Role of Endothelial Cell Metabolism in Vessel Sprouting

Cell Metabolism Review Role of Endothelial Cell Metabolism in Vessel Sprouting Katrien De Bock,1,2 Maria Georgiadou,1,2 and Peter Carmeliet1,2,* 1Department of Oncology, University of Leuven 2VIB Laboratory of Angiogenesis and Neurovascular Link, Vesalius Research Center, Leuven 3000, Belgium *Correspondence: [email protected] http://dx.doi.org/10.1016/j.cmet.2013.08.001 Endothelial cells (ECs) are quiescent for years but can plastically switch to angiogenesis. Vascular sprouting relies on the coordinated activity of migrating tip cells at the forefront and proliferating stalk cells that elongate the sprout. Past studies have identified genetic signals that control vascular branching. Prominent are VEGF, activating tip cells, and Notch, which stimulates stalk cells. After the branch is formed and perfused, ECs become quiescent phalanx cells. Now, emerging evidence has accumulated indicating that ECs not only adapt their metabolism when switching from quiescence to sprouting but also that metabolism regulates vascular sprouting in parallel to the control by genetic signals. Blood vessels arose in evolution for various reasons. First and foremost, they supply oxygen, nutrients, and growth factors to tissues while draining toxic metabolic waste. They also ensure immune surveillance, thus allowing immune cells to patrol the organism for foreign antigens or invaders. Interestingly, vessels are evolutionarily closely associated with organismal metabolism. Indeed, in primitive invertebrates, blood vessels were initially hollow matrix tubes that were not lined by endothelial cells (ECs), thus allowing only slow, sluggish, turbulent blood flow and limited tissue perfusion (Muñoz-Chápuli et al., 2005). Only when organisms required a more rapid metabolism (for instance, to predate) did vessels become lined by ECs in order to establish faster laminar blood flow and more efficient perfusion (Muñoz-Chápuli et al., 2005). However, how ECs rewire their own metabolism when switching from quiescence to vascular branching and whether such metabolic adaptations affect vascular branching remain much less studied. ECs are highly plastic cells and can rapidly switch from a long-term quiescent state to active growth upon stimulation by hypoxia or growth factors. According to the prevalent model of vascular sprouting (Potente et al., 2011), an endothelial tip cell takes the lead by navigating at the vascular forefront. Following the tip cell, endothelial stalk cells elongate the branch by proliferating, whereas endothelial phalanx cells line quiescent perfused vessels. The process of tip and stalk cell differentiation is under the tight control of VEGF and Notch signaling and other genetic signals (Potente et al., 2011). VEGF promotes tip cell induction and filopodia formation and induces the expression of the Notch ligand Delta-like 4 (DLL4), which activates Notch signaling in neighboring cells and thereby suppresses VEGF receptor 2 (VEGFR-2) expression and tip cell behavior (Figure 1A). Tip and stalk cells do not exhibit permanently fixed cell fates but dynamically switch between tip and stalk cell phenotypes (Jakobsson et al., 2010). In a matter of hours, a tip cell that lacks the fitness to compete for the leading position can be overtaken by a stalk cell, which then acquires a tip position. This mechanism may ensure that vessel branching relies on the fittest cells. However, little is known about the different meta634 Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. bolic characteristics and requirements of these various EC subtypes and whether Notch controls metabolism in ECs. First, we will overview our current understanding of the various metabolic pathways in ECs, and then we will discuss how these pathways regulate vessel sprouting, illustrating a major role for glycolysis in this process. Metabolic Pathways in Endothelial Cells Glucose Uptake and Transport Glucose delivery to peripheral organs occurs via paracellular transport as well as a transcellular route. In fact, only a small fraction of the glucose that is taken up by ECs is phosphorylated for further internal metabolization. ECs take up glucose through facilitated diffusion, an energy-independent process facilitated by glucose transporters (GLUT), mainly by GLUT-1. VEGF increases GLUT-1 expression in ECs through the activation of PI3K-AKT signaling (Yeh et al., 2008). Reduced GLUT-1 levels in ECs decrease glucose uptake in peripheral organs (Huang et al., 2012). In humans, impaired glucose transport across the blood-brain barrier due to GLUT-1 mutations causes the glucose transporter protein syndrome, which is characterized by infantile seizures, developmental delay, and microcephaly (Klepper et al., 1999). GLUT-1 mutations have also been linked to learning disability and Alzheimer’s disease (Guo et al., 2005; Shulman et al., 2011). In ECs of intact coronary arteries, glucose is taken up at the periphery of the cell and accumulates close to cell-tocell junctions, where the majority of glucose transporters are anchored. This compartmentalization of glucose produces a concentration gradient between the cytosol and the interstitial space that might facilitate transcellular transport of glucose (Gaudreault et al., 2008). Divergent effects of insulin on glucose uptake and metabolism in ECs have been reported (Artwohl et al., 2007; Gaudreault et al., 2008; Gerritsen et al., 1988; Wu et al., 1994). Insulin signaling and insulin-induced phosphorylation of endothelial nitric-oxide synthase (eNOS) in ECs control glucose uptake via skeletal muscle cells (Kubota et al., 2011). The vascular effects of insulin rely on the production of nitric oxide (NO), which promotes capillary Cell Metabolism Review A B C recruitment, vasodilation, and perfusion, altogether enhancing glucose disposal in skeletal muscle (Muniyappa and Quon, 2007). Insulin also signals in ECs in order to facilitate its own transendothelial transport to perivascular organs (Barrett and Liu, 2013; Kubota et al., 2011). Endothelial Cells Are Addicted to Glycolysis After glucose is taken up inside the cell, it is metabolized to pyruvate in the glycolytic pathway (Figure 2). ECs line blood vessels and have immediate access to oxygen in the blood, which could promote mitochondrial respiration. Nonetheless, most studies report that ECs do not rely on oxidative metabolism but are highly glycolytic, generating more than 80% of their ATP in this pathway (Culic et al., 1997; De Bock et al., 2013; Krützfeldt et al., 1990). In the presence of physiological glucose concentrations, only <1% of pyruvate generated in glycolysis is oxidized in the tricarboxylic acid (TCA) cycle (De Bock et al., 2013). However, when glucose and glycolysis levels drop, the oxidation of glucose (as well as of palmitate and amino acids) is enhanced, indicating that ECs switch to oxidative metabolism when anaerobic glycolysis is impaired (known as the Crabtree effect) (Krützfeldt et al., 1990). ECs increase their glycolytic flux when switching from quiescence to proliferation and migration (De Bock et al., 2013). In pathological conditions, such as pulmonary hypertension or latent infection with Kaposi’s sarcoma-associated herpesvirus, glycolysis is increased while oxygen consumption is reduced in ECs (Delgado et al., 2010; Fijalkowska et al., 2010). Thus, ECs metabolically resemble other rapidly proliferating healthy and malignant cell types (Dang, 2012; Marelli-Berg et al., 2012; Mullen and DeBerardinis, 2012; Vander Heiden et al., 2011). Consequently, reducing glycolysis by silencing phosphofructokinase-2/fructose-2,6-bisphosphatase 3 (PFKFB3), which generates fructose-2,6-bisphosphate, a potent allosteric acti- Figure 1. Metabolic Fitness Influences Vessel Branching (A) A schematic of a tip (green) and a stalk (yellow) cell in a vascular sprout. The tip cell extends numerous filopodia in order to sense the environment. In response to the VEGF gradient (orange), the tip cell upregulates Notch ligand Delta-like 4 (DLL4), which activates Notch signaling in the stalk cell and promotes stalk cell formation by downregulating VEGFR-2. The activation of VEGFR signaling by VEGF upregulates PFKFB3 levels and glycolysis, whereas Notch intracellular domain (NCID, active Notch) lowers PFKFB3 expression and glycolytic flux. (B) A schematic of mosaic spheroid assay, in which ECs compete for the tip cell position when forming vascular sprouts. By using spheroids containing ECs of two different genotypes, it is possible to determine which EC is at the tip cell position. (C) A schematic illustrating a tip and stalk cell in a vascular branch of a mosaic spheroid (top) and zebrafish intersomitic vessels (ISVs, bottom) showing that, in comparison to wild-type (WT) cells (green), cells overexpressing NICD (yellow cell, left) are more frequently present in the stalk. However, when NICD-overexpressing cells also overexpress PFKFB3, these cells can compete again for the tip position (yellow cell, right). Thus, PFKFB3 overexpression overrules the stalk-cellinducing activity of NICD. vator of phosphofructokinase-1 (PFK1), impairs EC proliferation, migration, and vascular sprouting in vitro (De Bock et al., 2013). Also, the genetic deficiency of PFKFB3 in ECs causes vascular hypobranching in mice (De Bock et al., 2013). Similar to fibroblasts (Lemons et al., 2010; Valcourt et al., 2012), ECs have substantial baseline glycolysis levels when they are quiescent and only double their glycolysis flux when they are activated to divide and migrate (De Bock et al., 2013). Accordingly, when vessel sprouting is stimulated in hypoxic conditions, ECs enhance glycolysis by no more than 50% (Dobrina and Rossi, 1983). Thus, ECs differ from immune cells, which have negligible glycolysis in their quiescent nonactivated state and upregulate glycolysis by 20- to 30-fold upon activation (Frauwirth et al., 2002; Wang et al., 2011b). Rather, like quiescent fibroblasts (Lemons et al., 2010; Valcourt et al., 2012), ECs need a high baseline glycolysis level for homeostatic maintenance, and blocking glycolysis by 80% by 2-deoxy-Dglucose is toxic for these cells (Merchan et al., 2010; Wang et al., 2011a). Glycolysis levels are subject to environmental conditions and molecular signals. Arterial, venous, microvascular, and lymphatic ECs are all glycolytic (De Bock et al., 2013), but, in comparison to rapidly proliferating, highly glycolytic microvascular ECs, arterial ECs that grow more slowly are less glycolytic but consume more oxygen, though it remains to be determined to what extent adaptation to cell culture conditions influences these results (Parra-Bonilla et al., 2010). Hemodynamic forces such as blood flow also stimulate glycolysis through shear forces acting on the EC glycocalyx (Suárez and Rubio, 1991). The proangiogenic molecules VEGF and FGF2 increase PFKFB3driven glycolysis, whereas DLL4, which activates Notch signaling and decreases branching, reduces glycolysis in ECs (De Bock et al., 2013). Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. 635 Cell Metabolism Review Figure 2. Schematic of Endothelial Cell Metabolism A simplified schematic of EC metabolism showing the known metabolic pathways in ECs and their ratelimiting metabolic enzymes. a-KG, a-ketoglutarate; Ac-CoA, acetyl coenzyme A; AR, aldolase reductase; F2,6P2, fructose-2,6-bisphosphate; F6P, fructose-6-phosphate; FBP, fructose-1,6-bisphosphate; G6P, glucose-6-phosphate; G6PD, glucose6-phosphate dehydrogenase; GFAT, glutamine fructose-6-phosphate amino-transferase; Gln, glutamine; GLS, glutaminase; Glu, glutamate; GSH, glutathione; GSSG, glutathione disulfite; 3PG, glyceraldehyde-3-phosphate; HK, hexokinase; LDH, lactate dehydrogenase; MCT, monocarboxylate transporter; OAA, oxaloacetate; Orn, ornithine; PFK, phosphofructokinase; PFKFB3, phosphofructokinase-2/fructose-2,6-bisphosphatase isoform 3; R5P, ribose-5-phosphate; TKT, transketolase; TCA, tricarboxylic acid; UDP-GlcNAc, uridine diphosphate N-acetylglucosamine. At first sight, it may seem paradoxical that quiescent ECs rely on glycolysis, given that they could take advantage of the available oxygen in their immediate environment in the blood to more efficiently generate ATP via oxidative phosphorylation. Indeed, per glucose molecule, glycolysis produces a net total of only two molecules of ATP, whereas glucose oxidation yields up to 36 molecules of ATP. Nonetheless, one of the prime tasks of ECs is to vascularize avascular tissues through sprouting. If they relied primarily (or solely) on oxidative metabolism, then ECs would be unable to generate ATP in oxygen-depleted areas. In fact, given that interstitial oxygen levels drop faster than glucose levels over a distance away from a blood vessel, ECs can continue to rely on anaerobic glycolysis in such conditions (Buchwald, 2011; Gatenby and Gillies, 2004). Indeed, ECs are resistant to hypoxia as long as glucose is available but become sensitive to oxygen deprivation when glucose is limiting (Mertens et al., 1990). Another reason is that glycolysis rapidly generates ATP, which ECs need in order to form highly motile and rapidly moving lamellipodia and filopodia. Moreover, as long as glucose is not limiting in the extracellular milieu, glycolysis can generate similar amounts of ATP as glucose oxidation (Locasale and Cantley, 2011). Another advantage of glycolytic metabolism is that glycolysis and its side pathways generate the necessary precursors for macromolecules needed in order for ECs to grow, divide, and migrate (see below). Also, a low-oxidative metabolism generates fewer reactive oxygen species (ROS) and less oxidative stress in the high-oxygen environment that quiescent ECs are exposed to. Finally, by consuming less oxygen, they can transfer more oxygen to perivascular cells, thereby improving tissue oxygenation. ECs also store intracellular glucose reserves as glycogen (Amemiya, 1983; Numano et al., 1974; Vizán et al., 2009). However, glycogen breakdown only becomes significant in glucose-deprived conditions and not in hypoxia, although it is not clear whether glycogenolysis is used for bioenergetic pur636 Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. poses alone (Krützfeldt et al., 1990; Vizán et al., 2009). In cerebral microvascular ECs, norepinephrine induces glycogenolysis, whereas 5-hydroxytryptamine stimulates glycogenesis (Spatz et al., 1986). Overall, little is known about the role and importance of glycogen in ECs, but the inhibition of glycogen phosphorylase impairs EC viability and migration (Vizán et al., 2009). This raises the question of whether ECs use this endogenous glucose storage to sprout into avascular glucose-deprived areas. Pentose Phosphate Pathway The pentose phosphate pathway (PPP) is a side branch of glycolysis that cells use for divergent purposes (Figure 2). In this pathway, glucose-6-phosphate is oxidized to pentose sugars and reduces equivalents in two phases. The irreversible oxidative branch (oxPPP) generates NADPH and ribose-5-phosphate (R5P), whereas the reversible nonoxidative arm (non-oxPPP) produces only R5P. The latter is used for the synthesis of nucleotides, whereas NADPH is used for the reductive biosynthesis of lipids, production of NO, or reconversion of oxidized glutathione (GSSG) to reduced glutathione (GSH), a major cellular redox buffer. Depending on the cellular needs and context, the PPP can serve to promote cellular growth and division by increasing the biosynthesis of macromolecules (Cairns et al., 2011; Lunt and Vander Heiden, 2011; Vander Heiden et al., 2009), or the oxPPP can ensure redox homeostasis (Anastasiou et al., 2011). As rate-limiting enzymes, glucose-6-phosphate dehydrogenase (G6PD) controls the oxPPP arm, whereas transketolase (TKT) regulates the non-oxPPP branch. A recent study highlighted that ECs possess additional mechanisms for maintaining the redox balance. Indeed, ECs express UBIAD1, a nonmitochondrial prenyltransferase that synthesizes the electron carrier CoQ10 in the Golgi membrane compartment in order to prevent lipid peroxidation and protect membranes from oxidative damage (Mugoni et al., 2013). Of all the glucose utilized by ECs, only 1%–3% normally enters the PPP in physiological conditions (Dobrina and Rossi, 1983; Jongkind et al., 1989; Krützfeldt et al., 1990; Spolarics and Spitzer, 1993; Vizán et al., 2009). However, in conditions of increased Cell Metabolism Review oxidative stress, such as after treatment with lipopolysaccharide in vivo (Spolarics and Spitzer, 1993) or methylene blue in vitro (Dixit et al., 2008; Krützfeldt et al., 1990), up to 80% of glucose can enter the PPP, allowing cells to sustain GSH levels in order to reduce possibly harmful ROS (Spolarics and Wu, 1997). G6PD overexpression in ECs also increases NADPH and NO production and maintains intracellular glutathione stores when exposed to oxidants (Leopold et al., 2003b). The ability to activate the oxPPP enables quiescent ECs to better survive and tolerate oxidative stress during hypoxia reoxygenation events (Buderus et al., 1989). By activating protein kinase A, high glucose levels impair G6PD activity in ECs, thereby decreasing cell survival because of insufficient redox control (Zhang et al., 2000). A reduction of G6PD expression in vivo increases ROS levels and decreases eNOS activity in the aorta, thereby reducing vascular reactivity (Leopold et al., 2007). The PPP might also influence vascular sprouting via other mechanisms. First, G6PD can modulate VEGF signaling, as revealed by findings that inhibition of G6PD impairs and G6PD overexpression promotes angiogenesis in vitro by regulating NO production via VEGF and tyrosine phosphorylation of VEGFR-2 (Leopold et al., 2003a; Vizán et al., 2009). In a positive feedback, the proangiogenic factor VEGF increases the oxPPP flux (Vizán et al., 2009) and enhances G6PD activity and localization at the plasma membrane (Pan et al., 2009). Second, the non-oxPPP can promote angiogenesis via the production of macromolecules, explaining why the inhibition of TKT reduces EC viability and migration (Vizán et al., 2009). Third, because low amounts of ROS can be proangiogenic (Okuno et al., 2012), hereditary G6PD deficiency in diabetes patients promotes the development of ocular neovascularization, presumably by increasing ROS levels as a result of the decreased production of NADPH (Cappai et al., 2011). Fourth, insulin may regulate NO generation in ECs by stimulating oxPPP and NADPH production, which is required for NO synthesis (Wu et al., 1994). Overall, the role of the oxPPP in vessel sprouting and maintenance is contextual, and understanding its role and regulation requires further study. Hexosamine Biosynthesis Pathway A fraction of glucose can also flux through the hexosamine biosynthesis pathway (HBP), where it is used for protein glycosylation (Figure 2). The rate-limiting step is catalyzed by glutamine:fructose-6-phosphate amidotransferase, which regulates the HBP in order to produce UDP-N-acetylglucosamine (UDPGlcNAc), a substrate used for N-linked and O-linked glycosylation (Hart et al., 2007; Helenius, 1994; Love and Hanover, 2005). Because the HBP depends on the availability of glucose, glutamine, acetyl-CoA, and ATP, it is considered to be a ‘‘nutrient sensor’’ (Zachara and Hart, 2004a, b). Despite its presumed importance in regulating glycosylation, only a few reports studied the role of the HBP or glycosylation in angiogenesis. For instance, 2-deoxy-D-glucose inhibits angiogenesis by interfering with N-linked glycosylation (Merchan et al., 2010), whereas elevated protein O-GlcNAc modification in ECs impairs angiogenesis, possibly by inhibiting AKT signaling (Luo et al., 2008). Furthermore, by interacting with N-glycans of VEGFR-2, galectin-3 facilitates VEGFR-2 plasma membrane retention and phosphorylation and thereby stimulates VEGF-mediated angiogenesis (Markowska et al., 2011). Glycoproteins bearing N-linked oligosaccharides are essential for capillary tube formation (Nguyen et al., 1992) and the formation of the glycocalyx layer, which acts as a mechanosensor and controls EC permeability (Curry and Adamson, 2012). Also, glycosylation of the Notch receptor determines its responsiveness toward its ligands DLL4 and Jagged1, thereby playing a crucial role in tip-stalk cell differentiation during vessel branching (Benedito et al., 2009). Whether glycosylation of the angiogenic receptor is dynamically controlled through the HBP and whether those changes are dependent on nutrient availability and sensing awaits further insight. Polyol Pathway Hyperglycemia promotes vascular complications in diabetes. When glucose is present in excess of what the glycolytic pathway can handle, glucose enters the polyol pathway, a two-step pathway in which aldose reductase reduces glucose to sorbitol, which is then converted to fructose (Lorenzi, 2007; Tang et al., 2012) (Figure 2). Because the aldose reductase reaction converts NADPH to NADP+, the activation of the polyol pathway can deplete stores of NADPH, which are necessary for maintaining reduced GSH levels for redox homeostasis, thus leading to the accumulation of ROS. It is unknown to what extent the accumulation of polyols themselves can also be toxic. In humans, high levels of aldose reductase are associated with toxicity. Human aldose reductase transgene expression in ECs in low-density lipoprotein receptor knockout mice (a model of atherosclerosis) aggravated vascular disease in diabetic conditions (Vedantham et al., 2011). Conversely, pharmacologic inhibition of the aldose reductase reduced EC oxidative damage and apoptosis in vitro and retinal vascular overgrowth through the upregulation of VEGF in diabetic rats and mice in vivo (Obrosova and Kador, 2011; Oyama et al., 2006), whereas aldose reductase deficiency diminished retinal vascular changes in a model of oxygen-induced retinopathy (Fu et al., 2012). Whether aldose reductase is a target in humans remains debated. Normal polyol pathway activity is also required for physiological angiogenesis by affecting VEGFR-2 and FGF signaling (Tammali et al., 2011; Yadav et al., 2012). Mitochondria and Respiration in ECs Unlike other glycolysis-addicted cell types such as red blood cells (lacking mitochondria) or embryonic stem cells (containing few inactive mitochondria) (Kondoh et al., 2007), ECs have active mitochondria, but they contain fewer mitochondria than oxidative cell types. Indeed, mitochondria make up only 5% of the cellular volume, in contrast to 30% in hepatocytes (Blouin et al., 1977). With gestational age during embryonic development, respiratory chain complexes and oxidative phosphorylation are reduced, whereas glycolytic activity is increased in ECs, presumably to cope with the metabolic stress during birth and render ECs less susceptible to peripartum hypoxic damage (Illsinger et al., 2011). Vascular branching signals also regulate mitochondrial biogenesis. Indeed, VEGF stimulates mitochondrial biogenesis via AKT-dependent signaling (Wright et al., 2008), whereas the silencing of SIRT1, a negative regulator of the Notch pathway (Guarani et al., 2011) (thus leading to enhanced Notch signaling and reduced vessel branching), inhibits mitochondrial biogenesis in ECs (Csiszar et al., 2009). However, whether mitochondrial biogenesis influences vessel sprouting and how it may do so remain to be assessed. Notably, Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. 637 Cell Metabolism Review Figure 3. Mitochondria and Respiration in Endothelial Cells Despite having immediate access to oxygen in the blood, ECs rely on glycolysis to generate ATP. Mitochondria in ECs are not considered important bioenergetic powerhouses but, rather, act primarily as signaling organelles by generating proangiogenic reactive oxygen species (ROS) and nitric oxide (NO). Moreover, by utilizing low amounts of oxygen, they ensure oxygen diffusion across the endothelial barrier to the perivascular tissues. ATP derived from glycolysis is essential for maintaining the mitochondrial network (Giedt et al., 2012). In vitro, ECs consume relatively low amounts of oxygen, which allows them to transfer most of the oxygen that enters these cells to perivascular cells in vivo (Figure 3) (Helmlinger et al., 2000). However, in vivo measurements documented a drop of oxygen levels across the arteriolar vessel wall (Tsai et al., 1998; Tsai et al., 2003). Though the cellular source consuming oxygen (ECs versus smooth muscle cells) was not identified, these studies postulated that the vessel wall can act as an ‘‘oxygen sink’’ to prevent the exposure of perivascular tissues to high oxygen levels and oxidative damage, though these observations are debated (Golub et al., 2011). Primarily, mitochondria in ECs are generally considered to have a signaling function (via the production of proangiogenic ROS levels and NO) rather than serving as a bioenergetic powerhouse (Davidson, 2010; Quintero et al., 2006) (Figure 3). For instance, ECs detect mechanical stress by transmitting force via the cytoskeleton to the mitochondria and triggering mitochondrial ROS signaling, leading to an increase in the expression of NF-kB and VCAM-1 (Ali et al., 2004). Mitochondria have also been implicated as regulators of ion (H+ and Ca2+) homeostasis and apoptosis in ECs. Indeed, exposure of ECs to high levels of glucose (as occurs in diabetes) enhances mitochondrial fission and/or reduces mitochondrial fusion, resulting in mitochondrial fragmentation, ROS production, and Ca2+ overload, altogether leading to EC dysfunction and death (Pangare and Makino, 2012). Prohibitin 1 (PHB1), a protein localized to the inner mitochondrial membrane, inhibits mitochondrial complex I and thereby prevents ROS-induced senescence as well as AKT-dependent Rac1 hyperactivation, which leads to cytoskeletal rearrangements, decreased EC motility, and impaired capillary tube formation. Thus, PHB1 is important for proper mitochondrial function and maintaining the angiogenic capacity of ECs (Schleicher et al., 2008). Because of their role in signaling rather than in bioenergetics, mitochondria in ECs have been considered targets for angiogenesis inhibition (Park and Dilda, 2010). 638 Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. In line with findings that mitochondria in ECs are not bioenergetic powerhouses, oxidative pathways only account for 15% of the total amount of ATP generated in ECs (De Bock et al., 2013). Also, mitochondrial respiration poisons that reduce oxygen consumption do not impair vessel branching, whereas the supplementation of NADH or pyruvate, which increase oxygen consumption, do not stimulate vascular sprouting (De Bock et al., 2013). Nonetheless, mitochondria in ECs have a high bioenergetic reserve capacity and can increase respiration substantially in stress conditions of glucose deprivation or oxidative stress (Dranka et al., 2010; Mertens et al., 1990). An exception may be the quiescent ECs of the blood-brain barrier, which have up to 5-fold more mitochondria than ECs in peripheral organs, presumably to provide large amounts of energy for transport of nutrients and ions across the blood-brain barrier in order to ensure brain homeostasis (Oldendorf and Brown, 1975). Fatty Acid Metabolism The role of other metabolic pathways in ECs remains poorly characterized. Some studies report that fatty acid oxidation (FAO) is a critical bioenergetic supply pathway for ECs (Figure 2), especially when carnitine is supplemented (Dagher et al., 1999, 2001; Hülsmann and Dubelaar, 1988, 1992) or glucose is removed from the medium (Dagher et al., 2001; Krützfeldt et al., 1990). In the latter condition, an increase in FAO compensates for the lack of glycolytic ATP production. Carnitine palmitoyl transferase 1 is a rate-limiting enzyme of FAO whose activity is inhibited by malonyl-CoA, itself produced by acetyl-CoA carboxylase (ACC). By inactivating ACC, AMPK stimulates FAO in ECs (Dagher et al., 2001; Fisslthaler and Fleming, 2009). ECs can oxidize both extra- and intra-cellular fatty acids, though the relative contribution of FAO versus glycolysis to ATP generation in ECs is debated (Dagher et al., 1999, 2001; Delgado et al., 2010; Dobrina and Rossi, 1983; Polet and Feron, 2013; Spolarics et al., 1991). In glucose-deprived or matrix-detached tumor cells, FAO produces metabolites for the TCA cycle, and these metabolites generate citrate and malate; i.e., substrates of the NAPDH-producing isocitrate dehydrogenase and malic enzyme, respectively (Jeon et al., 2012; Pike et al., 2011). Hence, by indirectly generating NADPH needed to convert GSSG to its reduced form (GSH), FAO regulates redox homeostasis and prevents excessive oxidative stress (which is antiangiogenic) (Jeon et al., 2012; Pike et al., 2011), but such a role has not been Cell Metabolism Review described in ECs. The bioenergetic importance of FAO for vessel branching remains unknown. Fatty-acid-binding protein 4 (FABP4), an intracellular lipid chaperone, is a target of VEGF and stimulates EC proliferation in vitro (Elmasri et al., 2009), whereas the loss of FABP4 impairs VEGF transgene-induced neovascularization in airways, in part by decreasing VEGF-induced EC proliferation and lowering eNOS and stem cell factor expression (Ghelfi et al., 2013). Also, more FABP4-immunoreactive vessels are detected in bronchial biopsies of patients with asthma (Ghelfi et al., 2013). Notably, VEGF-B promotes endothelial lipid uptake and transport to peripheral tissues (heart and skeletal muscle) through the upregulation of the expression of fatty acid transporters FATP3 and FATP4 (Hagberg et al., 2010). In addition, VEGF-B stimulates AMPK required for aortic EC proliferation in vitro independently of an increase in FAO (Reihill et al., 2011). In agreement with findings that ectopic lipid deposition is associated with the pathogenesis of type II diabetes, neutralizing VEGF-B antibody restored insulin sensitivity and glucose tolerance by reducing endothelial-to-tissue lipid transport, thereby creating a novel option for diabetes therapy (Carmeliet et al., 2012; Hagberg et al., 2012). Lipids are also required for the formation of membranes and act as intracellular signaling molecules. The upregulation of lipid metabolism is a hallmark of multiple cancer types (Biswas et al., 2012; Santos and Schulze, 2012; Schug et al., 2012). The expression of fatty acid synthase (FAS), which catalyzes de novo lipid synthesis, is generally low or undetectable in adult healthy tissues, given that the majority of fatty acids are taken up from dietary sources. In contrast, even with an adequate nutritional lipid supply, FAS is highly upregulated in cancer cells to provide these rapidly proliferating cells with sufficient amounts of lipids for membrane biogenesis and to confer them a growth and survival advantage (Pandey et al., 2012; Santos and Schulze, 2012). The role of lipogenesis in vascular branching remains poorly studied. Pharmacological inhibition of FAS inhibits tumor angiogenesis (Seguin et al., 2012), whereas the genetic loss of endothelial FAS impairs pathological angiogenesis by decreasing plasma membrane targeting of eNOS (via reduced palmitoylation) and VEGFR-2 (Wei et al., 2011). A recent study showed that maintaining efficient cholesterol efflux from ECs is essential for angiogenesis (Fang et al., 2013). In order to prevent cholesterol overload in cells, ATPbinding cassette transporters mediate cholesterol efflux from cells to apolipoprotein A-I (apoA-I) and the apoA-I-containing high-density lipoprotein. In ECs, cholesterol efflux reduces membrane lipid rafts, which interferes with VEGFR-2 membrane localization, dimerization, and endocytosis and impairs VEGFinduced angiogenesis (Fang et al., 2013). Another apolipoprotein (e.g., apoB), impairs angiogenesis by upregulating the antiangiogenic VEGF trap VEGFR-1 (Avraham-Davidi et al., 2012). The effects of apoB-containing lipoproteins on vessel growth were not induced by a decreased delivery of fatty acids to tissues or due to global lipid starvation, given that apoC-II deficiency did not phenocopy the vascular defects resulting from the deficiency of microsomal triglyceride transfer protein, which is involved in the biosynthesis of apoB-containing lipoproteins. It remains to be determined whether this mechanism underlies the EC dysfunction that precedes the formation of atherosclerotic pla- ques or impairs collateral vessel growth in hypercholesterolemic patients. Amino Acid Metabolism Besides glucose, rapidly growing cancer cells and embryonic stem cells also metabolize various types of amino acids, such as glutamine, proline, and serine (Dang, 2012; DeBerardinis and Thompson, 2012; Kalhan and Hanson, 2012; Phang and Liu, 2012; Shyh-Chang et al., 2013) (Figure 2). Of these amino acids, glutamine is a key metabolic fuel for proliferating cells. The role of glutamine metabolism has not been studied extensively in ECs, and, therefore, its role in angiogenesis remains unclear. ECs take up glutamine via Na+-dependent transport mechanisms but also have the capacity to produce this amino acid via glutamine synthetase, though the physiological relevance of this process for vessel branching remains unknown (Lohmann et al., 1999). Glutamine taken up by ECs can be converted to glutamate and ammonia (Wu et al., 2000) (Figure 2). The activity of glutaminase 1, the first step in the glutaminolysis pathway, is higher in ECs than it is in lymphocytes (Leighton et al., 1987). Pharmacological inhibition of glutaminolysis impairs the proliferative capacity of ECs and induces a senescent-like phenotype in ECs (Unterluggauer et al., 2008). Nonetheless, it is still debated whether glutamine oxidation contributes substantially to ATP production in ECs (De Bock et al., 2013; Krützfeldt et al., 1990; Spolarics et al., 1991; Wu et al., 2000). However, glutamine contributes more significantly to ATP production and promotes survival when oxidative stress impairs glucose-dependent pathways of ATP production (Hinshaw and Burger, 1990), indicating that the contribution of glutamine metabolism to ATP production in ECs is contextual (De Bock et al., 2013; Krützfeldt et al., 1990; Spolarics et al., 1991; Wu et al., 2000). Glutamine also serves as a carbon source for the biosynthesis of macromolecules, but stable isotope-tracer-based metabolomic flux analysis of glutamine in ECS has not been performed yet. Glutamine metabolism by ECs could also be important for the provision of nitrogen for biosynthetic purposes. Indeed, the synthesis of the polyamine precursor from glutamine sustains EC growth (Wu et al., 2000) (Figure 2). Furthermore, glutamine inhibits the endothelial production of NO, in part via the conversion of glutamine to glucosamine in the HBP, which inhibits the oxPPP activity and thereby reduces the availability of NADPH, an essential cofactor for eNOS (Wu et al., 2001). Glutamine also impairs NO production, inhibiting the formation of arginine from citrulline through reducing citrulline transport (Kawaguchi et al., 2005; Meininger and Wu, 1997; Sessa et al., 1990). Also, arginine controls angiogenesis by regulating the levels of ROS in ECs (Park et al., 2003; Zhuo et al., 2011). No studies have been reported on the possible role of proline, serine, and threonine metabolism in ECs. Metabolic Changes during Vascular Sprouting Migrating Tip and Proliferating Stalk Cells In ECs, the tip-cell-activating signal VEGF increases glycolysis by upregulating PFKFB3 levels, suggesting that tip cells require elevated levels of PFKFB3-driven glycolysis (De Bock et al., 2013) (Figure 1A). Accordingly, PFKFB3 silencing and/or deletion impairs the formation of distal sprouts with tip cell filopodia and the number of filopodia in retinal sprouting vessels and reduces the lamellipodia area of cultured ECs (De Bock et al., 2013). Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. 639 Cell Metabolism Review Figure 4. PFKFB3 Blockade Impairs Tip and Stalk Cell Behavior A B However, aside from a role for PFKFB3 in tip cells, PFKFB3 also regulates stalk cell functions. Consistent with a reported increase in glycolysis when cells enter S-phase (Almeida et al., 2010), PFKFB3 silencing and/or deletion reduced EC proliferation in various angiogenesis assays in vitro and sprouting retinal vessels in vivo (De Bock et al., 2013). DLL4-mediated Notch activation also lowered PFKFB3 levels and glycolysis (De Bock et al., 2013) (Figure 1A). Yet, stalk cells proliferate in order to elongate the stalk, and cell proliferation is known to require increased levels of glycolysis (Vander Heiden et al., 2011). This paradox is resolved by findings that the growth-inhibitory activity of Notch is overruled by other genetic (Wnt) signals (Phng et al., 2009). Notably, in mosaic sprouting assays (Figure 1B), the overexpression of PFKFB3 is able to overcome the pro-stalk-cell activity of Notch and favors the tip localization of ECs overexpressing both PFKFB3 and the transcriptionally active Notch domain NICD when using an endothelial spheroid sprouting model in vitro or analyzing vascular branching in zebrafish embryos in vivo (De Bock et al., 2013) (Figure 1C). This is remarkable, given that no other genetic signal has been shown to be able to overrule the prostalk activity of Notch. Conversely, PFKFB3-silenced ECs are less capable of competing for the tip position in mosaic sprouting assays (De Bock et al., 2013). The consequences of PFKFB3 blockade on tip and stalk cells is illustrated in Figure 4. Overall, in parallel to genetic signals, PFKFB3-driven glycolysis also regulates vascular branching. Quiescent Phalanx Cells Little is known about the metabolic changes that accompany, promote, or are necessary for inducing EC quiescence. Glycolysis is decreased in quiescent ECs, which might serve several purposes. First, given that ECs rely on glycolysis in order to divide, thus lowering glycolysis reduces proliferation and promotes quiescence. Second, quiescent ECs are exposed to high oxygen levels in the blood, which may cause oxidative damage. Thus, similar to erythrocytes, quiescent ECs must protect themselves against oxidative damage. By maintaining a low oxidative metabolism, ECs minimize ROS production, thereby providing protection against their high-oxygen milieu. Third, of the total amount of glycolysis, ECs use 40% to proliferate and migrate, whereas they use the remaining 60% for maintenance homeostasis (De Bock et al., 2013), which is in line with findings 640 Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. (A) A schematic model demonstrating that, in control WT conditions, stalk cells proliferate, and tip cells migrate directionally, extend long motile filopodia, and compete for the tip. (B) Upon PFKFB3 blockade, stalk cells are hypoproliferative, whereas tip cells have impaired migration and directional movement with short immobile filopodia so that tip cells lose their competitive advantage for the tip position. that proliferation only requires a 30% increase in ATP production (Kilburn et al., 1969; Locasale and Cantley, 2011). Quiescent ECs need this residual 60% of glycolysis in order to perform energydemanding homeostatic maintenance functions such as transendothelial transport, barrier formation, glycocalyx deposition, and matrix production (Curry and Adamson, 2012; Potente et al., 2011)—resembling quiescent fibroblasts, which also have an active metabolism in baseline conditions (Lemons et al., 2010). It has been postulated that cellular quiescence requires a higher baseline metabolism than previously anticipated to ensure maintenance of ion gradients, protein and RNA synthesis, and other processes (Locasale and Cantley, 2011). Overall, because of their particular milieu and cellular activities, quiescent ECs adapt their metabolism to optimally accommodate the need for reduced proliferation on one side with the homeostatic needs for redox control and baseline maintenance activities on the other side. Compartmentalization of Metabolism Tip cells extend lamellipodia and filopodia in order to migrate. The formation of these motile structures relies on the remodeling of the actin cytoskeleton, a process that requires the rapid production of high amounts of ATP. In motile ECs, a large fraction of the total amount of glycolytic ATP generated is utilized by the actomyosin ATPase (Culic et al., 1997). In quiescent contactinhibited ECs, enzymes of the glycolytic cascade are primarily present in the perinuclear cytosol. However, once they become motile and start migrating, these glycolytic enzymes also become translocated to lamellipodia, where they are highly concentrated along with F-actin in membrane ruffles at the leading front to generate high levels of ATP in lamellipodial ‘‘ATP hot spots’’—mitochondria are excluded from lamellipodia and filopodia (De Bock et al., 2013) (Figures 5A–5D). Furthermore, biochemical analysis indicates that PFKFB3 is present in F-actin-enriched fractions and to higher levels in proliferating and migrating ECs (De Bock et al., 2013). Various glycolytic enzymes are inactive as dimers but become more active in a tetrameric configuration. Through actin binding sites, these enzymes bind to actin, which stabilizes their tetrameric configuration and enhances their enzymatic activity (Real-Hohn et al., 2010) (Figure 5E). Such compartmentalization of glycolysis with the actin cytoskeleton offers various advantages. High levels of ATP are rapidly generated in lamellipodia and filopodia at the site where energy is needed during EC migration (Figures 5E and 5F). In addition, the rapid Cell Metabolism Review A D E B F C Figure 5. Compartmentalization of Metabolism (A–C) Representative images of ECs expressing the ATP biosensor GO-ATeam (A), the glycolytic enzyme phosphoglycerate kinase (B), and the mitochondrial marker TOMM20 (C), revealing high ATP levels and glycolytic enzymes in the perinuclear cytosol as well as at the membrane ruffles of lamellipodia, whereas mitochondria are present around the nucleus but excluded from lamellipodia. Lamellipodia are denoted with a white dotted line. (D) A schematic showing the localization of glycolytic enzymes in the perinuclear cytosol and the compartmentalization in lamellipodia and filopodia of the tip cell. The mitochondria are excluded from the filopodia because they are too large to fit into the thin cytosolic protrusions. (E) A schematic illustrating the compartmentalization of the glycolytic regulator PFK1 with F-actin. Actin binding stabilizes PFK1 in its tetrameric active configuration. (F) A schematic illustrating that the glycolytic enzymes are arranged in an assembly line, creating a ‘‘glycolytic hub.’’ extension-retraction of filopodia and lamellipodia may create an ATP drain for the cell body. Localizing ATP supply to compartments where ATP is consumed can prevent catastrophic ATP depletion for the cell. Moreover, through binding to actin, an assembly line of the glycolytic machinery is generated wherein the product of one glycolytic enzyme becomes the substrate of its neighboring glycolytic enzyme because of their close proximity (al-Habori, 1995; Fulgenzi et al., 2001; Lagana et al., 2000; Real-Hohn et al., 2010). Motile structures in other organisms, such as flagella in sperm and predatory tentacles in hydra, also concentrate glycolytic enzymes (Baquer et al., 1975; Hereng et al., 2011; Mitchell et al., 2005; Pavlova, 2010). The functional relevance of this link is underscored by findings that a mutant fruit fly expressing an aldolase variant that cannot bind actin is unable to fly (Wojtas et al., 1997). In vascular smooth muscle cells, glycolysis and gluconeogenesis occur in separate ‘‘compartments’’ because of the spatial separation of glycolytic and gluconeogenic enzymes in distinct plasma membrane microdomains (Lloyd and Hardin, 2001). Whether a comparable compartmentalization of glycolysis versus gluconeogenesis occurs in ECs is unknown. Feedback Regulation of Vascular Branching by Metabolism An intriguing question is whether metabolism provides a feedback for the genetic signals that control vascular branching. One example is how Notch is subject to regulation by metabolic signals in ECs. Indeed, Notch is a direct target of the NAD+dependent deacytelase SIRT1, a key regulator of cellular metabolism that is activated by nutrient deprivation (Chalkiadaki and Guarente, 2012; Guarani and Potente, 2010). By deacetylating NICD and thereby reducing its protein stability, SIRT1 reduces the amplitude and duration of the Notch response in a negative feedback loop (Guarani et al., 2011). This promotes the vascularization of the nutrient-deprived tissue. Accordingly, in the absence of SIRT1 (a condition mimicking nutrient abundance), ECs are sensitized to Notch signaling, resulting in a stalk-cell-like phenotype with impaired vessel outgrowth (Potente et al., 2007). Another mechanism by which SIRT1 controls vascular sprouting is via deacetylating the transcription factor FOXO1, which controls cell growth and metabolism (Eijkelenboom and Burgering, 2013). FOXO1 is activated by nutrient stress (and is a target Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. 641 Cell Metabolism Review of SIRT1 and AMPK), but, unlike SIRT1, this transcription factor inhibits vessel branching (Oellerich and Potente, 2012). Indeed, loss of FOXO1 results in embryonic lethality because of vascular abnormalities (Furuyama et al., 2004), and the inducible postnatal deletion of FOXO1, FOXO2, and FOXO3 together induces hemangioma formation (Paik et al., 2007). These data suggest that a finely tuned balance between SIRT1 and FOXO1 is required in order to orchestrate the vessel-branching response to nutrient deprivation. Metabolism has been shown to control growth factor signaling in cancer cells (DeBerardinis and Thompson, 2012). Interestingly, the response of Notch to its ligands DLL4 and Jagged1 is determined by the glycosylation state of its extracellular domain in a tip-versus-stalk cell manner (Benedito et al., 2009), thus providing another example of how metabolism impacts on angiogenic signal transduction. In particular, glycosylation by FRINGE glycosyltransferases favors the activation of Notch by DLL4 over Jagged1 (Eilken and Adams, 2010). Even though stalk cells primarily express Jagged1 (and tip cells predominantly express DLL4), Jagged1 is only able to weakly activate Notch1 in comparison to DLL4 (Eilken and Adams, 2010). As a result, DLL4 causes hypobranching, whereas Jagged1 induces opposite effects. An outstanding question is whether the glycosylation of other key angiogenic receptors (such as VEGFR-2) provides another level of metabolic control of angiogenesis. Metabolites as Angiogenic Signals In the brain, muscle, and tumors, lactate is a fuel for neighboring cells (Bergersen, 2007; Brooks, 2009; Draoui and Feron, 2011; Whitaker-Menezes et al., 2011). This metabolite is generated through the conversion of pyruvate, itself produced in the glycolytic pathway (or other pathways) by lactate dehydrogenase A (LDH-A). However, more than being a waste product, lactate also serves as a fuel for oxidative metabolism after conversion to pyruvate by LDH-B (Figure 2). For instance, in tumors, lactate is oxidized by oxygenated tumor cells, thereby sparing glucose for more hypoxic glycolytic cancer cells (Draoui and Feron, 2011). Cancer-associated fibroblasts also have aerobic glycolysis and extrude lactate to ‘‘feed’’ adjacent tumor cells (Whitaker-Menezes et al., 2011). In the brain, a cell-to-cell lactate shuttle between astrocytes and neurons is linked to glutamatergic signaling (Brooks, 2007). However, in ECs, only <1% of glucose is oxidized in the TCA cycle, and glucose oxidation generates only 6% of the total amount of ATP in ECs (De Bock et al., 2013). Also, lactate is only a significant substrate for oxidation in the absence of glucose in ECs (Krützfeldt et al., 1990). Lactate in ECs can act as a signaling molecule rather than a metabolic substrate. Indeed, lactate inhibits the activity of the oxygen-sensing prolyl-hydroxylase domain protein PHD2, thereby activating the hypoxia-inducible transcription factor HIF-1a in normoxic oxidative tumor cells and triggering tumor angiogenesis by upregulating VEGF and other proangiogenic cytokines (De Saedeleer et al., 2012; Hunt et al., 2007). By inhibiting PHD2, lactate also triggers the phosphorylation and degradation of the inhibitory subunit IkBa, thus stimulating an autocrine proangiogenic NF-kB-IL-8 pathway (Végran et al., 2011). Lactate also accelerates EC progenitor recruitment and differentiation via the release of HIF-1a-dependent angiogenic factors (Milovanova et al., 2008). Blockade of lactate influx in ECs by monocar642 Cell Metabolism 18, November 5, 2013 ª2013 Elsevier Inc. boxylate transporter 1 blockers, inhibiting lactate uptake in tumor ECs, impedes HIF-1a-dependent angiogenesis (Sonveaux et al., 2012). Another paracrine metabolic signal is glutaminolysis-derived ammonia, which induces autophagy and enhances tumor cell survival at the expense of cell growth and proliferation (Eng et al., 2010). Glutamine metabolism not only enhances the proliferation and survival of oxygenated nutrient-rich tumor cells via the anabolic replenishment of TCA cycle intermediates, but it also helps nutritionally stressed neighbor tumor cells cope with nutrient deprivation by inducing autophagy in order to preserve cellular functions (Eng et al., 2010). The importance of autophagy in EC biology remains poorly studied, though haplodeficiency of the autophagy mediator Beclin-1 increases hypoxia-induced angiogenesis associated with an increase in HIF-2a expression and erythropoietin production (Lee et al., 2011). Moreover, during retinal vascular development, the inhibition of autophagy reduces hyaloid regression (Kim et al., 2010). Another metabolic interaction between the tumor and its stroma was recently described. ECs and tumor cells make direct cell-cell contact and can exchange cellular components by generating tunneling nanotubes. The transfer of mitochondria from ECs to tumor cells via these nanotubes conferred tumor resistance against chemotherapy (Pasquier et al., 2013). Thus, besides providing the tumor with nutrients and oxygen, ECs might also feed the tumor with their own metabolic machinery, though further studies will be required to validate these findings. Conclusions and Perspectives In a field where >44,000 papers have been published on a single molecule such as VEGF, it is surprising that <100 papers on how ECs rewire their metabolism during vascular branching have been reported. In order to grasp the importance of how metabolism might influence EC behavior and vascular sprouting, it will be necessary to first establish a metabolic roadmap of the different metabolic pathways in the different EC subtypes involved in vascular branching and to characterize how these various metabolic pathways adapt during the various steps in vessel sprouting. This will require state-of-the-art metabolic flux analytic methods with stable isotope tracers in combination with measurements of radioactive tracer flux analyses and steady-state metabolite levels. It will also be intriguing to characterize the metabolism of transformed hemangiomas and angiosarcomas or study the effects of diabetes and hypercholesterolemia on EC metabolism and sprouting. Another appealing question is how ECs can regulate organismal metabolism by differentiating into metabolically active adipocytes (Gupta et al., 2012; Tran et al., 2012). Yet another unexplored field is the metabolism-epigenome interaction. Does metabolism epigenetically regulate vascular branching, similar to cancer cells (Mazzarelli et al., 2007; Teperino et al., 2010; Yun et al., 2012), or can the epigenome influence EC metabolism? Finding an answer to these questions promises to be an exciting endeavor. ACKNOWLEDGMENTS We apologize for not being able to cite the work of all other studies related to this topic because of space restrictions. We acknowledge the work of Cell Metabolism Review S. Vinckier for help with the confocal imaging. K.D.B. was funded by a postdoctoral fellowship of the Research Foundation Flanders (FWO) and is now an academic staff member at the Department of Kinesiology (KU Leuven). M.G. received funding as an Emmanuel Vanderschueren fellow of the Flemish Association against Cancer. The work of P.C. is supported by a Federal Government Belgium grant (IUAP07/03), long-term structural Methusalem funding by the Flemish Government, a Concerted Research Activities Belgium grant (GOA2006/11), grants from the FWO (G.0652.08, G.0692.09, G.0532.10, G.0817.11, and 1.5.202.10.N.00 Krediet aan navorsers), the Foundation Leducq Transatlantic Network, and an European Research Council Advanced Research Grant (EU-ERC269073). P.C. declares to be named as an inventor on patent applications, claiming subject matter related to the results described in this paper. REFERENCES al-Habori, M. (1995). Microcompartmentation, metabolic channelling and carbohydrate metabolism. Int. J. Biochem. Cell Biol. 27, 123–132. Cairns, R.A., Harris, I.S., and Mak, T.W. (2011). Regulation of cancer cell metabolism. Nat. Rev. Cancer 11, 85–95. 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