A modified protein called citrullinated vimentin (cit-vim), released by immune cells in response to harmful pollutants, may contribute to lung scarring in people with idiopathic pulmonary fibrosis (IPF), a study reports.
These findings suggest that cadmium and carbon black, which can be found in cigarette smoke as well as in polluted air, can trigger the production and secretion of cit-vim by immune cells in the lungs called macrophages. This modified protein, in turn, activates cells responsible for the thickening and stiffening (scarring) of lung tissue.
“Altogether, these studies support a role for cit-vim as a damage-associated molecular pattern molecule, or DAMP, that is generated by lung macrophages in response to environmental cadmium/carbon black exposure,” Veena Antony, MD, the study’s senior author and endowed professor of Environmental Medicine at the University of Alabama at Birmingham (UAB) Department of Medicine, said in a UAB press release.
The study, “Citrullinated vimentin mediates development and progression of lung fibrosis,” was published in Science Translational Medicine.
IPF is characterized by scarring, or fibrosis, of lung tissue due to unknown causes, which ultimately leads to difficulties in breathing. Fibrosis is defined by the deposition of extracellular matrix proteins, including collagen, with the contribution of macrophages.
Air pollution and cigarette smoke have been previously linked with IPF development. However, the mechanisms by which toxic environmental elements can contribute to lung fibrosis are not clear.
A team of researchers led by Antony set out to investigate how carbon black and cadmium, two pollutants, can cause fibrosis in IPF.
Cadmium, a toxic heavy metal, can attach to carbon black and form particles that are eaten, in a sense, by macrophages — the sentinel host defense cells in the lung.
Researchers analyzed the levels of cadmium and cit-vim in lung biopsies from 25 IPF patients; of these, eight had never smoked and 17 were smokers. The team also recruited 14 healthy volunteers as a control group, eight people who had never smoked and six smokers.
Vimentin is a structural protein that can be citrullinated, meaning that it can be modified by the addition of the amino acid citrulline. Researchers showed that cadmium/carbon black induces citrullination of vimentin, and that the release of cit-vim from lung macrophages was dependent on the activation of two enzymes, Akt1 and PAD2.
High levels of both cadmium and carbon black were found in the lung tissue samples from IPF patients. In addition, the amounts of cit-vim paralleled the concentrations of cadmium. Lung macrophages from these patients had increased cit-vim expression compared with macrophages from control tissue samples.
Cit-vim levels were also greater in lung tissue from smokers than non-smokers, and higher levels were associated with worse lung function in IPF patients.
“Our data demonstrate that cadmium/carbon black is a risk factor, not only in subjects with IPF who smoked, but also in nonsmokers. Higher concentrations of cadmium in patients who do not smoke may be caused by exposure to cadmium through food and/or occupation or environment,” Antony said.
In the next set of experiments, researchers clarified the role of cit-vim in lung fibrosis. Fibroblast cells (involved in tissue repair and fibrosis) were incubated with cit-vim isolated from IPF or control lung macrophages. They observed that cit-vim increased significantly the expression of collagen on control lung fibroblasts, and to a higher extent on fibroblasts from IPF patients. Results also showed that cit-vim could induce fibroblast invasion abilities.
Of note, fibroblast proliferation and excessive collagen deposition are part of the fibrotic process.
Researchers suggested that cit-vim could be working as a damage-associated molecular pattern molecule (DAMP), which are molecules released from damaged or dying cells that can trigger an immune response. According to the team, cit-vim from lung macrophages could then activate fibroblasts, through toll-like receptor 4 (TLR4) signaling, which then produce profibrotic cytokines, or chemical messengers, that may contribute to the development of IPF.
The team observed that cit-vim, but not the normal vimentin protein, could induce lung fibrosis in mice. Interestingly, when mice lacked a functional PAD2 or TLR4, they were protected from developing lung fibrosis.
“This finding is important,” Antony said, “because cit-vim is sufficient to provoke fibroblast activation in vitro and elicit profibrotic cytokine/chemokine production and TLR4-dependent lung fibrosis in vivo.”
Based on the findings, the researchers suggested that “PAD2 might represent a promising target to attenuate [cadmium/carbon black]-induced fibrosis.”
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