Abstract 5058: STRAP-induced Wnt/ß-catenin signaling is important in inactivated Apc-mediated tumorigenicity and stemness of colorectal cancer
Authors: Vu Trung , Datta Arunima , Jin Lin , Datta Pran K. 
Published: August 2018

Aberrant Inactivation of the Apc gene and activation of Wnt/β-catenin signaling in colorectal cancer (CRC) has been implicated in tumorigenesis and homeostasis of cancer stem-like cells (CSCs), albeit the mechanism remains mostly obscure. We have previously reported the identification of a WD-domain protein, STRAP (Serine-Threonine Kinase Receptor-Associated Protein) that inhibits TGF-ß signaling, interacts with a variety of proteins, and influences a wide range of cellular processes. Upregulation of STRAP in CRC patients had a worse survival with adjuvant therapy. In contrast, patients carrying tumors with normal or low STRAP expression benefited from the treatment. Our preliminary data suggest that homozygous deletion of STRAP causes embryonic lethality at embryonic day E9.5. Therefore, to study the role of STRAP in vivo in CRC, we have generated Strap-conditional knockout mice Villin-Cre;Strapfl/fl(Strap-cko) and used two mouse models of CRC: Apc+/min and AOM/DSS. Conditional deletion of Strap in both mouse models resulted in a dramatic decrease in tumor formation. Our results demonstrate that STRAP is required for tumor formation through activation of Wnt/β-catenin signaling pathway in both models. Immunoblotting and immunohistochemistry analyses of tumors derived from the Strap-cko;Apc+/min mice showed decreased levels of β-catenin and of its nuclear staining. Furthermore, mRNA levels of several Wnt target genes are also decreased in tumors derived from Strap-cko;Apc+/min mice compared to those from Apc+/min mice. Formation and sustained propagation of intestinal enteroids require cells from the stem cell niche. APC mutation accompanied by elevated Wnt signaling promotes spheroid-shaped enteroids as indicated by the absence of crypt buds. We found that enteroids prepared from Strap-cko;Apc+/min mice exhibited more budding compared with the Apc+/min enteroids, indicating the role of STRAP in stemness. Strap deletion in enteroids decrease stem cell markers such as CD133, CD44, and LGR5. By using chromatin immunoprecipitation (ChIP) assay, we have found that STRAP knockdown can affect the recruitment of β-catenin/TCF4 complex on target promoter regions. Interestingly, STRAP protein and mRNA levels were found to be upregulated in Apc+/min induced tumors. STRAP is upregulated by Wnt/β-catenin signaling pathway, as evidenced by downregulation of STRAP in β-catenin/TCF4 inhibitor iCRT3 treated cells. In addition, ChIP assay has identified TCF DNA binding sites in the Strap promoter through which Wnt signaling regulates Strap gene expression. Altogether, our findings reveal that STRAP is a novel target of Wnt/β-catenin signaling and it is required for inactivated Apc- and Wnt/β-catenin-induced stemness and tumorigenesis through a complex feedforward loop.

Citation Format: Trung Vu, Arunima Datta, Lin Jin, Pran K. Datta. STRAP-induced Wnt/ß-catenin signaling is important in inactivated Apc-mediated tumorigenicity and stemness of 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 5058.