Splicing Regulator p54nrb/Non–POU Domain–Containing Octamer‐Binding Protein Enhances Carcinogenesis Through Oncogenic Isoform Switch of MYC Box–Dependent Interacting Protein 1 in Hepatocellular Carcinoma

Abstract

Background and Aims Alternative splicing (AS) is a key step that increases the diversity and complexity of the cancer transcriptome. Recent evidence has highlighted that AS has an increasingly crucial role in cancer. Nonetheless, the mechanisms underlying AS and its dysregulation in hepatocellular carcinoma (HCC) remain elusive. Here, we report that the expression of RNA‐binding protein p54 nrb /non‐POU domain‐containing octamer‐binding protein (NONO) is frequently increased in patients with HCC and is associated with poor outcomes. Approach and Results Knockdown of NONO significantly abolished liver cancer cell proliferation, migration, and tumor formation. RNA‐sequencing revealed that NONO regulates MYC box–dependent interacting protein 1 (or bridging integrator 1 [BIN1]; also known as amphiphysin 2 3P9) exon 12a splicing. In the normal liver, BIN1 generates a short isoform (BIN1‐S) that acts as a tumor suppressor by inhibiting the binding of c‐Myc to target gene promoters. In HCC, NONO is highly up‐regulated and produces a long isoform (BIN1‐L, which contains exon 12a) instead of BIN1‐S. High levels of BIN1‐L promote carcinogenesis by binding with the protein polo‐like kinase 1 to enhance its stability through the prevention of ubiquitin/proteasome‐dependent cullin 3 degradation. Further analysis revealed that NONO promotes BIN1 exon 12a inclusion through interaction with DExH‐box helicase 9 (DHX9) and splicing factor proline and glutamine–rich (SFPQ). Notably, frequent coexpression of DHX9–NONO–SFPQ is observed in patients with HCC. Conclusions Taken together, our findings identify the DHX9–NONO–SFPQ complex as a key regulator manipulating the oncogenic splicing switch of BIN1 and as a candidate therapeutic target in liver cancer.