USP9X is a mechanosensitive deubiquitinase that controls tumor cell invasiveness and drug response through YAP stabilization

Post-translational modification by ubiquitin is crucial for protein turnover. Deubiquitinases (DUBs) remove ubiquitin chains from target proteins to prevent their degradation by the proteasome, thus acting as gatekeepers of protein homeostasis alongside the ubiquitin-proteasome system (UPS). Tumor cells exhibit remarkable plasticity, enabling them to adapt to anticancer treatments and the conditions of the tumor microenvironment, including mechanical cues from the extracellular matrix (ECM). However, the role of DUBs in mechanotransduction remains unexplored. To identify DUBs involved in cancer cell mechanosignaling, we used melanoma cells grown on collagen matrices with varying stiffnesses and an activity-based ubiquitin probe to profile DUB activities. Our approach, combined with quantitative proteomics, revealed that ubiquitin-specific protease 9X (USP9X) is sensitive to ECM stiffness through discoidin domain receptors (DDR)/actomyosin signaling pathway. In silico analysis further indicated that the mechanosensor YAP is part of the USP9X interactome, and USP9X expression correlates with the YAP transcriptional signature in melanoma. We hypothesized that mechanical signals regulate YAP levels through USP9X DUB activity. Consistently, low collagen stiffness reduced YAP expression, and siRNA-mediated depletion or pharmacological inhibition of USP9X decreased YAP protein expression in tumor cells. Conversely, knockdown of the ubiquitin E3 ligase {beta}TrCP increased YAP protein levels. Affinity purification of polyubiquitinated proteins using Tandem Ubiquitin Binding Entities (TUBEs) showed that combined USP9X and proteasome inhibition increased YAP poly-ubiquitination, revealing that USP9X deubiquitinates YAP to prevent its proteasomal degradation. Targeting USP9X impaired stiffness-mediated responses, including YAP nuclear translocation and transcriptional activity, cell migration and invasion, and drug resistance. An experimental metastasis assay showed that stable knockdown of USP9X impaired melanoma cell lung colonization. Finally, targeting USP9X in a syngeneic BRAF-mutant melanoma model counteracted targeted therapy-induced ECM remodeling, enhanced treatment efficacy, and delayed tumor relapse. Our findings reveal a novel role of USP9X in cancer cell mechanobiology and drug resistance through stiffness-dependent stabilization of the oncoprotein YAP, proposing USP9X as a targetable "mechano-DUB" in cancer.