Activation of cytosolic phospholipase A₂ by opsonized zymosan in human neutrophils requires both ERK and p38 MAP-kinase

The present study demonstrates that stimulation of human neutrophils with opsonized zymosan (OZ), which binds to Fc gamma receptors (FcγRs) and C3b receptors, activates both ERK and p38 MAP-kinase. Thus, the relative role of both types of MAP-kinase, ERK and p38, in activation of cPLA₂ by OZ was studied. cPLA₂ activation by OZ was detected 15 sec after stimulation, maintained a plateau for 10 min and decreased thereafter. p38 MAP-kinase activation exhibited kinetics similar to that of cPLA₂, while ERK activation was detected within 15 sec but decreased significantly in less than 5 min after stimulation. Pretreatment of the cells with the MEK inhibitor, PD-098059, or the p38 MAP-kinase inhibitor, SB-203580 resulted in total inhibition of ERK or p38 MAP-kinase activity, respectively. Each inhibitor caused a partial inhibition during the time course of cPLA₂ activity, while their combination caused a total inhibition. Compared to OZ, inactivated OZ, which does not contain the complement proteins, induced an identical time-dependent stimulation of ERK and p38 MAP-kinase as well as a similar cPLA₂ activity, suggesting that the role of the C3b receptors in this system is negligible. It is concluded that OZ activates both ERK and p38 MAP-kinase and that the two isotypes are required for the onset and maintenance of cPLA₂ activity.