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.     

Biochemical and molecular characterization of two phosphatidic acid-selective phospholipase A1s,mPA-PLA1alpha and mPA-PLA1beta

We have identified a novel phospholipase A1, named mPA-PLA1beta, which is specifically expressed in human testis and characterized it biochemically together with previously identified mPA-PLA1alpha. The sequence of mPAPLA1beta encodes a 460-amino acid protein containing a lipase domain with significant homology to the previously identified phosphatidic acid (PA)-selective PLA1, mPA-PLA1alpha. mPA-PLA1beta contains a short lid and deleted beta9 loop, which are characteristics of PLA1 molecules in the lipase family, and is a member of a subfamily in the lipase family that includes mPA-PLA1alpha and phosphatidylserine-specific PLA1. Both mPA-PLA1beta and mPA-PLA1alpha recombinant proteins exhibited PA-specific PLA1 activity and were vanadate-sensitive. When mPAPLA1beta-expressing cells were treated with bacterial phospholipase D, the cells produced lysophosphatidic acid (LPA). In both mPA-PLA1alpha and beta-expressing cells, most of the PA generated by the phospholipase D (PLD) treatment was converted to LPA, whereas in control cells it was converted to diacylglycerol. When expressed in HeLa cells most mPA-PLA1alpha protein was recovered from the cell supernatant. By contrast, mPA-PLA1beta was recovered almost exclusively from cells. Consistent with this observation, we found that mPA-PLA1beta has higher affinity to heparin than mPA-PLA1alpha. We also found that the membrane-associated mPA-PLA1s were insoluble in solubilization by 1% Triton X-100 and were detected in Triton X-100-insoluble buoyant fractions of sucrose gradients. The present study raises the possibility that production of LPA by mPA-PLA1alpha and -beta occurs on detergent-resistant membrane domains of the cells where they compete with lipid phosphate phosphatase for PA.     

Two pathways for lysophosphatidic acid production

Lysophosphatidic acid (LPA, 1- or 2-acyl-sn-glycerol 3-phosphate) is a simple phospholipid but displays an intriguing cell biology that is mediated via interactions with G protein-coupled seven transmembrane receptors (GPCRs). So far, five GPCRs, designated LPA(1-5), and, more recently, two additional GPCRs, GPR87 and P2Y5, have been identified as receptors for LPA. These LPA receptors can be classified into two families, the EDG and P2Y families, depending on their primary structures. Recent studies on gene targeting mice and family diseases of these receptors revealed that LPA is involved in both pathological and physiological states including brain development (LPA(1)), neuropathy pain (LPA(1)), lung fibrosis (LPA(1)), renal fibrosis (LPA(1)) protection against radiation-induced intestinal injury (LPA(2)), implantation (LPA(3)) and hair growth (P2Y5). LPA is produced both in cells and biological fluids, where multiple synthetic reactions occur. There are at least two pathways for LPA production. In serum or plasma, LPA is predominantly produced by a plasma enzyme called autotaxin (ATX). ATX is a multifunctional ectoenzyme and is involved in many patho-physiological conditions such as cancer, neuropathy pain, lymphocyte tracking in lymph nodes, obesity, diabetes and embryonic blood vessel formation. LPA is also produced from phosphatidic acid (PA) by its deacylation catalyzed by phospholipase A (PLA)-type enzymes. However, the physiological roles of this pathway as well as the enzymes involved remained to be solved. A number of phospholipase A(1) and A(2) isozymes could be involved in this pathway. One PA-selective PLA(1) called mPA-PLA(1)alpha/LIPH is specifically expressed in hair follicles, where it has a critical role in hair growth by producing LPA through a novel LPA receptor called P2Y5.     

Hydrolysis of phosphatidylserine-exposing red blood cells by secretory phospholipase A2 generates lysophosphatidic acid and results in vascular dysfunction

Secretory phospholipase A(2) (sPLA(2)) type IIa, elevated in inflammation, breaks down membrane phospholipids and generates arachidonic acid. We hypothesized that sPLA(2) will hydrolyze red blood cells that expose phosphatidylserine (PS) and generate lysophosphatidic acid (LPA) from phosphatidic acid that is elevated in PS-exposing red blood cells. In turn, LPA, a powerful lipid mediator, could affect vascular endothelial cell function. Although normal red blood cells were not affected by sPLA(2), at levels of sPLA(2) observed under inflammatory conditions (100 ng/ml) PS-exposing red blood cells hemolyzed and generated LPA (1.2 nM/10(8) RBC). When endothelial cell monolayers were incubated in vitro with LPA, a loss of confluence was noted. Moreover, a dose-dependent increase in hydraulic conductivity was identified in rat mesenteric venules in vivo with 5 microM LPA, and the combination of PS-exposing red blood cells with PLA(2) caused a similar increase in permeability. In the presence of N-palmitoyl L-serine phosphoric acid, a competitive inhibitor for the endothelial LPA receptor, loss of confluence in vitro and the hydraulic permeability caused by 5 microM LPA in vivo were abolished. The present study demonstrates that increased sPLA(2) activity in inflammation in the presence of cells that have lost their membrane phospholipid asymmetry can lead to LPA-mediated endothelial dysfunction and loss of vascular integrity.     

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.     

Phosphatidic acid synthesis in mitochondria. Topography of formation and transmembrane migration.

The topography of formation and migration of phosphatidic acid (PA) in the transverse plane of rat liver mitochondrial outer membrane (MOM) were investigated. Isolated mitochondria and microsomes, incubated with sn-glycerol 3-phosphate and an immobilized substrate palmitoyl-CoA-agarose, synthesized both lyso-PA and PA. The mitochondrial and microsomal acylation of glycerophosphate with palmitoyl-CoA-agarose was 80-100% of the values obtained in the presence of free palmitoyl-CoA. In another series of experiments, both free polymyxin B and polymyxin B-agarose stimulated mitochondrial glycerophosphate acyltransferase activity approximately 2-fold. When PA loaded mitochondria were treated with liver fatty acid binding protein, a fifth of the phospholipid left the mitochondria. The amount of exportable PA reduced with the increase in the time of incubation. In another approach, PA-loaded mitochondria were treated with phospholipase A(2). The amount of phospholipase A(2)-sensitive PA reduced when the incubation time was increased. Taken together, the results suggest that lysophosphatidic acid (LPA) and PA are synthesized on the outer surface of the MOM and that PA moves to the inner membrane presumably for cardiolipin formation.     

Lysophosphatidic acid stimulates inflammatory cascade in airway epithelial cells

Nasal polyps are benign outgrowths originating from the anterior ethmoid and maxillary sinuses. The events leading to polyp formation are unknown but evidence points to damage of the mucousal epithelium. Lysophosphatidic acid (LPA) is a water-soluble phospholipid that has been implicated in the development of allergic inflammation. We hypothesized LPA may be an important mediator in the initiation and maintenance of the inflammatory milieu of the polyp. Data was compared from unstimulated lung epithelial and when possible nasal polyp-derived epithelial cells with LPA stimulated cells. LPA receptors 1 and 2 were constitutively expressed on lung and nasal polyp-derived epithelial cells and receptor mRNA expression was decreased upon stimulation with IL-13 and IFN-gamma. When cells were treated with LPA, cellular proliferation was stimulated 2.2 fold. Supernatants from LPA stimulated cells displayed decreases in the levels of VEGF, GM-CSF, and TNF-alpha at 24h which returned to normal or increased at 48h. Our results suggest epithelial cells undergo rapid proliferation in response to LPA resulting in a transient decrease in inflammatory cytokines followed by an upregulation of these cytokines that could lead to increased inflammation.     

Multiple mechanisms linked to platelet activation result in lysophosphatidic acid and sphingosine 1-phosphate generation in blood

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (Sph1P) production was examined in vitro under conditions that simulated blood clotting. Several approaches were utilized to elucidate the metabolic pathways. 1) Platelet phospholipids were labeled using [32P]orthophosphate, and the production of [32P]Sph1P and LPA was examined. Thrombin stimulation of platelets resulted in rapid secretion of Sph1P stored within the platelet. In contrast, LPA was neither stored within nor secreted from platelets. Nonetheless, extracellular levels of LPA gradually increased following stimulation. 2) Stable-isotope dilution mass spectrometry was used to quantify the molecular species of LPA generated from platelets in vitro. Only 10% of the LPA generated following thrombin stimulation was associated with platelets, the remaining 90% was contained within the extracellular medium. The acyl composition of LPA produced by platelets differed depending on the presence or absence of plasma in the incubation. 3) The fate of exogenously added fluorescent phospholipid analogs was determined. Incubation of [(7-nitro-2-1,3-benzoxadiazol-4-yl)amino]dodecanoyl-(NBD)-labeled phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine with the supernatant fractions from thrombin-stimulated platelets yielded no LPA production. However, these lipids were converted to the corresponding lysolipids by released PLA1 and PLA2 activities. When incubated with plasma or serum the NBD-labeled lysophospholipids were readily converted to LPA. Inhibitors of lysophospholipase D and the biological activity of LPA were detected in plasma. These results suggest that the bulk of LPA produced through platelet activation results from the sequential cleavage of phospholipids to lysophospholipids by released phospholipases A1 and A2 and then to LPA by plasma lysophospholipase D.     

Inositol 1,4,5-trisphosphate-independent Ca(2+) mobilization triggered by a lipid factor isolated from vitreous body.

A complex phospholipid from bovine vitreous body with a strong Ca(2+)-mobilizing activity has been recently isolated to homogeneity by our group. In this work, a sequential analysis of its transmembrane signaling pathway has been undertaken to characterize the intracellular mechanisms responsible for the Ca(2+) rise. The results show that this phospholipid induces, in a dose-dependent manner (ED(50) of around 0.25 microgram/ml), a Ca(2+) mobilization from inositol 1,4,5-trisphosphate-insensitive intracellular stores, with no participation of extracellular Ca(2+). Upon repeated administration, it shows no signs of autologous desensitization, does not induce heterologous desensitization of the L-alpha-lysophosphatidic acid (LPA) receptor but is desensitized by the previous administration of LPA. The Ca(2+)-mobilizing activity requires a membrane protein, is blocked after preincubation of the cells with pertussis toxin and phorbol esters, as well as by U73122 (an inhibitor of phospholipases C/D), R59022 (a diacylglycerol kinase inhibitor), and D609 (which inhibits phosphatidylcholine-specific phospholipase C). Upon administration of this phospholipid, the intracellular levels of phosphatidic acid (PA) rise with a time course that parallels that of the Ca(2+) mobilization, suggesting that PA could be responsible for this Ca(2+) signal. Exposure to AACOCF(3) (a specific inhibitor of phospholipase A(2)) does not modify the Ca(2+) rise, ruling out the possibility that the PA generated could be further converted to LPA by the action of phospholipase A(2). Based on the experimental data obtained, a signaling pathway involving a phosphatidylcholine-specific phospholipase C coupled to diacylglycerol kinase is proposed. This compound may represent a new class of bioactive lipids with a putative role in the physiology of the vitreous body.     

Lysophosphatidic acid increases intracellular H2O2 by phospholipase D and RhoA in rat-2 fibroblasts.

We have investigated the possible roles of phospholipase D (PLD) and RhoA in the production of intracellular H2O2 and actin polymerization in response to lysophosphatidic acid (LPA) in Rat-2 fibroblasts. LPA increased intracellular H2O2, with a maximal increase at 30 min, which was blocked by the catalase from Aspergillus niger. The LPA-stimulated production of H2O2 was inhibited by 1-butanol or PKC-downregulation, but not by 2-butanol. Purified phosphatidic acid (PA) also increased intracellular H2O2 and the increase was inhibited by the catalase. The role of RhoA was studied by the scrape-loading of C3 transferase into the cells. The C3 toxin, which inhibited stress fiber formation stimulated by LPA, blocked the H2O2 production in response to LPA or PA, but had no inhibitory effect on the activation of PLD by LPA. Exogenous H2O2 increased F-actin content by stress fiber formation. In addition, catalase inhibited actin polymerization activated by LPA, PA, or H2O2, indicated the role of H2O2 in actin polymerization. These results suggest that LPA increased intracellular H2O2 by the activation of PLD and RhoA, and that intracellular H2O2 was required for the LPA-stimulated stress fiber formation.     

Multiple lysophosphatidic acid acyltransferases in Neisseria meningitidis

Lysophosphatidic acid (LPA) and phosphatidic acid (PA) are critical phospholipid intermediates in the biosynthesis of cell membranes. In Escherichia coli, LPA acyltransferase (1-acyl-sn-glycerol-3-phosphate acyltransferase; EC 2.3.1.51) catalyses the transfer of an acyl chain from either acyl-coenzyme A or acyl-acyl carrier protein onto LPA to produce PA. While E. coli possesses one essential LPA acyltransferase (PlsC), Neisseria meningitidis possesses at least two LPA acyltransferases. This study describes the identification and characterization of nlaB (neisserial LPA acyltransferase B), the second LPA acyltransferase identified in N. meningitidis. The gene was located downstream of the Tn916 insertion in N. meningitidis mutant 469 and differed in nucleotide and predicted amino acid sequence from the previously characterized neisserial LPA acyltransferase homologue nlaA. NlaB has specific LPA acyltransferase activity, as demonstrated by complementation of an E. coli plsC(Ts) mutant in trans, by decreased levels of LPA acyltransferase activity in nlaB mutants and by lack of complementation of E. coli plsB26,X50, a mutant defective in the first acyltransferase step in phospholipid biosynthesis. Meningococcal nlaA mutants accumulated LPA and demonstrated alterations in membrane phospholipid composition, yet retained LPA acyltransferase activity. In contrast, meningococcal nlaB mutants exhibited decreased LPA acyltransferase activity, but did not accumulate LPA or display any other observable membrane changes. We propose that N. meningitidis possesses at least two LPA acyltransferases to provide for the production of a greater diversity of membrane phospholipids.     

Phospholipase D stimulation is required for sphingosine-1-phosphate activation of actin stress fibre assembly in human airway epithelial cells.

In human airway epithelial cells, sphingosine-1-phosphate (SPP) and lysophosphatidic acid (LPA) stimulated the production of phosphatidic acid (PA), which was inhibited by the primary alcohol butan-1-ol, but not by the inactive butan-2-ol, clearly indicating phospholipase D (PLD) involvement. Both SPP and LPA stimulated actin stress fibre formation, which was also butan-2-ol-insensitive and inhibited by butan-1-ol. SPP-induced PLD activation and cytoskeletal remodelling were insensitive to brefeldin A and toxin B from Clostridium difficile, which conversely blocked the effect of LPA, suggesting that the monomeric GTPases ADP ribosylation factor (ARF) and Rho are involved in LPA, but not in SPP responses. Pertussis toxin inhibited SPP- but not LPA-induced effects. PLD activation and stress fibre formation by both lysolipids were abolished by the tyrosine kinase inhibitor genistein. Addition of PA to cells caused a massive stress fibre assembly. In conclusion, PLD is one of the signalling components linking SPP-receptor activation to assembly of actin stress fibres.     

Lysophosphatidic acid activates peroxisome proliferator activated receptor-γ in CHO cells that over-express glycerol 3-phosphate acyltransferase-1

Lysophosphatidic acid (LPA) is an agonist for peroxisome proliferator activated receptor-γ (PPARγ). Although glycerol-3-phosphate acyltransferase-1 (GPAT1) esterifies glycerol-3-phosphate to form LPA, an intermediate in the de novo synthesis of glycerolipids, it has been assumed that LPA synthesized by this route does not have a signaling role. The availability of Chinese Hamster Ovary (CHO) cells that stably overexpress GPAT1, allowed us to analyze PPARγ activation in the presence of LPA produced as an intracellular intermediate. LPA levels in CHO-GPAT1 cells were 6-fold higher than in wild-type CHO cells, and the mRNA abundance of CD36, a PPARγ target, was 2-fold higher. Transactivation assays showed that PPARγ activity was higher in the cells that overexpressed GPAT1. PPARγ activity was enhanced further in CHO-GPAT1 cells treated with the PPARγ ligand troglitazone. Extracellular LPA, phosphatidic acid (PA) or a membrane-permeable diacylglycerol had no effect, showing that PPARγ had been activated by LPA generated intracellularly. Transient transfection of a vector expressing 1-acylglycerol-3-phosphate acyltransferase-2, which converts endogenous LPA to PA, markedly reduced PPARγ activity, as did over-expressing diacylglycerol kinase, which converts DAG to PA, indicating that PA could be a potent inhibitor of PPARγ. These data suggest that LPA synthesized via the glycerol-3-phosphate pathway can activate PPARγ and that intermediates of de novo glycerolipid synthesis regulate gene expression.     

Phospholipase D/phosphatidic acid signal transduction: role and physiological significance in lung

Phospholipase D (PLD), a phospholipid phosphohydrolase, catalyzes the hydrolysis of phosphatidylcholine and other membrane phospholipids to phosphatidic acid (PA) and choline. PLD, ubiquitous in mammals, is a critical enzyme in intracellular signal transduction. PA generated by agonist- or reactive oxygen species (ROS)-mediated activation of the PLDI and PLD2 isoforms can be subsequently converted to lysoPA (LPA) or diacylglycerol (DAG) by phospholipase A1/A2 or lipid phosphate phosphatases. In pulmonary epithelial and vascular endothelial cells, a wide variety of agonists stimulate PLD and involve Src kinases, p-38 mitogen activated protein kinase, calcium and small G proteins. PA derived from the PLD pathway has second-messenger functions. In endothelial cells, PA regulates NAD[P]H oxidase activity and barrier function. In airway epithelial cells, sphingosine-1-phosphate and PA-induced IL-8 secretion and ERKI/2 phosphorylation is regulated by PA. PA can be metabolized to LPA and DAG, which function as first- and second-messengers, respectively. Signaling enzymes such as Raf 1, protein kinase Czeta and type I phosphatidylinositol-4-phosphate 5-kinase are also regulated by PA in mammalian cells. Thus, PA and its metabolic products play a central role in modulating endothelial and epithelial cell functions.     

Dioleoyl phosphatidic acid increases intracellular Ca2+ through endogenous LPA receptors in C6 glioma and L2071 fibroblasts

Phosphatidic acid (PA) increased intracellular Ca(2+) concentration ([Ca(2+)](i)) in C6 rat glioma and L2071 mouse fibroblast cells. Dioleoyl PA (PA, 18:1) was the most efficacious, followed by dipalmitoyl PA (16:0 PA) and dimyristoyl PA (14:0 PA). Lysophosphatidic acid (LPA) also increased the [Ca(2+)](i) in the both cells. PA desensitized LPA-induced Ca(2+) response completely in C6 cells, but partly in L2071 cells. Treatment of pertussis toxin (PTX), a specific inhibitor of G(i/o)-type G proteins, completely ameliorated LPA- and PA-induced Ca(2+) response in C6 cells. However, in L2071 cells, PTX inhibited PA-induced Ca(2+) increase by 80% and LPA-induced one by 20%. Ki16425, a specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited both LPA- and PA-induced Ca(2+) responses in C6 cells. On the other hand, in L2071 cells, Ki16425 completely inhibited PA-induced Ca(2+) response, but partly LPA-induced one. VPC32183, another specific inhibitor of LPA(1)/LPA(3) receptors, completely inhibited LPA- and PA-induced Ca(2+) responses in both C6 and L2071 cells. Therefore, PA and LPA appear to increase [Ca(2+)](i) through Ki16425/VPC32183-sensitive LPA receptor coupled to PTX-sensitive G proteins in C6 cells. In L2071 cells, however, LPA increases [Ca(2+)](i) through Ki16425-insensitive LPA receptor coupled to PTX-insensitive G proteins and Ki16425-sensitive LPA receptor coupled to PTX-sensitive G protein, whereas PA utilized only the latter pathway. Our results suggest that PA acts as a partial agonist on endogenous LPA receptors, which are sensitive to Ki16425 and coupled to PTX-sensitive G protein, but not on LPA receptors, which are not sensitive to Ki16425 and coupled to PTX-insensitive G protein.     

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.     

Age-related changes in the metabolization of phosphatidic acid in rat cerebral cortex synaptosomes

In this study, phosphatidic acid (PA) metabolization is found to generate diacylglycerol (DAG), monoacylglycerol (MAG) and glycerol by the sequential action of lipid phosphate phosphatase (LPP), diacylglycerol lipase (DAGL), and monoacylglycerol lipase (MAGL) in cerebral cortex (CC) synaptosomes. It is also demonstrated that PA is metabolized by phospholipases A (PLA)/lysophosphatidic acid phosphohydrolase (LPAPase) in synaptic endings. Age-related changes in the metabolization of PA have been observed in rat cerebral cortex synaptosomes in the presence of the alternative substrates for LPP, namely LPA, sphingosine 1-phosphate (S1P) and ceramide 1-phosphate (C1P). In addition, LPA and C1P up to concentrations of about 50 μM favor the metabolism in the direction of MAG and glycerol in aged and adult synaptosomes, respectively. At equimolecular concentrations with PA, LPA decreases DAG formation in adult and aged synaptosomes, whereas S1P decreases it and C1P increases it only in aged synaptosomes. Sphingosine (50 μM) or ceramide (100 μM) increase PA metabolism by the pathway that involves LPP/DAGL/MAGL action in aged membranes. Using RHC-80267, a DAGL inhibitor, we could observe that 50% and 33% of MAG are produced as a result of DAGL action in adult and aged synaptosomes, respectively. Taken together, our findings indicate that the ageing modifies the different enzymatic pathways involved in PA metabolization.     

Phospholipase D/phosphatidic acid signal transduction: Role and physiological significance in lung

Phospholipase D (PLD), a phospholipid phosphohydrolase, catalyzes the hydrolysis of phosphatidylcholine and other membrane phospholipids to phosphatidic acid (PA) and choline. PLD, ubiquitous in mammals, is a critical enzyme in intracellular signal transduction. PA generated by agonist- or reactive oxygen species (ROS)-mediated activation of the PLD1 and PLD2 isoforms can be subsequently converted to lysoPA (LPA) or diacylglycerol (DAG) by phospholipase A₁/A₂ or lipid phosphate phosphatases. In pulmonary epithelial and vascular endothelial cells, a wide variety of agonists stimulate PLD and involve Src kinases, p-38 mitogen activated protein kinase, calcium and small G proteins. PA derived from the PLD pathway has second messenger functions. In endothelial cells, PA regulates NAD[P]H oxidase activity and barrier function. In airway epithelial cells, sphingosine-1-phosphate and PA-induced IL-8 secretion and ERK1/2 phosphorylation is regulated by PA. PA can be metabolized to LPA and DAG, which function as first- and second-messengers, respectively. Signaling enzymes such as Raf 1, protein kinase C and type I phosphatidylinositol-4-phosphate 5-kinase are also regulated by PA in mammalian cells. Thus, PA and its metabolic products play a central role in modulating endothelial and epithelial cell functions.     

Lysophosphatidic acid (LPA) stimulates mouse mammary epithelial cell growth

LPA (lysophosphatidic acid) is a bioactive phospholipid having diverse effects on various types of tissues. When NMuMG (normal murine mammary gland) cells were cultured in the presence of 0-10 μM LPA, cell numbers were increased by dose dependency for the 6-day culture periods (P<0.05). In DNA synthesis assay, 10 μM LPA induced 4.5-fold more DNA synthesis compared with control (P<0.05). In addition, the cultured cell density in the given area was increased by LPA treatment. MMP (matrix metalloproteinase) inhibitor GM6001 and EGFR [EGF (epidermal growth factor) receptor] tyrosine kinase inhibitor AG1478 [tyrphostin AG1478, 4-(3-chloroanilino)-6,7-dimethoxyquinazoline] significantly decreased LPA-induced DNA synthesis and cell growth without cell death (P<0.05). To test the hypothesis that LPA-induced cell growth is mediated through LPA subtype receptors, LPA subtype receptor gene expressions were amplified by PCR. NMuMG cells expressed LPA1 and LPA2 receptor genes in the presence of 10% FBS (fetal bovine serum). LPA treatments increased ERK1/2 (extracellular-signal-regulated kinase) phosphorylation at 30 min and then dephosphorylated at 2 h after treatment. LPA treatment phosphorylated at tyrosine residues on a variety of Gi and PI3-dependent signal transducers in NMuMG cells. These results suggest that LPA subtype receptors play a role as the active transactivator of EGFR-associated kinases as well as direct growth regulator in mammary tissues.