SRSF3 and HNRNPH1 regulate alternative splicing of PRMT5 induced by ionizing radiation in hepatocellular carcinoma

Background Aberrant splice variants play different roles in the formation of tumors. We observed the splice isoform of Protein arginine methyltransferase 5 (PRMT5-ISO5) increases in HCC patients undergoing stereotactic body radiotherapy, which is associated with improvement of poor prognosis. However, the mechanism of alternative splicing of PRMT5-ISO5 induced by ionizing radiation (IR) is still unclear. Methods The transcriptional changes of PRMT5-ISO5 induced by IR were validated by reverse transcription quantitative polymerase chain reaction (RT-qPCR) assay. Bioinformatic analyses were performed to identify potential splicing factors involved in regulating PRMT5 splicing. Small interferring RNA and overexpressing plasmids for SRSF3 and HNRNPH1 were introduced into HCC cell lines, followed by in vitro functional experiments in regulating PRMT5 splicing by RT-qPCR, western blot and RNA-immunoprecipitation assay in vitro. The roles of IR-induced PRMT5-ISO5 and hepatocyte-specific Prmt5 knockout on HCC progression were evaluated in vivo. Results we indicated IR could induce PRMT5-ISO5 transcript in HCC cells by virtue of splicing factors SRSF3 and HNRNPH1. Mechanistically, HNRNPH1 silencing resulted in the decrease of PRMT5-ISO5 while SRSF3 silencing led to the increase of PRMT5-ISO5. In addition, both SRSF3 and HNRNPH1 bound to PRMT5 precursor mRNA on the region around 3' splicing site of intron 2 and alternative 3’ splicing site on exon 4, leading to their opposite functions on regulating PRMT5 splicing. In vivo, the increase of PRMT5-ISO5 induced by IR led to tumor regression, and liver-specific Prmt5 depletion decelerated the progression of Akt/N-Ras-derived spontaneous HCC. Conclusion Our study not only provides mechanistic views that IR-induced SRSF3 downregulation leads to the imbalance of SRSF3 and HNRNPH1 in regulating PRMT5-ISO5 transcript, but also indicates a potential radiotherapeutic of PRMT5-ISO5 in HCC formation since liver-specific Prmt5 knockout inhibits spontaneous HCC tumorigenesis.