Abstract 5226: Tiam1 is a key modulator of chemoresistance in colorectal cancer
Authors: Izumi Daisuke 
Published: August 2018

Abstract
Purpose: Emerging evidence indicates that a small subpopulation of cancer cells with stem cell-like properties are responsible for acquisition of drug resistance in colorectal cancer (CRC). Intriguingly, the acquisition of such a stem-like state and subsequent drug resistance has been shown to be induced by the tumor microenvironment. In particular, cancer associated fibroblasts (CAFs) have been identified as a key component of tumor microenvironment which promote cancer stemness. Wnt signaling is a well-recognized pathway that drives self-renewal, and recently one of the Wnt-responsive genes, Tiam1, a guanine nucleotide exchange factor specific for Rac1, was shown to be overexpressed in CRC and associated with an aggressive cancer phenotype. Herein, we have investigated Tiam1 as a key modulator of chemoresistance in CRC.
Methods: Tiam1 expression was assessed in resected CRC tissues from 300 patients who did or did not respond to chemotherapy. We used siRNA and CRISPR/CAS9 approach in three cell lines and animal models to examine whether inhibition of Tiam1 affects chemosensitivity in CRC. In addition, we established CRC patient derived-CAFs and examined whether the drug sensitivity of CRC cells was altered by supplementation of CAF-derived conditioned medium.
Results: High Tiam1 expression significantly correlated with poor prognosis in CRC patients (P=0.032), and emerged as an independent prognostic factor (HR 5.06, 95% CI 1.10-23.2, P=0.038). In addition, Tiam1 overexpression is associated with patients who did not respond to chemotherapy (P=0.0005). We demonstrated that siRNA-mediated inhibition of Tiam1 enhanced sensitivity to three chemotherapeutic drugs (5-fluorouracil, oxaliplatin and irinotecan), and reduced tumor invasiveness in HCT116, SW480 and SW620 cell lines. Silencing of Tiam1 resulted in downregulation of stemness-related genes and spheroid formation ability. Furthermore, we validated that Tiam1-knockdown enhanced drug sensitivity and decreased the expression of stemness-related genes in xenograft tumors. Finally, we illustrated that the conditioned medium (CM) derived from CAFs led to increased stemness and chemoresistance in CRC cells through Tiam1 overexpression, and silencing of Tiam1 in CAFs resulted in reversal of stemness and chemoresistance properties. From a mechanistic standpoint, the target genes of Wnt signaling were also upregulated during co-culture with CM in CRC cells.
Conclusion: We have firstly demonstrated that Tiam1 is overexpressed in CRC patients who did not respond to chemotherapeutic drugs; and furthermore, high Tiam1 expression emerged as an independent prognostic factor in CRC. Mechanistically, CAFs enhanced CRC chemoresistance through Tiam1 overexpression. Collectively, these results suggest that Tiam1 suppression in stroma may sensitize CRC cells to chemotherapeutic agents, suggesting that Tiam1 may be an attractive therapeutic target in colorectal cancer.
Citation Format: Daisuke Izumi. Tiam1 is a key modulator of chemoresistance in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5226.