Stabilization of TGF-β Receptor 1 by a Receptor-Associated Adaptor Dictates Feedback Activation of the TGF-β Signaling Pathway to Maintain Liver Cancer Stemness and Drug Resistance

Dysregulation of the transforming growth factor-beta (TGF-beta) signaling pathway regulates cancer stem cells (CSCs) and drug sensitivity, whereas it remains largely unknown how feedback regulatory mechanisms are hijacked to fuel drug-resistant CSCs. Through a genome-wide CRISPR activation screen utilizing stem-like drug-resistant properties as a readout, the TGF-beta receptor-associated binding protein 1 (TGFBRAP1) is identified as a TGF-beta-inducible positive feedback regulator that governs sensitivity to tyrosine kinase inhibitors (TKIs) and promotes liver cancer stemness. By interacting with and stabilizing the TGF-beta receptor type 1 (TGFBR1), TGFBRAP1 plays an important role in potentiating TGF-beta signaling. Mechanistically, TGFBRAP1 competes with E3 ubiquitin ligases Smurf1/2 for binding to TGF Beta R1, leading to impaired receptor poly-ubiquitination and proteasomal degradation. Moreover, hyperactive TGF-beta signaling in turn up-regulates TGFBRAP1 expression in drug-resistant CSC-like cells, thereby constituting a previously uncharacterized feedback mechanism to amplify TGF-beta signaling. As such, TGFBRAP1 expression is correlated with TGF Beta R1 levels and TGF-beta signaling activity in hepatocellular carcinoma (HCC) tissues, as well as overall survival and disease recurrence in multiple HCC cohorts. Therapeutically, blocking TGFBRAP1-mediated stabilization of TGFBR1 by selective inhibitors alleviates Regorafenib resistance via reducing CSCs. Collectively, targeting feedback machinery of TGF-beta signaling pathway may be an actionable approach to mitigate drug resistance and liver cancer stemness. TGFBRAP1 protects TGFBR1 from ubiquitination and degradation by competing with SMURF1/2 for binding to TGFBR1, thereby promoting the activation of the TGF-beta signaling pathway and enhancing cancer stemness and resistance to regorafenib in HCC. Moreover, TGFBRAP1 is transcriptionally up-regulated by the SMAD2/3 complex, thus forming a positive feedback regulation of the TGF-beta signaling pathway. image