Ceramide synthase 4 interferes with replication of influenza virus but is downregulated by infection.

Influenza continues to pose a serious threat to humans. Influenza-host interaction is incompletely understood, requiring identification of host factors that regulate viral pathogenicity. Ceramide synthases (CerSs) are responsible for producing and controlling ceramide levels within cells. Ceramides are structural and signaling sphingolipid components that mediate various biological functions and affect the infectivity of multiple viruses. However, the role of CerSs during virus infections remains unclear. In this study, we investigated the possible function of CerSs in host defense against influenza virus infection. Cells stably expressing CerS4 poorly supported influenza virus replication, whereas CerS1 did not affect replication. Transient overexpression of CerS4 also impaired the efficient production of viral proteins as well as infectious progeny viruses. In support of these results, knockdown of endogenous CerS4 in cells enhanced virus replication. Intriguingly, CerS4 impeded virus-induced activation of cellular c-Jun N-terminal kinase (JNK), which interfered with influenza viral replication. On the other hand, influenza virus infection was shown to induce CerS4 ubiquitination and downregulation, which could limit the antiviral activity of CerS4. Collectively, these findings reveal a new function of CerS4 that restricts influenza virus infection and provides valuable insights into influenza-host defense interactions.IMPORTANCESeasonal influenza causes serious public health problems in the world with substantial annual morbidity and mortality. Further, there have been persistent concerns about potential development of an influenza pandemic. Current antiviral drugs are limited in their efficacy, especially due to the rapid emergence of drug-resistant variants. Host protein-directed therapy is an alternative or complementary approach to broadly controlling influenza virus infections but requires a deeper understanding of influenza-host interplay. Host ceramide synthase 4 regulates the level of ceramides that possess both structural and signaling mediator functions. Our study reveals that ceramide synthase 4 displays an antiviral activity against influenza virus infection by regulating JNK activation. However, influenza virus triggers degradation of ceramide synthase 4, which could favor virus replication. The findings advance our knowledge about the ceramide network interaction with influenza and provide a framework for developing a host-targeted therapy to cure influenza.