Conditional deletion of FAK in mice endothelium disrupts lung vascular barrier function due to destabilization of RhoA and Rac1 activities

Loss of lung-fluid homeostasis is the hallmark of acute lung injury (ALI). Association of catenins and actin cytoskeleton with vascular endothelial (VE)-cad-herin is generally considered the main mechanism for stabilizing adherens junctions (AJs), thereby preventing disruption of lung vascular barrier function. The present study identifies endothelial focal adhesion kinase (FAK), a nonreceptor tyrosine kinase that canonically regulates focal adhesion turnover, as a novel AJ-stabilizing mechanism. In wild-type mice, induction of ALI by intraperitoneal administration of lipopolysaccharide or cecal ligation and puncture markedly decreased FAK expression in lungs. Using a mouse model in which FAK was conditionally deleted only in endothelial cells (ECs), we show that loss of EC-FAK mimicked key features of ALI (diffuse lung hemorrhage, increased transvascular albumin influx, edema, and neutrophil accumulation in the lung). EC-FAK deletion disrupted AJs due to impairment of the fine balance between the activities of RhoA and Rac1 GTPases. Deletion of EC-FAK facilitated RhoA's interaction with p115-RhoA guanine exchange factor, leading to activation of RhoA. Activated RhoA antagonized Rac1 activity, destabilizing AJs. Inhibition of Rho kinase, a downstream effector of RhoA, reinstated normal endothelial barrier function in FAK^-/- ECs and lung vascular integrity in EC-FAK^-/- mice. Our findings demonstrate that EC-FAK plays an essential role in maintaining AJs and thereby lung vascular barrier function by establishing the normal balance between RhoA and Rac1 activities.