Selectivity of the diacylglycerol kinase inhibitor 3-[2-(4-[bis-(4-fluorophenyl)methylene]-1-piperidinyl)ethyl]-2, 3-dihydro-2-thioxo-4(1H)quinazolinone (R59949) among diacylglycerol kinase subtypes

Diacylglycerol kinases are essential for hepatocyte growth factor-dependent proliferation and motility of Kaposi's sarcoma cells

Hepatocyte growth factor (HGF) is involved in the pathogenesis of Kaposi's sarcoma (KS), the most frequent neoplasia in patients with AIDS, characterized by proliferating spindle cells, infiltrating inflammatory cells, angiogenesis, edema, and invasiveness. In vitro, this factor sustains the biological behavior of KS derived cells, after activation of its receptor and the downstream MAPK and AKT signals. In other cell types, namely endothelial and epithelial cells, movement, proliferation, and survival stimulated by HGF and other growth factors and cytokines depend on diacylglycerol kinases (DGK). In an effort to identify new intracellular transducers operative in KS cells, which could represent therapeutic targets, we investigated the role of DGK in KS cell movement and proliferation by treating cells with the DGK pharmacological inhibitor R59949. We report that R59949 strongly inhibits HGF-induced KS motility, proliferation, and anchorage-independent growth with only a partial effect on cell adhesion and spreading. R59949 does not affect cell survival, HGF receptor activation, or the classical MAPK and AKT signalling pathways. Furthermore, we carried out an siRNA screen to characterize the DGK isoforms involved in KS motility and anchorage independent growth. Our data indicate a strong involvement of DGK-δ in KS motility and of DGK-ι in anchorage-independent growth. These results indicate that DGK inhibition is sufficient to impair in vitro KS cell proliferation and movement and suggest that selected DGK represent new pharmacological targets to interfere with the malignant properties of KS, independently from the well-known RAS/MAPK and PI3K/AKT pathways.

Diacylglycerol kinase α regulates the formation and polarisation of mature multivesicular bodies involved in the secretion of Fas ligand-containing exosomes in T lymphocytes

Multivesicular bodies (MVBs) are endocytic compartments that contain intraluminal vesicles formed by inward budding from the limiting membrane of endosomes. In T lymphocytes, these vesicles contain pro-apoptotic Fas ligand (FasL), which may be secreted as 'lethal exosomes' upon fusion of MVBs with the plasma membrane. Diacylglycerol kinase α (DGKα) regulate the secretion of exosomes, but it is unclear how this control is mediated. T-lymphocyte activation increases the number of MVBs that contain FasL. DGKα is recruited to MVBs and to exosomes in which it has a double function. DGKα kinase activity exerts a negative role in the formation of mature MVBs, as we demonstrate by the use of an inhibitor. Downmodulation of DGKα protein resulted in inhibition of both the polarisation of MVBs towards immune synapse and exosome secretion. The subcellular location of DGKα together with its complex role in the formation and polarised traffic of MVBs support the notion that DGKα is a key regulator of the polarised secretion of exosomes.

Insulin modifies aging-related inhibition of 1-stearoyl, 2-arachidonoylglycerol phosphorylation in rat synaptic terminals

The purpose of the present study was to analyze diacylglycerol kinase (DAGK) activity in synaptic terminals from cerebral cortex (CC) and hippocampus (Hp) from adult (3-4 month-old) and aged (26-28 month-old) rats. The effect of insulin through DAGK activity on synaptosomes from adult and aged rats was also analyzed under conditions favoring saturated or unsaturated phosphatidic acid (PA) formation, using exogenous di-palmitoil glycerol (DPG) or 1-stearoyl-2-arachidonoylglycerol (SAG) as substrates. Results showed that the enzymatic activity preferentially uses SAG as substrate, thus indicating the presence of ɛ-type DAGK. A significant decrease in DAGK activity transforming SAG into PA was also observed in both tissues from aged rats. Western blot detection of DAGKɛ showed that enzyme content undergoes no changes with aging. [3H] inositol incorporation into phosphoinosites was also analyzed to evaluate the role of DAGKɛ in their synthesis. Data obtained from 3H-inositol incorporation into phosphoinositides revealed that in synaptosomes from aged rats phosphatidylinositol (PI) synthesis is lower than in adult animals. Interestingly, in the presence of SAG, PI synthesis was restored to adult values. DAGK activity over SAG was more highly stimulated by insulin in CC and Hp synaptosomes of aged rats with respect to adult rats. On the other hand, insulin exerted a stimulatory effect on PI and phosphatidylinositol 4 phosphate (PI(4)P) synthesis in synaptosomal CC from aged rats. Taken together, our findings indicate that in aged rats insulin triggers a stimulatory mechanism that reverts the diminished synaptosomal ability to synthesize arachidonoyl phosphatidic acid (20:4 PA). The recovery of this PA species indicates that insulin positively regulates phosphoinositide synthesis.

Phosphatidic acid plays a regulatory role in clathrin-mediated endocytosis

We have manipulated the activities of PLD and DGK, enzymes that regulate PA biosynthesis, and directly measured their effects on cellular PA levels and on clathrin-mediated endocytosis (CME). We report a previously unappreciated complexity in PA regulation and show that PA selectively regulates CME of EGF but not transferrin.

Clathrin-mediated endocytosis (CME) is the main route of internalization of receptor-ligand complexes. Relatively little is known about the role of specific lipids in CME, in particular that of phosphatidic acid (PA). We examined the effect of altering cellular PA levels on CME by manipulating the activities and/or levels of either phospholipase D (PLD1 and PLD2) or diacylglycerol kinase (DGK), two enzyme classes involved in PA production. DGK inhibition resulted in a dramatic reduction of cellular PA, measured directly using an enzyme-coupled reaction, which resulted in a decreased rate of EGFR internalization measured biochemically. This corresponded to a decreased rate of clathrin-coated pit (CCP) initiation and increased lifetimes of productive CCPs, as determined by quantitative live-cell total internal reflection fluorescence microscopy. Unexpectedly, PLD inhibition caused an increase in cellular PA, suggesting that PLD activity negatively regulates PA synthesis by other more productive pathways. Consistent with opposite effects on cellular PA levels, PLD inhibition had opposite effects on EGFR internalization and CCP dynamics, compared with DGK inhibition. Importantly, the constitutive internalization of transferrin receptors was unaffected by either treatment. These findings demonstrate that PA plays a regulatory rather than obligatory role in CME and differentially regulates ligand-stimulated CME of EGFR.

Role of phospholipase D2/phosphatidic acid signal transduction in micro- and delta-opioid receptor endocytosis

We demonstrated recently that opioid-induced activation of phospholipase D2 (PLD2) enhances mu- (MOPr) and delta-opioid receptor endocytosis/recycling and thus reduces the development of opioid receptor desensitization and tolerance. However, the mechanistic basis for the PLD2-mediated induction of opioid receptor endocytosis is currently unknown. Here we show that PLD2-generated phosphatidic acid (PA) might play a key role in facilitating the endocytosis of opioid receptors. However, PLD2-derived PA is known to be further converted to diacylglycerol (DAG) by PA phosphohydrolase (PPAP2). In fact, blocking of PA phosphohydrolase activity by propranolol or PPAP2-short interfering RNA (siRNA) transfection significantly attenuated agonist-induced opioid receptor endocytosis. The primary importance of PA-derived DAG in the induction of opioid receptor endocytosis was further supported by the finding that increasing the DAG level by inhibiting the reconversion of DAG into PA with the DAG kinase inhibitor 3-[2-(4-[bis-(4-fluorophenyl)methylene]-1-piperidinyl)ethyl]-2,3-dihydro-2-thioxo-4(1H)quinazolinone (R59949) or the addition of the synthetic cell-permeable DAG analog 1,2-dioctanoyl-sn-glycerol (DOG), further increased the agonist-induced opioid receptor endocytosis. Moreover, the addition of DOG bypasses the PLD2-siRNA- or PPAP2-siRNA-mediated impairment of DAG synthesis and resulted in a restoration of agonist-induced opioid receptor internalization. Further studies established a functional link between PA-derived DAG and the activation of p38 mitogen-activated protein kinase (MAPK) and the subsequent phosphorylation of the Rab5 effector early endosome antigen 1, which has been demonstrated recently to be required for the induction of MOPr endocytosis. Taken together, our results revealed that the regulation of opioid receptor endocytosis by PLD2 involves the conversion of its product PA to DAG resulting in an activation of the p38 MAPK pathway.

Diacylglycerol kinase alpha mediates HGF-induced Rac activation and membrane ruffling by regulating atypical PKC and RhoGDI

Diacylglycerol kinases (DGKs) convert diacylglycerol (DAG) into phosphatidic acid (PA), acting as molecular switches between DAG- and PA-mediated signaling. We previously showed that Src-dependent activation and plasma membrane recruitment of DGKalpha are required for growth-factor-induced cell migration and ruffling, through the control of Rac small-GTPase activation and plasma membrane localization. Herein we unveil a signaling pathway through which DGKalpha coordinates the localization of Rac. We show that upon hepatocyte growth-factor stimulation, DGKalpha, by producing PA, provides a key signal to recruit atypical PKCzeta/iota (aPKCzeta/iota) in complex with RhoGDI and Rac at ruffling sites of colony-growing epithelial cells. Then, DGKalpha-dependent activation of aPKCzeta/iota mediates the release of Rac from the inhibitory complex with RhoGDI, allowing its activation and leading to formation of membrane ruffles, which constitute essential requirements for cell migration. These findings highlight DGKalpha as the central element of a lipid signaling pathway linking tyrosine kinase growth-factor receptors to regulation of aPKCs and RhoGDI, and providing a positional signal regulating Rac association to the plasma membrane.

Negative regulation of diacylglycerol kinase theta mediates adenosine-dependent hepatocyte preconditioning

In liver ischemic preconditioning (IP), stimulation of adenosine A2a receptors (A2aR) prevents ischemia/reperfusion injury by promoting diacylglycerol-mediated activation of protein kinase C (PKC). By concerting diacylglycerol to phosphatidic acid, diacylglycerol kinases (DGKs) act as terminator of diacylglycerol signalling. This study investigates the role of DGK in the development of hepatocyte IP. DGK activity and cell viability were evaluated in isolated rat hepatocytes preconditioned by 10 min hypoxia followed by 10 min re-oxygenation or by the treatment with the A2aR agonist, CGS21680, and subsequently exposed to prolonged hypoxia. We observed that after IP or A2aR activation, a decrease in DGK activity was associated with the onset of hepatocyte tolerance to hypoxia. CGS21680-induced stimulation of A2aR specifically inhibited DGK isoform theta by activating RhoA-GTPase. Consistently, both siRNA-mediated downregulation of DGK theta and hepatocyte pretreatment with the DGK inhibitor R59949 induced cell tolerance to hypoxia. The pharmacological inhibition of DGK was associated with the diacylglycerol-dependent activation of PKC delta and epsilon and of their downstream target p38 MAPK. In conclusion, we unveil a novel signalling pathway contributing to the onset of hepatocyte preconditioning, which through RhoA-GTPase, couples A2aR to the downregulation of DGK. Such an inhibition is essential for the sustained accumulation of diacylglycerol required for triggering PKC-mediated survival signals.

Diacylglycerol Kinase Inhibition and Vascular Function

Diacylglycerol kinases (DGKs), a family of lipid kinases, convert diacylglycerol (DG) to phosphatidic acid (PA). Acting as a second messenger, DG activates protein kinase C (PKC). PA, a signaling lipid, regulates diverse functions involved in physiological responses. Since DGK modulates two lipid second messengers, DG and PA, regulation of DGK could induce related cellular responses. Currently, there are 10 mammalian isoforms of DGK that are categorized into five groups based on their structural features. These diverse isoforms of DGK are considered to activate distinct cellular functions according to extracellular stimuli. Each DGK isoform is thought to play various roles inside the cell, depending on its subcellular localization (nuclear, ER, Golgi complex or cytoplasm). In vascular smooth muscle, vasoconstrictors such as angiotensin II, endothelin-1 and norepinephrine stimulate contraction by increasing inositol trisphosphate (IP(3)), calcium, DG and PKC activity. Inhibition of DGK could increase DG availability and decrease PA levels, as well as alter intracellular responses, including calcium-mediated and PKC-mediated vascular contraction. The purpose of this review is to demonstrate a role of DGK in vascular function. Selective inhibition of DGK isoforms may represent a novel therapeutic approach in vascular dysfunction.

Mammalian diacylglycerol kinases: molecular interactions and biological functions of selected isoforms

The mammalian diacylglycerol kinases (DGK) are a group of enzymes having important roles in regulating many biological processes. Both the product and the substrate of these enzymes, i.e. diacylglycerol and phosphatidic acid, are important lipid signalling molecules. Each DGK isoform appears to have a distinct biological function as a consequence of its location in the cell and/or the proteins with which it associates. This review discusses three of the more extensively studied forms of this enzyme, DGKalpha, DGKvarepsilon, and DGKzeta. DGKalpha has an important role in immune function and its activity is modulated by several mechanisms. DGKvarepsilon has several unique features among which is its specificity for arachionoyl-containing substrates, suggesting its importance in phosphatidylinositol cycling. DGKzeta is expressed in many tissues and also has several mechanisms to regulate its functions. It is localized in several subcellular organelles, including the nucleus. The current state of our understanding of the properties and functions of these proteins is reviewed.

Insulin action on polyunsaturated phosphatidic acid formation in rat brain: an "in vitro" model with synaptic endings from cerebral cortex and hippocampus

The highly efficient formation of phosphatidic acid from exogenous 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) in rat brain synaptic nerve endings (synaptosomes) from cerebral cortex and hippocampus is reported. Phosphatidic acid synthesized from SAG or 1,2-dipalmitoyl-sn-glycerol (DPG) was 17.5 or 2.5 times higher, respectively, than from endogenous synaptosomal diacylglycerides. Insulin increased diacylglycerol kinase (DAGK) action on endogenous substrate in synaptic terminals from hippocampus and cerebral cortex by 199 and 97%, respectively. Insulin preferentially increased SAG phosphorylation from hippocampal membranes. In CC synaptosomes insulin increased phosphatidic acid (PA) synthesis from SAG by 100% with respect to controls. Genistein (a tyrosine kinase inhibitor) inhibited this stimulatory insulin effect. Okadaic acid or cyclosporine, used as Ser/Threo protein phosphatase inhibitors, failed to increase insulin effect on PA formation. GTP gamma S and particularly NaF were potent stimulators of PA formation from polyunsaturated diacylglycerol but failed to increase this phosphorylation when added after 5 min of insulin exposure. GTP gamma S and NaF increased phosphatidylinositol 4,5 bisphosphate (PIP2) labeling with respect to controls when SAG was present. On the contrary, they decreased polyphosphoinositide labeling with respect to controls in the presence of DPG. Our results indicate that a DAGK type 3 (DAGKepsilon) which preferentially, but not selectively, utilizes 1-acyl-2-arachidonoyl-sn-glycerol and which could be associated with polyphosphoinositide resynthesis, participates in synaptic insulin signaling. GTP gamma S and NaF appear to be G protein activators related to insulin and the insulin receptor, both affecting the signaling mechanism that augments phosphatidic acid formation.

Massive Ca-induced membrane fusion and phospholipid changes triggered by reverse Na/Ca exchange in BHK fibroblasts

Baby hamster kidney (BHK) fibroblasts increase their cell capacitance by 25-100% within 5 s upon activating maximal Ca influx via constitutively expressed cardiac Na/Ca exchangers (NCX1). Free Ca, measured with fluo-5N, transiently exceeds 0.2 mM with total Ca influx amounting to approximately 5 mmol/liter cell volume. Capacitance responses are half-maximal when NCX1 promotes a free cytoplasmic Ca of 0.12 mM (Hill coefficient approximately 2). Capacitance can return to baseline in 1-3 min, and responses can be repeated several times. The membrane tracer, FM 4-64, is taken up during recovery and can be released at a subsequent Ca influx episode. Given recent interest in signaling lipids in membrane fusion, we used green fluorescent protein (GFP) fusions with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and diacylglycerol (DAG) binding domains to analyze phospholipid changes in relation to these responses. PI(4,5)P(2) is rapidly cleaved upon activating Ca influx and recovers within 2 min. However, PI(4,5)P(2) depletion by activation of overexpressed hM1 muscarinic receptors causes only little membrane fusion, and subsequent fusion in response to Ca influx remains massive. Two results suggest that DAG may be generated from sources other than PI(4,5)P in these protocols. First, acylglycerols are generated in response to elevated Ca, even when PI(4,5)P(2) is metabolically depleted. Second, DAG-binding C1A-GFP domains, which are brought to the cell surface by exogenous ligands, translocate rapidly back to the cytoplasm in response to Ca influx. Nevertheless, inhibitors of PLCs and cPLA2, PI(4,5)P(2)-binding peptides, and PLD modification by butanol do not block membrane fusion. The cationic agents, FM 4-64 and heptalysine, bind profusely to the extracellular cell surface during membrane fusion. While this binding might reflect phosphatidylserine (PS) "scrambling" between monolayers, it is unaffected by a PS-binding protein, lactadherin, and by polylysine from the cytoplasmic side. Furthermore, the PS indicator, annexin-V, binds only slowly after fusion. Therefore, we suggest that the luminal surfaces of membrane vesicles that fuse to the plasmalemma may be rather anionic. In summary, our results provide no support for any regulatory or modulatory role of phospholipids in Ca-induced membrane fusion in fibroblasts.

Regulation of clathrin-dependent endocytosis by diacylglycerol kinase delta: importance of kinase activity and binding to AP2alpha

DGKdelta (diacylglycerol kinase delta), which phosphorylates DAG (diacylglycerol) and converts it into PA (phosphatidic acid), has an important role in signal transduction. In the present study, we have demonstrated the molecular mechanism of DGKdelta-mediated regulation of clathrin-dependent endocytosis that controls the internalization, recycling and degradation of receptors. Involvement of DGKdelta in the regulation of clathrin-dependent endocytosis was previously proposed following genome-wide RNAi (RNA interference) screening. Clathrin-coated pits are mainly formed by clathrin and AP-2 (adaptor protein 2) complex. These proteins assemble a polyhedral lattice at the membrane and gather several endocytic accessory proteins. As the intracellular localization of DGKdelta2 overlapped with clathrin-coated pits, we predicted the possible regulation of clathrin-dependent endocytosis by DGKdelta2 and its interaction with some endocytosis-regulatory proteins. DGKdelta2 contained the DXF-type binding motifs, and DGKdelta2 bound to AP2alpha, a subunit of the AP-2 complex. DGKdelta2 interacted with the platform subdomain in the AP2alpha ear domain via F369DTFRIL and D746PF sequences in the catalytic domain of DGKdelta2. For further insight into the role for DGKdelta2 in clathrin-dependent endocytosis, we measured the transferrin and EGF (epidermal growth factor) uptake-expressing wild-type or mutant DGKdelta2 under knockdown of endogenous DGKdelta. Mutants lacking binding ability to AP2alpha as well as kinase-negative mutants could not compensate for the uptake of transferrin inhibited by siRNA (small interfering RNA) treatment, whereas overexpression of wild-type DGKdelta2 completely recovered the transferrin uptake. These results demonstrate that binding between DGKdelta2 and AP2alpha is involved in the transferrin internalization and that DGK activity is also necessary for the regulation of the endocytic process.

Cyclic AMP-stimulated interaction between steroidogenic factor 1 and diacylglycerol kinase theta facilitates induction of CYP17

In the human adrenal cortex, adrenocorticotropin (ACTH) activates CYP17 transcription by promoting the binding of the nuclear receptor steroidogenic factor 1 (SF1) (Ad4BP, NR5A1) to the promoter. We recently found that sphingosine is an antagonist for SF1 and inhibits cyclic AMP (cAMP)-dependent CYP17 gene transcription. The aim of the current study was to identify phospholipids that bind to SF1 and to characterize the mechanism by which ACTH/cAMP regulates the biosynthesis of this molecule(s). Using tandem mass spectrometry, we show that in H295R human adrenocortical cells, SF1 is bound to phosphatidic acid (PA). Activation of the ACTH/cAMP signal transduction cascade rapidly increases nuclear diacylglycerol kinase (DGK) activity and PA production. PA stimulates SF1-dependent transcription of CYP17 reporter plasmids, promotes coactivator recruitment, and induces the mRNA expression of CYP17 and several other steroidogenic genes. Inhibition of DGK activity attenuates the binding of SF1 to the CYP17 promoter, and silencing of DGK-theta expression inhibits cAMP-dependent CYP17 transcription. LXXLL motifs in DGK-theta mediate a direct interaction of SF1 with the kinase and may facilitate binding of PA to the receptor. We conclude that ACTH/cAMP stimulates PA production in the nucleus of H295R cells and that this increase in PA concentrations facilitates CYP17 induction.

Diacylglycerol kinases: Why so many of them?

Diacylglycerol (DAG) kinase (DGK) modulates the balance between the two signaling lipids, DAG and phosphatidic acid (PA), by phosphorylating DAG to yield PA. To date, ten mammalian DGK isozymes have been identified. In addition to the C1 domains (protein kinase C-like zinc finger structures) conserved commonly in all DGKs, these isoforms possess a variety of regulatory domains of known and/or predicted functions, such as a pair of EF-hand motifs, a pleckstrin homology domain, a sterile alpha motif domain and ankyrin repeats. Beyond our expectations, recent studies have revealed that DGK isozymes play pivotal roles in a wide variety of signal transduction pathways conducting development, neural and immune responses, cytoskeleton reorganization and carcinogenesis. Moreover, there has been rapidly growing evidence indicating that individual DGK isoforms exert their specific roles through interactions with unique partner proteins such as protein kinase Cs, Ras guanyl nucleotide-releasing protein, chimaerins and phosphatidylinositol-4-phosphate 5-kinase. Therefore, an emerging paradigm for DGK is that the individual DGK isoforms assembled in their own signaling complexes should carry out spatio-temporally segregated tasks for a wide range of biological processes via regulating local, but not global, concentrations of DAG and/or PA.

HCN pacemaker channel activation is controlled by acidic lipids downstream of diacylglycerol kinase and phospholipase A2

Hyperpolarization-activated pacemaker currents (I(H)) contribute to the subthreshold properties of excitable cells and thereby influence behaviors such as synaptic integration and the appearance and frequency of intrinsic rhythmic activity. Accordingly, modulation of I(H) contributes to cellular plasticity. Although I(H) activation is regulated by a plethora of neurotransmitters, including some that act via phospholipase C (PLC), the only second messengers known to alter I(H) voltage dependence are cAMP, internal protons (H+(I)s), and phosphatidylinositol-4,5-phosphate. Here, we show that 4beta-phorbol-12-myristate-13-acetate (4betaPMA), a stereoselective C-1 diacylglycerol-binding site agonist, enhances voltage-dependent opening of wild-type and cAMP/H+(I)-uncoupled hyperpolarization-activated, cyclic nucleotide-regulated (HCN) channels, but does not alter gating of the plant hyperpolarization-activated channel, KAT1. Pharmacological analysis indicates that 4betaPMA exerts its effects on HCN gating via sequential activation of PKC and diacylglycerol kinase (DGK) coupled with upregulation of MAPK (mitogen-activated protein kinase) and phospholipase A2 (PLA2), but its action is independent of phosphoinositide kinase 3 (PI3K) and PI4K. Demonstration that both phosphatidic acid and arachidonic acid (AA) directly facilitate HCN gating suggests that these metabolites may serve as the messengers downstream of DGK and PLA2, respectively. 4BetaPMA-mediated suppression of the maximal HCN current likely arises from channel interaction with AA coupled with an enhanced membrane retrieval triggered by the same pathways that modulate channel gating. These results indicate that regulation of excitable cell behavior by neurotransmitter-mediated modulation of I(H) may be exerted via changes in three signaling lipids in addition to the allosteric actions of cAMP and H+(I)s.

Antineutrophil Cytoplasm Antibody–Stimulated Neutrophil Adhesion Depends on Diacylglycerol Kinase–Catalyzed Phosphatidic Acid Formation

Patients with certain forms of systematic vasculitis, such as Wegener's granulomatosis, have circulating antineutrophil cytoplasmic antibodies (ANCA). These inappropriately stimulate circulating neutrophils adhere to and thereby obstruct small vessels. This, together with ANCA-induced degranulation and an oxidative burst, leads to local tissue damage. The signaling pathways that are activated by ANCA IgG are distinct from those that are involved in normal neutrophil activation. This study shows that diacylglycerol kinase is selectively activated by ANCA and that the generated phosphatidic acid is responsible for promoting neutrophil adhesion, in part through integrin activation. The data presented point to diacylglycerol kinase alpha as a novel but selective target for the development of drugs to treat this potentially fatal disorder.

Persistent activation of protein kinase Calpha is not necessary for expression of cerebellar long-term depression

Protein kinase Calpha (PKCalpha) plays a major role in the induction of long-term depression (LTD) in a cerebellar Purkinje cell (PC). The sequential activation model for classical PKC states that PKCalpha translocates to the plasma membrane by binding Ca(++) and then becomes fully activated by binding diacylglycerol (DAG), which enables estimation of the activity by monitoring its localization. Here, we performed simultaneous electrophysiological recording and fluorescence imaging in a cultured PC expressing GFP-tagged PKCalpha. When a PC was depolarized, PKCalpha transiently translocated to the plasma membrane in a Ca(++)-dependent manner. Application of membrane permeable DAG or the blocker of DAG lipase prolonged the translocation. These results suggest that the sequential activation model is applicable to PCs. Conjunctive applications of glutamate and depolarization pulse induced LTD, but did not prolong the translocation. Thus, our results imply that persistent activation of PKCalpha is not necessary for the expression of LTD.

Insulin promotes diacylglycerol kinase activation by different mechanisms in rat cerebral cortex synaptosomes

The mechanism by which insulin increases diacylglycerol kinase (DAGK) activity has been studied in cerebral cortex (CC) synaptosomes from adult (3-4 months of age) rats. The purpose of this study was to identify the role of phospholipases C and D (PLC and PLD) in DAGK activation by insulin. Neomycin, an inhibitor of PLC phosphatidylinositol-bisphosphate (PIP2) specific; ethanol, an inhibitor of phosphatidic acid (PA) formation by the promotion of a transphosphatidyl reaction of phosphatidylcholine phospholipase D (PC-PLD); and DL propranolol, an inhibitor of phosphatidate phosphohydrolase (PAP), were used in this study. Insulin (0.1 microM) shielded an increase in PA synthesis by [32P] incorporation using [gamma-32P]ATP as substrate and endogenous diacylglycerol (DAG) as co-substrate. This activated synthesis was strongly inhibited either by ethanol or DL propranolol. Pulse chase experiments also showed a PIP2-PLC activation within 1 min exposure to insulin. When exogenous unsaturated 18:0-20:4 DAG was present, insulin increased PA synthesis significantly. However, this stimulatory effect was not observed in the presence of exogenous saturated (di-16:0). In the presence of R59022, a selective DAGK inhibitor, insulin exerted no stimulatory effect on [32P]PA formation, suggesting a strong relationship between increased PA formation by insulin and DAGK activity. These data indicate that the increased synthesis of PA by insulin could be mediated by the activation of both a PC-PLD pathway to provide DAG and a direct DAGK activation that is associated to the use of 18:0-20:4 DAG species. PIP2-PLC activation may contribute at least partly to the insulin effect on DAGK activity.

Disruption of diacylglycerol metabolism impairs the induction of T cell anergy

Anergic T cells have altered diacylglycerol metabolism, but whether that altered metabolism has a causative function in the induction of T cell anergy is not apparent. To test the importance of diacylglycerol metabolism in T cell anergy, we manipulated diacylglycerol kinases (DGKs), which are enzymes that terminate diacylglycerol-dependent signaling. Overexpression of DGK-alpha resulted in a defect in T cell receptor signaling that is characteristic of anergy. We generated DGK-alpha-deficient mice and found that DGK-alpha-deficient T cells had more diacylglycerol-dependent T cell receptor signaling. In vivo anergy induction was impaired in DGK-alpha-deficient mice. When stimulated in anergy-producing conditions, T cells lacking DGK-alpha or DGK-zeta proliferated and produced interleukin 2. Pharmacological inhibition of DGK-alpha activity in DGK-zeta-deficient T cells that received an anergizing stimulus proliferated similarly to wild-type T cells that received CD28 costimulation and prevented anergy induction. Our findings suggest that regulation of diacylglycerol metabolism is critical in determining whether activation or anergy ensues after T cell receptor stimulation.

T cell anergy is reversed by active Ras and is regulated by diacylglycerol kinase-alpha

T cell anergy has been correlated with defective signaling by the GTPase Ras, but causal and mechanistic data linking defective Ras activity with T cell anergy are lacking. Here we used adenoviral transduction to genetically manipulate nonproliferating T cells and show that active Ras restored interleukin 2 production and mitogen-activated protein kinase signaling in T cells that were made anergic in vitro or in vivo. Diacylglycerol kinases (DGKs), which negatively regulate Ras activity, were upregulated in anergic T cells, and a DGK inhibitor restored interleukin 2 production in anergic T cells. Both anergy and DGK-alpha overexpression were associated with defective translocation of the Ras guanine nucleotide-exchange factor RasGRP1 to the plasma membrane. Our data support a causal function for excess DGK activity and defective Ras signaling in T cell anergy.

A new role of diacylglycerol kinase alpha on the secretion of lethal exosomes bearing Fas ligand during activation-induced cell death of T lymphocytes

Exosomes are small membrane vesicles that intracellularly accumulate into late or multivesicular endosomes (multivesicular bodies, MVB). Exosomes have a particular lipid and protein content, reflecting their origin as intraluminal vesicles of late endosomes. The stimulation of several hematopoietic cells induces the fusion of the limiting membrane of the MVB with the plasma membrane, leading to the release of exosomes towards the extracellular environment. In T lymphocytes, stimulation of the T cell receptor (TCR) induces the fusion of the MVBs with the plasma membrane and exosomes carrying several bio-active proteins are secreted. Among these proteins, the pro-apoptotic protein Fas ligand (FasL) is released as a non-proteolysed form (mFasL), associated to the exosomes. These mFasL-bearing exosomes may trigger the apoptosis of T lymphocytes. Here, we present evidences supporting a role of diacylglycerol kinase alpha (DGKalpha), a diacylglycerol (DAG)-consuming enzyme, on the secretion of exosomes carrying mFasL, and the subsequent activation-induced cell death (AICD) on a T cell line and primary T lymphoblasts.

Signaling roles of diacylglycerol kinases

Diacylglycerol kinases (DGKs) attenuate diacylglycerol signaling by converting this lipid to phosphatidic acid (PA). The nine mammalian DGKs that have been identified are widely expressed, but each isoform has a unique tissue and subcellular distribution. Their kinase activity is regulated by mechanisms that modify their access to diacylglycerol, directly affect their kinase activity, or alter their ability to bind to other proteins. In many cases, these enzymes regulate the activity of proteins that are modulated by either diacylglycerol or PA. Experiments using cultured cells and model organisms have demonstrated that DGKs have prominent roles in neuronal transmission, lymphocyte signaling, and carcinogenesis.

Arabidopsis AtDGK7, the smallest member of plant diacylglycerol kinases (DGKs), displays unique biochemical features and saturates at low substrate concentration: the DGK inhibitor R59022 differentially affects AtDGK2 and AtDGK7 activity in vitro and alters plant growth and development

Diacylglycerol kinase (DGK) regulates the level of the second messenger diacylglycerol and produces phosphatidic acid (PA), another signaling molecule. The Arabidopsis thaliana genome encodes seven putative diacylglycerol kinase isozymes (named AtDGK1 to -7), structurally falling into three major clusters. So far, enzymatic activity has not been reported for any plant Cluster II DGK. Here, we demonstrate that a representative of this cluster, AtDGK7, is biochemically active when expressed as a recombinant protein in Escherichia coli. AtDGK7, encoded by gene locus At4g30340, contains 374 amino acids with an apparent molecular mass of 41.2 kDa. AtDGK7 harbors an N-terminal catalytic domain, but in contrast to various characterized DGKs (including AtDGK2), it lacks a cysteine-rich domain at its N terminus, and, importantly, its C-terminal DGK accessory domain is incomplete. Recombinant AtDGK7 expressed in E. coli exhibits Michaelis-Menten type kinetics with 1,2-dioleoyl-sn-glycerol as substrate. AtDGK7 activity was affected by pH, detergents, and the DGK inhibitor R59022. We demonstrate that both AtDGK2 and AtDGK7 phosphorylate diacylglycerol molecular species that are typically found in plants, indicating that both enzymes convert physiologically relevant substrates. AtDGK7 is expressed throughout the Arabidopsis plant, but expression is strongest in flowers and young seedlings. Expression of AtDGK2 is transiently induced by wounding. R59022 at approximately 80 mum inhibits root elongation and lateral root formation and reduces plant growth, indicating that DGKs play an important role in plant development.

Activation of HIF-prolyl Hydroxylases by R59949, an Inhibitor of the Diacylglycerol Kinase

Hypoxia-inducible factors (HIF) are heterodimeric (alpha/beta) transcription factors that play a fundamental role in cellular adaptation to low oxygen tension. In the presence of oxygen, the HIF-alpha subunit becomes hydroxylated at specific prolyl residues by prolyl hydroxylases. This post-translational modification is recognized by the von Hippel-Lindau (VHL) protein, which targets HIF-alpha for degradation. In the absence of oxygen, HIF-alpha hydroxylation is compromised and this subunit is stabilized. We have previously shown that the hypoxia-induced accumulation of HIF-alpha protein is strongly impaired by the inhibitor of diacylglycerol kinase, R59949. Here, we have investigated the mechanisms through which this inhibitor exerts its effect. We found that R59949 inhibits the accumulation of HIF-1/2alpha protein without affecting the expression of their mRNAs. We also determined that R59949 could only block the accumulation of HIF-alpha in the presence of VHL protein. In agreement with this, the binding of VHL to endogenous HIF-alpha was significantly enhanced after R59949 treatment, even under hypoxic conditions. In addition, we found that R59949 could stimulate prolyl hydroxylase both at 21% O2 as well as at 1% O2. Taken together, these results reveal that R59949 is an activator of HIF prolyl hydroxylases. This is of particular interest when we consider that, to date, mainly inhibitors of these enzymes have been described.

Diacylglycerol kinase alpha regulates the secretion of lethal exosomes bearing Fas ligand during activation-induced cell death of T lymphocytes

Fas ligand (FasL) mediates both apoptotic and inflammatory responses in the immune system. FasL function critically depends on the different forms of FasL; soluble Fas ligand lacking the transmembrane and cytoplasmic domains is a poor mediator of apoptosis, whereas full-length, membrane-associated FasL (mFasL) is pro-apoptotic. mFasL can be released from T lymphocytes, via the secretion of mFasL-bearing exosomes. mFasL in exosomes retains its activity in triggering Fas-dependent apoptosis, providing an alternative mechanism of cell death that does not necessarily imply cell-to-cell contact. Diacylglycerol kinase alpha (DGKalpha), a diacylglycerol (DAG)-consuming enzyme, is involved in the attenuation of DAG-derived responses initiated at the plasma membrane that lead to T lymphocyte activation. Here we studied the role of DGKalpha on activation-induced cell death on a T cell line and primary T lymphoblasts. The inhibition of DGKalpha increases the secretion of lethal exosomes bearing mFas ligand and subsequent apoptosis. On the contrary, the overactivation of the DGKalpha pathway inhibits exosome secretion and subsequent apoptosis. DGKalpha was found associated with the trans-Golgi network and late endosomal compartments. Our results support the hypothesis that the DGKalpha effect on apoptosis occurs via the regulation of the release of lethal exosomes by the exocytic pathway, and point out that the spatial orchestration of the different pools of DAG (plasma membrane and Golgi membranes) by DGKalpha is crucial for the control of cell activation and also for the regulation of the secretion of lethal exosomes, which in turn controls cell death.

Nuclear diacylglycerol kinase-theta is activated in response to nerve growth factor stimulation of PC12 cells

Previous evidence from independent laboratories has shown that the nucleus contains diacylglycerol kinase (DGK) isoforms, i.e., the enzymes, which yield phosphatidic acid from diacylglycerol, thus terminating protein kinase C-mediated signaling events. A DGK isoform, which resides in the nucleus of PC12 cells, is DGK-theta. Here, we show that nerve growth factor (NGF) treatment of serum-starved PC12 cells results in the stimulation of both a cytoplasmic and a nuclear DGK activity. However, time course analysis shows that cytoplasmic DGK activity peaked earlier than its nuclear counterpart. While nuclear DGK activity was dramatically down-regulated by a monoclonal antibody known for selectively inhibiting DGK-theta, cytoplasmic DGK activity was not. Moreover, nuclear DGK activity was stimulated by phosphatidylserine, an anionic phospholipid that had no effect on cytoplasmic DGK activity. Upon NGF stimulation, the amount and the activity of DGK-theta, which was bound to the insoluble nuclear matrix fraction, substantially increased. Epidermal growth factor up-regulated a nuclear DGK activity insensitive to anti-DGK-theta monoclonal antibody. Overall, our findings identify nuclear DGK-theta as a down-stream target of NGF signaling in PC12 cells.

Diacylglycerol kinase activity in purified basolateral membranes of kidney tubules. I. Evidence for coupling with phospholipase C

The diacylglycerol kinase (DGK) catalyzes the phosphorylation of diacylglycerol (DAG) yielding phosphatidic acid (PA) signaling molecules which are involved in the modulation of different cell responses. The aim of this work was to characterize the DGK activity associated to the basolateral membranes (BLM) of kidney proximal tubules, in a native preparation that preserves the membrane microenvironment. The Arrhenius plot of DGK activity was non-linear, indicating a complex influence of the lipid environment of the native membrane. The formation of PA was strongly impaired by U73122, an inhibitor of PLC, whereas remained unmodified when exogenous DAG or PLC were added. The Mg.ATP2- complex is the true phosphoryl-donor substrate, and the very narrow peak of activation at pH 7.0 suggests that amino acids that dissociate at this pH, i.e. hystidine residues, play a role by acting in the coordination of the Mg2+ atoms. The renal DGK is almost completely blocked by 0.1 mM sphingosine, but it is insensitive to micromolar free Ca2+ concentrations and to R59499, the most potent inhibitor of the classical DGKs. Taken as a whole, these data suggest that the DGK isoform present in BLM of proximal tubules is different from those included in the type I family, and that membranous PLC could be the main source of DAG for DGK catalysis.

Differentiation of HL-60 cells to granulocytes involves regulation of select diacylglycerol kinases (DGKs)

Diacylglycerol Kinases (DGKs) are a family of enzymes that regulate the levels of different pools of diacylglycerol (DAG), affecting DAG-mediated signal transduction. Since DAG is known to play several important regulatory roles in granulocyte physiology, we investigated the expression pattern of DGK isoforms throughout differentiation of HL-60 cells to granulocytes. HL-60 cells were incubated with 1.25% dimethyl-sulfoxide (DMSO) to initiate differentiation and total RNA isolated at different time points. DGK expression was assessed through Northern blot, end-point PCR, and real-time PCR. The non-selective inhibitors R59022 and R59949 were used to block DGK at different time points throughout differentiation. CD11b and GPI-80, reactive oxygen species (ROS) generation, changes in the cell cycle, and apoptosis were used as markers of differentiation. Of the nine isoforms of DGK evaluated (alpha, delta, epsilon, gamma, zeta, beta, theta;, iota, eta), only five (alpha, delta, epsilon, gamma, and zeta) were expressed in HL-60 cells. DGKalpha was virtually absent in non-differentiated cells, but was markedly upregulated throughout differentiation. The other isoforms (delta, epsilon, gamma, and zeta) were expressed in undifferentiated HL-60 cells but were substantially decreased throughout differentiation. Non-selective blocking of DGK with R59022 and R59949 led to acceleration of differentiation, reducing the time necessary to observe upregulation of CD11b, GPI-80 and generation of ROS by 50%. Likewise, the cell cycle was disrupted when DGK isoforms were inhibited. These results provide evidence that DGK levels are dynamically regulated throughout differentiation and that expression of DGKs play an important regulatory function during the differentiation of neutrophils.

Diacylglycerol Kinase γ Serves as an Upstream Suppressor of Rac1 and Lamellipodium Formation

Nine diacylglycerol kinase (DGK) isozymes have been identified. However, our knowledge of their individual functions is still limited. Here, we demonstrate the role of DGKgamma in regulating Rac1-governed cell morphology. We found that the expression of kinase-dead DGKgamma, which acts as a dominant-negative mutant, and inhibition of endogenous DGKgamma activity with R59949 induced lamellipodium and membrane ruffle formation in NIH3T3 fibroblasts in the absence of growth factor stimulation. Reciprocally, lamellipodium formation induced by platelet-derived growth factor was significantly inhibited upon expression of constitutively active DGKgamma. Moreover, the constitutively active DGKgamma mutant suppressed integrin-mediated cell spreading. These effects are isoform-specific because, in the same experiments, none of the corresponding mutants of DGKalpha and DGKbeta, closely related isoforms, affected cell morphology. These results suggest that DGKgamma specifically participates in the Rac1-mediated signaling pathway leading to cytoskeletal reorganization. In support of this, DGKgamma co-localized with dominant-active Rac1 especially in lamellipodia. Moreover, we found that endogenous DGKgamma was physically associated with cellular Rac1. Dominant-negative Rac1 expression blocked the lamellipodium formation induced by kinase-dead DGKgamma, indicating that DGKgamma acts upstream of Rac1. This model is supported by studies demonstrating that kinase-dead DGKgamma selectively activated Rac1, but not Cdc42. Taken together, these results strongly suggest that DGKgamma functions through its catalytic action as an upstream suppressor of Rac1 and, consequently, lamellipodium/ruffle formation.

Activation of diacylglycerol kinase α is required for VEGF-induced angiogenic signaling in vitro

Vascular endothelial growth factor-A (VEGF-A) promotes angiogenesis by stimulating migration, proliferation and organization of endothelium, through the activation of signaling pathways involving Src tyrosine kinase. As we had previously shown that Src-mediated activation of diacylglycerol kinase-alpha (Dgk-alpha) is required for hepatocytes growth factor-stimulated cell migration, we asked whether Dgk-alpha is involved in the transduction of angiogenic signaling. In PAE-KDR cells, an endothelial-derived cell line expressing VEGFR-2, VEGF-A165, stimulates the enzymatic activity of Dgk-alpha: activation is inhibited by R59949, an isoform-specific Dgk inhibitor, and is dependent on Src tyrosine kinase, with which Dgk-alpha forms a complex. Conversely in HUVEC, VEGF-A165-induced activation of Dgk is only partially sensitive to R59949, suggesting that also other isoforms may be activated, albeit still dependent on Src tyrosine kinase. Specific inhibition of Dgk-alpha, obtained in both cells by R59949 and in PAE-KDR by expression of Dgk-alpha dominant-negative mutant, impairs VEGF-A165-dependent chemotaxis, proliferation and in vitro angiogenesis. In addition, in HUVEC, specific downregulation of Dgk-alpha by siRNA impairs in vitro angiogenesis on matrigel, further suggesting the requirement for Dgk-alpha in angiogenic signaling in HUVEC. Thus, we propose that activation of Dgk-alpha generates a signal essential for both proliferative and migratory response to VEGF-A165, suggesting that it may constitute a novel pharmacological target for angiogenesis control.

PPARgamma agonists ameliorate endothelial cell activation via inhibition of diacylglycerol-protein kinase C signaling pathway: role of diacylglycerol kinase

Subject- Peroxisome proliferator-activated receptor (PPAR)-gamma agonists are emerging as potential protectors against inflammatory cardiovascular diseases including atherosclerosis and diabetic complications. However, their molecular mechanism of action within vasculature remains unclear. We report here that PPARgamma agonists, thiazolidinedione class drugs (TZDs), or 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) were capable of activating diacylglycerol (DAG) kinase (DGK), resulting in attenuation of DAG levels and inhibition of protein kinase C (PKC) activation. The PPARgamma agonist-induced DGK was completely blocked by a dominant-negative mutant of PPARgamma, indicating an essential receptor-dependent action. Importantly, the suppression of DAG-PKC signaling pathway was functional linkage to the anti-inflammatory properties of PPARgamma agonists in endothelial cells (EC), characterized by the inhibition of proinflammatory adhesion molecule expression and adherence of monocytes to the activated EC induced by high glucose. These findings thus demonstrate a novel molecular action of PPARgamma agonists to suppress the DAG-PKC signaling pathway via upregulation of an endogenous attenuator, DGK.

AtDGK2, a novel diacylglycerol kinase from Arabidopsis thaliana, phosphorylates 1-stearoyl-2-arachidonoyl-sn-glycerol and 1,2-dioleoyl-sn-glycerol and exhibits cold-inducible gene expression

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol (DAG) to generate phosphatidic acid (PA). Both DAG and PA are implicated in signal transduction pathways. DGKs have been widely studied in animals, but their analysis in plants is fragmentary. Here, we report the cloning and biochemical characterization of AtDGK2, encoding DGK from Arabidopsis thaliana. AtDGK2 has a predicted molecular mass of 79.4 kDa and, like AtDGK1 previously reported, harbors two copies of a phorbol ester/DAG-binding domain in its N-terminal region. AtDGK2 belongs to a family of seven DGK genes in A. thaliana. AtDGK3 to AtDGK7 encode approximately 55-kDa DGKs that lack a typical phorbol ester/DAG-binding domain. Phylogenetically, plant DGKs fall into three clusters. Members of all three clusters are widely expressed in vascular plants. Recombinant AtDGK2 was expressed in Escherichia coli and biochemically characterized. The enzyme phosphorylated 1,2-dioleoyl-sn-glycerol to yield PA, exhibiting Michaelis-Menten type kinetics. Estimated K(m) and V(max) values were 125 microm for DAG and 0.25 pmol of PA min(-1) microg(-1), respectively. The enzyme was maximally active at pH 7.2. Its activity was Mg(2+)-dependent and affected by the presence of detergents, salts, and the DGK inhibitor R59022, but not by Ca(2+). AtDGK2 exhibited substrate preference for unsaturated DAG analogues (i.e. 1-stearoyl-2-arachidonoyl-sn-glycerol and 1,2-dioleoyl-sn-glycerol). The AtDGK2 gene is expressed in various tissues of the Arabidopsis plant, including leaves, roots, and flowers, as shown by Northern blot analysis and promoter-reporter gene fusions. We found that AtDGK2 is induced by exposure to low temperature (4 degrees C), pointing to a role in cold signal transduction.

Site-directed mutagenesis of the active site of diacylglycerol kinase alpha: calcium and phosphatidylserine stimulate enzyme activity via distinct mechanisms

Diacylglycerol kinases (DAGKs) catalyse ATP-dependent phosphorylation of sn-1,2-diacylglycerol that arises during stimulated phosphatidylinositol turnover. DAGKa is activated in vitro by Ca2+ and by acidic phospholipids. The regulatory region of DAGKa includes an N-terminal RVH motif and EF hands that mediate Ca2+-dependent activation. DAGKa also contains tandem C1 protein kinase C homology domains. We utilized yeast, Saccharomyces cerevisiae, which lacks an endogenous DAGK, to express DAGKa and to determine the enzymic activities of different mutant forms of pig DAGKa in vitro. Six aspartate residues conserved in all DAGKs were individually examined by site-directed mutagenesis. Five of these aspartate residues reside in conserved blocks that correspond to sequences in the catalytic site of phosphofructokinases. Mutation of D434 (Asp434) or D650 abolished all DAGKa activity, whereas substitution of one among D465, D497, D529 and D697 decreased the activity to 6% or less of that for wild-type DAGKa. Roles of homologous residues in phosphofructokinases suggested that the N-terminal half of the DAGK catalytic domain binds Mg-ATP and the C-terminal half binds diacylglycerol. A DAGKa mutant with its entire regulatory region deleted showed a much decreased activity that was not activated by Ca2+, but still exhibited PS (phosphatidylserine)-dependent activation. Moreover, mutations of aspartate residues at the catalytic domain had differential effects on activation by Ca2+ and PS. These results indicate that Ca2+ and PS stimulate DAGKa via distinct mechanisms.

Foreign body-type multinucleated giant cell formation is potently induced by alpha-tocopherol and prevented by the diacylglycerol kinase inhibitor R59022

Multinucleated foreign body giant cells (FBGCs) form by monocyte-derived macrophage fusion on implanted biomedical devices and are believed to mediate oxidative damage to biomaterial surfaces. Our in vitro system of human macrophage culture and interleukin (IL)-4-induced FBGC formation was developed to study the macrophage fusion mechanism and the physiological significance of FBGCs on implanted biomaterials and at other sites of chronic inflammation. Here, we demonstrate that the antioxidant vitamin E (90% alpha-tocopherol) moderately induces macrophage fusion and increases IL-4-induced FBGC formation. Moreover, purified alpha-tocopherol, but not beta-, gamma-, or delta-tocopherol, most remarkably induces macrophage fusion, leading to cultures of confluent FBGCs below normal plasma concentrations. This is not observed with the similar antioxidants probucol or Trolox, suggesting that the alpha-tocopherol effects on FBGC formation are independent of its antioxidant activity. Consistent with the reported activation of diacylglycerol kinase by alpha-tocopherol, the diacylglycerol kinase inhibitor R59022 completely abrogates FBGC formation. R59022 inhibition of IL-4-induced FBGC formation is reversed by alpha-tocopherol, suggesting that FBGC formation involves diacylglycerol kinase activation. This study suggests a novel role for diacylglycerol kinase in the mechanism of macrophage fusion/FBGC formation at sites of chronic inflammation and reveals that the pleiotropic lipophilic compound, alpha-tocopherol, is a highly potent macrophage fusion factor.

Regulation of canonical transient receptor potential (TRPC) channel function by diacylglycerol and protein kinase C

The mechanism of receptor-induced activation of the ubiquitously expressed family of mammalian canonical transient receptor potential (TRPC) channels has been the focus of intense study. Primarily responding to phospholipase C (PLC)-coupled receptors, the channels are reported to receive modulatory input from diacylglycerol, endoplasmic reticulum inositol 1,4,5-trisphosphate receptors and Ca2+ stores. Analysis of TRPC5 channels transfected within DT40 B cells and deletion mutants thereof revealed efficient activation in response to PLC-beta or PLC-gamma activation, which was independent of inositol 1,4,5-trisphoshate receptors or the content of stores. In both HEK293 cells and DT40 cells, TRPC5 and TRPC3 channel responses to PLC activation were highly analogous, but only TRPC3 and not TRPC5 channels responded to the addition of the permeant diacylglycerol (DAG) analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG). However, OAG application or elevated endogenous DAG, resulting from either DAG lipase or DAG kinase inhibition, completely prevented TRPC5 or TRPC4 activation. This inhibitory action of DAG on TRPC5 and TRPC4 channels was clearly mediated by protein kinase C (PKC), in distinction to the stimulatory action of DAG on TRPC3, which is established to be PKC-independent. PKC activation totally blocked TRPC3 channel activation in response to OAG, and the activation was restored by PKC-blockade. PKC inhibition resulted in decreased TRPC3 channel deactivation. Store-operated Ca2+ entry in response to PLC-coupled receptor activation was substantially reduced by OAG or DAG-lipase inhibition in a PKC-dependent manner. However, store-operated Ca2+ entry in response to the pump blocker, thapsigargin, was unaffected by PKC. The results reveal that each TRPC subtype is strongly inhibited by DAG-induced PKC activation, reflecting a likely universal feedback control on TRPCs, and that DAG-mediated PKC-independent activation of TRPC channels is highly subtype-specific. The profound yet distinct control by PKC and DAG of the activation of TRPC channel subtypes is likely the basis of a spectrum of regulatory phenotypes of expressed TRPC channels.

Programmed cell death in the developing inner ear is balanced by nerve growth factor and insulin-like growth factor I

Nerve growth factor induces cell death in organotypic cultures of otic vesicle explants. This cell death has a restricted pattern that reproduces the in vivo pattern of apoptosis occurring during inner ear development. In this study, we show that binding of nerve growth factor to its low affinity p75 neurotrophin receptor is essential to achieve the apoptotic response. Blockage of binding to p75 receptor neutralized nerve-growth-factor-induced cell death, as measured by immunoassays detecting the presence of cytosolic oligonucleosomes and by TUNEL assay to visualize DNA fragmentation. Nerve growth factor also induced a number of cell-death-related intracellular events including ceramide generation, caspase activation and poly-(ADP ribose) polymerase cleavage. Again, p75 receptor blockade completely abolished all of these effects. Concerning the intracellular pathway, ceramide increase depended on initiator caspases, whereas its actions depended on both initiator and effector caspases, as shown by using site-specific caspase inhibitors. Conversely, insulin-like growth factor I, which promotes cell growth and survival in the inner ear, abolished apoptosis induced by nerve growth factor. Insulin-like growth factor cytoprotective actions were accomplished, at least in part, by decreasing endogenous ceramide levels and activating Akt. Taken together, these results strongly suggest that regulation of nerve-growth-factor-induced apoptosis in the otocysts occurs via p75 receptor binding and is strictly controlled by the interaction with survival signalling pathways.

Monocytic activation of protein tyrosine kinase, protein kinase A and protein kinase C induced by porins isolated from Salmonella enterica serovar Typhimurium

OBJECTIVES

In the present study a monocytic cell line, U937, was used to investigate the possible involvement of protein tyrosine kinases (NT-PTKs), protein kinase A (PKA) and protein kinase C (PKC) in cell signaling pathways following Salmonella enterica serovar Typhimurium porin stimulation.

METHODS

Different concentrations of porins and lipopolysaccharide (LPS) were analysed to evaluate changes in PTK activity by a non radioactive tyrosine kinase assay and in PKA and PKC phosphorylation by Western blotting analysis. The inhibitors of PTK, PKA and PKC activation used, were: 3,4-dihydroxybenzylidene-malononitrile (tyrphostin 23), inhibitor of epidermal growth factor (EGF) receptor tyrosine kinase activity; dihychloride (H-89), a selective inhibitor of PKA which is useful to discriminate between the effects of PKC and PKA; diacylglycerol kinase inhibitor II (R59949), which is useful for elucidating roles of PKC; calphostin C, a specific inhibitor of PKC.

RESULTS

Porins of the outer membrane of the ST were isolated to be used as a stimulus in the performed experiments. Following porin treatment, a dose-dependent increase in PTK, PKA and PKC activation was observed. U937 monocytes pretreated with inhibitors induced an evident decrease in PTK activity and PKA and PKC phosphorylation pattern in porin stimulated monocytes.

CONCLUSIONS

Our data support the important role played by NT-PTK, PKA and PKC in transducing the activating signal in macrophages stimulated with porins through the activation of the mitogen-activated protein kinase (MAPK) pathway that participate in the regulation of gene expression.

Effect of novel modulators of protein kinase C activity upon chemotherapy-induced differentiation and apoptosis in myeloid leukemic cells

Modulation of protein kinase C (PKC) activity has been demonstrated to either prevent or enhance drug-induced apoptosis in various tissue types. We tested four novel modulators of PKC activity in comparison to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) for the capability to affect differentiation, cell cycle progression and apoptosis in the human myeloid leukemia cell lines U937 and HL-60. Farnesyl thiotriazole (FTT) and N-(n-heptyl)-5-chloro-1-naphthalenesulfonamide (SC-10) are both direct activators of PKC, whereas 6-(2-(4-[(4-fluorophe-nyl)phenylmethylene]-1-piperidinyl)ethyl)-7-methyl-5H-thiazolo[3,2-a]pyrimidin-5-one (R59022) and [3-[2-[4-(bis-(4-fluorophenyl)methylene]piperidin-1-yl)ethyl]-2,3-dihydro-2-thioxo-4(1H)-quin-azolinone (R59949) are diacyl glycerol kinase inhibitors that activate PKC by enhancing the levels of the endogenous ligand diacyl glycerol. U937 cells displayed a slight reduction in the number of cells in G(2)/M cell cycle phase after exposure to FTT, SC-10, R59022 and R59949, respectively. In contrast, HL-60 cells demonstrated a largely unaltered cell cycle distribution. Whereas TPA treatment resulted in a strong induction of p21(WAF/CIP1), c-Fos and c-Jun levels, neither one of the novel PKC activators altered expression of these proteins. Consequently, we tested the ability of the activators to cause membrane translocation of PKC. While TPA treatment resulted in translocation of the PKC isoforms alpha, delta and epsilon, SC-10 and FTT failed to induce alterations in the PKC content of the membrane and cytosolic fractions, respectively. Expression of the beta(2)-integrin CD11c that is induced during TPA-mediated differentiation remained unaltered after exposure to SC-10 and was partly reduced after treatment with FTT. To further investigate the effect of these activators upon apoptosis in leukemic cells, HL-60 and U937 cells were treated with 1-beta-D-arabinofuranosylcytosine (Ara-C) or etoposide (VP-16). Whereas TPA strongly reduced apoptosis in Ara-C- or VP-16-treated U937 cells, little if any reduction was observed after pretreatment with either FTT, SC-10, R59022 or R59949, respectively, in these cells. In contrast, TPA enhanced apoptosis in Ara-C- or VP-16-treated HL-60 cells. Interestingly, FTT and SC-10 demonstrated a protective effect in Ara-C-treated HL-60 cells. Taken together, these data suggest that the novel PKC activators FTT, SC-10, R59022 and R59949 exhibit modest biological effects upon leukemic blast cells, and are not capable of enhancing the apoptotic response of these cells to cytotoxic drugs.

PC and PE synthesis: mixed micellar analysis of the cholinephosphotransferase and ethanolaminephosphotransferase activities of human choline/ethanolamine phosphotransferase 1 (CEPT1)

The human choline/ethanolamine phosphotransferase 1 (CEPT1) gene codes for a dual-specificity enzyme that catalyzes the de novo synthesis of the two major phospholipids through the transfer of a phosphobase from CDP-choline or CDP-ethanolamine to DAG to form PC and PE. We used an expression system devoid of endogenous cholinephosphotransferase and ethanolaminephosphotransferase activities to assess the diradylglycerol specificity of CEPT1. A mixed micellar assay was used to ensure that the diradylglycerols delivered were not affecting the membrane environment in which CEPT1 resides. The CEPT1 enzyme displayed an apparent Km of 36 microM for CDP-choline and 4.2 mol% for di-18:1 DAG with a Vmax of 14.3 nmol min(-1) mg(-1). When CDP-ethanolamine was used as substrate, the apparent Km was 98 microM for CDP-ethanolamine and 4.3 mol% for di-18:1 DAG with a Vmax of 8.2 nmol min(-1) mg(-1). The preferred diradylglycerol substrates used by CEPT1 with CDP-choline as the phosphobase donor were di-18:1 DAG, di-16:1 DAG, and 16:0/18:1 DAG. A major difference between previous emulsion-based assay results and the mixed micelle results was a complete inability to use 16:0(O)/2:0 as a substrate for the de novo synthesis of platelet-activating factor when the mixed micelle assay was used. When CDP-ethanolamine was used as the phosphobase donor, 16:0/18:1 DAG, di-18:1 DAG, and di-16:1 DAG were the preferred substrates. The mixed micelle assay also allowed the lipid activation of CEPT to be measured, and both the cholinephosphotransferase and ethanolaminephosphotransferase activities displayed the unusual property of product activation at 5 mol%, implying that specific lipid activation binding sites exist on CEPT1. The protein kinase C inhibitor chelerythrine and the human DAG kinase inhibitor R59949 both inhibited CEPT1 activity with IC50 values of 40 microM.

Diacylglycerol kinase alpha activity promotes survival of CD4+ 8+ double positive cells during thymocyte development

The diacylglycerol kinases (DGK) form a family of isoenzymes that catalyse the conversion of diacylglycerol (DAG) to phosphatidic acid (PA), both powerful second messengers in the cell. DGKalpha is expressed in brain, peripheral T cells and thymocytes and has been shown to translocate to the nuclear matrix upon T-cell receptor (TCR) engagement. Here, we show that high level expression of DGKalpha is induced following a signal transmitted through the pre-TCR and the protein tyrosine kinase, lck. Activity of DGKalpha contributes to survival in CD4+ 8+ (DP) thymocytes as pharmacological inhibition of DGK activity results in death of this cell population both in cell suspension and thymic explants. DGKalpha promotes survival in these thymocytes through a Bcl-regulated pathway. A consequence of inhibition of DGKalpha is the specific down-regulation of Bcl-xl, whereas in transgenic mice that over-express Bcl-2, death induced by the inhibitor is partially blocked. Thus we report a novel activity of DGKalpha in survival of thymocytes immediately after entry into the DP stage in development.

Expression of a catalytically inactive form of diacylglycerol kinase alpha induces sustained signaling through RasGRP

Regulating the generation and clearance of lipid second messengers, such as diacylglycerol (DAG), is critical for the correct propagation of intracellular signaling pathways. DAGK type alpha acts as a negative modulator of the DAG levels generated during T cell activation, which is initiated by triggering of the endogenous T cell receptor (TCR), as well as by stimulation of an ectopically expressed human muscarinic type 1 receptor. Here we show that stimulation of either of these receptors causes rapid, transient membrane translocation of the recently discovered Ras guanyl nucleotide release protein (RasGRP), followed by activation of mitogen-activated protein kinase (MAPK). When cells expressing a catalytically inactive form of DAGKalpha were analyzed, however, similar agonist stimulation resulted in sustained signaling through RasGRP and MAPK. Biochemical analysis showed that expression of kinase-dead diacylglycerol kinase a (DGKalpha) led to a greater, more sustained, DAG accumulation following receptor stimulation. These results suggest that, in T cells, agonist-stimulated DAG generation is the key factor controlling activation of the Ras/MAPK signaling pathway through membrane translocation of RasGRP. Moreover, we demonstrate that through the modulation of the intracellular level of agonist-stimulated DAG, DGKalpha alters Ras activation and downstream signaling dramatically, a process of utmost importance for sound immunological function.

T lymphocyte nuclear diacylglycerol is derived from both de novo synthesis and phosphoinositide hydrolysis

Novel phospholipid metabolism in T lymphocytes and the generation of biologically active lipid second messengers (LSMs) has attracted much attention in recent years. Despite this interest, no reports have attempted to characterise such events in the nuclei of these cells. In order to gain insight into the structural relationships between the lipids diglyceride (DG) and phosphatidic acid (PtdOH) and their structural precursors phosphatidylcholine (PtdCho) and phosphatidylinositides (PtdIns) in the nuclei of CTLL-2 T lymphocytes, an analysis of their molecular species was performed. The results clearly indicated that there were two pools of DG. The major pool consisted primarily of saturated and monunsaturated structures whereas the minor pool consisted of more unsaturated species, most likely derived from PtdIns. Only the latter pool was found to be accessible to endogenous nuclear diacylglycerol kinase (DGK) activity which showed partial inhibition with the recognised DGK inhibitor R59949. Molecular species analysis of the endogenous nuclear PtdOH revealed it to be distinct from that generated by the endogenous DGK, but instead resembled that of PtdCho species. We were unable to detect enzymatic activities which targeted PtdCho (PtdCho-phospholipase C (PtdCho-PLC), PtdCho-phospholipase D (PtdCho-PLD) and sphingomyelin synthase (SMS)) but instead a detectable PtdOH phosphatase (PAP) activity. We propose that, in exponentially growing CTLL-2 cells, synthesis de novo represents one of the routes for the biosynthesis of structural phospholipids which may be the source of biologically active LSMs in the nucleus.

Gene expression and in situ localization of diacylglycerol kinase isozymes in normal and infarcted rat hearts: effects of captopril treatment

Diacylglycerol (DG) kinase (DGK) terminates signaling from DG, which serves as an activator of protein kinase C (PKC), by converting DG to phosphatidic acid. DGK is thus regarded as an attenuator of the PKC activity. In rats, five DGK isozymes have been cloned, but little is known about their role in the heart. In this study, the spatiotemporal expression of DGK isozymes was investigated in rat hearts under a normal condition and after myocardial infarction (MI) by in situ hybridization histochemistry and immunohistochemistry. In normal left ventricular myocardium, DGKalpha, DGKepsilon, and DGKzeta mRNAs were expressed evenly throughout the myocardium, although the DGKalpha expression was very low. In infarcted hearts, the expression of DGKzeta was enhanced in the peripheral zone of the necrotic area and at the border zone 3 and 7 days after MI, and to a lesser extent in the middle layer of the granulation tissue 21 days after MI. The enhanced DGKzeta expression in the infarcted and border areas could be attributed to granulocytes and macrophages. In contrast, the expression of DGKepsilon in the infarcted and border areas was lower than that in the viable left ventricle (LV) throughout the postoperation period. Furthermore, DGKepsilon expression in the viable myocardium 21 days after MI decreased significantly compared with left ventricular myocardium in the sham-operated rats and was completely restored by treatment with captopril. Our results demonstrate that three DGK isozymes are expressed in the heart and that each isozyme might have different functional characteristics in the healing and LV remodeling after MI.

Fatty acids inhibit growth-factor-induced diacylglycerol kinase alpha activation in vascular smooth-muscle cells

We have previously shown that unsaturated fatty acids amplify platelet-derived-growth-factor (PDGF)-induced protein kinase C (PKC) activation in vascular smooth-muscle cells (VSMCs). Diacylglycerol-induced PKC activation is normally terminated by diacylglycerol kinases (DGKs). We thus hypothesized that fatty acids act by inhibiting a DGK. Fractionation of VSMC extracts demonstrated that the DGK alpha isoform was the major DGK activity present. PDGF markedly increased the DGK activity of cultured cells. An inhibitor selective for the DGK alpha isoform, R59949 [3-[2-[4-(bis-(4-fluorophenyl)methylene]piperidin-1-yl)ethyl]-2,3-dihydro-2-thioxo-4(1H)-quinazolinone], abolished the growth-factor-induced increase in DGK activity, but had little effect on basal activity. PDGF thus selectively activates DGKalpha. Epidermal growth factor and alpha-thrombin stimulated total DGK activity similarly to PDGF. Activation by epidermal growth factor was sensitive to R59949, again suggesting involvement of DGKalpha. However, the alpha-thrombin-induced activity was unaffected by this agent. Unsaturated fatty acids inhibited growth-factor-induced DGKalpha activation, but had no effect on basal activity. Fatty acids also amplified the PDGF-induced increase in cell diacylglycerol content. These results indicate that inhibition of DGKalpha contributes to fatty-acid-induced amplification of PKC activation. Increased levels of fatty acids in diabetes may thus contribute to chronic PKC activation associated with this disorder.

Role of diacylglycerol kinase alpha in the attenuation of receptor signaling

Diacylglycerol kinase (DGK) is suggested to attenuate diacylglycerol-induced cell responses through the phosphorylation of this second messenger to phosphatidic acid. Here, we show that DGKalpha, an isoform highly expressed in T lymphocytes, translocates from cytosol to the plasma membrane in response to two different receptors known to elicit T cell activation responses: an ectopically expressed muscarinic type I receptor and the endogenous T cell receptor. Translocation in response to receptor stimulation is rapid, transient, and requires calcium and tyrosine kinase activation. DGKalpha-mediated phosphatidic acid generation allows dissociation of the enzyme from the plasma membrane and return to the cytosol, as demonstrated using a pharmacological inhibitor and a catalytically inactive version of the enzyme. The NH(2)-terminal domain of the protein is shown to be responsible for receptor-induced translocation and phosphatidic acid-mediated membrane dissociation. After examining induction of the T cell activation marker CD69 in cells expressing a constitutively active form of the enzyme, we present evidence of the negative regulation that DGKalpha exerts on diacylglycerol-derived cell responses. This study is the first to describe DGKalpha as an integral component of the signaling cascades that link plasma membrane receptors to nuclear responses.

Evidence for the involvement of diacylglycerol kinase in the activation of hypoxia-inducible transcription factor 1 by low oxygen tension

Hypoxia-inducible factor 1 (HIF-1) induces a gene expression program essential for the cellular adaptation to lowered oxygen environments. The intracellular mechanisms by which hypoxia induces HIF-1 remain poorly understood. Here we show that exposure of various cell types to hypoxia raises the intracellular level of phosphatidic acid primarily through the action of diacylglycerol kinase (DGK). Pharmacological inhibition of DGK activity through use of the specific DGK inhibitors and abrogated specifically HIF-1-dependent transcription analyzed with a HIF-1-responsive reporter plasmid. A more detailed analysis revealed that pharmacological inhibition of DGK activity prevented the hypoxia-dependent accumulation of the HIF-1alpha subunit and the subsequent HIF-1-DNA complex formation as well as hypoxia-induced activity of the HIF-1 transactivation domains localized to amino acids 530-582 and 775-826 of the HIF-1alpha subunit. Our results demonstrate for the first time that accumulation of phosphatidic acid through DGK underlines oxygen sensing and provide evidence for the involvement of this lipid kinase in the intracellular signaling that leads to HIF-1 activation.

Phosphatidic acid and diacylglycerol directly activate NADPH oxidase by interacting with enzyme components

The enzyme NADPH oxidase is regulated by phospholipase D in intact neutrophils and is activated by phosphatidic acid (PA) plus diacylglycerol (DG) in cell-free systems. We showed previously that cell-free NADPH oxidase activation by these lipids involves both protein kinase-dependent and -independent pathways. Here we demonstrate that only the protein kinase-independent pathway is operative in a cell-free system of purified and recombinant NADPH oxidase components. Activation by PA + DG was ATP-independent and unaffected by the protein kinase inhibitor staurosporine, indicating the lack of protein kinase involvement. Both PA and DG were required for optimal activation to occur. The drug reduced activation of NADPH oxidase by either arachidonic acid or PA + DG, with IC(50) values of 46 and 25 microm, respectively. The optimal concentration of arachidonic acid or PA + DG for oxidase activation was shifted to the right with, indicating interference of the drug with the interaction of lipid activators and enzyme components. inhibited the lipid-induced aggregation/sedimentation of oxidase components p47(phox) and p67(phox), suggesting a disruption of the lipid-mediated assembly process. The direct effects of on NADPH oxidase activation complicate its use as a "specific" inhibitor of DG kinase. We conclude that the protein kinase-independent pathway of NADPH oxidase activation by PA and DG involves direct interaction with NADPH oxidase components. Thus, NADPH oxidase proteins are functional targets for these lipid messengers in the neutrophil.

A domain with homology to neuronal calcium sensors is required for calcium-dependent activation of diacylglycerol kinase alpha

Diacylglycerol kinases (DGKs) phosphorylate diacylglycerol produced during stimulus-induced phosphoinositide turnover and attenuate protein kinase C activation. Diacylglycerol kinase alpha is an 82-kDa DGK isoform that is activated in vitro by Ca(2+). The DGK alpha regulatory region includes tandem C1 protein kinase C homology domains and Ca(2+)-binding EF hand motifs. It also contains an N-terminal recoverin homology (RVH) domain that is related to the N termini of the recoverin family of neuronal calcium sensors. To probe the structural basis of Ca(2+) regulation, we expressed a series of DGK alpha deletions spanning its regulatory domain in COS-1 cells. Deletion of the RVH domain resulted in loss of Ca(2+)-dependent activation. Further deletion of the EF hands resulted in a constitutively active enzyme, suggesting that sequences in or near the EF hands are sufficient for autoinhibition. Binding of Ca(2+) to the EF hands protected sites within both the RVH domain and EF hands from trypsin cleavage and increased the phenyl-Sepharose binding of a recombinant DGK alpha fragment that included both the RVH domain and EF hands. These observations suggested that Ca(2+) elicits a concerted conformational change of these two domains. A cationic amphiphile, octadecyltrimethylammonium chloride, also activated DGK alpha. As with Ca(2+), this activation required the RVH domain. However, this agent did not protect the EF hands and RVH domain from trypsin cleavage. These findings indicate that the EF hands and RVH domain act as a functional unit during Ca(2+)-induced DGK alpha activation.