V.II. Studies in acute lung injury
Acute lung injury (ALI) may lead to the development of acute respiratory distress syndrome (ARDS) which is the major cause of respiratory failure in ICU. To our knowledge, GLP-1-based drugs have not been utilized in clinical trials for ALI. Nevertheless, as mentioned above, a very recent retrospective study has shown that the utilization of GLP-1 based drugs reduced trends in the risks of pneumonia, in addition to asthma and COPD (104). Intensive investigations have, however, been conducted in ALI animal models, mainly with intratracheally LPS administration in mice (123).
In 2011, Lim and colleagues have developed a “nanomedicine” designated as GLP1-SSM, in which human GLP-1 (7-36) is self-associated with PEGylated phospholipid micelles (SSM). They then demonstrated that in LPS induce ALI mouse model, subcutaneous GLP1-SSM administration decreased lung neutrophil influx, myeloperoxidase activity and IL-6 levels in a dose-dependent manner (124). In 2017, GLP-1-SSM was shown by this team to alleviate gut inflammation in a dextran sodium sulfate induced mouse colitis model (125).
Several recent studies have explored mechanisms underlying the attenuating effect of GLP-1R agonists in ALI animal models. Reduction of pulmonary surfactant is tightly associated with decreased pulmonary compliance and edema in ALI. Thyroid transcription factor-1 (TTF-1) is known to play an important role in regulating levels of surfactant protein-A (SP-A), the most abundant protein component of pulmonary surfactant. Romaní-Pérez and colleagues have reported that in rats, administering of exenatide or liraglutide to the mother from gestational day 14 to the birth increased SP-A and SP-B mRNA levels and the amount of SPs in the amniotic fluid at the end of pregnancy (126). Furthermore, they have reported that lung GLP-1R mRNA level increased 4-fold at the 1st day of life in both male and female rats, while the level of expression was subsequently maintained into the adulthood (126). In 2018, Zhu et al found that in the ALI mouse model, LPS administration reduced lung SP-A and TTF-1 levels, while the reduction was reversed by simultaneous administration of liraglutide with LPS challenge (127). In 2019, in a similar mouse model, Xu and colleagues found that LPS challenge induced polymorphonuclear neutrophil (PMN) extravasation, lung injury, along with alveolar-capillary barrier dysfunction. Concomitant liraglutide administration prevented PMN-endothelial adhesion by inhibiting expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) (128). Other documented functions of GLP-1-based drugs in ALI models include the stimulation of eNOS/sGC/PKG signalling cascade, the induction of vasorelaxant expression, and the inactivation of the NF-κB inflammatory signalling (129-131). However, none of these investigations have directly assessed the involvement of pulmonary GLP-1R.
In 2020, our team has directly assessed the involvement of GLP-1R in mediating effect of liraglutide treatment in LPS-induced ALI mouse model. In this study, conducted by Zhou and colleagues, liraglutide was not administrated simultaneously with LPS challenge, but as a “preventative agent” which was subcutaneously administrated 2 hours before intratracheal LPS delivery (103). In such experimental settings, we observed that liraglutide pre-treatment significantly reduced LPS-induced acute lung injury, including the reduction on lung injury score, wet/dry lung weight ratio, immune cell counts, protein concentration in bronchoalveolar lavage fluid (BALF), and cell apoptosis in the lung. Those effects were highly associated with reduced pulmonary mRNA expression of genes that encode inflammatory chemokines and cytokines. Importantly, none of those “preventative” effects were observed in GLP-1R knockout (KO) mice, highlighting the essential role of lung GLP-1R in mediating the effect of liraglutide in preventing lung injury (103). Based on such “preventative” effect observed, we suggested that retrospective studies should be conducted in T2D subjects received with or without GLP-1 based drugs, asking whether T2D patients are less vulnerable to ALI as well as chronic lung inflammatory injury after receiving GLP-1 based drug treatment (103, 132, 133).
The study conducted by Zhou and colleagues has also revealed that liraglutide treatment attenuated LPS induced pulmonary thioredoxin-interacting protein (TxNIP) over-expression, and such attenuation is also GLP-1R dependent (103). TxNIP is a member of the NLR family pyrin domain containing 3 (NLRP3) inflammasome component (134-136), a mediator of glucotoxicity (137, 138), and a therapeutic target of T2D and other disorder (135, 138-141). In addition to high glucose challenge, TxNIP level in pancreatic β-cells was also shown to be stimulated by dexamethasone and streptozotocin (STZ), an antibiotic utilized in generating the T1D rodent model. Importantly, LPS challenge caused approximately 2.5-fold elevation in lung TxNIP levels in wild type littermates, while in GLP-1R KO mice, lung TxNIP increased about 7-fold after the challenge with the same amount of LPS. Thus, lung GLP-1R itself may represent a native defensing system. In contrast to the observation made by Balk-Moller and colleagues in their COPD model, we did not see a stimulatory effect of liraglutide treatment on pulmonary nppa (which encodes ANP) expression (120). However, we observed that LPS challenge led to a 3-fold activation on pulmonarynppa level. Whether such activation represents a protective or defensive response remains to be further explored (103). Figure 3 summarizes our current understanding on pulmonary GLP-1R mediated protection in ALI mouse model, in response to GLP-1R agonist treatment, involving the attenuation of the inflammasome component TxNIP.