1 Introduction
Metabolic reprogramming is a hallmark of cancer [1]. Cancer cells
utilise a range of nutrient sensing pathways to coordinate the
uptake of glucose, lipids and amino acids that are triaged into
energy production, biosynthesis pathways and redox control to
support cell proliferation and survival. For example, many tumours
dramatically increase glucose consumption and this is exploited
clinically by fluorodeoxyglucose-positron emission tomography
(FDG-PET), to image highly metabolically active cancer lesions.
Glucose is metabolised to pyruvate via the process of glycolysis,
where it can either enter the mitochondria for oxidative phosphorylation or generate ATP via its conversion to lactate in a process
referred to as aerobic glycolysis. This latter pathway, while producing less ATP, circumvents the product inhibition of biosynthetic
pathways and so allows more efficient cell proliferation. Aerobic
glycolysis, first observed by Warburg, has been considered the
dominant metabolic phenotype of cancer cells [2–4], however
evidence now supports an important role of functional mitochondria and oxidative phosphorylation (OXPHOS) in enhancing
metastasis [5–9]. Metastatic spread is attributed to dynamic alterations in carcinoma cells associated with the loss of epithelial features
and the acquisition of more migratory, mesenchymal properties,
referred to as epithelial–mesenchymal transition (EMT). The
reverse process, mesenchymal–epithelial transition (MET), is then
adopted for seeding into new niches [10–12]. The phenotypic
cellular hallmarks that define this epithelial-mesenchymal plasticity
(EMP) are well characterised [13], but the underlying metabolism
remains to be comprehensively studied.
The interrelationship of EMP and metabolic reprogramming
has been identified as a promising therapeutic strategy [14–16]. A
genome-wide computational study of glycolytic dependency from
NCI-60 cell lines identified metabolic targets to inhibit cellular
migration [17]. Using gene signatures in The Cancer Genome
Atlas (TCGA) database, the association between downregulation
of mitochondrial genes, induction of EMT and metastatic potential
was found to be directly associated with patient prognosis
[18]. The activation of EMT was also correlated with reduced
mtDNA content in prostate [19] and colorectal cancers [20], and
a study of 207 primary breast tumour specimens also reflected a
direct correlation between low mtDNA content and presence of
distant metastases [21]. However, these conclusions were observed
using DNA or RNA as reference, so validation studies are needed to
correlate EMP with the metabolic phenotypes of cells.
Bioenergetic profiling of cancer cells can be studied on a Seahorse XF Analyzer. This platform contains a single steady-state
probe which is lowered over the cell monolayer or spheroids
328
Sugandha Bhatia et al.
Metabolic reprogramming is a hallmark of cancer [1]. Cancer cells
utilise a range of nutrient sensing pathways to coordinate the
uptake of glucose, lipids and amino acids that are triaged into
energy production, biosynthesis pathways and redox control to
support cell proliferation and survival. For example, many tumours
dramatically increase glucose consumption and this is exploited
clinically by fluorodeoxyglucose-positron emission tomography
(FDG-PET), to image highly metabolically active cancer lesions.
Glucose is metabolised to pyruvate via the process of glycolysis,
where it can either enter the mitochondria for oxidative phosphorylation or generate ATP via its conversion to lactate in a process
referred to as aerobic glycolysis. This latter pathway, while producing less ATP, circumvents the product inhibition of biosynthetic
pathways and so allows more efficient cell proliferation. Aerobic
glycolysis, first observed by Warburg, has been considered the
dominant metabolic phenotype of cancer cells [2–4], however
evidence now supports an important role of functional mitochondria and oxidative phosphorylation (OXPHOS) in enhancing
metastasis [5–9]. Metastatic spread is attributed to dynamic alterations in carcinoma cells associated with the loss of epithelial features
and the acquisition of more migratory, mesenchymal properties,
referred to as epithelial–mesenchymal transition (EMT). The
reverse process, mesenchymal–epithelial transition (MET), is then
adopted for seeding into new niches [10–12]. The phenotypic
cellular hallmarks that define this epithelial-mesenchymal plasticity
(EMP) are well characterised [13], but the underlying metabolism
remains to be comprehensively studied.
The interrelationship of EMP and metabolic reprogramming
has been identified as a promising therapeutic strategy [14–16]. A
genome-wide computational study of glycolytic dependency from
NCI-60 cell lines identified metabolic targets to inhibit cellular
migration [17]. Using gene signatures in The Cancer Genome
Atlas (TCGA) database, the association between downregulation
of mitochondrial genes, induction of EMT and metastatic potential
was found to be directly associated with patient prognosis
[18]. The activation of EMT was also correlated with reduced
mtDNA content in prostate [19] and colorectal cancers [20], and
a study of 207 primary breast tumour specimens also reflected a
direct correlation between low mtDNA content and presence of
distant metastases [21]. However, these conclusions were observed
using DNA or RNA as reference, so validation studies are needed to
correlate EMP with the metabolic phenotypes of cells.
Bioenergetic profiling of cancer cells can be studied on a Seahorse XF Analyzer. This platform contains a single steady-state
probe which is lowered over the cell monolayer or spheroids
328
Sugandha Bhatia et al.