Ism1 deficiency in mice exacerbates bleomycin-induced pulmonary fibrosis with enhanced cellular senescence and delayed fibrosis resolution

Idiopathic pulmonary fibrosis (IPF) is a progressive fibrosing interstitial lung disease with high morbidity and mortality but unclear etiology and incomplete pathophysiological understandings, making the discovery of effective therapeutics arduous. Currently, two drugs, Nintedanib and Pirfenidone, are available for IPF treatment which can slow down the fibrotic scarring in the lung but are unable to provide disease resolution. Hence, further revelation of the molecular mechanisms of IPF is critical for the development of novel therapeutics. Isthmin-1 (ISM1) is a secreted anti-inflammatory protein highly expressed in the mouse and human lung. Ism1-/- mice presented spontaneous and progressive lung emphysema, as well as heightened acute lung injury (ALI) upon lipopolysaccharide (LPS) treatment with an accompanied increase of post-LPS-ALI pulmonary fibrosis. ISM1 is important for lung homeostasis with airway-delivered recombinant ISM1 (rISM1) suppressing cigarette smoke-induced emphysema, LPS-ALI, and house-dust mites (HDM)-induced asthma-like symptoms in mice. However, the role of ISM1 in pulmonary fibrosis is yet to be clearly understood. In this work, we show that Ism1-/- mice presented heightened bleomycin-induced pulmonary fibrosis (BIPF), with enhanced immune cell infiltration, myofibroblast accumulation, and collagen deposition. ISM1 deficiency also led to increased cellular senescence in mouse lungs, isolated primary alveolar type II epithelial cells, and primary lung fibroblasts upon bleomycin treatment. Ism1-/- mice also showed delayed resolution of pulmonary fibrosis with reduced lipofibroblasts and downregulation of lipid synthesis-related genes. These results are in congruence with the RNA-seq data which demonstrated gene expression alterations in Ism1-/- lung that are linked to predisposition to pulmonary fibrosis and dysregulation of lipid metabolism pathways. Gene expression analyses showed that Ism1 is similarly dysregulated in the lungs of BIPF and human IPF patients. These findings reveal an anti-fibrotic role of ISM1 in mouse lungs and provide the foundation to further investigate possible therapeutic applications of ISM1 for pulmonary fibrosis in the future.