ADP-ribosylation factor translocation correlates with potentiation of GTP gamma S-stimulated phospholipase D activity in membrane fractions of HL-60 cells

Early stages of Golgi vesicle and tubule formation require diacylglycerol

We have investigated the role for diacylglycerol (DAG) in membrane bud formation in the Golgi apparatus. Addition of propranolol to specifically inhibit phosphatidate phosphohydrolase (PAP), an enzyme responsible for converting phosphatidic acid into DAG, effectively prevents formation of membrane buds. The effect of PAP inhibition on Golgi membranes is rapid and occurs within 3 min. Removal of the PAP inhibitor then results in a rapid burst of buds, vesicles, and tubules that peaks within 2 min. The inability to form buds in the presence of propranolol does not appear to be correlated with a loss of ARFGAP1 from Golgi membranes, as knockdown of ARFGAP1 by RNA interference has little or no effect on actual bud formation. Rather, knockdown of ARFGAP1 results in an increase in membrane buds and a decrease of vesicles and tubules suggesting it functions in the late stages of scission. How DAG promotes bud formation is discussed.

The PGE2-induced inhibition of the PLD activation pathway stimulated by fMLP in human neutrophils is mediated by PKA at the PI3-Kgamma level

Prostaglandin E2 (PGE2), an eicosanoid that modulates inflammation, inhibits several chemoattractant-elicited functions in neutrophils such as chemotaxis, production of superoxide anions, adhesion, secretion of cytotoxic enzymes and synthesis of leukotriene B4. We previously reported that PGE2 inhibits the fMLP signaling pathway that leads to PLD activation through suppression of PI3-Kgamma activity and the decreased recruitment to membranes of PLD activation factors, PKC, Rho and Arf-GTPases. This effect is mediated via the EP2 receptors known to raise cAMP in cells. The inhibition of most fMLP-induced functional responses by PGE2 via EP2 receptors is mediated by PKA, except the chemotactic response. We have investigated the role of PKA in the EP2-mediated inhibition of the PLD activation pathway. H-89, a selective PKA pharmacological inhibitor suppressed the inhibitory effects of PGE2 at all stages of the PLD pathway activated by fMLP, i.e. PLD activity, translocation to membranes of PKCalpha, Rho and Arf-GTPases, calcium influx, tyrosine phosphorylation of proteins and finally translocation of p110gamma catalytic subunit of PI3-K to membranes. However, neither PLD nor PI3-Kgamma was substrate of PKA. These data provide evidence that PGE2-stimulated PKA activity regulates the PLD pathway stimulated by fMLP at the level of PI3-Kgamma and that the inhibition of PI3-Kgamma activation by PKA is a complex mechanism that remains to be completely elucidated.

Induction of cell cycle changes and modulation of apoptogenic/anti-apoptotic and extracellular signaling regulatory protein expression by water extracts of I'm-Yunity (PSP)

Background

I'm-Yunity™ (PSP) is a mushroom extract derived from deep-layer cultivated mycelia of the patented Cov-1 strain of Coriolus versicolor (CV), which contains as its main bioactive ingredient a family of polysaccharo-peptide with heterogeneous charge properties and molecular sizes. I'm-Yunity™ (PSP) is used as a dietary supplement by cancer patients and by individuals diagnosed with various chronic diseases. Laboratory studies have shown that I'm-Yunity™ (PSP) enhances immune functions and also modulates cellular responses to external challenges. Recently, I'm-Yunity™ (PSP) was also reported to exert potent anti-tumorigenic effects, evident by suppression of cell proliferation and induction of apoptosis in malignant cells. We investigate the mechanisms by which I'm-Yunity™ (PSP) elicits these effects.

Methods

Human leukemia HL-60 and U-937 cells were incubated with increasing doses of aqueous extracts of I'm-Yunity™ (PSP). Control and treated cells were harvested at various times and analyzed for changes in: (1) cell proliferation and viability, (2) cell cycle phase transition, (3) induction of apoptosis, (4) expression of cell cycle, apoptogenic/anti-apoptotic, and extracellular regulatory proteins.

Results

Aqueous extracts of I'm-Yunity™ (PSP) inhibited cell proliferation and induced apoptosis in HL-60 and U-937 cells, accompanied by a cell type-dependent disruption of the G₁/S and G₂/M phases of cell cycle progression. A more pronounced growth suppression was observed in treated HL-60 cells, which was correlated with time- and dose-dependent down regulation of the retinoblastoma protein Rb, diminution in the expression of anti-apoptotic proteins bcl-2 and survivin, increase in apoptogenic proteins bax and cytochrome c, and cleavage of poly(ADP-ribose) polymerase (PARP) from its native 112-kDa form to the 89-kDa truncated product. Moreover, I'm-Yunity™ (PSP)-treated HL-60 cells also showed a substantial decrease in p65 and to a lesser degree p50 forms of transcription factor NF-κB, which was accompanied by a reduction in the expression of cyclooxygenase 2 (COX2). I'm-Yunity™ (PSP) also elicited an increase in STAT1 (signal transducer and activator of transcription) and correspondingly, decrease in the expression of activated form of ERK (extracellular signal-regulated kinase).

Conclusion

Aqueous extracts of I'm-Yunity™ (PSP) induces cell cycle arrest and alterations in the expression of apoptogenic/anti-apoptotic and extracellular signaling regulatory proteins in human leukemia cells, the net result being suppression of proliferation and increase in apoptosis. These findings may contribute to the reported clinical and overall health effects of I'm-Yunity™ (PSP).

Role of the conserved NPxxY motif of the 5-HT2A receptor in determining selective interaction with isoforms of ADP-ribosylation factor (ARF)

In this study we have shown that N376 to D mutation in the conserved NPxxY motif within the carboxy terminal tail domain (CT) of the 5-HT2A receptor alters the binding preference of GST-fusion protein constructs of the CT domain from ARF1 to an alternative isoform, ARF6. These findings were corroborated by experiments investigating co-immunoprecipitation of the wild type (WT) and N376D mutant of the 5-HT2A receptor with ARF1 or 6 or dominant negative ARF1/6 constructs co-expressed in COS7 cells. In functional assays of 5-HT-induced phospholipase D (PLD) activation responses of the WT receptor were inhibited by a dominant negative mutant of ARF1 but not ARF6, whereas responses of the N376D mutant were strongly inhibited by negative mutant ARF6. No equivalent effect of the ARF mutants was seen on phospholipase C activation. In experiments assaying 5-HT-induced increases in [35S]GTPgammaS binding to ARF 1/6 immunoprecipitates as a measure of ARF activation, increased ARF6 activation was seen only with the mutant receptor. When cellular PLD responses of other NPxxY- or a DPxxY-containing GPCRs were measured in the presence of dominant negative ARF1/6 constructs, the majority, but not all, fitted the pattern exemplified by the 5-HT2A receptor and its N376D mutant. These data suggest that the presence of the N or a D in this highly conserved motif is an important, but not exclusive, determinant of which ARF isoform interacts with the GPCR.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Prostaglandin E2 inhibits the phospholipase D pathway stimulated by formyl-methionyl-leucyl-phenylalanine in human neutrophils. Involvement of EP2 receptors and phosphatidylinositol 3-kinase gamma

Prostaglandin E(2) (PGE(2)), originally discovered as a pro-inflammatory mediator, also inhibits several chemoattractant-elicited neutrophil functions, including adhesion, secretion of cytotoxic enzymes, production of superoxide anions, and chemotaxis. In this study, we have examined the effects of PGE(2) and prostaglandin E (EP) receptor-selective agonists/antagonists on several steps of the formyl-methionyl-leucyl-phenylalanine (fMLP)-induced phospholipase D (PLD) activation pathway in human neutrophils to elucidate the PGE(2) inhibitory mechanism. PGE(2) and EP(2) receptor agonists inhibited the stimulation of the activity of PLD induced by fMLP in a concentration-dependent manner. The fMLP-stimulated translocation to membranes of protein kinase C alpha, Rho, and Arf GTPases was diminished in the presence of PGE(2) or EP(2) agonists. Moreover, PGE(2) and EP(2) agonists decreased the activation of phosphatidylinositol 3-kinase gamma (PI3Kgamma) and Tec kinases as well as the tyrosine phosphorylation of proteins stimulated by fMLP. These data provide strong evidence that 1) the inhibitory effects of PGE(2) on the fMLP-induced PLD activation pathway were mediated via EP(2) receptors and that 2) the suppression of PI3Kgamma activity was the crucial step in the EP(2)-mediated inhibition of the fMLP-induced signaling cascade.

Ceramide inhibition of phospholipase D and its relationship to RhoA and ARF1 translocation in GTP gamma S-stimulated polymorphonuclear leukocytes

Phospholipase D (PLD) regulates the polymorphonuclear leukocyte (PMN) functions of phagocytosis, degranulation, and oxidant production. Ceramide inhibition of PLD suppresses PMN function. In streptolysin O-permeabilized PMNs, PLD was directly activated by guanosine 5'-[gamma-thio]triphosphate (GTP gamma S) stimulation of adenosine diphosphate (ADP)-ribosylation factor (ARF) and Rho, stimulating release of lactoferrin from specific granules of permeabilized PMNs; PLD activation and degranulation were inhibited by C2-ceramide but not dihydro-C2-ceramide. To investigate the mechanism of ceramide's inhibitory effect on PLD, we used a cell-free system to examine PLD activity and translocation from cytosol to plasma membrane of ARF, protein kinase C (PKC)alpha and beta, and RhoA, all of which can activate PLD. GTP gamma S-activated cytosol stimulated PLD activity and translocation of ARF, PKC alpha and beta, and RhoA when recombined with cell membranes. Prior incubation of PMNs with 10 microM C2-ceramide inhibited PLD activity and RhoA translocation, but not ARF1, ARF6, PKC alpha, or PKC beta translocation. However, in intact PMNs stimulated with N-formyl-1-methionyl-1-leucyl-1-phenylalamine (FMLP) or permeabilized PMNs stimulated with GTP gamma S, C2-ceramide did not inhibit RhoA translocation. Exogenous RhoA did not restore ceramide-inhibited PLD activity but bound to membranes despite ceramide treatment. These observations suggest that, although ceramide may affect RhoA in some systems, ceramide inhibits PLD through another mechanism, perhaps related to the ability of ceramide to inhibit phosphatidylinositol-bisphosphate (PIP2) interaction with PLD.

ADP-ribosylation factor-dependent phospholipase D activation by the M3 muscarinic receptor

G protein-coupled receptors can potentially activate phospholipase D (PLD) by a number of routes. We show here that the native M3 muscarinic receptor in 1321N1 cells and an epitope-tagged M3 receptor expressed in COS7 cells substantially utilize an ADP-ribosylation factor (ARF)-dependent route of PLD activation. This pathway is activated at the plasma membrane but appears to be largely independent of G, phospholipase C, Ca2+ q/11, protein kinase C, tyrosine kinases, and phosphatidyl inositol 3-kinase. We report instead that it involves physical association of ARF with the M3 receptor as demonstrated by co-immunoprecipitation and by in vitro interaction with a glutathione S-transferase fusion protein of the receptor's third intracellular loop domain. Experiments with mutant constructs of ARF1/6 and PLD1/2 indicate that the M3 receptor displays a major ARF1-dependent route of PLD1 activation with an additional ARF6-dependent pathway to PLD1 or PLD2. Examples of other G protein-coupled receptors assessed in comparison display alternative pathways of protein kinase C- or ARF6-dependent activation of PLD2.

Chemotactic factor-induced recruitment and activation of Tec family kinases in human neutrophils. II. Effects of LFM-A13, a specific Btk inhibitor

Tyrosine phosphorylation events play major roles in the initiation and regulation of several functional responses of human neutrophils stimulated by chemotactic factors such as the bacterially derived tripeptide formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). However, the links between the G protein-coupled receptors, the activation of the tyrosine kinases, and the initiation of neutrophil functional responses remain unclear. In the present study we assessed the effects of a Btk inhibitor, leflunomide metabolite analog (LFM-A13), on neutrophils. LFM-A13 decreased the tyrosine phosphorylation induced by fMet-Leu-Phe and inhibited the production of superoxide anions and the stimulation of adhesion, chemotaxis, and phospholipase D activity. We observed a decreased accumulation of phosphatidylinositol-3,4,5-trisphosphate in response to fMet-Leu-Phe in LFM-A13-pretreated cells even though the inhibitor had no direct effect on the lipid kinase activity of the p110 gamma or p85/p110 phosphatidylinositol 3-kinases or on the activation of p110 gamma by fMet-Leu-Phe. The phosphorylation of Akt and of extracellular signal-regulated kinases 1/2 and p38 were similarly inhibited by LFM-A13. LFM-A13 also negatively affected the translocation of Rac-2, RhoA, ADP ribosylation factor-1, Tec, Bmx, and Btk induced by fMet-Leu-Phe. The results of this study provide evidence for an involvement of Btk and possibly other Tec kinase family members in the regulation of the functional responsiveness of human neutrophils and link these events, in part at least, to the modulation of levels of phosphatidylinositol-3,4,5-trisphosphate.

Inhibition of formyl-methionyl-leucyl-phenylalanine-stimulated phospholipase D activation in rat neutrophils by the synthetic isoquinoline DMDI

The expression of phospholipase D (PLD) isoenzymes in neutrophils was investigated using reverse transcription-polymerase chain reaction analysis. Amplification products of predicted size were obtained from rat neutrophils with nucleotide sequences corresponding to PLD1a and PLD2. 1-(3',4'-Dimethoxybenzyl)-6,7-dichloroisoquinoline (DMDI) inhibited the formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated PLD activation in rat neutrophils. The underlying cellular signaling mechanism of DMDI inhibition was investigated. The fMLP-induced protein tyrosine phosphorylation and the membrane translocation of ADP-ribosylation factor (ARF) and Rho A in neutrophils was attenuated by DMDI in a concentration-dependent manner. However, neither the membrane association of protein kinase C-alpha and -beta isoenzymes in fMLP-stimulated cells nor the GTPgammaS- and phorbol 12-myristate 13-acetate-stimulated membrane translocation of ARF and Rho A in a cell-free system was affected significantly by DMDI. These results indicate that the expression of PLD1a and PLD2 mRNA in neutrophils. Attenuation of protein tyrosine phosphorylation and the membrane association of ARF and Rho A probably play a concerted role in the inhibition of PLD by DMDI in rat neutrophils in response to fMLP.

ADP-ribosylation factor 4 small GTPase mediates epidermal growth factor receptor-dependent phospholipase D2 activation

The epidermal growth factor receptor (EGFR) plays a critical role in the development, proliferation, and differentiation of cells of epithelial and mesenchymal origin. These EGFR-dependent cellular processes are mediated by a repertoire of intracellular signaling pathways triggered by the activation of the EGFR cytoplasmic domain, which originates from ligand binding of its extracellular domain. To understand the molecular mechanisms by which the intracellular domain of EGFR transmits mitogenic messages to the downstream signaling pathways, we used the cytoplasmic region of EGFR as bait in yeast two-hybrid screening. We found that ADP-ribosylation factor 4 (ARF4) interacts with the intracellular part of EGFR and mediates the EGF-dependent cellular activation of phospholipase D2 (PLD2) but does not mediate the activation of PLD1. In addition, ARF4-mediated PLD2 activation leads to dramatic activation of the transcription factor activator protein 1 (AP-1), and, importantly, ARF4 activity is required for EGF-induced activation of cellular AP-1. Our findings indicate that ARF4 is a critical molecule that directly regulates cellular PLD2 activity and that this ARF4-mediated PLD2 activation stimulates AP-1-dependent transcription in the EGF-induced cellular response.

ARF6: a newly appreciated player in G protein-coupled receptor desensitization

The luteinizing hormone/choriogonadotropin hormone receptor (LH/CG R) signals to regulate ovulation, corpus luteum formation, and fetal survival during pregnancy. Agonist binding to the LH/CG R is poorly reversible, emphasizing the importance of a cellular mechanism to temper signaling by a potentially persistently active receptor. Like other G protein-coupled receptors (GPCRs), signaling by this receptor is modulated by its binding of an arrestin. We have identified ADP ribosylation factor 6 (ARF6) as a protein whose activation state is regulated by the LH/CG R and which functions to regulate the availability of plasma membrane-docked arrestin 2 to this receptor. We hypothesize that ARF6 might also serve GPCRs other than the LH/CG R to regulate the availability of arrestin 2 for receptor desensitization.

Aspartic acid 564 in the third cytoplasmic loop of the luteinizing hormone/choriogonadotropin receptor is crucial for phosphorylation-independent interaction with arrestin2

Arrestin2 binding to the active but unphosphorylated luteinizing hormone/choriogonadotropin receptor (LH/CG R) in ovarian follicles is triggered by activation of ADP-ribosylation factor 6 (ARF6) and leads to uncoupling of this receptor from cAMP signaling. We sought to determine how arrestin2 binds to LH/CG R, if binding is of high affinity, and if the receptor also binds arrestin3. Desensitization of intact LH/CG R was equally sensitive to ectopic constructs of arrestin2 that bind other G protein-coupled receptors (GPCRs) either in a phosphorylation-independent or -dependent manner. Intact LH/CG R was not desensitized by ectopic arrestin3 constructs. Surface plasmon resonance studies showed that arrestin2 bound a synthetic third intracellular (3i) LH/CG R loop peptide with picomolar affinity; arrestin3 bound with millimolar affinity. To determine whether Asp-564 in the 3i loop mimicked the phosphorylated residue of other GPCRs, human embryonic kidney (HEK) cells were transfected with wild-type (WT) and D564G LH/CG R. An agonist-stimulated ARF6-dependent arrestin2 undocking pathway to drive desensitization of WT receptor was recapitulated in HEK cell membranes, and ectopic arrestin2 promoted desensitization of WT LH/CG R. However, D564G LH/CG R in HEK cells was not desensitized, and synthetic 3i D564G peptide did not bind arrestin2. Synthetic 3i loop peptides containing D564E, D564V, or D564N also did not bind arrestin2. We conclude that the ARF6-mediated mechanism to release a pool of membrane-delimited arrestin to bind GPCRs may be a widespread mechanism to deliver arrestin to GPCRs for receptor desensitization. Unlike other GPCRs that additionally require receptor phosphorylation, LH/CG R activation is sufficient to expose a conformation in which Asp-564 in the 3i loop confers high affinity binding selectively to arrestin2.

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.

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.

Ceramide inhibition of mammalian phospholipase D1 and D2 activities is antagonized by phosphatidylinositol 4,5-bisphosphate

Ceramides inhibit phospholipase D (PLD) activity in several mammalian cell types. These effects have been related to preventing activation by ARF1, RhoA, and protein kinase C-alpha and -beta and therefore indicate that PLD1 is inhibited. In the present work, we investigated the effects of ceramides in inhibiting both PLD1 and PLD2 and the interaction with another activator, phosphatidylinositol 4,5-bisphosphate (PIP2). PLD1 and PLD2 were overexpressed separately in Sf9 insect cells using baculovirus vectors. In our cell-free system, PLD1 activity was inhibited completely by C2-ceramide at sub-optimum concentrations of PIP2 (3 and 6 microM), whereas at supra-optimum PIP2 concentrations (18 and 24 microM) C2-ceramide did not inhibit PLD1 activity. Partially purified PLD2 exhibited an absolute requirement for PIP2 when the activity was measured using Triton X-100 micelles. Ceramides inhibited PLD2 activity, and this inhibition was decreased as PIP2 concentrations increased. However, C2-ceramide also reversibly inhibited the activity of PLD1 and PLD2 mutants in which binding of PIP2 was decreased, indicating that ceramides are interacting with the catalytic core of the mammalian PLDs. By contrast, C2-ceramide failed to produce a significant inhibition of PLDs from bacteria and plants. Our results provide a novel demonstration that ceramides reversibly inhibit mammalian PLD2 as well as PLD1 activities and that both of these actions are more pronounced when PIP2 concentrations are rate-limiting.

Activation of the luteinizing hormone/choriogonadotropin hormone receptor promotes ADP ribosylation factor 6 activation in porcine ovarian follicular membranes

Previously we demonstrated in a cell-free ovarian follicular plasma membrane model that agonist-dependent desensitization of the luteinizing hormone/choriogonadotropin receptor (LH/CG R) is GTP-dependent, mimicked by the addition of ADP-ribosylation factor (ARF) nucleotide binding site opener, which acts as a guanine nucleotide exchange factor for ARFs 1 and 6, and selectively inhibited by synthetic N-terminal ARF6 peptides. We therefore sought direct evidence that activation of the LH/CG R promotes activation of ARF1 and/or ARF6. Using a classic ARF activation assay, the cholera toxin-catalyzed ADP-ribosylation of G alpha(s), results show that LH/CG R activation stimulates an ARF protein by a brefeldin A-independent mechanism. Synthetic N-terminal inhibitory ARF6 but not ARF1 peptide blocks LH/CG R-stimulated ARF activity. LH/CG R activation also promotes the binding of a photoaffinity GTP analog to a protein that migrates on one- and two-dimensional polyacrylamide gel electrophoresis with ARF6. These results suggest that ARF6 is the predominant ARF activated by the LH/CG R. To activate ARF6, the LH/CG R does not appear to signal through the C-terminal regions of G alpha(i) or G alpha(q) or through the second or third intracellular loops or the N terminus of the cytoplasmic tail of the LH/CG R. Although exogenous recombinant ARNO promotes only a small increase in ARF6 activation in the presence of activated LH/CG R, hCG-stimulated ARF6 activation is reduced to basal levels by catalytically inactive ARF nucleotide binding-site opener. These results provide direct evidence that LH/CG R activation leads to the activation of membrane-delimited ARF6.

Characterization of cytohesin-1 monoclonal antibodies: expression in neutrophils and during granulocytic maturation of HL-60 cells

ADP-ribosylation factors (Arf) are small GTP-binding proteins involved in vesicular transport and the activation of phospholipase D (PLD). The conversion of Arf-GDP to Arf-GTP is promoted in vivo by guanine nucleotide exchange factors such as ARNO or cytohesin-1. In order to examine the expression of ARNO and cytohesin-1 in human granulocytes, we generated specific polyclonal and monoclonal antibodies (mAbs). We also overexpressed GFP-ARNO and GFP-cytohesin-1 in RBL-2H3 cells to characterize the specificity and the ability of cytohesin-1 mAbs to immunoprecipitate cytohesin-1. Among the hybridomas secreting cytohesin-1 mAbs, only the clones 2E11, 1E4, 3C8, 6F5, 4C7, 7A3 and 8F7 were found to be specific for cytohesin-1. Furthermore, mAb 2E11 immunoprecipitated GFP-cytohesin-1 but not GFP-ARNO under native conditions. In contrast, mAbs 5D8, 4C3, 2G8, 6G11, 4C3, 6D4, 7B4 and 6F8 detected both cytohesin-1 and ARNO as monitored by immunoblotting. Although mAb 6G11 detected both proteins, this antibody immunoprecipitated GFP-ARNO but not GFP-cytohesin-1 under native conditions. Another antibody, mAb 10A12, also selectively immunoprecipitated GFP-ARNO under native conditions, but the epitope recognized by this mAb is unlikely to be linear as no signal was obtained by immunoblotting. Immunoprecipitation with a cytohesin-1 polyclonal antibody and blotting with cytohesin-1 specific mAbs revealed that cytohesin-1 is highly expressed in neutrophils. Cytohesin-1 can be detected in HL-60 cells but the endogenous protein levels were low in undifferentiated cells. Using the specific cytohesin-1 mAb 2E11 we observed a marked increase in levels of cytohesin-1 expression during dibutyryl-cyclic AMP-induced granulocytic differentiation of HL-60 cells. These data suggest that cytohesin-1, which may have important functions in neutrophil physiology, can be useful as a potential marker for granulocytic differentiation.

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

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

A regulatory role for ADP-ribosylation factor 6 (ARF6) in activation of the phagocyte NADPH oxidase

In activated neutrophils NADPH oxidase is regulated through various signaling intermediates, including heterotrimeric G proteins, kinases, GTPases, and phospholipases. ADP-ribosylation factor (ARF) describes a family of GTPases associated with phospholipase D (PLD) activation. PLD is implicated in NADPH oxidase activation, although it is unclear whether activation of PLD by ARF is linked to receptor-mediated oxidase activation. We explored whether ARF participates in NADPH oxidase activation by formyl-methionine-leucine-phenylalanine (fMLP) and whether this involves PLD. Using multicolor forward angle light scattering analyses to measure superoxide production in differentiated neutrophil-like PLB-985 cells, we tested enhanced green fluorescent fusion proteins of wild-type ARF1 or ARF6, or their mutant counterparts. The ARF6(Q67L) mutant defective in GTP hydrolysis caused increased superoxide production, whereas the ARF6(T27N) mutant defective in GTP binding caused diminished responses to fMLP. The ARF1 mutants had no effect on fMLP responses, and none of the ARF proteins affected phorbol 12-myristate 13-acetate-elicited oxidase activity. PLD inhibitors 1-butanol and 2, 3-diphosphoglycerate, or the ARF6(N48R) mutant assumed to be defective in PLD activation, blocked fMLP-elicited oxidase activity in transfected cells. The data suggest that ARF6 but not ARF1 modulates receptor-mediated NADPH oxidase activation in a PLD-dependent mechanism. Because PMA-elicited NADPH oxidase activation also appears to be PLD-dependent, but ARF-independent, ARF6 and protein kinase C may act through distinct pathways, both involving PLD.

The characterization of phospholipase D in FRTL-5 thyroid cells

We previously demonstrated that TSH activates phospholipase D (PLD) in Fischer rat thyroid line (FRTL)-5 cells. To date, two types of mammalian phosphatidylcholine-specific PLD cDNAs, designated as PLD-1 and PLD-2, have been cloned. The present study determined the PLD isoform composition in FRTL-5 thyroid cells and which isoform is regulated by TSH. PLD-1 is activated by small molecular weight G-proteins, such as ADP-ribosylation factor (ARF) and RhoA family members, while PLD-2 is relatively independent of such stimuli. We established the presence of PLD-1 and PLD-2 by Western blot analysis and compared PLD activity in cytosol, membranes and combined fractions in the presence and absence of GTPgammaS. The membrane fraction showed very little activity in the absence of GTPgammaS, but this activity increased approximately 5-fold (P<0.05, ANOVA) in the presence of GTPgammaS. Maximal PLD activity was seen with the combination of membrane plus cytosolic fractions (which contained ARF and RhoA) where the addition of GTPgammaS increased PLD activity approximately 8-fold (P<0.05, ANOVA). To determine the relative activities of PLD-1 and PLD-2 in FRTL-5 thyroid cells, cell-free PLD assays were performed in the presence of GTPgammaS or GDPbetaS with varying concentrations of phosphatidylinositol 4,5-bisphosphate (PIP(2)). PLD-2 contributed only approximately 19% of the total amount of PLD activity in the membranes and PLD-1 was the predominant PLD isoform. TSH stimulated PLD-1 activity by up to 2. 3-fold over control values (P<0.01, ANOVA). To establish the dependence of PLD-1 on small molecular weight G-proteins, the translocations of ARF and RhoA to the membrane fractions was determined after stimulation by TSH. Both ARF and RhoA were maximally translocated to the membrane fraction after 10 min incubation with 100 microU/ml TSH by approximately 1.7- and 2.3-fold over control values, respectively (P<0.02 and P<0.03, ANOVA). It is concluded that TSH stimulates PLD-1 activity in FRTL-5 thyroid cells and this is accompanied by the translocation of ARF and RhoA to the membrane fraction.

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.

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.

Effects of local anesthetics on phospholipase D activity in differentiated human promyelocytic leukemic HL60 cells

Local anesthetics impair certain functions of neutrophils, and phospholipase D (PLD) is considered to play an important role in the regulation of these functions. To understand the mechanisms by which local anesthetics suppress the functions of neutrophils, we examined the effects of local anesthetics on PLD in neutrophil-like differentiated human promyelocytic leukemic HL60 cells. Tetracaine, a local anesthetic, inhibited formyl-methionyl-leucyl-phenylalanine (fMLP)- and 4beta-phorbol 12-myristate 13-acetate (PMA)-induced PLD activation, but potentiated fMLP-stimulated phospholipase C activity. All four local anesthetics tested suppressed PMA-induced PLD activation to different extents, and the order of their potency was tetracaine > bupivacaine > lidocaine > procaine. In a cell-free system, tetracaine suppressed guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-induced PLD activation as well as PMA-induced PLD activation. Western blot analysis revealed that tetracaine prevented the membrane translocation of PLD-activating factors, ADP-ribosylation factor, RhoA, and protein kinase Calpha. Tetracaine also inhibited the activity of recombinant hPLD1a in vitro. These results suggest that local anesthetics suppress PLD activation in differentiated HL60 cells by preventing the membrane translocation of PLD-activating factors, and/or by directly inhibiting the enzyme per se. Therefore, it could be assumed that local anesthetics would suppress the functions of neutrophils by inhibition of PLD activation.

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.

Differentiation of U937 cells enables a phospholipase D-dependent pathway of cytosolic phospholipase A2 activation

Treatment with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line results in differentiation toward a monocyte/granulocyte-like cell. This differentiation enables the cell to activate cytosolic phospholipase A2 (cPLA2) to release arachidonate upon stimulation. In contrast, undifferentiated cells are unable to release arachidonate even when stimulated with calcium ionophores. In the present research, a role for phospholipase D (PLD) in the regulation of cPLA2 was shown based on a number of observations. First, the ionomycin- and fMLP-stimulated production of arachidonate in differentiated cells was sensitive to ethanol (2% (v/v)). Ethanol acts as an alternate substrate in place of water for PLD producing phosphatidylethanol (PEt) instead of phosphatidic acid. Indeed, ionomycin stimulation of differentiated cells produced a 14-fold increase in PEt levels. Further evidence for the involvement of PLD in the regulation of cPLA2 came from the observation that the stimulated production of diacylglycerol (for which phosphatidic acid is a major source) was greatly diminished in undifferentiated cells as compared to differentiated cells. Moreover, the normally deficient activation of cPLA2 in undifferentiated cells could be stimulated to release arachidonate if the cells were electroporated in the presence of GTP[gamma]S and MgATP. This treatment stimulates phosphatidylinositol-4,5-bisphosphate (PIP2) production which appears to activate PLD and cPLA2 in subsequent steps. The phosphatidic acid (and diacylglycerol derived from phosphatidic acid) appears to greatly regulate the action of cPLA2 by an unknown mechanism, and undifferentiated cells lack the ability to stimulate PLD activity due to a dysfunction of PIP2 production.

Compartmentalization of the mitogen-activated protein kinase (MAPK) in hepatic endosomes: association with the internalized epidermal growth factor (EGF) receptor

A pool of MAPK was found in hepatic plasma membrane (PM) and endosomes (ENs). After injection of a single dose of EGF (10 microg/100 g body weight), MAPK was detected in EGF receptor (EGFR) immunoprecipitates prepared from ENs. MAPK was detected in a time-dependent manner in EGFR immunoprecipitates that was coincident with the progressive concentration of the EGFR. The EGFR-associated MAPK was also detected by using an anti-phospho-MAPK suggesting that it was active. MAPK was present in wheat-germ agglutinin (WGA) eluates prepared from ENs and was maximally tyrosine-phosphorylated at the time peak of EGFR internalization. MAPK therefore is compartmentalized in PM and ENs of rat liver. A fraction of the endosomal MAPK was found to be associated with the internalized EGFR complexes, suggesting that it plays a role in the control of the EGFR activity at this locus.

ADP-ribosylation factor 1 dependent clathrin-coat assembly on synthetic liposomes

The assembly of clathrin-coated vesicles on Golgi membranes is initiated by the GTP-binding protein ADP ribosylation factor (ARF), which generates high-affinity membrane-binding sites for the heterotetrameric AP-1 adaptor complex. Once bound, the AP-1 recruits clathrin triskelia, which polymerize to form the coat. We have found that ARF.GTP also recruits AP-1 and clathrin onto protein-free liposomes. The efficiency of this process is modulated by the composition of the liposomes, with phosphatidylserine being the most stimulatory phospholipid. There is also a requirement for cytosolic factor(s) other than ARF. Thin-section electron microscopy shows the presence of clathrin-coated buds and vesicles that resemble those formed in vivo. These results indicate that AP-1-containing clathrin-coated vesicles can form in the absence of integral membrane proteins. Thus, ARF.GTP, appropriate lipids, and cytosolic factor(s) are the minimal components necessary for AP-1 clathrin-coat assembly.

1,25-dihydroxyvitamin D3 but not TPA activates PLD in Caco-2 cells via pp60(c-src) and RhoA

In the accompanying paper [Khare et al., Am. J. Physiol. 276 (Gastrointest. Liver Physiol. 39): G993-G1004, 1999], activation of protein kinase C-alpha (PKC-alpha) was shown to be involved in the stimulation of phospholipase D (PLD) by 1,25-dihydroxyvitamin D3 [1, 25(OH)2D3] and 12-O-tetradecanoylphorbol 13-acetate (TPA) in Caco-2 cells. Monomeric or heterotrimeric G proteins, as well as pp60(c-src) have been implicated in PLD activation. We therefore determined whether these signal transduction elements were involved in PLD stimulation by 1,25(OH)2D3 or TPA. Treatment with C3 transferase, which inhibits members of the Rho family of monomeric G proteins, markedly diminished the ability of 1,25(OH)2D3, but not TPA, to stimulate PLD. Brefeldin A, an inhibitor of ADP-ribosylation factor proteins, did not, however, significantly reduce the stimulation of PLD by either of these agents. Moreover, 1,25(OH)2D3, but not TPA, activated pp60(c-src) and treatment with PP1, a specific inhibitor of the pp60(c-src) family, blocked the ability of 1,25(OH)2D3 to activate PLD. Pretreatment of cells with pertussis toxin (PTx) markedly reduced the stimulation of PLD by either agonist. PTx, moreover, inhibited the stimulation of pp60(c-src) and PKC-alpha by 1,25(OH)2D3. PTx did not, however, block the membrane translocation of RhoA induced by 1,25(OH)2D3 or inhibit the stimulation of PKC-alpha by TPA. These findings, taken together with those of the accompanying paper, indicate that although 1,25(OH)2D3 and TPA each activate PLD in Caco-2 cells in part via PKC-alpha, their stimulation of PLD differs in a number of important aspects, including the requirement for pp60(c-src) and RhoA in the activation of PLD by 1,25(OH)2D3, but not TPA. Moreover, the requirement for different signal transduction elements by 1,25(OH)2D3 and TPA to induce the stimulation of PLD may potentially underlie differences in the physiological effects of these agents in Caco-2 cells.

Expression and distribution of adenosine diphosphate-ribosylation factors in the rat kidney

BACKGROUND

Adenosine diphosphate (ADP)-ribosylation factors (ARFs) are small guanosine triphosphatases involved in membrane traffic regulation. Aiming to explore the possible involvement of ARF1 and ARF6 in the reabsorptive properties of the nephron, we evaluated their distribution along the different renal epithelial segments.

METHODS

ARFs were detected by immunofluorescence and immunogold cytochemistry on renal sections, using specific anti-ARF antibodies.

RESULTS

ARF1 was detected in proximal and distal tubules, thick ascending limbs of Henle's loops, and cortical and medullary collecting ducts. By immunofluorescence, labeling was mostly localized to the cell cytoplasm, particularly in Golgi areas. By electron microscopy, the Golgi apparatus and the endosomal compartment of proximal and distal tubular cells were labeled. ARF6 immunofluorescence was observed in brush border membranes and the cytoplasm of proximal convoluted tubular cells, whereas it was restricted to the apical border of proximal straight tubules. ARF6 immunogold labeling was detected over microvilli and endocytic compartments of proximal tubular cells.

CONCLUSIONS

This study demonstrates the following: (a) the heterogeneous distributions of ARF1 and ARF6 along the nephron, (b) the existence of cytosolic and membrane-bound forms for both ARFs, and (c) their association with microvilli and endocytic compartments, suggesting an active participation in renal reabsorption.

Upregulation of macrophage plasma membrane and nuclear phospholipase D activity on ligation of the alpha2-macroglobulin signaling receptor: involvement of heterotrimeric and monomeric G proteins

The effect of ligating the alpha2-macroglobulin signaling receptor (alpha2MSR) with receptor-recognized forms of alpha2M (alpha2M*) was studied with respect to phospholipase D (PLD) activity in murine macrophages, their plasma membranes, and nuclei. PLD activity in plasma membranes and nuclei increased linearly up to a ligand concentration of about 100 pM of either alpha2M* or a cloned and expressed receptor binding fragment (RBF). The RBF binding site mutant K1370A, which binds with high affinity to alpha2MSR, also increased nuclear PLD activity comparable to RBF and alpha2M*. Phorbol dibutyrate caused a two- to threefold stimulation of membrane and nuclear PLD activity, whereas PLD activity was nearly abolished by downregulation of protein kinase C; prior treatment with staurosporin, genestein, cyclosporin A, actinomycin D; or chelation of intracellular Ca2+. In permeabilized macrophages, isolated plasma membranes, and nuclei, GTP-gamma-S increased alpha2M*-stimulated PLD activity via a pertussis toxin-insensitive G protein and this effect was abolished on preincubation with GDP-beta-S. Incubation of plasma membranes with polyclonal antibody against sARFII, or the addition of cytosol which was immunoprecipitated with antibody against sARFII, greatly reduced alpha2M*-stimulated PLD activity in the presence of GTP-gamma-S. Preincubation of plasma membranes with GDP-beta-S prior to the addition of GTP-gamma-S and recombinant ARF1 significantly inhibited alpha2M*-stimulation of PLD activity. Nuclear PLD activity was maximally stimulated in the presence of both GTP-gamma-S and rARF1, whereas plasma membrane PLD activity was maximally stimulated in the presence of rARF1, GTP-gamma-S, RhoA, and ATP. In contrast, nuclear PLD activity was not affected by RhoA either alone or in combination with GTP-gamma-S or ATP.

Characterization of a novel ADP-ribosylation factor-like protein (yARL3) in Saccharomyces cerevisiae

ADP-ribosylation factors (ARFs) are highly conserved, approximately 20-kDa guanine nucleotide-binding proteins that enhance the ADP-ribosyltransferase activity of cholera toxin and have an important role in vesicular transport. Several cDNAs for ARF-like proteins (ARLs) have been cloned from human, Drosophila, rat, and yeast, although the biological function(s) of ARLs is unknown. We have identified a yeast gene (yARL3) encoding a protein that is structurally related (>43% identical) to the mammalian ARF-like protein ARP. Biochemical studies of purified recombinant yARL3 protein revealed properties similar to those of ARF and ARL proteins, including the ability to bind and hydrolyze GTP. Like other ARLs, recombinant yARL3 did not stimulate cholera toxin-catalyzed auto-ADP-ribosylation. Anti-yARL3 antibodies did not cross-react with yARFs or yARL1. yARL3 was not essential for cell viability, but disruption of yARL3 resulted in cold-sensitive cell growth. At the nonpermissive temperature, processing of alkaline phosphatase and carboxypeptidase Y in arl3 mutant was slowed. yARL3 might be required for protein transport from endoplasmic reticulum to Golgi or from Golgi to vacuole at nonpermissive temperatures. On subcellular fractionation, unlike its mammalian homologue ARP, yARL3 was detected in the soluble fraction but not in the plasma membrane. Indirect immunofluorescence analysis revealed that yARL3 when overexpressed was associated in part with the endoplasmic reticulum-nuclear envelope. Thus, the structural and functional characteristics of yARL3 indicate that it may have a unique role(s) in vesicular trafficking.

3T3-L1 adipocytes express two isoforms of phospholipase D in distinct subcellular compartments

Phospholipase D has been implicated as an important enzyme in a range of cellular responses, including regulated secretion and the formation of secretory vesicles, cell proliferation and control of cell morphology. As insulin treatment of adipocytes has been shown to stimulate a phosphatidylcholine-specific phospholipase D and also modulates membrane trafficking, we wished to determine which isoform(s) of phospholipase D were present within adipocytes, to identify their subcellular distribution, and examine how this distribution may change in response to insulin. Using RT-PCR, 3T3-L1 adipocytes were found to express two isoforms of phospholipase D, specifically PLD1b and PLD2a. Using isoform-specific antibodies, PLD1 and PLD2 were found to be present predominantly in intracellular membranes, unlike the situation reported in other cells. Detailed analysis of the intracellular localisation of PLD1 and PLD2 revealed that these isoforms are differentially localised within adipocytes, implying functionally distinct roles for PLD activity in distinct subcellular compartments.

Association of phospholipase D activity with the detergent-insoluble cytoskeleton of U937 promonocytic leukocytes

Phospholipase D (PLD) regulates cytoskeletal-dependent antimicrobial responses of myeloid leukocytes, including phagocytosis and oxidant generation. However, the mechanisms responsible for this association between PLD activity and the actin cytoskeleton are unknown. We utilized a cell-free system from U937 promonocytes to test the hypothesis that stimulation of PLD results in stable association of the activated lipase with the detergent-insoluble membrane skeleton. Plasma membrane and cytosol were incubated +/- guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), followed by re-isolation and extraction of the washed membranes with octyl glucoside. The detergent-insoluble fraction derived from membranes incubated with GTPgammaS (DIFGTPgammaS) exhibited 22-fold greater PLD activity than that derived from control membranes (DIF0), when both were assayed in the presence of GTPgammaS. The DIF contained PLD1, RhoA, and ARF, and the level of each was increased by GTPgammaS in a dose-dependent manner. The DIF also contained F-actin, vinculin, talin, paxillin, and alpha-actinin, consistent with its identification as the membrane skeleton. The physiologic relevance of these findings was demonstrated by a similar increase in DIF-associated PLD activity after stimulation of intact U937 cells with opsonized zymosan. These results indicate that stimulation of PLD1 is accompanied by stable association of the activated lipase, RhoA, and ADP-ribosylation factor with the actin-based membrane skeleton.

Remodeling of the actin cytoskeleton is coordinately regulated by protein kinase C and the ADP-ribosylation factor nucleotide exchange factor ARNO

ARNO is a member of a family of guanine-nucleotide exchange factors with specificity for the ADP-ribosylation factor (ARF) GTPases. ARNO possesses a central catalytic domain with homology to yeast Sec7p and an adjacent C-terminal pleckstrin homology (PH) domain. We have previously shown that ARNO localizes to the plasma membrane in vivo and efficiently catalyzes ARF6 nucleotide exchange in vitro. In addition to a role in endocytosis, ARF6 has also been shown to regulate assembly of the actin cytoskeleton. To determine whether ARNO is an upstream regulator of ARF6 in vivo, we examined the distribution of actin in HeLa cells overexpressing ARNO. We found that, while expression of ARNO leads to disassembly of actin stress fibers, it does not result in obvious changes in cell morphology. However, treatment of ARNO transfectants with the PKC agonist phorbol 12-myristate 13-acetate results in the dramatic redistribution of ARNO, ARF6, and actin into membrane protrusions resembling lamellipodia. This process requires ARF activation, as actin rearrangement does not occur in cells expressing a catalytically inactive ARNO mutant. PKC phosphorylates ARNO at a site immediately C-terminal to its PH domain. However, mutation of this site had no effect on the ability of ARNO to regulate actin rearrangement, suggesting that phosphorylation of ARNO by PKC does not positively regulate its activity. Finally, we demonstrate that an ARNO mutant lacking the C-terminal PH domain no longer mediates cytoskeletal reorganization, indicating a role for this domain in appropriate membrane localization. Taken together, these data suggest that ARNO represents an important link between cell surface receptors, ARF6, and the actin cytoskeleton.

Role of rho proteins in agonist regulation of phospholipase D in HL-60 cells

Rho family GTP-binding proteins have been demonstrated to play a role in the regulation of phospholipase D (PLD) activity. In the present study, we examined the role of Rho proteins in PLD activation in differentiated HL-60 cells using C3 exoenzyme from Clostridium botulinum, which ADP-ribosylates and inactivates Rho proteins. Introduction of C3 exoenzyme into differentiated HL-60 cells by electroporation resulted in complete inhibition of PLD activity stimulated by formyl methionine-leucine-phenylalanine (fMLP) and ATP, two receptor agonists. Phorbol myristate acetate-induced PLD activation was also inhibited in C3 exoenzyme-treated cells, but the inhibition was only partial. GTPgammaS-dependent activation of PLD, measured in the absence or presence of ATP in permeabilized cells, was also partially affected by C3 exoenzyme treatment. Thus, these results indicate that Rho proteins play a key role in receptor-mediated PLD regulation in differentiated HL-60 cells, but play a partial role in the in vivo action of PMA and in vitro action of GTPgammaS on PLD. ATP produced a significant enhancement of the in vitro effect of GTPgammaS on PLD activity, but the effect of ATP was not altered by inhibitors of serine/threonine and tyrosine kinases. However, it was markedly reduced by neomycin and accompanied by an increase in phosphatidylinositol 4,5-bisphosphate (PtdInsP2) synthesis. These data indicate that in permeabilized HL-60 cells, the stimulatory effect of ATP on PLD does not involve protein phosphorylation but is due to an increase in PtdInsP2.

Phospholipase D: a novel major player in signal transduction

The role of the mammalian phospholipase D (PLD) in the control of key cellular responses has been recognised for a long time, but only recently have there been the reagents to properly study this very important enzyme in the signalling pathways, linking cell agonists with intracellular targets. With the recent cloning of PLD isoenzymes, their association with low-molecular-weight G proteins, protein kinase C and tyrosine kinases, the availability of antibodies and an understanding of the role of PLD product, phosphatidic acid (PA), in cell physiology, the field is gaining momentum. In this review, we will explore the molecular properties of mammalian PLD and its gene(s), the complexity of this enzyme regulation and the myriad physiological roles for PLD and PA and related metabolic products, with particular emphasis on a role in the activation of NADPH oxidase, or respiratory burst, leading to the generation of oxygen radicals.

Exocytotic stimulation promotes association of the ADP-ribosylation factor with PC12 cell membranes

ADP-ribosylation factors (ARFs) are a family of small molecular, monomeric GTP-binding (G) proteins, initially identified by their ability to enhance cholera toxin (CTX) ADP-ribosyltransferase activity. ARFs have been implicated in protein transport and vesicle and endosome fusion. Although several reports show that synthetic peptides of the N-terminus of ARF inhibited Ca(2+)-dependent exocytosis in permeabilized adrenal chromaffin cells, the role of ARFs in exocytosis has not been established. In this study, we investigated the translocation of ARFs to the membrane fraction from the cytosol fraction in PC12 cells after exocytotic stimulation by measuring the immunoreactivity of ARFs (with anti-ARF anti-serum and with anti-ARF3 antibodies) and enzymatic ARF activity, which enhances the CTX effect. Both the immunoreactivity and the enzymatic activity of ARF in the membrane fraction increased about twofold, significantly, after exocytotic stimulation with ATP and KCl. The translocation of ARF and noradrenaline release was observed in the presence of extracellular CaCl2, but not in the absence of CaCl2. The ARF translocated to the membrane fraction after stimulation in intact cells seemed to be an inactive, perhaps is the GDP form, because ARF did not activate CTX in the absence of guanosine 5'-O-(thiotriphosphate) (GTP gamma S). As previously reported, ARF in the active, GTP gamma S-bound state bound to the membrane fractions. Thus ARF may have been active during translocation and inactivated later. The immunoreactivity of Gs alpha, one of the trimeric G proteins, was not changed before or after stimulation. These findings suggest that ARFs translocate to membranes from the cytosolic fraction after exocytotic stimulation in PC12 cells, and raise the possibility that ARFs regulate exocytosis.

The distribution and translocation of the G protein ADP-ribosylation factor 1 in live cells is determined by its GTPase activity

ADP-ribosylation factors (ARF) are small G proteins that play key roles in vesicular transport processes. We have studied the distribution of ARF1 in live cells using chimeras of ARF1 mutants (wild type (wt) ARF1; Q71L-ARF1 (reduced GTPase); T31N (low affinity for GTP); and (Delta)Nwt (deletion of amino acids 2–18)) with green fluorescent protein (GFP). Confocal microscopy studies showed that the wt and Q71L proteins were localized in the Golgi and cytoplasm. The (Delta)Nwt and the T31N mutants were exclusively cytoplasmic. The behavior of the wt and Q71L proteins was studied in detail. About 15% of wt-ARF1-GFP was bound to the Golgi. Bound wt-ARF1-GFP dissociated rapidly after addition of Brefeldin A (BFA). This process did not appear to be a consequence of BFA-induced disappearance of the Golgi. Photobleaching recovery showed that essentially all the ARF-GFP was mobile, although it diffused very slowly. In contrast, about 40–50% of the Q71L mutant was found in the Golgi, and its rate of dissociation in the presence of BFA was slow and biphasic. Q71L-ARF1-GFP diffused more slowly than the wt. We conclude that ARF1 proteins exist in a dynamic equilibrium between Golgi-bound and cytosolic pools, and that the translocation of ARF in live cells requires the hydrolysis of GTP by the Golgi-bound protein.

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

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