| Metastasis is the leading cause of death in patients with breast cancer. The epithelial-to-mysenchymal transition (EMT) enables epithelial cells loss of epithelial state and gain of mysenchymal cell state, which is critical for cancer cells metastasis. Elucidating mechanisms that govern EMT can help uncover the underlying principles of breast cancer metastasis and development of new drugs for breast cancer. The conserved Musashi (Msi) family of RNA binding proteins include two homologues in mammals:Msi1 and Msi2. During development, Msi (Msi1 and Msi2) are mainly expressed in epithelial tissues, especially in stem/progenitor cells. Msi are highly expressed in variety types of cancer tissues, hightlighting the importance of Msi in tumorigenesis. However, it is still unknown whether Msi proteins are important for breast cancer development and whether they regulate EMT. In this study, we utilized an inducible transgenic mouse, knockout mouse model and MMTV-PyVT breast cancer model, combined with in vitro cells culture method to investigate the role of Msi in mammary development and breast cancer.Here we found that the expression level of Msi 1 and Msi2 are closely associated with metastasis of breast cancer subtypes. Msi proteins show higher expression in luminal-like tumors and lower in invasive basal-like tumors. Further, the expression of Msi negatively associated with EMT marker genes. In vitro results showed that knockdown of Msi enforced breast cancer cells into a state of mesenchymal cells. In order to investigate its physiological role in vivo, we generated an inducible transgenic mice, which allowed Msi2 is ecotopically expressed in mammary myoepithelial cells in response to Dox treatment. Ecotopical Msi2 overexpression leads to defects of ductal elongation and mammary branching, increase in luminal epithelial cells, and repression of EMT. In contrast, loss of Msi promotes EMT and increases number of myoepithelial cells. These findings demonstrate that Msi proteins repress EMT in mammary epithelium. To study the role of Msi for breast cancer in vivo, we bred the inducible Msi2 transgenic mice to MMTV-PyVT breast cancer model. Msi2 overexpression represses mammary tumor growth and lung metastasis by inhibiting EMT. Msi are important post-transcriptional regulators of EMT in mammary epithelium and play important roles in breast cancer metastasis.In molecular mechanism, we found that Jaggedl (Jag1) is a direct target of Msi2. Msi proteins inhibited translation of Jag1 through directly binding to 31 UTR of Jag1 using molecular biological and biochemistrical method. Jagl is a ligand of Notch signaling pathways, activating the Notch pathway. The activation of Notch pathway is sufficient to induce EMT. Msi proteins maintain mammary epithelial cells in a state of epithelium. Msi2 represses EMT by inhibiting Jag1-Notch pathway.Taken together, Msi proteins repress EMT and breast cancer metastasis by inhibiting Jagl-Notch signaling pathway in mammary development and breast cancer development. It is the first time to report a novel post-transcriptional mechanism of EMT, broading our understanding of the regualtional network of EMT. The study provides new insight for drug development and potential strategies for therapeutic intervention in breast cancer. |
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