Role of Monocarboxylate Transporters (MCTs) in the Regulation of the Metabolic Profile of Cervical Cancer Cells by Hypoxia

Abstract

Background: Due to the high growth rates of some tumors, cells are exposed to hypoxia, leading to a metabolic switch from oxidative phosphorylation to glycolysis. High glycolytic rates, in turn, lead to production of important amounts of lactate, which are exported to the extracellular milieu, contributing to the acidic microenvironment. Low oxygen concentrations lead to stabilization of hypoxia-inducible factor 1 alpha (HIF-1a), which regulates the expression of several glycolytic markers, as well as pH regulators. Thus, monocarboxylate transporters (MCTs) by mediating the co-transport of lactate with a proton, allow the maintenance of the glycolytic phenotype and intracellular pH, contributing to the tumour acidic microenvironment. However, MCT regulation by hypoxia is not fully understood, being sometimes controversial such as the case of MCT1. Aims: We aimed to characterize the expression of MCT1 and MCT4 and other glycolytic markers, as well as lactate transport activity under hypoxia conditions. The sensitivity of uterine cervix tumour cells to the MCT1 inhibitor CHC (a-cyano-4-hydroxycinnamic acid) was compared between normoxic and hypoxic conditions. Material and Methods: Hypoxia was induced by incubation of human cervical cancer cell lines in a hypoxic chamber (<1% O2). Expression of MCT1, MCT4, CD147, GLUT-1, CAIX and LDH was evaluated by immunocytochemistry and Western blot. Glycolytic metabolism was assessed through quantification of glucose consumption and lactate production. The effect of CHC on cell biomass was performed through the Sulforhodamine B assay. Results and Discussion: In general, the expression of the glycolytic metabolic markers GLUT-1, CAIX and LDH increased with hypoxia, which was accompanied by an increase in MCT1 expression in C33 cells and increased plasma membrane expression of MCT1 of SiHa cells. However, the expected increase in MCT4 expression was not observed. Interestingly, the expression of the MCT1/4 chaperone CD147 decreased in C33 and SiHa cells, pointing to the possible association with a different chaperone. Consistent with the increase in MCT1 expression, the sensitivity to CHC decreased in hypoxic conditions. Conclusion: We showed that hypoxia upregulates the expression of MCT1 in cervical cancer cells, rather than MCT4, and decreased the sensitivity to the MCT1 inhibitor CHC. These findings provide evidence for MCT regulation by hypoxia in cervical cancer and for the role of MCT1 in glycolytic metabolism. Acknowledgements: Part of this work was supported by FAPESP-São Paulo Research Foundation: 2008/03232−1 and the FCT grant ref. PTDC/SAU-FCF/104347/2008, under the scope of 'Programa Operacional Temático Factores de Competitividade' (COMPETE) of ‘Quadro Comunitário de Apoio III’ and co-financed by Fundo Comunitário Europeu FEDER.