The receptors for ATP and fMetLeuPhe are independently coupled to phospholipases C and A2 via G-protein(s). Relationship between phospholipase C and A2 activation and exocytosis in HL60 cells and human neutrophils

Abstract

The relationship between phospholipase A2 and C activation and secretion was investigated in intact human neutrophils and differentiated HL60 cells. Activation by either ATP or fMetLeuPhe leads to [3H]arachidonic acid release into the external medium from prelabelled cells. This response was inhibited when the cells were pretreated with pertussis toxin. When the [3H]arachidonic acid-labelled cells were stimulated with fMetLeuPhe, ATP or Ca2+ ionophore A23187, and the lipids analysed by t.l.c., the increase in free fatty acid was accompanied by decreases in label from phosphatidylinositol and phosphatidylcholine. Moreover, incorporation of label into triacylglycerol and to a lesser extent phosphatidylethanolamine was evident. Activation of secretion was evident with ATP and fMetLeuPhe but not with A23187. The pharmacological specificity of the ATP receptor in HL60 cells was investigated by measuring secretion of beta-glucuronidase, formation of inositol phosphatases and release of [3H]arachidonic acid. External addition of ATP, UTP, ITP, adenosine 5'-[gamma-thio]triphosphate (ATP[S]), adenosine 5'-[beta gamma-imido]triphosphate (App[NH]p), XTP, CTP, GTP, 8-bromo-ATP and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) to intact HL60 cells stimulated inositol phosphate production, but only the first five nucleotides were effective at stimulating secretion or [3H]arachidonic acid release. In human neutrophils, addition of ATP, ITP, UTP and ATP[S] also stimulated secretion from specific and azurophilic granules, and this was accompanied by increases in cytosolic Ca2+ and in [3H]arachidonic acid release. The addition of phorbol 12-myristate 13-acetate (PMA; 1 nM) prior to the addition of either fMetLeuPhe or ATP led to inhibition of phospholipase C activity. In contrast, this had no effect on phospholipase A2 activation, whilst secretion was potentiated. Phospholipase A2 activation by either agonist was dependent on an intact cell metabolism, as was secretion. It is concluded that (1) activation of phospholipase C does not always lead to activation of phospholipase A2, (2) phospholipase A2 is coupled to the receptor independently of phospholipase C via a pertussis-toxin-sensitive G-protein and (3) for secretion to take place, the receptor has to activate both phospholipases C and A2.