Subcellular localization of phospholipids and enzymes involved in PAF-acether metabolism

Multidrug-resistance P-glycoprotein (MDR1) secretes platelet-activating factor

The human multidrug-resistance (MDR1) P-glycoprotein (Pgp) is an ATP-binding-cassette transporter (ABCB1) that is ubiquitously expressed. Often its concentration is high in the plasma membrane of cancer cells, where it causes multidrug resistance by pumping lipophilic drugs out of the cell. In addition, MDR1 Pgp can transport analogues of membrane lipids with shortened acyl chains across the plasma membrane. We studied a role for MDR1 Pgp in transport to the cell surface of the signal-transduction molecule platelet-activating factor (PAF). PAF is the natural short-chain phospholipid 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine. [(14)C]PAF synthesized intracellularly from exogenous alkylacetylglycerol and [(14)C]choline became accessible to albumin in the extracellular medium of pig kidney epithelial LLC-PK1 cells in the absence of vesicular transport. Its translocation across the apical membrane was greatly stimulated by the expression of MDR1 Pgp, and inhibited by the MDR1 inhibitors PSC833 and cyclosporin A. Basolateral translocation was not stimulated by expression of the basolateral drug transporter MRP1 (ABCC1). It was insensitive to the MRP1 inhibitor indomethacin and to depletion of GSH which is required for MRP1 activity. While efficient transport of PAF across the apical plasma membrane may be physiologically relevant in MDR1-expressing epithelia, PAF secretion in multidrug-resistant tumours may stimulate angiogenesis and thereby tumour growth.

Multidrug-resistance P-glycoprotein (MDR1) secretes platelet-activating factor

The human multidrug-resistance (MDR1) P-glycoprotein (Pgp) is an ATP-binding-cassette transporter (ABCB1) that is ubiquitously expressed. Often its concentration is high in the plasma membrane of cancer cells, where it causes multidrug resistance by pumping lipophilic drugs out of the cell. In addition, MDR1 Pgp can transport analogues of membrane lipids with shortened acyl chains across the plasma membrane. We studied a role for MDR1 Pgp in transport to the cell surface of the signal-transduction molecule platelet-activating factor (PAF). PAF is the natural short-chain phospholipid 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine. [(14)C]PAF synthesized intracellularly from exogenous alkylacetylglycerol and [(14)C]choline became accessible to albumin in the extracellular medium of pig kidney epithelial LLC-PK1 cells in the absence of vesicular transport. Its translocation across the apical membrane was greatly stimulated by the expression of MDR1 Pgp, and inhibited by the MDR1 inhibitors PSC833 and cyclosporin A. Basolateral translocation was not stimulated by expression of the basolateral drug transporter MRP1 (ABCC1). It was insensitive to the MRP1 inhibitor indomethacin and to depletion of GSH which is required for MRP1 activity. While efficient transport of PAF across the apical plasma membrane may be physiologically relevant in MDR1-expressing epithelia, PAF secretion in multidrug-resistant tumours may stimulate angiogenesis and thereby tumour growth.

The subcellular localization of phospholipase D activities in rat Leydig cells

Rat Leydig cells contain a phospholipase D (PLD), which can be activated by vasopressin and phorbol ester. In order to clarify which Leydig cell organelles that express PLD activity, the subcellular localization of two differently regulated PLD activities was investigated by subcellular fractionation on a 40% (v/v) self-generating Percoll gradient. PLD activities in broken cells were estimated using radiolabeled didecanoylphosphatidylcholine as a substrate. Initial experiments revealed the presence of an oleate Mg2+ -activated PLD and a phosphatidylinositol 4,5-bisphosphate-activated PLD (PIP2-PLD) in the microsomal fraction of Leydig cells. The latter activity could be further stimulated by recombinant nonmyristoylated ADP ribosylating factor 1 (ARF1) plus GTPgammaS. The peak of oleate Mg2+ -PLD activity colocalized with the plasma membrane marker, whereas the highest specific activity of the PIP2-PLD activity was found in fractions with a slightly lower density than those containing the plasma membrane and trans-Golgi marker enzymes. In order to localize phorbol ester-stimulated PLD activity in intact Leydig cells, the cells were prelabeled with [14C]-palmitate and then stimulated for 15 min with 100 nM 4-beta-phorbol-12-myristate-13-acetate (PMA) in the presence of ethanol or butanol. The PLD product [14C]-phosphatidylethanol, expressed as the percentage of total labeled phospholipids in the fraction, was slightly increased in all Percoll fractions and showed a prominent peak in the fractions containing plasma membrane, trans-Golgi, and fractions of slightly lower density. The PMA-induced formation of [14C]-phosphatidylbutanol could be inhibited dose-dependently with brefeldin A suggesting that the activation of PLD by the phorbol ester was mediated by ARF.

Subcellular localization of enzyme activities involved in the metabolism of platelet-activating factor in rainbow trout leukocytes

The subcellular distribution of an alkyllyso-GPC: acetyl-CoA acetyltransferase (EC 2.3.1.67) and transacylase, two important enzyme activities involved in the remodeling pathway for the biosynthesis of platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) have been examined in leukocytes isolated from the pronephros of the rainbow trout, Oncorhynchus mykiss. Contrary to mammalian systems, in which the acetyltransferase is localized to intracellular membranes, the subcellular distribution of an acetyltransferase activity in rainbow trout leukocytes was localized to the plasma membrane. Analysis of the acetyltransferase products by thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC) confirmed synthesis of two subclasses of PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and 1-acyl-2-acetyl-sn-glycero-3-phosphocholine. The transacylase activity in this study was detected in membrane fractions in two domains of the intermediate density region which also contained the NADH dehydrogenase activity, a marker enzyme for the endoplasmic reticulum. Acylation of lysoPAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) exhibited approximately 95% specificity for omega-3 fatty acids. Acylation patterns were not significantly different in either domain of the endoplasmic reticulum. A model is proposed herein for the metabolism of PAF in rainbow trout leukocytes.

Acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase is directly activated by p38 kinase

Acetyl-CoA:1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase, along with phospholipase A2, is a key regulator of platelet-activating factor biosynthesis via the remodeling pathway. We have now obtained evidence in human neutrophils indicating that this enzyme is regulated by a specific member of the mitogen-activated protein kinases, namely the p38 kinase. We earlier demonstrated that tumor necrosis factor-alpha (TNF-alpha) as well as N-formyl-methionyl-leucyl-phenylalanine treatment leads to increased phosphorylation and activation of p38 kinase in human neutrophils. Strikingly, in the present study these stimuli increased the catalytic activity of acetyltransferase up to 3-fold, whereas 4-phorbol 12-myristate 13-acetate, which activates the extracellular-regulated kinases (ERKs) but not p38 kinase, had no effect. Furthermore, a selective inhibitor of p38 kinase, SB 203580, was able to abolish the TNF-alpha- and N-formyl-methionyl-leucyl-phenylalanine-induced activation of acetyltransferase. The same effect was not observed in the presence of an inhibitor that blocked ERK activation (PD 98059). Complementing the findings in intact cells, we have shown that recombinant, activated p38 kinase added to microsomes in the presence of Mg2+ and ATP increased acetyltransferase activity to the same degree as in microsomes obtained from TNF-alpha-stimulated cells. No activation of acetyltransferase occurred upon treatment of microsomes with either recombinant, activated ERK-1 or ERK-2. Finally, the increases in acetyltransferase activity induced by TNF-alpha could be ablated by treating the microsomes with alkaline phosphatase. Thus acetyltransferase appears to be a downstream target for p38 kinase but not ERKs. These data from whole cells as well as cell-free systems fit a model wherein stimulus-induced acetyltransferase activation is mediated by a phosphorylation event catalyzed directly by p38 kinase.

Alkylglycerophosphate acetyltransferase and lyso platelet activating factor acetyltransferase, two key enzymes in the synthesis of platelet activating factor, are found in neuronal nuclei isolated from cerebral cortex

Neuronal nuclear fractions (N1) isolated from cerebral cortices of 15-day-old rabbits were enriched in two acetyltransferases involved in biosynthetic pathways leading to platelet activating factor (PAF). Alkylglycerophosphate (AGP) acetyltransferase of the de novo biosynthetic path had specific activities in fraction N1 which were 3-times those of the microsomal fraction (P3D) from cerebral cortex. Lyso PAF acetyltransferase of the remodelling path had specific activities in N1 which were 16-times those of P3D and 51-times those of the homogenate. The maximum specific activity observed for the N1 AGP acetyltransferase was 1.4-times the corresponding N1 lyso PAF acetyltransferase value. The pH optimum for the N1 AGP acetyltransferase was within the alkaline range (pH 8-9), while the N1 lyso PAF acetyltransferase showed a much broader pH optimal range which extended over the neutral and physiological pH values. Both acetyltransferases were inhibited by MgATP (0.125-1 mM) or oleoyl CoA (2-10 microM). However, the N1 AGP acetyltransferase could be distinguished from the N1 lyso PAF acetyltransferase by a greater sensitivity to MgATP inhibition. When NaF was not present in the assays, less of the product of N1 AGP acetyltransferase was recovered, likely indicating a hydrolysis of the acetylated AGP. When the AGP and lyso PAF substrates were combined in acetyltransferase assays, the two N1 acetylations appeared to proceed independently. The enrichment of the acetyltransferases, and particularly the lyso PAF acetyltransferase, within the neuronal nuclear fraction is of particular interest with respect to the intracellular effects of PAF which are considered to be involved in nuclear signalling mechanisms.

Transacylase-mediated alkylacyl-GPC synthesis and its hydrolysis by phospholipase D occur in separate cell compartments in the human neutrophil

Subcellular localizations of CoA-independent transacylase and phospholipase D enzymes have been investigated in human neutrophils performing a two-step gradient system to separate plasma membranes from internal membranes and from the bulk of granules. The internal membranes were constituted by endoplasmic reticulum and by a subpopulation of specific and tertiary granules. The enzymes activities were assayed in vitro on gradient fractions using exogenous substrates. Following cell prelabelling with [3H]alkyllyso-GPC, we also analyzed the in situ localization of labelled products involving the action of both enzymes. The CoA-independent transacylase activity, together with the CoA-dependent transacylase and acyltransferase activities were only located in the internal membranes. Following 15 min cell labelling, part of the [3H]alkylacyl-GPC was recovered in plasma membranes indicating a rapid redistribution of the acylated compound. Very high contents in arachidonate containing [3H]alkylacyl-GPC were recovered both in plasma membranes and internal membranes. Phospholipase D activity being assayed in the presence of cytosol, GTP gamma S and gradient fractions, only the plasma membrane fractions from resting or stimulated cells allowed the enzyme to be active. The [3H]alkylacyl-GP and [3H]alkylacyl-GPethanol, phospholipase D breakdown products from [3H]alkylacyl-GPC, obtained after neutrophil prelabelling and activation by phorbol myristate acetate, were exclusively present in the plasma membranes. In contrast, the secondary generated [3H]alkylacylglycerols were equally distributed between plasma and internal membranes. No labelled product was recovered on azurophil granules. These data demonstrate that internal membranes are the site of action of the CoA-independent transacylase and plasma membranes are the site of action of the phospholipase D. This topographical separation between CoA-independent transacylase which generated substrate and phospholipase D which degraded it, suggested that subcellular localisation and traffic of substrates within the cell can be important to regulate the enzymes.

Colocalization of cytosolic phospholipase A2, 5-lipoxygenase, and 5-lipoxygenase-activating protein at the nuclear membrane of A23187-stimulated human neutrophils

The distribution of cytosolic phospholipase A2 (cPLA2), arachidonate 5-lipoxygenase, and 5-lipoxygenase-activating protein (5-LAP) was investigated in subcellular fractions of human neutrophils disrupted by three techniques. As determined by immunoblot analysis, the bulk of cPLA2 and 5-lipoxygenase was detected in cytosolic fractions of unstimulated neutrophils disrupted by sonication or cavitation. After cell stimulation with the calcium ionophore A23187, both proteins accumulated primarily in nuclei-containing fractions; this accumulation was accompanied by a loss of these enzymes from cytosolic fractions. Further resolution of nuclear fractions revealed that 5-lipoxygenase and cPLA2 were localized in a fraction that contained nuclear membranes. In comparison, 5-LAP was localized to the nuclear-membrane fraction of resting and activated neutrophils, as determined by immunoblotting and photoaffinity labeling. In agreement with the immunoblot data, A23187 stimulation markedly enhanced 5-lipoxygenase enzymatic activity in the nuclear-membrane fraction, which was accompanied by decreased cytosolic 5-lipoxygenase activity. Similarly, neutrophil activation caused increased phosphorylation of cPLA2, a process that is known to result in enhanced catalytic activity. Our data demonstrate that in activated human neutrophils, the key proteins involved in leukotriene synthesis colocalize at the nuclear membrane, in a catalytically active state.

Platelet-activating factor: the biosynthetic and catabolic enzymes

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Excitable membranes, lipid messengers, and immediate-early genes. Alteration of signal transduction in neuromodulation and neurotrauma

The physical nature of neuronal cells, particularly in the functional and morphological segregation of synapse, soma, and dendrites, imparts special importance on the integrity of their cell membranes for the localization of function, generation of intrinsic second messengers, and plasticity required for adaptation and repair. The component phospholipids of neural membranes are important sources of bioactive mediators that participate in such diverse phenomena as memory formation and cellular damage following trauma. A common role for PAF in these processes is established through the suppressive effects of its antagonists. Furthermore, being both an extracellular and intracellular agonist of phospholipase activation, in addition to being a product of phospholipase activity, PAF assumes a centralized role in the cellular metabolism following neural stimulation. The linkage of PAF to neural immediate-early gene expression, both in vitro and in vivo, suggests that its effects are initiating to long-term formative and reparative processes. Such a common link between destructive and plastic responses provides an important view of cellular and tissue maintenance in the nervous system.

Nitrovasodilators inhibit thrombin-induced platelet-activating factor synthesis in human endothelial cells

In response to inflammatory agents such as thrombin, cultured endothelial cells produce platelet-activating factor (PAF), which has been linked with most inflammatory and immune processes, and is a potent coronary constrictor. Sodium nitroprusside (SNP) and SIN-1 (3-morpholinosydnonimine), which spontaneously release the free radical nitric oxide (NO), cause direct relaxation of blood vessels and inhibition of platelet aggregation by activating soluble guanylate cyclase. In the present study we report that in human umbilical vein endothelial cells (HUVEC) these compounds stimulate the production of cGMP and inhibit thrombin-induced PAF synthesis in a concentration-dependent manner. 8-bromo-cGMP, a permeant non-hydrolysable analogue of cGMP, mimics the inhibitory effect of NO-generating vasodilators. PAF synthesis requires phospholipase A2-mediated hydrolysis of membrane precursors to lyso-PAF, which is in turn converted into PAF by an acetyltransferase. The thrombin-elicited activation of both enzymes is inhibited in a dose-dependent way in HUVEC pretreated with SNP and SIN-1. The inhibitory effect of SNP and SIN-1 on the thrombin-mediated PAF synthesis suggests a new mechanism of action whereby the endogenous NO can affect vascular tone and endothelium-dependent intercellular adhesion. Moreover, PAF production in endothelial cells appears to be an important target for the pharmacological action of nitrovasodilators.

Platelet-activating factor in stroke and brain injury

Platelet-activating factor, an endogenous phospholipid of proinflammatory, hemostatic, and vasoactive properties, is synthesized by neurons and in injured brain. Platelet-activating factor is released together with eicosanoids such as thromboxane A2, prostacyclin, and leukotrienes. Its effects in neurons are mediated through a specific receptor coupled to phospholipase C and phosphoinositol metabolism. The cerebrovascular effects of platelet-activating factor include disruption of the blood-brain barrier, edema formation, and vasospasm. It has also been described to possess direct toxicity to neuronal cells in culture. Discovery and development of several highly potent and selective antagonists to platelet-activating factor receptors facilitated experimental studies underscoring the role of this factor as an endogenous mediator in cerebral disorders, particularly cerebral ischemia and trauma. Significant biochemical, microvascular, functional, and behavioral recovery has been demonstrated using these antagonists in an array of experimental models of focal and global ischemia in the central nervous system (CNS). Clearly, studies of platelet-activating factor in experimental models of CNS ischemia and reperfusion injury open a new perspective on phospholipid metabolism in stroke and offer an exceptionally promising therapeutic prospect. Data supporting this factor as a mediator of specific pathological sequelae in stroke and neuroinjury are surveyed in this review. We discuss the mechanisms and significance of platelet-activating factor-mediated effects and propose directions for future studies.

The effect of a phorbol ester on the lipid microviscosity of two endoplasmic reticulum membrane fractions isolated from Krebs II ascites cells

This paper deals with microviscosity parameters and thermoinduced structural transitions in the lipids of smooth and heavy rough endoplasmic reticulum membranes isolated from Krebs II ascites cells incubated with the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate. The phorbol ester was found to bring about a threefold increase in the microviscosity of the lipids in heavy rough membranes. Spin probe I (2,2,6,6-tetrahydro-4-capryloyl-oxypiperidine-1-oxyl), localized in the surface layer of the membrane lipids, gave results which indicate an increased number of thermoinduced structural transitions in the smooth membranes in the treated cells due to the transitions occurring at relatively low temperature and a decreased number of such transitions in the heavy rough fraction especially at high temperature. For 5,6-benzo-2,2,4,4-tetramethyl-1,2,3,4-tetrahydro-gamma-carboline-oxyl, probe II, mainly distributed in the annular lipids, a decrease in the number of low temperature transitions in the smooth fraction was observed, while an increase occurred in the heavy rough one. The results obtained are discussed in terms of the effect of phorbol esters as promoters of tumor progression.

Subcellular distribution of fatty acids, phospholipids and phospholipase A2 in human neutrophils

We have examined the putative relationship between the presence of neutral pH optimum phospholipase A2 activities and the phospholipid and fatty acid composition in the distinct fractions obtained after subcellular fractionation in a continuous sucrose gradient of human neutrophils. Neutral phospholipase A2 activity was found mainly in the plasma membrane, the azurophilic granules and an unidentified fraction which was a little denser than that containing specific granules and endoplasmic reticulum. These differently located phospholipase A2 activities resulted in activation upon calcium ionophore A23187-cell treatment, but the phospholipase A2 activity found in the unidentified region showed the highest degree of activation after cell treatment with A23187 and hydrolyzed preferentially ethanolamine-containing glycerophospholipids. The subcellular distribution of inositol-containing phosphoglycerides was bimodal, with a main peak at the plasma membrane and another peak at the azurophilic granule region. Choline- and ethanolamine-containing glycerophospholipids showed a broad distribution throughout the gradient, with preponderance in the denser part of the gradient, where the intracellular organelle phospholipase A2 activities were located. Interestingly, ethanolamine-containing glycerophospholipids were shown to be enriched in fractions containing the phospholipase A2 which hydrolyzes preferentially this phospholipid class. Most of the arachidonic acid present in the postnuclear fraction of human neutrophils was found in the plasma membrane and resulted to be the only fatty acid released during A23187 stimulation. Thus, plasma membrane phospholipase A2 seems to be involved mainly in the release of arachidonic acid in A23187-stimulated cells. These findings indicate that differences in substrate specificity found in vitro among the phospholipase A2 activities located in distinct subcellular particulates might be a reflection of the distinct phospholipid and fatty acid composition in these particulates.