Metabolism of D-myo-inositol 1,3,4,5-tetrakisphosphate by rat liver, including the synthesis of a novel isomer of myo-inositol tetrakisphosphate

A tale of two inositol trisphosphates

Between spring 1982 and autumn 1984 the physiological role of Ins(1,4,5)P3 as a calcium-mobilizing second messenger was first suggested and then experimentally established. At the same time the unexpected complexity of inositide metabolism began to be exposed by the discovery of Ins(1,3,4)P3. This article recalls my entanglement with these two inositol phosphates.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Defining signal transduction by inositol phosphates

Ins(1,4,5)P(3) is a classical intracellular messenger: stimulus-dependent changes in its levels elicits biological effects through its release of intracellular Ca(2+) stores. The Ins(1,4,5)P(3) response is "switched off" by its metabolism to a range of additional inositol phosphates. These metabolites have themselves come to be collectively described as a signaling "family". The validity of that latter definition is critically examined in this review. That is, we assess the strength of the hypothesis that Ins(1,4,5)P(3) metabolites are themselves "classical" signals. Put another way, what is the evidence that the biological function of a particular inositol phosphate depends upon stimulus dependent changes in its levels? In this assessment, examples of an inositol phosphate acting as a cofactor (i.e. its function is not stimulus-dependent) do not satisfy our signaling criteria. We conclude that Ins(3,4,5,6)P(4) is, to date, the only Ins(1,4,5)P(3) metabolite that has been validated to act as a second messenger.

How versatile are inositol phosphate kinases?

This review assesses the extent and the significance of catalytic versatility shown by several inositol phosphate kinases: the inositol phosphate multikinase, the reversible Ins(1,3,4) P (3)/Ins(3,4,5,6) P (4) kinase, and the kinases that synthesize diphosphoinositol polyphosphates. Particular emphasis is placed upon data that are relevant to the situation in vivo. It will be shown that catalytic promiscuity towards different inositol phosphates is not typically an evolutionary compromise, but instead is sometimes exploited to facilitate tight regulation of physiological processes. This multifunctionality can add to the complexity with which inositol signalling pathways interact. This review also assesses some proposed additional functions for the catalytic domains, including transcriptional regulation, protein kinase activity and control by molecular 'switching', all in the context of growing interest in 'moonlighting' (gene-sharing) proteins.

Divergent syntheses of all possible optically active regioisomers of myo-inositol tris- and tetrakisphosphates

Since the discovery of D-myo-inositol 1,4,5-trisphosphate, which plays a pivotal role as a second messenger in transmembrane signaling, the scope of the phosphoinositide-based signaling processes has been continually expanding. However, the clear understanding of the molecular signal transduction mechanisms including the functions of newly found IP(n) is still lacking. As a continuing effort to our previously reported syntheses of all possible 39 optically inactive regioisomers of myo-inositol phosphates (IP(n); n = 1-6), we synthesized all possible optically active regioisomers of myo-IP(3) and myo-IP(4) using chiral IBz(3)s and IBz(2)s, respectively. A series of procedures involving CRL-catalyzed enzymatic resolution of racemic 1,2:5,6-di-O-isopropylidene-myo-inositol and base-catalyzed benzoyl migration in tri- and dibenzoyl-isopropylidene-myo-inositol afforded eight enantiomeric pairs of IBz(3) and six enantiomeric pairs of IBz(2), respectively. Phosphorylation of these intermediates by the phosphitylation and oxidation procedure gave the target products.

High affinity association of myo-inositol trisphosphates with phytase and its effect upon the catalytic potential of the enzyme

A neutral phytase from germinating mung bean (Vigna radiata) seeds dephosphorylates myo-inositol hexakisphosphate sequentially to myo-inositol. The enzyme also binds with higher affinity to myo-inositol trisphosphates (1,4,5), (2,4,5), and (1,3,4) isomers without catalysis. The high affinity complex elicits Ca(2+) mobilization in vitro from microsomes/vacuoles via the formation of a ternary complex with the receptor for Ins(1,4,5)P(3). As a sequel to our previous report, we have carried out a detailed characterization of the two sites and examined the mutual interactions between them. Presaturation of the high affinity site leads to an increase in the affinity of the enzyme for phytic acid and its rate of dephosphorylation as well. From the products of limited tryptic cleavage of phytase, two peptides, each with one activity, have been isolated. The larger peptide ( approximately 66 kDa) contains the catalytic site, and the smaller peptide ( approximately 5 kDa) has the high affinity myo-inositol trisphosphate-binding site. The interaction between the dual activities of phytase has been observed also at the level of the two peptides. A sequence homology search using N-terminal 12 amino acid residues of the 5-kDa fragment has revealed significant homology with the Homer class of proteins implicated in signaling pathways involving metabotropic glutamate receptor and myo-inositol 1,4,5-trisphosphate receptor. These results indicate a second role of phytase in Ca(2+) mobilization during germination of mung been seed via a salvage pathway that involves allosteric activation by myo-inositol trisphosphate.

Inositol 1,3,4-trisphosphate 5/6-kinase is a protein kinase that phosphorylates the transcription factors c-Jun and ATF-2

Phosphorylation of inositol 1,3,4-trisphosphate by inositol 1,3,4-trisphosphate 5/6-kinase is the first committed step in the formation of higher phosphorylated forms of inositol. We have shown that the eight proteins called the COP9 signalosome complex copurify with calf brain 5/6-kinase. Because the complex has been shown to phosphorylate c-Jun in vitro, we tested both the complex and 5/6-kinase and found that both are able to phosphorylate c-Jun and ATF-2 on serine/threonine residues. These findings establish a link between two major signal transduction systems: the inositol phosphates and the stress response system.

Synthesis of the enantiomers of 6-deoxy-myo-inositol 1,3,4,5-tetrakisphosphate, structural analogues of myo-inositol 1,3,4,5-tetrakisphosphate

D-myo-Inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] is produced rapidly from the established second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P4] in stimulated cells. Despite extensive investigations, in particular concerning its potential role in mediating cellular Ca2+ influx, no exact cellular function has been described for this inositol phosphate; however, binding sites have been identified in a number of tissues and it has been shown to act synergistically with Ins(1,4,5)P3. To assist in the elucidation of the mechanism of action and structural requirements within the Ins(1,3,4,5)P4 moiety that are necessary for recognition and activation of the receptor, structural analogues of this tetrakisphosphate are required. Routes for the synthesis of racemic 6-deoxy-myo-inositol 1,3,4,5-tetrakisphosphate [6-deoxy-DL-Ins(1,3,4,5)P4] and the chiral antipodes D- and L-6-deoxy-myo-inositol 1,3,4,5-tetrakisphosphate are described here. The racemic tetrakisphosphate was synthesised from DL-1,2-O-isopropylidene-myo-inositol in eight steps. Deoxygenation at C-6 was achieved following the Barton-McCombie procedure. Both chiral tetrakisphosphates were synthesised through resolution of racemic cis-diol 6-deoxy-1,4,5-tri-O-p-methoxybenzyl-myo-inositol with the chiral auxiliary (S)-(+)-O-acetylmandelic acid. Absolute configuration was confirmed by synthesis of the known D-6-deoxy-myo-inositol. Both D-6-deoxy-Ins(1,3,4,5)P4 and its enantiomer will be useful tools to unravel the enigmatic role of Ins(1,3,4,5)P4 in the polyphosphoinositide pathway of signal transduction.

Characterization of a cDNA encoding Arabidopsis thaliana inositol 1,3,4-trisphosphate 5/6-kinase

We have sequenced and recombinantly expressed as a fusion protein an expressed sequence tag clone (GB Z25963) from Arabidopsis thaliana that represents the plant homologue of human inositol 1,3,4 trisphosphate 5/6-kinase. The 1365 base pair clone has an open reading frame of 960 base pairs that predicts a protein product of 36.2 kDa, with a pI of 6.1. There is no polyadenylation signal or poly (A) tail, suggesting that additional 3' sequence remains to be identified. The amino acid sequence is 30% identical to the human protein. There are several short regions with particularly high degrees of identity between the human and Arabidopsis protein sequences, and these may be useful in identifying the active site of the enzyme. The expressed sequence tag was expressed as a fusion protein in Escherichia coli, with a carboxyl terminal deletion removing one region of high identity between the two proteins. The protein product of this construct was found to have inositol 1,3,4-trisphosphate 5/6-kinase activity. The Arabidopsis enzyme produced both inositol 1,3,4,6-tetrakisphosphate and inositol 1,3,4,5-tetrakisphosphate as products in a ratio of 1:3, in contrast with the human enzyme which gives a product ratio of 3:1.

Isolation of inositol 1,3,4-trisphosphate 5/6-kinase, cDNA cloning and expression of the recombinant enzyme

Inositol 1,3,4-trisphosphate 5/6-kinase was purified 12,900-fold from calf brain using chromatography on heparin-agarose and affinity elution with inositol hexakisphosphate. The final preparation contained proteins of 48 and 36-38 kDa. All of these proteins had the same amino-terminal sequence and were enzymatically active. The smaller species represent proteolysis products with carboxyl-terminal truncation. The Km of the enzyme for inositol 1,3,4-trisphosphate was 80 nM with a Vmax of 60 nmol of product/min/mg of protein. The amino acid sequence of the tryptic peptide HSKLLARPAGGLVGERTCNAXP matched the protein sequence encoded by a human expressed sequence tag clone (GB T09063) at 16 of 22 residues. The expressed sequence tag clone was used to screen a human fetal brain cDNA library to obtain a cDNA clone of 1991 base pairs (bp) that predicts a protein of 46 kDa. The clone encodes the amino-terminal amino acid sequence obtained from the purified calf brain preparation, suggesting that it represents its human homologue. The cDNA was expressed as a fusion protein in Escherichia coli and was found to have inositol 1,3,4-trisphosphate 5/6-kinase activity. Remarkably, both the purified calf brain and recombinant proteins produced both inositol 1,3,4,6-tetrakisphosphate and inositol 1,3,4,5-tetrakisphosphate as products in a ratio of 2.3-5:1. This finding proves that a single kinase phosphorylates inositol in both the D5 and D6 positions. Northern blot analysis identified a transcript of 3.6 kilobases in all tissues with the highest levels in brain. The composite cDNA isolated contains 3054 bp with a poly(A) tail, suggesting that 500-600 bp of 5' sequence remains to be identified.

Interaction of myoinositoltrisphosphate-phytase complex with the receptor for intercellular Ca2+ mobilization in plants

One of the myoinositol trisphosphates produced by the phytase-myoinositol hexakisphosphate (InsP6) reaction is Ins(2,4,5)P3. That Ins(2,4,5)P3 can elicit Ca2+ mobilization from intracellular stores in plants [Samanta, S., Dalal, B., Biswas, S., & Biswas, B.B.(1993) Biochem. Biophys. Res. Commun. 191,427] prompted us to elucidate the mechanism. The InsP3 [Ins(1,4,5)P3/Ins(2,4,5)P3]-phytase complex has been found to interact with the receptor for InsP3 in vitro forming a ternary complex, and a nanomolar concentration of InsP3 is required. For enzymatic cleavage of InsP3 by phytase, micromolar concentrations are needed, and the affinities of the phytase for different myoinositol phosphates have been found to depend upon the number of phosphate groups present in the substrate. Fraction accessibility of tryptophan residues to a neutral fluorescence quencher, acrylamide in free and myoinositol phosphates bound phytase, as determined by Stern-Volmer plot, records a progressive decrease starting from InsP6 to InsP with the notable exceptions of both Ins (1,4,5)P3 and Ins(2,4,5)P3. This deviation from the trend of change in the accessibility of tryptophan residues in myoinositol phosphate bound phytase is recorded from the fact that there is a high affinity (dissociation constant of the nanomolar order) and noncatalytic binding site in phytase for the two isomers of InsP3. In the nanomolar range of concentrations, both isomers of InsP3 bind to a second site of phytase having about 40-fold higher affinity than the normal substrate binding site. InsP3, when bound to noncatalytic site in phytase is not hydrolyzed but induces a significant change in the conformation of phytase as assayed from the relative accessibility of tryptophan residues. This conformational change in phytase is recognized by the receptor for InsP3, because in absence of InsP3 no interaction between the receptor and phytase is detected. However, InsP3-phytase complex is a better elicitor of Ca2+ efflux from microsomal/vacuolar fractions than free InsP3. This is further confirmed by the fact that when Ins(1,3,4)P3-phytase complex can elicit Ca2+ efflux from intracellular stores, Ins(1,3,4)P3 per se is minimally effective.

The stimulatory effect of L-glutamate and related agents on inositol 1,4,5-trisphosphate production in the cestode Hymenolepis diminuta

The effects of L-glutamate, acetylcholine, and serotonin (5HT) were examined on generation of inositol 1,4,5-triphosphate [Ins(1,4,5)P3], in membrane preparations of the cestode Hymenolepis diminuta. Only L-glutamate and acetylcholine stimulated a significant elevation in Ins(1,4,5)P3. The response to L-glutamate was stereospecific; D-glutamate or L-aspartate were not as potent. A role for G-protein(s) was supported by the observations that sodium fluoride stimulated Ins(1,4,5)P3 generation, and the L-glutamate response was potentiated by GTP and GTP-S and was suppressed by GDPS. However, studies with pertussis and cholera toxins indicated that the putative G-protein(s) was not pertussis or cholera toxin sensitive. The pharmacological profile of the L-glutamate response was examined partially. Trans-ACPD was a very effective agonist at 10(-5)M. While 10(-3)M L-glutamate, NMDA, and AMPA significantly elevated Ins(1,4,5)P3 levels, quisqualate and kainate did not. The elevation of Ins(1,4,5)P3 levels by L-glutamate and NMDA was antagonized by the specific glutamatergic antagonists AP-5, AP-7, CNQX, and CPP. While the response to ACPD was antagonized by AP5, CPP and CPG, CNQX was without effect. Collectively, the data support the hypothesis that in the cestode H. diminuta, L-glutamate activation of a metabotropic (ACPD) and/or ionotropic-like AMPA/NMDA receptor subtypes proceeds via a G protein(s) to enhance phospholipase C activity, ultimately resulting in the elevation of Ins(1,4,5)P3 levels in the tissues.

Tissue distribution and intracellular localisation of the 75-kDa inositol polyphosphate 5-phosphatase

The 75-kDa inositol polyphosphate 5-phosphatase (75-kDa 5-phosphatase) hydrolyses several important mediators of intracellular calcium homeostasis, including inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. Northern analysis of various human tissues revealed the 75-kDa 5-phosphatase has a ubiquitous expression, where differential splicing may occur in specific tissues. Prominent expression of a 4.4-kb transcript was noted in human lung, thymus, testes and placenta, and a 4.6-kb transcript was observed in heart, brain, kidney, ovary and colon. Determination of the intracellular location of the enzyme by indirect immunofluorescence, demonstrated that the 75-kDa 5-phosphatase was associated with mitochondrial and cytosolic cellular compartments. Immunoprecipitation of the total cell homogenate of human lung carcinoma cells (A549) with anti-(recombinant 75-kDa 5-phosphatase) antibodies revealed that the 75-kDa 5-phosphatase is the major PtdIns(4,5)P2 5-phosphatase in this cell line. Analysis of PtdIns(4,5)P2 5-phosphatase activity in subcellular fractions of A549 cells revealed peak 75-kDa 5-phosphatase enzyme activity in the cytosolic and mitochondrial enriched fractions. Immunoblot analysis further confirmed the mitochondrial location of the enzyme. This study demonstrates the tissue distribution and intracellular location of the 75-kDa 5-phosphatase and reveals a novel location for an enzyme involved in phosphatidylinositol turnover.

Inositol trisphosphate metabolism in Saccharomyces cerevisiae: identification, purification and properties of inositol 1,4,5-trisphosphate 6-kinase

Ins(1,4,5)P3 metabolism was examined in Saccharomyces cerevisiae extracts. S. cerevisiae contains readily detectable Ins(1,4,5)P3 kinase activity that is predominantly soluble, but phosphomonoesterase activity acting on Ins(1,4,5)P3 was not detected in either soluble or particulate preparations from this organism. We have purified the kinase activity approximately 685-fold in a rapid four-step process, and obtained a stable preparation. The enzyme has an apparent native molecular mass of approximately 40 kDa, and displays Michaelis-Menten kinetics with respect to its two substrates, ATP and Ins(1,4,5)P3. The Km for ATP was 2.1 mM, and that for Ins(1,4,5)P3 was 7.1 microM. The enzyme appeared to be the first step in the conversion of Ins(1,4,5)P3 into an InsP5, and the partially purified preparation contained another activity that converted the InsP4 product into an InsP5. The InsP4 product of the partially purified kinase was not metabolized by human erythrocyte ghosts and co-chromatographed with an Ins(3,4,5,6)P4 [L-Ins(1,4,5,6)P4] standard, identifying it as D-Ins(1,4,5,6)P4. The yeast enzyme is thus an Ins(1,4,5)P3 6-kinase. This activity may be an important step in the production of inositol polyphosphates such as InsP5 and InsP6 in S. cerevisiae.

Phosphoinositides and calcium signaling New aspects and diverse functions in cell regulation

Numerous circulating and locally produced hormones bind to specific cell-surface receptors and activate a variety of second-messenger pathways that evoke characteristic phenotypic responses in their target cells. One of the most ubiquitous signal transduction mechanisms is the phosphoinositide-calcium messenger system, which is activated by hormones, neurotransmitters, and growth factors. Stimulation of these receptors by their ligands causes a characteristic change in the metabolism of membrane phospholipids with production of diacylglycerol and a rapid increase in cytoplasmic Ca(2+) concentration, due to the release of stored intracellular Ca(2+) and stimulated Ca(2+) entry from the extracellular space. These intracettular signals act in concert to activate protein kinases that phosphorylate a variety of regulatory proteins. The link between phosphoinositide turnover and Ca(2+) mobilization is inositol 1,4,5-trisphosphate, the major Ca(2+)-mobilizing second messenger, which is produced from membrane phosphoinositides by activated phospholipase C enzymes. The mechanisms of ligand-regulated Ca(2+) influx and the additional regulatory role(s) of phosphoinositides and inositol phosphates are still being unfolded. This review and the following article summarize some recent developments and unsolved issues about this major signal transduction cascade that links calcium-mobilizing hormone receptors to the regulation of endocrine cell function.

Studies on the properties of myo-inositol-1,4,5-trisphosphate 5-phosphatase and myo-inositol monophosphatase in bovine iris sphincter smooth muscle: effects of okadaic acid and protein phosphorylation

In bovine iris sphincter, myo-inositol 1,4,5-trisphosphate (IP3) 5-phosphatase and myo-inositol 1-phosphate (IP1) monophosphatase are mainly localized in the microsomal and soluble fractions, respectively. Studies on the properties of these enzymes can be summarized as follows. (1) The microsomal IP3 5-phosphatase hydrolyzed IP3 to myo-inositol 1,4-bisphosphate with an apparent Km of 28 microM and Vmax of 32 nmol/min per mg protein. The IP1 monophosphatase in the soluble fraction hydrolyzed IP1 into free inositol with an apparent Km of 89 microM and Vmax of 7 nmol/min per mg protein. (2) IP3 5-phosphatase and IP1 monophosphatase had optimal pH values at 8.0 and 7.0, respectively. (3) Both enzymes required Mg2+ and their highest specific activities were at a cation concentration of 2 mM. (4) Ca2+ (> 0.5 microM) exerted an inhibitory effect on IP3 5-phosphatase activity, and marked inhibition (47%) was observed at a concentration of 10 microM. Higher concentrations of the cation (> 100 microM) were required to inhibit IP1 monophosphatase. (5) IP1 monophosphatase, but not IP3 5-phosphatase, was inhibited by Li+. Li+ had no effect on the contractile response in this smooth muscle. (6) Both enzymes were inhibited by ATP and by the thiol-blocking agent, disulfiram. In addition, thimerosal, a thiol reagent, also inhibited the IP3 5-phosphatase activity. (7) Protein phosphorylation of the microsomal and soluble fractions with PKA or PKC had no effect on the activities of these enzymes. (8) Okadaic acid, a protein phosphatase inhibitor, had no effect on the activity of IP3 5-phosphatase. However, in the intact iris sphincter the toxin significantly reduced the carbachol-induced IP3 production, 1,2-diacylglycerol formation, measured as phosphatidic acid, and caused muscle relaxation.

Bovine testis and human erythrocytes contain different subtypes of membrane-associated Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphomonoesterases

1. We have purified membrane-associated Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatases from bovine testis and human erythrocytes by chromatography on several media, including a novel 2,3-bisphosphoglycerate affinity column. 2. The enzymes have apparent molecular masses of 42 kDa (testis) and 70 kDa (erythrocyte), as determined by SDS/PAGE, and affinities for Ins(1,4,5)P3 of 14 microM and 22 microM respectively. 3. The two enzymes hydrolyse both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 and are therefore type I Ins(1,4,5)P3 5-phosphatases [nomenclature of Hansen, Johanson, Williamson and Williamson (1987) J. Biol. Chem. 262, 17319-17326]. 4. On chromatofocusing, the partially purified testicular enzyme migrates as two peaks of activity, with pI values of about 5.8 and 5.5. The erythrocyte enzyme exhibits only the latter peak. 5. The testis 5-phosphatase is labile at 37 degrees C, but its activity can be maintained in the presence of 50 mM phorbol dibutyrate (PdBu). After PdBu treatment, a third form of the enzyme, with pI about 6.2, appears on chromatofocusing, but without change in its Km or Vmax. 6. Consideration of the properties of these enzymes and of the 5-phosphatases from other tissues suggests that type I Ins(1,4,5)P3 5-phosphatases are of two well-defined subtypes. We propose that these be termed type Ia [typified by the testis enzyme: approximately 40 kDa, higher affinity for Ins(1,4,5)P3] and Type Ib [typified by the erythrocyte enzyme: approximately 70 kDa, lower affinity for Ins(1,4,5)P3].

Inositol polyphosphates are not increased by overexpression of Ins(1,4,5)P3 3-kinase but show cell-cycle dependent changes in growth factor-stimulated fibroblasts

NIH 3T3 fibroblasts were stably transfected with rat brain inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) 3-kinase to explore the relationship between increased production of Ins(1,3,4,5)P4 and the formation of InsP5 and InsP6. Mass measurements of InsP5 and InsP6 revealed no significant difference between kinase- and vector-transfected fibroblasts. However, such 3-kinase-transfected cells, when labeled with [3H]inositol for 48-72 h, showed lower levels of [3H]InsP5 and [3H]InsP6, as well as [3H]Ins(1,3,4,6)P4 and D/L[3H]Ins(1,4,5,6)P4, than their vector-transfected counterparts. Because Ins(1,4,5)P3 3-kinase-transfected cells grew less rapidly than vector-transfected controls, we determined whether the synthesis of InsP5 and InsP6 was related to a specific phase of the cell cycle. When NIH 3T3 cells prelabeled with [3H]inositol were synchronized by serum deprivation followed by stimulation with platelet-derived growth factor (PDGF), the amounts of labeled InsP5 and InsP6 began to increase only after 12 h of stimulation, when cells entered the S-phase as indicated by increased [3H]thymidine incorporation. The enhanced synthesis of these inositol polyphosphates was preceded by an early increase in Ins(1,4,5)P3 and its metabolites that was no longer evident by the fifth hour of PDGF action. There was also a prominent and biphasic increase in the level of D/L-Ins(1,4,5,6)P4 with an early peak at approximately 3 h and a second rise that paralleled the increases in InsP5 and InsP6. These results indicate that the formation of highly phosphorylated inositols is not tightly coupled to the receptor-mediated formation of Ins(1,4,5)P3 and its metabolites but is mainly determined by other factors that operate at specific points of the cell cycle.

Pathways of dephosphorylation of 1-D-myo-inositol 1,4,5-trisphosphate in GH3 pituitary tumor cells

Previous work in [3H]inositol-labelled GH3 pituitary tumor cells stimulated with thyrotropin-releasing hormone (TRH) reported the existence of at least ten distinct [3H]inositol-containing substances which were identified as different inositol mono-, bis- and tris-phosphate isomers [1]. Here a complete kinetic study of the dephosphorylation pathways of the second messenger Ins(1,4,5)P3 is reported in GH3 cell homogenates, identifying a new intermediate, Ins(4,5)P2, in the metabolism of the second messenger. in vitro results obtained with exogenous substrates are compared with in vivo results obtained measuring levels of the endogenous [3H]inositol-labelled isomers that participate in the dephosphorylation pathways of Ins(1,4,5)P3 in resting and TRH-stimulated GH3 cells. The effect of Li+ on the activity of the different phosphatases involved in these pathways is studied as well.

LPS inhibits PI-phospholipase C but not PC-phospholipase D or phosphorylase activation by vasopressin and norepinephrine

Rats were infused with endotoxin (50 micrograms/100 g body wt) for 3 h, and the parenchymal cells of the liver were maintained in primary culture for 1-3 h. The effects of vasopressin, norepinephrine, and glucagon on the activation of phosphatidylinositol (PI)-phospholipase C, phosphatidylcholine (PC)-phospholipase D, and glycogen phosphorylase a were investigated. Activation of PI-phospholipase C was markedly reduced, particularly with norepinephrine. This confirms that one of the early metabolic impairments seen in acute endotoxin treatment is inhibition of PI-phospholipase C activity. However, the ability of vasopressin, norepinephrine, and glucagon to stimulate glycogen phosphorylase a and PC-phospholipase D was not affected by this endotoxin treatment. We conclude that activation of phosphorylase a by vasopressin and norepinephrine is not entirely dependent on the activation of PI-phospholipase C and inositol trisphosphate formation.

The effects of thimerosal on calcium uptake and inositol 1,4,5-trisphosphate-induced calcium release in cerebellar microsomes

Thimerosal inhibits calcium uptake in skeletal muscle sarcoplasmic reticulum and rat cerebellar microsomes by inhibiting the Ca(2+)-ATPase. In the presence of 5 mM dithiothreitol (DTT), Ca2+ uptake and ATPase activity were not inhibited by thimerosal, indicating that thimerosal modifies cysteine residues of the Ca(2+)-ATPase. Low thimerosal concentrations (2 microM) sensitize the inositol 1,4,5-trisphosphate (InsP3)-sensitive Ca2+ channel, making it open at lower InsP3 concentrations. Higher concentrations of thimerosal, however, cause inhibition of InsP3-induced Ca2+ release. Both sensitization and inhibition of the InsP3 receptor by thimerosal can be prevented by DTT. The binding and metabolism of InsP3 by cerebellar microsomes is not affected by thimerosal. The amount of InsP3-induced Ca2+ release is co-operatively linked to the InsP3 concentration with a Hill coefficient of 2.0 +/- 0.3. This is decreased to 1.0 +/- 0.2 at inhibitory concentrations of thimerosal. Under our experimental conditions, we observed no dependence of quantal Ca2+ release on intraluminal Ca2+ concentration.

The interrelationships of the inositol phosphates formed in vasopressin-stimulated WRK-1 rat mammary tumour cells

1. Temporal changes in the levels of many inositol phosphates, whose structural characterization is presented in the preceding paper [Wong, Barker, Morris, Craxton, Kirk & Michell (1991) Biochem. J. 286, 459-468], have been monitored in vasopressin-stimulated WRK-1 cells. 2. Upon stimulation, Ins(1,4,5)P3 accumulated within 1 s, consistent with its role as a rapidly acting second messenger produced by receptor activation of phosphoinositidase C. Ins(1,4)P2 and Ins(1,3,4,5)P4, both of which are immediate products of Ins(1,4,5)P3 metabolism, also accumulated quickly. Ins4P, Ins(1,3,4)P3, Ins(3,4)P2, Ins(1,3)P2, Ins1P and Ins3P, which are intermediates in the metabolism of Ins(1,4)P2 and Ins(1,3,4,5)P4 to inositol, accumulated after seconds or within a few minutes, and in a temporal sequence consistent with their known metabolic interrelationships. 3. The stimulated accumulation of Ins(1,3,4,6)P4 was delayed, as expected if it is formed by phosphorylation of Ins(1,3,4)P3. 4. Ins(3,4,5,6)P4 accumulated 2-3-fold in a few minutes, and mainly before Ins(1,3,4,6)P4. 5. Using a [3H]-/[14C]-inositol double-labelling protocol, we obtained evidence that all of the compounds that accumulated upon stimulation, except Ins(3,4,5,6)P4, originated from lipid-derived Ins(1,4,5)P3, but that the newly formed Ins(3,4,5,6)P4 came from a different source. 6. There were no consistent changes in the levels of Ins(1,3,4,5,6)P5 and InsP6 during stimulation. 7. Alongside the gradual accumulation of Ins(1:2-cyclic,4,5)P3 during stimulation [Wong, Barker, Shears, Kirk & Michell (1988) Biochem. J. 252, 1-5], there was an accumulation of Ins(1:2-cyclic,4)P2 and Ins(1:2-cyclic)P, probably as either minor side products of phosphoinositidase C action or metabolites of Ins(1:2-cyclic,4,5)P3. 8. When Li+ was present during stimulation, it redirected the dephosphorylation pathways downstream of Ins(1,4,5)P3 in the manner expected from its inhibition of inositol monophosphatase and Ins(1,4)P2/Ins(1,3,4)P3 1-phosphatase: there were marked increases in the accumulation of Ins(1,4)P2 and Ins(1,3,4)P3 and of monophosphates. Moreover, Li+ shifted the Ins1P/Ins3P balance in favour of Ins1P, thus demonstrating redirection of the metabolism of the accumulated Ins(1,3,4)P3 towards Ins(1,3)P2 rather than Ins(3,4)P2.

The inositol phosphates in WRK1 rat mammary tumour cells

1. A detailed structural survey has been made of the inositol phosphates of unstimulated and vasopressin-stimulated WRK-1 rat mammary tumour cells. Inositol phosphate peaks were separated by h.p.l.c., and structural assignments were made for more than 20 compounds by combinations of: (a) co-chromatography with labelled standards; (b) site-specific enzymic dephosphorylation; (c) complete and partial periodate oxidation, followed by h.p.l.c. of polyols and their stereospecific oxidation by dehydrogenases; and (d) ammoniacal hydrolysis. 2. The 'inositol monophosphates' fraction from unstimulated cells included an uncharacterized peak, probably containing some glycerophosphoinositol, and Ins(1:2-cyclic)P. Stimulation provoked accumulation of both Ins1P and Ins3P, of Ins2P, and of Ins5P and/or the enantiomers Ins4P and Ins6P. The proportions of Ins1P and Ins3P were determined by partial periodate oxidation and enantiomeric identification of the resulting glucitols. 3. Three inositol bisphosphate peaks were detected in unstimulated cells: Ins(1,4)P2 [this was distinguished chemically from its enantiomer Ins(3,6)P2], Ins(3,4)P2 and/or Ins(1,6)P2, and Ins(4,5)P2 and/or Ins(5,6)P2. On stimulation, Ins(1,4)P2 and Ins(3,4)P2 [and/or Ins(1,6)P2] levels increased, and Ins(1:2-cyclic,4)P2 and Ins(1,3)P2 were also formed. 4. Three inositol trisphosphate peaks were obtained from unstimulated cells: all increased during stimulation. These were Ins(1,3,4)P3 [with some Ins(1:2-cyclic,4,5)P3], Ins(1,4,5)P3 and Ins(3,4,5)P3 [and/or Ins(1,5,6)P3]. During stimulation, another compound, probably Ins(1,4,6)P3, appeared in the 'Ins(1,4,5)P3 peak'. The 'Ins(3,4,5)P3 peak' contained a second trisphosphate, probably Ins(2,4,5)P3. 5. Three inositol tetrakisphosphates, namely Ins(1,3,4,6)P4, Ins(1,3,4,5)P4, were present in unstimulated cells, and all accumulated during stimulation. 6. Ins(1,3,4,5,6)P5, which is the most abundant inositol polyphosphate in these cells, a less abundant inositol pentakisphosphate and inositol hexakisphosphate were all unresponsive to stimulation.

InsP3 and Ins(1,3,4,5)P4 act in synergy to stimulate influx of extracellular Ca2+ in Xenopus oocytes

To investigate the role of D-myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] in the regulation of Ca2+ influx, we injected inositol phosphates into Xenopus oocytes and measured Ca(2+)-gated Cl- current to assay intracellular free Ca2+ concentration ([Ca2+]i). To assess Ca2+ influx, we removed extracellular Ca2+ or added the inorganic Ca2+ channel blocker Mn2+ to the extracellular bath and measured the resulting change in Cl- current. Ins(1,3,4,5)P4 did not cause Ca2+ influx when injected alone or when preceded by an injection of Ca2+. In contrast, Ins(1,3,4,5)P4 stimulated Ca2+ influx when injected after the poorly metabolized inositol trisphosphate (InsP3) analogues D-myo-inositol 1,4,5-trisphosphorothioate [Ins(1,4,5)P3S3] or D-myo-inositol 2,4,5-trisphosphate [Ins(2,4,5)P3]. These results indicate that Ins(1,3,4,5)P4 is not sufficient to stimulate Ca2+ influx but acts in synergy with InsP3s to cause Ca2+ influx. We also studied the effect of Ca2+ influx on the immediate metabolism of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in single oocytes. Ca2+ influx shunted the metabolism of Ins(1,4,5)P3 toward the formation of Ins(1,3,4,5)P4 and away from D-myo-inositol 1,4-bisphosphate [Ins(1,4)P2]. These results suggest that there is a positive feedback regulatory mechanism in which Ca2+ influx stimulates Ins(1,3,4,5)P4 production and Ins(1,3,4,5)P4 stimulates further Ca2+ influx.

Calcium-sensitivity of inositol 1,4,5-trisphosphate metabolism in exocrine cells from the avian salt gland

The generation of inositol phosphates upon muscarinic-receptor activation was studied in [3H]inositol-loaded exocrine cells from the nasal salt glands of the duck Anas platyrhynchos, and the metabolism of different inositol phosphates in vitro was studied in tissue homogenates, with particular reference to the possible interaction of changes in intracellular [Ca2+] ([Ca2+]i) with the metabolic processes. In intact cells, there was a rapid (within 15 s) generation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, followed by an accumulation of their breakdown products, Ins(1,3,4)P3 and inositol bis- and monophosphates. Ca(2+)-sensitivity of the Ins(1,4,5)P3 3-kinase was demonstrated in tissue homogenates, with the rate of phosphorylation increasing 2-fold at free Ca2+ concentrations greater than 1 microM. However, addition of calmodulin or the presence of the calmodulin inhibitor W-7 (up to 100 microM) had no effect. 3-Kinase activity increased proportionally with the initial Ins(1,4,5)P3 concentration up to 1 microM, but a 10-fold higher substrate concentration produced only a doubling in the phosphorylation rate. Ins(1,3,4,5)P4 was dephosphorylated to Ins(1,3,4)P3, which accumulated in the homogenate assays as well as in intact cells. Depending on its concentration, Ins(1,3,4)P3 was phosphorylated [in part to Ins(1,3,4,6)P4] or dephosphorylated. To investigate the Ca(2+)-sensitivity of the 3-kinase in intact cells, excess quin2 was used to buffer the receptor-mediated transient changes in [Ca2+]i in [3H]inositol-loaded cells. These experiments revealed that increasing [Ca2+]i from less than 100 to approx. 400 nM (i.e. within the physiological range) has no effect on the partitioning of Ins(1,4,5)P3 metabolism (phosphorylation versus dephosphorylation) and on the accumulation of Ins(1,4,5)P3 and Ins(1,3,4,5)P4. This indicates that activation of the 3-kinase by physiologically relevant Ca2+ concentrations may not play a major role in the generation of Ins(1,3,4,5)P4 signals upon receptor activation in these cells. The latter are mainly achieved by the receptor-mediated increase in Ins(1,4,5)P3 in the cell and its phosphorylation by the 3-kinase in a substrate-concentration-dependent manner.

The opening of the inositol 1,4,5-trisphosphate-sensitive Ca2+ channel in rat cerebellum is inhibited by caffeine

Ins(1,4,5)P3(InsP3)-induced Ca2+ release and [3H]InsP3 binding were measured in rat cerebellar microsomes in the presence or absence of caffeine. The quantal Ca2+ release was shown to occur in an apparently co-operative fashion with a Hill coefficient (h) of 2.2. Half-maximal Ca2+ release was observed at 900 nM-InsP3. Addition of caffeine caused changes both to the concentration of InsP3 required to cause half-maximal Ca2+ release (3.9 microM at 50 mM-caffeine) and to the apparent co-operativity (h = 1.0 at 50 mM-caffeine). Under standard conditions for [3H]InsP3 binding, caffeine had no effect, and it had no effect on InsP3 metabolism. Cyclic AMP also had no effect on the quantal release induced by InsP3. These results are consistent with the view that caffeine affects the opening (Ca2+ release) events rather than the ligand-binding events in the operation of the InsP3-sensitive Ca2+ channel.

Determination of inositol polyphosphates from human T-lymphocyte cell lines by anion-exchange high-performance liquid chromatography and post-column derivatization

The intracellular amounts of several inositol tris-, tetrakis- and pentakisphosphates and inositol hexakisphosphate were determined in resting and stimulated cells from human T-lymphocyte lines. The inositol polyphosphates were separated by anion-exchange high-performance liquid chromatography and were detected on-line by a recently developed post-column dye system. In the human T-lymphocyte cell line Jurkat, basal intracellular concentrations ranged between 25 +/- 10 pmol per 10(9) cells for inositol 1,4,5-trisphosphate to 6380 +/- 355 pmol per 10(9) cells for inositol hexakisphosphate. Similar basal concentrations were observed in the human T-lymphocyte cell line HPB.ALL, with the exception that inositol hexakisphosphate was approximately 665 +/- 10 pmol per 10(9) cells. Stimulation of the human T-lymphocyte cell line Jurkat via the T-cell receptor by a monoclonal antibody directed against the T-cell receptor-CD3 complex induced time-dependent changes in the intracellular concentrations of multiple inositol polyphosphate isomers, including inositol 1,3,4-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, inositol 1,3,4,6-tetrakisphosphate, an as yet unidentified inositol tetrakisphosphate isomer, inositol 1,3,4,5,6-pentakisphosphate, inositol 1,2,3,4,6-pentakisphosphate and DL-inositol 1,2,4,5,6-pentakisphosphate. Inositol 1,4,5-trisphosphate increased only transiently after 5 min, whereas DL-inositol 1,4,5,6-tetrakisphosphate (determined as the enantiomeric mixture) increased after 20 min.

Inositol phosphates and intracellular calcium after bradykinin stimulation in fibroblasts from young, normal aged and Alzheimer donors

Several studies suggest that alterations in the receptor-mediated phosphoinositide cascade and cytosolic free calcium concentration ([Ca2+]i) are involved in the pathophysiology of aging and Alzheimer's disease. Therefore, the phosphoinositide cascade and [Ca2+]i were determined under resting conditions and after stimulation with bradykinin (100 nM) in cultured human skin fibroblasts from young (21 +/- 3 years), normal aged (59 +/- 6 years) and Alzheimer subjects (58 +/- 6 years). The inositol polyphosphates (IP3, IP2 and IP) were monitored after prelabeling the cells with [3H]inositol in serum free medium. [Ca2+]i was determined with the fluorescent probe, fura-2AM, under exactly analogous conditions. The bradykinin-induced formation of IP3 and IP2 increased significantly in fibroblasts from normal aged and Alzheimer donors compared to young subjects, but did not differ from each other. Bradykinin-induced IP3 formation was 63-117% above the young group at time points between 10-60 s in normal aged or Alzheimer donors. Bradykinin-induced IP2 formation was 49-59% above the young group at time points between 10-60 s in normal aged or Alzheimer subjects. Neither the basal [Ca2+]i, nor the bradykinin-stimulated [Ca2+]i, differed among fibroblasts from young, normal aged and Alzheimer donors. The precise molecular basis and pathophysiological significance of the enhanced bradykinin-induced phosphoinositide cascade in fibroblasts from aged donors remains to be determined.

Metabolism of inositol phosphates in ATP-stimulated vascular endothelial cells

The accumulation of InsP1, InsP2, InsP3 and InsP4 isomers was investigated in bovine aortic endothelial cells labelled with [3H]inositol and stimulated with ATP. The separation of these isomers was performed by ion-pairing reverse-phase h.p.l.c. on a mu Bondapack C18 column for the InsP3 and InsP4 isomers and by ion-exchange h.p.l.c. on a Partisil SAX column for the InsP1 and InsP2 isomers. In unstimulated endothelial cells, a large amount of material was co-eluted with InsP5 and InsP6, whereas amounts of InsP3 and InsP4 were small. The addition of ATP (100 microM) induced a striking (35-fold stimulation) and transient increase of Ins(1,4,5)P3 that was maximal around 15 s. This peak was followed by a more sustained accumulation of Ins(1,3,4,5)P4 and Ins(1,3,4)P3, but the amounts of these two metabolites accumulated in response to ATP were much smaller than that of Ins(1,4,5)P3. The increase in InsP2 isomers in response to ATP had similar characteristics: a rapid and transient accumulation of Ins(1,4)P2, followed by an increase of Ins(3,4)P2 and Ins(1,3)P2, which was more sustained but had a smaller magnitude. ATP also induced the accumulation of both Ins1P and Ins4P, but with different time courses: the level of Ins4P was maximal at 1 min (60 times the control value) and returned to baseline after 5 min, whereas the increase in Ins1P was undetectable at 1 min and reached a maximum after 5 min, which represented 240% of the basal level. These data indicate that Ins(1,4,5)P3, which is rapidly formed in aortic endothelial cells as a result of activation of P2Y receptors, is preferentially metabolized at early times (less than 1 min) by a 5-phosphatase, with the sequential formation of Ins(1,4)P2 and Ins4P. Afterwards, a small but sustained increase in the content of Ins(1,3,4)P3, Ins(1,3)P2, Ins(3,4)P2 and Ins1P was observed, reflecting the activation of the Ins(1,4,5)P3 3-kinase.

Effects of guanine nucleotides and parathyroid hormone on inositol 1,4,5-trisphosphate metabolism in canine renal cortical tubular cell membranes

Parathyroid hormone (PTH) and guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma S) increase levels of the second messenger inositol 1,4,5-triphosphate (IP3) and other inositol phosphates (IP) in several membrane preparations of PTH-responsive cells. We present evidence here indicating that in a membrane preparation of canine renal cortical tubular cells bPTH-(1-84), bPTH-(1-34), [N-Leu8,18Tyr34]bPTH-(3-34)NH2, and the human PTH related peptide fragment hPTHrP-(1-34)NH2 all increase levels of inositol phosphate (IP) but [Tyr34]-bPTH-(7-34)NH2 and hPTHrP-(7-34)NH2 have no significant effects on IP accumulation. Increases in IPs are generally attributed to increased formation of IPs and appear to be mediated by a G protein. However, increased levels of IPs may also result from inhibition of the phosphatases are responsible for their metabolism. We investigated the effect of PTH and GTP-gamma S on the metabolism of IP3 in canine renal cortical tubular membranes. These membranes rapidly metabolize [3H]IP3 (47% at 15 s). Decreases in [3H]IP3 at all time points are accounted for quantitatively by increases in the sum of its breakdown products: [3H]IP2, [3H]IP1, and [3H]inositol. After 5 minutes of exposure to membranes, the vast majority of [3H]IP3 (84%) is converted to its terminal metabolite, [3H]inositol. GTP-gamma S (100 microM) inhibits the amount of [3H]IP3 metabolized in 15 s by 70% and reduces the amount of [3H]inositol ultimately formed in 5 minutes by 64%. ATP-gamma S, ATP, and 2,3-bisphosphoglycerate (100 microM) also inhibit [3H]IP3 hydrolysis in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between inositol tris- and tetrakis-phosphates. Effects on intracellular Ca2+ mobilization in SH-SY5Y cells

The potential Ca2(+)-releasing activity of the inositol tetrakisphosphates Ins(1,3,4,6)P4 and DL-Ins(1,4,5,6)P4 and the inositol pentakisphosphate Ins(1,3,4,5,6)P5 and their effect on Ins(1,4,5)P3- and DL-Ins (1,3,4,5)P4-mediated Ca2+ release were examined in permeabilized SH-SY5Y human neuroblastoma cells. Neither DL-Ins(1,4,5,6)P4 nor Ins(1,3,4,5,6)P5 exhibit Ca2(+)-releasing activity at concentrations up to 10 microM, but Ins(1,3,4,6)P4 releases Ca2+ dose-dependently, with an EC50 value (conen, giving half-maximal effect) of 5.92 +/- 0.47 microM. Maximal response by this tetrakisphosphate (49 +/- 2.5%) is significantly less than that seen with Ins(1,4,5)P3 (60 +/- 3%) and is achieved at a concentration of 30 microM. In the presence of this concentration of Ins(1,3,4,6)P4 the EC50 value for Ins(1,4,5)P3-mediated Ca2+ release increases from 0.12 +/- 0.02 microM to 2.11 +/- 0.51 microM, providing evidence that this naturally occurring inositol tetrakisphosphate may recognize and exhibit its Ca2(+)-releasing activity via the Ins(1,4,5)P3 receptor. DL-Ins(1,3,4,5)P4, however, at its maximally effective concentration (10 microM) does not significantly affect Ins(1,4,5)P3-mediated Ca2+ release, and therefore appears to mediate its Ca2(+)-mobilizing action through a receptor distinct from that for Ins(1,4,5)P3.

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.

Second messengers derived from inositol lipids

Many hormones, growth factors, and neurotransmitters stimulate their target cells by promoting the hydrolysis of plasma-membrane phosphoinositides to form the two second messengers, diacylglycerol and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In such cells, ligand-receptor interaction stimulates specific phospholipases that are activated by guanyl nucleotide regulatory G proteins or tyrosine phosphorylation. In many cells, the initial rise in cytoplasmic calcium due to Ins(1,4,5)P3-induced mobilization of calcium from agonist-sensitive stores is followed by a sustained phase of cytoplasmic calcium elevation that maintains the target-cell response, and is dependent on influx of extracellular calcium. Numerous inositol phosphates are formed during metabolism of the calcium-mobilizing messenger, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], to lower and higher phosphorylated derivatives. The cloning of several phospholipase-C isozymes, as well as the Ins(1,4,5)P3-5 kinase and the Ins(1,4,5)P3 receptor, have clarified several aspects of the diversity and complexity of the phosphoinositide-calcium signaling system. In addition to their well-established roles in hormonal activation of cellular responses such as secretion and contraction, phospholipids and their hydrolysis products have been increasingly implicated in the actions of growth factors and oncogenes on cellular growth and proliferation.

Relationship between the calcium-mobilizing action of inositol 1,4,5-trisphosphate in permeable AR4-2J cells and the estimated levels of inositol 1,4,5-trisphosphate in intact AR4-2J cells

Various experimental strategies were employed in an effort to explain the previously reported [Horstman, Takemura & Putney (1988) J. Biol. Chem. 263, 15297-15303] paradoxically high levels of inositol 1,4,5-trisphosphate [(1,4,5)IP3] in resting and substance-P-stimulated AR4-2J cells. The concentration-effect curves for substance-P-induced [3H](1,4,5)IP3 formation in [3H]inositol-labelled cells and substance-P-induced increase in intracellular [Ca2+] were essentially superimposable, suggesting that formation of (1,4,5)IP3 is limiting for cellular Ca2+ mobilization. In electrically permeabilized AR4-2J cells, (1,4,5)IP3 and other inositol polyphosphates stimulated Ca2+ release with potencies similar to those reported for other cell types, including the parent pancreatic acinar cell. Compartmentalization of basal (1,4,5)IP3 was suggested by the fact that this material was stable in the presence of antimycin A, although this toxin completely blocked agonist stimulation of phospholipase C. However, subcellular fractionation as well as permeabilization of the cells with Staphylococcus aureus alpha-toxin failed to provide evidence for binding or sequestration of [3H](1,4,5)IP3 in AR4-2J cells. The density of (1,4,5)IP3 receptors in AR4-2J cells was not sufficiently large to impose non-linearity in the relationship between (1,4,5)IP3 concentration and (1,4,5)IP3-induced Ca2+ release. Thus the apparent high concentrations of (1,4,5)IP3 in resting and stimulated AR4-2J cells are not indicative of atypically low sensitivity or high concentration of (1,4,5)IP3 receptors, nor is there evidence for compartmentalization of (1,4,5)IP3 outside of the cytoplasm in these cells. It is possible that soluble factors in the cytoplasm of AR4-2J cells regulate the free concentration of (1,4,5)IP3 or the sensitivity of receptors to (1,4,5)IP3.

The effects of inositol trisphosphates and inositol tetrakisphosphate on Ca2+ release and Cl- current pattern in the Xenopus laevis oocyte

We report that Ins(1,3,4,5)P4 releases calcium from intracellular stores of intact Xenopus laevis oocytes, as indicated by two different techniques, Ca2(+)-sensitive microelectrodes and a fura-2 imaging system. Ins(1,3,4,5)P4 releases only 20% as much Ca2+ as the same amount of Ins(1,4,5)P3. This effect is not due to the conversion of the injected Ins(1,3,4,5)P4 to Ins(1,4,5)P3, which is known to release Ca2+, because the amount of [3H]Ins(1,3,4,5)P4 that is converted to Ins(1,4,5)P3 is extremely small, as determined using HPLC. Examination of the different current patterns induced by Ins(1,4,5)P3 and Ins(1,3,4,5)P4, when injected into voltage-clamped oocytes, provided further evidence that the Ins(1,3,4,5)P4 was not being converted back to Ins(1,4,5)P3. We investigated the effects of four compounds, three inositol trisphosphates (Ins(1,4,5)P3, Ins(2,4,5)P3, and Ins(1,3,4)P3), and Ins(1,3,4,5)P4, on Cl- current conductance in order to examine (1) the possible role of Ins(1,3,4,5)P4 in cell activation and (2) the relationships between intracellular Ca2+ and the activation of Cl- currents. Immature stage VI Xenopus laevis oocytes were voltage-clamped and injected with Ins(1,4,5)P3, Ins(2,4,5)P3, and Ins(1,3,4)P3. Ins(1,4,5)P3 and Ins(2,4,5)P3 triggered Ca2(+)-dependent Cl- currents, but Ins(1,3,4)P3 did not trigger currents nor did it release intracellular Ca2+. Ins(2,4,5)P3 was fourfold less effective at inducing the immediate Cl- current pulse than Ins(1,4,5)P3. The Cl- current pattern was quite dependent on the amount of Ins(1,4,5)P3 injected into the oocyte. Low amounts of Ins(1,4,5)P3 triggered only an immediate single Cl- current pulse, whereas large amounts triggered the immediate single pulse, followed by a quiescent period, followed by oscillating Cl- currents. In contrast to the response of Ins(1,4,5)P3, injection of Ins(1,3,4,5)P4 triggered only oscillating Cl- currents whose magnitude, but not pattern, was dependent on the amount injected into the cell. The currents generated by Ins(1,3,4,5)P4 resemble the oscillating Cl- currents triggered by large amounts of Ins(1,4,5)P3 and Ins(2,4,5)P3. Ins(1,3,4,5)P4, unlike Ins(1,4,5)P3 and Ins(2,4,5)P3, rarely caused an immediate Cl- current pulse, but caused an immediate release of calcium. Therefore, we suggest that the oscillating currents are only indirectly dependent on calcium. These [Ca2+]i and conductance measurements suggest that both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 have roles in intracellular Ca2+ regulation.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Properties of a soluble inositol 1,3,4,5-tetrakisphosphate 3-phosphatase from porcine brain

We have previously shown that Ins(1,3,4,5)P4 is degraded to Ins(1,4,5)P3 by a soluble Ins(1,3,4,5)P4 3-phosphatase from pig brain [Höer, Kwiatkowski, Seib, Rosenthal, Schultz & Oberdisse (1988) Biochem. Biophys. Res. Commun. 154, 668-675]. Here we present some properties of this enzyme using [5-32P]Ins(1,3,4,5)P4 as substrate. The molecular mass, estimated by gel filtration chromatography on a Superose 6 column, was determined to be 36 kDa. The 3-phosphatase showed a high affinity towards the substrate Ins(1,3,4,5)P4 (Km approximately 400 nM); the Vmax. of the freshly prepared enzyme was 2 nmol/min per mg of protein. The influence of Ins(1,4,5)P3 and Ins(1,3,4)P3, the reaction products of Ins(1,3,4,5)P4 hydrolysis by either 3- or 5-phosphatase respectively, on the 3-phosphatase was tested. Both isomers inhibited the enzyme, with Ki values of about 2 microM and 1.75 microM for Ins(1,3,4)P3 and Ins(1,4,5)P3 respectively. Enzyme activity was not influenced by Mg2+ up to 30 mM or Ca2+ up to 1 mM. Commercially available Ins(3,4,5,6)P4 from turkey erythrocytes produced a marked inhibition of the 3-phosphatase (Ki approximately 500 nM). Significant inhibitory effects on enzyme activity were also found with GTP and the pyrimidine nucleotides UTP and CTP. The kinetic data presented here suggest that the Ins(1,3,4,5)P4 3-phosphatase may be regulated by the intracellular concentrations of inositol tris- and tetrakis-phosphates.

Ca2(+)-stimulatable and protein kinase C-inhibitable accumulation of inositol 1,3,4,6-tetrakisphosphate in human platelets

Thrombin-stimulated (10 s) human platelets produce Ins(1,4,5)P3 and an additional inositol trisphosphate (InsP3), in approximately a 1:20 ratio. The major InsP3 co-migrates with Ins(1,3,4)P3 on strong-anion-exchange h.p.l.c. To identify this species unequivocally, we treated putative Ins(1,3,4)P3 obtained from thrombin-stimulated myo-[3H]inositol-labelled platelets with NaIO4/NaBH4 or 4-phosphomonoesterase. The products indicate that the major InsP3 is at least 90% D-Ins(1,3,4)P3. D-[3H]Ins(1,3,4)P3 added to saponin-permeabilized platelets is hydrolysed to an InsP2 (7.8%) and phosphorylated by a kinase to yield an inositol polyphosphate (0.9%) in 5 min. The phosphorylation product co-migrates with Ins(1,3,4,6)P4 on Partisphere WAX h.p.l.c. Under similar conditions, L-[3H]Ins(1,3,4)P3 is dephosphorylated but not phosphorylated. Relative phosphatase:kinase ratios are 8.7:1 (Vmax. values) and 0.86:1 (Km values) with respect to D-Ins(1,3,4)P3. The kinase activity is predominantly cytosolic (96.8% of total activity) in freeze-thaw-disrupted platelets, and the accumulation of its product is Ca2(+)-dependent. The activity is identified as a 6-kinase on the basis of its product's insensitivity to 5-phosphomonoesterase, resistance to periodate oxidation and co-migration with standard Ins(1,3,4,6)P4 on h.p.l.c. Incubation of platelets with beta-phorbol dibutyrate (beta-PDBu, 76 nM), causing activation of protein kinase C, results in a 57.5% inhibition (reversible by the protein kinase C inhibitor staurosporine) of Ins(1,3,4,6)P4 accumulation. alpha-PDBu, which does not stimulate protein kinase C, has no effect. Stimulation of intact platelets with thrombin results in the production of Ins(1,3,4,6)P4 (1.4-fold rise in 30 s) and Ins(1,3,4,5)P4, with the latter being the major InsP4 species. Accumulation of Ins(1,3,4,6)P4 is slightly delayed in comparison with Ins(1,3,4)P3 and is relatively small. We propose that the major route of Ins(1,3,4)P3 metabolism in stimulated human platelets is via phosphatase action.