Activated protein C-cleaved protease activated receptor-1 is retained on the endothelial cell surface even in the presence of thrombin

Activated protein C (APC) signals in endothelial cells ex vivo through protease activated receptor-1 (PAR1). However, it is controversial whether PAR1 can mediate APC's protective effects in sepsis because the inflammatory response results in thrombin generation and thrombin proteolytically activates PAR1 much more efficiently than APC. Here we show that APC can induce powerful barrier protective responses in an endothelial cell monolayer in the presence of thrombin. Using cell surface immunoassays with conformation sensitive monoclonal anti-PAR1 antibodies we analyzed cleavage of endogenous PAR1 on the endothelial cell surface by APC in the absence and presence of thrombin. Incubation with APC caused efficient PAR1 cleavage and upon coincubation with thrombin APC supported additional PAR1 cleavage. Thrombin-cleaved PAR1 rapidly disappeared from the cell surface whereas, unexpectedly, the APC-cleaved PAR1 remained and could be detected on the cell surface, even when thrombin at concentrations of up to 1 nM was also present. Our findings demonstrate for the first time directly that APC can generate a distinct PAR1 population on endothelial cells in the presence of thrombin. The data suggest that different trafficking of activated PAR1 might explain how PAR1 signaling by APC can be relevant when thrombin is present.