Granulocyte-macrophage colony-stimulating factor (GM-CSF) primes human neutrophils for enhanced phosphatidylcholine breakdown by phospholipase D

A phosphatidic acid-activated protein kinase and conventional protein kinase C isoforms phosphorylate p22(phox), an NADPH oxidase component

Using a phosphorylation-dependent cell-free system to study NADPH oxidase activation (McPhail, L. C., Qualliotine-Mann, D., and Waite, K. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7931-7935), we previously showed that p47(phox), a cytosolic NADPH oxidase component, is phosphorylated. Now, we show that p22(phox), a subunit of the NADPH oxidase component flavocytochrome b(558), also is phosphorylated. Phosphorylation is selectively activated by phosphatidic acid (PA) versus other lipids and occurs on a threonine residue in p22(phox). We identified two protein kinase families capable of phosphorylating p22(phox): 1) a potentially novel, partially purified PA-activated protein kinase(s) known to phosphorylate p47(phox) and postulated to mediate the phosphorylation-dependent activation of NADPH oxidase by PA and 2) conventional, but not novel or atypical, isoforms of protein kinase C (PKC). In contrast, all classes of PKC isoforms could phosphorylate p47(phox). In a gel retardation assay both the phosphatidic acid-dependent kinase and conventional PKC isoforms phosphorylated all molecules of p22(phox). These findings suggest that phosphorylation of p22(phox) by conventional PKC and/or a novel PA-activated protein kinase regulates the activation/assembly of NADPH oxidase.

Regulation of phospholipase D by phosphorylation-dependent mechanisms

The rapid production of phosphatidic acid following receptor stimulation has been demonstrated in a wide range of mammalian cells. Virtually every cell uses phosphatidylcholine as substrate to produce phosphatidic acid in a controlled reaction catalyzed by specific PLD isoforms. Considerable effort has been directed at studying the regulation of PLD activities and subsequent work has characterized a family of proteins including PLD1 and PLD2. Whereas both PLD enzymes are dependent on phosphatidylinositol 4, 5-bisphosphate for activity only the PLD1 isoform was strongly stimulated by the small GTPases ARF and RhoA and by protein kinase Calpha as well. A role for tyrosine kinase activities in the membrane recruitment of small GTPases, in the synthesis of phosphatidylinositol 4,5-bisphosphate and tyrosine phosphorylation of PLD1 and PLD2 has been uncovered. However, it still not clear exactly how tyrosine phosphorylation of proteins contributes to PLD activation in cells. Here we review the data linking tyrosine phosphorylation of proteins to the activation of PLD and describe recent finding on the sites and possible mechanisms of action of tyrosine kinases in receptor-mediated PLD activation. Finally, a model illustrating the potential complex interplay linking these signaling events with the activation of PLD is presented.

A novel protein kinase target for the lipid second messenger phosphatidic acid

Activation of phospholipase D occurs in response to a wide variety of hormones, growth factors, and other extracellular signals. The initial product of phospholipase D, phosphatidic acid (PA), is thought to serve a signaling function, but the intracellular targets for this lipid second messenger are not clearly identified. The production of PA in human neutrophils is closely correlated with the activation of NADPH oxidase, the enzyme responsible for the respiratory burst. We have developed a cell-free system, in which the activation of NADPH oxidase is induced by the addition of PA. Characterization of this system revealed that a multi-functional cytosolic protein kinase was a target for PA, and that two NADPH oxidase components were substrates for the enzyme. Partial purification of the PA-activated protein kinase separated the enzyme from known protein kinase targets of PA. The partially purified enzyme was selectively activated by PA, compared to other phospholipids, and phosphorylated the oxidase component p47-phox on both serine and tyrosine residues. PA-activated protein kinase activity was present in a variety of hematopoietic cells and cell lines and in rat brain, suggesting it has widespread distribution. We conclude that this protein kinase may be a novel target for the second messenger function of PA.

Stimulation of primed neutrophils by soluble immune complexes: priming leads to enhanced intracellular Ca2+ elevations, activation of phospholipase D, and activation of the NADPH oxidase

Soluble immune complexes activate a rapid burst of reactive oxidant secretion from neutrophils that have previously been primed with GM-CSF. Binding of these complexes to the cell surface of unprimed neutrophils results in the generation of intracellular Ca2+ transients, but the NADPH oxidase fails to become activated. No phospholipase D activity was observed following the addition of soluble immune complexes to unprimed cells. Upon priming with GM-CSF, the intracellular Ca2+ signal generated following soluble complex binding was greatly extended and phospholipase D was activated: there was also increased phosphorylation of proteins on tyrosine residues and the NADPH oxidase was activated. When Ca2+ influx was prevented, this phospholipase D activity was not observed. This primed oxidase activity was completely inhibited by erbstatin. Treatment of unprimed neutrophils with pervanadate (to inhibit protein tyrosine phosphatases) mimicked the effects of priming in that pervanadate-treated neutrophils secreted reactive oxidants in response to soluble immune complexes. The data indicate that during priming a new signaling pathway is activated that involves Ca2+ influx, phosphorylation on tyrosine residues, phospholipase D activity, and NADPH oxidase activation.

Immunoprecipitation of a phospholipase D activity with antiphosphotyrosine antibodies

When granulocyte-macrophage colony-stimulating factor (GM-CSF)-treated human neutrophils were challenged with the chemotactic factor fMet-Leu-Phe, it was possible to detect a time-dependent increase in the hydrolytic (as measured by the production of phosphatidic acid, PA) and the transphosphatidylation (as measured by the production of phosphatidylethanol, PEt) activities of phospholipase D in intact cells prelabeled with a radioactive fatty acid. Both activities were inhibited by preincubation of cells with genistein. Appropriate conditions were developed to test the PLD transphosphatidylation activity against exogenous phosphatidylcholine (PCho) in an in vitro system. As in intact cells, increased PLD activity could be detected in cell lysates obtained from fMet-Leu-Phe-treated cells compared with controls. When lysates were immunoprecipitated with antiphosphotyrosine antibodies, a PLD activity was found only in immune complexes that were prepared from fMet-Leu-Phe-treated cells. Conversely, no activity was found in lysates immunoprecipitated with an irrelevant antibody (GTPase-activating protein, GAP) that nevertheless was able to recognize a tyrosylphosphorylated form of GAP, as demonstrated by western blotting. These data suggest that a PCho-PLD, or a tightly bound protein, is tyrosine phosphorylated during cell activation.

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

New pathways of phagocyte activation: the coupling of receptor-linked phospholipase D and the role of tyrosine kinase in primed neutrophils

Protein kinase C (PKC) appears to have a central role in the O2- response of neutrophils following stimulation of membrane receptors. The second messenger, diacylglycerol (DG), that activates PKC is derived from membrane phospholipids via activation of phosphatidylinositol 4,5-bisphosphate (PIP2)-phospholipase C (PLC) and phospholipase D (PLD), with the latter pathway being more prominent in primed cells. In resting cells receptor coupling to PLD is through a G-protein. Priming brings a cytoplasmic tyrosine kinase into the transducer sequence which, through protein phosphorylation, increases the efficiency of coupling between membrane receptors and PLD. Phosphatidic acid (PA), the initial product of the PLD pathway, also appears to act as a second messenger by directly activating the NADPH oxidase responsible for generating O2-. Interconversion of PA and DG by phosphatidate phosphohydrolase and DG kinase determines which of these second messengers has the dominant role.