Lysosomes, now known to take part in multiple cellular functions, also respond to various stress stimuli. These include biogenesis in response to nanomolar concentrations of hydrophobic weak-base anticancer drugs. However, since lysosomal stress mediated by accumulation of weak-base drugs at such concentrations has never been proven and these drugs have diverse effects on malignant cells, we investigated whether the interpretation of the data was true. We found that lysosomal accumulation of the drugs daunorubicin, doxorubicin, mitoxantrone, symadex, chloroquine, clomipramine and sunitinib alone, was insufficient to induce lysosomal alkalization i.e., lysosomal stress-mediated biogenesis at nanomolar concentrations. Instead, we found that some of the drugs used induced G2 phase arrest and lysosomal biogenesis that is associated with activation of transcription factor EB (TFEB). Similarly, cantharidin, a control compound that does not belong to the weak base drugs, induced cell cycle arrest in the G2 phase associated with TFEB-driven lysosomal biogenesis. Overall none of the tested drugs caused stress-induced lysosomal biogenesis at nanomolar concentrations. However, daunorubicin, doxorubicin, mitoxantrone, symadex and cantharidin induced a massive block in the G2 phase of the cell cycle which is naturally associated with TFEB-driven lysosomal biogenesis.

Department of Anatomy Faculty of Medicine and Dentistry Palacky University Olomouc Hnevotinska 3 Olomouc 77515 Czech Republic

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