Divergent effects of propranolol on neutrophil superoxide release: involvement of phosphatidic acid and diacylglycerol as second messengers

Propranolol Is an Effective Topical and Systemic Treatment Option for Experimental Epidermolysis Bullosa Acquisita

Propranolol is an ADRB2 blocker that regulates heart muscle contractions, smooth muscle relaxation, and glycogenolysis. In addition, an increasing number of applications in dermatology have been described, most prominently, the use as a first-line treatment for infantile hemangiomas. We here show that propranolol enhances IL-8-induced neutrophil chemotaxis and reduces the release of ROS after immune complex stimulation. To obtain further molecular insights into the modulatory effects of propranolol in activated neutrophils, we performed RNA sequencing of immune complex-stimulated neutrophils in the absence and presence of the drug. We identified the transcriptomic signature of propranolol and demonstrated an ADR2-independent immunomodulatory effect. To determine if the anti-inflammatory transcriptomic signature of propranolol also translates into clinical effects, we next evaluated the impact of propranolol in a prototypical neutrophil-dependent skin disease, specifically, antibody transfer-induced epidermolysis bullosa acquisita in mice. To validate the identified propranolol gene signature obtained in human neutrophils, we analyzed a selection of genes by RT-PCR in mouse epidermolysis bullosa acquisita skin and confirmed TNF, among others, to be differentially regulated by propranolol treatment. Our data clearly indicate that, based on its molecular impact on immune complex-activated neutrophils, propranolol is a potential treatment option for neutrophil-mediated inflammatory skin diseases.

An essential role for phospholipase D in the recruitment of vesicle amine transport protein-1 to membranes in human neutrophils

Although phosphatidic acid (PA) regulates a wide variety of physiological processes, its targets remain poorly characterized in human neutrophils. By co-sedimentation with PA-containing vesicles we identified several PA-binding proteins including vesicle amine transport protein-1 (VAT-1), Annexin A3 (ANXA3), Rac2, Cdc42 and RhoG in neutrophil cytosol. Except for ANXA3, protein binding to PA-containing liposomes was calcium-independent. Cdc42 and RhoG preferentially interacted with PA whereas VAT-1 bound to PA or phosphatidylserine with the same affinity. VAT-1 translocated to neutrophil membranes upon N-formyl-methionyl-leucyl-phenylalanine (fMLF) stimulation. Inhibition of fMLF-induced PLD activity with the Src kinase inhibitor PP2, the selective inhibitor of PLD FIPI, or of PA formation with primary alcohols reduced VAT-1 translocation. In contrast, inhibition of PA hydrolysis with propranolol enhanced fMLF-mediated VAT-1 recruitment to membranes. PMA also redistributed VAT-1 to membranes in a PKC- and PLD-dependent manner. Though fMLF and PMA increased VAT-1 phosphorylation, different kinases appear to be involved. Cell fractionation revealed that a pool of VAT-1 was co-localized with primary, secondary and tertiary granules and plasma membrane markers in resting neutrophils. Stimulation with fMLF enhanced VAT-1 co-localization with CD32a, a plasma membrane marker. Confocal microscopy revealed that VAT-1 decorates granular structures at the cell periphery and double labeling with VAT-1/lactoferrin antibodies showed a partial co-localization with secondary granules in control and fMLF-stimulated cells. Characterization of these putative PA-binding proteins constitutes another step forward for a better understanding of the role of PLD-derived PA in neutrophil physiology.

Ca(2+)-dependent production of reactive oxygen metabolites by human neutrophils in response to fluorinated propranolol analogues

Fluorinated analogues of propranolol, namely trifluoroethyl propranolol (F3), pentafluoropropyl propranolol (F5), and heptafluorobutyl propranolol (F7), were found to induce reactive oxygen metabolite (ROM) production in human neutrophils in a dose-dependent manner. Preincubation of neutrophils with the calcium chelator BAPTA-AM or the tyrosine kinase inhibitor genistein inhibited this ROM production. Direct measurements of intracellular calcium revealed that these analogues caused a transient increase in intracellular calcium. In addition, these fluorinated analogues of propranolol caused a transient increase in actin polymerization. The effects of these compounds were found to be dependent upon the degree of fluorination of the parent compound. Propranolol, on the other hand, had no direct effect on ROM, calcium, or actin polymerization when added alone to neutrophils, although it did modify responses of cells to various stimuli. Whereas ROM production induced by the chemotactic peptide formyl-methionyl-leucyl-phenylalanine was enhanced in a dose-dependent manner, the response to the particulate stimulus, latex beads, was abolished.

Independent functioning of cytosolic phospholipase A2 and phospholipase D1 in Trp-Lys-Tyr-Met-Val-D-Met-induced superoxide generation in human monocytes

Recently, a novel peptide (Trp-Lys-Tyr-Met-Val-D-Met, WKYMVm) has been shown to induce superoxide generation in human monocytes. The peptide stimulated phospholipase A2 (PLA2) activity in a concentration- and time-dependent manner. Superoxide generation as well as arachidonic acid (AA) release evoked by treatment with WKYMVm could be almost completely blocked by pretreatment of the cells with cytosolic PLA2 (cPLA2)-specific inhibitors. The involvement of cPLA2 in the peptide-induced AA release was further supported by translocation of cPLA2 to the nuclear membrane of monocytes incubated with WKYMVm. WKYMVm-induced phosphatidylbutanol formation was completely abolished by pretreatment with PKC inhibitors. Immunoblot showed that monocytes express phospholipase D1 (PLD1), but not PLD2. GF109203X as well as butan-1-ol inhibited peptide-induced superoxide generation in monocytes. Furthermore, the interrelationship between the two phospholipases, cPLA2 and PLD1, and upstream signaling molecules involved in WKYMVm-dependent activation was investigated. The inhibition of cPLA2 did not blunt peptide-stimulated PLD1 activation or vice versa. Intracellular Ca2+ mobilization was indispensable for the activation of PLD1 as well as cPLA2. The WKYMVm-dependent stimulation of cPLA2 activity was partially dependent on the activation of PKC and mitogen-activated protein kinase, while PKC activation, but not mitogen-activated protein kinase activation, was an essential prerequisite for stimulation of PLD1. Taken together, activation of the two phospholipases, which are absolutely required for superoxide generation, takes place through independent signaling pathways that diverge from a common pathway at a point downstream of Ca2+.

Novel effects of propranolol. Release of internal Ca(2+) followed by activation of capacitative Ca(2+) entry in Madin Darby canine kidney cells

The effect of propranolol on Ca(2+) signalling in Madin Darby canine kidney (MDCK) cells was investigated by using fura-2 as a Ca(2+) probe. Propranolol increased cytosolic free Ca(2+) levels ([Ca(2+)](i)) in a concentration-dependent manner between 0.1 and 1 mM. The response was partly inhibited by external Ca(2+) removal. In Ca(2+)-free medium pretreatment with 0.2 mM propranolol partly inhibited the [Ca(2+)](i) rise induced by 1 microM thapsigargin, an inhibitor of the endoplasmic reticulum Ca(2+) pump; but pretreatment with thapsigargin abolished propranolol-induced Ca(2+) release. Addition of 3 mM Ca(2+) induced a [Ca(2+)](i) rise after pretreatment with 0.2 mM propranolol in Ca(2+)-free medium. Propranolol (0.2 mM) inhibited 25% of thapsigargin-induced capacitative Ca(2+) entry. Suppression of 1,4,5-trisphosphate (IP(3)) formation by 2 microM U73122, a phospholipase C inhibitor, did not alter 0.2 mM propranolol-induced internal Ca(2+) release. Propranolol (1 mM) also increased [Ca(2+)](i) in human neutrophils. Collectively, we have found that 0.2 mM propranolol increased [Ca(2+)](i) in MDCK cells by releasing Ca(2+) from thapsigargin-sensitive Ca(2+) stores in an IP(3)-independent manner, followed by Ca(2+) influx from external space. Independently, propranolol was able to inhibit thapsigargin-induced capacitative Ca(2+) entry.

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.

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.

Modulation of luminol chemiluminescence of fMet-Leu-Phe-stimulated neutrophils by affecting dephosphorylation and the metabolism of phosphatidic acid

This paper is addressed to study how PKC-mediated effects and phosphatidic acid interact together in activation of NADPH-oxidase in formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) stimulated neutrophils as detected by luminol chemiluminescence. The early luminescence response in fMet-Leu-Phe-stimulated cells (up to 5 min after stimulation) depends mainly on reactive oxygen species generated extracellularly, whereas all later events are caused by oxidation of luminol inside the cells. The two protein phosphatase inhibitors, okadaic acid and calyculin A, dramatically increased the late luminescence of cells. This enhancement was totally inhibited by the phospholipase D modulator butanol, while the protein kinase C (PKC) inhibitor bisindolylmaleimide I was insensitive. The early luminescence response of the cells was slightly inhibited by both protein phosphatase inhibitors and depended on protein kinase C as well as on phospholipase D activities. Propranolol, an inhibitor of phosphatidate phosphohydrolase, enhanced all parts of luminescence response of fMet-Leu-Phe-stimulated neutrophils at concentrations up to 2.5 x 10(-5) mol/L. While the late luminescence response of propranolol-treated cells was not inhibited by the PKC inhibitor bisindolylmaleimide I, the first response depended on protein kinase C. The inhibitor of diacylglycerol kinase R59949 enhanced the luminescence signal only during the first 4 min in fMet-Leu-Phe-stimulated cells. Only diacylglycerols derived from phospholipase C, such as 1-stearoyl-2-arachidonoyl-sn-glycerol, were able to initiate an oxidative burst in cells. Saturated diacylglycerols (e.g. 1,2-dipalmitoyl-sn-glycerol or 1,2-distearoyl-sn-glycerol) did not yield any luminol chemiluminescence, although they were incorporated into the plasma membrane, as evidenced by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Our results demonstrate that phosphatidic acid produced by phospholipase D is responsible for NADPH-oxidase activity in fMet-Leu-Phe-stimulated neutrophils over the entire measuring time, whereas PKC-mediated processes are only involved during the first 5 min.

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.

Effects of propranolol on phosphatidate phosphohydrolase and mitogen-activated protein kinase activities in A7r5 vascular smooth muscle cells

High doses of propranolol inhibit phosphatidate phosphohydrolase (PAP) activity in intact cells, thus blocking metabolism of phosphatidic acid (PA), product of the phospholipase D (PLD) reaction. Vasopressin and phorbol ester activate PLD and ERK (extracellular signal-regulated protein kinase) mitogen-activated protein kinases in A7r5, a rat vascular smooth muscle cell line. Propranolol increased PA levels in intact A7r5 cells and inhibited cytosolic PAP and membrane calcium-independent phospholipase A2 but did not activate PLD or enhance agonist-induced PA accumulation. Incubation of cells with 200 microM propranolol for 10-45 min markedly elevated PA but caused only partial activation of ERKs. Propranolol and other lipophilic amines caused a time- and dose-dependent detachment of cells from their substrate. These results confirm that elevation of PA is not a strong signal for ERK activation and emphasize that caution should be exercised in using propranolol as a PAP inhibitor in intact cells.

Activation of phospholipase D in FRTL-5 thyroid cells by forskolin and dibutyryl-cyclic adenosine monophosphate

We demonstrated previously that TSH activates phospholipase D (PLD) via stimulation of protein kinase C (PKC) in Fischer rat thyroid line (FRTL)-5 thyroid cells. To examine the role of the cAMP pathway in the regulation of PLD, we studied the effects of forskolin (0-100 microM; 30 min) and dibutyryl cAMP (dbcAMP; 0-1 mM; 30 min) on PLD activation. FRTL-5 thyroid cells were labeled mainly in phosphatidylcholine with [3H]myristate followed by incubation with 200 mM ethanol before the addition of agonist. PLD was assessed by the measurement of [3H]phosphatidylethanol. Forskolin (100 nM to 100 microM) and dbcAMP (100 pM to 100 microM) increased PLD activity significantly. Maximal responses to forskolin and dbcAMP exceed the PLD responses produced by 100 microU/ml of TSH. To determine whether the effects of forskolin and dbcAMP on PLD occurred as a consequence of PKC activation, FRTL-5 thyroid cells were preincubated for 10 min with the PKC inhibitors, chelerythrine (1 microM) or calphostin C (1 microM), or they were pretreated for 24 h with phorbol myristate acetate (100 nM) to down-regulate PKC. Unlike TSH-mediated PLD activation, these treatments had no effect on PLD activation by cAMP agonists. Forskolin (10 microM; 30 min) had no effect on the subcellular distribution of PKC alpha-, epsilon-, or zeta-isoforms, confirming the lack of involvement of PKC. The protein kinase A (PKA) inhibitors, H-89 (10 microM; 30 min) and dideoxyadenosine (5 nM; 10 min) significantly decreased the forskolin- and dbcAMP-mediated PLD activation without any effect on the phorbol ester-mediated PLD response. Following pretreatment with H-89 or dideoxyadenosine, the TSH-mediated PLD response was also significantly reduced. These studies indicate that forskolin and dbcAMP stimulate PLD in FRTL-5 thyroid cells directly via PKA without involvement of PKC. Studies of cells in the presence and absence of ethanol revealed approximately 60% of the phosphatidate plus diacylglycerol produced via TSH occurs via PLD activation. Although TSH-mediated inositol phosphate generation occurred with similar concentrations of TSH that led to PLD activation, 10-fold higher TSH concentrations were required to increase intracellular Ca2+. These results and the lack of a rapid Ca2+ transient following physiological TSH concentrations suggest that alternatives to conventional hydrolysis of phosphatidylinositol 4,5-bisphosphate may initiate PKC activation. Thus, the two major signal transduction systems in the FRTL-5 thyroid cell (PKA and PKC) appear to converge on PLD activation. Stimulation of both of these pathways by TSH may be required for optimal physiological activation of PLD.

Effects of human low-density lipoproteins on superoxide production by formyl-methionyl-leucyl-phenylalanine activated polymorphonuclear leukocytes

Neutrophils play a major role in the host defence by producing reactive oxygen species. These products are liberated by activated cells and are known to cause endothelial cell injury and damage. The present study shows that low-density lipoproteins increase superoxide anion production by twofold in polymorphonuclear leukocytes stimulated by formyl-Met-Leu-Phe in vitro. Moreover, LDL induced a large increase in phosphoinositides and cytosolic-free calcium. Data from experiments performed on neutrophils treated with pertussis toxin, staurosporine, propranolol or niflumic acid suggest that modulation of phospholipase D and A2 activities could be involved in the modification by LDL of leukocyte response to formyl-Met-Leu-Phe. LDL lipid moiety could play a key role in their action on polymorphonuclear functions because cholesterol was exchanged between lipoproteins and cells that can modify membrane fluidity and interact with the formyl-Met-Leu-Phe receptor.

Exogenous phosphatidic acid with saturated short-chain fatty acyl groups induces superoxide anion release from guinea pig peritoneal polymorphonuclear leukocytes by three different mechanisms

Treatment of suspensions of guinea pig peritoneal polymorphonuclear leukocytes (PMN) with four species of phosphatidate (PA) containing short-chain fatty acids induced sustained superoxide anion (O2-) production after a lag time. The rank order of efficiency of these PAs in triggering O2- production was PA8:0 [1,2-dioctanoyl-sn-glycerol-3-phosphate (GP)] > PA10:0 (1,2-didecanoyl-GP) > PA6:0 (1,2-dicaproyl-GP) > > PA12:0 (1,2-dilauroyl-GP). The O2- release from PMN stimulated with PA10:0 or PA12:0, but not with PA6:0 or PA8:0, was lowered by the addition of 1 mM extracellular Ca2+. Studies with various inhibitors showed that the mechanism of multiphasic O2- production induced by PA8:0 depended on its concentration: 1 and 3 microM PA8:0 induced O2- production constantly after a lag time through a protein kinase-dependent mechanism that was inhibited by 100 nM staurosporine. With concentrations of PA of 10 microM or more, an additional mechanism that was independent of protein kinase became operative and predominant over the protein kinase-dependent one. This protein kinase-independent mechanism was inhibited selectively by 80 microM TMB-8. Concentrations of 30, 60 and 100 microM PA first elicited transient O2- production via another protein kinase-dependent mechanism that was more sensitive to H-7 than to staurosporine, and then sustained O2- production, mainly driven by the protein kinase-independent mechanism. Metabolism of exogenously added [14C]PA8:0 in intact PMN was examined in the presence and absence of propranolol. Results suggest that PA itself is more important rather than its degradation products such as diacylglycerol, in inducing O2- production via three different mechanisms described above.

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.

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.

A comparison of the effects of signal transduction inhibitors on oxidative burst and degranulation in IL-I beta stimulated bovine neutrophils

There is little information available on IL-1 mediated signal transduction in neutrophils from species other than humans. In this study, signal transduction pathway inhibitors were used to compare signaling pathways for the oxidative burst and degranulation in bovine neutrophils stimulated with rBoIL-1 beta. Protein kinase C inhibitors (staurosporine and chelerythine), DL-propranolol, pertussis toxin (PT), genistein and verapamil significantly inhibited rBoIL-1 beta (10 ng/ml) stimulated luminol-dependent chemiluminescence (LDCL) in a dose-dependent manner, while indomethacin and zileuton had no effect. Propranolol significantly decreased both primary and secondary granule release in response to rBoIL-1 beta. Staurosporine enhanced secondary but not primary granule release, and PT increased primary and secondary granule release. In addition, propranolol inhibited the shape change induced by rBoIL-1 beta and zymosan-activated serum, whereas PT markedly decreased the response induced by zymosan-activated serum, but not rBoIL-1 beta. These findings suggest that rBoIL-1 beta stimulation of the oxidative burst, degranulation, and shape change of bovine neutrophils are mediated through distinct signal transduction pathways.

Neutrophil release of arachidonic acid, oxidants, and proteinases: causally related or independent

This investigation examined the concept that arachidonic acid (AA) serves as a second messenger in stimulation of the respiratory burst and degranulation of polymorphonuclear neutrophils (PMN). The main support for this idea is from observations that reagent AA, added to cell suspensions, stimulates the respiratory burst and degranulation and these events are blocked by PLA2 inhibitors. We verified that exogenously-added AA stimulated release of O2-, myeloperoxidase (MPO), and lysozyme (LZ), but this required amounts of AA which approximated the critical micellar concentration. This suggested that such administration of AA might act as an extracellular agonist, similar to particulate stimuli, rather than acting as a second messenger as might occur following mobilization of AA from cellular membranes. To investigate the role of fatty acids released by hydrolysis of cellular phospholipids, exogenously-added group I, II or III PLA2's were used to mobilize fatty acids from cellular membranes. Mole quantities of cell-associated free fatty acids were measured by negative ion chemical ionization gas chromatography/mass spectrometry. AA mobilization in response to exogenous PLA2 was dose- (0.1 to 10 U/ml PLA2) and time-dependent (peak at 1 to 2 min with a reduction by 4 min). Resting neutrophils contained < 10 pmol free AA/10(7) PMN; the receptor-mediated agonist N-formyl-methionyl-leucyl-phenylalanine (fMLP) alone did not increase these values. Exogenously-added PLA2 generated large quantities of free AA in control and fMLP-treated cells (462 +/- 122 and 2097 +/- 176 pmol/10(7) PMN, respectively); however, this did not induce O2-, nor did it augment the level of O2- stimulated by fMLP. Also, PLA2 caused no degranulation and did not alter degranulation induced by fMLP. PLA2 also did not alter O2- or degranulation responses in primed PMN. The data indicate that mobilization of AA from cellular phospholipids neither stimulates nor modulates the respiratory burst or degranulation of PMN.

Purification and characterization of N-ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP2) from rat liver

N-Ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP; EC 3.1.3.4) was purified 5900-fold from rat liver. The enzyme was solubilized from membranes with octylglucoside, fractionated with (NH4)2SO4, and purified in the presence of Triton X-100 by chromatography on Sephacryl S300, hydroxyapatite, heparin-Sepharose and Affi-Gel Blue. Silver-stained SDS/PAGE indicated that the enzyme was an 83 kDa polypeptide. Sephacryl S-300 gel filtration also produced a second peak of enzyme activity, which was eluted from all of the chromatography columns at a different position from the purified enzyme. SDS/PAGE indicated that it contained three polypeptides (83 kDa, 54 kDa and 34 kDa), and gel filtration suggested that it was not an aggregate of the purified enzyme. Both forms were sensitive to inhibition by amphiphilic amines, Mn2+ and Zn2+, but not by N-ethylmaleimide. Purified PAP required detergent for activity, but was not activated by Mg2+, fatty acids or phospholipids. The enzyme was able to dephosphorylate lysophosphatidic acid or phosphatidic acid, and was inhibited by diacylglycerol and monoacylglycerol. No evidence was obtained for regulation of PAP by reversible phosphorylation.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Phospholipases and activation of the NADPH oxidase

The signal transductional mechanisms regulating the activation of NADPH oxidase, the respiratory burst enzyme in phagocytic cells, are not completely understood. Receptors for most physiologic stimuli trigger the activation of various phospholipases, including phospholipases A2, C, and D. The lipid mediators formed (arachidonic acid, 1,2-diacylglycerol, and phosphatidic acid) have been implicated as second messengers in the induction of the respiratory burst. In intact cells, we have correlated phospholipase D activation and the production of phosphatidic acid with the activation of NADPH oxidase, using the drug propranolol. Phosphatidic acid activated NADPH oxidase in a cell-free system, but the level of activation was low. 1,2-Diacylglycerol markedly enhanced NADPH oxidase activation by phosphatidic acid. The synergistic effect required the diacyl species, since mono- or tri-acylglycerols were ineffective. Phosphatidic acid could be replaced by either lysophosphatidic acid or phosphatidylserine, but not by phosphatidylcholine, phosphatidylethanolamine, or phosphatidylinositol, suggesting specificity for an anionic phospholipid. Since other cell-free activators of NADPH oxidase (arachidonic acid, sodium dodecyl sulfate) are also anionic amphiphiles, phosphatidic acid may directly interact with an enzyme component(s). The targets for phosphatidic acid and diacylglycerol in the cell-free system are currently under investigation. These results emphasize the critical importance of phospholipases, particularly phospholipase D, in the regulation of the respiratory burst.

Evidence for coupling of Clostridium perfringens alpha-toxin-induced hemolysis to stimulated phosphatidic acid formation in rabbit erythrocytes

When rabbit erythrocytes were exposed to low concentrations of Clostridium perfringens alpha-toxin, hot-cold hemolysis was observed. The toxin induced production of phosphatidic acid (PA) in a dose-dependent manner when incubated with erythrocytes at 37 degrees C. When erythrocyte membranes were incubated with the toxin and [gamma-32P]ATP in the presence or absence of ethanol, [32P]PA formation was maximal within 30 s, then sharply decreased, and began again after 5 min of incubation. Ethanol had no effect on the early appearance (at approximately 5 min) of PA formation induced by the toxin but significantly inhibited formation of PA over 10 min of incubation. Treatment of erythrocyte membranes with alpha-toxin resulted in the biphasic formation of 1,2-diacylglycerol and PA as well as an increase of inositol-1,4,5-trisphosphate (IP3) and decrease of phosphatidylinositol-4,5-bisphosphate (PIP2) within 30 s. Neomycin inhibited the toxin-induced increase in turbidity of egg yolk suspensions but did not inhibit the toxin-induced hemolysis of intact erythrocytes. On the other hand, neomycin inhibited the toxin-induced hemolysis of saponin-treated erythrocytes. In addition, neomycin inhibited PA formation induced by the toxin in erythrocyte membranes. IP3 was released by incubation of PIP2 with erythrocyte membranes but not by incubation of PIP2 with the toxin. The toxin stimulated the membrane-induced release of IP3 from PIP2. These data suggest that the toxin-induced hemolysis is dependent on the action of phospholipase C in erythrocyte membranes.

Phosphatidic acid induces the respiratory burst of electropermeabilized human neutrophils by acting on a downstream step of protein kinase C

Phosphatidic acid (PA) dose-dependently induced superoxide (O2-) production of electropermeabilized human neutrophils but not of intact neutrophils, indicating that PA induces the activation of NADPH oxidase by acting on an intracellular target. The O2- production by PA was not inhibited by protein kinase C (PKC) inhibitors, such as staurosporine and calphostin C, and an inhibitor of PA phosphohydrolase, propranolol. These observations suggest that the activation of the oxidase by PA is independent of the activity of PKC and may dominate the activation by diacylglycerol which is formed from PA via the action of PA phosphohydrolase. Furthermore, the production by PA, as well as that by phorbol myristate acetate, was inhibited by cyclic AMP and GDP beta S. Therefore, PA seems to act at a site downstream of PKC.

Characterization of phosphatidic acid phosphohydrolase in neutrophil subcellular fractions

This investigation was designed to confirm the presence of PA phosphohydrolase in human neutrophils and to determine the distribution and characteristics of the enzyme in soluble and particulate subcellular fractions of disrupted neutrophils. Enzyme activity was detected in unseparated extracts of sonicated neutrophils. The majority of the recovered activity was recovered in a particulate fraction rich in neutrophil plasma-membrane markers; moderate levels (20%) of the total activity were recovered in the cytosol. While Mg2+ markedly potentiated the cytosolic but not the particulate activity, Ca2+ moderately inhibited both the cytosolic and particulate enzymes. The plasma-membrane-associated activity was absolutely dependent on detergent (0.5% Triton X-100) and displayed an apparent Km of 62 microM for phosphatidic acid. Enzyme activity was markedly inhibited by NaF, not influenced by excess glycerophosphate and slightly attenuated by propranolol, an inhibitor of PA phosphohydrolase in other systems. Preincubation of plasma membranes with N-ethylmaleimide at concentrations up to 25 mM had little effect on enzyme activity. However, activity in cytosolic and microsomal fractions of neutrophils were completely abolished by preincubation with N-ethylmaleimide at concentrations of less than 5 mM. We conclude that neutrophils possess a potent PA phosphohydrolase localized in their plasma membranes. Metabolism of cellular second-messengers by this enzyme may exert a profound effect on the functions of stimulated neutrophils.

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.

Crystal-induced neutrophil activation. II. Evidence for the activation of a phosphatidylcholine-specific phospholipase D

OBJECTIVE

To investigate the involvement of phospholipase D in the signaling pathways activated by 2 pathologically relevant inflammatory microcrystals, monosodium urate (MSU) and calcium pyrophosphate dihydrate (CPPD).

METHODS

Human peripheral blood neutrophils were used throughout. Phospholipase D activity was monitored by measuring 3 separate indices: 1) the mass of phosphatidic acid, 2) the levels of alkyl-phosphatidic acid, and 3) the levels of formation, in the presence of ethanol, of phosphatidylethanol. The latter 2 parameters were measured in cells labeled with 1-0-3H-alkyl-2-acetyl-sn-glycero-3-phosphocholine. The cells were stimulated with microcrystals of triclinic morphology.

RESULTS

Both MSU and CPPD crystals induced a time- and concentration-dependent accumulation of phosphatidic acid mass and elevation in levels of alkyl-phosphatidic acid and phosphatidylethanol in prelabeled cells. The activation of phospholipase D by the microcrystals was partially sensitive to colchicine and largely resistant to pertussis toxin. Inhibition of phosphatidic acid formation by wortmannin or ethanol reduced the microcrystal-stimulated production of superoxide anions.

CONCLUSION

These results indicate that microcrystals stimulate phospholipase D in human neutrophils and that at least some of the functional consequences of neutrophil-microcrystal interactions may be dependent on this biochemical pathway.

Effects of niflumic acid on polyphosphoinositide and oxidative metabolism in polymorphonuclear leukocytes from healthy and thermally injured rats

Thermal injury in rats leads to an impairment of polymorphonuclear leukocyte (PMN) functions, particularly oxidative metabolism and phosphoinositide turnover. As prostaglandin E2, which has immunosuppressive properties, is released in high levels after burn trauma, we investigated the in vitro and in vivo effects of a nonsteroidal antiinflammatory drug, niflumic acid, on oxidative and phosphoinositide metabolism in PMNs from healthy and burned rats. Given the role of fluoride ions on PMN, the influence of niflumic acid was compared with that of sodium fluoride (NaF) at equivalent doses of F-. In vitro, niflumic acid and sodium fluoride had no effect on oxidative metabolism in stimulated by formyl methionyl-leucyl-phenylalanine (FMLP) or opsonized zymosan (OZ) or nonstimulated PMNs from healthy and burned rats. Niflumic acid slightly increased the production of inositol phosphate by nonstimulated PMNs from healthy and burned rats. Niflumic acid and NaF partly restored the stimulating effect of FMLP on inositol phosphate production by PMNs from burned rats. In vivo treatment with niflumic acid and NaF increased the oxidative metabolism of PMNs from burned rats but not healthy rats. Niflumic acid, more than NaF, restored the activity of both stimulants on phosphoinositide metabolism in PMNs from burned rats. In conclusion, at non-antiinflammatory doses, while inhibiting cyclooxygenase activity, niflumic acid exerts a complex effect on the burn-induced depression of PMN functions. The fluoride anion induces similar but generally weaker effects and seems to be involved in the restoring effects of niflumic acid on PMN functions in burned rats.

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.

Differential effects of propranolol on responses to receptor-dependent and receptor-independent stimuli in human neutrophils

Recent evidence suggests that the hydrolysis of phosphatidylcholine (PC) by phospholipase D (PLD) may mediate superoxide anion (O2-) production in human neutrophils. To define the role of the PC-specific PLD products phosphatidic acid (PA) and diacylglycerol (DAG) in O2- production in response to agonists which activate the PLD pathway, we blocked the metabolism of PA to DAG with propranolol, an inhibitor of PA phosphohydrolase. Propranolol (150 microM) enhanced the production of O2- in response to the receptor agonists n-formyl-methionyl-leucyl-phenylalanine (FMLP, 292 +/- 94% of controls), platelet-activating factor (PAF, 932 +/- 215%) and leukotriene B4 (LTB4, 1305 +/- 475%). In the presence of propranolol, total O2- production in response to PAF and LTB4, which are potent priming stimuli but very weak direct agonists, was similar to that obtained with FMLP. IN contrast, responses to receptor-independent agonists phorbol myristate acetate (PMA) and ionomycin were inhibited (81 +/- 8% and 87 +/- 5% inhibition, respectively). The effects of propranolol were demonstrable in the absence of cellular calcium and were shared by both stereoisomers of the drug. These data are consistent with the hypothesis that PA produced through the hydrolysis of PC by PLD is an important mediator of O2- production in response to receptor-dependent agonists. However, the inhibitory effects of propranolol on receptor-independent stimuli suggest that PA generated through the PLD pathway plays a different role in the signal transduction mechanisms of these agonists or that propranolol may have additional effects beyond inhibition of PA phosphohydrolase.

Phosphatidic acid as a second messenger in human polymorphonuclear leukocytes. Effects on activation of NADPH oxidase

Receptor-mediated agonists, such as FMLP, induce an early, phospholipase D (PLD)-mediated accumulation of phosphatidic acid (PA) which may play a role in the activation of NADPH oxidase in human PMN. We have determined the effect of changes in PA production on O2 consumption in intact PMN and the level of NADPH oxidase activity measured in a cell-free assay. Pretreatment of cells with various concentrations of propranolol enhanced (less than or equal to 200 microM) or inhibited (greater than 300 microM) PLD-induced production of PA (mass and radiolabel) in a manner that correlated with enhancement or inhibition of O2 consumption in PMN stimulated with 1 microM FMLP in the absence of cytochalasin B. The concentration-dependent effects of propranolol on FMLP-induced NADPH oxidase activation was confirmed by direct assay of the enzyme in subcellular fractions. In PA extracted from cells pretreated with 200 microM propranolol before stimulation with 1 microM FMLP, phospholipase A1 (PLA1)-digestion for 90 min, followed by quantitation of residual PA, showed that a minimum of 44% of PA in control (undigested) sample was diacyl-PA; alkylacyl-PA remained undigested by PLA1. Propranolol was also observed to have a concentration-dependent enhancement of mass of 1,2-DG formed in PMN stimulated with FMLP. DG levels reached a maximum at 300 microM propranolol and remained unchanged up to 500 microM propranolol. However, in contrast to PA levels, the level of DG produced did not correlate with NADPH oxidase activation. Exogenously added didecanoyl-PA activated NADPH oxidase in a concentration-dependent manner (1-300 microM) in a reconstitution assay using membrane and cytosolic fractions from unstimulated PMN. In addition, PA synergized with SDS for oxidase activation. Taken together, these results indicate that PA plays a second messenger role in the activation of NADPH oxidase in human PMN and that regulation of phospholipase D is a key step in the activation pathway.