Turnover of the phosphomonoester groups of polyphosphoinositol lipids in unstimulated human platelets

Counterion-mediated pattern formation in membranes containing anionic lipids

Most lipid components of cell membranes are either neutral, like cholesterol, or zwitterionic, like phosphatidylcholine and sphingomyelin. Very few lipids, such as sphingosine, are cationic at physiological pH. These generally interact only transiently with the lipid bilayer, and their synthetic analogs are often designed to destabilize the membrane for drug or DNA delivery. However, anionic lipids are common in both eukaryotic and prokaryotic cell membranes. The net charge per anionic phospholipid ranges from -1 for the most abundant anionic lipids such as phosphatidylserine, to near -7 for phosphatidylinositol 3,4,5 trisphosphate, although the effective charge depends on many environmental factors. Anionic phospholipids and other negatively charged lipids such as lipopolysaccharides are not randomly distributed in the lipid bilayer, but are highly restricted to specific leaflets of the bilayer and to regions near transmembrane proteins or other organized structures within the plane of the membrane. This review highlights some recent evidence that counterions, in the form of monovalent or divalent metal ions, polyamines, or cationic protein domains, have a large influence on the lateral distribution of anionic lipids within the membrane, and that lateral demixing of anionic lipids has effects on membrane curvature and protein function that are important for biological control.

Inositol phosphate metabolism and platelet activation

Platelet activation by thrombin and most other agonists appears to require two second messenger systems that are both initiated by phospholipase C-catalysed cleavage of phosphatidylinositol phosphates leading to: 1. formation of inositol phosphates with a subsequent rise in intracellular calcium from intracellular stores and from outside the cell; 2. formation of diacylglycerol with subsequent activation of protein kinase C. This review examines inositol phosphate metabolism in platelets and its involvement in calcium metabolism.

Rapid turnover rate of phosphoinositides at the front of migrating MDCK cells

Phosphoinositides (PtdInss) play key roles in cell polarization and motility. With a series of biosensors based on Förster resonance energy transfer, we examined the distribution and metabolism of PtdInss and diacylglycerol (DAG) in stochastically migrating Madin-Darby canine kidney (MDCK) cells. The concentrations of phosphatidylinositol (4,5)-bisphosphate, phosphatidylinositol (3,4,5)-trisphosphate (PIP(3)), phosphatidylinositol (3,4)-bisphosphate, and DAG were higher at the plasma membrane in the front of the cell than at the plasma membrane of the rear of the cell. The difference in the concentrations of PtdInss was estimated to be less than twofold between the front and rear of the migrating MDCK cells. To decode the spatial activities of PtdIns metabolic enzymes from the obtained concentration maps of PtdInss, we developed a one-dimensional reaction diffusion model of PtdIns metabolism. In this model, the activities of phosphatidylinositol monophosphate 5-kinase, phosphatidylinositol 3-kinase, phospholipase C, and PIP(3) 5-phosphatases were higher at the plasma membrane of the front than at the plasma membrane of the rear of the cell. This result suggests that, although the difference in the steady-state level of PtdInss is less than twofold, PtdInss were more rapidly turned over at the front than the rear of the migrating MDCK cells.

Protein kinase A mediates inhibition of the thrombin-induced platelet shape change by nitric oxide

The thrombin-induced platelet shape change was blocked by nitric oxide (NO), as revealed by scanning electron microscopy, light transmission, and resistive-particle volume determination. The inhibitory effect of NO was accompanied by an increase in levels of both cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) and phosphorylation of the vasodilator-stimulated phosphoprotein (VASP). However, the inhibition of the shape change was only mimicked by cAMP analogs (Sp-5,6-DClcBIMPS, 8-AHA-cAMP, and 8-CPT-cAMP) and not by cGMP analogs (8-Br-PET-cGMP, 8-Br-cGMP, and 8-pCPT-cGMP). The effect of NO on the thrombin-induced shape change was prevented by the protein kinase A (PKA) antagonists Rp-8-Br-cAMPS and Rp-cAMPS. The protein kinase G (PKG) antagonist Rp-8-CPT-cGMPS strongly inhibited PKG-mediated 46-kDa VASP Ser239 phosphorylation, but did not inhibit the thrombin-induced shape change or the PKA-mediated VASP Ser157 phosphorylation. Whereas an inhibitor of cyclic nucleotide phosphodiesterase (PDE) 3A (milrinone) mimicked the effect of NO, inhibitors of PDE2 (erythro-9-(2-hydroxy-3-nonyl)adenine) and PDE5 (dipyridamole) were poorly effective. We concluded that (1) NO was a potent and reversible inhibitor of the platelet shape change, (2) the shape change was reversible, (3) the inhibitory effect of NO was mediated through activation of PKA, (4) the onset of the NO effect coincided with VASP Ser157 phosphorylation, and (5) removal of NO and platelet shape change coincided with VASP Ser157 dephosphorylation. These findings are compatible with elevation of cGMP by NO in a compartment close to PDE3A, PKA, and VASP, leading to a local increase of cAMP able to block thrombin-induced shape change.

Energy consumption by phospholipid metabolism in mammalian brain

Until recently, brain phospholipid metabolism was thought to consume only 2% of the ATP consumed by the mammalian brain as a whole. In this paper, however, we calculate that 1.4% of total brain ATP consumption is consumed for the de novo synthesis of ether phospholipids and that another 5% is allocated to the phosphatidylinositide cycle. When added to previous estimates that fatty acid recycling within brain phospholipids and maintenance of membrane lipid asymmetries of acidic phospholipids consume, respectively, 5% and 8% of net brain ATP consumption, it appears that phospholipid metabolism can consume up to 20% of net brain ATP consumption. This new estimate is consistent with recent evidence that phospholipids actively participate in brain signaling and membrane remodeling, among other processes.

Phosphoinositides: key players in cell signalling, in time and space

Over the last few years, many reports have extended our knowledge of the inositol lipid metabolism and brought out some exciting information about the location, the variety and the role of phosphoinositides (PIs). Besides the so-called "canonical PI pathway" leading to the production of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), the precursor of the intracellular second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG), many other metabolic pathways have been identified to produce seven different polyphosphoinositides. Several of these quantitatively minor lipid molecules appear to be specifically involved in the control of cellular events, such as the spatial and temporal organisation of key signalling pathways, the rearrangement of the actin cytoskeleton or the intracellular vesicle trafficking. This is consistent with the fact that many of the enzymes, such as kinases and phosphatases, involved in the tight control of the intracellular level of polyphosphoinositides, are regulated and/or relocated through cell surface receptors for extracellular ligands. The remarkable feature of PIs, which can be rapidly synthesised and degraded in discrete membrane domains or even subnuclear structures, places them as ideal regulators and integrators of very dynamic mechanisms of cell regulation. In this review, we will summarise recent studies on the potential location, the metabolic pathways and the role of the different PIs. Some aspects of the temporal synthesis of D3 PIs will also be discussed.

Role of autocrine stimulation on the effects of cyclic AMP on protein and lipid phosphorylation in collagen-activated and thrombin-activated platelets

We compared several responses in thrombin-stimulated and collagen (type I)-stimulated platelets with and without forskolin and inhibitors of autocrine stimulation (IAS: an ADP-removing system of creatine phosphate/creatine phosphokinase, Arg-Gly-Asp-Ser peptide to prevent fibrinogen/fibronectin binding to GPIIb/IIIa, SQ 29.548 as a thromboxane A2 receptor antagonist, cyproheptadine as a serotonin receptor antagonist, BN 52021 as a platelet-activating factor receptor antagonist). The pattern of tyrosine-phosphorylated proteins, the phosphorylation of lipids in the polyphosphoinositide cycle and phosphorylation of pleckstrin (P47) were studied as markers for signal-transducing responses, exposure of CD62 (P-selectin) and CD63 (Glycoprotein 53), as well as secretion of ADP + ATP and beta-N-acetyl-glycosaminidase were studied as final activation responses. Clear differences between thrombin-stimulated and collagen-stimulated platelets were observed. First, practically all protein-tyrosine phosphorylation induced by thrombin was inhibited by IAS, while a partial inhibition was observed for collagen; the phosphorylation due to collagen alone was apparently stimulated by elevation of cAMP. Secondly, the other responses to thrombin were inhibited by increased levels of cAMP, independent of autocrine stimulation. In contrast, only the autocrine part of the collagen-induced responses was inhibited by elevation of cAMP. Thus, the inhibition by elevated cAMP seen in collagen-stimulated platelets seems to be due to removal of the G-protein-mediated activation from secreted autocrine stimulators either by IAS or forskolin. The remaining activity is a pure collagen effect which is not affected by elevated levels of cAMP.

Elevation of cyclic AMP decreases phosphoinositide turnover and inhibits thrombin-induced secretion in human platelets

Elevation of cyclic AMP (cAMP) in platelets inhibits agonist-induced, G protein-mediated responses and activation of polyphosphoinositide-specific phospholipase C (PLC) by ill-defined mechanism(s). Signal transduction steps downstream of PLC are inhibited by elevated cAMP, suggesting an inhibitory effect of cAMP, via protein kinase A, on PLC. In [32P]i-prelabeled platelets, forskolin increased intracellular cAMP (104 nmol/1011 cells at 10-5 M forskolin) and [32P]phosphatidylinositol 4-phosphate (Delta[32P]PIP) (30% at 10-7-10-6 M forskolin). The thrombin-induced (0.1 U/ml) increase in production of [32P]PA, 'overshoots' in [32P]PIP and [32P]PIP2 ([32P]phosphatidylinositol 4,5-bisphosphate), and the increase in [32P]PI and secretion of ADP+ATP were abolished by forskolin (10-7 M). Forskolin stimulated total [32P]Pi uptake in resting platelets (48%), increased 32P incorporation into PIP (110%), and inhibited 32P incorporation into PI (50%). The latter inhibition was most likely considerably greater due to the forskolin-induced stimulation of [32P]Pi uptake. The changes in radioactive PA, PIP and PIP2 are regarded as being proportional with their masses in the prelabeled platelets, while the increase in PI (phosphatidylinositol) is regarded as a change in specific radioactivity, and hence in its synthesis. The results suggest that cAMP elevation inhibits the flux in the polyphosphoinositide cycle through both inhibition of PIP 5-kinase and PI synthesis. The inverse relation between forskolin-produced DeltaPIP and [32P]PA production suggests that the PLC reaction is inhibited by elevated cAMP through reduction of substrate (PIP2) resynthesis, and not by inhibition of the PLC enzyme.

Energy requirements for two aspects of phospholipid metabolism in mammalian brain

Previous estimates have placed the energy requirements of total phospholipid metabolism in mammalian brain at 2% or less of total ATP consumption. This low estimate was consistent with the very long half-lives (up to days) reported for fatty acids esterified within phospholipids. However, using an approach featuring analysis of brain acyl-CoA, which takes into account dilution of the precursor acyl-CoA pool by recycling of fatty acids, we reported that half-lives of fatty acids in phospholipids are some 100 times shorter (min-h) than previously thought. Based on these new estimates of short half-lives, palmitic acid and arachidonic acid were used as prototype fatty acids to calculate energy consumption by fatty acid recycling at the sn-1 and sn-2 positions of brain phospholipids. We calculated that the energy requirements for reacylation of fatty acids into lysophospholipids are 5% of net brain ATP consumption. We also calculated ATP requirements for maintaining asymmetry of the aminophospholipids, phosphatidylserine and phosphatidylethanolamine across brain membrane bilayers. This asymmetry is maintained by a translocase at a stoichiometry of 1 mol of ATP per mol of phospholipid transferred in either direction across the membrane. The energy cost of maintaining membrane bilayer asymmetry of aminophospholipids at steady-state was calculated to be 8% of total ATP consumed. Taken together, deacylation-reacylation and maintenance of membrane asymmetry of phosphatidylserine and phosphatidylethanolamine require about 13% of ATP consumed by brain as a whole. This is a lower limit for energy consumption by processes involving phospholipids, as other processes, including phosphorylation of polyphosphoinositides and de novo phospholipid biosynthesis, were not considered.

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.

Organic Solvents Activate Human Platelets Through the Inositol Lipid-linked Signal Transduction System

It has recently been proposed that occupational exposure to organic solvents in vivo may lead to platelet activation and this has been substantiated by exposure of platelets to solvents in vitro. The present work was undertaken to study the effects of organic solvents on the platelet inositol lipid signal transduction system. Human platelets that had been prelabelled with [(32)P] P, were exposed to a saturated atmosphere of the organic solvents toluene, xylene or hexane. Extracts were analyzed for metabolites of the polyphosphoinositide cycle and ATP. All solvents studied induced a decrease in radioactivity in phosphatidylinositol 4,5-bisphosphate together with an increase in radioactivities in phosphatidylinositol 4-phosphate and phosphatidic acid. This is compatible with solvent-induced activation of the cells through the inositol lipid pathway. In cells exposed to toluene or xylene we could detect an increased level in inositol trisphosphates at 3 min of exposure. The solvent-induced changes in metabolic ATP could not explain the solvent-induced effects on the inositol lipid metabolism. It is concluded that the organic solvents toluene, xylene and hexane can activate human platelets through the inositol lipid-linked transmembrane signal system.

Tyrosine kinases and phosphoinositide metabolism in thrombin-stimulated human platelets

In this study we have examined the implication of tyrosine kinase activities in aggregation, 5-hydroxytryptamine secretion and mainly phosphoinositide metabolism in response to human platelet stimulation by thrombin. Using the potent tyrosine kinase inhibitor tyrphostin AG-213, we have observed a significant inhibition of aggregation and 5-hydroxytryptamine release; however, this percentage inhibition was lower at high thrombin concentrations. On the other hand, tyrphostin treatment of metabolically 32P-labelled platelets significantly inhibited the thrombin-dependent accumulation of PtdIns(3,4)P2, which involves at least a PtdIns 3-kinase and/or a PtdIns3P 4-kinase, whereas the synthesis of phosphatidic acid (PtdOH), a good reflection of the phospholipase C (PLC) activation in platelets, was partially blocked. Inositol phosphate production was also inhibited by about 40% when tyrphostin-treated platelets were stimulated with thrombin. In addition, we show by Western-blot analysis that PLC gamma 1, as well as the regulatory subunit (p85) of the PtdIns 3-kinase, were present in the anti-phosphotyrosine immunoprecipitate isolated from thrombin-stimulated platelets. Furthermore, tyrphostin treatment clearly decreased the PLC gamma 1 and p85 contents in such an anti-phosphotyrosine immunoprecipitate. Our results provide the first evidence for a direct or indirect regulation of PtdIns(3,4)P2 accumulation and PLC gamma 1 activity by tyrosine phosphorylation during thrombin stimulation of human platelets.

Stimulation of phospholipase D activity and phosphatidic acid production by norepinephrine in rat aorta

We sought to relate norepinephrine (NE) stimulation of phosphatidic acid (PA) production to functional responses of rat aorta and pathways for PA production. The time course for changes in PA was closely related to Ca-dependent tonic responses in 42K efflux and contraction. NE (30 microM for 1 min) increased PA and reduced phosphatidylcholine (PC) and phosphatidylinositol (PI) based on Pi analyses and 32P labeling of phospholipids. The 32P-to-Pi ratio in PA (0.8 +/- 0.2, n = 13) was similar to PC (0.8 +/- 0.1, n = 14) but was significantly lower (P < 0.001) than PI (4.6 +/- 0.5, n = 14). The 32P-to-Pi ratio in PA was also lower (P < 0.02) than phosphatidylinositol phosphate and phosphatidylinositol bisphosphate. NE also increased [3H]PA twofold (P < 0.05) when PC was selectively labeled with [3H]myristic acid. These observations are more consistent with PA being formed from the hydrolysis of PC by phospholipase D (PLD) than by the phosphorylation of diacylglycerol produced by the action of phospholipase C. PLD was assayed by the formation of phosphatidylethanol (PEt) via a transphosphatidylation reaction with ethanol (half-maximal stimulation at 0.4-0.5% vol/vol). The time course for PLD stimulation by NE was similar to PA, with significant increases (P < 0.002) during 10 s to 30 min exposure. Once formed, PEt was degraded slowly, with a half time > 3 h. It is concluded that NE stimulates PLD in rat aorta, which forms a significant amount of PA from the hydrolysis of PC.(ABSTRACT TRUNCATED AT 250 WORDS)

Chlorpromazine increases the turnover of metabolically active phosphoinositides and elevates the steady-state level of phosphatidylinositol-4-phosphate in human platelets

Non-permeabilizing concentrations (< 40 microM) of chlorpromazine (CPZ) increase the radioactivity of phosphatidylinositol-4-phosphate (PIP) in platelets pre-labelled with [32P]Pi, but the biochemical mechanisms underlying this increase are poorly understood. Incubation of [32P]Pi-labelled, gel-filtered platelets with 25 microM CPZ for 10 min increased: (1) the mass of PIP from 315 to 476 nmol/10(11) platelets but not the total inositol phospholipid mass, (2) the specific phosphodiester radioactivities in phosphatidylinositol (PI), PIP and phosphatidylinositol-4,5-bisphosphate (PIP2) by 34, 63 and 37%, respectively, and (3) the specific phosphomonoester radioactivities in PIP and PIP2 by 53 and 10%, respectively. In control platelets (no CPZ) the specific radioactivity of the phosphodiester was the same in PI, PIP and PIP2, and the specific radioactivity in the phosphomonoester in PIP and PIP2 was 55% of that of the gamma-phosphoryl in ATP, measured as metabolically active, actin-bound ADP. These results suggest that 55% of each of PI, PIP and PIP2 constitutes a metabolic pool which is labelled by 32P in the platelets, while the remainder is in a metabolically inactive pool and not labelled. CPZ has two major effects: (1) CPZ interferes with the kinase and phosphohydrolase reactions that maintain the steady-state level of PIP in the metabolic phosphoinositide pool, resulting in a 92% increase in the PIP level of this pool, and (2) CPZ causes synthesis (45% in 10 min) of new phosphodiester in the metabolically active phosphoinositides by tentative stimulation of the turnover of the phosphoinositide cycle, de novo phosphoinositide synthesis and/or diacylglycerol formation through phospholipases C and D. The marked alteration by CPZ of phosphoinositide metabolism may be part of the mechanism by which this drug effects its psychotropic action.

Trichothecene mycotoxins inhibit phosphoinositide hydrolysis in bovine platelets stimulated with platelet activating factor

The effects of the trichothecene mycotoxins, acetyl T-2 toxin, T-2 toxin, HT-2 toxin, diacetoxyscirpenol (DAS), deoxynivalenol (DON) and T-2 tetraol on phospholipid turnover were determined in bovine platelets prelabelled with [1-14C]arachidonic acid (AA). In resting, non-stimulated platelets exposed to acetyl T-2 toxin, a marked decrease in [1-14C]phosphatidylinositol (PI) along with a marked increase in [1-14C]phosphatidic acid (PA) were observed, whereas T-2 toxin, and HT-2 toxin only induced a significant increase in [1-14C]PA. In contrast, in platelet activating factor (PAF)-stimulated platelets, the mycotoxins were found to suppress both the agonist-induced loss of [1-14C]PI and the appearance of [1-14C]PA with acetyl T-2 toxin being the most effective and T-2 toxin, HT-2 toxin, and DAS essentially equally effective. T-2 tetraol and DON did not affect phospholipid metabolism either in unstimulated or PAF stimulated platelets. The alterations in [1-14C]PI and [1-14C]PA suggest that the inhibitory toxins may activate a specific phospholipase C (PLC) in the unstimulated platelets and then impede further PLC activation in PAF-stimulated platelets.

BW755C or staurosporine inhibits collagen-stimulated phosphoinositide phosphorylation in platelets

Stimulation of platelets by collagen results in increased formation of the polyphosphoinositides, phosphatidylinositol phosphate (PtdInsP) and phosphatidylinositol bisphosphate (PtdInsP2) through stimulation of phosphoinositide kinase activities. We investigated a possible regulatory role of endogenous thromboxane formation and protein kinase C (PKC) activation in the induction of phosphoinositide phosphorylation following collagen stimulation, as well as following stimulation by the thromboxane mimetic, U-46619. Human platelets were prelabeled with [3H]inositol and stimulated with collagen (2 micrograms/mL) or U-46619 (1 microM), in the absence or presence of either the cyclo-oxygenase/lipoxygenase inhibitor, BW755C, or staurosporine, a putative inhibitor or PKC. Collagen stimulation resulted in a time-dependent increase in [3H]inositol-labeled PtdInsP and PtdInsP2 which was completely inhibited in the presence of BW755C. Addition of U-46619 to BW755C-treated, collagen-stimulated platelets restored the increased polyphosphoinositide formation. Stimulation of platelets with U-46619 alone also resulted in increased formation of [3H]PtdInsP and [3H]PtdInsP2, but this was not affected by the presence of BW755C. These results suggest that the collagen-induced activation of phosphoinositide kinases was dependent upon thromboxane formation, but that U-46619-induced phosphoinositide formation was rather independent of further thromboxane production. Pretreatment of platelets with staurosporine, prior to agonist addition, completely blocked the collagen-stimulated rise in radiolabeled PtdInsP and the U-46619-induced PtdInsP and PtdInsP2 generations, suggesting that protein kinase, possibly PKC, may play a role in the activation of phosphoinositide kinases by these agonists.

The novel inositol lipid phosphatidylinositol 3,4-bisphosphate is produced by human blood platelets upon thrombin stimulation

Radioactive PtdIns(3)P was detected in human platelets incubated with [32P]Pi, but remained unaffected by thrombin treatment. In contrast, [32P]PtdIns(3,4)P2 was absent from resting platelets, but was produced by thrombin-activated platelets in a dose- and time-dependent manner. [32P]PtdInsP3 was never found under these conditions. These changes are similar to those elicited in other cells by platelet-derived growth factor or the oncogene product pp60c-src.

Effect of dual agonists on phosphoinositide pools in WRK-1 cells

Both vasopressin and bradykinin activate the phosphoinositide cycle in WRK-1 rat mammary tumour cells. When the two agonists are added simultaneously, partial additivity is observed with respect to disappearance of prelabelled phosphoinositides and accumulation of inositol phosphates; no additivity is observed with respect to resynthesis of phosphatidylinositol as assessed by monitoring [32P]Pi incorporation. Lack of complete additivity can be explained, at least in part, by heterologous desensitization. In order to determine whether the two agonists were accessing a common or individual hormone-sensitive phosphoinositide pools, cells were incubated with [32P]Pi in the presence of either vasopressin or bradykinin and subsequently restimulated with the alternative agonist. The lipid pool labelled in the presence of either agonist was sensitive to subsequent treatment by the other ligand, suggesting a common phosphoinositide pool. However, when cells were incubated with [32P]Pi in the absence of agonists, the time course of labelling of the hormone-sensitive pool was different for bradykinin and vasopressin, with that for bradykinin becoming labelled within a much shorter time. Thus although there is a significant overlap between the phosphoinositide pools responding to vasopressin and bradykinin, there is a small fraction of the hormone-sensitive lipid which responds only to bradykinin.

Functional heterogeneity of polyphosphoinositides in human erythrocytes

After labelling of erythrocytes with [32P]P1 for 23 h, the specific radioactivities of the phosphomonoester groups of PtdIns4P and of PtdIns(4,5)P2 approached equilibrium values which were close to that of the gamma-phosphate of ATP (78-85%), showing that almost all of these phosphate groups were metabolically active. Phosphoinositidase C (PIC) activation, using Ca2+ and the ionophore A23187, of 32P-prelabelled erythrocytes was used to investigate a possible functional heterogeneity of the phosphoinositides. Hydrolysis of PtdIns(4,5)P2, measured from its radioactivity, decreased as function of the time of prelabelling up to a constant value equal to that measured from its content. In contrast, hydrolysis of PtdIns4P, determined both from radioactivity and from content, was always the same. These data suggest that newly labelled molecules of PtdIns(4,5)P2, initially accessible to PIC, then moved towards a PIC-resistant pool. This was further confirmed by measuring the fraction of labelled PtdIns(4,5)P2 molecules accessible to PIC after a prelabelling period of 5 min and different times of reincubation. Hydrolysis by PIC was also measured in erythrocytes in which the phosphoinositide content had been modified by activation (Mg2+-enriched cells) or inhibition (ATP-depleted cells) of the phosphoinositide kinases. The sizes of the PIC-resistant pools of polyphosphoinositides were not affected by these treatments, indicating that the kinases (and the phosphatases) act on the PIC-sensitive pools. This was also shown by the decrease in the production of Ins(1,4,5)P3 upon PIC activation in ATP-depleted erythrocytes. A model is presented in which the PIC-sensitive pools of polyphosphoinositides are those which are accessible to the kinases and the phosphatases and are rapidly turned over.

Evidence for tight metabolic control of the receptor-activated polyphosphoinositide cycle in human platelets

The [32P]PIP2/[32P]PA and the [32P]PIP/[32P]PA relationships were demonstrated to be remarkably similar after stimulation of [32P]Pi-prelabelled platelets for 90 s with various combinations and concentrations of agonists and inhibitors. Thus the activity of the PI and PIP kinases with the corresponding phosphomonoesterases may be tightly controlled during receptor-mediated platelet stimulation involving phospholipase C activation.

Platelet-derived-growth-factor-stimulated heterogeneous polyphosphoinositide metabolism and phosphate uptake in C3H fibroblasts

Pig platelet-derived growth factor (PDGF) increased the rate of [32P]Pi uptake by murine fibroblasts, resulting in a 3-9-fold elevation of the specific radioactivity of ATP, PtdInsP, PtdInsP2, PtdIns and phosphatidic acid. The specific radioactivity was 10-60-fold higher in ATP than in the four phospholipids. These substances are therefore not in metabolic equilibrium, which complicates determination of inositol phospholipid turnover.

Inositol lipid turnover and compartmentation in canine trachealis smooth muscle

We established conditions for the study of metabolism and compartmentation of inositol phospholipids in canine trachealis muscle. Unstimulated muscle was incubated with myo-[3H]inositol for 30 min at 37 degrees C which resulted in labeling of the tissue free myo-inositol pool, whereas only a small amount of radioactivity was incorporated into inositol phospholipids or inositol phosphates. After addition of 5.5 microM carbachol, phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PIP), and phosphatidylinositol-4,5-bisphosphate (PIP2), specific radioactivities increased exponentially, reaching apparent constant values in 180-240 min. Initial rates of increases in PI, PIP, and PIP2 specific radioactivities were 39, 32, and 66 times that measured in unstimulated muscle. Metabolic flux rates (nmol.100 nmol total lipid Pi-1.min-1) during development of force averaged 0.42 +/- 0.09 and during force maintenance averaged 0.14 +/- 0.01. Fractions of total PI, PIP, and PIP2 pools that were linked to muscarinic cholinergic activation were estimated to be 0.97, 0.85, and 0.65, respectively. Initial rates of increase in specific radioactivities and specific radioactivities during carbachol activation were similar in PI, PIP, and PIP2 fast active compartments, suggesting metabolic flux from PI to PIP to PIP2 was in near chemical equilibrium. Turnover times for PI, PIP, and PIP2 fast active compartments were estimated to be 21, 1.6, and 4.0 min, respectively.

Use of actin-bound adenosine 5'-diphosphate as a method to determine the specific 32P-radioactivity of the gamma-phosphoryl group of adenosine 5'-triphosphate in a highly compartmentalized cell, the platelet

Determination of the specific 32P-radioactivity of cytoplasmic ATP in 32P-Pi-labeled platelets is complicated by the presence of a large pool of metabolically inactive, granule-stored nucleotides. Moreover, our data show that the specific 32P-radioactivity of cytoplasmic ATP is severely underestimated when determined in platelets after the complete secretion of granule-stored nucleotides, possibly due to isotopic dilution with granule-stored phosphate. As F-actin-bound ADP is ethanol-insoluble, this pool can be readily separated from the other nucleotide pools in platelets. Here we show that the specific 32P-radioactivity of F-actin-bound ADP accurately reflects that of the gamma-phosphoryl group of cytoplasmic ATP. During uptake of 32P-Pi by human platelets the specific 32P-radioactivity of F-actin-bound ADP equals that of the monoester phosphates of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, which are in metabolic equilibrium with cytoplasmic ATP. Therefore, this method enables the determination of the specific 32P-radioactivity of the gamma-phosphoryl group of cytoplasmic ATP in platelets even under short-term labeling conditions.

Calcium modulates the generation of inositol 1,3,4-trisphosphate in human platelets by the activation of inositol 1,4,5-trisphosphate 3-kinase

We observed that more total inositol trisphosphate (InsP3) was formed when human platelets were stimulated with agonists (15-hydroxy-9,11-azo-prosta-5,13-dienoic acid or thrombin) in the presence of extracellular Ca2+ than in its absence. Analysis of the InsP3 by h.p.l.c. indicated that the increased InsP3 formed in the presence of extracellular Ca2+ was primarily the 1,3,4-trisphosphate [Ins(1,3,4)P3]. In addition, more inositol 1,3,4,5-tetrakisphosphate (InsP4) was formed in the presence of extracellular Ca2+. Experiments conducted with electrically permeabilized platelets demonstrated that conversion of [3H]Ins(1,4,5)P3 to [3H]InsP4 in platelets was Ca2+-dependent, with half-maximal conversion observed at approx. 2.5 microM-Ca2+. By contrast, dephosphorylation of [3H]InsP4 to [3H]Ins(1,3,4)P3 was not activated by Ca2+. A partially purified preparation of Ins(1,4,5)P3 3-kinase from human platelets was found to be insensitive to Ca2+, but addition of calmodulin restored Ca2+-sensitivity to the kinase, increasing its activity about 5-fold. These results show that in human platelets the metabolism of Ins(1,4,5)P3 is regulated by Ca2+-calmodulin, and suggest that the metabolites of Ins(1,4,5)P3 may also have important second-messenger functions in platelets, and are consistent with the hypothesis that the activation of phospholipase C is not dependent on extracellular Ca2+.

Rates of production and consumption of phosphatidic acid upon thrombin stimulation of human platelets

Human platelets were labelled with [32P]Pi and [3H]glycerol before gel filtration. In unstimulated cells, the specific 32P radioactivity in phosphatidic acid (PtdOH) was similar to that of phosphatidylinositol (PtdIns) but only 4% of that of the gamma-phosphate of ATP. Upon 3 min of stimulation with 0.5 U/ml of thrombin, there was a 20-fold increase in specific 32P radioactivity of PtdOH which approached that of the ATP gamma-phosphate. Based on constant rates of synthesis and removal, this thrombin-induced increase in specific 32P radioactivity in PtdOH allowed us to calculate the flux of phosphate through PtdOH upon stimulation. Synthesis and removal occurred at rates of 107 and 52 nmol min-1/10(11) cells, respectively. The specific [3H]glycerol radioactivity was similar in PtdIns, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate in unstimulated platelets. In PtdOH, it was 50% of that of the inositol phospholipids. Thrombin stimulation induced no changes in the specific 3H radioactivity of the inositol phospholipids whereas specific [3H]PtdOH increased to the level of these lipids. It is concluded that PtdIns, PtdInsP and PtdInsP2 exist in a metabolic homogenous pool in human platelets.