Ionizing Radiation Promotes Epithelial-to-Mesenchymal Transition in Lung Epithelial Cells by TGF-β-producing M2 Macrophages

Background/Aim: Ionizing radiation induces pulmonary fibrosis, which is a common dose-limiting complication in patients receiving radiotherapy. Fibrosis occurs through the accumulation of large amounts of ECM components, synthesized by myofibroblasts in damaged lung tissue. Epithelial cells serve as one of the cellular sources of myofibroblasts via the epithelial-to-mesenchymal transition (EMT) process. In this study, we investigated the role of TGF-beta-secreting M2 macrophages in association with ionizing radiation-induced EMT. Materials and Methods: The lung epithelial cell line MLE12, was irradiated and the expression of EMT markers and chemokines was examined. Moreover, the mouse lung macrophage MH-S cell line was cultured with conditioned media from irradiated MLE12 cells, to examine the effects of the secreted factors on the migration ability of macrophages. For the murine pulmonary fibrosis model, mice were locally irradiated and the levels of M1 or M2 macrophage-related markers and cytokines were measured in bronchoalvelolar lavage (BAL) fluid and lung tissue. Results: In MLE12 cells, irradiation directly induced expression of EMT-related markers and secretion of various chemokines, which lead to macrophage migration. Interestingly, the sub-population of macrophages recruited in the lung of mice after thoracic irradiation was M2 macrophages that expressed Arg-1 and CD206. M2 macrophages induced the MLE12 to undergo phenotypic conversion to form fibroblast-like cells, which leads to a down-regulation of epithelial markers and an up-regulation of new EMT-related markers. In thoracic irradiated mice, proinflammatory cytokines such as IL-1 beta, IL-4 and IL-10 were increased at 2 weeks, but returned to normal levels from 16 weeks or 24 weeks after irradiation. However, thoracic irradiation led to a rapid increase of TGF-beta and IGF-1 levels, which lasted up to 24 weeks. It was confirmed that M2 macrophages secreted the high levels of TGF-beta. Moreover, the elimination of TGF-beta from M2 macrophages attenuated mesenchymal transition of MLE12. Conclusion: TGF-beta-secreting M2 macrophages play an important regulatory role in mesenchymal transition of epithelial cells in the lung of irradiated mice, thus contributing to radiation-induced pulmonary fibrosis.