Kinetic analysis of a mammalian phospholipase D: allosteric modulation by monomeric GTPases, protein kinase C, and polyphosphoinositides

Antimicrobial cetylpyridinium chloride suppresses mast cell function by targeting tyrosine phosphorylation of Syk kinase

Cetylpyridinium chloride (CPC) is a quaternary ammonium antimicrobial used in numerous personal care products, human food, cosmetic products, and cleaning solutions. Yet, there is minimal published data on CPC effects on eukaryotes, immune signaling, and human health. Previously, we showed that low-micromolar CPC inhibits rat mast cell function by inhibiting antigen (Ag)-stimulated Ca 2+ mobilization, microtubule polymerization, and degranulation. In this study, we extend the findings to human mast cells (LAD2) and present data indicating that CPC's mechanism of action centers on its positively-charged quaternary nitrogen in its pyridinium headgroup. CPC's inhibitory effect is independent of signaling platform receptor architecture. Tyrosine phosphorylation events are a trigger of Ca 2+ mobilization necessary for degranulation. CPC inhibits global tyrosine phosphorylation in Ag-stimulated mast cells. Specifically, CPC inhibits tyrosine phosphorylation of specific key players Syk kinase and LAT, a substrate of Syk. In contrast, CPC does not affect Lyn kinase phosphorylation. Thus, CPC's root mechanism is electrostatic disruption of particular tyrosine phosphorylation events essential for signaling. This work outlines the biochemical mechanisms underlying the effects of CPC on immune signaling and allows the prediction of CPC effects on cell types, like T cells, that share similar signaling elements.

Structural insights into PA3488-mediated inactivation of Pseudomonas aeruginosa PldA

PldA, a phospholipase D (PLD) effector, catalyzes hydrolysis of the phosphodiester bonds of glycerophospholipids-the main component of cell membranes-and assists the invasion of the opportunistic pathogen Pseudomonas aeruginosa. As a cognate immunity protein, PA3488 can inhibit the activity of PldA to avoid self-toxicity. However, the precise inhibitory mechanism remains elusive. We determine the crystal structures of full-length and truncated PldA and the cryogenic electron microscopy structure of the PldA-PA3488 complex. Structural analysis reveals that there are different intermediates of PldA between the "open" and "closed" states of the catalytic pocket, accompanied by significant conformational changes in the "lid" region and the peripheral helical domain. Through structure-based mutational analysis, we identify the key residues responsible for the enzymatic activity of PldA. Together, these data provide an insight into the molecular mechanisms of PldA invasion and its neutralization by PA3488, aiding future design of PLD-targeted inhibitors and drugs.

PLD3 is a neuronal lysosomal phospholipase D associated with β-amyloid plaques and cognitive function in Alzheimer's disease

Phospholipase D3 (PLD3) is a protein of unclear function that structurally resembles other members of the phospholipase D superfamily. A coding variant in this gene confers increased risk for the development of Alzheimer's disease (AD), although the magnitude of this effect has been controversial. Because of the potential significance of this obscure protein, we undertook a study to observe its distribution in normal human brain and AD-affected brain, determine whether PLD3 is relevant to memory and cognition in sporadic AD, and to evaluate its molecular function. In human neuropathological samples, PLD3 was primarily found within neurons and colocalized with lysosome markers (LAMP2, progranulin, and cathepsins D and B). This colocalization was also present in AD brain with prominent enrichment on lysosomal accumulations within dystrophic neurites surrounding β-amyloid plaques. This pattern of protein distribution was conserved in mouse brain in wild type and the 5xFAD mouse model of cerebral β-amyloidosis. We discovered PLD3 has phospholipase D activity in lysosomes. A coding variant in PLD3 reported to confer AD risk significantly reduced enzymatic activity compared to wild-type PLD3. PLD3 mRNA levels in the human pre-frontal cortex inversely correlated with β-amyloid pathology severity and rate of cognitive decline in 531 participants enrolled in the Religious Orders Study and Rush Memory and Aging Project. PLD3 levels across genetically diverse BXD mouse strains and strains crossed with 5xFAD mice correlated strongly with learning and memory performance in a fear conditioning task. In summary, this study identified a new functional mammalian phospholipase D isoform which is lysosomal and closely associated with both β-amyloid pathology and cognition.

Structure and regulation of human phospholipase D

Mammalian phospholipase D (PLD) generates phosphatidic acid, a dynamic lipid secondary messenger involved with a broad spectrum of cellular functions including but not limited to metabolism, migration, and exocytosis. As a promising pharmaceutical target, the biochemical properties of PLD have been well characterized. This has led to the recent crystal structures of human PLD1 and PLD2, the development of PLD specific pharmacological inhibitors, and the identification of cellular regulators of PLD. In this review, we discuss the PLD1 and PLD2 structures, PLD inhibition by small molecules, and the regulation of PLD activity by effector proteins and lipids.

Phospholipase D as a Potential Modulator of Metabolic Syndrome: Impact of Functional Foods

Significance: Cardiometabolic disorders (CMD) are composed of a plethora of metabolic dysfunctions such as dyslipidemia, nonalcoholic fatty liver disease, insulin resistance, and hypertension. The development of these disorders is highly linked to inflammation and oxidative stress (OxS), two metabolic states closely related to physiological and pathological conditions. Given the drastically rising CMD prevalence, the discovery of new therapeutic targets/novel nutritional approaches is of utmost importance. Recent Advances: The tremendous progress in methods/technologies and animal modeling has allowed the clarification of phospholipase D (PLD) critical roles in multiple cellular processes, whether directly or indirectly via phosphatidic acid, the lipid product mediating signaling functions. In view of its multiple features and implications in various diseases, PLD has emerged as a drug target. Critical Issues: Although insulin stimulates PLD activity and, in turn, PLD regulates insulin signaling, the impact of the two important PLD isoforms on the metabolic syndrome components remains vague. Therefore, after outlining PLD1/PLD2 characteristics and functions, their role in inflammation, OxS, and CMD has been analyzed and critically reported in the present exhaustive review. The influence of functional foods and nutrients in the regulation of PLD has also been examined. Future Directions: Available evidence supports the implication of PLD in CMD, but only few studies emphasize its mechanisms of action and specific regulation by nutraceutical compounds. Therefore, additional investigations are first needed to clarify the functional role of nutraceutics and, second, to elucidate whether targeting PLDs with food compounds represents an appropriate therapeutic strategy to treat CMD. Antioxid. Redox Signal. 34, 252-278.

Recifin A, Initial Example of the Tyr-Lock Peptide Structural Family, Is a Selective Allosteric Inhibitor of Tyrosyl-DNA Phosphodiesterase I

Tyrosyl-DNA phosphodiesterase 1 (TDP1) is a molecular target for the sensitization of cancer cells to the FDA-approved topoisomerase inhibitors topotecan and irinotecan. High-throughput screening of natural product extract and fraction libraries for inhibitors of TDP1 activity resulted in the discovery of a new class of knotted cyclic peptides from the marine sponge Axinella sp. Bioassay-guided fractionation of the source extract resulted in the isolation of the active component which was determined to be an unprecedented 42-residue cysteine-rich peptide named recifin A. The native NMR structure revealed a novel fold comprising a four strand antiparallel β-sheet and two helical turns stabilized by a complex disulfide bond network that creates an embedded ring around one of the strands. The resulting structure, which we have termed the Tyr-lock peptide family, is stabilized by a tyrosine residue locked into three-dimensional space. Recifin A inhibited the cleavage of phosphodiester bonds by TDP1 in a FRET assay with an IC₅₀ of 190 nM. Enzyme kinetics studies revealed that recifin A can specifically modulate the enzymatic activity of full-length TDP1 while not affecting the activity of a truncated catalytic domain of TDP1 lacking the N-terminal regulatory domain (Δ1-147), suggesting an allosteric binding site for recifin A on the regulatory domain of TDP1. Recifin A represents both the first of a unique structural class of knotted disulfide-rich peptides and defines a previously unseen mechanism of TDP1 inhibition that could be productively exploited for potential anticancer applications.

Structural insights into phospholipase D function

Phospholipase D (PLD) and its metabolic active product phosphatidic acid (PA) engage in a wide range of physiopathologic processes in the cell. PLDs have been considered as a potential and promising drug target. Recently, the crystal structures of PLDs in mammalian and plant have been solved at atomic resolution. These achievements allow us to understand the structural differences among different species of PLDs and the functions of their key domains. In this review, we summarize the sequence and structure of different species of PLD isoforms, and discuss the structural mechanisms for PLD interactions with their binding partners and the functions of each key domain in the regulation of PLDs activation and catalytic reaction.

PLD2-PI(4,5)P2 interactions in fluid phase membranes: Structural modeling and molecular dynamics simulations

Interaction of phospholipase D2 (PLD2) with phosphatidylinositol (4,5)-bisphosphate (PIP2) is regarded as the critical step of numerous physiological processes. Here we build a full-length model of human PLD2 (hPLD2) combining template-based and ab initio modeling techniques and use microsecond all-atom molecular dynamics (MD) simulations of the protein in contact with a complex membrane to determine hPLD2-PIP2 interactions. MD simulations reveal that the intermolecular interactions preferentially occur between specific PIP2 phosphate groups and hPLD2 residues; the most strongly interacting residues are arginine at the pbox consensus sequence (PX) and pleckstrin homology (PH) domain. Interaction networks indicate formation of clusters at the protein-membrane interface consisting of amino acids, PIP2, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid (POPA); the largest cluster was in the PH domain.

Mechanistic models of PLC/PKC signaling implicate phosphatidic acid as a key amplifier of chemotactic gradient sensing

Chemotaxis of fibroblasts and other mesenchymal cells is critical for embryonic development and wound healing. Fibroblast chemotaxis directed by a gradient of platelet-derived growth factor (PDGF) requires signaling through the phospholipase C (PLC)/protein kinase C (PKC) pathway. Diacylglycerol (DAG), the lipid product of PLC that activates conventional PKCs, is focally enriched at the up-gradient leading edge of fibroblasts responding to a shallow gradient of PDGF, signifying polarization. To explain the underlying mechanisms, we formulated reaction-diffusion models including as many as three putative feedback loops based on known biochemistry. These include the previously analyzed mechanism of substrate-buffering by myristoylated alanine-rich C kinase substrate (MARCKS) and two newly considered feedback loops involving the lipid, phosphatidic acid (PA). DAG kinases and phospholipase D, the enzymes that produce PA, are identified as key regulators in the models. Paradoxically, increasing DAG kinase activity can enhance the robustness of DAG/active PKC polarization with respect to chemoattractant concentration while decreasing their whole-cell levels. Finally, in simulations of wound invasion, efficient collective migration is achieved with thresholds for chemotaxis matching those of polarization in the reaction-diffusion models. This multi-scale modeling framework offers testable predictions to guide further study of signal transduction and cell behavior that affect mesenchymal chemotaxis.

Cell movement directed by external gradients of chemical composition is critical for immune responses, wound healing, and development. Although theoretical concepts explaining how shallow external gradients might definitively polarize a cell’s motility have been offered over the past two decades, mathematical models cast in terms of defined molecules and mechanisms are uncommon in this context. Based on both recent and older insights from the literature, we offer mechanistic models that are able to explain experimentally observed polarization of signal transduction elicited by shallow attractant gradients. A novel insight of our models is the implicated role of phosphatidic acid, a membrane lipid produced by at least two enzymatic pathways, in two positive feedback loops that amplify signal transduction locally. In separate simulations, we explored the implications of polarization for efficient cell invasion during wound healing. We expected that the ability to polarize in response to shallow gradients would enhance the speed of wound invasion, but an unexpected finding is that this property can promote intermittent polarization throughout the wound.

Crystal structure of human PLD1 provides insight into activation by PI(4,5)P2 and RhoA

The signal transduction enzyme phospholipase D1 (PLD1) hydrolyzes phosphatidylcholine to generate the lipid second-messenger phosphatidic acid, which plays roles in disease processes such as thrombosis and cancer. PLD1 is directly and synergistically regulated by protein kinase C, Arf and Rho GTPases, and the membrane lipid phosphatidylinositol-4,5-bisphosphate (PIP₂). Here, we present a 1.8 Å-resolution crystal structure of the human PLD1 catalytic domain, which is characterized by a globular fold with a funnel-shaped hydrophobic cavity leading to the active site. Adjacent is a PIP₂-binding polybasic pocket at the membrane interface that is essential for activity. The C terminus folds into and contributes part of the catalytic pocket, which harbors a phosphohistidine that mimics an intermediate stage of the catalytic cycle. Mapping of PLD1 mutations that disrupt RhoA activation identifies the RhoA-PLD1 binding interface. This structure sheds light on PLD1 regulation by lipid and protein effectors, enabling rationale inhibitor design for this well-studied therapeutic target.

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.

Crystal structure of plant PLDα1 reveals catalytic and regulatory mechanisms of eukaryotic phospholipase D

Phospholipase D (PLD) hydrolyzes the phosphodiester bond of glycerophospholipids and produces phosphatidic acid (PA), which acts as a second messenger in many living organisms. A large number of PLDs have been identified in eukaryotes, and are viewed as promising targets for drug design because these enzymes are known to be tightly regulated and to function in the pathophysiology of many human diseases. However, the underlying molecular mechanisms of catalysis and regulation of eukaryotic PLD remain elusive. Here, we determined the crystal structure of full-length plant PLDα1 in the apo state and in complex with PA. The structure shows that the N-terminal C2 domain hydrophobically interacts with the C-terminal catalytic domain that features two HKD motifs. Our analysis reveals the catalytic site, substrate-binding mechanism, and a new Ca²⁺-binding site that is required for the activation of PLD. In addition, we tested several efficient small-molecule inhibitors against PLDα1, and suggested a possible competitive inhibition mechanism according to structure-based docking analysis. This study explains many long-standing questions about PLDs and provides structural insights into PLD-targeted inhibitor/drug design.

Antimicrobial agent triclosan suppresses mast cell signaling via phospholipase D inhibition

Humans are exposed to the antimicrobial agent triclosan (TCS) through use of TCS-containing products. Exposed tissues contain mast cells, which are involved in numerous biological functions and diseases by secreting various chemical mediators through a process termed degranulation. We previously demonstrated that TCS inhibits both Ca²⁺ influx into antigen-stimulated mast cells and subsequent degranulation. To determine the mechanism linking the TCS cytosolic Ca²⁺ depression to inhibited degranulation, we investigated the effects of TCS on crucial signaling enzymes activated downstream of the Ca²⁺ rise: protein kinase C (PKC; activated by Ca²⁺ and reactive oxygen species [ROS]) and phospholipase D (PLD). We found that TCS strongly inhibits PLD activity within 15 minutes post-antigen, a key mechanism of TCS mast cell inhibition. In addition, experiments using fluorescent constructs and confocal microscopy indicate that TCS delays antigen-induced translocations of PKCβII, PKCδ and PKC substrate myristoylated alanine-rich C-kinase. Surprisingly, TCS does not inhibit PKC activity or overall ability to translocate, and TCS actually increases PKC activity by 45 minutes post-antigen; these results are explained by the timing of both TCS inhibition of cytosolic Ca²⁺ (~15+ minutes post-antigen) and TCS stimulation of ROS (~45 minutes post-antigen). These findings demonstrate that it is incorrect to assume that all Ca²⁺ -dependent processes will be synchronously inhibited when cytosolic Ca²⁺ is inhibited by a toxicant or drug. The results offer molecular predictions of the effects of TCS on other mammalian cell types, which share these crucial signal transduction elements and provide biochemical information that may underlie recent epidemiological findings implicating TCS in human health problems.

CDC42-related genes are upregulated in helper T cells from obese asthmatic children

BACKGROUND

Pediatric obesity-related asthma is more severe and less responsive to medications than asthma in normal-weight children. Obese asthmatic children have nonatopic TH1-polarized systemic inflammation that correlates with pulmonary function deficits, but the pathways underlying TH1-polarized inflammation are not well understood.

OBJECTIVE

We compared the CD4+ T-cell transcriptome in obese children with asthma with that in normal-weight children with asthma to identify key differentially expressed genes associated with TH1-polarized inflammation.

METHODS

CD4+ T-cell transcriptome-wide differential gene expression was compared between 21 obese and 21 normal-weight children by using directional RNA sequencing. High-confidence differentially expressed genes were verified in the first cohort and validated in a second cohort of 20 children (10 obese and 10 normal-weight children) by using quantitative RT-PCR.

RESULTS

Transcriptome-wide differential gene expression among obese asthmatic children was enriched for genes, including VAV2, DOCK5, PAK3, PLD1, CDC42EP4, and CDC42PBB, which are associated with CDC42, a small guanosine triphosphate protein linked to T-cell activation. Upregulation of MLK3 and PLD1, genes downstream of CDC42 in the mitogen-activated protein kinase and mammalian target of rapamycin pathways and the inverse correlation of CDC42EP4 and DOCK5 transcript counts with FEV1/FVC ratio together support a role of CDC42 in the TH1 polarization and pulmonary function deficits found in patients with obesity-related asthma.

CONCLUSIONS

Our study identifies the CDC42 pathway as a novel target that is upregulated in TH cells of obese asthmatic children, suggesting its role in nonatopic TH1-polarized systemic inflammation and pulmonary function deficits found in patients with pediatric obesity-related asthma.

Mechanistic Aspects of Phosphate Diester Cleavage Assisted by Imidazole. A Template Reaction for Obtaining Aryl Phosphoimidazoles

Phosphoimidazole-containing compounds are versatile players in biological and chemical processes. We explore catalytic and mechanistic criteria for the efficient formation of cyclic aryl phosphoimidazoles in aqueous solution, viewed as a template reaction for the in situ synthesis of related compounds. To provide a detailed analysis for this reaction a series of o-(2'-imidazolyl)naphthyl (4-nitrophenyl) phosphate isomers were examined to provide a basis for analysis of both mechanism and the influence of structural factors affecting the nucleophilic attack of the imidazolyl group on the phosphorus center of the substrate. Formation of the cyclic aryl phosphoimidazoles was probed by NMR and ESI-MS techniques. Kinetic experiments show that cyclization is faster under alkaline conditions, with an effective molarity up to 2900 M for the imidazolyl group, ruling out competition from external nucleophiles. Heavy atom isotope effect and computational studies show that the reaction occurs through a S_N2(P)-type mechanism involving a pentacoordinated phosphorus TS, with apical positions occupied by the incoming imidazolyl nucleophile and the p-nitrophenolate leaving group. The P-O bond to the leaving group is about 50-60% broken in the transition state.

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.

Discovery of desketoraloxifene analogues as inhibitors of mammalian, Pseudomonas aeruginosa, and NAPE phospholipase D enzymes

Phospholipase D (PLD) hydrolyses cellular lipids to produce the important lipid second messenger phosphatidic acid. A PLD enzyme expressed by Pseudomonas aeruginosa (PldA) has been shown to be important in bacterial infection, and NAPE-PLD has emerged as being key in the synthesis of endocannabinoids. In order to better understand the biology and therapeutic potential of these less explored PLD enzymes, small molecule tools are required. Selective estrogen receptor modulators (SERMs) have been previously shown to inhibit mammalian PLD (PLD1 and PLD2). By targeted screening of a library of SERM analogues, additional parallel synthesis, and evaluation in multiple PLD assays, we discovered a novel desketoraloxifene-based scaffold that inhibited not only the two mammalian PLDs but also structurally divergent PldA and NAPE-PLD. This finding represents an important first step toward the development of small molecules possessing universal inhibition of divergent PLD enzymes to advance the field.

Biochemical characterization of a Pseudomonas aeruginosa phospholipase D

Phospholipase D is a ubiquitous protein in eukaryotes that hydrolyzes phospholipids to generate the signaling lipid phosphatidic acid (PtdOH). PldA, a Pseudomonas aeruginosa PLD, is a secreted protein that targets bacterial and eukaryotic cells. Here we have characterized the in vitro factors that modulate enzymatic activity of PldA, including divalent cations and phosphoinositides. We have identified several similarities between the eukaryotic-like PldA and the human PLD isoforms, as well as several properties in which the enzymes diverge. Notable differences include the substrate preference and transphosphatidylation efficiency for PldA. These findings offer new insights into potential regulatory mechanisms of PldA and its role in pathogenesis.

Two sites of action for PLD2 inhibitors: The enzyme catalytic center and an allosteric, phosphoinositide biding pocket

Phospholipase D (PLD) has been implicated in many physiological functions, such as chemotaxis and phagocytosis, as well as pathological functions, such as cancer cell invasion and metastasis. New inhibitors have been described that hamper the role of PLD in those pathologies but their site of action is not known. We have characterized the biochemical and biological behavior of the PLD1/2 dual inhibitor 5-Fluoro-2-indolyl des-chlorohalopemide (FIPI), and the specific PLD2 inhibitor, N-[2-[1-(3-Fluorophenyl)-4-oxo-1,3,-8-triazaspiro[4.5]dec-8-yl]ethyl]-2-naphthalenecarboxamide (NFOT), and found that both FIPI and NFOT are mixed-kinetics inhibitors. Mutagenesis studies indicate that FIPI binds at S757 of PLD2, which is within the HKD2 catalytic site of the enzyme, whereas NFOT binds to PLD2 at two different sites, one being at S757/S648 and another to an allosteric site that is a natural site occupied by PIP2 (R210/R212). This latter site, along with F244/L245/L246, forms a hydrophobic pocket in the PH domain. The mechanism of action of FIPI is a direct effect on the catalytic site (and as such inhibits both PLD1 and PLD2 isoforms), whereas PLD2 affects both the catalytic site (orthosteric) and blocks PIP2 binding to PLD2 (allosteric), which negates the natural enhancing role of PIP2. Moreover, NFOT prevents cell invasion of cancer cells, which does not occur in cells overexpressing PLD2-F244A/L245A/L246A, or PLD2-R210A/R212A, or PLD2-S757/S648 mutants. This study provides new specific knowledge of enzyme regulation and mechanisms of activation and inhibition of PLD2 that are necessary to understand its role in cell signaling and to develop new inhibitors for cancer cell invasion and metastasis.

Role of phospholipase d in g-protein coupled receptor function

Prolonged agonist exposure of many G-protein coupled receptors induces a rapid receptor phosphorylation and uncoupling from G-proteins. Resensitization of these desensitized receptors requires endocytosis and subsequent dephosphorylation. Numerous studies show the involvement of phospholipid-specific phosphodiesterase phospholipase D (PLD) in the receptor endocytosis and recycling of many G-protein coupled receptors e.g., opioid, formyl or dopamine receptors. The PLD hydrolyzes the headgroup of a phospholipid, generally phosphatidylcholine (PC), to phosphatidic acid (PA) and choline and is assumed to play an important function in cell regulation and receptor trafficking. Protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families regulate the two mammalian PLD isoforms 1 and 2. Mammalian and yeast PLD are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. The PA product is an intracellular lipid messenger. PLD and PA activities are implicated in a wide range of physiological processes and diseases including inflammation, diabetes, oncogenesis or neurodegeneration. This review discusses the characterization, structure, and regulation of PLD in the context of membrane located G-protein coupled receptor function.

Detection of choline and phosphatidic acid (PA) catalyzed by phospholipase D (PLD) using MALDI-QIT-TOF/MS with 9-aminoacridine matrix

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine (PC), the most abundant phospholipids of plasma membrane, resulting in the production of choline and phosphatidic acid (PA). Choline is a precursor of the neurotransmitter acetylcholine, whereas PA functions as an intracellular lipid mediator of diverse biological functions. For assessing PLD activity in vitro, PLD-derived choline has been often analyzed with radioactive or non-radioactive methods. In this study, we have developed a new method for detecting choline and PA with MALDI-QIT-TOF/MS by using 9-aminoacridine as a matrix. The standard calibration curves showed that choline and PA could be detected with linearity over the range from 0.05 and 1 pmol, respectively. Importantly, this method enables the concomitant detection of choline and PA as a reaction product of PC hydrolysis by PLD2 proteins. Thus, our simple and direct method would be useful to characterize the enzymatic properties of PLD, thereby providing insight into mechanisms of PLD activation.

A model for the interfacial kinetics of phospholipase D activity on long-chain lipids

The membrane-active enzyme phospholipase D (PLD) catalyzes the hydrolysis of the phosphodiester bond in phospholipids and plays a critical role in cell signaling. This catalytic reaction proceeds on lipid-water interfaces and is an example of heterogeneous catalysis in biology. Recently we showed that planar lipid bilayers, a previously unexplored model membrane for these kinetic studies, can be used for monitoring interfacial catalytic reactions under well-defined experimental conditions with chemical and electrical access to both sides of the lipid membrane. Employing an assay that relies on the conductance of the pore-forming peptide gramicidin A to monitor PLD activity, the work presented here reveals the kinetics of hydrolysis of long-chain phosphatidylcholine lipids in situ. We have developed an extension of a basic kinetic model for interfacial catalysis that includes product activation and substrate depletion. This model describes the kinetic behavior very well and reveals two kinetic parameters, the specificity constant and the interfacial quality constant. This approach results in a simple and general model to account for product accumulation in interfacial enzyme kinetics.

Chemical modulation of glycerolipid signaling and metabolic pathways

Thirty years ago, glycerolipids captured the attention of biochemical researchers as novel cellular signaling entities. We now recognize that these biomolecules occupy signaling nodes critical to a number of physiological and pathological processes. Thus, glycerolipid-metabolizing enzymes present attractive targets for new therapies. A number of fields-ranging from neuroscience and cancer to diabetes and obesity-have elucidated the signaling properties of glycerolipids. The biochemical literature teems with newly emerging small molecule inhibitors capable of manipulating glycerolipid metabolism and signaling. This ever-expanding pool of chemical modulators appears daunting to those interested in exploiting glycerolipid-signaling pathways in their model system of choice. This review distills the current body of literature surrounding glycerolipid metabolism into a more approachable format, facilitating the application of small molecule inhibitors to novel systems. This article is part of a Special Issue entitled Tools to study lipid functions.

The role of lipids in the control of autophagy

Macroautophagy is an essential cellular pathway mediating the lysosomal degradation of defective organelles, long-lived proteins and a variety of protein aggregates. Similar to other intracellular trafficking pathways, macroautophagy involves a complex sequence of membrane remodeling and trafficking events. These include the biogenesis of autophagosomes, which engulf portions of cytoplasm at specific subcellular locations, and their subsequent maturation into autophagolysosomes through fusion with the endo-lysosomal compartment. Although the formation and maturation of autophagosomes are controlled by molecular reactions occurring at the membrane-cytosol interface, little is known about the role of lipids and their metabolizing enzymes in this process. Historically dominated by studies on class III phosphatidylinositol 3-kinase (also known as Vps34) and its product phosphatidylinositol-3-phosphate, as well as on the lipidation of Atg8/LC3-like proteins, this area of research has recently expanded, implicating a variety of other lipids, such as phosphatidic acid and diacylglycerol, and their metabolizing enzymes in macroautophagy. This review summarizes this progress and highlights the role of specific lipids in the various steps of macroautophagy, including the signaling processes underlying macroautophagy initiation, autophagosome biogenesis and maturation.

Regulation of phospholipase D activity and phosphatidic acid production after purinergic (P2Y6) receptor stimulation

Phosphatidic acid (PA) is a lipid second messenger located at the intersection of several lipid metabolism and cell signaling events including membrane trafficking, survival, and proliferation. Generation of signaling PA has long been primarily attributed to the activation of phospholipase D (PLD). PLD catalyzes the hydrolysis of phosphatidylcholine into PA. A variety of both receptor-tyrosine kinase and G-protein-coupled receptor stimulations have been shown to lead to PLD activation and PA generation. This study focuses on profiling the PA pool upon P2Y6 receptor signaling manipulation to determine the major PA producing enzymes. Here we show that PLD, although highly active, is not responsible for the majority of stable PA being produced upon UDP stimulation of the P2Y6 receptor and that PA levels are tightly regulated. By following PA flux in the cell we show that PLD is involved in an initial increase in PA upon receptor stimulation; however, when PLD is blocked, the cell compensates by increasing PA production from other sources. We further delineate the P2Y6 signaling pathway showing that phospholipase Cβ3 (PLCβ3), PLCδ1, DGKζ and PLD are all downstream of receptor activation. We also show that DGKζ is a novel negative regulator of PLD activity in this system that occurs through an inhibitory mechanism with PKCα. These results further define the downstream events resulting in PA production in the P2Y6 receptor signaling pathway.

Biochemical analysis of the interaction between phospholipase Dα1 and GTP-binding protein α-subunit from Arabidopsis thaliana

Phospholipase Ds (PLDs) play diverse roles in plant lipid metabolism and cellular signaling processes. The sole canonical G-protein α-subunit (Gα) in Arabidopsis also plays multiple roles in plant growth and cellular signaling processes. Interestingly, overlapping functions of PLD and Gα have been indicated in many cellular processes, including abscisic acid (ABA)-mediated stomata movement and water loss, gibberellic acid (GA)-regulated seed germination, and auxin signaling. This interaction between PLD and Gα has also been suggested in biochemical and physiological studies. Here we described the methods used for studying the interaction between the major PLD form PLDα1 and Gα. From pulldown experiments with purified bacterially expressed PLDα1 and Gα, co-immunoprecipitation of plant protein extract, and yeast two-hybrid assay, we showed that there is a physical interaction between PLDα1 and Gα, and identified a key DRY motif in PLDα1, which is an essential element for the interaction. The interaction of PLDα1 and Gα can be affected by factors like GTP or GDP, but it also affected PLD phospholipase activity and Gα GTPase activity in turn.

Chemically reducible lipid bilayer coated mesoporous silica nanoparticles demonstrating controlled release and HeLa and normal mouse liver cell biocompatibility and cellular internalization

A controlled release system composed of mesoporous silica nanoparticles with covalently bound dipalmitoyl moieties supporting phosphorylated lipids has been successfully synthesized and characterized. This MSN system demonstrates controlled release of fluorescein molecules under disulfide reducing conditions. Flow cytometry analyses confirm increased biocompatibility of the resulting lipid bilayer MSNs (LB-MSNs) from nonfunctional MSNs. Fluorescently labeled LB-MSNs are examined via confocal fluorescent microscopy ex vivo and were found to enter both normal and cancer cell lines. The LB-MSNs presented here have potential to be used as rapid and diverse functionalized, stable liposome analogues for drug delivery.

The dual effect of Rac2 on phospholipase D2 regulation that explains both the onset and termination of chemotaxis

We document a biphasic effect of Rac2 on the activation and inhibition of PLD2. Cells overexpressing Rac2 and PLD2 simultaneously show a robust initial (<10 min) response toward a chemoattractant that is later (>30 min) greatly diminished over PLD2-only controls. The first phase is due to the presence of a Rac2-PLD2 positive-feedback loop. To explain the mechanism for the Rac2-led PLD2 inhibition (the second phase), we used leukocytes from wild-type (WT) and Rac2(-/-) knockout mice. Rac2(-/-) cells displayed an enhanced PLD2 (but not PLD1) enzymatic activity, confirming the inhibitory role of Rac2. Late inhibitory responses on PLD2 due to Rac2 were reversed in the presence of phosphatidylinositol 4,5-bisphosphate (PIP(2)) both in vitro (purified GST-PH-PLD2, where GST is glutathione S-transferase and PH is pleckstrin homology) and in vivo. Coimmunoprecipitation and immunofluorescence microscopy indicated that PLD2 and Rac2 remain together. The presence of an "arc" of Rac2 at the leading edge of leukocyte pseudopodia and PLD2 physically posterior to this wave of Rac2 was observed in late chemotaxis. We propose Rac-led inhibition of PLD2 function is due to sterical interference of Rac with PLD2's PH binding site to the membrane and deprivation of the PIP(2). This work supports the importance of functional interactions between PLD and Rac in the biological response of cell migration.

The molecular basis of phospholipase D2-induced chemotaxis: elucidation of differential pathways in macrophages and fibroblasts

We report the molecular mechanisms that underlie chemotaxis of macrophages and cell migration of fibroblasts, cells that are essential during the body's innate immune response and during wound repair, respectively. Silencing of phospholipase D1 (PLD1) and PLD2 reduced cell migration (both chemokinesis and chemotaxis) by approximately 60% and >80%, respectively; this migration was restored by cell transfection with PLD2 constructs refractory to small interfering RNA (siRNA). Cells overexpressing active phospholipase D1 (PLD1) but, mostly, active PLD2 exhibited cell migration capabilities that were elevated over those elicited by chemoattractants alone. The mechanism for this enhancement is complex. It involves two pathways: one that is dependent on the activity of the lipase (and signals through its product, phosphatidic acid [PA]) and another that involves protein-protein interactions. The first is evidenced by partial abrogation of chemotaxis with lipase activity-defective constructs (PLD2-K758R) and by n-butanol treatment of cells. The second is evidenced by PLD association with the growth factor receptor-bound protein 2 (Grb2) through residue Y(169), located within a Src homology 2 (SH2) consensus site. The association Grb2-PLD2 could be visualized by fluorescence microscopy in RAW/LR5 macrophages concentrated in actin-rich membrane ruffles, making possible that Grb2 serves as a docking or intermediary protein. The Grb2/PLD2-mediated chemotaxis process also depends on Grb2's ability to recognize other motility proteins, like the Wiskott-Aldrich syndrome protein (WASP). Cell transfection with WASP, PLD2, and Grb2 constructs yields the highest levels of cell migration response, particularly in a macrophage cell line (RAW/LR5) and only modestly in the fibroblast cell line COS-7. Further, RAW/LR5 macrophages utilize for cell migration an additional pathway that involves S6 kinase (S6K) through PLD2-Y(296), known to be phosphorylated by epidermal growth factor receptor (EGFR) kinase. Thus, both fibroblasts and macrophages use activity-dependent and activity-independent signaling mechanisms. However, highly mobile cells like macrophages use all signaling machinery available to them to accomplish their required function in rapid immune response, which sets them apart from fibroblasts, cells normally nonmobile that are only briefly involved in wound healing.

Comparative analysis of normal versus CLL B-lymphocytes reveals patient-specific variability in signaling mechanisms controlling LFA-1 activation by chemokines

Activation of lymphocyte function-associated antigen-1 (LFA-1) by chemokines is fine-tuned by inside-out signaling mechanisms responsible for integrin-mediated adhesion modulation. In the present study, we investigated the possibility of qualitative variability of signaling mechanisms controlling LFA-1 activation in chronic lymphocytic leukemia (CLL) cells. We pursued a multiplexed comparative analysis of the role of the recently described chemokine-triggered rho-signaling module in human normal versus CLL B-lymphocytes. We found that the rho-module of LFA-1 affinity triggering is functionally conserved in normal B-lymphocytes. In contrast, in malignant B-lymphocytes isolated from patients with B-CLL, the role of the rho-module was not maintained, showing remarkable differences and variability. Specifically, RhoA and phospholipase D1 were crucially involved in LFA-1 affinity triggering by CXCL12 in all analyzed patients. In contrast, Rac1 and CDC42 involvement displayed a consistent patient-by-patient variability, with a group of patients showing LFA-1 affinity modulation totally independent of Rac1 and CDC42 signaling activity. Finally, phosphatidylinositol-4-phosphate 5-kinase isoform 1gamma (PIP5KC) was found without any regulatory role in all patients. The data imply that the neoplastic progression may completely bypass the regulatory role of Rac1, CDC42, and PIP5KC, and show a profound divergence in the signaling mechanisms controlling integrin activation in normal versus neoplastic lymphocytes, suggesting that patients with CLL can be more accurately evaluated on the basis of the analysis of signaling mechanisms controlling integrin activation. Our findings could potentially affect the diagnosis, prognosis, and therapy of CLL disorders.

Irreversible platelet activation requires protease-activated receptor 1-mediated signaling to phosphatidylinositol phosphates

Thrombin induces platelet activation through an early, reversible stage of platelet aggregation, which is followed by a later, irreversible stage of platelet aggregation. Without intervention, events leading to pathological platelet activation can result in vessel occlusion, acute coronary syndrome, and stroke. Therefore, a better understanding of events leading to platelet-mediated clot formation may provide insight into new therapeutic targets. Once activated, protease activated receptors (PARs) are essential in regulating events leading to platelet aggregation. We have determined a signaling cascade through PAR1, which involves phosphatidylinositol (PI) kinases, phosphatidylinositol bisphosphate (PIP(2)), and Rap1 activation (independent of P2Y12) in the formation of a stable platelet aggregate. The putative phosphatidylinositol-3 kinase (PI3K) inhibitor LY294002 was found to reduce basal and PAR-stimulated PIP(2) levels by mass spectrometry and to inhibit PAR1-mediated stable platelet aggregation. Rap1 activation in platelets (during time points corresponding to the late, irreversible phase of aggregation) was found to require the PI signaling pathway. Perturbation of PI3K signaling by isoform-selective inhibitors had differential effects on Rap1 activation through PAR1 and PAR4. Hence, it is possible to disrupt lipid signaling pathways involved in stable clot formation without inhibiting early clot formation, offering a new potential target for antiplatelet therapy.

Ca2+ influx and phosphoinositide signalling are essential for the establishment and maintenance of cell polarity in monospores from the red alga Porphyra yezoensis

The asymmetrical distribution of F-actin directed by cell polarity has been observed during the migration of monospores from the red alga Porphyra yezoensis. The significance of Ca2+ influx and phosphoinositide signalling during the formation of cell polarity in migrating monospores was analysed pharmacologically. The results indicate that the inhibition of the establishment of cell polarity, as judged by the ability of F-actin to localize asymmetrically, cell wall synthesis, and development into germlings, occurred when monospores were treated with inhibitors of the Ca2+ permeable channel, phospholipase C (PLC), diacylglycerol kinase, and inositol-1,4,5-trisphosphate receptor. Moreover, it was also found that light triggered the establishment of cell polarity via photosynthetic activity but not its direction, indicating that the Ca2+ influx and PLC activation required for the establishment of cell polarity are light dependent. By contrast, inhibition of phospholipase D (PLD) prevented the migration of monospores but not the asymmetrical localization of F-actin. Taken together, these findings suggest that there is functional diversity between the PLC and PLD signalling systems in terms of the formation of cell polarity; the former being critical for the light-dependent establishment of cell polarity and the latter playing a role in the maintenance of established cell polarity.

Evidence that alpha-synuclein does not inhibit phospholipase D

Alpha-synuclein (alphaSyn) is a small cytosolic protein of unknown function, which is highly enriched in the brain. It is genetically linked to Parkinson's disease (PD) in that missense mutations or multiplication of the gene encoding alphaSyn causes early onset familial PD. Furthermore, the neuropathological hallmarks of both sporadic and familial PD, Lewy bodies and Lewy neurites, contain insoluble aggregates of alphaSyn. Several studies have reported evidence that alphaSyn can inhibit phospholipase D (PLD), which hydrolyzes phosphatidylcholine to form phosphatidic acid and choline. Although various hypotheses exist regarding the roles of alphaSyn in health and disease, no other specific biochemical function for this protein has been reported to date. Because PLD inhibition could represent an important function of alphaSyn, we sought to extend existing reports on this interaction. Using purified proteins, we tested the ability of alphaSyn to inhibit PLD activity in cell-free assays. We also examined several cell lines and transfection conditions to assess whether alphaSyn inhibits endogenous or overexpressed PLD in cultured mammalian cells. In yeast, we extended our previous report of an interaction between alphaSyn and PLD-dependent phenotypes, for which PLD activity is absolutely necessary. Despite testing a range of experimental conditions, including those previously published, we observed no significant inhibition of PLD by alphaSyn in any of these systems. We propose that the previously reported effects of alphaSyn on PLD activity could be due to increased endoplasmic reticulum-related stress associated with alphaSyn overexpression in cells, but are not likely due to a specific and direct interaction between alphaSyn and PLD.

Phospholipase D1 is required for angiogenesis of intersegmental blood vessels in zebrafish

Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid and choline. Studies in cultured cells and Drosophila melanogaster have implicated PLD in the regulation of many cellular functions, including intracellular vesicle trafficking, cell proliferation and differentiation. However, the function of PLD in vertebrate development has not been explored. Here we report cloning and characterization of a zebrafish PLD1 (pld1) homolog. Like mammalian PLDs, zebrafish Pld1 contains two conservative HKD motifs. Maternally contributed pld1 transcripts are uniformly distributed in early embryo. Localized expression of pld1 is observed in the notochord during early segmentation, in the somites during later segmentation and in the liver at the larval stages. Studies in intact and cell-free preparations demonstrate evolutionary conservation of regulation. Inhibition of Pld1 expression using antisense morpholino oligonucleotides (MO) interfering with the translation or splicing of pld1 impaired intersegmental vessel (ISV) development. Incubating embryos with 1-butanol, which diverts production of phosphatidic acid to a phosphatidylalcohol, caused similar ISV defects. To determine where Pld1 is required for ISV development we performed transplantation experiments. Analyses of the mosaic Pld1 deficient embryos showed partial suppression of ISV defects in the segments containing transplanted wild-type notochord cells but not in the ones containing wild-type somitic cells. These results provide the first evidence that function of Pld1 in the developing notochord is essential for vascular development in vertebrates.

Regulation of conformer-specific activation of the integrin LFA-1 by a chemokine-triggered Rho signaling module

Regulation of the affinity of the beta(2) integrin LFA-1 by chemokines is critical to lymphocyte trafficking, but the signaling mechanisms that control this process are not well understood. Here we investigated the signaling events controlling LFA-1 affinity triggering by chemokines in human primary T lymphocytes. We found that the small GTPase Rac1 mediated chemokine-induced LFA-1 affinity triggering and lymphocyte arrest in high endothelial venules. Unexpectedly, another Rho family member, Cdc42, negatively regulated LFA-1 activation. The Rho effectors PLD1 and PIP5KC were also critical to LFA-1 affinity modulation. Notably, PIP5KC was found to specifically control the transition of LFA-1 from an extended low-intermediate state to a high-affinity state, which correlated with lymphocyte arrest. Thus, chemokines control lymphocyte trafficking by triggering a Rho-dependent signaling cascade leading to conformer-specific modulation of LFA-1 affinity.

Design of isoform-selective phospholipase D inhibitors that modulate cancer cell invasiveness

Phospholipase D (PLD) is an essential enzyme responsible for the production of the lipid second messenger phosphatidic acid. Phosphatidic acid participates in both G protein-coupled receptor and receptor tyrosine kinase signal transduction networks. The lack of potent and isoform-selective inhibitors has limited progress in defining the cellular roles of PLD. We used a diversity-oriented synthetic approach and developed a library of PLD inhibitors with considerable pharmacological characterization. Here we report the rigorous evaluation of that library, which contains highly potent inhibitors, including the first isoform-selective PLD inhibitors. Specific members of this series inhibit isoforms with >100-fold selectivity both in vitro and in cells. A subset of inhibitors was shown to block invasiveness in metastatic breast cancer models. These findings demonstrate the power of diversity-oriented synthesis combined with biochemical assays and mass spectrometric lipid profiling of cellular responses to develop the first isoform-selective PLD inhibitors--a new class of antimetastatic agents.

Honokiol suppresses survival signals mediated by Ras-dependent phospholipase D activity in human cancer cells

PURPOSE

Elevated phospholipase D (PLD) activity provides a survival signal in several human cancer cell lines and suppresses apoptosis when cells are subjected to the stress of serum withdrawal. Thus, targeting PLD survival signals has potential to suppress survival in cancer cells that depend on PLD for survival. Honokiol is a compound that suppresses tumor growth in mouse models. The purpose of this study was to investigate the effect of honokiol on PLD survival signals and the Ras dependence of these signals.

EXPERIMENTAL DESIGN

The effect of honokiol upon PLD activity was examined in human cancer cell lines where PLD activity provides a survival signal. The dependence of PLD survival signals on Ras was investigated, as was the effect of honokiol on Ras activation.

RESULTS

We report here that honokiol suppresses PLD activity in human cancer cells where PLD has been shown to suppress apoptosis. PLD activity is commonly elevated in response to the stress of serum withdrawal, and, importantly, the stress-induced increase in PLD activity is selectively suppressed by honokiol. The stress-induced increase in PLD activity was accompanied by increased Ras activation, and the stress-induced increase in PLD activity in MDA-MB-231 breast cancer cells was dependent on a Ras. The PLD activity was also dependent on the GTPases RalA and ADP ribosylation factor. Importantly, honokiol suppressed Ras activation.

CONCLUSION

The data provided here indicate that honokiol may be a valuable therapeutic reagent for targeting a large number of human cancers that depend on Ras and PLD for their survival.

The regulation of cell motility and chemotaxis by phospholipid signaling

Phosphoinositide 3-kinase (PI3K), PTEN and localized phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3] play key roles in chemotaxis, regulating cell motility by controlling the actin cytoskeleton in Dictyostelium and mammalian cells. PtdIns(3,4,5)P3, produced by PI3K, acts via diverse downstream signaling components, including the GTPase Rac, Arf-GTPases and the kinase Akt (PKB). It has become increasingly apparent, however, that chemotaxis results from an interplay between the PI3K-PTEN pathway and other parallel pathways in Dictyostelium and mammalian cells. In Dictyostelium, the phospholipase PLA2 acts in concert with PI3K to regulate chemotaxis, whereas phospholipase C (PLC) plays a supporting role in modulating PI3K activity. In adenocarcinoma cells, PLC and the actin regulator cofilin seem to provide the direction-sensing machinery, whereas PI3K might regulate motility.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

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

Protease-activated receptors differentially regulate human platelet activation through a phosphatidic acid-dependent pathway

Pathological conditions such as coronary artery disease are clinically controlled via therapeutic regulation of platelet activity. Thrombin, through protease-activated receptor (PAR) 1 and PAR4, plays a central role in regulation of human platelet function in that it is known to be the most potent activator of human platelets. Currently, direct thrombin inhibitors used to block platelet activation result in unwanted side effects of excessive bleeding. An alternative therapeutic strategy would be to inhibit PAR-mediated intracellular platelet signaling pathways. To elucidate the best target, we are studying differences between the two platelet thrombin receptors, PAR1 and PAR4, in mediating thrombin's action. In this study, we show that platelet activation by PAR1-activating peptide (PAR1-AP) requires a phospholipase D (PLD)-mediated phosphatidic acid (PA) signaling pathway. We show that this PAR1-specific PA-mediated effect is not regulated through differential granule secretion after PAR-induced platelet activation. Perturbation of this signaling pathway via inhibition of lipid phosphate phosphatase-1 (LPP-1) by propranolol or inhibition of the phosphatidylcholine-derived phosphatidic acid (PA) formation by PLD with a primary alcohol significantly attenuated platelet activation by PAR1-AP. Platelet activation by thrombin or PAR4-AP was insensitive to these inhibitors. Furthermore, these inhibitors significantly attenuated activation of Rap1 after stimulation by PAR1-AP but not thrombin or PAR4-AP. Because PA metabolites such as diacylglycerol play an important role in intracellular signaling, identifying crucial differences in PA regulation of PAR-induced platelet activation may lead to a greater understanding of the role of PAR1 versus PAR4 in progression of thrombosis.

Direct Modulation of Phospholipase D Activity by Gβγ

Phospholipase D-mediated hydrolysis of phosphatidylcholine is stimulated by protein kinase C and the monomeric G proteins Arf, RhoA, Cdc42, and Rac1, resulting in complex regulation of this enzyme. Using purified proteins, we have identified a novel inhibitor of phospholipase D activity, Gbetagamma subunits of heterotrimeric G proteins. G protein-coupled receptor activation alters affinity between Galpha and Gbetagamma subunits, allowing subsequent interaction with distinct effectors. Gbeta1gamma1 inhibited phospholipase D1 and phospholipase D2 activity, and both Gbeta1gamma1 and Gbeta1gamma2 inhibited stimulated phospholipase D1 activity in a dosedependent manner in reconstitution assays. Reconstitution assays suggest this interaction occurs through the amino terminus of phospholipase D, because Gbeta1gamma1 is unable to inhibit an amino-terminally truncated phospholipase D construct, PLD1.d311, which like full-length phospholipase D isoforms, requires phosphatidylinositol-4,5-bisphosphate for activity. Furthermore, a truncated protein consisting of the amino-terminal region of phospholipase D containing the phox/pleckstrin homology domains was found to interact with Gbeta1gamma1, unlike the PLD1.d311 recombinant protein, which lacks this domain. In vivo, expressed recombinant Gbeta1gamma2 was also found to inhibit phospholipase D activity under basal and stimulated conditions in MDA-MB-231 cells, which natively express both phospholipase D1 and phospholipase D2. These data demonstrate that Gbetagamma directly regulates phospholipase D activity in vitro and suggest a novel mechanism to negatively regulate phospholipase D signaling in vivo.