# 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

## TL;DR

This study identifies TGFBRAP1 as a key protein that stabilizes TGFBR1, enhancing TGF-β signaling and promoting liver cancer stemness and drug resistance.

## Contribution

The paper discovers a novel positive feedback mechanism involving TGFBRAP1 in TGF-β signaling that drives liver cancer progression and resistance.

## Key findings

- TGFBRAP1 stabilizes TGFBR1 by blocking its ubiquitination and degradation.
- TGFBRAP1 is upregulated by TGF-β signaling, forming a feedback loop in drug-resistant liver cancer cells.
- Blocking TGFBRAP1 reduces Regorafenib resistance and cancer stemness in hepatocellular carcinoma.

## Abstract

Dysregulation of the transforming growth factor‐β (TGF‐β) 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‐β receptor‐associated binding protein 1 (TGFBRAP1) is identified as a TGF‐β‐inducible positive feedback regulator that governs sensitivity to tyrosine kinase inhibitors (TKIs) and promotes liver cancer stemness. By interacting with and stabilizing the TGF‐β receptor type 1 (TGFBR1), TGFBRAP1 plays an important role in potentiating TGF‐β signaling. Mechanistically, TGFBRAP1 competes with E3 ubiquitin ligases Smurf1/2 for binding to TGFΒR1, leading to impaired receptor poly‐ubiquitination and proteasomal degradation. Moreover, hyperactive TGF‐β signaling in turn up‐regulates TGFBRAP1 expression in drug‐resistant CSC‐like cells, thereby constituting a previously uncharacterized feedback mechanism to amplify TGF‐β signaling. As such, TGFBRAP1 expression is correlated with TGFΒR1 levels and TGF‐β 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‐β 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‐β 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‐β signaling pathway.