New patterns of diacylglycerol metabolism in intact cells

Identification and characterization of human PEIG-1/GPRC5A as a 12-O-tetradecanoyl phorbol-13-acetate (TPA) and PKC-induced gene

Homo sapiens orphan G protein-coupling receptor PEIG-1 was first cloned and characterized by applying differential display to T84 colonic carcinoma cells incubated in the presence of phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) (GenBank AF506289.1). Later, Lotan's laboratory found the same gene product in response to retinoic acid analogues, naming it with the symbol RAIG1. Now the official HGNC symbol is GPRC5A. Here, we report the extension of its original cDNA fragment towards the 5' and 3' end. In addition, we show that TPA (100 ng/ml, 162 nM) strongly stimulated GPRC5A mRNA in T84 colonic carcinoma cells, with maximal expression at 4 h and 100 ng/ml (162 nM). Western blots showed several bands between 35 and 50 kDa, responding to TPA stimulation. Confocal microscopy confirmed its TPA upregulation and the location in the plasma membrane. The PKC inhibitor Gö 6983 (10 μM), and the Ca²⁺ chelator BAPTA-AM (150 μM), strongly inhibited its TPA induced upregulation. The PKA inhibitor H-89 (10 μM), and the MEK1/2 inhibitor U0126 (10 μM), also produced a significant reduction in the TPA response (~50%). The SGK1 inhibitor GSK650394 stimulated GPRC5A basal levels at low doses and inhibit its TPA-induced expression at concentrations ≥10 μM. The IL-1β autocrine loop and downstream signalling did not affect its expression. In conclusion, RAIG1/RAI3/GPRC5A corresponds to the originally reported PEIG-1/TIG1; the inhibition observed in the presence of Gö 6983, BAPTA and U0126, suggests that its TPA-induced upregulation is mediated through a PKC/Ca²⁺ →MEK1/2 signalling axis. PKA and SGK1 kinases are also involved in its TPA-induced upregulation.

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

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

13C-labelling studies indicate compartmentalized synthesis of triacylglycerols in C6 rat glioma cells

NMR-visible mobile lipid (ML) signals have been detected in (1)H-NMR spectra of tissues in vivo, ex vivo and in vitro, and have been shown to change in apparent intensity in association with pathology (necrosis in brain tumours) and normal processes (cell differentiation, cell growth arrest and apoptosis). Although it is widely accepted that ML signals originate mainly from fatty-acyl chains in triacylglycerols (TAG) contained in cytosolic lipid droplets (LD), the dynamics of TAG in LD is not yet fully understood. In order to better understand the synthesis of cellular TAG and its relationship to ML dynamics we carried out a set of labelling experiments with C6 rat glioma cells in culture. TAG and phospholipid metabolism was monitored by incubating C6 cells with [1-(13)C]-glucose at two time points during cell growth curve -24 h incubation starting at log-phase; 48 h incubation starting at saturation density- and by acquiring the 2D-HMQC NMR spectra of the respective total lipid extracts. The resulting TAG, diacylglycerol (DAG) and phospholipid labelling patterns can only be explained if TAG synthesis takes place in two different subcellular compartments. One compartment would be the endoplasmic reticulum, which is known to be involved in TAG metabolism, while the other compartment could be the plasma membrane and/or the LD. This possible role of LD is further supported by the recent description of diacylglycerolacyltranferase-activity associated with LD. Accordingly, we postulate the existence of a carbon-shuttling mechanism between plasma membrane phospholipids and endoplasmic reticulum by way of LD content. The results we have obtained with C6 cells may also apply to other cellular systems and should be taken into account when interpreting ML dynamics detected by NMR in vivo.

Pharmacological characterization of the ergot alkaloid receptor in the salivary gland of the ixodid tickAmblyomma hebraeum

Female ticks of the family Ixodidae osmoregulate by secreting the excess fluid of the blood meal back into the host's circulation via the salivary glands. At least three receptors control salivary fluid secretion in the tick Amblyomma hebraeum: (1) dopamine (DA) stimulates fluid secretion via a DA receptor, (2) ergot alkaloids (ErAs) stimulate fluid secretion via an ErA-sensitive receptor (the natural ligand of which has not been identified), and (3) a GABA receptor potentiates the action of DA and ErAs. Here we present some pharmacological properties of the ErA-sensitive receptor. Of the 11 ErAs we tested, (i) four were complete agonists (approximate concentration eliciting 50% maximum response is given in parentheses): dihydroergotamine (0.02 μmol l–1),ergonovine (ErN; 0.06 μmol l–1), methylergonovine (0.1μmol l–1) and α-ergocriptine (0.9 μmol l–1); (ii) three were `incomplete agonists' (approximate concentration eliciting 20% maximum response is given in parentheses):ergocorninine (3.5 μmol l–1), ergocristinine (7.5 μmol l–1) and ergocristine (10 μmol l–1); (C)three were partial agonists (approximate concentration eliciting the respective maximum response in parentheses): ergocornine (50% maximum by 1μmol l–1), methysergide (28% maximum by 10 μmol l–1) and bromocriptine (22% maximum by 10 μmol l–1); and (D) one had no activity up to 1 mmol l–1: ergothioneine. Bromocriptine and methysergide did not antagonize the action of DA, but were effective competitive antagonists of ErN, with Kis of ∼0.3 μmol l–1 and 11 μmol l–1, respectively. Ergothioneine was not an antagonist at either the DA- or ErA-sensitive receptor. The putative protein kinase C activators, 1-oleoyl-2-acetyl-sn-glycerol (OAG) and 1,2-dioctanoyl-sn-glycerol (DiC8), neither stimulated salivary fluid secretion nor potentiated the action of DA or ErN. The putative protein kinase C inhibitors, bisindolymaleimide (BIM) and calphostin C did not inhibit the action of DA or ErN, although low concentrations of calphostin C(10 nmol l–1) appeared to potentiate the action of DA but not ErN. The ion transport inhibitors, furosemide and amiloride (both up to 1 mmol l–1), had no significant effect on DA-stimulated or ErN-stimulated fluid secretion.

Protein kinase Cα requirement in the activation of p38 mitogen-activated protein kinase, which is linked to the induction of tumor necrosis factor α in lipopolysaccharide-stimulated microglia

Activated microglia have been suggested to produce a cytotoxic cytokine, tumor necrosis factor alpha (TNF alpha), in many pathological brains. Thus, determining the molecular mechanism of this induction and suppression has been the focus of a great deal of research. Using lipopolysaccharide (LPS) as an experimental inducer of TNF alpha, we investigated the regulatory mechanism by which TNFalpha is induced or suppressed in microglia. We found that LPS-induced TNF alpha is suppressed by pretreatment with the p38 mitogen-activated protein kinase (p38MAPK) inhibitor SB203580. Similar suppression was achieved by pretreatment with specific protein kinase C (PKC) inhibitors, Gö6976, myristoylated pseudosubstrate (20-28), and bisindolylmaleimide. These results suggest that PKC alpha activity as well as p38MAPK activity is associated with TNF alpha induction in LPS-stimulated microglia. The requirement of PKC alpha in LPS-dependent TNFalpha induction was verified in PKC alpha-downregulated microglia which could be induced by phorbol-12-myristate-13-acetate pretreatment. Simultaneously, PKC alpha was found to be requisite for the activation of p38MAPK in LPS-stimulated microglia. In addition, the PKC alpha levels in the LPS-stimulated microglia were observed to decrease in response to the p38MAPK inhibitor, indicating that the PKC alpha levels are regulated by the p38MAPK activity. We therefore concluded that PKC alpha and p38MAPK are interactively linked to the signaling cascade inducing TNFalpha in LPS-stimulated microglia, and that in this cascade, PKC alpha is requisite for the activation of p38MAPK, leading to the induction of TNF alpha.

Free fatty acids induce cell differentiation to infective forms in Trypanosoma cruzi

Intestinal extracts of Triatoma infestans induce cell differentiation of Trypanosoma cruzi epimastigotes into the infective metacyclic form. Part of this effect can be explained by the presence of haemoglobin fragments, which stimulate trypanosomal adenylate cyclase. In this work we examined the metacyclogenic activity of lipids present in this intestinal extract. We found that lipid extracts of the intestinal extract have significant stimulatory effects that reside with the free-fatty-acid fraction, especially oleic acid. These compounds stimulate de novo diacylglycerol formation and protein kinase C activity in the parasite. Moreover, metacyclogenesis is stimulated by phorbol esters and cell-permeant diacylglycerol, while protein kinase C down-regulation or incubation with inhibitors of this kinase abrogates this effect. These results indicate that free fatty acids are a novel signal, inducing metacyclogenesis, acting through a pathway involving diacylglycerol biosynthesis and protein kinase C activation.

Neurotrophin secretion from cultured microglia

Because microglia have been suggested to produce neurotrophins, we tested this ability in vitro. Rat primary microglia were found to constitutively secrete a limited amount of brain-derived neurotrophic factor (BDNF), but nerve growth factor (NGF) and neurotrophin-3 (NT-3) were undetectable in the conditioned medium. Stimulation of the cells with lipopolysaccharide (LPS) increased BDNF secretion, and induced NGF secretion. As a first step to examine this regulation system, the association of protein kinase C (PKC) was pharmacologically analyzed. A PKC activator, phorbol-12-myristate-13-acetate, enhanced the secretion of BDNF. Pre-treatment of microglia with a PKC inhibitor, bisindolylmaleimide, suppressed LPS-stimulated BDNF secretion as well as the constitutive one. These results suggest that the PKC signaling cascade is closely associated with BDNF secretion. Among PKC isoforms, PKCalpha probably plays a role in BDNF secretion, based on the results of experiments using a specific PKC activator, 1-oleoyl-2-acetyl-sn-glycerol, and a specific PKC inhibitor, Gö 6976, and by immunoblotting. Taken together, these findings suggest that the secretion of BDNF from microglia is regulated through PKCalpha-associated signal transduction mechanism.

A unique phospholipid organization in bovine erythrocyte membranes

Ruminant erythrocytes are remarkable for their choline-phospholipid anomalies; namely, low or absent phosphatidylcholine (PC) along with high sphingomyelin levels. Here, we report another anomaly in bovine erythrocytes that affects aminophospholipids: phosphatidylethanolamine (PE) shows an extreme asymmetry, with only 2% of the total present in the outer leaflet. Furthermore, we found that phospholipase A(2), an enzyme located on the external surface of the erythrocytes, shows higher activity against PC than against PE. In addition, we observed that acylation of PE is by far the most important biosynthetic event in this system. We propose that deacylation of PE and PC by phospholipase A(2) to generate lysocompounds, followed by selective reacylation of lyso-PE in the inner leaflet, can account for the compositional and architectural peculiarities of bovine erythrocyte membranes.

Lysosomal phospholipase A1 in Trypanosoma cruzi: an enzyme with a possible role in the pathogenesis of Chagas' disease

We found that, as in African trypanosomes, endogenous phospholipase A(1) (Plase A(1)) activity can catalyse extensive deacylation of phospholipids upon cell death in all life stages of Trypanosoma cruzi. A major lysosomal Plase A(1) was purified and characterized. The enzyme products can explain the lesions surrounding degenerating T. cruzi cells in host tissues. Thus Plase A(1) emerges as a target to block pathogenesis in trypanosomal infections.

A mechanism for the partial insertion of protein kinase C into membranes

We propose that the principle driving force allowing protein kinase C (PKC) to insert partway into membranes is the transient creation of an interior hydrophilic phase within the membrane. We further suggest that this phase is composed of non-bilayer-forming elements, such as diacylglycerol or phorbol esters. We used the combination of fluorescence resonance energy transfer (using fluorescently labeled phospholipid molecules and the endogenous tryptophan residues of PKC) and fluorescence quenching by the water-soluble reagent potassium iodide. The experimental system used micelles and purified PKC. Our model accounts for both the established kinetic data on PKC as well as the physical requirements of protein-membrane interaction. Moreover, it establishes PKC as the first example of a partially embedded membrane protein, and provides a mechanism to account for its activation.

Ceramide-enhanced urokinase-type plasminogen activator (uPA) release is mediated by protein kinase C in cultured microglia

As described previously, a relatively high dose of neurotrophins increased the release of urokinase-type plasminogen activator (uPA) from cultured microglia. This biological response is suggested to be caused by ceramide, which is a metabolite of nerve growth factor low-affinity receptor (NGFRp75)-associated sphingomyelin turnover. Therefore, in the present study, we examined the effect of ceramide on the release of uPA from cultured microglia. Treatment of the cells with permeable C8-ceramide (D-erythro-Sphingosine, N-octanoyl-) enhanced uPA release in a dose-dependent manner. This effect of C8-ceramide was mimicked by treatment with bacterial sphingomyelinase. A pharmacological study using a specific PKC activator, phorbol-12-myristate-13-acetate, and a protein kinase C (PKC) inhibitor, bisindolylmaleimide, showed that PKC activation is required in order to release uPA from ceramide-stimulated microglia as well as from nonstimulated microglia. Further study using a specific conventional PKC (cPKC) activator, 1-oleoyl-2-acetyl-sn-glycerol (OAG), and a specific cPKC inhibitor, Gö 6976, suggested that PKC-delta and/or -epsilon is involved in uPA release. As opposed to the apoptotic pathway, however, no activation of c-Jun N-terminal kinase and nuclear factor kappa B was observed in C8-ceramide-stimulated microglia. The findings suggest that uPA release from microglia is regulated by a mechanism in which PKC-delta and/or -epsilon are activated and further signals are transduced subsequently.

Phosphatidylcholine formation is the predominant lipid biosynthetic event in the hemoparasite Babesia bovis

This work examines the lipid composition and metabolism of bovine red blood cells infected by apicomplexan Babesia parasites, organisms closely related to Plasmodium sp. We found that erythrocytes infected with Babesia bovis (i-RBC) accumulate lipids and show striking increases in phosphatidylcholine, phosphatidic acid, diacylglycerol and cholesteryl esters as compared to uninfected erythrocytes cultured under the same conditions (n-RBC). A similar pattern was observed in cultures of erythrocytes infected with Babesia bigemina. The lipid profile of purified B. bovis merozoites showed that phosphatidylcholine is the most abundant phospholipid in this parasite (31.8% +/- 2.8 of total phospholipid), markedly differing from bovine n-RBC, in which it is only a minor component (4.8% +/- 0.6). B. bovis cultures incorporate radiolabeled choline into complex lipids, especially phosphatidylcholine, with minor amounts recovered in sphingomyelin and lysophosphatidylcholine. When [14C] stearate was used as precursor, the labeling pattern again gave the highest incorporation into phosphatidylcholine, with lesser incorporation in sphingomyelin, phosphatidylinositol, phosphatidylethanolamine and phosphatidic acid. Diacylglycerol and small amounts of cholesteryl esters were the only labeled neutral lipids found. B. bovis also incorporates [3H] myo-inositol into phosphatidylinositol. Parallel incubations with n-RBC as a control yielded no incorporation into either polar or neutral lipids with any precursor. These results indicate that the lipid changes observed in i-RBC can be explained on the basis of the lipid biosynthetic activities of the babesial parasite. Gas chromatography-mass spectrometry (GC-MS) analysis of fatty acid methyl esters from phospholipids of i-RBC and n-RBC showed the same qualitative composition in both. However, i-RBC had higher ratios of saturated to unsaturated fatty acids and B. bovis cultures did not desaturate [14C] stearate. Cholesterol was the only sterol detected by GC-MS. Phospholipase A2 treatment of i-RBC and n-RBC revealed no enhanced hemolytic effects in i-RBC, suggesting that the erythrocyte membrane phospholipid composition is essentially unaltered by the parasite. Labeling of i-RBC or n-RBC with [125I] Bolton-Hunter resulted in an enhanced phosphatidylserine labeling in i-RBC. This study provides the first data on B. bovis lipid constitution and biosynthesis. They show that phosphatidylcholine formation is the main biosynthetic process in these cells. The striking differences in the contents of phosphatidylcholine between host erythrocytes and the parasite suggests that it may be a useful target for both chemotherapy and immunoprophylaxis against bovine babesiosis.

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.

Micellar lipoproteins as the possible storage and translocation form of intracellular diacylglycerol

Previous work indicated that diacylglycerol (DG) molecules translocate across the cytoplasm of mammalian cells, a process relevant to the signalling role of this lipid as protein kinase C activator. Here we investigated the possible mechanism underlying DG translocation. We examined the interaction between 1,2-di-[1-14C]oleoyl-sn-glycerol and rat liver cytosol (rlc) using assays based on Lipidex-1000 and on coelution on Sepharose CL 6B. We measured high DG binding activity and found that it resides in cytosolic proteins and not in cytosolic lipids. Chromatography of rlc proteins on Sepharose CL 6B showed profiles in which the activity measured by either method coincided. Further, we showed that the DG-rlc protein interaction results in the stabilization of DG in a micellar form, eluting in the void volume of Sepharose CL 6B. Such stabilized micelles are reminiscent of insect lipophorins and may represent a new, thus far unrecognized, mode of lipid transport within living cells.

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.

Protein kinase C and receptor kinase gene expression in olfactory receptor neurons

Recent biochemical evidence indicates that protein kinase C (PKC) and G-protein-coupled receptor kinases (GRKs) are involved in olfactory signal termination and desensitization. The polymerase chain reaction (PCR) was used to investigate the expression of PKC and GRK genes in olfactory tissue and in isolated olfactory receptor neurons from channel catfish (Ictalurus punctatus). Sequence analysis of cloned PKC PCR products showed that the alpha, beta, delta, epsilon, and theta isotypes were expressed in olfactory tissue. Sequence analysis of PCR products obtained from isolated olfactory receptor neurons showed that PKC beta and PKC delta were expressed in the receptor cells. A 600-bp GRK PCR product was obtained from isolated olfactory neurons that shared 86% and 92% amino acid sequence identity to the mammalian beta-adrenergic receptor kinase gene products beta ARK1 and beta ARK2, respectively. Go6976, a specific inhibitor of calcium-regulated PKC activity, completely inhibited odorant-stimulated PKC activity in isolated olfactory cilia. This result suggested that odorant-stimulated PKC activity is mediated by the calcium-sensitive PKC beta isotype. Taken together, these results are consistent with the conclusion that PKC beta and beta ARK mediate odorant receptor phosphorylation and olfactory signal termination.

Stimulation of phosphatidylcholine turnover by beta-phorbol ester and diacylglycerol in the primordial human placenta: the suggested role of phospholipase D activation

The effects of 4beta-phorbol-12-myristate-13-acetate (PMA) and 1,2-(sn)-dioctanoylglycerol (DOCG) on the phosphatidylcholine (PC) turnover (defined as degradation to diacylglycerol followed by PC resynthesis) and on the activity of PC-specific phospholipase D were investigated in placental mince incubated with various radiolabelled precursors in vitro. Experiments with [32P]phosphate indicated that 1 microM PMA and 125-250 microM DOCG were the lowest concentrations that led to maximal and selective stimulation of PC labelling. Moreover, PMA and DOCG acted along different time courses: PMA enhanced labelling after 60 min incubation, with a lag period of at least 30 min, whereas DOCG stimulated PC labelling after only 30 min with no further increase in the next 30 min. The following findings suggest that increased labelling of PC with [32P]phosphate in PMA-treated tissue reflects an increased rate of PC turnover: (1) the effects of PMA and DOCG were additive and PMA did not have any effect on the labelling of PC(DOCG) indicating that it stimulated PC labelling even if it did not activate CTP:choline cytidylyl transferase, the regulatory enzyme of PC synthesis de novo; (2) PMA did not increase the labelling of PC from [3H]glycerol or [3H]glucose ruling out a PMA-promoted availability of glycolytic and/or lipolytic intermediates for PC formation; and (3) the PMA effect was attended by an increased labelling of phosphatidic acid whereas there was no change in the labelling of lyso-PC, indicating the activation of phospholipase D. Experiments in which the transphosphatidylation reaction between [3H]myristic acid-labelled PC and ethanol was used to estimate phospholipase D activity showed 2.4-fold and 1.4-1.8-fold activations by PMA and DOCG, respectively, with no additivity noted. These results suggest that PMA stimulates PC turnover in the early human placenta via the activation of phospholipase D. Rapid metabolic conversion decreases the capacity of DOCG to accelerate PC-turnover and to activate phospholipase D. The early DOCG-induced stimulation of PC labelling with [32P]phosphate is attributed mainly to its known activating effect on CTP: choline cytidylyl transferase.

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.

Diacylglycerols derived from membrane phospholipids are metabolized by lipases in A10 smooth muscle cells

The metabolic fate of endogenous diacylglycerol (DAG) in cultured A10 smooth muscle cells was determined. Preincubation of A10 cells with [3H]myristic acid or [3H]arachidonic acid resulted in preferential labeling of phosphatidylcholine (PC) or phosphatidylinositol (PI), respectively. Addition of PC-specific phospholipase C (PC-PLC) to [3H]myristate-labeled A10 cells resulted in a 10-fold increase in radiolabeled DAG, which was converted to monoacylglycerol (MG) and fatty acid (FA). DAG degradation and MG formation was inhibited by tetrahydrolipstatin, a DAG lipase inhibitor. PC-derived DAG was not converted to phosphatidic acid; in addition, PC resynthesis or triacylglycerol synthesis was not observed. Addition of PI-specific PLC (PI-PLC) to [3H]arachidonate-labeled A10 cells resulted in a modest increase in radiolabeled DAG that was also hydrolyzed to MG and FA. Therefore, the principal metabolic fate of endogenous DAG generated from membrane phospholipids by treatment of A10 cells with PC-PLC and PI-PLC was hydrolysis by a DAG lipase pathway.

Alteration of glycerolipid and sphingolipid-derived second messenger kinetics in ras transformed 3T3 cells

The effect of ras transformation (rasB fibroblasts) on basal and serum-stimulated diacylglycerol (DAG) composition and mass was examined over time with respect to changes in membrane phospholipid composition and ceramide mass. RasB cells vs. nontransformed control cells (rasD and NR6) had chronically elevated DAG levels (up to 240 min) following serum stimulation, indicating a defect in the recovery phase of the intracellular DAG pulse. Ras transformation also had a dramatic effect on DAG composition. Molecular species analysis revealed that DAG from unstimulated rasB cells was enriched in the delta 9 desaturase fatty acyl species (monoenoate 18:1(n - 7) and 18:1(n - 9)), and depleted in arachidonic acid (20:4(n - 6)). With the exception of glycerophosphoinositol (GPI), DAG remodeling paralleled the compositional alterations in individual phospholipid classes. Importantly, ras transformation altered the fatty acyl composition of sphingomyelin, a precursor to the ceramide second messenger. With the addition of serum, control cells (rasD) had a progressive increase in ceramide mass with levels approximately 5-fold higher by 240 min. In contrast, ceramide levels did not increase in rasB cells at either 4 or 240 min. These results demonstrate that ras-oncogene, in addition to its effects on DAG metabolism, can also abolish the cellular increase in ceramide mass in response to serum stimulation. Since DAG and ceramide may have opposing biological functions, the prolonged elevation of DAG and the suppression of ceramide levels would be consistent with an enhanced proliferative capacity.

Characterization of a partially purified diacylglycerol lipase from bovine aorta

A partially-purified diacylglycerol (DG) lipase from bovine aorta has been characterized with respect to the effects of lipid metabolites and two lipase inhibitors, phenylboronic acid and tetrahydrolipstatin (THL). DG lipase activity was determined by the hydrolysis of the sn-1 position of 1-[1-14C]palmitoyl-2-oleoyl-sn-glycerol. The products of the lipase reaction, 2-monoacylglycerol (2-monoolein) and non-esterified fatty acids (oleate, archidonate) produced a concentration-dependent (20-200 microM) inhibition of DG lipase activity. Oleoyl-CoA and dioleoylphosphatidic acid also inhibited aortic DG lipase activity, but lysophosphatidylcholine had little or no effect. The inhibition of aortic DG lipase by phenylboronic acid was competitive, with a Ki of approx. 4 mM. THL was a very potent inhibitor of aortic DG lipase; the concentration required for inhibition to 50% of control was 2-6 nM. THL inhibition was reduced when the concentration of substrate in the assay was increased. Attempts to identify the aortic DG lipase by covalent-labelling with [14C]THL were unsuccessful. Immunoblotting experiments revealed that hormone-sensitive triacylglycerol lipase (HSL) could not be detected in bovine aorta.

Signal transduction in vascular smooth muscle: diacylglycerol second messengers and PKC action

Agonist-stimulated phospholipid turnover can generate diacylglycerol (DAG), an intracellular second messenger that activates protein kinase C (PKC). DAG can be produced from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by a phosphoinositide-specific phospholipase C and by the degradation of phosphatidylcholine (PC) by a phospholipase C or the concerted actions of phospholipase D and phosphatidate phosphohydrolase. In vascular smooth muscle, agonist-stimulated DAG accumulation is biphasic; PIP2 hydrolysis produces a transient increase in DAG, which is followed by a sustained phase of DAG accumulation from PC degradation. Metabolism of DAG attenuates PKC activation and thus results in signal termination. The metabolic fates for DAG include 1) ATP-dependent phosphorylation to form phosphatidic acid (DAG kinase), 2) hydrolysis to release fatty acids and glycerol (DAG and monoacylglycerol lipases), 3) synthesis of triacylglycerol (DAG acyltransferase), and 4) synthesis of PC (choline phosphotransferase). Hydrolysis through the lipase pathway is the predominant metabolic fate of DAG in vascular smooth muscle. Activation of PKC in vascular smooth muscle modulates agonist-stimulated phospholipid turnover, produces an increase in contractile force, and regulates cell growth and proliferation. Further research is required to investigate cross talk between signal transduction mechanisms involving lipid second messengers. In addition, spatial considerations such as nuclear PKC activation and the influence of diradylglycerol generation on the duration of PKC activation are important issues.

Partial purification of a diacylglycerol lipase from bovine aorta

A diacylglycerol (DG) lipase has been purified from a soluble subcellular fraction of bovine aorta by (NH4)2SO4 precipitation in the presence of 5.0% (w/v) Triton X-100, followed by chromatography on DEAE-Sephacel, heparin-Sepharose and octyl-Sepharose in the presence of either CHAPS or Triton X-100 detergents. Under basal conditions, the hydrolysis of a short-chain [3H]dioctanoylglycerol ([3H]diC8) substrate was much greater than that of a long-chain 1-[1-14C]palmitoyl-2-oleoyl-sn-glycerol (1-[14C]POG) substrate. Lipase activity measured with 1-[14C]POG was markedly enhanced by Triton X-100. In the presence of 0.1% Triton X-100, specific enzyme activities in the octyl-Sepharose fraction determined with 1-[14C]POG or 1-stearoyl-2-[1-14C]-arachidonoyl-sn-glycerol as substrates were the same as that measured with [3H]diC8. MgCl2 (5mM) or CaCl2 (2 mM) also selectively stimulated lipase activity (up to 10-13-fold) measured with the long-chain (1-[14C]POG) substrate only. The increase in relative specific activity in the octyl-Sepharose fraction was 60-fold and 155-fold, based on hydrolysis of [3H]diC8 and 1-[14C]POG (+ Triton X-100), respectively. Unlabelled diC8 was a competitive inhibitor of 1-[14C]POG hydrolysis, suggesting that a single lipase hydrolyses both the short-chain and long-chain DG substrates; selective stimulatory effects of non-ionic detergents and bivalent cations on the hydrolysis of 1-[14C]POG may be due to effects on the physical properties of the substrate preparation. Monoacylglycerol lipase, DG kinase and cholesterol esterase activities could not be detected in the partially purified lipase preparation.

Attenuation of diacylglycerol signal in the primordial human placenta: role of phosphatidylcholine formation

Metabolic conversion of the synthetic DAG, dioctanoylglycerol (DOCG) into the dioctanoyl species of phosphatidic acid (PADOCG) and phosphatidylcholine (PCDOCG) in minced human primordial placenta incubated with [32P]phosphate was studied. Time-course experiments performed with 0.25 mM DOCG revealed a much higher capacity of the placenta to synthetize PCDOCG than PADOCG and indicated the rapid metabolism of PCDOCG. In addition, DOCG stimulated the labeling of PC whereas no such effects on the labeling of lyso-PC and PA were observed. This effect of DOCG is not related to the synthesis of PC de novo, because DOCG did not exhibit any stimulating effect on the synthesis of PC from [3H]glycerol or [3H]glucose. Experiments with varying concentrations of DOCG (0.025-0.5 mM) showed increasing rate of formation of PCDOCG over a concentration range from 0.05 to 0.5 mM whereas 0.5 mM DOCG was the lowest concentration where significant formation of PADOCG was found. Labeling of PC was about 30% over control at 0.125 mM as well as at 0.25 mM DOCG, and decreased below the control level at 0.5 mM DOCG. Labeling of PA and PI was not influenced by these DOCG concentrations. Labeling of PC, either stimulated by DOCG or not, was more sensitive to the inhibitory effect of 1 mM of the DAG analog: dioctanoylethyleneglycol (DOEG) than that of PA. Moreover, 1 mM DOEG inhibited the formation of PCDOCG without such an effect on the labeling of PADOCG. These findings indicate that in the primordial human placenta PC synthesis represents a more efficient attenuation pathway of DAG signal than the formation of PA by DAG kinase.