Targeted subendothelial matrix oxidation by myeloperoxidase triggers myosin II-dependent de-adhesion and alters signaling in endothelial cells

During inflammation, myeloperoxidase (MPO) released by circulating leukocytes accumulates within the subendothelial matrix by binding to and transcytosing the endothelium. Oxidative reactions catalyzed by subendothelial-localized MPO are implicated as a key cause of endothelial dysfunction in inflammatory vascular diseases. Whilst the subendothelial matrix is a reactive target for MPO-derived oxidants in disease, the functional implications of oxidative matrix modification for the endothelium are largely unknown. Here we show that hypochlorous acid (HOCl) produced by endothelial-transcytosed MPO oxidizes the subendothelial matrix, involving covalent crosslinking of the adhesive matrix protein fibronectin. Real-time biosensor and live cell imaging studies showed that HOCl-mediated matrix oxidation triggers rapid membrane retraction from the substratum and adjacent cells (de-adhesion). This de-adhesion was linked with the alteration of Tyr-118 phosphorylation of paxillin, a key focal adhesion-dependent signaling process, as well as Rho kinase-dependent myosin light chain-2 phosphorylation. De-adhesion dynamics were dependent on the contractile state of cells, with myosin II inhibition with blebbistatin markedly attenuating the rate of membrane retraction. Rho kinase inhibition with Y-27632 also conferred protection, but not during the initial phase of membrane retraction, which was driven by pre-existing actomyosin tensile stress. Notably, diversion of MPO from HOCl production by thiocyanate and nitrite attenuated de-adhesion and associated signaling responses, despite the latter substrate supporting MPO-catalyzed fibronectin nitration. This study indicates that HOCl-mediated matrix oxidation by subendothelial MPO deposits may play an important and previously unrecognized role in altering endothelial adhesion, signaling and integrity during inflammatory vascular disorders