Abstract

Therehasbeenlong-standinginterestintargetingpro-survivalautophagyasacombinationalcancertherapeuticstrategy.Clinical trials are in progress testing chloroquine (CQ) or its derivatives in combination with chemo- or radiotherapy for solid and haematologicalcancers.AlthoughCQhasshownefficacyinpreclinicalmodels,itsmechanismofactionremainsequivocal.Here, we tested how effectively CQ sensitises metastatic breast cancer cells to further stress conditions such as ionising irradiation, doxorubicin,PI3K-Aktinhibitionandserumwithdrawal.Contrarytotheconventionalmodel,thecytotoxiceffectsofCQwerefound to be autophagy-independent, as genetic targeting of ATG7 or the ULK1/2 complex could not sensitise cells, like CQ, to serum depletion. Interestingly, although CQ combined with serum starvation was robustly cytotoxic, further glucose starvation under these conditions led to a full rescue of cell viability. Inhibition of hexokinase using 2-deoxyglucose (2DG) similarly led to CQ resistance.Asthisformofcelldeathdidnotresembleclassicalcaspase-dependentapoptosis,wehypothesisedthatCQ-mediated cytotoxicity was primarily via a lysosome-dependent mechanism. Indeed, CQ treatment led to marked lysosomal swelling and recruitment of Galectin3 to sites of membrane damage. Strikingly, glucose starvation or 2DG prevented CQ from inducing lysosomal damage and subsequent cell death. Importantly, we found that the related compound, amodiaquine, was more potent than CQ for cell killing and not susceptible to interference from glucose starvation. Taken together, our data indicate that CQ effectively targets the lysosome to sensitise towards cell death but is prone to a glucose-dependent resistance mechanism, thus providing rationale for the related compound amodiaquine (currently used in humans) as a better therapeutic option for cancer. Cell Death and Disease (2017) 8, e3014; doi:10.1038/cddis.2017.416; published online 24 August 2017