New research has discovered how senescent cells and their secretion of IL-33 can encourage immunosuppression and tumour expansion in obesity-linked liver cancer.
A recent study at the Department of Applied Biological Science in Tokyo University of Science, investigated a pathway in senescent hepatic cells (HSCs), potentially responsible for the development of obesity associated hepatocellular carcinoma (HCC). This exciting mechanistic link provides a foundation for future research, treatment and prevention against HCC.
The study, published in Science Immunology, looked at the relationship between a specific inflammatory cytokine, IL-33, and its immunosuppressive effects in the liver tumour microenvironment.
The Hayflick limit describes the natural event of cellular inactivity upon reaching approximately 50 doublings. However, this seemingly innocent phenomenon, known as cellular senescence, might just play a key role in the development of liver cancer.
Characterised by their inactivity and cell cycle arrest, senescent cells are not to be mistaken for ‘resting’ cells. They secrete a variety of biological molecules, termed the senescence-associated secretory phenotype (SASP).
The team used HCC associated mouse models and microarray analysis. The research sampled mice liver tumour tissues and identified IL-33 as a key inflammatory cytokine important in HCC development. Quantitative PCR confirmed the inflammatory cytokine’s expression in tumour tissue.
Next, the researchers found that CELA1, an IL-33 processing protease, was upregulated in HCC associated mice. In-vitro experiments demonstrated IL-33’s cleavage mechanisms by CELA1 where the protease shortened and activated the cytokine.
Acids causing SASP production
A “leaky gut” describes intestinal permeability and dysbiosis due to high-fat diets. It is therefore no surprise that the gut-microbiome and gut liver axis serve as key sources of SASP triggering compounds.
The study confirmed this by observing an upregulation in IL-33 upon treating HSCs with deoxycholic acid (DCA), a circulating secondary bile acid produced by our gut microbiota, and lipoteichoic acid (LTA), a cell wall component of gram-positive bacteria.
Gasdermin D pore formation
Using isolated HSCs from high-fat diet fed mice, the researchers investigated Gasdermin D pore formation on IL-33 release. They observed a blockage in cleaved IL-33 release in a Gasdermin D knockout, which indicated its role in facilitating IL-33 secretion.
Next, the team identified the key enzymes responsible for Gasdermin D formation. Through shRNA gene silencing, caspase 11 was knocked down and the Gasdermin D N-terminal cluster was consequently diminished, showing that caspase 11 role helps process Gasdermin D, and subsequently releases activated IL-33.
The effect of IL-33 on Treg cells
The team investigated immunosuppressive effects of IL-33 on surrounding cells and found that an abundance of T regulatory cells promoted HCC through CD8 T cell exhaustion.
Flow cytometry analysis found that ST2+ T regulatory cells and total T regulatory cells were most abundant in the liver of high-fat fed mice. This highlighted ST2+ T regulatory cells’ relationship to obesity associated HCC.
The researchers also injected IL-33 in-vivo to observe the effects on ST2 positive T regulatory cells. Here, IL-33 administration resulted in the significant enhancement of ST2+ T regulatory cells.
The authors reported, “Once IL-33 entered the tumour microenvironment, it bound to a receptor highly expressed on liver regulatory T (Treg) cells, supercharging Treg cells’ immunosuppressive functions.”
To investigate the effects of Gasdermin D inhibition on SASP release, high-fat diet fed mice were treated with Disulfiram. The team found that upon Disulfiram treatment, tumour formation was supressed via the effective inhibition of IL-33 secretion.
Overall, this study identified the IL-33 release mechanism from senescent cells. However, further studies are needed to conclude similar molecular mechanisms in the case of humans.
The authors write, “Our findings highlight the therapeutic potential of inhibitors of gasdermin D-mediated pore formation for HCC treatment.”
Written by Harry Yuen, Science Writer Intern
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