Protein Arginine Methyltransferase-5 Inhibition Induces Growth Arrest and Death in Triple-Negative Breast Cancer Cells

Background: PRMT5, or protein arginine methyltransferase 5, is recognized as an epigenetic regulator that suppresses gene transcription through symmetric dimethylation of histone arginine residues, including histone H4 arginine 3 (H4R3me2s) and histone H3 arginine 8 (H3R8me2s), modifications associated with chromatin condensation and transcriptional repression. PRMT5-mediated methylation has been associated with recruitment of polycomb repressive complex 2 (PRC2) and enhancer of zeste homolog 2 (EZH2)-mediated H3K27me3 deposition, contributing to stable repression of tumor suppressor genes and apoptosis-related effectors in breast cancer. Methods: The molecular and functional impacts of PRMT5 inhibition were studied in TNBC cell lines with a pharmacological inhibitor (CMP5). Cellular responses were evaluated using a viability assay, qPCR, Western blotting, Annexin V/PI staining, and transwell migration/proliferation assays. Results: PRMT5 inhibition substantially reduced TNBC viability in a time- and dose-dependent manner. EZH2 was downregulated, whereas the tumor suppressor retinoblastoma-like protein 2 (RBL2) was induced, concomitant with low expression of Cyclin D1. These changes were accompanied by upregulation of pro-apoptotic effectors (Caspase-3, Caspase-10, death-associated protein 1 (DAP1), and BCL2-associated x protein (BAX) and repression of the pro-survival B-cell lymphoma 2 (BCL2), consistent with apoptosis-associated molecular responses. Functionally, CMP5 treatment was associated with reduced migratory behavior in TNBC cells under the experimental conditions tested. Conclusions: These findings suggest that PRMT5 inhibition by CMP5 is associated with reduced TNBC cell viability, impaired migration, increased expression of apoptosis-associated regulators and enhanced apoptotic cell death as measured by Annexin V/PI analysis in vitro. Further mechanistic and in vivo studies are required to clarify the therapeutic relevance of PRMT5 inhibition in TNBC.