Introduction
The primary hurdle for effective chemotherapy is the inherent or acquired drug resistance of cancer cells. The glycolytic phenotype (or Warburg effect) is a major metabolic signature of cancer cells. Dichloroacetic acid (DCA) is a non-toxic drug, which can reverse the glycolytic phenotype by inhibiting pyruvate dehydrogenase kinases (PDKs). Recent studies have linked changes in cancer cell metabolism to multi-drug resistance (MDR) phenotypes. We have examined whether reversal of the glycolytic phenotype with DCA can restore sensitivity in drug resistant cancer cells.
Methodology
We examined DCA’s effects in doxorubicin (DOX)-resistant MDA-MB-231 and MCF7 cells (made by continuous exposure to increasing DOX concentrations for one month) and their DOX-sensitive counterparts by neutral red cell viability assay. Intracellular drug uptake was measured by flow cytometry and expression of PDKs and drug transporters was measured by western blotting.
Results
DCA treatment (1mM for 48hrs) significantly (p<0.05) enhanced DOX toxicity in DOX-resistant cells restoring sensitivity to a level similar to parental cells. This is despite DCA having no effect on DOX toxicity in parental cells. PDK2 expression was higher in the drug-selected cells compared to parental cells suggesting the sensitising effect of DCA on drug resistant cells could be an on-target effect. DCA pre-treatment increased the intracellular retention of DOX and mitoxantrone. DCA effectively reduced the expression of ATP-binding cassette (ABC) drug transporters such as P-glycoprotein and ABCG2 in DOX-resistant MDA-MB-231 and MCF7 cells and in vivo in 4T1 mouse mammary tumors. DCA pre-treatment also sensitised the MDR cell line MCF7-FLV1000 towards mitoxantrone induced apoptosis.
Conclusion
DCA may be an effective chemo-sensitizing agent in a wide range of MDR cancers, acting by reducing drug transporter expression and inhibiting PDKs.