Scission of the p53-MDM2 Loop by Ribosomal Proteins

Ribosomal proteins and human diseases: pathogenesis, molecular mechanisms, and therapeutic implications

Ribosomes are essential components of the protein synthesis machinery. The process of ribosome biogenesis is well organized and tightly regulated. Recent studies have shown that ribosomal proteins (RPs) have extraribosomal functions that are involved in cell proliferation, differentiation, apoptosis, DNA repair, and other cellular processes. The dysfunction of RPs has been linked to the development and progression of hematological, metabolic, and cardiovascular diseases and cancer. Perturbation of ribosome biogenesis results in ribosomal stress, which triggers activation of the p53 signaling pathway through RPs-MDM2 interactions, resulting in p53-dependent cell cycle arrest and apoptosis. RPs also regulate cellular functions through p53-independent mechanisms. We herein review the recent advances in several forefronts of RP research, including the understanding of their biological features and roles in regulating cellular functions, maintaining cell homeostasis, and their involvement in the pathogenesis of human diseases. We also highlight the translational potential of this research for the identification of molecular biomarkers, and in the discovery and development of novel treatments for human diseases.

Ribosomal protein S27-like is a physiological regulator of p53 that suppresses genomic instability and tumorigenesis

Cell-based studies showed that several Mdm2-binding ribosomal proteins, upon overexpression, stabilize and activate p53. In contrast, here we show in a mouse knockout study that Mdm2-binding ribosomal protein S27-like (Rps27l), upon disruption, activates p53. Germline inactivation of Rps27l triggers ribosomal stress to stabilize Mdm2, which degrades Mdm4 to reduce Mdm2-Mdm4 E3 ligase towards p53, leading to p53-dependent apoptotic depletion of hematopoietic stem cells and postnatal death, which is rescued by Trp53 deletion. Paradoxically, while increased p53 is expected to inhibit tumorigenesis, Rps27l^−/−;Trp53^+/− mice develop lymphomas at higher incidence with p53 loss-of-heterozygosity and severe genome aneuploidy, suggesting that Rps27l disruption impose a selection pressure against p53. Thus, Rps27l has dual functions in p53 regulation: under Trp53^+/+ background, Rps27l disruption triggers ribosomal stress to induce p53 and apoptosis, whereas under Trp53^+/− background, Rps27l disruption triggers genomic instability and Trp53 deletion to promote tumorigenesis. Our study provides a new paradigm of p53 regulation.

DOI:http://dx.doi.org/10.7554/eLife.02236.001

There are over a hundred different types of cancer that can affect humans; but, in general, all cancers are caused by mutations that cause cells to grow and divide abnormally. ‘Tumor suppressor genes’ are genes that normally protect a cell from genetic changes that can lead a cell towards becoming cancerous.

About half of all cancers in humans have a mutation in one of the two copies of a tumor suppressor gene that encodes a protein called p53, which helps to control how and when cells grow and divide. In normal cells, the p53 protein can be activated in various ways. Damage to a cell's DNA triggers p53 to stop the cell growing, which gives the cell time to repair the DNA damage. However, if the damage is too severe and cannot be repaired, p53 essentially causes the cell to kill itself, via a process called apoptosis. Furthermore, if a cell has problems building new copies of its protein-making machinery, some of the parts (called ribosomal proteins) that make up these molecular machines can also lead to p53 being activated.

By deleting the gene for a protein called Rps27l that is a newly characterized ribosomal protein, Xiong et al. have discovered that, in mice, Rps27I regulates the p53 protein in two different ways. In normal cells, Rps27l appears to inhibit p53, which is likely to encourage cancer to develop. But, if a cell has already lost a copy of the p53 gene—a situation that would normally encourage the cells to accrue further mutations and become cancerous—Rps27l acts as a tumor suppressor. In these mutated cells, the Rps27l protein helps to maintain the stability of the genome and prevent the loss of the second copy of gene for p53, and so protects the cell from becoming cancerous.

Thus Rps27l can either activate or inactivate p53 activity depending on how many copies of the gene for p53 remain intact. The next challenge is to investigate if Rps27l levels determine the early-onset of tumor development in cancer-prone cells seen in patients with Li-Fraumeni syndrome, who are born with a mutated copy of the p53 gene.

DOI:http://dx.doi.org/10.7554/eLife.02236.002

Ribosomopathies: mechanisms of disease

Ribosomopathies are diseases caused by alterations in the structure or function of ribosomal components. Progress in our understanding of the role of the ribosome in translational and transcriptional regulation has clarified the mechanisms of the ribosomopathies and the relationship between ribosomal dysfunction and other diseases, especially cancer. This review aims to discuss these topics with updated information.

Does not hUTP14a promoter form a regulation feedback loop with P53?

OBJECTIVE

We previously found that hUTP14a binds P53 and promotes P53 degradation. However, if hUTP14a is a downstream gene of P53 remains to be determined. This study aimed to identify the promoter of hUTP14a and investigate if hUTP14a is regulated by P53.

METHODS

The hUTP14a promoter region was cloned into pGL3-Basic-luciferase reporter plasmid to get pGL3-hUTP14a-luc. The reporter plasmid was transfected into 293T cells and luciferase activity was evaluated by the Dual-Luciferase Reporter Assay System. Putative transcription factors were identified through searching MatInspector Professional and Algorismica i Genetica databases. Either pGL3-hUTP14a-luc or p21 promoter reporter plasmid was co-transfected with increasing dose of p53 plasmid, and luciferase activity was evaluated. A series of deletion constructs of pGL3-hUTP14a-luc were constructed and minimal promoter region of hUTP14a was determined. Differences of the luciferase activities between different groups were assessed by statistical analysis.

RESULTS

The hUTP14a gene promoter reporter construct was correctly cloned and was demonstrated to possess promoter activity. The transcription of hUTP14a was not regulated by P53. The minimal promoter region of hUTP14a gene is located between -203 to -100 of the transcription initiation site.

CONCLUSION

Unlike other P53-interacting proteins such as MDM2, Pirh2 and Cop I which promote P53 degradation and whose transcriptions are regulated by P53, does not hUTP14a transcription form a regulation feedback loop with P53.

Connecting the nucleolus to the cell cycle and human disease

Long known as the center of ribosome synthesis, the nucleolus is connected to cell cycle regulation in more subtle ways. One is a surveillance system that reacts promptly when rRNA synthesis or processing is impaired, halting cell cycle progression. Conversely, the nucleolus also acts as a first-responder to growth-related stress signals. Here we review emerging concepts on how these "infraribosomal" links between the nucleolus and cell cycle progression operate in both forward and reverse gears. We offer perspectives on how new cancer therapeutic designs that target this infraribosomal mode of cell growth control may shape future clinical progress.

Nucleostemin stabilizes ARF by inhibiting the ubiquitin ligase ULF

Up-regulated expression of nucleolar GTPase Nucleostemin (NS) has been associated with increased cellular proliferation potential and tumor malignancy during cancer development. Recent reports attribute the growth regulatory effects of NS protein to its role in facilitating ribosome production. However, the oncogenic potential of NS remains unclear since imbalanced levels of NS have been reported to exert growth inhibitory effect by modulating p53 tumor suppressor activity. It also remains in questions if aberrant NS levels might play a p53-independent role in regulation of cell proliferation and growth. In this study, we performed affinity purification and mass spectrometry analysis to explore protein-protein interactions influencing NS growth regulatory properties independently of p53 tumor suppressor. We identified the Alternative Reading Frame (ARF) protein as a key protein associating with NS and further verified the interaction through in vitro and in vivo assays. We demonstrated that NS is able to regulate cell cycle progression by regulating the stability of the ARF tumor suppressor. Furthermore, overexpression of NS suppressed ARF polyubiquitination by its E3 ligase ULF and elongated its half-life, while knockdown of NS led to the decrease of ARF levels. Also, we found that NS can enhance NPM stabilization of ARF. Thus, we propose that in the absence of p53, ARF can be stabilized by NS and NPM to serve as an alternative tumor suppressor surveillance, preventing potential cellular transformation resulting from the growth inducing effects of NS overexpression.

New insights into p53 functions through its target microRNAs

The tumor suppressor p53 pathway, whose alterations are highly associated with all types of human cancers, plays an essential role in preventing tumor development and progression mostly through its downstream target genes. Over the last decade, a growing list of p53 microRNA (miRNA) targets has been identified as additional downstream players of this pathway. Further studies of these miRNAs have revealed their more complicated regulations and functions in executing and/or regulating p53 activity. Here, we review the p53 miRNA targets identified thus far, and discuss how they fine-tune p53 stress responses, mediate the crosstalk between p53 and other signaling pathways, and expand the role of p53 in other human diseases in addition to cancers.

Antitumor effect of miR-197 targeting in p53 wild-type lung cancer

Lung cancer is the leading cause of tumor-related death. The lack of effective treatments urges the development of new therapeutic approaches able to selectively kill cancer cells. The connection between aberrant microRNA (miRNA - miR) expression and tumor progression suggests a new strategy to fight cancer by interfering with miRNA function. In this regard, LNAs (locked nucleic acids) have proven to be very promising candidates for miRNA neutralization. Here, we employed an LNA-based anti-miR library in a functional screening to identify putative oncogenic miRNAs in non-small-cell lung cancer (NSCLC). By screening NIH-H460 and A549 cells, miR-197 was identified as a new functional oncomiR, whose downregulation induces p53-dependent lung cancer cell apoptosis and impairs the capacity to establish tumor xenografts in immunodeficient mice. We further identified the two BH3-only proteins NOXA and BMF as new miR-197 targets responsible for induction of apoptosis in p53 wild-type cells, delineating miR-197 as a key survival factor in NSCLC. Thus, we propose the inhibition of miR-197 as a novel therapeutic approach against lung cancer.

Targeting p53-MDM2-MDMX loop for cancer therapy

The tumor suppressor p53 plays a central role in anti-tumorigenesis and cancer therapy. It has been described as "the guardian of the genome", because it is essential for conserving genomic stability by preventing mutation, and its mutation and inactivation are highly related to all human cancers. Two important p53 regulators, MDM2 and MDMX, inactivate p53 by directly inhibiting its transcriptional activity and mediating its ubiquitination in a feedback fashion, as their genes are also the transcriptional targets of p53. On account of the importance of the p53-MDM2-MDMX loop in the initiation and development of wild type p53-containing tumors, intensive studies over the past decade have been aiming to identify small molecules or peptides that could specifically target individual protein molecules of this pathway for developing better anti-cancer therapeutics. In this chapter, we review the approaches for screening and discovering efficient and selective MDM2 inhibitors with emphasis on the most advanced synthetic small molecules that interfere with the p53-MDM2 interaction and are currently on Phase I clinical trials. Other therapeutically useful strategies targeting this loop, which potentially improve the prospects of cancer therapy and prevention, will also be discussed briefly.

Activation of the tumor suppressor p53 upon impairment of ribosome biogenesis

Errors in ribosome biogenesis can result in quantitative or qualitative defects in protein synthesis and consequently lead to improper execution of the genetic program and the development of specific diseases. Evidence has accumulated over the last decade suggesting that perturbation of ribosome biogenesis triggers a p53-activating checkpoint signaling pathway, often referred to as the ribosome biogenesis stress checkpoint pathway. Although it was originally suggested that p53 has a prominent role in preventing diseases by monitoring the fidelity of ribosome biogenesis, recent work has demonstrated that p53 activation upon impairment of ribosome biogenesis also mediates pathological manifestations in humans. Perturbations of ribosome biogenesis can trigger a p53-dependent checkpoint signaling pathway independent of DNA damage and the tumor suppressor ARF through inhibitory interactions of specific ribosomal components with the p53 negative regulator, Mdm2. Here we review the recent advances made toward understanding of this newly-recognized checkpoint signaling pathway, its role in health and disease, and discuss possible future directions in this exciting research field. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.