Translocation of diacylglycerol kinase theta from cytosol to plasma membrane in response to activation of G protein-coupled receptors and protein kinase C

Effect of leaflet asymmetry on the stretching elasticity of lipid bilayers with phosphatidic acid

The asymmetry of membranes has a significant impact on their biophysical characteristics and behavior. This study investigates the composition and mechanical properties of symmetric and asymmetric membranes in giant unilamellar vesicles (GUVs) made of palmitoyloleoyl phosphatidylcholine (POPC) and palmitoyloleoyl phosphatidic acid (POPA). A combination of fluorescence quantification, zeta potential measurements, micropipette aspiration, and bilayer molecular dynamics simulations are used to characterize these membranes. The outer leaflet composition in vesicles is found consistent across the two preparation methods we employed, namely electroformation and inverted emulsion transfer. However, characterizing the inner leaflet poses challenges. Micropipette aspiration of GUVs show that oil residues do not substantially alter membrane elasticity, but simulations reveal increased membrane thickness and decreased interleaflet coupling in the presence of oil. Asymmetric membranes with a POPC:POPA mixture in the outer leaflet and POPC in the inner leaflet display similar stretching elasticity values to symmetric POPC:POPA membranes, suggesting potential POPA insertion into the inner leaflet during vesicle formation and suppressed asymmetry. The inverse compositional asymmetry, with POPC in the outer leaflet and POPC:POPA in the inner one yield less stretchable membranes with higher compressibility modulus compared with their symmetric counterparts. Challenges in achieving and predicting compositional correspondence highlight the limitations of phase-transfer-based methods. In addition, caution is advised when using fluorescently labeled lipids (even at low fractions of 0.5 mol %), as unexpected gel-like domains in symmetric POPC:POPA membranes were observed only with a specific type of labeled DOPE (dioleoylphosphatidylethanolamine) and the same fraction of unlabeled DOPE. The latter suggest that such domain formation may result from interactions between lipids and membrane fluorescent probes. Overall, this study underscores the complexity of factors influencing GUV membrane asymmetry, emphasizing the need for further research and improvement of characterization techniques.

The role of N-terminal phosphorylation of DGK-θ

Diacylglycerol kinases (DGKs) are lipid kinases that mediate the phosphorylation of diacylglycerol (DAG) leading to the production of phosphatidic acid (PtdOH). To examine the role of phosphorylation on DGK-θ, we first identified the phosphorylated sites on endogenous DGK-θ from mouse brain and found four sites: S15, S17, which we refer to phosphomotif-1 sites, and S22 and S26 which we refer to as phosphomotif-2 sites. This study focused on the role of these phosphorylated sites on enzyme activity, membrane binding, thermal stability, and cellular half-life of DGK-θ. After generating a construct devoid of all non-catalytic phosphorylation sites (4A), we also generated other constructs to mimic phosphorylation of these residues by mutating them to glutamate (E). Our data demonstrate that an increase in membrane affinity requires the phosphorylation of all four endogenous sites as the phosphomimetic 4E but not other phosphomimietics. Furthermore, 4E also shows an increase in basal activity as well as an increase in the Syt1-induced activity compared to 4A. It is noteworthy that these phosphorylations had no effect on the thermal stability or cellular half-life of this enzyme. Interestingly, when only one phosphorylation domain (phosphomotif-1 or phosphomotif-2) contained phosphomimetics (S15E/S17E or S22E/S26E), the basal activity was also increased but membrane binding affinity was not increased. Furthermore, when only one residue in each domain mimicked an endogenous phosphorylated serine (S15E/S22E or S17E/S26E), the Syt1-induced activity as well as membrane binding affinity decreased relative to 4A. These results indicate that these endogenous phosphorylation sites contribute differentially to membrane binding and enzymatic activity.

Discovery of a potent allosteric activator of DGKQ that ameliorates obesity-induced insulin resistance via the sn-1,2-DAG-PKCε signaling axis

sn-1,2-diacylglycerol (sn-1,2-DAG)-mediated activation of protein kinase Cε (PKCε) is a key pathway that is responsible for obesity-related lipid metabolism disorders, which induces hepatic insulin resistance and type 2 diabetes. No small molecules have been previously reported to ameliorate these diseases through this pathway. Here, we screened and identified the phytochemical atractylenolide II (AT II) that reduces the hepatic sn-1,2-DAG levels, deactivates PKCε activity, and improves obesity-induced hyperlipidemia, hepatosteatosis, and insulin resistance. Furthermore, using the ABPP strategy, the diacylglycerol kinase family member DGKQ was identified as a direct target of AT II. AT II may act on a novel drug-binding pocket in the CRD and PH domains of DGKQ to thereby allosterically regulate its kinase activity. Moreover, AT II also increases weight loss by activating DGKQ-AMPK-PGC1α-UCP-1 signaling in adipose tissue. These findings suggest that AT II is a promising lead compound to improve obesity-induced insulin resistance.

Phosphorylation of DGK

Diacylglycerol (DAG) and phosphatidic acid (PtdOH) play important roles in a variety of signaling cascades (Carrasco and Merida, 2007; Stace and Ktistakis, 2006). Therefore, the physiological roles and regulatory mechanisms controlling the levels of these lipids are important. One class of enzymes capable of coordinating the levels of these two lipids are the diacylglycerol kinases (DGKs). DGKs catalyze the transfer of the γ-phosphate of ATP to the hydroxyl group of DAG which generates PtdOH(Merida et al., 2008; Sakane et al., 2007). As DGKs reciprocally modulate the relative levels of these two signaling lipids, it is not surprising that there is increasing interest in understanding the mechanism underlying the catalysis and regulation of these kinases. While post-translational modifications (PTMs) are often involved in enzyme regulation, there is surprisingly little information regarding the PTMs on these enzymes and their roles in modulating their activity and function. In this review, we will summarize what is known about one PTM on DGKs, phosphorylation, and the possible functions of this modification.

Subcellular Localization Relevance and Cancer-Associated Mechanisms of Diacylglycerol Kinases

An increasing number of reports suggests a significant involvement of the phosphoinositide (PI) cycle in cancer development and progression. Diacylglycerol kinases (DGKs) are very active in the PI cycle. They are a family of ten members that convert diacylglycerol (DAG) into phosphatidic acid (PA), two-second messengers with versatile cellular functions. Notably, some DGK isoforms, such as DGKα, have been reported to possess promising therapeutic potential in cancer therapy. However, further studies are needed in order to better comprehend their involvement in cancer. In this review, we highlight that DGKs are an essential component of the PI cycle that localize within several subcellular compartments, including the nucleus and plasma membrane, together with their PI substrates and that they are involved in mediating major cancer cell mechanisms such as growth and metastasis. DGKs control cancer cell survival, proliferation, and angiogenesis by regulating Akt/mTOR and MAPK/ERK pathways. In addition, some DGKs control cancer cell migration by regulating the activities of the Rho GTPases Rac1 and RhoA.

DGKγ Knock-Out Mice Show Impairments in Cerebellar Motor Coordination, LTD, and the Dendritic Development of Purkinje Cells through the Activation of PKCγ

Diacylglycerol kinase γ (DGKγ) regulates protein kinase C (PKC) activity by converting DG to phosphatidic acid (PA). DGKγ directly interacts with PKCγ and is phosphorylated by PKCγ, resulting in the upregulation of lipid kinase activity. PKC dysfunction impairs motor coordination, indicating that the regulation of PKC activity is important for motor coordination. DGKγ and PKC are abundantly expressed in cerebellar Purkinje cells. However, the physiological role of DGKγ has not been elucidated. Therefore, we developed DGKγ knock-out (KO) mice and tested their cerebellar motor coordination. In DGKγ KO mice, cerebellar motor coordination and long-term depression (LTD) were impaired, and the dendrites of Purkinje cells from DGKγ KO mice were significantly retracted. Interestingly, treatment with the cPKC inhibitor Gö6976 (Gö) rescued the dendritic retraction of primary cultured Purkinje cells from DGKγ KO mice. In contrast, treatment with the PKC activator 12-o-tetradecanoylphorbol 13-acetate (TPA) reduced morphologic alterations in the dendrites of Purkinje cells from wild-type (WT) mice. In addition, we confirmed the upregulation of PKCγ activity in the cerebellum of DGKγ KO mice and rescued impaired LTD in DGKγ KO mice with a PKCγ-specific inhibitor. Furthermore, impairment of motor coordination observed in DGKγ KO mice was rescued in tm1c mice with DGKγ reexpression induced by the FLP-flippase recognition target (FRT) recombination system. These results indicate that DGKγ is involved in cerebellar LTD and the dendritic development of Purkinje cells through the regulation of PKCγ activity, and thus contributes to cerebellar motor coordination.

Comprehensive Genomic Analysis and Expression Profiling of Diacylglycerol Kinase (DGK) Gene Family in Soybean (Glycine max) under Abiotic Stresses

Diacylglycerol kinase (DGK) is an enzyme that plays a pivotal role in abiotic and biotic stress responses in plants by transforming the diacylglycerol into phosphatidic acid. However, there is no report on the characterization of soybean DGK genes in spite of the availability of the soybean genome sequence. In this study, we performed genome-wide analysis and expression profiling of the DGK gene family in the soybean genome. We identified 12 DGK genes (namely GmDGK1-12) which all contained conserved catalytic domains with protein lengths and molecular weights ranging from 436 to 727 amino acids (aa) and 48.62 to 80.93 kDa, respectively. Phylogenetic analyses grouped GmDGK genes into three clusters-cluster I, cluster II, and cluster III-which had three, four, and five genes, respectively. The qRT-PCR analysis revealed significant GmDGK gene expression levels in both leaves and roots coping with polyethylene glycol (PEG), salt, alkali, and salt/alkali treatments. This work provides the first characterization of the DGK gene family in soybean and suggests their importance in soybean response to abiotic stress. These results can serve as a guide for future studies on the understanding and functional characterization of this gene family.

Diacylglycerol kinases: Relationship to other lipid kinases

Lipid kinases regulate a wide variety of cellular functions and have emerged as one the most promising targets for drug design. Diacylglycerol kinases (DGKs) are a family of enzymes that catalyze the ATP-dependent phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PtdOH). Despite the critical role in lipid biosynthesis, both DAG and PtdOH have been shown as bioactive lipids mediating a number of signaling pathways. Although there is increasing recognition of their role in signaling systems, our understanding of the key enzyme which regulate the balance of these two lipid messages remain limited. Solved structures provide a wealth of information for understanding the function and regulation of these enzymes. Solving the structures of mammalian DGKs by traditional NMR and X-ray crystallography approaches have been challenging and so far, there are still no three-dimensional structures of these DGKs. Despite this, some insights may be gained by examining the similarities and differences between prokaryotic DGKs and other mammalian lipid kinases. This review focuses on summarizing and comparing the structure of prokaryotic and mammalian DGKs as well as two other lipid kinases: sphingosine kinase and phosphatidylinositol-3-kinase. How these known lipid kinases structures relate to mammalian DGKs will also be discussed.

Diacylglycerol kinase (DGKA) regulates the effect of the epilepsy and bipolar disorder treatment valproic acid in Dictyostelium discoideum

Valproic acid (VPA) provides a common treatment for both epilepsy and bipolar disorder; however, common cellular mechanisms relating to both disorders have yet to be proposed. Here, we explore the possibility of a diacylglycerol kinase (DGK) playing a role in regulating the effect of VPA relating to the treatment of both disorders, using the biomedical model Dictyostelium discoideum. DGK enzymes provide the first step in the phosphoinositide recycling pathway, implicated in seizure activity. They also regulate levels of diacylglycerol (DAG), thereby regulating the protein kinase C (PKC) activity that is linked to bipolar disorder-related signalling. Here, we show that ablation of the single Dictyostelium dgkA gene results in reduced sensitivity to the acute effects of VPA on cell behaviour. Loss of dgkA also provides reduced sensitivity to VPA in extended exposure during development. To differentiate a potential role for this DGKA-dependent mechanism in epilepsy and bipolar disorder treatment, we further show that the dgkA null mutant is resistant to the developmental effects of a range of structurally distinct branched medium-chain fatty acids with seizure control activity and to the bipolar disorder treatment lithium. Finally, we show that VPA, lithium and novel epilepsy treatments function through DAG regulation, and the presence of DGKA is necessary for compound-specific increases in DAG levels following treatment. Thus, these experiments suggest that, in Dictyostelium, loss of DGKA attenuates a common cellular effect of VPA relating to both epilepsy and bipolar disorder treatments, and that a range of new compounds with this effect should be investigated as alternative therapeutic agents.

This article has an associated First Person interview with the first author of the paper.

Editor's choice: Here, using a tractable model system, Dictyostelium discoideum, we show that diacylglycerol kinase activity might contribute to the cellular mechanism of action of the epilepsy and bipolar disorder treatment, valproic acid.

Bortezomib induces neuropathic pain through protein kinase C-mediated activation of presynaptic NMDA receptors in the spinal cord

Chemotherapeutic drugs, including bortezomib, often cause painful peripheral neuropathy, which is a severe dose-limiting adverse effect experienced by many cancer patients. The glutamate N-methyl-d-aspartate receptors (NMDARs) at the spinal cord level are critically involved in the synaptic plasticity associated with neuropathic pain. In this study, we determined whether treatment with bortezomib, a proteasome inhibitor, affects the NMDAR activity of spinal dorsal horn neurons. Systemic treatment with bortezomib in rats did not significantly affect postsynaptic NMDAR currents elicited by puff application of NMDA directly to dorsal horn neurons. Bortezomib treatment markedly increased the baseline frequency of miniature excitatory postsynaptic currents (EPSCs), which was completely normalized by the NMDAR antagonist 2-amino-5-phosphonopentanoic acid (AP5). AP5 also reduced the amplitude of monosynaptic EPSCs evoked by dorsal root stimulation in bortezomib-treated, but not vehicle-treated, rats. Furthermore, inhibition of protein kinase C (PKC) with chelerythrine fully reversed the increased frequency of miniature EPSCs and the amplitude of evoked EPSCs in bortezomib-treated rats. Intrathecal injection of AP5 and chelerythrine both profoundly attenuated mechanical allodynia and hyperalgesia induced by systemic treatment with bortezomib. In addition, treatment with bortezomib induced striking membrane translocation of PKC-βII, PKC-δ, and PKC-ε in the dorsal root ganglion. Our findings indicate that bortezomib treatment potentiates nociceptive input from primary afferent nerves via PKC-mediated tonic activation of presynaptic NMDARs. Targeting presynaptic NMDARs and PKC at the spinal cord level may be an effective strategy for treating chemotherapy-induced neuropathic pain.

Label-Free Profiling of Ligands for Endogenous Gpcrs Using a Cell-Based High-Throughput Screening Technology

This article reports the use of Corning Epic system— a label-free and noninvasive optical system that is centered on resonant waveguide grating biosensors—to profile endogenous G protein-coupled receptors (GPCRs) in living cells under physiologically relevant conditions. The endogenous GPCRs examined were bradykinin B2 receptor in A431 cells and protease-activated receptor subtype 1 (PAR1) in Chinese hamster ovary (CHO) cells. The activation of either receptor led to Gq-mediated signaling in the respective cells, as confirmed by Fluo-3 assays. Stimulation of CHO cells with thrombin, a PAR1 natural agonist, resulted in an optical response relating to dynamic mass redistribution that is similar to that induced by bradykinin in A431 cells. Based on the kinetics of agonist-mediated optical signatures, two time points, one before and another 5 min after the stimulation, were chosen to develop high-throughput (HT) screening assays. Results showed that such endpoint measurements enable not only HT screening of compounds using endogenous GPCRs, but also determining the efficacies of agonists. Those results suggested that the Corning Epic label-free system is an easily scaleable biosensor, amenable as an HTS platform for GPCR drug discovery and deorphanization. (JALA 2006;11:181–7)

Phosphorylation of Dgk1 Diacylglycerol Kinase by Casein Kinase II Regulates Phosphatidic Acid Production in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, Dgk1 diacylglycerol (DAG) kinase catalyzes the CTP-dependent phosphorylation of DAG to form phosphatidic acid (PA). The enzyme in conjunction with Pah1 PA phosphatase controls the levels of PA and DAG for the synthesis of triacylglycerol and membrane phospholipids, the growth of the nuclear/endoplasmic reticulum membrane, and the formation of lipid droplets. Little is known about how DAG kinase activity is regulated by posttranslational modification. In this work, we examined the phosphorylation of Dgk1 DAG kinase by casein kinase II (CKII). When phosphate groups were globally reduced using nonspecific alkaline phosphatase, Triton X-100-solubilized membranes from DGK1-overexpressing cells showed a 7.7-fold reduction in DAG kinase activity; the reduced enzyme activity could be increased 5.5-fold by treatment with CKII. Dgk1(1-77) expressed heterologously in Escherichia coli was phosphorylated by CKII on a serine residue, and its phosphorylation was dependent on time as well as on the concentrations of CKII, ATP, and Dgk1(1-77). We used site-specific mutagenesis, coupled with phosphorylation analysis and phosphopeptide mapping, to identify Ser-45 and Ser-46 of Dgk1 as the CKII target sites, with Ser-46 being the major phosphorylation site. In vivo, the S46A and S45A/S46A mutations of Dgk1 abolished the stationary phase-dependent stimulation of DAG kinase activity. In addition, the phosphorylation-deficient mutations decreased Dgk1 function in PA production and in eliciting pah1Δ phenotypes, such as the expansion of the nuclear/endoplasmic reticulum membrane, reduced lipid droplet formation, and temperature sensitivity. This work demonstrates that the CKII-mediated phosphorylation of Dgk1 regulates its function in the production of PA.

DGK-θ: Structure, Enzymology, and Physiological Roles

Diacylglycerol kinases (DGKs) are a family of enzymes that catalyze the ATP-dependent phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PtdOH). The recognition of the importance of these enzymes has been increasing ever since it was determined that they played a role in the phosphatidylinositol (PtdIns) cycle and a number of excellent reviews have already been written [(see van Blitterswijk and Houssa, 2000; Kanoh et al., 2002; Mérida et al., 2008; Tu-Sekine and Raben, 2009, 2011; Shulga et al., 2011; Tu-Sekine et al., 2013) among others]. We now know there are ten mammalian DGKs that are organized into five classes. DGK-θ is the lone member of the Type V class of DGKs and remains as one of the least studied. This review focuses on our current understanding of the structure, enzymology, regulation, and physiological roles of this DGK and suggests some future areas of research to understand this DGK isoform.

Molecular Pathways: Targeting Diacylglycerol Kinase Alpha in Cancer

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

Inhibition of diacylglycerol kinases as a physiological way to promote diacylglycerol signaling

Diacylglycerol is a key regulator of cell physiology, controlling the membrane recruitment and activation of signaling molecules. Accordingly, diacylglycerol generation and metabolism are strictly controlled, allowing for localized regulation of its concentration. While the increased production of diacylglycerol upon receptor triggering is well recognized, the modulation of diacylglycerol metabolism by diacylglycerol kinases (DGKs) is less characterized. Some agonists induce DGK activation and recruitment to the plasma membrane, promoting diacylglycerol metabolism to phosphatidic acid. Conversely, several reports indicate that signaling pathways that selectively inhibits DGK isoforms can enhance cellular diacylglycerol levels and signal transduction. For example, the impairment of DGKθ activity by RhoA binding to the catalytic domain represents a conserved mechanism controlling diacylglycerol signaling from Caenorhabditis elegans motoneurons to mammalian hepatocytes. Similarly, DGKα activity is inhibited in lymphocytes by TCR signaling, thus contributing to a rise in diacylglycerol concentration for downstream signaling. Finally, DGKμ activity is inhibited by ischemia-reperfusion-generated reactive oxygen species in airway endothelial cells, promoting diacylglycerol-mediated ion channel opening and edema. In those systems, DGKs provide a gatekeeper function by blunting diacylglycerol levels or possibly establishing permissive domains for diacylglycerol signaling. In this review, I discuss the possible general relevance of DGK inhibition to enhanced diacylglycerol signaling.

Silencing diacylglycerol kinase-theta expression reduces steroid hormone biosynthesis and cholesterol metabolism in human adrenocortical cells

Diacylglycerol kinase theta (DGKθ) plays a pivotal role in regulating adrenocortical steroidogenesis by synthesizing the ligand for the nuclear receptor steroidogenic factor 1 (SF1). In response to activation of the cAMP signaling cascade nuclear DGK activity is rapidly increased, facilitating PA-mediated, SF1-dependent transcription of genes required for cortisol and dehydroepiandrosterone (DHEA) biosynthesis. Based on our previous work identifying DGKθ as the enzyme that produces the agonist for SF1, we generated a tetracycline-inducible H295R stable cell line to express a short hairpin RNA (shRNA) against DGKθ and characterized the effect of silencing DGKθ on adrenocortical gene expression. Genome-wide DNA microarray analysis revealed that silencing DGKθ expression alters the expression of multiple genes, including steroidogenic genes, nuclear receptors and genes involved in sphingolipid, phospholipid and cholesterol metabolism. Interestingly, the expression of sterol regulatory element binding proteins (SREBPs) was also suppressed. Consistent with the suppression of SREBPs, we observed a down-regulation of multiple SREBP target genes, including 3-hydroxy-3-methylglutary coenzyme A reductase (HMG-CoA red) and CYP51, concomitant with a decrease in cellular cholesterol. DGKθ knockdown cells exhibited a reduced capacity to metabolize PA, with a down-regulation of lipin and phospholipase D (PLD) isoforms. In contrast, suppression of DGKθ increased the expression of several genes in the sphingolipid metabolic pathway, including acid ceramidase (ASAH1) and sphingosine kinases (SPHK). In summary, these data demonstrate that DGKθ plays an important role in steroid hormone production in human adrenocortical cells.

Diacylglycerol kinase θ couples farnesoid X receptor-dependent bile acid signalling to Akt activation and glucose homoeostasis in hepatocytes

DGKs (diacylglycerol kinases) catalyse the conversion of diacylglycerol into PA (phosphatidic acid), a positive modulator of mTOR (mammalian target of rapamycin). We have found that chenodeoxycholic acid and the synthetic FXR (farnesoid X receptor) ligand GW4064 induce the mRNA and protein expression of DGKθ in the HepG2 cell line and in primary human hepatocytes. Reporter gene studies using 1.5 kB of the DGKθ promoter fused to the luciferase gene revealed that bile acids increase DGKθ transcriptional activity. Mutation of putative FXR-binding sites attenuated the ability of GW4046 to increase DGKθ luciferase activity. Consistent with this finding, ChIP (chromatin immunoprecipitation) assays demonstrated that bile acid signalling increased the recruitment of FXR to the DGKθ promoter. Furthermore, GW4064 evoked a time-dependent increase in the cellular concentration of PA. We also found that GW4064 and PA promote the phosphorylation of mTOR, Akt and FoxO1 (forkhead box O1), and that silencing DGKθ expression significantly abrogated the ability of GW4046 to promote the phosphorylation of these PA-regulated targets. DGKθ was also required for bile-acid-dependent decreased glucose production. Taken together, our results establish DGKθ as a key mediator of bile-acid-stimulated modulation of mTORC2 (mTOR complex 2), the Akt pathway and glucose homoeostasis.

The expression of diacylglycerol kinase theta during the organogenesis of mouse embryos

Background

Diacylglycerol kinase (DGK) is a key enzyme that regulates diacylglycerol (DG) turnover and is involved in a variety of physiological functions. The isoform DGKθ has a unique domain structure and is the sole member of type V DGK. To reveal the spatial and temporal expression of DGKθ we performed immunohistochemical staining on paraffin sections of mouse embryos.

Results

At an early stage of development (E10.5 and 11.5), the expression of DGKθ was prominently detected in the brain, spinal cord, dorsal root ganglion, and limb bud, and was also moderately detected in the bulbus cordis and the primordium of the liver and gut. At later stages (E12.5 and 14.5), DGKθ expression persisted or increased in the neocortex, epithalamus, hypothalamus, medulla oblongata, and pons. DGKθ was also evident in the epidermis, and nearly all epithelia of the oropharyngeal membrane, digestive tract, and bronchea. At prenatal developmental stages (E16.5 and E18.5), the expression pattern of DGKθ was maintained in the central nervous system, intestine, and kidney, but was attenuated in the differentiated epidermis.

Conclusion

These results suggest that DGKθ may play important physiological roles not only in the brain, but also in diverse organs and tissues during the embryonic stages.

Desmoglein 3 promotes cancer cell migration and invasion by regulating activator protein 1 and protein kinase C-dependent-Ezrin activation

Desmoglein 3 (Dsg3), the pemphigus vulgaris antigen, has recently been shown to be upregulated in squamous cell carcinoma (SCC) and has been identified as a good tumor-specific marker for clinical staging of cervical sentinel lymph nodes in head and neck SCC. However, little is known about its biological function in cancer. The actin-binding protein Ezrin and the activator protein 1 (AP-1) transcription factor are implicated in cancer progression and metastasis. Here, we report that Dsg3 regulates the activity of c-Jun/AP-1 as well as protein kinase C (PKC)-mediated phosphorylation of Ezrin-Thr567, which contributes to the accelerated motility of cancer cells. Ectopic expression of Dsg3 in cancer cell lines caused enhanced phosphorylation at Ezrin-Thr567 with concomitant augmented membrane protrusions, cell spreading and invasive phenotype. We showed that Dsg3 formed a complex with Ezrin at the plasma membrane that was required for its proper function of interacting with F-actin and CD44 as Dsg3 knockdown impaired these associations. The increased Ezrin phosphorylation in Dsg3-overexpressing cells could be abrogated substantially by various pharmacological inhibitors for Ser/Thr kinases, including PKC and Rho kinase that are known to activate Ezrin. Furthermore, a marked increase in c-Jun S63 phosphorylation, among others, was found in Dsg3-overexpressing cells and the activation of c-Jun/AP-1 was further supported by a luciferase reporter assay. Taken together, our study identifies a novel Dsg3-mediated c-Jun/AP-1 regulatory mechanism and PKC-dependent Ezrin phosphorylation that could be responsible for Dsg3-associated cancer metastasis.

cAMP-stimulated transcription of DGKθ requires steroidogenic factor 1 and sterol regulatory element binding protein 1

Diacylglycerol kinase (DGK)θ is a lipid kinase that phosphorylates diacylglycerol to form phosphatidic acid (PA). We have previously shown that PA is a ligand for the nuclear receptor steroidogenic factor 1 (SF1) and that cAMP-stimulated expression of SF1 target genes requires DGKθ. In this study, we sought to investigate the role of cAMP signaling in regulating DGKθ gene expression. Real time RT-PCR and Western blot analysis revealed that dibutyryl cAMP (Bt2cAMP) increased the mRNA and protein expression, respectively, of DGKθ in H295R human adrenocortical cells. SF1 and sterol regulatory element binding protein 1 (SREBP1) increased the transcriptional activity of a reporter plasmid containing 1.5 kb of the DGKθ promoter fused to the luciferase gene. Mutation of putative cAMP responsive sequences abolished SF1- and SREBP-dependent DGKθ reporter gene activation. Consistent with this finding, chromatin immunoprecipitation assay demonstrated that Bt2cAMP signaling increased the recruitment of SF1 and SREBP1 to the DGKθ promoter. Coimmunoprecipitation assay revealed that SF1 and SREBP1 interact, suggesting that the two transcription factors form a complex on the DGKθ promoter. Finally, silencing SF1 and SREBP1 abolished cAMP-stimulated DGKθ expression. Taken together, we demonstrate that SF1 and SREBP1 activate DGKθ transcription in a cAMP-dependent manner in human adrenocortical cells.

Diacylglycerol kinases: regulated controllers of T cell activation, function, and development

Diacylglycerol kinases (DGKs) are a diverse family of enzymes that catalyze the conversion of diacylglycerol (DAG), a crucial second messenger of receptor-mediated signaling, to phosphatidic acid (PA). Both DAG and PA are bioactive molecules that regulate a wide set of intracellular signaling proteins involved in innate and adaptive immunity. Clear evidence points to a critical role for DGKs in modulating T cell activation, function, and development. More recently, studies have elucidated factors that control DGK function, suggesting an added complexity to how DGKs act during signaling. This review summarizes the available knowledge of the function and regulation of DGK isoforms in signal transduction with a particular focus on T lymphocytes.

The emerging role of protein kinase Cθ in cytoskeletal signaling

Cytoskeletal rearrangements often occur as the result of transduction of signals from the extracellular environment. Efficient awakening of this powerful machinery requires multiple activation and deactivation steps, which usually involve phosphorylation or dephosphorylation of different signaling units by kinases and phosphatases, respectively. In this review, we discuss the signaling characteristics of one of the nPKC isoforms, PKCθ, focusing on PKCθ-mediated signal transduction to cytoskeletal elements, which results in cellular rearrangements critical for cell type-specific responses to stimuli. PKCθ is the major PKC isoform present in hematopoietic and skeletal muscle cells. PKCθ plays roles in T cell signaling through the IS, survival responses in adult T cells, and T cell FasL-mediated apoptosis, all of which involve cytoskeletal rearrangements and relocation of this enzyme. PKCθ has been linked to the regulation of cell migration, lymphoid cell motility, and insulin signaling and resistance in skeletal muscle cells. Additional roles were suggested for PKCθ in mitosis and cell-cycle regulation. Comprehensive understanding of cytoskeletal regulation and the cellular "modus operandi" of PKCθ holds promise for improving current therapeutic applications aimed at autoimmune diseases.

Dual regulation of diacylglycerol kinase (DGK)-θ: polybasic proteins promote activation by phospholipids and increase substrate affinity

Diacylglycerol kinases are important mediators of lipid signaling cascades, and insight into their regulation is of increasing interest. Using purified DGK-θ, we show that this isoform is subject to dual regulation and that the previously characterized stimulation by acidic phospholipids is dependent on the presence of a positively charged protein or peptide. Polybasic cofactors lowered the K(m) for diacylglycerol at the membrane surface (K(m)((surf))), and worked synergistically with acidic phospholipids to increase activity 10- to 30-fold, suggesting that the purified enzyme is autoinhibited. Vesicle pulldown studies showed that acidic phospholipids recruit polybasic cofactors to the vesicle surface but have little effect on the membrane association of DGK-θ, suggesting that a triad of enzyme, acidic lipid and basic protein are necessary for interfacial activity. Importantly, these data demonstrate that the interfacial association and catalytic activity of DGK-θ are independently regulated. Finally, we show that DGK-θ directly interacts with, and is activated by, basic proteins such as histone H1 and Tau with nm affinity, consistent with a potential role for a polybasic protein or protein domain in the activation of this enzyme.

Diacylglycerol kinase θ: regulation and stability

Given the well-established roles of diacylglycerol (DAG) and phosphatidic acid (PtdOH) in a variety of signaling cascades, it is not surprising that there is an increasing interest in understanding their physiological roles and mechanisms that regulate their cellular levels. One class of enzymes capable of coordinately regulating the levels of these two lipids is the diacylglycerol kinases (DGKs). These enzymes catalyze the transfer of the γ-phosphate of ATP to the hydroxyl group of DAG, which generates PtdOH while reducing DAG. As these enzymes reciprocally modulate the relative levels of these two signaling lipids, it is essential to understand the regulation and roles of these enzymes in various tissues. One system where these enzymes play important roles is the nervous system. Of the ten mammalian DGKs, eight of them are readily detected in the mammalian central nervous system (CNS): DGK-α, DGK-β, DGK-γ, DGK-η, DGK-ζ, DGK-ι, DGK-ε, and DGK-θ. Despite the increasing interest in DGKs, little is known about their regulation. We have focused some attention on understanding the enzymology and regulation of one of these DGK isoforms, DGK-θ. We recently showed that DGK-θ is regulated by an accessory protein containing polybasic regions. We now report that this accessory protein is required for the previously reported broadening of the pH profile observed in cell lysates in response to phosphatidylserine (PtdSer). Our data further reveal DGK-θ is regulated by magnesium and zinc, and sensitive to the known DGK inhibitor R599022. These data outline new parameters involved in regulating DGK-θ.

Diacylglycerol kinase θ counteracts protein kinase C-mediated inactivation of the EGF receptor

Epidermal growth factor receptor (EGFR) activation is negatively regulated by protein kinase C (PKC) signaling. Stimulation of A431 cells with EGF, bradykinin or UTP increased EGFR phosphorylation at Thr654 in a PKC-dependent manner. Inhibition of PKC signaling enhanced EGFR activation, as assessed by increased phosphorylation of Tyr845 and Tyr1068 residues of the EGFR. Diacylglycerol is a physiological activator of PKC that can be removed by diacylglycerol kinase (DGK) activity. We found, in A431 and HEK293 cells, that the DGKθ isozyme translocated from the cytosol to the plasma membrane, where it co-localized with the EGFR and subsequently moved into EGFR-containing intracellular vesicles. This translocation was dependent on both activation of EGFR and PKC signaling. Furthermore, DGKθ physically interacted with the EGFR and became tyrosine-phosphorylated upon EGFR stimulation. Overexpression of DGKθ attenuated the bradykinin-stimulated, PKC-mediated EGFR phosphorylation at Thr654, and enhanced the phosphorylation at Tyr845 and Tyr1068. SiRNA-induced DGKθ downregulation enhanced this PKC-mediated Thr654 phosphorylation. Our data indicate that DGKθ translocation and activity is regulated by the concerted activity of EGFR and PKC and that DGKθ attenuates PKC-mediated Thr654 phosphorylation that is linked to desensitisation of EGFR signaling.

Regulation and roles of neuronal diacylglycerol kinases: a lipid perspective

Diacylglycerol kinases (DGKs) are a class of enzymes that catalyze the ATP-dependent conversion of diacylglycerol (DAG) to phosphatidic acid (PtdOH), resulting in the coordinate regulation of these two lipid second messengers. This regulation is particularly important in the nervous system where it is now well-established that DAG and PtdOH serve very important roles in modulating a variety of neurological functions. There are currently 10 identified mammalian DGKs, organized into five classes or "Types" based upon similarities in their primary sequences. A number of studies have identified eight of these isoforms in various regions of the mammalian central nervous system (CNS): DGK-α, DGK-β, DGK-γ, DGK-η, DGK-ζ, DGK-ι, DGK-ϵ, and DGK-θ. Further studies have provided compelling evidence supporting roles for these enzymes in neuronal spine density, myelination, synaptic activity, neuronal plasticity, epileptogenesis and neurotransmitter release. The physiological regulation of these enzymes is less clear. Like all interfacial enzymes, DGKs metabolize their hydrophobic substrate (DAG) at a membrane-aqueous interface. Therefore, these enzymes can be regulated by alterations in their subcellular localization, enzymatic activity, and/or membrane association. In this review, we summarize what is currently understood about the localization and regulation of the neuronal DGKs in the mammalian CNS.

Mechanisms regulating the nuclear translocation of p38 MAP kinase

p38 mitogen-activated protein kinase (MAPK) is of fundamental importance in a cell's response to environmental stresses, cytokines and DNA damage. p38 resides in the cytoplasm of resting cells, and translocates into the nucleus upon activation, yet the exact mechanisms remain largely unclear. We show here that the phosphorylation-dependent nuclear translocation of p38 is a common phenomenon when cells are stimulated with various stresses. On the other hand, the nuclear export of p38 requires its dephosphorylation, and it is exported both in a MK2-dependent and a nuclear export signal (NES)-independent manner. Although different p38-regulated/activated protein kinase (PRAK) mutants all dictate the intracellular localization of p38, results from a PRAK-deficient cell line indicate that it plays no role in this process. Microtubule depolymerizing reagent nocodazole and dynein inhibitor EHNA both block the nuclear translocation of p38, demonstrating roles for microtubules and dynein in p38 transport. Taken together, stress-induced nuclear accumulation of p38 is a phosphorylation-dependent, microtubule- and dynein-associated process.

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.

Lipid second messengers and related enzymes in vertebrate rod outer segments

Rod outer segments (ROSs) are specialized light-sensitive organelles in vertebrate photoreceptor cells. Lipids in ROS are of considerable importance, not only in providing an adequate environment for efficient phototransduction, but also in originating the second messengers involved in signal transduction. ROSs have the ability to adapt the sensitivity and speed of their responses to ever-changing conditions of ambient illumination. A major contributor to this adaptation is the light-driven translocation of key signaling proteins into and out of ROS. The present review shows how generation of the second lipid messengers from phosphatidylcholine, phosphatidic acid, and diacylglycerol is modulated by the different illumination states in the vertebrate retina. Findings suggest that the light-induced translocation of phototransduction proteins influences the enzymatic activities of phospholipase D, lipid phosphate phosphatase, diacylglyceride lipase, and diacylglyceride kinase, all of which are responsible for the generation of the second messenger molecules.

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.

Diacylglycerol kinases as sources of phosphatidic acid

There are ten mammalian diacylglycerol kinases (DGKs) whose primary role is to terminate diacylglycerol (DAG) signaling. However, it is becoming increasingly apparent that DGKs also influence signaling events through their product, phosphatidic acid (PA). They do so in some cases by associating with proteins and then modifying their activity by generating PA. In other cases, DGKs broadly regulate signaling events by virtue of their ability to provide PA for the synthesis of phosphatidylinositols (PtdIns).

Signaling at the membrane interface by the DGK/SK enzyme family

The sphingosine (SK) and diacylglycerol (DGK) kinases have become the subject of considerable focus recently due to their involvement as signaling enzymes in a variety of important biological processes. These lipid signaling kinases are closely related by sequence as well as functional properties. These enzymes are soluble, yet their substrates are hydrophobic. Therefore, they must act at the membrane interface. Second, for both of these enzyme families, their substrates (diacylglycerol for DGKs, sphingosine for SKs) as well as their products (phosphatidic acid for DGK, sphingosine-1-phosphate for SK) have signaling function. To understand how the signaling processes emanating from these kinases are regulated it is critical to understand the fundamental mechanisms that control their enzymatic activity. This is particularly true for the rational design of small molecules that would be useful as therapeutic compounds. Here we summarize enzymological properties of the diacylglycerol and SKs. Further, because the three-dimensional structure of the eukaryotic members of this family has yet to be determined, we discuss what can be gleaned from the recently reported structures of related prokaryotic members of this enzyme family.

Diacylglycerol kinases in immune cell function and self-tolerance

Both diacylglycerol (DAG) and phosphatidic acid (PA) are important second messengers involved in signal transduction from many immune cell receptors and can be generated and metabolized through multiple mechanisms. Recent studies indicate that diacylglycerol kinases (DGKs), the enzymes that catalyze phosphorylation of DAG to produce PA, play critical roles in regulating the functions of multiple immune cell lineages. In T cells, two DGK isoforms, alpha and zeta, inhibit DAG-mediated signaling following T-cell receptor engagement and prevent T-cell hyperactivation. DGK alpha and zeta synergistically promote T-cell anergy and are critical for T-cell tolerance. In mast cells, DGKzeta plays differential roles in their activation by promoting degranulation but attenuating cytokine production following engagement of the high affinity receptor for immunoglobulin E. In dendritic cells and macrophages, DGKzeta positively regulates Toll-like receptor-induced proinflammatory cytokine production through its product PA and is critical for host defense against Toxoplasma gondii infection. These studies demonstrate pivotal roles of DGKs in regulating immune cell function by acting both as signal terminator and initiator.

Dishevelled-induced phosphorylation regulates membrane localization of Par1b

Par1b is an evolutionarily conserved kinase that plays crucial roles in cell polarity. Controlling intracellular localization of Par1b is important for its biological activity. We previously reported that Wnt stimulation or expression of Dvl promotes accumulation of Par1b in the membrane (T. Terabayashi, T.J. Itoh, H. Yamaguchi, Y. Yoshimura, Y. Funato, S. Ohno, H. Miki, Polarity-Regulating Kinase Partitioning-Defective 1/Microtubule Affinity-Regulating Kinase 2 Negatively Regulates Development of Dendrites on Hippocampal Neurons, J. Neurosci. 27 (2007) 13098-13107). However, its molecular mechanism remains unclear. Here we show the importance of Par1b phosphorylation in the regulation of membrane localization. We find that Thr-324 is phosphorylated in a Dvl-dependent manner. Interestingly, the conversion of Thr-324 to Glu results in a significant accumulation of Par1b in the membrane, without any effects on the kinase activity. Moreover, the phospho-mimicking Par1b mutant does not antagonistically function against Dvl in microtubule stabilization and neurite extension, although wildtype Par1b does. These results suggest that membrane accumulation of Par1b induced by Dvl is regulated by its phosphorylation status, which is important for Par1b to regulate the microtubule dynamics.

The life and death of protein kinase C

Protein kinase C (PKC) is a family of kinases that plays diverse roles in many cellular functions, notably proliferation, differentiation, and cell survival. PKC is processed by phosphorylation and regulated by cofactor binding and subcellular localization. Extensive detail is available on the molecular mechanisms that regulate the maturation, activation, and signaling of PKC. However, less information is available on how signaling is terminated both from a global perspective and isozyme-specific differences. To target PKC therapeutically, various ATP-competitive inhibitors have been developed, but this method has problems with specificity. One possible new approach to developing novel, specific therapeutics for PKC would be to target the signaling termination pathways of the enzyme. This review focuses on the new developments in understanding how PKC signaling is terminated and how current drug therapies as well as information obtained from the recent elucidation of various PKC structures and down-regulation pathways could be used to develop novel and specific therapeutics for PKC.

Diacylglycerol kinases: at the hub of cell signalling

DGKs (diacylglycerol kinases) are members of a unique and conserved family of intracellular lipid kinases that phosphorylate DAG (diacylglycerol), catalysing its conversion into PA (phosphatidic acid). This reaction leads to attenuation of DAG levels in the cell membrane, regulating a host of intracellular signalling proteins that have evolved the ability to bind this lipid. The product of the DGK reaction, PA, is also linked to the regulation of diverse functions, including cell growth, membrane trafficking, differentiation and migration. In multicellular eukaryotes, DGKs provide a link between lipid metabolism and signalling. Genetic experiments in Caenorhabditis elegans, Drosophila melanogaster and mice have started to unveil the role of members of this protein family as modulators of receptor-dependent responses in processes such as synaptic transmission and photoreceptor transduction, as well as acquired and innate immune responses. Recent discoveries provide new insights into the complex mechanisms controlling DGK activation and their participation in receptor-regulated processes. After more than 50 years of intense research, the DGK pathway emerges as a key player in the regulation of cell responses, offering new possibilities of therapeutic intervention in human pathologies, including cancer, heart disease, diabetes, brain afflictions and immune dysfunctions.

Non-invasive Optical Biosensor for Probing Cell Signaling

Cell signaling mediated through a cellular target is encoded by spatial andtemporal dynamics of downstream signaling networks. The coupling of temporal dynamicswith spatial gradients of signaling activities guides cellular responses upon stimulation.Monitoring the integration of cell signaling in real time, if realized, would provide a newdimension for understanding cell biology and physiology. Optical biosensors includingresonant waveguide grating (RWG) biosensor manifest a physiologically relevant andintegrated cellular response related to dynamic redistribution of cellular matters, thusproviding a non-invasive means for cell signaling study. This paper reviews recentprogresses in biosensor instrumentation, and theoretical considerations and potentialapplications of optical biosensors for whole cell sensing.

Role of the diacylglycerol kinase alpha-conserved domains in membrane targeting in intact T cells

Diacylglycerol kinase (DGK) phosphorylates diacylglycerol to phosphatidic acid, modifying the cellular levels of these two lipid mediators. Ten 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. DGKalpha is a type I enzyme that acts as a negative modulator of diacylglycerol-based signals during T cell activation. Here we studied the functional role of the DGKalpha domains using mutational analysis to investigate membrane binding in intact cells. We show that the two atypical C1 domains are essential for plasma membrane targeting of the protein in intact cells but unnecessary for catalytic activity. We also identify the C-terminal sequence of the protein as essential for membrane binding in a phosphatidic acid-dependent manner. Finally we demonstrate that, in the absence of the calcium binding domain, receptor-dependent translocation of the truncated protein is regulated by phosphorylation of Tyr(335). This functional study provides new insight into the role of the so-called conserved domains of this lipid kinase family and demonstrates the existence of additional domains that confer specific plasma membrane localization to this particular isoform.

Inflammation and inflammatory agents activate protein kinase C epsilon translocation and excite guinea-pig submucosal neurons

BACKGROUND & AIMS

Properties of enteric neurons are transformed by inflammation and protein kinase C (PKC) isoforms are involved both in long-term changes in enteric neurons, and in transducing the effects of substances released during inflammation. We investigated roles of PKCepsilon in submucosal neurons by studying translocation in response to inflammatory mediators, effects on neuron excitability, and the changes in PKCepsilon distribution in a trinitrobenzene sulphonate model of ileitis.

METHODS

Immunohistochemical detection and analysis of association with membrane and cytosolic fractions, and Western blot analysis of cytosolic and particulate fractions were used to quantify translocation. Electrophysiology methods were used to measure effects on neuron excitability.

RESULTS

All submucosal neurons were immunoreactive for the novel PKC, PKCepsilon, and direct PKC activators, phorbol 12,13-dibutyrate, ingenol 3,20-dibenzoate, and the PKCepsilon-specific activator, transactivator of transduction-Psiepsilon receptor for activated C kinase, all caused PKCepsilon translocation from cytoplasm to surfaces of the neurons. Electrophysiologic studies showed that the stimulant of novel PKCs, ingenol (1 micromol/L), increased excitability of all neurons. Stimulation of protease-activated receptors caused PKCepsilon translocation selectively in vasoactive intestinal peptide secretomotor neurons, whereas a neurokinin 3 tachykinin receptor agonist caused translocation in neuropeptide Y and calretinin neurons. In all cases translocation was reduced significantly by a PKCepsilon-specific translocation inhibitor peptide. Increased PKCepsilon at the plasma membrane occurred in all neurons 6-7 days after an inflammatory stimulus.

CONCLUSIONS

Major targets for PKCepsilon include ion channels near the plasma membrane. PKCepsilon is likely to have a significant role in controlling the excitability of submucosal neurons and is probably an intermediate in causing hyperexcitability after inflammation.

PMA-induced differentiation of a bone marrow progenitor cell line activates HIV-1 LTR-driven transcription

Cells of the monocyte-macrophage lineage play an important role in human immunodeficiency virus type 1 (HIV-1)-associated disease. Infected myeloid precursor cells of the bone marrow are thought to be a viral reservoir that may repopulate the peripheral blood, central nervous system (CNS), and other organ systems throughout the course of disease. To model select aspects of HIV-1 infection of the bone marrow compartment in vitro, the erythro-myeloid precursor cell line, TF-1, was used. Phorbol 12-myristate 13-acetate (PMA) was found to induce the TF-1 cell line to differentiate through the myeloid lineage and become activated, as demonstrated by cellular morphologic changes and surface expression of differentiation and activation markers. Herein we demonstrate that HIV-1 long terminal repeats (LTRs) from T-, M-, and dual-tropic molecular clones have similar basal LTR activity in TF-1 cells and that differentiation of these cells by PMA resulted in increased LTR activity. Examination of specific cis-acting elements involved in basal and PMA-induced LTR activity demonstrated that the transcription factor families nuclear factor-kappa B (NF-kappaB) and specificity protein (Sp) contributed to the LTR activity of TF-1 cells, the Sp family being the most critical. These studies elucidate the impact of infected bone marrow monocytic cell differentiation on LTR activity and its potential impact on HIV-1-associated disease.

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.

Optical biosensor differentiates signaling of endogenous PAR1 and PAR2 in A431 cells

Background

Protease activated receptors (PARs) consist of a family of four G protein-coupled receptors. Many types of cells express several PARs, whose physiological significance is mostly unknown.

Results

Here, we show that non-invasive resonant waveguide grating (RWG) biosensor differentiates signaling of endogenous protease activated receptor subtype 1 (PAR₁) and 2 (PAR₂) in human epidermoid carcinoma A431 cells. The biosensor directly measures dynamic mass redistribution (DMR) resulted from ligand-induced receptor activation in adherent cells. In A431, both PAR₁ and PAR₂ agonists, but neither PAR₃ nor PAR₄ agonists, trigger dose-dependent Ca²⁺ mobilization as well as G_q-type DMR signals. Both Ca²⁺ flux and DMR signals display comparable desensitization patterns upon repeated stimulation with different combinations of agonists. However, PAR₁ and PAR₂ exhibit distinct kinetics of receptor re-sensitization. Furthermore, both trypsin- and thrombin-induced Ca²⁺ flux signals show almost identical dependence on cell surface cholesterol level, but their corresponding DMR signals present different sensitivities.

Conclusion

Optical biosensor provides an alternative readout for examining receptor activation under physiologically relevant conditions, and differentiates the signaling of endogenous PAR₁ and PAR₂ in A431.

Non-invasive optical biosensor for assaying endogenous G protein-coupled receptors in adherent cells

INTRODUCTION

Screening drugs against G protein-coupled receptors (GPCRs) - the single largest family of drug targets in the human genome - is still a major effort in pharmaceutical and biotech industries. Conventional cell-based assays generally measure a single cellular event, such as the generation of a second messenger or the relocation of a specific protein target. However, manipulation or engineering of cells is often a prerequisite for these technologies to achieve desired sensitivities. The present study is focused on the use of non-invasive and manipulation-free optical biosensors for assaying endogenous GPCRs in adherent cells.

METHODS

Resonant waveguide grating (RWG) biosensor was applied to manifest ligand-induced dynamic mass redistribution (DMR) within the bottom portion of adherent cell layer. The DMR signatures mediated through the activation of several endogenous GPCRs in cells were characterized. Endogenous receptor panning was examined at cell system level by using a panel of agonists known to activate many GPCRs, and also at family receptor level by determining the efficacies of a set of family-specific agonists.

RESULTS

Three major types of optical signatures were identified; each was correlated with the activation of a class of GPCRs, depending on the G protein with which the receptor is coupled (i.e., G(q), G(s) and G(i)). The characteristics of DMR signals, mostly the amplitude and kinetics of a DMR event, were dependent on the doses of agonists and the expression levels of endogenous receptors. All three classes of endogenous receptors were found in human epidermoid carcinoma A431 cells. Interestingly, the dose-dependent switching from one type of DMR signal to another was observed for several GPCR agonists examined. A small panel of P2Y receptor agonists exhibited distinct efficacies in three cell lines examined.

DISCUSSIONS

The RWG biosensors were applicable to study the activation of endogenous GPCRs. Like second messengers or gene expression, the DMR signals obtained could be considered as novel and quantifiable physiological responses of living cells mediated through GPCRs and used for studying receptor biology.

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.