Ribosomal S6 protein kinase 4 promotes radioresistance in esophageal squamous cell carcinoma

Esophageal squamous cell carcinoma (ESCC) is one of the most aggressive cancers and is highly resistant to current treatments. ESCC harbors a subpopulation of cells exhibiting cancer stem-like cell (CSC) properties that contribute to therapeutic resistance including radioresistance, but the molecular mechanisms in ESCC CSCs are currently unknown. Here, we report that ribosomal S6 protein kinase 4 (RSI(4) plays a pivotal role in promoting CSC properties and radioresistance in ESCC. RSK4 was highly expressed in ESCC CSCs and associated with radioresistance and poor survival in patients with ESCC. RSK4 was found to be a direct downstream transcriptional target of Delta Np63 alpha, the main p63 isoform, which is frequently amplified in ESCC. RSK4 activated the beta-catenin signaling pathway through direct phosphorylation of GSK-3 beta at Ser9. Pharmacologic inhibition of RSK4 effectively reduced CSC properties and improved radiosensitivity in both nude mouse and patient-derived xenograft models. Collectively, our results strongly suggest that the Delta Np63 alpha/RSK4/GSK-3 beta axis plays a Icey role in driving CSC properties and radioresistance in ESCC, indicating that RSK4 is a promising therapeutic target for ESCC treatment.