The TBX18/SIX1 Transcriptional Circuit Maintains Stemness and EMT States to Promote Radioresistance in ESCC

Background: As a member of the T-box transcription factor family, TBX18 was found to be involved in ESCC progression, while its role in regulating radiotherapy resistance in ESCC remains unclear. This study was designed to investigate the molecular mechanisms underlying the regulation of radioresistance in ESCC by TBX18. Methods: Sphere formation assay, Transwell invasion assay, and wound healing assay were conducted to show the influence of TBX18 on tumor stemness and epithelial–mesenchymal transition (EMT). Western blot, immunofluorescence, chromatin immunoprecipitation-qPCR (ChIP-qPCR) and dual-luciferase reporter assay were preformed to identify regulatory networks. A nude mouse xenograft tumor model was established to assess the regulatory effect of TBX18 and SIX1 on radioresistance of ESCC in vivo. Results: TBX18 expression was positively associated with stemness markers, including CD44, CD271, and SOX2. TBX18 promoted stemness-associated phenotypes, EMT, migration, invasion, and radioresistance in ESCC cells. Mechanistically, TBX18 directly bound to the SIX1 promoter and transcriptionally activated SIX1 expression. In turn, SIX1 enhanced TBX18 protein stability by suppressing ubiquitin–proteasome-mediated degradation, thereby forming a positive feedback loop. Functional rescue experiments demonstrated that the TBX18/SIX1 axis coordinately maintained stemness and EMT phenotypes and attenuated radiotherapy-induced apoptosis. In vivo studies further confirmed that TBX18 knockdown enhanced radiosensitivity, whereas SIX1 overexpression partially reversed this effect. In addition, immunohistochemical analysis revealed that TBX18 and SIX1 were significantly upregulated in ESCC tissues and positively correlated with each other.