S119N Mutation of the E3 Ubiquitin Ligase SPOP Suppresses SLC7A1 Degradation to Regulate Hepatoblastoma Progression

A previous study on hepatoblastoma revealed novel mutations and cancer genes in the Wnt pathway and ubiquitin ligase complex, including the tumor suppressor speckle-type BTB/POZ (SPOP). Moreover, the SPOP gene affected cell growth, and its S119N mutation was identified as a loss-of-function mutation in hepatoblastoma. This study aimed to explore more functions and the potential mechanism of SPOP and its S119N mutation. The in vitro effects of SPOP on cell proliferation, invasion, apoptosis, and in vivo tumor growth were investigated by western blot analysis, Cell Counting Kit-8, colony formation assay, flow cytometry, and xenograft animal experiments. The substrate of SPOP was discovered by a protein quantification assay and quantitative ubiquitination modification assay. The present study further proved that SPOP functioned as an anti-oncogene through the phosphatidylinositol 3-kinase/Akt signaling pathway to affect various malignant biological behaviors of hepatoblastoma both in vitro and in vivo. Furthermore, experimental results also suggested that solute carrier family 7 member 1 (SLC7A1) might be a substrate of SPOP and influence cell phenotype by regulating arginine metabolism. In conclusion, these findings demonstrated the function of SPOP and revealed a potential substrate related to hepatoblastoma tumorigenesis, which might thus provide a novel therapeutic target for hepatoblastoma.