Matrix abnormalities in pulmonary fibrosis

Abstract

Idiopathic pulmonary fibrosis (IPF) is a devastating, progressive disease, marked by excessive scarring, which leads to increased tissue stiffness, loss in lung function and ultimately death. IPF is characterised by progressive fibroblast and myofibroblast proliferation, and extensive deposition of extracellular matrix (ECM). Myofibroblasts play a key role in ECM deposition. Transforming growth factor (TGF)-β1 is a major growth factor involved in myofibroblast differentiation, and the creation of a profibrotic microenvironment. There is a strong link between increased ECM stiffness and profibrotic changes in cell phenotype and differentiation. The activation of TGF-β1 in response to mechanical stress from a stiff ECM explains some of the influence of the tissue microenvironment on cell phenotype and function. Understanding the close relationship between cells and their surrounding microenvironment will ultimately facilitate better management strategies for IPF.

FIGURE 1

The dynamic interplay between the lung extracellular matrix (ECM) and resident cells, with transforming growth factor (TGF)-β1 at its centre. The active TGF-β1 protein in red remains trapped within the latency associated peptide (LAP) to form the small latent complex (SLC). The SLC is attached to the ECM via the latent TGF-β binding protein (LTBP). Integrins found on the surface of cells link the LAP with the cell's cytoskeleton to allow for the transduction of the mechanical signal within the cell. In addition, the integrin attaches to the LAP and serves as an anchor; upon mechanical stress the SLC is cleaved and releases the bioactive TGF-β1 homodimer. The active TGF-β1 protein is then able to bind to the TGF-β1 receptor located on neighbouring cells, i.e. fibroblasts/epithelial cells. The activation of this receptor results in the phosphorylation of SMAD2/3, causing its translocation to the nucleus where it promotes the transcription of ACTA2 (the mRNA for α-SMA), resulting in the differentiation of these cells into myofibroblasts. The cyclical relationship between TGF-β1 activation via myofibroblasts and the resulting myofibroblast differentiation can be visualised through the red arrows. Mechanical stress also causes contraction of F-actin via integrins, which results in the phosphorylation of focal adhesion kinase (FAK) to promote further actin polymerisation. FAK phosphorylation also activates Rho kinase (ROCK) via complexing with RhoA, which results in increased actomyosin contractility. This cyclical relationship of cell contractility promoting further stress-fibre polymerisation and contractility is outlined by the bright green arrows. In response to these mechanical signals, the mechanosensitive proteins YAP (yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif) are translocated into the nucleus of the cell to promote the transcription of profibrotic genes. Genes such as collagen and other ECM proteins are produced, promoting ECM deposition. This causes the cells' surroundings to become rigid and stiff, therefore further adding to the mechanical stress placed on the cells (indicated by the blue arrow). TG: transglutaminase; LOX/LOXL: lysyl oxidase/lysyl oxidase-like.