Abstract 46: The impact of monocarboxylate transporter expression on metabolic function in prostate cancer cells

INTRODUCTION The tumor microenvironment is subjected to variations in oxygen tension which results in hypoxic regions. Hypoxia in tumors is associated with radioresistance and a metastatic phenotype. Hypoxic conditions cause tumor cells to favor anaerobic metabolism, however this switch to glycolysis is also found in tumor cells under aerobic conditions, known as the ‘Warburg effect’. An increased reliance on glycolysis results in large quantities of lactate, which needs to be transported out of tumor cells to maintain the intracellular pH. The monocarboxylate transporters, MCT1 and MCT4, regulate the transport of lactate in tumor cells and here we report the effect of overexpression or knockdown of these transporters on metabolism, growth and response to therapy in prostate cancer cell lines. METHODS Human prostate cancer cell lines were stably transfected, using lentiviral vectors, to show overexpression of MCT1 (DU145, LNCaP and PC3), overexpression of MCT4 (LNCaP), or knockdown of MCT4 (DU145 and PC3). Using these stable cell lines, the effect of overexpression or silencing of MCT1 or MCT4 was assessed in vitro. A lactate assay was used to investigate the effect on intra- and extracellular lactate under varying oxygen tensions. Effect on cell cycle progression was assessed using flow cytometry and toxicity of docetaxel was determined using MTT and SRB assays. Finally, a metabolome siRNA screen was carried out to determine if MCT4 expression was related to sensitivity to toxicity resulting from knockdown of a range of metabolic proteins. RESULTS Expression levels of MCT1 and MCT4 were confirmed by western blot and immunofluorescence. Silencing of MCT4 in DU145 and PC3 cell lines increased intracellular lactate under hypoxic conditions. No effect on intra- or extracellular lactate was observed in cell lines with overexpression of MCT1. Changes in expression of MCT1 or MCT4 did not appear to result in sensitization or resistance to docetaxel treatment under either normoxic or hypoxic conditions. Overexpression of MCT1 in PC3 cells caused cell cycle arrest in the G2/M phase in both normoxia and hypoxia. The metabolome screen indicated that, in wild type LNCaP cells, down-regulation of the protein product of the genes encoding 2,4-Dehydrocholesterol Reductase or Pyruvate Dehydrogenase Phosphatase Catalytic Subunit 2 was synthetically lethal. Overexpression of MCT4 resulted in protection against these lethal effects. CONCLUSIONS Altering the expression of MCT1 and MCT4 in human prostate cancer cells had a variety of effects on cell function. Overexpression of MCT1 caused cell cycle arrest in PC3 cells which may impact on radiosensitivity as cells are most vulnerable to radiation treatment when in the G2/M phase. The identification of two proteins that cause toxicity when silenced in LNCaP wild-type cells, but not LNCaP cells showing overexpression of MCT4, is an interesting lead for further investigation. Citation Format: Laura Hutchinson, Amy Boyers, Amy Chadwick, Ian Stratford. The impact of monocarboxylate transporter expression on metabolic function in prostate cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 46.