Mycophenolic acid activation of p53 requires ribosomal proteins L5 and L11

The ribosomal protein S26 regulates p53 activity in response to DNA damage

Ribosomal proteins have emerged as novel regulators of the Mdm2-p53 feedback loop, especially in the context of ribosomal stress. RPS26 is a recently identified Diamond-Blackfan Anemia-related ribosomal protein and its role in p53 activation has not been previously explored. In this study we found knockdown of RPS26 induced p53 stabilization and activation via a RPL11-dependent mechanism, resulting in p53-dependent cell growth inhibition. Moreover, RPS26 has the ability to interact with Mdm2 and inhibits Mdm2-mediated p53 ubiquitination that leads to p53 stabilization upon overexpression. Importantly, we discovered that RPS26 knockdown impaired p53's ability to transcriptionally activate its target genes in response to DNA damage, without affecting its stability. Accordingly, the cells lost the ability to induce G2/M cell cycle arrest. We further found that upon RPS26 knockdown, the DNA damage induced recruitment of p53 to the promoters of its target genes and p53 acetylation were both greatly reduced. In addition, RPS26 can interact with p53 independent of Mdm2 and coexist in a complex with p53 and p300. These data establish a role of RPS26 in DNA damage response by directly influencing p53 transcriptional activity, and suggest that RPS26 acts distinctively in different scenarios of p53 activation. Our finding also implicates p53 transcriptional activity control as an important mechanism of p53 regulation by ribosomal proteins.

Scission of the p53-MDM2 Loop by Ribosomal Proteins

The oncoprotein MDM2 is both the transcriptional target and the predominant antagonist of the tumor suppressor p53. MDM2 inhibits the functions of p53 via a negative feedback loop that can be circumvented by several ribosomal proteins in response to nucleolar or ribosomal stress. Stress conditions in the nucleolus can be triggered by a variety of extracellular and intracellular insults that impair ribosomal biogenesis and function, such as chemicals, nutrient deprivation, DNA damaging agents, or genetic alterations. The past decade has witnessed a tremendous progress in understanding this previously underinvestigated ribosomal stress-MDM2-p53 pathway. Here, we review the recent progress in understanding this unique signaling pathway, discuss its biological and pathological significance, and share with readers our insight into the research in this field.

p53 -Dependent and -Independent Nucleolar Stress Responses

The nucleolus has emerged as a cellular stress sensor and key regulator of p53-dependent and -independent stress responses. A variety of abnormal metabolic conditions, cytotoxic compounds, and physical insults induce alterations in nucleolar structure and function, a situation known as nucleolar or ribosomal stress. Ribosomal proteins, including RPL11 and RPL5, become increasingly bound to the p53 regulatory protein MDM2 following nucleolar stress. Ribosomal protein binding to MDM2 blocks its E3 ligase function leading to stabilization and activation of p53. In this review we focus on a number of novel regulators of the RPL5/RPL11-MDM2-p53 complex including PICT1 (GLTSCR2), MYBBP1A, PML and NEDD8. p53-independent pathways mediating the nucleolar stress response are also emerging and in particular the negative control that RPL11 exerts on Myc oncoprotein is of importance, given the role of Myc as a master regulator of ribosome biogenesis. We also briefly discuss the potential of chemotherapeutic drugs that specifically target RNA polymerase I to induce nucleolar stress.

Identification of ribosomal protein S25 (RPS25)-MDM2-p53 regulatory feedback loop

There is an increasing interest in determining the role of ribosomal proteins (RPs) in the regulation of MDM2-p53 pathway in coordinating cellular response to stress. Herein, we report a novel regulatory role of ribosomal protein S25 (RPS25) in MDM2-mediated p53 degradation and a feedback regulation of S25 by p53. We demonstrated that S25 interacted with MDM2 and inhibited its E3 ligase activity, resulting in the reduction of MDM2-mediated p53 ubiquitination and the stabilization and activation of p53. S25, MDM2 and p53 formed a ternary complex following ribosomal stress. The nucleolar localization and MDM2-binding domains of S25 were critical for its role in MDM2-mediated p53 regulation. Knockdown of S25 by siRNA attenuated the induction and activation of p53 following ribosomal stress. S25 stabilized and cooperated with MDMX to regulate MDM2 E3 ligase activity. Furthermore, S25 was identified to be a transcriptional target of p53; p53 directly bound to S25 promoter and suppressed S25 expression. Our results suggest that there is a S25-MDM2-p53 regulatory feedback loop, which may have an important role in cancer development and progression.

Characterization of an HIV-targeted transcriptional gene-silencing RNA in primary cells

Small antisense RNAs targeted to the HIV-1 promoter have been shown to remodel the surrounding chromatin to a state unfavorable for transcriptional activation, yet transcriptional gene silencing (TGS) of HIV-1 has, to date, not been shown in primary human cells. We demonstrate here that TGS can reduce viral transcription in primary human CD4(+) T cells; however, increasing viral burden results in the loss of this antiviral effect. This observation suggests a critical level at which viral RNA can dilute out effective targeting by TGS-based RNAs. Furthermore, studies into off-target effects have identified a potential interaction between the small nucleolar RNA pathway and the TGS-based antisense RNA, resulting in activation of p53. Although not overtly toxic to primary cells, this represents a novel interaction between antisense RNAs and a cellular pathway that should be considered when pursuing small antisense RNA-based therapeutics.

Physical and functional interaction between ribosomal protein L11 and the tumor suppressor ARF

The ARF tumor suppressor protein activates p53 in response to oncogenic stress, whereas ribosomal protein L11 induces p53 following ribosomal stress. Both proteins bind to central, albeit non-overlapping, regions of MDM2 and suppress MDM2 activity toward p53. However, it is not known whether the two pathways are functionally connected. Here we show that ARF directly binds to L11 in vitro and in cells, which then forms a complex with MDM2 and p53. L11 collaboratively enhances ARF-induced p53 transcriptional activity and cell cycle arrest. Supporting these results, knocking down L11 reduces ARF-mediated p53 accumulation and alleviates ARF-induced cell cycle arrest. Interestingly, overexpression of ARF increases the levels of ribosome-free L11 and enhances the interaction of L11 with MDM2 and p53. These results demonstrate that ARF activates p53, at least partly by induction of ribosomal stress, which results in L11 suppression of MDM2, and suggest that the ARF-MDM2-p53 and the L11-MDM2-p53 pathways are functionally connected.

A new PICTure of nucleolar stress

Cell growth demands new protein synthesis, which requires nucleolar ribosomal functions. Ribosome biogenesis consumes a large proportion of the cell's resources and energy, and so is tightly regulated through an intricate signaling network to guarantee fidelity. Thus, events that impair ribosome biogenesis cause nucleolar stress. In response to this stress, several nucleolar ribosomal proteins (RPs) translocate to the nucleoplasm and bind to MDM2. MDM2-mediated ubiquitination and degradation of the tumor suppressor p53 is then blocked, resulting in p53 accumulation and the induction of p53-dependent cell cycle arrest and apoptosis. Nucleolar stress is therefore a quality control surveillance mechanism that monitors the synthesis and assembly of the rRNA and protein components of ribosomes. Although nucleolar stress signaling pathways have been extensively analyzed, critical questions remain about their regulatory mechanisms. For example, how do RPs translocate from the nucleolus to the nucleoplasm to exert their functions, and do these p53-regulating RPs influence the prognosis of human cancer patients? Our laboratory recently identified the nucleolar protein PICT1 as a novel regulator of nucleolar stress. PICT1 sequesters the ribosomal protein RPL11 in the nucleolus, preventing it from binding to MDM2. MDM2 is then free to degrade p53, favoring tumor cell growth. Accordingly, the level of PICT1 in a tumor is becoming a useful prognostic marker for human cancers. This review summarizes the evidence that links nucleolar stress to tumorigenesis, and casts PICT1 as an oncogenic player in human cancer biology.

Ribosomal protein S14 unties the MDM2-p53 loop upon ribosomal stress

The MDM2-p53 feedback loop is crucially important for restricting p53 level and activity during normal cell growth and proliferation, and is thus subjected to dynamic regulation in order for cells to activate p53 upon various stress signals. Several ribosomal proteins, such as RPL11, RPL5, RPL23, RPL26 or RPS7, have been shown to have a role in regulation of this feedback loop in response to ribosomal stress. Here, we identify another ribosomal protein S14, which is highly associated with 5q-syndrome, as a novel activator of p53 by inhibiting MDM2 activity. We found that RPS14, but not RPS19, binds to the central acidic domain of MDM2, similar to RPL5 and RPL23, and inhibits its E3 ubiquitin ligase activity toward p53. This RPS14-MDM2 binding was induced upon ribosomal stress caused by actinomycin D or mycophenolic acid. Overexpression of RPS14, but not RPS19, elevated p53 level and activity, leading to G1 or G2 arrest. Conversely, knockdown of RPS14 alleviated p53 induction by these two reagents. Interestingly, knockdown of either RPS14 or RPS19 caused a ribosomal stress that led to p53 activation, which was impaired by further knocking down the level of RPL11 or RPL5. Together, our results demonstrate that RPS14 and RPS19 have distinct roles in regulating the MDM2-p53 feedback loop in response to ribosomal stress.

Ubiquitin- and MDM2 E3 ligase-independent proteasomal turnover of nucleostemin in response to GTP depletion

Nucleostemin (NS) is a nucleolar GTP-binding protein essential for ribosomal biogenesis, proliferation, and animal embryogenesis. It remains largely unclear how this protein is regulated. While working on its role in suppression of MDM2 and activation of p53, we observed that NS protein (but not mRNA) levels decreased drastically in response to GTP depletion. When trying to further elucidate the molecular mechanism(s) underlying this unusual phenomenon, we found that NS was degraded independently of ubiquitin and MDM2 upon GTP depletion. First, depletion of GTP by treating cells with mycophenolic acid decreased the level of NS without apparently affecting the levels of other nucleolar proteins. Second, mutant NS defective in GTP binding and exported to the nucleoplasm was much less stable than wild-type NS. Although NS was ubiquitinated in cells, its polyubiquitination was independent of Lys-48 or Lys-63 in the ubiquitin molecule. Inactivation of E1 in E1 temperature-sensitive mouse embryonic fibroblast (MEF) cells failed to prevent the proteasomal degradation of NS. The proteasomal turnover of NS was also MDM2-independent, as its half-life in p53/MDM2 double knock-out MEF cells was the same as that in wild-type MEF cells. Moreover, NS ubiquitination was MDM2-independent. Mycophenolic acid or doxorubicin induced NS degradation in various human cancerous cells regardless of the status of MDM2. Hence, these results indicate that NS undergoes a ubiquitin- and MDM2-independent proteasomal degradation when intracellular GTP levels are markedly reduced and also suggest that ubiquitination of NS may be involved in regulation of its function rather than stability.

The design, synthesis and in vitro immunosuppressive evaluation of novel isobenzofuran derivatives

The synthesis and biological evaluation of a series of novel isobenzofuran-based compounds are described. The compounds were evaluated for their immunosuppressive effects of T-cell proliferation and IMPDH type II inhibitor activity in vitro, as well as their structure-activity relationships were assessed. Several compounds demonstrated highly efficacious immunosuppressive properties, especially compounds 2d, 2e, 2h and 2j, which were superior to MPA, while compounds 2k, 2m, 2n, 4c and 5d exhibited an equipotent inhibitory activity compared to MPA. Generally, it was obviously demonstrated that α,β-unsaturated amides proved more potent than the diamide and urea series. The present study provides a guide for further research on development of safe and effective immunosuppressive agents.

The Ribosomal Protein-Mdm2-p53 Pathway and Energy Metabolism: Bridging the Gap between Feast and Famine

Cellular growth and division are two fundamental processes that are exquisitely sensitive and responsive to environmental fluctuations. One of the most energetically demanding functions of these processes is ribosome biogenesis, the key component to regulating overall protein synthesis and cell growth. Perturbations to ribosome biogenesis have been demonstrated to induce an acute stress response leading to p53 activation through the inhibition of Mdm2 by a number of ribosomal proteins. The energy status of a cell is a highly dynamic variable that naturally contributes to metabolic fluctuations, which can affect both the rates of ribosome biogenesis and p53 function. This, in turn, determines whether a cell is in an anabolic, growth-promoting state or a catabolic, growth-suppressing state. Here the authors integrate the known functions of p53 to postulate how changes in nutrient availability may induce the ribosomal protein-Mdm2-p53 signaling pathway to modulate p53-dependent metabolic regulation.

Changes in ribosome biogenesis may induce cancer by down-regulating the cell tumor suppressor potential

Many human pathological conditions, not linked to genetic alterations of oncogenes or tumor suppressors, are nevertheless associated with an increased risk of developing cancer, and some of them are characterized by quantitative and/or qualitative changes in ribosome biogenesis. Indeed, there is evidence that both an up-regulation of ribosome biogenesis, such as that occurring during the abnormal stimulation of cell growth, and intrinsic dysfunctions of ribosomes, such as those characterizing a series of inherited disorders, show an increased incidence of tumor onset. Here we discuss some recent insights into the mechanisms by which these alterations in ribosome biogenesis may facilitate tumorigenesis.

GTP depletion synergizes the anti-proliferative activity of chemotherapeutic agents in a cell type-dependent manner

Mycophenolic acid (MPA) depletes intracellular GTP by blocking de novo guanine nucleotide synthesis. GTP is used ubiquitously for DNA/RNA synthesis and as a signaling molecule. Here, we made a surprising discovery that the anti-proliferative activity of MPA acts synergistically with specific chemotherapeutic agents in a cell type-dependent manner. In MDA-MB-231 cells, MPA shows an extremely potent synergy with 5-FU but not with doxorubicin or etoposide. The synergy between 5-FU and MPA works most effectively against the highly tumorigenic mammary tumor cells compared to the less tumorigenic ones, and does not work in the non-breast cancer cell types that we tested, with the exception of PC3 cells. On the contrary, MPA shows the highest synergy with paclitaxel but not with 5-FU in SCC-25 cells, derived from oral squamous cell carcinomas. Mechanistically, the synergistic effect of MPA on 5-FU in MDA-MB-231 cells can be recapitulated by inhibiting the RNA polymerase-I activity and requires the expression of nucleostemin. This work reveals that the synergy between MPA and anti-proliferative agents is determined by cell type-dependent factors.

Differential proteome analysis of human embryonic kidney cell line (HEK-293) following mycophenolic acid treatment

BACKGROUND

Mycophenolic acid (MPA) is widely used as a post transplantation medicine to prevent acute organ rejection. In the present study we used proteomics approach to identify proteome alterations in human embryonic kidney cells (HEK-293) after treatment with therapeutic dose of MPA. Following 72 hours MPA treatment, total protein lysates were prepared, resolved by two dimensional gel electrophoresis and differentially expressed proteins were identified by QTOF-MS/MS analysis. Expressional regulations of selected proteins were further validated by real time PCR and Western blotting.

RESULTS

The proliferation assay demonstrated that therapeutic MPA concentration causes a dose dependent inhibition of HEK-293 cell proliferation. A significant apoptosis was observed after MPA treatment, as revealed by caspase 3 activity. Proteome analysis showed a total of 12 protein spots exhibiting differential expression after incubation with MPA, of which 7 proteins (complement component 1 Q subcomponent-binding protein, electron transfer flavoprotein subunit beta, cytochrome b-c1 complex subunit, peroxiredoxin 1, thioredoxin domain-containing protein 12, myosin regulatory light chain 2, and profilin 1) showed significant increase in their expression. The expression of 5 proteins (protein SET, stathmin, 40S ribosomal protein S12, histone H2B type 1 A, and histone H2B type 1-C/E/F/G/I) were down-regulated. MPA mainly altered the proteins associated with the cytoskeleton (26%), chromatin structure/dynamics (17%) and energy production/conversion (17%). Both real time PCR and Western blotting confirmed the regulation of myosin regulatory light chain 2 and peroxiredoxin 1 by MPA treatment. Furthermore, HT-29 cells treated with MPA and total kidney cell lysate from MMF treated rats showed similar increased expression of myosin regulatory light chain 2.

CONCLUSION

The emerging use of MPA in diverse pathophysiological conditions demands in-depth studies to understand molecular basis of its therapeutic response. The present study identifies the myosin regulatory light chain 2 and peroxiredoxin 1 along with 10 other proteins showing significant regulation by MPA. Further characterization of these proteins may help to understand the diverse cellular effects of MPA in addition to its immunosuppressive activity.

Hydrophilic residues are crucial for ribosomal protein L11 (RPL11) interaction with zinc finger domain of MDM2 and p53 protein activation

Ribosomal protein L11 (RPL11) has been shown to activate p53 by binding to MDM2 and negating its p53 suppression activity in response to ribosomal stress. Although a mutation at Cys-305 within the zinc finger domain of MDM2 has been shown to drastically impair MDM2 interaction with RPL11 and thus escapes the inhibition by this ribosomal protein, it still remains elusive whether RPL11 inactivates MDM2 via direct action on this zinc finger domain and what is the chemical nature of this specific interaction. To define the roles of the MDM2 zinc finger in association with RPL11, we conducted hydrogen-deuterium exchange mass spectrometry, computational modeling, circular dichroism, and mutational analyses of the zinc finger domain of MDM2 and human RPL11. Our study reveals that RPL11 forms a stable complex with MDM2 in vitro through direct contact with its zinc finger. This binding is disrupted by single mutations of non-cysteine amino acids within the zinc finger domain of MDM2. Basic residues in RPL11 are crucial for the stable binding and RPL11 suppression of MDM2 activity toward p53. These results provide the first line of evidence for the specific interaction between RPL11 and the zinc finger of MDM2 via hydrophilic residues as well as a molecular foundation for better understanding RPL11 inhibition of MDM2 function.

Mycophenolic acid differentially impacts B cell function depending on the stage of differentiation

Production of pathogenic Abs contributes to disease progression in many autoimmune disorders. The immunosuppressant agent mycophenolic acid (MPA) has shown clinical efficacy for patients with autoimmunity. The goal of these studies was to elucidate the mechanisms of action of MPA on B cells isolated from healthy individuals and autoimmune patients. In this study, we show that MPA significantly inhibited both proliferation and differentiation of primary human B cells stimulated under various conditions. Importantly, MPA did not globally suppress B cell responsiveness or simply induce cell death, but rather selectively inhibited early activation events and arrested cells in the G0/G1 phase of the cell cycle. Furthermore, MPA blocked expansion of both naive and memory B cells and prevented plasma cell (PC) differentiation and Ab production from healthy controls and individuals with rheumatoid arthritis. Finally, whereas MPA potently suppressed Ig secretion from activated primary B cells, terminally differentiated PCs were not susceptible to inhibition by MPA. The target of MPA, IMPDH2, was found to be downregulated in PCs, likely explaining the resistance of these cells to MPA. These results suggest that MPA provides benefit in settings of autoimmunity by directly preventing activation and PC differentiation of B cells; however, MPA is unlikely to impact autoantibody production by preexisting, long-lived PCs.

Autoregulatory suppression of c-Myc by miR-185-3p

The expression of the c-myc oncogene at both protein and mRNA levels is transient and begins to be turned off 3-6 h after growth stimulation of cultured cells. The exact mechanism(s) underlying this down-regulation of c-Myc remains incompletely understood. Here we report the identification of miR-185-3p as a novel feedback regulator of c-Myc. This microRNA (miRNA) was initially identified as one of the c-Myc target miRNA transcripts through analysis of RNA samples isolated from cells prior to and after serum stimulation and further verified by real-time PCR, luciferase reporter, and ChIP assays. Interestingly, overexpression of wild type, but not mutant, miR-185-3p decreased the protein, but not mRNA, level of c-Myc in a dose-dependent fashion and also drastically abated the serum induction of c-Myc level in human cancer cells by targeting the coding sequence of c-Myc mRNA, consequently suppressing c-Myc-mediated proliferation. A miR-185-3p inhibitor rescued the inhibition of c-Myc expression by endogenous miR-185-3p. Thus, our results unveil miR-185-3p as the first miRNA that monitors c-Myc levels via an autoregulatory feedback mechanism in response to serum stimulation.

Ribosomal protein L11 recruits miR-24/miRISC to repress c-Myc expression in response to ribosomal stress

c-Myc promotes cell growth by enhancing ribosomal biogenesis and translation. Deregulated expression of c-Myc and aberrant ribosomal biogenesis and translation contribute to tumorigenesis. Thus, a fine coordination between c-Myc and ribosomal biogenesis is vital for normal cell homeostasis. Here, we show that ribosomal protein L11 regulates c-myc mRNA turnover. L11 binds to c-myc mRNA at its 3' untranslated region (3'-UTR), the core component of microRNA-induced silencing complex (miRISC) argonaute 2 (Ago2), as well as miR-24, leading to c-myc mRNA reduction. Knockdown of L11 drastically increases the levels and stability of c-myc mRNA. Ablation of Ago2 abrogated the L11-mediated reduction of c-myc mRNA, whereas knockdown of L11 rescued miR-24-mediated c-myc mRNA decay. Interestingly, treatment of cells with the ribosomal stress-inducing agent actinomycin D or 5-fluorouracil significantly decreased the c-myc mRNA levels in an L11- and Ago2-dependent manner. Both treatments enhanced the association of L11 with Ago2, miR-24, and c-myc mRNA. We further show that ribosome-free L11 binds to c-myc mRNA in the cytoplasm and that this binding is enhanced by actinomycin D treatment. Together, our results identify a novel regulatory paradigm wherein L11 plays a critical role in controlling c-myc mRNA turnover via recruiting miRISC in response to ribosomal stress.

Regulation of the MDM2-P53 pathway and tumor growth by PICT1 via nucleolar RPL11

PICT1 (also known as GLTSCR2) is considered a tumor suppressor because it stabilizes phosphatase and tensin homolog (PTEN), but individuals with oligodendrogliomas lacking chromosome 19q13, where PICT1 is located, have better prognoses than other oligodendroglioma patients. To clarify the function of PICT1, we generated Pict1-deficient mice and embryonic stem (ES) cells. Pict1 is a nucleolar protein essential for embryogenesis and ES cell survival. Even without DNA damage, Pict1 loss led to p53-dependent arrest of cell cycle phase G(1) and apoptosis. Pict1-deficient cells accumulated p53, owing to impaired Mdm2 function. Pict1 binds Rpl11, and Rpl11 is released from nucleoli in the absence of Pict1. In Pict1-deficient cells, increased binding of Rpl11 to Mdm2 blocks Mdm2-mediated ubiquitination of p53. In human cancer, individuals whose tumors express less PICT1 have better prognoses. When PICT1 is depleted in tumor cells with intact P53 signaling, the cells grow more slowly and accumulate P53. Thus, PICT1 is a potent regulator of the MDM2-P53 pathway and promotes tumor progression by retaining RPL11 in the nucleolus.

Interplay between ribosomal protein S27a and MDM2 protein in p53 activation in response to ribosomal stress

Ribosomal proteins play a critical role in tightly coordinating p53 signaling with ribosomal biogenesis. Several ribosomal proteins have been shown to induce and activate p53 via inhibition of MDM2. Here, we report that S27a, a small subunit ribosomal protein synthesized as an 80-amino acid ubiquitin C-terminal extension protein (CEP80), functions as a novel regulator of the MDM2-p53 loop. S27a interacts with MDM2 at the central acidic domain of MDM2 and suppresses MDM2-mediated p53 ubiquitination, leading to p53 activation and cell cycle arrest. Knockdown of S27a significantly attenuates the p53 activation in cells in response to treatment with ribosomal stress-inducing agent actinomycin D or 5-fluorouracil. Interestingly, MDM2 in turn ubiquitinates S27a and promotes proteasomal degradation of S27a in response to actinomycin D treatment, thus forming a mutual-regulatory loop. Altogether, our results reveal that S27a plays a non-redundant role in mediating p53 activation in response to ribosomal stress via interplaying with MDM2.

Depletion of ribosomal protein L37 occurs in response to DNA damage and activates p53 through the L11/MDM2 pathway

Perturbation of ribosomal biogenesis has recently emerged as a relevant p53-activating pathway. This pathway can be initiated by depletion of certain ribosomal proteins, which is followed by the binding and inhibition of MDM2 by a different subset of ribosomal proteins that includes L11. Here, we report that depletion of L37 leads to cell cycle arrest in a L11- and p53-dependent manner. DNA damage can initiate ribosomal stress, although little is known about the mechanisms involved. We have found that some genotoxic insults, namely, UV light and cisplatin, lead to proteasomal degradation of L37 in the nucleoplasm and to the ensuing L11-dependent stabilization of p53. Moreover, ectopic L37 overexpression can attenuate the DNA damage response mediated by p53. These results support the concept that DNA damage-induced proteasomal degradation of L37 constitutes a mechanistic link between DNA damage and the ribosomal stress pathway, and is a relevant contributing signaling pathway for the activation of p53 in response to DNA damage.

An ARF-independent c-MYC-activated tumor suppression pathway mediated by ribosomal protein-Mdm2 Interaction

In vitro studies have shown that inhibition of ribosomal biogenesis can activate p53 through ribosomal protein (RP)-mediated suppression of Mdm2 E3 ligase activity. To study the physiological significance of the RP-Mdm2 interaction, we generated mice carrying a cancer-associated cysteine-to-phenylalanine substitution in the zinc finger of Mdm2 that disrupted its binding to RPL5 and RPL11. Mice harboring this mutation, retain normal p53 response to DNA damage, but lack of p53 response to perturbations in ribosome biogenesis. Loss of RP-Mdm2 interaction significantly accelerates Eμ-Myc-induced lymphomagenesis. Furthermore, ribosomal perturbation-induced p53 response does not require tumor suppressor p19ARF. Collectively, our findings establish RP-Mdm2 interaction as a genuine p53 stress-signaling pathway activated by aberrant ribosome biogenesis and essential for safeguarding against oncogenic c-MYC-induced tumorigenesis.

Ubiquitin and ubiquitin-like proteins in the nucleolus: multitasking tools for a ribosome factory

Synthesis of new ribosomes is an essential process upregulated during cell growth and proliferation. Here, we review our current understanding of the role that ubiquitin and ubiquitin-like proteins (UBLs) play in ribosome biogenesis, with a focus on mammalian cells. One important function of the nuclear ubiquitin-proteasome system is to control the supply of ribosomal proteins for the assembly of new ribosomal subunits in the nucleolus. Mutations in ribosomal proteins or ribosome assembly factors, stress, and many anticancer drugs have been shown to disrupt normal ribosome biogenesis, triggering a p53-dependent response. We discuss how p53 can be activated by the aberrant ribosome formation, centering on the current models of the interaction between ribosomal proteins released from the nucleolus and the ubiquitin ligase Mdm2. Recent studies also revealed multiple ubiquitin- and UBL-conjugated forms of nucleolar proteins with largely unknown functions, indicating that many new details about the role of these modifications in the nucleolus await to be discovered.

Overexpression of inosine 5'-monophosphate dehydrogenase type II mediates chemoresistance to human osteosarcoma cells

Background

Chemoresistance is the principal reason for poor survival and disease recurrence in osteosarcoma patients. Inosine 5′-monophosphate dehydrogenase type II (IMPDH2) encodes the rate-limiting enzyme in the de novo guanine nucleotide biosynthesis and has been linked to cell growth, differentiation, and malignant transformation. In a previous study we identified IMPDH2 as an independent prognostic factor and observed frequent IMPDH2 overexpression in osteosarcoma patients with poor response to chemotherapy. The aim of this study was to provide evidence for direct involvement of IMPDH2 in the development of chemoresistance.

Methodology/Principal Findings

Stable cell lines overexpressing IMPDH2 and IMPDH2 knock-down cells were generated using the osteosarcoma cell line Saos-2 as parental cell line. Chemosensitivity, proliferation, and the expression of apoptosis-related proteins were analyzed by flow cytometry, WST-1-assay, and western blot analysis. Overexpression of IMPDH2 in Saos-2 cells induced strong chemoresistance against cisplatin and methotrexate. The observed chemoresistance was mediated at least in part by increased expression of the anti-apoptotic proteins Bcl-2, Mcl-1, and XIAP, reduced activation of caspase-9, and, consequently, reduced cleavage of the caspase substrate PARP. Pharmacological inhibition of IMPDH induced a moderate reduction of cell viability and a strong decrease of cell proliferation, but no increase in chemosensitivity. However, chemoresistant IMPDH2-overexpressing cells could be resensitized by RNA interference-mediated downregulation of IMPDH2.

Conclusions

IMPDH2 is directly involved in the development of chemoresistance in osteosarcoma cells, suggesting that targeting of IMPDH2 by RNAi or more effective pharmacological inhibitors in combination with chemotherapy might be a promising means of overcoming chemoresistance in osteosarcomas with high IMPDH2 expression.

Nucleostemin: Another nucleolar "Twister" of the p53-MDM2 loop

Several nucleolar proteins, such as ARF, ribosomal protein (RP) L5, L11, L23 and S7, have been shown to induce p53 activation by inhibiting MDM2 E3 ligase activity and consequently to trigger cell cycle arrest and/or apoptosis. Our recent study revealed another nucleolar protein called nucleostemin (NS), a nucleolar GTP binding protein, as a novel regulator of the p53-MDM2 feedback loop. However, unlike other known nucleolar regulators of this loop, NS surprisingly plays a dual role, as both up and downregulations of its levels could turn on p53 activity. Here, we try to offer some prospective views for this unusual phenomenon by reconciling previously and recently published studies in the field in hoping to better depict the role of NS in linking the p53 pathway with ribosomal biogenesis during cell growth and proliferation as well as to propose NS as another potential molecular target for anti-cancer drug development.

Perturbation of 60 S ribosomal biogenesis results in ribosomal protein L5- and L11-dependent p53 activation

Ribosomal proteins play an important role in p53 activation in response to nucleolar stress. Multiple ribosomal proteins, including L5, L11, L23, and S7, have been shown to bind to and inhibit MDM2, leading to p53 activation. However, it is not clear whether ribosomal protein regulation of MDM2 is specific to some, but not all ribosomal proteins. Here we show that L29 and L30, two ribosomal proteins from the 60 S ribosomal subunit, do not bind to MDM2 and do not inhibit MDM2-mediated p53 suppression, indicating that the ribosomal protein regulation of the MDM2-p53 feedback loop is specific. Interestingly, direct perturbation of the 60 S ribosomal biogenesis by knocking down either L29 or L30 drastically induced the level and activity of p53, leading to p53-depedent cell cycle arrest. This p53 activation was drastically inhibited by knockdown of L11 or L5. Consistently, knockdown of L29 or L30 enhanced the interaction of MDM2 with L11 and L5 and markedly inhibited MDM2-mediated p53 ubiquitination, suggesting that direct perturbation of 60 S ribosomal biogenesis activates p53 via L11- and L5-mediated MDM2 suppression. Mechanistically, knockdown of L30 or L29 significantly increased the NEDDylation and nuclear retention of L11. Knocking down endogenous NEDD8 suppressed p53 activation induced by knockdown of L30. These results demonstrate that NEDDylation of L11 plays a critical role in mediating p53 activation in response to perturbation of ribosomal biogenesis.

Negative regulation of HDM2 to attenuate p53 degradation by ribosomal protein L26

HDM2 is a p53-specific E3 ubiquitin ligase. Its overexpression leads to excessive inactivation of tumor protein p53, diminishing its tumor suppressor function. HDM2 also affects the cell cycle, apoptosis and tumorigenesis through interacting with other molecules, including several ribosomal proteins. To identify novel HDM2 regulators, we performed a yeast two-hybrid screening using HDM2 as bait. Among the candidates, ribosomal protein L26 (RPL26) was characterized as a novel HDM2-interactor. The interaction between HDM2 and RPL26 was further validated by in vivo and in vitro assays. RPL26 modulates the HDM2–p53 interaction by forming a ternary complex among RPL26, HDM2 and p53, which stabilize p53 through inhibiting the ubiquitin ligase activity of HDM2. The ribosomal stress caused by a low dose of Act D enhances RPL26–HDM2 interaction and activates p53. Overexpression of RPL26 results in activating of p53, inhibits cell proliferation and induces a p53-dependent cell cycle arrest. These results provide a novel regulatory mechanism of RPL26 to activate p53 by inhibiting HDM2.

Ribosome biogenesis surveillance: probing the ribosomal protein-Mdm2-p53 pathway

The dynamic processes of cell growth and cell division remain under constant surveillance. As one of the primary 'gatekeepers' of the cell, p53 has a major role in sensing a variety of stressors to maintain cellular homeostasis. Growth is driven by new protein synthesis, a process that requires robust manufacture of ribosomes in the nucleolus. Ribosome biogenesis is a complex process comprising transcription, modification, and processing of ribosomal RNA, production of ribosomal proteins (RPs) and auxiliary factors, and coordinated assembly of ribonucleoprotein particles to produce mature ribosomes. As the major function of the nucleolus, ribosome biogenesis demands a considerable amount of resources and must be maintained in a coordinated manner to ensure fidelity of the process. Perturbations to many aspects of ribosome biogenesis are thought to contribute to 'nucleolar stress' and trigger a RP-Mdm2-p53 stress response pathway. In this review, we will clarify how disruption to three major components of ribosome biogenesis can trigger nucleolar stress and activate p53, thereby lending support to a RP-Mdm2-p53 ribosome biogenesis surveillance pathway.

Regulation of p53--insights into a complex process

The p53 protein is one of the most important tumor suppressor proteins. Normally, the p53 protein is in a latent state. However, when its activity is required, e.g. upon DNA damage, nucleotide depletion or hypoxia, p53 becomes rapidly activated and initiates transcription of pro-apoptotic and cell cycle arrest-inducing target genes. The activity of p53 is regulated both by protein abundance and by post-translational modifications of pre-existing p53 molecules. In the 30 years of p53 research, a plethora of modifications and interaction partners that modulate p53's abundance and activity have been identified and new ones are continuously discovered. This review will summarize our current knowledge on the regulation of p53 abundance and activity.

Awakening guardian angels: drugging the p53 pathway

Currently, around 11 million people are living with a tumour that contains an inactivating mutation of TP53 (the human gene that encodes p53) and another 11 million have tumours in which the p53 pathway is partially abrogated through the inactivation of other signalling or effector components. The p53 pathway is therefore a prime target for new cancer drug development, and several original approaches to drug discovery that could have wide applications to drug development are being used. In one approach, molecules that activate p53 by blocking protein-protein interactions with MDM2 are in early clinical development. Remarkable progress has also been made in the development of p53-binding molecules that can rescue the function of certain p53 mutants. Finally, cell-based assays are being used to discover compounds that exploit the p53 pathway by either seeking targets and compounds that show synthetic lethality with TP53 mutations or by looking for non-genotoxic activators of the p53 response.

Signaling to p53: ribosomal proteins find their way

Inherently disparate cell growth and division, which are intimately coupled through a delicate network of intracellular and extracellular signaling, require ribosomal biogenesis. A number of events imparting instability to ribosomal biogenesis can cause nucleolar stress. In response to this stress, several ribosomal proteins bind to MDM2 and block MDM2-mediated p53 ubiquitination and degradation, resulting in p53-dependent cell cycle arrest. By doing so, the ribosomal proteins play a crucial role in connecting deregulated cell growth with inhibition of cell division. The ribosomal protein-MDM2-p53 signaling pathway provides a molecular switch that may constitute a surveillance network monitoring the integrity of ribosomal biogenesis.

Recent advances in validating MDM2 as a cancer target

The MDM2 oncogene is overexpressed in various human cancers. Its expression correlates with the phenotypes of high-grade, late-stage, and more resistant tumors. The auto-regulatory loop between MDM2 and the tumor suppressor p53 has long been considered the epitome of a rational target for cancer therapy. As such, many novel agents have been generated to interfere with the interaction of the two proteins, which results in the activation of p53. Among these agents are several small molecule inhibitors synthesized based upon the crystal structures of the MDM2-p53 complex. With use of high-throughput screening, several specific and effective agents for inhibition of the protein-protein interaction were discovered. Recent investigations, however, have demonstrated that many proteins regulate the MDM2-p53 interaction, and that MDM2 may have p53-independent oncogenic functions. In order for novel MDM2 inhibitors to be translated to the clinic, it is necessary to obtain a better understanding of the regulation of MDM2 and of the MDM2-p53 interaction. In particular, the implications of various interactions between certain regulator(s) and MDM2/p53 under different circumstances need to be elucidated to determine which pathway(s) represent the best targets for therapy. Targeting both MDM2 itself and regulators of MDM2 and the MDM2-p53 interaction, or use of MDM2 inhibitors in combination with conventional treatments, may improve prospects for tumor eradication.

Regulon-specific control of transcription elongation across the yeast genome

Transcription elongation by RNA polymerase II was often considered an invariant non-regulated process. However, genome-wide studies have shown that transcriptional pausing during elongation is a frequent phenomenon in tightly-regulated metazoan genes. Using a combination of ChIP-on-chip and genomic run-on approaches, we found that the proportion of transcriptionally active RNA polymerase II (active versus total) present throughout the yeast genome is characteristic of some functional gene classes, like those related to ribosomes and mitochondria. This proportion also responds to regulatory stimuli mediated by protein kinase A and, in relation to cytosolic ribosomal-protein genes, it is mediated by the silencing domain of Rap1. We found that this inactive form of RNA polymerase II, which accumulates along the full length of ribosomal protein genes, is phosphorylated in the Ser5 residue of the CTD, but is hypophosphorylated in Ser2. Using the same experimental approach, we show that the in vivo–depletion of FACT, a chromatin-related elongation factor, also produces a regulon-specific effect on the expression of the yeast genome. This work demonstrates that the regulation of transcription elongation is a widespread, gene class–dependent phenomenon that also affects housekeeping genes.

Transcription of DNA–encoded information into RNA is the first step in gene regulation. RNA polymerases initiate transcription at the promoter region and elongate the transcripts traveling throughout the gene until reaching the termination sequences. Classical models of transcriptional regulation were focused on the initiation step, but there is increasing evidence for gene regulation after initiation. We have investigated the importance of elongation in gene regulation using the yeast Saccharomyces cerevisiae, one of the main experimental systems in modern biology. By comparing the genomic distribution of RNA polymerase molecules with the actual transcriptional signal across the genome, we have detected that many genes are regulated at the elongation level. We show that yeast cells use this step to modulate the expression of several groups of genes, which have to be simultaneously regulated in a very coordinated manner. Genes encoding essential functions, like those related to protein synthesis and respiration, change their transcriptional activities in response to environmental stimuli, without changing in the same extension the amount of RNA polymerase that is physically associated to them. We also show that this kind of regulation, in spite of taking place during the elongation step, can be mediated by promoter-binding transcription factors.

How common are extraribosomal functions of ribosomal proteins?

Ribosomal proteins are ubiquitous, abundant, and RNA binding: prime candidates for recruitment to extraribosomal functions. Indeed, they participate in balancing the synthesis of the RNA and protein components of the ribosome itself. An exciting new story is that ribosomal proteins are sentinels for the self-evaluation of cellular health. Perturbation of ribosome synthesis frees ribosomal proteins to interface with the p53 system, leading to cell-cycle arrest or to apoptosis. Yet in only a few cases can we clearly identify the recruitment of ribosomal proteins for other extraribosomal functions. Is this due to a lack of imaginative evolution by cells and viruses, or to a lack of imaginative experiments by molecular biologists?

Loss of ribosomal protein L11 affects zebrafish embryonic development through a p53-dependent apoptotic response

Ribosome is responsible for protein synthesis in all organisms and ribosomal proteins (RPs) play important roles in the formation of a functional ribosome. L11 was recently shown to regulate p53 activity through a direct binding with MDM2 and abrogating the MDM2-induced p53 degradation in response to ribosomal stress. However, the studies were performed in cell lines and the significance of this tumor suppressor function of L11 has yet to be explored in animal models. To investigate the effects of the deletion of L11 and its physiological relevance to p53 activity, we knocked down the rpl11 gene in zebrafish and analyzed the p53 response. Contrary to the cell line-based results, our data indicate that an L11 deficiency in a model organism activates the p53 pathway. The L11-deficient embryos (morphants) displayed developmental abnormalities primarily in the brain, leading to embryonic lethality within 6-7 days post fertilization. Extensive apoptosis was observed in the head region of the morphants, thus correlating the morphological defects with apparent cell death. A decrease in total abundance of genes involved in neural patterning of the brain was observed in the morphants, suggesting a reduction in neural progenitor cells. Upregulation of the genes involved in the p53 pathway were observed in the morphants. Simultaneous knockdown of the p53 gene rescued the developmental defects and apoptosis in the morphants. These results suggest that ribosomal dysfunction due to the loss of L11 activates a p53-dependent checkpoint response to prevent improper embryonic development.

Crosstalk between c-Myc and ribosome in ribosomal biogenesis and cancer

Protein production is driven by protein translation and relies on ribosomal biogenesis, globally essential for cell growth, proliferation, and animal development. Deregulation of these sophisticated cellular processes leads to abnormal homeostasis and carcinogenesis. Thus, their tight regulation is vitally important for a cell to warrant normal growth and proliferation. One newly identified key regulator for ribosomal biogenesis and translation is the oncoprotein c-Myc, whose aberrantly excessive level and activity are highly associated with human cancers, too. Recently, we have shown that ribosomal protein L11 functions as a feedback regulator of c-Myc. Hence, in this review, we will provide some prospects on the interplay between c-Myc and ribosomal proteins during ribosomal biogenesis and discuss its implications in cancer.

RYBP stabilizes p53 by modulating MDM2

The mouse double minute 2 (MDM2)-p53 interaction regulates the activity of p53 and is a potential target for human cancer therapy. Here, we report that RYBP (RING1- and YY1-binding protein), a member of the polycomb group (PcG), interacts with MDM2 and decreases MDM2-mediated p53 ubiquitination, leading to stabilization of p53 and an increase in p53 activity. RYBP induces cell-cycle arrest and is involved in the p53 response to DNA damage. Expression of RYBP is decreased in human cancer tissues compared with adjacent normal tissues. These results show that RYBP is a new regulator of the MDM2-p53 loop and that it has tumour suppressor activity.