Protein kinase C is not involved in secretion by permeabilized human neutrophils

The generally accepted sequence of intracellular signal transduction involves: (1) cell surface receptor-ligand interactions; (2) activation of G-proteins; (3) activation of phospholipase C, leading to inositol phosphate (IP3), and diacylglycerol production; (4) parallel mobilization of intracellular Ca2+ by IP3, and; (5) activation of protein kinase C (PKC) by diacylglycerol and Ca2+, leading to; (6) cellular responses. Human neutrophils appear to utilize this cascade, at least in general, and some, but not all, elements of the intracellular signal cascade known to be operating in intact cells also function in permeabilized cell systems. We have previously shown that permeabilized neutrophils can be induced to secrete lysosomal enzymes in response to elevated levels of Ca2+ alone and this secretion can be synergistically enhanced by the presence of guanine nucleotides. We now show that Ca2+, in the presence and absence of guanine nucleotides, can stimulate the production of soluble inositol phosphates. Furthermore, neomycin, a putative inhibitor of phospholipase C, can block Ca2(+)-induced secretion. These data thus suggest a role for phospholipase C activity or its products in the transduction process. The next enzymatic activity 'downstream' is PKC. Consequently, we looked at the role Mg-ATP, one of the substrates of PKC, plays in degranulation by permeabilized neutrophils, We found no obligatory role for this nucleotide in the secretory process. We then looked at the activity of oleoyl-acetyl-glycerol (OAG), a synthetic diacylglycerol and PKC agonist, on degranulation. We found that OAG was largely additive with Ca2+. Another PKC agonist, phorbol myristate acetate (PMA), also did not display notable synergy. Finally, inhibitors of PKC activity were not capable of blocking secretion, either in the presence or absence of guanine nucleotides. Thus, while circumstantial evidence seems to point towards a requirement for phospholipase C activation and diacylglycerol production in secretion, we were unable to demonstrate the next putative step in signal transduction, namely activation of PKC.