# O-GlcNAcylation stabilizes RSK4 by antagonizing GSK3β-mediated phosphorylation to enhance radioresistance in esophageal squamous cell carcinoma

## TL;DR

This study reveals how O-GlcNAcylation stabilizes RSK4 in esophageal cancer, making it more resistant to radiation, and suggests targeting this process could improve treatment.

## Contribution

The study identifies a novel regulatory mechanism involving O-GlcNAcylation and phosphorylation of RSK4 in ESCC radioresistance.

## Key findings

- GSK3β phosphorylates RSK4 at Thr402/Ser406, leading to its degradation via the FBXW7-dependent proteasomal pathway.
- O-GlcNAcylation at Thr405 by OGT stabilizes RSK4 and enhances cancer stem cell properties and radioresistance.
- Targeting RSK4 O-GlcNAcylation with OSMI-4 sensitizes ESCC to radiotherapy in xenograft and organoid models.

## Abstract

Esophageal squamous cell carcinoma (ESCC) is a highly lethal malignancy characterized by significant radioresistance and poor prognosis. We previously reported that ribosomal S6 protein kinase 4 (RSK4) plays a pivotal role in promoting cancer stem cell (CSC) properties and radioresistance in ESCC. This study focuses on the regulation of post-translational modifications (PTMs) of RSK4 and their effects on CSC properties and radioresistance. We demonstrate that RSK4 stability and activity are tightly regulated by phosphorylation and O-GlcNAcylation. GSK3β phosphorylates RSK4 at Thr402/Ser406, promoting its degradation via the FBXW7-dependent proteasomal pathway. Additionally, O-GlcNAcylation of RSK4 at Thr405 by OGT inhibits GSK3β-mediated phosphorylation, stabilizing RSK4 and enhancing CSC properties and radioresistance. This antagonistic relationship between phosphorylation and O-GlcNAcylation highlights a novel regulatory mechanism of RSK4 in ESCC. Moreover, targeting RSK4 O-GlcNAcylation with OSMI-4 destabilizes RSK4 and sensitizes ESCC to radiotherapy in both patient-derived xenograft and organoid models. Collectively, this study provides critical insights into the molecular mechanisms underlying ESCC radioresistance and identifies RSK4 O-GlcNAcylation as a potential therapeutic target to improve radiotherapy efficacy and overcome treatment resistance.