Outstanding Questions
Cancer metabolism: current perspectives and future directions
Cell Death and Disease (2012) 3, e248; doi:10.1038/cddis.2011.123
l Is there a specific metabolic profile of tumor cells, different to that of non-transformed proliferating cells?
l Is there a general metabolic profile of tumor cells which is different from metabolism of noncancerous but proliferating tissues?
l Aerobic glycolysis has also been observed in non-transformed cells, including activated lymphocytes and embryonic stem cells. The reasons why cancer cells switch to aerobic glycolysis are still debated.
l Which enzymes and metabolic pathways are regulated by specific oncogenes?
l Which nutrients and metabolites are essential for which type of cancer?
l What is the influence of diet on cancer development and treatment?
l Can we develop non-toxic inhibitors of metabolic pathways with clinical efficacy?
l Can we exploit the metabolic particularities of cancer cells without affecting normal tissues?
l However, it should be investigated more thoroughly whether lactate production under aerobic conditions is truly a selective feature of cancer cells and not of other highly proliferating tissues.
l We need to identify which specific oncogenic mutations confer sensitivity to inhibition of specific pathways (synthetic lethal interactions).
l Metabolism is probably the research area that would benefit the most from application of systems biology methodology to identify the key pathways.
Metabolic Signaling to the Nucleus in Cancer
https://doi.org/10.1016/j.molcel.2018.07.015
l In order to mechanistically understand how metabolites modulate cellular phenotypes and activities, it will be crucial to further elucidate how specific genes are regulated in response to changes in the availability of a metabolite.
l Might metabolic alterations in cancer cells enhance sensitivity to epigenetic therapies?
l Could targeting metabolic pathways promote differentiation or suppress EMT and metastasis?
l Further, given that acetylation, methylation, and glycosylation can be influenced by nutrient availability, might a patient’s diet also modulate tumor formation and growth at least in part via metabolic signaling mechanisms?
l Could targeting the tumor microenvironment influence metabolite availability to cancer cells and thereby metabolic signaling?
l Future work will continue to elucidate the mechanisms linking metabolite availability to gene regulation, with the hope that understanding these mechanisms will aid in developing new cancer therapies or in identifying cancer prevention strategies.
l Further investigation is necessary to identify metabolic pathways that promote metastasis in specific cancer subtypes or stages and develop an effective paradigm for treating metastatic cancer.
Metabolic Hallmarks of Metastasis Formation
https://doi.org/10.1016/j.tcb.2018.04.002
l What is the importance and extent of metabolic priming in the premetastatic niche?
l Is there a metabolic priming of the premetastatic niche beyond glucose?
l What are the metabolic requirements that allow cancer cells to shape the metastatic niche?
l How do immune and stromal cells impact the metabolism of secondary tumors?
Understanding the Intersections between Metabolism and Cancer Biology
http://dx.doi.org/10.1016/j.cell.2016.12.039
l The next phase of cancer metabolism research will need to address increasingly complex questions about how intrinsic and extrinsic influences integrate to create exploitable metabolic phenotypes in cancer.
l What products of metabolism are limiting for proliferation?
l Further work is needed to understand how metabolite levels affect signalling and metabolism.
Oncogene-Directed Alterations in Cancer Cell Metabolism How do oncogenes rewire metabolism in cancer cells?
http://dx.doi.org/10.1016/j.trecan.2016.06.002
l Does the tumor microenvironment has a role in determining how oncogenes shape cancer metabolism?
l Do metastatic tumors have unique metabolic requirements different from those of primary tumors?
l Do different cancer types have distinct metabolic requirements?
l How do oncogene-induced metabolic alterations promote cancer initiation and progression?
l Which metabolic alterations represent viable clinical targets for precision cancer therapy and how can we discover such alterations?
The Warburg Effect: How Does it Benefit Cancer Cells?
https://doi.org/10.1016/j.tibs.2015.12.001
l How does the Warburg Effect promote the development of Cancer?
l How does the Warburg Effect impose dependencies on tumor growth?
l How can experimental systems be devised that can conclusively test the proposals for the function of the Warburg Effect?
l Does resolution of any given function of the Warburg Effect have immediate therapeutic consequences?
l Does the function of the Warburg Effect provide insights into its role in tumor evolution?
l Do the requirements of the Warburg Effect provide clues for its function?
Cancer metabolism at a glance
J Cell Sci 2016 129: 3367-3373; doi: 10.1242/jcs.181016
l What is the functional relevance and selective advantage of mitochondrial one-carbon metabolism?
l What is the role of acetate in the interaction between metabolism and epigenetics in cancer?
l Under which genetic and environmental conditions does the requirement of tumors for glutamine-derived carbon outweigh their demand for glutamine-derived nitrogen?
l How is macropinocytosis regulated and how important is it in supporting growth in vivo?
l What are the downstream factors mediating the tumorigenic activity of oncometabolites?
Metabolic Enzymes Moonlighting in the Nucleus: Metabolic Regulation of Gene Transcription http://dx.doi.org/10.1016/j.tibs.2016.05.013
l What is the full list of canonical and non-canonical functions of metabolic enzymes in the nucleus? Are these functions a part of an unappreciated comprehensive metabolic program in the nucleus?
l Could functionally related metabolic enzymes translocate together to the nucleus, as part of a program that promotes specific cell decisions (e.g., proliferation, differentiation)?
l How do metabolic enzymes translocate into the nucleus? Because many lack a nuclear localization sequence, other transportation mechanisms are needed. Do mitochondria-derived vesicles transport mitochondrial enzymes to the nucleus, as they do in peroxisomes?
Mitochondrial Metabolism: Yin and Yang for Tumor Progression
https://doi.org/10.1016/j.tem.2017.06.004
l What are the molecular or microenvironmental factors that determine the optimal metabolic state for tumor progression?
l Does the tissue of origin define the preferential mitochondrial activity of cancer cells?
l Does mitochondrial function contribute to the organotropism of cancer cells during the dissemination process?
l How does a given mitochondrial metabolic state favor survival of cancer cells upon therapeutic challenge?
Serine and glycine metabolism in cancer
https://doi.org/10.1016/j.tibs.2014.02.004
l How are expression levels of serine/glycine biosynthetic enzymes regulated under basal and stress conditions in cancer cells?
l How is serine/glycine biosynthesis coordinated with the mutational landscape (oncogene signalling) in cancer cells?
l How do serine, glycine, and one-carbon metabolism integrate and balance their contribution to the antioxidant response, anabolism, anaplerosis, or epigenetic status to sustain a cancer cell?
l Do serine and glycine equally contribute to the same phenomenon or do they retain exclusive functions?
l Does serine/glycine biosynthesis pathway affect cancer onset or only contribute to cancer progression?
l Would dietary intervention synergise with classic chemotherapeutic protocols in cancer patient treatments?
Famine versus feast: understanding the metabolism of tumors in vivo
https://doi.org/10.1016/j.tibs.2015.01.004
l How does prolonged culture shape the metabolic dependencies of cancer cells over time?
l What is the importance of different metabolic substrates such as cysteine, acetate, BCAAs, and protein relative to glucose and glutamine across a variety of cancer types?
l How do the metabolic requirements of non-proliferating tumor cells differ from proliferating tumor cells, and can conditions be developed to study these non-proliferating cells in culture?
l What are the metabolic phenotypes of ‘unculturable’ tumor cell populations?
l What is the metabolic relationship between tumor cell and normal cell populations present within the tumor and in distant tissues, and can systems be developed to investigate these symbioses in vivo?
l How does tissue of origin and tissue site of tumor growth affect the metabolic phenotype of cancer cells in vivo?
l How faithfully do cancer models recapitulate the metabolic alterations observed in human cancers?
Fueling the Cell Division Cycle
https://doi.org/10.1016/j.tcb.2016.08.009
l Is there a general need for both OXPHOS and glycolysis during the cell cycle, or one of these processes is dispensable for cell-cycle progression?
l What metabolic pathways are linked to normal cell-cycle transitions versus those in response to specific checkpoint mechanisms?
l Do OXPHOS or glycolysis specifically contribute to DNA replication of chromosome segregation by providing specific intermediates?
l Are the requirements for energy generation similar among all types of cell cycles across higher eukaryotes?
l Can we extrapolate the data generated in cultured cell lines to physiological conditions in vivo? What is the impact of culture conditions (e.g., continuous presence of growth factors) in energy generation during the cell cycle? How does oxygen availability in vivo affect cell-cycle progression of specific cells in tissues?
l Does autophagy or mitophagy control energy generation during the cell cycle?
l What is the difference in energy generation between ‘highly’ proliferating cells (e.g., cancer cells) and other proliferating cells?
l How do oncogenic mutations impose the glycolytic switch? Are these changes required for regulation of the cell cycle alone, or do they provide tumor cells with other oncogenic properties?
Epithelial–Mesenchymal Plasticity: A Central Regulator of Cancer Progression
https://doi.org/10.1016/j.tcb.2015.07.012
l What are the functionally important, distinct intermediate states along the epithelial–mesenchymal axis that naturally arise under physiological and pathological conditions, in particular during carcinoma progression?
l What are the features and/or markers that would allow identification of these intermediate states?
l Which of these states favors the formation of epithelial stem cells?
l Which combinations of EMT-TFs are responsible for organizing these distinct states?
l Which types of contextual signal, acting in various combinations, serve physiologically to activate expression of EMT-TFs and, in turn, the entrance into more mesenchymal cell states?
l Are the close functional connections between the stem cell state and the EMT program, which have been most thoroughly documented in the context of the mammary epithelium, also apparent in other epithelial tissues or do other principles govern EMT and stemness in other tissues?
l Do still-unidentified cell biological programs act together with the EMT program to orchestrate entrance into and out of the epithelial stem cell state?
l Is the MET actively induced by specific signals, or is it simply a default process that occurs when EMT-inducing signals are absent? Why is epithelial–mesenchymal plasticity mechanistically critical for residence in normal and neoplastic stem cell states?
l How can the EMT program be reversed to eliminate therapy-resistance CSCs?
ROS in Cancer: The Burning Question
https://doi.org/10.1016/j.molmed.2017.03.004
The concept that ROS arbitrate unique signaling events inspires the following questions, both immediate and long term in the field:
l In addition to the amino acid cysteine, methionine is another sulfur-containing amino acid that may be susceptible to oxidative modification. However, methods to bioconjugate methionines to study its oxidative changes are largely underdeveloped. A pending question is whether methionine oxidation mediates redox signaling. An innovative approach designed by Christopher Chang’s team at UC Berkeley may shed light on this topic and expand the scope of redox-dependent signaling mechanisms in the cell.
l While this review focuses extensively on ROS, reactive nitrogen species can also signal through S-nitrosation of cysteine residues. While the soluble isoform of guanylate cyclase (sGC) is believed to be the primary effector of nitric oxide (NO), a pending question is whether NO signaling involves protein targets other than sGC. If so, what are these targets and how is specificity of NO signaling conferred?
l Given the ability of redox-sensitive probes to reflect cellular redox changes, is it possible to marry this with intravital imaging and super-resolution microscopy to monitor subcellular redox dynamics in real-time? Obvious technical hurdles will involve the development of surgical procedures to allow exposure and proper positioning of the tissue of interest, as well as overcoming the nuance of endogenous tissue autofluorescence through the development of redox-sensitive far-red wavelength probes. The possibility of coupling these tools with magnetic resonance spectroscopic imaging for in vivo measurements of redox alterations will also be of interest.
l In addition to monitoring H2O2 and glutathione redox systems, can we develop similar redox-sensitive probes to assess other central redox couples, such as NADPH/NADP, thioredoxin systems, or reactive nitrogen species (RNS)?
l Vitamin C is synthesized by most invertebrates and vertebrates, including mice. However, humans and other anthropoid primates have lost the ability to synthesize vitamin C, due to mutations in the L-gulono-g-lactone (Gulo) gene, which encodes the enzyme responsible for catalyzing the last step of vitamin C biosynthesis. Does this difference between humans and experimental animals contribute to some of the disparities regarding the role of ROS in tumorigenesis? The Gulo-KO mouse should be evaluated in different murine experimental models of tumorigenesis to test this possibility.
l How do ROS and/or RNS initiate cell signaling? What are the dynamics of each event and how do they correlate with oncogenic insults in cancer cells?
l How is specificity achieved in ROS and/or RNS signaling? What determines the reactivity of independent cysteine and/or methionine residues towards reactive species? Is specificity mediated through elevated local concentrations of unique reactive species or through intermediate sensors that pass on the oxidation state to target proteins through thiol-disulfide exchange?
l What determines whether ROS and/or RNS act as deleterious oxidants versus signaling molecules? Is it a quantitative or qualitative difference of reactive species?
l How do ROS and/or RNS-mediated signaling events contribute to the process of tumorigenesis? Can these events be leveraged therapeutically?
Molecularly targeted cancer therapy: some lessons from the past decade
https://doi.org/10.1016/j.tips.2013.11.004
For the future of molecularly targeted cancer therapy, some important questions that need to be addressed include:
l How can we best select ‘biomarker sets’ and properly apply them in clinical treatment of patients to identify optimal target patient subsets, to predict a patient's response, resistance, and toxicity, and to rapidly distinguish between responders and non-responders?
l Is it possible to screen biomarkers using non-invasive approaches, such as circulating tumor cells, circulating DNA, cytokines, and chemokines? If not, how can we make technical breakthroughs to fully interpret the information of very limited patients’ biopsies?
l Is biomarker-based combinational therapy, that is, a ‘cocktail’ of highly-specific targeted drugs customized to individual patients according to their genetic aberrations, sufficient to largely overcome the resistance of targeted therapy?
l How can innovative biomarker-based clinical design, that is, stratification of patients, assignment of specific drug therapy, and adaptive trial designs, increase the translation of targeted drugs from bench to bedside?
l Given that the tumor microenvironment has an enormous impact on tumor development, how can we develop models that accurately reflect the tumor microenvironment, in particular the human immune system, for drug discovery?
