The F2-Isoprostane:Thromboxane-prostanoid receptor signaling axis drives persistent fibroblast activation in pulmonary fibrosis

Abstract While transient fibroblast activation is a normal and adaptive aspect of injury-repair in many contexts, persistent fibroblast activation is a hallmark of idiopathic pulmonary fibrosis (IPF) and other chronic fibrotic lung diseases. The mechanisms regulating persistent fibroblast activation in IPF have not been fully elucidated. In the lungs of IPF patients and in mice with experimental lung fibrosis, we observed that expression of the thromboxane-prostanoid receptor (TBXA2R) was increased in fibroblasts. Genetic deletion of TBXA2R, but not inhibition of thromboxane synthase, protected mice from bleomycin-induced lung fibrosis, suggesting an alternative ligand activates profibrotic TBXA2R signaling. We found that F2-isoprostanes (F2-IsoPs), nonezymatic products of arachidonic acid metabolism generated in the setting of reactive oxygen species (ROS), are persistently elevated in experimental lung fibrosis and act as an alternative TBXA2R ligand. Further studies demonstrated that F2-isoprostanes signal through TBXA2R to activate Smad signaling, revealing TBXA2R as a previously unrecognized regulator of the transforming growth factor beta (TGF-β) pathway. Further, treatment with the small-molecule TBXA2R antagonist ifetroban protected mice from lung fibrosis in three pre-clinical models: bleomycin treatment, Hermansky-Pudlak Syndrome, and radiation-induced fibrosis. Importantly, treatment with ifetroban during the fibrotic phase of bleomycin injury markedly enhanced fibrotic resolution. Together, these studies implicate TBXA2R signaling as a crucial mediator linking oxidative stress to fibroblast activation and indicate that TBXA2R antagonists could be efficacious for treatment of IPF and other chronic fibrotic disorders.