Properties and functions of diacylglycerol kinases

Modulation of the mammalian target of rapamycin pathway by diacylglycerol kinase-produced phosphatidic acid

The protein known as mammalian target of rapamycin (mTOR) regulates cell growth by integrating different stimuli, such as available nutrients and mitogenic factors. The lipid messenger phosphatidic acid (PA) binds and positively regulates the mitogenic response of mTOR. PA generator enzymes are consequently potential regulators of mTOR. Here we explored the contribution to this pathway of the enzyme diacylglycerol kinase (DGK), which produces PA through phosphorylation of diacylglycerol. We found that overexpression of the DGKzeta, but not of the alpha isoform, in serum-deprived HEK293 cells induced mTOR-dependent phosphorylation of p70S6 kinase (p70S6K). After serum addition, p70S6K phosphorylation was higher and more resistant to rapamycin treatment in cells overexpressing DGKzeta. The effect of this DGK isoform on p70S6K hyperphosphorylation required the mTOR PA binding region. Down-regulation of endogenous DGKzeta by small interfering RNA in HEK293 cells diminished serum-induced p70S6K phosphorylation, highlighting the role of this isoform in the mTOR pathway. Our results confirm a role for PA in mTOR regulation and describe a novel pathway in which DGKzeta-derived PA acts as a mediator of mTOR signaling.

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.

Selective translocation of diacylglycerol kinase ζ in hippocampal neurons under transient forebrain ischemia

The molecular mechanisms responsible for differential neuronal vulnerability to ischemic injury are incompletely understood. Previous studies have reported that the expression and activity of protein kinase C (PKC), some subtypes of which are activated by Ca(2+) and diacylglycerol (DG), are altered after ischemic insults. Therefore, DG kinase (DGK), which is responsible for controlling PKC activity through DG metabolism, may also be involved in this process. DGKzeta, which is abundantly expressed in the brain, contains a nuclear localization signal (NLS), suggesting its involvement in some nuclear processes in neuronal cells. To elucidate the functional implications of DGKzeta in ischemia, we examined detailed localization of DGKzeta in rat brain after ischemic insults. We used an ischemic model of global cerebral ischemia for 20 min by bilateral common carotid artery occlusion combined with hypotension and followed time-points of reperfusion. DGKzeta expression was evaluated by immunohistochemistry using affinity-purified anti-DGKzeta antibody. In sham-operated rats, a strong DGKzeta-immunoreactivity was observed in the nucleus of neurons in various parts of the brain. In the global ischemic model DGKzeta-immunoreactivity was reduced in intensity in the hippocampal formation and detected in the cytoplasm of CA1 pyramidal neurons throughout reperfusion time courses. Change in the subcellular localization was restricted to the pyramidal cells in CA1 and later in CA3, but not observed in other areas of hippocampus. No change was observed in the cerebral and cerebellar cortices. The present study suggests that DGKzeta might be involved in the process of selective vulnerability of hippocampal pyramidal neurons in postischemic brain.

Domains, amino acid residues, and new isoforms of Caenorhabditis elegans diacylglycerol kinase 1 (DGK-1) important for terminating diacylglycerol signaling in vivo

Diacylglycerol kinases (DGKs) inhibit diacylglycerol (DAG) signaling by phosphorylating DAG. DGK-1, the Caenorhabditis elegans ortholog of human neuronal DGK, inhibits neurotransmission to control behavior. DGK-1, like DGK, has three cysteine-rich domains (CRDs), a pleckstrin homology domain, and a kinase domain. To identify DGK domains and amino acid residues critical for terminating DAG signaling in vivo, we analyzed 20 dgk-1 mutants defective in DGK-1-controlled behaviors. We found by sequencing that the mutations included nine amino acid substitutions and seven premature stop codons that impair the physiological functions of DGK-1. All nine amino acid substitutions are in the second CRD, the third CRD, or the kinase domain. Thus, these domains are important for the termination of DAG signaling by DGK-1 in vivo. Seven of the substituted amino acid residues are present in all human DGKs and likely define key residues required for the function of all DGKs. An ATP-binding site mutation expected to inactivate the kinase domain retained very little physiological function, but we found two stop codon mutants predicted to truncate DGK-1 before its kinase domain that retained significantly more function. We detected novel splice forms of dgk-1 that can reconcile this apparent conflict, as they skip exons containing the stop codons to produce DGK-1 isoforms that contain the kinase domain. Two of these isoforms lack an intact pleckstrin homology domain and yet appear to have significant function. Additional novel isoform(s) account for all of the DGK-1 function necessary for one behavior, dopamine response.

Novel diacylglycerol kinase inhibitor selectively suppressed an U46619-induced enhancement of mouse portal vein contraction under high glucose conditions

1. Diacylglycerol kinase (DG kinase) is a key enzyme in vascular contraction; however, alterations of the regulatory mechanisms in vascular dysfunction are poorly understood. In this study, the effect of a novel DG kinase inhibitor, stemphone, on vascular contraction was investigated. 2. The conventional DG kinase inhibitor, 6-[2-(4-[(4-fluorophenyl)phenyl-methylene]-1-piperidinyl)ethyl]-7-methyl-5H-thiazolo [3,2-alpha] pyrimidine-5-one (R59022) (0.1-30 microm), inhibited thromboxane A(2) analogue 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F(2alpha) (U46619)-induced sustained contractions in mouse aorta and porcine coronary artery in a dose-dependent manner. Treatment with stemphone did not affect contractions in these tissues. However, stemphone significantly inhibited (>0.3 microm) U46619-induced spontaneous phasic contraction in mouse portal vein. This inhibitory effect was not detected following R59022 treatment in portal vein. Therefore, stemphone demonstrated selectivity in terms of portal vein contraction. 3. Under high glucose (22.2 mm) conditions, U46619-induced contraction was enhanced in these three types of vascular tissue. Inhibitory effects of R59022 were attenuated under these conditions; however, effects of stemphone were observed. These results indicated that stemphone could inhibit portal vein contraction under high glucose conditions, for example, diabetes. These data suggested the possibility that DG kinase may be a target of hyperportal pressure. 4. Total mass of DG was enhanced under high glucose conditions. DG was derived from incorporated glucose via de novo synthesis in the absence of phospholipase C pathway mediation. This enhanced DG under high glucose conditions activated a calcium-independent protein kinase C (PKC). This PKC was associated with calcium-independent DG kinase activation. Treatment with stemphone also inhibited calcium-independent DG kinase. These signal transduction pathways were distinguishable from a DG-PKC pathway under normal glucose conditions. 5. The present investigation suggested that stemphone selectively inhibited overcontraction of portal vein induced by high glucose levels. This phenomenon was attributable to inhibition of calcium-independent DG kinase activation that occurred under high glucose conditions mediated by both DG synthesized from glucose and calcium-independent PKC activation.

phgABC, a three-gene operon required for growth of Streptococcus pneumoniae in hyperosmotic medium and in vivo

To cause disease, bacterial pathogens need to be able to adapt to the physiological conditions found within the host, including an osmolality of approximately 290 mosmol kg(-1). While investigating Streptococcus pneumoniae genes contained within pneumococcal pathogenicity island 1, we identified a three-gene operon of unknown function termed phgABC. PhgC has a domain with similarity to diacylglycerol kinases of eukaryotes and is the first described member of a family of related proteins found in many gram-positive bacteria. phgA and phgC mutant strains were constructed by insertional duplication mutagenesis and found to have impaired growth under conditions of high osmotic and oxidative stress. The compatible solutes proline and glycine betaine improved growth of the wild-type and the phgA mutant strains in hyperosmolar medium, and when analyzed by electron microscopy, the cellular morphology of the phgA mutant strain was unaffected by osmotic stress. The phgA and phgC mutant strains were reduced in virulence in models of both systemic and pulmonary infection. As the virulence of the phgA mutant strain was not restored in gp91phox(-/-) mice and the phgA and phgC mutant strains had reduced growth in both blood and serum, the reduced virulence of these strains is unlikely to be due to increased sensitivity to the respiratory burst of phagocytes but is, instead, due to impaired growth at physiological osmolality.

Divergence and complexities in DAG signaling: looking beyond PKC

For many years protein kinase C (PKC) has been the subject of extensive studies as a molecular target for the treatment of cancer and other diseases. To better define the role of PKC isozymes in the control of cell proliferation, survival and transformation, the examination of PKC-mediated signal transduction pathways by isozyme-specific intervention has become essential. However, issues related to the selectivity of activators and inhibitors of PKC isozymes, in addition to convoluted cross-talks between phorbol ester-regulated pathways, have greatly complicated our understanding of PKC-mediated responses. An additional level of complexity is provided by the fact diacylglycerol (DAG) signals can be transduced by phorbol ester receptors other than PKC. These receptors include chimaerins, RasGRPs, MUNC13s, PKD (PKC mu) and DAG kinases beta and gamma. Thus, it is conceivable that some of the effects that were originally attributed to PKC isozymes in response to phorbol esters might be mediated by PKC-independent pathways. A key issue for the design of novel therapeutic strategies that target PKC isozymes is a comprehensive analysis of isozyme-specific signal transduction pathways in different cell types and the development of pharmacological and molecular tools that can distinguish between the various PKC and 'non-PKC' phorbol ester receptors.

MuLK, a eukaryotic multi-substrate lipid kinase

We report the identification and characterization of a novel lipid kinase that phosphorylates multiple substrates. This enzyme, which we term MuLK for multi-substrate lipid kinase, does not belong to a previously described lipid kinase family. MuLK has orthologs in many organisms and is broadly expressed in human tissues. Although predicted to be a soluble protein, MuLK co-fractionates with membranes and localizes to an internal membrane compartment. Recombinant MuLK phosphorylates diacylglycerol, ceramide, and 1-acylglycerol but not sphingosine. Although its affinity for diacylglycerol and ceramide are similar, MuLK exhibits a higher V(max) toward diacylglycerol in vitro, consistent with it acting primarily as a diacylglycerol kinase. MuLK activity was inhibited by sphingosine and enhanced by cardiolipin. It was stimulated by calcium when magnesium concentrations were low and inhibited by calcium when magnesium concentrations were high. The effects of charged lipids and cations on MuLK activity in vitro suggest that its activity in vivo is tightly regulated by cellular conditions.

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.

Structure-activity relationship of diacylglycerol kinase theta

Diacylglycerol kinase (DGK) phosphorylates the second messenger diacylglycerol (DAG) to phosphatidic acid (PA). Among the nine mammalian isotypes identified, DGKtheta is the only one with three cysteine-rich domains (CRDs) (instead of two) in its N-terminal regulatory region. We previously reported that DGKtheta binds to and is negatively regulated by active RhoA. We now report that RhoA strongly binds to the C-terminal catalytic domain, which would explain its inhibition of DGK activity. To help finding a physiological function of DGKtheta, we further determined its activity in vitro as a function of 15 different truncations and point mutations in the primary structure. Most of these alterations, located throughout the protein, inactivated the enzyme, suggesting that catalytic activity depends on all of its conserved domains. The most C-terminal CRD is elongated with a stretch of 15 amino acids that is highly conserved among DGK isotypes. Mutation analysis revealed a number of residues in this region that were essential for enzyme activity. We suggest that this CRD extension plays an essential role in the correct folding of the protein and/or in substrate presentation to the catalytic region of the protein.

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.

Cloning and Characterization of Diacylglycerol Kinase ι Splice Variants in Rat Brain

Diacylglycerol kinase (DGK) catalyzes phosphorylation of a second messenger diacylglycerol (DG) to phosphatidic acid in cellular signal transduction. Previous studies have revealed that DGK consists of a family of isozymes including our rat clones. In this study we isolated from rat brain cDNA library the cDNA clones for a rat homologue of DGKiota (rDGKiota-1) that contains two zinc finger-like sequences, the highly conserved DGK catalytic domain, a bipartite nuclear localization signal, and four ankyrin repeats at the carboxyl terminus. In addition, we found novel splice variants, which contain either insertion 1 (71 bp) or insertion 2 (19 bp) or both in the carboxyl-terminal portion. Each of the insertions causes a frameshift, and the resultant premature stop codons produce two truncated forms (termed rDGKiota-2 and -iota-3), the former lacking the ankyrin repeats at the carboxyl terminus and the latter lacking a part of the catalytic domain and the ankyrin repeats. Truncation of the carboxyl-terminal portion clearly exerts effects on the detergent solubility and enzymatic activity of the splice variants, although all three variants showed similar cytoplasmic localization in cDNA-transfected cultured neurons despite the continued presence of the nuclear localization signal sequence. Immunoblot analysis using anti-rDGKiota antibody raised against the common amino-terminal portion clearly shows that these rDGKiota variants are indeed expressed in the brain. These results suggest that the carboxyl-terminal truncated forms of rDGKiota-2 and -iota-3 that exhibit reduced enzymatic activities might show a dominant negative effect against the intact rDGKiota-1, and that the modulation of signal transduction by the splice variants may play some roles in the physiologic and/or pathologic conditions of neurons.

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.

[Analysis of molecular mechanism regulating spatio-temporal localization and activity of protein kinase C and diacylglycerol kinase using live imaging]

Protein kinase C (PKC) changes its subcellular localization depending on extracellular signals including hormones and neurotransmitters. Such translocation is referred to as "targeting" in this review. The live imaging technique using GFP has allowed the dynamic movement of PKC to be visualized in living cells and revealed a remarkable diversity in PKC targeting. These studies indicate an importance of targeting in regulating the physiological and isotype-specific function of PKC. Like PKC, diacylglycerol kinase (DGK), which phosphorylates diacylglycerol resulting in attenuation of PKC, subtype-specifically translocates to particular subcellular compartments including the plasma membrane and Golgi complex. In addition, it has been shown that the localization and activation of the two functionally-related kinases are well organized by direct interaction and phosphorylation. This review summarizes diversity in targeting of PKC and DGK and the molecular mechanisms regulating their targeting.

A diacylglycerol-gated cation channel in vomeronasal neuron dendrites is impaired in TRPC2 mutant mice: mechanism of pheromone transduction

Vomeronasal sensory neurons play a crucial role in detecting pheromones, but the chemoelectrical transduction mechanism remains unclear and controversial. A major barrier to the resolution of this question has been the lack of an activation mechanism of a key transduction component, the TRPC2 channel. We have identified a Ca(2+)-permeable cation channel in vomeronasal neuron dendrites that is gated by the lipid messenger diacylglycerol (DAG), independently of Ca(2+) or protein kinase C. We demonstrate that ablation of the TRPC2 gene causes a severe deficit in the DAG-gated channel, indicating that TRPC2 encodes a principal subunit of this channel and that the primary electrical response to pheromones depends on DAG but not Ins(1,4,5)P(3), Ca(2+) stores, or arachidonic acid. Thus, a previously unanticipated mechanism involving direct channel opening by DAG underlies the transduction of sensory cues in the accessory olfactory system.

Novel regulatory mechanisms of mTOR signaling

As a master regulator of cellular processes ranging from cell growth and proliferation to differentiation, the mammalian target of rapamycin (mTOR) is critically involved in a complex signaling network. mTOR appears to govern an amino acid sensing pathway that integrates with a phosphatidylinositol 3-kinase-dependent mitogenic pathway to activate the downstream effectors. Recent findings have revealed some unexpected regulatory mechanisms of mTOR signaling. A direct link between mTOR and mitogenic signals is found to be mediated by the lipid second messenger phosphatidic acid. In addition, cytoplasmic-nuclear shuttling of mTOR appears to be required for the cytoplasmic functions of this protein. A new picture of the rapamycin-sensitive signaling network is emerging, with implications in putative upstream regulators and additional downstream targets for mTOR.

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.

Protein kinase C alpha phosphorylates and negatively regulates diacylglycerol kinase zeta

Diacylglycerol kinase (DGK) terminates diacylglycerol (DAG) signaling by phosphorylating DAG to produce phosphatidic acid, which also has signaling properties. Thus, precise control of DGK activity is essential for proper signal transduction. We demonstrated previously that a peptide corresponding to the myristoylated alanine-rich C kinase substrate (MARCKS) phosphorylation site domain (PSD) in DGK zeta was phosphorylated in vitro by an active fragment of protein kinase C (PKC). In the present study, we tested full-length DGK zeta and found that PKC alpha phosphorylated DGK zeta on serines within the MARCKS PSD in vitro and in vivo. DGK zeta also coimmunoprecipitated with PKC alpha, suggesting that they reside in a regulated signaling complex. We then tested whether phosphorylation affected DAG kinase activity. We found that a mutant (DGK zeta S/D) in which serines within the MARCKS PSD were altered to aspartates (to mimic phosphorylation) had lower activity compared with wild-type DGK zeta or a control mutant (DGK zeta S/N) in which the same serines were changed to asparagines. Furthermore, activation of PKC alpha by phorbol 12-myristate 13-acetate inhibited the activity of wild-type DGK zeta, but not DGK zeta S/D, in human embryonic kidney 293 cells. These results suggest that by phosphorylating the MARCKS PSD, PKC alpha attenuates DGK zeta activity. Supporting this, we found that cells expressing DGK zeta S/D had higher DAG levels and grew more rapidly compared with cells expressing DGK zeta S/N that could not be phosphorylated. Taken together, these results indicate that PKC alpha phosphorylates DGK zeta in cells, and this phosphorylation inhibits its kinase activity to remove cellular DAG, thereby affecting cell growth.

A functional genomic analysis of cell morphology using RNA interference

Background

The diversity of metazoan cell shapes is influenced by the dynamic cytoskeletal network. With the advent of RNA-interference (RNAi) technology, it is now possible to screen systematically for genes controlling specific cell-biological processes, including those required to generate distinct morphologies.

Results

We adapted existing RNAi technology in Drosophila cell culture for use in high-throughput screens to enable a comprehensive genetic dissection of cell morphogenesis. To identify genes responsible for the characteristic shape of two morphologically distinct cell lines, we performed RNAi screens in each line with a set of double-stranded RNAs (dsRNAs) targeting 994 predicted cell shape regulators. Using automated fluorescence microscopy to visualize actin filaments, microtubules and DNA, we detected morphological phenotypes for 160 genes, one-third of which have not been previously characterized in vivo. Genes with similar phenotypes corresponded to known components of pathways controlling cytoskeletal organization and cell shape, leading us to propose similar functions for previously uncharacterized genes. Furthermore, we were able to uncover genes acting within a specific pathway using a co-RNAi screen to identify dsRNA suppressors of a cell shape change induced by Pten dsRNA.

Conclusions

Using RNAi, we identified genes that influence cytoskeletal organization and morphology in two distinct cell types. Some genes exhibited similar RNAi phenotypes in both cell types, while others appeared to have cell-type-specific functions, in part reflecting the different mechanisms used to generate a round or a flat cell morphology.

Identification of mRNAs for the various diacylglycerol kinase isoforms in neutrophils from patients with localized aggressive periodontitis

BACKGROUND

Diacylglycerol kinase (DGK) metabolizes diacylglycerol (DAG), an endogenous activator of protein kinase C, to phosphatidic acid. We have previously reported increased DAG in neutrophils from patients with localized aggressive periodontitis (LAP) associated with reduced DGK activity. This reduction could be related to a mutation, post-translational modification, differential expression, or lack of expression of a particular isoform(s).

OBJECTIVE

The aim of this study was to identify the mRNAs for DGK isoforms in normal and LAP neutrophils.

METHODS

The alpha-, gamma-, and delta-isoforms of DGK were identified by polymerase chain reaction (PCR) using specific oligonucleotide primers for each isoform. The PCR products were isolated and sequenced for comparison to published sequences to confirm the validity of the PCR reaction. Total RNA was isolated from LAP and normal neutrophils, and northern blotting and semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) was used to examine the level of mRNA for each isoform.

RESULTS

No major differences were found in the isoform pattern between resting normal and LAP neutrophils. However, the levels of mRNA for the alpha- and gamma-isoforms of DGK were increased in normal neutrophils while slightly decreased in LAP cells upon stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP).

CONCLUSION

These data suggest that alterations in the mRNAs for the various isoforms of DGK during cell stimulation and the involvement of DGK that is expressed in multiple forms are subject to a variety of regulatory/control mechanisms and these mechanisms may explain the role of the 'primed' neutrophil phenotype associated with LAP.

Identification and characterization of two splice variants of human diacylglycerol kinase eta

Diacylglycerol kinase (DGK) participates in regulating the intracellular concentrations of two bioactive lipids, diacylglycerol and phosphatidic acid. DGK eta (eta 1, 128 kDa) is a type II isozyme containing a pleckstrin homology domain at the amino terminus. Here we identified another DGK eta isoform (eta 2, 135 kDa) that shared the same sequence with DGK eta 1 except for a sterile alpha motif (SAM) domain added at the carboxyl terminus. The DGK eta 1 mRNA was ubiquitously distributed in various tissues, whereas the DGK eta 2 mRNA was detected only in testis, kidney, and colon. The expression of DGK eta 2 was suppressed by glucocorticoid in contrast to the marked induction of DGK eta 1. DGK eta 2 was shown to form through its SAM domain homo-oligomers as well as hetero-oligomers with other SAM-containing DGKs (delta 1 and delta 2). Interestingly, DGK eta 1 and DGK eta 2 were rapidly translocated from the cytoplasm to endosomes in response to stress stimuli. In this case, DGK eta 1 was rapidly relocated back to the cytoplasm upon removal of stress stimuli, whereas DGK eta 2 exhibited sustained endosomal association. The experiments using DGK eta mutants suggested that the oligomerization of DGK eta 2 mediated by its SAM domain was largely responsible for its sustained endosomal localization. Similarly, the oligomerization of DGK eta 2 was suggested to result in negative regulation of its catalytic activity. Taken together, alternative splicing of the human DGK eta gene generates at least two isoforms with distinct biochemical and cell biological properties responding to different cellular metabolic requirements.

Enhanced T cell responses due to diacylglycerol kinase zeta deficiency

Much is known about how T cell receptor (TCR) engagement leads to T cell activation; however, the mechanisms terminating TCR signaling remain less clear. Diacylglycerol, generated after TCR ligation, is essential in T cells. Its function must be controlled tightly to maintain normal T cell homeostasis. Previous studies have shown that diacylglycerol kinase zeta (DGKzeta), which converts diacylglycerol to phosphatidic acid, can inhibit TCR signaling. Here we show that DGKzeta-deficient T cells are hyperresponsive to TCR stimulation both ex vivo and in vivo. Furthermore, DGKzeta-deficient mice mounted a more robust immune response to lymphocytic choriomeningitis virus infection than did wild-type mice. These results demonstrate the importance of DGKzeta as a physiological negative regulator of TCR signaling and T cell activation.

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.

Diacylglycerol kinase-zeta localization in skeletal muscle is regulated by phosphorylation and interaction with syntrophins

Syntrophins are scaffolding proteins that link signaling molecules to dystrophin and the cytoskeleton. We previously reported that syntrophins interact with diacylglycerol kinase-zeta (DGK-zeta), which phosphorylates diacylglycerol to yield phosphatidic acid. Here, we show syntrophins and DGK-zeta form a complex in skeletal muscle whose translocation from the cytosol to the plasma membrane is regulated by protein kinase C-dependent phosphorylation of the DGK-zeta MARCKS domain. DGK-zeta mutants that do not bind syntrophins were mislocalized, and an activated mutant of this sort induced atypical changes in the actin cytoskeleton, indicating syntrophins are important for localizing DGK-zeta and regulating its activity. Consistent with a role in actin organization, DGK-zeta and syntrophins were colocalized with filamentous (F)-actin and Rac in lamellipodia and ruffles. Moreover, extracellular signal-related kinase-dependent phosphorylation of DGK-zeta regulated its association with the cytoskeleton. In adult muscle, DGK-zeta was colocalized with syntrophins on the sarcolemma and was concentrated at neuromuscular junctions (NMJs), whereas in type IIB fibers it was found exclusively at NMJs. DGK-zeta was reduced at the sarcolemma of dystrophin-deficient mdx mouse myofibers but was specifically retained at NMJs, indicating that dystrophin is important for the sarcolemmal but not synaptic localization of DGK-zeta. Together, our findings suggest syntrophins localize DGK-zeta signaling complexes at specialized domains of muscle cells, which may be critical for the proper control of lipid-signaling pathways regulating actin organization. In dystrophic muscle, mislocalized DGK-zeta may cause abnormal cytoskeletal changes that contribute to disease pathogenesis.

Phosphatidic acid production, required for cholecystokinin octapeptide-stimulated amylase secretion from pancreatic acinar AR42J cells, is regulated by a wortmannin-sensitive process

To investigate the role of phospholipids in exocytotic secretory events, we utilized rat pancreatic acinar AR42J cells that secreted amylase in response to cholecystokinin octapeptide (CCK-8). Wortmannin, an inhibitor of phosphoinositide 3-kinase (PI3K), was found to inhibit the secretion in a dose-dependent manner. When changes in cell membrane phospholipids were investigated before and after CCK-8 stimulation using [32P]orthophosphoric acid-labeled AR42J cells, we observed a rapid increase in phosphatidic acid (PtdOH) levels right after stimulation, which was not observed in non-stimulated cells. The increase, however, was suppressed by wortmannin pre-treatment, which also inhibited amylase secretion. Changes in other major phospholipids were not significant. These results indicate that CCK-8 induces amylase secretion through PI3K-regulated production of PtdOH in cell membranes.

Diacylglycerol kinase-theta is localized in the speckle domains of the nucleus

It is well established that the nucleus is endowed with enzymes that are involved in lipid-dependent signal transduction pathways. Diacylglycerol (DAG) is a fundamental lipid second messenger that is produced in the nucleus. Previous reports have shown that the nucleus contains diacylglycerol kinases (DGKs), i.e., the enzymes that, by converting DAG into phosphatidic acid (PA), terminate DAG-dependent events. Here, we show, by immunofluorescence staining and confocal analysis, that DGK-theta localizes mainly to the nucleus of various cell lines, such as MDA-MB-453, MCF-7, PC12, and HeLa. Nuclear DGK-theta co-localizes with phosphatidylinositol 4,5-bisphosphate (PIP(2)) in domains that correspond to nuclear speckles, as revealed by the use of an antibody to the splicing factor SC-35, a well-established marker for these structures. The spatial distribution of nuclear DGK-theta was dynamic in that it was affected by inhibition of mRNA transcription with alpha-amanitin. Immuno-electron microscopy analysis demonstrated that DGK-theta, PIP(2), and phosphoinositide-specific phospholipase Cbeta1 (PLCbeta1) associated with electron-dense particles within the nucleus that correspond to interchromatin granule clusters. Cell fractionation experiments performed in MDA-MB-453, HeLa, and PC12 cells showed a preferential association of DGK-theta with the nucleus. Western blots demonstrated that DGK-theta was enriched in the nuclear matrix fraction prepared from MDA-MB-453 cells. Immunoprecipitation experiments with an antibody to PLCbeta1 revealed in MDA-MB-453 cells an association between this enzyme and both DGK-theta and phosphatidylinositol phosphate kinase Ialpha (PIPKIalpha). Our findings strengthen the contention that speckles represent a crucial site for the nuclear-based inositol lipid cycle. We may speculate that nuclear speckle-located DGK-theta, on cell stimulation with an agonist, converts to PA the DAG derived from PLCbeta1-dependent PIP(2) hydrolysis.

Regulatory role of diacylglycerol kinase gamma in macrophage differentiation of leukemia cells

Although nine diacylglycerol kinase (DGK) isozymes have been identified, our knowledge of their individual functions is still limited. Here we report that the levels of DGKgamma mRNA/protein in human leukemia HL-60 and U937 cells were rapidly and markedly decreased upon cellular differentiation into macrophages. In contrast, the enzyme expression remained almost unchanged in granulocytic differentiation pathway. Interestingly, the overexpression of wild-type or constitutively active DGKgamma, but not its kinase-dead mutant, markedly inhibited phorbol ester-induced cell attachment and nonspecific esterase activity, which are hallmarks of macrophage differentiation. We noted in this case that no effects were observed for the corresponding constructs of a closely related isozyme, DGKalpha. Prior to the cell attachment, phorbol ester induced translocation of DGKgamma from the cytoplasm to the cell periphery, resulting in its co-localization with F-actin together with protein kinase Cdelta. The results suggest that DGKgamma negatively regulates macrophage differentiation through its catalytic action operating on the cytoskeleton.

Synthesis and phorbol ester binding of the cysteine-rich domains of diacylglycerol kinase (DGK) isozymes. DGKgamma and DGKbeta are new targets of tumor-promoting phorbol esters

Diacylglycerol kinase (DGK) and protein kinase C (PKC) are two distinct enzyme families associated with diacylglycerol. Both enzymes have cysteine-rich C1 domains (C1A, C1B, and C1C) in the regulatory region. Although most PKC C1 domains strongly bind phorbol esters, there has been no direct evidence that DGK C1 domains bind phorbol esters. We synthesized 11 cysteine-rich sequences of DGK C1 domains with good sequence homology to those of the PKC C1 domains. Among them, only DGKgamma-C1A and DGKbeta-C1A exhibited significant binding to phorbol 12,13-dibutyrate (PDBu). Scatchard analysis of rat-DGKgamma-C1A, human-DGKgamma-C1A, and human-DGKbeta-C1A gave K(d) values of 3.6, 2.8, and 14.6 nm, respectively, suggesting that DGKgamma and DGKbeta are new targets of phorbol esters. An A12T mutation of human-DGKbeta-C1A enhanced the affinity to bind PDBu, indicating that the beta-hydroxyl group of Thr-12 significantly contributes to the binding. The K(d) value for PDBu of FLAG-tagged whole rat-DGKgamma (4.4 nm) was nearly equal to that of rat-DGKgamma-C1A (3.6 nm). Moreover, 12-O-tetradecanoylphorbol 13-acetate induced the irreversible translocation of whole rat-DGKgamma and its C1B deletion mutant, not the C1A deletion mutant, from the cytoplasm to the plasma membrane of CHO-K1 cells. These results indicate that 12-O-tetradecanoylphorbol 13-acetate binds to C1A of DGKgamma to cause its translocation.

T cell activation in vivo targets diacylglycerol kinase alpha to the membrane: a novel mechanism for Ras attenuation

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol to produce phosphatidic acid, leading to decreased and increased levels, respectively, of these two lipid messengers that play a central role in T cell activation. Nine DGK isoforms, grouped into five subtypes, are found in higher organisms; all contain a conserved C-terminal domain and at least two cysteine-rich motifs of unknown function. In this study, we have researched in vivo the regulation of DGK alpha, using a transgenic mouse model in which injection of an antigenic peptide activates the majority of peripheral T cells. We demonstrate that DGK alpha, highly expressed in resting T lymphocytes, is subject to complex control at the mRNA and protein levels during in vivo T cell activation. Subcellular fractionation of T lymphocytes shortly after in vivo engagement of the TCR shows rapid translocation of cytosolic DGK alpha to the membrane fraction. At early time points, DGK alpha translocation to the membrane correlates with rapid translocation of Ras guanyl nucleotide-releasing protein (RasGRP), a nucleotide exchange activator for Ras that associates to the membrane through a diacylglycerol-binding domain. To demonstrate a causal relationship between DGK alpha activity and RasGRP relocation to the membrane, we determined RasGRP translocation kinetics in a T cell line transiently transfected with constitutive active and dominant-negative DGK alpha mutants. We show that membrane localization of DGK alpha is associated with a negative regulatory signal for Ras activation by reversing RasGRP translocation. This study is the first demonstration of in vivo regulation of DGK alpha, and provides new insight into the functional role of a member of this family of lipid kinases in the regulation of the immune response.

Dictyostelium discoideum has a single diacylglycerol kinase gene with similarity to mammalian theta isoforms

Diacylglycerol kinases (DGKs) phosphorylate the neutral lipid diacylglycerol (DG) to produce phosphatidic acid (PA). In mammalian systems DGKs are a complex family of at least nine isoforms that are thought to participate in down-regulation of DG-based signalling pathways and perhaps activation of PA-stimulated signalling events. We report here that the simple protozoan amoeba Dictyostelium discoideum appears to contain a single gene encoding a DGK enzyme. This gene, dgkA, encodes a deduced protein that contains three C1-type cysteine-rich repeats, a DGK catalytic domain most closely related to the theta subtype of mammalian DGKs and a C-terminal segment containing a proline/glutamine-rich region and a large aspargine-repeat region. This gene corresponds to a previously reported myosin II heavy chain kinase designated myosin heavy chain-protein kinase C (MHC-PKC), but our analysis clearly demonstrates that this protein does not, as suggested by earlier data, contain a protein kinase catalytic domain. A FLAG-tagged version of DgkA expressed in Dictyostelium displayed robust DGK activity. Earlier studies indicating that disruption of this locus alters myosin II assembly levels in Dictyostelium raise the intriguing possibility that DG and/or PA metabolism may play a role in controlling myosin II assembly in this system.

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.

Phospholipid-based signaling in plants

Phospholipids are emerging as novel second messengers in plant cells. They are rapidly formed in response to a variety of stimuli via the activation of lipid kinases or phospholipases. These lipid signals can activate enzymes or recruit proteins to membranes via distinct lipid-binding domains, where the local increase in concentration promotes interactions and downstream signaling. Here, the latest developments in phospholipid-based signaling are discussed, including the lipid kinases and phospholipases that are activated, the signals they produce, the domains that bind them, the downstream targets that contain them and the processes they control.

The nucleotide-binding site of human sphingosine kinase 1

Sphingosine kinase catalyzes the formation of sphingosine 1-phosphate, a lipid second messenger that has been implicated in a number of agonist-driven cellular responses including mitogenesis, anti-apoptosis, and expression of inflammatory molecules. Despite the importance of sphingosine kinase, very little is known regarding its structure or mechanism of catalysis. Moreover, sphingosine kinase does not contain recognizable catalytic or substrate-binding sites, based on sequence motifs found in other kinases. Here we have elucidated the nucleotide-binding site of human sphingosine kinase 1 (hSK1) through a combination of site-directed mutagenesis and affinity labeling with the ATP analogue, FSBA. We have shown that Gly(82) of hSK1 is involved in ATP binding since mutation of this residue to alanine resulted in an enzyme with an approximately 45-fold higher K(m)((ATP)). We have also shown that Lys(103) is important in catalysis since an alanine substitution of this residue ablates catalytic activity. Furthermore, we have shown that this residue is covalently modified by FSBA. Our data, combined with amino acid sequence comparison, suggest a motif of SGDGX(17-21)K is involved in nucleotide binding in the sphingosine kinases. This motif differs in primary sequence from all previously identified nucleotide-binding sites. It does, however, share some sequence and likely structural similarity with the highly conserved glycine-rich loop, which is known to be involved in anchoring and positioning the nucleotide in the catalytic site of many protein kinases.

Metabolism of sphingosine 1-phosphate and lysophosphatidic acid: a genome wide analysis of gene expression in Drosophila

Lipids, in addition to being structural components of cell membranes, can act as signaling molecules. Bioactive lipids, such as sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA), may act intracellularly as second messengers or be secreted and act as intercellular signaling molecules. Such molecules can affect a variety of cellular processes including apoptosis, proliferation, differentiation and motility. To investigate possible sources of bioactive lipids during development we have searched the Drosophila genome for homologs of genes involved in mammalian S1P and LPA metabolism. Here we report the developmental expression of 31 such genes by in situ hybridization to Drosophila embryos. Most show expression in specific tissues, with expression in the gut and nervous system being recurring patterns.

Alternative splicing of the human diacylglycerol kinase delta gene generates two isoforms differing in their expression patterns and in regulatory functions

Diacylglycerol kinase (DGK) plays an important role in signal transduction through modulating the balance between two signaling lipids, diacylglycerol and phosphatidic acid. DGKdelta (type II isozyme) contains a pleckstrin homology domain at the N terminus and a sterile alpha motif domain at the C terminus. We identified another DGKdelta isoform (DGKdelta2, 135 kDa) that shared the same sequence with DGKdelta previously cloned (DGKdelta1, 130 kDa) except for the 52 residues N-terminally extended. Analysis of panels of human normal and tumor tissue cDNAs revealed that DGKdelta2 was ubiquitously expressed in all normal and tumor tissues examined, whereas the transcript of DGKdelta1 was detected only in ovary and spleen, and in a limited set of tumor-derived cells. The expression of DGKdelta2 was induced by treating cells with epidermal growth factor and tumor-promoting phorbol ester. In contrast, the levels of mRNA and protein of DGKdelta1 were suppressed by phorbol ester treatment. It thus becomes clear that the two DGKdelta isoforms are expressed under distinct regulatory mechanisms. DGKdelta1 was translocated through its pleckstrin homology domain from the cytoplasm to the plasma membrane in response to phorbol ester stimulation, whereas DGKdelta2 remained in the cytoplasm even after stimulation. Further experiments showed that the delta2-specific N-terminal sequence blocks the phorbol ester-dependent translocation of this isoform. Co-immunoprecipitation analysis of differently tagged DGKdelta1 and DGKdelta2 proteins showed that they were able to form homo- as well as hetero-oligomers. Taken together, alternative splicing of the human DGKdelta gene generates at least two isoforms, differing in their expressions and regulatory functions.

The role of phosphatidic acid in the regulation of the Ras/MEK/Erk signaling cascade

Phosphatidic acid (PA) is an important second messenger produced by the activation of numerous cell surface receptors. Recent data have suggested that PA regulates multiple cellular processes. This review addresses primarily the role of PA in the regulation of the Erk1/2 cascade pathway. A model for the regulation of Erk1/2 phosphorylation by cell surface receptors is presented. According to this model, agonists stimulate the binding of GTP to Ras and the activation of phospholipase D to generate phosphatidic acid. PA promotes the binding of cRaf-1 kinase to the membrane, where it interacts with Ras.GTP and other regulatory components of the pathway. Ras-Raf complexes remain bound to the surface of endosomes, where scaffolding complexes involving Ras, cRaf-1, MEK and Erk are formed. Complete activation and coupling of the cascade requires endocytosis, a process that is also modulated by PA.

A novel pathway regulating the mammalian target of rapamycin (mTOR) signaling

Originally discovered as an anti-fungal agent, the bacterial macrolide rapamycin is a potent immunosuppressant and a promising anti-cancer drug. In complex with its cellular receptor, the FK506-binding protein (FKBP12), rapamycin binds and inhibits the function of the mammalian target of rapamycin (mTOR). By mediating amino acid sufficiency, mTOR governs signaling to translational regulation and other cellular functions by converging with the phosphatidylinositol 3-kinase (PI3K) pathway on downstream effectors. Whether mTOR receives mitogenic signals in addition to nutrient-sensing has been an unresolved issue, and the mechanism of action of rapamycin remained unknown. Our recent findings have revealed a novel link between mitogenic signals and mTOR via the lipid second messenger phosphatidic acid (PA), and suggested a role for mTOR in the integration of nutrient and mitogen signals. A molecular mechanism for rapamycin inhibition of mTOR signaling is proposed, in which a putative interaction between PA and mTOR is abolished by rapamycin binding. Collective evidence further implicates the regulation of the rapamycin-sensitive signaling circuitry by phospholipase D, and potentially by other upstream regulators such as the conventional protein kinase C, the Rho and ARF families of small G proteins, and calcium ions. As the mTOR pathway has been demonstrated to be an important anti-cancer target, the identification of new components and novel regulatory modes in mTOR signaling will facilitate the future development of diagnostic and therapeutic strategies.

Phorbol ester-regulated oligomerization of diacylglycerol kinase delta linked to its phosphorylation and translocation

Diacylglycerol kinase (DGK) plays an important role in signal transduction through modulating the balance between two signaling lipids, diacylglycerol and phosphatidic acid. In yeast two-hybrid screening, we unexpectedly found a self-association of the C-terminal part of DGKdelta containing a sterile alpha-motif (SAM) domain. We then bacterially expressed the SAM domain fused with maltose-binding protein and confirmed the formation of dimeric and tetrameric structures. Moreover, gel filtration and co-immunoprecipitation analyses demonstrated that DGKdelta formed homo-oligomeric structures in intact cells and that the SAM domain was critically involved in the oligomerization. Interestingly, phorbol ester stimulation induced dissociation of the oligomeric structures with concomitant phosphorylation of DGKdelta. Furthermore, we found that DGKdelta was translocated from cytoplasmic vesicles to the plasma membrane upon phorbol ester stimulation. In this case, DGKdelta mutants lacking the ability of self-association were localized at the plasma membranes even in the absence of phorbol ester. A protein kinase C inhibitor, staurosporine, blocked all of the effects of phorbol ester, i.e. oligomer dissociation, phosphorylation, and translocation. We confirmed that tumor-promoting phorbol esters did not directly bind to DGKdelta. The present studies demonstrated that the formation and dissociation of oligomers serve as the regulatory mechanisms of DGKdelta and that DGKdelta is a novel downstream effector of phorbol ester/protein kinase C signaling pathway.

Expression and characterization of a human cDNA that complements the temperature-sensitive defect in dolichol kinase activity in the yeast sec59-1 mutant: the enzymatic phosphorylation of dolichol and diacylglycerol are catalyzed by separate CTP-mediated kinase activities in Saccharomyces cerevisiae

Dolichol kinase (DK) catalyzes the CTP-mediated phosphorylation of dolichol in eukaryotic cells, the terminal step in dolichyl monophosphate (Dol-P) biosynthesis de novo. In S. cerevisiae, the SEC59 gene encodes a protein essential for the expression of DK, an enzyme activity that is required for cell viability and normal rates of lipid intermediate synthesis and protein N-glycosylation. This study identifies a cDNA clone from human brain that encodes the mammalian homolog of DK (hDK1p). hDK1 is capable of complementing the growth defect, elevating DK activity, and consequently increasing Dol-P levels in vivo and restoring normal N-glycosylation of carboxypeptidase Y at the restrictive temperature in the temperature-sensitive mutant sec59-1. The CTP-mediated phosphorylation of diacylglycerol (DAG) is unaffected by either the temperature-sensitive mutation in the sec59-1 strain, overexpression of the SEC59 gene, or the mammalian homolog hDK1 under conditions that produced a loss or elevation in the level of DK activity. Additionally, overexpression of hDK1p in Sf-9 cells resulted in a 15-fold increase in DK activity but not DAG kinase activity in crude microsomal fractions. The cloned cDNA contains an open reading frame that would encode a protein with 538 amino acids and a molecular weight of 59,268 kDa. Consistent with this prediction, new polypeptides were detected with an apparent molecular weight of 59-60 kDa when His(6)-tagged constructs of hDK1 or the SEC59 gene were expressed in Sf-9 cells or the temperature-sensitive sec59-1 mutant cells, respectively. These results identify the first cDNA clone encoding a protein required for the expression of DK activity, possibly the catalytic subunit, in a mammalian cell, and establish that the phosphorylation of dolichol and DAG are catalyzed by separate kinase activities in yeast.

Regulation of T cell receptor-induced activation of the Ras-ERK pathway by diacylglycerol kinase zeta

T cell development in the thymus and activation of mature T cells in the periphery depend on signals stimulated by engagement of the T cell antigen receptor (TCR). Among the second messenger cascades initiated by TCR ligation include the phosphatidylinositol pathway where the membrane phospholipid, phosphatidylinositol 4,5-bisphosphate, is hydrolyzed to inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Inositol 1,4,5-trisphosphate signals a rise in intracellular free calcium, leading to translocation of nuclear factor of activated T cells into the nucleus. DAG activates RasGRP and protein kinase C theta. Because both RasGRP and protein kinase C theta are essential for thymocyte and T cell function, it is critical to understand how DAG is regulated. In this report, we demonstrate expression of DAG kinase zeta (DGKzeta, the enzyme that catalyzes the conversion of DAG to phosphatidic acid) in multiple lymphoid organs, with highest expression observed within the T cell compartment. Overexpression studies in Jurkat T cells indicate that DGKzeta interferes with TCR-induced Ras and ERK activation, AP-1 induction, and expression of the activation marker CD69. In contrast, TCR-stimulated calcium influx is not altered. Mutational analysis indicates that the kinase and DAG binding domains, but not the ankyrin repeats of DGKzeta, are required for its inhibitory effects. Collectively these studies demonstrate a potential role of DGKzeta to function as a selective negative regulator of DAG signaling on T cell activation and provide the first structure/function analysis of this enzyme in T cells.

Dynamics of diacylglycerol kinase zeta translocation in living T-cells. Study of the structural domain requirements for translocation and activity

The diacylglycerol kinases (DGK) regulate diacylglycerol-based signals by phosphorylating this key lipid intermediate to phosphatidic acid. Here, we have investigated the spatial and temporal regulation of diacylglycerol kinase zeta (DGK zeta) in living Jurkat T-cells expressing a muscarinic type I receptor. Using real time confocal videomicroscopy, we show the rapid translocation of a green fluorescent protein-tagged enzyme from the cytosol to the plasma membrane following receptor stimulation. The generation of a panel of truncations, deletions, and point mutations of the enzyme allowed us to examine the requirements of the different structural motifs for both activity and receptor-regulated translocation. The data show that DGK zeta has strict requirements for intact zinc fingers and the conserved catalytic domain for full enzymatic activity. Protein kinase C-driven myristoylated alanine-rich C kinase substrate domain phosphorylation and intact zinc fingers are in turn essential for plasma membrane translocation. DGK zeta does not translocate to the membrane following stimulation of the endogenous T-cell receptor, and our data demonstrate that the specificity in terms of receptor response is provided by the regulatory motifs present at the C-terminal domain of the protein. This is the first report that shows in vivo DGK zeta translocation in response to agonist stimulation and establishes the role of the different domains in enzymatic activity and the selectivity of the response to receptors.

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.

Phosphatidic acid modulates G protein regulation of phospholipase C-beta1 activity in membranes

Regulation of G protein stimulated phospholipase C-beta1 (PLC-beta1) activity by phosphatidic acid (PA) was determined in membranes. In cerebral cortical membranes, PLC-beta1 is under dual regulation by G protein stimulatory and inhibitory mechanisms. PA stimulated basal activity and was synergistic with G protein activation in increasing PLC-beta1 activity. Lysophosphatidic acid (LPA) also stimulated PLC-beta1 activity, but was less effective then PA. PA stimulation of PLC-beta1 activity was relatively independent of acyl chain length. PA decreased the Ca2+ dependence for G protein stimulation of PLC-beta1 activity. PA modulated the dual G protein regulation of PLC-beta1 activity, increasing stimulatory regulation and reducing inhibitory G protein regulation. The sensitivity to guanosine 5'-[gamma-thio]trisphosphate (GTP-gamma-S) and carbachol stimulation of PLC-beta1 activity was increased by PA. These results demonstrate that PA regulates both basal activity and G protein stimulation of PLC-beta1 activity. The data indicates that PA regulates the PLC-beta1 signaling pathway and thus may have an important role in the modulation of cell activation.

Molecular cloning and characterization of the human diacylglycerol kinase beta (DGKbeta) gene: alternative splicing generates DGKbeta isotypes with different properties

Diacylglycerol kinases are key modulators of levels of diacylglycerol, a second messenger involved in a variety of cellular responses to extracellular stimuli. A number of diacylglycerol kinases encoded by separate genes are present in mammalian genomes. We have cloned cDNAs encoding several isoforms of the human homologue of the rat diacylglycerol kinase beta gene and characterized two such isoforms that differ at their carboxyl terminus through alternative splicing and the usage of different polyadenylation signals. Quantitative analysis of gene expression in a panel of human tissue cDNAs revealed that transcripts corresponding to both isoforms are co-expressed in central nervous system tissues and in the uterus, with one variant being expressed at relatively higher levels. As green fluorescent protein fusions, the two isoforms displayed localization to different subcellular compartments, with one variant being associated with the plasma membrane, while the other isoform was predominantly localized within the cytoplasm. Differences were also observed in their subcellular localization in response to phorbol ester stimulation. Enzymatic assays demonstrated that the two isoforms display comparable diacylglycerol kinase activities. Therefore, the human diacylglycerol kinase beta gene can generate several enzyme isoforms, which can display different expression levels and subcellular localization but similar enzymatic activities in vitro.

Diacylglycerol kinase delta suppresses ER-to-Golgi traffic via its SAM and PH domains

We report here that the anterograde transport from the endoplasmic reticulum (ER) to the Golgi was markedly suppressed by diacylglycerol kinase delta (DGKdelta) that uniquely possesses a pleckstrin homology (PH) and a sterile alpha motif (SAM) domain. A low-level expression of DGKdelta in NIH3T3 cells caused redistribution into the ER of the marker proteins of the Golgi membranes and the vesicular-tubular clusters (VTCs). In this case DGKdelta delayed the ER-to-Golgi traffic of vesicular stomatitis virus glycoprotein (VSV G) and also the reassembly of the Golgi apparatus after brefeldin A (BFA) treatment and washout. DGKdelta was demonstrated to associate with the ER through its C-terminal SAM domain acting as an ER-targeting motif. Both of the SAM domain and the N-terminal PH domain of DGKdelta were needed to exert its effects on ER-to-Golgi traffic. Kinase-dead mutants of DGKdelta were also effective as the wild-type enzyme, suggesting that the catalytic activity of DGK was not involved in the present observation. Remarkably, the expression of DGKdelta abrogated formation of COPII-coated structures labeled with Sec13p without affecting COPI structures. These findings indicate that DGKdelta negatively regulates ER-to-Golgi traffic by selectively inhibiting the formation of ER export sites without significantly affecting retrograde transport.

Phospholipase D1b and D2a generate structurally identical phosphatidic acid species in mammalian cells

Mammalian cells contain different phospholipase D enzymes (PLDs) whose distinct physiological roles are poorly understood and whose products have not been characterized. The development of porcine aortic endothelial (PAE) cell lines able to overexpress PLD-1b or -2a under the control of an inducible promoter has enabled us to characterize both the substrate specificity and the phosphatidic acid (PtdOH) product of these enzymes under controlled conditions. Liquid chromatography-MS analysis showed that PLD1b- and PLD2a-transfected PAE cells, as well as COS7 and Rat1 cells, generate similar PtdOH and, in the presence of butan-1-ol, phosphatidylbutanol (PtdBut) profiles, enriched in mono- and di-unsaturated species, in particular 16:0/18:1. Although PtdBut mass increased, the species profile did not change in cells stimulated with ATP or PMA. Overexpression of PLD made little difference to basal or stimulated PtdBut formation, indicating that activity is tightly regulated in vivo and that factors other than just PLD protein levels limit hydrolytic function. In vitro assays using PLD-enriched lysates showed that the enzyme could utilize both phosphatidylcholine and, much less efficiently, phosphatidylethanolamine, with slight selectivity towards mono- and di-unsaturated species. Phosphatidylinositol was not a substrate. Thus PLD1b and PLD2a hydrolyse a structurally similar substrate pool to generate an identical PtdOH product enriched in mono- and di-unsaturated species that we propose to function as the intracellular messenger forms of this lipid.

A point mutant of human sphingosine kinase 1 with increased catalytic activity

Sphingosine kinase (SK) catalyses the formation of sphingosine 1-phosphate, a lipid second messenger that has been implicated in mediating such fundamental biological processes as cell growth and survival. Very little is currently known regarding the structure or mechanisms of catalysis and activation of SK. Here we have tested the functional importance of Gly(113), a highly conserved residue of human sphingosine kinase 1 (hSK), by site-directed mutagenesis. Surprisingly, a Gly(113)-->Ala substitution generated a mutant that had 1.7-fold greater catalytic activity than wild-type hSK (hSK(WT)). Our data suggests that the Gly(113)-->Ala mutation increases catalytic efficiency of hSK, probably by inducing a conformational change that increases the efficiency of phosphoryl transfer. Interestingly, hSK(G113A) activity could be stimulated in HEK293T cells by cell agonists to a comparable extent to hSK(WT).