Fluoride activates diradylglycerol and superoxide generation in human neutrophils via PLD/PA phosphohydrolase-dependent and -independent pathways

Anionic lipid-induced conformational changes in human phagocyte flavocytochrome b precede assembly and activation of the NADPH oxidase complex

Phagocyte NADPH oxidases generate superoxide at high rates in defense against infectious agents, a process regulated by second messenger anionic lipids using incompletely understood mechanisms. We reconstituted the catalytic core of the human neutrophil NADPH oxidase, flavocytochrome b (Cyt b) in 99% phosphatidylcholine vesicles in order to correlate anionic lipid-dependent conformational changes in membrane-bound Cyt b and oxidase activity. The anionic lipid 10:0 phosphatidic acid (10:0 PA) specifically induced conformational changes in Cyt b as measured by a combination of fluorescence resonance energy transfer methods and size exclusion chromatography. The fluorescence lifetime of a complex between Cyt b and Cascade Blue-derivatized anti-p22(phox) antibody (CCB-CS9), increased after exposure to 10:PA by ∼50% of the change observed when the complex is dissociated, indicating a structural rearrangement of p22(phox) and/or the Cyt b heme prosthetic groups. Half of the quenching relaxation occurred at 10:0 PA concentrations permissive to less than 10% full NADPH oxidase activity, but saturated near the saturation in activity in a matched cell-free oxidase assay. We conclude that anionic lipids modulate the conformation of Cyt b in the membrane and suggest they may serve to modulate the structure of Cyt b as a control mechanism for the NADPH oxidase.

Phospholipase D2‐derived phosphatidic acid binds to and activates ribosomal p70 S6 kinase independently of mTOR

The product of phospholipase D (PLD) enzymatic action in cell membranes, phosphatidic acid (PA), regulates kinases implicated in NADPH oxidase activation, as well as the mammalian target of rapamycin (mTOR) kinase. However, other protein targets for this lipid second messenger must exist in order to explain other key PA-mediated cellular functions. In this study, PA was found to specifically and saturably bind to and activate recombinant and immunoprecipitated endogenous ribosomal S6 kinase (S6K) with a stoichiometry of 94:1 lipid/protein. Polyphosphoinositides PI4-P and PI4,5P2 and cardiolipin could also bind to and activate S6K, albeit with different kinetics. Conversely, PA with at least one acyl side chain saturated (10:0) was ineffective in binding or activating the enzyme. Transfection of COS-7 cells with a wild-type myc-(pcDNA)-PLD2 construct resulted in high PLD activity, concomitantly with an increase in ribosomal p70S6K enzyme activity and phosphorylation in T389 and T421/S424 as well as phosphorylation of p70S6K's natural substrate S6 protein in S235/S236. Overexpression of a lipase inactive mutant (K758R), however, failed to induce an increase in both PLD and S6K activity or phosphorylation, indicating that the enzymatic activity of PLD2 (i.e., synthesis of PA) must be present to affect S6K. Neither inhibiting mTOR kinase activity with rapamycin nor silencing mTOR gene expression altered the augmentative effect of PLD2 exerted on p70S6K activity. This finding indicates that PA binds to and activates p70S6K, even in the absence of mTOR. Lastly, COS-7 transfection with PLD2 changed the pattern of subcellular expression, and a colocalization of S6K and PLD2 was observed by immunofluorescence microscopy. These results show for the first time a direct (mTOR-independent) participation of PLD in the p70S6K pathway and implicate PA as a nexus that brings together cell phospholipases and kinases.

Signal transduction mechanism involved in Clostridium perfringens alpha-toxin-induced superoxide anion generation in rabbit neutrophils

Clostridium perfringens alpha-toxin induces the generation of superoxide anion (O2(-)) via production of 1,2-diacylglycerol (DG) in rabbit neutrophils. The mechanism of the generation, however, remains poorly understood. Here we report a novel mechanism for the toxin-induced production of O2(-) in rabbit neutrophils. Treatment of the cells with the toxin resulted in tyrosine phosphorylation of a protein of about 140 kDa. The protein reacted with anti-TrkA (nerve growth factor high-affinity receptor) antibody and bound nerve growth factor. Anti-TrkA antibody inhibited the production of O2(-) and binding of the toxin to the protein. The toxin induced phosphorylation of 3-phosphoinositide-dependent protein kinase 1 (PDK1). K252a, an inhibitor of TrkA receptor, and LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), reduced the toxin-induced production of O2(-) and phosphorylation of PDK1, but not the formation of DG. These inhibitors inhibited the toxin-induced phosphorylation of protein kinase C theta (PKCtheta). U73122, a phospholipase C (PLC) inhibitor, and pertussis toxin inhibited the toxin-induced generation of O2(-) and formation of DG, but not the phosphorylation of PDK1. These observations show that the toxin independently induces production of DG through activation of endogenous PLC and phosphorylation of PDK1 via the TrkA receptor signaling pathway and that these events synergistically activate PKCtheta in stimulating an increase in O2(-). In addition, we show the participation of mitogen-activated protein kinase-associated signaling events via activation of PKCtheta in the toxin-induced generation of O2(-).

Phospholipase D (PLD) gene expression in human neutrophils and HL-60 differentiation

Human neutrophils exhibit a regulated phospholipase D (PLD) activity that can be measured biochemically in vitro. However, the precise expression pattern of PLD isoforms and their specific biological role(s) are not well understood. Neutrophil mRNA is intrinsically difficult to isolate as a result of the extremely high content of lytic enzymes in the cell's lysosomal granules. Reverse transcription coupled to polymerase chain reaction indicated that pure populations of human neutrophils had the CD16b(+)/CD115(-)/CD20(-)/CD3zeta(-)/interleukin-5 receptor alpha(-) phenotype. These cells expressed the following splice variants of the PLD1 isoform: PLD1a, PLD1b, PLD1a2, and PLD1b2. As for the PLD2 isoform, neutrophils expressed the PLD2a but not the PLD2b mRNA variant. The relative amount of PLD1/PLD2 transcripts exists in an approximate 4:1 ratio. The expression of PLD isoforms varies during granulocytic differentiation, as demonstrated in the promyelocytic leukemia HL-60 cell line. Further, the pattern of mRNA expression is dependent on the differentiation-inducing agent, 1.25% dimethyl sulfoxide causes a dramatic increase in PLD2a and PLD1b transcripts, and 300 nM all-trans-retinoic acid induced PLD1a expression. These results demonstrate for the first time that human neutrophils express five PLD transcripts and that the PLD genes undergo qualitative changes in transcription regulation during granulocytic differentiation.

Age-related changes in membrane lipid composition, fluidity and respiratory burst in rat peritoneal neutrophils

The O2*(-) production has been studied in rat peritoneal neutrophils of different age (3, 12 and 24 months), in order to analyse whether the neutrophil respiratory burst is modified with increasing age. To stimulate NADPH oxidase, the enzyme responsible for the respiratory burst, two stimuli that act in different way have been used: phorbol myristate acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (N-FMLP). Production of O2*(-) decreased with age in neutrophils stimulated with N-FMLP (about 40%), but not in the stimulated with PMA. No difference in NADPH oxidase activity was found with age. The NADPH is supplied to the respiratory burst mainly by the pentose phosphate shunt. A progressive and significant decrease in the two most important enzymes of this route, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, was detected as a function of age; in spite of this reduction, the NADPH produced by cells from old animals seems not limiting for the O2*(-) production. The N-FMLP-induced decrease in the O2*(-) production may be related to the age-dependent increase in the membrane fluidity observed. A decline in the cholesterol/phospholipid ratio and a rise in the total polyunsaturated fatty acids content were found, that correlated well with the increase in the membrane fluidity. The decrease (50%) of phosphatidylinositols in the 24-month-old animals may be also related to the age-impairment in the respiratory burst found after stimulation with N-FMLP. These studies suggest that the age-related alterations in neutrophil may result in diminished neutrophil function and increased susceptibility to infection in the ageing.

A phosphatidic acid-activated protein kinase and conventional protein kinase C isoforms phosphorylate p22(phox), an NADPH oxidase component

Using a phosphorylation-dependent cell-free system to study NADPH oxidase activation (McPhail, L. C., Qualliotine-Mann, D., and Waite, K. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7931-7935), we previously showed that p47(phox), a cytosolic NADPH oxidase component, is phosphorylated. Now, we show that p22(phox), a subunit of the NADPH oxidase component flavocytochrome b(558), also is phosphorylated. Phosphorylation is selectively activated by phosphatidic acid (PA) versus other lipids and occurs on a threonine residue in p22(phox). We identified two protein kinase families capable of phosphorylating p22(phox): 1) a potentially novel, partially purified PA-activated protein kinase(s) known to phosphorylate p47(phox) and postulated to mediate the phosphorylation-dependent activation of NADPH oxidase by PA and 2) conventional, but not novel or atypical, isoforms of protein kinase C (PKC). In contrast, all classes of PKC isoforms could phosphorylate p47(phox). In a gel retardation assay both the phosphatidic acid-dependent kinase and conventional PKC isoforms phosphorylated all molecules of p22(phox). These findings suggest that phosphorylation of p22(phox) by conventional PKC and/or a novel PA-activated protein kinase regulates the activation/assembly of NADPH oxidase.

Activation of human neutrophil NADPH oxidase by phosphatidic acid or diacylglycerol in a cell-free system. Activity of diacylglycerol is dependent on its conversion to phosphatidic acid

The superoxide-generating neutrophil NADPH oxidase can be activated in cell-free reconstitution systems by several agonists, most notably arachidonic acid and the detergent sodium dodecyl sulfate. In this study, we show that both phosphatidic acids and diacylglycerols can serve separately as potent, physiologic activators of NADPH oxidase in a cell-free system. Stimulation of superoxide generation by these lipids was dependent upon both Mg(2+) and agonist concentration. Activation of NADPH oxidase by phosphatidic acids did not appear to require their conversion to corresponding diacylglycerols by phosphatidate phosphohydrolase, since diacylglycerols were much slower than phosphatidic acids to activate the system and required the presence of ATP. Stimulation of the oxidase by dioctanoylglycerol proved to be by a means other than the activation of protein kinase C. Instead, dioctanoylglycerol was converted to dioctanoylphosphatidic acid by an endogenous diacylglycerol kinase present in the cell-free reaction system. This conversion was sensitive to the diacylglycerol kinase inhibitor R59949 and explains the markedly slower kinetics of activation and the novel ATP requirement seen with dioctanoylglycerol. The level of dioctanoylphosphatidic acid formed was suboptimal for NADPH oxidase activation but could synergize with the unmetabolized dioctanoylglycerol to activate superoxide generation.

Phospholipase D, tumor promoters, proliferation and prostaglandins

Phosphatidylcholine hydrolysis by phospholipase D is a widespread response to cellular stimulation. However, the downstream signaling events subsequent to phosphatidylcholine hydrolysis are just beginning to be determined. Initially it was proposed that diglyceride formation by phospholipase D and phosphatidate phosphohydrolase resulted in long-term stimulation of protein kinase C. However, recent studies indicate that phosphatidic acid is the relevant signaling molecule in some signaling pathways. The present review will summarize studies of phospholipase D in the response of cells to the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, which causes cells to mimic the phenotype of oncogenic transformation. The role of phospholipase D in stimulation of Raf-1 and prostaglandin H synthase type-2 is emphasized.

A novel protein kinase target for the lipid second messenger phosphatidic acid

Activation of phospholipase D occurs in response to a wide variety of hormones, growth factors, and other extracellular signals. The initial product of phospholipase D, phosphatidic acid (PA), is thought to serve a signaling function, but the intracellular targets for this lipid second messenger are not clearly identified. The production of PA in human neutrophils is closely correlated with the activation of NADPH oxidase, the enzyme responsible for the respiratory burst. We have developed a cell-free system, in which the activation of NADPH oxidase is induced by the addition of PA. Characterization of this system revealed that a multi-functional cytosolic protein kinase was a target for PA, and that two NADPH oxidase components were substrates for the enzyme. Partial purification of the PA-activated protein kinase separated the enzyme from known protein kinase targets of PA. The partially purified enzyme was selectively activated by PA, compared to other phospholipids, and phosphorylated the oxidase component p47-phox on both serine and tyrosine residues. PA-activated protein kinase activity was present in a variety of hematopoietic cells and cell lines and in rat brain, suggesting it has widespread distribution. We conclude that this protein kinase may be a novel target for the second messenger function of PA.

Increased phospholipase D activity in butyrate-induced differentiation of HT-29 cells

Phospholipids are important constituents of biomembrane components and are supposed to function as enzyme activators or precursors of bioactive substances. Our earlier work has shown an increased esterification of neutral lipids of HT-29 cells during butyrate-induced differentiation (M. Madesh, O. Benard, K.A. Balasubramanian, Butyrate-induced alteration in lipid composition of human colon cell line HT-29, Biochem. Mol. Biol. Int. 38 (1996) 659-664). In this report we show that there is an increase in phospholipase D (PLD) activity during butyrate-induced differentiation of HT-29 cells as indicated by the formation of phosphatidic acid (PA). When the control and butyrate-treated cell homogenates were incubated in vitro with 1 mM Ca2+, the increase in PA formation was higher than in butyrate-treated cells. This PA was formed due to PLD activity that was confirmed by the generation of phosphatidylethanol by in vitro incubation of HT-29 cell homogenates in the presence of ethanol. The formation of PA was associated with a decrease in phosphatidylcholine (PC) and phosphatidylethanolamine (PE). This study has shown an increase in PLD activity associated with the differentiation of HT-29 cells.

Phospholipase D: a novel major player in signal transduction

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

Glucose activates the carboxyl methylation of gamma subunits of trimeric GTP-binding proteins in pancreatic beta cells. Modulation in vivo by calcium, GTP, and pertussis toxin

The gamma subunits of trimeric G-proteins (gamma1, gamma2, gamma5, and gamma7 isoforms) were found to be methylated at their carboxyl termini in normal rat islets, human islets and pure beta [HIT-T15] cells. Of these, GTPgammaS significantly stimulated the carboxyl methylation selectively of gamma2 and gamma5 isoforms. Exposure of intact HIT cells to either of two receptor-independent agonists--a stimulatory concentration of glucose or a depolarizing concentration of K+--resulted in a rapid (within 30 s) and sustained (at least up to 60 min) stimulation of gamma subunit carboxyl methylation. Mastoparan, which directly activates G-proteins (and insulin secretion from beta cells), also stimulated the carboxyl methylation of gamma subunits in intact HIT cells. Stimulatory effects of glucose or K+ were not demonstrable after removal of extracellular Ca2+ or depletion of intracellular GTP, implying regulatory roles for calcium fluxes and GTP; however, the methyl transferase itself was not directly activated by either. The stimulatory effects of mastoparan were resistant to removal of extracellular Ca2+, implying a mechanism of action that is different from glucose or K+ but also suggesting that dissociation of the alphabetagamma trimer is conducive to gamma subunit carboxyl methylation. Indeed, pertussis toxin also markedly attenuated the stimulatory effects of glucose, K+ or mastoparan without altering the rise in intracellular calcium induced by glucose or K+. Glucose-induced carboxyl methylation of gamma2 and gamma5 isoforms was vitiated by coprovision of any of three structurally different cyclooxygenase inhibitors. Conversely, exogenous PGE2, which activates Gi and Go in HIT cells and which thereby would dissociate alpha from beta(gamma), stimulated the carboxyl methylation of gamma2 and gamma5 isoforms and reversed the inhibition of glucose-stimulated carboxyl methylation of gamma subunits elicited by cyclooxygenase inhibitors. These data indicate that gamma subunits of trimeric G-proteins undergo a glucose- and calcium-regulated methylation-demethylation cycle in insulin-secreting cells, findings that may imply an important role in beta cell function. Furthermore, this is the first example of the regulation of the posttranslational modification of G-protein gamma subunits via nonreceptor-mediated activation mechanisms, which are apparently dependent on calcium influx and the consequent activation of phospholipases releasing arachidonic acid.

Tyrosine phosphorylation of 100-115 kDa proteins by phosphatidic acid generated via phospholipase D activation in HL60 granulocytes

In HL60 granulocytes, 4beta-phorbol 12-myristate 13-acetate (PMA) induced tyrosine phosphorylation of several proteins with molecular weight of 100-115 kDa and 45 kDa. Furthermore, PMA-mediated phosphatidic acid (PA) production via phospholipase D (PLD) activation. In the presence of either butanol or ethanol, PMA-induced PA production was markedly reduced and instead a metabolically stable phosphatidylbutanol (PBut) or phosphatidylethanol (PEt) was produced by transphosphatidylation by PLD. Under the same incubation condition, these primary alcohols inhibited PMA-induced tyrosine phosphorylation of the 100-115 kDa proteins. Propranolol, which is often used as a selective inhibitor of PA phosphohydrolase (PAP) involving diacylglycerol (DG) formation from PA, did not affect tyrosine phosphorylation of the 100-115 kDa proteins. Moreover, incubation of HL60 granulocytes with Streptomyces chromofuscus PLD caused both PA production and tyrosine phosphorylation of the above proteins. Exogenous PA treatment also induced tyrosine phosphorylation of the same proteins. Thus, the results presented here suggest that PA produced via PLD activation is involved in tyrosine phosphorylation of the 100-115 kDa proteins in HL60 granulocytes.

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.

Studies on ether-phospholipids of vascular smooth muscle cells. Identification of a rapid Ca(2+)-dependent hydrolysis of alkyl-phosphatidylethanolamine promoted by endothelin-l

We have investigated the metabolism of 1-O-[3H]octadecyl-sn-glycero-3-phosphocholine ([3H]lyso PAF) and [3H] myristic acid in secondary cultures of aortic smooth muscle cells (SMC) to characterize the origin of second messengers generated upon stimulation with endothelin-l (ET-l). When cells were labelled with [3H]lyso PAF, we observed a transfer of the label from phosphatidylcholine (PC) to phosphatidylethanolamine (PE) In contrast, incubation with [3H]lyso PAF labelled mainly alkyl-subclasses while [3H]myristate was associated with diacyl-subclasses. Using these specific labelling procedures, we have shown that ET-l induced a strong hydrolysis of PE. This hydrolysis was specific for alkyl-PE with a maximum after 5 s of stimulation. We have also observed an extracellular Ca(2+)-dependent increase in diglyceride (DG), phosphatidic acid (PA) and mainly triglyceride (TG) concomitant to alkyl-PE hydrolysis. Thus, alkyl-DG generated from alkyl-PE appears to be a major product in ET-l stimulation of SMC. These results suggest a new level of complexity in the signal transduction cascade involving a specificity for phospholipid subclasses.

Phospholipase D-derived products in the regulation of 12-O-tetradecanoylphorbol-13-acetate-stimulated prostaglandin synthesis in madin-darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells stimulated with 12-O-tetradecanoylphorbol-13-acetate (TPA) in the presence of ethanol synthesize phosphatidylethanol (PEt) instead of phosphatidic acid (PA) and diglyceride (DG). We have used ethanol to block the production of phospholipase D (PLD)-derived PA and DG (from PA hydrolysis) to study their role in signal transduction. In MDCK cells, TPA-stimulated prostaglandin E2 (PGE2) synthesis was inhibited by ethanol at concentrations which inhibit PA and DG formation. In addition, TPA elicited a prolonged increase in PGE2 synthesis that is dependent upon continuous activation of PLD. The TPA-stimulated translocation of protein kinase Calpha (PKCalpha) from cytosol to membrane was unaffected by ethanol. This suggests that PLD-derived products act downstream of PKC in TPA-stimulated prostaglandin synthesis. The calcium ionophore, A23187, did not activate PLD, and PGE2 synthesis in response to A23187 was unaffected by ethanol. TPA increased prostaglandin endoperoxide H synthase (PGHS) activity and increased the amount of immunodetectable prostaglandin endoperoxide H synthase 2 (PGHS-2). A23187 did not induce PGHS-2 and A23187-stimulated PGE2 synthesis appears to be due to the constitutively expressed PGHS-1. Blocking the formation of PLD-derived products, PA and DG, inhibited the induction of PGHS-2 by TPA. These results indicate that prolonged PGE2 synthesis in response to TPA is due to the continuous induction of PGHS-2, which is dependent upon PLD activation. In contrast, induction of PGHS-2 by epidermal growth factor was not affected by ethanol. Epidermal growth factor did not induce PKCalpha translocation nor activate PLD. Taken together, these data suggest that PLD-derived PA or DG act as second messengers in the induction of PGHS-2 by PKC-dependent pathways. The demonstration that inhibition of TPA-induced PA formation inhibits Raf-1 translocation in MDCK cells (Ghosh, S., Strum, J. C., Sciorra, V. A., Daniel, L. W. , and Bell, R. M. (1996) J. Biol. Chem. 271, 8472-8480) suggests that PA is the active PLD metabolite in TPA-stimulated signaling.

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.

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.

Impaired activation of phospholipase D in polycythaemia vera-implications for the pathogenesis of the disease?

A series of studies have demonstrated a stimulus-specific defect in PMN oxidative metabolism after stimulation with surface receptor dependent stimuli such as fMLP, leukotriene B4 and platelet activating factor (PAF), whereas the response to phorbol myristate acetate was normal. Having discovered this defect, studies of the stimulus response coupling for oxidative responses were performed showing a normal interaction of fMLP with it's receptor, as well as an intact activation of phospholipase C, as measured by the generation of 1,4,5-inositoltrisphosphate, and the subsequent rise of intracellular calcium. In contrast, the formation of diacylglycerol and phosphatidylethanol was decreased in PV PMN, denoting an impaired activation of phospholipase D (PLD). It was shown by flow cytometry analyses that the hampered oxidative response was present both in single PMN and monocytes. Moreover, platelets from PV patients, whose PMN exhibit a lower oxidative response to PAF, also have a diminished aggregatory response to PAF. Thus three different cell lineages in PV have been revealed to respond abnormally to surface receptor dependent stimuli, indicating that the proposed impairment of PLD might be relevant for changes in the malignant stem cell clone. Since phosphatidic acid, produced as a result of PLD activation, may be implicated in the regulation of several oncogenes, perturbations of the PLD system could theoretically be important for the development of PV.

Leukotriene B4 stimulation of an early elevation of phosphatidic acid mass in human neutrophils

The signal transduction pathway of leukotriene B4 involves phospholipase D activation in cytochalasin B-primed neutrophils, but leukotriene B4 stimulation of increased phosphatidic acid mass in neutrophils has not been demonstrated. Employing the NIH Image program, we have examined the effect of leukotriene B4 on phosphatidic acid mass in human neutrophils incubated with or without cytochalasin B. Our results show that 0.15 microM leukotriene B4 without cytochalasin B was capable of increasing phosphatidic acid mass in neutrophils by 2-fold after 5 s, 2.5-fold after 1 min, and 2-fold after 5 min incubation. Leukotriene B3, leukotriene B4, and leukotriene B5 were equipotent stimuli for phosphatidic acid mass elevation. Leukotriene B4 induced phosphatidylethanol formation at the expense of phosphatidic acid in cells preincubated with 0.25-1% ethanol, indicating phospholipase D activation. Cytochalasin B enhanced leukotriene B4 stimulation of phosphatidic acid mass elevation and phosphatidylethanol formation. There were no measurable changes in 1,2-diglyceride mass after 5 s, but a 1.7-fold increase occurred after 1 min and declined thereafter. Leukotriene B4 stimulation of [3H]glycerol incorporation into phosphatidic acid, diglyceride and phosphatidylinositol was detectable after a 1-min incubation, suggesting increased de novo synthesis of these lipids. These results suggest that leukotriene B4 stimulation of phospholipase D activity contributes to part of the early increased phosphatidic acid mass and that combined actions of stimulated phospholipases C and D, and de novo phosphatidic acid synthesis contribute to the total increased phosphatidic acid mass.

Activation of phospholipase D by guanosine 5'[gamma-thio]triphosphate and AlF4- in bovine corneal epithelial cells

We have investigated the regulation of phospholipase D (PLD) by guanine nucleotides and AlF4- in bovine corneal epithelial cells (BCEC) prelabeled with [3H]myristic acid. In the presence of ethanol, AlF4- increased the production of [3H]PA and [3H]PET indicating activation of PLD in these cells. The effects of AlF4- were time- and dose-dependent. Addition of guanosine 5[gamma-thio]triphosphate (GTP gamma S), to streptolysin O-permeabilized cells also resulted in increased accumulation of [3H]PA and [3H]PEt. Other guanine and adenine nucleotides were ineffective, and guanosine thiodiphosphate inhibited the GTP gamma S-induced activation of PLD. Direct addition of GTP gamma S to microsomal fraction prepared from [3H]myristate-labeled BCEC resulted in increased formation of [3H]PEt in a time- and dose-dependent manner. The activation of PLD by GTP gamma S in the microsomal fraction was absolutely dependent on the presence of Ca2+ > 0.5 microM. Addition of Ca2+ (10-100 microM) alone dose-dependently stimulated the PLD activity. Treatment of the microsomal fraction with phorbol esters had no effect on the ability of GTP gamma S to stimulate PLD. Addition of isoproterenol to BCEC resulted in several-fold stimulation of cAMP, but it had no effect on basal or PDBu-induced stimulation of PLD. Taken together, the data suggest that a GTP-binding protein is involved in regulation of PLD in BCEC, and that maximal stimulation of PLD probably results from an interaction between Ca2+, PKC and G-protein in BCEC.

A proposal for purification of salivary polymorphonuclear leukocytes by combination of nylon mesh filtration and density-gradient method: a validation by superoxide- and cyclic AMP-generating responses

Isolation and purification of salivary polymorphonuclear leukocytes (SPMNs) from accompanying epithelial cells was presented by using a density-gradient method with Ficoll. SPMNs samples prepared by already established methods (nylon mesh filtration) was compared with SPMNs samples after further purification by Ficoll (d = 1.083). Microscopically, SPMNs samples after nylon mesh filtration contain higher percentage of epithelial cells than SPMNs samples after Ficoll centrifugation. In response to stimulation of superoxide generation, both samples showed the same pattern of response. However, in response to forskolin and prostaglandin E1, cyclic AMP levels in samples after nylon mesh purification were significantly higher than in samples after Ficoll purification because of the presence of contaminating epithelial cells. We can conclude that, although nylon mesh filtration is satisfactory when we need to examine superoxide generation but further purification is necessary when we want to measure factors like intracellular cyclic AMP formation.

Phospholipase D: regulation and functional significance

PLD is a major route for hydrolysis of PC in most tissues, consistent with it playing an important role in signal transduction. The enzyme appears to be activated by a variety of different mechanisms in different tissues, suggesting there might be several different isoforms. Little, however, is known at present about its enzymology and molecular biology. There is little direct evidence to indicate the functional significance of PLD activation but an accumulation of indirect evidence links PLD with prolonged changes in cell function. In particular, two areas where there is strong evidence for a role for PLD are mitogenesis and leukocyte hyperresponsiveness. An important area for future work will be the investigation of how products from the PLD pathway exert these effects. Current evidence suggests an important role for Ca(2+)-independent PKC isoforms and probably also for novel cellular targets for the putative second messenger PA.

Glucagon modulates superoxide generation in human polymorphonuclear leucocytes

It has been found that leucocytes possess receptor sites for glucagon and glucagon was shown to increase during bacterial infection. To verify the interconnection between glucagon, leucocytes and bacterial infection we studied the effect of glucagon on superoxide generation and second messenger transduction in PMNs. We found that glucagon could not stimulate chemiluminescence by itself but it could enhance FMLP- but not PMA-induced chemiluminescence in a concentration (50-800 pg/ml) dependent manner. However, after incubation of PMNs with 10 microM of ST-638 (a tyrosine kinase inhibitor) the enhancement effect converted into inhibitory effect. We also found that glucagon treatment of PMNs increased both IP3 and cyclic AMP levels as second messengers. ST-638 greatly attenuated the IP3 increment in the glucagon-treated FMLP-stimulated PMNs. From these results we can conclude that glucagon could enhance superoxide generation from FMLP-stimulated PMNs by elevating IP3. Inhibition of IP3 increment by tyrosine kinase blockade uncover the inhibitory effect of the increasing cyclic AMP on superoxide production.

New pathways of phagocyte activation: the coupling of receptor-linked phospholipase D and the role of tyrosine kinase in primed neutrophils

Protein kinase C (PKC) appears to have a central role in the O2- response of neutrophils following stimulation of membrane receptors. The second messenger, diacylglycerol (DG), that activates PKC is derived from membrane phospholipids via activation of phosphatidylinositol 4,5-bisphosphate (PIP2)-phospholipase C (PLC) and phospholipase D (PLD), with the latter pathway being more prominent in primed cells. In resting cells receptor coupling to PLD is through a G-protein. Priming brings a cytoplasmic tyrosine kinase into the transducer sequence which, through protein phosphorylation, increases the efficiency of coupling between membrane receptors and PLD. Phosphatidic acid (PA), the initial product of the PLD pathway, also appears to act as a second messenger by directly activating the NADPH oxidase responsible for generating O2-. Interconversion of PA and DG by phosphatidate phosphohydrolase and DG kinase determines which of these second messengers has the dominant role.

Further studies of the defective stimulus-response coupling for the oxidative burst in neutrophils in polycythemia vera

We have previously reported that the chemiluminescence (CL) response of neutrophils (PMN) from patients with polycythemia vera (PV) was abnormally low when induced by surface receptor-dependent stimuli, fMLP and leukotriene B4, but normal when elicited by phorbol myristate acetate (PMA). This study documents that this discrepancy of the CL response to fMLP and PMA remained over a wide range of stimuli concentrations, was not due to iron-deficient PV cells and was also observed with the nitroblue tetrazolium assay. Moreover, another surface receptor-dependent agonist, platelet-activating factor, conferred a significantly lower CL response in PV PMN relative to controls. Treatment with alpha interferon or GM-CSF, to increase fMLP receptors, resulted in a similar enhancement of fMLP-induced CL in PV and controls. CL was normal when induced by a number of non-surface receptor-dependent stimuli. Release of lactoferrin in response to fMLP (and PMA) was normal (as was previously reported fMLP-induced chemotaxis and adherence). Thus, this defect is highly specific for oxidative metabolism, and localized to discrete step(s) of the stimulus-response coupling for fMLP, leukotriene B4 and PAF, but conceivably not due to impairment of the dynamic interaction of fMLP with its receptor.

Glucocorticoids inhibit formation of inositol phosphates in macrophages

Glucocorticoids inhibited the zymosan-induced formation of inositol phosphates in macrophages. No inhibition was observed with progesterone. Inhibitors of protein (cycloheximide) and RNA (actinomycin D) synthesis exhibited similar inhibitory effects. The activity of phospholipase C in subcellular fractions was not altered by hormone treatment of the cells. However, the incorporation of inositol into membrane lipids was reduced by dexamethasone. These data indicate that glucocorticoids are able to inhibit the formation of inositol phosphates; the effect of the hormone is rather due to an inhibition of the incorporation of inositol in membrane lipids than to an inhibition of phospholipase C. The anti-inflammatory action of glucocorticoids may, therefore, also be attributed to their effect on the polyphosphoinositide cycle and inositol phosphate-mediated processes.