Stimulation of actin stress fibre formation mediated by activation of phospholipase D

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

A newly developed PLD1 inhibitor ameliorates rheumatoid arthritis by regulating pathogenic T and B cells and inhibiting osteoclast differentiation

Phospholipase D1 (PLD1), which catalyzes the hydrolysis of phosphatidylcholine to phosphatidic acid and choline, plays multiple roles in inflammation. We investigated the therapeutic effects of the newly developed PLD1 inhibitors A2998, A3000, and A3773 in vitro and in vivo rheumatoid arthritis (RA) model. A3373 reduced the levels of LPS-induced TNF-α, IL-6, and IgG in murine splenocytes in vitro. A3373 also decreased the levels of IFN-γ and IL-17 and the frequencies of Th1, Th17 cells and germinal-center B cells, in splenocytes in vitro. A3373 ameliorated the severity of collagen-induced arthritis (CIA) and suppressed infiltration of inflammatory cells into the joint tissues of mice with CIA compared with vehicle-treated mice. Moreover, A3373 prevented systemic bone demineralization in mice with CIA and suppressed osteoclast differentiation and the mRNA levels of osteoclastogenesis markers in vitro. These results suggest that A3373 has therapeutic potential for RA.

Roles for Phospholipase D1 in the Tumor Microenvironment

The lipid-modifying signal transduction enzyme phospholipase D (PLD) has been proposed to have roles in oncogenic processes for well-on 30 years, with most of the early literature focused on potential functions for PLD in the biology of the tumor cells themselves. While such roles remain under investigation, evidence has also now been generated to support additional roles for PLD, in particular PLD1, in the tumor microenvironment, including effects on neoangiogenesis, the supply of nutrients, interactions of platelets with circulating cancer cells, the response of the immune system, and exosome biology. Here, we review these lines of investigation, accompanied by a discussion of the limitations of the existing studies and some cautionary notes regarding the study and interpretation of PLD function using model systems.

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

Syntrophins entangled in cytoskeletal meshwork: Helping to hold it all together

Syntrophins are a family of 59 kDa peripheral membrane-associated adapter proteins, containing multiple protein-protein and protein-lipid interaction domains. The syntrophin family consists of five isoforms that exhibit specific tissue distribution, distinct sub-cellular localization and unique expression patterns implying their diverse functional roles. These syntrophin isoforms form multiple functional protein complexes and ensure proper localization of signalling proteins and their binding partners to specific membrane domains and provide appropriate spatiotemporal regulation of signalling pathways. Syntrophins consist of two PH domains, a PDZ domain and a conserved SU domain. The PH1 domain is split by the PDZ domain. The PH2 and the SU domain are involved in the interaction between syntrophin and the dystrophin-glycoprotein complex (DGC). Syntrophins recruit various signalling proteins to DGC and link extracellular matrix to internal signalling apparatus via DGC. The different domains of the syntrophin isoforms are responsible for modulation of cytoskeleton. Syntrophins associate with cytoskeletal proteins and lead to various cellular responses by modulating the cytoskeleton. Syntrophins are involved in many physiological processes which involve cytoskeletal reorganization like insulin secretion, blood pressure regulation, myogenesis, cell migration, formation and retraction of focal adhesions. Syntrophins have been implicated in various pathologies like Alzheimer's disease, muscular dystrophy, cancer. Their role in cytoskeletal organization and modulation makes them perfect candidates for further studies in various cancers and other ailments that involve cytoskeletal modulation. The role of syntrophins in cytoskeletal organization and modulation has not yet been comprehensively reviewed till now. This review focuses on syntrophins and highlights their role in cytoskeletal organization, modulation and dynamics via its involvement in different cell signalling networks.

Phospholipase D: A new mediator during high phosphate-induced vascular calcification associated with chronic kidney disease

Vascular calcification (VC) is the pathological accumulation of calcium phosphate crystals in one of the layers of blood vessels, leading to loss of elasticity and causing severe calcification in vessels. Medial calcification is mostly seen in patients with chronic kidney disease (CKD) and diabetes. Identification of key enzymes and their actions during calcification will contribute to understand the onset of pathological calcification. Phospholipase D (PLD1, PLD2) is active at the earlier steps of mineralization in osteoblasts and chondrocytes. In this study, we aimed to determine their effects during high-phosphate treatment in mouse vascular smooth muscle cell line MOVAS, in the ex vivo model of the rat aorta, and in the in vivo model of adenine-induced CKD. We observed an early increase in PLD1 gene and protein expression along with the increase in the PLD activity in vascular muscle cell line, during calcification induced by ascorbic acid and β-glycerophosphate. Inhibition of PLD1 by the selective inhibitor VU0155069, or the pan-PLD inhibitor, halopemide, prevented calcification. The mechanism of PLD activation is likely to be protein kinase C (PKC)-independent since bisindolylmaleimide X hydrochloride, a pan-PKC inhibitor, did not affect the PLD activity. In agreement, we found an increase in Pld1 gene expression and PLD activity in aortic explant cultures treated with high phosphate, whereas PLD inhibition by halopemide decreased calcification. Finally, an increase in both Pld1 and Pld2 expression occurred simultaneously with the appearance of VC in a rat model of CKD. Thus, PLD, especially PLD1, promotes VC in the context of CKD and could be an important target for preventing onset or progression of VC.

MARCKS and MARCKS-like proteins in development and regeneration

BACKGROUND

The Myristoylated Alanine-Rich C-kinase Substrate (MARCKS) and MARCKS-like protein 1 (MARCKSL1) have a wide range of functions, ranging from roles in embryonic development to adult brain plasticity and the inflammatory response. Recently, both proteins have also been identified as important players in regeneration. Upon phosphorylation by protein kinase C (PKC) or calcium-dependent calmodulin-binding, MARCKS and MARCKSL1 translocate from the membrane into the cytosol, modulating cytoskeletal actin dynamics and vesicular trafficking and activating various signal transduction pathways. As a consequence, the two proteins are involved in the regulation of cell migration, secretion, proliferation and differentiation in many different tissues.

MAIN BODY

Throughout vertebrate development, MARCKS and MARCKSL1 are widely expressed in tissues derived from all germ layers, with particularly strong expression in the nervous system. They have been implicated in the regulation of gastrulation, myogenesis, brain development, and other developmental processes. Mice carrying loss of function mutations in either Marcks or Marcksl1 genes die shortly after birth due to multiple deficiencies including detrimental neural tube closure defects. In adult vertebrates, MARCKS and MARCKL1 continue to be important for multiple regenerative processes including peripheral nerve, appendage, and tail regeneration, making them promising targets for regenerative medicine.

CONCLUSION

This review briefly summarizes the molecular interactions and cellular functions of MARCKS and MARCKSL1 proteins and outlines their vital roles in development and regeneration.

Features of the Phosphatidylinositol Cycle and its Role in Signal Transduction

The phosphatidylinositol cycle (PI-cycle) has a central role in cell signaling. It is the major pathway for the synthesis of phosphatidylinositol and its phosphorylated forms. In addition, some lipid intermediates of the PI-cycle, including diacylglycerol and phosphatidic acid, are also important lipid signaling agents. The PI-cycle has some features that are important for the understanding of its role in the cell. As a cycle, the intermediates will be regenerated. The PI-cycle requires a large amount of metabolic energy. There are different steps of the cycle that occur in two different membranes, the plasma membrane and the endoplasmic reticulum. In order to complete the PI-cycle lipid must be transferred between the two membranes. The role of the Nir proteins in the process has recently been elucidated. The lipid intermediates of the PI-cycle are normally highly enriched with 1-stearoyl-2-arachidonoyl molecular species in mammals. This enrichment will be retained as long as the intermediates are segregated from other lipids of the cell. However, there is a significant fraction (>15 %) of lipids in the PI-cycle of normal cells that have other acyl chains. Phosphatidylinositol largely devoid of arachidonoyl chains are found in cancer cells. Phosphatidylinositol species with less unsaturation will not be as readily converted to phosphatidylinositol-3,4,5-trisphosphate, the lipid required for the activation of Akt with resulting effects on cell proliferation. Thus, the cyclical nature of the PI-cycle, its dependence on acyl chain composition and its requirement for lipid transfer between two membranes, explain many of the biological properties of this cycle.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Dual function of a bacterial protein as an adhesin and extracellular effector of host GTPase signaling

Bacterial pathogens often target conserved cellular mechanisms within their hosts to rewire signaling pathways and facilitate infection. Rho GTPases are important nodes within eukaryotic signaling networks and thus constitute a common target of pathogen-mediated manipulation. A diverse array of microbial mechanisms exists to interfere with Rho GTPase signaling. While targeting of GTPases by secreted bacterial effectors is a well-known strategy bacterial pathogens employ to interfere with the host, we have recently described pathogen adhesion as a novel extracellular stimulus that hijacks host GTPase signaling. The Multivalent Adhesion Molecule MAM7 from Vibrio parahaemolyticus directly binds host cell membrane lipids. The ensuing coalescence of phosphatidic acid ligands in the host membrane leads to downstream activation of RhoA and actin rearrangements. Herein, we discuss mechanistic models of lipid-mediated Rho activation and the implications from the infected host's and the pathogen's perspective.

Inhibitory effects of α-cyperone on adherence and invasion of avian pathogenic Escherichia coli O78 to chicken type II pneumocytes

Avian pathogenic Escherichia coli (APEC) are extra-intestinal pathogenic E. coli, and usually cause avian septicemia through breaching the blood-gas barrier. Type II pneumocytes play an important role of maintaining the function of the blood-gas barrier. However, the mechanism of APEC injuring type II pneumocytes remains unclear. α-cyperone can inhibit lung cell injury induced by Staphylococcus aureus. In order to explore whether α-cyperone regulates the adherence and invasion of APEC-O78 to chicken type II pneumocytes, we successfully cultured chicken type II pneumocytes. The results showed that α-cyperone significantly decreased the adherence of APEC-O78 to chicken type II pneumocytes. In addition, α-cyperone inhibited actin cytoskeleton polymerization induced by APEC-O78 through down regulating the expression of Nck-2, Cdc42 and Rac1. These results provide new evidence for the prevention of colibacillosis in chicken.

Activation of Src and release of intracellular calcium by phosphatidic acid during Xenopus laevis fertilization

We report a new step in the fertilization in Xenopus laevis which has been found to involve activation of Src tyrosine kinase to stimulate phospholipase C-γ (PLC-γ) which increases inositol 1,4,5-trisphosphate (IP3) to release intracellular calcium ([Ca](i)). Molecular species analysis and mass measurements suggested that sperm activate phospholipase D (PLD) to elevate phosphatidic acid (PA). We now report that PA mass increased 2.7 fold by 1 min after insemination and inhibition of PA production by two methods inhibited activation of Src and PLCγ, increased [Ca](i) and other fertilization events. As compared to 14 other lipids, PA specifically bound Xenopus Src but not PLCγ. Addition of synthetic PA activated egg Src (an action requiring intact lipid rafts) and PLCγ as well as doubling the amount of PLCγ in rafts. In the absence of elevated [Ca](i), PA addition elevated IP3 mass to levels equivalent to that induced by sperm (but twice that achieved by calcium ionophore). Finally, PA induced [Ca](i) release that was blocked by an IP3 receptor inhibitor. As only PLD1b message was detected, and Western blotting did not detect PLD2, we suggest that sperm activate PLD1b to elevate PA which then binds to and activates Src leading to PLCγ stimulation, IP3 elevation and [Ca](i) release. Due to these and other studies, PA may also play a role in membrane fusion events such as sperm-egg fusion, cortical granule exocytosis, the elevation of phosphatidylinositol 4,5-bisphosphate and the large, late increase in sn 1,2-diacylglycerol in fertilization.

Phospholipase D signaling mediates reactive oxygen species-induced lung endothelial barrier dysfunction

Reactive oxygen species (ROS) have emerged as critical players in the pathophysiology of pulmonary disorders and diseases. Earlier, we have demonstrated that ROS stimulate lung endothelial cell (EC) phospholipase D (PLD) that generates phosphatidic acid (PA), a second messenger involved in signal transduction. In the current study, we investigated the role of PLD signaling in the ROS-induced lung vascular EC barrier dysfunction. Our results demonstrated that hydrogen peroxide (H₂O₂), a typical physiological ROS, induced PLD activation and altered the barrier function in bovine pulmonary artery ECs (BPAECs). 1-Butanol, the quencher of PLD, generated PA leading to the formation of physiologically inactive phosphatidyl butanol but not its biologically inactive analog, 2-butanol, blocked the H₂O₂-mediated barrier dysfunction. Furthermore, cell permeable C2 ceramide, an inhibitor of PLD but not the C2 dihydroceramide, attenuated the H₂O₂-induced PLD activation and enhancement of paracellular permeability of Evans blue conjugated albumin across the BPAEC monolayers. In addition, transfection of BPAECs with adenoviral constructs of hPLD₁ and mPLD₂ mutants attenuated the H₂O₂-induced barrier dysfunction, cytoskeletal reorganization and distribution of focal adhesion proteins. For the first time, this study demonstrated that the PLD-generated intracellular bioactive lipid signal mediator, PA, played a critical role in the ROS-induced barrier dysfunction in lung vascular ECs. This study also underscores the importance of PLD signaling in vascular leak and associated tissue injury in the etiology of lung diseases among critically ill patients encountering oxygen toxicity and excess ROS production during ventilator-assisted breathing.

A new signaling pathway (JAK-Fes-phospholipase D) that is enhanced in highly proliferative breast cancer cells

The products of the oncogene Fes and JAK3 are tyrosine kinases, whose expressions are elevated in tumor growth, angiogenesis, and metastasis. Phosphatidic acid, as synthesized by phospholipase D (PLD), enhances cancer cell survival. We report a new signaling pathway that integrates the two kinases with the lipase. A new JAK3-Fes-PLD2 axis is responsible for the highly proliferative phenotype of MDA-MB-231 breast cancer cells. Conversely, this pathway is maintained at a low rate of expression and activity levels in untransformed cells such as MCF10A. We also deciphered the inter-regulation that exists between the two kinases (JAK3 and the oncogene Fes) and between these two kinases and the lipase (PLD2). Whereas JAK3 and Fes marginally activate PLD2 in non-transformed cells, these kinases greatly enhance (>200%) PLD activity following protein-protein interaction through the SH2 domain and the Tyr-415 residue of PLD2. We also found that phosphatidic acid enhances Fes activity in MDA-MB-231 cells providing a positive activation loop between Fes and PLD2. In summary, the JAK3, Fes and PLD2 interactions in transformed cells maintain PLD2 at an enhanced level that leads to abnormal cell growth. Modulating this new JAK3-Fes-PLD2 pathway could be important to control the highly invasive phenotype of breast cancer cells.

Deficiencies of the lipid-signaling enzymes phospholipase D1 and D2 alter cytoskeletal organization, macrophage phagocytosis, and cytokine-stimulated neutrophil recruitment

Cell migration and phagocytosis ensue from extracellular-initiated signaling cascades that orchestrate dynamic reorganization of the actin cytoskeleton. The reorganization is mediated by effector proteins recruited to the site of activity by locally-generated lipid second messengers. Phosphatidic acid (PA), a membrane phospholipid generated by multiple enzyme families including Phospholipase D (PLD), has been proposed to function in this role. Here, we show that macrophages prepared from mice lacking either of the classical PLD isoforms PLD1 or PLD2, or wild-type macrophages whose PLD activity has been pharmacologically inhibited, display isoform-specific actin cytoskeleton abnormalities that likely underlie decreases observed in phagocytic capacity. Unexpectedly, PA continued to be detected on the phagosome in the absence of either isoform and even when all PLD activity was eliminated. However, a disorganized phagocytic cup was observed as visualized by imaging PA, F-actin, Rac1, an organizer of the F-actin network, and DOCK2, a Rac1 activator, suggesting that PLD-mediated PA production during phagocytosis is specifically critical for the integrity of the process. The abnormal F-actin reorganization additionally impacted neutrophil migration and extravasation from the vasculature into interstitial tissues. Although both PLD1 and PLD2 were important in these processes, we also observed isoform-specific functions. PLD1-driven processes in particular were observed to be critical in transmigration of macrophages exiting the vasculature during immune responses such as those seen in acute pancreatitis or irritant-induced skin vascularization.

Mechanism of sperm capacitation and the acrosome reaction: role of protein kinases

Mammalian sperm must undergo a series of biochemical and physiological modifications, collectively called capacitation, in the female reproductive tract prior to the acrosome reaction (AR). The mechanisms of these modifications are not well characterized though protein kinases were shown to be involved in the regulation of intracellular Ca(2+) during both capacitation and the AR. In the present review, we summarize some of the signaling events that are involved in capacitation. During the capacitation process, phosphatidyl-inositol-3-kinase (PI3K) is phosphorylated/activated via a protein kinase A (PKA)-dependent cascade, and downregulated by protein kinase C α (PKCα). PKCα is active at the beginning of capacitation, resulting in PI3K inactivation. During capacitation, PKCα as well as PP1γ2 is degraded by a PKA-dependent mechanism, allowing the activation of PI3K. The activation of PKA during capacitation depends mainly on cyclic adenosine monophosphate (cAMP) produced by the bicarbonate-dependent soluble adenylyl cyclase. This activation of PKA leads to an increase in actin polymerization, an essential process for the development of hyperactivated motility, which is necessary for successful fertilization. Actin polymerization is mediated by PIP(2) in two ways: first, PIP(2) acts as a cofactor for phospholipase D (PLD) activation, and second, as a molecule that binds and inhibits actin-severing proteins such as gelsolin. Tyrosine phosphorylation of gelsolin during capacitation by Src family kinase (SFK) is also important for its inactivation. Prior to the AR, gelsolin is released from PIP(2) and undergoes dephosphorylation/activation, resulting in fast F-actin depolymerization, leading to the AR.

ADP-ribosylation factor 6 expression and activation are reduced in myometrium in complicated pregnancies

Background

ARF6 (ADP-ribosylation factor 6) small GTP binding protein plays critical roles in actin cytoskeleton rearrangements and membrane trafficking, including internalisation of G protein coupled receptors (GPCR). ARF6 operates by cycling between GDP-bound (inactive) and GTP-bound (active) forms and is a potential regulator of GPCR-mediated uterine activity during pregnancy and labour. ARF6 contains very low intrinsic GTP binding activity and depends on GEFs (guanine nucleotide exchange factors) such as CYTH3 (cytohesin 3) to bind GTP. ARF6 and CYTH3 were originally cloned from human placenta, but there is no information on their expression in other reproductive tissues.

Methods

The expression of ARF6, ARF1, and CYTH1-4 was investigated by measuring mRNA (using RT-PCR) and protein levels (using immunoblotting) in samples of myometrium obtained from non-pregnant women, and women with normal pregnancies, before or after the spontaneous onset of labour. We also analysed myometrial samples from women with spontaneous preterm labour and from women with complicated pregnancies requiring emergency preterm delivery. The GST)-effector pull down assay was used to study the presence of active ARF6 and ARF1 in all myometrial extracts.

Results

ARF6, ARF1 and CYTH3 but not CYTH1, CYTH2 and CYTH4 were expressed in all samples and the levels did not change with pregnancy or labour. However, ARF6 and CYTH3 but not ARF1 levels were significantly reduced in complicated pregnancies. The alterations in the expression of ARF6 and its GEF in human myometrium indicate a potential involvement of this signalling system in modulating the response of myometrial smooth muscle in complicated pregnancies. The levels of ARF6-GTP or ARF1-GTP did not change with pregnancy or labour but ARF6-GTP levels were significantly decreased in women with severe complications of pregnancy.

Conclusions

We have demonstrated a functional ARF6 system in human myometrium and a correlation between ARF6 level and activity in uterine and abnormal pregnancy.

Diacylglycerol kinase β in neurons: functional implications at the synapse and in disease

Phosphoinositide cycle plays a pivotal role in neuronal signal transduction. In this cycle, diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to yield phosphatidic acid (PA). DG and PA acts as important second messengers that regulate distinct cascade of cellular events. Previous studies have disclosed that DGK consists of a family of isozymes that differ in their structure, enzymatic property, gene expression, subcellular localization, and binding partner. Intriguingly, most if not all DGK isozymes are abundantly expressed in the brain, suggesting important roles of this enzyme family in brain function. Of DGKs, DGKβ was the first enzyme identified as being expressed in a neuronal population in the brain. This review focuses on recent findings of DGKβ at the molecular, cellular, and organismal levels together with pathological implications in brain function and disease.

Hyper-activated motility in sperm capacitation is mediated by phospholipase D-dependent actin polymerization

In order to fertilize the oocyte, sperm must undergo a series of biochemical changes in the female reproductive tract, known as capacitation. Once capacitated, spermatozoon can bind to the zona pellucida of the egg and undergo the acrosome reaction (AR), a process that enables its penetration and fertilization of the oocyte. Important processes that characterize sperm capacitation are actin polymerization and the development of hyper-activated motility (HAM). Previously, we showed that Phospholipase D (PLD)-dependent actin polymerization occurs during sperm capacitation, however the role of this process in sperm capacitation is not yet known. In the present study, we showed for the first time the involvement of PLD-dependent actin polymerization in sperm motility during mouse and human capacitation. Sperm incubated under capacitation conditions revealed a time dependent increase in actin polymerization and HAM. Inhibition of Phosphatidic Acid (PA) formation by PLD using butan-1-ol, inhibited actin polymerization and motility, as well as in vitro fertilization (IVF) and the ability of the sperm to undergo the AR. The inhibition of sperm HAM by low concentration of butan-1-ol is completely restored by adding PA, further indicating the involvement of PLD in these processes. Furthermore, exogenous PA enhanced rapid actin polymerization that was followed by a rise in the HAM, as well as an increased in IVF rate. In conclusion, our results demonstrate that PLD-dependent actin polymerization is a critical step needed for the development of HAM during mouse and human sperm capacitation.

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.

β-1,3-Glucan-induced host phospholipase D activation is involved in Aspergillus fumigatus internalization into type II human pneumocyte A549 cells

The internalization of Aspergillus fumigatus into lung epithelial cells is a process that depends on host cell actin dynamics. The host membrane phosphatidylcholine cleavage driven by phospholipase D (PLD) is closely related to cellular actin dynamics. However, little is known about the impact of PLD on A. fumigatus internalization into lung epithelial cells. Here, we report that once germinated, A. fumigatus conidia were able to stimulate host PLD activity and internalize more efficiently in A549 cells without altering PLD expression. The internalization of A. fumigatus in A549 cells was suppressed by the downregulation of host cell PLD using chemical inhibitors or siRNA interference. The heat-killed swollen conidia, but not the resting conidia, were able to activate host PLD. Further, β-1,3-glucan, the core component of the conidial cell wall, stimulated host PLD activity. This PLD activation and conidia internalization were inhibited by anti-dectin-1 antibody. Indeed, dectin-1, a β-1,3-glucan receptor, was expressed in A549 cells, and its expression profile was not altered by conidial stimulation. Finally, host cell PLD1 and PLD2 accompanied A. fumigatus conidia during internalization. Our data indicate that host cell PLD activity induced by β-1,3-glucan on the surface of germinated conidia is important for the efficient internalization of A. fumigatus into A549 lung epithelial cells.

InlB-mediated Listeria monocytogenes internalization requires a balanced phospholipase D activity maintained through phospho-cofilin

Internalization of Listeria monocytogenes into non-phagocytic cells is tightly controlled by host cell actin dynamics and cell membrane alterations. However, knowledge about the impact of phosphatidylcholine cleavage driven by host cell phospholipase D (PLD) on Listeria internalization into epithelial cells is limited. Here, we report that L. monocytogenes activates PLD in Vero cells during the internalization. With immunostaining it was shown that both PLD1 and PLD2 surrounded partially or completely the phagocytic cup of most L. monocytogenes. Either up- or down-regulation of PLD expression (activity) diminished Listeria internalization. Both PLD1 and PLD2 in Vero cells were required for efficient Listeria internalization, and could substitute for each other in the regulation of Listeria internalization. Further, exogenous InlB activated host cell PLD1 and PLD2 via the Met receptor, and restored host PLD activation by InlB-deficient L. monocytogenes. InlB-induced PLD activation and Listeria internalization were tightly controlled by phospho-cycling of cofilin. PLD1, but not PLD2, was involved in cofilin-mediated PLD activation and Listeria internalization. These data indicate that cofilin-dependent PLD activation induced by InlB may represent a novel regulation mechanism for efficient Listeria internalization into epithelial cells.

The dual effect of Rac2 on phospholipase D2 regulation that explains both the onset and termination of chemotaxis

We document a biphasic effect of Rac2 on the activation and inhibition of PLD2. Cells overexpressing Rac2 and PLD2 simultaneously show a robust initial (<10 min) response toward a chemoattractant that is later (>30 min) greatly diminished over PLD2-only controls. The first phase is due to the presence of a Rac2-PLD2 positive-feedback loop. To explain the mechanism for the Rac2-led PLD2 inhibition (the second phase), we used leukocytes from wild-type (WT) and Rac2(-/-) knockout mice. Rac2(-/-) cells displayed an enhanced PLD2 (but not PLD1) enzymatic activity, confirming the inhibitory role of Rac2. Late inhibitory responses on PLD2 due to Rac2 were reversed in the presence of phosphatidylinositol 4,5-bisphosphate (PIP(2)) both in vitro (purified GST-PH-PLD2, where GST is glutathione S-transferase and PH is pleckstrin homology) and in vivo. Coimmunoprecipitation and immunofluorescence microscopy indicated that PLD2 and Rac2 remain together. The presence of an "arc" of Rac2 at the leading edge of leukocyte pseudopodia and PLD2 physically posterior to this wave of Rac2 was observed in late chemotaxis. We propose Rac-led inhibition of PLD2 function is due to sterical interference of Rac with PLD2's PH binding site to the membrane and deprivation of the PIP(2). This work supports the importance of functional interactions between PLD and Rac in the biological response of cell migration.

Phospholipase D2 induces stress fiber formation through mediating nucleotide exchange for RhoA

Phospholipase D (PLD) is involved in diverse cellular processes including cell movement, adhesion, and vesicle trafficking through cytoskeletal rearrangements. However, the mechanism by which PLD induces cytoskeletal reorganization is still not fully understood. Here, we describe a new link to cytoskeletal changes that is mediated by PLD2 through direct nucleotide exchange on RhoA. We found that PLD2 induces RhoA activation independent of its lipase activity. PLD2 directly interacted with RhoA, and the PX domain of PLD2 specifically recognized nucleotide-free RhoA. Finally, we found that the PX domain of PLD2 has guanine nucleotide-exchange factor (GEF) activity for RhoA in vitro. In addition, we verified that overexpression of the PLD2-PX domain induces RhoA activation, thereby provoking stress fiber formation. Together, our findings suggest that PLD2 functions as an upstream regulator of RhoA, which enables us to understand how PLD2 regulates cytoskeletal reorganization in a lipase activity-independent manner.

An essential role for phospholipase D in the recruitment of vesicle amine transport protein-1 to membranes in human neutrophils

Although phosphatidic acid (PA) regulates a wide variety of physiological processes, its targets remain poorly characterized in human neutrophils. By co-sedimentation with PA-containing vesicles we identified several PA-binding proteins including vesicle amine transport protein-1 (VAT-1), Annexin A3 (ANXA3), Rac2, Cdc42 and RhoG in neutrophil cytosol. Except for ANXA3, protein binding to PA-containing liposomes was calcium-independent. Cdc42 and RhoG preferentially interacted with PA whereas VAT-1 bound to PA or phosphatidylserine with the same affinity. VAT-1 translocated to neutrophil membranes upon N-formyl-methionyl-leucyl-phenylalanine (fMLF) stimulation. Inhibition of fMLF-induced PLD activity with the Src kinase inhibitor PP2, the selective inhibitor of PLD FIPI, or of PA formation with primary alcohols reduced VAT-1 translocation. In contrast, inhibition of PA hydrolysis with propranolol enhanced fMLF-mediated VAT-1 recruitment to membranes. PMA also redistributed VAT-1 to membranes in a PKC- and PLD-dependent manner. Though fMLF and PMA increased VAT-1 phosphorylation, different kinases appear to be involved. Cell fractionation revealed that a pool of VAT-1 was co-localized with primary, secondary and tertiary granules and plasma membrane markers in resting neutrophils. Stimulation with fMLF enhanced VAT-1 co-localization with CD32a, a plasma membrane marker. Confocal microscopy revealed that VAT-1 decorates granular structures at the cell periphery and double labeling with VAT-1/lactoferrin antibodies showed a partial co-localization with secondary granules in control and fMLF-stimulated cells. Characterization of these putative PA-binding proteins constitutes another step forward for a better understanding of the role of PLD-derived PA in neutrophil physiology.

Janus-faced enzymes yeast Tgl3p and Tgl5p catalyze lipase and acyltransferase reactions

The yeast triacylglycerol (TAG) lipases Tgl3p and Tgl5p not only contain a TAG lipase, but also an acyltransferase motif. We show that purified Tgl3p and Tgl5p indeed exhibit lysophospholipid acyltransferase activity. Both Tgl3p and Tgl5p affect the level of glycerophospholipids, and Tgl3p is also important for efficient sporulation of yeast.

In the yeast, mobilization of triacylglycerols (TAGs) is facilitated by the three TAG lipases Tgl3p, Tgl4p, and Tgl5p. Motif search analysis, however, indicated that Tgl3p and Tgl5p do not only contain the TAG lipase motif GXSXG but also an H-(X)₄-D acyltransferase motif. Interestingly, lipid analysis revealed that deletion of TGL3 resulted in a decrease and overexpression of TGL3 in an increase of glycerophospholipids. Similar results were obtained with TGL5. Therefore, we tested purified Tgl3p and Tgl5p for acyltransferase activity. Indeed, both enzymes not only exhibited lipase activity but also catalyzed acylation of lysophosphatidylethanolamine and lysophosphatidic acid, respectively. Experiments using variants of Tgl3p created by site-directed mutagenesis clearly demonstrated that the two enzymatic activities act independently of each other. We also showed that Tgl3p is important for efficient sporulation of yeast cells, but rather through its acyltransferase than lipase activity. In summary, our results demonstrate that yeast Tgl3p and Tgl5p play a dual role in lipid metabolism contributing to both anabolic and catabolic processes.

Phospholipase D2 regulates endothelial permeability through cytoskeleton reorganization and occludin downregulation

Endothelial permeability is controlled by adhesive strengths which connect cells to each other through interendothelial junctions and by contractile forces associated with cytoskeleton reorganization. Phospholipase D (PLD) activation resulting in the generation of phosphatidic acid (PA) is increasingly recognized as a key event in the initiation of various cell responses. In human umbilical vein endothelial cells (HUV-EC), enhancement of intracellular PA by a variety of approaches increased the permeability of endothelial cell monolayers and induced stress fibre formation. Using adenovirus-mediated overexpression and siRNA silencing, we showed that PLD2 but not PLD1 was involved in the enhancement of basal permeability through cytoskeleton reorganization. Furthermore, PLD2 overexpression induced ERK1/2 activation and downregulated the expression of occludin, a major component of tight junctions. A substantial part of PLD2 protein was associated with the low-density caveolin-rich fractions isolated on sucrose gradients. The Raf-1 specific inhibitor GW-5074 drastically reduced hyperpermeability induced by PLD2 overexpression, and inhibited PA-mediated increase of endothelial permeability and ERK1/2 activation. On the whole, the present results demonstrate the selective role of PLD2 isoform in the control of endothelial permeability through a mechanism involving both stress fibre formation and contraction, and occludin downregulation, possibly resulting from PA-mediated activation of Raf-1.

5-Fluoro-2-indolyl des-chlorohalopemide (FIPI), a phospholipase D pharmacological inhibitor that alters cell spreading and inhibits chemotaxis

The signaling enzyme phospholipase D (PLD) and the lipid second messenger it generates, phosphatidic acid (PA), are implicated in many cell biological processes, including Ras activation, cell spreading, stress fiber formation, chemotaxis, and membrane vesicle trafficking. PLD production of PA is inhibited by the primary alcohol 1-butanol, which has thus been widely employed to identify PLD/PA-driven processes. However, 1-butanol does not always effectively reduce PA accumulation, and its use may result in PLD-independent deleterious effects. Consequently, identification of potent specific small-molecule PLD inhibitors would be an important advance for the field. We examine one such here, 5-fluoro-2-indolyl des-chlorohalopemide (FIPI), which was identified recently in an in vitro chemical screen for PLD2 inhibitors, and show that it rapidly blocks in vivo PA production with subnanomolar potency. We were surprised to find that several biological processes blocked by 1-butanol are not affected by FIPI, suggesting the need for re-evaluation of proposed roles for PLD. However, FIPI does inhibit PLD regulation of F-actin cytoskeleton reorganization, cell spreading, and chemotaxis, indicating potential utility for it as a therapeutic for autoimmunity and cancer metastasis.

Identification of caspase 3 motifs and critical aspartate residues in human phospholipase D1b and phospholipase D2a

Stimulation of mammalian cells frequently initiates phospholipase D-catalyzed hydrolysis of phosphatidylcholine in the plasma membrane to yield phosphatidic acid (PA) a novel lipid messenger. PA plays a regulatory role in important cellular processes such as secretion, cellular shape change, and movement. A number of studies have highlighted that PLD-based signaling also plays a pro-mitogenic and pro-survival role in cells and therefore anti-apoptotic. We show that human PLD1b and PLD2a contain functional caspase 3 cleavage sites and identify the critical aspartate residues within PLD1b that affect its activation by phorbol esters and attenuate phosphatidylcholine hydrolysis during apoptosis.

Protein kinase C-epsilon regulates sphingosine 1-phosphate-mediated migration of human lung endothelial cells through activation of phospholipase D2, protein kinase C-zeta, and Rac1

The signaling pathways by which sphingosine 1-phosphate (S1P) potently stimulates endothelial cell migration and angiogenesis are not yet fully defined. We, therefore, investigated the role of protein kinase C (PKC) isoforms, phospholipase D (PLD), and Rac in S1P-induced migration of human pulmonary artery endothelial cells (HPAECs). S1P-induced migration was sensitive to S1P(1) small interfering RNA (siRNA) and pertussis toxin, demonstrating coupling of S1P(1) to G(i). Overexpression of dominant negative (dn) PKC-epsilon or -zeta, but not PKC-alpha or -delta, blocked S1P-induced migration. Although S1P activated both PLD1 and PLD2, S1P-induced migration was attenuated by knocking down PLD2 or expressing dnPLD2 but not PLD1. Blocking PKC-epsilon, but not PKC-zeta, activity attenuated S1P-mediated PLD stimulation, demonstrating that PKC-epsilon, but not PKC-zeta, was upstream of PLD. Transfection of HPAECs with dnRac1 or Rac1 siRNA attenuated S1P-induced migration. Furthermore, transfection with PLD2 siRNA, infection of HPAECs with dnPKC-zeta, or treatment with myristoylated PKC-zeta peptide inhibitor abrogated S1P-induced Rac1 activation. These results establish that S1P signals through S1P(1) and G(i) to activate PKC-epsilon and, subsequently, a PLD2-PKC-zeta-Rac1 cascade. Activation of this pathway is necessary to stimulate the migration of lung endothelial cells, a key component of the angiogenic process.

Novel structural and functional findings of the ehFLN protein from Entamoeba histolytica

The ehFLN protein (previously known as EhABP-120) is the first filamin to be identified in the parasitic protozoan Entamoeba histolytica. Filamins are a family of cross-linking actin-binding proteins that organize filamentous actin in networks and stress fibers. It has been reported that filamins of different organisms directly interact with more than 30 cellular proteins and some PPIs. The biochemical consequences of such interactions may have either positive or negative effects on the cross-linking function. Besides, filamins form a link between cytoskeleton and plasma membrane. In this work, the ehFLN protein was biochemically characterized; amoebae filamin was found to associate with both PA and PI(3)P in vitro, new lipid targets for a member of the filamins. By molecular modeling analysis and protein-lipid overlay assays, K-609, 709, and 710 were determined to be essential for the PA-ehFLN1 complex stability. Also, the integrity of the 4th repeat of ehFLN is essential to keep interaction with the PI(3)P. Transfected trophozoites that overexpressed the d100, d50NH(2), and d50COOH regions of ehFLN1 displayed both increased motility and chemotactic response to TYI-S-33 media. Together, these results suggest that short regions of ehFLN are involved in signaling events that, in cooperation with phosphatidic acid, EhPLD2 and EhPI3K, could promote cell motility.

Screening of genes involved in cell migration in Dictyostelium

A single cell of wild-type Dictyostelium discoideum forms a visible colony on a plastic dish in several days, but due to enhanced cell migration, amiB-null mutant cells scatter over a large area and do not form noticeable colonies. Here, with an aim to identify genes involved in cell migration, we isolated suppresser mutants of amiB-null mutants that restore the ability to form colonies. From REMI (restriction enzyme-mediated integration)-mutagenized pool of double-mutants, we identified 18 responsible genes from them. These genes can be categorized into several biological processes. One cell line, Sab16 (Suppressor of amiB) was chosen for further analysis, which had a disrupted phospholipase D pldB gene. To confirm the role of pldB gene in cell migration, we knocked out the pldB gene and over-expressed gfp-pldB in wild-type cells. GFP-PLDB localized to plasma membrane and on vesicles, and in migrating cells, at the protruding regions of pseudopodia. Migration speed of vegetative pldB-null cells was reduced to 73% of that of the wild-type. These results suggest that PLDB plays an important role in migration in Dictyostelium cells, and that our screening system is useful for the identification of genes involved in cell migration.

Requirement of phospholipase D for ilimaquinone-induced Golgi membrane fragmentation

Although organelles such as the endoplasmic reticulum and Golgi apparatus are highly compartmentalized, these organelles are interconnected through a network of vesicular trafficking. The marine sponge metabolite ilimaquinone (IQ) is known to induce Golgi membrane fragmentation and is widely used to study the mechanism of vesicular trafficking. Although IQ treatment causes protein kinase D (PKD) activation, the detailed mechanism of IQ-induced Golgi membrane fragmentation remains unclear. In this work, we found that IQ treatment of cells caused a robust activation of phospholipase D (PLD). In the presence of 1-butanol but not 2-butanol, IQ-induced Golgi membrane fragmentation was completely blocked. In addition, IQ failed to induce Golgi membrane fragmentation in PLD knock-out DT40 cells. Furthermore, IQ-induced PKD activation was completely blocked by treatment with either 1-butanol or propranolol. Notably, IQ-induced Golgi membrane fragmentation was also blocked by propranolol treatment. These results indicate that PLD-catalyzed formation of phosphatidic acid is a prerequisite for IQ-induced Golgi membrane fragmentation and that enzymatic conversion of phosphatidic acid to diacylglycerol is necessary for subsequent activation of PKD and IQ-induced Golgi membrane fragmentation.

Direct stimulation of receptor-controlled phospholipase D1 by phospho-cofilin

The activity state of cofilin, which controls actin dynamics, is driven by a phosphorylation-dephosphorylation cycle. Phosphorylation of cofilin by LIM-kinases results in its inactivation, a process supported by 14-3-3zeta and reversed by dephosphorylation by slingshot phosphatases. Here we report on a novel cellular function for the phosphorylation-dephosphorylation cycle of cofilin. We demonstrate that muscarinic receptor-mediated stimulation of phospholipase D1 (PLD1) is controlled by LIM-kinase, slingshot phosphatase as well as 14-3-3zeta, and requires phosphorylatable cofilin. Cofilin directly and specifically interacts with PLD1 and upon phosphorylation by LIM-kinase1, stimulates PLD1 activity, an effect mimicked by phosphorylation-mimic cofilin mutants. The interaction of cofilin with PLD1 is under receptor control and encompasses a PLD1-specific fragment (aa 585-712). Expression of this fragment suppresses receptor-induced cofilin-PLD1 interaction as well as PLD stimulation and actin stress fiber formation. These data indicate that till now designated inactive phospho-cofilin exhibits an active cellular function, and suggest that phospho-cofilin by its stimulatory effect on PLD1 may control a large variety of cellular functions.

alpha(1)-Adrenergic receptor stimulation of cell motility requires phospholipase D-mediated extracellular signal-regulated kinase activation

Phospholipase D is suspected to play a role in tumorigenesis, and the inhibition of phospholipase D has been associated with changes in several cellular events including invasion and migration. We report here that the specific alpha(1)-adrenergic receptor agonist, phenylepherine, signals to a growth factor pathway in a manner that requires phospholipase D activity in CCL39 fibroblasts. Phenylepherine increased extracellular signal-regulated kinase phosphorylation eightfold and promoted stress fiber formation threefold. Stress fiber formation was blocked when extracellular signal-regulated kinase activation was inhibited. Stimulation of CCL39 fibroblasts by phenylepherine increased the rate of wound healing fourfold in a wounding assay, while treatment with the MEK inhibitor, PD98059 reduced the closure of phenylepherine-induced wound healing to control levels. Addition of 1-butanol but not 2-butanol inhibited extracellular signal-regulated kinase activation by phenylepherine, presumably by blocking the formation of phosphatidic acid. Exogenously added cell permeable phosphatidic acid increased extracellular signal-regulated kinase activation in a time- and dose-dependent manner as well as stimulated the formation of stress fibers. 1-butanol also significantly inhibited the ability of phenylepherine to stimulate stress fiber formation and wound healing. Taken together, these results indicate a novel role for phospholipase D in the activation of the extracellular signal-regulated kinase growth factor pathway to stimulate early cellular events induced by phenylepherine.

Stable adhesion and migration of human neutrophils requires phospholipase D-mediated activation of the integrin CD11b/CD18

The pathways regulating integrin-mediated adhesion during neutrophil migration are incompletely defined. Using a flow-based model in which human neutrophils rolling on P-selectin were activated to migrate by the chemoattractant peptide fMLP, we investigated the role of phospholipase D (PLD). fMLP-stimulated PLD generation of phosphatidate (PtdOH); while inhibition of PtdOH production with butan-1-ol had no effect on the initial immobilisation of rolling neutrophils (supported by activation of constitutively surface-expressed beta(2)-integrin CD11b/CD18) it impaired longer-term stability of adhesion and reduced the rate of migration (supported by activation of de novo-exocytosed CD11b/CD18). PtdOH regulated these processes by controlling activation of exocytosed CD11b/CD18, and appeared to act by directly stimulating phosphatidylinositol 4-phosphate 5-kinase type I to generate phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)). Cell-permeable PtdIns(4,5)P(2) recovered migration of neutrophils after PLD inhibition; PtdIns(4,5)P(2) appeared to act by promoting talin binding to CD18 and hence activating CD11b/CD18, as migration was inhibited when neutrophils were loaded with peptides previously shown to block the interaction between PtdIns(4,5)P(2) and talin or talin and CD18. Thus, these data indicate that PLD-synthesised PtdOH stimulates the generation of PtdIns(4,5)P(2), which in turn mediates talin binding to, and activation of, CD11b/CD18 required for neutrophil stable adhesion and migration.

Phosphatidic acid regulates the affinity of the murine phosphatidylinositol 4-phosphate 5-kinase-Ibeta for phosphatidylinositol-4-phosphate

Type I phosphatidylinositol 4-phosphate 5-kinase (PI4P5K) catalyzes the phosphorylation of phosphatidylinositol 4 phosphate [PI(4)P] at carbon 5, producing phosphatidylinositol 4,5 bisphosphate [PI(4,5)P2]. Phosphatidic acid (PA) activates PI4P5K in vitro and plays a central role in the activation of PIP5K pathways in vivo. This report demonstrates that actin fiber formation in murine fibroblasts involves PA activation of PIP5Ks and defines biochemical interactions between PA and the PIP5Ks. Inhibition of phospholipase D production of PA results in the loss of actin fibers. Overexpression of the beta isoform of the type I murine phosphatidylinositol 4-phosphate 5-kinase (mPIP5K-Ibeta) maintains actin fiber structure in the face of phospholipase D inhibition. PA activates mPIP5K-Ibeta by direct binding to mPIP5K-Ibeta through both electrostatic and hydrophobic interactions, with the fatty acid acyl chain length and degree of saturation acting as critical determinants of binding and activation. Furthermore, kinetic analysis suggests that phosphorylation of the PI(4)P substrate does not follow classical Michaelis-Menten kinetics. Instead, the kinetic data are consistent with a model in which mPIP5K-Ibeta initially binds to the lipid micelle and subsequently binds the PI(4)P substrate. In addition, the kinetics indicate substrate inhibition, suggesting that mPIP5K-Ibeta contains an inhibitory PI(4)P-binding site. These results suggest a model in which mPIP5K-Ibeta is surrounded by PI(4)P, but is unable to catalyze its conversion to PI(4,5)P2 unless PA is bound.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

Phospholipase D1 as a key enzyme for decidualization in human endometrial stromal cells

Using primary cell cultures of human endometrial stromal cells (ES cells), we investigated the role of phospholipase D (PLD) in 8-Br-cAMP-induced decidualization, which involves morphological and biological differentiation processes. When treated with 0.5 mM 8-Br-cAMP for 12 days, ES cells were transformed into a decidualized morphology and produced significant amounts of prolactin (PRL) and insulin-like growth factor-binding protein 1 (IGFBP1). Simultaneously, the activity and expression levels of PLD1 increased. In addition, removal of 8-Br-cAMP from decidualized ES cells restored the undifferentiated state, and this was accompanied by decreases in PLD1 promoter activity and PLD1 expression. Overexpression of dominant negative (DN)-PLD1 inhibited the morphological changes induced by 0.5 mM 8-Br-cAMP, whereas PLD1 overexpression induced morphological changes in the absence of 0.5 mM 8-Br-cAMP treatment. Moreover, knockdown of PLD1 by siRNA and blockage of PLD by treatment with 0.3% 1-butanol decreased PRL/IGFBP1 mRNA expression, whereas PLD1 overexpression increased PRL/IGFBP1 mRNA expression. Treatment of ES cells with phosphatidic acid (PA) for 3 days induced PRL mRNA expression and morphological changes, which implies that PA is an end-product of PLD activation-induced decidualization. In addition, pretreatment of ES cells with mepacrine decreased PRL/IGFBP1 expression and inhibited morphological change, whereas pretreatment with propranolol caused no changes, as compared to cAMP-treated cells, which suggests that PA induces decidualization through phospholipase A2 (PLA2G1B). Taken together, these results suggest that PLD1 regulates 8-Br-cAMP-induced decidualization through PLA2G1B, and that PLD1 upregulation is essential for the decidualization of ES cells.

Lipid phosphate phosphatase-1 regulates lysophosphatidate-induced fibroblast migration by controlling phospholipase D2-dependent phosphatidate generation

Lysophosphatidate (LPA) stimulates cell migration and division through a family of G-protein-coupled receptors. Lipid phosphate phosphatase-1 (LPP1) regulates the degradation of extracellular LPA as well as the intracellular accumulation of lipid phosphates. Here we show that increasing the catalytic activity of LPP1 decreased the pertussis toxin-sensitive stimulation of fibroblast migration by LPA and an LPA-receptor agonist that could not be dephosphorylated. Conversely, knockdown of endogenous LPP1 activity increased LPA-induced migration. However, LPP1 did not affect PDGF- or endothelin-induced migration of fibroblasts in Transwell chamber and "wound healing" assays. Thus, in addition to degrading exogenous LPA, LPP1 controls signaling downstream of LPA receptors. Consistent with this conclusion, LPP1 expression decreased phospholipase D (PLD) stimulation by LPA and PDGF, and phosphatidate accumulation. This LPP1 effect was upstream of PLD activation in addition to the possible metabolism of phosphatidate to diacylglycerol. PLD(2) activation was necessary for LPA-, but not PDGF-induced migration. Increased LPP1 expression also decreased the LPA-, but not the PDGF-induced activation of important proteins involved in fibroblast migration. These included decreased LPA-induced activation of ERK and Rho, and the basal activities of Rac and Cdc42. However, ERK and Rho activation were not downstream targets of LPA-induced PLD(2) activity. We conclude that the intracellular actions of LPP1 play important functions in regulating LPA-induced fibroblast migration through PLD2. LPP1 also controls PDGF-induced phosphatidate formation. These results shed new light on the roles of LPP1 in controlling wound healing and the growth and metastasis of tumors.

Phospholipase D couples survival and migration signals in stress response of human cancer cells

MDA-MB-231 human breast cancer cells belong to a highly invasive metastatic cell line that depends on phospholipase D (PLD) activity for survival when deprived of serum growth factors. In response to the stress of serum withdrawal, there is a rapid and dramatic increase in PLD activity. Concomitant with increased PLD activity, there was an increase in the ability of MDA-MB-231 cells to both migrate and invade Matrigel. The ability of MDA-MB-231 cells to both migrate and invade Matrigel was dependent on both PLD and mTOR, a downstream target of PLD signals. Serum withdrawal also led to a PLD-dependent increase in the expression of the stress factor, hypoxia-inducible factor-1alpha. These data reveal that PLD survival signals not only prevent apoptosis but also stimulate cell migration and invasion, linking the ability to suppress apoptosis with the ability to metastasize.

Epithelial cell motility is triggered by activation of the EGF receptor through phosphatidic acid signaling

Phospholipase D catalyzes the hydrolysis of phosphatidylcholine to generate phosphatidic acid, and there is currently much interest in elucidating messenger functions for this molecule. We report here that wounding sheets of corneal epithelial and Madin Darby canine kidney cells induces strong activation of phospholipase D, and we provide evidence that activation is amplified through a positive feed-back loop. Short-chain analogues of phosphatidic acid induce motility robustly in corneal and other epithelial cell types. The effects of these analogues were not the result of their conversion to the corresponding diacylglycerol or lysophosphatidic acid, implying that phosphatidic acid acts directly on one or more cellular targets. Strikingly, phosphatidic acid signaling was found to stimulate the epidermal growth factor receptor (EGFR) through a transactivation process. Healing of wounds in sheets of corneal epithelial cells is absolutely dependent on epidermal growth factor receptor signaling, and the present data suggest that its activation is a result of wound-induced phospholipase D activation.

Phospholipase D1 Regulates Phagocyte Adhesion

Adhesion is a fundamental cellular response that is essential to the physiologic processes of development, differentiation, proliferation, and motility, as well as to the pathology of inflammation, transformation, and metastasis. Adhesion of phagocytic leukocytes is a critical modulator of antimicrobial and cytotoxic functions, including the respiratory burst, secretion, and apoptosis. Because phospholipase D (PLD) is linked to several signaling pathways implicated in these processes, we tested the hypothesis that PLD regulates phagocyte adhesion. Adhesion of primary human neutrophils and monocyte-derived macrophages to fibronectin was accompanied by marked stimulation of PLD activity. Similarly, adhesion of both human (PLB, THP-1) and murine (RAW) myeloid-macrophage cell lines to fibronectin, fibrinogen, collagen, or plastic resulted in significant activation of PLD. Stimulation of PLD activity was rapid and persisted for at least 90 min. Confocal microscopy indicated that PLD1 exhibited partial colocalization with actin filaments at the adherent interface, in proximity to the focal adhesion protein, paxillin. Reductions in PLD activity by chemical inhibitors or specific short-interfering RNA-induced knockdown of PLD1 resulted in significant inhibition of phagocyte adhesion and was accompanied by reductions in total cellular F-actin. These data support the hypotheses that adhesion stimulates PLD activity, and that PLD1 regulates the initial stages of phagocyte adhesion. Stimulation of PLD activity may promote adhesion-dependent phagocyte effector responses.

Phospholipase D activity is essential for actin localization and actin-based motility in Dictyostelium

PLD (phospholipase D) activity catalyses the generation of the lipid messenger phosphatidic acid, which has been implicated in a number of cellular processes, particularly the regulation of membrane traffic. In the present study, we report that disruption of PLD signalling causes unexpectedly profound effects on the actin-based motility of Dictyostelium. Cells in which PLD activity is inhibited by butan-1-ol show a complete loss of actin-based structures, accompanied by relocalization of F-actin into small clusters, and eventually the nucleus, without a visible fall in levels of F-actin. Addition of exogenous phosphatidic acid reverses the effects of butan-1-ol, confirming that these effects are caused by inhibition of PLD. Loss of motility correlates with complete inhibition of endocytosis and a reduction in phagocytosis. Inhibition of PLD caused a major decrease in the synthesis of PtdIns(4,5)P2, which could again be reversed by exogenously applied phosphatidic acid. Thus the essential role of PLD signalling in both motility and endocytosis appears to be mediated directly via regulation of PtdIns(4)P kinase activity. This implies that localized PLD-regulated synthesis of PtdIns(4,5)P2 is essential for Dictyostelium actin function.

Phosphatidylinositol (3,4,5)-trisphosphate specifically interacts with the phox homology domain of phospholipase D1 and stimulates its activity

Phospholipase D (PLD), which catalyzes the hydrolysis of phosphatidylcholine to phosphatidic acid and choline, plays key roles in cellular signal transduction by mediating extracellular stimuli including hormones, growth factors, neurotransmitters, cytokines and extracellular matrix molecules. The molecular mechanisms by which domains regulate the activity of PLD - especially the phox homology (PX) domain - have not been fully elucidated. In this study, we have examined the properties of the PX domains of PLD1 and PLD2 in terms of phosphoinositide binding and PLD activity regulation. Interestingly, the PX domain of PLD1, but not that of PLD2, was found to specifically interact with phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3). We found that mutation of the conserved arginine at position 179 of the PLD1 PX domain to lysine or to alanine (R179A or R179K, respectively) disrupts PtdIns(3,4,5)P3 binding. In NIH-3T3 cells, the EGFP-PLD1 PX wild-type domain, but not the two mutants, localized to the plasma membrane after 5-minute treatment with platelet-derived growth factor (PDGF). The enzymatic activity of PLD1 was stimulated by adding PtdIns(3,4,5)P3 in vitro. Treatment with PDGF resulted in the significant increase of PLD1 activity and phosphorylation of the downstream extracellular signal-regulated kinases (ERKs), which was blocked by pre-treatment of HEK 293 cells with phosphoinositide 3-kinase (PI3K) inhibitor after the endogenous PLD2 had been depleted by siRNA specific for PLD2. Nevertheless, both PLD1 mutants (which cannot interact with PtdIns(3,4,5)P3) did not respond to treatment with PDGF. Moreover, PLD1 was activated in HepG2 cells stably expressing the Y40/51 mutant of PDGF receptor that is required for the binding with PI3K. Our results suggest that the PLD1 PX domain enables PLD1 to mediate signal transduction via ERK1/2 by providing a direct binding site for PtdIns(3,4,5)P3 and by activating PLD1.

Induction of autotaxin by the Epstein-Barr virus promotes the growth and survival of Hodgkin lymphoma cells

A proportion of patients with Hodgkin lymphoma carry Epstein-Barr virus (EBV), an oncogenic herpesvirus, in their tumor cells. Although it is generally assumed that EBV contributes to the malignant phenotype of Hodgkin lymphoma cells, direct evidence in support of this is lacking. Here we show that EBV infection of Hodgkin lymphoma cells results in the induction of autotaxin, a secreted tumor-associated factor with lysophospholipase-D activity. Up-regulation of autotaxin increased the generation of lysophosphatidic acid (LPA) and led to the enhanced growth and survival of Hodgkin lymphoma cells, whereas specific down-regulation of autotaxin decreased LPA levels and reduced cell growth and viability. In lymphoma tissues, autotaxin expression was mainly restricted to CD30+ anaplastic large-cell lymphomas and Hodgkin lymphoma; in the latter, high levels of autotaxin were strongly associated with EBV positivity (P = .006). Our results identify the induction of autotaxin and the subsequent generation of LPA as key molecular events that mediate the EBV-induced growth and survival of Hodgkin lymphoma cells and suggest that this pathway may provide opportunities for novel therapeutic intervention. (Blood. 2005;106:2138-2146)