The Combinational Effects of Gamma Secretase Inhibition and Radiation on the Cancer Stem Cell Population in Glioblastoma

Gamma secretase (GS) is an intramembraneous protease that cleaves over 91 different membrane substrates. GS is responsible for the final S3 cleavage of the notch receptor, thereby releasing the notch intracellular domain (NICD) into the cytoplasm. Upon translocation into the nucleus NICD activates the transcription of notch effector proteins that maintain cell stemness. Due to GS activity on the notch pathway, it has become an attractive target for cancer stem cells. The cancer stem cell (CSC) hypothesis states that cancers are generated and maintained by a group of cells that share similarities with normal adult stem cells. CSCs have been shown to be resistant to most current anti-cancer treatment approaches, including radiation therapy, thus contributing to tumor repopulation after therapy. A combinational therapy that targets both cancer cells and inhibits cancer stem cell growth is highly desirable. Unfortunately, there is inconsistent data determining the combinational effects of GS inhibitors (GSI) with radiation. In this study, the efficacy of GSI treatment with radiation therapy in reducing the cancer stem cell population in glioblastoma multiforme (GBM) was evaluated. Utilizing a panel of GBM cell lines varying in PTEN, p53, and EGFR status, we evaluated the effects of GSI plus radiation treatment on the cancer stem cell population, using sphere-forming capacity assays, cell cycle analysis, and γH2AX and Hoechst/PY staining. Our data demonstrates that PTEN status plays a role in the sensitivity to GSI treatment in combination with radiation treatment. In addition, we observed that treating PTEN-wt cell lines with GSI improved survival among the stem cell population while PTEN-mutant lines showed a reduced survival. We believe this glioma stem cell protection is mediated through FOXO, or the Forkhead class O transcription factors, which is positively regulated by functioning PTEN. In conclusion, this study demonstrates that the effectiveness of combinational treatment of GSI and radiation on glioma stem cells depends on the genetic background of the tumor. Specifically, PTENwt neurosphere cell lines are radioprotected under GSI treatment while PTEN-null neurosphere cell lines become more radiosensitivecycle analysis, and γH2AX and Hoechst/PY staining. Our data demonstrates that PTEN status plays a role in the sensitivity to GSI treatment in combination with radiation treatment. In addition, we observed that treating PTEN-wt cell lines with GSI improved survival among the stem cell population while PTEN-mutant lines showed a reduced survival. We believe this glioma stem cell protection is mediated through FOXO, or the Forkhead class O transcription factors, which is positively regulated by functioning PTEN. In conclusion, this study demonstrates that the effectiveness of combinational treatment of GSI and radiation on glioma stem cells depends on the genetic background of the tumor. Specifically, PTENwt neurosphere cell lines are radioprotected under GSI treatment while PTEN-null neurosphere cell lines become more radiosensitive