Phospholipase D activity in phagocytic leucocytes is synergistically regulated by G-protein- and tyrosine kinase-based mechanisms

β-1,3-Glucan-induced host phospholipase D activation is involved in Aspergillus fumigatus internalization into type II human pneumocyte A549 cells

The internalization of Aspergillus fumigatus into lung epithelial cells is a process that depends on host cell actin dynamics. The host membrane phosphatidylcholine cleavage driven by phospholipase D (PLD) is closely related to cellular actin dynamics. However, little is known about the impact of PLD on A. fumigatus internalization into lung epithelial cells. Here, we report that once germinated, A. fumigatus conidia were able to stimulate host PLD activity and internalize more efficiently in A549 cells without altering PLD expression. The internalization of A. fumigatus in A549 cells was suppressed by the downregulation of host cell PLD using chemical inhibitors or siRNA interference. The heat-killed swollen conidia, but not the resting conidia, were able to activate host PLD. Further, β-1,3-glucan, the core component of the conidial cell wall, stimulated host PLD activity. This PLD activation and conidia internalization were inhibited by anti-dectin-1 antibody. Indeed, dectin-1, a β-1,3-glucan receptor, was expressed in A549 cells, and its expression profile was not altered by conidial stimulation. Finally, host cell PLD1 and PLD2 accompanied A. fumigatus conidia during internalization. Our data indicate that host cell PLD activity induced by β-1,3-glucan on the surface of germinated conidia is important for the efficient internalization of A. fumigatus into A549 lung epithelial cells.

InlB-mediated Listeria monocytogenes internalization requires a balanced phospholipase D activity maintained through phospho-cofilin

Internalization of Listeria monocytogenes into non-phagocytic cells is tightly controlled by host cell actin dynamics and cell membrane alterations. However, knowledge about the impact of phosphatidylcholine cleavage driven by host cell phospholipase D (PLD) on Listeria internalization into epithelial cells is limited. Here, we report that L. monocytogenes activates PLD in Vero cells during the internalization. With immunostaining it was shown that both PLD1 and PLD2 surrounded partially or completely the phagocytic cup of most L. monocytogenes. Either up- or down-regulation of PLD expression (activity) diminished Listeria internalization. Both PLD1 and PLD2 in Vero cells were required for efficient Listeria internalization, and could substitute for each other in the regulation of Listeria internalization. Further, exogenous InlB activated host cell PLD1 and PLD2 via the Met receptor, and restored host PLD activation by InlB-deficient L. monocytogenes. InlB-induced PLD activation and Listeria internalization were tightly controlled by phospho-cycling of cofilin. PLD1, but not PLD2, was involved in cofilin-mediated PLD activation and Listeria internalization. These data indicate that cofilin-dependent PLD activation induced by InlB may represent a novel regulation mechanism for efficient Listeria internalization into epithelial cells.

Understanding phospholipase D (PLD) using leukocytes: PLD involvement in cell adhesion and chemotaxis

Phospholipase D (PLD) is an enzyme that catalyzes the conversion of membrane phosphatidylcholine to choline and phosphatidic acid (PA; a second messenger). PLD is expressed in nearly all types of leukocytes and has been associated with phagocytosis, degranulation, microbial killing, and leukocyte maturation. With the application of recently developed molecular tools (i.e., expression vectors, silencing RNA, and specific antibodies), the demonstration of a key role for PLD in those and related cellular actions has contributed to a better awareness of its importance. A case in point is the recent findings that RNA interference-mediated depletion of PLD results in impaired leukocyte adhesion and chemotaxis toward a gradient of chemokines, implying that PLD is necessary for leukocyte movement. We forecast that based on results such as those, leukocytes may prove to be useful tools to unravel still-unresolved mechanistic issues in the complex biology of PLD. Three such issues are considered here: first, whether the cellular actions of PLD are mediated entirely by PA (the product of its enzymatic reaction) or whether PLD by itself interacts with other protein signaling molecules; second, the current difficulty of defining a "PA consensus site" in the various intracellular protein targets of PA; and third, the resolution of specific PLD location (upstream or downstream) in a particular effector signaling cascade. There are reasons to expect that leukocytes and their leukemic cell line counterparts will continue yielding invaluable information to cell biologists to resolve standing molecular and functional issues concerning PLD.

Phospholipase D1 Regulates Phagocyte Adhesion

Adhesion is a fundamental cellular response that is essential to the physiologic processes of development, differentiation, proliferation, and motility, as well as to the pathology of inflammation, transformation, and metastasis. Adhesion of phagocytic leukocytes is a critical modulator of antimicrobial and cytotoxic functions, including the respiratory burst, secretion, and apoptosis. Because phospholipase D (PLD) is linked to several signaling pathways implicated in these processes, we tested the hypothesis that PLD regulates phagocyte adhesion. Adhesion of primary human neutrophils and monocyte-derived macrophages to fibronectin was accompanied by marked stimulation of PLD activity. Similarly, adhesion of both human (PLB, THP-1) and murine (RAW) myeloid-macrophage cell lines to fibronectin, fibrinogen, collagen, or plastic resulted in significant activation of PLD. Stimulation of PLD activity was rapid and persisted for at least 90 min. Confocal microscopy indicated that PLD1 exhibited partial colocalization with actin filaments at the adherent interface, in proximity to the focal adhesion protein, paxillin. Reductions in PLD activity by chemical inhibitors or specific short-interfering RNA-induced knockdown of PLD1 resulted in significant inhibition of phagocyte adhesion and was accompanied by reductions in total cellular F-actin. These data support the hypotheses that adhesion stimulates PLD activity, and that PLD1 regulates the initial stages of phagocyte adhesion. Stimulation of PLD activity may promote adhesion-dependent phagocyte effector responses.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Phospholipases D1 and D2 Coordinately Regulate Macrophage Phagocytosis

Phagocytosis is a fundamental feature of the innate immune system, required for antimicrobial defense, resolution of inflammation, and tissue remodeling. Furthermore, phagocytosis is coupled to a diverse range of cytotoxic effector mechanisms, including the respiratory burst, secretion of inflammatory mediators and Ag presentation. Phospholipase D (PLD) has been linked to the regulation of phagocytosis and subsequent effector responses, but the identity of the PLD isoform(s) involved and the molecular mechanisms of activation are unknown. We used primary human macrophages and human THP-1 promonocytes to characterize the role of PLD in phagocytosis. Macrophages, THP-1 cells, and other human myelomonocytic cells expressed both PLD1 and PLD2 proteins. Phagocytosis of complement-opsonized zymosan was associated with stimulation of the activity of both PLD1 and PLD2, as demonstrated by a novel immunoprecipitation-in vitro PLD assay. Transfection of dominant-negative PLD1 or PLD2 each inhibited the extent of phagocytosis (by 55-65%), and their combined effects were additive (reduction of 91%). PLD1 and PLD2 exhibited distinct localizations in resting macrophages and those undergoing phagocytosis, and only PLD1 localized to the phagosome membrane. The COS-7 monkey fibroblast cell line, which has been used as a heterologous system for the analysis of receptor-mediated phagocytosis, expressed PLD2 but not PLD1. These data support a model in which macrophage phagocytosis is coordinately regulated by both PLD1 and PLD2, with isoform-specific localization. Human myelomonocytic cell lines accurately model PLD-dependent signal transduction events required for phagocytosis, but the heterologous COS cell system does not.

ATP concentrations and muscle tension increase linearly with muscle contraction

Previous studies have suggested that activation of ATP-sensitive P2X receptors in skeletal muscle play a role in mediating the exercise pressor reflex (Li J and Sinoway LI. Am J Physiol Heart Circ Physiol 283: H2636-H2643, 2002). To determine the role ATP plays in this reflex, it is necessary to examine whether muscle interstitial ATP (ATPi) concentrations rise with muscle contraction. Accordingly, in this study, muscle contraction was evoked by electrical stimulation of the L7 and S1 ventral roots of the spinal cord in 12 decerebrate cats. Muscle ATPi was collected from microdialysis probes inserted in the muscle. ATP concentrations were determined by the HPLC method. Electrical stimulation of the ventral roots at 3 and 5 Hz increased mean arterial pressure by 13 +/- 2 and 16 +/- 3 mmHg (P < 0.05), respectively, and it increased ATP concentration in contracting muscle by 150% (P < 0.05) and 200% (P < 0.05), respectively. ATP measured in the opposite control limb did not rise with ventral root stimulation. Section of the L7 and S1 dorsal roots did not affect the ATPi seen with 5-Hz ventral root stimulation. Finally, ventral roots stimulation sufficient to drive motor nerve fibers did not increase ATP in previously paralyzed cats. Thus ATPi is not largely released from sympathetic or motor nerves and does not require an intact afferent reflex pathway. We conclude that ATPi is due to the release of ATP from contracting skeletal muscle cells.

Regulation of phospholipase D activity by actin. Actin exerts bidirectional modulation of Mammalian phospholipase D activity in a polymerization-dependent, isoform-specific manner

Many critical cellular processes, including proliferation, vesicle trafficking, and secretion, are regulated by both phospholipase D (PLD) and the actin microfilament system. Stimulation of human PLD1 results in its association with the detergent-insoluble actin cytoskeleton, but the molecular mechanisms and functional consequences of PLD-actin interactions remain incompletely defined. Biochemical and pharmacologic modulation of actin polymerization resulted in complex bidirectional effects on PLD activity, both in vitro and in vivo. Highly purified G-actin inhibited basal and stimulated PLD activity, whereas F-actin produced the opposite effects. Actin-induced modulation of PLD activity was independent of the activating stimulus. The efficacy and potency of the effects of actin were isoform-specific but broadly conserved among actin family members. Human betagamma-actin was only 45% as potent and 40% as efficacious as rabbit skeletal muscle alpha-actin, whereas its inhibitory profile was similar to the single actin species from the yeast, Saccharomyces cerevisiae. Use of actin polymerization-specific reagents indicated that PLD1 binds both monomeric G-actin, as well as actin filaments. These data are consistent with a model in which the physical state of the actin cytoskeleton is a critical determinant of its regulation of PLD activity.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Presence of a phospholipase D (PLD) distinct from PLD1 or PLD2 in human neutrophils: immunobiochemical characterization and initial purification

Utilizing the transphosphatidylation reaction catalyzed by phospholipase D (PLD) in the presence of a primary alcohol and the short-chain phospholipid PC8, we have characterized the enzyme from human neutrophils. A pH optimum of 7.8-8.0 was determined. PIP(2), EDTA/EGTA, and ATP were found to enhance basal PLD activity in vitro. Inhibitory elements were: oleate, Triton X-100, n-octyl-beta-glucopyranoside, divalent cations, GTPgammaS and H(2)O(2). The apparent K(m) for the butanol substrate was 0.1 mM and the V(max) was 6.0 nmol mg(-1) h(-1). Immunochemical analysis by anti-pan PLD antibodies revealed a neutrophil PLD of approximately 90 kDa and other bands recognized minimally by anti-PLD1 or anti-PLD2 antibodies. The 90-kDa protein is tyrosine-phosphorylated upon cell stimulation with GM-CSF and formyl-Met-Leu-Phe. Protein partial purification using column liquid chromatography was performed after cell subfractionation. Based on the enzyme's regulatory and inhibitory factors, and its molecular weight, these data indicate an enzyme isoform that might be different from the mammalian PLD1/2 forms described earlier. The present results lay the foundation for further purification of this granulocyte PLD isoform.

Regulation of phospholipase D2 by H(2)O(2) in PC12 cells

Phospholipase D2 (PLD2) is expressed in brain and inhibited by synuclein, which is involved in Parkinson's and Alzheimer's diseases. However, the activation mechanism of PLD2 in neuronal cells has not been defined clearly. Hydrogen peroxide (H(2)O(2)) plays roles in the neurodegenerative diseases and also acts as a second messenger of various molecules such as nerve growth factor. To study regulation mechanisms of PLD2 by H(2)O(2) in neuronal cells, we have made stable PC12 cell lines expressing PLD2 (PLD2-PC12 cells). H(2)O(2) treatment stimulated PLD activity in PLD2-PC12 cells in a dose- and time-dependent manner. This activation was inhibited by the treatment with protein kinase C (PKC) inhibitors or by depletion of PKCalpha, -delta, and -epsilon. Phorbol ester markedly activated PLD2. Co-treatment with phorbol ester and H(2)O(2) did not show an additive effect. Chelation of extracellular calcium substantially blocked the H(2)O(2)-induced activation of PLD2. A calcium ionophore induced PLD2 activation in a PKC-dependent manner. Protein-tyrosine kinase inhibitors inhibited H(2)O(2)-induced PLD activation slightly. These data indicate that H(2)O(2) can activate PLD2 in PC12 cells and that this activation is largely dependent on PKC and Ca(2+) ions and minimally dependent on tyrosine phosphorylation.

Phosphatidylcholine-specific phospholipase C-mediated induction of phospholipase D activity in Fas-expressing murine cells

We have previously reported that Fas cross-linking resulted in the activation of phosphatidylcholine-specific phospholipase C (PC-PLC) and the subsequent activation of protein kinase C (PKC) and phospholipase D (PLD) in A20 cells. In an attempt to correlate the existence of PC-PLC activity and activation of PLD by Fas activation among various Fas-expressing murine cell lines, we have investigated the effect of anti-Fas monoclonal antibody on PC-PLC and PLD activities in A20, P388D1 and YAC-1 cell lines. Upon treatment of anti-Fas monoclonal antibody to these three cell lines, the activation of PLD was only observed in A20 cells. When the effect of anti-Fas monoclonal antibody on PKC and PC-PLC activities in Fas-expressing clones were investigated, the activation of PKC and PC-PLC was detected only in A20 clones. Results presented here also show that exogenous addition of Bacillus cereus PC-PLC activates PC hydrolysis, PKC and PLD in all three murine cell lines. These findings suggest that the activation of PC-PLC is a necessary requirement for the activation of PLD by Fas cross-linking and cell lines devoid of functional PC-PLC activity could exhibit enhanced PLD activity by exogenous addition of PC-PLC.

Regulation of phospholipase D from human hepatocarcinoma cell line by purine nucleotides and protein kinase A

The regulation of phosphatidylcholine-specific phospholipase D by purine nucleotides and protein kinase A were studied in vitro using an enzyme preparation partially purified from the membranous fraction of 7721 hepatocarcinoma cells. It was found that the enzyme activity was elevated by low concentrations of some purine nucleotides, but the activating effects were decreased when the concentrations of the nucleotides were higher. The optimal concentrations of GTP, GTPgamma[S], GDP and ATP for maximal activation were 0.1 mM, 5 microM, 1 mM and 1 mM respectively. The activation caused by 1 mM ADP was lower. The enzyme was not activated by 1 mM AMP, but significant activation was observed by the addition of 1 mM cAMP. The latter was mediated by protein kinase A, as a specific inhibitor of protein kinase A abolished the activation. There were synergic effects between ATP and GTP, ATP and PIP2, but not between ATP and GTPgamma[S], or PIP2 and GTPgamma[S]. The activating effects of GTP and ATP were abolished by neomycin, a PIP2 scavenger. These results suggest that phospholipase D is regulated by GTP-binding protein and the presence of PIP2 is required for the activation induced by GTP. Protein kinase A may be another protein kinase in addition to protein kinase C and protein tyrosine kinase which regulate the activity of phospholipase D, when the intracellular concentration of cAMP is increased.

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.

Differentiation of U937 cells enables a phospholipase D-dependent pathway of cytosolic phospholipase A2 activation

Treatment with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line results in differentiation toward a monocyte/granulocyte-like cell. This differentiation enables the cell to activate cytosolic phospholipase A2 (cPLA2) to release arachidonate upon stimulation. In contrast, undifferentiated cells are unable to release arachidonate even when stimulated with calcium ionophores. In the present research, a role for phospholipase D (PLD) in the regulation of cPLA2 was shown based on a number of observations. First, the ionomycin- and fMLP-stimulated production of arachidonate in differentiated cells was sensitive to ethanol (2% (v/v)). Ethanol acts as an alternate substrate in place of water for PLD producing phosphatidylethanol (PEt) instead of phosphatidic acid. Indeed, ionomycin stimulation of differentiated cells produced a 14-fold increase in PEt levels. Further evidence for the involvement of PLD in the regulation of cPLA2 came from the observation that the stimulated production of diacylglycerol (for which phosphatidic acid is a major source) was greatly diminished in undifferentiated cells as compared to differentiated cells. Moreover, the normally deficient activation of cPLA2 in undifferentiated cells could be stimulated to release arachidonate if the cells were electroporated in the presence of GTP[gamma]S and MgATP. This treatment stimulates phosphatidylinositol-4,5-bisphosphate (PIP2) production which appears to activate PLD and cPLA2 in subsequent steps. The phosphatidic acid (and diacylglycerol derived from phosphatidic acid) appears to greatly regulate the action of cPLA2 by an unknown mechanism, and undifferentiated cells lack the ability to stimulate PLD activity due to a dysfunction of PIP2 production.

Upregulation of macrophage plasma membrane and nuclear phospholipase D activity on ligation of the alpha2-macroglobulin signaling receptor: involvement of heterotrimeric and monomeric G proteins

The effect of ligating the alpha2-macroglobulin signaling receptor (alpha2MSR) with receptor-recognized forms of alpha2M (alpha2M*) was studied with respect to phospholipase D (PLD) activity in murine macrophages, their plasma membranes, and nuclei. PLD activity in plasma membranes and nuclei increased linearly up to a ligand concentration of about 100 pM of either alpha2M* or a cloned and expressed receptor binding fragment (RBF). The RBF binding site mutant K1370A, which binds with high affinity to alpha2MSR, also increased nuclear PLD activity comparable to RBF and alpha2M*. Phorbol dibutyrate caused a two- to threefold stimulation of membrane and nuclear PLD activity, whereas PLD activity was nearly abolished by downregulation of protein kinase C; prior treatment with staurosporin, genestein, cyclosporin A, actinomycin D; or chelation of intracellular Ca2+. In permeabilized macrophages, isolated plasma membranes, and nuclei, GTP-gamma-S increased alpha2M*-stimulated PLD activity via a pertussis toxin-insensitive G protein and this effect was abolished on preincubation with GDP-beta-S. Incubation of plasma membranes with polyclonal antibody against sARFII, or the addition of cytosol which was immunoprecipitated with antibody against sARFII, greatly reduced alpha2M*-stimulated PLD activity in the presence of GTP-gamma-S. Preincubation of plasma membranes with GDP-beta-S prior to the addition of GTP-gamma-S and recombinant ARF1 significantly inhibited alpha2M*-stimulation of PLD activity. Nuclear PLD activity was maximally stimulated in the presence of both GTP-gamma-S and rARF1, whereas plasma membrane PLD activity was maximally stimulated in the presence of rARF1, GTP-gamma-S, RhoA, and ATP. In contrast, nuclear PLD activity was not affected by RhoA either alone or in combination with GTP-gamma-S or ATP.

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.

Association of phospholipase D activity with the detergent-insoluble cytoskeleton of U937 promonocytic leukocytes

Phospholipase D (PLD) regulates cytoskeletal-dependent antimicrobial responses of myeloid leukocytes, including phagocytosis and oxidant generation. However, the mechanisms responsible for this association between PLD activity and the actin cytoskeleton are unknown. We utilized a cell-free system from U937 promonocytes to test the hypothesis that stimulation of PLD results in stable association of the activated lipase with the detergent-insoluble membrane skeleton. Plasma membrane and cytosol were incubated +/- guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), followed by re-isolation and extraction of the washed membranes with octyl glucoside. The detergent-insoluble fraction derived from membranes incubated with GTPgammaS (DIFGTPgammaS) exhibited 22-fold greater PLD activity than that derived from control membranes (DIF0), when both were assayed in the presence of GTPgammaS. The DIF contained PLD1, RhoA, and ARF, and the level of each was increased by GTPgammaS in a dose-dependent manner. The DIF also contained F-actin, vinculin, talin, paxillin, and alpha-actinin, consistent with its identification as the membrane skeleton. The physiologic relevance of these findings was demonstrated by a similar increase in DIF-associated PLD activity after stimulation of intact U937 cells with opsonized zymosan. These results indicate that stimulation of PLD1 is accompanied by stable association of the activated lipase, RhoA, and ADP-ribosylation factor with the actin-based membrane skeleton.

Involvement of tyrosine phosphorylation and protein kinase C in the activation of phospholipase D by H2O2 in Swiss 3T3 fibroblasts

We have investigated the mechanisms involved in H2O2-mediated phospholipase D (PLD) activation in Swiss 3T3 fibroblasts. In the presence of vanadate, H2O2 induced tyrosine phosphorylation of PLD as well as the platelet-derived growth (PDGF) factor receptor, protein kinase Calpha (PKCalpha), and a 62-kDa protein in rat brain PLD1 (rPLD1) immune complexes. PDGF also induced tyrosine phosphorylation of PLD, but this was abolished by catalase, indicating that it was mediated by H2O2 generation. Interestingly, PLD was found to be constitutively associated with the PDGF receptor and PKCalpha. Stimulation by H2O2 showed a concentration- and time-dependent tyrosine phosphorylation of the proteins in rPLD1 immunoprecipitates and activation of PLD in the cells. Pretreatment of the cells with the protein-tyrosine kinase inhibitors genistein and herbimycin A resulted in a concentration-dependent inhibition of H2O2-induced tyrosine phosphorylation and PLD activation. Activation of PLD by H2O2 was also inhibited dose-dependently by the PKC inhibitors Ro 31-8220 and calphostin C. Down-regulation of PKC by prolonged treatment with 4beta-phorbol 12-myristate 13-acetate also abolished H2O2-stimulated PLD activity. H2O2 or vanadate alone did not induce tyrosine phosphorylation of proteins in the rPLD1 immune complex or PLD activation. Reduction of intracellular H2O2 levels by pretreatment of the cells with catalase dramatically abrogated tyrosine phosphorylation of proteins in the rPLD1 immune complex and PLD activation, suggesting the potential role of intracellular H2O2 in H2O2-mediated PLD signaling. Taken together, these results suggest that both protein-tyrosine kinase(s) and protein kinase C participate in H2O2-induced PLD activation in Swiss 3T3 cells.

Characterization of human PLD2 and the analysis of PLD isoform splice variants

Phospholipase D (PLD) cleaves phosphatidylcholine in response to a variety of cell stimuli to release phosphatidic acid, which is associated with a number of cellular responses including regulated secretion, mitogenesis, and cytoskeletal changes. Recent advances in this field include the reports of cDNA sequences for two mammalian PLD isoforms: human PLD1 and rodent PLD2. We report the characterization of cDNA encoding human PLD2. In these experiments, we uncovered alternate splice variants of both human isoforms and evaluated the relative abundance of these messages by reverse transcriptase polymerase chain reaction, thereby indicating the physiologically relevant forms. Further, Northern hybridization experiments defined the tissue distribution of the human PLD messages. Human PLD1 does not appear to be an abundant message in any tissue tested whereas levels of human PLD2 mRNA apparently were higher and more variable. The specific activity and regulation of recombinant human PLD2 are indistinguishable from that of recombinant mouse PLD2. Analysis of the amino acid sequences of both human isoforms revealed important putative Pleckstrin homology domains and identified additional members of the PLD gene family that help to delimit the catalytic domain. The presence of Pleckstrin homology domains in the PLDs resolves several contradictory observations regarding PLD regulation and the domain structure of the proteins.

Tyrosine-phosphorylation-dependent and rho-protein-mediated control of cellular phosphatidylinositol 4,5-bisphosphate levels

The polyphosphoinositide PtdIns(4,5)P2, best known as a substrate for phospholipase C isozymes, has recently been recognized to be involved in a variety of other cellular processes. The aim of this study was to examine whether the cellular levels of this versatile phospholipid are controlled by tyrosine phosphorylation. The studies were performed in human embryonic kidney (HEK)-293 cells stably expressing the M3 muscarinic acetylcholine receptor. Inhibition of tyrosine phosphatases by pervanadate induced an up-to-approx.-2. 5-fold increase in the total cellular level of PtdIns(4,5)P2, which was both time- and concentration-dependent. In contrast, the tyrosine kinase inhibitors, genistein and tyrphostin 23, caused a rapid and specific fall in the cellular PtdIns(4,5)P2 level and prevented the stimulatory effect of pervanadate on PtdIns(4,5)P2 formation. Inactivation of Rho proteins by Clostridium difficile toxin B caused a similar fall in the HEK-293 cell PtdIns(4,5)P2 level, which was not altered by additional genistein treatment. Furthermore, toxin B treatment abolished the pervanadate-induced increase in PtdIns(4,5)P2 levels. As PtdIns(4,5)P2 is an essential stimulatory cofactor for phospholipase D (PLD) enzymes, we finally examined the effects of the agents regulating PtdIns(4,5)P2 levels on PLD activity in HEK-293 cells. Inhibition of tyrosine phosphatases by pervanadate caused an increase in PLD activity, which was susceptible to genistein and tyrphostin 23, and which was abolished by prior treatment with toxin B. In conclusion, the data presented indicate that the cellular level of the multifunctional phospholipid, PtdIns(4,5)P2, in HEK-293 cells is controlled by a tyrosine-kinase-dependent mechanism and that this process apparently involves Rho proteins, as found similarly for tyrosine-phosphorylation-induced PLD activation.

Protein kinase C eta mediates lipopolysaccharide-induced nitric-oxide synthase expression in primary astrocytes

The signaling pathway involved in protein kinase C (PKC) activation and role of PKC isoforms in lipopolysaccharide (LPS)-induced nitric oxide (NO) release were studied in primary cerebellar astrocytes. LPS caused a dose- and time-dependent increase in NO release and inducible NO synthase (iNOS) expression. The tyrosine kinase inhibitor, genestein, the phosphatidylcholine-phospholipase C inhibitor, D609, and the phosphatidate phosphodrolase inhibitor, propranolol, attenuated the LPS effects, whereas the PI-PLC inhibitor, U73122, had no effect. The PKC inhibitors (staurosporine, Ro 31-8220, Go 6976, and calphostin C) also inhibited LPS-induced NO release and iNOS expression. However, long term (24 h) pretreatment of cells with 12-O-tetradecanoyl phorbol-13-acetate (TPA) did not affect the LPS response. Previous results have shown that TPA-induced translocation, but not down-regulation, of PKCeta occurs in astrocytes (Chen, C. C., and Chen, W. C. (1996) Glia 17, 63-71), suggesting possible involvement of PKCeta in LPS-mediated effects. Treatment with antisense oligonucleotides for PKCeta or delta, another isoform abundantly expressed in astrocytes, demonstrated the involvement of PKCeta, but not delta, in LPS-mediated effects. Stimulation of cells for 1 h with LPS caused activation of nuclear factor (NF)-kB in the nuclei as detected by the formation of a NF-kB-specific DNA-protein complex; this effect was inhibited by genestein, D609, propranolol, or Ro 31-8220 or by PKCeta antisense oligonucleotides, but not by long term TPA treatment. These data suggest that in astrocytes, LPS might activate phosphatidylcholine-phospholipase C and phosphatidylcholine-phospholipase D through an upstream protein tyrosine kinase to induce PKC activation. Of the PKC isoforms present in these cells, only activation of PKCeta by LPS resulted in the stimulation of NF-kB-specific DNA-protein binding and then initiated the iNOS expression and NO release. This is further evidence demonstrating that different members of the PKC family within a single cell are involved in specific physiological responses.

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.

Phospholipase D activity in L1210 cells: a model for oleate-activated phospholipase D in intact mammalian cells

Phospholipase D (PLD) in lymphocytic mouse leukemic L1210 cells has been found to be activated by oleate both in vitro and in intact cells. The PLD activity was measured by phosphatidylethanol produced from radiolabeled phosphatidylcholine or myristic acid in the presence of ethanol. This oleate-activated PLD was further characterized in intact cells and compared with that in HL60 cells. Unlike PLD in HL60 cells, the PLD in L1210 cells was activated by unsaturated fatty acids, stimulated by melittin, insensitive to guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), ADP-ribosylation factor (ARF) and phosphatidylinositol 4,5-bisphosphate (PIP2), independent of phorbol 12-myristate 13-acetate (PMA) and staurosporine, and inhibited by pervanadate. These observations indicate that the PLD present in L1210 cells is distinct from that in HL60 cells. Key PLD properties of L1210 cells such as insensitivity to GTP gamma S, ARF, PIP2, or PMA were in good agreement with currently known in vitro properties of the oleate-activated PLD found in mammalian sources. Therefore, the L1210 cells could be used as an intact-cell source for an oleate-activated PLD.

Human phospholipase D1 can be tyrosine-phosphorylated in HL-60 granulocytes

The human phospholipase D1 (hPLD1) has recently been cloned. Although recent data have implicated PLD in receptor-stimulated secretion, the regulation of the activity of PLD enzymes remains to be clarified. Purified hPLD1 is activated by several cytosolic cofactors among which are protein kinase Calpha, ARF, and RhoA. In human granulocytes, a strong correlation between tyrosine phosphorylation of proteins and PLD activity has been established. In this study, the presence of hPLD1 in HL-60 granulocytes and its phosphorylation on tyrosine residues have been studied. We generated antipeptide antibodies (Abs) specific for hPLD1 but not PLD2 as shown by Western blotting (WB) of recombinant PLD1 and PLD2. These Abs identified the presence of hPLD1 in HL-60 cells with the bulk of it being detected in the membranes and only a minor fraction in the cytosol. The hPLD1 Abs detected a major band at 120 kDa (PLD1a) and a minor band at 115 kDa (PLD1b). The specificity of the Abs was confirmed using PLD antisera neutralized with the immunizing peptides. The two forms of hPLD1 were consistently detected by immunoprecipitation under nondenaturing and denaturing conditions following a WB analysis with hPLD1 Abs. Following exposure of HL-60 cells to peroxides of vanadate (V4+-OOH), an inhibitor of tyrosine phosphatases, hPLD1 was immunoprecipitated under nondenaturing conditions from HL-60 cell lysates and assayed for tyrosine phosphorylation by WB. hPLD1 comigrated with a 120-kDa tyrosine phosphorylated protein by gel electrophoresis. Other tyrosine-phosphorylated peptides of 160, 140, 135, 90, and 75-80 kDa were also detected in hPLD1 immune complexes. hPLD1 and the associated tyrosine-phosphorylated proteins were not immunoprecipitated by neutralized hPLD1 Abs. Using denaturing conditions, the PLD immunoprecipitates were sequentially immunoblotted with anti-PLD1 and anti-phosphotyrosine Abs. PLD1a and PLD1b were detected, and the major PLD1a protein was superimposable with a major tyrosine-phosphorylated protein detected at 120 kDa. Conversely, PLD1a and PLD1b were recovered, at least in part, in the anti-phosphotyrosine immunoprecipitates. These results provide evidence that two PLD1 forms are expressed in human granulocytes. Furthermore, in response to stimulation by V4+-OOH, PLD1 was tyrosine-phosphorylated and associated with several, presently undefined, tyrosine-phosphorylated proteins.

Phospholipase D stimulates release of nascent secretory vesicles from the trans-Golgi network

Phospholipase D (PLD) is a phospholipid hydrolyzing enzyme whose activation has been implicated in mediating signal transduction pathways, cell growth, and membrane trafficking in mammalian cells. Several laboratories have demonstrated that small GTP-binding proteins including ADP-ribosylation factor (ARF) can stimulate PLD activity in vitro and an ARF-activated PLD activity has been found in Golgi membranes. Since ARF-1 has also been shown to enhance release of nascent secretory vesicles from the TGN of endocrine cells, we hypothesized that this reaction occurred via PLD activation. Using a permeabilized cell system derived from growth hormone and prolactin-secreting pituitary GH3 cells, we demonstrate that immunoaffinity-purified human PLD1 stimulated nascent secretory vesicle budding from the TGN approximately twofold. In contrast, a similarly purified but enzymatically inactive mutant form of PLD1, designated Lys898Arg, had no effect on vesicle budding when added to the permeabilized cells. The release of nascent secretory vesicles from the TGN was sensitive to 1% 1-butanol, a concentration that inhibited PLD-catalyzed formation of phosphatidic acid. Furthermore, ARF-1 stimulated endogenous PLD activity in Golgi membranes approximately threefold and this activation correlated with its enhancement of vesicle budding. Our results suggest that ARF regulation of PLD activity plays an important role in the release of nascent secretory vesicles from the TGN.

Phosphatase inhibitors potentiate 4-hydroxynonenal-induced phospholipase D activation in vascular endothelial cells

We have previously reported that endothelial cell phospholipase D (PLD), activated by 4-hydroxynonenal (4-HNE), was independent of protein kinase C activation. To determine whether PLD stimulation by 4-HNE is related to protein tyrosine phosphorylation, the effects of tyrosine kinase (Tyrk) and protein tyrosine phosphatase (PTPase) inhibitors on PLD activation were investigated. Pretreatment of bovine pulmonary artery endothelial cells (BPAEC) with Tyrk inhibitors, such as genistein, erbstatin, and herbimycin attenuated 4-HNE-induced PLD activation. Furthermore, vanadate, phenylarsine oxide, and diamide, inhibitors of PTPases, markedly increased the 4-HNE-induced PLD activation. The effects of Tyrk and PTPase inhibitors were specific towards the 4-HNE, as these agents had no effect on the agonist- or TPA-induced PLD activation. In addition to PLD activation, treatment of BPAEC with 4-HNE increased tyrosine phosphorylation of proteins including bands of molecular weights 40,000-60,000, 70,000-90,000, and 110,000-130,000. The 4-HNE-mediated increase in protein tyrosine phosphorylation was partly inhibited by genistein (100 microM). Vanadate (10 microM) pretreatment also potentiated 4-HNE-induced protein tyrosine phosphorylation. These data suggest that 4-HNE-mediated stimulation of PLD may occur as a result of activation of tyrosine kinases.

Inhibition of GTP gamma S-dependent phospholipase D and Rho membrane association by calphostin is independent of protein kinase C catalytic activity

We studied the relationships between the activation of phospholipase D (PLD) by guanine nucleotides and phorbol esters in permeabilized U937 promonocytes and in solubilized extracts prepared from U937 cell membranes. Treatment of permeabilized cells with phorbol myristate acetate (PMA) strongly potentiated GTP gamma S-dependent PLD activity at free Ca2+ < 100 nM. In the absence of GTP gamma S, PMA stimulated only minor PLD activity. This suggested synergistic interaction between regulatory G-proteins and a protein kinase C (PKC) family kinase. The potential role of PKC was evaluated by testing two mechanistically distinct PKC inhibitors, bisindolylmaleimide (BIM) and calphostin. BIM inhibits PKC enzymes via competition with ATP for binding to the catalytic domain, while calphostin competes with PMA or diglyceride for binding to the regulatory domain. The ability of PMA to potentiate the GTP gamma S-dependent PLD was not inhibited by BIM. In contrast, calphostin strongly inhibited the GTP gamma S-dependent PLD activity, both in the presence and absence of PMA as a potentiating agent. Calphostin also produced complete inhibition of a GTP gamma S-dependent PLD activity, present in solubilized membrane extracts, which was assayed using phospholipid vesicles of defined composition. Treatment of reconstituted membrane/cytosol mixtures with calphostin also produced complete inhibition of the GTP gamma S-induced translocation of Rho A from cytosol to membrane. In contrast to its effects on the U937 cell PLD, calphostin did not inhibit the activity of purified PLD from cabbage. These results suggest that the assembly of active RhoA/PLD signaling complexes on membranes involves a phorbol ester/calphostin-binding protein, but is not dependent on PKC-type catalytic activity.

Role of Rho family proteins in phospholipase D activation by growth factors

Treatment of fibroblasts with growth factors results in activation of phospholipase D (PLD). In order to determine the role of the Rho family of small GTPases in growth factor-mediated PLD activation, we used cells transfected with wild type and mutant Rac1. In response to epidermal growth factor (EGF), PLD activity was greatly increased in Rat1 fibroblasts expressing wild type Rac1 (wtRac1), and completely abrogated in cells expressing dominant negative N17Rac1, consistent with Rac1 mediating the action of this growth factor. In contrast, in cells treated with platelet-derived growth factor (PDGF) or phorbol ester, the wtRac1 cells showed little or no enhancement of PLD activity, and the response was not affected in the N17Rac1 cells, implying that Rac1 played a minimal role in the activation of PLD by PDGF or protein kinase C. Both growth factors produced an attenuated PLD response in cells expressing constitutively active V12Rac1, but these cells showed other changes, including altered morphology, increased basal PLD, and decreased growth factor receptor autophosphorylation. The effects of EGF and PDGF on phosphoinositide phospholipase C activity were not enhanced in cells expressing wtRac1 or inhibited in those expressing N17Rac1. In cells expressing constitutively active V12Rac1, basal phosphoinositide phospholipase C was elevated, but there were no significant effects of EGF or PDGF. We used C3 transferase of Clostridium botulinum, which ADP-ribosylates and inactivates RhoA, to investigate the involvement of RhoA in the activation of PLD by PDGF. Cells expressing wtRac1 and N17Rac1 showed a decreased PLD in response to PDGF when treated with C3 transferase, indicating a role for RhoA. In summary, these data indicate a major role for Rac1 in the activation of PLD by EGF, but not PDGF or protein kinase C.

Phospholipase D hydrolysis of plasmalogen and diacyl ethanolamine phosphoglycerides by protein kinase C dependent and independent mechanisms

Ethanolamine phosphoglycerides (EPG) are potential sources of lipid second messengers in signal transduction pathways. We investigated EPG turnover, including both 1-alkenyl-2-acyl- (plasmalogen) and diacyl-classes, in response to stimulation of protein kinase C (PKC) by phorbol ester (4 beta-12-O-tetradecanoylphorbol-13-acetate (TPA)) in cultured C6 rat glioma cells. Release of ethanolamine to the medium from EPG prelabeled with [14C]ethanolamine indicated that initial (< 60 min) TPA-stimulated hydrolysis of EPG was predominantly by phospholipase D (PLD). Effects of TPA on PLD activity specifically with EPG was confirmed using trans-phosphatidylation by incubating cells prelabeled with [14C]eicosapentaenoic acid (20:5n-3) with 100 nM TPA and 1% butanol. Analysis of acid-labile phosphatidylbutanol and remaining EPG showed utilization of both plasmalogen and non-plasmalogen EPG. Staurosporine (STS) inhibited PKC at 200-500 nM but stimulated PLD activity 2-fold at > or = 1 microM. However, STS did not eliminate all TPA-stimulated PLD activity, even when PKC was > 98% inhibited. Bis-indolylmaleimide (BIM) fully inhibited PKC activity but had no independent effects on PLD and did not completely inhibit TPA- or bryostatin-stimulated PLD activity. Down-regulation of PKC by chronic exposure to TPA eliminated stimulation of PLD by TPA but not by STS. Thus, PLD hydrolysis of both plasmalogen and diacyl-EPG is a source of potential lipid second messengers in C6 glioma cells. PLD is stimulated by activation of PKC and by PKC-independent action of STS. Further, the possibility that TPA may also elicit responses through a mechanism independent of PKC activity is suggested.

ADP-ribosylation factor 1-regulated phospholipase D activity is localized at the plasma membrane and intracellular organelles in HL60 cells

ADP-ribosylation factor (ARF), a small GTPase required for vesicle formation, has been identified as an activator of phospholipase D (PLD), thus implying that PLD is localized at intracellular organelles. HL60 cells were prelabelled with [14C]acetate for 72 h and, after disruption, fractionated on a linear sucrose gradient. ARF1-regulated PLD activity in each fraction was assessed by measurement of phosphatidylethanol production. Two peaks of activity were identified, coincident with markers for Golgi/endoplasmic reticulum/granules (endomembranes) and plasma membrane respectively. Analysis of the fractions using exogenous phosphatidylcholine as substrate confirmed the presence of ARF1-dependent PLD activity in endomembranes and plasma membrane, and also identified an additional activity in the cytosol. In formyl-Met-Leu-Phe-stimulated cells, PLD activity as assessed by phosphatidylethanol formation was also associated with both the plasma membrane and endomembranes. Since ARF1-regulated PLD activity requires phosphatidylinositol 4,5-bisphosphate (PIP2), the distributions of inositol lipids and the kinases responsible for lipid phosphorylation were examined. PIP2 was highly enriched at the plasma membrane, whereas phosphatidylinositol (PI) and phosphatidylinositol 4-phosphate (PI4P), the precursors for PIP2 synthesis, were found predominantly at endomembranes. The distribution of PI 4-kinase and PI4P 5-kinase activities confirmed the plasma membrane as the major site of PIP2 production. However, endomembranes possessed substantial PI 4-kinase activity and some PI4P 5-kinase activity, illustrating the potential for PIP2 synthesis. It is concluded that:(1) ARF1-regulated PLD activity is localized at endomembranes and the plasma membrane, (2) PIP2 is available at both membrane compartments to function as a cofactor for ARF-regulated PLD, and (3) in intact cells, formyl-Met-Leu-Phe stimulates PLD activity at endomembranes as well as plasma membrane.

Restoration of Clostridium difficile toxin-B-inhibited phospholipase D by phosphatidylinositol 4,5-bisphosphate

Receptor signalling to phospholipase D (PLD) in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor apparently involves Rho proteins. Since phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] has been recognized as an essential cofactor for PLD activity and since activated Rho proteins have been reported to stimulate the synthesis of PtdIns(4,5)P2, we studied whether in HEK cells PLD activity is regulated by PtdIns(4,5)P2 and, in particular, whether PtdIns(4,5)P2 can restore PLD activity inhibited by Clostridium difficile toxin B, which inactivates Rho proteins. Addition of MgATP to permeabilized HEK cells increased basal PLD activity and potentiated PLD stimulation by the stable GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), concomitant with a large increase in PtdIns(4,5)P2. On the other hand, neomycin, which binds to PtdIns(4,5)P2, inhibited basal and GTP[S]-stimulated PLD activities. Addition of PtdIns(4,5)P2 increased PLD activity in HEK cell membranes by 2-3-fold, whereas various other phospholipids were ineffective. Prior treatment of HEK cells with toxin B reduced the level of PtdIns(4,5)P2, measured either in intact cells or in membrane preparations, by about 40%. In membranes of toxin-B-treated cells, basal and GTP[S]-stimulated PLD activities were reduced, when measured with exogenous phosphatidylcholine as enzyme substrate. Inclusion of PtdIns(4,5)P2 with phosphatidylcholine in the substrate vesicles or addition of PtdIns(4,5)P2 fully restored basal and GTP[S]-stimulated PLD activities in membranes of toxin-B-treated cells. In conclusion, the data indicate that PtdIns(4,5)P2 is an essential cofactor for PLD activity in HEK cells and that inhibition of PLD activity by the Rho-inactivating toxin B is apparently caused by depletion of the PLD cofactor, PtdIns(4,5)P2.

Activation of phospholipase D is tightly coupled to the phagocytosis of Mycobacterium tuberculosis or opsonized zymosan by human macrophages

Phagocytosis of Mycobacterium tuberculosis by human mononuclear phagocytes is mediated primarily by complement receptors (CRs) but the transmembrane signaling mechanisms that regulate phagocytosis of the bacterium are unknown. We have analyzed the activation of phospholipase D (PLD) during phagocytosis of the virulent Erdman and attenuated H37Ra strains of M. tuberculosis by human monocyte-derived macrophages (MDMs), radiolabeled with [3H]-lyso-phosphatidylcholine. Phagocytosis of either Erdman or H37Ra M. tuberculosis in the presence of autologous non-immune serum was associated with a 2.5-3-fold increase in phosphatidic acid (PA). Definitive evidence for activation of PLD by M. tuberculosis was provided by markedly increased generation of the PLD-specific product phosphatidylethanol (PEt) (9.9-fold increases in [3H]-PEt for both Erdman and H37Ra strains compared to control, P < 0.001, n = 12), in the presence of 0.5% ethanol. Phagocytosis of opsonized zymosan (OZ), which is also mediated by CRs, was similarly associated with activation of PLD (12.2-fold increase in PEt, P < 0.001, n = 12). The competitive PLD inhibitor 2,3-diphosphoglycerate (2,3-DPG) produced concentration-dependent inhibition of PLD activity stimulated by either M. tuberculosis (-78 +/- 8%) or OZ (-73 +/- 6%). Inhibition of PLD by 2,3-DPG was associated with concentration-dependent reductions in phagocytosis of M. tuberculosis (-74 +/- 4%) and OZ (-68 +/- 5%). Addition of purified PLD from Streptomyces chromofuscus to 2,3-DPG-treated macrophages restored phagocytosis of M. tuberculosis to control levels. Inhibition of M. tuberculosis- or OZ-stimulated PA generation by ethanol was associated with concentration-dependent reductions in phagocytosis of both particles. Incubation of MDMs with either Erdman or H37Ra M. tuberculosis, or OZ, resulted in rapid (onset 1 min) and sustained (60 min) increases in the tyrosine phosphorylation (Tyr-P) of multiple MDM proteins. Prominent Tyr-P was noted in proteins of 150, 95, 72, 56, and 42 kD. The protein tyrosine kinase (PTK) inhibitors genistein and herbimycin A reduced M. tuberculosis-stimulated PLD activity by 66-84%. Inhibition of PLD activity by genistein or herbimycin A was associated with inhibition of phagocytosis of M. tuberculosis and OZ. These data demonstrate that PLD is activated during macrophage phagocytosis of M. tuberculosis or OZ, that PTKs are involved in this stimulation of PLD, and that the extent of phagocytosis of these particles is tightly coupled to activation of PLD.

Evidence for Rho-mediated agonist stimulation of phospholipase D in rat1 fibroblasts. Effects of Clostridium botulinum C3 exoenzyme

Small GTP-binding proteins of the Rho family are implicated in the in vitro regulation of phosphatidylcholine hydrolysis by phospholipase D (PLD). However, their role in agonist-stimulated PLD activity in whole cells is not clear. The ribosyltransferase C3 from Clostridium botulinum modifies Rho proteins and inhibits their function. When introduced into rat1 fibroblasts by scrape-loading, C3 inhibited PLD activity stimulated by lysophosphatidic acid (LPA), endothelin-1, or phorbol ester. Neither the time course nor agonist dose response for LPA-stimulated PLD activity was altered in C3-treated cells. In contrast to the effects of C3 on PLD activity, agonist-stimulated phosphatidylinositol-phospholipase C activity was not altered in C3-treated cells. Surprisingly, C3 treatment led to a decrease in the amount of RhoA protein, indicating that the loss of PLD activity in response to agonist was partly due to the loss of Rho proteins. As described previously, C3 treatment led to the inhibition of LPA-stimulated actin filament formation. However, disruption of actin filaments with cytochalasin D caused only a minor inhibition of LPA-stimulated PLD activity. Interestingly, stimulation of cells with LPA caused a rapid enrichment of RhoA in the particulate fraction of cell lysates. These data support an in vivo role for RhoA in agonist-stimulated PLD activity that is separate from its role in actin fiber formation.

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.

Regulation of phospholipase D by tyrosine kinases

Activation of phospholipase D (PLD) represents part of an important signalling pathway in mammalian cells. Phospholipase D catalyzed hydrolysis of phospholipids generates phosphatidic acid (PA) which is subsequently metabolized to lyso-PA (LPA) or diacylglycerol (DAG). While DAG is an endogenous activator of protein kinase C (PKC), PA and LPA have been recognized as second messengers as well. Activation of PLD in response to an external stimulus may involve PKC, Ca2+, G-proteins and/or tyrosine kinases. In this review, we will address the role of protein tyrosine phosphorylation in growth factor-, agonist- and oxidant-mediated activation of PLD. Furthermore, a possible link between PKC, Ca2+, G-proteins and tyrosine kinases is discussed to indicate the complexity involved in the regulation of PLD in mammalian cells.

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.

A tyrosine kinase regulates alpha-adrenoceptor-stimulated contraction and phospholipase D activation in the rat aorta

Since previous studies had indicated a role for tyrosine kinases in alpha 2-adrenoceptor-induced contractile responses in other blood vessels, as well as in the activation of phospholipase D, we examined the sensitivity of these responses in rat aorta to the tyrosine kinase inhibitor genistein. Contractions induced by both noradrenaline and the alpha 2-adrenoceptor-selective agonist UK14304 (5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline) were fully inhibited by genistein, with the latter responses being more sensitive. Contractions induced by high K+ buffer were also inhibited, but to a lesser extent. Both agonists caused a stimulation of phospholipase D activity, which could be blocked by pretreatment with pertussis toxin, indicating involvement of either Gi or Go. Genistein completely inhibited the agonist-induced phospholipase D activity and also substantially reduced the basal level of phospholipase D activity. Pretreatment with either the alpha 1-adrenoceptor antagonist prazosin or the alpha 2-adrenoceptor antagonist rauwolscine was also effective in eliminating the agonist-induced increase of phospholipase D. These results indicate that a tyrosine kinase-regulated phospholipase D plays a critical role in alpha-adrenoceptor-induced contractions of the rat aorta and that stimulation of both alpha 1- and alpha 2-adrenoceptors is essential to allow phospholipase activation.

Phospholipase D regulation by a physical interaction with the actin-binding protein gelsolin

Increases in intracellular phosphatidic acid levels caused by receptor- mediated activation of phospholipase D (PLD) have been implicated in many signal transduction pathways leading to cellular activation. PLD is known to be regulated by several means, including tyrosine kinase activity, increases in Ca2+, receptor-coupled G proteins, small GTP binding proteins, ceramide metabolisms, and protein kinase C. We have investigated a additional regulatory effect on PLD activity involving nucleoside triphosphates (NTPs). A NTP binding protein copurifies with LPD activity from rabbit brains using a GTP-agarose affinity column, and this protein stimulates PLD activity only in the absence of NPTs. The NTP effect is reversible and labile, and the binding protein is separable from the PLD activity by heparin-agarose chromatography. We identified this protein as the actin- binding protein gelsolin by amino acid sequencing following peptide mapping. This finding was verified by the co-immunoprecipitation of gelsolin and PLD activity as well as by the reconstitution of gelsolin- dependent nucleotide sensitive PLD activity by the addition of purified gelsolin-free PLD. Our data indicate that actin rearrangements and PLD signaling are coordinately regulated through the physical association between PLD and gelsolin and that this interaction may also serve to amplify both PLD signaling and actin reorganization.

Effects of selective phosphodiesterase type IV inhibitor, rolipram, on signal transducing phospholipases in neutrophil: inhibition of phospholipases A2, D but not C

The effects of a selective phosphodiesterase (PDE) type IV inhibitor, rolipram, on activation of neutrophil phospholipases in response to the chemotactic peptide formyl-methiony-leucyl-phenylalanine (fMLP) were investigated. fMLP caused liberation of arachidonic acid, a precursor of eicosanoids and in the presence of 0.3% butanol, production of phosphatidylbutanol, an indicator of phospholipase D activation. Rolipram inhibited arachidonic acid release and phosphatidylbutanol formation. The inhibition was considered to be mediated through a cAMP-dependent mechanism, probably protein kinase A, because selective inhibitors for protein kinase A, H-8 or H-89 overcame the action of rolipram. The concentration-dependent inhibitory profile for phospholipase D activation was similar to that for lysosomal enzyme release, providing additional evidence for the functional link of these two events. In contrast, rolipram was without effect on fMLP-induced inositol trisphosphate production. These results indicate that this selective PDE IV inhibitor had no effect on phosphatidylinositol-specific phospholipase C activation but effectively prevented activation of phospholipases A2 and D coupled to arachidonic acid liberation and lysosomal enzyme release, respectively.

Mastoparan-induced phosphatidylcholine hydrolysis by phospholipase D activation in human astrocytoma cells

1. The effect of mastoparan on phosphatidylcholine hydrolysis was examined in 1321N1 human astrocytoma cells. Mastoparan (3-30 microM) caused an accumulation of diacylglycerol (DG) and phosphatidic acd (PA) accompanied by choline release in a concentration- and time-dependent manner. 2. In the presence of 2% n-butanol, mastoparan (3-100 microM) induced phosphatidylbutanol (PBut) accumulation in a concentration- and time-dependent manner, suggesting that mastoparan activates phospholipase D (PLD). Propranolol (30-300 microM), a phosphatidate phosphohydrolase inhibitor, inhibited DG accumulation induced by mastoparan, supporting this idea. 3. Depletion of extracellular free calcium ion did not alter the effect of mastoparan on PLD activity. 4. A protein kinase C (PKC) inhibitor, calphostin C (1 microM), did not inhibit mastoparan-induce PLD activation but the ability of mastoparan to stimulate phospholipase D activity was decreased in the PKC down regulated cells. 5. PLD activity stimulated by mastoparan was not prevented by pretreatment of the cells with pertussis toxin (PT) or C3 ADP-ribosyltransferase. Furthermore, guanine nucleotides did not affect PLD activity stimulation by mastoparan in membrane preparations. 6. Mastoparan stimulated PLD in several cell lines such as RBL-2H3, RBL-1, HL-60, P388, endothelial cells, as well as 1321N1 human astrocytoma cells. 7. These results suggest that mastoparan induces phosphatidylcholine (PC) hydrolysis by activation of PLD, not by activation of phosphatidylcholine-specific phospholipase C (PC-PLC); mastoparan-induced PLD activation is not mediated by G proteins.

ADP-ribosylation factor translocation correlates with potentiation of GTP gamma S-stimulated phospholipase D activity in membrane fractions of HL-60 cells

Phospholipase D (PLD) activation by guanine nucleotides requires protein cofactors from both the membrane and the cytosol. The small GTP-binding protein ADP-ribosylation factor (ARF) has been established as one important component of PLD activation. By stimulating HL-60 cells with various agonists and then isolating the membrane fraction and assaying PLD activity in the presence and absence of GTP gamma S, we observed that fMet-Leu-Phe (fMLP) and phorbol esters induced a potentiation of GTP gamma S-stimulated PLD activity in the membrane fractions of these cells. Inactive phorbol esters induced no such potentiation. Both fMLP and active phorbol esters induced a 2-3-fold increase in GTP gamma S-stimulated PLD in HL-60 membranes. Membranes derived from stimulated HL-60 cells contained 60-70% more ARF as compared with membranes derived from control cells. Membrane contents of ARF were assessed by Western blotting with the anti-ARF monoclonal antibody 1D9 followed by densitometric evaluation. Therefore, ARF translocation correlates with the potentiation of the GTP gamma S-stimulated PLD activity. The effect on PLD activity and ARF membrane content achieved through fMLP stimulation was greatly enhanced by prior treatment of the cells with cytochalasin B. Membranes derived from control and fMLP-stimulated cells were assayed for PLD activity in the presence of exogenous ARF and a 50-kDa fraction known to contain elements implicated in PLD activation. The ability of ARF and the 50-kDa fraction to enhance GTP gamma S-sensitive PLD activity was significantly reduced when the membranes were derived from fMLP-stimulated cells. The data indicate that, in addition to ARF, elements of the 50-kDa PLD-inducing factors were likely already translocated to the membranes upon stimulation. We propose that ARF, upon stimulation with agonists such as fMLP or phorbol esters, is translocated to the membrane and in concert with other protein components of the 50-kDa fraction enhances PLD activity.

Role of phospholipase D in the cAMP signal transduction pathway activated during fibroblast contraction of collagen matrices

Fibroblast contraction of stressed collagen matrices results in activation of a cAMP signal transduction pathway. This pathway involves influx of extracellular Ca2+ ions and increased production of arachidonic acid. We report that within 5 min after initiating contraction, a burst of phosphatidic acid release was detected. Phospholipase D was implicated in production of phosphatidic acid based on observation of a transphosphatidylation reaction in the presence of ethanol that resulted in formation of phosphatidylethanol at the expense of phosphatidic acid. Activation of phospholipase D required extracellular Ca2+ ions and was regulated by protein kinase C. Ethanol treatment of cells also inhibited by 60-70% contraction-dependent release of arachidonic acid and cAMP but had no effect on increased cAMP synthesis after addition of exogenous arachidonic acid or on phospholipase A2 activity measured in cell extracts. Moreover, other treatments that inhibited the burst of phosphatidic acid release after contraction--chelating extracellular Ca2+ or down-regulating protein kinase C--also blocked contraction activated cyclic AMP signaling. These results were consistent with the idea that phosphatidic acid production occurred upstream of arachidonic acid in the contraction-activated cAMP signaling pathway.

Lipid signal transduction pathways in angiotensin II type 1 receptor-transfected fibroblasts

A stable Chinese hamster ovary fibroblast line expressing the rat vascular type 1a angiotensin II (ANG II) receptor was used to study the lipid-derived signal transduction pathways elicited by type 1a ANG II receptor activation. ANG II caused a biphasic and dose-dependent increase in diacylglycerol (DAG) accumulation with an initial peak at 15 s (181 +/- 11% of control, P < 0.02) and a second sustained peak at 5-10 min (214 +/- 10% of control, P < 0.02). The late DAG peak was derived from phosphatidylcholine (PC), and the formation was blocked by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. ANG II also increased phosphatidic acid (PA) production nearly fourfold by 7.5 min. In the presence of ethanol, ANG II markedly increased phosphatidylethanol (PEt) formation, indicating activation of phospholipase D (PLD). ANG II was shown to increase the mass of three separate PA species, one of which apparently originated from DAG kinase action on PC-phospholipase C (PLC)-produced DAG, providing evidence for PC-PLC activity. ANG II also formed a third PA species, which originated neither from PLD nor from DAG kinase. These results demonstrate that multiple lipid signals propagated via collateral stimulation of PLC and PLD are generated by specific activation of the vascular type 1a ANG II receptor.

Tyrosine kinase-mediated signal transduction pathways and the actions of polypeptide growth factors and G-protein-coupled agonists in smooth muscle

This synopsis focuses on the role that tyrosine kinase pathways may play in the acute regulation of smooth muscle contractility by receptor-kinase-activating growth factors, such as epidermal growth factor-urogastrone (EGF-URO) and by G-protein-coupled agonists, such as angiotensin-II. Growth factor-activated response paradigms that modulate smooth muscle contractility are summarized and the parallels between the actions of G-protein-coupled agonists and growth factors in these response systems are pointed out. A possible dynamic interplay between tyrosine kinase and tyrosine phosphatase activities to modulate tissue tension is also hypothesized. Finally, a model is proposed, wherein an intermediary tyrosine kinase pathway is suggested as a point of convergence for the regulation of smooth muscle contractility by agonists as diverse as EGF-URO and angiotensin-II.

Thrombin induces the preproendothelin-1 gene in endothelial cells by a protein tyrosine kinase-linked mechanism

Thrombin stimulates synthesis and secretion of endothelin-1 (ET-1), a vasoactive peptide that triggers responses in the vascular endothelium and smooth muscle. We investigated the signal transduction pathways by which thrombin stimulates preproET-1 gene expression and ET-1 peptide secretion in macrovascular cells (human umbilical vein endothelial cells [HUVECs] and bovine pulmonary artery endothelial cells [BPAECs]) and microvascular cells (human microvascular endothelial cell line [HMEC-1]). Thrombin (4 U/mL) stimulated maximal induction of ET-1 peptide secretion and preproET-1 mRNA after 2 hours in HUVECs and BPAECs and after 1 hour in HMEC-1. A synthetic thrombin receptor activator peptide confirmed ligand-specific receptor actions to induce preproET-1 mRNA. Protein kinase C (PKC) activation by phorbol ester transiently induced preproET-1 mRNA but had no effect on ET-1 peptide synthesis. PKC inhibitors sangivamycin and calphostin C and PKC depletion failed to suppress thrombin-stimulated preproET-1 mRNA. Adenylate cyclase and cAMP-dependent protein kinase did not participate in thrombin-induced preproET-1 gene activation. Thrombin stimulated a rapid increase in phosphotyrosine-containing proteins, suggesting a role for tyrosine phosphorylation in thrombin signaling. These data demonstrate that thrombin induces the preproET-1 gene and ET-1 peptide synthesis by a PKC-independent PTK-dependent pathway in macrovascular and microvascular endothelial cells. Protein tyrosine kinase inhibitors herbimycin A and genistein blocked thrombin-stimulated preproET-1 mRNA and peptide secretion, whereas daidzein, which lacks inhibitory activity, did not suppress thrombin-induced ET-1.