S1PR1 attenuates pulmonary fibrosis by inhibiting EndMT and improving endothelial barrier function

Idiopathic pulmonary fibrosis (IPF) is a chronic fatal disease of unknown etiology. Its pathological manifestations include excessive proliferation and activation of fibroblasts and deposition of extracellular matrix. Endothelial cell-mesenchymal transformation (EndMT), a novel mechanism that generates fibroblast during IPF, is responsible for fibroblast-like phenotypic changes and activation of fibroblasts into hypersecretory cells. However, the exact mechanism behind EndMT-derived fibroblasts and activation is uncertain. Here, we investigated the role of sphingosine 1-phosphate receptor 1 (S1PR1) in EndMT-driven pulmonary fibrosis. We treated C57BL/6 mice with bleomycin (BLM) in vivo and pulmonary microvascular endothelial cells with TGF-β1 in vitro. Western blot, flow cytometry, and immunofluorescence were used to detect the expression of S1PR1 in endothelial cells. To evaluate the effect of S1PR1 on EndMT and endothelial barrier and its role in lung fibrosis and related signaling pathways, S1PR1 agonist and antagonist were used in vitro and in vivo. Endothelial S1PR1 protein expression was downregulated in both in vitro and in vivo models of pulmonary fibrosis induced by TGF-β1 and BLM, respectively. Downregulation of S1PR1 resulted in EndMT, indicated by decreased expression of endothelial markers CD31 and VE-cadherin, increased expression of mesenchymal markers α-SMA and nuclear transcription factor Snail, and disruption of the endothelial barrier. Further mechanistic studies found that stimulation of S1PR1 inhibited TGF-β1-mediated activation of the Smad2/3 and RhoA/ROCK1 pathways. Moreover, stimulation of S1PR1 attenuated Smad2/3 and RhoA/ROCK1 pathway-mediated damage to endothelial barrier function. Endothelial S1PR1 provides protection against pulmonary fibrosis by inhibiting EndMT and attenuating endothelial barrier damage. Accordingly, S1PR1 may be a potential therapeutic target in progressive IPF.