Fibrosis: How collagen becomes ‘stiff’
Oxidative stress following a lung injury can alter the structure of collagen, causing it to stiffen and trigger the formation of a fibrotic scar that further hardens the tissue.
- Department of Medicine, University of Pennsylvania, United States
When our bodies heal following an injury, fibroblasts and other cells deposit components of the extracellular matrix, such as collagen, that form a ‘scar’ over the damaged area. This process, known as fibrosis, leads to a build-up of stiff fibrotic tissue that interferes with the activity of the underlying organ.
In healthy organs, fibroblasts lay down just enough collagen to maintain the integrity of the tissue (D’Urso and Kurniawan, 2020). However, if the tissue begins to stiffen, these cells produce more collagen, disrupting this equilibrium (Tschumperlin et al., 2018). This increases the rigidity of the organ, triggering the fibroblasts to release even more collagen. While this explains how fibrosis progresses, it is less clear how the cycle starts.
Previous studies showed that fibrosis is initiated by changes in the mechanical properties of collagen that arise from modifications to its structure (Jones et al., 2018; Georges et al., 2007). The experiments found that, after a tissue injury, a family of enzymes called lysyl oxidases (or LOXs for short) increase the bonds between individual collagen fibers in the organ. This modified structure makes the tissue stiffer, driving the fibrotic process. Now, in eLife, Mark Jones from the University of Southampton and co-workers – including Christopher Brereton and Liudi Yao as joint first authors – report on the pathway that leads to this altered collagen architecture in patients with pulmonary fibrosis (Brereton et al., 2022).
First, the team (who are based at various institutes in the United Kingdom, the United States, Austria, Italy and Ireland) studied lung tissue from individuals with idiopathic pulmonary fibrosis, a disease of unknown cause in which patient’s lungs are severely scarred (Richeldi et al., 2017). They found that the genes coding for two enzymes that sequentially modify collagen (Yamauchi and Sricholpech, 2012) – lysyl hydroxylase PLOD2 and LOXL2 – were highly expressed in the same lung cells at the same time. The gene for the most commonly found type of collagen in fibrosis (called type I) was also activated in the lung tissue, but its pattern of expression did not correlate with the genes for PLOD2 or LOXL2. This suggests that it is regulated independently from these two enzymes.
Next, Brereton, Yao et al. extracted fibroblasts from patients with idiopathic pulmonary fibrosis and cultured them in the laboratory for six weeks, treating the cells with several growth factors associated with fibrosis. The experiments showed that a family of proteins called Hypoxia Inducible Factors (HIFs), which regulate the body’s response to varying oxygen levels, activated the genes for PLOD2 and LOXL2. This increased the number of cross-links between the collagen fibers and made the fibers stiffer. Imaging the collagen produced by these cells with an electron microscope revealed that the fibers had a smaller diameter, as had previously been observed in patients with idiopathic pulmonary fibrosis (Jones et al., 2018). This suggests that HIFs regulate tissue mechanics not by increasing the amount of collagen, but by altering collagen’s structure.
HIF is negatively regulated by another protein called FIH (short for Factor Inhibiting HIF), which is inactivated by the unstable byproducts of oxygen metabolism, also known as oxidative stress (Lando et al., 2002). Loss of FIH causes cells to enter ‘pseudohypoxia’, a state in which cells behave as if oxygen levels are low despite being in normal conditions (Masson et al., 2012). Brereton, Yao et al. found that reducing FIH – both in cells cultured in the laboratory as well as in lung tissue from patients with idiopathic pulmonary fibrosis – led to higher levels of HIF, which increased the activity of PLOD2 and LOX2, causing changes to collagen’s architecture and fibrosis.
These findings suggest that the pseudohypoxia state induced by oxidative stress is central to the pathology of idiopathic pulmonary fibrosis, and may help to explain how the fibrosis cycle starts (Figure 1): injuring or damaging the lung leads to a rise in oxidative stress, which dampens the level of FIH. This allows HIF to activate the genes for the enzymes PLOD2 and LOXL2, which then sequentially alter the structure of collagen. These changes increase the stiffness of the tissue, initiating the fibrosis cycle.
Figure 1
The pathway to fibrosis.
When cells in the lung become injured or damaged (left), the resulting increase in oxidative stress leads to a decrease in the expression of the genes that code for a protein called FIH (purple triangle). This results in a pseudohypoxia state which increases the activity of the protein HIF (pink circle), allowing it to upregulate the expression of the genes that code for two enzymes: PLOD2 and LOXL2. These enzymes act in sequence to increase and strengthen the bonds, or ‘cross-links’, between fibers of collagen. The altered structure stiffens the tissue, triggering cells in the lung to deposit more collagen and start the process of fibrosis. HIF: Hypoxia Inducible Factor; FIH: Factor Inhibiting HIF.
Image credit: Iris de Jong (CC BY 4.0).
This work provides new insights in to how fibrosis is triggered. Particularly intriguing is the suggestion that intervening at the level FIH or HIF may be better at treating idiopathic pulmonary fibrosis than targeting the machinery that synthesizes collagen, which is currently considered to be the most effective approach. Further work is needed to fully understand how the architecture of collagen becomes dysregulated and to test how substances that block this newly discovered pathway impact fibrosis.
References
-
Mechanical and physical regulation of fibroblast-myofibroblast transition: from cellular mechanoresponse to tissue pathologyFrontiers in Bioengineering and Biotechnology 8:609653.https://doi.org/10.3389/fbioe.2020.609653
-
Increased stiffness of the rat liver precedes matrix deposition: implications for fibrosisAmerican Journal of Physiology. Gastrointestinal and Liver Physiology 293:G1147–G1154.https://doi.org/10.1152/ajpgi.00032.2007
-
Mechanosensing and fibrosisThe Journal of Clinical Investigation 128:74–84.https://doi.org/10.1172/JCI93561
-
Lysine post-translational modifications of collagenEssays in Biochemistry 52:113–133.https://doi.org/10.1042/bse0520113
Article and author information
Author details
Publication history
Copyright
© 2022, Wells
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 7,576
- views
-
- 414
- downloads
-
- 15
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
A two-part list of links to download the article, or parts of the article, in various formats.
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Fibrosis: How collagen becomes ‘stiff’
eLife 11:e77041.
https://doi.org/10.7554/eLife.77041
Further reading
-
- Cell Biology
Dynamic interactions between gut mucosal cells and the external environment are essential to maintain gut homeostasis. Enterochromaffin (EC) cells transduce both chemical and mechanical signals and produce 5-hydroxytryptamine to mediate disparate physiological responses. However, the molecular and cellular basis for functional diversity of ECs remains to be adequately defined. Here, we integrated single-cell transcriptomics with spatial image analysis to identify 14 EC clusters that are topographically organized along the gut. Subtypes predicted to be sensitive to the chemical environment and mechanical forces were identified that express distinct transcription factors and hormones. A Piezo2+ population in the distal colon was endowed with a distinctive neuronal signature. Using a combination of genetic, chemogenetic, and pharmacological approaches, we demonstrated Piezo2+ ECs are required for normal colon motility. Our study constructs a molecular map for ECs and offers a framework for deconvoluting EC cells with pleiotropic functions.
-
- Cell Biology
Niemann–Pick disease type C (NPC) is a devastating lysosomal storage disease characterized by abnormal cholesterol accumulation in lysosomes. Currently, there is no treatment for NPC. Transcription factor EB (TFEB), a member of the microphthalmia transcription factors (MiTF), has emerged as a master regulator of lysosomal function and promoted the clearance of substrates stored in cells. However, it is not known whether TFEB plays a role in cholesterol clearance in NPC disease. Here, we show that transgenic overexpression of TFEB, but not TFE3 (another member of MiTF family) facilitates cholesterol clearance in various NPC1 cell models. Pharmacological activation of TFEB by sulforaphane (SFN), a previously identified natural small-molecule TFEB agonist by us, can dramatically ameliorate cholesterol accumulation in human and mouse NPC1 cell models. In NPC1 cells, SFN induces TFEB nuclear translocation via a ROS-Ca2+-calcineurin-dependent but MTOR-independent pathway and upregulates the expression of TFEB-downstream genes, promoting lysosomal exocytosis and biogenesis. While genetic inhibition of TFEB abolishes the cholesterol clearance and exocytosis effect by SFN. In the NPC1 mouse model, SFN dephosphorylates/activates TFEB in the brain and exhibits potent efficacy of rescuing the loss of Purkinje cells and body weight. Hence, pharmacological upregulating lysosome machinery via targeting TFEB represents a promising approach to treat NPC and related lysosomal storage diseases, and provides the possibility of TFEB agonists, that is, SFN as potential NPC therapeutic candidates.
