Abnormal protein arginine methylation triggers pathological changes in the lung. In the normal lung, methylarginines are generated via intracellular proteolysis and released to the intraalveolar, interstitial, and intravascular compartments of the lung. Pathological tissue injury, in particular alveolar epithelial cell injuries, leads to an increase of reactive oxygen species (ROS), growth factor production. This perpetuates epithelial cell damage and leads to increases of alveolar ADMA via increased proteolysis. Secondary pathological events of lung tissue injury include, but are not restricted to, fibroproliferation and deposition of extracellular matrix, as well as vasoconstriction and pulmonary artery smooth muscle cell (PASMC) proliferation. ATI; type I alveolar epithelial cell, ATII; type II alveolar epithelial cell type II, EC; endothelial cell.