The Anticancer Drug Mitoxantrone Triggers the Formation of Ribosome-enriched Stress Granules Independently of the Classical Translational Control Pathways

Mitoxantrone (MIT) is a chemotherapeutic drug widely used for its DNA intercalation and inhibition of topoisomerase. In this work, we show that MIT also affects cytoplasmic RNA-ribosome organization. In human cancer cells, MIT induced stress granules (SGs) that contained large ribosomal subunit proteins, including eL8, together with polyadenylated mRNA. These MIT-induced SGs were different from arsenite-induced SGs: they formed without eIF2α phosphorylation, mTOR inhibition, or 4E-BP1 activity, and they remained stable in the presence of cycloheximide and after drug withdrawal. In vitro assays further demonstrated that MIT promotes ribosome aggregation in a concentration- and salt-dependent manner. Taken together, our results identify a distinct type of ribosome-enriched SGs that form through RNA-ribosome condensation rather than classical translational stress pathways. This mechanism provides a direct example of how a clinically used drug can reorganize cytoplasmic RNA-protein complexes, with possible consequences for mRNA regulation, cancer therapy, and neurodegenerative disease.