Abstract 3078: IGF1/N-cadherin/b-catenin/Clusterin signaling axis can mediates adaptive radioresistance in glioblastoma

Glioblastoma (GBM) is composed of heterogeneous tumor cell populations including those with stem cell properties, termed glioma stem cells (GSCs). GSCs are innately less radiation sensitive than the tumor bulk and are believed to drive GBM formation and recurrence after repeated irradiation. However, it is unclear how GSCs adapt to escape the toxicity of repeated irradiation used in clinical practice. To identify important mediators of adaptive radioresistance, we generated radioresistant human and mouse GSCs by exposing them to repeated cycles of irradiation. Surviving subpopulations acquired strong radioresistance in vivo, which was accompanied by increased cell-cell adhesion, slower proliferation, an elevation of stemness properties and N-cadherin expression. Stably transfecting N-cadherin in parental GSC rendered them radioresistant, reduced their proliferation, and increased their stemness and intercellular adhesive properties. Conversely, radioresistant GSCs lost their acquired phenotypes upon CRISPR/Cas9-mediated knockout of N-cadherin. Mechanistically, elevated N-cadherin expression resulted in the accumulation of b-catenin at the cell surface, which suppressed Wnt/b-catenin proliferative signaling, and reduced neural differentiation. Transfection of wild type N-cadherin, but not mutant N-cadherin lacking the b-catenin binding region, restored radioresistance in N-cadherin knockout GSCs, indicating the importance of the binding between N-cadherin and b-catenin. Moreover, N-cadherin increased Clusterin secretion, which protected GSCs against apoptosis after radiation treatment. N-cadherin knockout decreased Clusterin secretion and sensitized the cells to radiation therapy. We also demonstrated that N-cadherin upregulation was induced by radiation-induced IGF1 secretion, which induced an EMT-like phenotype change in GSCs. The N-cadherin-mediated radioresistance phenotype could be reverted with picropodophyllin (PPP), a clinically applicable blood-brain-barrier permeable IGF1 receptor inhibitor. Adjuvant PPP combined with irradiation significantly extended the survival of orthotopically xenografted mice versus irradiation-only or drug alone controls, supporting clinical translation. Moreover, elevated N-cadherin and Clusterin mRNA expression is related to prognosis of GBM in the TCGA dataset. In conclusion, our data indicate that IGF1R inhibition can block the N-cadherin-mediated resistance pathway. Our study deepens our understanding of adaptive radioresistance during repeated irradiation in GBM, and validates the IGF1/N-cadherin/β-catenin/Clusterin signaling axis as a novel target for radio-sensitization, which has direct therapeutic applicability. Citation Format: Satoru Osuka, Dan Zhu, Zhaobin Zhang, Chaoxi Li, Christian T. Stackhouse, Oltea Sampetrean, Jeffrey J. Olson, G. Yancey Gillespie, Hideyuki Saya, Christopher D. Willey, Erwin G. Van Meir. IGF1/N-cadherin/b-catenin/Clusterin signaling axis can mediates adaptive radioresistance in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3078.