Defects in 18 S or 28 S rRNA Processing Activate the p53 Pathway

Abstract

The p53 tumor suppressor pathway is activated by defective ribosome synthesis. Ribosomal proteins are released from the nucleolus and block human double minute-2 (Hdm2) that targets p53 for degradation. However, it remained elusive how abrogation of individual rRNA processing pathways contributes to p53 stabilization. Here, we show that selective inhibition of 18 S rRNA processing provokes accumulation of p53 as efficiently as abrogated 28 S rRNA maturation. We describe hUTP18 as a novel mammalian rRNA processing factor that is specifically involved in 18 S rRNA production. hUTP18 was essential for the cleavage of the 5-external transcribed spacer leader sequence from the primary polymerase I transcript, but was dispensable for rRNA transcription. Because maturation of the 28 S rRNA was unaffected in hUTP18-depleted cells, our results suggest that the integrity of both the 18 S and 28 S rRNA synthesis pathways can be monitored independently by the p53 pathway. Interestingly, accumulation of p53 after hUTP18 knock down required the ribosomal protein L11. Therefore, cells survey the maturation of the small and large ribosomal subunits by separate molecular routes, which may merge in an L11-dependent signaling pathway for p53 stabilization.Mammalian RNA polymerase I (pol I) 3 transcribes a single 47 S precursor ribosomal RNA (rRNA). Endonucleolytic, exonucleolytic, and other RNA modification steps generate the mature 5.8 S, 18 S, and 28 S rRNAs (1). Processing of rRNA is exerted within preribosomal particles and therefore tightly connected with 40 S and 60 S subunit assembly. Ribosome biogenesis and thus successful doubling of the translational machinery are prerequisites for ongoing cell proliferation. Interestingly, the integrity of mammalian ribosome biogenesis is monitored by the p53 tumor suppressor pathway. A truncated version of Bop1 blocked 28 S rRNA maturation and elicited a p53-dependent cell cycle arrest (2). Bop1 is the core component of the PeBoW complex, consisting of Pes1, Bop1, and WDR12, which plays a crucial role in 28 S rRNA maturation (3). Moreover, selective abrogation of pol I transcription by low doses of actinomycin D (5-10 nM) provokes stabilization of p53. The large ribosomal subunit protein L11 was identified as an endogenous inhibitor of Hdm2/Mdm2 (4, 5). Hdm2 is an E3 ubiquitin ligase that targets p53 for degradation and is a key negative regulator of p53 (6). L11 was no longer incorporated into nascent ribosomes in actinomycin D-treated cells because of defective pol I transcription. L11 binds Hdm2 and inhibits its function resulting in accumulation of p53. Subsequently, the ribosomal proteins L5 and L23 were described as additional inhibitors of Hdm2 (7, 8). Similar results were obtained in 5-fluorouracil (5-FU)-treated cells (9, 10). The chemotherapeutic agent 5-FU is incorporated efficiently in newly synthesized RNA and potently blocks rRNA processing. 5-FU treatment caused an increase of ribosome-free L5, L11, and L23 mediating Mdm2 inhibition. L5, L11, and L23 are all components o...

The maternal nucleolus plays a key role in centromere satellite maintenance during the oocyte to embryo transition

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ATM-dependent E2F1 accumulation in the nucleolus is an indicator of ribosomal stress in early response to DNA damage

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“…This was achieved either by knockdown of the ribosomal S6 or S7 proteins [10][11][12], or by knockdown of the 18S rRNA specific processing factor UTP18 [13]. In all instances p53 became stabilized and unexpectedly, this stabilization required the presence of L11 protein of the large subunit in mammalian cell culture cells [11,13]. Therefore, cells survey the maturation of the small and large ribosomal subunits by separate molecular routes, which may merge in an L11-dependent signaling pathway for p53 stabilization.…”

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