Inositol polyphosphate 1-phosphatase from calf brain. Purification and inhibition by Li+, Ca2+, and Mn2+

Modulation of Gq/PLC-Mediated Signaling by Acute Lithium Exposure

Although lithium has long been one of the most widely used pharmacological agents in psychiatry, its mechanisms of action at the cellular and molecular levels remain poorly understood. One of the targets of Li+ is the phosphoinositide pathway, but whereas the impact of Li+ on inositol lipid metabolism is well documented, information on physiological effects at the cellular level is lacking. We examined in two mammalian cell lines the effect of acute Li+ exposure on the mobilization of internal Ca2+ and phospholipase C (PLC)-dependent membrane conductances. We first corroborated by Western blots and immunofluorescence in HEK293 cells the presence of key signaling elements of a muscarinic PLC pathway (M1AchR, Gq, PLC-β1, and IP3Rs). Stimulation with carbachol evoked a dose-dependent mobilization of Ca, as determined with fluorescent indicators. This was due to release from internal stores and proved susceptible to the PLC antagonist U73122. Li+ exposure reproducibly potentiated the Ca response in a concentration-dependent manner extending to the low millimolar range. To broaden those observations to a neuronal context and probe potential Li modulation of electrical signaling, we next examined the cell line SHsy5y. We replicated the potentiating effects of Li on the mobilization of internal Ca, and, after characterizing the basic properties of the electrical response to cholinergic stimulation, we also demonstrated an equally robust upregulation of muscarinic membrane currents. Finally, by directly stimulating the signaling pathway at different links downstream of the receptor, the site of action of the observed Li effects could be narrowed down to the G protein and its interaction with PLC-β. These observations document a modulation of Gq/PLC/IP3-mediated signaling by acute exposure to lithium, reflected in distinct physiological changes in cellular responses.

Astroglial Serotonin Receptors as the Central Target of Classic Antidepressants

Major depressive disorder (MDD) presents multiple clinical phenotypes and has complex underlying pathological mechanisms. Existing theories cannot completely explain the pathophysiological mechanism(s) of MDD, while the pharmacology of current antidepressants is far from being fully understood. Astrocytes, the homeostatic and defensive cells of the central nervous system, contribute to shaping behaviors, and regulating mood and emotions. A detailed introduction on the role of astrocytes in depressive disorders is thus required, to which this chapter is dedicated. We also focus on the interactions between classic antidepressants and serotonin receptors, overview the role of astrocytes in the pharmacological mechanisms of various antidepressants, and present astrocytes as targets for the treatment of bipolar disorder. We provide a foundation of knowledge on the role of astrocytes in depressive disorders and astroglial 5-HT_2B receptors as targets for selective serotonin reuptake inhibitors in vivo and in vitro.

A structural basis for lithium and substrate binding of an inositide phosphatase

Inositol polyphosphate 1-phosphatase (INPP1) is a prototype member of metal-dependent/lithium-inhibited phosphomonoesterase protein family defined by a conserved three-dimensional core structure. Enzymes within this family function in distinct pathways including inositide signaling, gluconeogenesis, and sulfur assimilation. Using structural and biochemical studies, we report the effect of substrate and lithium on a network of metal binding sites within the catalytic center of INPP1. We find that lithium preferentially occupies a key site involved in metal-activation only when substrate or product is added. Mutation of a conserved residue that selectively coordinates the putative lithium-binding site results in a dramatic 100-fold reduction in the inhibitory constant as compared with wild-type. Furthermore, we report the INPP1/inositol 1,4-bisphosphate complex which illuminates key features of the enzyme active site. Our results provide insights into a structural basis for uncompetitive lithium inhibition and substrate recognition and define a sequence motif for metal binding within this family of regulatory phosphatases.

Is There Justification to Treat Neurodegenerative Disorders by Repurposing Drugs? The Case of Alzheimer's Disease, Lithium, and Autophagy

Lithium is the prototype mood-stabilizer used for acute and long-term treatment of bipolar disorder. Cumulated translational research of lithium indicated the drug's neuroprotective characteristics and, thereby, has raised the option of repurposing it as a drug for neurodegenerative diseases. Lithium's neuroprotective properties rely on its modulation of homeostatic mechanisms such as inflammation, mitochondrial function, oxidative stress, autophagy, and apoptosis. This myriad of intracellular responses are, possibly, consequences of the drug's inhibition of the enzymes inositol-monophosphatase (IMPase) and glycogen-synthase-kinase (GSK)-3. Here we review lithium's neurobiological properties as evidenced by its neurotrophic and neuroprotective properties, as well as translational studies in cells in culture, in animal models of Alzheimer's disease (AD) and in patients, discussing the rationale for the drug's use in the treatment of AD.

INPP1 up-regulation by miR-27a contributes to the growth, migration and invasion of human cervical cancer

Inositol polyphosphate‐1‐phosphatase (INPP1) is an enzyme that is responsible for glycolysis and lipid metabolism. Here, we discovered that INPP1 expression was up‐regulated in CC tissues compared to that in adjacent normal tissues by RT‐qPCR. Inositol polyphosphate‐1‐phosphatase overexpression promoted and INPP1 knockdown suppressed cell viability, cellular migration/invasion and EMT in CC cells. To explore the mechanism of dysregulation, INPP1 was predicted to be a target of miR‐27a, and a pmiRGLO dual‐luciferase reporter assay showed that miR‐27a bound to the 3′ UTR of INPP1. RT‐qPCR revealed that miR‐27a was also up‐regulated and had a positive correlation with INPP1 expression in CC tissues. Furthermore, shR‐INPP1 could favour the malignant phenotype reversion induced by miR‐27a, suggesting that miR‐27a up‐regulates INPP1 to promote tumorigenic activities. Altogether, our findings show that the up‐regulation of INPP1 by miR‐27a contributes to tumorigenic activities and may provide a potential biomarker for CC.

Coordination, microprotonation equilibria and conformational changes of myo-inositol hexakisphosphate with pertinence to its biological function

Within all the eukaryotic cells there is an important group of biomolecules that has been potentially related to signalling functions: the myo-inositol phosphates (InsPs). In nature, the most abundant member of this family is the so called InsP6 (phytate, L(12-)), for which our group has strived in the past to elucidate its intricate chemical behaviour. In this work we expand on our earlier findings, shedding light on the inframolecular details of its protonation and complexation processes. We evaluate systematically the chemical performance of InsP6 in the presence and absence of alkali and alkaline earth metal ions, through (31)P NMR measurements, in a non-interacting medium and over a wide pH range. The analysis of the titration curves by means of a model based on the cluster expansion method allows us to describe in detail the distribution of the different protonated microspecies of the ligand. With the aid of molecular modelling tools, we assess the energetic and geometrical characteristics of the protonation sequence and the conformational transition suffered by InsP6 as the pH changes. By completely characterizing the protonation pattern, conformation and geometry of the metal complexes, we unveil the chemical and structural basis behind the influence that the physiologically relevant cations, Na(+), K(+), Mg(2+) and Ca(2+) have over the phytate chemical reactivity. This information is essential in the process of gaining reliable structural knowledge about the most important InsP6 species in the in vitro and in vivo experiments, and how these features modulate their probable biological functions.

Interaction of myo-inositol hexakisphosphate with biogenic and synthetic polyamines

myo-Inositol hexakisphosphate (phytate) forms very stable adducts with biogenic and synthetic polyamines in aqueous solution.

Inositol phosphate recycling regulates glycolytic and lipid metabolism that drives cancer aggressiveness

Cancer cells possess fundamentally altered metabolism that supports their pathogenic features, which includes a heightened reliance on aerobic glycolysis to provide precursors for synthesis of biomass. We show here that inositol polyphosphate phosphatase 1 (INPP1) is highly expressed in aggressive human cancer cells and primary high-grade human tumors. Inactivation of INPP1 leads to a reduction in glycolytic intermediates that feed into the synthesis of the oncogenic signaling lipid lysophosphatidic acid (LPA), which in turn impairs LPA signaling and further attenuates glycolytic metabolism in a feed-forward mechanism to impair cancer cell motility, invasiveness, and tumorigenicity. Taken together these findings reveal a novel mode of glycolytic control in cancer cells that can serve to promote key oncogenic lipid signaling pathways that drive cancer pathogenicity.

A gene trap mutagenesis screen for genes underlying cellular response to the mood stabilizer lithium

Identifying the biological pathways mediating the action of a therapeutic compound may help the development of more specific treatments while also increasing our understanding of the underlying disease pathology. Salts of the metal lithium are commonly used as a front-line mood stabilizing treatment for bipolar disorder. Lithium's action has been variously linked to inositol phosphate metabolism and the WNT/Glycogen Synthase Kinase 3β (GSK3β)/β-Catenin signalling cascade, but, to date, little is known about which of these provides the principal therapeutic benefit for patients and, more specifically, which constituent genes, through presumed sequence variation, determine differences in patient response to treatment. Here, we describe a functional screen in which SH-SY5Y neuroblastoma cells were randomly mutated through genomic integration of the pMS1 poly A ‘gene trap’ plasmid vector. Lithium normally induces differentiation of neuroblastoma cells, but a small proportion of mutated cells continued to proliferate and formed colonies. Rapid amplification of cDNA ends (RACE)-PCR was used to identify the ‘trapped’ gene in each of these lithium-resistant colonies. Heterozygous, gene trap integrations were identified within ten genes, eight of which are likely to produce loss-of-function mutations including MED10, MSI2 and three long intergenic non-coding (LINC) RNAs. Both MED10 and MSI2 have been previously linked with WNT/GSK3β/β-Catenin pathway function suggesting that this is an important mediator of lithium action in this screen. The methodology applied here provides a rapid, objective and economic approach to define the genetic contribution to drug action, but could also be readily adapted to any desired in vitro functional selection/screening paradigm.

Inhibition of GSK3β-mediated BACE1 expression reduces Alzheimer-associated phenotypes

Deposition of amyloid β protein (Aβ) to form neuritic plaques in the brain is the pathological hallmark of Alzheimer's disease (AD). Aβ is generated from sequential cleavages of the β-amyloid precursor protein (APP) by the β- and γ-secretases, and β-site APP-cleaving enzyme 1 (BACE1) is the β-secretase essential for Aβ generation. Previous studies have indicated that glycogen synthase kinase 3 (GSK3) may play a role in APP processing by modulating γ-secretase activity, thereby facilitating Aβ production. There are two highly conserved isoforms of GSK3: GSK3α and GSK3β. We now report that specific inhibition of GSK3β, but not GSK3α, reduced BACE1-mediated cleavage of APP and Aβ production by decreasing BACE1 gene transcription and expression. The regulation of BACE1 gene expression by GSK3β was dependent on NF-κB signaling. Inhibition of GSK3 signaling markedly reduced Aβ deposition and neuritic plaque formation, and rescued memory deficits in the double transgenic AD model mice. These data provide evidence for regulation of BACE1 expression and AD pathogenesis by GSK3β and that inhibition of GSK3 signaling can reduce Aβ neuropathology and alleviate memory deficits in AD model mice. Our study suggests that interventions that specifically target the β-isoform of GSK3 may be a safe and effective approach for treating AD.

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.

A role for a lithium-inhibited Golgi nucleotidase in skeletal development and sulfation

Sulfation is an important biological process that modulates the function of numerous molecules. It is directly mediated by cytosolic and Golgi sulfotransferases, which use 3'-phosphoadenosine 5'-phosphosulfate to produce sulfated acceptors and 3'-phosphoadenosine 5'-phosphate (PAP). Here, we identify a Golgi-resident PAP 3'-phosphatase (gPAPP) and demonstrate that its activity is potently inhibited by lithium in vitro. The inactivation of gPAPP in mice led to neonatal lethality, lung abnormalities resembling atelectasis, and dwarfism characterized by aberrant cartilage morphology. The phenotypic similarities of gPAPP mutant mice to chondrodysplastic models harboring mutations within components of the sulfation pathway lead to the discovery of undersulfated chondroitin in the absence of functional enzyme. Additionally, we observed loss of gPAPP leads to perturbations in the levels of heparan sulfate species in lung tissue and whole embryos. Our data are consistent with a model that clearance of the nucleotide product of sulfotransferases within the Golgi plays an important role in glycosaminoglycan sulfation, provide a unique genetic basis for chondrodysplasia, and define a function for gPAPP in the formation of skeletal elements derived through endochondral ossification.

The mood stabilizer valproate inhibits both inositol- and diacylglycerol-signaling pathways in Caenorhabditis elegans

The antiepileptic valproate (VPA) is widely used in the treatment of bipolar disorder, although the mechanism of its action in the disorder is unclear. We show here that VPA inhibits both inositol phosphate and diacylglycerol (DAG) signaling in Caenorhabditis elegans. VPA disrupts two behaviors regulated by the inositol-1,4,5-trisphosphate (IP(3)): defecation and ovulation. VPA also inhibits two activities regulated by DAG signaling: acetylcholine release and egg laying. The effects of VPA on DAG signaling are relieved by phorbol ester, a DAG analogue, suggesting that VPA acts to inhibit DAG production. VPA reduces levels of DAG and inositol-1-phosphate, but phosphatidylinositol-4,5-bisphosphate (PIP(2)) is slightly increased, suggesting that phospholipase C-mediated hydrolysis of PIP(2) to form DAG and IP(3) is defective in the presence of VPA.

Alterations in an inositol phosphate code through synergistic activation of a G protein and inositol phosphate kinases

In mammals, many cellular stimuli evoke a response through G protein activation of phospholipase C, which results in the lipid-derived production of inositol 1,4,5-trisphosphate (IP(3)). Although it is well established that IP(3) is converted to numerous inositol phosphates (IPs) and pyrophosphates (PP-IPs) through the action of up to six classes of inositol phosphate kinases (IPKs), it is not clear that these metabolites are influenced by G protein signaling. Here we report that activation of Galpha(q) leads to robust stimulation of IP(3) to IP(8) metabolism. To expose flux through these pathways, genetic perturbation was used to alter IP homeostasis. Coupled expression of a constitutively active Galpha(q)QL and one or more IPK gene products synergistically generated dramatic changes in the patterns of intracellular IP messengers. Many distinct IP profiles were observed through the expression of different combinations of IPKs, including changes in previously unappreciated pools of IP(5) and IP(6), two molecules widely viewed as stable metabolites. Our data link the activation of a trimeric G protein to a plethora of metabolites downstream of IP(3) and provide a framework for suggesting that cells possess the machinery to produce an IPK-dependent IP code. We imply, but do not prove, that agonist-induced alterations in such a code would theoretically be capable of enhancing signaling complexity and specificity. The essential roles for IPKs in organism development and cellular adaptation are consistent with our hypothesis that such an IP code may be relevant to signaling pathways.

Effect of LiCl on phosphoenolpyruvate carboxylase kinase and the phosphorylation of phosphoenolpyruvate carboxylase in leaf disks and leaves of Sorghum vulgare

In the present work, the effect of LiCl on phosphoenolpyruvate carboxylase kinase (PEPCase-k), C4 phosphoenolpyruvate carboxylase (PEPCase: EC 4.1.1.31) and its phosphorylation process has been investigated in illuminated leaf disks and leaves of the C4 plant Sorghum vulgare. Although this salt induced severe damages to older leaves, it did not significantly alter the physiological parameters (photosynthesis, transpiration rate, intercellular CO2 concentration) of young leaves. An immunological approach was used to demonstrate that the PEPCase-k protein accumulated rapidly in illuminated leaf tissues, consistent with the increase in its catalytic activity. In vivo, LiCl was shown to strongly enhance the light effect on PEPCase-k protein content, this process being dependent on protein synthesis. In marked contrast, the salt was found to inhibit the PEPCase-k activity in reconstituted assays and to decrease the C4 PEPCase content and phosphorylation state in LiCl treated plants. Short-term (15 min) LiCl treatment increased IP3 levels, PPCK gene expression, and PEPCase-k accumulation. Extending the treatment (1 h) markedly decreased IP3 and PPCK gene expression, while PEPCase-k activity was kept high. The cytosolic protein synthesis inhibitor cycloheximide (CHX), which blocked the light-dependent up-regulation of the kinase in control plants, was found not to be active on this process in preilluminated, LiCl-treated leaves. This suggested that the salt causes the kinase turnover to be altered, presumably by decreasing degradation of the corresponding polypeptide. Taken together, these results establish PEPCase-k and PEPCase phosphorylation as lithium targets in higher plants and that this salt can provide a means to investigate further the organization and functioning of the cascade controlling the activity of both enzymes.

d-myo-Inositol 1-phosphate as a surrogate of d-myo-inositol 1,4,5-tris phosphate to monitor G protein-coupled receptor activation

Phospholipase C beta (PLC-beta)-coupled G protein-coupled receptor (GPCR) activities traditionally are assessed by measuring Ca2+ triggered by D-myo-inositol 1,4,5-trisphosphate (IP3), a PLC-beta hydrolysis product, or by measuring the production of inositol phosphate using cumbersome radioactive assays. A specific detection of IP3 production was also established using IP3 binding proteins. The short lifetime of IP3 makes this detection very challenging in measuring GPCR responses. Indeed, this IP3 rapidly enters the metabolic inositol phosphate cascade. It has been known for decades that lithium chloride (LiCl) leads to D-myo-inositol 1-phosphate accumulation on GPCR activation by inhibiting inositol monophosphatase, the final enzyme of the IP3 metabolic cascade. We show here that IP1 can be used as a surrogate of IP3 to monitor GPCR activation. We developed a novel homogeneous time-resolved fluorescence (HTRF) assay that correlates perfectly with existing methods and is easily amenable to high-throughput screening. The IP-One assay was validated on various GPCR models. It has the advantage over the traditional Ca2+ assay of allowing the measurement of inverse agonist activity as well as the analysis of PLC-beta activity in any nontransfected primary cultures. Finally, the high assay specificity for D-myo-inositol 1 monophosphate (IP1(1)) opens new possibilities in developing selective assays to study the functional roles of the various isoforms of inositol phosphates.

Identification of mitogen-activated protein kinase docking sites in enzymes that metabolize phosphatidylinositols and inositol phosphates

BACKGROUND

Reversible interactions between the components of cellular signaling pathways allow for the formation and dissociation of multimolecular complexes with spatial and temporal resolution and, thus, are an important means of integrating multiple signals into a coordinated cellular response. Several mechanisms that underlie these interactions have been identified, including the recognition of specific docking sites, termed a D-domain and FXFP motif, on proteins that bind mitogen-activated protein kinases (MAPKs). We recently found that phosphatidylinositol-specific phospholipase C-gamma1 (PLC-gamma1) directly binds to extracellular signal-regulated kinase 2 (ERK2), a MAPK, via a D-domain-dependent mechanism. In addition, we identified D-domain sequences in several other PLC isozymes. In the present studies we sought to determine whether MAPK docking sequences could be recognized in other enzymes that metabolize phosphatidylinositols (PIs), as well as in enzymes that metabolize inositol phosphates (IPs).

RESULTS

We found that several, but not all, of these enzymes contain identifiable D-domain sequences. Further, we found a high degree of conservation of these sequences and their location in human and mouse proteins; notable exceptions were PI 3-kinase C2-gamma, PI 4-kinase type IIbeta, and inositol polyphosphate 1-phosphatase.

CONCLUSION

The results indicate that there may be extensive crosstalk between MAPK signaling and signaling pathways that are regulated by cellular levels of PIs or IPs.

Lithium and bipolar mood disorder: the inositol-depletion hypothesis revisited

Inositol, a simple six-carbon sugar, forms the basis of a number of important intracellular signaling molecules. Over the last 35 years, a series of biochemical and cell biological experiments have shown that lithium (Li(+)) reduces the cellular concentration of myo-inositol and as a consequence attenuates signaling within the cell. Based on these observations, inositol-depletion was proposed as a therapeutic mechanism in the treatment of bipolar mood disorder. Recent results have added significant new dimensions to the original hypothesis. However, despite a number of clinical studies, this hypothesis still remains to be either proven or refuted. In this review of our current knowledge, I will consider where the inositol-depletion hypothesis stands today and how it may be further investigated in the future.

Alteration of lithium pharmacology through manipulation of phosphoadenosine phosphate metabolism

Bisphosphate 3'-nucleotidase (BPNT1 in mammals and Met22/Hal2 in yeast) is one of five members of a family of signaling phosphatases united through a common tertiary structure and inhibition by subtherapeutic doses of the antibipolar drug lithium. Here we report a role for 3'-nucleotidase and its substrate, 3'-phosphoadenosine 5'-phosphate (PAP), in mediating the cellular effects of lithium. Lithium-induced inhibition of growth in yeast cells may be overcome by dose-dependent heterologous expression of human BPNT1. Disruption of the yeast 3'-nucleotidase gene or treatment of cells with lithium results in a >80-fold accumulation of PAP and leads to potent growth inhibition. These data indicate that the accumulation of a 3'-nucleotidase substrate, such as PAP, mediates the toxicity of lithium. To further probe this model we examined the growth inhibitory effects of lithium under conditions in which PAP biosynthetic machinery was concomitantly down-regulated. Disruption of met3 or met14 genes (ATP sulfurylase or phosphosulfate kinase), transcriptional down-regulation of MET3 through methionine addition, or administration of chlorate, a widely used cell-permeable sulfurylase inhibitor, function to reduce lithium-induced intracellular PAP accumulation and lithium toxicity; all of these effects were reversed by heterologous expression of human sulfurylase and kinase. Collectively, our data support a role for 3'-nucleotidase activity and PAP metabolism in aspects of lithium's mechanism of action and provide a platform for development of novel pharmacological modulators aimed at improving therapies for the treatment of bipolar disorder.

Phospholipase Cdelta(1) does not mediate Ca(2+) responses in neonatal rat cardiomyocytes

Phospholipase C (PLC) activation in neonatal rat ventricular cardiomyocytes (NRVM) generates inositol(1,4,5)trisphosphate (Ins(1,4,5)P(3)) in response to elevations in Ca(2+) or inositol(1,4)bisphosphate in response to G protein stimulation. Overexpression of PLCdelta(1) increased total [(3)H]inositol phosphate (InsP) content and elevated [(3)H]Ins(1,4,5)P(3), but failed to increase [(3)H]InsP responses to the Ca(2+) ionophore A23187. Antisense PLCdelta(1) expression reduced endogenous PLCdelta(1) content but did not decrease the A23187 response. In permeabilized NRVM, [(3)H]InsP responses to elevated Ca(2+) were not inhibited by Ins(1,4,5)P(3), even at concentrations 1000-fold greater than required for selective inhibition of PLCdelta(1). Taken together these data provide evidence that PLCdelta(1) does not mediate the InsP response to elevated Ca(2+) in NRVM.

Protection against glutamate toxicity through inhibition of the p44/42 mitogen-activated protein kinase pathway in neuronally differentiated P19 cells

Excessive levels of the neurotransmitter glutamate trigger excitotoxic processes in neurons that lead to cell death. N-Methyl-D-aspartate (NMDA) receptor over-activation is a key excitotoxic stimulus that leads to increases in intracellular calcium and activation of downstream signaling pathways, including the p44/42 mitogen-activated protein (MAP) kinase pathway. In the present study, we have demonstrated that 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (U0126), a potent and selective inhibitor of the p44/42 MAP kinase signaling pathway, prevents glutamate-induced death in neuronally differentiated P19 cells. In addition, we show that differentiated, but not undifferentiated, P19 cells expressed zeta1, epsilon1, and epsilon2 subunits of the NMDA receptor. Differentiated P19 cells exhibited specific NMDA receptor binding and intracellular calcium responses to glutamate that were blocked by the selective NMDA receptor antagonist [5R,10S]-[+]-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), but not U0126. Glutamate treatment of differentiated P19 cells triggered a rapid and sustained induction in p42 MAP kinase phosphorylation that was blocked by U0126. Pretreatment of differentiated P19 cells with U0126, but not other classes of protein kinase inhibitors, protected against glutamate-induced cell death. Post-treatment with U0126, even as late as 6 hr after glutamate application, also protected against glutamate toxicity. These results suggest that the p44/42 MAP kinase pathway may be a critical downstream signaling pathway in glutamate receptor-activated toxicity.

Transcription of the inositol polyphosphate 1-phosphatase gene (INPP1) is upregulated in human colorectal cancer

We have used suppression subtractive hybridization to demonstrate significant overexpression of the inositol polyphosphate 1-phosphatase gene (INPP1) in colorectal cancer compared with matched normal colon epithelium. Its gene product catalyses the hydrolysis of inositol 1,3,4-trisphosphate and inositol 1, 4-bisphosphate, a key molecule in the phosphoinositide metabolic and signaling pathways. Following confirmation of the differential expression by reverse Northern dot blot analysis, fully quantitative Taqman reverse transcriptase-polymerase chain reaction assays showed that its transcription was upregulated in 42/49 colorectal tumors. There was no significant difference in four tumors and reduced transcription was observed in three. This is the first study to report the upregulation of the INPP1 gene in a human cancer and should facilitate further studies looking at the role of phosphatidylinositol signaling reactions in human colorectal cancer.

Inositol 1,3,4-trisphosphate acts in vivo as a specific regulator of cellular signaling by inositol 3,4,5,6-tetrakisphosphate

Ca2+-activated Cl- channels are inhibited by inositol 3,4,5, 6-tetrakisphosphate (Ins(3,4,5,6)P4) (Xie, W., Kaetzel, M. A., Bruzik, K. S., Dedman, J. R., Shears, S. B., and Nelson, D. J. (1996) J. Biol. Chem. 271, 14092-14097), a novel second messenger that is formed after stimulus-dependent activation of phospholipase C (PLC). In this study, we show that inositol 1,3,4-trisphosphate (Ins(1,3,4)P3) is the specific signal that ties increased cellular levels of Ins(3,4,5,6)P4 to changes in PLC activity. We first demonstrated that Ins(1,3,4)P3 inhibited Ins(3,4,5,6)P4 1-kinase activity that was either (i) in lysates of AR4-2J pancreatoma cells or (ii) purified 22,500-fold (yield = 13%) from bovine aorta. Next, we incubated [3H]inositol-labeled AR4-2J cells with cell permeant and non-radiolabeled 2,5,6-tri-O-butyryl-myo-inositol 1,3, 4-trisphosphate-hexakis(acetoxymethyl) ester. This treatment increased cellular levels of Ins(1,3,4)P3 2.7-fold, while [3H]Ins(3, 4,5,6)P4 levels increased 2-fold; there were no changes to levels of other 3H-labeled inositol phosphates. This experiment provides the first direct evidence that levels of Ins(3,4,5,6)P4 are regulated by Ins(1,3,4)P3 in vivo, independently of Ins(1,3,4)P3 being metabolized to Ins(3,4,5,6)P4. In addition, we found that the Ins(1, 3,4)P3 metabolites, namely Ins(1,3)P2 and Ins(3,4)P2, were >100-fold weaker inhibitors of the 1-kinase compared with Ins(1,3,4)P3 itself (IC50 = 0.17 microM). This result shows that dephosphorylation of Ins(1,3,4)P3 in vivo is an efficient mechanism to "switch-off" the cellular regulation of Ins(3,4,5,6)P4 levels that comes from Ins(1,3, 4)P3-mediated inhibition of the 1-kinase. We also found that Ins(1,3, 6)P3 and Ins(1,4,6)P3 were poor inhibitors of the 1-kinase (IC50 = 17 and >30 microM, respectively). The non-physiological trisphosphates, D/L-Ins(1,2,4)P3, inhibited 1-kinase relatively potently (IC50 = 0.7 microM), thereby suggesting a new strategy for the rational design of therapeutically useful kinase inhibitors. Overall, our data provide new information to support the idea that Ins(1,3,4)P3 acts in an important signaling cascade.

Cloning and characterization of a mammalian lithium-sensitive bisphosphate 3'-nucleotidase inhibited by inositol 1,4-bisphosphate

Discovery of a structurally conserved metal-dependent lithium-inhibited phosphomonoesterase protein family has identified several potential cellular targets of lithium as used to treat manic depression. Here we describe identification of a novel family member using a "computer cloning" strategy. Human and murine cDNA clones encoded proteins sharing 92% identity and were highly expressed in kidney. Native and recombinant protein harbored intrinsic magnesium-dependent bisphosphate nucleotidase activity (BPntase), which removed the 3'-phosphate from 3'-5' bisphosphate nucleosides and 3'-phosphoadenosine 5'-phosphosulfate with Km and Vmax values of 0.5 microM and 40 micromol/min/mg. Lithium uncompetitively inhibited activity with a Ki of 157 microM. Interestingly, BPntase was competitively inhibited by inositol 1,4-bisphosphate with a Ki of 15 microM. Expression of mammalian BPntase complemented defects in hal2/met22 mutant yeast. These data suggest that BPntase's physiologic role in nucleotide metabolism may be regulated by inositol signaling pathways. The presence of high levels of BPntase in the kidney are provocative in light of the roles of bisphosphorylated nucleotides in regulating salt tolerance, sulfur assimilation, detoxification, and lithium toxicity. We propose that inhibition of human BPntase may account for lithium-induced nephrotoxicity, which may be overcome by supplementation of current therapeutic regimes with inhibitors of nucleotide biosynthesis, such as methionine.

The versatility of inositol phosphates as cellular signals

Cells from across the phylogenetic spectrum contain a variety of inositol phosphates. Many different functions have been ascribed to this group of compounds. However, it is remarkable how frequently several of these different inositol phosphates have been linked to various aspects of signal transduction. Therefore, this review assesses the evidence that inositol phosphates have evolved into a versatile family of second messengers.

Effect of concomitant beta-adrenoceptor stimulation on alpha 1-adrenoceptor-mediated increase of inositol-1,4,5-trisphosphate mass in adult rat cardiomyocytes

The aim of the present study was to investigate the accumulation of inositol-1,4,5-trisphosphate (IP3) in isolated adult rat ventricular cardiomyocytes after alpha 1- and beta-adrenoceptor stimulation, separate and in combination, in order to elucidate a possible influence of concomitant beta-adrenoceptor stimulation on the alpha 1-adrenoceptor stimulated response. IP3 was measured by a radioligand binding assay based on an (1,4,5)IP3-specific binding protein from bovine adrenal cortex. The basal IP3 content was 4.06 +/- 0.31 pmol/mg protein (N = 56). alpha 1-Adrenoceptor stimulation resulted in a rapid increase in the IP3 level, which reached a plateau, 50-80% above basal level, at 10-30 sec. The plateau lasted at least up to 120 sec., while at 300 sec. there was no significant difference between control values and values after alpha 1-adrenoceptor stimulation. Li+ did not affect either the basal IP3 level, or the magnitude or time course of alpha 1-adrenoceptor-stimulated IP3 accumulation. Combined adrenoceptor stimulation gave a similar response as separate alpha 1-adrenoceptor stimulation, whereas there was no significant change in the IP3 level after beta-adrenoceptor stimulation. No inhibitory influence of simultaneous beta-adrenoceptor stimulation on the alpha 1-adrenoceptor-stimulated increase of IP3 mass was revealed.

A role for phospholipase C activity in GLUT4-mediated glucose transport

Overexpression of surrogate receptors [epidermal growth factor (EGF) receptor (EGFR) and platelet-derived growth factor receptor] in adipocytes has demonstrated that multiple signaling pathways may lead to GLUT4-mediated glucose uptake. These implicated pathways function independently of IRS-1 phosphorylation and PI3-kinase activation. In addition, we previously demonstrated that EGFR tyrosyl autophosphorylation is required to stimulate GLUT4-mediated glucose transport in 3T3-L1 adipocytes. This observation suggests that signaling molecules that are dependent on EGFR autophosphorylation, such as phospholipase C (PLC), may lie in the signaling pathway to glucose transport. As PLC has been implicated in glucose transport by several clinical and basic mechanistic studies, we investigated whether EGFR signaling may promote glucose transport via modulation of PLC activity. Activation of EGFR overexpressing 3T3-L1 adipocytes leads to a 3.4 +/- 1.2-fold stimulation of PLC activity over basal levels vs. only 1.06 +/- 0.01-fold stimulation by insulin. Pharmacological inhibition of PLC by 50 microM U73122 reduced phosphoinositide accumulation by 79.2 +/- 16.9% and resulted in a concomitant 56.0 +/- 12.7% decrease in EGF-induced glucose transport. This inhibition of glucose transport by U73122 was specific, because the inactive congener, U73343, failed to block EGF-induced glucose transport. Despite the low levels of insulin-induced PLC activity, insulin-stimulated glucose transport activity was similarly inhibited by U73122 (55.9 +/- 13.1% inhibition). Inhibition of PLC activation did not impair either EGF- or insulin-induced activation of glycogen synthase or incorporation of glucose into lipid, supporting the hypothesis that both EGF- and insulin-induced glucose disposal can be independent of GLUT4-mediated glucose transport. The diminution of glucose transport secondary to inhibition of PLC activity was reflected by a decrease in GLUT4 translocation to the plasma membrane upon either EGF or insulin stimulation. These results are consistent with either a permissive or an active role for PLC activity in the translocation of GLUT4 to the plasma membrane.

A salt-sensitive 3'(2'),5'-bisphosphate nucleotidase involved in sulfate activation

Overexpression of a yeast gene, HAL2, allows the cells to tolerate higher than normal extracellular salt concentrations. HAL2 encodes a 3'(2')5'-bisphosphate nucleotidase that serves to remove the end products of sulfate transfer during cellular metabolism. The enzyme is inhibited by lithium and sodium and is activated by potassium. Metabolic systems that are sensitive to salt, as well as those governing osmolyte synthesis and ion transport, offer routes by which genetic engineering can be used to improve the tolerance of various organisms to salt.

Crystal structure of inositol polyphosphate 1-phosphatase at 2.3-A resolution

Bovine inositol polyphosphate 1-phosphatase (1-ptase), M(r) = 44,000, is a Mg(2+)-dependent/Li(+)-sensitive enzyme that catalyzes the hydrolysis of the 1-position phosphate from inositol 1,4-bisphosphate and inositol 1,3,4-trisphosphate. We have determined the crystal structure of recombinant bovine 1-ptase in the presence of Mg2+ by multiple isomorphous replacement. The structure is currently refined to an R value of 0.198 for 15,563 reflections within a resolution range of 8.0-2.3 A. 1-Ptase is monomeric in the crystal, consistent with biochemical data, and folds into an alternatively layered alpha/beta/alpha/beta sandwich. The central core of 1-ptase consists of a six-stranded antiparallel beta sheet perpendicular to two parallel three-turn alpha-helices. The beta sheet is flanked by two antiparallel six-turn alpha-helices aligned parallel to the beta sheet, and the central helices are flanked by a five-stranded largely parallel beta sheet. Two neighboring metal binding sites are located in adjacent acidic pockets formed by the intersection of several secondary structure elements including an unusual kink structure formed by the "DPIDST" sequence motif. The fold of 1-ptase is similar to that of two other metal-dependent/Li(+)-sensitive phosphatases, inositol monophosphate phosphatase and fructose 1,6-bisphosphatase despite minimal amino acid identity. Comparison of the active-site pockets of these proteins will likely provide insight into substrate binding and the mechanisms of metal-dependent catalysis and Li+ inhibition.

Cellular actions of inositol phosphates and other natural calcium and magnesium chelators

Naturally occurring chelators of Ca2+ and Mg2+ have largely been unrecognized due to their low binding affinities. They include carbohydrate and cyclitol phosphates, nucleotides and nucleic acids. The calciotrophic inositol phosphates Ins(1,4,5)P3 and Ins(1,3,4,5)P4 form chelates within the range of Ca2+ concentrations found in biological systems. As well as being a likely source of experimental artifact where these compounds have been investigated at unphysiological cation concentrations, chelation may have important physiological roles. The autoregulation of Ca2+ entry into the cell cytosol is one, whereas the coupling of chelation with enzyme or receptor interactions offers a general mechanism for divalent cation control of diverse biological processes. Inositol monophosphate 1-phosphatase and inositol polyphosphate 1-phosphatase are two related enzymes which may conform to this mechanism. If so, it would provide a possible explanation for their sensitivity to divalent cations and for their non-competitive inhibition by lithium ion.

Biochemical and molecular properties of lithium-sensitive myo-inositol monophosphatase

Myo-inositol monophosphatase is a pivotal enzyme of the inositol second messenger system which is specifically inhibited by therapeutic levels of lithium salts, implicating inhibition of this enzyme as a potential site of its action in bipolar disease. This enzyme has a native molecular weight of 59,000, and has traditionally been found in the cytosolic fraction, although a membrane-bound form has also been identified. Possessing two identical subunits, this enzyme hydrolyzes those monophosphates which are equatorially located within the inositol ring, and several nucleoside monophosphates phosphorylated at the 2-position. Each subunit of the native enzyme contains an active site with unusually large caverns as revealed by crystallographic studies, which may explain the accommodation of these structurally unrelated substrates. We have suggested that the uncompetitive inhibition of this phosphatase by lithium ions may prevent the formation of an enzyme-bound non-isomeric (meso) intermediate, Mg(2+)-inositol 1,3 or 4,6 cyclic monophosphate when this enzyme hydrolyzes its respective isomeric substrates.

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.

Hydrolysis of inositol trisphosphate by purified rat brain myelin

Highly purified rat brain myelin was found to hydrolyze inositol 1,4,5-trisphosphate to inositol 1,4-bisphosphate, but subsequent hydrolysis of the latter, characteristic of whole brainstem, did not occur. Inositol 1,4,5-trisphosphate 5-phosphatase in myelin was approximately 33% of the level in microsomes and 127% that of the cytosolic fraction from brainstem. The myelin and microsomal enzymes had similar properties, as follows: activation by saponin, requirement for Mg2+ and similar Kact (0.16 and 0.13 mM), Km (8.7 +/- 2.5 and 7.0 +/- 1.0 microM), and pH optima (6.6-6.8). Vmax values were 11.2 +/- 1.0 and 26.3 +/- 2.0 nmol/mg/min for myelin and microsomes, respectively. A possible role for this enzyme in phosphoinositide-mediated signal transduction within myelin and its subcompartments is discussed.

Lithium enhances muscarinic receptor-stimulated CDP-diacylglycerol formation in inositol-depleted SK-N-SH neuroblastoma cells

The psychotherapeutic action of Li+ in brain has been proposed to result from the depletion of cellular inositol secondary to its block of inositol monophosphatase. This action is thought to slow phosphoinositide resynthesis, thereby attenuating stimulated phosphoinositidase-mediated signal transduction in affected cells. In the present study, the effect of Li+ on muscarinic receptor-stimulated formation of the immediate precursor of phosphatidylinositol, CDP-diacylglycerol (CDP-DAG), has been examined in human SK-N-SH neuroblastoma cells that have been cultured under conditions that alter the cellular content of myo-inositol. Resting neuroblastoma cells, like brain cells in vivo, were found to concentrate inositol from the culture medium, achieving an intracellular level of 60.0 +/- 4 nmol/mg of protein. The addition of carbachol to [3H]cytidine-prelabeled cells elicited a four- to fivefold increase in the accumulation of labeled CDP-DAG. This stimulated formation of [3H]CDP-DAG was completely blocked by the addition of 10 microM atropine, was not dependent on the presence of Li+, nor was it affected by co-incubation with myo-inositol. This result was in sharp contrast to findings in rat brain slices, in which carbachol-stimulated formation of [3H]CDP-DAG was potentiated approximately 10-fold by Li+ and substantially reduced by coincubation with inositol. The formation of [3H]CDP-DAG in labeled SK-N-SH cells by carbachol was both concentration and time dependent. The order of efficacy of muscarinic ligands in stimulating [3H]-CDP-DAG accumulation paralleled that established in these cells for inositol phosphate accumulation, i.e., carbachol > or = oxotremorine-M > bethanecol > or = arecoline > oxotremorine > pilocarpine.(ABSTRACT TRUNCATED AT 250 WORDS)

Li(+)-induced structural changes of synaptic ribbons are related to the phosphoinositide metabolism in photoreceptor synapses

Synaptic ribbons are specialized cytoskeletal components of the presynaptic exocytotic machinery in photoreceptors. In cone photoreceptors, these structures are highly dynamic disappearing during darkness and reforming in the light phase. In this study we wanted to characterize the second messengers involved in the regulation of these cyclic changes. We show that synaptic ribbons in cone photoreceptors are very sensitive to both Li+ and inositol 1,4,5-trisphosphate suggesting that inositol polyphosphates might play a physiological role in the disassembly of synaptic ribbons. The presence of the phosphoinositide pathway was demonstrated in isolated photoreceptor synapses. The phosphoinositide metabolism in photoreceptor synapses was dark-activated and correlated with the disappearance of synaptic ribbons during dark-adaptation. Since Li+ and inositol 1,4,5-trisphosphate only influenced synaptic ribbons in cones but not in rods the dark-activated phosphoinositide metabolism should be largely ascribed to cone synapses.

Restoration of brain myo-inositol levels in rats increases latency to lithium-pilocarpine seizures

Lithium pretreatment in rats potentiates the epileptogenic effects of pilocarpine and other cholinergic agonists. In order to determine if this effect of lithium could be reversed by myo-inositol, rats were pretreated with intracerebroventricular (ICV) injections of myoinositol, artificial CSF or L-chiro-inositol. Lithium chloride, 3 meq/kg was administered intraperitoneally 20-24 h prior to the subcutaneous injection of pilocarpine, 20 or 30 mg/kg. In both experiments, myo-inositol significantly prolonged the latency to the appearance of clonic seizures and lowered the pilocarpine seizure score. myo-Inositol prevented the development of clonic seizures in 50% of the rats receiving pilocarpine, 20 mg/kg. The levels of cortical myo-inositol in rats injected with myo-inositol were approximately double those of the CSF and L-chiro-inositol groups.