Novel function of phosphatidylinositol 4,5-bisphosphate as a cofactor for brain membrane phospholipase D

Growth factor-dependent phosphoinositide signalling

A wide variety of messages, in the form of diffusible growth factors, hormones and cytokines, are carried throughout multicellular organisms to coordinate important physiological properties of target cells, such as proliferation, differentiation, migration, apoptosis and metabolism. Most messengers bind to cognate receptors on target cells, which initiate a characteristic cascade of reactions within the cell, ultimately leading to the desired response. The cellular response is defined by the combination of signalling components whose individual activity depends upon the number and type of surface receptors. Consequently the responses of different cell types to one or more stimuli can be quite disparate. A molecular understanding of the signalling pathways employed by each type of receptor therefore underlies the ability to rationalize many cellular functions and to correct disfunctions. As a well studied example of the primary signalling events that take place on the cytoplasmic leaflet of the plasma membrane following receptor activation, we will discuss how the widely expressed receptor for epidermal growth factor (EGF) causes the phosphorylation and hydrolysis of a signalling precursor, the membrane lipid phosphatidylinositol. This paradigm will be used to illustrate certain general principles of signalling, including formation of multienzyme complexes, compartmentation of second messengers and intermediates, and cross-talk between different signalling pathways.

Phospholipase D activity in the Tetrahymena pyriformis GL

Phospholipase D (PLD) is an enzyme which participates in the signaling mechanism cleaving phosphatidylcholine (PC) to choline and phosphatidic acid (PA). In Tetrahymena pyriformis GL this enzyme activity is enhanced by different kinds of agonists (sodium orthovanadate, sodium fluoride and phorbol 12-myristate 13-acetate), and its activity can be inhibited by inhibitors such as pertussis toxin, calphostin C, genistein, trifluoperazine. These results suggest that the PLD signalling pathway is connected with the tyrosine kinase, phospholipase C, phosphatidylinositol and G-protein coupled signalling pathways. By demonstrating the PLD activity in Tetrahymena our knowledge on the signaling mechanisms at a unicellular level has been extended. The results support our view that most transducing mechanisms that are characteristic of mammalian cells are also in the protozoan Tetrahymena.

Cloning and characterization of a wortmannin-sensitive human phosphatidylinositol 4-kinase

Phosphatidylinositol (PtdIns) 4-kinases catalyze the synthesis of PtdIns-4-P, the immediate precursor of PtdIns-4,5-P2. Here we report the cloning of a novel, ubiquitously expressed PtdIns 4-kinase (PI4Kbeta). The 2.4-kilobase pair cDNA encodes a putative translation product of 801 amino acids which shows greatest homology to the yeast PIK1 gene. The recombinant protein exhibits lipid kinase activity when expressed in Escherichia coli, and specific antibodies recognize a 110-kDa PtdIns 4-kinase in cell lysates. The biochemical properties of PI4Kbeta are characteristic of a type III enzyme. Interestingly, both recombinant PI4Kbeta and the endogenous protein are inhibited by 150 nM wortmannin, suggesting that we have cloned the previously described PtdIns 4-kinase that is responsible for regulating the synthesis of agonist-sensitive pools of polyphosphoinositides (Nakanishi, S., Catt, J. K., and Balla, T. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 5317-5321).

Increased activity of small GTP-binding protein-dependent phospholipase D during differentiation in human promyelocytic leukemic HL60 cells

In response to dibutyryl cyclic AMP (dbcAMP) and all-trans retinoic acid, human promyelocytic leukemic HL60 cells differentiate into granulocyte-like cells. In cell lysate and in vitro reconstitution system, phospholipase D (PLD) activity in response to guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) was up-regulated by dbcAMP or all-trans retinoic acid treatment. In the present study, the mechanism(s) for increased PLD activity during differentiation was examined. Western blot analysis revealed that the contents of ADP-ribosylation factor, Rac2, and Cdc42Hs but not RhoA and Rac1 in the cytosolic fraction were elevated during differentiation. However, the cytosolic fraction from undifferentiated cells was almost equally potent as the cytosolic fraction from differentiated cells in the ability to stimulate membrane PLD activity. It was shown that the GTPgammaS-dependent PLD activity in membranes from differentiated cells was much higher than that in membranes from undifferentiated cells, suggesting that the increased PLD activity during differentiation was due to alterations in some membrane component(s). Clostridium botulinum ADP-ribosyltransferase C3 and C. difficile toxin B, which are known as inhibitors of RhoA and Rho family proteins, respectively, effectively suppressed PLD activity in membranes from differentiated cells. In fact, the amount of membrane-associated RhoA was increased during differentiation. Furthermore, the extent of GTPgammaS-dependent PLD activity partially purified from membranes from differentiated cells was greater than that from membranes from undifferentiated cells in the presence of recombinant ADP-ribosylation factor 1. The PLD (hPLD1) mRNA level was observed to be up-regulated during differentiation, as inferred by reverse transcription-polymerase chain reaction. Our results suggest the possibility that the increased Rho proteins in membranes and the changed level of PLD itself may be, at least in part, responsible for the increase in GTPgammaS-dependent PLD activity during granulocytic differentiation of HL60 cells.

Identification of a novel Ca2+-dependent, phosphatidylethanolamine-hydrolyzing phospholipase D in yeast bearing a disruption in PLD1

We have previously reported the identification and partial characterization of a gene encoding a phospholipase D activity (PLD1) in the yeast, Saccharomyces cerevisiae. Here we report the existence of a second phospholipase D activity, designated PLD2, in yeast cells bearing disruption at the PLD1 locus. PLD2 is a Ca2+-dependent enzyme which preferentially utilizes phosphatidylethanolamine over phosphatidylcholine as a substrate. In contrast to PLD1, the activity of PLD2 is insensitive to phosphatidylinositol 4,5-bisphosphate, and the enzyme is incapable of catalyzing the transphosphatidylation reaction with short chain alcohols as acceptors. Subcellular fractionation shows that PLD2 localizes mainly to the cytosol, but could also be detected in the particulate fraction. Thus, the biochemical properties of PLD2 appear to be substantially different from those of PLD1. PLD2 activity is significantly and transiently elevated upon exit of wild type yeast cells from stationary phase, suggesting that it may play a role in the initiation of mitotic cell division in yeast. In view of the significantly different properties of PLD1 and PLD2, and because the yeast genome contains PLD1 as the sole member of the recently defined PLD gene family, it may be concluded that PLD2 is structurally unrelated to PLD1. Thus, the novel PLD2 activity described herein is likely to represent the first identified member of a new PLD gene family.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Activation of toxin ADP-ribosyltransferases by the family of ADP-ribosylation factors

ADP-ribosylation factors or ARFs are 20-kDa guanine nucleotide-binding proteins, initially identified as stimulators of cholera toxin-catalyzed ADP-ribosylation of Gs alpha. We now know that ARFs play a critical role in many vesicular trafficking events and ARF activation of a membrane-associated phospholipase D (PLD) has been recognized. ARF is active and associates with membranes when GTP is bound. The active state is terminated by hydrolysis of bound GTP, producing inactive ARF-GDP. The nucleotide effect on ARF association with membranes is related to alteration in orientation of the N-terminal myristoyl moiety that is important for ARF function. Cycling of ARF between active and inactive states involves guanine nucleotide-exchange proteins (GEPs) that accelerate replacement of bound GDP with GTP and GTPase-activating proteins (GAPS) that are responsible for ARF inactivation. Six mammalian ARFs have been identified by cDNA cloning. Class I ARFs 1 and 3 have been studied most extensively. Their activation (GTP binding) is catalyzed by a GEP now purified from spleen cytosol. In crude preparations, GEP was inhibited by brefeldin A (BFA), which in cells causes apparent disintegration of Golgi. Demonstration that the approximately 60 kDa purified GEP was not inhibited by BFA means that contrary to earlier belief, there must be another protein to mediate BFA inhibition. GEP activity was greatly enhanced by phosphatidyl serine. The purified GEP, equally active with ARFs 1 and 3, was inactive with ARFs 5 and 6 (Classes II and III); myristoylated ARFs were better substrates than were their non-myristoylated counterparts. ARF GAP purified from bovine spleen cytosol in our laboratory had much broader substrate specificity than the GEP. It used both ARFs 5 and 6 at least as well as ARFs 1 and 3; myristoylation was without effect. It also accelerated GTP hydrolysis by certain ARF mutants and an ARF-like protein (ARL1) that does not have ARF activity. The purified GAP also differed from the GEP in its rather specific requirement for phosphatidylinositol bisphosphate. This was also observed with a seemingly different ARF GAP that was purified and subsequently cloned in Cassel's laboratory. Activation and inactivation of ARFs present many potential sites for physiological regulation and, therefore, for pathological disruption of ARF function.

Regulation of phospholipase C and D activities by small molecular weight G proteins and muscarinic receptors

The role of small molecular weight guanine nucleotide-binding proteins (G proteins) of the Rho family in muscarinic acetylcholine receptor (mAChR) signaling to phospholipase C (PLC) and phospholipase D (PLD) was studied in human embryonic kidney (HEK) cells, stably expressing the human m3 receptor subtype. Evidence for the involvement of Rho proteins in m3 mAChR signaling to both phospholipases is based on findings obtained with Clostridium (C.) difficile toxin B and C. botulinum C3 exoenzyme, both of which specifically, although by different mechanisms, inactivate Rho family G proteins. Toxin B potently inhibited both the mAChR-stimulated PLC and PLD activities in intact cells as well as the stimulation of both phospholipases by the stable GTP analog GTPgammaS in permeabilized cells, the latter effect being mimicked by C3 exoenzyme. In contrast, PLC and PLD activities, measured in the presence of exogenous phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], a substrate and cofactor for PLC and PLD, respectively, were not altered. These data suggested that the Rho-inactivating toxins inhibit stimulation of PLC and PLD by reducing the cellular level of PtdIns(4,5)P2, which was indeed found with both toxin B and C3 exoenzyme. In agreement with a crucial role of cellular PtdIns(4,5)P2 supply for PLC signaling, we observed that short-term agonist (carbachol) treatment of HEK cells caused a long-lasting increase in PtdIns(4,5)P2 level, accompanied by a potentiation of receptor- and G protein-stimulated inositol phosphate formation. Finally, studies with tyrosine kinase and tyrosine phosphatase inhibitors strongly suggest that PtdIns(4,5)P2 synthesis and mAChR-stimulated PLD activity in HEK cells apparently also involve a tyrosine phosphorylation-dependent mechanism(s). Thus, m3 mAChR signaling to PLC and PLD in HEK cells requires the concerted action of various intracellular components, most notably the complex regulation of PtdIns(4,5)P2 synthesis.

Type I phosphatidylinositol-4-phosphate 5-kinases are distinct members of this novel lipid kinase family

Phosphatidylinositol-4-phosphate 5-kinases (PIP5K) synthesize phosphatidylinositol-4,5-bisphosphate, a key precursor in phosphoinositide signaling that also regulates some proteins and cellular processes directly. Two distinct PIP5Ks have been characterized in erythrocytes, the 68-kDa type I (PIP5KI) and 53-kDa type II (PIP5KII) isoforms. Using peptide sequences from the erythroid 68-kDa PIP5KI, we have isolated cDNAs encoding PIP5KIalpha from human brain. Partial cDNAs obtained for a second isoform, PIP5KIbeta, established that the human STM7 gene encoded a previously unrecognized PIP5KI. However, the peptide sequences demonstrated that erythroid PIP5KI corresponded to PIP5KIalpha. Recombinant, bacterially expressed PIP5KIalpha possessed PIP5K activity and was immunoreactive with erythroid PIP5KI antibodies. By Northern analysis, PIP5KIalpha and PIP5KIbeta had wide tissue distributions, but their expression levels differed greatly. PIP5KIs had homology to the kinase domains of PIP5KIIalpha, yeast Mss4p and Fab1p, and a new Caenorhabditis elegans Fab1-like protein identified in the data base. These new isoforms have refined the sequence requirements for PIP5K activity and, potentially, regulation of these enzymes. Furthermore, the limited homology between PIP5KIs and PIP5KIIalpha, which was almost exclusively within the kinase domain core, provided a molecular basis for distinction between type I and II PIP5Ks.

Effects of Clostridium difficile toxin A and toxin B on phospholipase D activation in human promyelocytic leukemic HL60 cells

The possible involvement of Rho family GTP-binding proteins in the regulation of phospholipase D (PLD) activity has recently been demonstrated. In the present study, to further examine the role of Rho family proteins in PLD activation of human promyelocytic leukemic HL60 cells, we used toxin A and toxin B from the anaerobic bacterium Clostridium difficile, which was shown to glucosylate Rho family proteins and inhibit their interaction with effectors. Pretreatment of [3H]oleic acid-labeled HL60 cell lysates with either one of the toxins resulted in a remarkable inhibition of membrane PLD activity stimulated by guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). The magnitude of inhibition of PLD activity was correlated well with the extent of toxin A- or B-induced glucosylation of 22-kDa RhoA in HL60 cells, toxin B being more effective than toxin A. GTPgammaS-stimulated PLD activation measured with the exogenous substrate containing phosphatidylinositol 4,5-bisphosphate was also inhibited by toxin B. Toxin B had no effect on GTP-gammaS-induced translocation of RhoA from cytosol to membranes. Furthermore, the toxin B pretreatment also suppressed PLD activation induced by 4beta-phorbol 12-myristate 13-acetate in HL60 cell lysates. Thus, it was indicated that Rho family proteins play a key role in GTPgammaS- and 40-phorbol 12-myristate 13-acetate-induced PLD activity in HL60 cells. In addition, the results obtained here indicate that C. difficile toxins are a useful tool for researching the regulation of the Rho family protein-mediated PLD activation and also provide a clue toward understanding the pathogenic background of pseudomembranous colitis from the viewpoint of signal transduction.

Inhibition of phospholipase D by a protein factor from bovine brain cytosol. Partial purification and characterization of the inhibition mechanism

A specific protein inhibitor of partially purified bovine brain phospholipase D (PLD) was identified from bovine brain cytosol. The PLD inhibitor has been enriched through several chromatographic steps and characterized with respect to size and mechanism of inhibition. The inhibitor showed an apparent molecular mass of 30 kDa by Superose 12 gel exclusion chromatography and inhibited PLD activity with an IC50 of 7 nM. The inhibitor had neither proteolytic activity nor phospholipid-hydrolyzing activity. Because phosphatidylinositol 4,5-bisphosphate (PIP2), which is included in substrate vesicles, is an essential cofactor for PLD, we examined whether the inhibition might be mediated by sequestration of PIP2. PIP2 hydrolysis by phospholipase C (PLC)-beta1 was not affected by the inhibitor and the inhibitor did not bind to substrate vesicles containing PIP2. In contrast, a PH domain derived from PLC-delta1, which could bind to PIP2, showed a nearly identical inhibition of both PLC-beta1 and PLD activities. Thus, the PLD inhibition by the inhibitor is due to the specific interaction with not PIP2 but PLD. The suppression of PLD activity by the inhibitor was largely eliminated by the addition of ADP-ribosylation factor (ARF) and GTPgammaS. We propose that the inhibitor plays a negative role in regulation of PLD activity by PIP2 and ARF.

Phosphatidylinositol 4,5-bisphosphate binding to the pleckstrin homology domain of phospholipase C-delta1 enhances enzyme activity

The pleckstrin homology (PH) domain is a newly recognized protein module believed to play an important role in signal transduction. While the tertiary structures of several PH domains have been determined, some co-complexed with ligands, the function of this domain remains elusive. In this report, the PH domain located in the N terminus of human phospholipase C-delta1 (PLCdelta1) was found to regulate enzyme activity. The hydrolysis of phosphatidylinositol (PI) was stimulated by phosphatidylinositol 4,5-bisphosphate (PIP2) in a dose-dependent manner with an EC50 = 1 microM (0.3 mol%), up to 9-fold higher when 5 microM (1.5 mol%) of PIP2 was incorporated into the PI/phosphatidylserine (PS)/phosphatidylcholine (PC) vesicles (30 microM of PI with a molar ratio of PI:PS:PC = 1:5:5). Stimulation was specific for PIP2, since other anionic phospholipids including phosphatidylinositol 4-phosphate had no stimulatory effect. PIP2-mediated stimulation was, however, inhibited by inositol 1,4, 5-triphosphate (IP3) in a dose-dependent manner, suggesting a modulatory role for this inositol. When a nested set of PH domain deletions up to 70 amino acids from the N terminus of PLCdelta1 were constructed, the deletion mutant enzymes all catalyzed the hydrolysis of the micelle forms of PI and PIP2 with specific activities comparable with those of the wild type enzyme. However, the stimulatory effect of PIP2 was greatly diminished when more than 20 amino acid residues were deleted from the N terminus. To identify the specific residues involved in PIP2-mediated enzyme activation, amino acids with functional side chains between residues 20 and 40 were individually changed to glycine. While all these mutations had little effect on the ability of the enzyme to catalyze the hydrolysis of PI or PIP2 micelles, the catalytic activity of mutants K24G, K30G, K32G, R38G, or W36G was markedly unresponsive to PIP2. Analysis of PIP2-stimulated PI hydrolysis by a dual substrate binding model of catalysis revealed that the micellar dissociation constant (Ks) of PLCdelta1 for the PI/PS/PC vesicles was reduced from 558 microM to 53 microM, and the interfacial Michaelis constant (Km) was reduced from 0.21 to 0.06 by PIP2. The maximum rate of PI hydrolysis (Vmax) was not affected by PIP2. These results demonstrate that a major function of the PH domain of PLCdelta1 is to modulate enzyme activity. Further, our results identify PIP2 as a functional ligand for a PH domain and suggest a general mechanism for the regulation of other proteins by PIP2.

Inhibition of phospholipase D activity by fodrin. An active role for the cytoskeleton

Phospholipase D (PLD) is a major enzyme implicated in important cellular processes such as secretion and proliferation. The knowledge of its regulation is essential to understand the control of these phenomena. Several proteins activating PLD have been described in the last years. In this report, we chromatographed bovine brain cytosolic proteins to identify fodrin, the non-erythroid spectrin, as the first described inhibitor of PLD. A cytosolic fraction with an inhibitory effect on PLD activity loses its capacity after immunoprecipitation of fodrin. Moreover, at 1 nM, purified fodrin blocks fully and quickly PLD activity, whatever the stimuli used. In contrast, fodrin has no effect on adenylate cyclase activity. Fodrin-analogous proteins like dimeric or tetrameric erythroid spectrin have the same inhibitory effect on PLD, at higher concentrations. Other cytoskeletal proteins, actin and vimentin, are inefficient on PLD inhibition. The mechanisms implicated in PLD modulation such as post-translational modifications of fodrin and the role of small G-proteins on the cytoskeleton regulation are discussed. In conclusion, this study reveals that fodrin is involved in the control of PLD activity, suggesting that the cytoskeleton could have an active role in control of secretion and proliferation.

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.

Phospholipase D: role in signal transduction and membrane traffic

The activation of phospholipase D (PLD) in response to cell stimulation by extracellular signal molecules is a widespread phenomenon. A variety of extracellular signal molecules cause a rapid and dramatic stimulation of PLD activity. G proteins and protein kinases appear to be involved in the receptor-mediated regulation of PLD. There is indirect evidence for the existence of multiple PLD subtypes, both membrane-associated and cytosolic. Recent studies indicate that PLD activities require a lipid cofactor, phosphatidylinositol 4,5-bisphosphate (PIP2). Addition of PIP2 at physiological concentrations stimulates both membrane-associated and partially purified PLD activity. Other acidic phospholipids have little or no effect. Neomycin, a high affinity ligand of PIP2, inhibits membrane PLD activity, presumably by binding to endogenous PIP2. A monoclonal antibody to phosphatidylinositol 4-kinase inhibits PIP2 synthesis in permeabilized U937 cells and blocks PLD activation by GTP gamma S and TPA. These results indicate that PIP2 synthesis is required for G protein- and protein kinase C-mediated activation of PLD in the cells. Recent evidence has implicated PLD and phosphoinositide kinases in vesicular trafficking. The main lipid mediator produced by PLD, phosphatidic acid, could regulate membrane traffic events by direct regulation of target proteins involved in vesicle targeting, docking and fusion. In addition, under certain circumstances, the formation of phosphatidic acid may lead to changes in lipid bilayer properties that would facilitate vesicle budding and fusion events in the course of intracellular membrane traffic.

Membrane-associated phospholipase D activity in neural cell line PC12

Stimulation of PC12 cells with carbachol (Cch) or phorbol 12-myristate 13-acetate (PMA) induced [3H]phosphatidylbutanol (PBut) production. Depletion of extracellular Ca2+ abolished the Cch-mediated phospholipase D (PLD) activation, indicating the requirement of Ca2+ influx. Two different types of PLD activity, oleate-dependent and GTP gamma S-dependent, were examined by using exogenous [3H]phosphatidylcholine substrate. PLD activity of the membrane fraction of PC12 cells was highly dependent on oleate and independent of GTP gamma S. This profile is in sharp contrast to that observed in HL60 cells showing the profound GTP gamma S-induced activation of PLD. The oleate-dependent PLD activity of PC12 membrane was inhibited by high concentrations of Ca2+ and Mg2+. These results indicate that Ca2+ may not directly activate PLD but through some Ca(2+)-dependent mechanism(s) in Cch-stimulated cells.

Regulation of phospholipase D by low molecular weight GTP-binding proteins

Phospholipase D (PLD) is believed to play an important role in cell signal transduction: PLD catalyzes the hydrolysis primarily of phosphatidylcholine (PC) to produce phosphatidic acid that may serve as a lipid second messenger. Although the mechanism of PLD activation has not yet been fully understood, a member of the low molecular weight GTP-binding protein (small G protein) superfamily, ADP-ribosylation factor (ARF), has been identified as a PLD-activating factor. In addition to ARF, we found that RhoA, another member of the small G proteins, activated rat brain PLD, and that ARF and RhoA synergistically stimulated the enzyme activity. When proteins of bovine brain cytosol were subjected to anion exchange column chromatography and then reconstituted with rat brain PLD partially purified from the membranes, fractions eluted at 60 mM NaCl, where ARF was not detected, activated the enzyme in a guanosine 5'-O-(3-thiotriphosphate)-dependent manner. This PLD-stimulating activity seemed to be attributed to a small G protein RhoA. Evidence provided includes the findings that: (1) the partially purified preparation of the PLD-activating factor by subsequent column chromatographies contained a 22 kDa substrate for botulinum C3 exoenzyme ADP-ribosyltransferase; (2) the 22 kDa protein strongly reacted with anti-RhoA antibody; (3) the treatment of the partially purified PLD-activating factor with C3 exoenzyme and NAD together, but not individually, significantly inhibited the PLD-stimulating activity; and (4) recombinant isoprenylated RhoA activated the PLD. On the contrary, recombinant nonisoprenylated RhoA failed to activate the PLD. Interestingly, the partially purified PLD-activating factor and ARF synergistically activated rat brain PLD, and recombinant isoprenylated RhoA could substitute for the partially purified preparation. These results conclude that rat brain PLD is regulated by RhoA in concert with ARF, and that the post-translational modification of RhoA is essential for its function as the PLD activator.

Regulation of cardiac Na+,Ca2+ exchange and KATP potassium channels by PIP2

Cardiac Na+,Ca2+ exchange is activated by a mechanism that requires hydrolysis of adenosine triphosphate (ATP) but is not mediated by protein kinases. In giant cardiac membrane patches, ATP acted to generate phosphatidylinositol-4,5-bisphosphate (PIP2) from phosphatidylinositol (PI). The action of ATP was abolished by a PI-specific phospholipase C (PLC) and recovered after addition of exogenous PI; it was reversed by a PIP2-specific PLC; and it was mimicked by exogenous PIP2. High concentrations of free Ca2+ (5 to 20 microM) accelerated reversal of the ATP effect, and PLC activity in myocyte membranes was activated with a similar Ca2+ dependence. Aluminum reversed the ATP effect by binding with high affinity to PIP2. ATP-inhibited potassium channels (KATP) were also sensitive to PIP2, whereas Na+,K+ pumps and Na+ channels were not. Thus, PIP2 may be an important regulator of both ion transporters and channels.

Phosphatidic acid: a lipid messenger involved in intracellular and extracellular signalling

Generated during the initial phases of cell signalling, phosphatidic acid has been implicated as a messenger involved in the activation of cellular kinases and phospholipases as well as certain proto-oncogene products and low-molecular-weight G-proteins. Although many of the reported effects of phosphatidic acid can be attributed to metabolites generated by cellular hydrolases, the parent compound clearly possesses important biological activities. However, instead of acting as a ubiquitous second messenger mediating signalling events shared by a wide variety of cells, in many systems the phospholipid seems to function specifically, regulating unique functions confined to specialized groupings of cells. One such function is neutrophil superoxide generation, which is induced when phosphatidic acid, generated by activated phospholipase D (PLD), facilitates the interaction of a cytoplasmic low-molecular-weight G-protein with dormant, membrane-bound reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Positioned on the outer surface of the plasma membrane of triggering cells, phosphatidic acid potentially mediates the "juxtacrine" stimulation of cells in direct contact. This review critically evaluates the known biological effects of phosphatidic acid as opposed to functions induced by its metabolites and addresses the mechanisms by which these effects are specifically induced by this phospholipid.

Activation of phospholipase D and phosphatidylinositol 4-phosphate 5-kinase in HL60 membranes is mediated by endogenous Arf but not Rho

Membrane-associated phospholipase D (PLD) in HL60 cells can be activated by the small GTP-binding proteins Arf and RhoA, but polyphosphorylated inositol lipids were required for maximum activity. The intact lipid was required because neither inositol 1,4, 5-trisphosphate nor stearoyl-arachidonyl glycerol could substitute for phosphatidylinositol 4,5-bisphosphate (PIP2). Arf-stimulated but not Rho-stimulated PLD activity was increased by the inclusion of Mg2+ and ATP. ATP-dependent PLD activation occurred when phosphatidylinositol 4-phosphate (PIP), PIP2, or phosphatidylinositol 3,4,5-trisphosphate (PIP3) were included, but PIP2 formation was only detected with PIP; no PIP3 production was detected under any conditions. Therefore, the ATP-dependent increase in PLD activity cannot be explained by PIP2 or PIP3 formation. Association of endogenous Arf and RhoA with membranes was increased by incubation with GTPgammaS. This treatment increased membrane PLD and PIP kinase activities in the absence of exogenous p21 proteins. Reduction of Arf translocation suppressed the increase in PLD and PIP kinase activities, whereas complete removal of Rho but not Arf from membranes with RhoGDI was without effect on PLD activity but increased PIP kinase activity. Therefore, although recombinant Arf and Rho can activate PLD and PIP kinase in HL60 cells, it is the endogenous Arf but not Rho that regulates PLD, and thus a role for Rho in the physiological regulation of PLD in HL60 cells is unlikely.

Identification of diacylglycerol pyrophosphate as a novel metabolic product of phosphatidic acid during G-protein activation in plants

We provide evidence that phosphatidic acid (PtdOH) formed during signaling in plants is metabolized by a novel pathway. In much of this study, 32Pi-labeled Chlamydomonas cells were used, and signaling was activated by adding the G-protein activator mastoparan. Within seconds of activation, large amounts of [32P]PtdOH were formed, with peak production at about 4 min, when the level was 5-25-fold higher than the control. As the level of [32P]PtdOH subsequently decreased, an unknown phospholipid (PLX) increased in radiolabeling; before activation it was barely detectable. The chromatographic properties of PLX resembled those of lyso-PtdOH and CMP.PtdOH but on close inspection were found to be different. PLX was shown to be diacylglycerol pyrophosphate (DGPP), the product of a newly discovered enzyme, phosphatidate kinase, whose in vitro activity was described recently (Wissing, J. B., and Behrbohm, H. (1993) Plant Physiol. 102, 1243-1249). The identity of DGPP was established by co-chromatrography with a standard and by degradation analysis as follows: [32P]DGPP was deacylated, and the product (glycerolpyrophosphate, GroPP) was hydrolyzed by mild acid treatment or pyrophosphatase to produce GroP and Pi as the only radioactive products. Since DGPP is the pyrophosphate derivative of PtdOH and is formed as the concentration of PtdOH decreases, we assumed that PtdOH was converted in vivo to DGPP. This was confirmed by showing that during a short labeling protocol while the specific radioactivity of DGPP was increasing, the specific radioactivity of the 32Pi derived from DGPP as above was higher than that of [32P]GroP. DGPP was also formed in suspension cultures of tomato and potato cells, and its synthesis was activated by mastoparan. Moreover, it was also found in intact tissues of a number of higher plants, for example, carnation flower petals, vetch roots, leaves of fig-leaved goosefoot, and common persicaria and microspores of rape seed. Our results suggest that DGPP is a common but minor plant lipid that increases in concentration when signaling is activated. Possible functions of DGPP in phospholpase C and D signaling cascades are discussed.

Sphingoid bases and phospholipase D activation

There is increased interest in physiological functions and mechanisms of action of sphingolipids metabolites, ceramide, sphingosine, and sphingosine-1-phosphate (SPP), members of a new class of lipid second messengers. This review summarizes current knowledge regarding the role of these sphingolipids metabolites in the actions of growth factors and focuses on the second messenger roles of sphingosine and its metabolite, SPP, in the regulation of cell growth. We also discuss possible interactions with intermediates of the well known glycerophospholipid cycle. Sphingosine and SPP generally provide positive mitogenic signals whereas ceramide has been reported to induce apoptosis and cell arrest in several mammalian cell lines. Stimulation of phospholipase D leading to an increase in phosphatidic acid, a positive regulator of cell growth, by sphingosine and SPP, and its inhibition by ceramide, might be related to their opposite effects on cell growth. This also indicates that sphingolipid turnover could regulate the diacylglycerol cycle. Cross-talk between sphingolipid turnover pathways and the diacylglycerol cycle increases complexity of signaling pathways leading to cellular proliferation and adds additional sites of regulation.

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.

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.

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.

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.

Cloning, expression, and localization of 230-kDa phosphatidylinositol 4-kinase

A phosphatidylinositol (PI) 4-kinase cDNA was cloned from a rat brain cDNA library. This cDNA encoded a protein of 2041 amino acids with a calculated molecular weight of 231,317. The deduced amino acid sequence shared the identity of 52.3 and 34.4% in the presumed catalytic domain with two yeast PI 4-kinases, STT4 and PIK1, respectively, and showed 31.7% identity to p110alpha subunit of rat PI 3-kinase in the same domain. In addition, a 3' half coding region of the present cDNA was 89.6% identical to and its deduced amino acid sequence was 98.2% identical to the sequence for P14Kalpha, a recently reported human PI 4-kinase of type II, suggesting that P14Kalpha is an alternative form of the present PI 4-kinase molecule. The present cDNA contained sequences encoding the ankyrin repeat domain, lipid kinase unique domain, pleckstrin homology domain, presumed lipid kinase/protein kinase homology domain, proline-rich region, and SH3 domain. By examining PI kinase activity in transfected COS-7 cells using the epitope tag immunoprecipitation as well as the conventional way, the product phosphatidylinositol phosphate was identified as phosphatidylinositol 4-phosphate but not phosphatidylinositol 3-phosphate. This PI 4-kinase activity was markedly enhanced in the presence of Triton X-100 but relatively insensitive to inhibition by adenosine. By epitope tag immunohistochemistry, the immunoreactivity for this PI 4-kinase molecule was largely localized in close association with the membranes of the Golgi vesicles and vacuoles. By in situ hybridization analysis, the expression of mRNA for this PI 4-kinase was evident throughout the gray matter of entire brain with higher expression intensity in fetal brain. These data imply that this novel PI 4-kinase is involved in some processes essential to neuronal differentiation and maturation including the synaptogenesis and synaptic plasticity.

Multiple forms of phospholipase D inhibitor from rat brain cytosol. Purification and characterization of heat-labile form

Rat brain cytosol contains proteins that markedly inhibit the activity of partially purified brain membrane phospholipase D (PLD) stimulated by ADP-ribosylation factor (Arf) and phosphatidylinositol 4,5-bisphosphate (PIP2). Sequential chromatography of the brain cytosol yielded four inhibitor fractions, which exhibited different kinetics to heat treatment at 70 degrees C. Purification of the most heat-labile inhibitor to homogeneity yielded two preparations, which displayed apparent molecular masses of 150 kDa and 135 kDa, respectively, on SDS-polyacrylamide gels. Tryptic digests of the 150- and 135-kDa proteins yielded similar elution profiles on a C18 reverse-phase column, suggesting that the 135-kDa form is a truncated form of the 150-kDa form. Sequences of two tryptic peptides were determined. A data base search revealed no proteins with these sequences. The purified 150-kDa inhibitor negated the PLD activity stimulated by Arf, RhoA, or Cdc42. The concentration required for half-maximal inhibition was 0.4 nM. Concentration dependence on the 150-kDa inhibitor was not affected by changes in the concentrations of Arf, PIP2, or phosphatidylcholine used in the assays, suggesting that the inhibition is not due to competition with the activators or substrate for PLD. The purified inhibitor did not affect the PIP2-hydrolyzing activity of a phospholipase C isozyme that was measured with substrate vesicles of lipid composition identical with that used for the PLD assay. Thus, the mechanism of inhibition appears to be a specific allosteric modification of PLD rather than disruption of substrate vesicles.

Insulin-mediated targeting of phosphatidylinositol 3-kinase to GLUT4-containing vesicles

Phosphatidylinositol (PI) 3-kinase is hypothesized to be a signaling element in the acute redistribution of intracellular GLUT4 glucose transporters to the plasma membrane in response to insulin. However, some receptors activate PI 3-kinase without causing GLUT4 translocation, suggesting specific cellular localization may be critical to this PI 3-kinase function. Consistent with this idea, complexes containing PI 3-kinase bound to insulin receptor substrate 1 (IRS-1) in 3T3-L1 adipocytes are associated with intracellular membranes (Heller-Harrison, R., Morin, M. and Czech, M. (1995) J. Biol. Chem. 270, 24442-24450). We report here that in response to insulin, activated complexes of IRS-1.PI 3-kinase can be immunoprecipitated with anti-IRS-1 antibody from detergent extracts of immunoadsorbed GLUT4-containing vesicles prepared from 3T3-L1 adipocytes. The targeting of PI 3-kinase to rat adipocyte GLUT4-containing vesicles using vesicles prepared by sucrose velocity gradient ultracentrifugation was also demonstrated. Insulin treatment caused a 2.3-fold increase in immunoreactive p85 protein in these GLUT4-containing vesicles while anti-p85 immunoprecipitates of PI 3-kinase activity in GLUT4-containing vesicle extracts increased to a similar extent. HPLC analysis of the GLUT4 vesicle-associated PI 3-kinase activity showed insulin-mediated increases in PI 3-P, PI 3,4-P2, and PI 3,4,5-P3 when PI, PI 4-P, and PI 4,5-P2 were used as substrates. Our data demonstrate that insulin directs the association of PI 3-kinase with GLUT4-containing vesicles in 3T3-L1 and rat adipocytes, consistent with the hypothesis that PI 3-kinase is involved in the insulin-regulated movement of GLUT4 to the plasma membrane.

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.

Characterization of a soluble adrenal phosphatidylinositol 4-kinase reveals wortmannin sensitivity of type III phosphatidylinositol kinases

Phosphorylation of phosphatidylinositol (PtdIns) by PtdIns 4-kinases is the first step in the synthesis of polyphosphoinositides, the lipid precursors of intracellular signaling molecules. We have recently identified a cytosolic PtdIns 4-kinase (cPI4K) in the bovine adrenal cortex that is distinguished from previously known PtdIns 4-kinases by its sensitivity to the PtdIns 3-kinase inhibitor wortmannin (WT). The present study has further characterized this soluble enzyme and compared its properties to those of the membrane-bound, type II PtdIns 4-kinase activity of the adrenal cortex and the type III enzyme of bovine brain. The enzymatic activity of adrenal cPI4K was inhibited not only by WT (IC50 approximately 50 nM) but also by LY-294002 (IC50 approximately 100 microM), another inhibitor of PtdIns 3-kinase, and neither compound affected type II PtdIns 4-kinase at concentrations that inhibited cPI4K. In contrast to the type II enzyme, cPI4K had a significantly higher Km for ATP, was relatively insensitive to inhibition by adenosine (Ki approximately 800 microM vs approximately 40 microM), had lower affinity for PtdIns, and was not inhibited by Ca2+ ions. These properties identify the WT-sensitive adrenal cPI4K as a type III PtdIns 4-kinase that is distinct from the tightly membrane-bound, Ca2+- and adenosine-sensitive, type II PtdIns 4-kinase. The type III PtdIns 4-kinase prepared from bovine brain exhibited similar kinetic parameters as the adrenal cPI4K, and was also inhibited by WT with an IC50 of 30-50 nM. Since WT inhibits the synthesis of agonist-regulated phosphoinositide pools in intact cells at micromolar concentrations, these findings indicated that type III rather than type II PtdIns 4-kinases are responsible for the maintenance of the precursor phospholipids required for intracellular signaling through the inositol phosphate/Ca2+ pathway.

Phosphoinositides as regulators in membrane traffic

Phosphorylated products of phosphatidylinositol play critical roles in the regulation of membrane traffic, in addition to their classical roles as second messengers in signal transduction at the cell surface. Growing evidence suggests that phosphorylation-dephosphorylation of the polar heads of phosphoinositides (polyphosphorylated inositol lipids) in specific intracellular locations signals either the recruitment or the activation of proteins essential for vesicular transport. Cross talk between phosphatidylinositol metabolites and guanosine triphosphatases is an important feature of these regulatory mechanisms.

ADP-ribosylation factor-1 stimulates formation of nascent secretory vesicles from the trans-Golgi network of endocrine cells

ADP-ribosylation factor (ARF) is a small GTP-binding protein that has been implicated in intracellular vesicular transport. ARF regulates the budding of vesicles that mediate endoplasmic reticulum to Golgi and intra-Golgi transport. It also plays an important role in maintaining the function and morphology of the Golgi apparatus. Using a permeabilized cell system derived from GH3 cells, we provide evidence that ARF-1 regulates the formation of nascent secretory vesicles from the trans-Golgi network. Both myristoylated and non-myristoylated forms of recombinant human ARF-1 enhanced secretory vesicle budding about 2-fold. A mutant lacking the first 17 N-terminal residues, as well as one that preferentially binds GDP (T31N) did not stimulate vesicle formation. In contrast, a mutant defective in GTP hydrolysis (Q71L) promoted vesicle budding. Strikingly, a peptide corresponding to the N terminus of human ARF-1 (amino acids 2-17) also stimulated vesicle budding from the trans-Golgi network, in marked contrast to its inhibitory effect on vesicular transport from the endoplasmic reticulum to Golgi. These data demonstrate that in endocrine cells, ARF-1 and in particular its N terminus play an essential role in the formation of secretory vesicles.

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.

Identification and characterization of a gene encoding phospholipase D activity in yeast

We have identified an open reading frame on chromosome XI of the yeast, Saccharomyces cerevisiae, as encoding a protein with phospholipase D (PLD) activity. We have named this open reading frame, PLD1, and show that yeast bearing a disruption in this gene are unable to catalyze the hydrolysis of phosphatidylcholine. PLD1 encodes a hypothetical protein of 1683 amino acids and has a predicted molecular mass of 195 kDa. Yeast bearing disruptions at the PLD1 locus are morphologically normal and grow vegetatively like wild-type cells. In contrast, homozygous delta pld1 diploid cells are unable to sporulate and do not produce asci under conditions that induce meiosis and sporulation in wild-type cells. Thus, PLD1 is likely to be essential for the meiotic cycle in yeast cells. This is the first identification of a eukaryotic, nonplant, phosphatidylcholine-hydrolyzing phospholipase D gene. Because the biological role of PLD is not well understood, we expect that delta pld1 yeast will become a useful tool for the characterization of PLD functions as well as for the identification of mammalian PLD homologs.

Inhibition of receptor signaling to phospholipase D by Clostridium difficile toxin B. Role of Rho proteins

Rho proteins have been reported to activate phospholipase D (PLD) in in vitro preparations. To examine the role of Rho proteins in receptor signaling to PLD, we studied the effect of Clostridium difficile toxin B, which glucosylates Rho proteins, on the regulation of PLD activity in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor (mAChR). Toxin B treatment of HEK cells potently and efficiently blocked mAChR-stimulated PLD. In contrast, basal and phorbol ester-stimulated PLD activities were not or only slightly reduced. Cytochalasin B and Clostridium botulinum C2 toxin, mimicking the effect of toxin B on the actin cytoskeleton but without involving Rho proteins, had no effect on mAChR-stimulated PLD. Toxin B did not alter cell surface mAChR number and mAChR-stimulated binding of (guanosine 5'-O-(thio)triphosphate (GTP gamma S) to G proteins. In addition to mAChR-stimulated PLD, toxin B treatment also inhibited PLD activation by the direct G protein activators, AlF4- and GTP gamma S, studied in intact and permeabilized cells, respectively. Finally, C. botulinum C3 exoenzyme, which ADP-ribosylates Rho proteins, mimicked the inhibitory effect of toxin B on GTP gamma S-stimulated PLD activity. In conclusion, the data presented indicate that toxin B potently and selectively interferes with receptor coupling mechanisms to PLD, and furthermore suggest an essential role for Rho proteins in receptor signaling to PLD.

Identification of a phosphatidylinositol 4,5-bisphosphate-binding site in chicken skeletal muscle alpha-actinin

We previously reported that phosphatidylinositol 4,5-bisphosphate (PIP2) dramatically increases the gelating activity of smooth muscle alpha-actinin (Fukami, K., Furuhashi, K., Inagaki, M., Endo, T., Hatano, S., and Takenawa, T. (1992) Nature 359, 150-152) and that the hydrolysis of PIP2 on alpha-actinin by tyrosine kinase activation may be important in cytoskeletal reorganization (Fukami, K., Endo, T., Imamura, M., and Takenawa, T. (1994) J. Biol. Chem. 269, 1518-1522). Here we report that a proteolytic fragment with lysylendopeptidase comprising amino acids 168-184 (TAPYRNVNIQNFHLSWK) from striated muscle alpha-actinin contains a PIP2-binding site. A synthetic peptide composed of the 17 amino acids remarkably inhibited the activities of phospholipase C (PLC)-gamma 1 and -delta 1. Furthermore, we detected an interaction between PIP2 and a bacterially expressed alpha-actinin fragment (amino acids 137-259) by PLC inhibition assay. Point mutants in which arginine 172 or lysine 184 of alpha-actinin were replaced by isoleucine reduced the inhibitory effect on PLC activity by nearly half. Direct interactions between PIP2 and the peptide (amino acids 168-184) or the bacterially expressed protein (amino acids 137-259) were confirmed by enzyme-linked immunosorvent assay. We also found this region homologous to the sequence of the PIP2-binding site in spectrin and the pleckstrin homology domains of PLC-delta 1 and Grb7. Synthetic peptides from the homologous regions in spectrin and PLC-delta 1 inhibited PLC activities. These results indicate that residues 168-184 comprise a binding site for PIP2 in alpha-actinin and that similar sequences found in spectrin and PLC-delta 1 may be involved in the interaction with PIP2.

The phosphatidylinositol 4-phosphate 5-kinase family

The existence of a PIP5K family of enzymes has been suggested by Western blotting and purification of numerous PIP5Ks from various tissues and cell types. The erythrocyte has at least two PIP5Ks, named PIP5KI and PIP5KII, while the brain appears to have even more isoforms. The cloning of the first PIP5K, the PIP5KII alpha, is just the beginning of the molecular classification of this protein family. The PIP5KII alpha sequence has shown that these enzymes lack obvious homology to protein, sugar and other lipid kinases. The identification of two S. cerevisiae homologues, Mss4p and Fab1p, confirms that this family of kinases is widely distributed in eukaryotes. Not surprisingly, cloning experiments have identified additional isoforms. By cloning additional isoforms, insights into the structure and functions of this family of enzymes will be gained. One reason for a large family of PIP5Ks is that many forms of regulation and cellular functions have been ascribed to PIP5Ks, as summarized in Figure 10. Some of these functional links result from PtdIns[4,5]P2 being required for a given process, but the direct involvement of specific PIP5Ks is not well defined. Which PIP5K isoforms are regulated by a specific mechanism or are involved in a cellular process often is not clear. For example, which PIP5Ks produce PtdIns[4,5]P2 that is hydrolyzed by PLC or phosphorylated by the PI 3-kinase is not known. A few exceptions are PIP5KII not being able to phosphorylate PtdIns[4,5]P2 in native membranes, and PIP5KIs being stimulated by PtdA, required for secretion, and possibly regulated by G proteins of the Rho subfamily. The multiplicity of regulation and functions of each PIP5K isoform remains to be elucidated. Another factor governing the number of isoforms may be presence of multiple pools of polyphosphoinositides and the localizing of PIP5K function within cells. The polyphosphoinositides appear to be compartmentalized within cells and each pool appears to be sensitive to specific signals. These polyphosphoinositide pools may include those in the plasma membrane that are used by PLC, nuclear pools that appear to turn over separately from cytoplasmic pools and a small pool at sites of vesicle fusion with the plasma membrane. Each pool may be controlled by a specific PIP5K isoform. This would explain the diversity of PIP5K cellular roles. Another possibility is that the PIP5Ks are localized to certain areas of the cell by being part of a protein or proteolipid complex. Furthermore, the presence of PITP or PLC in the complex would potentially impart specificity and speed on the use of PtdIns[4]P and PtdIns[4,5]P2 because these lipids could be channeled quickly from one enzyme to the next. The concept of localized complexes containing particular PIP5K isoforms that control the composition of different polyphosphoinositide pools will likely be important as the family of PIP5K isoforms grows.

Phosphatidylinositol (3,4,5)P3 interacts with SH2 domains and modulates PI 3-kinase association with tyrosine-phosphorylated proteins

Src homology 2 (SH2) domains on the regulatory subunit of phosphoinositide 3-kinase (PI 3-kinase) mediate its binding to specific tyrosine-phosphorylated proteins in stimulated cells. Using a pharmacological and genetic approach, we show that the amount of PI 3-kinase associated with tyrosine-phosphorylated proteins inversely correlates with the amount of PI 3-kinase lipid products present in the cell. An explanation for this observation is provided by our finding that phosphatidylinositol (3,4,5)trisphosphate (Ptdlns [3,4,5]P3) binds directly and selectively to the SH2 domains of the 85 kDa subunit of PI 3-kinase and thereby blocks binding to tyrosine-phosphorylated proteins. The SH2 domain of pp60C-STC also specifically bound Ptdlns (3,4,5)P3, and the binding was competed by a phosphopeptide specific for the Src SH2 domain. These results indicate that production of Ptdlns (3,4,5)P3 at the membrane disrupts the binding of PI 3-kinase to phosphoproteins. This lipid may also recruit other SH2-containing proteins to the membrane to initiate downstream signaling.

Arachidonoyl-diacylglycerol kinase. Specific in vitro inhibition by polyphosphoinositides suggests a mechanism for regulation of phosphatidylinositol biosynthesis

We previously described the purification of a membrane-bound diacylglycerol kinase highly selective for sn-1-acyl-2-arachidonoyl diacylglycerols (Walsh, J. P., Suen, R., Lemaitre, R. N., and Glomset, J. A. (1994) J. Biol. Chem. 269, 21155-21164). This enzyme appears to be responsible for the rapid clearance of the arachidonate-rich pool of diacylglycerols generated during stimulus-induced phosphoinositide turnover. We have now shown phosphatidylinositol 4,5-bisphosphate to be a potent and specific inhibitor of arachidonoyl-diacylglycerol kinase. Kinetic analyses indicated a Ki for phosphatidylinositol 4,5-bisphosphate of 0.04 mol %. Phosphatidic acid also was an inhibitor with a Ki of 0.7 mol %. Other phospholipids had only small effects at these concentrations. A series of multiply phosphorylated lipid analogs also inhibited the enzyme, indicating that the head group phosphomonoesters are the primary determinants of the polyphosphoinositide effect. However, these compounds were not as potent as phosphatidylinositol 4,5-bisphosphate, indicating some specificity for the polyphosphoinositide additional to its total charge. Five other diacylglycerol kinases were activated to varying degrees by phosphatidylinositol 4,5-bisphosphate and phosphatidic acid, suggesting that inhibition by acidic lipids may be specific for the arachidonoyl-DAG kinase isoform. Given the presumed role of arachidonoyl-diacylglycerol kinase in the phosphoinositide cycle, this inhibition may represent a mechanism for polyphosphoinositides to regulate their own synthesis.

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits transcytosis in polarized epithelial cells

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits both basolateral to apical and apical to basolateral transcytosis of ricin in Fisher rat thyroid (FRT) cells by 50% at 100 nM in a continuous transcytosis assay. In MDCK cells, a similar effect of wortmannin on basolateral to apical transcytosis of ricin was found, whereas apical to basolateral transcytosis was inhibited to a lesser degree. Transcytosis of dimeric IgA in MDCK cells expressing the polymeric immunoglobulin receptor was also reduced to 50% of controls, suggesting that wortmannin inhibits membrane translocation rather than sorting of specific proteins in the transcytotic pathway. This effect of wortmannin is selective, however, in that endocytosis at the basolateral domain and recycling at both the basolateral and apical membrane domains are unaffected, and apical endocytosis and apical secretion are only moderately reduced. We have shown previously that cAMP stimulates a late stage in basolateral to apical transcytosis in MDCK cells through activation of protein kinase A (Hansen, S. H., and Casanova, J.E. (1994) J. Cell Biol. 126, 677-687). Elevation of cellular cAMP still induced a 100% increase in transcytosis in wortmannin-treated cells, but transcytosis was no longer increased when compared to cells which received no drugs. In contrast, in experiments using a 17 degrees C block to accumulate ricin internalized from the basolateral surface in the apical compartment of MDCK cells, wortmannin had little effect on the stimulation of transcytosis by activators of protein kinase A observed under these conditions. The data thus suggest the existence of a wortmannin-sensitive step in the transcytotic pathway, positioned after endocytosis but prior to translocation into the protein kinase A-sensitive apical compartment, implying a role for phosphoinositide 3-kinase in an intermediate step in transcytosis in polarized epithelial cells.

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.

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.

Characterization and partial purification of phospholipase D from human placenta

We report the existence in the human placenta of a phosphatidylcholine-hydrolyzing phospholipase D (PLD) activity, which has been characterized and partially purified. Triton X-100 effectively solubilized PLD from the particulate fraction of human placenta in a dose-dependent manner. However, Triton X-100 caused decreasing enzyme activities. Maximum transphosphatidylation was obtained with 2% ethanol. The enzyme was found to have a pH optimum of 7.0-7.5 and an apparent Km of 33 mol% (or 0.8 mM). Ca2+ and Mg2+ was not required for the enzyme activity. Addition of phosphatidyl-4,5-bisphosphate, but not phosphatidylethanolamine, to the substrate mixture gave rise to a pronounced dose-dependent increase in PLD activity (EC50 = 0.3 mol%), suggesting a regulatory role of this phospholipid in PLD action. The enzyme was inhibited by sodium oleate when partly or fully substituting for octylglucoside in the substrate mixture. The PLD activity was enriched 15-fold by solubilization and purification on a DEAE-Sepharose column. N-Ethylmaleimide (10 mM) markedly inhibited the purified enzyme, indicating the presence of free thiol groups on PLD. Sphingosine (20 microM) and (+/-) propranolol (53 microM) had no direct effect on PLD activity. The present results form the basis for further purification of a PLD from human tissue.

Activation induces dephosphorylation of cofilin and its translocation to plasma membranes in neutrophil-like differentiated HL-60 cells

We suggested that a cytosolic 21-kDa phosphoprotein played an important role in opsonized zymosan-trigered activation of superoxide-generating enzyme in neutrophil-like HL-60 cells through dephosphorylation (Suzuki, K., Yamaguchi, T., Oshizawa, T., Yamamoto, Y., Nishimaki-Mogami, T., Hayakawa, T., and Takahashi, A (1995) Biochim. Biophys. Acta 1266, 261-267). In the present study, we characterized the phosphoprotein and studied changes in it localization upon activation of phagocytes. The 21-kDa phosphoprotein was rapidly dephosphorylated upon activation not only wit opsonized zymosan but also with formyl-Met-Leu-Phe and arachidonic acid. The peptide fragments derived from the 21-kDa phosphoprotein were found to have the same amino acid sequences as those of cofilin, an actin-binding protein. The phosphoprotein reacted exclusively with anti-cofilin antibody on two dimensional immunoblots. Accordingly, together with its apparent molecular weight, isoelectric point, and detection of phosphoserine as a phosphoamino acid, we concluded that the 21-kDa phosphoprotein was a phosphorylated form of cofilin. The amount of cofilin in membranous fractions was increased upon activation. Furthermore, confocal laser scanning microscopy showed that cofilin existed diffusely in the cytosol and nuclear region of the resting cells, while in the activated cells, it was accumulated at the plasma membrane area, forming ruffles or endocytic vesicles on which O2.- should be produced. These results suggested that in resting cells cofilin exists as a soluble phosphoprotein in the cytosol and nuclei, while upon stimulation a large portion of cofilin is dephosphorylated and translocated to the plasma membrane regions.

Differential effects of spermine on phosphatidylinositol 3-kinase and phosphatidylinositol phosphate 5-kinase

The metabolism of phosphoinositides plays an important role in the signal transduction pathways. We report here that naturally occurring polyamines affect the activities of phosphatidylinositol (PI) 3-kinase and PI 4-phosphate (PIP) 5-kinase differently. While polyamines inhibited the PI 3-kinase activity, they stimulated the activity of PIP 5-kinase in the order of spermine > spermidine > putrescine. Spermine inhibited the PI 3-kinase activity in a concentration-dependent manner with an IC50 of 100 microM. On the other hand, spermine (5 mM) stimulated the activity of PIP 5-kinase 2-3 fold. Kinetic studies of spermine-mediated inhibition of PI 3-kinase revealed that it was noncompetitive with respect to ATP. The effect of Mg2+ and PIP2 concentration on kinase activity was sigmoidal, with spermine inhibiting PI 3-kinase activity at all PIP2 concentrations. While 1 mM calcium stimulated PI 3-kinase activity at submaximal concentrations of Mg2+ (1.25 mM), inhibition was observed at optimal concentration of Mg2+ (2 mM). We propose that spermine may modulate the cellular signal by virtue of its differential effects on phosphoinositide kinases.