New Concepts in Phospholipase D Signaling in Inflammation and Cancer

Phospholipase D (PLD) catalyzes the hydrolysis of phosphatidylcholine to generate the lipid second messenger phosphatidic acid (PA) and choline. PLD regulation in cells falls into two major signaling categories. One is via growth factors/mitogens, such as EGF, PDGF, insulin, and serum, and implicates tyrosine kinases; the other is via the small GTPase proteins Arf and Rho. We summarize here our lab's and other groups' contributions to those pathways and introduce several novel concepts. For the mitogen-induced signaling, new data indicate that an increase in cell transformation in PLD2-overexpressing cells is due to an increase of de novo DNA synthesis induced by PLD2, with the specific tyrosine residues involved in those functions being Y179 and Y511. Recent research has also implicated Grb2 in tyrosine phosphorylation of PLD2 that also involves Sos and the ERK pathway. The targets of phosphorylation within the PLD2 molecule that are key to its regulation have recently been precisely mapped. They are Y296, Y415, and Y511 and the responsible kinases are, respectively, EGFR, JAK3, and Src. Y296 is an inhibitory site and its phosphorylation explains the low PLD2 activity that exists in low-invasive MCF-7 breast cancer cells. Advances along the small GTPase front have implicated cell migration, as PLD1 and PLD2 cause an increase in chemotaxis of leukocytes and inflammation. PA is necessary for full chemotaxis. PA enriches the localization of the atypical guanine exchange factor (GEF), DOCK2, at the leading edge of polarized neutrophils. Further, extracellular PA serves as a neutrophil chemoattractant; PA enters the cell and activates the mTOR/S6K pathway (specifically, S6K). A clear connection between PLD with the mTOR/S6K pathway has been established, in that PA binds to mTOR and also binds to S6K independently of mTOR. Lastly, there is evidence in the upstream direction of cell signaling that mTOR and S6K keep PLD2 gene expression function down-regulated in basal conditions. In summary, the involvement of PLD2 in cell signaling continues to expand geometrically. It involves gene transcription, mitogenic and cell migration effects as seen in normal growth, tumor development, and inflammation

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

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

A feedback mechanism between PLD and deadenylase PARN for the shortening of eukaryotic poly(A) mRNA tails that is deregulated in cancer cells

ABSTRACT The removal of mRNA transcript poly(A) tails by 3′→5′ exonucleases is the rate-limiting step in mRNA decay in eukaryotes. Known cellular deadenylases are the CCR4-NOT and PAN complexes, and poly(A)-specific ribonuclease (PARN). The physiological roles and regulation for PARN is beginning to be elucidated. Since phospholipase D (PLD2 isoform) gene expression is upregulated in breast cancer cells and PARN is downregulated, we examined whether a signaling connection existed between these two enzymes. Silencing PARN with siRNA led to an increase in PLD2 protein, whereas overexpression of PARN had the opposite effect. Overexpression of PLD2, however, led to an increase in PARN expression. Thus, PARN downregulates PLD2 whereas PLD2 upregulates PARN. Co-expression of both PARN and PLD2 mimicked this pattern in non-cancerous cells (COS-7 fibroblasts) but, surprisingly, not in breast cancer MCF-7 cells, where PARN switches from inhibition to activation of PLD2 gene and protein expression. Between 30 and 300 nM phosphatidic acid (PA), the product of PLD enzymatic reaction, added exogenously to culture cells had a stabilizing role of both PARN and PLD2 mRNA decay. Lastly, by immunofluorescence microscopy, we observed an intracellular co-localization of PA-loaded vesicles (0.1-1 nm) and PARN. In summary, we report for the first time the involvement of a phospholipase (PLD2) and PA in mediating PARN-induced eukaryotic mRNA decay and the crosstalk between the two enzymes that is deregulated in breast cancer cells

PLD-Specific Small-Molecule Inhibitors Decrease Tumor-Associated Macrophages and Neutrophils Infiltration in Breast Tumors and Lung and Liver Metastases

Phospholipase D-2 (PLD2) has a key role in breast cancer formation and metastasis formation with PLD small inhibitors reducing primary tumor growth. This study aimed to evaluate the importance of targeting PLD on the tumor microenvironment. We provide evidence about the beneficial effect of PLD inhibitors [FIPI (dual PLD1/PLD2) or VU0155072-2 (PLD2 inhibitor)] on avoiding infiltration of tumor-helping macrophages and neutrophils. Tumor growth and metastasis within the primary tumors had low (100 μm) clusters in lungs. However, PLD inhibitors, particularly FIPI, were able to diminish leukocyte presence. Ex vivo chemotaxis and PLD activity of peripheral blood neutrophils (PMN) and peritoneal macrophages was also determined. Whereas PMN had impaired functionality, macrophages did not. This significantly increased (“emboldened”) macrophage function was due to PLD inhibition. Since tumor-associated leukocytes in primary tumors and metastases were targeted via PLD inhibition, we posit that these inhibitors have a key role in cancer regression, while still affording an appropriate inflammatory response at least from off-site innate immunity macrophages

A Comprehensive Model That Explains the Regulation of Phospholipase D2 Activity by Phosphorylation-Dephosphorylation ▿

We report here that the enzymatic activity of phospholipase D2 (PLD2) is regulated by phosphorylation-dephosphorylation. Phosphatase treatment of PLD2-overexpressing cells showed a biphasic nature of changes in activity that indicated the existence of “activator” and “inhibitory” sites. We identified three kinases capable of phosphorylating PLD2 in vitro—epidermal growth factor receptor (EGFR), JAK3, and Src (with JAK3 reported for the first time in this study)—that phosphorylate an inhibitory, an activator, and an ambivalent (one that can yield either effect) site, respectively. Mass spectrometry analyses indicated the target of each of these kinases as Y296 for EGFR, Y415 for JAK3, and Y511 for Src. The extent to which each site is activated or inhibited depends on the cell type considered. In COS-7, cells that show the highest level of PLD2 activity, the Y415 is a prominent site, and JAK3 compensates the negative modulation by EGFR on Y296. In MCF-7, cells that show the lowest level of PLD2 activity, the converse is the case, with Y296 unable to compensate the positive modulation by Y415. MTLn3, with medium to low levels of lipase activity, show an intermediate pattern of regulation but closer to MCF-7 than to COS-7 cells. The negative effect of EGFR on the two cancer cell lines MTLn3 and MCF-7 is further proven by RNA silencing experiments that yield COS-7 showing lower PLD2 activity, and MTLn3 and MCF-7 cells showing an elevated activity. MCF-7 is a cancer cell line derived from a low-aggressive/invasive form of breast cancer that has relatively low levels of PLD activity. We propose that PLD2 activity is low in the breast cancer cell line MCF-7 because it is kept downregulated by tyrosyl phosphorylation of Y296 by EGFR kinase. Thus, phosphorylation of PLD2-Y296 could be the signal for lowering the level of PLD2 activity in transformed cells with low invasive capabilities

The hepatic cellular environment is altered with PLD inhibitors.

<p>Liver sections were incubated with hematoxylin and eosin (<b>A-D</b>) or antibodies specific for Arg1 (<b>E-H</b>), F4/80 (<b>J-M</b>) or LY6G (<b>N-Q</b>) for immunofluorescence microscopy. Xenotransplanted MDA-MB-231 cells in the presence of vehicle, FIPI or VU0155072-2, respectively, and no xenotransplantation in the presence of vehicle. (<b>A-D</b>) Scale bar = 50 μm. (<b>E-Q</b>) Scale bar = 200 μm. (<b>I</b>) ImageJ quantification of the negative effect of PLD inhibitors on the total area of Arg1 positive foci in liver tissues in terms of μm² x 10³. Sample sizes were n = 5 for each set of animals used and 6–8 fields were viewed for each section. Statistical analyses for the increases (*, p<0.05, ANOVA) in xenotransplants and the decreases (#, p<0.05, ANOVA) in PLD inhibitors-treated samples.</p

Presence of tumor-associated macrophage (TAM) F4/80 and tumor-associated neutrophil (TAN) Ly6G in xenotransplanted mice are suppressed by PLD inhibitors.

<p>(<b>A,C</b>) Immunofluorescence staining of mouse primary breast tumor sections using either a rat anti-mouse F4/80 AlexaFluor 488 IgG antibody specific for TAMs (<b>A</b>) or a rat anti-mouse Ly6G-R-PE IgG antibody specific for TANs (<b>C</b>). Each set of three panels represents results from Xenotransplanted MDA-MB-231 cells in the presence of vehicle, FIPI or VU0155072-2, respectively. Scale bar = 200 μm. (<b>B,D</b>) ImageJ quantification of the negative effect of PLD inhibitors on the total mean fluorescence of each field of view for either F4/80 (<b>B</b>) or Ly6G (<b>D</b>) in primary metastatic breast tumors. Sample sizes were n = 5 for each set of animals used and 6–8 fields were viewed for each section. Statistical analyses for the increases (*, p<0.05, ANOVA) in xenotransplants and the decreases (#, p<0.05, ANOVA) in PLD inhibitors-treated samples.</p