Identification of Proteins Involved in Airway Remodeling in Human Lung Fibroblasts

Asthma is an inflammatory disease associated with chronic perturbation of homeostatic mechanisms, leading to alterations in the structure of the airway walls, termed airway remodeling (AR). Lung fibrosis is characterized by increased deposition of extracellular matrix (ECM) proteins, especially collagens, and enhanced proliferation and activation of fibroblasts, and, ultimately, distortion of normal lung architecture and loss of respiratory function. L-arginine is a key precursor of nitric oxide and proline, an amino acid enriched in collagen. We hypothesized that L-arginine metabolism is altered in AR, ultimately affecting collagen synthesis. In this study we have analyzed the expression of key regulatory enzymes in the arginine pathway (Figure 1). Arginase, which converts L-arginine into L-ornithine and urea, is a key enzyme of the urea cycle in the liver, but it is also expressed in cells and tissues that lack a complete urea cycle, including the lung. Arginase exists as two distinct isoenzymes, arginase I and II, which are encoded by different genes. Arginase I is a cytosolic enzyme and is the predominant isoform in the liver, where it is highly expressed. Even though low levels of arginase II have been detected in liver as well, this mitochondrial enzyme is mainly expressed in extrahepatic tissue. Arginase isoenzymes and the downstream enzymes ornithine decarboxylase (ODC) and ornithine aminotransferase (OAT) are key regulators of critical processes associated with asthma including AR, cell hyperplasia and collagen deposition and have been implicated in the hyperplastic and fibrotic changes of AR, respectively. One of the biological functions of arginase II may be regulation of the synthesis of nitric oxide (NO) by means of competition with NO synthase (NOS) for the common substrate, L-arginine (Figure 1). In addition, extrahepatic arginase may be involved in tissue repair processes through the synthesis of L-ornithine. Thus, L-ornithine is a precursor of polyamines and L-proline, which are involved in cell proliferation and collagen synthesis, respectively. We investigated expressions of arginase I, arginase II, ODC, and OAT proteins by Western blot and cell immunochemistry in human lung fibroblast (HLF) from normal (NHLF) and diseased (DHLF) asthmatic donors