Abstract: Objective To explore the mechanism by which protein arginine methyltransferase 5(PRMT5)up-regulates the expression of ubiquitin-specific protease 15(USP15)through methylation, thereby promoting the occurrence and development of breast cancer. Methods Gene Expression Profiling Interactive Analysis(GEPIA2)online database and Kaplan-Meier plotter survival curve analysis were used to explore the difference in the expression of PRMT5 in tumor tissue and normal tissue and its relationship with prognosis. The protein bound with PRMT5 was obtained with immunoaffinity purification and silver staining combined with mass spectrometry analysis. After PRMT5 was overexpressed or knocked down in MCF-7 cells with lentiviral stabilizing technology, the expressions of related proteins were detected with quantitative real-time PCR(qRT-PCR)and Western blotting. After USP15 was overexpressed or knocked down with plasmid or small interfering RNA(siRNA)transfection, the expressions of related proteins were determined with qRT-PCR and Western blotting, and the proliferation and invasion ability of breast cancer cells were detected with CCK8, colony formation and Transwell assay. After the activity of PRMT5 methyltransferase was inhibited with EPZ015666(GSK3235025)small molecule inhibitor, the changes of related proteins were determined with Western blotting, and the effects on the proliferation and invasion ability of breast cancer cells were assessed with CCK8 and Transwell assay. Results PRMT5 was highly expressed in breast cancer patients and was associated with poor prognosis. PRMT5 and USP15 interacted with each other. In breast cancer cell MCF-7, after overexpression of FLAG-PRMT5, the protein expression of USP15 was significantly increased, while the mRNA level remained unchanged. After the expression of PRMT5 was knocked down, the protein level of USP15 decreased, but the mRNA level did not change. In MCF-7 cells, overexpression of Myc-USP15 significantly decreased the mRNA and protein levels of FBXW7, but increased the proliferation and invasion ability, while knockdown of USP15 had the opposite results. After EPZ015666 inhibited the methyltransferase activity of PRMT5, the proliferation and invasion ability of cells were decreased by reducing the protein level of USP15. Conclusion PRMT5 interacts with USP15 in breast cancer cells and upregulates the protein level of USP15 in a methyltransferase activity-dependent manner. The increase of USP15 can inhibit the transcriptional activation of F-box and WD repeat domain-containing 7(FBXW7)gene by catalyzing the deubiquitination modification of histone H2BK120ub, promote breast cancer cell proliferation and epithelial to mesenchymal transition(EMT), and promote the occurrence and progress of breast cancer. These findings suggest that PRMT5 is a potential diagnostic and therapeutic target for breast cancer.

Key words: Breast cancer, PRMT5, USP15, FBXW7, Proliferation, Epithelial to mesenchymal transition