Biomechanical Force and Cellular Stiffness in Lung Fibrosis

Abstract

Lung fibrosis is characterized by the continuous accumulation of extracellular matrix (ECM) proteins produced by apoptosis-resistant (myo)fibroblasts. Lung epithelial injury promotes the recruitment and activation of fibroblasts, which are necessary for tissue repair and restoration of homeostasis. However, under pathologic conditions, a vicious cycle generated by profibrotic growth factors/cytokines, multicellular interactions, and matrix-associated signaling propagates the wound repair response and promotes lung fibrosis characterized not only by increased quantities of ECM proteins but also by changes in the biomechanical properties of the matrix. Importantly, changes in the biochemical and biomechanical properties of the matrix itself can serve to perpetuate fibroblast activity and propagate fibrosis, even in the absence of the initial stimulus of injury. The development of novel experimental models and methods increasingly facilitates our ability to interrogate fibrotic processes at the cellular and molecular levels. The goal of this review is to discuss the impact of ECM conditions in the development of lung fibrosis and to introduce new approaches to more accurately model the in vivo fibrotic microenvironment. This article highlights the pathologic roles of ECM in terms of mechanical force and the cellular interactions while reviewing in vitro and ex vivo models of lung fibrosis. The improved understanding of the fundamental mechanisms that contribute to lung fibrosis holds promise for identification of new therapeutic targets and improved outcomes.

Figure 1

Histologic and ultrastructural images of fibrotic lung tissue in idiopathic pulmonary fibrosis. A: Hematoxylin and eosin stain showing honeycomb change with cystic airway dilation corresponding to distended bronchioles in scarred, fibrotic lung.B: Electron microscopy from idiopathic pulmonary fibrosis lung showing disruption of the alveolar–capillary barrier with fragments of the original alveolar basal lamina is displayed (arrows). Panel A reprinted from Visscher et al with permission of the American Thoracic Society. Panel B reprinted from Kuhn and McDonald with permission from The American Society for Investigative Pathology. Original magnification: ×40 (A); ×4000 (B).

Figure 2

Extracellular matrix (ECM) and fibroblast stiffness in lung fibrosis. Stiff ECM influences the cellular network of structural and signaling molecules while fibroblasts sense and respond to the extracellular environment. The dysregulated interplay causes the formation of scar tissues and the emergence of apoptosis-resistant fibroblasts, promoting lung fibrosis.

Figure 3

Mechanotransduction pathways engaged by stiff extracellular matrix (ECM). The external biomechanical forces promote the clustering of integrins to focal adhesions that integrate the extracellular inputs with the intracellular cytoskeleton. Focal adhesion kinase (FAK) can be activated by a stiff ECM as well as by transforming growth factor-β (TGF-β). Rho-associated protein kinase (ROCK), downstream of FAK, promotes actin polymerization, which facilitates the nuclear translocation of myocardin-related transcription factor A (MRTF-A) implicated in myofibroblast differentiation, survival, and fibrosis. Nuclear translocation of the mechanosensitive transcription factors Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) also increases transcription of profibrotic genes. Extracellular enzymes such as lysyl oxidase (LOX) or transglutaminase 2 (TG2) initiate covalent intramolecular and intermolecular cross-linking of collagen, increasing ECM stiffness. These changes collectively translate into profibrotic phenotypic changes in fibroblasts. TGF-βR1/2, transforming growth factor-β receptor 1 and 2.