Prostaglandin E2 inhibits profibrotic function of human pulmonary fibroblasts by disrupting Ca2+ signaling

We have shown that calcium (Ca²⁺) oscillations in human pulmonary fibroblasts (HPFs) contribute to profibrotic effects of transforming growth factor-β (TGF-β) and that disruption of these oscillations blunts features of pulmonary fibrosis. Prostaglandin E₂ (PGE₂) exerts antifibrotic effects in the lung, but the mechanisms for this action are not well defined. We thus sought to explore interactions between PGE₂ and the profibrotic agent TGF-β in pulmonary fibroblasts (PFs) isolated from patients with or without idiopathic pulmonary fibrosis (IPF). PGE₂ inhibited TGF-β-promoted [Ca²⁺] oscillations and prevented the activation of Akt and Ca²⁺/calmodulin-dependent protein kinase-II (CaMK-II) but did not prevent activation of Smad-2 or ERK. PGE₂ also eliminated TGF-β-stimulated expression of collagen A1, fibronectin, and α-smooth muscle actin and reduced stress fiber formation in the HPFs. RNA sequencing revealed that HPFs preferentially express EP₂ receptors relative to other prostanoid receptor subtypes: EP₂ expression is ~10-fold higher than that of EP₄ receptors; EP₁ and EP₃ receptors are barely detectable; and EP₂-receptor expression is ~3.5-fold lower in PFs from IPF patients than in normal HPFs. The inhibitory effects of PGE₂ on synthetic function and stress fiber formation were blocked by selective EP₂ or EP₄ antagonists and mimicked by selective EP₂ or EP₄ agonists, the phosphodiesterase inhibitor isobutylmethylxanthine and forskolin, all of which elevate cellular cAMP concentrations. We conclude that PGE₂, likely predominantly via EP₂ receptors, interferes with Ca²⁺ signaling, CaMK-II activation, and Akt activation in IPF-HPFs and HPFs treated with TGF-β. Moreover, a decreased expression of EP₂ receptors in pulmonary fibroblasts from IPF patients may contribute to the pathophysiology of this disease.