ARF1(2-17) does not specifically interact with ARF1-dependent pathways. Inhibition by peptide of phospholipases C beta, D and exocytosis in HL60 cells

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Poliovirus proteins induce membrane association of GTPase ADP-ribosylation factor

Poliovirus infection results in the disintegration of intracellular membrane structures and formation of specific vesicles that serve as sites for replication of viral RNA. The mechanism of membrane rearrangement has not been clearly defined. Replication of poliovirus is sensitive to brefeldin A (BFA), a fungal metabolite known to prevent normal function of the ADP-ribosylation factor (ARF) family of small GTPases. During normal membrane trafficking in uninfected cells, ARFs are involved in vesicle formation from different intracellular sites through interaction with numerous regulatory and coat proteins as well as in regulation of phospholipase D activity and cytoskeleton modifications. We demonstrate here that ARFs 3 and 5, but not ARF6, are translocated to membranes in HeLa cell extracts that are engaged in translation of poliovirus RNA. The accumulation of ARFs on membranes correlates with active replication of poliovirus RNA in vitro, whereas ARF translocation to membranes does not occur in the presence of BFA. ARF translocation can be induced independently by synthesis of poliovirus 3A or 3CD proteins, and we describe mutations that abolished this activity. In infected HeLa cells, an ARF1-enhanced green fluorescent protein fusion redistributes from Golgi stacks to the perinuclear region, where poliovirus RNA replication occurs. Taken together, the data suggest an involvement of ARF in poliovirus RNA replication.

Mastoparan selectively activates phospholipase D2 in cell membranes

Both known isoforms of phospholipase (PL) D, PLD1 and PLD2, require phosphatidylinositol 4,5-bisphosphate for activity. However, PLD2 is fully active in the presence of this phospholipid, whereas PLD1 activation is dependent on additional factors such as ADP-ribosylation factor-1 (ARF-1) and protein kinase Calpha. We find that mastoparan, an activator of G(i) and mast cells, stimulates an intrinsic PLD activity, most likely PLD2, in fractions enriched in plasma membranes from rat basophilic leukemia 2H3 mast cells. Overexpression of PLD2, but not of PLD1, results in a large increase in the mastoparan-inducible PLD activity in membrane fractions, particularly those enriched in plasma membranes. As in previous studies, expressed PLD2 is localized primarily in the plasma membrane and PLD1 in granule membranes. Studies with pertussis toxin and other agents indicate that mastoparan stimulates PLD2 independently of G(i), ARF-1, protein kinase C, and calcium. Kinetic studies indicate that mastoparan interacts synergistically with phosphatidylinositol 4,5-bisphosphate and that oleate, itself a weak stimulant of PLD2 at low concentrations, is a competitive inhibitor of mastoparan stimulation of PLD2. Therefore, mastoparan may be useful for investigating the regulation of PLD2, particularly in view of the well studied molecular interactions of mastoparan with certain other strategic signaling proteins.

Calcium-regulated exocytosis of dense-core vesicles requires the activation of ADP-ribosylation factor (ARF)6 by ARF nucleotide binding site opener at the plasma membrane

The ADP ribosylation factor (ARF) GTP binding proteins are believed to mediate cytoskeletal remodeling and vesicular trafficking along the secretory pathway. Here we show that ARF6 is specifically associated with dense-core secretory granules in neuroendocrine PC12 cells. Stimulation with a secretagogue triggers the recruitment of secretory granules to the cell periphery and the concomitant activation of ARF6 by the plasma membrane-associated guanine nucleotide exchange factor, ARF nucleotide binding site opener (ARNO). Expression of the constitutively inactive ARF6(T27N) mutant inhibits secretagogue-dependent exocytosis from PC12 cells. Using a mutant of ARF6 specifically impaired for PLD1 stimulation, we find that ARF6 is functionally linked to phospholipase D (PLD)1 in the exocytotic machinery. Finally, we show that ARNO, ARF6, and PLD1 colocalize at sites of exocytosis, and we demonstrate direct interaction between ARF6 and PLD1 in stimulated cells. Together, these results provide the first direct evidence that ARF6 plays a role in calcium-regulated exocytosis in neuroendocrine cells, and suggest that ARF6-stimulated PLD1 activation at the plasma membrane and consequent changes in membrane phospholipid composition are critical for formation of the exocytotic fusion pore.

Involvement of ADP-ribosylation factor 1 in cholera toxin-induced morphological changes of Chinese hamster ovary cells

ADP-ribosylation factor 1 (ARF1) was originally found as a cofactor in CT-catalyzed ADP-ribosylation of Galpha(s) but is now known to participate in vesicle trafficking. We asked whether ARF1 function in vesicular trafficking is necessary for CT-induced morphological changes in Chinese hamster ovary (CHO) cells, which result from increased intracellular cAMP. Brefeldin A treatment of cells suppressed CT action, confirming a requirement for Golgi integrity. Overexpression of a GFP-ARF1 fusion protein did not affect the morphological changes induced by CT, but changes were reduced in cells overexpressing guanine nucleotide exchange-defective ARF1(T31N) or GTP hydrolysis-deficient ARF1(Q71L) mutants. In cells expressing these mutants, 8-bromo-cAMP induced changes similar to those seen in cells transfected with ARF1 or vector. Inhibition of CT action was specific for mutants of ARF1 and not reproduced by analogous mutants of ARF5 or ARF6. ARF1(Q71L) was mostly colocalized with betaCOP, but ARF5(Q71L) less so. ARF6(Q67L) did not colocalize with betaCOP and was partially associated with the plasma membrane. These data are consistent with the conclusion that ARF1 influenced CT action in cells by its specific function in the vesicular transport pathway used by CT to travel from plasma membrane to Golgi to ER.

Responsiveness of beta-escin-permeabilized rabbit gastric gland model: effects of functional peptide fragments

We established a beta-escin-permeabilized gland model with the use of rabbit isolated gastric glands. The glands retained an ability to secrete acid, monitored by [14C]aminopyrine accumulation, in response to cAMP, forskolin, and histamine. These responses were all inhibited by cAMP-dependent protein kinase inhibitory peptide. Myosin light-chain kinase inhibitory peptide also suppressed aminopyrine accumulation, whereas the inhibitory peptide of protein kinase C or that of calmodulin kinase II was without effect. Guanosine-5'-O-(3-thiotriphosphate) (GTPgammaS) abolished cAMP-stimulated acid secretion concomitantly, interfering with the redistribution of H+-K+-ATPase from tubulovesicles to the apical membrane. To identify the targets of GTPgammaS, effects of peptide fragments of certain GTP-binding proteins were examined. Although none of the peptides related to Rab proteins showed any effect, the inhibitory peptide of Arf protein inhibited cAMP-stimulated secretion. These results demonstrate that our new model, the beta-escin-permeabilized gland, allows the introduction of relatively large molecules, e.g., peptides, into the cell, and will be quite useful for analyzing signal transduction of parietal cell function.

N-acylphosphatidylethanolamine-hydrolysing phospholipase D lacks the ability to transphosphatidylate

The N-acylphosphatidylethanolamine-hydrolysing phospholipase D (NAPE-PLD) generates N-acylethanolamines, including N-arachidonoyl-ethanolamine (anandamide), that may be neuroprotective and analgesic. The properties of NAPE-PLD from rat heart and brain microsomes are investigated and compared to those of other PLDs. NAPE-PLD is inhibited by the fatty acid aminohydrolase inhibitor MAFP in high concentrations (> or = 100 microM) while PMSF in high concentrations (10 mM) tends to stabilise NAPE-PLD activity. Oleate inhibits NAPE-PLD but the enzyme is not affected by PIP2, alpha-synuclein or mastoparan. Furthermore, it is for the first time reported that NAPE-PLD is not capable of catalysing a transphosphatidylation reaction like most other known PLDs.

Brefeldin A increases the quantal size and alters the kinetics of catecholamine release from rat adrenal chromaffin cells

The fungal metabolite, brefeldin A (BFA), is known to inhibit guanine nucleotide exchange on the ADP-ribosylating factors that are involved in vesicle membrane trafficking. Here, we investigated the action of BFA on Ca2+-regulated exocytosis in single rat adrenal chromaffin cells. Incubation of chromaffin cells with BFA (1 or 10 microM) for 2 h effectively disrupted the Golgi membranes but did not affect the pattern of catecholamine release triggered by high extracellular K+, which was monitored with carbon fiber amperometry along with cytosolic Ca2+ measurement. The BFA treatment, however, increased the mean quantal size of catecholamine-containing vesicles and the occurrence of amperometric events with a "foot" or "stand alone" signal (which reflects sluggish or incomplete dilation of the fusion pore). To examine whether BFA altered the Ca2+-dependence of exocytosis, we employed the whole-cell recording technique in conjunction with the capacitance measurement to measure exocytosis evoked from the entire cell during voltage-gated Ca2+ entry. Our results suggested that BFA treatment did not alter either the initial rate of capacitance increase or the total amount of capacitance increase. Therefore, in chromaffin cells, BFA treatment affects Ca2+-regulated exocytosis predominantly by increasing the quantal size and by slowing the fusion kinetics of some vesicles.

Analysis of recombinant human ADP-ribosylation factors by reversed-phase high-performance liquid chromatography and electrospray mass spectrometry

Two complementary approaches utilizing reverse-phase high-performance liquid chromatography and liquid chromatography/mass spectrometry were developed to analyze recombinantly produced Group I and Group II human ADP-ribosylation factors (ARFs). We observe that the NH2 termini of Group II ARFs (ARF4 and ARF5) are efficiently processed by removal of the initiating methionine. In contrast, the NH2 termini of Group I ARFs (ARF1 and ARF3), although fully deformylated, undergo only partial methionine cleavage. This result is unexpected as ARFs are canonical substrates for methionine processing in both bacterial and eukaryotic systems, but it may explain the difficulties encountered by many researchers attempting to produce myristoylated ARFs in Escherichia coli. Additionally, we observe that a significant fraction of purified ARF4 contains a modification which we demonstrate to be consistent with mono-glutathionation. Both methionine retention and glutathione modification may impact ARF function and the methods presented here should be employed to determine the quality of recombinant ARFs.

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

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

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.

Rapid ATP-dependent priming of secretory granules precedes Ca(2+)-induced exocytosis in mouse pancreatic B-cells

1. The glucose and ATP dependence of exocytosis were investigated in single mouse pancreatic B-cells by monitoring changes in cell capacitance evoked by voltage-clamp depolarizations, infusion of high [Ca2+]i buffers or photorelease of caged Ca2+ or ATP. 2. In intact B-cells, using the perforated patch whole-cell technique, glucose (5 mM) increased exocytotic responses evoked by membrane depolarization 5-fold over that observed in the absence of the sugar. Increasing the glucose concentration to 20 mM produced a further doubling of exocytosis. The stimulatory action of glucose was attributable to glucose metabolism and abolished by mannoheptulose, an inhibitor of glucose phosphorylation. 3. Exocytosis triggered by infusion of high [Ca2+]i and ATP was reduced by 80% when ATP was replaced by its non-hydrolysable analogue adenosine 5'-[beta, gamma-methylene]triphosphate (AMP-PCP) in standard whole-cell experiments. Exocytosis elicited by GTP gamma S was similarly affected by replacement of ATP with the stable analogue. 4. Photoreleasing ATP in the presence of 170 nM [Ca2+]i, following the complete wash-out of endogenous ATP produced a prompt (latency, < 400 ms) and biphasic stimulation of exocytosis. 5. Elevation of [Ca2+]i to exocytotic levels by photorelease from Ca(2+)-nitrophenyl EGTA preloaded into the cell evoked a biphasic stimulation in the presence of Mg-ATP. The response consisted of an initial rapid (completed in < 200 ms) phase followed by a slower (lasting > or = 10 s) sustained component. Replacement of ATP with AMP-PCP abolished the late component but did not affect the initial phase. The latency between elevation of [Ca2+]i and exocytosis was determined as < 45 ms. Inclusion of N-ethylmaleimide (NEM; 0.5 mM for 3 min) in the intracellular solution exerted effects similar to those obtained by substituting AMP-PCP for ATP. 6. We conclude that the B-cell contains a small pool (40 granules) of primed granules which are immediately available for release and which are capable of undergoing exocytosis in an ATP-independent fashion. We propose that this pool of granules is preferentially released during first phase glucose-stimulated insulin secretion. The short latency between the application of ATP and the onset of exocytosis finally suggests that priming takes place with sufficient speed to participate in the rapid adjustment of the secretory capacity of the B-cell.

ADP ribosylation factor 1 is required for synaptic vesicle budding in PC12 cells

Carrier vesicle generation from donor membranes typically progresses through a GTP-dependent recruitment of coats to membranes. Here we explore the role of ADP ribosylation factor (ARF) 1, one of the GTP-binding proteins that recruit coats, in the production of neuroendocrine synaptic vesicles (SVs) from PC12 cell membranes. Brefeldin A (BFA) strongly and reversibly inhibited SV formation in vivo in three different PC12 cell lines expressing vesicle-associated membrane protein-T Antigen derivatives. Other membrane traffic events remained unaffected by the drug, and the BFA effects were not mimicked by drugs known to interfere with formation of other classes of vesicles. The involvement of ARF proteins in the budding of SVs was addressed in a cell-free reconstitution system (Desnos, C., L. Clift-O'Grady, and R.B. Kelly. 1995. J. Cell Biol. 130:1041-1049). A peptide spanning the effector domain of human ARF1 (2-17) and recombinant ARF1 mutated in its GTPase activity, both inhibited the formation of SVs of the correct size. During in vitro incubation in the presence of the mutant ARFs, the labeled precursor membranes acquired different densities, suggesting that the two ARF mutations block at different biosynthetic steps. Cell-free SV formation in the presence of a high molecular weight, ARF-depleted fraction from brain cytosol was significantly enhanced by the addition of recombinant myristoylated native ARF1. Thus, the generation of SVs from PC12 cell membranes requires ARF and uses its GTPase activity, probably to regulate coating phenomena.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

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

Phospholipase D (PLD) activation by guanine nucleotides requires protein cofactors from both the membrane and the cytosol. The small GTP-binding protein ADP-ribosylation factor (ARF) has been established as one important component of PLD activation. By stimulating HL-60 cells with various agonists and then isolating the membrane fraction and assaying PLD activity in the presence and absence of GTP gamma S, we observed that fMet-Leu-Phe (fMLP) and phorbol esters induced a potentiation of GTP gamma S-stimulated PLD activity in the membrane fractions of these cells. Inactive phorbol esters induced no such potentiation. Both fMLP and active phorbol esters induced a 2-3-fold increase in GTP gamma S-stimulated PLD in HL-60 membranes. Membranes derived from stimulated HL-60 cells contained 60-70% more ARF as compared with membranes derived from control cells. Membrane contents of ARF were assessed by Western blotting with the anti-ARF monoclonal antibody 1D9 followed by densitometric evaluation. Therefore, ARF translocation correlates with the potentiation of the GTP gamma S-stimulated PLD activity. The effect on PLD activity and ARF membrane content achieved through fMLP stimulation was greatly enhanced by prior treatment of the cells with cytochalasin B. Membranes derived from control and fMLP-stimulated cells were assayed for PLD activity in the presence of exogenous ARF and a 50-kDa fraction known to contain elements implicated in PLD activation. The ability of ARF and the 50-kDa fraction to enhance GTP gamma S-sensitive PLD activity was significantly reduced when the membranes were derived from fMLP-stimulated cells. The data indicate that, in addition to ARF, elements of the 50-kDa PLD-inducing factors were likely already translocated to the membranes upon stimulation. We propose that ARF, upon stimulation with agonists such as fMLP or phorbol esters, is translocated to the membrane and in concert with other protein components of the 50-kDa fraction enhances PLD activity.

The assembly of neutrophil NADPH oxidase: effects of mastoparan and its synthetic analogues

Detergent-mediated activation of the phagocyte superoxide-generating NADPH oxidase requires the participation of at least four proteins: the membrane-bound heterodimeric cytochrome b558 and three cytosolic components, p47-phox, p67-phox and a Rac1/Rac2 protein. Peptides corresponding to sequences of different subunits of NADPH oxidase have been used as probes of the mechanism and sequence of assembly of the active complex. In the present study effects of mastoparans on activation of NADPH oxidase were investigated. Mastoparans are wasp venom cationic amphiphilic tetradecapeptides capable of modulation of various cellular activities. Natural mastoparans, as well as several synthetic mastoparan analogues, unrelated to oxidase components, blocked activation of the oxidase in the cell-free system (EC50 = 1.5 microM) and in guanosine 5'-[gamma-thio]triphosphate (GTP[S])/ATP-stimulated neutrophils permeabilized with streptolysin O. In the cell-free system the effect was not relieved by raising the detergent concentration and could not be ascribed to changes in critical micellar concentration values of the activating SDS or arachidonate. Chromatography of neutrophil cytosol on an immobilized mastoparan column suggested interaction of cytosolic p47-phox and p67-phox with the peptide. In spite of this interaction mastoparan did not interfere with translocation of p47-phox and p67-phox to the cell membranes.

Cytosolic ADP-ribosylation factors are not required for endosome-endosome fusion but are necessary for GTP gamma S inhibition of fusion

A specific role for ADP-ribosylation factors (ARFs) in in vitro endosome-endosome fusion has been proposed (Lenhard, J. M., Kahn, R. A., and Stahl, P. D. (1992) J. Biol. Chem. 267, 13047-13052). However, in vivo studies have failed to support a function for ARFs in the endocytic pathway, since an antagonist of ARF activities, brefeldin A, does not interfere with receptor internalization (Schonhorn, J. E., and Wessling-Resnick, M. (1994) Mol. Cell. Biochem. 135, 159-164). This controversy surrounding the exact function of ARF in endocytic vesicle traffic prompted us to critically re-examine the involvement of ARFs in cell-free endosome fusion. Cytosol depleted of ARF activity was capable of supporting in vitro endocytic vesicle fusion but failed to support inhibition of this reaction in the presence of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S). Addition of purified ARF1 restored the ability of the ARF-depleted cytosol to inhibit endosome fusion when incubated with GTP gamma S. Both endocytic vesicle fusion and the GTP gamma S-mediated inhibition of vesicle fusion were unaffected by brefeldin A. Moreover, the ATP requirement and kinetics of cell-free fusion are not altered by brefeldin A or depletion of cytosolic ARFs. These results suggest that cytosolic ARFs are not necessary for endosomal vesicle fusion in vitro but are responsible for inhibition of fusion in the presence of GTP gamma S and cytosolic factors in a brefeldin A-resistant manner.