Over the past 25 years, the oncogene revolution has stim-ulated research, revealing that the crucial phenotypes that are characteristic of tumour cells result from a host of mutational events that combine to alter multiple signalling pathways. Moreover, high-throughput sequencing data suggest that the mutations leading to tumorigenesis are even more numerous and heterogeneous than previously thought 1,2. It is now clear that there are thousands of point mutations, translocations, amplifications and deletions that may contribute to cancer development, and that the mutational range can differ even among histopathologi-cally identical tumours. Detailed bioinformatic analyses have suggested that cancer-related driver mutations affect a dozen or more core signalling pathways and processes responsible for tumorigenesis 3. These findings have led to questions about the usefulness of targeting individual signalling molecules as a practical therapeutic strategy. However, it is becoming clear that many key oncogenic signalling pathways converge to adapt tumour cell metab-olism in order to support their growth and survival. Furthermore, some of these metabolic alterations seem to be absolutely required for malignant transformation. In view of these fundamental discoveries, we propose that alterations to cellular metabolism should be considered a crucial hallmark of cancer.
Multiple molecular mechanisms, both intrinsic and extrinsic, converge to alter core cellular metabolism and provide support for the three basic needs of dividing cells: rapid ATP generation to maintain energy s
tatus; increased biosynthesis of macromolecules; and tightened maintenance of appropriate cellular redox status (FIG. 1). To meet these needs, cancer cells acquire alterations to the metabolism of all four major classes of macromolecules:
carbohydrates, proteins, lipids and nucleic acids. Many similar alterations are also observed in rapidly prolifer-ating normal cells, in which they represent appropriate responses to physiological growth signals as opposed to constitutive cell autonomous adaptations 4,5. In the case of cancer cells, these adaptations must be implemented in the stressful and dynamic microenvironment of the solid tumour, where concentrations of crucial nutrients such as glucose, glutamine and oxygen are spatially and temporally heterogeneous 6. The nature and importance of metabolic restriction in cancer has often been masked owing to the use of tissue culture conditions in which both oxygen and nutrients are always in excess.
The link between cancer and altered metabolism is not new, as many observations made during the early period of cancer biology research identified metabolic changes as a common feature of cancerous tissues (such as the Warburg effect; discussed below)7. As much of the work in the field to date has focused on rapidly prolif-erating tumour models and cells in vitro , it is unclear to what extent these metabolic changes are important in low-grade slow growing tumours in which metabolic demand
s are not as extreme. Future clinical data describing the metabolic profiles of human tumours will be required to determine which metabolic alterations are most preva-lent in specific tumour types. However, despite the lack of comprehensive clinical data, there has been substantial recent progress in understanding the molecular events that regulate some of these metabolic phenotypes.
The Warburg effect
In addition to the ATP that is required to maintain nor-mal cellular processes, proliferating tumour cells must
The Campbell Family Cancer Research Institute, 610 University Avenue, T oronto, ON M56 2M9, Canada.
*These authors contributed equally to this work.
Correspondence to T .W.M. e-mail:
tmak@uhnres.utoronto.ca doi:10.1038/nrc2981
Redox status
Balance of the reduced state versus the oxidized state of a biochemical system. This balance is influenced by the level of reactive oxygen and nitrogen species (ROS and RNS) relative to the capacity of antioxidant systems to eliminate ROS and RNS.
Regulation of cancer cell metabolism
Rob A. Cairns*, Isaac S. Harris* and T ak W. Mak
Abstract | Interest in the topic of tumour metabolism has waxed and waned over the past century of cancer research. The early observations of Warburg and his contemporaries established that there are fundamental differences in the central metabolic pathways operating in malignant tissue. However, the initial hypotheses that were based on these observations proved inadequate to explain tumorigenesis, and the oncogene revolution pushed tumour metabolism to the margins of cancer research. In recent years, interest has been renewed as it has become clear that many of the signalling pathways that are affected by genetic mutations and the tumour microenvironment have a profound effect on core metabolism, making this topic once again one of the most intense areas of research in cancer biology.
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Oxidative phosphorylation Oxygen-dependent process coupling the oxidation of macromolecules and the electron transport chain with ATP synthesis. In eukaryotic cells, it occurs within the mitochondria and is a source of ROS production. Glycolysis
Oxygen-independent metabolism of glucose and other sugars into pyruvate to produce energy in the form of ATP and intermediate substrates for other metabolic pathways.also generate the energy that is required to support rapid
cell division. Furthermore, tumour cells must evade the
checkpoint controls that would normally block prolif-
eration under the stressful metabolic conditions that are
characteristic of the abnormal tumour microenviron-
ment. Tumour cells reprogramme their metabolic path-
ways to meet these needs during the process of tumour
progression. The best characterized metabolic phenotype
observed in tumour cells is the Warburg effect (FIG. 2),
which is a shift from ATP generation through oxidative
phosphorylation to ATP generation through glycolysis, even
under normal oxygen concentrations7. As a result, unlike
most normal cells, many transformed cells derive a sub-
stantial amount of their energy from aerobic glycolysis,
converting most incoming glucose to lactate rather than
metabolizing it in the mitochondria through oxidative
phosphorylation7,8. Although ATP production by glyco-
lysis can be more rapid than by oxidative phosphorylation,
it is far less efficient in terms of ATP generated per unit
of glucose consumed. This shift therefore demands that
tumour cells implement an abnormally high rate of glu-
cose uptake to meet their increased energy, biosynthesis
and redox needs.
There is some debate about the most important selec-
tive advantage that glycolytic metabolism provides to
proliferating tumour cells. Initial work focused on the con-
cept that tumour cells develop defects in mitochondrial
function, and that aerobic glycolysis is therefore a necessary
adaptation to cope with a lack of ATP generation by oxi-
dative phosphorylation. However, it was later appreci-
ated that mitochondrial defects are rare9 and that most
tumours retain the capacity for oxidative phosphorylation
and consume oxygen at rates similar to those observed in
normal tissues10. In fact, mitochondrial function is crucial
for transformation in some systems11–13. Other explana-
tions include the concept that glycolysis has the capacity to
generate ATP at a higher rate than oxidative phosphory-
lation and so would be advantageous as long as glucose
supplies are not limited. Alternatively, it has been pro-
posed that glycolytic metabolism arises as an adaptation
to hypoxic conditions during the early avascular phase of
tumour development, as it allows for ATP production in
the absence of oxygen. Adaptation to the resulting acidic
microenvironment that is caused by excess lactate pro-
duction may further drive the evolution of the glycolytic
phenotype14,15. Finally, most recently, it has been proposed
that aerobic glycolysis provides a biosynthetic advantage
for tumour cells, and that a high flux of substrate through
glycolysis allows for effective shunting of carbon to key
subsidiary biosynthetic pathways4,5.
The reliance of cancer cells on increased glucose
uptake has proved useful for tumour detection and
monitoring, with this phenotype serving as the basis for
clinical [18F]fluorodeoxyglucose positron emission tom-
ography (FDG–PeT) imaging. FDG–PeT uses a radio-
active glucose analogue to detect regions of high glucose
uptake, and has proved highly effective for the identifica-
tion and monitoring of many tumour types. Accordingly,
there is now a substantial body of useful clinical data
regarding the importance of glucose as a fuel for malig-
nancies16–19. Although there have been attempts to block
aerobic glycolysis in tumour cells using compounds such
as 2-deoxyglucose, effective therapeutic strategies have
not yet been devised. several new therapeutic approaches
targeting numerous points in the glycolytic process are
currently under evaluation, including the inhibition
of lactate dehydrogenase and the inactivation of the
monocarboxylate transporters that are responsible for
conveying lactate across the plasma membrane20,21.
The PI3K pathway. The PI3K pathway is one of the most
commonly altered signalling pathways in human can-
cers. This pathway is activated by mutations in tumour
suppressor genes, such as PTEN, mutations in the com-
ponents of the PI3K complex itself or by aberrant signal-
ling from receptor tyrosine kinases22. Once activated, the
PI3K pathway not only provides strong growth and sur-
vival signals to tumour cells but also has profound effects
on their metabolism. Indeed, it seems that the integra-
tion of growth and proliferation signals with alterations
to central metabolism is crucial for the oncogenic effects
of this signalling pathway23.
The best-studied effector downstream of PI3K is
AKT1 (also known as PKb). AKT1 is an important
driver of the tumour glycolytic phenotype and stimulates
ATP generation through multiple mechanisms, ensur-
ing that cells have the bioenergetic capacity required to
respond to growth signals24,25. AKT1 stimulates glycolysis
by increasing the expression and membrane transloca-tion of glucose transporters and by phosphorylating key glycolytic enzymes, such as hexokinase and phospho-fructokinase 2 (also known as PFKFb3)24,26 (FIG. 2). The increased and prolonged AKT1 signalling that is asso-ciated with transformation inhibits forkhead box sub-family O (FOXO) transcription factors, resulting in a host of complex transcriptional changes that increase glyco-lytic capacity 27. AKT1 also activates ectonucleoside tri-phosphate diphosphohydrolase 5 (enTPD5), an enzyme that supports increased protein glycosylation in the endoplasmic reticulum and indirectly increases glyco-lysis by creating an ATP hydrolysis cycle 28. Finally, AKT1 strongly stimulates signalling through the kinase mTOr by phos
phorylating and inhibiting its negative regulator tuberous sclerosis 2 (TsC2; also known as tuberin)26. mTOr functions as a key metabolic integration point, coupling growth signals to nutrient availability. Activated mTOr stimulates protein and lipid biosynthesis and cell growth in response to sufficient nutrient and energy conditions and is often constitutively activated during tumorigenesis 29. At the molecular level, mTOr directly stimulates mrnA translation and ribosome biogenesis, and indirectly causes other metabolic changes by acti-vating transcription factors such as hypoxia-inducible factor 1 (HIF1) even under normoxic conditions. The subsequent HIF1-dependent metabolic changes are a major determinant of the glycolytic phenotype downstream of PI3K, AKT1 and mTOr (FIG. 2).
HIF1 and MYC. The HIF1 and HIF2 complexes are the major transcription factors that are responsible for gene expression changes during the cellular response to low oxygen conditions. They are heterodimers that are com-posed of the constitutively expressed HIF1β (also known conditions, the HIFα subunits undergo oxygen-dependent hydroxylation by prolyl hydroxylase enzymes, which results in their recognition by von Hippel–lindau tumour suppressor (vHl), an e3 ubiquitin ligase, and subsequent degradation. HIF1α is ubiquitously expressed, whereas the expression of HIF2α is restricted to a more limited subset of cell types 30. Although these two transcription factors transactivate an overlapping set of genes, the effects on central metabolism have been bet-ter characterized for HIF1, and therefore our discussion is limited to HIF1 specifically.
In addition to its stabilization under hypoxic con-ditions, HIF1 can also be activated under normoxic conditions by oncogenic signalling pathways, including PI3K 23,31, and by mutations in tumour suppressor pro-teins such as vHl 32,33, succinate dehydrogenase (sDH)34 and fumarate hydratase (FH)35. Once activated, HIF1 amplifies the transcription of genes encoding glucose transporters and most glycolytic enzymes, increasing the capacity of the cell to carry out glycolysis 36. In addi-tion, HIF1 activates the pyruvate dehydrogenase kinases (PDKs), which inactivate the mitochondrial pyruvate dehydrogenase complex and thereby reduce the flow of glucose-derived pyruvate into the tricarboxylic acid (TCA) cycle 37–39 (FIG. 2). This reduction in pyruvate flux into the TCA cycle decreases the rate of oxidative phos-phorylation and oxygen consumption, reinforcing the glycolytic phenotype and sparing oxygen under hypoxic conditions.
Inhibitors of HIF1 or the PDKs could potentially
reverse some of the metabolic effects of tumorigenic HIF1 signalling and several such candidates, including the PDK inhibitor dichloroacetic acid (DCA), are currently under evaluation for their therapeutic utility 40–43.In addition to its well-described role in controlling cell growth and proliferation, the oncogenic transcrip-tion factor MyC also has several important effects on cell metabolism 44. With respect to glycolysis, highly expressed
oncogenic MyC has been shown to collaborate with HIF in the activation of several glucose transporters and
glycolytic enzymes, as well as lactate dehydrogenase A (lDHA) and PDK1 (ReFS 45,46). However, MyC also activates the transcription of targets that increase mito-chondrial biogenesis and mitochondrial function, espe-cially the metabolism of glutamine, which is discussed in further detail below 47.
AMP-activated protein kinase. AMP-activated protein
kinase (AMPK ) is a crucial sensor of energy status and has an important pleiotropic role in cellular responses to metabolic stress. The AMPK pathway couples energy status to growth signals; biochemically, AMPK opposes the effects of AKT1 and functions as a potent inhibitor of mTOr (FIG. 2). The AMPK complex thus functions as a metabolic checkpoint, regulating the cellular response to energy availability. During periods of energetic stress, AMPK becomes activated in response to an increased AMP/ATP ratio, and is responsible for shifting cells to an oxidative metabolic phenotype and inhibiting cell prolif-eration 48–50. Tumour cells must overcome this checkpoint in order to proliferate in response to activated growth
responses to the tumour microenvironment. Oncogenic signalling pathways controlling growth and survival are often activated by the loss of tumour suppressors (such as p53) or the activation of oncoproteins (such as PI3K). The resulting altered signalling modifies cellular metabolism to match the requirements of cell division. Abnormal
microenvironmental conditions such as hypoxia, low pH and/or nutrient deprivation elicit responses from tumour cells, including autophagy, which further affect metabolic activity. These adaptations optimize tumour cell metabolism for proliferation by
providing appropriate levels of energy in the form of ATP , biosynthetic capacity and the maintenance of balanced redox status. AMPK, AMP-activated protein kinase; HIF1, hypoxia-inducible factor 1.
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Nature Reviews | Cancer Quiescent normal cell
a signalling pathways, even in a less than ideal microen-vironment 49. several oncogenic mutations and signal-ling pathways can suppress AMPK signalling 49, which uncouples fuel signals from growth signals, allowing tumour cells to divide under abnormal nutrient condi-tions. This uncoupling permits tumour cells to respond to inappropriate growth signalling pathways that are
activated by oncogenes and the loss of tumour sup-pressors. Accordingly, many cancer cells exhibit a loss of appropriate AMPK signalling: an event that may also contribute to their glycolytic phenotype.
Given the role of AMPK, it is not surprising that STK11, which encodes liver kinase b1 (lKb1) — the upstream kinase necessary for AMPK activation — has been identified as a tumour suppressor gene and is mutated in Peutz–Jeghers syndrome 51. This syndrome is characterized by the development of benign gastro-intestinal and oral lesions and an increased risk of developing a broad range of malignancies. lKb1 is also frequently mutated in sporadic cases of non-small-cell lung cancer 52 and cervical carcinoma 53. recent evidence suggests that lKb1 mutations are tumorigenic owing to the resulting decrease in AMPK signalling and loss of mTOr inhibition 49. The loss of AMPK signalling allows the activation of mTOr and HIF1, and therefore might also support the shift towards glycolytic metabolism. Clinically, there is currently considerable interest in eval-uating whether AMPK agonists can be used to re-couple fuel and growth signals in tumour cells and to shut down cell growth. Two suc
h agonists are the commonly used antidiabetic drugs metformin and phenformin 49,54–56. It remains to be seen whether these agents represent a useful class of metabolic modifiers with antitumour activity.p53 and OCT1. Although the transcription factor and tumour suppressor p53 is best known for its functions in the DnA damage response (DDr) and apoptosis, it is becoming clear that p53 is also an important regulator of metabolism 57. p53 activates the expression of hexokinase 2 (HK2), which converts glucose to glucose-6-phosphate (G6P)58. G6P then either enters glycolysis to produce ATP , or enters the pentose phosphate pathway (PPP), which supports macromolecular biosynthesis by produc-ing reducing potential in the form of reduced nicotina-mide adenine dinucleotide phosphate (nADPH) and/or ribose, the building blocks for nucleotide synthesis. However, p53 inhibits the glycolytic pathway by upreg-ulating the expression of TP53-induced glycolysis and apoptosis regulator (TIGAr), an enzyme that decreases the levels of the glycolytic activator fructose-2,6- bisphosphate 59 (FIG. 2). Wild-type p53 also supports the expression of PTen, which inhibits the PI3K pathway, thereby suppressing glycolysis (as discussed above)60. Furthermore, p53 promotes oxidative phosphorylation by activating the expression of sCO2, which is required for the assembly of the cytochrome c oxidase complex of the electron transport chain 61. Thus, the loss of p53 might also be a major force behind the acquisition of the glycolytic phenotype.
OCT1 (also known as POu2F1) is a transcription factor, the expression of which is increased in several human cancers, and it may cooperate with p53 in regu-lating the balance between oxidative and glycolytic metabolism 62–64. The transcriptional programme that is initiated by OCT1 supports resistance to oxidative stress and this may cooperate with the loss of p53 during trans-formation 64. Data from studies of knockout mice and human cancer cell lines show that OCT1 regulates a set
(part a ) the shift to aerobic glycolysis in tumour cells (part b ) is driven by multiple
oncogenic signalling pathways. PI3K activates AKT , which stimulates glycolysis by directly regulating glycolytic enzymes and by activating mTOR. The liver kinase B1 (LKB1) tumour suppressor, through AMP-activated protein kinase (AMPK) activation, opposes the glycolytic phenotype by inhibiting mTOR. mTOR alters metabolism in a variety of ways, but it has an effect on the glycolytic phenotype by enhancing hypoxia-inducible factor 1 (HIF1) activity, which engages a hypoxia-adaptive transcriptional programme. HIF1
increases the expression of glucose transporters (GLUT), glycolytic enzymes and pyruvate dehydrogenase kinase, isozyme 1 (PDK1), which blocks the entry of pyruvate into the tricarboxylic acid (TCA) cycle. MYC cooperates with HIF in activating several genes that encode glycolytic proteins,
but also increases mitochondrial metabolism. The tumour suppressor p53 opposes the glycolytic phenotype by suppressing glycolysis through TP53-induced glycolysis and apoptosis regulator (TIGAR), increasing mitochondrial
metabolism via SCO2 and supporting expression of PTEN. OCT1 (also known as POU2F1) acts in an opposing manner to activate the transcription of genes that drive glycolysis and suppress oxidative phosphorylation. The switch to the pyruvate kinase M2 (PKM2) isoform affects glycolysis by slowing the pyruvate kinase reaction and diverting substrates into alternative biosynthetic and reduced nicotinamide adenine dinucleotide phosphate (NADPH)-generating pathways. MCT , monocarboxylate transporter; PDH, pyruvate dehydrogenase. The dashed lines indicate loss of p53 function.
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Pentose phosphate pathway
PPP . Biochemical pathway converting glucose into substrates for nucleotide
biosynthesis and redox control, such as ribose and NADPH. Owing to multiple connections to the glycolytic pathway, the PPP can operate in various modes to allow the production of NADPH and/or ribose as required.
Macromolecular biosynthesis
Biochemical synthesis of the carbohydrates, nucleotides, proteins and lipids that make up cells and tissues. These pathways require energy, reducing power and appropriate substrates.
Reduced nicotinamide adenine dinucleotide phosphate
NADPH. Cofactor that drives anabolic biochemical reactions and provides reducing capacity to combat oxidative stress.
reactive oxygen species名词解释of genes that increase glucose metabolism and reduce mitochondrial respiration. One of these genes encodes an isoform of PDK (PDK4) that has the same function as the PDK enzymes that are activated
by HIF1 (ReF . 64) (FIG. 2). Although the mechanisms by which OCT1 is upregulated in tumour cells are poorly understood, its downstream effectors may be potential targets for therapeutic intervention.
Beyond the Warburg effect Metabolic adaptation in tumours extends beyond the Warburg effect. It is becoming clear that alterations to metabolism balance the need of the cell for energy with
its equally important need for macromolecular building blocks and maintenance of redox balance.
Pyruvate kinase (PK). As previously discussed, the gen-eration of energy in the form of ATP through aerobic glycolysis is required for unrestricted cancer cell pro-liferation 7. However, studies of the M2 isoform of PK (PKM2) have shown that ATP generation by aerobic glycolysis is not the sole metabolic requirement of a cancer cell, and that alterations to metabolism not only bolster ATP resources but also stimulate macromolecular biosynthesis and redox control.
PK catalyses the rate-limiting, ATP-generating step of glycolysis in which phosphoenolpyruvate (PeP) is con-verted to pyruvate 65. Multiple isoenzymes of PK exist in mammals: type l, which is found in the liver and kid-neys; type r, which is expressed in erythrocytes; type M1, which is found in tissues such as muscle and brain; and type M2, which is present in self-renewing cells such as embryonic and adult stem cells 65. Intriguingly, PKM2 is also expressed by many tumour cells. Furthermore, it was discover
ed that although PKM1 could efficiently promote glycolysis and rapid energy generation, PKM2 is characteristically found in an inactive state and is
ineffective at promoting glycolysis 66–68
.
This observation was ignored by the scientific com-munity for several years owing to its shear counterin-tuitive nature: a tumour-specific glycolytic enzyme that inhibits ATP generation and antagonizes the Warburg effect. Only on closer examination of the full metabolic requirements of a cancer cell was the advantage of PKM2 expression revealed. A cancer cell, like any normal cell, must obtain the building blocks that are required for the synthesis of lipids, nucleotides and amino acids. Without sufficient precursors available for this purpose, rapid cell proliferation will halt, no matter how vast a supply of ATP is present. PKM2 provides an advantage to cancer cells because, by slowing glycolysis, this isozyme allows carbohydrate metabolites to enter other subsidiary pathways, including the hexosamine pathway, uridine diphosphate (uDP)–glucose synthesis, glycerol syn-thesis and the PPP, which generate macromolecule precursors, that are necessary to support cell prolif-eration, and reducing equivalents such as nADPH 4,28,69 (FIG. 3). subsequent studies have confirmed that PKM2
expression by lung cancer cells confers a tumorigenic advantage over cells expressing the PKM1 isoform 70. Interestingly, the classical oncoprotein MyC has been
found to promote preferential expression of PKM2 over PKM1 by modulating exon splicing. The inclusion of exon 9 in the PK mrnA leads to translation of the PKM1 isoform, whereas inclusion of exon 10 produces PKM2 (ReF. 71). MyC upregulates the expression of heteroge-neous nuclear ribonucleoproteins (hnrnPs) that bind to exon 9 of the PK mrnA and lead to the preferen-tial inclusion of exon 10 and thus to the predominant production of PKM2. by promoting PKM2 expression, MyC promotes the production of nADPH in order to match the increased ATP production and to satisfy the auxiliary needs required for increased proliferation.At the clinical level, increased PKM2 expression has been documented in patient samples of various cancer types, leading to the proposal that PKM2 might be a use-ful biomarker for the early detection of tumours 65,72–74. However, further study of the prevalence of PKM2 in cancers and the effect of PKM2 on tumorigenesis is still required.
NADPH. A key molecule produced as a result of the promotion of the oxidative PPP by PKM2 is nADPH (FIG. 4). nADPH functions as a cofactor and provides reducing power in many enzymatic reactions that are crucial for macromolecular biosynthesis. Although other metabolites are produced as a result of increased PPP activity, including ribose, which can be converted
M2 (PKM2) is present in very few types of proliferating normal cells but is present at high levels in cancer cells. PKM2 catalyses the rate-limiting step of glycolysis,
controlling the conversion of phosphoenolpyruvate (PEP) to pyruvate, and thus ATP generation. Although
counterintuitive, PKM2 opposes the Warburg effect by inhibiting glycolysis and the generation of ATP in tumours. Although such an effect might at first seem to be
detrimental to tumour growth, the opposite is true. By slowing the passage of metabolites through glycolysis, PKM2 promotes the shuttling of these substrates through the pentose phosphate pathway (PPP) and other
alternative pathways so that large quantities of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and other macromolecules are produced. These molecules are required for macromolecule biosynthesis and the maintenance of redox balance that is needed to support the rapid cell division that occurs within a tumour. G6P , glucose-6-phosphate.
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