TRIM55 restricts the progression of hepatocellular carcinoma through ubiquitin-proteasome-mediated degradation of NF90

Hepatocellular carcinoma (HCC) is a prevalent malignant tumor worldwide. Tripartite motif containing 55 (TRIM55), also known as muscle-specific ring finger 2 (Murf2), belongs to the TRIM protein family and serves as an E3 ligase. Recently, the function and mechanism of TRIM55 in the advancement of solid tumors have been elucidated. However, the role of TRIM55 and its corresponding protein substrates in HCC remains incompletely explored. In this study, we observed a significant reduction in TRIM55 expression in HCC tissues. The downregulation of TRIM55 expression correlated with larger tumor size and elevated serum alpha-fetoprotein (AFP), and predicted unfavorable overall and tumor-free survival. Functional experiments demonstrated that TRIM55 suppressed the proliferation, migration, and invasion of HCC cells in vitro, as well as hindered HCC growth and metastasis in vivo. Additionally, TRIM55 exhibited a suppressive effect on HCC angiogenesis. Mechanistically, TRIM55 interacted with nuclear factor 90 (NF90), a double-stranded RNA-binding protein responsible for regulating mRNA stability and gene transcription, thereby facilitating its degradation via the ubiquitin-proteasome pathway. Furthermore, TRIM55 attenuated the association between NF90 and the mRNA of HIF1α and TGF-β2, consequently reducing their stability and inactivating the HIF1α/VEGF and TGFβ/Smad signaling pathways. In conclusion, our findings unveil the important roles of TRIM55 in suppressing the progression of HCC partly by promoting the degradation of NF90 and subsequently modulating its downstream pathways, including HIF1α/VEGF and TGFβ/Smad signaling.