The Ligand Occupancy of Endothelial Protein C Receptor Switches the PAR-1 Dependent Signaling Specificity of Thrombin from a Disruptive to a Protective Response in Endothelial Cells.

It has been hypothesized that activated protein C (APC) exerts its cytoprotective and antiinflammatory activities through the endothelial protein C receptor (EPCR)-dependent cleavage of protease activated receptor 1 (PAR-1) on vascular endothelial cells. Noting that

• the activation of protein C on endothelial cells requires thrombin,

• relative to APC, thrombin cleaves PAR-1 with ∼3–4-orders of magnitude higher catalytic efficiency, and

• PAR-1 is a target for the proinflammatory activity of thrombin, it is not understood how APC can elicit a protective signaling response through the cleavage of PAR-1 when thrombin is present.

In this study, we demonstrate that EPCR is associated with caveolin-1 in lipid rafts of endothelial cells and that its occupancy by the Gla-domain of protein C/APC leads to its dissociation from caveolin-1 and recruitment of PAR-1 to a protective signaling pathway through coupling of PAR-1 to the pertussis toxin sensitive G_i-protein. Thus, when EPCR is bound by protein C/APC, the PAR-1 cleavage-dependent protective signaling responses in endothelial cells can be mediated by either thrombin or APC. These results provide a new paradigm for understanding how PAR-1 and EPCR participate in protective signaling events in endothelial cells.