Cancer metabolism: New insights into classic characteristics

Abstract

Initial studies of cancer metabolism in the early 1920s found that cancer cells were phenotypically characterized by aerobic glycolysis, in that these cells favor glucose uptake and lactate production, even in the presence of oxygen. This property, called the Warburg effect, is considered a hallmark of cancer. The mechanism by which these cells acquire aerobic glycolysis has been uncovered. Acidic extracellular fluid, secreted by cancer cells, induces a malignant phenotype, including invasion and metastasis. Cancer cells survival depends on a critical balance of redox status, which is regulated by amino acid metabolism. Glutamine is extremely important for oxidative phosphorylation and redox regulation. Cells highly dependent on glutamine and that cannot survive with glutamine are called glutamine-addicted cells. Metabolic reprogramming has been observed in cancer stem cells, which have the property of self-renewal and are resistant to chemotherapy and radiotherapy. These findings suggest that studies of cancer metabolism can reveal methods of preventing cancer recurrence and metastasis.

Keywords: Acidic extracellular pH; Glutamine metabolism; Glycolysis; Warburg effect.

Figure 1

Oncogene and tumor suppressor gene products regulate glucose and glutamine metabolism in cancer. Glycolysis is the main source of ATP production rather than oxidative phosphorylation (OXPHOS) in tumor cells. Glucose transporters and glycolysis metabolic enzymes are up-regulated by oncogene product c-Myc. It was believed that mutation of p53 causes loss of function. More recently, p53’s mutation-based “gain of function” has been accepted: e.g., IκB kinase (IKK) is inhibited by wild type p53 (wtp53) but activated by mutant p53 (mutp53). Glucose transporter 4 (GLUT4) and phosphoglycerate mutase (PGM1) activities are also regulated by p53 in the same way. This means reprogramming of which metabolic pathway is directed to lactate when cellular transformation occurs. This is a significant reprogramming of metabolic pathways during carcinogenesis. Hypoxia accelerates glycolysis dependency for energy production through activation of hypoxia-inducible transcription factor 1 (HIF1). Malate and oxaloacetate (OAA) in the TCA cycle can be metabolized to pyruvate in cytosol. Especially, this pathway is important for metabolism of glutamine, rather than glucose, through α-ketoglutarate (α-KG) (see Fig. 6). Two isozymes of glutamine-OAA transaminase (GOT) are closely associated in this pathway. ASCT2, neutral amino acid transporter; SIRT6, distant mammalian Sir2 homolog (sirtuin 6); NEK2, never in mitosis gene A-related kinase 2; NF-κB, nuclear factor-κB; HK2, hexokinase 2, TIGAR, TP-53-induced glycolysis and apoptosis regulator; PFK1/2, 6-phosphofructo 1-kinase 1/2; AMPK, AMP-activated protein kinase; ALDA/C, aldolase A/C; TPI, triosephosphate isomerase; GAPDH, glyceraldehyde-3-phosphate-dehydrogenase; PGK1, phosphoglycerate kinase 1; PGM1, phosphoglycerate mutase 1; ENO1, enolase 1; PKM1/M2, pyruvate kinase M1/M2; LDHA, lactate dehydrogenase A; Nrf2, NF-E2-related factor 2; PDH, pyruvate dehydrogenase; PDK1, pyruvate dehydrogenase kinase 1; PTEN, tensin homolog on chromosome ten; PINK1, PTEN-induced putative kinase 1; SCO2, cytochrome c oxidase assembly factor 2; GLS1/2, glutaminase 1/2; GLUD1, glutamate dehydrogenase 1; TCA cycle, tricarboxylic acid cycle.

Figure 2

Increase in pyruvate kinase M2 (PKM2)/PKM1 ratio by phosphorylation of tyrosine residue directs to glycine production. (A) PKM2 activity is regulated by phosphorylation in contrast to constitutively active PKM1. Phosphorylation of tyrosine (Tyr) residue activates it whereas that of serine (Ser) residue inhibits it. (B) When PKM2/PKM1 ratio increases, the metabolic pathway directs to pyruvate (continues glycolysis). When the ratio decreases, glycolysis is prevented and metabolic direction changes to serine followed by glycine. Glycine condensates with γ-glutamylcysteine for glutathione synthesis (see Fig. 6).

Figure 3

Cell to cell communication by proton and acidic metabolites (lactate and β-hydroxybutyrate). Carbonic anhydrase (CA) catalyzes H₂O and CO₂ yielding H₂CO₃ followed by H⁺ and HCO₃⁻. CA II and CA IX are located on the cytosol and plasma membrane, respectively. Intracellular H⁺ is secreted by vacuolar type-ATPase (v-ATPase), Na⁺/H⁺ exchanger 1 (NHE1). Monocarboxylate transporter (MCT) functions as the lactate/H⁺ or β-hydroxybutyrate (βOHB)/H⁺ co-transporter. MCT1 and MCT4 are associated with their up-take and secretion, respectively. Intracellular HCO₃⁻ can be secreted by Cl⁻/HCO₃⁻ exchanger, which is not shown in this figure. Upper cell: tumor cells in normoxia and sufficient nutrition due to proximity to blood vessels. Lower cell: cancer-associated fibroblasts (CAFs) or tumor cells in hypoxic and inadequate nutrition due to distance from blood vessels.

Figure 4

Acidic pH_e signaling. In acidic pH_e signaling, Ca²⁺ influx may be common in various tumor cells. Increase in intracellular Ca²⁺ causes activation of phospholipase D and two mitogen-activated protein kinases (MAPKs) (extracellular signal-regulated kinase (ERK) 1/2 and p38) followed by nuclear factor-κB (NF-κB) activation. NF-κB is also activated by acidic sphingomyelinase (aSMase) independent of Ca²⁺ influx.

Figure 5

Cell cycle dependent glutamine metabolism. Glutamine to α-ketoglutarate is metabolized by different enzymes depending on cell cycle status. (A) Oncogenic molecules such as c-Myc and K-ras activate glutaminase 1 (GLS1) and glutamine-oxaloacetate transaminase 2 (GOT2) in proliferating cells. K-ras inhibits glutamate dehydrogenase 1 (GLUD1). Thus, GLS1 and GOT2 are major metabolic enzymes in proliferating cells. (B) Wild type p53 (wtp53) not only increase in the cyclin-dependent kinase inhibitor p21 but also glutaminase 2 (GLS2). GLUD1 is not inhibited by K-ras in quiescent cells, thereby metabolizing by GLS2 and GLUD1.

Figure 6

Glucose and glutamine metabolism in redox prevention. Production of NADPH is mainly obtained from glucose metabolism (the pentose phosphate pathway) and glutamine metabolism (pathway from malate to pyruvate) through part of the TCA cycle. Glutathione is a tripeptide comprising glutamate, cysteine, and glycine. OAA, oxaloacetate; Asp, aspartate; α-KG, α-ketoglutarate.

Figure 7

Regulation of mTORC1. mTORC1 comprises five molecules: mTOR; the regulatory associated protein of mTOR (RAPTOR); the DEP domain containing mTOR interacting protein (DEPTOR); the proline-rich Akt substrate of 40-kDa (PRAS40); and the mammalian lethal with SEC13 protein 8 (mLST8). Glutamine activates mTORC1 through ADP-ribosylation factor 1 (ARF1) but leucine and arginine does through Rag small G proteins. mTORC1 promotes mRNA translation and protein synthesis through inhibition of the eukaryotic translation initiation factor 4E-binding protein (4E-BP1) and activation of the ribosomal protein S6 kinase (S6 K). It also inhibits ULK1 and HIF1α which induce autophagy and glycolysis, respectively. mTORC1 induces inflammation and lipid synthesis through NF-κB and the sterol regulatory element binding protein 1 (SREBP1), respectively.