Targeting phospholipase D with small-molecule inhibitors as a potential therapeutic approach for cancer metastasis

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

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

Phospholipase D1 Attenuation Therapeutics Promotes Resilience against Synaptotoxicity in 12-Month-Old 3xTg-AD Mouse Model of Progressive Neurodegeneration

Abrogating synaptotoxicity in age-related neurodegenerative disorders is an extremely promising area of research with significant neurotherapeutic implications in tauopathies including Alzheimer's disease (AD). Our studies using human clinical samples and mouse models demonstrated that aberrantly elevated phospholipase D1 (PLD1) is associated with amyloid beta (Aβ) and tau-driven synaptic dysfunction and underlying memory deficits. While knocking out the lipolytic PLD1 gene is not detrimental to survival across species, elevated expression is implicated in cancer, cardiovascular conditions and neuropathologies, leading to the successful development of well-tolerated mammalian PLD isoform-specific small molecule inhibitors. Here, we address the importance of PLD1 attenuation, achieved using repeated 1 mg/kg of VU0155069 (VU01) intraperitoneally every alternate day for a month in 3xTg-AD mice beginning only from ~11 months of age (with greater influence of tau-driven insults) compared to age-matched vehicle (0.9% saline)-injected siblings. A multimodal approach involving behavior, electrophysiology and biochemistry corroborate the impact of this pre-clinical therapeutic intervention. VU01 proved efficacious in preventing in later stage AD-like cognitive decline affecting perirhinal cortex-, hippocampal- and amygdala-dependent behaviors. Glutamate-dependent HFS-LTP and LFS-LTD improved. Dendritic spine morphology showed the preservation of mushroom and filamentous spine characteristics. Differential PLD1 immunofluorescence and co-localization with Aβ were noted.

Phospholipase D and cancer metastasis: A focus on exosomes

In mammals, phospholipase D (PLD) enzymes involve 6 isoforms, of which only three have established lipase activity to produce the signaling lipid phosphatidic acid (PA). This phospholipase activity has been postulated to contribute to cancer progression for over three decades now, but the exact mechanisms involved have yet to be uncovered. Indeed, using various models, an altered PLD activity has been proposed altogether to increase cell survival rate, promote angiogenesis, boost rapamycin resistance, and favor metastasis. Although for some part, the molecular pathways by which this increase in PA is pro-oncogenic are partially known, the pleiotropic functions of PA make it quite difficult to distinguish which among these simple signaling pathways is responsible for each of these PLD facets. In this review, we will describe an additional potential contribution of PA generated by PLD1 and PLD2 in the biogenesis, secretion, and uptake of exosomes. Those extracellular vesicles are now viewed as membrane vehicles that carry informative molecules able to modify the fate of receiving cells at distance from the original tumor to favor homing of metastasis. The perspectives for a better understanding of these complex role of PLDs will be discussed.

Tricyclodecan-9-yl-Xanthogenate (D609): Mechanism of Action and Pharmacological Applications

Tricyclodecan-9-yl xanthogenate (D609) is a synthetic tricyclic compound possessing a xanthate group. This xanthogenate compound is known for its diverse pharmacological properties. Over the last three decades, many studies have reported the biological activities of D609, including antioxidant, antiapoptotic, anticholinergic, anti-tumor, anti-inflammatory, anti-viral, anti-proliferative, and neuroprotective activities. Its mechanism of action is extensively attributed to its ability to cause the competitive inhibition of phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) and sphingomyelin synthase (SMS). The inhibition of PCPLC or SMS affects secondary messengers with a lipidic nature, i.e., 1,2-diacylglycerol (DAG) and ceramide. Various in vitro/in vivo studies suggest that PCPLC and SMS inhibition regulate the cell cycle, block cellular proliferation, and induce differentiation. D609 acts as a pro-inflammatory cytokine antagonist and diminishes Aβ-stimulated toxicity. PCPLC enzymatic activity essentially requires Zn²⁺, and D609 might act as a potential chelator of Zn²⁺, thereby blocking PCPLC enzymatic activity. D609 also demonstrates promising results in reducing atherosclerotic plaque formation, post-stroke cerebral infarction, and cancer progression. The present compilation provides a comprehensive mechanistic insight into D609, including its chemistry, mechanism of action, and regulation of various pharmacological activities.

Aldolase A and Phospholipase D1 Synergistically Resist Alkylating Agents and Radiation in Lung Cancer

Exposure to alkylating agents and radiation may cause damage and apoptosis in cancer cells. Meanwhile, this exposure involves resistance and leads to metabolic reprogramming to benefit cancer cells. At present, the detailed mechanism is still unclear. Based on the profiles of several transcriptomes, we found that the activity of phospholipase D (PLD) and the production of specific metabolites are related to these events. Comparing several particular inhibitors, we determined that phospholipase D1 (PLD1) plays a dominant role over other PLD members. Using the existing metabolomics platform, we demonstrated that lysophosphatidylethanolamine (LPE) and lysophosphatidylcholine (LPC) are the most critical metabolites, and are highly dependent on aldolase A (ALDOA). We further demonstrated that ALDOA could modulate total PLD enzyme activity and phosphatidic acid products. Particularly after exposure to alkylating agents and radiation, the proliferation of lung cancer cells, autophagy, and DNA repair capabilities are enhanced. The above phenotypes are closely related to the performance of the ALDOA/PLD1 axis. Moreover, we found that ALDOA inhibited PLD2 activity and enzyme function through direct protein–protein interaction (PPI) with PLD2 to enhance PLD1 and additional carcinogenic features. Most importantly, the combination of ALDOA and PLD1 can be used as an independent prognostic factor and is correlated with several clinical parameters in lung cancer. These findings indicate that, based on the PPI status between ALDOA and PLD2, a combination of radiation and/or alkylating agents with regulating ALDOA-PLD1 may be considered as a new lung cancer treatment option.

Phospholipase D2 is a positive regulator of sirtuin 1 and modulates p53-mediated apoptosis via sirtuin 1

Sirtuin 1 (SIRT1) is a nicotinamide adenine dinucleotide-dependent histone deacetylase that plays diverse physiological roles. However, little is known about the regulation of SIRT1 activity. Here, we show that phospholipase D2 (PLD2), but not PLD1, selectively interacts with SIRT1 and increases the deacetylase activity of SIRT1. PLD2 does not interact with the other isozymes of SIRT (SIRT2–7). Two leucine residues in the LXXLL motif (L173 and L174) in the phox domain of PLD2 interact with the region essential for SIRT1 activity. PLD2 stimulates the SIRT1-mediated deacetylation of p53 independent of its lipase activity. In our study, mutagenesis of the LXXLL motif suppressed the interaction of PLD2 with SIRT1 and inhibited SIRT1-mediated p53 deacetylation and p53-induced transactivation of proapoptotic genes. Ultimately, overexpression of wild-type PLD2 but not that of LXXLL-mutant PLD2 protected cells against etoposide-induced apoptosis. Moreover, PLD2 did not protect against apoptosis induced by SIRT1 depletion under genotoxic stress. Collectively, our results suggest that PLD2 is a positive regulator of SIRT1 and modulates p53-mediated apoptosis via SIRT1.

New details about the regulatory mechanisms that prevent tumor cell death could be exploited to increase the effectiveness of chemotherapy. The sirtuin (SIRT) protein family has been linked to both promotion and suppression of tumors in different cancers. The enzyme SIRT1 in particular deacetylates, and thereby deactivates, the key tumor-suppressing antigen p53, stopping p53 from inducing apoptosis (controlled cell death) in tumors. Do Sik Min at Yonsei University, Incheon, South Korea, and co-workers revealed that this SIRT1 deacetylation of p53 is greatly enhanced by the activity of the enzyme phospholipase D2 (PLD2). A particular region on PLD2 is required to activate SIRT1, this activation leading to protection of tumor cells from apoptosis induced by the chemotherapy drug etoposide. Therapies that target that region on PLD2 might therefore suppress a tumor’s natural resistance to chemotherapy.

Dopamine receptors in emesis: Molecular mechanisms and potential therapeutic function

Dopamine is a member of the catecholamine family and is associated with multiple physiological functions. Together with its five receptor subtypes, dopamine is closely linked to neurological disorders such as schizophrenia, Parkinson's disease, depression, attention deficit-hyperactivity, and restless leg syndrome. Unfortunately, several dopamine receptor-based agonists used to treat some of these diseases cause nausea and vomiting as impending side-effects. The high degree of cross interactions of dopamine receptor ligands with many other targets including G-protein coupled receptors, transporters, enzymes, and ion-channels, add to the complexity of discovering new targets for the treatment of nausea and vomiting. Using activation status of signaling cascades as mechanism-based biomarkers to foresee drug sensitivity combined with the development of dopamine receptor-based biased agonists may hold great promise and seems as the next step in drug development for the treatment of such multifactorial diseases. In this review, we update the present knowledge on dopamine and dopamine receptors and their potential roles in nausea and vomiting. The pre- and clinical evidence provided in this review supports the implication of both dopamine and dopamine receptor agonists in the incidence of emesis. Besides the conventional dopaminergic antiemetic drugs, potential novel antiemetic targeting emetic protein signaling cascades may offer superior selectivity profile and potency.

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.

Antimicrobial agent triclosan suppresses mast cell signaling via phospholipase D inhibition

Humans are exposed to the antimicrobial agent triclosan (TCS) through use of TCS-containing products. Exposed tissues contain mast cells, which are involved in numerous biological functions and diseases by secreting various chemical mediators through a process termed degranulation. We previously demonstrated that TCS inhibits both Ca²⁺ influx into antigen-stimulated mast cells and subsequent degranulation. To determine the mechanism linking the TCS cytosolic Ca²⁺ depression to inhibited degranulation, we investigated the effects of TCS on crucial signaling enzymes activated downstream of the Ca²⁺ rise: protein kinase C (PKC; activated by Ca²⁺ and reactive oxygen species [ROS]) and phospholipase D (PLD). We found that TCS strongly inhibits PLD activity within 15 minutes post-antigen, a key mechanism of TCS mast cell inhibition. In addition, experiments using fluorescent constructs and confocal microscopy indicate that TCS delays antigen-induced translocations of PKCβII, PKCδ and PKC substrate myristoylated alanine-rich C-kinase. Surprisingly, TCS does not inhibit PKC activity or overall ability to translocate, and TCS actually increases PKC activity by 45 minutes post-antigen; these results are explained by the timing of both TCS inhibition of cytosolic Ca²⁺ (~15+ minutes post-antigen) and TCS stimulation of ROS (~45 minutes post-antigen). These findings demonstrate that it is incorrect to assume that all Ca²⁺ -dependent processes will be synchronously inhibited when cytosolic Ca²⁺ is inhibited by a toxicant or drug. The results offer molecular predictions of the effects of TCS on other mammalian cell types, which share these crucial signal transduction elements and provide biochemical information that may underlie recent epidemiological findings implicating TCS in human health problems.

Preparation and Evaluation of Liposomes Co-Loaded with Doxorubicin, Phospholipase D Inhibitor 5-Fluoro-2-Indolyl Deschlorohalopemide (FIPI) and D-Alpha Tocopheryl Acid Succinate (α-TOS) for Anti-Metastasis

Tumor metastasis has become a key obstacle to cancer treatment, which causes high mortality. Nowadays, it involves multiple complex pathways, and conventional treatments are not effective due to fewer targets. The aims of the present study were to construct a novel liposome delivery system co-loading a specific PLD inhibitor 5-fluoro-2-indolyldes-chlorohalopemide (FIPI) in combination with antitumor drug doxorubicin (DOX) and functional excipient D-alpha tocopheryl acid succinate (α-TOS) for anti-metastasis. In this study, the liposomes containing three components (DFT-Lip) with different physicochemical properties were successfully prepared by film dispersion method combined with pH-gradient method. Physicochemical parameters such as particles size, potential, encapsulation efficiency, stability, and release profiles were investigated. In vitro and in vivo anti-metastasis effectiveness against highly metastatic breast cancer MDA-MB-231 cell line was evaluated. The liposomes showed uniform particle size (approximately 119 nm), high drug encapsulation efficiency (> 90%), slow release characteristics and stability. In vitro anti-tumor cell metastasis study demonstrated DFT-Lip could greatly inhibit motility, migration and invasion of MDA-MB-231 cells compared to other liposomes, predicting a synergistic anti-tumor metastasis effect between FIPI with α-TOS in liposomes. In vivo anti-metastasis study showed that DFT-Lip prevented the initiation and the progression of metastasis of high metastatic breast cancer. These results suggested that the liposomes containing DOX, FIPI, and α-TOS might be a promising strategy for metastatic tumor therapy in clinics.

Determination of the formation rate of phosphatidylethanol by phospholipase D (PLD) in blood and test of two selective PLD inhibitors

Phosphatidylethanol (PEth) is an alcohol biomarker formed from phosphatidylcholine (PC) by the enzyme phospholipase D (PLD) in the presence of ethanol. A drinking study revealed individual differences in maximum PEth levels after drinking to a targeted blood alcohol concentration (BAC) of 0.1%. This seemed to be due to different PLD activities in the tested persons. Furthermore, post-sampling formation of PEth occurred in blood samples, still containing alcohol. Therefore, a standardized in vitro test for measuring individual PEth formation rates was developed. Two PLD inhibitors were tested for their potency to inhibit post-sampling PEth formation. PEth-negative blood samples were collected from a volunteer. Ethanol was added in different concentrations (0.01-0.3% BAC) directly after blood sampling. The specimens were incubated at 37 °C. Aliquots were taken at the start of the incubation, and every hour until 8 h after start of incubation, and one sample was taken on subsequent days over 1 week. PEth 16:0/18:1 and PEth 16:0/18:2 were determined by online SPE-LC-MS/MS. Furthermore, this test system was applied to blood samples of 12 volunteers. For the inhibition tests, fresh blood (spiked with 0.1% ethanol) was spiked with 30, 300, 3000, or 30,000 nM of either halopemide or 5-fluoro-2-indolyl-deschlorohalopemide (FIPI), and incubated at 37 °C. PEth concentrations were determined hourly over 5 h on the first day and once on day 2 and day 3. PEth formation was linear in the first 7 h of incubation and dependent on the alcohol concentration. The formation rates of PEth 16:0/18:1 were 0.002 μmol L^-1 h^-1 (0.01% BAC), 0.016 μmol L^-1 h^-1 (0.1% BAC), 0.025 μmol L^-1 h^-1 (0.2% BAC), and 0.029 μmol L^-1 h^-1 (0.3% BAC). For PEth 16:0/18:2, the formation rates were 0.002 μmol L^-1 h^-1 (0.01% BAC), 0.019 μmol L^-1 h^-1 (0.1% BAC), 0.025 μmol L^-1 h^-1 (0.2% BAC), and 0.030 μmol L^-1 h^-1 (0.3% BAC). Maximum concentrations reached 431 ng/mL (PEth 16:0/18:1) and 496 ng/mL (PEth 16:0/18:2) at 0.3% BAC after 3 days. Maximum velocity (v_max) was not reached under these conditions. PEth formation in blood of the 12 volunteers ranged between 0.011 and 0.025 μmol L^-1 h^-1 for PEth 16:0/18:1 and between 0.014 and 0.021 μmol L^-1 h^-1 for PEth 16:0/18:2. PEth formation in human blood was inhibited by halopemide in a concentration-dependent manner. However, a complete inhibition was not achieved by the applied maximum concentration of 30,000 nM. FIPI showed a better inhibition of PEth formation. A complete inhibition could be achieved by a concentration of 30,000 nM for the first 24 h (for PEth 16:0/18:1) and for 48 h (for PEth 16:0/18:2). Formation of PEth was found to be dependent on the BAC. As a consequence, it is essential to inhibit PLD activity after blood collection to avoid post-sampling formation of PEth in blood samples with a positive BAC. Inhibition of PEth formation was more effective using FIPI, compared to halopemide.

The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes

Phosphoinositides are key regulators of a large number of diverse cellular processes that include membrane trafficking, plasma membrane receptor signaling, cell proliferation, and transcription. How a small number of chemically distinct phosphoinositide signals are functionally amplified to exert specific control over such a diverse set of biological outcomes remains incompletely understood. To this end, a novel mechanism is now taking shape, and it involves phosphatidylinositol (PtdIns) transfer proteins (PITPs). The concept that PITPs exert instructive regulation of PtdIns 4-OH kinase activities and thereby channel phosphoinositide production to specific biological outcomes, identifies PITPs as central factors in the diversification of phosphoinositide signaling. There are two evolutionarily distinct families of PITPs: the Sec14-like and the StAR-related lipid transfer domain (START)-like families. Of these two families, the START-like PITPs are the least understood. Herein, we review recent insights into the biochemical, cellular, and physiological function of both PITP families with greater emphasis on the START-like PITPs, and we discuss the underlying mechanisms through which these proteins regulate phosphoinositide signaling and how these actions translate to human health and disease.

Long Noncoding RNAs: Advances in Lipid Metabolism

Long noncoding RNAs (lncRNAs) are an important group of pervasive noncoding RNAs (>200nt) proposed to be crucial regulators of numerous physiological and pathological processes. Through interactions with RNA, chromatin, and protein, lncRNAs modulate mRNA stability, chromatin structure, and the function of proteins (including transcription factors). In addition, to their well-known roles in the modulation of cell growth, apoptosis, neurological disease progression and cancer metastasis, these large molecules have also been identified as likely mediators of lipid metabolism. In particular, lncRNAs orchestrate adipogenesis; fatty acid, cholesterol, and phospholipid metabolism and transport; and the formation of high-density and low-density lipoproteins (HDLs and LDLs). LncRNAs also appear to target several transcription factors that play essential roles in the regulation of lipid metabolism, such as liver X receptors (LXRs), sterol regulatory element binding proteins (SREBPs), and peroxisome proliferator-activated receptor γ (PPARγ). Better understanding the regulatory roles of lncRNAs in dyslipidemia, atherosclerosis, and adipogenesis will reveal appropriate strategies to treat these diseases. In this review, we review recent progress in lncRNA-mediated regulation of lipid metabolism, as well as its role in the regulation of adipogenesis.

Phospholipase D2 promotes disease progression of renal cell carcinoma through the induction of angiogenin

A hallmark of clear cell renal cell carcinoma (ccRCC) is the presence of intracellular lipid droplets (LD) and it is assumed that phosphatidic acid (PA) produced by phospholipase D (PLD) plays some role in the LD formation. However, little is known about the significance of PLD in ccRCC. In this study, we examined the expression levels of PLD in ccRCC. The classical mammalian isoforms of PLD are PLD1 and PLD2, and the levels of both mRNA were higher at the primary tumor sites than in normal kidney tissues. Similarly, both PLD were significantly abundant in tumor cells as determined by analysis using immunohistochemical staining. Importantly, a higher level of PLD was significantly associated with a higher tumor stage and grade. Because PLD2 knockdown effectively suppressed the cell proliferation and invasion of ccRCC as compared with PLD1 in vitro, we examined the effect of PLD2 in vivo. Notably, shRNA-mediated knockdown of PLD2 suppressed the growth and invasion of tumors in nude mouse xenograft models. Moreover, the higher expression of PLD2 was significantly associated with poorer prognosis in 67 patients. As for genes relating to the tumor invasion of PLD2, we found that angiogenin (ANG) was positively regulated by PLD2. In fact, the expression levels of ANG were elevated in tumor tissues as compared with normal kidney and the inhibition of ANG activity with a neutralizing antibody significantly suppressed tumor invasion. Overall, we revealed for the first time that PLD2-produced PA promoted cell invasion through the expression of ANG in ccRCC cells.

Targeting Phospholipase D Genetically and Pharmacologically for Studying Leukocyte Function

Phospholipase D (PLD), is a protein that breaks down phospholipids, maintaining structural integrity and remodeling of cellular or intracellular membranes, as well as mediating protein trafficking and cytoskeletal dynamics during cell motility. One of the reaction products of PLD action is phosphatidic acid (PA). PA is a mitogen involved in a large variety of physiological cellular functions, such as cell growth, cell cycle progression, and cell motility. We have chosen as cell models the leukocyte polymorphonuclear neutrophil and the macrophage as examples of cell motility. We provide a three-part method for targeting PLD genetically and pharmacologically to study its role in cell migration. In the first part, we begin with genetically deficient mice PLD1-KO and PLD2-KO. We describe bone marrow neutrophil (BMN) isolation; BMN is labeled fluorescently and can be used for studying tissue-damaging neutrophilia in ischemia-reperfusion injury (IRI). In the second part, we begin also with PLD1-KO and PLD2-KO and prepare bone marrow-derived macrophages (BMDM), first from monocytes and then inducing macrophage differentiation in culture with continuous incubation of cytokines. We use BMDM to find experimentally if PLD woul play a role in cholesterol phagocytosis, which is the first step in atherosclerosis progression. In the third part, we study PLD function in BMN and BMDM with PLD enzyme pharmacological inhibitors instead of genetically deficient mice, to ascertain the particular contributions of isoforms PLD1 and PLD2 on leukocyte function. By using the three-step thorough approach, we could understand the molecular underpinning of PLD in the pathological conditions indicated above, IRI-neutrophilia and atherosclerosis.

Phospholipase D and Its Essential Role in Cancer

The role of phospholipase D (PLD) in cancer development and management has been a major area of interest for researchers. The purpose of this mini-review is to explore PLD and its distinct role during chemotherapy including anti-apoptotic function. PLD is an enzyme that belongs to the phospholipase super family and is found in a broad range of organisms such as viruses, yeast, bacteria, animals, and plants. The function and activity of PLD are widely dependent on and regulated by neurotransmitters, hormones, small monomeric GTPases, and lipids. A growing body of research has shown that PLD activity is significantly increased in cancer tissues and cells, indicating that it plays a critical role in signal transduction, cell proliferation, and anti-apoptotic processes. In addition, recent studies show that PLD is a downstream transcriptional target of proteins that contribute to inflammation and carcinogenesis such as Sp1, NFκB, TCF4, ATF-2, NFATc2, and EWS-Fli. Thus, compounds that inhibit expression or activity of PLD in cells can be potentially useful in reducing inflammation and sensitizing resistant cancers during chemotherapy.

Expression proteomics study to determine metallodrug targets and optimal drug combinations

The emerging technique termed functional identification of target by expression proteomics (FITExP) has been shown to identify the key protein targets of anti-cancer drugs. Here, we use this approach to elucidate the proteins involved in the mechanism of action of two ruthenium(II)-based anti-cancer compounds, RAPTA-T and RAPTA-EA in breast cancer cells, revealing significant differences in the proteins upregulated. RAPTA-T causes upregulation of multiple proteins suggesting a broad mechanism of action involving suppression of both metastasis and tumorigenicity. RAPTA-EA bearing a GST inhibiting ethacrynic acid moiety, causes upregulation of mainly oxidative stress related proteins. The approach used in this work could be applied to the prediction of effective drug combinations to test in cancer chemotherapy clinical trials.

Targetting PED/PEA-15 for diabetes treatment

INTRODUCTION

PED/PEA-15 is an ubiquitously expressed protein, involved in the regulation of proliferation and apoptosis. It is commonly overexpressed in Type 2 Diabetes (T2D) and in different T2D-associated comorbidities, including cancer and certain neurodegenerative disorders. Areas covered: In mice, Ped/Pea-15 overexpression impairs glucose tolerance and, in combination with high fat diets, further promotes insulin resistance and T2D. It also controls β-cell mass, altering caspase-3 activation and the expression of pro- and antiapoptotic genes. These changes are mediated by PED/PEA-15-PLD1 binding. Overexpression of PLD1 D4 domain specifically blocks Ped/Pea-15-PLD1 interaction, reverting the effect of Ped/Pea-15 in vivo. D4α, a D4 N-terminal peptide, is able to displace Ped/Pea-15-PLD1 binding, but features greater stability in vivo compared to the entire D4 peptide. Here, we review early mechanistic studies on PED/PEA-15 relevance in apoptosis before focusing on its role in cancer and T2D. Finally, we describe potential therapeutic opportunities for T2D based on PED/PEA-15 targeting. Expert opinion: T2D is a major problem for public health and economy. Thus, the identification of new molecules with pharmacological activity for T2D represents an urgent need. Further studies with D4α will help to identify smaller pharmacologically active peptides and innovative molecules of potential pharmacological interest for T2D treatment.

Emerging roles of lipid metabolism in cancer metastasis

Cancer cells frequently display fundamentally altered cellular metabolism, which provides the biochemical foundation and directly contributes to tumorigenicity and malignancy. Rewiring of metabolic programmes, such as aerobic glycolysis and increased glutamine metabolism, are crucial for cancer cells to shed from a primary tumor, overcome the nutrient and energy deficit, and eventually survive and form metastases. However, the role of lipid metabolism that confers the aggressive properties of malignant cancers remains obscure. The present review is focused on key enzymes in lipid metabolism associated with metastatic disease pathogenesis. We also address the function of an important membrane structure-lipid raft in mediating tumor aggressive progression. We enumerate and integrate these recent findings into our current understanding of lipid metabolic reprogramming in cancer metastasis accompanied by new and exciting therapeutic implications.

A Unique High-Throughput Assay to Identify Novel Small Molecule Inhibitors of Chemotaxis and Migration

Chemotaxis and cell migration play pivotal roles in normal physiological processes such as embryogenesis, inflammation, and wound healing, as well as in pathological processes including chronic inflammatory disease and cancer metastasis. Novel chemotaxis/migration inhibitors are desirable for developing effective therapeutics and probing molecular mechanisms. We describe a fluorescence-based phenotypic assay in a 1536-well plate format for high-throughput screening of novel inhibitors of chemotaxis/migration within complex libraries of thousands of compounds. Although the assay utilizes the unique cellular response properties of Dictyostelium, the compounds identified are able to inhibit chemotaxis of mammalian cells. In addition, a parallel cell cytotoxicity counter-screen with an ATP content assay is described that eliminates cytotoxic compounds from the screen. This novel compound screening approach enables rapid identification of novel lead compounds that inhibit chemotaxis in human and other cells for drug development and research tools. © 2017 by John Wiley & Sons, Inc.

Phospholipase D1 Acts through Akt/TopBP1 and RB1 to Regulate the E2F1-Dependent Apoptotic Program in Cancer Cells

The RB1/E2F1 signaling pathway is frequently deregulated in colorectal cancer and has been suggested to intersect with Wnt/β-catenin and PI3K/Akt pathways, but molecular evidence for this link is lacking. In this study, we demonstrate that phospholipase D1 (PLD1), a transcriptional target of β-catenin/TCF4, orchestrates functional interactions between these pathways during intestinal tumor development. Overexpression of PLD1 in intestinal epithelial cells protected cells from apoptosis induced by PLD1 ablation in the Apc^min/+ mouse model of intestinal tumorigenesis. Mechanistic investigations revealed that genetic and pharmacologic targeting of PLD1 promote the E2F1-dependent apoptotic program via both miR-192/4465-mediated downregulation of RB1 and inhibition of Akt-TopBP1 pathways. Moreover, the miRNA-RB1 axis and Akt pathway also contributed to the PLD1-mediated self-renewal capacity of colon cancer-initiating cells. Finally, PLD1-driven E2F1 target gene expression positively correlated with tumor stage in patients with colorectal cancer. Overall, our findings suggest that PLD1 mediates cross-talk between multiple major signaling pathways to promote the survival and malignancy of colon cancer cells and may therefore represent an ideal signaling node for therapeutic targeting. Cancer Res; 77(1); 142-52. ©2016 AACR.

A central role for phosphatidic acid as a lipid mediator of regulated exocytosis in apicomplexa

Lipids are commonly known for the structural roles they play, however, the specific contribution of different lipid classes to wide-ranging signalling pathways is progressively being unravelled. Signalling lipids and their associated effector proteins are emerging as significant contributors to a vast array of effector functions within cells, including essential processes such as membrane fusion and vesicle exocytosis. Many phospholipids have signalling capacity, however, this review will focus on phosphatidic acid (PA) and the enzymes implicated in its production from diacylglycerol (DAG) and phosphatidylcholine (PC): DGK and PLD respectively. PA is a negatively charged, cone-shaped lipid identified as a key mediator in specific membrane fusion and vesicle exocytosis events in a variety of mammalian cells, and has recently been implicated in specialised secretory organelle exocytosis in apicomplexan parasites. This review summarises the recent work implicating a role for PA regulation in exocytosis in various cell types. We will discuss how these signalling events are linked to pathogenesis in the phylum Apicomplexa.

Therapeutic inhibition of phospholipase D1 suppresses hepatocellular carcinoma

Hepatocellular carcinoma (HCC) represents a leading cause of deaths worldwide. Novel therapeutic targets for HCC are needed. Phospholipase D (PD) is involved in cell proliferation and migration, but its role in HCC remains unclear. In the present study, we show that PLD1, but not PLD2, was overexpressed in HCC cell lines (HepG2, Bel-7402 and Bel-7404) compared with the normal human L-02 hepatocytes. PLD1 was required for the proliferation, migration and invasion of HCC cells without affecting apoptosis and necrosis, and PLD1 overexpression was sufficient to promote those effects. By using HCC xenograft models, we demonstrated that therapeutic inhibition of PLD1 attenuated tumour growth and epithelial-mesenchymal transition (EMT) in HCC mice. Moreover, PLD1 was found to be highly expressed in tumour tissues of HCC patients. Finally, mTOR (mechanistic target of rapamycin) and Akt (protein kinase B) were identified as critical pathways responsible for the role of PLD1 in HCC cells. Taken together, the present study indicates that PLD1 activation contributes to HCC development via regulation of the proliferation, migration and invasion of HCC cells, as well as promoting the EMT process. These observations suggest that inhibition of PLD1 represents an attractive and novel therapeutic modality for HCC.

A High-Throughput, Multi-Cell Phenotype Assay for the Identification of Novel Inhibitors of Chemotaxis/Migration

Chemotaxis and cell migration are fundamental, universal eukaryotic processes essential for biological functions such as embryogenesis, immunity, cell renewal, and wound healing, as well as for pathogenesis of many diseases including cancer metastasis and chronic inflammation. To identify novel chemotaxis inhibitors as probes for mechanistic studies and leads for development of new therapeutics, we developed a unique, unbiased phenotypic chemotaxis-dependent Dictyostelium aggregation assay for high-throughput screening using rapid, laser-scanning cytometry. Under defined conditions, individual Dictyostelium secrete chemoattractants, migrate, and aggregate. Chemotaxis is quantified by laser-scanning cytometry with a GFP marker expressed only in cells after chemotaxis/multi-cell aggregation. We applied the assay to screen 1,280 known compounds in a 1536-well plate format and identified two chemotaxis inhibitors. The chemotaxis inhibitory activities of both compounds were confirmed in both Dictyostelium and in human neutrophils in a directed EZ-TAXIscan chemotaxis assay. The compounds were also shown to inhibit migration of two human cancer cell lines in monolayer scratch assays. This test screen demonstrated that the miniaturized assay is extremely suited for high-throughput screening of very large libraries of small molecules to identify novel classes of chemotaxis/migratory inhibitors for drug development and research tools for targeting chemotactic pathways universal to humans and other systems.

Inhibition of phospholipaseD2 increases hypoxia-induced human colon cancer cell apoptosis through inactivating of the PI3K/AKT signaling pathway

Hypoxia is a common feature of solid tumor, and is a direct stress that triggers apoptosis in many human cell types. As one of solid cancer, hypoxia exists in the whole course of colon cancer occurrence and progression. Our previous studies shown that hypoxia induce high expression of phospholipase D2 (PLD2) and survivin in colon cancer cells. However, the correlation between PLD2 and survivin in hypoxic colon cancer cells remains unknown. In this study, we observed significantly elevated PLD2 and survivin expression levels in colon cancer tissues and cells. This is a positive correlation between of them, and co-expression of PLD2 and survivin has a positive correlation with the clinicpatholic features including tumor size, TNM stage, and lymph node metastasis. We also found that hypoxia induced the activity of PLD increased significant mainly caused by PLD2 in colon cancer cells. However, inhibition the activity of PLD2 induced by hypoxia promotes the apoptosis of human colon cancer cells, as well as decreased the expression of apoptosis markers including survivin and bcl2. Moreover, the pharmacological inhibition of PI3K/AKT supported the hypothesis that promotes the apoptosis of hypoxic colon cancer cells by PLD2 activity inhibition may through inactivation of the PI3K/AKT signaling pathway. Furthermore, interference the PLD2 gene expression leaded to the apoptosis of hypoxic colon cancer cells increased and also decreased the expression level of survivin and bcl2 may through inactivation of PI3K/AKT signaling pathway. These results indicated that PLD2 play antiapoptotic role in colon cancer under hypoxic conditions, inhibition of the activity, or interference of PLD2 gene expression will benefit for the treatment of colon cancer patients.

Targeting phospholipase D1 attenuates intestinal tumorigenesis by controlling β-catenin signaling in cancer-initiating cells

Kang et al. show that genetic or pharmacological inactivation of the enzyme phospholipase D1 (PLD1) disrupts colitis-associated intestinal tumorigenesis by suppressing the self-renewal capacity of colon cancer stem cells.

Expression of the Wnt target gene phospholipase D1 (PLD1) is up-regulated in various carcinomas, including colorectal cancer (CRC). However, the mechanistic significance of its elevated expression in intestinal tumorigenesis remains unknown. In this study, we show that genetic and pharmacological targeting of PLD1 disrupts spontaneous and colitis-associated intestinal tumorigenesis in Apc^Min/+ and azoxymethane/dextran sodium sulfate mice models. Intestinal epithelial cell–specific PLD1 overexpression in Apc^Min/+ mice accelerated tumorigenesis with increased proliferation and nuclear β-catenin levels compared with Apc^Min/+ mice. Moreover, PLD1 inactivation suppressed the self-renewal capacity of colon cancer–initiating cells (CC-ICs) by decreasing expression of β-catenin via E2F1-induced microRNA (miR)-4496 up-regulation. Ultimately, low expression of PLD1 coupled with a low level of CC-IC markers was predictive of a good prognosis in CRC patients, suggesting in vivo relevance. Collectively, our data reveal that PLD1 has a crucial role in intestinal tumorigenesis via its modulation of the E2F1–miR-4496–β-catenin signaling pathway. Modulation of PLD1 expression and activity represents a promising therapeutic strategy for the treatment of intestinal tumorigenesis.

Two sites of action for PLD2 inhibitors: The enzyme catalytic center and an allosteric, phosphoinositide biding pocket

Phospholipase D (PLD) has been implicated in many physiological functions, such as chemotaxis and phagocytosis, as well as pathological functions, such as cancer cell invasion and metastasis. New inhibitors have been described that hamper the role of PLD in those pathologies but their site of action is not known. We have characterized the biochemical and biological behavior of the PLD1/2 dual inhibitor 5-Fluoro-2-indolyl des-chlorohalopemide (FIPI), and the specific PLD2 inhibitor, N-[2-[1-(3-Fluorophenyl)-4-oxo-1,3,-8-triazaspiro[4.5]dec-8-yl]ethyl]-2-naphthalenecarboxamide (NFOT), and found that both FIPI and NFOT are mixed-kinetics inhibitors. Mutagenesis studies indicate that FIPI binds at S757 of PLD2, which is within the HKD2 catalytic site of the enzyme, whereas NFOT binds to PLD2 at two different sites, one being at S757/S648 and another to an allosteric site that is a natural site occupied by PIP2 (R210/R212). This latter site, along with F244/L245/L246, forms a hydrophobic pocket in the PH domain. The mechanism of action of FIPI is a direct effect on the catalytic site (and as such inhibits both PLD1 and PLD2 isoforms), whereas PLD2 affects both the catalytic site (orthosteric) and blocks PIP2 binding to PLD2 (allosteric), which negates the natural enhancing role of PIP2. Moreover, NFOT prevents cell invasion of cancer cells, which does not occur in cells overexpressing PLD2-F244A/L245A/L246A, or PLD2-R210A/R212A, or PLD2-S757/S648 mutants. This study provides new specific knowledge of enzyme regulation and mechanisms of activation and inhibition of PLD2 that are necessary to understand its role in cell signaling and to develop new inhibitors for cancer cell invasion and metastasis.

Phospholipase D activity underlies very-low-density lipoprotein (VLDL)-induced aldosterone production in adrenal glomerulosa cells

Aldosterone is the mineralocorticoid responsible for sodium retention, thus increased blood volume and pressure. Excessive production of aldosterone results in high blood pressure as well as renal disease, stroke, and visual loss via both direct effects and effects on blood pressure. Weight gain is often associated with increased blood pressure, but it remains unclear how obesity increases blood pressure. Obese patients typically have higher lipoprotein levels; moreover, some studies have suggested that aldosterone levels are also elevated and represent a link between obesity and hypertension. Very-low-density lipoprotein (VLDL) functions to transport triglycerides from the liver to peripheral tissues. Although previous studies have demonstrated that VLDL can stimulate aldosterone production, the mechanisms underlying this effect are largely unclear. Here we show for the first time that phospholipase D (PLD) is involved in VLDL-induced aldosterone production in both a human adrenocortical cell line (HAC15) and primary cultures of bovine zona glomerulosa cells. Our data also reveal that PLD mediates steroidogenic acute regulatory (StAR) protein and aldosterone synthase (CYP11B2) expression via increasing the phosphorylation (activation) of their regulatory transcription factors. Finally, by using selective PLD inhibitors, our studies suggest that both PLD1 and PLD2 isoforms play an important role in VLDL-induced aldosterone production.

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.

The role of phospholipase D1 in liver fibrosis induced by dimethylnitrosamine in vivo

BACKGROUND

Phospholipase D (PLD) has been proved to be involved in regulating function of fibroblasts and might play a role in mediating organic fibrosis.

AIMS

To investigate the role and mechanism of PLD on dimethylnitrosamine (DMN)-induced rat liver fibrosis.

METHODS

Fifty-five male Wistar rats were divided into normal control group, DMN model group, N-methylethanolamine (MEA) control group, and MEA-intervention group. We observed the effects of MEA, a PLD inhibitor on the development and progression of rat liver fibrosis by comparing the physical and biochemical indexes, tissue pathology, PLD activity, and typical markers and cytokines related to fibrosis in the four groups.

RESULTS

Accompanied by the down-regulation of PLD1 expression, the MEA-intervention group had improved outcomes compared with the DMN model group in terms of spleen weight, spleen/weight index, serum and tissue biochemical indexes, tissue hydroxyproline, and tissue pathology. The MEA-intervention group had lower TIMP1, COL1A1, and higher MMPs expression level than the DMN model group. The activity of PLD and PLD1, α-SMA expression level in the MEA-intervention group was much lower than those in the DMN model group. There was no significant difference between the two groups in the expression level of TGF-β1 and MCP1. Meanwhile, there were no significant differences between normal control group and MEA control group in the parameters stated above.

CONCLUSION

Phospholipase D1 may play an important role in the development and progression of rat liver fibrosis. Inhibition of PLD may become a new strategy to prevent or alleviate liver fibrosis.

Mitogenic effects of phosphatidylcholine nanoparticles on MCF-7 breast cancer cells

Lecithins, mainly composed of the phospholipids phosphatidylcholines (PC), have many different uses in the pharmaceutical and clinical field. PC are involved in structural and biological functions as membrane trafficking processes and cellular signaling. Considering the increasing applications of lecithin-based nanosystems for the delivery of therapeutic agents, the aim of the present work was to determine the effects of phosphatidylcholine nanoparticles over breast cancer cellular proliferation and signaling. PC dispersions at 0.01 and 0.1% (w/v) prepared in buffer pH 7.0 and 5.0 were studied in the MCF-7 breast cancer cell line. Neutral 0.1% PC-derived nanoparticles induced the activation of the MEK-ERK1/2 pathway, increased cell viability and induced a 1.2 fold raise in proliferation. These biological effects correlated with the increase of epidermal growth factor receptor (EGFR) content and its altered cellular localization. Results suggest that nanoparticles derived from PC dispersion prepared in buffer pH 7.0 may induce physicochemical changes in the plasma membrane of cancer cells which may affect EGFR cellular localization and/or activity, increasing activation of the MEK-ERK1/2 pathway and inducing proliferation. Results from the present study suggest that possible biological effects of delivery systems based on lecithin nanoparticles should be taken into account in pharmaceutical formulation design.

Epidermal growth factor receptor endocytic traffic perturbation by phosphatidate phosphohydrolase inhibition: new strategy against cancer

Epidermal growth factor receptor (EGFR) exaggerated (oncogenic) function is currently targeted in cancer treatment with drugs that block receptor ligand binding or tyrosine kinase activity. Because endocytic trafficking is a crucial regulator of EGFR function, its pharmacological perturbation might provide a new anti-tumoral strategy. Inhibition of phosphatidic acid (PA) phosphohydrolase (PAP) activity has been shown to trigger PA signaling towards type 4 phosphodiesterase (PDE4) activation and protein kinase A inhibition, leading to internalization of empty/inactive EGFR. Here, we used propranolol, its l- and d- isomers and desipramine as PAP inhibitors to further explore the effects of PAP inhibition on EGFR endocytic trafficking and its consequences on EGFR-dependent cancer cell line models. PAP inhibition not only made EGFR inaccessible to stimuli but also prolonged the signaling lifetime of ligand-activated EGFR in recycling endosomes. Strikingly, such endocytic perturbations applied in acute/intermittent PAP inhibitor treatments selectively impaired cell proliferation/viability sustained by an exaggerated EGFR function. Phospholipase D inhibition with FIPI (5-fluoro-2-indolyl des-chlorohalopemide) and PDE4 inhibition with rolipram abrogated both the anti-tumoral and endocytic effects of PAP inhibition. Prolonged treatments with a low concentration of PAP inhibitors, although without detectable endocytic effects, still counteracted cell proliferation, induced apoptosis and decreased anchorage-independent growth of cells bearing EGFR oncogenic influences. Overall, our results show that PAP inhibitors can counteract EGFR oncogenic traits, including receptor overexpression or activating mutations resistant to current tyrosine kinase inhibitors, perturbing EGFR endocytic trafficking and perhaps other as yet unknown processes, depending on treatment conditions. This puts PAP activity forward as a new suitable target against EGFR-driven malignancy.

Phospholipase D1 and choline kinase-α are interactive targets in breast cancer

A consistent metabolic hallmark observed in multiple cancers is the increase of cellular phosphocholine (PC) and total choline-containing compounds (tCho), which is closely related to malignant transformation, invasion, and metastasis. Enzymes in choline phospholipid metabolism present attractive targets to exploit for treatment, but require a clear understanding of the mechanisms underlying the altered choline phospholipid metabolism observed in cancer. Choline kinase-α (Chk-α) is an enzyme in the Kennedy pathway that phosphorylates free choline (Cho) to PC, and its upregulation in several cancers is a major contributor to increased PC levels. Similarly, increased expression and activity of phospholipase D1 (PLD1), which converts phosphatidylcholine (PtdCho) to phosphatidic acid (PA) and Cho, has been well documented in gastric, ovarian and breast cancer. Here we report a strong correlation between expression of Chk-α and PLD1 with breast cancer malignancy. Data from patient samples established an association between estrogen receptor (ER) status and Chk-α and PLD1 expression. In addition, these two enzymes were found to be interactive. Downregulation of Chk-α with siRNA increased PLD1 expression, and downregulation of PLD1 increased Chk-α expression. Simultaneous silencing of PLD1 and Chk-α in MDA-MB-231 cells increased apoptosis as detected by the TUNEL assay. These data provide new insights into choline phospholipid metabolism of breast cancer, and support multiple targeting of enzymes in choline phospholipid metabolism as a strategy for treatment.

Chemical modulation of glycerolipid signaling and metabolic pathways

Thirty years ago, glycerolipids captured the attention of biochemical researchers as novel cellular signaling entities. We now recognize that these biomolecules occupy signaling nodes critical to a number of physiological and pathological processes. Thus, glycerolipid-metabolizing enzymes present attractive targets for new therapies. A number of fields-ranging from neuroscience and cancer to diabetes and obesity-have elucidated the signaling properties of glycerolipids. The biochemical literature teems with newly emerging small molecule inhibitors capable of manipulating glycerolipid metabolism and signaling. This ever-expanding pool of chemical modulators appears daunting to those interested in exploiting glycerolipid-signaling pathways in their model system of choice. This review distills the current body of literature surrounding glycerolipid metabolism into a more approachable format, facilitating the application of small molecule inhibitors to novel systems. This article is part of a Special Issue entitled Tools to study lipid functions.

Preparation of primary cultures of mouse epidermal keratinocytes and the measurement of phospholipase D activity

In this chapter information is provided about the outer layer of the skin, the epidermis, and the predominant cells comprising this epithelium, the keratinocytes. The evidence supporting a possible role for the lipid-metabolizing enzyme phospholipase D in regulating keratinocyte differentiation is also discussed. A detailed protocol for the preparation of primary cultures of epidermal keratinocytes from neonatal mice is described, to allow other investigators to obtain data concerning these important cells involved in forming and maintaining the mechanical and water permeability of the skin. Finally, a complete protocol for monitoring phospholipase D activity in intact cells is supplied in the hope that additional research will result in a better understanding of the role of phospholipase D in controlling keratinocyte proliferation and differentiation.

Caffeic acid phenethyl ester downregulates phospholipase D1 via direct binding and inhibition of NFκB transactivation

Upregulation of phospholipase D (PLD) is functionally linked with oncogenic signals and tumorigenesis. Caffeic acid phenethyl ester (CAPE) is an active compound of propolis extract that exhibits anti-proliferative, anti-inflammatory, anti-oxidant, and antineoplastic properties. In this study, we demonstrated that CAPE suppressed the expression of PLD1 at the transcriptional level via inhibition of binding of NFκB to PLD1 promoter. Moreover, CAPE, but not its analogs, bound to a Cys837 residue of PLD1 and inhibited enzymatic activity of PLD. CAPE also decreased activation of matrix metalloproteinases-2 induced by phosphatidic acid, a product of PLD activity. Ultimately, CAPE-induced downregulation of PLD1 suppressed invasion and proliferation of glioma cells. Taken together, the results of this study indicate that CAPE might contribute to anti-neoplastic effect by targeting PLD1.

Phosphatidic acid, phospholipase D and tumorigenesis

Phospholipase D (PLD) is a membrane protein with a double role: maintenance of the structural integrity of cellular or intracellular membranes and involvement in cell signaling through the product of the catalytic reaction, PA, and through protein-protein interaction with a variety of partners. Cross-talk during PLD signaling occurs with other cancer regulators (Ras, PDGF, TGF and kinases). Elevation of either PLD1 or PLD2 (the two mammalian isoforms of PLD) is able to transform fibroblasts and contribute to cancer progression. Elevated total PLD activity, as well as overexpression, is present in a wide variety of cancers such as gastric, colorectal, renal, stomach, esophagus, lung and breast. PLD provides survival signals and is involved in migration, adhesion and invasion of cancer cells, and all are increased during PLD upregulation or, conversely, they are decreased during PLD loss of function. Eventhough the end results of PLD action as relates to downstream signaling mechanisms are still currently being elucidated, invasion, a pre-requisite for metastasis, is directly affected by PLD. This review will introduce the classical mammalian PLD's, PLD1 and PLD2, followed by the mechanisms of intracellular regulation and a status of current investigation in the crucial involvement of PLD in cancer, mostly through its role in cell migration, invasion and metastasis, that has grown exponentially in the last few years.

Pharmacological Inhibition of Phospholipase D Protects Mice From Occlusive Thrombus Formation and Ischemic Stroke—Brief Report

Objective—

We recently showed that mice lacking the lipid signaling enzyme phospholipase (PL) D1 or both PLD isoforms (PLD1 and PLD2) were protected from pathological thrombus formation and ischemic stroke, whereas hemostasis was not impaired in these animals. We sought to assess whether pharmacological inhibition of PLD activity affects hemostasis, thrombosis, and thrombo-inflammatory brain infarction in mice.

Approach and Results—

Treatment of platelets with the reversible, small molecule PLD inhibitor, 5-fluoro-2-indolyl des-chlorohalopemide (FIPI), led to a specific blockade of PLD activity that was associated with reduced α-granule release and integrin activation. Mice that received FIPI at a dose of 3 mg/kg displayed reduced occlusive thrombus formation upon chemical injury of carotid arteries or mesenterial arterioles. Similarly, FIPI-treated mice had smaller infarct sizes and significantly better motor and neurological function 24 hours after transient middle cerebral artery occlusion. This protective effect was not associated with major intracerebral hemorrhage or prolonged tail bleeding times.

Conclusions—

These results provide the first evidence that pharmacological PLD inhibition might provide a safe therapeutic strategy to prevent arterial thrombosis and ischemic stroke.

Phospholipase D inhibitor enhances radiosensitivity of breast cancer cells

Radiation and drug resistance remain the major challenges and causes of mortality in the treatment of locally advanced, recurrent and metastatic breast cancer. Dysregulation of phospholipase D (PLD) has been found in several human cancers and is associated with resistance to anticancer drugs. In the present study, we evaluated the effects of PLD inhibition on cell survival, cell death and DNA damage after exposure to ionizing radiation (IR). Combined IR treatment and PLD inhibition led to an increase in the radiation-induced apoptosis of MDA-MB-231 metastatic breast cancer cells. The selective inhibition of PLD1 and PLD2 led to a significant decrease in the IR-induced colony formation of breast cancer cells. Moreover, PLD inhibition suppressed the radiation-induced activation of extracellular signal-regulated kinase and enhanced the radiation-stimulated phosphorylation of the mitogen-activated protein kinases p38 and c-Jun N-terminal kinase. Furthermore, PLD inhibition, in combination with radiation, was very effective at inducing DNA damage, when compared with radiation alone. Taken together, these results suggest that PLD may be a useful target molecule for the enhancement of the radiotherapy effect.

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.

Regulation of phospholipase D activity and phosphatidic acid production after purinergic (P2Y6) receptor stimulation

Phosphatidic acid (PA) is a lipid second messenger located at the intersection of several lipid metabolism and cell signaling events including membrane trafficking, survival, and proliferation. Generation of signaling PA has long been primarily attributed to the activation of phospholipase D (PLD). PLD catalyzes the hydrolysis of phosphatidylcholine into PA. A variety of both receptor-tyrosine kinase and G-protein-coupled receptor stimulations have been shown to lead to PLD activation and PA generation. This study focuses on profiling the PA pool upon P2Y6 receptor signaling manipulation to determine the major PA producing enzymes. Here we show that PLD, although highly active, is not responsible for the majority of stable PA being produced upon UDP stimulation of the P2Y6 receptor and that PA levels are tightly regulated. By following PA flux in the cell we show that PLD is involved in an initial increase in PA upon receptor stimulation; however, when PLD is blocked, the cell compensates by increasing PA production from other sources. We further delineate the P2Y6 signaling pathway showing that phospholipase Cβ3 (PLCβ3), PLCδ1, DGKζ and PLD are all downstream of receptor activation. We also show that DGKζ is a novel negative regulator of PLD activity in this system that occurs through an inhibitory mechanism with PKCα. These results further define the downstream events resulting in PA production in the P2Y6 receptor signaling pathway.

Phospholipase D1 has a pivotal role in interleukin-1β-driven chronic autoimmune arthritis through regulation of NF-κB, hypoxia-inducible factor 1α, and FoxO3a

Interleukin-1β (IL-1β) is a potent proinflammatory and immunoregulatory cytokine playing an important role in the progression of rheumatoid arthritis (RA). However, the signaling network of IL-1β in synoviocytes from RA patients is still poorly understood. Here, we show for the first time that phospholipase D1 (PLD1), but not PLD2, is selectively upregulated in IL-1β-stimulated synoviocytes, as well as synovium, from RA patients. IL-1β enhanced the binding of NF-κB and ATF-2 to the PLD1 promoter, thereby enhancing PLD1 expression. PLD1 inhibition abolished the IL-1β-induced expression of proinflammatory mediators and angiogenic factors by suppressing the binding of NF-κB or hypoxia-inducible factor 1α to the promoter of its target genes, as well as IL-1β-induced proliferation or migration. However, suppression of PLD1 activity promoted cell cycle arrest via transactivation of FoxO3a. Furthermore, PLD1 inhibitor significantly suppressed joint inflammation and destruction in IL-1 receptor antagonist-deficient (IL-1Ra(-/-)) mice, a model of spontaneous arthritis. Taken together, these results suggest that the abnormal upregulation of PLD1 may contribute to the pathogenesis of IL-1β-induced chronic arthritis and that a selective PLD1 inhibitor might provide a potential therapeutic molecule for the treatment of chronic inflammatory autoimmune disorders.

Identification of potential biomarkers from microarray experiments using multiple criteria optimization

Microarray experiments are capable of determining the relative expression of tens of thousands of genes simultaneously, thus resulting in very large databases. The analysis of these databases and the extraction of biologically relevant knowledge from them are challenging tasks. The identification of potential cancer biomarker genes is one of the most important aims for microarray analysis and, as such, has been widely targeted in the literature. However, identifying a set of these genes consistently across different experiments, researches, microarray platforms, or cancer types is still an elusive endeavor. Besides the inherent difficulty of the large and nonconstant variability in these experiments and the incommensurability between different microarray technologies, there is the issue of the users having to adjust a series of parameters that significantly affect the outcome of the analyses and that do not have a biological or medical meaning. In this study, the identification of potential cancer biomarkers from microarray data is casted as a multiple criteria optimization (MCO) problem. The efficient solutions to this problem, found here through data envelopment analysis (DEA), are associated to genes that are proposed as potential cancer biomarkers. The method does not require any parameter adjustment by the user, and thus fosters repeatability. The approach also allows the analysis of different microarray experiments, microarray platforms, and cancer types simultaneously. The results include the analysis of three publicly available microarray databases related to cervix cancer. This study points to the feasibility of modeling the selection of potential cancer biomarkers from microarray data as an MCO problem and solve it using DEA. Using MCO entails a new optic to the identification of potential cancer biomarkers as it does not require the definition of a threshold value to establish significance for a particular gene and the selection of a normalization procedure to compare different experiments is no longer necessary.

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

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

Key roles for the lipid signaling enzyme phospholipase d1 in the tumor microenvironment during tumor angiogenesis and metastasis

Angiogenesis inhibitors, which target tumor cells, confer only short-term benefits on tumor growth. We report that ablation of the lipid signaling enzyme phospholipase D1 (PLD1) in the tumor environment compromised the neovascularization and growth of tumors. PLD1 deficiency suppressed the activation of Akt and mitogen-activated protein kinase signaling pathways by vascular endothelial growth factor in vascular endothelial cells, resulting in decreased integrin-dependent cell adhesion to, and migration on, extracellular matrices, as well as reduced tumor angiogenesis in a xenograft model. In addition, mice lacking PLD1 incurred fewer lung metastases than did wild-type mice. Bone marrow transplantation and binding studies identified a platelet-derived mechanism involving decreased tumor cell-platelet interactions, in part because of impaired activation of αIIbβ3 integrin in platelets, which decreased the seeding of tumor cells into the lung parenchyma. Treatment with a small-molecule inhibitor of PLD1 phenocopied PLD1 deficiency, efficiently suppressing both tumor growth and metastasis in mice. These findings reveal that PLD1 in the tumor environment promotes tumor growth and metastasis and, taken together with previous reports on the roles of PLD in tumor cell-intrinsic adaptations to stress, suggest the potential use of PLD inhibitors as cancer therapeutics.

Choline-releasing glycerophosphodiesterase EDI3 links the tumor metabolome to signaling network activities

Recently, EDI3 was identified as a key factor for choline metabolism that controls tumor cell migration and is associated with metastasis in endometrial carcinomas. EDI3 cleaves glycerophosphocholine (GPC) to form choline and glycerol-3-phosphate (G3P). Choline is then further metabolized to phosphatidylcholine (PtdC), the major lipid in membranes and a key player in membrane-mediated cell signaling. The second product, G3P, is a precursor molecule for several lipids with central roles in signaling, for example lysophosphatidic acid (LPA), phosphatidic acid (PA) and diacylglycerol (DAG). LPA activates intracellular signaling pathways by binding to specific LPA receptors, including membrane-bound G protein-coupled receptors and the intracellular nuclear receptor, PPARγ. Conversely, PA and DAG mediate signaling by acting as lipid anchors that bind and activate several signaling proteins. For example, binding of GTPases and PKC to PA and DAG, respectively, increases the activation of signaling networks, mediating processes such as migration, adhesion, proliferation or anti-apoptosis—all relevant for tumor development. We present a concept by which EDI3 either directly generates signaling molecules or provides “membrane anchors” for downstream signaling factors. As a result, EDI3 links choline metabolism to signaling activities resulting in a more malignant phenotype.