The E3-ubiquitin ligase CSA drives AKT activation through K63-linked ubiquitination: implications for cancer and Cockayne syndrome

Abstract The serine/threonine kinase AKT is a central node driving cell proliferation, survival and migration, and its hyperactivation promotes cancer progression and chemoresistance. The E3-ubiquitin ligase CSA (Cockayne syndrome group A protein) ubiquitinates a range of substrates to control DNA repair, transcription and cell division; loss-of-function mutations in the CSA gene cause the degenerative disorder Cockayne syndrome, whereas CSA overexpression is found in breast and other cancers. Here we identify CSA as a novel upstream regulator of AKT. We show that CSA binds the pleckstrin-homology (PH) domain of AKT and catalyzes a non-proteolytic, K63-linked polyubiquitination of AKT. This modification promotes the recruitment of AKT to the plasma membrane and its activating phosphorylation on T308 and S473 upon stimulation with either IGF-1 or EGF. In CSA-overexpressing breast cancer cells, CSA silencing impairs AKT ubiquitination and membrane recruitment, blunting its activation resulting in reduced proliferation, clonogenicity and migration of the cells; expression of a constitutively active AKT rescues these defects, placing AKT genetically downstream of CSA. Conversely, re-expression of CSA, in patient-derived CS-A cells, restores AKT phosphorylation and activity. Transcriptomic profiling further shows that CSA depletion phenocopies a coherent, core subset of the AKT-dependent gene-expression program. Collectively, our results suggest that the CSA–AKT axis behaves as a double-edged sword: its hyperactivation supports tumorigenesis, whereas its impairment may contribute to the degenerative and aging features of Cockayne syndrome, highlighting CSA as a potential therapeutic target.