Mammalian Target of Rapamycin Inhibition With Rapamycin Mitigates Radiation-Induced Pulmonary Fibrosis in a Murine Model

Abstract

Radiation-induced pulmonary fibrosis (RIPF) is a late toxicity of therapeutic radiation. Signaling of the mammalian target of rapamycin drives several processes implicated in RIPF, including inflammatory cytokine production, fibroblast proliferation, and epithelial senescence. We sought to determine if mammalian target of rapamycin inhibition with rapamycin would mitigate RIPF. C57BL/6NCr mice received a diet formulated with rapamycin (14mg/kg food) or a control diet 2days before and continuing for 16weeks after exposure to 5 daily fractions of 6Gy of thoracic irradiation. Fibrosis was assessed with Masson trichrome staining and hydroxyproline assay. Cytokine expression was evaluated by quantitative real-time polymerase chain reaction.Senescence was assessed by staining for β-galactosidase activity. Administration of rapamycin extended the median survival of irradiated mice compared with the control diet from 116days to 156days (P=.006, log-rank test). Treatment with rapamycin reduced hydroxyproline content compared with the control diet (irradiation plus vehicle, 45.9±11.8μg per lung; irradiation plus rapamycin, 21.4±6.0μg per lung; P=.001) and reduced visible fibrotic foci.Rapamycin treatment attenuated interleukin 1β and transforming growth factor β induction in irradiated lungs compared with the control diet. Type II pneumocyte senescence after irradiation was reduced with rapamycin treatment at 16weeks (3-fold reduction at 16weeks, P<.001). Rapamycin protected against RIPF in a murine model. Rapamycin treatment reduced inflammatory cytokine expression, extracellular matrix production, and senescence in type II pneumocytes.