Abstract 6170: Newly emerging CD44 negative cells after irradiation reproduce CD44 positive cells and promote radioresistance
Authors: Sasaki Masaru , Miyoshi Norikatsu , Fujino Shiki , Ogino Takayuki , Takahashi Hidekazu , Uemura Mamoru , Matsuda Chu , Yamamoto Hirofumi , Mizushima Tsunekazu , Mori Masaki , Doki Yuichiro 
Published: September 2020

Abstract
Backgrounds: Although radiation therapy is effective for many cancers, tumor regrowth and recurrence can occur after irradiation. Various mechanisms have been reported regarding the resistance, including the involvement of cancer stem cells and radioresistance genes. However, acquisition of resistance and recurrence are still crucial problems in clinical field. Tumor heterogeneity is thought to play an important role and greatly involve in the acquisition of therapeutic resistance. It is difficult to examine heterogeneous population using cancer cell lines consisted of homogenous cells. Therefore, we have developed primary cultured cells, named 2D organoid (2DO) which maintained clinical tumor heterogeneity. The purpose of this study is to reveal a novel mechanism of radiation resistance by focusing on changes in various populations and persister cells after irradiation using 2DO.
Materials and Methods: Colorectal cancer cell lines and 2DOs established from surgery resected specimens were irradiated with various doses (2-10Gy once, 1-4Gy for 5 days consecutive, etc.). Cancer stem cell markers were evaluated before and after irradiation using flow cytometry.
Results: In the cell lines, CD44+ cells remained after irradiation, and no significant change was observed in stem cell population before and after irradiation. In the 2DOs, CD44+ cells remained after irradiation, and in addition CD44− cells also newly emerged. The CD44− cells were sorted by cell sorter and cultured. The CD44− cells reproduced CD44+ cells and reconstructed the heterogenous population similar to the parent 2DOs. However, CD44− cells sorted from the parent cells not irradiated did not reconstructed the heterogenous population. Furthermore, the cells reproduced from the survived CD44− cells had more resistance to anti-tumor drugs than cells from the survived CD44+ cells. Other stem cell markers such as CD24 and CD133 were also examined, but no significant changes were observed before and after irradiation.
Conclusions: In addition to CD44+ cells, CD44− cells also survived after irradiation. The survived CD44− cells after irradiation had more resistance to anti-tumor drugs compared with the survived CD44+ cells. These newly emerged CD44− cells may be important persister cells in tumor regrowth and recurrence. We report on the functional analysis of these cells.
Citation Format: Masaru Sasaki, Norikatsu Miyoshi, Shiki Fujino, Takayuki Ogino, Hidekazu Takahashi, Mamoru Uemura, Chu Matsuda, Hirofumi Yamamoto, Tsunekazu Mizushima, Masaki Mori, Yuichiro Doki. Newly emerging CD44 negative cells after irradiation reproduce CD44 positive cells and promote radioresistance [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6170.