Glutamine reliance in cell metabolism

Abstract

As knowledge of cell metabolism has advanced, glutamine has been considered an important amino acid that supplies carbon and nitrogen to fuel biosynthesis. A recent study provided a new perspective on mitochondrial glutamine metabolism, offering mechanistic insights into metabolic adaptation during tumor hypoxia, the emergence of drug resistance, and glutaminolysis-induced metabolic reprogramming and presenting metabolic strategies to target glutamine metabolism in cancer cells. In this review, we introduce the various biosynthetic and bioenergetic roles of glutamine based on the compartmentalization of glutamine metabolism to explain why cells exhibit metabolic reliance on glutamine. Additionally, we examined whether glutamine derivatives contribute to epigenetic regulation associated with tumorigenesis. In addition, in discussing glutamine transporters, we propose a metabolic target for therapeutic intervention in cancer.

Fig. 1. Glutamine metabolic pathways.

Glutamine enters through several plasma membrane glutamine transporters and is then utilized in the cytosol in processes such as the biosynthesis of nucleotides, asparagine, and UDP-GlcNAc. For glutaminolysis, glutamine is transported into the mitochondrial matrix through the SLC1A5 variant and subsequently converted to glutamate by GLS. Next, GLUD1 or several aminotransferases catalyze the deamidation of glutamate, producing α-KG. Glutamine-derived α-KG supplies metabolites for the TCA cycle and fuels the generation of 2-HG under conditions of IDH2 mutation or hypoxia. Citrate derived from glutamine via reductive carboxylation supports fatty acid synthesis under conditions of hypoxia or HIF-2α transcription factor stabilization. Glutamine-derived α-KG also activates the mTORC1 pathway. Α-KG and 2-HG affect epigenetic modification through α-KG-dependent dioxygenases. Gln glutamine, Glu glutamate, Asn asparagine, Cys cystine, Asp aspartate, αKG α-ketoglutarate, PRA 5-phosphoribosyl-1-amine, CP carbamoyl phosphate, GFAT glutamine-fructose-6-phosphate transaminase, ASNS asparagine synthetase, PPAT phosphoribosyl pyrophosphate amidotransferase, CPS carbamoyl phosphate synthetase, GLS glutaminase, GLUD glutamate dehydrogenase, GOT glutamic-oxaloacetic transaminase, GPT glutamic-pyruvate transaminase, IDH isocitrate dehydrogenase, 2-HG 2-hydroxyglutarate, Me methylation.

Fig. 2. Nucleotide biosynthesis from glutamine.

In purine biosynthesis, two glutamine molecules are consumed to synthesize AMP, and three glutamine molecules are used to synthesize GMP. Similarly, in pyrimidine biosynthesis, one glutamine molecule is consumed to synthesize UMP, and two glutamine molecules are spent to convert UTP into CTP. The initial step in de novo pyrimidine synthesis is the condensation reaction between glutamine and bicarbonate catalyzed by CPS to produce CP. In cells with an oncogenic mutational status, including K-Ras mutation, glutaminolysis sustains mitochondrial generation of CP by providing enough nitrogen fuel as ammonium ions, and mitochondrial CP then participates in cytosolic de novo pyrimidine synthesis. Glutamine-induced nucleotide biosynthesis is also enhanced by MYC or growth signals such as mTORC1 activation. PPAT phosphoribosyl pyrophosphate amidotransferase, PFAS phosphoribosylformylglycinamidine synthase, GMPS GMP synthetase, CPS carbamoyl phosphate synthetase, CTPS CTP synthetase, GLS glutaminase, PRPP 5-phosphoribosyl-1-pyrophosphate, PRA 5-phosphoribosyl-1-amine, FGAR N2-formyl-N1-(5-phospho-d-ribosyl)glycinamide, FGAM 2-(formamido)-N1-(5-phospho-d-ribosyl)acetamidine, IMP inosine monophosphate, SAMP adenylosuccinate, XMP xanthosine monophosphate, AMP adenosine monophosphate, GMP guanosine monophosphate, CP carbamoyl phosphate, UMP uridine monophosphate, UTP uridine triphosphate, CTP cytidine, Glu glutamine, Glu glutamate, αKG α-ketoglutarate.

Fig. 3. NEAAs synthesized from glutamine.

Intracellular glutamine is converted into diverse NEAAs and supports protein translation and amino acid signaling. Glutamine-derived glutamate plays a central role as a substrate for several aminotransferases producing aspartate, alanine, proline, arginine, serine, cysteine, and glycine. ASNS directly utilizes cytosolic glutamine to synthesize Asn, which plays a distinct role in glutamine-related metabolism. Collectively, glutamine-derived NEAAs suppress ATF4, which is a master transcriptional regulator stimulated under stress conditions. NEAAs nonessential amino acids, GLS glutaminase, GLUD glutamate dehydrogenase, GOT glutamic-oxaloacetic transaminase, GPT glutamic-pyruvate transaminase, PSAT phosphoserine aminotransferase, ATF activating transcription factor, ASNS asparagine synthetase, Gln glutamine, Glu glutamate, Pro proline, Asp aspartate, Ala alanine, Ser serine, Gly glycine, Cys cystine, Asn asparagine, Lys lysine, Thr threonine, Met methionine, aKG α-ketoglutarate, OAA oxaloacetate, Pyr pyruvate, PHP phosphohydroxypyruvate, PS phosphoserine.

Fig. 4. Control of glutamine metabolism by hypoxia.

Hypoxia stabilizes HIF-α proteins such as HIF-1α and HIF-2α. HIF-1α enhances glucose uptake and increases the level of glycolytic enzymes. Under hypoxic conditions, most glucose-derived pyruvate is converted into lactate via LDHA and exported to the extracellular space through the lactate transporters SLC16A1 and SLC16A4. Under these conditions, HIF-2α-mediated glutaminolysis becomes essential to support the adaptation to hypoxia, altering the metabolic fate of glutamine via reductive carboxylation to generate citrate. Then, citrate participates in fatty acid synthesis in the cytosol, which is also activated by stabilized HIF-2α. Hypoxia-induced acidic pH also plays a crucial role in the production of L-2-HG by affecting the substrate affinities of LDHA and MDH. Next, L-2-HG can control DNA or histone methylation levels by regulating α-KG-dependent dioxygenases. HIF hypoxia-inducible factor, GLS glutaminase, GLUD glutamate dehydrogenase, IDH isocitrate dehydrogenase, MDH malate dehydrogenase, L-2HGDH L-2-hydroxyglutarate dehydrogenase, LDHA lactate dehydrogenase, TETs ten-eleven translocation enzymes, JHDMs JmjC domain-containing histone demethylases, Gln glutamine, Glu glutamate, α-KG α-ketoglutarate, L-2-HG L-2-hydroxyglutarate, Me methylation.

Fig. 5. Glutamine oncometabolites and energy production from glutamine.

a Several mutations in enzymes in the glutaminolysis pathway are responsible for the production of oncometabolites. Mutation of IDH1 and IDH2 produces R-2-HG from α-KG, which, when accumulated, leads to the inhibition of dioxygenases, in turn leading to the activation of TET and JHDM enzymes inside the nucleus. Mutation of SDH arrests the TCA cycle, resulting in an increase in the succinate concentration. A high concentration of succinate has an effect similar to the oncometabolite effect of R-2-HG. Additionally, impaired function of FH prevents further metabolism of fumarate, leading to its accumulation. FH impairment inhibits the function of Keap1 and PHD, which stimulates the transcription of protooncogenes. Gln glutamine, Glu glutamate, α-KG α-ketoglutarate, IDH isocitrate dehydrogenase, 2OGDH 2-oxoglutarate dehydrogenase, SDH succinate dehydrogenase, FH fumarate hydratase, R-2-HG R-2-hydroxyglutarate, Keap1 Kelch-like ECH-associated protein 1, PHD prolyl hydroxylase, TETs ten-eleven translocation enzymes, JHDMs JmjC domain-containing histone demethylases, Me methylation. b. Glutamine anaplerosis is a key mitochondrial metabolic pathway for cancer cell growth and survival. Influx of glutamine-derived α-KG into the TCA cycle replenishes the intermediates and consequently generates NADH, FADH₂, and GTP. The generated GTP can be readily converted to an equal amount of ATP. Additionally, glutamate and α-KG produced via glutaminolysis participate in the malate-aspartate shuttle, promoting the transport of NADH from the cytosol into mitochondria. Elevated mitochondrial NADH and FADH₂ levels collectively contribute to enhanced ATP production via OXPHOS through the ETC. Gln glutamine, Glu glutamate, Asp aspartate, αKG α-ketoglutarate, GOT1/2 glutamic-oxaloacetic transaminase 1/2, MDH1/2 malate dehydrogenase 1/2, OAA oxaloacetate, OGC 2-oxoglutarate carrier, AGC aspartate-glutamate carrier, ETC electron transport chain.

Fig. 6. Metabolic reprogramming induced by glutamine metabolism.

a Aerobic glycolysis is a hallmark of cancer metabolism. During this process, most glucose-derived pyruvate is secreted extracellularly as lactate, and glutamine becomes a conditionally essential amino acid. Glutaminolysis sustains mitochondrial function, supplying TCA cycle metabolites such as αKG and generating diverse biomolecules, including NEAAs, NADPH, and nucleotides. Increased glutamine flux into the mitochondrial matrix via the SLC1A5 variant can enhance glutaminolysis and lead to metabolic reprogramming toward enhanced aerobic glycolysis. b Glutamine-derived α-KG activates the mTORC1 signaling pathway, resulting in aerobic glycolysis and protein translation, which are crucial for tumor proliferation. c During glutaminolysis, ammonium ions are generated via a deamidation reaction catalyzed by glutaminase and glutamate dehydrogenase. Most ammonium ions are used as a nitrogen source for nucleotide biosynthesis and are disposed of via the urea cycle, but an excess of ammonium ions promotes autophagy. Augmented autophagy is associated with drug resistance by enhancing aerobic glycolysis and is involved in cancer cell survival, progression, and metastasis. Gln glutamine, Glu glutamate, α-KG a-ketoglutarate, PHD prolyl hydroxylase.

Fig. 7. Oncogenic control of glutamine metabolism.

Oncogenes such as MYC, K-Ras, and PI3KCA modulate cancer metabolic reprogramming, favoring cancer cell growth and survival partially via the promotion of glutamine metabolism. Glutamine uptake is enhanced in MYC- and K-Ras-driven cells in which the expression of the glutamine transporter SLC1A5 is upregulated. Deamination of glutamine to form glutamate in mitochondria is enhanced by MYC-mediated upregulation of GLS1. Conversion of glutamate into α-KG is mediated by GLUD1 or aminotransferases such as GOT1/2 and GPT2. The expression of these enzymes is upregulated in cancer cells with MYC-driven, K-Ras-driven, and PI3KCA-driven signaling activation. Gln glutamine, Glu glutamate, Ala alanine, Asp aspartate, α-KG α-ketoglutarate, GLS1 glutaminase 1, GLUD1 glutamate dehydrogenase 1, GOT1/2 glutamic-oxaloacetic transaminase 1/2, GPT1/2 glutamic-pyruvate transaminase 1/2, MDH1 malate dehydrogenase 1, ME1 malic enzyme 1.

Fig. 8. Inhibitors of glutamine transporters and glutaminolysis.

For the principal inhibition of glutaminolysis, attempts have been made to target the amino acid transporters related to these pathways. SLC6A14 and SLC38A1 are inhibited by α-Me-Trp and MeIAB, respectively. The most intensely researched topic is inhibitors of SLC1A5, a major glutamine transporter, which include substrate analog competitive inhibitors such as GPNA, benzylserine, and V-9302 and the inhibitory antibody MEDI7247. Although they exhibit low potency, inhibitors of SLC7A11 include erastin and SSZ. Inhibitors of glutaminolytic enzymes are agents that target GLS1, GOT2, and GLUD1. CB-839, an agent in its 2nd clinical trial, inhibits GLS1 similarly to BPTES and 968. AOA inhibits GOT2 activity, and EGCG, purpurin, and R162 inactivate GLUD1. However, the SLC1A5 variant, the sole glutamine transporter discovered to date, is expected to be a much more effective target for cancer therapeutics than previously studied glutaminolysis inhibitors. Cys cysteine, Glu glutamate, α-KG α-ketoglutarate, GLS glutaminase, GOT2 glutamic-oxaloacetic transaminase 2, GPT2 glutamic-pyruvate transaminase 2, GLUD1 glutamate dehydrogenase 1, α-Me-Trp alpha-methyl-tryptophan, MeAIB methylaminoisobutyric acid, GPNA L-γ-glutamyl-p-nitroanilide, SSZ sulfasalazine, DON 6-diazo-5-oxo-l-norleucine, AOA aminooxyacetate, EGCG epigallocatechin-3-gallate.