USP21-mediated SMARCB1 stabilization under hypoxia may influence tumor progression and immune response in HCC.

Hepatocellular carcinoma (HCC) remains a highly lethal malignancy despite therapeutic advances. SMARCB1 exhibits context-dependent functions across different cancer types. While it is frequently inactivated as a tumor suppressor in various malignancies, our previous work demonstrated that SMARCB1 acts as an oncogene in HCC by engaging the NUP210-P300 transcriptional axis. However, the upstream mechanisms that regulate SMARCB1 stability in HCC remain unexplored. Because SMARCB1 undergoes ubiquitin-mediated degradation under hypoxic conditions in other contexts, we investigated whether this regulation occurs in HCC. Cycloheximide (CHX) chase assays revealed that SMARCB1 remained highly stable under hypoxia in HCC cells, suggesting the presence of an active stabilization mechanism. Our screening for post-translational regulators identified USP21 as a deubiquitinase modulating SMARCB1 turnover. TCGA-LIHC analysis showed that USP21 is upregulated in HCC and positively correlated with SMARCB1 expression. Loss- and gain-of-function experiments confirmed that USP21 inhibition promotes SMARCB1 degradation, while USP21 overexpression prevents it. Co-immunoprecipitation demonstrated a physical interaction between USP21 and SMARCB1 that blocks ubiquitin-mediated proteasomal degradation. Combined inhibition of USP21 and P300, a downstream effector of SMARCB1, more effectively suppressed HCC cell proliferation under hypoxia than either treatment alone. Transcriptomic analysis further revealed that USP21-SMARCB1 axis contributes to immune-tolerant features in HCC. USP21 stabilizes SMARCB1 under hypoxic conditions, thereby sustaining its oncogenic and immunosuppressive activities in HCC. Targeting the USP21-SMARCB1 axis may inhibit tumor growth and enhance immunotherapy responsiveness, offering a potential therapeutic strategy for overcoming resistance in HCC treatment.