A novel protein C–factor VII chimera provides new insights into the structural requirements for cytoprotective protease‐activated receptor 1 signaling

Abstract

Background Activated proteinC (APC) cell signaling is largely reliant upon its ability to mediate protease-activated receptor (PAR)1 proteolysis when bound to the endothelial cell (EC) proteinC (PC) receptor (EPCR). Furthermore, EPCR-bound PC modulates PAR1 signaling by thrombin to induce APC-like EC cytoprotection. Objective The molecular determinants of EPCR-dependent cytoprotective PAR1 signaling remain poorly defined. To address this, a PC-factorVII chimera (PCFVII-82 ) possessing FVII N-terminal domains and conserved EPCR binding was characterized. Methods Activated PC-FVII chimera (APCFVII-82 ) anticoagulant activity was measured with calibrated automated thrombography and activated FV degradation assays. APCFVII-82 signaling activity was characterized by the use of reporter assays of PAR1 proteolysis and EC barrier integrity. APCFVII-82 anti-inflammatory activity was assessed according to its inhibition of nuclear factor-κB (NF-κB) activation and cytokine secretion from monocytes. Results PCFVII-82 was activated normally by thrombin on ECs, but was unable to inhibit plasma thrombin generation. Surprisingly, APCFVII-82 did not mediate EPCR-dependent PAR1 proteolysis, confer PAR1-dependent protection of thrombin-induced EC barrier disruption, or limit PAR1-dependent attenuation of interleukin-6 release from lipopolysaccharide (LPS)-stimulated macrophages. Interestingly, EPCR occupation by active site-blocked APCFVII-82 was, like FVII, unable to mimic EC barrier stabilization induced by PC upon PAR1 proteolysis by thrombin. APCFVII-82 did, however, diminish LPS-induced NF-κB activation and tumor necrosis factor-α release from monocytes in an apolipoproteinE receptor2-dependent manner, with similar efficacy as wild-type APC. Conclusions These findings identify a novel role for APC light chain amino acid residues outside the EPCR-binding site in enabling cytoprotective PAR1 signaling.