FSTL-1: A New Player in the Prevention of Emphysema

Emphysema is an incurable destructive lung disease that causes impairment in gas exchange, gas trapping, hyperinflation, and ultimately shortness of breath. Cigarette smoking is a major cause of emphysema, but there are also less common genetic causes of emphysema, such as alpha-1 antitrypsin deficiency (1). The mechanisms underlying the induction and progression of emphysema are believed to involve an imbalance in lung proteases and antiproteases, chronic inflammation and oxidative stress, alveolar wall cell death, and failure of alveolar wall maintenance (2, 3). A better understanding of the cellular and molecular mechanisms that drive emphysema may lead to novel therapeutic strategies to prevent its development or halt its progression, resulting in better health outcomes for patients.

There is a growing body of evidence showing that FSTL-1 (follistatin-like 1) plays an important role in lung development and respiratory diseases, including asthma and pulmonary fibrosis (4–8). In a study presented in this issue of the Journal, Henkel and colleagues (pp. 934–945) examined the consequences of reduced FSTL-1 expression on postnatal lung homeostasis (9). For this purpose, they used mice with reduced Fstl1 gene expression, termed FSTL-1 hypomorphic (FSTL-1 Hypo) mice. The authors found that FSTL-1 Hypo mice spontaneously developed emphysema and that pulmonary function was impaired. Micro–computed tomography scanning showed that FSTL-1 Hypo mice had increased lung volumes and decreased lung density. Collectively, these findings indicate that reduced FSTL-1 expression was sufficient to cause the mice to develop histologic, functional, and radiographic findings consistent with pulmonary emphysema. A very interesting finding in this study was that FSTL-1–dependent emphysema was not exacerbated by chronic cigarette smoke exposure, suggesting that FSTL-1 deficiency protects mice from cigarette smoke–induced emphysema. Given that emphysema is a clinical condition of cigarette smoke–induced chronic obstructive pulmonary disease (COPD), and that novel treatments are desperately needed, it would have been worth expanding on this intriguing aspect of the study.

Because FSTL-1 protects against emphysema, the authors went on to investigate the cellular sources of FSTL-1 in the lung. Interestingly, FSTL-1 was highly expressed in endothelial and mesenchymal cells, both significantly more so than in epithelial and immune cells. To determine whether murine FSTL-1 expression correlated with human lung cell FSTL-1 expression, the authors analyzed data from the Lung Gene Expression Analysis Web Portal and found that in 20-month-old human lung tissue, FSTL1 expression was evident in the endothelium, epithelium, and (most highly) mesenchyme. Together, these observations identify endothelial and mesenchymal cells as the primary producers of FSTL-1 in the postnatal lung.

RNA sequencing was used to determine the mechanism(s) by which FSTL-1 Hypo mice develop emphysema. This identified 33 genes that were significantly differentially expressed between wild-type (WT) and FSTL1 Hypo mice, irrespective of cigarette smoke exposure. Of particular note was that several gene regulation and macrophage antiinflammatory genes were differentially expressed in an FSTL-1–dependent manner, including the nuclear orphan receptor Nr4a1, also known as Nur77. Using qRT-PCR, the authors validated this finding by showing that Nr4a1 gene expression was significantly decreased in FSTL-1 Hypo mice, as well as cigarette smoke–exposed WT mice, compared with WT mice exposed to room air. They then used fluorescence-activated cell sorting to show that reduced FSTL-1 expression was associated with decreased Nr4α1 expression and Nur77⁺ staining within the lung, which corresponded with increases in myeloid cell abundance and reduced myeloid Nur77 positivity. Taken together, these data suggest that FSTL-1 may act directly on macrophages to influence Nr4α1/Nur77 function.

Because lung cell apoptosis is a known cause of emphysema (2, 3), and Nr4α1 and FSTL-1 have been shown to influence cell survival via apoptosis (10, 11), the authors investigated whether endothelial or epithelial cell apoptosis could explain the observed FSTL-1 Hypo phenotype. Surprisingly, FSTL-1 Hypo mice showed no difference in the percentage of apoptosis in endothelial, epithelial, or mesenchymal cells, suggesting that this mechanism did not account for the observed emphysema in FSTL-1 Hypo mice.

Genome-wide association studies have provided new insights into the molecular mechanisms of COPD and lung function (12, 13), which can be used to develop new drugs against molecular targets and inform population-based preventive strategies for targeting these previously unidentified molecular pathways. To investigate whether FSTL1 SNPs were associated with COPD-related phenotypes, the authors analyzed genotype data from non-Hispanic white participants in the COPDGene project. The authors show for the first time that genetic polymorphisms in the FSTL1 locus may influence COPD and lung function in a subset of individuals.

There are some potential limitations to this study that bear some discussion. First, it would have been worth exploring whether Nr4a1 can rescue the FSTL-1 Hypo emphysema phenotype. Second, to investigate whether FSTL1 SNPs were associated with COPD-related phenotypes, the authors analyzed genotype data from non-Hispanic white participants in the COPDGene project. It would have been worth examining FSTL1 SNPs in other cohorts and populations, as well as measuring FSTL-1 levels/expression in other sample types (e.g., blood, serum, and lung tissue) from COPD study populations. However, despite these potential limitations, this valuable study will inform and prompt further studies in the field to verify and expand on the outcomes of the present study.

In summary, Henkel and colleagues have identified a novel role of FSTL-1 in protecting against the development of emphysema in mice, which is independent of cigarette smoke exposure. This appears to be achieved by FSTL-1 affecting NF-κB (nuclear factor-κB) signal transduction in macrophages via modulation of Nr4α1. Although the exact nature of an FSTL-1/Nr4a1/NF-κB pathway in human emphysema remains to be defined, this study has important clinical implications, as it may pave the way for novel therapeutics that can prevent or halt the progression of emphysema.