Exogenous phospholipase D generates lysophosphatidic acid and activates Ras, Rho and Ca2+ signaling pathways

Unlocking the signaling potential of GPI-anchored proteins through lipolytic cleavage

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) regulate numerous biological processes through interaction with signaling effectors at the cell surface. As a unique feature, GPI-APs can be released from their anchors by multi-pass GPI-specific phospholipases (types A2, C, and D) to impact signaling networks, phenotype, and cell fate; however, many questions remain outstanding. Here, we discuss and expand our current understanding of the distinct GPI-specific phospholipases, their substrates, effector pathways, and emerging physiological roles, with a focus on the six-transmembrane ecto-phospholipases GDE2 (GDPD5) and GDE3 (GDPD2). We provide structural insight into their AlphaFold-predicted inner workings, revealing how transmembrane (TM) domain plasticity may enable GPI-anchor binding and hydrolysis. Understanding lipolytic cleavage of GPI-APs adds a new dimension to their signaling capabilities and biological functions.

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

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.

The effect of lysophosphatidic acid using a hydrogel or collagen sponge carrier on bone healing in dogs

Objectives: The purposes of this study were to determine: 1) the efficacy of polycaprolac-tone-g-polyethylene glycol (PCL-g-PEG) and polylactic-co-glycolic acid (PLGA-g-PEG) hydrogels and an absorbable collagen sponge (ACS) as carriers for lysophosphatidic acid (LPA), 2) the effect of LPA on bone healing in dogs, and 3) the ideal dose of LPA to maximally stimulate bone healing.

Methods: Bilateral ulnar ostectomies were performed on purpose bred dogs. Control defects were filled with a PCL-g-PEG or PLGA-g-PEG hydrogel, or a saline soaked ACS. Contralateral defects were filled with a PCL-g-PEG or PLGA-g-PEG hydrogel, or an ACS with each carrying differing concentrations of an LPA solution. Dual-energy X-ray absorptiometry (DXA) was performed. Total bone area (TBA), mineral density (BMD), and mineral content (BMC) were determined at each time point. Relationships between the effect of treatment over time on TBA, BMC and BMD were determined.

Results: Phase 1 - There was no significant difference in DXA-based TBA (p = 0.09), BMC (p = 0.33), or BMD (p = 0.74) over time between LPA treatments, or between the LPA treated and control groups TBA (p = 0.95), BMC (p = 0.99), or BMD (p = 0.46). Phase 2 - There was no significant difference over time between LPA treatments in DXA-based TBA (p = 0.33), BMC (p = 0.45), or BMD (p = 0.43), or between the LPA treated and control groups TBA (p = 0.94), BMC (p = 0.38), or BMD (p = 0.17). Phase 3 - There was no significant difference over time between LPA treatments in DXA-based TBA (p = 0.78), BMC (p = 0.88), or BMD (p = 0.35), or between the LPA treated and control groups TBA (p = 0.07), BMC (p = 0.85), or BMD (p = 0.06). There was a significant increase in TBA (p <0.0001) and BMC (p = 0.0014), but a significant decrease in BMD (p <0.0001) was noted over time when all groups were combined.

Clinical significance: Although LPA has shown promise as an osteoinductive agent in research, its performance as a bone graft substitute, as utilized in this study, is unsupported. Further studies are necessary to determine the incorporation and elution kinetics of LPA from the PLGA-g-PEG hydrogel and from an ACS. Hydrogels may have clinical applications for delaying or preventing bone formation.

Acinetobacter baumannii Virulence Is Mediated by the Concerted Action of Three Phospholipases D

Acinetobacter baumannii causes a broad range of opportunistic infections in humans. Its success as an emerging pathogen is due to a combination of increasing antibiotic resistance, environmental persistence and adaptation to the human host. To date very little is known about the molecular basis of the latter. Here we demonstrate that A. baumannii can use phosphatidylcholine, an integral part of human cell membranes, as sole carbon and energy source. We report on the identification of three phospholipases belonging to the PLD superfamily. PLD1 and PLD2 appear restricted to the bacteria and display the general features of bacterial phospholipases D. They possess two PLDc_2 PFAM domains each encompassing the HxKx₄Dx₆GS/GGxN (HKD) motif necessary for forming the catalytic core. The third candidate, PLD3, is found in bacteria as well as in eukaryotes and harbours only one PLDc_2 PFAM domain and one conserved HKD motif, which however do not overlap. Employing a markerless mutagenesis system for A. baumannii ATCC 19606^T, we generated a full set of PLD knock-out mutants. Galleria mellonella infection studies as well as invasion experiments using A549 human lung epithelial cells revealed that the three PLDs act in a concerted manner as virulence factors and are playing an important role in host cell invasion.

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.

Autotaxin: structure-function and signaling

Autotaxin (ATX), or ecto-nucleotide pyrophosphatase/phosphodiesterase-2, is a secreted lysophospholipase D (lysoPLD) that hydrolyzes extracellular lysophospholipids into the lipid mediator lysophosphatidic acid (LPA), a ligand for specific G protein-coupled receptors. ATX-LPA signaling is essential for development and has been implicated in a great diversity of (patho)physiological processes, ranging from lymphocyte homing to tumor progression. Structural and functional studies have revealed what makes ATX a unique lysoPLD, and how secreted ATX binds to its target cells. The ATX catalytic domain shows a characteristic bimetallic active site followed by a shallow binding groove that can accommodate nucleotides as well as the glycerol moiety of lysophospholipids, and by a deep lipid-binding pocket. In addition, the catalytic domain has an open tunnel of unknown function adjacent to the active site. Here, we discuss our current understanding of ATX structure-function relationships and signaling mechanisms, and how ATX isoforms use distinct mechanisms to target LPA production to the plasma membrane, notably binding to integrins and heparan sulfate proteoglycans. We also briefly discuss the development of drug-like inhibitors of ATX.

Lysophosphatidic acid (LPA) signalling in cell migration and cancer invasion: a focussed review and analysis of LPA receptor gene expression on the basis of more than 1700 cancer microarrays

Lysophosphatidic acid (LPA) is a ubiquitously present signalling molecule involved in diverse cellular processes such as cell migration, proliferation and differentiation. LPA acts as an autocrine and/or paracrine signalling molecule via different G-protein-coupled LPA receptors (LPARs) that trigger a broad range of intracellular signalling cascades, especially the RHOA pathway. Mounting evidence suggests a crucial role of the LPA/LPAR-axis in cancer cell metastasis and promising studies are underway to investigate the therapeutic potential of LPAR-antagonists. This review summarises current knowledge on how LPA promotes cytoskeletal remodelling to enhance the migratory and invasive properties of cells, which may ultimately contribute to cancer metastasis. Furthermore, we provide comprehensive transcriptome analyses of published microarrays of more than 350 normal tissues and more than 1700 malignant tissues to define the expression signatures of LPARs and the LPA-generating enzymes autotaxin (ATX) and lipase member 1 (LIPI). These analyses demonstrate that ATX is highly expressed in a variety of carcinomas and sarcomas, whereas LIPI is almost exclusively overexpressed in highly aggressive Ewing's sarcomas, which underscores the potential contribution of LPA in metastatic disease. In addition, these analyses show that different cancer entities display distinct expression signatures of LPARs that distinguish them from one another. Finally, we discuss current approaches to specifically target the LPA/LPAR circuits in experimental cancer therapy.

Legionella phospholipases implicated in virulence

Phospholipases are diverse enzymes produced in eukaryotic hosts and their bacterial pathogens. Several pathogen phospholipases have been identified as major virulence factors acting mainly in two different modes: on the one hand, they have the capability to destroy host membranes and on the other hand they are able to manipulate host signaling pathways. Reaction products of bacterial phospholipases may act as secondary messengers within the host and therefore influence inflammatory cascades and cellular processes, such as proliferation, migration, cytoskeletal changes as well as membrane traffic. The lung pathogen and intracellularly replicating bacterium Legionella pneumophila expresses a variety of phospholipases potentially involved in disease-promoting processes. So far, genes encoding 15 phospholipases A, three phospholipases C, and one phospholipase D have been identified. These cell-associated or secreted phospholipases may contribute to intracellular establishment, to egress of the pathogen from the host cell, and to the observed lung pathology. Due to the importance of phospholipase activities for host cell processes, it is conceivable that the pathogen enzymes may mimic or substitute host cell phospholipases to drive processes for the pathogen's benefit. The following chapter summarizes the current knowledge on the L. pneumophila phospholipases, especially their substrate specificity, localization, mode of secretion, and impact on host cells.

Lysophosphatidic acid and sphingosine-1-phosphate promote morphogenesis and block invasion of prostate cancer cells in three-dimensional organotypic models

Normal prostate and some malignant prostate cancer (PrCa) cell lines undergo acinar differentiation and form spheroids in three-dimensional (3-D) organotypic culture. Acini formed by PC-3 and PC-3M, less pronounced also in other PrCa cell lines, spontaneously undergo an invasive switch, leading to the disintegration of epithelial structures and the basal lamina, and formation of invadopodia. This demonstrates the highly dynamic nature of epithelial plasticity, balancing epithelial-to-mesenchymal transition against metastable acinar differentiation. This study assessed the role of lipid metabolites on epithelial maturation. PC-3 cells completely failed to form acinar structures in delipidated serum. Adding back lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) rescued acinar morphogenesis and repressed invasion effectively. Blocking LPA receptor 1 (LPAR1) functions by siRNA (small interference RNA) or the specific LPAR1 inhibitor Ki16425 promoted invasion, while silencing of other G-protein-coupled receptors responsive to LPA or S1P mainly caused growth arrest or had no effects. The G-proteins Gα(12/13) and Gα(i) were identified as key mediators of LPA signalling via stimulation of RhoA and Rho kinases ROCK1 and 2, activating Rac1, while inhibition of adenylate cyclase and accumulation of cAMP may be secondary. Interfering with these pathways specifically impeded epithelial polarization in transformed cells. In contrast, blocking the same pathways in non-transformed, normal cells promoted differentiation. We conclude that LPA and LPAR1 effectively promote epithelial maturation and block invasion of PrCa cells in 3-D culture. The analysis of clinical transcriptome data confirmed reduced expression of LPAR1 in a subset of PrCa's. Our study demonstrates a metastasis-suppressor function for LPAR1 and Gα(12/13) signalling, regulating cell motility and invasion versus epithelial maturation.

Acrosin activity regulation by protein kinase C and tyrosine kinase in bovine sperm acrosome exocytosis induced by lysophosphatidylcholine

Acrosin is an important proteolytic enzyme that is capable of hydrolysing the zona pellucida in bovine oocyte. Lysophosphatydic acid (LPA) derivated from lysophosphatidylcholine (LPC) is known to trigger the acrosome exocytosis. The present study was aimed at examining the acrosin activity variations in LPC-induced acrosome exocytosis and its regulation by tyrosine kinase, protein kinase C (PKC) and voltage-dependent calcium channels (VDCC) in spermatozoa previously capacitated with heparin or quercetin. The enzyme activities were spectrophotometrically measured using N-α-benzoyl-DL-arginine p-nitroanilide as an acrosin-specific substrate. The capacitation and acrosomal reaction were evaluated by chlorotetracycline assay, and the viability and acrosome integrity were evaluated by the trypan blue stain/differential interference contrast. It was observed that LPC induced acrosome exocytosis and increased the activity of acrosin in spermatozoa previously capacitated with heparin. In heparin/LPC-treated samples, it was observed that the inhibition of tyrosine kinase and PKC blocked the acrosome exocytosis and the acrosin activity (p < 0.05). Under these conditions, in heparin-capacitated spermatozoa, the LPC provokes an acrosin activity increase that is independent of calcium influx through VDCC Type L. In cryopreserved bovine spermatozoa, LPC might require modulation, mainly tyrosine kinase participation with respect to PKC activity to induce acrosome exocytosis and increase acrosin activity.

ATX-LPA receptor axis in inflammation and cancer

Lysophosphatidic acid (LPA, 1- or 2-acyl-sn-glycerol 3-phosphate) mediates a plethora of physiological and pathological activities via interactions with a series of high affinity G protein-coupled receptors (GPCR). Both LPA receptor family members and autotaxin (ATX/LysoPLD), the primary LPA-producing enzyme, are aberrantly expressed in many human breast cancers and several other cancer lineages. Using transgenic mice expressing either an LPA receptor or ATX, we recently demonstrated that the ATX-LPA receptor axis plays a causal role in breast tumorigenesis and cancer-related inflammation, further validating the ATX-LPA receptor axis as a rich therapeutic target in cancer.

Phosphoinositide signalling and cardiac arrhythmias

Arrhythmias arise from a complex interaction between structural changes in the myocardium and changes in cellular electrophysiology. Electrophysiological balance requires precise control of sarcolemmal ion channels and exchangers, many of which are regulated by phospholipid, phosphatidylinositol(4,5)bisphosphate. Phosphatidylinositol(4,5)bisphosphate is the immediate precursor of inositol(1,4,5)trisphosphate, a regulator of intracellular Ca2+ signalling and, therefore, a potential contributor to arrhythmogenesis by altering Ca2+ homeostasis. The aim of the present review is to outline current evidence that this signalling pathway can be a player in the initiation or maintenance of arrhythmias.

Lysophospholipid metabolism facilitates Toll-like receptor 4 membrane translocation to regulate the inflammatory response

Sepsis, an overwhelming inflammatory response to infection, is a major cause of morbidity and mortality worldwide and has no specific therapy. Phospholipid metabolites, such as lysophospholipids, have been shown to regulate inflammatory responses in sepsis, although their mechanism of action is not well understood. The phospholipid-metabolizing enzymes, lysophospholipid acyltransferases, control membrane phospholipid composition, function, and the inflammatory responses of innate immune cells. Here, we show that lysophosphatidylcholine acyltransferase (LPCAT) regulates inflammatory responses to LPS and other microbial stimuli. Specific inhibition of LPCAT down-regulated inflammatory cytokine production in monocytes and epithelial cells by preventing translocation of TLR4 into membrane lipid raft domains. Our observations demonstrate a new regulatory mechanism that facilitates the innate immune responses to microbial molecular patterns and provide a basis for the anti-inflammatory activity observed in many phospholipid metabolites. This provides the possibility of the development of new classes of anti-inflammatory and antisepsis agents.

Role of lysophosphatidic acid in the regulation of uterine leiomyoma cell proliferation by phospholipase D and autotaxin

Phospholipase D (PLD) hydrolyzes phosphatidylcholine into phosphatidic acid (PA), a lipidic mediator that may act directly on cellular proteins or may be metabolized into lysophosphatidic acid (LPA). We previously showed that PLD contributed to the mitogenic effect of endothelin-1 (ET-1) in a leiomyoma cell line (ELT3 cells). In this work, we tested the ability of exogenous PA and PLD from Streptomyces chromofuscus (scPLD) to reproduce the effect of endogenous PLD in ELT3 cells and the possibility that these agents acted through LPA formation. We found that PA, scPLD, and LPA stimulated thymidine incorporation. LPA and scPLD induced extracellular signal-regulated kinase (ERK(1/2)) mitogen-activated protein kinase activation. Using Ki16425, an LPA(1)/LPA(3) receptor antagonist and small interfering RNA targeting LPA(1) receptor, we demonstrated that scPLD acted through LPA production and LPA(1) receptor activation. We found that scPLD induced LPA production by hydrolyzing lysophosphatidylcholine through its lysophospholipase D (lysoPLD) activity. Autotaxin (ATX), a naturally occurring lysoPLD, reproduced the effects of scPLD. By contrast, endogenous PLD stimulated by ET-1 failed to produce LPA. These results demonstrate that scPLD stimulated ELT3 cell proliferation by an LPA-dependent mechanism, different from that triggered by endogenous PLD. These data suggest that in vivo, an extracellular lysoPLD such as ATX may participate in leiomyoma growth through local LPA formation.

Upregulation of Phospholipase D Expression and Activation in Ventricular Pressure-Overload Hypertrophy

Evidence for a role of phospholipase D (PLD) in cellular proliferation and differentiation is accumulating. We studied PLD activity and expression in normal and hypertrophic rat and human hearts. In rat heart, abdominal aortic banding (constriction to 50% of original lumen) caused hypertrophy in the left ventricle (as shown by weight index and ANP expression) by about 15% after 30 days without histological evidence of fibrosis or signs of decompensation and in the right ventricle after 100 days. The hypertrophy was accompanied by small increases of basal PLD activity and strong potentiation of stimulated PLD activity caused by 4beta-phorbol-12beta,13alpha-dibutyrate (PDB) and by phenylephrine. The mRNA expressions of both PLD1 and PLD2 determined by semiquantitative competitive RT-PCR were markedly enhanced after aortic banding. In the caveolar fraction of the rat heart, PLD2 protein determined by Western blot analysis was upregulated in parallel with the expression of caveolin-3. A similar induction of PLD mRNA and protein expression was observed in hypertrophied human hearts of individuals (39-45-year-old) who had died from non-cardiac causes. In conclusion, PLD1 and PLD2 expressions were strongly enhanced both in rat and human heart hypertrophy, which may be responsible for the coincident potentiation of the PLD activation by alpha-adrenoceptor and protein kinase C stimulation. These results are compatible with a significant role of PLD activation in cell signaling of ventricular pressure-overload hypertrophy.

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.

Lysophosphatidic acid is a mediator of Trp-Lys-Tyr-Met-Val-d-Met-induced calcium influx

Intracellular calcium (Ca(2+)) homeostasis is very strictly regulated, and the activation of G-protein-coupled receptor (GPCR) can cause two different calcium changes, intracellular calcium release, and calcium influx. In this study, we investigated the possible role of lysophosphatidic acid (LPA) on GPCR-induced Ca(2+) signaling. The addition of exogenous LPA induced dramatic Ca(2+) influx but not intracellular Ca(2+) release in U937 cells. LPA-induced Ca(2+) influx was not affected by pertussis toxin and phospholipase C inhibitor (U73122), ruling out the involvement of pertussis toxin-sensitive G-proteins, and phospholipase C. Stimulation of U937 cells with Trp-Lys-Tyr-Met-Val-D-Met (WKYMVm), which binds to formyl peptide receptor like 1, enhanced phospholipase A(2) and phospholipase D activation, indicating LPA formation. The inhibition of LPA synthesis by phospholipase A(2)-specific inhibitor (MAFP) or n-butanol significantly inhibited WKYMVm-induced Ca(2+) influx, suggesting a crucial role for LPA in the process. Taken together, we suggest that LPA mediates WKYMVm-induced Ca(2+) influx.

Mechanisms of lysophosphatidic acid production

Lysophosphatidic acid is one of the most attractive phospholipid mediator with multiple biological functions and is implicated in various human diseases. In the past ten years much has been learned about the physiological roles of LPA through series of studies on LPA actions and its receptors. However, the molecular mechanisms of LPA have been poorly understood. LPA is produced in various conditions both in cells and in biological fluids, where multiple synthetic reactions occur. At least two pathways are postulated. In serum and plasma, LPA is mainly converted from lysophospholipids. By contrast, in platelets and some cancer cells, LPA is converted from phosphatidic acid. In each pathway, at least two phospholipase activities are required: phospholipase A1 (PLA1)/PLA2 plus lysophospholipase D (lysoPLD) activities are involved in the first pathway and phospholipase D (PLD) plus PLA1/PLA2 activities are involved in the second pathway. Now multiple phospholipases are identified that account for PLA1, PLA2, PLD, and lysoPLD activities. In the absence of specific inhibitors and genetically modified animals and individuals, the contribution of each phospholipase to LPA production can not be easily determined. However, apparently certain extracellular phospholipases such as secretory PLA2 (sPLA2-IIA), membrane-associated PA-selective PLA1 (mPA-PLA1), lecithin-cholesterol acyltransferase (LCAT), and lysoPLD are involved in LPA production.

Overexpression of phospholipase D prevents actinomycin D-induced apoptosis through potentiation of phosphoinositide 3-kinase signalling pathways in Chinese-hamster ovary cells

To examine the roles of PLD (phospholipase D) in the regulation of the apoptotic process, PLD1 and PLD2 were stably overexpressed in S1P3-CHO cells [CHO (Chinese-hamster ovary) cells expressing the S1P (sphingosine 1-phosphate) receptor S1P3]. Treatment of S1P3-CHO cells with ActD (actinomycin D) induced apoptosis, as shown by the occurrence of nuclear fragmentation and the caspase-dependent proteolytic cleavage of PARP [poly(ADP-ribose) polymerase] and protein kinase Cd. Overexpression of either PLD1 or PLD2 protected S1P3-CHO cells from ActD-induced apoptosis, as demonstrated by an increased number of viable cells and inhibition of PARP and protein kinase Cd cleavage. However, in the early phase of apoptosis, ActD induced an increase in PLD activity and activation of key factors in the cell-survival signalling pathways, such as PI3K (phosphoinositide 3-kinase), Akt, p70S6K (p70 S6 kinase) and ERK (extracellular-signal-regulated kinase). Furthermore, the ActD-induced activation of these survival signalling enzymes was potentiated by overexpression of either PLD1 or PLD2. The PI3K inhibitor LY294002 inhibited the ActD-induced activation of Akt and p70S6K, and completely abolished the effects of PLD1 or PLD2, whereas inhibition of ERK activity by the MEK inhibitor U0126 had a milder effect. The ActD-induced activation of p70S6K and ERKs was blocked by 1-butanol, but not by t-butanol; similar to S1P, exogenous PLD suppressed the ActD-induced events in the apoptosis signalling pathways. These results show that, in S1P3-CHO cells, increased expression of PLDs prevents ActD-induced apoptosis by enhanced activation of the PI3K signalling pathways.

Spider and Bacterial Sphingomyelinases D Target Cellular Lysophosphatidic Acid Receptors by Hydrolyzing Lysophosphatidylcholine

Bites by Loxosceles spiders can produce severe clinical symptoms, including dermonecrosis, thrombosis, vascular leakage, hemolysis, and persistent inflammation. The causative factor is a sphingomyelinase D (SMaseD) that cleaves sphingomyelin into choline and ceramide 1-phosphate. A similar enzyme, showing comparable bioactivity, is secreted by certain pathogenic corynebacteria and acts as a potent virulence factor. However, the molecular basis for SMaseD toxicity is not well understood, which hampers effective therapy. Here we show that the spider and bacterial SMases D hydrolyze albumin-bound lysophosphatidylcholine (LPC), but not sphingosylphosphorylcholine, with K(m) values ( approximately 20-40 microm) well below the normal LPC levels in blood. Thus, toxic SMases D have intrinsic lysophospholipase D activity toward LPC. LPC hydrolysis yields the lipid mediator lysophosphatidic acid (LPA), a known inducer of platelet aggregation, endothelial hyperpermeability, and pro-inflammatory responses. Introduction of LPA(1) receptor cDNA into LPA receptor-negative cells renders non-susceptible cells susceptible to SmaseD, but only in LPC-containing media. Degradation of circulating LPC to LPA with consequent activation of LPA receptors may have a previously unappreciated role in the pathophysiology of secreted SMases D.

The ins and outs of lysophosphatidic acid signaling

Lysophosphatidic acid (LPA) is a lipid mediator with a wide variety of biological actions, particularly as an inducer of cell proliferation, migration and survival. LPA binds to specific G-protein-coupled receptors and thereby activates multiple signal transduction pathways, including those initiated by the small GTPases Ras, Rho, and Rac. LPA signaling has been implicated in such diverse processes as wound healing, brain development, vascular remodeling and tumor progression. Knowledge of precisely how and where LPA is produced has long proved elusive. Excitingly, it has recently been discovered that LPA is generated from precursors by 'autotaxin', a once enigmatic exo-phosphodiesterase implicated in tumor cell motility. Exogenous phospholipases D can also produce LPA, which may contribute to their toxicity. Here we review recent progress in our understanding of LPA bioactivity, signaling and synthesis.

Gonococcal phospholipase d modulates the expression and function of complement receptor 3 in primary cervical epithelial cells

CR3-mediated endocytosis is a primary mechanism by which Neisseria gonorrhoeae elicits membrane ruffling and cellular invasion of the cervical epithelia. Our data indicate that, upon infection of cervical epithelia, N. gonorrhoeae specifically releases proteins, including a phospholipase D (PLD) homolog, which facilitate membrane ruffling. To elucidate the function of gonococcal PLD in infection of the cervical epithelia, we constructed an N. gonorrhoeae PLD mutant. By comparative association and/or invasion assays, we demonstrated that PLD mutant gonococci are impaired in their ability to adhere to and to invade primary cervical cells. This defect can be rescued by the addition of supernatants obtained from wild-type-infected cell monolayers but not by exogenously added Streptomyces PLD. The decreased level of total cell association (i.e., adherence and invasion) observed for mutant gonococci is, in part, attributed to the inability of these bacteria to recruit CR3 to the cervical cell surface with extended infection. Using electron microscopy, we demonstrate that gonococcal PLD may be necessary to potentiate membrane ruffling and clustering of gonococci on the cervical cell surface. These data may be indicative of the inability of PLD mutant gonococci to recruit CR3 to the cervical cell surface. Alternatively, in the absence of gonococcal PLD, signal transduction events required for CR3 clustering may not be activated. Collectively, our data indicate that PLD augments CR3-mediated gonococcus invasion of and survival within cervical epithelia.

Different modes of internalization of apoptotic alkyl-lysophospholipid and cell-rescuing lysophosphatidylcholine

The synthetic alkyl-lysophospholipid (ALP), Et-18-OCH3 (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine), can induce apoptosis in tumour cells. Unlike conventional chemotherapeutic drugs, ALP acts at the cell-membrane level. We have reported previously that ALP is internalized, and interferes with phosphatidylcholine (PC) biosynthesis de novo, which appeared to be essential for survival in lymphoma cells [Van der Luit, Budde, Ruurs, Verheij and Van Blitterswijk (2002) J. Biol. Chem. 277, 39541-39547]. Here, we report that, in HeLa cells, ALP accumulates in lipid rafts, and that internalization is inhibited by low temperature, monensin, disruption of lipid rafts and expression of a dominant-negative mutant of dynamin bearing a replacement of Lys44 with alanine (K44A). Thus ALP is internalized via raft- and dynamin-mediated endocytosis. Dynamin-K44A alleviated the ALP-induced inhibition of PC synthesis and rescued the cells from apoptosis induction. Additional cell rescue was attained by exogenous lysoPC, which after internalization serves as an alternative substrate for PC synthesis (through acylation). Unlike ALP, and despite the high structural similarity to ALP, lysoPC uptake did not occur via lipid rafts and did not depend on functional dynamin, indicating no involvement of endocytosis. Albumin back-extraction experiments suggested that (radiolabelled) lysoPC undergoes transbilayer movement (flipping). We conclude that ALP is internalized by endocytosis via lipid rafts to cause apoptosis, while exogenous cell-rescuing lysoPC traverses the plasma membrane outside rafts by flipping. Additionally, our data imply the importance of ether bonds in lyso-phospholipids, such as in ALP, for partitioning in lipid rafts.

Mechanisms in LPA-induced tumor cell migration: critical role of phosphorylated ERK

Lysophosphatidic acid (LPA) is a serum-borne phospholipid with hormone and growth factor-like properties. LPA has been shown to modulate tumor cell invasion and malignant cell growth. Here, we report that two human pancreatic carcinoma cell lines, PANC-1 and BxPC-3, express functionally active LPA receptors coupled to pertussis toxin-sensitive Gi/o-proteins. In contrast to other cell types, LPA does not act as a mitogen, but is an efficacious stimulator of cell migration of these tumor cells. LPA-induced chemotaxis is markedly dependent on activation of PTX-sensitive heterotrimeric G-proteins, on activation of the small GTPases Ras, Rac and RhoA, and on GTPase-dependent activation of ERK. LPA-induced ERK activation results in a transient translocation of the phosphorylated ERK to newly forming focal contact sites at the leading edge of the migrating cells. Inhibition of ERK activation and its subsequent translocation impaired LPA-induced chemotaxis and LPA-induced actin reorganization. Thus, pancreatic tumor cell migration in response to LPA is essentially controlled by activation of a Gi/o-ERK pathway and requires the LPA-induced activation of Ras, Rac1 and RhoA.

The emerging role of lysophosphatidic acid in cancer

The bioactive phospholipid lysophosphatidic acid (LPA) stimulates cell proliferation, migration and survival by acting on its cognate G-protein-coupled receptors. Aberrant LPA production, receptor expression and signalling probably contribute to cancer initiation, progression and metastasis. The recent identification of ecto-enzymes that mediate the production and degradation of LPA, as well as the development of receptor-selective analogues, indicate mechanisms by which LPA production or action could be modulated for cancer therapy.

Alkyl-lysophospholipid accumulates in lipid rafts and induces apoptosis via raft-dependent endocytosis and inhibition of phosphatidylcholine synthesis

The synthetic alkyl-lysophospholipid (ALP), 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine, is an antitumor agent that acts on cell membranes and can induce apoptosis. We investigated how ALP is taken up by cells, how it affects de novo biosynthesis of phosphatidylcholine (PC), and how critical this is to initiate apoptosis. We compared an ALP-sensitive mouse lymphoma cell line, S49, with an ALP-resistant variant, S49(AR). ALP inhibited PC synthesis at the CTP:phosphocholine cytidylyltransferase (CT) step in S49 cells, but not in S49(AR) cells. Exogenous lysophosphatidylcholine, providing cells with an alternative way (acylation) to generate PC, rescued cells from ALP-induced apoptosis, indicating that continuous rapid PC turnover is essential for cell survival. Apoptosis induced by other stimuli that do not target PC synthesis remained unaffected by lysophosphatidylcholine. Using monensin, low temperature and albumin back-extraction, we demonstrated that ALP is internalized by endocytosis, a process defective in S49(AR) cells. This defect neither involved clathrin-coated pit- nor fluid-phase endocytosis, but depended on lipid rafts, because disruption of these microdomains with methyl-beta-cyclodextrin or filipin (sequestering cholesterol) or bacterial sphingomyelinase reduced uptake of ALP. Furthermore, ALP was found accumulated in isolated rafts and disruption of rafts also prevented the inhibition of PC synthesis and apoptosis induction in S49 cells. In summary, ALP is internalized by raft-dependent endocytosis to inhibit PC synthesis, which triggers apoptosis.

A novel phosphatidic acid-selective phospholipase A1 that produces lysophosphatidic acid

Lysophosphatidic acid (LPA) is a lipid mediator with diverse biological properties, although its synthetic pathways have not been completely solved. We report the cloning and characterization of a novel phosphatidic acid (PA)-selective phospholipase A(1) (PLA(1)) that produces 2-acyl-LPA. The PLA(1) was identified in the GenBank(TM) data base as a close homologue of phosphatidylserine (PS)-specific PLA(1) (PS-PLA(1)). When expressed in insect Sf9 cells, this enzyme was recovered from the Triton X-100-insoluble fraction and did not show any catalytic activity toward exogenously added phospholipid substrates. However, culture medium obtained from Sf9 cells expressing the enzyme was found to activate EDG7/LPA(3), a cellular receptor for 2-acyl-LPA. The activation of EDG7 was further enhanced when the cells were treated with phorbol ester or a bacterial phospholipase D, suggesting involvement of phospholipase D in the process. In the latter condition, an increased level of LPA, but not other lysophospholipids, was confirmed by mass spectrometry analyses. Expression of the enzyme is observed in several human tissues such as prostate, testis, ovary, pancreas, and especially platelets. These data show that the enzyme is a membrane-associated PA-selective PLA(1) and suggest that it has a role in LPA production.

LPA as a determinant of mesangial growth and apoptosis

Mesangial cell proliferation is a prominent feature of progression in many forms of renal diseases, including immunoglobulin A nephropathy, lupus nephritis, hemolytic uremic syndrome, and diabetic nephropathy. Platelet-derived growth factor (PDGF) has received much attention as the major mediator of mesangial cell proliferation by autocrine/paracrine mechanisms involving up-regulation of mesangial PDGF and its receptor on mesangial cells. In this review, we wish to spotlight lysophosphatidic acid (LPA), which in combination with PDGF, undoubtedly plays a key role as an autocrine and paracrine mediator in regulating mesangial cell growth. We not only showed that PDGF acts as a bimodal molecule for mesangial cells, inducing mesangial cell proliferation and death simultaneously, but also showed that LPA is a survival factor suppressing PDGF-induced mesangial cell death, thereby remarkably enhancing mesangial mitogenic response by PDGF. We believe that a better understanding of the mechanisms of mesangial cell proliferation by the combined action of PDGF and LPA could lead to novel diagnostic as well as therapeutic strategies, and thus help to better control proliferative glomerulonephritis.

Autotaxin has lysophospholipase D activity leading to tumor cell growth and motility by lysophosphatidic acid production

Autotaxin (ATX) is a tumor cell motility-stimulating factor, originally isolated from melanoma cell supernatants. ATX had been proposed to mediate its effects through 5'-nucleotide pyrophosphatase and phosphodiesterase activities. However, the ATX substrate mediating the increase in cellular motility remains to be identified. Here, we demonstrated that lysophospholipase D (lysoPLD) purified from fetal bovine serum, which catalyzes the production of the bioactive phospholipid mediator, lysophosphatidic acid (LPA), from lysophosphatidylcholine (LPC), is identical to ATX. The Km value of ATX for LPC was 25-fold lower than that for the synthetic nucleoside substrate, p-nitrophenyl-tri-monophosphate. LPA mediates multiple biological functions including cytoskeletal reorganization, chemotaxis, and cell growth through activation of specific G protein-coupled receptors. Recombinant ATX, particularly in the presence of LPC, dramatically increased chemotaxis and proliferation of multiple different cell lines. Moreover, we demonstrate that several cancer cell lines release significant amounts of LPC, a substrate for ATX, into the culture medium. The demonstration that ATX and lysoPLD are identical suggests that autocrine or paracrine production of LPA contributes to tumor cell motility, survival, and proliferation. It also provides potential novel targets for therapy of pathophysiological states including cancer.

Lysophospholipids in the limelight: autotaxin takes center stage

Lysophosphatidic acid (LPA) is a serum phospholipid that evokes growth factor-like responses in many cell types through the activation of its G protein-coupled receptors. Although much is known about LPA signaling, it has remained unclear where and how bioactive LPA is produced. Umezu-Goto et al. (2002)(this issue, page 227) have purified a serum lysophospholipase D that generates LPA from lysophosphatidylcholine and found it to be identical to autotaxin, a cell motility-stimulating ectophosphodiesterase implicated in tumor progression. This result is surprising, as there was previously no indication that autotaxin could act as a phospholipase.

Lysophosphatidic acid and receptor-mediated activation of endothelial nitric-oxide synthase

Both lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are platelet-derived phospholipids that elicit diverse biological responses. In endothelial cells, S1P stimulates the EDG-1 receptor-mediated activation of the endothelial isoform of nitric oxide synthase (eNOS), but the role of LPA in eNOS regulation is less well understood. We now report that LPA treatment of bovine aortic endothelial cells (BAEC) activates eNOS enzyme activity in a pathway that involves phosphorylation of eNOS on serine 1179 by protein kinase Akt. In contrast to the cellular responses elicited by S1P in COS-7 cells, LPA can stimulate the activation of eNOS and Akt independently of EDG-1 receptor transfection. LPA-stimulated enzyme activation was significantly attenuated in an eNOS mutant lacking the site that is phosphorylated by kinase Akt (eNOS S1179A). In BAEC, activation of eNOS by LPA is completely blocked by pertussis toxin, by the intracellular calcium chelator BAPTA (1,2-bis(aminophenoxy) ethane-N,N,N',N'-tetraacetic acid), and by the phosphoinositide 3-kinase (PI3-K) inhibitor wortmannin, but is unaffected by U0126, an inhibitor of mitogen-activated protein (MAP) kinase pathways. Analysis of the LPA dose response for eNOS activation reveals an EC(50) of approximately 40 nM, a concentration well below the potency of LPA at the EDG-1 receptor. Taken together, these results indicate that LPA potently activates eNOS in BAEC in a pathway distinct from the EDG-1 receptor, but mediated by a similar receptor-mediated pathway dependent on pertussis toxin-sensitive G proteins and involving activation of the PI3-K/Akt pathway. These studies have identified a role for the phospholipid LPA in eNOS activation, and point out the complementary role of distinct platelet-derived lipids in endothelial signaling pathways.

Lysophosphatidic acid stimulates proliferation of cultured smooth muscle cells from human BPH tissue: sildenafil and papaverin generate inhibition

BACKGROUND

The endogenous substance lysophosphatidic acid (LPA) has been found to generate proliferation of cultured smooth muscle cells (SMC). Therefore, the effect of LPA on human benign prostate hyperplasia (BPH) could be of interest.

METHODS

The proliferative effect of LPA on cultured human prostatic SMC from specimens obtained at trans-urethral resection of the prostate (TURP) because of BPH, was analyzed by [3H]-thymidine and [35S]-methionine incorporation. In addition, LPA stimulated BPH SMC were treated with papaverin, forskolin, sildenafil or zaprinast, well known to increase the intracellular level of cAMP or cGMP.

RESULTS

LPA produced a dose-dependent increase in BPH SMC, both regarding DNA- and protein-synthesis with EC50 values of 3 and 10 microM, respectively. Furthermore, both papaverin, a general phosphodiesterase inhibitor regarding cAMP hydrolyzes, and forskolin, an adenylyl cyclase stimulating agent, inhibited the LPA-stimulated DNA replication in a dose dependent manner with IC50 = 2.5, and 0.35 microM, respectively. cGMP increasing agents, such as the NO-donors SIN-1 and SNAP, produced a weak anti-proliferative response. However, both phosphodiesterase 5 inhibitors sildenafil (Viagra) and zaprinast efficiently blocked DNA replication. In addition, when the protein synthesis was examined, we found that the LPA response was significantly inhibited by forskolin and papaverin.

CONCLUSIONS

The major conclusion of this investigation is that the endogenous serum component LPA, is able to promote human BPH SMC growth. In addition, our study indicates that cyclic nucleotides can inhibit this effect. Future clinical studies will be needed to determine if different specific phosphodiesterase inhibitors per se or in combination could represent a new therapeutic possibility for the treatment of BPH.

Pharmacological importance of phospholipase D and phosphatidic acid in the regulation of the mitogen-activated protein kinase cascade

The stimulation of cells with many extracellular agonists leads to the activation of phospholipase (PL)D. PLD metabolizes phosphatidylcholine to generate phosphatidic acid (PA). Neither the mechanism through which cell surface receptors regulate PLD activation nor the functional consequences of PLD activity in mitogenic signaling are completely understood. PLD is activated by protein kinase C, phospholipids, and small GTPases of the ADP-ribosylation factor and Rho families, but the mechanisms linking cell surface receptors to the activation of PLD still require detailed analysis. Furthermore, the latest data on the functional consequences of the generation of cellular PA suggest an important role for this lipid in the regulation of membrane traffic and on the activation of the mitogen-activated protein kinase cascade. This review addresses these issues, examining some novel models for the physiological role of PLD and PA and discussing their potential usefulness as specific targets for the development of new therapies.

Lysophosphatidic acid and its role in reproduction

Lysophosphatidic acid (LPA) belongs to a new family of lipid mediators that are endogenous growth factors and that elicit diverse biological effects, usually via the activation of G protein-coupled receptors. LPA can be generated after cell activation through the hydrolysis of preexisting phospholipids in the membranes of stimulated cells. A dramatic elevation of LPA levels was found in serum of patients suffering from ovarian carcinoma. Because these high LPA amounts can be detected as early as stage I of the disease, LPA has been introduced as a new marker for ovarian cancer. Progression of the malignancy is correlated with a differential expression of various LPA receptor subtypes. The presence of LPA in the follicular fluid of healthy individuals implicates that this biological mediator may be relevant to normal ovarian physiology. LPA induces proliferation and mitogenic signaling of prostate cancer cells, and a novel LPA receptor isoform has been recognized in healthy prostate tissues. This evidence indicates multiple roles for LPA in both male and female reproductive physiology and pathology. In this review, we summarize the literature on LPA generation, the way it is degraded, and the mechanisms by which signals are transduced by various LPA receptors in reproductive tissues, and we discuss possible future research directions in these areas.

Lysophosphatidic acid synthesis and release

Lysophosphatidic acid (LPA) is a bioactive phospholipid controlling numerous cellular responses through the activation of specific G-protein coupled transmembrane receptors. LPA is present in several biological fluids (serum, plasma, aqueous humor) and can be secreted by several cell types (platelets, fibroblasts, adipocytes, cancer cells). Whereas, multiple pathways of synthesis and degradation of LPA have been described, their relative contribution in extracellular secretion and biodisponibility is still a matter of debate. The first part of the present review is devoted to the description of the different enzymes involved in LPA synthesis (acyltransferases, phospholipases, kinases) and degradation (lysophospholipases, lipid-phosphatases), as well as to the molecules involved in LPA transport (albumin, fatty acid binding proteins, gelsolin, lipoproteins). In a second part, the different physio-pathological situations (aggregation, cancer, injuries) associated with LPA production, as well as the potential role played by LPA in genesis of certain diseases (cancer, obesity, arteriosclerosis) are listed and analyzed.

Physiological responses to lysophosphatidic acid and related glycero-phospholipids

1-Acyl-2-hydroxy(lyso)-sn-glycero-3-phosphate (lysophosphatidic acid, LPA) has attracted a lot of attention in recent years due to the wide range of its biological effects that span the phylogenetic tree from slime mold to human. LPA can be viewed as a pleiotropic phospholipid growth factor that utilizes the same signal transduction mechanisms as traditional polypeptide growth factors; however, LPA activates these mechanism via specific G protein-coupled receptors. The concentration of LPA in serum is in the high micromolar range, making it the most abundant mitogen/survival factor present in serum, one that is often unknowingly utilized in tissue culture. The present review gives a historical perspective and a critical analysis of the LPA literature with a special emphasis on the physiological implications of its effects.

Ras-MAP kinase signaling by lysophosphatidic acid and other G protein-coupled receptor agonists

Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are extracellular lipid mediators that signal through distinct members of the Edg/LP subfamily of G protein-coupled receptors (GPCRs). LPA and S1P receptors are expressed in almost every cell type and can couple to multiple G proteins (G(i), G(q) and G(12/13)) to mediate a great variety of responses, ranging from rapid morphological changes to long-term stimulation of cell proliferation. LPA serves as the prototypic GPCR agonist that activates the small GTPases Ras (via G(i)) and RhoA (via G(12/13)), leading to activation of the mitogen-activated protein kinase (MAPK) cascade and reorganization of the actin cytoskeleton, respectively. This review focuses on our current insights into how Ras-MAPK signaling is regulated by GPCR agonists in general, and by LPA in particular.

Enhancement of lysophosphatidic acid-induced ERK phosphorylation by phospholipase D1 via the formation of phosphatidic acid

We made stable cell lines overexpressing PLD1 (GP-PLD1) from GP+envAm12 cell, a derivative of NIH 3T3 cell. PLD1 activity and extracellular signal-regulated kinase (ERK) phosphorylation were enhanced in GP-PLD1 cells by the treatment of lysophosphatidic acid (LPA). In contrast, these LPA-induced effects were attenuated with the pretreatment of pertussis toxin (PTX) or protein kinase C (PKC) inhibitor. Moreover, accumulation of phosphatidic acid (PA), a product of PLD action, potentiated the LPA-induced ERK activation in GP-PLD1 cells while blocking of PA production with the treatment of 1-butanol attenuated LPA-induced ERK phosphorylation. From these results, we suggest that LPA activate PLD1 through pertussis toxin-sensitive G protein and PKC-dependent pathways, then PA produced from PLD1 activation facilitate ERK phosphorylation.

Topical application of the phospholipid growth factor lysophosphatidic acid promotes wound healing in vivo

The lipid mediator lysophosphatidic acid (LPA) regulates cell proliferation and enhances cell motility in vitro, both of which are important events in wound healing. To evaluate the effects of LPA in vivo, it was applied to a full-thickness wound of rat skin. LPA in micromolar concentrations, or solvent, was applied daily. Animals were killed at 1, 3, 6, and 9 days after wounding and processed for histological evaluation, including hematoxylin-eosin staining and histochemical markers for macrophage-histiocytes, proliferating cells, and capillary endothelial cells. LPA treatment accelerated wound closing and increased neoepithelial thickness. Cytological evaluation showed no evidence for a secondary inflammation-mediated injury, infection, or increased keloid formation. Whereas LPA caused only a modest dose-dependent increase in proliferating cells, a marked increase in the immigration of histiocyte-macrophage cells was observed as early as day 1. The peaks of several cytological features and immunohistological markers preceded those of the untreated side. Our data suggest that exogenously applied LPA in this model promotes healing and that macrophage-histiocytes are the primary LPA-responsive cells in vivo.