Understanding the Warburg effect: the metabolic requirements of cell proliferation

The molecular pathology of cancer

Rapid technical advances in DNA sequencing and genome-wide association studies are driving the discovery of the germline and somatic mutations that are present in different cancers. Mutations in genes involved in cellular signaling are common, and often shared by tumors that arise in distinct anatomical locations. Here we review the most important molecular changes in different cancers from the perspective of what should be analyzed on a routine basis in the clinic. The paradigms are EGFR mutations in adenocarcinoma of the lung that can be treated with gefitinib, KRAS mutations in colon cancer with respect to treatment with EGFR antibodies, and the use of gene-expression analysis for ER-positive, node-negative breast cancer patients with respect to chemotherapy options. Several other examples in both solid and hematological cancers are also provided. We focus on how disease subtypes can influence therapy and discuss the implications of the impending molecular diagnostic revolution from the point of view of the patients, clinicians, and the diagnostic and pharmaceutical companies. This paradigm shift is occurring first in cancer patient management and is likely to promote the application of these technologies to other diseases.

Phosphate-activated glutaminase (GLS2), a p53-inducible regulator of glutamine metabolism and reactive oxygen species

We identified a p53 target gene, phosphate-activated mitochondrial glutaminase (GLS2), a key enzyme in conversion of glutamine to glutamate, and thereby a regulator of glutathione (GSH) synthesis and energy production. GLS2 expression is induced in response to DNA damage or oxidative stress in a p53-dependent manner, and p53 associates with the GLS2 promoter. Elevated GLS2 facilitates glutamine metabolism and lowers intracellular reactive oxygen species (ROS) levels, resulting in an overall decrease in DNA oxidation as determined by measurement of 8-OH-dG content in both normal and stressed cells. Further, siRNA down-regulation of either GLS2 or p53 compromises the GSH-dependent antioxidant system and increases intracellular ROS levels. High ROS levels following GLS2 knockdown also coincide with stimulation of p53-induced cell death. We propose that GLS2 control of intracellular ROS levels and the apoptotic response facilitates the ability of p53 to protect cells from accumulation of genomic damage and allows cells to survive after mild and repairable genotoxic stress. Indeed, overexpression of GLS2 reduces the growth of tumor cells and colony formation. Further, compared with normal tissue, GLS2 expression is reduced in liver tumors. Thus, our results provide evidence for a unique metabolic role for p53, linking glutamine metabolism, energy, and ROS homeostasis, which may contribute to p53 tumor suppressor function.

Alterations in bioenergetics due to changes in mitochondrial DNA copy number

All mitochondrial DNA (mtDNA)-encoded genes are involved in mitochondrial electron transport and ATP production. Alterations of mtDNA due to dysfunctional mitochondrial DNA polymerase gamma (POLG) induce loss of mitochondrial oxidative phosphorylation (OXPHOS) and mitochondrial ATP generation. Total intracellular ATP is generated by two energetic pathways, glycolysis and mitochondrial OXPHOS. Decreased ATP generation from mitochondria due to mitochondrial dysfunction induces compensatory upregulation of cytoplasmic glycolysis process, thus increasing the contribution of glycolysis to the total cellular ATP generation. Decreased mitochondrial respiration and ATP generation with concomitant enhanced glycolysis is associated with mitochondrial disease and cancer. This chapter introduces a novel assay using a pharmacological profiling strategy in combination with a Seahorse XF24 instrument, which quantifies mitochondrial oxygen consumption rate and extracellular acidification rate for the measurement of OXPHOS and glycolysis, respectively. This assay combined with an analysis of steady-state ATP levels was used to study the bioenergetics of cells depleted of mtDNA (rho0 cells).

Inhibition of trans-plasma membrane electron transport: a potential anti-leukemic strategy

The recently demonstrated reliance of glycolytic cancer cells on trans-plasma membrane electron transport (tPMET) for survival raises the question of its suitability as a target for anticancer drug development. In this study, the effects of several new and known compounds on proliferation, tPMET activity and NAD(P)H intrinsic fluorescence in human myelogenous leukemic cell lines were investigated. The whole data confirm the importance of tPMET in leukemic cell survival and suggest this activity as a new potential anti-leukemic target.

Ethyl pyruvate stabilizes hypoxia-inducible factor 1 alpha via stimulation of the TCA cycle

Ethyl pyruvate is a simple derivative of the endogenous metabolite pyruvate. Pyruvate is the starting substrate for the tricarboxylic acid (TCA) cycle and plays a central role in intermediary metabolism. The present study was to determine whether ethyl pyruvate affects the expression of hypoxia-inducible factor 1 alpha (HIF-1alpha) and to explore the mechanism of HIF-1alpha regulation. We found that ethyl pyruvate increased HIF-1alpha stability via inhibition of pVHL-mediated degradation. Furthermore, ethyl pyruvate enhanced the reactive oxygen species (ROS) generated through the TCA cycle in mitochondria. Taken together, our results support a novel role for ethyl pyruvate in HIF-1alpha stabilization by which high rates of the TCA cycle can promote ROS production.

Is (18)F-FDG a surrogate tracer to measure tumor hypoxia? Comparison with the hypoxic tracer (14)C-EF3 in animal tumor models

INTRODUCTION

Fluorodeoxyglucose (FDG) has been reported as a surrogate tracer to measure tumor hypoxia with positron emission tomography (PET). The hypothesis is that there is an increased uptake of FDG under hypoxic conditions secondary to enhanced glycolysis, compensating the hypoxia-induced loss of cellular energy production. Several studies have already addressed this issue, some with conflicting results. This study aimed to compare the tracers (14)C-EF3 and (18)F-FDG to detect hypoxia in mouse tumor models.

MATERIALS AND METHODS

C3H, tumor-bearing mice (FSAII and SCCVII tumors) were injected iv with (14)C-EF3, and 1h later with (18)F-FDG. Using a specifically designed immobilization device with fiducial markers, PET (Mosaic®, Philips) images were acquired 1h after the FDG injection. After imaging, the device containing mouse was frozen, transversally sliced and imaged with autoradiography (AR) (FLA-5100, Fujifilm) to obtain high resolution images of the (18)F-FDG distribution within the tumor area. After a 48-h delay allowing for (18)F decay a second AR was performed to image (14)C-EF3 distribution. AR images were aligned to reconstruct the full 3D tumor volume, and were compared with the PET images. Image segmentation with threshold-based methods was applied on both AR and PET images to derive various tracer activity volumes. The matching index DSI (dice similarity index) was then computed. The comparison was performed under normoxic (ambient air, FSAII: n=4, SCCVII, n=5) and under hypoxic conditions (10% O(2) breathing, SCCVII: n=4).

RESULTS

On AR, under both ambient air and hypoxic conditions, there was a decreasing similarity between (14)C-EF3 and FDG with higher activity sub-volumes. Under normoxic conditions, when comparing the 10% of tumor voxels with the highest (18)F-FDG or (14)C-EF3 activity, a DSI of 0.24 and 0.20 was found for FSAII and SCCVII, respectively. Under hypoxic conditions, a DSI of 0.36 was observed for SCCVII tumors. When comparing the (14)C-EF3 distribution in AR with the corresponding (18)F-FDG-PET images, the DSI reached values of 0.26, 0.22 and 0.21 for FSAII and SCCVII under normoxia and SCCVII under hypoxia, respectively.

CONCLUSION

This study showed that FDG is not a good surrogate tracer for tumor hypoxia under either ambient or hypoxic conditions. Only specific hypoxia tracers should be used to measure tumor hypoxia.

The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

Blood and tissue biomarkers in prostate cancer: state of the art

The prevalence of prostate cancer (PCa) is high and increases with age. PCa is the most common cutaneous cancer in American men. Prostate-specific antigen (PSA) screening has impacted the detection of PCa and is directly responsible for a dramatic decrease in stage at diagnosis. Gleason score and stage at the time of diagnosis remain the mainstay to predict prognosis, in the absence of more accurate and reliable tissue or blood biomarkers. Despite extensive research efforts, very few biomarkers of PCa have been introduced to date in clinical practice. Even screening with PSA has recently been questioned. A thorough analysis of all tissue and serum biomarkers in prostate cancer research cannot be easily synthesized, and goes beyond the scope of the present article. Therefore the authors focus here on the most recently reported tissue and circulating biomarkers for PCa whose application in clinical practice is either current or expected in the near future.

Wnt signalling and the control of cellular metabolism

At the cellular level, the biological processes of cell proliferation, growth arrest, differentiation and apoptosis are all tightly coupled to appropriate alterations in metabolic status. In the case of cell proliferation, this requires redirecting metabolic pathways to provide the fuel and basic components for new cells. Ultimately, the successful co-ordination of cell-specific biology with cellular metabolism underscores multicellular processes as diverse as embryonic development, adult tissue remodelling and cancer cell biology. The Wnt signalling network has been implicated in all of these areas. While each of the Wnt-dependent signalling pathways are being individually delineated in a range of experimental systems, our understanding of how they integrate and regulate cellular metabolism is still in its infancy. In the present review we reassess the roles of Wnt signalling in functionally linking cellular metabolism to tissue development and function.

MicroRNA-451 regulates LKB1/AMPK signaling and allows adaptation to metabolic stress in glioma cells

To sustain tumor growth, cancer cells must be able to adapt to fluctuations in energy availability. We have identified a single microRNA that controls glioma cell proliferation, migration, and responsiveness to glucose deprivation. Abundant glucose allows relatively high miR-451 expression, promoting cell growth. In low glucose, miR-451 levels decrease, slowing proliferation but enhancing migration and survival. This allows cells to survive metabolic stress and seek out favorable growth conditions. In glioblastoma patients, elevated miR-451 is associated with shorter survival. The effects of miR-451 are mediated by LKB1, which it represses through targeting its binding partner, CAB39 (MO25 alpha). Overexpression of miR-451 sensitized cells to glucose deprivation, suggesting that its downregulation is necessary for robust activation of LKB1 in response to metabolic stress. Thus, miR-451 is a regulator of the LKB1/AMPK pathway, and this may represent a fundamental mechanism that contributes to cellular adaptation in response to altered energy availability.

Pioglitazone modulates tumor cell metabolism and proliferation in multicellular tumor spheroids

The anti-diabetic thiazolidinedione compound pioglitazone, a peroxisome proliferator-activated receptor-gamma agonist, and selective cyclooxygenase-2 inhibitors are clinically used in patients with advanced malignancies. Several previously published in vivo and in vitro studies showed growth inhibitory effects on different cancer cell lines. However, the underlying mechanisms are fairly unclear. Here, we analyzed the effects of pioglitazone in combination with other drugs in a three-dimensional multicellular tumor spheroid culture system (MCTS) generated from the two prostate carcinoma cell lines PC3 and LNCaP. As expected, pioglitazone also inhibited tumor cell proliferation in the MCTS system. Further studies revealed that pioglitazone lowered the pH of the culture medium, decreased oxygen consumption and increased lactate secretion in both tumor cell lines. Other glitazones, troglitazone and ciglitazone, had similar effects. The combination of pioglitazone with 2-deoxyglucose, a potent inhibitor of glycolysis, had an additive effect on the inhibition of cell proliferation and led to MCTS disintegration. Our data propose a new mechanism of growth inhibition by pioglitazone through modulation of the tumor cell metabolism.

Ex vivo expansion of human mesenchymal stem cells: a more effective cell proliferation kinetics and metabolism under hypoxia

The low bone marrow (BM) MSC titers demand a fast ex vivo expansion process to meet the clinically relevant cell dosage. Attending to the low oxygen tension of BM in vivo, we studied the influence of hypoxia on human BM MSC proliferation kinetics and metabolism. Human BM MSC cultured under 2% (hypoxia) and 20% O(2) (normoxia) were characterized in terms of proliferation, cell division kinetics and metabolic patterns. BM MSC cultures under hypoxia displayed an early start of the exponential growth phase, and cell numbers obtained at each time point throughout culture were consistently higher under low O(2), resulting in a higher fold increase after 12 days under hypoxia (40 +/- 10 vs. 30 +/- 6). Cell labeling with PKH26 allowed us to determine that after 2 days of culture, a significant higher cell number was already actively dividing under 2% compared to 20% O(2) and BM MSC expanded under low oxygen tension displayed consistently higher percentages of cells in the latest generations (generations 4-6) until the 5th day of culture. Cells under low O(2) presented higher specific consumption of nutrients, especially early in culture, but with lower specific production of inhibitory metabolites. Moreover, 2% O(2) favored CFU-F expansion, while maintaining BM MSC characteristic immunophenotype and differentiative potential. Our results demonstrated a more efficient BM MSC expansion at 2% O(2), compared to normoxic conditions, associated to an earlier start of cellular division and supported by an increase in cellular metabolism efficiency towards the maximization of cell yield for application in clinical settings.

Hypoxia-inducible factors 1alpha and 2alpha exert both distinct and overlapping functions in long bone development

The hypoxia-inducible factors have recently been identified as critical regulators of angiogenic-osteogenic coupling. Mice overexpressing HIFalpha subunits in osteoblasts produce abundant VEGF and develop extremely dense, highly vascularized long bones. In this study, we investigated the individual contributions of Hif-1alpha and Hif-2alpha in angiogenesis and osteogenesis by individually disrupting each Hifalpha gene in osteoblasts using the Cre-loxP method. Mice lacking Hif-1alpha demonstrated markedly decreased trabecular bone volume, reduced bone formation rate, and altered cortical bone architecture. By contrast, mice lacking Hif-2alpha had only a modest decrease in trabecular bone volume. Interestingly, long bone blood vessel development measured by angiography was decreased by a similar degree in both DeltaHif-1alpha and DeltaHif-2alpha mice suggesting a common role for these Hifalpha subunits in skeletal angiogenesis. In agreement with this idea, osteoblasts lacking either Hif-1alpha or Hif-2alpha had profound reductions in VEGF mRNA expression but only the loss of Hif-1alpha impaired osteoblast proliferation. These findings indicate that expression of both Hif-1alpha and Hif-2alpha by osteoblasts is required for long bone development. We propose that both Hif-1alpha and Hif-2alpha function through cell non-autonomous modes to promote vascularization of bone and that Hif-1alpha also promotes bone formation by exerting direct actions on the osteoblast.

Lipid droplet-associated PAT-proteins show frequent and differential expression in neoplastic steatogenesis

In many human cancers, lipogenic pathways are activated; in some tumors, such as hepatocellular carcinoma, this is reflected by the presence of visible lipid droplets. Yet, the biology of steatogenesis in malignant tumors is largely unknown. We have recently shown that lipid droplet-associated proteins of the PAT-family, named after their constituents perilipin (perilipin 1), adipophilin (perilipin 2), and TIP47 (perilipin 3) are differentially expressed in hepatic steatogenesis. We have comprehensively investigated PAT-expression in neoplastic steatogenesis as well as in respective normal tissues with immunohistology and electron microscopy as well as protein biochemical and molecular biological methods. By staining for PAT-proteins, we found lipid droplet accumulation to be a frequent phenomenon of carcinoma cells. Although adipophilin and TIP47 stained almost ubiquitously the rim of lipid droplets in various tumor types, especially those with clear cell phenotype, perilipin was restricted to lipid droplets of hepatocellular adenoma and carcinoma, sebaceous adenoma and carcinoma, and lipomatous tumors. In hepatocellular carcinoma, perilipin, adipophilin, and TIP47 were coexpressed, and showed regional heterogeneity with a predominantly mutually exclusive localization pattern. In step-wise carcinogenesis, adipophilin expression correlated with the proliferation rate and was upregulated during early tumorigenesis, whereas perilipin was often lost during hepatocarcinogenesis. In conclusion, expression analysis of PAT-proteins showed that by far more carcinomas contain (PAT-positive) lipid droplets than expected by conventional light microscopy. PAT-proteins, such as perilipin, are differentially expressed in different tumor types and thus may support diagnostic considerations. Because inhibition of lipogenesis has been shown to exert antineoplastic effects, PAT-proteins may represent targets for interventive strategies.

Metabolism in T cell activation and differentiation

When naïve or memory T cells encounter foreign antigen along with proper co-stimulation they undergo rapid and extensive clonal expansion. In mammals, this type of proliferation is fairly unique to cells of the adaptive immune system and requires a considerable expenditure of energy and cellular resources. While research has often focused on the roles of cytokines, antigenic signals, and co-stimulation in guiding T cell responses, data indicate that, at a fundamental level, it is cellular metabolism that regulates T cell function and differentiation and therefore influences the final outcome of the adaptive immune response. This review will focus on some earlier fundamental observations regarding T cell bioenergetics and its role in regulating cellular function, as well as recent work that suggests that manipulating the immune response by targeting lymphocyte metabolism could prove useful in treatments against infection and cancer.

Insulin analogues and cancer risk: cause for concern or cause célèbre?

People with diabetes, particularly those with type 2 diabetes, may be at an increased risk of cancer. Furthermore, their cancer risk may be modified by treatment choices. In this respect, metformin may be protective, whereas insulin and insulin analogues can function as growth factors and therefore have theoretical potential to promote tumour proliferation. Analogues causing inappropriate prolonged stimulation of the insulin receptor, or excess stimulation of the IGF-1 receptor, are the most likely to show mitogenic properties in laboratory studies. Some recent epidemiological studies appear to be consistent with these experimental findings, suggesting that there could be different relative risks for cancer associated with different insulins, although these studies have attracted some methodological criticism. However, it is biologically plausible that hormonal factors that influence neoplasia could begin to manifest their effects in surprisingly short timescales (within 2 years) and hence these epidemiological studies justify further research. Even if future research were to document an increase in cancer risk among insulin users, this would be unlikely to significantly diminish the favourable benefit-risk ratio for patients requiring insulin therapy. There is a need for further population studies and for the development of new laboratory models that are more sophisticated than previous experimental methods employed to assess potential tumour growth-promoting properties of insulins.

Evaluation of substituted N,N'-diarylsulfonamides as activators of the tumor cell specific M2 isoform of pyruvate kinase

The metabolism of cancer cells is altered to support rapid proliferation. Pharmacological activators of a tumor cell specific pyruvate kinase isozyme (PKM2) may be an approach for altering the classic Warburg effect characteristic of aberrant metabolism in cancer cells yielding a novel antiproliferation strategy. In this manuscript, we detail the discovery of a series of substituted N,N'-diarylsulfonamides as activators of PKM2. The synthesis of numerous analogues and the evaluation of structure-activity relationships are presented as well as assessments of mechanism and selectivity. Several agents are found that have good potencies and appropriate solubility for use as chemical probes of PKM2 including 55 (AC(50) = 43 nM, maximum response = 84%; solubility = 7.3 microg/mL), 56 (AC(50) = 99 nM, maximum response = 84%; solubility = 5.7 microg/mL), and 58 (AC(50) = 38 nM, maximum response = 82%; solubility = 51.2 microg/mL). The small molecules described here represent first-in-class activators of PKM2.

Chewing the fat on tumor cell metabolism

Tumor cells undergo a metabolic shift toward specific bioenergetic (glycolysis) and anabolic (protein and lipid synthesis) processes that promote rapid growth. Nomura et al. (2010) now demonstrate that an increase in monoacylglycerol lipase (MAGL) drives tumorigenesis through the lipolytic release and remodeling of free fatty acids.

Common corruption of the mTOR signaling network in human tumors

The mammalian target of rapamycin (mTOR) is responsive to numerous extracellular and intracellular cues and, through the formation of two physically and functionally distinct complexes, has a central role in the homeostatic control of cell growth, proliferation and survival. Through the aberrant activation of mTOR signaling, the perception of cellular growth signals becomes disconnected from the processes promoting cell growth, and this underlies the pathophysiology of a number of genetic tumor syndromes and cancers. Here, we review the oncogenes and tumor suppressors comprising the regulatory network upstream of mTOR, highlight the human cancers in which mTOR is activated and discuss how dysregulated mTOR signaling provides tumors a selective growth advantage. In addition, we discuss why activation of mTOR, as a consequence of distinct oncogenic events, results in diverse clinical outcomes, and how the complexity of the mTOR signaling network might dictate therapeutic approaches.

A microRNA expression signature for cervical cancer prognosis

Invasive cervical cancer is a leading cause of cancer death in women worldwide, resulting in about 300,000 deaths each year. The clinical outcomes of cervical cancer vary significantly and are difficult to predict. Thus, a method to reliably predict disease outcome would be important for individualized therapy by identifying patients with high risk of treatment failures before therapy. In this study, we have identified a microRNA (miRNA)-based signature for the prediction of cervical cancer survival. miRNAs are a newly identified family of small noncoding RNAs that are extensively involved in human cancers. Using an established PCR-based miRNA assay to analyze 102 cervical cancer samples, we identified miR-200a and miR-9 as two miRNAs that could predict patient survival. A logistic regression model was developed based on these two miRNAs and the prognostic value of the model was subsequently validated with independent cervical cancers. Furthermore, functional studies were done to characterize the effect of miRNAs in cervical cancer cells. Our results suggest that both miR-200a and miR-9 could play important regulatory roles in cervical cancer control. In particular, miR-200a is likely to affect the metastatic potential of cervical cancer cells by coordinate suppression of multiple genes controlling cell motility.

Allosteric mechanism of pyruvate kinase from Leishmania mexicana uses a rock and lock model

Allosteric regulation provides a rate management system for enzymes involved in many cellular processes. Ligand-controlled regulation is easily recognizable, but the underlying molecular mechanisms have remained elusive. We have obtained the first complete series of allosteric structures, in all possible ligated states, for the tetrameric enzyme, pyruvate kinase, from Leishmania mexicana. The transition between inactive T-state and active R-state is accompanied by a simple symmetrical 6 degrees rigid body rocking motion of the A- and C-domain cores in each of the four subunits. However, formation of the R-state in this way is only part of the mechanism; eight essential salt bridge locks that form across the C-C interface provide tetramer rigidity with a coupled 7-fold increase in rate. The results presented here illustrate how conformational changes coupled with effector binding correlate with loss of flexibility and increase in thermal stability providing a general mechanism for allosteric control.

Metabolic genes in cancer: their roles in tumor progression and clinical implications

Re-programming of metabolic pathways is a hallmark of physiological changes in cancer cells. The expression of certain genes that directly control the rate of key metabolic pathways including glycolysis, lipogenesis and nucleotide synthesis are drastically altered at different stages of tumor progression. These alterations are generally considered as an adaptation of tumor cells; however, they also contribute to the progression of tumor cells to become more aggressive phenotypes. This review summarizes the recent information about the mechanistic link of these genes to oncogenesis and their potential utility as diagnostic markers as well as for therapeutic targets. We particularly focus on three groups of genes; GLUT1, G6PD, TKTL1 and PGI/AMF in glycolytic pathway, ACLY, ACC1 and FAS in lipogenesis and RRM2, p53R2 and TYMS for nucleotide synthesis. All these genes are highly up-regulated in a variety of tumor cells in cancer patients, and they play active roles in tumor progression rather than expressing merely as a consequence of phenotypic change of the cancer cells. Molecular dissection of their orchestrated networks and understanding the exact mechanism of their expression will provide a window of opportunity to target these genes for specific cancer therapy. We also reviewed existing database of gene microarray to validate the utility of these genes for cancer diagnosis.

Cancer as a metabolic disease

Emerging evidence indicates that impaired cellular energy metabolism is the defining characteristic of nearly all cancers regardless of cellular or tissue origin. In contrast to normal cells, which derive most of their usable energy from oxidative phosphorylation, most cancer cells become heavily dependent on substrate level phosphorylation to meet energy demands. Evidence is reviewed supporting a general hypothesis that genomic instability and essentially all hallmarks of cancer, including aerobic glycolysis (Warburg effect), can be linked to impaired mitochondrial function and energy metabolism. A view of cancer as primarily a metabolic disease will impact approaches to cancer management and prevention.

Circadian proteins and genotoxic stress response

The circadian clock is an evolutionarily conserved time-keeping system that coordinates the physiology of the organism with daily changes in the environment. A growing body of evidence gradually leads to the conception that virtually all aspects of the biochemical, physiological, and behavioral functions of the animal are linked to circadian regulation. Moreover, proper synchronization of various processes through the activity of circadian components is important for the well-being of many organisms, including humans. The focus of this review is the circadian control of an organism's response to genotoxic stress, which is a major contributor to life-threatening human pathologies such as cancer and cardiovascular disease.

The histone deacetylase Sirt6 regulates glucose homeostasis via Hif1alpha

SIRT6 is a member of a highly conserved family of NAD(+)-dependent deacetylases with various roles in metabolism, stress resistance, and life span. SIRT6-deficient mice develop normally but succumb to a lethal hypoglycemia early in life; however, the mechanism underlying this hypoglycemia remained unclear. Here, we demonstrate that SIRT6 functions as a histone H3K9 deacetylase to control the expression of multiple glycolytic genes. Specifically, SIRT6 appears to function as a corepressor of the transcription factor Hif1alpha, a critical regulator of nutrient stress responses. Consistent with this notion, SIRT6-deficient cells exhibit increased Hif1alpha activity and show increased glucose uptake with upregulation of glycolysis and diminished mitochondrial respiration. Our studies uncover a role for the chromatin factor SIRT6 as a master regulator of glucose homeostasis and may provide the basis for novel therapeutic approaches against metabolic diseases, such as diabetes and obesity.

Cancer proliferation and therapy: the Warburg effect and quantum metabolism

Background

Most cancer cells, in contrast to normal differentiated cells, rely on aerobic glycolysis instead of oxidative phosphorylation to generate metabolic energy, a phenomenon called the Warburg effect.

Model

Quantum metabolism is an analytic theory of metabolic regulation which exploits the methodology of quantum mechanics to derive allometric rules relating cellular metabolic rate and cell size. This theory explains differences in the metabolic rates of cells utilizing OxPhos and cells utilizing glycolysis. This article appeals to an analytic relation between metabolic rate and evolutionary entropy - a demographic measure of Darwinian fitness - to: (a) provide an evolutionary rationale for the Warburg effect, and (b) propose methods based on entropic principles of natural selection for regulating the incidence of OxPhos and glycolysis in cancer cells.

Conclusion

The regulatory interventions proposed on the basis of quantum metabolism have applications in therapeutic strategies to combat cancer. These procedures, based on metabolic regulation, are non-invasive, and complement the standard therapeutic methods involving radiation and chemotherapy

Rethinking the Warburg effect with Myc micromanaging glutamine metabolism

The MYC oncogene, which is frequently deregulated in human cancers, encodes a master transcription factor c-Myc (herein termed Myc) that integrates cell proliferation with metabolism through its regulation of thousands of genes including microRNAs (miRNA). In addition to its known function in regulating the cell cycle and glucose metabolism, recent studies document a role for Myc in stimulating glutamine catabolism, in part through the repression of miRNAs miR-23a and miR-23b. These observations suggest an additional level of complexity in tumor metabolism, which includes the commensal metabolic relationship between hypoxic and nonhypoxic regions of tumors as well as the surrounding stroma. Thus, a reevaluation of cancer metabolism considering glutamine catabolism with a better understanding of the tumor histological complexity is needed before cancer metabolism can be effectively targeted in therapy.

Inhibition of lactate dehydrogenase A induces oxidative stress and inhibits tumor progression

As the result of genetic alterations and tumor hypoxia, many cancer cells avidly take up glucose and generate lactate through lactate dehydrogenase A (LDHA), which is encoded by a target gene of c-Myc and hypoxia-inducible factor (HIF-1). Previous studies with reduction of LDHA expression indicate that LDHA is involved in tumor initiation, but its role in tumor maintenance and progression has not been established. Furthermore, how reduction of LDHA expression by interference or antisense RNA inhibits tumorigenesis is not well understood. Here, we report that reduction of LDHA by siRNA or its inhibition by a small-molecule inhibitor (FX11 [3-dihydroxy-6-methyl-7-(phenylmethyl)-4-propylnaphthalene-1-carboxylic acid]) reduced ATP levels and induced significant oxidative stress and cell death that could be partially reversed by the antioxidant N-acetylcysteine. Furthermore, we document that FX11 inhibited the progression of sizable human lymphoma and pancreatic cancer xenografts. When used in combination with the NAD(+) synthesis inhibitor FK866, FX11 induced lymphoma regression. Hence, inhibition of LDHA with FX11 is an achievable and tolerable treatment for LDHA-dependent tumors. Our studies document a therapeutical approach to the Warburg effect and demonstrate that oxidative stress and metabolic phenotyping of cancers are critical aspects of cancer biology to consider for the therapeutical targeting of cancer energy metabolism.

Selection of cancer cells with repressed mitochondria triggers colon cancer progression

The contribution that mitochondrial bioenergetics could have in cancer development is debated. Here, we have generated HCT116-derived colocarcinoma cell lines expressing different levels of the beta catalytic subunit of the mitochondrial H+-adenosine triphosphate synthase to assess the contribution of mitochondrial bioenergetics in colon cancer progression. The generated cells exhibit large ultrastructural, transcriptomic, proteomic and functional differences in their mitochondria and in their in vivo tumor forming capacity. We show that the activity of oxidative phosphorylation defines the rate of glucose utilization by aerobic glycolysis. The aggressive cellular phenotype, which is highly glycolytic, is bound to the deregulated expression of genes involved in metabolic processes, the regulation of the cell cycle, apoptosis, angiogenesis and cell adhesion. Remarkably, the molecular and ultrastructural analysis of the tumors derived from the three HCT116 cell lines under study highlight that tumor promotion inevitably requires the selection of cancer cells with a repressed biogenesis and functional activity of mitochondria, i.e. the highly glycolytic phenotype is selected for tumor development. The tumor forming potential of the cells is a non-genetically acquired condition that provides the cancer cell with a cell-death resistant phenotype. An abrogated mitochondrial respiration contributes to a diminished potential for reactive oxygen species signaling in response to 5-fluorouracil treatment. Treatment of cancer cells with dichloroacetate partially restores the functional differentiation of mitochondria and promotes tumor regression, emphasizing the reversible nature of the metabolic trait of cancer.

Intracellular succinylation of 8-chloroadenosine and its effect on fumarate levels

8-Chloroadenosine (8-Cl-Ado) is a ribosyl nucleoside analog currently in phase I testing for the treatment of chronic lymphocytic leukemia (CLL). 8-Cl-Ado activity is dependent on adenosine kinase and requires intracellular accumulation of 8-Cl-Ado as mono-, di-, and tri-phosphates. In the current study with four mantle cell lymphoma cell lines, we report a new major metabolic pathway for 8-Cl-Ado intracellular metabolism, the formation of succinyl-8-chloro-adenosine (S-8-Cl-Ado) and its monophosphate (S-8-Cl-AMP). 8-Cl-AMP levels were highly associated with S-8-Cl-AMP levels and reached a steady-state prior to the secondary metabolites, 8-Cl-ATP and S-8-Cl-Ado. Consistent with fumarate as a required substrate for formation of succinyl-8-Cl-adenylate metabolites, the S-8-Cl-adenylate concentrations in multiple cell lines were associated with fumarate loss. The distribution of metabolites was also altered using the energy metabolism modifiers, metformin and oligomycin. The rates of succinyl-8-Cl-adenylate metabolism were enhanced by increasing the intracellular fumarate concentrations after metformin co-treatment. In addition, the S-8-Cl-AMP concentrations were increased after acute inhibition of ATP synthase by oligomycin. We conclude that 8-Cl-Ado metabolism not only affects intracellular purine metabolism; 8-Cl-Ado conversion to succinyl analogs ties its metabolism to the citric acid cycle by reduction of the fumarate pool.

Metabolic systems biology

The first full genome sequences were established in the mid-1990s. Shortly thereafter, genome-scale metabolic network reconstructions appeared. Since that time, we have witnessed an exponential growth in their number and uses. Here I discuss, from a personal point of view, four topics: (1) the placement of metabolic systems biology in the context of broader scientific developments, (2) its foundational concepts, (3) some of its current uses, and (4) some of the expected future developments in the field.

Mutant metabolic enzymes are at the origin of gliomas

Mutations of the isocitrate dehydrogenase (IDH) metabolic enzymes IDH1 and IDH2 have been found to be frequent and early genetic alterations in astrocytomas and oligodendrogliomas. All mutations identified to date affect a single amino acid located within the isocitrate binding site (R132 of IDH1 and the analogous R172 residue of IDH2). IDH1 and IDH2 mutations define a specific subtype of gliomas and may have significant utility for the diagnosis, prognosis, and treatment of patients with these tumors.

Oxygen sensing: a common crossroad in cancer and neurodegeneration

Prolyl hydroxylase domain (PHD) proteins are cellular oxygen sensors that orchestrate an adaptive response to hypoxia and oxidative stress, executed by hypoxia-inducible factors (HIFs). By increasing oxygen supply, reducing oxygen consumption, and reprogramming metabolism, the PHD/HIF pathway confers tolerance towards hypoxic and oxidative stress. This review discusses the involvement of the PHD/HIF response in two, at first sight, entirely distinct pathologies with opposite outcome, i.e. cancer leading to cellular growth and neurodegeneration resulting in cell death. However, these disorders share common mechanisms of sensing oxygen and oxidative stress. We will focus on how PHD/HIF signaling is pathogenetically implicated in metabolic and vessel alterations in these diseases and how manipulation of this pathway might offer novel treatment opportunities.

Roles of mitochondria in human disease

The chapters throughout this volume illustrate the many contributions of mitochondria to the maintenance of normal cell and tissue function, experienced as the health of the individual. Mitochondria are essential for maintaining aspects of physiology as fundamental as cellular energy balance, the modulation of calcium signalling, in defining cellular redox balance, and they house significant biosynthetic pathways. Mitochondrial numbers and volume within cells are regulated and have an impact on their functional roles, while, especially in the CNS (central nervous system), mitochondrial trafficking is critical to ensure the cellular distribution and strategic localization of mitochondria, presumably driven by local energy demand. Maintenance of a healthy mitochondrial population involves a complex system of quality control, involving degrading misfolded proteins, while damaged mitochondria are renewed by fusion or removed by autophagy. It seems evident that mechanisms that impair any of these processes will impair mitochondrial function and cell signalling pathways, leading to disordered cell function which manifests as disease. As gatekeepers of cell life and cell death, mitochondria regulate both apoptotic and necrotic cell death, and so at its most extreme, disturbances involving these pathways may trigger untimely cell death. Conversely, the lack of appropriate cell death can lead to inappropriate tissue growth and development of tumours, which are also characterized by altered mitochondrial metabolism. The centrality of mitochondrial dysfunction to a surprisingly wide range of major human diseases is slowly becoming recognized, bringing with it the prospect of novel therapeutic approaches to treat a multitude of unpleasant and pervasive diseases.

A new view of carcinogenesis and an alternative approach to cancer therapy

During the last few decades, cancer research has focused on the idea that cancer is caused by genetic alterations and that this disease can be treated by reversing or targeting these alterations. The small variations in cancer mortality observed during the previous 30 years indicate, however, that the clinical applications of this approach have been very limited so far. The development of future gene-based therapies that may have a major impact on cancer mortality may be compromised by the high number and variability of genetic alterations recently found in human tumors. This article reviews evidence that tumor cells, in addition to acquiring a complex array of genetic changes, develop an alteration in the metabolism of oxygen. Although both changes play an essential role in carcinogenesis, the altered oxygen metabolism of cancer cells is not subject to the high genetic variability of tumors and may therefore be a more reliable target for cancer therapy. The utility of this novel approach for the development of therapies that selectively target tumor cells is discussed.

E3 ubiquitin ligase APC/C-Cdh1 accounts for the Warburg effect by linking glycolysis to cell proliferation

Cell proliferation is known to be accompanied by activation of glycolysis. We have recently discovered that the glycolysis-promoting enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, isoform 3 (PFKFB3), is degraded by the E3 ubiquitin ligase APC/C-Cdh1, which also degrades cell-cycle proteins. We now show in two different cell types (neoplastic and nonneoplastic) that both proliferation and aerobic glycolysis are prevented by overexpression of Cdh1 and enhanced by its silencing. Furthermore, we have coexpressed Cdh1 with PFKFB3--either wild-type or a mutant form resistant to ubiquitylation by APC/C-Cdh1--or with the glycolytic enzyme 6-phosphofructo-1-kinase and demonstrated that whereas glycolysis is essential for cell proliferation, its initiation in the presence of active Cdh1 does not result in proliferation. Our experiments indicate that the proliferative response, regardless of whether it occurs in normal or neoplastic cells, is dependent on a decrease in the activity of APC/C-Cdh1, which activates both proliferation and glycolysis. These observations have implications for cell proliferation, neoplastic transformation, and the prevention and treatment of cancer.

Pharmacologic inhibition of fatty acid oxidation sensitizes human leukemia cells to apoptosis induction

The traditional view is that cancer cells predominately produce ATP by glycolysis, rather than by oxidation of energy-providing substrates. Mitochondrial uncoupling--the continuing reduction of oxygen without ATP synthesis--has recently been shown in leukemia cells to circumvent the ability of oxygen to inhibit glycolysis, and may promote the metabolic preference for glycolysis by shifting from pyruvate oxidation to fatty acid oxidation (FAO). Here we have demonstrated that pharmacologic inhibition of FAO with etomoxir or ranolazine inhibited proliferation and sensitized human leukemia cells--cultured alone or on bone marrow stromal cells--to apoptosis induction by ABT-737, a molecule that releases proapoptotic Bcl-2 proteins such as Bak from antiapoptotic family members. Likewise, treatment with the fatty acid synthase/lipolysis inhibitor orlistat also sensitized leukemia cells to ABT-737, which supports the notion that fatty acids promote cell survival. Mechanistically, we generated evidence suggesting that FAO regulates the activity of Bak-dependent mitochondrial permeability transition. Importantly, etomoxir decreased the number of quiescent leukemia progenitor cells in approximately 50% of primary human acute myeloid leukemia samples and, when combined with either ABT-737 or cytosine arabinoside, provided substantial therapeutic benefit in a murine model of leukemia. The results support the concept of FAO inhibitors as a therapeutic strategy in hematological malignancies.

Activating mutations in TOR are in similar structures as oncogenic mutations in PI3KCalpha

TOR (Target of Rapamycin) is a highly conserved Ser/Thr kinase and a central controller of cell growth. Using the crystal structure of the related lipid kinase PI3KCγ, we built a model of the catalytic region of TOR, from the FAT domain to near the end of the FATC domain. The model reveals that activating mutations in TOR, identified in yeast in a genetic selection for Rheb-independence, correspond to hotspots for oncogenic mutations in PI3KCα. The activating mutations are in the catalytic domain (helices kα3, kα9, kα11) and the helical domain of TOR. Docking studies with small molecule inhibitors (PP242, NVP-BEZ235, and Ku-0063794) show that drugs currently in development utilize a novel pharmacophore space to achieve specificity. Thus, our model provides insight on the regulation of TOR and may be useful in the design of new anticancer drugs.

Mechanisms underlying metabolic and neural defects in zebrafish and human multiple acyl-CoA dehydrogenase deficiency (MADD)

In humans, mutations in electron transfer flavoprotein (ETF) or electron transfer flavoprotein dehydrogenase (ETFDH) lead to MADD/glutaric aciduria type II, an autosomal recessively inherited disorder characterized by a broad spectrum of devastating neurological, systemic and metabolic symptoms. We show that a zebrafish mutant in ETFDH, xavier, and fibroblast cells from MADD patients demonstrate similar mitochondrial and metabolic abnormalities, including reduced oxidative phosphorylation, increased aerobic glycolysis, and upregulation of the PPARG-ERK pathway. This metabolic dysfunction is associated with aberrant neural proliferation in xav, in addition to other neural phenotypes and paralysis. Strikingly, a PPARG antagonist attenuates aberrant neural proliferation and alleviates paralysis in xav, while PPARG agonists increase neural proliferation in wild type embryos. These results show that mitochondrial dysfunction, leading to an increase in aerobic glycolysis, affects neurogenesis through the PPARG-ERK pathway, a potential target for therapeutic intervention.

Akt and c-Myc differentially activate cellular metabolic programs and prime cells to bioenergetic inhibition

The high glucose consumption of tumor cells even in an oxygen-rich environment, referred to as the Warburg effect, has been noted as a nearly universal biochemical characteristic of cancer cells. Targeting the glycolysis pathway has been explored as an anti-cancer therapeutic strategy to eradicate cancer based on this fundamental biochemical property of cancer cells. Oncoproteins such as Akt and c-Myc regulate cell metabolism. Accumulating studies have uncovered various molecular mechanisms by which oncoproteins affect cellular metabolism, raising a concern as to whether targeting glycolysis will be equally effective in treating cancers arising from different oncogenic activities. Here, we established a dual-regulatable FL5.12 pre-B cell line in which myristoylated Akt is expressed under the control of doxycycline, and c-Myc, fused to the hormone-binding domain of the human estrogen receptor, is activated by 4-hydroxytamoxifen. Using this system, we directly compared the effect of these oncoproteins on cell metabolism in an isogenic background. Activation of either Akt or c-Myc leads to the Warburg effect as indicated by increased cellular glucose uptake, glycolysis, and lactate generation. When cells are treated with glycolysis inhibitors, Akt sensitizes cells to apoptosis, whereas c-Myc does not. In contrast, c-Myc but not Akt sensitizes cells to the inhibition of mitochondrial function. This is correlated with enhanced mitochondrial activities in c-Myc cells. Hence, although both Akt and c-Myc promote aerobic glycolysis, they differentially affect mitochondrial functions and render cells susceptible to the perturbation of cellular metabolic programs.

Glycolysis: a bioenergetic or a survival pathway?

Following inhibition of mitochondrial respiration neurons die rapidly, whereas astrocytes utilize glycolytically-generated ATP to increase their mitochondrial membrane potential, thus becoming more resistant to pro-apoptotic stimuli. Neurons are unable to increase glycolysis due to the lack of activity of the glycolysis-promoting enzyme 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase, isoform 3 (PFKFB3). In neurons, PFKFB3 is degraded constantly via the E3 ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C)- CDH1. Glucose metabolism in neurons is directed mainly to the pentose phosphate pathway, leading to regeneration of reduced glutathione. In addition to their relevance to brain physiology and pathophysiology, these observations suggest that APC/C-CDH1 might link activation of glycolysis and cell proliferation as it is also involved in the regulation of cell cycle proteins.

Science Signaling Podcast: 15 December 2009

Inhibiting fatty acid synthesis may be effective in controlling glioblastomas driven by EGFR signaling.