No abstract

Respiratory syncytial virus (RSV) preferentially infects lung epithelial cells. Infected cells remain viable well into the infection. This prolonged survival results from RSV-induced activation of pro-survival pathways, including Akt and extracellular signal-related kinase (ERK). Sphingosine 1-phosphate (S1P) is a sphingolipid metabolite with demonstrated links to cell survival. It is enzymatically generated by sequential activation of ceramidase (generation of sphingosine) and sphingosine kinase (generation of S1P). In these studies, we found that RSV stimulated neutral ceramidase and sphingosine kinase activities in lung epithelial cells. The combined effect of activation of these two enzymes would decrease proapoptotic ceramide and increase antiapoptotic S1P. S1P activated Akt and ERK within minutes, and inhibition of sphingosine kinase blocked RSV-induced ERK and Akt activation, leading to accelerated cell death after viral infection. RSV infection does eventually kill infected cells but activation of cell survival pathways significantly delays cell death. The studies are the first evidence linking sphingolipid metabolites to cell survival mechanisms in the context of a viral infection.

Human alveolar macrophages are unique in that they have an extended life span in contrast to precursor monocytes. In evaluating the role of sphingolipids in alveolar macrophage survival, we found high levels of sphingosine, but not sphingosine-1-phosphate. Sphingosine is generated by the action of ceramidase(s) on ceramide, and alveolar macrophages have high constitutive levels of acid ceramidase mRNA, protein, and activity. The high levels of acid ceramidase were specific to alveolar macrophages, because there was little ceramidase protein or activity (or sphingosine) in monocytes from matching donors. In evaluating prolonged survival of alveolar macrophages, we observed a requirement for constitutive activity of ERK MAPK and the PI3K downstream effector Akt. Blocking acid ceramidase but not sphingosine kinase activity in alveolar macrophages led to decreased ERK and Akt activity and induction of cell death. These studies suggest an important role for sphingolipids in prolonging survival of human alveolar macrophages via distinct survival pathways.