Silencing of monocarboxylate transporters via small interfering ribonucleic acid inhibits glycolysis and induces cell death in malignant glioma: An in vitro study

OBJECTIVE: Dependence on glycolysis is a hallmark of malignant tumors. As a consequence, these tumors generate more lactate, which is effluxed from cells by monocarboxylate transporters (MCTs). We hypothesized that 1) MCT expression in malignant tumors may differ from normal tissue in quantity, isoform, or both; and 2) silencing MCT expression would induce intracellular acidification, resulting in decreased proliferation and/or increased cell death. METHODS: We quantified expression of MCT isoforms in human glioblastoma multiforme and glioma-derived cells lines by Western blot analysis. MCTs that were abundant or specific to glioma then were targeted in the model U-87 MG glioma cell line via small interfering ribonucleic acid-mediated gene silencing and tested for inhibition of lactate efflux, intracellular pH changes, reduced proliferation, and/or induction of cell death. RESULTS: MCT 1 and 2 were the primary isoforms expressed in human glioblastoma multiforme and glioma-derived cell lines. In contrast, MCT 3 was the predominantly expressed isoform in normal brain. Small interfering ribonucleic acid specific for MCT 1 and 2 reduced expression of these isoforms in U-87 MG cells to barely detectable levels and reduced lactate efflux by 30% individually and 85% in combination, with a concomitant decrease of intracellular pH by 0.6 units (a fourfold increase in intracellular H+). Prolonged silencing of both MCTs reduced viability by 75% individually and 92% in combination, as measured by both phenotypic and flow cytometric analyses. CONCLUSION: MCT targeting significantly reduced the viability of U-87 MG cells mediated by both apoptosis and necrosis. This indicates that the strategy may be a useful therapeutic avenue for treatment of patients with malignant glioma.