Increased expression of c‐Ski as a co‐repressor in transforming growth factor‐β signaling correlates with progression of esophageal squamous cell carcinoma

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Abstract
Transforming growth factor‐β (TGF‐β) regulates cell growth inhibition, and inactivation of the TGF‐β signaling pathway contributes to tumor development. In our previous study, altered expression of TGF‐β, TGF‐β‐specific receptors and Smad4 was shown to correlate with tumor progression in esophageal squamous cell carcinoma (SCC). These components, however, were maintained normally in some patients with esophageal SCC. In our study, the mechanism by which aggressive esophageal SCC maintains these components was investigated, with particular emphasis on the participation of c‐Ski and SnoN as transcriptional co‐repressors in TGF‐β signaling. Immunohistochemistry for c‐Ski and SnoN was carried out on surgical specimens obtained from 80 patients with esophageal SCC. The expression of c‐Ski and SnoN was also studied in 6 established cell lines derived from esophageal SCC and compared to an immortalized human esophageal cell line by Western blotting. High levels of expression of c‐Ski, detected immunohistologically, were found to correlate with depth of invasion (p = 0.0080) and pathologic stage (p = 0.0447). There was, however, no significant correlation between expression of SnoN and clinicopathologic characteristics. A significant correlation between c‐Ski and TGF‐β expression was observed. Moreover, in patients with TGF‐β negative expression, the survival rates of patients with c‐Ski positive expression were significantly lower than those of patients with c‐Ski negative expression (p = 0.0486). c‐Ski was expressed at a high level in 5 of 6 cell lines derived from esophageal SCC compared to immortalized esophageal keratinocytes. Furthermore, the cyclin‐dependent kinase (CDK) inhibitor, p21 that was up‐regulated by TGF‐β signaling was expressed at a low level in the 5 cell lines. The expression of c‐Ski protein as a transcriptional co‐repressor in TGF‐β signaling seems to be correlated with tumor progression of esophageal SCC.