Fig. 4.

Inhibition of RNA polymerase I transcription by CX-5461 induces both p53-dependent and -independent responses. A schematic representation of CX-5461’s mode of action and its downstream stress response pathways. CX-5461 inhibits the initiation of Pol I-mediated transcription by disrupting the association between SL-1 and Pol I, thus preventing Pol I recruitment to the rDNA promoter. This displacement leads to “exposed” rDNA repeats devoid of Pol I and the presence of defects associated with an open chromatin structure and the recruitment and phosphorylation of RPA to single-stranded rDNA, a marker for replication stress. CX-5461-mediated alterations in rRNA synthesis and rDNA chromatin and topology in turn trigger the downstream activation of two major signaling pathways: (i) a canonical p53-dependent nucleolar stress response leading to accumulation of p53 and/or (ii) a p53-independent DNA damage response (DDR) involving the activation of ATM/ATR kinase signaling. Each pathway induces various cellular responses including G1/S and G2/M cell cycle defects, apoptosis and senescence. Pol I, RNA polymerase I; SL-1, selectivity factor 1; rDNA, ribosomal RNA gene; TBP, TATAbinding protein; UBF, upstream binding factor; UCE, upstream control element; RRN3, RNA polymerase I-specific transcription initiation factor; Mdm2, mouse double minute 2; CHK, checkpoint kinases; CDK, cyclin-dependent kinases; ATM indicates ataxia telangiectasia mutated; ATR, ataxia telangiectasia and Rad3-related