Abstract 3446: STRAP is involved in mutated Apc-induced stemness and tumorigenesis in colon cancer

Abstract

Abstract Inactivation of the Apc gene is a critical early event in the development of sporadic colorectal cancer (CRC). Loss of its normal function leads to constitutive activation of β-catenin/T-cell factor 4 signaling and hence transcription of Wnt target genes. The signaling scaffold protein STRAP is upregulated in several cancers, where it promotes tumorigenicity and stemness. Here, we tested the effect of conditional deletion of Strap on intestinal tumor formation and growth by crossing ApcMin/+ mice with mice that are null for Strap in the intestinal epithelium, Strapfl/fl;Vil-Cre mice (StrapIEKO). We observed a dramatic decrease in the number of intestinal polyps in the ApcMin/+;Strapfl/fl;Vil-Cre mice (called StrapIEKO;ApcMin/+) compared to mice with intact Strap (Strapfl/fl;ApcMin/+). In an attempt to understand the mechanism of function of STRAP, we have observed that colon tumors from StrapIEKO;ApcMin/+ mice displayed a decrease in β-catenin nuclear localization and downregulation of Wnt targets such as c-Myc, AXIN2, and CCND1. Enteroids prepared from StrapIEKO;ApcMin/+ mice exhibited more budding compared to the ApcMin/+ enteroids, indicating the role of STRAP in stemness. STRAP knockdown in human CRC cells significantly diminished β-catenin/TCF4 transcriptional activity. Specifically, STRAP knockdown impaired the assembly of β-catenin/TCF4 complex and recruitment of the complex to the promoters of β-catenin target genes. Functional proteomic analyses of crypt cells from different genetic mice suggested that the MEK/ERK signaling pathway is suppressed in StrapIEKO;ApcMin/+ mice that is required for the effect of STRAP on Wnt/β-catenin signaling. We have shown that STRAP associates with MEK1/2 and promotes binding between MEK1/2 and ERK1/2 and subsequently induces the phosphorylation of ERK1/2. Given that MEK/ERK signaling is found to promote Wnt/β-catenin signaling, our results suggest that STRAP indirectly induce Wnt/β-catenin signaling through activating MEK/ERK pathway. Importantly, STRAP is found to be a novel target of Wnt/β-catenin signaling as ChIP assays determined putative binding sites of β-catenin/TCF4 complex on Strap promoter. Finally, we showed that forced expression of STRAP in colon cancer organoids derived from patients with colon cancer can induce the growth of colon tumor organoids through promoting Wnt/β-catenin signaling. Altogether, these results suggest that STRAP is involved in mutated APC-induced tumor initiation through a novel feed-forward STRAP/MEK-ERK/β-catenin/STRAP regulatory axis. Citation Format: Trung Vu. STRAP is involved in mutated Apc-induced stemness and tumorigenesis in colon cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3446.