Manuscript title: C/EBPa is methylated by PRMT1 to regulate Cyclin D1 gene transcription in human breast cancer

C/EBPa is an essential transcription factor involved in regulating the expression or function of certain cell-cycle regulators. However, little is known about the role of methylation in regulating the antiproliferation activity of C/EBPa. Here, we report that knockdown of protein arginine methyltransferases 1 (PRMT1) leads to cellular growth arrest accompanied by a decrease in Cyclin D1 gene expression in breast cancer cells. Furthermore, we reveal that C/EBPa is methylated by PRMT1 at three arginine residues (R35, R156, and R165), which impairs the interaction of C/EBPa with HDAC3 and modulates the transcription activity of C/EBPa and Cyclin D1 gene expression. PRMT1 is upregulated in human breast cancer, and elevated PRMT1 is correlated with cancer malignancy. Most importantly, a specific inhibitor of PRMT1 significantly impedes the growth of cancer cells from triple-negative breast cancer patients. Our data demonstrate that high expression of PRMT1 promotes the expression of Cyclin D1 through methylation of C/EBPa to interfere with the repressive function of HDAC3, which leads to rapid growth of tumor cells during the pathogenesis of breast cancer. This evidence that PRMT1 mediates C/EBPa methylation sheds light on a novel therapeutic pathway for breast cancer. Overall design: mRNA profiles of sh1, sh2 and shGFP cells, sh1 and sh2 cells were obtained by infecting MDA-MB-231 cell using lentivirus expressing shPRMT1, while the control sample shGFP cell was generated by infecting MDA-MB-231 cell with lentivirus expressing shGFP. Independent biology triplicate for each sample was performed.