Dichotomy of Ca2+ signals triggered by different phospholipid pathways in antigen stimulation of human mast cells

Mast cell activation triggers Ca2+ signals and the release of enzyme-containing granules, events that play a major role in allergic/hypersensitivity reactions. However, the precise molecular mechanisms that regulate antigen-triggered degranulation and Ca2+ fluxes in human mast cells are still poorly understood. Here we show, for the first time, that a receptor can trigger Ca2+ via two separate molecular mechanisms. Using an antisense approach, we show that IgE-antigen stimulation of human bone marrow-derived mast cells triggers a sphingosine kinase (SPHK) 1-mediated fast and transient Ca2+ release from intracellular stores. However, phospholipase C (PLC) gamma1 triggers a second (slower) wave of calcium release from intracellular stores, and it is this PLCgamma1-generated signal that is responsible for Ca2+ entry. Surprisingly, FcepsilonRI (a high affinity receptor for IgE)-triggered mast cell degranulation depends on the first, sphingosine kinase-mediated Ca2+ signal. These two pathways act independently because antisense knock down of either enzyme does not interfere with the activity of the other enzyme. Of interest, similar to PLCgamma1, SPHK1 translocates rapidly to the membrane after FcepsilonRI cross-linking. Here we also show that SPHK1 activity depends on phospholipase D1 and that FcepsilonRI-triggered mast cell degranulation depends primarily on the activation of both phospholipase D1 and SPHK1.