Endotoxemia and sepsis mortality reduction by non-anticoagulant–activated protein C

Abstract

Activated protein C (APC) reduces mortality of severe sepsis patients but increases the risk of serious bleeding. APC exerts anticoagulant activity by proteolysis of factors Va/VIIIa. APC also exerts antiinflammatory and antiapoptotic effects and stabilizes endothelial barrier function by APC-initiated cell signaling that requires two receptors, endothelial cell protein C receptor (EPCR) and protease-activated receptor 1 (PAR1). The relative importance of APC's various activities for efficacy in sepsis is unknown. We used protein engineering of mouse APC and genetically altered mice to clarify mechanisms for the efficacy of APC in mouse sepsis models. Mortality reduction in LPS-induced endotoxemia required the enzymatic active site of APC, EPCR, and PAR-1, highlighting a key role for APC's cytoprotective actions. A recombinant APC variant with normal signaling but <10% anticoagulant activity (5A-APC) was as effective as wild-type APC in reducing mortality after LPS challenge, and enhanced the survival of mice subjected to peritonitis induced by gram-positive or -negative bacteria or to polymicrobial peritoneal sepsis triggered by colon ascendens stent implantation. Thus, APC's efficacy in severe sepsis is predominantly based on EPCR- and PAR1-dependent cell signaling, and APC variants with normal cell signaling but reduced anticoagulant activities retain efficacy while reducing the risk of bleeding.

The Endothelial Protein C Receptor Supports Tissue Factor Ternary Coagulation Initiation Complex Signaling through Protease-activated Receptors

“…The role of EPCR as a crucial co-receptor for PC activation and aPC-mediated cytoprotective signaling in vascular and immune cells is supported by in vitro and in vivo data (57)(58)(59)(60)(61)(62)(63). However, TF is normally not expressed in the vascular endothelium but co-expressed with EPCR in extravascular cells.…”

Activated protein C ligation of ApoER2 (LRP8) causes Dab1-dependent signaling in U937 cells

“…In addition to reducing death by sepsis in humans and murine models (1,14), APC has remarkable in vivo neuroprotective activities in murine brain injury model studies (11-13, 62, 63). Identification of an ApoER2-dependent, Reelin-like signaling pathway for APC on U937 cells that activates the PI3K-Akt pathway raises questions about potential roles for this new APC signaling mechanism for neurons and other brain cells.…”

“…Central to this paradigm is the binding of APC's ␥-carboxyglutamic acidrich N-terminal domain to the EPCR with subsequent cleavage of PAR1 by APC's protease domain, followed by PAR1-induced direct effects of APC on cells. Strong support for the in vivo relevance of this mechanism for triggering cell signaling by pharmacologic doses of APC comes from studies of murine ischemic stroke, NMDA excitotoxicity injury, and severe sepsis models (11)(12)(13)(14) .…”

Activated protein C promotes protease-activated receptor-1 cytoprotective signaling through β-arrestin and dishevelled-2 scaffolds

“…APC cytoprotective signaling also requires the compartmentalization of PAR1 and its coreceptor EPCR in cholesterol-enriched caveolar microdomains (5,6). A previous study further showed that the cytoprotective activity of a noncoagulant APC is critical for reducing mortality associated with sepsis in an experimental mouse model (7). However, the mechanism by which APC-activated PAR1 promotes cytoprotective cellular responses is poorly understood.…”