Arachidonic acid cycloxygenase and lipoxygenase pathways are differently activated by platelet activating factor and the calcium-ionophore A23187 in a primary culture of astroglial cells

Comparison of docosahexaenoic acid uptake in murine cardiomyocyte culture and tissue: significance to physiologically relevant studies

Long-chain n-3 (or omega 3) fatty acids, namely docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3) have been attributed cardioprotective properties. In this study, we evaluated the incorporation of DHA into cardiomyocytes and the shift in the omega 3/omega 6 ratio after supplementation of primary cardiomyocyte culture. Results are compared with atrial tissue concentrations attained after prolonged feeding of rats. The major difference between in vitro vs. in vivo supplementation is the paradoxical accumulation of arachidonic acid in cultured cardiomyocyte. However, this increase does not give rise to a higher PGE2 production after cellular stimulation, as compared with controls, possibly because of the associated inhibition of sPLA2 by DHA. Notably, in vitro supplementations with DHA 10 to 25μM approximate in vivo pharmacological treatments.

Cysteinyl-leukotrienes are released from astrocytes and increase astrocyte proliferation and glial fibrillary acidic protein via cys-LT1 receptors and mitogen-activated protein kinase pathway

Cysteinyl-leukotrienes (cys-LTs), potent mediators in inflammatory diseases, are produced by nervous tissue, but their cellular source and role in the brain are not very well known. In this report we have demonstrated that rat cultured astrocytes express the enzymes (5'-lipoxygenase and LTC(4) synthase) required for cys-LT production, and release cys-LTs in resting condition and, to a greater extent, in response to calcium ionophore A23187, 1 h combined oxygen-glucose deprivation or 2-methyl-thioATP, a selective P2Y(1)/ATP receptor agonist. MK-886, a LT synthesis inhibitor, prevented basal and evoked cys-LT release. In addition, 2-methyl-thioATP-induced cys-LT release was abolished by suramin, a P2 receptor antagonist, or by inhibitors of ATP binding cassette proteins involved in cys-LT release. We also showed that astrocytes express cys-LT(1) and not cys-LT(2) receptors. The stimulation of these receptors by LTD(4) activated the mitogen-activated protein kinase (MAPK) pathway. This effect was: (i) insensitive to inhibitors of receptor-coupled Gi protein (pertussis toxin) or tyrosine kinase receptors (genistein); (ii) abolished by MK-571, a cys-LT(1) selective receptor antagonist, or PD98059, a MAPK inhibitor; (iii) reduced by inhibitors of calcium/calmodulin-dependent kinase II (KN-93), Ca(2+)-dependent and -independent (GF102903X) or Ca(2+)-dependent (Gö6976) protein kinase C isoforms. LTD(4) also increased astrocyte proliferation and glial fibrillary acidic protein content, which are considered hallmarks of reactive astrogliosis. Both effects were counteracted by cell pretreatment with MK-571 or PD98059. Thus, cys-LTs released from astrocytes might play an autocrine role in the induction of reactive astrogliosis that, in brain injuries, contributes to the formation of a reparative glial scar.

Platelet-activating factor induces cell death in cultured astrocytes and oligodendrocytes: involvement of caspase-3

The biologically active lipid metabolite, platelet-activating factor (PAF), is thought to contribute to inflammatory processes and tissue damage in a variety of central nervous system (CNS) injuries. In previous studies, we found that after contusion spinal cord injury, treatment with a PAF antagonist led to significantly increased white matter tissue sparing as well as decreased mRNA levels for pro-inflammatory cytokines. Some studies suggest that PAF can also have toxic effects on neurons in vitro. Few studies, however, have examined the effects of PAF on glial cells of the CNS. In the present study, the potential for PAF to act as a toxin to cultured astrocytes was examined. Also investigated were the effects of PAF on oligodendrocytes at two different stages of development. Treatment with 0.02-2 microM PAF for 72 h resulted in significant levels of cell death in both cell types (P < 0.05), an effect that was blocked by the PAF receptor antagonists, WEB 2170 and BN 52021. To investigate PAF-induced glial cell death further, we looked for activation of the enzyme, caspase-3, which can be indicative of apoptosis. Immunocytochemistry demonstrated that PAF at all concentrations caused activation of caspase-3 at 24, 48, and 72 h after treatment in both cell types. Caspase-3-dependent cell death was further confirmed using knockout mice (-/-) deficient in the caspase-3 gene. Toxicity was lost when astrocytes (-/-) were exposed to 0.02-2 microM PAF (P < 0.01). Oligodendrocytes (-/-) were not susceptible to toxicity at 2 microM PAF (P < 0.001). The results demonstrate that the pro-inflammatory molecule, PAF, induces cell death in cultured CNS glial cells and that this effect is, in part, dependent on caspase-3 activation.

Production of leukotrienes in a model of focal cerebral ischaemia in the rat

1. The aim of this work was to evaluate the role of leukotrienes in brain damage in vivo in a model of focal cerebral ischaemia in the rat, obtained by permanent occlusion of middle cerebral artery. 2. A significant (P < 0.01) elevation of LTC(4), LTD(4) and LTE(4) (cysteinyl-leukotrienes) levels occurred 4 h after ischaemia induction in the ipsilateral cortices of ischaemic compared to sham-operated animals (3998 +/- 475 and 897 +/- 170 fmol g(-1) tissue, respectively, P < 0.01). 3. The NMDA receptor antagonist MK-801 and the adenosine A(2A) receptor antagonist SCH 58261 were administered in vivo at doses known to reduce infarct size and compared with the leukotriene biosynthesis inhibitor MK-886. 4. MK-886 (0.3 and 2 mg kg(-1) i.v.) and MK-801 (3 mg kg(-1) i.p.) decreased cysteinyl-leukotriene levels (-78%, P < 0.05; -100%, P < 0.01; -92%, P < 0.01, respectively) 4 h after permanent occlusion of the middle cerebral artery, whereas SCH 58261 (0.01 mg kg(-1) i.v.) had no significant effects. 5. MK-886 (2 mg kg(-1) i.v.) was also able to significantly reduce the cortical infarct size by 30% (P < 0.05). 6. We conclude that cysteinyl-leukotriene formation is associated with NMDA receptor activation, and that it represents a neurotoxic event, the inhibition of which is able to reduce brain infarct area in a focal ischaemic event.

Effects of ATP on phosphoinositide hydrolysis and prostaglandin E2 generation in rabbit astrocytes

Extracellular ATP secreted from stimulated nerves plays a role in neurotransmission. This study examined the effects of extracellular ATP on phospholipase A2 and C signalling pathways in rabbit astrocytes. ATP caused prostaglandin E2 (PGE2) generation and phosphoinositide hydrolysis in a time- and concentration-dependent manner. A P2y purinoceptor-selective agonist, 2-methylthio-ATP also caused phosphoinositide hydrolysis, but not PGE2 generation. A P2x purinoceptor-selective agonist, alpha, beta-methylene-ATP did not cause either phosphoinositide hydrolysis or PGE2 generation. Although pertussis toxin had no effect on 2-methylthio-ATP-induced phosphoinositide hydrolysis, it markedly decreased ATP-induced PGE2 generation, with significant inhibition of phosphoinositide hydrolysis. Dexamethasone and indomethacin which potently inhibited ATP-induced PGE2 generation, caused partial inhibition of phosphoinositide hydrolysis, suggesting that pertussis toxin-sensitive component of ATP-induced phospholipase C activation is mediated by cyclo-oxygenase metabolites of arachidonic acid. These results suggest that a stimulation of P2y receptor results in phospholipase C activation in a pertussis toxin-insensitive manner, and that a P2 receptor other than the P2y or P2x subtypes is involved in ATP-induced phospholipase A2 activation via a pertussis toxin-sensitive G protein.

Production of eicosanoids derived from 20:4n-6 and 20:5n-3 in primary cultures of turbot (Scophthalmus maximus) brain astrocytes in response to platelet activating factor, substance P and interleukin-1 beta

Primary cultures of turbot (Scophthalmus maximus) brain astroglial cells established in medium containing fetal bovine serum contain increased proportions of 18:1(n-9), total (n-9) and (n-6) polyunsaturated fatty acids (PUFA) and greatly reduced (n-3) PUFA in comparison with turbot brain. Supplementation with a mixture of 5 microM eicosapentaenoic [20:5(n-3)] and 25 microM docosahexaenoic [22:6(n-3)] acids for 4 days significantly increased the percentages of these acids in total cellular lipid of turbot astrocytes and restored the (n-3) PUFA composition of the cells to that found in turbot brain. The production of prostaglandins (PG) E and F of the 2- and 3-series and leukotrienes (LT) C4 and C5 in response to various agonists was determined in PUFA-supplemented astrocytes. Calcium ionophore A23187, platelet activating factor and substance P stimulated the production of both PGF and PGE. Interleukin-1 beta significantly stimulated the production of PGF only. There were differences between the agonists in their effects on the relative levels of 2- and 3-series PGs produced. Only very low amounts of LTC were produced by the turbot astrocytes, with only substance P showing a minor stimulatory effect.

Immunofluorescent localization of constitutive and inducible prostaglandin H synthase in ovine astroglia

We have used immunofluorescent techniques to examine the distribution of prostaglandin H synthase (PGHS) in ovine astrocyte-enriched secondary cultures and in mixed cortical cells in primary culture. A battery of monoclonal and polyclonal antibodies specific for the constitutive (PGHS-1) or inducible (PGHS-2) forms of the enzyme were used to examine the cells in culture. Varying levels of PGHS-1 and PGHS-2-specific immunofluorescence were seen in astrocytes as well as in other cells. The fluorescent pattern and localization seen with antisera to both PGHS-1 and PGHS-2 were similar but were not identical. Both immunoreactive species were confined to nuclear and perinuclear regions of the cell, with no immunoreactivity evident in plasmalemma. In addition, PGHS-2-specific fluorescence was concentrated often as a homogeneous ring around the nucleus in heavily stained astrocytes. Mixed cortical glia/fibroblasts in primary culture were double labeled with antibodies to glial fibrillary acidic protein (GFAP) and to PGHS-2. GFAP and PGHS-2 were colocalized in clusters of astrocytes, but PGHS-2 was evident in GFAP- cells as well. Cells treated with the mitogenic agent phorbol dibutyrate displayed more PGHS-2+ immunofluorescence compared to either vehicle control or cells pretreated with dexamethasone. We conclude that astrocytes cultured in serum express both constitutive and inducible forms of PGHS and that PGHS-2 is induced by mitogens in this cell type.

A comparative study of arachidonic acid metabolism in rabbit cultured astrocytes and human astrocytoma cells (1321N1)

1. ATP, bradykinin (BK), and A-23187 activated the generation of prostaglandin (PG) E2 and thromboxane (TX) B2 in rabbit astrocytes, but not in human astrocytoma cells (1321N1). 2. In human astrocytoma cells, ATP, BK, and A-23187 could not release [3H]arachidonic acid (AA) from [3H]AA-labeled cells and exogenous AA was not converted to TXB2 and PGE2, suggesting the lack of phospholipase (PL) A2 and cyclooxygenase activities in 1321N1 human astrocytoma cells, although they express TXA2 receptors. 3. In rabbit astrocytes, ATP and BK, but not A-23187, showed increased accumulation of inositol phosphates, indicating that an increase in intracellular Ca2+ concentration alone would not be enough to activate PLC. Furthermore, indomethacin, a cyclooxygenase inhibitor, partially attenuated ATP-induced phosphoinositide hydrolysis, indicating that cyclooxygenase product(s) would secondarily activate PLC in response to ATP.

Evidence for the involvement of 5-lipoxygenase products in the regulation of the expression of the proenkephalin gene in cultured astroglial cells

Cultured astroglial cells secrete eicosanoids which are produced by the cyclooxygenase and lipoxygenases. These cells also transcribe the proenkephalin gene. In the present study, it was investigated whether agents which inhibit the metabolism of arachidonic acid affect the basal and stimulated expression of the gene. Tetradecanoyl phorbol acetate (TPA; 1-1000 nmol/l) increases the concentration of proenkephalin mRNA in these cells by activating protein kinase C. The enhancement in proenkephalin mRNA caused by TPA (10 nmol/l) was not affected by the cyclooxygenase inhibitor indomethacin (5 mumol/l). However, nordihydroguaiaretic acid, which blocks cyclooxygenase and lipoxygenases, potentiated the effect of TPA on proenkephalin mRNA, when used at concentrations of 0.5-50 mumol/l. Two selective inhibitors of 5-lipoxygenase, i.e. MK886 (5 mumol/l) and BAY X1005 (1 mumol/l), also enhanced the effect of TPA (10 nmol/l) without affecting the basal expression of the gene. When added to the incubation medium, leukotriene E4 (10-1000 nmol/l) diminished in a dose-dependent manner the basal and TPA-induced expression of the proenkephalin gene. It is concluded that in astroglial cells derived from cortex of new-born rats products of 5-lipoxygenase can diminish the action of protein kinase C on the proenkephalin gene.

Arachidonic acid as a neurotoxic and neurotrophic substance

In this article we summarize a wide variety of properties of arachidonic acid (AA) in the mammalian nervous system especially in the brain. AA serves as a biologically-active signaling molecule as well as an important component of membrane lipids. Esterified AA is liberated from the membrane by phospholipase activity which is stimulated by various signals such as neurotransmitter-mediated rise in intracellular Ca2+. AA exerts many biological actions which include modulation of the activities of protein kinases and ion channels, inhibition of neurotransmitter uptake, and enhancement of synaptic transmission. AA serves also as a precursor of a variety of eicosanoids, which are formed by oxidative metabolism of AA. AA cascade is activated under several pathological conditions in the brain such as ischemia and seizures, and may be involved in irreversible tissue damage. On the other hand, AA can show beneficial influences on brain tissues and cells in several situations. In a recent study using cultured brain neurons, we have found that AA shows quite distinct actions at a narrow concentration range, such as induction of cell death, promotion of cell survival and enhancement of neurite extension. The neurotoxic action is mediated by free radicals generated by AA metabolism, whereas the neurotrophic actions are exerted by AA itself. The observed in vitro actions of AA might be related to important roles of AA in brain pathogenesis and neural development.

Eicosanoids are produced by microglia, not by astrocytes, in rat glial cell cultures

To determine principal sources of eicosanoid production in glial cells, we analyzed the metabolites of arachidonic acid in cultured rat glial cells by use of reversed-phase, high-performance liquid chromatography and an on-line radioisotope detector. Prostaglandin D2, leukotriene B4, leukotriene C4, and 5-hydroxyeicosatetraenoic acid were present in cultures in which microglia appeared on a monolayer astrocytes. None were detected in culture dishes that contained only astrocytes, although astrocytes have been believed to be a main source of eicosanoid production in brain.

Inhibition of platelet aggregation and eicosanoid production by phenolic components of olive oil

This study was designed to investigate the in vitro effects of phenolic compounds extracted from olive oil and from olive derived fractions. More specifically, we investigated the effects on platelets of 2-(3,4-di-hydroxyphenyl)-ethanol (DHPE), a phenol component of extra-virgin olive oil with potent antioxidant properties. The following variables were studied: aggregation of platelet rich plasma (PRP) induced by ADP or collagen, and thromboxane B2 production by collagen or thrombin-stimulated PRP. In addition, thromboxane B2 and 12-hydroxyeicosatetraenoic acid (12-HETE) produced during blood clotting were measured in serum. Preincubation of PRP with DHPE for at least 10 min resulted in maximal inhibition of the various measured variables. The IC50s (concentration resulting in 50% inhibition) of DHPE for ADP or collagen-induced PRP aggregations were 23 and 67 microM, respectively. At 400 microM DHPE, a concentration which completely inhibited collagen-induced PRP aggregation, TxB2 production by collagen- or thrombin-stimulated PRP was inhibited by over 80 percent. At the same DHPE concentration, the accumulation of TxB2 and 12-HETE in serum was reduced by over 90 and 50 percent, respectively. We also tested the effects of PRP aggregation of oleuropein, another typical olive oil phenol, and of selected flavnoids (luteolin, apigenin, quercetin) and found them to be much less active. On the other hand a partially characterized phenol-enriched extract obtained from aqueous waste from olive oil showed rather potent activities. Our results are the first evidence that components of the phenolic fraction of olive oil can inhibit platelet function and eicosanoid formation in vitro, and that other, partially characterized, olive derivatives share these biological activities.

Inhibition by n-3 fatty acids of arachidonic acid metabolism in a primary culture of astroglial cells

Docosahexaenoic acid (22:6 n-3) was present in low concentrations in a primary culture of rat brain astroglial cells, when compared to brain cortex. We have thus supplemented these cells with this fatty acid and investigated the effects of its incorporation in cell phospholipids on the conversion of arachidonic acid, 20:4 n-6, through the cyclo and lipoxygenase pathways, after cell stimulation. Docosahexaenoic acid-enriched cells produced less thromboxane B2 and 6-keto-Prostaglandin F1 alpha and markedly less 12-hydroxyeicosatetraenoic acid than unsupplemented cells, after stimulation with the Ca(2+)-ionophore A23187. The production of 15-hydroxyeicosatetraenoic acid from arachidonic acid was slightly increased in docosahexaenoic acid-supplemented cells. We have also supplemented these cells with eicosapentaenoic acid (20:5 n-3) and, in addition to accumulation of this fatty acid in cell phospholipids, we found elevation of 22:5 n-3 and some increment of 22:6, confirming that glial cells are able to convert eicosapentaenoic acid to the long chain, more unsaturated derivatives. In conclusion, n-3 fatty acids, when supplemented to glial cells, appear to modulate the arachidonic acid cascade and to be converted through the elongation and desaturation pathways.

Astrocyte-derived lipoxygenase product evokes endothelium-dependent relaxation of the basilar artery

The goal of this study was to examine the possible production of vasoactive factors by astrocytes. We consistently observe that rat astroglial cells in suspension produce marked relaxation when added to precontracted rings of intact (but not endothelium-denuded) rabbit basilar artery. The ultimate mediator of this relaxation was endothelium-derived nitric oxide whose synthesis is activated by an as yet unidentified factor(s) produced tonically by astrocytes. The factor is relatively stable, and is not arachidonate, or a product of cyclooxygenase or P450 metabolism. Based upon studies with selective inhibitors, the factor appears to result from 12- or 15-lipoxygenase metabolism, the products of which are known to be vasoactive. In a separate series of experiments, astrocyte-conditioned medium stimulated the production of citrulline from L-arginine by nitric oxide synthase in bovine aortic endothelial cells. The possible significance for central nervous system (CNS) pathophysiology of an astrocyte-derived vasodilator is discussed.

Eicosanoid and inositol phosphate response to platelet-activating factor (PAF) and to a PAF antagonist in rat astroglial cells

The effects of different concentrations of exogenous platelet-activating factor (PAF) on the formation of arachidonic acid-cyclooxygenase metabolites and on the production of inositol phosphates have been investigated in a primary culture of rat astroglial cells. The cells were used at confluence and the purity was checked by immunostaining of the culture with specific antibodies against glial fibrillary acidic protein. Incubation of the cells with PAF (range 10(-9) to 10(-6) M) resulted in maximal accumulation of total inositol phosphate (620 +/- 60% increment over basal values, P < 0.001) at the concentration of 10(-8) M, after 1 min of stimulation. Smaller inositol phosphate accumulation occurred at higher concentrations of the agonist and at longer stimulation time. After 1 min of stimulation with PAF, the accumulation of the cyclooxygenase metabolites, thromboxane B2 (630 +/- 58 vs 20 +/- 2 pg/mg protein in non-stimulated samples) and 6-keto-prostaglandin F1 alpha (132 +/- 15 vs 55 +/- 7 pg/mg protein in non-stimulated samples) was also maximal at 10(-8) M concentration of the agonist. When the cultures were stimulated with PAF or Ca(2+)-ionophore after preincubation with equimolar concentration of the PAF inhibitor BN 52021, a significant inhibition in the synthesis of both inositol phosphates and cyclooxygenase metabolites occurred only in the PAF-stimulated cells.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Molecular cloning of a 12-lipoxygenase cDNA from rat brain

A cDNA encoding an arachidonate 12-lipoxygenase from rat brain was obtained by polymerase chain reaction cloning. Primers specific for porcine leukocyte 12-lipoxygenase cDNA were used to isolate the initial polymerase-chain-reaction product (395 bp). The final sequence of the rat 12-lipoxygenase cDNA coding region (1989 bp) was verified by analysis of several separate polymerase-chain-reaction products. The open reading frame corresponded to a protein of 662 amino acid residues, with a calculated molecular mass of 75,305 Da. Also the rat 12-lipoxygenase contained the six conserved histidines, characteristic for all cloned lipoxygenases. It displayed the highest degree of identity to porcine leukocyte 12-lipoxygenase (71%) and to human 15-lipoxygenase (75%), with less resemblance to human platelet 12-lipoxygenase (59%) or rat leukocyte 5-lipoxygenase (41%). The recombinant enzyme was expressed in Escherichia coli and incubated with arachidonic acid. Primarily 12-lipoxygenase (but also some 15-lipoxygenase) enzyme activity was obtained. A part of the brain 12-lipoxygenase cDNA was used as probe in Northern blots. A 2.7-kb mRNA was more abundant in RNA from rat leukocytes, lung, and aorta, than in RNA from rat brain. Sequencing of parts of the corresponding cDNAs (from leukocytes and lung), and comparison to the brain 12-lipoxygenase sequence, indicated that these mRNAs from the different rat tissues were identical.