Nuclear localization of phospholipase D1 mediates the activation of nuclear protein kinase C(alpha) and extracellular signal-regulated kinase signaling pathways

Nuclear PLD1 combined with NPM1 induces gemcitabine resistance through tumorigenic IL7R in pancreatic adenocarcinoma

OBJECTIVE

Pancreatic ductal adenocarcinoma (PDAC) is a highly malignant gastrointestinal cancer with a 5-year survival rate of only 9%. Of PDAC patients, 15%-20% are eligible for radical surgery. Gemcitabine is an important chemotherapeutic agent for patients with PDAC; however, the efficacy of gemcitabine is limited due to resistance. Therefore, reducing gemcitabine resistance is essential for improving survival of patients with PDAC. Identifying the key target that determines gemcitabine resistance in PDAC and reversing gemcitabine resistance using target inhibitors in combination with gemcitabine are crucial steps in the quest to improve survival prognosis in patients with PDAC.

METHODS

We constructed a human genome-wide CRISPRa/dCas 9 overexpression library in PDAC cell lines to screen key targets of drug resistance based on sgRNA abundance and enrichment. Then, co-IP, ChIP, ChIP-seq, transcriptome sequencing, and qPCR were used to determine the specific mechanism by which phospholipase D1 (PLD1) confers resistance to gemcitabine.

RESULTS

PLD1 combines with nucleophosmin 1 (NPM1) and triggers NPM1 nuclear translocation, where NPM1 acts as a transcription factor to upregulate interleukin 7 receptor (IL7R) expression. Upon interleukin 7 (IL-7) binding, IL7R activates the JAK1/STAT5 signaling pathway to increase the expression of the anti-apoptotic protein, BCL-2, and induce gemcitabine resistance. The PLD1 inhibitor, Vu0155069, targets PLD1 to induce apoptosis in gemcitabine-resistant PDAC cells.

CONCLUSIONS

PLD1 is an enzyme that has a critical role in PDAC-associated gemcitabine resistance through a non-enzymatic interaction with NPM1, further promoting the downstream JAK1/STAT5/Bcl-2 pathway. Inhibiting any of the participants of this pathway can increase gemcitabine sensitivity.

Non-Vesicular Lipid Transport Machinery in Leishmania donovani: Functional Implications in Host-Parasite Interaction

Eukaryotic cells have distinct membrane-enclosed organelles, each with a unique biochemical signature and specialized function. The unique identity of each organelle is greatly governed by the asymmetric distribution and regulated intracellular movement of two important biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid-transfer proteins (LTPs) plays essential roles in intra-cellular lipid trafficking and cellular lipid homeostasis, while vesicular transport regulates protein trafficking. A comparative analysis of non-vesicular lipid transport machinery in protists could enhance our understanding of parasitism and basis of eukaryotic evolution. Leishmania donovani, the trypanosomatid parasite, greatly depends on receptor-ligand mediated signalling pathways for cellular differentiation, nutrient uptake, secretion of virulence factors, and pathogenesis. Lipids, despite being important signalling molecules, have intracellular transport mechanisms that are largely unexplored in L. donovani. We have identified a repertoire of sixteen (16) potential lipid transfer protein (LTP) homologs based on a domain-based search on TriTrypDB coupled with bioinformatics analyses, which signifies the presence of well-organized lipid transport machinery in this parasite. We emphasized here their evolutionary uniqueness and conservation and discussed their potential implications for parasite biology with regards to future therapeutic targets against visceral leishmaniasis.

Differential expression patterns of phospholipase D isoforms 1 and 2 in the mammalian brain and retina

Phosphatidic acid is a key signaling molecule heavily implicated in exocytosis due to its protein-binding partners and propensity to induce negative membrane curvature. One phosphatidic acid-producing enzyme, phospholipase D (PLD), has also been implicated in neurotransmission. Unfortunately, due to the unreliability of reagents, there has been confusion in the literature regarding the expression of PLD isoforms in the mammalian brain which has hampered our understanding of their functional roles in neurons. To address this, we generated epitope-tagged PLD1 and PLD2 knockin mice using CRISPR/Cas9. Using these mice, we show that PLD1 and PLD2 are both localized at synapses by adulthood, with PLD2 expression being considerably higher in glial cells and PLD1 expression predominating in neurons. Interestingly, we observed that only PLD1 is expressed in the mouse retina, where it is found in the synaptic plexiform layers. These data provide critical information regarding the localization and potential role of PLDs in the central nervous system.

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.

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.

Non-vesicular Lipid Transport Machinery in Entamoeba histolytica

Eukaryotic cells are organized into separate membrane-bound compartments that have specialized biochemical signature and function. Maintenance and regulation of distinct identity of each compartment is governed by the uneven distribution and intra-cellular movement of two essential biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid transfer proteins plays a pivotal role in intra-cellular lipid trafficking and homeostasis whereas vesicular transport plays a central role in protein trafficking. Comparative study of lipid transport machinery in protist helps to better understand the pathogenesis and parasitism, and provides insight into eukaryotic evolution. Amebiasis, which is caused by Entamoeba histolytica, is one of the major enteric infections in humans, resulting in 40–100 thousand deaths annually. This protist has undergone remarkable alterations in the content and function of its sub-cellular compartments as well represented by its unique diversification of mitochondrion-related organelle, mitosome. We conducted domain-based search on AmoebaDB coupled with bioinformatics analyses and identified 22 potential lipid transfer protein homologs in E. histolytica, which are grouped into several sub-classes. Such in silico analyses have demonstrated the existence of well-organized lipid transport machinery in this parasite. We summarized and discussed the conservation and unique features of the whole repertoire of lipid transport proteins in E. histolytica.

Phospholipase D1 downregulation by α-synuclein: Implications for neurodegeneration in Parkinson's disease

We have previously shown that phospholipase D (PLD) pathways have a role in neuronal degeneration; in particular, we found that PLD activation is associated with synaptic injury induced by oxidative stress. In the present study, we investigated the effect of α-synuclein (α-syn) overexpression on PLD signaling. Wild Type (WT) α-syn was found to trigger the inhibition of PLD1 expression as well as a decrease in ERK1/2 phosphorylation and expression levels. Moreover, ERK1/2 subcellular localization was shown to be modulated by WT α-syn in a PLD1-dependent manner. Indeed, PLD1 inhibition was found to alter the neurofilament network and F-actin distribution regardless of the presence of WT α-syn. In line with this, neuroblastoma cells expressing WT α-syn exhibited a degenerative-like phenotype characterized by a marked reduction in neurofilament light subunit (NFL) expression and the rearrangement of the F-actin organization, compared with either the untransfected or the empty vector-transfected cells. The gain of function of PLD1 through the overexpression of its active form had the effect of restoring NFL expression in WT α-syn neurons. Taken together, our findings reveal an unforeseen role for α-syn in PLD regulation: PLD1 downregulation may constitute an early mechanism in the initial stages of WT α-syn-triggered neurodegeneration.

Phospholipase D is involved in the formation of Golgi associated clathrin coated vesicles in human parotid duct cells

Phospholipase D (PLD) has been implicated in many cellular functions, such as vesicle trafficking, exocytosis, differentiation, and proliferation. The aim of this study was to characterize the role of PLD in HSY cells, a human cell line originating from the intercalated duct of the parotid gland. As the function and intracellular localization of PLD varies according to cell type, initially, the intracellular localization of PLD1 and PLD2 was determined. By immunofluorescence, PLD1 and PLD2 both showed a punctate cytoplasmic distribution with extensive co-localization with TGN-46. PLD1 was also found in the nucleus, while PLD2 was associated with the plasma membrane. Treatment of cells with the primary alcohol 1-butanol inhibits the hydrolysis of phosphatidylcoline by PLD thereby suppressing phosphatidic acid (PA) production. In untreated HSY cells, there was only a slight co-localization of PLD with the clathrin coated vesicles. When HSY cells were incubated with 1-butanol the total number of clathrin coated vesicles increased, especially in the juxtanuclear region and the co-localization of PLD with the clathrin coated vesicles was augmented. Transmission electron microscopy confirmed that the number of Golgi-associated coated vesicles was greater. Treatment with 1-butanol also affected the Golgi apparatus, increasing the volume of the Golgi saccules. The decrease in PA levels after treatment with 1-butanol likewise resulted in an accumulation of enlarged lysosomes in the perinuclear region. Therefore, in HSY cells PLD appears to be involved in the formation of Golgi associated clathrin coated vesicles as well as in the structural maintenance of the Golgi apparatus.

Phospholipase D-mediated autophagic regulation is a potential target for cancer therapy

Autophagy is a catabolic process in which cell components are degraded to maintain cellular homeostasis by nutrient limitations. Defects of autophagy are involved in numerous diseases, including cancer. Here, we demonstrate a new role of phospholipase D (PLD) as a regulator of autophagy. PLD inhibition enhances autophagic flux via ATG1 (ULK1), ATG5 and ATG7, which are essential autophagy gene products critical for autophagosome formation. Moreover, PLD suppresses autophagy by differentially modulating phosphorylation of ULK1 mediated by mTOR and adenosine monophosphate-activated protein kinase (AMPK), and by suppressing the interaction of Beclin 1 with vacuolar-sorting protein 34 (Vps34), indicating that PLD coordinates major players of the autophagic pathway, AMPK-mTOR-ULK1 and Vps34/Beclin 1. Ultimately, PLD inhibition significantly sensitized in vitro and in vivo cancer regression via genetic and pharmacological inhibition of autophagy, providing rationale for a new therapeutic approach to enhancing the anticancer efficacy of PLD inhibition. Collectively, we show a novel role for PLD in the molecular machinery regulating autophagy.

The hydrophobic amino acids involved in the interdomain association of phospholipase D1 regulate the shuttling of phospholipase D1 from vesicular organelles into the nucleus

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine to generate the lipid second messenger, phosphatidic acid. PLD is localized in most cellular organelles, where it is likely to play different roles in signal transduction. PLD1 is primarily localized in vesicular structures such as endosomes, lysosomes and autophagosomes. However, the factors defining its localization are less clear. In this study, we found that four hydrophobic residues present in the N-terminal HKD catalytic motif of PLD1, which is involved in intramolecular association, are responsible for vesicular localization. Site-directed mutagenesis of the residues dramatically disrupted vesicular localization of PLD1. Interestingly, the hydrophobic residues of PLD1 are also involved in the interruption of its nuclear localization. Mutation of the residues increased the association of PLD1 with importin-β, which is known to mediate nuclear importation, and induced the localization of PLD1 from vesicles into the nucleus. Taken together, these data suggest that the hydrophobic amino acids involved in the interdomain association of PLD1 are required for vesicular localization and disturbance of its nuclear localization.

Intermolecular association between caspase-mediated cleavage fragments of phospholipase D1 protects against apoptosis

Phospholipase D plays an anti-apoptotic role but little is known about dynamics of phospholipase D turnover during apoptosis. We have recently identified phospholipase D1 as a new substrate of caspases which generates the N-terminal and C-terminal fragment of phospholipase D1. In the present study, we tried to investigate whether association of the caspase cleavage fragments may be involved in regulation of apoptosis. Ectopically expressed C-terminal fragment, but not N-terminal fragment of phospholipase D1, is exclusively imported into the nucleus via a nuclear localization sequence; however, endogenous C-terminal fragment of phospholipase D1 from etoposide-induced apoptotic cells and Alzheimer's disease brain tissues with active caspase-3, was localized in the cytosolic fraction as well as the nuclear fraction. Intermolecular association between the two fragments of phospholipase D1 through hydrophobic residues within the catalytic motif inhibited nuclear localization of C-terminal fragment of phospholipase D1, and two catalytic motif and nuclear localization sequence regulated nuclocytoplasmic shuttling of phospholipase D1. Moreover, hydrophobic residues involved in the intermolecular association are also required for both its enzymatic activity and anti-apoptotic function. Taken together, we demonstrate that interdomain association and dissociation of phospholipase D1 might provide new insights into modulation of apoptosis.