David R. Wise

The Common Feature of Leukemia-Associated IDH1 and IDH2 Mutations Is a Neomorphic Enzyme Activity Converting α-Ketoglutarate to 2-Hydroxyglutarate

The somatic mutations in cytosolic isocitrate dehydrogenase 1 (IDH1) observed in gliomas can lead to the production of 2-hydroxyglutarate (2HG). Here, we report that tumor 2HG is elevated in a high percentage of patients with cytogenetically normal acute myeloid leukemia (AML). Surprisingly, less than half of cases with elevated 2HG possessed IDH1 mutations. The remaining cases with elevated 2HG had mutations in IDH2, the mitochondrial homolog of IDH1. These data demonstrate that a shared feature of all cancer-associated IDH mutations is production of the oncometabolite 2HG. Furthermore, AML patients with IDH mutations display a significantly reduced number of other well characterized AML-associated mutations and/or associated chromosomal abnormalities, potentially implicating IDH mutation in a distinct mechanism of AML pathogenesis.

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.

Glutamine addiction: a new therapeutic target in cancer

Most cancers depend on a high rate of aerobic glycolysis for their continued growth and survival. Paradoxically, some cancer cell lines also display addiction to glutamine despite the fact that glutamine is a nonessential amino acid that can be synthesized from glucose. The high rate of glutamine uptake exhibited by glutamine-dependent cells does not appear to result solely from its role as a nitrogen donor in nucleotide and amino acid biosynthesis. Instead, glutamine plays a required role in the uptake of essential amino acids and in maintaining activation of TOR (target of rapamycin) kinase. Moreover, in many cancer cells, glutamine is the primary mitochondrial substrate and is required for maintenance of mitochondrial membrane potential and integrity and for support of the NADPH production needed for redox control and macromolecular synthesis.

Hypoxia promotes isocitrate dehydrogenase-dependent carboxylation of α-ketoglutarate to citrate to support cell growth and viability.

Citrate is a critical metabolite required to support both mitochondrial bioenergetics and cytosolic macromolecular synthesis. When cells proliferate under normoxic conditions, glucose provides the acetyl-CoA that condenses with oxaloacetate to support citrate production. Tricarboxylic acid (TCA) cycle anaplerosis is maintained primarily by glutamine. Here we report that some hypoxic cells are able to maintain cell proliferation despite a profound reduction in glucose-dependent citrate production. In these hypoxic cells, glutamine becomes a major source of citrate. Glutamine-derived α-ketoglutarate is reductively carboxylated by the NADPH-linked mitochondrial isocitrate dehydrogenase (IDH2) to form isocitrate, which can then be isomerized to citrate. The increased IDH2-dependent carboxylation of glutamine-derived α-ketoglutarate in hypoxia is associated with a concomitant increased synthesis of 2-hydroxyglutarate (2HG) in cells with wild-type IDH1 and IDH2. When either starved of glutamine or rendered IDH2-deficient by RNAi, hypoxic cells are unable to proliferate. The reductive carboxylation of glutamine is part of the metabolic reprogramming associated with hypoxia-inducible factor 1 (HIF1), as constitutive activation of HIF1 recapitulates the preferential reductive metabolism of glutamine-derived α-ketoglutarate even in normoxic conditions. These data support a role for glutamine carboxylation in maintaining citrate synthesis and cell growth under hypoxic conditions.

ATF4 Regulates MYC-Mediated Neuroblastoma Cell Death upon Glutamine Deprivation

Oncogenic Myc alters mitochondrial metabolism, making it dependent on exogenous glutamine (Gln) for cell survival. Accordingly, Gln deprivation selectively induces apoptosis in MYC-overexpressing cells via unknown mechanisms. Using MYCN-amplified neuroblastoma as a model, we identify PUMA, NOXA, and TRB3 as executors of Gln-starved cells. Gln depletion in MYC-transformed cells induces apoptosis through ATF4-dependent, but p53-independent, PUMA and NOXA induction. MYC-transformed cells depend on both glutamate-oxaloacetate transaminase and glutamate dehydrogenase to maintain Gln homeostasis and suppress apoptosis. Consequently, either ATF4 agonists or glutaminolysis inhibitors potently induce apoptosis in vitro and inhibit tumor growth in vivo. These results reveal mechanisms whereby Myc sensitizes cells to apoptosis, and validate ATF4 agonists and inhibitors of Gln metabolism as potential Myc-selective cancer therapeutics.

Hypoxic Reactive Oxygen Species Regulate the Integrated Stress Response and Cell Survival

Under hypoxic conditions, cells suppress energy-intensive mRNA translation by modulating the mammalian target of rapamycin (mTOR) and pancreatic eIF2α kinase (PERK) pathways. Much is known about hypoxic inhibition of mTOR activity; however, the cellular processes activating PERK remain unclear. Since hypoxia is known to increase intracellular reactive oxygen species (ROS), we hypothesized that hypoxic ROS regulate mTOR and PERK to control mRNA translation and cell survival. Our data indicate that although exogenous ROS inhibit mTOR, eIF2α, and eEF2, mTOR and eEF2 were largely refractory to ROS generated under moderate hypoxia (0.5% O2). In direct contrast, the PERK/eIF2α/ATF4 integrated stress response (ISR) was activated by hypoxic ROS and contributed to global protein synthesis inhibition and adaptive ATF4-mediated gene expression. The ISR as well as exogenous growth factors were critical for cell viability during extended hypoxia, since ISR inhibition decreased the viability of cells deprived of O2 and growth factors. Collectively, our data support an important role for ROS in hypoxic cell survival. Under conditions of moderate hypoxia, ROS induce the ISR, thereby promoting energy and redox homeostasis and enhancing cellular survival.

PET Imaging of Glutaminolysis in Tumors by 18F-(2S,4R)4-Fluoroglutamine

Changes in gene expression, metabolism, and energy requirements are hallmarks of cancer growth and self-sufficiency. Upregulation of the PI3K/Akt/mTor pathway in tumor cells has been shown to stimulate aerobic glycolysis, which has enabled 18F-FDG PET tumor imaging. However, of the millions of 18F-FDG PET scans conducted per year, a significant number of malignant tumors are 18F-FDG PET–negative. Recent studies suggest that several tumors may use glutamine as the key nutrient for survival. As an alternative metabolic tracer for tumors, 18F-(2S,4R)4-fluoroglutamine was developed as a PET tracer for mapping glutaminolytic tumors. Methods: A series of in vitro cell uptake and in vivo animal studies were performed to demonstrate tumor cell addiction to glutamine. Cell uptake studies of this tracer were performed in SF188 and 9L glioblastoma tumor cells. Dynamic small-animal PET studies of 18F-(2S,4R)4-fluoroglutamine were conducted in 2 animal models: xenografts produced in F344 rats by subcutaneous injection of 9L tumor cells and transgenic mice with M/tomND spontaneous mammary gland tumors. Results: In vitro studies showed that both transformed 9L and SF188 tumor cells displayed a high rate of glutamine uptake (maximum uptake, ≈16% dose/100 μg of protein). The cell uptake of 18F-(2S,4R)4-fluoroglutamine by SF188 cells is comparable to that of 3H-l-glutamine but higher than that of 18F-FDG. The tumor cell uptake can be selectively blocked. Biodistribution and PET studies showed that 18F-(2S,4R)4-fluoroglutamine localized in tumors with a higher uptake than in surrounding muscle and liver tissues. Data suggest that certain tumor cells may use glutamine for energy production. Conclusion: The results support that 18F-(2S,4R)4-fluoroglutamine is selectively taken up and trapped by tumor cells. It may be useful as a novel metabolic tracer for tumor imaging.

Preparation and Characterization of l-[5-11C]-Glutamine for Metabolic Imaging of Tumors

Recently, there has been a renewed interest in the study of tumor metabolism above and beyond the Warburg effect. Studies on cancer cell metabolism have provided evidence that tumor-specific activation of signaling pathways, such as the upregulation of the oncogene myc, can regulate glutamine uptake and its metabolism through glutaminolysis to provide the cancer cell with a replacement of energy source. Methods: We report a convenient procedure to prepare l-[5-11C]-glutamine. The tracer was evaluated in 9L and SF188 tumor cells (glioma and astrocytoma cell lines). The biodistribution of l-[5-11C]-glutamine in rodent tumor models was investigated by dissection and PET. Results: By reacting 11C-cyanide ion with protected 4-iodo-2-amino-butanoic ester, the key intermediate was obtained in good yield. After hydrolysis with trifluoroacetic and sulfonic acids, the desired optically pure l-[5-11C]-glutamine was obtained (radiochemical yield, 5% at the end of synthesis; radiochemical purity, >95%). Tumor cell uptake studies showed maximum uptake of l-[5-11C]-glutamine reached 17.9% and 22.5% per 100 μg of protein, respectively, at 60 min in 9L and SF188 tumor cells. At 30 min after incubation, more than 30% of the activity appeared to be incorporated into cellular protein. Biodistribution in normal mice showed that l-[5-11C]-glutamine had significant pancreas uptake (7.37 percentage injected dose per gram at 15 min), most likely due to the exocrine function and high protein turnover within the pancreas. Heart uptake was rapid, and there was 3.34 percentage injected dose per gram remaining at 60 min after injection. Dynamic small-animal PET studies in rats bearing xenografted 9L tumors and in transgenic mice bearing spontaneous mammary gland tumors showed a prominent tumor uptake and retention. Conclusion: The data demonstrated that this tracer was favorably taken up in the tumor models. The results suggest that l-[5-11C]-glutamine might be useful for probing in vivo tumor metabolism in glutaminolytic tumors.