Enzyme driving lung scarring seen as potential new therapy target
Preclinical study finds blocking caspase-9 eased fibrosis in mice and cell models
Blocking caspase-9 — an enzyme involved in programmed cell death, or apoptosis — may halt or reverse tissue scarring in pulmonary fibrosis (PF), according to a new preclinical study.
Researchers found elevated levels of caspase-9 in lung tissue from people with PF and in a mouse model of the disease, particularly localized to diseased areas. When caspase-9 activity was blocked, signs of cell death (apoptosis) and lung fibrosis were reduced in the PF mouse model.
In addition, caspase-9 was found to promote lung fibrosis not only by triggering apoptosis but also by activating beta-catenin signaling.
“Our results provide significant insights into the contribution of Caspase-9 to fibrotic processes in the lung, highlighting its potential as a novel therapeutic target,” the researchers wrote.
The study, “Caspase-9 activates β-catenin signaling to promote pulmonary fibrosis,” was published in the Journal of Translational Medicine.
Pulmonary fibrosis symptoms and treatment options
PF is characterized by excessive scarring in the lungs, changes in the architecture of the alveoli — the tiny sacs in the lungs where oxygen passes into the bloodstream — and a decline in lung function.
Current antifibrotic therapies “can slow disease progression to some extent but are unable to reverse established fibrosis or significantly improve long-term outcomes,” the researchers wrote. “There is an urgent need to explore novel molecular targets and develop more effective therapeutic strategies to halt or even reverse the fibrotic process.”
Apoptosis is one way the body gets rid of unneeded or abnormal cells. In PF, however, excessive or abnormal apoptosis can contribute to scarring in the lungs, particularly affecting the epithelial cells that line the alveoli.
A key trigger of the apoptotic pathway is the enzyme caspase-9, whose levels and activation are increased in experimental models of kidney, liver, and heart fibrosis.
Data suggest “caspase-9 may not merely be a passive bystander during tissue injury, but rather a key [disease] mediator driving fibrotic remodeling, potentially through both apoptosis-dependent and -independent mechanisms,” the researchers wrote.
Still, the molecular mechanisms linking caspase-9 activity to lung fibrosis remain unclear.
Study finds higher caspase-9 levels in lung tissue from PF patients and mice
To learn more, scientists in China studied caspase-9’s role in lung fibrosis by examining lung tissue from 10 people with PF and 10 people without the condition, as well as lab-grown mouse alveolar epithelial cells and an induced mouse model of PF.
The researchers first found that levels of caspase-9 and its active form, called cleaved caspase-9, were elevated in lung tissue from people with PF relative to tissue from unaffected controls. Tissue staining showed both forms of the enzyme were most abundant in scarred, fibrotic areas of the lungs.
Additional tests confirmed the elevated caspase-9 level was driven by higher activity of the gene that encodes for the enzyme.
When lab-grown alveolar epithelial cells were treated with TGF-beta — a pro-fibrotic signaling protein — they showed more signs of cell death and fibrosis. Suppressing caspase-9 production in these cells significantly reversed these changes.
In addition, boosting caspase-9 production in these cells not only increased apoptosis but also promoted fibrosis.
“Caspase-9 contributes to both fibrotic and apoptotic responses in alveolar epithelial cells, and its [suppression] confers protective effects, highlighting its potential as a therapeutic target in pulmonary fibrosis,” the team wrote.
Similar to what was seen in human lung tissues, levels of caspase-9 and active caspase-9 were significantly higher in the fibrotic lungs of mice with induced PF.
Treating this mouse model with a molecule that blocks caspase-9 significantly reduced several markers of fibrosis and PF-related lung abnormalities. This was accompanied by significantly reduced levels of apoptotic markers and lower counts of apoptotic cells, along with lower levels of caspase-9/cleaved-caspase-9 in alveolar regions.
“Collectively, these data demonstrate that Caspase-9 [suppression] mitigates … pulmonary fibrosis and epithelial apoptosis in mice, supporting its potential as a therapeutic target,” the scientists wrote.
Caspase-9 found to activate beta-catenin pathway that drives fibrosis
The researchers next set out to identify the molecular mechanisms underlying caspase-9-induced fibrosis. In both human and mouse fibrotic lung tissue, caspase-9 and its active form, cleaved caspase-9, were found to directly interact with the beta-catenin protein.
Beta-catenin is involved in regulating cell-cell adhesion, as well as gene activity as part of a major signaling pathway. Abnormalities in the Wnt/beta-catenin signaling pathway have been implicated in human cancers.
The team found significantly elevated levels of beta-catenin in both human and mouse fibrotic lung tissues, as well as in TGF-beta-treated alveolar epithelial cells. Increasing caspase-9 production in these cells raised fibrosis markers, but suppressing beta-catenin eliminated that effect.
When beta-catenin was re-activated in cells where caspase-9-had been suppressed, fibrosis returned — “suggesting that Caspase-9 acts upstream of [beta-catenin] in the fibrotic pathway,” the researchers wrote.
“These findings provide a novel mechanistic insight into the [underlying mechanisms] of pulmonary fibrosis,” showing that “caspase-9 drives pulmonary fibrosis by promoting epithelial apoptosis and activating [beta-catenin] signaling,” the team concluded. “Targeting the Caspase-9/[beta-catenin] axis may offer a promising therapeutic strategy for PF.”
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