Phagocyte cell migration is mediated by phospholipases PLD1 and PLD2

Phospholipase D2 controls bone homeostasis by modulating M-CSF-dependent osteoclastic cell migration and microtubule stability

Phospholipase D2 (PLD2), a signaling protein, plays a central role in cellular communication and various biological processes. Here, we show that PLD2 contributes to bone homeostasis by regulating bone resorption through osteoclastic cell migration and microtubule-dependent cytoskeletal organization. Pld2-deficient mice exhibited a low bone mass attributed to increased osteoclast function without altered osteoblast activity. While Pld2 deficiency did not affect osteoclast differentiation, its absence promoted the migration of osteoclast lineage cells through a mechanism involving M-CSF-induced activation of the PI3K–Akt–GSK3β signaling pathway. The absence of Pld2 also boosted osteoclast spreading and actin ring formation, resulting in elevated bone resorption. Furthermore, Pld2 deletion increased microtubule acetylation and stability, which were later restored by treatment with a specific inhibitor of Akt, an essential molecule for microtubule stabilization and osteoclast bone resorption activity. Interestingly, PLD2 interacted with the M-CSF receptor (c-Fms) and PI3K, and the association between PLD2 and c-Fms was reduced in response to M-CSF. Altogether, our findings indicate that PLD2 regulates bone homeostasis by modulating osteoclastic cell migration and microtubule stability via the M-CSF-dependent PI3K–Akt–GSK3β axis.

A signaling protein that regulates bone resorption may prove a useful target in treating skeletal conditions such as osteoporosis and rheumatoid arthritis. Bone is synthesized by cells called osteoblasts, while osteoclasts trigger bone resorption, keeping the skeleton healthy. Imbalances in this recycling process are common in bone disorders. Je-Young Choi and Hyun-Ju Kim at Kyungpook National University in Daegu, South Korea, and co-workers demonstrated that phospholipase D2 (PLD2), a membrane protein, directly regulates bone resorption in mice. Mice without the Pld2 gene had increased osteoclast activity, resulting in low bone mass. The absence of PLD2 promotes the migration of osteoclasts via a particular signaling pathway. This increased the organization of microtubules, polymers that help form the cytoskeleton. The results suggest that regulating PLD2 activity could form the basis of a future treatment method.

Single-Cell Transcriptome Analysis Reveals the Role of Pancreatic Secretome in COVID-19 Associated Multi-organ Dysfunctions

The SARS-CoV-2 infection affects the lungs, heart, kidney, intestine, olfactory epithelia, liver, and pancreas and brings forward multi-organ dysfunctions (MODs). However, mechanistic details of SARS-CoV-2-induced MODs are unclear. Here, we have investigated the role of pancreatic secretory proteins to mechanistically link COVID-19 with MODs using single-cell transcriptome analysis. Secretory proteins were identified using the Human Protein Atlas. Gene ontology, pathway, and disease enrichment analyses were used to highlight the role of upregulated pancreatic secretory proteins (secretome). We show that SARS-CoV-2 infection shifts the expression profile of pancreatic endocrine cells to acinar and ductal cell-specific profiles, resulting in increased expression of acinar and ductal cell-specific genes. Among all the secretory proteins, the upregulated expression of IL1B, AGT, ALB, SPP1, CRP, SERPINA1, C3, TFRC, TNFSF10, and MIF was mainly associated with disease of diverse organs. Extensive literature and experimental evidence are used to validate the association of the upregulated pancreatic secretome with the coagulation cascade, complement activation, renin-angiotensinogen system dysregulation, endothelial cell injury and thrombosis, immune system dysregulation, and fibrosis. Our finding suggests the influence of an upregulated secretome on multi-organ systems such as nervous, cardiovascular, immune, digestive, and urogenital systems. Our study provides evidence that an upregulated pancreatic secretome is a possible cause of SARS-CoV-2-induced MODs. This finding may have a significant impact on the clinical setting regarding the prevention of SARS-CoV-2-induced MODs.

Olfactomedin-4 improves cutaneous wound healing by promoting skin cell proliferation and migration through POU5F1/OCT4 and ESR1 signalling cascades

Olfactomedin-4 (OLFM4) is an olfactomedin-domain-containing glycoprotein, which regulates cell adhesion, proliferation, gastrointestinal inflammation, innate immunity and cancer metastasis. In the present study we investigated its role in skin regeneration. We found that OLFM4 expression is transiently upregulated in the proliferative phase of cutaneous wound healing in humans as well as in mice. Moreover, a significant increase in OLFM4 expression was detected in the skin of lesional psoriasis, a chronic inflammatory disease characterized by keratinocyte hyperproliferation. In vitro experiments demonstrated that OLFM4 selectively stimulated keratinocyte proliferation and increased both keratinocyte and fibroblast migration. Using proteotranscriptomic pathway analysis we revealed that transcription factors POU5F1/OCT4 and ESR1 acted as hubs for OLFM4-induced signalling in keratinocytes. In vivo experiments utilizing mouse splinted full-thickness cutaneous wound healing model showed that application of recombinant OLFM4 protein can significantly improve wound healing efficacy. Taken together, our results suggest that OLFM4 acts as a transiently upregulated inflammatory signal that promotes wound healing by regulating both dermal and epidermal cell compartments of the skin.

CXCL2 Impairs Functions of Bone Marrow Mesenchymal Stem Cells and Can Serve as a Serum Marker in High-Fat Diet-Fed Rats

In modern society excessive consumption of a high-fat diet (HFD) is a significant risk factor for many diseases such as diabetes, osteoarthritis and certain cancers. Resolving cellular and molecular mechanisms underlying HFD-associated disorders is of great importance to human health. Mesenchymal stem cells (MSCs) are key players in tissue homeostasis and adversely affected by prolonged HFD feeding. Low-grade systemic inflammation induced by HFD is characterized by increased levels of pro-inflammatory cytokines and alters homeostasis in many organs. However, whether, which and how HFD associated inflammatory cytokines impair MSCs remain unclear. Here we demonstrated that HFD induced serum cytokines disturbances, especially a continuous elevation of serum CXCL2 level in rats. Coincidentally, the differentially expressed genes (DEGs) of bone marrow MSCs (BMSCs) which functions were impaired in HFD rats were enriched in cytokine signaling. Further mechanism analysis revealed that CXCL2 treatment in vitro suppresses the adipogenic potential of BMSCs via Rac1 activation, and promoted BMSC migration and senescence by inducing over-production of ELMO1 and reactive oxygen species (ROS) respectively. Moreover, we found that although glycolipid metabolism indicators can be corrected, the CXCL2 elevation and BMSC dysfunctions cannot be fully rescued by diet correction and anti-inflammatory aspirin treatment, indicating the long-lasting deleterious effects of HFD on serum CXCL2 levels and BMSC functions. Altogether, our findings identify CXCL2 as an important regulator in BMSCs functions and may serve as a serum marker to indicate the BMSC dysfunctions induced by HFD. In addition, our findings underscore the intricate link among high-fat intake, chronic inflammation and BMSC dysfunction which may facilitate development of protective strategies for HFD associated diseases.

Potential role of diacylglycerol kinases in immune-mediated diseases

The mechanism promoting exacerbated immune responses in allergy and autoimmunity as well as those blunting the immune control of cancer cells are of primary interest in medicine. Diacylglycerol kinases (DGKs) are key modulators of signal transduction, which blunt diacylglycerol (DAG) signals and produce phosphatidic acid (PA). By modulating lipid second messengers, DGK modulate the activity of downstream signaling proteins, vesicle trafficking and membrane shape. The biological role of the DGK α and ζ isoforms in immune cells differentiation and effector function was subjected to in deep investigations. DGK α and ζ resulted in negatively regulating synergistic way basal and receptor induced DAG signals in T cells as well as leukocytes. In this way, they contributed to keep under control the immune response but also downmodulate immune response against tumors. Alteration in DGKα activity is also implicated in the pathogenesis of genetic perturbations of the immune function such as the X-linked lymphoproliferative disease 1 and localized juvenile periodontitis. These findings suggested a participation of DGK to the pathogenetic mechanisms underlying several immune-mediated diseases and prompted several researches aiming to target DGK with pharmacologic and molecular strategies. Those findings are discussed inhere together with experimental applications in tumors as well as in other immune-mediated diseases such as asthma.

D-series Resolvins activate Phospholipase D in phagocytes during inflammation and resolution

A successful acute inflammatory response results in the elimination of infectious agents by neutrophils and monocytes, followed by resolution and repair through tissue-resident and recruited macrophages. Resolvins (D-series and E-series) are pro-resolving lipid mediators involved in resolution and tissue repair, whose intracellular signaling remains of interest. Here, we report that D-series resolvins (RvD1- RvD5) activate phospholipase D (PLD), a ubiquitously expressed membrane lipase enzyme activity in modulating phagocyte functions. The mechanism for PLD-mediated actions of Resolvin-D5 (RvD5) in polarizing macrophages (M1-like toward M2-like) was found to be two-pronged: (a) RvD5 inhibits post-transcriptional modifications, by miRs and 3'exonucleases that process PLD2 mRNA, thus increasing PLD2 expression and activity; and (b) RvD5 enhances PLD2-S6Kinase signaling required for membrane expansion and efferocytosis. In an in vivo model of second organ reflow injury, we found that RvD5 did not reduce lung neutrophil myeloperoxidase levels in PLD2^-/- mice compared to WT and PLD1^-/- mice, confirming a novel role of PLD2 as the isoform in RvD5-mediated resolution processes. These results demonstrate that RvD5-PLD2 are attractive targets for therapeutic interventions in vascular inflammation such as ischemia-reperfusion injury and cardiovascular diseases.

BCALM (AC099524.1) Is a Human B Lymphocyte-Specific Long Noncoding RNA That Modulates B Cell Receptor-Mediated Calcium Signaling

Of the thousands of long noncoding RNAs (lncRNA) identified in lymphocytes, very few have defined functions. In this study, we report the discovery and functional elucidation of a human B cell-specific lncRNA with high levels of expression in three types of B cell cancer and normal B cells. The AC099524.1 gene is upstream of the gene encoding the B cell-specific phospholipase C γ 2 (PLCG2), a B cell-specific enzyme that stimulates intracellular Ca2+ signaling in response to BCR activation. AC099524.1 (B cell-associated lncRNA modulator of BCR-mediated Ca+ signaling [BCALM]) transcripts are localized in the cytoplasm and, as expected, CRISPR/Cas9 knockout of AC099524.1 did not affect PLCG2 mRNA or protein expression. lncRNA interactome, RNA immunoprecipitation, and coimmunoprecipitation studies identified BCALM-interacting proteins in B cells, including phospholipase D 1 (PLD1), and kinase adaptor proteins AKAP9 (AKAP450) and AKAP13 (AKAP-Lbc). These two AKAP proteins form signaling complexes containing protein kinases A and C, which phosphorylate and activate PLD1 to produce phosphatidic acid (PA). BCR stimulation of BCALM-deficient B cells resulted in decreased PLD1 phosphorylation and increased intracellular Ca+ flux relative to wild-type cells. These results suggest that BCALM promotes negative feedback that downmodulates BCR-mediated Ca+ signaling by promoting phosphorylation of PLD1 by AKAP-associated kinases, enhancing production of PA. PA activates SHP-1, which negatively regulates BCR signaling. We propose the name BCALM for B-Cell Associated LncRNA Modulator of BCR-mediated Ca+ signaling. Our findings suggest a new, to our knowledge, paradigm for lncRNA-mediated modulation of lymphocyte activation and signaling, with implications for B cell immune response and BCR-dependent cancers.

Phosphatidic acid regulates subcellular distribution of RA-GEFs critical for chemokine-dependent migration

Integrin activation by Rap1-GTP is pivotal for lymphocyte trafficking. In this study, we show the phosphatidic acid (PA)-dependent membrane distribution of RA-GEF-1 and -2 (also known as Rapgef2 and 6), which are guanine nucleotide exchange factors for Rap1, plays important roles in lymphocyte migration. RA-GEF-1 associates with PA through 919-967 aa within CDC25 homology domain, and the deletion of this region of RA-GEF-1 inhibits chemokine-dependent migration. Chemokine stimulation induces temporal production of PA on the plasma membrane, which is not necessary for Rap1 activation, but the translocation of RA-GEFs. Thus, chemokine-dependent generation of PA is critical for lymphocyte migration through membrane localization of RA-GEFs.

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.

DGKα in Neutrophil Biology and Its Implications for Respiratory Diseases

Diacylglycerol kinases (DGKs) play a key role in phosphoinositide signaling by removing diacylglycerol and generating phosphatidic acid. Besides the well-documented role of DGKα and DGKζ as negative regulators of lymphocyte responses, a robust body of literature points to those enzymes, and specifically DGKα, as crucial regulators of leukocyte function. Upon neutrophil stimulation, DGKα activation is necessary for migration and a productive response. The role of DGKα in neutrophils is evidenced by its aberrant behavior in juvenile periodontitis patients, which express an inactive DGKα transcript. Together with in vitro experiments, this suggests that DGKs may represent potential therapeutic targets for disorders where inflammation, and neutrophils in particular, plays a major role. In this paper we focus on obstructive respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), but also rare genetic diseases such as alpha-1-antitrypsin deficiency. Indeed, the biological role of DGKα is understudied outside the T lymphocyte field. The recent wave of research aiming to develop novel and specific inhibitors as well as KO mice will allow a better understanding of DGK's role in neutrophilic inflammation. Better knowledge and pharmacologic tools may also allow DGK to move from the laboratory bench to clinical trials.

Manuka Honey Modulates the Inflammatory Behavior of a dHL-60 Neutrophil Model under the Cytotoxic Limit

Recent work has shown that Manuka honey, an increasingly popular wound additive with potent antibacterial properties, also has anti-inflammatory properties. However, little research has been done examining its effect on neutrophils. This study investigates the hypothesis that Manuka honey reduces neutrophil superoxide release and chemotaxis and reduces the activation of the inflammatory nuclear factor-κB (NF-κB) signaling pathway under honey's cytotoxic limit. A differentiated HL-60 cell line was used as a neutrophil model and cultured in various concentrations of Manuka honey for 3 and 24 hours to measure cytotoxicity via mitochondrial activity and visual trypan-exclusion count. Cytochrome C and Boyden chamber assays were used to measure the effect of Manuka honey on superoxide release and chemotaxis toward fMLP, respectively. Additionally, a Western blot for NF-κB inhibitor α (IκBα) was performed to measure Manuka honey's effect on the NF-κB pathway via IκBα phosphorylation. The results indicate a cytotoxic limit of 3-5% v/v. The presence of 1% honey decreased superoxide release at 24 hours. The 0.5, 1, and 3% honey concentrations reduced chemotaxis and IκBα phosphorylation in a dose-dependent fashion. These results suggest that Manuka honey significantly reduces neutrophil recruitment and inflammatory behavior in the wound site in a dose-dependent fashion under the cytotoxic limit.

Oxidized LDL phagocytosis during foam cell formation in atherosclerotic plaques relies on a PLD2-CD36 functional interdependence

The uptake of cholesterol carried by low-density lipoprotein (LDL) is tightly controlled in the body. Macrophages are not well suited to counteract the cellular consequences of excess cholesterol leading to their transformation into "foam cells," an early step in vascular plaque formation. We have uncovered and characterized a novel mechanism involving phospholipase D (PLD) in foam cell formation. Utilizing bone marrow-derived macrophages from genetically PLD deficient mice, we demonstrate that PLD2 (but not PLD1)-null macrophages cannot fully phagocytose aggregated oxidized LDL (Agg-Ox-LDL), which was phenocopied with a PLD2-selective inhibitor. We also report a role for PLD2 in coupling Agg-oxLDL phagocytosis with WASP, Grb2, and Actin. Further, the clearance of LDL particles is mediated by both CD36 and PLD2, via mutual dependence on each other. In the absence of PLD2, CD36 does not engage in Agg-Ox-LDL removal and when CD36 is blocked, PLD2 cannot form protein-protein heterocomplexes with WASP or Actin. These results translated into humans using a GEO database of microarray expression data from atheroma plaques versus normal adjacent carotid tissue and observed higher values for NFkB, PLD2 (but not PLD1), WASP, and Grb2 in the atheroma plaques. Human atherectomy specimens confirmed high presence of PLD2 (mRNA and protein) as well as phospho-WASP in diseased arteries. Thus, PLD2 interacts in macrophages with Actin, Grb2, and WASP during phagocytosis of Agg-Ox-LDL in the presence of CD36 during their transformation into "foam cells." Thus, this study provides new molecular targets to counteract vascular plaque formation and atherogenesis.

Phospholipase D in TCR-Mediated Signaling and T Cell Activation

Phospholipase D (PLD) is an enzyme that catalyzes the hydrolysis of phosphatidylcholine, the major phospholipid in the plasma membrane, to generate an important signaling lipid, phosphatidic acid. Phosphatidic acid is a second messenger that regulates vesicular trafficking, cytoskeletal reorganization, and cell signaling in immune cells and other cell types. Published studies, using pharmacological inhibitors or protein overexpression, indicate that PLD plays a positive role in TCR-mediated signaling and cell activation. In this study, we used mice deficient in PLD1, PLD2, or both to assess the function of these enzymes in T cells. Our data showed that PLD1 deficiency impaired TCR-mediated signaling, T cell expansion, and effector function during immune responses against Listeria monocytogenes; however, PLD2 deficiency had a minimal impact on T cells. Biochemical analysis indicated that PLD1 deficiency affected Akt and PKCθ activation. In addition, it impaired TCR downregulation and the secondary T cell response. Together, our results suggested that PLD1 plays an important role in T cell activation.

A Phosphatidic Acid (PA) conveyor system of continuous intracellular transport from cell membrane to nucleus maintains EGF receptor homeostasis

The intracellular concentration of the mitogen phosphatidic acid (PA) must be maintained at low levels until the need arises for cell proliferation. How temporal and spatial trafficking of PA affects its target proteins in the different cellular compartments is not fully understood. We report that in cancer cells, PA cycles back and forth from the cellular membrane to the nucleus, affecting the function of epidermal growth factor (EGF), in a process that involves PPARα/LXRα signaling. Upon binding to its ligand, EGF receptor (EGFR)-initiated activation of phospholipase D (PLD) causes a spike in intracellular PA production that forms vesicles transporting EGFR from early endosomes (EEA1 marker) and prolonged internalization in late endosomes and Golgi (RCAS marker). Cells incubated with fluorescent-labeled PA (NBD-PA) show PA in “diffuse” locations throughout the cytoplasm, punctae (small, <0.1 μm) vesicles) and large (>0.5 μm) vesicles that co-localize with EGFR. We also report that PPARα/LXRα form heterodimers that bind to new Responsive Elements (RE) in the EGFR promoter. Nuclear PA enhances EGFR expression, a role compatible with the mitogenic ability of the phospholipid. Newly made EGFR is packaged into PA recycling vesicles (Rab11 marker) and transported back to the cytoplasm and plasma membrane. However, a PLD+PA combination impedes binding of PPARα/LXRα to the EGFR promoter. Thus, if PA levels inside the nucleus reach a certain threshold (>100 nM) PA outcompetes the nuclear receptors and transcription is inhibited. This new signaling function of PLD-PA targeting EGFR trafficking and biphasically modulating its transcription, could explain cell proliferation initiation and its maintenance in cancer cells.

Targeting Phospholipase D4 Attenuates Kidney Fibrosis

Phospholipase D4 (PLD4), a single-pass transmembrane glycoprotein, is among the most highly upregulated genes in murine kidneys subjected to chronic progressive fibrosis, but the function of PLD4 in this process is unknown. Here, we found PLD4 to be overexpressed in the proximal and distal tubular epithelial cells of murine and human kidneys after fibrosis. Genetic silencing of PLD4, either globally or conditionally in proximal tubular epithelial cells, protected mice from the development of fibrosis. Mechanistically, global knockout of PLD4 modulated innate and adaptive immune responses and attenuated the upregulation of the TGF-β signaling pathway and α1-antitrypsin protein (a serine protease inhibitor) expression and downregulation of neutrophil elastase (NE) expression induced by obstructive injury. In vitro, treatment with NE attenuated TGF-β-induced accumulation of fibrotic markers. Furthermore, therapeutic targeting of PLD4 using specific siRNA protected mice from folic acid-induced kidney fibrosis and inhibited the increase in TGF-β signaling, decrease in NE expression, and upregulation of mitogen-activated protein kinase signaling. Immunoprecipitation/mass spectrometry and coimmunoprecipitation experiments confirmed that PLD4 binds three proteins that interact with neurotrophic receptor tyrosine kinase 1, a receptor also known as TrkA that upregulates mitogen-activated protein kinase. PLD4 inhibition also prevented the folic acid-induced upregulation of this receptor in mouse kidneys. These results suggest inhibition of PLD4 as a novel therapeutic strategy to activate protease-mediated degradation of extracellular matrix and reverse fibrosis.

PLD Protein-Protein Interactions With Signaling Molecules and Modulation by PA

We describe methods for studying phospholipase D (PLD) interactions with signaling proteins and modulation of these interactions by the PLD reaction product, phosphatidic acid (PA). PLD is fundamental to the physiological maintenance of cellular/intracellular membranes, protein trafficking, cytoskeletal dynamics, membrane remodeling, cell proliferation, meiotic division and sporulation. PA is an acidic phospholipid involved in the biosynthesis of many other lipids that affects the enzymatic activities of many different signaling proteins via protein-lipid interactions or as a substrate. The involvement of PLD as an effector of protein-protein interactions and downstream signaling via PA-mediated processes has led to the investigation of PA-binding domains in target protein partners. We present here data and protocols detailing the interaction between PLD2-Rac2 interaction and modulation of this interaction by PA. We describe biochemical techniques to measure interactions between PLD, PA, and the small GTPase Rac2, which are associated in the cell. We found two maxima concentrations of PA that contributed to association or dissociation of Rac2 with PLD2, as well as the PLD2 lipase and guanine nucleotide exchange factor (GEF) activities. Fluctuations in the Rac2-PLD2 protein-protein binding interaction facilitate shuttling of Rac2 and/or PLD2 within the cell dependent on local cellular PA concentration. Fluorescence resonance emission transfer stoichiometry for PLD2 and Rac2 binding yielded a 3:1 ratio of Rac2:PLD2. Detection of PA in mammalian cells with a new biosensor showed colocalization in and around the nucleus. We also described methods for quantitation of PA in biological materials by HPLC electrospray ionization tandem mass spectrometry.

Accumulating insights into the role of phospholipase D2 in human diseases

Phospholipase D2 (PLD2) is a lipid-signaling enzyme that produces the signaling molecule phosphatidic acid (PA) by catalyzing the hydrolysis of phosphatidylcholine (PC). The molecular characteristics of PLD2, the mechanisms of regulation of its activity, its functions in the signaling pathway involving PA and binding partners, and its role in cellular physiology have been extensively studied over the past decades. Although several potential roles of PLD2 have been proposed based on the results of molecular and cell-based studies, the pathophysiological functions of PLD2 in vivo have not yet been fully investigated at the organismal level. Here, we address accumulated evidences that provide insight into the role of PLD2 in human disease. We summarize recent studies using animal models that provide direct evidence of the function of PLD2 in several pathological conditions such as vascular disease, immunological disease, and neurological disease. In light of the use of recently developed PLD2-specific inhibitors showing potential in alleviating pathological conditions, improving our understanding of the role of PLD2 in human disease would be necessary to target the regulation of PLD2 activity as a therapeutic strategy.

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.

Differential Roles of Phospholipase D Proteins in FcεRI-Mediated Signaling and Mast Cell Function

Phospholipase D (PLD) proteins are enzymes that catalyze the hydrolysis of phosphatidylcholine to generate an important signaling lipid, phosphatidic acid. Phosphatidic acid is a putative second messenger implicated in the regulation of vesicular trafficking and cytoskeletal reorganization. Previous studies using inhibitors and overexpression of PLD proteins indicate that PLD1 and PLD2 play positive roles in FcεRI-mediated signaling and mast cell function. We used mice deficient in PLD1, PLD2, or both to study the function of these enzymes in mast cells. In contrast to published studies, we found that PLD1 deficiency impaired FcεRI-mediated mast cell degranulation; however, PLD2 deficiency enhanced it. Biochemical analysis showed that PLD deficiency affected activation of the PI3K pathway and RhoA. Furthermore, our data indicated that, although PLD1 deficiency impaired F-actin disassembly, PLD2 deficiency enhanced microtubule formation. Together, our results suggested that PLD1 and PLD2, two proteins that catalyze the same enzymatic reaction, regulate different steps in mast cell degranulation.

The molecular basis of leukocyte adhesion involving phosphatidic acid and phospholipase D

Defining how leukocytes adhere to solid surfaces, such as capillary beds, and the subsequent migration through the extracellular matrix, is a central biological issue. We show here that phospholipase D (PLD) and its enzymatic reaction product, phosphatidic acid (PA), regulate cell adhesion of immune cells (macrophages and neutrophils) to collagen and have defined the underlying molecular mechanism in a spatio-temporal manner that coincides with PLD activity timing. A rapid (t½ = 4 min) and transient activation of the PLD1 isoform occurs upon adhesion, and a slower (t½ = 7.5 min) but prolonged (>30 min) activation occurs for PLD2. Importantly, PA directly binds to actin-related protein 3 (Arp3) at EC50 = 22 nm, whereas control phosphatidylcholine did not bind. PA-activated Arp3 hastens actin nucleation with a kinetics of t½ = 3 min at 300 nm (compared with controls of no PA, t½ = 5 min). Thus, PLD and PA are intrinsic components of cell adhesion, which reinforce each other in a positive feedback loop and react from cues from their respective solid substrates. In nascent adhesion, PLD1 is key, whereas a sustained adhesion in mature or established focal points is dependent upon PLD2, PA, and Arp3. A prolonged adhesion could effectively counteract the reversible intrinsic nature of this cellular process and constitute a key player in chronic inflammation.

Caffeic acid phenethyl ester inhibits the inflammatory effects of interleukin-1β in human corneal fibroblasts

CONTEXT

Expression of various inflammatory mediators in corneal fibroblasts contributes to corneal inflammation.

OBJECTIVE

The purpose of this study was to assess the possible effects of caffeic acid phenethyl ester (CAPE) on the expression of inflammatory mediators during an inflammatory response in human corneal fibroblasts.

MATERIALS AND METHODS

The levels of interleukin (IL)-6, monocyte chemotactic protein (MCP)-1, and intercellular adhesion molecule-1 (ICAM-1) from IL-1β-exposed human corneal fibroblasts were measured with enzyme-linked immunosorbent assays (ELISA). The regulatory mechanisms of CAPE on cellular signaling pathways were examined using Western blot and electrophoretic mobility shift assays. A functional validation was carried out by evaluating the inhibitory effects of CAPE on neutrophil and monocyte migration in vitro.

RESULTS

CAPE inhibited the expression of IL-6, MCP-1 and ICAM-1 induced by the pro-inflammatory cytokine IL-1β in corneal fibroblasts. The activation of AKT and NF-κB by IL-1β was markedly inhibited by CAPE, whereas the activity of mitogen-activated protein kinases (MAPKs) was not affected. CAPE significantly suppressed the IL-1β-induced migration of differentiated (d)HL-60 and THP-1 cells.

DISCUSSION

These anti-inflammatory effects of CAPE may be expected to inhibit the infiltration of leukocytes into the corneal stroma in vivo.

Phospholipase D in cell signaling: from a myriad of cell functions to cancer growth and metastasis

Phospholipase D (PLD) enzymes play a double vital role in cells: they maintain the integrity of cellular membranes and they participate in cell signaling including intracellular protein trafficking, cytoskeletal dynamics, cell migration, and cell proliferation. The particular involvement of PLD in cell migration is accomplished: (a) through the actions of its enzymatic product of reaction, phosphatidic acid, and its unique shape-binding role on membrane geometry; (b) through a particular guanine nucleotide exchange factor (GEF) activity (the first of its class assigned to a phospholipase) in the case of the mammalian isoform PLD2; and (c) through protein-protein interactions with a wide network of molecules: Wiskott-Aldrich syndrome protein (WASp), Grb2, ribosomal S6 kinase (S6K), and Rac2. Further, PLD interacts with a variety of kinases (PKC, FES, EGF receptor (EGFR), and JAK3) that are activated by it, or PLD becomes the target substrate. Out of these myriads of functions, PLD is becoming recognized as a major player in cell migration, cell invasion, and cancer metastasis. This is the story of the evolution of PLD from being involved in a large number of seemingly unrelated cellular functions to its most recent role in cancer signaling, a subfield that is expected to grow exponentially.

Pivotal role of phospholipase D1 in tumor necrosis factor-α-mediated inflammation and scar formation after myocardial ischemia and reperfusion in mice

Myocardial inflammation is critical for ventricular remodeling after ischemia. Phospholipid mediators play an important role in inflammatory processes. In the plasma membrane they are degraded by phospholipase D1 (PLD1). PLD1 was shown to be critically involved in ischemic cardiovascular events. Moreover, PLD1 is coupled to tumor necrosis factor-α signaling and inflammatory processes. However, the impact of PLD1 in inflammatory cardiovascular disease remains elusive. Here, we analyzed the impact of PLD1 in tumor necrosis factor-α-mediated activation of monocytes after myocardial ischemia and reperfusion using a mouse model of myocardial infarction. PLD1 expression was highly up-regulated in the myocardium after ischemia/reperfusion. Genetic ablation of PLD1 led to defective cell adhesion and migration of inflammatory cells into the infarct border zone 24 hours after ischemia/reperfusion injury, likely owing to reduced tumor necrosis factor-α expression and release, followed by impaired nuclear factor-κB activation and interleukin-1 release. Moreover, PLD1 was found to be important for transforming growth factor-β secretion and smooth muscle α-actin expression of cardiac fibroblasts because myofibroblast differentiation and interstitial collagen deposition were altered in Pld1(-/-) mice. Consequently, infarct size was increased and left ventricular function was impaired 28 days after myocardial infarction in Pld1(-/-) mice. Our results indicate that PLD1 is crucial for tumor necrosis factor-α-mediated inflammation and transforming growth factor-β-mediated collagen scar formation, thereby augmenting cardiac left ventricular function after ischemia/reperfusion.

Cellular and physiological roles for phospholipase D1 in cancer

Phospholipase D enzymes have long been proposed to play multiple cell biological roles in cancer. With the generation of phospholipase D1 (PLD1)-deficient mice and the development of small molecule PLD-specific inhibitors, in vivo roles for PLD1 in cancer are now being defined, both in the tumor cells and in the tumor environment. We review here tools now used to explore in vivo roles for PLD1 in cancer and summarize recent findings regarding functions in angiogenesis and metastasis.

Tulathromycin exerts proresolving effects in bovine neutrophils by inhibiting phospholipases and altering leukotriene B4, prostaglandin E2, and lipoxin A4 production

The accumulation of neutrophils and proinflammatory mediators, such as leukotriene B4 (LTB4), is a classic marker of inflammatory disease. The clearance of apoptotic neutrophils, inhibition of proinflammatory signaling, and production of proresolving lipids (including lipoxins, such as lipoxin A4 [LXA4]) are imperative for resolving inflammation. Tulathromycin (TUL), a macrolide used to treat bovine respiratory disease, confers immunomodulatory benefits via mechanisms that remain unclear. We recently reported the anti-inflammatory properties of TUL in bovine phagocytes in vitro and in Mannheimia haemolytica-challenged calves. The findings demonstrated that this system offers a powerful model for investigating novel mechanisms of pharmacological immunomodulation. In the present study, we examined the effects of TUL in a nonbacterial model of pulmonary inflammation in vivo and characterized its effects on lipid signaling. In bronchoalveolar lavage (BAL) fluid samples from calves challenged with zymosan particles (50 mg), treatment with TUL (2.5 mg/kg of body weight) significantly reduced pulmonary levels of LTB4 and prostaglandin E2 (PGE2). In calcium ionophore (A23187)-stimulated bovine neutrophils, TUL inhibited phospholipase D (PLD), cytosolic phospholipase A2 (PLA2) activity, and the release of LTB4. In contrast, TUL promoted the secretion of LXA4 in resting and A23187-stimulated neutrophils, while levels of its precursor, 15(S)-hydroxyeicosatetraenoic acid [15(S)-HETE], were significantly lower. These findings indicate that TUL directly modulates lipid signaling by inhibiting the production of proinflammatory eicosanoids and promoting the production of proresolving lipoxins.

Macrophage migration arrest due to a winning balance of Rac2/Sp1 repression over β-catenin-induced PLD expression

Monocytes and neutrophils infiltrate into tissues during inflammation and stay for extended periods of time until the initial insult is resolved or sometimes remain even longer in the case of chronic inflammation. The mechanism as to why phagocytes become immobilized after the initial cell migration event is not understood completely. Here, we show that overexpression or hyperactivation of Rac2 decreases sustained chemotactic responses of macrophages to MCP-1/CCL2. The resulting leukocyte arrest is not caused by a diminished availability of the cytokine receptor CCR2 that remains intact during MCP-1 stimulation. We show a novel mechanism that links the Rac2-dependent arrest of chemotaxis to decreased expression of PLD2 through the transcription regulator Sp1. Prolonged Rac2 activity leads to nuclear overactivation of Sp1, which acts as a repressor for PLD2. Also, another signaling component plays a regulatory role: β-catenin. Although early times of stimulation (≈ 20 min) with MCP-1/CCL2 resulted in activation of β-catenin with a positive effect on PLD2, after ≈ 3 h of stimulation, the levels of β-catenin were reduced and not able to prevent the negative effect of Rac2 on PLD2 activity. This is a novel molecular mechanism underlying immobilization of monocyte/macrophage migration that is important for the physiological maintenance of leukocytes at the site of inflammation. If this immobilization is prolonged enough, it could lead to chronic inflammation.

A GEF-to-phospholipase molecular switch caused by phosphatidic acid, Rac and JAK tyrosine kinase that explains leukocyte cell migration

Phospholipase D2 (PLD2) is a cell-signaling molecule that bears two activities: a guanine-nucleotide exchange factor (GEF) and a lipase that reside in the PX/PH domains and in two HKD domains, respectively. Upon cell stimulation, the GEF activity yields Rac2-GTP and the lipase activity yields phosphatidic acid (PA). In the present study, we show for the first time that these activities regulate one another. Upon cell stimulation, both GEF and lipase activities are quickly (within ∼3 min) elevated. As soon as it is produced, PA positively feeds back on the GEF and further activates it. Rac2-GTP, on the other hand, is inhibitory to the lipase activity. PLD2 would remain downregulated if it were not for the contribution of the tyrosine kinase Janus kinase 3 (JAK3), which restores lipase action (by phosphorylation at Y415). Conversely, the GEF is inhibited upon phosphorylation by JAK3 and is effectively terminated by this action and by the increasing accumulation of PA at >15 min of cell stimulation. This PA interferes with the ability of the GEF to bind to its substrate (Rac2-GTP). Thus, both temporal inter-regulation and phosphorylation-dependent mechanisms are involved in determining a GEF-lipase switch within the same molecule. Human neutrophils stimulated by interleukin-8 follow a biphasic pattern of GEF and lipase activation that can be explained by such an intramolecular switch. This is the first report of a temporal inter-regulation of two enzymatic activities that reside in the same molecule with profound biological consequences in leukocyte cell migration.

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.

Serum deprivation confers the MDA-MB-231 breast cancer line with an EGFR/JAK3/PLD2 system that maximizes cancer cell invasion

Our laboratory has reported earlier that in leukocytes, phospholipase D2 (PLD2) is under control of Janus kinase 3 (JAK3), which mediates chemotaxis. Investigating JAK3 in cancer cells led to an important discovery as exponentially growing MDA-MB-231 human breast cancer cells, which are highly proliferative and metastatic, did not substantially use JAK3 to activate PLD2. However, in 2-h or 16-h starved cell cultures, JAK3 switches to a PLD2-enhancing role, consistent with the needs of those cells to enter a "survival state" that relies on an increase in PLD2 activity to withstand serum deprivation. Using a small-molecule tyrosine kinase inhibitor, the flavonoid 4',5,7-trihydroxyflavone (apigenin), as well as RNA silencing, we found that the invasive phenotype of MDA-MB-231 cells is mediated by PLD2 under direct regulation of both JAK3 and the tyrosine kinase, epidermal growth factor receptor (EGFR). Furthermore, serum-deprived cells in culture show an upregulated EGFR/JAK3/PLD2-PA system and are especially sensitive to a combination of JAK3 and PLD2 enzymatic activity inhibitors (30nM apigenin and 300nM 5-fluoro-2-indolyl des-chlorohalopemide (FIPI), respectively). Thus, a multi-layered activation of cell invasion by two kinases (EGFR and JAK3) and a phospholipase (PLD2) provides regulatory flexibility and maximizes the aggressively invasive power of MDA-MB-231 breast cancer cells. This is especially important in the absence of growth factors in serum, coincidental with migration of these cells to new locations.

Structure analysis between the SWAP-70 RHO-GEF and the newly described PLD2-GEF

Small GTPases like Rac2 are crucial regulators of many cell functions central to life itself. Our laboratory has recently found that phospholipase D2 (PLD2) can act as a guanine nucleotide exchange factor (GEF) for Rac2. PLD2 has a Pleckstrin Homology (PH) domain but does not bear a Dbl homology (DH) or DOCK homology region (DHR) domain. It has, however, a Phox (PX) domain upstream of its PH domain. To better understand the novel finding of PLD2 as an enhancer of GDP/GTP exchange, we modeled the N-terminal portion of PLD2 (as the crystal structure of this protein has not as of yet been resolved), and studied the correlation with two known GEFs, SWAP-70 and the Leukemic Associated RhoGEF (LARG). Structural similarities between PLD2's PH and SWAP-70s or LARG's PH domain are very extensive, while similarities between PLD2's PX and SWAP-70s or LARG's DH domains are less evident. This indicates that PLD functions as a GEF utilizing its PH domain and part of its PX domain and possibly other regions. All this makes PLD unique, and an entirely new class of GEF. By bearing two enzymatic activities (break down of PC and GDP/GTP exchange), it is realistic to assume that PLD is an important signaling node for several intracellular pathways. Future experiments will ascertain how the newly described PLD2's GEF is regulated in the context of cell activation.

Biochemical and cellular implications of a dual lipase-GEF function of phospholipase D2 (PLD2)

PLD2 plays a key role in cell membrane lipid reorganization and as a key cell signaling protein in leukocyte chemotaxis and phagocytosis. Adding to the large role for a lipase in cellular functions, recently, our lab has identified a PLD2-Rac2 binding through two CRIB domains in PLD2 and has defined PLD2 as having a new function, that of a GEF for Rac2. PLD2 joins other major GEFs, such as P-Rex1 and Vav, which operate mainly in leukocytes. We explain the biochemical and cellular implications of a lipase-GEF duality. Under normal conditions, GEFs are not constitutively active; instead, their activation is highly regulated. Activation of PLD2 leads to its localization at the plasma membrane, where it can access its substrate GTPases. We propose that PLD2 can act as a "scaffold" protein to increase efficiency of signaling and compartmentalization at a phagocytic cup or the leading edge of a leukocyte lamellipodium. This new concept will help our understanding of leukocyte crucial functions, such as cell migration and adhesion, and how their deregulation impacts chronic inflammation.

Phosphoinositides in chemotaxis

Phosphatidylinositol lipids generated through the action of phosphinositide 3-kinase (PI3K) are key mediators of a wide array of biological responses. In particular, their role in the regulation of cell migration has been extensively studied and extends to amoeboid as well as mesenchymal migration. Through the emergence of fluorescent probes that target PI3K products as well as the use of specific inhibitors and knockout technologies, the spatio-temporal distribution of PI3K products in chemotaxing cells has been shown to represent a key anterior polarity signal that targets downstream effectors to actin polymerization. In addition, through intricate cross-talk networks PI3K products have been shown to regulate signals that control posterior effectors. Yet, in more complex environments or in conditions where chemoattractant gradients are steep, a variety of cell types can still chemotax in the absence of PI3K signals. Indeed, parallel signal transduction pathways have been shown to coordinately regulate cell polarity and directed movement. In this chapter, we will review the current role PI3K products play in the regulation of directed cell migration in various cell types, highlight the importance of mathematical modeling in the study of chemotaxis, and end with a brief overview of other signaling cascades known to also regulate chemotaxis.

Cloning of PLD2 from baculovirus for studies in inflammatory responses

The enzyme PLD hydrolyzes phosphodiester bonds of lipids in cell membranes. Phosphatidic acid, a chief product of PLD enzymatic activity, is a pleiotropic second messenger with key roles in membrane trafficking, cell invasion, cell growth, and anti-apoptosis. We describe in the present study molecular, cellular, and physiological methods to understand the mechanism of how the PLD2 isozyme regulates the process of inflammation. We describe here (1) a method that details phospholipase D2 (PLD2) cloning in the pBac expression vector, (2) the large-scale infection of Sf21 insect cells for protein production, (3) protein purification by TALON cobalt metal affinity matrix and subsequent assessment of PLD2 protein and lipase activity, (4) application of purified PLD2 protein for the study of Rac2 GTPase biology involving GTP binding by a pull-down assay and GTP/GDP exchange activity, (5) a method of PLD2 expression that involves mammalian cells, (6) a physiological application as relates to adhesion, chemotaxis, and phagocytosis, and (7) a model that integrates the results of a PLD-GTPase interaction from the molecular to the physiological contexts.

Phospholipase D2 (PLD2) is a guanine nucleotide exchange factor (GEF) for the GTPase Rac2

We have discovered that the enzyme phospholipase D2 (PLD2) binds directly to the small GTPase Rac2, resulting in PLD2 functioning as a guanine nucleotide exchange factor (GEF), because it switches Rac2 from the GDP-bound to the GTP-bound states. This effect is large enough to be meaningful (∼72% decrease for GDP dissociation and 300% increase for GTP association, both with PLD2), it has a half-time of ∼7 min, is enhanced with increasing PLD2 concentrations, and compares favorably with other known GEFs, such as Vav-1. The PLD2-Rac2 protein-protein interaction is sufficient for the GEF function, because it can be demonstrated in vitro with just recombinant proteins without lipid substrates, and a catalytically inactive lipase (PLD2-K758R) has GEF activity. Apart from this function, exogenous phosphatidic acid by itself (300 pM) increases GTP binding and enhances PLD2-K758R-mediated GTP binding (by ∼34%) but not GDP dissociation. Regarding the PLD2-Rac2 protein-protein association, it involves, for PLD2, residues 263-266 within a Cdc42/Rac interactive binding region in the PH domain, as well as the PX domain, and it involves, for Rac2, residue N17 within its Switch-1 region. PLD2's GEF function is demonstrated in living cells, because silencing PLD2 results in reduced Rac2 activity, whereas PLD2-initiated Rac2 activation enhances cell adhesion, chemotaxis, and phagocytosis. There are several known GEFs, but we report that this GEF is harbored in a phospholipase. The benefit to the cell is that PLD2 brings spatially separated molecules together in a membrane environment, ready for fast intracellular signaling and cell function.

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 (PLD2) shortens the time required for myeloid leukemic cell differentiation: mechanism of action

Cell differentiation is compromised in acute leukemias. We report that mammalian target of rapamycin (mTOR) and S6 kinase (S6K) are highly expressed in the undifferentiated promyelomonocytic leukemic HL-60 cell line, whereas PLD2 expression is minimal. The expression ratio of PLD2 to mTOR (or to S6K) is gradually inverted upon in vitro induction of differentiation toward the neutrophilic phenotype. We present three ways that profoundly affect the kinetics of differentiation as follows: (i) simultaneous overexpression of mTOR (or S6K), (ii) silencing of mTOR via dsRNA-mediated interference or inhibition with rapamycin, and (iii) PLD2 overexpression. The last two methods shortened the time required for differentiation. By determining how PLD2 participates in cell differentiation, we found that PLD2 interacts with and activates the oncogene Fes/Fps, a protein-tyrosine kinase known to be involved in myeloid cell development. Fes activity is elevated with PLD2 overexpression, phosphatidic acid or phosphatidylinositol bisphosphate. Co-immunoprecipitation indicates a close PLD2-Fes physical interaction that is negated by a Fes-R483K mutant that incapacitates its Src homology 2 domain. All these suggest for the first time the following mechanism: mTOR/S6K down-regulation→PLD2 overexpression→PLD2/Fes association→phosphatidic acid-led activation of Fes kinase→granulocytic differentiation. Differentiation shortening could have a clinical impact on reducing the time of return to normalcy of the white cell counts after chemotherapy in patients with acute promyelocytic leukemia.

A novel phospholipase D2-Grb2-WASp heterotrimer regulates leukocyte phagocytosis in a two-step mechanism

Phagocytosis is a primary innate response of both macrophages and neutrophils involving the formation of filamentous actin (F-actin)-rich protrusions that are extended around opsonized pathogens to form a phagocytic cup, resulting in their subsequent internalization. The molecular mechanism for this is still not completely understood. We now show for the first time that phospholipase D2 (PLD2) binds to growth factor receptor-bound protein 2 (Grb2) and to the Wiskott-Aldrich syndrome protein (WASp) to form a heterotrimer complex, PLD2-Grb2-WASp, and present the mechanism of interaction. Grb2 binds to the Y169/Y179 residues of PLD2 using its only SH2 domain, and it interacts with the poly-proline region of WASp using its two SH3 domains. The PLD2-Grb2-WASp heterotrimer can be visualized in early phagocytic cups of macrophages ingesting opsonized red blood cells, where it associates with polymerized actin. Cup colocalization and phagocytosis are disrupted with mutants that alter binding at either of the two proteins or by silencing Grb2 with RNA interference (RNAi). WASp association to PLD2-K758R, a lipase-inactive mutant, still occurs, albeit at lower levels, indicating that PLD2 plays a second role in phagocytosis, which is the production of phosphatidic acid (PA) and activation of phosphatidylinositol 5-kinase (PI5K) with subsequent synthesis of phosphatidylinositol 4,5-bisphosphate (PIP(2)). The latter can be blocked with RNAi, which negates phagocytosis. Lastly, a constitutively "open" active form of WASp (WASp-L270P) brings phagocytosis to its maximum level, which can be mimicked with WASp-WT plus PLD2 or plus PA. Since neither a protein-protein disruption nor lack of PLD activity completely negates cup formation or phagocytosis, we posit a two-step mechanism: PLD2 anchors WASp at the phagocytic cup through Grb2 following protein-protein interactions and also activates it, making key lipids available locally. The heterotrimer PLD2-Grb2-WASp then enables actin nucleation at the phagocytic cup and phagocytosis, which are at the center of the innate immune system function.

The mechanism of cell membrane ruffling relies on a phospholipase D2 (PLD2), Grb2 and Rac2 association

Membrane ruffling is the formation of actin rich membrane protrusions, essential for cell motility. The exact mechanism of ruffling is not fully known. Using YFP and CFP fluorescent chimeras, we show for the first time a co-localization of Phospholipase D2 (PLD2) and Growth factor Receptor Bound protein-2 (Grb2) with actin-rich membrane protrusions of macrophages. Grb2 cooperates with PLD2 in enhancing membrane ruffling, whether in resting cells or in cells stimulated with the growth factor M-CSF, although in the latter an increase in dorsal ruffles was observed, consistent with receptor-ligand internalization. Cells transfected with PLD2 mutated in the PH domain (Y169F) or with Grb2 mutated in the SH2 site (R86K) negate this effect, indicating an association PLD2(Y169)-SH2-Grb2 that was confirmed by immunoprecipitation and Western blotting. The association results in enhanced PLD activity, but the lipase activity can only partially explain the formation of membrane ruffles in vivo. A third component involves the Rho-GTPase Rac2 and it is only when Rac2 is overexpressed along with PLD2 and Rac2 that a full biological effect, including actin polymerization in vivo, is obtained. The mechanism involved is, then, as follows: PLD enzymatic action, after having been increased due to the binding to Grb2-SH2 via Y169, cooperates with Rac2, and the three molecules stimulate actin polymerization and consequently, membrane ruffle formation. Since membrane ruffling precedes cell migration, the results herein provide a novel mechanism for control of membrane dynamics, crucial for the physiology of leukocytes.

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.

PLD1 rather than PLD2 regulates phorbol-ester-, adhesion-dependent and Fc{gamma}-receptor-stimulated ROS production in neutrophils

The signalling lipid phosphatidic acid (PA) is generated by the hydrolysis of phosphatidylcholine (PC), which is catalysed by phospholipase D (PLD) enzymes. Neutrophils, important cells of the innate immune system, maintain the body's defence against infection. Previous studies have implicated PLD-generated PA in neutrophil function; these have relied heavily on the use of primary alcohols to act as inhibitors of PA production. The recent development of isoform-selective small molecule inhibitors and the generation of a knockout mouse model provide us with accurate tools to study the role of PLDs in neutrophil responses. We show that PLD1 is a regulator of phorbol-ester-, chemoattractant, adhesion-dependent and Fcγ-receptor-stimulated production of reactive oxygen species (ROS) in neutrophils. Significantly we found that this role of PLD is isoform specific: the absence of PLD2 does not negatively affect these processes. Contrary to expectation, other functions required for an efficient immune response operate effectively in Pld2-deficient neutrophils or when both isoforms are inhibited pharmacologically. We conclude that although PLD1 does have important regulatory roles in neutrophils, the field has been confused by the use of primary alcohols; now that gold standard Pld-knockout mouse models are available, previous work might need to be reassessed.

Evidence for two CRIB domains in phospholipase D2 (PLD2) that the enzyme uses to specifically bind to the small GTPase Rac2

Phospholipase D (PLD) and small GTPases are vital to cell signaling. We report that the Rac2 and the PLD2 isoforms exist in the cell as a lipase-GTPase complex that enables the two proteins to elicit their respective functionalities. A strong association between the two molecules was demonstrated by co-immunoprecipitation and was confirmed in living cells by FRET with CFP-Rac2 and YFP-PLD2 fluorescent chimeras. We have identified the amino acids in PLD2 that define a specific binding site to Rac2. This site is composed of two CRIB (Cdc42-and Rac-interactive binding) motifs that we have named "CRIB-1" and "CRIB-2" in and around the PH domain in PLD2. Deletion mutants PLD2-ΔCRIB-1/2 negate co-immunoprecipitation with Rac2 and diminish the FRET signal in living cells. The PLD2-Rac2 association was further confirmed in vitro using affinity-purified recombinant proteins. Binding was saturable with an apparent K(d) of 3 nm and was diminished with PLD2-ΔCRIB mutants. Furthermore, PLD2 bound more efficiently to Rac2-GTP than to Rac2-GDP or to a GDP-constitutive Rac2-N17 mutant. Increasing concentrations of recombinant Rac2 in vitro and in vivo during cell adhesion inhibit PLD2. Conversely, Rac2 activity is increased in the presence of PLD2-WT but not in PLD2-ΔCRIB. We propose that in activated cells PLD2 affects Rac2 in an initial positive feedback, but as Rac2-GTP accumulates in the cell, this constitutes a "termination signal" leading to PLD2 inactivation.

Activation of cytosolic phospholipase A2 downstream of the Src-phospholipase D1 (PLD1)-protein kinase C γ (PKCγ) signaling axis is required for hypoxia-induced pathological retinal angiogenesis

In view of understanding the mechanisms of retinal neovascularization, we had reported previously that vascular endothelial growth factor (VEGF)-induced pathological retinal angiogenesis requires the activation of Src-PLD1-PKCγ signaling. In the present work, we have identified cytosolic phospholipase A(2) (cPLA(2)) as an effector molecule of Src-PLD1-PKCγ signaling in the mediation of VEGF-induced pathological retinal angiogenesis based on the following observations. VEGF induced cPLA(2) phosphorylation in a time-dependent manner in human retinal microvascular endothelial cells (HRMVECs). VEGF also induced arachidonic acid (AA) release in a dose-, time-, and cPLA(2)-dependent manner. Depletion of cPLA(2) levels inhibited VEGF-induced HRMVEC DNA synthesis, migration, and tube formation. In addition, the exogenous addition of AA rescued VEGF-induced HRMVEC DNA synthesis, migration, and tube formation from inhibition by down-regulation of cPLA(2). Inhibition of Src, PLD1, or PKCγ attenuated VEGF-induced cPLA(2) phosphorylation and AA release. Consistent with these findings, hypoxia induced cPLA(2) phosphorylation and activity in VEGF-Src-PLD1-PKCγ-dependent manner in a mouse model of oxygen-induced retinopathy. In addition, siRNA-mediated down-regulation of cPLA(2) levels in the retina abrogated hypoxia-induced retinal endothelial cell proliferation and neovascularization. These observations suggest that cPLA(2)-dependent AA release is required for VEGF-induced Src-PLD1-PKCγ-mediated pathological retinal angiogenesis.

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.

Cell invasion of highly metastatic MTLn3 cancer cells is dependent on phospholipase D2 (PLD2) and Janus kinase 3 (JAK3)

MTLn3 cells are highly invasive breast adenoacarcinoma cells. The relative level of the epidermal-growth-factor-stimulated invasion of this cell line is greater than two other breast cancer cell lines (MDA-MB-231 and MCF-7) and one non-small cell lung cancer cell line (H1299). We have determined that the mechanism of cancer cell invasion involves the presence of an enzymatically active phospholipase D (PLD), with the PLD2 isoform being more relevant than PLD1. PLD2 silencing abrogated invasion, whereas ectopic expression of PLD2 augmented cell invasion in all four cell lines, with an efficacy (MTLn3±MDA-MB-231>H1299±MCF-7) that correlated well with their abilities to invade Matrigel in vitro. We also report that PLD2 is under the control of Janus kinase 3 (JAK3), with the kinase phosphorylating PLD2 at the Y415 residue, thus enabling its activation. Y415 is located downstream of a PH domain and upstream of the catalytic HKD-1 domain of PLD2. JAK3 knockdown abrogated lipase activity and epidermal-growth-factor-stimulated cell invasion directly. For the purposes of activating PLD2 for cell invasion, JAK3 operates via an alternative pathway that is independent of STAT, at least in MTLn3 cells. We also consistently found that JAK3 and PLD2 pathways are utilized at the maximum efficiency (phosphorylation and activity) in highly invasive MTLn3 cells versus a relatively low utilization in the less invasive MCF-7 cell line. In summary, a high level of cell invasiveness of cancer cells can be explained for the first time by combined high JAK3/PLD2 phosphorylation and activity involving PLD2's Y415 residue, which might constitute a novel target to inhibit cancer cell invasion.

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.

Loss of cofilin 1 disturbs actin dynamics, adhesion between enveloping and deep cell layers and cell movements during gastrulation in zebrafish

During gastrulation, cohesive migration drives associated cell layers to the completion of epiboly in zebrafish. The association of different layers relies on E-cadherin based cellular junctions, whose stability can be affected by actin turnover. Here, we examined the effect of malfunctioning actin turnover on the epibolic movement by knocking down an actin depolymerizing factor, cofilin 1, using antisense morpholino oligos (MO). Knockdown of cfl1 interfered with epibolic movement of deep cell layer (DEL) but not in the enveloping layer (EVL) and the defect could be specifically rescued by overexpression of cfl1. It appeared that the uncoordinated movements of DEL and EVL were regulated by the differential expression of cfl1 in the DEL, but not EVL as shown by in situ hybridization. The dissociation of DEL and EVL was further evident by the loss of adhesion between layers by using transmission electronic and confocal microscopy analyses. cfl1 morphants also exhibited abnormal convergent extension, cellular migration and actin filaments, but not involution of hypoblast. The cfl1 MO-induced cell migration defect was found to be cell-autonomous in cell transplantation assays. These results suggest that proper actin turnover mediated by Cfl1 is essential for adhesion between DEL and EVL and cell movements during gastrulation in zebrafish.

IL-8-induced neutrophil chemotaxis is mediated by Janus kinase 3 (JAK3)

Janus kinase 3 (JAK3) is a non-receptor tyrosine kinase vital to the regulation of T-cells. We report that JAK3 is a mediator of interleukin-8 (IL-8) stimulation of a different class of hematopoietic relevant cells: human neutrophils. IL-8 induced a time- and concentration-dependent activation of JAK3 activity in neutrophils and differentiated HL-60 leukemic cells. JAK3 was more robustly activated by IL-8 than other kinases: p70S6K, mTOR, MAPK or PKC. JAK3 silencing severely inhibited IL-8-mediated chemotaxis. Thus, IL-8 stimulates chemotaxis through a mechanism mediated by JAK3. Further, JAK3 activity and chemotaxis were inhibited by the flavonoid apigenin (4',5,7-trihydroxyflavone) at ∼5nM IC(50). These new findings lay the basis for understanding the molecular mechanism of cell migration as it relates to neutrophil-mediated chronic inflammatory processes.

Protein kinase Cdelta-mediated phosphorylation of phospholipase D controls integrin-mediated cell spreading

Integrin signaling plays critical roles in cell adhesion, spreading, and migration, and it is generally accepted that to regulate these integrin functions accurately, localized actin remodeling is required. However, the molecular mechanisms that control the targeting of actin regulation molecules to the proper sites are unknown. We previously demonstrated that integrin-mediated cell spreading and migration on fibronectin are dependent on the localized activation of phospholipase D (PLD). However, the mechanism underlying PLD activation by integrin is largely unknown. Here we demonstrate that protein kinase Cδ (PKCδ) is required for integrin-mediated PLD signaling. After integrin stimulation, PKCδ is activated and translocated to the edges of lamellipodia, where it colocalizes with PLD2. The abrogation of PKCδ activity inhibited integrin-induced PLD activation and cell spreading. Finally, we show that Thr566 of PLD2 is directly phosphorylated by PKCδ and that PLD2 mutation in this region prevents PLD2 activation, PLD2 translocation to the edge of lamellipodia, Rac translocation, and cell spreading after integrin activation. Together, these results suggest that PKCδ is a primary regulator of integrin-mediated PLD activation via the direct phosphorylation of PLD, which is essential for directing integrin-induced cell spreading.