Dissociation of protein kinase C activation and sn-1,2-diacylglycerol formation. Comparison of phosphatidylinositol- and phosphatidylcholine-derived diglycerides in alpha-thrombin-stimulated fibroblasts

Phosphatidylinositol 4-phosphate is a major source of GPCR-stimulated phosphoinositide production

Phospholipase C (PLC) enzymes hydrolyze the plasma membrane (PM) lipid phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) to generate the second messengers inositol trisphosphate (IP₃) and diacylglycerol (DAG) in response to receptor activation in almost all mammalian cells. We previously found that stimulation of G protein-coupled receptors (GPCRs) in cardiac cells leads to the PLC-dependent hydrolysis of phosphatidylinositol 4-phosphate (PI4P) at the Golgi, a process required for the activation of nuclear protein kinase D (PKD) during cardiac hypertrophy. We hypothesized that GPCR-stimulated PLC activation leading to direct PI4P hydrolysis may be a general mechanism for DAG production. We measured GPCR activation-dependent changes in PM and Golgi PI4P pools in various cells using GFP-based detection of PI4P. Stimulation with various agonists caused a time-dependent reduction in PI4P-associated, but not PI4,5P₂-associated, fluorescence at the Golgi and PM. Targeted depletion of PI4,5P₂ from the PM before GPCR stimulation had no effect on the depletion of PM or Golgi PI4P, total inositol phosphate (IP) production, or PKD activation. In contrast, acute depletion of PI4P specifically at the PM completely blocked the GPCR-dependent production of IPs and activation of PKD but did not change the abundance of PI4,5P₂ Acute depletion of Golgi PI4P had no effect on these processes. These data suggest that most of the PM PI4,5P₂ pool is not involved in GPCR-stimulated phosphoinositide hydrolysis and that PI4P at the PM is responsible for the bulk of receptor-stimulated phosphoinositide hydrolysis and DAG production.

Defining lipid mediators of insulin resistance - controversies and challenges

Essential elements of all cells, lipids play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects of insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

Sexual dimorphism in hepatic lipids is associated with the evolution of metabolic status in mice

Ectopic lipid accumulation in the liver is implicated in metabolic disease in an age- and sex-dependent manner. The role of hepatic lipids has been well established within the scope of metabolic insults in mice, but has been insufficiently characterized under standard housing conditions, where age-related metabolic alterations are known to occur. We studied a total of 10 male and 10 female mice longitudinally. At 3, 7 and 11 months of age, non-invasive ¹ H-magnetic resonance spectroscopy (¹ H-MRS) was used to monitor hepatic lipid content (HLC) and fatty acid composition in vivo, and glucose homeostasis was assessed with glucose and insulin challenges. At the end of the study, hepatic lipids were comprehensively characterized by nuclear magnetic resonance (NMR) and liquid chromatography-mass spectrometric analyses of liver tissue samples. In males, HLC increased from 1.4 ± 0.1% at 3 months to 2.9 ± 0.3% at 7 months (p < 0.01) and 2.7 ± 0.3% at 11 months (p < 0.05), in correlation with fasting insulin levels (p < 0.01, r = 0.51) and parameters from the insulin tolerance test (ITT; p < 0.001, r = -0.69 versus area under the curve; p < 0.01, r = -0.57 versus blood glucose drop at 1 h post-ITT; p < 0.01, r = 0.55 versus blood glucose at 3 h post-ITT). The metabolic performance of females remained the same throughout the study, and HLC was higher than that of males at 3 months (2.7 ± 0.2%, p < 0.01), but comparable at 7 months (2.2 ± 0.2%) and 11 months (2.2 ± 0.1%). Strong sexual dimorphism in bioactive lipid species, including diacylglycerols (higher in males, p < 0.0001), phosphatidylinositols (higher in females, p < 0.001) and omega-3 polyunsaturated fatty acids (higher in females, p < 0.01), was found to be in good correlation with metabolic scores at 11 months. Therefore, in mice housed under standard conditions, sex-specific composition of bioactive lipids is associated with metabolic protection in females, whose metabolic performance was independent of hepatic cytosolic lipid content.

Lipid levels in sperm, eggs, and during fertilization in Xenopus laevis

Critical developmental periods, such as fertilization, involve metabolic activation, membrane fusion events such as sperm-egg or plasma membrane-cortical granule merger, and production and hydrolysis of phospholipids. However, there has been no large-scale quantification of phospholipid changes during fertilization. Using an enzymatic assay, traditional FA analysis by TLC and gas chromatography, along with a new method of phospholipid measurement involving HPLC separation and evaporative light-scattering detection, we report lipid levels in eggs, sperm, and during fertilization in Xenopus laevis. Sperm were found to contain different amounts of phospholipids as compared with eggs. During fertilization, total phosphatidylinositol, lysophosphatidylcholine, sphingomyelin, and phosphatidylserine decreased, and ceramide increased, whereas there was no change in phosphatidylcholine, cardiolipin, or phosphatidylethanolamine. FA analysis of phospholipids found numerous changes during fertilization. Because there is an increase in sn-1,2-diacylglycerol at fertilization, the FAs associated with this increase and the source of the increase in this neutral lipid were examined. Finally, activation of phospholipase C, phospholipase D, phospholipase A2, autotoxin, and sphingomyelinase at fertilization is discussed.

Regulation of phosphoinositide and phosphatidylcholine phospholipases by G proteins

Two G proteins that regulate phosphoinositide phospholipase C in liver plasma membranes have been purified to homogeneity in both the heterotrimeric and dissociated forms. The heterotrimers contain a 42 kDa or 43 kDa alpha subunit and a 35 kDa beta subunit. The alpha subunits are not ADP-ribosylated by pertussis toxin and are closely related immunologically to members of the recently identified Gq class of G proteins. The specific phosphoinositide phospholipase C isozyme that responds to the G proteins has been determined to the beta 1 isozyme. GTP analogues stimulate phosphatidylcholine hydrolysis in rat liver plasma membranes. The nucleotide specificity and Mg2+ dependency of the response indicate that it is mediated by a G protein. Phosphatidic acid, diacylglycerol, choline and phosphorylcholine are the products, indicating that both phospholipase D and C activities are involved. Activation of phospholipase D is also indicated by the enhanced production of phosphatidyl-ethanol in the presence of ethanol.

The signal-induced phospholipid degradation cascade and protein kinase C activation

Acting in synergy with diacylglycerol, unsaturated free fatty acids such as arachidonic, oleic, linoleic, linolenic and docosahexaenoic acids dramatically activate some members of the protein kinase C family at the basal level of Ca2+ concentration. It is plausible that phospholipase C and phospholipase A2, and possibly phospholipase D as well, are involved in the activation of protein kinase C. Presumably, this enzyme activation is integrated into the signal-induced membrane phospholipid degradation cascade, prolonging the activation of protein kinase C. The sustained activity of this enzyme appears to be of importance for long-term cellular responses such as development of neuronal plasticity and gene activation.

Phosphatidylcholine-specific phospholipase C in mitogen-stimulated fibroblasts

To investigate expression, subcellular localization and mechanisms of translocation of phosphatidylcholine-specific phospholipase C (PC-PLC) during the cell proliferative response, biochemical, immunoblotting, and immunofluorescence analyses were performed on quiescent and mitogen-stimulated NIH-3T3 fibroblasts. Platelet-derived growth factor (PDGF), insulin and 12-O-tetradecanoylphorbol-13-acetate induced, in 10-60 min, PC-PLC translocation from a perinuclear cytoplasmic area to the plasma membrane. Following cell exposure to PDGF (60 min), the overall PC-PLC expression increased up to 2-3x, while the enzyme activity increased 5x in total cell lysates, 2x in the plasma membrane, and 4x in the nucleus; moreover, confocal laser scanning microscopy showed a progressive externalization of PC-PLC on the outer plasma membrane surface and its accumulation in the nuclear matrix. Pre-incubation of cells with the PC-PLC inhibitor tricyclodecan-9-yl potassium xanthate (D609), before PDGF-stimulation, not only reduced the enzyme activity in total cell lysates as well as in plasma membrane and nuclear fractions, but also blocked the mechanisms of PC-PLC subcellular redistribution. These effects were associated with a D609-induced long-lasting cell cycle block in Go.

Isoenzyme-specific Translocation of Protein Kinase C (PKC)βII and not PKCβI to a Juxtanuclear Subset of Recycling Endosomes

Elucidation of isoenzyme-specific functions of individual protein kinase C (PKC) isoenzymes has emerged as an important goal in the study of this family of kinases, but this task has been complicated by modest substrate specificity and high homology among the individual members of each PKC subfamily. The classical PKCbetaI and PKCbetaII isoenzymes provide a unique opportunity because they are the alternatively spliced products of the beta gene and are 100% identical except for the last 50 of 52 amino acids. In this study, it is shown that green fluorescent protein-tagged PKCbetaII and not PKCbetaI translocates to a recently described juxtanuclear site of localization for PKCalpha and PKCbetaII isoenzymes that arises with sustained stimulation of PKC. Mechanistically, translocation of PKCbetaII to the juxtanuclear region required kinase activity. PKCbetaII, but not PKCbetaI, was found to activate phospholipase D within this time frame. Inhibitors of phospholipase D (1-butanol and a dominant negative construct) prevented the translocation of PKCbetaII to the juxtanuclear region but not to the plasma membrane, thus demonstrating a role for phospholipase D in the juxtanuclear translocation of PKCbetaII. Taken together, these results define specific biochemical and cellular actions of PKCbetaII when compared with PKCbetaI.

Diacylglycerols Containing Omega 3 and Omega 6 Fatty Acids Bind to RasGRP and Modulate MAP Kinase Activation

We elucidated the effects of different diacylglycerols (DAGs), i.e. 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG), 1-stearoyl-2-docosahexaenoyl-sn-glycerol (SDG), and 1-stearoyl-2-eicosapentaenoyl-sn-glycerol (SEG), on [3H]PDBu binding to RasGRP. The competition studies with these DAGs on [3H]PDBu binding to RasGRP revealed different Ki values for these DAG molecular species. Furthermore, we transfected human Jurkat T cells by a plasmid containing RasGRP and assessed the implication of endogenous DAGs on activation of MAP kinases ERK1/ERK2, induced by phorbol-12-myristate-13-acetate (PMA). In control cells, GF109203X, a protein kinase C inhibitor, inhibited ERK1/ERK2 activation. However, this agent curtailed but failed to completely diminish ERK1/ERK2 phosphorylation in RasGRP-overexpressing cells, though calphostin C, a DAG binding inhibitor, suppressed the phosphorylation of MAP kinases in these cells. In cells incubated with arachidonic acid (AA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA), PMA induced the production of endogenous DAGs containing these fatty acids, respectively: DAG-AA, DAG-DHA, and DAG-EPA. The inhibition of production of DAG-AA and DAG-DHA significantly inhibited MAP kinase activation in RasGRP overexpressing, but not in control, cells. Our study demonstrates that three DAG molecular species bind to RasGRP, but only DAG-AA and DAG-DHA participate in the modulation of RasGRP-mediated activation of MAP kinases in Jurkat T cells.

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.

Non-methylene-interrupted polyunsaturated fatty acids: effective substitute for arachidonate of phosphatidylinositol

In mammalian tissues and cells, a characteristic of phosphatidylinositol (PI) is a high abundance of arachidonic acid (AA) relative to the other phospholipids. In this study, we investigated the effects of supplementation of several polyunsaturated fatty acids (PUFAs) on the AA concentration of the PI fraction using a cultured cell system. Neither alpha-linolenic acid nor eicosapentaenoic acid supplement reduced the level of AA in PI of HepG2 cells. In contrast to the n-3 series PUFAs, adding podocarpic acid (20:3, Delta-5,11,14) and pinolenic acid (18:3, Delta-5,9,12) reduced the AA content of the PI fraction from a control value of 15.9% to 7.0 and 8.7%, respectively. In the experiments with pinolenic acid, selective and significant accumulation of 20:3 (Delta-7,11,14), the chain-elongated metabolite of pinolenic acid, was observed in the PI fraction. On the other hand, adding columbinic acid (18:3, Delta-5t,9,12) had no effect on AA content of the PI fraction. Because both podocarpic acid and pinolenic acid are non-methylene-interrupted fatty acids (NMIFAs) that are not converted to AA metabolically, these NMIFAs may be interesting experimental tools for research on the function of PI-origin bioactive lipids.

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.

Diacylglycerol--when is it an intracellular messenger?

Distinct, structurally different forms of sn-1,2-diacylglycerol are found in cells, these are polyunsaturated, mono- or di-unsaturated and saturated. The pathways that generate or metabolise sn-1, 2-diacylglycerol are reviewed. The evidence that it is the polyunsaturated forms of sn-1,2-diacylglycerol, but the more saturated forms of phosphatidate which function as intracellular signals is considered.

Lipid second messengers derived from glycerolipids and sphingolipids, and their role in smooth muscle function

The processes that link activation of an external receptor to the internal mechanisms that elicit a physiological response have been the subject of extensive investigation. It has been established that rather than just being an inert barrier to protect the cell from environmental damage, there are populations of phospholipids located within the plasma membrane that act as a reservoir for signalling molecules and when a receptor binds its appropriate activating ligand a chain of events is initiated which leads to the breakdown of these lipids and the release of second messengers. Such processes are rapid enough for physiological responses to be effected. The purpose of this review is to examine the profile of lipid second messengers derived from glycerophospholipids and sphingolipids. In the former class are included phosphoinositide and phosphatidylcholine and the latter includes sphingomyelin. Hydrolysis of such parent compounds is mediated by phospholipases and the profile of metabolites appears to be agonist specific and modulated by a number of mechanisms including heterotrimeric G-protein subunits, small G-proteins, alterations in intracellular calcium concentration, protein kinase C and tyrosine kinases. The recent interest in sphingolipids, particularly in vascular smooth muscle cells, has been provoked by the observation that ceramide and sphingoid base formation is observed in response to vasoconstrictor hormones.

Diacylglycerols and phosphatidates: which molecular species are intracellular messengers?

In eukaryotes, many receptor agonists use phospholipase-generated lipids as intracellular messengers. Receptor occupation stimulates the production of polyunsaturated 1,2-diacylglycerols by phosphatidylinositol-4,5-bisphosphate specific phospholipases C and/or of mono-unsaturated and saturated phosphatidates by phospholipase-D-catalysed phosphatidylcholine breakdown. The primary phospholipase products are rapidly metabolized: polyunsaturated 1,2-diacylglycerols are converted to polyunsaturated phosphatidates by diacylglycerol kinase; mono-unsaturated and saturated phosphatidates are dephosphorylated to give mono-unsaturated and saturated 1,2-diacylglycerols by phosphatidate phosphohydrolase. The phospholipase-generated polyunsaturated 1,2-diacylglycerols and mono-unsaturated and saturated phosphatidates appear to be intracellular messengers, whereas their immediate metabolites probably do not have signalling functions.

Activation of ADP-ribosylation factor 1 GTPase-activating protein by phosphatidylcholine-derived diacylglycerols

Disassembly of the coatomer from Golgi vesicles requires that the small GTP-binding protein ADP-ribosylation factor 1 (ARF1) hydrolyzes its bound GTP by the action of a GTPase-activating protein. In vitro, the binding of the ARF1 GTPase-activating protein to lipid vesicles and its activity on membrane-bound ARF1GTP are increased by diacylglycerols with monounsaturated acyl chains, such as those arising in vivo as secondary products from the hydrolysis of phosphatidylcholine by ARF-activated phospholipase D. Thus, the phospholipase D pathway may provide a feedback mechanism that promotes GTP hydrolysis on ARF1 and the consequent uncoating of vesicles.

Diacylglycerol and phosphatidate generated by phospholipases C and D, respectively, have distinct fatty acid compositions and functions. Phospholipase D-derived diacylglycerol does not activate protein kinase C in porcine aortic endothelial cells

Stimulation of cells with certain agonists often activates both phospholipases C and D. These generate diacylglycerol and phosphatidate, respectively, although the two lipids are also apparently interconvertable through the actions of phosphatidate phosphohydrolase and diacylglycerol kinase. Diacylglycerol activates protein kinase C while one role for phosphatidate is the activation of actin stress fiber formation. Therefore, if the two lipids are interconvertable, it is theoretically possible that an uncontrolled signaling loop could arise. To address this issue structural analysis of diacylglycerol, phosphatidate, and phosphatidylbutanol (formed in the presence of butan-1-ol) from both Swiss 3T3 and porcine aortic endothelial cells was performed. This demonstrated that phospholipase C activation generates primarily polyunsaturated species while phospholipase D activation generates saturated/monounsaturated species. In the endothelial cells, where phospholipase D was activated by lysophosphatidic acid independently of phospholipase C, there was no activation of protein kinase C. Thus we propose that only polyunsaturated diacylglycerols and saturated/monounsaturated phosphatidates function as intracellular messengers and that their interconversion products are inactive.

Pemphigus IgG activates and translocates protein kinase C from the cytosol to the particulate/cytoskeleton fractions in human keratinocytes

We have demonstrated previously that pemphigus vulgaris (PV)-IgG induces activation of phospholipase C (PLC), production of inositol 1,4,5-trisphosphate, and a rapid transient increase in [Ca2+]i in cultured human keratinocytes, leading to secretion of plasminogen activator and cell-cell detachment in cell culture. In the current study, to examine the involvement of protein kinase C (PKC) in the mechanism of blister formation in PV, we studied the PV-IgG-induced translocation of PKC isozymes from the cytosol to the particulate/cytoskeleton (p/c) fractions and the activation of PKC in human keratinocytes. Cells cultured in Eagle's minimum essential medium were incubated with PV-IgGs for 30 s, 1 min, 5 min, or 30 min. PV-IgG binding to the cell surface antigen (desmoglein III) induced translocation of PKC-alpha from the cytosol to the p/c fractions within 30 s, with a peak at 1 min that lasted at least 30 min. PKC-delta also was translocated within 1 min and reached a peak at 5 min but was reduced to basal levels at 30 min. Alternatively, PKC-eta translocation to the p/c fraction was induced slowly, taking more than 5 min, and was reduced to approximately half-maximum at 30 min, whereas PKC-zeta translocation reached a maximum at 30 s, rapidly returning to baseline by 5 min after PV-IgG stimulation. The total PKC activity in the p/c fraction also was increased after PV-IgG exposure, peaked at 1 min, and was sustained for at least 30 min. These findings suggest that a unique activation profile of PKC isomers may be involved in mediating the intracellular signaling events induced by PV-IgG binding to desmoglein III in cultured human keratinocytes.

The enhancement by wortmannin of protein kinase C-dependent activation of phospholipase D in vascular endothelial cells

Phosphatidic acid generation by phospholipase D (PLD) activation has been implicated in agonist- and oxidant-mediated endothelial cell signal transduction. We examined the effect of wortmannin on PLD activation in pulmonary artery endothelial and smooth muscle cells in culture. Pretreatment of bovine pulmonary artery endothelial cells (BPAECs) with wortmannin potentiated TPA- (100 nM), ATP- (100 microM), and bradykinin- (1 microM) induced [32P]PEt formation, an index of PLD activation. However, wortmannin by itself had no effect on PLD activity. The potentiating effect of wortmannin on TPA-induced PLD activation was dose- (1-10 microM) and time-dependent (5-30 min) and was inhibited by bisindoylmalemide, an inhibitor of protein kinase C (PKC). Furthermore, down-regulation of PKC by prolonged treatment with TPA (100 nM, 18 h) attenuated the wortmannin effect. This effect of wortmannin was specific for TPA- or agonist-induced PLD activation as no potentiation of [32P]PEt formation was observed with H2O2 (1 mM) or ionomycin (1 microM). The effect of wortmannin was not due to activation of PKC alpha as determined by western blot analysis of PKC alpha in the cytosol and membrane fractions. Also, genistein, an inhibitor of tyrosine kinases, did not attenuate the wortmannin-mediated potentiation of PLD thereby suggesting non-involvement of protein tyrosine phosphorylation. These results indicate that wortmannin potentiates PKC-dependent stimulation of PLD in vascular endothelial cells.

Phosphatidylinositol- and phosphatidylcholine-dependent phospholipases C are involved in the mechanism of action of atrial natriuretic factor in cultured rat aortic smooth muscle cells

We have investigated the involvement of specific phospholipase systems and their possible mutual relationship with the mechanism by which atrial natriuretic factor (ANF) increases phosphatidate (PA) and diacylglycerol (DAG) in rat aortic smooth muscle cells (RASMC), one of the major targets of this hormone. Our results indicate that ANF initially stimulates a phosphatidylinositol-dependent phospholipase C (PI-PLC) with a significant increase of DAG, enriched in arachidonate, and inositol trisphosphate (IP3) and then a phosphatidylcholine-dependent phospholipase C (PC-PLC) with formation of DAG, enriched in myristate, and phosphocholine (Pcho). Moreover, ANF stimulates PA formation at an intermediate stage between early and late DAG formation. The transphosphatidylation reaction, as well as its labeling ratio, demonstrate that phosphatidylcholine-dependent phospholipase D (PC-PLD) is not involved. Our experiments with R59022, a DAG kinase (DAGK) inhibitor, indicate that such an increase may be due to the phosphorylation of DAG derived from phosphatidylinositol (PI) hydrolysis. Our results show that phorbol 12-myristate 13 acetate (PMA) plays a significant role in late DAG formation and that Pcho is released concomitantly, suggesting there is a relationship between the two phospholipase Cs (PLCs) that occurs through a protein kinase C (PKC) translocation from cytosol to the plasma membrane. These findings are confirmed by the use of PKC inhibitors calphostin, H7, and staurosporine. The involvement of membrane phospholipid hydrolysis and the ensuing production of second messengers might explain the vasorelaxant effect of ANF.

Nuclear translocation of RhoA mediates the mitogen-induced activation of phospholipase D involved in nuclear envelope signal transduction

In this paper we demonstrate for the first time a mitogen-induced activation of a nuclear acting phosphatidylcholine-phospholipase D (PLD) which is mediated, at least in part, by the translocation of RhoA to the nucleus. Addition of alpha-thrombin to quiescent IIC9 cells results in an increase in PLD activity in IIC9 nuclei. This is indicated by an increase in the alpha-thrombin-induced production of nuclear phosphatidylethanol in quiescent cells incubated in the presence of ethanol as well as an increase in PLD activity in isolated nuclei. Consistent with our previous report (Wright, T. M., Willenberger, S., and Raben, D. M. (1992) Biochem. J. 285, 395-400), the presence of ethanol decreases the alpha-thrombin-induced production of phosphatidic acid without affecting the induced increase in nuclear diglyceride, indicating that the increase in nuclear PLD activity is responsible for the effect on phosphatidic acid, but not that on diglyceride. Our data further demonstrate that RhoA mediates the activation of nuclear PLD. RhoA translocates to the nucleus in response to alpha-thrombin. Additionally, PLD activity in nuclei isolated from alpha-thrombin-treated cells is reduced in a concentration-dependent fashion by incubation with RhoGDI and restored by the addition of prenylated RhoA in the presence of guanosine 5'-3-O-(thio)triphosphate. Western blot analysis indicates that this RhoGDI treatment results in the extraction of RhoA from the nuclear envelope. These data support a role for a RhoA-mediated activation of PLD in our recently described hypothesis, which proposes that a signal transduction cascade exists in the nuclear envelope and represents a novel signal transduction cascade that we have termed NEST (nuclear envelope signal transduction).

Humoral factors of ascites sarcoma 180 stimulate osteoblastic UMR 106-01 cell proliferation and bone resorption via signal transduction pathways, which are clearly different from those of parathyroid hormone

Ascites sarcoma 180 (S180A) is a transplantable tumor that induces hypercalcemia in tumor-bearing mice and stimulates bone resorption in cultured neonatal mouse calvaria without parathyroid hormone (PTH)-like activity. The serum-free conditioned media of S180A cell cultures (S180A-CM) stimulated [3H]thymidine incorporation (178.3% of the control) and inhibited alkaline phosphatase activity (39.0% of the control) in the osteoblastic osteosarcoma cell line UMR 106-01, contrary to PTH. To investigate signal transduction by S180A-CM, we determined the levels of intracellular free calcium ([Ca2+]i), inositol 1,4,5-triphosphate (IP3), 1,2-diacylglycerol (DAG), phosphatidylcholine (PC) and protein kinase (PK) C activity in UMR 106-01 cells. PTH and PTH-related protein (PTHrP), both potent bone-resorbing factors (BRFs), caused an increase in [Ca2+]i and stimulated IP3 production, whereas S180A-CM had little or no effect on these parameters. On the other hand, S180A-CM stimulated DAG production, accompanied by PC breakdown, and the translocation of PKC activity from the cytosol to the membrane fraction. Sphingosine, a specific PKC inhibitor, inhibited bone-resorbing activity (BRA) in S180A-CM more effectively than PTH or PTHrP-stimulated resorption. H-7, an inhibitor of both cAMP-dependent PKA and PKC, completely inhibited BRA in S180A-CM. These results suggest that BRFs of S180A-CM stimulate osteoblastic cell proliferation and bone resorption via two signal transduction pathways, which are different from those of PTH: 1) activation of PKC by DAG resulting from PC hydrolysis and 2) activation of PKA subsequent to prostaglandin E2 production by bone.

Phospholipase D-derived products in the regulation of 12-O-tetradecanoylphorbol-13-acetate-stimulated prostaglandin synthesis in madin-darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells stimulated with 12-O-tetradecanoylphorbol-13-acetate (TPA) in the presence of ethanol synthesize phosphatidylethanol (PEt) instead of phosphatidic acid (PA) and diglyceride (DG). We have used ethanol to block the production of phospholipase D (PLD)-derived PA and DG (from PA hydrolysis) to study their role in signal transduction. In MDCK cells, TPA-stimulated prostaglandin E2 (PGE2) synthesis was inhibited by ethanol at concentrations which inhibit PA and DG formation. In addition, TPA elicited a prolonged increase in PGE2 synthesis that is dependent upon continuous activation of PLD. The TPA-stimulated translocation of protein kinase Calpha (PKCalpha) from cytosol to membrane was unaffected by ethanol. This suggests that PLD-derived products act downstream of PKC in TPA-stimulated prostaglandin synthesis. The calcium ionophore, A23187, did not activate PLD, and PGE2 synthesis in response to A23187 was unaffected by ethanol. TPA increased prostaglandin endoperoxide H synthase (PGHS) activity and increased the amount of immunodetectable prostaglandin endoperoxide H synthase 2 (PGHS-2). A23187 did not induce PGHS-2 and A23187-stimulated PGE2 synthesis appears to be due to the constitutively expressed PGHS-1. Blocking the formation of PLD-derived products, PA and DG, inhibited the induction of PGHS-2 by TPA. These results indicate that prolonged PGE2 synthesis in response to TPA is due to the continuous induction of PGHS-2, which is dependent upon PLD activation. In contrast, induction of PGHS-2 by epidermal growth factor was not affected by ethanol. Epidermal growth factor did not induce PKCalpha translocation nor activate PLD. Taken together, these data suggest that PLD-derived PA or DG act as second messengers in the induction of PGHS-2 by PKC-dependent pathways. The demonstration that inhibition of TPA-induced PA formation inhibits Raf-1 translocation in MDCK cells (Ghosh, S., Strum, J. C., Sciorra, V. A., Daniel, L. W. , and Bell, R. M. (1996) J. Biol. Chem. 271, 8472-8480) suggests that PA is the active PLD metabolite in TPA-stimulated signaling.

Regulation of phospholipase D by tyrosine kinases

Activation of phospholipase D (PLD) represents part of an important signalling pathway in mammalian cells. Phospholipase D catalyzed hydrolysis of phospholipids generates phosphatidic acid (PA) which is subsequently metabolized to lyso-PA (LPA) or diacylglycerol (DAG). While DAG is an endogenous activator of protein kinase C (PKC), PA and LPA have been recognized as second messengers as well. Activation of PLD in response to an external stimulus may involve PKC, Ca2+, G-proteins and/or tyrosine kinases. In this review, we will address the role of protein tyrosine phosphorylation in growth factor-, agonist- and oxidant-mediated activation of PLD. Furthermore, a possible link between PKC, Ca2+, G-proteins and tyrosine kinases is discussed to indicate the complexity involved in the regulation of PLD in mammalian cells.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

Nuclear phospholipase D in Madin-Darby canine kidney cells. Guanosine 5'-O-(thiotriphosphate)-stimulated activation is mediated by RhoA and is downstream of protein kinase C

We have recently demonstrated the existence of an ATP-activated phospholipase D (PLD) in the nuclei of MDCK-D1 cells (Balboa, M. A., Balsinde, J., Dennis, E. A., and Insel, P. A. (1995) J. Biol. Chem. 270, 11738-11740). We have now found that nuclear PLD is synergistically activated by guanosine 5'-O-(thiotriphosphate) (GTP gamma S) and ATP in a time- and concentration-dependent manner, but these compounds do not alter the sensitivity of the enzyme to activation by Ca2+. The synergistic stimulation of PLD activity could be blocked by addition of the protein kinase C inhibitors chelerythrine and calphostin C. Stimulation by GTP gamma S was abolished by guanosine 5'-O-(2-thiodiphosphate). Incubation of isolated nuclei with Clostridium botulinum C3 exoenzyme inhibited the potentiating effect of GTP gamma S on ATP-dependent nuclear PLD activity. Moreover, use of the Rho GDP dissociation inhibitor to extract Rho family G proteins from cell nuclei also inhibits PLD activity. Western blot analyses of isolated nuclei revealed the presence of the small G protein RhoA, but not of RhoB or the ADP-ribosylation factor. GTP gamma S-stimulated ATP-dependent PLD activity could be reconstituted in Rho GDP dissociation inhibitor-washed nuclei by addition of recombinant prenylated RhoA, but not by addition of non-prenylated RhoA. Taken together, these results indicate that nuclear PLD activity is modulated via a RhoA-dependent activation that occurs downstream of protein kinase C. Nuclear PLD, which appears to be a previously unrecognized effector regulated by protein kinase C and G proteins, may be involved in the regulation of nuclear function or structure.

Phosphorylation of MARCKS, neuromodulin, and neurogranin by protein kinase C exhibits differential responses to diacylglycerols

Diacylglycerols (DG) derived from brain phosphatidylinositol (PI) and phosphatidylcholine (PC) and synthetic 1,2-dioleoylglycerol (diC18:1) and 1,2-dioctanoylglycerol (diC8) were tested for their efficacy in stimulating PKC-catalyzed phosphorylation of three physiological substrates in the brain, namely, MARCKS, neuromodulin (Nm), and neurogranin (Ng). The A0.5 of these DGs for PKC were variable dependent on the protein substrates; the values were lowest with MARCKS and highest with Ng. With Ng as a substrate the A0.5 of these DGs for PKC gamma were PI- and PC-DGs < diC18:1 < diC8. Both PI- and PC-DGs, in spite of their differences in unsaturated fatty acids content, were similarly effective in stimulating PKC. Since the phosphorylation of MARCKS, as compared to those of Nm and Ng, has the lowest A0.5 with the various DGs, it seems that among these three PKC substrates MARCKS is most readily phosphorylated by PKCs following DG formation in vivo.

The molecular selectivity of phospholipase D in HL60 granulocytes

The molecular selectivity of PLD in PMA-stimulated HL60 granulocytes was determined by HPLC analysis of [3H]butanol incorporation into phosphatidyl[3H]butanol (Ptd[3H]But) molecular species. Comparison with phospholipid compositions confirmed that PLD acted primarily on phosphatidylcholine (PtdCho). Apparent enzyme selectivity was suggested by negligible formation of PB16:0/16:0 and preferential synthesis of Ptd[3H]But species containing sn-1 18:0. Culture with exogenous 18:2n-6 or 20:4n-6 readily modified both PtdCho and Ptd[3H]But compositions, and accentuated the apparent selectivity of stimulated PLD for sn-1 18:0 species of PtdCho. Such modifications to PLD-based signalling mechanisms may contribute to the modulatory effects of altered dietary lipid intakes on cellular functions.

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.

Evidence that ceramide selectively inhibits protein kinase C-alpha translocation and modulates bradykinin activation of phospholipase D

Sphingomyelinase (SMase) treatment (0.1 unit/ml for up to 30 min) of mouse epidermal (HEL-37) or human skin fibroblast (SF 3155) cells preincubated with [3H]serine to label the sphingomyelin pool caused the accumulation of labeled ceramide but not sphingosine or ceramide 1-phosphate. Incubation of HEL-37 cells with dioctanoylglycerol (diC8) or SF 3155 cells with bradykinin caused translocation of calcium/phosphatidylserine-dependent protein kinase C (PKC) activity to particulate material. In both cell lines the translocation was blocked by SMase treatment of the cells or by incubation with the cell-permeable ceramide analogue N-acetylsphingosine (C2-Cer). Western blot analysis indicated that treatment of HEL-37 cells with diC8 or SF 3155 cells with bradykinin resulted in the translocation of both PKC-alpha and PKC-espilon to particulate material. Treatment with SMase or C2-Cer specifically blocked the translocation of PKC-alpha but not that of PKC-epsilon. Pretreatment of cells with SMase or C2-Cer also inhibited the activation of phospholipase D activity induced by either diC8 (HEL-37 cells) or bradykinin (SF 3155 cells). The data provide strong evidence that ceramide can negatively regulate the translocation of PKC-alpha but not PKC-epsilon and further suggest that PKC-alpha may be involved in regulating phospholipase D activity.

Effect of insulin on SN-1,2-diacylglycerol species and de novo synthesis in rat skeletal muscle

Insulin treatment increases the SN-1,2-diacylglycerol (DAG) concentration in skeletal muscle. Because DAG may participate in transmission or modulation of the insulin receptor signal, we examined the effect of insulin on total DAG and on different DAG species in isolated rat hemidiaphragms incubated with 5 mmol/L glucose. Five DAG species (16:0-18:1 omega 9, 16:0-18:1 omega 7, 18:0-18:1 omega 9, 18:0-18:2 omega 6, and 18:1-18:2) were identified and quantified. After a 5-minute incubation with 60 nmol/L insulin, neither total DAG nor a DAG species increased; exposure to insulin for 10 or 20 minutes increased the concentration of total DAG and of several DAG species. Insulin did not increase DAG in muscles incubated without glucose. Two sources for the insulin-mediated DAG increase were considered: phosphatidylcholine (PC) hydrolysis and de novo DAG synthesis from glucose. Concentrations of choline and phosphocholine in muscle were not increased after 10-minute incubations with insulin. However, insulin increased 14C incorporation from [U-14C]glucose into DAG, triacylglycerol (TAG), and total lipids approximately threefold. Okadaic acid (OKA), an inhibitor of phosphoprotein phosphatases 1 and 2A, increased muscle DAG content and synthesis from glucose, similar to the effect of insulin. Doses of OKA or insulin that increased DAG mass greatly exceeded those required for stimulation of glucose transport. The insulin-mediated, relatively slow increase in muscle DAG observed here likely reflects primarily de novo synthesis from glucose. This effect would be downstream of insulin stimulation of glucose transport. However, a possible insulin-mediated, rapid transient increase in muscle DAG content and PC hydrolysis cannot be ruled out by our studies.

T-cell antigen receptor-induced signal-transduction pathways--activation and function of protein kinases C in T lymphocytes

CONTENTS. T-cell activation--Structure of the T-cell antigen receptor--Modular organisation of the T-cell antigen receptor--T-cell antigen receptor-coupled signaling pathways: Activation of protein-tyrosine kinase by the T-cell antigen receptor; Signal transduction in lymphoid cells involves several protein-tyrosine kinases in parallel; Regulation of T-cell antigen receptor signaling by the phosphoprotein phosphatase CD45--Consequences of T-cell antigen receptor-induced tyrosine phosphorylation: Activation of phosphoinositol-lipid-turnover pathways--Activation of phospholipase C-gamma-1: p59fyn or p56lck?--G-protein motif of CD3-gamma: relevance for signal transduction--Association of lipid kinase with the T-cell antigen receptor--Intracellular signaling by phospholipid metabolites and calcium: activation of protein kinase C--Protein kinase C isoenzymes--Heterogenity of protein kinase C and mode of activation--Phospholipid-derived mediators in activation of protein kinase C in T-cells--Role of phospholipase D metabolites in activation of protein kinase C--Polyunsaturated fatty acids and lysophosphatidylcholine as activators of protein kinase C--Potein kinase C and p21ras function in interdependent and distinct signaling pathways during T-cell activation--Raf-1 kinase: regulator or target of protein kinase C?--Summary and perspectives.

Oxidant-induced activation of protein kinase C in UC11MG cells

Free radical formation and subsequent lipid peroxidation may participate in the pathogenesis of tissue injury, including the brain injury induced by hypoxia or trauma and cardiac injury arising from ischemia and reperfusion. However, the exact cellular mechanisms by which the initial oxidative insult leads to the ultimate tissue damage are not known. A number of reports have indicated that protein kinase C (PKC) may be activated following oxidative stress and that this enzyme may play an important role in the steps leading to cellular damage. In this work, we have examined in a cell model whether PKC is activated following oxidative exposure. UC11MG cells, a human astrocytoma cell line, were treated with H2O2. Incubation with 0.5 mM H2O2 increased malondialdehyde levels by as early as 15 minutes. To assess the effects of H2O2 treatment on PKC activation, we measured phosphorylation of an endogenous PKC substrate, the MARCKS (myristoylated alanine-rich C kinase substrate) protein. Treatment of cells with 0.2-1.0 mM H2O2 resulted in a rapid increase in MARCKS phosphorylation. Phosphorylation was stimulated approximately 2.5-fold following treatment with 0.5 mM H2O2 for ten minutes. Treatment with phorbol 12-myristate 13-acetate, a PKC activator, increased MARCKS phosphorylation approximately 4-fold. The H2O2-induced MARCKS phosphorylation was inhibited by the addition of the kinase inhibitors H-7 and staurosporine. Furthermore, specific down-regulation of PKC by phorbol ester also inhibited H2O2-induced MARCKS phosphorylation. These results indicate that PKC is rapidly activated in cells following an oxidative exposure and that this cell system may be a good model to further investigate the role of PKC in regulating oxidative damage in the cell.

Effects of Ca2+ on the activation of conventional and new PKC isozymes and on TPA and endothelin-1 induced translocations of these isozymes in intact cells

The effects of Ca2+ on the translocation of conventional and new protein kinase C isozymes in intact cells were studied by using C6 glioma cells as a model system. Two conditions which monitor intracellular Ca2+ were performed: one is extracellular Ca(2+)-depletion by treating the cells with physiological saline solution (PSS) without Ca2+ but containing 0.5 mM EGTA, the other is treating the cells with 1 microM ionomycin to induce Ca(2+)-influx. In addition, the TPA and endothelin-1 induced translocations of conventional and new PKC isozymes under these two conditions were also comparatively studied. When the intact cells were treated with Ca(2+)-free, EGTA containing PSS, the membrane-bound conventional PKC alpha (cPKC alpha) was greatly reduced and cytosolic cPKC alpha was slightly increased. However, neither membrane bound nor cytosolic new PKC delta (nPKC delta) was affected by extracellular Ca(2+)-depletion. On the other hand, when the cells were treated with 1 microM ionomycin, the translocation of cPKC alpha itself was observed while nPKC delta was not affected. In extracellular Ca(2+)-depletion, the translocation of cPKC alpha induced by 100 nM TPA still occurred although the extent of translocation was smaller than that induced by TPA under normal Ca2+ conditions; however, that induced by 30 nM ET-1 was blocked. After the cells were treated with 1 microM ionomycin, the translocation of cPKC alpha induced by 30 nM TPA was further increased compared to 1 microM ionomycin or 30 nM TPA alone, while that induced by ET-1 was only slightly further increased. All these results suggested that in intact cells, the activation of cPKC alpha was operated by both the intracellular Ca2+ level and diacylglycerol and that of nPKC delta was operated by diacylglycerol alone as predicted by their properties from purified enzyme or cDNA. In addition, the translocation of cPKC alpha induced by the natural activator ET-1 seemed to be more dependent on Ca2+ than TPA in intact cells.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphoinositide metabolism in airway smooth muscle

Agonist-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, which generates inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol, is thought to be one of the major mechanisms underlying pharmacomechanical coupling in airway smooth muscle. This article is a review of the currently available information on phosphoinositide and inositol 1,4,5-trisphosphate metabolism in this tissue and includes data on inositol 1,4,5-trisphosphate-induced Ca2+ release and the receptor mediating this effect. The final section outlines the potential mechanisms underlying physiological regulation of phosphoinositide metabolism by other second-messenger pathways operative in this tissue.

A role for phospholipase D in control of mitogenesis

How do growth factors that act on G protein-coupled cell-surface receptors communicate with the nucleus? These receptors commonly activate phospholipase C, and it has been assumed that the consequent rise in cytosolic Ca2+ concentrations and activation of protein kinase C mediates the mitogenic response. Recent evidence has demonstrated that phospholipase D (PLD) might be capable of eliciting mitogenesis. This enzyme is stimulated by a variety of growth factors, including those that act on receptors that possess intrinsic tyrosine kinase activity as well as those acting on G protein-coupled receptors. In this review, Michael Boarder considers the evidence that PLD, activated downstream of tyrosine protein kinases by both classes of cell-surface growth factor receptor, is implicated in the mitogenic response. This evidence is related to the possibility of PLD involvement in the regulation of vascular smooth muscle cell proliferation by endothelin-1 and platelet-derived growth factor.

Alpha-thrombin-induced nuclear sn-1,2-diacylglycerols are derived from phosphatidylcholine hydrolysis in cultured fibroblasts

Diglycerides play an important role in a number of agonist-induced signal transduction pathways. We have recently demonstrated that alpha-thrombin induces a rapid increase in the level of diglyceride mass in the nucleus and a selective increase in nuclear PKC-alpha [Leach, K.L., Ruff, V.A., Jarpe, M.B., Fabbro, D., Adams, L.D., & Raben, D.M. (1992) J. Biol. Chem. 267, 21816-21822]. In the present report, we examined the potential source of the induced nuclear diglycerides by examining the molecular species profiles of both the induced diglycerides and nuclear phospholipids by capillary gas chromatography. The molecular species profiles of the nuclear diglycerides generated resemble the species profiles of PC, and not PI species, at all times. In addition, while our previous data indicated that the molecular species of whole-cell phospholipids did not change in response to alpha-thrombin, nuclear PE was altered in a dramatic and selective manner in response to this agonist. These results demonstrate that PC hydrolysis is the predominant, if not exclusive, source of the alpha-thrombin-induced nuclear diglycerides in these fibroblasts.

Decreased myo-inositol uptake is associated with reduced bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol content in cultured neuroblastoma cells exposed to L-fucose

L-Fucose is a potent, competitive inhibitor of myo-inositol transport by cultured mammalian cells. Chronic exposure of neuroblastoma cells to L-fucose causes a concentration-dependent decrease in myo-inositol content, accumulation, and incorporation into phosphoinositides. In these studies, L-fucose supplementation of culture medium was used to assess the effect of decreased myo-inositol metabolism and content on bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol production. Chronic exposure of cells to 30 mM L-fucose caused a sustained decrease in bradykinin-stimulated, but not basal, 3H-inositol phosphate release and 32P incorporation into phosphatidylinositol in cells incubated in serum-free, unsupplemented medium. In addition, 32P incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was not altered in L-fucose-conditioned cells. Acute exposure of cells to serum-free medium containing 30 mM L-fucose did not affect either basal or bradykinin-stimulated 32P incorporation into phosphatidylinositol. Basal diacylglycerol content was decreased by 20% in cells chronically exposed to 30 mM L-fucose, although analysis of the molecular species profile revealed no compositional change. Bradykinin stimulated diacylglycerol production in neuroblastoma cells by increasing the hydrolysis of both phosphoinositides and phosphatidylcholine. Bradykinin-stimulated production of total diacylglycerol was similar for control and L-fucose-conditioned cells. However, there was a decrease in the bradykinin-induced generation of the 1-stearoyl-2-arachidonoyl diacylglycerol molecular species in the cells chronically exposed to 30 mM L-fucose. This molecular species accounts for about 70% of the composition of phosphoinositides, but only 10% of phosphatidylcholine. The results suggest that a decrease in myo-inositol uptake results in diminished agonist-induced phosphatidylinositol synthesis and phosphoinositide hydrolysis in cultured neuroblastoma cells grown in L-fucose-containing medium.