Ca2+-dependent conversion of phosphatidylinositol to phosphatidate in neutrophils stimulated with fMet-Leu-Phe or ionophore A23187

CDP-diacylglycerol, a critical intermediate in lipid metabolism

The roles of lipids expand beyond the basic building blocks of biological membranes. In addition to forming complex and dynamic barriers, the thousands of different lipid species in the cell contribute to essentially all the processes of life. Specific lipids are increasingly identified in cellular processes, including signal transduction, membrane trafficking, metabolic control and protein regulation. Tight control of their synthesis and degradation is essential for homeostasis. Most of the lipid molecules in the cell originate from a small number of critical intermediates. Thus, regulating the synthesis of intermediates is essential for lipid homeostasis and optimal biological functions. Cytidine diphosphate diacylglycerol (CDP-DAG) is an intermediate which occupies a branch point in lipid metabolism. CDP-DAG is incorporated into different synthetic pathways to form distinct phospholipid end-products depending on its location of synthesis. Identification and characterization of CDP-DAG synthases which catalyze the synthesis of CDP-DAG has been hampered by difficulties extracting these membrane-bound enzymes for purification. Recent developments have clarified the cellular localization of the CDP-DAG synthases and identified a new unrelated CDP-DAG synthase enzyme. These findings have contributed to a deeper understanding of the extensive synthetic and signaling networks stemming from this key lipid intermediate.

A short history of inositol lipids

The diverse family of inositol lipids is now known to be central to many aspects of cell biology. The route from the first discovery of inositol to our present day knowledge of inositol lipids spans more than 150 years and is long and complex. This is a brief account of some of the most important stages along that route.

Genome-wide and molecular evolution analyses of the phospholipase D gene family in Poplar and Grape

BACKGROUND

The Phospholipase D (PLD) family plays an important role in the regulation of cellular processes in plants, including abscisic acid signaling, programmed cell death, root hair patterning, root growth, freezing tolerance and other stress responses. PLD genes constitute an important gene family in higher plants. However, until now our knowledge concerning the PLD gene family members and their evolutionary relationship in woody plants such as Poplar and Grape has been limited.

RESULTS

In this study, we have provided a genome-wide analysis of the PLD gene family in Poplar and Grape. Eighteen and eleven members of the PLD gene family were identified in Poplar and Grape respectively. Phylogenetic and gene structure analyses showed that the PLD gene family can be divided into 6 subgroups: alpha, beta/gamma, delta, epsilon, zeta, and phi, and that the 6 PLD subgroups originated from 4 original ancestors through a series of gene duplications. Interestingly, the majority of the PLD genes from both Poplar (76.5%, 13/17) and Grape (90.9%, 10/11) clustered closely together in the phylogenetic tree to the extent that their evolutionary relationship appears more tightly linked to each other, at least in terms of the PLD gene family, than it does to either Arabidopsis or rice. Five pairs of duplicated PLD genes were identified in Poplar, more than those in Grape, suggesting that frequent gene duplications occurred after these species diverged, resulting in a rapid expansion of the PLD gene family in Poplar. The majority of the gene duplications in Poplar were caused by segmental duplication and were distinct from those in Arabidopsis, rice and Grape. Additionally, the gene duplications in Poplar were estimated to have occurred from 11.31 to 13.76 million years ago, which are later than those that occurred in the other three plant species. Adaptive evolution analysis showed that positive selection contributed to the evolution of the PXPH- and SP-PLDs, whereas purifying selection has driven the evolution of C2-PLDs that contain a C2 domain in their N-terminal. Analyses have shown that the C2-PLDs generally contain 23 motifs, more than 17 motifs in PXPH-PLDs that contain PX and PH domains in N-terminal. Among these identified motifs, eight, (6, 8, 5, 4, 3, 14, 1 and 19) were shared by both the C2- and PXPH-PLD subfamilies, implying that they may be necessary for PLD function. Five of these shared motifs are located in the central region of the proteins, thus strongly suggesting that this region containing a HKD domain (named after three conserved H, K and D residues) plays a key role in the lipase activity of the PLDs.

CONCLUSION

As a first step towards genome wide analyses of the PLD genes in woody plants, our results provide valuable information for increasing our understanding of the function and evolution of the PLD gene family in higher plants.

Phosphatidic acid regulation of phosphatidylinositol 4-phosphate 5-kinases

Phosphatidic acid (PA) production by receptor-stimulated phospholipase D is believed to play an important role in the regulation of cell function. The second messenger function of PA remains to be elucidated. PA can bind and affect the activities of different enzymes and here we summarise the current status of activation of Type I phosphatidylinositol 4-phosphate 5-kinase by PA. Type 1 phosphatidylinositol 4-phosphate 5-kinase is also regulated by ARF proteins as is phospholipase D and we discuss the contributions of ARF and PA towards phosphatidylinositol(4,5)bisphosphate synthesis at the plasma membrane.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

20 years of Ins(1,4,5)P3, and 40 years before

This year marks the 20th birthday of the discovery of inositol-1,4,5-trisphosphate as a second messenger. The background to this discovery is a complex story that goes back more than 50 years and involves a large cast of characters, both chemical and human.

Continual production of phosphatidic acid by phospholipase D is essential for antigen-stimulated membrane ruffling in cultured mast cells

Phospholipase Ds (PLDs) are regulated enzymes that generate phosphatidic acid (PA), a putative second messenger implicated in the regulation of vesicular trafficking and cytoskeletal reorganization. Mast cells, when stimulated with antigen, show a dramatic alteration in their cytoskeleton and also release their secretory granules by exocytosis. Butan-1-ol, which diverts the production of PA generated by PLD to the corresponding phosphatidylalcohol, was found to inhibit membrane ruffling when added together with antigen or when added after antigen. Inhibition by butan-1-ol was completely reversible because removal of butan-1-ol restored membrane ruffling. Measurements of PLD activation by antigen indicate a requirement for continual PA production during membrane ruffling, which was maintained for at least 30 min. PLD1 and PLD2 are both expressed in mast cells and green fluorescent protein-tagged proteins were used to identify PLD2 localizing to membrane ruffles of antigen-stimulated mast cells together with endogenous ADP ribosylation factor 6 (ARF6). In contrast, green fluorescent protein-PLD1 localized to intracellular vesicles and remained in this location after stimulation with antigen. Membrane ruffling was independent of exocytosis of secretory granules because phorbol 12-myristate 13-acetate increased membrane ruffling in the absence of exocytosis. Antigen or phorbol 12-myristate 13-acetate stimulation increased both PLD1 and PLD2 activity when expressed individually in RBL-2H3 cells. Although basal activity of PLD2-overexpressing cells is very high, membrane ruffling was still dependent on antigen stimulation. In permeabilized cells, antigen-stimulated phosphatidylinositol(4,5)bisphosphate synthesis was dependent on both ARF6 and PA generated from PLD. We conclude that both activation of ARF6 by antigen and a continual PLD2 activity are essential for local phosphatidylinositol(4,5)bisphosphate generation that regulates dynamic actin cytoskeletal rearrangements.

Differential expression of genes encoding Arabidopsis phospholipases after challenge with virulent or avirulent Pseudomonas isolates

Phospholipase D (PLD; EC 3.1.4.4) has been linked to a number of cellular processes, including Tran membrane signaling and membrane degradation. Four PLD genes (alpha, beta, gamma1, and gamma2) have been cloned from Arabidopsis thalami. They encode isoforms with distinct regulatory and catalytic properties but little is known about their physiological roles. Using cDNA amplified fragment length polymorphism display and RNA blot analysis, we identified Arabidopsis PLDgamma1 and a gene encoding a lysophospholipase (EC 3.1.1.5), lysoPL1, to be differentially expressed during host response to virulent and avirulent pathogen challenge. Examination of the expression pattern of phospholipase genes induced in response to pathogen challenge was undertaken using the lysoPL1 and gene-specific probes corresponding to the PLD isoforms a, beta, and gamma1. Each mRNA class exhibited different temporal patterns of expression after infiltration of leaves with Pseudomonas syringae pv. tomato with or without avrRpm1. PLDalpha was rapidly induced and remained constitutively elevated regardless of treatment. PLDbeta was transiently induced upon pathogen challenge. However, mRNA for the lysoPL1 and PLDgamma1 genes showed enhanced and sustained elevation during an incompatible interaction, in both ndr1 and overexpressing NahG genetic backgrounds. Further evidence for differential engagement of these PLD mRNA during defense responses, other than gene-for-gene interactions, was demonstrated by their response to salicylic acid treatment or wounding. Our results indicate that genes encoding lysoPL1, PLDgamma1, and PLDbeta are induced during early responses to pathogen challenge and, additionally, PLDyl and lysoPL1 are specifically upregulated during gene-for-gene interactions, leading to the hypersensitive response. We discuss the possible role of these genes in plant-pathogen interactions.

The Arabidopsis Phospholipase D Family. Characterization of a Calcium-Independent and Phosphatidylcholine-Selective PLDζ1 with Distinct Regulatory Domains1

Four types of phospholipase D (PLD), PLDα, β, γ, and δ, have been characterized in Arabidopsis, and they display different requirements for Ca²⁺, phosphatidylinositol 4,5-bisphosphate (PIP₂), substrate vesicle composition, and/or free fatty acids. However, all previously cloned plant PLDs contain a Ca²⁺-dependent phospholipid-binding C2 domain and require Ca²⁺ for activity. This study documents a new type of PLD, PLDζ1, which is distinctively different from previously characterized PLDs. It contains at the N terminus a Phox homology domain and a pleckstrin homology domain, but not the C2 domain. A full-length cDNA for Arabidopsis PLDζ1 has been identified and used to express catalytically active PLD in Escherichia coli. PLDζ1 does not require Ca²⁺ or any other divalent cation for activity. In addition, it selectively hydrolyzes phosphatidylcholine, whereas the other Arabidopsis PLDs use several phospholipids as substrates. PLDζ1 requires PIP₂ for activity, but unlike the PIP₂-requiring PLDβ or γ, phosphatidylethanolamine is not needed in substrate vesicles. These differences are described, together with a genomic analysis of 12 putative Arabidopsis PLD genes that are grouped into α, β, δ, γ, and ζ based on their gene architectures, sequence similarities, domain structures, and biochemical properties.

Involvement of phospholipase D in store-operated calcium influx in vascular smooth muscle cells

In non-excitable cells, sustained intracellular Ca2+ increase critically depends on influx of extracellular Ca2+. Such Ca2+ influx is thought to occur by a 'store-operated' mechanism, i.e. the signal for Ca2+ entry is believed to result from the initial release of Ca2+ from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Here we show that the depletion of cellular Ca2+ stores by thapsigargin or bradykinin is functionally linked to a phosphoinositide-specific phospholipase D (PLD) activity in cultured vascular smooth muscle cells (VSMC), and that phosphatidic acid formed via PLD enhances sustained calcium entry in this cell type. These results suggest a regulatory role for PLD in store-operated Ca2+ entry in VSMC.

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.

Phospholipase D and membrane traffic. Potential roles in regulated exocytosis, membrane delivery and vesicle budding

It is now well-established that phospholipase D is transiently stimulated upon activation by G-protein-coupled and receptor tyrosine kinase cell surface receptors in mammalian cells. Over the last 5 years, a tremendous effort has gone to identify the major intracellular regulators of mammalian phospholipase D and to the cloning of two mammalian phospholipase D enzymes (phospholipase D1 and D2). In this chapter, we review the physiological function of mammalian phospholipase D1 that is synergistically stimulated by ADP ribosylation factor, Rho and protein kinase Calpha. We discuss the function of this enzyme in membrane traffic, emphasising the possible integrated relationships between consumption of vesicles in regulated exocytosis, membrane delivery and constitutive membrane traffic.

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.

Phospholipase C-delta3 binds with high specificity to phosphatidylinositol 4,5-bisphosphate and phosphatidic acid in bilayer membranes

In order to acquire an understanding of phospholipase C-delta3 (PLC-delta3) action on substrate localized in lipid membrane we have studied the binding of human recombinant PLC-delta3 to large, unilamellar phospholipid vesicles (LUVs). PLC-delta3 bound weakly to vesicles composed of phosphatidylcholine (PtdCho) or PtdCho plus phosphatidylethanolamine (PtdEtn) or phosphatidylinositol (PtdIns). The enzyme bound strongly to LUVs composed of PtdEtn + PtdCho and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The binding affinity (molar partition coefficient) of PLC-delta3 to PtdEtn + PtdCho + PtdInsP2 vesicles was 7.7 x 105 m-1. High binding of PLC-delta3 was also observed for LUVs composed of phosphatidic acid (PA). Binding of PLC-delta3 to phosphatidylserine (PtdSer) vesicles was less efficient. Calculated molar partition coefficient for binding of PLC-delta3 to PA and PtdSer vesicles was 1.6 x 104 m-1 and 9.4 x 102 m-1, respectively. Presence of PA in the LUVs containing PtdInsP2 considerably enhanced the binding of PLC-delta3 to the phospholipid membrane. Binding of PLC-delta3 to phospholipid vesicles was not dependent on Ca2+ presence. In the liposome assay PA caused a concentration-dependent increase in activity of PLC-delta3. The stimulatory effect of PA on PLC-delta3 was calcium-dependent. At Ca2+ concentrations lower than 1 microm, no effect of PA on the activity of PLC-delta3 was observed. PA enhanced PLC-delta3 activity by increasing the Vmax and lowering Km for PtdInsP2. As the mol fraction of PA increased from 0-40 mol% the enzyme Vmax increased 2.3-fold and Km decreased threefold. Based on the results presented, we assume that PA supports binding of PLC-delta3 to lipid membranes by interaction with the PH domain of the enzyme. The stimulatory effect of PA depends on calcium-dependent interaction with the C2 domain of PLC-delta3. We propose that binding of PLC-delta3 to PA may serve as a mechanism for dynamic membrane association and modulation of PLC-delta3 activity.

Binding of GM-CSF to adherent neutrophils activates phospholipase D

When the hematopoietic growth factor granulocyte-macrophage colony-stimulating factor was incubated with neutrophils adherent to plastic tissue culture plates or plates coated with extracellular matrix proteins, a rapid (3 min) but transient formation of phosphatidic acid was observed. This stimulation was dependent on the dose of GM-CSF, with an EC50 of 140 pM, and was further enhanced (up to 350%) with the PA phosphatase inhibitor propranolol in a dose-dependent manner. Conversely, GM-CSF was unable to trigger any PA formation in neutrophils maintained in suspension, even in the presence of soluble fibronectin. However, GM-CSF did prime the cells for enhanced PA formation in the presence of a secondary stimulus (fMet-Leu-Phe or PAF). GM-CSF also caused a time-dependent stimulation of diacylglycerol formation in adherent, but not suspended, cells and elicited a time-dependent stimulation of phosphatidylethanol formation, with a concomitant decrease in the formation of PA only at early (< 7 min) times. These observations were consistent with a rapid activation of the enzyme phospholipase D in adherent cells stimulated with GM-CSF. Additional data indicated that the source of DAG was PLD coexisting with PLC, especially at later times ( > 7 min) of stimulation with GM-CSF. Finally, the formation of PA and PEt, and to a minor extent, DAG, were inhibited by the protein tyrosine kinase inhibitor erbstatin in conditions in which tyrosine phosphorylation occurred. Taken together the data indicate that GM-CSF rapidly activates PLD in adherent cells, which is responsible for the generation of PA. Thus, PLD activation is an early event in neutrophil signal transduction following exposure of adherent cells to GM-CSF.

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.

Phospholipase D: a novel major player in signal transduction

The role of the mammalian phospholipase D (PLD) in the control of key cellular responses has been recognised for a long time, but only recently have there been the reagents to properly study this very important enzyme in the signalling pathways, linking cell agonists with intracellular targets. With the recent cloning of PLD isoenzymes, their association with low-molecular-weight G proteins, protein kinase C and tyrosine kinases, the availability of antibodies and an understanding of the role of PLD product, phosphatidic acid (PA), in cell physiology, the field is gaining momentum. In this review, we will explore the molecular properties of mammalian PLD and its gene(s), the complexity of this enzyme regulation and the myriad physiological roles for PLD and PA and related metabolic products, with particular emphasis on a role in the activation of NADPH oxidase, or respiratory burst, leading to the generation of oxygen radicals.

Diacylglycerols and phosphatidates: which molecular species are intracellular messengers?

In eukaryotes, many receptor agonists use phospholipase-generated lipids as intracellular messengers. Receptor occupation stimulates the production of polyunsaturated 1,2-diacylglycerols by phosphatidylinositol-4,5-bisphosphate specific phospholipases C and/or of mono-unsaturated and saturated phosphatidates by phospholipase-D-catalysed phosphatidylcholine breakdown. The primary phospholipase products are rapidly metabolized: polyunsaturated 1,2-diacylglycerols are converted to polyunsaturated phosphatidates by diacylglycerol kinase; mono-unsaturated and saturated phosphatidates are dephosphorylated to give mono-unsaturated and saturated 1,2-diacylglycerols by phosphatidate phosphohydrolase. The phospholipase-generated polyunsaturated 1,2-diacylglycerols and mono-unsaturated and saturated phosphatidates appear to be intracellular messengers, whereas their immediate metabolites probably do not have signalling functions.

ADP-ribosylation factor and Rho proteins mediate fMLP-dependent activation of phospholipase D in human neutrophils

Activation of intact human neutrophils by fMLP stimulates phospholipase D (PLD) by an unknown signaling pathway. The small GTPase, ADP-ribosylation factor (ARF), and Rho proteins regulate the activity of PLD1 directly. Cell permeabilization with streptolysin O leads to loss of cytosolic proteins including ARF but not Rho proteins from the human neutrophils. PLD activation by fMLP is refractory in these cytosol-depleted cells. Readdition of myr-ARF1 but not non-myr-ARF1 restores fMLP-stimulated PLD activity. C3 toxin, which inactivates Rho proteins, reduces the ARF-reconstituted PLD activity, illustrating that although Rho alone does not stimulate PLD activity, it synergizes with ARF. To identify the signaling pathway to ARF and Rho activation by fMLP, we used pertussis toxin and wortmannin to examine the requirement for heterotrimeric G proteins of the Gi family and for phosphoinositide 3-kinase, respectively. PLD activity in both intact cells and the ARF-restored response in cytosol-depleted cells is inhibited by pertussis toxin, indicating a requirement for Gi2/Gi3 protein. In contrast, wortmannin inhibited only fMLP-stimulated PLD activity in intact neutrophils, but it has no effect on myr-ARF1-reconstituted activity. fMLP-stimulated translocation of ARF and Rho proteins to membranes is not inhibited by wortmannin. It is concluded that activation of Gi proteins is obligatory for ARF/Rho activation by fMLP, but activation of phosphoinositide 3-kinase is not required.

Regulated exocytosis in chromaffin cells. Translocation of ARF6 stimulates a plasma membrane-associated phospholipase D

The ADP-ribosylation factor (ARF) GTP-binding proteins have been implicated in a wide range of vesicle transport and fusion steps along the secretory pathway. In chromaffin cells, ARF6 is specifically associated with the membrane of secretory chromaffin granules. Since ARF6 is an established regulator of phospholipase D (PLD), we have examined the intracellular distribution of ARF6 and PLD activity in resting and stimulated chromaffin cells. We found that stimulation of intact chromaffin cells or direct elevation of cytosolic calcium in permeabilized cells triggered the rapid translocation of ARF6 from secretory granules to the plasma membrane and the concomitant activation of PLD in the plasma membrane. To probe the existence of an ARF6-dependent PLD in chromaffin cells, we measured the PLD activity in purified plasma membranes. PLD could be activated by a nonhydrolyzable analogue of GTP and by recombinant myristoylated ARF6 and inhibited by specific anti-ARF6 antibodies. Furthermore, a synthetic myristoylated peptide corresponding to the N-terminal domain of ARF6 inhibited both PLD activity and catecholamine secretion in calcium-stimulated chromaffin cells. The possibility that ARF6 participates in the exocytotic reaction by controlling a plasma membrane-bound PLD and thereby generating fusogenic lipids at the exocytotic sites is discussed.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Two phosphatidylethanol classes separated by thin layer chromatography are produced by phospholipase D in rat brain hippocampal slices

Noradrenaline- and ionomycin-stimulated as well as basal phospholipase D activity from rat hippocampus produced, in the presence of ethanol, two different classes of [32P]phosphatidylethanol (designated I and II), which were separated by thin layer chromatography. Endogenous labeling experiments using 3H-fatty acids showed that two different classes of phosphatidylcholine, separated by two-dimensional TLC, one enriched with high incorporation of [3H]arachidonic acid (B) and the other with [3H]myristic acid (A), were the most likely sources for the two classes of phosphatidylethanol. Experiments where individual 32P-phospholipids extracted from [32P]Pi-labeled hippocampal slices were incubated with cabbage phospholipase D, in the presence of ethanol, showed that each class of [32P]phosphatidylcholine, i.e. A and B, produced a different band of [32P]phosphatidylethanol, with the same mobility in TLC as phosphatidylethanol II and I, respectively.

Formyl-methionyl-leucyl-phenylalanine and a calcium ionophore A23187 reverse the inhibition of phorbol myristate acetate-induced oxidative burst by linoleic and oleic acid anilides

Linoleic and oleic acid anilides profoundly inhibited the production of reactive oxygen metabolites (ROM) in human polymorphonuclear leukocytes (PMNL) induced by a tumor promoter, phorbol myristate acetate (PMA). The addition of a Ca2+ ionophore, A23187, or a chemotactic peptide, formyl-methionyl-leucyl-phenylalanine (fMLP), readily reversed linoleic and oleic acid anilide-induced inhibiton of PMA-evoked respiratory burst in PMNL without affecting PMA-induced respiratory burst. fMLP or A23187 caused a marked increase in the production of ROM in PMNL that did not produce ROM after their co-exposure to PMA and cis-fatty acid anilides. This suggests a role for Ca2+ in this restoration of respiratory burst activity in PMNL. Oleic and linoleic acid anilides enhanced also respiratory burst in PMNL subsequent to their stimulation with fMLP. Interestingly, corresponding fatty acids, linoleic and oleic acid, also inhibited PMA-induced production of ROM in PMNL, but this inhibition was not reversed by A23187 or fMLP. These findings suggest that the aniline moiety of cis-fatty acids significantly modifies the effects of linoleic and oleic acids in the production of ROM in PMNL. Moreover, free intracellular Ca2+ may play a critical role in the activation of PMNL to produce ROM, and in the modulation of the effects of cis-fatty acid anilides.

ARF and PITP restore GTP gamma S-stimulated protein secretion from cytosol-depleted HL60 cells by promoting PIP2 synthesis

BACKGROUND

In many cell types, including neutrophils and HL60 cells, there is an absolute requirement for a GTP-dependent step to elicit Ca(2+)-regulated secretion. Neutrophils and HL60 cells secrete lysosomal enzymes from azurophilic granules; this secretion is inhibited by 1% ethanol, indicating that phosphatidate (PA) produced by phospholipase D (PLD) activity may be involved. PLD can use primary alcohols in preference to water during the hydrolytic step, generating the corresponding phosphatidylalcohol instead of PA, its normal product. As ARF (ADP-ribosylation factor) proteins regulate PLD activity and are implicated in constitutive vesicular traffic, we have investigated whether ARF is also required for GTP-dependent secretion in HL60 cells.

RESULTS

We have used a cell-permeabilization protocol that allows HL60 cells to become refractory to stimulation with GTP gamma S plus 10 microM Ca2+ with regard to secretion and PLD activity. Permeabilization with streptolysin O for 10 minutes permitted the loss of freely diffusable cytosolic proteins, including ARF proteins. Fractions derived from brain cytosol, enriched in ARF proteins, restored secretory function and PLD activity. The major contaminating protein present in these ARF-enriched fractions was identified as phosphatidylinositol transfer protein (PITP). Unexpectedly, PITP was also found to restore GTP gamma S-dependent secretion. Restoration of secretory function was characterized using recombinant proteins, rARF1 and rPITP alpha and rPITP beta. The rARF1 protein restored both secretory function and PLD activity, whereas PITP only restored secretory function. However, both ARF and PITP were capable of stimulating phosphatidylinositol bis phosphate (PIP2) synthesis.

CONCLUSIONS

ARF and PITP restore secretory function in cytosol-depleted cells when stimulated with GTP gamma S plus Ca2+. We have previously shown that PITP participates in the synthesis of PIP2. In comparison, ARF1 activates PLD, producing PA, which is a known activator of phosphatidylinositol-4-phosphate 5 kinase, the enzyme responsible for PIP2 synthesis. We propose that ARF and PITP both restore exocytosis by a common mechanism-promoting PIP2 synthesis.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

Biochemistry and cell biology of phospholipase D in human neutrophils

Neutrophils play a major role host defense against invading microbes. Recent studies have emphasized the importance of the phospholipase D (PLD) in the signalling cascade leading to neutrophil activation. Phospholipase D catalyzes the hydrolysis of phospholipids to generate phosphatidic acid with secondarily generation of diradylglycerol; both of these products have been implicated as second messengers. Herein, we discuss the regulation and the biochemistry of the receptor-regulated PLD in human neutrophils. In vivo and in vitro studies suggest an activation mode in which initial receptor-linked activation of phospholipase C generates diacylglycerol and inositol trisphosphate. The resulting calcium flux along with the diacylglycerol activate a conventional isoform of protein kinase C (PKC), probably PKC beta 1. This PKC, in turn phosphorylates a plasma membrane component resulting in PLD activation and a second outpouring of diradylglycerol. The small GTP-binding proteins, RhoA and ARF, also participate in this process, and synergize with a 50 kDa cytosolic regulatory factor.

Release of gelatinase A (matrix metalloproteinase 2) induced by photolysis of caged phosphatidic acid in HT 1080 metastatic fibrosarcoma cells

Phosphatidic acid (PA) is a putative novel messenger in signal transduction and membrane traffic. We have synthesized a photolyzable derivative of PA, termed caged PA (cPA), which may be utilized as a new tool in studies of PA-mediated cellular events. 1-(2-Nitrophenyl)diazoethane, synthesized from 2-nitroacetophenone, was reacted with dipalmitoyl-PA to yield a 1-(2-nitrophenyl)ethyl ester of PA. Photolysis of the compound by ultraviolet light resulted in the formation of phosphatidic acid. The structure of the compound and of its photolytic products was verified by NMR spectroscopy. The utility of cPA was examined in HT 1080 metastatic fibrosarcoma cells, in which the formation of PA by phospholipase D was implicated in laminin-induced release of gelatinase A (matrix metalloproteinase 2 (MMP-2)). The uptake of cPA by HT 1080 cells reached a plateau after 120 min of incubation. Ultraviolet illumination of cPA-loaded cells for 5 s resulted in photolysis of 1.8% of the cell-incorporated cPA. The photolysis of cPA caused a 2-fold elevation in the release of MMP-2 to the medium, whereas nonphotolyzed cPA caused no change in MMP-2 release. Moreover, the effect of cPA photolysis was significantly higher than that obtained with extracellularly introduced PA. Thus, the effect of laminin on MMP-2 secretion can be mimicked by photolysis of cPA, suggesting a pivotal role for phospholipase D in laminin-induced cancer cell invasiveness and metastasis. These results indicate that cPA could serve as a unique tool for studying the cellular roles of PA.

Stimulation of protein and DNA synthesis in mouse C2C12 satellite cells: evidence for phospholipase D-dependent and -independent pathways

In C2C12 myoblasts, 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated a phospholipase D (PLD) to degrade phosphatidylcholine (PC) as measured by the release of choline and an increase in the formation of phosphatidic acid (PA) (or phosphatidylbutanol [PtdBuOH] in the presence of 0.5% butanol). Exogenous PLD also stimulated choline release, PA and PtdBuOH formation. The protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC downregulation significantly inhibited the effects of TPA but Ro-31-8220 had no effect on PLD action. Neither basic Fibroblast Growth Factor (bFGF) or Epidermal Growth Factor (EGF) increased PLD activity. All agonists stimulated protein synthesis during both a 90 min and a 6 hr incubation and increased RNA accretion after 6 hr. The response at 90 min was not inhibited by the transcription inhibitor, actinomycin D. Ro-31-8220 and PKC downregulation significantly inhibited all the effects of TPA. In contrast, Ro-31-8220 significantly inhibited the increase in RNA accretion elicited by PLD but had no effect on the ability of agonists other than TPA to enhance protein synthesis. All agonists also stimulated thymidine incorporation into DNA. The effects of EGF, bFGF, and PLD were rapid and transient whereas that of TPA was delayed and sustained. Ro-31-8220 and PKC downregulation significantly inhibited the response due to TPA. Furthermore, Ro-31-8220 also significantly inhibited the effects elicited by EGF and PLD but not that induced by bFGF. In differentiated myotubes, TPA and PLD, but not bFGF or EGF, again stimulated choline release and PtdBuOH formation. However, all agents failed to stimulate protein synthesis and RNA accretion. The data demonstrate the presence in C2C12 myoblasts, but not differentiated myotubes, of both a PLD-dependent and PLD-independent pathway(s) leading to the stimulation of protein synthesis, RNA accretion, and DNA synthesis.

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.

Bradykinin stimulates phospholipase D in PC12 cells by a mechanism which is independent of increases in intracellular Ca2+

These experiments were designed to learn the role of bradykinin induced changes in intracellular Ca2+ in the activation of phospholipase D activity in PC12 cells. Ionomycin at a concentration of 0.1 microM caused an increase in intracellular Ca2+ comparable to bradykinin, but had no effect on phospholipase D activity. Carbachol, ATP, and thapsigargin also increased intracellular Ca2+ but had no effect on phospholipase D activity. Increases in intracellular Ca2+ may be a necessary but not a sufficient factor in the activation of phospholipase D. To investigate this issue, the bradykinin induced increase in intracellular Ca2+ was blocked by preincubating the cells in Ca(2+)-free media plus EGTA or in media containing the intracellular Ca2+ chelator BAPTA/AM. These preincubations completely blocked the bradykinin induced increase in intracellular Ca2+ but only attenuated the bradykinin mediated activation of phospholipase D. Physiological increases in intracellular Ca2+ apparently do not mediate the effect of bradykinin on phospholipase D.

EGF-induced increase in diacylglycerol, choline release, and DNA synthesis is extracellular calcium dependent

Previous studies have demonstrated a strict extracellular Ca2+ dependence for the G0 to G1 and G1 to S transition in growth factor-treated T51B rat liver cells that is associated with increased levels of protein kinase C activity. Consequently, we have examined these cells for changes in phospholipid-derived second messengers in response to epidermal growth factor (EGF) and thrombin in order to determine which signals are generated during the initiation of the G0 to G1 transition. Thrombin is coupled to a phosphoinositide hydrolyzing phospholipase C, as we have found a rapid Ca(2+)-independent increase in the levels of inositol 1,4,5-trisphosphate (Ins[1,4,5]P3), inositol 1,4-bisphosphate (Ins[1,4]P2), and inositol 4-monophosphate (Ins[4]P), as well as a concomitant, transient elevation in diacylglycerol. No changes in either intracellular or extracellular choline metabolites, or an increase in DNA synthesis, were found in response to thrombin. By contrast, treatment of T51B cells with EGF results in a slower, more prolonged extracellular Ca(2+)-dependent increase in both [3H]-glycerol radiolabeled diacyl-glycerol, and diacylglycerol mass, an increase in choline release into the extracellular medium, and eventually a substantial DNA synthesis. We were, however, unable to detect any changes in phosphatidylinositol (PtdIns) turnover, either by accumulation of inositol phosphates or by changes in phospholipids in response to EGF. These results indicate that DNA synthesis can readily occur in the absence of stimulated PtdIns turnover, and that PtdIns turnover is not sufficient in itself or necessary to induce DNA synthesis and is not necessary for a Ca(2+)-dependent increase in diacylglycerol. Moreover, we have demonstrated that the extracellular Ca(2+)-dependent increase in diacylglycerol levels in response to EGF is associated with an increase in extracellular choline release, which is indicative of an activation of a phosphatidylcholine-linked phospholipase D. These results suggest that diacylglycerol sources other than PtdIns's may be important in the extracellular Ca(2+)-dependent regulation of EGF-mediated cell replication.

Vasopressin Vla receptor-stimulated phospholipase D: differential regulation of transphosphatidylation and phospholipid hydrolysis by protein kinase C [corrected]

Phospholipase D belongs to a group of membrane associated phospholipases which have been shown to be activated by G-protein coupled neurotransmitter receptors. Phosphatidylcholine is the primary substrate for phospholipase D generating phosphatidic acid (PA) and choline. In the presence of 1% ethanol, phospholipase D catalyzes a transphosphatidylation reaction generating phosphatidylethanol (PEt) which is an indicator of phospholipase D activation. In the present study, we utilized Chinese hamster ovary (CHO) cells stably transfected with and expressing a rat V1a vasopressin receptor to study the regulation of phospholipase D by protein kinase C and calcium. Arginine-vasopressin (AVP) stimulated the release of 3H-PEt and 3H-PA in cells pre-labelled overnight with 3H-palmitic acid. The phorbol ester, phorbol 12-myristate 13-acetate (PMA), stimulated the release of PEt and PA that was additive with AVP over 15 min. However, long-term stimulation with PMA, which desensitizes protein kinase C, decreased PEt production while simultaneously increasing PA production. Differential regulation of PEt and PA production by PMA suggests the existence of more than one phospholipase D isoenzyme. Though differentially regulated by protein kinase C, both AVP-stimulated PEt and PA production required extracellular and not intracellular calcium.

Role of phospholipase D in laminin-induced production of gelatinase A (MMP-2) in metastatic cells

Metastatic spread depends critically upon the invasiveness of tumor cells, i.e. their ability to breach basement membranes by elaborating and secreting specific proteolytic enzymes such as gelatinase A (MMP-2). Laminin is a major constituent of the extracellular matrix that can trigger production of MMP-2 in metastatic cells, but not in non-metastatic cells. The present study was designed to examine the role of phospholipase D (PLD) and its product, phosphatidic acid, in the intracellular signal transduction mechanisms that mediate induction of MMP-2 by laminin. Here we show that stimulation of tumor cells with laminin results in a time- and dose-dependent activation of PLD. Laminin-induced production of MMP-2 is attenuated by 1-butanol, a competitive substrate of PLD that reduces PLD-catalyzed production of PA. Moreover, phosphatidic acid itself can induce production of MMP-2 in metastatic tumor cells. MMP-2 can also be induced by exposing the cells to exogenous bacterial PLD. Elevated cellular phosphatidic acid induces MMP-2 in metastatic ras-transformed 3T3 fibroblasts but, like laminin, fails to do so in normal cells. These data indicate that laminin-induced activation of PLD and consequent generation of phosphatidic acid are involved in a signal propagation pathway leading to induction of MMP-2 and enhanced invasiveness of metastatic tumor cells.

Phosphatidic acid activation of protein kinase C-zeta overexpressed in COS cells: comparison with other protein kinase C isotypes and other acidic lipids

Phosphatidic acid (PA) is produced rapidly in agonist-stimulated cells, but the physiological function of this PA is unknown. We have examined the effects of PA on distinct isoforms of protein kinase C (PKC) using a new cell-free assay system. Addition of PA to cytosol from COS cells overexpressing PKC-alpha, -epsilon or -zeta differentially-activated all three isotypes, as shown by PKC autophosphorylation, and prominent phosphorylation of multiple endogenous substrates. In the absence of Ca2+, the diacylglycerol-insensitive zeta-isotype of PKC was most strongly activated by both PA and bisPA, a newly identified product of activated phospholipase D, with each lipid inducing its own profile of protein phosphorylation. BisPA was also a strong activator of PKC-epsilon, but a weak activator of PKC-alpha. Ca2+, at > or = 0.1 microM, inhibited PA and bisPA activation of PKC-zeta, but did not affect PKC-epsilon activation. In contrast, PKC-alpha was strongly activated by PA only in the presence of Ca2+. BisPA-induced phosphorylations mediated by PKC-zeta could be mimicked in part by other acidic phospholipids and unsaturated fatty acids. PA activation of PKC-zeta was unique in that PA not only stimulated PKC-zeta-mediated phosphorylation of distinctive substrates, but also caused an upward shift in electrophoretic mobility of PKC-zeta, which was not observed with other acidic lipids or with PKC-alpha or -epsilon. We have presented evidence that this mobility shift is not caused by PKC-zeta autophosphorylation, but it coincides with physical binding of PA to PKC-zeta. These results suggest that in cells stimulated under conditions where intracellular Ca2+ is at (or has returned to) basal level, PA may be a physiological activator of PKC-zeta.

ADP-ribosylation factor, a small GTP-dependent regulatory protein, stimulates phospholipase D activity

The hydrolysis of phosphatidylcholine by phospholipase D (PLD) results in the production of phosphatidic acid and choline. An assay that uses an exogenous substrate was developed to measure this activity in membranes and solubilized preparations from HL60 cells. A cytosolic factor markedly enhanced PLD activity in membranes and was essential for GTP gamma S-dependent stimulation of an enriched preparation of PLD. The factor was purified to homogeneity from bovine brain cytosol and identified as a member of the ADP-Ribosylation Factor (ARF) subfamily of small G proteins. Subsequently, recombinant myristoylated ARF1 was found to be a better activator of PLD activity than was the nonmyristoylated form. ARF proteins have been implicated recently as factors for regulation of intracellular vesicle traffic. The current finding suggests that PLD activity plays a prominent role in the action of ARF and that ARF may be a key component in the generation of second messengers via phospholipase D.

Phospholipases and activation of the NADPH oxidase

The signal transductional mechanisms regulating the activation of NADPH oxidase, the respiratory burst enzyme in phagocytic cells, are not completely understood. Receptors for most physiologic stimuli trigger the activation of various phospholipases, including phospholipases A2, C, and D. The lipid mediators formed (arachidonic acid, 1,2-diacylglycerol, and phosphatidic acid) have been implicated as second messengers in the induction of the respiratory burst. In intact cells, we have correlated phospholipase D activation and the production of phosphatidic acid with the activation of NADPH oxidase, using the drug propranolol. Phosphatidic acid activated NADPH oxidase in a cell-free system, but the level of activation was low. 1,2-Diacylglycerol markedly enhanced NADPH oxidase activation by phosphatidic acid. The synergistic effect required the diacyl species, since mono- or tri-acylglycerols were ineffective. Phosphatidic acid could be replaced by either lysophosphatidic acid or phosphatidylserine, but not by phosphatidylcholine, phosphatidylethanolamine, or phosphatidylinositol, suggesting specificity for an anionic phospholipid. Since other cell-free activators of NADPH oxidase (arachidonic acid, sodium dodecyl sulfate) are also anionic amphiphiles, phosphatidic acid may directly interact with an enzyme component(s). The targets for phosphatidic acid and diacylglycerol in the cell-free system are currently under investigation. These results emphasize the critical importance of phospholipases, particularly phospholipase D, in the regulation of the respiratory burst.

Phosphatidic acid and phospholipase D both stimulate phosphoinositide turnover in cultured human keratinocytes

Phosphatidic acid (PA) induced a rapid dose-dependent increase in production of inositol phosphates in cultured adult human keratinocytes, peaking at 30 s. Natural and dioleoyl PA were equally effective, while other phospholipid classes had no effect. Lipid A was also active. Lyso-PA also induced inositol phosphate production, but contamination of the PA preparation by lyso-PA could not account for the effect of PA. The effect of PA could not be reproduced by treatment of cells with calcium ionophore. PA-induced inositol phosphate production could be inhibited (> 50%) by pre-treatment of cells with either pertussis toxin or 12-O-tetradecanoylphorbol 13-acetate, suggesting the involvement of a GTP-binding protein and a protein kinase C-mediated negative feedback mechanism. PA also stimulated release of arachidonic acid from keratinocytes. Treatment of cells with exogenous phospholipase D similarly induced inositol phosphate production in the keratinocytes. Since PA may be formed by receptor-mediated activation of phospholipase D, or by phosphorylation of diacylglycerol, the results suggest that PA may play a significant role in signalling mechanisms of human keratinocytes.

Correlation between secretion and phospholipase D activation in differentiated HL60 cells

Receptor-directed agonists including N-formylmethionyl-leucyl-phenylalanine (fMetLeuPhe), C5a, ATP and UTP all activate phospholipase D (PLD), which is accompanied by secretion in differentiated HL60 cells. Interference in the production of phosphatidase (PA) by the PLD pathway by diverting it towards the production of phosphatidylethanol (PEt) in the presence of ethanol leads to near-total inhibition of the secretion evoked by ATP and UTP and a partial inhibition of that evoked by fMetLeuPhe and C5a. In streptolysin-O-permeabilized cells, fMetLeuPhe is able to activate PLD, and this is dependent on the presence of a low concentration of guanosine 5'-[gamma-thio]-triphosphate (GTP[S]). Ca2+ (10 microM) and GTP[S] individually or in combination are also able to activate PLD and secretion. The stimulation of secretion in permeabilized cells stimulated by Ca2+ alone or fMetLeuPhe or GTP[S] is also abrogated when the production of PA is diverted to PEt by the presence of ethanol. Activation of PLD by GTP[S] or fMetLeuPhe is decreased if the cells are permeabilized first and GTP[S] or fMetLeuPhe is added subsequently. This corresponds well with the loss of the secretory response. We conclude that the ability of GTP[S] or fMetLeuPhe to stimulate secretion from permeabilized cells is dependent on a prior activation of the PLD signalling pathway. PA, generated as a consequence of PLD activation, acts as second messenger that can provide an initiating signal for secretion and is not required for exocytosis itself.

Association between leukotriene B4-induced phospholipase D activation and degranulation of human neutrophils

We have explored the role of phospholipase D (PLD) activation in leukotriene B4 (LTB4)-induced Ca2+ mobilization and degranulation of human neutrophils. Stimulation of [3H]alkyl-acyl-phosphatidylcholine-labeled neutrophils with LTB4 resulted in a rapid accumulation of [3H]alkyl-phosphatidic acid (PA) as well as a somewhat slower accumulation of [3H]alkyl-diglyceride (DG). In the presence of ethanol, PLD catalyzed a transphosphatidylation reaction in which LTB4 increased [3H]alkyl-phosphatidylethanol formation and simultaneously decreased LTB4-induced PA and DG accumulation. This pattern of lipid metabolism is consistent with the conclusion that LTB4 stimulates PLD activity in human neutrophils. Additional studies in which the extracellular and intracellular concentrations of Ca2+ were varied indicated that maximal LTB4-induced PLD activation was dependent upon Ca2+ and potentiated by inhibitors of protein kinase C. The time-course and concentration-response curves for LTB4-induced PLD activation were different from those for LTB4-induced Ca2+ mobilization, as measured by fura-2 fluorescence. On the other hand, the concentration-response curve for LTB4-induced PLD activation was similar to that for LTB4-induced degranulation. Preincubation of the cells with ethanol inhibited LTB4-induced PA and DG accumulation, as well as degranulation, suggesting that one or both of these metabolites were important for this response. In contrast, ethanol had no effect on LTB4-induced Ca2+ mobilization. Propranolol, an inhibitor of phosphatidate phosphohydrolase, abolished DG accumulation in response to LTB4 but had no effect on degranulation, suggesting that PA is more important than DG as a mediator of degranulation. Taken collectively, these data indicate that LTB4-induced activation of PLD in human neutrophils is mediated by a Ca(2+)-dependent mechanism, but not by protein kinase C. In addition, PLD activation in these cells may induce degranulation, but not Ca2+ mobilization.

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.