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Key Event Title
Increased, extracellular matrix deposition
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|Substance interaction with the lung cell membrane leading to lung fibrosis||KeyEvent||Cataia Ives (send email)||Under development: Not open for comment. Do not cite||EAGMST Under Review|
|ACE2 inhibition, liver fibrosis||KeyEvent||Evgeniia Kazymova (send email)||Under development: Not open for comment. Do not cite||Under Development|
|AHR activation leading to lung fibrosis via TGF-β dependent fibrosis tox path||KeyEvent||Allie Always (send email)||Under development: Not open for comment. Do not cite|
|AHR activation leading to lung fibrosis via IL-6 tox path||KeyEvent||Evgeniia Kazymova (send email)||Under development: Not open for comment. Do not cite|
Key Event Description
ECM is a macromolecular structure that provides physical support to tissues and is essential for organ function. The composition of ECM is tissue specific and consists mainly of fibrous proteins, glycoproteins, and proteoglycans. The ECM in lung is compartmentalised to basement membrane and the interstitial space. Fibroblasts found in the interstitial space are the main sources of ECM in lung (White, 2015). Altered composition of ECM is observed in several lung diseases of inflammatory origin in humans including chronic obstructive pulmonary disease, asthma and idiopathic lung fibrosis. The composition and architecture of the ECM determines 1) the open sites of attachment that are available to cells, 2) the mechanical properties of the ECM and 3) the mechanical loading (breathing) experienced by the cells. Thus, changes in the ECM composition during the exaggerated wound healing process determines if an organism commits to fibrotic process or completes the wound healing (Blaauboer et al., 2014).
Evidence for its perturbation in the context of pulmonary fibrosis:
In lung fibrosis, an exaggerated amount of ECM is distributed in the alveolar parenchyma in a non-heterogenous manner, leading to lower spirometry readings implying occlusion of alveolar regions and reduced gas exchange. Collagen 1 and Collagen III are suggested to be the main components of the ECM in the thickened alveolar septa in fibrosis with other constituents such as fibronectin, elastin and tenacin C (Zhang et al., 1994; Hinz, 2006; Kuhn & McDonald, 1991; Crabb et al., 2006; Bensadoun et al., 1996; Klingberg et al., 2012; McKleroy et al., 2013). It is suggested that ECM composition dramatically changes during the fibrotic process. The early fibrotic process is characterised by collagen III deposition and collagen 1 predominates the later stages of the fibrosis. Excessive collagen production by myofibroblasts is necessary for the development of fibrosis (scarred tissue), with established areas of scar formation containing almost exclusively Type I collagen (Bateman et al., 1981; McKleroy et al., 2013; Zhang et al., 1994). Studies have demonstrated that while total collagen increases in IPF, there is also a shift toward the less elastic type I collagen, which contributes to the stiffness of the scar tissue within the lung (Nimni, 1983; Rozin et al., 2005; McKleroy et al., 2013).
The fibrotic ECM contains characteristic accumulation of fibroblasts and myofibroblasts, which are the major contributors of ECM synthesised. The proliferation of fibroblasts and their differentiation into myofibroblasts is, in turn, guided by the composition and structure of the ECM. For example, studies have demonstrated that cytokines secreted in response to inflammation are capable of activating fibroblasts, and that these changes could cause alterations in the fibroblasts that lead to excessive proliferation and ECM deposition (Sivakumar et al., 2012; Wynn, 2011).
How It Is Measured or Detected
qRT-PCR, Immunosorbant assays, and immunohistochemistry:
The qRT-PCR, ELISA, and immunohistochemistry are routinely used to assess the levels of protein and mRNA levels. The various genes and proteins that are assessed include, collagen I, collagen III, elastin and tenacin C. Histological staining with stains such as Masson Trichrome, Picro-sirius red are used to identify the tissue/cellular distribution of collagen, which can be quantified using morphometric analysis both in vivo and in vitro. The assays are routinely used and are quantitative.
Sircol Collagen Assay for collagen quantification:
The Serius dye has been used for many decades to detect collagen in histology samples. The Serius Red F3BA selectively binds to collagen and the signal can be read at 540 nm (Chen & Raghunath, 2009; Nikota et al., 2017).
Hydroxyproline is a non-proteinogenic amino acid formed by the prolyl-4-hydroxylase. Hydroxyproline is only found in collagen and thus, it serves as a direct measure of the amount of collagen present in cells or tissues. Colorimetric methods are readily available and have been extensively used to quantify collagen using this assay (Chen & Raghunath, 2009; Nikota et al., 2017).
Ex vivo and in vitro models of ECM deposition:
No models currently exist which allow for in vitro assessment of ECM deposition. Using single, or co-cultures containing fibroblasts, the production of soluble ECM components can be assessed after exposure to a stressor of interest using either ELISA or qRT-PCR experiments as a proxy.
Domain of Applicability
1. Bateman, E., Turner-Warwick, M. and Adelmann-Grill, B. (1981). Immunohistochemical study of collagen types in human foetal lung and fibrotic lung disease. Thorax, 36(9), pp.645-653.
2. Bensadoun, E., Burke, A., Hogg, J. and Roberts, C. (1996). Proteoglycan deposition in pulmonary fibrosis. American Journal of Respiratory and Critical Care Medicine, 154(6), pp.1819-1828.
3. Blaauboer M et al. Extracellular matrix proteins: A positive feedback loop in lung fibrosis. Matrix Biology, 2014, 34, 170-178
4. Chen, C. and Raghunath, M. (2009). Focus on collagen: in vitro systems to study fibrogenesis and antifibrosis _ state of the art. Fibrogenesis & Tissue Repair, 2(1).
5. Crabb, R., Chau, E., Decoteau, D. and Hubel, A. (2006). Microstructural Characteristics of Extracellular Matrix Produced by Stromal Fibroblasts. Annals of Biomedical Engineering, 34(10), pp.1615-1627.
6. HINZ, B. (2006). Masters and servants of the force: The role of matrix adhesions in myofibroblast force perception and transmission. European Journal of Cell Biology, 85(3-4), pp.175-181.
7. Kuhn C, McDonald JA. The roles of the myofibroblast in idiopathic pulmonary fibrosis. Ultrastructural and immunohistochemical features of sites of active extracellular matrix synthesis. Am J Pathol. 1991;138(5):1257–1265.
8. Klingberg, F., Hinz, B. and White, E. (2012). The myofibroblast matrix: implications for tissue repair and fibrosis. The Journal of Pathology, 229(2), pp.298-309.
9. McKleroy, W., Lee, T. and Atabai, K. (2013). Always cleave up your mess: targeting collagen degradation to treat tissue fibrosis. American Journal of Physiology-Lung Cellular and Molecular Physiology, 304(11), pp.L709-L721.
10. Nikota, J., Banville, A., Goodwin, L., Wu, D., Williams, A., Yauk, C., Wallin, H., Vogel, U. and Halappanavar, S. (2017). Stat-6 signaling pathway and not Interleukin-1 mediates multi-walled carbon nanotube-induced lung fibrosis in mice: insights from an adverse outcome pathway framework. Particle and Fibre Toxicology, 14(1).
11. Nimni, M. (1983). Collagen: Structure, function, and metabolism in normal and fibrotic tissues. Seminars in Arthritis and Rheumatism, 13(1), pp.1-86.
12. Rozin, G., Gomes, M., Parra, E., Kairalla, R., de Carvalho, C. and Capelozzi, V. (2005). Collagen and elastic system in the remodelling process of major types of idiopathic interstitial pneumonias (IIP). Histopathology, 46(4), pp.413-421.
13. Sivakumar, P., Ntolios, P., Jenkins, G. and Laurent, G. (2012). Into the matrix. Current Opinion in Pulmonary Medicine, 18(5), pp.462-469.
14. White, E. (2015). Lung Extracellular Matrix and Fibroblast Function. Annals of the American Thoracic Society, 12(Supplement 1), pp.S30- S33.
15. Wynn, T. (2011). Integrating mechanisms of pulmonary fibrosis. The Journal of Experimental Medicine, 208(7), pp.1339-1350.
16. Zhang K, Rekhter MD, Gordon D, Phan SH. Myofibroblasts and their role in lung collagen gene expression during pulmonary fibrosis. A combined immunohistochemical and in situ hybridization study. Am J Pathol. 1994;145(1):114–125