rhMFGE8 Attenuates Endothelial-to-Mesenchymal Transition in Acute Lung Injury Induced by Lipopolysaccharide Through the BMP/Smad1/4/5/Snail Signalling Pathway

Background and Objectives: Acute lung injury (ALI) is an inflammatory response in the lung caused by bacteria, viruses, trauma and other factors that results in lack of alveolar surfactant and subsequently respiratory distress. Endothelial-to-mesenchymal transition (EndoMT) is closely related to ALI-induced pulmonary fibrosis development and progression. This study examined the effect of milk fat globulin factor 8 (MFGE8) knockdown and overexpression on lipopolysaccharide (LPS)-induced EndoMT. Methods: Total MFGE8 activity in patients with acute respiratory distress syndrome (ARDS) and healthy volunteers was assessed using a colorimetric kit. In vitro, cell morphology was observed by microscopy, and invasion and migration were tested by Transwell and scratch assays. Specific siRNAs were transfected into HLMECs to downregulate MFGE8, and CD31 and α-SMA protein expression was detected by fluorescence microscopy and FCM. MFGE8, BMP, Smad 1, Smad4, Smad5, Snail, CD31 and α-SMA protein and gene expression was assessed by western blotting (WB) and qRT‒PCR. In vivo, the changes in cell morphology and alveolar septum in the lung of mice (C57BL/6, aged 7-8 weeks, male) were observed by H&E staining. In addition, ECM deposition in lung tissue was detected by Masson staining. CD31 and α-SMA protein expression in lung tissue was measured by WB. Results: The serum level of MFGE8 was lower in the ARDS group than in the control group. MFGE8 was identified as a protective factor associated with hospital survival. In vitro, the treatment of human lung microvascular endothelial cells (HLMECs) with LPS for 96 h clearly changed the cells from the typical cobblestone shape of ECs to spindle-like fibroblasts. The scratch and Transwell assay results indicated that endothelial cell migration and invasion were enhanced in the LPS group compared with the control group. Fluorescence microscopy, FCM, WB and PCR showed that LPS greatly inhibited CD31 (endothelial marker) expression and increased α-SMA (mesenchymal marker) expression. These data showed that LPS could induce EndoMT in HLMECs. Notably, siRNA-treated HLMECs yielded the same results. The administration of rhMFGE8 to HLMECs in the LPS or LPS+siRNA group ameliorated the changes in cell morphology and decreased cell migration and invasion. rhMFGE8 attenuated the effect of LPS or LPS+siRNA on EndoMT induction by increasing CD31 and decreasing α-SMA protein and gene expression. Moreover, activation of BMP/Smad1/5-Smad4 signalling in response to LPS and Snail (related to EndoMT transcription factors) expression were increased by MFGE8 knockdown but inhibited by rhMFGE8. In vivo, H&E staining revealed a thickened alveolar septum in the LPS group, and the thickness increased over time; in contrast, rhMFGE8 reversed this effect. ECM deposition occurred early in ALI induced by LPS and increased over time, and the administration of rhMFGE8 reversed this effect. WB showed that LPS inhibited CD31 protein expression and increased α-SMA protein expression in lung tissue. Conclusions: rhMFGE8 exerts a protective effect early in LPS-induced EndoMT through BMP/Smad1/5/Smad4 signalling and could be a therapeutic target in ALI.