REGULATION OF PHOSPHOLIPASE-A(2) ACTIVITY IN UNDIFFERENTIATED AND NEUTROPHIL-LIKE HL-60 CELLS - LINKAGE BETWEEN IMPAIRED RESPONSES TO AGONISTS AND ABSENCE OF PROTEIN-KINASE C-DEPENDENT PHOSPHORYLATION OF CYTOSOLIC PHOSPHOLIPASE-A(2)

We compared the regulation of cytosolic phospholipase A2 (cPLA2) activity in undifferentiated and neutrophil-like HL60 cells. Although Ca2+-mobilizing P2-purinergic receptors are expressed in both cell types, arachidonic acid (AA) release stimulated by P2-purinergic agonists was 5-7-fold higher in the differentiated cells. Similarly, the stimulation of AA release by AlF4- in intact cells or by ATP and guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) in electropermeabilized cells was significantly higher in the differentiated cells. Treatment with phorbol 12-myristate 13-acetate (PMA) enhanced A23187-stimulated AA release in intact HL60 granulocytes with minimal effects in the undifferentiated cells. Immunoblotting experiments showed similar levels of cPLA2 and of agonist-mediated activation of mitogen-activated protein kinase in both cell types. Experiments measuring stimulation of AA release by either melittin, using endogenously labeled intact cells, or Ca2+, using homogenates and exogenous substrate, indicated that undifferentiated cells do not lack an activatable PLA2. The stimulatory effects of GTPgammaS and Ca2+ on AA release in homogenates from endogenously labeled cells suggested that undifferentiated cells display G protein-cPLA2 coupling. Basal and PMA-stimulated phosphorylation of cPLA2 was detected in differentiated, but not in undifferentiated cells. However, the two cell types displayed only subtle differences in the time courses of phosphorylation of mitogen-activated protein kinase triggered by agonists and PMA. The observed defect in cPLA2 phosphorylation may represent the alteration preventing agonist-mediated stimulation of AA release in undifferentiated HL60 cells.