Mitogenic phospholipase D activity is restricted to caveolin-enriched membrane microdomains

A Dominant-Negative Mutant of ANXA7 Impairs Calcium Signaling and Enhances the Proliferation of Prostate Cancer Cells by Downregulating the IP3 Receptor and the PI3K/mTOR Pathway

Annexin A7/ANXA7 is a calcium-dependent membrane fusion protein with tumor suppressor gene (TSG) properties, which is located on chromosome 10q21 and is thought to function in the regulation of calcium homeostasis and tumorigenesis. However, whether the molecular mechanisms for tumor suppression are also involved in the calcium- and phospholipid-binding properties of ANXA7 remain to be elucidated. We hypothesized that the 4 C-terminal endonexin-fold repeats in ANXA7 (GX(X)GT), which are contained within each of the 4 annexin repeats with 70 amino acids, are responsible for both calcium- and GTP-dependent membrane fusion and the tumor suppressor function. Here, we identified a dominant-negative triple mutant (DNTM/DN-ANXA7J) that dramatically suppressed the ability of ANXA7 to fuse with artificial membranes while also inhibiting tumor cell proliferation and sensitizing cells to cell death. We also found that the [DNTM]ANA7 mutation altered the membrane fusion rate and the ability to bind calcium and phospholipids. In addition, in prostate cancer cells, our data revealed that variations in phosphatidylserine exposure, membrane permeabilization, and cellular apoptosis were associated with differential IP3 receptor expression and PI3K/AKT/mTOR modulation. In conclusion, we discovered a triple mutant of ANXA7, associated with calcium and phospholipid binding, which leads to the loss of several essential functions of ANXA7 pertinent to tumor protection and highlights the importance of the calcium signaling and membrane fusion functions of ANXA7 for preventing tumorigenesis.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

Paxillin phosphorylation and complexing with Erk and FAK are regulated by PLD activity in MDA-MB-231 cells

MDA-MB-231 cells are highly aggressive human breast adenocarcinoma cells that depend on PLD activity for survival. In response to the stress of serum withdrawal, there is increased motility and invasiveness of these cells that is associated with a rapid increase in PLD activity. In addition, PLD activity is elevated in response to most mitogenic signals. Similar to PLD, paxillin, a focal adhesion adaptor protein, and Erk, mitogen-activated protein kinase, play vital roles in cell motility through regulation of focal adhesion dynamics. Here, we addressed whether there is a functional correlation between paxillin and PLD that may influence cancer cell motility. We investigated the role of PLD activity on paxillin regulation, Erk activation and formation of a paxillin-Erk and paxillin-FAK association. Inhibition of PLD activity led to an increase in paxillin tyrosine phosphorylation, a decrease in Erk activation, as measured by phosphorylation, and enhanced association of paxillin with Erk. In addition, we found that paxillin tyrosine phosphorylation depends upon Erk activity and may be a consequence of an increased association with FAK. Taken together, these results suggest that Erk activity is governed by PLD activity and regulates the tyrosine phosphorylation of paxillin, potentially explaining its role in cell motility. This study indicated that PLD, Erk, paxillin and FAK participate in the same signaling pathway in this breast cancer cell line.

The exquisite regulation of PLD2 by a wealth of interacting proteins: S6K, Grb2, Sos, WASp and Rac2 (and a surprise discovery: PLD2 is a GEF)

Phospholipase D (PLD) catalyzes the conversion of the membrane phospholipid phosphatidylcholine to choline and phosphatidic acid (PA). PLD's mission in the cell is two-fold: phospholipid turnover with maintenance of the structural integrity of cellular/intracellular membranes and cell signaling through PA and its metabolites. Precisely, through its product of the reaction, PA, PLD has been implicated in a variety of physiological cellular functions, such as intracellular protein trafficking, cytoskeletal dynamics, chemotaxis of leukocytes and cell proliferation. The catalytic (HKD) and regulatory (PH and PX) domains were studied in detail in the PLD1 isoform, but PLD2 was traditionally studied in lesser detail and much less was known about its regulation. Our laboratory has been focusing on the study of PLD2 regulation in mammalian cells. Over the past few years, we have reported, in regards to the catalytic action of PLD, that PA is a chemoattractant agent that binds to and signals inside the cell through the ribosomal S6 kinases (S6K). Regarding the regulatory domains of PLD2, we have reported the discovery of the PLD2 interaction with Grb2 via Y169 in the PX domain, and further association to Sos, which results in an increase of de novo DNA synthesis and an interaction (also with Grb2) via the adjacent residue Y179, leading to the regulation of cell ruffling, chemotaxis and phagocytosis of leukocytes. We also present the complex regulation by tyrosine phosphorylation by epidermal growth factor receptor (EGF-R), Janus Kinase 3 (JAK3) and Src and the role of phosphatases. Recently, there is evidence supporting a new level of regulation of PLD2 at the PH domain, by the discovery of CRIB domains and a Rac2-PLD2 interaction that leads to a dual (positive and negative) effect on its enzymatic activity. Lastly, we review the surprising finding of PLD2 acting as a GEF. A phospholipase such as PLD that exists already in the cell membrane that acts directly on Rac allows a quick response of the cell without intermediary signaling molecules. This provides only the latest level of PLD2 regulation in a field that promises newer and exciting advances in the next few years.

Phospholipase D2 Enhances Epidermal Growth Factor-Induced Akt Activation in EL4 Lymphoma Cells

Phospholipase D2 (PLD2) generates phosphatidic acid through hydrolysis of phosphatidylcholine. PLD2 has been shown to play a role in enhancing tumorigenesis. The epidermal growth factor receptor (EGFR) can both activate and interact with PLD2. Murine lymphoma EL4 cells lacking endogenous PLD2 present a unique model to elucidate the role of PLD2 in signal transduction. In the current study, we investigated effects of PLD2 on EGF response. Western blotting and RT-PCR were used to establish that both parental cells and PLD2 transfectants express endogenous EGFR. Levels of EGFR protein are increased in cells expressing active PLD2, as compared to parental cells or cells expressing inactive PLD2. EGF stimulates proliferation of EL4 cells transfected with active PLD2, but not parental cells or cells transfected with inactive PLD2. EGF-mediated proliferation in cells expressing active PLD2 is dependent on the activities of both the EGFR and the PI3K/Akt pathway, as demonstrated by studies using protein kinase inhibitors. EGF-induced invasion through a synthetic extracellular matrix is enhanced in cells expressing active PLD2, as compared to parental cells or cells expressing inactive PLD2. Taken together, the data suggest that PLD2 acts in concert with EGFR to enhance mitogenesis and invasion in lymphoma cells.

A comprehensive model that explains the regulation of phospholipase D2 activity by phosphorylation-dephosphorylation

We report here that the enzymatic activity of phospholipase D2 (PLD2) is regulated by phosphorylation-dephosphorylation. Phosphatase treatment of PLD2-overexpressing cells showed a biphasic nature of changes in activity that indicated the existence of "activator" and "inhibitory" sites. We identified three kinases capable of phosphorylating PLD2 in vitro-epidermal growth factor receptor (EGFR), JAK3, and Src (with JAK3 reported for the first time in this study)-that phosphorylate an inhibitory, an activator, and an ambivalent (one that can yield either effect) site, respectively. Mass spectrometry analyses indicated the target of each of these kinases as Y(296) for EGFR, Y(415) for JAK3, and Y(511) for Src. The extent to which each site is activated or inhibited depends on the cell type considered. In COS-7, cells that show the highest level of PLD2 activity, the Y(415) is a prominent site, and JAK3 compensates the negative modulation by EGFR on Y(296). In MCF-7, cells that show the lowest level of PLD2 activity, the converse is the case, with Y(296) unable to compensate the positive modulation by Y(415). MTLn3, with medium to low levels of lipase activity, show an intermediate pattern of regulation but closer to MCF-7 than to COS-7 cells. The negative effect of EGFR on the two cancer cell lines MTLn3 and MCF-7 is further proven by RNA silencing experiments that yield COS-7 showing lower PLD2 activity, and MTLn3 and MCF-7 cells showing an elevated activity. MCF-7 is a cancer cell line derived from a low-aggressive/invasive form of breast cancer that has relatively low levels of PLD activity. We propose that PLD2 activity is low in the breast cancer cell line MCF-7 because it is kept downregulated by tyrosyl phosphorylation of Y(296) by EGFR kinase. Thus, phosphorylation of PLD2-Y(296) could be the signal for lowering the level of PLD2 activity in transformed cells with low invasive capabilities.

PLD2 has both enzymatic and cell proliferation-inducing capabilities, that are differentially regulated by phosphorylation and dephosphorylation

Phospholipase D2 (PLD2) overexpression in mammalian cells results in cell transformation. We have hypothesized that this is due to an increase of de novo DNA synthesis. We show here that overexpression of PLD2-WT leads to an increased DNA synthesis, as measured by the expression levels of the proliferation markers PCNA, p27(KIP1) and phospho-histone-3. The enhancing effect was even higher with phosphorylation-deficient PLD2-Y179F and PLD2-Y511F mutants. The mechanism for this did not involve the enzymatic activity of the lipase, but, rather, the presence of the protein tyrosine phosphatase CD45, as silencing with siRNA for CD45 abrogated the effect. The two Y-->F mutants had in common a YxN consensus site that, in the phosphorylated counterparts, could be recognized by SH2-bearing proteins, such as Grb2. Even though Y179F and Y511F cannot bind Grb2, they could still find other protein partners, one of which, we have reasoned, could be CD45 itself. Affinity purified PLD2 is indeed activated by Grb2 and deactivated by CD45 in vitro. We concluded that phosphorylated PLD2, aided by Grb2, mediates lipase activity, whereas dephosphorylated PLD2 mediates an induction of cell proliferation, and the specific residues involved in this newly discovered regulation of PLD2 are Y(179) and Y(511).

Subunit composition and role of Na+,K+-ATPases in ventricular myocytes

Na+,K+-ATPases are composed of one alpha and one beta subunit; four alpha and three beta isoforms have been found to date. We elucidated which alpha and beta subunits were present in the ventricular myocytes of rat and guinea-pig and what roles the Na+,K(+)-ATPase isozymes play in cardiac contraction. The presence of the alpha1, alpha2, and alpha3 subunits and the beta1 and beta2 subunits in rat and guinea-pig hearts were confirmed at the protein or mRNA level. Immunocytochemistry showed a patchy presence of alpha1 in the transverse tubules and surface sarcolemma, whereas alpha2 was distributed continuously in the transverse tubules alone. The alpha3 isoform was expressed prominently in the guinea-pig intercalated disc and slightly in the rat. On the other hand, the beta1 isoform was located in the transverse tubules and surface sarcolemma, whereas the beta2 was mainly located in the intercalated disc. The immunocytochemistry and immunoprecipitation findings indicated that the alpha1 and alpha2 form heterodimers with beta1 and the alpha3 with beta2 in ventricular myocytes. The application of low concentrations of ouabain enhanced the amplitudes of twitch without a change in resting tension in rat and guinea-pig ventricular stripts, whereas that of high concentrations resulted in a decrease in twitch with an increase in the resting tension. We thus conclude that the alpha2beta1 and alpha3beta2 isozymes are selectively located in the transverse tubules and intercalated disc of the ventricular myocytes, respectively, and the alpha2beta1 is involved in the regulation of the Ca2+ contents in the SR.

Direct Modulation of Phospholipase D Activity by Gβγ

Phospholipase D-mediated hydrolysis of phosphatidylcholine is stimulated by protein kinase C and the monomeric G proteins Arf, RhoA, Cdc42, and Rac1, resulting in complex regulation of this enzyme. Using purified proteins, we have identified a novel inhibitor of phospholipase D activity, Gbetagamma subunits of heterotrimeric G proteins. G protein-coupled receptor activation alters affinity between Galpha and Gbetagamma subunits, allowing subsequent interaction with distinct effectors. Gbeta1gamma1 inhibited phospholipase D1 and phospholipase D2 activity, and both Gbeta1gamma1 and Gbeta1gamma2 inhibited stimulated phospholipase D1 activity in a dosedependent manner in reconstitution assays. Reconstitution assays suggest this interaction occurs through the amino terminus of phospholipase D, because Gbeta1gamma1 is unable to inhibit an amino-terminally truncated phospholipase D construct, PLD1.d311, which like full-length phospholipase D isoforms, requires phosphatidylinositol-4,5-bisphosphate for activity. Furthermore, a truncated protein consisting of the amino-terminal region of phospholipase D containing the phox/pleckstrin homology domains was found to interact with Gbeta1gamma1, unlike the PLD1.d311 recombinant protein, which lacks this domain. In vivo, expressed recombinant Gbeta1gamma2 was also found to inhibit phospholipase D activity under basal and stimulated conditions in MDA-MB-231 cells, which natively express both phospholipase D1 and phospholipase D2. These data demonstrate that Gbetagamma directly regulates phospholipase D activity in vitro and suggest a novel mechanism to negatively regulate phospholipase D signaling in vivo.

Upregulation of Phospholipase D Expression and Activation in Ventricular Pressure-Overload Hypertrophy

Evidence for a role of phospholipase D (PLD) in cellular proliferation and differentiation is accumulating. We studied PLD activity and expression in normal and hypertrophic rat and human hearts. In rat heart, abdominal aortic banding (constriction to 50% of original lumen) caused hypertrophy in the left ventricle (as shown by weight index and ANP expression) by about 15% after 30 days without histological evidence of fibrosis or signs of decompensation and in the right ventricle after 100 days. The hypertrophy was accompanied by small increases of basal PLD activity and strong potentiation of stimulated PLD activity caused by 4beta-phorbol-12beta,13alpha-dibutyrate (PDB) and by phenylephrine. The mRNA expressions of both PLD1 and PLD2 determined by semiquantitative competitive RT-PCR were markedly enhanced after aortic banding. In the caveolar fraction of the rat heart, PLD2 protein determined by Western blot analysis was upregulated in parallel with the expression of caveolin-3. A similar induction of PLD mRNA and protein expression was observed in hypertrophied human hearts of individuals (39-45-year-old) who had died from non-cardiac causes. In conclusion, PLD1 and PLD2 expressions were strongly enhanced both in rat and human heart hypertrophy, which may be responsible for the coincident potentiation of the PLD activation by alpha-adrenoceptor and protein kinase C stimulation. These results are compatible with a significant role of PLD activation in cell signaling of ventricular pressure-overload hypertrophy.

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.

Targeting mTOR-mediated survival signals in anticancer therapeutic strategies

An important component of tumor progression is the generation of survival signals that overcome default apoptotic programs. In principle, survival signals are ideal targets for anticancer therapeutic strategies because blocking these signals leads to the death of cells that are dependent upon them. A common target of survival signals is mTOR. Survival signals generated by both phosphatidylinositol-3-kinase and phospholipase D target mTOR. Suppression of these mTOR-mediated survival signals provides the opportunity to reactivate default apoptotic pathways in cancer cells and allow the cancer cells to die on their own. In this review, the potential for anticancer strategies that target mTOR-mediated survival signals is explored.

Protein kinase Calpha translocates to the perinuclear region to activate phospholipase D1

The inhibition of phorbol ester activation of phospholipase D1 (PLD1) by protein kinase C (PKC) inhibitors has been considered proof of phosphorylation-dependent activation of PLD1 by PKCalpha. We studied the effect of the PKC inhibitors Ro-31-8220 and bisindolylmaleimide I on PLD1 activation and found that they inhibited the activation by interfering with PKCalpha binding to PLD1. Further studies showed that only unphosphorylated PKCalpha could bind to and activate PLD1 and that both inhibitors induced phosphorylation of PKCalpha. The phosphorylation status of either PLD1 or PKCalpha per se did not affect PLD1 activation in vitro. Immunofluorescence studies showed that PLD1 remained in the perinuclear region after phorbol ester treatment, whereas PKCalpha translocated from cytosol to both plasma membrane and perinuclear regions. Both Ro-31-8220 and bisindolylmaleimide I blocked the translocation of PKCalpha to the perinuclear region but not to the plasma membrane. Studies with okadaic acid suggested that phosphorylation regulated the relocation of PKCalpha from the plasma membrane to the perinuclear region. It is proposed that localization and interaction of PKCalpha with PLD1 in the perinuclear region is required for PLD1 activation and that PKC inhibitors inhibit this through phosphorylation of PKCalpha, which blocks its translocation.

The direct interaction of phospholipase C-gamma 1 with phospholipase D2 is important for epidermal growth factor signaling

The epidermal growth factor (EGF) receptor has an important role in cellular proliferation, and the enzymatic activity of phospholipase C (PLC)-gamma1 is regarded to be critical for EGF-induced mitogenesis. In this study, we report for the first time a phospholipase complex composed of PLC-gamma1 and phospholipase D2 (PLD2). PLC-gamma1 is co-immunoprecipitated with PLD2 in COS-7 cells. The results of in vitro binding analysis and co-immunoprecipitation analysis in COS-7 cells show that the Src homology (SH) 3 domain of PLC-gamma1 binds to the proline-rich motif within the Phox homology (PX) domain of PLD2. The interaction between PLC-gamma1 and PLD2 is EGF stimulation-dependent and potentiates EGF-induced inositol 1,4,5-trisphosphate (IP(3)) formation and Ca(2+) increase. Mutating Pro-145 and Pro-148 within the PX domain of PLD2 to leucines disrupts the interaction between PLC-gamma1 and PLD2 and fails to potentiate EGF-induced IP(3) formation and Ca(2+) increase. However, neither PLD2 wild type nor PLD2 mutant affects the EGF-induced tyrosine phosphorylation of PLC-gamma1. These findings suggest that, upon EGF stimulation, PLC-gamma1 directly interacts with PLD2 and this interaction is important for PLC-gamma1 activity.

Elevated phospholipase D activity in H-Ras- but not K-Ras-transformed cells by the synergistic action of RalA and ARF6

Phospholipase D (PLD) activity is elevated in response to the oncogenic stimulus of H-Ras but not K-Ras. H-Ras and K-Ras have been reported to localize to different membrane microdomains, with H-Ras localizing to caveolin-enriched light membrane fractions. We reported previously that PLD activity elevated in response to mitogenic stimulation is restricted to the caveolin-enriched light membrane fractions. PLD activity in H-Ras-transformed cells is dependent upon RalA, and consistent with a lack of elevated PLD activity in K-Ras-transformed cells, RalA was not activated in K-Ras-transformed cells. Although H-Ras-induced PLD activity is dependent upon RalA, an activated mutant of RalA is not sufficient to elevate PLD activity. We reported previously that RalA interacts with PLD activating ADP ribosylation factor (ARF) proteins. In cells transformed by H-Ras, we found increased coprecipitation of ARF6 with RalA. Moreover, ARF6 colocalized with RalA in light membrane fractions. Interestingly, ARF6 protein levels were elevated in H-Ras- but not K-Ras-transformed cells. A dominant-negative mutant of ARF6 inhibited PLD activity in H-Ras-transformed NIH 3T3 cells. Activated mutants of either ARF6 or RalA were not sufficient to elevate PLD activity in NIH 3T3 cells; however, expression of both activated RalA and activated ARF6 in NIH 3T3 cells led to increased PLD activity. These data suggest a model whereby H-Ras stimulates the activation of both RalA and ARF6, which together lead to the elevation of PLD activity.

The mechanism of docosahexaenoic acid-induced phospholipase D activation in human lymphocytes involves exclusion of the enzyme from lipid rafts

Docosahexaenoic acid (DHA), an n-3 polyunsaturated fatty acid that inhibits T lymphocyte activation, has been shown to stimulate phospholipase D (PLD) activity in stimulated human peripheral blood mononuclear cells (PBMC). To elucidate the mechanisms underlying the DHA-induced PLD activation, we first characterized the PLD expression pattern of PBMC. We show that these cells express PLD1 and PLD2 at the protein and mRNA level and are devoid of oleate-dependent PLD activity. DHA enrichment of PBMC increased the DHA content of cell phospholipids, which was directly correlated with the extent of PLD activation. The DHA-induced PLD activation was independent of conventional protein kinase C but inhibited by brefeldin A, which suggests ADP-ribosylation factor (ARF)-dependent mechanism. Furthermore, DHA enrichment dose-dependently stimulated ARF translocation to cell membranes. Whereas 50% of the guanosine 5'-3-O-(thio)triphosphate plus ARF-dependent PLD activity and a substantial part of PLD1 protein were located to the detergent-insoluble membranes, so-called rafts, of non-enriched PBMC, DHA treatment strongly displaced them toward detergent-soluble membranes where ARF is present. Collectively, these results suggest that the exclusion of PLD1 from lipid rafts, due to their partial disorganization by DHA, and its relocalization in the vicinity of ARF, is responsible for its activation. This PLD activation might be responsible for the immunosuppressive effect of DHA because it is known to transmit antiproliferative signals in lymphoid cells.

Detecting protein-phospholipid interactions. Epidermal growth factor-induced activation of phospholipase D1b in situ

Phospholipase D (PLD) proteins have been identified in secretory and endocytic vesicles, consistent with their proposed role in regulating membrane traffic. However, their sites of catalytic action remain obscure. We have developed here a novel, analytical approach to monitor PLD activation in intact cells employing lifetime imaging microscopy to measure fluorescence resonance energy transfer between protein and membrane phospholipid. Verification and application of this technique demonstrates a dispersed endosomal, epidermal growth factor-induced activation of the PLD1b isoform. Application of this approach will facilitate the spatial resolution of many protein-phospholipid interactions that are key events in the regulation of cellular processes.

Transformation of cells overexpressing a tyrosine kinase by phospholipase D1 and D2

Phospholipase D (PLD) activity is elevated in response to most mitogenic signals. Two mammalian PLD genes (PLD1 and PLD2) have been cloned and their gene products have been characterized. PLD1 is a downstream target of the Ras/RalA GTPase cascade implicated in mitogenic and oncogenic signaling. Consistent with a role in mitogenic signaling, elevated expression of PLD1 transforms cells overexpressing the epidermal growth factor (EGF) receptor (EGFR). However, PLD2 colocalizes with the EGFR in caveolin-enriched light membrane microdomains. We therefore investigated whether PLD2 could also contribute to the transformation of cells overexpressing a tyrosine kinase. We report here that elevated expression of PLD2 transforms rat fibroblasts overexpressing either the EGFR or c-Src. Since overexpression of a tyrosine kinase is a common genetic alteration in several human cancers, these data suggest that elevation of either PLD1 or PLD2 may contribute to the progression to a malignant phenotype in cells with elevated tyrosine kinase activity.

Role for phospholipase D in receptor-mediated endocytosis

In response to epidermal growth factor (EGF), the EGF receptor is endocytosed and degraded. A substantial lag period exists between endocytosis and degradation, suggesting that endocytosis is more than a simple negative feedback. Phospholipase D (PLD), which has been implicated in vesicle formation in the Golgi, is activated in response to EGF and other growth factors. We report here that EGF receptor endocytosis is dependent upon PLD and the PLD1 regulators, protein kinase C alpha and RalA. EGF-induced receptor degradation is accelerated by overexpression of either wild-type PLD1 or PLD2 and retarded by overexpression of catalytically inactive mutants of either PLD1 or PLD2. EGF-induced activation of mitogen-activated protein kinase, which is dependent upon receptor endocytosis, is also dependent upon PLD. These data suggest a role for PLD in signaling that facilitates receptor endocytosis.