Inositol phospholipids are probably not the source of arachidonic acid for eicosanoid synthesis in astrocytes

Effect of Glutamate and Ionomycin on the Release of Arachidonic Acid, Prostaglandins and HETEs from Cultured Neurons and Astrocytes

The release of arachidonic acid (ArA) metabolites from mouse neurons and astrocytes in primary culture has been studied in response to ionomycin or glutamate stimulation. Cells were preincubated with [3H]ArA for 24 h and the radioactivity released was examined by HPLC. In striatal, cortical and hippocampal neurons, glutamate and ionomycin strongly stimulated the release of ArA, but neither prostaglandins (PGs) nor hydroxyeicosatetraenoic acids (HETEs) could be detected. If they were released, these latter compounds represented < 0.02% of the amount of ArA. In contrast, in astrocyte cultures, ionomycin (but not glutamate) strongly stimulated the release of PGs and HETEs as well as ArA. Reversed- and straight-phase HPLC analysis revealed the presence of PGD2, PGE2, PGF2alpha, 12-hydroxyheptadeca-5,8,10-trienoic acid (HHT) and HETEs (15-HETE, 11-HETE and 5-HETE). Indomethacin inhibited the release of PGs and HHT, but also that of 11- and 15-HETE, indicating that these two HETEs may be produced through the cyclooxygenase pathway. Metabolism of [3H]ArA was also examined in cellular homogenates. Although > 50% of the [3H]ArA was metabolized to PGF2alpha, PGE2, PGD2, HHT, 15- and 11-HETE in cultured astrocyte homogenates, no [3H]ArA metabolism could be detected in cultured striatal neuron homogenates. Moreover, neuronal homogenates did not inhibit the metabolism of [3H]ArA observed in either astrocyte or platelet homogenates. These results indicate that central neurons in primary culture possess very low lipoxygenase and cyclooxygenase activities. They emphasize the need to identify the cellular source of ArA metabolites in the brain, particularly when considering the multiple new messenger roles proposed for these molecules, such as that of retrograde messengers involved in synaptic plasticity phenomena.

Endothelin-3 attenuates the cyclic GMP responses to C-type natriuretic peptide in cultured mouse astrocytes

The effect of endothelin-3 (ET-3) on cyclic GMP (cGMP) responses to C-type natriuretic peptide (CNP) was studied in primary cultures of mouse astrocytes. Attenuation of CNP-stimulated cGMP formation by ET-3 was time-dependent, with maximum inhibition achieved at 30 min of preincubation. ET-3 suppressed cGMP production in response to 10 nM CNP in a dose-dependent fashion, with an IC50 of 0.04 nM and a maximal inhibitory concentration of 1 microM, which led to a 66% reduction of the cGMP increment from 45.0 +/- 4.2 pmol/mg protein to 15.4 +/- 2.6 pmol/mg protein. ET-1, ET-2, and ET-3 were equipotent in suppressing the CNP-induced cGMP response, suggesting that this effect was mediated by ETB receptors. Staurosporine, Ro 31-8220, calcium-free medium, nifedipine, verapamil, lanthanum, thapsigargin, BAPTA, W7, calmidazolium, U-73122, neomycin, quinacrine, wortmannin, herbimycin-A, okadaic acid, and sodium orthovanadate failed to block the effect of ET-3. Cycloheximide (100 microM), however, partially but significantly reversed the inhibitory effect of ET-3 on CNP-induced cGMP from 48.2 to 73.3% of the control value. The results support the premise that ET-3 and CNP interact within the central nervous system. The data also suggest that cGMP accumulation in mouse astrocytes is mediated by activation of certain kinases through as yet undefined mechanisms and not by protein kinase C, increased intracellular calcium, or other second messenger pathways such as phospholipases A2, C, D, tyrosine kinase, or protein phosphatases.

Altered lipid metabolism in the presence and absence of extracellular Ca2+ during combined oxygen-glucose deprivation in primary astrocyte cultures

The effect of combined oxygen-glucose deprivation (COGD) on lipid metabolism in primary rat cortical astrocyte cultures was studied in both the presence and absence of extracellular Ca2+. In this study, increases in intracellular Ca2+ from internal Ca2+ stores were not inhibited nor were internal Ca2+ levels buffered. Combined oxygen-glucose deprivation resulted in a quantitative reduction in phospholipid levels and an increase in free fatty acid and lysophospholipid levels. Four hours after the onset of COGD, ethanolamine- and choline glycerophospholipid levels were decreased by 40 and 46% from control levels in the presence of Ca2+, respectively. A similar decrease was found 6 hr after onset of COGD in the absence of Ca2+. These changes were accompanied by elevated levels of the corresponding lysophospholipids. However, the increases in lysophospholipid content did not account for the entire loss of ethanolamine- or choline glycerophospholipid. Phosphatidylserine was reduced in both the presence and absence of extracellular Ca2+ but phosphatidylinositol was only decreased in the absence of Ca2+. Statistically significant increases in total fatty acid (FA) and polyunsaturated fatty acid (PUFA) levels occurred at 30 min and 3 hr after the onset of COGD in the absence and presence of Ca2+, respectively. Arachidonic acid levels were increased in both groups by 1 hr. These increases in FA, PUFA, and specifically arachidonic acid were time-dependent and increased over the 12 hr of COGD. Collectively, these results indicate the activation of an acylhydrolase mechanism in the possible presence of an inhibited reacylation pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Regulation of free arachidonic acid levels in isolated salivary glands from the lone star tick: a role for dopamine

An important regulatory step for prostaglandin synthesis is the availability of the precursor, free arachidonic acid (AA). In isolated salivary glands of the lone star tick, Amblyomma americanum (L.), the level of free AA appears to depend on higher phospholipase A2 (PLA2) activity rather than decreased rates of re-esterification by lysophosphatide acyl transferase (LAT). This conclusion is supported by experiments where inhibition of LAT with merthiolate was without effect, while the calcium ionophore A23187, a PLA2 stimulant, increased levels of free AA. The PLA2 activity in A. americanum was reduced by the substrate analog, PLA2 inhibitor, oleyloxyethyl phosphorylcholine in a dose-dependent manner, but was insensitive to the other mammalian PLA2 inhibitors mepacrine (20 microM), aristolochic acid (45 microM), and dexamethasone (50 microM). No substrate preference was observed for the functional group of the phospholipid, with phosphatidylcholine and phosphatidylethanolamine being equal sources of AA in A23187-stimulated glands. Compared to phospholipids containing other fatty acids, only arachidonyl-phospholipid (arachidonyl-PL) was significantly hydrolyzed by PLA2 activity in A23187-stimulated glands. Dopamine was as effective as A23187 as a stimulant of PLA2 activity in isolated glands, but this effect was abolished in the presence of the calcium channel blocking agent verapamil. It is concluded that free AA levels in tick salivary glands are increased through activation of a Type IV-like PLA2 following an increase of intracellular calcium caused by the opening of voltage-dependent calcium channels due to dopamine stimulation.

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.

Purinergic P2Y receptors on astrocytes are directly coupled to phospholipase A2

ATP stimulates arachidonic acid mobilization and eicosanoid production in cultured astrocytes via P2Y-purinergic receptors. To assist in determining the mechanism of phospholipase A2 activation and the role of calcium in eicosanoid production, cultures were pretreated with pertussis toxin (PTx). ATP-evoked eicosanoid release was inhibited by PTx in a concentration-dependent fashion. Inositol phospholipid hydrolysis was partially attenuated by PTx, but the concentrations required were approximately 50 times greater than those for inhibition of eicosanoid production, suggesting that phospholipase C activation is not necessary for eicosanoid synthesis. Stimulation of eicosanoid release by other P2Y-purinergic receptor agonists was also inhibited by PTx; however, PTx had no effect on eicosanoid release evoked by ionomycin or thapsigargin, nor did it affect ATP-stimulated calcium influx or mobilization from intracellular stores. Increases in intracellular free calcium concentration alone were insufficient to stimulate eicosanoid production, but maximal production was dependent upon the concentration of extracellular calcium. These results suggest that the P2Y-purinergic receptor is coupled to phospholipase A2 via a guanine nucleotide-binding protein, and that extracellular calcium may also be involved in the synthesis of eicosanoids by astrocytes.

Metabolism of [14C]arachidonic acid-labeled lipids in quiescent and OAG-stimulated ascites tumor cells

1. A rapid uptake and esterification of [14C]arachidonic acid during the first 4 hr of cultivation of ascites cells in serum-deprived medium was observed followed by a fast turnover of the fatty acid. 2. Labeling and turnover of esterified arachidonate in individual phospholipid classes was in the order: phosphatidylcholine (PC) greater than phosphatidylinositol (PI) much greater than phosphatidylinositol-4-phosphate (PIP) and -4,5-bisphosphate (PIP2) greater than phosphatidylethanolamine (PE) greater than PE-plasmalogens. 3. In cells stimulated with 1-oleoyl-2-acetyl-sn-glycerol a transient course of arachidonic acid incorporation into PC, PI, PIP and PIP2 was determined peaking 30 min after stimulation, indicating both esterification and release under these conditions. 4. The release of arachidonate was blocked by quinacrine which is a specific inhibitor of phospholipase A2.

ATP-evoked arachidonic acid mobilization in astrocytes is via a P2Y-purinergic receptor

To reveal more of the mechanism whereby ATP induces arachidonic acid (AA) mobilization in astrocytes, primary cell cultures prelabeled with [3H]AA were exposed to ATP and various analogs. Release of 3H was dose and time dependent and was inhibited by blocking ATP binding. The potencies of a range of ATP analogs in mobilizing AA were consistent with that predicted for the involvement of a P2Y-purinergic receptor. Mobilization of AA was not due to non-specific cell permeabilization, as assessed by leakage of cytoplasmic lactate dehydrogenase. AA mobilization by ATP was reduced when mobilization of intracellular calcium was inhibited and in the absence of extracellular calcium. Thapsigargin, which induces release of intracellular calcium, evoked mobilization of AA and thromboxane formation, findings similar to the effects of ATP. These results suggest that ATP stimulates AA mobilization via a P2Y-purinergic receptor and that, although extracellular calcium is involved, mobilization of intracellular calcium activates phospholipase A2.

Formation and function of eicosanoids in the central nervous system

Neuronal firing during experimental convulsions triggered a large increase in brain eicosanoid synthesis. Mature astrocytes are an important source of cerebral prostanoids. Endogenously formed prostaglandins possess anticonvulsive properties of biological relevance. These conclusions suggest new ideas that might explain the formation and functions of prostanoids in the brain. First, as augmented neuronal discharge is a prerequisite for enhanced prostanoid synthesis during seizures, a functional coupling between firing neurons and prostanoid-forming astrocytes may be expected. Second, the anticonvulsive effects of endogenous prostanoids suggest that astroglia-derived substances might regulate neuronal activity. The phenomenon of convulsion-induced prostanoid synthesis may, therefore, represent a new example of neuron-glia interaction. Neither K+-induced membrane depolarization nor receptor activation by drugs with affinity to alpha or beta adrenoceptors, dopamine, serotonin, muscarine, histamine, GABA, glutamate, aspartate, adenosine, and opioid receptors evoked eicosanoid synthesis in astrocytes. The only physiologically relevant ligand that induced prostanoid synthesis concentration dependently in astrocytes was ATP and related nucleotide triphosphates, as well as nucleotide disphosphates. In peripheral nerves ATP serves as a cotransmitter. The effect of the P2 agonists was reduced by pertussis toxin. The mechanism by which eicosanoids regulate neuronal activity remains to be elucidated.

ATP-evoked Ca2+ mobilisation and prostanoid release from astrocytes: P2-purinergic receptors linked to phosphoinositide hydrolysis

Astrocyte cultures prelabelled with either [3H]inositol or 45Ca2+ were exposed to ATP and its hydrolysis products. ATP and ADP, but not AMP and adenosine, produced increases in the accumulation of intracellular 3H-labelled inositol phosphates (IP), efflux of 45Ca2+, and release of thromboxane A2 (TXA2). Whereas ATP-stimulated 3H-IP accumulation was unaffected, its ability to promote TXA2 release was markedly reduced by mepacrine, an inhibitor of phospholipase A2 (PLA2). ATP-evoked 3H-IP production was also spared following treatment with the cyclooxygenase inhibitor, indomethacin. We conclude that ATP-induced phosphoinositide (PPI) breakdown and 45 Ca2+ mobilisation occurred in parallel with, if not preceded, the release of TXA2. Following depletion of intracellular Ca2+ with a brief preexposure to ATP in the absence of extracellular Ca2+, the release of TXA2 in response to a subsequent ATP challenge was greatly reduced when compared with control. These results suggest that mobilisation of cytosolic Ca2+ may be the stimulus for PLA2 activation and, thus, TXA2 release. Stimulation of alpha 1-adrenoceptors also caused PPI breakdown and 45 Ca2+ efflux but not TXA2 release. The effects of ATP and noradrenaline (NA) on 3H-IP accumulation were additive, but their combined ability to increase 45Ca2+ efflux was not. Interestingly, in the presence of NA, ATP-stimulated TXA2 release was reduced. Our data provide evidence that functional P2-purinergic receptors are present on astrocytes and that ATP is the first physiologically relevant stimulus found to initiate prostanoid release from these cells.

Astrocytes as eicosanoid-producing cells

A variety of prostaglandins and leukotrienes, together with thromboxane and prostacyclin metabolites, can be detected in central nervous tissues and in cerebrospinal fluid. Defined cultures of astrocytes have revealed these cells to be a major source of eicosanoids. In common with other eicosanoid-producing cells, agents such as calcium ionophores and phorbol esters are potent stimuli for promoting release. While in other tissues agonists for receptors linked to calcium mobilisation prompt eicosanoid release, this does not seem to be the case in astrocytes, though a range of such receptors are present. The notable exceptions to this observation are adenosine triphosphate and adenosine diphosphate, presumably acting through P2 purinergic receptors. Many cell types in the CNS are targets for eicosanoids, possessing receptors linked to adenylate cyclase or phospholipase C. An appreciation of the functional significance of activation of these receptors is just now beginning. Eicosanoids have effects in the CNS that involve not only the vascular supply but also synaptic modulation and immune regulation.

Morphological changes in cultured astrocytes following exposure to calcium ionophores

Induced differentiation of cultured astrocytes to a process-bearing morphology has been described with a variety of agents known to increase intracellular cyclic AMP. The ionophores A23187 and ionomycin were found to induce a reversible change in astrocyte morphology to a process-bearing form. Cell death also occurs following exposure to either A23187 or ionomycin. This change in morphology which occurs following exposure to ionomycin in calcium- and serum-containing Dulbecco's Modified Eagle's medium also occurs in the absence of extracellular calcium. These findings suggest a role for other second messengers than cyclic AMP in controlling astrocyte morphology.

The morphological differentiation of cultured astrocytes induced by ionomycin: lack of dependence on protein kinase C activation

In a previous paper we have shown that the calcium ionophores, A23187 and ionomycin, induce a morphological differentiation of cultured astrocytes to a process-bearing form. As A23187 is known to induce the turnover of inositol phospholipids in astrocytes, and this response is dependent on extracellular calcium, we examined the morphological effects of exposure of cells to protein kinase C activators. 1,2-Dioctanoyl-sn-glycerol, mezerein and various phorbol esters are shown to result in no alteration in the morphology of cultured astrocytes. The results suggest that the morphological response of cultured astrocytes to calcium ionophores is not dependent on activation of protein kinase C.

Phorbol ester stimulation of prostanoid synthesis by cultured astrocytes

The role of protein kinase C in mediating the synthesis and release of various prostanoids (prostaglandins E2, I2, F2 alpha and thromboxane A2) from astroglial cells derived from neonatal rat cerebrum and maintained in primary culture was investigated using phorbol ester. Phorbol myristate acetate stimulated the release of arachidonic acid from prelabelled cells and all 4 prostanoids in a dose-dependent manner (EC50 = 300 nM). This effect was inhibited by the protein kinase inhibitor 1-(-5-isoquinolinylsulfonyl)-2-methylpiperizine (IC50 = 25 microM) and the phospholipase A2 inhibitor, mepacrine (IC50 = 5 microM). In addition, the stimulatory effect of the phorbol ester was not apparent in cells which had been depleted specifically of protein kinase C. In the presence of the calcium ionophore A23187, phorbol ester-stimulated prostanoid release was enhanced. In the absence of extracellular calcium, there was no prostanoid-stimulation by phorbol ester, but the calcium channel blocker verapamil did not mimic this effect. We conclude that stimulation of protein kinase C by phorbol ester elicits prostanoid synthesis and release by a process that involves calcium influx and the activation of phospholipase A2.