Focal Adhesion Kinase and Regulation of Endothelial Barrier

Unresolved or persistent increases in lung vascular permeability result in the leakage of fluid and macromolecules into the interstitial spaces, causing devastating disease known as acute lung injury (ALI). Adherens junctions (AJs) and focal adhesions (FAs) at sites of EC-extracellular matrix contact primarily maintain endothelial barrier function. Thus, a cellular mechanism that intersects with both AJs and FAs may be required to maintain quiescent endothelium. Focal adhesion kinase (FAK), a non-receptor tyrosine kinase initially described to regulate FAs, also interacts with AJs to regulate endothelial permeability. We conditionally induced the deletion of FAK in the endothelium of adult mice to determine the role of FAK in regulating lung vascular injury. We found that FAK not only maintains lung-fluid homeostasis, but suppresses inflammatory signaling through the downstream target molecule, p38 mitogen activated protein kinase (MAPK). We show that FAK negatively regulates the activity of p38 MAPK and thereby shifts the balance between RhoA and Rac1 GTPase activities in the favor of Rac1 and suppresses activating transcription factor (ATF)-2 activities, to maintain the restrictive and un-inflamed endothelium. Endogenous mechanisms exist to restore endothelial monolayer integrity. Through sphingosine-1-phosphate receptor (S1PR)-1, S1P induces signaling that strengthens endothelial barrier and limits lung vascular permeability. We found S1P failed to enhance AJ assembly and monolayer integrity in FAK depleted cells, or reverse the leaky phenotype observed in EC-FAK-/- mice. We also show S1P induces tyrosine phosphorylation of S1PR1 and also enhances S1PR1 interaction with FAK. However, FAK negatively regulates S1PR1 tyrosine phosphorylation, and that the tyrosine phosphorylation of S1PR1 at Y143 impairs its cell surface localization. Thus, we identified a novel mechanism by which FAK regulates S1PR1, which may also play an important role in ALI, as persistent increases in permeability exist even in the presence of high S1P plasma concentrations. Therefore, our studies identify FAK as a critical regulator for maintaining quiescent endothelial barrier function by suppressing p38 MAPK and S1PR1 tyrosine phosphorylation. These findings have far-reaching implications for understanding FAK related signaling in endothelium and for identifying therapeutics against ALI. Mechanisms which maintain FAK expression could therefore represent attractive approaches to prevent ALI.