Cadherin engagement regulates Rho family GTPases

The formation of cell-cell adherens junctions is a cadherin-mediated proces s associated with reorganization of the actin cytoskeleton. Because Rho fam ily GTPases regulate actin dynamics, we investigated whether cadherin-media ted adhesion regulates the activity of RhoA, Rac1, and Cdc42. Confluent epi thelial cells were found to have elevated Rac1 and Cdc42 activity but decre ased RhoA activity when compared with low density cultures. Using a calcium switch method to manipulate junction assembly, we found that induction of cell-cell junctions increased Rac1 activity, and this was inhibited by E-ca dherin function-blocking antibodies. Using the same calcium switch procedur e, we found little effect on RhoA activity during the first hour of junctio n assembly. However, over several hours, RhoA activity significantly decrea sed. To determine whether these effects are mediated directly through cadhe rins or indirectly through engagement of other surface proteins downstream from junction assembly, we used a model system in which cadherin engagement is induced without cell-cell contact. For these experiments, Chinese hamst er ovary cells expressing C-cadherin were plated on the extracellular domai n of C-cadherin immobilized on tissue culture plates. Whereas direct cadher in engagement did not stimulate Cdc42 activity, it strongly inhibited RhoA activity but increased Rac1 activity. Deletion of the C-cadherin cytoplasmi c domain abolished these effects.