Hypofractionated radiation therapy promotes distinct resistant cellular fates as a survival mechanism in two colorectal cell lines: senescence or transient dormancy followed by heterogeneous repopulation

Abstract Radiotherapy (RT) is an effective non-surgical treatment used in colorectal cancer (CRC), which is one of the major causes of cancer mortality. In patients with advanced rectal cancer, this approach is used as neoadjuvant therapy to reduce local recurrence risk. However, RT has a small impact on metastasis formation, which remains an obstacle in clinical practice, being responsible for the high mortality rates. Also, local tumor recurrence after RT tends to be more aggressive, with shorter survival expectancy. This suggests that RT surviving cells develop resistance mechanisms that allow their survival and increase metastatic potential. Despite efforts in this field, the knowledge concerning the post-irradiation behavior and the radioresistant cancer cells phenotype is still limited. Therefore, it is important to elucidate the mechanisms that contribute to the acquisition of this resistant phenotype to improve therapeutic success. In this study, we investigated the characteristics of surviving cells that were submitted to a clinically relevant treatment protocol of hypofractionated irradiation. The cell lines were chosen due to the different levels of genomic instability and genetic mutations and were exposed to 25 Gy delivery in 5 daily fractions. Irradiation surviving cells (ISC) from both cell lines, SW-480 and HCT-116, gain resistance to re-irradiation, increasing their autophagic process and assuming a polyploid giant cancer cell phenotype. In addition, HCT-116 ISC became senescent, while SW-480 ISC show an increase in mesenchymal markers and a decrease in epithelial markers. Long-term evaluation of the ISCs confirmed senescence in HCT-116, while SW-480 exhibited a transient dormancy state, resuming its growth after 14 days. SW-480 ISC also generates a heterogeneous progeny with three distinct phenotypes: rounded and highly proliferative cells, elongated mesenchymal cells, and polyploid giant cancer cells. Furthermore, both ISC enhance the secretion of extracellular vesicles and cytokines (like IL-8, MCP-1, VEGF, and GM-CSF) in a cell-specific manner. Based on these results and the identification of proteins and pathways differentially regulated in ISCs we can contribute to the identification of therapeutic targets for neoadjuvant therapies, in order to improve the prognosis of CRC patients. Citation Format: Josiane Weber Tessmann, Murilo Ramos Rocha, Renata Ivo Vasconcelos, Danielle Lazarin-Bidoia, Raphael Rodrigues Corrêa, Rafael Soares Lindoso, Paulo Emílio Corrêa Leite, Celso Vataru Nakamura, Jose Andres Morgado-Diaz. Hypofractionated radiation therapy promotes distinct resistant cellular fates as a survival mechanism in two colorectal cell lines: senescence or transient dormancy followed by heterogeneous repopulation [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 211.