An Essential Role for Fibronectin Extra Type III Domain A in Pulmonary Fibrosis

Abstract

Rationale: Tissue fibrosis is considered a dysregulated wound-healing response. Fibronectin containing extra type III domain A (EDA) is implicated in the regulation of wound healing. EDA-containing fibronectin is deposited during wound repair, and its presence precedes that of collagen. Objectives: To investigate the role of EDA-containing fibronectin in lung fibrogenesis. Methods: Primary lung fibroblasts from patients with idiopathic pulmonary fibrosis or from patients undergoing resection for lung cancer were assessed for EDA-containing fibronectin and a-smooth muscle actin (a-SMA) expression. Mice lacking the EDA domain of fibronectin and their wild-type littermates were challenged with the bleomycin model of lung fibrosis. Primary lung fibroblasts from these mice were assayed in vitro to determine the contribution of EDA-containing fibronectin to fibroblast phenotypes. Measurements and Main Results: Idiopathic pulmonary fibrosis lung fibroblasts produced markedly more EDA-containing fibronectin and a-SMA than control fibroblasts. EDA-null mice failed to develop significant fibrosis 21 days after bleomycin challenge, whereas wildtype controls developed the expected increase in total lung collagen. Histologic analysis of EDA-null lungs after bleomycin showed less collagen and fewer a-SMA-expressing myofibroblasts compared with that observed in wild-type mice. Failure to develop lung fibrosis in EDA-null mice correlated with diminished activation of latent transforming growth factor (TGF)-b and decreased lung fibroblast responsiveness to active TGF-b in vitro. Conclusions: The data show that EDA-containing fibronectin is essential for the fibrotic resolution of lung injury through TGF-b activation and responsiveness, and suggest that EDA-containing fibronectin plays a critical role in tissue fibrogenesis.Keywords: fibrosis; fibronectin; TGF-b; myofibroblast Fibroproliferative disorders, characterized by the increased production and deposition of extracellular matrix (ECM) proteins in tissues, are not well understood despite major efforts to elucidate pathogenic mechanisms. Recent attention has focused on ECM components and mesenchymal cell phenotypes as being critical to the development of fibrosis (1, 2). The ECM is a highly dynamic complex that varies in composition according to its tissue localization and physiologic circumstances. Collagens are the predominant ECM proteins identified in fibrotic lesions and are the hallmark of fibroproliferative diseases, but fibronectins (FNs) are also present in abnormally large quantities and localize to areas of active fibrogenesis (3, 4).FNs are multifunctional glycoproteins found in the ECM of tissues and plasma. Two main forms of FN exist: plasma FN, a dimeric and soluble form produced by hepatocytes that lacks the EDA and EDB sequences, and cellular FN (cFN), a multimeric form synthesized by mesenchymal, epithelial, and inflammatory cells, which is deposited in ECM fibrils and that contains variable proportions of the extra type III domains A and B (EDA and EDB) (5...

Telomere Shortening in Familial and Sporadic Pulmonary Fibrosis

“…This same mutation has been reported in a 34-year-old man with moderate aplastic anemia that did not respond to immunosuppression and has short telomere lengths as measured by flow-fluorescence in situ hybridization (8). The development of pulmonary fibrosis versus bone marrow dysfunction in telomerase mutation carriers may be related to secondary ''hits,'' such as environmental toxins (cigarette smoking), fibrosis-prone intrinsic host mesenchymal responses to injury (34,35), or other susceptibilities. Understanding the influences on the development of lung disease in telomerase mutation carriers will be important to delineate.…”

Matrix, Mesenchyme, and Mechanotransduction

“…Mechanistically, the authors identified a critical role for matrix-dependent suppression of miR-29 in engaging increased translation of matrix proteins when fibroblasts were seeded on IPF-derived matrices (5), elucidating a mechanism by which fibrotic matrix programs cells to produce greater amounts of matrix (Figure 1). Delineating the profibrotic cues that exist within pathological matrices will require intensive efforts, but several matrix or matrixassociated protein candidates have been identified that confer profibrotic signals and correlate with fibrotic pathologies (2), including fibulin-1 (10), osteopontin (11,12), periostin (13,14), connective tissue growth factor (15), and fibronectin (16,17).…”

Regulation of Matrix Assembly through Rigidity-dependent Fibronectin Conformational Changes

“…The protein fibronectin (FN) is a major component of the ECM, and an excessive and disordered FN matrix is present in fibrotic diseases (15) and hypertrophic scars (16). FN matrix assembly is mediated by integrin receptors and regulated by intracellular signals, cytoskeletal organization, and availability of FN (17).…”