Targeting of hexokinase 1 to liver and hepatoma mitochondria

Apoptotic proteins with non-apoptotic activity: expression and function in cancer

Apoptosis is a process of programmed cell death in which a cell commits suicide while maintaining the integrity and architecture of the tissue as a whole. Apoptosis involves activation of one of two major pathways: the extrinsic pathway, where extracellular pro-apoptotic signals, transduced through plasma membrane death receptors, activate a caspase cascade leading to apoptosis. The second, the intrinsic apoptotic pathway, where damaged DNA, oxidative stress, or chemicals, induce the release of pro-apoptotic proteins from the mitochondria, leading to the activation of caspase-dependent and independent apoptosis. However, it has recently become apparent that proteins involved in apoptosis also exhibit non-cell death-related physiological functions that are related to the cell cycle, differentiation, metabolism, inflammation or immunity. Such non-conventional activities were predominantly reported in non-cancer cells although, recently, such a dual function for pro-apoptotic proteins has also been reported in cancers where they are overexpressed. Interestingly, some apoptotic proteins translocate to the nucleus in order to perform a non-apoptotic function. In this review, we summarize the unconventional roles of the apoptotic proteins from a functional perspective, while focusing on two mitochondrial proteins: VDAC1 and SMAC/Diablo. Despite having pro-apoptotic functions, these proteins are overexpressed in cancers and this apparent paradox and the associated pathophysiological implications will be discussed. We will also present possible mechanisms underlying the switch from apoptotic to non-apoptotic activities although a deeper investigation into the process awaits further study.

Mapping the N-Terminal Hexokinase-I Binding Site onto Voltage-Dependent Anion Channel-1 To Block Peripheral Nerve Demyelination

The voltage-dependent anion channel (VDAC), the most abundant protein on the outer mitochondrial membrane, is implicated in ATP, ion and metabolite exchange with cell compartments. In particular, the VDAC participates in cytoplasmic and mitochondrial Ca²⁺ homeostasis. Notably, the Ca²⁺ efflux out of Schwann cell mitochondria is involved in peripheral nerve demyelination that underlies most peripheral neuropathies. Hexokinase (HK) isoforms I and II, the main ligands of the VDAC, possess a hydrophobic N-terminal structured in α-helix (NHKI) that is necessary for the binding to the VDAC. To gain further insight into the molecular basis of HK binding to the VDAC, we developed and optimized peptides based on the NHKI sequence. These modifications lead to an increase of the peptide hydrophobicity and helical content that enhanced their ability to prevent peripheral nerve demyelination. Our results provide new insights into the molecular basis of VDAC/HK interaction that could lead to the development of therapeutic compounds for demyelinating peripheral neuropathies.

Hepatic HKDC1 Expression Contributes to Liver Metabolism

Glucokinase (GCK) is the principal hexokinase (HK) in the liver, operating as a glucose sensor to regulate glucose metabolism and lipid homeostasis. Recently, we proposed HK domain-containing 1 (HKDC1) to be a fifth HK with expression in the liver. Here, we reveal HKDC1 to have low glucose-phosphorylating ability and demonstrate its association with the mitochondria in hepatocytes. As we have shown previously that genetic deletion of HKDC1 leads to altered hepatic triglyceride levels, we also explored the influence of overexpression of HKDC1 in hepatocytes on cellular metabolism, observing reduced glycolytic capacity and maximal mitochondrial respiration with concurrent reductions in glucose oxidation and mitochondrial membrane potential. Furthermore, we found that acute in vivo overexpression of HKDC1 in the liver induced substantial changes in mitochondrial dynamics. Altogether, these findings suggest that overexpression of HKDC1 causes mitochondrial dysfunction in hepatocytes. However, its overexpression was not enough to alter energy storage in the liver but led to mild improvement in glucose tolerance. We next investigated the conditions necessary to induce HKDC1 expression, observing HKDC1 expression to be elevated in human patients whose livers were at more advanced stages of nonalcoholic fatty liver disease (NAFLD) and similarly, found high liver expression in mice on diets causing high levels of liver inflammation and fibrosis. Overall, our data suggest that HKDC1 expression in hepatocytes results in defective mitochondrial function and altered hepatocellular metabolism and speculate that its expression in the liver may play a role in the development of NAFLD.

Targeting Mitochondrial Ion Channels to Fight Cancer

In recent years, several experimental evidences have underlined a new role of ion channels in cancer development and progression. In particular, mitochondrial ion channels are arising as new oncological targets, since it has been proved that most of them show an altered expression during tumor development and the pharmacological targeting of some of them have been demonstrated to be able to modulate cancer growth and progression, both in vitro as well as in vivo in pre-clinical mouse models. In this scenario, pharmacology of mitochondrial ion channels would be in the near future a new frontier for the treatment of tumors. In this review, we discuss the new advances in the field, by focusing our attention on the improvements in new drug developments to target mitochondrial ion channels.

VDAC1 as Pharmacological Target in Cancer and Neurodegeneration: Focus on Its Role in Apoptosis

Cancer and neurodegeneration are different classes of diseases that share the involvement of mitochondria in their pathogenesis. Whereas the high glycolytic rate (the so-called Warburg metabolism) and the suppression of apoptosis are key elements for the establishment and maintenance of cancer cells, mitochondrial dysfunction and increased cell death mark neurodegeneration. As a main actor in the regulation of cell metabolism and apoptosis, VDAC may represent the common point between these two broad families of pathologies. Located in the outer mitochondrial membrane, VDAC forms channels that control the flux of ions and metabolites across the mitochondrion thus mediating the organelle's cross-talk with the rest of the cell. Furthermore, the interaction with both pro-apoptotic and anti-apoptotic factors makes VDAC a gatekeeper for mitochondria-mediated cell death and survival signaling pathways. Unfortunately, the lack of an evident druggability of this protein, since it has no defined binding or active sites, makes the quest for VDAC interacting molecules a difficult tale. Pharmacologically active molecules of different classes have been proposed to hit cancer and neurodegeneration. In this work, we provide an exhaustive and detailed survey of all the molecules, peptides, and microRNAs that exploit VDAC in the treatment of the two examined classes of pathologies. The mechanism of action and the potential or effectiveness of each compound are discussed.

Voltage-Dependent Anion Channel 1 As an Emerging Drug Target for Novel Anti-Cancer Therapeutics

Cancer cells share several properties, high proliferation potential, reprogramed metabolism, and resistance to apoptotic cues. Acquiring these hallmarks involves changes in key oncogenes and non-oncogenes essential for cancer cell survival and prosperity, and is accompanied by the increased energy requirements of proliferating cells. Mitochondria occupy a central position in cell life and death with mitochondrial bioenergetics, biosynthesis, and signaling are critical for tumorigenesis. Voltage-dependent anion channel 1 (VDAC1) is situated in the outer mitochondrial membrane (OMM) and serving as a mitochondrial gatekeeper. VDAC1 allowing the transfer of metabolites, fatty acid ions, Ca²⁺, reactive oxygen species, and cholesterol across the OMM and is a key player in mitochondrial-mediate apoptosis. Moreover, VDAC1 serves as a hub protein, interacting with diverse sets of proteins from the cytosol, endoplasmic reticulum, and mitochondria that together regulate metabolic and signaling pathways. The observation that VDAC1 is over-expressed in many cancers suggests that the protein may play a pivotal role in cancer cell survival. However, VDAC1 is also important in mitochondria-mediated apoptosis, mediating release of apoptotic proteins and interacting with anti-apoptotic proteins, such as B-cell lymphoma 2 (Bcl-2), Bcl-xL, and hexokinase (HK), which are also highly expressed in many cancers. Strategically located in a “bottleneck” position, controlling metabolic homeostasis and apoptosis, VDAC1 thus represents an emerging target for anti-cancer drugs. This review presents an overview on the multi-functional mitochondrial protein VDAC1 performing several functions and interacting with distinct sets of partners to regulate both cell life and death, and highlights the importance of the protein for cancer cell survival. We address recent results related to the mechanisms of VDAC1-mediated apoptosis and the potential of associated proteins to modulate of VDAC1 activity, with the aim of developing VDAC1-based approaches. The first strategy involves modification of cell metabolism using VDAC1-specific small interfering RNA leading to inhibition of cancer cell and tumor growth and reversed oncogenic properties. The second strategy involves activation of cancer cell death using VDAC1-based peptides that prevent cell death induction by anti-apoptotic proteins. Finally, we discuss the potential therapeutic benefits of treatments and drugs leading to enhanced VDAC1 expression or targeting VDAC1 to induce apoptosis.

Mitochondrial Ca2+ and regulation of the permeability transition pore

The mitochondrial permeability transition pore was originally described in the 1970's as a Ca2+ activated pore and has since been attributed to the pathogenesis of many diseases. Here we evaluate how each of the current models of the pore complex fit to what is known about how Ca2+ regulates the pore, and any insight that provides into the molecular identity of the pore complex. We also discuss the central role of Ca2+ in modulating the pore's open probability by directly regulating processes, such as ATP/ADP balance through the tricarboxylic acid cycle, electron transport chain, and mitochondrial membrane potential. We review how Ca2+ influences second messengers such as reactive oxygen/nitrogen species production and polyphosphate formation. We discuss the evidence for how Ca2+ regulates post-translational modification of cyclophilin D including phosphorylation by glycogen synthase kinase 3 beta, deacetylation by sirtuins, and oxidation/ nitrosylation of key residues. Lastly we introduce a novel view into how Ca2+ activated proteolysis through calpains in the mitochondria may be a driver of sustained pore opening during pathologies such as ischemia reperfusion injury.

Hexokinase I N-terminal based peptide prevents the VDAC1-SOD1 G93A interaction and re-establishes ALS cell viability

Superoxide Dismutase 1 mutants associate with 20–25% of familial Amyotrophic Lateral Sclerosis (ALS) cases, producing toxic aggregates on mitochondria, notably in spinal cord. The Voltage Dependent Anion Channel isoform 1 (VDAC1) in the outer mitochondrial membrane is a docking site for SOD1 G93A mutant in ALS mice and the physiological receptor of Hexokinase I (HK1), which is poorly expressed in mouse spinal cord. Our results demonstrate that HK1 competes with SOD1 G93A for binding VDAC1, suggesting that in ALS spinal cord the available HK1-binding sites could be used by SOD1 mutants for docking mitochondria, producing thus organelle dysfunction. We tested this model by studying the action of a HK1-N-terminal based peptide (NHK1). This NHK1 peptide specifically interacts with VDAC1, inhibits the SOD1 G93A binding to mitochondria and restores the viability of ALS model NSC34 cells. Altogether, our results suggest that NHK1 peptide could be developed as a therapeutic tool in ALS, predicting an effective role also in other proteinopathies.

High rates of glucose utilization in the gas gland of Atlantic cod (Gadus morhua) are supported by GLUT1 and HK1b

The gas gland of physoclistous fish utilizes glucose to generate lactic acid that leads to the off-loading of oxygen from haemoglobin. This study addresses characteristics of the first two steps in glucose utilization in the gas gland of Atlantic cod (Gadus morhua). Glucose metabolism by isolated gas gland cells was 12- and 170-fold higher, respectively, than that in heart and red blood cells (RBCs) as determined by the production of (3)H2O from [2-(3)H]glucose. In the gas gland, essentially all of the glucose consumed was converted to lactate. Glucose uptake in the gas gland shows a very high dependence upon facilitated transport as evidenced by saturation of uptake of 2-deoxyglucose at a low extracellular concentration and a requirement for high levels of cytochalasin B for uptake inhibition despite the high efficacy of this treatment in heart and RBCs. Glucose transport is via glucose transporter 1 (GLUT1), which is localized to the glandular cells. GLUT1 western blot analysis from whole-tissue lysates displayed a band with a relative molecular mass of 52 kDa, consistent with the deduced amino acid sequence. Levels of 52 kDa GLUT1 in the gas gland were 2.3- and 33-fold higher, respectively, than those in heart and RBCs, respectively. Glucose phosphorylation is catalysed by hexokinase Ib (HKIb), a paralogue that cannot bind to the outer mitochondrial membrane. Transcript levels of HKIb in the gas gland were 52- and 57-fold more abundant, respectively, than those in heart and RBCs. It appears that high levels of GLUT1 protein and an unusual isoform of HKI are both critical for the high rates of glycolysis in gas gland cells.

Identification of a mitochondrial-binding site on the N-terminal end of hexokinase II

Hexokinase II (HKII) is responsible for the first step in the glycolysis pathway by adding a phosphate on to the glucose molecule so it can proceed down the pathway to produce the energy for continuous cancer cell growth. Tumour cells overexpress the HKII enzyme. In fact, it is the overexpression of the HKII enzyme that makes the diagnosis of cancer possible when imaged by positron emission tomography (PET). HKII binds to the voltage-dependent anion channel (VDAC) located on the mitochondrial outer membrane (MOM). When bound to the MOM, HKII is blocking a major cell death pathway. Thus, HKII is responsible for two characteristics of cancer cells, rapid tumour growth and inability of cancer cells to undergo apoptosis. One method to identify novel compounds that may interfere with the HKII-VDAC-binding site is to create a molecular model using the crystal structure of HKII. However, the amino acid(s) responsible for HKII binding to VDAC are not known. Therefore, a series of truncations and point mutations were made to the N-terminal end of HKII to identify the binding site to VDAC. Deletions of the first 10 and 20 amino acids indicated that important amino acid(s) for binding were located within the first 10 amino acids. Next, a series of point mutations were made within the first 10 amino acids. It is clear from the immunofluorescence images and immunoblot results that mutating the fifth amino acid from histidine to proline completely abolished binding to the MOM.

Analysis and interpretation of transcriptomic data obtained from extended Warburg effect genes in patients with clear cell renal cell carcinoma

Background

Many cancers adopt a metabolism that is characterized by the well-known Warburg effect (aerobic glycolysis). Recently, numerous attempts have been made to treat cancer by targeting one or more gene products involved in this pathway without notable success. This work outlines a transcriptomic approach to identify genes that are highly perturbed in clear cell renal cell carcinoma (CCRCC).

Methods

We developed a model of the extended Warburg effect and outlined the model using Cytoscape. Following this, gene expression fold changes (FCs) for tumor and adjacent normal tissue from patients with CCRCC (GSE6344) were mapped on to the network. Gene expression values with FCs of greater than two were considered as potential targets for treatment of CCRCC.

Results

The Cytoscape network includes glycolysis, gluconeogenesis, the pentose phosphate pathway (PPP), the TCA cycle, the serine/glycine pathway, and partial glutaminolysis and fatty acid synthesis pathways. Gene expression FCs for nine of the 10 CCRCC patients in the GSE6344 data set were consistent with a shift to aerobic glycolysis. Genes involved in glycolysis and the synthesis and transport of lactate were over-expressed, as was the gene that codes for the kinase that inhibits the conversion of pyruvate to acetyl-CoA. Interestingly, genes that code for unique proteins involved in gluconeogenesis were strongly under-expressed as was also the case for the serine/glycine pathway. These latter two results suggest that the role attributed to the M2 isoform of pyruvate kinase (PKM2), frequently the principal isoform of PK present in cancer: i.e. causing a buildup of glucose metabolites that are shunted into branch pathways for synthesis of key biomolecules, may not be operative in CCRCC. The fact that there was no increase in the expression FC of any gene in the PPP is consistent with this hypothesis. Literature protein data generally support the transcriptomic findings.

Conclusions

A number of key genes have been identified that could serve as valid targets for anti-cancer pharmaceutical agents. Genes that are highly over-expressed include ENO2, HK2, PFKP, SLC2A3, PDK1, and SLC16A1. Genes that are highly under-expressed include ALDOB, PKLR, PFKFB2, G6PC, PCK1, FBP1, PC, and SUCLG1.

The role of hexokinase in cardioprotection - mechanism and potential for translation

Mitochondrial permeability transition pore (mPTP) opening plays a critical role in cardiac reperfusion injury and its prevention is cardioprotective. Tumour cell mitochondria usually have high levels of hexokinase isoform 2 (HK2) bound to their outer mitochondrial membranes (OMM) and HK2 binding to heart mitochondria has also been implicated in resistance to reperfusion injury. HK2 dissociates from heart mitochondria during ischaemia, and the extent of this correlates with the infarct size on reperfusion. Here we review the mechanisms and regulations of HK2 binding to mitochondria and how this inhibits mPTP opening and consequent reperfusion injury. Major determinants of HK2 dissociation are the elevated glucose‐6‐phosphate concentrations and decreased pH in ischaemia. These are modulated by the myriad of signalling pathways implicated in preconditioning protocols as a result of a decrease in pre‐ischaemic glycogen content. Loss of mitochondrial HK2 during ischaemia is associated with permeabilization of the OMM to cytochrome c, which leads to greater reactive oxygen species production and mPTP opening during reperfusion. Potential interactions between HK2 and OMM proteins associated with mitochondrial fission (e.g. Drp1) and apoptosis (B‐cell lymphoma 2 family members) in these processes are examined. Also considered is the role of HK2 binding in stabilizing contact sites between the OMM and the inner membrane. Breakage of these during ischaemia is proposed to facilitate cytochrome c loss during ischaemia while increasing mPTP opening and compromising cellular bioenergetics during reperfusion. We end by highlighting the many unanswered questions and discussing the potential of modulating mitochondrial HK2 binding as a pharmacological target.

Linked Articles

This article is part of a themed section on Conditioning the Heart – Pathways to Translation. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue‐8

Hexokinases and cardioprotection

As mediators of the first enzymatic step in glucose metabolism, hexokinases (HKs) orchestrate a variety of catabolic and anabolic uses of glucose, regulate antioxidant power by generating NADPH for glutathione reduction, and modulate cell death processes by directly interacting with the voltage-dependent anion channel (VDAC), a regulatory component of the mitochondrial permeability transition pore (mPTP). Here we summarize the current state-of-knowledge about HKs and their role in protecting the heart from ischemia/reperfusion (I/R) injury, reviewing: 1) the properties of different HK isoforms and how their function is regulated by their subcellular localization; 2) how HKs modulate glucose metabolism and energy production during I/R; 3) the molecular mechanisms by which HKs influence mPTP opening and cellular injury during I/R; and 4) how different metabolic and HK profiles correlate with susceptibility to I/R injury and cardioprotective efficacy in cancer cells, neonatal hearts, and normal, hypertrophied and failing adult hearts, and how these difference may guide novel therapeutic strategies to limit I/R injury in the heart. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

AKAP350C targets to mitochondria via a novel amphipathic alpha helical domain

Mitochondria regulate metabolism and homeostasis within cells. Mitochondria are also very dynamic organelles, constantly undergoing fission and fusion. The importance of maintaining proper mitochondrial dynamics is evident in the various diseases associated with defects in these processes. Protein kinase A (PKA) is a key regulator of mitochondrial dynamics. PKA is spatially regulated by A-Kinase Anchoring Proteins (AKAPs). We completed cloning of a novel AKAP350 isoform, AKAP350C. Immunostaining for endogenous AKAP350C showed localization to mitochondria. The carboxyl-terminal 54-amino acid sequence unique to AKAP350C contains a novel amphipathic alpha helical mitochondrial-targeting domain. AKAP350C co-localizes with Mff (mitochondrial fission protein) and mitofusins 1 and 2 (mitochondrial fusion proteins), and likely regulates mitochondrial dynamics by scaffolding PKA and mitochondrial fission and fusion proteins.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Hexokinase 1 blocks apoptotic signals at the mitochondria

To coordinate a meaningful response to infection or tissue damage, Tumor Necrosis Factor (TNF) triggers a spectrum of reactions in target cells that includes cell activation, differentiation, proliferation and death. Deregulated TNF signaling can lead to tissue damage and organ dysfunction during inflammation. Previously, we identified hexokinase 1 (HK1) as a potent pro-survival factor that counters TNF-induced apoptosis in type II cells. Here we used HK1 siRNA and clotrimazole to generate mitochondrial depletion phenotypes of HK1 to test if HK1 acts at the mitochondria to block TNF-induced apoptosis. We found that HK1 is predominantly mitochondrial in type II cells and that its depletion at the mitochondria decreased the inner mitochondrial membrane potential and accelerated TNF-induced apoptosis. In addition, we showed that the decrease of the mitochondrial membrane potential after HK1 depletion depended on the presence of Bak and Bax and was blocked by Bcl-2 overexpression. From these findings, we conclude that HK1 counters TNF-induced apoptosis through antagonization of pro-apoptotic Bcl-2 proteins at the outer mitochondrial membrane.

Light activation of the insulin receptor regulates mitochondrial hexokinase. A possible mechanism of retinal neuroprotection

The serine/threonine kinase Akt has been shown to mediate the anti-apoptotic activity through hexokinase (HK)-mitochondria interaction. We previously reported that Akt activation in retinal rod photoreceptor cells is mediated through the light-dependent insulin receptor (IR)/PI3K pathway. Our data indicate that light-induced activation of IR/PI3K/Akt results in the translocation of HK-II to mitochondria. We also found that PHLPPL, a serine/threonine phosphatase, enhanced the binding of HK-II to mitochondria. We found a mitochondrial targeting signal in PHLPPL and our study suggests that Akt translocation to mitochondria could be mediated through PHLPPL. Our results suggest that the light-dependent IR/PI3K/Akt pathway regulates hexokinase-mitochondria interaction in photoreceptors. Down-regulation of IR signaling has been associated with ocular diseases of retinitis pigmentosa, diabetic retinopathy, and Leber Congenital Amaurosis-type 2, and agents that enhance the binding interaction between hexokinase and mitochondria may have therapeutic potential against these ocular diseases.

CD8 memory T cells have a bioenergetic advantage that underlies their rapid recall ability

A characteristic of memory T (TM) cells is their ability to mount faster and stronger responses to reinfection than naïve T (TN) cells do in response to an initial infection. However, the mechanisms that allow this rapid recall are not completely understood. We found that CD8 TM cells have more mitochondrial mass than CD8 TN cells and, that upon activation, the resulting secondary effector T (TE) cells proliferate more quickly, produce more cytokines, and maintain greater ATP levels than primary effector T cells. We also found that after activation, TM cells increase oxidative phosphorylation and aerobic glycolysis and sustain this increase to a greater extent than TN cells, suggesting that greater mitochondrial mass in TM cells not only promotes oxidative capacity, but also glycolytic capacity. We show that mitochondrial ATP is essential for the rapid induction of glycolysis in response to activation and the initiation of proliferation of both TN and TM cells. We also found that fatty acid oxidation is needed for TM cells to rapidly respond upon restimulation. Finally, we show that dissociation of the glycolysis enzyme hexokinase from mitochondria impairs proliferation and blocks the rapid induction of glycolysis upon T-cell receptor stimulation in TM cells. Our results demonstrate that greater mitochondrial mass endows TM cells with a bioenergetic advantage that underlies their ability to rapidly recall in response to reinfection.

Pathophysiological consequences of TAT-HKII peptide administration are independent of impaired vascular function and ensuing ischemia

RATIONALE

We have shown that partial dissociation of hexokinase II (HKII) from mitochondria in the intact heart using low-dose transactivating transcriptional factor (TAT)-HKII (200 nmol/L) prevents the cardioprotective effects of ischemic preconditioning, whereas high-dose TAT-HKII (10 μmol/L) administration results in rapid myocardial dysfunction, mitochondrial depolarization, and disintegration. In this issue of Circulation Research, Pasdois et al argue that the deleterious effects of TAT-HKII administration on cardiac function are likely because of vasoconstriction and ensuing ischemia.

OBJECTIVE

To investigate whether altered vascular function and ensuing ischemia recapitulate the deleterious effects of TAT-HKII in intact myocardium.

METHODS AND RESULTS

Using a variety of complementary techniques, including mitochondrial membrane potential (ΔΨm) imaging, high-resolution optical action potential mapping, analysis of lactate production, nicotinamide adenine dinucleotide epifluorescence, lactate dehydrogenase release, and electron microscopy, we provide direct evidence that refutes the notion that acute myocardial dysfunction by high-dose TAT-HKII peptide administration is a consequence of impaired vascular function. Moreover, we demonstrate that low-dose TAT-HKII treatment, which abrogates the protective effects of ischemic preconditioning, is not associated with ischemia or ischemic injury.

CONCLUSIONS

Our findings challenge the notion that the effects of TAT-HKII are attributable to impaired vascular function and ensuing ischemia, thereby lending further credence to the role of mitochondria-bound HKII as a critical regulator of cardiac function, ischemia-reperfusion injury, and cardioprotection by ischemic preconditioning.

Hexokinase II and reperfusion injury: TAT-HK2 peptide impairs vascular function in Langendorff-perfused rat hearts

RATIONALE

Mitochondrial-bound hexokinase II (HK2) was recently proposed to play a crucial role in the normal functioning of the beating heart and to be necessary to maintain mitochondrial membrane potential. However, our own studies confirmed that mitochondria from ischemic rat hearts were HK2-depleted, yet showed no indication of depolarization and responded normally to ADP.

OBJECTIVE

To establish whether the human TAT-HK2 peptide used to dissociate mitochondrial-bound HKII in the Langendorff-perfused heart may exert its effects indirectly by impairing coronary function.

METHODS AND RESULTS

Ischemic preconditioning was blocked in rat hearts perfused with 2.5 µmol/L TAT-HK2 before ischemia or at the onset of reperfusion. However, TAT-HK2 also decreased the phosphocreatine:ATP ratio that correlated with reduced rate pressure product and increased diastolic pressure. These effects were preceded by increased aortic pressure (Langendorff constant flow) or decreased coronary flow (Langendorff constant pressure), which was also observed, albeit less pronounced, at 200 nmol/L TAT-HK2 and was prevented by coperfusion with the NO-donor diethylamine NONOate. Mitochondria from TAT-HK2-perfused hearts showed no loss of bound HK2, unlike mitochondria from ischemic hearts where the expected loss was prevented by ischemic preconditioning.

CONCLUSIONS

In the perfused rat heart, TAT-HK2 should be used with caution and careful attention to dosage because some of its effects may be mediated by vasoconstriction of the coronary vasculature rather than dissociation of HK2 from myocyte mitochondria.

VDAC1: from structure to cancer therapy

Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to cancer. Found at the outer mitochondrial membrane, VDAC1 assumes a crucial position in the cell, controlling the metabolic cross-talk between mitochondria and the rest of the cell. Moreover, its location at the boundary between the mitochondria and the cytosol enables VDAC1 to interact with proteins that mediate and regulate the integration of mitochondrial functions with other cellular activities. As a metabolite transporter, VDAC1 contributes to the metabolic phenotype of cancer cells. This is reflected by VDAC1 over-expression in many cancer types, and by inhibition of tumor development upon silencing VDAC1 expression. Along with regulating cellular energy production and metabolism, VDAC1 is also a key protein in mitochondria-mediated apoptosis, participating in the release of apoptotic proteins and interacting with anti-apoptotic proteins. The involvement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space is discussed, as is VDAC1 oligomerization as an important step in apoptosis induction. VDAC also serves as an anchor point for mitochondria-interacting proteins, some of which are also highly expressed in many cancers, such as hexokinase (HK), Bcl2, and Bcl-xL. By binding to VDAC, HK provides both metabolic benefit and apoptosis-suppressive capacity that offers the cell a proliferative advantage and increases its resistance to chemotherapy. VDAC1-based peptides that bind specifically to HK, Bcl2, or Bcl-xL abolished the cell’s abilities to bypass the apoptotic pathway. Moreover, these peptides promote cell death in a panel of genetically characterized cell lines derived from different human cancers. These and other functions point to VDAC1 as a rational target for the development of a new generation of therapeutics.

shRNA-mediated silencing of hexokinase II inhibits proliferation but promotes apoptosis in SGC7901 cells

AIM: To observe the changes in cell proliferation and apoptosis in gastric cancer cell line SGC7901 after silencing the hexokinase II (HK-II) gene with an HK-II-specific shRNA, and to assess the potential application of HK-II-targeted gene therapy for gastric cancer.

METHODS: Gastric cancer cell line SGC7901 and gastric epithelial cell line GES-1 were used in this study. Both SGC7901 cells and GES-1 cells were divided into 5 groups: HK-II shRNA group, positive control group, negative control group, empty liposome group, and blank control group. After transfection of HK-II-specific shRNA into SGC7901 and GES-1 cells, the change in the expression of HK-II mRNA was detected by reverse transcription-polymerase chain reaction (RT-PCR), and the changes in cell proliferation and apoptosis were assessed by MTT assay and flow cytometry, respectively.

RESULTS: The expression level of HK-II mRNA in SGC7901 cells was significantly higher than that in GES-1 cells (t = 12.119, P < 0.01). The expression of HK-II mRNA was obviously silenced after transfection of HK-II-specific shRNA (P < 0.01). HK-II knockdown could significantly inhibit proliferation (F = 159.811, P < 0.01) and promote apoptosis (χ² = 21.324, P < 0.01) in SGC7901 cells, but had no significant effect on proliferation and apoptosis in GES-1 cells (F = 0.704, P = 0.592; χ² = 1.007, P = 0.909).

CONCLUSION: HK-II knockdown significantly inhibits proliferation and promotes apoptosis in SGC7901 cells, but has no significant impact in GES-1 cells. HK-II may be a potential target for the treatment of gastric cancer.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

Two-color STED microscopy reveals different degrees of colocalization between hexokinase-I and the three human VDAC isoforms

The voltage-dependent anion channel (VDAC, also known as mitochondrial porin) is the major transport channel mediating the transport of metabolites, including ATP, across the mitochondrial outer membrane. Biochemical data demonstrate the binding of the cytosolic protein hexokinase-I to VDAC, facilitating the direct access of hexokinase-I to the transported ATP. In human cells, three hVDAC isoforms have been identified. However, little is known on the distribution of these isoforms within the outer membrane of mitochondria and to what extent they colocalize with hexokinase-I. In this study we show that whereas hVDAC1 and hVDAC2 are localized predominantly within the same distinct domains in the outer membrane, hVDAC3 is mostly uniformly distributed over the surface of the mitochondrion. We used two-color stimulated emission depletion (STED) microscopy enabling a lateral resolution of ~40 nm to determine the detailed sub-mitochondrial distribution of the three hVDAC isoforms and hexokinase-I. Individual hVDAC and hexokinase-I clusters could thus be resolved which were concealed in the confocal images. Quantitative colocalization analysis of two-color STED images demonstrates that within the attained resolution, hexokinase-I and hVDAC3 exhibit a higher degree of colocalization than hexokinase-I with either hVDAC1 or hVDAC2. Furthermore, a substantial fraction of the mitochondria-bound hexokinase-I pool does not colocalize with any of the three hVDAC isoforms, suggesting a more complex interplay of these proteins than previously anticipated. This study demonstrates that two-color STED microscopy in conjunction with quantitative colocalization analysis is a powerful tool to study the complex distribution of membrane proteins in organelles such as mitochondria.PACS: 87.16.Tb, 87.85.Rs.

Uncovering the role of VDAC in the regulation of cell life and death

Proper cell activity requires an efficient exchange of molecules between mitochondria and cytoplasm. Lying in the outer mitochondrial membrane, VDAC assumes a crucial position in the cell, forming the main interface between the mitochondrial and the cellular metabolisms. As such, it has been recognized that VDAC plays a crucial role in regulating the metabolic and energetic functions of mitochondria. Indeed, down-regulation of VDAC1 expression by shRNA leads to a decrease in energy production and cell growth. VDAC has also been recognized as a key protein in mitochondria-mediated apoptosis through its involvement in the release of apoptotic proteins located in the inter-membranal space and as the proposed target of pro- and anti-apoptotic members of the Bcl2-family and of hexokinase. Questions, however, remain as to if and how VDAC mediates the transfer of apoptotic proteins from the inter-membranal space to the cytosol. The diameter of the VDAC pore is only about 2.5-3 nm, insufficient for the passage of a folded protein like cytochrome c. New work, however, suggests that pore formation involves the assembly of homo-oligomers of VDAC or hetero-oligomers composed of VDAC and pro-apoptotic proteins, such as Bax. Thus, VDAC appears to represent a convergence point for a variety of cell survival and cell death signals. This review provides insight into the central role of VDAC in mammalian cell life and death, emphasizing VDAC function in the regulation of mitochondria-mediated apoptosis and, as such, its potential as a rational target for new therapeutics.

Enzymatic properties of the N- and C-terminal halves of human hexokinase II

Although previous studies on hexokinase (HK) II indicate both the N- and C-terminal halves are catalytically active, we show in this study the N-terminal half is significantly more catalytic than the C-terminal half in addition to having a significantly higher Km for ATP and Glu. Furthermore, truncated forms of intact HK II lacking its first N-terminal 18 amino acids (delta18) and a truncated N-terminal half lacking its first 18 amino acids (delta18N) have higher catalytic activity than other mutants tested. Similar results were obtained by PET-scan analysis using (18)FFDG. Our results collectively suggest that each domain of HK II possesses enzyme activity, unlike HK I, with the N-terminal half showing higher enzyme activity than the C-terminal half.

A mutation in an alternative untranslated exon of hexokinase 1 associated with hereditary motor and sensory neuropathy -- Russe (HMSNR)

Hereditary Motor and Sensory Neuropathy -- Russe (HMSNR) is a severe autosomal recessive disorder, identified in the Gypsy population. Our previous studies mapped the gene to 10q22-q23 and refined the gene region to approximately 70 kb. Here we report the comprehensive sequencing analysis and fine mapping of this region, reducing it to approximately 26 kb of fully characterised sequence spanning the upstream exons of Hexokinase 1 (HK1). We identified two sequence variants in complete linkage disequilibrium, a G>C in a novel alternative untranslated exon (AltT2) and a G>A in the adjacent intron, segregating with the disease in affected families and present in the heterozygote state in only 5/790 population controls. Sequence conservation of the AltT2 exon in 16 species with invariable preservation of the G allele at the mutated site, strongly favour the exonic change as the pathogenic mutation. Analysis of the Hk1 upstream region in mouse mRNA from testis and neural tissues showed an abundance of AltT2-containing transcripts generated by extensive, developmentally regulated alternative splicing. Expression is very low compared with ubiquitous Hk1 and all transcripts skip exon1, which encodes the protein domain responsible for binding to the outer mitochondrial membrane, and regulation of energy production and apoptosis. Hexokinase activity measurement and immunohistochemistry of the peripheral nerve showed no difference between patients and controls. The mutational mechanism and functional effects remain unknown and could involve disrupted translational regulation leading to increased anti-apoptotic activity (suggested by the profuse regenerative activity in affected nerves), or impairment of an unknown HK1 function in the peripheral nervous system (PNS).

Expression levels of genes associated with oxygen utilization, glucose transport and glucose phosphorylation in hypoxia exposed Atlantic cod (Gadus morhua)

Expression level of genes associated with oxygen [cytochrome oxidase 1 (COX1) and myoglobin (Mb)] and glucose utilization [glucose transporters (GLUTs) and hexokinases (HKs)] along with metabolic indices were determined in Atlantic cod (Gadus morhua) subjected to an hypoxic challenge of <45% oxygen saturation for 24 days. There were two closely related HKs considered to be homologues of mammalian HKIs. HKIa and HKIb share 86% sequence identity and are both ubiquitously expressed. Mb was also expressed in many tissues with highest levels occurring in heart. Over the first 15 days of hypoxia there were transient increases in plasma lactate in hypoxic relative to normoxic fish associated with a significant decrease in liver glycogen. Over days 1-6, there were in ten of eleven cases, increased average (with a number of conditions being statistically significant) expression levels of GLUTs (1, 2, 4) and HKs (1a and b) in gill, heart, liver, and white muscle in hypoxic relative to normoxic fish. There were significant increases in COX1 and Mb expression levels in gill by day 24 but no changes in these aerobic indicators in heart or liver. Overall the data suggest a transient increase in genes associated with glucose utilization during the early part of the hypoxic challenge followed by alterations in gene expression in gill.

Key regions of VDAC1 functioning in apoptosis induction and regulation by hexokinase

The voltage-dependent anion channel (VDAC), located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, and thus controls cross-talk between mitochondria and the cytosol. VDAC also serves as a site for the docking of cytosolic proteins, such as hexokinase, and is recognized as a key protein in mitochondria-mediated apoptosis. The role of VDAC in apoptosis has emerged from various studies showing its involvement in cytochrome c release and apoptotic cell death as well as its interaction with proteins regulating apoptosis, including the mitochondria-bound isoforms of hexokinase (HK-I, HK-II). Recently, the functional HK-VDAC association has shifted from being considered in a predominantly metabolic light to the recognition of its major impact on the regulation of apoptotic responsiveness of the cell. Here, we demonstrate that the HK-VDAC1 interaction can be disrupted by mutating VDAC1 and by VDAC1-based peptides, consequently leading to diminished HK anti-apoptotic activity, suggesting that disruption of HK binding to VDAC1 can decrease tumor cell survival. Indeed, understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of differing functions, all important for cell life and death. By expressing VDAC1 mutants and VDAC1-based peptides, we have identified VDAC1 amino acid residues and domains important for interaction with HK and protection against apoptosis. These include negatively- and positively-charged residues, some of which are located within beta-strands of the protein. The N-terminal region of VDAC1 binds HK-I and prevents HK-mediated protection against apoptosis induced by STS, while expression of a VDAC N-terminal peptide detaches HK-I-GFP from mitochondria. These findings indicate that the interaction of HK with VDAC1 involves charged residues in several beta-strands and in the N-terminal domain. Displacing HK, serving as the 'guardian of the mitochondrion', from its binding site on VDAC1 may thus be exploited as an approach to cancer therapy.

Hexokinase-I protection against apoptotic cell death is mediated via interaction with the voltage-dependent anion channel-1: mapping the site of binding

In brain and tumor cells, the hexokinase isoforms HK-I and HK-II bind to the voltage-dependent anion channel (VDAC) in the outer mitochondrial membrane. We have previously shown that HK-I decreases murine VDAC1 (mVDAC1) channel conductance, inhibits cytochrome c release, and protects against apoptotic cell death. Now, we define mVDAC1 residues, found in two cytoplasmic domains, involved in the interaction with HK-I. Protection against cell death by HK-I, as induced by overexpression of native or mutated mVDAC1, served to identify the mVDAC1 amino acids required for interaction with HK-I. HK-I binding to mVDAC1 either in isolated mitochondria or reconstituted in a bilayer was inhibited upon mutation of specific VDAC1 residues. HK-I anti-apoptotic activity was also diminished upon mutation of these amino acids. HK-I-mediated inhibition of cytochrome c release induced by staurosporine was also diminished in cells expressing VDAC1 mutants. Our results thus offer new insights into the mechanism by which HK-I promotes tumor cell survival via inhibition of cytochrome c release through HK-I binding to VDAC1. These results, moreover, point to VDAC1 as a key player in mitochondrially mediated apoptosis and implicate an HK-I-VDAC1 interaction in the regulation of apoptosis. Finally, these findings suggest that interference with the binding of HK-I to mitochondria by VDAC1-derived peptides may offer a novel strategy by which to potentiate the efficacy of conventional chemotherapeutic agents.

Prediction of protein orientation upon immobilization on biological and nonbiological surfaces

We report on a rapid simulation method for predicting protein orientation on a surface based on electrostatic interactions. New methods for predicting protein immobilization are needed because of the increasing use of biosensors and protein microarrays, two technologies that use protein immobilization onto a solid support, and because the orientation of an immobilized protein is important for its function. The proposed simulation model is based on the premise that the protein interacts with the electric field generated by the surface, and this interaction defines the orientation of attachment. Results of this model are in agreement with experimental observations of immobilization of mitochondrial creatine kinase and type I hexokinase on biological membranes. The advantages of our method are that it can be applied to any protein with a known structure; it does not require modeling of the surface at atomic resolution and can be run relatively quickly on readily available computing resources. Finally, we also propose an orientation of membrane-bound cytochrome c, a protein for which the membrane orientation has not been unequivocally determined.

Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt

Cell survival has been closely linked to both trophic growth factor signaling and cellular metabolism. Such couplings have obvious physiologic and pathophysiologic implications, but their underlying molecular bases remain incompletely defined. As a common mediator of both the metabolic and anti-apoptotic effects of growth factors, the serine/threonine kinase Akt - also known as protein kinase B or PKB - is capable of regulating and coordinating these inter-related processes. The glucose dependence of the antiapoptotic effects of growth factors and Akt plus a strong correlation between Akt-regulated mitochondrial hexokinase association and apoptotic susceptibility suggest a major role for hexokinases in these effects. Mitochondrial hexokinases catalyse the first obligatory step of glucose metabolism and directly couple extramitochondrial glycolysis to intramitochondrial oxidative phosphorylation, and are thus well suited to play this role. The ability of Akt to regulate energy metabolism appears to have evolutionarily preceded the capacity to control cell survival. This suggests that Akt-dependent metabolic regulatory functions may have given rise to glucose-dependent antiapoptotic effects that evolved as an adaptive sensing system involving hexokinases and serve to ensure mitochondrial homeostasis, thereby coupling metabolism to cell survival. We hypothesize that the enlistment of Akt and hexokinase in the control of mammalian cell apoptosis evolved as a response to the recruitment of mitochondria to the apoptotic cascade. The central importance of mitochondrial hexokinases in cell survival also suggests that they may represent viable therapeutic targets in cancer.

A mitochondrial paradigm of metabolic and degenerative diseases, aging, and cancer: a dawn for evolutionary medicine

Life is the interplay between structure and energy, yet the role of energy deficiency in human disease has been poorly explored by modern medicine. Since the mitochondria use oxidative phosphorylation (OXPHOS) to convert dietary calories into usable energy, generating reactive oxygen species (ROS) as a toxic by-product, I hypothesize that mitochondrial dysfunction plays a central role in a wide range of age-related disorders and various forms of cancer. Because mitochondrial DNA (mtDNA) is present in thousands of copies per cell and encodes essential genes for energy production, I propose that the delayed-onset and progressive course of the age-related diseases results from the accumulation of somatic mutations in the mtDNAs of post-mitotic tissues. The tissue-specific manifestations of these diseases may result from the varying energetic roles and needs of the different tissues. The variation in the individual and regional predisposition to degenerative diseases and cancer may result from the interaction of modern dietary caloric intake and ancient mitochondrial genetic polymorphisms. Therefore the mitochondria provide a direct link between our environment and our genes and the mtDNA variants that permitted our forbears to energetically adapt to their ancestral homes are influencing our health today.

Compartmentalization of transport and phosphorylation of glucose in a hepatoma cell line

The first steps of glucose metabolism are carried out by members of the families of GLUTs (glucose transporters) and HKs (hexokinases). Previous experiments using the inhibitor of glucose transport, CB (cytochalasin B), revealed that compartmentalization of GLUTs and HKs is a major factor in the control of glucose uptake in L6 myotubes [Whitesell, Ardehali, Printz, Beechem, Knobel, Piston, Granner, Van Der Meer, Perriott and May (2003) Biochem. J. 370, 47-56]. In the present paper, we evaluate compartmentalization of GLUTs and HKs in a hepatoma cell line, H4IIE, which is characterized by excess GLUT activity, HKI in a particulate and a cytosolic fraction, and insignificant G6Pase (glucose-6-phosphatase) activity. The measured activity of glucose transport exceeded the rate of phosphorylation approx. 30-fold. Treatment with 25 microM CB (K(i) approximately 3 microM in H4IIE cells) paradoxically increased the excess of GLUTs over phosphorylation (GLUTs are inhibited 80%, while phosphorylation is inhibited 98%). The global relationships of the data could be reconciled most simply by a two-compartment model. In this model, phosphorylation of glucose is carried out by a subset of HK molecules supplied by a subset of GLUTs that are more sensitive to CB than the other GLUTs. The agent, DCC (dicyclohexylcarbodi-imide) caused HKI to translocate from the particulate compartment to the cytosolic compartment and potently inhibited glucose phosphorylation. The particulate compartment may represent the mitochondria, to which the more CB-sensitive GLUTs may control the transport of glucose.

Bovine hexokinase type I: full-length cDNA sequence and characterisation of the recombinant enzyme

This study reports the revised and full-length cDNA sequence of bovine hexokinase type I obtained from bovine brain. Since dissimilarities have been observed between the published bovine hexokinase type I coding sequence (GenBank accession no. M65140) (Genomics 11: 1014-1024, 1991) and an analysed portion of bovine hexokinase type I gene, the entire open reading frame was re-sequenced and the ends of cDNA isolated by rapid amplification of cDNA ends. The coding sequences, when compared with the published bovine hexokinase type I, contained a large number of mismatches that lead to changes in the resulting amino acid sequence. The revisions result in a hexokinase type I cDNA of 3619 bp that encodes a protein of 917 amino acids highly homologous to human hexokinase type I. The expression of the recombinant full-length enzyme demonstrated that it was a catalytically active hexokinase. When characterised for its kinetic and regulatory properties, it displayed the same affinity for glucose and MgATP as the human hexokinase type I and was inhibited by glucose 6-phosphate competitively versus MgATP. The production of the N- and C-terminal recombinant halves of the enzyme followed by comparison with the full-length hexokinase indicated that the catalytic activity is located in the C-terminal domain.

Hexokinase-Mitochondria Interaction Mediated by Akt Is Required to Inhibit Apoptosis in the Presence or Absence of Bax and Bak

The serine/threonine kinase Akt inhibits mitochondrial cytochrome c release and apoptosis induced by a variety of proapoptotic stimuli. The antiapoptotic activity of Akt is coupled, at least in part, to its effects on cellular metabolism. Here, we provide genetic evidence that Akt is required to maintain hexokinase association with mitochondria. Targeted disruption of this association impairs the ability of growth factors and Akt to inhibit cytochrome c release and apoptosis. Targeted disruption of mitochondria-hexokinase (HK) interaction or exposure to proapoptotic stimuli that promote rapid dissociation of hexokinase from mitochondria potently induce cytochrome c release and apoptosis, even in the absence of Bax and Bak. These effects are inhibited by activated Akt, but not by Bcl-2, implying that changes in outer mitochondrial membrane (OMM) permeability leading to apoptosis can occur in the absence of Bax and Bak and that Akt inhibits these changes through maintenance of hexokinase association with mitochondria.

A developmentally regulated, neuron-specific splice variant of the variable subunit Bbeta targets protein phosphatase 2A to mitochondria and modulates apoptosis

Heterotrimeric protein phosphatase 2A (PP2A) is a major Ser/Thr phosphatase composed of catalytic, structural, and regulatory subunits. Here, we characterize Bbeta2, a novel splice variant of the neuronal Bbeta regulatory subunit with a unique N-terminal tail. Bbeta2 is expressed predominantly in forebrain areas, and PP2A holoenzymes containing Bbeta2 are about 10-fold less abundant than those containing the Bbeta1 (previously Bbeta) isoform. Bbeta2 mRNA is dramatically induced postnatally and in response to neuronal differentiation of a hippocampal progenitor cell line. The divergent N terminus of Bbeta2 does not affect phosphatase activity but encodes a subcellular targeting signal. Bbeta2, but not Bbeta1 or an N-terminal truncation mutant, colocalizes with mitochondria in neuronal PC12 cells. Moreover, the Bbeta2 N-terminal tail is sufficient to target green fluorescent protein to this organelle. Inducible or transient expression of Bbeta2, but neither Bbeta1, Bgamma, nor a Bbeta2 mutant defective in holoenzyme formation, accelerates apoptosis in response to growth factor deprivation. Thus, alternative splicing of a mitochondrial localization signal generates a PP2A holoenzyme involved in neuronal survival signaling.

Isozymes of mammalian hexokinase: structure, subcellular localization and metabolic function

The first step in metabolism of glucose (Glc) is usually phosphorylation, catalyzed by hexokinase. However, the Glc-6-P produced can then enter one or more of several alternative pathways. Selective expression of isozymic forms of hexokinase, differing in catalytic and regulatory properties as well as subcellular localization, is likely to be an important factor in determining the pattern of Glc metabolism in mammalian tissues/cells. Despite their overall structural similarity, the Type I, Type II and Type III isozymes differ in important respects. All three isozymes are inhibited by the product, Glc-6-P, but with the Type I isozyme, this inhibition is antagonized by P(I), whereas with the Type II and Type III isozymes, P(i) actually causes additional inhibition. Reciprocal changes in intracellular levels of Glc-6-P and P(i) are closely associated with cellular energy status, and it is proposed that the response of the Type I isozyme to these effectors adapts it for catabolic function, introducing Glc into glycolytic metabolism for energy production. In contrast, the Type II, and probably the Type III, isozymes are suggested to serve primarily anabolic functions, e.g. to provide Glc-6-P for glycogen synthesis or metabolism via the pentose phosphate pathway for lipid synthesis. Type I hexokinase binds to mitochondria through interaction with porin, the protein that forms channels through which metabolites traverse the outer mitochondrial membrane. Several experimental approaches have led to the conclusion that the Type I isozyme, bound to actively phosphorylating mitochondria, selectively uses intramitochondrial ATP as substrate. Such interactions are thought to facilitate coordination of the introduction of Glc into glycolysis, via the hexokinase reaction, with the terminal oxidative stages of Glc metabolism occurring in the mitochondria, thus ensuring an overall rate of Glc metabolism commensurate with cellular energy demands and avoiding excessive production of lactate. The Type II isozyme also binds to mitochondria. Whether such coupling occurs with mitochondrially bound Type II hexokinase in normal tissues, and how it might be related to the proposed anabolic role of this isozyme, remain to be determined. The Type III isozyme lacks the hydrophobic N-terminal sequence known to be critical for binding of the Type I and Type II isozymes to mitochondria. Immunolocalization studies have indicated that, in many cell types, the Type III has a perinuclear localization, the possible metabolic consequences of which remain unclear.

A 15-residue bifunctional element in D-AKAP1 is required for both endoplasmic reticulum and mitochondrial targeting

The cAMP-dependent protein kinase anchoring protein, d-AKAP1, has two N-terminal splice variants. The shorter forms (N0, d-AKAP1a, and -1c) target to mitochondria, and the longer forms (N1, d-AKAP1b, and -1d) with 33 additional residues N-terminal to N0 target to the endoplasmic reticulum (ER) (Huang, L. J., Wang, L., Ma, Y., Durick, K., Perkins, G., Deerinck, T. J., Ellisman, M. H., and Taylor, S. S. (1999) J. Cell Biol. 145, 951-959). In d-AKAP1a, translation may initiate from both Met-34 or Met-49 producing two molecules both targeted to mitochondria. The shorter molecule contains the 15-residue targeting motif, homologous to the N-terminal mitochondrial targeting motif of hexokinase I. Extensive mutagenesis showed that one hydrophobic surface of the 15-residue hexokinase-homologous segment contained the key elements for mitochondrial targeting. The same 15 residues are also part of the ER-targeting signal, but for ER targeting multiple hydrophobic residues are required that encompass both surfaces of the helix. The different involvement of the same helical motif for targeting to the two organelles appears to reflect different modes of interaction with the two organelles. This is the first example of a bifunctional helical element that is required for both ER and mitochondrion targeting.

Characterisation of hexokinase in Toxoplasma gondii tachyzoites

We have cloned the hexokinase [E.C. 2.7.1.1] gene of Toxoplasma gondii tachyzoite and obtained an active recombinant enzyme with a calculated molecular mass of 51,465Da and an isoelectric point of 5.82. Southern blot analysis indicated that the hexokinase gene existed as a single copy in the tachyzoites of T. gondii. The sequence of T. gondii hexokinase exhibited the highest identity (44%) to that of Plasmodium falciparum hexokinase and lower identity of less than 35% to those of hexokinases from other organisms. The specific activity of the homogeneously purified recombinant enzyme was 4.04 micromol/mg protein/min at 37 degrees C under optimal conditions. The enzyme could use glucose, fructose, and mannose as substrates, though it preferred glucose. Adenosine triphosphate was exclusively the most effective phosphorus donor, and pyrophosphate did not serve as a substrate. K(m) values for glucose and adenosine triphosphate were 8.0+/-0.8 microM and 1.05+/-0.25mM, respectively. No allosteric effect of substrates was observed, and the products, glucose 6-phosphate and adenosine diphosphate, had no inhibitory effect on T. gondii hexokinase activity. Other phosphorylated hexoses, fructose 6-phosphate, trehalose 6-phosphate which is an inhibitor of yeast hexokinase, and pyrophosphate, also did not affect T. gondii hexokinase activity. Native hexokinase activity was recovered in both the cytosol and membrane fractions of the whole lysate of T. gondii tachyzoites. This result suggests that T. gondii hexokinase weakly associates with the membrane or particulate fraction of the tachyzoite cell.

Mouse models for mitochondrial disease

Mutations in mitochondrial genes encoded by both mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) genes have been implicated in a wide range of neuromuscular diseases. MtDNA base substitution and rearrangement mutations generally inactivate one or more tRNA or rRNA genes and can cause myopathy, cardiomyopathy, cataracts, growth retardation, diabetes, etc. nDNA mutations can cause Leigh syndrome, cardiomyopathy, and nephropathy, due to defects in oxidative phosphorylation (OXPHOS) enzyme complexes; cartilage-hair hypoplasia (CHH) and mtDNA depletion syndrome, through defects in mitochondrial nucleic acid metabolism; and ophthalmoplegia with multiple mtDNA deletions, caused by adenine nucleotide translocator-1 (ANT1) mutations. Mouse models have been prepared that recapitulate a number of these diseases. The mtDNA 16S rRNA chloramphenicol (CAP) resistance mutation was introduced into the mouse female germline and caused cataracts and rod and cone abnormalities in chimeras and neonatal lethal myopathy and cardiomyopathy in mutant animals. A mtDNA deletion was introduced into the mouse germline and caused myopathy, cardiomyopathy, and nephropathy. Conditional inactivation of the nDNA mitochondrial transcription factor (Tfam) gene in the heart resulted in neonatal lethal cardiomyopathy, while its inactivation in the pancreatic beta-cells caused diabetes. The ATP/ADP ratio was implicated in mitochondrial diabetes through transgenic modification of the beta-cell ATP-sensitive K(+) channel (K(ATP)). Mutational inactivation of the mouse Ant1 gene resulted in myopathy, cardiomyopathy, and multiple mtDNA deletions in association with elevated reactive oxygen species (ROS) production. Inactivation of uncoupler proteins (Ucp) 1-3 revealed that mitochondrial Delta Psi regulated ROS production. The role of mitochondrial ROS toxicity in disease and aging was confirmed by inactivating glutathione peroxidase (GPx1), resulting in growth retardation, and by total and partial inactivation of Mn superoxide dismutase (MnSOD; Sod2), resulting in neonatal lethal dilated cardiomyopathy and accelerated apoptosis in aging, respectively. The importance of mitochondrial ROS in degenerative diseases and aging was confirmed by treating Sod2 -/- mice and C. elegans with catalytic antioxidant drugs.

Interaction of hexokinase with the outer mitochondrial membrane and a hydrophobic matrix

The major portion of rat brain hexokinase (HK type 1) is bound to the outer membrane of mitochondria and glucose-6-phosphate (G6P) can release the bound enzyme. In an attempt to look at the 'hydrophobic' component of binding, interaction of the enzyme with a purely hydrophobic matrix, palmityl-substituted Sepharose-4B (Sepharose-lipid) was investigated. Hexokinase readily bound to this matrix with retention of its catalytic activity. Glucose-6-phosphate which has a releasing effect on the mitochondrially bound enzyme, enhanced binding of the enzyme on the hydrophobic matrix. Chymotrypsin treatment of hexokinase which causes loss of binding to mitochondria, also results in loss of adsorption to the hydrophobic matrix, thus demonstrating that the 'hydrophobic tail' present at its N-terminal end is essential for binding in both cases. Data presented provide some new information relevant to understanding how hexokinase interacts with its natural binding matrix, the mitochondrion.

Hexokinase 'binding sites' of normal and tumoral human brain mitochondria

Interaction of type I hexokinase (HK-I) with the mitochondria obtained from the biopsy specimens of normal and tumoral human brain tissues was studied in the present investigation. This effort was undertaken with the aim of exploring possible differences in the mode of association of the enzyme with the outer mitochondrial membrane in the described sources. Results indicate that the two 'sites' for binding of HK-I suggested in the literature, based on extensive studies carried out on rat brain mitochondria, are similarly present in the human brain mitochondria. Differences in the microenvironments of HK binding, as reflected by the presented data, are suggested to be of importance in regulation of the catalytic potential of the bound enzyme. The real metabolic significance of this association in relation to cancer and its practical importance would need further investigation.

Interaction of insulin-like growth factor binding protein-4, Miz-1, leptin, lipocalin-type prostaglandin D synthase, and granulin precursor with the N-terminal half of type III hexokinase

Insulin-like growth factor binding protein-4, Miz-1, leptin, prostaglandin D synthase, and granulin precursor were identified as proteins interacting with the N-terminal half of mammalian Type III hexokinase (HKIII) in the yeast two-hybrid method. These interactions were confirmed by in vitro binding studies. All five of these proteins, and their mRNAs, were present in PC12 cells, as shown by immunoblotting and RT-PCR, respectively. All were coimmunoprecipitated from PC12 extracts with an antibody against HKIII, but not with anti-Type I hexokinase. Moreover, all of these proteins were coimmunoprecipitated using antileptin as precipitating antibody, indicating the existence of a macromolecular complex including these five proteins and HKIII. Transfection of M+R 42 cells with HKIII-green fluorescent protein (GFP) reporter constructs gave a diffuse intracellular fluorescence. Cotransfection with leptin or Miz-1 resulted in distinctly different localization of the HKIII-GFP fusion protein, at intracellular sites coincident with localization of leptin-GFP or Miz-1-GFP reporter constructs.

Characterization of porin isoforms expressed in tumor cells

Mitochondria from malignant tumor cell lines show a higher capability for hexokinase binding than those from normal liver. To explore possible differences in hexokinase binding sites of mitochondria between tumor cells and normal liver, we characterized porin isoforms expressed in tumor cells. Cloning experiments on the three porin isoforms, VDAC1, VDAC2 and VDAC3 from malignant tumor cell line AH130 clearly showed that their primary structures were completely identical to those of the corresponding VDACs of normal liver cells. Possible expression of the fourth porin isoform in AH130 cells was excluded by degenerate primer-based RT-PCR. However, the transcript levels of the three VDAC isoforms in AH130 cells were significantly higher than those in normal liver. These results suggest that the high hexokinase-binding capability of malignant tumor cell mitochondria was not due to any structural difference, but due to a quantitative difference in binding sites.

Structure of the 5' region of the human hexokinase type I (HKI) gene and identification of an additional testis-specific HKI mRNA

We previously reported the structure of the human hexokinase type I (HKI) gene and provided direct evidence of an alternative red blood cell-specific exon 1 located in the 5' flanking region of the gene. Three unique HKI mRNA species have also been described in human spermatogenic cells. These mRNAs contain a testis-specific sequence not present in somatic cell HKI, but lack the sequence for the porin-binding domain necessary for HKI to bind to porin on the outer mitochondrial membrane. The present study reports a new mRNA isoform, hHKI-td, isolated from human sperm. hHKI-td mRNA contains both a testis-specific sequence at the 5' end common to the three other mRNA isoforms and an additional unique sequence. Screening of a cosmid library and analysis of the cosmids containing the HKI gene revealed that testis-specific sequences are encoded by six different exons. Five of these exons are located upstream from the somatic exon 1 (5.6-30 kb) and one within intron 1. This study shows that a single human HKI gene spanning at least 100 kb encodes multiple transcripts that are generated by alternative splicing of different 5' exons. Testis-specific transcripts are probably produced by a separate promoter that induces the expression of the HKI gene in spermatogenic cells.