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Tafur et al. J Cancer Metastasis Treat 2018;5:xx I http://dx.doi.org/10.20517/2394-4722.2018.102 Page 11 of 14

DISCUSSION
The metabolic changes that cancer cells undergo in order to support macromolecule biosynthesis, growth
and survival, are collectively referred to as “metabolic reprogramming”, now considered a hallmark of cancer
cell biology [36,37] . Increasing evidence has linked metabolic reprogramming to metastatic transformation.
For example, it has been shown that the EMT-transcription factor SNAIL mediates the suppression of
mitochondrial respiration and enhances glycolysis in breast cancer [38,39] . In the accompanying report in
this issue, we demonstrated that EMT induced a metabolic shift to a less oxidative and more glycolytic
metabolism in two in epithelial cell lines MCF-7 and BT-474, as evidenced by increased glucose uptake and
lactate production along with increased expression of enzymes and transporters. These studies suggest that
metastatic transformation or EMT facilitates the metabolic swift toward glycolysis in some forms of breast
cancer.

We also observed that metabolic reprogramming induced by EMT was associated with a striking switch in the
expression of the lactate transporters MCT1 and MCT4. The two epithelial breast cancer cell lines expressed
high levels of MCT1 and imported lactate as an alternative energy source. In contrast, the mesenchymal
cells showed a marked decreased expression of MCT1 and upregulation of the lactate exporter MCT4. Our
[40]
findings are consistent with those of Hussein and Brooks , who reported MCT1 was expressed in MCF7
cells, but not in mesenchymal, triple-negative MDA-MB-231 cells, that were maintained in 2-dimensional
culture containing high glucose medium. Similarly, MCT4 expression was lower in MCF7 cells as compared
to the MDA-MB-231 cells. Using 2-dimensional culture and DMEM/F12 media with high glucose levels,
[41]
Baenke et al. examined MCT4 expression and function in 17 breast cancer cell lines. Although their findings
differed from ours in that MCT4 expression was highest in HER2-positive cell lines, these authors reported
that MCT4 suppression increased mitochondrial oxidation, linking MCT4 to a more glycolytic phenotype.
This is consistent with MCT4 promoting a glycolytic phenotype, and also raises the prospect that breast cancer
cells can adapt to targeted therapies in part through metabolic reprogramming. In contrast to our finding of
reciprocal expression between MCT1 and MCT4, Pinheiro et al. used immunohistochemistry to analyze
[42]
primary breast carcinomas for MCT1 and MCT4, and found MCT1 expression occurring most frequently in
the Basal-like subtype and in higher grades of breast cancer. It is also important to note that expression of
MCTs does not always correlate to lactate/H transport. For example, hypoxia-induced carbonic anhydrase
+
IX enhanced the activity of MCT1 to export lactate and H in MCF7 cells in 2-dimensional culture with
+
physiological (5 mmol/L) glucose levels. MCT4 upregulation was previously reported in triple negative breast
cancer and high levels of this lactate exporter were associated with poor prognosis and survival [41,43] .

Lactate has been identified as the ligand for the endogenous cell-surface G-protein-coupled receptor 81
(GPR81). GPR81 was first discovered and mostly studied in adipocytes, in which lactate activates GPR81 to
decrease the cAMP production to ultimately reduce lipolysis . GPR81 is highly expressed in neurons ,
[44]
[24]
[45]
and when activated, it is able to modify electrical activity of primary neuronal cells . Also, the activation of
GPR81 by lactate has been identified as the initial step for an anti-inflammatory response in pancreas, liver
and uterus [46-48] .
More recently, GPR81 expression has been found to be upregulated in several types of cancer, including
pancreatic, colon, liver, breast, lung and cervical cancers, and in several cases high GPR81 expression
was associated with tumor growth, chemoresistance and metastasis [25-28] . It should also be noted that
previous studies indicate that lactate activates GPR81 in the millimolar range (1-5 mmol/L), and in human
[14]
breast cancer tissues lactate was found to be at concentrations as high as 8 mmol/L , suggesting that
GPR81 and downstream signaling pathways could be constantly activated by lactate in the breast tumor
microenvironment. Our study provides the first examination of GPR81 expression in the context of EMT,
and indicates that GPR81 expression is dependent on the type of breast cancer cell. We showed that
hormone-positive epithelial breast cancer cells highly expressed GPR81. In contrast, EMT induction and