It has almost become a truism to say that cancer cells have acquired distinctive characteristics that distinguish them from their normal counterparts, but it is worth remembering that among the very first of these differ-ences to be recognized were the changes in tumour cell metabolism. Early oncologists noted dramatic altera-tions in the way malignant cells organize catabolic and anabolic processes. For example, glucose uptake was found to be much higher in tumours than in most nor-mal tissue, and the persistence of glycolysis even under normal aerobic conditions led Otto Warburg to propose that these metabolic changes were at the heart of cancer development — leading to, rather than resulting from, malignant transformation1. In the intervening years, as the importance of oncogenes, tumour suppressors, proliferation and apoptosis was unravelled, the role of metabolism in cancer development became somewhat sidelined, with the general feeling that the metabolic changes were simply a by-product of malignant trans-formation2. However, an increasing understanding of the molecular mechanisms that control metabolism has led to a resurgence of interest in this topic, along with a growing realization that metabolic transforma-tion can have a crucial role in the maintenance of the tumorigenic state.

The importance of metabolism in cancer has revived enthusiasm for the study of how these pathways are con-trolled, revealing some interesting contributions from well-known oncoproteins and tumour suppressor pro-teins. Among these is p53, one of the most important defenders against tumour development, which is now also emerging as an important player in the response to and the regulation of metabolic stress. Compared with our understanding of the functions of p53 in con-trolling cell cycle progression and apoptosis, this is a new and burgeoning area of influence for p53, with some confusing and apparently contradictory results

highlighting gaps in our understanding. However, these activities of p53 are indisputably interesting and prob-ably extremely important. Furthermore, it is clear that the role of p53 in responding to and effecting altera-tions in metabolism will have consequences beyond cancer, influencing various other aspects of disease and normal life (TABLE 1).

Metabolic changes in cancer developmentMetabolic pathways in normal cells are tightly regu-lated to allow cell growth or survival, depending on the conditions. Nutrient availability supports the synthesis of proteins, lipids and nucleic acids for cell growth and proliferation, whereas starvation triggers a series of responses to restrict cell proliferation, maximize energy production (by switching to the breakdown rather than the synthesis of macromolecules) and help cell survival. One important node in these responses is mTOR (also known as FRAP), which promotes protein synthesis and suppresses the induction of autophagy (a mechanism that can mobilize alternative energy sources and is dis-cussed in more detail below). Key regulators of mTOR are AKT, which is stimulated by growth factors to acti-vate mTOR, and AMP-activated protein kinase (AMPK), which responds to an increased AMP/ATP ratio under conditions of low energy to repress mTOR (FIG. 1). These growth regulatory cascades intersect with the metabolic pathways that control energy production and biosynthe-sis, in which AKT can promote the anabolic, energy-consuming pathways (such as fatty acid synthesis) that are necessary for cell growth3 and AMPK drives the catabolic, energy-producing responses (such as fatty acid oxidation) that are needed under conditions of metabolic stress4. Layered over this complexity are the pathways that regulate cell division and survival; virtu-ally all parts of this intricate network can be profoundly perturbed in cancer cells.

The Beatson Institute for Cancer Research, Garscube Estate, Glasgow G61 1BD, UK. e-mails: k.vousden@beatson.gla.ac.uk; k.ryan@beatson.gla.ac.ukdoi:10.1038/nrc2715Published online 17 September 2009

GlycolysisThe stepwise pathway that converts glucose into pyruvate with the net generation of two molecules of ATP.

AutophagyLiterally translated from greek as ‘self eating’. The cellular trafficking process whereby cytoplasmic constituents are targeted to lysosomes for degradation.

p53 and metabolismKaren H. Vousden and Kevin M. Ryan

Abstract | Although metabolic alterations have been observed in cancer for almost a century, only recently have the mechanisms underlying these changes been identified and the importance of metabolic transformation realized. p53 has been shown to respond to metabolic changes and to influence metabolic pathways through several mechanisms. The contributions of these activities to tumour suppression are complex and potentially rather surprising: some reflect the function of basal p53 levels that do not require overt activation and others might even promote, rather than inhibit, tumour progression.

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HypoxiaA decrease in ambient O2 availability and levels.

Oxidative stressThe accumulation of ROS owing to increased production, the inability of the cell to counter ROS production or both.

GlutaminolysisThe metabolic pathway that breaks down glutamine.

The importance of metabolic transformation for tumour progression is a topic that has been covered by many outstanding reviews (for example REFS 5–10). As already mentioned, one of the most obvious changes is the shift to aerobic glycolysis. In most normal cells, the tricarboxylic acid (TCA) cycle drives the generation of ATP in the presence of O2, a process known as oxi-dative phosphorylation. However, under conditions of limiting O2 or when energy is needed rapidly, glycolysis becomes the preferred route of energy production. This seems to reflect the observation that although oxidative phosphorylation is a more efficient process (producing 36 molecules of ATP per molecule of glucose, compared with only 2 ATP molecules for glycolysis), glycolysis can produce ATP at a higher rate than oxidative phosphory-lation11. The predilection of cancer cells to use glycolysis may partly reflect a response to hypoxia, which occurs as the tumour outgrows the blood supply. However, can-cer cells use glycolysis even under normoxic conditions, thereby adopting a metabolic programme that favours fast rather than efficient energy production and leads to the extremely high rate of glucose uptake seen in most malignant tumours.

but how do these changes in metabolism help cancer development? Malignant progression inflicts a spiral-ling number of demands on the incipient tumour cell, including increased energy requirements, the produc-tion of macromolecules to support tumour cell growth, and the need to survive under hostile conditions that include a poor O2 supply and increased oxidative stress. Alterations in metabolism can help to ameliorate each of these potential obstructions to tumour development and, although there is still some debate as to whether such changes can be a founding cause of cancer development, there is compelling evidence that cancer cells become dependent on these changes for continued growth and survival (for example see REFS 12–15). Indeed, it is pos-sible that alterations in metabolism contribute to all the major hallmarks of cancer cell behaviour6 and several specific advantages for the cancer cell are associated with reprogrammed metabolism7,16. The recurring theme seems to be that the increase in glycolysis at the expense

of mitochondrial energy production helps to support unbridled growth, provide precursors for the biosyn-thesis of macromolecules and protect cells from exces-sive and toxic levels of reactive oxygen species (ROs) and oxidative stress. In addition to increased glycolysis, cancer cells show increased use of glutamine. This is not only another route for energy production but also pro-vides an important anapleurotic mechanism for replen-ishing the TCA cycle intermediates that are necessary precursors for the anabolic processes required for cancer cell growth17,18.

Accompanying our increased appreciation of the importance of metabolic transformation is a growing understanding of the complexity of the mechanisms that underlie these changes. The reduced dependence on oxidative phosphorylation for energy production shown by cancer cells is not generally due to a defect in components of the TCA cycle or the electron transport chain, but reflects an ability of proteins associated with oncogenic transformation to promote glycolysis. These include not only AKT, which is frequently activated in human cancers through various mechanisms19, but also other oncoproteins associated with deregulated prolif-eration (such as MYC) or the response to O2 starvation (such as hypoxia-inducible factor (HIF))20–22. In addition to increasing the expression of numerous enzymes in the glycolytic cascade, MYC has recently been shown to promote glutaminolysis23,24. by contrast, several tumour suppressor proteins oppose the metabolic pathways that contribute to cell growth and proliferation. These include PTEN (which inhibits AKT), tuberous sclerosis 1 (TsC1) and TsC2 (negative regulators of mTOR), and liver kinase b1 (LKb1, also known as sTK11; an activa-tor of AMPK). Taken together, metabolic remodelling is likely to be a key requirement for the success of a cancer cell, and understanding the mechanisms that might con-tribute to or oppose such changes will be crucial for the development of new treatments for malignant disease. It is within this context that we consider some newly described activities of p53.

Activation of p53 by metabolic stressThe role of p53 as a central component of the stress response machinery is well established, and numerous forms of stress — many of which are encountered during malignant transformation — lead to the activation of p53 (REF. 25). In this way acute or persistent hazards, such as genotoxic damage or the activation of proliferative onco-genes, can trigger p53-mediated senescence or cell death to ensure that the damaged cell cannot persist. However, under conditions of low or basal stress, p53 can also help to prevent or repair damage — functions that are associated with an ability of p53 to promote cell survival26.

virtually any stress signal, whether extrinsic or intrinsic to the cell, can activate p53, so it is not surpris-ing that the metabolic responses to limited nutrient, energy or O2 availability can also involve p53 (FIG. 1). As mentioned above, reduced nutrient or energy levels result in a failure to stimulate the AKT–mTOR path-way and in the activation of AMPK, both of which can lead to the induction of p53. AKT activates MDM2,

At a glance

•Metabolicalterationsarecommonfeaturesofcancercellsandhaverecentlybeenshowntohaveanimportantroleinthemaintenanceofmalignancies.

•p53isakeytumoursuppressorproteinthathasadiverserangeoffunctions—includingtheabilitytopromoteapoptosis,senescenceandDNArepair—eachofwhichhelpstopreventcancerdevelopment.Aroleforp53inregulatingmetabolicpathwayshasalsorecentlybeenidentified,suggestingthatthisisanothermechanismbywhichp53helpstostallmalignantprogression.

•Severalfunctionsofp53promoteoxidativephosphorylationanddampenglycolysisincells;disruptionofthisbalanceisassociatedwithmutationsinp53andoncogenictransformation.

•p53alsohasakeyroleinregulatingcellgrowthandautophagy,therebyhelpingtocoordinatethecell’sresponsetonutrientstarvation.

•Alteredmetabolismcancontributetomalignanttransformation,andcancercellsbecomedependentonthesechanges.Understandingtheroleofp53intheregulationofmetabolismmayprovidesomeinterestingpotentialtargetsforthedevelopmentofnewcancertherapies.

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a ubiquitin ligase that mediates the degradation and therefore inhibition of p53; a reduction of AKT func-tion (such as through the action of PTEN) would acti-vate p53 by removing the negative regulation normally imposed by MDM2 (REF. 27). Interestingly, in some cells this response can be amplified by the ability of p53 to activate the expression of PTEN28. Conversely, activa-tion of AMPK leads to the induction of p53 both by increasing transcription of TP53 (the human gene that encodes p53) and through direct phosphorylation that stabilizes p53 (REFS 29,30). Nucleocytoplasmic malate dehydrogenase (a metabolic enzyme that regu-lates the TCA cycle by controlling the conversion of malate to oxaloacetic acid) has also been shown to bind to and activate p53 in response to glucose starvation31. In addition, ADP and ATP can directly modulate the ability of p53 to bind DNA, with ADP promoting and ATP inhibiting this interaction32. This provides another mechanism through which low energy can signal to induce a p53 response. Interestingly, although mTOR signalling can inhibit p53 through the action of a phosphatase that dephosphorylates p53 (REF. 33), constitutive activation of mTOR signalling (as occurs in cancers through increased AKT activity or loss of the tumour suppressors TsC1 and TsC2) has also been shown to activate p53 through increased trans-lation34. These results indicate that a lack of nutrients and excessive or deregulated signalling through the nutrient-sensing pathways can each activate a p53 response and that combinations of these abnormali-ties during tumour progression amplify the protective p53 response. However, increased glucose metabolism stimulated by the expression of the glucose transporter GLuT1 or hexokinase has also been reported to sup-press p53 activity35, suggesting that the high levels of glycolysis seen in many cancers may help evade the tumour-suppressive effects of p53.

Decreased O2 availability is another stress associated with solid tumours, so it is no surprise that hypoxia acti-vates p53 and that p53 is an important component of hypoxic responses that often invoke programmed cell death. However, the way in which hypoxia signals to p53 is complex and there are conflicting reports as to how a lack of available O2 increases p53 activity. In the simplest scenario it has been shown that hypoxia causes a decrease in MDM2 levels, thereby alleviating the autoregulatory negative feedback on p53 (REF. 36). Responses to hypoxia involve the HIF transcription factor; the induction of p53 in response to low O2 can also involve crosstalk with HIF and may even involve direct interaction at very low O2 tensions37. However, at moderately low O2 tensions HIF is induced without p53 stabilization, indicating that the stabilization of HIF and p53 in response to hypoxia is not necessarily epistatically linked. p53 activation in hypoxia usually requires more acute (near anoxic) con-ditions, and therefore it has been proposed that p53 does not respond to hypoxia per se but rather to the effects of hypoxia, such as DNA damage or the nutrient depriva-tion that is associated with the poorly vascularized state of many tumours38,39.

It seems very likely that there are further mechanisms that signal to p53 in response to metabolic abnormalities. For example, p53 is efficiently activated by ribosomal stress through the ability of several ribosomal proteins to bind and inactivate MDM2 (REF. 40), a response that might be induced by alterations in cell growth that result in perturbations of ribosome biogenesis. Oxidative stress can also promote the activation of p53 by inducing DNA damage, or can regulate p53 function by directly affecting the redox state41 and oxidation of p53 (REF. 42). Indeed, in some cells mitochondrial ROs were found to be an important component of the stress-induced acti-vation of p53 (REF. 43). The most obvious advantage of p53 activation under conditions of metabolic stress is a

Table 1 | Contribution of p53 to aspects of health and disease besides cancer

Phenotype Observation(s) refs

Ageing Contributions to both ageing and longevity have been reported — this may reflect regulation of ROS

112–114

Development As shown by exencephaly in Trp53-deficient mouse models and evidence from other animal models

115–117

Regulation of stem cells

A role of p53 has been demonstrated in controlling self-renewal and quiescence in adult stem cells

118,119

Endurance Promotion of aerobic respiration by p53 is important for endurance during exercise 52

Fecundity p53 is an important activator of leukaemia inhibitory factor, which is required for implantation of blastocytes

120,121

Sun tanning p53 induces the expression of pro-opiomelanocortin 122

Neurodegeneration Roles for p53 reported in models of Alzheimer’s, Parkinson’s and Huntington’s disease 123–125

Ischaemia Inhibition of p53 can be protective in models of stroke, myocardial infarction, kidney ischaemia and reperfusion injury

126–129

Ribosomal syndromes

Role for p53 in Diamond-Blackfan anaemia, Treacher Collins syndrome and dark skin 130–132

Diabetes p53 senescence activity contributes to the development of insulin resistance, and additional functions for p53 in diabetes also seem possible, given the role of p53 in regulating metabolism and autophagy

133

ROS, reactive oxygen species.

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Starvation and nutrientdeprivation

AKT

MDM2AMPK

AMP

LKB1

mTOR

Autophagy

Growth Sestrins

TSC1 andTSC2

p53p53

p53p53

Mitogens andgrowth factors

p21 Proliferation

PTEN

CatabolismThe metabolic breakdown of relatively complex molecules into simpler parts.

coordinated inhibition of cell proliferation and growth, as p53 can arrest both of these processes. but additional consequences of p53 activation are being uncovered, including an ability of p53 to promote the use of certain metabolic pathways and to increase cell survival.

Power play: p53 and energy productionseveral studies have shown that p53 has a role in the reg-ulation of both glycolysis and oxidative phosphorylation (FIG. 2). several mechanisms have now been described through which p53 can slow glycolysis and therefore counteract the increase in glycolysis that is characteris-tic of cancers. p53 can inhibit the expression of the glu-cose transporters GLuT1 and GLuT4 (REF. 44) and can decrease the levels of phosphoglycerate mutase (PGM)45 while increasing the expression of TIGAR46. The effect of each of these is to impede flux through various steps of the glycolytic pathway. p53 also indirectly regulates gly-colysis by modulating the nuclear factor-κb (NF-κb) pathway47. Expression of p53 can limit the activity of Iκb kinase-α (IKKα) and IKKβ, thereby restricting the activation of NF-κb and dampening the expression of glycolysis-promoting genes such as GLUT3. The exact mechanism by which p53 functions in this pathway is not clear but relates to the ability of p53 to oppose the activating O-linked β-N-acetyl glucosamine modifica-tion of IKKβ48. The restraint on glycolytic rate imposed by p53 is paralleled by the ability of p53 to help maintain mitochondria49,50 and drive oxidative phosphorylation. These effects are likely to be the consequence of several p53-dependent functions, including the transcriptional activation of subunit I of cytochrome c oxidase51; acti-vation of expression of synthesis of cytochrome c oxi-dase 2 (sCO2)52, a key regulator of the cytochrome c

oxidase complex; and the induction of expression of the ribonucleotide reductase subunit p52R2, a protein that contributes to the maintenance of mitochondrial DNA53. The ability of p53 to promote oxidative phosphorylation is also demonstrated by the effect of reducing expres-sion of cytoplasmic polyadenylation element-binding protein (CPEb), a protein that increases translation of mRNA by promoting polyadenylation54. One of the targets of CPEb is TP53 mRNA, and cells with reduced CPEb levels express only half the normal levels of p53. Intriguingly, although this does not affect overall ATP production, the reduction of p53 expression is accom-panied by a switch from oxidative phosphorylation to glycolysis54. Interestingly, several of these p53 activities, such as the regulation of sCO2 and IKK, seem to func-tion under normal growth conditions in the absence of an acute stress signal, suggesting that p53 can help to maintain the aerobic respiration that is characteristic of most normal cells.

However, making sense of the regulation of metabolic pathways by p53 is confounded by other studies showing apparently completely opposing activities: for example, the presence of p53-responsive elements in the pro-moters of PGM55 and hexokinase II (HK2)56,57 suggests that p53 can promote at least some steps in glycolysis (FIG. 2). Although these activities could result in increased survival signalling by helping to limit ROs production (as discussed below), it is difficult to reconcile an abil-ity of p53 to decrease glucose transport with an ability to increase the next step of glucose metabolism in the same system. It seems likely that these different activi-ties reflect, at least to some extent, context- or tissue-dependent differences in the metabolic functions of p53. For example, although p53-dependent transcriptional activation of PGM seems to contribute to muscle cell differentiation55, p53 downregulates PGM activity in fibroblasts45. Furthermore, it is clear that p53 can play an important part in helping cells adapt to and survive ener-getic stress by promoting catabolism. Notwithstanding the role of p53 in enhancing oxidative phosphorylation (as discussed above), increased glycolysis in response to pharmacological inhibition of oxidative phosphorylation has also been shown to be dependent on p53 (REF. 58). Which genes are regulated by p53 can also depend on the tissue or cell type (an effect that has also been noted in the participation of p53 in the AKT–mTOR pathway59), introducing a further level of complexity.

Weight-watchers: p53 and cell growthAlthough p53-mediated inhibition of proliferation is an important facet of the response to nutrient depriva-tion, more recently several activities of p53 that directly inhibit cell growth (meaning an increase in biomass) have also been described (FIG. 1). These include the abil-ity of p53 to directly activate the expression of several components of the AMPK pathway, including AMPK itself and TsC2 (REF. 60), and the p53-dependent acti-vation of the sestrins, which interact with and activate AMPK directly61. These activities suggest that p53 has a key role in coordinating the cessation of proliferation and growth under times of starvation62. The intimacy of

Figure 1 | nutrient deprivation signals to p53. The activation of p53 in response to a lack of nutrients signals through the activation of AMP-activated protein kinase (AMPK) and the inhibition of AKT. p53 further induces AMPK (both directly and indirectly through the sestrins) and activates the expression of tuberous sclerosis 2 (TSC2), resulting in the inhibition of mTOR. This leads to a decrease in cell growth, which coordinates with the inhibition of proliferation that is also mediated through the activation of p53, together with liver kinase B1 (LKB1). This pathway also contributes to p53-mediated activation of autophagy (FIG. 3).

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p53p53

p53p53

NF-κB

IKK

Glucose

Pyruvate

SCO2

NADH

NAD+ Oxidative phosphorylation

NADHdehydrogenase

Cytochromeoxidoreductase

Cytochrome coxidase

Glycolysis

Hexokinase

TIGAR

PGM

Cytoplasm

GLUT1 GLUT4 GLUT3

O2

TCAcycle

ATP

C

H2O

NecrosisA less ordered form of cell death that is characterized by cell rupture and, in an organismal setting, an inflammatory response.

this coordination is further increased by the observation that LKb1, which mediates the activation of AMPK in response to starvation, also cooperates with p53 to acti-vate the expression of p21, the principal effector of p53-mediated cell cycle arrest63 (FIG. 1). The presence of p53 is also necessary for the AMPK-dependent activation of fatty acid catabolism through β-oxidation, providing another mechanism by which p53 can help cells endure nutrient deprivation58.

In addition to decreasing cell growth, p53-dependent inhibition of mTOR signalling promotes autophagy — another response that might help the cell survive short-term nutrient deprivation. However, further studies have suggested a much deeper complexity to the role of p53 in the regulation of the autophagic response, as discussed below.

Eat me: p53 and autophagyAutophagy (or more strictly the form of autophagy termed macroautophagy, but hereafter referred to as autophagy) is a membrane trafficking process that mediates the delivery of cytoplasmic constituents to the lysosome for degradation64. under basal conditions autophagy operates in most cells as a homeostatic mech-anism to monitor the integrity of long-lived proteins and organelles65. On lysosomal degradation, the compo-nents of the cargo, such as amino acids and fatty acids, can then be either further catabolized or recycled into biosynthetic pathways64,65. The rate of autophagic deg-radation and the specific cargo can change in response to various forms of cellular stress. For example, as first defined in yeast, autophagy can be induced in response to nutrient deprivation and provides a self-limited supply of ATP that can prolong viability or bridge hiatuses of nutrient supply66. Therefore, it is easy to see

how the promotion of autophagy could be oncogenic in the environment of a poorly vascularized (and so nutrient-deprived) tumour. In contrast to the role of autophagy in promoting cell survival, reports have also indicated that autophagy may promote cell death in cer-tain settings either when apoptosis is compromised or when it is activated in combination with other signals, such as classic apoptotic signals involving caspases67–69. supporting this, evidence from human cancer samples and from animal models with deficiencies in crucial autophagy genes indicates that autophagy also has a potent tumour-suppressive role during tumorigene-sis70–72. Put simply, this may reflect a role for autophagy in the promotion of cell death as outlined above. It has also been postulated, however, that the generation of ATP through autophagy may be tumour suppressive by preventing necrosis and the associated immunological response that could exacerbate tumour development73. Inhibition of autophagy during metabolic stress has also been shown to promote DNA damage74, and a separate study has reported an essential role for autophagy in oncogene-induced senescence75.

Owing to reports of autophagy modulation during tumour development and in the control of meta-bolic stress, it is perhaps no surprise that several reports have now indicated that p53 can directly acti-vate autophagy62,76–78 (FIG. 3). However, the control of autophagy by p53 is both complex and often context specific. As described above, the activation of AMPK by p53 and the subsequent inhibition of mTOR can result in the induction of autophagy62. However, the whole story is less simple and other p53 target genes have also been shown to have a role. Damage-regulated autophagy modulator (DRAM) is a lysosomal protein induced by p53 that positively regulates autophagy. Although this protein does not induce cell death when expressed alone, it seems to be crucial for an effective death response that has both autophagic and apop-totic components76,79. Other p53 target genes, such as bCL2-associated X protein (Bax) and p53-upregulated modulator of apoptosis (PUMa) have also recently been shown to be positive regulators of autophagy80, and (as with DRAM) this response seems to contribute to the induction of apoptosis. Interestingly, both DRAM and bAX have been shown to be perturbed in human can-cer76,81, but whether they are targeted owing to their role in apoptosis, autophagy or both is yet to be determined. Moreover, in the broader sense, understanding the way in which autophagy contributes to cell death and how and when this is relevant to tumour development are clearly areas that require further investigation.

In addition to promoting autophagy, p53 is an inhibi-tor of this process78. In contrast to the induction of autophagy that has been reported in many contexts fol-lowing activation of p53, the ability to inhibit autophagy is a facet of basal levels of p53. This may therefore be a role for p53 in the control of autophagy that is distinct from that in tumour settings, in which there are many cellular stresses and oncogenic signals that can result in the activation of p53. Nonetheless, depletion of p53 either in vitro by RNA interference or pharmacological

Figure 2 | regulation of energy production by p53. Several functions of p53 reduce the flux through the glycolytic pathway and increase oxidative phosphorylation, thereby opposing the Warburg effect, in which cancer cells predominantly use glycolysis for energy production. However, there are also activities of p53, such as the activation of hexokinase and phosphoglycerate mutase (PGM), which could increase glycolysis under some circumstances. GLUT, glucose transporter; IKK, IκB kinase; NF-κB, nuclear factor-κB; SCO2, synthesis of cytochrome c oxidase 2; TCA, tricarboxylic acid.

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Mt-p53

Basalp53

DNA damage andcellular stress

DRAMSESN2BAXPUMA

Nucleus

Biosynthesis

Amino acidsFatty acids

ATP

p53p53

p53p53

Autophagosome

Autolysosome

MitophagyThe selective degradation of mitochondria by autophagy.

inhibition in cell lines or in vivo by genetic deletion of p53 in mice and Caenorhabditis elegans was found to cause an increase in autophagy78. The increased activa-tion of autophagy promoted the survival of p53-deficient cells under conditions of nutrient deprivation or low O2. Whether the autophagy induced by loss of p53 contrib-utes to the tumour-prone nature of p53-null animals is yet to be resolved, but crosses of Trp53–/– animals with animals conditional for loss of essential autophagy genes, such as atg5 or atg7, would certainly be interesting.

The mechanisms by which p53 inhibits autophagy also seem to be different from how it activates autophagy. In this context, the activation of target genes, such as DRaM or SESN2 (which also affects autophagy through mTOR regulation)61,76,82, does not seem to be a component of the response. Instead, basal levels of p53 regulate autophagy directly at the endoplasmic reticu-lum78,79. Loss of p53 in this context causes endoplasmic reticulum stress, presumably through the accumulation of misfolded proteins, and the cells respond by direct-ing selective autophagy of the endoplasmic reticulum — known as reticulophagy — to effect their removal. Interestingly, the observed effect is not solely associ-ated with wild-type p53, as several tumour-associated mutants, including gain-of-function mutants, can also inhibit autophagy83. For both mutant and wild-type p53, the crucial feature for this activity is subcellular localization, with only cytoplasmic p53 inhibiting autophagy.

The increased autophagic response to loss of p53 might also be related to an increase in the expression of ARF, a protein with disparate activities in regulating proliferation, the expression of which is negatively regu-lated by p53 (REF. 84). Although the best known function of ARF is to stabilize p53 (REF. 85), a recent study has

shown that ARF can induce autophagy by inhibiting the interaction of bCL-XL with beclin 1, a positive regulator of autophagy86.

Even in its infancy, the study of the ability of p53 to regulate autophagy and how this relates to the regula-tion of tumour metabolism raises many issues. First, one has to question whether the reports of both posi-tive and negative effects of p53 on autophagy can be reconciled. Certainly the two effects are mechanisti-cally distinct and perhaps the difference, as outlined above, merely reflects the ability of p53 to promote cell survival during low levels of stress or damage and its ability to induce cell death when the damage is more severe. One would predict that the autophagic cargoes in these two contexts would be distinct, and it is noteworthy that the activation of autophagy in response to p53-mediated upregulation of PuMA and bAX seems to be predominantly mitophagy rather than reticulophagy80. The role of mutant p53 in the control of autophagy is also intriguing. Many tumour-derived mutants of p53 are highly penetrant oncogenes, rather than simply mutants with loss of wild-type p53 func-tions87,88; therefore, some mutants may be more onco-genic than others owing to differences in their ability to regulate autophagy. It is also possible that wild-type p53 represses autophagy through a different mechanism than mutant p53. The function of p53 mutants may reflect their ability to bind and inhibit p73, which is a p53-related protein that can also drive an autophagic response89,90. Clearly more studies are required to answer these questions, particularly regarding com-binatorial targeting to activate p53 and to modulate autophagy either positively or negatively for tumour therapy.

Radical solutions: p53 and oxidative stressOur recent appreciation of the importance of basal levels of p53 in regulating metabolic pathways is mirrored by the clear importance of the antioxidant functions of low or uninduced levels of p53 (REF. 91). Indeed, the regula-tion of metabolism and oxidative stress by p53 is often a consequence of the same activities (FIG. 4). The sestrin proteins, the expression of which is regulated by p53, not only activate AMPK to regulate growth and autophagy but also function as antioxidants, protecting cells from hydrogen peroxide-induced damage92. p53-dependent activation of glutathione peroxidase93, aldehyde dehy-drogenase94 and tumour protein p53-inducible nuclear protein 1 (TP53INP1)95 provides further antioxidant functions. Regulation of the glycolytic pathway by p53 can also help to modulate oxidative stress by increasing flux through the pentose phosphate pathway, an alterna-tive route for glucose metabolism. The activation of both TIGAR and a form of glucose-6-phosphate dehydroge-nase — hexose-6-phosphate dehydrogenase (H6PD)46,96 — would promote the pentose phosphate pathway, lead-ing to the generation of NADPH for use in both anabolic and antioxidant pathways.

Despite these antioxidant activities of p53, the role of p53 in the regulation of oxidative stress is complex and p53 can also show strong pro-oxidative effects (FIG. 4).

Figure 3 | regulation of autophagy by p53. Several transcriptional targets of p53 can promote autophagy, a response that has a tumour-suppressive role. However, basal levels of p53 function directly in the cytoplasm to inhibit autophagy, an activity that is shared by cytoplasmic tumour-derived p53 mutants (mt-p53). BAX, BCL2-associated X protein; DRAM, damage-regulated autophagy modulator; PUMA, p53-upregulated modulator of apoptosis; SESN2, sestrin 2.

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p53p53

p53p53

p53p53

p53p53

Basal orlow stress

Acute orpersistent stress

Sestrins,TIGAR andTP53INP1

Antioxidant

Protect,repair andsurvive

Eradicateand purge

Pro-oxidant

PIG3,BAX andPUMA

AnabolismThe aspect of metabolism in which more complex molecules are built from their constituent parts, such as proteins from amino acids.

The ability of p53 to induce senescence and apoptosis is clearly related to the induction of oxidative stress97, and even necrotic cell death is activated by the cooperation between ROs and p53-induced cathepsin Q expres-sion98. Many p53-inducible proteins that are activated during the apoptotic response promote ROs produc-tion, including p53-induced gene 3 (PIG3)99, proline oxidase100, bAX, PuMA101 and P66sHC102. To further increase this effect, p53 can also inhibit or modulate the expression of antioxidant genes such as superoxide dis-mutase 2 (SOD2), aldehyde dehydrogenase 4 (ALDH4) and glutathione peroxidase 1 (GPx1), which increases oxidative stress94,103–105. Less directly, the ability of p53 to drive oxidative phosphorylation through the activation of sCO2 will also promote the generation of ROs from the mitochondria52,54.

What the consequences of ROs regulation by p53 might be is extremely difficult to predict, as ROs can contribute to almost every aspect of cell behaviour. Depending on the cell type and the levels of ROs, this can include proliferation, migration, genotoxic dam-age, senescence and cell death. Mice lacking p53 exhibit increased levels of ROs in normal tissue91, and the ability of this increased oxidative stress to drive the accumula-tion of oncogenic mutations is entirely consistent with the accompanying increased tumour susceptibility of these animals. Indeed, mice lacking TP53INP1 also showed an increased rate of lymphoma development that cor-related with increased levels of ROs95. It seems likely that the antioxidant activities of p53 help to both prevent the acquisition of tumour-promoting changes and promote cell survival to allow the repair of any moderate damage that has been accrued. However, although the antioxidant activities of p53 are clearly helpful under normal growth conditions (and these beneficial effects may extend beyond tumour suppression (TABLE 1)), these activities of p53 can be superseded under conditions in which p53 induces apoptosis partly by activating the expression of pro-oxidant genes. Quite what determines this switch is not known, and to some extent this is likely to represent cell type-dependent variations in response. For example, p53 seems to be pro-oxidant in brain tissue under all con-ditions, reflecting a transcriptional repression by p53 of antioxidant genes that are normally regulated by the poly-comb group oncogene BMi1 (REF. 106). However, in cells in which both pro-oxidant and antioxidant responses are possible an interesting model suggests that the difference lies in the extent and persistence of the damage-inducing stress. The regulation of ROs by p53 might be different under basal or low-stress conditions (in which p53 func-tions as an antioxidant) compared with high-damage and high-stress conditions (in which p53 contributes to cell death by increasing ROs levels)26.

The role of p53’s metabolic functions in cancerIt is clear that alterations in metabolism can have a role in cancer development, and that p53 can regulate vari-ous aspects of metabolism. Although the implications of these two statements are tantalizingly obvious, fitting the metabolic activities of p53 into a simple paradigm of how cancers are regulated is less straightforward.

some activities of p53 are obviously tumour suppressive — for example, the ability to inhibit cell proliferation and cell growth, and the potential to drive apoptosis or senescence under various conditions of oncogenic stress, including oncogene activation and hypoxia26. We now also understand that p53 can function in several ways to counteract the metabolic transformation that seems to be so crucial for successful cancer progression. The ability of p53 to suppress glycolysis and to promote oxidative phos-phorylation might help to prevent the unrestrained glyco-lytic flux that is associated with malignant cell growth, and so represents another manifestation of the tumour-suppressive activity of p53 (REF. 107). The activation of p53 in response to the various forms of metabolic stress that might be associated with malignant development, such as the depletion of nutrients or O2, could also help to prevent tumour progression by driving senescence or apoptosis in the affected cell. Activation of p53 in response to abnor-mal mTOR signalling would also help to arrest the malig-nant progression of incipient cancer cells.

However, p53 also has some functions that seem counterintuitive and may even be predicted to drive cancer-associated metabolic changes. The ability of p53 to increase flux through the pentose phosphate pathway could, for example, help to protect developing cancer cells from toxic levels of ROs and promote anabolism, which is needed for tumour cell growth. The presence of p53 is also required for tumour cells to adopt metabolic changes that allow them to withstand treatment with drugs that induce metabolic stress58. These paradoxical functions of p53 are reflected in its general ability to participate in different and apparently opposing groups of responses, each of which would normally contribute to tumour suppression108. On the one hand, p53 can contribute to a ‘protect and survive’ response by defending cells from the accumulation of genotoxic damage through lowering

Figure 4 | regulation of oxidative stress by p53. In common with many other p53 responses, oxidative stress can be both decreased and increased by p53. It is possible that low or basal levels of p53 have an antioxidant role, protecting cells from the accumulation of damaging levels of reactive oxygen species while also allowing survival and repair of moderate damage. A more robust activation of p53 — suggesting higher or more persistent stress levels — results in a pro-oxidant activity that contributes to the effective removal of the damaged or stressed cell through cell death or senescence. BAX, BCL2-associated X protein; PIG3, p53-induced gene 3; PUMA, p53-upregulated modulator of apoptosis; TP53INP1, tumour protein p53-inducible nuclear protein 1.

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the intracellular levels of ROs, imposing a transient cell cycle arrest and helping in the repair of DNA damage. In combination with the ability of p53 to inhibit cell death109, these responses allow a damaged or stressed cell to sur-vive until the problems have been resolved. Each of these activities of p53 can limit the acquisition of changes that might promote malignant progression, but clearly they are not infallible, and cancer progression can occur despite this monitoring by p53. On the other hand, under con-ditions in which the stress or damage are not resolved, it seems that p53 can adopt a different strategy — now inducing a permanent loss of proliferative ability through the induction of either senescence or cell death. This is more of a ‘scorched earth’ approach to simply eliminate any potentially dangerous cells. In this case, the cell is lost but the organism is saved.

At first glance, this dual function of p53 in preventing cancer development might seem to provide a double layer of protection from malignant development, a proposal that is certainly supported by the high level of defence afforded by p53. Mutation of only one TP53 allele results in an appalling increase in cancer susceptibility — as illus-trated by individuals with Li-Fraumeni syndrome who inherit one faulty TP53 allele110. However, there do seem to be some inherent dangers in the duality of p53 activity, which are manifested when the response is inappropriate to the situation. Most obviously, maintenance of the pro-tect and survive response under conditions of sustained stress (such as tumour progression) might help to safe-guard the developing cancer cell. This conflict also leads to a difficulty in ascribing a prognostic advantage to can-cers that retain wild-type p53. In most cases the predic-tion would be that the presence of wild-type p53 should make a tumour more sensitive to therapy through the

induction of p53-driven apoptotic and senescent pathways that effectively induce tumour regression in mouse models. but, as a quick look at the literature will attest, retention of wild-type p53 can predict a good response in some cases but a poor response in others — and the latter cases are likely to reflect the protective functions of p53 in allowing cells to survive stress and damage111. The consequences of autophagy modulation by p53 for cancer prognosis are similarly difficult to predict. Clearly, we are being hampered by our incomplete understanding of how metabolic changes contribute to cancer and when they might help or hinder the malignant process.

Future directionsMetabolic pathways are providing an exciting new hunt-ing ground to explore for potential therapeutic targets in the treatment of malignant disease. There is now ample evidence that tumour cells depend on metabolic altera-tions for their continued growth and survival, and that these changes make cancer cells peculiarly addicted to the rapacious uptake of glucose and glutamine. We now have some indication of the role of wild-type p53 in averting metabolic transformation, although the possible role of mutant p53 is as yet largely unexplored. TP53 incurs point mutations in many cancers, leading to the expression of mutant p53 proteins that have acquired activities that con-tribute to malignant progression independently of the loss of wild-type p53 function. A role for these mutant p53 pro-teins in promoting the metabolic transformation would be extremely interesting. Model systems provide excit-ing examples of the potential of metabolic targets for the treatment of cancer6, and we can now start to consider the potential of important new targets based on the loss or deregulation of certain aspects of the p53 response.

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AcknowledgementsWe would like to thank E. Gottlieb and E. Cheung for reading the manuscript and support from Cancer Research UK.

DATABASESentrez Gene: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=geneAtg5 | Atg7 | BAX | BMi1 | GLUT3 | HK2 | PUMA | SESN2 | SOD2oMIM: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIMLi-Fraumeni syndromepathway Interaction Database: http://pid.nci.nih.gov/AKT | AMPK | ARFuniprotKB: http://www.uniprot.orgbeclin1 | GLUT1 | GLUT4 | H6PD | IKKα | IKKβ | LKB1 | MDM2 | mTOR | MYC | P21 | p53 | PTEN | SCO2 | TIGAR | TP53INP1 | TSC1 | TSC2

FURTHER INFORMATIONKaren H. Vousden’s homepage: http://www.beatson.gla.ac.uk/Regulation-of-Cancer-Cell-Death-and-Survival/Karen-Vousden-Tumour-Suppression.html Kevin M. ryan’s homepage: http://www.beatson.gla.ac.uk/Regulation-of-Cancer-Cell-Death-and-Survival/Kevin-Ryan-Tumour-Cell-Death.html

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