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TUMOUR METABOLISM
www.beatson.gla.ac.uk/eyal_gottlieb
Our lab utilises state-of-the-art metabolomics capabilities
to study metabolic transformations and to identify metabolic
vulnerabilities in cancer. An early hallmark of cancer tissues
is metabolic reprogramming, first noted by Otto Warburg
who found that cancer cells rely on glycolysis under
aerobic conditions.
Group Leader
Eyal Gottlieb
Research Scientists
Simone Cardaci
Nadja Pfetzer1
Zachary Schug2
Saverio Tardito
Johan Vande Voorde2
Liang (Leon) Zheng
Scientific Officer
Elaine MacKenzie
Clinical Research Fellows
Henry Dabritz3
Stefan Nowicki4
Graduate Students
Elodie Kuntz
Jiska van der Reest
1
Janssen Pharmaceutica
2
CRUK Discovery Committee
Beatson West of Scotland
Cancer Centre
Endowment Fund
3
4
CRUK Glasgow Centre
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More recent research showed that metabolic
alterations in cancer involve many additional
pathways, potentially increasing the number
of clinical targets. In fact, most, if not all
tumour suppressors and oncogenes regulate
metabolism. Furthermore, tumours are typically
placed in a metabolically stressful environment,
leading to essential metabolic adaptations. Our
major interest is in metabolic enzymes that also
function as tumour suppressors or oncogenes,
or that regulate the essential metabolic
requirements of cancer cells.
Pyruvate carboxylation enables growth
of SDH-deficient cells by supporting
aspartate biosynthesis
The tricarboxylic acid (TCA) cycle is a key source
for mitochondrial NADH and the core metabolic
route for production of many biosynthetic
precursors. Despite their pivotal metabolic role,
oncogenic mutations in three TCA cycle-related
enzymes, succinate dehydrogenase (SDH),
fumarate hydratase (FH) and isocitrate
dehydrogenase (IDH) have been identified.
While these discoveries implicated a causal link
between altered metabolism and neoplastic
transformation, they left the question of how
these cancer cells fulfil bioenergetic and
anabolic demands largely unresolved. SDH is a
hetero-tetrameric, nuclear-encoded protein
complex responsible for oxidation of succinate
to fumarate in the TCA cycle and for feeding
electrons into the mitochondrial respiratory
chain for ATP production. Inactivating mutations
in the human genes for any of the SDH subunits,
or the SDH complex assembly factor (SDHAF2),
are associated with susceptibility to develop
neuroendrocrine neoplasms, gastrointestinal
stromal tumours and renal cell carcinoma. We
and others have previously demonstrated that
the loss of SDH causes succinate accumulation
in cells, which activates hypoxia-inducible
SCIENTIFIC REPORT 2015 CANCER RESEARCH UK BEATSON INSTITUTE
Figure 1.
Schematic representation of
metabolic rewiring observed in
Sdhb-null cells. Lack of SDH
activity commits cells to
consume extracellular pyruvate,
which sustains Warburg-like
bioenergetic features. Moreover,
pyruvate carboxylation diverts
glucose-derived carbons into
aspartate biosynthesis, thus
sustaining cell growth of
SDH-deficient cells. The size of
arrows is proportional to the
relative contribution/rate of the
corresponding metabolic
pathway. Red arrows indicate
metabolic pathways required to
support proliferation of Sdhbnull cells.
factors at normal oxygen tension and inhibits
α-ketoglutarate-dependent histone and
DNA demethylases, thereby establishing,
respectively, a pseudohypoxic and
hypermethylator phenotype in tumours.
Although SDH is the first discovered TCA cycle
enzyme with tumour suppressor properties,
the molecular mechanisms that enable the
survival and growth of SDH defective cells
remain largely unexplored. We generated
Sdhb-ablated, immortalised kidney mouse cells
and by unsupervised metabolomic screenings
and isotope tracing approaches, we identified
metabolic pathways essential to support their
proliferation. We found that SDHB loss is
sufficient to ensure a complete block of the
TCA cycle and to drive Warburg-like
bioenergetic features of aerobic glycolysis in
proliferating cells. We demonstrated that
ablation of SDH activity commits cells to
consume extracellular pyruvate needed to
sustain maximal glycolytic flux and support the
diversion of glucose-derived carbons into
aspartate biosynthesis via pyruvate carboxylase
activity (Fig. 1). This study unveiled a metabolic
vulnerability for potential treatment of SDHassociated neoplasms (Cardaci et al, 2015).
Glutamine synthetase activity fuels
nucleotide biosynthesis and supports growth
of glutamine-restricted glioblastoma
Glutamine and glutamate constitute a
metabolic hub in cellular physiology. An
increased demand for glutamine by
transformed cells has been recognised for
almost a century and has been linked to its role
as an abundant circulating respiratory fuel.
Notably, glutamine carbons can support
anabolism through entering the TCA cycle via
glutaminolysis. In certain cancer models, the
inhibition of glutaminase, which deaminates
glutamine to glutamate, reduces proliferation
and tumourigenicity. Glutamine addiction has
been proposed as a mark of glioblastoma, the
most aggressive glioma. Using isotope tracing
(13C- and 15N-labelled glutamine) in vitro and in
vivo, we dissected the differential metabolic
roles of glutamine-derived carbon and nitrogen
atoms in sustaining anabolism and growth in six
human glioblastoma cell lines, primary
glioblastoma stem-like cells and normal
astrocytes. Additionally, glutamine-related
metabolism was investigated in vivo utilising
isotope tracing in both primary orthotopic
murine xenografts and glioblastoma patients.
These studies led to the discovery of a
metabolic crosstalk between glioblastoma cells
and normal astrocytes in the brain (Fig. 2).
Figure 2.
Glutamine metabolism in
glioblastoma. (a) Glutaminerestricted glioblastoma cells
utilise glucose-derived carbons
to produce alpha-ketoglutarate
(solid arrows). This keto acid is
transaminated to glutamate,
mostly by the glutamate pyruvate
transaminases (GPT). Glutamate
is then either secreted by the
Xc- antiporter or converted to
glutamine by glutamine
synthetase (GS). GS supplies
glutamine to nitrogendemanding catabolic pathways
essential for growth, such as
nucleotides biosynthesis. (b) A
proposed model for a novel
tumour-glia interaction.
Glioblastoma cells expressing
low levels of GS rely on the
glutamine produced and
released from GS-positive
astrocytes to synthesise
nucleotides for growth under
glutamine-restricted conditions.
In contrast to the current view that in cancer
cells that undergo aerobic glycolysis,
accelerated anabolism is sustained by
glutamine-derived carbons, which replenish the
TCA cycle (anaplerosis), we have demonstrated
that in glioblastoma cells, almost half of the
glutamine-derived carbon is secreted as
glutamate and does not enter the TCA cycle.
Furthermore, the inhibition of glutaminolysis
did not block proliferation of glioblastoma
cells. Instead, the conversion of glutamate
to glutamine by glutamine synthetase
(cataplerosis) confers glutamine prototrophy
and fuels de novo purine biosynthesis in
glutamine-deprived cells. In both orthotopic
glioblastoma mouse models and in patients,
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C-glucose tracing showed that glutamine
synthetase produces glutamine from TCA
cycle-derived carbons. Finally, while glutamine
is only marginally supplied by the circulation to
the brain, the glutamine required for the growth
of glioblastoma tumours is either autonomously
synthesised by glutamine synthetase-positive
glioma stem cells or supplied locally by
astrocytes (Tardito et al, 2015).
Publications listed on page 85
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TUMOUR METABOLISM