The separation of myo-inositol phosphates by ion-pair chromatography

Leptin stimulates endogenous cholesterol synthesis in human monocytes: New role of an old player in atherosclerotic plaque formation. Leptin-induced increase in cholesterol synthesis

The role of leptin in the pathomechanism of atherosclerosis, through its free radical generating ability is established. Its effect however, on the regulation of intracellular cholesterol synthesis has not been studied. The aim of the present study was to elucidate whether leptin influences endogenous cholesterol synthesis in monocytes. Furthermore, leptin signaling to HMG CoA reductase in control and hypercholesterolemic monocytes were compared. The in vitro effect of leptin was studied on freshly isolated human monocytes obtained from healthy control volunteers and patients with hypercholesterolemia. Our results can be summarized as follows: (1) Leptin is able to increase endogenous cholesterol synthesis in human monocytes in vitro. (2) The cholesterol synthesis increasing effect of the hormone is more pronounced in hypercholesterolemic monocytes with high basal cholesterol biosynthesis. (3) The leptin-induced Ca(2+) signal was involved in the enhancement of HMG CoA reductase activation in monocytes from both controls and hypercholesterolemic patients. (4) In control monocytes the Ca(2+) signal originated from intracellular pools, whereas in patients, Ca(2+)-influx and protein kinase C activation were found to be responsible for the leptin-effect. Mevalonate cycle inhibiting fluvastatin and 25-hydroxycholesterol decreased cholesterol production in leptin-stimulated monocytes. Our present study provides the first proof of the cholesterol synthesis enhancing effect of leptin through a statin-sensitive pathway in circulating monocytes. Furthermore our results suggest that leptin can be involved in the pathomechanism of atherosclerotic plaque formation also through its effect on cholesterol biosynthesis in monocytes.

Crosstalk of sterol-dependent and non-sterol-dependent signaling in human monocytes after in vitro addition of LDL

The aim of the present study was to investigate low density lipoprotein (LDL)-induced, non-sterol-dependent signaling and its possible role in cholesterol balance. LDL in 10 microg ml(-1) concentration could induce inositol trisphosphate (IP3) and Ca2+ signal generation through a pertussis toxin (PT) sensitive G protein in human monocytes. The increase in [Ca2+]i was derived from the intracellular pools. LDL also induced activation and translocation of protein kinase C (PKC) into the cell membrane, by processes, which were significantly inhibited in the first 20 min by preincubation with PT and PKC-inhibitor H-7. The PKC-activating phorbol-12-myristate-13-acetate (PMA), differently from LDL, enhanced the LDL-receptor (LDL-R)-mediated binding and degradation of [125I]LDL, but inhibited endogenous cholesterol synthesis, and both effects were inhibited by H-7. The LDL-induced inhibition of binding and degradation of [125I]LDL was not affected by H-7, whereas decreased cholesterol synthesis was counteracted by H-7. These results suggest the existence of a non-sterol-dependent signal pathway of LDL-Rs, by which endogenous cholesterol synthesis, that is, the [14C]acetate incorporation, is regulated through PKC activation.

Altered signal pathway in angiotensin II-stimulated neutrophils of patients with hypercholesterolaemia

Angiotensin II (AII) in 1-10 nM concentrations has an in vivo immunostimulating effect on human neutrophils. The release of superoxide anions and leukotrienes (LTs) is significantly increased by 10 nM AII-stimulated neutrophils of patients with hypercholesterolaemia (HCH). These oxidizing agents may be involved in the damage of vessel walls, i.e., in atherosclerotic plaque formation. To clarify the receptor types and signal pathways in neutrophils of healthy controls and patients, inositol trisphosphate (IP(3)) production and Ca(2+) signalling were studied. Neutrophils were pretreated before AII stimulation with different inhibitory drugs. In control cells, the stimulation occurred predominantly through pertussis toxin-sensitive, type angiotensin 1 receptors. This induced IP(3) production and Ca(2+) signalling from intracellular pools. In neutrophils of hypercholesterolaemic patients, the enhanced release of oxidizing agents was dependent more on type angiotensin 2 than type angiotensin 1 receptors. After stimulation, there was no IP(3) production detected. The Ca(2+) signalling was lower than in control cells and was dependent on extracellular Ca(2+).

Metabolism of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate by the oocytes of Xenopus laevis

The pathway and kinetics of inositol 1,4,5-trisphosphate (IP3) metabolism were measured in Xenopus laevis oocytes and cytoplasmic extracts of oocytes. Degradation of microinjected IP3 in intact oocytes was similar to that in the extracts containing comparable concentrations of IP3 ([IP3]). The rate and route of metabolism of IP3 depended on the [IP3] and the intracellular free Ca2+ concentration ([Ca2+]). At low [IP3] (100 nM) and high [Ca2+] (>/=1 microM), IP3 was metabolized predominantly by inositol 1,4, 5-trisphosphate 3-kinase (3-kinase) with a half-life of 60 s. As the [IP3] was increased, inositol polyphosphate 5-phosphatase (5-phosphatase) degraded progressively more IP3. At a [IP3] of 8 microM or greater, the dephosphorylation of IP3 was the dominant mode of IP3 removal irrespective of the [Ca2+]. At low [IP3] and low [Ca2+] (both </=400 nM), the activities of the 5-phosphatase and 3-kinase were comparable. The calculated range of action of IP3 in the oocyte was approximately 300 micron suggesting that IP3 acts as a global messenger in oocytes. In contrast to IP3, inositol 1,3,4, 5-tetrakisphosphate (IP4) was metabolized very slowly. The half-life of IP4 (100 nM) was 30 min and independent of the [Ca2+]. IP4 may act to sustain Ca2+ signals initiated by IP3. The half-life of both IP3 and IP4 in Xenopus oocytes was an order of magnitude or greater than that in small mammalian cells.

Altered postreceptor signal transduction of formyl-Met-Leu-Phe receptors in polymorphonuclear leukocytes of patients with non-insulin-dependent diabetes mellitus

The signal transduction of the formyl-Met-Leu-Phe (FMLP) receptor in polymorphonuclear leukocytes (PMNLs) from patients with non-insulin-dependent diabetes mellitus (NIDDM) was compared to that of PMNLs obtained from healthy volunteers. According to our previous studies in this group of patients neither the decrease in insulin binding capacity nor the enhanced insulin-degrading enzyme activity was involved. In control PMNLs, 10 nM FMLP induced a pertussis toxin-sensitive increase in phosphatidyl inositol (PI) cleavage and a subsequent Ca2+ signaling from the intracellular pools. On the other hand, the FMLP-induced protein kinase C (PKC) activation and translocation into the membrane could not be detected in these cells via the measurement of 32P incorporation into histone. In contrast, in PMNLs of this special group of patients suffering from NIDDM the FMLP stimulus produced a significantly low increase in PI cleavage and Ca2+ signaling from the intracellular pools. Moreover, in resting PMNLs of these patients with NIDDM, not only the [Ca2+]i but also the membrane-bound PKC activity was found to be significantly increased. In addition, PKC translocation into the cell membrane of diabetic PMNLs could be further triggered with FMLP as judged by the measurement of 32P incorporation into histone. Based on these results, it appears that the signaling of FMLP receptors in PMNLs of some NIDDM patients may have an alternative pathway through Ca2+ influx from extracellular medium, arachidonic acid cascade, and PKC activation.

Range of messenger action of calcium ion and inositol 1,4,5-trisphosphate

The range of messenger action of a point source of Ca2+ or inositol 1,4,5-trisphosphate (IP3) was determined from measurements of their diffusion coefficients in a cytosolic extract from Xenopus laevis oocytes. The diffusion coefficient (D) of [3H]IP3 injected into an extract was 283 microns 2/s. D for Ca2+ increased from 13 to 65 microns 2/s when the free calcium concentration was raised from about 90 nM to 1 microM. The slow diffusion of Ca2+ in the physiologic concentration range results from its binding to slowly mobile or immobile buffers. The calculated effective ranges of free Ca2+ before it is buffered, buffered Ca2+, and IP3 determined from their diffusion coefficients and lifetimes were 0.1 micron, 5 microns, and 24 microns, respectively. Thus, for a transient point source of messenger in cells smaller than 20 microns, IP3 is a global messenger, whereas Ca2+ acts in restricted domains.

Production of inositol phosphates and reactive oxygen metabolites in quartz-dust-stimulated human polymorphonuclear leukocytes

The present paper explores phosphoinositide turnover in quartz-stimulated human polymorphonuclear leukocytes. Separation of inositol phosphates was carried out with a new ion-pair, reverse-phase high performance liquid chromatographic method applying a gentle tetrabutyl ammonium phosphate buffer gradient. The method separates inositol monophosphates, inositol 1,4-bisphosphate, inositol trisphosphates and inositol 1,3,4,5-tetrakisphosphate. Reactive oxygen metabolites, indices for leukocyte activation, were measured with a luminometric assay. Quartz increased the production of reactive oxygen metabolites, preceded by facilitated inositol phosphate turnover. This finding provides evidence that inositol phosphate second messengers may be involved in quartz-induced leukocyte activation and subsequent production of reactive oxygen metabolites.

Regulation of endothelin-mediated calcium mobilization in vascular smooth muscle cells by isoproterenol

We have investigated the effect of isoproterenol on endothelin-induced Ca2+ mobilization in A10 vascular smooth muscle cells. Endothelin (ET) stimulates a rapid and sustained elevation of intracellular Ca2+ mediated by production of inositol phosphates, release of intracellular Ca2+, and activation of a plasmalemmal Ca2+ influx pathway. This influx pathway appears to be a L-type channel because it is inhibited by nicardipine and activated by BAY K 8644. Depolarization of the cells, by elevating extracellular K+, activated a pharmacologically similar channel and produced a similar change in intracellular Ca2+ concentration. Preincubation of cells with isoproterenol reduced the peak Ca2+ response to endothelin and blocked the sustained elevation. However, isoproterenol did not alter K(+)-induced Ca2+ entry. Thus it appears that ET-induced entry is mediated by intracellular signals and not by depolarization. With the use of cells incubated in Ca2(+)-free medium containing 1 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, isoproterenol was shown to inhibit Ca2+ release from intracellular pools by 36 +/- 3%. Furthermore, isoproterenol pretreatment or addition of adenosine 3',5'-cyclic monophosphate (cAMP) to saponin-permeabilized cells inhibited inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]-induced Ca2+ release from intracellular sites. Similar effects were seen with forskolin. Propranolol reversed the inhibitory effects of isoproterenol. Isoproterenol pretreatment also inhibited the rapid formation of Ins(1,4,5)P3 and [2-3H]inositol 1,3,4,5-tetrakisphosphate stimulated by endothelin and reduced the sustained formation of these compounds. Finally, isoproterenol and forskolin led to a greater than 10-fold increase in intracellular cAMP levels. This stimulation of adenylate cyclase by isoproterenol was completely blocked by propranolol. It appears then that the beta-agonist isoproterenol interacts with a beta-adrenergic receptor, elevates cAMP, and thereby alters endothelin-induced Ca2+ mobilization. Inhibition of Ins(1,4,5)P3 formation, reduction in the responsiveness of the Ins(1,4,5)P3 intracellular receptor, and perhaps inhibition of ET-induced Ca2+ entry appear to be involved.

Kinetics of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate generation in dog-thyroid primary cultured cells stimulated by carbachol

The action of carbachol on the generation of inositol trisphosphate and tetrakisphosphate isomers was investigated in dog-thyroid primary cultured cells radiolabelled with [3H]inositol. The separation of the inositol phosphate isomers was performed by reverse-phase high pressure liquid chromatography. The structure of inositol phosphates co-eluting with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] standards was determined by enzymatic degradation using a purified Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatase. The data indicate that Ins(1,3,4,5)P4 was the only [3H]inositol phosphate which co-eluted with a [32P]Ins(1,3,4,5)P4 standard, whereas 80% of the [3H]InsP3 co-eluting with an Ins(1,4,5)P3 standard was actually this isomer. In the presence of Li+, carbachol led to rapid increases in [3H]Ins(1,4,5)P4. The level of Ins(1,4,5)P3 reached a peak at 200% of the control after 5-10 s of stimulation and fell to a plateau that remained slightly elevated for 2 min. The level of Ins(1,3,4,5)P4 reached its maximum at 20s. The level of inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] increased continuously for 2 min after the addition of carbachol. Inositol-phosphate generation was also investigated under different pharmacological conditions. Li+ largely increased the level of Ins(1,3,4)P3 but had no effect on Ins(1,4,5)P3 and Ins(1,3,4,5)P4. Forskolin, which stimulates dog-thyroid adenylate cyclase and cyclic-AMP accumulation, had no effect on the generation of inositol phosphates. The absence of extracellular Ca2+ largely decreased the level of Ins(1,3,4,5)P4 as expected considering the Ca2(+)-calmodulin sensitivity of the Ins(1,4,5)P3 3-kinase. Staurosporine, an inhibitor of protein kinase C, increased the levels of Ins(1,4,5)P3, Ins(1,3,4,5)P4 and Ins(1,3,4)P3. This supports a negative feedback control of diacyglycerol on Ins(1,4,5)P3 generation.

High-performance reversed-phase ion-pair chromatographic study of myo-inositol phosphates. Separation of myo-inositol phosphates, some common nucleotides and sugar phosphates

A detailed study of all the major chromatographic variables affecting the retention behaviour and separation of myo-inositol phosphates in reversed-phase ion-pair chromatographic systems was carried out. The parameters studied included the eluent concentration of the pairing ion, the eluent concentration of the organic modifier and the buffer salt, the pH of the eluent, the minimum column plate count necessary for the separation of the inositol trisphosphate isomers and isocratic and gradient modes of separation. The retention behaviour of some common nucleotides and sugar phosphates was also investigated as these phosphates present chromatographic interference problems in biochemical studies based on the cellular incorporation of [32P]Pi. The separation methods developed appear to be superior to established anion-exchange separation techniques in terms of separation speed and "mildness" of the chromatographic conditions.

Inositol metabolism during neuroblastoma B50 cell differentiation: effects of differentiating agents on inositol uptake

Inositol uptake was studied in the rat CNS neuroblastoma B50 cell line. Eadie-Hofstee analysis of the uptake pattern reveals two defined modes of inositol entry into the cell. The high-affinity uptake component requires the presence of extracellular sodium and is inhibited by phloridzin. Analysis of the uptake velocities of the high-affinity uptake component provided the following apparent kinetic parameters: Km = 13.7 microM and Vmax = 14.7 pmol/mg of protein/min (without correcting for residual diffusion) and Km = 12.9 microM and Vmax = 12.3 pmol/mg of protein/min (with correction). At physiological concentrations, the high-affinity transport process contributes approximately 70% to total uptake; the remainder is due to a low-affinity diffusion-like process. Uptake inhibition studies reveal that the uptake process is sensitive to ouabain, amiloride, and dichlorobenzamil inhibition but relatively insensitive to cytochalasin B or phloretin. When neuroblastoma B50 cells are induced to differentiate morphologically with high extracellular calcium or with dibutyryl cyclic AMP, a significant decrease in inositol uptake is observed. The dibutyryl cyclic AMP-mediated inhibition of uptake affects only the high-affinity uptake component and is noncompetitive in nature. The high extracellular calcium-mediated inhibition is less specific; it involves "disappearance" of the high-affinity process, some inhibition of the low-affinity process, and an increase of inositol efflux. The significance of these observations is discussed in the context of neuroblastoma B50 cell differentiation.

Ion-pair chromatography of inositol polyphosphates with N-methylimipramine

A novel counter-ion, N-methylimipramine, was synthesized and utilized in the separation of inositol phosphates by ion-pair chromatography. The structural identity of the counter-ion was documented by nuclear magnetic resonance spectroscopy. This counter-ion was capable of resolving inositol phosphates isocratically by reversed-phase high-performance liquid chromatography. Solvent polarity and ionic strength markedly affected the retention of the polyphosphorylated inositides. pH, however, was less significant in its effects. Injection of inositol trisphosphate paired to N-methylimipramine into a mobile phase containing tetrabutylammonium ions demonstrated free exchange of the inositide between the counter-ions. This counter-ion may therefore prove useful in defining empirically the mechanisms of ion-pair chromatography.

The inositol 1,4,5-trisphosphate receptor binding sites of platelet membranes. pH-dependency, inhibition by polymeric sulphates, and the possible presence of arginine at the binding site

The present study was initiated to characterize the inositol 1,4,5-trisphosphate (InsP3)-binding site in human platelets that is involved in Ca2+ release. InsP3 binding to platelet membranes was measured in two ways; (1) by displacement of labelled InsP3 with unlabelled InsP3, as in previous studies, and (2) directly, using only radioactive InsP3 as ligand, over the concentration range 0.25-100 nM. At physiological pH (7.1) the binding data were best fitted by a model for a single saturable binding site, with KD = 11.8 nM and Bmax. = 1.4 pmol/mg of protein. At alkaline pH values (8.3 and 9.4) binding was best fitted by a two-site model, the second site being of higher affinity (KD = 0.75-1.2 nM) but lower concentration (Bmax. = 0.195-0.6 pmol/mg of protein). All binding of InsP3 was blocked by polymeric sulphates (heparin, dextran sulphate, polyvinyl sulphate) regardless of pH. The specific arginine-modifying reagent p-hydroxyphenylglyoxal irreversibly blocked InsP3 binding, suggesting the presence of arginine at the recognition site for InsP3 binding. NN'-dicyclohexylcarbodi-imide (DCCD) and 1-ethyl-3-(3-dimethylaminopropyl)carbodi-imide (ECCD), which are carboxy-group-specific reagents, blocked Ca2+ release, but not InsP3 binding, indicating the existence of another site that regulates Ca2+ release apart from the active centre for InsP3.

Methods for the analysis of inositol phosphates

Interest in the inositol phospholipids was stimulated by the simultaneous discoveries that the products of hydrolysis of these lipids could serve as messengers to activate to synergistic signaling pathways in hormonally responsive cells, namely, inositol 1,4,5-trisphosphate which causes the release of Ca2+ from intracellular stores and diacylglycerol which promotes the activation of protein kinase C. At the same time, Berridge and co-workers introduced relatively simple approaches to study the inositol phospholipid cycle. These included the use of [3H]inositol to label the inositol metabolites, all of which are confined to this cycle, and of Li+ to decrease the rate of degradation of the inositol phosphates. Water-soluble inositol phosphates and chloroform-soluble inositol phospholipids could then be separated by solvent partition and the inositol phosphates further separated by use of an anion-exchange resin. However, the subsequent application of high-performance liquid chromatography as a separation technique indicated the existence of many isomers of the inositol phosphates formed by different pathways of dephosphorylation and phosphorylation. Mapping of these metabolic pathways may be substantially complete, but novel pathways may still be discovered. We review both old and new methods of analysis of the inositol phosphates for the measurement of mass and radioactivity. Although the complexity of the cycle sometimes demands the use of sophisticated methods of separation and rigorous identification, older and inexpensive methods may still be useful for some purposes.

Automated isocratic high-performance liquid chromatography of inositol phosphate isomers

Isomers of inositol phosphates from biological samples can be analysed by anion-exchange h.p.l.c., by using isocratic elution with phosphate buffers. The method involves the preliminary processing of the extracted samples with conventional soft-gel anion-exchange resins, including the commonly used Dowex resins, followed by direct analysis with h.p.l.c. of a portion of relevant fractions. Run times (up to 20 min) and collected fraction numbers (up to 24) are minimal, so that if the method is used in conjunction with automated h.p.l.c. injection a high throughput of samples is achieved.

The separation of [32P]inositol phosphates by ion-pair chromatography: optimization of the method and biological applications

We have developed an ion-pair reverse-phase HPLC method to measure inositol phosphates in 32P-labeled cells. The different chromatographic parameters were analyzed to optimize the resolution of the 32P-labeled metabolites. Analysis of inositol phosphates in biological samples was improved by a single charcoal pretreatment which eliminated interfering nucleotides without removing inositol phosphates. The kinetics of production of inositol phosphates in calcium-activated erythrocytes, vasopressin-stimulated hepatocytes, and thrombin-activated platelets were analyzed. Original data on the activation of phosphoinositide phospholipase C were obtained in intact erythrocytes by direct measurement of inositol (1,4,5)P3. Data from agonist-stimulated hepatocytes and platelets were consistent with those from previous studies. In conclusion, this technique offers many advantages over the methodologies currently employed involving anion-exchange chromatography and [3H]inositol labeling: (i) 32P labeling is less expensive and more efficient than 3H labeling and can be used with all types of cells without permeabilization treatments and (ii) ion-pair HPLC gives good resolution of inositol phosphates from nucleotides with shorter retention times, and long reequilibration periods are not required.