STAMBP-Mediated K63 Deubiquitination of E2F1 Release E2F1 from RB Repressive Complex to Drive Bladder Cancer Progression

The canonical RB1-E2F regulatory model depends on RB phosphorylation-induced allosteric changes during the cell cycle. However, extensive RB mutations across cancers indicate the existence of phosphorylation-independent mechanisms governing RB–E2F complex stability. Here, we report a novel regulatory axis where enhanced E2F1 activity inversely correlates with K63-linked ubiquitination levels, independent of changes in RB1 phosphorylation status. Through systematic deubiquitinase profiling, we identify STAM Binding Protein (STAMBP)—a K63-specific deubiquitinase overexpressed in bladder tumors and correlated with advanced disease and poor survival—as the key enzymatic regulator. Mechanistically, STAMBP binds E2F1 and removes K63 chains at lysines 161/164, destabilizing the RB1-E2F1 repressive complex while maintaining RB1 phosphorylation homeostasis. This enhances E2F1 transcriptional activity, driving cell cycle target gene expression and promoting malignant progression through proliferation and invasion. Genetic loss of STAMBP suppresses tumor growth in vitro and in vivo. Bladder-specific Stambp knockout delays carcinogen-induced tumor progression and improves survival, while pharmacological inhibition with BC1471 selectively blocks proliferation in STAMBP-high cells without toxicity. Together, these findings establish a 'Dual-Lock' paradigm: K63-linked ubiquitin chains act as a molecular scaffold stabilizing the RB1-E2F1-HDAC1 complex, whereas STAMBP-mediated deubiquitination triggers oncogenic E2F1 activation. This work nominates STAMBP as a biomarker-driven therapeutic target for precision oncology in bladder cancer.