Source of arachidonic acid for prostaglandin synthesis in Madin-Darby canine kidney cells

Altered Peroxisome Proliferator-Activated Receptor Alpha Signaling in Variably Diseased Peripheral Arterial Segments

Objective

Peripheral atherosclerosis that accumulates in the extracranial carotid and lower extremity arteries can lead to significant morbidity and mortality. However, atherosclerotic disease progression is often not homogenous and is accelerated by diabetes. We previously observed increased phospholipid content in minimally (Min)-diseased arterial segments compared to maximally (Max)-diseased segments. Since Peroxisome Proliferator-Activated Receptor alpha (PPARα) is a key regulator of lipid metabolism, we hypothesized that it may have differential expression and signaling in Min vs. Max-diseased peripheral arterial segments.

Methods

Eighteen patients who underwent carotid endarterectomy (CEA), and 34 patients who underwent major lower extremity amputation were prospectively enrolled into a vascular tissue biobank. Min and Max-diseased segments were obtained in real-time from CEA plaque and amputated lower extremity arterial segments. mRNA and protein were isolated from specimens and the relative expression of ppara, and its downstream genes Acyl-CoA Oxidase 1 (acox1) and Carnitine Palmitoyltransferase 1A (cpt1a) were also evaluated. We evaluated gene expression and protein content relative to atherosclerotic disease severity and clinical diabetes status. Gene expression was also evaluated relative to Hemoglobin A1c and serum lipid profiles.

Results

In CEA segments of patients with diabetes, we observed significantly higher ppara and acox1 gene expression (p < 0.01 and p < 0.001 respectively), and higher PPARα protein content (p < 0.05). Hemoglobin A1c significantly correlated with expression of ppara (R² = 0.66, p < 0.001), acox1 (R² = 0.31, p < 0.05), and cpt1a (R² = 0.4, p < 0.05). There was no significant difference in gene expression between Min vs. Max-diseased CEA plaque segments. Conversely, in lower extremity arterial segments of patients with diabetes, we observed significantly lower ppara, acox1, and cpt1a expression (p < 0.05, p < 0.001, and p < 0.0001 respectively). Interestingly, CPT1A content was lower in arterial segments of patients with diabetes (p < 0.05). Hemoglobin A1c and HDL-cholesterol had negative correlations with ppara (R² = 0.44, p < 0.05; R² = 0.42, p < 0.05; respectively).

Conclusion

This study demonstrates the significant differential expression of ppara and its immediate downstream genes in human carotid and lower extremity arteries relative to disease severity and diabetes. These findings highlight that mechanisms that influence atheroprogression in the carotid and lower extremities peripheral arteries are not homogenous and can be impacted by patient diabetes status and serum cholesterol profiles. Further elucidating these differential molecular mechanisms can help improve targeted therapy of atherosclerosis in different peripheral arterial beds.

Protein kinase C-independent activation of a novel nonspecific phospholipase C pathway by phorbol myristate acetate releases arachidonic acid for prostaglandin synthesis in MC3T3-E1 osteoblasts

The effects of phorbol myristate acetate, an activator of protein kinase C, on the release of [3H]arachidonic acid and prostaglandin synthesis were studied in an osteoblast cell line (MC3T3-E1). Phorbol myristate acetate (20 uM) liberated 16 and 55% of the [3H]arachidonate in prelabeled phosphatidylinositol and phosphatidylethanolamine, respectively; and evoked a 19-fold stimulation in the synthesis of prostaglandin E2. Phorbol myristate acetate doubled the cellular mass of 1,2-diacylglycerol and stimulated the liberation of [3H]arachidonate from the diacylglycerol pool in prelabeled cells. The diacylglycerol lipase inhibitor RHC 80267 blocked 75-80% of the phorbol ester-promoted (total) cellular liberation of [3H]arachidonic acid and production of prostaglandin E2. In comparison, the release of [3H]arachidonate from phosphatidylethanolamine (but not phosphatidylinositol) was only partially antagonized (to the same degree) by the PLA2 inhibitor p-bromophenacylbromide and the protein kinase C inhibitor Et-18-OMe, PMA-induced formation of diacylglycerol or synthesis of PGE2 was not affected by the prior inhibition of protein kinase C. Therefore, we have shown a novel pathway for the liberation of arachidonic acid in osteoblasts involving the nonspecific hydrolysis of phosphatidylinositol and phosphatidylethanolamine by phospholipase C followed by the deesterification of diacylglycerol. This pathway can be activated by a phorbol ester through a protein kinase C-independent mechanism.

Regulation of bradykinin-stimulated phospholipase C and arachidonic acid release by protein kinase A in MDCK-D1 cells

Regulation of phospholipases C (PLC) and arachidonic acid (AA) release by cAMP-dependent protein kinase (PKA) was investigated in MDCK-D1 cells. Bradykinin (BDK) was used to stimulate PLC and AA release, while arginine vasopressin (AVP), forskolin (FSK), isobutylmethylxanthine (IBMX) were used to increase cAMP levels and stimulate PKA. When cells were preincubated for 20 min with 10 microM FSK + 0.5 mM IBMX, and subsequently treated with 1 microM BDK or control medium for 40 min, the basal and BDK-stimulated PLC activity, measured as accumulated labelled inositol phosphate (InsP) after 40 min and inositol trisphosphate (InsP3) after 10 s, were significantly inhibited. In a parallel manner, FSK + IBMX also significantly decreased both basal and BDK-stimulated diacylglycerol (DAG) production. The basal and BDK-enhanced AA release into the media was also significantly inhibited by pretreatment with FSK + IBMX. In parallel experiments, H-89, a specific inhibitor of PKA, was preincubated for 60 min prior to addition of BDK and this resulted in a reversal of FSK+IBMX-induced inhibition of basal and BDK-stimulated PLC activity and AA release. An inhibitor of inositide-hydrolysing PLC, U73122, (1 microM) was also found to blunt BDK-stimulated PLC activity and BDK-enhanced AA release which indicated that stimulation of AA release by the nonapeptide was second to PLC activation. The ionophore, A23187, (10 microM) greatly stimulated AA release and to a much lesser extent, PLC activity. Its effect on AA release however was not blocked by inhibiting protein kinase C (PKC) with staurosporine (SSP) and consequently did not notably involve the PLC-PKC cascade. Activation of PKA with FSK + IBMX was found to significantly inhibit the enhancement of AA release by ionophore. With 12-tetradecanoyl-phorbol-13-acetate (TPA) also present there was a synergistic increase in the A23187-stimulated AA release and activation of PKA under such conditions inhibited AA release to a similar extent though the synergistic effect remained. The results strongly suggest a role for PKA in the regulation of PLC activity and AA release in MDCK-D1 cells. Control of AA release by PKA, is mediated both by mechanisms which involve blunting of PLC activity and mechanisms which are downstream from the PLC-PKC cascade.

Effects of Ca++ and glycine on lipid breakdown and death of ATP-depleted MDCK cells

The relationships between cytosolic free calcium (Caf), cell associated glycine, phospholipid hydrolysis and cell death were investigated in Madin-Darby canine kidney (MDCK) cells injured by depletion of adenosine triphosphate (ATP). Glucose free incubation for three hours with a mitochondrial uncoupler resulted in progressive loss of glycine from cells. However, they were not lethally injured unless a perturbation of Ca++ homeostasis was also induced. Exposure to a Ca++ ionophore and uncoupler in 1.25 mM Ca++ medium (+Ca) resulted in accelerated cell death. ATP depleted cells with ionophore in 100 nM Ca++ medium (-Ca) were also lethally injured, but after a significant delay. Depletion of glycine preceded death in both groups of cells. Exogenous glycine (5 mM) protected +Ca cells against lethal membrane damage, but the beneficial effects were lost over a period of time. In contrast, -Ca cells were completely protected throughout. Phospholipid mass and radioactive label in lipid fractions of cells prelabeled with 3H-oleic acid were measured. Accelerated death of +Ca cells was accompanied by large decreases of phospholipid mass, loss of phospholipid label, and accumulation of unesterified labeled fatty acid. These changes were greatly decreased by incubation in -Ca medium. On the other hand, protection by glycine could not be attributed to modifications of either the massive breakdown of phospholipids that occurred in +Ca cells, or the modest changes seen in -Ca cells. In +Ca cells, the deleterious effects of increased Caf and phospholipid breakdown ultimately prevailed over protection by the amino acid. Thus, separate pathways of cell death associated with increased Caf and decreased glycine were defined in ATP depleted, Ca(+)- permeabilized MDCK cells. Calcium excess and massive phospholipid loss are features of a damage process that occurs independently of whether cells are protected by glycine or not. Conversely, the glycine sensitive component of injury is expressed regardless of whether intracellular Ca++ is increased, or large phospholipid losses occur. ATP depletion in -Ca medium provides a system to study mechanisms of glycine cytoprotection uncomplicated by Ca++ toxicity.

Interactions between the monocyte/macrophage and the vascular smooth muscle cell. Stimulation of mitogenesis by a soluble factor and of prostanoid synthesis by cell-cell contact

The effect of soluble factors from the monocyte/macrophage (M phi) on cell proliferation and the functional effects of cell-cell contact on the arachidonic acid (AA) cascade were studied with vascular smooth muscle cells (SMCs). Peripheral blood M phi s were isolated by adherence or in a Percoll gradient, and alveolar M phi s were obtained by lavage. Conditioned medium (CM) was prepared by preincubating M phi s with medium alone or by separating SMC and M phi cocultures by a membrane insert. Cell proliferation (image analysis) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha, radioimmunoassay) were measured in SMCs. Labeled prostanoids and other eicosanoid metabolites were isolated by high-performance liquid chromatography from SMCs prelabeled with 14C-AA. M phi s did not synthesize 6-keto-PGF1 alpha. The CM enhanced proliferation but did not stimulate 6-keto-PGF1 alpha synthesis in SMCs. However, cell-cell contact in cocultures of SMCs with the same concentration of M phi s used to generate CM resulted in increased 6-keto-PGF1 alpha synthesis by SMCs. Since the stimulatory effect of cell contact was not blocked by butylated hydroxytoluene, it could not be attributed to an oxidative burst from M phi s. Functional studies showed that the stimulatory effect of cell contact was enhanced by exogenous free AA and by endogenous AA release through A23187. Release of total radioactivity from prelabeled SMCs was enhanced by cell contact, and this effect was blocked by indomethacin (IM). Cell contact did not increase the release of free AA from prelabeled SMCs, even in the presence of IM. Finally, cell contact only stimulated the formation of prostanoids (IM-sensitive eicosanoid metabolites) from prelabeled SMCs. Lipoxygenase and other products of AA were not formed through cell-cell contact. These data showed that M phi s express a soluble factor that enhances SMC proliferation without affecting prostanoid synthesis. Subsequent cell contact between SMCs and M phi s stimulates prostanoid synthesis, which may possibly serve as a local and focal homeostatic mechanism for the regulation of uncontrolled SMC proliferation in atherogenesis.

Stimulation of enterocyte protein kinase C by laxatives in-vitro

To elucidate the role of protein kinase C in the mechanism of action of stimulatory laxatives, experiments were performed with preparations of rat lysed enterocytes. The phorbol ester 4-beta-phorbol 12-myristate 13-acetate (PMA) concentration-dependently (2-200 micrograms mL-1) stimulated the activity of protein kinase C in this preparation. Ricinoleic acid, the active principle of castor oil, deacetylbisacodyl, the active principle of bisacodyl, and deoxycholic acid exerted the same effect, although less efficiently. This reflects their potency for inducing intestinal fluid secretion and prostaglandin release, effects that are also induced more potently by PMA. Accordingly, the potency of the three C18 fatty acids, ricinoleic acid, stearic acid and oleic acid on protein kinase C activity in-vitro, on prostaglandin E2 release and on net fluid secretion in-vivo runs in parallel. It is therefore concluded that stimulatory laxatives activate protein kinase C, leading to prostaglandin E2 release, thus resulting in net fluid secretion.

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

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

Cellular mechanism of synergistic stimulation of PGE2 production by phorbol diester and Ca2+ ionophore A23187 in cultured Madin-Darby canine kidney cells

The synergistic stimulation of arachidonic acid release and prostaglandin (PG) E2 production was observed when quiescent Madin-Darby canine kidney (MDCK) cells were exposed to phorbol 12-myristate 13-acetate (PMA) and Ca2+ ionophore, A23187. Addition of PMA or A23187 by itself was not effective. When cells were treated with 50 nM PMA and 0.1 microM A23187 at a suboptimal concentration, there was a marked increase in the production of PGE2. In the presence of higher concentrations of A23187 (5-10 microM), 50 nM PMA was only synergistic in potentiating the liberation of free arachidonic acid, but failed to stimulate the PGE2 production. The amount of free arachidonic acid liberated by 50 nM PMA and 10 microM A23187 reached a maximum level within several hours, whereas PGE2 synthesis induced by 50 nM PMA and 0.1 microM A23187 proceeded with a slower process requiring more than 24 h to reach a maximum. The stimulated PGE2 synthesis was blocked by transcription and translation inhibitors. The addition of 50 nM PMA alone or the mixture of 50 nM PMA and 0.1 microM A23187 was found similarly to increase the cellular PG endoperoxide synthase activity, suggesting that PMA was responsible for the increased enzyme activity. However, these agents failed to enhance the activities of phospholipases and PGE2 synthase from PGH2. Northern blot analysis confirmed the increased level of PG endoperoxide synthase mRNA in the cells treated with PMA. The effect of PMA was mimicked by other protein kinase C activators. The pretreatment with PMA caused a down-regulation in the PGE2 production. The stimulated PGE2 production was abolished in the presence of selective protein kinase C inhibitors such as staurosporine and H-7. In addition, sphingosine, dihyrosphingosine, and psychosine, recently found to be protein kinase C inhibitors, blocked the effect of PMA in intact MDCK cells. Thus, the results indicate that the synergistically stimulated PGE2 production with phorbol diesters and 0.1 microM A23187 occurred principally through the de novo synthesis of PG endoperoxide synthase, also implying a role for protein kinase C.

Prostaglandin and thromboxane biosynthesis

We describe the enzymological regulation of the formation of prostaglandin (PG) D2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2 (prostacyclin), and thromboxane (Tx) A2 from arachidonic acid. We discuss the three major steps in prostanoid formation: (a) arachidonate mobilization from monophosphatidylinositol involving phospholipase C, diglyceride lipase, and monoglyceride lipase and from phosphatidylcholine involving phospholipase A2; (b) formation of prostaglandin endoperoxides (PGG2 and PGH2) catalyzed by the cyclooxygenase and peroxidase activities of PGH synthase; and (c) synthesis of PGD2, PGE2, PGF2 alpha, 9 alpha, 11 beta-PGF2, PGI2, and TxA2 from PGH2. We also include information on the roles of aspirin and other nonsteroidal anti-inflammatory drugs, dexamethasone and other anti-inflammatory steroids, platelet-derived growth factor (PDGF), and interleukin-1 in prostaglandin metabolism.

Phorbol myristate enhanced specific incorporation of arachidonic acid into phospholipids through lysophospholipid acyltransferase in cultured smooth muscle cells

The effect of stimulation of phospholipase with phorbol 12-myristate 13-acetate and lipopolysaccharide on 1-acyl-glycerophospholipid acyltransferase was studied in cultured rabbit aorta smooth muscle cells. The acyltransferase in smooth muscle cells without stimulation was active on a wide range of unsaturated fatty acids and was not arachidonic acid specific. Upon increase in phospholipase activity, acyltransferase activity only with arachidonic acid as substrate increased in a time-dependent fashion. Apparent acyltransferase activity was increased most upon increase in phospholipase activity when lysophosphatidylcholine was used as acceptor. These results suggest that arachidonic acid specific acyltransferase was induced in smooth muscle cells by increase in phospholipase activity. The role of this acyltransferase is postulated to be the specific incorporation of endogenously released arachidonic acid.

Effects of eicosapentaenoic and docosahexaenoic acid supplements on phospholipid composition and plasmalogen biosynthesis in P388D1 cells

This investigation describes the influence of n-3 fatty acid supplements on the phospholipid composition and the metabolism of plasmalogens in P388D1 cells. The cellular content of phospholipid classes and subclasses was unchanged in P388D1 cells (a macrophage-like cell) grown for 24 h in media supplemented with 10 microM sodium eicosapentaenoate or sodium docosahexaenoate. However, phospholipids from these cells were highly enriched in acyl groups of the corresponding fatty acid supplement, with the largest increases occurring in the ethanolamine plasmalogens (e.g., 46% of the ethanolamine plasmalogens from cells supplemented with docosahexaenoate contained this acyl group at the sn-2 position). Eicosapentaenoate supplements lowered the levels of oleate in phosphatidylinositol/serine, diacyl-sn-glycero-3-phosphoethanolamine (GroPEtn), and alk-1-enylacyl-GroPEtn in the P388D1 cells but had little or no effect on the amounts of arachidonate in the cellular phospholipids. In contrast, supplementation of the cells with docosahexaenoic acid not only reduced the level of oleate but also decreased the amount of arachidonate by one-third in the alk-1-enylacyl-GroPEtn. When P388D1 cells were incubated for 1 h with [3H]alkyllyso-GroPEtn both [3H]alkylacyl-GroPEtn and [3H]alk-1-enylacyl-GroPEtn were formed. The sn-2 acyl composition of these two ether-containing GroPEtn lipids reflected the fatty acid supplement that the cells had received (e.g., 68% of the [3H]alk-1-enylacyl-GroPEtn from cells supplemented with docosahexaenoate contained this acyl group at the sn-2 position). Cells from both the controls and supplemented groups contained greater amounts of docosahexaenoate in the [3H]alk-1-enylacyl-GroPEtn (plasmalogen) than in the [3H]alkylacyl-GroPEtn subclass. Analysis of molecular species from pulse-chase experiments with intact cells and examination of the molecular species of [3H]alk-1-enylacyl-GroPEtn produced by the delta 1-desaturase system in cell-free membrane fractions suggest that the docosahexaenoate-containing species of [3H]alk-1-enylacyl-GroPEtn have a higher turnover rate than other molecular species. Possible biological implications of our findings are also discussed.

Distribution of omega-3 and omega-6 fatty acids in the ether- and ester-linked phosphoglycerides from tissues of the rainbow trout, Salmo gairdneri

1. Data presented here demonstrate that polyunsaturated fatty acids in the phospholipids of rainbow trout tissues are compartmentalized differently than in mammalian tissues. 2. We have determined the distribution of omega-3 (n-3) and omega-6 (n-6) fatty acids in the alkyl-, alk-1-enyl-, and diacyl- subclasses of phosphatidylcholines (PC), phosphatidyl-ethanolamines (PE), phosphatidylinositols (PI), and phosphatidylserines (PS) from gill, kidney and spleen of rainbow trout. 3. Alkyl-linked PC and alk-1-enyl-linked PE were the most abundant ether-containing phospholipids, amounting to 10-15% of each class; no ether-linked PI or PS was detected. 4. C20:4 n-6 was found in high concentrations only in PI; the n-3 fatty acids were found in highest concentration in the ether-linked phospholipids as compared with the diacyl subclasses and C20:5 n-3 was especially prevalent in 1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine and C22:6 n-3 was prevalent in PS.

Protein kinase C modulates phospholipase C and increases arachidonic acid release in bradykinin stimulated MDCK cells

The tumor promoter phorbol ester (PMA) has been shown to stimulate protein kinase C (PKC) in MDCK cells. At the concentrations that produce stimulation of PKC, PMA (100 microM) inhibits BK-induced I1,4,5P3 (IP3) formation and calcium transients in these cells. 1-5-isoquinolinyl-2-methyl-piperazine (H7) a known inhibitor of PKC in MDCK cells reverses the effect of PMA on BK-stimulated IP3 formation and Ca2+ transients in these cells. PMA also stimulates arachidonate release which can be inhibited by preincubation with H7. A dual mechanism of regulation by PKC at the level of phospholipase C (down regulation) and phospholipase A2 (stimulation) is suggested in these cells.

Phospholipase D activity of Vibrio damsela cytolysin and its interaction with sheep erythrocytes

Exposure of sheep erythrocytes to sublytic amounts of Vibrio damsela cytolysin markedly reduced their membrane sphingomyelin content and their sensitivity to lysis by the sphingomyelin-dependent cytolysins staphylococcal sphingomyelinase C (beta-toxin) and helianthin. The toxin was found to be a phospholipase D active against sphingomyelin.

Ether lipids inhibit the effects of phorbol diester tumor promoters

Recent studies have shown that the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulates protein kinase C (PKC), whereas the ether-linked phospholipid 1-O-octadecyl-2-O-methyl-rac-glycerol-3-phosphocholine (ET-18-OCH3) inhibits PKC activity in vitro. Therefore, the antitumor effects of ET-18-OCH3 could be due to its inhibition of PKC activity and the effects of tumor promotion. TPA stimulates arachidonic acid release, prostaglandin synthesis, phosphatidylcholine synthesis and the degradation of phosphatidylcholine by phospholipase C in Madin Darby canine kidney (MDCK) cells. Therefore, we have determined the effects of ET-18-OCH3 on these consequences of TPA stimulation. Preliminary experiments determined that ET-18-OCH3 inhibited PKC partially purified from MDCK cells by ion-exchange chromatography on DEAE-cellulose. In addition, ET-18-OCH3 inhibited the TPA-stimulated phosphorylation of a 40,000-dalton protein in intact MDCK cells. These data indicate that ET-18-OCH3 is an effective inhibitor of PKC activity in MDCK cells. In addition, ET-18-OCH3 was found to inhibit arachidonic acid release and prostaglandin synthesis. The inhibition of prostaglandin synthesis appears to be secondary to inhibition of arachidonic acid release, since ET-18-OCH3 does not inhibit TPA-stimulated synthesis of prostaglandin H synthase or the activity of the enzyme directly (Parker, J., Daniel, L. W., and Waite, M. [1987] J. Biol. Chem. 262, 5385-5393). ET-18-OCH3 also inhibits TPA-stimulated phosphatidylcholine synthesis and phosphatidylcholine degradation by phospholipase C.(ABSTRACT TRUNCATED AT 250 WORDS)

Ionophore-induced metabolism of phospholipids and eicosanoid production in porcine aortic endothelial cells: selective release of arachidonic acid from diacyl and ether phospholipids

Confluent cultures of porcine aortic endothelial cells were prelabeled with 1 microM [14C]arachidonic acid complexed to 1 microM bovine serum albumin. After washing, the cells were stimulated with 1 microM A23187 for time intervals between 30 s and 30 min. Cellular lipids were extracted and separated into major lipid classes and phospholipid subclasses. The external medium was analyzed for released radioactive eicosanoids. The time-course of total release of 14C radioactivity demonstrated a biphasic nature of A23187-induced changes in endothelial cell lipids. Early, from 30 s to 5 min, substantial losses of [14C]arachidonic acid from diacylphosphatidylethanolamine and phosphatidylinositol, as well as an abrupt increase in diacylphosphatidylcholine-associated radioactivity were observed. These initial changes coincided with the release of 14C-labeled cyclooxygenase products. Later changes (5-30 min) included a sustained progressive loss of 14C radioactivity from alkenyl (alk-1-enyl) acylphosphatidylethanolamine and diacylphosphatidylcholine. These later changes coincided with the elaboration of 14C-labeled lipoxygenase products. Although unequivocal assignments cannot be made, the data suggest that specific pools of arachidonic acid provide precursors for individual classes of eicosanoids.

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.

Role of prostanoids and lipid peroxides as mediators of the 12-O-tetradecanoylphorbol-13-acetate effect on cell growth

Confluent cultures of guinea pig smooth muscle cells (SMC) or human fibroblasts (HNF) were treated with 12-tetradecanoylphorbol-13-acetate (TPA). Prostanoid levels were measured by the radioimmunoassay of 6-keto-PGF1 alpha and PGE2, and lipid peroxides were measured by the thiobarbituric acid test for malondialdehyde (MDA). Cells were seeded at low densities, and growth was calculated both from the cell count (Coulter Counter) and the colony number (image analysis). When confluent SMC and HNF were incubated in media alone, 6-keto-PGF1 alpha levels were a function of the TPA concentration, increasing to a maximum at 10(-8) M TPA and then decreasing at higher TPC concentrations. When confluent SMC and HNF were incubated in media containing exogenous arachidonic acid, 6-keto-PGF1 alpha levels again increased to a maximum at 10(-8) M TPA but decreased at higher TPA concentrations only with SMC. The increase in 6-keto-PGF1 alpha levels was much greater in HNF (1310%) than SMC (680%). SMC synthesized similar amounts of 6-keto-PGF1 alpha and PGE2, and the stimulatory effect of TPA was similar with 6-keto-PGF1 alpha and PGE2. Indomethacin (IM) blocked prostanoid synthesis at all TPA concentrations. TPA did not have a significant effect on MDA levels in either cell line. The lipid antioxidants alpha-tocopherol and alpha-tocopherylquinone blocked lipid peroxidation without affecting the stimulation of prostanoid synthesis with TPA. Cell number decreased to a minimum at 10(-8) M TPA in both cell lines. The decrease in cell number was much greater in HNF (72%) than SMC (30%). SMC colony number decreased at 10(-8) TPA and then increased at 10(-6) M TPA. IM did not block the TPA effect on cell number in either cell line.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Regulation of arachidonic acid metabolism in Madin-Darby canine kidney cells: stimulation of synthesis of the cyclooxygenase system by 12-O-tetradecanoyl-phorbol-13-acetate

Stimulation of Madin-Darby canine kidney cells by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) results in an increase in prostaglandin synthesis. We have measured the specific activity of the cyclooxygenase system in a cell-free assay and demonstrate that a fourfold induction occurs by 6 h poststimulation. This induction could be prevented by continuous treatment with either cycloheximide or actinomycin D. When cycloheximide was added to cells 6 h post-TPA stimulation we found only a 50% reduction in prostaglandin synthesis over the ensuing 6 h in vitro; this decrease in activity was not observed when actinomycin was used. On the other hand, cells stimulated with TPA for 6 h and subsequently treated with cycloheximide or actinomycin D ceased prostaglandin synthesis completely within 1 h. These results suggest that TPA stimulation of prostaglandin synthesis requires both transcriptional and translational events but that a factor(s) in addition to or in place of the cyclooxygenase might be crucial.

Human vascular endothelial cells synthesize and release 24- and 26-carbon polyunsaturated fatty acids

Vascular endothelial cells from human umbilical vein readily incorporate [14C]eicosatrienoate (20:3 (n - 6) and desaturate it to synthesize [14C]arachidonate (20:4) and [14C]docosatetraenoate (22:4). Both substrate and metabolites are extensively esterified in cellular phospholipids and triacylglycerol. After these cells are incubated for 24-48 h with 4.5 microM [14C]20:3 in culture medium plus 10% fetal bovine serum, the medium contains a number of radiolabeled free fatty acids. In addition to arachidonate and docosatetraenoate, these include still longer-chain polyunsaturated fatty acids. We have identified these as 24:4, 24:5, 26:4 and 26:5 by both radio-gas chromatography and HPLC. Although the 24- and 26-carbon polyunsaturated fatty acids represent a negligible percentage of cellular 14C-labeled fatty acids, they are each present in the medium at a concentration of 10-40 nM, whereas [14C]arachidonate is 60-100 nM. In particular, products of delta 4 desaturation are a significant component of radiolabeled polyunsaturated fatty acids in medium but not in the cells. Since docosapolyenoic fatty acids have recently been shown to give rise to biologically active oxygenated derivatives, the selective release and possible subsequent metabolism of even longer polyunsaturated fatty acids warrants further investigation.

Extracellular phospholipase A2 and lysophospholipase produced by Vibrio vulnificus

Phospholipase A2 and lysophospholipase activities were detected in the culture supernatant fluids of a virulent strain of Vibrio vulnificus. The phospholipase A2 was inactivated by heating at 56 degrees C for 30 min, had an apparent molecular weight of greater than or equal to 80,000 (estimated by gel filtration with Sephadex G-75), and a pI of ca. 5.0. Phospholipid hydrolysis was unaffected by Ca2+ or ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid and was optimal at pH 5.0 to 5.5. The lysophospholipase was not affected by heating at 56 degrees C for 30 min but was inactivated at 100 degrees C and had an apparent molecular weight of greater than or equal to 80,000 and a pI of ca. 4.0. The enzymes were detected coincidentally with a previously described extracellular cytolysin of V. vulnificus; however, they were physically separable from the toxin (which did not possess phospholipase A, C, or D activity) by gel filtration with Sephadex G-75.

Regulation of phosphatidylinositol turnover, calcium metabolism and enzyme secretion by phorbol dibutyrate in neutrophils

The action of the tumor promoter, phorbol 12,13-dibutyrate (PDBu), on rabbit peritoneal and human neutrophils is associated with stimulation of 14C-arachidonic acid incorporation into phospholipids within 1-2 min. Stimulated 14C-arachidonate incorporation was relatively selective for phosphatidylinositol (PI) in rabbit neutrophils. In contrast, the secretory response of human neutrophils to PDBu coincided with stimulated label incorporation into phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA) and PI. Significant increases in label incorporation were observed with PDBu concentrations as low as 2 nM, and the dose response of stimulated label incorporation paralleled that of evoked lysozyme secretion. A parallel, but partial, inhibition of PDBu-stimulated PI labeling and enzyme release was observed after exposing rabbit neutrophils to calcium-deprived medium, whereas calcium deprivation failed to significantly depress either of these stimulant actions of PDBu in human neutrophils. Further, in rabbit neutrophils PDBu elicited an increase in cell associated 45Ca. However, PDBu was unable to promote the incorporation of 32P orthophosphate into PI or enhance phospholipase A2 activity in broken cells. These findings suggest that one expression of the interaction between phorbol esters and their receptors on neutrophils involves the turnover of arachidonic acid in phospholipids. This stimulated turnover of arachidonate may be a critical step in the cascade of events associated with neutrophil activation.

Accumulation of unesterified arachidonic acid in ischemic canine myocardium. Relationship to a phosphatidylcholine deacylation-reacylation cycle and the depletion of membrane phospholipids

Studies in ischemic canine left ventricle have shown that the depletion of membrane phospholipids is a critical event in the development of a sarcolemmal calcium permeability defect and associated irreversible myocyte injury. The mechanism of phospholipid loss is unclear, but may be due to the activation of endogenous phospholipases. Since arachidonic acid is a fatty acid found almost entirely in phospholipid, increases in arachidonate provide evidence for increased phospholipase activity. The present study was designed to examine the temporal relationship of the accumulation of free arachidonate with the onset of phospholipid depletion during fixed ligation of the left anterior descending coronary artery in canine myocardium. The following results were demonstrated in ischemic canine myocardium: (1) the accumulation of unesterified arachidonate is minimal during 10-30 minutes of ischemia, but is significantly increased after prolonging the duration of ischemia to 1-3 hours; (2) significant increases in arachidonate precede the development of a significant decrease in total phospholipid content; (3) the decrease in the arachidonate content of phosphatidylcholine is accompanied by similar decreases in all of the fatty acyl moieties; (4) the arachidonate content of lysophosphatidylcholine and diacylglycerol are unchanged during myocardial ischemia; (5) there is evidence of a deacylation-reacylation cycle in phosphatidylcholine prior to the accumulation of free arachidonate; (6) the fatty acyl specificity of the lysophosphatidylcholine acyltransferase corresponds to the pattern of fatty acyl remodeling of phosphatidylcholine during early myocardial ischemia. These data suggest that the accumulation of arachidonate may be a more sensitive measure of phospholipid degradation than the decrease in total phospholipid content in ischemic canine myocardium. It is postulated that the defective reacylation of arachidonate into phosphatidylcholine may contribute to the net loss of membrane phospholipid during myocardial ischemia.

Regulation of arachidonic acid metabolism in Madin-Darby canine kidney cells. Comparison of A23187 and 12-O-tetradecanoyl-phorbol-13-acetate

Challenge of Madin-Darby canine kidney (MDCK) cells with the divalent cation ionophore A23187 caused a marked increase in the deacylation of [3H]arachidonic acid but not of [14C]palmitic acid. When the cells were treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and A23187, there was an additional increase in the deacylation of [3H]arachidonic acid compared to that observed with either agent alone. In contrast to deacylation, the stimulation of prostaglandin production by A23187 was small compared to the stimulation by TPA. Cycloheximide inhibited synthesis of prostaglandins in TPA-treated cells, but did not block the stimulated deacylation caused by either TPA or A23187. These data indicate that, while both TPA and A23187 stimulated the deacylation of [3H]arachidonic acid, TPA had an additional, cycloheximide-sensitive effect that was required for efficient conversion of the release fatty acids to prostaglandins. Thus, although required, deacylation appeared to be independent of and insufficient to stimulate maximum prostaglandin synthesis in these cells.

Stimulation of deacylation in Madin-Darby canine kidney cells. 12-O-tetradecanoyl-phorbol-13-acetate stimulates rapid phospholipid deacylation

The tumor-promoting phorbol diester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), stimulates Madin-Darby canine (MDCK) cells to deacylate cellular phospholipid and to produce prostaglandins. We have used this system to characterize the kinetics of deacylation of [3H]arachidonate and the further metabolism of arachidonate by the cyclooxygenase system. Stimulation of the appearance of [3H]arachidonic acid in extracellular fluids was found to be maximal 2 h after treatment with TPA and its subsequent removal. The production of prostaglandins then followed for up to 24 h. Phospholipase activity was not inhibited by indomethacin over the range of 0.01-100 micrograms/ml. In contrast, prostaglandin synthesis was inhibited at 1 microgram/ml indomethacin. Further, there was a significant stimulation of deacylation within 15 min in the presence of TPA that increased to nearly 30% of the total radioactivity within 1 h. Likewise, stimulation of prostaglandin production was detected within 15 min, but, unlike the deacylation process, did not increase significantly during TPA treatment. The source of arachidonic acid in the early stimulation period was found to be primarily phosphatidylethanolamine, but phosphatidylcholine and phosphatidylinositol were also deacylated. The results presented here argue that the phospholipase and cyclooxygenase are not tightly coupled in this system. Furthermore, we conclude that the earliest effect of TPA with regard to increased prostaglandin production in the MDCK cell is the direct stimulation of phospholipase activity.

Selective utilization of omega 6 and omega 3 polyunsaturated fatty acids by human skin fibroblasts

Incorporation of exogenous [14C] arachidonate by human skin fibroblasts was found to be significantly greater than that of either [14C]linoleate or alpha-[14C] linolenate. Arachidonate was preferentially esterified in the PI + PS and PE classes of phospholipids. Over 40% of the incorporated [14C] arachidonate was chain elongated in 24 hours. Cells were also grown in lipid-free medium to enhance PUFA desaturation and elongation and the utilization of various omega 6 and omega 3 metabolites examined. Whereas [14C] linoleate partitioned approximately 50:50 between PL and TAG, eicosatrienoate (20:3 omega 6) was selectively sequestered in TAG. Arachidonate and docosatetraenoate (22:4 omega 6) were preferentially incorporated into phospholipids; the PI + PS fraction was most highly enriched with arachidonate. Modification of alpha-[14C] linolenate was more extensive than that of [14C] linoleate. Docosapentaenoate (22:5 omega 3) was the major omega 3 [14C] PUFA of PI + PS and PE. Eicosapentaeonate was not selectively incorporated into phospholipids; within phospholipids the 20:5 omega 3 was primarily in PC. These results indicate that human skin fibroblasts exhibit acyl specificity in the esterification of polyunsaturated fatty acids, including preferential utilization of arachidonate rather than other prostaglandin precursors in the PI + PS fraction.