Endothelial cell barrier enhancement by ATP is mediated by the small GTPase Rac and cortactin

ATP is a physiologically relevant agonist released by various sources, including activated platelets, with complex effects mediated via activation of P2 purinergic receptors. ATP-induced endothelial cell (EC) production of prostacyclin and nitric oxide is recognized, and EC barrier enhancement evoked by ATP has been described. ATP effects on EC barrier function and vascular permeability, however, remain poorly characterized. Although the mechanisms involved are unclear, we previously identified activation of the small GTPase Rac and translocation of cortactin, an actin-binding protein, as key to EC barrier augmentation induced by simvastatin and sphingosine 1-phosphate and therefore examined the role of these molecules in ATP-induced EC barrier enhancement. ATP induced rapid, dose-dependent barrier enhancement in human pulmonary artery EC as measured by transendothelial electrical resistance, with a peak effect appreciable at 25 min (39% increase, 10 μM) and persisting at 2 h. These effects were associated with rearrangement of the EC actin cytoskeleton, early myosin light chain phosphorylation, and spatially defined (cell periphery) translocation of both Rac and cortactin. ATP (10 μM)-treated EC demonstrated a significant increase in Rac activation relative to controls, with a maximal effect (∼4-fold increase) at 10 min. Finally, ATP-induced barrier enhancement was markedly attenuated by reductions of either Rac or cortactin (small interfering RNA) relative to controls. Our results suggest for the first time that ATP-mediated barrier protection is associated with cytoskeletal activation and is dependent on both Rac activation and cortactin.