The development of COPD involves inducible responses to inhaled particles, including cigarette smoke, and stress triggered by cigarette smoke exposure can induce autophagy in lung tissue. In this study, we report that FSTL1 is highly expressed in COPD patients together with autophagy activation, airway inflammation and airway remodeling. In both COPD patients and CS exposed animal model, we found the level of FSTL1 was positively related to overactive autophagy. Moreover, autophagy inhibition with 3-MA can attenuate airway remodeling and inflammation to a certain extent in CS exposed mice. By establishing FSTL1+/− mice model, we proved that the haplodeletion of FSTL1 had a repressive effect on autophagy, indicating that FSTL1 might play a vital role in the activation of autophagy. In conclusion, our study has found a link between FSTL1, autophagy and the pathogenesis of COPD, supporting our hypothesis that FSTL1 modulates CS-induced autophagy and is involved in COPD process.
FSTL1 is implicated in various biological and pathological processes, including apoptosis, autophagy, proliferation, differentiation and migration 19. Generated mainly by mesenchymal cells(fibroblasts, chondrocytes, osteocytes, etc.), FSTL1 affects immunomodulation, tumorigenesis, fibrogenesis and organ development via multiple signal pathways27. As an antagonist of BMP4 signaling pathway, FSTL1 plays a crucial part in the embryogenesis of lung and alveolar ,thus FSTL1−/− mice showed postnatal lethality due to respiratory failure28,29. There is growing evidence that FSTL-1 plays an essential role in respiratory diseases. Recent studies prove that FSTL1 participates in lung injury as it is secreted in response to bleomycin, silica, radiation and other lung injury inducers 30–32. Besides, FSTL1 have an association with airway remodeling in chronic airway diseases33. Our previous study suggested that FSTL1 was positively linked to thickened RBM and smooth muscle mass in asthmatics. Moreover, FSTL1 can induce EMT and airway remodeling in asthma34. Several FSTL1 SNPs were found corresponded to COPD and lung function, and FSTL-1 deficiency might protect mice from cigarette smoke induced emphysema35. In the present study, we further unravel the role of FSTL1 in the pathogenesis of COPD. The increased level of FSTL1 in COPD is linked to overactive autophagy, aggravating airway remodeling and airway inflammation. Besides, haplodeletion of FSTL1 in CS exposed mice abate airway remodeling and airway inflammation through inhibiting CS-induced autophagy.
Autophagy is a lysosome-dependent degradation of microorganisms ,cellular proteins and organelles in order to keep homeostatic balance, consisting of macroautophagy, microautophagy and selected autophagy36,37. LC3B (microtubule associated protein 1 light chain 3 beta), as one of autophagy-related (ATG) proteins, represents autophagosome formation by the conversion from LC3B-I to LC3B-II38,39. Z.H. Chen et al reported that LC3B exerted a critical propathogenic role in the development of CS-induced emphysema40. P62, also known as sequestosome 1 (SQSTM1), which directly conjugated to LC3B, is self-degraded by autophagy and indicates insufficient autophagic clearance 41,42. Our data suggest a higher LC3B II/I ratio and P62 expression in CS exposed animal model. Also, the autophagosome formation was more obvious after CS exposure. These results revealed that CS triggered overactive autophagy and insufficient autophagic clearance in lung. Meanwhile, pretreatment with 3-MA significantly reduced CS-induced autophagy and disease progression. It is well known that autophagy plays a critical role in pulmonary inflammation and pathogenesis of numerous chronic lung disease13. However, in COPD which undergoes prolonged inflammation and stress, the function of autophagy is more complicated.
There is growing evidence that dysregulated autophagy is tied to pathogenesis of COPD. Some studies suggest that autophagy contributes to COPD progress. Increased autophagic proteins were found in lung tissue from COPD patients43. In vitro, autophagy was considered as an early event in COPD progression 43. CS-induced autophagy in epithelial cells promotes the production of TNF-α, IL-6 and IL-8 and lymphocyte recruitment into the lung44. It appears that CS initiate and prompt airway inflammation by increasing autophagosomal turnover (flux) and therefore result in epithelial cell apoptosis and death16,17,40,43,45.Moreover, recent advances deem autophagy has a potential impact on airway fibrosis46. Deficiency of autophagy alleviates CS-induced cilia shortening in vitro and reduces profibrotic signaling pathway and ECM release in vitro45. In vivo experiment, we validated that inhibition of autophagy activation with 3-MA mitigated CS-induced airway remodeling and airway inflammation, further enriching evidence that overactive autophagy exacerbates COPD progression. On the other hand, there exist other mechanisms of dysregulated autophagy in COPD. Functional autophagy is critical to degrade damaged organelles and proteins and maintain homeostasis. Some research indicates that autophagy impairment induced by ROS from CS and mitochondria can accelerate lung aging and emphysema exacerbations47. Inadequate autophagy can induce senescence in COPD and also contribute to development of idiopathic pulmonary fibrosis48,49. Autophagy augmentation appears to be a therapeutic target for alleviating aging and COPD progress50. In our study, P62 increased after CS exposure, suggesting impaired autophagy while overactive autophagy was found. Dysregulated autophagy in COPD pathogenesis may operate in two extremes. Since autophagy is a well-known dynamic and complicated process, innovative methods and specific regulators should be developed to investigate autophagy in “real-time”.
In our research, FSTL1 was elevated together with autophagosome formation and autophagic proteins in both COPD patients and CS exposed animal model. Pretreatment with autophagic inhibitor in WT mice could alleviate CS-induced airway inflammation, airway remodeling and impaired lung function. In FSTL1+/− mice, deficiency of FSTL1 also bated CS-induced autophagy activation and adverse response to CS in lung tissue. These results indicate that FSTL1 may modulate autophagy by certain signaling pathways in COPD. From the existing study, we can learn that FSTL1 have crosstalk with several autophagic signaling pathways. Via DIP2A receptor, FSTL1 is able to activate Akt pathway and finally attenuate apoptosis after MCAO in Rats51. In cardiovascular disease, FSTL1 has been implicated in the activation of PI3K/Akt signaling, exhibiting protective effects52–54. Furthermore, FSTL1 is involved in the activation of AMPK signaling pathway in cardiac and renal disease55,56. These studies shed light on the correlation between FSTL1 and regulation of autophagy. However, there exists some limitations. The present study cannot demonstrate a complete mechanism of FSTL1, dysregulated autophagy and COPD. We still require additional work including in vitro experiments with more accurate autophagic regulators to study autophagy, and investigate the mechanism of how FSTL1 exerts the effect on autophagy and leads to COPD.