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 conse- quently lead to improper execution of the genetic program and the development of speci fi c diseases. Evidence has accumulated over the last decade suggesting that perturbation of ribosome biogenesis triggers a p53- activating checkpointsignaling pathway,often referred to as the ribosome biogenesis stress checkpoint pathway. Although it was originally suggested that p53 has a pro minent role in preventing diseases by monitoring the fi delity 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 speci fi c 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 fi eld. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease

Nucleolus as an emerging hub in maintenance of genome stability and cancer pathogenesis

The nucleolus is the major site for synthesis of ribosomes, complex molecular machines that are responsible for protein synthesis. A wealth of research over the past 20 years has clearly indicated that both quantitative and qualitative alterations in ribosome biogenesis can drive the malignant phenotype via dysregulation of protein synthesis. However, numerous recent proteomic, genomic, and functional studies have implicated the nucleolus in the regulation of processes that are unrelated to ribosome biogenesis, including DNA-damage response, maintenance of genome stability and its spatial organization, epigenetic regulation, cell-cycle control, stress responses, senescence, global gene expression, as well as assembly or maturation of various ribonucleoprotein particles. In this review, the focus will be on features of rDNA genes, which make them highly vulnerable to DNA damage and intra- and interchromosomal recombination as well as built-in mechanisms that prevent and repair rDNA damage, and how dysregulation of this interplay affects genome-wide DNA stability, gene expression and the balance between euchromatin and heterochromatin. We will also present the most recent insights into how malfunction of these cellular processes may be a central driving force of human malignancies, and propose a promising new therapeutic approach for the treatment of cancer

Regulatory module involving FGF13, miR-504, and p53 regulates ribosomal biogenesis and supports cancer cell survival

The microRNA miR-504 targets TP53 mRNA encoding the p53 tumor suppressor. miR-504 resides within the fibroblast growth factor 13 (FGF13) gene, which is overexpressed in various cancers. We report that the FGF13 locus, comprising FGF13 and miR-504, is transcriptionally repressed by p53, defining an additional negative feedback loop in the p53 network. Furthermore, we show that FGF13 1A is a nucleolar protein that represses ribosomal RNA transcription and attenuates protein synthesis. Importantly, in cancer cells expressing high levels of FGF13, the depletion of FGF13 elicits increased proteostasis stress, associated with the accumulation of reactive oxygen species and apoptosis. Notably, stepwise neoplastic transformation is accompanied by a gradual increase in FGF13 expression and increased dependence on FGF13 for survival ("nononcogene addiction"). Moreover, FGF13 overexpression enables cells to cope more effectively with the stress elicited by oncogenic Ras protein. We propose that, in cells in which activated oncogenes drive excessive protein synthesis, FGF13 may favor survival by maintaining translation rates at a level compatible with the protein quality- control capacity of the cell. Thus, FGF13 may serve as an enabler, allowing cancer cells to evade proteostasis stress triggered by oncogene activation

Mutual protection of ribosomal proteins L5 and L11 from degradation is essential for p53 activation upon ribosomal biogenesis stress

Impairment of ribosomal biogenesis can activate the p53 protein independently of DNA damage. The ability of ribosomal proteins L5, L11, L23, L26, or S7 to bind Mdm2 and inhibit its ubiquitin ligase activity has been suggested as a critical step in p53 activation under these conditions. Here, we report that L5 and L11 are particularly important for this response. Whereas several other newly synthesized ribosomal proteins are degraded by proteasomes upon inhibition of Pol I activity by actinomycin D, L5 and L11 accumulate in the ribosome-free fraction where they bind to Mdm2. This selective accumulation of free L5 and L11 is due to their mutual protection from proteasomal degradation. Furthermore, the endogenous, newly synthesized L5 and L11 continue to be imported into nucleoli even after nucleolar disruption and colocalize with Mdm2, p53, and promyelocytic leukemia protein. This suggests that the disrupted nucleoli may provide a platform for L5- and L11-dependent p53 activation, implying a role for the nucleolus in p53 activation by ribosomal biogenesis stress. These findings may have important implications with respect to understanding the pathogenesis of diseases caused by impaired ribosome biogenesis

New insights into HEATR1 functions

Human HEAT repeat containing 1 protein (HEATR1) is the structural and functional homolog of yeast U3 small nucleolar RNA-associated protein 10 (UTP 10). Human HEATR1 is a nucleolar protein that plays a prominent positive role in regulation of rRNA synthesis. Depletion of HEATR1 cause disruption of nucleolar structure and activate the ribosomal biogenesis stress pathway – RPL5 / RPL11 dependent stabilization and activation of the tumor suppressor protein p53

New insights into HEATR1 functions

Human HEAT repeat containing 1 protein (HEATR1) is the structural and functional homolog of yeast U3 small nucleolar RNA-associated protein 10 (UTP 10). Human HEATR1 is a nucleolar protein that plays a prominent positive role in regulation of rRNA synthesis. Depletion of HEATR1 cause disruption of nucleolar structure and activate the ribosomal biogenesis stress pathway – RPL5 / RPL11 dependent stabilization and activation of the tumor suppressor protein p53

Dysregulated Ribosome Biogenesis Reveals Therapeutic Liabilities in Cancer

Ribosome biogenesis (RiBi) is one of the most complex and energy demandingprocesses in human cells, critical for cell growth and proliferation. Strong causallinks between inherited and acquired impairment in RiBi with cancer pathogene-sis are emerging, pointing to RiBi as an attractive therapeutic target for cancer.Here, we will highlight new knowledge about causes of excessive or impairedRiBi and the impact of these changes on protein synthesis. We will also discusshow new knowledge about secondary consequences of dysregulated RiBi andprotein synthesis, including proteotoxic stress, metabolic alterations, adaptivetranscriptional and translational programs, and the impaired ribosome biogenesischeckpoint (IRBC) provide a foundation for the development of new anticancertherapie

Inactivation of S6 ribosomal protein gene in T lymphocytes activates a p53-dependent checkpoint response

Ribosome biogenesis has been associated with regulation of cell growth and cell division, but the molecular mechanisms that integrate the effect of ribosome biogenesis on these processes in mammalian cells remain unknown. To study the effect of impaired ribosome functions in vivo, we conditionally deleted one or two alleles of the 40S ribosomal protein S6 gene in T cells in the mouse. While complete deletion of S6 abrogated T-cell development, hemizygous expression did not have any effect on T-cell maturation in the thymus, but inhibited the accumulation of T cells in the spleen and lymph nodes, as a result of their decreased survival in the peripheral lymphoid organs. Additionally, TCR-mediated stimulation of S6-heterozygous T cells induced a normal increase in their size, but cell cycle progression was impaired. Genetic inactivation of p53 tumor suppressor rescued development of S6-homozygous null thymocytes and proliferative defect of S6-heterozygous T cells. These results demonstrate the existence of a p53-dependent checkpoint mechanism that senses changes in the fidelity of the translational machinery to prevent aberrant cell division or eliminate defective T cells in vivo. Failure to activate this checkpoint response could potentially lead to a development of pathological processes such as tumors and autoimmune diseases