Factor VIIa bound to endothelial cell protein C receptor activates protease activated receptor-1 and mediates cell signaling and barrier protection

Abstract

IntroductionRecent studies from our laboratory 1,2 and others 3,4 have shown that factor VIIa (FVIIa), a clotting protease that binds to tissue factor (TF) and initiates the activation of the coagulation cascade, also binds to the endothelial cell protein C receptor (EPCR), a receptor for anticoagulant protein C/activated protein C (APC). EPCR controls coagulation by promoting the activation of protein C by thrombin-thrombomodulin complexes. 5 In addition to controlling coagulation, EPCR has been shown to modulate several nonhemostatic functions by supporting APC-induced protease activated receptor-1 (PAR1)-mediated cell signaling. [6][7][8][9][10][11][12][13] Although direct evidence for an association of FVIIa with EPCR in vivo is yet to come, several recent observations are a strong indication that FVIIa does in fact interact with EPCR in vivo. Both human and murine FVIIa administered to mice were shown to associate with endothelium, and blockade of EPCR with EPCR-specific antibodies was shown to prolong the human FVIIa circulatory-half life in mice. 2,14 Analysis of FVII, FVIIa, and soluble EPCR levels in a large group of healthy individuals revealed that those with the EPCR Gly variants, whose circulating levels of soluble EPCR were higher, had higher levels of circulating FVII and FVIIa, suggesting that EPCR in vivo serves as a reservoir for FVII. 15,16 At present, the physiologic importance of FVIIa's interaction with EPCR is not entirely clear. Our recent studies suggest that EPCR may play a role in the clearance and/or transport of FVIIa. 2 Although we are unable to find evidence for the modulation of FVIIa's coagulant activity by EPCR, 1 others have shown that FVIIa binding to EPCR on endothelial cells downregulates FVIIa's coagulant activity. 4 Similarly, EPCR was shown to down-regulate FVIIa generation on endothelial cells by reducing FVII accessibility to phospholipids at the cell surface. 17 Despite divergent views on the potential mechanisms by which APC binding to EPCR provides cytoprotective activity through PAR1-mediated cell signaling, it is generally believed that complex formation of APC with EPCR potentiates APC cleavage of PAR1, and that PAR1 activation is responsible for eliciting protective signaling responses. 6,13,[18][19][20] In agreement with this notion, APC was shown to cleave PAR1 on endothelial cells, and EPCRblocking antibodies that prevent APC binding to EPCR inhibited APC cleavage of PAR1. 18 In studies performed in a heterologous cell model system expressing transfected EPCR and PAR1 or PAR2 reporter constructs, we found no evidence that the FVIIa bound to EPCR was capable of cleaving either PAR1 or PAR2 or of inducing cell signaling. 1 In earlier studies, APC was shown to cleave PAR1 reporter constructs expressed in endothelial cells (EA.hy926 cells), but this cleavage required high concentrations of APC (75nM or higher) and was EPCR independent. 10,21 In the same studies, an APC-mediated protective effect was seen with much lower concentrations of APC, and this effect was E...

Prophylaxis in congenital factor VII deficiency: indications, efficacy and safety. Results from the Seven Treatment Evaluation Registry (STER)

“…7 One explanation for this apparent paradox is offered by the possible presence of infused FVII bound extravascularly to pericytes, which may allow FVIIa to be available for a longer time than expected. 31 Another possibility is that suggested by a recent study demonstrating binding of excess rFVIIa to activated protein C receptor on undamaged endothelial cells, 32,33 which saturates the receptor and may keep excess rFVIIa in circulation for longer, or may allow a late subsequent release of the factor.…”

The endothelial protein C receptor enhances hemostasis of FVIIa administration in hemophilic mice in vivo

“…In humans, support for an EPCR role in the engagement of FVIIa in inflammation has been previously provided. 44 Additionally, an EPCR role in the FVII/FVIIa tissue distribution and, possibly, its catabolism has also been suggested. 18 Such EPCR-dependent processes are not expected to be present in the mouse.…”

Tissue Factor Signals Airway Epithelial Basal Cell Survival via Coagulation and Protease-Activated Receptor Isoforms 1 and 2

“…Binding of active Factor VII causes cytosolic calcium mobilization and signal transduction via mitogen-activated protein kinase phosphorylation (49). Factor VII-dependent PAR1 signaling promotes barrier function in endothelium (50). PAR2 can also be protective against virus-induced lung injury (51).…”