Phospholipase D1 localises to secretory granules and lysosomes and is plasma-membrane translocated on cellular stimulation

The regulation of phospholipase D by inositol phospholipids and small GTPases

Phospholipase D1 and D2 (PLD1, PLD2) both have PX and PH domains in their N-terminal regions with these inositol lipid binding domains playing key roles in regulating PLD activity and localisation. The activity of PLD1 is also regulated by protein kinase C and members of the Rho and Arf families of GTPases. Each of these proteins binds to unique sites; however, there appears to be little in vitro discrimination between individual family members. In agonist-stimulated cells, however, there is specificity, with, for example in RBL-2H3 cells, antigen stimulating the activation of PLD1 by association with Arf6, Rac1 and protein kinase Calpha. PLD2 appears to be less directly regulated by GTPases and rather is primarily controlled through interaction with phosphatidylinositol 4-phosphate 5-kinase that generates the activating phosphatidylinositol 4,5-bisphosphate.

Continual production of phosphatidic acid by phospholipase D is essential for antigen-stimulated membrane ruffling in cultured mast cells

Phospholipase Ds (PLDs) are regulated enzymes that generate phosphatidic acid (PA), a putative second messenger implicated in the regulation of vesicular trafficking and cytoskeletal reorganization. Mast cells, when stimulated with antigen, show a dramatic alteration in their cytoskeleton and also release their secretory granules by exocytosis. Butan-1-ol, which diverts the production of PA generated by PLD to the corresponding phosphatidylalcohol, was found to inhibit membrane ruffling when added together with antigen or when added after antigen. Inhibition by butan-1-ol was completely reversible because removal of butan-1-ol restored membrane ruffling. Measurements of PLD activation by antigen indicate a requirement for continual PA production during membrane ruffling, which was maintained for at least 30 min. PLD1 and PLD2 are both expressed in mast cells and green fluorescent protein-tagged proteins were used to identify PLD2 localizing to membrane ruffles of antigen-stimulated mast cells together with endogenous ADP ribosylation factor 6 (ARF6). In contrast, green fluorescent protein-PLD1 localized to intracellular vesicles and remained in this location after stimulation with antigen. Membrane ruffling was independent of exocytosis of secretory granules because phorbol 12-myristate 13-acetate increased membrane ruffling in the absence of exocytosis. Antigen or phorbol 12-myristate 13-acetate stimulation increased both PLD1 and PLD2 activity when expressed individually in RBL-2H3 cells. Although basal activity of PLD2-overexpressing cells is very high, membrane ruffling was still dependent on antigen stimulation. In permeabilized cells, antigen-stimulated phosphatidylinositol(4,5)bisphosphate synthesis was dependent on both ARF6 and PA generated from PLD. We conclude that both activation of ARF6 by antigen and a continual PLD2 activity are essential for local phosphatidylinositol(4,5)bisphosphate generation that regulates dynamic actin cytoskeletal rearrangements.

Serine/threonine protein kinases synergistically regulate phospholipase D1 and 2 and secretion in RBL-2H3 mast cells

The role of phospholipase (PL) D in secretion was examined in RBL-2H3 mast cells which contain both PLD1 and 2. The effects of pharmacologic stimulants and inhibitors of Ca(2+)/calmodulin-dependent kinase II, protein kinase C, and protein kinase A suggested that all three kinases synergistically stimulate PLD and, when associated with a calcium signal, secretion as well to indicate a possible linkage between these two events. Overexpression of either PLD1 or 2 markedly enhanced the activation of PLD by pharmacologic stimulants as well as antigen and both isoforms thus appear co-ordinately regulated. As the expressed PLD1 was associated with secretory granules and PLD2 with the plasma membrane, the two isoforms may serve distinct but complementary functions in secretion.

GTP gamma S-stimulated lysosomal lysis dependent on the assembly of adaptor protein on lysosome

Cytosol treated with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) destroys the dextran-loaded lysosomes (J. Biochem., 123, 637 (1998)). The transfer of the ADP-ribosylation factor (ARF) from the cytosol to the lysosomal membrane is necessary for this lysis to occur. The role of ARF in the biogenesis of the Golgi complex is to generate high-affinity membrane-binding sites for the heterotetrameric adaptor protein complex in Golgi membranes. We have found that ARF also recruits the adaptor protein to lysosomes. The recruitment of protein coats for vesicles is necessary for the GTPgammaS-stimulated lysis of lysosomes. The GTPgammaS-induced lysis proceeded via a process similar to that for the assembly of coated proteins to coated vesicles, which serve to transport proteins between intracellular organelles.

Similarities in the induction of post-Golgi vesicles by the vaccinia virus F13L protein and phospholipase D

Intracellular mature vaccinia virions are wrapped by cisternae, derived from virus-modified trans-Golgi or endosomal membranes, and then transported via microtubules to the cell periphery. Two viral proteins, encoded by the F13L and B5R open reading frames, are essential for the membrane-wrapping step. Previous transfection studies indicated that F13L induces the formation of post-Golgi vesicles that incorporate the B5R protein and that this activity depends on an intact F13L phospholipase motif. Here we show that the F13L protein has a general effect on the trafficking of integral membrane proteins from the Golgi apparatus, as both the vaccinia virus A36R protein and the vesicular stomatitis virus G protein also colocalized with the F13L protein in vesicles. In addition, increased expression of cellular phospholipase D, which has a similar phospholipase motif as, but little amino acid sequence identity with, F13L, induced post-Golgi vesicles that contained B5R and A36R proteins. Butanol-1, which prevents the formation of phosphatidic acid by phospholipase D and specifically inhibits phospholipase D-mediated vesicle formation, also inhibited F13L-induced vesicle formation, whereas secondary and tertiary alcohols had no effect. Moreover, inhibition of phospholipase activity by butanol-1 also reduced plaque size and decreased the formation of extracellular vaccinia virus without affecting the yield of intracellular mature virus. Phospholipase D, however, could not complement a vaccinia virus F13L deletion mutant, indicating that F13L has additional virus-specific properties. Taken together, these data support an important role for F13L in inducing the formation of vesicle precursors of the vaccinia virus membrane via phospholipase activity or activation.

A noninvasive cell-based assay for monitoring proteolytic activity within a specific subcellular compartment

A noninvasive cell-based assay has been developed to monitor the proteolytic activity of cathepsin L within a specific subcellular compartment, the lysosome. The green fluorescent protein (GFP) of Aequorea victoria was selected as a substrate. Targeting to lysosomes was achieved by fusing GFP to preprocathepsin L, which also ensures colocalization of the enzyme and the substrate. Stably transfected HeLa-rtTA (reverse tetracycline-controlled transactivator) cells were induced with doxycycline and cultured in the presence of various concentrations of cysteine protease inhibitors for 48 h. In the absence of inhibitor, proteolytic degradation of GFP leads to loss of fluorescence, which is due almost exclusively to the action of recombinant cathepsin L. However, a dose-dependent increase of GFP fluorescence is observed for cells treated with the potent cathepsin L inhibitor benzyloxycarbonyl-LeuLeuTyr-CHN(2). Fluorescence is also observed when GFP is fused to an inactive preprocathepsin L (C25A mutant). Targeting of GFP to an acidic cellular compartment can destabilize the protein and render it susceptible to proteolytic degradation. The approach should be generally applicable for proteases localized in acidic environments. Such an assay can be of great value in validating the participation of a specific enzyme in a given process or in testing the ability of putative inhibitors to reach their intracellular target.

Detecting protein-phospholipid interactions. Epidermal growth factor-induced activation of phospholipase D1b in situ

Phospholipase D (PLD) proteins have been identified in secretory and endocytic vesicles, consistent with their proposed role in regulating membrane traffic. However, their sites of catalytic action remain obscure. We have developed here a novel, analytical approach to monitor PLD activation in intact cells employing lifetime imaging microscopy to measure fluorescence resonance energy transfer between protein and membrane phospholipid. Verification and application of this technique demonstrates a dispersed endosomal, epidermal growth factor-induced activation of the PLD1b isoform. Application of this approach will facilitate the spatial resolution of many protein-phospholipid interactions that are key events in the regulation of cellular processes.

Regulation of phospholipase D and secretion in mast cells by protein kinase A and other protein kinases

Functions attributed to phospholipase (PL) D include the regulation of intracellular trafficking of Golgi-derived vesicles and secretion of granules from mast cells. We have reported that activation of PLD and secretion in a rat mast cell (RBL-2H3) line is substantially enhanced by cholera toxin, a known activator of protein kinase (PK) A. Here we review the evidence that (1) the synergistic interactions of cholera toxin and other pharmacological agents on mast cell secretion are attributable to the synergistic activation of PLD via PKA, CaM kinase II, and PKC and (2) both PLD1 and PLD2 participate in this process. For example, treatment with cholera toxin, thapsigargin, and phorbol 12-myristate 13-acetate (which activate PKA, CaM kinase II, and PKC, respectively) exhibit synergy in the stimulation of both PLD and secretion. These kinases and PLD are likely confined to membrane components, as similar synergistic interactions could be demonstrated in permeabilized cells. The regulation of PLD and secretion by these kinases is also apparent from studies of inhibitors of PKA and other kinases. Also, by overexpression of either PLD1 or PLD2 it is apparent that both isoforms respond to the same stimuli as endogenous PLD, although PLD1 is largely associated with secretory granules and PLD2 with plasma membrane. The studies reveal interesting differences in the regulation of the translocation of granules (regulated by PKA) and the fusion of these granules with the plasma membrane (regulated by Ca(2+) and PKC). The pathological/physiological implications of the regulation of PLD by PKA require further evaluation in other cell systems.

Phospholipases D1 and D2 regulate different phases of exocytosis in mast cells

The rat mast cell line RBL-2H3 contains both phospholipase D (PLD)1 and PLD2. Previous studies with this cell line indicated that expressed PLD1 and PLD2 are both strongly activated by stimulants of secretion. We now show by use of PLDs tagged with enhanced green fluorescent protein that PLD1, which is largely associated with secretory granules, redistributes to the plasma membrane in stimulated cells by processes reminiscent of exocytosis and fusion of granules with the plasma membrane. These processes and secretion of granules are suppressed by expression of a catalytically inactive mutant of PLD1 or by the presence of 50 mM 1-butanol but not tert-butanol, an indication that these events are dependent on the catalytic activity of PLD1. Of note, cholera toxin induces translocation of PLD1-labeled granules to the plasma membrane but not fusion of granules with plasma membrane or secretion. Subsequent stimulation of calcium influx with Ag or thapsigargin leads to rapid redistribution of PLD1 to the plasma membrane and accelerated secretion. Also of note, PLD1 is recycled from plasma membrane back to granules within 4 h of stimulation. PLD2, in contrast, is largely confined to the plasma membrane, but it too participates in the secretory process, because expression of catalytically inactive PLD2 also blocks secretion. These data indicate a two-step process: translocation of granules to the cell periphery, regulated by granule-associated PLD1, and a calcium-dependent fusion of granules with the plasma membrane, regulated by plasma membrane-associated PLD2 and possibly PLD1.

Dichotomy of Ca2+ signals triggered by different phospholipid pathways in antigen stimulation of human mast cells

Mast cell activation triggers Ca(2+) signals and the release of enzyme-containing granules, events that play a major role in allergic/hypersensitivity reactions. However, the precise molecular mechanisms that regulate antigen-triggered degranulation and Ca(2+) fluxes in human mast cells are still poorly understood. Here we show, for the first time, that a receptor can trigger Ca(2+) via two separate molecular mechanisms. Using an antisense approach, we show that IgE-antigen stimulation of human bone marrow-derived mast cells triggers a sphingosine kinase (SPHK) 1-mediated fast and transient Ca(2+) release from intracellular stores. However, phospholipase C (PLC) gamma1 triggers a second (slower) wave of calcium release from intracellular stores, and it is this PLCgamma1-generated signal that is responsible for Ca(2+) entry. Surprisingly, FcepsilonRI (a high affinity receptor for IgE)-triggered mast cell degranulation depends on the first, sphingosine kinase-mediated Ca(2+) signal. These two pathways act independently because antisense knock down of either enzyme does not interfere with the activity of the other enzyme. Of interest, similar to PLCgamma1, SPHK1 translocates rapidly to the membrane after FcepsilonRI cross-linking. Here we also show that SPHK1 activity depends on phospholipase D1 and that FcepsilonRI-triggered mast cell degranulation depends primarily on the activation of both phospholipase D1 and SPHK1.

Antigen-stimulated activation of phospholipase D1b by Rac1, ARF6, and PKCalpha in RBL-2H3 cells

Phospholipase D (PLD) activity can be detected in response to many agonists in most cell types; however, the pathway from receptor occupation to enzyme activation remains unclear. In vitro PLD1b activity is phosphatidylinositol 4,5-bisphosphate dependent via an N-terminal PH domain and is stimulated by Rho, ARF, and PKC family proteins, combinations of which cooperatively increase this activity. Here we provide the first evidence for the in vivo regulation of PLD1b at the molecular level. Antigen stimulation of RBL-2H3 cells induces the colocalization of PLD1b with Rac1, ARF6, and PKCalpha at the plasma membrane in actin-rich structures, simultaneously with cooperatively increasing PLD activity. Activation is both specific and direct because dominant negative mutants of Rac1 and ARF6 inhibit stimulated PLD activity, and surface plasmon resonance reveals that the regulatory proteins bind directly and independently to PLD1b. This also indicates that PLD1b can concurrently interact with a member from each regulator family. Our results show that in contrast to PLD1b's translocation to the plasma membrane, PLD activation is phosphatidylinositol 3-kinase dependent. Therefore, because inactive, dominant negative GTPases do not activate PLD1b, we propose that activation results from phosphatidylinositol 3-kinase-dependent stimulation of Rac1, ARF6, and PKCalpha.

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

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

Phospholipase D and immune receptor signalling

Immune receptors are coupled to the activation of phosphatidylcholine phospholipase D (PC-PLD) that hydrolyses phosphatidylcholine to generate phosphatidic acid and choline. As these receptors are also coupled to other signalling cascades, it has been difficult to define the precise cell activation events resulting from PLD activation in the absence of specific inhibitors. There is increasing evidence that phosphatidic acid acts as an intracellular signalling molecule regulating release of calcium from intracellular stores, sphingosine kinase and protein kinase C activation and membrane budding. Phosphatidic acid can also be rapidly converted into lysophosphatidic acid, diacylglycerol and arachidonates.

Cell-permeable ceramides preferentially inhibit coated vesicle formation and exocytosis in Chinese hamster ovary compared with Madin-Darby canine kidney cells by preventing the membrane association of ADP-ribosylation factor

Differential effects of acetyl(C2-) ceramide (N-acetylsphingosine) were studied on coated vesicle formation from Golgi-enriched membranes of Chinese hamster ovary (CHO) and Madin-Darby canine kidney (MDCK) cells. C2-ceramide blocked the translocation of ADP-ribosylation factor-1 (ARF-1) and protein kinase C-alpha (PKC-alpha) to the membranes from CHO cells, but not those of MDCK cells. Consequently, C2-ceramide blocked the stimulation of phospholipase D1 (PLD1) by the cytosol and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in membranes from CHO cells. Basal specific activity of PLD1 and the concentration of ARF-1 were 3-4 times higher in Golgi-enriched membranes from MDCK cells compared with CHO cells. Moreover, PLD1 activity in MDCK cells was stimulated less by cytosol and GTP[S]. PLD2 was not detectable in the Golgi-enriched membranes. Incubation of intact CHO cells or their Golgi-enriched membranes with C2-ceramide also inhibited COP1 vesicle formation by membranes from CHO, but not MDCK, cells. Specificity was demonstrated, since dihydro-C2-ceramide had no significant effect on ARF-1 translocation, PLD1 activation or vesicle formation in membranes from both cell types. C2-ceramide also decreased the secretion of virus-like particles to a greater extent in CHO compared with MDCK cells, whereas dihydro-C2-ceramide had no significant effect. The results demonstrate a biological effect of C2-ceramide in CHO cells by decreasing ARF-1 and PKC-alpha binding to Golgi-enriched membranes, thereby preventing COP1 vesicle formation.

Dimerization of phospholipase d isozymes

Two mammalian phospholipase D (PLD) isozymes (PLD1 and PLD2) have been reported. In this study, we differentially tagged these isozymes with enhanced green fluorescent protein (EGFP-rPLD1 and EGFP-rPLD2) or Xpress peptide epitope (Xpress-rPLD1 and Xpress-rPLD2) to examine the association between these isozymes. Overexpressed EGFP-rPLD1 coimmunoprecipitated with Xpress-rPLD1 using anti-Xpress antibody. However, the coimmunoprecipitation was independent of the activity of rPLD1. Xpress-rPLD2 also bound to EGFP-rPLD1 although the binding was less efficient than observed with Xpress-rPLD1. The association between rPLD2 and rPLD1 was confirmed by coimmunoprecipitation of EGFP-rPLD2 with Xpress-rPLD1. EGFP-rPLD2 also bound to Xpress-rPLD2 as shown by coimmunoprecipitation. Immunofluorescence staining of COS-7 cells coexpressing EGFP-rPLDs and Xpress-rPLDs showed that the PLD isozymes colocalized in the perinuclear and plasma membrane regions, suggesting that they could associate in a cellular setting. These results suggest that rPLD1 and rPLD2 can exist as homodimers and can form heterodimers.

Novel mastoparan analogs induce differential secretion from mast cells

Cationic amphiphilic peptides stimulate secretion via a receptor-independent action upon G proteins. We have previously utilized chimeric analogs of mastoparan (MP), including galparan (galanin(1-13)-MP ), as molecular probes of secretion. Here, we further resolve the structure-activity relationship of peptidyl secretagogs, including rationally designed chimeric MP analogs. The secretory efficacies of 10 MP analogs were significantly higher than 45 unrelated basic peptides. Comparative studies identified MP analogs that are differential secretagogs for 5-hydroxytryptamine (5-HT) and beta-hexosaminidase. Peptide-induced activation of phospholipase D (PLD), an enzyme intimately involved in regulated exocytosis [5], correlated with the secretion of beta-hexosaminidase but not 5-HT. Thus, these data indicate that different mechanisms are responsible for the exocytosis of 5-HT and beta-hexosaminidase, respectively. Moreover, mastoparan analogs are novel tools for probing the molecular details of exocytosis and other biological phenomena.

Regulation of constitutive protein transit by phospholipase D in HT29-cl19A cells

Phospholipase D (PLD) plays a central role in the control of vesicle budding and protein transit. We previously showed that in resting epithelial HT29-cl19A cells, PLD is implicated in the control of constitutive protein transit, from the trans-Golgi network to the plasma membrane, and that phorbol ester stimulation of protein transit is correlated with PLD activation (Auger, R., Robin, P., Camier, B., Vial, G., Rossignol, B., Tenu, J.-P., and Raymond, M.-N. (1999) J. Biol. Chem. 274, 28652-28659). In this paper we demonstrate that: 1) PLD is not implicated in the earliest phases of protein transit; 2) PLD controls apical but not basolateral protein transit; 3) HT29-cl19A cells express PLD1b and PLD2a mRNAs and proteins; 4) the expression of a catalytically inactive mutant of PLD2 (mPLD2-K758R) significantly inhibited apical constitutive protein transit whereas expression of a catalytically inactive mutant of PLD1 (hPLD1b-K898R) prevented increases in the rate of apical transit as triggered by phorbol esters; 5) PLD2 appears to be located in a perinuclear region containing the Golgi whereas PLD1, which is scattered in the cytoplasm in resting cells, is translocated to the plasma membrane after phorbol ester stimulation. Taken together, these data lead to the conclusion that in HT29-cl19A cells, both PLDs regulate protein transit between the trans-Golgi network and the apical plasma membrane, but that they do so at different steps in the pathway.

Functional coupling of FcgammaRI to nicotinamide adenine dinucleotide phosphate (reduced form) oxidative burst and immune complex trafficking requires the activation of phospholipase D1

Immunoglobulin G (IgG) receptors (FcgammaRs) on myeloid cells are responsible for the internalization of immune complexes. Activation of the oxidase burst is an important component of the integrated cellular response mediated by Fc receptors. Previous work has demonstrated that, in interferon-gamma-primed U937 cells, the high-affinity receptor for IgG, FcgammaRI, is coupled to a novel intracellular signaling pathway that involves the sequential activation of phospholipase D (PLD), sphingosine kinase, and calcium transients. Here, it is shown that both known PLD isozymes, PLD1 and PLD2, were present in these cells. With the use of antisense oligonucleotides to specifically reduce the expression of either isozyme, PLD1, but not PLD2, was found to be coupled to FcgammaRI activation and be required to mediate receptor activation of sphingosine kinase and calcium transients. In addition, coupling of FcgammaRI to activation of the nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH) oxidase burst was inhibited by pretreating cells with 0.3% butan-1-ol, indicating an absolute requirement for PLD. Furthermore, use of antisense oligonucleotides to reduce expression of PLD1 or PLD2 demonstrated that PLD1 is required to couple FcgammaRI to the activation of NADPH oxidase and trafficking of internalized immune complexes for degradation. These studies demonstrate the critical role of PLD1 in the intracellular signaling cascades initiated by FcgammaRI and its functional role in coordinating the response to antigen-antibody complexes.

ATP-mediated killing of intracellular mycobacteria by macrophages is a P2X(7)-dependent process inducing bacterial death by phagosome-lysosome fusion

Mycobacterium tuberculosis survives within host macrophages by actively inhibiting phagosome fusion with lysosomes. Treatment of infected macrophages with ATP induces both cell apoptosis and rapid killing of intracellular mycobacteria. The following studies were undertaken to characterize the effector pathway(s) involved. Macrophages were obtained from p47(phox) and inducible NO synthase gene-disrupted mice (which are unable to produce reactive oxygen and nitrogen radicals, respectively) and P2X(7) gene-disrupted mice. RAW murine macrophages transfected with either the natural resistance-associated macrophage protein gene 1 (Nramp1)-resistant or Nramp1-susceptible gene were also used. The cells were infected with bacille Calmette-Guérin (BCG), and intracellular mycobacterial trafficking was analyzed using confocal and electron microscopy. P2X(7) receptor activation was essential for effective ATP-induced mycobacterial killing, as its bactericidal activity was radically diminished in P2X(7)(-/-) macrophages. ATP-mediated killing of BCG within p47(phox-/-), inducible NO synthase(-/-), and Nramp(s) cells was unaffected, demonstrating that none of these mechanisms have a role in the ATP/P2X(7) effector pathway. Following ATP stimulation, BCG-containing phagosomes rapidly coalesce and fuse with lysosomes. Blocking of macrophage phospholipase D activity with butan-1-ol blocked BCG killing, but not macrophage death. ATP stimulates phagosome-lysosome fusion with concomitant mycobacterial death via P2X(7) receptor activation. Macrophage death and mycobacterial killing induced by the ATP/P2X(7) signaling pathway can be uncoupled, and diverge proximal to phospholipase D activation.

Immunolocalisation of phospholipase D1 on tubular vesicular membranes of endocytic and secretory origin

We have examined the localisation of overexpressed phospholipase D1 (PLD1) using antibodies against its amino- and carboxyl-terminal domains. PLD1 overexpressed in COS-7 cells showed variable distribution by immunofluorescence but was mainly in punctate structures in the perinuclear region and at the plasma membrane. Downregulation by an anti-sense plasmid resulted in almost exclusively perinuclear distribution in punctate structures that contained immunoreactivity for the endogenous KDEL receptor and the early endosomal antigen EEA1 protein. Influenza haemagglutinin (HA) and HA-derived mutants designed to locate primarily to secretory or endocytic membranes were present in PLD1-positive membranes. Immunofluorescence analysis in permanent CHO cell lines that express PLD1 inducibly confirmed the presence of PLD1 on both endocytic and secretory membranes. Analysis of PLD1 distribution by immunocytochemistry and electron microscopy of intact CHO cells and of isolated membranes revealed that PLD1 was present in tubulovesicular elements and multivesicular bodies. Some of these were close to the Golgi region whereas others stained positive for endocytic cargo proteins. Morphometric analysis assigned the majority of PLD1 immunoreactivity on endosomal membranes and a smaller amount on membranes of secretory origin. PLD1, via signals that are currently not understood, is capable of localising in tubulovesicular membranes of both endocytic and secretory origin.

Phospholipase D1: a key factor for the exocytotic machinery in neuroendocrine cells

Phospholipase D (PLD) has been proposed to mediate cytoskeletal remodeling and vesicular trafficking along the secretory pathway. We recently described the activation of an ADP ribosylation factor-regulated PLD at the plasma membrane of chromaffin cells undergoing secretagogue-stimulated exocytosis. We show here that the isoform involved is PLD1b, and, using a real-time assay for individual cells, that PLD activation and exocytosis are closely correlated. Moreover, overexpressed PLD1, but not PLD2, increases stimulated exocytosis in a phosphatidylinositol 4,5-bisphosphate-dependent manner, whereas catalytically inactive PLD1 inhibits it. These results provide the first direct evidence that PLD1 is an important component of the exocytotic machinery in neuroendocrine cells.

Endosomal localization of phospholipase D 1a and 1b is defined by the C-termini of the proteins, and is independent of activity

The factors regulating the activity of cellular phospholipase D (PLD) have been well characterized; however, the cellular distribution of specific PLD isoforms and the factors defining localization are less clear. Two specific PLD1 isoforms, PLD1a and PLD1b, are shown in the present study to be localized in endosomal compartments with early endosomal autoantigen 1, internalizing epidermal growth factor receptor (ErbB1) and lysobisphosphatidic acid. Novel C-terminal splice variants of PLD1, PLD1a2 and PLD1b2, do not exhibit this endosomal localization. Studies using catalytically inactive and C-terminal deletion mutants of the four PLD1 isoforms led to the conclusion that the C-terminus plays an important part in the catalytic activity of PLD1, but that the endosomal localization of PLD1a and PLD1b is defined by the C-terminus and not catalytic activity.

PKN regulates phospholipase D1 through direct interaction

The association of phospholipase (PLD)-1 with protein kinase C-related protein kinases, PKNalpha and PKNbeta, was analyzed. PLD1 interacted with PKNalpha and PKNbeta in COS-7 cells transiently transfected with PLD1 and PKNalpha or PKNbeta expression constructs. The interactions between endogenous PLD1 and PKNalpha or PKNbeta were confirmed by co-immunoprecipitation from mammalian cells. In vitro binding studies using the deletion mutants of PLD1 indicated that PKNalpha directly bound to residues 228-598 of PLD1 and that PKNbeta interacted with residues 1-228 and 228-598 of PLD1. PKNalpha stimulated the activity of PLD1 in the presence of phosphatidylinositol 4,5-bisphosphate in vitro, whereas PKNbeta had a modest effect on the stimulation of PLD1 activity. The stimulation of PLD1 activity by PKNalpha was slightly enhanced by the addition of arachidonic acid. These results suggest that the PKN family functions as a novel intracellular player of PLD1 signaling pathway.

ATP-mediated killing of Mycobacterium bovis bacille Calmette-Guérin within human macrophages is calcium dependent and associated with the acidification of mycobacteria-containing phagosomes

We previously demonstrated that extracellular ATP stimulated macrophage death and mycobacterial killing within Mycobacterium bovis Bacille Calmette-Guérin (BCG)-infected human macrophages. ATP increases the cytosolic Ca(2+) concentration in macrophages by mobilizing intracellular Ca(2+) via G protein-coupled P2Y receptors, or promoting the influx of extracellular Ca(2+) via P2X purinoceptors. The relative contribution of these receptors and Ca(2+) sources to ATP-stimulated macrophage death and mycobacterial killing was investigated. We demonstrate that 1) ATP mobilizes Ca(2+) in UTP-desensitized macrophages (in Ca(2+)-free medium) and 2) UTP but not ATP fails to deplete the intracellular Ca(2+) store, suggesting that the pharmacological properties of ATP and UTP differ, and that a Ca(2+)-mobilizing P2Y purinoceptor in addition to the P2Y(2) subtype is expressed on human macrophages. ATP and the Ca(2+) ionophore, ionomycin, promoted macrophage death and BCG killing, but ionomycin-mediated macrophage death was inhibited whereas BCG killing was largely retained in Ca(2+)-free medium. Pretreatment of cells with thapsigargin (which depletes inositol (1,4,5)-trisphosphate-mobilizable intracellular stores) or 1,2-bis-(2-aminophenoxy)ethane-N, N, N',N'-tetraacetic acid acetoxymethyl ester (an intracellular Ca(2+) chelator) failed to inhibit ATP-stimulated macrophage death but blocked mycobacterial killing. Using the acidotropic molecular probe, 3-(2,4-dinitroanilino)-3'-amino-N-methyl dipropylamine, it was revealed that ATP stimulation promoted the acidification of BCG-containing phagosomes within human macrophages, and this effect was similarly dependent upon Ca(2+) mobilization from intracellular stores. We conclude that the cytotoxic and bactericidal effects of ATP can be uncoupled and that BCG killing is not the inevitable consequence of death of the host macrophage.

Intracellular localization of phospholipase D1 in mammalian cells

Phospholipase D (PLD) hydrolyzes phosphatidylcholine to generate phosphatidic acid. In mammalian cells this reaction has been implicated in the recruitment of coatomer to Golgi membranes and release of nascent secretory vesicles from the trans-Golgi network. These observations suggest that PLD is associated with the Golgi complex; however, to date, because of its low abundance, the intracellular localization of PLD has been characterized only indirectly through overexpression of chimeric proteins. We have used highly sensitive antibodies to PLD1 together with immunofluorescence and immunogold electron microscopy as well as cell fractionation to identify the intracellular localization of endogenous PLD1 in several cell types. Although PLD1 had a diffuse staining pattern, it was enriched significantly in the Golgi apparatus and was also present in cell nuclei. On fragmentation of the Golgi apparatus by treatment with nocodazole, PLD1 closely associated with membrane fragments, whereas after inhibition of PA synthesis, PLD1 dissociated from the membranes. Overexpression of an hemagglutinin-tagged form of PLD1 resulted in displacement of the endogenous enzyme from its perinuclear localization to large vesicular structures. Surprisingly, when the Golgi apparatus collapsed in response to brefeldin A, the nuclear localization of PLD1 was enhanced significantly. Our data show that the intracellular localization of PLD1 is consistent with a role in vesicle trafficking from the Golgi apparatus and suggest that it also functions in the cell nucleus.

Protein kinase C activation by 12-0-tetradecanoylphorbol 13-acetate in CG-4 line oligodendrocytes stimulates turnover of choline and ethanolamine phospholipids by phospholipase D and induces rapid process contraction

Treatment of [3H]-choline- or [14C]-ethanolamine-labelled undifferentiated bipolar and differentiated multipolar CG-4 line oligodendrocytes with 12-0-tetradecanoylphorbol 13-acetate (TPA) to activate protein kinase C stimulated the release of choline or ethanolamine metabolites to the medium over controls. Ro31-8220, a PKC inhibitor, reduced TPA-stimulated release of choline- and ethanolamine-metabolites to basal levels. TPA treatment of both bipolar and multipolar cells caused rapid contraction of processes leaving rounded up cells: this effect was blocked by Ro31-8220. After 12-15 h exposure to TPA, bipolar undifferentiated CG-4 line cells extended short processes again and the cells became multipolar. Nocodozole, an agent which disrupts microtubules and caused CG-4 line cells to round up, caused increased choline or ethanolamine-metabolite release to the medium over basal levels suggesting that some release during TPA-treatment might occur due to process fragmentation. However, the transphosphatidylation reaction confirmed that phospholipase D was active in these cells. Exposure of bipolar undifferentiated CG-4 line cells to TPA resulted in down-regulatation of PKC-alpha and PKC-beta which could not be detected by Western blotting after a few hours; PKC-epsilon was down-regulated much more slowly but PKCs delta, zeta and iota were not influenced by 48 h exposure of cells to TPA. Formation of phosphatidylethanol in the transphosphatidylation reaction was markedly reduced in TPA down-regulated cells indicating a role for PKCs alpha and beta in phospholipase D activation in CG-4 line oligodendrocytes.

Cholecystokinin octapeptide CCK-8 and carbachol reduce [(32)P]orthophosphate labeling of phosphatidylcholine without modifying phospholipase D activity in rat pancreatic acini

We have studied phospholipase D activation in [(32)P]orthophosphoric acid-prelabeled rat pancreatic acini by measuring the formation of (32)P-phosphatidylalcohols as stimulated in the presence of ethanol or butanol. A small but significant and time-dependent basal accumulation of [(32)P]phosphatidylethanol and [(32)P]phosphatidylbutanol was detected, which was further stimulated by phorbol myristate acetate, orthovanadate and pervanadate. However, the secretagogues cholecystokinin octapeptide and carbachol did not enhance basal accumulation of (32)P-phosphatidylalcohol, yet they decreased [(32)P]phosphatidylcholine content and stimulated the generation of [(32)P]phosphatidic acid. Our results stress the need to examine the transphosphatidylation reaction as well as agonist effects on the synthesis of phosphatidylcholine in order to assess unambiguously phospholipase D activity.

Interaction of the type Ialpha PIPkinase with phospholipase D: a role for the local generation of phosphatidylinositol 4, 5-bisphosphate in the regulation of PLD2 activity

Phosphoinositides are localized in various intracellular compartments and can regulate a number of intracellular functions, such as cytoskeletal dynamics and membrane trafficking. Phospholipase Ds (PLDs) are regulated enzymes that hydrolyse phosphatidylcholine (PtdCho) to generate the putative second messenger phosphatidic acid (PtdOH). In vitro, PLDs have an absolute requirement for higher phosphorylated inositides, such as phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)]. Whether this lipid is able to regulate the activity of PLD in vivo is contentious. To examine this hypothesis we studied the relationship between PLD and an enzyme critical for the intracellular synthesis of PtdIns(4,5)P(2): phosphatidylinositol 4-phosphate 5-kinase alpha (Type Ialpha PIPkinase). We find that both PLD1 and PLD2 interact with the Type Ialpha PIPkinase and that PLD2 activity in vivo can be regulated solely by the expression of this lipid kinase. Moreover, PLD2 is able to recruit the Type Ialpha PIPkinase to its intracellular location. We show that the physiological requirement of PLD enzymes for PtdIns(4,5)P(2) is critical and that PLD2 activity can be regulated solely by the levels of this key intracellular lipid.

Interleukin-1β Regulates Phospholipase D-1 Expression in Rat Pancreatic β-Cells

The cytokine interleukin (IL)-1beta induces a biphasic effect in rat pancreatic islets, with an early and transitory stimulation of insulin release followed by progressive functional suppression. To clarify the mechanisms involved in these effects, we have recently performed a differential display of messenger RNA (mRNA) by RT-PCR (DDRT-PCR) on rat beta-cells exposed for 6 or 24 h to IL-1beta. Among the different IL-1beta-induced genes, there was an early and transient increase in phospholipase D-1 (PLD1) expression. PLD1 can induce phosphatidic acid formation and subsequent activation of protein kinase C, a process which stimulates insulin release. In the present study, we characterized the regulation of PLD isoforms by IL-1beta in pancreatic beta-cells. By using different combinations of primers and RT-PCR, we observed that IL-1beta induces an early increase (2 and 6 h) in the expression of both alternatively spliced isoforms of PLD1 (PLD1alpha and 1b). Prolonged exposure to IL-1beta (12 and 24 h) caused a decrease of PLD1a mRNA expression compared with control beta-cells, and lead to a return of PLD1b mRNA to basal level. NG-methyl-L-arginine (LMA), a blocker of the inducible form of nitric oxide synthase (iNOS), prevented this late inhibitory effect of IL-1beta, suggesting that IL-1beta-induced decrease in PLD1a expression is NO-mediated. IL-1beta induced an early (2-6 h) and sustained (16-24 h) increase in PLD1a mRNA expression in insulin-producing RINm5F cells. This was paralleled by a cytokine-induced increase in PLD1 protein expression and enzyme activity. RINm5F cells, but not primary beta-cells, expressed PLD2, and the expression of this gene was not affected by IL-1beta. In conclusion, we have shown that the cytokine IL-1beta regulates PLD1 expression in primary and clonal beta-cells. The early induction of PLD1 probably contributes to the early stimulatory effects of IL-1beta on islet insulin release.

Inhibition of phospholipase D by amphiphysins

Two distinct proteins inhibiting phospholipase D (PLD) activity in rat brain cytosol were previously purified and identified as synaptojanin and AP180, which are specific to nerve terminals and associate with the clathrin coat. Two additional PLD-inhibitory proteins have now been purified and identified as the amphiphysins I and II, which forms a heterodimer that also associates with the clathrin coat. Bacterially expressed recombinant amphiphysins inhibited both PLD1 and PLD2 isozymes in vitro with a potency similar to that of brain amphiphysin (median inhibitory concentration of approximately 15 nm). Expressions of either amphiphysin in COS-7 cells reduced activity of endogenous PLD as well as exogenously expressed PLD1 and PLD2. Coprecipitation experiments suggested that the inhibitory effect of amphiphysins results from their direct interaction with PLDs. The NH(2) terminus of amphiphysin I was critical for both inhibition of and binding to PLD. Phosphatidic acid formed by signal-induced PLD is thought to be required for the assembly of clathrin-coated vesicles during endocytosis. Thus, the inhibition of PLD by amphiphysins, synaptojanin, and AP180 might play an important role in synaptic vesicle trafficking.

Phospholipase D1 is phosphorylated and activated by protein kinase C in caveolin-enriched microdomains within the plasma membrane

Activities of phospholipase D (PLD) in diverse subcellular organelles have been identified but the details of regulatory mechanisms in such locations are unknown. Protein kinase C (PKC) is a major regulator of PLD. Serine 2, threonine 147, and serine 561 residues of phospholipase D1 (PLD1) were determined as sites of phosphorylation by PKC (Kim, Y., Han, J. M., Park, J. B., Lee, S. D., Oh, Y. S., Chung, C., Lee, T. G., Kim, J. H., Park, S. K., Yoo, J. S., Suh, P. G., Ryu, S. H. (1999) Biochemistry 38, 10344-10351). In our present study, a triple mutation of these phosphorylation sites diminished markedly phorbol 12-myristate 13-acetate (PMA)-induced PLD1 activity in COS-7 cells. We looked at the location of the PLD1 phosphorylation by PKC by observing PMA induced band shifts and by use of anti-phospho-PLD1 monoclonal antibody. The shifted PMA-induced proteins and the immunoreactivity of the anti-phospho-PLD1 antibody were mainly found in the caveolin-enriched membrane (CEM) fraction. Depletion of cellular cholesterol led to a loss of this compartmentalization of phosphorylated PLD1 in the CEM. Replacement of the cellular cholesterol led to the restoration of phosphorylated PLD1 in the CEM. Immunocytochemical studies of COS-7 cells revealed that PLD1 was localized in the plasma membrane as well as in the vesicular structures in the cytoplasm, but the phosphorylation of PLD1 occurred only in the plasma membrane. Our results, therefore, show that phosphorylation, and thereby activation, of PLD1 by PKC occurs in the caveolin and cholesterol-enriched low density domain of the plasma membrane in COS-7 cells.

Inhibition of phosphatidic acid synthesis alters the structure of the Golgi apparatus and inhibits secretion in endocrine cells

In mammalian cells, activation of a Golgi-associated phospholipase D by ADP-ribosylation factor results in the hydrolysis of phosphatidylcholine to form phosphatidic acid (PA). This reaction stimulates the release of nascent secretory vesicles from the trans-Golgi network of endocrine cells. To understand the role of PA in mediating secretion, we have exploited the transphosphatidylation activity of phospholipase D. Rat anterior pituitary GH3 cells, which secrete growth hormone and prolactin, were treated with 1-butanol resulting in the synthesis of phosphatidylbutanol rather than PA. Under these conditions transport from the ER through the Golgi apparatus and secretion of polypeptide hormones were inhibited quantitatively. Furthermore, the in vitro synthesis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) by Golgi membranes was inhibited quantitatively. Most significantly, in the presence of 1-butanol the architecture of the Golgi apparatus was disrupted, resulting in its disassembly and fragmentation. Removal of the alcohol resulted in the rapid restoration of Golgi structure and secretion of growth hormone and prolactin. Our results suggest that PA stimulation of PtdIns(4,5)P(2) synthesis is required for maintaining the structural integrity and function of the Golgi apparatus.

Inhibition of the CD8+ T cell-mediated cytotoxicity reaction by hypericin: potential for treatment of T cell-mediated diseases

The cytotoxicity reaction of murine CD8 T lymphocytes has been found to be strongly inhibited by nanomolar concentrations of hypericin, a lipophilic dianthraquinone with photodynamic properties. Cytotoxic T lymphocyte (CTL)-induced target cell apoptosis, as well as exocytosis of cytolytic granules from these cells, were ablated by hypericin, administered at the onset of the reaction, without affecting CTL viability. The inhibition of cytolysis occurred without the light irradiation which is essential for photosensitization. The findings suggest that the action of hypericin targets the effector CTL; however, apoptosis induced in murine L-cells with recombinant tumor necrosis factor (TNF)-alpha was also prevented by hypericin. Since hypericin is a known inhibitor of protein kinase C, MAP kinase and at least one other tyrosine kinase, this inhibitory activity could play a role in the down-modulation of CTL-induced cytotoxicity. Furthermore, our studies show that the action of hypericin induces rapid dephosphorylation of phospholipids associated with low-density membranes in CTL, but not with membranes of the cytotoxic granules. The ability of hypericin to interfere with cytotoxicity may render it useful in the treatment of T cell-mediated diseases.

Phospholipase D

Phospholipase D is an ubiquitous enzyme that hydrolyzes phosphatidylcholine to phosphatidic acid and choline. Its cellular actions are related to the production of phosphatidic acid and include alterations to cell growth, shape, and secretion. There are two mammalian phospholipase D genes whose products (PLD1 and PLD2) are alternatively spliced. Both forms have two highly conserved HKD motifs that are essential for catalysis and dimerization. PLD1 is regulated in vitro and in vivo by protein kinase C and small GTPases of the Rho and ARF families, whereas PLD2 shows a higher basal activity with little or no response to these proteins. The cellular locations and specific functions of the two PLD isoforms remain to be established.

Multiple forms of phospholipase D in plants: the gene family, catalytic and regulatory properties, and cellular functions

Multiple Phospholipase D (PLD) genes have been identified in plants and encode isoforms with distinct regulatory and catalytic properties. Elucidation of the genetic and biochemical heterogeneity has provided important clues as to the regulation and function of this family of enzymes. Polyphosphoinositides, Ca(2+), and G-proteins are possible cellular regulators for PLD activation. PLD-mediated hydrolysis of membrane lipids increases in response to various stresses. Recent studies suggest that PLD plays a role in the signaling and production of hormones involved in plant stress responses.

Phospholipase D2: functional interaction with caveolin in low-density membrane microdomains

Low-density detergent-insoluble membrane domains contain caveolin-1 and are enriched in a phospholipase D activity that is not PLD1. Here we show that caveolin-rich fractions, prepared from HaCaT human keratinocytes by either detergent-based or detergent-free methods, contain PLD2. Caveolar membrane PLD activity is stimulated 2-fold by low concentrations (10-30 microM) of the caveolin-1 and caveolin-2 scaffolding domain peptides, whereas it is inhibited at higher concentrations of the peptides. Immunoisolated HA-tagged PLD1 and PLD2 are not stimulated by the peptides, although both enzymes retain sensitivity to their inhibitory effect. Down-regulation of caveolin-1 expression by treatment of the cells with acetyl-leucyl-leucyl-norleucinal decreased caveolar PLD activity by 50%. Similarly, expression of an active form of the sterol regulatory element-binding protein (SREBP(1-490)) down-regulated caveolin-1 expression by 50% and decreased caveolar PLD activity by 60%. These data identify the PLD activity in caveolin-rich membranes as PLD2 and provide in vivo evidence suggesting that caveolin-1 regulates PLD2 activity.

Adipsin and the glucose transporter GLUT4 traffic to the cell surface via independent pathways in adipocytes

Insulin increases the exocytosis of many soluble and membrane proteins in adipocytes. This may reflect a general effect of insulin on protein export from the trans Golgi network. To test this hypothesis, we have compared the trafficking of the secreted serine protease adipsin and the integral membrane proteins GLUT4 and transferrin receptors in 3T3-L1 adipocytes. We show that adipsin is secreted from the trans Golgi network to the endosomal system, as ablation of endosomes using transferrin-HRP conjugates strongly inhibited adipsin secretion. Phospholipase D has been implicated in export from the trans Golgi network, and we show that insulin stimulates phospholipase D activity in these cells. Inhibition of phospholipase D action with butan-1-ol blocked adipsin secretion and resulted in accumulation of adipsin in trans Golgi network-derived vesicles. In contrast, butan-1-ol did not affect the insulin-stimulated movement of transferrin receptors to the plasma membrane, whereas this was abrogated following endosome ablation. GLUT4 trafficking to the cell surface does not utilise this pathway, as insulin-stimulated GLUT4 translocation is still observed after endosome ablation or inhibition of phospholipase D activity. Immunolabelling revealed that adipsin and GLUT4 are predominantly localised to distinct intracellular compartments. These data suggest that insulin stimulates the activity of the constitutive secretory pathway in adipocytes possibly by increasing the budding step at the TGN by a phospholipase D-dependent mechanism. This may have relevance for the secretion of other soluble molecules from these cells. This is not the pathway employed to deliver GLUT4 to the plasma membrane, arguing that insulin stimulates multiple pathways to the cell surface in adipocytes.

Phospholipase D regulation and localisation is dependent upon a phosphatidylinositol 4,5-biphosphate-specific PH domain

The signalling pathway leading, for example, to actin cytoskeletal reorganisation, secretion or superoxide generation involves phospholipase D (PLD)-catalysed hydrolysis of phosphatidylcholine to generate phosphatidic acid, which appears to mediate the messenger functions of this pathway. Two PLD genes (PLD1 and PLD2) with similar domain structures have been doned and progress has been made in identifying the protein regulators of PLD1 activation, for example Arf and Rho family members. The activities of both PLD isoforms are dependent on phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) and our sequence analysis suggested the presence of a pleckstrin homology (PH) domain in PLD1, although its absence has also been daimed. Investigation of the inositide dependence showed that a bis-phosphorylated lipid with a vicinal pair of phosphates was required for PLD1 activity. Furthermore, PLD1 bound specifically and with high affinity to lipid surfaces containing PI(4,5)P2 independently of the substrate phosphatidylcholine, suggesting a key role for the PH domain in PLD function. Importantly, a glutathione-S-transferase (GST) fusion protein comprising GST and the PH domain of PLD1 (GST-PLD1-PH) also bound specifically to supported lipid monolayers containing PI(4,5)P2. Point mutations within the PLD1 PH domain inhibited enzyme activity, whereas deletion of the domain both inhibited enzyme activity and disrupted normal PLD1 localisation. Thus, the functional PH domain regulates PLD by mediating its interaction with polyphosphoinositide-containing membranes; this might also induce a conformational change, thereby regulating catalytic activity.

Expression and regulation of phospholipase D isoforms in mammalian cell lines

Phospholipase D (PLD) is activated in mammalian cells in response to diverse stimuli that include growth factors, activators of protein kinase C, and agonists binding to G-protein-coupled receptors. Two forms of mammalian PLD, PLD1 and PLD2, have been identified. Expression of mRNA and protein for PLD1 and PLD2 was analyzed in the following cell lines: A7r5 (rat vascular smooth muscle); EL4 (mouse thymoma); HL-60 (human myeloid leukemia); Jurkat (human leukemia); PC-3 (human prostate adenocarcinoma); PC-12K (rat phaeochromocytoma); and Rat-1 HIR (rat fibroblast). All, with the exception of EL4, express agonist-activated PLD activity. PLD1 is expressed in A7r5, HL-60, PC-3, and Rat-1, while PLD2 is expressed in A7r5, Jurkat, PC12K, PC-3, and Rat-1. Neither isoform is expressed in EL4. Guanine nucleotide-independent PLD activity is present in membranes from all cells expressing PLD2. In PC12K cells, which express only PLD2, treatment with nerve growth factor causes neurite outgrowth and increases expression of PLD2 mRNA and protein within 6-12 h. A corresponding increase is observed in membrane PLD activity and in phorbol-12-myristate-13-acetate (PMA)-stimulated PLD activity in intact cells. These results show that PLD2 can be regulated both pretranslationally and posttranslationally by agonists.

Ethanol inhibits astroglial cell proliferation by disruption of phospholipase D-mediated signaling

The activation of phospholipase D (PLD) is a common response to mitogenic stimuli in various cell types. As PLD-mediated signaling is known to be disrupted in the presence of ethanol, we tested whether PLD is involved in the ethanol-induced inhibition of cell proliferation in rat cortical primary astrocytes. Readdition of fetal calf serum (FCS) to serum-deprived astroglial cultures caused a rapid, threefold increase of PLD activity and a strong mitogenic response; both effects were dependent on tyrosine kinases but not on protein kinase C. Ethanol (0.1-2%) suppressed the FCS-induced, PLD-mediated formation of phosphatidic acid (PA) as well as astroglial cell proliferation in a concentration-dependent manner. Moreover, exogenous bacterial PLD increased astroglial proliferation in an ethanol-sensitive manner, whereas exogenous PA or lysophosphatidic acid was less effective. Formation of PA and astroglial proliferation were strongly inhibited by 1-butanol (0.1-1%), a substrate of PLD, but were unaffected by t-butanol, a non-substrate; 2-butanol had intermediate effects. Platelet-derived growth factor and endothelin-1 mimicked the mitogenic effect of FCS; their effects were also inhibited by the butanols in the potency order 1-butanol > 2-butanol > tert-butanol. Our results, in particular, the differential effects of 1-, 2-, and tert-butanol with respect to PA formation and astroglial proliferation, strongly suggest that the antiproliferative effects of ethanol in glial cells are due to the disruption of the PLD signaling pathway. This mechanism may also contribute to the inhibition of astroglial growth and brain development observed in alcoholic embryopathy.

Sequential actions of phospholipase D and phosphatidic acid phosphohydrolase 2b generate diglyceride in mammalian cells

Phosphatidylcholine (PC) is a major source of lipid-derived second messenger molecules that function as both intracellular and extracellular signals. PC-specific phospholipase D (PLD) and phosphatidic acid phosphohydrolase (PAP) are two pivotal enzymes in this signaling system, and they act in series to generate the biologically active lipids phosphatidic acid (PA) and diglyceride. The identity of the PAP enzyme involved in PLD-mediated signal transduction is unclear. We provide the first evidence for a functional role of a type 2 PAP, PAP2b, in the metabolism of PLD-generated PA. Our data indicate that PAP2b localizes to regions of the cell in which PC hydrolysis by PLD is taking place. Using a newly developed PAP2b-specific antibody, we have characterized the expression, posttranslational modification, and localization of endogenous PAP2b. Glycosylation and localization of PAP2b appear to be cell type and tissue specific. Biochemical fractionation and immunoprecipitation analyses revealed that PAP2b and PLD2 activities are present in caveolin-1-enriched detergent-resistant membrane microdomains. We found that PLD2 and PAP2b act sequentially to generate diglyceride within this specialized membrane compartment. The unique lipid composition of these membranes may provide a selective environment for the regulation and actions of enzymes involved in signaling through PC hydrolysis.

Phospholipase D and membrane traffic. Potential roles in regulated exocytosis, membrane delivery and vesicle budding

It is now well-established that phospholipase D is transiently stimulated upon activation by G-protein-coupled and receptor tyrosine kinase cell surface receptors in mammalian cells. Over the last 5 years, a tremendous effort has gone to identify the major intracellular regulators of mammalian phospholipase D and to the cloning of two mammalian phospholipase D enzymes (phospholipase D1 and D2). In this chapter, we review the physiological function of mammalian phospholipase D1 that is synergistically stimulated by ADP ribosylation factor, Rho and protein kinase Calpha. We discuss the function of this enzyme in membrane traffic, emphasising the possible integrated relationships between consumption of vesicles in regulated exocytosis, membrane delivery and constitutive membrane traffic.

Regulation of phospholipase D by phosphorylation-dependent mechanisms

The rapid production of phosphatidic acid following receptor stimulation has been demonstrated in a wide range of mammalian cells. Virtually every cell uses phosphatidylcholine as substrate to produce phosphatidic acid in a controlled reaction catalyzed by specific PLD isoforms. Considerable effort has been directed at studying the regulation of PLD activities and subsequent work has characterized a family of proteins including PLD1 and PLD2. Whereas both PLD enzymes are dependent on phosphatidylinositol 4, 5-bisphosphate for activity only the PLD1 isoform was strongly stimulated by the small GTPases ARF and RhoA and by protein kinase Calpha as well. A role for tyrosine kinase activities in the membrane recruitment of small GTPases, in the synthesis of phosphatidylinositol 4,5-bisphosphate and tyrosine phosphorylation of PLD1 and PLD2 has been uncovered. However, it still not clear exactly how tyrosine phosphorylation of proteins contributes to PLD activation in cells. Here we review the data linking tyrosine phosphorylation of proteins to the activation of PLD and describe recent finding on the sites and possible mechanisms of action of tyrosine kinases in receptor-mediated PLD activation. Finally, a model illustrating the potential complex interplay linking these signaling events with the activation of PLD is presented.

Regulation of phospholipase D

Phospholipase D (PLD) is a widely distributed enzyme that is under elaborate control by hormones, neurotransmitters, growth factors and cytokines in mammalian cells. Protein kinase C (PKC) plays a major role in the regulation of the PLD1 isozyme through interaction with its N-terminus. PKC activates this isozyme by a non-phosphorylation mechanism in vitro, but phosphorylation plays a role in the action of PKC on the enzyme in vivo. Although PLD1 can be phosphorylated by PKC in vitro, it is unclear that this occurs in vivo. Small GTPases of the ADP-ribosylation factor (ARF) and Rho families directly activate PLD1 in vitro and there is evidence that Rho proteins are involved in agonist regulation of PLD1 in vivo. ARF proteins stimulate PLD activity in the Golgi apparatus, but the role of these proteins in agonist regulation of the enzyme is less clear. PLD1 undergoes tyrosine phosphorylation in response to H(2)O(2) treatment of cells. The functional consequence of this phosphorylation and soluble tyrosine kinase(s) involved are presently unknown.

Localization and possible functions of phospholipase D isozymes

The activation of PLD is believed to play an important role in the regulation of cell function and cell fate by extracellular signal molecules. Multiple PLD activities have been characterized in mammalian cells and, more recently, several PLD genes have been cloned. Current evidence indicates that diverse PLD activities are localized in most, if not all, cellular organelles, where they are likely to subserve different functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

Coupled inositide phosphorylation and phospholipase D activation initiates clathrin-coat assembly on lysosomes

Adaptors appear to control clathrin-coat assembly by determining the site of lattice polymerization but the nucleating events that target soluble adaptors to an appropriate membrane are poorly understood. Using an in vitro model system that allows AP-2-containing clathrin coats to assemble on lysosomes, we show that adaptor recruitment and coat initiation requires phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) synthesis. PtdIns(4,5)P2 is generated on lysosomes by the sequential action of a lysosome-associated type II phosphatidylinositol 4-kinase and a soluble type I phosphatidylinositol 4-phosphate 5-kinase. Phosphatidic acid, which potently stimulates type I phosphatidylinositol 4-phosphate 5-kinase activity, is generated on the bilayer by a phospholipase D1-like enzyme located on the lysosomal surface. Quenching phosphatidic acid function with primary alcohols prevents the synthesis of PtdIns(4, 5)P2 and blocks coat assembly. Generating phosphatidic acid directly on lysosomes with exogenous bacterial phospholipase D in the absence of ATP still drives adaptor recruitment and limited coat assembly, indicating that PtdIns(4,5)P2 functions, at least in part, to activate the PtdIns(4,5)P2-dependent phospholipase D1. These results provide the first direct evidence for the involvement of anionic phospholipids in clathrin-coat assembly on membranes and define the enzymes responsible for the production of these important lipid mediators.

EGF-and NGF-stimulated translocation of cytohesin-1 to the plasma membrane of PC12 cells requires PI 3-kinase activation and a functional cytohesin-1 PH domain

ADP-ribosylation factors (ARFs) are small GTP-binding proteins that function as regulators of eukaryotic vesicle trafficking. Cytohesin-1 is a member of a family of ARF guanine nucleotide-exchange factors that contain a C-terminal pleckstrin homology (PH) domain which has been proposed to bind the lipid second messenger phosphatidylinositol 3,4,5-trisphosphate (PIP3). Here we demonstrate that in vitro, recombinant cytohesin-1 binds, via its PH domain, the inositol head group of PIP3, inositol 1,3,4, 5-tetrakisphosphate (IP4), with an affinity greater than 200-fold higher than the inositol head group of either phosphatidylinositol 4, 5-bisphosphate or phosphatidylinositol 3,4-bisphosphate. Moreover, addition of glycerol or diacetylglycerol to the 1-phosphate of IP4 does not alter the ability to interact with cytohesin-1, data which is entirely consistent with cytohesin-1 functioning as a putative PIP3 receptor. To address whether cytohesin-1 binds PIP3 in vivo, we have expressed a chimera of green fluorescent protein (GFP) fused to the N terminus of cytohesin-1 in PC12 cells. Using laser scanning confocal microscopy we demonstrate that either EGF- or NGF-stimulation of transiently transfected PC12 cells results in a rapid translocation of GFP-cytohesin-1 from the cytosol to the plasma membrane. This translocation is dependent on the cytohesin-1 PH domain and occurs with a time course that parallels the rate of plasma membrane PIP3 production. Furthermore, the translocation requires the ability of either agonist to activate PI 3-kinase, since it is inhibited by wortmannin (100 nM), LY294002 (50 microM) and by coexpression with a dominant negative p85. This data therefore suggests that in vivo cytohesin-1 can interact with PIP3 via its PH domain.

The ADP-ribosylation factor (ARF)-related GTPase ARF-related protein binds to the ARF-specific guanine nucleotide exchange factor cytohesin and inhibits the ARF-dependent activation of phospholipase D

ADP-ribosylation factor-related protein (ARP) is a membrane-associated GTPase with remote similarity to the family of ADP-ribosylation factors (ARF). In a yeast two-hybrid screen designed to identify proteins interacting with ARP, we isolated a partial cDNA of the ARF-specific guanine nucleotide exchange factor mSec7-1/cytohesin encoding its N terminus and most of the Sec7 domain (codons 1-200). ARP and ARP-Q79L (GTPase-negative ARP) exhibited a higher affinity to mSec7-1-(1-200) than ARP-T31N (nucleotide exchange-defective ARP) in the two-hybrid assay. Similarly, full-length [35S]mSec7-1/cytohesin was specifically adsorbed to glutathione-Sepharose loaded with glutathione S-transferase (GST)-ARP-Q79L, GST-ARP, or GST-ARP-T31N, the latter exhibiting the lowest binding affinity. Overexpression of ARP-Q79L, but not of ARP-T31N, in COS-7 cells reduced the fluorescence from co-expressed green fluorescent protein fused with mSec7-1/cytohesin or mSec7-2/ARNO in plasma membranes as detected by deconvolution microscopy. Recombinant ARP and ARP-Q79L, but not ARP-T31N, inhibited the phospholipase D (PLD) activity stimulated by mSec7-2/ARNO and ARF in a system of isolated membranes. Furthermore, transfection of HEK-293 cells with ARP or ARP-Q79L, but not ARP-T31N, inhibited the muscarinic acetylcholine receptor-3 induced PLD stimulation and translocation of ARF from cytosol to membranes. These data suggest that the GTP-bound form of ARP specifically binds mSec7-1/cytohesin, and that ARP may be involved in a pathway inhibiting the ARF-controlled activity of PLD.