Fyn limits the protease activating receptor‐1 (PAR‐1)‐induced increase in lung microvascular permeability by inducing FAK activation

Abstract

We showed that focal adhesion kinase (FAK) is required to reverse the thrombin‐induced increase in endothelial permeability. Fyn is a non‐receptor Src family tyrosine kinase, which phosphorylates FAK. We investigated the role of Fyn in regulating lung microvascular permeability using Fyn deficient mice. First, we determined whether activation of PAR‐1 by specific activating peptide (TFLLRN), alters FAK activity in lungs. Lungs of WT or Fyn−/− mice received i.v. injection of either control or PAR‐1 activating peptide were homogenized for determination of FAK phosphorylation. We observed that PAR‐1 agonist peptide increased FAK phosphorylation at Y397 and Y576 in WT mice lungs, but failed to induce FAK phosphorylation in lungs of Fyn−/− mice. Next, we determined microvessel filtration coefficient (Kf,c) in lungs isolated from WT and Fyn−/− mice. We observed that basal Kf,,c was significantly higher in Fyn−/− lungs than WT lungs. We also determined Evans blue albumin extravasation and lung wet‐ to dry weight ratio following PAR‐1 activation to assess the role of Fyn in regulating lung edema formation. Injection of PAR‐1 peptide produced significantly greater increase in lung vascular permeability in Fyn−/− mice, indicating that Fyn limits the PAR‐1‐induced pulmonary edema. We next addressed whether rescuing FAK activation could restore lung vascular permeability in Fyn−/− mice. Liposomes conjugated with FAK double phospho‐mimicing mutant (Y397D/Y576D) were injected in Fyn−/− mice and lung vascular permeability was determined after confirmation of FAK mutant expression. We show that rescuing FAK phosphorylation markedly suppressed PAR‐1 induced lung vascular permeability in Fyn−/− mice. These studies identify Fyn as the critical tyrosine kinase that by inducing FAK activation maintains lung vascular barrier function.