Tiam1 and βPIX mediate Rac‐dependent endothelial barrier protective response to oxidized phospholipids

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Abstract
Oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC) exhibits potent barrier protective effects on pulmonary endothelium, which are mediated by small GTPases Rac and Cdc42. However, upstream mechanisms of OxPAPC-induced small GTPase activation are not known. We studied involvement of Rac/Cdc42-specific guanine nucleotide exchange factors (GEFs) Tiam1 and βPIX in OxPAPC-induced Rac activation, cytoskeletal remodeling, and barrier protective responses in the human pulmonary endothelial cells (EC). OxPAPC induced membrane translocation of Tiam1, βPIX, Cdc42, and Rac, but did not affect intracellular distribution of Rho and Rho-specific GEF p115-RhoGEF. Protein depletion of Tiam1 and βPIX using siRNA approach abolished OxPAPC-induced activation of Rac and its effector PAK1. EC transfection with Tiam1-, βPIX-, or PAK1-specific siRNA dramatically attenuated OxPAPC-induced barrier enhancement, peripheral actin cytoskeletal enhancement, and translocation of actin-binding proteins cortactin and Arp3. These results show for the first time that Tiam1 and βPIX mediate OxPAPC-induced Rac activation, cytoskeletal remodeling, and barrier protective response in pulmonary endothelium. J. Cell. Physiol. 211: 608–617, 2007.