Inositol hexakisphosphate kinase-2 non-catalytically regulates mitophagy by attenuating PINK1 signaling.

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 various cellular functions including neuroprotection. Absence of IP6K2 causes impairment of oxidative phosphorylation regulated by creatine kinase-B. In the present study, we show that IP6K2 is involved in attenuation of PINK1-mediated mitochondrial autophagy (mitophagy) in the brain. Up-regulation of dynamin-related protein (Drp-1), as well as increased expression of mitochondrial biogenesis markers (PGC1-α and NRF-1) in the cerebella of IP6K2-deleted mice (IP6K2-knockout), point to the involvement of IP6K2 in the regulation of mitochondrial fission. Knockdown of IP6K2 also leads to augmented glycolysis, potentially as a compensatory mechanism for decreased mitochondrial respiration. Overexpressing IP6K2 as well as IP6K2-kinase dead mutant in IP6K2-knockdown N2A cells reverses the expression of mitophagy markers, demonstrating that IP6K2-induced mitoprotection is catalytically/kinase independent. IP6K2 supplementation in K2-PINK1 double-knockdown N2A cells fails to reverse the expression of the mitophagic marker, LC3-II, indicating that the mitoprotective effect of IP6K2 is dependent on PINK1. Overall, our study reveals a key neuroprotective role of IP6K2 in the prevention of PINK1-mediated mitophagy in the brain.

Phosphate Starvation by Energy Metabolism Disturbance in Candida albicansvip1Δ/Δ Induces Lipid Droplet Accumulation and Cell Membrane Damage.

Phosphorus in the form of phosphate (Pi) is an essential element for metabolic processes, including lipid metabolism. In yeast, the inositol polyphosphate kinase vip1 mediated synthesis of inositol heptakisphosphate (IP7) regulates the phosphate-responsive (PHO) signaling pathway, which plays an important role in response to Pi stress. The role of vip1 in Pi stress and lipid metabolism of Candida albicans has not yet been studied. We found that when vip1Δ/Δ was grown in glucose medium, if Pi was supplemented in the medium or mitochondrial Pi transporter was overexpressed in the strain, the lipid droplet (LD) content was reduced and membrane damage was alleviated. However, further studies showed that neither the addition of Pi nor the overexpression of the Pi transporter affected the energy balance of vip1Δ/Δ. In addition, the LD content of vip1Δ/Δ grown in Pi limitation medium PNMC was lower than that grown in SC, and the metabolic activity of vip1Δ/Δ grown in PNMC was also lower than that grown in SC medium. This suggests that the increase in Pi demand by a high energy metabolic rate is the cause of LD accumulation in vip1Δ/Δ. In addition, in the vip1Δ/Δ strains, the core transcription factor PHO4 in the PHO pathway was transported to the vacuole and degraded, which reduced the pathway activity. However, this does not mean that knocking out vip1 completely blocks the activation of the PHO pathway, because the LD content of vip1Δ/Δ grown in the medium with ß-glycerol phosphate as the Pi source was significantly reduced. In summary, the increased Pi demand and the decreased PHO pathway activity in vip1Δ/Δ ultimately lead to LD accumulation and cell membrane damage.

Vip1 is a kinase and pyrophosphatase switch that regulates inositol diphosphate signaling.

Inositol diphosphates (PP-IPs), also known as inositol pyrophosphates, are high-energy cellular signaling codes involved in nutrient and regulatory responses. We report that the evolutionarily conserved gene product, Vip1, possesses autonomous kinase and pyrophosphatase domains capable of synthesis and destruction of D-1 PP-IPs. Our studies provide atomic-resolution structures of the PP-IP products and unequivocally define that the Vip1 gene product is a highly selective 1-kinase and 1-pyrophosphatase enzyme whose activities arise through distinct active sites. Kinetic analyses of kinase and pyrophosphatase parameters are consistent with Vip1 evolving to modulate levels of 1-IP7 and 1,5-IP8 Individual perturbations in kinase and pyrophosphatase activities in cells result in differential effects on vacuolar morphology and osmotic responses. Analogous to the dual-functional key energy metabolism regulator, phosphofructokinase 2, Vip1 is a kinase and pyrophosphatase switch whose 1-PP-IP products play an important role in a cellular adaptation.

Inorganic polyphosphate accumulation suppresses the dormancy response and virulence in Mycobacterium tuberculosis.

Stringent response pathways involving inorganic polyphosphate (PolyP) play an essential role in bacterial stress adaptation and virulence. The intracellular levels of PolyP are modulated by the activities of polyphosphate kinase-1 (PPK1), polyphosphate kinase-2 (PPK2), and exopolyphosphatases (PPXs). The genome of Mycobacterium tuberculosis encodes two functional PPXs, and simultaneous deletion of ppx1 and ppx2 results in a defect in biofilm formation. We demonstrate here that these PPXs cumulatively contribute to the ability of M. tuberculosis to survive in nutrient-limiting, low-oxygen growth conditions and also in macrophages. Characterization of single (Δppx2) and double knockout (dkppx) strains of M. tuberculosis indicated that PPX-mediated PolyP degradation is essential for establishing bacterial infection in guinea pigs. RNA-Seq-based transcriptional profiling revealed that relative to the parental strain, the expression levels of DosR regulon-regulated dormancy genes were significantly reduced in the dkppx mutant strain. In concordance, we also provide evidence that PolyP inhibits the autophosphorylation activities associated with DosT and DosS sensor kinases. The results in this study uncover that enzymes involved in PolyP homeostasis play a critical role in M. tuberculosis physiology and virulence and are attractive targets for developing more effective therapeutic interventions.

Inositol Hexakisphosphate Kinase-2 in Cerebellar Granule Cells Regulates Purkinje Cells and Motor Coordination via Protein 4.1N.

Inositol hexakisphosphate kinases (IP6Ks) regulate various biological processes. Among pyrophosphates generated by IP6Ks, diphosphoinositol pentakisphosphate (IP7), and bis-diphosphoinositol tetrakisphosphate have been extensively characterized. IP7 is produced in mammals by a family of inositol hexakisphosphate kinases, IP6K1, IP6K2, and IP6K3, which have distinct biological functions. We report that IP6K2 binds protein 4.1.N with high affinity and specificity. Nuclear translocation of 4.1N, which is required for its principal functions, is dependent on IP6K2. Both of these proteins are highly expressed in granule cells of the cerebellum where their interaction regulates Purkinje cell morphology and cerebellar synapses. The deletion of IP6K2 in male/female mice elicits substantial defects in synaptic influences of granule cells upon Purkinje cells as well as notable impairment of locomotor function. Moreover, the disruption of IP6K2-4.1N interactions impairs cell viability. Thus, IP6K2 and its interaction with 4.1N appear to be major determinants of cerebellar disposition and psychomotor behavior.SIGNIFICANCE STATEMENT Inositol phosphates are produced by a family of inositol hexakisphosphate kinases (IP6Ks)-IP6K1, IP6K2, and IP6K3. Of these, the physiological roles of IP6K2 in the brain have been least characterized. In the present study, we report that IP6K2 binds selectively to the neuronal protein 4.1N. Both of these proteins are highly expressed in granule cells of the cerebellum. Using IP6K2 knock-out (KO) mice, we establish that IP6K2-4.1N interactions in granule cells regulate Purkinje cell morphology, the viability of cerebellar neurons, and psychomotor behavior.

Inositol Hexakisphosphate Kinase 3 Regulates Metabolism and Lifespan in Mice.

Inositol hexakisphosphate kinase 3 (IP6K3) generates inositol pyrophosphates, which regulate diverse cellular functions. However, little is known about its own physiological role. Here, we show the roles of IP6K3 in metabolic regulation. We detected high levels of both mouse and human IP6K3 mRNA in myotubes and muscle tissues. In human myotubes, IP6K3 was upregulated by dexamethasone treatment, which is known to inhibit glucose metabolism. Furthermore, Ip6k3 expression was elevated under diabetic, fasting, and disuse conditions in mouse skeletal muscles. Ip6k3(-/-) mice demonstrated lower blood glucose, reduced circulating insulin, deceased fat mass, lower body weight, increased plasma lactate, enhanced glucose tolerance, lower glucose during an insulin tolerance test, and reduced muscle Pdk4 expression under normal diet conditions. Notably, Ip6k3 deletion extended animal lifespan with concomitant reduced phosphorylation of S6 ribosomal protein in the heart. In contrast, Ip6k3(-/-) mice showed unchanged skeletal muscle mass and no resistance to the effects of high fat diet. The current observations suggest novel roles of IP6K3 in cellular regulation, which impact metabolic control and lifespan.

PPIP5K1 interacts with the exocyst complex through a C-terminal intrinsically disordered domain and regulates cell motility.

Cellular signaling involves coordinated regulation of many events. Scaffolding proteins are crucial regulators of cellular signaling, because they are able to affect numerous events by coordinating specific interactions among multiple protein partners in the same pathway. Scaffolding proteins often contain intrinsically disordered regions (IDR) that facilitate the formation and function of distinct protein complexes. We show that PPIP5K1 contains an unusually long and evolutionarily conserved IDR. To investigate the biological role(s) of this domain, we identified interacting proteins using affinity purification coupled with mass spectrometry. Here, we report that PPIP5K1 is associated with a network of proteins that regulate vesicle-mediated transport. We further identified exocyst complex component 1 as a direct interactor with the IDR of PPIP5K1. Additionally, we report that knockdown of PPIP5K1 decreases motility of HeLa cells in a wound-healing assay. These results suggest that PPIP5K1 might play an important role in regulating function of exocyst complex in establishing cellular polarity and directional migration of cells.

Developmental accumulation of inorganic polyphosphate affects germination and energetic metabolism in Dictyostelium discoideum.

Inorganic polyphosphate (polyP) is composed of linear chains of phosphate groups linked by high-energy phosphoanhydride bonds. However, this simple, ubiquitous molecule remains poorly understood. The use of nonstandardized analytical methods has contributed to this lack of clarity. By using improved polyacrylamide gel electrophoresis we were able to visualize polyP extracted from Dictyostelium discoideum. We established that polyP is undetectable in cells lacking the polyphosphate kinase (DdPpk1). Generation of this ppk1 null strain revealed that polyP is important for the general fitness of the amoebae with the mutant strain displaying a substantial growth defect. We discovered an unprecedented accumulation of polyP during the developmental program, with polyP increasing more than 100-fold. The failure of ppk1 spores to accumulate polyP results in a germination defect. These phenotypes are underpinned by the ability of polyP to regulate basic energetic metabolism, demonstrated by a 2.5-fold decrease in the level of ATP in vegetative ppk1. Finally, the lack of polyP during the development of ppk1 mutant cells is partially offset by an increase of both ATP and inositol pyrophosphates, evidence for a model in which there is a functional interplay between inositol pyrophosphates, ATP, and polyP.

ubiA (Rv3806c) encoding DPPR synthase involved in cell wall synthesis is associated with ethambutol resistance in Mycobacterium tuberculosis.

OBJECTIVES: Ethambutol (EMB) is a frontline antituberculosis drug for the treatment of tuberculosis (TB). The embB gene is responsible for EMB resistance in only about 50-60% clinical isolates of Mycobacterium tuberculosis, and the mechanism of resistance in the remaining EMB-resistant strains is not clear. We assessed the role of the ubiA gene encoding 5-phospho-α-d-ribose-1-diphosphate: decaprenyl-phosphate 5-phosphoribosyltransferase (DPPR synthase, UbiA) involved in decaprenylphosphoryl-d-arabinose (DPA) pathway for cell wall synthesis in EMB resistance. METHODS: Antimicrobial susceptibility testing was performed using broth colorimetric method or an agar proportion method to compare the MICs of recombinant M. tuberculosis strains that overexpressed the wild type or mutant ubiA gene. PCR and DNA sequencing were used to detect ubiA mutations in EMB-resistant M. tuberculosis clinical isolates with or without embB mutations. ubiA mRNA expression levels were measured by qRT-PCR. Enzymatic activity of UbiA in different M. tuberculosis strains was analyzed by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). RESULTS: Overexpression of the wild type M. tuberculosis ubiA led to increased resistance to EMB in M. tuberculosis. In addition, novel ubiA mutations were found in some EMB-resistant XDR-TB isolates without common embB mutations. Topological modeling analysis showed that the mutation sites in ubiA were mainly localized in the sixth transmembrane domain of the UbiA protein. Moreover, mutated ubiA when overexpressed led to higher level of EMB resistance and increased DPA levels, compared to wild type ubiA in M. tuberculosis. CONCLUSIONS: Our results indicate that ubiA is involved in EMB resistance in M. tuberculosis and that ubiA mutations that caused elevated DPA levels may be responsible for EMB resistance. The essentiality of UbiA, its involvement in EMB resistance, and lack of human homologs make UbiA an attractive target for new drug development.

Inositol hexakisphosphate kinase-1 regulates behavioral responses via GSK3 signaling pathways.

Glycogen synthase kinase 3 (GSK3), a prominent enzyme in carbohydrate metabolism, also has a major role in brain function. It is physiologically regulated by the kinase Akt, which phosphorylates GSK3 to inhibit catalytic activity. Inositol hexakisphosphate-1 (IP6K1) generates the inositol pyrophosphate diphosphoinositol pentakisphosphate (IP7), which physiologically inhibits Akt leading to enhanced GSK3 activity. We report that IP6K1 binds and stimulates GSK3 enzymatic activity in a non-catalytic fashion. Physiological relevance is evident in the inhibition of GSK3 activity in the brains of IP6K1-deleted mice. Behavioral alterations of IP6K1 knockout mice resemble those of GSK3 mutants. Accordingly, modulation of IP6K1-GSK3ß interaction may exert beneficial effects in psychiatric disorders involving GSK3.

One inositol ring to rule thrombosis.

In this issue of Blood, Ghosh and colleagues report that knocking out an inositol kinase in mice diminishes polyphosphate in platelet dense granules, thereby reducing hemostasis and protecting against thrombosis.

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.

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.

[Progress in polyphosphate and related metabolizing enzymes].

Inorganic polyphosphate (Poly P) is a polymer consisting of ten to hundreds of phosphate residues linked by "high-energy" phosphoanhydride bonds, which is abundantly found in all organisms and nature. Here the basic facts of PolyP are summarized: genes regulated by polyP, role in DNA uptake, motility of microorganism, function in stress response, the virulence of pathogens, as well as the proliferation of mammary cancer cells, blood coagulation, cell calcification and the modulation of mitochondrial activity. Enzymes with activities requiring polyP, such as endopolyphosphatase, glucokinase, NAD kinase, AMP phosphotransferase are outlined too. The structure and activity of enzymes regulating polyP level such as polyphosphate kinase and exopolyphosphatase are noted. A thorough analysis of the mycobacterium tuberculosis PPX protein homologs and their biochemical activity is presented.

Polyphosphate--an ancient energy source and active metabolic regulator.

There are a several molecules on Earth that effectively store energy within their covalent bonds, and one of these energy-rich molecules is polyphosphate. In microbial cells, polyphosphate granules are synthesised for both energy and phosphate storage and are degraded to produce nucleotide triphosphate or phosphate. Energy released from these energetic carriers is used by the cell for production of all vital molecules such as amino acids, nucleobases, sugars and lipids. Polyphosphate chains directly regulate some processes in the cell and are used as phosphate donors in gene regulation. These two processes, energetic metabolism and regulation, are orchestrated by polyphosphate kinases. Polyphosphate kinases (PPKs) can currently be categorized into three groups (PPK1, PPK2 and PPK3) according their functionality; they can also be divided into three groups according their homology (EcPPK1, PaPPK2 and ScVTC). This review discusses historical information, similarities and differences, biochemical characteristics, roles in stress response regulation and possible applications in the biotechnology industry of these enzymes. At the end of the review, a hypothesis is discussed in view of synthetic biology applications that states polyphosphate and calcium-rich organelles have endosymbiotic origins from ancient protocells that metabolized polyphosphate.

p53-mediated apoptosis requires inositol hexakisphosphate kinase-2.

Inositol pyrophosphates have been implicated in numerous biological processes. Inositol hexakisphosphate kinase-2 (IP6K2), which generates the inositol pyrophosphate, diphosphoinositol pentakisphosphate (IP7), influences apoptotic cell death. The tumor suppressor p53 responds to genotoxic stress by engaging a transcriptional program leading to cell-cycle arrest or apoptosis. We demonstrate that IP6K2 is required for p53-mediated apoptosis and modulates the outcome of the p53 response. Gene disruption of IP6K2 in colorectal cancer cells selectively impairs p53-mediated apoptosis, instead favoring cell-cycle arrest. IP6K2 acts by binding directly to p53 and decreasing expression of proarrest gene targets such as the cyclin-dependent kinase inhibitor p21.

Importance of polyphosphate kinase 1 for Campylobacter jejuni viable-but-nonculturable cell formation, natural transformation, and antimicrobial resistance.

Campylobacter jejuni, a gram-negative, microaerophilic bacterium, is a predominant cause of bacterial gastroenteritis in humans. Although considered fragile and fastidious and lacking many classical stress response mechanisms, C. jejuni exhibits a remarkable capacity for survival and adaptation, successfully infecting humans and persisting in the environment. Consequently, understanding the physiological and genetic properties that allow C. jejuni to survive and adapt to various stress conditions is crucial for therapeutic interventions. Of importance is polyphosphate (poly-P) kinase 1 (PPK1), which is a key enzyme mediating the synthesis of poly-P, an essential molecule for survival, mediating stress responses, host colonization, and virulence in many bacteria. Therefore, we investigated the role of PPK1 in C. jejuni pathogenesis, stress survival, and adaptation. Our findings demonstrate that a C. jejuni Deltappk1 mutant was deficient in poly-P accumulation, which was associated with a decreased ability to form viable-but-nonculturable cells under acid stress. The Deltappk1 mutant also showed a decreased frequency of natural transformation and an increased susceptibility to various antimicrobials. Furthermore, the Deltappk1 mutant was characterized by a dose-dependent deficiency in chicken colonization. Complementation of the Deltappk1 mutant with the wild-type copy of ppk1 restored the deficient phenotypes to levels similar to those of the wild type. Our results suggest that poly-P plays an important role in stress survival and adaptation and might contribute to genome plasticity and the spread and development of antimicrobial resistance in C. jejuni. These findings highlight the potential of PPK1 as a novel target for therapeutic interventions.

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.

Mutated ATP synthase induces oxidative stress and impaired insulin secretion in beta-cells of female BHE/cdb rats.

BACKGROUND: Adenosine triphosphate (ATP) is a critical determinant of beta-cell insulin secretion in response to glucose. BHE/cdb rats have a mutation in ATP synthase that limits ATP production, yet develop mild diabetes only with ageing. We investigated the cellular basis for reduced insulin secretion and compensatory mechanisms that mitigate the effects of the ATP synthase mutation. METHODS: In vitro beta-cell function in isolated islets and expression of key regulatory genes was compared with in vivo oral glucose tolerance and insulin sensitivity in BHE/cdb and control rats. RESULTS: BHE/cdb rat islets had reduced responsiveness to glucose stimulation and ATP content was 35% lower than in control islets. Oral glucose tolerance was impaired at both 21 and 43 weeks of age because of a reduction in glucose-stimulated insulin secretion (GSIS). An increase in inducible nitric oxide synthase (INOS, 3-fold) and manganese superoxide dismutase (MnSOD, 1.6-fold), detection of nitrotyrosine, beta-cell apoptosis, and nucleocytoplasmic translocation of pancreas duodenum homeobox-1 (PDX-1) in beta-cells indicated increased oxygen radical formation. However, BHE/cdb rats partially compensated for low glucose responsiveness by increasing the number of small islets and beta-cell hypertrophy. There was also an increase in the proportion of mature insulin relative to proinsulin (PI) detected within beta-cell granules. Increased activation of AMP-dependent kinase (AMPK)-regulated pathways was consistent with increased oxidative stress and with induction of apoptosis and reduction of preproinsulin gene transcription. CONCLUSIONS: The findings are consistent with impaired but partially compensated mechanisms of insulin secretion early in life, but progressive non-compensated impairments due to oxidative stress occurs by age 43 weeks.

HSP90 regulates cell survival via inositol hexakisphosphate kinase-2.

Heat-shock proteins (HSPs) are abundant, inducible proteins best known for their ability to maintain the conformation of proteins and to refold damaged proteins. Some HSPs, especially HSP90, can be antiapoptotic and the targets of anticancer drugs. Inositol hexakisphosphate kinase-2 (IP6K2), one of a family of enzymes generating the inositol pyrophosphate IP7 [diphosphoinositol pentakisphosphate (5-PP-IP5)], mediates apoptosis. Increased IP6K2 activity sensitizes cancer cells to stressors, whereas its depletion blocks cell death. We now show that HSP90 physiologically binds IP6K2 and inhibits its catalytic activity. Drugs and selective mutations that abolish HSP90-IP6K2 binding elicit activation of IP6K2, leading to cell death. Thus, the prosurvival actions of HSP90 reflect the inhibition of IP6K2, suggesting that selectively blocking this interaction could provide effective and safer modes of chemotherapy.