Neutrophils undergo switch of apoptosis to NETosis during murine fatty liver injury via S1P receptor 2 signaling

Inappropriate neutrophil infiltration and subsequent neutrophil extracellular trap (NET) formation have been confirmed to be involved in chronic inflammatory conditions. Fatty liver disease is an increasingly severe health problem worldwide and currently considered the most common cause of chronic liver disease. Sphingosine 1-phosphate (S1P), a product of membrane sphingolipid metabolism, regulates vital physiological and pathological actions by inducing infiltration and activation of various cell types through S1P receptors (S1PRs). Here, we seek to determine the S1PR-mediated effects on neutrophil activation during chronic liver inflammation. In this study, NETs are detected in the early stage of methionine-choline-deficient and a high-fat (MCDHF) diet-induced liver injury. NET depletion by deoxyribonuclease I intraperitoneal injection significantly protects liver from MCDHF-induced liver injury in vivo. Meanwhile, we show that levels of myeloperoxidase-DNA complex (NET marker) in the serum present positive correlation with sphingosine kinase1 (S1P rate-limiting enzyme) messenger RNA expression or S1P levels in the injured liver of MCDHF-fed mice. In vitro, S1PR(2) participates in the redirection of neutrophil apoptosis to NETosis via Gα(i/o), extracellular signal-regulated kinase, p38 mitogen-activated protein kinase, and reactive oxygen species signaling pathways. Moreover, S1PR(2) knockdown in MCDHF-fed mice by S1PR(2)-siRNA intravenous injection significantly inhibits NET formation in damaged liver tissue and then alleviates hepatic inflammation and fibrosis. Conclusion: In the early stage of fatty liver disease, S1PR(2)-mediated neutrophil activation plays an important role in the evolvement of liver injury.