HGF attenuates thrombin‐induced endothelial permeability by Tiaml‐mediated activation of the Rac pathway and by Tiam1/Rac‐dependent inhibition of the Rho pathway

Abstract

Reorganization of the endothelial cell (EC) cytoskeleton and cell adhesive complexes provides a structural basis for increased vascular permeability implicated in the pathogenesis of many diseases, including asthma, sepsis, and acute respiratory distress syndrome (ARDS). We have recently described the barrier-protective effects of hepatocyte growth factor (HGF) on the human pulmonary EC. In the present study, we explored the involvement of Rac-GTPase and Rac-specific nucleotide exchange factor Tiam1 in the mechanisms of EC barrier protection by HGF. HGF protected EC monolayers from thrombin-induced hyperpermeability, disruption of intercellular junctions, and formation of stress fibers and paracellular gaps by inhibiting thrombin-induced activation of Rho GTPase, Rho association with nucleotide exchange factor p115-RhoGEF, and myosin light chain phosphorylation, which was opposed by stimulation of Rac-dependent signaling. The pharmacological Rac inhibitor or silencing RNA (siRNA) based depletion of either Rac or Tiam1 significantly attenuated HGF-induced peripheral translocation of Rac effector cortactin, cortical actin ring formation, and EC barrier enhancement. Moreover, Tiam1 knockdown using the siRNA approach, attenuated the protective effect of HGF against thrombin-induced activation of Rho signaling, monolayer disruption, and EC hyperpermeability. This study demonstrates the Tiam1/Rac-dependent mechanism of HGF-induced EC barrier protection and provides novel mechanistic insights into regulation of EC permeability via dynamic interactions between Rho- and Tiam1/Rac-mediated pathways.

Rnd3 as a Novel Target to Ameliorate Microvascular Leakage

“…As alternatives, we investigated Rac1 and Cdc42, 2 other key Rho family small GTPases that contribute to the control of endothelial barrier function 22, 24, 77, 78, 79. Thrombin caused a rapid, significant decrease in Rac1‐GTP in agreement with several previous reports 17, 78, 80, 81. However, thrombin did not activate Cdc42 in HUVEC, in contrast to one previous report that described a delayed activation of Cdc42 in immortalized human dermal microvascular endothelial cells 71.…”

Hepatocyte Growth Factor-induced Asef-IQGAP1 Complex Controls Cytoskeletal Remodeling and Endothelial Barrier

“…Previous studies reporting an additional role of another Rac GEF, Tiam1, in the mediation of HGF-protective responses in vascular endothelium (8,48) may explain the incomplete inhibition of HGF responses by Asef knockdown shown in this study.…”

“…Although HGF also activates Tiam1 (8,48), our results suggest that IQGAP1 redistribution and interaction with the actin cytoskeleton effectors are at least in part regulated by Asef. These data suggest that Asef and Tiam1 may be involved in different aspects of the HGFinduced effects, and further studies are warranted to investigate the interplay between Tiam1 and Asef in the HGF-induced regulation of peripheral cytoskeletal remodeling and the EC permeability response.…”

“…IQGAP1 at the Cell Periphery-HGF stimulation activates peripheral cytoskeletal dynamics in vascular EC in a Rac-dependent fashion (8). This study used human pulmonary artery endothelial cells to characterize functional interactions between the Rac-specific guanine nucleotide exchange factor Asef and Rac/ Cdc42 effector IQGAP1.…”

“…This interaction promotes IQGAP1 association with cortactin and Arp3, proteins known to activate peripheral cytoskeletal remodeling and EC barrier enhancement (8,42,52). IQGAP1 interacts with Rac and Cdc42 GTPases via GRD, and this interaction leads to IQGAP1 activation and interaction with effector proteins (23).…”