Extracellular release of free fatty acids by rat T lymphocytes is stimulus-dependent and is affected by dietary lipid manipulation

[(3)H]-Arachidonic acid-labelled rat T lymphocytes released radioactivity extracellularly when stimulated by the calcium ionophore A23187 or by monoclonal antibodies to some cell surface structures (CD2, CD5, CD11a, CD18, CD54, T-cell receptor) but not to others (CD49d, CD62L); release was greater with the calcium ionophore. Almost all of the radioactivity released from anti-CD2-stimulated lymphocytes was recovered in the free fatty acid fraction, whereas only about 50 per cent of that released after A23187 stimulation was recovered in this fraction. A23187 stimulation resulted in release of arachidonic acid from a variety of phospholipids (phosphatidylinositol, phosphatidylcholine and perhaps phosphatidylethanolamine), while the monoclonal antibody stimulation released arachidonic acid from phosphatidylinositol and perhaps phosphatidylcholine. Unstimulated lymphocytes released a range of fatty acids extracellularly, with palmitic acid accounting for 35-40 per cent and arachidonic acid for 5 per cent of released fatty acid. Stimulation of lymphocytes with either anti-CD2 or A23187 increased total fatty acid release 1.5- to 1.8-fold. In both cases palmitic acid remained the most predominant fatty acid released but the contribution of arachidonic acid increased. The type of lipid fed to the rats significantly influenced the amount and type of fatty acid released. Fish oil feeding significantly reduced extracellular fatty acid release by stimulated lymphocytes.     

Pathways of arachidonic acid liberation in thrombin and calcium ionophore A23187-stimulated human endothelial cells: respective roles of phospholipids and triacylglycerol and evidence for diacylglycerol generation from phosphatidylcholine

Cultured endothelial cells from human umbilical vein were incubated for 20 h at 37 degrees C in the presence of [U-14C]arachidonic acid. Around 60-70% of the radioactive fatty acid was incorporated into cell lipids and was predominantly found in phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and triacylglycerol (39%, 33%, 13% and 6.5% of total incorporated radioactivity, respectively). Stimulation of the cells with human thrombin (2 U/ml) or calcium ionophore A23187 (5 microM) promoted the release into supernatants of arachidonic acid, 6-ketoprostaglandin F1 alpha, prostaglandins E2 and F2 alpha, in decreasing order of importance. The amount of secreted material was 4-fold higher with A23187, compared to thrombin. Parallel to the liberation process, phosphatidylcholine underwent a rapid decrease of radioactivity with both agonists, suggesting the involvement of a Ca2+-dependent phospholipase A2. Phosphatidylethanolamine displayed a minor decrease with A23187, whereas some reacylation was observed at 10 min with thrombin. Phosphatidylinositol was non-significantly affected in thrombin-stimulated cells, whereas A23187 promoted an early but minor decrease, followed by resynthesis. In contrast to A23187, thrombin was also able to promote a significant hydrolysis of triacylglycerol, which might thus be implicated in the process of arachidonate liberation. Finally, radioactive phosphatidic acid and diacylglycerol appeared in endothelial cells, in response to the two agonists. However, diacylglycerol formation did not parallel that of phosphatidic acid, especially with A23187. Determination of the 14C/3H ratio of the different lipids upon cell labelling with both [14C]arachidonic acid and [3H]palmitic acid revealed that diacylglycerol and phosphatidic acid are hardly derived from inositol-phospholipid breakdown by phospholipase C. Other possible pathways involving for instance phospholipase C splitting of phosphatidylcholine are discussed.     

Release of arachidonic acid from vascular endothelial cells: fatty acyl specificity is observed with receptor-mediated agonists and with the calcium ionophore A23187 but not with melittin

Vascular endothelial cells respond to a variety of physiological and pharmacological stimuli by releasing free arachidonic acid from membrane phospholipids, thus initiating synthesis of prostacyclin. Previous work in our laboratory has demonstrated that the thrombin-stimulated deacylation is specific for arachidonate and structurally similar polyunsaturated fatty acids that contain a delta-5 double bond. We now report that histamine, bradykinin, and the calcium ionophore A23187 exhibit the same fatty acid specificity as does thrombin. Experiments with both human umbilical vein and calf pulmonary artery endothelial cells indicate that these agonists stimulate the release of previously incorporated [14C]arachidonate but not 8,11,14-[14C]eicosatrienoate or [14C]docosatetraenoate. By contrast, melittin stimulates the release of 8,11,14-eicosatrienoate, docosatetraenoate, and oleate as well as arachidonate. These results suggest that histamine, bradykinin, and A23187 activate a common calcium-dependent phospholipase A2. Melittin appears either to alter the substrate specificity of the receptor-linked phospholipase A2 activity or to activate additional enzymes as well.     

Uptake, release and novel species-dependent oxygenation of arachidonic acid in human and animal airway epithelial cells

To determine identities of mediators and mechanisms for their release from pulmonary airway epithelial cells, we examined the capacities of epithelial cells from human, dog and sheep airways to incorporate, release and oxygenate arachidonic acid. Purified cell suspensions were incubated with radiolabeled arachidonic acid and/or ionophore A23187; fatty acid esterification and hydrolysis were traced chromatographically, and oxygenated metabolites were identified using high-pressure liquid chromatography and mass-spectrometry. In each species, cellular uptake of 10 nM arachidonic acid was concentrated in the phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine fractions, and subsequent incubation with 5 microM A23187 caused release of 10-12% of the radiolabeled pool selectively from phosphatidylcholine and phosphatidylinositol. By contrast, the products of arachidonic acid oxygenation were species-dependent and in the case of human cells were also novel: A23187-stimulated human epithelial cells converted arachidonic acid predominantly to 15-hydroxyeicosatetraenoic acid (15-HETE) and two distinct 8,15-diols in addition to prostaglandin (PG) E2 and PGF2 alpha. Cell incubation with exogenous arachidonic acid (2.0-300 microM) led to progressively larger amounts of 15-HETE and the dihydroxy, epoxyhydroxy and keto acids characteristic of arachidonate 15-lipoxygenase. Both dog and sheep cells converted exogenous or endogenous arachidonic acid to low levels of 5-lipoxygenase products, including leukotriene B4 without significant 15-lipoxygenase activity. In the cyclooxygenase series, sheep cells selectively released PGE2, while dog cells generated predominantly PGD2. The findings demonstrate that stereotyped esterification and phospholipase activities are expressed at uniform levels among airway epithelial cells from these species, but pathways for oxygenating arachidonic acid allow mediator diversity depending greatly on species and little on arachidonic acid presentation.     

Arachidonic acid release in BW755C-pretreated rat peritoneal mast cells stimulated with A23187, concanavalin A and compound 48/80

Rat peritoneal mast cells respond to various secretagogues, such as ionophore A23187, concanavalin A (Ig E receptor cross-bridging) and compound 48/80 (membrane perturbing), to secrete histamine and to liberate arachidonic acid. Arachidonic acid release was made identifiable by pretreatment with BW755C, an inhibitor of both lipoxygenase and cyclo-oxygenase. The extent of arachidonic acid release varied among these three secretagogues. A23187 appeared to be most potent, whereas compound 48/80 was weakest. The sources of released arachidonic acids may be different depending on the types of stimulants. The stimulation with A23187 released arachidonic acid mainly from phosphatidylcholine and triacylglycerol. After treatment with concanavalin A and compound 48/80, in addition to phosphatidylcholine, phosphatidylinositol also appeared to serve as a donor of arachidonic acid.     

The role of polyphosphoinositides and their breakdown products in A23187-induced release of arachidonic acid from rabbit polymorphonuclear leucocytes.

Stimulation of rabbit polymorphonuclear leucocytes with A23187 causes phospholipase C mediated breakdown of polyphosphoinositides, as evidenced by accumulation of [3H]inositol-labelled inositol bisphosphate and inositol trisphosphate. At the same time the polyphosphoinositides and the products of their breakdown, diacylglycerol and phosphatidic acid, label rapidly with radioactive arachidonic acid. Enhancement of polyphosphoinositide labelling is not as great as enhancement of diacylglycerol or phosphatidic acid labelling, suggesting additional early activation of a second independent synthetic pathway to the last named lipids. Experiments using double (3H/14C) labelling, to distinguish pools with different rates of turnover, suggest the major pool of arachidonic acid used for synthesis of lipoxygenase metabolites turns over more slowly than arachidonic acid in diacylglycerol, but at about the same rate as arachidonic acid esterified in phosphatidylcholine or phosphatidylinositol. Further, when cells are prelabelled with [14C]arachidonic acid, then stimulated for 5 min, it is only from phosphatidylcholine, and to a lesser extent phosphatidylinositol, that radiolabel is lost. Release of arachidonic acid is probably via phospholipase A2, since it is blocked by the phospholipase A2 inhibitor manoalide. The absence of accumulated lysophosphatides can be explained by reacylation and, in the case of lysophosphatidylinositol, deacylation. The importance of phospholipase A2 in phosphatidylinositol breakdown contrasts with the major role of phospholipase C in polyphosphoinositide hydrolysis. Measurements of absolute free fatty acid levels, as well as studies showing a correlation between production of radiolabelled hydroxyeicosatetraenoic acids and release of radiolabel from the phospholipid pool, both suggest that hydrolysis of arachidonic acid esterified into phospholipids is the limiting factor regulating formation of lipoxygenase metabolites. By contrast with A23187, fMet-Leu-Phe (a widely used polymorphonuclear leucocyte activator) is a poor stimulant for arachidonic acid release unless a 'second signal' (e.g. cytochalasin B, or a product of A23187-stimulated cells) is also present. In the presence of cytochalasin B, fMet-Leu-Phe, like A23187, stimulates release of radiolabelled arachidonic acid principally from phosphatidylcholine.     

Substrate specificity of the agonist-stimulated release of polyunsaturated fatty acids from vascular endothelial cells

Stimulation of vascular endothelial cells with agonists such as histamine and thrombin results in release of arachidonic acid from membrane lipids and subsequent eicosanoid synthesis. As shown previously, the agonist-stimulated deacylation is specific for arachidonate, eicosapentaenoate, and 5,8,11-eicosatrienoate. This study has utilized radiolabeled fatty acids differing in chain length and position of double bonds to further elucidate the fatty acyl specificity of agonist-stimulated deacylation. Replicate wells of confluent human umbilical vein endothelial cells were incubated with 14C-labeled fatty acids and then challenged with histamine, thrombin, or the calcium ionophore A23187. Comparison of the results obtained with isomeric eicosatetraenoic fatty acids with initial double bonds at carbons 4, 5, or 6 indicated that the deacylation induced by all three agonists exhibited marked specificity for the cis-5 double bond. Lack of stringent chain length specificity was indicated by agonist-stimulated release of 5,8,11,14- tetraenoic fatty acids with 18, 19, 20, and 21 carbons. Release of 5,8,14-[14C]eicosatrienoate was two-to threefold that of 5,11,14-[14C]eicosatrienoate, thus indicating that the cis-8 double bond may also contribute to the stringent recognition by the agonist-sensitive phospholipase. The present study has also demonstrated that histamine, thrombin, and A23187 do not stimulate release of docosahexaenoate from endothelial cells.     

Cyclic adenosine-3',5'-monophosphate alters hydrolysis of phospholipids by mouse embryo palate mesenchyme cells

The purpose of this study was to determine whether inhibition of release of arachidonic acid from mouse embryo palate mesenchyme (MEPM) cells in response to cAMP is due to a selected or generalized inhibition of hydrolysis of esterified pools of arachidonic acid. The calcium ionophore A23187 proved to be a useful probe of phospholipid hydrolases in MEPM cells, since it stimulated release of radiolabeled fatty acids from phospholipids of prelabeled MEPM cells as a function of the length of exposure, concentration, and concentration of Ca2+ in the medium. Elevation of intracellular levels of cAMP by treatment with (-) isoproterenol resulted in the inhibition of release of radiolabeled arachidonic acid in response to A23187. Analysis by quantitative gas-liquid chromatography revealed the source of the arachidonic acid released in response to the ionophore to be 1,2-diradyl-sn-glycero-3-phosphoethanolamine; elevation of intracellular levels of cAMP inhibited hydrolysis of this substrate, but may have stimulated hydrolysis of 1,2-diradyl-sn-glycero-3-phosphocholine. These findings permit the conclusions that 1) the ionophore stimulates activities of selected phospholipases A in MEPM cells and 2) cAMP modulates certain phospholipases A in MEPM cells in a specific manner.     

Efflux of γ-aminobutyric acid from and appearance of free arachidonic acid inside synaptosomes

When synaptosomes were depolarized in the presence of Ca²⁺, or when Ca²⁺ was added to synaptosomes pretreated with Ca²⁺ ionophore (A23187), free arachidonic acid was clearly increased within synaptosomes, and at the same time an efflux of γ-aminobutyric acid from synaptosomes was observed. Moreover, when synaptosomes labelled with [¹⁴C]arachidonic acid were depolarized in the presence of Ca²⁺, there was a significant decrease in the radioactivity of the fatty acid of phosphatidylinositol and phosphatidylcholine. Exogenously added arachidonic acid, but not other fatty acids, stimulated the efflux of γ-aminobutyric acid in the absence of Ca²⁺. These observations suggest that the release of arachidonic acid from phospholipids is an intrinsic part of the biochemical mechanism that modulates the γ-aminobutyric acid efflux.     

Liberation and oxygenation of polyenoic acids in stimulated platelets

Radiolabeled polyenoic acids were incorporated into human platelet lipids using albumin as vector. Platelets were then triggered with 0.1 or 1 U/ml thrombin, and 0.5 or 2 x 10(-6) M calcium ionophore A23187. Lipid extracts were analyzed for neutral lipids, free fatty acids, monohydroxylated acids, prostanoids and glycocerophospholipid subclasses. During platelet activation induced by thrombin or by ionophore, arachidonic and eicosapentaenoic acids were liberated from phospholipids in large amounts and were subsequently oxygenated via platelet oxygenases. Substantial amounts of lipoxygenase products and thromboxanes were produced from these acids. Liberation and oxygenation of linoleic, alpha-linolenic, and docosahexaenoic acids were much less pronounced. Polyenoic acid liberation from phospholipid subclasses also behaved quite differently. Apart from alpha-linolenic and adrenic acids, which were poorly liberated, all the others were freed from phosphatidylinositol. In addition, arachidonic, eicosapentaenoic, and 5, 8, 11-eicosatrienoic acids were liberated from phosphatidylcholine at high concentrations of agonists and partially reincorporated into phosphatidylethanolamine. Finally, linoleic acid was deacylated from phosphatidylinositol and phosphatidylserine and almost entirely reacylated into phosphatidylcholine, whereas docosahexaenoic acid was deacylated from phosphatidylcholine and phosphatidylinositol reacylated into phosphatidylethanolamine, respectively. It is concluded that these polyenoic acids, all for which modulate platelet functions, exhibit very different metabolisms. They may act via their oxygenated derivatives and/or at the membrane phospholipid level.     

Effect of unsaturated fatty acids and Ca2+ on phosphatidylinositol synthesis and breakdown

CDP-diglyceride : inositol transferase was inhibited by unsaturated fatty acids. The inhibitory activity decreased in the following order: arachidonic acid greater than linolenic acid greater than linoleic acid greater than oleic acid greater than or equal to palmitoleic acid. Saturated fatty acids such as myristic acid, palmitic acid, and stearic acid had no effect. Calcium ion also inhibited the activity of CDP-diglyceride : inositol transferase. In rat hepatocytes, arachidonic acid inhibited 32P incorporation into phosphatidylinositol and phosphatidic acid without any significant effect on 32P incorporation into phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine. Ca2+ ionophore A23187 also inhibited 32P incorporation into phosphatidylinositol. However, 32P incorporation into phosphatidic acid was stimulated with Ca2+ ionophore A23187. Phosphatidylinositol-specific phospholipase C was activated by unsaturated fatty acids. Polyunsaturated fatty acids such as arachidonic acid and linolenic acid had a stronger effect than di- and monounsaturated fatty acids. Saturated fatty acids had no effect on the phospholipase C activity. The phospholipase C required Ca2+ for activity. Arachidonic acid and Ca2+ had synergistic effects. These results suggest the reciprocal regulation of phosphatidylinositol synthesis and breakdown by unsaturated fatty acids and Ca2+.     

Stimulation of deacylation in Madin-Darby canine kidney cells. Specificity of deacylation and prostaglandin production in 12-O-tetradecanoylphorbol-13-acetate-treated cells

Madin-Darby canine kidney cells deacylate arachidonic acid from cellular phospholipid in response to 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and convert the free arachidonic acid to prostaglandins. We have used this system to characterize the acyl specificity of deacylation. Cells were labeled with either [14C]linoleic, [14C]eicosatrienoic (delta 8,11,14 or delta 5,8,11), or [14C]arachidonic acid and stimulated with 10 nM TPA. We found that TPA stimulated the deacylation of all four acids, primarily from phosphatidylethanolamine and phosphatidylcholine.l Only products from linoleic (presumably through chain elongation and desaturation), eicosatrienoic (delta 8,11,14), and arachidonic acids produced prostaglandins. Those produced from linoleic and eicosatrienoic acid (delta 8,11,14)-labeled cells were determined to be primarily of the 1-series, while arachidonic acid-labeled cells produced prostaglandins of the 2-series. Together these results indicate that the stimulated deacylation of phospholipids is not specific for arachidonic acid and that the membrane acyl composition controls the particular series of prostaglandin which is produced.     

Calcium ionophore A 23187 induces arachidonic acid release from phosphatidylcholine in cultured human endothelial cells

Cultured endothelial cells from human umbilical vein were incubated with (³H)arachidonic acid for 24 hours. The label was incorporated into phospholipids (79.3 %), neutral lipids (15.6 %) and non-esterified fatty acids (4.7 %). Upon challenge with the calcium ionophore A 23187, 5.3 % of the total radioactivity were found in supernatant and corresponded to 6-keto-prostaglandin F_1α (1.6 %) and free arachidonic acid (3.7 %). This release was accompanied by a concomitant and selective decrease of phosphatidylcholine. It is concluded that the entry of calcium promoted by A 23187 activates a phospholipase A₂ regulating the availability of arachidonic acid to the prostacyclin synthetase.     

Association of calmodulin inhibition, erythrocyte membrane stabilization and pharmacological effects of drugs

Isolated liver cells from rats fed a diet deficient in essential fatty acids were used to study the oxidation, esterification and, especially, the desaturation and chain elongation of [1-14C]linoleic acid. 14C-labelled arachidonic acid (20:4) and smaller amounts of eicosatrienoic acid (20:3) were recovered mainly in the phospholipids, while gamma-linolenic acid (18:3) was found in both the phospholipids and the triacylglycerol fraction. Lactate strongly increased the formation of arachidonic acid, which was found mainly in the phosphatidylcholine and the phosphatidylinositol fractions. Lactate reduced the amounts of gamma-linolenic acid. Glucagon and (+)-decanoylcarnitine reduced the formation of arachidonic acid, and (+)-decanoylcarnitine increased the incorporation of gamma-linolenic acid especially, in the triacylglycerol fraction. Increasing concentrations of the [1-14C]linoleic acid substrate increased the formation of arachidonic acid and of the other chain-elongated or desaturated fatty acids. Lactate also stimulated the formation of arachidonic acid in liver cells from animals fed adequate amounts of essential fatty acids. It is suggested that dietary and hormonal factors which can change the intracellular levels of malonyl-CoA may influence both the ratio of arachidonic acid/gamma-linolenic acid formed and the total amounts of desaturated and chain-elongated fatty acids formed from linoleic acid.     

Efflux of gamma-aminobutyric acid from and appearance of free arachidonic acid inside synaptosomes

When synaptosomes were depolarized in the presence of Ca2+, or when Ca2+ was added to synaptosomes pretreated with Ca2+ ionophore (A23187), free arachidonic acid was clearly increased within synaptosomes, and at the same time an efflux of gamma-aminobutyric acid from synaptosomes was observed. Moreover, when synaptosomes labelled with [14C]arachidonic acid were depolarized in the presence of Ca2+, there was a significant decrease in the radioactivity of the fatty acid of phosphatidylinositol and phosphatidylcholine. Exogenously added arachidonic acid, but not other fatty acids, stimulated the efflux of gamma-aminobutyric acid in the absence of Ca2+. These observations suggest that the release of arachidonic acid from phospholipids is an intrinsic part of the biochemical mechanism that modulates the gamma-aminobutyric acid efflux.     

Incorporation and distribution of epoxyeicosatrienoic acids into cellular phospholipids.

The different regioisomers of epoxyeicosatrienoic acids derived from cytochrome P-450 monooxygenase are readily esterified into phospholipids of mastocytoma cells. Incorporation of 14,15-epoxyeicosatrienoic acid was concentration-dependent, with Km = 1.1 microM and Vmax = 36 pmol/min/10(7) cells. Half-maximal incorporation occurred in 30 min, reaching a steady-state concentration of 470 pmol/10(6) cells. This was slightly lower than the values for arachidonic acid (665 pmol/10(6) cells) or 5-hydroxyeicosatetraenoic acid (554 pmol/10(6) cells). The distribution of 14,15-epoxyeicosatrienoic acid was preferential in the order phosphatidylethanolamine greater than phosphatidylcholine greater than phosphatidylinositol greater than phosphatidyl serine much greater than neutral lipids plus fatty acids. This contrasted with 5(S)-hydroxyeicosatetraenoic acid, which was distributed primarily into phosphatidylcholine. Fast atom bombardment/tandem mass spectrometry facilitated identification of molecular species containing epoxyeicosatrienoic acids without relying on radioisotopes. Phosphatidylethanolamine plasmalogens with 16:1 or 18:2 at the sn-1 position, or an 18:0 acyl group, and phosphatidylcholine with 16:0 alkyl ether or an acyl group at the sn-1 position incorporated all possible epoxyeicosatrienoic acid regioisomers. Under basal conditions, cells eliminated 14,15-cis-epoxyeicosatrienoic acid slowly with a half-life of 34.9 +/- 7 h. Cells stimulated with calcium ionophore A23187 eliminated 14,15-epoxyeicosatrienoic acid rapidly. It was notable that its rate of release from phosphatidylcholine and phosphatidylinositol exceeded that for arachidonic acid. A coenzyme A-independent transacylase also catalyzed the transfer of epoxyeicosatrienoic acids from mastocytoma cell membranes into 1-palmitoyl-2-lysophosphatidylcholine. The cellular incorporation, release, and distribution of epoxyeicosatrienoic acids is distinctive and contrasts with most other eicosanoids, suggesting that these compounds may have both autocoid and nonautocoid functions.     

Evidence for multiple metabolic pools of phosphatidylinositol in stimulated platelets

Stimulation of platelets with ionophore A23187 or thrombin indicates the existence of three distinct metabolic fractions of phosphatidylinositol. Two of those pools of phosphatidylinositol are degraded by phosphatidylinositol-specific phospholipase C and the third one by a phospholipase A2 activity. Low concentrations of ionophore A23187 (100 nM) or thrombin (0.25 units/ml) induce the degradation by phospholipase C of a minor fraction of phosphatidylinositol which is involved in the phosphatidylinositol cycle. In addition, thrombin, but not ionophore A23187, leads to the degradation by phospholipase C of a larger fraction of phosphatidylinositol and the subsequent accumulation of phosphatidic acid. A third fraction of phosphatidylinositol, sensitive to thrombin (0.5-2 units/ml) or ionophore A23187 (0.5-2 microM), can be degraded by phospholipase A2 to lysophosphatidylinositol with the concomitant liberation of arachidonic acid. Degradation of phosphatidylinositol by the phospholipase C pathway precedes that of the phospholipase A2 pathway. The results also suggest that the phosphatidylinositol cycle is sensitive to a small rise in cytosolic Ca2+ concentration. A further mobilization of cytosolic Ca2+ interrupts the phosphatidylinositol cycle by inhibiting conversion of phosphatidic acid to phosphatidylinositol and also activates phospholipases of the A2 type.     

The receptors for ATP and fMetLeuPhe are independently coupled to phospholipases C and A2 via G-protein(s). Relationship between phospholipase C and A2 activation and exocytosis in HL60 cells and human neutrophils.

The relationship between phospholipase A2 and C activation and secretion was investigated in intact human neutrophils and differentiated HL60 cells. Activation by either ATP or fMetLeuPhe leads to [3H]arachidonic acid release into the external medium from prelabelled cells. This response was inhibited when the cells were pretreated with pertussis toxin. When the [3H]arachidonic acid-labelled cells were stimulated with fMetLeuPhe, ATP or Ca2+ ionophore A23187, and the lipids analysed by t.l.c., the increase in free fatty acid was accompanied by decreases in label from phosphatidylinositol and phosphatidylcholine. Moreover, incorporation of label into triacylglycerol and to a lesser extent phosphatidylethanolamine was evident. Activation of secretion was evident with ATP and fMetLeuPhe but not with A23187. The pharmacological specificity of the ATP receptor in HL60 cells was investigated by measuring secretion of beta-glucuronidase, formation of inositol phosphatases and release of [3H]arachidonic acid. External addition of ATP, UTP, ITP, adenosine 5'-[gamma-thio]triphosphate (ATP[S]), adenosine 5'-[beta gamma-imido]triphosphate (App[NH]p), XTP, CTP, GTP, 8-bromo-ATP and guanosine 5'-[gamma-thio]triphosphate (GTP[S]) to intact HL60 cells stimulated inositol phosphate production, but only the first five nucleotides were effective at stimulating secretion or [3H]arachidonic acid release. In human neutrophils, addition of ATP, ITP, UTP and ATP[S] also stimulated secretion from specific and azurophilic granules, and this was accompanied by increases in cytosolic Ca2+ and in [3H]arachidonic acid release. The addition of phorbol 12-myristate 13-acetate (PMA; 1 nM) prior to the addition of either fMetLeuPhe or ATP led to inhibition of phospholipase C activity. In contrast, this had no effect on phospholipase A2 activation, whilst secretion was potentiated. Phospholipase A2 activation by either agonist was dependent on an intact cell metabolism, as was secretion. It is concluded that (1) activation of phospholipase C does not always lead to activation of phospholipase A2, (2) phospholipase A2 is coupled to the receptor independently of phospholipase C via a pertussis-toxin-sensitive G-protein and (3) for secretion to take place, the receptor has to activate both phospholipases C and A2.     

Phosphatidate and oxidized fatty acids are calcium ionophores. Studies employing arsenazo III in liposomes

Liposomes which have entrapped the metallochromic dye, arsenazo III, constitute a sensitive assay system for ionophoresis of divalent cations. By this means we have compared known calcium ionophores (A23187, ionomycin) with membrane phospholipids, fatty acids, prostanoids, and retinoids. Added at micromolar concentrations to preformed multilamellar liposomes (phosphatidylcholine 7:dicetyl phosphate 2: cholesterol 1) both A23187 and ionomycin, as well as phosphatidic acid and products derived from linoleic acid, linolenic acid, and two eicosatrienoic acids provoked Ca influx (e.g. phosphatidic acid: 0.13 mol of Ca2+/mol of membrane lipid/5 min). A variety of other phospholipids (e.g. phosphatidylinositol), fatty acids (e.g. arachidonic acid), prostanoids (e.g. PGE1) retinoids (e.g. retinoic acid), and glyceryl ether phosphorylcholines ("platelet-activating factors") were without effect. Phosphatidic acid and oxidized fatty acids translocated divalent cations selectively, demonstrating the same rank order as A23187 or ionomycin: Mn greater than Ca greater than Sr much greater than Mg. Membrane lysis did not contribute to the perceived translocation; the liposomes remained impermeable to EDTA, EGTA, arsenazo III, or Mg. Liposomes with phosphatidic acid or oxidized trienoic acids preincorporated at 1-5 mole % of total lipids also permitted translocation of Ca but not Mg. Reduction of ionophoretic fatty acids or ionomycin with stannous chloride abolished their ionophoretic activity. Release of Ca from liposomes which had entrapped arsenazo III-Ca complexes into a medium rich in EGTA permitted calculation of efflux induced by ionophores, whether these were added to the outside of liposomes or preincorporated. Data suggest that phosphatidic acid and oxidized di- and trienoic fatty acids, which act as calcium ionophores in model bilayers, could serve as "endogenous ionophores" in cells.     

Inhibition of lymphocyte activation by cyclosporin A: interference with the early activation of the membrane phospholipid metabolism in rabbit lymphocytes stimulated with concanavalin A, anti-rabbit immunoglobulin or the Ca2+ ionophore A 23187

Rabbit lymphocytes from the mesenteric lymph nodes were stimulated with concanavalin A, goat anti-rabbit immunoglobulin, or the Ca2+ ionophore A 23187. The stimulated incorporation of labeled uridine into RNA as well as of labeled thymidine into DNA was suppressed within a dose range of 40-1000 ng/ml cyclosporin A in both Con A-stimulated T lymphocytes and in anti-immunoglobulin-stimulated B lymphocytes, without affecting the resting cells. A 23187-stimulated rabbit lymphocytes proved to be more sensitive to cyclosporin A. At 40 ng/ml the immunosuppressive drug was effective in inhibiting elevated incorporation of labeled nucleosides into macromolecules in ionophore-stimulated cells. Cyclosporin A, at the same concentrations that were effective in inhibiting stimulated RNA and DNA synthesis, suppressed one of the earliest events occurring in stimulated lymphocytes, i.e., enhanced incorporation of unsaturated fatty acids into membrane phospholipids. Whereas cyclosporin A significantly inhibited the incorporation of arachidonic acid into phosphatidylcholine and phosphatidylethanolamine in concanavalin A-, anti-immunoglobulin-, and A 23187-stimulated cells, it proved to be ineffective in inhibiting the incorporation of arachidonate into phosphatidylinositol. The data indicate that cyclosporin A inhibits both T- and B-cell stimulation by interfering with a common target, e.g., the early activation of membrane phospholipid metabolism of rabbit lymphocytes.