Metformin treatment impairs the adenine nucleotide translocator activity and energy metabolism in human clear cell renal carcinoma cells

Although metformin (MET), the well-known antidiabetic drug, exhibits clear antineoplastic effects and is reported to target mitochondria, several issues are still open in this regard, thus limiting its utilization as an anticancer drug alone or in combination with other molecules. Here a functional investigation was carried out to reveal how MET impacted on mitochondrial functions and cell energy metabolism in human cultured clear cell renal carcinoma cells (ccRCCs), in which the anticancer effect of MET is already known. The in vitro effect of increasing MET concentrations on cell viability, necrosis and apoptosis of ccRCCs was checked and compared to normal immortalized HK2 cells. At the same time, the effect of MET on mitochondrial functions, ATP synthesis via oxidative phosphorylation, cellular ATP level, L-lactate (L-LAC) production and export, glucose consumption and key mitochondrial and cytosolic enzyme activities was also investigated in cancer cells. MET affected ccRCC viability and impaired mitochondrial respiration, membrane potential generation and ATP production by targeting complex I (CI), III and IV of the respiratory chain at a concentration near to the IC50 value (25 mM). Importantly, we first identified a significant inhibition of the adenine nucleotide translocator (ANT) activity in response to MET treatment. Notably, the sensitivity of ANT and CI activity to increasing MET concentrations differed markedly, the former being considerably inhibited already at a low, near-clinically relevant concentrations, while the latter only at concentrations ≥ 1 mM. The drug also induced a glycolytic shift in ccRCCs and increased the activity of the mitochondrial flavoenzymes succinate dehydrogenase (SDH) and D-lactate dehydrogenase (D-LDH), and of the key enzymes of the pay-off phase of glycolysis, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), phosphoglycerate kinase (PGK) and pyruvate kinase (PK). Nevertheless, cellular ATP level dropped markedly, and the intracellular L-LAC amount was almost doubled in the presence of MET. Interestingly, MET-induced glycolytic shift showed a drug concentration dependence similar to that seen for CI inhibition, suggesting not ANT but rather CI inhibition may be the trigger for metabolic rewiring. These findings give new insights into MET mechanisms of action which may potentially improve its application and outcome in cancer as well as in other pathologies.