Renal cell carcinoma (RCC) accounts for 2–3% of all adult malignant tumors, having
the highest incidence in Western countries. It is estimated that in 2016, about 62,700
new
cases will be diagnosed (3.7% of all new cancer cases) in the United States, and nearly
14,240 patients will die of this cancer (2.4% of all cancer deaths) (1). Patients
with low stage, localized disease have an excellent
prognosis after surgical treatment, but a significant percentage of subjects still
present
with a surgically unresectable tumor or will subsequently develop metastatic disease
(2,3). Recent
studies have provided novel insights into the molecular mechanisms involved in the
RCC
pathogenesis, leading to the identification of potential biomarkers for early diagnosis,
risk assessment, and outcome prediction. Many molecular markers such as C-reactive
protein,
CA15-3, αKlotho and some metabolic enzymes have been investigated, although none of
these molecules has improved the predictive accuracy of current prognostic systems
and
their use is not recommended in clinical practice (4-8). The recent introduction of
large-scale methods to purify, identify, and characterize DNA, RNA, proteins, metabolites
and other molecules, has led to an in-depth exploration of the molecular bases underlying
the development of urological cancer, together with the identification of new biomarkers
and potential therapeutic targets (9,10). In this scenario, analysis of cancer cell
metabolism has shown that tumor cells exhibit an atypical reprogramming of the energy
metabolism that serves to promote cell growth and proliferation (11). RCC is fundamentally
a metabolic disease (12) and many studies have suggested that in some diseases characterized
by an altered metabolism there is an increased risk to develop this tumor (13-15).
Moreover, a bird’s eye view of the genetic alterations involved in the RCC
pathogenesis shows that many genes play an important role in controlling cell metabolism
(16).
The recent comprehensive molecular characterization of clear cell RCC by the cancer
genome
atlas (TCGA) research network has confirmed that oncogenic metabolism and epigenetic
reprogramming are two main features of RCC (17). In
particular, an inverse correlation has been shown between patients’ survival and the
activation of a metabolic shift characterized by a Warburg effect-like state (i.e.,
the
shift to aerobic glycolysis with lactate production), an increased dependence on the
pentose phosphate pathway (PPP), a reduced Krebs cycle activity, increased glutaminolysis
and fatty acid production (17). A later study
explored the specific role of glycolysis and PPP in sustaining RCC cell proliferation,
the
maintenance of NADPH levels, production of reactive oxygen species (ROS) and in reducing
chemotherapy-induced cytotoxicity (7). Clear cell RCC
was characterized by high levels of glucose and other sugars, in association with
an
increase in upstream glycolytic intermediates (glucose 6-phosphate and fructose
6-phosphate), a reduction in downstream intermediates (3-phosphoglycerate,
2-phosphoglycerate, and phosphoenolpyruvate), and an increased lactate production.
Other
important findings were the higher expression in neoplastic tissue of two cancer-specific
isoenzymes, namely Pyruvate kinase isoform M2 (PKM2) and L-lactate dehydrogenase isoform
5
(LDH5). These metabolic alterations, in association with high levels of PPP enzymes
(Glucose-6-phosphate dehydrogenase-G6PDH and Transketolase-TKT) and metabolic intermediates
(sedoheptulose 7-phosphate, ribose 5-phosphate, and ribulose 5-phosphate/xylulose
5-phosphate), suggested a rerouting of the sugar metabolism toward the PPP, with the
aim of
promoting both anabolic reactions and redox homeostasis in RCC (7). Ribose-5-phosphate
is a sugar used in the synthesis of nucleotides,
so the increased flux of metabolites through the PPP provides an advantage for cell
growth
and survival. Since G6PDH is the rate-limiting enzyme of the PPP, its inhibition causes
a
significant decrease in cancer cell growth, confirming the importance of this pathway
in
RCC. Another important byproduct of PPP is NADPH, a reducing compound that is used
for
biosynthetic reactions and to control the redox state. Cancer cells pre-treatment
with a
G6PDH inhibitor induces a significant reduction in NADPH levels and an increased production
of ROS in renal cancer cells, suggesting that G6PDH, and hence the PPP, have a fundamental
role in maintaining redox homeostasis in RCC (7).
These findings were recently confirmed by Hakimi et al. that showed how
clear cell RCC is characterized by a reprogramming of central carbon metabolism, one-carbon
metabolism, and the antioxidant response (18). In
particular, consistent with previous studies (7,17), these authors showed increased
levels of
metabolites in upper glycolysis, and a reduction of metabolic derivatives in lower
glycolysis, downstream of fructose 6-phosphate. This different abundance in metabolites
production between upper and lower glycolysis suggests that the metabolic flux through
this
pathway may be differentially partitioned. In fact, while the sugars produced in the
upper
part of glycolysis are diverted to the PPP, the triose phosphates generated in the
lower
part are rerouted towards the Krebs cycle or one-carbon metabolism. In accordance
with
these results, analysis of tricarboxylic acid (Krebs) cycle metabolites (reduced levels
of
malate and fumarate, and accumulation of succinate) suggests that in RCC the mitochondrial
bioenergetics and oxidative phosphorylation processes are impaired. It has recently
been
demonstrated that NADH dehydrogenase (ubiquinone) 1 alpha subcomplex 4-like 2 (NDUFA4L2)
is
a HIF-1 target gene that encodes for a component of the electron transport chain (ETC)
Complex I (19). In particular, Complex I catalyzes
the first step of mitochondrial respiratory chain reactions, and it couples the Krebs
cycle
to oxidative phosphorylation. Tello et al. have shown that hypoxia-induced
NDUFA4L2 attenuates mitochondrial oxygen consumption through inhibiting Complex I
activity,
and reduces intracellular ROS production under low-oxygen conditions (19). The most
surprising fact is that NDUFA4L2 is one of the most
highly expressed genes in clear cell RCC, while its knockdown impairs cell proliferation,
alters metabolic pathways and induces autophagy in renal cancer cells (20). Taken
together, these findings outline a clear
cell RCC metabolic signature characterized by an anaerobic switch that favors the
upper
part of glycolysis and the rerouting of the sugar metabolism toward the PPP, and attenuates
the mitochondrial activity though the overexpression of NDUFA4L2.
Another important advance highlighted by the Hakimi study (18) was the development
of a metabologram, a web-based application that
integrates transcriptomic and metabolomics data (http://sanderlab.org/kidneyMetabProject),
with the aim of providing a rapid
evaluation of metabolic changes in RCC. This visual tool explores the metabolic pathways
using both gene expression (derived from TCGA database) and metabolite abundance data
(derived from MSK Institute). Using metabolograms, it is possible to simultaneously
evaluate changes in gene expression and metabolite abundance between tumor and normal
tissues, as well as between different tumor stages. In addition, this web platform
makes it
possible to explore the association between the abundance of particular metabolic
intermediates and 24 clinical parameters such as gender, age, the BMI, pathological
characteristics, the onset of recurrence or metastasis.
In conclusion, the field of cancer metabolomics is evolving very rapidly. Recent amazing
discoveries have cast new light on the molecular regulation of cancer cells metabolism,
and
novel biochemical pathways are now under investigation. In the next years we may expect
a
progressive integration of data derived from different platforms and other omics approaches
and a more extensive use of the systems biology approach to identify a panel of molecular
factors as biomarkers for diagnosis, prognosis, and as potential targets for specific
therapies.