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Akt expression in 3D GSC before and after radiation treatment. A, Bar charts representing mRNA expression of Akt, PARP and PTEN genes in G7 and E2 3D cells treated with 0 or 5 Gy. RNA extraction was performed 4 h after irradiation treatment. B, Protein expression of G7 and E2 3D GSC for Akt1, Akt2, Akt3, PARP1 and PTEN treated with 0 or 5 Gy. Cell lysates were prepared 4 h after irradiation treatment. Actin serves as loading control. C, Enrichment analysis of RNASeq data for G7 or E2 3D cells grown in the presence of VEGF comparing no radiation v radiation treatment. Analysis was performed using Biocarta_Akt_Pathway. D. Perifosine dose response (0.1 microM to 10 microM) at 0 and 3 Gy in G7 3D GSC. Cells were treated with respective inhibitor 2 h prior to ionizing radiation. Values were normalized to vehicle (DMSO) against their respective radiation dose. Curves were fitted with a median enhancement effect model. Statistical analysis with one-way ANOVA, *p

Statistical analysis of MK2206 efficacy study

Glioblastoma is resistant to conventional treatments and has dismal prognosis. Despite promising in vitro data, molecular targeted agents have failed to improve outcomes in patients, indicating that conventional two-dimensional (2D) in vitro models of GBM do not recapitulate the clinical scenario. Responses of primary glioblastoma stem-like cells (GSC) to radiation in combination with EGFR, VEGF, and Akt inhibition were investigated in conventional 2D cultures and a three-dimensional (3D) in vitro model of GBM that recapitulates key GBM clinical features. VEGF deprivation had no effect on radiation responses of 2D GSCs, but enhanced radiosensitivity of GSC cultures in 3D. The opposite effects were observed for EGFR inhibition. Detailed analysis of VEGF and EGF signaling demonstrated a radioprotective role of Akt that correlates with VEGF in 3D and with EGFR in 2D. In all cases, positive correlations were observed between increased radiosensitivity, markers of unrepaired DNA damage and persistent phospho-DNA-PK nuclear foci. Conversely, increased numbers of Rad51 foci were observed in radioresistant populations, indicating a novel role for VEGF/Akt signaling in influencing radiosensitivity by regulating the balance between nonhomologous end-joining and homologous recombination–mediated DNA repair. Differential activation of tyrosine kinase receptors in 2D and 3D models of GBM explains the well documented discrepancy between preclinical and clinical effects of EGFR inhibitors. Data obtained from our 3D model identify novel determinants and mechanisms of DNA repair and radiosensitivity in GBM, and confirm Akt as a promising therapeutic target in this cancer of unmet need.

Supplementary Figure S2 shows additional data obtained during aphidicolin studies detailed in Fig 1 and additional data detailing expression of genes associated with ongoing and stalled replication forks from figure 3.

Legends for supplementary figures and tables

Glioblastoma (GBM) is a lethal primary brain tumor characterized by treatment resistance and inevitable tumor recurrence, both of which are driven by a subpopulation of GBM cancer stem–like cells (GSC) with tumorigenic and self-renewal properties. Despite having broad implications for understanding GSC phenotype, the determinants of upregulated DNA-damage response (DDR) and subsequent radiation resistance in GSC are unknown and represent a significant barrier to developing effective GBM treatments. In this study, we show that constitutive DDR activation and radiation resistance are driven by high levels of DNA replication stress (RS). CD133+ GSC exhibited reduced DNA replication velocity and a higher frequency of stalled replication forks than CD133− non-GSC in vitro; immunofluorescence studies confirmed these observations in a panel of orthotopic xenografts and human GBM specimens. Exposure of non-GSC to low-level exogenous RS generated radiation resistance in vitro, confirming RS as a novel determinant of radiation resistance in tumor cells. GSC exhibited DNA double-strand breaks, which colocalized with “replication factories” and RNA: DNA hybrids. GSC also demonstrated increased expression of long neural genes (>1 Mbp) containing common fragile sites, supporting the hypothesis that replication/transcription collisions are the likely cause of RS in GSC. Targeting RS by combined inhibition of ATR and PARP (CAiPi) provided GSC-specific cytotoxicity and complete abrogation of GSC radiation resistance in vitro. These data identify RS as a cancer stem cell–specific target with significant clinical potential.Significance: These findings shed new light on cancer stem cell biology and reveal novel therapeutics with the potential to improve clinical outcomes by overcoming inherent radioresistance in GBM. Cancer Res; 78(17); 5060–71. ©2018 AACR.

Supplemental Table 1 Summary table of in vivo growth characteristics of E2, G7, R10, R15, R24, R9 and S2 GSC primary GBM cultures after intracranial injection in CD1 nude mice. Supplemental Table 2 List of primary antibodies utilised Supplemental Table 3 List of secondary antibodies utilised