Regulation of cadherin function by Rho and Rac: Modulation by junction maturation and cellular context

Cadherins are cell-cell adhesion receptors whose adhesive function requires their association with the actin cytoskeleton via proteins called catenins . The small guanosine triphosphatases (GTPases), Rho and Rac, are intracell ular proteins that regulate the formation of distinct actin structures in d ifferent eel types. Ln keratinocytes and in other epithelial cells, Rho and Rac activities are required for E-cadherin function. Here we show that the regulation of cadherin adhesiveness by the small GTPases is influenced by the maturation status of the junction and the cellular context. E-cadherin localization was disrupted in mature keratinocyte junctions after inhibitio n of Rho and Rac. However, an incubation of 2 h was required after GTPase i nhibition, when compared with newly established E-cadherin contacts (30 min ). Regarding other cadherin receptors, P-cadherin was effectively removed f rom mature keratinocytes junctions by blocking Rho or Rac. In contrast, VE- cadherin localization at endothelial junctions was independent of Rho/Rac a ctivity. We demontrate that the insensitivity of VE-cadherin to inhibition of Rho and Rac was not due to the maturation status of endothelial junction , but rather the cellular background: when transfected into CHO cells, the localization of VE-cadherin was perturbed by inhibition of Rho proteins. Ou r results suggest that the same stimuli may have different activity in regu lating the paracellular activity in endothelial and epithelial cells. In ad dition, we uncovered possible roles for the small GTPases during the establ ishment of E-cadherin-dependent contacts. In keratinocytes, Rac activation by itself cannot promote accumulation of actin at the cell periphery in the absence of cadherin-dependent contacts. Moreover, neither Rho nor Rac acti vation was sufficient to redistribute cadherin molecules to cell borders, i ndicating that redistribution results mostly from the homophilic binding of the receptors. Our results point out the complexity of the regulation of c adherin-mediated adhesion by the small GTPases, Rho and Rac.