TIGAR, a p53-inducible regulator of glycolysis and apoptosis

Oxygen, a source of life and stress

Oxygen is an essential element in the survival of complex organisms, however the level of oxygen, low or high, can be a source of stress depending on the biological context. Low levels of oxygen in tissues (hypoxia) can be the consequence of a number of pathophysiological conditions including ischemic disorders and cancer while relative, higher levels (hyperoxia) can lead to retinopathy of prematurity. The local oxygen environment and oxygen consumption dictate vascular homeostasis, vaso-proliferation and vaso-cessation, which is deregulated in these diseases through oxygen-dependent growth factors. In this review, we will introduce aspects of the physiology and biology of oxygen partial pressure and the molecular mechanisms implicated in oxygen sensing. We will outline the regulation and function of the key operator in cellular signalling of hypoxia, the transcription factor, hypoxia-inducible factor. In addition, we will focus on cancer cell hypoxia and on its role in driving cell metabolism, pH regulation and survival.

The Warburg effect and its cancer therapeutic implications

Increased aerobic glycolysis in cancer, a phenomenon known as the Warburg effect, has been observed in various tumor cells and represents a major biochemical alteration associated with malignant transformation. Although the exact molecular mechanisms underlying this metabolic change remain to be elucidated, the profound biochemical alteration in cancer cell energy metabolism provides exciting opportunities for the development of therapeutic strategies to preferentially kill cancer cells by targeting the glycolytic pathway. Several small molecules capable of inhibiting glycolysis in experimental systems have been shown to have promising anticancer activity in vitro and in vivo. This review article provides a brief summary of our current understanding of the Warburg effect, the underlying mechanisms, and its influence on the development of therapeutic strategies for cancer treatment.

Bcl-2 induces pro-oxidant state by engaging mitochondrial respiration in tumor cells

Mitochondrial respiration, the key process behind cellular energy production, is critical for cell proliferation, growth and survival. However, the regulation of mitochondrial respiratory function in tumor cells is not well understood. In this study, we propose a model whereby tumor cells possess the capacity to fine-tune the balance between energy demands and mitochondrial reactive oxygen species (ROS) status, to maintain a milieu optimal for survival. This is achieved through the moderation of mitochondrial respiration, depending on the ROS context within the organelle, with the main players being Bcl-2 and cytochrome c oxidase (COX). We report a higher level of COX activity, oxygen consumption and mitochondrial respiration in tumor cells overexpressing Bcl-2. Transient overexpression, gene silencing and pharmacological inhibition of Bcl-2 corroborate these findings. Interestingly, Bcl-2 is also able to regulate mitochondrial respiration and COX activity in the face of mounting ROS levels, triggered by mitochondrial complex inhibitors. In this respect, it is plausible to suggest that Bcl-2 may be able to create an environment, most suited for survival by adjusting mitochondrial respiration accordingly to meet energy requirements, without incurring an overwhelming, detrimental increase in intracellular ROS.

TP53 mutations and hepatocellular carcinoma: insights into the etiology and pathogenesis of liver cancer

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide and the major risk factors include chronic infections with the hepatitis B (HBV) or C (HCV) virus, and exposure to dietary aflatoxin B(1) (AFB(1)) or alcohol consumption. Multiple genetic and epigenetic changes are involved in the molecular pathogenesis of HCC, for example, somatic mutations in the p53 tumor suppressor gene (TP53) and the activation of the WNT signal transduction pathway. AFB(1) frequently induces G:C to T:A transversions at the third base in codon 249 of TP53 and cooperates with HBV in causing p53 mutations in HCC. The detection of TP53 mutant DNA in plasma is a biomarker of both AFB(1) exposure and HCC risk. Chronic infection with HBV and HCV viruses, and oxyradical disorders including hemochromatosis, also generate reactive oxygen/nitrogen species that can both damage DNA and mutate cancer-related genes such as TP53. Certain mutant p53 proteins may exhibit a 'gain of oncogenic function'. The p53 biological network is a key responder to this oxidative and nitrosative stress. Depending on the extent of the DNA damage, p53 regulates the transcription of protective antioxidant genes and with extensive DNA damage, transactivates pro-oxidant genes that contribute to apoptosis. The X gene of HBV (HBx) is the most common open reading frame integrated into the host genome in HCC and the integrated HBx is frequently mutated. Mutant HBx proteins still retain their ability to bind to p53, and attenuate DNA repair and p53-mediated apoptosis. In summary, both viruses and chemicals are implicated in the etiology of TP53 mutations during the molecular pathogenesis of HCC.

p53: new roles in metabolism

Virtually all cancers show metabolic changes that result in upregulation of glycolysis and glucose consumption. Although discovered in the 1920s, how this glycolytic switch happens, and whether it is a cause or a consequence of the malignant process, has remained a matter of debate. The p53 tumor suppressor gene, discovered some 30 years ago, has a key role in preventing cancer development. Recent discoveries revealing new functions for p53 in the regulation of glucose metabolism and oxidative stress have brought together these two venerable fields of cancer biology. These activities of p53 appear to be key in tumor suppression, and shed some light on the pathways that underlie the metabolic changes in cancer cells.

Transcription factor p53 in degenerating spinal cords

The causes of spinal cord cell loss in motor neuron disorders such as amyotrophic lateral sclerosis (ALS) are currently unknown. A role can be postulated for the transcription factor p53, which can induce apoptosis via upregulation of proapoptotic genes (e.g., Bax) and inhibition of antiapoptotic genes (e.g., Bcl-2). A model of motor neuron loss is the wobbler mouse that exhibits rapid motor neuron cell death as well as motor deficit from 21 days after birth. Affymetrix microarray data from wobbler mice demonstrate a 2.2-fold increase in p53 signal compared with their normal littermates, whereas qRT-PCR of RNA from laser capture microdissected ventral horns of normal and wobbler mice reveals a larger 6.6-fold increase in gene expression and this was supported by western blotting. Human ventral horns obtained from ALS and age-matched normal spinal cords also demonstrated an increase (2.7-fold) in p53 expression as determined by qRT-PCR. Evidence of a causative role for p53 in spinal cord cell death was provided by use of a p53 inhibitor, pifithrin-alpha, in organotypic slice cultures of mouse spinal cord. A 24-h pretreatment with pifithrin-alpha (and continuing in the presence of insult), significantly reduced the toxicity of a 48-h treatment with FeSO(4), tested with the MTT viability assay. These results indicate that p53 plays a functional role in oxidative stress-induced cell death and supports the possibility that elevated p53 could be involved in motor neuron death in ALS and the wobbler mouse.

Outcomes of p53 activation--spoilt for choice

The p53 tumour suppressor protein can efficiently inhibit tumour development. This activity reflects its ability to induce a number of different responses, including cell cycle arrest and apoptosis. Recent studies have revealed some interesting insights into how the choice of response to p53 is regulated, highlighting a correlation between the activation of cell cycle arrest and survival with the ability of p53 to reduce oxidative stress and protect cells from genotoxic damage. Understanding the molecular mechanisms that determine which response is selected may allow us to modulate these pathways so that therapeutic reactivation of p53 favours apoptotic cell death in tumour cells, but a reversible--and therefore far less toxic--induction of cell cycle arrest in normal cells.

Mitochondrial oxidative stress: implications for cell death

In addition to the established role of the mitochondria in energy metabolism, regulation of cell death has emerged as a second major function of these organelles. This seems to be intimately linked to their generation of reactive oxygen species (ROS), which have been implicated in mtDNA mutations, aging, and cell death. Mitochondrial regulation of apoptosis occurs by mechanisms, which have been conserved through evolution. Thus, many lethal agents target the mitochondria and cause release of cytochrome c and other pro-apoptotic proteins into the cytoplasm. Cytochrome c release is initiated by the dissociation of the hemoprotein from its binding to the inner mitochondrial membrane. Oxidation of cardiolipin reduces cytochrome c binding and increases the level of soluble cytochrome c in the intermembrane space. Subsequent release of the hemoprotein occurs by pore formation mediated by pro-apoptotic Bcl-2 family proteins, or by Ca(2+) and ROS-triggered mitochondrial permeability transition, although the latter pathway might be more closely associated with necrosis. Taken together, these findings have placed the mitochondria in the focus of current cell death research.

Metabolic targeting as an anticancer strategy: dawn of a new era?

As a result of a spectrum of mitochondrial defects, tumor cells often preferentially use glycolysis to generate adenosine triphosphate (ATP), even in the presence of oxygen, a phenomenon known as aerobic glycolysis, or the "Warburg effect." Dichloroacetate (DCA) is an inhibitor of mitochondrial pyruvate dehydrogenase kinase (PDK), which inhibits pyruvate dehydrogenase (PDH), a gatekeeping enzyme for the entry of pyruvate into the mitochondrial tricarboxylic acid (TCA) cycle. In mice, DCA treatment appears to reactivate mitochondrial respiration in tumor cells, induces their selective killing, and suppresses cancer growth. These observations provide intriguing insights into the plasticity of tumor metabolism that may offer new opportunities for therapeutic intervention.

Hypoxia signalling controls metabolic demand

It has been known for quite some time that cancer cells undergo far-reaching modifications in their metabolism, yet a full understanding of these changes and how they come about remains elusive. Even under conditions of plentiful oxygen, cancer cells choose to switch glucose metabolism from respiration to lactic acid formation. The mystery behind the molecular mechanisms of this phenomenon, known as the Warburg effect, is now being unravelled. The reduced respiration rate and increased glucose uptake associated with lactic acid production, and acidosis of the micro-environment, are primarily due to activation of the alpha/beta hypoxia-inducible transcription factor. This distinctive metabolic nature of cancer cells is already being exploited as a diagnostic tool but is yet to be harnessed as a therapeutic intervention.

Coping with stress: multiple ways to activate p53

Over the years, p53 has been shown to sit at the centre of an increasingly complex web of incoming stress signals and outgoing effector pathways. The number and diversity of stress signals that lead to p53 activation illustrates the breadth of p53's remit - responding to a wide variety of potentially oncogenic insults to prevent tumour development. Interestingly, different stress signals can use different and independent pathways to activate p53, and there is some evidence that different stress signals can mediate different responses. How each of the responses to p53 contributes to inhibition of malignant progression is beginning to be clarified, with the hope that identification of responses that are key to tumour suppression will allow a more focused and effective search for new therapeutic targets. In this review, we will highlight some recently identified roles for p53 in tumour suppression, and discuss some of the numerous mechanisms through which p53 can be regulated and activated.

Transformation of human mesenchymal stem cells increases their dependency on oxidative phosphorylation for energy production

An increased dependency on glycolysis for ATP production is considered to be a hallmark of tumor cells. Whether this increase in glycolytic activity is due mainly to inherent metabolic alterations or to the hypoxic microenvironment remains controversial. Here we have transformed human adult mesenchymal stem cells (MSC) using genetic alterations as described for differentiated cells. Our data suggest that MSC require disruption of the same pathways as have been shown for differentiated cells to confer a fully transformed phenotype. Furthermore, we found that MSC are more glycolytic than primary human fibroblasts and, in contrast to differentiated cells, do not depend on increased aerobic glycolysis for ATP production during transformation. These data indicate that aerobic glycolysis (the Warburg effect) is not an intrinsic component of the transformation of adult stem cells, and that oncogenic adaptation to bioenergetic requirements, in some circumstances, may also rely on increases in oxidative phosphorylation. We did find, however, a reversible increase in the transcription of glycolytic enzymes in tumors generated by transformed MSC, indicating this is a secondary phenomenon resulting from adaptation of the tumor to its microenvironment.

Cell-permeating alpha-ketoglutarate derivatives alleviate pseudohypoxia in succinate dehydrogenase-deficient cells

Succinate dehydrogenase (SDH) and fumarate hydratase (FH) are components of the tricarboxylic acid (TCA) cycle and tumor suppressors. Loss of SDH or FH induces pseudohypoxia, a major tumor-supporting event, which is the activation of hypoxia-inducible factor (HIF) under normoxia. In SDH- or FH-deficient cells, HIF activation is due to HIF1alpha stabilization by succinate or fumarate, respectively, either of which, when in excess, inhibits HIFalpha prolyl hydroxylase (PHD). To reactivate PHD, we focused on its substrate, alpha-ketoglutarate. We designed and synthesized cell-permeating alpha-ketoglutarate derivatives, which build up rapidly and preferentially in cells with a dysfunctional TCA cycle. This study shows that succinate- or fumarate-mediated inhibition of PHD is competitive and is reversed by pharmacologically elevating intracellular alpha-ketoglutarate. Introduction of alpha-ketoglutarate derivatives restores normal PHD activity and HIF1alpha levels to SDH-suppressed cells, indicating new therapy possibilities for the cancers associated with TCA cycle dysfunction.

p53 downstream target DDA3 is a novel microtubule-associated protein that interacts with end-binding protein EB3 and activates beta-catenin pathway

We have previously identified mouse DDA3 as a p53-inducible gene. To explore the functional role of DDA3, we screened a mouse brain cDNA library by the yeast two-hybrid assay, and identified the microtubule plus-end binding protein EB3 as a DDA3-interacting protein. Binding of DDA3 to EB3 was verified by glutathione S-transferase (GST) pull-down assay and subcellular colocalization; co-immunoprecipitation further indicated that interaction of these two proteins within cells required intact microtubules. Domains of DDA3-EB3 interaction were mapped by GST pull-down assay to amino acids 118-241 and 242-329 of DDA3 and the N- and C-termini of EB3. Immunofluorescence analysis revealed colocalization of DDA3 with microtubules in various cell phases, and regions encompassing aa 118-241 and 242-329 contained microtubule-interacting and bundling activities. In vitro microtubule-binding assay showed that DDA3 and EB3 associated directly with microtubules, and cooperated with each other for microtubule binding. In addition, DDA3 bound to the EB3 interacting partner adenomatous polyposis coli 2 (APC2), a homolog of the tumor suppressor APC, which is a component of the beta-catenin destruction complex. Ectopic expression of DDA3 and EB3 enhanced beta-catenin-dependent transactivation and cyclin D1 production, whereas knockdown of endogenous DDA3 or EB3 inhibited beta-catenin-mediated transactivation and the ability of cells to form colonies. Together, our results identify DDA3 as a novel microtubule-associated protein that binds to EB3, and implicate DDA3 and EB3 in the beta-catenin-mediated growth signaling.

Mitochondria, oxidative stress and cell death

In addition to the well-established role of the mitochondria in energy metabolism, regulation of cell death has recently emerged as a second major function of these organelles. This, in turn, seems to be intimately linked to their role as the major intracellular source of reactive oxygen species (ROS), which are mainly generated at Complex I and III of the respiratory chain. Excessive ROS production can lead to oxidation of macromolecules and has been implicated in mtDNA mutations, ageing, and cell death. Mitochondria-generated ROS play an important role in the release of cytochrome c and other pro-apoptotic proteins, which can trigger caspase activation and apoptosis. Cytochrome c release occurs by a two-step process that is initiated by the dissociation of the hemoprotein from its binding to cardiolipin, which anchors it to the inner mitochondrial membrane. Oxidation of cardiolipin reduces cytochrome c binding and results in an increased level of "free" cytochrome c in the intermembrane space. Conversely, mitochondrial antioxidant enzymes protect from apoptosis. Hence, there is accumulating evidence supporting a direct link between mitochondria, oxidative stress and cell death.

The STAT3 oncogene as a predictive marker of drug resistance

Constitutive activation of STAT3 (signal transducer and activator of transcription) has been reported in several primary cancers and tumor cell lines where it induces cell transformation through a combined inhibition of apoptosis and cell-cycle activation. Several studies have suggested that STAT3 prevents cell-cycle arrest and cell death through upregulation of survival proteins and downregulation of tumor suppressors. As a consequence of anti-apoptotic and proliferative lesions, we propose that this oncogenic pathway is also involved in intrinsic drug resistance and that STAT3-expressing tumors are resistant to chemotherapeutic agents. If this hypothesis is correct, the detection of the activated form of this protein should help to define subsets of tumors that fail to respond to chemotherapy. Furthermore, interfering with the STAT3 oncogenic pathway might restore the sensitivity to anticancer drugs.

Withdrawal symptoms on display: Bcl-2 members under investigation

The balance between survival and apoptosis of lymphocytes is considered to be regulated by specific signals delivered through cell surface receptors binding either antigen (fragments) or homeostatic cytokines. Expanding lymphocytes must also compete for nutrients. For growth factors and nutrients, recent data indicate how these generic environmental signals couple to members of the apoptosis-regulating Bcl-2 family. The prosurvival molecule Mcl-1 is engaged by lethal BH3-only proteins Puma and Noxa under these circumstances. We propose that Puma and Noxa have specific roles in tipping the balance towards apoptosis after growth factor withdrawal and nutrient shortage, respectively. These complementary mechanisms tune survival in the various niches when lymphocytes compete for resources during selection and expansion.

Models of reactive oxygen species in cancer

Increased generation of reactive oxygen species (ROS) has been observed in cancer, degenerative diseases, and other pathological conditions. ROS can stimulate cell proliferation, promote genetic instability, and induce adaptive responses that enable cancer cells to maintain their malignant phenotypes. However, when cellular redox balance is severely disturbed, high levels of ROS may cause various damages leading to cell death. The studies of ROS effects on biological systems, their underlying mechanisms and therapeutic implications largely depend on proper experimental models. Here we review several in vitro and in vivo models for ROS research.

Metabolic regulation of Akt: roles reversed

The respiration-deficient, highly glycolytic metabolic phenotype of cancer cells known as the “Warburg effect” has been appreciated for many years. A new study (see Pelicano et al. on p. 913 of this issue) demonstrates that respiration deficiency caused by mitochondrial mutation or hypoxia may directly promote the enormous survival advantage observed in cancer cells by activation of the phosphatidylinositol 3-kinase–Akt survival pathway. We discuss these and other recent findings that show how metabolic changes associated with cancer can play a significant role in tumor biology.

PGAM5, a Bcl-XL-interacting Protein, Is a Novel Substrate for the Redox-regulated Keap1-dependent Ubiquitin Ligase Complex

Keap1 is a BTB-Kelch substrate adaptor protein for a Cul3-dependent ubiquitin ligase complex that functions as a sensor for thiol-reactive chemopreventive compounds and oxidative stress. Inhibition of Keap1-dependent ubiquitination of the bZIP transcription factor Nrf2 enables Nrf2 to activate a cyto-protective transcriptional program that counters the damaging effects of oxidative stress. In this report we have identified a member of the phosphoglycerate mutase family, PGAM5, as a novel substrate for Keap1. The N terminus of the PGAM5 protein contains a conserved NXESGE motif that binds to the substrate binding pocket in the Kelch domain of Keap1, whereas the C-terminal PGAM domain binds Bcl-X(L). Keap1-dependent ubiquitination of PGAM5 results in proteasome-dependent degradation of PGAM5. Quinone-induced oxidative stress and the chemopreventive agent sulforaphane inhibit Keap1-dependent ubiquitination of PGAM5. The identification of PGAM5 as a novel substrate of Keap1 suggests that Keap1 regulates both transcriptional and post-transcriptional responses of mammalian cells to oxidative stress.

Glucose metabolism and cancer

The first identified biochemical hallmark of tumor cells was a shift in glucose metabolism from oxidative phosphorylation to aerobic glycolysis. We now know that much of this metabolic conversion is controlled by specific transcriptional programs. Recent studies suggest that activation of the hypoxia-inducible factor (HIF) is a common consequence of a wide variety of mutations underlying human cancer. HIF stimulates expression of glycolytic enzymes and decreases reliance on mitochondrial oxidative phosphorylation in tumor cells, which occurs even under aerobic conditions. In addition, recent efforts have also connected the master metabolic regulator AMP-activated protein kinase (AMPK) to several human tumor suppressors. Several promising therapeutic strategies based on modulation of AMPK, HIF and other metabolic targets have been proposed to exploit the addiction of tumor cells to increased glucose uptake and glycolysis.

Hdmx modulates the outcome of p53 activation in human tumor cells

Tumors that express wild-type P53 provide a target for therapies designed to reactivate P53 function. This is supported by the potent activation of P53 in tumor cells by Nutlin, a cis-imidazoline that inhibits the Hdm2-P53 interaction. The efficacy of Hdm2.P53 antagonists could be compromised if they do not antagonize Hdmx, an Hdm2 homolog that inhibits P53 transactivation. We evaluated the role of Hdmx expression in sensitivity to Nutlin in a range of cancer cell lines. Nutlin reduced Hdmx levels in normal cells and some cancer cell lines, whereas other cancer cells were refractory to such down-regulation. Strikingly, Nutlin did not disrupt Hdmx.P53 complexes, and in cell lines where no Hdmx degradation occurred, Nutlin failed to induce apoptosis. shRNA-mediated reduction of Hdmx sensitized cells to apoptosis, but caspase-3 was neither required nor sufficient for Hdmx degradation or apoptosis. Our data imply that Hdmx is an important determinant of the outcome of P53 activation. Thus, targeting Hdmx may be a therapeutic strategy that complements drugs such as Nutlin.