Biochemical responses in activated human neutrophils mediated by protein kinase C and a Ca2+-requiring proteinase

Low concentrations of phorbol 12-myristate 13-acetate (PMA) elicit a specific response in human neutrophils, characterized by the production of oxygen radicals and the release into the medium of a membrane-bound serine proteinase (Pontremoli, S., Melloni, E., Michetti, M., Sacco, O., Sparatore, B., Salamino, F., Damiani, G. and Horecker, B. L. (1986) Proc. Natl. Acad. Sci. U. S. A., 83, 1685-1689). The following evidence indicates that this response is mediated by membrane-bound protein kinase C: 1) it is blocked by inhibitors of protein kinase C; and 2) it is enhanced in cells preloaded with leupeptin which prevents proteolysis of protein kinase C and its subsequent dissociation from the cell membrane. This response is not accompanied by significant exocytosis of granule enzymes. With higher concentrations of PMA, and more particularly on stimulation with formylmethionyl-leucyl-phenylalanine (fMLP) plus cytochalasin B, a substantial exocytosis of constituents of both specific and azurophil granules is observed. With fMLP, exocytosis of granule enzymes is the predominant event, with little production of H2O2 and negligible release of membrane-bound serine proteinase. Exocytosis promoted either by a high concentration of PMA or by fMLP is inhibited by leupeptin, indicating that it is due to the action of an intracellular Ca2+-dependent thiol proteinase (calpain), either directly or by conversion by calpain of membrane-bound protein kinase C to the soluble Ca2+/phospholipid-independent form. Intracellular mobilization of Ca2+ is also observed following stimulation with either PMA or fMLP, but only the latter results in a net increase in the intracellular concentration of free Ca2+; under these conditions maximum exocytosis of granule contents is observed.