SIX1 overexpression predicts poor prognosis and induces radioresistance through AKT signaling in esophageal squamous cell carcinoma

The Sineoculis homeobox homolog 1 (SIX1) protein has been found to be overexpressed in several human cancers. However, its expression pattern and biological roles in esophageal squamous cell carcinoma (ESCC) remain unexplored. This study examined the clinical significance of SIX1 in 119 ESCC tissues. It was found that SIX1 protein was upregulated in 36.9% (44/119) cases. SIX1 overexpression was an independent predictor for short survival of ESCC patients. siRNA knockdown and plasmid transfection were carried out in ESCC cell lines. SIX1 depletion inhibited cell growth, invasion, and colony formation, whereas its overexpression facilitated in vivo and in vitro cell growth, invasion, and colony formation. The apoptosis rate induced by X-ray irradiation was substantially increased by SIX1 knockdown in Eca-109 cells. Ectopic overexpression of SIX1 in TE-1 cells dramatically enhanced resistance to irradiation. Western blot analysis showed that SIX1 depletion downregulated cyclin E, matrix metalloproteinase-2 (MMP-2), Bcl-2 expression and upregulated Bim expression. SIX1 overexpression exhibited the opposite effect on these proteins. In addition, it was found that SIX1 could positively regulate extracellular signal-regulated kinase (ERK) and AKT signaling pathway. ERK inhibitor abolished the effect of SIX1 on MMP-2 expression. AKT inhibitor treatment blocked the role of SIX1 on anti-apoptotic protein Bcl-2. In conclusion, this study demonstrates that SIX1 overexpression predicts poor survival in ESCC patients and confers radioresistance through activation of AKT signaling pathways.