Expression of SphK1 reduces degranulation. (A) IgE-sensitized RBL-2H3 cells stably expressing vector (□) or SphK1 (▪) were cultured in the presence of 15% serum (A-B) and then treated for 1 hour with the indicated concentrations of DNP-HSA (Ag) or 1 μM ionomycin. Degranulation was assessed by measuring release of β-hexosaminidase. (B) IgE-sensitized RBL-2H3 cells expressing vector (□), SphK1 (▪), or catalytically inactive SphK1 (▦; SphK1G82D) were treated for 1 hour with Ag (100 ng/mL) and degranulation was measured. (C-D) Effect of serum on SphK1-modulated degranulation. IgE-sensitized vector-overexpressing (○) or SphK1-overexpressing (•) RBL-2H3 cells were cultured for 24 hours in the presence of 15% serum (C) or in the absence of serum (D) and then degranulation was induced by treatment with DNP-HSA (100 ng/mL) for the indicated times. ^*P < .05; Student t test. NS indicates not significantly different. Data are means ± standard deviation (SD) of triplicate determinations.