Phosphatidic acid-dependent recruitment and function of the Rac activator DOCK1 during dorsal ruffle formation

Phosphatidic acid: from biophysical properties to diverse functions

Phosphatidic acid (PA), the simplest phospholipid, acts as a key metabolic intermediate and second messenger that impacts diverse cellular and physiological processes across species ranging from microbes to plants and mammals. The cellular levels of PA dynamically change in response to stimuli, and multiple enzymatic reactions can mediate its production and degradation. PA acts as a signalling molecule and regulates various cellular processes via its effects on membrane tethering, enzymatic activities of target proteins, and vesicular trafficking. Because of its unique physicochemical properties compared to other phospholipids, PA has emerged as a class of new lipid mediators influencing membrane structure, dynamics, and protein interactions. This review summarizes the biosynthesis, dynamics, and cellular functions and properties of PA.

Macropinocytosis: mechanisms and regulation

Macropinocytosis is defined as an actin-dependent but coat- and dynamin-independent endocytic uptake process, which generates large intracellular vesicles (macropinosomes) containing a non-selective sampling of extracellular fluid. Macropinocytosis provides an important mechanism of immune surveillance by dendritic cells and macrophages, but also serves as an essential nutrient uptake pathway for unicellular organisms and tumor cells. This review examines the cell biological mechanisms that drive macropinocytosis, as well as the complex signaling pathways - GTPases, lipid and protein kinases and phosphatases, and actin regulatory proteins - that regulate macropinosome formation, internalization, and disposition.

Tandem C2 domains mediate dynamic organelle targeting of a DOCK family guanine nucleotide exchange factor

Multicellular organisms use DOCK family guanine nucleotide exchange factors to activate Rac/Rho-of-Plants small GTPases and coordinate cell shape change. In developing tissues, DOCK signals integrate cell-cell interactions with cytoskeleton remodeling, and the GEFs cluster reversibly at specific organelle surfaces to orchestrate cytoskeletal reorganization. The domain organizations among DOCK orthologs are diverse, and the mechanisms of localization control are poorly understood. Here we use combinations of transgene complementation and live cell imaging assays to uncover an evolutionarily conserved and essential localization determinant in the DOCK-GEF named SPIKE1. The SPIKE1-DHR3 domain is sufficient for organelle association in vivo, and displays a complicated lipid binding selectivity for both phospholipid head groups and fatty acid chain saturation. SPIKE1-DHR3 is predicted to adopt a C2-domain structure and functions as part of tandem C2 array that enables reversible clustering at the cell apex. This work provides mechanistic insight into how DOCK GEFs sense compositional and biophysical membrane properties at the interface of two organelle systems.

SRC kinase activator CDCP1 promotes hepatocyte growth factor-induced cell migration/invasion of a subset of breast cancer cells

Cancer invasion and metastasis are the major causes of cancer patient mortality. Various growth factors, including hepatocyte growth factor (HGF), are known to promote cancer invasion and metastasis, but the regulatory mechanisms involved are not fully understood. Here, we show that HGF-promoted migration and invasion of breast cancer cells are regulated by CUB domain–containing protein 1 (CDCP1), a transmembrane activator of SRC kinase. In metastatic human breast cancer cell line MDA-MB-231, which highly expresses the HGF receptor MET and CDCP1, we show that CDCP1 knockdown attenuated HGF-induced MET activation, followed by suppression of lamellipodia formation and cell migration/invasion. In contrast, in the low invasive/nonmetastatic breast cancer cell line T47D, which had no detectable MET and CDCP1 expression, ectopic MET expression stimulated the HGF-dependent activation of invasive activity, and concomitant CDCP1 expression activated SRC and further promoted invasive activity. In these cells, CDCP1 expression dramatically activated HGF-induced membrane remodeling, which was accompanied by activation of the small GTPase Rac1. Analysis of guanine nucleotide exchange factors revealed that ARHGEF7 was specifically required for CDCP1-dependent induction of HGF-induced invasive ability. Furthermore, immunofluorescence staining demonstrated that CDCP1 coaccumulated with ARHGEF7. Finally, we confirmed that the CDCP1-SRC axis was also crucial for HGF and ARHGEF7-RAC1 signaling in MDA-MB-231 cells. Altogether, these results demonstrate that the CDCP1-SRC-ARHGEF7-RAC1 pathway plays an important role in the HGF-induced invasion of a subset of breast cancer cells.

PI3K in T Cell Adhesion and Trafficking

PI3K signalling is required for activation, differentiation, and trafficking of T cells. PI3Kδ, the dominant PI3K isoform in T cells, has been extensively characterised using PI3Kδ mutant mouse models and PI3K inhibitors. Furthermore, characterisation of patients with Activated PI3K Delta Syndrome (APDS) and mouse models with hyperactive PI3Kδ have shed light on how increased PI3Kδ activity affects T cell functions. An important function of PI3Kδ is that it acts downstream of TCR stimulation to activate the major T cell integrin, LFA-1, which controls transendothelial migration of T cells as well as their interaction with antigen-presenting cells. PI3Kδ also suppresses the cell surface expression of CD62L and CCR7 which controls the migration of T cells across high endothelial venules in the lymph nodes and S1PR1 which controls lymph node egress. Therefore, PI3Kδ can control both entry and exit of T cells from lymph nodes as well as the recruitment to and retention of T cells within inflamed tissues. This review will focus on the regulation of adhesion receptors by PI3Kδ and how this contributes to T cell trafficking and localisation. These findings are relevant for our understanding of how PI3Kδ inhibitors may affect T cell redistribution and function.

Fe65: A Scaffolding Protein of Actin Regulators

The scaffolding protein family Fe65, composed of Fe65, Fe65L1, and Fe65L2, was identified as an interaction partner of the amyloid precursor protein (APP), which plays a key function in Alzheimer’s disease. All three Fe65 family members possess three highly conserved interaction domains, forming complexes with diverse binding partners that can be assigned to different cellular functions, such as transactivation of genes in the nucleus, modulation of calcium homeostasis and lipid metabolism, and regulation of the actin cytoskeleton. In this article, we rule out putative new intracellular signaling mechanisms of the APP-interacting protein Fe65 in the regulation of actin cytoskeleton dynamics in the context of various neuronal functions, such as cell migration, neurite outgrowth, and synaptic plasticity.

Genetic analyses in mouse fibroblast and melanoma cells demonstrate novel roles for PDGF-AB ligand and PDGF receptor alpha

Platelet Derived Growth Factor Receptor (PDGFR) signaling is a central mitogenic pathway in development, as well as tissue repair and homeostasis. The rules governing the binding of PDGF ligand to the receptor to produce activation and downstream signaling have been well defined over the last several decades. In cultured cells after a period of serum deprivation, treatment with PDGF leads to the rapid formation of dramatic, actin-rich Circular Dorsal Ruffles (CDRs). Using CDRs as a robust visual readout of early PDGFR signaling, we have identified several contradictory elements in the widely accepted model of PDGF activity. Employing CRISPR/Cas9 gene editing to disrupt the Pdgfra gene in two different murine cell lines, we show that in addition to the widely accepted function for PDGFR-beta in CDR formation, PDGFR-alpha is also clearly capable of eliciting CDRs. Moreover, we demonstrate activity for heterodimeric PDGF-AB ligand in the vigorous activation of PDGFR-beta homodimers to produce CDRs. These findings are key to a more complete understanding of PDGF ligand-receptor interactions and their downstream signaling consequences. This knowledge will allow for more rigorous experimental design in future studies of PDGFR signaling and its contributions to development and disease.

New Era of Diacylglycerol Kinase, Phosphatidic Acid and Phosphatidic Acid-Binding Protein

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DG) to generate phosphatidic acid (PA). Mammalian DGK consists of ten isozymes (α–κ) and governs a wide range of physiological and pathological events, including immune responses, neuronal networking, bipolar disorder, obsessive-compulsive disorder, fragile X syndrome, cancer, and type 2 diabetes. DG and PA comprise diverse molecular species that have different acyl chains at the sn-1 and sn-2 positions. Because the DGK activity is essential for phosphatidylinositol turnover, which exclusively produces 1-stearoyl-2-arachidonoyl-DG, it has been generally thought that all DGK isozymes utilize the DG species derived from the turnover. However, it was recently revealed that DGK isozymes, except for DGKε, phosphorylate diverse DG species, which are not derived from phosphatidylinositol turnover. In addition, various PA-binding proteins (PABPs), which have different selectivities for PA species, were recently found. These results suggest that DGK–PA–PABP axes can potentially construct a large and complex signaling network and play physiologically and pathologically important roles in addition to DGK-dependent attenuation of DG–DG-binding protein axes. For example, 1-stearoyl-2-docosahexaenoyl-PA produced by DGKδ interacts with and activates Praja-1, the E3 ubiquitin ligase acting on the serotonin transporter, which is a target of drugs for obsessive-compulsive and major depressive disorders, in the brain. This article reviews recent research progress on PA species produced by DGK isozymes, the selective binding of PABPs to PA species and a phosphatidylinositol turnover-independent DG supply pathway.

The first DEP domain of the RhoGEF P-Rex1 autoinhibits activity and contributes to membrane binding

Phosphatidylinositol (3,4,5)-trisphosphate (PIP₃)-dependent Rac exchanger 1 (P-Rex1) catalyzes the exchange of GDP for GTP on Rac GTPases, thereby triggering changes in the actin cytoskeleton and in transcription. Its overexpression is highly correlated with the metastasis of certain cancers. P-Rex1 recruitment to the plasma membrane and its activity are regulated via interactions with heterotrimeric Gβγ subunits, PIP₃, and protein kinase A (PKA). Deletion analysis has further shown that domains C-terminal to its catalytic Dbl homology (DH) domain confer autoinhibition. Among these, the first dishevelled, Egl-10, and pleckstrin domain (DEP1) remains to be structurally characterized. DEP1 also harbors the primary PKA phosphorylation site, suggesting that an improved understanding of this region could substantially increase our knowledge of P-Rex1 signaling and open the door to new selective chemotherapeutics. Here we show that the DEP1 domain alone can autoinhibit activity in context of the DH/PH-DEP1 fragment of P-Rex1 and interacts with the DH/PH domains in solution. The 3.1 Å crystal structure of DEP1 features a domain swap, similar to that observed previously in the Dvl2 DEP domain, involving an exposed basic loop that contains the PKA site. Using purified proteins, we show that although DEP1 phosphorylation has no effect on the activity or solution conformation of the DH/PH-DEP1 fragment, it inhibits binding of the DEP1 domain to liposomes containing phosphatidic acid. Thus, we propose that PKA phosphorylation of the DEP1 domain hampers P-Rex1 binding to negatively charged membranes in cells, freeing the DEP1 domain to associate with and inhibit the DH/PH module.

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.

TBOPP enhances the anticancer effect of cisplatin by inhibiting DOCK1 in renal cell carcinoma

The treatment of renal cell carcinoma (RCC) with chemotherapy remains a challenge; therefore, improving the knowledge of the molecular mechanisms underlying RCC chemoresistance and developing novel therapeutic strategies is important. Dedicator of cytokinesis 1 (DOCK1), the first member of the DOCK family to be discovered, displays various roles during tumorigenesis; however, its role during RCC progression is not completely understood. Therefore, the present study aimed to clarify the function of DOCK1 and 1-[2-(3′-(trifluoromethyl)-(1,1′-biphenyl)-4-yl)-2-oxoethyl]-5-pyrrolidinylsulfonyl-2 (1H)-pyridone (TBOPP), a DOCK1-sensitive inhibitor, during RCC development and chemoresistance. The results of CCK-8 and EdU assay indicated that TBOPP decreased RCC cell viability and proliferation compared with the control group, and sensitized RCC cells to cisplatin. Moreover, RCC cells with high DOCK1 expression levels displayed increased resistance to cisplatin, whereas DOCK1 knockdown enhanced the lethal effects of cisplatin on RCC cells. Furthermore, the results determined by western blotting, CCK-8 and cell apoptosis assay indicated that TBOPP effectively reduced DOCK1 expression levels compared with the control group, and the TBOPP-mediated cisplatin sensitizing effect was mediated by DOCK1 inhibition. The present study suggests that DOCK1 plays a vital role in RCC cell chemoresistance to cisplatin; therefore, TBOPP may serve as a novel therapeutic agent for RCC chemoresistance.

Amyotrophic Lateral Sclerosis Modifiers in Drosophila Reveal the Phospholipase D Pathway as a Potential Therapeutic Target

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder lacking effective treatments. ALS pathology is linked to mutations in several different genes indicating...

Amyotrophic lateral sclerosis (ALS), commonly known as Lou Gehrig’s disease, is a devastating neurodegenerative disorder lacking effective treatments. ALS pathology is linked to mutations in >20 different genes indicating a complex underlying genetic architecture that is effectively unknown. Here, in an attempt to identify genes and pathways for potential therapeutic intervention and explore the genetic circuitry underlying Drosophila models of ALS, we carry out two independent genome-wide screens for modifiers of degenerative phenotypes associated with the expression of transgenic constructs carrying familial ALS-causing alleles of FUS (hFUS^R521C) and TDP-43 (hTDP-43^M337V). We uncover a complex array of genes affecting either or both of the two strains, and investigate their activities in additional ALS models. Our studies indicate the pathway that governs phospholipase D activity as a major modifier of ALS-related phenotypes, a notion supported by data we generated in mice and others collected in humans.

DOCK5 regulates energy balance and hepatic insulin sensitivity by targeting mTORC1 signaling

The dedicator of cytokinesis 5 (DOCK5) is associated with obesity. However, the mechanism by which DOCK5 contributes to obesity remains completely unknown. Here, we show that hepatic DOCK5 expression significantly decreases at a state of insulin resistance (IR). Deletion of DOCK5 in mice reduces energy expenditure, promotes obesity, augments IR, dysregulates glucose metabolism, and activates the mTOR (Raptor)/S6K1 pathway under a high-fat diet (HFD). The overexpression of DOCK5 in hepatocytes inhibits gluconeogenic gene expression and increases the level of insulin receptor (InsR) and Akt phosphorylation. DOCK5 overexpression also inhibits mTOR/S6K1 phosphorylation and decreases the level of raptor protein expression. The opposite effects were observed in DOCK5-deficient hepatocytes. Importantly, in liver-specific Raptor knockout mice and associated hepatocytes, the effects of an adeno-associated virus (AAV8)- or adenovirus-mediated DOCK5 knockdown on glucose metabolism and insulin signaling are largely eliminated. Additionally, DOCK5-Raptor interaction is indispensable for the DOCK5-mediated regulation of hepatic glucose production (HGP). Therefore, DOCK5 acts as a regulator of Raptor to control hepatic insulin activity and glucose homeostasis.

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.

Phosphatidic acid in membrane rearrangements

Phosphatidic acid (PA) is the simplest cellular glycerophospholipid characterized by unique biophysical properties: a small headgroup; negative charge; and a phosphomonoester group. Upon interaction with lysine or arginine, PA charge increases from -1 to -2 and this change stabilizes protein-lipid interactions. The biochemical properties of PA also allow interactions with lipids in several subcellular compartments. Based on this feature, PA is involved in the regulation and amplification of many cellular signalling pathways and functions, as well as in membrane rearrangements. Thereby, PA can influence membrane fusion and fission through four main mechanisms: it is a substrate for enzymes producing lipids (lysophosphatidic acid and diacylglycerol) that are involved in fission or fusion; it contributes to membrane rearrangements by generating negative membrane curvature; it interacts with proteins required for membrane fusion and fission; and it activates enzymes whose products are involved in membrane rearrangements. Here, we discuss the biophysical properties of PA in the context of the above four roles of PA in membrane fusion and fission.

A real-time, click chemistry imaging approach reveals stimulus-specific subcellular locations of phospholipase D activity

The fidelity of signal transduction requires spatiotemporal control of the production of signaling agents. Phosphatidic acid (PA) is a pleiotropic lipid second messenger whose modes of action differ based on upstream stimulus, biosynthetic source, and site of production. How cells regulate the local production of PA to effect diverse signaling outcomes remains elusive. Unlike other second messengers, sites of PA biosynthesis cannot be accurately visualized with subcellular precision. Here, we describe a rapid, chemoenzymatic approach for imaging physiological PA production by phospholipase D (PLD) enzymes. Our method capitalizes on the remarkable discovery that bulky, hydrophilic trans-cyclooctene-containing primary alcohols can supplant water as the nucleophile in the PLD active site in a transphosphatidylation reaction of PLD's lipid substrate, phosphatidylcholine. The resultant trans-cyclooctene-containing lipids are tagged with a fluorogenic tetrazine reagent via a no-rinse, inverse electron-demand Diels-Alder (IEDDA) reaction, enabling their immediate visualization by confocal microscopy in real time. Strikingly, the fluorescent reporter lipids initially produced at the plasma membrane (PM) induced by phorbol ester stimulation of PLD were rapidly internalized via apparent nonvesicular pathways rather than endocytosis, suggesting applications of this activity-based imaging toolset for probing mechanisms of intracellular phospholipid transport. By instead focusing on the initial 10 s of the IEDDA reaction, we precisely pinpointed the subcellular locations of endogenous PLD activity as elicited by physiological agonists of G protein-coupled receptor and receptor tyrosine kinase signaling. These tools hold promise to shed light on both lipid trafficking pathways and physiological and pathological effects of localized PLD signaling.

Lack of effective translational regulation of PLD expression and exosome biogenesis in triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer that is difficult to treat since cells lack the three receptors (ES, PR, or HER) that the most effective treatments target. We have used a well-established TNBC cell line (MDA-MB-231) from which we found evidence in support for a phospholipase D (PLD)-mediated tumor growth and metastasis: high levels of expression of PLD, as well as the absence of inhibitory miRs (such as miR-203) and 3'-mRNA PARN deadenylase activity in these cells. Such findings are not present in a luminal B cell line, MCF-7, and we propose a new miR•PARN•PLD node that is not uniform across breast cancer molecular subtypes and as such TNBC could be pharmacologically targeted differentially. We review the participation of PLD and phosphatidic acid (PA), its enzymatic product, as new "players" in breast cancer biology, with the aspects of regulation of the tumor microenvironment, macrophage polarization, regulation of PLD transcripts by specific miRs and deadenylases, and PLD-regulated exosome biogenesis. A new signaling miR•PARN•PLD node could serve as new biomarkers for TNBC abnormal signaling and metastatic disease staging, potentially before metastases are able to be visualized using conventional imaging.

Phosphatidic Acid: From Pleiotropic Functions to Neuronal Pathology

Among the cellular lipids, phosphatidic acid (PA) is a peculiar one as it is at the same time a key building block of phospholipid synthesis and a major lipid second messenger conveying signaling information. The latter is thought to largely occur through the ability of PA to recruit and/or activate specific proteins in restricted compartments and within those only at defined submembrane areas. Furthermore, with its cone-shaped geometry PA locally changes membrane topology and may thus be a key player in membrane trafficking events, especially in membrane fusion and fission steps, where lipid remodeling is believed to be crucial. These pleiotropic cellular functions of PA, including phospholipid synthesis and homeostasis together with important signaling activity, imply that perturbations of PA metabolism could lead to serious pathological conditions. In this mini-review article, after outlining the main cellular functions of PA, we highlight the different neurological diseases that could, at least in part, be attributed to an alteration in PA synthesis and/or catabolism.

Phospholipase D and the Mitogen Phosphatidic Acid in Human Disease: Inhibitors of PLD at the Crossroads of Phospholipid Biology and Cancer

Lipids are key building blocks of biological membranes and are involved in complex signaling processes such as metabolism, proliferation, migration, and apoptosis. Extracellular signaling by growth factors, stress, and nutrients is transmitted through receptors that activate lipid-modifying enzymes such as the phospholipases, sphingosine kinase, or phosphoinositide 3-kinase, which then modify phospholipids, sphingolipids, and phosphoinositides. One such important enzyme is phospholipase D (PLD), which cleaves phosphatidylcholine to yield phosphatidic acid and choline. PLD isoforms have dual role in cells. The first involves maintaining cell membrane integrity and cell signaling, including cell proliferation, migration, cytoskeletal alterations, and invasion through the PLD product PA, and the second involves protein-protein interactions with a variety of binding partners. Increased evidence of elevated PLD expression and activity linked to many pathological conditions, including cancer, neurological and inflammatory diseases, and infection, has motivated the development of dual- and isoform-specific PLD inhibitors. Many of these inhibitors are reported to be efficacious and safe in cells and mouse disease models, suggesting the potential for PLD inhibitors as therapeutics for cancer and other diseases. Current knowledge and ongoing research of PLD signaling networks will help to evolve inhibitors with increased efficacy and safety for clinical studies.

Macropinocytosis: New Insights Into Its Underappreciated Role in Innate Immune Cell Surveillance

Macropinocytosis has received increasing attention in recent years for its various roles in nutrient acquisition, immune surveillance, and virus and cancer pathologies. In most cases macropinocytosis is initiated by the sudden increase in an external stimulus such as a growth factor. This "induced" form of macropinocytosis has been the subject of much of the work addressing its mechanism and function over the years. An alternative, "constitutive" form of macropinocytosis restricted to primary innate immune cells also exists, although its mechanism has remained severely understudied. This mini-review focuses on the very recent advances that have shed new light on the initiation, formation and functional relevance of constitutive macropinocytosis in primary innate immune cells. An emphasis is placed on how this new understanding of constitutive macropinocytosis is helping to define the sentinel function of innate immune cells including polarized macrophages and dendritic cells.

Of local translation control and lipid signaling in neurons

Fine-tuned regulation of new proteins synthesis is key to the fast adaptation of cells to their changing environment and their response to external cues. Protein synthesis regulation is particularly refined and important in the case of highly polarized cells like neurons where translation occurs in the subcellular dendritic compartment to produce long-lasting changes that enable the formation, strengthening and weakening of inter-neuronal connection, constituting synaptic plasticity. The changes in local synaptic proteome of neurons underlie several aspects of synaptic plasticity and new protein synthesis is necessary for long-term memory formation. Details of how neuronal translation is locally controlled only start to be unraveled. A generally accepted view is that mRNAs are transported in a repressed state and are translated locally upon externally cued triggering signaling cascades that derepress or activate translation machinery at specific sites. Some important yet poorly considered intermediates in these cascades of events are signaling lipids such as diacylglycerol and its balancing partner phosphatidic acid. A link between these signaling lipids and the most common inherited cause of intellectual disability, Fragile X syndrome, is emphasizing the important role of these secondary messages in synaptically controlled translation.

Targeting Ras-Driven Cancer Cell Survival and Invasion through Selective Inhibition of DOCK1

Oncogenic Ras plays a key role in cancer initiation but also contributes to malignant phenotypes by stimulating nutrient uptake and promoting invasive migration. Because these latter cellular responses require Rac-mediated remodeling of the actin cytoskeleton, we hypothesized that molecules involved in Rac activation may be valuable targets for cancer therapy. We report that genetic inactivation of the Rac-specific guanine nucleotide exchange factor DOCK1 ablates both macropinocytosis-dependent nutrient uptake and cellular invasion in Ras-transformed cells. By screening chemical libraries, we have identified 1-(2-(3'-(trifluoromethyl)-[1,1'-biphenyl]-4-yl)-2-oxoethyl)-5-pyrrolidinylsulfonyl-2(1H)-pyridone (TBOPP) as a selective inhibitor of DOCK1. TBOPP dampened DOCK1-mediated invasion, macropinocytosis, and survival under the condition of glutamine deprivation without impairing the biological functions of the closely related DOCK2 and DOCK5 proteins. Furthermore, TBOPP treatment suppressed cancer metastasis and growth in vivo in mice. Our results demonstrate that selective pharmacological inhibition of DOCK1 could be a therapeutic approach to target cancer cell survival and invasion.

G-Protein Gα13 Functions with Abl Kinase to Regulate Actin Cytoskeletal Reorganization

Heterotrimeric G-proteins are essential cellular signal transducers. One of the G-proteins, Gα₁₃, is critical for actin cytoskeletal reorganization, cell migration, cell proliferation, and apoptosis. Previously, we have shown that Gα₁₃ is essential for both G-protein-coupled receptor and receptor tyrosine kinase-induced actin cytoskeletal reorganization such as dynamic dorsal ruffle turnover and cell migration. However, the mechanism by which Gα₁₃ signals to actin cytoskeletal reorganization is not completely understood. Here we show that Gα₁₃ directly interacts with Abl tyrosine kinase, which is a critical regulator of actin cytoskeleton. This interaction is critical for Gα₁₃-induced dorsal ruffle turnover, endothelial cell remodeling, and cell migration. Our data uncover a new molecular signaling pathway by which Gα₁₃ controls actin cytoskeletal reorganization.

Proliferative and metastatic roles for Phospholipase D in mouse models of cancer

Phospholipase D (PLD) activity has been proposed to facilitate multiple steps in cancer progression including growth, metabolism, angiogenesis, and mobility. The canonical enzymes PLD1 and PLD2 enact their diverse effects through hydrolyzing the membrane lipid phosphatidylcholine to generate the second messenger and signaling lipid phosphatidic acid (PA). However, the widespread expression of PLD1 and PLD2 in normal tissues and the additional distinct enzymatic mechanisms through which PA can be generated have produced uncertainty regarding the optimal settings in which PLD inhibition might ameliorate cancer. Recent studies in mouse model systems have demonstrated that inhibition or elimination of PLD activity reduces tumor growth and metastasis. One mechanism proposed for this outcome involves proliferative signaling mediated by receptor tyrosine kinases (RTK) and G protein-coupled receptors (GPCR), which is attenuated when downstream PLD signal propagation is suppressed. The reduced proliferative signaling has been reported to be compounded by dysfunctional energetic metabolism in the tumor cells under conditions of nutrient deprivation. Moreover, cancer cells lacking PLD activity display inefficiencies across multiple steps of the metastatic cascade, limiting the tumor's lethal spread. Using PLD isoform knockout mice, recent studies have reported on the net effects of inhibition and ablation in multiple cancer models through examining the role of PLD in the non-tumor cells comprising the stroma and microenvironment. The promising results of such in vivo studies, combined with the apparent low toxicity of highly-specific and potent inhibitors, highlights PLD as an attractive target for therapeutic inhibition in cancer. We discuss here the array of anti-tumor effects produced by PLD inhibition and ablation in cancer models with a focus on animal studies.

Phosphatidic acid binds to and regulates guanine nucleotide exchange factor 8 (GEF8) activity in Arabidopsis

Phosphatidic acid (PA) forms part of plant lipid metabolism and is a signalling molecule used in response to various external stresses. Guanine nucleotide exchange factors (GEFs) activate small GTPase ROPs, serving as molecular switches in a wide range of signalling pathways. However, the interaction between PA and GEFs in plants has not yet been reported. Here we show that PA bound specifically to GEF8 by using fat-Western blot and isothermal titration calorimetry assays. A C-terminal truncation of GEF8 exhibited strong PA binding, and mutation of lysines 13 and 18 in GEF8 PRONE domain caused a total loss of binding to PA. Two ROPs, ROP7 and ROP10, were identified as preferred substrates of GEF8 by pull-down and bimolecular fluorescence complementation assays. GEF8 activity towards ROP7, but not ROP10, was stimulated by PA both in vitro and in cells. Moreover, the PA- or ABA-induced activation of GEF8 was completely lost in the mutant GEF8, which did not bind to PA. Together, these findings identify a direct interconnection between PA-mediated GEFs activity and small GTPase signalling in plants and provide evidence for a synergistic activation of GEF8 by direct PA-binding to its PRONE domain.

ELMO proteins transduce G protein-coupled receptor signal to control reorganization of actin cytoskeleton in chemotaxis of eukaryotic cells

Chemotaxis, which is chemoattractant-guided directional cell migration, plays major roles in recruitment of neutrophils, the metastasis of cancer cells, and the development of the model organism Dictyostelium discoideum. These cells share remarkable similarities in the signaling pathways by which they control chemotaxis. They all use a G protein-coupled receptor (GPCR)-mediated signal transduction pathway to sense the chemotactic gradient to guide cell migration. Diverse chemokines activate Rac through conserved GPCR signaling pathways. ELMO proteins are an evolutionarily conserved, essential component of the ELMO/Dock complex, which functions as a guanine nucleotide exchange factor (GEF) for small G protein Rac activation. The linkages between the GPCR-initiated gradient sensing compass and the Rac-mediated migrating machinery have long been missing. Here, we summarize recent findings on ELMO proteins that directly interact with G protein and transduce GPCR signaling to control the reorganization of actin-based cytoskeleton through regulating Rac activation during chemotaxis, first in D. discoideum and then in mammalian cancer cells. This represents an evolutionarily conserved signaling shortcut from GPCR to the actin cytoskeleton.

The life cycle of phagosomes: formation, maturation, and resolution

Phagocytosis, the regulated uptake of large particles (>0.5 μm in diameter), is essential for tissue homeostasis and is also an early, critical component of the innate immune response. Phagocytosis can be conceptually divided into three stages: phagosome, formation, maturation, and resolution. Each of these involves multiple reactions that require exquisite spatial and temporal orchestration. The molecular events underlying these stages are being unraveled and the current state of knowledge is briefly summarized in this article.

Diacylglycerol Kinases: Shaping Diacylglycerol and Phosphatidic Acid Gradients to Control Cell Polarity

Diacylglycerol kinases (DGKs) terminate diacylglycerol (DAG) signaling and promote phosphatidic acid (PA) production. Isoform specific regulation of DGKs activity and localization allows DGKs to shape the DAG and PA gradients. The capacity of DGKs to constrain the areas of DAG signaling is exemplified by their role in defining the contact interface between T cells and antigen presenting cells: the immune synapse. Upon T cell receptor engagement, both DGK α and ζ metabolize DAG at the immune synapse thus constraining DAG signaling. Interestingly, their activity and localization are not fully redundant because DGKζ activity metabolizes the bulk of DAG in the cell, whereas DGKα limits the DAG signaling area localizing specifically at the periphery of the immune synapse. When DGKs terminate DAG signaling, the local PA production defines a new signaling domain, where PA recruits and activates a second wave of effector proteins. The best-characterized example is the role of DGKs in protrusion elongation and cell migration. Indeed, upon growth factor stimulation, several DGK isoforms, such as α, ζ, and γ, are recruited and activated at the plasma membrane. Here, local PA production controls cell migration by finely modulating cytoskeletal remodeling and integrin recycling. Interestingly, DGK-produced PA also controls the localization and activity of key players in cell polarity such as aPKC, Par3, and integrin β1. Thus, T cell polarization and directional migration may be just two instances of the general contribution of DGKs to the definition of cell polarity by local specification of membrane identity signaling.

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.

The focal adhesion-associated proteins DOCK5 and GIT2 comprise a rheostat in control of epithelial invasion

DOCK proteins are guanine nucleotide exchange factors for Rac and Cdc42 GTPases. DOCK1 is the founding member of the family and acts downstream of integrins via the canonical Crk-p130Cas complex to activate Rac GTPases in numerous contexts. In contrast, DOCK5, which possesses the greatest similarity to DOCK1, remains sparingly studied. Here we establish that DOCK5 has a non-redundant role in regulating motile and invasive capacities of epithelial cells. DOCK1 is constitutively associated with sites of integrin attachment termed focal adhesions (FAs). In contrast, we demonstrate that DOCK5 recruitment to FAs in Hela cells is restricted by GIT2, an established regulator of FA signaling. We determine that GIT2 is targeted to FAs in response to Rho-ROCK signaling and actomyosin contractility. Accordingly, inhibition of ROCK activity or MLC function promotes enrichment of DOCK5 in membrane protrusions and nascent cell–substratum adhesions. We further demonstrate that GIT2 inhibits the interaction of DOCK5 with Crk. Moreover, we show that depletion of GIT2 promotes DOCK5-dependent activation of the Crk-p130Cas signaling cascade to promote Rac1-mediated lamellipodial protrusion and FA turnover. The antagonism between GIT2 and DOCK5 extends to non-transformed MCF10A mammary epithelial cells, with DOCK5 ‘dialing-up' and GIT2 ‘dialing-down' invasiveness. Finally, we determine that DOCK5 inhibition attenuates invasion and metastasis of MDA-MB-231 cells and prolongs life span of mice injected with these cells. Collectively, our work identifies DOCK5 as a key regulator of epithelial invasion and metastasis, and demonstrates that suppression of DOCK5 by GIT2 represents a previously unappreciated mechanism for coordination of Rho and Rac GTPases.

The transcription factors Slug (SNAI2) and Snail (SNAI1) regulate phospholipase D (PLD) promoter in opposite ways towards cancer cell invasion

Slug (SNAI2) and Snail (SNAI1) are master regulatory transcription factors for organogenesis and wound healing, and they are involved in the epithelial to mesenchymal transition (EMT) of cancer cells. We found that the activity of phospholipase D isoform 2 (PLD2) is highly increased in cancers with larger size and poor prognosis (MDA-MB-231 versus MCF-7 cells), so we determined if Snail or Slug were responsible for PLD2 gene transcription regulation. Unexpectedly, we found that PLD2 expression was positively regulated by Slug but negatively regulated by Snail. The differential effects are amplified in breast cancer cells over normal cells and with MDA-MB-231 more robustly than MCF-7. Slug putatively binds to the PLD2 promoter and transactivates it, which is negated when Slug and Snail compete with each other. Meanwhile, PLD2 has a negative effect on Snail expression and a positive effect on Slug, thus closing a feedback loop between the lipase and the transcription factors. Further, PA, the product of PLD2 enzymatic reaction, has profound effects on its own and it further regulates the transcription factors. Thus, we show for the first time that the overexpressed PLD2 in human breast tumors is regulated by Slug and Snail transcription factors. The newly uncovered feedback loops in highly invasive cancer cells have important implications in the process of EMT.

The phospholipase D superfamily as therapeutic targets

The phospholipase D (PLD) lipid-signaling enzyme superfamily has long been studied for its roles in cell communication and a wide range of cell biological processes. With the advent of loss-of-function genetic mouse models that have revealed that PLD1 and PLD2 ablation is overtly tolerable, small-molecule PLD1/2 inhibitors that do not cause unacceptable clinical toxicity, a PLD2 polymorphism that has been linked to altered physiology, and growing delineation of processes that are subtly altered in mice lacking PLD1/2 activity, the stage is being set for assessment of PLD1/2 inhibition for therapeutic purposes. Based on findings to date, PLD1/2 inhibition may be of more utility in acute rather than chronic settings, although this generalization will depend on the specific risks and benefits in each disease setting.

DOCK2 and DOCK5 act additively in neutrophils to regulate chemotaxis, superoxide production, and extracellular trap formation

Neutrophils are highly motile leukocytes that play important roles in the innate immune response to invading pathogens. Neutrophils rapidly migrate to the site of infections and kill pathogens by producing reactive oxygen species (ROS). Neutrophil chemotaxis and ROS production require activation of Rac small GTPase. DOCK2, an atypical guanine nucleotide exchange factor (GEF), is one of the major regulators of Rac in neutrophils. However, because DOCK2 deficiency does not completely abolish fMLF-induced Rac activation, other Rac GEFs may also participate in this process. In this study, we show that DOCK5 acts with DOCK2 in neutrophils to regulate multiple cellular functions. We found that fMLF- and PMA-induced Rac activation were almost completely lost in mouse neutrophils lacking both DOCK2 and DOCK5. Although β2 integrin-mediated adhesion occurred normally even in the absence of DOCK2 and DOCK5, mouse neutrophils lacking DOCK2 and DOCK5 exhibited a severe defect in chemotaxis and ROS production. Similar results were obtained when human neutrophils were treated with CPYPP, a small-molecule inhibitor of these DOCK GEFs. Additionally, we found that DOCK2 and DOCK5 regulate formation of neutrophil extracellular traps (NETs). Because NETs are involved in vascular inflammation and autoimmune responses, DOCK2 and DOCK5 would be a therapeutic target for controlling NET-mediated inflammatory disorders.

p53 Down regulates PDGF-induced formation of circular dorsal ruffles in rat aortic smooth muscle cells

The tumor suppressor, p53, negatively regulates cell migration and invasion in addition to its role in apoptosis, cell cycle regulation and senescence. Here, we study the roles of p53 in PDGF-induced circular dorsal ruffle (CDR) formation in rat aortic smooth muscle (RASM) cells. In primary and immortalized RASM cells, up-regulation of p53 expression or increase in activity with doxorubicin inhibits CDR formation. In contrast, shRNA-knockdown of p53 or inhibition of its activity with pifithrin α promotes CDR formation. p53 acts by up-regulating PTEN expression, which antagonizes Rac and Cdc42 activation. Both lipid and protein phosphatase activities of PTEN are required for maximal suppression of CDR, but the lipid activity clearly plays the dominant role. N-WASP, the downstream effector of Cdc42, is the major positive contributor to CDR formation in RASM, and is an indirect target of p53. The Rac effector, WAVE2, appears to also play a minor role, while WAVE1 has no significant effect in CDR formation. In sum, we propose that p53 suppresses PDGF-induced CDR formation in RASM cells by upregulating PTEN leading mainly to the inhibition of the Cdc42-N-WASP pathway.

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.

DOCK5 functions as a key signaling adaptor that links FcεRI signals to microtubule dynamics during mast cell degranulation

Mast cells play a key role in the induction of anaphylaxis, a life-threatening IgE-dependent allergic reaction, by secreting chemical mediators that are stored in secretory granules. Degranulation of mast cells is triggered by aggregation of the high-affinity IgE receptor, FcεRI, and involves dynamic rearrangement of microtubules. Although much is known about proximal signals downstream of FcεRI, the distal signaling events controlling microtubule dynamics remain elusive. Here we report that DOCK5, an atypical guanine nucleotide exchange factor (GEF) for Rac, is essential for mast cell degranulation. As such, we found that DOCK5-deficient mice exhibit resistance to systemic and cutaneous anaphylaxis. The Rac GEF activity of DOCK5 is surprisingly not required for mast cell degranulation. Instead, DOCK5 associated with Nck2 and Akt to regulate microtubule dynamics through phosphorylation and inactivation of GSK3β. When DOCK5-Nck2-Akt interactions were disrupted, microtubule formation and degranulation response were severely impaired. Our results thus identify DOCK5 as a key signaling adaptor that orchestrates remodeling of the microtubule network essential for mast cell degranulation.

Insights into the biological functions of Dock family guanine nucleotide exchange factors

Rho GTPases regulate many essential processes during development, yet the full impact of their upstream regulation through guanine nucleotide exchange factors (GEFs) is only beginning to be appreciated. In this review, Laurin and Côté focus on emerging biological functions of the mammalian Dock family of GEFs in development and disease and discuss how recent discoveries might be exploited for novel therapeutic strategies.

Rho GTPases play key regulatory roles in many aspects of embryonic development, regulating processes such as differentiation, proliferation, morphogenesis, and migration. Two families of guanine nucleotide exchange factors (GEFs) found in metazoans, Dbl and Dock, are responsible for the spatiotemporal activation of Rac and Cdc42 proteins and their downstream signaling pathways. This review focuses on the emerging roles of the mammalian DOCK family in development and disease. We also discuss, when possible, how recent discoveries concerning the biological functions of these GEFs might be exploited for the development of novel therapeutic strategies.

Phospholipase D (PLD) drives cell invasion, tumor growth and metastasis in a human breast cancer xenograph model

Breast cancer is one of the most common malignancies in human females in the world. One protein that has elevated enzymatic lipase activity in breast cancers in vitro is phospholipase D (PLD), which is also involved in cell migration. We demonstrate that the PLD2 isoform, which was analyzed directly in the tumors, is crucial for cell invasion that contributes critically to the growth and development of breast tumors and lung metastases in vivo. We used three complementary strategies in a SCID mouse model and also addressed the underlying molecular mechanism. First, the PLD2 gene was silenced in highly metastatic, aggressive breast cancer cells (MDA-MB-231) with lentivirus-based shRNA, which were xenotransplanted in SCID mice. The resulting mouse primary mammary tumors were reduced in size (65%, p<0.05) and their onset delayed when compared to control tumors. Second, we stably overexpressed PLD2 in low-invasive breast cancer cells (MCF-7) with a biscistronic MIEG retroviral vector and observed that these cells were converted into a highly aggressive phenotype, as primary tumors that formed following xenotransplantation were larger, grew faster and developed lung metastases more readily. Third, we implanted osmotic pumps into SCID xenotransplanted mice that delivered two different small-molecule inhibitors of PLD activity (FIPI and NOPT). These inhibitors led to significant (>70%, p<0.05) inhibition of primary tumor growth, metastatic axillary tumors and lung metastases. In order to define the underlying mechanism, we determined that the machinery of PLD-induced cell invasion is mediated by phosphatidic acid (PA), WASp, Grb2 and Rac2 signaling events that ultimately affect actin polymerization and cell invasion. In summary, this study shows that PLD has a central role in the development, metastasis and level of aggressiveness of breast cancer, raising the possibility that PLD2 could be used as a new therapeutic target.