medium thereafter. The present study explores the impact of inhibitor and IR schedule on the radiosensitizing ability of NVP-BEZ235 in four human glioblastoma cell lines. Two different drug-IR treatment schedules were compared. In schedule I, cells were treated with NVP-BEZ235 for 24 hours before IR and the drug was removed before IR. In schedule II, the cells were exposed to NVP-BEZ235 1 hour before, during, and up to 48 hours after IR. The cellular response was analyzed by colony counts, expression of marker proteins of the PI3K/AKT/mTOR pathway, cell cycle, and DNA damage. We found that under schedule I, NVP-BEZ235 did not radiosensitize cells, which were mostly arrested in G1 phase during IR exposure. In addition, the drug-pretreated and irradiated cells exhibited less DNA damage but increased expressions of phospho-AKT and phospho-mTOR, compared to controls. In contrast, NVP-BEZ235 strongly enhanced the radiosensitivity of cells treated according to schedule II. Possible reasons of radiosensitization by NVP-BEZ235 under schedule II might be the protracted DNA repair, prolonged G2/M arrest, and, to some extent, apoptosis. In addition, the PI3K pathway was downregulated by the NVP-BEZ235 at the time of irradiation under schedule II, as contrasted with its activation in schedule I. We found that, depending on the drug-IR schedule, the NVP-BEZ235 can act either as a strong radiosensitizer or as a cytostatic agent in glioblastoma cells.

Dual PI3K- and mTOR-inhibitor PI-103 can either enhance or reduce the radiosensitizing effect of the Hsp90 inhibitor NVP-AUY922 in tumor cells: The role of drug-irradiation schedule

“…Thirty minutes after IR, Rad51 was reduced in samples treated with NVP-AUY922 alone or in combination with PI-103 under Schedule I. Twenty four hours post-IR the protein was reduced also under Schedule II after Hsp90 inhibition. In contrast to the published data on NVP-BEZ235 [26, 27], we found that PI-103 alone suppressed the expression of Rad51 only in 2 (GaMG and SW480) out of 4 tested cell lines (Figure 7). This means that the impaired DNA repair capacity revealed by the high residual histone γH2AX levels observed under Schedule II in the majority of cell lines tested cannot be explained by the depletion of Rad51 (Figure 7).…”

“…The reason for the inhibition of Rad51 by PI-103 can be the drug-mediated G 1 arrest, similar to that reported for a different PI3K and mTOR inhibitor NVP-BEZ35 [27, 48]. It is known that Rad51 operates mostly during Homologous Recombination (HR) of DNA, which appears to be active only from the mid-S to G 2 phases of the cell cycle [49].…”

“…Recently, the expression of this protein has been found to be abrogated by a different PI3K and mTOR inhibitor, NVP-BEZ235 [26, 27]. Figure 7 shows representative Western blot detections of Rad51 protein in 4 cell lines treated with drugs and IR according to both schedules.…”

Dual Phosphoinositide 3-kinase/mammalian target of rapamycin inhibitor is an effective radiosensitizer for colorectal cancer

“…To prevent radiationinducing PI3K/AKT/mTOR signaling pathway, we treated CRC cells with BEZ235 one hour before radiation and found that BEZ235 synergistically inhibited the cell viability and increased the apoptosis. Our findings are in accordance with a recent observation that the radiosensitivity of glioblastoma cells was strongly enhanced when cells were exposed to BEZ235 1 hour before radiation [35].…”

MR Studies of Glioblastoma Models Treated with Dual PI3K/mTOR Inhibitor and Temozolomide:Metabolic Changes Are Associated with Enhanced Survival

“…The PI3K pathway can be activated at different steps of the signaling cascade, most notably by loss of the phosphatase and tensin homologue (PTEN) protein, and by amplification and/or mutation of the epidermal growth factor receptor (EGFR), which occur in 40% and 50% of GBM cases, respectively (8). Accordingly, several drugs that inhibit signaling downstream of these mutations are currently in phase I and II clinical trials, either in combination with other FDA-approved chemotherapeutic agents such TMZ, or as single agents (9–13). Importantly, whereas first generation PI3K pathway inhibitors target mostly the downstream node of mTOR, the observation of feedback loops that result in enhanced PI3K signaling has led to the development of second-generation drugs (14–17).…”