Dual targeting of energy metabolism and lysosomes as an anticancer strategy; It is not all about autophagy

Abstract

Background: Intensive proliferation of tumor cells consumes a lot of energy. In nutrient deficiency substrates for energy metabolism are obtained by lysosomal degradation of unnecessary/dysfunctional intracellular organelles/molecules in the process of autophagy. Leakage of enlarged unstable lysosomes, which characterize tumor cells, causes cell death. We investigated antitumor effect of combined targeting of lysosomes/autophagy and energy metabolism. Material and Methods: Toxicity against U251 human glioma and B16 mouse melanoma cells was measured by viability tests. Type/mechanisms of cell death were determined by flow cytometry, immunoblot, fluorescent/electron microscopy and confirmed by appropriate genetic/pharmacological inhibitors. Therapeutic potential was estimated in B16 melanoma bearing C57Bl/6 mice. Results: In the first study, lysosomotropic autophagy inhibitor chloroquine (CQ) rapidly killed tumor cells incubated in the absence of serum. CQ-induced lysosomal destabilization triggered: oxidative stress, mitochondrial depolarization, and mixed apoptosis/necrosis of serum-deprived cells. In the second study, lysosomal detergent N-dodecylimidazole (NDI) synergized in antitumor activity with the glycolytic inhibitor 2-deoxy-D-glucose (2DG). NDI-triggered release of lysosomal enzymes into the cytoplasm caused mitochondrial damage and blocked oxidative phosphorylation, which synergized with 2DG-mediated glycolysis block in ATP reduction, oxidative stress, and necrosis. Interestingly, although both serum deprivation and 2DG stimulated autophagy, CQ- and NDI-induced autophagy suppression was irrelevant for their cytotoxicity. Importantly, CQ+food restriction and 2DG+NDI reduced melanoma growth in vivo. Conclusion: Autophagy independent antitumor effects of combined energy metabolism suppression and lysosomal destabilization might be exploited in cancer therapy.