Phosphatidic Acid Increases Epidermal Growth Factor Receptor Expression by Stabilizing mRNA Decay and by Inhibiting Lysosomal and Proteasomal Degradation of the Internalized Receptor

Phosphatidic Acid Promoting the Generation of Interleukin-17A Producing Double-Negative T Cells by Enhancing mTORC1 Signaling in Lupus

OBJECTIVE

The goal was to investigate the role and intracellular regulatory mechanisms of double-negative T (DNT) cells in the pathogenesis of systemic lupus erythematosus (SLE).

METHODS

DNT cells were assessed in murine models, patients with SLE, and controls using flow cytometry (FCM). DNT cells from either resiquimod (R848) or vehicle-treated C57BL/6 (B6) mice were cultured with B cells from R848-treated mice to explore functions. Differential mechanistic target of rapamycin (mTOR) pathway signaling in DNT cells measured using FCM and quantitative polymerase chain reaction was validated by rapamycin inhibition. Candidate lipid metabolites detected using liquid chromatography with electrospray ionization mass spectrometry/mass spectrometry were functionally assessed in DNT cell cultures.

RESULTS

DNT cells were markedly increased in both spontaneous and induced mouse lupus models and in patients with SLE. Expanded DNT cells from R848-treated B6 mice produced elevated interleukin (IL)-17A and IgG with increased germinal center B (GCB) cells. Expansion of DNT cells associated with activation of mTORC1 pathway that both IL-17A levels and the number of DNT cells exhibited dose-dependent reduction with rapamycin treatment. Lipidomics studies revealed differential patterns of lipid metabolites in T cells of R848-treated mice. Among candidate metabolites, elevated phosphatidic acid (PA) that was partially controlled by phospholipase D2 increased the expression of the mTORC1 downstream target p-S6 and positively expanded IL-17A-producing DNT cells. Similarly, elevated proportions of circulating DNT cells in patients with SLE correlated with disease activity and proteinuria, and IL-17A secretion was elevated after in vitro PA stimulation.

CONCLUSION

The accumulation of PA in T cells could activate the mTORC1 pathway, promoting DNT cell expansion and IL-17A secretion, resulting in GCB cell abnormalities in lupus.

[Basic Research on Bullfrog Egg-derived Sialic Acid-binding Lectin for Cancer Treatment]

Sialic acid-binding lectin from Rana catesbeiana (cSBL) is a multifunctional protein with both lectin and ribonuclease activity and is, therefore, called a leczyme. It exerts cancer cell-selective antitumor effects on a variety of cancer cells in vitro and in vivo under conditions where no undesired side effects are observed. cSBL elicits antitumor effects by degrading cellular RNA and subsequently inducing apoptosis via a pathway mediated by mitochondria and endoplasmic reticulum stress. Further, it exerts synergistic antitumor effects with other molecules such as tumor necrosis factor-related apoptosis-inducing ligand and pemetrexed. Recent studies have revealed that long-term treatment of cancer cells with cSBL causes significant pleiotropic changes in the expression profiles of several genes, including multiple genes involved in metabolic pathways. Furthermore, cSBL reduces the expression of some cancer-related molecules such as human epidermal growth factor receptors, aldo-keto reductase 1B10, and ATP-binding cassette transporter C2. The information described above is expected to lead to useful applications, such as effective regimens comprising cSBL and other drugs. These findings reveal favorable properties of cSBL as an anticancer drug, which may contribute to the development of new therapeutic strategies for cancer treatment.

Multi-omics analysis unravels dysregulated lysosomal function and lipid metabolism involved in sub-chronic particulate matter-induced pulmonary injury

Particulate matter (PM) is a huge environmental threat and is of major public concern. Oxidative stress and systemic inflammation are known factors that contribute to PM- related damage; however, a systematic understanding of the deleterious pulmonary effects of PM using multi-omics analysis is lacking. In this study, we performed transcriptomic, proteomic, and metabolomic analyses in a mouse model exposed to PM for three months to identify molecular changes in lung tissues. We identified 1690 genes, 326 proteins, and 67 metabolites exhibiting significant differences between PM-challenged and control mice (p < 0.05). Differentially expressed genes and proteins regulated in PM-challenged mice were involved in lipid metabolism and in the immune and inflammatory response processes. Moreover, a comprehensive analysis of transcript, protein, and metabolite datasets revealed that the genes, proteins, and metabolites in the PM-treated group were involved in lysosomal function and lipid metabolism. Specifically, Cathepsin D (Ctsd), Ferritin light chain (Ftl), Lactotransferrin (Ltf), Lipocalin 2 (Lcn2), and Prosaposin (Psap) were major proteins/genes associated with PM-induced pulmonary damage, while two lipid molecules PC (18:1(11Z)/16:0) and PA (16:0/18:1(11Z)) were major metabolites related to PM-induced pulmonary injury. In summary, lipid metabolism might be used as successful precautions and therapeutic targets in PM-induced pulmonary injury to maintain the stability of cellular lysosomal function.

Phosphatidic acid inhibits inositol synthesis by inducing nuclear translocation of kinase IP6K1 and repression of myo-inositol-3-P synthase

Inositol is an essential metabolite that serves as a precursor for structural and signaling molecules. Although perturbation of inositol homeostasis has been implicated in numerous human disorders, surprisingly little is known about how inositol levels are regulated in mammalian cells. A recent study in mouse embryonic fibroblasts demonstrated that nuclear translocation of inositol hexakisphosphate kinase 1 (IP6K1) mediates repression of myo-inositol-3-P synthase (MIPS), the rate-limiting inositol biosynthetic enzyme. Binding of IP6K1 to phosphatidic acid (PA) is required for this repression. Here, we elucidate the role of PA in IP6K1 repression. Our results indicate that increasing PA levels through pharmacological stimulation of phospholipase D (PLD) or direct supplementation of 18:1 PA induces nuclear translocation of IP6K1 and represses expression of the MIPS protein. We found that this effect was specific to PA synthesized in the plasma membrane, as endoplasmic reticulum–derived PA did not induce IP6K1 translocation. Furthermore, we determined that PLD-mediated PA synthesis can be stimulated by the master metabolic regulator 5′ AMP-activated protein kinase (AMPK). We show that activation of AMPK by glucose deprivation or by treatment with the mood-stabilizing drugs valproate or lithium recapitulated IP6K1 nuclear translocation and decreased MIPS expression. This study demonstrates for the first time that modulation of PA levels through the AMPK-PLD pathway regulates IP6K1-mediated repression of MIPS.

The phospholipase D inhibitor FIPI potently blocks EGF-induced calcium signaling in human breast cancer cells

BACKGROUND

Phosphotyrosine kinase (PTK)-mediated phospholipase C-γ1 (PLC-γ1) signaling plays a crucial role in the release of the universal second messenger calcium from intracellular stores, which is mandatory for several cellular processes, including cell migration. However, PLC-γ1 could also be activated in a PTK-independent manner by phospholipase D (PLD)-derived phosphatidic acid (PA). Because both higher PLD expression levels and PLD activity have also been associated with breast cancer cell invasion and migration, we wondered whether there might be a link between PLD and PLC-γ1, which was investigated in this study.

MATERIALS

MDA-MB-468-NEO (EGFR positive) and MDA-MB-468-HER2 (EGFR and HER2 positive) human breast cancer cells were used in this study. The migratory behavior of the cells in the presence of epidermal growth factor (EGF) and the PLD inhibitor 5-fluoro-2-indolyl-des-chlorohalopemide (FIPI) was analyzed using the 3D collagen matrix migration assay. Changes in cytosolic calcium levels in the presence of EGF, FIPI and Sig-1R agonists and antagonists as well as in PLD1 siRNA knockdown cells were determined by flow cytometry. Western blot analyses were performed to determine the basal expression levels and phosphorylation patterns of EGFR, HER2, AKT, MAPKp42/44, PLC-γ1 and Sig-1R.

RESULTS

The EGF-induced migration of MDA-MB-468-NEO and MDA-MB-468-HER2 cells was significantly impaired by FIPI. Likewise, FIPI also significantly abolished EGF-induced calcium release in both cell lines. However, neither the expression levels nor the phosphorylation patterns of EGFR, HER2, AKT, MAPKp42/44 and PLC-γ1 were markedly changed by FIPI. Knockdown of PLD1 expression by siRNA also significantly impaired EGF-induced calcium release in both cell lines. Targeting Sig-1R, which interacts with IP3R, with the antagonist BD1047 also abrogated EGF-induced calcium release. However, EGF-induced calcium release was also impaired if cells were treated with the Sig-1R agonists PRE084 and PPBP maleate.

CONCLUSION

In summary, blocking PLD activity with the specific inhibitor FIPI or knocking down PDL1 expression by siRNA significantly impaired EGF-induced calcium release in MDA-MB-468-NEO and MDA-MB-468-HER2 cells, likely indicating a connection between PLD activity and PLC-γ1-mediated calcium signaling. However, how PLD activity interferes with the release of calcium from intracellular stores remains unclear. Video Abstract.

Musashi-2 (MSI2) regulates epidermal growth factor receptor (EGFR) expression and response to EGFR inhibitors in EGFR-mutated non-small cell lung cancer (NSCLC)

Non-small cell lung cancer (NSCLC) has limited treatment options. Expression of the RNA-binding protein (RBP) Musashi-2 (MSI2) is elevated in a subset of non-small cell lung cancer (NSCLC) tumors upon progression, and drives NSCLC metastasis. We evaluated the mechanism of MSI2 action in NSCLC to gain therapeutically useful insights. Reverse phase protein array (RPPA) analysis of MSI2-depleted versus control Kras^LA1/+; Trp53^R172HΔG/+ NSCLC cell lines identified EGFR as a MSI2-regulated protein. MSI2 control of EGFR expression and activity in an NSCLC cell line panel was studied using RT-PCR, Western blots, and RNA immunoprecipitation. Functional consequences of MSI2 depletion were explored for cell growth and response to EGFR-targeting drugs, in vitro and in vivo. Expression relationships were validated using human tissue microarrays. MSI2 depletion significantly reduced EGFR protein expression, phosphorylation, or both. Comparison of protein and mRNA expression indicated a post-transcriptional activity of MSI2 in control of steady state levels of EGFR. RNA immunoprecipitation analysis demonstrated that MSI2 directly binds to EGFR mRNA, and sequence analysis predicted MSI2 binding sites in the murine and human EGFR mRNAs. MSI2 depletion selectively impaired cell proliferation in NSCLC cell lines with activating mutations of EGFR (EGFR^mut). Further, depletion of MSI2 in combination with EGFR inhibitors such as erlotinib, afatinib, and osimertinib selectively reduced the growth of EGFR^mut NSCLC cells and xenografts. EGFR and MSI2 were significantly co-expressed in EGFR^mut human NSCLCs. These results define MSI2 as a direct regulator of EGFR protein expression, and suggest inhibition of MSI2 could be of clinical value in EGFR^mut NSCLC.

Phosphatidic acid increases Notch signalling by affecting Sanpodo trafficking during Drosophila sensory organ development

Organ cell diversity depends on binary cell-fate decisions mediated by the Notch signalling pathway during development and tissue homeostasis. A clear example is the series of binary cell-fate decisions that take place during asymmetric cell divisions that give rise to the sensory organs of Drosophila melanogaster. The regulated trafficking of Sanpodo, a transmembrane protein that potentiates receptor activity, plays a pivotal role in this process. Membrane lipids can regulate many signalling pathways by affecting receptor and ligand trafficking. It remains unknown, however, whether phosphatidic acid regulates Notch-mediated binary cell-fate decisions during asymmetric cell divisions, and what are the cellular mechanisms involved. Here we show that increased phosphatidic acid derived from Phospholipase D leads to defects in binary cell-fate decisions that are compatible with ectopic Notch activation in precursor cells, where it is normally inactive. Null mutants of numb or the α-subunit of Adaptor Protein complex-2 enhance dominantly this phenotype while removing a copy of Notch or sanpodo suppresses it. In vivo analyses show that Sanpodo localization decreases at acidic compartments, associated with increased internalization of Notch. We propose that Phospholipase D-derived phosphatidic acid promotes ectopic Notch signalling by increasing receptor endocytosis and inhibiting Sanpodo trafficking towards acidic endosomes.

Synergistic effect between LH and estrogen in the acceleration of cumulus expansion via GPR30 and EGFR pathways

The estrogen membrane receptor GPR30 (also known as G-protein coupled receptor 30) has recently been shown to be involved in the regulation of oocyte maturation and cumulus expansion. However, whether GPR30 expression is regulated by gonadotropin stimulation and how it participates in the regulation of the maturation process is still not clear. In this study, we explored the mechanism underlying the synergy between luteinizing hormone and 17β-estradiol (17β-E₂) to improve the epidermal growth factor (EGF) response in cumulus oocyte complexes (COCs) during oocyte maturation in mice. The expression and distribution of GPR30, EGFR, and EGF-like growth factors were examined by real-time quantitative PCR, western blot, and immunofluorescence staining. Lyso-Tracker Red labeling was performed to detect the lysosomal activity in follicle granular cells (FGCs). Cumulus expansion of COCs was evaluated after in vitro maturation for 16 h. We found that EGF-like growth factors transmit LH signals to increase GRP30 levels by inhibiting protein degradation in lysosomes. Meanwhile, 17β-E₂ stimulates the GPR30 signaling pathway to increase EGF receptor levels, enhancing the response ability of EGF signaling in COCs and thus promoting cumulus expansion. In conclusion, our study reveals the synergistic mechanism between LH and estrogen in the regulation of cumulus expansion during oocyte maturation process.

D-series Resolvins activate Phospholipase D in phagocytes during inflammation and resolution

A successful acute inflammatory response results in the elimination of infectious agents by neutrophils and monocytes, followed by resolution and repair through tissue-resident and recruited macrophages. Resolvins (D-series and E-series) are pro-resolving lipid mediators involved in resolution and tissue repair, whose intracellular signaling remains of interest. Here, we report that D-series resolvins (RvD1- RvD5) activate phospholipase D (PLD), a ubiquitously expressed membrane lipase enzyme activity in modulating phagocyte functions. The mechanism for PLD-mediated actions of Resolvin-D5 (RvD5) in polarizing macrophages (M1-like toward M2-like) was found to be two-pronged: (a) RvD5 inhibits post-transcriptional modifications, by miRs and 3'exonucleases that process PLD2 mRNA, thus increasing PLD2 expression and activity; and (b) RvD5 enhances PLD2-S6Kinase signaling required for membrane expansion and efferocytosis. In an in vivo model of second organ reflow injury, we found that RvD5 did not reduce lung neutrophil myeloperoxidase levels in PLD2^-/- mice compared to WT and PLD1^-/- mice, confirming a novel role of PLD2 as the isoform in RvD5-mediated resolution processes. These results demonstrate that RvD5-PLD2 are attractive targets for therapeutic interventions in vascular inflammation such as ischemia-reperfusion injury and cardiovascular diseases.

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.

Sialic Acid-Binding Lectin from Bullfrog Eggs Exhibits an Anti-Tumor Effect Against Breast Cancer Cells Including Triple-Negative Phenotype Cells

Sialic acid-binding lectin from Rana catesbeiana eggs (cSBL) is a multifunctional protein that has lectin and ribonuclease activity. In this study, the anti-tumor activities of cSBL were assessed using a panel of breast cancer cell lines. cSBL suppressed the cell growth of all cancer cell lines tested here at a concentration that is less toxic, or not toxic at all, to normal cells. The growth suppressive effect was attributed to the cancer-selective induction of apoptosis. We assessed the expressions of several key molecules associated with the breast cancer phenotype after cSBL treatment by western blotting. cSBL decreased the expression level of estrogen receptor (ER) α, while it increased the phosphorylation level of p38 mitogen-activated protein kinase (MAPK). cSBL also suppressed the expression of the progesterone receptor (PgR) and human epidermal growth factor receptor type 2 (HER2). Furthermore, it was revealed that cSBL decreases the expression of the epidermal growth factor receptor (EGFR/HER1) in triple-negative breast cancer cells. These results indicate that cSBL induces apoptosis with decreasing ErbB family proteins and may have great potential for breast cancer chemotherapy, particularly in triple-negative phenotype cells.

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

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

A Phosphatidic Acid (PA) conveyor system of continuous intracellular transport from cell membrane to nucleus maintains EGF receptor homeostasis

The intracellular concentration of the mitogen phosphatidic acid (PA) must be maintained at low levels until the need arises for cell proliferation. How temporal and spatial trafficking of PA affects its target proteins in the different cellular compartments is not fully understood. We report that in cancer cells, PA cycles back and forth from the cellular membrane to the nucleus, affecting the function of epidermal growth factor (EGF), in a process that involves PPARα/LXRα signaling. Upon binding to its ligand, EGF receptor (EGFR)-initiated activation of phospholipase D (PLD) causes a spike in intracellular PA production that forms vesicles transporting EGFR from early endosomes (EEA1 marker) and prolonged internalization in late endosomes and Golgi (RCAS marker). Cells incubated with fluorescent-labeled PA (NBD-PA) show PA in “diffuse” locations throughout the cytoplasm, punctae (small, <0.1 μm) vesicles) and large (>0.5 μm) vesicles that co-localize with EGFR. We also report that PPARα/LXRα form heterodimers that bind to new Responsive Elements (RE) in the EGFR promoter. Nuclear PA enhances EGFR expression, a role compatible with the mitogenic ability of the phospholipid. Newly made EGFR is packaged into PA recycling vesicles (Rab11 marker) and transported back to the cytoplasm and plasma membrane. However, a PLD+PA combination impedes binding of PPARα/LXRα to the EGFR promoter. Thus, if PA levels inside the nucleus reach a certain threshold (>100 nM) PA outcompetes the nuclear receptors and transcription is inhibited. This new signaling function of PLD-PA targeting EGFR trafficking and biphasically modulating its transcription, could explain cell proliferation initiation and its maintenance in cancer cells.

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

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.

Summary: Cell signaling enzyme phospholipase D2 (PLD2) and its reaction product, phospholipid phosphatidic acid (PA), are involved in mediating PARN-induced eukaryotic mRNA decay.

Novel Ribonuclease Activity Differs between Fibrillarins from Arabidopsis thaliana

Fibrillarin is one of the most important nucleolar proteins that have been shown as essential for life. Fibrillarin localizes primarily at the periphery between fibrillar center and dense fibrillar component as well as in Cajal bodies. In most plants there are at least two different genes for fibrillarin. In Arabidopsis thaliana both genes show high level of expression in transcriptionally active cells. Here, we focus on two important differences between A. thaliana fibrillarins. First and most relevant is the enzymatic activity by AtFib2. The AtFib2 shows a novel ribonuclease activity that is not seen with AtFib1. Second is a difference in the ability to interact with phosphoinositides and phosphatidic acid between both proteins. We also show that the novel ribonuclease activity as well as the phospholipid binding region of fibrillarin is confine to the GAR domain. The ribonuclease activity of fibrillarin reveals in this study represents a new role for this protein in rRNA processing.

Biochemical, Biophysical and Cellular Techniques to Study the Guanine Nucleotide Exchange Factor, GIV/Girdin

Canonical signal transduction via heterotrimeric G proteins is spatiotemporally restricted, i.e., triggered exclusively at the plasma membrane, only by agonist activation of G protein-coupled receptors via a finite process that is terminated within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a noncanonical pathway for activation of heterotrimeric G proteins via the nonreceptor guanidine-nucleotide exchange factor, GIV/Girdin. Biochemical, biophysical, and functional studies evaluating this pathway have unraveled its unique properties and distinctive spatiotemporal features. As in the case of any new pathway/paradigm, these studies first required an in-depth optimization of tools/techniques and protocols, governed by rationale and fundamentals unique to the pathway, and more specifically to the large multimodular GIV protein. Here we provide the most up-to-date overview of protocols that have generated most of what we know today about noncanonical G protein activation by GIV and its relevance in health and disease. © 2016 by John Wiley & Sons, Inc.

PLD Protein-Protein Interactions With Signaling Molecules and Modulation by PA

We describe methods for studying phospholipase D (PLD) interactions with signaling proteins and modulation of these interactions by the PLD reaction product, phosphatidic acid (PA). PLD is fundamental to the physiological maintenance of cellular/intracellular membranes, protein trafficking, cytoskeletal dynamics, membrane remodeling, cell proliferation, meiotic division and sporulation. PA is an acidic phospholipid involved in the biosynthesis of many other lipids that affects the enzymatic activities of many different signaling proteins via protein-lipid interactions or as a substrate. The involvement of PLD as an effector of protein-protein interactions and downstream signaling via PA-mediated processes has led to the investigation of PA-binding domains in target protein partners. We present here data and protocols detailing the interaction between PLD2-Rac2 interaction and modulation of this interaction by PA. We describe biochemical techniques to measure interactions between PLD, PA, and the small GTPase Rac2, which are associated in the cell. We found two maxima concentrations of PA that contributed to association or dissociation of Rac2 with PLD2, as well as the PLD2 lipase and guanine nucleotide exchange factor (GEF) activities. Fluctuations in the Rac2-PLD2 protein-protein binding interaction facilitate shuttling of Rac2 and/or PLD2 within the cell dependent on local cellular PA concentration. Fluorescence resonance emission transfer stoichiometry for PLD2 and Rac2 binding yielded a 3:1 ratio of Rac2:PLD2. Detection of PA in mammalian cells with a new biosensor showed colocalization in and around the nucleus. We also described methods for quantitation of PA in biological materials by HPLC electrospray ionization tandem mass spectrometry.

Molecular Pathways: Targeting Diacylglycerol Kinase Alpha in Cancer

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