Characterization of a rat brain phospholipase D isozyme

Determination of interaction sites of phospholipase D1 for RhoA

Phospholipase D (PLD) is regulated by many factors, including protein kinase C (PKC) and small G-proteins of the Rho and ADP-ribosylation factor families. Previous studies revealed that the interaction site of human PLD(1) for RhoA is located in its C-terminus, but the exact locus has not been determined. The purpose of the present study was to determine the interaction site of rat PLD(1) (rPLD(1)) with RhoA. Selection with phage display of different peptides of rPLD(1) confirmed that GTP-bound RhoA interacted with a site in the amino acid sequence 873-1024 at the C-terminus of rPLD(1). RhoA also associated with this peptide in a GTP-dependent manner in COS-7 cell lysates and the peptide inhibited RhoA stimulation of PLD activity in membranes from COS-7 cells expressing rPLD(1). A series of alanine mutations of non-conserved residues were made in this sequence, and the enzymes were expressed in COS-7 cells and checked for responses to activation of PKC, which interacts with the N-terminus of PLD(1), and also to the constitutively active V14RhoA. Mutations in the C-terminus of rPLD(1) (K946A, V950A, R955A and K962A) caused partial loss of V14RhoA stimulation, and double mutations (K946A/K962A, K946A/V950A and K962A/V950A) caused an almost total loss. Co-immunoprecipitation studies also showed that the mutated forms of rPLD(1) described above failed to bind V14RhoA compared with wild-type rPLD(1), whereas rPLD(1) with mutations outside the region K946-K962 bound V14RhoA normally. It is concluded that basic amino acids in a restricted C-terminal region of rPLD(1) are important for binding of RhoA and its activation of PLD activity.

Interaction of phospholipase D1 with a casein-kinase-2-like serine kinase

Phospholipase D (PLD)1 was phosphorylated in vivo and by an associated kinase in vitro following immunoprecipitation. Both phosphorylation events were greatly reduced in a catalytically inactive point mutant in which the serine residue at position 911 was converted into alanine (S911A). The kinase could be enriched from detergent-extracted brain membranes and bind and phosphorylate PLD1 that was immunoprecipitated from COS-7 cells. Using in-gel kinase assays we determined that the size of the kinase is approximately 40 kDa and that PLD1 is more effective than S911A in binding the kinase. Preliminary analysis of the phosphorylation sites on PLD1 suggested that the kinase belongs to the casein kinase 2 (CK2) family. Consistent with this, we found that the kinase could utilize GTP, and could be inhibited by heparin and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). Membrane fractions from Chinese hamster ovary (CHO) cell lines that inducibly express PLD1 contained an endogenous kinase activity that phosphorylated PLD1 using GTP and was inhibited by DRB. Direct evidence that the kinase is CK2 came from observations that immunoprecipitates using PLD1 antibodies contained immunoreactive CK2alpha, and immunoprecipitates using CK2alpha antibodies contained immunoreactive PLD1. Co-expression of PLD1 in COS-7 cells with the two recombinant CK2 subunits, alpha or beta, suggests that the association of PLD1 with the kinase is through the beta subunit. Supporting this, phosphorylation of PLD1 by purified recombinant CK2alpha was enhanced by purified recombinant CK2beta. Assays measuring PLD1 catalytic activity following phosphorylation by CK2 suggest that this phosphorylation event does not influence PLD1-mediated hydrolysis of phosphatidylcholine in vitro.

Presence of a phospholipase D (PLD) distinct from PLD1 or PLD2 in human neutrophils: immunobiochemical characterization and initial purification

Utilizing the transphosphatidylation reaction catalyzed by phospholipase D (PLD) in the presence of a primary alcohol and the short-chain phospholipid PC8, we have characterized the enzyme from human neutrophils. A pH optimum of 7.8-8.0 was determined. PIP(2), EDTA/EGTA, and ATP were found to enhance basal PLD activity in vitro. Inhibitory elements were: oleate, Triton X-100, n-octyl-beta-glucopyranoside, divalent cations, GTPgammaS and H(2)O(2). The apparent K(m) for the butanol substrate was 0.1 mM and the V(max) was 6.0 nmol mg(-1) h(-1). Immunochemical analysis by anti-pan PLD antibodies revealed a neutrophil PLD of approximately 90 kDa and other bands recognized minimally by anti-PLD1 or anti-PLD2 antibodies. The 90-kDa protein is tyrosine-phosphorylated upon cell stimulation with GM-CSF and formyl-Met-Leu-Phe. Protein partial purification using column liquid chromatography was performed after cell subfractionation. Based on the enzyme's regulatory and inhibitory factors, and its molecular weight, these data indicate an enzyme isoform that might be different from the mammalian PLD1/2 forms described earlier. The present results lay the foundation for further purification of this granulocyte PLD isoform.

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.

Activation of phospholipase D by PKC and GTPgammaS in human neuroblastoma cells overexpressing MARCKS

Regulation of phospholipase D (PLD) activity participating in signal transduction involves complex interactions with small G-proteins (ARF, Rho) and protein kinase C isoforms (PKCalpha). In SK-N-MC human neuroblastoma cells, phorbol ester (TPA) activation of PLD was enhanced by overexpressing myristoylated alanine-rich C kinase substrate (MARCKS). To study MARCKS interactions with PLD, we investigated PLD isoform expression and activation by TPA and GTPgammaS in intact and digitonin-permeabilized clones transfected with MARCKS (M22). PLD2 was in both cytosol and membrane fractions while PLD1 was primarily membrane-associated in both vector control and M22 cells; location or quantities were unaltered by TPA treatment. TPA-stimulated PLD activity was higher in both intact and digitonin-permeabilized M22 cells than in vector controls. In contrast, GTPgammaS-stimulated PLD activity was independent of MARCKS expression but was additive with MARCKS-PKC-dependent activation in permeabilized cells. Combinations of PKC inhibition and down-regulation in intact and permeabilized (with GTPgammaS present) cells indicated that a PKC-mediated phosphorylation event was necessary in intact cells without access to GTPgammaS, stimulation of PLD mediated by GTPgammaS was independent of PKC, and PLD activation by PKC in permeabilized cells was kinase-independent. Western blot analysis showed that MARCKS, PKCalpha, PLD1 and PLD2 were present in a detergent-insoluble fraction (DIF); GTPgammaS increased recovery of PLD2 in DIF. Disruption of cholesterol-rich DIFs with digitonin, cyclodextrin or filipin potentiated activation of PLD by TPA. Our studies suggest that activation of PLD by PKC requires MARCKS and can involve both phosphorylation-independent and -dependent processes. As PLD activation by GTPgammaS is PKC-MARCKS-independent, MARCKS may provide a fine tuning component in conjunction with G-protein-mediated mechanisms for regulation of PLD.

Association of the N- and C-terminal domains of phospholipase D. Contribution of the conserved HKD motifs to the interaction and the requirement of the association for Ser/Thr phosphorylation of the enzyme

Rat brain phospholipase D1 (rPLD1) belongs to a superfamily defined by the highly conserved catalytic motif (H(X)K(X)(4)D, denoted HKD. rPLD1 contains two HKD domains, located in the N- and C-terminal regions. The integrity of the two HKD domains is essential for enzymatic activity. Our previous studies showed that the N-terminal half of rPLD1 containing one HKD motif can associate with the C-terminal half containing the other HKD domain to reconstruct wild type PLD activity (Xie, Z., Ho, W.-T. and Exton, J. H. (1998) J. Biol. Chem. 273, 34679-34682). In the present study, we have shown by mutagenesis that conserved amino acids in the HKD domains are important for both the catalytic activity and the association between the two halves of rPLD1. Furthermore, we found that rPLD1 could be modified by Ser/Thr phosphorylation. The modification occurred at the N-terminal half of the enzyme, however, the association of the N-terminal domain with the C-terminal domain was required for the modification. The phosphorylation of the enzyme was not required for its catalytic activity or response to PKCalpha and small G proteins in vitro, although the phosphorylated form of rPLD1 was localized exclusively in the crude membrane fraction. In addition, we found that the individually expressed N- and C-terminal fragments did not interact when mixed in vitro and were unable to reconstruct PLD activity under these conditions. It is concluded that the association of the N- and C-terminal halves of rPLD1 requires their co-expression in vivo and depends on conserved residues in the HKD domains. The association is also required for Ser/Thr phosphorylation of the enzyme.

Different regulation of phospholipase D activity in glioma C6 cells by sphingosine, propranolol, imipramine and phorbol ester

In has been found that sphingosine, propranolol, imipramine and phorbol ester (12-O-tetradecanoylphorbol-13-acetate, TPA) have a stimulatory effect on phospholipase D activity in glioma C6 cells. The cells were prelabelled with [1-(14)C]palmitic acid and phospholipase D-mediated synthesis of [(14)C]phosphatidylethanol was measured. The enhancing effect of TPA was almost completely blocked by a specific protein kinase C inhibitor, GF 109203X. In contrast, GF 109203X failed to inhibit the sphingosine, imipramine and propranolol stimulatory effects, indicating that their stimulation was independent of protein kinase C. The effect of TPA on phospholipase D was also blocked by imipramine and propranolol, whereas sphingosine additively potentiated TPA-mediated phospholipase D activity, both at shorter and longer (2-60 min) times of incubation. These results suggest that in glioma C6 cells, sphingosine is not only involved in a different phospholipase D activation than the TPA regulatory system, but also that it operates in a different compartment of the cell.

Protein kinase C-stimulated formation of ethanolamine from phosphatidylethanolamine involves a protein phosphorylation mechanism: negative regulation by p21 Ras protein

Mammalian cells express a phospholipase D (PLD)-like enzyme which forms ethanolamine from phosphatidylethanolamine (PtdEtn) by a protein kinase C-alpha (PKC-alpha)-activated, presently unknown, mechanism. Now we report that addition of a PKC-alpha-enriched purified PKC preparation or recombinant PKC-alpha to a plasma membrane-enriched membrane fraction, isolated from leukemic HL60 cells, greatly ( approximately 6.5-fold stimulation) enhanced PtdEtn hydrolysis if the PKC activator phorbol 12-myristate 13-acetate (PMA) and ATP were both present; this was accompanied by PKC-mediated phosphorylation of several membrane proteins. The combined effects of PKC-alpha, ATP, and PMA on [(14)C]PtdEtn hydrolysis were inhibited by GF 109203X (10 microM), an inhibitor of catalytic activity of PKC. In this membrane fraction, PMA alone also had a smaller ( approximately 3.5-fold) stimulatory effect on PtdEtn hydrolysis which was not affected by adding ATP or GF 109203X to the membranes. These results suggest that PMA can stimulate PtdEtn hydrolysis via a PKC-catalyzed phosphorylation mechanism as well as by a phosphorylation-independent process. Transformation of NIH 3T3 fibroblasts by H-ras reduced the effect of PMA on PtdEtn hydrolysis. Furthermore, in NIH 3T3 fibroblasts, scrape-loaded Y13-259 anti Ras antibody enhanced PMA-stimulated hydrolysis of PtdEtn. These results suggest that activation of the PtdEtn-hydrolyzing PLD enzyme by PKC-alpha is inhibited by p21 Ras.

Subcellular distribution and characterization of rat pancreatic phospholipase D isoforms

This study was undertaken to characterize the biochemical properties of rat pancreatic phospholipase D (PLD). Based on Western blot analysis of pancreas subcellular fractions, PLD1 was detected as a protein of 120 kDa associated with the microsomal fraction, whereas PLD2 appeared as a 105-kDa protein enriched in the microvesicular fraction. In these fractions, a low level of PLD activity was measured with an exogenous substrate containing phosphatidylinositol-4,5-bisphosphate (PIP2), unresponsive to guanosine triphosphate (GTP)gammaS and adenosine diphosphate (ADP)-ribosylation factor (ARF). Addition of unsaturated but not saturated fatty acids stimulated an oleate-dependent PLD activity that colocalized with the PLD1 enzyme in the crude plasma membrane and microsomal fractions. The transphosphatidylation reaction was maximal with either 200-400 mM (1.2-2.3%) ethanol or 25 mM (0.23%) 1-butanol, with an optimal pH between 6.5 and 7.2. Lipids extracted from the pancreatic membranes were potent inhibitors of the HL60 cell PLD activity when compared with those isolated from HL60 cells. Oleate-dependent PLD activity was less susceptible to these inhibitions. A phospholipase A1 (PLA1) activity hydrolyzing phosphatidylethanol also was found in the pancreatic membrane fractions and was nearly absent in the HL60 cells. This activity was completely inhibited by 400 nM tetrahydrolipstatin (THL), a lipase inhibitor. Pancreatic PLD1 and PLD2 activities could be measured after a chromatographic separation from microsomal membranes and high-speed supernatants, respectively. Activities of both enzymes were inhibited by oleate and required the presence of PIP2 in the substrate vesicles. ARF1 strongly activated PLD1 in a dose-dependent manner, and PLD2 was slightly responsive. Indirect immunofluorescence revealed that PLD2 is distributed throughout the pancreas, with a more intense staining in the islets. This study presents for the first time biochemical characteristics of the pancreatic PLD activities and shows the presence of oleate-dependent PLD1 and PLD2 activities, as well as PLD1 and PLD2 proteins in this gland.

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.

Adenosine receptor occupancy suppresses chemoattractant-induced phospholipase D activity by diminishing membrane recruitment of small GTPases

Adenosine (Ado) is an important autocrine modulator of neutrophil functions. In this study, we determined the effects of endogenous Ado on fMet-Leu-Phe (fMLP)–induced phospholipase D (PLD) activity in neutrophils. The removal of extracellular Ado by Ado deaminase (ADA) or the blockade of its action by the A2a receptor antagonists 8-(3-chlorostyryl) caffeine (CSC) or CGS15943 markedly increased fMLP-induced PLD activation. The concentration-dependent stimulatory effects of CSC and CGS15943 were abolished by a pretreatment of neutrophil suspensionswith ADA. In contrast, the selective A2a receptor agonist CGS21680 suppressed fMLP-induced PLD activation. Furthermore, inhibition by CGS21680 of fMLP-induced PLD activity was reversed by CSC or CGS15943. The removal of Ado by ADA or the blockade of its action by CSC or CGS15943, markedly increased the membrane recruitment of cytosolic protein kinase C (PKC), RhoA, and ADP-ribosylation factor (ARF) in response to fMLP. As shown for PLD activity, the stimulatory effect of Ado receptor antagonists on PLD cofactors translocation was abolished by a pretreatment of the cells with ADA. Moreover, the membrane translocation of both PKC, RhoA, and ARF in response to fMLP was attenuated by CGS21680 and this effect of the A2a receptor agonist was antagonized by CSC or CGS15943. These data demonstrate that Ado released by neutrophils in the extracellular milieu inhibits PLD activation by blocking membrane association of ARF, RhoA, and PKC through Ado A2a receptor occupancy.

Cytosolic Phospholipase A2-Mediated Regulation of Phospholipase D2 in Leukocyte Cell Lines

Phospholipase D (PLD) has been implicated in a variety of cellular processes, including inflammation, secretion, and respiratory burst. Two distinct PLD isoforms, designated PLD1 and PLD2, have been cloned; however, the regulatory mechanism for each PLD isoform is not clear. In our present study we investigated how PLD2 activity is regulated in mouse lymphocytic leukemia L1210 cells, which mainly contain PLD2 , and in PLD2 -transfected COS-7 cells. Intriguingly, A23187, a calcium ionophore that induces calcium influx, potently stimulates PLD activity in these two cell lines, suggesting that Ca2+ might be implicated in the regulation of the PLD2 activity. In addition to the A23187-induced PLD2 activation, A23187 also increases PLA2-mediated arachidonic acid release, and the A23187-stimulated PLD2 and PLA2 activities could be blocked by pretreatment of the cells with cytosolic calcium-dependent PLA2 (cPLA2) inhibitors, such as arachidonyl trifluoromethyl ketone and methyl arachidonyl fluorophosphonate in these two cell lines. Moreover, the A23187-induced PLD2 and PLA2 activities could be inhibited by cotransfection with antisense cPLA2 oligonucleotide. These results suggest a role for cPLA2 in the regulation of PLD2 activity in vivo. The inhibitory effect of arachidonyl trifluoromethyl ketone on the A23187-induced PLD2 activity could be recovered by addition of exogenous lysophosphatidylcholine. This study is the first to demonstrate that PLD2 activity is up-regulated by Ca2+ influx and that cPLA2 may play a key role in the Ca2+-dependent regulation of PLD2 through generation of lysophosphatidylcholine.

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.

Expression of phospholipase D isoforms in mammalian cells

Two mammalian isoforms of phospholipase D, PLD1 and PLD2, have recently been characterized at the molecular level. Effects of physiologic agonists on PLD activity in intact cells, as characterized in earlier studies, have generally not been attributed to specific PLD isoforms. Recent work has established that expression of PLD1 and PLD2 varies within tissues and between cell lines. A single cell type can express one, both, or neither isoform, although most cells co-express PLD1 and PLD2. Lymphocytes often lack expression of one or both isoforms of PLD. Relative levels of PLD mRNA expression vary considerably between established cell lines. Expression of transcripts for both PLD1 and PLD2 can be regulated at the transcriptional level by growth and differentiation factors in cultured cells. Thus, it is apparent that the known mammalian PLD isoforms are subject to regulation at the transcriptional level. The available data do not conclusively establish whether PLD1 and PLD2 are the only isoforms responsible for agonist-mediated PLD activation. Further studies of the regulation of expression of PLD isoforms should provide insight into the roles of PLD1 and PLD2 in physiologic responses, and may suggest whether additional forms of PLD remain to be characterized.

Arfaptin 1 forms a complex with ADP-ribosylation factor and inhibits phospholipase D

ADP-ribosylation factors (ARFs) regulate coatomer assembly on the Golgi as well as recruitment of clathrin adapter proteins and are therefore involved in vesicle budding from the Golgi and vesicular transport. They are also regulators of phospholipase D (PLD) activity. Arfaptin 1 is an ARF binding protein that inhibits PLD activation, vesicular trafficking and secretion. In the present report, we show that arfaptin 1 interacts with 'high speed' membranes independently of ARF. However, addition of myristoylated ARF3 (myrARF3) increases the association of arfaptin 1 with the membranes, suggesting that arfaptin 1 and ARF form a complex on the Golgi. Utilizing several deletion mutants of arfaptin 1 it is shown that the association of arfaptin 1 with myrARF3 is mediated via two binding sites on arfaptin 1. These two domains are needed for arfaptin 1 inhibition of PLD activation by myrARF3 in vitro.

Selective activation of phospholipase D2 by unsaturated fatty acid

Although oleate has been implicated in the regulation of phospholipase D (PLD) activity, the molecular identity of the oleate-stimulated PLD is still poorly understood. We now report that oleate selectively stimulates the enzymatic activity of PLD2 but not of PLD1, with an optimal concentration of 20 microM in vitro. Intriguingly, phosphatidylinositol 4,5-bisphosphate (PIP2) synergistically stimulates the oleate-dependent PLD2 activity with an optimal concentration of 2.5 microM. These results provide the first evidence that oleate is a PLD2-specific activating factor and PLD2 activity is synergistically stimulated by oleate and PIP2.

A role for rho-kinase in rho-controlled phospholipase D stimulation by the m3 muscarinic acetylcholine receptor

Stimulation of phospholipase D (PLD) by membrane receptors is now recognized as a major signal transduction pathway involved in diverse cellular functions. Rho proteins control receptor signaling to PLD, and these GTPases have been shown to directly stimulate purified recombinant PLD1 enzymes in vitro. Here we report that stimulation of PLD activity, measured in the presence of phosphatidylinositol 4,5-bisphosphate, by RhoA in membranes of HEK-293 cells expressing the m3 muscarinic acetylcholine receptor (mAChR) is phosphorylation-dependent. Therefore, the possible involvement of the RhoA-stimulated serine/threonine kinase, Rho-kinase, was investigated. Overexpression of Rho-kinase and constitutively active Rho-kinase (Rho-kinase-CAT) but not of kinase-deficient Rho-kinase-CAT markedly increased m3 mAChR-mediated but not protein kinase C-mediated PLD stimulation, similar to overexpression of RhoA. Expression of the Rho-inactivating C3 transferase abrogated the stimulatory effect of wild-type Rho-kinase, but not of Rho-kinase-CAT. Recombinant Rho-kinase-CAT mimicked the phosphorylation-dependent PLD stimulation by RhoA in HEK-293 cell membranes. Finally, the Rho-kinase inhibitor HA-1077 largely inhibited RhoA-induced PLD stimulation in membranes as well as PLD stimulation by the m3 mAChR but not by protein kinase C in intact HEK-293 cells. We conclude that Rho-kinase is involved in Rho-dependent PLD stimulation by the G protein-coupled m3 mAChR in HEK-293 cells. Thus, our findings identify Rho-kinase as a novel player in the receptor-controlled PLD signaling pathway.

1,25-dihydroxyvitamin D3 and TPA activate phospholipase D in Caco-2 cells: role of PKC-alpha

1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] and 12-O-tetradecanoylphorbol 13-acetate (TPA) both activated phospholipase D (PLD) in Caco-2 cells. GF-109203x, an inhibitor of protein kinase C (PKC) isoforms, inhibited this activation by both of these agonists. 1,25(OH)2D3 activated PKC-alpha, but not PKC-beta1, -betaII, -delta, or -zeta, whereas TPA activated PKC-alpha, -beta1, and -delta. Chronic treatment with TPA (1 microM, 24 h) significantly reduced the expression of PKC-alpha, -betaI, and -delta and markedly reduced the ability of 1,25(OH)2D3 or TPA to acutely stimulate PLD. Removal of Ca2+ from the medium, as well as preincubation of cells with Gö-6976, an inhibitor of Ca2+-dependent PKC isoforms, significantly reduced the stimulation of PLD by 1,25(OH)2D3 or TPA. Treatment with 12-deoxyphorbol-13-phenylacetate-20-acetate, which specifically activates PKC-betaI and -betaII, however, failed to stimulate PLD. In addition, the activation of PLD by 1,25(OH)2D3 or TPA was markedly reduced or accentuated in stably transfected cells with inhibited or amplified PKC-alpha expression, respectively. Taken together, these observations indicate that PKC-alpha is intimately involved in the stimulation of PLD in Caco-2 cells by 1,25(OH)2D3 or TPA.

Overexpression of protein kinase C-epsilon and its regulatory domains in fibroblasts inhibits phorbol ester-induced phospholipase D activity

In fibroblasts, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) stimulates phospholipase D (PLD)-mediated hydrolysis of both phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) by PKC-alpha-mediated nonphosphorylating and phosphorylating mechanisms. Here we have used NIH 3T3 fibroblasts overexpressing holo PKC-epsilon and its regulatory, catalytic, and zinc finger domain fragments to determine if this isozyme also regulates PLD activity. Overexpression of holo PKC-epsilon inhibited the stimulatory effects of PMA (5-100 nM) on both PtdCho and PtdEtn hydrolysis. Overexpression of PKC-epsilon also was found to inhibit platelet-derived growth factor-induced PLD activity. Expression of the catalytic unit of PKC-epsilon had no effect on PMA-induced PLD activity. In contrast, expression of both the regulatory domain fragment and the zinc finger domain of PKC-epsilon resulted in significant inhibition of PMA-stimulated PtdCho and PtdEtn hydrolysis. Interestingly, although PKC-alpha also mediates the stimulatory effect of PMA on the synthesis of PtdCho by a phosphorylation mechanism, overexpression of holo PKC-epsilon or its regulatory domain fragments did not affect PMA-induced PtdCho synthesis. These results indicate that the PKC-epsilon system can act as a negative regulator of PLD activity and that this inhibition is mediated by its regulatory domain.

Association of N- and C-terminal domains of phospholipase D is required for catalytic activity

Rat brain phospholipase D1 (rPLD1) belongs to a superfamily defined by the highly conserved catalytic motif (H(X)K(X)4D, denoted HKD. RPLD1 contains two HKD domains, located in the N- and C-terminal regions. Deletion mutants of rPLD1 that contained only an N- or C-terminal HKD domain exhibited no catalytic activity when expressed in COS 7 cells. However, when N-terminal fragments containing one of the HKD domains were cotransfected with a C-terminal fragment containing the other HKD domain, PLD activity was restored. Furthermore, immunoprecipitation assays showed that the N- and C-terminal halves of rPLD1 were physically associated when expressed in COS 7 cells. In addition, deletion of 168 amino acids from the N terminus of rPLD1 significantly enhanced basal PLD activity while inhibiting the response to phorbol ester. Likewise, the coexpression of this truncated N-terminal half with the C-terminal half resulted in increased PLD activity. In summary, this study provides direct evidence that the enzymatic activity of rPLD1 requires the presence of the HKD domains in both the N- and C-terminal regions of the molecule. More importantly, the two halves of rPLD1 can associate, and this may be essential to bring the two HKD domains together to form an active catalytic center. These findings provide new insights into the catalytic mechanism of enzymes of the PLD superfamily.

Involvement of tyrosine phosphorylation and protein kinase C in the activation of phospholipase D by H2O2 in Swiss 3T3 fibroblasts

We have investigated the mechanisms involved in H2O2-mediated phospholipase D (PLD) activation in Swiss 3T3 fibroblasts. In the presence of vanadate, H2O2 induced tyrosine phosphorylation of PLD as well as the platelet-derived growth (PDGF) factor receptor, protein kinase Calpha (PKCalpha), and a 62-kDa protein in rat brain PLD1 (rPLD1) immune complexes. PDGF also induced tyrosine phosphorylation of PLD, but this was abolished by catalase, indicating that it was mediated by H2O2 generation. Interestingly, PLD was found to be constitutively associated with the PDGF receptor and PKCalpha. Stimulation by H2O2 showed a concentration- and time-dependent tyrosine phosphorylation of the proteins in rPLD1 immunoprecipitates and activation of PLD in the cells. Pretreatment of the cells with the protein-tyrosine kinase inhibitors genistein and herbimycin A resulted in a concentration-dependent inhibition of H2O2-induced tyrosine phosphorylation and PLD activation. Activation of PLD by H2O2 was also inhibited dose-dependently by the PKC inhibitors Ro 31-8220 and calphostin C. Down-regulation of PKC by prolonged treatment with 4beta-phorbol 12-myristate 13-acetate also abolished H2O2-stimulated PLD activity. H2O2 or vanadate alone did not induce tyrosine phosphorylation of proteins in the rPLD1 immune complex or PLD activation. Reduction of intracellular H2O2 levels by pretreatment of the cells with catalase dramatically abrogated tyrosine phosphorylation of proteins in the rPLD1 immune complex and PLD activation, suggesting the potential role of intracellular H2O2 in H2O2-mediated PLD signaling. Taken together, these results suggest that both protein-tyrosine kinase(s) and protein kinase C participate in H2O2-induced PLD activation in Swiss 3T3 cells.

Characterization of phospholipase D activity in bovine photoreceptor membranes

Phospholipase D (E.C. 3.1.4.4.) was detected in isolated bovine rod outer segments (ROS) and its properties determined. The enzyme activity was assayed using either a sonicated microdispersion of 1,2-diacyl-sn-[2(3)H]glycerol-3-phosphocholine (PC), or [14C]ethanol. Using [3H]PC and ethanol as a substrate, we were able to detect the hydrolytic properties as well as the transphosphatidylation reaction catalyzed by phospholipase D (PLD): formation of [3H]phosphatidic acid and phosphatidylethanol [3H]PtdEt; whereas with [14C]ethanol or [3H]glycerol in the absence of exogenous PC, only transphosphatidylation reactions were detected (formation of [14C]PtdEt or [3H]phosphatidylglycerol, respectively). The use of varying concentrations of [3H]PC and 400 mM of ethanol gave an apparent Km value for PC of 0.51 mM and a Vmax value of 111 nmol x h(-1) x (mg protein)(-1). The activity was linear up to 60 min of incubation and up to 0.2 mg of protein. The optimal ethanol concentration was determined to be 400 mM, with an apparent Km of 202 mM and a Vmax value for ethanol of 125 nmol x h(-1) x (mg protein)(-1). A clear pH optimum was observed around 7. PLD activity was increased in the presence of 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate or sodium deoxycholate and inhibited with Triton X-100. The enzyme activity was also activated in the presence of Ca2+ or Mg2+ (1 mM) although these ions were not required for measuring PLD activity. The high specific activity of PLD found in purified ROS compared to the activity found in other subcellular fractions of the bovine retina suggests that this enzymatic activity is native to ROS. The present report is the first evidence of PLD activity associated with photoreceptor ROS.

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

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

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

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

Determination of interaction sites on the small G protein RhoA for phospholipase D

Phospholipase D (PLD) has been identified as a target of small G proteins of the Rho family. The present study was directed at defining the interaction sites of RhoA with rat brain PLD in vitro using chimeric proteins between RhoA and Ha-Ras or Cdc42Hs and point mutations. The switch I region of RhoA, which is the common effector domain of Ras-like G proteins, was a crucial interaction site for PLD. Mutations in conserved amino acids (Tyr34, Thr37, Phe39) totally abolished PLD activation, while mutations in Val38 or Tyr42 caused partial loss. Two additional sites were responsible for the differential PLD activation ability between RhoA and Cdc42Hs. Changing Asp76 in the switch II region of RhoA to the corresponding amino acid in Cdc42Hs led to partial loss of PLD activation. A chimeric protein with the N-terminal third of Cdc42Hs changed to RhoA showed enhanced PLD activation. Analysis of other Rho/Ha-Ras chimeric proteins and mutations indicated that Gln52 adjacent to the switch II region is responsible for this gain of function. In conclusion, the present study shows that conserved amino acids in the switch I region of RhoA are major PLD interaction sites and that residues in the switch II and internal regions are responsible for the differential activation of PLD by RhoA and Cdc42Hs.