Phosphorylation-dependent regulation of phospholipase D2 by protein kinase C delta in rat Pheochromocytoma PC12 cells

Repair of traumatic lesions to the plasmalemma of neurons and other cells: Commonalities, conflicts, and controversies

Neuroscientists and Cell Biologists have known for many decades that eukaryotic cells, including neurons, are surrounded by a plasmalemma/axolemma consisting of a phospholipid bilayer that regulates trans-membrane diffusion of ions (including calcium) and other substances. Cells often incur plasmalemmal damage via traumatic injury and various diseases. If the damaged plasmalemma is not rapidly repaired within minutes, activation of apoptotic pathways by calcium influx often results in cell death. We review publications reporting what is less-well known (and not yet covered in neuroscience or cell biology textbooks): that calcium influx at the lesion sites ranging from small nm-sized holes to complete axonal transection activates parallel biochemical pathways that induce vesicles/membrane-bound structures to migrate and interact to restore original barrier properties and eventual reestablishment of the plasmalemma. We assess the reliability of, and problems with, various measures (e.g., membrane voltage, input resistance, current flow, tracer dyes, confocal microscopy, transmission and scanning electron microscopy) used individually and in combination to assess plasmalemmal sealing in various cell types (e.g., invertebrate giant axons, oocytes, hippocampal and other mammalian neurons). We identify controversies such as plug versus patch hypotheses that attempt to account for currently available data on the subcellular mechanisms of plasmalemmal repair/sealing. We describe current research gaps and potential future developments, such as much more extensive correlations of biochemical/biophysical measures with sub-cellular micromorphology. We compare and contrast naturally occurring sealing with recently-discovered artificially-induced plasmalemmal sealing by polyethylene glycol (PEG) that bypasses all natural pathways for membrane repair. We assess other recent developments such as adaptive membrane responses in neighboring cells following injury to an adjacent cell. Finally, we speculate how a better understanding of the mechanisms involved in natural and artificial plasmalemmal sealing is needed to develop better clinical treatments for muscular dystrophies, stroke and other ischemic conditions, and various cancers.

Ca2+ homeostasis in brain microvascular endothelial cells

Blood brain barrier (BBB) is formed by the brain microvascular endothelial cells (BMVECs) lining the wall of brain capillaries. Its integrity is regulated by multiple mechanisms, including up/downregulation of tight junction proteins or adhesion molecules, altered Ca²⁺ homeostasis, remodeling of cytoskeleton, that are confined at the level of BMVECs. Beside the contribution of BMVECs to BBB permeability changes, other cells, such as pericytes, astrocytes, microglia, leukocytes or neurons, etc. are also exerting direct or indirect modulatory effects on BBB. Alterations in BBB integrity play a key role in multiple brain pathologies, including neurological (e.g. epilepsy) and neurodegenerative disorders (e.g. Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis etc.). In this review, the principal Ca²⁺ signaling pathways in brain microvascular endothelial cells are discussed and their contribution to BBB integrity is emphasized. Improving the knowledge of Ca²⁺ homeostasis alterations in BMVECa is fundamental to identify new possible drug targets that diminish/prevent BBB permeabilization in neurological and neurodegenerative disorders.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Phospholipase D2 in prostate cancer: protein expression changes with Gleason score

BACKGROUND

Phospholipases D1 and D2 (PLD1/2) are implicated in tumorigenesis through their generation of the signalling lipid phosphatidic acid and its downstream effects. Inhibition of PLD1 blocks prostate cell growth and colony formation. Here a role for PLD2 in prostate cancer (PCa), the major cancer of men in the western world, is examined.

METHODS

PLD2 expression was analysed by immunohistochemistry and western blotting. The effects of PLD2 inhibition on PCa cell viability and cell motility were measured using MTS, colony forming and wound-healing assays.

RESULTS

PLD2 protein is expressed about equally in luminal and basal prostate epithelial cells. In cells from different Gleason-scored PCa tissue PLD2 protein expression is generally higher than in non-tumorigenic cells and increases in PCa tissue scored Gleason 6-8. PLD2 protein is detected in the cytosol and nucleus and had a punctate appearance. In BPH tissue stromal cells as well as basal and luminal cells express PLD2. PLD2 protein co-expresses with chromogranin A in castrate-resistant PCa tissue. PLD2 inhibition reduces PCa cell viability, colony forming ability and directional cell movement.

CONCLUSIONS

PLD2 expression correlates with increasing Gleason score to GS8. PLD2 inhibition has the potential to reduce PCa progression.

Amygdala-Hippocampal Phospholipase D (PLD) Signaling As Novel Mechanism of Cocaine-Environment Maladaptive Conditioned Responses

BACKGROUND

Drug-environment associative memory mechanisms and the resulting conditioned behaviors are key contributors in relapse to cocaine dependence. Recently, we reported rat amygdala phospholipase D as a key convergent downstream signaling partner in the expression of cocaine-conditioned behaviors mediated by glutamatergic and dopaminergic pathways. In the present study, 1 of the 2 known upstream serotonergic targets of phospholipase D, the serotonin (5-hydroxytryptamine) 2C receptor, was investigated for its role in recruiting phospholipase D signaling in cocaine-conditioned behaviors altered in the rat amygdala and dorsal hippocampus.

METHODS

Using Western-blot analysis, amygdala phospholipase D phosphorylation and total expression of phospholipase D/5-hydroxytryptamine 2C receptor were observed in early (Day-1) and late (Day-14) withdrawal (cocaine-free) states among male Sprague-Dawley rats subjected to 7-day cocaine-conditioned hyperactivity training. Functional studies were conducted using Chinese Hamster Ovary cells with stably transfected human unedited isoform of 5-hydroxytryptamine 2C receptor.

RESULTS

Phosphorylation of phospholipase D isoforms was altered in the Day-1 group of cocaine-conditioned animals, while increased amygdala and decreased dorsal hippocampus phospholipase D/5-hydroxytryptamine 2C receptor protein expression were observed in the Day-14 cocaine-conditioned rats. Functional cellular studies established that increased p phospholipase D is a mechanistic response to 5-HT2CR activation and provided the first evidence of a biased agonism by specific 5-hydroxytryptamine 2C receptor agonist, WAY163909 in phospholipase D phosphorylation 2, but not phospholipase D phosphorylation 1 activation.

CONCLUSIONS

Phospholipase D signaling, activated by dopaminergic, glutamatergic, and serotonergic signaling, can be a common downstream element recruited in associative memory mechanisms altered by cocaine, where increased expression in amygdala and decreased expression in dorsal hippocampus may result in altered anxiety states and increased locomotor responses, respectively.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Regulation of aldosterone synthesis and secretion

Aldosterone is a steroid hormone synthesized in and secreted from the outer layer of the adrenal cortex, the zona glomerulosa. Aldosterone is responsible for regulating sodium homeostasis, thereby helping to control blood volume and blood pressure. Insufficient aldosterone secretion can lead to hypotension and circulatory shock, particularly in infancy. On the other hand, excessive aldosterone levels, or those too high for sodium status, can cause hypertension and exacerbate the effects of high blood pressure on multiple organs, contributing to renal disease, stroke, visual loss, and congestive heart failure. Aldosterone is also thought to directly induce end-organ damage, including in the kidneys and heart. Because of the significance of aldosterone to the physiology and pathophysiology of the cardiovascular system, it is important to understand the regulation of its biosynthesis and secretion from the adrenal cortex. Herein, the mechanisms regulating aldosterone production in zona glomerulosa cells are discussed, with a particular emphasis on signaling pathways involved in the secretory response to the main controllers of aldosterone production, the renin-angiotensin II system, serum potassium levels and adrenocorticotrophic hormone. The signaling pathways involved include phospholipase C-mediated phosphoinositide hydrolysis, inositol 1,4,5-trisphosphate, cytosolic calcium levels, calcium influx pathways, calcium/calmodulin-dependent protein kinases, diacylglycerol, protein kinases C and D, 12-hydroxyeicostetraenoic acid, phospholipase D, mitogen-activated protein kinase pathways, tyrosine kinases, adenylate cyclase, and cAMP-dependent protein kinase. A complete understanding of the signaling events regulating aldosterone biosynthesis may allow the identification of novel targets for therapeutic interventions in hypertension, primary aldosteronism, congestive heart failure, renal disease, and other cardiovascular disorders.

Phorbol ester stimulates ethanolamine release from the metastatic basal prostate cancer cell line PC3 but not from prostate epithelial cell lines LNCaP and P4E6

Background:

Malignancy alters cellular complex lipid metabolism and membrane lipid composition and turnover. Here, we investigated whether tumorigenesis in cancer-derived prostate epithelial cell lines influences protein kinase C-linked turnover of ethanolamine phosphoglycerides (EtnPGs) and alters the pattern of ethanolamine (Etn) metabolites released to the medium.

Methods:

Prostate epithelial cell lines P4E6, LNCaP and PC3 were models of prostate cancer (PCa). PNT2C2 and PNT1A were models of benign prostate epithelia. Cellular EtnPGs were labelled with [1-³H]-Etn hydrochloride. PKC was activated with phorbol ester (TPA) and inhibited with Ro31-8220 and GF109203X. D609 was used to inhibit PLD (phospholipase D). [³H]-labelled Etn metabolites were resolved by ion-exchange chromatography. Sodium oleate and mastoparan were tested as activators of PLD2. Phospholipase D activity was measured by a transphosphatidylation reaction. Cells were treated with ionomycin to raise intracellular Ca²⁺ levels.

Results:

Unstimulated cell lines release mainly Etn and glycerylphosphorylEtn (GPEtn) to the medium. Phorbol ester treatment over 3h increased Etn metabolite release from the metastatic PC3 cell line and the benign cell lines PNT2C2 and PNT1A but not from the tumour-derived cell lines P4E6 and LNCaP; this effect was blocked by Ro31-8220 and GF109203X as well as by D609, which inhibited PLD in a transphosphatidylation reaction. Only metastatic PC3 cells specifically upregulated Etn release in response to TPA treatment. Oleate and mastoparan increased GPEtn release from all cell lines at the expense of Etn. Ionomycin stimulated GPEtn release from benign PNT2C2 cells but not from cancer-derived cell lines P4E6 or PC3. Ethanolamine did not stimulate the proliferation of LNCaP or PC3 cell lines but decreased the uptake of choline (Cho).

Conclusions:

Only the metastatic basal PC3 cell line specifically increased the release of Etn on TPA treatment most probably by PKC activation of PLD1 and increased turnover of EtnPGs. The phosphatidic acid formed will maintain a cancer phenotype through the regulation of mTOR. Ethanolamine released from cells may reduce Cho uptake, regulating the membrane PtdEtn:PtdCho ratio and influencing the action of PtdEtn-binding proteins such as RKIP and the anti-apoptotic hPEBP4. The work highlights a difference between LNCaP cells used as a model of androgen-dependent early stage PCa and androgen-independent PC3 cells used to model later refractory stage disease.

A river runs through it: how autophagy, senescence, and phagocytosis could be linked to phospholipase D by Wnt signaling

Neutrophils and macrophages are professional phagocytic cells, extremely efficient at the process of engulfing and killing bacteria. Autophagy is a similar process, by which phagosomes recycle internal cell structures during nutrient shortages. Some pathogens are able to subvert the autophagy process, funneling nutrients for their own use and for the host's detriment. Additionally, a failure to mount an efficient autophagy is a deviation on the cell's part from normal cellular function into cell senescence and cessation of the cell cycle. In spite of these reasons, the mechanism of autophagy and senescence in leukocytes has been under studied. We advance here the concept of a common thread underlying both autophagy and senescence, which implicates PLD. Such a PLD-based autophagy mechanism would involve two positive inputs: the generation of PA to help the initiation of the autophagosome and a protein-protein interaction between PLD and PKC that leads to enhanced PA. One negative input is also involved in this process: down-regulation of PLD gene expression by mTOR. Additionally, a dual positive/negative input plays a role in PLD-mediated autophagy, β-catenin increase of autophagy through PLD up-regulation, and a subsequent feedback termination by Dvl degradation in case of excessive autophagy. An abnormal PLD-mTOR-PKC-β-catenin/Wnt network function could lead to faulty autophagy and a means for opportunistic pathogens to survive inside of the cell.

Mechanisms of neuronal membrane sealing following mechanical trauma

Membrane integrity is crucial for maintaining the intricate signaling and chemically-isolated intracellular environment of neurons; disruption risks deleterious effects, such as unregulated ionic flux, neuronal apoptosis, and oxidative radical damage as observed in spinal cord injury and traumatic brain injury. This paper, in addition to a discussion of the current understanding of cellular tactics to seal membranes, describes two major factors involved in membrane repair. These are line tension, the hydrophobic attractive force between two lipid free-edges, and membrane tension, the rigidity of the lipid bilayer with respect to the tethered cortical cytoskeleton. Ca(2+), a major mechanistic trigger for repair processes, increases following flux through a membrane injury site, and activates phospholipase enzymes, calpain-mediated cortical cytoskeletal proteolysis, protein kinase cascades, and lipid bilayer microdomain modification. The membrane tension appears to be largely modulated through vesicle dynamics, cytoskeletal organization, membrane curvature, and phospholipase manipulation. Dehydration of the phospholipid gap edge and modification of membrane packaging, as in temperature variation, experimentally impact line tension. Due to the time-sensitive nature of axonal sealing, increasing the efficacy of axolemmal sealing through therapeutic modification would be of great clinical value, to deter secondary neurodegenerative effects. Better therapeutic enhancement of membrane sealing requires a complete understanding of its intricate underlying neuronal mechanism.

Preparation of primary cultures of mouse epidermal keratinocytes and the measurement of phospholipase D activity

In this chapter information is provided about the outer layer of the skin, the epidermis, and the predominant cells comprising this epithelium, the keratinocytes. The evidence supporting a possible role for the lipid-metabolizing enzyme phospholipase D in regulating keratinocyte differentiation is also discussed. A detailed protocol for the preparation of primary cultures of epidermal keratinocytes from neonatal mice is described, to allow other investigators to obtain data concerning these important cells involved in forming and maintaining the mechanical and water permeability of the skin. Finally, a complete protocol for monitoring phospholipase D activity in intact cells is supplied in the hope that additional research will result in a better understanding of the role of phospholipase D in controlling keratinocyte proliferation and differentiation.

Understanding of the roles of phospholipase D and phosphatidic acid through their binding partners

Phospholipase D (PLD) is a phosphatidyl choline (PC)-hydrolyzing enzyme that generates phosphatidic acid (PA), a lipid second messenger that modulates diverse intracellular signaling. Through interactions with signaling molecules, both PLD and PA can mediate a variety of cellular functions, such as, growth/proliferation, vesicle trafficking, cytoskeleton modulation, development, and morphogenesis. Therefore, systemic approaches for investigating PLD networks including interrelationship between PLD and PA and theirs binding partners, such as proteins and lipids, can enhance fundamental knowledge of roles of PLD and PA in diverse biological processes. In this review, we summarize previously reported protein-protein and protein-lipid interactions of PLD and PA and their binding partners. In addition, we describe the functional roles played by PLD and PA in these interactions, and provide PLD network that summarizes these interactions. The PLD network suggests that PLD and PA could act as a decision maker and/or as a coordinator of signal dynamics. This viewpoint provides a turning point for understanding the roles of PLD-PA as a dynamic signaling hub.

Protein kinase Cdelta-mediated phosphorylation of phospholipase D controls integrin-mediated cell spreading

Integrin signaling plays critical roles in cell adhesion, spreading, and migration, and it is generally accepted that to regulate these integrin functions accurately, localized actin remodeling is required. However, the molecular mechanisms that control the targeting of actin regulation molecules to the proper sites are unknown. We previously demonstrated that integrin-mediated cell spreading and migration on fibronectin are dependent on the localized activation of phospholipase D (PLD). However, the mechanism underlying PLD activation by integrin is largely unknown. Here we demonstrate that protein kinase Cδ (PKCδ) is required for integrin-mediated PLD signaling. After integrin stimulation, PKCδ is activated and translocated to the edges of lamellipodia, where it colocalizes with PLD2. The abrogation of PKCδ activity inhibited integrin-induced PLD activation and cell spreading. Finally, we show that Thr566 of PLD2 is directly phosphorylated by PKCδ and that PLD2 mutation in this region prevents PLD2 activation, PLD2 translocation to the edge of lamellipodia, Rac translocation, and cell spreading after integrin activation. Together, these results suggest that PKCδ is a primary regulator of integrin-mediated PLD activation via the direct phosphorylation of PLD, which is essential for directing integrin-induced cell spreading.

P2Y nucleotide receptor signaling through MAPK/ERK is regulated by extracellular matrix: involvement of beta3 integrins

Extracellular matrix influences cell behavior through receptors such as integrins and through transmission of mechanical forces. Nucleotides are released in response to mechanical stimuli and bind to P2 nucleotide receptors. As chondrocytes are subjected to frequent mechanical stimulation within a rich extracellular matrix, they are an excellent model for studying integration of signals induced by matrix and nucleotides. We investigated signaling of G protein-coupled P2Y receptors to MAPK/ERK and how this is influenced by matrix. Rat articular chondrocytes expressed transcripts for P2Y1, P2Y2, P2Y4, and P2Y6 receptors and responded to extracellular nucleotides by transient elevation of cytosolic calcium and MAPK/ERK phosphorylation. ERK1/2 activation was suppressed by the protein kinase C (PKC) inhibitors bisindolylmaleimide I and rottlerin, and by the phospholipase D inhibitor 1-butanol. Thus, nucleotides stimulate P2Y receptors to activate ERK1/2 through a mechanism dependent on PKC and phospholipase D. We next examined the involvement of integrins. Both an RGD-containing pentapeptide and a beta3 integrin blocking antibody, but not a beta1 integrin blocking antibody, abolished nucleotide-induced ERK1/2 phosphorylation. Moreover, chondrocytes adhering to fibronectin (which binds to beta1 and beta3 containing integrins in an RGD-dependent manner) displayed prolonged ERK1/2 signaling compared to cells grown on type I or II collagen (which bind to beta1-containing integrins in an RGD-independent manner). In conclusion, P2Y receptor signaling through ERK1/2 is gated selectively by matrix proteins. Thus, nucleotides released in response to mechanical stimulation will have differing effects on cell function due to changes in the composition of the extracellular matrix during development and disease.

PLD2 forms a functional complex with mTOR/raptor to transduce mitogenic signals

Mammalian target-of-rapamycin (mTOR), which is a master controller of cell growth, senses a mitogenic signal in part through the lipid second messenger phosphatidic acid (PA), generated by phospholipase D (PLD). To understand further which isozymes of PLD are involved in this process, we compared the effect of PLD isozymes on mTOR activation. We found that PLD2 has an essential role in mitogen-induced mTOR activation as the siRNA-mediated knockdown of PLD2, not of PLD1, profoundly reduced the phosphorylations of S6K1 and 4EBP1, well-known mTOR effectors. Furthermore, exogenous PA-induced mTOR activation was abrogated by PLD2 knockdown, but not by PLD1 knockdown. This abrogation was found to be the result of complex formation between PLD2 and mTOR/raptor. PLD2 possesses a TOS-like motif (Phe-Glu-Val-Gln-Val, a.a. 265-269), through which it interacts with raptor independently of the other TOS motif-containing proteins, S6K1 and 4EBP1. PLD2-dependent mTOR activation appears to require PLD2 binding to mTOR/raptor with lipase activity, since lipase-inactive PLD2 cannot trigger mTOR activation despite its ability to interact with mTOR/raptor. Abrogation of mitogen-dependent mTOR activation by PLD2 knockdown was rescued only by wild type PLD2, but not by raptor binding-deficient and lipase-inactive PLD2. Our results demonstrate the importance of localized PA generation for the mitogen-induced activation of mTOR, which is achieved by a specific interaction between PLD2 and mTOR/raptor.

Localization and regulation of phospholipase D2 by ARF6

Phospholipase D (PLD) and ADP-ribosylation factor 6 (ARF6) have been implicated in vesicular trafficking and rearrangement of the actin cytoskeleton. We have explored the co-localization of rat PLD1b and rat PLD2 with wild type and mutant forms of ARF6 in HeLa cells and studied their activation by ARF6 and the role of the actin cytoskeleton. GFP-tagged PLD1 had a similar pattern to multivesicular and late endosomes and the trans-Golgi apparatus, but not to other organelles. When wild type or dominant negative ARF6 and PLD1 or PLD2 were co-expressed, they had a similar localization in cytosolic particles and at the cell periphery. In contrast, dominant active ARF6 caused cell shrinkage and had a similar localization with PLD1 and PLD2 in dense structures, containing the trans-Golgi apparatus and actin. Disruption of the actin cytoskeleton with cytochalasin D did not induce the formation of these structures. To determine, if ARF6 selectively activated PLD1 or PLD2, wild type and mutant forms of the ARF isoform were transfected together with PLD1 or PLD2. Wild type ARF6 did not affect either PLD isozyme, but dominant active ARF6 selectively activated PLD2 and dominant negative ARF6 selectively inhibited PLD2. In contrast, dominant active ARF1 or Rac1 stimulated both PLD isozymes but the ARF1 effect on PLD2 was very small. Cytochalasin D did not affect the activation of PLD by phorbol ester. The localizations of PLD and ARF6 were also analyzed by fractionation after methyl-beta-cyclodextrin extraction to deplete cholesterol. The results showed that all PLD isoforms and ARF6 mutants existed in the membrane fraction, but only wild type ARF6 was dependent on the presence of cholesterol. These experiments showed that wild type ARF6 had a similar location with PLD isoforms on cell staining, but it did not colocalize with PLD isoforms in fractionation experiments. It is proposed that activated ARF6 translocates to the cholesterol independent microdomain and then activates PLD2 there. It is further concluded that PLD2 is selectively activated by ARF6 in vivo and that disruption of the actin cytoskeleton does not affect this activation.

Sites on phospholipase D2 phosphorylated by PKCα

The phosphorylation sites in phospholipase D2 (PLD2) induced by activation of protein kinase Calpha (PKCalpha) in COS 7 cells were analyzed by mass spectrometry. Ser134, 146, and 243, and Thr72, 99/100, and 252 were identified. These sites were mutated to Ala and the double mutation of Ser243 and Thr252 eliminated the phosphorylation. However, the PLD2 activity, and the binding between PKCalpha and PLD2 were unaffected by the mutations. We conclude that phosphorylation of these residues is not required for PLD2 activation by PKCalpha, and that protein-protein interaction between PLD2 and PKCalpha is sufficient to activate PLD2.

Functions and pathophysiological roles of phospholipase D in the brain

Ten years after the isoforms of mammalian phospholipase D (PLD), PLD1 and 2, were cloned, their roles in the brain remain speculative but several lines of evidence now implicate these enzymes in basic cell functions such as vesicular trafficking as well as in brain development. Many mitogenic factors, including neurotransmitters and growth factors, activate PLD in neurons and astrocytes. Activation of PLD downstream of protein kinase C seems to be a required step for astroglial proliferation. The characteristic disruption of the PLD signaling pathway by ethanol probably contributes to the delay of brain growth in fetal alcohol syndrome. The post-natal increase of PLD activities concurs with synapto- and myelinogenesis in the brain and PLD is apparently involved in neurite formation. In the adult and aging brain, PLD activity has antiapoptotic properties suppressing ceramide formation. Increased PLD activities in acute and chronic neurodegeneration as well as in inflammatory processes are evidently due to astrogliosis and may be associated with protective responses of tissue repair and remodeling. ARF-regulated PLD participates in receptor endocytosis as well as in exocytosis of neurotransmitters where PLD seems to favor vesicle fusion by modifications of the shape and charge of lipid membranes. Finally, PLD activities contribute free choline for the synthesis of acetylcholine in the brain. Novel tools such as RNA interference should help to further elucidate the roles of PLD isoforms in brain physiology and pathology.

The uncovering of a novel regulatory mechanism for PLD2: formation of a ternary complex with protein tyrosine phosphatase PTP1B and growth factor receptor-bound protein GRB2

The regulation of PLD2 activation is poorly understood at present. Transient transfection of COS-7 with a mycPLD2 construct results in elevated levels of PLD2 enzymatic activity and tyrosyl phosphorylation. To investigate whether this phosphorylation affects PLD2 enzymatic activity, anti-myc immunoprecipitates were treated with recombinant protein tyrosine phosphatase PTP1B. Surprisingly, lipase activity and PY levels both increased over a range of PTP1B concentrations. These increases occurred in parallel to a measurable PTP1B-associated phosphatase activity. Inhibitor studies demonstrated that an EGF-receptor type kinase is involved in phosphorylation. In a COS-7 cell line created in the laboratory that stably expressed myc-PLD2, PTP1B induced a robust (>6-fold) augmentation of myc-PLD2 phosphotyrosine content. The addition of growth factor receptor-bound protein 2 (Grb2) to cell extracts also elevated PY levels of myc-PLD (>10-fold). Systematic co-immunoprecipitation-immunoblotting experiments pointed at a physical association between PLD2, Grb2, and PTP1B in both physiological conditions and in overexpressed cells. This is the first report of a demonstration of the mammalian isoform PLD2 existing in a ternary complex with a protein tyrosine phosphatase, PTP1b, and the docking protein Grb2 which greatly enhances tyrosyl phosphorylation of the lipase.

Sphingosylphosphorylcholine generates reactive oxygen species through calcium-, protein kinase Cδ- and phospholipase D-dependent pathways

Sphingosylphosphorylcholine (SPC) is a bioactive lipid molecule involved in numerous biological processes. Treatment of MS1 pancreatic islet endothelial cells with SPC increased phospholipase D (PLD) activity in a time- and dose-dependent manner. In addition, treatment of the MS1 cells with 10 microM SPC induced stimulation of phospholipase C (PLC) activity and transient elevation of intracellular Ca2+. The SPC-induced PLD activation was prevented by pretreatment of the MS1 cells with a PLC inhibitor, U73122, and an intracellular Ca2+-chelating agent, BAPTA-AM. This suggests that PLC-dependent elevation of intracellular Ca2+ is involved in the SPC-induced activation of PLD. The SPC-dependent PLD activity was also almost completely prevented by pretreatment with pan-specific PKC inhibitors, GF109203X and RO-31-8220, and with a PKCdelta-specific inhibitor, rottlerin, but not by pretreatment with GO6976, a conventional PKC isozymes-specific inhibitor. Adenoviral overexpression of a kinase-deficient mutant of PKCdelta attenuated the SPC-induced PLD activity. These results suggest that PKCdelta plays a crucial role for the SPC-induced PLD activation. The SPC-induced PLD activation was preferentially potentiated in COS-7 cells transfected with PLD2 but not with PLD1, suggesting a specific implication of PLD2 in the SPC-induced PLD activation. SPC treatment induced phosphorylation of PLD2 in COS-7 cells, and overexpression of the kinase-deficient mutant of PKCdelta prevented the SPC-induced phosphorylation of PLD2. Furthermore, SPC treatment generated reactive oxygen species (ROS) in MS1 cells and the SPC induced production of ROS was inhibited by pretreatment with U73122, BAPTA-AM, and rottlerin. In addition, pretreatment with a PLD inhibitor 1-butanol and overexpression of a lipase-inactive mutant of PLD2 but not PLD1 attenuated the SPC-induced generation of ROS. These results suggest that PLC-, Ca2+-, PKCdelta-, and PLD2-dependent pathways are essentially required for the SPC induced ROS generation.

Novel functions of the phospholipase D2-Phox homology domain in protein kinase Czeta activation

It has been established that protein kinase Czeta (PKCzeta) participates in diverse signaling pathways and cellular functions in a wide variety of cells, exhibiting properties relevant to cellular survival and proliferation. Currently, however, the regulation mechanism of PKCzeta remains elusive. Here, for the first time, we determine that phospholipase D2 (PLD2) enhances PKCzeta activity through direct interaction in a lipase activity-independent manner. This interaction of the PLD2-Phox homology (PX) domain with the PKCzeta-kinase domain also induces the activation loop phosphorylation of PKCzeta and downstream signal stimulation, as measured by p70 S6 kinase phosphorylation. Furthermore, only the PLD2-PX domain directly stimulates PKCzeta activity in vitro, and it is necessary for the formation of the ternary complex with phosphoinositide-dependent kinase 1 and PKCzeta. The mutant that substitutes the triple lysine residues (Lys101, Lys102, and Lys103) within the PLD2-PX domain with alanine abolishes interaction with the PKCzeta-kinase domain and activation of PKCzeta. Moreover, breast cancer cell viability is significantly affected by PLD2 silencing. Taken together, these results suggest that the PLD2-mediated PKCzeta activation is induced by its PX domain performing both direct activation of PKCzeta and assistance of activation loop phosphorylation. Furthermore, we find it is an important factor in the survival of breast cancer cells.

Studies of the roles of ADP-ribosylation factors and phospholipase D in phorbol ester-induced membrane ruffling

In this study, we have explored the roles of ADP-ribosylation factors (ARFs), phospholipase D (PLD) isozymes, and arfaptins in phorbol ester (PMA)-induced membrane ruffling in HeLa cells. PMA stimulation induced ruffling and translocated cortactin to the plasma membrane. The cortactin translocation was inhibited by dominant negative (DN)-ARF6, DN-ARF1, and DN-Rac1, but not by DN-RhoA and DN-Cdc42. The inability of DN-forms of ARF6, ARF1, and Rac1 to affect PLD activity in response to PMA indicated that this enzyme was not activated via these small G proteins and that its activation was not essential for the induction of ruffling. Endogenous-ARF1, -ARF6, and -Rac1 existed in the ruffling region along with cortactin after PMA stimulation. DN-ARF1 had no effect on the ruffling induced by DA-ARF6 or DA-Rac1, and DN-ARF6 had no effect on that induced by DA-ARF1 or DA-Rac1. On the other hand DN-Rac1 suppressed the effect of DA-ARF6 but not that of DA-ARF1. These results suggest that PMA causes membrane ruffling via an ARF6-Rac1 pathway and also an ARF1 pathway operating in parallel. Overexpression of PLD1 and PLD2 inhibited PMA-induced cortactin translocation and actin-cortactin complex formation, supporting the view that these enzymes are not required for ruffling, but actually suppress it. We conclude that PMA-induced membrane ruffling is caused via ARF6-Rac1 and ARF1 pathways operating in parallel and that PLD may be inhibitory.

Phorbol ester-induced differentiation of L6 myogenic cells involves phospholipase D activation

TPA, a potent PKC activator, inhibits myogenic differentiation and activates phospholipase D (PLD). We evaluated the involvement of PLD in the TPA effects on L6 myoblasts differentiation. TPA, at concentrations inhibiting differentiation of L6 cells, induced a strong, though transient, PLD activation. Surprisingly, at nanomolar concentration, TPA induced both myogenic differentiation and sustained activation of PLD. Differential effect of TPA can be ascribed to PKC downregulation induced by highest TPA concentrations. TPA-induced differentiation was inhibited by 1-butanol, confirming the involvement of PLD in this effect. These data suggest that prolonged elevation of PLD activity is required for myogenic differentiation.

Oxygen-dependent PAF receptor binding and intracellular signaling in ovine fetal pulmonary vascular smooth muscle

Circulating levels of platelet-activating factor (PAF) are high in the fetus, and PAF is active in maintaining high PVR in fetal hypoxia (Ibe BO, Hibler S, Raj J. J Appl Physiol 85: 1079-1085, 1998). PAF synthesis by fetal pulmonary vascular smooth muscle cells (PVSMC) is high in hypoxia, but how oxygen tension affects PAF receptor (PAF-r) binding in PVSMC is not known. We studied the effect of oxygen tension on PAF-r binding and signaling in fetal PVSMC. PAF binding was saturable. PAF-r density (B(max): fmol/10(6) cells; means +/- SE, n = 6), 25.2 +/- 0.77 during hypoxia (Po(2) <40 Torr), was higher than 13.9 +/- 0.44 during normoxia (Po(2) approximately 100 Torr). K(d) was twofold lower in hypoxia than normoxia. PAF-r protein expression, 35-40% greater in hypoxia, was inhibited by cycloheximide, a protein synthesis inhibitor, suggesting translational regulation. IP(3) release, an index of PAF-r-mediated cell signaling, was greater in hypoxia (EC(50): hypoxia, 2.94 +/- 0.61; normoxia, 5.85 +/- 0.51 nM). Exogenous PAF induced 50-90% greater intracellular calcium flux in cells during hypoxia, indicating hypoxia augments PAF-r-mediated cell signaling. PAF-r phosphorylation, with or without 5 nM PAF, was 40% greater in hypoxia. These data show 1) hypoxia upregulates PAF-r binding, PAF-r phosphorylation, and PAF-r-mediated intracellular signaling, evidenced by augmented IP(3) production and intracellular Ca(2+) flux; and 2) hypoxia-induced PAF-r phosphorylation results in activation of PAF-r-mediated signal transduction. The data suggest the fetal hypoxic environment facilitates PAF-r binding and signaling, thereby promoting PAF-mediated pulmonary vasoconstriction and maintenance of high PVR in utero.

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.

Regulation of acetylcholine-induced phosphorylation of PLD1 in porcine tracheal smooth muscle

The muscarinic agonist, acetylcholine (ACh), stimulates phospholipase D (PLD) activity in tracheal smooth muscle cells. Direct activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) also stimulates PLD in this tissue. Activation of ACh-induced PLD was inhibited by the tyrosine kinase inhibitor genistein in a concentration-dependent manner. Presently known isoforms of PLD, PLD1 and PLD2, were identified in tracheal smooth muscle and their activation-induced phosphorylation status studied. Both ACh and PMA increased phosphorylation of PLD1 that was significantly blocked by genistein or the PKC inhibitor calphostin C. PLD2 phosphorylation was not detected in the present experiments. Western blots probed with an anti-phosphotyrosine antibody indicate that PLD1 in this tissue is phosphorylated on tyrosine residues after ACh or PMA stimulation. Tyrosine phosphorylation of PLD1 was blocked by genistein and calphostin C. No tyrosine residues were phosphorylated on PLD2. Taken together, these results demonstrate that porcine tracheal smooth muscle cells express both isoforms PLD1 and PLD2. However, on muscarinic activation only PLD1 in this tissue is phosphorylated by PKC via a tyrosine-kinase-dependent pathway.

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.

Localization of VEGFR-2 and PLD2 in endothelial caveolae is involved in VEGF-induced phosphorylation of MEK and ERK

To clarify the role of caveolae in VEGF/VEGF receptor-2 (VEGFR-2)-mediated signaling cascades, primary cultured human umbilical vein endothelial cells (HUVECs) were fractionated to isolate caveolae-enriched cell membranes. Interestingly, VEGFR-2, phospholipase D2 (PLD2), and Ras were enriched in caveolae-enriched fractions. Moreover, VEGF increased PLD activity in a time- and dose-dependent manner in HUVECs, whereas a ligand specific for VEGFR-1 placental growth factor did not change PLD activity. A PLD inhibitor, 1-butanol, almost completely suppressed VEGF-induced ERK phosphorylation and cellular proliferation, whereas the negative control for 1-butanol, 3-butanol, did not produce significant changes. Addition of phosphatidic acid negated the 1-butanol-induced suppression. Pharmacological analyses using several inhibitors indicated that PKC-δ regulates the VEGF-induced activation of PLD/ERK. Thus PLD2 could be involved in MEK/ERK signaling cascades that are induced by the VEGF/VEGFR-2/PKC-δ pathway in endothelial cells. Pretreatment with the cholesterol depletion agent methyl-β-cyclodextrin (MβCD) almost completely disassembled caveolar structures, whereas the addition of cholesterol to MβCD-treated cells restored caveolar structures. Pretreatment with MβCD largely abolished phosphorylation of MEK/ERK by VEGF, whereas the addition of cholesterol restored VEGF-induced MEK/ERK phosphorylations. These results indicate that intact caveolae are required for the VEGF/VEGFR-2-mediated MEK/ERK signaling cascade.

Endothelial dysfunction and altered bradykinin response due to oxidative stress induced by serum deprivation in the bovine cerebral artery

Oxidative stress plays a critical role in the pathogenesis of vasospasm after a subarachnoid hemorrhage. We demonstrate that 24-h incubation of the isolated bovine middle cerebral arteries in the serum-free media at 37 degrees C converted the response to bradykinin from relaxation to contraction, in a manner sensitive to free radical scavengers. In the freshly prepared strips, bradykinin induced an endothelium-dependent relaxation, while having no direct effect on the smooth muscle. However, in the strips treated in serum-free media, bradykinin failed to induce endothelium-dependent relaxation, but did demonstrate a direct contractile effect on smooth muscle. The addition of superoxide dismutase and ascorbic acid or 5% serum during the 24-h incubation in the serum-free media prevented the loss of endothelium-dependent relaxation and the development of a contractile response to bradykinin. SB203580 (4-(4-Fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole), a p38 mitogen-activated protein kinase inhibitor, and genistein (4',5,7-Trihydroxyisoflavone), a tyrosine kinase inhibitor, also demonstrated a similar preventive effect. In conclusion, serum-deprivation induced endothelial dysfunction and the responsiveness of smooth muscle to bradykinin due to failure of eliminating oxidative stress. p38 mitogen-activated protein kinase and tyrosine kinase were suggested to play a critical role in this endothelial dysfunction.

Regulation of phospholipase D2 activity by protein kinase C alpha

It has been well documented that protein kinase C (PKC) plays an important role in regulation of phospholipase D (PLD) activity. Although PKC regulation of PLD1 activity has been studied extensively, the role of PKC in PLD2 regulation remains to be established. In the present study it was demonstrated that phorbol 12-myristate 13-acetate (PMA) induced PLD2 activation in COS-7 cells. PLD2 was also phosphorylated on both serine and threonine residues after PMA treatment. PKC inhibitors Ro-31-8220 and bisindolylmaleimide I inhibited both PMA-induced PLD2 phosphorylation and activation. However, Gö 6976, a PKC inhibitor relatively specific for conventional PKC isoforms, almost completely abolished PLD2 phosphorylation by PMA but only slightly inhibited PLD2 activation. Furthermore, time course studies showed that phosphorylation of PLD2 lagged behind its activation by PMA. Concentration curves for PMA action on PLD2 phosphorylation and activation also showed that PLD2 was activated by PMA at concentrations at which PMA didn't induce phosphorylation. A kinase-deficient mutant of PKCalpha stimulated PLD2 activity to an even higher level than wild type PKCalpha. Co-expression of wild type PKCalpha, but not PKCdelta, greatly enhanced both basal and PMA-induced PLD2 phosphorylation. A PKCdelta-specific inhibitor, rottlerin, failed to inhibit PMA-induced PLD2 phosphorylation and activation. Co-immunoprecipitation studies indicated an association between PLD2 and PKCalpha under basal conditions that was further enhanced by PMA. Time course studies of the effects of PKCalpha on PLD2 showed that as the phosphorylation of PLD2 increased, its activity declined. In summary, the data demonstrated that PLD2 is activated and phosphorylated by PMA and PKCalpha in COS-7 cells. However, the phosphorylation is not required for PKCalpha to activate PLD2. It is suggested that interaction rather than phosphorylation underscores the activation of PLD2 by PKC in vivo and that phosphorylation may contribute to the inactivation of the enzyme.

Modulation of phospholipase D-mediated phosphatidylglycerol formation by differentiating agents in primary mouse epidermal keratinocytes

The major component of the epidermis, keratinocytes, must continuously proliferate and differentiate to form the mechanical and water permeability barrier of the skin. Our previous data have suggested a potential role in these processes for phospholipase D (PLD), an enzyme that hydrolyzes phospholipids to generate phosphatidic acid. In the presence of primary alcohols, PLD also catalyzes a transphosphatidylation reaction to produce phosphatidylalcohols, and this characteristic has been exploited to monitor the activity of PLD in intact cells. In this report, PLD was demonstrated to utilize the physiological alcohol glycerol to form phosphatidylglycerol (PG) in vitro. In intact primary murine epidermal keratinocytes treated for 24 h with elevated extracellular Ca(2+) levels, but not 1,25-dihydroxyvitamin D(3), incubation with radioactive glycerol resulted in an increase in PLD-mediated radiolabeled PG production. This effect was dose-dependent and biphasic, with maximal PG formation detected after exposure to an intermediate (125 microM) Ca(2+) concentration. Furthermore, the biphasic nature of the response was due, in part, to a corresponding biphasic change in glycerol uptake. Finally, short-term treatment of keratinocytes with phorbol 12-myristate 13-acetate (PMA) failed to increase PG synthesis and inhibited glycerol uptake. Since (1) PMA is reported to activate PLD-1 to a greater extent than PLD-2, (2) 1,25-dihydroxyvitamin D(3) increases the expression/activity of PLD-1 in keratinocytes, and (3) PLD-2 is co-localized with a glycerol channel in keratinocyte membrane microdomains, we speculate that radiolabeled PG production from radioactive glycerol is a measure of PLD-2 activation in these cells. Our results also suggest that PLD-mediated PG synthesis may be regulated at the level of both PLD activity and alcohol substrate availability via changes in glycerol uptake.

AngII induces transient phospholipase D activity in the H295R glomerulosa cell model

In this report we demonstrate that in human adrenocortical carcinoma NCI H295R cells, a model for adrenal glomerulosa cells, PLD was activated both by AngII and protein kinase C (PKC)-activating phorbol 12-myristate 13-acetate (PMA). However, while PMA triggered sustained PLD activation, AngII induced transient PLD activation, in contrast to results in bovine glomerulosa cells in primary culture. Despite the transient effect of AngII on PLD activity, PLD-derived lipid signals were required for maximal AngII-elicited aldosterone secretion. AngII-induced PLD activation was inhibited by PKC inhibitors, but not by tyrosine kinase or calcium/calmodulin-dependent kinase inhibitors or a calmodulin antagonist. Both AngII- and PMA-stimulated PLD activity was enhanced by phosphoinositide 3-kinase (PI3K) inhibitors. Akt, a downstream protein kinase activated by the products of PI3K, was constitutively active in H295R cells, and this activity was blocked by PI3K inhibitors. These results suggested that in H295R adrenocortical carcinoma cells, AngII-induced PLD activation was promoted by PKC and inhibited by the constitutively active PI3K pathway.

Protein kinase C alpha associates with phospholipase D1 and enhances basal phospholipase D activity in a protein phosphorylation-independent manner in human melanoma cells

It is well known that phospholipase D plays a crucial part in the signal transduction of many types of cells, and is activated by protein kinase C alpha when cells are stimulated. To elucidate the role of phospholipase D in melanoma, the expression of phospholipase D1 and protein kinase C alpha in primary and metastatic lesions of acral lentiginous melanoma and superficial spreading melanoma was investigated using immunohistologic techniques. In addition, the mechanism of regulation of phospholipase D1 by protein kinase C alpha was examined in a human melanoma cell line HM3KO using an adenovirus-mediated gene transfer technique. Both phospholipase D1 and protein kinase C alpha were strongly expressed in primary and metastatic lesions of superficial spreading melanoma. Conversely, in acral lentiginous melanoma lesions, the expression of these two proteins increased dramatically with tumor progression; the expression of both phospholipase D1 and protein kinase C alpha was almost negative in the radial growth phase of primary acral lentiginous melanoma lesions, and increased synchronously in a progression-related manner in advanced acral lentiginous melanoma lesions, including vertical growth phase and metastatic lesions. Immunoprecipitation study showed that phospholipase D1 and protein kinase C alpha are associated physiologically in resting melanoma cells. Further immunoprecipitation study using HM3KO cells after adenovirus-mediated simultaneous overexpression of phospholipase D1 and protein kinase C alpha, or phospholipase D1 and the kinase-negative mutant of protein kinase C alpha revealed that both protein kinase C alpha and the kinase-negative mutant of protein kinase C alpha are associated with phospholipase D1 in melanoma cells in the absence of an external signal. Overexpression of protein kinase C alpha or the kinase-negative mutant of protein kinase C alpha in melanoma cells by the adenovirus vectors resulted in the enhancement of basal phospholipase D activity in a viral concentration-dependent manner. Furthermore, enhanced basal phospholipase D activity increased the in vitro invasive potential of HM3KO cells. These results suggest that upregulation of phospholipase D1 and protein kinase C alpha plays a part in the progression of acral lentiginous melanoma from the radial growth phase to the vertical growth phase. The present results also suggest that protein kinase C alpha associates with phospholipase D1 and enhances basal phospholipase D activity in a protein phosphorylation-independent manner in melanoma cells, which contributes to the cell's high invasive potential.

Endogenous phospholipase D2 localizes to the plasma membrane of RBL-2H3 mast cells and can be distinguished from ADP ribosylation factor-stimulated phospholipase D1 activity by its specific sensitivity to oleic acid

We have examined the specificity of oleate as an activator of phospholipase D2 (PLD2) and whether it can be used to study PLD2 localization and its involvement in cell function. Oleate stimulates PLD activity in intact RBL-2H3 mast cells. Comparing PLD1- with PLD2-overexpressing cells, oleate enhanced PLD activity only in PLD2-overexpressing cells. Membranes were also sensitive to oleate and when membranes prepared from PLD1- and PLD2-overexpressing cells were examined, oleate further increased PLD activity only in membranes from PLD2-overexpressing cells. Overexpressed green fluorescent protein (GFP)-PLD2 fusion protein was localized at the plasma membrane and GFP-PLD1 was found in an intracellular vesicular compartment. Oleate was used to examine whether overexpressed PLD2 co-localized with endogenous PLD2. RBL-2H3 mast cell homogenates were fractionated on a linear sucrose gradient and analysed for both oleate-stimulated activity and ADP ribosylation factor 1-stimulated PLD1 activity. The oleate-stimulated activity co-localized with markers of the plasma membrane including the beta-subunit of the FcepsilonRI and linker for activation of T cells. Fractionation of homogenates from PLD2-overexpressing cells demonstrated that the overexpressed PLD2 fractionated in an identical location to the endogenous oleate-stimulated activity and this activity was greatly enhanced in comparison with control membranes. Examination of membranes prepared from COS-7, Jurkat and HL60 cells indicated a relationship between oleate-stimulated PLD2 activity and PLD2 immunoreactivity. We examined whether oleate could be used to activate secretion and membrane ruffling in adherent RBL-2H3 mast cells. Oleate did not stimulate secretion but did stimulate membrane ruffling, which was short-lived. We conclude that oleic acid is a selective activator of PLD2 and can be used for localization studies, but its use as an activator of PLD2 in intact cells to study function is limited due to toxicity.

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.

Proteomic analysis in the neurosciences

Proteomics is a field of study directed toward providing a comprehensive view of the characteristics and activity of every cellular protein. Rapid innovations in the core technologies required to characterize proteins on a global scale are poised to bring about a comprehensive understanding of how dynamic changes in protein expression, post-translational modification, and function affect complex signaling and regulatory networks. These advances have significant implications for understanding the multitude of pathways that govern behavior and cognition and the response of the nervous system to injury and disease.