Tissue factor- and factor X-dependent activation of protease-activated receptor 2 by factor VIIa

Protease-activated receptor 2 (PAR2) is expressed by vascular endothelial c ells and other cells in which its function and physiological activator(s) a re unknown. Unlike PAR1, PAR3, and PAR4, PAR2 is not activatable by thrombi n. Coagulation factors VIIa (FVIIa) and Xa (FXa) are proteases that act ups tream of thrombin in the coagulation cascade and require cofactors to inter act with their substrates. These proteases elicit cellular responses, but t heir receptor(s) have not been identified. We asked whether FVIIa and FXa m ight activate PARs if presented by their cofactors. Coexpression of tissue factor (TF), the cellular cofactor for FVIIa, together with PAR1, PAR2, PAR 3, or PAR4 conferred TF-dependent FVIIa activation of PAR2 and, to lesser d egree, PAR1. Responses to FXa were also observed but were independent of ex ogenous cofactor. The TF/FVIIa complex converts the inactive zymogen Factor X (FX) to FXa. Strikingly, when FX was present, low picomolar concentratio ns of FVIIa caused robust signaling in cells expressing TF and PAR2. Respon ses in keratinocytes and cytokine-treated endothelial cells suggested that PAR2 may be activated directly by TF/FVIIa and indirectly by TF/FVIIa-gener ated FXa at naturally occurring expression levels of TF and PAR2. These res ults suggest that PAR2, although not activatable by thrombin, may nonethele ss function as a sensor for coagulation proteases and contribute to endothe lial activation in the setting of injury and inflammation. More generally, these findings highlight the potential importance of cofactors in regulatin g PAR function and specificity.