A cross-link between protein kinase A and Rho-family GTPases signaling mediates cell-cell adhesion and actin cytoskeleton organization in epithelial cancer cells.

Disassembly of the apical junctional complex (AJC) together with actin cytoskeleton alterations are among the initial events for the development of epithelial cancer. The cell signaling pathways for these processes have been analyzed separately. However, the existence of a link between these two events has not been defined. In this study, using the extracellular calcium depletion model, we analyzed the signaling pathways regulating AJC disassembly together with actin cytoskeleton organization in colon adenocarcinoma cells (Caco-2). Changes in the location of AJC proteins were examined by immunofluorescence and immunoblotting, and tight junction (TJ) functionality was observed by measuring the transepithelial electrical resistance and permeation to ruthenium red. The actin cytoskeleton was stained with rhodamine-phalloidin and analyzed by confocal microscopy. Rho-GTPase activation was assessed by its translocation to the membrane (a hallmark of RhoA activation) and immunoblotting. Pharmacological inhibition of protein kinase A (PKA) with H-89 [N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide)] prevented AJC disassembly and actin disorganization at the apical and medial regions caused by calcium depletion. Rho inhibition using toxin A induced AJC disassembly and actin cytoskeleton reorganization. Y-27632 [(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-ciclohexanecarboxamide], a Rho-associated kinase inhibitor, reversed redistribution of E-cadherin, but not of TJ proteins and actin disorganization caused by calcium depletion. Calcium depletion and forskolin treatment caused activation of Rho, as evidenced by their translocation to the membrane, an event concurrent to Rac and RhoGDI translocation, and this effect was also reverted by H-89. Thus, our findings demonstrate a central role of a regulatory cascade that integrates PKA and Rho-family GTPases in the AJC disassembly and actin organization in tumor epithelial cells.