Detection of resistance to imatinib by metabolic profiling: clinical and drug development implications

Imatinib therapy of chronic myeloid leukemia significantly reduces carnitine cell intake, resulting in adverse events

OBJECTIVE

A prominent, safe and efficient therapy for patients with chronic myeloid leukemia (CML) is inhibiting oncogenic protein BCR::ABL1 in a targeted manner with imatinib, a tyrosine kinase inhibitor. A substantial part of patients treated with imatinib report skeletomuscular adverse events affecting their quality of life. OCTN2 membrane transporter is involved in imatinib transportation into the cells. At the same time, the crucial physiological role of OCTN2 is cellular uptake of carnitine which is an essential co-factor for the mitochondrial β-oxidation pathway. This work investigates the impact of imatinib treatment on carnitine intake and energy metabolism of muscle cells.

METHODS

HTB-153 (human rhabdomyosarcoma) cell line and KCL-22 (CML cell line) were used to study the impact of imatinib treatment on intracellular levels of carnitine and vice versa. The energy metabolism changes in cells treated by imatinib were quantified and compared to changes in cells exposed to highly specific OCTN2 inhibitor vinorelbine. Mouse models were used to test whether in vitro observations are also achieved in vivo in thigh muscle tissue. The analytes of interest were quantified using a Prominence HPLC system coupled with a tandem mass spectrometer.

RESULTS

This work showed that through the carnitine-specific transporter OCTN2, imatinib and carnitine intake competed unequally and intracellular carnitine concentrations were significantly reduced. In contrast, carnitine preincubation did not influence imatinib cell intake or interfere with leukemia cell targeting. Blocking the intracellular supply of carnitine with imatinib significantly reduced the production of most Krebs cycle metabolites and ATP. However, subsequent carnitine supplementation rescued mitochondrial energy production. Due to specific inhibition of OCTN2 activity, the influx of carnitine was blocked and mitochondrial energy metabolism was impaired in muscle cells in vitro and in thigh muscle tissue in a mouse model.

CONCLUSIONS

This preclinical experimental study revealed detrimental effect of imatinib on carnitine-mediated energy metabolism of muscle cells providing a possible molecular background of the frequently occurred side effects during imatinib therapy such as fatigue, muscle pain and cramps.

Telomere loss is accompanied by decreased pool of shelterin proteins TRF2 and RAP1, elevated levels of TERRA and enhanced glycolysis in imatinib-resistant CML cells

Telomere length may be maintained by telomerase nucleoprotein complex and shelterin complex, namely TRF1, TRF2, TIN2, TPP1, POT1 and RAP1 proteins and modulated by TERRA expression. Telomere loss is observed during progression of chronic myeloid leukemia (CML) from the chronic phase (CML-CP) to the blastic phase (CML-BP). The introduction of tyrosine kinase inhibitors (TKIs), such as imatinib (IM), has changed outcome for majority of patients, however, a number of patients treated with TKIs may develop drug resistance. The molecular mechanisms underlying this phenomenon are not fully understood and require further investigation. In the present study, we demonstrate that IM-resistant BCR::ABL1 gene-positive CML K-562 and MEG-A2 cells are characterized by decreased telomere length, lowered protein levels of TRF2 and RAP1 and increased expression of TERRA in comparison to corresponding IM-sensitive CML cells and BCR::ABL1 gene-negative HL-60 cells. Furthermore, enhanced activity of glycolytic pathway was observed in IM-resistant CML cells. A negative correlation between a telomere length and advanced glycation end products (AGE) was also revealed in CD34⁺ cells isolated from CML patients. In conclusion, we suggest that affected expression of shelterin complex proteins, namely TRF2 and RAP1, TERRA levels, and glucose consumption rate may promote telomere dysfunction in IM-resistant CML cells.

Importance of lactate dehydrogenase (LDH) and monocarboxylate transporters (MCTs) in cancer cells

Background

In most regions, cancer ranks the second most frequent cause of death following cardiovascular disorders.

Aim

In this article, we review the various aspects of glycolysis with a focus on types of MCTs and the importance of lactate in cancer cells.

Results and Discussion

Metabolic changes are one of the first and most important alterations in cancer cells. Cancer cells use different pathways to survive, energy generation, growth, and proliferation compared to normal cells. The increase in glycolysis, which produces substances such as lactate and pyruvate, has an important role in metastases and invasion of cancer cells. Two important cellular proteins that play a role in the production and transport of lactate include lactate dehydrogenase and monocarboxylate transporters (MCTs). These molecules by their various isoforms and different tissue distribution help to escape the immune system and expansion of cancer cells under different conditions.

Mitochondrial determinants of chemoresistance

Chemoresistance constitute nowadays the major contributor to therapy failure in most cancers. There are main factors that mitigate cell response to therapy, such as target organ, inherent sensitivity to the administered compound, its metabolism, drug efflux and influx or alterations on specific cellular targets, among others. We now know that intrinsic properties of cancer cells, including metabolic features, substantially contribute to chemoresistance. In fact, during the last years, numerous reports indicate that cancer cells resistant to chemotherapy demonstrate significant alterations in mitochondrial metabolism, membrane polarization and mass. Metabolic activity and expression of several mitochondrial proteins are modulated under treatment to cope with stress, making these organelles central players in the development of resistance to therapies. Here, we review the role of mitochondria in chemoresistant cells in terms of metabolic rewiring and function of key mitochondria-related proteins.

Emerging roles of Myc in stem cell biology and novel tumor therapies

The pathophysiological roles and the therapeutic potentials of Myc family are reviewed in this article. The physiological functions and molecular machineries in stem cells, including embryonic stem (ES) cells and induced pluripotent stem (iPS) cells, are clearly described. The c-Myc/Max complex inhibits the ectopic differentiation of both types of artificial stem cells. Whereas c-Myc plays a fundamental role as a "double-edged sword" promoting both iPS cells generation and malignant transformation, L-Myc contributes to the nuclear reprogramming with the significant down-regulation of differentiation-associated genetic expression. Furthermore, given the therapeutic resistance of neuroendocrine tumors such as small-cell lung cancer and neuroblastoma, the roles of N-Myc in difficult-to-treat tumors are discussed. N-Myc and p53 exhibit the co-localization in the nucleus and alter p53-dependent transcriptional responses which are necessary for DNA repair, anti-apoptosis, and lipid metabolic reprogramming. NCYM protein stabilizes N-Myc, resulting in the stimulation of Oct4 expression, while Oct4 induces both N-Myc and NCYM via direct transcriptional activation of N-Myc, [corrected] thereby leading to the enhanced metastatic potential. Importantly enough, accumulating evidence strongly suggests that c-Myc can be a promising therapeutic target molecule among Myc family in terms of the biological characteristics of cancer stem-like cells (CSCs). The presence of CSCs leads to the intra-tumoral heterogeneity, which is mainly responsible for the therapeutic resistance. Mechanistically, it has been shown that Myc-induced epigenetic reprogramming enhances the CSC phenotypes. In this review article, the author describes two major therapeutic strategies of CSCs by targeting c-Myc; Firstly, Myc-dependent metabolic reprogramming is closely related to CD44 variant-dependent redox stress regulation in CSCs. It has been shown that c-Myc increases NADPH production via enhanced glutaminolysis with a finely-regulated mechanism. Secondly, the dormancy of CSCs due to FBW7-depedent c-Myc degradation pathway is also responsible for the therapeutic resistance to the conventional anti-tumor agents, the action points of which are largely dependent on the operation of the cell cycle. That is why the loss-of-functional mutations of FBW7 gene are expected to trigger "awakening" of dormant CSCs in the niche with c-Myc up-regulation. Collectively, although the further research is warranted to develop the effective anti-tumor therapeutic strategy targeting Myc family, we cancer researchers should always catch up with the current advances in the complex functions of Myc family in highly-malignant and heterogeneous tumor cells to realize the precision medicine.

Structural homologies between phenformin, lipitor and gleevec aim the same metabolic oncotarget in leukemia and melanoma

Phenformin's recently demonstrated efficacy in melanoma and Gleevec's demonstrated anti-proliferative action in chronic myeloid leukemia may lie within these drugs' significant pharmacokinetics, pharmacodynamics and structural homologies, which are reviewed herein. Gleevec's success in turning a fatal leukemia into a manageable chronic disease has been trumpeted in medical, economic, political and social circles because it is considered the first successful targeted therapy. Investments have been immense in omics analyses and while in some cases they greatly helped the management of patients, in others targeted therapies failed to achieve clinically stable recurrence-free disease course or to substantially extend survival. Nevertheless protein kinase controlling approaches have persisted despite early warnings that the targeted genomics narrative is overblown. Experimental and clinical observations with Phenformin suggest an alternative explanation for Gleevec's mode of action. Using 13C-guided precise flux measurements, a comparative multiple cell line study demonstrated the drug's downstream impact on submolecular fatty acid processing metabolic events that occurred independent of Gleevec's molecular target. Clinical observations that hyperlipidemia and diabetes are both reversed in mice and in patients taking Gleevec support the drugs' primary metabolic targets by biguanides and statins. This is evident by structural data demonstrating that Gleevec shows pyridine- and phenyl-guanidine homology with Phenformin and identical phenylcarbamoyl structural and ligand binding homology with Lipitor. The misunderstood mechanism of action of Gleevec is emblematic of the pervasive flawed reasoning that genomic analysis will lead to targeted, personalized diagnosis and therapy. The alternative perspective for Gleevec's mode of action may turn oncotargets towards metabolic channel reaction architectures in leukemia and melanoma, as well as in other cancers.

Exploring cancer metabolism using stable isotope-resolved metabolomics (SIRM)

Metabolic reprogramming is a hallmark of cancer. The changes in metabolism are adaptive to permit proliferation, survival, and eventually metastasis in a harsh environment. Stable isotope-resolved metabolomics (SIRM) is an approach that uses advanced approaches of NMR and mass spectrometry to analyze the fate of individual atoms from stable isotope-enriched precursors to products to deduce metabolic pathways and networks. The approach can be applied to a wide range of biological systems, including human subjects. This review focuses on the applications of SIRM to cancer metabolism and its use in understanding drug actions.

Computational Model Predicts the Effects of Targeting Cellular Metabolism in Pancreatic Cancer

Reprogramming of energy metabolism is a hallmark of cancer that enables the cancer cells to meet the increased energetic requirements due to uncontrolled proliferation. One prominent example is pancreatic ductal adenocarcinoma, an aggressive form of cancer with an overall 5-year survival rate of 5%. The reprogramming mechanism in pancreatic cancer involves deregulated uptake of glucose and glutamine and other opportunistic modes of satisfying energetic demands in a hypoxic and nutrient-poor environment. In the current study, we apply systems biology approaches to enable a better understanding of the dynamics of the distinct metabolic alterations in KRAS-mediated pancreatic cancer, with the goal of impeding early cell proliferation by identifying the optimal metabolic enzymes to target. We have constructed a kinetic model of metabolism represented as a set of ordinary differential equations that describe time evolution of the metabolite concentrations in glycolysis, glutaminolysis, tricarboxylic acid cycle and the pentose phosphate pathway. The model is comprised of 46 metabolites and 53 reactions. The mathematical model is fit to published enzyme knockdown experimental data. We then applied the model to perform in silico enzyme modulations and evaluate the effects on cell proliferation. Our work identifies potential combinations of enzyme knockdown, metabolite inhibition, and extracellular conditions that impede cell proliferation. Excitingly, the model predicts novel targets that can be tested experimentally. Therefore, the model is a tool to predict the effects of inhibiting specific metabolic reactions within pancreatic cancer cells, which is difficult to measure experimentally, as well as test further hypotheses toward targeted therapies.

ITD mutation in FLT3 tyrosine kinase promotes Warburg effect and renders therapeutic sensitivity to glycolytic inhibition

Internal tandem duplication (ITD) mutation in Fms-like tyrosine kinase 3 gene (FLT3/ITD) represents an unfavorable genetic change in acute myeloid leukemia (AML) and is associated with poor prognosis. Metabolic alterations have been involved in tumor progression and attracted interest as a target for therapeutic intervention. However, few studies analyzed the adaptations of cellular metabolism in the context of FLT3/ITD mutation. Here, we report that FLT3/ITD causes a significant increase in aerobic glycolysis through AKT-mediated upregulation of mitochondrial hexokinase (HK2), and renders the leukemia cells highly dependent on glycolysis and sensitive to pharmacological inhibition of glycolytic activity. Inhibition of glycolysis preferentially causes severe ATP depletion and massive cell death in FLT3/ITD leukemia cells. Glycolytic inhibitors significantly enhances the cytotoxicity induced by FLT3 tyrosine kinase inhibitor sorafenib. Importantly, such combination provides substantial therapeutic benefit in a murine model bearing FLT3/ITD leukemia. Our study suggests that FLT3/ITD mutation promotes Warburg effect, and such metabolic alteration can be exploited to develop effective therapeutic strategy for treatment of AML with FLT3/ITD mutation via metabolic intervention.

Network pharmacology of cancer: From understanding of complex interactomes to the design of multi-target specific therapeutics from nature

Despite massive investments in drug research and development, the significant decline in the number of new drugs approved or translated to clinical use raises the question, whether single targeted drug discovery is the right approach. To combat complex systemic diseases that harbour robust biological networks such as cancer, single target intervention is proved to be ineffective. In such cases, network pharmacology approaches are highly useful, because they differ from conventional drug discovery by addressing the ability of drugs to target numerous proteins or networks involved in a disease. Pleiotropic natural products are one of the promising strategies due to their multi-targeting and due to lower side effects. In this review, we discuss the application of network pharmacology for cancer drug discovery. We provide an overview of the current state of knowledge on network pharmacology, focus on different technical approaches and implications for cancer therapy (e.g. polypharmacology and synthetic lethality), and illustrate the therapeutic potential with selected examples green tea polyphenolics, Eleutherococcus senticosus, Rhodiola rosea, and Schisandra chinensis). Finally, we present future perspectives on their plausible applications for diagnosis and therapy of cancer.

Mcl-1 downregulation leads to the heightened sensitivity exhibited by BCR-ABL positive ALL to induction of energy and ER-stress

BCR-ABL positive (+) acute lymphoblastic leukemia (ALL) accounts for ∼30% of cases of ALL. We recently demonstrated that 2-deoxy-d-glucose (2-DG), a dual energy (glycolysis inhibition) and ER-stress (N-linked-glycosylation inhibition) inducer, leads to cell death in ALL via ER-stress/UPR-mediated apoptosis. Among ALL subtypes, BCR-ABL+ ALL cells exhibited the highest sensitivity to 2-DG suggesting BCR-ABL expression may be linked to this increased vulnerability. To confirm the role of BCR-ABL, we constructed a NALM6/BCR-ABL stable cell line and found significant increase in 2-DG-induced apoptosis compared to control. We found that Mcl-1 was downregulated by agents inducing ER-stress and Mcl-1 levels correlated with ALL sensitivity. In addition, we showed that Mcl-1 expression is positively regulated by the MEK/ERK pathway, dependent on BCR-ABL, and further downregulated by combining ER-stressors with TKIs. We determined that energy/ER stressors led to translational repression of Mcl-1 via the AMPK/mTOR and UPR/PERK/eIF2α pathways. Taken together, our data indicate that BCR-ABL+ ALL exhibits heightened sensitivity to induction of energy and ER-stress through inhibition of the MEK/ERK pathway, and translational repression of Mcl-1 expression via AMPK/mTOR and UPR/PERK/eIF2α pathways. This study supports further consideration of strategies combining energy/ER-stress inducers with BCR-ABL TKIs for future clinical translation in BCR-ABL+ ALL patients.

Potential of metabolomics in preclinical and clinical drug development

Metabolomics is an upcoming technology system which involves detailed experimental analysis of metabolic profiles. Due to its diverse applications in preclinical and clinical research, it became an useful tool for the drug discovery and drug development process. This review covers the brief outline about the instrumentation and interpretation of metabolic profiles. The applications of metabolomics have a considerable scope in the pharmaceutical industry, almost at each step from drug discovery to clinical development. These include finding drug target, potential safety and efficacy biomarkers and mechanisms of drug action, the validation of preclinical experimental models against human disease profiles, and the discovery of clinical safety and efficacy biomarkers. As we all know, nowadays the drug discovery and development process is a very expensive, and risky business. Failures at any stage of drug discovery and development process cost millions of dollars to the companies. Some of these failures or the associated risks could be prevented or minimized if there were better ways of drug screening, drug toxicity profiling and monitoring adverse drug reactions. Metabolomics potentially offers an effective route to address all the issues associated with the drug discovery and development.

Pattern recognition and biomarker validation using quantitative1H-NMR-based metabolomics

The collection of global metabolic data and their interpretation (both spectral and biochemical) using modern spectroscopic techniques and appropriate statistical approaches, are known as 'metabolic profiling', 'metabonomics' or 'metabolomics'. This review addresses 1H-nuclear magnetic resonance (NMR)-based metabolomic principles and their application in biomedical science, with special emphasis on their potential in translational research in transplantation, oncology, and drug toxicity or discovery. Various steps in metabolomics analysis are described in order to illustrate the types of biological samples, their respective handling and preparation for 1H-NMR analysis; provide a rationale for using pattern-recognition techniques (spectral database concept) versus quantitative 1H-NMR-based metabolomics (metabolite database concept); and identify necessary technological and logistical future developments that will allow 1H-NMR-based metabolomics to become an established tool in biomedical research and patient care.

Inhibition of transketolase by oxythiamine altered dynamics of protein signals in pancreatic cancer cells

Oxythiamine (OT), an analogue of anti-metabolite, can suppress the nonoxidative synthesis of ribose and induce cell apoptosis by causing a G1 phase arrest in vitro and in vivo. However, the molecular mechanism remains unclear yet. In the present study, a quantitative proteomic analysis using the modified SILAC method (mSILAC) was performed to determine the effect of metabolic inhibition on dynamic changes of protein expression in MIA PaCa-2 cancer cells treated with OT at various doses (0 μM, 5 μM, 50 μM and 500 μM) and time points (0 h, 12 h and 48 h). A total of 52 differential proteins in MIA PaCa-2 cells treated with OT were identified, including 14 phosphorylated proteins. Based on the dynamic expression pattern, these proteins were categorized in three clusters, straight down-regulation (cluster 1, 37% of total proteins), upright "V" shape expression pattern (cluster 2, 47.8% total), and downright "V" shape pattern (cluster 3, 15.2% total). Among them, Annexin A1 expression was significantly down-regulated by OT treatment in time-dependent manner, while no change of this protein was observed in OT dose-dependent fashion. Pathway analysis suggested that inhibition of transketolase resulted in changes of multiple cellular signaling pathways associated with cell apoptosis. The temporal expression patterns of proteins revealed that OT altered dynamics of protein expression in time-dependent fashion by suppressing phosphor kinase expression, resulting in cancer cell apoptosis. Results from this study suggest that interference of single metabolic enzyme activity altered multiple cellular signaling pathways.

Linking vitamin B1 with cancer cell metabolism

The resurgence of interest in cancer metabolism has linked alterations in the regulation and exploitation of metabolic pathways with an anabolic phenotype that increases biomass production for the replication of new daughter cells. To support the increase in the metabolic rate of cancer cells, a coordinated increase in the supply of nutrients, such as glucose and micronutrients functioning as enzyme cofactors is required. The majority of co-enzymes are water-soluble vitamins such as niacin, folic acid, pantothenic acid, pyridoxine, biotin, riboflavin and thiamine (Vitamin B1). Continuous dietary intake of these micronutrients is essential for maintaining normal health. How cancer cells adaptively regulate cellular homeostasis of cofactors and how they can regulate expression and function of metabolic enzymes in cancer is underappreciated. Exploitation of cofactor-dependent metabolic pathways with the advent of anti-folates highlights the potential vulnerabilities and importance of vitamins in cancer biology. Vitamin supplementation products are easily accessible and patients often perceive them as safe and beneficial without full knowledge of their effects. Thus, understanding the significance of enzyme cofactors in cancer cell metabolism will provide for important dietary strategies and new molecular targets to reduce disease progression. Recent studies have demonstrated the significance of thiamine-dependent enzymes in cancer cell metabolism. Therefore, this review discusses the current knowledge in the alterations in thiamine availability, homeostasis, and exploitation of thiamine-dependent pathways by cancer cells.

Therapeutic implications of targeting energy metabolism in breast cancer

PPARs are ligand activated transcription factors. PPARγ agonists have been reported as a new and potentially efficacious treatment of inflammation, diabetes, obesity, cancer, AD, and schizophrenia. Since cancer cells show dysregulation of glycolysis they are potentially manageable through changes in metabolic environment. Interestingly, several of the genes involved in maintaining the metabolic environment and the central energy generation pathway are regulated or predicted to be regulated by PPARγ. The use of synthetic PPARγ ligands as drugs and their recent withdrawal/restricted usage highlight the lack of understanding of the molecular basis of these drugs, their off-target effects, and their network. These data further underscores the complexity of nuclear receptor signalling mechanisms. This paper will discuss the function and role of PPARγ in energy metabolism and cancer biology in general and its emergence as a promising therapeutic target in breast cancer.

Metabolomic approach to evaluating adriamycin pharmacodynamics and resistance in breast cancer cells

Continuous exposure of breast cancer cells to adriamycin induces high expression of P-gp and multiple drug resistance. However, the biochemical process and the underlying mechanisms for the gradually induced resistance are not clear. To explore the underlying mechanism and evaluate the anti-tumor effect and resistance of adriamycin, the drug-sensitive MCF-7S and the drug-resistant MCF-7Adr breast cancer cells were used and treated with adriamycin, and the intracellular metabolites were profiled using gas chromatography mass spectrometry. Principal components analysis of the data revealed that the two cell lines showed distinctly different metabolic responses to adriamycin. Adriamycin exposure significantly altered metabolic pattern of MCF-7S cells, which gradually became similar to the pattern of MCF-7Adr, indicating that metabolic shifts were involved in adriamycin resistance. Many intracellular metabolites involved in various metabolic pathways were significantly modulated by adriamycin treatment in the drug-sensitive MCF-7S cells, but were much less affected in the drug-resistant MCF-7Adr cells. Adriamycin treatment markedly depressed the biosynthesis of proteins, purines, pyrimidines and glutathione, and glycolysis, while it enhanced glycerol metabolism of MCF-7S cells. The elevated glycerol metabolism and down-regulated glutathione biosynthesis suggested an increased reactive oxygen species (ROS) generation and a weakened ability to balance ROS, respectively. Further studies revealed that adriamycin increased ROS and up-regulated P-gp in MCF-7S cells, which could be reversed by N-acetylcysteine treatment. It is suggested that adriamycin resistance is involved in slowed metabolism and aggravated oxidative stress. Assessment of cellular metabolomics and metabolic markers may be used to evaluate anti-tumor effects and to screen for candidate anti-tumor agents.

Cancer cell growth and survival as a system-level property sustained by enhanced glycolysis and mitochondrial metabolic remodeling

Systems Biology holds that complex cellular functions are generated as system-level properties endowed with robustness, each involving large networks of molecular determinants, generally identified by “omics” analyses. In this paper we describe four basic cancer cell properties that can easily be investigated in vitro: enhanced proliferation, evasion from apoptosis, genomic instability, and inability to undergo oncogene-induced senescence. Focusing our analysis on a K-ras dependent transformation system, we show that enhanced proliferation and evasion from apoptosis are closely linked, and present findings that indicate how a large metabolic remodeling sustains the enhanced growth ability. Network analysis of transcriptional profiling gives the first indication on this remodeling, further supported by biochemical investigations and metabolic flux analysis (MFA). Enhanced glycolysis, down-regulation of TCA cycle, decoupling of glucose and glutamine utilization, with increased reductive carboxylation of glutamine, so to yield a sustained production of growth building blocks and glutathione, are the hallmarks of enhanced proliferation. Low glucose availability specifically induces cell death in K-ras transformed cells, while PKA activation reverts this effect, possibly through at least two mitochondrial targets. The central role of mitochondria in determining the two investigated cancer cell properties is finally discussed. Taken together the findings reported herein indicate that a system-level property is sustained by a cascade of interconnected biochemical pathways that behave differently in normal and in transformed cells.

Stable isotope-resolved metabolomics and applications for drug development

Advances in analytical methodologies, principally nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), during the last decade have made large-scale analysis of the human metabolome a reality. This is leading to the reawakening of the importance of metabolism in human diseases, particularly cancer. The metabolome is the functional readout of the genome, functional genome, and proteome; it is also an integral partner in molecular regulations for homeostasis. The interrogation of the metabolome, or metabolomics, is now being applied to numerous diseases, largely by metabolite profiling for biomarker discovery, but also in pharmacology and therapeutics. Recent advances in stable isotope tracer-based metabolomic approaches enable unambiguous tracking of individual atoms through compartmentalized metabolic networks directly in human subjects, which promises to decipher the complexity of the human metabolome at an unprecedented pace. This knowledge will revolutionize our understanding of complex human diseases, clinical diagnostics, as well as individualized therapeutics and drug response. In this review, we focus on the use of stable isotope tracers with metabolomics technologies for understanding metabolic network dynamics in both model systems and in clinical applications. Atom-resolved isotope tracing via the two major analytical platforms, NMR and MS, has the power to determine novel metabolic reprogramming in diseases, discover new drug targets, and facilitates ADME studies. We also illustrate new metabolic tracer-based imaging technologies, which enable direct visualization of metabolic processes in vivo. We further outline current practices and future requirements for biochemoinformatics development, which is an integral part of translating stable isotope-resolved metabolomics into clinical reality.

Preclinical activity of the rational combination of selumetinib (AZD6244) in combination with vorinostat in KRAS-mutant colorectal cancer models

PURPOSE

Despite the availability of several active combination regimens for advanced colorectal cancer (CRC), the 5-year survival rate remains poor at less than 10%, supporting the development of novel therapeutic approaches. In this study, we focused on the preclinical assessment of a rationally based combination against KRAS-mutated CRC by testing the combination of the MEK inhibitor, selumetinib, and vorinostat, a histone deacetylase (HDAC) inhibitor.

EXPERIMENTAL DESIGN

Transcriptional profiling and gene set enrichment analysis (baseline and posttreatment) of CRC cell lines provided the rationale for the combination. The activity of selumetinib and vorinostat against the KRAS-mutant SW620 and SW480 CRC cell lines was studied in vitro and in vivo. The effects of this combination on tumor phenotype were assessed using monolayer and 3-dimensional cultures, flow cytometry, apoptosis, and cell migration. In vivo, tumor growth inhibition, (18)F-fluoro-deoxy-glucose positron emission tomography (FDG-PET), and proton nuclear magnetic resonance were carried out to evaluate the growth inhibitory and metabolic responses, respectively, in CRC xenografts.

RESULTS

In vitro, treatment with selumetinib and vorinostat resulted in a synergistic inhibition of proliferation and spheroid formation in both CRC cell lines. This inhibition was associated with an increase in apoptosis, cell-cycle arrest in G(1), and reduced cellular migration and VEGF-A secretion. In vivo, the combination resulted in additive tumor growth inhibition. The metabolic response to selumetinib and vorinostat consisted of significant inhibition of membrane phospholipids; no significant changes in glucose uptake or metabolism were observed in any of the treatment groups.

CONCLUSION

These data indicate that the rationally based combination of the mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor, selumetinib, with the HDAC inhibitor vorinostat results in synergistic antiproliferative activity against KRAS-mutant CRC cell lines in vitro. In vivo, the combination showed additive effects that were associated with metabolic changes in phospholipid turnover, but not on FDG-PET, indicating that the former is a more sensitive endpoint of the combination effects.

MRS and MRSI guidance in molecular medicine: targeting and monitoring of choline and glucose metabolism in cancer

MRS and MRSI are valuable tools for the detection of metabolic changes in tumors. The currently emerging era of molecular medicine, which is shaped by molecularly targeted anticancer therapies combined with molecular imaging of the effects of such therapies, requires powerful imaging technologies that are able to detect molecular information. MRS and MRSI are such technologies that are able to detect metabolites arising from glucose and choline metabolism in noninvasive in vivo settings and at higher resolution in tissue samples. The roles played by MRS and MRSI in the diagnosis of different types of cancer, as well as in the early monitoring of the tumor response to traditional chemotherapies, are reviewed. The emerging roles of MRS and MRSI in the development and detection of novel targeted anticancer therapies that target oncogenic signaling pathways or markers in choline or glucose metabolism are discussed.

MR evaluation of response to targeted treatment in cancer cells

The development of molecular technologies, together with progressive sophistication of molecular imaging methods, has allowed the further elucidation of the multiple mutations and dysregulatory effects of pathways leading to oncogenesis. Acting against these pathways by specifically targeted agents represents a major challenge for current research efforts in oncology. As conventional anatomically based pharmacological endpoints may be inadequate to monitor the tumor response to these targeted treatments, the identification and use of more appropriate, noninvasive pharmacodynamic biomarkers appear to be crucial to optimize the design, dosage and schedule of these novel therapeutic approaches. An aberrant choline phospholipid metabolism and enhanced flux of glucose derivatives through glycolysis, which sustain the redirection of mitochondrial ATP to glucose phosphorylation, are two major hallmarks of cancer cells. This review focuses on the changes detected in these pathways by MRS in response to targeted treatments. The progress and limitations of our present understanding of the mechanisms underlying MRS-detected phosphocholine accumulation in cancer cells are discussed in the light of gene and protein expression and the activation of different enzymes involved in phosphatidylcholine biosynthesis and catabolism. Examples of alterations induced in the MRS choline profile of cells exposed to different agents or to tumor environmental factors are presented. Current studies aimed at the identification in cancer cells of MRS-detected pharmacodynamic markers of therapies targeted against specific conditional or constitutive cell receptor stimulation are then reviewed. Finally, the perspectives of present efforts addressed to identify enzymes of the phosphatidylcholine cycle as possible novel targets for anticancer therapy are summarized.

Metabolic assessment of a novel chronic myelogenous leukemic cell line and an imatinib resistant subline by H NMR spectroscopy

The goal of this study was to examine metabolic differences between a novel chronic myelogenous leukemic (CML) cell line, MyL, and a sub-clone, MyL-R, which displays enhanced resistance to the targeted Bcr-Abl tyrosine kinase inhibitor imatinib. (1)H nuclear magnetic resonance (NMR) spectroscopy was carried out on cell extracts and conditioned media from each cell type. Both principal component analysis (PCA) and specific metabolite identification and quantification were used to examine metabolic differences between the cell types. MyL cells showed enhanced glucose removal from the media compared to MyL-R cells with significant differences in production rates of the glycolytic end-products, lactate and alanine. Interestingly, the total intracellular creatine pool (creatine + phosphocreatine) was significantly elevated in MyL-R compared to MyL cells. We further demonstrated that the MyL-R cells converted the creatine to phosphocreatine using non-invasive monitoring of perfused alginate-encapsulated MyL-R and MyL cells by in vivo (31)P NMR spectroscopy and subsequent HPLC analysis of extracts. Our data demonstrated a clear difference in the metabolite profiles of drug-resistant and sensitive cells, with the biggest difference being an elevation of creatine metabolites in the imatinib-resistant MyL-R cells.

Use of the 1-mm micro-probe for metabolic analysis on small volume biological samples

Endogenous metabolites are promising diagnostic end-points in cancer research. Clinical application of high-resolution NMR spectroscopy is often limited by extremely low volumes of human specimens. In the present study, the use of the Bruker 1-mm high-resolution TXI micro-probe was evaluated in the elucidation of metabolic profiles for three different clinical applications with limited sample sizes (body fluids, isolated cells and tissue biopsies). Sample preparation and (1)H-NMR metabolite quantification protocols were optimized for following oncology-oriented applications: (i) to validate the absolute concentrations of citrate and spermine in human expressed prostatic specimens (EPS volumes 5 to 10 microl: prostate cancer application); (ii) to establish the metabolic profile of isolated human lymphocytes (total cell count 4 x 10(6): chronic myelogenous leukaemia application); (iii) to assess the metabolic composition of human head-and-neck cancers from mouse xenografts (biopsy weights 20 to 70 mg: anti-cancer treatment application). In this study, the use of the Bruker 1-mm micro-probe provides a convenient way to measure and quantify endogenous metabolic profiles of samples with a very low volume/weight/cell count.

Metabolic characteristics of imatinib resistance in chronic myeloid leukaemia cells

BACKGROUND AND PURPOSE

Early detection of resistance development is crucial for imatinib-based treatment in chronic myeloid leukaemia (CML) patients. We aimed to distinguish metabolic markers of cell resistance to imatinib.

EXPERIMENTAL APPROACH

Two human imatinib-sensitive CML cell lines: LAMA84-s and K562-s, and their resistant counterparts: LAMA84-r and K562-r (both resistant to 1 microM imatinib), and K562-R (5 microM) were analysed by nuclear magnetic resonance spectroscopy to assess global metabolic profiling, including energy state, glucose and phospholipid metabolism.

KEY RESULTS

We found, by Western blotting and flow cytometry, that the levels of Bcr-Abl tyrosine kinase and multi-drug resistance p-glycoprotein were inconsistent among resistant clones. On the other hand, phospholipid metabolism and lactate production were highly predictive for cell response to imatinib. As previously reported, sensitive cells showed significantly decreased glycolytic activity (lactate) and phospholipid synthesis (phosphocholine) as well as increased phospholipid catabolism (glycerophosphocholine) after 24 h of 1 microM imatinib treatment, which correlated with inhibition of cell proliferation and induction of apoptosis. In contrast to their sensitive counterparts, the K562-r, K562-R and LAMA84-r maintained increased phospholipid synthesis and glycolytic lactate production in the presence of 1 microM (K562-r and LAMA84-r) and 5 microM (K562-R) imatinib.

CONCLUSIONS AND IMPLICATIONS

Specific metabolic markers for early detection of imatinib resistance, including increased glycolytic activity and phospholipid turnover, can be identified in resistant clones. Once validated in human isolated leukocytes, they may be used to monitor the responsiveness of CML patients to treatment.

Clinical applications of metabolomics in oncology: a review

Metabolomics, an omic science in systems biology, is the global quantitative assessment of endogenous metabolites within a biological system. Either individually or grouped as a metabolomic profile, detection of metabolites is carried out in cells, tissues, or biofluids by either nuclear magnetic resonance spectroscopy or mass spectrometry. There is potential for the metabolome to have a multitude of uses in oncology, including the early detection and diagnosis of cancer and as both a predictive and pharmacodynamic marker of drug effect. Despite this, there is lack of knowledge in the oncology community regarding metabolomics and confusion about its methodologic processes, technical challenges, and clinical applications. Metabolomics, when used as a translational research tool, can provide a link between the laboratory and clinic, particularly because metabolic and molecular imaging technologies, such as positron emission tomography and magnetic resonance spectroscopic imaging, enable the discrimination of metabolic markers noninvasively in vivo. Here, we review the current and potential applications of metabolomics, focusing on its use as a biomarker for cancer diagnosis, prognosis, and therapeutic evaluation.

Use of nuclear magnetic resonance-based metabolomics in detecting drug resistance in cancer

Cancer cells possess a highly unique metabolic phenotype, which is characterized by high glucose uptake, increased glycolytic activity, decreased mitochondrial activity, low bioenergetic and increased phospholipid turnover. These metabolic hallmarks can be readily assessed by metabolic technologies - either in vitro or in vivo - to monitor responsiveness and resistance to novel targeted drugs, where specific inhibition of cell proliferation (cytostatic effect) occurs rather than direct induction of cell death (cytotoxicity). Using modern analytical technologies in combination with statistical approaches, 'metabolomics', a global metabolic profile on patient samples can be established and validated for responders and nonresponders, providing additional metabolic end points. Discovered metabolic end points should be translated into noninvasive metabolic imaging protocols.

Abnormalities in glucose uptake and metabolism in imatinib-resistant human BCR-ABL-positive cells

The development of imatinib resistance has become a significant therapeutic problem in which the etiology seems to be multifactorial and poorly understood. As of today, clinical criteria to predict the development of imatinib resistance in chronic myelogenous leukemia (CML), other than rebound of the myeloproliferation, are under development. However, there is evidence that the control of glucose-substrate flux is an important mechanism of the antiproliferative action of imatinib because imatinib-resistant gastrointestinal stromal KIT-positive tumors reveal highly elevated glucose uptake in radiologic images. We used nuclear magnetic resonance spectroscopy and gas chromatography mass spectrometry to assess (13)C glucose uptake and metabolism (glycolysis, TCA cycle, and nucleic acid ribose synthesis) during imatinib treatment in CML cell lines with different sensitivities to imatinib. Our results show that sensitive K562-s and LAMA84-s BCR-ABL-positive cells have decreased glucose uptake, decreased lactate production, and an improved oxidative TCA cycle following imatinib treatment. The resistant K562-r and LAMA84-r cells maintained a highly glycolytic metabolic phenotype with elevated glucose uptake and lactate production. In addition, oxidative synthesis of RNA ribose from (13)C-glucose via glucose-6-phosphate dehydrogenase was decreased, and RNA synthesis via the nonoxidative transketolase pathway was increased in imatinib-resistant cells. CML cells which exhibited a (oxidative/nonoxidative) flux ratio for nucleic acid ribose synthesis of >1 were sensitive to imatinib. The resistant K562-r and LAMA84-r exhibited a (oxidative/nonoxidative) flux ratio of <0.7. The changes in glucose uptake and metabolism were accompanied by intracellular translocation of GLUT-1 from the plasma membrane into the intracellular fraction in sensitive cells treated with imatinib, whereas GLUT-1 remained located at the plasma membrane in LAMA84-r and K562-r cells. The total protein load of GLUT-1 was unchanged among treated sensitive and resistant cell lines. In summary, elevated glucose uptake and nonoxidative glycolytic metabolic phenotype can be used as sensitive markers for early detection of imatinib resistance in BCR-ABL-positive cells.

Modulation of caspase-independent cell death leads to resensitization of imatinib mesylate-resistant cells

Imatinib mesylate is widely used for the treatment of patients with chronic myelogenous leukemia (CML). This compound is very efficient in killing Bcr-Abl-positive cells in a caspase-dependent manner. Nevertheless, several lines of evidence indicated that caspase-mediated cell death (i.e., apoptosis) is not the only type of death induced by imatinib. The goal of our study was to evaluate the importance of the newly described caspase-independent cell death (CID) in Bcr-Abl-positive cells. We established in several CML cell lines that imatinib, in conjunction with apoptosis, also induced CID. CID was shown to be as efficient as apoptosis in preventing CML cell proliferation and survival. We next investigated the potential implication of a recently identified mechanism used by cancer cells to escape CID through overexpression of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). We showed here, in several CML cell lines, that GAPDH overexpression was sufficient to induce protection from CID. Furthermore, imatinib-resistant Bcr-Abl-positive cell lines were found to spontaneously overexpress GAPDH. Finally, we showed that a GAPDH partial knockdown, using specific short hairpin RNAs, was sufficient to resensitize those resistant cells to imatinib-induced cell death. Taken together, our results indicate that CID is an important effector of imatinib-mediated cell death. We also established that GAPDH overexpression can be found in imatinib-resistant Bcr-Abl-positive cells and that its down-regulation can resensitize those resistant cells to imatinib-induced death. Therefore, drugs able to modulate GAPDH administered together with imatinib could find some therapeutic benefits in CML patients.

Time-dependent effects of imatinib in human leukaemia cells: a kinetic NMR-profiling study

The goal of this study was to evaluate the time course of metabolic changes in leukaemia cells treated with the Bcr-Abl tyrosine kinase inhibitor imatinib. Human Bcr-Abl⁺ K562 cells were incubated with imatinib in a dose-escalating manner (starting at 0.1 μM with a weekly increase of 0.1 μM imatinib) for up to 5 weeks. Nuclear magnetic resonance spectroscopy and liquid-chromatography mass spectrometry were performed to assess a global metabolic profile, including glucose metabolism, energy state, lipid metabolism and drug uptake, after incubation with imatinib. Initially, imatinib treatment completely inhibited the activity of Bcr-Abl tyrosine kinase, followed by the inhibition of cell glycolytic activity and glucose uptake. This was accompanied by the increased mitochondrial activity and energy production. With escalating imatinib doses, the process of cell death rapidly progressed. Phosphocreatine and NAD⁺ concentrations began to decrease, and mitochondrial activity, as well as the glycolysis rate, was further reduced. Subsequently, the synthesis of lipids as necessary membrane precursors for apoptotic bodies was accelerated. The concentrations of the Kennedy pathway intermediates, phosphocholine and phosphatidylcholine, were reduced. After 4 weeks of exposure to imatinib, the secondary necrosis associated with decrease in the mitochondrial and glycolytic activity occurred and was followed by a shutdown of energy production and cell death. In conclusion, monitoring of metabolic changes in cells exposed to novel signal transduction modulators supplements molecular findings and provides further mechanistic insights into longitudinal changes of the mitochondrial and glycolytic pathways of oncogenesis.

Myc regulates a transcriptional program that stimulates mitochondrial glutaminolysis and leads to glutamine addiction

Mammalian cells fuel their growth and proliferation through the catabolism of two main substrates: glucose and glutamine. Most of the remaining metabolites taken up by proliferating cells are not catabolized, but instead are used as building blocks during anabolic macromolecular synthesis. Investigations of phosphoinositol 3-kinase (PI3K) and its downstream effector AKT have confirmed that these oncogenes play a direct role in stimulating glucose uptake and metabolism, rendering the transformed cell addicted to glucose for the maintenance of survival. In contrast, less is known about the regulation of glutamine uptake and metabolism. Here, we report that the transcriptional regulatory properties of the oncogene Myc coordinate the expression of genes necessary for cells to engage in glutamine catabolism that exceeds the cellular requirement for protein and nucleotide biosynthesis. A consequence of this Myc-dependent glutaminolysis is the reprogramming of mitochondrial metabolism to depend on glutamine catabolism to sustain cellular viability and TCA cycle anapleurosis. The ability of Myc-expressing cells to engage in glutaminolysis does not depend on concomitant activation of PI3K or AKT. The stimulation of mitochondrial glutamine metabolism resulted in reduced glucose carbon entering the TCA cycle and a decreased contribution of glucose to the mitochondrial-dependent synthesis of phospholipids. These data suggest that oncogenic levels of Myc induce a transcriptional program that promotes glutaminolysis and triggers cellular addiction to glutamine as a bioenergetic substrate.

Regulation of the Warburg effect in early-passage breast cancer cells

Malignancy in cancer is associated with aerobic glycolysis (Warburg effect) evidenced by increased trapping of [(18)F]deoxyglucose (FdG) in patients imaged by positron emission tomography (PET). [(18)F]deoxyglucose uptake correlates with glucose transporter (GLUT-1) expression, which can be regulated by hypoxia-inducible factor 1 alpha (HIF-1alpha). We have previously reported in established breast lines that HIF-1alpha levels in the presence of oxygen leads to the Warburg effect. However, glycolysis and GLUT-1 can also be induced independent of HIF-1alpha by other factors, such as c-Myc and phosphorylated Akt (pAkt). This study investigates HIF-1alpha, c-Myc, pAkt, and aerobic glycolysis in low-passage breast cancer cells under the assumption that these represent the in vivo condition better than established lines. Similar to in vivo FdG-PET or primary breast cancers, rates of glycolysis were diverse, being higher in cells expressing both c-Myc and HIF-1alpha and lower in cell lines low or negative in both transcription factors. No correlations were observed between glycolytic rates and pAkt levels. Two of 12 cell lines formed xenografts in mice. Both were positive for HIF-1alpha and phosphorylated c-Myc, and only one was positive for pAkt. Glycolysis was affected by pharmacological regulation of c-Myc and HIF-1alpha. These findings suggest that c-Myc and/or HIF-1alpha activities are both involved in the regulation of glycolysis in breast cancers.

Stable isotope-assisted metabolomics in cancer research

Stable Isotopes are nontoxic, naturally occurring elemental surrogates that are fully compatible with live organisms, including humans in a clinical setting. The ability to enrich common compounds with rare isotopes such as carbon ((13)C) and nitrogen ((15)N) is the only practical means by which metabolic pathways can be traced, performed by following the fate of individual atoms from the source molecules to products via metabolic transformation. Changes in regulation of pathways are therefore captured by this approach, which leads to deeper understanding of fundamental biochemistry of cancer compared with non-cancerous cells, which can lead to new diagnostic tools.

Metabolomics: a global biochemical approach to drug response and disease

Metabolomics is the study of metabolism at the global level. This rapidly developing new discipline has important potential implications for pharmacologic science. The concept that metabolic state is representative of the overall physiologic status of the organism lies at the heart of metabolomics. Metabolomic studies capture global biochemical events by assaying thousands of small molecules in cells, tissues, organs, or biological fluids-followed by the application of informatic techniques to define metabolomic signatures. Metabolomic studies can lead to enhanced understanding of disease mechanisms and to new diagnostic markers as well as enhanced understanding of mechanisms for drug or xenobiotic effect and increased ability to predict individual variation in drug response phenotypes (pharmacometabolomics). This review outlines the conceptual basis for metabolomics as well as analytical and informatic techniques used to study the metabolome and to define metabolomic signatures. It also highlights potential metabolomic applications to pharmacology and clinical pharmacology.

Adaptive landscapes and emergent phenotypes: why do cancers have high glycolysis?

Investigating the causes of increased aerobic glycolysis in tumors (Warburg Effect) has gone in and out of fashion many times since it was first described almost a century ago. The field is currently in ascendance due to two factors. Over a million FDG-PET studies have unequivocally identified increased glucose uptake as a hallmark of metastatic cancer in humans. These observations, combined with new molecular insights with HIF-1alpha and c-myc, have rekindled an interest in this important phenotype. A preponderance of work has been focused on the molecular mechanisms underlying this effect, with the expectation that a mechanistic understanding may lead to novel therapeutic approaches. There is also an implicit assumption that a mechanistic understanding, although fundamentally reductionist, will nonetheless lead to a more profound teleological understanding of the need for altered metabolism in invasive cancers. In this communication, we describe an alternative approach that begins with teleology; i.e. adaptive landscapes and selection pressures that promote emergence of aerobic glycolysis during the somatic evolution of invasive cancer. Mathematical models and empirical observations are used to define the adaptive advantage of aerobic glycolysis that would explain its remarkable prevalence in human cancers. These studies have led to the hypothesis that increased consumption of glucose in metastatic lesions is not used for substantial energy production via Embden-Meyerhoff glycolysis, but rather for production of acid, which gives the cancer cells a competitive advantage for invasion. Alternative hypotheses, wherein the glucose is used for generation of reducing equivalents (NADPH) or anabolic precursors (ribose) are also discussed.

Pharmacokinetic investigation of imatinib using accelerator mass spectrometry in patients with chronic myeloid leukemia

PURPOSE

To investigate the potential use of accelerator mass spectrometry (AMS) in the study of the clinical pharmacology of imatinib.

EXPERIMENTAL DESIGN

Six patients who were receiving imatinib (400 mg/d) as part of their ongoing treatment for chronic myeloid leukemia (CML) received a dose containing a trace quantity (13.6 kBq) of (14)C-imatinib. Blood samples were collected from patients before and at various times up to 72 h after administration of the test dose and were processed to provide samples of plasma and peripheral blood lymphocytes (PBL). Samples were analyzed by AMS, with chromatographic separation of parent compound from metabolites. In addition, plasma samples were analyzed by liquid chromatography/mass spectrometry (LCMS).

RESULTS

Analysis of the AMS data indicated that imatinib was rapidly absorbed and could be detected in plasma up to 72 h after administration. Imatinib was also detectable in PBL at 24 h after administration of the (14)C-labeled dose. Comparison of plasma concentrations determined by AMS with those derived by LCMS analysis gave similar average estimates of area under plasma concentration time curve (26 +/- 3 versus 27 +/- 11 microg/mL.h), but with some variation within each individual.

CONCLUSIONS

Using this technique, data were obtained in a small number of patients on the pharmacokinetics of a single dose of imatinib in the context of chronic dosing, which could shed light on possible pharmacologic causes of resistance to imatinib in CML.

The Warburg effect and its cancer therapeutic implications

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

Application of a metabolomic method combining one-dimensional and two-dimensional gas chromatography-time-of-flight/mass spectrometry to metabolic phenotyping of natural variants in rice

We have developed a comprehensive method combining analytical techniques of one-dimensional (1D) and two-dimensional (GC x GC) gas chromatography-time-of-flight (TOF)-mass spectrometry. This method was applied to the metabolic phenotyping of natural variants in rice for the 68 world rice core collection (WRC) and two other varieties. Ten metabolites were selected as metabolite representatives, and the selected ion current of each metabolite peak obtained from both techniques were statistically compared. Our method of combining 1D- and GC x GC-TOF/MS is useful for the metabolic phenotyping of natural variants in rice for further studies in breeding programs.

Glycolysis in cancer: a potential target for therapy

Clinical imaging of primary and metastatic cancers with Fluoro deoxy-d-Glucose Positron Emission Tomography (FdG PET) has clearly demonstrated that increased glucose flux compared to normal tissue is a common trait of human malignancies (Gambhir, 2002) This is a consequence of a shift of glucose metabolism to less efficient glycolytic pathways in response to regional hypoxia and evolution of aerobic glycolysis in many cancer phenotypes. This distinctive metabolic profile presents an inviting target for cancer treatment and prevention. Here, we summarize the therapeutic strategies under investigation to exploit or interrupt tumor glycolytic metabolism. Although a number of approaches are under investigation, none has been sufficiently successful to warrant widespread clinical application. We point out that the environmental heterogeneity and evolutionary capacity of tumor cells that likely led to development of upregulated glycolysis could also promote adaptive strategies that confer resistance to therapies designed to inhibit glucose metabolism.

Therapeutic targets and biomarkers identified in cancer choline phospholipid metabolism

Choline phospholipid metabolism is altered in a wide variety of cancers. The choline metabolite profile of tumors and cancer cells is characterized by an elevation of phosphocholine and total choline-containing compounds. Noninvasive magnetic resonance spectroscopy can be used to detect this elevation as an endogenous biomarker of cancer, or as a predictive biomarker for monitoring tumor response to novel targeted therapies. The enzymes directly causing this elevation, such as choline kinase, phospholipase C and phospholipase D may provide molecular targets for anticancer therapies. Signal transduction pathways that are activated in cancers, such as those mediated by the receptor tyrosine kinases breakpoint cluster region-abelson (Bcr-Abl), c-KIT or epidermal growth factor receptor (EGFR), correlate with the alterations in choline phospholipid metabolism of cancers, and also offer molecular targets for specific anticancer therapies. This review summarizes recently discovered molecular targets in choline phospholipid metabolism and signal transduction pathways, which may lead to novel anticancer therapies potentially being monitored by magnetic resonance spectroscopy techniques.

Glucose metabolism and cancer

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

Glycolysis inhibition for anticancer treatment

Most cancer cells exhibit increased glycolysis and use this metabolic pathway for generation of ATP as a main source of their energy supply. This phenomenon is known as the Warburg effect and is considered as one of the most fundamental metabolic alterations during malignant transformation. In recent years, there are significant progresses in our understanding of the underlying mechanisms and the potential therapeutic implications. Biochemical and molecular studies suggest several possible mechanisms by which this metabolic alteration may evolve during cancer development. These mechanisms include mitochondrial defects and malfunction, adaptation to hypoxic tumor microenvironment, oncogenic signaling, and abnormal expression of metabolic enzymes. Importantly, the increased dependence of cancer cells on glycolytic pathway for ATP generation provides a biochemical basis for the design of therapeutic strategies to preferentially kill cancer cells by pharmacological inhibition of glycolysis. Several small molecules have emerged that exhibit promising anticancer activity in vitro and in vivo, as single agent or in combination with other therapeutic modalities. The glycolytic inhibitors are particularly effective against cancer cells with mitochondrial defects or under hypoxic conditions, which are frequently associated with cellular resistance to conventional anticancer drugs and radiation therapy. Because increased aerobic glycolysis is commonly seen in a wide spectrum of human cancers and hypoxia is present in most tumor microenvironment, development of novel glycolytic inhibitors as a new class of anticancer agents is likely to have broad therapeutic applications.