Effect of S1P receptor agonists on prostaglandin and thromboxane secretion. (A,C) Sensitized RBL-2H3 cells (5 × 10⁴) were treated without or with 1 μM each of FTY720, FTY720-P, S1P, SEW2871, or sphingosine (A) or 100 nM FTY720 or S1P (B) for 2 hours, and then stimulated with Ag for 15 minutes. PGD₂ secretion was measured and results were expressed as percent inhibition of Ag-induced release ± SEM (n = 4). (C) Sensitized RBL-2H3 cells were treated without or with 1 μM FTY720 or S1P for 2 hours, then stimulated with Ag for 15 minutes, and secreted thromboxane was measured by an ELISA for TXB₂. Data are means ± SD of quadruplicate determinations. (D) RBL-2H3 cells (2 × 10⁶) were serum-starved for 8 hours and then stimulated with 1 μM S1P, FTY720, or FTY720-P for the indicated times. Equal amounts of lysate proteins were immunoblotted with anti–phospho-ERK1/2. Blots were stripped and reprobed with antitubulin as a loading control. (E) Sensitized RBL-2H3 cells were treated with S1P or FTY720 (1 μM) for the indicated times prior to stimulation with Ag. PGD₂ secretion was measured, and results were expressed as percent inhibition of Ag-induced release ± SEM.