Evidence of p53-Dependent Cross-Talk between Ribosome Biogenesis and the Cell Cycle: Effects of Nucleolar Protein Bop1 on G₁/S Transition

Abstract

Bop1 is a novel nucleolar protein involved in rRNA processing and ribosome assembly. We have previously shown that expression of Bop1⌬, an amino-terminally truncated Bop1 that acts as a dominant negative mutant in mouse cells, results in inhibition of 28S and 5.8S rRNA formation and deficiency of newly synthesized 60S ribosomal subunits (Z. Strezoska, D. G. Pestov, and L. F. Lau, Mol. Cell. Biol. 20:5516-5528, 2000). Perturbation of Bop1 activities by Bop1⌬ also induces a powerful yet reversible cell cycle arrest in 3T3 fibroblasts. In the present study, we show that asynchronously growing cells are arrested by Bop1⌬ in a highly concerted fashion in the G 1 phase. Kinase activities of the G 1 -specific Cdk2 and Cdk4 complexes were downregulated in cells expressing Bop1⌬, whereas levels of the Cdk inhibitors p21 and p27 were concomitantly increased. The cells also displayed lack of hyperphosphorylation of retinoblastoma protein (pRb) and decreased expression of cyclin A, indicating their inability to progress through the restriction point. Inactivation of functional p53 abrogated this Bop1⌬-induced cell cycle arrest but did not restore normal rRNA processing. These findings show that deficiencies in ribosome synthesis can be uncoupled from cell cycle arrest and reveal a new role for the p53 pathway as a mediator of the signaling link between ribosome biogenesis and the cell cycle. We propose that aberrant rRNA processing and/or ribosome biogenesis may cause "nucleolar stress," leading to cell cycle arrest in a p53-dependent manner.Proliferating cells can delay or block cell cycle transitions in response to a variety of extracellular regulatory signals as well as to perturbations in intracellular processes. Several types of stress, such as DNA damage, defects in replication and chromosome segregation, and accumulation of misfolded proteins in the endoplasmic reticulum are now known to elicit checkpoint responses that prevent progression through the cell cycle (16,25,69). These responses are often altered in neoplastic cells, suggesting that the regulatory mechanisms involved play important roles in tumor development (24).In a previous study, we applied a genetic selection procedure to search for sequences in a cDNA library that can cause reversible arrest of the cell cycle (45). One cDNA clone (Bop1⌬) that induced a particularly strong inhibition of DNA synthesis in NIH 3T3 fibroblasts encoded an amino-terminally truncated form of a novel WD40 repeat protein, named Bop1 (block of proliferation). Expression of Bop1⌬ interfered with the functions of the endogenous Bop1 in a dominant manner, which likely accounted for the strong growth-inhibitory potential of this clone.Subsequent studies revealed that Bop1 was predominantly localized to the nucleolus and cofractionated with preribosomal particles (58). Bop1⌬ exhibited a similar localization but lacked some of the critical functions of the wild-type protein, leading to a dominant negative phenotype. Expression of this mutant form of Bop1 in LAP3 cells completely bloc...

PIM1 kinase is destabilized by ribosomal stress causing inhibition of cell cycle progression

“…It is now well documented that ribosomal stress, defined as an alteration of ribosome synthesis, can induce the activation of the tumor suppressor p53 blocking cell proliferation and activating apoptosis (Pestov et al, 2001;Sulic et al, 2005;Anderson et al, 2007;Panic et al, 2007;Danilova et al, 2008;McGowan et al, 2008;Fumagalli et al, 2009). Here, we report the analysis of the effect of RPS19 deficiency on the metabolism of two erythroid cell lines.…”

“…It involves four ribosomal RNA molecules, about 80 RPs and nearly 200 non-ribosomal factors that are required for the synthesis, maturation and export of the two ribosomal subunits (Fatica and Tollervey, 2002). A number of reports suggest that perturbations of ribosome biogenesis, owing to a variety of causes (ribosomal stress), can activate a specific checkpoint and block cell proliferation mostly through a p53-dependent mechanism (Pestov et al, 2001;Rubbi and Milner, 2003;Anderson et al, 2007;Panic et al, 2007;Danilova et al, 2008;McGowan et al, 2008;Fumagalli et al, 2009). This occurs, for example, in the case of conditional deletion of RPS6 (Volarevic et al, 2000;Sulic et al, 2005) or in response to drugs that disrupt nucleolar structures (Rubbi and Milner, 2003).…”

The balance between rRNA and ribosomal protein synthesis up- and downregulates the tumour suppressor p53 in mammalian cells

“…There is also evidence that defective production of specific ribosomal proteins may stabilise p53 (Pestov et al, 2001;Sulic et al, 2005;Panic´et al, 2007;Fumagalli et al, 2009). These studies were mainly based on models in which a derangement of ribosome biogenesis was developed more than a selective rRNA synthesis inhibition.…”

Translational research? Ribosome integrity and a new p53 tumor suppressor checkpoint

“…The existence of a p53-dependent signaling mechanism between ribosomal biogenesis and cell cycle control was established when a nucleolar protein involved in rRNA processing and assembly known as BOP1 was identified. 21 Expression of a dominant negative version of BOP1 causes defects in rRNA processing that induces a cell cycle arrest, which was alleviated by deletion of p53. Therefore, p53 can functionally link ribosome biogenesis to cell cycle progression.…”