Mannan-Binding Lectin Reduces Epithelial-Mesenchymal Transition in Pulmonary Fibrosis via Inactivating the Store-Operated Calcium Entry Machinery

Idiopathic pulmonary fibrosis (IPF) is a type of idiopathic interstitial pneumonia with a poor clinical prognosis. Increasing evidence has demonstrated that epithelial-mesenchymal transition (EMT) contributes to the production of pathogenic myofibroblasts and plays a pivotal role in the development of pulmonary fibrosis. Mannan-binding lectin (MBL) is a soluble calcium-dependent complement molecule. Several studies have reported associations between serum MBL levels and lung diseases; however, the effect of MBL on IPF remains unknown. The present study observed aggravated pulmonary fibrosis in bleomycin-treated MBL−/− mice compared with their wild-type counterparts. Lung tissues from bleomycin-treated MBL−/− mice displayed a more severe EMT phenotype. In vitro studies determined that MBL inhibited the EMT process through attenuating store-operated calcium entry (SOCE) signaling. It was further demonstrated that MBL promoted the ubiquitination of Orai1, an essential component of SOCE, via pyruvate dehydrogenase kinase 1 (PDK1)-serum glucocorticoid-regulated kinase 1 signaling. PDK1 inhibition abolished the MBL-mediated regulation of SOCE activity and the EMT process. Notably, biochemical analysis showed that MBL interacted with PDK1 and contributed to PDK1 ubiquitination. In summary, the present findings suggested that MBL limited the EMT phenotype in human alveolar epithelial cells through regulation of SOCE, and MBL could be recognized as a potential therapeutic target for IPF.