Deubiquitinating enzyme Josephin-2 stabilizes PHGDH to promote a cancer stem cell phenotype in hepatocellular carcinoma

Deubiquitinating enzymes (DUBs) have been shown to be possible targets for hepatocellular carcinoma (HCC) treatment. This study was designed to reveal the effect and underlying mechanism of Josephin-2, a relatively newly defined DUB, in HCC progression. SNU-387 and PLC/PRF/5 cells were used for in vitro functional assays. The levels of Josephin-2 and phosphoglycerate dehydrogenase (PHGDH) were determined using RT-qPCR and western blotting. Cell proliferation, migration and invasion were assessed by CCK-8, colony formation and Transwell. Spheroid-forming assay was performed to assess the cancer stem cell (CSC)-phenotype of HCC cells. A xenograft mice model was applied to verify the effect of Josephin-2 on HCC cell growth in vivo. Herein, we showed that Josephin-2 expression was negatively correlated with HCC patient survival in data from the online database. Cell experiments indicated that knockdown of Josephin-2 attenuated HCC cell malignant biological behaviors. Besides, Josephin-2 silencing also decreased the spheroid-formation while inhibited the expression of CSC biomarkers (CD133, OCT4, SOX2 and EpCAM) in HCC cells. Mechanistically, Josephin-2 had a deubiquitinating activity towards the regulation of PHGDH protein, the rate-limiting enzyme in the first step of serine biosynthesis pathway. Depletion of Josephin-2 enhanced the ubiquitination degradation of PHGDH and ultimately inhibited the proliferation and CSC-phenotype of HCC in vitro and in vivo. Our work uncovered the regulatory effects of Josephin-2 on PHGDH protein stability and profiled its contribution in HCC malignant progression, which might provide a potential therapeutic target for HCC.