Characterization of a selective inhibitor of inositol hexakisphosphate kinases: use in defining biological roles and metabolic relationships of inositol pyrophosphates

The significance of the 1-kinase/1-phosphatase activities of the PPIP5K family

The inositol pyrophosphates (diphosphoinositol polyphosphates), which include 1-InsP₇, 5-InsP₇, and InsP₈, are highly 'energetic' signaling molecules that play important roles in many cellular processes, particularly with regards to phosphate and bioenergetic homeostasis. Two classes of kinases synthesize the PP-InsPs: IP6Ks and PPIP5Ks. The significance of the IP6Ks - and their 5-InsP₇ product - has been widely reported. However, relatively little is known about the biological significance of the PPIP5Ks. The purpose of this review is to provide an update on developments in our understanding of key features of the PPIP5Ks, which we believe strengthens the hypothesis that their catalytic activities serve important cellular functions. Central to this discussion is the recent discovery that the PPIP5K is a rare example of a single protein that catalyzes a kinase/phosphatase futile cycle.

A High-Throughput Screening-Compatible Strategy for the Identification of Inositol Pyrophosphate Kinase Inhibitors

Pharmacological tools-'chemical probes'-that intervene in cell signaling cascades are important for complementing genetically-based experimental approaches. Probe development frequently begins with a high-throughput screen (HTS) of a chemical library. Herein, we describe the design, validation, and implementation of the first HTS-compatible strategy against any inositol phosphate kinase. Our target enzyme, PPIP5K, synthesizes 'high-energy' inositol pyrophosphates (PP-InsPs), which regulate cell function at the interface between cellular energy metabolism and signal transduction. We optimized a time-resolved, fluorescence resonance energy transfer ADP-assay to record PPIP5K-catalyzed, ATP-driven phosphorylation of 5-InsP7 to 1,5-InsP8 in 384-well format (Z' = 0.82 ± 0.06). We screened a library of 4745 compounds, all anticipated to be membrane-permeant, which are known-or conjectured based on their structures-to target the nucleotide binding site of protein kinases. At a screening concentration of 13 μM, fifteen compounds inhibited PPIP5K >50%. The potency of nine of these hits was confirmed by dose-response analyses. Three of these molecules were selected from different structural clusters for analysis of binding to PPIP5K, using isothermal calorimetry. Acceptable thermograms were obtained for two compounds, UNC10112646 (Kd = 7.30 ± 0.03 μM) and UNC10225498 (Kd = 1.37 ± 0.03 μM). These Kd values lie within the 1-10 μM range generally recognized as suitable for further probe development. In silico docking data rationalizes the difference in affinities. HPLC analysis confirmed that UNC10225498 and UNC10112646 directly inhibit PPIP5K-catalyzed phosphorylation of 5-InsP7 to 1,5-InsP8; kinetic experiments showed inhibition to be competitive with ATP. No other biological activity has previously been ascribed to either UNC10225498 or UNC10112646; moreover, at 10 μM, neither compound inhibits IP6K2, a structurally-unrelated PP-InsP kinase. Our screening strategy may be generally applicable to inhibitor discovery campaigns for other inositol phosphate kinases.

Adipocyte-specific deletion of Ip6k1 reduces diet-induced obesity by enhancing AMPK-mediated thermogenesis

Enhancing energy expenditure (EE) is an attractive strategy to combat obesity and diabetes. Global deletion of Ip6k1 protects mice from diet-induced obesity (DIO) and insulin resistance, but the tissue-specific mechanism by which IP6K1 regulates body weight is unknown. Here, we have demonstrated that IP6K1 regulates fat accumulation by modulating AMPK-mediated adipocyte energy metabolism. Cold exposure led to downregulation of Ip6k1 in murine inguinal and retroperitoneal white adipose tissue (IWAT and RWAT) depots. Adipocyte-specific deletion of Ip6k1 (AdKO) enhanced thermogenic EE, which protected mice from high-fat diet-induced weight gain at ambient temperature (23°C), but not at thermoneutral temperature (30°C). AdKO-induced increases in thermogenesis also protected mice from cold-induced decreases in body temperature. UCP1, PGC1α, and other markers of browning and thermogenesis were elevated in IWAT and RWAT of AdKO mice. Cold-induced activation of sympathetic signaling was unaltered, whereas AMPK was enhanced, in AdKO IWAT. Moreover, beige adipocytes from AdKO IWAT displayed enhanced browning, which was diminished by AMPK depletion. Furthermore, we determined that IP6 and IP6K1 differentially regulate upstream kinase-mediated AMPK stimulatory phosphorylation in vitro. Finally, treating mildly obese mice with the IP6K inhibitor TNP enhanced thermogenesis and inhibited progression of DIO. Thus, IP6K1 regulates energy metabolism via a mechanism that could potentially be targeted in obesity.

Inositol Polyphosphate Kinases, Fungal Virulence and Drug Discovery

Opportunistic fungi are a major cause of morbidity and mortality world-wide, particularly in immunocompromised individuals. Developing new treatments to combat invasive fungal disease is challenging given that fungal and mammalian host cells are eukaryotic, with similar organization and physiology. Even therapies targeting unique fungal cell features have limitations and drug resistance is emerging. New approaches to the development of antifungal drugs are therefore needed urgently. Cryptococcus neoformans, the commonest cause of fungal meningitis worldwide, is an accepted model for studying fungal pathogenicity and driving drug discovery. We recently characterized a phospholipase C (Plc1)-dependent pathway in C. neoformans comprising of sequentially-acting inositol polyphosphate kinases (IPK), which are involved in synthesizing inositol polyphosphates (IP). We also showed that the pathway is essential for fungal cellular function and pathogenicity. The IP products of the pathway are structurally diverse, each consisting of an inositol ring, with phosphate (P) and pyrophosphate (PP) groups covalently attached at different positions. This review focuses on (1) the characterization of the Plc1/IPK pathway in C. neoformans; (2) the identification of PP-IP₅ (IP₇) as the most crucial IP species for fungal fitness and virulence in a mouse model of fungal infection; and (3) why IPK enzymes represent suitable candidates for drug development.

TNP [N2-(m-Trifluorobenzyl), N6-(p-nitrobenzyl)purine] ameliorates diet induced obesity and insulin resistance via inhibition of the IP6K1 pathway

Objective

Obesity and type 2 diabetes (T2D) lead to various life-threatening diseases such as coronary heart disease, stroke, osteoarthritis, asthma, and neurodegeneration. Therefore, extensive research is ongoing to identify novel pathways that can be targeted in obesity/T2D. Deletion of the inositol pyrophosphate (5-IP7) biosynthetic enzyme, inositol hexakisphosphate kinase-1 (IP6K1), protects mice from high fat diet (HFD) induced obesity (DIO) and insulin resistance. Yet, whether this pathway is a valid pharmacologic target in obesity/T2D is not known. Here, we demonstrate that TNP [N2-(m-Trifluorobenzyl), N6-(p-nitrobenzyl)purine], a pan-IP6K inhibitor, has strong anti-obesity and anti-diabetic effects in DIO mice.

Methods

Q-NMR, GTT, ITT, food intake, energy expenditure, QRT-PCR, ELISA, histology, and immunoblot studies were conducted in short (2.5-week)- and long (10-week)-term TNP treated DIO C57/BL6 WT and IP6K1-KO mice, under various diet and temperature conditions.

Results

TNP, when injected at the onset of HFD-feeding, decelerates initiation of DIO and insulin resistance. Moreover, TNP facilitates weight loss and restores metabolic parameters, when given to DIO mice. However, TNP does not reduce weight gain in HFD-fed IP6K1-KO mice. TNP specifically enhances insulin sensitivity in DIO mice via Akt activation. TNP decelerates weight gain primarily by enhancing thermogenic energy expenditure in the adipose tissue. Accordingly, TNP's effect on body weight is partly abolished whereas its impact on glucose homeostasis is preserved at thermoneutral temperature.

Conclusion

Pharmacologic inhibition of the inositol pyrophosphate pathway has strong therapeutic potential in obesity, T2D, and other metabolic diseases.

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Pharmacologic inhibition of IP6K by TNP, at the onset of high fat feeding, decelerates initiation of DIO and insulin resistance in mice.

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TNP, when treated to DIO mice, promotes weight loss and restores metabolic homeostasis.

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TNP does not reduce high fat diet induced weight gain in IP6K1-KO mice.

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TNP promotes insulin sensitivity by stimulating Akt activity, whereas it reduces body weight primarily by enhancing thermogenic energy expenditure.

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Long-term TNP treatment does not display deleterious side effects.

Inositol Hexakisphosphate Kinase 1 (IP6K1) Regulates Inositol Synthesis in Mammalian Cells

myo-Inositol, the precursor of all inositol compounds, has pivotal roles in cell metabolism and signaling pathways. Although physiological studies indicate a strong correlation between abnormal intracellular inositol levels and neurological disorders, very little is known about the regulation of inositol synthesis in mammalian cells. In this study, we report that IP6K1, an inositol hexakisphosphate kinase that catalyzes the synthesis of inositol pyrophosphate, regulates inositol synthesis in mammalian cells. Ip6k1 ablation led to profound changes in DNA methylation and expression of Isyna1 (designated mIno1), which encodes the rate-limiting enzyme inositol-3-phosphate synthase. Interestingly, IP6K1 preferentially bound to the phospholipid phosphatidic acid, and this binding was required for IP6K1 nuclear localization and the regulation of mIno1 transcription. This is the first demonstration of IP6K1 as a novel negative regulator of inositol synthesis in mammalian cells.

Identification of a functional nuclear translocation sequence in hPPIP5K2

BACKGROUND

Cells contain several inositol pyrophosphates (PP-InsPs; also known as diphosphoinositol polyphosphates), which play pivotal roles in cellular and organismic homeostasis. It has been proposed that determining mechanisms of compartmentation of the synthesis of a particular PP-InsP is key to understanding how each of them may exert a specific function. Human PPIP5K2 (hPPIP5K2), one of the key enzymes that synthesizes PP-InsPs, contains a putative consensus sequence for a nuclear localization signal (NLS). However, such in silico analysis has limited predictive power, and may be complicated by phosphorylation events that can dynamically modulate NLS function. We investigated if this candidate NLS is functional and regulated, using the techniques of cell biology, mutagenesis and mass spectrometry.

RESULTS

Multiple sequence alignments revealed that the metazoan PPIP5K2 family contains a candidate NLS within a strikingly well-conserved 63 amino-acid domain. By analyzing the distribution of hPPIP5K2-GFP in HEK293T cells with the techniques of confocal microscopy and imaging flow cytometry, we found that a distinct pool of hPPIP5K2 is present in the nucleus. Imaging flow cytometry yielded particular insight into the characteristics of the nuclear hPPIP5K2 sub-pool, through a high-throughput, statistically-robust analysis of many hundreds of cells. Mutagenic disruption of the candidate NLS in hPPIP5K2 reduced its degree of nuclear localization. Proximal to the NLS is a Ser residue (S1006) that mass spectrometry data indicate is phosphorylated inside cells. The degree of nuclear localization of hPPIP5K2 was increased when S1006 was rendered non-phosphorylatable by its mutation to Ala. Conversely, a S1006D phosphomimetic mutant of hPPIP5K2 exhibited a lower degree of nuclear localization.

CONCLUSIONS

The current study describes for the first time the functional significance of an NLS in the conserved PPIP5K2 family. We have further demonstrated that there is phosphorylation of a Ser residue that is proximal to the NLS of hPPIP5K2. These conclusions draw attention to nuclear compartmentation of PPIP5K2 as being a physiologically relevant and covalently-regulated event. Our study also increases general insight into the consensus sequences of other NLSs, the functions of which might be similarly regulated.

Fungal Inositol Pyrophosphate IP7 Is Crucial for Metabolic Adaptation to the Host Environment and Pathogenicity

Inositol pyrophosphates (PP-IPs) comprising inositol, phosphate, and pyrophosphate (PP) are essential for multiple functions in eukaryotes. Their role in fungal pathogens has never been addressed. Cryptococcus neoformans is a model pathogenic fungus causing life-threatening meningoencephalitis. We investigate the cryptococcal kinases responsible for the production of PP-IPs (IP₇/IP₈) and the hierarchy of PP-IP importance in pathogenicity. Using gene deletion and inositol polyphosphate profiling, we identified Kcs1 as the major IP₆ kinase (producing IP₇) and Asp1 as an IP₇ kinase (producing IP₈). We show that Kcs1-derived IP₇ is the most crucial PP-IP for cryptococcal drug susceptibility and the production of virulence determinants. In particular, Kcs1 kinase activity is essential for cryptococcal infection of mouse lungs, as reduced fungal burdens were observed in the absence of Kcs1 or when Kcs1 was catalytically inactive. Transcriptome and carbon source utilization analysis suggested that compromised growth of the KCS1 deletion strain (Δkcs1 mutant) in the low-glucose environment of the host lung is due to its inability to utilize alternative carbon sources. Despite this metabolic defect, the Δkcs1 mutant established persistent, low-level asymptomatic pulmonary infection but failed to elicit a strong immune response in vivo and in vitro and was not readily phagocytosed by primary or immortalized monocytes. Reduced recognition of the Δkcs1 cells by monocytes correlated with reduced exposure of mannoproteins on the Δkcs1 mutant cell surface. We conclude that IP₇ is essential for fungal metabolic adaptation to the host environment, immune recognition, and pathogenicity.

Cryptococcus neoformans is responsible for 1 million cases of AIDS-associated meningitis and ~600,000 deaths annually. Understanding cellular pathways responsible for pathogenicity might have an impact on new drug development. We characterized the inositol polyphosphate kinases Kcs1 and Asp1, which are predicted to catalyze the production of inositol pyrophosphates containing one or two diphosphate moieties (PP-IPs). Using gene deletion analysis and inositol polyphosphate profiling, we confirmed that Kcs1 and Asp1 are major IP₆ and IP₇ kinases, respectively. Kcs1-derived IP₇, but not Asp1-derived IP₈, is crucial for pathogenicity. Global expression profiling and carbon source utilization testing suggest that IP₇-deficient cryptococci cannot adapt their metabolism to allow growth in the glucose-poor environment of the host lung, and consequently, fungal burdens are significantly reduced. Persistent asymptomatic Δkcs1 mutant infection correlated with decreased mannoprotein exposure on the Δkcs1 mutant surface and reduced phagocytosis. We conclude that IP₇ is crucial for the metabolic adaptation of C. neoformans to the host environment and for pathogenicity.

VIH2 Regulates the Synthesis of Inositol Pyrophosphate InsP8 and Jasmonate-Dependent Defenses in Arabidopsis

Diphosphorylated inositol polyphosphates, also referred to as inositol pyrophosphates, are important signaling molecules that regulate critical cellular activities in many eukaryotic organisms, such as membrane trafficking, telomere maintenance, ribosome biogenesis, and apoptosis. In mammals and fungi, two distinct classes of inositol phosphate kinases mediate biosynthesis of inositol pyrophosphates: Kcs1/IP6K- and Vip1/PPIP5K-like proteins. Here, we report that PPIP5K homologs are widely distributed in plants and that Arabidopsis thaliana VIH1 and VIH2 are functional PPIP5K enzymes. We show a specific induction of inositol pyrophosphate InsP8 by jasmonate and demonstrate that steady state and jasmonate-induced pools of InsP8 in Arabidopsis seedlings depend on VIH2. We identify a role of VIH2 in regulating jasmonate perception and plant defenses against herbivorous insects and necrotrophic fungi. In silico docking experiments and radioligand binding-based reconstitution assays show high-affinity binding of inositol pyrophosphates to the F-box protein COI1-JAZ jasmonate coreceptor complex and suggest that coincidence detection of jasmonate and InsP8 by COI1-JAZ is a critical component in jasmonate-regulated defenses.

Inositol hexakisphosphate kinase-1 mediates assembly/disassembly of the CRL4-signalosome complex to regulate DNA repair and cell death

Inositol polyphosphates containing an energetic pyrophosphate bond are formed primarily by a family of three inositol hexakisphosphate (IP6) kinases (IP6K1-3). The Cullin-RING ubiquitin ligases (CRLs) regulate diverse biological processes through substrate ubiquitylation. CRL4, comprising the scaffold Cullin 4A/B, the E2-interacting Roc1/2, and the adaptor protein damage-specific DNA-binding protein 1, is activated by DNA damage. Basal CRL4 activity is inhibited by binding to the COP9 signalosome (CSN). UV radiation and other stressors dissociate the complex, leading to E3 ligase activation, but signaling events that trigger signalosome dissociation from CRL4 have been unclear. In the present study, we show that, under basal conditions, IP6K1 forms a ternary complex with CSN and CRL4 in which IP6K1 and CRL4 are inactive. UV dissociates IP6K1 to generate IP7, which then dissociates CSN-CRL4 to activate CRL4. Thus, IP6K1 is a novel CRL4 subunit that transduces UV signals to mediate disassembly of the CRL4-CSN complex, thereby regulating nucleotide excision repair and cell death.

Inositol pyrophosphates: why so many phosphates?

The inositol pyrophosphates (PP-InsPs) are a specialized group of "energetic" signaling molecules found in yeasts, plants and animals. PP-InsPs boast the most crowded three dimensional phosphate arrays found in Nature; multiple phosphates and diphosphates are crammed around the six-carbon, inositol ring. Yet, phosphate esters are also a major energy currency in cells. So the synthesis of PP-InsPs, and the maintenance of their levels in the face of a high rate of ongoing turnover, all requires significant bioenergetic input. What are the particular properties of PP-InsPs that repay this investment of cellular energy? Potential answers to that question are discussed here, against the backdrop of a recent hypothesis that signaling by PP-InsPs is evolutionarily ancient. The latter idea is extended herein, with the proposal that the primordial origins of PP-InsPs is reflected in the apparent lack of isomeric specificity of certain of their actions. Nevertheless, there are other aspects of signaling by these polyphosphates that are more selective for a particular PP-InsP isomer. Consideration of the nature of both specific and non-specific effects of PP-InsPs can help rationalize why such molecules possess so many phosphates.

Discovery of InsP6-kinases as InsP6-dephosphorylating enzymes provides a new mechanism of cytosolic InsP6 degradation driven by the cellular ATP/ADP ratio

InsP6 (inositol hexakisphosphate), the most abundant inositol phosphate in metazoa, is pyrophosphorylated to InsP7 [5PP-InsP5 (diphosphoinositol pentakisphosphate)] by cytosolic and nuclear IP6Ks (InsP6 kinases) and to 1PP-InsP5 by another InsP6/InsP7 kinase family. MINPP1 (multiple inositol-polyphosphate phosphatase 1), the only known InsP6 phosphatase, is localized in the ER (endoplasmic reticulum) and lysosome lumina. A mechanism of cytosolic InsP6 dephosphorylation has remained enigmatic so far. In the present study, we demonstrated that IP6Ks change their kinase activity towards InsP6 at a decreasing ATP/ADP ratio to an ADP phosphotransferase activity and dephosphorylate InsP6. Enantio-selective analysis revealed that Ins(2,3,4,5,6)P5 is the main InsP5 product of the IP6K reaction, whereas the exclusive product of MINPP1 activity is the enantiomer Ins(1,2,4,5,6)P5. Whereas lentiviral RNAi-based depletion of MINPP1 at falling cellular ATP/ADP ratios had no significant impact on Ins(2,3,4,5,6)P5 production, the use of the selective IP6K inhibitor TNP [N2-(m-trifluorobenzyl),N6-(p-nitrobenzyl)purine] abolished the production of this enatiomer in different types of cells. Furthermore, by analysis of rat tissue and human blood samples all (main and minor) dephosphorylation products of InsP6 were detected in vivo. In summary, we identified IP6Ks as novel nuclear and cytosolic InsP6- (and InsP5-) dephosphorylating enzymes whose activity is sensitively driven by a decrease in the cellular ATP/ADP ratio, thus suggesting a role for IP6Ks as cellular adenylate energy 'sensors'.

Inositol phosphates induce DAPI fluorescence shift

The polymer inorganic polyP (polyphosphate) and inositol phosphates, such as IP6 (inositol hexakisphosphate; also known as phytic acid), share many biophysical features. These similarities must be attributed to the phosphate groups present in these molecules. Given the ability of polyP to modify the excitation-emission spectra of DAPI we decided to investigate whether inositol phosphates possess the same property. We discovered that DAPI-IP6 complexes emit at approximately 550 nm when excited with light of wavelength 410-420 nm. IP5 (inositol pentakisphosphate) is also able to induce a similar shift in DAPI fluorescence. Conversely, IP3 (inositol trisphosphate) and IP4 (inositol tetrakisphosphate) are unable to shift DAPI fluorescence. We have employed this newly discovered feature of DAPI to study the enzymatic activity of the inositol polyphosphate multikinase and to monitor phytase phosphatase reactions. Finally, we used DAPI-IP6 fluorescence to determine the amount of IP6 in plant seeds. Using an IP6 standard curve this straight-forward analysis revealed that among the samples tested, borlotti beans possess the highest level of IP6 (9.4 mg/g of dry mass), whereas the Indian urad bean the lowest (3.2 mg/g of dry mass). The newly identified fluorescence properties of the DAPI-IP5 and DAPI-IP6 complexes allow the levels and enzymatic conversion of these two important messengers to be rapidly and reliably monitored.

IP6K structure and the molecular determinants of catalytic specificity in an inositol phosphate kinase family

Inositol trisphosphate kinases (IP3Ks) and inositol hexakisphosphate kinases (IP6Ks) each regulate specialized signalling activities by phosphorylating either InsP3 or InsP6 respectively. The molecular basis for these different kinase activities can be illuminated by a structural description of IP6K. Here we describe the crystal structure of an Entamoeba histolytica hybrid IP6K/IP3K, an enzymatic parallel to a 'living fossil'. Through molecular modelling and mutagenesis, we extrapolated our findings to human IP6K2, which retains vestigial IP3K activity. Two structural elements, an α-helical pair and a rare, two-turn 310 helix, together forge a substrate-binding pocket with an open clamshell geometry. InsP6 forms substantial contacts with both structural elements. Relative to InsP6, enzyme-bound InsP3 rotates 55° closer to the α-helices, which provide most of the protein's interactions with InsP3. These data reveal the molecular determinants of IP6K activity, and suggest an unusual evolutionary trajectory for a primordial kinase that could have favored efficient bifunctionality, before propagation of separate IP3Ks and IP6Ks.

Inositol pyrophosphates mediate the effects of aging on bone marrow mesenchymal stem cells by inhibiting Akt signaling

Introduction

Bone marrow-derived mesenchymal stem cells (BM-MSCs) have been proposed as an ideal autologous stem cell source for cell-based therapy for myocardial infarction (MI). However, decreased viability and impaired function of aged MSCs hampered the therapeutic efficacy of engrafted MSCs, and the underlying mechanisms remain unclarified. Here, we investigated the role of inositol phosphates 6 kinase (IP6Ks) inhibition on the therapeutic efficacy of BM-MSCs and its underlying mechanism.

Methods

BM-MSCs isolated from young (8-week-old) or aged (18-month-old) donor male C57BL/6 mice, were subjected to hypoxia and serum deprivation (H/SD) injury with or without administration of inositol phosphates 6 kinase (IP6Ks) inhibitor TNP (10 μM). MSC apoptosis induced by H/SD was determined by flow cytometry and TUNEL assays. Protein expressions were evaluated by Western blot assay. Furthermore, the paracrine effects of MSCs were measured by reverse transcriptase–polymerized chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA) analyses.

Results

Aged BM-MSCs exhibited more Inositol pyrophosphate 7 (IP7) production, compared with young BM-MSCs. Meanwhile, the expression of phospho-Akt (Thr308) was significantly decreased in the aged MSCs, resulting in enhanced Bad activation and decreased Bax/Bcl-2 ratio. Moreover, the apoptosis in aged BM-MSCs was increased, compared with young BM-MSCs. Furthermore, TNP administration significantly inhibited IP7 production and increased the phosphorylation of Akt under both normoxic and hypoxic conditions. Meanwhile, IP6Ks inhibition reduced apoptotic index of aged MSCs, associated with decreased expressions of pro-apoptotic proteins Bax and Bad and increased anti-apoptotic protein Bcl-2. The expressions of angiogenic factors, including VEGF, bFGF, IGF-1 and HGF, were decreased in MSCs from aged mice. In addition, TNP administration enhanced the paracrine efficiency of aged BM-MSCs under normoxic and hypoxic conditions.

Conclusions

This study demonstrates for the first time that IP6Ks and IP7 play critical role in the aging related vulnerability to hypoxic injury and impaired paracrine efficiency of BM-MSCs, which is associated with impaired Akt activation.

Inositol pyrophosphates mediate the DNA-PK/ATM-p53 cell death pathway by regulating CK2 phosphorylation of Tti1/Tel2

The apoptotic actions of p53 require its phosphorylation by a family of phosphoinositide-3-kinase-related-kinases (PIKKs), which include DNA-PKcs and ATM. These kinases are stabilized by the TTT (Tel2, Tti1, Tti2) cochaperone family, whose actions are mediated by CK2 phosphorylation. The inositol pyrophosphates, such as 5-diphosphoinositol pentakisphosphate (IP7), are generated by a family of inositol hexakisphosphate kinases (IP6Ks), of which IP6K2 has been implicated in p53-associated cell death. In the present study we report an apoptotic signaling cascade linking CK2, TTT, the PIKKs, and p53. We demonstrate that IP7, formed by IP6K2, binds CK2 to enhance its phosphorylation of the TTT complex, thereby stabilizing DNA-PKcs and ATM. This process stimulates p53 phosphorylation at serine 15 to activate the cell death program in human cancer cells and in murine B cells.

Human genome-wide RNAi screen identifies an essential role for inositol pyrophosphates in Type-I interferon response

The pattern recognition receptor RIG-I is critical for Type-I interferon production. However, the global regulation of RIG-I signaling is only partially understood. Using a human genome-wide RNAi-screen, we identified 226 novel regulatory proteins of RIG-I mediated interferon-β production. Furthermore, the screen identified a metabolic pathway that synthesizes the inositol pyrophosphate 1-IP7 as a previously unrecognized positive regulator of interferon production. Detailed genetic and biochemical experiments demonstrated that the kinase activities of IPPK, PPIP5K1 and PPIP5K2 (which convert IP5 to1-IP7) were critical for both interferon induction, and the control of cellular infection by Sendai and influenza A viruses. Conversely, ectopically expressed inositol pyrophosphate-hydrolases DIPPs attenuated interferon transcription. Mechanistic experiments in intact cells revealed that the expression of IPPK, PPIP5K1 and PPIP5K2 was needed for the phosphorylation and activation of IRF3, a transcription factor for interferon. The addition of purified individual inositol pyrophosphates to a cell free reconstituted RIG-I signaling assay further identified 1-IP7 as an essential component required for IRF3 activation. The inositol pyrophosphate may act by β-phosphoryl transfer, since its action was not recapitulated by a synthetic phosphonoacetate analogue of 1-IP7. This study thus identified several novel regulators of RIG-I, and a new role for inositol pyrophosphates in augmenting innate immune responses to viral infection that may have therapeutic applications.

The innate immune system is critical for viral infection control by host organisms. The type I interferons are a family of major antiviral cytokines produced upon the activation of innate immune pattern recognition receptors (PRRs) by viruses. The RIG-I is a major PRR that uniquely detects RNA viruses within the cytoplasm. In this study, we aimed to discover cellular genes and pathways that play regulatory roles in the transcriptional induction of type I interferon-β (IFNβ). Using a human genome wide RNA interference (RNAi) screening, we identified 226 genes whose expression is important for proper IFNβ production. Through bioinformatics-based mining of the RNAi screen results, we identified that the cellular pathway synthesizing inositol pyrophosphates, a class of inositol phosphates with high-energy diphosphates, is a key positive regulator of RIG-I mediated IFNβ production. The kinases IPPK, PPIP5K1 and PPIP5K2, that synthesize inositol pyrophosphate 1-IP7, regulated IFNβ response in a catalytically dependent manner. Mechanistic studies identified that 1-IP7 synthesis pathway was needed for efficient phosphorylation of IRF3. The DIPP family of inositol pyrophosphate hydrolases negatively regulated the IFNβ response, upon ectopic expression. In summary, this study generated a global view of the regulation of RIG-I signaling, and identified inositol pyrophosphates as important regulators of antiviral response.

Akt/PKB activation and insulin signaling: a novel insulin signaling pathway in the treatment of type 2 diabetes

Type 2 diabetes is a metabolic disease categorized primarily by reduced insulin sensitivity, β-cell dysfunction, and elevated hepatic glucose production. Treatments reducing hyperglycemia and the secondary complications that result from these dysfunctions are being sought after. Two distinct pathways encourage glucose transport activity in skeletal muscle, ie, the contraction-stimulated pathway reliant on Ca(2+)/5'-monophosphate-activated protein kinase (AMPK)-dependent mechanisms and an insulin-dependent pathway activated via upregulation of serine/threonine protein kinase Akt/PKB. Metformin is an established treatment for type 2 diabetes due to its ability to increase peripheral glucose uptake while reducing hepatic glucose production in an AMPK-dependent manner. Peripheral insulin action is reduced in type 2 diabetics whereas AMPK signaling remains largely intact. This paper firstly reviews AMPK and its role in glucose uptake and then focuses on a novel mechanism known to operate via an insulin-dependent pathway. Inositol hexakisphosphate (IP6) kinase 1 (IP6K1) produces a pyrophosphate group at the position of IP6 to generate a further inositol pyrophosphate, ie, diphosphoinositol pentakisphosphate (IP7). IP7 binds with Akt/PKB at its pleckstrin homology domain, preventing interaction with phosphatidylinositol 3,4,5-trisphosphate, and therefore reducing Akt/PKB membrane translocation and insulin-stimulated glucose uptake. Novel evidence suggesting a reduction in IP7 production via IP6K1 inhibition represents an exciting therapeutic avenue in the treatment of insulin resistance. Metformin-induced activation of AMPK is a key current intervention in the management of type 2 diabetes. However, this treatment does not seem to improve peripheral insulin resistance. In light of this evidence, we suggest that inhibition of IP6K1 may increase insulin sensitivity and provide a novel research direction in the treatment of insulin resistance.

The enzymes of human diphosphoinositol polyphosphate metabolism

Diphospho-myo-inositol polyphosphates have many roles to play, including roles in apoptosis, vesicle trafficking, the response of cells to stress, the regulation of telomere length and DNA damage repair, and inhibition of the cyclin-dependent kinase Pho85 system that monitors phosphate levels. This review focuses on the three classes of enzymes involved in the metabolism of these compounds: inositol hexakisphosphate kinases, inositol hexakisphosphate and diphosphoinositol-pentakisphosphate kinases and diphosphoinositol polyphosphate phosphohydrolases. However, these enzymes have roles beyond being mere catalysts, and their interactions with other proteins have cellular consequences. Through their interactions, the three inositol hexakisphosphate kinases have roles in exocytosis, diabetes, the response to infection, and apoptosis. The two inositol hexakisphosphate and diphosphoinositol-pentakisphosphate kinases influence the cellular response to phosphatidylinositol (3,4,5)-trisphosphate and the migration of pleckstrin homology domain-containing proteins to the plasma membrane. The five diphosphoinositol polyphosphate phosphohydrolases interact with ribosomal proteins and transcription factors, as well as proteins involved in membrane trafficking, exocytosis, ubiquitination and the proteasomal degradation of target proteins. Possible directions for future research aiming to determine the roles of these enzymes are highlighted.

Understanding inositol pyrophosphate metabolism and function: kinetic characterization of the DIPPs

We illuminate the metabolism and the cell-signaling activities of inositol pyrophosphates, by showing that regulation of yeast cyclin-kinase by 1-InsP7 is not conserved for mammalian CDK5, and by kinetically characterizing Ddp1p/DIPP-mediated dephosphorylation of 1-InsP7, 5-InsP7 and InsP8. Each phosphatase exhibited similar Km values for every substrate (range: 35-148 nM). The rank order of kcat values (1-InsP7>5-InsP7=InsP8) was identical for each enzyme, although DIPP1 was 10- to 60-fold more active than DIPP2α/β and DIPP3α/β. We demonstrate InsP8 dephosphorylation preferentially progresses through 1-InsP7. Conversely, we conclude that the more metabolically and functionally significant steady-state route of InsP8 synthesis proceeds via 5-InsP7.

Inositol hexakisphosphate kinase 1 maintains hemostasis in mice by regulating platelet polyphosphate levels

Polyphosphate (polyP), a polymer of orthophosphate moieties released from the dense granules of activated platelets, is a procoagulant agent. Inositol pyrophosphates, another group of phosphate-rich molecules, consist of mono- and diphosphates substituted on an inositol ring. Diphosphoinositol pentakisphosphate (IP7), the most abundant inositol pyrophosphate, is synthesized on phosphorylation of inositol hexakisphosphate (IP6) by IP6 kinases, of which there are 3 mammalian isoforms (IP6K1/2/3) and a single yeast isoform. Yeast lacking IP6 kinase are devoid of polyP, suggesting a role for IP6 kinase in maintaining polyP levels. We theorized that the molecular link between IP6 kinase and polyP is conserved in mammals and investigated whether polyP-dependent platelet function is altered in IP6K1 knockout (Ip6k1(-/-)) mice. We observe a significant reduction in platelet polyP levels in Ip6k1(-/-) mice, along with slower platelet aggregation and lengthened plasma clotting time. Incorporation of polyP into fibrin clots was reduced in Ip6k1(-/-) mice, thereby altering clot ultrastructure, which was rescued on the addition of exogenous polyP. In vivo assays revealed longer tail bleeding time and resistance to thromboembolism in Ip6k1(-/-) mice. Taken together, our data suggest a novel role for IP6K1 in regulation of mammalian hemostasis via its control of platelet polyP levels.

The regulation of runt-related transcription factor 2 by fibroblast growth factor-2 and connexin43 requires the inositol polyphosphate/protein kinase Cδ cascade

Connexin43 (Cx43) plays a critical role in osteoblast function and bone mass accrual, yet the identity of the second messengers communicated by Cx43 gap junctions, the targets of these second messengers and how they regulate osteoblast function remain largely unknown. We have shown that alterations of Cx43 expression in osteoblasts can impact the responsiveness to fibroblast growth factor-2 (FGF2), by modulating the transcriptional activity of runt-related transcription factor 2 (Runx2). In this study, we examined the contribution of the phospholipase Cγ1/inositol polyphosphate/protein kinase C delta (PKCδ) cascade to the Cx43-dependent transcriptional response of MC3T3 osteoblasts to FGF2. Knockdown of expression and/or inhibition of function of phospholipase Cγ1, inositol polyphosphate multikinase, which generates inositol 1,3,4,5-tetrakisphosphate (InsP₄) and InsP₅, and inositol hexakisphosphate kinase 1/2, which generates inositol pyrophosphates, prevented the ability of Cx43 to potentiate FGF2-induced signaling through Runx2. Conversely, overexpression of phospholipase Cγ1 and inositol hexakisphosphate kinase 1/2 enhanced FGF2 activation of Runx2 and the effect of Cx43 overexpression on this response. Disruption of these pathways blocked the nuclear accumulation of PKCδ and the FGF2-dependent interaction of PKCδ and Runx2, reducing Runx2 transcriptional activity. These data reveal that FGF2-signaling involves the inositol polyphosphate cascade, including inositol hexakisphosphate kinase (IP6K), and demonstrate that IP6K regulates Runx2 and osteoblast gene expression. Additionally, these data implicate the water-soluble inositol polyphosphates as mediators of the Cx43-dependent amplification of the osteoblast response to FGF2, and suggest that these low molecular weight second messengers may be biologically relevant mediators of osteoblast function that are communicated by Cx43-gap junctions.

Inositol pyrophosphates: between signalling and metabolism

The present review will explore the insights gained into inositol pyrophosphates in the 20 years since their discovery in 1993. These molecules are defined by the presence of the characteristic ‘high energy’ pyrophosphate moiety and can be found ubiquitously in eukaryotic cells. The enzymes that synthesize them are similarly well distributed and can be found encoded in any eukaryote genome. Rapid progress has been made in characterizing inositol pyrophosphate metabolism and they have been linked to a surprisingly diverse range of cellular functions. Two decades of work is now beginning to present a view of inositol pyrophosphates as fundamental, conserved and highly important agents in the regulation of cellular homoeostasis. In particular it is emerging that energy metabolism, and thus ATP production, is closely regulated by these molecules. Much of the early work on these molecules was performed in the yeast Saccharomyces cerevisiae and the social amoeba Dictyostelium discoideum, but the development of mouse knockouts for IP6K1 and IP6K2 [IP6K is IP6 (inositol hexakisphosphate) kinase] in the last 5 years has provided very welcome tools to better understand the physiological roles of inositol pyrophosphates. Another recent innovation has been the use of gel electrophoresis to detect and purify inositol pyrophosphates. Despite the advances that have been made, many aspects of inositol pyrophosphate biology remain far from clear. By evaluating the literature, the present review hopes to promote further research in this absorbing area of biology.

The kinetic properties of a human PPIP5K reveal that its kinase activities are protected against the consequences of a deteriorating cellular bioenergetic environment

We obtained detailed kinetic characteristics--stoichiometry, reaction rates, substrate affinities and equilibrium conditions--of human PPIP5K2 (diphosphoinositol pentakisphosphate kinase 2). This enzyme synthesizes 'high-energy' PP-InsPs (diphosphoinositol polyphosphates) by metabolizing InsP₆ (inositol hexakisphosphate) and 5-InsP₇ (5-diphosphoinositol 1,2,3,4,6-pentakisphosphate) to 1-InsP₇ (1-diphosphoinositol 2,3,4,5,6-pentakisphosphate) and InsP₈ (1,5-bis-diphosphoinositol 2,3,4,6-tetrakisphosphate), respectively. These data increase our insight into the PPIP5K2 reaction mechanism and clarify the interface between PPIP5K catalytic activities and cellular bioenergetic status. For example, stochiometric analysis uncovered non-productive, substrate-stimulated ATPase activity (thus, approximately 2 and 1.2 ATP molecules are utilized to synthesize each molecule of 1-InsP₇ and InsP₈, respectively). Impaired ATPase activity of a PPIP5K2-K248A mutant increased atomic-level insight into the enzyme's reaction mechanism. We found PPIP5K2 to be fully reversible as an ATP-synthase in vitro, but our new data contradict previous perceptions that significant 'reversibility' occurs in vivo. PPIP5K2 was insensitive to physiological changes in either [AMP] or [ATP]/[ADP] ratios. Those data, together with adenine nucleotide kinetics (ATP Km=20-40 μM), reveal how insulated PPIP5K2 is from cellular bioenergetic challenges. Finally, the specificity constants for PPIP5K2 revise upwards by one-to-two orders of magnitude the inherent catalytic activities of this enzyme, and we show its equilibrium point favours 80-90% depletion of InsP₆/₅-InsP₇.

Cell signalling by inositol pyrophosphates

Inositol serves as a module for the generation of a high level of molecular diversity through the combinatorial attachment and removal of phosphate groups. The array of potential inositol-containing molecules is further expanded by the generation of diphospho inositol polyphosphates, commonly referred as inositol pyrophosphates. All eukaryotic cells possess inositol pyrophosphates containing one or more diphospho- moieties. The metabolism of this class of molecules is highly dynamic, and the enzymes responsible for their metabolism are evolutionary conserved. This new, exciting class of molecules are uniquely chracterized by a high energetic diphospho- bound that is able to participate in phosphotrasfer reactions thereby generating pyrophosphorylation of protein. However, allosteric mechanisms of action have been also proposed. In the past decade several disparate nuclear and cytoplasmic functions have been attributed to inositol pyrophosphates, ranging from intracellular trafficking to telomere length control and from regulating apoptotic process to stimulating insulin secretion. The extraordinary range of cellular function controlled by inositol pyrophosphate underline their great importance.

Inositol pyrophosphates as mammalian cell signals

Inositol pyrophosphates are highly energetic inositol polyphosphate molecules present in organisms from slime molds and yeast to mammals. Distinct classes of enzymes generate different forms of inositol pyrophosphates. The biosynthesis of these substances principally involves phosphorylation of inositol hexakisphosphate (IP₆) to generate the pyrophosphate IP₇. Initial insights into functions of these substances derived primarily from yeast, which contain a single isoform of IP₆ kinase (yIP₆K), as well as from the slime mold Dictyostelium. Mammalian functions for inositol pyrophosphates have been investigated by using cell lines to establish roles in various processes, including insulin secretion and apoptosis. More recently, mice with targeted deletion of IP₆K isoforms as well as the related inositol polyphosphate multikinase (IPMK) have substantially enhanced our understanding of inositol polyphosphate physiology. Phenotypic alterations in mice lacking inositol hexakisphosphate kinase 1 (IP₆K1) reveal signaling roles for these molecules in insulin homeostasis, obesity, and immunological functions. Inositol pyrophosphates regulate these processes at least in part by inhibiting activation of the serine-threonine kinase Akt. Similar studies of IP₆K2 establish this enzyme as a cell death inducer acting by stimulating the proapoptotic protein p53. IPMK is responsible for generating the inositol phosphate IP₅ but also has phosphatidylinositol 3-kinase activity--that participates in activation of Akt. Here, we discuss recent advances in understanding the physiological functions of the inositol pyrophosphates based in substantial part on studies in mice with deletion of IP₆K isoforms. These findings highlight the interplay of IPMK and IP₆K in regulating growth factor and nutrient-mediated cell signaling.

Inositol hexakisphosphate kinase 1 regulates neutrophil function in innate immunity by inhibiting phosphatidylinositol-(3,4,5)-trisphosphate signaling

Inositol phosphates (InsP) are widely produced throughout animal and plant tissues. Diphosphoinositol pentakisphosphate (InsP7) contains an energetic pyrophosphate bond. Here, we demonstrate that disruption of InsP6K1, one of the three mammalian InsP6Ks that convert InsP6 to InsP7, confers enhanced PtdIns(3,4,5)P3-mediated membrane translocation of Akt pleckstrin homology (PH) domain and thus augments downstream PtdIns(3,4,5)P3 signaling in murine neutrophils. Consequently, these neutrophils exhibited elevated phagocytic and bactericidal capabilities and amplified NADPH oxidase-mediated superoxide production. These phenotypes were replicated in human primary neutrophils with pharmacologically inhibited InsP6Ks. By contrast, increasing intracellular InsP7 amounts blocked chemoattractant-elicited PH domain membrane translocation and dramatically suppressed PtdIns(3,4,5)P3-mediated cellular events in neutrophils. These findings establish a role for InsP7 in signal transduction and provide a mechanism for modulating PtdIns(3,4,5)P3 signaling in neutrophils.

Receptor-dependent compartmentalization of PPIP5K1, a kinase with a cryptic polyphosphoinositide binding domain

The inositol pyrophosphates are multifunctional signalling molecules. One of the families of enzymes that synthesize the inositol pyrophosphates are the Vip1/PPIP5Ks (PP-InsP5 kinases). The kinase domains in Vip1/PPIP5Ks have been mapped to their N-terminus. Each of these proteins also possess a phosphatase-like domain of unknown significance. In the present study, we show that this phosphatase-like domain is not catalytically active. Instead, by using SPR (surface plasmon resonance) to study protein binding to immobilized lipid vesicles, we show that this domain is specialized for binding PtdIns(3,4,5)P3 (PPIP5K1 K(d)=96 nM; PPIP5K2 K(d)=705 nM). Both PtdIns(3,4)P2 and PtdIns(4,5)P2 are significantly weaker ligands, and no significant binding of PtdIns(3,5)P2 was detected. We confirm the functional importance of this domain in inositol lipid binding by site-directed mutagenesis. We present evidence that the PtdIns(3,4,5)P3-binding domain is an unusual hybrid, in which a partial PH (pleckstrin homology) consensus sequence is spliced into the phosphatase-like domain. Agonist-dependent activation of the PtdIns 3-kinase pathway in NIH 3T3 cells drives translocation of PPIP5K1 from the cytosol to the plasma membrane. We have therefore demonstrated receptor-regulated compartmentalization of inositol pyrophosphate synthesis in mammalian cells.

Inositol pyrophosphates inhibit Akt signaling, thereby regulating insulin sensitivity and weight gain

The inositol pyrophosphate IP7 (5-diphosphoinositolpentakisphosphate), formed by a family of three inositol hexakisphosphate kinases (IP6Ks), modulates diverse cellular activities. We now report that IP7 is a physiologic inhibitor of Akt, a serine/threonine kinase that regulates glucose homeostasis and protein translation, respectively, via the GSK3β and mTOR pathways. Thus, Akt and mTOR signaling are dramatically augmented and GSK3β signaling reduced in skeletal muscle, white adipose tissue, and liver of mice with targeted deletion of IP6K1. IP7 affects this pathway by potently inhibiting the PDK1 phosphorylation of Akt, preventing its activation and thereby affecting insulin signaling. IP6K1 knockout mice manifest insulin sensitivity and are resistant to obesity elicited by high-fat diet or aging. Inhibition of IP6K1 may afford a therapeutic approach to obesity and diabetes.

The signaling role of inositol hexakisphosphate kinases (IP6Ks)

The past ten years have seen a contained explosion of interest in inositol pyrophosphates. The early cloning of the IP6Ks and the more recent identification of the PP-IP5Ks have allowed the development of essential experimental tools to investigate the physiological role of inositol pyrophosphates. However, for this exciting field of research to gain momentum, simpler and more reliable research protocols need to be further developed. The ability to resolve and quantify inositol pyrophosphates using gel electrophoresis (Losito et al., 2009) has dramatically altered the way we are studying this class of molecules, opening new avenues for research. The use of this technology to resolve, detect and characterize inositol pyrophosphates extracted from cells certainly represents one desirable aim. The most crucial objective, however, is to obtain definite proof of the new mechanism of post-translational modification by identifying with biophysical methods the presence in vivo of pyrophosphorylated serines. This will hopefully precipitate the development of new ways to detect this modification, for example through the production of antibodies that specifically recognize pyrophosphorylated serines.

Diphosphoinositol polyphosphates: what are the mechanisms?

In countries where adulthood is considered to be attained at age eighteen, 2011 can be the point at which the diphosphoinositol polyphosphates might formally be described as "coming of age", since these molecules were first fully defined in 1993 (Menniti et al., 1993; Stephens et al., 1993b). But from a biological perspective, these polyphosphates cannot quite be considered to have matured into the status of being independently-acting intracellular signals. This review has discussed several of the published proposals for mechanisms by which the diphosphoinositol polyphosphates might act. We have argued that all of these hypotheses need further development.We also still do not know a single molecular mechanism by which a change in the levels of a particular diphosphoinositol polyphosphate can be controlled. Yet, despite all these gaps in our understanding, there is an enduring anticipation that these molecules have great potential in the signaling field. Reflecting our expectations of all teenagers, it should be our earnest hope that in the near future the diphosphoinositol polyphosphates will finally grow up.

Inositol hexakisphosphate kinase-2 acts as an effector of the vertebrate Hedgehog pathway

Inositol phosphate (IP) kinases constitute an emerging class of cellular kinases linked to multiple cellular activities. Here, we report a previously uncharacterized cellular function in Hedgehog (Hh) signaling for the IP kinase designated inositol hexakisphosphate kinase-2 (IP6K2) that produces diphosphoryl inositol phosphates (PP-IPs). In zebrafish embryos, IP6K2 activity was required for normal development of craniofacial structures, somites, and neural crest cells. ip6k2 depletion in both zebrafish and mammalian cells also inhibited Hh target gene expression. Inhibiting IP(6) kinase activity using N(2)-(m-(trifluoromethy)lbenzyl) N(6)-(p-nitrobenzyl)purine (TNP) resulted in altered Hh signal transduction. In zebrafish, restoring IP6K2 levels with exogenous ip6k2 mRNA reversed the effects of IP6K2 depletion. Furthermore, overexpression of ip6k2 in mammalian cells enhanced the Hh pathway response, suggesting IP6K2 is a positive regulator of Hh signaling. Perturbations from IP6K2 depletion or TNP were reversed by overexpressing smoM2, gli1, or ip6k2. Moreover, the inhibitory effect of cyclopamine was reversed by overexpressing ip6k2. This identified roles for the inositol kinase pathway in early vertebrate development and tissue morphogenesis, and in Hh signaling. We propose that IP6K2 activity is required at the level or downstream of Smoothened but upstream of the transcription activator Gli1.

Inositol trisphosphate 3-kinases: focus on immune and neuronal signaling

The localized control of second messenger levels sculpts dynamic and persistent changes in cell physiology and structure. Inositol trisphosphate [Ins(1,4,5)P(3)] 3-kinases (ITPKs) phosphorylate the intracellular second messenger Ins(1,4,5)P(3). These enzymes terminate the signal to release Ca(2+) from the endoplasmic reticulum and produce the messenger inositol tetrakisphosphate [Ins(1,3,4,5)P(4)]. Independent of their enzymatic activity, ITPKs regulate the microstructure of the actin cytoskeleton. The immune phenotypes of ITPK knockout mice raise new questions about how ITPKs control inositol phosphate lifetimes within spatial and temporal domains during lymphocyte maturation. The intense concentration of ITPK on actin inside the dendritic spines of pyramidal neurons suggests a role in signal integration and structural plasticity in the dendrite, and mice lacking neuronal ITPK exhibit memory deficits. Thus, the molecular and anatomical features of ITPKs allow them to regulate the spatiotemporal properties of intracellular signals, leading to the formation of persistent molecular memories.

Inositol pyrophosphates: structure, enzymology and function

The stereochemistry of the inositol backbone provides a platform on which to generate a vast array of distinct molecular motifs that are used to convey information both in signal transduction and many other critical areas of cell biology. Diphosphoinositol phosphates, or inositol pyrophosphates, are the most recently characterized members of the inositide family. They represent a new frontier with both novel targets within the cell and novel modes of action. This includes the proposed pyrophosphorylation of a unique subset of proteins. We review recent insights into the structures of these molecules and the properties of the enzymes which regulate their concentration. These enzymes also act independently of their catalytic activity via protein-protein interactions. This unique combination of enzymes and products has an important role in diverse cellular processes including vesicle trafficking, endo- and exocytosis, apoptosis, telomere length regulation, chromatin hyperrecombination, the response to osmotic stress, and elements of nucleolar function.

Inositol pyrophosphates modulate hydrogen peroxide signalling

Inositol pyrophosphates are involved in a variety of cellular functions, but the specific pathways and/or downstream targets remain poorly characterized. In the present study we use Saccharomyces cerevisiae mutants to examine the potential roles of inositol pyrophosphates in responding to cell damage caused by ROS (reactive oxygen species). Yeast lacking kcs1 [the S. cerevisiae IP6K (inositol hexakisphosphate kinase)] have greatly reduced IP7 (diphosphoinositol pentakisphosphate) and IP8 (bisdiphosphoinositol tetrakisphosphate) levels, and display increased resistance to cell death caused by H2O2, consistent with a sustained activation of DNA repair mechanisms controlled by the Rad53 pathway. Other Rad53-controlled functions, such as actin polymerization, appear unaffected by inositol pyrophosphates. Yeast lacking vip1 [the S. cerevisiae PP-IP5K (also known as IP7K, IP7 kinase)] accumulate large amounts of the inositol pyrophosphate IP7, but have no detectable IP8, indicating that this enzyme represents the physiological IP7 kinase. Similar to kcs1Delta yeast, vip1Delta cells showed an increased resistance to cell death caused by H2O2, indicating that it is probably the double-pyrophosphorylated form of IP8 [(PP)2-IP4] which mediates the H2O2 response. However, these inositol pyrophosphates are not involved in directly sensing DNA damage, as kcs1Delta cells are more responsive to DNA damage caused by phleomycin. We observe in vivo a rapid decrease in cellular inositol pyrophosphate levels following exposure to H2O2, and an inhibitory effect of H2O2 on the enzymatic activity of Kcs1 in vitro. Furthermore, parallel cysteine mutagenesis studies performed on mammalian IP6K1 are suggestive that the ROS signal might be transduced by the direct modification of this evolutionarily conserved class of enzymes.

Diphosphoinositol polyphosphates: metabolic messengers?

The diphosphoinositol polyphosphates ("inositol pyrophosphates") are a specialized subgroup of the inositol phosphate signaling family. This review proposes that many of the current data concerning the metabolic turnover and biological effects of the diphosphoinositol polyphosphates are linked by a common theme: these polyphosphates act as metabolic messengers. This review will also discuss the latest proposals concerning possible molecular mechanisms of action of this intriguing class of molecules.