mTORC1-USP30-LEF1 Cascade Regulates Cancer Stemness and Malignant Progression Through Mitonuclear Crosstalk.

USP30, a ubiquitin-specific protease, primarily characterized as a mitochondrial deubiquitinase regulating mitophagy, has not been previously reported to have nuclear functions. In this study, we demonstrate that USP30 is present in both mitochondrial and nuclear compartments. Nutrient deprivation triggers USP30 nuclear translocation via an N-terminal nuclear localization signal (NLS), mediated through suppression of mTORC1-dependent phosphorylation at serine 104, a modification constraining nuclear entry. Nuclear USP30 acts as a tumor suppressor by inhibiting cancer stemness and chemoresistance in triple-negative breast cancer (TNBC) cells. Mechanistically, USP30 directly interacts with and deubiquitinates the transcription factor TCF/LEF1 at K379 and K382 residues, disrupting recruitment of CBP/P300 co-activators to the β-catenin/LEF1 complex. This abolishes β-catenin/LEF1 transactivation and suppresses WNT signaling. Clinically, USP30 is downregulated in TNBC and cancer stem cells (CSCs), with notably reduced nuclear levels in cancer tissues. Overexpression of nuclear USP30 markedly reduces lung metastatic burden in TNBC mouse models. These findings uncover a novel role for nuclear USP30 in regulating cancer stemness and suggest that targeting the dynamic relocalization of USP30 from mitochondria to the nucleus could offer new therapeutic strategies for breast cancer metastasis.