The statins are a class of 3-hydroxy-3-methylglutaryl-coenzyme A-reductase inhibitors that are recognized to have pleiotropic properties. We previously reported the attenuation of LPS-induced murine acute lung injury (ALI) by simvastatin in vivo and identified relevant effects of simvastatin on endothelial cell (EC) signaling, activation, and barrier function in vitro. In particular, simvastatin induces the upregulation of integrin-4, which in turn inhibits EC inflammatory responses via attenuation of MAPK signaling. The role of integrin-4 in murine ALI protection by simvastatin, however, is unknown. We initially confirmed a time-and dose-dependent effect of simvastatin on increased integrin-4 mRNA expression in human lung EC with peak protein expression evident at 16 h. Subsequently, reciprocal immunoprecipitation demonstrated an attenuation of LPS-induced integrin-4 tyrosine phosphorylation by simvastatin (5 M, 16 h). Increased expression of EC inflammatory cytokines [IL-6, IL-8, monocyte chemoattractant protein (MCP)-1, regulated on activation normal T cell expressed and secreted (RANTES)] by LPS (500 ng/ml, 4 h) was also significantly attenuated by simvastatin pretreatment (5 M, 16 h), but this effect was reversed by cotreatment with an integrin-4-blocking antibody. Finally, although simvastatin (20 mg/kg) conferred significant protection in murine ALI as evidenced by decreased bronchoalveolar lavage fluid cell counts, protein, inflammatory cytokines (IL-6, IL-1, MCP-1, RANTES), decreased Evans blue dye albumin extravasation in lung tissue, and changes on lung histology, these effects were reversed by the integrin-4-blocking antibody (IV, 1 mg/kg, 2 h before LPS). These findings support integrin-4 as an important mediator of ALI protection by simvastatin and implicate signaling by integrin-4 as a novel therapeutic target in patients with ALI.statins; integrins; endothelial cells THE STATINS ARE A CLASS OF 3-hydroxy-3-methylglutaryl-coenzyme A-reductase inhibitors recognized to have potent antiinflammatory and vascular-protective properties independent of their ability to lower serum cholesterol levels. In this regard, we previously reported the attenuation of murine acute lung injury (ALI) by pretreatment with simvastatin (12). The mechanisms underlying these effects, however, are complex and not fully characterized. For example, we have identified several distinct effects of statins on endothelial cell (EC) signaling and activation that are relevant to the pathophysiology of ALI including actin cytoskeletal rearrangement and EC barrier protection via differential effects on small GTPases RhoA and Rac1 (13) and inhibition of superoxide generation via NADPH oxidase inhibition (4). We have also reported the marked upregulation of integrin-4 (13), a molecule that attenuates EC inflammatory responses via effects on MAPK signaling (3). This dramatic upregulation of EC integrin-4 by statins has subsequently been corroborated (16); however, the role of integrin-4 in murine ALI protection by simvastatin ...