Dual regulation of cytosolic phospholipase A2 in mast cells after cross-linking of FC epsilon-receptor.

We have reported previously that cultured mast cells (MC) express three discrete phospholipases A2 (PLA2s), one of which corresponds to arachidonoyl-preferential cytosolic PLA2 (cPLA2). In the present study, we investigated the possible role of cPLA2 in eicosanoid synthesis by activating mouse bone marrow-derived mast cells (BMMC) through cross-linking of the high affinity IgE receptor (Fc epsilon RI) with a specific Ag. BMMC released arachidonic acid within 2 min after Fc epsilon RI cross-linking. A rapid, transient phosphorylation of cPLA2 was observed after Fc epsilon RI cross-linking, reaching the maximum within 2 min, and accompanied by an increase of cPLA2 activity in the cell lysate. Exposure of BMMC to the IgE-Ag for longer periods resulted in a time-dependent increase of the cPLA2 protein. The increase was detected within 10 h after stimulation and reached the maximum within 30 h. Dexamethasone inhibited the Ag-stimulated cPLA2 induction significantly. cPLA2 activity in cells stimulated for 24 h was increased significantly, and suppressed in cells treated with dexamethasone. When the cells were exposed to IgE-Ag for 36 h and then challenged with a secondary agonist, thrombin, arachidonate release was augmented significantly in comparison with cells without the Ag pretreatment. Thus, cPLA2 activation in BMMC by short term exposure to the Ag might be regulated by post-Fc epsilon RI modification (phosphorylation) of pre-existing enzyme, whereas that observed after long term exposure might be explained by the increase in cPLA2 protein.