FcϵRI-induced PLD activation requires Syk and is enhanced by overexpression of wild-type PLD1 and PLD2, but is inhibited only by catalytically inactive PLD1. Cells expressing wild-type PLD1 and PLD2 (A) or their catalytically inactive forms (B), were labeled with [¹⁴C]lysoPC for 90 minutes, cultured overnight with antigen-specific IgE, then stimulated for 20 minutes with the indicated concentrations of antigen in the presence of 0.25% 1-butanol. Production of [¹⁴C]PBut induced by antigen is expressed as the percentage of total labeled lipids. Significant change in PLD activity compared with the corresponding control-treated cells is indicated as follows: ^*P < .05, ^**P < .005. (C) Syk-deficient RBL-2H3 cells (Syk^Neg) and Syk-reconstituted cells (Syk^wt) were used to measure PLD activation. Stimulation was with 100 ng/mL antigen or 300 nM A23187. Significant change in PLD activity compared with Syk^Neg cells is indicated as follows: ^**P < .005. Data in all panels are the mean ± SD from 3 independent experiments.