Idiopathic Pulmonary Fibrosis (IPF) is the most common form of interstitial lung disease with a 3-year median survival upon diagnosis. The lack of effective therapies in treating this disease highlights our incomplete understanding in the pathogenesis of IPF. The prevailing hypothesis is that IPF is a result of abnormal wound healing which consists of persistent injury and apoptosis to alveolar epithelial cells (AECs), aberrant fibroblast proliferation, and the accumulation of extracellular matrix proteins. Our laboratory has implicated a role of the angiotensin (ANG) system in these events. In IPF, both angiotensinogen (AGT), the only known precursor to angiotensin II (ANGII), and Transforming Growth Factor-Beta (TGF-β1) mRNA and protein are up-regulated, as well as the profibrotic peptide, ANGII. In human pulmonary fibroblasts, TGF-β1-inducible AGT transcription is mediated by the core promoter spanning from -46 to +22. At the -20, -18, and -6 positions lies single nucleotide polymorphisms (SNPs) that have been shown to influence its transcription rate in hepatocytes. Our results in human pulmonary fibroblasts parallel those observed in hepatocytes where the CA haplotype at -20 and -6 respectively, had about a 1.5-fold increase in AGT transcription compared to the AG haplotype (p = 0.011). The increase in AGT transcription would result in an increase in ANGII, which we predict to be associated with greater severity of IPF as measured by pulmonary function tests. Studies in IPF cohorts from the United States and Spain demonstrated that the CC genotype at -20 (p = 0.0028 for U.S. and p = 0.017 for Spain), the AA genotype at -6 (p = 0.021 for U.S.), and the CA haplotype (p = 0.0048 for U.S. and p = 0.014 for Spain) predicted lower diffusing capacity. Additionally, the Proline/Proline variant at codon 10 in TGF-β1 was also associated with lower diffusing capacity (p = 0.0014). Surprisingly, the results of both studies were only significant in males, reflecting the male bias of this disease. Preliminary data indicates that in addition to inducing AGT transcription, TGF-β1 also up-regulates cathepsin D and down-regulates ACE-2. Cathepsin D and AGT are both part of the rate-limiting step in the generation of ANGII whereas ACE-2 functions in its removal. This suggests that TGF-β1 may cause an imbalance in the ANG system by favoring the ANGII producing axis. The mechanism by which ACE-2 is down-regulated has not been well studied. However, results from our lab suggests that this down-regulation may be related to ACE-2 ectodomain shedding or through a JNK-mediated mechanism as seen with inducers of ER-stress and cell-cycling in AECs.
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