Protease activated receptor 2 deficiency in alveolar macrophages impairs cAMP generation leading to NFAT‐dependent pro‐inflammatory signalling and lung injury

Abstract

Endotoxin (lipopolysaccharide [LPS]) is well known to induce intravascular coagulation during sepsis which may lead to multi‐organ failure and death. Protease Activated Receptor (Par) 1 and 2 expressed on several lung cell types can mediate crosstalk between coagulation and inflammation during endotoxemia but the mechanism remains unclear. Here, we exposed Par1−/−, Par2−/− and WT mice to nebulized LPS to address the relative roles of PAR1 versus PAR2 in modulating lung vascular injury caused by endotoxin LPS. We observed that LPS failed to induce lung injury in Par1−/− mice while it resulted in unresolvable edema formation and neutrophilic injury in Par2−/− mice. Also, Par2−/− mice, showed significant increase in the expression of pro‐inflammatory cytokines in bronchoalveolar lavage. We showed that transplantation of bone marrow from WT mice into Par2−/− mice resolved inflammatory injury in Par2−/− mice indicating haematopoietic PAR2 as a critical factor mediating resolution of inflammatory lung injury. Additionally, depletion of alveolar macrophages using intratracheally (i.t) clodronate following injury augmented lung injury in WT mice indicating PAR2 functioned by dampening alveolar macrophage activity. Since PAR2 has been shown to reverse PAR1 signalling and can be activated by thrombin directly, we next assessed if LPS‐induced thrombin generation activated PAR2 that then resolved edema formation. We delivered WT‐Par2 cDNA or mutated Par2 cDNA resistant to thrombin cleavage in Par2−/− mice i.t. using liposomes and assessed lung edema following LPS challenge. We found that Par2−/− mice transducing WT‐PAR2 resolved edema formation but this response was absent in mice expressing mutant PAR2 cDNA indicating that LPS induced thrombin generation activates PAR1 to mediate lung injury which is resolved following delayed activation of PAR2 by thrombin. Because PAR2 induces the generation of cAMP and Ca2+ mobilization we assessed the generation of these second messengers in bone marrow derived macrophages (BMDM) from WT and Par2−/− mice. We found that thrombin significantly increased cAMP levels in WT‐BMDM but not in Par2−/− BMDM. This reduction was not due to insufficient activation of Gs or phosphodiesterase as foskolin and rollipram similarly altered cAMP levels in WT and Par2−/− BMDM. Additionally, we found that loss of Par2 activated Ca2+ entry into macrophages through transient receptor potential vanniloid channel (TRPV4) which reverted back to WT levels following addition of dibutryl cAMP. Since Ca2+ entry induces NFAT transcriptional activity, we assessed the role of PAR2 in dampening inflammatory signaling and noted that loss of PAR2 increased phosphorylation of p65‐NFκB via NFAT activation. Rescuing cAMP levels diminished the phosphorylation of p65‐NFκB and NFAT activity and thereby inflammatory cytokine generation. These studies demonstrate the critical role of PAR2 activation in alveolar macrophages in resolving lung injury by inducing cAMP generation which in turn suppresses Ca2+‐dependent inflammatory signalling by NFAT.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.