Featured article: Treating Fibrosis may not just be a DREAM anymore!

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Autoimmune diseases cover a wide range of disorders that involve a dysregulated immune system that wreaks havoc within our bodies. Systemic sclerosis (SSc), one of the rarer autoimmune diseases, starts with excessive collagen production (fibrosis) on the skin but soon spreads to other internal organs like the kidneys, lungs, and heart. The excessive collagen synthesized during SSc tightens and scars the skin. As the disease progresses, it increases the risk of organ (especially kidney) failures, causing death in patients suffering from SSc. Women are more prone to suffer from SSc than men. Reports have also shown that it is more prevalent in African-American populations.

Although the cause of SSc is still unclear, genetic and environmental factors have been implicated. Activated immune cells ranging from adaptive (T and B cells) and innate (monocytes, macrophages & dendritic cells) characterize SSc in its early stages wherein they produce inflammatory cytokines that activate fibroblasts. Fibroblasts in turn transdifferentiate into myofibroblasts and produce an excessive extracellular matrix (ECM). The overproduction of ECM can have grave consequences on internal organs during SSc progression since it makes them stiff and unable to carry out their function efficiently.

Studies have shown that patients suffering from SSc harbor multiple single nucleotide polymorphisms (SNPs) on a gene locus called TNFAIP3. This locus codes for a deubiquitinating enzyme called A20. One of the most important functions of A20 is the inhibition of the NFkB signaling pathway – a major inflammatory pathway in immune cells. A20 is a rather infamous enzyme – having been implicated in various autoimmune disorders including rheumatoid arthritis. Dr. Varga, in an interview with Antibuddies, explained that the original name for A20 was “tumor necrosis factor (TNF) activated inducible protein” as TNF is the key signal to turn on A20. At the same time, A20 is the brake for TNF and when it is dysregulated you get inflammation and in some cases fibrosis. Hence, it is pivotal to understand the regulation of A20. Wang and colleagues, with the assistance of Dr. Varga, set out to explore the role of A20 in SSc, especially in the context of fibrosis. It is currently thought that A20 is not a druggable target due to its ubiquitous role in preventing inflammation, but the team may have found a novel way of inducing the expression of A20 in a tissue-specific manner that can prevent or reverse fibrosis only in that tissue. 

The researchers found lowered A20 transcript and protein expression in SSc patient-derived skin and lung fibroblasts. This reduction in A20 was accompanied by an increase in profibrotic proteins like fibronectin and collagens. Mice deficient in A20 had severe organ inflammation to the extent that they could not survive, but haploinsufficient mice (A20 knocked out on only one allele) were relatively stable and mimicked the human genotype better. Dr. Varga added that these mice, with the tissue-specific knockouts, took a very long time to generate and this project took around 4-5 years due to the slow process of generating these mice. 

Injecting bleomycin into mice is one of the most common models of studying fibrosis. When WT and haploinsufficient mice were treated with bleomycin, skin fibrosis was significantly worsened in the mice lacking normal A20 levels, accompanied by an increased number of activated fibroblasts and myofibroblasts. A similarly accentuated profibrotic response along with an influx of inflammatory immune cells was observed in the lung of bleomycin-treated haploinsufficient mice. Lastly, when A20 was specifically knocked out from mice fibroblasts, fibrosis in the skin and lungs was significantly exacerbated. This solidified the hypothesis that A20 acts as a negative regulator of fibrosis in SSc and plays a key role in fibroblast-driven pathogenesis. 

The same researchers previously determined a protein called DREAM (Downstream Regulatory Element Antagonist Modulator) that acts as a transcriptional repressor for A20. As the DRE in DREAM suggests, DREAM binds to the A20 DRE and downregulates its expression. They showed high DREAM expression in SSc skin fibroblasts and it correlated negatively with A20 expression. Mice lacking DREAM had high expression of A20, and subsequently, much lower fibrosis and inflammatory response on bleomycin treatment. Based on their results, the authors suggest that the augmentation of endogenous A20 could be a potential novel therapeutic for fibrosis. Reconfirming their previous work, they showed that adiponectin can stimulate cellular A20 expression in vitro and in vivo. A 4-week treatment targeting adiponectin receptors was able to upregulate A20 in the skin and reverse bleomycin-induced fibrosis.

This study provided a novel mechanistic link between A20 and DREAM in SSc that can regulate fibrosis. Through this link, a potential pharmacological therapy is offered by stimulating A20 expression by targeting adiponectin receptors. This therapy has the potential to help thousands of people experiencing SSc and other fibrosing conditions. 

According to Dr. Varga, these results could have far-reaching applications since A20 is involved in many such chronic and acute inflammatory conditions. The next steps for this team of researchers are to look further into the mechanism of action including how A20 is regulated and why it might be dysregulated in SSc and look even further into the DREAM factor of SSc regulation. They would also like to validate these results in multiple systems. Another question that came up during this investigation was whether A20 levels can be restored in SSc patients by upregulating adiponectin. Hopefully, the answers will continue to help people suffering and will inspire more scientists to continue searching for such therapies.

P.S. Dr. John Varga’s lab is currently looking for postdocs if any readers are particularly interested in this research. 

Sources:

Wang, W., Bale, S., Wei, J. et al. Fibroblast A20 governs fibrosis susceptibility and its repression by DREAM promotes fibrosis in multiple organs. Nat Commun 13, 6358 (2022).

Wei, J. et al. Wnt/beta-catenin signaling is hyperactivated in systemic sclerosis and induces Smad-dependent fibrotic responses in mesenchymal cells. Arthritis Rheum. 64, 2734–2745 (2012).

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