Cancer Cell Metabolism: Unique Features Inform New Therapeutic Opportunities

Abstracts

Beyond Glucose and Glutamine: Some Cancer Cells Forage for Food
Craig B. Thompson, Memorial Sloan-Kettering Cancer Center, New York, NY

Under conditions of an abundant supply of extracellular glucose and glutamine, cancer cells synthesize lipids, nucleic acids, and non-essential amino acids from these precursors. However, when cancer grows to exceed its vascular supply, cells must adapt to lower availability of essential nutrients and oxygen. In the past year, we have seen a major breakthrough in the discovery of novel mechanisms by which cancer cells adapt to metabolic stress. It was previously determined that glutamine-dependent lipid synthesis could support de novo lipid synthesis under hypoxia but metabolic tracing studies uncovered that over 50% of lipids used in phospholipid biosynthesis in hypoxic cells come from exogenous sources. We were able to determine that hypoxia-induced macropinocytosis facilitates the uptake and incorporation of unsaturated phospholipids to maintain effective unsaturated lipid levels under conditions in which stearoyl-CoA desaturase is inhibited by oxygen limitation. Follow up studies have demonstrated that this property of cellular scavenging of extracellular macromolecules is induced in a Ras-dependent fashion in transformed cells and provides a potent mechanism by which Ras-transformed cells can adapt to metabolic stress in their environment. This pathway of cellular acquisition of bioenergetic substrates has previously escaped detection because, in complete medium, the utilization of macropinocytosed proteins as metabolic substrates is actively suppressed. The molecular details of this suppression and the implication of these studies to understanding both cancer and wound repair will be discussed.
 

mTOR, Cancer Metabolism and Therapeutic Opportunities
John Blenis, PhD¹

The mTOR Complex 1 (mTORC1) signaling pathway has evolved to sense and respond to cellular energy status, nutrient availability and surrounding oxygen concentrations.  In addition, mTORC1 can be further activated by mitogen- and hormone-stimulated kinases including Akt, ERK and RSK, and suppressed by mTORC1-regulated S6K1 via a variety of negative feedback loops.  The integration of these multiple inputs control the strength and duration of downstream signaling, which is important in differentially regulating mTORC1-dependent processes such as protein synthesis and cellular metabolism.  I will discuss how mTORC1 and S6K1 regulate aspects of nutrient metabolism, mRNA metabolism and protein production; biological processes critical to the control of cell growth while at the same time creating vulnerabilities that may provide important therapeutic opportunities in cancers with activated mTORC1 signaling.
 
Coauthors: Sang Gyun Kim, PhD¹, Sanguine Byun, PhD², Jing Li, PhD¹, Andy Choo, PhD¹, Alfredo Csibi, PhD¹, Gina Lee, PhD¹, Sam Lee, PhD²
1 Sandra and Edward Meyer Cancer Center, Department of Pharmacology, Belfer Research Building, Weill Cornell Medical College, New York, NY
2 Cutaneous Biology Research Center, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts, USA
 

Mitochondrial Amino Acid Metabolism and Nutrient Sensing
Christian Metallo, PhD, University of California, San Diego

Metabolism is central to virtually all cellular functions and contributes to a range of diseases. A detailed understanding of how biochemical pathways are regulated is necessary to understand and control cell function. Quantitative information describing the flow of metabolites through biochemical networks provides critical insights into how different nutrients contribute to energy metabolism and biosynthesis. To this end we apply stable isotope tracers, mass spectrometry, and metabolic flux analysis (MFA) to study central metabolism in mammalian cells. Using these approaches we have characterized how proliferating and differentiated adipocytes regulate flux of glucose and essential amino acids into mitochondria for biosynthesis of fatty acids and glutamine. Through this analysis we have also identified fatty acids synthesized by adipocytes that integrate carbohydrate, fatty acid, and protein availability. The abundance of these metabolites changes in response to changes in diet or cellular microenvironments and influences metabolic signaling pathways. Ultimately, the application of MFA to these cellular models has improved our ability to characterize intracellular metabolic processes, providing a mechanistic understanding of cellular physiology and metabolic function.
 

Role of Metabolism in Supporting Cell Proliferation
Matthew G. Vander Heiden, MD, PhD, Koch Institute for Integrative Cancer Research at Massachusetts Institute of Technology, Cambridge, MA, USA

Cells adapt metabolism to meet distinct physiological needs, and metabolic regulation influences tumor progression. To proliferate, cancer cells must adapt metabolism to support anabolic processes and allow the accumulation of biomass. However, those nutrients with the highest consumption by cancer cells are not necessarily the fuels that contribute directly to cell mass. Cell culture provides a system to study how metabolism supports proliferation, but understanding non-proliferating cell populations requires an analysis of metabolism in patients and in tumor tissue. Use of mass spectrometry to trace nutrient use both in cell culture models and mouse cancer models suggests that both glucose and serine metabolism are critical to support nucleotide synthesis. Regulation of these pathways determines whether sufficient nucleotides are available to support cell proliferation and begins to clarify how metabolism is regulated to control cell proliferation.
 

Metabolic Regulation of Cell Fate Decisions
Lydia W.S. Finley, PhD¹

The deposition and removal of DNA and histone modifications depends on the availability of specific metabolites, suggesting that cellular metabolic pathways may tightly regulate chromatin modifications and gene expression. However, whether changes in metabolic flux can influence cell state is poorly understood. Here we show that mouse embryonic stem cells (ESCs) grown under conditions that maintain naïve pluripotency exhibit unique metabolic properties, including reduced reliance on catabolism of extracellular glutamine. Despite this, ESCs consume high levels of exogenous glutamine when the metabolite is available. As a result, naïve embryonic stem cells utilize both glucose and glutamine to maintain high levels of intracellular a-ketoglutarate, a metabolite that is critical for the activity of histone and DNA demethylases. High a-ketoglutarate levels promote an open chromatin landscape, thereby inhibiting differentiation and promoting stem cell self-renewal. This work reveals that intracellular a-ketoglutarate levels can contribute to the maintenance of cellular identity and play a mechanistic role in the transcriptional and epigenetic state of stem cells.
 
Coauthors: Bryce W. Carey, PhD², C. David Allis, PhD² and Craig B. Thompson, MD¹
1 Memorial Sloan Kettering Cancer Center, New York, New York, United States
2 The Rockefeller University, New York, New York, United States
 

A Post-transcriptionally Regulated Metabolic Pathway that enables Liver Metastatic Colonization by Colon Cancer
Sohail Tavazoie, MD, PhD, Associate Professor and Head, Laboratory of Systems Cancer Biology
Jiamin Loo & Alexander Nguyen, Graduate Students
Rockefeller University

For cancer cells to form metastatic colonies, rare cells must achieve optimal gene expression states that enables them to adapt to, and progress within, distal organs. We have found that one mechanism by which cancer cells achieve such pro-metastatic gene expression states is through the modulation of specific miRNAs. By functionally screening 661 miRNAs in parallel during the process of liver colonization, we identified miR-483 and miR-551a as endogenous suppressors of colon cancer liver metastatic colonization. By using these miRNAs as molecular probes, we have identified a metabolic pathway that is utilized by colon cancer cells during liver colonization. MiR-483 and miR-551a convergently target the metabolic enzyme creatine kinase brain-type (CKB). The silencing of these miRNAs enables rare cells to over-express CKB. CKB is known to phosphorylate the metabolite creatine. Loss-of-function, gain-of-function, and epistasis studies reveal CKB to be required for efficient colonization of the liver. Colon cancer cells enter the liver via the portal circulation, which is hypoxemic. We find that CKB is release by colon cancer cells experiencing hypoxia. Previous clinical studies have also detected circulating CKB in the plasma of patients, though the reason for this was not understood. The substrate for CKB, creatine, is produced by the liver. Moreover, CKB-driven metastasis requires extracellular ATP, which is present in the tumor microenvironment. Our observations support a model whereby CKB is release into the extracellular space, where it catalyzes phosphorylation of creatine, yielding phosphocreatine. Phosphocreatine is then imported into the cell, used to generate ATP for fueling metastatic survival. Indeed, phosphocreatine is taken up by colon cancer cells, converted to creatine and ATP, and found to promote colon cancer survival during hypoxia. Analysis of human clinical samples reveals that the expression levels of these miRNAs and CKB supports their experimentally inferred roles in metastatic progression. We furthermore provide proof-of-concept for targeting of this pathway as a means of suppressing colon cancer metastatic progression.
 

Reversible Metabolic Changes in Human Melanoma Cells Enable Distant Metastasis
Elena Piskounova, PhD¹

Metastasis is a complex multistep process that requires many cellular adaptations. We developed a mouse model of metastasis, utilizing patient-derived xenografts, which is predictive of metastasis in patients. Functional analysis of melanoma cells from primary subcutaneous tumors, peripheral blood, and metastatic nodules from visceral organs from this model showed that these populations differ in their capacity to form tumors at different sites. Metabolomic profiling of these populations indicated that efficiently metastasizing melanomas undergo reversible metabolic changes as they progress through the metastatic cascade. Specifically, compared to cells from primary subcutaneous tumors, both circulating tumor cells (CTCs) and cells from metastatic nodules had lower Glutathione to Oxidized Glutathione (GSH/GSSG) ratio and higher levels of reactive oxygen species (ROS). Treatment of tumor-bearing mice with antioxidants caused a significant increase in CTC frequency and metastatic burden in distant organs, suggesting that the ability of tumor cells to detoxify ROS is directly linked to their ability to metastasize. Additionally, we observed increased de novo serine synthesis in metastasizing cells, implicating folate/one-carbon metabolism in melanoma metastasis. NADPH production through the folate pathway has been shown to play a key role in maintaining the cellular redox state. Depletion of NADPH-producing enzymes in the one-carbon/folate pathway had a detrimental effect on both CTC frequency and distant metastases, without affecting primary tumor growth. This suggests that successful metastasizers rely on the folate pathway, as a NADPH source, to progress through the metastatic cascade. This work provides insight into the metabolism of metastasis and suggests that enzymes in the folate pathway represent novel therapeutic targets.
 
Coauthors: Michalis Agathocleous, PhD¹, Sara E. Mann¹, Zeping Hu, PhD¹, Zhiyu Zhao, PhD¹, A. Marilyn Leitch, MD², Timothy M. Johnson, MD³, Ralph J. DeBerardinis, MD, PhD¹, Sean J. Morrison, PhD^1,2
1 Children’s Research Institute at University of Texas Southwestern Medical Center, Dallas, Texas, United States
2 University of Texas Southwestern Medical Center, Dallas, Texas, United States
3 Department of Dermatology, University of Michigan, Ann Arbor, Michigan, United States
 

Identifying Metabolic Dependencies in Pancreatic Cancer
Alec Kimmelman, MD, PhD, Dana Farber Cancer Institute, Harvard Medical School, Boston, MA USA

Pancreatic cancers have an intense resistance to currently available therapeutics which results in a 5-year survival rate of approximately 6%. This resistance points toward altered cell metabolic pathways. In this regard we have previously shown that that oncogenic Kras promotes a rewiring of pancreatic cancer metabolism allowing glucose and glutamine to be utilized in a variety of biosynthetic pathways. Importantly, several of these metabolic pathways are critical for tumor growth and therefore represent potential therapeutic targets. Additional studies from our group have demonstrated pancreatic cancers have elevated basal autophagy which is required for their continued growth. Importantly, inhibition of autophagy pharmacologically or genetically leads to decreased oxidative phosphorylation, a drop in ATP production, and ultimately growth inhibition. These findings have implicated autophagy as a key component of pancreatic cancer metabolism and have motivated the opening of multiple clinical trials assessing the efficacy of hydroxychloroquine as an autophagy inhibitor in pancreatic cancer. Ongoing work form our group seeks to understand the metabolic contributions that autophagy makes in pancreatic tumors. These and other aspects of pancreatic cancer metabolism will be discussed.
 

Cell Cycle Control of Cancer Cell Metabolism
Selina Chen-Kiang, PhD

Cell cycle dysregulation is a hallmark of human cancer. How the cell cycle controls metabolism in cancer cells, however, is not understood. To address this question, we have developed a novel strategy to reprogram cancer cells in prolonged early G1 arrest (pG1) induced by selective inhibition of CDK4/CDK6. pG1, but not normal G1 arrest, results in the expression of genes scheduled in early G1 only and depletion of glycolytic metabolites. This is exacerbated in synchronous S phase entry upon release of the G1 block, due to incomplete restoration of cell cycle-coupled gene expression. We investigated pG1-induced metabolic perturbation in the context of clinical response in a phase I study targeting CDK4/6 with a selective inhibitor palbociclib in combination with the proteasome inhibitor bortezomib in recurrent mantle cell lymphoma (n=16), in which CDK4 is overactivated and cyclin D1 is aberrantly expressed. This therapy achieved a durable response including one complete remission (>900 days) and only one progression at optimal doses. CDK4 inhibition induced pG1 in all patients initially but inactivated PI3K only in clinically responding patients (R). Longitudinal integrative transcriptome and whole exome sequencing of serial biopsies further revealed that <1% of the 1500 genes suppressed (not programmed) in pG1 in Rs were activated in non-responding patients. These genes were critical for metabolism and redox homeostasis.  This study provides the first evidence for cell cycle control of PI3K and metabolism in the context of clinical response, suggesting that metabolic perturbation and redox stress mediates cell cycle reprogramming in cancer cells.
 

Coauthors: Maurizio DiLiberto, PhD, Peter Martin, MD, Jihye Paik, PhD, Costas Lyssiotis, PhD, Qiuying Chen, PhD, Priyanka Vijay, BS, Xiangao Huang, PhD, Olivier Elemento, PhD, Christopher Mason, PhD, Timothy McGraw, PhD, Steven Gross, PhD, John P. Leonard, MD, Lewis Cantley, PhD