TM9SF1 Promotes YAP‐Mediated EMT of Alveolar Epithelial Cells in VILI by Enhancing Autophagic Degradation of CLDN18

ABSTRACT

Ventilator‐induced lung injury (VILI) is a severe complication of mechanical ventilation, in which alveolar epithelial–mesenchymal transition (EMT) plays a critical role. However, the upstream regulatory mechanisms remain unclear. The mouse VILI model was established through high tidal volume mechanical ventilation, and the VILI in vitro model was constructed through cell stretching experiments. Lung injury was assessed by H&E staining, wet/dry ratio, TUNEL assay, ELISA, and Masson's trichrome staining. EMT markers, CLDN18, YAP and TM9SF1 were detected by western blot, immunofluorescence, Co‐IP and qRT‐PCR. Autophagy inhibitors (3‐MA, and CQ) and proteasome inhibitor (MG132) were used to determine the degradation pathway. VILI mice showed increased lung injury, inflammation, fibrosis, and EMT, accompanied by decreased CLDN18 expression. CLDN18 overexpression attenuated VILI and EMT, and suppressed YAP activation, while YAP overexpression reversed the protective effect of CLDN18. TM9SF1 was upregulated in VILI models, and it interacted with CLDN18. Overexpression of TM9SF1 accelerated CLDN18 degradation via autophagy, as evidenced by the inhibitory effects of 3‐MA and CQ, whereas MG132 had no such effect. Furthermore, TM9SF1 knockdown increased CLDN18 expression and suppressed YAP activation and EMT, which was reversed by simultaneous CLDN18 knockdown in vitro. In conclusion, TM9SF1 promotes alveolar epithelial EMT in VILI by mediating the autophagic degradation of CLDN18, thereby activating YAP signaling. The TM9SF1‐CLDN18‐YAP axis represents a potential therapeutic target for VILI.