The Role of Sphingosine Kinase in the Signaling Initiated at the High-Affinity Receptor for IgE (FcεRI) in Mast Cells

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Abstract
Over the last few years, sphingolipids have emerged as an additional class of lipids participating in signaling events in various cell types. The best-investigated examples so far are ceramide and sphingosine-1-phosphate. Ceramide-activated protein kinase and sphingosine kinase are two enzymes which respond to and generate these mediators. In particular, sphingosine kinase, its substrate sphingosine and the product sphingosine-1-phosphate have recently been implicated in the signaling cascades initiated at the FcεRI of mast cells. High intracellular levels of sphingosine seem to serve as an ‘intracellular’ inhibitor which is ‘deactivated’ by the action of sphingosine kinase, due to the conversion to sphingosine-1-phosphate. One mode of action of the inhibitory process in this cell type is prevention of the activation of the mitogen-activated protein (MAP) kinase pathway. Sphingosine-1-phosphate itself, the product of this enzymatic reaction, is believed to lead to Ca2+ mobilization and to stimulate the MAP kinase pathway. The existence and function of this second messenger explains the ‘IP3 gap’ described in mast cells after FcεRI activation. Therefore, a picture emerges whereby the balance of these two lipid molecules seems to be decisive for the activation of mast cells by IgE plus antigen, with sphingosine kinase acting as a permissive switch for stimulation.