**5. Metabolism and cancer**

Broadly defined, cellular metabolism involves a series of catabolic or anabolic chemical reactions which generate or use energy as part of this process. In chemotrophs this energy is obtained through the oxidation of nutrients, with the energy typically stored in the form of ATP. Whilst in higher organisms a plethora of enzymatically catalyzed metabolic reactions occur, which are all part of different interconnecting metabolic pathways with multitudinous feedback mechanisms. These pathways are evolutionarily highly conserved with the citric acid cycle, for example, essentially a feature in all terrestrial life. There are three main classes of molecules involved in metabolism; carbohydrates, proteins and lipids that are either catabolized to generate energy or energy stores or used by anabolic pathways

in the synthesis of, for example, nucleotides and structural molecules such as cell membranes. In mammals, a triumvirate of glycolysis, citric acid cycle and the ETC are central to the generation of ATP, with glycolysis and the citric acid cycle contributing 2 ATP molecules each and the ETC generating up to 34 ATP molecules in a process collectively termed aerobic respiration (reviewed in [66]).

Given the skew towards ATP production in the ETC, Otto Warburg's observation in 1956 that aerobic glycolysis was a hallmark feature of cancer cells [15], was initially attributed to being the result of defective mitochondria in malignant cells, and initially raised little interest. However, this hypothesis is now known in most cases to be incorrect (reviewed in [67]) and instead, it has been shown that mitochondrial respiration is often necessary in tumorigenesis [68]. However, given its ubiquity and despite its inefficiency when compared with ETC, it is clear that the phenomenon of increased aerobic glycolysis (eponymously titled 'The Warburg Effect'), must offer cancer cells some competitive advantage, although its exact ontology remains unclear. One hypothesis contends that whilst inefficient, aerobic glycolysis generates ATP at a rate 10–100 times faster than oxidative phosphorylation, therefore supplying cancer cells with energy at a faster rate. This increased glycolytic flux could then, potentially generate more nucleotides, amino acids and lipids for biosynthesis as well as generating the reducing agent NADPH, to deal with the increased levels of ROS common in many cancer cells [69]. Alternatively, increases in excreted lactate as a result of aerobic glycolysis would likely generate a more acidic microenvironment, breaking down stromal membrane structures and potentially increasing cancer cell motility and metastasis [70].

#### **5.1 NADPH: a link between ROS and metabolism in cancer**

It has been shown that activation of the tumor suppressor protein ATM by ROS promotes glucose-6-phosphate dehydrogenase (G-6-PD) activity, the first step of the pentose phosphate pathway (PPP), which in turn generates NADPH [71]. Given that major cellular antioxidant systems, ultimately rely on NADPH to provide their reducing power, it is perhaps not surprising that ROS in both normal and aberrant cellular processes is inextricably linked with metabolism. In the cytosol, NADPH is primarily generated through the PPP, whilst a number of mechanisms exist for mitochondrial NADPH generation [72], which include the serine synthesis pathway (SSP) (via the folate cycle) [73] and the action of the citric acid cycle enzyme isocitrate dehydrogenase (IDH). IDH1 and IDH2 are commonly mutated in AML [74], although in this context NADPH is consumed, and the D-2-hydroxyglutarate generated leads to stabilization of the hypoxia regulator, hypoxia inducible factor alpha (HIF-1α) [75].

HIF-1α as a target of ROS is controversial [76], however it is overexpressed in many cancers where it induces expression of numerous glycolytic genes. The ROS regulated transcription factor nuclear-related factor 2 (NRF2) has also been shown to modulate metabolism in lung cancer cell lines, through the upregulation of enzymes involved in the NADPH production, notably G-6-PD, IDH1 and malic enzyme 1 [77] and high NRF2 levels have previously been reported in AML [78]. Furthermore, the tumor suppressor protein TP53 is also important in regulating metabolism. Homozygous deletion of TP53 in mice results in decreased oxygen consumption arising from decreased mitochondrial respiration [79]. TP53 expression has been shown to inhibit, both glucose transporter (GLUT) 1 and 4 and the glycolytic enzyme phosphoglycerate mutase (PGAM) (reviewed in [80]) leading to decreased glycolysis and potentially increased metabolism via the PPP and SSP. Finally, TP53 also upregulates the apoptosis regulator (TIGAR) an enzyme which has an active domain similar to 6-Phosphofructo-2-kinase/fructoste-2,6-bisphosphatase

**43**

**Figure 4.**

*hydroxylase domain (PHD).*

*Reactive Oxygen Species and Metabolic Re-Wiring in Acute Leukemias*

shown to upregulate GLUT3 in mouse embryonic fibroblasts [86].

Nuclear localization of the glycolytic enzyme pyruvate kinase muscle 2 (PKM2) is also ROS mediated, where it acts as a co-factor in the activation of the transcription factor, c-MYC. RAS also activates c-MYC which is overexpressed in greater than 50% of human cancers and c-MYC has been shown to activate glycolysis via the upregulation of GLUT, the glycolytic enzymes hexokinase (HK),

*Regulation of metabolic pathways. Schematic illustration outlining some of the regulatory mechanism involved in glycolysis and other key metabolic pathways. Transcription factors are in pink and signalling pathways in blue. Reactive oxygen species (ROS), forkhead box O (FOXO), pyruvate kinase muscle 2 (PKM2), signal transducer and activator of transcription (STAT), nuclear factor kappa-light-chain-enhancer of activated B-cells (NF-*κ*B), glucose transporter (GLUT) hypoxia inducible factor-1 alpha (HIF-1*α*), tumour suppressor protein 53 (TP53), glycogen synthase kinase 3*β *(GSK-3*β*), isocitrate dehydrogenase (IDH), succinate dehydrogenase (SDH), fumarate hydratase (FH), protein kinase B (AKT), mammalian target of rapamycin (mTOR), phosphoinositide 3-kinase (PI3K), synthesis of cytochrome c oxidase 2 (SCO2) and prolyl-*

(PFKFB). TIGAR catalyzes the reaction of fructose-2,6-bisphosphate (F-2,6-BP) to fructose-6-phosphate (F-6-P), which inhibits glycolysis, redirects metabolites into

Changes of cellular ROS levels in both normal signalling as well cell signalling following cellular transformation result in changes in numerous signalling pathways controlling multiple cellular functions including growth, proliferation and differentiation. A number of these signalling pathways, exercise regulatory control over various metabolic pathways, which in turn modulate ROS levels via several feedback mechanisms (**Figure 4**). In leukemia, mutations in the *RAS* gene are present in about 15% of hematological malignancies [82]. RAS activates the PI3K/AKT/mTOR pathway which promotes nucleotide biosynthesis and lipid synthesis (reviewed in [83]) as well as HIF-1α, which upregulate glycolysis via the activation of numerous glycolytic genes. In addition to HIF-1α, other ROS activated transcription factors are important in metabolic regulation such as STAT3, which has been shown to promote glycolysis in hepatocellular carcinoma cell lines [84], FOXO3A, which inhibits glycolysis via activation of tuberous sclerosis 1 protein [85] and NF-κB which was

*DOI: http://dx.doi.org/10.5772/intechopen.94829*

**5.2 ROS regulation of metabolic pathways**

the PPP, generating NADPH [81].

(PFKFB). TIGAR catalyzes the reaction of fructose-2,6-bisphosphate (F-2,6-BP) to fructose-6-phosphate (F-6-P), which inhibits glycolysis, redirects metabolites into the PPP, generating NADPH [81].
