Targeting Cancer Metabolism - Revisiting the Warburg Effects

Abstract

After more than half of century since the Warburg effect was described, this atypical metabolism has been standing true for almost every type of cancer, exhibiting higher glycolysis and lactate metabolism and defective mitochondrial ATP production. This phenomenon had attracted many scientists to the problem of elucidating the mechanism of, and reason for, this effect. Several models based on oncogenic studies have been proposed, such as the accumulation of mitochondrial gene mutations, the switch from oxidative phosphorylation respiration to glycolysis, the enhancement of lactate metabolism, and the alteration of glycolytic genes. Whether the Warburg phenomenon is the consequence of genetic dysregulation in cancer or the cause of cancer remains unknown. Moreover, the exact reasons and physiological values of this peculiar metabolism in cancer remain unclear. Although there are some pharmacological compounds, such as 2-deoxy-D-glucose, dichloroacetic acid, and 3-bromopyruvate, therapeutic strategies, including diet, have been developed based on targeting the Warburg effect. In this review, we will revisit the Warburg effect to determine how much scientists currently understand about this phenomenon and how we can treat the cancer based on targeting metabolism.

Keywords: Cancer metabolism; Energy metabolism; Mitochondria; Warburg effects.

Fig. 1

The PI3K/PKB/mTOR signaling pathway regulates cancer cell metabolism. PKB upregulates glycolysis by affecting glucose transporter 1 (GLUT1) and activating hexokinase 2 (HK2) association with the mitochondria. Moreover, PKB regulates de novo fatty acid synthesis and the usage of fatty acid for β-oxidation. It phosphorylates ATP citratelyase (ACL) to supply downstream de novo fatty acid synthesis (64). Phosphoinositide 3-kinase (PI3K) and PKB suppress the expression of the β-oxidation enzyme carnitine palmitoyltransferase 1A (CPT1A), suppressing β-oxidation and impairing mitochondria. mTOR, a downstream effector of the PI3K/PKB pathway, is regulated by AMP-activated protein kinase (AMPK; the cellular energy sensor), the tuberous sclerosis 1 & 2 (TSC1/TSC2) complex, and Ras homolog enriched in brain (RHEB). Most importantly, mTOR is an upstream activator of HIF-1α in cancer cells (69), which is a subunit of a transcription factor that upregulates the expression of nearly all of the genes involved in the glycolytic pathway (See details in the text (Section 1-4-1)). Arrows represent stimulation/activation, and ends represent inhibition.

Fig. 2

c-MYC, HIF-1 and p53 regulate glycolytic metabolism. The Warburg phenomenon is due, at least in part, to the upregulation of genes coding for glucose transporters and glycolytic and regulatory enzymes mediated by the increased activity of the transcription factors c-MYC and HIF-1 in cancer cells, and the coordinated loss of regulatory proteins due to the loss of p53 function. Loss of p53 function also leads to the activation of GLUT-3 transcription via NFκB. Arrows represent stimulation/activation, and ends represent inhibition. + indicates synergism. HK2, hexokinase type 2; GPI, glucose phosphate isomerase; PFK1, phosphofructokinase 1; PFK2, phosphofructokinase 2; ALDA, aldolase A; TPI, triose phosphate isomerase; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; PGK1, phosphoglycerate kinase 1; PGM, phosphoglycerate mutase; ENO1, enolase 1; PKM2, pyruvate kinase type M2; LDH-A, lactate dehydrogenase type A: PDK1, pyruvate dehydrogenase kinase-1; TIGAR, TP53-induced glycolysis and apoptosis regulator; SCO2, synthesis of cytochrome c oxidase-2; IKK, I-kappa-B kinase; NF-κB, nuclear factor-kappa-B; GLUT, glucose transporter (–158).

Fig. 3

The targets of 2-DG. Similar to glucose, 2-DG is taken up through GLUTs and then is phosphorylated by HK to form 2-DG-6-P, and 2-DG competitively inhibits GLUTs. 2-DG-6-P cannot be further metabolized via glycolysis but accumulates in the cell and non-competitively inhibits HK and competitively inhibits PGI. NADPH generation is inhibited. Only one molecule of NADPH can be generated from the conversion of 2-DG-6-P to 2-DG-6-phosphogluconolactone. 2-DG structurally resembles mannose and undergoes conversion into 2-DG-GDP, which interferes with the N-linked glycosylation of proteins. The inhibition of N-linked glycosylation induces the accumulation of unfold/misfolded proteins in the ER, resulting in ER stress and constant cell apoptosis. Intracellular glucose can promote glycosylation because its metabolic product F-6-P can be used in the mannose glycosylation pathway. However, upstream glucose metabolism is inhibited by 2-DG, which may not allow exogenous glucose to restore the interrupted N-linked glycosylation. GLUTs, glucose transporters; HK, hexokinase; PGI, phosphoglucose isomerase; G-6-PD, glucose-6-phosphate dehydrogenase; GDP, guanosine diphosphate; MCT, monocarboxylate transporter.

Fig. 4

DCA “switch on” mitochondria in cancer. The multi enzyme pyruvate dehydrogenase complex (PDC) is located in the mitochondrial matrix and catalyzes the rate-limiting step in the aerobic oxidation of glucose, pyruvate, alanine and lactate to acetyl CoA, a substrate of the TCA cycle. Thus, PDC is the key mediator of OxPhos. The upstream effectors of the PDC include the following: the family of pyruvate dehydrogenase kinase (PDK) isoforms that phosphorylate and inactivate PDC; the pyruvate dehydrogenase phosphatase (PDP) isoforms that dephosphorylate the PDC and restore catalytic activity. Dichloroacetic acid (DCA), a structural analog of pyruvate, stimulates PDC activation by inhibiting PDKs, thereby maintaining the PDC in its unphosphorylated form. Moreover, DCA increases PDC activity by inhibiting the turnover of the complex, although the mechanism remains unclear.

Fig. 5

Effect of 3-bromopyruvate (3-BP) on cell metabolism and survival. 3-BP dampens glycolysis by inhibiting hexokinase II and impeding the production of ATP. Furthermore, glycolysis inhibition caused by 3-BP leads to the dephosphorylation of Bcl-2-associated death promoter protein (BAD) at Ser112. Consequently, BAX, a protein required by BAD, is displaced and localized to the mitochondria, altering the mitochondrial membrane permeability and resulting in the release of cytochrome c and subsequent cell death. In addition, a main target of 3-BP is the pyruvylation of glyceraldehydes-3-phosphate dehydrogenase (GAPDH), which is associated with the loss of GAPDH enzymatic activity, resulting in the anti-glycolytic and anti-cancer effects.