Barrier dysfunction of pulmonary endothelial monolayer is associated with dramatic cytoskeletal reorganization, activation of actomyosin contractility, and gap formation. The linkage between the microtubule (MT) network and the contractile cytoskeleton has not been fully explored, however, clinical observations suggest that intravenous administration of anti-cancer drugs and MT inhibitors (such as the vinca alkaloids) can lead to the sudden development of pulmonary edema in breast cancer patients. In this study, we investigated the crosstalk between MT and actomyosin cytoskeleton and characterized specific molecular mechanisms of endothelial cells (EC) barrier dysfunction induced by MT inhibitor nocodazole (ND). Our results demonstrate that MT disassembly by ND induced rapid decreases in transendothelial electrical resistance (TER) and actin cytoskeletal remodeling, indicating EC barrier dysfunction. These effects involved ND-induced activation of Rho GTPase. Rho-mediated activation of its downstream target, Rho-kinase, induced phosphorylation of Rho-kinase effector EC MLC phosphatase (MYPT1) at Thr696 and Thr850 resulting in MYPT1 inactivation. Phosphatase inhibition leaded to accumulation of diphospho-MLC, which induced acto-myosin polymerization, stress fiber formation and gap formation. Inhibition of Rho-kinase by Y27632 abolished ND-induced MYPT1 phosphorylation, MLC phosphorylation, and stress fiber formation. In addition, MT preservation via the MT stabilizer paclitaxel, Rho inhibition (via C3 exotoxin, or dominant negative (DN)-Rho, or DN-Rho-kinase) attenuated ND-induced TER decreases, stress fiber formation and MLC phosphorylation. Collectively, our results demonstrate a leading role for Rho-dependent mechanisms in crosstalk between the MT and actomyosin cytoskeleton, and suggest Rho-kinase and MYPT1 as major Rho effectors mediating pulmonary EC barrier disruption in response to ND-induced MT disassembly.
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