JP-1302 inhibits RNA polymerase I transcription and induces nucleolar stress in cancer cell lines

Cancer cells upregulate ribosomal RNA (rRNA) synthesis to sustain the elevated ribosome production that fuels rapid proliferation, creating a dependency that renders RNA polymerase I (Pol I) transcription a pharmacological vulnerability. JP-1302 is a blood-brain barrier-penetrant 9-anilinoacridine derivative developed as an α2C-adrenergic receptor antagonist, but also recently identified as an RNA Pol II inhibitor at micromolar concentrations (~5-10 μM). Here we show that JP-1302 also suppresses Pol I-driven 47S pre-rRNA synthesis at submicromolar-to-low-micromolar concentrations, evidenced by loss of nascent EU-labelled rRNA and reduced activity of an rDNA promoter-driven luciferase reporter. This was accompanied by preferential proteasome-dependent degradation of the Pol I catalytic subunit POLR1A, independent of α2C-adrenergic receptor signalling, displacement of POLR1A from the rDNA promoter and progressive disruption of nucleolar morphology. Growth inhibitory IC50 values across a panel of normal and cancer cell lines, including glioblastoma, ranged from submicromolar to low micromolar. Nucleolar stress stabilised p53 and induced p21 expression at low-to-intermediate concentrations, without evidence of DNA damage at these doses (no γH2AX increase at concentrations sufficient to abrogate Pol I transcription). Interestingly, p53-null and p53-wild type cells showed comparable S-phase depletion in response to JP-1302. At higher concentrations, JP-1302 additionally caused covalent trapping of TOP2A/TOP2B, accompanied by delayed γH2AX induction, and enrichment of the FACT subunits SSRP1 and SPT16 on chromatin. Together, these findings identify JP-1302 as a pharmacologically distinct inhibitor of ribosome biogenesis with broader, dose-dependent chromatin-disruptive activity.