Choline-linked Phosphoglycerides

Recent advances in our understanding of the biochemical interactions between platelet-activating factor and arachidonic acid

In the last few years, it has become increasingly apparent that the biochemistry of PAF (platelet-activating factor) and that of arachidonic acid are interrelated in a number of inflammatory cells. Experiments presented here further point out that arachidonic acid plays a crucial role in the catabolism and biosynthesis of PAF. In addition, they suggest that the same phospholipid molecular species may serve as a source for both arachidonic acid and 1-alkyl-2-lyso-sn-glycero-3-phosphocholine during cell activation. Finally, they reveal that there may be common regulatory mechanisms for the biosynthesis of PAF and arachidonic acid metabolites. Taken together, studies examining the relationship between PAF and arachidonic acid suggest it may be difficult to consider the biochemistry of PAF without considering arachidonic acid metabolism and vice versa.

The superoxide-generating oxidase of phagocytic cells. Physiological, molecular and pathological aspects

Professional phagocytes (neutrophils, eosinophils, monocytes and macrophages) possess an enzymatic complex, the NADPH oxidase, which is able to catalyze the one-electron reduction of molecular oxygen to superoxide, O2-. The NADPH oxidase is dormant in non-activated phagocytes. It is suddenly activated upon exposure of phagocytes to the appropriate stimuli and thereby contributes to the microbicidal activity of these cells. Oxidase activation in phagocytes involves the assembly, in the plasma membrane, of membrane-bound and cytosolic components of the oxidase complex, which were diassembled in the resting state. One of the membrane-bound components in resting phagocytes has been identified as a low-potential b-type cytochrome, a heterodimer composed of two subunits of 22-kDa and 91-kDa. The link between NADPH and cytochrome b is probably a flavoprotein whose subcellular localization in resting phagocytes remains to be determined. Genetic defects in the cytochrome b subunits and in the cytosolic factors have been shown to be the molecular basis of chronic granulomatous disease, a group of inherited disorders in the host defense, characterized by severe, recurrent bacterial and fungal infections in which phagocytic cells fail to generate O2- upon stimulation. The present review is focused on recent data concerning the signaling pathway which leads to oxidase activation, including specific receptors, the production of second messengers, the organization of the oxidase complex and the molecular defects responsible for granulomatous disease.

A method for the simultaneous determination of alkylacylglycerol, diacylglycerol, monoalkylglycerol, monoacylglycerol, and cholesterol by high-performance liquid chromatography

We describe a method for the quantitative analysis of the individual subclasses (1-O-alkyl and 1-acyl) of diradylglycerols and monoradylglycerols. These lipids, along with cholesterol, were separated from other neutral and polar lipids on silica columns and analyzed by normal-phase high-performance liquid chromatography (HPLC) as their benzoate derivatives. Cholesterylbenzoate, alkylacylglycerolbenzoate, diacylglycerolbenzoate, monoalkylglyceroldibenzoate, and monoacylglyceroldibenzoate eluted from HPLC in five distinct zones. The derivatives of diradylglycerols and monoradylglycerols were further separated within each discrete zone on the basis of the total number of aliphatic carbons at the sn-1 and sn-2 positions. Radiolabeled cholesterol and dihexadecanoylglycerol were used to monitor recovery. Amounts of synthetic alkylacylglycerol, diacylglycerol, monoalkylglycerol, and monoacylglycerol as low as 0.2 nmol per subclass could be accurately quantified. The technique was used to determine the content of diradylglycerol and monoradylglycerol subclasses in Madin-Darby canine kidney and CFTL-12 mast cells. This method should prove useful for the quantitation of lipid second messengers in cultured cells.

Membrane-associated phospholipase D activity in rat sciatic nerve

Rat sciatic nerve contains a membrane-bound phospholipase D that catalyzes the hydrolysis of exogenous phosphatidylcholine (PC) to phosphatidic acid (PA) and choline. The enzyme is associated with a particulate fraction consisting primarily of microsomes and myelin. This fraction also contains phosphatidate phosphohydrolase activity leading to the production of diacylglycerols (DAG). The phosphohydrolase activity can be completely inhibited by NaF. Hydrolysis of exogenous PC requires detergent and is linear up to about 40 micrograms of protein at a pH optimum of 6.5. In the absence of NaF, the sum of PA and DAG increases linearly for 40 min, whereas in its presence, PA production is linear for only 15 min. At optimum conditions, PC hydrolysis proceeds at 15 nmol/h/mg of protein. Addition of increasing amounts of ethanol to the incubation system leads to the generation of increasing amounts of phosphatidylethanol, indicating transphosphatidylation activity. At an ethanol concentration of 0.4 M, phosphatidylethanol represents about one-half of the reaction products generated at approximately the same rate of enzymic activity observed in the absence of ethanol. Higher ethanol concentrations are inhibitory.

Changes in inositol 1,4,5-trisphosphate mass in agonist-stimulated human neutrophils

Changes in the endogenous synthesis of inositol 1,4,5-trisphosphate (IP3) mass have been quantitated in human peripheral neutrophils stimulated with FMLP, LTB4 and PAF using a recently described, highly specific radioreceptor assay. Each agonist induced a concentration-dependent synthesis of IP3 which was detectable within 10 seconds after stimulation. IP3 production was short-lived, returning to basal levels within 90 seconds. The maximal stimulated level of IP3 in response to FMLP and LTB4 was 30-50 50 pmoles/10(7) neutrophils. PAF was more effective (approximately 100 pmoles IP3/10(7) neutrophils). The response to FMLP was inhibited by pertussis toxin, but was unaffected by cholera toxin. Pretreatment with cytochalasin B did not enhance IP3 synthesis. These findings are generally consistent with previous studies employing [3H]myo-inositol-prelabeled cells, and provide one of the first measurements of IP3 synthesis by mass in agonist-stimulated human neutrophils.

Sources of diradylglycerols generated during cell growth and phorbol ester stimulation in Madin-Darby canine kidney cells

The molecular species of diacylglycerol and alkylacylglycerol of Madin-Darby canine Kidney (MDCK) cells were analyzed to determine the sources of diradylglycerols generated during cell growth and phorbol ester stimulation. MDCK cells in log phase growth contained higher levels of diacylglycerol and alkylacylglycerol than confluent cells. Both subclasses of diradylglycerol showed higher levels of saturated and monoenoic species during log phase. Glycerol incorporation into diradylglycerols was increased during growth, consistent with an increase in their synthesis de novo. Stimulation with 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent activator of protein kinase C, caused an increase in the level of diacylglycerol but not alkylacylglycerol. Log phase MDCK cells showed a greater response to TPA treatment than confluent cells. The molecular species of diacylglycerol generated during stimulation with either TPA or dioctanoylglycerol closely resembled the species of phosphatidylcholine. These results indicate that TPA and synthetic diacylglycerol stimulate endogenous diacylglycerol production through the hydrolysis of phosphatidylcholine. In contrast, the higher content of diacylglycerol and alkylacylglycerol in replicating MDCK cells is the result of an increase in their synthesis de novo.

Molecular species analysis of 1,2-diacylglycerols and phosphatidic acid formed during bombesin stimulation of Swiss 3T3 cells

Swiss 3T3 cells were labelled with [3H]glycerol and stimulated with bombesin over a time course of 20 min. The individual 1,2-diacylglycerols produced were quantified by acetylation followed by analysis by HPLC and argentation chromatography. The major phospholipids and phosphatidic acid were acetolysed and then analysed in the same manner. The data show that even at an early time of stimulation (30 s), stimulated diacylglycerol formation comes from at least two sources--phosphoinositides and phosphatidylcholine.

Phospholipase D-catalyzed hydrolysis of phosphatidylcholine occurs with P-O bond cleavage

Mammalian phospholipase D has been implicated in signal-transduction mechanisms, most recently in association with stimuli that enhance phosphatidylcholine (PC) turnover. It was previously unknown whether hydrolysis of PC by phospholipase D proceeds via P-O or C-O bond cleavage. Commercially available phospholipase D isolated from Streptomyces chromofuscus was used to hydrolyse distearoyl phosphatidylcholine (PC) in a detergent-containing buffer consisting of 90% 18O-water. The product of hydrolysis, phosphatidic acid (PA), was purified by thin-layer chromatography and analyzed using californium-252 plasma desorption mass spectrometry. An increase of two mass units was observed, compared to a distearoyl PA control, consistent with a reaction mechanism involving cleavage of the P-O bond.

Differential actions of diacyl- and alkylacylglycerols in priming phospholipase A2, 5-lipoxygenase and acetyltransferase activation in human neutrophils

One aspect of human neutrophil (PMN) function during inflammation is formation of platelet-activating factor (PAF), leukotriene B4 (LTB4), and 5-hydroxyeicosatetraenoic acid (5-HETE), but production of these lipid mediators is limited if PMN are directly stimulated with soluble, physiologic agonists. In vitro, PMN activities can be enhanced by the process of primed-stimulation where cells are sequentially treated with non-stimulatory concentrations of different agonists. Many agents that prime PMN also induce production of 1,2-diacyl- and 1-O-alkyl-2-acylglycerols. Therefore, we investigated whether diglycerides were involved in priming PMN for production of lipid mediators. We previously described the ability of the diacylglycerol, 1-oleoyl-2-acetylglycerol (OAG), and its alkylacylglycerol analog, 1-O-octadecenyl-2-acetylglycerol (EAG), to prime phospholipase A2 (PLA2) for subsequent activation by a second stimulus. However, while OAG also primed 5-lipoxygenase activity (LTB4 and 5-HETE production), EAG priming inhibited LTB4 and 5-HETE formation. We now report the effects of diglyceride priming on acetyltransferase activation (PAF formation). PMN, prelabeled with 1-O-[9',10'-3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine, were primed with OAG or EAG before stimulation. Neither OAG nor EAG induced formation of labeled PAF. Treatment of PMN with the chemotactic peptide, N-formyl-met-leu-phe (FMLP), induced low but significant production of PAF; PAF formation doubled in PMN primed with 20 microM OAG before FMLP stimulation while priming with 20 microM EAG more than tripled the level of PAF. Calcium ionophore strongly induced PAF formation; OAG priming before ionophore challenge had no effect but EAG priming further enhanced PAF formation. These results suggests a role for alkylacylglycerols in modulating the production of lipid mediators of inflammation.

Phorbol ester inhibits 13-cis-retinoic acid-induced hydrolysis of phosphatidylinositol 4,5-bisphosphate in cultured murine keratinocytes: a possible negative feedback via protein kinase C-activation

The incubation of double-labelled [( 14C]-glycerol and [3H]-myoinositol) keratinocytes with 13-cis retinoic acid induced the transient and simultaneous release of [3H]-inositol trisphosphate ([3H]-InsP3) and [14C]-diacylglycerol ([14C]-DAG) indicating that a possible mode of action of this retinoid on murine keratinocytes may be at least in part the early transient release of the two putative messengers (InsP3 and DAG) from phosphatidylinositol-4,5 bisphosphate (PtdIns4, 5P2). In contrast, the preincubation of the keratinocytes with 12-O-tetradecanoylphorbol-13-acetate (TPA) prior to incubation with 13-cis-RA suppressed the 13-cis-RA-induced release of [3H]-InsP3 and [14C]-DAG. The specificity of the TPA effect was established by the lack of effect of the biologically inactive 4 alpha-phorbol 12, 13-didecanoate. Furthermore, the incubation of the TPA-primed keratinocytes with 13-cis-RA caused a delayed and sustained accumulation of [14C]-DAG. An exploration of the source of this late release of [14C]-DAG revealed that this [14C]-DAG was released from non-inositol containing phospholipids, particularly, phosphatidylcholine. This latter DAG released in the TPA-primed cells correlated with the translocation of the cytoplasmic protein kinase C (PKC) activity to the membrane associated PKC activity. Taken together, these results suggest that alteration of PKC activity, presumably induced by DAG released from non-inositol phospholipids, may play a major role in the TPA-induced negative feedback inhibition of 13-cis RA-induced hydrolysis of keratinocyte PtdIns4, 5P2.

Source and role of diacylglycerol formed during phagocytosis of opsonized yeast particles and associated respiratory burst in human neutrophils

The results presented in this paper demonstrate that in human neutrophils phagocytosis of C3b/bi and IgG-opsonized yeast particles is associated with activation of phospholipase D and that this reaction is the main source of diglycerides. The demonstration is based upon the following findings: 1) the challenge of neutrophils with these opsonized particles was followed by a rapid formation of [3H]alkyl-phosphatidic acid [( 3H]alkyl-PA) and [3H]alkyl-diglyceride [( 3H]alkyl-DG) in cells labeled with [3H]alkyl-lyso-phosphatidylcholine; 2) in the presence of ethanol [3H]alkyl-phosphatidylethanol was formed, and accumulation of [3H]alkyl-PA and [3H]alkyl-DG was depressed; 3) propranolol, by inhibiting the dephosphorylation of [3H]alkyl-PA, completely inhibited the accumulation of [3H]alkyl-DG and depressed by about 75% the formation of diglyceride mass. Evidence is also presented that phagocytosis of C3b/bi and IgG-opsonized yeast particles and associated respiratory burst can take place independently of diglyceride formation and of the activity of this second messenger on protein kinase C. In fact: a) propranolol while completely inhibited the formation of diglyceride mass did not modify either the phagocytosis or respiratory burst; b) these two processes were insensitive to staurosporine.

Cytidylyltransferase translocation onto endoplasmic reticulum and increased de novo synthesis without phosphatidylcholine accumulation in Krebs-II ascite cells

Addition of oleic acid to Krebs-II cells stimulated by 9-fold [3H]choline incorporation into choline glycerophospholipids without affecting the selective incorporation of the precursor into diacyl subclass (90% of total [3H]choline glycerophospholipids). The total activity of cytidylyltransferase (E.C. 2.7.7.15), the regulatory enzyme of choline glycerophospholipid synthesis, was increased in the particulate fraction at the expense of cytosol. Free [3H]oleic acid was also associated with the particulate fraction. Subcellular fractionation of membranes on Percoll gradient, indicated that the endoplasmic reticulum, which contained 90% of total cell free oleic acid, was the unique target for the translocation of cytidylyltransferase. [3H]oleic acid was incorporated almost exclusively into phosphatidylcholine and corresponded to a synthesis of 9 nmol/h per 10(6) cells. Based on [3H]choline incorporation a net synthesis of 22 nmol/h per 10(6) cells was determined. However, oleic acid treatment did not change the total amount of phosphatidylcholine (45 nmol/10(6) cells) and other phospholipids; also no modification in the subcellular distribution of phospholipids was observed. It is concluded that the stimulation of the de novo synthesis of phosphatidylcholine which involves translocation of cytidylyltransferase onto the endoplasmic reticulum, is accompanied by a renewal of their polar head group. Also exogenous oleic acid induces an enhanced fatty acid turnover, highly specific for phosphatidylcholine. Therefore, Krebs-II cells exhibited a high degree of regulation of their phosphatidylcholine content, suggesting a parallel stimulation of both synthesis and degradation.

The protein kinase C inhibitor, K252a, inhibits superoxide production in human neutrophils activated by both PIP2-dependent and -independent mechanisms

We report that the putative protein kinase C inhibitor, K252a, at concentrations of 0.2 and 1 microM, inhibited the respiratory burst induced by fluoride and formyl-methionyl-leucyl-phenyl-alanine respectively, both in human neutrophils primed with a subthreshold dose of phorbol myristate acetate and in non-primed neutrophils. In addition, K252a effectively inhibited ConA-zymosan-mediated superoxide generation in Ca2(+)-depleted neutrophils, with virtually maximal inhibition seen at 1 microM. These results suggest that protein kinase C is involved in the putative phosphatidylinositol bisphosphate-independent signal transduction mechanism of the respiratory burst as well as the pathway dependent on phosphatidylinositol bisphosphate hydrolysis.

Activation of phospholipase D in rabbit neutrophils by fMet-Leu-Phe is mediated by a pertussis toxin-sensitive GTP-binding protein that may be distinct from a phospholipase C-regulating protein

Stimulation by N-formyl-Met-Leu-Phe (fMLP) of rabbit peritoneal neutrophils, in which phosphatidylcholine was preferentially labeled with 1-O-[3H]octadecyl lyso platelet-activating factor, activated phospholipase D, resulting in the formation of [3H]PA from [3H]PC. A direct activator of GTP-binding proteins (G-proteins), NaF, also stimulated [3H]PA formation. fMLP-stimulated [3H]PA formation was inhibited by pertussis toxin (IAP) in a time- and dose-dependent manner. IAP also inhibited fMLP-stimulated IP3 formation, but the inhibition of IP3 formation was significantly greater than that of [3H]PA formation. These results indicate that activation of phospholipase D by fMLP in rabbit neutrophils is mediated by an IAP-sensitive G-protein that may be distinct from a phospholipase C-regulating protein.

Phospholipase D in cell signalling and its relationship to phospholipase C

Phospholipases C and D are phosphodiesterases which act on phospholipid head groups. Although the presence of these enzymes in living organisms has long been known, it is only recently that their role in cell signal transduction has been appreciated. The new developments on phospholipases D (PLD) are especially noteworthy, since these enzymes catalyze a novel pathway for second messenger generation. In a variety of mammalian cell systems, several biological or chemical agents have recently been shown to stimulate PLD activity. Depending on the system, activation of PLD has been suggested to be either dependent on, or independent of, Ca2+ and protein kinase C. PLD primarily hydrolyses phosphatidylcholine (PC) but phosphatidylinositol and phosphatidylethanolamine have also been reported as substrates. Different forms of endogenous PLD may also exist in cells. Exogenous addition of PLD causes alterations in cellular functions. In many instances, Ca2+ mobilizing agonists may stimulate both PLC and PLD pathways. Interestingly, several metabolites of these two enzymes are second messengers and are common to both pathways (e.g. phosphatidic acid, diglyceride). This has raised the issue of the interrelationship between these pathways. The regulation of either PLC or PLD by cellular components, e.g. guanine nucleotide binding proteins or protein kinases, is under intense investigation. These recent advances are providing novel information on the significance of phospholipase C and D mediated phospholipid turnover in cellular signalling. This review highlights some of these new discoveries and emerging issues, as well as challenges for future research on phospholipases.

Antineoplastic ether-linked phospholipid induces differentiation of acute myelogenous leukemic KG-1 cells into macrophage-like cells

The culture of a human acute myelogenous leukemic cell line (KG-1) with a synthetic ether-linked phospholipid: 1-0-octadecyl-2-0-methylglycerol phosphocholine (ET-18-OCH3), suppressed the growth of the KG-1 cells while the variant subline, (KG-1a cells) similarly treated was unresponsive. The growth inhibition of the KG-1 cells was accompanied by morphological changes into cells of the monocyte/macrophage lineage. Histochemically, the ET-18-OCH3-treated KG-1 cells increase 17-fold in the nonspecific esterase activity when compared to control non-treated cells, whereas they responded negatively in the assay for the reduction of soluble nitroblue tetrazolium into insoluble blue formazan deposits (a marker for cells of the granulocytic lineage). Taken together, our data revealed that the synthetic ether-lipid inhibited the growth of the KG-1 acute myelogenous leukemic cells while inducing the differentiation of these cells into cells of the monocyte/macrophage-lineage. These effects of the synthetic ether lipid raise the possibility that naturally occurring ether-linked phospholipids may likewise function in vivo to modulate hyperproliferative processes and thus warrant further explorations.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

Diacylglycerol production induced by growth hormone in Ob1771 preadipocytes arises from phosphatidylcholine breakdown

Growth Hormone has recently been shown to stimulate the formation of diacylglycerol in Ob1771 mouse preadipocyte cells without increasing inositol lipid turnover. Addition of growth hormone to Ob1771 cells prelabelled with [3H]glycerol or [3H]choline led to a rapid, transient and stoechiometric formation of labelled diacylglycerol and phosphocholine, respectively. In contrast, no change was observed in the level of choline and phosphatidic acid whereas the release of water-soluble metabolites in [3H]ethanolamine prelabelled cells exposed to growth hormone was hardly detectable. Stimulation by growth hormone of cells prelabelled with (2-palmitoyl 9, 10 [3H])phosphatidylcholine also induced the production of labelled diacyglycerol. Pertussis toxin abolished both diacylglycerol and phosphocholine formation induced by growth hormone. It is concluded that growth hormone mediates diacylglycerol production in Ob1771 cells by means of phosphatidylcholine breakdown involving a phospholipase C which is likely coupled to the growth hormone receptor via a pertussis toxin-sensitive G-protein.

Production of phosphatidylethanol by phospholipase D phosphatidyl transferase in intact or dispersed pancreatic islets: evidence for the in situ metabolism of phosphatidylethanol

To determine if phospholipase D is present in intact adult islets, we took advantage of the fact that, in the presence of ethanol, this enzyme generates phosphatidylethanol via transphosphatidylation. Extracts of cells prelabeled with [14C]arachidonate, [14C]myristate, or [14C]stearate were analyzed via three TLC systems; the identify of phosphatidylethanol was further confirmed via incorporation of [14C]ethanol into the same phospholipid bands. The phorbol ester 12-O-tetradecanoylphorbol-13-acetate stimulated phosphatidylethanol (to 603% of basal by 60 min) both in intact adult islets and in dispersed neonatal islet cells. A nonphorbol activator of protein kinase C (mezerein) also stimulated phospholipase D, whereas a phorbol which does not activate protein kinase C (4 alpha-phorbol-12,13-didecanoate) was virtually inactive. The effects of the active phorbol ester or of mezerein were reduced by the protein kinase C inhibitor H-7 and were virtually eliminated by prior down-regulation of that enzyme. In addition, a calcium-selective ionophore (ionomycin) or fluoroaluminate also activated the islet phospholipase D. When accumulation of phosphatidylethanol (labeled with any of three fatty acids) was induced by a preincubation in the presence of ethanol plus agonist, which then were removed, phosphatidylethanol declined by 34-47% over a subsequent 60-min incubation. Thus, while phosphatidylethanol is relatively stable metabolically, it is detectably degraded (a variable overlooked in previous studies). In the absence of ethanol, stimulated islet cells generated phosphatidic acid, although such hydrolysis was less evident than transphosphatidylation. Ethanol provision distinguished phosphatidate formed via phospholipase D (inhibition, via phosphatidylethanol formation) from that due predominantly to phospholipase C (phosphatidate not inhibited). In view of our recent findings that phosphatidic acid (or exogenous phospholipase D) has potent insulinotropic effects, this pathway could play a role in stimulus-secretion coupling; conversely, stimulation of transphosphatidylation at the expense of hydrolysis could contribute to the inhibition of secretion caused by ethanol.

A method for the quantitative analysis of molecular species of alkylacylglycerol and diacylglycerol

We describe a method for the quantitative analysis of molecular species of diacylglycerol and alkylacylglycerol as their diradylglycerobenzoate derivatives. Synthetic internal standards were used to provide quantitative determinations of the low levels of diacylglycerol and alkylacylglycerol and their individual molecular species in cultured cells. Diradylglycerols were isolated by thin-layer chromatography (TLC), converted to their benzoate derivatives and separated into subclasses by TLC. The molecular species of each subclass were analyzed by reversed-phase high performance liquid chromatography. Thirty-six species of diglyceride-type molecules were identified in Madin-Darby canine kidney cells. These cells were shown to contain 7.88 nmoles of diacylglycerol and 3.97 nmoles of alkylacylglycerol per mumole of phospholipid. Both subclasses contain predominantly monoenoic and saturated species. This technique should be valuable for studies examining the origin and metabolism of these important intracellular mediators.

ATP-evoked arachidonic acid mobilization in astrocytes is via a P2Y-purinergic receptor

To reveal more of the mechanism whereby ATP induces arachidonic acid (AA) mobilization in astrocytes, primary cell cultures prelabeled with [3H]AA were exposed to ATP and various analogs. Release of 3H was dose and time dependent and was inhibited by blocking ATP binding. The potencies of a range of ATP analogs in mobilizing AA were consistent with that predicted for the involvement of a P2Y-purinergic receptor. Mobilization of AA was not due to non-specific cell permeabilization, as assessed by leakage of cytoplasmic lactate dehydrogenase. AA mobilization by ATP was reduced when mobilization of intracellular calcium was inhibited and in the absence of extracellular calcium. Thapsigargin, which induces release of intracellular calcium, evoked mobilization of AA and thromboxane formation, findings similar to the effects of ATP. These results suggest that ATP stimulates AA mobilization via a P2Y-purinergic receptor and that, although extracellular calcium is involved, mobilization of intracellular calcium activates phospholipase A2.

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

In contrast to the rapid, ethanol-inhibited superoxide generation by the receptor-linked agonist formyl-methionyl-leucyl-phenylalanine (fMLP), fluoride-activated superoxide generation occurs after a prolonged lag, and as shown herein is relatively ethanol-insensitive. We have investigated fluoride-activation of diradylglycerol generation and phospholipase D activity. Fluoride induces a very large increase in diradylglycerol mass (both 1,2-diacylglycerol (DAG) and 1-O-alkyl,2-acylglycerol (EAG)), with kinetics similar to superoxide generation. Unlike fMLP-activated diglyceride generation which is completely inhibited by ethanol, that produced by fluoride is only partially (30%) blocked. When the phosphatidylcholine pool is 3H-prelabeled, fluoride activates both [3H]phosphatidic acid (PA) and [3H]diglyceride generation with similar kinetics. Partial inhibition of the production of these species by ethanol was seen, coincident with the appearance of [3H]phosphatidylethanol, indicating phospholipase D-dependent transphosphatidylation had occurred. The data are consistent with the fluoride activation of PA and diglyceride generation by both phospholipase D-dependent and -independent (presumably phospholipase C) mechanisms.

Verapamil inhibits phosphatidic acid formation and modifies phosphoinositide metabolism in stimulated platelets

Rabbit platelet-rich plasma was incubated with [32P]orthophosphate, after which the platelets were washed, further incubated in the absence or presence of verapamil and subsequently stimulated with PAF-acether or thrombin. In the absence of verapamil, a rapid increase in radioactivity in phosphatidic acid was observed in platelets stimulated with PAF-acether or thrombin. This was inhibited by verapamil over the concentration range 10(-7) to 10(-4) M, at which concentration the rise in phosphatidic acid was completely abolished. In unstimulated platelets, 10(-4) M verapamil induced an increase in radioactivity in polyphosphoinositides but not significantly in phosphatidylinositol. When these verapamil-treated platelets were stimulated with PAF-acether or thrombin, there was a rapid, sustained loss of the additional radioactivity induced in the polyphosphoinositides by verapamil. Polyphosphoinositide radioactivity remained unchanged in platelets stimulated in the absence of verapamil. Verapamil may stimulate formation of a separate pool of polyphosphoinositide which is susceptible to agonist-induced phospholipase C, and failure to re-synthesize this polyphosphoinositide could result from inhibition of phosphatidic acid synthesis.

Phospholipase C-mediated hydrolysis of phosphatidylcholine is an important step in PDGF-stimulated DNA synthesis

Recent evidence suggests the involvement of phosphatidylcholine (PC) hydrolysis both in the control of normal cell growth and in transformation. We show here that the simple exogenous addition of Bacillus cereus PC-hydrolyzing phospholipase C (PC-PLC) is sufficient to elicit a potent mitogenic response in Swiss 3T3 fibroblasts by a mechanism that is independent of protein kinase C. Our results on the additivity and synergism between B. cereus PC-PLC, PDGF, and insulin in the mitogenic response indicate that this novel phospholipid degradative pathway may be important in the mitogenic signaling cascade activated by PDGF.

Phorbol esters modulate the turnover of both ether- and ester-linked phospholipids in cultured mammalian cells

The effects of 12-O-tetradecanoylphorbol 13-acetate (TPA) on the metabolism of ester- and ether derivatives of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were studied in HeLa and HEL-37 cells. TPA stimulated the incorporation of [3H]choline into diacyl-, alkylacyl- and alkenylacy/PC in HeLa cells, but inhibited the incorporation of [3H]ethanolamine into the corresponding derivatives of PE. TPA also stimulated the incorporation of [3H]ethanolamine into lysoPE and the release of labelled ethanolamine and phosphoethanolamine from HeLa cells prelabelled with [3H]ethanolamine. All responses to TPA were abolished in HeLa cells preincubated with the phorbol ester and which were deficient in protein kinase C. In HEL-37 cells TPA stimulated label incorporation into both ester- and ether-forms of PE. The marked effects of TPA on ether-lipid metabolism raises the possibility that hydrolysis products of this class of lipid are important in transmembrane signalling pathways.

Phosphatidic acid and not diacylglycerol generated by phospholipase D is functionally linked to the activation of the NADPH oxidase by FMLP in human neutrophils

It is widely accepted that the activation of the NADPH oxidase of phagocytes is linked to the stimulation of protein kinase C by diacylglycerol formed by hydrolysis of phospholipids. The main source would be choline containing phospholipid via phospholipase D and phosphatidate phosphohydrolase. This paper presents a condition where the activation of the respiratory burst by FMLP correlates with the formation of phosphatidic acid, via phospholipase D, and not with that of diacylglycerol. In fact: 1) in neutrophils treated with propranolol, an inhibitor of phosphatidate phosphohydrolase, FMLP plus cytochalasin B induces a respiratory burst associated with a stimulation of phospholipase D, formation of phosphatidic acid and complete inhibition of that of diacylglycerol. 2) The respiratory burst by FMLP plus cytochalasin B lasts a few minutes and may be restimulated by propranolol which induces an accumulation of phosphatidic acid. 3) In neutrophils stimulated by FMLP in the absence of cytochalasin B propranolol causes an accumulation of phosphatidic acid and a marked enhancement of the respiratory burst without formation of diacylglycerol. 4) The inhibition of the formation of phosphatidic acid via phospholipase D by butanol inhibits the respiratory burst by FMLP.