New players in the sepsis-protective activated protein C pathway

Sepsis affects as many as 500,000 individuals in the United States each year, and in many of these cases it is lethal. Uncontrolled inflammation and coagulation are two hallmarks of severe sepsis. The latter is a result of increased coagulation mediated by tissue factor and impaired anticoagulation following cellular downregulation or depletion of thrombomodulin, endothelial cell protein C receptor (EPCR), and protein C (PC). The fact that sepsis-induced uncontrolled coagulation leads to microvascular thrombosis, which in turn can result in organ failure, provided the rationale for the clinical development of recombinant activated PC (aPC) as a therapy for sepsis. This approach markedly improved overall 28-day mortality in patients, specifically in the subgroup of patients with severe sepsis. After the discovery that aPC could improve the survival of patients with severe sepsis, researchers sought to understand the molecular and cellular mechanisms underlying its effects. Initial observations that aPC attenuated inflammatory NF-κB signaling in endothelial and monocytic cells led to the hypothesis that the effects of aPC were not mediated simply by its ability to promote anticoagulation. Subsequent analysis identified an aPC signaling complex in which aPC binds to EPCR, facilitating the proteolytic cleavage of protease-activated receptor 1 (PAR1) on endothelial cells by aPC (1). Endothelial EPCR/aPC/PAR1 signaling typically counterbalances the detrimental effects of inflammation as well as those of thrombin/PAR1 signaling, which promote apoptosis, barrier disruption, and endothelial activation (2, 3). Although the paradoxical, opposing roles of PAR1 signaling are incompletely understood, several contributing factors have been identified, including the inability of EPCR/aPC-cleaved PAR1 to cross-activate PAR2 (3); coupling of EPCR/aPC signaling to sphingosine-1-phosphate receptor signaling (2); localization of EPCR/aPC signaling to caveolae, resulting in PAR1 desensitization (4); and ligand occupancy of EPCR, leading to EPCR/caveolin dissociation and a broad switch of PAR1 signaling specificity (5). The development of mutants of aPC that were either permissive for signaling but had minimal anticoagulant activity (6) or potent anticoagulants with drastically reduced signaling function (7) provided the decisive tools to investigate whether aPC-mediated signaling, rather than its anticoagulant activity, was crucial for mortality reduction following aPC therapy in mouse models of sepsis, and this was demonstrated to be the case (6, 7). Furthermore, mice with very low levels of EPCR or deficiency of PAR1 did not benefit from aPC administration (6). However, these studies did not provide insight into relevant cellular targets for sepsis-protective aPC signaling (6, 7). Notably, the effects of aPC in other cell types can be independent of EPCR or PAR1 and may involve additional receptors. EPCR also supports PAR2 cleavage by aPC (1), aPC requires PAR3 for neuroprotection in stroke (8), and aPC utilizes the endocytic receptor LDL receptor–related protein 8 (LRP8; also known as ApoER2) to directly trigger disabled 1 (Dab1) and glycogen synthase kinase 3β phosphorylation in myeloid cells (9). Data published recently in the JCI by Cao et al. (10) and in this issue of the JCI by Kerschen et al. (11) elucidate new non-anticoagulant mechanisms by which aPC can protect against lethal sepsis in mice. Specifically, these reports indicate that aPC can control immune responses and are consistent with another recent article showing that aPC suppresses neutrophil migration (12).