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Page 2 of 16 Tu et al. J Cancer Metastasis Treat 2018;4:58 I http://dx.doi.org/10.20517/2394-4722.2018.67

highly metastatic daughter line. The elevated metabolic rate is only partially reflected by transcript levels of rel-
evant metabolic regulators. Heightened mitochondrial respiration is potentially underpinned by increased expres-
sion mitochondrial electron transport chain components. However, increased glycolysis was not underpinned by
up-regulation of metabolic genes encoding enzymes participating in glycolysis.

Conclusion: Our results indicate breast tumour cells with elevated metastatic propensity are more metabolic ac-
tive. We also identified differentially expressed metabolic genes, such as IDH2, that may play a part in the meta-
static process beyond energy reprogramming.

Keywords: Breast cancer, energy reprogramming, cancer metabolism, metastasis, RNA-seq

INTRODUCTION
The majority of breast cancer-related deaths are not caused by the primary tumor itself, but are due to the
[1]
results of metastasis to vital organs . Although only a small percentage of patients are initially diagnosed
with late stage or metastatic breast cancer, the 5-year survival for these patients is 25% compared with 99%
[2]
for patients diagnosed with localized disease . In addition, current prognostic markers are unable to ac-
curately predict the risk of metastasis development and approximately 30% of patients first diagnosed with
[3]
earlier-stage breast cancer will eventually develop recurrent metastatic disease . Therefore, despite current
[4]
advances in therapies and the gradual decline in breast cancer-related mortality , the diagnosis and man-
agement of metastatic disease remains a major therapeutic challenge for breast cancer treatment.

[5]
The dysregulation of cellular energetics is now regarded as one of the hallmarks of cancer . The metabolic
[6]
phenomenon describing increased glycolytic capacity in cancer cells, known as “the Warburg effect” ,
stimulated decades of research directed towards the characterization of the reprogramming of energy me-
tabolism during cellular transformation and its role in tumor development. The Warburg effect emerged as
just one component of global changes in energy metabolism occurring in both cancer cells and the tumor
[7,8]
microenviroment . Additionally, an increasing number of studies suggest that metabolic reprogramming
plays an important role not only in the process of malignant transformation, but also in the growth and
survival of tumor cells within a hostile environment, such as the often limited nutrient and oxygen sup-
ply in solid tumours [9-11] . However, despite the significant number of studies that investigated the metabolic
programming of primary cancer cells, less is known about metabolic alterations in the context of metastatic
disease, especially in breast cancer.

Comparison of breast cancer cell lines panel reveals that cell lines with molecular subtypes associated with
more aggressive disease progression exhibit an overall increase in energy metabolic processes, including
glycolysis and oxidative phosphorylation (OXPHOS) [12-15] . Studies using metastases derived from the same
primary tumour reported more puzzling metabolic changes. In a xenograft model using circulating tumor
cells from a breast cancer patient, a proteomic comparison between parental cells and cells that metastasized
to the brain demonstrated up-regulation in enzymes involved in both glycolysis and mitochondrial respira-
[16]
tion pathways . Moreover, compared to primary tumour cells, circulating tumour cells derived from 4T1
mouse mammary tumors exhibited elevated expression of mitochondrial respiration pathway genes, but not
[17]
glycolytic genes, while lung metastasis from the same primary tumour revealed modest metabolic change .
Consistent with this finding, increased OXPHOS, were observed with increased metastatic potential in sev-
eral metastatic cell line variants derived from the same primary breast cancer [13,18] . These findings provide
evidence that energy reprogramming may be an important feature of the complex process of breast cancer
metastasis, but also raise the question of whether the metabolic profiles of metastatic cells vary depending
on the stage of metastasis and site of distant metastasis.

To gain a better understanding of the metabolic changes underlying the process of breast cancer metastasis,
we characterized a highly metastatic variant line of the commonly used triple-negative human breast ad-