**5.2 ROS regulation of metabolic pathways**

*Acute Leukemias*

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

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

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

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

process collectively termed aerobic respiration (reviewed in [66]).

potentially increasing cancer cell motility and metastasis [70].

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

**42**

alpha (HIF-1α) [75].

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 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),

#### **Figure 4.**

*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 prolylhydroxylase domain (PHD).*

phosphoglucose isomerase (PGI), phosphofructokinase (PFK), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK), PKM2, as well as lactate dehydrogenase A (LDHA), pyruvate dehydrogenase kinase 1 (PDK1) and PFKFB3 (reviewed in [87]). Increased glutaminolysis is also a target of c-MYC, which upregulates the glutamine transporter ASCT2 and a key enzyme glutaminase. Additionally, c-MYC was shown to upregulate both phosphoglycerate dehydrogenase (PHGDH) which catalyzes the first step of the SSP, serine hydroxymethyltransferase, part of the folate cycle as well as several genes involved in fatty acid metabolism and the citric acid cycle (reviewed in [67]). In contrast TP53 is known to inhibit glycolysis through inhibition of GLUT1, GLUT4 and PGAM and through activation of TIGAR and synthesis of cytochrome c oxidase 2 (SCO2). Inhibition of glycolysis also occurs due to the regulatory role of miRNA. For example, miR-195-5p inhibits GLUT3, miR-143 inhibits HK2 and miR-155 inhibits HIF-1α. Furthermore, TP53 induces miR-34a which suppresses HK1, HK2, GPI and PDK1, as well as sirtuin 1, which activates FOXO1, NF-κB and in a positive feedback loop TP53 (reviewed in [80]).

#### **5.3 Metabolism and leukemia**

Given the role that ROS plays in regulating metabolism, it is not surprising that expression of nearly all enzymes associated with glycolysis have been shown to be altered in solid tumors, a pattern also observed in leukemia. In ALL, micro-array analysis showed significant upregulation of PFK as well as the glucose transporters GLUT1 and GLUT4 in pediatric B-ALL samples [88], whilst deletion of GLUT1 in primary human B-ALL cells suppressed leukemic progression *in vivo* [89]. In AML, upregulation of *GLUT1* mRNA [90] and the fructose transporter *GLUT5* [91] have also been reported to be associated with poor outcome in AML patients. Furthermore, NOX generated ROS has previously been reported to modulate cellular glucose uptake through increased GLUT1 activity, in leukemic cell lines [92]. In Philadelphia<sup>+</sup> ALL (Ph+ ALL) GLUT5 has been found to be upregulated at both the mRNA and protein level [93]. Song *et al* have identified HK2 overexpression as a feature of AML patients who failed to show remission [90], whilst decreased proliferation in the AML cell line, KG-1, was observed upon knock-down of PGI with shRNA [94]. The HK inhibitors 2-deoxy-D-glucose and 3-bromopyruvate have both been shown to be cytotoxic in AML patient samples harboring a FLT3-ITD mutation both alone and in combination with sorafenib [90, 95]. In chronic lymphocytic leukemia (CLL), a study by Ryland et al., 2013 showed increased expression of glyceraldehyde phosphate dehydrogenase (GAPDH) in CLL patients compared to healthy controls [96]. Proteomic studies revealed elevated levels of aldolase A (ALDO(A)), ALDO(C) and enolase 1 (ENO1) in the chemoresistant leukemia cell line K562/ A02 when compared with parental K562 cells and in the case of ENO1 this was confirmed by western blot [97]. Elevated levels of ENO2 have also been reported in patients with ALL where it is associated with lower overall survival [98], whilst PGAM is upregulated in both AML and CML patient samples [99]. LDH is a tetramer which exists as five isoforms, comprising of two subunits LDHA and LDHB in different combinations and encoded by the LDHA and LDHB genes [100], with LDHA strongly catalyzing pyruvate to lactate and LDHB preferentially catalyzing the reverse reaction. In B-ALL, mRNA expression levels of *LDHB* were shown to be decreased [88], suggesting increased lactate production, whilst more recently increased serum levels of LDH were found in patients with B-ALL in conjunction with increased levels of total oxidant status and decreased total anti-oxidant status [101]. Another recent study involving 204 patients with acute leukemia's also reported that LDH plasma levels were significantly elevated compared to healthy

**45**

conversion of NADP+

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

controls and were also increased in relapse patients compared to those in complete remission [102]. Recently, it was shown that ROS dependent proliferative increases observed in hematopoietic models [103] were also accompanied by increased glucose uptake and expression of the regulatory glycolytic enzyme PFKFB3 [53], whilst downregulation of this enzyme suppressed growth both *in vivo* and *in vitro* [16]. This study also reported that metabolomic analysis comparing AML patient samples with high/low levels of ROS, which showed significantly elevated levels of glucose, glucose-6-phosphate (G-6-P) and F-6-P in the ROS high patients. Another metabolomic study involving serum from 400 AML patients compared with 446 healthy controls, identified elevated levels of the glycolytic intermediates 3-phosphoglycerate (3-PG), pyruvate and lactate as conferring a poor prognosis for survival [104]. Interestingly, a recent study showed that the bromodomain and extra-terminal protein inhibitor JQ1, which has shown promise in ALL by targeting c-myc, downregulates expression of HK2, PKM2 and LDHA both at the transcrip-

The citric acid cycle is a series of metabolic reactions involving oxidation/reduction reactions, which generate nicotinamide adenine dinucleotide (NAD)H and flavin adenine dinucleotide (FAD)H via the transfer of hydride ions, thus providing electrons for the ETC which is a major source of cellular ROS (reviewed in [106]). Mutations of IDH, which catalyzes the decarboxylation of isocitrate to alphaketoglutarate are frequently reported in AML (reviewed in [107]). Characterization of the inhibitor AG-221, which has been shown to inhibit mutant IDH2 in AML cells *in vitro* and *in vivo* and is currently undergoing phase I/II clinical trials [108], as is the IDH1 inhibitor, AG-120 [109]. A metabolomic study which examined a cohort of 183 patients with *de novo* AML matched with 232 healthy controls showed significant differences in citrate levels between AML patients and controls [110]. In pediatric ALL a recent metabolomic study revealed increased metabolites of glycolysis, the citric acid cycle and the PPP in patients testing positive for measurable residual disease compared to those testing negative [111]. Interestingly use of nicotinamide phosphoribosyltransferase (NAMPT) inhibitors on ALL cell lines and patient samples showed cytotoxicity *in vitro*. NAMPT is a key enzyme in the

The SSP branches from the glycolytic pathway at the glycolytic intermediate 3-PG, where it is converted into 3-phosphohydroxypyruvate by the enzyme PHGDH, followed by conversion to phosphoserine by phosphoserine aminotransferase 1 and finally to serine by the action of the enzyme phosphoserine phosphatase (reviewed in [73]). Regulation of the SSP is achieved through 2-phosphoglycerate (2-PG) which activates PHGDH whilst serine activates the tetrameric form of PKM2 leading to increased glycolysis and decreased levels of 2-PG. Importantly serine can enter the folate cycle, which provides another route for the generation of NADPH, which has been shown to contribute to tumor growth *in vivo* [112]. Whilst overexpression of PHGDH has been reported in melanomas, colorectal and breast cancers, little has been published from a leukemia perspective. Knock-down of PHGDH has been shown to inhibit the growth of the leukemia cell line, HL-60 [113], and in multiple myeloma increased expression of PHGDH led to increased SSP activity and antioxidant capacity in cells resistant to treatment with

The PPP generate nucleotides for biosynthesis and is a major source of cellular NADPH, an important cellular antioxidant. The first step involves the dehydrogenation of G-6-P to 6-phosphogluconolactone (6-PG) catalyzed by G-6-PD and the

to 6-phosphogluconate, which is then catalyzed by 6-phosphogluconate dehydrogenase (6-PGD) to ribulose-5-phosphate (Ru-5-P) alongside the generation of a

to [115]. Gluconolactonase catalyzes the hydrolysis of 6-PG

, in both glycolysis and the citric acid cycle.

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

tional and protein level [105].

synthesis of the oxidizing agent NAD+

the proteasome inhibitor bortezomib [114].

#### *Reactive Oxygen Species and Metabolic Re-Wiring in Acute Leukemias DOI: http://dx.doi.org/10.5772/intechopen.94829*

*Acute Leukemias*

loop TP53 (reviewed in [80]).

**5.3 Metabolism and leukemia**

ALL (Ph+

In Philadelphia<sup>+</sup>

phosphoglucose isomerase (PGI), phosphofructokinase (PFK), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK), PKM2, as well as lactate dehydrogenase A (LDHA), pyruvate dehydrogenase kinase 1 (PDK1) and PFKFB3 (reviewed in [87]). Increased glutaminolysis is also a target of c-MYC, which upregulates the glutamine transporter ASCT2 and a key enzyme glutaminase. Additionally, c-MYC was shown to upregulate both phosphoglycerate dehydrogenase (PHGDH) which catalyzes the first step of the SSP, serine hydroxymethyltransferase, part of the folate cycle as well as several genes involved in fatty acid metabolism and the citric acid cycle (reviewed in [67]). In contrast TP53 is known to inhibit glycolysis through inhibition of GLUT1, GLUT4 and PGAM and through activation of TIGAR and synthesis of cytochrome c oxidase 2 (SCO2). Inhibition of glycolysis also occurs due to the regulatory role of miRNA. For example, miR-195-5p inhibits GLUT3, miR-143 inhibits HK2 and miR-155 inhibits HIF-1α. Furthermore, TP53 induces miR-34a which suppresses HK1, HK2, GPI and PDK1, as well as sirtuin 1, which activates FOXO1, NF-κB and in a positive feedback

Given the role that ROS plays in regulating metabolism, it is not surprising that expression of nearly all enzymes associated with glycolysis have been shown to be altered in solid tumors, a pattern also observed in leukemia. In ALL, micro-array analysis showed significant upregulation of PFK as well as the glucose transporters GLUT1 and GLUT4 in pediatric B-ALL samples [88], whilst deletion of GLUT1 in primary human B-ALL cells suppressed leukemic progression *in vivo* [89]. In AML, upregulation of *GLUT1* mRNA [90] and the fructose transporter *GLUT5* [91] have also been reported to be associated with poor outcome in AML patients. Furthermore, NOX generated ROS has previously been reported to modulate cellular glucose uptake through increased GLUT1 activity, in leukemic cell lines [92].

the mRNA and protein level [93]. Song *et al* have identified HK2 overexpression as a feature of AML patients who failed to show remission [90], whilst decreased proliferation in the AML cell line, KG-1, was observed upon knock-down of PGI with shRNA [94]. The HK inhibitors 2-deoxy-D-glucose and 3-bromopyruvate have both been shown to be cytotoxic in AML patient samples harboring a FLT3-ITD mutation both alone and in combination with sorafenib [90, 95]. In chronic lymphocytic leukemia (CLL), a study by Ryland et al., 2013 showed increased expression of glyceraldehyde phosphate dehydrogenase (GAPDH) in CLL patients compared to healthy controls [96]. Proteomic studies revealed elevated levels of aldolase A (ALDO(A)), ALDO(C) and enolase 1 (ENO1) in the chemoresistant leukemia cell line K562/ A02 when compared with parental K562 cells and in the case of ENO1 this was confirmed by western blot [97]. Elevated levels of ENO2 have also been reported in patients with ALL where it is associated with lower overall survival [98], whilst PGAM is upregulated in both AML and CML patient samples [99]. LDH is a tetramer which exists as five isoforms, comprising of two subunits LDHA and LDHB in different combinations and encoded by the LDHA and LDHB genes [100], with LDHA strongly catalyzing pyruvate to lactate and LDHB preferentially catalyzing the reverse reaction. In B-ALL, mRNA expression levels of *LDHB* were shown to be decreased [88], suggesting increased lactate production, whilst more recently increased serum levels of LDH were found in patients with B-ALL in conjunction with increased levels of total oxidant status and decreased total anti-oxidant status [101]. Another recent study involving 204 patients with acute leukemia's also reported that LDH plasma levels were significantly elevated compared to healthy

ALL) GLUT5 has been found to be upregulated at both

**44**

controls and were also increased in relapse patients compared to those in complete remission [102]. Recently, it was shown that ROS dependent proliferative increases observed in hematopoietic models [103] were also accompanied by increased glucose uptake and expression of the regulatory glycolytic enzyme PFKFB3 [53], whilst downregulation of this enzyme suppressed growth both *in vivo* and *in vitro* [16]. This study also reported that metabolomic analysis comparing AML patient samples with high/low levels of ROS, which showed significantly elevated levels of glucose, glucose-6-phosphate (G-6-P) and F-6-P in the ROS high patients. Another metabolomic study involving serum from 400 AML patients compared with 446 healthy controls, identified elevated levels of the glycolytic intermediates 3-phosphoglycerate (3-PG), pyruvate and lactate as conferring a poor prognosis for survival [104]. Interestingly, a recent study showed that the bromodomain and extra-terminal protein inhibitor JQ1, which has shown promise in ALL by targeting c-myc, downregulates expression of HK2, PKM2 and LDHA both at the transcriptional and protein level [105].

The citric acid cycle is a series of metabolic reactions involving oxidation/reduction reactions, which generate nicotinamide adenine dinucleotide (NAD)H and flavin adenine dinucleotide (FAD)H via the transfer of hydride ions, thus providing electrons for the ETC which is a major source of cellular ROS (reviewed in [106]). Mutations of IDH, which catalyzes the decarboxylation of isocitrate to alphaketoglutarate are frequently reported in AML (reviewed in [107]). Characterization of the inhibitor AG-221, which has been shown to inhibit mutant IDH2 in AML cells *in vitro* and *in vivo* and is currently undergoing phase I/II clinical trials [108], as is the IDH1 inhibitor, AG-120 [109]. A metabolomic study which examined a cohort of 183 patients with *de novo* AML matched with 232 healthy controls showed significant differences in citrate levels between AML patients and controls [110]. In pediatric ALL a recent metabolomic study revealed increased metabolites of glycolysis, the citric acid cycle and the PPP in patients testing positive for measurable residual disease compared to those testing negative [111]. Interestingly use of nicotinamide phosphoribosyltransferase (NAMPT) inhibitors on ALL cell lines and patient samples showed cytotoxicity *in vitro*. NAMPT is a key enzyme in the synthesis of the oxidizing agent NAD+ , in both glycolysis and the citric acid cycle.

The SSP branches from the glycolytic pathway at the glycolytic intermediate 3-PG, where it is converted into 3-phosphohydroxypyruvate by the enzyme PHGDH, followed by conversion to phosphoserine by phosphoserine aminotransferase 1 and finally to serine by the action of the enzyme phosphoserine phosphatase (reviewed in [73]). Regulation of the SSP is achieved through 2-phosphoglycerate (2-PG) which activates PHGDH whilst serine activates the tetrameric form of PKM2 leading to increased glycolysis and decreased levels of 2-PG. Importantly serine can enter the folate cycle, which provides another route for the generation of NADPH, which has been shown to contribute to tumor growth *in vivo* [112]. Whilst overexpression of PHGDH has been reported in melanomas, colorectal and breast cancers, little has been published from a leukemia perspective. Knock-down of PHGDH has been shown to inhibit the growth of the leukemia cell line, HL-60 [113], and in multiple myeloma increased expression of PHGDH led to increased SSP activity and antioxidant capacity in cells resistant to treatment with the proteasome inhibitor bortezomib [114].

The PPP generate nucleotides for biosynthesis and is a major source of cellular NADPH, an important cellular antioxidant. The first step involves the dehydrogenation of G-6-P to 6-phosphogluconolactone (6-PG) catalyzed by G-6-PD and the conversion of NADP+ to [115]. Gluconolactonase catalyzes the hydrolysis of 6-PG to 6-phosphogluconate, which is then catalyzed by 6-phosphogluconate dehydrogenase (6-PGD) to ribulose-5-phosphate (Ru-5-P) alongside the generation of a

second NADPH. Ru-5-P can then be converted into ribose-5-phosphate (R-5-P) by the enzymatic action of ribulose-5-phosphate isomerase. R-5-P can then be used in the synthesis of nucleotides. Alternatively, where redox homeostasis and not nucleotide synthesis is the major requirement of the cell Ru-5-P can be catalyzed by ribulose-5-phosphate epimerase, into xyulose-5-phosphate (X-5-P) and via a series of further metabolic reactions back into the glycolytic intermediates F-6-P and glyceraldehyde-3-phosphate. G-6-PD is the rate limiting step of the PPP and is regulated by the NADP+/NADPH ratio, RAS/PI3K signalling and phosphorylation by Src, whilst 6-PGD is inhibited by 3-PG [99]. In cancer, aberrant RAS signalling or activation of Src can promote activation of the PPP. In AML, a recent study showed upregulation of *G-6-PD* mRNA in approximately 60% of patients, although it was not correlated with overall survival or relapse [116]. Targeting of xenograft mice injected with the leukemic cell line K562, with the antimalarial drug dihydroartemisinin and the 6-PGD inhibitor Physicon resulted in decreased tumor growth, whilst primary leukemia cells isolated from the PB of AML patients showed significantly decreased viability, with no toxicity observed in hematopoietic cells isolated from healthy individuals [117]. A metabolomic study comparing primary AML samples with either high or low levels of ROS, have also shown increased levels of the PPP metabolites sedoheptulose-7-phosphate and Ru-5-P in the samples with higher ROS levels [16]. Another study, using both AML cell lines and patient material, showed increased glucose metabolism and increased flux through the PPP, alongside increased *G-6-PD* mRNA expression [118]. Importantly, this study showed that use of the G-6-PD inhibitor 6-aminonictoinamide (6-AN) in AML cell lines induced both *in vitro* and *in vivo* cytotoxicity, and induced apoptosis in primary AML cells but not normal HPCs. In B-ALL, redirection of carbon from the glycolytic pathway to the PPP by the serine/threonine-protein phosphatase 2A (PP2A), has been shown to occur to combat cellular oxidative stress. Synergistic inhibition of G-6-PD by 6-AN and PP2A inhibitor LB100 induced cell death in patient derived Ph+ ALL [119].

Lipid metabolism has also been shown to be dysregulated in both solid tumors and hematological malignancies (reviewed in [120]). Increased fatty acid oxidation (FAO) allows cancer cells to overcome metabolic and oxidative stress through the generation of ATP and NADPH. Significant changes to lipid metabolite levels are seen in AML patient samples with either high levels or low levels of ROS [16], whilst suppression of NOX2 has also been shown to increase FAO [121]. Furthermore, inhibition of the FAO using Avocatin B results in decreased NADPH levels and ROS dependent cell death in primary human AML samples but not normal mononuclear cells [122]. In ALL, use of L-asparaginase has been shown to increase FAO activity as a metabolic escape mechanism, however use of the FAO inhibitor etomoxir in combination with L-asparaginase has been shown to increase sensitivity of both leukemic cell lines and patient samples [123].

#### **6. Conclusions**

In the last twenty years, it has become increasingly clear that ROS play a significant role in cellular signalling, particularly pathways associated with growth, differentiation and survival, whilst its roles in HSC quiescence and normal hematopoiesis have started to be delineated. In many cancers including hematological malignancies, ROS levels have been shown to be elevated, leading to aberrant signalling in these pathways. Previously, arguments for both the use of anti-oxidant and pro-oxidant treatments in leukemia have been made (reviewed in [124]). Despite the transformation of survival rates in patients with acute promyelocytic leukemia

**47**

**Author details**

**Acknowledgements**

of our studies.

Cardiff, UK

Andrew J. Robinson, Richard L. Darley and Alex Tonks\*

\*Address all correspondence to: tonksa@cf.ac.uk

provided the original work is properly cited.

Department of Haematology, Division of Cancer and Genetics, Cardiff University,

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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

using arsenic trioxide [125] cancer cells often upregulate the production of antioxidants, and downregulate pro-apoptotic pathways such as TP53, as a response to high ROS, allowing them to escape apoptosis. In addition, it has been shown that both cancer stem cells [126, 127] and leukemic stem cells [128] exhibit low ROS levels, suggesting that even if treatment with pro-oxidants eliminates the bulk of cancer cells, cancer/leukemic stem cells may survive and relapse occur. Conversely, studies involving the use of antioxidants in treatment and epidemiological studies of antioxidant use, have shown mixed results (reviewed in [129, 130]). Increasingly it is becoming apparent that increased levels of ROS are leading to changes in signalling pathways directly or indirectly controlling metabolism, as a mechanism for managing oxidative stress. Whilst, it has long been known that cancer cells exhibit greatly altered metabolism, only recently have the purposes behind this altered metabolism, started to be elucidated. Consequently, synergistic treatments involving the use of metabolic inhibitors, alongside classical treatments for leukemias are being explored. Future work, elucidating the intricate mechanisms governing the interplay between ROS and metabolism, alongside new and more specific metabolic

inhibitors provide much promise for the future treatment of leukemia.

We are grateful to Blood Cancer UK for programmatic funding and to Tenovus Cancer Care for funding Andrew Robinson. We are grateful to Wellcome ISSF for funding aspects of ROS research. We are grateful for support from the NCRI AML trials cell bank and the AML patients for providing primary samples used in several

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

*Reactive Oxygen Species and Metabolic Re-Wiring in Acute Leukemias DOI: http://dx.doi.org/10.5772/intechopen.94829*

using arsenic trioxide [125] cancer cells often upregulate the production of antioxidants, and downregulate pro-apoptotic pathways such as TP53, as a response to high ROS, allowing them to escape apoptosis. In addition, it has been shown that both cancer stem cells [126, 127] and leukemic stem cells [128] exhibit low ROS levels, suggesting that even if treatment with pro-oxidants eliminates the bulk of cancer cells, cancer/leukemic stem cells may survive and relapse occur. Conversely, studies involving the use of antioxidants in treatment and epidemiological studies of antioxidant use, have shown mixed results (reviewed in [129, 130]). Increasingly it is becoming apparent that increased levels of ROS are leading to changes in signalling pathways directly or indirectly controlling metabolism, as a mechanism for managing oxidative stress. Whilst, it has long been known that cancer cells exhibit greatly altered metabolism, only recently have the purposes behind this altered metabolism, started to be elucidated. Consequently, synergistic treatments involving the use of metabolic inhibitors, alongside classical treatments for leukemias are being explored. Future work, elucidating the intricate mechanisms governing the interplay between ROS and metabolism, alongside new and more specific metabolic inhibitors provide much promise for the future treatment of leukemia.
