The Regulation of Musashi RNA Binding Proteins and the Implications for Cancer Therapy

Abstract

RNA-binding protein and stem cell regulator, Musashi-1 (MSI1) is overexpressed in a broad range of human cancers and associated with poor clinical outcome. MSI1 encourages tumorigenesis by promoting Notch and Wnt signaling, along with cell cycle progression. In vitro and in vivo data have demonstrated that MSI1 is an attractive therapeutic target. Accordingly, there is great interest in identifying and exploiting MSI1 regulating mechanisms as anti-cancer therapy. We successfully determined that non-coding RNA, miR-137, acts as a negative regulator of MSI1 by binding to the 3′UTR of MSI1 mRNA. Using numerous in vitro and in vivo studies, we show that miR-137 acts as a tumor suppressive miRNA in colon cancer, in part by inhibiting MSI1. To understand the clinical relevance of our observations, we measured the expression of miR-137 and MSI1 in tissue samples from patients with rectal cancer. miR-137 expression is decreased in approximately 84% of rectal tumor samples as compared to paired normal rectal mucosal samples. Inversely, MSI1 protein was found to be highly expressed in 79% of primary rectal tumors. In addition to this study, we show preliminary data that miR-137 also negatively regulates Musashi-2 (MSI2). We provide a comprehensive review on the expression of MSI1 and MSI2 in human cancers and correlate this expression with clinical outcome and evidence for MSI-induced malignancy. In addition to using noncoding RNAs to inhibit MSI1, we are also identifying small molecules that inhibit the RNA-binding function of MSI1. Our preliminary studies identified (−)-Gossypol, gossypolone and azaphilone-9 as potent MSI1 inhibitors using a fluorescence polarization-based screening of our in-house library of compounds. In this study, we validated and characterized the binding event between hit compounds and MSI1 RNA-binding domain 1 (RBD1) using nuclear magnetic resonance. In summary, we discovered two novel means of inhibiting the function of MSI1 in cancer, using miR-137 and small molecule inhibitors. Our study supports the idea of exploiting miRNAs and small molecules as novel cancer therapy targeting the RNA-binding protein, Musashi.