Activated protein C action in inflammation

Abstract

Activated protein C (APC) is a natural anticoagulant that plays an important role in coagulation homeostasis by inactivating the procoagulation factor Va and VIIIa. In addition to its anticoagulation functions, APC also has cytoprotective effects such as anti-inflammatory, anti-apoptotic, and endothelial barrier protection. Recently, a recombinant form of human APC (rhAPC or drotrecogin alfa activated; known commercially as 'Xigris') was approved by the US Federal Drug Administration for treatment of severe sepsis associated with a high risk of mortality. Sepsis, also known as systemic inflammatory response syndrome (SIRS) resulting from infection, is a serious medical condition in critical care patients. In sepsis, hyperactive and dysregulated inflammatory responses lead to secretion of pro- and anti-inflammatory cytokines, activation and migration of leucocytes, activation of coagulation, inhibition of fibrinolysis, and increased apoptosis. Although initial hypotheses focused on antithrombotic and profibrinolytic functions of APC in sepsis, other agents with more potent anticoagulation functions were not effective in treating severe sepsis. Furthermore, APC therapy is also associated with the risk of severe bleeding in treated patients. Therefore, the cytoprotective effects, rather than the anticoagulant effect of APC are postulated to be responsible for the therapeutic benefit of APC in the treatment of severe sepsis.

Figure 1

A. Crystal structure of activated protein C (APC; Heavy-chain; purple, Light-chain; green, Gla-domain; red) with endothelial protein C receptor (EPCR; yellow). Structures of Gla-less APC (1AUT.pdb) and Gla-domain (1LQV.pdb) were aligned by superimposing with FVIIa crystal structure (1dan.pdb). 15 missing residues between the two structures of APC were filled with corresponding residues in FVIIa. Critical residues are represented in the ball-and-stick presentation. B, C, and D. The local structures of the selected residues were enlarged. Highlighted residues in B, C and D are critical for interaction with FVa, PAR-1 and integrins, respectively.

Figure 2

APC anticoagulation pathway. When tissue factor (TF) is exposed to the bloodstream, it activates FVIIa, and the TF-FVIIa complex in turn activates FIX and FX. Then, FXa, along with FVa, forms prothrombinase, which converts prothrombin (FII) into thrombin (FIIa), which in turn activates FV and FVIII. FVIIIa and FIXa form the tenase complex also activating FX. Thrombin, after binding to thrombomodulin (TM), activates PC into activated PC (APC) and this process is accelerated in the presence of endothelial protein C receptor (EPCR). Thrombin also activates fibrinogen, which forms the fibrin clot. After dissociation from EPCR, APC can cleave FVa and FVIIIa, shutting down the coagulation pathway. Additionally, APC also inactivates plasminogen activator inhibitor, which results in increased fibrinolysis. T-PA, tissue plasminogen activator.

Figure 3

Anti-inflammatory functions of APC. The pathways involved in the anti-inflammatory and other cytoprotective effects of activated protein C (APC) on endothelial cells and leucocytes are illustrated. Cytoprotective effects of APC are mediated by both endothelial protein C receptor – protease-activated receptor 1 (EPCR-PAR-1)-dependent as well as independent pathways. In leucocytes, soluble EPCR-APC complexes can interact with the proteinase 3 (PR3)-Mac-1 complex and regulate neutrophil extravasations. APC binding to its receptors, downregulates (dotted lines) the expression of inflammatory cytokines and chemokines by blocking nuclear factor (NF)κB transcription factor. APC can also block leucocyte trafficking by decreasing the expression of adhesion molecules such as intercellular adhesion molecule 1 (ICAM-1), E selectin and vascular adhesion molecule 1 (VCAM-1) on the endothelium. Additionally, direct binding of APC to β₁ and β₃ integrins expressed on leucocyte inhibits their interstitial migration and infiltration into various tissues. Micro particle-bound APC (MP-APC) can also exert similar anti-inflammatory effects as non-bound APC. The role of soluble EPCR (sEPCR) in mediating anti-inflammatory effects of APC is not yet completely understood. In endothelial cells, APC can also inhibit the expression of monocytes chemoattractant protein-1 and prostaglandin I₂. Apart from anti-inflammatory effects, APC also have beneficial effects like anti-apoptotic and barrier protection in endothelial cells.

Figure 4

Cytoprotective effects of APC. Schematic overview of the anti-apoptotic, anti-inflammatory and barrier protective functions of APC. Cytoprotective effects of APC are mediated by both EPCR-PAR-1 dependent as well as independent pathways. APC, activated protein C; EPCR, endothelial protein C receptor; PAR-1, protease-activated receptor 1; PR3, proteinase 3; SphK-1, sphingosine kinase-1; S1P(1), sphingosine-1-phosphate(-1); PI3Kinase, phosphoinositide-3 kinase; NFκB, nuclear factor κB; IL, interleukin; TNF-α, tumour necrosis factor-α; MCP-1, monocytes chemoattractant protein-1; ICAM-1, intercellular adhesion molecule 1; FV, factor V; FVIII, factor VIII; TF, tissue factor.