Summary.  Coagulation and inflammation are intimately linked and cellular signaling by coagulation proteases through protease‐activated receptors (PARs) may affect pro‐ and anti‐inflammatory responses. Permeability of the endothelial cell barrier at the blood–tissue interface plays a key role in inflammatory disorders such as sepsis. We have recently shown that PAR1 signaling by activated protein C or low concentrations of thrombin can enhance endothelial barrier integrity. In the present study, we analyzed effects of coagulation factor Xa (FXa), which is known to activate both endothelial cell PAR1 and PAR2, on monolayer integrity using a transformed human umbilical vein endothelial cell (HUVEC) line in a dual‐chamber system. Preincubation with FXa potently reduced high‐dose thrombin‐mediated hyperpermeability and basal permeability. FXa was protective at concentrations of 5 nm or higher and proteolytic activity was required. Barrier protective FXa signaling was not affected by cleavage‐blocking anti‐PAR1 antibodies or by a PAR1 antagonist. Similarly, cleavage‐blocking anti‐PAR2 alone had no effect, but blocking both PAR1 and PAR2 inhibited barrier protection by FXa. Incubation of the cell layer with a PAR2‐specific agonist peptide reduced thrombin‐mediated hyperpermeability and basal permeability similar to FXa. In conclusion, not only PAR1, but also PAR2 can mediate barrier protection in endothelial cells and FXa can use either receptor to enhance barrier integrity. Although it is currently unknown whether PAR signaling by FXa has a physiological role, the results suggest a potential protective effect of FXa and other agonists of endothelial PAR2, which should be explored in models of local and systemic inflammation in vivo.

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