Regulation of sphingosine 1-phosphate-induced endothelial cytoskeletal rearrangement and barrier enhancement by S1P₁ receptor, PI3 kinase, Tiam1/Rac1, and α-actinin

Endothelial cell (EC) barrier dysfunction results in increased vascular permeability observed in inflammation, tumor angiogenesis, and atherosclerosis. The platelet-derived phospholipid sphingosine-1-phosphate (S1P) decreases EC permeability in vitro and in vivo and thus has obvious therapeutic potential. We examined S1P-mediated human pulmonary artery EC signaling and barrier regulation in caveolin-enriched microdomains (CEM). Immunoblotting from S1P-treated EC revealed S1P-mediated rapid recruitment (1 μM, 5 min) to CEMs of the S1P receptors S1P₁ and S1P₃, p110 PI3 kinase α and β catalytic subunits, the Rac1 GEF, Tiam1, and α-actinin isoforms 1 and 4. Immunoprecipitated p110 PI3 kinase catalytic subunits from S1P-treated EC exhibited PIP₃ production in CEMs. Immunoprecipitation of S1P receptors from CEM fractions revealed complexes containing Tiam1 and S1P₁. PI3 kinase inhibition (LY294002) attenuated S1P-induced Tiam1 association with S1P₁, Tiam1/Rac1 activation, α-actinin-1/4 recruitment, and EC barrier enhancement. Silencing of either S1P₁ or Tiam1 expression resulted in the loss of S1P-mediated Rac1 activation and α-actinin-1/4 recruitment to CEM. Finally, silencing S1P₁, Tiam1, or both α-actinin isoforms 1/4 inhibits S1P-induced cortical F-actin rearrangement and S1P-mediated barrier enhancement. Taken together, these results suggest that S1P-induced recruitment of S1P₁ to CEM fractions promotes PI3 kinase-mediated Tiam1/Rac1 activation required for α-actinin-1/4-regulated cortical actin rearrangement and EC barrier enhancement.