TI - Cross talk between paxillin and Rac is critical for mediation of barrier-protective effects by oxidized phospholipids . AB - We previously reported that the barrier-protective effects of oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine ( OxPAPC ) on pulmonary endothelial cells ( ECs ) delineate the role of Rac- and Cdc42 -dependent mechanisms and described the involvement of the focal adhesion ( FA ) protein paxillin in enhancement of the EC barrier upon OxPAPC challenge . This study examined a potential role of paxillin in the feedback mechanism of Rac regulation by FAs in OxPAPC -stimulated ECs . Our results demonstrate that OxPAPC induced Rac -dependent , Rho-independent peripheral accumulation of paxillin-containing FAs and time -dependent paxillin phosphorylation . Molecular inhibition of Rac decreased association of paxillin with the Rac-specific guanine nucleotide exchange factor beta-PIX . Molecular inhibition of paxillin also attenuated OxPAPC -induced enhancement of adherens junctions critical for the EC barrier-protective response , accumulation of vascular endothelial cadherin in the membrane fractions , and decreased activation of Rac and its effector p21-activated kinase ( PAK1 ) . Expression of paxillin with a mutated PAK1 -dependent phosphorylation site (S273A) attenuated OxPAPC -induced PAK1 activation and the EC barrier-protective response . These results suggest that PAK1-specific paxillin phosphorylation at Ser(273) is critically involved in the positive-feedback regulation of the Rac-PAK1 pathway and may contribute to sustained enhancement of the EC barrier caused by oxidized phospholipids .