Regulation of cell-cell adhesion and actin cytoskeleton in non-transformed and transformed epithelial cells

Abstract Epithelial cell-cell adhesions have a critical role in morphogenesis, establishment and maintenance of tissue architecture, cell-cell communication, normal cell growth and differentiation. These adhesions are disrupted during malignant transformation and tumour cell invasion. Several kinases, phosphatases and small GTPases regulate cell-cell contacts. In the present work we investigated the dynamics of cell-cell adhesion structures after microinjection of fluorophore tagged vinculin, during transformation caused by an active Src tyrosine kinase and during Helicobacter pylori infection. The regulatory role of Rac GTPase as well as the behaviour of actin and cadherin were analysed in all these conditions. Microinjection of vinculin into bovine kidney epithelial MDBK cells induced release of actin, cadherin and plakoglobin to cytoplasm of the cells, caused disruption of protein complexes at adherens and tight junctions that finally led to formation of polykaryons. Activated Rac GTPase, in turn, enhanced accumulation of cadherin to membranes and thereby diminished the formation of polykaryons, whereas inactive Rac removed cadherin from membranes. Incorporation of vinculin to lateral membranes took place also in acidifying and depolarising conditions where cell fusions were prevented. Thus, the membrane potential seemed to control fusion ability. In src-MDCK cells, activation of Src kinase led to disintegration of adherens junctions. Clusters of junctional components and bundles of actin were seen at the basal surface already within 30 min after Src activation. p120ctn was the only component of adherens junction whose relocation correlated to its phosphorylation. Inhibition of Src by a specific inhibitor PP2 restored the cubic morphology of the cells and accumulated cadherin back to lateral walls. Still p120ctn remained in cytoplasm and thereby was not responsible for the epithelial phenotype. Activation of Rac GTPase by Tiam1 also increased the amount of cadherin at lateral membranes and maintained the morphology of src-MDCK cells practically normal after activation of Src kinase. In the same way, actin cytoskeleton was reorganised in gastric carcinoma cells in response to infection with H. pylori via activation of Rac signalling pathway. Hence, Rac and cadherin seem to be the major players in the maintenance of epithelial cell morphology