Epithelial-mesenchymal transition (EMT) plays an important role in development and also in initiation of metastasis during cancer. Disruption of cell-cell contacts during EMT allowing cells to detach from and migrate away from their neighbors remains poorly understood. Using immunofluorescent staining and live-cell imaging, we analyzed early events during EMT induced by epidermal growth factor (EGF) in IAR-20 normal epithelial cells. Control cells demonstrated stable adherens junctions (AJs) and robust contact paralysis, whereas addition of EGF caused rapid dynamic changes at the cell-cell boundaries: fragmentation of the circumferential actin bundle, assembly of actin network in lamellipodia, and retrograde flow. Simultaneously, an actin-binding protein EPLIN was phosphorylated, which may have decreased the stability of the circumferential actin bundle. Addition of EGF caused gradual replacement of linear E-cadherin–based AJs with dynamic and unstable punctate AJs, which, unlike linear AJs, colocalized with the mechanosensitive protein zyxin, confirming generation of centripetal force at the sites of cell-cell contacts during EMT. Our data show that early EMT promotes heightened dynamics at the cell-cell boundaries—replacement of stable AJs and actin structures with dynamic ones—which results in overall weakening of cell-cell adhesion, thus priming the cells for front-rear polarization and eventual migration.

Using DIC and confocal microscopy, changes in morphology, migratory characteristics and adherence junctions (AJs) were analyzed in the mammary carcinoma cell line MCF-7-SNAI1  after activation of the EMT transcription factor SNAI1. Western Blot analysis showed that  after removal of tetracycline from the cell culture medium expression of SNAI1 reached its  peak in 24 hours and then plateaued for 7 days. During the 7 days the cells continued to  express E-cadherin; however, tangential AJs typical for cells with stable cell-cell adhesion,  changed into radial AJs. The radial AJs continued to accumulate E-cadherin during 24‑72  hours after tetracycline removal. As a result of SNAI1 activation, the cells underwent  epithelial-mesenchymal transition (EMT) and became migratory. On a two-dimensional  substrate, cells exhibited both individual and collective migration. As the tetracycline  washout period progressed, the fraction of the cells capable of migrating through migration chamber membranes increased; on the contrary, cells’ ability to invade an epithelial  monolayer decreased. These results demonstrate that retaining a hybrid epithelial/mesenchymal  phenotype and accumulation of E-cadherin in AJs during early stages of EMT do not impede  disruption of stable cell-cell adhesion and cells’ acquisition of migratory activity.

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