Direct effects of protein S in ameliorating acute lung injury

Summary. Objective: Protein S may exert an anticoagulant activity by enhancing the anticoagulant activity of activated protein C and/or by directly inhibiting the prothrombinase complex. Protein S itself may also directly regulate inflammatory responses and apoptosis. The role of protein S in acute lung injury (ALI) was unknown. This study evaluated the effect of protein S on ALI in the mouse. Methods: Animal ALI was induced in C57/BL6 mice by intratracheal instillation of lipopolysaccharide (LPS). Mice were treated with protein S or saline by intraperitoneal injection 1 h before LPS instillation. Results: Activated protein or protein S alone and combined activated protein C + protein S therapy decreased inflammatory markers and cytokines in mice with acute lung injury. In LPS‐treated mice compared with controls ALI was induced as shown by significantly increased levels of total protein, tumor necrosis factor‐α, interleukin‐6 and monocyte chemoattractant protein‐1 in the bronchoalveolar lavage fluid. Mice with ALI treated with protein S had significantly decreased concentrations of tumor necrosis factor‐α and interleukin‐6 in the lung compared with untreated animals. Thrombin‐antithrombin III, a marker of the activity of the coagulation cascade, was unchanged. Protein S inhibited the expression of cytokines in vitro and increased activation of the Axl tyrosine kinase pathway in A549 epithelial cells. Conclusion: Protein S protects against LPS‐induced ALI, possibly by directly inhibiting the local expression of inflammatory cytokines without affecting coagulation.