Abstract
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Objectives The Warburg effect of cancer is exploited by FDG PET imaging and is becoming a promising target for tumor therapy. Recent research points to a crucial role of cancer stem cells (CSCs) for cancer metastasis, resistance, and recurrence. However, little is known regarding CSC glucose metabolism and its potential for targeted therapy.
Methods HT29 colon cancer cells were grown in CSCs selection media for 7 days to form tumor-spheres, which were dissociated and plated for experiments. Stemness was confirmed by CD133 immunoblotting and FACS. Cells were analyzed for FDG uptake, ROS production, and cell proliferation (SRB assays).
Results Metformin acutely stimulated FDG uptake of cancer cells (2.5 fold), and also of CSCs, but to a lower magnitude. Lactate production was increased, indicating enhanced glycolysis by inhibition of oxidative respiration. Metformin also increased ROS production in cancer cells and less potently in CSCs (240.1 ± 16.9% vs. 150.2 ± 9.8% of controls; P < 0.0001). This effect was more pronounced in low (0.5 g/L) compared to high glucose (2.0 g/L) conditions. Blocking of mitochondrial proton leakage by the specific uncoupling protein-2 (UCP-2) inhibitor genipin also increased ROS production in cancer cells and CSS, and its combination with metformin had an additive effect. Following 72 h treatment with metformin, reduction of cancer cell number was greater than that of CSCs (31.9 ± 3.1% vs. 45.9 ± 5.0% of controls; P = 0.01). Genipin did not reduce cell viability when used alone, but its addition significantly potentiated the suppressive effect of metformin on CSC survival. This indicates that the anti-CSC effect of metformin benefits from UCP-2 blockade.
Conclusions Metformin stimulates FDG uptake and ROS production, and suppresses survival of cancer cells more potently than CSCs. UCP2 blockade by genipin further augments cellular ROS levels and potentiates the anti-cancer effect of metformin in CSCs.