Protein arginine methyltransferase 5 (PRMT5) is a type II methyltransferase that is associated with several protein complexes and regulates a wide spectrum of target genes. PRMT5 modulates chromatin structure by symmetrically methylating histone H4 at arginine 3 (H4R3me2S) and histone H3 at arginine 8 (H3R8me2s), and has also been shown to methylate many non-histone proteins both in the nucleus and cytoplasm. Accordingly it is involved in a diverse range of cellular processes including transcription, RNA processing, development, differentiation, cell cycle control and apoptosis.
Evidence is also accumulating that implicates PRMT5 in tumourigenesis. PRMT5 is involved in the transcriptional repression of a number of tumour suppressors including p53, Rb, Nme1, ST7, STAT3 and PDCD4 and is involved in cyclin-D1-mediated cellular transformation. Knockdown of PRMT5 has been shown to potentiate TRAIL-induced apoptosis in cancer, inhibit growth of lung cancer xenografts and impair hypoxic cellular growth by preventing de novo synthesis of HIF-1alpha which plays a key role in tumour adaptation to hypoxia. PRMT5 expression is elevated in a number of cancers including melanoma, mantle cell lymphoma and also in breast cancer where high levels of expression correlate with poor prognosis. This suggests that PRMT5 is a potential therapeutic target.
In this study we wanted to assess the effect of shRNA-mediated knock-down of PRMT5 expression in a panel of human tumour cell lines and to determine if the results could be recapitulated using pharmacological inhibition of PRMT5. Knockdown of PRMT5 resulted in a concomitant reduction of symmetrically dimethylated H4R3. Similar results were observed using a selective, small molecule inhibitor of PRMT5. Additionally, the ability of tumour cells to form colonies was significantly reduced upon knockdown or inhibition of PRMT5 for 7 days. These results justify therapeutic targeting of PRMT5.