Abstract 2413: Glioblastoma stem-like cells are resistant to the negative effects of increased aneuploidy on in vitro survival and radiosensitivity

Glioblastomas (GBM) have long been classified as radioresistant tumors. Glioblastoma stem-like cells (GSCs) are a portion of GBM’s heterogenous cell population and are generally considered to be crucial drivers of tumorigenesis and radioresistance. As for most tumor cells, GSCs display a moderate level of chromosomal instability and aneuploidy. However, whether aneuploidy promotes or inhibits GSC survival and its effects on radiosensitivity remain unclear. In this study we compared the radiation response of GSC lines (NSC11 and NSC20) and the standard glioma cell line U251 after an increase in aneuploidy. This increase was induced by treatment with CFI-402257, a selective inhibitor of the mitotic kinase TTK, which plays a key role in spindle-assembly checkpoint (SAC) regulation. After an initial drug exposure of 24h, the drug was removed and cells were allowed to grow for 5 days. In both GSC lines and U251, as demonstrated by interphase chromosome FISH, this protocol resulted in approximately 150%, 50%, and 125% increases in aneuploidy of chromosomes 2, 7 and 10 respectively. After these increases in aneuploidy, the clonogenicity (in vitro survival) of U251 cells was reduced by 80%, whereas it was reduced by 50% and 30% in NSC11 and NSC20 respectively. Moreover, the increase in aneuploidy significantly enhanced the radiosensitivity of U251 cells, whereas it had no effect on the radiosensitivity of the GSCs. These changes could not be attributed to residual inhibition of TTK or deficiencies in the SAC: at 5 days after CFI inhibition no changes in nocodazole-induced SAC arrest in mitosis were detected. Additionally, after aneuploid induction no changes in the expression of the stem cell marker CD133 were detected in the GSC lines, indicating no change in differentiation status. To begin to investigate the mechanism behind these differences in radiosensitivity, γH2AX foci dispersal, a surrogate marker for DNA double strand break (DSB) repair was evaluated. After the increase in aneuploidy, cells were irradiated (2Gy) and collected 1-24h after for analysis of γH2AX. In U251 cells, the increase in aneuploidy resulted in slower foci dispersal, indicative of a decrease in DSB repair; whereas in GSCs the increased aneuploidy had no effect on foci dispersal as compared to control. Mitotic catastrophe, the primary mechanism of solid tumor cell death after irradiation, was also evaluated. The increase in aneuploidy significantly increased the frequency of radiation-induced mitotic catastrophe in U251 cells, but no changes in either GSC line were detected. Overall, these results suggest that GSCs have an enhanced ability to tolerate the negative consequences of aneuploidy on survival as well as on radiosensitivity. Such aneuploid tolerance may provide a mechanism through which GBMs exploit karyotype diversity to survive under harsh environmental conditions and after treatment.

Citation Format: Ian C. Sutton, Charlotte M. Degorre, Komal Rawal, Philip J. Tofilon. Glioblastoma stem-like cells are resistant to the negative effects of increased aneuploidy on in vitro survival and radiosensitivity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2413.

Glioblastoma cells have increased capacity to repair radiation-induced DNA damage after migration to the olfactory bulb

BACKGROUND

The invasive nature of GBM combined with the diversity of brain microenvironments creates the potential for a topographic heterogeneity in GBM radioresponse. Investigating the mechanisms responsible for a microenvironment-induced differential GBM response to radiation may provide insights into the molecules and processes mediating GBM radioresistance.

METHODS

Using a model system in which human GBM stem-like cells implanted into the right striatum of nude mice migrate throughout the right hemisphere (RH) to the olfactory bulb (OB), the radiation-induced DNA damage response was evaluated in each location according to γH2AX and 53BP1 foci and cell cycle phase distribution as determined by flow cytometry and immunohistochemistry. RNAseq was used to compare transcriptomes of tumor cells growing in the OB and the RH. Protein expression and neuron-tumor interaction were defined by immunohistochemistry and confocal microscopy.

RESULTS

After irradiation, there was a more rapid dispersal of γH2AX and 53BP1 foci in the OB versus in the RH, indicative of increased double strand break repair capacity in the OB and consistent with the OB providing a radioprotective niche. With respect to the cell cycle, by 6 h after irradiation there was a significant loss of mitotic tumor cells in both locations suggesting a similar activation of the G2/M checkpoint. However, by 24 h post-irradiation there was an accumulation of G2 phase cells in the OB, which continued out to at least 96 h. Transcriptome analysis showed that tumor cells in the OB had higher expression levels of DNA repair genes involved in non-homologous end joining and genes related to the spindle assembly checkpoint. Tumor cells in the OB were also found to have an increased frequency of soma-soma contact with neurons.

CONCLUSION

GBM cells that have migrated to the OB have an increased capacity to repair radiation-induced double strand breaks and altered cell cycle regulation. These results correspond to an upregulation of genes involved in DNA damage repair and cell cycle control. Because the murine OB provides a source of radioresistant tumor cells not evident in other experimental systems, it may serve as a model for investigating the mechanisms mediating GBM radioresistance.

Abstract 6057: Mechanisms mediating the radioresistance of human glioma cells growing in the murine olfactory bulb

We have previously reported (Int J Radiat Oncol Biol Phys. 107(1):194-201, 2020) that human GBM-stem like cells (GSCs) implanted into the right striatum of nude mice migrate/invade throughout the right hemisphere (RH) as well as into the olfactory bulb (OB). The highly invasive nature of the GSCs suggested a model system for evaluating the effects of specific brain microenvironments on glioma radioresponse and the potential for topographic heterogeneity in radioresistance. Subsequent analyses revealed that GSCs in the OB were significantly more radioresistant than those in the RH. Because the OB is highly enriched in neurons and given recent data suggesting that neuron/glioma interactions drive GBM growth, in this study we first defined contact between GSCs and neurons in the OB and RH using immunohistochemistry (IHC) and confocal microscopy. In the OB 56% of tumor cells were in direct contact with neurons as compared to 24% of tumor cells in the RH. Moreover, among the tumor cells in contact with neurons 25% had 2 or more neuronal contacts in the OB versus 2% in the RH. These data suggest a significantly greater level of glioma-neurons interaction in the OB. To further investigate the OB as a radioresistant niche, RNAseq was used to define the transcriptome of NSC11 GSCs located in the OB versus the RH. Of the greater than 15,000 genes evaluated, 1887 genes were differentially expressed between the 2 sites: 872 upregulated and 1015 down regulated in the OB versus the RH. Functional analyses (IPA and GSEA) indicated that among the genes overexpressed in the OB GSCs were a group associated with the cell cycle, specifically mitosis and the spindle assembly checkpoint (SAC). Comparison of the SAC genes overexpressed in the OB with GBM clinical outcomes (TCGA) showed a significant correlation with decreased patient survival. These analyses suggested that cell cycle regulation may play a role in the radioresistance of OB GSCs. Accordingly, radiation-induced cell cycle redistribution of GSCs in the OB and the RH was determined by IHC analysis of cell cycle phase markers and flow cytometry (propidium iodide) performed on dissociated tumor cells. Results showed a significantly higher percentage of cells in G2 in the OB at 24h and 48h after 10Gy, suggestive of prolonged G2 arrest. In addition, analysis of radiation-induced DNA DSBs using γH2AX and 53BP1 nuclear foci showed faster and more complete foci dispersal after 10Gy in the OB GSCs versus those in the RH, indicative of increased DSBs repair in the OB. These data suggest that GSC radioresistance in the OB is the result of a more effective DNA damage response. Moreover, results suggest that the murine OB may provide a model system for investigating GBM radioresistance as well as the potential role of glioma/neuron interactions.

Citation Format: Charlotte M. Degorre, Ian C. Sutton, Stacey L. Lehman, Uma T. Shankavaram, Kevin A. Camphausen, Philip J. Tofilon. Mechanisms mediating the radioresistance of human glioma cells growing in the murine olfactory bulb [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6057.

Galectin-3 protects against ischemic stroke by promoting neuro-angiogenesis via apoptosis inhibition and Akt/Caspase regulation

Post-stroke neurological deficits and mortality are often associated with vascular disruption and neuronal apoptosis. Galectin-3 (Gal3) is a potent pro-survival and angiogenic factor. However, little is known about its protective role in the cerebral ischemia/reperfusion (I/R) injury. We have previously shown significant up-regulation of Gal3 in the post-stroke rat brain, and that blocking of Gal3 with neutralizing antibody decreases the cerebral blood vessel density. Our current study demonstrates that intracerebral local delivery of the Gal3 into rat brain at the time of reperfusion exerts neuroprotection. Ischemic lesion volume and neuronal cell death were significantly reduced as compared with the vehicle-treated MCAO rat brains. Gal3 increased vessel density and neuronal survival after I/R in rat brains. Importantly, Gal3-treated groups showed significant improvement in motor and sensory functional recovery. Gal3 increased neuronal cell viability under in vitro oxygen-glucose deprivation conditions in association with increased phosphorylated-Akt, decreased phosphorylated-ERK1/2, and reduced caspase-3 activity. Gene expression analysis showed down regulation of pro-apoptotic and inflammatory genes including Fas-ligand, and upregulation of pro-survival and pro-angiogenic genes including Bcl-2, PECAM, and occludin. These results indicate a key role for Gal3 in neuro-vascular protection and functional recovery following ischemic stroke through modulation of angiogenic and apoptotic pathways.