Leukotriene B4 stimulation of an early elevation of phosphatidic acid mass in human neutrophils

Polyisoprenyl phosphate signaling: topography in human neutrophils

To determine the relationship of polyisoprenyl phosphate (PIPP) remodeling and signaling to the activation state of human neutrophils (PMN), we examined the impact of leukotriene B(4) (LTB(4)) on the conversion of a unique bioactive isoprenoid (presqualene diphosphate: PSDP), recently identified as a novel endogenous signaling molecule. LTB(4) initiated rapid decrements in total PSDP that were concurrent with the respiratory burst (e.g., O(-2) formation). PSDP was identified in nuclear (39%)-, granule (36%)-, and plasma membrane (16%)-containing fractions of PMN. LTB(4) receptor (BLT) activation led to a decrease in nuclear PSDP and concomitant increase in granule-associated PSDP. In addition, PMN nuclei displayed PSDP associated with chromatin as established by mass spectrometry. Together, these results indicate that PSDP is present in membranes and receptor activation rapidly initiates subcellular PIPP remodeling (i.e., conversion) and distribution predominantly to granule membranes. Moreover, identification of nuclear PSDP provides the basis for novel roles for PIPP and PSDP in nuclear-associated signaling events.

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.

Formation of phosphatidic acid and subclasses of phosphatidylethanol in human neutrophils upon interleukin-8 stimulation

Previous studies showed that interleukin-8 (IL-8) stimulates phospholipase D hydrolysis of phosphatidylcholine to generate phosphatidic acid in human neutrophils. Phosphatidylcholine in these cells contains diacyl, alkylacyl and alkenylacyl subclasses. No studies have examined phospholipase D hydrolysis of the three subclasses of phosphatidylcholine in interleukin-8-stimulated neutrophils. We used a non-radioactive but very sensitive method to assess the relative distribution of the subclasses in phosphatidylethanol, which is derived from phospholipase D activity in ethanol-exposed neutrophils. We present evidence that the relative abundance of diacyl and alkylacyl subclasses in phosphatidylethanol is similar to that in phosphatidylcholine. Alkenylacyl subclass was also detectable in the phosphatidylethanol fraction, albeit as a minor subclass. Our findings suggest that phospholipase D catalyses the hydrolysis of diacyl, alkylacyl and alkenylacyl subclasses of phosphatidylcholine in neutrophils upon IL-8 stimulation.

Polyisoprenyl phosphates in intracellular signalling

In response to environmental stimuli, leukocyte membrane remodelling generates biologically active lipids that can serve as both intra- and extracellular mediators. There are several classes of lipids that can mediate inflammatory reactions. We report here on a new intracellular lipid signal that regulates oxygen-radical formation in neutrophils, a key response in microbial killing, inflammation and tissue injury. Screening of neutrophil-derived extracts rich in phosphorylated, non-saponifiable lipids revealed a potent inhibitor of superoxide anion (O2-) production. Structural analysis of biologically active fractions gave four major phosphorylated lipids: most abundant was presqualene diphosphate (PSDP). Upon activation of neutrophil receptors, PSDP and its monophosphate form, presqualene monophosphate (PSMP), undergo rapid remodelling. At submicromolar concentrations, PSDP but not PSMP inhibit O2- production by human neutrophil cell-free oxidase preparations. We prepared PSDP and PSMP by total organic synthesis and matched both the physical properties and biological activity of the neutrophil-derived compounds. Our results indicate that PSDP, a recognized intermediate of cholesterol biosynthesis, is present in immune effector cells and is a potent regulator of the cellular response in host defence.

Phosphatase activity regulates superoxide anion generation and intracellular signaling in human neutrophils

Phosphorylation of components of the neutrophil NADPH oxidase plays a critical role in activation and maintenance of superoxide anion (O2-) generation. To investigate the role of dephosphorylation by phosphatases in regulating O2- production, human neutrophils were treated with calyculin A, a potent inhibitor of protein phosphatases 1 and 2A, prior to stimulation. Calyculin A alone did not stimulate O2- production. However, neutrophils exposed to 50 nM calyculin A and the chemotactic peptide formyl-met-leu-phe (FMLP, 100 nM) displayed markedly enhanced O2- production in comparison to cells stimulated with FMLP alone (28.63 +/- 7.00 versus 8.69 +/- 3.69 nmol O2-/1.5 x 10(6) neutrophils/5 min, respectively, n = 18, p < 0.001), with an increased duration of O2- production. In contrast, phosphatase-inhibition decreased oxidative responsiveness to phorbol myristate acetate (PMA, > or = 16 nM). We next examined the effect of calyculin A on products of the phosphatidylcholine-specific phospholipase D (PLD) pathway by assaying the mass levels of phosphatidic acid (PA), choline and diacylglycerol (DAG). Calyculin A increased both PA and choline production to 224 +/- 28% and 315 +/- 61% of FMLP-stimulated controls, respectively (p < 0.01, n = 7) without significantly increasing DAG. Also, membrane protein kinase C activity increased more than 10-fold in FMLP-stimulated cells exposed to calyculin A but decreased in cells stimulated with PMA following calyculin A pre-treatment. These results suggest that phosphatases exert variable and stimulus-dependent effects on pathways leading to O2- production. Further, it appears that phospholipase D activity and PA generation represent important steps in the pathway for NADPH activation triggered by FMLP.