The soluble but not mitochondrially bound hexokinase is a substrate for the ATP- and ubiquitin-dependent proteolytic system

Human Red Blood Cells Modulate Cytokine Expression in Monocytes/Macrophages Under Anoxic Conditions

In the bone marrow (BM) hematopoietic niche, the oxygen tension is usually very low. Such condition affects stem and progenitor cell proliferation and differentiation and, at cellular level regulates hematopoietic growth factors, chemokines and adhesion molecules expression. In turn, these molecules affect the proliferation and maturation of other cellular components of the niche. Due to the complexity of the system we started the in vitro investigations of the IL-6, IL-8, TNFα cytokines expression and the vascular endothelial growth factor (VEGF), considered key mediators of the hematopoietic niche, in human macrophages and macrophage cell line. Since in the niche the oxygen availability is mediated by red blood cells (RBCs), we have influenced the anoxic cell cultures by the administration of oxygenated or deoxygenated RBCs (deoxy RBCs). The results reported in this brief paper show that the presence of RBCs up-regulates IL-8 mRNA while IL-6 and VEGF mRNA expression appears down-regulated. This does not occur when deoxy RBCs are used. Moreover, it appears that the administration of RBCs leads to an increase of TNFα expression levels in MonoMac 6 (MM6). Interestingly, the modulation of these factors likely occurs in a hypoxia-inducible factor-1α (HIF-1α) independent manner. Considering the role of oxygen in the hematopoietic niche further studies should explore these preliminary observations in more details.

Inhibition of glycolysis by a novel EGFR/HER2 inhibitor KU004 suppresses the growth of HER2+ cancer

Upregulation of glycolysis was often observed in human HER2-overexpressing cancers. In this study, we demonstrated that KU004, a dual novel EGFR/HER2 inhibitor, disrupted cancer cell proliferation via modulation of glycolysis. KU004, inhibited the Warburg effect by suppressing hexokinase II (HK2) expression at the transcriptional and post-translational levels. Further study demonstrated that the downregulation of HKII by KU004 was mainly mediated by the PI3K/Akt signaling pathway. Furthermore, the role of HKII downregulation in KU004-mediated antitumor effect was also confirmed in our in vivo xenograft model. Collectively, these data suggest that multifaceted targeting the aberrant glucose metabolism along with the upstream HER2 may be an effective approach for clinical treatment against HER2+ cancer.

Hexokinase regulates Bax-mediated mitochondrial membrane injury following ischemic stress

Hexokinase (HK), the rate-limiting enzyme in glycolysis, controls cell survival by promoting metabolism and/or inhibiting apoptosis. Since HK isoforms I and II have mitochondrial targeting sequences, we attempted to separate the protective effects of HK on cell metabolism from those on apoptosis. We exposed renal epithelial cells to metabolic stress causing ATP depletion in the absence of glucose and found that this activated glycogen synthase kinase 3β (GSK3β) and Bax caused mitochondrial membrane injury and apoptosis. ATP depletion led to a progressive HK II dissociation from mitochondria, released mitochondrial apoptosis inducing factor and cytochrome c into the cytosol, activated caspase-3, and reduced cell survival. Compared with control, adenoviral-mediated HK I or II overexpression improved cell survival following stress, but did not prevent GSK3β or Bax activation, improve ATP content, or reduce mitochondrial fragmentation. HK I or HK II overexpression increased mitochondria-associated isoform-specific HK content, and decreased mitochondrial membrane injury and apoptosis after stress. In vivo, HK II localized exclusively to the proximal tubule. Ischemia reduced total renal HK II content and dissociated HK II from proximal tubule mitochondria. In cells overexpressing HK II, Bax and HK II did not interact before or after stress. While the mechanism by which HK antagonizes Bax-mediated apoptosis is unresolved by these studies, one possible scenario is that the two proteins compete for a common binding site on the outer mitochondrial membrane.

Glucose metabolism and proliferation in glia: role of astrocytic gap junctions

Astrocytes play a well-established role in brain metabolism, being a key element in the capture of energetic compounds from the circulation and in their delivery to active neurons. Their metabolic status is affected in many pathological situations, such as gliomas, which are the most common brain tumors. This proliferative dysfunction is associated with changes in gap junctional communication, a property strongly developed in normal astrocytes studied both in vitro and in vivo. Here, we summarize and discuss the findings that have lead to the identification of a link between gap junctions, glucose uptake, and proliferation. Indeed, the inhibition of gap junctional communication is associated with an increase in glucose uptake due to a rapid change in the localization of both GLUT-1 and type I hexokinase. This effect persists due to the up-regulation of GLUT-1 and type I hexokinase and to the induction of GLUT-3 and type II hexokinase. In addition, cyclins D1 and D3 have been found to act as sensors of the inhibition of gap junctions and have been proposed to play the role of mediators in the mitogenic effect observed. Conversely, in C6 glioma cells, characterized by a low level of intercellular communication, an increase in gap junctional communication reduces glucose uptake by releasing type I and type II hexokinases from the mitochondria and decreases the exacerbated rate of proliferation due to the up-regulation of the Cdk inhibitors p21 and p27. Identification of the molecular actors involved in these pathways should allow the determination of potential therapeutic targets that could lead to the testing of alternative strategies to prevent, or at least slow down, the proliferation of glioma cells.

RanBP2 modulates Cox11 and hexokinase I activities and haploinsufficiency of RanBP2 causes deficits in glucose metabolism

The Ran-binding protein 2 (RanBP2) is a large multimodular and pleiotropic protein. Several molecular partners with distinct functions interacting specifically with selective modules of RanBP2 have been identified. Yet, the significance of these interactions with RanBP2 and the genetic and physiological role(s) of RanBP2 in a whole-animal model remain elusive. Here, we report the identification of two novel partners of RanBP2 and a novel physiological role of RanBP2 in a mouse model. RanBP2 associates in vitro and in vivo and colocalizes with the mitochondrial metallochaperone, Cox11, and the pacemaker of glycolysis, hexokinase type I (HKI) via its leucine-rich domain. The leucine-rich domain of RanBP2 also exhibits strong chaperone activity toward intermediate and mature folding species of Cox11 supporting a chaperone role of RanBP2 in the cytosol during Cox11 biogenesis. Cox11 partially colocalizes with HKI, thus supporting additional and distinct roles in cell function. Cox11 is a strong inhibitor of HKI, and RanBP2 suppresses the inhibitory activity of Cox11 over HKI. To probe the physiological role of RanBP2 and its role in HKI function, a mouse model harboring a genetically disrupted RanBP2 locus was generated. RanBP2^−/− are embryonically lethal, and haploinsufficiency of RanBP2 in an inbred strain causes a pronounced decrease of HKI and ATP levels selectively in the central nervous system. Inbred RanBP2^+/− mice also exhibit deficits in growth rates and glucose catabolism without impairment of glucose uptake and gluconeogenesis. These phenotypes are accompanied by a decrease in the electrophysiological responses of photosensory and postreceptoral neurons. Hence, RanBP2 and its partners emerge as critical modulators of neuronal HKI, glucose catabolism, energy homeostasis, and targets for metabolic, aging disorders and allied neuropathies.

The Ran-binding protein 2 (RanBP2) is a large protein with several domains. Although several protein partners were found to interact with selective domains of RanBP2, none to this date were found toward its large leucine-rich domain (LD). Cell-based experiments support several roles of RanBP2 in cell function, such as the production of functional proteins, control of protein trafficking between the nuclear and cytosol compartments, and control of multiple facets underlying cell division. Still, the genetic and physiological implications of the interactions between RanBP2 and its partners and of the function of RanBP2 in a whole-animal model remain elusive. The authors report the identification of two novel mitochondrial partners of the LD of RanBP2, Cox11 and hexokinase type I (HKI); and with multidisciplinary approaches probe the role of RanBP2 and its LD on Cox11, HKI, and functions allied to these. The authors found that RanBP2 exhibits chaperone activity toward HKI and Cox11. RanBP2 and Cox11 profoundly modulate HKI activity. Moreover, partial loss-of-function of RanBP2 in a mouse model induces deficits in growth rates and breakdown of glucose, promotes the down-regulation of HKI and ATP levels selectively in the central nervous system, and impairs visual function. These findings support a critical role of RanBP2 and its partners in metabolic processes and allied disease states.

Suppression of apoptosis by cyclophilin D via stabilization of hexokinase II mitochondrial binding in cancer cells

The permeability transition pore is involved in the mitochondrial pathway of apoptosis. Cyclophilin D, a pore component, has catalytic activity as a peptidyl prolyl cis, trans-isomerase (PPIase), which is essential to the pore opening. It has been reported that cyclophilin D overexpression suppresses apoptosis in cancer cells. To clarify the mechanism of this effect, we generated glioma cells overexpressing wild-type or a PPIase-deficient mutant of cyclophilin D. Interestingly, we found that the PPIase-dependent apoptosis suppression by cyclophilin D correlated with the amounts of mitochondrial-bound hexokinase II, which has anti-apoptotic activity. Inactivation of endogenous cyclophilin D by small interference RNA or a cyclophilin inhibitor was found to release hexokinase II from mitochondria and to enhance Bax-mediated apoptosis. The anti-apoptotic effects of cyclophilin D were canceled out by the detachment of hexokinase II from mitochondria, demonstrating that mitochondrial binding of hexokinase II is essential to the apoptosis suppression by cyclophilin D. Furthermore, cyclophilin D dysfunction appears to abrogate hexokinase II-mediated apoptosis suppression, indicating that cyclophilin D is required for the anti-apoptotic activity of hexokinase II. Based on the above, we propose here that cyclophilin D suppresses apoptotic cell death via a mitochondrial hexokinase II-dependent mechanism in cancer cells.

The increase in gap junctional communication decreases the rate of glucose uptake in C6 glioma cells by releasing hexokinase from mitochondria

We have previously shown that the enhancement of glucose uptake caused by the inhibition of gap junctional communication is a consequence of the increase in astrocyte proliferation. Since C6 glioma cells are highly proliferative and are poorly coupled through gap junctions, we used these cells to investigate the effect of increasing gap junctional communication on the rate of glucose uptake. Previous work by us had shown that tolbutamide increases gap junctional communication in C6 glioma cells, as does dbcAMP, a classical activator of gap junctional communication. In this work, our results show that both tolbutamide and dbcAMP reduce the rate of glucose uptake in C6 glioma cells and that their effects are additive. The main glucose transporters expressed in C6 glioma cells are GLUT-1 and GLUT-3. Neither the expression nor the cellular localization of either GLUT-1 or GLUT-3 were modified by increasing gap junctional communication. The estimation of glucose uptake with 2-deoxyglucose includes not only glucose transport but also glucose phosphorylation, which in C6 glioma cells is mainly catalyzed by type I and type II hexokinase. Our results reveal that the increase in gap junctional communication caused by tolbutamide and dbcAMP is associated with a decrease in the activity of hexokinase. In agreement with this, tolbutamide and dbcAMP caused a rapid change in the localization of both type I and type II hexokinase, which were detached from the mitochondria to the cytosol.

Involvement of porin N,N-dicyclohexylcarbodiimide-reactive domain in hexokinase binding to the outer mitochondrial membrane

The proportion of hexokinase that is bound to the outer mitochondrial membrane is tissue specific and metabolically regulated. This study examined the role of the N,N-dicyclohexylcarbodiimide-binding domain of mitochondrial porin in binding to hexokinase 1. Selective proteolytic cleavage of porin protein was performed and peptides were assayed for their, effect on hexokinase I binding to isolated mitochondria. Specificity of DCCD-reactive domain binding to hexokinase I was demonstrated by competition of the peptides for porin binding sites on hexokinase as well as by blockage hexokinase binding by N,N-dicyclohexylcarbodiimide. One of the peptides, designated as 5 kDa (the smallest of the porin peptides, which contains a DCCD-reactive site), totally blocked binding of the enzyme to the mitochondrial membrane, and significantly enhanced the release of the mitochondrially bound enzyme. These experiments demonstrate that there exists a direct and specific interaction between the DCCD-reactive domain of VDAC and hexokinase I. The peptides were further characterized with respect to their effects on certain functional properties of hexokinase I. None had any detectable effect on catalytic properties, including inhibition by glucose 6-phosphate. To evaluate further the outer mitochondrial membrane's role in the hexokinase binding, insertion of VDAC was examined using isolated rat mitochondria. Preincubation of mitochondria with purified porin strongly increases hexokinase I binding to rat liver mitochondria. Collectively, the results imply that the high hexokinase-binding capability of porin-enriched mitochondria was due to a quantitative difference in binding sites.

Tubulin and microtubule are potential targets for brain hexokinase binding

The metabolite-modulated association of a fraction of hexokinase to mitochondria in brain is well documented, however, the involvement of other non-mitochondrial components in the binding of the hexokinase is controversial. Now we present evidence that the hexokinase binds both tubulin and microtubules in brain in vitro systems. The interaction of tubulin with purified bovine brain hexokinase was characterized by displacement enzyme-linked immunosorbent assay using specific anti-brain hexokinase serum (IC(50)=4.0+/-1.4 microM). This value virtually was not affected by specific ligands such as ATP or glucose 6-phosphate. Microtubule-bound hexokinase obtained in reconstituted systems using microtubule and purified hexokinase or brain extract was visualized by transmission and immunoelectron microscopy on the surface of tubules. The association of purified bovine brain hexokinase with either tubulin or microtubules caused about 30% increase in the activity of the enzyme. This activation was also observed in brain, but not in muscle cell-free extract. The possible physiological relevance of the multiple heteroassociation of brain hexokinase is discussed.

Binding of rat brain hexokinase to recombinant yeast mitochondria: identification of necessary molecular determinants

The association in vitro of rat brain hexokinase to mitochondria from rat liver or yeast (wild type, porinless, or expressing recombinant human porin) was studied in an effort to identify minimal requirements for each component. A short hydrophobic N-terminal peptide of hexokinase, readily cleavable by proteases, is absolutely required for its binding to all mitochondria. Mammalian porins are significantly cleaved at two positions in putative cytoplasmic loops around residues 110 and 200, as determined by proteolytic-fragment identification using antibodies. Recombinant human porin in yeast mitochondria is more sensitive to proteolysis than wild-type porin in rat liver mitochondria. Recombinant yeast mitochondria, harboring several natural or engineered porins from various sources, bind hexokinase to variable extent with marked preference for the mammalian porin1 isoform. Genetic alteration of this isoform at the C-, but not the N-terminal, results in a significant reduction of hexokinase binding ability. Macromolecular crowding (dextran) promotes a stronger association of the enzyme to all recombinant mitochondria, as well as to proteolytically digested organelles. Consequently, brain hexokinase association with heterologous mitochondria (yeast) in these conditions occurs to an extent comparable to that with homologous (rat) mitochondria. The study, also pertinent to the topology and organization of porin in the membrane, represents a necessary first step in the functional investigation of the physiological role of mammalian hexokinase binding to mitochondria in reconstituted heterologous recombinant systems, as models to cellular metabolism.

Activation of the ubiquitin proteolytic system in murine acquired immunodeficiency syndrome affects IkappaBalpha turnover

Murine acquired immunodeficiency syndrome (MAIDS) is a complex immunopathology caused by a defective murine leukemia virus (LP-BM5) that mainly targets B-lymphocytes. Lymphadenophathy, splenomegaly, hypergammaglobulinemia and progressive immunodeficiency are prominent features of MAIDS. Previously, we showed that the ubiquitin proteolytic system was upregulated in infected lymph nodes [Crinelli, R., Fraternale, A., Casabianca, A. & Magnani, M. (1997) Eur. J. Biochem. 247, 91-97]. In this report, we demonstrate that increased 26S proteasome activity is responsible for accelerated turnover of the IkappaBalpha inhibitor in lymph node extracts derived from animals with MAIDS. The molecular mechanisms mediating IkappaBalpha proteolysis involved constitutive phosphorylation of IkappaBalpha at Ser32 and Ser36 and subsequent ubiquitination, suggesting persistent activation of an NF-kappaB inducing pathway. Interestingly, enhanced IkappaBalpha degradation did not result in enhanced NF-kappaB DNA binding activity, but rather in a different subunit composition. The modulation of NF-kappaB/IkappaB system may affect multiple immunoregulatory pathways and may in part explain the mechanisms leading to the profound immune dysregulation involved in MAIDS pathogenesis.

NF-kappa B p105 processing via the ubiquitin-proteasome pathway

The p50 subunit of NF-kappa B is generated by proteolytic processing of a 105-kDa precursor (p105) in yeast and mammalian cells. Here we show that yeast mutants in the ubiquitin-proteasome pathway inhibit or abolish p105 processing. Specifically, p105 processing is inhibited by a mutation in a 20 S proteasome subunit (pre1-1), by mutations in the ATPases located in the 19 S regulatory complexes of the proteasome (yta1, yta2/sug1, yta5, cim5), and by a mutation in a proteasome-associated isopeptidase (doa4). A ubiquitinated intermediate of the p105 processing reaction accumulates in some of these mutants, strongly suggesting that ubiquitination is required for processing. However, none of the ubiquitin conjugating enzyme mutants tested (ubc1, -2, -3, -4/5, -6/7, -8, -9, -10, -11) had an effect on p105 processing, suggesting that more than one of these enzymes is sufficient for p105 processing. Interestingly, a mutant "N-end rule" ligase does not adversely affect p105 processing, showing that the N-end rule pathway is not involved in degrading the C-terminal region of p105. Unexpectedly, we found that a glycine-rich region of p105 that is required for p105 processing in mammalian cells is not required for processing in yeast. Thus, p105 processing in both yeast and mammalian cells requires the ubiquitin-proteasome pathway, but the mechanisms of processing, while similar, are not identical.

The endomembrane sheath: a key structure for understanding the plant cell?

Recent evidence suggests that integrin is abundant in endomembranes of plant cells, and the endomembranes are clad by a sheath of cytoskeleton including F-actin. A role for endomembrane integrin and the endomembrane sheath is proposed: this system might orchestrate metabolic regulation by providing and modulating loci for channelling, and might accelerate channeling as needed by dragging the endoplasmic reticulum (ER) and organelles through the cytoplasm. To accomplish this "streaming", F-actin might lever against the rest of the endomembrane sheath and the ER might also lever against adhesion sites (i.e., plasmodesmata and plasmalemmal control centers). As an important agent in the control of cellular activities, according to this model, the endomembrane sheath would play a major part in responses to diverse signals and stresses, and under extreme stress cell survival would depend on the ability of the system to maintain enough integrity to direct critical syntheses and degradations.