Lysosomal membrane permeabilization enhances the anticancer effects of POLR1 (RNA polymerase I) transcription inhibitors

Lysosomes contribute to the development of drug resistance through various mechanisms that include drug sequestration and the activation of adaptive stress pathways. While inhibitors of DNA-to-RNA transcription exhibit potent anticancer effects, the role of lysosomes in modulating responses to such transcription inhibitors remains largely unexplored. This study investigates this aspect in the context of two potent POLR1 (RNA polymerase I) transcription inhibitors, CX-3543 (quarfloxin) and CX-5461 (pidnarulex). Unexpectedly, CX-3543 was found to accumulate within lysosomes, leading to lysosomal membrane permeabilization (LMP) and the subsequent activation of cellular stress adaptation pathways, including those regulated by the transcription factor TFEB and autophagy. Disrupting TFEB or autophagy increased cell sensitivity to CX-3543, highlighting the cytoprotective role of these processes in counteracting CX-3543-induced cell death. Moreover, targeting lysosomal membranes with chloroquine derivatives or blue light exposure induced substantial LMP, releasing compound CX-3543 from lysosomes. This effect enhanced both the inhibition of DNA-to-RNA transcription and CX-3543-induced cell death. Similar effects were observed when chloroquine derivatives were combined with CX-5461. Additionally, combining CX-3543 with the chloroquine derivative DC661 more effectively reduced the fibrosarcoma growth in immunocompetent mice than either agent alone. Altogether, our results reveal an unanticipated lysosome-related mechanism that contributes to cancer cell resistance to POLR1 inhibitors and propose a strategy to overcome this resistance.Abbreviations: ATG7: autophagy related 7; ATG13: autophagy related 13; Baf A1: bafilomycin A1; CTSB: cathepsin B; DKO: double knockout; G4: Guanine quadruplex; KO: knockout; LAMP1: lysosomal associated membrane protein 1; LAMP2: lysosomal associated membrane protein 2; LGALS3: galectin 3; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MTORC1: mechanistic target of rapamycin kinase complex 1; NCL: nucleolin; POLR1: RNA polymerase I; SQSTM1/p62: sequestosome 1; TFEB: transcription factor EB; TFE3: transcription factor E3; ULK1: unc-51 like autophagy activating kinase 1.