Human neutrophil protein kinase C: calcium-induced changes in the solubility of the enzyme do not always correlate with enzymatic activity

We hypothesized that calcium and 1,2-diacylglycerols stimulated human neutrophil (PMN) protein kinase C (EC 2.7.1.37) in a two-step mechanism. The proposed mechanism entails (1) increased insoluble protein kinase C activity and (2) endogenous protein phosphorylation, events which have not been biochemically dissociated. PMN which were treated with 100 nM ionomycin shifted protein kinase C activity from being mostly soluble to insoluble. Concentrations of ionomycin greater than 300 nM stimulated a doubling of total cellular (soluble + insoluble) protein kinase activity and stimulated increased endogenous phosphorylation of PMN proteins. Intracellular calcium (measured with fura-2) increased from 65 nM (basal) to 680 nM using 500 nM ionomycin; calcium increases were dose-dependent. The anti-inflammatory agents acetylsalicylic acid and sodium salicylate (but not ibuprophen, indomethacin or acetaminophen) inhibited ionomycin-induced protein kinase C activation and protein phosphorylation in a dose-dependent manner by inhibiting the production of diacylglycerols. 1-Oleoyl-2-acetylglycerol reversed the inhibitory effect of salicylates. In contrast to the effect of acetylsalicylates on protein kinase C functional activity the distribution of phorbol receptors was unaffected in acetylsalicylate-treated, ionomycin-stimulated PMN using a phorbol-binding assay. Our results show that ionomycin increased intracellular diacylglycerol levels 3.5-fold over those present in control PMN, while acetylsalicylate decreased diacylglycerol production in ionomycin-stimulated PMN below baseline values. These results support the hypothesis that increased intracellular calcium activated protein kinase C leading to protein phosphorylation in two distinct dissociable events: (1) increased intracellular calcium; and (2) increased 1,2-diacylglycerol levels.