Mammalian phospholipase D: activation by ammonium sulfate and nucleotides

Phospholipase D is Dispensable for Epidermal Growth Factor-Induced Chemotaxis

α-Synuclein (α-Syn) is implicated in several neurodegenerative disorders, including Parkinson's disease, known collectively as the synucleinopathies. α-Syn is known to be secreted from the cells and may contribute to the progression of the disease. Although extracellular α-Syn is shown to impair platelet-derived growth factor-induced chemotaxis, molecular mechanism of α-Syn-induced motility failure remains elusive. Here we have aimed at phospholipase D (PLD) as a potential target for α-Syn and examined the involvement of this enzyme in α-Syn action. Indeed, extracellular α-Syn caused inhibition of agonist-induced PLD activation. However, inhibition of hydrolytic activity of PLD by 1-butanol treatment showed little or no effect on agonist-induced chemotaxis. These results suggest that some signaling pathways other than PLD may be involved in α-Syn-induced inhibition of chemotaxis.

Antisense 2'-Deoxy, 2'-Fluroarabino Nucleic Acids (2'F-ANAs) Oligonucleotides: In Vitro Gymnotic Silencers of Gene Expression Whose Potency Is Enhanced by Fatty Acids

Gymnosis is the process of the delivery of antisense oligodeoxynucleotides to cells, in the absence of any carriers or conjugation, that produces sequence-specific gene silencing. While gymnosis was originally demonstrated using locked nucleic acid (LNA) gapmers, 2′-deoxy-2′fluoroarabino nucleic acid (2′F-ANA) phosphorothioate gapmer oligonucleotides (oligos) when targeted to the Bcl-2 and androgen receptor (AR) mRNAs in multiple cell lines in tissue culture, are approximately as effective at silencing of Bcl-2 expression as the iso-sequential LNA congeners. In LNCaP prostate cancer cells, gymnotic silencing of the AR by a 2′F-ANA phosphorothioate gapmer oligo led to downstream silencing of cellular prostate-specific antigen (PSA) expression even in the presence of the androgenic steroid R1881 (metribolone), which stabilizes cytoplasmic levels of the AR. Furthermore, gymnotic silencing occurs in the absence of serum, and silencing by both LNA and 2′F-ANA oligos is augmented in serum-free (SF) media in some cell lines when they are treated with oleic acid and a variety of ω-6 polyunsaturated fatty acids (ω-6 PUFAs), but not by an aliphatic (palmitic) fatty acid. These results significantly expand our understanding of and ability to successfully manipulate the cellular delivery of single-stranded oligos in vitro.

CtBP1/BARS is an activator of phospholipase D1 necessary for agonist-induced macropinocytosis

Vesicular trafficking such as macropinocytosis is a dynamic process that requires coordinated interactions between specialized proteins and lipids. A recent report suggests the involvement of CtBP1/BARS in epidermal growth factor (EGF)-induced macropinocytosis. Detailed mechanisms as to how lipid remodelling is regulated during macropinocytosis are still undefined. Here, we show that CtBP1/BARS is a physiological activator of PLD1 required in agonist-induced macropinocytosis. EGF-induced macropinocytosis was specifically blocked by 1-butanol but not by 2-butanol. In addition, stimulation of cells by serum or EGF resulted in the association of CtBP1/BARS with PLD1. Finally, CtBP1/BARS activated PLD1 in a synergistic manner with other PLD activators, including ADP-ribosylation factors as demonstrated by in vitro and intact cell systems. The present results shed light on the molecular basis of how the ‘fission protein' CtBP1/BARS controls vesicular trafficking events including macropinocytosis.

Requirement of phospholipase D for ilimaquinone-induced Golgi membrane fragmentation

Although organelles such as the endoplasmic reticulum and Golgi apparatus are highly compartmentalized, these organelles are interconnected through a network of vesicular trafficking. The marine sponge metabolite ilimaquinone (IQ) is known to induce Golgi membrane fragmentation and is widely used to study the mechanism of vesicular trafficking. Although IQ treatment causes protein kinase D (PKD) activation, the detailed mechanism of IQ-induced Golgi membrane fragmentation remains unclear. In this work, we found that IQ treatment of cells caused a robust activation of phospholipase D (PLD). In the presence of 1-butanol but not 2-butanol, IQ-induced Golgi membrane fragmentation was completely blocked. In addition, IQ failed to induce Golgi membrane fragmentation in PLD knock-out DT40 cells. Furthermore, IQ-induced PKD activation was completely blocked by treatment with either 1-butanol or propranolol. Notably, IQ-induced Golgi membrane fragmentation was also blocked by propranolol treatment. These results indicate that PLD-catalyzed formation of phosphatidic acid is a prerequisite for IQ-induced Golgi membrane fragmentation and that enzymatic conversion of phosphatidic acid to diacylglycerol is necessary for subsequent activation of PKD and IQ-induced Golgi membrane fragmentation.

D5 dopamine receptor regulation of phospholipase D

D(1)-like receptors have been reported to decrease oxidative stress in vascular smooth muscle cells by decreasing phospholipase D (PLD) activity. However, the PLD isoform regulated by D(1)-like receptors (D(1) or D(5)) and whether abnormal regulation of PLD by D(1)-like receptors plays a role in the pathogenesis of hypertension are unknown. The hypothesis that the D(5) receptor is the D(1)-like receptor that inhibits PLD activity and serves to regulate blood pressure was tested using D(5) receptor mutant mice (D(5)(-/-)). We found that in the mouse kidney, PLD2, like the D(5) receptor, is mainly expressed in renal brush-border membranes, whereas PLD1 is mainly expressed in renal vessels with faint staining in brush-border membranes and collecting ducts. Total renal PLD activity is increased in D(5)(-/-) mice relative to congenic D(5) wild-type (D(5)(+/+)) mice. PLD2, but not PLD1, expression is greater in D(5)(-/-) than in D(5)(+/+) mice. The D(5) receptor agonist fenoldopam decreases PLD2, but not PLD1, expression and activity in human embryonic kidney-293 cells heterologously expressing the human D(5) receptor, effects that are blocked by the D(5) receptor antagonist SCH-23390. These studies show that the D(5) receptor regulates PLD2 activity and expression. The hypertension in the D(5)(-/-) mice is associated with increased PLD expression and activity. Impaired D(5) receptor regulation of PLD2 may play a role in the pathogenesis of hypertension.

Down-regulation of melanogenesis by phospholipase D2 through ubiquitin proteasome-mediated degradation of tyrosinase

The involvement of phospholipase D (PLD) in the regulation of melanogenesis was examined. Treatment of B16 mouse melanoma cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) resulted in the activation of PLD and a decrease in melanin content. 1-Butanol, but not 2-butanol, completely blocked the TPA-induced inhibition of melanogenesis, suggesting the involvement of PLD in this event. Reverse transcription-PCR and immunoblot analyses revealed the existence of both PLD isozymes, PLD1 and PLD2, in B16 cells. When PLD1 or PLD2 was introduced into those cells by an adenoviral gene-transfer technique, both PLD1 and PLD2 were activated by TPA. When PLD1 and PLD2 were overexpressed, PLD2 potently caused a decrease in melanin content, whereas the effect of PLD1 expression on melanin content was minimal. Over-expression of PLD2 itself did not affect protein kinase C activity, as assessed by the intracellular distribution and levels of expression of each isoform expressed in B16 cells. The effects of TPA on the down-regulation of basal or alpha-melanocyte-stimulating hormone-enhanced melanogenesis were almost completely blocked by expressing a lipase activity-negative mutant, LN-PLD2, but not by LN-PLD1. Further, the PLD2-induced decrease in melanin content was accompanied by a decrease in the amount and activity of tyrosinase, a key enzyme in melanogenesis, whereas the mRNA level of tyrosinase was unchanged by the over-expression of PLD2. Moreover, treatment with proteasome inhibitors completely blocked the PLD2-induced down-regulation of melanogenesis. Taken together, the present results indicate that the TPA-induced down-regulation of melanogenesis is mediated by PLD2 but not by PLD1 through the ubiquitin proteasome-mediated degradation of tyrosinase. This suggests that PLD2 may play an important role in regulating pigmentation in vivo.

Regulation of mammalian phospholipase D2: interaction with and stimulation by G(M2) activator

We have previously reported that a heat-stable activator for ganglioside metabolism, G(M2) activator, potently stimulates ADP-ribosylation factor (ARF)-dependent phospholipase D (PLD) activity (presumably PLD1) in an in vitro system [Nakamura, Akisue, Jinnai, Hitomi, Sarkar, Miwa, Okada, Yoshida, Kuroda, Kikkawa and Nishizuka (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 12249-12253]. However, little is known about the regulation of PLD2. In the present studies we have investigated the regulation of PLD2 by G(M2) activator and various other regulators including ARF. PLD2 was potently stimulated in vitro by G(M2) activator in a time- and dose-dependent manner. Neither ARF nor protein kinase C caused any significant changes in PLD2 activity. Importantly, PLD2 responsiveness to ARF was greatly enhanced by G(M2) activator, suggesting a possible role for G(M2) activator as a coupling factor. G(M2) activator was also demonstrated to physically associate with PLD2 in a stoichiometric manner. Further, PMA stimulation of COS-7 cells overexpressing both G(M2) activator and PLD2 resulted in a marked increase in the association of the two molecules. Interestingly, ARF association with PLD2 was greatly increased by G(M2) activator. Moreover, G(M2) activator enhanced PMA-induced PLD activity in a synergistic manner with ARF in streptolysin-O-permeabilized, cytosol-depleted HL-60 cells, suggesting that G(M2) activator may regulate PLD in a concerted manner with other factors, including ARF, inside the cells.

GLUT-4 translocation in skeletal muscle studied with a cell-free assay: involvement of phospholipase D

GLUT-4-containing membranes immunoprecipitated from insulin-stimulated rat skeletal muscle produce the phospholipase D (PLD) product phosphatidic acid. In vitro stimulation of PLD in crude membrane with ammonium sulfate (5 mM) resulted in transfer of GLUT-4 (3.0-fold vs. control) as well as transferrin receptor proteins from large to small membrane structures. The in vitro GLUT-4 transfer could be blocked by neomycin (a PLD inhibitor), and neomycin also reduced insulin-stimulated glucose transport in intact incubated soleus muscles. Furthermore, protein kinase B(beta) (PKB(beta)) was found to associate with the GLUT-4 protein and was transferred to small vesicles in response to ammonium sulfate in vitro. Finally, addition of cytosolic proteins, prepared from basal skeletal muscle, and GTP nucleotides to an enriched GLUT-4 membrane fraction resulted in in vitro transfer of GLUT-4 to small membranes (6.8-fold vs. unstimulated control). The cytosol and nucleotide-induced GLUT-4 transfer could be blocked by neomycin and N-ethylmaleimide. In conclusion, we have developed a cell-free assay that demonstrates in vitro GLUT-4 transfer. This transfer may suggest release of GLUT-4-containing vesicles from donor GLUT-4 membranes involving PLD activity and binding of PKB(beta) to GLUT-4.

Identification of novel membrane-bound phospholipase D from Streptoverticillium cinnamoneum, possessing only hydrolytic activity

A membrane-bound phospholipase D (PLD) has been identified and isolated in a soluble form from an actinomycete, Streptoverticillium cinnamoneum. The enzyme has a monomeric structure with a molecular size of about 37 kDa, being the smallest among the enzymes so far reported. The enzyme catalyzes the hydrolysis of phosphatidylethanolamine and phosphatidylserine as preferred substrates, but not the transphosphatidylation reaction of their phospholipid groups to ethanol. Together with the absence of immunochemical cross-reactivity, these enzymatic properties demonstrate that the membrane-bound enzyme is distinct from the extracellular enzyme recently characterized and cloned from the same bacterial strain [C. Ogino et al., J. Biochem. 125 (1999) 263-269] and is therefore regarded as a novel prokaryotic PLD.

Phospholipase D in rat myometrium: occurrence of a membrane-bound ARF6 (ADP-ribosylation factor 6)-regulated activity controlled by betagamma subunits of heterotrimeric G-proteins

Both protein kinase C and protein tyrosine kinases have been shown to be involved in phospholipase D (PLD) activation in intact rat myometrium [Le Stunff, Dokhac and Harbon (2000) J. Pharmacol. Exp. Ther. 292, 629-637]. In this study we assessed the involvement of monomeric G-proteins in PLD activation in a cell-free system derived from myometrial tissue. Both the PLD1 and PLD2 isoforms were detected. Two forms of PLD activity, essentially membrane-bound, were found in myometrial preparations. One form was stimulated by oleate and insensitive to guanosine 5'-[gamma-thio] triphosphate (GTP[S]). The second required ammonium sulphate to be detected and was stimulated by GTP[S]. ADP-ribosylation factors (ARF1 and ARF6) and RhoA were immunodetected in myometrial preparations. ARF1 and RhoA were present in the membrane and cytosolic fractions whereas ARF6 was detected exclusively in the membrane fraction. A synthetic myristoylated peptide corresponding to the N-terminal domain of ARF6 [myrARF6((2-13))] totally abolished PLD activation in the presence of ammonium sulphate and GTP[S], whereas myrARF1((2-17)) and the inhibitory GDP/GTP-exchange factor, Rho GDI, did not. These data are consistent with a membrane-bound ARF6-regulated PLD activity. Finally, the stimulation of PLD by ARF6 was inhibited by AlF(-)(4) and this inhibition was counteracted by the fusion protein glutathione S-transferase-beta-adrenergic receptor kinase 1 (495-689) and by the QEHA peptide (from adenylate cyclase ACII), which act as G-protein betagamma-subunit scavengers. It is concluded that G-protein subunits betagamma are involved in a pathway modulating PLD activation by ARF6, illustrating cross-talk between heterotrimeric and monomeric G-proteins.

Influence of phosphatidylinositol 4,5-bisphosphate on human phospholipase D1 wild-type and deletion mutants: is there evidence for an interaction of phosphatidylinositol 4,5-bisphosphate with the putative pleckstrin homology domain?

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is an essential cofactor of phospholipase D (PLD) enzymes. In order to further characterize its role in PLD activation, we have constructed N-terminal deletion mutants of the human PLD1 (hPLD1) and a mutant lacking the putative pleckstrin homology domain (delta PH), which has been proposed to be involved in PIP(2) binding. For the N-terminal deletion mutants (up to 303 amino acids) and the delta PH mutant we found no significant differences compared to the hPLD1 wild-type, except changes in the specific activities: the K(m) values were about 20 microM for the substrate phosphatidylcholine, and PIP(2) activated the PLD enzymes maximally between 5 and 10 microM. In contrast, preincubation of the PLD proteins with 5-10 microM PIP(2) or PIP(2)-containing lipid vesicles inhibited the PLD activity. This inhibition was neither abolished by n-octyl-beta-D-glucopyranoside or neomycin nor by the ADP-ribosylation factor, another activator of PLD enzymes. All tested PLD proteins were active without PIP(2) in the presence of 1 M ammonium sulfate. The 303 N-terminal amino acids of hPLD1 are not involved in substrate binding or the interaction with PIP(2). Our data indicate further that the putative PH domain of hPLD1 is not responsible for the essential effects of PIP(2) on PLD activity.

Effect of dietary lipids on phospholipase D activity in rat brain

Phospholipase D (PLD) is emerging as a major player in many novel signaling pathways. Based on recent studies correlating membrane composition with enzyme function, we speculated that feeding of dietary lipids to the newborns has a major impact on brain PLD activity. To test this hypothesis, the rat dams were fed fat-free powder containing either safflower oil or fish oil, and a control powdered chow. The pups were weaned onto the diet and sacrificed at 30 days of age. PLD activity was measured by transphosphatidylation assays using rat brain membranes. This study shows that microsome GTPgammaS-dependent PLD activity in rats fed safflower oil or fish oil was significantly reduced by 38% and 30% respectively compared to controls. Oleate-dependent PLD activity in the safflower oil group, however, was significantly increased by 38%. In contrast, synaptosome membrane (P2) GTPgammaS-dependent PLD activity in rats consuming safflower oil was significantly increased by 29%, but there was no difference in oleate-dependent PLD activity. Likewise, no difference was observed in microsome oleate-dependent PLD and P2 GTPgammaS-dependent PLD activity between the fish oil and the control groups. These results indicate that dietary lipid intake appears to modulate phospholipid metabolism and differential expression of PLD isozymes in the brain.

Requirement of GM2 ganglioside activator for phospholipase D activation

Sequence analysis of a heat-stable protein necessary for the activation of ADP ribosylation factor-dependent phospholipase D (PLD) reveals that this protein has a structure highly homologous to the previously known GM2 ganglioside activator whose deficiency results in the AB-variant of GM2 gangliosidosis. The heat-stable activator protein indeed has the capacity to enhance enzymatic conversion of GM2 to GM3 ganglioside that is catalyzed by beta-hexosaminidase A. Inversely, GM2 ganglioside activator purified separately from tissues as described earlier [Conzelmann, E. & Sandhoff, K. (1987) Methods Enzymol. 138, 792-815] stimulates ADP ribosylation factor-dependent PLD in a dose-dependent manner. At higher concentrations of ammonium sulfate, the PLD activator protein apparently substitutes for protein kinase C and phosphatidylinositol 4,5-bisphosphate, both of which are known as effective stimulators of the PLD reaction. The mechanism of action of the heat-stable PLD activator protein remains unknown.

Purification of a heat-stable activator protein for ADP-ribosylation factor-dependent phospholipase D

A heat-stable activator for ADP-ribosylation factor (ARF)-dependent phospholipase D (PLD) was purified to near homogeneity from rat kidney cytosol by a sequential column chromatography. The purified activator has a molecular mass of 23 kDa on SDS-PAGE. Using a partially purified ARF-dependent PLD from rat kidney, the activator synergistically stimulates PLD with ARF in time- and dose-dependent manner. In the absence of ARF, the activator has little or no effect. The purified activator also stimulates PLD under several conditions including permeabilized cell system, suggesting that the activator is a physiologically relevant regulator of PLD.

Different mechanisms regulate phosphatidylserine synthesis in rat cerebral cortex

Transduction of extracellular signals through the membrane involves both the lipid and protein moiety. Phosphatidylserine participates to these processes as a cofactor for protein kinase C activity and thus the existence of a regulatory mechanism for its synthesis ought to be expected. In plasma membranes from rat cerebral cortex, the activity of serine base exchange enzyme, that is mainly responsible for phosphatidylserine synthesis in mammalian tissues, was reduced by the addition to the incubation mixture of AlF4- or GTP-gamma-S, known activators of G proteins, whereas ATP was almost uneffective. GTP-gamma-S inhibited the enzyme activity only at relatively high concentration (> 0.5 mM). When the synthesis of phosphatidylserine in the same cerebral area was investigated by measuring the incorporation of labelled serine into the phospholipid in the homogenate buffered at pH 7.6, ATP had an inhibitory effect as GTP-gamma-S and AlF4-. Heparin activated both serine base exchange enzyme in plasma membranes and phosphatidylserine synthesis in the homogenate. The preincubation of plasma membranes in the buffer without any other addition at 37 degrees C for 15 min reduced by 30% serine base exchange enzyme activity. The remaining activity responded to the addition of GTP-gamma-S but was insensitive to 5 mM AlF-4, a concentration that inhibited by 60% the enzyme assayed without preincubation. These results indicate the existence of different regulatory mechanisms, involving ATP and G proteins, possibly acting on different enzymes responsible for the synthesis of phosphatidylserine. Since previous studies have shown that hypoxia increases the synthesis of this phospholipid in brain slices or homogenate (Mozzi et al. Mol Cell Biochem 126: 101-107, 1993), it is possible that hypoxia may interfere with at least one of these mechanisms. This hypothesis is supported by the observation that in hypoxic homogenate 20 mM AlF-4 was not able to reduce the synthesis of phosphatidylserine as in normoxic samples. A similar difference between oxygenated and hypoxic samples, concerning their response to AlF4-, was observed when the incorporation of ethanolamine into phosphatidylethanolamine was studied. The incorporation of choline into phosphatidilcholine was, on the contrary, inhibited at a similar extent in both experimental conditions.

Inhibitory effects of thymus-independent type 2 antigens on MHC class II-restricted antigen presentation: comparative analysis of carbohydrate structures and the antigen presenting cell

The role of thymus-independent type 2 (TI-2) antigens (polysaccharides) on the MHC-II-restricted processing of protein antigens was studied in vitro. In general, antigen presentation is inhibited when both peritoneal and splenic macrophages (M phi) as well as Küpffer cells (KC) are preincubated with acidic polysaccharides or branched dextrans. However, the inhibitory effect of neutral polysaccharides was minimal when KC were used as antigen presenting cells (APC). Morphological evaluation of the uptake of fluoresceinated polysaccharides clearly correlates with this selective and differential interference. Polysaccharides do not block MHC-I-restricted antigen presentation. Some chemical characteristics shared by different saccharides seem to be specially related to their potential inhibitory abilities: (i) those where two anomeric carbon atoms of two interlinked sugars and (ii) those containing several sulfate groups per disaccharide repeating unit. No polysaccharide being inhibitory in M phi abrogated antigen processing in other APC: lipopolysaccharide-activated B cells, B lymphoma cells, or dendritic cells (DC). Using radiolabeled polysaccharides it was observed that DC and B cells incorporated less radioactivity as a function of time than M phi. Morphological evaluation of these different APC incubated for extended periods of time with inhibitory concentrations of polysaccharides revealed intense cytoplasmic vacuolization in M phi but not in B cells or DC. The large majority of M phi lysosomes containing polysaccharides fail to fuse with incoming endocytic vesicles and delivery of fluid-phase tracers was reduced, suggesting that indigestible carbohydrates reduced the fusion of these loaded lysosomes with endosomes containing recently internalized tracers. It is suggested that the main causes of this antigen presentation blockade are (i) the chemical characteristics of certain carbohydrates and whether the specific enzymatic machinery for their intracellular degradation exists; and (ii) the different phagocytic abilities of distinct APC populations, fluid-phase pinocytosis and receptor-mediated saccharide uptake, and existence of a differential antigen-processing pathway in M phi and DC or B cells, which could be based on a polysaccharide-inhibited step present in M phi but unaffected or irrelevant in both B cells and DC.

Didecanoyl phosphatidylcholine is a superior substrate for assaying mammalian phospholipase D

Phospholipase D (PLD) activity in crude or solubilized membranes from mammalian tissues is difficult to detect with the current assay techniques, unless a high radioactive concentration of substrate and/or long incubation times are employed. Generally, the enzyme has to be extracted and partially purified on one column before easy detection of activity. Furthermore, PLD activity in cultured cells can only be detected by the available assay techniques in the presence of guanosine 5'-[gamma-thio]-triphosphate (GTP[S]) and a cytosolic factor [usually ADP-ribosylation factor (Arf)]. In this paper we report that the use of didecanoyl phosphatidylcholine (C10-PC) in mammalian PLD assays considerably increases the detection limit. C10-PC was compared with the commonly used dipalmitoyl phosphatidylcholine (C16-PC) as a substrate for PLD activity from membranes of human neutrophils, human placenta and pig brain, and from placental cytosol. C10-PC was superior to C16-PC by a factor of 2-28 depending on assay conditions and tissue, and it allowed the detection of GTP[S]-and Arf-stimulated PLD activity without addition of phosphatidylinositol 4,5-bisphosphate.

Mammalian phospholipase D: phosphatidylethanolamine as an essential component

Bovine kidney phospholipase D (PLD) was assayed by measuring the formation of phosphatidylethanol from added radioactive phosphatidylcholine (PtdCho) in the presence of ethanol, guanosine 5'-[gamma-thio]triphosphate, ammonium sulfate, and cytosol factor that contained small GTP-binding regulatory proteins. The PLD enzyme associated with particulate fractions was solubilized by deoxycholate and partially purified by chromatography on a heparin-Sepharose column. This PLD preferentially used PtdCho as substrate. After purification, the enzyme per se showed little or practically no activity but required an additional factor for the enzymatic reaction. This factor was extracted with chloroform/methanol directly from particulate fractions of various tissues, including kidney, liver, and brain, and identified as phosphatidylethanolamine (PtdEtn), although this phospholipid did not serve as a good substrate. Plasmalogen-rich PtdEtn, dioleoyl-PtdEtn, and L-alpha-palmitoyl-beta-linoleoyl-PtdEtn were effective, but dipalmitoyl-PtdEtn was inert. Sphingomyelin was 30% as active as PtdEtn. The results suggest that mammalian PLD reacts nearly selectively with PtdCho in the form of mixed micelles or membranes with other phospholipids, especially PtdEtn.