A hypoxia-induced vascular endothelial-to-mesenchymal transition in development of radiation-induced pulmonary fibrosis

Purpose: Radiation-induced pulmonary fibrosis (RIPF) is a late side effect of thoracic radiotherapy. The purpose of our study was to gain further insight into the development of RIPF. Experimental Design/Results: Here, we observed that irradiation of mouse lungs induced collagen deposition, particularly around blood vessels, in the early phase of RIPF. Such deposition subsequently became evident throughout the irradiated tissues. Accompanied by the collagen deposition, vascular EndMT (endothelialto-mesenchymal transition) began to develop in the early phase of RIPF, before the appearance of EMT (epithelial-tomesenchymal transition) of alveolar epithelial (AE) II cells in the substantive fibrotic phase. Concomitant with the EndMT, we detected vascular endothelial cell (EC)-specific hypoxic damage in the irradiated lung tissues. In human pulmonary artery endothelial cells (HPAEC), the radiation-induced EndMT via activation of TGFβ-R1/Smad signaling was dependent on HIF1α expression. A novel HIF1α inhibitor, 2-methoxyestradiol (2-ME), inhibited the irradiation-induced EndMT via downregulation of HIF1α dependent Smad signaling. In vivo, 2-ME inhibited the vascular EndMT, and decreased the collagen deposition associated with RIPF. Furthermore, HIF1α-related EndMT was observed also in human RIPF tissues. Conclusions: We provide the first evidence that an EndMT occurs in RIPF development and that the EndMT may be effectively inhibited by modulating vascular EC-specific hypoxic damage.