Senescence-Associated Ribosome Biogenesis Defects Contributes to Cell Cycle Arrest Through the Rb Pathway
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ARTICLES https://doi.org/10.1038/s41556-018-0127-y Senescence-associated ribosome biogenesis defects contributes to cell cycle arrest through the Rb pathway Frédéric Lessard1, Sebastian Igelmann1, Christian Trahan2, Geneviève Huot1, Emmanuelle Saint-Germain1, Lian Mignacca1, Neylen Del Toro1, Stéphane Lopes-Paciencia1, Benjamin Le Calvé5, Marinieve Montero1, Xavier Deschênes-Simard4, Marina Bury6, Olga Moiseeva7, Marie-Camille Rowell1, Cornelia E. Zorca1, Daniel Zenklusen 1, Léa Brakier-Gingras1, Véronique Bourdeau1, Marlene Oeffinger1,2,3 and Gerardo Ferbeyre 1* Cellular senescence is a tumour suppressor programme characterized by a stable cell cycle arrest. Here we report that cellular senescence triggered by a variety of stimuli leads to diminished ribosome biogenesis and the accumulation of both rRNA pre- cursors and ribosomal proteins. These defects were associated with reduced expression of several ribosome biogenesis factors, the knockdown of which was also sufficient to induce senescence. Genetic analysis revealed that Rb but not p53 was required for the senescence response to altered ribosome biogenesis. Mechanistically, the ribosomal protein S14 (RPS14 or uS11) accu- mulates in the soluble non-ribosomal fraction of senescent cells, where it binds and inhibits CDK4 (cyclin-dependent kinase 4). Overexpression of RPS14 is sufficient to inhibit Rb phosphorylation, inducing cell cycle arrest and senescence. Here we describe a mechanism for maintaining the senescent cell cycle arrest that may be relevant for cancer therapy, as well as biomarkers to identify senescent cells. ellular senescence opposes neoplastic transformation by by cyclin-dependent kinases such as CDK2, CDK4 and CDK618. preventing the proliferation of cells that have experienced Genetic inactivation of CDK4 leads to p53-independent senes- oncogenic stimuli1. Senescence is efficient as a tumour sup- cence18 and CDK4 inhibitors can reactivate a latent senescence pro- C 19,20 pressor mechanism as long as it prevents cell cycle progression. gramme in cancer cells . Here, we show a mechanism that targets Some benign lesions contain senescent cells that remain out of the CDK4/6 kinases in senescent cells. We demonstrate that onco- the cell cycle2,3. Senescent cells also stimulate their own clearance genic signals interfere with rRNA biogenesis via the degradation of by the immune system, thereby strengthening the defenses against multiple ribosome biogenesis factors, leading to a p53-independent tumourigenesis4,5. However, the process is not infallible and lesions but Rb-dependent senescence response. Senescent cells accumu- containing senescent cells can progress into malignant cancers6,7. lated both rRNA precursors and ribosomal proteins in the nucleus To escape from senescence, cells need to increase protein biosyn- and nucleolus. Moreover, we found that the ribosomal protein S14 thesis and, more importantly, the machinery to do so. Indeed, ribo- (uS11) binds and inactivates the cyclin-dependent kinase CDK4, some biogenesis is upregulated in cancer cells8,9. Senescent cells thereby acting as a CKI and linking ribosome biogenesis to cell cycle exhibit reduced ribosome biogenesis and this has been explained control. Since p53 is inactivated in most human cancers, this work by a delay in ribosomal RNA (rRNA) processing eventually lead- provides a rationale to target ribosome biogenesis and restore senes- ing to the accumulation of the ribosomal proteins L5 (uL18) and cence in human tumours in a p53-independent manner. L11 (uL5), which form a complex with the 5S rRNA and disable the E3 ligase MDM2 (or HDM2), thus activating p5310. Conversely, Results drugs that reduce ribosome biogenesis can trigger senescence and Deficient ribosome biogenesis and accumulation of rRNA precur- are considered very promising anticancer therapeutics11–13. sors in senescent cells. Non-dividing cells as well as senescent cells Cellular senescence can be induced in cells in the absence have a limited demand for ribosomes and hence ribosome biogen- of p5314 through a pathway that involves several members of the esis. Paradoxically, induction of senescence by oncogenic Ras or by cyclin-dependent kinase inhibitor (CKI) family, such as p16INK4a telomere shortening (replicative senescence) in normal human fibro- (p16) and p15INK4b (or CDKN2B), and the retinoblastoma (Rb1 blasts induces nucleolar clustering into a single prominent nucleolus or Rb) tumour suppressor15,16. Rb controls cell cycle progression (Fig. 1a), reminiscent of big and abundant nucleoli of aggressive can- by repressing E2F activity, and in senescent cells, by promoting the cer cells9. To determine the relationship between decreased ribosome formation of heterochromatin at E2F target promoters3,17. In pro- biogenesis and the senescence programme, we first used the model of liferating cells or cells that escape from senescence, Rb is inhibited oncogene-induced senescence (OIS). Shortly after the introduction 1Department of Biochemistry and Molecular Medicine, Université de Montréal, Montreal, Quebec, Canada. 2Institut de Recherches Cliniques de Montréal, Montreal, Quebec, Canada. 3Faculty of Medicine, Division of Experimental Medicine, McGill University, Montreal, Quebec, Canada. 4Faculty of Medicine, Department of Medicine, McGill University, Montreal, Quebec, Canada. 5URBC-NARILIS, University of Namur, Namur, Belgium. 6Lady Davis Institute for Medical Research, McGill University, Montreal, Quebec, Canada. 7Generium 601125 Vladimirskaya obl, Petushinsky, Russia. *e-mail: [email protected] NATURE CELL BiologY | www.nature.com/naturecellbiology © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. ARTICLES NATURE CELL BIOLOGY a c FBL DAPI 5′-ETS ITS1 ITS2 3′-ETS 18S 5.8S 28S 01 A0 13EC 24′4b 4a 3′ 02 47S Vector 01 45S A0 43S 41S E 36S C 36S-C RAS 2 32.5S 32S 30S 26S 21S Old 18S-E 4a 12S 5.8S 5520 6512 7564 Starved d e 3 D6 D12 D18 P = 0.0020 2 VectRAS Vect RASVect RAS YoungOld P = 0.0025 P = 0.0106 o 45/47S rati Confluent 41S 1 45-47S/21S 30S 0 d b Day 20 26S 5520 S S S Ol Vect Vect Vect RA RA RA Young 21S D6 D12D18 VectRASPMLYoungOld 18S-E f 7 47S P = 0.0610 36S-C 6512 6 36S 5 32S 7564 o 4 P = 0.0150 rati 3 41S 12S 7564 P = 0.0176 P = 0.0052 2 28S 45-47S/18S-E 1 0 d 18S S Ol Vect Vect Vect RAS RAS RA 32S Young D6 D12D18 28S ghAntisense Sense Antisense Sense Vector RAS RAS Young OldOld ) ) FISH 18S FISH ′ 5.8S(5 ′ 5.8S(5 28S y Overla y (EtBr) Overla Fig. 1 | Senescence involves diminished ribosome biogenesis. a, Indirect immunofluorescence (IF) with a specific anti-fibrillarin (FBL) antibody showing the nucleolus. The nucleus was counterstained with DAPI. Images represent IMR90 cells expressing an empty control vector or H-RASV12 (RAS) at day 12 post-infection, IMR90 cells at replicative senescence (old) (passage 40), serum starved for 6 days or confluent arrested for 4 days (1 out of 3 independent experiments with similar results). Scale bar, 20 µ m. b, Autoradiography for de novo rRNA synthesis after a three-hour pulse labelling with [3H]-uridine. Total RNA was extracted after the pulse from IMR90 cells at day 20 post-infection with H-RASV12 (RAS), PML-IV (PML) or an empty control vector (Vect), or young (passage 24) and old (passage 38) IMR90 cells and compared to total 28S rRNA detected with ethidium bromide (EtBr) under UV light (1 out of 3 independent experiments with similar results). c, Schematic representation of the human 47S rRNA and different rRNA processing intermediates. The regions used for northern probes (yellow, position 5520; blue, position 6512; green, position 7564) and FISH (red) are indicated. d, Northern blots with total RNA extracted from IMR90 cells at day 6, day 12 and day 18 post-infection with H-RASV12 (RAS) or an empty control vector (Vect), or young (passage 24) and old (passage 40) IMR90 cells (1 out of 3 independent experiments with similar results). Precursor rRNAs detected with the probes described in c are indicated (right). e,f, Ratios of 45–47S/21S (e) and 45–47S/18S-E (f) rRNA levels quantified from experiments as in d. All signals were normalized to total 28S rRNA detected with methylene blue staining. Error bars are mean ± s.d., n = 3 independent experiments. P value as indicated, two-sided Student’s t-test. g,h, Fluorescence in situ hybridization (FISH) with an antisense or sense probe overlapping the 5.8S (5′ ) rRNA processing site 2 as shown in c. The nucleus was counterstained with DAPI. Images represent MR90 cells expressing H-RASV12 (RAS) or an empty control vector at day 20 post-infection (g) or young (passage 25) and old (passage 43) IMR90 cells (h) (1 out of 2 independent experiments with similar results). Scale bars (g,h), 10 µ m. See also Supplementary Figs. 1 and 2. of oncogenic Ras, there is an increase in both rRNA synthesis and We conclude that the synthesis of rRNA is largely reduced in senes- cell proliferation (days 2–6, Supplementary Fig. 1a). However, when cent cells irrespective of the stress that triggered the process. cells became senescent (days 8–20 after Ras expression, PML (pro- Next, we used probes complementary to the internal transcribed myelocytic leukaeumia) expression or telomere shortening), rRNA spacers ITS1 and ITS2 (Fig. 1c) to characterize pre-rRNA process- synthesis was dramatically reduced as measured using a three- ing intermediates in senescent cells. We loaded the gels normal- hour pulse of 3H-uridine (Fig. 1b and Supplementary Fig. 1a–d). ized against the amount of mature 28S rRNA as was done for pulse NATURE CELL BiologY | www.nature.com/naturecellbiology © 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. NATURE CELL BIOLOGY ARTICLES a Vector RAS PML c Ctrl Camptothecin β -gal - SA β -gal SA- 5.9% 89.5% 72.7% 4.3% 60.6% RPL29 RPL29 4.7% 89.3% 59.3% 13.7% 83.7% FBL FBL Overlay Overlay b Young Old Starved Confluent d Ctrl β-IFN β -gal SA- β -gal SA- 1.5% 86.6% 1.8% 7.1% 5.4% 25.9% 9 RPL2 9 RPL2 4.3% 78.7% 7.0% 8.0% 11.7% 53.3% FBL FBL y Overla y Overla Fig.