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Protein arginine methyltransferase-5 (PRMT5) is overexpressed in aggressive B-cell non-Hodgkin's lymphomas, including mantle cell lymphoma and diffuse large B-cell lymphoma, and supports constitutive expression of CYCLIN D1 and c-MYC. Here, we combined ChIP analysis with next-generation sequencing to identify microRNA (miRNA) genes that are targeted by PRMT5 in aggressive lymphoma cell lines. We identified enrichment of histone 3 dimethylation at Arg-8 (H3(Me2)R8) in the promoter regions of miR33b, miR96, and miR503. PRMT5 knockdown de-repressed transcription of all three miRNAs, accompanied by loss of recruitment of epigenetic repressor complexes containing PRMT5 and either histone deacetylase 2 (HDAC2) or HDAC3, enhanced binding of co-activator complexes containing p300 or CREB-binding protein (CBP), and increased acetylation of specific histones, including H2BK12, H3K9, H3K14, and H4K8 at the miRNA promoters. Re-expression of individual miRNAs in B-cell lymphoma cells down-regulated expression of PRMT5, CYCLIN D1, and c-MYC, which are all predicted targets of these miRNAs, and reduced lymphoma cell survival. Luciferase reporter assays with WT and mutant 3′UTRs of CYCLIN D1 and c-MYC mRNAs revealed that binding sites for miR33b, miR96, and miR503 are critical for translational regulation of the transcripts of these two genes. Our findings link altered PRMT5 expression to transcriptional silencing of tumor-suppressing miRNAs in lymphoma cells and reinforce PRMT5's relevance for promoting lymphoma cell growth and survival.

Epigenetic regulation by the type II protein arginine methyltransferase, PRMT5, plays an essential role in the control of cancer cell proliferation and tumorigenesis. In this report, we investigate the relationship between PRMT5 and WNT/B- CATENIN as well as AKT/GSK3B proliferative signaling in three different types of non-Hodgkin's lymphoma cell lines, clinical samples, and mouse primary lymphoma cells. We show that PRMT5 stimulates WNT/B-CATENIN signaling through direct epigenetic silencing of pathway antagonists, AXIN2 and WIF1, and indirect activation of AKT/GSK3 signaling.PRMT5 inhibition with either shRNA-mediated knockdown or a specific small molecule PRMT5 inhibitor, CMP-5, not only leads to derepression of WNT antagonists and decreased levels of active phospho-AKT (Thr-450 and Ser-473) and inactive phospho- GSK3B (Ser-9) but also results in decreased transcription of WNT/B-CATENIN target genes, CYCLIN D1, c-MYC, and SURVIVIN, and enhanced lymphoma cell death. Furthermore, PRMT5inhibition leads to reduced recruitment of co-activators CBP, p300, and MLL1, as well as enhanced recruitment of co-repressors HDAC2 and LSD1 to the WNT/B-CATENIN target gene promoters. These results indicate that PRMT5 governs expression of prosurvival genes by promoting WNT/B- CATENIN and AKT/GSK3 proliferative signaling and that its inhibition induces lymphoma cell death, which warrants further clinical evaluation. This work was supported by Leukemia and Lymphoma Society Award MCL7001-18 (to R. A. B.) and National Priorities Research Program Grant NPRP8-617-3-131 from the National Research Fund (a member of Qatar Foundation) (to S. S). Scopus