Cellular and physiological roles for phospholipase D1 in cancer

A positive feedback loop between PLD1 and NF-κB signaling promotes tumorigenesis of nasopharyngeal carcinoma

Phospholipase D (PLD) lipid-signaling enzyme superfamily has been widely implicated in various human malignancies, but its role and underlying mechanism remain unclear in nasopharyngeal carcinoma (NPC). Here, we analyze the expressions of 6 PLD family members between 87 NPC and 10 control samples through transcriptome analysis. Our findings reveal a notable upregulation of PLD1 in both NPC tumors and cell lines, correlating with worse disease-free and overall survival in NPC patients. Functional assays further elucidate the oncogenic role of PLD1, demonstrating its pivotal promotion of critical tumorigenic processes such as cell proliferation and migration in vitro, as well as tumor growth in vivo. Notably, our study uncovers a positive feedback loop between PLD1 and the NF-κB signaling pathway to render NPC progression. Specifically, PLD1 enhances NF-κB activity by facilitating the phosphorylation and nuclear translocation of RELA, which in turn binds to the promoter of PLD1, augmenting its expression. Moreover, RELA overexpression markedly rescues the inhibitory effects in PLD1-depleted NPC cells. Importantly, the application of the PLD1 inhibitor, VU0155069, substantially inhibits NPC tumorigenesis in a patient-derived xenograft model. Together, our findings identify PLD1/NF-κB signaling as a positive feedback loop with promising therapeutic and prognostic potential in NPC.

The wide world of non-mammalian phospholipase D enzymes

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

Murine leukemia virus (MLV) P50 protein induces cell transformation via transcriptional regulatory function

BACKGROUND

The murine leukemia virus (MLV) has been a powerful model of pathogenesis for the discovery of genes involved in cancer. Its splice donor (SD')-associated retroelement (SDARE) is important for infectivity and tumorigenesis, but the mechanism remains poorly characterized. Here, we show for the first time that P50 protein, which is produced from SDARE, acts as an accessory protein that transregulates transcription and induces cell transformation.

RESULTS

By infecting cells with MLV particles containing SDARE transcript alone (lacking genomic RNA), we show that SDARE can spread to neighbouring cells as shown by the presence of P50 in infected cells. Furthermore, a role for P50 in cell transformation was demonstrated by CCK8, TUNEL and anchorage-independent growth assays. We identified the integrase domain of P50 as being responsible for transregulation of the MLV promoter using luciferase assay and RTqPCR with P50 deleted mutants. Transcriptomic analysis furthermore revealed that the expression of hundreds of cellular RNAs involved in cancerogenesis were deregulated in the presence of P50, suggesting that P50 induces carcinogenic processes via its transcriptional regulatory function.

CONCLUSION

We propose a novel SDARE-mediated mode of propagation of the P50 accessory protein in surrounding cells. Moreover, due to its transforming properties, P50 expression could lead to a cellular and tissue microenvironment that is conducive to cancer development.

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.

Phosphatidylserine-Specific Phospholipase A1 Limits Aggressiveness of Lung Adenocarcinoma by Lysophosphatidylserine and Protein Kinase A-Dependent Pathway

Lipid metabolic abnormalities in cancer cells are increasingly being studied. Several studies have reported that phosphatidylserine-specific phospholipase A1 (PLA1A) might be involved in the pathogenesis of cancers. Nevertheless, the function and mechanistic details of PLA1A in lung adenocarcinoma (LUAD) progression remain largely undefined. In the present study, low PLA1A expression was correlated with poor prognosis in patients with LUAD. Results from in vitro and in vivo animal studies showed that overexpressed PLA1A suppressed the proliferation of LUAD cells in vitro and tumor growth in vivo through regulation of cyclin abundance, thereby inducing S-phase arrest. Meanwhile, PLA1A overexpression attenuated migration and invasion of LUAD cells, including by inhibiting the epithelial-mesenchymal transition. Mechanistically, PLA1A overexpression inhibited aggressiveness of LUAD cells through elevated lysophosphatidylserine, which acts via G-protein-coupled receptor 174, further activating cAMP/protein kinase A pathway. Activating G-protein-coupled receptor 174/protein kinase A pathway may involve effects on cell cycle regulators and transcription factors-regulated epithelial-mesenchymal transition. The study uncovered the mechanism through which PLA1A regulates LUAD proliferation, invasion, and migration. These results demonstrate the potential use of PLA1A as a biomarker for diagnosing LUAD, which may therefore potentially serve as a therapeutic target for LUAD.

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.

Blockade of PLD1 potentiates the antitumor effects of bortezomib in multiple myeloma cells by inhibiting the mTOR/NF-κB signal pathway

OBJECTIVE

Phospholipase D1 (PLD1) is an enzyme of the phospholipase D (PLD) superfamily. It is involved in the occurrence of various tumors. However, its role in multiple myeloma (MM) remained undefined. This study aimed to investigate the mechanism of PLD1 in the therapy of myeloma disease.

MATERIAL AND METHODS

Cell lines U266 and H929 were treated with PLD1 specific inhibitor VU0359595 combined bortezomib, a proteasome inhibitor. Their effects on MM cell proliferation, apoptosis, and relevant signal pathways of apoptosis were determined by cell counting kit-8 (CCK-8), real-time polymerase reaction chain (RT-PCR), ATP assay, and western blot.

RESULTS

PLD1 was highly expressed in U266 and H929 cells. VU0359595 didn't affect the proliferation and apoptosis of MM cells. However, VU0359595 could enhance growth inhibition, decreasing mitochondrial membrane potentials (MMPs) and ATP levels of bortezomib treated MM cells. VU0359595 also strengthened bortezomib-induced apoptosis via activating caspase-8, caspase-9, caspase-3; and down-regulating the expressions of anti-apoptosis proteins BCL-2. In addition, the bortezomib-induced cytotoxicity on MM cells was significantly augmented by VU0359595 through efficient suppression of the mTOR/NF-κB signal pathway.

CONCLUSION

PLD1 inhibition can remarkably exert antitumor effects with bortezomib on MM, which is a novel potentially targeting therapeutic agent, especially for drug-resistant MM patients.

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

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

Hormones Secretion and Rho GTPases in Neuroendocrine Tumors

Neuroendocrine tumors (NETs) belong to a heterogeneous group of neoplasms arising from hormone secreting cells. These tumors are often associated with a dysfunction of their secretory activity. Neuroendocrine secretion occurs through calcium-regulated exocytosis, a process that is tightly controlled by Rho GTPases family members. In this review, we compiled the numerous mutations and modification of expression levels of Rho GTPases or their regulators (Rho guanine nucleotide-exchange factors and Rho GTPase-activating proteins) that have been identified in NETs. We discussed how they might regulate neuroendocrine secretion.

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.

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.

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.

[Phospholipase D: its role in metabolism processes and disease development]

Phospholipase D (PLD) is one of the key enzymes that catalyzes the hydrolysis of cell membrane phospholipids. In this review current knowledge about six human PLD isoforms, their structure and role in physiological and pathological processes is summarized. Comparative analysis of PLD isoforms structure is presented. The mechanism of the hydrolysis and transphosphatidylation performed by PLD is described. The PLD1 and PLD2 role in the pathogenesis of some cancer, infectious, thrombotic and neurodegenerative diseases is analyzed. The prospects of PLD isoform-selective inhibitors development are shown in the context of the clinical usage and the already-existing inhibitors are characterized. Moreover, the formation of phosphatidylethanol (PEth), the alcohol abuse biomarker, as the result of PLD-catalyzed phospholipid transphosphatidylation is considered.

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.

Phospholipase D and Choline Metabolism

Phospholipases D (PLDs) catalyze hydrolysis of the diester bond of phospholipids to generate phosphatidic acid and the free lipid headgroup. In mammals, PLD enzymes comprise the intracellular enzymes PLD1 and PLD2 and possibly the proteins encoded by related genes, as well as a class of cell surface and secreted enzymes with structural homology to ectonucleotide phosphatases/phosphodiesterases as typified by autotaxin (ENPP2) that have lysoPLD activities. Genetic and pharmacological loss-of-function approaches implicate these enzymes in intra- and intercellular signaling mediated by the lipid products phosphatidic acid, lysophosphatidic acid, and their metabolites, while the possibility that the water-soluble product of their reactions is biologically relevant has received far less attention. PLD1 and PLD2 are highly selective for phosphatidylcholine (PC), whereas autotaxin has broader substrate specificity for lysophospholipids but by virtue of the high abundance of lysophosphatidylcholine (LPC) in extracellular fluids predominantly hydrolyses this substrate. In all cases, the water-soluble product of these PLD activities is choline. Although choline can be formed de novo by methylation of phosphatidylethanolamine, this activity is absent in most tissues, so mammals are effectively auxotrophic for choline. Dietary consumption of choline in both free and esterified forms is substantial. Choline is necessary for synthesis of the neurotransmitter acetylcholine and of the choline-containing phospholipids PC and sphingomyelin (SM) and also plays a recently appreciated important role as a methyl donor in the pathways of "one-carbon (1C)" metabolism. This review discusses emerging evidence that some of the biological functions of these intra- and extracellular PLD enzymes involve generation of choline with a particular focus on the possibility that these choline and PLD dependent processes are dysregulated in cancer.

Inhibition of Phospholipase D1 mRNA Expression Slows Down the Proliferation Rate of Prostate Cancer Cells That Have Transited to Androgen Independence

To explore the role of phospholipase D1 (PLD1) mRNA in transition of prostate cancer (PCa) cells to androgen independence, we used Arraystar Human LncRNA Microarray V3.0 to detect and compare the differential expression of PLD1 and its signaling pathway-related gene in standard androgen dependence prostate cancer (ADPC) cell line LNCaP before and after the occurrence of androgen independence prostate cancer (AIPC) transition. In addition, we used the shRNA lentiviral vector to inhibit the PLD1 mRNA expression and observed its effect on LNCaP cell proliferation after AIPC transition by using MTS method. The results showed that the expression level of PLD1 mRNA was increased by 373-fold after AIPC transition (P<0.05); the PI3K/AKT signaling pathway-related gene expression was also elevated (P<0.05); the growth rate of LNCaP cells that had transited to androgen independence was reduced by about 30% when the PLD1 mRNA expression was inhibited by the shRNA lentivirus as compared with the negative control group (P<0.05). All these results suggest that PLD1 mRNA and the related PI3K/AKT signaling pathway may play an important role in AIPC. Down-regulating the expression of PLD1 mRNA could to some extent inhibit the proliferation rate of PCa cells after AIPC transition.

A Chemoenzymatic Strategy for Imaging Cellular Phosphatidic Acid Synthesis

Phosphatidic acid (PA) is a potent lipid secondary messenger, the synthesis of which is tightly regulated in both space and time. Established tools for detecting PA involve ex vivo analysis and do not provide information on the subcellular locations where this lipid is synthesized. Here, a chemoenzymatic strategy for imaging sites of cellular PA synthesis by phospholipase D (PLD) enzymes is reported. PLDs were found to be able to catalyze phospholipid head-group exchange with alkynols to generate alkyne-labeled PA analogues within cells. Subsequent fluorophore tagging through Cu-catalyzed azide-alkyne cycloaddition enabled both visualization by fluorescence microscopy and quantification by HPLC. Our studies revealed several intracellular sites of PLD-mediated PA synthesis. We envision applications of this approach to dissect PA-dependent signaling pathways, image PLD activity in disease, and remodel intracellular membranes with new functionality.

Phospholipase D1 regulation of TNF-alpha protects against responses to LPS

Sepsis is a systemic inflammatory disorder with organ dysfunction and represents the leading cause of mortality in non-coronary intensive care units. A key player in septic shock is Tumor Necrosis Factor-alpha (TNF-α). Phospholipase (PL)D1 is involved in the regulation of TNF-α upon ischemia/reperfusion injury in mice. In this study we analyzed the impact of PLD1 in the regulation of TNF-α, inflammation and organ damage in experimental sepsis. PLD1 deficiency increased survival of mice and decreased vital organ damage after LPS injections. Decreased TNF-α plasma levels and reduced migration of leukocytes and platelets into lungs was associated with reduced apoptosis in lung and liver tissue of PLD1 deficient mice. PLD1 deficient platelets contribute to preserved outcome after LPS-induced sepsis because platelets exhibit an integrin activation defect suggesting reduced platelet activation in PLD1 deficient mice. Furthermore, reduced thrombin generation of PLD1 deficient platelets might be responsible for reduced fibrin formation in lungs suggesting reduced disseminated intravascular coagulation (DIC). The analysis of Pld1^fl/fl-PF4-Cre mice revealed that migration of neutrophils and cell apoptosis in septic animals is not due to platelet-mediated processes. The present study has identified PLD1 as a regulator of innate immunity that may be a new target to modulate sepsis.

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.

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.

Clickable Substrate Mimics Enable Imaging of Phospholipase D Activity

Chemical imaging techniques have played instrumental roles in dissecting the spatiotemporal regulation of signal transduction pathways. Phospholipase D (PLD) enzymes affect cell signaling by producing the pleiotropic lipid second messenger phosphatidic acid via hydrolysis of phosphatidylcholine. It remains a mystery how this one lipid signal can cause such diverse physiological and pathological signaling outcomes, due in large part to a lack of suitable tools for visualizing the spatial and temporal dynamics of its production within cells. Here, we report a chemical method for imaging phosphatidic acid synthesis by PLD enzymes in live cells. Our approach capitalizes upon the enzymatic promiscuity of PLDs, which we show can accept azidoalcohols as reporters in a transphosphatidylation reaction. The resultant azidolipids are then fluorescently tagged using the strain-promoted azide-alkyne cycloaddition, enabling visualization of cellular membranes bearing active PLD enzymes. Our method, termed IMPACT (Imaging Phospholipase D Activity with Clickable Alcohols via Transphosphatidylation), reveals pools of basal and stimulated PLD activities in expected and unexpected locations. As well, we reveal a striking heterogeneity in PLD activities at both the cellular and subcellular levels. Collectively, our studies highlight the importance of using chemical tools to directly visualize, with high spatial and temporal resolution, the subset of signaling enzymes that are active.

PLD1 overexpression promotes invasion and migration and function as a risk factor for Chinese glioma patients

Glioma is a lethal disease with few effective therapeutic options. Recently, insights into cancer biology had suggested that abnormal lipid metabolism was a risk factor for various human malignancies, including glioma. As a key enzyme implicated in lipid metabolism, PLD1 was overexpression in multiple human cancers, and it was stated to be responsible for aggressive phenotypes, such as angiogenesis and chemoresistance. However, there was still much to know about its expression and function in glioma. In the present study, we showed that PLD1 was overexpression in clinical samples of glioma. In addition, the correlation assay revealed that PLD1 overexpression was correlated with poor differentiation (p = 0.04), and it was responsible for a poor prognosis for the patients (p = 0.009). Furthermore, we showed in COX regression assay that PLD1 was a risk factor for glioma (p = 0.018, HR = 0.461, 95% CI = 0.243–0.887). Consistently, we found that PLD1 was overexpression in glioma cell lines, and it could facilitate the proliferation and migration. Taken together, our study suggested that PLD1 was pro-tumoral in glioma, and that further studies were urgently needed so as to define whether it was a novel therapeutic target for the disease.

24h-gene variation effect of combined bevacizumab/erlotinib in advanced non-squamous non-small cell lung cancer using exon array blood profiling

Background

The SAKK 19/05 trial investigated the safety and efficacy of the combined targeted therapy bevacizumab and erlotinib (BE) in unselected patients with advanced non-squamous non-small cell lung cancer (NSCLC). Although activating EGFR mutations were the strongest predictors of the response to BE, some patients not harboring driver mutations could benefit from the combined therapy. The identification of predictive biomarkers before or short after initiation of therapy is therefore paramount for proper patient selection, especially among EGFR wild-types. The first aim of this study was to investigate the early change in blood gene expression in unselected patients with advanced non-squamous NSCLC treated by BE. The second aim was to assess the predictive value of blood gene expression levels at baseline and 24h after BE therapy.

Methods

Blood samples from 43 advanced non-squamous NSCLC patients taken at baseline and 24h after initiation of therapy were profiled using Affymetrix’ exon arrays. The 24h gene dysregulation was investigated in the light of gene functional annotations using gene set enrichment analysis. The predictive value of blood gene expression levels was assessed and validated using an independent dataset.

Results

Significant gene dysregulations associated with the 24h-effect of BE were detected from blood-based whole-genome profiling. BE had a direct effect on “Pathways in cancer”, by significantly down-regulating genes involved in cytokine–cytokine receptor interaction, MAPK signaling pathway and mTOR signaling pathway. These pathways contribute to phenomena of evasion of apoptosis, proliferation and sustained angiogenesis. Other signaling pathways specifically reflecting the mechanisms of action of erlotinib and the anti-angiogenesis effect of bevacizumab were activated. The magnitude of change of the most dysregulated genes at 24h did not have a predictive value regarding the patients’ response to BE. However, predictive markers were identified from the gene expression levels at 24h regarding time to progression under BE.

Conclusions

The 24h-effect of the combined targeted therapy BE could be accurately monitored in advanced non-squamous NSCLC blood samples using whole-genome exon arrays. Putative predictive markers at 24h could reflect patients’ response to BE after adjusting for their mutational status.

Trial registration ClinicalTrials.gov: NCT00354549

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1174-z) contains supplementary material, which is available to authorized users.

Targeting phospholipase D in cancer, infection and neurodegenerative disorders

Phospholipase D (PLD) enzymes are one source of receptor-generated phosphatidic acid (PtdOH),which may subsequently be metabolized to diacylglycerol (DAG) and lysophosphatidic acid. There are other pathways that lead to PtdOH generation, but differences in pathways and in the acyl composition of the products seem to provide some specificity.

Both direct and indirect inhibitors of PLD activity have been identified despite a long-held suspicion that this pathway was undruggable. The identification of raloxifene and halopemide as direct inhibitors was followed by the systematic development of isoenzyme-preferring compounds that have been used to further differentiate the functions of PLD1 and PLD2.

PLD2 in host cells has been associated with viral entry processes and innate immune response pathways such that inhibition blocks efficient infection. This PLD2 pathway has been linked to autophagy via AKT kinases.

As a potential target in antiretroviral therapy, PLD1 works through the CAD enzyme (which contains carbamoyl aspartate synthase, aspartate transcarbamylase and dihydro-orotase domains) to modulate pyrimidine biosynthesis.

PLD activity and expression have been shown to be upregulated in several types of human cancers, in which PLD enzymes function downstream of a variety of known oncogenes. Inhibition of PtdOH production has a marked effect on tumorigenesis and malignant invasion.

PLD1, PLD2 and PLD3 have each been suggested to have a role in Alzheimer disease and other neurodegenerative conditions, but a mechanism has not yet emerged to explain the roles of these proteins in central nervous system pathophysiology.

Lipid second messengers such as phosphatidic acid (PtdOH) have a role in a wide range of pathological processes, and phospholipase D (PLD) enzymes are one of the major sources of signal-activated PtdOH generation. In this Review, Brown, Thomas and Lindsley discuss the development of PLD inhibitors, with a focus on isoform-specific inhibitors, and their potential applications in the treatment of cancer, neurodegeneration and infection.

Lipid second messengers have essential roles in cellular function and contribute to the molecular mechanisms that underlie inflammation, malignant transformation, invasiveness, neurodegenerative disorders, and infectious and other pathophysiological processes. The phospholipase D (PLD) isoenzymes PLD1 and PLD2 are one of the major sources of signal-activated phosphatidic acid (PtdOH) generation downstream of a variety of cell-surface receptors, including G protein-coupled receptors (GPCRs), receptor tyrosine kinases (RTKs) and integrins. Recent advances in the development of isoenzyme-selective PLD inhibitors and in molecular genetics have suggested that PLD isoenzymes in mammalian cells and pathogenic organisms may be valuable targets for the treatment of several human diseases. Isoenzyme-selective inhibitors have revealed complex inter-relationships between PtdOH biosynthetic pathways and the role of PtdOH in pathophysiology. PLD enzymes were once thought to be undruggable owing to the ubiquitous nature of PtdOH in cell signalling and concerns that inhibitors would be too toxic for use in humans. However, recent promising discoveries suggest that small-molecule isoenzyme-selective inhibitors may provide novel compounds for a unique approach to the treatment of cancers, neurodegenerative disorders and other afflictions of the central nervous system, and potentially serve as broad-spectrum antiviral and antimicrobial therapeutics.

Targeting Phospholipid Metabolism in Cancer

All cancers tested so far display abnormal choline and ethanolamine phospholipid metabolism, which has been detected with numerous magnetic resonance spectroscopy (MRS) approaches in cells, animal models of cancer, as well as the tumors of cancer patients. Since the discovery of this metabolic hallmark of cancer, many studies have been performed to elucidate the molecular origins of deregulated choline metabolism, to identify targets for cancer treatment, and to develop MRS approaches that detect choline and ethanolamine compounds for clinical use in diagnosis and treatment monitoring. Several enzymes in choline, and recently also ethanolamine, phospholipid metabolism have been identified, and their evaluation has shown that they are involved in carcinogenesis and tumor progression. Several already established enzymes as well as a number of emerging enzymes in phospholipid metabolism can be used as treatment targets for anticancer therapy, either alone or in combination with other chemotherapeutic approaches. This review summarizes the current knowledge of established and relatively novel targets in phospholipid metabolism of cancer, covering choline kinase α, phosphatidylcholine-specific phospholipase D1, phosphatidylcholine-specific phospholipase C, sphingomyelinases, choline transporters, glycerophosphodiesterases, phosphatidylethanolamine N-methyltransferase, and ethanolamine kinase. These enzymes are discussed in terms of their roles in oncogenic transformation, tumor progression, and crucial cancer cell properties such as fast proliferation, migration, and invasion. Their potential as treatment targets are evaluated based on the current literature.

Phosphatidic acid induces decidualization by stimulating Akt-PP2A binding in human endometrial stromal cells

Decidualization of human endometrial stromal cells (hESCs) is crucial for successful uterine implantation and maintaining pregnancy. We previously reported that phospholipase D1 (PLD1) is required for cAMP-induced decidualization of hESCs. However, the mechanism by which phosphatidic acid (PA), the product of PLD1 action, might regulate decidualization is not known. We confirmed that PA induced decidualization of hESCs by observing morphological changes and measuring increased levels of decidualization markers such as IGFBP1 and prolactin transcripts (P < 0.05). Treatment with PA reduced phosphorylation of Akt and consequently that of FoxO1, which led to the increased IGFBP1 and prolactin mRNA levels (P < 0.05). Conversely, PLD1 knockdown rescued Akt phosphorylation. Binding of PP2A and Akt increased in response to cAMP or PA, suggesting that their binding is directly responsible for the inactivation of Akt during decidualization. Consistent with this observation, treatment with okadaic acid, a PP2A inhibitor, also inhibited cAMP-induced decidualization by blocking Akt dephosphorylation.

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.

Molecular Pathways: Targeting Diacylglycerol Kinase Alpha in Cancer

Lipid kinases have largely been neglected as targets in cancer, and an increasing number of reports suggest diacylglycerol kinase alpha (DGKα) may be one with promising therapeutic potential. DGKα is one of 10 DGK family members that convert diacylglycerol (DAG) to phosphatidic acid (PA), and both DAG and PA are critical lipid second messengers in the plasma membrane. A host of important oncogenic proteins and pathways affect cancer cells in part through DGKα, including the c-Met and VEGF receptors. Others partially mediate the effects of DGKα inhibition in cancer, such as mTOR and HIF-1α. DGKα inhibition can directly impair cancer cell viability, inhibits angiogenesis, and notably may also boost T-cell activation and enhance cancer immunotherapies. Although two structurally similar inhibitors of DGKα were established decades ago, they have seen minimal in vivo usage, and it is unlikely that either of these older DGKα inhibitors will have utility for cancer. An abandoned compound that also inhibits serotonin receptors may have more translational potential as a DGKα inhibitor, but more potent and specific DGKα inhibitors are sorely needed. Other DGK family members may also provide therapeutic targets in cancer, but require further investigation.

MicroRNA-638 inhibits cell proliferation by targeting phospholipase D1 in human gastric carcinoma

MicroRNAs (miRNAs) are a type of small non-coding RNAs that are often play important roles in carcinogenesis, but the carcinogenic mechanism of miRNAs is still unclear. This study will investigate the function and the mechanism of miR-638 in carcinoma (GC). The expression of miR-638 in GC and the DNA copy number of miR-638 were detected by real-time PCR. The effect of miR-638 on cell proliferation was measured by counting kit-8 assay. Different assays, including bioinformatics algorithms (TargetScan and miRanda), luciferase report assay and Western blotting, were used to identify the target gene of miR-638 in GC. The expression of miR-638 target gene in clinical CRC tissues was also validated by immunohistochemical assay. From this research, we found that miR-638 was downregulated in GC tissues compared with corresponding noncancerous tissues (NCTs), and the DNA copy number of miR-638 was lower in GC than NCTs, which may induce the corresponding downregulation of miR-638 in GC. Ectopic expression of miR-638 inhibited GC cell growth in vitro. Subsequently, we identified that PLD1 is the target gene of miR-638 in GC, and silencing PLD1 expression phenocopied the inhibitory effect of miR-638 on GC cell proliferation. Furthermore, we observed that PLD1 was overexpressed in GC tissues, and high expression of PLD1 in GC predicted poor overall survival. In summary, we revealed that miR-638 functions as a tumor suppressor in GC through inhibiting PLD1.

Phosphatidic acid-mediated activation and translocation to the cell surface of sialidase NEU3, promoting signaling for cell migration

The plasma membrane-associated sialidase NEU3 plays crucial roles in regulation of transmembrane signaling, and its aberrant up-regulation in various cancers contributes to malignancy. However, it remains uncertain how NEU3 is naturally activated and locates to plasma membranes, because of its Triton X-100 requirement for the sialidase activity in vitro and its often changing subcellular location. Among phospholipids examined, we demonstrate that phosphatidic acid (PA) elevates its sialidase activity 4 to 5 times at 50 μM in vitro at neutral pH and promotes translocation to the cell surface and cell migration through Ras-signaling in HeLa and COS-1 cells. NEU3 was found to interact selectively with PA as assessed by phospholipid array, liposome coprecipitation, and ELISA assays and to colocalize with phospholipase D (PLD) 1 in response to epidermal growth factor (EGF) or serum stimulation. Studies using tagged NEU3 fragments with point mutations identified PA- and calmodulin (CaM)-binding sites around the N terminus and confirmed its participation in translocation and catalytic activity. EGF induced PLD1 activation concomitantly with enhanced NEU3 translocation to the cell surface, as assessed by confocal microscopy. These results suggest that interactions of NEU3 with PA produced by PLD1 are important for regulation of transmembrane signaling, this aberrant acceleration probably promoting malignancy in cancers.

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.

Thematic minireview series on phospholipase D and cancer

Phospholipase D (PLD) signaling plays a critical role in cell growth and proliferation, vesicular trafficking, secretion, and endocytosis. At the cellular level, PLD and its reaction product, phosphatidate, interact with a large number of protein partners that are directly related to the actin cytoskeleton and cell migration. Cancer invasion and metastasis rely heavily on cellular motility, and as such, they have put PLD at center stage in cancer research. This minireview series highlights some of the molecular mechanisms that provide evidence for the emerging tumorigenic potential of PLD, the role of the microenvironment, and putative connections with inflammation. PLD represents a potential target for the rational development of therapeutics against cancer and other diseases.