Distribution of distinct arachidonoyl-specific and non-specific isoenzymes of diacylglycerol kinase in baboon (Papio cynocephalus) tissues

Arachidonoyl-Specific Diacylglycerol Kinase ε and the Endoplasmic Reticulum

The endoplasmic reticulum (ER) comprises an interconnected membrane network, which is made up of lipid bilayer and associated proteins. This organelle plays a central role in the protein synthesis and sorting. In addition, it represents the synthetic machinery of phospholipids, the major constituents of the biological membrane. In this process, phosphatidic acid (PA) serves as a precursor of all phospholipids, suggesting that PA synthetic activity is closely associated with the ER function. One enzyme responsible for PA synthesis is diacylglycerol kinase (DGK) that phosphorylates diacylglycerol (DG) to PA. DGK is composed of a family of enzymes with distinct features assigned to each isozyme in terms of structure, enzymology, and subcellular localization. Of DGKs, DGKε uniquely exhibits substrate specificity toward arachidonate-containing DG and is shown to reside in the ER. Arachidonic acid, a precursor of bioactive eicosanoids, is usually acylated at the sn-2 position of phospholipids, being especially enriched in phosphoinositide. In this review, we focus on arachidonoyl-specific DGKε with respect to the historical context, molecular basis of the substrate specificity and ER-targeting, and functional implications in the ER.

Bacterial expression strategies for several Sus scrofa diacylglycerol kinase alpha constructs: solubility challenges

We pursued several strategies for expressing either full-length Sus scrofa diacylglycerol kinase (DGK) alpha or the catalytic domain (alphacat) in Escherichia coli. Alphacat could be extracted, refolded, and purified from inclusion bodies, but when subjected to analytical gel filtration chromatography, it elutes in the void volume, in what we conclude are microscopic aggregates unsuitable for x-ray crystallography. Adding glutathione S-transferase, thioredoxin, or maltose binding protein as N-terminal fusion tags did not improve alphacat's solubility. Coexpressing with bacterial chaperones increased the yield of alphacat in the supernatant after high-speed centrifugation, but the protein still elutes in the void upon analytical gel filtration chromatography. We believe our work will be of interest to those interested in the structure of eukaryotic DGKs, so that they know which expression strategies have already been tried, as well as to those interested in protein folding and those interested in chaperone/target-protein interactions.

Molecular and structural basis of metabolic diversity mediated by prenyldiphosphate converting enzymes

General thermodynamic calculations using the semiempiric PM3 method have led to the conclusion that prenyldiphosphate converting enzymes require at least one divalent metal cation for the activation and cleavage of the diphosphate-prenyl ester bond, or they must provide structural elements for the efficient stabilization of the intermediate prenyl cation. The most important common structural features, which guide the product specificity in both terpene synthases and aromatic prenyl transferases are aromatic amino acid side chains, which stabilize prenyl cations by cation-pi interactions. In the case of aromatic prenyl transferases, a proton abstraction from the phenolic hydroxyl group of the second substrate will enhance the electron density in the phenolic ortho-position at which initial prenylation of the aromatic compound usually occurs. A model of the structure of the integral transmembrane-bound aromatic prenyl transferase UbiA was developed, which currently represents the first structural insight into this group of prenylating enzymes with a fold different from most other aromatic prenyl transferases. Based on this model, the structure-activity relationships and mechanistic aspects of related proteins, for example those of Lithospermum erythrorhizon or the enzyme AuaA from Stigmatella aurantiaca involved in the aurachin biosynthesis, were elucidated. The high similarity of this group of aromatic prenyltransferases to 5-epi-aristolochene synthase is an indication of an evolutionary relationship with terpene synthases (cyclases). This is further supported by the conserved DxxxD motif found in both protein families. In contrast, there is no such relationship to the aromatic prenyl transferases with an ABBA-fold, such as NphB, or to any other known family of prenyl converting enzymes. Therefore, it is possible that these two groups might have different evolutionary ancestors.

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

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

Mechanisms of accumulation of arachidonate in phosphatidylinositol in yellowtail. A comparative study of acylation systems of phospholipids in rat and the fish species Seriola quinqueradiata

It is known that phosphatidylinositol (PtdIns) contains abundant arachidonate and is composed mainly of 1-stearoyl-2-arachidonoyl species in mammals. We investigated if this characteristic of PtdIns applies to the PtdIns from yellowtail (Seriola quinqueradiata), a marine fish. In common with phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and phosphatidylserine (PtdSer) from brain, heart, liver, spleen, kidney and ovary, the predominant polyunsaturated fatty acid was docosahexaenoic acid, and levels of arachidonic acid were less than 4.5% (PtdCho), 7.5% (PtdEtn) and 3.0% (PtdSer) in these tissues. In striking contrast, arachidonic acid made up 17.6%, 31.8%, 27.8%, 26.1%, 25.4% and 33.5% of the fatty acid composition of PtdIns from brain, heart, liver, spleen, kidney and ovary, respectively. The most abundant molecular species of PtdIns in all these tissues was 1-stearoyl-2-arachidonoyl. Assay of acyltransferase in liver microsomes of yellowtail showed that arachidonic acid was incorporated into PtdIns more effectively than docosahexaenoic acid and that the latter inhibited incorporation of arachidonic acid into PtdCho without inhibiting the utilization of arachidonic acid for PtdIns. This effect of docosahexaenoic acid was not observed in similar experiments using rat liver microsomes and is thought to contribute to the exclusive utilization of arachidonic acid for acylation to PtdIns in yellowtail. Inositolphospholipids and their hydrolysates are known to act as signaling molecules in cells. The conserved hydrophobic structure of PtdIns (the 1-stearoyl-2-arachidonoyl moiety) may have physiological significance not only in mammals but also in fish.

Metabolic characterization of sciadonic acid (5c,11c,14c-eicosatrienoic acid) as an effective substitute for arachidonate of phosphatidylinositol

Sciadonic acid (20:3 Delta-5,11,14) is an n-6 series trienoic acid that lacks the Delta8 double bond of arachidonic acid. This fatty acid is not converted to arachidonic acid in higher animals. In this study, we characterized the metabolic behavior of sciadonic acid in the process of acylation to phospholipid of HepG2 cells. One of the characteristics of fatty acid compositions of phospholipids in sciadonic acid-supplemented cells is a higher proportion of sciadonic acid in phosphatidylinositol (PtdIns) (27.4%) than in phosphatidylethanolamine (PtdEtn) (23.2%), phosphatidylcholine (PtdCho) (17.3%) and phosphatidylserine (PtdSer) (20.1%). Similarly, the proportion of arachidonic acid was higher in PtdIns (35.8%) than in PtdEtn (29.1%), PtdSer (18.2%) and PtdCho (20.2%) in arachidonic-acid-supplemented cells. The extensive accumulation of sciadonic acid in PtdIns resulted in the enrichment of newly formed 1-stearoyl-2-sciadonoyl molecular species (38%) in PtdIns and caused the reduction in the level of pre-existing arachidonic-acid-containing molecular species. The kinetics of incorporation of sciadonic acid to PtdEtn, PtdSer and PtdIns of cells were similar to those of arachidonic acid. In contrast to sciadonic acid, neither eicosapentaenoic acid (20:5 Delta-5,8,11,14,17) nor juniperonic acid (20:4 Delta-5,11,14,17) accumulated in the PtdIns fraction. Rather, these n-3 series polyunsaturated fatty acids, once incorporated into PtdIns, tended to be excluded from PtdIns. In addition, the level of arachidonic-acid-containing PtdIns molecular species remained unchanged by eicosapentaenoic-acid-supplementation. These results suggest that sciadonic acid or sciadonic-acid-containing glycerides are metabolized in a similar manner to arachidonic acid or arachidonic-acid-containing glyceride in the biosynthesis of PtdIns and that sciadonic acid can effectively modify the molecular species composition of PtdIns in HepG2 cells. In this regard, sciadonic acid will be an interesting experimental tool to clarify the significance of arachidonic acid-residue of PtdIns-origin bioactive lipids.

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

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

Molecular analysis of a novel Drosophila diacylglycerol kinase, DGKepsilon

Diacylglycerol kinase plays a central role in the metabolism of diacylglycerol by converting diacylglycerol into phosphatidic acid thus initiating resynthesis of phosphatidylinositols. Diacylglycerol is a known second messenger reversibly activating protein kinase C. In addition, diacylglycerol is a potential precursor for polyunsaturated fatty acids. We describe the identification and molecular analysis of a novel type III Drosophila diacylglycerol kinase isoform, DGKepsilon. Drosophila DGKepsilon is mapped to the cytological position 49C1-3. DGKepsilon mRNA is 1.9 kb in length and is broadly distributed throughout development in different cells, primordia and organs, including testes. In embryogenesis, the transcripts are enriched in the cells, which are in S-phase or undergoing endoreplication. Comparison of the Drosophila DGKepsilon with the human homologue revealed that the first zinc finger-like motif is specific for the type III isoform. Although the testis-specific diacylglycerol kinase activity is dependent upon the dose of DGKepsilon gene, the deletion of DGKepsilon does not modulate the total cellular diacylglycerol level. In spite of a proposed key role of diacylglycerol kinase in termination of the diacylglycerol signal, overexpression of a DGKepsilon transgene in flies under the control of a yeast upstream activating sequence promoter does not disrupt normal development in Drosophila.

Properties and functions of diacylglycerol kinases

Diacylglycerol kinases (DGKs) phosphorylate the second-messenger diacylglycerol (DAG) to phosphatidic acid (PA). The family of DGKs is well conserved among most species. Nine mammalian isotypes have been identified, and are classified into five subgroups based on their primary structure. DGKs contain a conserved catalytic domain and an array of other conserved motifs that are likely to play a role in lipid-protein and protein-protein interactions in various signalling pathways dependent on DAG and/or PA production. DGK is therefore believed to be activated at the (plasma) membrane where DAG is generated. Some isotypes are found associated with and/or regulated by small GTPases of the Rho family, presumably acting in cytoskeletal rearrangements. Others are (also) found in the nucleus, in association with other regulatory enzymes of the phosphoinositide cycle, and have an effect on cell cycle progression. Most DGK isotypes show high expression in the brain, often in distinct brain regions, suggesting that each individual isotype has a unique function.

Physical properties of the transmembrane signal molecule, sn-1-stearoyl 2-arachidonoylglycerol. Acyl chain segregation and its biochemical implications

sn-1,2-diacylglycerol (DAG), a key intermediate in lipid metabolism, activates protein kinase C and is a fusogen. Phosphoinositides, the main sources of DAG in cell signaling, contain mostly stearoyl and arachidonoyl in the sn-1 and -2 positions, respectively. The polymorphic behavior of sn-1-stearoyl-2-arachidonoylglycerol (SAG) was studied by differential scanning calorimetry, x-ray powder diffraction, and solid state magic angle spinning (MAS) (13)C NMR. Three alpha phases were found in the dry state. X-ray diffraction indicated that the acyl chains packed in a hexagonal array in the alpha phase, and the two sub-alpha phases packed with pseudo-hexagonal symmetry. In the narrow angle range strong diffractions of approximately 31 and approximately 62 A were present. High power proton-decoupled MAS (13)C NMR of isotropic SAG gave 16 distinct resonances of the 20 arachidonoyl carbons and 5 distinct resonances of the 18 stearoyl carbons. Upon cooling, all resonances of stearoyl weakened and vanished in the sub-alpha(2) phase, whereas arachidonoyl carbons from 8/9 to 20 gave distinct resonances in the frozen phases. Remarkably, the omega-carbon of the two acyl chains had different chemical shifts in alpha, sub-alpha(1), and sub-alpha(2) phases. Large differences in spin lattice relaxation of the stearoyl and arachidonoyl methene and methyl groups were demonstrated by contact time (cross-polarization) MAS (13)C NMR experiments in the solid phases alpha, sub-alpha(1), and sub-alpha(2). This shows that stearoyl and arachidonoyl in SAG have different environments in the solid states (alpha, sub-alpha(1), and sub-alpha(2) phases) and may segregate during cooling. The NMR and long spacing x-ray diffraction results suggest that SAG does not pack in a conventional double layer with the two acyls in a hairpin fashion. Our findings thus provide a physicochemical basis for DAG hexagonal phase domain separation within membrane bilayers.

Diacylglycerol kinases in signal transduction

Diacylglycerol kinase (DGK) phosphorylates the second messenger diacylglycerol (DAG) to phosphatidic acid. A family of nine mammalian isotypes have been identified. Their primary structure shows a diverse array of conserved domains, such as a catalytic domain, zinc fingers, pleckstrin homology domains and EF-hand structures, known to interact with other proteins, lipids or Ca2+, in signal transduction processes. DGK is believed to act in the phosphoinositide cycle in which DAG is enriched with arachidonoyl moieties, but the majority of DGK isotypes do not show specificity for this DAG species in vitro. This could imply that DGKs may also have other functions in the cell. DGK activity is not only found in membranes, but also in the nucleus and at the cytoskeleton. Agonist-induced translocations of DGK to or from these subcellular sites are known to occur. Some isotypes are contained in signaling complexes in specific association with members of the Rho family of small GTP binding proteins, suggesting that they are involved in Rho-mediated processes such as cytoskeletal reorganization.

Multiple factors influence the binding of a soluble, Ca2+-independent, diacylglycerol kinase to unilamellar phosphoglyceride vesicles

We studied the influence of membrane lipids, MgCl2, and ATP on the ability of a soluble diacylglycerol kinase to bind to 100-nm lipid vesicles. The enzyme did not bind detectably to vesicles that contained phosphatidylcholine alone or to vesicles that contained 50 mol % phosphatidylcholine + 50 mol % phosphatidylethanolamine. But it did bind to vesicles that contained anionic phosphoglycerides, and maximal binding occurred (in the presence of MgCl2) when the vesicles contained anionic phosphoglycerides alone. When increasing amounts of phosphatidylcholine were included in phosphatidylserine-containing vesicles, enzyme binding to the vesicles decreased by as much as 1000-fold. However, when increasing amounts of phosphatidylethanolamine were included in phosphatidylserine-containing vesicles, little change in binding occurred until the concentration of phosphatidylserine was reduced to below 25 mol %. These results and results obtained with vesicles that contained various mixtures of anionic phosphoglycerides, phosphatidylcholine, phosphatidylethanolamine, and unesterified cholesterol provided evidence that anionic phosphoglycerides were positive effectors of binding, phosphatidylcholine was a negative effector, and phosphatidylethanolamine and unesterified cholesterol were essentially neutral diluents. Other experiments showed that diacylglycerol and some of its structural analogues also were important, positive effectors of enzyme binding and that addition of ATP to the medium increased their effects. The combined results of the study suggest that the enzyme may bind to vesicles via at least two types of binding sites: one type that requires anionic phospholipids and is enhanced by Mg2+ but inhibited by phosphatidylcholine, and one type that requires diacylglycerol and is enhanced by ATP.

Alternations of diacylglycerol kinase in streptozotocin-induced diabetic rats

Dysfunction of organs has been reported in diabetic rats, suggesting an association with changes in intracellular signal transduction pathways including phosphatidylinositol (PI) turnover. Diacylglycerol (DG) kinase catalyses the phosphorylation of DG, which is considered to play a major physiological role in the metabolism of the intracellular messenger DG. However, no relation between DG kinase activity and any disease in mammalian tissue has been reported to date. In the present study, we investigated whether the changes in DG kinase activity are related to diabetes. Basal resting level of DG kinase activity changed in tissue isolated from diabetic rats. Decreases in resting activity detected in aorta and kidney and agonist-induced responses differed between these tissues. Submaximal increases in basal activity also were detected in vas deferens and hepatocytes. These changes in DG kinase activity resemble the functional changes associated with complications of diabetes, suggesting that changes in PI turnover followed by DG kinase activity are a key element in the complications. It is the first study about the changes in DG kinase activity in mammalian disease.

Metabolic fate of exogenous diacylglycerols in A10 smooth muscle cells

The metabolic fate of exogenous diacylglycerols, 1-palmitoyl-2-[1-14C]oleoyl-sn-glycerol (2-[14C]POG) and 1-stearoyl-2-[1-14C]arachidonoyl-sn-glycerol (2-[14C]SAG), was determined after incubation of A10 smooth muscle cells with liposomal suspensions. Hydrolysis through a diacylglycerol (DG) lipase pathway was the predominant metabolic fate; more than 80% of cell-associated radioactivity from 2-[14C]POG and 2-[14C]SAG was recovered in lipolytic products, monoacylglycerol (MG) and fatty acids (FA), which were present in the incubation medium. Hydrolysis of 2-[14C]POG was reduced completely by tetrahydrolipstatin, a lipase inhibitor. Very little radioactivity from either 2-[14C]POG or 2-[14C]SAG was incorporated into triacylglycerol or phospholipids. DG lipase and kinase activities were measured by in vitro enzyme assays. 1-[1-14C]Palmitoyl-2-oleoyl-sn-glycerol (1-[14C]POG) was phosphorylated (kinase activity) to a greater extent than 2-[14C]SAG in assays with both soluble and particulate subcellular fractions from A10 cells. DG lipase activity (hydrolysis of 1-[14C]POG and 2-[14C]SAG) was markedly stimulated by the addition of 20 mM MgCl2 and 20 mM ATP to the assay. Under optimal assay conditions, DG lipase activity exhibited little substrate specificity. Our findings indicate that exogenous DG are mainly hydrolyzed by DG and MG lipases in A10 smooth muscle cells; as a result, signalling mechanisms responding to DG second messengers will be attenuated.

Gene cloning and characterization of CDP-diacylglycerol synthase from rat brain

A cDNA encoded a 462-amino acid protein, which showed CDP-diacylglycerol synthase (CDS) activity was cloned for the first time as the vertebrate enzyme molecule from rat brain cDNA library. The deduced molecular mass of this rat CDS was 53 kDa, and putative primary structure included several possible membrane- spanning regions. At the amino acid sequence level, rat CDS shared 55.5%, 31. 7%, and 20.9% identity with already known Drosophila, Saccharomyces cerevisiae, and Escherichia coli CDS, respectively. This rat CDS preferred 1-stearoyl-2-arachidonoyl phosphatidic acid as a substrate, and its activity was strongly inhibited by phosphatidylglycerol 4, 5-bisphosphate. By immunoblotting analysis of COS cells overexpressed with the epitope-tagged for rat CDS, a 60-kDa band was detected. By epitope-tag immunocytochemistry, the CDS protein was mainly localized in close association with the membrane of the endoplasmic reticulum of the transfected cells. The intense mRNA expression of CDS was localized in the cerebellar Purkinje cells, the pineal body, and the inner segment of photoreceptor cells. Additionally, very intense expression was detected in postmitotic spermatocytes and spermatids.

Purification and characterization of sn-1-stearoyl-2-arachidonoylglycerol kinase from pig testes

1-Stearoyl-2-arachidonoylglycerol (SAG) kinase was identified in the particulate fraction of pig testes. This activity was enriched by hydroxyapatite and blue dye chromatography. The enzyme was selective for polyunsaturated diradylglycerol species and activity was not modulated by other diradylglycerol species or sphingomyelin metabolites. Further purification resulted in the isolation of 55 and 50 kDa proteins that corresponded with SAG kinase activity. These results support the view that the phosphorylation of polyunsaturated diradylglycerol is regulated by structural determinants in the molecule.

Cloning and characterization of a glucocorticoid-induced diacylglycerol kinase

Diacylglycerol kinase (DGK) plays a key role in cellular processes by regulating the intracellular concentration of the second messenger diacylglycerol. We screened a hamster DDT1 smooth muscle cell library and isolated a unique, glucocorticoid-inducible cDNA with substantial homology to known DGKs. DGK activity was increased in lysates of insect cells infected with recombinant baculovirus containing this cDNA. Antibodies raised against expressed sequences recognized a glucocorticoid-inducible 130-140-kDa protein on immunoblots of DDT1 cell lysates. Thus, this sequence appears to be a new member of the DGK family that we refer to as DGKeta. Homology to other DGKs was apparent in domains that are thought to be important for DGK function including the cysteine-rich motifs and potential catalytic domains. DGKeta shares substantial homology with DGKdelta including the N-terminal pleckstrin homology domain. The tissue distribution of DGKeta message (determined by ribonuclease protection assays) and protein (determined by immunoblots) was broader than reported for other DGKs, indicating that DGKeta may play a more general role in regulating cellular DG levels than other DGKs. Heterogeneity among DGK family members indicates that individual DGKs may have unique functions.

Ca2+ mobilizing action of sphingosine in Jurkat human leukemia T cells. Evidence that sphingosine releases Ca2+ from inositol trisphosphate- and phosphatidic acid-sensitive intracellular stores through a mechanism independent of inositol trisphosphate

Effects of sphingosine on Ca2+ mobilization in the human Jurkat T cell line were examined. Sphingosine increased the cytoplasmic Ca2+ concentration ([Ca2+]i) in a dose-dependent manner with an ED50 of around 8 microM. Sphingosine and OKT3, a CD3 monoclonal antibody, transiently increased [Ca2+]i, which declined to the resting level in the absence of extracellular Ca2+. Under the same conditions, pretreatment with sphingosine inhibited but did not abolish an increase in [Ca2+]i induced by the subsequent addition of OKT3 and vice versa. However, pretreatment with sphingosine did not affect an increase in [Ca2+]i induced by OKT3 in the presence of Ca2+. OKT3 increased IP3 formation, but sphingosine did not affect the level of IP3 by itself nor did it cause IP3 formation induced by OKT3. In permeabilized Jurkat cells, the addition of IP3 released Ca2+ from nonmitochondrial intracellular stores, but the addition of sphingosine did not. Sphingosine, stearylamine, and psychosine increased [Ca2+]i and diacylglycerol (DG) kinase activation; however, ceramide did not, whereas sphingosine 1-phosphate slightly activated DG kinase without elevation of [Ca2+]i. Pretreatment with R59022, a DG kinase inhibitor, abolished the peak but did not affect the sustained response to [Ca2+]i to sphingosine. Phosphatidic acid (PA) elevated [Ca2+]i, after which it declined to a resting level even in the presence of extracellular Ca2+. In accordance with this, PA did not stimulate 45Ca2+ uptake into cells, but sphingosine and OKT3 did. Pretreatment with PA partially inhibited a rise in [Ca2+]i induced by the subsequent addition of sphingosine and vice versa in the absence of extracellular Ca2+. Under similar conditions, pretreatment with PA affected an elevation of [Ca2+]i induced by OKT3 less, after which the subsequent addition of sphingosine did not increase [Ca2+]i. In permeabilized Jurkat cells, the addition of IP3 did not release Ca2+, but PA did in the presence of heparin. Pretreatment with thapsigargin, a microsomal Ca2+-ATPase inhibitor, abolished the rises of [Ca2+]i induced by the subsequent addition of sphingosine, OKT3, and PA in the absence of extracellular Ca2+. The present results suggest that at least two kinds of intracellular Ca2+ stores exist in Jurkat cells, both of which are IP3- and PA-sensitive, and that sphingosine mobilizes Ca2+ from both stores in an IP3-independent manner. Furthermore, the IP3- but not the PA-sensitive intracellular Ca2+ store seems to regulate Ca2+ entry induced by sphingosine.

Postnatal changes in activation of polyunsaturated fatty acid, acylation of lysophosphoglyceride, and phosphorylation of diacylglycerol in cortical gray matter of rat cerebrum

Activation of polyunsaturated fatty acid, acylation of lysophosphoglyceride, and phosphorylation of diacylglycerol in postnatal developing rat cerebrum were studied in vitro, using cortical gray matter homogenates. The rate of n-3 and n-6 fatty acid activation was maximal at ten days after birth and minimal at adulthood. Acylation of lysophosphoglyceride was most active at the neonatal stage and gradually decreased with age. In addition, the amount of phosphatidic acid formed from 1, 2-dioleoyl-glycerol was also maximum at the neonatal stage, and then gradually decreased. These results suggest that the specific activity of glycerolipid synthesis in cortical gray matter declines during postnatal development.

Arachidonoyl-diacylglycerol kinase. Specific in vitro inhibition by polyphosphoinositides suggests a mechanism for regulation of phosphatidylinositol biosynthesis

We previously described the purification of a membrane-bound diacylglycerol kinase highly selective for sn-1-acyl-2-arachidonoyl diacylglycerols (Walsh, J. P., Suen, R., Lemaitre, R. N., and Glomset, J. A. (1994) J. Biol. Chem. 269, 21155-21164). This enzyme appears to be responsible for the rapid clearance of the arachidonate-rich pool of diacylglycerols generated during stimulus-induced phosphoinositide turnover. We have now shown phosphatidylinositol 4,5-bisphosphate to be a potent and specific inhibitor of arachidonoyl-diacylglycerol kinase. Kinetic analyses indicated a Ki for phosphatidylinositol 4,5-bisphosphate of 0.04 mol %. Phosphatidic acid also was an inhibitor with a Ki of 0.7 mol %. Other phospholipids had only small effects at these concentrations. A series of multiply phosphorylated lipid analogs also inhibited the enzyme, indicating that the head group phosphomonoesters are the primary determinants of the polyphosphoinositide effect. However, these compounds were not as potent as phosphatidylinositol 4,5-bisphosphate, indicating some specificity for the polyphosphoinositide additional to its total charge. Five other diacylglycerol kinases were activated to varying degrees by phosphatidylinositol 4,5-bisphosphate and phosphatidic acid, suggesting that inhibition by acidic lipids may be specific for the arachidonoyl-DAG kinase isoform. Given the presumed role of arachidonoyl-diacylglycerol kinase in the phosphoinositide cycle, this inhibition may represent a mechanism for polyphosphoinositides to regulate their own synthesis.

Inhibition of diacylglycerol kinase by the antitumor agent calphostin C. Evidence for similarity between the active site of diacylglycerol kinase and the regulatory site of protein kinase C

Calphostin C is an anti-tumor agent that binds to the regulatory domain of protein kinase C and inhibits the binding of phorbol dibutyrate. Recent studies suggest that there may be structural similarities between protein kinase C (PKC) and diacylglycerol kinase (DGK). Both enzymes bind diacylglycerol and phosphatidylserine, and sequencing of the 80 kDa diacylglycerol kinase shows that it contains zinc finger-like sequences, similar to those occurring in PKC. Similarities in some enzymatic properties of PKC and DGK led us to examine whether regulatory-site inhibitors of PKC also might inhibit DGK. For these studies, the membrane-bound DGK was partially purified from porcine testis membranes. Calphostin C inhibited DGK with an IC50 in the micromolar range. The inhibition of DGK by calphostin C was competitive with respect to diacylglycerol and was not affected by the presence or absence of phosphatidylserine. Other inhibitors of protein kinase C were without effect, with the exception of Adriamycin, which inhibited at millimolar concentrations. Staurosporine, which binds to the catalytic domain of protein kinase C, did not inhibit DGK. The results suggest that there are functional similarities between the substrate binding site of DGK and the regulatory site of protein kinase C.

Diacylglycerol elevations in control platelets are unaccompanied by pleckstrin phosphorylation. Implications for the role of diacylglycerol in platelet activation

Several laboratories have reported that diacylglycerol levels in human platelets (approximately 100 pmol/10(9) platelets) increased severalfold in response to 0.5-1 U/ml thrombin. We report here fluctuations in diacylglycerol mass in control platelets, the magnitude of which were 60-90% of that measured in platelets treated with 0.2-0.5 U/ml of thrombin. These control platelets were not activated by such criteria as absence of aggregation, secretion, phosphatidic acid production and phosphorylation of the protein kinase C substrate, pleckstrin. Thrombin treatment evoked all of the above responses. Analysis of the diacylglycerol molecular species by reverse-phase HPLC of the dimethylated, phosphorylated derivatives showed that all of the molecular species that were present in control platelets were also present in thrombin-treated platelets. Most of the species appeared to fluctuate at random in control platelets with the exception of 1-stearoyl-2-arachidonoyl-sn-glycerol which was more or less stable and increased severalfold over control values only upon thrombin treatment. Furthermore, only this species accumulated as [32P]phosphorylated PtdOH in thrombin-treated platelets prelabelled with [32P]Pi. Our findings show that, in platelets, elevation of diacylglycerol molecular species other than the 1-stearoyl-2-arachidonoyl species occurs, but these changes are not necessarily linked to activation of protein kinase C as measured by pleckstrin phosphorylation which was observed only upon elevation of 1-stearoyl-2-arachidonoyl-sn-glycerol.

Activation of diacylglycerol kinase by carbachol in guinea pig taenia coli

Changes in diacylglycerol kinase (DG kinase) activity in carbachol (CCh)-stimulated guinea pig taenia coli were investigated. In a mixed micellar assay system, added 1,2-dioctanoyl-sn-glycerol (diC8) and endogenous DG were competitively bound to common DG kinase isozymes from guinea pig taenia coli and phosphorylated, suggesting that diC8 is useful as a probe of agonist effects on DG kinase activity. In phosphorus-32 ([32P]Pi)- and diC8-prelabeled guinea pig taenia coli, diC8 was phosphorylated by DG kinase to [32P]dioctanoyl-phosphatidic acid ([32P]diC8-PA). CCh increased the accumulation of both [32P]diC8-PA and endogenous [32P]phosphatidic acid ([32P]PA) in a time- and dose-dependent manner (0.1-100 microM CCh). CCh-induced increases in [32P]diC8-PA and [32P]PA were inhibited by 1 microM atropine and 3 microM DG kinase inhibitor (R59022). These findings indicated the activation of DG kinase by muscarinic receptor stimulation in guinea pig taenia coli. Therefore, DG kinase activation may play an important role in CCh-induced PA formation. CCh-induced [32P]diC8-PA and [32P]PA accumulation was dependent on intracellular calcium concentrations. However, a KCl-induced increase in intracellular calcium, without receptor stimulation, was ineffective. Moreover, treatment with phorbol ester also increased accumulation of both PA species in KCl-treated tissues. These findings suggest that muscarinic receptor mediated activation of DG kinase may require both an increase in intracellular calcium and PKC activation in guinea pig taenia coli.

Membrane-associated diacylglycerol kinase activity is increased by noradrenaline, but not by angiotensin II, in arterial smooth muscle

In rat small arteries, noradrenaline stimulates the sustained production of arachidonoyl-phosphatidic acid, whereas there is only a slight and transient increase with angiotensin II [Ohanian, Ollerenshaw, Collins and Heagerty (1990) J. Biol. Chem. 265, 8921-8928]. Diacylglycerol kinase (DGK) is the enzyme responsible for generating phosphatidic acid from 1,2-diacylglycerol (DAG). To investigate whether agonists influence DGK activity, we have studied this enzyme in both particulate and soluble fractions prepared from rat small arteries. Soluble DGK activity was inhibited by octyl glucoside. Therefore a deoxycholate assay was used for this fraction, whereas an octyl glucoside mixed-micelle assay was used to examine particulate fractions. Particulate DGK selectively phosphorylated long-chain DAG at a rate 2.5-3-fold higher than that for the synthetic substrate dioctanoylglycerol. In contrast, the substrate preference of the soluble isoenzyme(s) was: dioctanoylglycerol > arachidonoyl-DAG= dioleoylglycerol. Stimulation of intact arteries with noradrenaline (15 microM) increased membrane-associated DGK activity 3-fold, transiently. Angiotensin II (100 nM) stimulation did not alter the DGK activity of this fraction. The activity of the soluble DGK was increased by both agonists, but only transiently. These results demonstrate that rat small arteries contain a membrane-associated DGK which metabolizes arachidonoyl-containing substrate. Also, the activity of this enzyme is regulated differentially by vasoconstrictor hormones. It is concluded that modulation of DGK activity may represent one point at which agonists using the same signal-transduction pathway may tailor the cellular response.

Isolation and characterization of the human diacylglycerol kinase gene

The 80 kDa diacylglycerol kinase (DGK) is abundantly expressed in oligodendrocytes and lymphocytes but not to a detectable extent in other cells such as neurons and hepatocytes. As an initial attempt to delineate the mechanism of the transcriptional control of the DGK gene, we have cloned from a human genomic library a 22 kb genomic fragment. The genomic clone consists of the 5'-flanking region and 17 exons coding for approx. 53% of the total exons of human DGK, including those encoding EF-hand and zinc-finger regions. The translation initiation site is located in the second exon. S1 nuclease mapping and primer extension analysis of the human DGK mRNA identified a major transcription initiation site (position +1) at 264 bp upstream from the initiator ATG. In the 5'-flanking sequence we detected a single GC box at -35 but no canonical TATA and CAAT sequences. However, the sequence starting from the cap site (AGTTCCTGCCA) is very similar to the initiator element that specifies the transcription initiation site of some housekeeping genes. In addition, the 5'-upstream region contains several putative cis-elements. Jurkat and HepG2 cells were transfected with various 5'-deletion mutants of the upstream region fused to the structural gene of chloramphenicol acetyltransferase (CAT). The CAT assay revealed that among constructs containing up to 3.4 kb of the 5'-flanking region, a fragment of 263 bp from the transcription initiation site contains a basic promoter that is active in both types of cells. Moreover, the region between -263 and -850 contains a negative element that is active in HepG2 but not in Jurkat cells. This negative element may, at least in part, be responsible for the cell type-specific expression of the DGK gene.

Molecular cloning and expression of a 90-kDa diacylglycerol kinase that predominantly localizes in neurons

A diacylglycerol kinase cDNA was isolated from a rat brain cDNA library. This cDNA encoded an 801-amino acid protein of 90,287 Da. This 90-kDa diacylglycerol kinase showed 58% identity in deduced amino acid sequence with a previously isolated rat 80-kDa diacylglycerol kinase. EF-hand motifs, cysteine-rich zinc-finger-like sequences, and putative ATP-binding sites were all conserved between the two kinase species. However, mRNA encoding the 90-kDa kinase was confined to restricted neuronal populations such as the caudate-putamen, the accumbens nucleus, and the olfactory tubercle. Further, the 90-kDa kinase was found to exhibit high phosphorylation activity for long-chain diacylglycerols and was mainly associated with the membrane fraction when the cDNA was transfected into COS-7 cells.

Characterization of phosphatidylinositol-4-phosphate 5-kinase activities from bovine brain membranes

Phosphatidylinositol-4-phosphate (PtdIns(4)P) kinase activity associated with bovine brain membranes, was released by NaCl treatment and partially purified by chromatography on phosphocellulose, phenylsepharose, Ultrogel AcA44, DEAE-cellulose and ATP-agarose. The final preparation contained a 6333-fold purified protein fraction with a specific activity of 171 nmol.min-1 x mg-1. Under conditions where this PtdIns(4)P kinase activity (PtdIns(4)P kinase activity b) did not bind to DEAE-cellulose, a PtdIns(4)P kinase activity purified earlier (Moritz, A., De Graan, P.N.E., Ekhart, P.F., Gispen, W.H. and Wirtz, K.W.A. (1990) J. Neurochem. 54, 351-354) does bind (PtdIns(4)P kinase activity a). Both enzyme activities specifically used PtdIns(4)P as substrate and phosphorylated the inositol moiety at the 5'-position. PtdIns(4) kinase activity a has an apparent Km of 18 microM for PtdIns(4)P whereas PtdIns(4)P kinase activity b has a Km of 4 microM. All other measured kinetic parameters (i.e., Km for ATP, Mg(2+)-dependence, pH optimum, activation by phosphatidylserine and inhibition by phosphatidylinositol 4,5-bisphosphate) were similar for both enzyme activities.

Diacylglycerol kinase is phosphorylated in vivo upon stimulation of the epidermal growth factor receptor and serine/threonine kinases, including protein kinase C-epsilon

In signal transduction, diacylglycerol (DG) kinase attenuates levels of the second messenger DG by converting it to phosphatidic acid. A previously cloned full-length human 86 kDa DG kinase cDNA was expressed as fusion protein in Escherichia coli, to aid in the generation of DG-kinase-specific monoclonal antibodies suitable for immunoprecipitation experiments. To investigate whether phosphorylation of DG kinase is a possible mechanism for its regulation, COS-7 cells were transiently transfected with the DG kinase cDNA and phosphorylation of the expressed DG kinase was induced by various stimuli. Activation of both cyclic AMP-dependent protein kinase and protein kinase C (PKC) resulted in phosphorylation of DG kinase on serine residues in vivo, and both kinases induced this phosphorylation within the same tryptic phosphopeptide, suggesting that they may exert similar control over DG kinase. No phosphorylation was observed upon ionomycin treatment, intended to activate Ca2+/calmodulin-dependent kinases. Co-transfections of DG kinase with either PKC-alpha or PKC-epsilon cDNA revealed that both protein kinases, when stimulated, are able to phosphorylate DG kinase. For PKC-epsilon, DG kinase is the first in vivo substrate identified. Stimulation with epidermal growth factor (EGF) of COS-7 cells transfected with both DG kinase and EGF-receptor cDNA results mainly in phosphorylation of DG kinase on tyrosine. Since the EGF receptor has an intrinsic tyrosine kinase activity, this finding implies that DG kinase may be a direct substrate for the activated EGF receptor.

Glycerophospholipid metabolism: back to the future

It has become customary to regard the various glycerophospholipids as quite similar, and the acyl groups are considered to have little influence on the behaviour of the lipids in membranes or metabolism. Nevertheless, a number of recent observations by the authors and others indicate a high degree of metabolic compartmentation and substrate specificity with regard to the acyl substituents (acyl specificity) of glycerophospholipid metabolising enzymes in intact cells. 1. [32P]Orthophosphate and [3H]glycerol are incorporated into phosphatidylcholine (PC) and phosphatidylethanolamine (PE) of platelets and Swiss 3T3 fibroblasts with a [32P]/[3H]-ratio several fold lower than in glycerol-3-phosphate, phosphatidic acid (PA) and phosphatidylinositol (PI), suggesting distinct metabolic separation (probably by cellular compartmentation) of the glycerol and choline (or ethanolamine) branches of de novo phospholipid biosynthesis. 2. In fibroblasts the [32P]/[3H]-ratio varied 50-fold among the molecular species of PC, PE, PI and PA, which indicates that the enzymes involved in these conversions have some degree of acyl specificity. 3. In vitro assays for lipid-converting enzymes employ detergents, which affect acyl specificity of the enzymes (lipid kinases) both by their chemical nature and concentrations. 4. Thrombin stimulation of platelets causes formation of a multitude of diacylglycerol (DAG) molecular species, but only one major molecular species of PA is formed indicating that the DAG kinase may have distinct acyl specificity in the intact cell. 5. However, this specificity could also result from the net reactions of DAG kinase(s) and PA phosphohydrolase(s), which would constitute an ATP-utilising, paired regulation of the molecular species of PA and the inositol lipids on one hand, and PC, PE phosphatidylserine and triacylglycerol on the other. These findings indicate a high complexity of glycerophospholipid metabolism and a distinct acyl specificity in intact cells that are not apparent from studies in vitro. A major challenge for future research in this area is to bridge the apparent discrepancy between in vivo and in vitro observations regarding glycerophospholipid metabolism, an endeavour that will require more knowledge about the physical chemistry of naturally occurring molecular species than is available today. The most prevailing appreciation of glycerophospholipids among biological scientists to-day is that they can be distinguished functionally, topographically and metabolically only by their head groups and that they form the bilayer in biological membranes. Most of us know that the fatty acid in the sn-2 position is unsaturated and have been indoctrinated that the higher the degree of unsaturation, the greater the fluidity of the membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of diacylglycerol in the exocytosis of the sperm acrosome. Studies using diacylglycerol lipase and diacylglycerol kinase inhibitors and exogenous diacylglycerols

When ram spermatozoa were treated with Ca2+ and the ionophore A23187 to induce acrosomal exocytosis, a rise in diacylglycerol (DAG) mass was observed, concomitant with a rapid breakdown of [32P]P1-labelled phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate and a rise in [32P]Pi-labelled phosphatidate. Inclusion of the DAG lipase inhibitor RHC 80267 resulted in further but biphasic increases in DAG; there was an increasing accumulation of DAG with concentrations of RHC 80267 up to 10 microM, whereas higher concentrations produced lessening accumulation. Inclusion of RHC 80267 in the ionophore induction system also resulted in significant accelerations of the onset of exocytosis. In spermatozoa stimulated with Ca2+/A23187 and the DAG kinase inhibitor R59022, a similar increase in DAG levels together with stimulation of acrosomal exocytosis were observed. Preincubation of spermatozoa with sn-1-oleoyl-2-acetylglycerol, rac-1-oleoyl-2-acetylglycerol, sn-1,2-dioctanoylglycerol and sn-1,3-dioctanoylglycerol before treatment with Ca2+/A23187 resulted in a dose-dependent stimulation of exocytosis by all these isomers. Neomycin inhibited Ca2+/A23187-induced generation of DAG together with polyphosphoinositide breakdown, as well as acrosomal exocytosis. Inclusion of exogenous DAG, however, overcame the inhibitory effect of neomycin on exocytosis. Our results suggest that DAG has a key role in acrosomal exocytosis and that it acts as a messenger rather than as a substrate from which other active metabolites are generated. The lack of stereospecificity shown by the exogenous DAGs implies that DAG does not act by stimulating protein kinase C, but the metabolite's actual target in the sperm cell is as yet unclear.

Cytosolic rat brain synapsin I is a diacylglycerol kinase

The phosphorylation of diacylglycerol (DG), a reaction catalyzed by DG kinase, may be critical in the termination of effector-induced signals mediated by protein kinase C. Synapsin I is a principal target of intracellular protein kinases and is thought to be involved in the release of neurotransmitter from axon terminals. We present several lines of evidence which indicate that rat brain synapsin, in addition to this role, may function as a DG kinase. Purified rat brain DG kinase was digested with trypsin, which produced three major fragments whose sequence was identical to three regions in synapsin I. Using a rabbit anti-synapsin polyclonal antiserum, the elution profile of synapsin immunoreactivity coincided exactly with that of DG kinase activity in column fractions from the final step in the DG kinase purification procedure. As is the case with synapsin, the purified enzyme was a strongly basic protein with an isoelectric point greater than 10.0. Finally, incubating the DG kinase with highly purified bacterial collagenase, an enzyme that partially degrades the proline- and glycine-rich synapsin, resulted in the simultaneous loss of DG kinase activity and synapsin immunoreactivity. We conclude that cytosolic rat brain synapsin is capable of functioning as a DG kinase.

Purification, cDNA-cloning and expression of human diacylglycerol kinase

Diacylglycerol (DG) kinase attenuates the level of the second messenger DG in signal transduction, and therefore possibly modulates protein kinase C (PKC). DG kinase was purified to homogeneity from human white blood cells, showing an Mr of 86 kDa as determined by SDS-PAGE and gel filtration. Two amino acid sequences of tryptic peptides from DG kinase were determined and degenerate oligonucleotides were prepared and used in the polymerase chain reaction. An amplified DNA fragment was subsequently used to clone the full-length human DG kinase cDNA. This sequence is the human homolog of a porcine DG kinase cDNA sequence reported recently. The sequence contains a double EF-hand structure typical for Ca2+ binding proteins. DG kinase further contains a double cysteine repeat that is present in all PKC isoforms, where it constitutes the phorbol ester (and most likely diacylglycerol) binding site. Therefore we speculate that the double cysteine repeat in DG kinase is involved in DG binding. DG kinase is transcribed as a single mRNA of 3.2 kb, that is highly expressed in T-lymphocytes. The human DG kinase cDNA when transfected in mammalian cells (COS-7) results in a 6-7-fold increase of DG kinase activity.