Small-molecule CBLB inhibitor abolishes EGFR ubiquitination, reduces receptor endocytosis and diminishes cell motility signaling

Endocytosis of the epidermal growth factor receptor (EGFR) is considered a key regulator of the receptor signaling activity. However, the molecular mechanisms underlying EGFR endocytosis are incompletely understood. Although ligand-induced ubiquitination of EGFR is known to promote its endocytic trafficking, the importance of EGFR ubiquitination in clathrin-mediated endocytosis, the primary physiological route of EGFR internalization, remains debated, and the relative contributions of ubiquitination-dependent and - independent mechanisms are not defined. Hence, we used NX-1013, a novel small-molecule inhibitor of the CBLB E3 ubiquitin ligase, to dissect the role of EGFR ubiquitination in its endocytic trafficking and signaling. Strikingly, brief treatment with NX-1013 completely abolished EGF-induced EGFR ubiquitination, demonstrating that this process is exclusively mediated by the closely related CBLB and CBL ligases. NX-1013 inhibited clathrin-mediated internalization of activated EGFR by 60-70%. The remaining, ubiquitination-independent internalization required EGFR kinase activity, was highly clathrin-dependent, and was significantly impaired by depletion of the AP-2 clathrin adaptor complex. Interestingly, inhibition of CBLs and EGFR endocytosis by NX-1013 did not affect major downstream signaling pathways in human oral squamous cell carcinoma cells, with the exception of Rac1 activation and EGFR-dependent cell migration, both of which were suppressed. Significance StatementCBL E3 ubiquitin ligases mediate ubiquitin conjugation of EGFR but their functional contributions to EGFR endocytic trafficking and signaling remain poorly defined. Here, we describe a newly developed small-molecule inhibitor of CBL proteins that potently blocks EGFR ubiquitination. This tool allowed us to dissect ubiquitination-dependent versus - independent components of the clathrin-mediated endocytosis and ligand-induced downregulation of EGFR. Strikingly, while inhibition of CBLs suppressed EGFR-driven cell motility signaling, it spared other major downstream pathways in EGFR-dependent human oral squamous cell carcinoma cells. These findings establish acute inhibition of CBLs as a powerful approach to interrogate ubiquitin-mediated receptor regulation and highlight its potential for therapeutic targeting of cancer cell migration.