Abstract 1653: A novel role for Wnt/β-catenin signaling in mediating resistance of colorectal cancer to chemoradiotherapy

Background: Preoperative chemoradiotherapy represents the standard treatment for patients with rectal cancer. However, the clinical response of individual tumors to multimodal treatment is not uniform, and ranges from complete response to complete resistance. Therefore, the identification of novel therapeutic targets whose modification could be harnessed to sensitize a priori resistant tumors is exceedingly important. Previously, we demonstrated that the Wnt transcription factor TCF7L2 (also known as TCF4) was overexpressed in primary rectal cancers that were resistant to chemoradiotherapy (CRT), and that TCF7L2 functionally mediates resistance of CRC cells to clinically relevant doses of ionizing radiation (IR). Methods: Using siRNAs we silenced CTNNB1 (β-catenin), another key-component of canonical Wnt-signaling, in colorectal cancer cell lines LS1034, SW480, and SW837. To asses influence on CRT, cells were exposed to 0, 1, 2, 4, 6 and 8 Gy of X-rays and 5-FU. Wnt- signaling was stimulated in retinal pigment epithelial cells (RPE) either by adding Wnt-3A, or overexpressing non degradable β-catenin (S33Y-mutated) and analyzed changes in CRT. Finally we repetitively irradiated SW1463 (68Gy) to establish an isogenic radio-resistant cell line and examined changes in protein expression. Results: Silencing of CTNNB1 resulted in (chemo-) radiation-sensitization of all three CRC-cell lines. To further investigate the potential role of Wnt/β-catenin signaling in controlling therapeutic responsiveness, non-tumorigenic RPE cells were stimulated with Wnt-3A, which significantly increased resistance to CRT. This effect could be recapitulated by overexpression of β-catenin (S33Y-mutated), resulting in a significantly increased resistance to CRT. The effect could be rescued by siRNA mediated knockdown of β-catenin. Consistent with these findings, we observed higher expression levels of active (unphosphorylated) β-catenin as well as increased TCF reporter activity in SW1463 cells that were rendered radiation-resistant due to repeated IR treatment. Conclusion: Together, these findings strongly support the interpretation that Wnt/β-catenin signaling plays a central role in mediating resistance of CRC cells to CRT. Hence, pathway inhibition may represent a promising strategy to increase therapeutic responsiveness to CRT, which represents the standard treatment for locally advanced rectal cancers. This would have considerable clinical implications. Citation Format: Georg Emons, Melanie Spitzner, Sebastian Reineke, Frank Kramer, Margret Rave-Fraenk, Jochen Gaedcke, Michael Ghadimi, Thomas Ried, Marian Grade. A novel role for Wnt/β-catenin signaling in mediating resistance of colorectal cancer to chemoradiotherapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1653.