ADP-ribosylation factor functions synergistically with a 50-kDa cytosolic factor in cell-free activation of human neutrophil phospholipase D

Restoration of Clostridium difficile toxin-B-inhibited phospholipase D by phosphatidylinositol 4,5-bisphosphate

Receptor signalling to phospholipase D (PLD) in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor apparently involves Rho proteins. Since phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] has been recognized as an essential cofactor for PLD activity and since activated Rho proteins have been reported to stimulate the synthesis of PtdIns(4,5)P2, we studied whether in HEK cells PLD activity is regulated by PtdIns(4,5)P2 and, in particular, whether PtdIns(4,5)P2 can restore PLD activity inhibited by Clostridium difficile toxin B, which inactivates Rho proteins. Addition of MgATP to permeabilized HEK cells increased basal PLD activity and potentiated PLD stimulation by the stable GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), concomitant with a large increase in PtdIns(4,5)P2. On the other hand, neomycin, which binds to PtdIns(4,5)P2, inhibited basal and GTP[S]-stimulated PLD activities. Addition of PtdIns(4,5)P2 increased PLD activity in HEK cell membranes by 2-3-fold, whereas various other phospholipids were ineffective. Prior treatment of HEK cells with toxin B reduced the level of PtdIns(4,5)P2, measured either in intact cells or in membrane preparations, by about 40%. In membranes of toxin-B-treated cells, basal and GTP[S]-stimulated PLD activities were reduced, when measured with exogenous phosphatidylcholine as enzyme substrate. Inclusion of PtdIns(4,5)P2 with phosphatidylcholine in the substrate vesicles or addition of PtdIns(4,5)P2 fully restored basal and GTP[S]-stimulated PLD activities in membranes of toxin-B-treated cells. In conclusion, the data indicate that PtdIns(4,5)P2 is an essential cofactor for PLD activity in HEK cells and that inhibition of PLD activity by the Rho-inactivating toxin B is apparently caused by depletion of the PLD cofactor, PtdIns(4,5)P2.

Azotemia, TNF alpha, and LPS prime the human neutrophil oxidative burst by distinct mechanisms

The oxidative burst of neutrophils from azotemic patients (AzoPMNs) is primed for an enhanced response compared to neutrophils from normal subjects (NorPMNs). The mechanism for this priming is unknown, although TNF alpha does not further prime AzoPMNs. The present study examines the hypothesis that azotemia and TNF alpha prime neutrophils by the same mechanism. Formyl peptide receptor expression and degranulation were not primed in AzoPMNs, but were primed by both LPS and TNF alpha. LPS was also able to prime the AzoPMN oxidative burst. Guanine nucleotide exchange by multiple guanine nucleotide binding proteins, including heterotrimeric G-proteins and low molecular weight GTP-binding proteins (LMWGs), was increased in AzoPMNs, as demonstrated by GTP gamma S binding and azidoanilide GTP photoaffinity labeling. The plasma membrane density of G-protein alpha i2, alpha i3, and alpha s subunits and the density in the cytosol of the LMWG, Rap1A, was present in significantly greater amounts on plasma membranes from AzoPMNs. FMet-Leu-Phe-stimulated phospholipase D activity, but not basal activity, was significantly greater in AzoPMNs. Finally, incubation of NorPMNs in plasma from azotemic patients resulted in a significant increase in basal GTP gamma S binding. These results demonstrate that priming of AzoPMNs is restricted to oxidative burst activity and that it occurs by a mechanism distinct from that utilized by TNF alpha and LPS. While the exact mechanism remains unknown, it appears to involve a plasma factor and changes in LMWG expression or activity.

Small GTPase-regulated phospholipase D in granulocytes

This review examines the functional role of phospholipase D in the neutrophil. Phospholipase D is emerging as an important component in the signal transduction pathways leading to granulocyte activation. Through the second messenger it produces, phosphatidic acid, phospholipase D plays an active role in the regulation of granulocyte NADPH oxidase activation and granular secretion. Many factors from both the cytosol and the membrane are necessary for maximal phospholipase D activation. This paper will focus on the regulation of phospholipase D by low molecular weight GTP-binding proteins, tyrosine kinases, and protein kinase C.

Interspecies relationships among ADP-ribosylation factors (ARFs): evidence of evolutionary pressure to maintain individual identities

ADP-ribosylation factors (ARFs) are approximately 20-kDa guanine nucleotide-binding proteins that are allosteric activators of the NAD:arginine ADP-ribosyltransferase activity of cholera toxin and appear to play a role in intracellular vesicular trafficking. Although the physiological roles of these proteins have not been defined, it has been presumed that each has a specific intracellular function. To obtain genetic evidence that each ARF is under evolutionary pressure to maintain its structure, and presumably function, rat ARF cDNA clones were isolated and their nucleotide and deduced amino acid sequences were compared to those of other mammalian ARFs. Deduced amino acid sequences for rat ARFs 1, 2, 3, 5 and 6 were identical to those of the known cognate human and bovine ARFs; rat ARF4 was 96% identical to human ARF4. Nucleotide sequences of both the untranslated as well as the coding regions were highly conserved. These results indicate that the ARF proteins are, as a family, extraordinarily well conserved across mammalian species. The unusually high degree of conservation of the untranslated regions is consistent with these regions having important regulatory roles and that individual ARFs contain structurally unique elements required for specific functions.

Reconstitution of GTP-gamma-S-dependent phospholipase D activity with ARF, RhoA, and a soluble 36-kDa protein

For activation of kidney membrane phospholipase D (PLD), cytosol is absolutely needed in addition to GTP-gamma-S. The active component of cytosol consists of three protein factors: ADP-ribosylation factor, RhoA, and a soluble 36-kDa protein. Any combination of these two factors synergistically activates PLD to some extent, but the presence of the three factors causes full activation. The 36-kDa protein is stable at 60 degrees C but inactivated at 80 degrees C for 10 min. Tissue distribution of the 36-kDa protein roughly coincides with that of PLD, suggesting physiological relevance of the protein in the regulation of PLD.

Evidence for Rho-mediated agonist stimulation of phospholipase D in rat1 fibroblasts. Effects of Clostridium botulinum C3 exoenzyme

Small GTP-binding proteins of the Rho family are implicated in the in vitro regulation of phosphatidylcholine hydrolysis by phospholipase D (PLD). However, their role in agonist-stimulated PLD activity in whole cells is not clear. The ribosyltransferase C3 from Clostridium botulinum modifies Rho proteins and inhibits their function. When introduced into rat1 fibroblasts by scrape-loading, C3 inhibited PLD activity stimulated by lysophosphatidic acid (LPA), endothelin-1, or phorbol ester. Neither the time course nor agonist dose response for LPA-stimulated PLD activity was altered in C3-treated cells. In contrast to the effects of C3 on PLD activity, agonist-stimulated phosphatidylinositol-phospholipase C activity was not altered in C3-treated cells. Surprisingly, C3 treatment led to a decrease in the amount of RhoA protein, indicating that the loss of PLD activity in response to agonist was partly due to the loss of Rho proteins. As described previously, C3 treatment led to the inhibition of LPA-stimulated actin filament formation. However, disruption of actin filaments with cytochalasin D caused only a minor inhibition of LPA-stimulated PLD activity. Interestingly, stimulation of cells with LPA caused a rapid enrichment of RhoA in the particulate fraction of cell lysates. These data support an in vivo role for RhoA in agonist-stimulated PLD activity that is separate from its role in actin fiber formation.

Enzymology of mammalian phospholipases D: in vitro studies

The existence of multiple forms of phopholipase D was clearly established in a large number of biochemical studies that described and characterized the enzymological properties of the different PLD activities. This review summarizes the in vitro evidence showing differential subcellular localization and chromatographic properties of putative PLD isozymes, their phospholipid and alcohol substrate specificities, their modulation by various divalent cations, small G proteins and protein kinase c isozymes, and the role of phosphatidylinositol 4,5-bisphosphate as a cofactor of phospholipase D.

Regulation of phospholipase D by tyrosine kinases

Activation of phospholipase D (PLD) represents part of an important signalling pathway in mammalian cells. Phospholipase D catalyzed hydrolysis of phospholipids generates phosphatidic acid (PA) which is subsequently metabolized to lyso-PA (LPA) or diacylglycerol (DAG). While DAG is an endogenous activator of protein kinase C (PKC), PA and LPA have been recognized as second messengers as well. Activation of PLD in response to an external stimulus may involve PKC, Ca2+, G-proteins and/or tyrosine kinases. In this review, we will address the role of protein tyrosine phosphorylation in growth factor-, agonist- and oxidant-mediated activation of PLD. Furthermore, a possible link between PKC, Ca2+, G-proteins and tyrosine kinases is discussed to indicate the complexity involved in the regulation of PLD in mammalian cells.

Messenger functions of phosphatidic acid

Under physiological conditions, phosphatidic acid (PA) is an anionic phospholipid with moderate biological reactivity. Some of its biological effects can be attributed to lyso-PA and diacylglycerol generated by the action of cellular hydrolases. However, it is clear that the parent compound exhibits biological activities of its own. Early studies implicated PA in the transport of Ca++ across plasma membranes as well as in the mobilization of intracellular stored calcium. Both responses may be induced as a consequence of other cellular processes activated by PA, as opposed to being directly mediated by the lipid. PA may be involved in the activation of certain functions confined to specialized groupings of cells, such as the neutrophil superoxide-generating enzyme or actin polymerization. Recent studies implicate PA as an activator of intracellular protein kinases, and a PA-dependent superfamily of kinases involved in cellular signalling has been hypothesized. Deployed on the outer surface of the plasma membrane, PA potentially provides a method of communication between cells in direct contact. This review will explore the known functions of PA as an intracellular mediator and extracellular messenger of biological activities and address ways in which these functions are potentially regulated by cellular enzymes which hydrolyse the phospholipid.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

Biochemistry and cell biology of phospholipase D in human neutrophils

Neutrophils play a major role host defense against invading microbes. Recent studies have emphasized the importance of the phospholipase D (PLD) in the signalling cascade leading to neutrophil activation. Phospholipase D catalyzes the hydrolysis of phospholipids to generate phosphatidic acid with secondarily generation of diradylglycerol; both of these products have been implicated as second messengers. Herein, we discuss the regulation and the biochemistry of the receptor-regulated PLD in human neutrophils. In vivo and in vitro studies suggest an activation mode in which initial receptor-linked activation of phospholipase C generates diacylglycerol and inositol trisphosphate. The resulting calcium flux along with the diacylglycerol activate a conventional isoform of protein kinase C (PKC), probably PKC beta 1. This PKC, in turn phosphorylates a plasma membrane component resulting in PLD activation and a second outpouring of diradylglycerol. The small GTP-binding proteins, RhoA and ARF, also participate in this process, and synergize with a 50 kDa cytosolic regulatory factor.

Phospholipase D regulation by a physical interaction with the actin-binding protein gelsolin

Increases in intracellular phosphatidic acid levels caused by receptor- mediated activation of phospholipase D (PLD) have been implicated in many signal transduction pathways leading to cellular activation. PLD is known to be regulated by several means, including tyrosine kinase activity, increases in Ca2+, receptor-coupled G proteins, small GTP binding proteins, ceramide metabolisms, and protein kinase C. We have investigated a additional regulatory effect on PLD activity involving nucleoside triphosphates (NTPs). A NTP binding protein copurifies with LPD activity from rabbit brains using a GTP-agarose affinity column, and this protein stimulates PLD activity only in the absence of NPTs. The NTP effect is reversible and labile, and the binding protein is separable from the PLD activity by heparin-agarose chromatography. We identified this protein as the actin- binding protein gelsolin by amino acid sequencing following peptide mapping. This finding was verified by the co-immunoprecipitation of gelsolin and PLD activity as well as by the reconstitution of gelsolin- dependent nucleotide sensitive PLD activity by the addition of purified gelsolin-free PLD. Our data indicate that actin rearrangements and PLD signaling are coordinately regulated through the physical association between PLD and gelsolin and that this interaction may also serve to amplify both PLD signaling and actin reorganization.

Acidic phospholipids inhibit the phospholipase D activity of rat brain neuronal nuclei

An oleate dependent form of phospholipase D is present in rat brain neuronal nuclei and both the hydrolytic and transphosphatidylation activities measured. Several acidic phospholipids were found to inhibit this activity in a dose dependent manner. The IC50 values varied from 3.5 microM for PIP2 to 200 microM for phosphatidic acid. The hydrolysis of PIP2 by phospholipase C would be expected to result in the disinhibition of the oleate dependent phospholipase D activity.

Regulation of phospholipase D by protein kinase C is synergistic with ADP-ribosylation factor and independent of protein kinase activity

Phospholipase D (PLD) which was partially purified from membranes of porcine brain could be stimulated by multiple cytosolic components; these included ADP-ribosylation factor (Arf) and RhoA, which required guanine nucleotides for activity, and an unidentified factor which activated the enzyme in a nucleotide-independent manner (Singer, W. D., Brown, H. A., Bokoch, G. M., and Sternweis, P. C. (1995) J. Biol. Chem. 270, 14944-14950). Here, we report purification of the latter factor, its identification as the alpha isoform of protein kinase C (PKCalpha), and characterization of its regulation of PLD activity. Stimulation of PLD by purified PKCalpha or recombinant PKCalpha (rPKCalpha) occurred in the absence of any nucleotide and required activators such as Ca2+ or phorbol ester. This action was synergistic with stimulation of PLD evoked by either Arf or RhoA. Dephosphorylation of rPKC alpha with protein phosphatase 1 or 2A resulted in a loss of its kinase activity, but had little effect on its ability to stimulate PLD either alone or in conjunction with Arf. Staurosporine inhibited the kinase activity of PKCalpha without affecting activation of PLD. Finally, gel filtration of PKCalpha that had been cleaved with trypsin demonstrated that stimulatory activity for PLD coeluted with the regulatory domain of the enzyme. These data indicate that PKC may regulate signaling events through direct molecular interaction with downstream effectors as well as through its well characterized catalytic modification of proteins by phosphorylation.

Regulation of membrane-bound phospholipase D by protein kinase C in HL60 cells. Synergistic action of small GTP-binding protein RhoA

In HL60 cells, the membrane-bound phospholipase D (PLD) was stimulated by 4beta-phorbol 12-myristate 13-acetate (PMA) in the presence of the cytosolic fraction from HL60 cells or rat brain. The cytosolic factor for this PMA-induced PLD activation was subjected to purification from rat brain by sequential chromatographies. The PLD stimulating activity was found in protein kinase C (PKC) fraction containing alpha, betaI, betaII, and gamma isozymes. PKC isozymes were further separated by hydroxylapatite chromatography. PKCalpha and - beta, but not gamma, isozymes were found to activate membrane-bound PLD. PKCalpha was much more effective than PKCbeta for PLD activation. Millimolar concentrations of MgATP were required for the PKC-mediated PLD activation in HL60 membranes. MgATP is utilized to maintain the levels of phosphatidylinositol 4,5-bisphosphate (PIP2) under these assay conditions. The PKC-mediated PLD activation was completely inhibited by neomycin, a high affinity ligand for PIP2, and this suppression was recovered by the addition of exogenous PIP2. Thus, these results suggest that PIP2 is supposed to play a key role in PKC-mediated PLD activity in HL60 membranes. Furthermore, PKCalpha-mediated PLD activation was potentiated by the addition of recombinant RhoA protein in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). The results obtained here indicate that PKCalpha and RhoA (GTP form) exert a synergistic action in the membrane-bound PLD activation in HL60 cells.

Involvement of Ral GTPase in v-Src-induced phospholipase D activation

An early response to the tyrosine kinase activity of v-Src is an increase in phospholipase D (PLD) activity, which leads to the generation of biologically active lipid second messengers, including phosphatidic acid, lysophosphatidic acid and diacylglycerol. We have recently demonstrated that v-Src-induced PLD activity is mediated by Ras, although Ras involvement was indirect, requiring a cytosolic factor for PLD activation. Ras interacts with and activates Ral-GDS, the exchange factor responsible for the activation of Ral GTPases. Here we report that this newly identified Ras/Ral signalling pathway mediates PLD activation by v-Src. PLD activity could be precipitated from v-Src-transformed cell lysates with immobilized RalA protein and with an anti-Ral antibody. A mutation to the region of RalA analogous to the 'effector domain' of Ras did not reduce the ability of RalA to complex with PLD, although deletion of a Ral-specific amino-terminal region did. Overexpression of RalA potentiated PLD activation by v-Src, and expression of dominant negative RalA mutants inhibited both v-Src- and v-Ras-induced PLD activity. Thus RalA is involved in the tyrosine kinase activation of PLD through its unique N terminus, and that PLD is a downstream target of a Ras/Ral GTPase cascade.

RhoA and a cytosolic 50-kDa factor reconstitute GTP gamma S-dependent phospholipase D activity in human neutrophil subcellular fractions

Receptor activation of phospholipase D has been implicated in signal transduction in a variety of cells. Reconstitution of cell-free guanosine 5'-O-(3-thiotriphosphate)(GTP gamma S)-dependent phospholipase D activity from human neutrophils requires protein factors in both the plasma membrane and the cytosol. We previously proposed that one of the factors is a Ras-family small molecular weight GTPase of the Rho subtype (Bowman, E. P., Uhlinger, D. J., and Lambeth, J. D. (1993) J. Biol. Chem. 268, 21509-21512). Herein, we have used RhoGDI (GDP dissociation inhibitor), an inhibitory Rho-binding protein, to selectively extract Rho-type GTPases from the plasma membrane, and have used immunoprecipitation as well as chromatographic methods to remove cytosolic Rho. Depletion of RhoA from either the plasma membrane or the cytosol resulted in a partial loss in GTP gamma S dependent activity, while removal of RhoA from both fractions resulted in a nearly complete loss in activity. Activity was nearly completely restored by adding purified recombinant RhoA, which showed an EC50 of 52 nM, while Rac1 showed little activity. Cytosol fractionated using DEAE-cellulose chromatography separated ADP-ribosylation factor and Rho from the major activating fraction. Gel exclusion chromatography of this fraction revealed an activating factor of 50 kDa apparent molecular mass. Using RhoA-depleted membranes, reconstitution of phospholipase D activity required both RhoA and the 50-kDa factor. Thus, RhoA along with a non-Rho, non-ADP-ribosylation factor 50-kDa cytosolic factor are both required to reconstitute GTP gamma S-dependent phospholipase D activity by neutrophil plasma membranes.

Synergistic activation of rat brain phospholipase D by ADP-ribosylation factor and rhoA p21, and its inhibition by Clostridium botulinum C3 exoenzyme

An activator of rat brain phospholipase D (PLD) that is distinct from the already identified PLD activator, ADP-ribosylation factor (ARF), was partially purified from bovine brain cytosol by a series of chromatographic steps. The partially purified preparation contained a 22-kDa substrate for Clostridium botulinum C3 exoenzyme ADP-ribosyltransferase, which strongly reacted with anti-rhoA p21 antibody, but not with anti-rac1 p21 or anti-cdc42Hs p21 antibody. Treatment of the partially purified PLD-activating factor with both C3 exoenzyme and NAD significantly inhibited the PLD-stimulating activity. These results suggest that rhoA p21 is, at least in part, responsible for the PLD-stimulating activity in the preparation. Recombinant isoprenylated rhoA p21 expressed in and purified from Sf9 cells activated rat brain PLD in a concentration- and GTP gamma S (guanosine 5'-O-(3-thiotriphosphate))-dependent manner. In contrast, recombinant non-isoprenylated rhoA p21 (fused to glutathione S-transferase) expressed in Escherichia coli failed to activate the PLD. This difference cannot be explained by a lower affinity of non-isoprenylated rhoA p21 for GTP gamma S, as the rates of [35S]GTP gamma S binding were very similar for both recombinant preparations and the GTP gamma S-bound form of non-isoprenylated rhoA p21 did not induce PLD activation. Interestingly, recombinant isoprenylated rhoA p21 and ARF synergistically activated rat brain PLD; a similar pattern was seen with the partially purified PLD-activating factor. The synergistic activation was inhibited by C3 exoenzyme-catalyzed ADP-ribosylation of recombinant isoprenylated rhoA p21 in a NAD-dependent manner. Inhibition correlated with the extent of ADP-ribosylation. These findings suggest that rhoA p21 regulates rat brain PLD in concert with ARF, and that isoprenylation of rhoA p21 is essential for PLD regulation in vitro.

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.

Regulation of phospholipase D by protein kinase C in human neutrophils. Conventional isoforms of protein kinase C phosphorylate a phospholipase D-related component in the plasma membrane

In a variety of intact cells, phorbol esters are known to activate phospholipase D. In a cell-free system consisting of plasma membrane and cytosol from human neutrophils, phorbol esters activated phospholipase D in an adenosine nucleotide triphosphate-dependent manner. ATP gamma S (adenosine 5'-O-(thiotriphosphate)) was 2-3-fold more effective than ATP, while ADP and AppNHp (adenyl-5'-yl imidodiphosphate) were ineffective, and activation was blocked by the kinase inhibitor staurosporine. In cytosol deplete of protein kinase C by chromatography on threnoine-Sepharose, phorbol ester-dependent activation was lost, but was restored upon addition of purified rat brain protein kinase C. The target for phosphorylation was shown to be the plasma membrane plasma membrane was phosphorylated using ATP gamma S/phorbol 12,13-dibutyrate and protein kinase C and was reisolated to remove activators. Upon adding nucleotide-depleted cytosol, activator-independent phospholipase D activity was seen. Using this prephosphorylation protocol, PKC-dependent activation of plasma membranes was found to require micromolar calcium, implicating a conventional protein kinase C. Using recombinant isoforms of protein kinase C, only the conventional isoforms showed significant activation, with the following rank order of potency: beta 1 > alpha > gamma; the beta 2, delta, epsilon, eta, and sigma isoforms showed little or no activity. Thus, conventional isoform(s) of protein kinase C activate neutrophil phospholipase D by phosphorylating a target protein located in the plasma membrane.

Resolved phospholipase D activity is modulated by cytosolic factors other than Arf

Phospholipase D, which has been extracted from porcine brain membranes and chromatographically enriched 100-fold, was activated better by impure preparations of Arf than by purified or recombinant Arf. Examination of brain cytosol with this enriched preparation of PLD activity revealed at least three stimulatory components. One of these is Arf or the first cytoplasmic factor. A second peak of PLD-stimulating activity (cytoplasmic factor II, CFII) was resolved from Arf by anion exchange and gel filtration. This CFII can be further separated into multiple activities by chromatography with heparin-agarose. The activities were differentiated by their stimulatory properties as measured in the absence or presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) alone and in the presence of added Arf and GTP gamma S. While all of the CFII pools stimulated PLD activity to some degree and showed synergistic activation when administered in conjunction with Arf, they could be classified into two groups with distinct behavior. When used together, pools from the two respective groups showed synergistic activation of PLD. The first set of pools contained the RhoA monomeric G protein. Recombinant RhoA was used to show that it could indeed activate this enriched PLD activity and act synergistically with Arf proteins. A related monomeric G protein, Cdc42, was also effective. The second set of CFII pools were devoid of RhoA and, in contrast to the first group, demonstrated significant stimulating activity in the absence of guanine nucleotides. These data indicate that the PLD activity from brain can be modulated by several cytosolic factors and that Arf-sensitive PLD may represent a complex activity that can be regulated in an interactive fashion by a variety of cellular signaling events.