Cortactin is a ubiquitously expressed actin filament (F-actin)-binding protein that stabilizes F-actin networks and promotes actin polymerization by activating the actin-related protein 2/3 (Arp2/3) complex. Overexpression of cortactin in cancer cells stimulate cell migration, invasion, and experimental metastasis; however, the underlying mechanism in cortactin involvement in tumor progression is not fully understood. Recently, a direct interaction between zonula occludens-1 (ZO-1) and cortactin in epithelial cells was reported. The present study aimed to further clarify the significance of the interaction between cortactin and ZO-1 in cancer progression. Cortactin expression and localization in colorectal human cancer tissues were evaluated by immunohistochemistry and immunofluorescence. Co-immunoprecipitation and immunofluorescence analysis revealed cortactin and ZO-1 interaction and localization in cancer cells. In our study, the localization of cortactin is a crucial marker for lymph node metastasis in colorectal cancer. We show how the localization of cortactin effects cancer development. A molecular interaction between cortactin and ZO-1 in migrating or polarized cancer cells was revealed. This is the first report to show the interaction of cortactin and ZO-1 in colorectal cancer progression. We conclude that localization of cortactin in cancer cells and interaction between ZO-1 and cortactin are crucial for cancer progression.

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