Glutamine addiction: a new therapeutic target in cancer

Abstract

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.

Figure 1. Glucose and glutamine can provide the carbon and nitrogen for the synthesis of the nonessential amino acids

High throughput glucose and glutamine (gln) uptake provide the growing cell with a large pool of carbon and nitrogen for the biosynthesis of the nonessential amino acids. Compounds containing carbon, but not nitrogen, are shown in red, whereas those containing carbon and nitrogen are shown in blue. Carbon precursors derived from glycolysis (3-phosphoglycerate, 2-phosphoglycerate, pyruvate) and glutaminolysis (oxaloacetate, glutamic acid γ-semialdehyde) serve as the carbon substrate for amino acid biosynthesis. Glutamine-derived glutamic acid (glu) donates its amine group to these carbon substrates to produce nonessential amino acids (serine, alanine, aspartate, ornithine) and α-ketoglutarate (αkg). Alanine (ala) serves as the amine donor to produce serine and pyruvate (pyr) in the mitochondrion for the synthesis of glycine. Glutamine provides the carbon and nitrogen for the synthesis of proline, ornithine, and arginine. Glutamine can also serve as a direct nitrogen donor in the synthesis of asparagine from aspartic acid.

Figure 2. Pharmacologic Targets in Glutamine Metabolism

Myc enables cancer cells to maximize glutamine uptake from the extracellular space through upregulation of the glutamine importer, ASCT2. Uptake of glutamine can be suppressed both by depletion of glutamine from the extracellular space with L-asparaginase and phenylbutyrate or through inhibition of ASCT2-dependent uptake with L-γ-glutamyl-p-nitroanilide (GPNA). Once glutamine enters the cell, it can be metabolized through glutaminolysis to provide NADPH or exported to facilitate TOR kinase activation. Myc enables conversion of glutamine into glutamic acid via upregulation of glutaminase (GLS), an enzyme whose activity can be inhibited by treatment with 6-diazo-5-oxo-L-norleucine (L-DON) and Azaserine. Transamination of glutamic acid to α-ketoglutarate can be inhibited with amino-oxyacetic acid (AOA). Mitochondrial metabolism of α-ketoglutarate leads to the production of citrate, its cleavage into oxaloacetate (OAA), reduction of oxaloacetate into malate, and oxidation of malate to pyruvate via malic enzyme (ME) to produce pyruvate and NADPH. The high rate of glutamine metabolism through successive steps into oxaloacetate establishes glutamine as the primary anaplerotic substrate. This metabolism requires the regeneration of NAD⁺ through the electron transport chain, a process that can be inhibited with the biguanides, phenformin and metformin. Glutamine’s use as a substrate for the amino acid exchanger LAT1 can be suppressed with 2-aminobicyclo-(2,2,1)heptanecarboxylic acid (BCH) treatment and TOR activity can be suppressed by rapamycin treatment.