SIX 1 maintains tumor basal cells via transforming growth factor‐β pathway and associates with poor prognosis in esophageal cancer

Esophageal squamous cell carcinoma ( ESCC ) is one of the most common malignant tumors. Although improvement in both surgical techniques and neoadjuvant chemotherapy has been achieved, the 5‐year survival rate of locally advanced tumors was, at best, still 55%. Therefore, elucidation of mechanisms of the malignancy is eagerly awaited. Epithelial‐mesenchymal transition ( EMT ) by transforming growth factor‐β ( TGF ‐β) has been reported to have critical biological roles for cancer cell stemness, whereas little is known about it in ESCC . In the current study, a transcriptional factor SIX 1 was found to be aberrantly expressed in ESCC s. SIX 1 cDNA transfection induced overexpression of transforming growth factors ( TGFB 1 and TGFB 2 ) and its receptor ( TGFBR 2 ). Cell invasion was reduced by SIX 1 knockdown and was increased in stable SIX 1 ‐transfectants. Furthermore, the SIX 1 ‐transfectants highly expressed tumor basal cell markers such as NGFR , SOX 2 , ALDH 1A1 , and PDPN . Although mock‐transfectants had only a 20% PDPN ‐high population, SIX 1 ‐transfectants had 60–70%. In two sets of 42 and 85 ESCC patients receiving surgery alone or neoadjuvant chemoradiotherapy followed by surgery, the cases with high SIX 1 mRNA and protein expression level significantly showed a poor prognosis compared with those with low levels. These SIX 1 high cases also expressed the above basal cell markers, but suppressed the differentiation markers. Finally, TGF ‐β signaling blockade suppressed ESCC cell growth in association with the reduction of PDPN ‐positive tumor basal cell population. The present results suggest that SIX 1 accelerates self‐renewal of tumor basal cells, resulting in a poor prognosis for ESCC patients.