Glucose Metabolism

Glycolysis in Normal and Cancer Cells

Glucose is transported from the circulation into cells via glucose transporters; it then is phosphorylated to form glucose-6-phosphate (G6P). G6P is then further phosphorylated and, after a series of reactions, is broken up into dihydroacetone-phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P), which is converted to glycerol-3phosphate for lipid synthesis or sequentially transformed to produce pyruvate. Pyruvate can be converted to acetyl-coenzyme A (CoA) in the tricarboxylic acid (TCA) cycle in the mitochondria or converted to lactate in the cytosol. G6P can take an alternative metabolic pathway, the pentose phosphate pathway (PPP), which generates ribose5-phosphate for nucleotide synthesis and the byproduct NADPH for reductive biosynthesis. Finally, G6P can also be converted to glycogen for storage (Fig. 1.1).

Fig. 1.1 (continued) glycogen phosphorylase liver form; ROS reactive oxygen species; R5P ribose5-phosphate; SFA saturated fatty acid; SCD1 stearoyl-CoA desaturase 1; SDH succinate dehydrogenase; SCO2 synthesis of cytochrome C oxidase 2; TIGAR TP53-inducible glycolytic and apoptotic regulator; TCA tricarboxylic acid; TPI triose-phosphate isomerase; TG triglyceride; UDP-GlcNAc UDP-N-acetylglucosamine

Fig. 1.1 Regulation of cancer metabolism pathways by oncogenes and tumor suppressors. Metabolic enzymes are regulated by oncogenes – Myc, Akt, and receptor tyrosine kinases (RTKs) – and the tumor suppressors 5′ adenosine monophosphate-activated protein kinase (AMPK) and p53. Key metabolic pathways are represented within colored boxes: blue indicates pathways linked to glucose metabolism (glycolysis, pentose phosphate pathway, glycogen metabolism, hexosamine biosynthesis pathway, and serine metabolism); pink represents mitochondrial respiration, green represents glutamine metabolism, and yellow indicates lipid metabolism (lipid synthesis, lipolysis, and β-oxidation). pH regulation contributes to the control of intracellular acidity. The enzymes involved in metabolic pathways regulated by oncogenes or tumor suppressors are shown in bold and colored as indicated above. A circled plus or minus represents a positive or negative regulation by the indicated oncogenes or tumor suppressors. Dashed arrows represent multiple reaction pathways. ACC acetyl-CoA carboxylase: α-KG α-ketoglutarate; ACLY ATP citrate lyase; ACO aconitase; ALDOA aldolase A; ATP adenosine-5′-triphosphate; ATGL adipose triglycerides lipase; AIF apoptosis-inducing factor; CoA coenzyme A; CS citrate synthase; DHAP dihydroxyacetone phosphate; ENO1 enolase 1; FASN fatty acid synthase; FAT/CD36 fatty acid translocase; FADH2 flavin adenine dinucleotide; FFA free fatty acid; F1,6BP fructose-1,6-bisphosphate; F2,6BP fructose-2,6-bisphosphate; F6P fructose6-phosphate; FH fumarate hydratase; GFAT glucosamine fructose-6-phosphate amidotransferase; GAPDH glyceraldehyde 3-phosphate dehydrogenase; GLS glutaminase; GLUD glutamate dehydrogenase 1; GSH glutathione; G1P glucose-1-phosphate; G3P glyceraldehyde 3-phosphate; G6P glucose-6-phosphate; G6PDH G6P dehydrogenase; GLUT glucose transporter; GYS1 glycogen synthase 1; HK2 hexokinase 2; HSL hormone-sensitive lipase; IDH isocitrate dehydrogenase; LDHA lactate dehydrogenase A; MCD malonyl-CoA decarboxylase; MCT monocarboxylate transporters; MDH malate dehydrogenase; ME1 malic enzyme 1; miR microRNA; MAGL monoacylglycerol lipase; MUFA monounsaturated fatty acid; NHE1 Na+/H+ exchange protein 1; NADH nicotinamide adenine dinucleotide; NADPH nicotinamide adenine dinucleotide phosphate; OAA oxaloacetate; OXPHOS oxidative phosphorylation; PDH pyruvate dehydrogenase; PEP phosphoenolpyruvate; PFK-1 phosphofructokinase 1; PFK-2 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase; PGI phosphoglucose isomerase; PGK1 phosphoglycerate kinase 1; PGM phosphoglycerate mutase; PHGDH phosphoglycerate dehydrogenase; PKM2 pyruvate kinase M2; PL phospholipids; PYGL

The enzyme 6-phosphofructo-1-kinase (PFK-1) regulates a key step in glycolysis by controlling the conversion of fructose-6-phosphate (F6P) to fructose-1,6bisphosphate (F1,6BP). Four different genes (pfkfb1–4) encode another enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2), which has an essential role in regulating PFK-1 activity. PFK-2 is a bifunctional enzyme with both kinase and bisphosphatase activities that are catalyzed at different sites on each subunit of this protein. The kinase domain of this enzyme is localized within the NH2-terminal part of the enzyme, and the bisphosphatase domain is in the COOHterminal region. PFK-2/FBPase-2 regulates both the synthesis (through its kinase function) and the degradation (through its phosphatase function) of intracellular fructose-2,6-bisphosphate, the most potent positive allosteric activator of PFK-1. PFK-1 activity is increased in tumors and activated by oncogenes. Similarly, fructose-2,6-bisphosphate levels are also increased in tumors (Bartrons and Caro 2007; Yalcin et al. 2009).

Driver genetic mutations can directly regulate metabolic enzymes (Fig. 1.1). Oncogene status can drive some of these metabolic changes in tumor cells. Oncogenic Myc, oncogenic Ras, and Akt kinase (also known as protein kinase B (PKB)) promote an increase in glycolytic flux by upregulating the transcription of various metabolic genes (Levine and Puzio-Kuter 2010). Myc was the first oncogene to be linked to increased glycolysis in cancer cells, through the direct activation of almost all glycolytic enzymes, and lactate dehydrogenase A (LDHA), which converts pyruvate to lactate (Shim et al. 1997). Oncogenic Ras induces glycolysis by enhancing the stability of Myc (Sears et al. 1999). Akt kinase stimulates glycolytic flux through activation by mutated phosphoinositide 3-kinase (PI3K) (Elstrom et al. 2004). Mutations in tumor suppressor genes can also influence the glycolytic rate. More than 50 % of human tumors contain a mutation or deletion of the tumor suppressor gene p53. In addition to its role in cell cycle arrest and cell death, several recent studies have revealed a major role for p53 in the regulation of metabolism (Maddocks and Vousden 2011; Vousden and Ryan 2009). p53 can inhibit glycolysis by repressing the expression of the glucose transporters GLUT1 and GLUT4 and the glycolytic enzyme phosphoglycerate mutase (Kondoh et al. 2005; Schwartzenberg-Bar-Yoseph et al. 2004). p53 also induces the expression of the TP53-inducible glycolytic and apoptotic regulator (TIGAR). TIGAR is involved in the regulation of PFK-1 activity, the key enzyme in the glycolytic pathway (Bensaad et al. 2006, 2009). Therefore, TIGAR inhibits glycolysis and induces the PPP, leading to the removal of intracellular reactive oxygen species (ROS).

The increased uptake of glucose and its conversion into lactate causes lactate accumulation and intracellular acidification. While acidification of the tumor microenvironment promotes tumor cell invasion and metastasis formation, intracellular pH must remain alkaline for cancer cells to survive (Chiche et al. 2010). Several mechanisms have been implicated in the pH regulation of cancer cells (Fig. 1.1). The levels of lactate in the cytosol are dependent on the regulation and expression of monocarboxylate transporters (MCTs) on the membrane of tumor cells. Lactate transport across the plasma membrane through MCTs is coupled to the symport of protons (H+) (Halestrap and Wilson 2012). Expression of MCT1 and MCT4 has been shown to be elevated in several types of tumors as compared to normal tissues, and it correlates with poor prognosis and disease progression (Chen et al. 2010; Pinheiro et al. 2009, 2010). Furthermore, MCTs have a role in cellular pyruvate uptake to fuel mitochondrial respiration and support proliferation of breast cancer cells (Diers et al. 2012). Another pH regulation mechanism involves the Na+/H+ exchanger protein called NHE1. NHE1 has recently been shown to be important for tumor growth, cell migration, and metastasis formation (Amith and Fliegel 2013; Loo et al. 2012).

As previously mentioned, glycolysis can rapidly produce energy, especially under low oxygen tension, but cells also require precursors for biosynthesis for growth and proliferation, and reducing equivalents for antioxidant mechanisms. During the final step of glycolysis, phosphoenolpyruvate (PEP) is converted to pyruvate, a reaction driven by the rate-limiting enzyme pyruvate kinase (Fig. 1.1). There are two isoforms of pyruvate kinase, pyruvate kinase isozyme type M1 (PKM1) and pyruvate kinase isozyme type M2 (PKM2), which are differentially expressed in normal and cancer cells. While PKM1 is mainly expressed in normal tissue, PKM2 is mainly expressed in cancerous tissue (Christofk et al. 2008a, b). However, a more recent study has shown that both PKM1 and PKM2 are expressed in normal and cancer tissues, but PKM2 is the prominent isoform in cancer cell lines (Bluemlein et al. 2011). PKM2 can be phosphorylated by oncogenic tyrosine kinases. This leads to a switch from its active tetrameric form to a much less active dimeric form and therefore contributes to anabolic metabolism in proliferating cancer cells (Christofk et al. 2008a, b; Vander Heiden et al. 2010). PKM2 can also be inactivated by ROS and contributes to oxidative stress. This regulation contributes to cellular antioxidant response by increasing the flux of phosphorylated glucose through the PPP to generate NADPH and remove intracellular ROS (Anastasiou et al. 2011).