Endothelial cell motility, integrin receptor clustering, and microfilament organization are inhibited by agents that increase intracellular cAMP

Abstract

Adhesion of human umbilical vein endothelial cells (ECs) to various extracellular matrix proteins is mostly mediated by receptors of the integrin family. The interaction of ECs with extracellular matrix proteins is accompanied by cell spreading, cytoskeletal organization, and clustering of the specific integrin receptors in complex supramolecular structures known as adhesion plaques or focal contacts. Little is known on the functional role of focal contacts in EC adhesion and motility and on the possibility to modulate their organization. In this article we report that an increase in intracellular cAMP levels severely impaired focal contact formation. This process did not affect cell attachment, but increased cell adhesion and strongly inhibited cell motility. ECs were treated with the cAMP-increasing agents forskolin and 2-chloro-adenosine or with the cAMP analogue 8-bromo-cAMP. When treated cells were seeded on purified vitronectin, fibrinogen, or fibronectin little modification in the number of attached cell was observed. In contrast ECs showed impaired organization of microfilaments and poorly developed clusters of beta 3- and beta 1-integrin receptors. On a vitronectin substrate, vinculin followed the distribution of beta 3-receptors. It was typically enriched at the focal contacts in control cells but was fragmented in small dots at the cell periphery in treated cells, as were bundles of actin stress fibers. Similarly, when forskolin was added to ECs spread on vitronectin or on fibrinogen, there was a progressive but reversible disruption of actin microfilaments and diffusion of beta 3 receptors. This was accompanied by a tighter adhesion of the cells to substrata. Migration of ECs in response to different matrix proteins was severely inhibited by cAMP-increasing agents. These data indicate that EC adhesion can occur very efficiently in the absence of fully developed beta 3- or beta 1-integrin receptor-containing focal contacts but suggest that the capacity to normally assemble focal contacts and cytoskeletal proteins is required for full cell spreading and migration.