Cadherins are calcium-dependent cell–cell adhesion molecules that require the interaction of the cytoplasmic tail with the actin cytoskeleton for adhesive activity. Because of the functional relationship between cadherin receptors and actin filament organization, we investigated whether members of the Rho family of small GTPases are necessary for cadherin adhesion. In fibroblasts, the Rho family members Rho and Rac regulate actin polymerization to produce stress fibers and lamellipodia, respectively. In epithelial cells, we demonstrate that Rho and Rac are required for the establishment of cadherin-mediated cell–cell adhesion and the actin reorganization necessary to stabilize the receptors at sites of intercellular junctions. Blocking endogenous Rho or Rac selectively removed cadherin complexes from junctions induced for up to 3 h, while desmosomes were not perturbed. In addition, withdrawal of cadherins from intercellular junctions temporally precedes the removal of CD44 and integrins, other microfilament-associated receptors. Our data showed that the concerted action of Rho and Rac modulate the establishment of cadherin adhesion: a constitutively active form of Rac was not sufficient to stabilize cadherindependent cell–cell contacts when endogenous Rho was inhibited. Upon induction of calcium-dependent intercellular adhesion, there was a rapid accumulation of actin at sites of cell–cell contacts, which was prevented by blocking cadherin function, Rho or Rac activity. However, if cadherin complexes are clustered by specific antibodies attached to beads, actin recruitment to the receptors was perturbed by inhibiting Rac but not Rho. Our results provide new insights into the role of the small GTPases in the cadherin-dependent cell– cell contact formation and the remodelling of actin filaments in epithelial cells.

In epithelial tissues, cells are linked to their neighbors through specialized cell-cell adhesion proteins. E-cadherin is one of the most important membrane proteins for the establishment of intimate cell-cell contacts, but the molecular mechanism by which it is recruited to contact sites is largely unknown. We report here that the cytoplasmic domain of E-cadherin interacts with C3G, a guanine nucleotide exchange factor for Rap1. In epithelial cell cultures, ligation of the extracellular domain of E-cadherin enhances Rap1 activity, which in turn is necessary for the proper targeting of E-cadherin molecules to maturing cell-cell contacts. Furthermore, our data suggest that Cdc42 functions downstream of Rap1 in this process. We conclude that Rap1 plays a vital role in the establishment of E-cadherin-based cell-cell adhesion.

Previously published reports support the concept that, besides promoting homotypic intercellular adhesion, cadherins may transfer intracellular signals. However, the signaling pathways triggered by cadherin clustering and their biological significance are still poorly understood. We report herein that transfection of VE-cadherin (VEC) cDNA in VEC null endothelial cells induces actin rearrangement and increases the number of vinculin positive adhesion plaques. VEC expression augments the level of active Rac but decreases active Rho. Microinjection of a dominant negative Rac mutant altered stress fiber organization, whereas inhibition of Rho was ineffective. VEC expression increased protein and mRNA levels of the Rac-specific guanosine exchange factor Tiam-1 and induced its localization at intercellular junctions. In addition, in the presence of VEC, the amounts of Tiam, Rac, and the Rac effector PAK as well as the level of PAK phosphorylation were found increased in the membrane/cytoskeletal fraction. These observations are consistent with a role of VEC in localizing Rac and its signaling partners in the same membrane compartment, facilitating their reciprocal interaction. Through this mechanism VEC may influence the constitutive organization of the actin cytoskeleton.