Diminished Prostaglandin E2 Contributes to the Apoptosis Paradox in Idiopathic Pulmonary Fibrosis

Rationale: Patients with idiopathic pulmonary fibrosis (IPF), a progressive disease with a dismal prognosis, exhibit an unexplained disparity of increased alveolar epithelial cell (AEC) apoptosis but reduced fibroblast apoptosis. Objectives: To examine whether the failure of patients with IPF to up-regulate cyclooxygenase (COX)-2, and thus the antifibrotic mediator prostaglandin (PG)E2, accounts for this imbalance. Methods: Fibroblasts and primary type II AECs were isolated from control and fibrotic human lung tissue. The effects of COX-2 inhibition and exogenous PGE2 on fibroblast and AEC sensitivity to Fas ligand (FasL)-induced apoptosis were assessed. Measurements and Main Results: IPF lung fibroblasts are resistant to FasL-induced apoptosis compared with control lung fibroblasts. Inhibition of COX-2 in control lung fibroblasts resulted in an apoptosis-resistant phenotype. Administration of PGE2 almost doubled the rate of FasL-induced apoptosis in fibrotic lung fibroblasts compared with FasL alone. Conversely, in primary fibrotic lung type II AECs, PGE2 protected against FasL-induced apoptosis. In human control and, to a greater extent, fibrotic lung fibroblasts, PGE2 inhibits the phosphorylation of Akt, suggesting that regulation of this prosurvival protein kinase is an important mechanism by which PGE2 modulates cellular apoptotic responses. Conclusions: The observation that PGE2 deficiency results in increased AEC but reduced fibroblast sensitivity to apoptosis provides a novel pathogenic insight into the mechanisms driving persistent fibroproliferation in IPF.