The roles of sphingosine-1-phosphate in asthma

Asthma is a complex condition in which exposure to environmental antigens induces inflammatory reactions in the airway characterized by activation of mast cells and eosinophils. Mast cells are known to be the main effector cells in eliciting IgE-mediated allergic response. These cells secrete various substances that perpetuate inflammation and provoke airway smooth muscle (ASM) contraction. A newly recognized addition to the repertoire of FcepsilonRI-mediated signaling events is the activation of sphingosine kinase leading to the generation of the potent sphingolipid mediator, sphingosine-1-phosphate (S1P) from sphingosine. S1P secretion by the lung significantly increases after challenge with an allergen, adding this sphingolipid metabolite to the variety of mediators that are released during an allergic reaction [FASEB J. 15 (2001) 1212]. Indeed, similar to previous reports, we found that FcepsilonRI cross-linking not only increased cellular levels of S1P, it also markedly enhanced its secretion from rat basophilic leukemia RBL-2H3 cells. Moreover, S1P induced degranulation of RBL and bone marrow derived mast cells (BMMCs) cells as determined by hexosaminidase release. Treatment of BMMCs with the sphingosine kinase inhibitors, DL-threo-dihydrosphingosine and dimethylsphingosine, reduced IgE/Ag stimulated histamine release. RT-PCR analysis demonstrated that these mast cells express S1P receptors EDG-1 and EDG-5 but not EDG-3, EDG-6 or EDG-8 transcripts. Further studies are needed to determine whether IgE triggering results in transactivation of EDG-1 or EDG-5 present on mast cells and whether this is a critical event for mast cell activation.