p53 regulates mitochondrial respiration

Redox-based escape mechanism from death: the cancer lesson

We review here current evidence on the role of reactive oxygen species (ROS) and of the intracellular redox state in governing crucial steps of the metastatic process, from cell detachment from the primary tumor to final colonization of the distant site. In particular, we discuss the redox-dependent aspects of cell glycolytic metabolism (Warburg effect), of cell juggling between different motility styles (epithelial-to-mesenchymal and mesenchymal-to-amoeboid transition), of cell resistance to anoikis and of cell interaction with the stromal components of the metastatic niche. Central to this overview is the concept that metastasis can be viewed as an integrated "escape program" triggered by redox changes and instrumental at avoiding oxidative stress within the primary tumor. In this novel perspective, metabolic, motility, and prosurvival choices of the cell along the entire metastatic process can be interpreted as exploiting redox-signaling cascades to monitor oxidative/reductive environmental cues and escape oxidative damage. We also propose that this theoretic framework be applied to "normal" evasion/invasion programs such as in inflammation and development. Furthermore, we suggest that the intimate connection between metastasis, inflammation, and stem cells results, at least in part, by the sharing of a common redox-dependent strategy for infiltration, survival, dissemination, and patterning.

p53--a Jack of all trades but master of none

Cancers are rare because their evolution is actively restrained by a range of tumour suppressors. Of these p53 seems unusually crucial as either it or its attendant upstream or downstream pathways are inactivated in virtually all cancers. p53 is an evolutionarily ancient coordinator of metazoan stress responses. Its role in tumour suppression is likely to be a relatively recent adaptation, which is only necessary when large, long-lived organisms acquired the sufficient size and somatic regenerative capacity to necessitate specific mechanisms to reign in rogue proliferating cells. However, such evolutionary reappropriation of this venerable transcription factor entails compromises that restrict its efficacy as a tumour suppressor.

Involvement of cytochrome c oxidase subunits Va and Vb in the regulation of cancer cell metabolism by Bcl-2

Bcl-2 has been shown to promote survival of cancer cells by maintaining a slight pro-oxidant state through elevated mitochondrial respiration during basal conditions. On oxidative stress, Bcl-2 moderates mitochondrial respiration through cytochrome c oxidase (COX) activity to prevent an excessive buildup of reactive oxygen species (ROS) by-production from electron transport activities. However, the underlying molecular mechanism(s) of Bcl-2-mediated ROS regulation and its impact on carcinogenesis remain unclear. In this study, we show that Bcl-2 expression positively influences the targeting of nuclear-encoded COX Va and Vb to the mitochondria of cancer cells. In addition, evidence is presented in support of a protein-protein interaction between COX Va and Bcl-2, involving the BH2 domain of Bcl-2. Interestingly, episodes of serum withdrawal, glucose deprivation or hypoxia aimed at inducing early oxidative stress triggered Bcl-2-overexpressing cells to preserve mitochondrial levels of COX Va while depressing COX Vb, whereas the reverse was observed in mock-transfected cells. The unique manner in which Bcl-2 adjusted COX subunits during these physiological stress triggers had a profound impact on the resultant decrease in COX activity and maintenance of mitochondrial ROS levels, thus delineating a novel mechanism for the homeostatic role of Bcl-2 in the redox biology and metabolism of cancer cells.

Influence of chemoresistance and p53 status on fluoro-2-deoxy-D-glucose incorporation in cancer

Both mutant p53 and chemoresistance are poor prognostic factors in cancer. Many studies have examined the influence of these factors on fluoro-2-deoxy-D-glucose (FDG) incorporation. Whilst mutant p53 is associated with increased FDG incorporation, chemoresistance, especially when associated with P-glycoprotein, is associated with decreased FDG incorporation.

p53 is an important factor for the radiosensitization effect of 2-deoxy-D-glucose

Metabolic change in cancer cells by preferential production of energy through glycolysis is a well-documented characteristic of cancer. However, whether inhibition of glycolysis will enhance the efficacy of radiation therapy is a matter of debate. In this study which uses lung cancer as the model, we demonstrate that the improvement of radiotherapy by 2-deoxy-D-glucose (2DG) is p53-dependent. Based on clonogenic survival data, we show that p53-deficient lung cancer cells (H358) are more sensitive to 2DG treatment when compared to p53 wild-type lung cancer cells (A549). The effective doses of 2DG at 0.5-surviving fraction of A549 and H358 are 17.25 and 4.61 mM, respectively. Importantly, 2DG exhibits a significant radiosensitization effect in A549 cells but not in H358 cells. Treatment with 2DG increases radiation-induced p53 protein levels in A549 cells. siRNA inhibition of p53 in A549 cells reduces the radiosensitization effect of 2DG. Furthermore, ectopic expression of wild-type p53 in H358 cells significantly enhances the radiosensitization effect of 2DG as determined by colony formation assay. In nude mice injected with A549 cells, treatment of 2DG enhances the efficacy of radiation therapy. Together, these results suggest that inhibition of glycolysis may only be beneficial for radiation therapy of cancer expressing wild-type p53.

The expanding universe of p53 targets

The p53 tumour suppressor is modified through mutation or changes in expression in most cancers, leading to the altered regulation of hundreds of genes that are directly influenced by this sequence-specific transcription factor. Central to the p53 master regulatory network are the target response element (RE) sequences. The extent of p53 transactivation and transcriptional repression is influenced by many factors, including p53 levels, cofactors and the specific RE sequences, all of which contribute to the role that p53 has in the aetiology of cancer. This Review describes the identification and functionality of REs and highlights the inclusion of non-canonical REs that expand the universe of genes and regulation mechanisms in the p53 tumour suppressor network.

Tumour suppression by p53: a role for the DNA damage response?

Loss of p53 function occurs during the development of most, if not all, tumour types. This paves the way for genomic instability, tumour-associated changes in metabolism, insensitivity to apoptotic signals, invasiveness and motility. However, the nature of the causal link between early tumorigenic events and the induction of the p53-mediated checkpoints that constitute a barrier to tumour progression remains uncertain. This Review considers the role of the DNA damage response, which is activated during the early stages of tumour development, in mobilizing the tumour suppression function of p53. The relationship between these events and oncogene-induced p53 activation through the ARF pathway is also discussed.

p53 and metabolism

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.

Energetics, epigenetics, mitochondrial genetics

The epigenome has been hypothesized to provide the interface between the environment and the nuclear DNA (nDNA) genes. Key factors in the environment are the availability of calories and demands on the organism's energetic capacity. Energy is funneled through glycolysis and mitochondrial oxidative phosphorylation (OXPHOS), the cellular bioenergetic systems. Since there are thousands of bioenergetic genes dispersed across the chromosomes and mitochondrial DNA (mtDNA), both cis and trans regulation of the nDNA genes is required. The bioenergetic systems convert environmental calories into ATP, acetyl-Coenzyme A (acetyl-CoA), s-adenosyl-methionine (SAM), and reduced NAD(+). When calories are abundant, ATP and acetyl-CoA phosphorylate and acetylate chromatin, opening the nDNA for transcription and replication. When calories are limiting, chromatin phosphorylation and acetylation are lost and gene expression is suppressed. DNA methylation via SAM can also be modulated by mitochondrial function. Phosphorylation and acetylation are also pivotal to regulating cellular signal transduction pathways. Therefore, bioenergetics provides the interface between the environment and the epigenome. Consistent with this conclusion, the clinical phenotypes of bioenergetic diseases are strikingly similar to those observed in epigenetic diseases (Angelman, Rett, Fragile X Syndromes, the laminopathies, cancer, etc.), and an increasing number of epigenetic diseases are being associated with mitochondrial dysfunction. This bioenergetic-epigenomic hypothesis has broad implications for the etiology, pathophysiology, and treatment of a wide range of common diseases.

Improved swimming pool achieves higher reproducibility and sensitivity to effect of food components as ergogenic AIDS

A previously developed current swimming pool for mice has been used to evaluate many food components that enhance endurance exercise performance. In this article, to improve reproducibility, reliability and sensitivity of this assay system, we improved the spout part to generate a uniform current and divided the pool into six lanes to avoid physical interference between swimming mice. The stability of the current flow was assessed by measuring the surface current speed and water volume from the spout part. Maximum swimming times of ddY and BALB/c mice were measured to assess the reproducibility of the maximum swimming time. The improvement in sensitivity compared to the original equipment was estimated under three physiological conditions: low carbohydrate diet feeding, low blood hemoglobin level, and carbohydrate supplementation during exercise. The new spout part improved uniformity and quick adjustment of surface current, yielding an increase of workload in a stepwise manner during swimming. Exercise workload was increased in proportion to surface current speed, as evidenced by cadence of kicks and serum lactic acid levels. The improved swimming pool showed higher reproducibility of swimming time until fatigue (p<0.0001). Correspondence between blood hemoglobin concentration and swimming time was improved in the swimming pool. The improved swimming pool yielded higher sensitivity for low carbohydrate diet feeding (p<0.0001) and carbohydrate supplementation during exercise (p<0.01) compared to the original swimming pool. The improvement of the swimming pool achieved higher sensitivity and reproducibility in assessing various diet and food components compared to the original swimming pool.

The Warburg effect is genetically determined in inherited pheochromocytomas

The Warburg effect describes how cancer cells down-regulate their aerobic respiration and preferentially use glycolysis to generate energy. To evaluate the link between hypoxia and Warburg effect, we studied mitochondrial electron transport, angiogenesis and glycolysis in pheochromocytomas induced by germ-line mutations in VHL, RET, NF1 and SDH genes. SDH and VHL gene mutations have been shown to lead to the activation of hypoxic response, even in normoxic conditions, a process now referred to as pseudohypoxia. We observed a decrease in electron transport protein expression and activity, associated with increased angiogenesis in SDH- and VHL-related, pseudohypoxic tumors, while stimulation of glycolysis was solely observed in VHL tumors. Moreover, microarray analyses revealed that expression of genes involved in these metabolic pathways is an efficient tool for classification of pheochromocytomas in accordance with the predisposition gene mutated. Our data suggest an unexpected association between pseudohypoxia and loss of p53, which leads to a distinct Warburg effect in VHL-related pheochromocytomas.

Oxygen consumption can regulate the growth of tumors, a new perspective on the Warburg effect

Background

The unique metabolism of tumors was described many years ago by Otto Warburg, who identified tumor cells with increased glycolysis and decreased mitochondrial activity. However, “aerobic glycolysis” generates fewer ATP per glucose molecule than mitochondrial oxidative phosphorylation, so in terms of energy production, it is unclear how increasing a less efficient process provides tumors with a growth advantage.

Methods/Findings

We carried out a screen for loss of genetic elements in pancreatic tumor cells that accelerated their growth as tumors, and identified mitochondrial ribosomal protein L28 (MRPL28). Knockdown of MRPL28 in these cells decreased mitochondrial activity, and increased glycolysis, but paradoxically, decreased cellular growth in vitro. Following Warburg's observations, this mutation causes decreased mitochondrial function, compensatory increase in glycolysis and accelerated growth in vivo. Likewise, knockdown of either mitochondrial ribosomal protein L12 (MRPL12) or cytochrome oxidase had a similar effect. Conversely, expression of the mitochondrial uncoupling protein 1 (UCP1) increased oxygen consumption and decreased tumor growth. Finally, treatment of tumor bearing animals with dichloroacetate (DCA) increased pyruvate consumption in the mitochondria, increased total oxygen consumption, increased tumor hypoxia and slowed tumor growth.

Conclusions

We interpret these findings to show that non-oncogenic genetic changes that alter mitochondrial metabolism can regulate tumor growth through modulation of the consumption of oxygen, which appears to be a rate limiting substrate for tumor proliferation.

Upregulation of mitochondrial function and antioxidant defense in the differentiation of stem cells

Stem cell research has received increasing attention due to their invaluable potentials in the clinical applications to cure degenerative diseases, genetic disorders and even cancers. A great number of studies have been conducted with an aim to elucidate the molecular mechanisms involved in the regulation of self-renewal of stem cells and the mysterious circuits guiding them to differentiate into all kinds of progenies that can replenish the cell pools. However, little effort has been made in studying the metabolic aspects of stem cells. Mitochondria play essential roles in mammalian cells in the generation of ATP, Ca(2+) homeostasis, compartmentalization of biosynthetic pathways and execution of apoptosis. Considering the metabolic roles of mitochondria, they must be also critical in stem cells. This review is primarily focused on the biogenesis and bioenergetic function of mitochondria in the differentiation process and metabolic features of stem cells. In addition, the involvement of reactive oxygen species and hypoxic signals in the regulation of stem cell pluripotency and differentiation is also discussed.

Transitions at CpG dinucleotides, geographic clustering of TP53 mutations and food availability patterns in colorectal cancer

BACKGROUND

Colorectal cancer is mainly attributed to diet, but the role exerted by foods remains unclear because involved factors are extremely complex. Geography substantially impacts on foods. Correlations between international variation in colorectal cancer-associated mutation patterns and food availabilities could highlight the influence of foods on colorectal mutagenesis.

METHODOLOGY

To test such hypothesis, we applied techniques based on hierarchical clustering, feature extraction and selection, and statistical pattern recognition to the analysis of 2,572 colorectal cancer-associated TP53 mutations from 12 countries/geographic areas. For food availabilities, we relied on data extracted from the Food Balance Sheets of the Food and Agriculture Organization of the United Nations. Dendrograms for mutation sites, mutation types and food patterns were constructed through Ward's hierarchical clustering algorithm and their stability was assessed evaluating silhouette values. Feature selection used entropy-based measures for similarity between clusterings, combined with principal component analysis by exhaustive and heuristic approaches.

CONCLUSION/SIGNIFICANCE

Mutations clustered in two major geographic groups, one including only Western countries, the other Asia and parts of Europe. This was determined by variation in the frequency of transitions at CpGs, the most common mutation type. Higher frequencies of transitions at CpGs in the cluster that included only Western countries mainly reflected higher frequencies of mutations at CpG codons 175, 248 and 273, the three major TP53 hotspots. Pearson's correlation scores, computed between the principal components of the datamatrices for mutation types, food availability and mutation sites, demonstrated statistically significant correlations between transitions at CpGs and both mutation sites and availabilities of meat, milk, sweeteners and animal fats, the energy-dense foods at the basis of "Western" diets. This is best explainable by differential exposure to nitrosative DNA damage due to foods that promote metabolic stress and chronic inflammation.

Modulation of intracellular ROS levels by TIGAR controls autophagy

The p53-inducible TIGAR protein functions as a fructose-2,6-bisphosphatase, promoting the pentose phosphate pathway and helping to lower intracellular reactive oxygen species (ROS). ROS functions in the regulation of many cellular responses, including autophagy—a response to stress conditions such as nutrient starvation and metabolic stress. In this study, we show that TIGAR can modulate ROS in response to nutrient starvation or metabolic stress, and functions to inhibit autophagy. The ability of TIGAR to limit autophagy correlates strongly with the suppression of ROS, with no clear effects on the mTOR pathway, and is p53 independent. The induction of autophagy in response to loss of TIGAR can function to moderate apoptotic response by restraining ROS levels. These results reveal a complex interplay in the regulation of ROS, autophagy and apoptosis in response to TIGAR expression, and shows that proteins similar to TIGAR that regulate glycolysis can have a profound effect on the autophagic response through ROS regulation.

Polo-like kinases mediate cell survival in mitochondrial dysfunction

Cancer cells often display defects in mitochondrial respiration, thus the identification of pathways that promote cell survival under this metabolic state may have therapeutic implications. Here, we report that the targeted ablation of mitochondrial respiration markedly increases expression of Polo-like kinase 2 (PLK2) and that it is required for the in vitro growth of these nonrespiring cells. Furthermore, we identify PLK2 as a kinase that phosphorylates Ser-137 of PLK1, which is sufficient to mediate this survival signal. In vivo, knockdown of PLK2 in an isogenic human cell line with a modest defect in mitochondrial respiration eliminates xenograft formation, indicating that PLK2 activity is necessary for growth of cells with compromised respiration. Our findings delineate a mitochondrial dysfunction responsive cell cycle pathway critical for determining cancer cell outcome.

p53 improves aerobic exercise capacity and augments skeletal muscle mitochondrial DNA content

RATIONALE

Exercise capacity is a physiological characteristic associated with protection from both cardiovascular and all-cause mortality. p53 regulates mitochondrial function and its deletion markedly diminishes exercise capacity, but the underlying genetic mechanism orchestrating this is unclear. Understanding the biology of how p53 improves exercise capacity may provide useful insights for improving both cardiovascular as well as general health.

OBJECTIVE

The purpose of this study was to understand the genetic mechanism by which p53 regulates aerobic exercise capacity.

METHODS AND RESULTS

Using a variety of physiological, metabolic, and molecular techniques, we further characterized maximum exercise capacity and the effects of training, measured various nonmitochondrial and mitochondrial determinants of exercise capacity, and examined putative regulators of mitochondrial biogenesis. As p53 did not affect baseline cardiac function or inotropic reserve, we focused on the involvement of skeletal muscle and now report a wider role for p53 in modulating skeletal muscle mitochondrial function. p53 interacts with Mitochondrial Transcription Factor A (TFAM), a nuclear-encoded gene important for mitochondrial DNA (mtDNA) transcription and maintenance, and regulates mtDNA content. The increased mtDNA in p53(+/+) compared to p53(-/-) mice was more marked in aerobic versus glycolytic skeletal muscle groups with no significant changes in cardiac tissue. These in vivo observations were further supported by in vitro studies showing overexpression of p53 in mouse myoblasts increases both TFAM and mtDNA levels whereas depletion of TFAM by shRNA decreases mtDNA content.

CONCLUSIONS

Our current findings indicate that p53 promotes aerobic metabolism and exercise capacity by using different mitochondrial genes and mechanisms in a tissue-specific manner.

Mitochondrial gateways to cancer

Mitochondria are required for cellular survival, yet can also orchestrate cell death. The peculiar biochemical properties of these organelles, which are intimately linked to their compartmentalized ultrastructure, provide an optimal microenvironment for multiple biosynthetic and bioenergetic pathways. Most intracellular ATP is generated by mitochondrial respiration, which also represents the most relevant source of intracellular reactive oxygen species. Mitochondria participate in a plethora of anabolic pathways, including cholesterol, cardiolipin, heme and nucleotide biosynthesis. Moreover, mitochondria integrate numerous pro-survival and pro-death signals, thereby exerting a decisive control over several biochemical cascades leading to cell death, in particular the intrinsic pathway of apoptosis. Therefore, it is not surprising that cancer cells often manifest the deregulation of one or several mitochondrial functions. The six classical hallmarks of cancer (i.e., limitless replication, self-provision of proliferative stimuli, insensitivity to antiproliferative signals, disabled apoptosis, sustained angiogenesis, invasiveness/metastatic potential), as well as other common features of tumors (i.e., avoidance of the immune response, enhanced anabolic metabolism, disabled autophagy) may directly or indirectly implicate deregulated mitochondria. In this review, we discuss several mechanisms by which mitochondria can contribute to malignant transformation and tumor progression.

Cancer abolishes the tissue type-specific differences in the phenotype of energetic metabolism

Nowadays, cellular bioenergetics has become a central issue of investigation in cancer biology. Recently, the metabolic activity of the cancer cell has been shown to correlate with a proteomic index that informs of the relative mitochondrial activity of the cell. Within this new field of investigation, we report herein the production and characterization of high-affinity monoclonal antibodies against proteins of the "bioenergetic signature" of the cell. The use of recombinant proteins and antibodies against the mitochondrial beta-F1-ATPase and Hsp60 proteins and the enzymes of the glycolytic pathway glyceraldehyde-3-phosphate dehydrogenase and pyruvate kinase M2 in quantitative assays provide, for the first time, the actual amount of these proteins in normal and tumor surgical specimens of breast, lung, and esophagus. The application of this methodology affords a straightforward proteomic signature that quantifies the variable energetic demand of human tissues. Furthermore, the results show an unanticipated finding: tumors from different tissues and/or histological types have the same proteomic signature of energetic metabolism. Therefore, the results indicate that cancer abolishes the tissue-specific differences in the bioenergetic phenotype of mitochondria. Overall, the results support that energetic metabolism represents an additional hallmark of the phenotype of the cancer cell and a promising target for the treatment of diverse neoplasias.

Transcriptional and post-transcriptional regulation of mitochondrial biogenesis in skeletal muscle: effects of exercise and aging

Acute contractile activity of skeletal muscle initiates the activation of signaling kinases. This promotes the phosphorylation of transcription factors, leading to enhanced DNA binding and transcriptional activation and/or repression. The mRNA products of nuclear genes encoding mitochondrial proteins are translated in the cytosol and imported into pre-existing mitochondria. When contractile activity is repeated, the recapitulation of these cellular events progressively leads to an expansion of the mitochondrial reticulum within muscle. This has physiologically relevant health benefit, including enhanced lipid metabolism and reduced muscle fatigability. In aging skeletal muscle, the response to contractile activity appears to be attenuated, suggesting that a greater contractile stimulus is required to attain a similar phenotype adaptation. This review summarizes our current understanding of the effects of exercise on the gene expression pathway leading to organelle biogenesis in muscle.

Glucose deprivation contributes to the development of KRAS pathway mutations in tumor cells

Tumor progression is driven by genetic mutations, but little is known about the environmental conditions that select for these mutations. Studying the transcriptomes of paired colorectal cancer cell lines that differed only in the mutational status of their KRAS or BRAF genes, we found that GLUT1, encoding glucose transporter-1, was one of three genes consistently up-regulated in cells with KRAS or BRAF mutations. The mutant cells exhibited enhanced glucose uptake and glycolysis and survived in low-glucose conditions, phenotypes that all required GLUT1 expression. In contrast, when cells with wild-type KRAS alleles were subjected to a low-glucose environment, very few cells survived. Most surviving cells expressed high levels of GLUT1, and 4% of these survivors had acquired KRAS mutations not present in their parents. The glycolysis inhibitor 3-bromopyruvate preferentially suppressed the growth of cells with KRAS or BRAF mutations. Together, these data suggest that glucose deprivation can drive the acquisition of KRAS pathway mutations in human tumors.

Blinded by the Light: The Growing Complexity of p53

While the tumor suppressor functions of p53 have long been recognized, the contribution of p53 to numerous other aspects of disease and normal life is only now being appreciated. This burgeoning range of responses to p53 is reflected by an increasing variety of mechanisms through which p53 can function, although the ability to activate transcription remains key to p53's modus operandi. Control of p53's transcriptional activity is crucial for determining which p53 response is activated, a decision we must understand if we are to exploit efficiently the next generation of drugs that selectively activate or inhibit p53.

Mitochondrial dysfunction in human breast cancer cells and their transmitochondrial cybrids

Somatic mitochondrial DNA alterations have been found in all types of cancer. To better understand the role of mitochondria and their involvement in the pathogenic mechanisms of cancer development, the effects of cancer mitochondria were investigated in a defined nuclear background using a transmitochondrial cybrid system. Our results demonstrated that cancer mitochondria confer a significant reduction in cell growth when cells are metabolically stressed in a galactose medium. Activities of the respiratory chain complexes, cellular oxygen consumption, and ATP synthesis rates were found to be much lower in breast cancer cells, than those in normal breast epithelial cells of MCF-10A (10A). These results suggest that there is reduced mitochondrial function in the studied breast cancer cell lines. Similarly reduced mitochondrial function was observed in cybrids containing cancer mitochondria. Novel tRNA mutations were also identified in two breast cancer cell lines, possibly responsible for the observed mitochondrial dysfunction. We conclude that altered mitochondria in cancer cells may play a crucial role in tumor development.

p53/CEP-1 increases or decreases lifespan, depending on level of mitochondrial bioenergetic stress

Mitochondrial pathologies underlie a number of life-shortening diseases in humans. In the nematode Caenorhabditis elegans, severely reduced expression of mitochondrial proteins involved in electron transport chain-mediated energy production also leads to pathological phenotypes, including arrested development and/or shorter life; in sharp contrast, mild suppression of these same proteins extends lifespan. In this study, we show that the C. elegans p53 ortholog cep-1 mediates these opposite effects. We found that cep-1 is required to extend longevity in response to mild suppression of several bioenergetically relevant mitochondrial proteins, including frataxin - the protein defective in patients with Friedreich's Ataxia. Importantly, we show that cep-1 also mediates both the developmental arrest and life shortening induced by severe mitochondrial stress. These findings support an evolutionarily conserved function for p53 in modulating organismal responses to mitochondrial dysfunction and suggest that metabolic checkpoint responses may play a role in longevity control and in human mitochondrial-associated diseases.

A hyperfused mitochondrial state achieved at G1-S regulates cyclin E buildup and entry into S phase

Mitochondria undergo fission-fusion events that render these organelles highly dynamic in cells. We report a relationship between mitochondrial form and cell cycle control at the G(1)-S boundary. Mitochondria convert from isolated, fragmented elements into a hyperfused, giant network at G(1)-S transition. The network is electrically continuous and has greater ATP output than mitochondria at any other cell cycle stage. Depolarizing mitochondria at early G(1) to prevent these changes causes cell cycle progression into S phase to be blocked. Inducing mitochondrial hyperfusion by acute inhibition of dynamin-related protein-1 (DRP1) causes quiescent cells maintained without growth factors to begin replicating their DNA and coincides with buildup of cyclin E, the cyclin responsible for G(1)-to-S phase progression. Prolonged or untimely formation of hyperfused mitochondria, through chronic inhibition of DRP1, causes defects in mitotic chromosome alignment and S-phase entry characteristic of cyclin E overexpression. These findings suggest a hyperfused mitochondrial system with specialized properties at G(1)-S is linked to cyclin E buildup for regulation of G(1)-to-S progression.

Regulation of mitochondrial respiratory chain biogenesis by estrogens/estrogen receptors and physiological, pathological and pharmacological implications

There has been increasing evidence pointing to the mitochondrial respiratory chain (MRC) as a novel and important target for the actions of 17beta-estradiol (E(2)) and estrogen receptors (ER) in a number of cell types and tissues that have high demands for mitochondrial energy metabolism. This novel E(2)-mediated mitochondrial pathway involves the cooperation of both nuclear and mitochondrial ERalpha and ERbeta and their co-activators on the coordinate regulation of both nuclear DNA- and mitochondrial DNA-encoded genes for MRC proteins. In this paper, we have: 1) comprehensively reviewed studies that reveal a novel role of estrogens and ERs in the regulation of MRC biogenesis; 2) discussed their physiological, pathological and pharmacological implications in the control of cell proliferation and apoptosis in relation to estrogen-mediated carcinogenesis, anti-cancer drug resistance in human breast cancer cells, neuroprotection for Alzheimer's disease and Parkinson's disease in brain, cardiovascular protection in human heart and their beneficial effects in lens physiology related to cataract in the eye; and 3) pointed out new research directions to address the key questions in this important and newly emerging area. We also suggest a novel conceptual approach that will contribute to innovative regimens for the prevention or treatment of a wide variety of medical complications based on E(2)/ER-mediated MRC biogenesis pathway.

Open-system thermodynamic analysis of DNA polymerase fidelity

Replication inside a living cell, carried out by DNA polymerase, has an error rate far below that predicted by equilibrium thermodynamics from the affinities between nucleotides and a polymerase complex. The high fidelity is achieved through several distinctly different molecular mechanisms that include a nucleotide insertion checkpoint and 3'-5' exonuclease activity. The checkpoint mechanism has recently been articulated as a new paradigm for high specificity. A rigorous thermodynamic analysis of the bare DNA polymerization reaction, i.e., in the absence of exonuclease activity and proofreading, is developed in this paper. The analysis (a) reveals the important role of nonequilibrium steady-state (NESS) flux that drives high fidelity, (b) quantifies the error rate of the polymerization reaction as a function of free energy input through sustained non-equilibrium between chemical species, (c) bridges the 'thermodynamic' and 'kinetic' views of specificity and (d) generalizes the theory of kinetic checkpoints and provides it with a sound thermodynamic basis. The underlying mechanism again calls attention to the energy expenditure in heightened biomolecular specificity, a concept first developed by Hopfield and Ninio in the mid-1970s. The mechanism discussed in the present paper is not limited to DNA replication alone; it may be applicable to other biochemical systems.

Mitochondrial D-loop mutation is a common event in colorectal cancers with p53 mutations

PURPOSE

p53 is the most frequently mutated gene in colorectal cancer. In mitochondria, p53 protein is involved in regulation of transcription/replication and maintenance of genomic stability. Our aim was to examine the relationship between p53, D-loop mutation, and mitochondrial DNA content in colorectal cancer (CRC).

METHODS

A total of 194 patients with sporadic CRC without microsatellite instability who underwent surgery in Taipei Veterans General Hospital from January 1999 to December 2000 were included. The mitochondrial DNA content and D-loop mutation were quantified using real-time PCR and sequencing.

RESULTS

D-loop mutation occurred at significantly higher frequency in tumors with p53 mutation (34/88; 38.6%) than in tumors without p53 mutation (23/106; 21.7%). The frequency of the decreased mtDNA content was significantly associated with TNM stage (p = 0.009) and p53 mutation (p = 0.036). The 5-year DFS rate was 39% in patients exhibiting tumors with decreased mtDNA content, and was significantly poorer in these patients than in those exhibiting tumors with normal level of mtDNA content (61%, p = 0.01). The presence of D-loop mutations had no effect on 5-year DFS rate. In multivariate survival analysis, TNM stage, and p53 mutation, but not decreased mtDNA content and D-loop instability, had significant impacts on prognosis.

CONCLUSION

Change of mitochondrial DNA is a common event in colorectal cancer with p53 mutation, but is not associated with prognosis of CRC patients.

Mitochondria and energetic depression in cell pathophysiology

Mitochondrial dysfunction is a hallmark of almost all diseases. Acquired or inherited mutations of the mitochondrial genome DNA may give rise to mitochondrial diseases. Another class of disorders, in which mitochondrial impairments are initiated by extramitochondrial factors, includes neurodegenerative diseases and syndromes resulting from typical pathological processes, such as hypoxia/ischemia, inflammation, intoxications, and carcinogenesis. Both classes of diseases lead to cellular energetic depression (CED), which is characterized by decreased cytosolic phosphorylation potential that suppresses the cell's ability to do work and control the intracellular Ca(2+) homeostasis and its redox state. If progressing, CED leads to cell death, whose type is linked to the functional status of the mitochondria. In the case of limited deterioration, when some amounts of ATP can still be generated due to oxidative phosphorylation (OXPHOS), mitochondria launch the apoptotic cell death program by release of cytochrome c. Following pronounced CED, cytoplasmic ATP levels fall below the thresholds required for processing the ATP-dependent apoptotic cascade and the cell dies from necrosis. Both types of death can be grouped together as a mitochondrial cell death (MCD). However, there exist multiple adaptive reactions aimed at protecting cells against CED. In this context, a metabolic shift characterized by suppression of OXPHOS combined with activation of aerobic glycolysis as the main pathway for ATP synthesis (Warburg effect) is of central importance. Whereas this type of adaptation is sufficiently effective to avoid CED and to control the cellular redox state, thereby ensuring the cell survival, it also favors the avoidance of apoptotic cell death. This scenario may underlie uncontrolled cellular proliferation and growth, eventually resulting in carcinogenesis.

Targeting of cancer energy metabolism

The main purpose of this review is to update and analyze the effect of several antineoplastic drugs (adriamycin, apoptodilin, casiopeinas, cisplatin, clotrimazole, cyclophosphamide, ditercalinium, NSAIDs, tamoxifen, taxol, 6-mercaptopurine, and alpha-tocopheryl succinate) and energy metabolism inhibitors (2-DOG, gossypol, delocalized lipophilic cations, and uncouplers) on tumor development and progression. The possibility that these antineoplastic drugs currently used in in vitro cancer models, in chemo-therapy, or under study in phase I to III clinical trials induce tumor cellular death by altering also metabolite concentration (i.e., ATP), enzyme activities, and/or energy metabolism fluxes is assessed. It is proposed that the use of energy metabolic therapy, as an alternative or complementary strategy, might be a promising novel approach in the treatment of cancer.

Watching the watcher: regulation of p53 by mitochondria

p53 has been referred to as the 'guardian of the genome' because of its role in protecting the cell from DNA damage. p53 performs its duties by regulating cell-cycle progression and DNA repair and, in cases of irreparable DNA damage, by executing programmed cell death. Mitochondria are an important target of transcription-dependent and -independent actions of p53 to carry out the apoptotic function. However, increasing evidence suggests that p53 activity is regulated by mitochondria. Cellular insults that alter mitochondrial function can have important consequences on p53 activity. In light of these new findings, the following review focuses on p53/mitochondria connections, in particular how reactive oxygen species generated at mitochondria regulate p53 activity. A better understanding of the mechanisms by which mitochondria regulate p53 may have an impact on our understanding of the development and progression of many diseases, especially cancer.

Artemis is a negative regulator of p53 in response to oxidative stress

Artemis is a multifunctional phospho-protein with roles in V(D)J recombination, repair of double-strand breaks by nonhomologous end-joining, and regulation of cell cycle checkpoints after DNA damage. Here, we describe a novel function of Artemis as a negative regulator of p53 in response to oxidative stress in both primary cells and cancer cell lines. We show that depletion of Artemis under typical culture conditions (21% oxygen) leads to a spontaneous phosphorylation and stabilization of p53, and resulting cellular G1 arrest and apoptosis. These effects are suppressed by co-depletion of DNA-PKcs, but not ATM, indicating that Artemis is an inhibitor of DNA-PKcs-mediated stabilization of p53. Culturing of cells at 3% oxygen or treatment with an antioxidant abrogated p53 stabilization indicating that oxidative stress is the responsible cellular stimulus. Treatment with IR or hydrogen peroxide did not cause activation of this signaling pathway, while inhibitors of mitochondrial electron transport were effective in reducing its activation. In addition, we show that p53-inducible genes involved in reducing reactive oxygen species (ROS) are upregulated by Artemis depletion. These findings indicate that Artemis and DNA-PKcs participate in a novel, signaling pathway to modulate p53 function in response to oxidative stress produced by mitochondrial respiration.

Mitochondria and cancer

The authors review the role played by mutations in mitochondrial DNA and in nuclear genes encoding mitochondrial proteins in cancer development, with an emphasis on the alterations of the oxidative phosphorylation system and glycolysis.

Metabonomics: a useful tool for the future surgeon

BACKGROUND

In the past decade or so, a range of technologies have emerged that have shown promise in increasing our understanding of disease processes and progression. These advances are referred to as the "omics" technologies; genomics, transcriptomics, and proteomics. More recently, another "omics" approach has come to the fore: metabonomics, and this technology has the potential for significant clinical impact. Metabonomics refers to the analysis of the metabolome, that is, the metabolic profile of a system. The advantage of studying the metabolome is that the end points of biological events are elucidated.

RESULTS

Although still in its infancy, the metabonomics approach has shown immense promise in areas as diverse as toxicology studies to the discovery of biomarkers of disease. It has also been applied to studies of both renal and hepatic transplants. Metabolome analysis may be conducted on a variety of biological fluids and tissue types and may utilize a number of different technology platforms, mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy being the most popular. In this review, we cover the background to the evolution of metabonomics and its applications with particular emphasis on clinical applications.

CONCLUSIONS

We conclude with the suggestion that metabonomics offers a platform for further biomarker development, drug development, and in the field of medicine.

Winter temperature and UV are tightly linked to genetic changes in the p53 tumor suppressor pathway in Eastern Asia

The tumor suppressor p53 is a master sensor of stress. Two human-specific polymorphisms, p53 codon 72 and MDM2 SNP309, influence the activities of p53. There is a tight association between cold winter temperature and p53 Arg72 and between low UV intensity and MDM2 SNP309 G/G in a cohort of 4029 individuals across Eastern Asia that suggests causative selection. Moreover, the two polymorphisms are not coselected. Haplotype-based selection analysis further suggests that this is a striking example of two functional polymorphisms being strongly selected for in human populations in response to environmental stresses.

Metabolic transformation in cancer

In 2000, Douglas Hanahan and Robert Weinberg published a review detailing the six hallmarks of cancer. These are six phenotypes that a tumour requires in order to become a fully fledged malignancy: persistent growth signals, evasion of apoptosis, insensitivity to anti-growth signals, unlimited replicative potential, angiogenesis and invasion and metastasis. However, it is becoming increasingly clear that these phenotypes do not portray the whole story and that other hallmarks are necessary: one of which is a shift in cellular metabolism. The tumour environment creates a unique collection of stresses to which cells must adapt in order to survive. This environment is formed by the uncontrolled proliferation of cells, which ignore the cues that would create normal tissue architecture. As a result, the cells forming the tumour are exposed to low oxygen and nutrient levels, as well as high levels of toxic cellular waste products, which is thought to propel cells towards a more transformed phenotype, resistant to cell death and pro-metastatic.

Tumor suppressors and cell metabolism: a recipe for cancer growth

Growing tumors face two major metabolic challenges-how to meet the bioenergetic and biosynthetic demands of increased cell proliferation, and how to survive environmental fluctuations in external nutrient and oxygen availability when tumor growth outpaces the delivery capabilities of the existing vasculature. Cancer cells display dramatically altered metabolic circuitry that appears to directly result from the oncogenic mutations selected during the tumorigenic process. An emerging theme in cancer biology is that many of the genes that can initiate tumorigenesis are intricately linked to metabolic regulation. In turn, it appears that a number of well-established tumor suppressors play critical roles in suppressing growth and/or proliferation when intracellular supplies of essential metabolites become reduced. In this review, we consider the potential role of tumor suppressors as metabolic regulators.

Considering the role of pyruvate in tumor cells during hypoxia

Impairment of oxygen supply occurs in many pathological situations. In the case of cancer, both chronic and acute hypoxic areas are found in the tumor. Tumor hypoxia is associated with poor clinical prognoses and is correlated with tumor growth and metastasis development. Pyruvate is a common metabolite, as it is an end-product of glycolysis and an energy substrate for the mitochondrial Krebs cycle. It is also well known for its protective properties against stressful conditions, particularly hypoxia. Its presence determines cellular fate when there is a lack of oxygen. Interestingly, pyruvate metabolism is altered during cancer development. For years, this was assumed to be a consequence of malignant transformation. However, it now is becoming clear that pyruvate could contribute to cancer progression. The role of pyruvate during hypoxia has been widely studied in non-tumor tissues and cells; it is less documented whether or not the protective effect of pyruvate could also take place in cancer cells. If so, pyruvate might be deleterious for cancer patients. The present paper reviews data that highlight the role of pyruvate in cancer cells and tumors during hypoxic stress.

The regulation of the endosomal compartment by p53 the tumor suppressor gene

The endosomal compartment of the cell is involved in a number of functions including: (a) internalizing membrane proteins to multivesicular bodies and lysosomes; (b) producing vesicles that are secreted from the cell (exosomes); and (c) generating autophagic vesicles that, especially in times of nutrient deprivation, supply cytoplasmic components to the lysosome for degradation and recycling of nutrients. The p53 protein responds to various stress signals by initiating a transcriptional program that restores cellular homeostasis and prevents the accumulation of errors in a cell. As part of this process, p53 regulates the transcription of a set of genes encoding proteins that populate the endosomal compartment and impact upon each of these endosomal functions. Here, we demonstrate that p53 regulates transcription of the genes TSAP6 and CHMP4C, which enhance exosome production, and CAV1 and CHMP4C, which produce a more rapid endosomal clearance of the epidermal growth factor receptor from the plasma membrane. Each of these p53-regulated endosomal functions results in the slowing of cell growth and division, the utilization of catabolic resources and cell-to-cell communication by exosomes after a stress signal is detected by the p53 protein. These processes avoid errors during stress and restore homeostasis once the stress is resolved.

Role of p53 in mitochondrial biogenesis and apoptosis in skeletal muscle

p53 is a tumor suppressor protein that also plays a role in regulating aerobic metabolism. Since skeletal muscle is a major source of whole body aerobic respiration, it is important to delineate the effects of p53 on muscle metabolism. In p53 knockout (KO) mice, we observed diminished mitochondrial content in mixed muscle and lowered peroxisome proliferator-activated receptor-γ (PPARγ) coactivator (PGC)-1α protein levels in gastrocnemius muscle. In intermyofibrillar (IMF) mitochondria, lack of p53 was associated with reduced respiration and elevated reactive oxygen species production. Permeability transition pore kinetics remained unchanged; however, IMF mitochondrial cytochrome c release was reduced and DNA fragmentation was lowered, illustrating a resistance to mitochondrially driven apoptosis in muscle of KO mice. p53-null animals displayed similar muscle strength but greater fatigability and less locomotory endurance than wild-type (WT) animals. Surprisingly, the adaptive responses in mitochondrial content to running were similar in WT and KO mice. Thus p53 may be important, but not necessary, for exercise-induced mitochondrial biogenesis. In WT animals, acute muscle contractions induced the phosphorylation of p53 in concert with increased activation of upstream kinases AMP-activated protein kinase and p38, indicating a pathway through which p53 may initiate mitochondrial biogenesis in response to contractile activity. These data illustrate a novel role for p53 in maintaining mitochondrial biogenesis, apoptosis, and performance in skeletal muscle.

The bioenergetics of cancer: is glycolysis the main ATP supplier in all tumor cells?

The molecular mechanisms by which tumor cells achieve an enhanced glycolytic flux and, presumably, a decreased oxidative phosphorylation are analyzed. As the O(2) concentration in hypoxic regions of tumors seems not limiting for oxidative phosphorylation, the role of this mitochondrial pathway in the ATP supply is re-evaluated. Drugs that inhibit glycoysis and oxidative phosphorylation are analyzed for their specificity toward tumor cells and effect on proliferation. The energy metabolism mechanisms involved in the use of positron emission tomography are revised and updated. It is proposed that energy metabolism may be an alternative therapeutic target for both hypoxic (glycolytic) and oxidative tumors. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Silencing of elongation factor-2 kinase potentiates the effect of 2-deoxy-D-glucose against human glioma cells through blunting of autophagy

2-Deoxy-d-glucose (2-DG), a synthetic glucose analogue that acts as a glycolytic inhibitor, is currently being evaluated in the clinic as an anticancer agent. In this study, we observed that treatment of human glioma cells with 2-DG activated autophagy, a highly conserved cellular response to metabolic stress and a catabolic process of self-digestion of intracellular organelles for energy use and survival in stressed cells. The induction of autophagy by 2-DG was associated with activation of elongation factor-2 kinase (eEF-2 kinase), a structurally and functionally unique enzyme that phosphorylates eEF-2, leading to loss of affinity of this elongation factor for the ribosome and to termination of protein elongation. We also showed that inhibition of eEF-2 kinase by RNA interference blunted the 2-DG-induced autophagic response, resulted in a greater reduction of cellular ATP contents, and increased the sensitivity of tumor cells to the cytotoxic effect of 2-DG. Furthermore, the blunted autophagy and enhanced 2-DG cytotoxicity were accompanied by augmentation of apoptosis in cells in which eEF-2 kinase expression was knocked down. The results of this study indicate that the energy stress and cytotoxicity caused by 2-DG can be accelerated by inhibition of eEF-2 kinase, and suggest that targeting eEF-2 kinase-regulated autophagic survival pathway may represent a novel approach to sensitizing cancer cells to glycolytic inhibitors.

Mitochondrial DNA instability and metabolic shift in human cancers

A shift in glucose metabolism from oxidative phosphorylation to glycolysis is one of the biochemical hallmarks of tumor cells. Mitochondrial defects have been proposed to play an important role in the initiation and/or progression of various types of cancer. In the past decade, a wide spectrum of mutations and depletion of mtDNA have been identified in human cancers. Moreover, it has been demonstrated that activation of oncogenes or mutation of tumor suppressor genes, such as p53, can lead to the upregulation of glycolytic enzymes or inhibition of the biogenesis or assembly of respiratory enzyme complexes such as cytochrome c oxidase. These findings may explain, at least in part, the well documented phenomena of elevated glucose uptake and mitochondrial defects in cancers. In this article, we review the somatic mtDNA alterations with clinicopathological correlations in human cancers, and their potential roles in tumorigenesis, cancer progression, and metastasis. The signaling pathways involved in the shift from aerobic metabolism to glycolysis in human cancers are also discussed.