The efficacy of activated protein C in murine endotoxemia is dependent on integrin CD11b

Abstract

Activated protein C (APC), the only FDA-approved biotherapeutic drug for sepsis, possesses anticoagulant, antiinflammatory, and barrier-protective activities. However, the mechanisms underlying its antiinflammatory functions are not well defined. Here, we report that the antiinflammatory activity of APC on macrophages is dependent on integrin CD11b/CD18, but not on endothelial protein C receptor (EPCR). We showed that CD11b/CD18 bound APC within specialized membrane microdomains/lipid rafts and facilitated APC cleavage and activation of protease-activated receptor-1 (PAR1), leading to enhanced production of sphingosine-1-phosphate (S1P) and suppression of the proinflammatory response of activated macrophages. Deletion of the γ-carboxyglutamic acid domain of APC, a region critical for its anticoagulant activity and EPCR-dependent barrier protection, had no effect on its antiinflammatory function. Genetic inactivation of CD11b, PAR1, or sphingosine kinase-1, but not EPCR, abolished the ability of APC to suppress the macrophage inflammatory response in vitro. Using an LPS-induced mouse model of lethal endotoxemia, we showed that APC administration reduced the mortality of wild-type mice, but not CD11b-deficient mice. These data establish what we believe to be a novel mechanism underlying the antiinflammatory activity of APC in the setting of endotoxemia and provide clear evidence that the antiinflammatory function of APC is distinct from its barrier-protective function and anticoagulant activities.

Activated Protein C Attenuates Systemic Lupus Erythematosus and Lupus Nephritis in MRL-Fas(lpr) Mice

“…Aberrations of monocyte/macrophage phenotype and function are increasingly recognized in SLE and animal models of the disease. Cao et al (30) showed that the anti-inflammatory activity of activated protein C on macrophages is dependent on integrin CD11b/CD18, but not on EPCR. They showed that CD11b/CD18-bound activated protein C, facilitated cleavage and activation of PAR-1, leading to enhanced production of sphingosine 1-phosphate and suppression of the proinflammatory response of activated macrophages (30).…”

“…Cao et al (30) showed that the anti-inflammatory activity of activated protein C on macrophages is dependent on integrin CD11b/CD18, but not on EPCR. They showed that CD11b/CD18-bound activated protein C, facilitated cleavage and activation of PAR-1, leading to enhanced production of sphingosine 1-phosphate and suppression of the proinflammatory response of activated macrophages (30). Activated protein C decreases the release of the MIP-1a from the monocytic cell line THP-1 and from human monocytes of septic patients (31).…”

Activated protein C: biased for translation

“…Proof for the major role for APC's cell-signaling actions (causing cytoprotective and immunoregulatory actions rather than its anticoagulant actions) came after 2006. 22,44,81,102,104 Hence, a 96-hour, low-dose wt-APC infusion was not optimally suited to harness APC's cell-signaling actions. We believe that bolus dosing is preferred to promote potent endothelial-barrier stabilization via PAR1 signaling and Rac1 activation and to alter gene expression profiles that promote antiapoptotic and anti-inflammatory activities.…”

“…Thus, other PAR3-effector interactions are required for signal induction, diversification, and regulation ( Figure 3). PAR-effector complexes are hypothesized to involve the formation of PAR-PAR heterodimers and homodimers, 43 which may enable PARinduced transactivation of other PARs, integrate the transactivation of other GPCRs such as S1P1, 18,25,26 and incorporate cooperative cross talk with integrins such as Mac1 44,45 or other receptors such as ApoER2 46,47 or Tie2. 20,42,48 Formation of these complexes may achieve a signaling bias by promoting or discouraging the association of particular G-protein ensembles, 49,50 by recruiting b-arrestins, 30 or by incorporating nontraditional PAR signaling pathways via transactivations such as the activation of Tie2 by noncanonical activation of PAR3.…”

Anti-Inflammatory Actions of the Anticoagulant, Activated Protein C

“…Genetic inactivation of CD11b, PAR1, or sphingosine kinase-1, but not EPCR, abolished the ability of APC to suppress the macrophage inflammatory response in vitro. Using a LPS-induced mouse model of lethal endotoxemia, Cao et al (Cao, 2010) showed that APC administration reduced the mortality of wild-type mice, but not CD11b-deficient mice. Table 1.…”