SRSF7-mediated Alternative Splicing of USO1 Promotes Hepatocellular Carcinoma by Activating the MAPK/ERK1-ELK1 Pathway

We conducted a transcriptome-wide survey to identify novel hepatocellular carcinoma (HCC)-associated genes that have undergone aberrant alternative splicing. We revealed that the vesicle transport factor (USO1) is a major alternatively spliced target in HCC, mainly composed of a long wild-type isoform (USO1-L) and a short truncated isoform lacking exons 5 and 15 (USO1-S). A markedly increased isoform switching from USO1-L to USO1-S occurs in approximately 80% of HCCs and predicts poor clinical outcomes. USO1-L suppresses HCC cells growth and metastasis through weakening the activation of MAPK/ERK1-ELK1 signaling by interacting with phosphorylated ERK1 and anchoring it onto the Golgi apparatus to reduce its nuclear translocation; while USO1-S confers a loss-of-function effect. The splicing factor SRSF7, which is shown to be hypomethylated and upregulated in HCC, mediates the splicing of USO1-L to produce USO1-S. Notably, the loss of USO1-L in HCC cells induces their resistance to MEK inhibitors; however, restoring USO1-L through antisense oligodeoxynucleotide (ASO)-mediated blockade of switching from USO1-L to USO1-S can reverse this resistance. In summary, our findings highlight the crucial role of aberrantly alternative splicing of USO1 in HCC, as well as the SRSF7-USO1-MAPK axis as a potential target for this malignancy. To confirm the effects of SRSF7 and antisense oligonucleotides (ASOs) targeting the binding sites of SRSF7 within the USO1 pre-mRNA, MHCC97H cells under overexpression or HepG2 cells under ASO treatment (ASO-5 or ASO-15) were subjected to RNA extraction and short-read RNA sequencing analyses. Additionally, HepG2 cells under ASO treatment (ASO-5 or ASO-15) were subjected to RNA extraction and long-read RNA sequencing analyses.