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Epithelial cell adhesion molecule (EpCAM) (CD326) is a surface glycoprotein expressed by invasive carcinomas and some epithelia. Herein, we report that EpCAM regulates the composition and function of tight junctions (TJ). EpCAM accumulated on the lateral interfaces of human colon carcinoma and normal intestinal epithelial cells but did not co-localize with TJ. Knockdown of EpCAM in T84 and Caco-2 cells using shRNAs led to changes in morphology and adhesiveness. TJ formed readily after EpCAM knockdown; the acquisition of trans-epithelial electroresistance was enhanced, and TJ showed increased resistance to disruption by calcium chelation. Preparative immunoprecipitation demonstrated that EpCAM bound tightly to claudin-7. Co-immunoprecipitation documented associations of EpCAM with claudin-7 and claudin-1 but not claudin-2 or claudin-4. Claudin-1 associated with claudin-7 in co-transfection experiments, and claudin-7 was required for association of claudin-1 with EpCAM. EpCAM knockdown resulted in decreases in claudin-7 and claudin-1 proteins that were reversed with lysosome inhibitors. Immunofluorescence microscopy revealed that claudin-7 and claudin-1 continually trafficked into lysosomes. Although EpCAM knockdown decreased claudin-1 and claudin-7 protein levels overall, accumulations of claudin-1 and claudin-7 in TJ increased. Physical interactions between EpCAM and claudins were required for claudin stabilization. These findings suggest that EpCAM modulates adhesion and TJ function by regulating intracellular localization and degradation of selected claudins. [ABSTRACT FROM AUTHOR]

E-cadherins play an essential role in maintaining epithelial polarity by forming Ca2+-dependent adherens junctions between epithelial cells. Here, we report that Ca2+ depletion induces E-cadherin ubiquitination and lysosomal degradation and that Cdc42 plays an important role in regulating this process. We demonstrate that Ca2+ depletion induces activation of Cdc42. This in turn up-regulates epidermal growth factor receptor (EGFR) signaling to mediate Src activation, leading to E-cadherin ubiquitination and lysosomal degradation. Silencing Cdc42 blocks activation of EGFR and Src induced by Ca2+ depletion, resulting in a reduction in E-cadherin degradation. The role of Cdc42 in regulating E-cadherin ubiquitination and degradation is underscored by the fact that constitutively active Cdc42(F28L) increases the activity of EGFR and Src and significantly enhances E-cadherin ubiquitination and lysosomal degradation. Furthermore, we found that GTP-dependent binding of Cdc42 to E-cadherin is critical for Cdc42 to induce the dissolution of adherens junctions. Our data support a model that activation of Cdc42 contributes to mesenchyme-like phenotype by targeting of E-cadherin for lysosomal degradation. [ABSTRACT FROM AUTHOR]