Cytotoxic activity of high dose ascorbic acid is enhanced by 2-deoxy-d-glucose in glycolytic melanoma cells

Abstract

Although, numerous in vitro studies showed that cancer cells are killed after exposure to pharmacological doses of ascorbic acid (AA), significant clinical data proving the efficacy of AA is still absent. A hallmark of most tumor cells is an altered glucose metabolism characterized by an upregulation of glycolysis despite normoxic conditions (Warburg effect). Since pyruvate is capable of detoxifying hydrogen peroxide (H2O2), the alleged mediator of AA-induced toxicity, it seems likely that enhanced glycolysis and subsequent higher pyruvate formation might be an explanation for the attenuated effect of pharmacological AA in vivo. Therefore, inhibition of glycolysis might be a promising approach to enhance the anticancer effect of AA by diminishing the generation of pyruvate. Considering the altered metabolism of cancer cells, we examined the cytotoxic potential of 2-DG and/or AA using SRB assay in two different cell lines: a glycolytic human melanoma (451Lu) and a non-glycolytic breast cancer (MCF-7) cell line. Inhibition of glycolysis increased AA cytotoxicity in 451Lu cells, whereas same treatment had a marginal effect on MCF-7 cells. We also investigated the influence of glycolysis inhibition on H2O2 generation. H2O2 concentrations were higher in presence of 451Lu cells when pretreated with 2-DG, but not in MCF-7 cells. Treatment with 10 mM 2-DG decreased pyruvate and lactate concentrations in both cell lines in a concentration-dependent manner. In summary, 2-DG enhances the cytotoxic effect of AA in glycolytic 451Lu cells by increasing AA-induced H2O2 concentration. This result indicates that lower pyruvate levels, as a result of glycolysis inhibition, may be responsible for the enhanced effect of 2-DG on AA toxicity. Further experiments are needed to clarify the underlying mechanism and the potential use in cancer therapy.