Background & Aims: The activation of Wnt/β-catenin signaling causes the development of gastric and colon cancers. Sox17 represses Wnt/β-catenin signaling and is down-regulated in colon cancer. This study was designed to elucidate the role of Sox17 during the course of gastrointestinal tumorigenesis. Methods: Sox17 expression was examined in gastrointestinal tumors of mouse models and humans. The roles of Sox17 in gastric tumorigenesis were examined by cell culture experiments and by construction of Sox17 transgenic mice. Results: Sox17 was induced in K19-Wnt1/C2mE mouse gastric tumors and K19-Wnt1 preneoplastic lesions, where Wnt/β-catenin signaling was activated. Consistently, Wnt activation induced Sox17 expression in gastric cancer cells. In contrast, Sox17 was rarely detected by immunohistochemistry in gastric and colon cancers, whereas strong nuclear staining of Sox17 was found in >70% of benign gastric and intestinal tumors. Treatment with a demethylating agent induced Sox17 expression in gastric cancer cells, thus indicating the down-regulation of Sox17 by methylation. Moreover, transfection of Sox17 in gastric cancer cells suppressed both the Wnt activity and colony formation efficiency. Finally, transgenic expression of Sox17 suppressed dysplastic tumor development in K19-Wnt1/C2mE mouse stomach. Conclusions: Sox17 plays a tumor suppressor role through suppression of Wnt signaling. However, Sox17 is induced by Wnt activation in the early stage of gastrointestinal tumorigenesis, and Sox17 is down-regulated by methylation during malignant progression. It is therefore conceivable that Sox17 protects benign tumors from malignant progression at an early stage of tumorigenesis, and down-regulation of Sox17 contributes to malignant progression through promotion of Wnt activity.
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