Inositol phospholipid arachidonic acid metabolism in GH3 pituitary cells

Structural and molecular determinants of HIV-1 Gag binding to the plasma membrane

Targeting of the Gag polyprotein to the plasma membrane (PM) for assembly is a critical event in the late phase of immunodeficiency virus type-1 (HIV-1) infection. Gag binding to the PM is mediated by interactions between the myristoylated matrix (MA) domain and PM lipids. Despite the extensive biochemical and in vitro studies of Gag and MA binding to membranes over the last two decades, the discovery of the role of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P₂] in Gag binding to the PM has sparked a string of studies aimed at elucidating the molecular mechanism of retroviral Gag–PM binding. Electrostatic interactions between a highly conserved basic region of MA and acidic phospholipids have long been thought to be the main driving force for Gag–membrane interactions. However, recent studies suggest that the mechanism is rather complex since other factors such as the hydrophobicity of the membrane interior represented by the acyl chains and cholesterol also play important roles. Here we summarize the current understanding of HIV-1 Gag–membrane interactions at the molecular and structural levels and briefly discuss the underlying forces governing interactions of other retroviral MA proteins with the PM.

Structural basis for targeting HIV-1 Gag proteins to the plasma membrane for virus assembly

During the late phase of HIV type 1 (HIV-1) replication, newly synthesized retroviral Gag proteins are targeted to the plasma membrane of most hematopoietic cell types, where they colocalize at lipid rafts and assemble into immature virions. Membrane binding is mediated by the matrix (MA) domain of Gag, a 132-residue polypeptide containing an N-terminal myristyl group that can adopt sequestered and exposed conformations. Although exposure is known to promote membrane binding, the mechanism by which Gag is targeted to specific membranes has yet to be established. Recent studies have shown that phosphatidylinositol (PI) 4,5-bisphosphate [PI(4,5)P(2)], a factor that regulates localization of cellular proteins to the plasma membrane, also regulates Gag localization and assembly. Here we show that PI(4,5)P(2) binds directly to HIV-1 MA, inducing a conformational change that triggers myristate exposure. Related phosphatidylinositides PI, PI(3)P, PI(4)P, PI(5)P, and PI(3,5)P(2) do not bind MA with significant affinity or trigger myristate exposure. Structural studies reveal that PI(4,5)P(2) adopts an "extended lipid" conformation, in which the inositol head group and 2'-fatty acid chain bind to a hydrophobic cleft, and the 1'-fatty acid and exposed myristyl group bracket a conserved basic surface patch previously implicated in membrane binding. Our findings indicate that PI(4,5)P(2) acts as both a trigger of the myristyl switch and a membrane anchor and suggest a potential mechanism for targeting Gag to membrane rafts.

Activation of PAF receptors results in enhanced synthesis of 2-arachidonoylglycerol (2-AG) in immune cells

The endocannabinoid signaling system is believed to play a down-regulatory role in the control of cell functions. However, little is known about the factors activating endocannabinoid synthesis and which of two known endocannabinoids, 2-arachidonoylglycerol (2-AG) or N-arachidonoylethanolamine (20:4n-6 NAE, anandamide), is of physiological importance. We approached these questions by studying a possible link between cell activation with 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor, PAF) and the generation of 2-AG and anandamide in human platelets and mouse P388D1 macrophages. Human platelets responded to stimulation with the production of various 1- and 2-monoacylglycerols, including 2-AG, whereas stimulation of P388D1 macrophages induced the rapid and selective generation of 2-AG, which was immediately released into the medium. The effect of PAF was receptor mediated, as PAF receptor antagonist BN52021 blocked the effect. The treatment did not change the content of anandamide in either macrophages or platelet-rich plasma. The inhibitors of PI- and PC-specific phospholipases C (U73122 and D609) as well as PI3-kinase inhibitor (wortmannin) attenuated PAF-induced 2-AG production in macrophages. These data suggest a direct role for the endocannabinoid system in controlling immune cell activation status and indicate that 2-AG rather than anandamide is the endocannabinoid rapidly produced in response to proinflammatory stimulation of immune cells.

Essentiality of fatty acids

All fatty acids have important functions, but the term "essential" is applied only to those polyunsaturated fatty acids (PUFA) that are necessary for good health and cannot be completely synthesized in the body. The need for arachidonic acid, which is utilized for eicosanoid synthesis and is a constituent of membrane phospholipids involved in signal transduction, is the main reason why the n-6 class of PUFA are essential. Physiological data indicate that n-3 PUFA also are essential. Although eicosapentaenoic acid also is a substrate for eicosanoid synthesis, docosahexaenoic acid (DHA) is more likely to be the essential n-3 constituent because it is necessary for optimal visual acuity and neural development. DHA is present in large amounts in the ethanolamine and serine phospholipids, suggesting that its function involves membrane structure. Because the metabolism of n-6 PUFA is geared primarily to produce arachidonic acid, only small amounts of 22-carbon n-6 PUFA are ordinarily formed. Thus, the essentiality of n-3 PUFA may be due to their ability to supply enough 22-carbon PUFA for optimal membrane function rather than to a unique biochemical property of DHA.

TRH-evoked entry of extracellular calcium in GH4C1 cells: possible importance of arachidonic acid metabolites

Previous studies have shown that stimulating pituitary GH4C1 cells with thyrotropin-releasing hormone (TRH) evoked a biphasic change in cytosolic free Ca2+ concentration ([Ca2+]i): a rapid release of sequestered Ca2+ due to the production of inositol-1,4,5-trisphosphate, and Ca2+ entry via both voltage-operated Ca2+ channels and a presently unknown voltage-independent influx pathway. The aim of the present study was to further evaluate to which extent the TRH-evoked changes in [Ca2+]i were dependent on entry of extracellular Ca2+, and which mechanisms participated in regulating this Ca2+ entry. Pretreatment of the cells with 4-bromophenylacylbromide (an inhibitor of phospholipase A2), nordihydroguaiaretic acid (an inhibitor of lipoxygenase), and econazole (an inhibitor of both lipoxygenase and cytochrome P-450 enzymes), attenuated the TRH-evoked increase in [Ca2+]i, suggesting that noncyclooxygenase metabolites of arachidonic acid or cytochrome P-450 metabolites may participate in regulating the TRH-evoked entry of extracellular Ca2+. Both nordihydroguaiaretic acid and econazole showed a similar inhibition of the Ca2+ entry, as did SKF 96365, a compound previously shown to inhibit receptor-activated Ca2+ entry. We also showed that arachidonic acid per se increased [Ca2+]i, and acidified the cytosol in GH4C1 cells in a dose-dependent manner. The effects of arachidonic acid was reversed by addition of BSA to the cell suspension. The calcium entry and the activation of the metabolism of arachidonic acid may thus be important components of the TRH-evoked signal-transduction pathway in GH4C1 cells.

15-HETE: selective incorporation into inositol phospholipids of MDCK cells

The interaction of 15-hydroxyeicosatetraenoic acid (15-HETE) and cultured MDCK renal tubular epithelial cells was investigated to determine whether incorporation of this lipoxygenase product will affect polyphosphoinositide formation. MDCK cells were incubated with 1 microM [3H]-15-HETE for 15 to 120 minutes. Maximum uptake occurred between 15 and 30 minutes, and after 60 minutes, 70% of the incorporated 15-HETE was present in the phosphatidylinositol (PI) fraction. Some 15-HETE was also incorporated into phosphatidylinositol-4-monophosphate (PIP) and phosphatidylinositol-4,5-bisphosphate (PIP2). However, even though more 15-HETE than arachidonic acid was incorporated into PI, the fractional amount of 15-HETE present in the polyphosphoinositides was smaller than arachidonic acid. Therefore, although 15-HETE is selectively channeled into PI, conversion of PI species containing 15-HETE to PIP and PIP2 is relatively impaired. This suggests that either PI containing 15-HETE is a less effective substrate for phosphorylation, or PI containing arachidonic acid is a preferred substrate. MDCK cells converted 15-HETE to polar metabolites that were released into the extracellular fluid. This process may constitute a renal tubular mechanism for the clearance of 15-HETE and related lipoxygenase products.

Differential metabolism of hydroxyeicosatetraenoic acid isomers by mouse cerebromicrovascular endothelium

Hydroxyeicosatetraenoic acid (HETE) derivatives of arachidonic acid are produced in the brain and have been implicated as pathologic mediators in various types of brain injury. To understand better their fate in the brain, particularly in cerebral microvessels, several HETEs were incubated with cultured mouse cerebromicrovascular endothelium for 1, 2, and 4 h, followed by HPLC analysis of medium and cellular lipids. 5(S)-, 8(RS)-, and 9(RS)-HETE were not metabolized by the cells, but were extensively incorporated, unmodified, into cell lipids. On the other hand, 11(RS)-, 12(S)-, and 15(S)-HETE were extensively metabolized and only minimally incorporated into cell lipids. Previously, the major 12-HETE metabolite was identified as 8-hydroxyhexadecatrienoic acid. In the present study, we identified the major 11-HETE metabolite as 7-hydroxyhexadecatrienoic acid and the major 15-HETE metabolite as 11-hydroxyhexadecatrienoic acid. omega-3 compounds, 15(S)- and 12(S)-hydroxyeicosapentaenoic acids (HEPE), were also metabolized to more polar compounds, but to a lesser extent than their tetraenoic acid, omega-6 counterparts. Comparison of 5-, 12-, and 15-HETE enantiomers revealed no differences in metabolism or incorporation between the R and S stereoisomers. These data suggest that many isomers of HETE and HEPE can be incorporated into cell lipids or metabolized by pathways that do not distinguish between enantiomers. These pathways, however, are sensitive to the position or number of double bonds and are selective based on the position of the hydroxyl group.

Phorbol ester plus calcium ionophore induces release of arachidonic acid from membrane phospholipids of a human B cell line

Binding of LA350, a lymphoblastoid human B cell line, by phorbol myristate acetate (PMA) plus a calcium ionophore, either ionomycin or A23187, produced unique alterations in the release of arachidonic acid (AA) from cellular phospholipids. After equilibrium labeling of cells with radioactive fatty acids, [14C]AA demonstrated a selective enhanced release from the cells in response to the binding of PMA plus calcium ionophore as compared to the release of [14C]stearic acid (STE), [3H]oleic acid (OLE) and [3H]palmitic acid (PAL). The major phospholipid sources of the released [14C]AA were shown to be phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. The participation of protein kinase C (PKC) in the enhanced synergistic release of [14C]AA was demonstrated by the inhibition of the release by the PKC inhibitor, staurosporine. Approximately 2-6% of the labeled AA liberated was converted to 5-hydroxyeicosatetraenoic acid by an endogenous 5-lipoxygenase. Therefore during cell activation the B cell is capable of liberating AA via a PKC-dependent mechanism, implicating AA and/or its metabolites in signal transduction.

Astrocytes, not neurons, produce docosahexaenoic acid (22:6 omega-3) and arachidonic acid (20:4 omega-6)

Elongated, highly polyunsaturated derivatives of linoleic acid (18:2 omega-6) and linolenic acid (18:3 omega-3) accumulate in brain, but their sites of synthesis are not fully characterized. To investigate whether neurons themselves are capable of essential fatty acid elongation and desaturation or are dependent upon the support of other brain cells, primary cultures of rat neurons and astrocytes were incubated with [1-14C] 18:2 omega-6, [1-14C]20:4 omega-6, [1-14C]18:3 omega-3, or [1-14C]20:5 omega-3 and their elongation/desaturation products determined. Neuronal cultures were routinely incapable of producing significant amounts of delta 4-desaturase products. They desaturated fatty acids very poorly at every step of the pathway, producing primarily elongation products of the 18- and 20-carbon precursors. In contrast, astrocytes actively elongated and desaturated the 18- and 20-carbon precursors. The major metabolite of 18:2 omega-6 was 20:4 omega-6, whereas the primary products from 18:3 omega-3 were 20:5 omega-3, 22:5 omega-3, and 22:6 omega-3. The majority of the long-chain fatty acids formed by astrocyte cultures, particularly 20:4 omega-6 and 22:6 omega-3, was released into the extracellular fluid. Although incapable of producing 20:4 omega-6 and 22:6 omega-3 from precursor fatty acids, neuronal cultures readily took up these fatty acids from the medium. These findings suggest that astrocytes play an important supportive role in the brain by elongating and desaturating omega-6 and omega-3 essential fatty acid precursors to 20:4 omega-6 and 22:6 omega-3, then releasing the long-chain polyunsaturated fatty acids for uptake by neurons.

Growth hormone increases phosphoinositide turnover in rat adipocytes that are sensitive to the insulin-like action of the hormone

The effect of pituitary human growth hormone (hGH) on the 32P-labelling of phosphoinositides and phosphatidic acid was studied in noradrenaline-stimulated rat adipocytes which were either responsive or non-responsive to the antilipolytic (insulin-like) effect of hGH. In cells responsive to the insulin-like effect of hGH, hormone treatment resulted in a marked increase of the 32P-labelling of phosphatidic acid and phosphatidyl inositol in the plasma membrane, high density microsomal, and low density microsomal fractions. The increased 32P-labelling most likely reflects an activation of phosphoinositide phospholipase C.

ABBREVIATIONS

IP3 - Myo-Inositol 1,4,5-Triphosphate. BSA - Bovine Serum Albumin, HEPES - N-2-Hydroxyethylpiperazine-N'-2-Ethanesulphonic Acid, hGH - Human Pituitary Growth Hormone, LPC-Lysophosphatidylcholine, LPE - Lysophosphatidylethanolamine, LPI - Lysophosphatidylinositol, PA - Phosphatidic Acid, PC - Phosphatidylcholine, PE - Phosphatidylethanolamine, PI - Phosphatidylinositol, PIP - Phosphatidylinositol-4-Phosphate, PIP2-Phosphatidylinositol-4,5-Diphosphate, PS-Phosphatidylserine.

Role of the blood-brain barrier in the formation of long-chain omega-3 and omega-6 fatty acids from essential fatty acid precursors

Elongated, more highly polyunsaturated derivatives of linoleic acid (18:2 omega-6) and linolenic acid (18:3 omega-3) accumulate in brain, but their sites of synthesis and mechanism of entry are not well characterized. To investigate the role of the blood-brain barrier in this process, cultured murine cerebromicrovascular endothelia were incubated with [1-14C]18:2 omega-6 or [1-14C]18:3 omega-3 and their elongation/desaturation products determined. The major metabolite of 18:2 omega-6 was 20:4 omega-6, whereas the primary product from 18:3 omega-3 was 20:5 omega-3. Although these products were found primarily in cell lipids, they were also released from the cells and gradually accumulated in the extracellular fluid. Eicosanoid production was observed from the 20:4 omega-6 and 20:5 omega-3 that were formed. No 22:5 omega-6 or 22:6 omega-3 fatty acids were detected, suggesting that these endothelial cells are not the site of the final desaturation step. Although the uptake of 18:3 omega-3 and 18:2 omega-6 was nearly identical, 18:3 omega-3 was more extensively elongated and desaturated. Competition experiments demonstrated a preference for 18:3 omega-3 by the elongation/desaturation pathway. These findings suggest that the blood-brain barrier can play an important role in the elongation and desaturation of omega-3 and omega-6 essential fatty acids during their transfer from the circulation into the brain.

Ethanol alters the transfer of arachidonic acid to ethanolamine plasmalogens in C-6 glioma cells

In this study, the effects of ethanol exposure on uptake and metabolism of arachidonic acid by C-6 glioma cells in culture was examined. Labeled arachidonic acid was effectively taken up by the phospholipids of these cells and radioactivity was initially incorporated into phosphatidylinositols and phosphatidylcholines, reaching a peak between 4 and 6 hours. However, the labeling of ethanolamine plasmalogens continued to show an increase with time after labeled arachidonic acid has been exhausted in the medium. Since over 90% of labeled arachidonic acid was already taken up by the cells after 4 hours of exposure, the continued increase in labeling of ethanolamine plasmalogens is attributed to a transacylation mechanism. Cells grown in 150 mM ethanol for 2 days did not show a change in the overall incorporation of labeled arachidonic acid into phospholipids but showed a significant increase in labeling of ethanolamine plasmalogens, which was marked by a concomitant decrease in labeling of phosphatidylcholines. Ethanol exposure also resulted in a significant increase in the transfer of labeled arachidonic acid to triacylglycerols. Changes in phospholipid and triacylglycerol labeling pattern positively correlated with increasing ethanol concentration from 75 to 300 mM. Besides, most ethanol effects were readily noticeable after 24 hours of exposure. These data suggest a specific effect of ethanol on promoting the transacylase process for biosynthesis of ethanolamine plasmalogens as well as the acyltransferase for biosynthesis of triacylglycerols.

Effects of omega-3 fatty acids on vascular smooth muscle cells: reduction in arachidonic acid incorporation into inositol phospholipids

A rapid increase in arachidonic acid incorporation into phosphatidylinositol (PI) occurred following exposure of cultured porcine pulmonary artery smooth muscle cells to calcium ionophore A23187. This response was specific for PI and phosphatidic acid; none of the other phosphoglycerides showed any increase in arachidonic acid incorporation. The incorporation of [3H]inositol also was increased, indicating that complete synthesis of PI rather than only fatty acylation occurred in response to the ionophore. The presence of omega-3 fatty acids, especially eicosapentaenoic acid (EPA), reduced arachidonic acid but not inositol incorporation into PI. Stimulated incorporation of EPA also occurred under these conditions, suggesting that EPA replaces arachidonic acid in the newly synthesized pool of PI. Although much less arachidonic acid was incorporated into the polyphosphoinositides following exposure to the ionophore, arachidonic acid incorporation into these phosphorylated derivatives also decreased when EPA was present. These findings suggest that when omega-3 fatty acids are available, less arachidonic acid is channeled into the inositol phospholipids of activated smooth muscle cells because of replacement by EPA. This may represent a mechanism whereby omega-3 fatty acids, especially EPA, can accumulate in the metabolically active pools of inositol phospholipids and thereby possibly influence the properties or responsiveness of vascular smooth muscle.

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.

Diacylglycerol modulates action potential frequency in GH3 pituitary cells: correlative biochemical and electrophysiological studies

We have investigated the involvement of enhanced phosphoinositide metabolism in mediating TRH-induced alteration of electrophysiological events related to prolactin secretion by GH3 cells (a line of pituitary origin). Patch-clamp recording (in the current clamp, whole-cell configuration) showed that a few seconds after TRH application there was a brief period (about 30 s) of membrane hyperpolarization followed by several minutes of increased calcium-dependent action potential frequency. In parallel experiments cells were labeled for 24 h with either [3H]myo-inositol or [3H]arachidonate. Application of TRH resulted in rapid increases in levels of inositol phosphates and diacylglycerol. The time course of elevation of inositol 1,4,5-triphosphate (maximal by 5 s) is compatible with an initial burst of intracellular calcium mobilization associated with a transient phase of TRH-induced prolactin release. Application of TRH was also followed by a rapid but more sustained (several minutes) period of elevated diglyceride accumulation; a time course corresponding to a prolonged period of prolactin release which is dependent on the influx of external calcium. A causal relationship between diglyceride release and increased action potential frequency was demonstrated since local application (via a U-tube apparatus) of either 2 microM phorbol ester (phorbol 12,13-dibutyrate or phorbol 12-myristate 13-acetate) or 60 microM 1-oleoyl-2-acetyl-glycerol to patch-clamped cells could mimic this aspect of the TRH effect. In contrast, the inactive phorbol ester, 4 alpha-phorbol, was unable to elicit this response.(ABSTRACT TRUNCATED AT 250 WORDS)

Murine cerebral microvascular endothelium incorporate and metabolize 12-hydroxyeicosatetraenoic acid

Cultured murine cerebromicrovascular endothelial cells were employed to study the metabolism of 12-hydroxyeicosatetraenoic acid (12-HETE) in an in vitro model of the blood-brain barrier. These endothelial cells convert 12-HETE to at least four, more polar compounds. Analysis of the least polar and predominant metabolite by gas chromatography combined with chemical ionization and electron impact mass spectrometry of reduced and nonreduced derivatives indicate that the compound is 8-hydroxyhexadecatrienoic acid (8-HHDTrE). The uptake of 12-HETE into cell phospholipids peaks at 2 hr, and is not saturable up to the highest concentration tested, 5 microM. Seventy-five to 92% of this 12-HETE is incorporated into phosphatidylcholine, while the remainder is divided between the inositol and ethanolamine phospholipids. Incorporation into neutral lipids is slower, with radioactivity gradually accumulating in triglycerides over 24 hr. Saponification of cell lipids demonstrated that not only 12-HETE, but also its major metabolite, 8-HHDTrE, is incorporated into the cell lipids. Prostacyclin and prostaglandin E2 production by the cerebral endothelial cells is inhibited by up to 56% with 1 microM and 90% with 5 microM 12-HETE. These data demonstrate that 12-HETE is actively metabolized by cerebral endothelium and suggest at least two mechanisms through which 12-HETE may alter cerebromicrovascular function: 1) incorporation into cerebral endothelial membranes and 2) inhibition of cerebral endothelial prostaglandin production. Conversion of 12-HETE to more polar compounds, particularly 8-HHDTrE, may be interpreted as either the inactivation of 12-HETE or the production of additional, biological mediators.

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.

Depletion of arachidonic acid from GH3 cells. Effects on inositol phospholipid turnover and cellular activation

We have adapted rat pituitary GH3 cells to grow in delipidated culture medium. In response, esterfied linoleic acid and arachidonic acid become essentially undetectable, whereas eicosa-5,8,11-trienoic acid accumulates and oleic acid increases markedly. These changes occur in all phospholipid classes, but are particularly pronounced in inositol phospholipids, where the usual stearate/arachidonate profile is replaced with oleate/eicosatrienoate (n - 9) and stearate/eicosatrienoate (n - 9). Incubation of arachidonate-depleted cells with 10 microM-arachidonic acid for only 24 h results in extensive remodelling of phospholipid fatty acids, such that close-to-normal compositions and arachidonic acid content are achieved for the inositol phospholipids. In comparison studies with arachidonic acid-depleted or -repleted cells, it was found that the arachidonate content does not affect thyrotropin-releasing-hormone (TRH)-stimulated responses measured at long time points, including [32P]Pi labelling of phosphatidylinositol and phosphatidic acid, stimulation of protein phosphorylation, and basal or TRH-stimulated prolactin release. However, transient events such as stimulated breakdown of inositol phospholipids and an initial rise in diacylglycerol are enhanced by the presence of arachidonate. These results show that arachidonic acid itself is not required for operation of the phosphatidylinositol cycle and is not an obligatory intermediate in TRH-mediated GH3 cell activation. It is possible that any structural or functional role of arachidonic acid in these processes is largely met by replacement with eicosatrienoate (n - 9). However, since arachidonate in inositol phospholipids facilitates their hydrolysis upon stimulation by TRH, arachidonic acid apparently may have a specific role in the recognition of these lipids by phospholipase C.

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

In astrocyte-enriched cultures of the rat cerebral cortex the Ca2+ ionophore A23187 provoked the breakdown of inositol phospholipids, the liberation of arachidonic acid and the release of prostaglandins E2, F2 alpha, I2 and thromboxane A2. However, agonists for receptors also coupled to inositol phospholipid metabolism in these cells failed to produce an increase in the release of both arachidonic acid and eicosanoids. Results suggest that the A23187-stimulated release of arachidonic acid and eicosanoids is caused by a phospholipase A2-mediated attack on lipids other than the inositol phospholipids. Moreover, receptors linked to inositol lipid turnover are not involved in the control of eicosanoid release from astrocytes.