Circadian Clock of Cells in Lung Appears Troubled in Pulmonary Fibrosis, Study Finds
Poor sleeping habits also appear to upset the lungs’ circadian clockwork, with people who regularly sleep few or excessive hours being more likely to have the disease, its researchers report. Treatments that target core clock proteins and pathways may be a way of addressing PF.
The study, “The circadian clock protein REVERBα inhibits pulmonary fibrosis development,” was published in the journal Proceedings of the National Academy of Sciences.
The circadian clock is a system that organizes the internal and external activities of the body around the 24-hour day. At the molecular level, it allows for regular, orchestrated fluctuations in the levels of specific proteins which, in turn, allow time-dependent signals to be sent between and within cells.
Ultimately, this regulation allows for time-dependent bodily functions — getting tired in the evening, for example, or getting hungry at certain times of the day.
In the lungs, these fluctuations are known to help regulate inflammation. But their role in disease is not well understood.
Researchers examined the lungs of mice with pulmonary fibrosis (PF).
They found significant fluctuations in the levels of certain proteins associated with the circadian clock in fibrotic regions of the mice’s lungs. That is, levels of these clock proteins rose to higher peaks and fell to deeper lows than in fibrotic regions than they did lung areas unaffected by fibrosis.
Further examination showed these changes were most pronounced in fibroblasts, a type of cell that helps make connective and scar tissue and, as such, plays a key role in fibrosis (scarring). This suggested that dysregulation of the circadian machinery in fibroblasts could be involved in the development of PF.
To test this, researchers induced PF in wild-type (normal) mice, and in mice engineered to lack functional REVERBα, a core clock protein. Compared to the wild-type mice, mice lacking functional REVERBα had a significantly greater fibrotic response (that is, more scarring in their lungs).
Importantly, this difference was only found when REVERBα was lacking in fibroblasts; no differences were seen when it was selectively deleted from other types of cells in the lung.
Subsequent experiments with cells found that those without functional REVERBα were more active and produced more integrinβ1, a protein involved in fibrosis. Because REVERBα is a core circadian clock protein, this outcome further supported a role for a poorly regulated clock system in fibrotic fibroblasts.
Results like those above were also seen in experiments that removed a factor called TBPL1, suggesting that these two proteins may act through similar pathways.
Researchers then analyzed data from the UK Biobank — which stores medical information on more than 500,000 people — to determine whether altered circadian rhythms were associated with PF.
The team found that people with abnormal sleeping patterns are significantly more likely to have PF than those who follow a normal sleeping pattern, defined as seven hours of sleep per night.
Specifically, people who reported sleeping less than seven hours per night were significantly more likely to have the disease. A similar pattern was also seen for those who slept more than seven hours per night. These results were valid even after adjustment for known risk factors for PF, namely age, sex, smoking, and body mass index.
The PF risk was highest at the extremities of the spectrum. People who reported four or fewer hours of sleep each day were about twice as likely to have PF, and those sleeping 11 or more hours daily were almost three times more likely to have the disease, both compared to those sleeping seven hours each day.
“[I]t is clear that short sleep length is associated with pulmonary fibrosis, and this is as least as strong as existing risk factors for this disease, indicating potential clinical relevance,” the researchers wrote. “An association with long sleep duration was also found that may be biological or due to confounders.”
Shift work and staying up late at night were also found to be risk factors for PF, although to a lesser degree.
In an additional analysis, researchers treated samples of lung tissue taken from PF patients with GSK4112, which is an activator of REVERBα. Results showed that the treated tissue exhibited a diminished fibrotic response (lesser production of molecules promoting scarring), namely a reduced production of collagen (a key protein in fibrosis).
“Targeting REVERBα repressed collagen secretion from human fibrotic lung tissue, making this protein a promising therapeutic target,” the researchers wrote.
Taken together, these data suggest that a dysregulated circadian clock in fibroblasts in the lung is involved in PF’s development, and that targeting this molecular machinery could be a potential treatment strategy.
“Pulmonary fibrosis is a devastating condition which is incurable at present. Therefore, the discovery that the body clock is potentially a key player potentially opens new ways to treat or prevent the condition,” John Blaikley, PhD, a senior lecturer at the University of Manchester who led the project, said in a university press release.
“More work will need to be done around studying the association between pulmonary fibrosis and sleep duration to establish both causation and reproducibility. If these results are confirmed, then sleeping for the optimal time may reduce the impact of this devastating disease,” Blaikley added.