CBMS-7 IGF1/N-cadherin/Clusterin signaling axis mediates adaptive radioresistance of glioma stem cells

Glioblastoma (GBM) is composed of a variety of tumor cell populations including those with stem cell properties, known as glioma stem cells (GSCs). GSCs are innately less sensitive to radiation than the tumor bulk and are believed to drive GBM formation and recurrence following repeated irradiation. However, it is unclear how GSCs adapt to avoid the toxicity of repeated irradiation used in clinical practice. We established radioresistant human and mouse GSCs by exposing them to repeated rounds of irradiation in order to uncover critical mediators of adaptive radioresistance. 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. Increasing N-cadherin expression rendered parental GSCs radioresistant, reduced their proliferation, and increased their stemness and intercellular adhesive properties. Conversely, radioresistant GSCs reduced their acquired phenotypes upon CRISPR/Cas9-mediated knockout of N-cadherin. Mechanistically, elevated N-cadherin expression resulted in the accumulation of β-catenin at the cell surface, which decreased Wnt/ β-catenin proliferative signaling, reduced neural differentiation, and protected against apoptosis through Clusterin secretion. Restoration of wild type N-cadherin, but not mutant N-cad lacking β-catenin binding region, led to increased radioresistance in N-cadherin knockout GSCs, indicating the importance of the binding between N-cadherin and β-catenin. We also demonstrated that N-cadherin upregulation was induced by radiation-induced IGF1 secretion, and the radiation resistance phenotype can be reversed with picropodophyllin (PPP), a clinically applicable blood-brain-barrier permeable IGF1 receptor inhibitor, supporting clinical translation. Moreover, the elevation of N-cad and Clusterin are related to prognosis of GBM in the TCGA dataset. In conclusion, our data indicate that IGF1R inhibitor can block the N-cadherin-mediated resistance pathway. Our research provides a deeper understanding of adaptive radioresistance after repeated irradiation, and validates the IGF1/N-cadherin/β-catenin/Clusterin signaling axis as a novel target for radio-sensitization, which has direct therapeutic applicability.