The substrate specificity of brain microsomal phospholipase D

The substrate specificities of sunflower and soybean phospholipases D using transphosphatidylation reaction

BACKGROUND

Phospholipase D (PLD) belongs to a lipolytic enzyme subclass which catalyzes the hydrolysis and transesterification of glycerophospholipids at the terminal phosphodiester bond.

RESULTS

In this work, we have studied the substrate specificity of PLDs from germinating sunflower seeds and cultured-soybean cells, using their capacity of transphosphatidylation. In the presence of a nucleophilic acceptor, such as [¹⁴C]ethanol, PLD catalyzes the production of phosphatidyl-[¹⁴C]-ethanol. The resulting product is easily identified since it is well separated from the other lipids by thin-layer chromatography. The main advantage of this assay is that the phospholipid used as substrate does not need to be radiolabelled and thus allow us a large choice of polar heads and fatty acids. In vitro, we observed that sunflower and soybean cell PLD show the following decreasing order of specificity: phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol; while phosphatidylserine and phosphatidylinositol are utilized much less efficiently.

CONCLUSIONS

The substrate specificity is modulated by the fatty acid composition of the phosphatidylcholine used as well as by the presence of other charged phospholipids.

Phospholipase D mechanism using Streptomyces PLD

Phospholipase D (PLD) plays various roles in important biological processes and physiological functions, including cell signaling. Streptomyces PLDs show significant sequence similarity and belong to the PLD superfamily containing two catalytic HKD motifs. These PLDs have conserved catalytic regions and are among the smallest PLD enzymes. Therefore, Streptomyces PLDs are thought to be suitable models for studying the reaction mechanism among PLDs from other sources. Furthermore, Streptomyces PLDs present advantages related to their broad substrate specificity and ease of enzyme preparation. Moreover, the tertiary structure of PLD has been elucidated only for PLD from Streptomyces sp. PMF. This article presents a review of recently reported studies of the mechanism of the catalytic reaction, substrate recognition, substrate specificity and stability of Streptomyces PLD using various protein engineering methods and surface plasmon resonance analysis.

Sensor of phospholipids in Streptomyces phospholipase D

Recently, we identified Ala426 and Lys438 of phospholipase D from Streptomyces septatus TH-2 (TH-2PLD) as important residues for activity, stability and selectivity in transphosphatidylation. These residues are located in a C-terminal flexible loop separate from two catalytic HxKxxxxD motifs. To study the role of these residues in substrate recognition, we evaluated the affinities of inactive mutants, in which these residues were substituted with Phe and His, toward several phospholipids by SPR analysis. By substituting Ala426 and Lys438 with Phe and His, respectively, the inactive mutant showed a much stronger interaction with phosphatidylcholine and a weaker interaction with phosphatidylglycerol than the inactive TH-2PLD mutant. We demonstrated that Ala426 and Lys438 of TH-2PLD play a role in sensing the head group of phospholipids.

Phospholipase D

Phospholipase D catalyses the hydrolysis of the phosphodiester bond of glycerophospholipids to generate phosphatidic acid and a free headgroup. Phospholipase D activities have been detected in simple to complex organisms from viruses and bacteria to yeast, plants, and mammals. Although enzymes with broader selectivity are found in some of the lower organisms, the plant, yeast, and mammalian enzymes are selective for phosphatidylcholine. The two mammalian phospholipase D isoforms are regulated by protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families. Mammalian and yeast phospholipases D are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. This review discusses the identification, characterization, structure, and regulation of phospholipase D. Genetic and pharmacological approaches implicate phospholipase D in a diverse range of cellular processes that include receptor signaling, control of intracellular membrane transport, and reorganization of the actin cytoskeleton. Most ideas about phospholipase D function consider that the phosphatidic acid product is an intracellular lipid messenger. Candidate targets for phospholipase-D-generated phosphatidic acid include phosphatidylinositol 4-phosphate 5-kinases and the raf protein kinase. Phosphatidic acid can also be converted to two other lipid mediators, diacylglycerol and lyso phosphatidic acid. Coordinated activation of these phospholipase-D-dependent pathways likely accounts for the pleitropic roles for these enzymes in many aspects of cell regulation.

Dipalmitoyl-phosphatidylcholine/phospholipase D interactions investigated with polarization-modulated infrared reflection absorption spectroscopy

The hydrolysis of 1,2-dipalmitoylphosphatidylcholine (DPPC) catalyzed by Streptomyces chromofuscus phospholipase D (PLD) has been investigated using monolayer techniques and polarization-modulated infrared absorption reflection spectroscopy. The spectroscopic analysis of the phosphate groups provides a quantitative estimation of the hydrolysis yield. The hydrolysis kinetics was investigated in dependence on the phase state of the lipid monolayer. It was found that PLD exhibits maximum activity in the liquid-expanded phase, whereas PLA2 has its activity maximum in the two-phase region. A lag phase was observed in all experiments indicating that small amounts of the hydrolysis product 1,2-dipalmitoylphosphatidic acid (DPPA) are needed for initiating the fast hydrolysis reaction. Higher concentrations of DPPA inhibit the hydrolysis. The critical inhibition concentration of DPPA is a function of the monolayer pressure.

Phospholipase D: molecular and cell biology of a novel gene family

Interaction of extracellular-signal molecules with cell-surface receptors often activates a phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine and other phospholipids, generating phosphatidic acid. The activation of PLD is believed to play an important role in the regulation of cell function and cell fate. Multiple PLD activities were characterized in eukaryotic cells, and, more recently, several PLD genes have been cloned. A PLD gene superfamily, defined by a number of structural domains and sequence motifs, also includes phosphatidyltransferases and certain phosphodiesterases. Among the eukaryotic PLD genes are those from mammals, nematodes, fungi and plants. The present review focuses on the structure, localization, regulation and possible functions of cloned mammalian and yeast PLDs. In addition, an overview of plant PLD genes, and of several distinct PLD activities that have not yet been cloned, is provided. Emerging evidence from recent work employing new molecular tools indicates that different PLD isoforms are localized in distinct cellular organelles, where they are likely to serve diverse functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

A spectrophotometric assay for the transphosphatidylation activity of phospholipase D enzyme

We developed a specific spectrophotometric assay for the quantitative determination of phospholipase D-catalyzed transphosphatidylation activity. The assay measures p-nitrophenol liberated by phospholipase D-catalyzed reaction of phosphatidyl-p-nitrophenol and ethanol in an aqueous-organic emulsion system. The release of p-nitrophenol was linear to reaction time at an early stage of the reaction with phospholipase D from Streptomyces sp. In the spectrophotometric assay for the reaction with phospholipase D from Streptomyces chromofuscus, which has higher hydrolytic activity than transphosphatidylation activity, p-nitrophenol was not found. The advantages of this novel method for measuring the transphosphatidylation activity of phospholipase D are that (i) it does not use radioactive compounds, (ii) it can measure the initial velocity of the reaction, and (iii) it is rapid, easy, and accurate to perform.

Localization and possible functions of phospholipase D isozymes

The activation of PLD is believed to play an important role in the regulation of cell function and cell fate by extracellular signal molecules. Multiple PLD activities have been characterized in mammalian cells and, more recently, several PLD genes have been cloned. Current evidence indicates that diverse PLD activities are localized in most, if not all, cellular organelles, where they are likely to subserve different functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

Developmental changes in phospholipase D activity and mRNA levels in rat brain

Phospholipase D (PLD) activity and PLD1 mRNA levels were determined in rat brain at ages ranging from embryonic day (E) 19 to postnatal day (P) 49. Basal, oleate-, and phosphatidylinositol-4, 5-bisphosphate-stimulated PLD activity increased between E19 and P24 by approximately 3-fold and remained unaltered thereafter. A similar developmental pattern of mRNA levels of PLD1 isoform was found by Northern blotting. The development of PLD correlates with synaptogenesis and myelination suggesting that the enzyme might have an important function in these processes.

Increased levels of methylated intermediates of phosphatidylcholine lead to enhanced phospholipase D activity

Previous work from this laboratory and others has shown that neurotransmitters can activate phospholipase D. Unlike the phospholipase C that specifically hydrolyzes inositol-containing phospholipids, phospholipase D in neuronal tissue specifically hydrolyzes phosphatidylcholine. One route for the synthesis of phosphatidylcholine, is via methylation of phosphatidylethanolamine. Using an in vitro assay, we have previously shown that methylated intermediates are also good substrates for phospholipase D (1). In this manuscript we demonstrate that these intermediates are also substrates in the intact PC12 cells. Cells incubated with methyl and dimethylethanolamine incorporate more [3H]palmitic acid into the corresponding phospholipid, phosphatidyl-N-methylethanolamine and phosphatidyl-N,N-dimethylethanolamine. In these cells bradykinin causes a greater increase in [3H]phosphatidylethanol production. Elevated levels of [3H]phosphatidylcholine do not enhance bradykinin-stimulated [3H]phosphatidylethanol production, therefore, this effect is specific for the methylated intermediates. Finally, this effect is not due to some generalized enhancement of receptor coupling because incubation of the cells with methylethanolamine does not lead to an increase in bradykinin stimulated inositol phosphate production.

Phospholipase D hydrolyzes short-chain analogs of phosphatidylcholine in the absence of detergent

Phospholipase D is an important enzyme in signal transduction in neuronal tissue. A variety of assays have been used to measure phospholipase D activity in vitro. The most typical measure of phospholipase D activity is the production of phosphatidylethanol in the presence of ethanol. Phosphatidylethanol is a product of transphosphatidylation activity that is considered a unique property of phospholipase D. To support transphosphatidylation activity, high concentrations of ethanol may be required. Furthermore, most assays in the literature utilize a detergent. These extreme conditions, detergent and ethanol, may alter phospholipase D and hinder the study of its regulation. In this manuscript we describe an assay that eliminates these potentially confounding conditions. It utilizes high specific activity [3H]butanol as a nucleophilic receptor. This eliminates the need for high concentrations of alcohol. The substrate is an analog of phosphatidylcholine that contains short-chain fatty acids, 1,2-dioctanoyl-sn-glycero-3-phosphocholine. Phospholipase D readily hydrolyzes this substrate in the absence of detergent. This novel assay should be useful in the further characterization of phospholipase D.

N-arachidonylethanolamine (anandamide) formation from N-arachidonylphosphatidylethanolamine in rat brain membranes

Labeled L-N-arachidonylphosphatidylethanolamine (L-N-arachidonyl PE), a likely precursor of N-arachidonylethanolamine (anandamide), as well as its D-isomer, were synthesized using [14C]arachidonic acid. Anandamide was formed by incubating L-N-arachidonyl PE and rat brain membrane with phenylmethylsulfonyl fluoride (PMSF), an inhibitor of anandamide amidohydrolase. Formation of anandamide from L-N-arachidonyl PE was inhibited by p-chloromercuriphenylsulfonic acid (p-CMPS), sulfhydryl reagent, and heat inactivate pre-treatment. D-N-Arachidonyl PE, an unnatural analog for N-arachidonyl PE, did not form anandamide.

A phospholipase D and protein kinase C inhibitor blocks the spreading of murine mammary adenocarcinoma cells altering f-actin and beta1-integrin point contact distribution

Spreading is a critical process involved in motility and growth of tumor cells during the metastatic cascade. Focal adhesion kinase, src-proteins and PKC have been reported to participate in the regulation of cytoskeleton organization in both normal and transformed cells during spreading. The role of other signaling enzymes such as PLD and PAP has not been studied during spreading in tumor cells. We now show that the spreading of murine mammary adenocarcinoma LM3 cells was significantly reduced by n-butanol, a PLD and PKC inhibitor, with a maximal inhibition of 54% (p < 0.001) in both the presence and absence of serum, as measured by phase-contrast microscopy. PMA only stimulated cell spreading over the control in the absence of serum and n-butanol inhibition was completely reversed by PMA treatment in both conditions. PA, the product of PLD activity, stimulated LM3 cell spreading and the same effect was observed with staurosporine. Spreading was enhanced when cells were seeded on collagen-IV- or fibronectin-coated surfaces and n-butanol could inhibit both integrin-derived signals. Cell spreading inhibition correlated with the absence of f-actin bundles and fewer beta1-integrin point contacts as determined by double immunofluorescence microscopy. In addition, n-butanol inhibited the proliferation of LM3 cells in the presence of serum (p < 0.01). These results suggest that beta1-integrin and f-actin/point contact assembly, involved in spreading and proliferation, require the participation of PLD-PKC regulatory pathways in LM3 cells.

Overproduction of urokinase-type plasminogen activator is regulated by phospholipase D- and protein kinase C-dependent pathways in murine mammary adenocarcinoma cells

Urokinase-type plasminogen activator (uPA) initiates a proteolytic cascade with which invasive cells eliminate barriers to movement. The signaling pathways regulating uPA production in tumor cells remain unclear. We first studied the effects of n-butanol, a phospholipase D (PLD) and protein kinase C (PKC) inhibitor, on the production of uPA in murine mammary adenocarcinoma cells. Tumor cell monolayers treated during 24 h with 0.3% v/v n-butanol, secreted 45-50% less uPA to the culture medium than control monolayers (P < 0.001) as determined by radial caseinolysis, zymography and western blot. This inhibition occurred also with 5-h treatments and remained up to 5 h after the removal of the alcohol. Treatment with the phorbol ester PMA or with EGF, strongly increased uPA production (P < 0.001). Interestingly, a mild inhibition of uPA production was observed when PMA stimulation was assayed in cotreatments with n-butanol. In contrast EGF was unable to reverse the inhibition induced by n-butanol. H7 significantly inhibited uPA activity (P < 0.001) secreted to the culture media. Furthermore, phosphatidic acid significantly stimulated uPA production meanwhile propranolol, which blocks phosphatidic acid availability, reduced it, suggesting a main regulatory role for this intermediary metabolite. These results suggest for the first time that uPA production is regulated by PLD and PKC signal transduction pathways in murine mammary adenocarcinoma cells.

ADP-ribosylation factor 1-regulated phospholipase D activity is localized at the plasma membrane and intracellular organelles in HL60 cells

ADP-ribosylation factor (ARF), a small GTPase required for vesicle formation, has been identified as an activator of phospholipase D (PLD), thus implying that PLD is localized at intracellular organelles. HL60 cells were prelabelled with [14C]acetate for 72 h and, after disruption, fractionated on a linear sucrose gradient. ARF1-regulated PLD activity in each fraction was assessed by measurement of phosphatidylethanol production. Two peaks of activity were identified, coincident with markers for Golgi/endoplasmic reticulum/granules (endomembranes) and plasma membrane respectively. Analysis of the fractions using exogenous phosphatidylcholine as substrate confirmed the presence of ARF1-dependent PLD activity in endomembranes and plasma membrane, and also identified an additional activity in the cytosol. In formyl-Met-Leu-Phe-stimulated cells, PLD activity as assessed by phosphatidylethanol formation was also associated with both the plasma membrane and endomembranes. Since ARF1-regulated PLD activity requires phosphatidylinositol 4,5-bisphosphate (PIP2), the distributions of inositol lipids and the kinases responsible for lipid phosphorylation were examined. PIP2 was highly enriched at the plasma membrane, whereas phosphatidylinositol (PI) and phosphatidylinositol 4-phosphate (PI4P), the precursors for PIP2 synthesis, were found predominantly at endomembranes. The distribution of PI 4-kinase and PI4P 5-kinase activities confirmed the plasma membrane as the major site of PIP2 production. However, endomembranes possessed substantial PI 4-kinase activity and some PI4P 5-kinase activity, illustrating the potential for PIP2 synthesis. It is concluded that:(1) ARF1-regulated PLD activity is localized at endomembranes and the plasma membrane, (2) PIP2 is available at both membrane compartments to function as a cofactor for ARF-regulated PLD, and (3) in intact cells, formyl-Met-Leu-Phe stimulates PLD activity at endomembranes as well as plasma membrane.

Two phosphatidylethanol classes separated by thin layer chromatography are produced by phospholipase D in rat brain hippocampal slices

Noradrenaline- and ionomycin-stimulated as well as basal phospholipase D activity from rat hippocampus produced, in the presence of ethanol, two different classes of [32P]phosphatidylethanol (designated I and II), which were separated by thin layer chromatography. Endogenous labeling experiments using 3H-fatty acids showed that two different classes of phosphatidylcholine, separated by two-dimensional TLC, one enriched with high incorporation of [3H]arachidonic acid (B) and the other with [3H]myristic acid (A), were the most likely sources for the two classes of phosphatidylethanol. Experiments where individual 32P-phospholipids extracted from [32P]Pi-labeled hippocampal slices were incubated with cabbage phospholipase D, in the presence of ethanol, showed that each class of [32P]phosphatidylcholine, i.e. A and B, produced a different band of [32P]phosphatidylethanol, with the same mobility in TLC as phosphatidylethanol II and I, respectively.

Enzymology of mammalian phospholipases D: in vitro studies

The existence of multiple forms of phopholipase D was clearly established in a large number of biochemical studies that described and characterized the enzymological properties of the different PLD activities. This review summarizes the in vitro evidence showing differential subcellular localization and chromatographic properties of putative PLD isozymes, their phospholipid and alcohol substrate specificities, their modulation by various divalent cations, small G proteins and protein kinase c isozymes, and the role of phosphatidylinositol 4,5-bisphosphate as a cofactor of phospholipase D.

ARF-regulated phospholipase D: a potential role in membrane traffic

Phospholipase D activity is stimulated rapidly upon occupation of cell-surface receptors. One of the intracellular regulators of phospholipase D activity has been identified as ADP ribosylation factor (ARF). ARF is a small GTP binding protein whose function has been elucidated in vesicular traffic. This review puts into context the connection between the two fields of signal transduction and vesicular transport.

The transphosphatidylation activity of phospholipase D

Transphosphatidylation activity is a characteristic and remarkable property of phospholipase D (PLD) and has been studied in plants and mammalian tissues. This reaction is often used to confirm the properties and/or abnormalities of PLD activity. The mechanism for activating PLD transphosphatidylation seems multiple. Although significant changes of transphosphatidylation activity have been found in some pathological animal models, the biological significance of PLD transphosphatidylation remains largely unknown.

Ethanol potentiates the uptake of [14C]serine into phosphatidylserine by base-exchange reaction in NG 108-15 cells

Phospholipid base-exchange enzymes catalyze the incorporation of nitrogenous bases into phosphoglycerides by a calcium-dependent mechanism. In this study, we describe the effect of ethanol on the incorporation of radioactive serine, choline and ethanolamine into their respective phospholipids in a neuroblastoma x glioma hybrid cell line (NG 108-15). Long term ethanol exposure induced a potentiation of the incorporation of [14C]serine into phosphatidylserine. Moreover, the phosphorus content of PS was found to be increased after long-term ethanol exposure. No concomitant changes in the phosphorus content of other phospholipids were observed. The results indicate that in NG 108-15 cells, the incorporation of radiolabelled serine into PS is potentiated during chronic ethanol exposure.

Ethanolamine analogues stimulate DNA synthesis by a mechanism not involving phosphatidylethanolamine synthesis

Dimethylethanolamine (0.5-1 mM), added to serum-starved NIH 3T3 fibroblasts, stimulated DNA synthesis 11-32-fold, and it also greatly enhanced the relatively modest (15-20-fold) mitogenic effect of insulin. Ethanolamine and monomethylethanolamine alone had no effects on DNA synthesis, but they also enhanced the stimulatory effect of insulin, although less effectively than dimethylethanolamine did. Lower concentrations (2.5-5 microg/ml) of compound D 609 (tricyclo-9-yl-xanthogenate), which had no effects on phospholipase activities, synergistically enhanced the combined effects of ethanolamine analogs and insulin on DNA synthesis without affecting the synthesis of ethanolamine phospholipids. These results suggest that ethanolamine and its analogues, formed by phospholipase D-mediated hydrolysis of ethanolamine phospholipids, may have growth regulatory functions independent of their role as phospholipid precursors.

Characterization and partial purification of phospholipase D from human placenta

We report the existence in the human placenta of a phosphatidylcholine-hydrolyzing phospholipase D (PLD) activity, which has been characterized and partially purified. Triton X-100 effectively solubilized PLD from the particulate fraction of human placenta in a dose-dependent manner. However, Triton X-100 caused decreasing enzyme activities. Maximum transphosphatidylation was obtained with 2% ethanol. The enzyme was found to have a pH optimum of 7.0-7.5 and an apparent Km of 33 mol% (or 0.8 mM). Ca2+ and Mg2+ was not required for the enzyme activity. Addition of phosphatidyl-4,5-bisphosphate, but not phosphatidylethanolamine, to the substrate mixture gave rise to a pronounced dose-dependent increase in PLD activity (EC50 = 0.3 mol%), suggesting a regulatory role of this phospholipid in PLD action. The enzyme was inhibited by sodium oleate when partly or fully substituting for octylglucoside in the substrate mixture. The PLD activity was enriched 15-fold by solubilization and purification on a DEAE-Sepharose column. N-Ethylmaleimide (10 mM) markedly inhibited the purified enzyme, indicating the presence of free thiol groups on PLD. Sphingosine (20 microM) and (+/-) propranolol (53 microM) had no direct effect on PLD activity. The present results form the basis for further purification of a PLD from human tissue.

Accumulation of phosphatidylalcohol in cultured cells: use of subcellular fractionation to investigate phospholipase D activity during signal transduction

Phosphatidylalcohol accumulates as a product of a phospholipase D (PLD)-catalysed transphosphatidylation reaction in cells incubated in the presence of a primary alcohol. In the presence of ethanol the phorbol ester, phorbol 12-myristate 13-acetate (PMA), stimulated the accumulation of [3H]phosphatidylethanol (PEth) in HeLa cells prelabelled with [3H]palmitic acid. Radioactivity associated with PEth increased linearly during a 30 min incubation, indicating that a sustained activation of PLD is caused by PMA in these cells. This was accompanied by the membrane association of protein kinase C-alpha (PKC-alpha), the PKC isoform that recent studies indicate is involved in the activation of PLD. In similar experiments, the neuropeptide bradykinin stimulated an accumulation of PEth in 3T3 Li cells. The radioactivity associated with PEth increased to a maximal level at 30 s and plateaued after this time, suggesting that bradykinin induces only a transient activation of PLD in these cells. This is consistent with the effects of bradykinin on PKC-alpha, which underwent a rapid and transient association with cell membranes. The subcellular localization of PEth was examined using the technique of subcellular fractionation on Percoll density gradients to isolate organelle-enriched fractions from HeLa and 3T3 Li cells. An accumulation of [3H]PEth was measured in the plasma-membrane (PM)-enriched fractions of both HeLa and 3T3 Li cells after incubation with PMA and bradykinin respectively. This was accompanied by a time-dependent accumulation of [3H]PEth in the combined mitochondrial and endoplasmic reticulum (MER)-enriched fractions of both cell lines. PMA was also found to cause translocation of PKC-alpha to both the PM- and MER-enriched fractions in HeLa cells. However, bradykinin stimulated the translocation of PKC-alpha to the PM-enriched fractions only of 3T3 Li cells. The results show that PLD activation leads to the accumulation of PEth in both the PM and MER fractions. We therefore propose that either bradykinin activates a PM-associated PLD and the PLD reaction product is rapidly translocated to other membrane systems or it activates an MER-associated PLD by a mechanism that does not involve PKC-alpha.

Phorbol ester selectively stimulates the phospholipase D-mediated hydrolysis of phosphatidylethanolamine in multidrug-resistant MCF-7 human breast carcinoma cells

The phospholipase D (PLD)-mediated synthesis of phosphatidylethanol (PtdEtOH) and the hydrolysis of phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho) were examined in drug-sensitive and multidrug-resistant lines of MCF-7 human breast carcinoma cells. In drug-sensitive (MCF-7/WT) cells, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) failed to enhance either the synthesis of PtdEtOH or the hydrolysis of either phospholipid. In the drug-resistant (MCF-7/MDR) cells, 100 nM PMA greatly enhanced both the synthesis of PtdEtOH (approximately 21-fold) and the hydrolysis of PtdEtn (approximately 29-fold), but had no effect on the hydrolysis of PtdCho. The PLD activators sphingosine and H2O2 were found to elicit only a slight (1.28-1.4-fold) stimulatory effect on PtdCho hydrolysis in both the MCF-7/WT and MCF-7/MDR cell types, and had only a small effect on PtdEtn hydrolysis in the MCF-7/WT cells as well. However, these agents significantly (approximately 2.6-3.5-fold) stimulated PtdEtn hydrolysis in the MCF-7/MDR cells. These data indicate that MCF-7/MDR cells contain a PtdEtn-specific PLD activity which can be selectively stimulated by PMA, sphingosine and H2O2.