The pathogenesis of sepsis-associated ALI is manifested by inflammatory cascade and apoptosis accumulation. Our findings in this study identify Serinc2 as a novel protective regulator in the development of ALI pathologies both in vitro and in vivo. While the upregulation of Serinc2 during ALI might contribute as an endogenous protection feedback.
Serinc family is highly conserved in all eukaryotes and has similar transmembrane topology. This transmembrane topology has 10 transmembrane segments and contains a functional domain "helix-loop-helix" 30. The Serinc family contains five members: Serinc1, Serinc2, Serinc3, Seinc4 and Serinc5. By integrating serine, a non-essential amino acid, into the cell membrane, its crucial function is to promote the production of phosphatidylserine and sphingomyelin, and regulate the biosynthesis of membrane lipid molecules 11. Serinc5 has been proved to be the most powerful antiviral factor and can inhibit HIV, MLV, EIAV and so on 31. It selectively inactivates envelope glycoprotein, interferes with the remaining env activity, prevents the folding of envelope, and then limits the infection of offspring virus to new target cells 32. In addition, serinc1 and serinc3 have also been observed to have limited HIV inhibition. Interestingly, serinc2, also known as tumor differentially expressed 2, has been firstly identified in non-small cell lung cancer cells 12, and has no effects on the infectivity of HIV-1 32. Here we, for the first time, reveal a protective role of Serinc2 in sepsis-associated ALI through inhibiting apoptosis and inflammation.
The plasma membrane is of great significance for the extracellular to intracellular transmembrane signal transduction of membrane receptors. Plasma membrane microcapsules and lipid rafts are lipid ordered domains rich in cholesterol and sphingomyelin, and enrich a variety of intracellular proteins involved in signal transduction 33. Intact plasma membrane microcapsules and lipid rafts are important molecular basis for signal transduction, especially transmembrane signal transduction 33. Emerging evidence reveals that the production of phosphatidylserine, “apoptosis clearance signaling”, inhibits leukocyte migration and promotes inflammation resolution 34. As a cofactor, phosphatidylserine is required for the activation of TAM family, and further plays a counter-inflammation response in macrophages 35, 36. The main function of Serinc2 is to assist in lipid synthesis on plasma membrane 11. Our findings indicate that Serinc2 might contribute to the quench of the activated inflammation signaling, which depends on lipid raft integrity and mobility. As a key molecule in the synthesis of membrane lipids, whether Serinc2 also plays a protective role in ALI via the synthesis of phosphatidylserine and sphingomyelin needs to be investigated in future.
Production of inflammatory factors is mainly mediated by transcription factors, especially STAT3, which is responsible for transducing extracellular stimuli to nuclear gene expression through translocation from cytosol to nucleus after phosphorylation 37. Persistent activation of STAT3 mediates both the release of proinflammatory cytokines and the suppression of anti-immune response in M1 pro-inflammatory macrophages 38–40. This was further confirmed by our findings that the increasing phosphorylation lever of STAT3 according to time the of LPS challenge and the expression of p-STAT3 was downregulated while inflammatory response was alleviated.
As the center of signal transduction pathways, MAPK (mitogen-activated protein kinase, MAPK) pathway can be activated by various stimuli such as cytokines, radiation, osmotic pressure 17. Activated MAPK receives signals that are converted and transmitted by membrane receptors and carries them into the nucleus, playing a pivotal role in cell proliferation and other biofunctions. ERK widely exists in various tissues, is involved in cell proliferation and differentiation. p38, mediating inflammation and apoptosis, many of whose specific inhibitor have shown early anti-inflammatory efficacy, even the p38γ inhibitor have been used in clinical treatment of idiopathic pulmonary fibrosis 22, 23, 41. In the present study, we found that p38 and ERK were evoked by inflammatory stimuli including CLP and LPS, which could be modified by Serinc2. Considering the function of Serinc2 in membrane trafficking, our data implicate a crucial role of Serinc2-mediated membrane event in organizing transmembrane signaling transduction.
The activation of AKT mainly occurs on the cell membrane. When cells suffered from extracellular signals, activated PI3K generates PIP3 and transposes it to the cell membrane, which not only enables AKT itself to obtain catalytic activity, but also enables Both AKT and PDK-1 to be co-located on the cell membrane. PDK-1 can further catalyse AKT phosphorylation and make it fully activated, thereby regulating the phenotypes of cell proliferation, differentiation, apoptosis and migration 28, 42. Corresponding to in vivo experiments, we used an inflammatory response model of ALI induced by LPS. It was confirmed that up-regulation of serinc2 can lead to a decrease the apoptosis associated protein and reduced of TUNEL cells. Furthermore, our in vitro analyses further verify that serinc2 could inhibits apoptosis by activation of Akt, as evidenced by negligible changes in LPS-induced effects, containing elevated cleaved-caspase3, Bax and decreased Bcl2 protein expression, along with increases in TUNEL cells after AKTin pre-treatment. Additionally, previous studies demonstrating activation of PI3K/Akt signaling pathway suppresses the LPS-induced inflammatory caspases in ALI 43, 44. In the present study, we found the inhibitor of Akt blocked the protective effect of serinc2 against inflammation in ALI, implying that serinc2 prevents LPS-induced inflammation and apoptosis partially by the Akt pathway. However, it is unclear whether the special membrane trafficking function of serinc2 can activate Akt or there is a spatial interaction between serinc2 and Akt, which needs further experimental proof.
COVID-19 pandemic and its subsequent development into ALI/ARDS threaten the lives of millions of people worldwide, and the adverse progress of COVID-19 is related to the severe inflammatory response and cell injury by virus infection 45, 46. COVID-19, as a specific intracellular parasitic pathogen, from the initial adsorption, penetration into host cells to the final release of progeny cells, is inseparable from the cell membrane of host cells. As an important regulator of membrane lipid synthesis, serinc2 can not only regulate inflammatory signal pathway, promote cell survival and inhibit cell apoptosis, but also can be a promising candidate in the development of new anti-COVID-19 approaches and may be an effective target to treat COVID-19 clinical complications.