Protein kinase C activation precedes arachidonic acid release during IgG-mediated phagocytosis.

Previous studies demonstrated that both protein kinase C (PKC) and arachidonic acid (AA) are required for IgG-mediated phagocytosis by human monocytes. We have characterized a calcium-independent "phagocytic" phospholipase A2 (designated pPL) that mediates arachidonic acid release. The present studies were designed to order PKC and pPL in the phagocytic signaling pathway. The PKC inhibitors staurosporine and calphostin C caused a coordinated decrease in phagocytosis of IgG-opsonized erythrocytes and arachidonic acid release. The PLA2 activators mastoparan and melittin restored phagocytosis to PKC-inhibited cells, but were ineffective in monocytes pretreated with the pPL inhibitor bromoenol lactone. Similarly, PKC activation with PMA and diacylglycerol enhanced phagocytosis in the absence, but not in the presence, of bromoenol lactone. These results indicate that pPL may be regulated by an upstream phosphorylation event. Thus, we examined the effects of Ab-opsonized glass bead ingestion, okadaic acid-mediated inhibition of phosphatases, and PMA treatment on the activity of pPL and on its distribution between the cytosolic and membrane-associated compartments. IgG-opsonized erythrocytes and okadaic acid caused an overall increase in pPL activity, with a twofold increase in membrane-associated pPL. PMA treatment caused a 1.8-fold increase in membrane-associated pPL activity. Okadaic acid and PMA mimic IgG-opsonized erythrocytes with respect to membrane activation of pPL, suggesting that pPL activity may be regulated by PKC. Collectively, these results indicate that pPL activity is modulated by PKC during IgG-mediated phagocytosis, and that the PKC requirement can be bypassed by direct activation of pPL.