Smad3 Null Mice Develop Airspace Enlargement and Are Resistant to TGF-β-Mediated Pulmonary Fibrosis

Abstract

Transforming growth factor-β1 plays a key role in the pathogenesis of pulmonary fibrosis, mediating extracellular matrix (ECM) gene expression through a series of intracellular signaling molecules, including Smad2 and Smad3. We show that Smad3 null mice (knockout (KO)) develop progressive age-related increases in the size of alveolar spaces, associated with high spontaneous presence of matrix metalloproteinases (MMP-9 and MMP-12) in the lung. Moreover, transient overexpression of active TGF-β1 in lungs, using adenoviral vector-mediated gene transfer, resulted in progressive pulmonary fibrosis in wild-type mice, whereas no fibrosis was seen in the lungs of Smad3 KO mice up to 28 days. Significantly higher levels of matrix components (procollagen 3A1, connective tissue growth factor) and antiproteinases (plasminogen activator inhibitor-1, tissue inhibitor of metalloproteinase-1) were detected in wild-type lungs 4 days after TGF-β1 administration, while no such changes were seen in KO lungs. These data suggest a pivotal role of the Smad3 pathway in ECM metabolism. Basal activity of the pathway is required to maintain alveolar integrity and ECM homeostasis, but excessive signaling through the pathway results in fibrosis characterized by inhibited degradation and enhanced ECM deposition. The Smad3 pathway is involved in pathogenic mechanisms mediating tissue destruction (lack of repair) and fibrogenesis (excessive repair).

Progressive pulmonary fibrosis is mediated by TGF-β isoform 1 but not TGF-β3

“…At the level of gene expression, we noticed that the duration of TIMP-1 upregulation following AdTGF-β3 was transient and returned to baseline by day 14, while this gene was persistently induced in AdTGF-β1 treated rats. These findings strongly support previous reports showing that transient fibrosis in the lungs following CTGF overexpression or using bleomycin or AdTGF-β1 in fibrosis resistant mice were accompanied by low levels of TIMP-1 gene induction (Bonniaud et al, 2004;Kolb et al, 2002).…”

Combinatorial activation of FAK and AKT by transforming growth factor-β1 confers an anoikis-resistant phenotype to myofibroblasts

“…Although studies by our group [12] and others [36] have shown the capacity of TGF-β1 to activate FAK in certain cell types, a role for SMAD3 in mediating this effect has not been previously demonstrated. The requirement for SMAD3 in FAK activation is consistent with a role for SMAD3 in myofibroblast differentiation both in-vitro [37,38] and in-vivo [39][40][41].…”

Fibroblast growth factor-2 mediates transforming growth factor-β action in prostate cancer reactive stroma

“…Data TGF-b signaling in prostate cancer reactive stroma F Yang et al presented here are consistent with the implication of Smad3 in mediating wound healing and fibrosis. As might be predicted, Smad3 null mice were refractory to TGF-b1-induced pulmonary fibrosis (Bonniaud et al, 2004). However, the function of Smad3 in mediating TGF-b-induced responses in cancer-associated reactive stroma biology is poorly understood.…”