PAR2-Mediated cAMP Generation Suppresses TRPV4-Dependent Ca2+ Signaling in Alveolar Macrophages to Resolve TLR4-Induced Inflammation

Abstract

SUMMARY Alveolar macrophages (AMs), upon sensing pathogens, trigger host defense by activating toll-like receptor 4 (TLR4), but the counterbalancing mechanisms that deactivate AM inflammatory signaling and prevent lethal edema, the hallmark of acute lung injury (ALI), remain unknown. Here, we demonstrate the essential role of AM protease-activating receptor 2 (PAR2) in rapidly suppressing inflammation to prevent long-lasting injury. We show that thrombin, released during TLR4-induced lung injury, directly activates PAR2 to generate cAMP, which abolishes Ca2+ entry through the TRPV4 channel. Deletion of PAR2 and thus the accompanying cAMP generation augments Ca2+ entry via TRPV4, causing sustained activation of the transcription factor NFAT to produce long-lasting TLR4-mediated inflammatory lung injury. Rescuing thrombin-sensitive PAR2 expression or blocking TRPV4 activity in PAR2-null AMs restores their capacity to resolve inflammation and reverse lung injury. Thus, activation of the thrombin-induced PAR2-cAMP cascade in AMs suppresses TLR4 inflammatory signaling to reinstate tissue integrity.

Is Toll-like receptor 4 involved in the severity of COVID-19 pathology in patients with cardiometabolic comorbidities?

“…On the other hand, alveolar macrophages are activated by TLR4 signaling and play an important role in the clearance of pathogens within the lung compartment. Appropriate resolution of inflammation, however, is modulated by calcium signaling via TRPV4 [ 65 ] and TRPV6 [ 66 ]. Further evaluation of strategies to promote successful and appropriate resolution of the inflammatory response in severe COVID-19 patients via suppression of TLR4 signaling could be beneficial for improving prognosis in these patients.…”

TRPV4—A Missing Link Between Mechanosensation and Immunity

“…In gram-negative infections with bacteria such as Escherichia coli and Pseudomonas aeruginosa, TRPV4 activation by ECM stiffening during infection synergizes with LPS-stimulated TLR4 activation of p38 and thereby promotes host defense and resolution from lung injury (15,73). Conversely, activation of protease-activated receptor (PAR)-2 by thrombin suppresses TRPV4 activity in macrophages and resolves lung injury (74). Similarly, PARs are also activated by neutrophil elastase (NE), matrix metalloproteases (MMPs) or other microbial proteases (33,(75)(76)(77)(78)(79) which has been implicated to degrade ECM and thereby causing remodeling and matrix stiffening during infection (33,63,73,80).…”

Macrophage TLR4 and PAR2 Signaling: Role in Regulating Vascular Inflammatory Injury and Repair

“…Originally, PAR2 was thought to be cleaved only by trypsin, tryptase, factor VIIa, factor Xa and elastase (28,29). However, recent studies show that thrombin can also cleave PAR2, albeit at higher concentrations (30,31). PAR3 is cleaved by thrombin only.…”

“…PAR2 couples to G α s, G α q, G α i, and G α 12/13 and triggers several signaling cascades to mediate its diverse cellular functions (31,123,124). The canonical activation of PAR2 by its proteases occurs after hydrolysis at the R 36 /S 37 position.…”

“…Inflammatory signaling induces the expression of tissue factor (TF) and elastase in leukocytes and monocytes, which facilitates activation of the coagulation pathway in part through the production of thrombin (31,177). TF is also constitutively expressed by cells segregated from blood, mostly epithelial cells and macrophages (115,178).…”