How is the level of free arachidonic acid controlled in mammalian cells?

Characterization of the arachidonate-containing molecular species of phosphoglycerides in the human neutrophil

Neutrophils were labeled with high specific activity, [3H]-arachidonate and separated into their arachidonate-containing phosphoglycerides using a slight modification of the normal phase and reverse phase HPLC described by Patton et al. The individual molecular species of PC, PE and PI containing arachidonate were characterized by fast atom bombardment/mass spectrometry techniques. Eight major arachidonate-containing molecular species of PC, six major arachidonate-containing molecular species of PE and two major arachidonate-containing molecular species of PI could be found. With the structures of the phosphoglycerides eluting from the reverse phase HPLC column known, general trends in the HPLC elution behavior of 1-0-alkyl, 1-alk-1-enyl and 1-acyl-2-arachidonoyl phosphoglycerides could be observed.

Stimulation of phosphoinositide hydrolysis by oxytocin and the mechanism by which oxytocin controls prostaglandin synthesis in the ovine endometrium

Slices of caruncular endometrium from steroid-treated ovariectomized sheep were incubated with myo-[2-3H]inositol to label tissue phosphatidylinositol. Effects of oxytocin were determined on the rate of incorporation of radioactivity into phosphatidylinositol and on the hydrolysis of phosphoinositides to inositol phosphates and diacylglycerol. Incorporation of radioactivity into phosphatidylinositol was linear during 2 h incubations; 10(-7) M (100 nM)-oxytocin caused a 2.8-fold increase in the rate of incorporation. In the presence of Li+, addition of 10(-7) M-oxytocin to slices in which phosphatidylinositol was pre-labelled caused mean increase of 40-fold in the incorporation of radioactivity into inositol mono-, bis- and tris-phosphates. Inositol 1,3,4-trisphosphate was quantitatively the major trisphosphate formed. The action of oxytocin on phosphoinositide hydrolysis was dose- and time-dependent, occurring at concentrations within the range observed in plasma during episodes of secretion in vivo, and with a time course comparable with that of the action of oxytocin on uterine prostaglandin production. The effect of oxytocin on incorporation of radioactivity into inositol phosphates was not affected by inhibitors of prostaglandin synthesis. Diacylglycerol 1- and 2-lipases in caruncular endometrium converted up to 72% of added 2-[3H]arachidonyldiacylglycerol into [3H]arachidonic acid during 30 min incubations at pH 7.0. Caruncular endometrium contained 1.49 mumol of phosphatidylinositol/g, representing approx. 0.2 mumol/g of phosphatidylinositol arachidonic acid. It is proposed that the stimulation of endometrial prostaglandin synthesis by oxytocin is accounted for by increased hydrolysis of phosphoinositides to diacylglycerol and inositol phosphates with subsequent release of arachidonic acid from diacylglycerol.

Effects of substance P on inositol triphosphate accumulation, on contractile responses and on arachidonic acid release and prostaglandin biosynthesis in rabbit iris sphincter muscle

Addition of substance P (10(-7) to 10(-6) M) to rabbit iris sphincter muscle induced: (a) a rapid phosphodiesteratic breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2) into 1,2-diacylglycerol (DG), measured as phosphatidic acid, and inositol triphosphate (IP3), measured by anion-exchange chromatography; (b) a rapid and strong contractile response, and (c) a rapid release of prostaglandin E2 (PGE2), measured by radioimmunoassay, and rapid release of 14C-labeled arachidonic acid, measured by radiochromatography. These substance P actions are concentration and time-dependent, and are blocked by substance P antagonist, [D-Pro2, D-Trp7,9]SP. The effects of substance P on arachidonic acid release and PG synthesis are not mediated through the cyclo-oxygenase and lipoxygenase pathways. Substance P exerted little effect on PGE2 release by the iris dilator muscle. We conclude that substance P, which is liberated from the sensory nerves that innervate the sphincter region of the iris and plays a role in miosis, may function as a Ca2+-mobilizing agonist in this tissue. Thus, a substance P-induced release of IP3 and formation of DG, a source for arachidonic acid in PG synthesis, followed by Ca2+ mobilization could underlie the mechanism for the biological actions, such as muscle contraction, of the neuropeptide reported in the eye. However, the precise relationship remains to be established.

Phospholipase C and diacylglycerol lipase activities associated with plasma membranes of chromaffin cells isolated from bovine adrenal medulla

The plasma membranes of bovine adrenal chromaffin cells were isolated and the activities of enzymes involved in arachidonic acid liberation were investigated. Only a minute activity of phospholipase A2 (phosphatide 2-acylhydrolase, EC 3.1.1.4) could be detected using externally added phosphatidylcholine (PC) and phosphatidylethanolamine (PE) as substrate. When membranes were treated with exogenous phospholipase C (orthophosphoric acid diester phosphohydrolase, EC 3.1.4.1) there was a liberation of free fatty acids from the sn-2 position of PC. The enzyme responsible for this effect could be demonstrated to be a diacylglycerol lipase (glycerol ester hydrolase, EC 3.1.1.3) localized in the plasma membrane. Using phosphatidylinositol (PI) as a substrate, it was found that an endogenous phospholipase C exists which co-purifies with the membrane preparation. The produced diacylglycerol is subsequently hydrolyzed by diacylglycerol lipase liberating arachidonic acid. The two enzymes, phospholipase C and diacylglycerol lipase were characterized. Phospholipase C was found to be calcium dependent and PI specific, showing an activity of 60 pmol/micrograms protein per h (1.2 mM Ca2+), whereas the diacylglycerol lipase was calcium independent hydrolyzing diacylglycerol at a rate of 7.2 pmol/micrograms protein per h. The lipase but not the phospholipase C was inhibited 50% by 1.7 mM para-bromophenacylbromide.

Dexamethasone-induced stimulation of arachidonic acid release by U937 cells grown in defined medium

Since the presence of serum in culture media has been shown to alter prostaglandin production, as well as to interfere with the action of anti-inflammatory drugs, we have studied the effect of dexamethasone, a potent steroidal anti-inflammatory drug, on the metabolism of arachidonic acid by human monocyte-like cells (U937) grown in a fully defined medium. Under these culture conditions, dexamethasone (10(-6) M, 24 h) induced a marked stimulation of the release of unmetabolized arachidonic acid into the culture medium. The steroid also induced an inhibition of cell proliferation which became significant only after 48 h of treatment. The accumulation of arachidonic acid in the medium after steroid treatment was associated with a significant inhibition of cell acyltransferase activity, suggesting that steroids may also act upon arachidonic acid metabolism at sites other than those of phospholipase activity.

Phospholipase A2 activity in T-lymphocytes

Phospholipase A2 activity was shown indirectly in T-lymphocytes from rat thymus and a permanent T-cell line by the liberation of arachidonic acid from phospholipids. In addition, phospholipase A2 activity was measured directly with two different substrates, phosphatidylcholine and labelled E. coli.

Effect of estradiol and progesterone on the rate of incorporation of radiolabeled arachidonate into phospholipids and triglycerides of the rat uterus

To obtain information on the effect of estradiol (E) and progesterone (P) on the overall rate of the acylation and deacylation reactions with [3H]arachidonate ([3H]AA) in the rat uterus, we investigated the effect of chronic treatment of ovariectomized rats with 2-200 micrograms/day E-dipropionate (EPP) and with 2 mg P alone or with the combination of these steroids on the extent and the time course of the in vitro incorporation of [3H]AA into various phospholipids (PLs) and the triacylglycerol fraction (TG). The results demonstrate that physiologic doses of E leads to a rapid equilibrium of the deacylation-acylation cycle only in the case of phosphatidylinositol (PI) whereas at high (200 micrograms/day) dose level it exerts the same effect on phosphatidylcholine (PC), phosphatidylethanolamine (PE) as well as on TG. P alone has no remarkable effects in the ovariectomized rat, but it decreases markedly the incorporation of [3H]AA into PI and TG in intact animals. Furthermore, P decreases the incorporation of [3H]AA into PI in the ovariectomized rat treated with 2 micrograms/day EPP as well as attenuating the enhanced labeling of PC, PE and PI caused by higher doses. Time-course studies provide evidence that all of these effects of progesterone can be accounted for by its ability to decrease the rate of deacylation and, therefore, to prolong the time needed to attain equilibrium in the rates of acylation and deacylation reactions of the various lipids with arachidonate. These data offer a novel outlook on the regulatory role of progesterone and estrogens in uterine function.

Evidence for a catalytic role of phospholipase A in phorbol diester- and zymosan-induced mobilization of arachidonic acid in mouse peritoneal macrophages

Inositol phospholipid degradation and release of phospholipid-bound arachidonic acid was induced in intact peritoneal macrophages by exposure to phorbol myristate acetate (PMA) or zymosan particles. PMA, known to activate protein kinase C, selectively enhanced the deacylation of phosphatidylinositol (i.e., degradation by phospholipase A), while zymosan particles enhanced degradation via both phospholipase A and inositol lipid phosphodiesterase (phospholipase C). The release of arachidonic acid was found to correlate with the degradation of phosphatidylinositol by the phospholipase A pathway and could be dissociated from the phospholipase C-catalyzed cleavage of inositol phospholipids in several experimental situations: (i) when PMA was the stimulus, (ii) by the difference in Ca2+ dependence between the two enzymatic processes when zymosan was the stimulus and (iii) by the parallel inhibition by chlorpromazine of the phospholipase A pathway and arachidonic acid release, but not inositol phospholipid phosphodiesterase. In addition, phloretin, a reported inhibitor of protein kinase C, was found to inhibit arachidonic acid release and the deacylation of phosphatidylinositol. The results are consistent with a model in which arachidonic acid release is mediated by phospholipase(s) A and in which PMA or the phosphodiesterase-catalyzed degradation of phosphoinositides causes activation of the phospholipase A pathway via protein kinase C.

Inositol phospholipid arachidonic acid metabolism in GH3 pituitary cells

Inositol phospholipids in cultured GH3 cells, a prolactin secreting, thyrotropin-releasing hormone (TRH) sensitive rat pituitary cell line, exhibit a preferential selectivity for incorporating arachidonic acid. Fatty acid composition data show that all inositol phospholipids are enriched in stearic and arachidonic acids to a much greater degree than other cellular phospholipids. Incubation of GH3 cells with radioactive stearate, oleate, arachidonate, eicosapentaenoate or docosahexaenoate also showed that much more stearate and arachidonate were incorporated into inositol phospholipids. In short term incubations with tracer amounts of radioactive arachidonate, incorporation was initially into phosphatidylinositol (PtdIns), with phosphatidylinositol 4-phosphate (PtdIns4P), and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] being labelled at later times. During longer incubations, all of the inositol phospholipids reach equilibrium at about 10 h, and the resulting specific activities of the three fractions were similar. These findings suggest that arachidonate is incorporated initially into PtdIns and that PtdIns is then phosphorylated. There was no release of either arachidonate or eicosanoid products when GH3 cells were incubated with TRH. However, TRH stimulation of 32P-labelled GH3 cells resulted in rapid breakdown of PtdIns(4,5)P2 and PtdIns4P, with concomitant increases in [32P]phosphatidic acid and [32P]PtdIns. When the [32P]PtdIns was further analysed by argentation chromatography to separate PtdIns molecular species, it was found that tetraenoic (stearate/arachidonate) species accounted for 80% of the stimulated labelling. The selectivity for arachidonate incorporation into inositol phospholipids coupled with turnover of the arachidonate-containing molecular species suggests that inositol phospholipids containing arachidonic acid or the diacylglycerol resulting therefrom may play a vital cellular role in GH3 cells. This role may involve the operation of the PtdIns cycle itself rather than a stimulated release of arachidonate for eicosanoid formation.

Liberation of [3H]arachidonic acid and changes in cytosolic free calcium in fura-2-loaded human platelets stimulated by ionomycin and collagen

Cytosolic Ca2+ levels and arachidonate liberation were investigated in platelets loaded with the fluorescent Ca2+ indicator dye fura-2, and labelled with [3H]arachidonate. Fura-2 was used in preference to quin2 because the latter interfered with [3H]arachidonate labelling of phospholipids. From a resting free Ca2+ level of around 100 nM, ionomycin (10-200 nM) evoked an instantaneous, concentration-dependent increase in cytosolic Ca2+ that only resulted in [3H]arachidonate liberation (up to 4-fold over control) at Ca2+ levels greater than 1 microM. Addition of collagen (10 micrograms/ml) evoked an elevation in Ca2+ up to 461 +/- 133 nM. These changes in Ca2+ were accompanied by a 2-4-fold elevation in [3H]arachidonate with depletion of [3H]phosphatidylcholine by 17 +/- 4% and [3H]phosphatidylinositol by 41 +/- 7%. Indomethacin (10 microM) reduced the elevation in Ca2+ by collagen to 115 +/- 18 nM but did not significantly inhibit the 2-4-fold increase in [3H]arachidonate. [3H]Phosphatidylcholine and [3H]phosphatidylinositol were decreased by 9 +/- 7% and 10 +/- 6%, respectively, with collagen in the presence of indomethacin. Stimulation of phosphoinositide turnover by collagen in the presence and absence of indomethacin was indicated by [32P]phosphatidate formation in cells prelabelled with [32P]Pi. This phosphatidate formation was decreased (75%) by the presence of indomethacin. In the presence of indomethacin, phorbol myristate acetate (20 nM) alone or in combination with ionomycin (30 nM) failed to stimulate arachidonate liberation despite a marked stimulation of aggregation. These results indicate that, whereas ionomycin requires Ca2+ in the microM range for arachidonate liberation, collagen, notably in the presence of indomethacin, does so at basal Ca2+ levels. The mechanisms underlying the regulation of arachidonate release by collagen are not clear, but do not appear to involve activation of protein kinase C, or an elevation of cytosolic free Ca2+.

Uptake of fatty acids and their mobilization from phospholipids in cultured monocyte-macrophages from rheumatoid arthritis patients

Prostaglandins (PG) and related eicosanoids which derive from essential fatty acids are important mediators and modulators of inflammation. Macrophages (M phi), which derive from peripheral blood monocytes (PBM), are prominent cells in the synovium of patients with rheumatoid arthritis (RA), and are a major source of synovial PGE2. In addition, fresh and cultured PBM from RA patients produce more PG than normal control cells. When allowed to mature in culture PBM exhibit many characteristics of macrophages (M-M phi). We examined uptake by M-M phi of eicosanoid precursor fatty acids (FA), their incorporation into cellular phospholipid (PL), and mobilization of FA after cell stimulation. Cultured M-M phi from treated and untreated RA patients (RA M-M phi) took up significantly more linoleic acid (LA), dihomogammalinolenic acid (DHLA) and arachidonic acid (AA) than M-M phi from normal volunteers (N M-M phi). The enhanced uptake of FA observed in 12-day cultures of RA M-M phi was similar to uptake seen in normal human peritoneal macrophages (PM phi). After uptake FA were incorporated mainly into phosphatidylcholine (PC). M-M phi from untreated RA patients incorporated a smaller proportion of [14C]LA into PC (37.0 +/- 12.7% of total PL label) than normal cells (86.0 +/- 4.2%), and a greater proportion of [3H]AA into PC (57.1 +/- 7.1%) than normals (23.9 +/- 6.9%). Stimulation of M-M phi with calcium ionophore A23187 resulted in significantly greater hydrolysis of LA and AA from PC in RA M-M phi from both treated and untreated patients than from PC in N M-M phi. The data indicate that M-M phi from RA patients mature more rapidly in vitro than M-M phi from controls as uptake of FA by RA M-M phi increases with duration of culture and by 12 days in culture equals uptake by normal human peritoneal M phi. Also, RA M-M phi exhibit differences from N M-M phi in uptake, PL distribution, and hydrolysis of eicosanoid precursor FA. Such changes in FA metabolism might influence cell function and inflammatory responses.

Enhanced inositide turnover in brain during bicuculline-induced status epilepticus

Because brain inositides are enriched in the 1-stearoyl-2-arachidonoyl species, they form a likely source for the tetraenoic free fatty acids (FFA) and diacylglycerols (DG) that are accumulated during seizures. To study inositide turnover during bicuculline-induced seizures, rats were injected intraventricularly and bilaterally with 10-20 microCi 32P, mechanically ventilated and sacrificed by 6.5 KW head-focused microwave irradiation. Seizure activity was recorded by electroencephalography. Bicuculline-induced seizure activity resulted in: a) almost 50% increase in 32P labeling of phosphatidic acid (PA); phosphatidylinositol (PI) and phosphatidylinositol 4,5-bisphosphate (PIP2) also increased (24% and 36%, respectively); b) no change in other lipids; and c) water-soluble phosphodiesteratic degradation products, analyzed by high voltage paper electrophoresis, increased 24% in the amount of radiotracer recovered as inositol 1,4-bisphosphate (IP2) and by 44% in the amount recovered as inositol 1,4,5-trisphosphate (IP3). These data indicate that during experimental status epilepticus the cerebral inositide cycle is accelerated: PIP2----(IP3----IP2----IP----I) + DG----PA----PI----PIP----PIP2.

Ether phospholipids as inhibitors of the arachidonoyl-CoA: 1-acyl-sn-glycero-3-phosphocholine acyltransferase in macrophages

1-Alkylglycerophosphatide analogs which are known to activate macrophages to enhanced tumor cytotoxicity are structurally closely related to 1-acyl-sn-glycero-3-phosphocholine. In this study we have examined the influence of some of these compounds and of platelet-activating factor (PAF-acether, 1-0-alkyl-2-0-acetyl-sn-glycero-3-phosphocholine) on the arachidonoyl-CoA: 1-acyl-sn-glycero-3-phosphocholine acyltransferase (EC 2.3.1.23) in homogenate of bone-marrow-derived murine macrophages. This enzyme is suggested to be involved in the control of the availability of the icosanoid precursor, arachidonic acid. Kinetic experiments revealed apparent Km and V values for 1-palmitoyl-sn-glycero-3-phosphocholine of 6.0 microM and 16.10 nmol/mg protein per min, respectively. When the 1-palmitoyl-sn-glycero-3-phosphocholine concentration was equal to Km, the enzyme was dose-dependently inhibited by 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine with a 50% inhibition at 30 microM. The kinetic parameters in the presence of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (K'm = 10.0 microM, V' = 11.40 nmol X mg-1 X min-1) suggest that this alkyl phospholipid is a mixed-type inhibitor. All other alkyl analogs tested (1-O-methyl-2-O-octadecyl-rac-glycerol-3-phosphocholine, racemic PAF-acether, L-PAF-acether, D-1-O-hexadecyl-sn-glycero-3-phosphocholine, 1-O-octadecyl-rac-glycero-3-phosphocholine) inhibited the enzyme to various degrees. Arachidonic acid transfer to the 1-alkylglycerophosphatide analogs themselves could be ruled out under the assay conditions used. Therefore, we conclude that the arachidonoyl-CoA: 1-acyl-sn-glycero-3-phosphocholine acyltransferase can be inhibited by synthetic and naturally occurring ether phospholipids in homogenate of bone-marrow-derived murine macrophages.

Influences of inactivity and indomethacin on soleus phosphatidylethanolamine and size

Results of previous investigations indicate that muscular prostaglandins (PG) E2 and F2 alpha are biosynthesized during changes in muscular activity and stimulate protein degradation and synthesis, respectively. The results of the present investigation demonstrates a reduction by indomethacin of soleus hypertrophy, which occurred during a one week recovery period following rat hindlimb suspension. While this hypertrophic model evoked significant, rapid increases in soleus myosin and weight in control groups, continuous systemic release of indomethacin (3.5 mg/Kg/day) reduced the gain in soleus weight, Type I and II hypertrophy. Body weights of all groups were not significantly different. Additional analysis found a selective reduction in phosphatidylethanolamine (PE) during soleus atrophy and hypertrophy which were elicited by hindlimb suspension and recovery from suspension, respectively. Indomethacin enhanced PE recovery. Data from previous and present studies support a hypothesis that a change in soleus activity elicits PE catabolism, changes in cytosolic and muscular calcium, in calcium-dependent phospholipase activity, in PG biosynthesis, and in soleus size.

Metabolism of arachidonic acid by hamster trachea lack of stimulation by A23187

The metabolism of arachidonic acid has been studied using hamster trachea in short-term organ culture. To study endogenous substrate utilization, tissue lipids were labeled with [3H]-arachidonic acid, whereas exogenous substrate turnover was assessed by the addition of 100 microM [14C]-arachidonic acid to the medium. Both exogenous and endogenous arachidonate were converted primarily to 6-keto-PGF1 alpha and PGE2, with varying amounts of an unidentified non-polar product noted. Production of the prostanoids increased steadily with time up to 24 hours. No significant generation of lipoxygenase products was found. Release of incorporated labeled arachidonic acid was nearly linear with time, resulting in the transfer of about 10% of the total label into the medium after 24 hours. About 3% of the total label was converted to prostaglandins. In the presence of 10 microM A23187, release of label was increased by only 25 to 60% relative to the control. Analysis of labeled compounds in the medium showed that this increase resulted from increased release of unchanged arachidonic acid, and that the yield of oxygenated products was the same as from the control incubations.

Coenzyme A-mediated transacylation of sn-2 fatty acids from phosphatidylcholine in rat lung microsomes

Evidence was obtained for a CoA-dependent transfer of linoleate from rat lung microsomal phosphatidylcholine to lysophosphatidylethanolamine without the intervention of a Ca2+-requiring phospholipase A2 activity and ATP. To study this CoA-mediated transacylation process, microsomes were prepared in which the endogenous phosphatidylcholine was labeled by protein-catalyzed exchange with phosphatidylcholines containing labeled fatty acids in the sn-2-position. The apparent Km for CoA in the transfer of arachidonate from phosphatidylcholine to 1-acyllysophosphatidylethanolamine was 1.5 microM. At saturating lysophosphatidylethanolamine concentrations, the transacylation was linear with the amount of microsomal protein, i.e., a fixed percentage of the labeled fatty acid was transferred independent of the amount of microsomal protein. A maximal transfer of 12.2% for arachidonate and 2.0% for linoleate from the respective phosphatidylcholines to lysophosphatidylethanolamine was observed in 30 min. With 1-acyl-2-[1-14C]arachidonoylphosphatidylcholine as acyl donor, lysophosphatidylethanolamine was the best acceptor followed by lysophosphatidylglycerol and lysophosphatidylserine. Lysophosphatidate barely functioned as acceptor. These data provide further evidence for the widespread occurrence of CoA-mediated transacylation reactions. The arachidonate transacylation from phosphatidylcholine to other phospholipids in lung tissue may contribute to the low level of arachidonate in pulmonary phosphatidylcholine.

Elongation of arachidonic and eicosapentaenoic acids limits their availability for thrombin-stimulated release from the glycerolipids of vascular endothelial cells

This study has examined the thrombin-stimulated release of polyunsaturated fatty acids from endothelial glycerolipids. Human umbilical vein endothelial cells were incubated with 1.25 microM [14C]arachidonate or [14C]eicosapentaenoate and then exposed to thrombin in buffered saline plus albumin. After an incorporation period of 0.5 h, the thrombin-stimulated release of the two radiolabeled fatty acids was quite similar. By contrast, after 24 h of fatty acid incorporation, the thrombin-stimulated release of radiolabeled fatty acid from cells incubated with [14C]eicosapentaenoate was only 25-30% of that from cells with [14C]arachidonate. Analysis of cellular glycerolipids indicated that 23 and 72%, respectively, of the incorporated [14C]arachidonate and [14C]eicosapentaenoate had been elongated to 22-carbon fatty acids in 24 h. Both 20- and 22-carbon 14C-labeled fatty acids were released to albumin in the medium in control incubations. Addition of thrombin stimulated the release of [14C]arachidonate and [14C]eicosapentaenoate, but not of their respective elongation products. Furthermore, endothelial cells incorporated exogenous [14C]docosatetraenoate into cellular glycerolipids but did not release it in response to thrombin. Thus, the thrombin-stimulated release of polyunsaturated fatty acids from vascular endothelial cells is highly selective for arachidonate and eicosapentaenoate. These results suggest that the extensive elongation of eicosapentaenoate by these cells serves to remove n - 3 polyunsaturated fatty acids from the pool of cellular acyl groups which are released in response to thrombin and are thus made available for metabolism by cyclooxygenase and lipoxygenase enzymes.

Selective inhibition of human platelet phospholipase A2 by buffering cytoplasmic calcium with the fluorescent indicator quin 2. Evidence for different calcium sensitivities of phospholipases A2 and C

Human platelets labelled with either [14C]arachidonic acid or [32P]orthophosphate were loaded or not with the Ca2+ fluorescent indicator quin 2. They were then incubated in the presence or in the absence of human thrombin (1 U/ml) in a medium where Ca2+ concentration was adjusted near zero or to 1 mM. Under these conditions, phospholipase A2 activity, as detected by the release of [14C]arachidonate and of its metabolites, or by the hydrolysis of [14C]phosphatidylcholine, was severely impaired in quin 2-loaded platelets upon removal of external Ca2+. However, Ca2+ was not required in non-loaded platelets, where a maximal phospholipase A2 activity was detected in the absence of external Ca2+. In contrast, phospholipase C action, as determined from the amounts of [14C]diacylglycerol, [14C]- or [32P]phosphatidic acid formed, appeared to be much less sensitive to the effects of quin 2 loading and of Ca2+ omission. By using various concentrations of quin 2, it was found that the inhibitory effect exerted against phospholipase A2 could be overcome by external Ca2+ only when the intracellular concentration of the calcium chelator did not exceed 2 mM. At higher concentrations averaging 3.5 mM of quin 2, phospholipase A2 activity was fully suppressed even in the presence of external Ca2+, whereas phospholipase C was still active, although partly inhibited. It is concluded that platelet phospholipase A2 requires higher Ca2+ concentrations than phospholipase C to display a maximal activity. By comparing platelet phospholipase A2 activity under various conditions with the values of cytoplasmic free Ca2+ as detected by quin 2 fluorescence, it is proposed that cytoplasmic free Ca2+ in control platelets stimulated with thrombin can attain concentrations above 1 microM, probably close to 5-10 microM, as recently determined with the photoprotein aequorin (Johnson, P.C., Ware, J.A., Cliveden, P.B., Smith, M., Dvorak, A.M. and Salzman, E.W. (1985) J. Biol. Chem. 260, 2069-2076).

Arachidonate mobilization in diacyl, alkylacyl and alkenylacyl phospholipids on stimulation of rat platelets by thrombin and the Ca2+ ionophore A23187

Platelet stimulation by thrombin or Ca2+ ionophore induces mobilization of arachidonate from lipid stores. We have previously shown that, in [14C]arachidonic acid-prelabelled resting platelets, [14C]arachidonate was transferred from diacyl-sn-glycerophosphocholine to ethanolamine and choline-containing ether phospholipids. This transfer reached an equilibrium after 5 h incubation [Colard, Breton & Bereziat (1984a) Biochem. J. 222, 657-662]. [14C]Arachidonate-prelabelled platelets having reached this transfer equilibrium were used to study the mobilization of arachidonate in etheracyl and diacyl phospholipids. Upon thrombin stimulation, arachidonate decreased in diacyl-sn-glycero-3-phosphoinositol, in alkylacyl- and diacyl-sn-glycero-3-phosphocholine and increased in alkenylacyl- and diacyl-sn-glycero-3-phosphoethanolamine. Upon challenge with Ca2+ ionophore A23187, arachidonate decreased in diacyl-sn-glycero-3-phosphoethanolamine, in diacyl- and alkylacyl-sn-glycero-3-phosphocholine and increased in alkenylacyl-sn-glycero-3-phosphoethanolamine. We also compared arachidonate mobilization in platelets stimulated immediately after [14C]arachidonic acid chase with platelets stimulated after 5 h reincubation. We observed that the arachidonate newly incorporated into diacyl-sn-glycero-3-phosphocholine and triacylglycerols was rapidly released upon stimulation. This suggests the presence in these two lipids of a rapidly-turning-over arachidonate pool.

Basal phosphatidylinositol turnover controls aortic Na+/K+ ATPase activity

To determine whether basal phosphoinositide turnover plays a role in metabolic regulation in resting rabbit aortic intima-media incubated under steady state conditions, we used deprivation of extracellular myo-inositol as a potential means of inhibiting basal phosphatidylinositol (PI) synthesis at restricted sites and of depleting small phosphoinositide pools with a rapid basal turnover. Medium myo-inositol in a normal plasma level was required to prevent inhibition of a specific component of basal de novo PI synthesis that is necessary to demonstrate a discrete rapidly turning-over [1,3-14C]glycerol-labeled PI pool. Medium myo-inositol was also required to label the discrete PI pool with [1-14C]arachidonic acid (AA). The rapid basal turnover of this PI pool, when labeled with glycerol or AA, was not attributable to its utilization for polyphosphoinositide formation, and it seems to reflect basal PI hydrolysis. Depleting endogenous free AA with medium defatted albumin selectively inhibits the component of basal de novo PI synthesis that replenishes the rapidly turning-over PI pool. A component of normal resting energy utilization in aortic intima-media also specifically requires medium myo-inositol in a normal plasma level and a free AA pool; its magnitude is unaltered by indomethacin, nordihydroguaiaretic acid, or Ca2+-free medium. This energy utilization results primarily from Na+/K+ ATPase activity (ouabain-inhibitable O2 consumption), and in Ca2+-free medium deprivation of medium myo-inositol or of free AA inhibits resting Na+/K+ ATPase activity to a similar degree (60%, 52%). In aortic intima-media basal PI turnover controls a major fraction of resting Na+/K+ ATPase activity.

Fast atom bombardment analysis of arachidonic acid-containing phosphatidylcholine molecular species

Fast atom bombardment (FAB) mass spectrometry was evaluated as a means for the characterization of molecular species of glycerophosphocholines (GPC) from HL60 cells. Previous reports of phospholipid molecular species analysis have suggested that several inherent problems with FAB could limit its analytical usefulness in such an application. The GPC-related secondary ions produced by the FAB experiment were found to be dependent on the matrix employed. Triethanolamine was found to minimize mass-related discrimination in ion emission when mixtures were studied; and furthermore, this matrix maintained 60% maximal ion current after 12 minutes as compared to a glycerol matrix which diminished to 10% maximal ion current. Using triethanolamine, the major GPC species in HL60 cells prelabeled with (2H8) arachidonic acid were found to be 16:0a16:1, 16:0e/18:1, 16:0a/18:1, 18:1a/18:1 and 18:0a/18:1. It was possible to identify the minor GPC species containing arachidonic acid only after partial purification by reverse-phase high-performance liquid chromatography. Comparison of the 2H8/2H0 enrichment data estimated by FAB with data obtained by gas chromatographic/mass spectral analysis of arachidonic acid following GPC hydrolysis revealed that the precision of FAB was less precise than gas chromatography/mass spectrometry. Yet the FAB technique did allow the observation of one unexpected molecular species (18:1a/20:4) due to the fact that the GPC was not degraded to simpler species prior to analysis. In this respect, the two strategies of molecular species analysis complement each other.

Inhibition by ethanol and mepacrine of phospholipase-dependent prostaglandin release from the isolated perfused rat lung

To investigate the possibility that millimolar concentrations of ethanol have a membrane-directed inhibitory effect on phospholipase A2 and prostanoid generation (suggested from previous platelet experiments), we studied the release of prostacyclin, thromboxane A2 and prostaglandin E2 from isolated perfused rat lung. Prostanoid release was evoked by arachidonic acid, bradykinin and ionophore A23187 and was measured after extraction by radioimmunoassay. In these experiments, prostanoid release is dependent upon biosynthesis from fatty acid precursors as there is no endogenous prostanoid storage pool. Arachidonic acid and bradykinin caused enhanced release of more prostacyclin than thromboxane A2 with much less prostaglandin E2 and no detectable prostaglandin F2 alpha, whereas A23187 released equal proportions of prostacyclin and thromboxane A2 with less prostaglandin E2. Ethanol at 50 mM resembled mepacrine (46 microM) in that prostanoid release in response to bradykinin and A23187 was highly significantly reduced with little effect on release induced by arachidonic acid. We suggest that ethanol, like mepacrine, interferes with prostaglandin generation by an action at the phospholipase step. This may be secondary to a physical effect on membrane configuration.

Formation of diacyl- and alkylacylphosphatidylcholine by the membranes of human platelets

We have investigated the distribution and fatty acid preference of two acyl-CoA transferase activities in a human platelet mixed membrane fraction and in well-characterised surface and intracellular membrane subfractions prepared from it by high-voltage free-flow electrophoresis. One transferase inserts long-chain unsaturated fatty acids into 1-acyllysophosphatidylcholine (1-acyl-LPC) and the other into lyso-platelet-activating factor (LPAF). Both transferase activities were approx. 4-fold enriched in the intracellular membranes with respect to their specific activities in the mixed membranes. The surface membrane activities were correspondingly depleted. Using 1-acyl-LPC as the acceptor, all the intracellular membrane preparations showed transferase preference for the CoA ester of 8,11,14-eicosatrienoic acid. In contrast when LPAF was the acceptor the CoA esters of linoleic and arachidonic acid were the preferred donors.

The role of polyunsaturated fatty acids in fish

The physical properties of polyunsaturated and saturated fatty acids are compared in relation to melting points and fluidity. The role of polyunsaturated fatty acids on membrane fluidity and membrane bound enzyme activity is discussed. The influence of the environment, particularly temperature, on poikilothermic animals is considered in relation to membrane fatty acid composition and metabolism. The metabolic role of polyunsaturated fatty acids of the (n-3) series and their interaction with arachidonate metabolism is discussed.

Mechanisms of action of gonadotropin releasing hormone on rat granulosa cells

To elucidate the mechanisms of stimulatory actions of GnRH on rat granulosa cells (GC), we have compared the actions of a GnRH agonist with those of a tumor-promoting phorbol ester, 12-0-tetradecanoylphorbol 13-acetate (TPA) and Ca+2 ionophore, A23187. GC were obtained from immature (28-29 days old) rats 48 h after injection of 20 IU PMSG. Following prelabeling with 3[H]arachidonic acid (AA), the cells were incubated with the test substances for 10 min and AA release determined. A GnRH agonist, [D-Ala6, des-Gly-NH2(10)] GnRH ethylamide (GnRHa; 10 ng/ml) increased AA release 175% compared to the control value. AA release in the presence of GnRHa was larger than that due to 1 microM A23187 or 40 nM TPA alone. A23187 or TPA increased GnRHa-stimulated AA release further. GC were incubated with the test substances for longer time periods, i.e., up to 5 h. GnRHa caused a 4-fold increase in prostaglandin (PG) synthase activity at 5 h. GnRHa increased PGE accumulation to the same extent as TPA, but only increased PG synthase activity about half as much. In combination with TPA, GnRHa had no influence on TPA-stimulated PG synthase activity, but increased PGE accumulation to levels comparable to those with A23187 plus TPA. GnRHa caused a 2.5 fold increase in progesterone (P) accumulation, which was the same as TPA. P accumulation in the presence of GnRHa was affected by neither A23187 nor TPA. These data indicate that the combination of TPA and A23187 can substitute for GnRH action on PGE and P accumulation in rat GC.

Mode of action of erythropoietin and glucocorticoids on the hepatic erythroid precursor cells: role of prostaglandins

The hypothesis that prostaglandins, and especially PGE2, are the second messengers of erythropoietin (Ep) and that glucocorticoids inhibit Ep action by inhibiting PG synthesis was tested on the erythroid cell line from fetal rat liver. The optimal (10(-9) M) stimulatory concentration of PGE2 did not reproduce, by far, the maximal effect of Ep on the growth of CFUE erythroid colonies. Ep did not increase PGE2 release in liquid culture media of cell suspensions made of the whole erythroid line or enriched (over 85%) in precursor cells. Ep did not modify the turnover rate of arachidonate. Nevertheless, indomethacin partially inhibited Ep effect on CFUE development, and this inhibition was abolished by PGE2. These results suggest that PGE2 potentiates Ep action but is not its second messenger. Spontaneous PGE2 release in liquid culture media brought about concentrations of the order of 10(-9) M, and 10(-7) M dexamethasone completely inhibited this release. Part of (but not all) the anti-Ep effects of glucocorticoids might thus be mediated this way. Dexamethasone effects required previous protein synthesis.

Arachidonic acid metabolism in human neutrophils: lack of effect of cyclosporine A

The metabolism of arachidonic acid (AA) in human neutrophils was studied by incorporation of 1-14C-AA, removal of excess 1-14C-AA, and stimulation of radiolabelled cells with A23187. Radiolabelled lipids were quantitated by extraction, thin-layer chromatography, autoradiography, and laser densitometry. Following 5 h of incubation with 1-14C-AA, the maximum amount of radioactivity was located in triglycerides, 70%, and phospholipids, 30%. Activation of the cells with calcium ionophore A23187 led to release of free AA, 58%, whereas AA-metabolites revealed mainly lipoxygenase (arachidonate 5 lipoxygenase, E.C. 1. 13. 11. 34) activity, 5-HETE 13%, LTB4 5%, with only small amounts of cyclooxygenase (prostaglandin synthase, E.C. 1. 14. 99. 1) metabolites, HHT 2%. Intra-assay coefficient of variation for release of metabolites was approximately 15%. A potent immunosuppressive agent, cyclosporine A (CS-A) was shown to be without any effect in AA-release and metabolism. This method is applicable to studies of both basic cell function in human disease and to further immunopharmacological investigations.

Involvement of acyl exchange between acyl-CoA and phosphatidylcholine in the remodelling of phosphatidylcholine in microsomal preparations of rat lung

Microsomal membrane preparations from rat lung catalyse the incorporation of radioactive linolenic acid from [14C]linolenoyl-CoA into position 2 of sn-phosphatidylcholine. The incorporation was stimulated by bovine serum albumin and free CoA. Free fatty acids in the incubation mixtures were not utilised in the incorporation into complex lipids. Fatty acids were transferred to the acyl-CoA pool during the incorporation of linolenic acid into phosphatidylcholine. An increase in lysophosphatidylcholine occurred in incubations containing both bovine serum albumin and free CoA and in the absence of acyl-CoA. The results were consistent with an acyl-CoA: lysophosphatidylcholine acyltransferase operating in both a forwards and backwards direction and thus catalysing the acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine. In incubations with mixed species of acyl-CoAs, palmitic acid was the major fatty acid substrate transferred to phosphatidylcholine in acyl exchange, whereas this acid was completely selected against in the acylation of added lysophosphatidylcholine. The selectivity for palmitoyl-CoA was particularly enhanced when the mixed acyl-CoA substrate was presented to the microsomes in molar concentrations equivalent to the molar ratios of the fatty acids in position 2 of sn-phosphatidylcholine. During acyl exchange, the predominant fatty acid transferred to phosphatidylcholine from acyl-CoA was palmitic acid, whereas arachidonic acid was particularly selected for in the reverse reaction from phosphatidylcholine to acyl-CoA. A hypothesis is presented to explain the differential selectivity for acyl species between the forward and backward reactions of the acyltransferase that is based upon different affinities of the enzyme for substrates at high and low concentrations of acyl donor. Acyl exchange between acyl-CoA and phosphatidylcholine offers, therefore, a possible mechanism for the acyl-remodelling of phosphatidylcholine for the production of lung surfactant.

Release of prostaglandins E and F from rat uterine strips incubated in depolarizing and hyperpolarizing solutions

The purpose of this study was investigate the effect of depolarizing (high K+, procaine 5 mM, high K+-Ca2+ free) or hyperpolarizing (K+ free) solutions on the generation and release of prostaglandins (PGs) E and F from the rat uterus. The output of PGs in the bathing solution was measured by bioassay. Results are expressed as ng/g dry weight/min. Under basal conditions in Krebs Ringer Bicarbonate medium, the total output of PGs (E + F) was 12.8 +/- 0.9, 20.5 +/- 2.5 and 39.1 +/- 4.3 ng/dw/min at 0-60, 60-80, 80-100 min intervals respectively, the major proportion being PGF like material in all cases. When the strips were bathed in depolarizing solution the total output increased significantly PGE being the most augmented. However, when K+ free solution was used there was a marked reduction in prostaglandins release as compared with basal conditions or depolarizing media. These results can not be attributed to osmotic effect, muscular work or neurotransmitter release from nerve endings. A close relationship between the variations in membrane potential and prostaglandins release seems to indicate that both parameters might correlate well.

Effect of contrast media on the formation of prostacyclin in isolated rat lungs

The synthesis of prostacyclin (PGI2) was studied in isolated perfused rat lungs during the infusion of radiographic contrast media into the pulmonary circulation. At the same molar concentration, diatrizoate, iopamidol, and NaCl fairly equally stimulated the generation of PGI2. A bolus injection of histamine also enhanced the formation of PGI2. A high dose of ionic diatrizoate and hypertonic saline (0.4 mol/l) caused considerable pulmonary edema, which was less marked with non-ionic iopamidol. Experiments with 125I-labeled contrast media indicated rapid efflux of contrast media from the lungs. The present investigation indicates that different contrast media stimulate the synthesis of prostacyclin mainly because of chemical irritation of the pulmonary endothelium. The enhanced formation of endothelium-derived prostacyclin may mediate some systemic and local side effects seen temporarily during intravascular contrast medium examinations.

Secretion of phospholipase A1 by bone marrow-derived macrophages

Bone marrow-derived macrophages contain phospholipase activity of the A1 and A2 types, active at acid or neutral pH and with different specificities for the fatty acid to be liberated. In contrast to this variety, only one single phospholipase could be detected in extracellular fluids of these cells. Surprisingly, this phospholipase was of the A1 type and active at about pH 8. It exhibited a restricted substrate specificity in that, of the various substrates tested, only phosphatidylcholine containing palmitic acid in position 2 was degraded. This total restriction was not detected with phosphatidylethanolamine substrates. In addition to phospholipase A1, extracellular fluids exhibited lipase activity. A modulation of enzyme secretion could not be achieved by lymphokines or phorbol esters. However, release could be blocked by treating cells with cycloheximide (5 micrograms/ml) or tunicamycin (0.5 micrograms/ml). Phospholipase A1 was also released by thioglycollate-induced peritoneal macrophages.

The effect of angiotensin II on arachidonate metabolism in adrenal glomerulosa cells

The effect of angiotensin II on arachidonate metabolism was examined in rat adrenal glomerulosa cells. Incorporation of both [3H]arachidonate and [32P]phosphate into phosphatidylinositol (PI) were significantly stimulated by angiotensin II. These effects were abolished by lithium, a cation, which was found suitable to prevent increased synthesis of PI in our previous study (T. Balla et al., FEBS Letters 171, 179, 1984). On the other hand, the phospholipase A2 inhibitor mepacrine failed to inhibit the increased labelling of PI. These observations suggest that the increased 3H labelling of PI occurs via CDP-diacylglycerol, and not via enhanced deacylation-reacylation cycle. The validity of this assumption was further supported, since angiotensin II failed to stimulate the formation of lyso-PI, as examined by both [32P]phosphate incorporation and pulse-chase techniques. Angiotensin II decreased the incorporation of [3H]arachidonate into phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Considering that we did not find arachidonate release either from phospholipids or from other possible arachidonate sources this decrease may not be due to dilution of the tracer. Thus we assume that angiotensin II may induce a shift in phospholipid synthesis from PC and PE to phosphoinositides. These observations indicate that the enhanced hydrolysis and synthesis of PI in response to angiotensin II is not associated with increased phospholipase A2 activity in adrenal glomerulosa cells.

Purification and characterization of a phosphatidylinositol transfer protein from human platelets

We report the purification of a phospholipid transfer protein from human platelets. This protein preferentially transfers phosphatidylinositol, with phosphatidylcholine and phosphatidylglycerol being transferred to a lesser extent. Phosphatidylethanolamine is not transferred. Transfer activity is detected by measuring the transfer of radiolabeled phospholipids between two populations of small unilamellar vesicles. The protein was purified approximately 1000-fold over the platelet cytosol by chromatography on Sephadex G-75, sulfooxyethyl cellulose, and hydroxylapatite. The molecular weight of this protein appears to be 28 000 as determined by gel filtration chromatography. When the purified protein is analyzed on sodium dodecyl sulfate-polyacrylamide gels, two major components and several minor ones are observed. The molecular weight of the two major bands are 28 600 and 29 200. Isoelectric focusing of the platelet cytosol yielded phosphatidylinositol and phosphatidylcholine transfer activity at pH 5.6 and 5.9. The platelet phospholipid transfer protein is able to catalyze the transfer of phosphatidylinositol and phosphatidylcholine between vesicles and human platelet plasma membranes. One possible physiological role for this transfer protein is an involvement in the rapid turnover of inositol-containing lipids which occurs upon exposure of platelets to various stimuli.

Studies on the enzymatic pathways of calcium ionophore-induced phospholipid degradation and arachidonic acid mobilization in peritoneal macrophages

Exposure of mouse peritoneal macrophages to ionophore A23187 caused a rapid and extensive Ca2+-dependent phospholipid degradation and mobilization of arachidonic acid. Phosphatidylinositol, phosphatidylcholine and phosphatidylethanolamine all contributed to the arachidonic acid release, although the ethanolamine phospholipids incorporated [3H]arachidonic acid more slowly during the prelabeling period, particularly the plasmalogen form. Several enzymatic pathways could be positively identified as contributing to the ionophore-induced phospholipid degradation by the use of several different radiolabeled phospholipid precursors: (i) a phospholipase A-mediated deacylation, (ii) a phosphodiesterase (phospholipase C) reaction, rapidly generating diacylglycerol units from inositol phospholipids, and (iii) enzymatic processes generating diacylglycerol and CDP- and phosphocholine/ethanolamine from phosphatidylcholine/ethanolamine. The diacylglycerol formed was in part phosphorylated and in part hydrolyzed to monoacylglycerol, with retention of its arachidonic acid. These, and other, results indicate that the Ca2+-ionophore activates several apparently distinct phospholipid-degrading processes, in contrast to stimuli acting via cellular receptors.

Phospholipase A activities in embryonic palate mesenchyme cells in vitro

Incubation of palate mesenchyme cells in vitro in the presence of [3H]arachidonate and [14C]palmitate resulted in incorporation of radiolabel into all major families of phospholipids. Almost all (more than 90%) of the 3H and about one-half of the 14C in the phospholipids were in the sn-2 position. The [14C] fatty acid in the sn-2 position was saturate. When [14C]stearate was added to the culture medium with [3H]arachidonate, 30-40% of the total 14C in phospholipids was in the sn-2 position. 80% of the 14C in the sn-2 position was found in unsaturated fatty acids. Hydrolysis of phospholipids could be demonstrated at acid, neutral and alkaline pH. Calcium stimulated phospholipase activity at neutral and alkaline pH, but inhibited hydrolytic activity at acid pH. Radiolabeled lysophospholipid indicative of phospholipase A2 activities accumulated at acid pH, whereas little, if any, radiolabeled lysophospholipids accumulated at neutral and alkaline pH. Quantitative analysis revealed the production of some lysophosphatidylethanolamine at alkaline pH.

The inhibition of diacylglycerol-stimulated intracellular phospholipases by phospholipids with a phosphocholine-containing polar group. A possible physiological role for sphingomyelin

Phosphatidylinositol phosphodiesterase activated by diacylglycerol is substantially inhibited by all phospholipids containing a phosphocholine head group, including phosphatidylcholine, hydrogenated phosphatidylcholine, choline plasmalogen, lysophosphatidylcholine, lysocholine plasmalogen, sphingomyelin and sphingosylphosphocholine. The sphingosine-containing phospholipids are the most inhibitory. Phosphatidic acid does not inhibit, and phosphatidylethanolamine activates the hydrolysis still further. Sphingomyelin is highly inhibitory to a diacylglycerol-stimulated intestinal mucosal phospholipase A2, or a liver lysosomal phospholipase A1 + A2, both hydrolysing a phosphatidylcholine substrate. Sphingomyelin [20% molar (20 mol of sphingomyelin/80 mol of phosphatidylethanolamine)] activates phosphatidylethanolamine hydrolysis by intestinal mucosal phospholipase A2, and then at higher concentrations (40% molar) substantially inhibits the activity. The results are discussed in relation to possible molecular reorganizations brought about in the hydrated phospholipid substrate complex, and in particular the possible stabilizing role of sphingomyelin in the maintenance of membrane structure, and hence in the modulation of phospholipase activity.