Role of phosphatidic acid phosphatase 2a in uptake of extracellular lipid phosphate mediators

The Emerging Role of LPA as an Oncometabolite

Lysophosphatidic acid (LPA) is a phospholipid that displays potent signalling activities that are regulated in both an autocrine and paracrine manner. It can be found both extra- and intracellularly, where it interacts with different receptors to activate signalling pathways that regulate a plethora of cellular processes, including mitosis, proliferation and migration. LPA metabolism is complex, and its biosynthesis and catabolism are under tight control to ensure proper LPA levels in the body. In cancer patient specimens, LPA levels are frequently higher compared to those of healthy individuals and often correlate with poor responses and more aggressive disease. Accordingly, LPA, through promoting cancer cell migration and invasion, enhances the metastasis and dissemination of tumour cells. In this review, we summarise the role of LPA in the regulation of critical aspects of tumour biology and further discuss the available pre-clinical and clinical evidence regarding the feasibility and efficacy of targeting LPA metabolism for effective anticancer therapy.

Lipid Phosphate Phosphatases and Cancer

Lipid phosphate phosphatases (LPPs) are a group of three enzymes (LPP1–3) that belong to a phospholipid phosphatase (PLPP) family. The LPPs dephosphorylate a wide spectrum of bioactive lipid phosphates, among which lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) are two important extracellular signaling molecules. The LPPs are integral membrane proteins, which are localized on plasma membranes and intracellular membranes, including the endoplasmic reticulum and Golgi network. LPPs regulate signaling transduction in cancer cells and demonstrate different effects in cancer progression through the breakdown of extracellular LPA and S1P and other intracellular substrates. This review is intended to summarize an up-to-date understanding about the functions of LPPs in cancers.

Transcript Abundance of Putative Lipid Phosphate Phosphatases During Development of Trypanosoma brucei in the Tsetse Fly

African trypanosomes (Trypanosoma brucei spp.) cause devastating diseases in sub-Saharan Africa. Trypanosomes differentiate repeatedly during development in tsetse flies before gaining mammalian infectivity in fly salivary glands. Lipid phosphate phosphatases (LPPs) are involved in diverse biological processes, such as cell differentiation and cell migration. Gene sequences encoding two putative T. brucei LPP proteins were used to search the T. brucei genome, revealing two additional putative family members. Putative structural features and transcript abundance during parasite development in tsetse fly were characterized. Three of the four LPP proteins are predicted to have six transmembrane domains, while the fourth shows only one. Semiquantitative gene expression revealed differential regulation of LPPs during parasite development. Transcript abundance for three of the four putative LPP genes was elevated in parasites infecting salivary glands, but not mammalian-infective metacyclic cells in fly saliva, indicating a potential role of this family in parasite establishment in tsetse salivary glands.

Lipid phosphate phosphatases and their roles in mammalian physiology and pathology

Lipid phosphate phosphatases (LPPs) are a group of enzymes that belong to a phosphatase/phosphotransferase family. Mammalian LPPs consist of three isoforms: LPP1, LPP2, and LPP3. They share highly conserved catalytic domains and catalyze the dephosphorylation of a variety of lipid phosphates, including phosphatidate, lysophosphatidate (LPA), sphingosine 1-phosphate (S1P), ceramide 1-phosphate, and diacylglycerol pyrophosphate. LPPs are integral membrane proteins, which are localized on plasma membranes with the active site on the outer leaflet. This enables the LPPs to degrade extracellular LPA and S1P, thereby attenuating their effects on the activation of surface receptors. LPP3 also exhibits noncatalytic effects at the cell surface. LPP expression on internal membranes, such as endoplasmic reticulum and Golgi, facilitates the metabolism of internal lipid phosphates, presumably on the luminal surface of these organelles. This action probably explains the signaling effects of the LPPs, which occur downstream of receptor activation. The three isoforms of LPPs show distinct and nonredundant effects in several physiological and pathological processes including embryo development, vascular function, and tumor progression. This review is intended to present an up-to-date understanding of the physiological and pathological consequences of changing the activities of the different LPPs, especially in relation to cell signaling by LPA and S1P.

Francisella tularensis LVS induction of prostaglandin biosynthesis by infected macrophages requires specific host phospholipases and lipid phosphatases

Francisella tularensis induces the synthesis of prostaglandin E(2) (PGE(2)) by infected macrophages to alter host immune responses, thus providing a survival advantage to the bacterium. We previously demonstrated that PGE(2) synthesis by F. tularensis-infected macrophages requires cytosolic phospholipase A2 (cPLA(2)), cyclooxygenase 2 (COX-2), and microsomal prostaglandin E synthase 1 (mPGES1). During inducible PGE(2) synthesis, cPLA(2) hydrolyzes arachidonic acid (AA) from cellular phospholipids to be converted to PGE(2). However, in F. tularensis-infected macrophages we observed a temporal disconnect between Ser505-cPLA(2) phosphorylation (a marker of activation) and PGE(2) synthesis. These results suggested to us that cPLA(2) is not responsible for the liberation of AA to be converted into PGE(2) by F. tularensis-infected macrophages. Utilizing small-molecule inhibitors, we demonstrated that phospholipase D and diacylglycerol lipase were required for providing AA for PGE(2) biosynthesis. cPLA(2), on the other hand, was required for macrophage cytokine responses to F. tularensis. We also demonstrated for the first time that lipin-1 and PAP2a contribute to macrophage inflammation in response to F. tularensis. Our results identify both an alternative pathway for inducible PGE(2) synthesis and a role for lipid-modifying enzymes in the regulation of macrophage inflammatory function.

Choline kinase-alpha by regulating cell aggressiveness and drug sensitivity is a potential druggable target for ovarian cancer

Background:

Aberrant choline metabolism has been proposed as a novel cancer hallmark. We recently showed that epithelial ovarian cancer (EOC) possesses an altered MRS-choline profile, characterised by increased phosphocholine (PCho) content to which mainly contribute over-expression and activation of choline kinase-alpha (ChoK-alpha).

Methods:

To assess its biological relevance, ChoK-alpha expression was downmodulated by transient RNA interference in EOC in vitro models. Gene expression profiling by microarray analysis and functional analysis was performed to identify the pathway/functions perturbed in ChoK-alpha-silenced cells, then validated by in vitro experiments.

Results:

In silenced cells, compared with control, we observed: (I) a significant reduction of both CHKA transcript and ChoK-alpha protein expression; (II) a dramatic, proportional drop in PCho content ranging from 60 to 71%, as revealed by ¹H-magnetic spectroscopy analysis; (III) a 35–36% of cell growth inhibition, with no evidences of apoptosis or modification of the main cellular survival signalling pathways; (IV) 476 differentially expressed genes, including genes related to lipid metabolism. Ingenuity pathway analysis identified cellular functions related to cell death and cellular proliferation and movement as the most perturbed. Accordingly, CHKA-silenced cells displayed a significant delay in wound repair, a reduced migration and invasion capability were also observed. Furthermore, although CHKA silencing did not directly induce cell death, a significant increase of sensitivity to platinum, paclitaxel and doxorubicin was observed even in a drug-resistant context.

Conclusion:

We showed for the first time in EOC that CHKA downregulation significantly decreased the aggressive EOC cell behaviour also affecting cells' sensitivity to drug treatment. These observations open the way to further analysis for ChoK-alpha validation as a new EOC therapeutic target to be used alone or in combination with conventional drugs.

Compartmentalizing the embryo: lipids and septate junction mediated barrier function

Lipid phosphate phosphatases (LPPs) are a class of enzymes that can dephosphorylate a number of lysophopholipids in vitro. Analysis of knockouts of LPP family members has demonstrated striking but diverse developmental roles for these enzymes. LPP3 is required for mouse vascular development while the Drosophila LPPs Wunen (Wun) and Wunen2 (Wun2) are required during embryogenesis for germ cell migration and survival. In a recent publication we examined if these fly LPPs have further developmental roles and found that Wun is required for proper tracheal formation. In particular we highlight a role for Wun in septate junction mediated barrier function in the tracheal system. In this paper we discuss further the possible mechanisms by which LPPs may influence barrier activity.

Cell autonomous lipin 1 function is essential for development and maintenance of white and brown adipose tissue

Through analysis of mice with spatially and temporally restricted inactivation of Lpin1, we characterized its cell autonomous function in both white (WAT) and brown (BAT) adipocyte development and maintenance. We observed that the lipin 1 inactivation in adipocytes of aP2(Cre/+)/Lp(fEx2)(-)(3/fEx2)(-)(3) mice resulted in lipodystrophy and the presence of adipocytes with multilocular lipid droplets. We further showed that time-specific loss of lipin 1 in mature adipocytes in aP2(Cre-ERT2/+)/Lp(fEx2)(-)(3/fEx2)(-)(3) mice led to their replacement by newly formed Lpin1-positive adipocytes, thus establishing a role for lipin 1 in mature adipocyte maintenance. Importantly, we observed that the presence of newly formed Lpin1-positive adipocytes in aP2(Cre-ERT2/+)/Lp(fEx2)(-)(3/fEx2)(-)(3) mice protected these animals against WAT inflammation and hepatic steatosis induced by a high-fat diet. Loss of lipin 1 also affected BAT development and function, as revealed by histological changes, defects in the expression of peroxisome proliferator-activated receptor alpha (PPARα), PGC-1α, and UCP1, and functionally by altered cold sensitivity. Finally, our data indicate that phosphatidic acid, which accumulates in WAT of animals lacking lipin 1 function, specifically inhibits differentiation of preadipocytes. Together, these observations firmly demonstrate a cell autonomous role of lipin 1 in WAT and BAT biology and indicate its potential as a therapeutical target for the treatment of obesity.

A hypomorphic mutation in Lpin1 induces progressively improving neuropathy and lipodystrophy in the rat

The Lpin1 gene encodes the phosphatidate phosphatase (PAP1) enzyme Lipin 1, which plays a critical role in lipid metabolism. In this study we describe the identification and characterization of a rat model with a mutated Lpin1 gene (Lpin1(1Hubr)), generated by N-ethyl-N-nitrosourea mutagenesis. Lpin1(1Hubr) rats are characterized by hindlimb paralysis and mild lipodystrophy that are detectable from the second postnatal week. Sequencing of Lpin1 identified a point mutation in the 5'-end splice site of intron 18 resulting in mis-splicing, a reading frameshift, and a premature stop codon. As this mutation does not induce nonsense-mediated decay, it allows the production of a truncated Lipin 1 protein lacking PAP1 activity. Lpin1(1Hubr) rats developed hypomyelination and mild lipodystrophy rather than the pronounced demyelination and adipocyte defects characteristic of Lpin1(fld/fld) mice, which carry a null allele for Lpin1. Furthermore, biochemical, histological, and molecular analyses revealed that these lesions improve in older Lpin1(1Hubr) rats as compared with young Lpin1(1Hubr) rats and Lpin1(fld/fld) mice. We observed activation of compensatory biochemical pathways substituting for missing PAP1 activity that, in combination with a possible non-enzymatic Lipin 1 function residing outside of its PAP1 domain, may contribute to the less severe phenotypes observed in Lpin1(1Hubr) rats as compared with Lpin1(fld/fld) mice. Although we are cautious in making a direct parallel between the presented rodent model and human disease, our data may provide new insight into the pathogenicity of recently identified human LPIN1 mutations.

Functional characterization of the atypical integral membrane lipid phosphatase PDP1/PPAPDC2 identifies a pathway for interconversion of isoprenols and isoprenoid phosphates in mammalian cells

The polyisoprenoid diphosphates farnesyl diphosphate (FPP) and geranylgeranyl diphosphate (GGPP) are intermediates in the synthesis of cholesterol and related sterols by the mevalonate pathway and precursors for the addition of isoprenyl anchors to many membrane proteins. We developed tandem mass spectrometry assays to evaluate polyisoprenoid diphosphate phosphatase activity of an unusual integral membrane lipid enzyme: type 1 polyisoprenoid diphosphate phosphatase encoded by the PPAPDC2 gene (PDP1/PPAPDC2). In vitro, recombinant PDP1/PPAPDC2 preferentially hydrolyzed polyisoprenoid diphosphates, including FPP and GGPP over a variety of glycerol- and sphingo-phospholipid substrates. Overexpression of mammalian PDP1/PPAPDC2 in budding yeast depletes cellular pools of FPP leading to growth defects and sterol auxotrophy. In mammalian cells, PDP1/PPAPDC2 localizes to the endoplasmic reticulum and nuclear envelope and, unlike the structurally related lipid phosphate phosphatases, is predicted to be oriented with key residues of its catalytic domain facing the cytoplasmic face of the membrane. Studies using synthetic isoprenols with chemical properties that facilitate detection by mass spectrometry identify a pathway for interconversion of isoprenols and isoprenoid diphosphates in intact mammalian cells and demonstrate a role for PDP1/PPAPDC2 in this process. Overexpression of PDP1/PPAPDC2 in mammalian cells substantially decreases protein isoprenylation and results in defects in cell growth and cytoskeletal organization that are associated with dysregulation of Rho family GTPases. Taken together, these results focus attention on integral membrane lipid phosphatases as regulators of isoprenoid phosphate metabolism and suggest that PDP1/PPAPDC2 is a functional isoprenoid diphosphate phosphatase.

Synaptic PRG-1 modulates excitatory transmission via lipid phosphate-mediated signaling

Plasticity related gene-1 (PRG-1) is a brain-specific membrane protein related to lipid phosphate phosphatases, which acts in the hippocampus specifically at the excitatory synapse terminating on glutamatergic neurons. Deletion of prg-1 in mice leads to epileptic seizures and augmentation of EPSCs, but not IPSCs. In utero electroporation of PRG-1 into deficient animals revealed that PRG-1 modulates excitation at the synaptic junction. Mutation of the extracellular domain of PRG-1 crucial for its interaction with lysophosphatidic acid (LPA) abolished the ability to prevent hyperexcitability. As LPA application in vitro induced hyperexcitability in wild-type but not in LPA(2) receptor-deficient animals, and uptake of phospholipids is reduced in PRG-1-deficient neurons, we assessed PRG-1/LPA(2) receptor-deficient animals, and found that the pathophysiology observed in the PRG-1-deficient mice was fully reverted. Thus, we propose PRG-1 as an important player in the modulatory control of hippocampal excitability dependent on presynaptic LPA(2) receptor signaling.

The expression of phosphatidic acid phosphatase 2a, which hydrolyzes lipids to generate diacylglycerol, is regulated by p73, a member of the p53 family

p73, a p53-related gene, is essential for a development of animals, while p53 is important for tumor formation. And little is known about the target genes specifically regulated by p73. Identifying the specific targets of p73 is important to understand the physiological roles of p73. To identify the genes specifically regulated by p73, we conducted serial analysis of gene expression to quantitatively evaluate messenger RNA populations. We found that the gene for phosphatidic acid phosphatase 2a (PAP2a), an enzyme that hydrolyzes lipids to generate diacylglycerol, was specifically upregulated by ectopic production of p73beta. The promoter region of this gene contains an element that is functionally responsive to p73beta. And the quantity of PAP2a protein was upregulated by ectopic production of p73beta. These results suggest that the expression of PAP2a is directly regulated by p73.

Enzymatic analysis of lipid phosphate phosphatases

Lipid phosphate monoesters including phosphatidic acid, lysophosphatidic acid, sphingosine 1-phosphate and ceramide 1-phosphate are intermediates in phosho- and sphingo-lipid biosynthesis and also play important roles in intra- and extra-cellular signaling. Dephosphorylation of these lipids terminates their signaling actions and, in some cases, generates products with additional biological activities or metabolic fates. The key enzymes responsible for dephosphorylation of these lipid phosphate substrates are collectively termed lipid phosphate phosphatases (LPPs). They are integral membrane enzymes with a core domain of six transmembrane spanning alpha-helices linked by extramembrane loops. LPPs are oriented in the membrane with their N- and C-termini facing the cytoplasm. LPPs exhibit isoform and cell specific localization patterns being variably distributed between endomembrane compartments (primarily the endoplasmic reticulum and Golgi apparatus) and the plasma membrane. The active site of these enzymes is formed from residues within two of the extramembrane loops and faces the lumen of endomembrane compartments or, when localized to the plasma membrane, towards, the extracellular space. Biochemical, pharmacological, cell biological and genetic studies identify roles for LPPs in both intracellular lipid metabolism and the regulation of both intra- and extra-cellular signaling pathways that control cell growth, survival and migration. This article describes procedures for the expression of LPPs in insect and mammalian cells and their analysis by SDS-PAGE and Western blotting. The most straightforward way to determine LPP activity is to measure release of the substrate phosphate group. We described methods for the synthesis and purification of [(32)P]-labeled LPP substrates. We describe the use of both radiolabeled and fluorescent lipid substrates for the detection, quantitation and analysis of the enzymatic activities of the LPPs measured using intact or broken cell preparations as the source of enzyme.

Lipid phosphate phosphatase-2 activity regulates S-phase entry of the cell cycle in Rat2 fibroblasts

Lipid phosphates are potent mediators of cell signaling and control processes including development, cell migration and division, blood vessel formation, wound repair, and tumor progression. Lipid phosphate phosphatases (LPPs) regulate the dephosphorylation of lipid phosphates, thus modulating their signals and producing new bioactive compounds both at the cell surface and in intracellular compartments. Knock-down of endogenous LPP2 in fibroblasts delayed cyclin A accumulation and entry into S-phase of the cell cycle. Conversely, overexpression of LPP2, but not a catalytically inactive mutant, caused premature S-phase entry, accompanied by premature cyclin A accumulation. At high passage, many LPP2 overexpressing cells arrested in G(2)/M and the rate of proliferation declined severely. This was accompanied by changes in proteins and lipids characteristic of senescence. Additionally, arrested LPP2 cells contained decreased lysophosphatidate concentrations and increased ceramide. These effects of LPP2 activity were not reproduced by overexpression or knock-down of LPP1 or LPP3. This work identifies a novel and specific role for LPP2 activity and bioactive lipids in regulating cell cycle progression.

Biological effects of lysophospholipids

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are potent biologically active lipid mediators that exert a wide range of cellular effects through specific G protein-coupled receptors. To date, four LPA receptors and five S1P receptors have been identified. These receptors are expressed in a large number of tissues and cell types, allowing for a wide variety of cellular responses to lysophospholipid signaling, including cell adhesion, cell motility, cytoskeletal changes, proliferation, angiogenesis, process retraction, and cell survival. In addition, recent studies in mice show that specific lysophospholipid receptors are required for proper cardiovascular, immune, respiratory, and reproductive system development and function. Lysophospholipid receptors may also have specific roles in cancer and other diseases. This review will cover identification and expression of the lysophospholipid receptors, as well as receptor signaling properties and function. Additionally, phenotypes of mice deficient for specific lysophospholipid receptors will be discussed to demonstrate how these animals have furthered our understanding of the role lysophospholipids play in normal biology and disease.

Targeted disruption of glycerol kinase gene in mice: expression analysis in liver shows alterations in network partners related to glycerol kinase activity

Glycerol kinase deficiency (GKD) is an X-linked inborn error of metabolism with metabolic and neurological crises. Liver shows the highest level of glycerol kinase (GK) activity in humans and mice. Absence of genotype-phenotype correlations in patients with GKD indicates the involvement of modifier genes, including other network partners. To understand the molecular pathogenesis of GKD, we performed microarray analysis on liver mRNA from neonatal glycerol kinase (Gyk) knockout (KO) and wild-type (WT) mice. Unsupervised learning revealed that the overall gene expression profile of the KO mice was different from that of WT. Real-time PCR confirmed the differences for selected genes. Functional gene enrichment analysis was used to find 56 increased and 37 decreased gene functional categories. PathwayAssist analysis identified changes in gene expression levels of genes involved in organic acid metabolism indicating that GK was part of the same metabolic network which correlates well with the patients with GKD having metabolic acidemia during their episodic crises. Network component analysis (NCA) showed that transcription factors sterol regulatory element-binding protein (SREBP)-1c, carbohydrate response element-binding protein (ChREBP), hepatocyte nuclear factor-4 alpha (HNF-4alpha) and peroxisome proliferative-activated receptor-alpha (PPARalpha) had increased activity in the Gyk KO mice compared with WT mice, whereas SREBP-2 was less active in the Gyk KO mice. These studies show that Gyk deletion causes alterations in expression of genes in several regulatory networks and is the first time NCA has been used to expand on microarray data from a mouse KO model of a human disease.

Cloning and characterization of a novel human phosphatidic acid phosphatase type 2, PAP2d, with two different transcripts PAP2d_v1 and PAP2d_v2

This study reports the cloning and characterization of a novel human phosphatidic acid phosphatase type 2 isoform cDNAs (PAP2d) from the foetal brain cDNA library. The PAP2d gene is localized on chromosome 1p21.3. It contains six exons and spans 112 kb of the genomic DNA. By large-scale cDNA sequencing we found two splice variants of PAP2d, PAP2d_v1 and PAP2d_v2. The PAP2d_v1 cDNA is 1722 bp in length and spans an open reading frame from nucleotide 56 to 1021, encoding a 321aa protein. The PAP2d_v2 cDNA is 1707 bp in length encoding a 316aa protein from nucleotide 56-1006. The PAP2d_v1 cDNA is 15 bp longer than the PAP2d_v2 cDNA in the terminal of the fifth exon and it creates different ORF. Both of the proteins contain a well-conserved PAP2 motif. The PAP2d_v1 is mainly expressed in human brain, lung, kidney, testis and colon, while PAP2d_v2 is restricted to human placenta, skeletal muscle, and kidney. The two splice variants are co-expressed only in kidney.

Integral membrane lipid phosphatases/phosphotransferases: common structure and diverse functions

Phospholipids and sphingolipids play critical roles in signal transduction, intracellular membrane trafficking, and control of cell growth and survival. We discuss recent progress in the identification and characterization of a family of integral membrane proteins with central roles in bioactive lipid metabolism and signalling. These five groups of homologous proteins, which we collectively term LPTs (lipid phosphatases/phosphotransferases), are characterized by a core domain containing six transmembrane-spanning alpha-helices connected by extramembrane loops, two of which interact to form the catalytic site. LPT family members are localized to all major membrane compartments of the cell. The transmembrane topology of these proteins places their active site facing the lumen of endomembrane compartments or the extracellular face of the plasma membrane. Sequence conservation between the active site of the LPPs (lipid phosphate phosphatases), SPPs (sphingosine phosphate phosphatases) and the recently identified SMSs (sphingomyelin synthases) with vanadium-dependent fungal oxidases provides a framework for understanding their common catalytic mechanism. LPPs hydrolyse LPA (lysophosphatidic acid), S1P (sphingosine 1-phosphate) and structurally-related substrates. Although LPPs can dephosphorylate intracellularly generated substrates to control intracellular lipid metabolism and signalling, their best understood function is to regulate cell surface receptor-mediated signalling by LPA and S1P by inactivating these lipids at the plasma membrane or in the extracellular space. SPPs are intracellularly localized S1P-selective phosphatases, with key roles in the pathways of sphingolipid metabolism linked to control of cell growth and survival. The SMS enzymes catalyse the interconversion of phosphatidylcholine and ceramide with sphingomyelin and diacylglycerol, suggesting a pivotal role in both housekeeping lipid synthesis and regulation of bioactive lipid mediators. The remaining members of the LPT family, the LPR/PRGs (lipid phosphatase-related proteins/plasticity-related genes) and CSS2s (type 2 candidate sphingomyelin synthases), are presently much less well studied. These two groups include proteins that lack critical amino acids within the catalytic site, and could therefore not use the conserved LPT reaction mechanism to catalyse lipid phosphatase or phosphotransferase reactions. In this review, we discuss recent ideas about their possible biological activities and functions, which appear to involve regulation of cellular morphology and, possibly, lipid metabolism and signalling in the nuclear envelope.

Lipid phosphate phosphatases and related proteins: signaling functions in development, cell division, and cancer

Lipid phosphates initiate key signaling cascades in cell activation. Lysophosphatidate (LPA) and sphingosine 1-phosphate (S1P) are produced by activated platelets. LPA is also formed from circulating lysophosphatidylcholine by autotaxin, a protein involved tumor progression and metastasis. Extracellular LPA and S1P stimulate families of G-protein coupled receptors that elicit diverse responses. LPA is involved in wound repair and tumor growth. Exogenous S1P is a potent stimulator of angiogenesis, a process vital in development, tissue repair and the growth of aggressive tumors. Inside the cell, phosphatidate (PA), ceramide 1-phosphate (C1P), LPA, and S1P act as signaling molecules with distinct functions including the stimulation of cell division, cytoskeletal rearrangement, Ca(2+) transients, and membrane movement. These observations imply that phosphatases that degrade lipid phosphates on the cell surface, or inside the cell, regulate cell signaling under physiological and pathological conditions. This occurs through attenuation of signaling by the lipid phosphates and by the production of bioactive products (diacylglycerol, ceramide, and sphingosine). Three lipid phosphate phosphatases (LPPs) and a splice variant dephosphorylate LPA, PA, CIP, and S1P. Two S1P phosphatases (SPPs) act specifically on S1P. In addition, there is family of four LPP-related proteins (LPRs, or plasticity-related genes, PRGs). PRG-1 expression in neurons has been reported to increase extracellular LPA breakdown and attenuate LPA-induced axonal retraction. It is unclear whether the LRPs dephosphorylate LPA directly, stimulate LPP activity, or bind LPA and S1P. Also, the importance of extra- versus intra-cellular actions of the LPPs and SPPs, and the individual roles of different isoforms is not firmly established. Understanding the functions and regulation of the LPPs, SPPs and related proteins will hopefully contribute to interventions to correct dysfunctions in conditions such as wound repair, inflammation, angiogenesis, tumor growth, and metastasis.

Lysophosphatidic acid and sphingosine 1-phosphate biology: the role of lipid phosphate phosphatases

The biological actions of the lysolipid agonists sphingosine 1-phosphate and lysophosphatidic acid, in addition to other bioactive lipid phosphates such as phosphatidic acid and ceramide 1-phosphate, can be influenced by a family of lipid phosphate phosphatases (LPP), including LPP1, LPP2, LPP3, the Drosophila homologues Wunen (Wun) and Wunen2 (Wun2) and sphingosine 1-phosphate phosphatases 1 and 2 (SPP1, SPP2). This review describes the characteristic of these enzymes and their potential physiological roles in regulating intracellular and extracellular actions and amounts of these lipids in addition to the involvement of these phosphatases in development.

Germ cell-autonomous Wunen2 is required for germline development in Drosophila embryos

In many animals, primordial germ cells (PGCs) migrate through the embryo towards the future gonad, a process guided by attractive and repulsive cues provided from surrounding somatic cells. In Drosophila, the two related lipid phosphate phosphatases (LPPs), Wunen (Wun) and Wun2, are thought to degrade extracellular substrates and to act redundantly in somatic cells to provide a repulsive environment to steer the migration of PGCs, or pole cells. Wun and Wun2 also affect the viability of pole cells, because overexpression of either one in somatic cells causes pole cell death. However, the means by which they regulate pole cell migration and survival remains elusive. We report that Wun2 has a maternal function required for the survival of pole cells during their migration to the gonad. Maternal wun2 RNA was found to be concentrated in pole cells and pole cell-specific expression of wun2 rescued the pole cell death phenotype of the maternal wun2 mutant, suggesting that wun2 activity in pole cells is required for their survival. Furthermore, we obtained genetic evidence that pole cell survival requires a proper balance of LPP activity in pole cells and somatic cells. We propose that Wun2 in pole cells competes with somatic Wun and Wun2 for a common lipid phosphate substrate, which is required by pole cells to produce their survival signal. In somatic cells, Wun and Wun2 may provide a repulsive environment for pole cell migration by depleting this extracellular substrate.

Germ Cell Specification and Migration in Drosophila and beyond

The passage of an individual's genome to future generations is essential for the maintenance of species and is mediated by highly specialized cells, the germ cells. Genetic studies in a number of model organisms have provided insight into the molecular mechanisms that control specification, migration and survival of early germ cells. Focusing on Drosophila, we will discuss the mechanisms by which germ cells initially form and remain transcriptionally silent while somatic cells are transcriptionally active. We will further discuss three separate attractive and repellent guidance pathways, mediated by a G-protein coupled receptor, two lipid phosphate phosphohydrolases, and isoprenylation. We will compare and contrast these findings with those obtained in other organisms, in particular zebrafish and mice. While aspects of germ cell specification are strikingly different between these species, germ cell specific gene functions have been conserved. In particular, mechanisms that sense directional cues during germ cell migration seem to be shared between invertebrates and vertebrates.

Translocation of lysophosphatidic acid phosphatase in response to gonadotropin-releasing hormone to the plasma membrane in ovarian cancer cell

OBJECTIVE

Lysophosphatidic acid mediates proliferative and/or morphologic effects on multiple cell lineages, which include ovarian cancer cells. Lysophosphatidic acid hydrolysis limits the duration of lysophosphatidic acid's action. We examined hormonal translocation of lipid phosphate phosphatase type 3 to the plasma membrane in gonadotropin-releasing hormone-responsive ovarian cancers.

STUDY DESIGN

Ovarian cancers that were removed surgically and the ovarian cancer cell lines Caov-3 and SK-OV-3 were examined. Lipid phosphate phosphatase type 3 protein and activity in plasma membranes were assessed by immunohistochemical staining with lipid phosphate phosphatase type 3-specific antibodies and by the measurement of the conversion of exogenous [(3)H-oleoyl]lysophosphatidic acid to mono[(3)H-oleoyl]glycerol, respectively.

RESULTS

In ovarian cancers that were removed surgically, the cell surface staining and activity measurements indicated that a portion of the enzyme was localized to the plasma membrane. In Caov-3 cells and SK-OV-3 cells, lipid phosphate phosphatase type 3 protein was present both in the cytoplasm and at the plasma membrane. Treatment of the cells with a gonadotropin-releasing hormone agonist buserelin produced a rapid and progressive translocation of lipid phosphate phosphatase type 3 protein to the plasma membrane, with a concomitant loss of cytoplasmic staining. The enzyme activity in plasma membrane was also increased when the cell lines were exposed to the gonadotropin-releasing hormone agonist in intact cells before the assay of the cell membranes.

CONCLUSION

These findings support the presence of lipid phosphate phosphatase type 3 in plasma membrane of ovarian cancers and provide for the ability of agonists (such as gonadotropin-releasing hormone) to induce the translocation of lipid phosphate phosphatase type 3 to plasma membrane in ovarian cancer cells.

Lipid phosphate phosphatases regulate lysophosphatidic acid production and signaling in platelets: studies using chemical inhibitors of lipid phosphate phosphatase activity

Blood platelets play an essential role in ischemic heart disease and stroke contributing to acute thrombotic events by release of potent inflammatory agents within the vasculature. Lysophosphatidic acid (LPA) is a bioactive lipid mediator produced by platelets and found in the blood and atherosclerotic plaques. LPA receptors on platelets, leukocytes, endothelial cells, and smooth muscle cells regulate growth, differentiation, survival, motility, and contractile activity. Definition of the opposing pathways of synthesis and degradation that control extracellular LPA levels is critical to understanding how LPA bioactivity is regulated. We show that intact platelets and platelet membranes actively dephosphorylate LPA and identify the major enzyme responsible as lipid phosphate phosphatase 1 (LPP1). Localization of LPP1 to the platelet surface is increased by exposure to LPA. A novel receptor-inactive sn-3-substituted difluoromethylenephosphonate analog of phosphatidic acid that is a potent competitive inhibitor of LPP1 activity potentiates platelet aggregation and shape change responses to LPA and amplifies LPA production by agonist-stimulated platelets. Our results identify LPP1 as a pivotal regulator of LPA signaling in the cardiovascular system. These findings are consistent with genetic and cell biological evidence implicating LPPs as negative regulators of lysophospholipid signaling and suggest that the mechanisms involve both attenuation of lysophospholipid actions at cell surface receptors and opposition of lysophospholipid production.

Lipid phosphate phosphatase-1 dephosphorylates exogenous lysophosphatidate and thereby attenuates its effects on cell signalling

The serum-derived phospholipid growth factor, lysophosphatidate (LPA), activates cells through a family of G-protein-coupled EDG receptors. The present article examines the role of lipid phosphate phosphatase-1 (LPP-1, or phosphatidate phosphate 2A) in regulating cell activation by LPA. Overexpressing LPP-1 approximately doubled the rate of dephosphorylation of exogenous LPA by Rat2 fibroblasts. The amount of LPA dephosphorylation was restricted to less than 10% of the total exogenous LPA. Over-expression of LPP-1 attenuated cell activation as indicated by diminished responses including cAMP, Ca2+, activation of phospholipase D and ERK, DNA synthesis and cell division. LPP-1 therefore provides a novel level of regulation for controlling cell signalling by exogenous LPA.