BRIX1 activated by mTORC1-SP1 signaling regulates ribosome biogenesis and alternative splicing to promote hepatocellular carcinoma progression.

[BACKGROUND] The treatment of hepatocellular carcinoma (HCC) remains thorny, due to that just few molecules have been defined as pathogenic drivers. Ribosome biogenesis and alternative splicing (AS) are dysregulated during the development of HCC, but how both processes coordinate remains unexplored.

[METHODS] Pan-cancer analyses and tissue microarray were used to assess the clinical relevance of BRIX1. Functional studies employed CCK-8 assays, colony formation assays, Transwell migration/invasion assays, flow cytometry analysis, and animal studies. Splicing networks were analyzed by RNA-seq and validated through isoform-specific qRT-PCR. Molecular mechanisms were dissected via ChIP-PCR, Co-IP/MS, immunofluorescence, EU RNA Synthesis, and puromycin incorporation assay.

[RESULTS] BRIX1 is upregulated in HCC, involved in the mTORC1-SP1 signaling, correlated with a poor prognosis, suggesting it as a critical oncogene. BRIX1 depletion suppresses the proliferation, migration and invasion, and induces the apoptosis of HCC cells. Mechanically, BRIX1 localizes to the nucleolus to interact with UBTF and POLR1A, thus promoting rDNA transcription and ribosomal biogenesis. BRIX1 depletion triggers ribosomal stress, disturbs pre-rRNA synthesis, and inhibits global protein translation. Furthermore, BRIX1 knockdown suppresses SRSF1-mediated oncogenic AS, reduces the production of carcinogenic isoforms MKNK2 and S6K1, and silences mTORC1-4EBP1 signaling. SRSF1 overexpression reverses the phenotypic and molecular changes induced by BRIX1 knockdown.

[CONCLUSION] BRIX1 activated by mTORC1-SP1 signaling regulates ribosome biogenesis and AS to promote HCC progression. Our findings establish BRIX1 as a governor on both ribosome biogenesis and AS in HCC, and a possible prognostic or therapeutic biomarker.