Interference with glucose metabolism confers resistance toward chloroquine-induced cytotoxicity in metastatic breast cancer cells

Lysosomes have been considered a 'suicide bag' of the cell since they contain an array of hydrolytic enzymes normally constrained within a membrane compartment. During lysosomal membrane permeabilisation (LMP), when integrity of the membrane is compromised, degradative enzymes leak into the cytosol initiating cell death. Our studies focused on the lysosomal inhibitor Chloroquine (CQ), a compound in clinical trials for a variety of blood and solid cancers. The form of cell death induced by CQ is still widely debated, ranging from inhibition of the pro-survival autophagy pathway to inducing apoptosis. Here, we set out to investigate mechanisms of CQ and furthermore, the potential of combining this drug with metabolic targeting strategies in 4T1 metastatic breast cancer cells. In doing this, we uncovered an unexpected resistance mechanism linking glucose metabolism and CQ. In clonogenic assays, CQ induced cell death that cooperated with other cellular stressors such as ionising irradiation, PI3K-Akt inhibition, and serum starvation. Unexpectedly, further glucose starvation fully rescued cell viability. The cytotoxic effects of CQ were found to be autophagy-independent as knockdown of ATG proteins did not mimic CQ. Cell death also did not resemble classical caspase-dependent apoptosis or necrosis. CQ treatments led to marked lysosomal stress and enlargement, suggestive of LMP. In contrast, while CQ was still able to enter and deacidify the lysosome in glucose starved cells, it failed to induce enlargement. However, related quinoline compounds, Primaquine (PQ) and Amodiaquine (AQ), were not susceptible to such metabolic interference. Our data indicate that glucose metabolic rate has a profound influence on the efficacy of CQ to target lysosomes and induce LMP-mediated death. These effects may be reducing clinical outcomes of CQ in cancer cells with reduced glucose metabolism.