Contributing factors in the trafficking of [3H]arachidonate between phospholipids

Phospholipases A₁

Phospholipase A(1) (PLA(1)) is an enzyme that hydrolyzes phospholipids and produces 2-acyl-lysophospholipids and fatty acids. This lipolytic activity is conserved in a wide range of organisms but is carried out by a diverse set of PLA(1) enzymes. Where their function is known, PLA(1)s have been shown to act as digestive enzymes, possess central roles in membrane maintenance and remodeling, or regulate important cellular mechanisms by the production of various lysophospholipid mediators, such as lysophosphatidylserine and lysophosphatidic acid, which in turn have multiple biological functions.

Influence of cellular arachidonic acid levels on phospholipid remodeling and CoA-independent transacylase activity in human monocytes and U937 cells

The availability of free arachidonic acid (AA) constitutes a limiting step in the synthesis of biologically active eicosanoids. Free AA levels in cells are regulated by a deacylation/reacylation cycle of membrane phospholipids, the so-called Lands cycle, as well as by further remodeling reactions catalyzed by CoA-independent transacylase. In this work, we have comparatively investigated the process of AA incorporation into and remodeling between the various phospholipid classes of human monocytes and monocyte-like U937 cells. AA incorporation into phospholipids was similar in both cell types, but a marked difference in the rate of remodeling was appreciated. U937 cells remodeled AA at a much faster rate than human monocytes. This difference was found not to be related to the differentiation state of the U937 cells, but rather to the low levels of esterified arachidonate found in U937 cells compared to human monocytes. Incubating the U937 cells in AA-rich media increased the cellular content of this fatty acid and led to a substantial decrease of the rate of phospholipid AA remodeling, which was due to reduced CoA-independent transacylase activity. Collectively, these findings provide the first evidence that cellular AA levels determine the amount of CoA-independent transacylase activity expressed by cells and provide support to the notion that CoA-IT is a major regulator of AA metabolism in human monocytes.

Blockade of arachidonic acid incorporation into phospholipids induces apoptosis in U937 promonocytic cells

Arachidonic acid (AA) participates in a reacylation/deacylation cycle of membrane phospholipids, the so-called Lands cycle, that serves to keep the concentration of this free fatty acid in cells at a very low level. To manipulate the intracellular AA level in U937 phagocytes, we have used several pharmacological strategies to interfere with the Lands cycle. We used inhibitors of the AA reacylation pathway, namely thimerosal and triacsin C, which block the conversion of AA into arachidonoyl-CoA, and a CoA-independent transacylase inhibitor that blocks the movement of AA within phospholipids. In addition, we used cells overexpressing group VIA phospholipase A(2), an enzyme with key roles in controlling basal fatty acid deacylation reactions in phagocytic cells. All of these different strategies resulted in the expected increase of cellular free AA but also in the induction of cell death by apoptosis. Moreover, when used in combination with any of the aforementioned drugs, AA itself was able to induce apoptosis at doses as low as 10 muM. Blocking cyclooxygenase or lipoxygenases had no effect on the induction of apoptosis by AA. Collectively, these results indicate that free AA levels within the cells may provide an important cellular signal for the onset of apoptosis and that perturbations of the mechanisms controlling AA reacylation, and hence free AA availability, may decisively affect cell survival.

Roles of various phospholipases A2 in providing lysophospholipid acceptors for fatty acid phospholipid incorporation and remodelling

In the present study the lysophospholipid sources for arachidonic (AA) and eicosapentaenoic acid (EPA) incorporation into and redistribution within the phospholipids of phorbol-ester-differentiated U937 cells was investigated. Initially, AA incorporated primarily into choline glycerophospholipids (PC), whereas EPA incorporated mainly into ethanolamine glycerophospholipids (PE). Bromoenol lactone (BEL), an inhibitor of the Group VI Ca2+-independent phospholipase A2 (iPLA2), diminished both lysophosphatidylcholine levels and the incorporation of AA into phospholipids. However BEL had little effect on EPA incorporation. In concanavalin A-activated cells, EPA, but not AA, incorporation was also affected by methyl arachidonyl fluorophosphonate (MAFP), suggesting an additional role for the group IV cytosolic phospholipase A2. In the activated cells AA and EPA did not compete with each other for incorporation, indicating that the pathways for AA and EPA incorporation are partially different. The AA and EPA initially incorporated into PC slowly moved to PE in a process that took several hours. The transfer of AA and EPA from PC to PE was not inhibited by BEL, MAFP or LY311727 [3-(3-acetamide 1-benzyl-2-ethylindolyl-5-oxy)propanesulphonic acid], raising the possibility that an as-yet-undetermined phospholipase A2 may be involved in fatty acid phospholipid remodelling. A strong candidate to be involved in these reactions is a novel Ca2+-independent phospholipase A2 that, unlike all known iPLA2s, is resistant to inhibition by BEL and also to MAFP and LY311727. The enzyme activity cleaves both PC and PE and is thus able to provide the lysoPC and lysoPE acceptors required for the fatty acid acylation reactions.

Delta6-, Stearoyl CoA-, and Delta5-desaturase enzymes are expressed in beta-cells and are altered by increases in exogenous PUFA concentrations

In the evolution of Type II diabetes, an initial period of hyper-fatty acidemia leads to an insulin secretory defect which triggers overt hyperglycemia and frank diabetes. The mechanism by which elevated free fatty acids contribute to beta-cell dysfunction, however, is not clearly understood. We recently reported that arachidonic acid (20:4) or linoleic acid (18:2) supplementations result in increases in abundances of long chain polyunsaturated fatty acids in INS-1 beta-cell membrane lipids, suggesting that beta-cells express desaturases that catalyze generation of unsaturated fatty acids. As expression of desaturases by beta-cells has not yet been addressed, we initiated studies to examine this issue using INS-1 beta-cells and find that they express messages for the Delta6-, stearoyl CoA-, and Delta5-desaturase. Supplementation of the INS-1 beta-cells with arachidonic acid leads to decreased expression of all three desaturases, presumably in response to the decreased need for endogenous generation of unsaturated fatty acids. In contrast, linoleic acid supplementation promoted minimal changes in the three desaturases. These findings demonstrate for the first time that beta-cells express regulatable desaturases. Additionally, reverse transcriptase-polymerase chain reaction analyses reveal expression of the desaturases in native pancreatic islets. It might be speculated that long-term elevations in fatty acids can also adversely influence desaturase activity in beta-cells and affect PUFA composition in beta-cell membranes contributing to beta-cell membrane structural abnormalities and altered secretory function.

Involvement of calcium-independent phospholipase A2 in uterine stromal cell phospholipid remodelling

The role of Ca2+-independent phospholipase A2 (iPLA2) in arachidonic (AA) and docosahexaenoic (DHA) acid incorporation and phospholipid remodelling in rat uterine stromal cells (UIII cells) was studied. Incorporation of AA and DHA into UIII cell phospholipids was Ca2+-independent. Bromoenollactone (BEL), a potent inhibitor of iPLA2, reduced lysophosphatidylcholine level and AA incorporation into phospholipids by approximately 20%. DHA incorporation was not affected by BEL, indicating that the pathways for AA and DHA incorporation are partially different. In control cells, the transfer of AA occurred mainly from diacyl-glycerophosphocholine (GroPCho) to alkenylacyl-glycerophosphoethanolamine (GroPEtn) and to a lesser extent from diacyl-GroPCho to diacyl-GroPEtn. [3H]DHA was redistributed from diacyl-GroPCho and alkylacyl-GroPEtn to alkenylacyl-GroPEtn. BEL treatment inhibited completely the redistributrion of AA within diacyl-GroPCho and diacyl -GroPEtn and reduced the [3H]DHA content of diacyl-GroPEtn, indicating that a BEL-sensitive iPLA2 controls the redistribution of polyunsaturated fatty acids to diacyl-GroPEtn. In contrast the redistribution of radioactive AA and DHA to alkenylacyl-GroPEtn was almost insensitive to BEL. The analysis of substrate specificity and BEL sensitivity of iPLA2 activity indicates that UIII cells exhibit at least two isoforms of iPLA2, one of which is BEL-sensitive and quite selective of diacyl species, and another one that is insensitive to BEL and selective for alkenylacyl-GroPEtn. Taken together, these results suggest that several iPLA2 participate independently in the remodelling of UIII cell phospholipids.

Calcium dependency of arachidonic acid incorporation into cellular phospholipids of different cell types

Ca2+ -independent phospholipase A2 (iPLA2) is involved in the incorporation of arachidonic acid (AA) into resting macrophages by the generation of the lysophospholipid acceptor. The role of iPLA2 in AA remodeling in different cells was evaluated by studying the Ca2+ dependency of AA uptake from the medium, the incorporation into cellular phospholipids, and the effect of the iPLA2 inhibitor bromoenol lactone on these events. Uptake and esterification of AA into phospholipids were not affected by Ca2+ depletion in human polymorphonuclear neutrophils and rat fibroblasts. The uptake was Ca2+ independent in chick embryo glial cells, but the incorporation into phospholipids was partially dependent on extracellular Ca2+. Both events were fully dependent on extra and intracellular Ca2+ in human platelets. In human polymorphonuclear neutrophils, the kinetics of incorporation in several isospecies of phospholipids was not affected by the absence of Ca2+ at short times (<30 min). The involvement of iPLA2 in the incorporation of AA from the medium was confirmed by the selective inhibition of this enzyme with bromoenol lactone, which reduced < or =50% of the incorporation of AA into phospholipids of human neutrophils. These data provide evidence that suggests iPLA2 plays a major role in regulating AA turnover in different cell types.

Studies of the role of group VI phospholipase A2 in fatty acid incorporation, phospholipid remodeling, lysophosphatidylcholine generation, and secretagogue-induced arachidonic acid release in pancreatic islets and insulinoma cells

An 84-kDa group VI phospholipase A2 (iPLA2) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet beta-cells. A housekeeping role for iPLA2 in generating lysophosphatidylcholine (LPC) acceptors for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA2 inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet beta-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA2 in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA2 with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [3H]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and BEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [14C]glucose incorporation into phospholipids. BEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA2 did not alter the phospholipid head-group classes into which [3H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.

Perturbations in the control of cellular arachidonic acid levels block cell growth and induce apoptosis in HL-60 cells

Our previous studies demonstrated that inhibitors of arachidonate-phospholipid remodeling [i.e. the enzyme CoA-independent transacylase (CoA-IT)] decrease cell proliferation and induce apoptosis in neoplastic cells. The goal of the current study was to elucidate the molecular events associated with arachidonate-phospholipid remodeling that influence cell proliferation and survival. Initial experiments revealed the essential nature of cellular arachidonate to the signaling process by demonstrating that HL-60 cells depleted of arachidonate were more resistant to apoptosis induced by CoA-IT inhibition. In cells treated with CoA-IT inhibitors a marked increase in free arachidonic acid and AA-containing triglycerides were measured. TG enrichment was likely due to acylation of arachidonic acid into diglycerides and triglycerides via de novo glycerolipid biosynthesis. To determine the potential of free fatty acids to affect cell proliferation, HL-60 cells were incubated with varying concentrations of free fatty acids; exogenously provided 20-carbon polyunsaturated fatty acids caused a dose-dependent inhibition of cell proliferation, whereas oleic acid was without effect. Blocking 5-lipoxygenase or cyclooxygenases had no effect on the inhibition of cell proliferation induced by arachidonic acid or CoA-IT inhibitors. An increase in cell-associated ceramides (mainly in the 16:0-ceramide fraction) was measured in cells exposed to free arachidonic acid or to CoA-IT inhibitors. This study, in conjunction with other recent studies, suggests that perturbations in the control of cellular arachidonic acid levels affect cell proliferation and survival.

Different contribution of phospholipid and triacylglycerol metabolism to esterification of free intracellular arachidonate: a study on SK-N-BE(2) human neuroblastoma cells

When SK-N-BE(2) human neuroblastoma cells were exposed for 1h to growth medium supplemented with [14C]arachidonic acid (AA) at final concentrations ranging from 1 microM to 100 microM, an amount of this fatty acid was uptaken ranging form a 2% to a 120% of that present in cells at steady state. As more [14C]AA was uptaken by cells, a larger fraction was progressively incorporated into triacylglycerols (TAG) in comparison to phospholipids (PL), with minor amounts remaining in a free form. By gas chromatographic analysis it was estimated that TAG from cells grown in ordinary medium contained about 2 nmoles AA per mg protein, but, after 1 h exposure to medium supplemented with 100 microM AA (label-free) this value rose to about 28 nmoles/mg protein; furthermore, as estimated on the basis of total fatty acid content, TAG mass was increased by a 16%. Cell exposure to medium enriched with 100 microM AA did not cause PL mass changes, whereas AA content was significantly increased only in phosphatidylcholine. Medium enrichment with 100 microM AA dramatically enhanced [3H]glycerol incorporation into TAG, as assessed after 1 h cell pulse, with minor but significant changes observed also for phosphatidylinositol and phosphatidylethanolamine, but not for phosphatidylcholine. In the light of these data, the contribution of PL and TAG to the removal of free intracellular AA is discussed.

Bromoenol lactone inhibits magnesium-dependent phosphatidate phosphohydrolase and blocks triacylglycerol biosynthesis in mouse P388D1 macrophages

Bromoenol lactone (BEL) has previously been identified as a potent, irreversible, mechanism-based phospholipase A2 (PLA2) inhibitor that possesses greater than 1000-fold selectivity for inhibition of Ca2+-independent PLA2 (iPLA2) versus the Ca2+-dependent ones. Thus, this compound has been used as a selective tool for studies aimed at elucidating the role of iPLA2 in certain cellular functions. Herein we report that BEL also inhibits cellular phosphatidic acid phosphohydrolase (PAP) activity in intact P388D1 macrophages with an IC50 of about 8 microM, which is very similar to that previously found for inhibition of iPLA2 under the same experimental conditions. This results in the blockage of the incorporation of exogenous arachidonate and palmitate into diacylglycerol and triacylglycerol. Thus, inhibition of PAP by BEL blocks triacylglycerol biosynthesis in P388D1 cells due to decreased diacylglycerol availability. Because two forms of PAP activity exist in mammalian cells, differential assays were performed to identify which of these forms was inhibited by BEL. The results of these experiments revealed that BEL selectively inhibits the cytosolic, Mg2+-dependent enzyme. No apparent effect of BEL on the membrane-bound Mg2+-independent PAP form could be detected. Collectively, the results reported herein establish that BEL inhibits two cellular phospholipases, namely iPLA2 and Mg2+-dependent PAP, with similar potency. Therefore, the inhibitory effect of BEL on Mg2+-dependent PAP might explain several cellular functions previously attributed to iPLA2.

The incorporation of arachidonic acid into triacylglycerol in P388D1 macrophage-like cells

When 388D1 cells are incubated in media containing 10 microM [3H]arachidonic acid (delta4Ach), the label is rapidly incorporated into phospholipids and triacylglycerol. However, incorporation of [3H]delta4Ach into phospholipids clearly precedes incorporation into triacylglycerol, indicating that the phospholipid pool constitutes the primary metabolic fate of the delta4Ach via a remodelling pathway. In contrast, [3H]delta4Ach is incorporated into triacylglycerol almost exclusively via de novo synthesis, as evidenced by studies using propranolol, a phosphatidate phosphohydrolase inhibitor. This compound induced a time-dependent and concentration-dependent increase in the levels of [3H]delta4Ach-containing phosphatidate that is directly correlated with a decrease in the levels of [3H]delta4Ach-containing triacylglycerol. In addition, no change in the levels of [3H]delta4Ach-containing diacylglycerol and monoacylglycerol were apparent along the time course of fatty acid incorporation into triacylglycerol. However, a sharp and transient accumulation of cell-associated free [3H]delta4Ach was detected shortly after exposure of the cells to the radioactive fatty acid. Collectively, the results reported herein suggest that free delta4Ach availability determines the patterns of incorporation and distribution of this fatty acid among the various lipid classes of P338D1 cells.

Alteration of arachidonate levels in tick salivary glands by dietary modification of host blood lipids

Tick saliva contains prostaglandins of the 2-series, believed to facilitate bloodmeal acquisition. Because ticks cannot synthesize the prostaglandin precursor, arachidonic acid, investigations were undertaken to study the uptake, incorporation, and distribution of arachidonic acid in the salivary glands of the lone star tick in vitro and in vivo. Uptake of [3H]arachidonate by isolated salivary glands was reduced in the presence of low concentrations of arachidonic or eicosapentaenoic acids, but much higher, non-physiological concentrations of oleic and linoleic acids were required to inhibit [3H]arachidonate uptake. The incorporation of [3H]arachidonate into triglycerides increased at high concentrations of arachidonic or eicosapentaenoic acid, but not at any concentration of oleic or linoleic acid. Eicosatetraynoic acid greatly inhibited [3H]arachidonic acid. Guinea pigs fed hydrogenated coconut oil, safflower/primrose oil, or fish oil exhibited altered blood lipids; notably increased levels of eicosapentaenoic acid when fed fish oil. Salivary gland lipids in ticks fed on these hosts were also altered. Ticks parasitizing fish oil-fed guinea pigs contained high levels of eicosapentaenoic acid with a 30% reduction in arachidonate levels. The results demonstrated that eicosapentaenoic acid in the host diet had profound effects on arachidonate assimilation by tick salivary glands, which could lead to altered prostaglandin content in tick saliva.

Inhibition of calcium-independent phospholipase A2 prevents arachidonic acid incorporation and phospholipid remodeling in P388D1 macrophages

Cellular levels of free arachidonic acid (AA) are controlled by a deacylation/reacylation cycle whereby the fatty acid is liberated by phospholipases and reincorporated by acyltransferases. We have found that the esterification of AA into membrane phospholipids is a Ca(2+)-independent process and that it is blocked up to 60-70% by a bromoenollactone (BEL) that is a selective inhibitor of a newly discovered Ca(2+)-independent phospholipase A2 (PLA2) in macrophages. The observed inhibition correlates with a decreased steady-state level of lysophospholipids as well as with the inhibition of the Ca(2+)-independent PLA2 activity in these cells. This inhibition is specific for the Ca(2+)-independent PLA2 in that neither group IV PLA2, group II PLA2, arachidonoyl-CoA synthetase, lysophospholipid:arachidonoyl-CoA acyltransferase, nor CoA-independent transacylase is affected by treatment with BEL. Moreover, two BEL analogs that are not inhibitors of the Ca(2+)-independent PLA2--namely a bromomethyl ketone and methyl-BEL--do not inhibit AA incorporation into phospholipids. Esterification of palmitic acid is only slightly affected by BEL, indicating that de novo synthetic pathways are not inhibited by BEL. Collectively, the data suggest that the Ca(2+)-independent PLA2 in P388D1 macrophages plays a major role in regulating the incorporation of AA into membrane phospholipids by providing the lysophospholipid acceptor employed in the acylation reaction.

Uptake, incorporation and redistribution of arachidonic acid in isolated salivary glands of the lone star tick

The ability of isolated salivary glands from the lone star tick, Amblyomma americanum, to take up, incorporate and redistribute [3H]arachidonic acid was examined. Uptake of arachidonic acid was concentration dependent--a single salivary gland incorporated up to approximately 2.8 micrograms arachidonic acid in 60 min. Over 90% of the [3H]arachidonate entering the glands was esterified and found only in the phospholipid (approximately 80%) and triglyceride (approximately 10%). Essentially no radioactivity was associated with the diglyceride fraction and none with phosphatidic acid indicating de novo phospholipid synthesis was negligible. Phospholipid synthesis via acylation of lysophospholipids (the Lands pathway) was indicated by the rapidity of the synthesis (< 2 min) and the sensitivity to sulfhydryl-blocking agents. Within the phospholipids, [3H]arachidonate was incorporated only into phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Initially [3H]arachidonate was incorporated primarily into PC, but as the incubation proceeded PE contained an increasing proportion of the label. The proportion of [3H]arachidonate incorporated into triglyceride increased at higher media concentrations of arachidonic acid. The roles of lysophosphatide acyltransferase, transacylase and diglycerol acyltransferase in the distribution of arachidonate in tick salivary glands are discussed.

Phospholipid metabolism of lymphocytes with inhibited glutathione synthesis using L-buthionine-S,R-sulfoximine

The potential role of phospholipid metabolism in restricting lymphocyte proliferation under conditions of oxidative stress was investigated using [1-14C]-arachidonic acid (14C-AA) and 32P-orthophosphoric acid. Human peripheral blood lymphocytes (PBL) and PBL depleted of glutathione with L-buthionine-S,R-sulfoximine (BSO-PBL) were compared. The relative uniformity of glutathione depletion in the PBL population was assessed by flow cytometry. BSO-PBL were 40 to 90% depleted of glutathione 1 to 3 days after activation, respectively, and the BSO-PBL had unimpaired early activation events based on 32P-phosphatidylinositol levels. However, unlike stimulated PBL, which showed a progressive decrease in radioactivity incorporated into phosphatidylcholine and a corresponding increase into phosphatidylethanolamine, no significant differences occurred with BSO-PBL. Prelabeled BSO-PBL showed considerably more 14C radioactivity in the supernatant following 72-120 h stimulation with anti-CD3 than control PBL, which was mostly in the form of unmetabolized 14C-AA. Higher levels of leukotriene B4, 5-hydroxyeicosatetraenoate and 12-hydroxy-5,8,10-heptadecatrienoate also were observed with L-buthionine-S,R-sulfoximine treatment, which could explain the impaired proliferation obtained with a depletion of cellular glutathione. Both lysophosphatidylcholine and liberated free 14C-AA increased with L-buthionine-S,R-sulfoximine treatment following 72 h stimulation, suggesting functional impairment in the reacylating enzymes. The increased release of 14C-AA by BSO-PBL also may contribute to the imparied proliferation that occurs with loss of glutathione.

Separation of agonist-stimulated arachidonate mobilization from subsequent leukotriene B4 synthesis in human neutrophils: different effects of oleoylacetylglycerol and phorbol myristate acetate as priming agents

Preincubation of human neutrophils with phorbol esters or soluble diglycerides enhances subsequent f-Met-Leu-Phe (fMLP)-stimulated arachidonate mobilization and leukotriene B4 (LTB4) synthesis. We have recently reported that 1,3-dioctanoylglycerol (1,3-diC8) is equipotent with 1,2-sn-dioctanoylglycerol (1,2-diC8) as priming agent, thus suggesting that the priming effects of diacylglycerols are protein kinase C (PKC) independent (Rosenthal et al., 1993, Biochim. Biophys. Acta 1177:79-86). In order to further investigate this question, the present study has directly compared the effects of oleoylacetylglycerol (OAG) and the PKC activator, phorbol 12-myristate 13-acetate (PMA), on agonist-stimulated lipid metabolism. The results indicate that both OAG and PMA dose dependently enhance f-Met-Leu-Phe (fMLP)-stimulated release of [3H]arachidonate. Optimal concentrations of OAG (5 microns) and PMA (10 nM) are equipotent in increasing fMLP-stimulated arachidonate mobilization as quantitated either with total radioactivity or by mass measurements of free arachidonate. By contrast OAG is sixfold more effective than PMA in enhancing synthesis of 5-lipoxygenase (5-LO) metabolites by mass and two to threefold more effective than PMA in enhancing synthesis of [3H]eicosanoids. Furthermore, OAG, but not PMA, enhances fMLP-stimulated synthesis of platelet-activating factor. By contrast, PMA directly stimulates [3H]arachidonate mobilization, while OAG (20 microM) does not; despite these differences, the combined effects of PMA + OAG on subsequent agonist-stimulated arachidonate release are not greater than those of PMA alone. In cells challenged with subthreshold concentrations (< 0.1 microM) of the calcium ionophore A23187, both OAG and PMA stimulate [3H]arachidonate release but not [3H]LTB4 synthesis. These findings suggest that OAG does not directly activate 5-LO, but instead couples arachidonate mobilization to leukotriene synthesis in a PKC-independent manner.

Arachidonate-containing triacylglycerols: biosynthesis and a lipolytic mechanism for the release and transfer of arachidonate to phospholipids in HL-60 cells

When HL-60 cells are incubated in media containing 10 microM [3H]arachidonic acid the label is immediately incorporated into both triacylglycerols and phospholipids. About one-half of the cellular tritium was associated with triacylglycerols after 2 h of incubation and this [3H]arachidonate was then transferred to phospholipids as soon as the labeled cells were placed in arachidonate-free media. A technique was devised to analyze the stereospecific distribution of [3H]arachidonate at the three sn-positions of glycerol in order to identify the mechanism(s) responsible for the biosynthesis of the labeled triacylglycerols. [3H]Arachidonate was found to be distributed in nearly equal amounts among all three glycerol positions of the triacylglycerols. In addition, analysis of intact triacylglycerols containing [3H]arachidonate revealed that 24% of the tritium eluted from reverse-phase HPLC with triarachidonoylglycerol. Both of these findings would be expected if a significant portion of the arachidonate-containing triacylglycerols were synthesized de novo. Homogenates prepared from [3H]arachidonate prelabeled HL-60 cells were capable of hydrolyzing the endogenous [3H]arachidonate-containing triacylglycerols to produce mainly free fatty acids and smaller amounts of monoacylglycerols. The relatively small amount of monoacyl- and diacylglycerols produced by the lipolytic activity of the homogenates indicated that [3H]arachidonate was hydrolyzed from all three sn-positions of the [3H]triacylglycerols. This lipase activity had a pH optimum of 4.5 and was associated to a greater extent with the soluble fraction than in the total membrane fraction. Although it is not known whether this lipolytic activity is the same as that expressed in the intact cells, the activity of the cell-free triacylglycerol lipase was of sufficient magnitude to have easily accounted for the decrease in [3H]triacylglycerols that was observed after transfer of the intact HL-60 cells (prelabeled with [3H]arachidonate) to fresh media. The data suggest that transfer of arachidonate from triacylglycerols to phospholipids probably occurs through an acyltransferase utilizing a lysophospholipid and arachidonoyl-CoA.

Metabolism of [1-14C]linoleic acid in human promyelocytic leukemia HL-60 cells grown and differentiated in serum-free medium

The metabolism of [1-14C]linoleic acid (LA) was studied in human promyelocytic leukemia cell line HL-60, grown and differentiated in serum-free medium. Both undifferentiated and dibutyryl cyclic AMP-differentiated HL-60 cells exhibited similar patterns of conversion of LA to four other major fatty acids (i.e. 18:3, 20:3, 20:4, and 22:4).

Possible mechanisms of action of cobra snake venom cardiotoxins and bee venom melittin

Cobra snake venom cardiotoxins and bee venom melittin share a number of pharmacological properties in intact tissues including hemolysis, cytolysis, contractures of muscle, membrane depolarization and activation of tissue phospholipase C and, to a far lesser extent, an arachidonic acid-associated phospholipase A2. The toxins have also been demonstrated to open the Ca2+ release channel (ryanodine receptor) and alter the activity of the Ca(2+)+Mg(2+)-ATPase in isolated sarcoplasmic reticulum preparations derived from cardiac or skeletal muscle. However, a relationship of these actions in isolated organelles to contracture induction has not yet been established. The toxins also bind to and, in some cases, alter the function of a number of other proteins in disrupted tissues. The most difficult tasks in understanding the mechanism of action of these toxins have been dissociating the primary from secondary effects and distinguishing between effects that only occur in disrupted tissues and those that occur in intact tissue. The use of cardiotoxin and melittin fractions contaminated with trace ('undetectable') amounts of venom-derived phospholipases A2 has continued to be common practice, despite the problems associated with the synergism between the toxins and enzymes and the availability of methods to overcome this problem. With adequate precautions taken with regard to methodology and interpretation of results, the cobra venom cardiotoxins and bee venom melittin may prove to be useful probes of a number of cell processes, including lipid metabolism and Ca2+ regulation in skeletal and cardiac muscle.

Incorporation of arachidonic acid in a human cancer gastric tumor cell line (HGT) at various stages of cell proliferation

Human cancer gastric cells (HGT) in culture were labeled with [14C]arachidonic acid. The kinetic of acylation of arachidonic acid was similar using cells after 3, 5 and 7 days of culture. However, the duration of the proliferation enhanced the acyltransfer to triglycerides and inhibited the transfer to phospholipids. After 8 h of incubation with [14C]arachidonic acid the radioactivity was predominantly recovered in the phosphatidylcholine and phosphatidylethanolamine species. Prelabeling-chase experiments indicated a preferential incorporation of arachidonic acid into phosphatidylcholine followed by a transfer of arachidonate to phosphatidylethanolamine.