Alterations of monocarboxylate transporter densities during hypoxia in brain and breast tumour cells.
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BACKGROUND
Tumour cells are characterized by aerobic glycolysis, which provides biomass for tumour proliferation and leads to extracellular acidification through efflux of lactate via monocarboxylate transporters (MCTs). Deficient and spasm-prone tumour vasculature causes variable hypoxia, which favours tumour cell survival and metastases. Brain metastases frequently occur in patients with advanced breast cancer.Effective treatment strategies are therefore needed against brain metastasis from breast carcinoma.
METHODS
In order to identify differences in the capacity for lactate exchange, human T-47D breast cancer cells and human glioblastoma T98G cells were grown under 4 % or 20 % oxygen conditions and examined for MCT1, MCT2 and MCT4 expression on plasma membranes by quantitative post embedding immunogold electron microscopy. Whereas previous studies on MCT expression in tumours have recorded mRNA and protein levels in cell extracts, we examined concentrations of the proteins in the microvillous plasma membrane protrusions specialized for transmembrane transport.
RESULTS
In normoxia, both tumour cell types highly expressed the low affinity transporter MCT4, which is thought to mainly mediate monocarboxylate efflux, while for high affinity transport the breast tumour cells preferentially expressed MCT1 and the brain tumour cells resembled brain neurons in expressing MCT2, rather than MCT1. The expressions of MCT1 and MCT4 were upregulated in hypoxic conditions in both breast and brain tumour cells. The expression of MCT2 also increased in hypoxic breast cancer cells, but decreased in hypoxic brain tumour cells. Quantitative immunoblots showed similar hypoxia induced changes in the protein levels.
CONCLUSIONS
The differential expression and regulation of MCTs in the surface membranes of hypoxic and normoxic tumour cells of different types provide a foundation for innovation in tumour therapy through the selective targeting of MCTs. Selective inhibition of various MCTs could be an efficient way to quench an important energy source in both original breast tumour and metastatic cancer tissue in the brain.
