Sphingosine 1-phosphate (S1P) lyase (SPL) is an intracellular enzyme that mediates the irreversible degradation of the bioactive lipid S1P. We have previously reported that overexpressed SPL displays anti-influenza viral activity; however， the underlying mechanism is incompletely understood. In this study， we demonstrate that SPL functions as a positive regulator of IKKε to propel type I IFN-mediated innate immune responses against viral infection. Exogenous SPL expression inhibited influenza A virus replication， which correlated with an increase in type I IFN production and IFN-stimulated gene accumulation upon infection. In contrast， the lack of SPL expression led to an elevated cellular susceptibili... More
Sphingosine 1-phosphate (S1P) lyase (SPL) is an intracellular enzyme that mediates the irreversible degradation of the bioactive lipid S1P. We have previously reported that overexpressed SPL displays anti-influenza viral activity; however， the underlying mechanism is incompletely understood. In this study， we demonstrate that SPL functions as a positive regulator of IKKε to propel type I IFN-mediated innate immune responses against viral infection. Exogenous SPL expression inhibited influenza A virus replication， which correlated with an increase in type I IFN production and IFN-stimulated gene accumulation upon infection. In contrast， the lack of SPL expression led to an elevated cellular susceptibility to influenza A virus infection. In support of this， SPL-deficient cells were defective in mounting an effective IFN response when stimulated by influenza viral RNAs. SPL augmented the activation status of IKKε and enhanced the kinase-induced phosphorylation of IRF3 and the synthesis of type I IFNs. However， the S1P degradation-incompetent form of SPL also enhanced IFN responses， suggesting that SPL's pro-IFN function is independent of S1P. Biochemical analyses revealed that SPL， as well as the mutant form of SPL， interacts with IKKε. Importantly， when endogenous IKKε was downregulated using a small interfering RNA approach， SPL's anti-influenza viral activity was markedly suppressed. This indicates that IKKε is crucial for SPL-mediated inhibition of influenza virus replication. Thus， the results illustrate the functional significance of the SPL-IKKε-IFN axis during host innate immunity against viral infection.
