Distinct roles in signal transduction for each of the phospholipase A(2) enzymes present in P388D(1) macrophages

Receptor-stimulated arachidonic acid (AA) mobilization in P388D(1) macrophages consists of a transient phase in which AA accumulates in the cell and a sustained phase in which AA accumulates in the incubation medium. We have shown previously that a secretory group II phospholipase A(2) (sPLA(2)) is the enzyme responsible for most of the AA released to the incubation medium. By using selective inhibitors for each of the PLA(2)s present in P388D(1) macrophages, we demonstrate herein that the cytosolic group IV PLA(2) (cPLA(2)) mediates accumulation of cell-associated AA during the early steps of P388D(1) cell activation. The contribution of both cPLA(2) and sPLA(2) to AA release can be distinguished on the basis of the different spatial and temporal characteristics of activation and substrate preferences of the two phospholipase A(2)s (PLA(2)s). Furthermore, the results suggest the possibility that a functionally active cPLA, may be necessary for sPLA(2) to act. cPLA(2) action precedes that of sPLA(2), and overcoming cPLA(2) inhibition by artificially increasing intracellular free AA levels restores extracellular AA release. Although this suggests cross-talk between cPLA(2) and sPLA(2), selective inhibition of one other PLA(2) present in these cells, namely the Ca2+-independent PLA(2), does not block, but instead enhances receptor-coupled AA release. These data indicate that Ca2+-independent PLA(2), does not mediate AA mobilization in P388D(1) macrophages. Collectively, the results of this work suggest that each of the PLA(2) present in P388D(1) macrophages serves a distinct role in cell activation and signal transduction.