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Page 4 of 19 Davidson et al. J Cancer Metastasis Treat 2021;7:45 https://dx.doi.org/10.20517/2394-4722.2021.77

JOURNEY TO PYRUVATE
Following glucose phosphorylation and isomerization via glucose phosphate isomerase,
phosphofructosekinase-1 generates fructose-1,6-bisphosphate (fructose-1,6-biP) [15,35] . Two molecules of
glyceraldehyde-3-phosphate (GA3P) are generated from fructose-1,6-biP via aldolase A and triose
phosphate isomerase [15,35] . Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) converts GA3P to 1,3-
bisphosphoglycerate (1,3-BPG) and forms ATP in the process. GAPDH is overexpressed in PTC and FTC
[31]
[36]
and is a promising drug target . Kumagai et al. reported that koningic acid, a GAPDH inhibitor,
inhibited proliferation of leukemia cells in addition to murine breast, Ehrlich ascites, and fibrosarcoma cells
in vitro as well as Ehrlich ascites in vivo. Interestingly, vitamin C has been found to inhibit PTC, FTC, and
[37]
ATC growth in vitro; Su et al. reported that pharmaceutical grade injections of vitamin C slowed ATC
tumor growth by inducing reactive oxygen species and inhibiting MAPK and PI3K signaling via GAPDH
inhibition.

1,3 BPG then generates ATP and 3-phosphoglycerate (3PG) by phosphoglycerate kinase (PGK1) which was
highly expressed in a PTC model (B-CPAP cells) as well as PTC tissue [38,39] . Following isomerization via
phosphoglycerate mutase, enolase generates phosphoenolpyruvate (PEP) [15,35] . Enolase 1 (ENO1) is
expressed ubiquitously while enolase 2 (ENO2) is typically found in neurons and has been recommended as
a tumor marker for its high expression in some cancers . ENO1 is overexpressed in the B-CPAP cell line
[40]
while ENO2 was highly expressed in B-CPAP and FTC-133 cells [31,39] . Since cancer cells rely so heavily on
glycolysis for ATP and the metabolic intermediate pyruvate, these reactions in glycolysis provide promising
targets for pharmacological intervention in TC.

Pyruvate kinase
The final rate-limiting step of glycolysis is pyruvate kinase (PKM1) [15,35] . While PKM1 is expressed in most
tissues and serves only to phosphorylate PEP, PKM2 is frequently overexpressed in cancers and acts as a
strict checkpoint to regulate the metabolic demands of cancer cells [41,42] . PKM2 is often found as a tetramer,
in which it forms pyruvate from PEP. Unlike PKM1, PKM2 is allosterically regulated by fructose-1,6-biP to
enhance enzymatic activity on PEP to increase the rate of glycolysis in cancer cells [42-44] . A key difference
between isozymes is that the PKM2 tetramer is stabilized by the presence of serine and succinyl-5-
aminoimidazole-4-carboxamide-1-ribose 5-phosphate (SAICAR), important intermediates of one carbon
metabolism and purine synthesis . When nucleotide levels are high, the cell is able to employ these two
[45]
metabolites to encourage glycolysis for ATP and NADH production. When serine and SAICAR levels are
low, PKM2 adopts its dimer form which has a dramatically reduced affinity for PEP. As a dimer, PKM2
moonlights as a protein kinase for over one hundred substrates such as EGFR, HER2, FGFR, cell-cycle
[46]
proteins, and notable to TC, ERK . ERK is not only phosphorylated by PKM2 but it phosphorylates PEP in
a positive feedback loop to encourage adopting the PKM2 dimer form. As a dimer, glycolysis bottlenecks as
PEP levels rise, favoring the reverse reactions via partial gluconeogenesis to 2PG and 3PG. 3PG is then
anabolized to serine via three reactions which will favor the tetrameric form of PKM2, continuing
glycolysis [15,35] . PKM2 represents a critical junction in cancer metabolism by simultaneously regulating
glycolysis and nucleotide metabolism while phosphorylating several tumor promoters. There is only one
PKM1/2 inhibitor available, shikonin. Shikonin inhibits multiple cell processes in addition to PKM2 but was
effective at slowing FTC growth in vivo [47,48] . Additionally, siRNA against PKM2 demonstrates robust activity
[49]
at inhibiting PTC growth in vitro and in vivo .

FATE OF PYRUVATE IN THE CYTOPLASM
Pyruvate can be funneled into the tricarboxylic acid (TCA) cycle for further ATP generation or may be
[15]
converted to lactate to replenish glycolytic intermediates . In highly glycolytic tumors such as aggressive,