Design and synthesis of isoform-selective phospholipase D (PLD) inhibitors. Part I: Impact of alternative halogenated privileged structures for PLD1 specificity

Chiral iron porphyrin (+)-D4-(por)FeCl catalyzes highly enantioselective cyclopropanation of alkenes using in situ generated diazoacetonitrile with up to 35 000 product turnover

Transition metal-catalyzed asymmetric cyclopropanation of alkenes is an important strategy to construct chiral cyclopropane skeletons of pharmaceutical interest, but highly enantioselective and practical carbene transfer reactions based on Earth abundant and bio-compatible metals are still a difficult challenge. In this work, we use a chiral iron porphyrin (+)-D4-(por)FeCl catalyst and in situ generated α-diazoacetonitrile for highly enantioselective cyclopropanation of arylalkene. This reaction is applicable to a wide range of arylalkenes (44 examples) with yield up to 99%, diastereomeric ratio (dr) up to 93 : 7, and enantiomeric excess (ee) values up to 98%. Importantly, for the cyclopropanation reaction of 3,4-difluorostyrene (1.40 g, 10.0 mmol) with α-diazoacetonitrile in the presence of 0.002 mol% of (+)-D4-(por)FeCl as a catalyst, the turnover number and enantioselectivity of the cyclopropyl nitrile product reached 31 000 and 88% ee, respectively. Using cyclopropyl nitriles as a starting material, downstream functionalization derivatives including cyclopropyl carboxylic acids, cyclopropylamines, and cyclopropylmethanamines can be produced as key intermediates for the preparation of a series of bioactive or drug-like molecules. In addition, the chiral Fe(ii)porphyrin-cyanocarbene intermediate [(-)-D4-(por)FeII(:CHCN)], which is directly responsible for the carbene transfer reaction, has been characterized by 1H NMR, HR ESI-MS, UV-vis and ATR-FTIR spectroscopy.

PLD1 is a key player in cancer stemness and chemoresistance: Therapeutic targeting of cross-talk between the PI3K/Akt and Wnt/β-catenin pathways

The development of chemoresistance is a major challenge in the treatment of several types of cancers in clinical settings. Stemness and chemoresistance are the chief causes of poor clinical outcomes. In this context, we hypothesized that understanding the signaling pathways responsible for chemoresistance in cancers is crucial for the development of novel targeted therapies to overcome drug resistance. Among the aberrantly activated pathways, the PI3K-Akt/Wnt/β-catenin signaling pathway is clinically implicated in malignancies such as colorectal cancer (CRC) and glioblastoma multiforme (GBM). Aberrant dysregulation of phospholipase D (PLD) has been implicated in several malignancies, and oncogenic activation of this pathway facilitates tumor proliferation, stemness, and chemoresistance. Crosstalk involving the PLD and Wnt/β-catenin pathways promotes the progression of CRC and GBM and reduces the sensitivity of cancer cells to standard therapies. Notably, both pathways are tightly regulated and connected at multiple levels by upstream and downstream effectors. Thus, gaining deeper insights into the interactions between these pathways would help researchers discover unique therapeutic targets for the management of drug-resistant cancers. Here, we review the molecular mechanisms by which PLD signaling stimulates stemness and chemoresistance in CRC and GBM. Thus, the current review aims to address the importance of PLD as a central player coordinating cross-talk between the PI3K/Akt and Wnt/β-catenin pathways and proposes the possibility of targeting these pathways to improve cancer therapy and overcome drug resistance.

GDE7 produces cyclic phosphatidic acid in the ER lumen functioning as a lysophospholipid mediator

Cyclic phosphatidic acid (cPA) is a lipid mediator, which regulates adipogenic differentiation and glucose homeostasis by suppressing nuclear peroxisome proliferator-activated receptor γ (PPARγ). Glycerophosphodiesterase 7 (GDE7) is a Ca2+-dependent lysophospholipase D that localizes in the endoplasmic reticulum. Although mouse GDE7 catalyzes cPA production in a cell-free system, it is unknown whether GDE7 generates cPA in living cells. Here, we demonstrate that human GDE7 possesses cPA-producing activity in living cells as well as in a cell-free system. Furthermore, the active site of human GDE7 is directed towards the luminal side of the endoplasmic reticulum. Mutagenesis revealed that amino acid residues F227 and Y238 are important for catalytic activity. GDE7 suppresses the PPARγ pathway in human mammary MCF-7 and mouse preadipocyte 3T3-L1 cells, suggesting that cPA functions as an intracellular lipid mediator. These findings lead to a better understanding of the biological role of GDE7 and its product, cPA.

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.

Allosteric Antagonism of the Pregnane X Receptor (PXR): Current-State-of-the-Art and Prediction of Novel Allosteric Sites

The pregnane X receptor (PXR, NR1I2) is a xenobiotic-activated transcription factor with high levels of expression in the liver. It not only plays a key role in drug metabolism and elimination, but also promotes tumor growth, drug resistance, and metabolic diseases. It has been proposed as a therapeutic target for type II diabetes, metabolic syndrome, and inflammatory bowel disease, and PXR antagonists have recently been considered as a therapy for colon cancer. There are currently no PXR antagonists that can be used in a clinical setting. Nevertheless, due to the large and complex ligand-binding pocket (LBP) of the PXR, it is challenging to discover PXR antagonists at the orthosteric site. Alternative ligand binding sites of the PXR have also been proposed and are currently being studied. Recently, the AF-2 allosteric binding site of the PXR has been identified, with several compounds modulating the site discovered. Herein, we aimed to summarize our current knowledge of allosteric modulation of the PXR as well as our attempt to unlock novel allosteric sites. We describe the novel binding function 3 (BF-3) site of PXR, which is also common for other nuclear receptors. In addition, we also mention a novel allosteric site III based on in silico prediction. The identified allosteric sites of the PXR provide new insights into the development of safe and efficient allosteric modulators of the PXR receptor. We therefore propose that novel PXR allosteric sites might be promising targets for treating chronic metabolic diseases and some cancers.

Discovery of Phospholipase D Inhibitors with Improved Drug-like Properties and Central Nervous System Penetrance

Phospholipase D (PLD) is a phospholipase enzyme responsible for hydrolyzing phosphatidylcholine into the lipid signaling molecule, phosphatidic acid, and choline. From a therapeutic perspective, PLD has been implicated in human cancer progression as well as a target for neurodegenerative diseases, including Alzheimer's. Moreover, knockdown of PLD rescues the ALS phenotype in multiple Drosophila models of ALS (amyotrophic lateral sclerosis) and displays modest motor benefits in an SOD1 ALS mouse model. To further validate whether inhibiting PLD is beneficial for the treatment of ALS, a brain penetrant small molecule inhibitor with suitable PK properties to test in an ALS animal model is needed. Using a combination of ligand-based drug discovery and structure-based design, a dual PLD1/PLD2 inhibitor was discovered that is single digit nanomolar in the Calu-1 cell assay and has suitable PK properties for in vivo studies. To capture the in vivo measurement of PLD inhibition, a transphosphatidylation pharmacodynamic LC-MS assay was developed, in which a dual PLD1/PLD2 inhibitor was found to reduce PLD activity by 15-20-fold.

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.

Lipid metabolism in cancer progression and therapeutic strategies

Dysregulated lipid metabolism represents an important metabolic alteration in cancer. Fatty acids, cholesterol, and phospholipid are the three most prevalent lipids that act as energy producers, signaling molecules, and source material for the biogenesis of cell membranes. The enhanced synthesis, storage, and uptake of lipids contribute to cancer progression. The rewiring of lipid metabolism in cancer has been linked to the activation of oncogenic signaling pathways and cross talk with the tumor microenvironment. The resulting activity favors the survival and proliferation of tumor cells in the harsh conditions within the tumor. Lipid metabolism also plays a vital role in tumor immunogenicity via effects on the function of the noncancer cells within the tumor microenvironment, especially immune-associated cells. Targeting altered lipid metabolism pathways has shown potential as a promising anticancer therapy. Here, we review recent evidence implicating the contribution of lipid metabolic reprogramming in cancer to cancer progression, and discuss the molecular mechanisms underlying lipid metabolism rewiring in cancer, and potential therapeutic strategies directed toward lipid metabolism in cancer. This review sheds new light to fully understanding of the role of lipid metabolic reprogramming in the context of cancer and provides valuable clues on therapeutic strategies targeting lipid metabolism in cancer.

Structure and regulation of human phospholipase D

Mammalian phospholipase D (PLD) generates phosphatidic acid, a dynamic lipid secondary messenger involved with a broad spectrum of cellular functions including but not limited to metabolism, migration, and exocytosis. As a promising pharmaceutical target, the biochemical properties of PLD have been well characterized. This has led to the recent crystal structures of human PLD1 and PLD2, the development of PLD specific pharmacological inhibitors, and the identification of cellular regulators of PLD. In this review, we discuss the PLD1 and PLD2 structures, PLD inhibition by small molecules, and the regulation of PLD activity by effector proteins and lipids.

Ral GTPases promote breast cancer metastasis by controlling biogenesis and organ targeting of exosomes

Cancer extracellular vesicles (EVs) shuttle at distance and fertilize pre-metastatic niches facilitating subsequent seeding by tumor cells. However, the link between EV secretion mechanisms and their capacity to form pre-metastatic niches remains obscure. Using mouse models, we show that GTPases of the Ral family control, through the phospholipase D1, multi-vesicular bodies homeostasis and tune the biogenesis and secretion of pro-metastatic EVs. Importantly, EVs from RalA or RalB depleted cells have limited organotropic capacities in vivoand are less efficient in promoting metastasis. RalA and RalB reduce the EV levels of the adhesion molecule MCAM/CD146, which favors EV-mediated metastasis by allowing EVs targeting to the lungs. Finally, RalA, RalB, and MCAM/CD146, are factors of poor prognosis in breast cancer patients. Altogether, our study identifies RalGTPases as central molecules linking the mechanisms of EVs secretion and cargo loading to their capacity to disseminate and induce pre-metastatic niches in a CD146-dependent manner.

Phospholipase D inhibitors screening: Probing and evaluation of ancient and novel molecules

Phospholipase D (PLD) is a ubiquitous enzyme that cleaves the distal phosphoester bond of phospholipids generating phosphatidic acid (PA). In plants, PA is involved in numerous cell responses triggered by stress. Similarly, in mammals, PA is also a second messenger involved in tumorigenesis. PLD is nowadays considered as a therapeutic target and blocking its activity with specific inhibitors constitutes a promising strategy to treat cancers. Starting from already described PLD inhibitors, this study aims to investigate the effect of their structural modifications on the enzyme's activity, as well as identifying new potent inhibitors of eukaryotic PLDs. Being able to purify the plant PLD from Vigna unguiculata (VuPLD), we obtained a SAXS model of its structure. We then used a fluorescence-based test suitable for high-throughput screening to review the effect of eukaryotic PLD inhibitors described in the literature. In this regard, we found that only few molecules were in fact able to inhibit VuPLD and we confirmed that vanadate is the most potent of all with an IC₅₀ around 58 μM. Moreover, the small-scale screening of a chemical library of 3120 compounds allowed us to optimize the different screening's steps and paved the way towards the discovery of new potent inhibitors.

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.

IMPACT: Imaging phospholipase d activity with clickable alcohols via transphosphatidylation

Phospholipase Ds (PLDs) are multifunctional and disease-relevant enzymes operating at the center of phospholipid metabolism and signaling. Physiologically, they hydrolyze abundant phospholipids into phosphatidic acid (PA), a potent lipid second messenger and central biosynthetic intermediate. Given the pleiotropic nature of PA, the multiple locations of PLD activity within single cells, and differences in PLD activities across cell types in vivo, tools with spatiotemporal precision are urgently needed to dissect the signaling functions of PLDs. Here, we describe a toolset for visualizing and quantifying cellular PLD activity with high spatial and temporal resolution. Our approach capitalizes on the ability of PLDs to catalyze transphosphatidylation reactions with exogenous alcohols to generate phosphatidyl alcohols, lipids whose location and abundance report on the extent of PLD-mediated PA synthesis. Our key innovation is to employ functionalized, "clickable," alcohols as PLD substrates, which enables subsequent tagging of the resultant phosphatidyl alcohols with fluorophores or other functional probes for detection via highly selective click chemistry reactions. In this chapter, we describe this method, termed IMPACT (Imaging PLD Activity with Clickable Alcohols via Transphosphatidylation), which can be coupled to downstream analysis by fluorescence microscopy, flow cytometry, HPLC, or mass spectrometry. We describe two variants of IMPACT, one with greater sensitivity, for detecting PLD activity at single-cell and population levels, and one with greater spatiotemporal resolution ("real-time," or RT-IMPACT), for accurately visualizing PLD activity at the subcellular, individual-organelle level. Together, IMPACT represents a major advance in our ability to dissect PLD-mediated PA signaling in native biological settings.

Manganese-Based Nanoactivator Optimizes Cancer Immunotherapy via Enhancing Innate Immunity

Cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING) are essential components of the innate immune sensors to cytosolic DNA and elicit type I interferon (IFN). Recent studies have revealed that manganese (Mn) can enhance cGAS and STING activation to viral infection. However, the role of Mn in antitumor immunity has not been explored. Here, we designed a nanoactivator, which can induce the presence of DNA in cytoplasm and simultaneously elevate Mn²⁺ accumulation within tumor cells. In detail, amorphous porous manganese phosphate (APMP) NPs that are highly responsive to tumor microenvironment were employed to construct doxorubicin (DOX)-loaded and phospholipid (PL)-coated hybrid nanoparticles (PL/APMP-DOX NPs). PL/APMP-DOX NPs were stably maintained during systemic circulation, but triggered to release DOX for inducing DNA damage and Mn²⁺ to augment cGAS/STING activity. We found that PL/APMP-DOX NPs with superior tumor-targeting capacity boosted dendritic cell maturation and increased cytotoxic T lymphocyte infiltration as well as natural killer cell recruitment into the tumor site. Furthermore, the NPs increased production of type I IFN and secretion of pro-inflammatory cytokines (for example, TNF-α and IL-6). Consequently, PL/APMP-DOX NPs exhibited excellent antitumor efficacy and prolonged the lifespan of the tumor-bearing mice. Collectively, we developed a PL-decorated Mn-based hybrid nanoactivator to intensify immune activation and that might provide therapeutic potential for caner immunotherapy.

Spatiotemporal control of phosphatidic acid signaling with optogenetic, engineered phospholipase Ds

Phosphatidic acid (PA) is both a central phospholipid biosynthetic intermediate and a multifunctional lipid second messenger produced at several discrete subcellular locations. Organelle-specific PA pools are believed to play distinct physiological roles, but tools with high spatiotemporal control are lacking for unraveling these pleiotropic functions. Here, we present an approach to precisely generate PA on demand on specific organelle membranes. We exploited a microbial phospholipase D (PLD), which produces PA by phosphatidylcholine hydrolysis, and the CRY2-CIBN light-mediated heterodimerization system to create an optogenetic PLD (optoPLD). Directed evolution of PLD using yeast membrane display and IMPACT, a chemoenzymatic method for visualizing cellular PLD activity, yielded a panel of optoPLDs whose range of catalytic activities enables mimicry of endogenous, physiological PLD signaling. Finally, we applied optoPLD to elucidate that plasma membrane, but not intracellular, pools of PA can attenuate the oncogenic Hippo signaling pathway. OptoPLD represents a powerful and precise approach for revealing spatiotemporally defined physiological functions of PA.

Human PLD structures enable drug design and characterization of isoenzyme selectivity

Phospholipase D enzymes (PLDs) are ubiquitous phosphodiesterases that produce phosphatidic acid (PA), a key second messenger and biosynthetic building block. Although an orthologous bacterial Streptomyces sp. strain PMF PLD structure was solved two decades ago, the molecular basis underlying the functions of the human PLD enzymes (hPLD) remained unclear based on this structure due to the low homology between these sequences. Here, we describe the first crystal structures of hPLD1 and hPLD2 catalytic domains and identify novel structural elements and functional differences between the prokaryotic and eukaryotic enzymes. Furthermore, structure-based mutation studies and structures of inhibitor-hPLD complexes allowed us to elucidate the binding modes of dual and isoform-selective inhibitors, highlight key determinants of isoenzyme selectivity and provide a basis for further structure-based drug discovery and functional characterization of this therapeutically important superfamily of enzymes.

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.

Suppressing aberrant phospholipase D1 signaling in 3xTg Alzheimer's disease mouse model promotes synaptic resilience

Current approaches in treatment of Alzheimer's disease (AD) is focused on early stages of cognitive decline. Identifying therapeutic targets that promote synaptic resilience during early stages may prevent progressive memory deficits by preserving memory mechanisms. We recently reported that the inducible isoform of phospholipase D (PLD1) was significantly increased in synaptosomes from post-mortem AD brains compared to age-matched controls. Using mouse models, we reported that the aberrantly elevated neuronal PLD1 is key for oligomeric amyloid driven synaptic dysfunction and underlying memory deficits. Here, we demonstrate that chronic inhibition using a well-tolerated PLD1 specific small molecule inhibitor is sufficient to prevent the progression of synaptic dysfunction during early stages in the 3xTg-AD mouse model. Firstly, we report prevention of cognitive decline in the inhibitor-treated group using novel object recognition (NOR) and fear conditioning (FC). Secondly, we provide electrophysiological assessment of better synaptic function in the inhibitor-treated group. Lastly, using Golgi staining, we report that preservation of dendritic spine integrity as one of the mechanisms underlying the action of the small molecule inhibitor. Collectively, these studies provide evidence for inhibition of PLD1 as a potential therapeutic strategy in preventing progression of cognitive decline associated with AD and related dementia.

Biased M1 receptor-positive allosteric modulators reveal role of phospholipase D in M1-dependent rodent cortical plasticity

Highly selective, positive allosteric modulators (PAMs) of the M₁ subtype of muscarinic acetylcholine receptor have emerged as an exciting new approach to potentially improve cognitive function in patients suffering from Alzheimer's disease and schizophrenia. Discovery programs have produced a structurally diverse range of M₁ receptor PAMs with distinct pharmacological properties, including different extents of agonist activity and differences in signal bias. This includes biased M₁ receptor PAMs that can potentiate coupling of the receptor to activation of phospholipase C (PLC) but not phospholipase D (PLD). However, little is known about the role of PLD in M₁ receptor signaling in native systems, and it is not clear whether biased M₁ PAMs display differences in modulating M₁-mediated responses in native tissue. Using PLD inhibitors and PLD knockout mice, we showed that PLD was necessary for the induction of M₁-dependent long-term depression (LTD) in the prefrontal cortex (PFC). Furthermore, biased M₁ PAMs that did not couple to PLD not only failed to potentiate orthosteric agonist-induced LTD but also blocked M₁-dependent LTD in the PFC. In contrast, biased and nonbiased M₁ PAMs acted similarly in potentiating M₁-dependent electrophysiological responses that were PLD independent. These findings demonstrate that PLD plays a critical role in the ability of M₁ PAMs to modulate certain central nervous system (CNS) functions and that biased M₁ PAMs function differently in brain regions implicated in cognition.

Phospholipase D2 in prostate cancer: protein expression changes with Gleason score

BACKGROUND

Phospholipases D1 and D2 (PLD1/2) are implicated in tumorigenesis through their generation of the signalling lipid phosphatidic acid and its downstream effects. Inhibition of PLD1 blocks prostate cell growth and colony formation. Here a role for PLD2 in prostate cancer (PCa), the major cancer of men in the western world, is examined.

METHODS

PLD2 expression was analysed by immunohistochemistry and western blotting. The effects of PLD2 inhibition on PCa cell viability and cell motility were measured using MTS, colony forming and wound-healing assays.

RESULTS

PLD2 protein is expressed about equally in luminal and basal prostate epithelial cells. In cells from different Gleason-scored PCa tissue PLD2 protein expression is generally higher than in non-tumorigenic cells and increases in PCa tissue scored Gleason 6-8. PLD2 protein is detected in the cytosol and nucleus and had a punctate appearance. In BPH tissue stromal cells as well as basal and luminal cells express PLD2. PLD2 protein co-expresses with chromogranin A in castrate-resistant PCa tissue. PLD2 inhibition reduces PCa cell viability, colony forming ability and directional cell movement.

CONCLUSIONS

PLD2 expression correlates with increasing Gleason score to GS8. PLD2 inhibition has the potential to reduce PCa progression.

Metal-Free Synthesis of N-(Carboselenoate) Benzimidazolones by Cascade Cyclization of ortho-Diisocyanoarenes and Selenosulfonates

A facile synthesis of N-(carboselenoate) benzimidazolones through metal-free reactions of ortho-diisocyanoarenes with selenosulfonates is reported here. The desired products are obtained in moderate to good yields with good functional group compatibility. The ortho-diisocyanoarenes are applied to the construction of 2-benzimidazolone derivatives for the first time.

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.

mTOR Signalling in Head and Neck Cancer: Heads Up

The mammalian target of rapamycin (mTOR) signalling pathway is a central regulator of metabolism in all cells. It senses intracellular and extracellular signals and nutrient levels, and coordinates the metabolic requirements for cell growth, survival, and proliferation. Genetic alterations that deregulate mTOR signalling lead to metabolic reprogramming, resulting in the development of several cancers including those of the head and neck. Gain-of-function mutations in EGFR, PIK3CA, and HRAS, or loss-of-function in p53 and PTEN are often associated with mTOR hyperactivation, whereas mutations identified from The Cancer Genome Atlas (TCGA) dataset that potentially lead to aberrant mTOR signalling are found in the EIF4G1, PLD1, RAC1, and SZT2 genes. In this review, we discuss how these mutant genes could affect mTOR signalling and highlight their impact on metabolic processes, as well as suggest potential targets for therapeutic intervention, primarily in head and neck cancer.

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

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

PLD-dependent phosphatidic acid microdomains are signaling platforms for podosome formation

Local membrane phospholipid enrichment serves as docking platform for signaling proteins involved in many processes including cell adhesion and migration. Tissue-resident dendritic cells (DCs) assemble actomyosin-based structures called podosomes, which mediate adhesion and degradation of extracellular matrix for migration and antigen sampling. Recent evidence suggested the involvement of phospholipase D (PLD) and its product phosphatidic acid (PA) in podosome formation, but the spatiotemporal control of this process is poorly characterized. Here we determined the role of PLD1 and PLD2 isoforms in regulating podosome formation and dynamics in human primary DCs by combining PLD pharmacological inhibition with a fluorescent PA sensor and fluorescence microscopy. We found that ongoing PLD2 activity is required for the maintenance of podosomes, whereas both PLD1 and PLD2 control the early stages of podosome assembly. Furthermore, we captured the formation of PA microdomains accumulating at the membrane cytoplasmic leaflet of living DCs, in dynamic coordination with nascent podosome actin cores. Finally, we show that both PLD1 and PLD2 activity are important for podosome-mediated matrix degradation. Our results provide novel insight into the isoform-specific spatiotemporal regulation of PLD activity and further our understanding of the role of cell membrane phospholipids in controlling localized actin polymerization and cell protrusion.

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

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

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.

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.

Isoform selective PLD inhibition by novel, chiral 2,8-diazaspiro[4.5]decan-1-one derivatives

This letter describes the on-going SAR efforts to develop PLD1, PLD2 and dual PLD1/2 inhibitors with improved physiochemical and disposition properties as well as securing intellectual property position. Previous PLD inhibitors, based on a triazaspiro[4.5]decanone core proved to be highly selective PLD2 inhibitors, but with low plasma free fraction (rat, human f_u < 0.03), high predicted hepatic clearance (rat CL_hep > 65 mL/min/kg) and very short half-lives in vivo (t_1/2 < 0.15 h). Removal of a nitrogen atom from this core generated a 2,8-diazaspiro[4.5]decanone core, harboring a new chiral center, as well as increased sp³ character. This new core demonstrated enantioselective inhibition of the individual PLD isoforms, enhanced free fraction (rat, human f_u < 0.13), engendered moderate predicted hepatic clearance (rat CL_hep ∼ 43 mL/min/kg), improved half-lives in vivo (t_1/2 > 3 h), and led to the first issued US patent claiming composition of matter for small molecule PLD inhibitors.

Elevated phospholipase D activity in androgen-insensitive prostate cancer cells promotes both survival and metastatic phenotypes

Prostate cells are hormonally driven to grow and divide. Typical treatments for prostate cancer involve blocking activation of the androgen receptor by androgens. Androgen deprivation therapy can lead to the selection of cancer cells that grow and divide independently of androgen receptor activation. Prostate cancer cells that are insensitive to androgens commonly display metastatic phenotypes and reduced long-term survival of patients. In this study we provide evidence that androgen-insensitive prostate cancer cells have elevated PLD activity relative to the androgen-sensitive prostate cancer cells. PLD activity has been linked with promoting survival in many human cancer cell lines; and consistent with the previous studies, suppression of PLD activity in the prostate cancer cells resulted in apoptotic cell death. Of significance, suppressing the elevated PLD activity in androgen resistant prostate cancer lines also blocked the ability of these cells to migrate and invade Matrigel™. Since survival signals are generally an early event in tumorigenesis, the apparent coupling of survival and metastatic phenotypes implies that metastasis is an earlier event in malignant prostate cancer than generally thought. This finding has implications for screening strategies designed to identify prostate cancers before dissemination.

Sonic Hedgehog Activates Phospholipase A2 to Enhance Smoothened Ciliary Translocation

The G protein-coupled receptor Smoothened (Smo) is the signal transducer of the Sonic Hedgehog (Shh) pathway. Smo signals through G protein-dependent and -independent routes, with G protein-independent canonical signaling to Gli effectors requiring Smo accumulation in the primary cilium. The mechanisms controlling Smo activation and trafficking are not yet clear but likely entail small-molecule binding to pockets in its extracellular cysteine-rich domain (CRD) and/or transmembrane bundle. Here, we demonstrate that the cytosolic phospholipase cPLA2α is activated through Gβγ downstream of Smo to release arachidonic acid. Arachidonic acid binds Smo and synergizes with CRD-binding agonists, promoting Smo ciliary trafficking and high-level signaling. Chemical or genetic cPLA2α inhibition dampens Smo signaling to Gli, revealing an unexpected contribution of G protein-dependent signaling to canonical pathway activity. Arachidonic acid displaces the Smo transmembrane domain inhibitor cyclopamine to rescue CRD agonist-induced signaling, suggesting that arachidonic acid may target the transmembrane bundle to allosterically enhance signaling by CRD agonist-bound Smo.

Ex Uno Plura: Differential Labeling of Phospholipid Biosynthetic Pathways with a Single Bioorthogonal Alcohol

Imaging approaches that track biological molecules within cells are essential tools in modern biochemistry. Lipids are particularly challenging to visualize, as they are not directly genetically encoded. Phospholipids, the most abundant subgroup of lipids, are structurally diverse and accomplish many cellular functions, acting as major structural components of membranes and as signaling molecules that regulate cell growth, division, apoptosis, cytoskeletal dynamics, and numerous other physiological processes. Cells regulate the abundance, and therefore bioactivity, of phospholipids by modulating the activities of their biosynthetic enzymes. Thus, techniques that enable monitoring of flux through individual lipid biosynthetic pathways can provide key functional information. For example, the choline analogue propargylcholine (ProCho) can report on de novo biosynthesis of phosphatidylcholine by conversion to an alkynyl lipid that can be imaged following click chemistry tagging with an azido fluorophore. We report that ProCho is also a substrate of phospholipase D enzymes-which normally hydrolyze phosphatidylcholine to generate the lipid second messenger phosphatidic acid-in a transphosphatidylation reaction, generating the identical alkynyl lipid. By controlling the activities of phosphatidylcholine biosynthesis and phospholipase D enzymes, we establish labeling conditions that enable this single probe to selectively report on two different biosynthetic pathways. Just as nature exploits the economy of common metabolic intermediates to efficiently diversify biosynthesis, so can biochemists in interrogating such pathways with careful probe design. We envision that ProCho's ability to report on multiple metabolic pathways will enable studies of membrane dynamics and improve our understanding of the myriad roles that lipids play in cellular homeostasis.

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.

Phospholipase D inhibitors reduce human prostate cancer cell proliferation and colony formation

Background:

Phospholipases D1 and D2 (PLD1/2) hydrolyse cell membrane glycerophospholipids to generate phosphatidic acid, a signalling lipid, which regulates cell growth and cancer progression through effects on mTOR and PKB/Akt. PLD expression and/or activity is raised in breast, colorectal, gastric, kidney and thyroid carcinomas but its role in prostate cancer (PCa), the major cancer of men in the western world, is unclear.

Methods:

PLD1 protein expression in cultured PNT2C2, PNT1A, P4E6, LNCaP, PC3, PC3M, VCaP, 22RV1 cell lines and patient-derived PCa cells was analysed by western blotting. PLD1 protein localisation in normal, benign prostatic hyperplasia (BPH), and castrate-resistant prostate cancer (CRPC) tissue sections and in a PCa tissue microarray (TMA) was examined by immunohistochemistry. PLD activity in PCa tissue was assayed using an Amplex Red method. The effect of PLD inhibitors on PCa cell viability was measured using MTS and colony forming assays.

Results:

PLD1 protein expression was low in the luminal prostate cell lines (LNCaP, VCaP, 22RV1) compared with basal lines (PC3 and PC3M). PLD1 protein expression was elevated in BPH biopsy tissue relative to normal and PCa samples. In normal and BPH tissue, PLD1 was predominantly detected in basal cells as well in some stromal cells, rather than in luminal cells. In PCa tissue, luminal cells expressed PLD1. In a PCa TMA, the mean peroxidase intensity per DAB-stained Gleason 6 and 7 tissue section was significantly higher than in sections graded Gleason 9. In CRPC tissue, PLD1 was expressed prominently in the stromal compartment, in luminal cells in occasional glands and in an expanding population of cells that co-expressed chromogranin A and neurone-specific enolase. Levels of PLD activity in normal and PCa tissue samples were similar. A specific PLD1 inhibitor markedly reduced the survival of both prostate cell lines and patient-derived PCa cells compared with two dual PLD1/PLD2 inhibitors. Short-term exposure of PCa cells to the same specific PLD1 inhibitor significantly reduced colony formation.

Conclusions:

A new specific inhibitor of PLD1, which is well tolerated in mice, reduces PCa cell survival and thus has potential as a novel therapeutic agent to reduce prostate cancer progression. Increased PLD1 expression may contribute to the hyperplasia characteristic of BPH and in the progression of castrate-resistant PCa, where an expanding population of neuroendocrine-like cells express PLD1.

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.

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.

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.

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.

An Evolutionarily Conserved PLC-PKD-TFEB Pathway for Host Defense

The mechanisms that tightly control the transcription of host defense genes have not been fully elucidated. We previously identified TFEB as a transcription factor important for host defense, but the mechanisms that regulate TFEB during infection remained unknown. Here, we used C. elegans to discover a pathway that activates TFEB during infection. Gene dkf-1, which encodes a homolog of protein kinase D (PKD), was required for TFEB activation in nematodes infected with Staphylococcus aureus. Conversely, pharmacological activation of PKD was sufficient to activate TFEB. Furthermore, phospholipase C (PLC) gene plc-1 was also required for TFEB activation, downstream of Gαq homolog egl-30 and upstream of dkf-1. Using reverse and chemical genetics, we discovered a similar PLC-PKD-TFEB axis in Salmonella-infected mouse macrophages. In addition, PKCα was required in macrophages. These observations reveal a previously unknown host defense signaling pathway, which has been conserved across one billion years of evolution.

Synthesis and Antitumor Evaluation of Novel 5-Hydrosulfonyl-1H-benzo[d]imidazol-2(3H)-one Derivatives

A series of novel 5-hydrosulfonyl-1H-benzo[d]imidazol-2(3H)-one derivatives bearing natural product substructures has been successfully synthesized and their antitumor activity studied. These newly synthesized derivatives were characterized by ¹H-NMR, (13)C-NMR and high resolution mass spectral data, then screened as antitumor agents against the A549, HCC1937, and MDA-MB-468 human tumor cell lines using MTT cell proliferation assays. The results show that some of these compounds can effectively inhibit the growth of these cancerous cells, with compound 5b being the best one (IC50 = 2.6 μM). Flow cytometry data revealed that compound 5b induced apoptosis of HCC1937 cells with increased solution concentration. The structure and activity relationships (SAR) of these compounds is summarized.

Expression of Phospholipase D in Periodontitis and Its Role in the Inflammatory and Osteoclastic Response by Nicotine- and Lipopolysaccharide-Stimulated Human Periodontal Ligament Cells

BACKGROUND

The aim of the present study is to investigate the expression of phospholipase D (PLD) 1 and PLD2 in periodontal patients and in human periodontal ligament cells (HPDLCs) exposed to nicotine plus lipopolysaccharide (LPS) from Porphyromonas gingivalis (Toll-like receptor 2 ligand). Furthermore, the effects of PLD isoform inhibition on the inflammatory response and osteoclast differentiation and its mechanisms were determined.

METHODS

Proinflammatory mediators were examined by reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay. To silence the gene expression of the PLD isoforms, cells were transfected with small interfering RNA (siRNA) targeting PLD1 or PLD2. Mouse bone marrow-derived macrophages (BMMs) were used as osteoclast precursor cells for in vitro osteoclastogenesis. Western blot analysis and immunofluorescence were used to assess signaling pathways.

RESULTS

Chronic smokers with periodontitis exhibited significantly higher PLD1 and PLD2 messenger RNA (mRNA) expression than non-smokers with periodontitis and healthy controls. Nicotine and LPS upregulated PLD1 and PLD2 mRNA expression in a dose-dependent manner in HPDLCs. Pharmacologic and siRNA-mediated inhibition of PLD1 and PLD2 attenuated the nicotine- and LPS-induced upregulation of inducible nitric oxide (NO) synthase and cyclooxygenase-2, production of NO, and prostaglandin E2, and mRNA expression and secretion of tumor necrosis factor-α, interleukin (IL)-1β, and IL-8. The conditioned media from HPDLCs treated with PLD isoform inhibitors or siRNA against PLD inhibited receptor activator of nuclear factor-κB (NF-κB) ligand-mediated osteoclast differentiation, as well as protein expression of nuclear factor of activated T cells c1 and c-Fos, in BMMs. In addition, PLD isoform inhibitors and siRNA inhibited the nicotine- and LPS-induced activation of phosphoinositide 3-kinase, protein kinase C, p38, extracellular signal-regulated kinase, c-Jun N-terminal protein kinase, mitogen-activated protein kinase, and NF-κB.

CONCLUSION

To the best of the authors' knowledge, this study is the first to demonstrate that PLD isoform inhibition has anti-inflammatory and antiosteoclastogenic effects and thus may be a therapeutic target for the treatment of periodontitis.

Human phospholipase D activity transiently regulates pyrimidine biosynthesis in malignant gliomas

Cancer cells reorganize their metabolic pathways to fuel demanding rates of proliferation. Oftentimes, these metabolic phenotypes lie downstream of prominent oncogenes. The lipid signaling molecule phosphatidic acid (PtdOH), which is produced by the hydrolytic enzyme phospholipase D (PLD), has been identified as a critical regulatory molecule for oncogenic signaling in many cancers. In an effort to identify novel regulatory mechanisms for PtdOH, we screened various cancer cell lines, assessing whether treatment of cancer models with PLD inhibitors altered production of intracellular metabolites. Preliminary findings lead us to focus on how deoxyribonucleoside triphosphates (dNTPs) are altered upon PLD inhibitor treatment in gliomas. Using a combination of proteomics and small molecule intracellular metabolomics, we show herein that PtdOH acutely regulates the production of these pyrimidine metabolites through activation of CAD via mTOR signaling pathways independently of Akt. These changes are responsible for decreases in dNTP production after PLD inhibitor treatment. Our data identify a novel regulatory role for PLD activity in specific cancer types.

Discovery of desketoraloxifene analogues as inhibitors of mammalian, Pseudomonas aeruginosa, and NAPE phospholipase D enzymes

Phospholipase D (PLD) hydrolyses cellular lipids to produce the important lipid second messenger phosphatidic acid. A PLD enzyme expressed by Pseudomonas aeruginosa (PldA) has been shown to be important in bacterial infection, and NAPE-PLD has emerged as being key in the synthesis of endocannabinoids. In order to better understand the biology and therapeutic potential of these less explored PLD enzymes, small molecule tools are required. Selective estrogen receptor modulators (SERMs) have been previously shown to inhibit mammalian PLD (PLD1 and PLD2). By targeted screening of a library of SERM analogues, additional parallel synthesis, and evaluation in multiple PLD assays, we discovered a novel desketoraloxifene-based scaffold that inhibited not only the two mammalian PLDs but also structurally divergent PldA and NAPE-PLD. This finding represents an important first step toward the development of small molecules possessing universal inhibition of divergent PLD enzymes to advance the field.