New developments in phospholipase D

Developmental changes in phospholipase D activity and mRNA levels in rat brain

Phospholipase D (PLD) activity and PLD1 mRNA levels were determined in rat brain at ages ranging from embryonic day (E) 19 to postnatal day (P) 49. Basal, oleate-, and phosphatidylinositol-4, 5-bisphosphate-stimulated PLD activity increased between E19 and P24 by approximately 3-fold and remained unaltered thereafter. A similar developmental pattern of mRNA levels of PLD1 isoform was found by Northern blotting. The development of PLD correlates with synaptogenesis and myelination suggesting that the enzyme might have an important function in these processes.

Analysis of platelet-derived growth factor-induced phospholipase D activation in mouse embryo fibroblasts lacking phospholipase C-gamma1

Platelet-derived growth factor (PDGF) activates phospholipase D (PLD) in mouse embryo fibroblasts (MEFs). In order to investigate a role for phospholipase C-gamma1 (PLC-gamma1), we used targeted disruption of the Plcg1 gene in the mouse to develop Plcg1(+/+) and Plcg1(-/-) cell lines. Plcg1(+/+) MEFs treated with PDGF showed a time- and dose-dependent increase in the production of total inositol phosphates that was substantially reduced in Plcg1(-/-) cells. Plcg1(+/+) cells also showed a PDGF-induced increase in PLD activity that had a similar dose dependence to the PLC response but was down-regulated after 15 min. Phospholipase D activity, however, was markedly reduced in Plcg1(-/-) cells. The PDGF-induced inositol phosphate formation and the PLD activity that remained in the Plcg1(-/-) cells could be attributed to the presence of phospholipase C-gamma2 (PLC-gamma2) in the Plcg1(-/-) cells. The PLC-gamma2 expressed in the Plcg1(-/-) cells was phosphorylated on tyrosine in response to PDGF treatment, and a small but significant fraction of the Plcg1(-/-) cells showed Ca2+ mobilization in response to PDGF, suggesting that the PLC-gamma2 expressed in the Plcg1(-/-) cells was activated in response to PDGF. The inhibition of PDGF-induced phospholipid hydrolysis in Plcg1(-/-) cells was not due to differences in the level of PDGF receptor or in the ability of PDGF to cause autophosphorylation of the receptor. Upon treatment of the Plcg1(-/-) cells with oleoylacetylglycerol and the Ca2+ ionophore ionomycin to mimic the effect of PLC-gamma1, PLD activity was restored. The targeted disruption of Plcg1 did not result in universal changes in the cell signaling pathways of Plcg1(-/-) cells, because the phosphorylation of mitogen-activated protein kinase was similar in Plcg1(+/+) and Plcg1(-/-) cells. Because increased plasma membrane ruffles occurred in both Plcg1(+/+) and Plcg1(-/-) cells following PDGF treatment, it is possible neither PLC nor PLD are necessary for this growth factor response. In summary, these data indicate that PLC-gamma is required for growth factor-induced activation of PLD in MEFs.

Effects of mastoparan B and its analogs on the phospholipase D activity in L1210 cells

Mastoparan B (MP-B), an amphiphilic alpha-helical peptide isolated from hornet venom, and its Ala-substituted analogs were examined for their effectiveness on phospholipase D (PLD) activity in L1210 cells. PLD activity was determined by measuring phosphatidylethanol produced from [3H]myristate-labelled cells in the presence of ethanol. PLD activity was stimulated by MP-B, 4MP-B (Lys4-->Ala), and 12MP-B (Lys12-->Ala), but not by 3MP-B (Leu3-->Ala) and 9MP-B (Trp9-->Ala). Other MPs including mastoparan 7 also stimulated the PLD activity, but inactive mastoparan 17 did not. The stimulatory effect of various MP analogs could be correlated with their alpha-helical contents. The PLD activity stimulated by MP-B was not affected by G-protein blocking chemicals. The extent of PLD stimulation by various MP-Bs, as well as by digitonin and beta-escin, correlated with the permeability of the membrane to ethidium bromide. These results suggest that the stimulation of PLD activity by MP-B in L1210 cells is probably coupled with membrane perturbation brought about by the peptide.

Phospholipase D is associated in a phorbol ester-dependent manner with protein kinase C-alpha and with a 220-kDa protein which is phosphorylated on serine and threonine

Many studies have shown that phospholipase D (PLD) is activated by protein kinase C (PKC) in vivo and in vitro. In this study, a PLD isoform (rPLD1) was shown to bind to PKC-alpha in Rat1 fibroblasts treated with phorbol ester. The PKC-alpha binding domain of rPLD1 was localized to its N-terminus. The phospholipase was shown to become associated also with a 220-kDa protein (p220) in the fibroblasts and in Sf9 cells infected with recombinant baculovirus coding rPLD1. This interaction was increased by phorbol myristate acetate (PMA) treatment. p220 was phosphorylated on serine/threonine in PMA-stimulated Rat1 cells, and rPLD1 expressed in Sf9 cells was also serine/threonine phosphorylated in response to PMA treatment. These data suggest the PMA induces the formation of a RPLD1/PKC alpha/P220 complex in cells, some components of which undergo serine/threonine phosphorylation.

GnRH signalling pathways and GnRH-induced homologous desensitization in a gonadotrope cell line (alphaT3-1)

Exposure of the gonadotrope cells to gonadotropin-releasing hormone (GnRH) reduces their responsiveness to a new GnRH stimulation (homologous desensitization). The time frame as well as the mechanisms underlying this phenomenon are yet unclear. We studied in a gonadotrope cell line (alphaT3-1) the effects of short as well as long term GnRH pretreatments on the GnRH-induced phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities, by measuring the production of IP3, total inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that although rapid desensitization of GnRH-induced IP3 formation did not occur in these cells, persistent stimulation of cells with GnRH or its analogue resulted in a time-dependent attenuation of GnRH-elicited IPs formation. GnRH-induced IPs desensitization was potentiated after direct activation of PKC by the phorbol ester TPA, suggesting the involvement of distinct mechanisms in the uncoupling exerted by either GnRH or TPA on GnRH-stimulated PI hydrolysis. The levels of individual phosphoinositides remained unchanged under any desensitization condition applied. Interestingly, while the GnRH-induced PLA2 activity was rapidly desensitized (2.5 min) after GnRH pretreatments, the neuropeptide-evoked PLD activation was affected at later times, indicating an important time-dependent contribution of these enzymatic activities in the sequential events underlying the GnRH-induced homologous desensitization processes in the gonadotropes. Under GnRH desensitization conditions, TPA was still able to induce PLD activation and to further potentiate the GnRH-evoked PLD activity. AlphaT3-1 cells possess several PKC isoforms which, except PKCzeta, were differentially down-regulated by TPA (PKCalpha, betaII, delta, epsilon, eta) or GnRH (PKCbetaII, delta, epsilon, eta). In spite of the presence of PKC inhibitors or down-regulation of PKC isoforms by TPA, the desensitizing effect of the neuropeptide on GnRH-induced IPs, AA and PEt formation remained unchanged. In conclusion, in alphaT3-1 cells the GnRH-induced homologous desensitization affects the GnRH coupling with PLC, PLA2 and PLD by mechanism(s) which do not implicate TPA-sensitive PKC isoforms, but likely reflect time-dependent modification(s) on the activation processes of the enzymes.

Secretory vesicle budding from the trans-Golgi network is mediated by phosphatidic acid levels

Phospholipid metabolism plays a central role in regulating vesicular traffic in the secretory pathway. In mammalian cells, activation of a Golgi-associated phospholipase D activity by ADP-ribosylation factor results in hydrolysis of phosphatidylcholine to phosphatidic acid (PA). This reaction has been proposed to stimulate nascent secretory vesicle budding from the trans-Golgi network. It is unclear whether PA itself or diacylglycerol (DAG), a metabolite implicated in yeast secretory vesicle formation, regulates budding. To distinguish between these possibilities we have used a permeabilized cell system supplemented with phospholipid-modifying enzymes that generate either DAG or PA. The data demonstrate that in mammalian cells accumulation of PA rather than DAG is a key step in regulating budding of secretory vesicles from the trans-Golgi network.

Molecular cloning of a gene encoding phospholipase D from the pathogenic and dimorphic fungus, Candida albicans

A phospholipase D gene (CaPLD) has been cloned from the Candida albicans genomic DNA library. The CaPLD is a member of a highly conserved gene family of PLD and has the highest homology to Saccharomyces cerevisiae PLD (SPO14) with an overall homology of 42%. Phylogenetic analysis indicated that fungus PLDs including CaPLD composed one of the three clusters of PLD genes.

A role for phospholipase D (Pld1p) in growth, secretion, and regulation of membrane lipid synthesis in yeast

The SEC14 gene encodes a phosphatidylinositol/phosphatidylcholine transfer protein essential for secretion and growth in yeast (1). Mutations (cki1, cct1, and cpt1) in the CDP-choline pathway for phosphatidylcholine synthesis suppress the sec14 growth defect (2), permitting sec14(ts) cki1, sec14(ts) cct1, and sec14(ts) cpt1 strains to grow at the sec14(ts) restrictive temperature. Previously, we reported that these double mutant strains also excrete the phospholipid metabolites, choline and inositol (3). We now report that these choline and inositol excretion phenotypes are eliminated when the SPO14 (PLD1) gene encoding phospholipase D1 is deleted. In contrast to sec14(ts) cki1 strains, sec14(ts) cki1 pld1 strains are not viable at the sec14(ts) restrictive temperature and exhibit a pattern of invertase secretion comparable with sec14(ts) strains. Thus, the PLD1 gene product appears to play an essential role in the suppression of the sec14(ts) defect by CDP-choline pathway mutations, indicating a role for phospholipase D1 in growth and secretion. Furthermore, sec14(ts) strains exhibit elevated Ca2+-independent, phophatidylinositol 4,5-bisphosphate-stimulated phospholipase D activity. We also propose that phospholipase D1-mediated phosphatidylcholine turnover generates a signal that activates transcription of INO1, the structural gene for inositol 1-phosphate synthase.

Activation of phospholipase D1 by direct interaction with ADP-ribosylation factor 1 and RalA

Phospholipase D1 (PLD1) is known to be activated by ADP-ribosylation factor 1 (ARF1). We report here that ARF1 co-immunoprecipitates with PLD1 and that the ARF1-dependent PLD activation is induced by the direct interaction between ARF1 and PLD1. We found that RalA, another member of the small GTP-binding proteins, synergistically enhances the ARF1-dependent PLD activity with an EC50 of about 30 nM. Using in vitro binding assay, we show that ARF1 and RalA directly interact with different sites of PLD1. The results suggest that the independent interactions of RalA and ARF1 with PLD1 are responsible for the synergistic activation.

Phospholipase D activity is altered in X-linked adrenoleukodystrophy heterozygous carriers, but not in hemizygous patients

Abnormalities in levels of choline and its metabolites have been reported in the lesions of brains of X-linked adrenoleukodystrophy (X-ALD) patients. We have examined the turnover of the major choline-containing phospholipid, phosphatidylcholine (PtdCho), in fibroblasts from hemizygous X-ALD, heterozygous X-ALD, Zellweger syndrome (ZW), and male and female control individuals to assess possible alterations in PtdCho metabolism mediated by activation of protein kinase C (PKC). Hydrolysis of PtdCho by phospholipase D (PLD) and resynthesis of PtdCho from labeled choline were stimulated 2- to 4-fold by PKC activation with the phorbol ester, 4beta-12-O-tetradecanoylphorbol-13-acetate (beta-TPA), in all cells except those from heterozygous X-ALD individuals. No differences in quantity or intracellular distribution of PKC activity, PKC isoforms by Western blot analysis, or of the PKC substrate, myristoylated alanine-rich C kinase substrate (MARCKS), were apparent in any of the cells. Thus, altered PtdCho metabolism was not directly linked to either of these inherited defects that result in abnormal peroxisomal functions. Further, altered responsiveness of PLD in X-ALD heterozygotes was independent of changes in PKC and MARCKS.

Effects of propranolol on phosphatidate phosphohydrolase and mitogen-activated protein kinase activities in A7r5 vascular smooth muscle cells

High doses of propranolol inhibit phosphatidate phosphohydrolase (PAP) activity in intact cells, thus blocking metabolism of phosphatidic acid (PA), product of the phospholipase D (PLD) reaction. Vasopressin and phorbol ester activate PLD and ERK (extracellular signal-regulated protein kinase) mitogen-activated protein kinases in A7r5, a rat vascular smooth muscle cell line. Propranolol increased PA levels in intact A7r5 cells and inhibited cytosolic PAP and membrane calcium-independent phospholipase A2 but did not activate PLD or enhance agonist-induced PA accumulation. Incubation of cells with 200 microM propranolol for 10-45 min markedly elevated PA but caused only partial activation of ERKs. Propranolol and other lipophilic amines caused a time- and dose-dependent detachment of cells from their substrate. These results confirm that elevation of PA is not a strong signal for ERK activation and emphasize that caution should be exercised in using propranolol as a PAP inhibitor in intact cells.

Overexpression of myristoylated alanine-rich C-kinase substrate enhances activation of phospholipase D by protein kinase C in SK-N-MC human neuroblastoma cells

Signal transduction can involve the activation of protein kinase C (PKC) and the subsequent phosphorylation of protein substrates, including myristoylated alanine-rich C kinase substrate (MARCKS). Previously we showed that stimulation of phosphatidylcholine (PtdCho) synthesis by PMA in SK-N-MC human neuroblastoma cells required overexpression of MARCKS, whereas PKCalpha alone was insufficient. We have now investigated the role of MARCKS in PMA-stimulated PtdCho hydrolysis by phospholipase D (PLD). Overexpression of MARCKS enhanced PLD activity 1.3-2.5-fold compared with vector controls in unstimulated cells, and 3-4-fold in cells stimulated with 100 nM PMA. PMA-stimulated PLD activity was blocked by the PKC inhibitor bisindolylmaleimide. Activation of PLD by PMA was linear with time to 60 min, whereas stimulation of PtdCho synthesis by PMA in clones overexpressing MARCKS was observed after a 15 min time lag, suggesting that the hydrolysis of PtdCho by PLD preceded synthesis. The formation of phosphatidylbutanol by PLD was greatest when PtdCho was the predominantly labelled phospholipid, indicating that PtdCho was the preferred, but not the only, phospholipid substrate for PLD. Cells overexpressing MARCKS had 2-fold higher levels of PKCalpha than in vector control cells analysed by Western blot analysis; levels of PKCbeta and PLD were similar in all clones. The loss of both MARCKS and PKCalpha expression at higher subcultures of the clones was paralleled by the loss of stimulation of PLD activity and PtdCho synthesis by PMA. Our results show that MARCKS is an essential link in the PKC-mediated activation of PtdCho-specific PLD in these cells and that the stimulation of PtdCho synthesis by PMA is a secondary response.

Diacylglycerols and phosphatidates: which molecular species are intracellular messengers?

In eukaryotes, many receptor agonists use phospholipase-generated lipids as intracellular messengers. Receptor occupation stimulates the production of polyunsaturated 1,2-diacylglycerols by phosphatidylinositol-4,5-bisphosphate specific phospholipases C and/or of mono-unsaturated and saturated phosphatidates by phospholipase-D-catalysed phosphatidylcholine breakdown. The primary phospholipase products are rapidly metabolized: polyunsaturated 1,2-diacylglycerols are converted to polyunsaturated phosphatidates by diacylglycerol kinase; mono-unsaturated and saturated phosphatidates are dephosphorylated to give mono-unsaturated and saturated 1,2-diacylglycerols by phosphatidate phosphohydrolase. The phospholipase-generated polyunsaturated 1,2-diacylglycerols and mono-unsaturated and saturated phosphatidates appear to be intracellular messengers, whereas their immediate metabolites probably do not have signalling functions.

ADP-ribosylation factor and Rho proteins mediate fMLP-dependent activation of phospholipase D in human neutrophils

Activation of intact human neutrophils by fMLP stimulates phospholipase D (PLD) by an unknown signaling pathway. The small GTPase, ADP-ribosylation factor (ARF), and Rho proteins regulate the activity of PLD1 directly. Cell permeabilization with streptolysin O leads to loss of cytosolic proteins including ARF but not Rho proteins from the human neutrophils. PLD activation by fMLP is refractory in these cytosol-depleted cells. Readdition of myr-ARF1 but not non-myr-ARF1 restores fMLP-stimulated PLD activity. C3 toxin, which inactivates Rho proteins, reduces the ARF-reconstituted PLD activity, illustrating that although Rho alone does not stimulate PLD activity, it synergizes with ARF. To identify the signaling pathway to ARF and Rho activation by fMLP, we used pertussis toxin and wortmannin to examine the requirement for heterotrimeric G proteins of the Gi family and for phosphoinositide 3-kinase, respectively. PLD activity in both intact cells and the ARF-restored response in cytosol-depleted cells is inhibited by pertussis toxin, indicating a requirement for Gi2/Gi3 protein. In contrast, wortmannin inhibited only fMLP-stimulated PLD activity in intact neutrophils, but it has no effect on myr-ARF1-reconstituted activity. fMLP-stimulated translocation of ARF and Rho proteins to membranes is not inhibited by wortmannin. It is concluded that activation of Gi proteins is obligatory for ARF/Rho activation by fMLP, but activation of phosphoinositide 3-kinase is not required.

Phosphatidic acid-induced elevation of intracellular Ca2+ is mediated by RhoA and H2O2 in Rat-2 fibroblasts

We have investigated possible roles of RhoA and H2O2 in the elevation of intracellular Ca2+ ([Ca2+]i) by phosphatidic acid (PA) in Rat-2 fibroblasts. PA induced a transient elevation of [Ca2+]i in the presence or absence of EGTA. Lysophosphatidic acid (LPA) also increased [Ca2+]i, but the sustained Ca2+ response was inhibited by EGTA. LPA stimulated the production of inositol phosphates, but PA did not. In the presence of EGTA, preincubation with LPA completely blocked the subsequent elevation of [Ca2+]i by PA, but not vice versa. PA stimulated the translocation of RhoA to the particulate fraction as did LPA. Scrape loading of C3 transferase inhibited the transient Ca2+ response to PA, but not to LPA, suggesting an essential role of RhoA in the elevation of [Ca2+]i by PA. H2O2 also induced a transient increase of [Ca2+]i as did PA. H2O2 scavengers, catalase and N-acetyl-L-cysteine, completely blocked the rise of [Ca2+]i stimulated by PA, but not by LPA. Furthermore, preincubation with PA blocked the subsequent Ca2+ response to H2O2, and the incubation with H2O2 also blocked the PA-induced rise of [Ca2+]i. Thus, it was suggested that PA stimulated Ca2+ release from PA-sensitive, but not inositol 1,4,5-trisphosphate-sensitive, Ca2+ stores by the activation of RhoA and intracellular H2O2.

Phorbol ester stimulation of phosphatidylcholine synthesis requires expression of both protein kinase C-alpha and phospholipase D

The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) stimulates both the synthesis and phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho). Here, attached and suspended NIH 3T3 fibroblasts as well as variants of the MCF-7 human breast carcinoma cell line expressing PKC-alpha and a PtdCho-specific PLD activity at widely different levels were used to determine the possible role of PKC-alpha, PtdCho hydrolysis, and choline uptake in the mediation of PMA effect on PtdCho synthesis. In wild-type MCF-7 cells, which express both PKC-alpha and PLD activities at very low levels, PMA had little effects on the uptake or incorporation [14C]choline into PtdCho. In multidrug resistant MCF-7/MDR1 cells, which highly express PKC-alpha but lack the PtdCho-specific PLD activity, 100-nM PMA had relatively small stimulatory effects on the uptake of [14C]choline (approximately 1.5-fold) and [14C]PtdCho synthesis (1.5- to 2-fold). In NIH 3T3 fibroblasts and MCF-7/PKC-alpha cells, both expressing PKC-alpha and PLD activities at high levels, 10-100-nM PMA enhanced [14C]choline uptake only slightly (1.7- to 2.2-fold), while it had much greater (approximately 4-9-fold) stimulatory effects on PtdCho synthesis. PMA significantly enhanced the formation of phosphatidic acid (PtdOH) in MCF-7/PKC-alpha cells (2.8-fold increase), but not in MCF-7/MDR1 cells (1.4-fold increase), while in both cell lines it had only small (1.3-1.5-fold) stimulatory effects on 1,2-diacylglycerol (1, 2-DAG) formation. In suspended NIH 3T3 cells, 200-300-mM ethanol blocked the stimulatory effect of PMA on PtdOH formation without affecting PtdCho synthesis indicating that neither PtdOH nor 1,2-DAG derived from it is a mediator of PMA effect on PtdCho synthesis. In attached NIH 3T3 cells, dimethylbenz[a]anthracene enhanced phosphocholine formation and, thus, choline uptake without increasing PtdCho synthesis or modifying the effect of PMA. While the results indicate that the stimulatory effect of PMA on PtdCho synthesis requires the expression of both PKC-alpha and a PtdCho-specific PLD, they do not support a role for 1,2-DAG, PtdOH or choline in the mediation of PMA effect.

Activation of the small GTPase Ral in platelets

Ral is a ubiquitously expressed Ras-like small GTPase which is abundantly present in human platelets. The biological function of Ral and the signaling pathway in which Ral is involved are largely unknown. Here we describe a novel method to measure Ral activation utilizing the Ral binding domain of the putative Ral effector RLIP76 as an activation-specific probe. With this assay we investigated the signaling pathway that leads to Ral activation in human platelets. We found that Ral is rapidly activated after stimulation with various platelet agonists, including alpha-thrombin. In contrast, the platelet antagonist prostaglandin I2 inhibited alpha-thrombin-induced Ral activation. Activation of Ral by alpha-thrombin could be inhibited by depletion of intracellular Ca2+, whereas the induction of intracellular Ca2+ resulted in the activation of Ral. Our results show that Ral can be activated by extracellular stimuli. Furthermore, we show that increased levels of intracellular Ca2+ are sufficient for Ral activation in platelets. This activation mechanism correlates with the activation mechanism of the small GTPase Rap1, a putative upstream regulator of Ral guanine nucleotide exchange factors.

Phosphatidylinositol transfer proteins: a requirement in signal transduction and vesicle traffic

Phosphatidylinositol transfer protein (PITP) was originally identified and named because of its ability to transport phosphatidylinositol through the aqueous phase from one membrane compartment to another. Recent data, however, indicate unanticipated roles for PITP in the coupling of PIP2 synthesis to signal transduction reactions and to membrane traffic in mammalian cells. PITP was recently purified on the basis of its ability to restore cellular functions in permeabilized cells depleted of cytosolic proteins. These functions include cell-surface receptor-regulated hydrolysis of PIP2 by phospholipases C beta- and gamma-isozymes, regulated release of secretory granules, and the budding of constitutive secretory vesicles and immature secretory granules from the trans-Golgi network. In the yeast Saccharomyces cerevisiae, a PITP was identified from a mutant strain with a defect in the secretory pathway (SEC14) and therefore required for cell viability; in Yarrowia lipolytica, PITP is required for differentiation from a yeast to a mycelial growth form. We are just beginning to unravel the intriguing mechanisms by which PITP/SEC14 may accomplish its function in eukaryotic cells in signal transduction and membrane trafficking.

Formation of lithiated adducts of glycerophosphocholine lipids facilitates their identification by electrospray ionization tandem mass spectrometry

Electrospray ionization (ESI) tandem mass spectrometry (MS) has simplified analysis of phospholipid mixtures, and, in negative ion mode, permits structural identification of picomole amounts of phospholipid species. Collisionally activated dissociation (CAD) of phospholipid anions yields negative ion tandem mass spectra that contain fragment ions representing the fatty acid substituents as carboxylate anions. Glycerophosphocholine (GPC) lipids contain a quaternary nitrogen moiety and more readily form cationic adducts than anionic species, and positive ion tandem mass spectra of protonated GPC species contain no abundant ions that identify fatty acid substituents. We report here that lithiated adducts of GPC species are readily formed by adding lithium hydroxide to the solution in which phospholipid mixtures are infused into the ESI source. CAD of [MLi+] ions of GPC species yields tandem mass spectra that contain prominent ions representing losses of the fatty acid substituents. These ions and their relative abundances can be used to assign the identities and positions of the fatty acid substituents of GPC species. Tandem mass spectrometric scans monitoring neutral losses of the head-group or of fatty acid substituents from lithiated adducts can be used to identify GPC species in tissue phospholipid mixtures. Similar scans monitoring parents of specific product ions can also be used to identify the fatty acid substituents of GPC species, and this facilitates identification of distinct isobaric contributors to ions observed in the ESI/MS total ion current.

Organization and alternative splicing of the murine phospholipase D2 gene

Phospholipase D (PLD) catalyses the hydrolysis of phosphatidylcholine, generating phosphatidic acid and choline. Mammalian PLD activity derives from a family of membrane-associated enzymes that are activated by a wide variety of signal transduction events. cDNA species encoding human, mouse and rat PLD1 and PLD2 have recently been reported. In this study we undertook to determine the organization of the mouse PLD2 gene. We report that the gene spans 17.1 kb and contains 25 exons. Mouse PLD2 is notable for a relatively GC-rich and large 5' untranslated region. Proximal promoter sequences upstream of the first exon contain several consensus SP1 sequences (GGGCGG) but lack TATA and CAAT boxes. Finally, alternatively spliced cDNA species identified for PLD1 and PLD2 are discussed in the context of the PLD2 genomic organization.

Regulation of phospholipase D2: selective inhibition of mammalian phospholipase D isoenzymes by alpha- and beta-synucleins

Two widely expressed mammalian phosphatidylcholine (PC)-specific phospholipases D (PLD), PLD1 and PLD2, have been identified. Recombinantly expressed PLD2 has high basal activity and is insensitive to GTP-binding protein activators of PLD1 [Colley, W. C., et al. (1997) Curr. Biol. 7, 191-201]. To investigate the regulation of PLD2 we isolated PLD2, from mouse brain by immunoaffinity chromatography. The native and recombinant proteins have indistinguishable properties: PLD2 is potently activated by phosphoinositides with a vicinal 4,5-phosphate pair but is not stimulated by guanosine 5'-O-(3-thio triphosphate)-activated ADP-ribosylation factor-1, Rho family GTP-binding proteins, or protein kinases C-alpha, or -beta1. We used recombinant PLD2 in a reconstitution assay to search for regulators in cell and tissue extracts. Bovine brain contains a heat-stable protein factor that inhibits PLD2 activity in vitro. This factor was purified to homogeneity and identified as a mixture of alpha- and beta-synucleins by microsequencing and Western blotting. Recombinantly expressed alpha- and beta-synucleins inhibit PLD2 activity in vitro (K0.5 10 nM). Inhibition is not overcome by the protein or lipid activators of PLD1. Synucleins have been implicated in Parkinson's and Alzheimer's diseases. Our findings suggest that inhibition of PLD2 may be a function of synucleins. Modulation of PLD2 activity by synucleins may play a role in some aspects of the pathophysiologies that characterize these neurodegenerative diseases.

Decreased phospholipase D (PLD) activity in ceramide-induced apoptosis of human keratinocyte cell line HaCaT

Ceramide is recognized as an intracellular lipid second messenger, which induces various kinds of cell function including apoptosis. To evaluate the competence of ceramide on the keratinocyte apoptosis, we examined effects of a cell-permeable ceramide, N-acetylsphingosine (C2-ceramide), on a human keratinocyte cell line, HaCaT. C2-ceramide induced a distinct apoptosis in HaCaT cells in a time-dependent manner, as inferred by morphologic hallmarks of apoptosis such as bleb formation, cell body shrinkage, nuclear chromatin condensation, and internucleosomal DNA fragmentation. In sharp contrast, an inactive C2-ceramide, dihydroC2-ceramide, which lacks the 4-5trans double bond, failed to induce the apoptosis. The apoptotic HaCaT cells induced by C2-ceramide showed a significant suppression of phospholipase D (PLD) activity, regardless of the presence or absence of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). This indicates that C2-ceramide inhibits both GTPgammaS dependent and GTPgammaS independent PLD. The membrane associated GTPgammaS dependent PLD activity was stimulated by recombinant adenosine diphosphate-ribosylation factor. The adenosine diphosphate-ribosylation factor dependent and independent PLD activities were inhibited by C2-ceramide in a concentration dependent manner, but not by the inactive C2-ceramide. The concentration of C2-ceramide to inhibit the membrane associated PLD activity was comparable with that required for apoptosis induction in HaCaT cells. It was thus suggested that downregulation of PLD activity may be involved in the mechanism underlying C2-ceramide induced keratinocyte apoptosis.

Functional association between Arf and RalA in active phospholipase D complex

Activation of phospholipase D1 (PLD1) by Arf has been implicated in vesicle transport and membrane trafficking. PLD1 has also been shown to be associated with the small GTPase RalA, which functions downstream from Ras in a Ras-RalA GTPase cascade that facilitates intracellular signal transduction. Although PLD1 associates directly with RalA, RalA has no effect upon the activity of PLD1. However, PLD1 precipitated from cell lysates with immobilized glutathione S-transferase-RalA fusion protein is active. This suggests the presence of an additional activating factor in the active RalA-PLD1 complexes. Because Arf stimulates PLD1, we looked for the presence of Arf in the active RalA-PLD1 complexes isolated from v-Src- and v-Ras-transformed cell lysates. Low levels of Arf protein were detected in RalA-PLD1 complexes; however, if guanosine 5'-[gamma-thio]triphosphate was added to activate Arf and stimulate translocation to the membrane, high levels of Arf were precipitated by RalA from cell lysates. Interestingly, deletion of 11 amino-terminal amino acids unique to Ral GTPases, which abolished the ability of RalA to precipitate PLD activity, prevented the association between RalA and Arf. Brefeldin A, which inhibits Arf GDP-GTP exchange, inhibited PLD activity in v-Src- and v-Ras-transformed cells but not in the nontransformed cells, suggesting that the association of Arf with RalA is required for the increased PLD activity induced by v-Src and v-Ras. These data implicate Arf in the transduction of intracellular signals activated by v-Src and mediated by the Ras-RalA GTPase cascade. Because both Arf and PLD1 stimulate vesicle formation in the Golgi, these data raise the possibility that vesicle formation and trafficking may play a role in the transduction of intracellular signals.

Lipid metabolism in fibroblast growth factor-stimulated L6 myoblasts: a receptor mutation (Y766F) abrogates phospholipase D and diacylglycerol kinase activities

Phosphatidylcholine (PC) hydrolysis induced by basic fibroblast growth factor (bFGF) was studied in rat L6 myoblasts expressing the wild-type FGF receptor-1 (FGFR-1) or a mutant (Y766F) that is incapable of activating phospholipase C-gamma (PLCgamma). Stimulation of FGFR-1 activated phospholipase D (PLD) rapidly and transiently, but did not induce PC-specific PLC activity. Downregulation of protein kinase C blocked bFGF-induced PLD activation but not phosphatidic acid formation by diacylglycerol (DG) kinase. Only phosphoinositide (PI)-derived DG, not PC-derived DG, appeared to be a substrate for DG kinase. Stimulation of FGFR-1(Y766F) did not activate PLD or DG kinase, both of which apparently require initial PLCgamma activation. The Y766F mutation reduced mitogen-activated protein kinase activation but not cell proliferation. We conclude that both PI turnover and PC hydrolysis are dispensable for bFGF-induced mitogenesis.

A role for phospholipase D activation in the lipid signalling cascade generated by bradykinin and thrombin in C2C12 myoblasts

In the present study evidence is provided for a rapid activation of lipid signalling pathways induced by thrombin and bradykinin (BK) in C2C12 myoblasts. Both agonists were able to increase [3H]inositol phosphates (InsP), 1,2-[3H]diacylglycerol (DAG) and [3H]phosphatidic acid (PtdOH) levels. In particular [3H]PtdOH values were rapidly increased and maintained at significantly high values at prolonged times of incubation. BK and thrombin were able to activate phospholipase D (PLD) in vivo as demonstrated by the accumulation of [3H]phosphatidylethanol (PtdEtOH) through the transphoshatidylation reaction catalyzed by the enzyme in the presence of ethanol. The observation that ethanol could significantly reduce [3H]PtdOH formation in myoblasts stimulated with BK and thrombin indicates that stimulation of PLD has a major role. The two agonists appear to stimulate PLD activity through a common molecular mechanism, involving the activation of protein kinase C (PKC). In addition, BK and thrombin appear able to activate DAG kinase at early times of incubation and also this pathway may contribute to determine the increase in [3H]PtdOH levels. This is the first report which describes activation of lipid signalling pathways by BK and thrombin in myoblast cells and it is possible that these early signals may have an important role in mediating the biological effects of the two agonists.

Phospholipase D activity in L1210 cells: a model for oleate-activated phospholipase D in intact mammalian cells

Phospholipase D (PLD) in lymphocytic mouse leukemic L1210 cells has been found to be activated by oleate both in vitro and in intact cells. The PLD activity was measured by phosphatidylethanol produced from radiolabeled phosphatidylcholine or myristic acid in the presence of ethanol. This oleate-activated PLD was further characterized in intact cells and compared with that in HL60 cells. Unlike PLD in HL60 cells, the PLD in L1210 cells was activated by unsaturated fatty acids, stimulated by melittin, insensitive to guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), ADP-ribosylation factor (ARF) and phosphatidylinositol 4,5-bisphosphate (PIP2), independent of phorbol 12-myristate 13-acetate (PMA) and staurosporine, and inhibited by pervanadate. These observations indicate that the PLD present in L1210 cells is distinct from that in HL60 cells. Key PLD properties of L1210 cells such as insensitivity to GTP gamma S, ARF, PIP2, or PMA were in good agreement with currently known in vitro properties of the oleate-activated PLD found in mammalian sources. Therefore, the L1210 cells could be used as an intact-cell source for an oleate-activated PLD.

Specific inhibition of phorbol ester-stimulated phospholipase D by Clostridium sordellii lethal toxin and Clostridium difficile toxin B-1470 in HEK-293 cells. Restoration by Ral GTPases

Activation of m3 muscarinic acetylcholine receptor (mAChR), stably expressed in human embryonic kidney (HEK)-293 cells, leads to phospholipase D (PLD) stimulation, a process apparently involving Rho GTPases, as shown by studies with Clostridium botulinum C3 exoenzyme and Clostridium difficile toxin B (TcdB). Direct activation of protein kinase C (PKC) by phorbol esters, such as phorbol 12-myristate 13-acetate (PMA), also induces PLD stimulation, which is additive to the mAChR action and which is only poorly sensitive to inactivation of Rho proteins by TcdB. To study whether Ras-like GTPases are involved in PLD regulation, we studied the effects of the TcdB variant TcdB-1470 and Clostridium sordellii lethal toxin (TcsL), known to inactivate Rac and some members of the Ras protein family, on PLD activities. TcdB-1470 and TcsL did not affect basal PLD activity and PLD stimulation by mAChR or direct G protein activation. In contrast, PMA-induced PLD stimulation was inhibited by TcdB-1470 and TcsL in a time- and concentration-dependent manner, without alteration in immunologically detectable PKC isozyme levels. In membranes of HEK-293 cells pretreated with TcdB-1470 or TcsL, basal and stable GTP analog-stimulated PLD activities measured with exogenous phosphatidylcholine, in the presence or absence of phosphatidylinositol 4,5-bisphosphate, were not altered. In contrast, pretreatment with TcdB-1470 and TcsL, but not TcdB, strongly reduced PMA-stimulated PLD activity. The addition of recombinant Rac1, serving as glucosylation substrate for TcdB, TcsL, and TcdB-1470, did not restore PLD stimulation by PMA. Furthermore, PMA-stimulated PLD activity, suppressed by prior treatment with TcdB-1470 or TcsL, was not rescued by the addition of recombinant Ras (RasG12V) or Rap proteins, acting as glucosylation substrates for TcsL only (Ras) or TcdB-1470 and TcsL (Rap). In contrast, the addition of recombinant Ral proteins (RalA and RalB), glucosylation substrates for TscL and TcdB-1470, but not for TcdB, to membranes of TcdB-1470- or TcsL-treated cells fully restored PLD stimulation by PMA without altering the strict MgATP dependence of PMA-induced PLD stimulation. RalA-mediated restoration of PMA-stimulated PLD activity in membranes of TcsL-treated cells was not enhanced by coaddition of RasG12V. In conclusion, the data presented indicate that TcdB-1470 and TcsL selectively interfere with phorbol ester stimulation of PLD and suggest an essential role of Ral proteins in PKC signaling to PLD in HEK-293 cells.

Involvement of phosphatidate phosphohydrolase in arachidonic acid mobilization in human amnionic WISH cells

Prostaglandins are known to play a central role in the initiation of labor in humans, and amnionic cells constitute a major source of these compounds. Prostaglandin synthesis and release by amnion cells in response to hormones and ligands takes place after a characteristic 4-5 h lag. However, we report herein that free arachidonic acid (AA), the metabolic precursor of prostaglandins, can be induced at much shorter times (1 h) in human amnionic WISH cells by phorbol 12-myristate 13-acetate (PMA) through activation of protein kinase Calpha (PKCalpha). WISH cells were found to possess both cytosolic group IV phospholipase A2 (cPLA2) and Group VI Ca2+-independent phospholipase A2 (iPLA2). Of these, the cPLA2 was found to be the likely mediator of AA mobilization in PMA-activated WISH cells. PMA also activates phospholipase D (PLD) in these cells and ethanol, a compound that inhibits PLD-mediated phosphatidic acid (PA) formation, blocked AA release. Moreover, prevention of PA dephosphorylation by the PA phosphohydrolase inhibitors propranolol and bromoenol lactone, resulted in inhibition of AA release by PMA-treated WISH cells. Collectively, these data suggest that activation of cPLA2 and attendant AA release by phorbol esters in WISH cells requires prior generation of DAG by phosphatidate phosphohydrolase.

Characterization of a rat brain phospholipase D isozyme

We have recently cloned a cDNA encoding a phospholipase D (PLD) from rat brain and named it rPLD1. It shows 90% amino acid identity with the human PLD isoform hPLD1b. We have expressed rPLD1 as a histidine-tagged fusion protein in insect (Sf9) cells using the expression vector pBlueBacHis and purified the recombinant protein to homogeneity by Ni2+-agarose affinity chromatography. Phosphatidylinositol 4,5-P2 and phosphatidylinositol 3,4,5-P3 activated the PLD equipotently, but other acidic phospholipids were ineffective. The activity of rPLD1 was dependent on both Mg2+ and Ca2+. It was specific for phosphatidylcholine and showed a broad dependence on pH with optimum activity at pH 6.5-7.5. The enzyme was inhibited by oleate and activated by the small G proteins ARF3 and RhoA in the presence of guanosine 5'-3-O-(thio)triphosphate. Protein kinase C (PKC)-alpha and -betaII, but not PKC-gamma, -delta, -epsilon, or -zeta, activated rPLD1 in a manner that was stimulated by phorbol ester but did not require ATP. Neither synergistic interactions between ARF3 and RhoA nor between these G proteins and PKC-alpha or -betaII were observed. Recombinant PKC-alpha and -betaII phosphorylated purified rPLD1 to high stoichiometry in vitro, and the phosphorylated PLD exhibited a mobility shift upon electrophoresis. Phosphorylation of the PLD by PKC was correlated with inhibition of its catalytic activity. rPLD1 bound to concanavalin A-Sepharose beads, and its electrophoretic mobility was altered by treatment with endoglycosidase F. The amount of PLD bound to the beads was decreased in a concentration-dependent manner when tunicamycin was added to the Sf9 expression system. Tunicamycin also decreased membrane localization of rPLD1. These results suggest that rPLD1 is a glycosylated protein and that it is negatively regulated by phosphorylation by PKC in vitro.

The alpha-subunit of the heterotrimeric G protein G13 activates a phospholipase D isozyme by a pathway requiring Rho family GTPases

G13 belongs to the G12 family of heterotrimeric G proteins, whose effectors are poorly defined. The present study was designed to test if phospholipase D (PLD) is regulated by G13 and if Rho-type small GTPases are involved. Expression of the constitutively active Q226L mutant of the alpha-subunit of G13 in COS-7 cells stimulated the activity of a rat brain phospholipase D isozyme (rPLD1) co-expressed in the cells. Wild type Galpha13 was ineffective unless the cells were incubated with AlF4-. rPLD1 was previously shown to be activated by constitutively active V14RhoA in COS-7 cells (Park, S. K., Provost, J. J., Bae, C. D., Ho, W. T., and Exton, J. H. (1997) J. Biol. Chem. 272, 29263-29272). When the endogenous Rho proteins of the cells were inactivated by treatment with C3 exoenzyme from Clostridium botulinum, the ability of Galpha13Q226L to activate rPLD1 was greatly attenuated. Co-transfection of dominant negative N19RhoA and N17Rac-1, but not N17Cdc42Hs or N17Ras, also inhibited the activation. Expression of constitutively active Galphaq in COS-7 cells also activated rPLD1, but constitutively active forms of Galphai2 and Galphas were without effect. These findings support an effector role for PLD in G13 signaling and demonstrate a requirement for Rho GTPases in this response.

Regulated exocytosis in chromaffin cells. Translocation of ARF6 stimulates a plasma membrane-associated phospholipase D

The ADP-ribosylation factor (ARF) GTP-binding proteins have been implicated in a wide range of vesicle transport and fusion steps along the secretory pathway. In chromaffin cells, ARF6 is specifically associated with the membrane of secretory chromaffin granules. Since ARF6 is an established regulator of phospholipase D (PLD), we have examined the intracellular distribution of ARF6 and PLD activity in resting and stimulated chromaffin cells. We found that stimulation of intact chromaffin cells or direct elevation of cytosolic calcium in permeabilized cells triggered the rapid translocation of ARF6 from secretory granules to the plasma membrane and the concomitant activation of PLD in the plasma membrane. To probe the existence of an ARF6-dependent PLD in chromaffin cells, we measured the PLD activity in purified plasma membranes. PLD could be activated by a nonhydrolyzable analogue of GTP and by recombinant myristoylated ARF6 and inhibited by specific anti-ARF6 antibodies. Furthermore, a synthetic myristoylated peptide corresponding to the N-terminal domain of ARF6 inhibited both PLD activity and catecholamine secretion in calcium-stimulated chromaffin cells. The possibility that ARF6 participates in the exocytotic reaction by controlling a plasma membrane-bound PLD and thereby generating fusogenic lipids at the exocytotic sites is discussed.

Lipids of the Golgi membrane

The thin membrane of the endoplasmic reticulum matures into the thick plasma membrane in the Golgi apparatus. Along the way, the concentrations of cholesterol and sphingolipids increase. Here, Gerrit van Meer discusses how this phenomenon may reflect an intricate lipid-protein sorting machinery. Synthesis of sphingolipids, translocation across the Golgi membrane and lateral segregation into lumenal domains seem to be key events. In addition, signalling lipids indicate the lipid status of the Golgi and interact with proteins of the transport machinery to regulate membrane flux.

An Eps homology (EH) domain protein that binds to the Ral-GTPase target, RalBP1

Ral proteins constitute a family of small GTPases that can be activated by Ras in cells. In the GTP-bound state, Ral proteins bind to RalBP1, a GTPase-activating protein for CDC42 and Rac GTPases. We have used the two-hybrid system in yeast to clone a cDNA for a novel approximately 85-kDa protein that can bind to an additional site on RalBP1. This newly identified protein contains an Eps homology (EH) domain, which was first detected in the epidermal growth factor (EGF) receptor substrate Eps15. Recently, the EH domain of Eps15 has been shown to bind to proteins containing an asparagine-proline-phenylalanine motif. Moreover, EH domains have been found in proteins involved in endocytosis and/or actin cytoskeleton regulation. The RalBP1 associated Eps-homology domain protein, Reps1, is tyrosine-phosphorylated in response to EGF stimulation of cells. In addition, Reps1 has the capacity to form a complex with the SH3 domains of the adapter proteins Crk and Grb2, which may link Reps1 to an EGF-responsive tyrosine kinase. Thus, Reps1 may coordinate the cellular actions of activated EGF receptors and Ral-GTPases.

The coatomer protein beta'-COP, a selective binding protein (RACK) for protein kinase Cepsilon

Distinct subcellular localization of activated protein kinase C (PKC) isozymes is mediated by their binding to isozyme-specific RACKs (receptors for activated C-kinase). Our laboratory has previously isolated one such protein, RACK1, and demonstrated that this protein displays specificity for PKCbeta. We have recently shown that at least part of the PKCepsilon RACK-binding site on PKCepsilon lies within the unique V1 region of this isozyme (Johnson, J. A., Gray, M. O., Chen, C.-H., and Mochly-Rosen, D. (1996) J. Biol. Chem. 271, 24962-24966). Here, we have used the PKCepsilon V1 region to clone a PKCepsilon-selective RACK, which was identified as the COPI coatomer protein, beta'-COP. Similar to RACK1, beta'-COP contains seven repeats of the WD40 motif and fulfills the criteria previously established for RACKs. Activated PKCepsilon colocalizes with beta'-COP in cardiac myocytes and binds to Golgi membranes in a beta'-COP-dependent manner. A role for PKC in control of secretion has been previously suggested, but this is the first report of direct protein/protein interaction of PKCepsilon with a protein involved in vesicular trafficking.

Effects of brefeldin A on phosphatidylcholine phospholipase D and inositolphospholipid metabolism in HL-60 cells

The involvement of the small GTP-binding protein ADP-ribosylation factor (ARF) in guanosine 5'-[gamma-thio]triphosphate (GTP[S])-dependent activation of phospholipase D (PLD) in HL-60 cells has been well established in vitro. In this study, we tested the effect of brefeldin A, which prevents ARF activation by inhibiting guanine-nucleotide-exchange activity, on PLD stimulation by receptor agonists (formyl-Met-Leu-Phe and ATP) and by the phorbol ester phorbol 12-myristate 13-acetate (PMA) in differentiated HL-60 cells. However, brefeldin A did not affect the activation of PLD at a time (1 h) when it eliminated the activity of the trans-Golgi enzyme galactosyltransferase. It also did not inhibit PLD activity in Golgi-enriched membranes treated with GTP[S] with or without ARF in vitro. However, longer times of brefeldin A treatment (>6 h), progressively and completely inhibited the activation of PLD by formyl-Met-Leu-Phe and partly inhibited (approximately 50%) the activation by PMA. In contrast, long-term brefeldin A treatment did not inhibit the effect of GTP[S] on PLD in permeabilized HL-60 cells. Long-term brefeldin A treatment completely inhibited inositol phosphate production in response to formyl-Met-Leu-Phe and ATP, indicating that it affected inositolphospholipid-specific phospholipase C activity. These data indicate that the rapid inhibitory effect of brefeldin A on Golgi function is not associated with inhibition of receptor-mediated or PMA-mediated PLD activation in HL-60 cells. However, longer-term effects, presumably arising from the disruption of the Golgi, lead to a total inhibition of agonist activation of PLD and inositolphospholipid-specific phospholipase C. In summary, these results do not support a role for brefeldin-A-sensitive ARF in agonist regulation of PLD in HL-60 cells.