Abstract 1430: N-cadherin is a driver of adaptive radioresistance in malignant brain tumors
Authors: Osuka Satoru , Zhu Dan , Zhang Zhaobin , Li Chaoxi , Stackhouse Christian T. , Sampetrean Oltea , Olson Jeffrey J. , Gillespie G. Yancey , Saya Hideyuki , Willey Christopher D. , Van Meir Erwin G. 
Published: June 2022

Abstract
Many subtypes of brain tumors are highly malignant and resistant to chemo- and radio- therapy. Tumor cells can shift their phenotype in response to treatments, the so-called adaptive resistance. Adaptive resistance mechanisms in malignant brain tumors are still poorly understood, and effective treatments have not yet been developed. To unveil such mechanisms, we have developed unique new experimental models to identify the adaptive resistance mechanisms to fractionated radiation in malignant brain tumors. We performed repeated irradiation (2-5Gy every 3-4 days, 3-6 weeks) on 6 human Glioma stem cells (GSCs), 2 mouse GSCs and 4 medulloblastomas (MB) cells in vitro and examined how tumor cells adapt to repeated irradiation. Brain tumor cells demonstrated dynamic adaptation to fractionated irradiation. They rapidly altered cell proliferation, intercellular adhesion, and stemness and acquired strong radioresistance. To identify genes responsible for radio-resistance, we performed RNA-seq analysis and CRISPR library screening using primary and radioresistant cells. We found that N-cadherin was upregulated in the majority of radioresistant GSCs. 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. Moreover, N-cadherin increased Clusterin secretion, which protected GSCs against apoptosis after radiation treatment. We also demonstrated that N-cadherin upregulation was induced by radiation-induced IGF1 secretion, which caused 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. In conclusion, our data indicate that IGF1R inhibition can block the N-cadherin-mediated resistance pathway. Our study deepens our understanding of adaptive resistance during repeated irradiation in GBM, and validates the IGF1/N-cadherin/b-catenin/Clusterin signaling axis as a novel target for radio-sensitization, which has direct therapeutic applicability. These findings also confirmed that our radioresistant models effectively identify new adaptive resistance mechanisms in malignant brain tumors. (References: Osuka S, et. al., J Clin Invest. 2021;131(6):e136098)
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. N-cadherin is a driver of adaptive radioresistance in malignant brain tumors [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 1430.