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Mitochondrial Amino Acid Metabolism Provides Vulnerabilities in Mutant KRAS-Driven Cancers
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Mitochondrial Amino Acid Metabolism Provides Vulnerabilities in Mutant KRAS-Driven Cancers See “SLC25A22 Promotes proliferation and survival of colorectal cancer cells with KRAS mutations, and xenograft tumor progression in mice, via intracellular synthesis of aspartate,” by Wong CC, Qian Y, Li X, et al, on page 000.
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olorectal cancer cells frequently carry Kras mutations.1 Despite its prevalence and significance in colorectal and other cancers, Kras has remained an elusive drug target owing to its biochemical and pharmacologic properties.2 Identifying surrogate effector functions regulated by mutant Kras are needed. It is now well-appreciated that reprogramed metabolism is a principle feature mediating the oncogenic activity of mutant Kras.3–5 In this issue of Gastroenterology, Wong et al6 describe a new metabolic vulnerability that is uniquely manifest in colon cancer cells harboring Kras mutations. Metabolic pathways and processes are reprogrammed in cancer cells to facilitate survival and growth in the presence of physical barriers, oxidative stress, nutrient and oxygen deprivation, and antitumor immune responses. Accordingly, there is renewed interest in detailing these metabolic alterations in an effort to identify new drug targets and therapies.7 Recently, the Achilles Project initiated by Broad Institute (www.broadinstitute.org/achilles) carried out a screen to systematically identify genetic vulnerabilities in a large cancer cell line panel. Interrogating this dataset for metabolic enzymes, Wong et al found that mutant Krasexpressing colon cancer cell lines are selectively sensitive to knockdown of the mitochondrial glutamate transporter SLC25A22. This observation was then explored in several patient data repositories and in patient tissue sections, which illustrated both that SLC25A22 is up-regulated in colon cancers relative to corresponding normal tissue, and that high expression of SLC25A22 in Kras mutant tumors is significantly predictive of worse survival outcomes. To verify the results obtained from the Achilles Project, the authors used colon cancer cell line models expressing wild-type or mutant Kras. Specifically, RNA interferencemediated SLC25A22 knockdown in mutant Kras expressing, but not Kras wild-type, cell lines impaired proliferation, colony outgrowth, cell migration, and induced cell cycle arrest and apoptosis. SLC25A22 knockdown was also examined in tumor models in vivo, where knockdown in 2 mutant Kras-expressing colon cancer cell lines impaired tumor growth. A limitation of these studies was the use of constitutive short hairpin RNAs that were active in the cells before their implantation. This prevents an assessment of how a “drug” would ultimately affect tumor dynamics in vivo. In addition, the generality of this synthetic lethal
interaction remains to be tested in wild-type Kras-expressing cells in vivo. Next, the authors sought to determine the mechanism behind SLC25A22 dependence. SLC25A22 is a mitochondrial importer for the nonessential amino acid glutamate (Figure 1). In most cancer cell lines grown in culture, the predominant source of glutamate is the amino acid glutamine. Indeed, numerous studies have illustrated the dependence on glutamine for cells grown in culture,4,8–11 and glutamine is a supplement in nearly all tissue culture media formulations. Previous studies using mutant Krasexpressing cancer cell lines have shown an even greater reliance on glutamine relative to other cancer cells.3,4,12 Consistent with these previous studies, the authors found that Kras mutant colon cancer cell lines required significantly higher concentrations of glutamine in the media for proliferation than Kras wild-type lines. Furthermore, the authors demonstrate that, similar to other systems, glutamine was the source of the glutamate and this was generated in large part by glutaminase, an enzyme that hydrolyzes the terminal amido nitrogen on glutamine. Given the ubiquitous role for glutamine and glutamate, the interesting question that arose from the growth and viability studies was how glutamine-derived glutamate is being differentially used in Kras wild-type versus mutant cell lines. Glutamate can be used for several purposes. As an amino acid, it is used in protein biosynthesis. Second, glutamate is 1 of 3 amino acids in glutathione, a central cellular antioxidant molecule. And, the carbon skeleton of glutamate is also a principle source of biosynthetic material in the form of other amino acids, lipids and reducing potential (Figure 1). Glutamate-derived building blocks are created in the mitochondria as products of the tricarboxylic acid cycle in a process referred to as glutamine/glutamate anaplerosis. Glutamate is not able to penetrate the mitochondrial matrix directly and must be transported. The authors demonstrate that in Kras mutant colon cancer cells, this is achieved by SLC25A22. Furthermore, in this system, a rate-limiting function of glutamate is to fuel anaplerotic mitochondrial metabolism, as opposed to protein or glutathione biosynthesis. Currently, it is unclear why Kras mutant colon cancer cells require this method. Moreover, the mechanisms used by Kras wild-type cancer cell lines remain outstanding and interesting questions. Once inside the mitochondria, glutamate is converted into alpha-ketoglutarate (the carbon skeleton of glutamate) by deamindating or transaminating enzymes where it can then enter the tricarboxylic acid cycle. Previous work in Kras mutant pancreatic cancer demonstrated that the transaminase-mediated pathway was predominant,4 which is distinct from deamidase-mediated pathways present in other systems.13 Based on similarities in the glutamate Gastroenterology 2016;-:1–3
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EDITORIAL Figure 1. Anaplerotic mitochondrial amino acid metabolism. Cancer cells consume glutamine (Gln), which can be used in protein biosynthesis or biosynthetic pathways involving nitrogen capture/ metabolism. Removal of nitrogen from Gln yields the amino acid glutamate (Glu). The transfer of Glu from the inner membrane space (IMS) of the mitochondria into the matrix enables anaplerotic tricarboxylic acid (TCA) cycle metabolism. Biosynthetic intermediates derived from Glu leave the TCA cycle and can be used in lipid and amino acid biosynthesis. An important intermediate is aspartate (Asp), which can be use in pyrimidine biosynthesis (to make DNA and RNA), asparagine (Asn) biosynthesis (which facilitates uptake of essential amino acids [EAA]), redox, and bioenergetic maintenance and protein biosynthesis. Mutant Kras expression drives Gln uptake and anaplerotic Gln metabolism, which is facilitated by enhanced SLC25A22 expression and activity.
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metabolism patterns, delineated using carbon-13 isotopelabeled glutamine tracing mass spectrometry, with those previously published,4 the authors imply that this occurs using the transaminase-mediated pathway, although this was not formally proven. A product of anaplerotic glutamate metabolism is the nonessential amino acid aspartate, which is released into the cytosol (Figure 1). Aspartate is an important biosynthetic substrate and, like glutamate, plays myriad roles in cellular biosynthesis. Namely, aspartate is used in protein biosynthesis, serves as the backbone for pyrimidine nucleotides made de novo, can be converted into other nonessential amino acids, and is involved in the storage and generation of biosynthetic reducing potential. In accordance with these important biosynthetic roles, cancer cells with a defect in the canonical aspartate biosynthetic pathway rewire their metabolism for survival.14 Indeed, 2 recent studies have revealed that one of the most important features of the mitochondria in cultured cancer cells is to generate aspartate, where all other activities of the mitochondria can be compensated by other pathways and processes.15,16 In Kras-transformed pancreatic cancer, we
found previously that a principle role for aspartate is to generate cellular reducing potential and that this is required to maintain redox balance and proliferation.4 Most recently, the Christofk laboratory found that aspartate-derived asparagine export is required to import essential amino acids, without which the cells cannot proliferate.17 In this study, the authors illustrated the aspartate is indeed a rate limiting product of glutamine metabolism where the defects observed upon SLC25A22 inhibition can be partially rescued with exogenous aspartate. Indeed, the cell cycle arrest, redox and bioenergetic defects noted upon SLC25A22 knockdown are likely owing to multiple metabolic roles of aspartate. The important functions for aspartate are in no way mutually exclusive, and a comprehensive assessment of the degree to which each is relevant has not been determined in this or other studies. The present work and other recent detailed studies on the reprogramming of metabolism by mutant Kras have suggested promising new drug targets that exploit this important effector function. However, several important questions still remain regarding the ultimate clinical utility of targeting tumor metabolism, and these are active areas of
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investigation. First, recent work has suggested that glutamine anaplerosis is significantly more relevant in tissue culture conditions.18–20 Accordingly, it will be critical to determine the role of glutamine anaplerosis and SLC25A22 dependence in colon cancer cells that arise in tumors in vivo. Second, normal cells and cancer cells often rely on many of the same metabolic pathways,7 not the least of which is aspartate biosynthesis. The observation that Kras wild-type colon cancer cells tolerate SLC25A22 provide promising data that this may be a target with a tractable therapeutic window. Last, like all targeted therapies, cancer cells rapidly develop resistance to pathway inhibition. Studies into the mechanisms of intrinsic and acquired resistance may provide insights that could be used to select target patient populations and/or reveal combination strategies to exploit this metabolic vulnerability. Q3
YATRIK M. SHAH COSTAS A. LYSSIOTIS Department of Molecular and Integrative Physiology and Department of Internal Medicine Division of Gastroenterology University of Michigan Ann Arbor, Michigan
References 1. Fearon ER. Molecular genetics of colon cancer. Ann Rev Pathol 2011;6:479–507. 2. Cox AD, Fesik SW, Kimmelman AC, et al. Drugging the undruggable RAS: mission possible? Nat Rev Drug Discov 2014;13:828–851. 3. Gaglio D, Metallo CM, Gameiro PA, et al. Oncogenic KRas decouples glucose and glutamine metabolism to support cancer cell growth. Mol Syst Biol 2011;7:523. 4. Son J, Lyssiotis CA, Ying HQ, et al. Glutamine supports pancreatic cancer growth through a KRAS-regulated metabolic pathway. Nature 2013;496:101–105. 5. Ying H, Kimmelman AC, Lyssiotis CA, et al. Oncogenic Kras maintains pancreatic tumors through regulation of anabolic glucose metabolism. Cell 2012;149:656–670. 6. Wong CC, Qian Y, Li X, et al. SLC25A22 Promotes proliferation and survival of colorectal cancer cells with KRAS mutations, and xenograft tumor progression in mice, via intracellular synthesis of aspartate. Gastroenterology 2016. 00:00–00. 7. Vander Heiden MG. Targeting cancer metabolism: a therapeutic window opens. Nat Rev Drug Discov 2011; 10:671–684. 8. Eagle H, Oyama VI, Levy M, et al. The growth response of mammalian cells in tissue culture to L-glutamine and Lglutamic acid. J Biol Chem 1956;218:607–616. 9. Medina MA, Sanchez-Jimenez F, Marquez J, et al. Relevance of glutamine metabolism to tumor cell growth. Mol Cell Biochem 1992;113:1–15.
10. Yuneva M, Zamboni N, Oefner P, et al. Deficiency in glutamine but not glucose induces MYC-dependent apoptosis in human cells. J Cell Biol 2007; 178:93–105. 11. Hensley CT, Wasti AT, DeBerardinis RJ. Glutamine and cancer: cell biology, physiology, and clinical opportunities. J Clin Invest 2013;123:3678–3684. 12. Weinberg F, Hamanaka R, Wheaton WW, et al. Mitochondrial metabolism and ROS generation are essential for Kras-mediated tumorigenicity. Proc Natl Acad Sci U S A 2010;107:8788–8793. 13. Choo AY, Kim SG, Vander Heiden MG, et al. Glucose addiction of TSC null cells is caused by failed mTORC1dependent balancing of metabolic demand with supply. Mol Cell 2010;38:487–499. 14. Cardaci S, Zheng L, MacKay G, et al. Pyruvate carboxylation enables growth of SDH-deficient cells by supporting aspartate biosynthesis. Nat Cell Biol 2015; 17:1317–1326. 15. Birsoy K, Wang T, Chen WW, et al. An essential role of the mitochondrial electron transport chain in cell proliferation is to enable aspartate synthesis. Cell 2015; 162:540–551. 16. Sullivan LB, Gui DY, Hosios AM, et al. Supporting aspartate biosynthesis is an essential function of respiration in proliferating cells. Cell 2015;162:552–563. 17. Krall AS, Xu S, Graeber TG, et al. Asparagine promotes cancer cell proliferation through use as an amino acid exchange factor. Nat Commun 2016;29:11457. 18. Davidson SM, Papagiannakopoulos T, Olenchock BA, et al. Environment impacts the metabolic dependencies of ras-driven non-small cell lung cancer. Cell Metab 2016;23:517–528. 19. Hensley CT, Faubert B, Yuan Q, et al. Metabolic heterogeneity in human lung tumors. Cell 2016; 164:681–694. 20. Sellers K, Fox MP, Bousamra M 2nd, et al. Pyruvate carboxylase is critical for non-small-cell lung cancer proliferation. J Clin Invest 2015;125:687–698.
Reprint requests Address requests for reprints to: Yatrik M. Shah, Department of Molecular and Integrative Physiology, Division of Gastroenterology, University of Michigan, Q1 Ann Arbor, Michigan 48109. e-mail: [email protected].
Y.S. is supported by grants from the NIH (CA148828 and DK095201) C.A.L. is supported by grants from the American Association for Cancer Research (16-80-44-LYSS), Pancreatic Cancer Action Network (13-70-25-LYSS), National Pancreas Foundation, Sidney Kimmel Foundation, Damon Runyon Cancer Research Foundation (DFS-09-14) and American Gastroenterological Association. Conflicts of interest The authors disclose no conflicts.
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© 2016 by the AGA Institute 0016-5085/$36.00 http://dx.doi.org/10.1053/j.gastro.2016.09.036
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Mitochondrial Amino Acid Metabolism Provides Vulnerabilities in Mutant KRAS-Driven Cancers See “SLC25A22 Promotes proliferation and survival of colorectal cancer cells with KRAS mutations, and xenograft tumor progression in mice, via intracellular synthesis of aspartate,” by Wong CC, Qian Y, Li X, et al, on page 000.
C
olorectal cancer cells frequently carry Kras mutations.1 Despite its prevalence and significance in colorectal and other cancers, Kras has remained an elusive drug target owing to its biochemical and pharmacologic properties.2 Identifying surrogate effector functions regulated by mutant Kras are needed. It is now well-appreciated that reprogramed metabolism is a principle feature mediating the oncogenic activity of mutant Kras.3–5 In this issue of Gastroenterology, Wong et al6 describe a new metabolic vulnerability that is uniquely manifest in colon cancer cells harboring Kras mutations. Metabolic pathways and processes are reprogrammed in cancer cells to facilitate survival and growth in the presence of physical barriers, oxidative stress, nutrient and oxygen deprivation, and antitumor immune responses. Accordingly, there is renewed interest in detailing these metabolic alterations in an effort to identify new drug targets and therapies.7 Recently, the Achilles Project initiated by Broad Institute (www.broadinstitute.org/achilles) carried out a screen to systematically identify genetic vulnerabilities in a large cancer cell line panel. Interrogating this dataset for metabolic enzymes, Wong et al found that mutant Krasexpressing colon cancer cell lines are selectively sensitive to knockdown of the mitochondrial glutamate transporter SLC25A22. This observation was then explored in several patient data repositories and in patient tissue sections, which illustrated both that SLC25A22 is up-regulated in colon cancers relative to corresponding normal tissue, and that high expression of SLC25A22 in Kras mutant tumors is significantly predictive of worse survival outcomes. To verify the results obtained from the Achilles Project, the authors used colon cancer cell line models expressing wild-type or mutant Kras. Specifically, RNA interferencemediated SLC25A22 knockdown in mutant Kras expressing, but not Kras wild-type, cell lines impaired proliferation, colony outgrowth, cell migration, and induced cell cycle arrest and apoptosis. SLC25A22 knockdown was also examined in tumor models in vivo, where knockdown in 2 mutant Kras-expressing colon cancer cell lines impaired tumor growth. A limitation of these studies was the use of constitutive short hairpin RNAs that were active in the cells before their implantation. This prevents an assessment of how a “drug” would ultimately affect tumor dynamics in vivo. In addition, the generality of this synthetic lethal
interaction remains to be tested in wild-type Kras-expressing cells in vivo. Next, the authors sought to determine the mechanism behind SLC25A22 dependence. SLC25A22 is a mitochondrial importer for the nonessential amino acid glutamate (Figure 1). In most cancer cell lines grown in culture, the predominant source of glutamate is the amino acid glutamine. Indeed, numerous studies have illustrated the dependence on glutamine for cells grown in culture,4,8–11 and glutamine is a supplement in nearly all tissue culture media formulations. Previous studies using mutant Krasexpressing cancer cell lines have shown an even greater reliance on glutamine relative to other cancer cells.3,4,12 Consistent with these previous studies, the authors found that Kras mutant colon cancer cell lines required significantly higher concentrations of glutamine in the media for proliferation than Kras wild-type lines. Furthermore, the authors demonstrate that, similar to other systems, glutamine was the source of the glutamate and this was generated in large part by glutaminase, an enzyme that hydrolyzes the terminal amido nitrogen on glutamine. Given the ubiquitous role for glutamine and glutamate, the interesting question that arose from the growth and viability studies was how glutamine-derived glutamate is being differentially used in Kras wild-type versus mutant cell lines. Glutamate can be used for several purposes. As an amino acid, it is used in protein biosynthesis. Second, glutamate is 1 of 3 amino acids in glutathione, a central cellular antioxidant molecule. And, the carbon skeleton of glutamate is also a principle source of biosynthetic material in the form of other amino acids, lipids and reducing potential (Figure 1). Glutamate-derived building blocks are created in the mitochondria as products of the tricarboxylic acid cycle in a process referred to as glutamine/glutamate anaplerosis. Glutamate is not able to penetrate the mitochondrial matrix directly and must be transported. The authors demonstrate that in Kras mutant colon cancer cells, this is achieved by SLC25A22. Furthermore, in this system, a rate-limiting function of glutamate is to fuel anaplerotic mitochondrial metabolism, as opposed to protein or glutathione biosynthesis. Currently, it is unclear why Kras mutant colon cancer cells require this method. Moreover, the mechanisms used by Kras wild-type cancer cell lines remain outstanding and interesting questions. Once inside the mitochondria, glutamate is converted into alpha-ketoglutarate (the carbon skeleton of glutamate) by deamindating or transaminating enzymes where it can then enter the tricarboxylic acid cycle. Previous work in Kras mutant pancreatic cancer demonstrated that the transaminase-mediated pathway was predominant,4 which is distinct from deamidase-mediated pathways present in other systems.13 Based on similarities in the glutamate Gastroenterology 2016;-:1–3
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EDITORIAL Figure 1. Anaplerotic mitochondrial amino acid metabolism. Cancer cells consume glutamine (Gln), which can be used in protein biosynthesis or biosynthetic pathways involving nitrogen capture/ metabolism. Removal of nitrogen from Gln yields the amino acid glutamate (Glu). The transfer of Glu from the inner membrane space (IMS) of the mitochondria into the matrix enables anaplerotic tricarboxylic acid (TCA) cycle metabolism. Biosynthetic intermediates derived from Glu leave the TCA cycle and can be used in lipid and amino acid biosynthesis. An important intermediate is aspartate (Asp), which can be use in pyrimidine biosynthesis (to make DNA and RNA), asparagine (Asn) biosynthesis (which facilitates uptake of essential amino acids [EAA]), redox, and bioenergetic maintenance and protein biosynthesis. Mutant Kras expression drives Gln uptake and anaplerotic Gln metabolism, which is facilitated by enhanced SLC25A22 expression and activity.
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metabolism patterns, delineated using carbon-13 isotopelabeled glutamine tracing mass spectrometry, with those previously published,4 the authors imply that this occurs using the transaminase-mediated pathway, although this was not formally proven. A product of anaplerotic glutamate metabolism is the nonessential amino acid aspartate, which is released into the cytosol (Figure 1). Aspartate is an important biosynthetic substrate and, like glutamate, plays myriad roles in cellular biosynthesis. Namely, aspartate is used in protein biosynthesis, serves as the backbone for pyrimidine nucleotides made de novo, can be converted into other nonessential amino acids, and is involved in the storage and generation of biosynthetic reducing potential. In accordance with these important biosynthetic roles, cancer cells with a defect in the canonical aspartate biosynthetic pathway rewire their metabolism for survival.14 Indeed, 2 recent studies have revealed that one of the most important features of the mitochondria in cultured cancer cells is to generate aspartate, where all other activities of the mitochondria can be compensated by other pathways and processes.15,16 In Kras-transformed pancreatic cancer, we
found previously that a principle role for aspartate is to generate cellular reducing potential and that this is required to maintain redox balance and proliferation.4 Most recently, the Christofk laboratory found that aspartate-derived asparagine export is required to import essential amino acids, without which the cells cannot proliferate.17 In this study, the authors illustrated the aspartate is indeed a rate limiting product of glutamine metabolism where the defects observed upon SLC25A22 inhibition can be partially rescued with exogenous aspartate. Indeed, the cell cycle arrest, redox and bioenergetic defects noted upon SLC25A22 knockdown are likely owing to multiple metabolic roles of aspartate. The important functions for aspartate are in no way mutually exclusive, and a comprehensive assessment of the degree to which each is relevant has not been determined in this or other studies. The present work and other recent detailed studies on the reprogramming of metabolism by mutant Kras have suggested promising new drug targets that exploit this important effector function. However, several important questions still remain regarding the ultimate clinical utility of targeting tumor metabolism, and these are active areas of
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investigation. First, recent work has suggested that glutamine anaplerosis is significantly more relevant in tissue culture conditions.18–20 Accordingly, it will be critical to determine the role of glutamine anaplerosis and SLC25A22 dependence in colon cancer cells that arise in tumors in vivo. Second, normal cells and cancer cells often rely on many of the same metabolic pathways,7 not the least of which is aspartate biosynthesis. The observation that Kras wild-type colon cancer cells tolerate SLC25A22 provide promising data that this may be a target with a tractable therapeutic window. Last, like all targeted therapies, cancer cells rapidly develop resistance to pathway inhibition. Studies into the mechanisms of intrinsic and acquired resistance may provide insights that could be used to select target patient populations and/or reveal combination strategies to exploit this metabolic vulnerability. Q3
YATRIK M. SHAH COSTAS A. LYSSIOTIS Department of Molecular and Integrative Physiology and Department of Internal Medicine Division of Gastroenterology University of Michigan Ann Arbor, Michigan
References 1. Fearon ER. Molecular genetics of colon cancer. Ann Rev Pathol 2011;6:479–507. 2. Cox AD, Fesik SW, Kimmelman AC, et al. Drugging the undruggable RAS: mission possible? Nat Rev Drug Discov 2014;13:828–851. 3. Gaglio D, Metallo CM, Gameiro PA, et al. Oncogenic KRas decouples glucose and glutamine metabolism to support cancer cell growth. Mol Syst Biol 2011;7:523. 4. Son J, Lyssiotis CA, Ying HQ, et al. Glutamine supports pancreatic cancer growth through a KRAS-regulated metabolic pathway. Nature 2013;496:101–105. 5. Ying H, Kimmelman AC, Lyssiotis CA, et al. Oncogenic Kras maintains pancreatic tumors through regulation of anabolic glucose metabolism. Cell 2012;149:656–670. 6. Wong CC, Qian Y, Li X, et al. SLC25A22 Promotes proliferation and survival of colorectal cancer cells with KRAS mutations, and xenograft tumor progression in mice, via intracellular synthesis of aspartate. Gastroenterology 2016. 00:00–00. 7. Vander Heiden MG. Targeting cancer metabolism: a therapeutic window opens. Nat Rev Drug Discov 2011; 10:671–684. 8. Eagle H, Oyama VI, Levy M, et al. The growth response of mammalian cells in tissue culture to L-glutamine and Lglutamic acid. J Biol Chem 1956;218:607–616. 9. Medina MA, Sanchez-Jimenez F, Marquez J, et al. Relevance of glutamine metabolism to tumor cell growth. Mol Cell Biochem 1992;113:1–15.
10. Yuneva M, Zamboni N, Oefner P, et al. Deficiency in glutamine but not glucose induces MYC-dependent apoptosis in human cells. J Cell Biol 2007; 178:93–105. 11. Hensley CT, Wasti AT, DeBerardinis RJ. Glutamine and cancer: cell biology, physiology, and clinical opportunities. J Clin Invest 2013;123:3678–3684. 12. Weinberg F, Hamanaka R, Wheaton WW, et al. Mitochondrial metabolism and ROS generation are essential for Kras-mediated tumorigenicity. Proc Natl Acad Sci U S A 2010;107:8788–8793. 13. Choo AY, Kim SG, Vander Heiden MG, et al. Glucose addiction of TSC null cells is caused by failed mTORC1dependent balancing of metabolic demand with supply. Mol Cell 2010;38:487–499. 14. Cardaci S, Zheng L, MacKay G, et al. Pyruvate carboxylation enables growth of SDH-deficient cells by supporting aspartate biosynthesis. Nat Cell Biol 2015; 17:1317–1326. 15. Birsoy K, Wang T, Chen WW, et al. An essential role of the mitochondrial electron transport chain in cell proliferation is to enable aspartate synthesis. Cell 2015; 162:540–551. 16. Sullivan LB, Gui DY, Hosios AM, et al. Supporting aspartate biosynthesis is an essential function of respiration in proliferating cells. Cell 2015;162:552–563. 17. Krall AS, Xu S, Graeber TG, et al. Asparagine promotes cancer cell proliferation through use as an amino acid exchange factor. Nat Commun 2016;29:11457. 18. Davidson SM, Papagiannakopoulos T, Olenchock BA, et al. Environment impacts the metabolic dependencies of ras-driven non-small cell lung cancer. Cell Metab 2016;23:517–528. 19. Hensley CT, Faubert B, Yuan Q, et al. Metabolic heterogeneity in human lung tumors. Cell 2016; 164:681–694. 20. Sellers K, Fox MP, Bousamra M 2nd, et al. Pyruvate carboxylase is critical for non-small-cell lung cancer proliferation. J Clin Invest 2015;125:687–698.
Reprint requests Address requests for reprints to: Yatrik M. Shah, Department of Molecular and Integrative Physiology, Division of Gastroenterology, University of Michigan, Q1 Ann Arbor, Michigan 48109. e-mail: [email protected].
Y.S. is supported by grants from the NIH (CA148828 and DK095201) C.A.L. is supported by grants from the American Association for Cancer Research (16-80-44-LYSS), Pancreatic Cancer Action Network (13-70-25-LYSS), National Pancreas Foundation, Sidney Kimmel Foundation, Damon Runyon Cancer Research Foundation (DFS-09-14) and American Gastroenterological Association. Conflicts of interest The authors disclose no conflicts.
Q2
© 2016 by the AGA Institute 0016-5085/$36.00 http://dx.doi.org/10.1053/j.gastro.2016.09.036
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