Properties and functions of diacylglycerol kinases

Diacylglycerol kinase δ phosphorylates phosphatidylcholine-specific phospholipase C-dependent, palmitic acid-containing diacylglycerol species in response to high glucose levels

Decreased expression of diacylglycerol (DG) kinase (DGK) δ in skeletal muscles is closely related to the pathogenesis of type 2 diabetes. To identify DG species that are phosphorylated by DGKδ in response to high glucose stimulation, we investigated high glucose-dependent changes in phosphatidic acid (PA) molecular species in mouse C2C12 myoblasts using a newly established liquid chromatography/MS method. We found that the suppression of DGKδ2 expression by DGKδ-specific siRNAs significantly inhibited glucose-dependent increases in 30:0-, 32:0-, and 34:0-PA and moderately attenuated 30:1-, 32:1-, and 34:1-PA. Moreover, overexpression of DGKδ2 also enhanced the production of these PA species. MS/MS analysis revealed that these PA species commonly contain palmitic acid (16:0). D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), significantly inhibited the glucose-stimulated production of the palmitic acid-containing PA species. Moreover, PC-PLC was co-immunoprecipitated with DGKδ2. These results strongly suggest that DGKδ preferably metabolizes palmitic acid-containing DG species supplied from the PC-PLC pathway, but not arachidonic acid (20:4)-containing DG species derived from the phosphatidylinositol turnover, in response to high glucose levels.

A novel light-dependent activation of DAGK and PKC in bovine photoreceptor nuclei

In this work, we describe a selective light-dependent distribution of the lipid kinase 1,2-diacylglycerol kinase (EC 2.7.1.107, DAGK) and the phosphorylated protein kinase C alpha (pPKCα) in a nuclear fraction of photoreceptor cells from bovine retinas. A nuclear fraction enriched in small nuclei from photoreceptor cells (PNF), was obtained when a modified nuclear isolation protocol developed by our laboratory was used. We measured and compared DAGK activity as phosphatidic acid (PA) formation in PNF obtained from retinas exposed to light and in retinas kept in darkness using [γ-(32)P]ATP or [(3)H]DAG. In the absence of exogenous substrates and detergents, no changes in DAGK activity were observed. However, when DAGK activity assays were performed in the presence of exogenous substrates, such as stearoyl arachidonoyl glycerol (SAG) or dioleoyl glycerol (DOG), and different detergents (used to make different DAGK isoforms evident), we observed significant light effects on DAGK activity, suggesting the presence of several DAGK isoforms in PNF. Under conditions favoring DAGKζ activity (DOG, Triton X-100, dioleoyl phosphatidylserine and R59022) we observed an increase in PA formation in PNF from retinas exposed to light with respect to those exposed to darkness. In contrast, under conditions favoring DAGKɛ (SAG, octylglucoside and R59022) we observed a decrease in its activity. These results suggest different physiological roles of the above-mentioned DAGK isoforms. Western blot analysis showed that whereas light stimulation of bovine retinas increases DAGKζ nuclear content, it decreases DAGKɛ and DAGKβ content in PNF. The role of PIP2-phospholipase C in light-stimulated DAGK activity was demonstrated using U73122. Light was also observed to induce enhanced pPKCα content in PNF. The selective distribution of DAGKζ and ɛ in PNF could be a light-dependent mechanism that in vertebrate retina promotes selective DAG removal and PKC regulation.

Regulation of diacylglycerol kinase δ2 expression in C2C12 skeletal muscle cells by free fatty acids

Decreased expression of diacylglycerol kinase (DGK) δ in skeletal muscles is closely related to the pathogenesis of type 2 diabetes. However, the regulation of DGKδ expression is not well understood. In this study, we found that myristic acid (14:0) significantly increased DGKδ2 protein expression in a dose-dependent manner (EC(50) = 0.16 mM) in mouse C2C12 myotubes. In contrast, oleic [18:1(n-9)], eicosenoic [20:1(n-9)] and erucic [22:1(n-9)] acids markedly decreased DGKδ2 expression. Myristic acid slowly enhanced DGKδ2 expression at the transcription level. Therefore, DGKδ2 expression is positively regulated by the relatively short-chain saturated fatty acid myristic acid but attenuated by n-9 monounsaturated fatty acids.

Mutations in the cholesterol transporter gene ABCA5 are associated with excessive hair overgrowth

Inherited hypertrichoses are rare syndromes characterized by excessive hair growth that does not result from androgen stimulation, and are often associated with additional congenital abnormalities. In this study, we investigated the genetic defect in a case of autosomal recessive congenital generalized hypertrichosis terminalis (CGHT) (OMIM135400) using whole-exome sequencing. We identified a single base pair substitution in the 5' donor splice site of intron 32 in the ABC lipid transporter gene ABCA5 that leads to aberrant splicing of the transcript and a decrease in protein levels throughout patient hair follicles. The homozygous recessive disruption of ABCA5 leads to reduced lysosome function, which results in an accumulation of autophagosomes, autophagosomal cargos as well as increased endolysosomal cholesterol in CGHT keratinocytes. In an unrelated sporadic case of CGHT, we identified a 1.3 Mb cryptic deletion of chr17q24.2-q24.3 encompassing ABCA5 and found that ABCA5 levels are dramatically reduced throughout patient hair follicles. Collectively, our findings support ABCA5 as a gene underlying the CGHT phenotype and suggest a novel, previously unrecognized role for this gene in regulating hair growth.

EF-hand motifs of diacylglycerol kinase α interact intra-molecularly with its C1 domains

•

We revealed the intra-molecular interaction of DGKα.

•

The EF-hand motifs of DGKα directly binds to its C1 domains.

•

The intra-molecular interaction was negatively regulated by Ca²⁺.

•

The intra-molecular interaction is important for the activation mechanism of DGKα.

Diacylglycerol kinase (DGK) α, which is activated by Ca²⁺, contains a recoverin homology (RVH) domain, tandem repeats of two Ca²⁺-binding EF-hand motifs, two cysteine-rich C1 domains and the catalytic domain. We previously found that a DGKα mutant lacking the RVH domain and EF-hands was constitutively active and that the N-terminal region of DGKα, consisting of the RVH domain and EF-hand motifs, interacted intra-molecularly with the C-terminal region containing the C1 and catalytic domains. In this study, we narrowed down the interaction regions of DGKα. At the C-terminal region, the C1 domains are responsible for the intra-molecular interaction. At the N-terminal region, the EF-hand motifs mainly contribute to the interaction. Moreover, using highly purified EF-hand motifs and C1 domains, we demonstrate that they directly bind to each other. The co-precipitation of these two domains was clearly attenuated by the addition of Ca²⁺. These results indicate that the Ca²⁺-induced dissociation of the intra-molecular interaction between the EF-hand motifs and the C1 domains of DGKα is the key event that regulates the activity of the enzyme.

Diacylglycerol kinase as a possible therapeutic target for neuronal diseases

Diacylglycerol kinase (DGK) is a lipid kinase converting diacylglycerol to phosphatidic acid, and regulates many enzymes including protein kinase C, phosphatidylinositol 4-phosphate 5-kinase, and mTOR. To date, ten mammalian DGK subtypes have been cloned and divided into five groups, and they show subtype-specific tissue distribution. Therefore, each DGK subtype is thought to be involved in respective cellular responses by regulating balance of the two lipid messengers, diacylglycerol and phosphatidic acid. Indeed, the recent researches using DGK knockout mice have clearly demonstrated the importance of DGK in the immune system and its pathophysiological roles in heart and insulin resistance in diabetes. Especially, most subtypes show high expression in brain with subtype specific regional distribution, suggesting that each subtype has important and unique functions in brain. Recently, neuronal functions of some DGK subtypes have accumulated. Here, we introduce DGKs with their structural motifs, summarize the enzymatic properties and neuronal functions, and discuss the possibility of DGKs as a therapeutic target of the neuronal diseases.

Overexpression of diacylglycerol kinase η enhances Gαq-coupled G protein-coupled receptor signaling

Multiple genome-wide association studies have linked diacylglycerol kinase η (DGKη) to bipolar disorder (BPD). Moreover, DGKη expression is increased in tissue from patients with BPD. How increased levels of this lipid kinase might affect cellular functions is currently unclear. Here, we overexpressed mouse DGKη in human embryonic kidney 293 cells to examine substrate specificity and signaling downstream of endogenous G protein-coupled receptors (GPCRs). We found that DGKη can phosphorylate diacylglycerol (DAG) with different acyl side chains (8:0, 12:0, 18:1). In addition, overexpression of DGKη enhanced calcium mobilization after stimulating muscarinic receptors with carbachol and after stimulating purinergic receptors with ATP. This effect required DGKη catalytic activity, as assessed using a kinase-dead (G389D) mutant and multiple truncation constructs. DGKη was localized throughout the cytosol and did not translocate to the plasma membrane after stimulation with carbachol. Since protein kinase C (PKC) can be activated by DAG and promotes receptor desensitization, we also examined functional interactions between PKC and DGKη. We found that acute activation of PKC with phorbol 12-myristate 13-acetate shortened carbachol-evoked calcium responses and occluded the effect of overexpressed DGKη. Moreover, inhibition of PKC activity with bisindolylmaleimide I (BIM I) produced the same enhancing effect on carbachol-evoked calcium mobilization as overexpressed DGKη, and overexpression of DGKη produced no additional effect on calcium mobilization in the presence of BIM I. Taken together, our data suggest that DGKη enhances GPCR signaling by reducing PKC activation.

Effects of dechlorane plus on the hepatic proteome of juvenile Chinese sturgeon (Acipenser sinensis)

Dechlorane Plus (DP), an alternative to decabromodiphenyl ether (BDE-209), is a widely used polychlorinated flame retardant that is frequently detected in aquatic ecosystems. While the mechanisms of toxicity of BDE-209 have been well documented, less is known about the toxicity of DP. In this study, juvenile Chinese sturgeon (Acipenser sinensis) were treated with DP at doses of 1, 10, and 100mg/kg wet weight for 14 days via a single intraperitoneal injection (i.p.). After 14 days, liver proteomes of juvenile Chinese sturgeon were analyzed using two-dimensional electrophoresis (2-DE) coupled matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF/TOF-MS). A total of 39 protein spots were significantly altered in abundance (>2-fold) and of these proteins, 27 were successfully identified. Proteins related to the stress response that included heat shock cognate protein 70 and T-complex protein 1 were significantly increased and decreased in abundance, respectively. Moreover, Ras-related protein Rab-6B and GDP dissociation inhibitor 2, proteins that are involved in small G-protein signal cascades, were decreased in abundance 2- to 5-fold. Annexin A4, which is associated with Ca(2+) signaling pathways, was also markedly decreased by 2-fold in the liver. Pathway analysis of differentially regulated proteins revealed that DP interfered with metabolism and was associated with proteins related to apoptosis and cell differentiation. Based upon protein responses, we suggest that DP has effects on the generalized stress response, small G-protein signal cascades, Ca(2+) signaling pathway, and metabolic process, and may induce apoptosis in the liver. This study offers novel mechanistic insight into the protein responses induced in the liver with DP, an increasingly used and understudied flame retardant.

Diacylglycerol kinase γ is a novel anionic phospholipid binding protein with a selective binding preference

There are ten isozymes of diacylglycerol kinase (DGK), and they regulate diverse patho-physiological functions. Here, we investigated the lipid-binding properties of DGK isozymes using protein-lipid overlay and liposome-binding assays. DGKγ showed a strong binding activity compared with other DGK isozymes for phosphatidic acid (PA) among the various glycerophospholipids tested. However, DGKγ failed to interact with DG and lyso-PA. Moreover, the isozyme was capable of binding to ceramide-1-phosphate but not to ceramide or sphingosine-1-phosphate. The isozyme bound more strongly to PA containing unsaturated fatty acid than to PA having only saturated fatty acid. An analysis using a series of deletion mutants of DGKγ revealed that the N-terminal region, which contains a recoverin homology domain and EF-hand motifs, is responsible for the PA binding activity of DGKγ. Taken together, these results indicate that DGKγ is an anionic phospholipid binding protein that preferably interacts with a small highly charged head group that is very close to the glycerol or sphingosine backbone.

Structure prediction of magnetosome-associated proteins

Magnetotactic bacteria (MTB) are Gram-negative bacteria that can navigate along geomagnetic fields. This ability is a result of a unique intracellular organelle, the magnetosome. These organelles are composed of membrane-enclosed magnetite (Fe₃O₄) or greigite (Fe₃S₄) crystals ordered into chains along the cell. Magnetosome formation, assembly, and magnetic nano-crystal biomineralization are controlled by magnetosome-associated proteins (MAPs). Most MAP-encoding genes are located in a conserved genomic region – the magnetosome island (MAI). The MAI appears to be conserved in all MTB that were analyzed so far, although the MAI size and organization differs between species. It was shown that MAI deletion leads to a non-magnetic phenotype, further highlighting its important role in magnetosome formation. Today, about 28 proteins are known to be involved in magnetosome formation, but the structures and functions of most MAPs are unknown. To reveal the structure–function relationship of MAPs we used bioinformatics tools in order to build homology models as a way to understand their possible role in magnetosome formation. Here we present a predicted 3D structural models’ overview for all known Magnetospirillum gryphiswaldense strain MSR-1 MAPs.

The role of diacylglycerol kinase ζ and phosphatidic acid in the mechanical activation of mammalian target of rapamycin (mTOR) signaling and skeletal muscle hypertrophy

The activation of mTOR signaling is essential for mechanically induced changes in skeletal muscle mass, and previous studies have suggested that mechanical stimuli activate mTOR (mammalian target of rapamycin) signaling through a phospholipase D (PLD)-dependent increase in the concentration of phosphatidic acid (PA). Consistent with this conclusion, we obtained evidence which further suggests that mechanical stimuli utilize PA as a direct upstream activator of mTOR signaling. Unexpectedly though, we found that the activation of PLD is not necessary for the mechanically induced increases in PA or mTOR signaling. Motivated by this observation, we performed experiments that were aimed at identifying the enzyme(s) that promotes the increase in PA. These experiments revealed that mechanical stimulation increases the concentration of diacylglycerol (DAG) and the activity of DAG kinases (DGKs) in membranous structures. Furthermore, using knock-out mice, we determined that the ζ isoform of DGK (DGKζ) is necessary for the mechanically induced increase in PA. We also determined that DGKζ significantly contributes to the mechanical activation of mTOR signaling, and this is likely driven by an enhanced binding of PA to mTOR. Last, we found that the overexpression of DGKζ is sufficient to induce muscle fiber hypertrophy through an mTOR-dependent mechanism, and this event requires DGKζ kinase activity (i.e. the synthesis of PA). Combined, these results indicate that DGKζ, but not PLD, plays an important role in mechanically induced increases in PA and mTOR signaling. Furthermore, this study suggests that DGKζ could be a fundamental component of the mechanism(s) through which mechanical stimuli regulate skeletal muscle mass.

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.

Alternative splicing generates a diacylglycerol kinase α transcript that acts as a dominant-negative modulator of superoxide production in localized aggressive periodontitis

BACKGROUND

Diacylglycerol (DAG), levels of which are tightly regulated by diacylglycerol kinases (DGKs), is a lipid mediator linked to key biologic functions. Members of the DGK family undergo alternative splicing, generating the protein diversity necessary to control different intracellular DAG pools. DGKα function is altered in polymorphonuclear neutrophils (PMNs) of patients with localized aggressive periodontitis (LAgP), suggesting a genetic basis. Here, the authors assess DGKα spliced transcripts in human LAgP neutrophils.

METHODS

In an expression library of a patient with LAgP, PMNs were screened for different DGKα transcripts. Real-time polymerase chain reaction and in vitro expression assays were performed to assess the fate of different transcripts on protein translocation and superoxide production in human leukemia cells (HL-60) and COS-7 cells.

RESULTS

A DGKα transcript that lacks exon 10 (DGKαΔ10) and generates a premature stop codon and a truncated protein was identified as being upregulated in LAgP neutrophils. In vitro assays revealed that DGKαΔ10 translocation occurred even in the absence of important regulatory motifs. Transfection of HL-60 neutrophil-like cells with the DGKαΔ10 spliced variant induced an increase in the stimulated production of superoxide anion replicating the phenotype of LAgP PMNs.

CONCLUSION

DGKαΔ10 can act as a dominant-negative transcript that can modulate superoxide production and provides an example of genetic regulation of the inflammatory response that may be relevant to human inflammatory diseases such as LAgP.

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.

Evaluations of the selectivities of the diacylglycerol kinase inhibitors R59022 and R59949 among diacylglycerol kinase isozymes using a new non-radioactive assay method

Ten mammalian diacylglycerol kinase (DGK) isozymes (α-κ) have been identified. Recent studies have revealed that DGK isozymes play pivotal roles in a wide variety of pathophysiological functions. Thus, it is important to be able to easily check DGK activity in each pathophysiological event. Moreover, the conventional DGK assay is quite laborious because it requires the use of a radioisotope and thin-layer chromatography including multiple extraction steps. In order to minimize the laborious procedures, we established a non-radioactive, single well, two-step DGK assay system. We demonstrated that, compared to the conventional method, the new assay system has comparable sensitivity and much higher efficiency, and is effective in detecting potential agents with high reliability (Z'-factor = 0.69 ± 0.12; n = 3). Using the newly developed assay, we comprehensively evaluated the DGK isozyme selectivities of commercially available DGK inhibitors, R59022 and R59949, in vitro. We found that among 10 isozymes, R59022 strongly inhibited type I DGKα and moderately attenuated type III DGKε and type V DGKθ, and that R59949 strongly inhibited type I DGK α and γ, and moderately attenuated type II DGK δ and κ.

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.

Induction of filopodia-like protrusions in N1E-115 neuroblastoma cells by diacylglycerol kinase γ independent of its enzymatic activity: potential novel function of the C-terminal region containing the catalytic domain of diacylglycerol kinase γ

Type I diacylglycerol kinase (DGK) isozymes (α, β, and γ) contain recoverin homology domains and calcium-binding EF-hand motifs at their N-termini. The γ-isoform of DGK is abundantly expressed in retinal and Purkinje cells; however, its function in neuronal cells remains unknown. Here, we report that the mRNA and protein levels of DGKγ, but not DGKα or β, were markedly increased in N1E-115 neuroblastoma cells upon cellular differentiation by serum starvation. Interestingly, overexpression of wild-type DGKγ, which was partially located at the plasma membrane, considerably induced the formation of slender, filopodia-like cytoplasmic projections from N1E-115 cell bodies. Deletion of the recoverin homology domain and the EF-hand motifs, which potentiated the plasma membrane localization of the isozyme, significantly enhanced the formation of the filopodia-like protrusions. Intriguingly, the catalytic activity of the isozyme is not essential for the protrusion formation. The N-terminal half of the catalytic domain and a short stretch of amino acid residues at the C-terminus are responsible for plasma membrane localization and filopodia-like process formation. Taken together, we have described a potentially novel morphological function of the C-terminal DGKγ catalytic region that is independent of its enzymatic activity.

Diacylglycerol kinase δ1 transiently translocates to the plasma membrane in response to high glucose

The type II diacylglycerol kinases (DGKs) contain several functional domains such as a pleckstrin homology (PH) domain, two C1 domains and a sterile α-motif (SAM) domain. It was previously revealed that DGKδ contributes to hyperglycemia-induced peripheral insulin resistance and thereby exacerbate the severity of type 2 diabetes. Moreover, a high extracellular concentration of glucose activated DGKδ in skeletal muscle cells, which was followed by a reduction in the intracellular diacylglycerol levels and the inactivation of protein kinase Cα, the enzyme that phosphorylates and inactivates the insulin receptor. However, the intracellular behavior of DGKδ upon high glucose stimulation remains unclear. In this study, we found that DGKδ1, but not a splice variant DGKδ2 or the other type II DGKη1/2, translocated from the cytoplasm to the plasma membrane in human embryonic kidney HEK293 and mouse myoblast C2C12 cells within 5 min in response to high glucose levels. The translocation was inhibited by phosphatidylinositol 3-kinase inhibitors, LY294002 and GDC-0941, suggesting that the event is regulated via the phosphatidylinositol 3-kinase pathway. Moreover, we revealed that the PH and C1 domains are responsible for the plasma membrane translocation and that the SAM domain negatively regulates the translocation. These results indicate that DGKδ1 is the sole type II DGK isoform that responds rapidly and dynamically to high glucose levels.

Diacylglycerol kinase-dependent formation of phosphatidic acid molecular species during interleukin-2 activation in CTLL-2 T-lymphocytes

Although effective liquid chromatography (LC)/mass spectrometry (MS) methods enabling the separation of phospholipid molecular species have been developed, there are still problems with an intracellular signaling molecule, phosphatidic acid (PA). In this study, we optimized LC/MS conditions to improve the quantitative detection of PA molecular species from a cellular lipid mixture. Using the newly developed LC/MS method, we showed that stimulation of CTLL-2 murine T-lymphocytes by interleukin-2 (IL-2) induced a significant increase of 36:1-, 36:2-, 40:5- and 40:6-diacyl-PA. A diacylglycerol kinase (DGK) inhibitor, R59949, attenuated the increase of 36:1-, 40:5-, 40:6-diacyl-PA, suggesting that DGK IL-2-dependently and selectively generated these diacyl-PA species.

Diacylglycerol kinase alpha enhances hepatocellular carcinoma progression by activation of Ras-Raf-MEK-ERK pathway

BACKGROUND & AIMS

Diacylglycerol kinases (DGKs) were recently recognized as key regulators in cell signaling pathways. We investigated whether DGKα is involved in human hepatocellular carcinoma (HCC) progression.

METHODS

We silenced or overexpressed DGKα in HCC cells and assessed its effect on tumor progression. DGKα expression in 95 surgical samples was analyzed by immunohistochemistry, and the expression status of each sample was correlated with clinicopathological features.

RESULTS

DGKα was detected in various HCC cell lines but at very low levels in the normal liver. Knockdown of DGKα significantly suppressed cell proliferation and invasion. Overexpression of wild type (WT) DGKα, but not its kinase-dead (KD) mutant, significantly enhanced cell proliferation. DGKα knockdown impaired MEK and ERK phosphorylation, but did not inhibit Ras activation in HCC cells. In a xenograft model, WT DGKα overexpression significantly enhanced tumor growth compared to the control, but KD DGKα mutant had no effect. Immunohistochemical studies showed that DGKα was expressed in cancerous tissue, but not in adjacent non-cancerous hepatocytes. High DGKα expression (≥20%) was associated with high Ki67 expression (p<0.05) and a high rate of HCC recurrence (p=0.033) following surgery. In multivariate analyses, high DGKα expression was an independent factor for determining HCC recurrence after surgery.

CONCLUSIONS

DGKα is involved in HCC progression by activation of the MAPK pathway. DGKα could be a novel target for HCC therapeutics as well as a prognostic marker.

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.

Calcium negatively regulates an intramolecular interaction between the N-terminal recoverin homology and EF-hand motif domains and the C-terminal C1 and catalytic domains of diacylglycerol kinase α

The type I diacylglycerol kinase (DGK) isozymes (α, β and γ) contain a shared recoverin homology (RVH) domain, a tandem repeat of Ca2+-binding EF-hand motifs, two cysteine-rich C1 domains, and the catalytic domain. We previously reported that a DGKα mutant lacking the RVH domain and EF-hands was constitutively active, implying that the N-terminal region (NTR) of DGKα, consisting of the RVH domain and EF-hand motifs, intramolecularly interacts with and masks the activity of the C-terminal region (CTR), containing the C1 and catalytic domains. In this study, we demonstrate that a glutathione S-transferase (GST)-fused DGKα-NTR construct physically binds to a green fluorescent protein (GFP)-fused DGKα-CTR construct. Moreover, co-precipitation of GFP-DGKα-CTR with GST-DGKα-NTR was clearly attenuated by the addition of 1 μM Ca2+. This result indicates that Ca2+ induces dissociation of the physical interaction between DGKα-NTR and DGKα-CTR. In addition to previously reported calcium-dependent changes in the hydrophobicity and net surface charge, Ca2+ also appeared to induce a decrease in the α-helical content of DGKα-NTR. These results suggest that Ca2+-induced conformational changes in the NTR release the intramolecular association between the NTR and the CTR of DGKα.

The role and regulation of mTOR in T-lymphocyte function

The conversion of naïve T cells into effector T cells is initiated by stimulation through the T-cell receptor (TCR). Upon activation, T cells undergo significant morphological and functional changes, putting new metabolic demands on the cell. Past research has identified the mammalian target of rapamycin (mTOR) as a critical regulator of cell metabolism, and the development of new genetic models has begun to reveal an important role for this pathway in the homeostasis and function of T lymphocytes. In this review, we focus on the most recent findings that demonstrate the ability of mTOR to regulate T-cell activation, CD8(+) memory cell formation and function, and helper T lineage differentiation. Furthermore, we highlight the importance of tight control of mTOR signaling by tuberous sclerosis complex 1 for T-cell homeostasis, and the regulation of mTOR signaling by diacylglycerol kinases and the RasGRP1-Ras-Erk1/2 pathway in the context of TCR signaling.

Differential regulation of primary and memory CD8 T cell immune responses by diacylglycerol kinases

The manipulation of signals downstream of the TCR can have profound consequences for T cell development, function, and homeostasis. Diacylglycerol (DAG) produced after TCR stimulation functions as a secondary messenger and mediates the signaling to Ras-MEK-Erk and NF-κB pathways in T cells. DAG kinases (DGKs) convert DAG into phosphatidic acid, resulting in termination of DAG signaling. In this study, we demonstrate that DAG metabolism by DGKs can serve a crucial function in viral clearance upon lymphocytic choriomeningitis virus infection. Ag-specific CD8(+) T cells from DGKα(-/-) and DGKζ(-/-) mice show enhanced expansion and increased cytokine production after lymphocytic choriomeningitis virus infection, yet DGK-deficient memory CD8(+) T cells exhibit impaired expansion after rechallenge. Thus, DGK activity plays opposing roles in the expansion of CD8(+) T cells during the primary and memory phases of the immune response, whereas consistently inhibiting antiviral cytokine production.

PKC-1 acts with the ERK MAPK signaling pathway to regulate Caenorhabditis elegans mechanosensory response

In most animals, multiple genes encode protein kinase C (PKC) proteins. Pharmacological studies have revealed numerous roles for this protein family, yet the in vivo roles of specific PKC proteins and the functional targets of PKC activation are poorly understood. We find that in Caenorhabditis elegans, two PKC genes, pkc-1 and tpa-1, are required for mechanosensory response; the role of the nPKCε/η ortholog, pkc-1, was examined in detail. pkc-1 function is required for response to nose touch in adult C. elegans and pkc-1 likely acts in the interneurons that regulate locomotion which are direct synaptic targets of mechanosensory neurons. Previous studies have suggested numerous possible targets of pkc-1; our analysis indicates that pkc-1 may act via the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway. We find that ERK/MAPK pathway function is required for mechanosensory response in C. elegans and that at least one component of this pathway, lin-45 Raf, acts in interneurons of the mechanosensory circuit. Genetic analysis indicates that lin-45 and pkc-1 act together to regulate nose touch response. Thus, these results functionally link two conserved signaling pathways in adult C. elegans neurons and define distinct roles for PKC genes in vivo.

Diacylglycerol kinase β knockout mice exhibit lithium-sensitive behavioral abnormalities

Background

Diacylglycerol kinase (DGK) is an enzyme that phosphorylates diacylglycerol (DG) to produce phosphatidic acid (PA). DGKβ is widely distributed in the central nervous system, such as the olfactory bulb, cerebral cortex, striatum, and hippocampus. Recent studies reported that the splice variant at the COOH-terminal of DGKβ was related to bipolar disorder, but its detailed mechanism is still unknown.

Methodology/Principal Findings

In the present study, we performed behavioral tests using DGKβ knockout (KO) mice to investigate the effects of DGKβ deficits on psychomotor behavior. DGKβ KO mice exhibited some behavioral abnormalities, such as hyperactivity, reduced anxiety, and reduced depression. Additionally, hyperactivity and reduced anxiety were attenuated by the administration of the mood stabilizer, lithium, but not haloperidol, diazepam, or imipramine. Moreover, DGKβ KO mice showed impairment in Akt-glycogen synthesis kinase (GSK) 3β signaling and cortical spine formation.

Conclusions/Significance

These findings suggest that DGKβ KO mice exhibit lithium-sensitive behavioral abnormalities that are, at least in part, due to the impairment of Akt-GSK3β signaling and cortical spine formation.

Essential role of neuron-enriched diacylglycerol kinase (DGK), DGKbeta in neurite spine formation, contributing to cognitive function

BACKGROUND

Diacylglycerol (DG) kinase (DGK) phosphorylates DG to produce phosphatidic acid (PA). Of the 10 subtypes of mammalian DGKs, DGKbeta is a membrane-localized subtype and abundantly expressed in the cerebral cortex, hippocampus, and caudate-putamen. However, its physiological roles in neurons and higher brain function have not been elucidated.

METHODOLOGY/PRINCIPAL FINDINGS

We, therefore, developed DGKbeta KO mice using the Sleeping Beauty transposon system, and found that its long-term potentiation in the hippocampal CA1 region was reduced, causing impairment of cognitive functions including spatial and long-term memories in Y-maze and Morris water-maze tests. The primary cultured hippocampal neurons from KO mice had less branches and spines compared to the wild type. This morphological impairment was rescued by overexpression of DGKbeta. In addition, overexpression of DGKbeta in SH-SY5Y cells or primary cultured mouse hippocampal neurons resulted in branch- and spine-formation, while a splice variant form of DGKbeta, which has kinase activity but loses membrane localization, did not induce branches and spines. In the cells overexpressing DGKbeta but not the splice variant form, DGK product, PA, was increased and the substrate, DG, was decreased on the plasma membrane. Importantly, lower spine density and abnormality of PA and DG contents in the CA1 region of the KO mice were confirmed.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that membrane-localized DGKbeta regulates spine formation by regulation of lipids, contributing to the maintenance of neural networks in synaptic transmission of cognitive processes including memory.

Diacylglycerol kinase alpha enhances protein kinase Czeta-dependent phosphorylation at Ser311 of p65/RelA subunit of nuclear factor-kappaB

We recently reported that diacylglycerol kinase (DGK) alpha enhanced tumor necrosis factor-alpha (TNF-alpha)-induced activation of nuclear factor-kappaB (NF-kappaB). However, the signaling pathway between DGKalpha and NF-kappaB remains unclear. Here, we found that small interfering RNA-mediated knockdown of DGKalpha strongly attenuated protein kinase C (PKC) zeta-dependent phosphorylation of a large subunit of NF-kappaB, p65/RelA, at Ser311 but not PKCzeta-independent phosphorylation at Ser468 or Ser536. Moreover, knockdown and overexpression of PKCzeta suppressed and synergistically enhanced DGKalpha-mediated NF-kappaB activation, respectively. These results strongly suggest that DGKalpha positively regulates TNF-alpha-dependent NF-kappaB activation via the PKCzeta-mediated Ser311 phosphorylation of p65/RelA.

Diacylglycerol kinase epsilon is selective for both acyl chains of phosphatidic acid or diacylglycerol

The phosphatidylinositol (PI) cycle mediates many cellular events by controlling the metabolism of many lipid second messengers. Diacylglycerol kinase epsilon (DGK epsilon) has an important role in this cycle. DGK epsilon is the only DGK isoform to show inhibition by its product phosphatidic acid (PA) as well as substrate specificity for sn-2 arachidonoyl-diacylglycerol (DAG). Here, we show that this inhibition and substrate specificity are both determined by selectivity for a combination of the sn-1 and sn-2 acyl chains of PA or DAG, respectively, preferring the most prevalent acyl chain composition of lipids involved specifically in the PI cycle, 1-stearoyl-2-arachidonoyl. Although the difference in rate for closely related lipid species is small, there is a significant enrichment of 1-stearoyl-2-arachidonoyl PI because of the cyclical nature of PI turnover. We also show that the inhibition of DGK epsilon by PA is competitive and that the deletion of the hydrophobic segment and cationic cluster of DGK epsilon does not affect its selectivity for the acyl chains of PA or DAG. Thus, this active site not only recognizes the lipid headgroup but also a combination of the two acyl chains in PA or DAG. We propose a mechanism of DGK epsilon regulation where its dual acyl chain selectivity is used to negatively regulate its enzymatic activity in a manner that ensures DGK epsilon remains committed to the PI turnover cycle. This novel mechanism of enzyme regulation within a signaling pathway could serve as a template for the regulation of enzymes in other pathways in the cell.

Diacylglycerol kinase delta associates with receptor for activated C kinase 1, RACK1

The delta-isozyme (type II) of diacylglycerol kinase (DGK) is known to positively regulate growth factor receptor signaling. DGKdelta, which is distributed to clathrin-coated vesicles, interacts with DGKdelta itself, protein kinase C and AP2alpha. To search for additional DGKdelta-interacting proteins, we screened a yeast two-hybrid cDNA library from HepG2 cells using aa 896-1097 of DGKdelta as a bait. We identified aa 184-317 (WD40 repeats 5-7) of receptor for activated C kinase 1 (RACK1), which interacts with various important signaling molecules, as a novel binding partner of DGKdelta. Co-immunoprecipitation analysis, using COS-7 cells co-expressing RACK1 and DGKdelta, revealed that RACK1 selectively interacted with DGKdelta, but not with type I DGKs, in mammalian cells. The interaction was dynamically regulated by phorbol ester. Intriguingly, DGKdelta appeared to recruit RACK1 to clathrin-coated vesicles and co-localized with RACK1. These results suggest that DGKdelta serves as an adaptor protein to regulate the localization of the versatile scaffold protein, RACK1.

Cell and molecular biology of the novel protein tyrosine-phosphatase-interacting protein 51

This chapter examines the current state of knowledge about the expression profile, as well as biochemical properties and biological functions of the evolutionarily conserved protein PTPIP51. PTPIP51 is apparently expressed in splice variants and shows a particularly high expression in epithelia, skeletal muscle, placenta, and germ cells, as well as during mammalian development and in cancer. PTPIP51 is an in vitro substrate of Src- and protein kinase A, the PTP1B/TCPTP protein tyrosine phosphatases and interacts with 14-3-3 proteins, the Nuf2 kinetochore protein, the ninein-interacting CGI-99 protein, diacylglycerol kinase alpha, and also with itself forming dimers and trimers. Although the precise cellular function remains to be elucidated, the current data implicate PTPIP51 in signaling cascades mediating proliferation, differentiation, apoptosis, and motility.

Molecular cloning and expression patterns of a putative olfactory diacylglycerol kinase from the noctuid moth Spodoptera littoralis

In the aim of the characterization of the molecular actors of insect olfactory transduction, we have cloned the full cDNA encoding a Spodoptera littoralis diacylglycerol kinase (DGK) named SlDGK. In male adults, SlDGK transcript was detected predominantly in the brain and in the olfactory sensilla trichodea located on the antennae. SlDGK expression was first detected at day 3 of the pupal stage, then reached a maximum at the end of this stage and was maintained at this level during the adult period. These data provide the first molecular characterization of a DGK potentially involved in the regulation of signalling pathways responsible for the establishment and/or the functioning of the olfactory system in Lepidoptera.

Diacylglycerol kinase regulation of protein kinase D during oxidative stress-induced intestinal cell injury

We recently demonstrated that protein kinase D (PKD) exerts a protective function during oxidative stress-induced intestinal epithelial cell injury; however, the exact role of DAG kinase (DGK)zeta, an isoform expressed in intestine, during this process is unknown. We sought to determine the role of DGK during oxidative stress-induced intestinal cell injury and whether DGK acts as an upstream regulator of PKD. Inhibition of DGK with R59022 compound or DGKzeta siRNA transfection decreased H2O2-induced RIE-1 cell apoptosis as measured by DNA fragmentation and increased PKD phosphorylation. Overexpression of kinase-dead DGKzeta also significantly increased PKD phosphorylation. Additionally, endogenous nuclear DGKzeta rapidly translocated to the cytoplasm following H2O2 treatment. Our findings demonstrate that DGK is involved in the regulation of oxidative stress-induced intestinal cell injury. PKD activation is induced by DGKzeta, suggesting DGK is an upstream regulator of oxidative stress-induced activation of the PKD signaling pathway in intestinal epithelial cells.

Analysis of the Staphylococcus aureus DgkB structure reveals a common catalytic mechanism for the soluble diacylglycerol kinases

Soluble diacylglycerol (DAG) kinases function as regulators of diacylglycerol metabolism in cell signaling and intermediary metabolism. We report the structure of a DAG kinase, DgkB from Staphylococcus aureus, both as the free enzyme and in complex with ADP. The molecule is a tight homodimer, and each monomer comprises two domains with the catalytic center located within the interdomain cleft. Two distinctive features of DkgB are a structural Mg2+ site and an associated Asp*water*Mg2+ network that extends toward the active site locale. Site-directed mutagenesis revealed that these features play important roles in the catalytic mechanism. The key active site residues and the components of the Asp*water*Mg2+ network are conserved in the catalytic cores of the mammalian signaling DAG kinases, indicating that these enzymes use the same mechanism and have similar structures as DgkB.

Lck-dependent tyrosine phosphorylation of diacylglycerol kinase alpha regulates its membrane association in T cells

TCR engagement triggers phospholipase Cgamma1 activation through the Lck-ZAP70-linker of activated T cell adaptor protein pathway. This leads to generation of diacylglycerol (DAG) and mobilization of intracellular Ca(2+), both essential for TCR-dependent transcriptional responses. TCR ligation also elicits transient recruitment of DAG kinase alpha (DGKalpha) to the lymphocyte plasma membrane to phosphorylate DAG, facilitating termination of DAG-regulated signals. The precise mechanisms governing dynamic recruitment of DGKalpha to the membrane have not been fully elucidated, although Ca(2+) influx and tyrosine kinase activation were proposed to be required. We show that DGKalpha is tyrosine phosphorylated, and identify tyrosine 335 (Y335), at the hinge between the atypical C1 domains and the catalytic region, as essential for membrane localization. Generation of an Ab that recognizes phosphorylated Y335 demonstrates Lck-dependent phosphorylation of endogenous DGKalpha during TCR activation and shows that pY335DGKalpha is a minor pool located exclusively at the plasma membrane. Our results identify Y335 as a residue critical for DGKalpha function and suggest a mechanism by which Lck-dependent phosphorylation and Ca(2+) elevation regulate DGKalpha membrane localization. The concerted action of these two signals results in transient, receptor-regulated DGKalpha relocalization to the site at which it exerts its function as a negative modulator of DAG-dependent signals.

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

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

Phorbol ester and hydrogen peroxide synergistically induce the interaction of diacylglycerol kinase gamma with the Src homology 2 and C1 domains of beta2-chimaerin

DGKgamma (diacylglycerol kinase gamma) was reported to interact with beta2-chimaerin, a GAP (GTPase-activating protein) for Rac, in response to epidermal growth factor. Here we found that PMA and H2O2 also induced the interaction of DGKgamma with beta2-chimaerin. It is noteworthy that simultaneous addition of PMA and H2O2 synergistically enhanced the interaction. In this case, PMA was replaceable by DAG (diacylglycerol). The beta2-chimaerin translocation from the cytoplasm to the plasma membrane caused by PMA plus H2O2 was further enhanced by the expression of DGKgamma. Moreover, DGKgamma apparently enhanced the beta2-chimaerin GAP activity upon cell stimulation with PMA. PMA was found to be mainly required for a conversion of beta2-chimaerin into an active form. On the other hand, H2O2 was suggested to induce a release of Zn2+ from the C1 domain of beta2-chimaerin. By stepwise deletion analysis, we demonstrated that the SH2 (Src homology 2) and C1 domains of beta2-chimaerin interacted with the N-terminal half of catalytic region of DGKgamma. Unexpectedly, the SH2 domain of beta2-chimaerin contributes to the interaction independently of phosphotyrosine. Taken together, these results suggest that the functional link between DGKgamma and beta2-chimaerin has a broad significance in response to a wide range of cell stimuli. Our work offers a novel mechanism of protein-protein interaction, that is, the phosphotyrosine-independent interaction of the SH2 domain acting in co-operation with the C1 domain.

Diacylglycerol kinase-alpha mediates hepatocyte growth factor-induced epithelial cell scatter by regulating Rac activation and membrane ruffling

Diacylglycerol kinases (Dgk) phosphorylate diacylglycerol (DG) to phosphatidic acid (PA), thus turning off and on, respectively, DG-mediated and PA-mediated signaling pathways. We previously showed that hepatocyte growth factor (HGF), vascular endothelial growth factor, and anaplastic lymphoma kinase activate Dgkalpha in endothelial and leukemia cells through a Src-mediated mechanism and that activation of Dgkalpha is required for chemotactic, proliferative, and angiogenic signaling in vitro. Here, we investigate the downstream events and signaling pathways regulated by Dgkalpha, leading to cell scatter and migration upon HGF treatment and v-Src expression in epithelial cells. We report that specific inhibition of Dgkalpha, obtained either pharmacologically by R59949 treatment, or by expression of Dgkalpha dominant-negative mutant, or by small interfering RNA-mediated down-regulation of endogenous Dgkalpha, impairs 1) HGF- and v-Src-induced cell scatter and migration, without affecting the loss of intercellular adhesions; 2) HGF-induced cell spreading, lamellipodia formation, membrane ruffling, and focal adhesions remodeling; and 3) HGF-induced Rac activation and membrane targeting. In summary, we provide evidence that Dgkalpha, activated downstream of tyrosine kinase receptors and Src, regulates crucial steps directing Rac activation and Rac-dependent remodeling of actin cytoskeleton and focal contacts in migrating epithelial cells.