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Page 128 Nguyen et al. Cancer Drug Resist 2018;1:126-38 I http://dx.doi.org/10.20517/cdr.2018.08

intermediary metabolites and reducing power as NADPH. Glycolysis can robustly provide these demands,
providing glycolytic intermediates which are diverted into branching pathways. A prominent case of a
pathway which uses glycolytic intermediates is the pentose phosphate pathway (PPP). Glucose-6-phosphate
produced from glucose can be oxidized by glucose-6-phosphate dehydrogenase to generate NADPH and
ribose-5-phosphate, necessary for nucleotide synthesis. PPP is often upregulated in tumors and their en-
zymes are frequently overexpressed in cancer [23,24] . Another important case is the use of glycolytic 3-phos-
phoglycerate as a precursor for the serine and glycine metabolism through the one-carbon cycle. Several
studies have revealed that the gene encoding 3-phosphoglycerate dehydrogenase, the rate-limiting serine
biosynthesis enzyme, is amplified in breast cancers and melanomas [25,26] . Serine and glycine metabolism,
derived from glycolytic 3-phosphoglycerate, provide advantages for cell growth, such as nucleotide synthe-
sis, DNA methylation, glutathione production and NADPH generation.

After feeding all branching pathways, the excess of glycolytic flux is converted to lactate to preserve a suf-
ficient pool of NAD+ for glycolysis and also to avoid the tricarboxylic acid (TCA) cycle inhibition due
to excess NADH. Still, a percentage of pyruvate enters the mitochondria, and a great portion of citrate
generated at the TCA cycle from this pyruvate will be secreted to the cytosol through the mitochondrial
tricarboxylate carrier. Once at the cytosol, citrate is transformed to acetyl-CoA and oxaloacetate, which is
converted to malate for mitochondrial anaplerosis [27,28] . Citrate-derived acetyl-CoA is used as a precursor
for lipid biosynthesis and protein acetylation.

In addition to glycolytic intermediates, TCA cycle intermediates are also used for biosynthetic precursors
accumulation. The first example is citrate-derived acetyl-CoA, whose production is increased by PI3K/
[29]
AKT-mediated ATP-citrate lyase (ACLY) enzyme . Secondly, the TCA cycle also provides metabolic pre-
cursors for the synthesis of nonessential amino acids, such as aspartate and asparagine from oxaloacetate,
or proline and arginine from α-ketoglutarate. Then, aspartate is used for nucleotide biosynthesis. Indeed,
enabling aspartate synthesis is an essential role of the oxidative phosphorylation in cell proliferation [30,31] .

Due to the release of citrate to the cytosol, the maintenance of the pool of TCA cycle intermediates needs
[32]
additional influx, called anaplerosis. The main anaplerotic source in growing cells is glutamine . In c-
myc-transformed cells, glutamine deprivation could disrupt the TCA cycle and induce cell death, which
[33]
can be rescued by the addition of oxaloacetate or α-ketoglutarate . Glutamine-derived α-ketoglutarate is
oxidized into oxaloacetate to maintain the production of citrate. During hypoxia or under certain onco-
genic conditions, α-ketoglutarate could be converted directly to citrate (following a reversed TCA cycle), in
order to generate the cytosolic acetyl-CoA when glucose-derived acetyl-CoA is insufficient .
[34]

GLUTAMINE UTILIZATION IN CANCER CELLS
Glutamine is the most abundant free amino acid in the blood, whose circulating concentration is around
[35]
0.5 mmol/L . Despite being a nonessential amino acid, glutamine is physiologically an essential source of
carbon and nitrogen for cancer cell proliferation. As discussed above, glutamine uptake is increased spe-
cifically in cancer cells that have dysregulated oncogenes and tumor suppressors, such as c-myc. Glutamine
is catabolized by different enzymes, including GLS, CAD or glutamine fructose-6-phosphate amidotrans-
ferase (GFAT). As an anaplerotic source, glutamine is converted to α-ketoglutarate through mitochondrial
glutaminolysis. Glutamine is first deamidated to glutamate, in an irreversible reaction catalysed by the
enzyme GLS. Then, glutamate is deaminated to α-ketoglutarate by the enzyme GLUD1/glutamate dehydro-
genase (GDH) or by several aminotransferases to produce other non-essential amino acids. Subsequently,
α-ketoglutarate enters the TCA cycle to replenish the mitochondrial citrate pool. GLS is the rate-limiting
enzyme of glutaminolysis, whose regulation is controlled tightly. There are two isoforms of GLS which are
encoded by two genes in mammals, the kidney-type GLS1 and the liver-type GLS2. GLS1 is the main iso-
form expressed in cancer cells and has been shown to be upregulated in a wide variety of cancers, includ-