Abstract 1110: Musashi 1 stabilizes and blocks translation of Adenomatous Polyposis Coli mRNA in intestinal cells.
Authors: Wolfe Andy R. , Spears Erick , Ernlund Amanda , Neufeld Kristi L. 
Published: August 2014

Abstract
Most colorectal cancers are thought to be initiated by mutation of the tumor suppressor Adenomatous polyposis coli gene (APC). APC mutations that result in loss of function lead to deregulation of Wnt signaling and inappropriate proliferation within intestinal crypts. Understanding APC regulation within normal intestinal cells is important for developing methods to restore function in a pathological state. Using cultured human colonocytes, we have previously shown that translation of APC can be blocked by the RNA binding protein, Musashi 1 (MSI1) and that MSI1 is a Wnt target. We propose that a double negative feedback loop between APC and MSI1 functions in maintenance of intestinal cell homeostasis. To explore the MSI1/APC interaction in vivo, we analyzed intestinal tissue from MSI1 knock-out mice (Msi1-/-). We found higher levels of APC and less Wnt signal activation in intestines from the Msi1-/- mice. There were also more enterocytes within the intestinal crypts of Msi1-/-mice, indicating an increase in differentiation. These three observations provide confirmation of the proposed APC/MSI1 double negative feedback loop in vivo.
Evidence from cultured human cells and mouse models supports a role for an APC/MSI1 double negative feedback loop in the response of intestinal cells to Wnt signaling. However, the underlying mechanisms remain to be determined. Proteins that bind to a specific motif in the 3’UTR of target mRNA can induce translation inhibition or promotion as well as stabilization or destabilization of the mRNA. We already established that MSI1 blocks APC translation; however, the effect of MSI1 on APC mRNA stability was unknown. Here, we show that MSI1 binding to the 3’ UTR of APC mRNA stabilizes the message. In contrast, Numb, another established MSI1 target, does not appear to be stabilized by MSI1 binding. If MSI1 bound to the 3’UTR of APC mRNA stabilizes the message, but inhibits translation, then something likely leads to MSI1 release when APC translation resumes. One possibility is that MSI1 protein is inherently unstable. In this case, MSI1 binds to APC mRNA and blocks translation but is rapidly degraded and therefore releases APC mRNA for translation unless additional MSI1 is made. Because MSI1 is a Wnt target, a cell stimulated by Wnt ligand should continually replace the MSI1, therefore APC mRNA translation would remain blocked until the Wnt signal is removed. Our data showing that MSI1 has a relatively long half-life does not support this model. Current work is aimed to explore whether MSI1 release from APC mRNA is regulated by post-translational modifications or other protein-protein interactions.
In conclusion, by acting as a Wnt antagonist, APC blocks MSI1 transcription. On the other hand, MSI1 binds to and stabilizes APC mRNA while also blocking APC translation. The MSI/APC double negative feedback loop appears important for maintaining homeostasis of intestinal epithelial cells.
Citation Format: Andy R. Wolfe, Erick Spears, Amanda Ernlund, Kristi L. Neufeld. Musashi 1 stabilizes and blocks translation of Adenomatous Polyposis Coli mRNA in intestinal cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1110. doi:10.1158/1538-7445.AM2013-1110