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The pleiotropic cytokine tumor necrosis factor-␣ (TNF-␣) and thrombin lead to increased endothelial permeability in sepsis. Numerous studies demonstrated the significance of intracellular cyclic nucleotides for the maintenance of endothelial barrier function. Actions of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are terminated by distinct cyclic nucleotide phosphodiesterases (PDEs). We hypothesized that TNF-␣ could regulate PDE activity in endothelial cells, thereby impairing endothelial barrier function. In cultured human umbilical vein endothelial cells (HUVECs), we found a dramatic increase of PDE2 activity following TNF-␣ stimulation, while PDE3 and PDE4 activities remained unchanged. Significant PDE activities other than PDE2, PDE3, and PDE4 were not detected. TNF-␣ increased PDE2 expression in a p38 mitogen-activated protein kinase (MAPK)-dependent manner. Endothelial barrier function was investigated in HUVECs and in isolated mice lungs. Selective PDE2 up-regulation sensitized HUVECs toward the permeability-increasing agent thrombin. In isolated mice lungs, we demonstrated that PDE2 inhibition was effective in preventing thrombin-induced lung edema, as shown with a reduction in both lung wet-to-dry ratio and albumin flux from the vascular to bronchoalveolar compartment. Our findings suggest that TNF-␣-mediated upregulation of PDE2 may destabilize endothelial barrier function in sepsis. Inhibition of PDE2 is therefore of potential therapeutic interest in sepsis and acute respiratory distress syndrome (ARDS). IntroductionSepsis-the systemic inflammatory response to infection-is the most common cause of death among patients in noncoronary intensive care units. 1 Laboratory markers of inflammation include high circulating levels of tumor necrosis factor ␣ (TNF-␣) and other cytokines, as well as activation of the coagulation cascade. 1 Endothelial hyperpermeability is a hallmark of sepsis. TNF-␣ increases endothelial cell permeability 2 with subsequent vascular leakage contributing to severe organ dysfunction such as adult respiratory distress syndrome (ARDS). 3 Half of the sepsis patients develop disseminated intravascular coagulation 2,4 due to a shift toward a procoagulant state with excessive thrombin and fibrin generation. 1 On a cellular level, thrombin changes the shape of endothelial cells and increases endothelial permeability, 5 thereby impairing endothelial barrier function synergistically with TNF-␣. 1,6 Thrombin, the main effector protease of the coagulation cascade, activates endothelial cells directly via protease-activated receptor 1 (PAR1) and PAR4, 7 thereby inducing hyperpermeability and promoting adhesion of platelets and leukocytes, as well as secretion of plateletactivating factor (PAF) and inflammatory cytokines. 8 After binding to one of the various receptors, TNF-␣ mediates activation of diverse signaling pathways, such as the p38 mitogenactivated protein kinase (p38 MAPK), JUN N-terminal kinase (JNK), and nuclear factor kappa B (NF-B). TNF recepto...