Stress-dependent nucleolin mobilization mediated by p53-nucleolin complex formation

Functions of the histone chaperone nucleolin in diseases

Alteration of nuclear morphology is often used by pathologist as diagnostic marker for malignancies like cancer. In particular, the staining of cells by the silver staining methods (AgNOR) has been proved to be an important tool for predicting the clinical outcome of some cancer diseases. Two major argyrophilic proteins responsible for the strong staining of cells in interphase are the nucleophosmin (B23) and the nucleolin (C23) nucleolar proteins. Interestingly these two proteins have been described as chromatin associated proteins with histone chaperone activities and also as proteins able to regulate chromatin transcription. Nucleolin seems to be over-expressed in highly proliferative cells and is involved in many aspect of gene expression: chromatin remodeling, DNA recombination and replication, RNA transcription by RNA polymerase I and II, rRNA processing, mRNA stabilisation, cytokinesis and apoptosis. Interestingly, nucleolin is also found on the cell surface in a wide range of cancer cells, a property which is being used as a marker for the diagnosis of cancer and for the development of anti-cancer drugs to inhibit proliferation of cancer cells. In addition to its implication in cancer, nucleolin has been described not only as a marker or as a protein being involved in many diseases like viral infections, autoimmune diseases, Alzheimer's disease pathology but also in drug resistance. In this review we will focus on the chromatin associated functions of nucleolin and discuss the functions of nucleolin or its use as diagnostic marker and as a target for therapy

Nucleolin inhibits Hdm2 by multiple pathways leading to p53 stabilization

Nucleolin is a c-Myc-induced gene product with defined roles in ribosomal RNA processing and the inhibition of chromosomal DNA replication following stress. Here we find that changes in nucleolin protein levels in unstressed cells cause parallel changes in the amount of p53 protein. Alterations in p53 levels arise from nucleolin binding to the p53 antagonist Hdm2, resulting in the inhibition of both p53 ubiquitination and Hdm2 auto-ubiquitination. Nucleolin does not alter p53 ubiquitination by human papillomavirus E6, indicating that the effect is specific for Hdm2. Although the inhibition of ligase activity would be expected to stabilize Hdm2, we instead find that nucleolin also reduces Hdm2 protein levels, demonstrating that nucleolin inhibits Hdm2 using multiple mechanisms. Increases in nucleolin levels in unstressed cells led to higher expression of p21(cip1/waf1), a reduced rate of cellular proliferation, and an increase in apoptosis. Thus, nucleolin has a number of properties in common with the tumor suppressor ARF (alternate reading frame). We propose that nucleolin, like ARF, responds to hyperproliferative signals by upregulation of p53 through Hdm2 inhibition.

Upon intracellular processing, the C-terminal death domain-containing fragment of the p53-inducible PIDD/LRDD protein translocates to the nucleoli and interacts with nucleolin

The p53-inducible and death domain-containing PIDD/LRDD protein has been described as an adaptor protein, which forms large protein complexes with RAIDD, another death domain-containing protein, leading to recruitment, and activation of the initiator caspase-2, and p53-mediated apoptosis. Here, we describe in further detail the proteolytic processing of PIDD/LRDD that occurs in healthy cells before induction of apoptosis. We could demonstrate that the C-terminal fragment containing the PIDD death domain shuttles into the nucleoli. This translocation is mediated by or leads to the interaction of the PIDD death domain with nucleolin, a protein important for rRNA processing within nucleoli and possibly involved in the DNA damage response. Ectopically expressed LRDD and endogenous nucleolin co-localized within the nucleoli, and overexpression of both full-length LRDD and the LRDD death domain sensitized cells for UV-induced apoptosis. When expressed alone, the PIDD/LRDD death domain tended to form large filamentous structures resembling so-called death filaments. The functional consequences of the identified PIDD/nucleolin interaction remain to be elucidated, but may be related to a recently discovered new role for PIDD in the activation of NF-kappaB upon genotoxic stress.

Nucleolin is involved in interferon regulatory factor-2-dependent transcriptional activation

We have previously shown that interferon regulatory factor-2 (IRF-2) is acetylated in a cell growth-dependent manner, which enables it to contribute to the transcription of cell growth-regulated promoters. To clarify the function of acetylation of IRF-2, we investigated the proteins that associate with acetylated IRF-2. In 293T cells, the transfection of p300/CBP-associated factor (PCAF) enhanced the acetylation of IRF-2. In cells transfected with both IRF-2 and PCAF, IRF-2 associated with endogenous nucleolin, while in contrast, minimal association was observed when IRF-2 was transfected with a PCAF histone acetyl transferase (HAT) deletion mutant. In a pull-down experiment using stable transfectants, acetylation-defective mutant IRF-2 (IRF-2K75R) recruited nucleolin to a much lesser extent than wild-type IRF-2, suggesting that nucleolin preferentially associates with acetylated IRF-2. Nucleolin in the presence of PCAF enhanced IRF-2-dependent H4 promoter activity in NIH3T3 cells. Nucleolin knock-down using siRNA reduced the IRF-2/PCAF-mediated promoter activity. Chromatin immunoprecipitation analysis indicated that PCAF transfection increased nucleolin binding to IRF-2 bound to the H4 promoter. We conclude that nucleolin is recruited to acetylated IRF-2, thereby contributing to gene regulation crucial for the control of cell growth.

Changes in nucleolar morphology and proteins during infection with the coronavirus infectious bronchitis virus

The nucleolus is a dynamic subnuclear structure involved in ribosome subunit biogenesis, cell cycle control and mediating responses to cell stress, among other functions. While many different viruses target proteins to the nucleolus and recruit nucleolar proteins to facilitate virus replication, the effect of infection on the nucleolus in terms of morphology and protein content is unknown. Previously we have shown that the coronavirus nucleocapsid protein will localize to the nucleolus. In this study, using the avian infectious bronchitis coronavirus, we have shown that virus infection results in a number of changes to the nucleolus both in terms of gross morphology and protein content. Using confocal microscopy coupled with fluorescent labelled nucleolar marker proteins we observed changes in the morphology of the nucleolus including an enlarged fibrillar centre. We found that the tumour suppressor protein, p53, which localizes normally to the nucleus and nucleolus, was redistributed predominately to the cytoplasm.

Cell cycle-controlled interaction of nucleolin with the retinoblastoma protein and cancerous cell transformation

Retinoblastoma protein (Rb) is a multifunctional tumor suppressor, frequently inactivated in certain types of human cancer. Nucleolin is an abundant multifunctional phosphoprotein of proliferating and cancerous cells, recently identified as cell cycle-regulated transcription activator, controlling expression of human papillomavirus type 18 (HPV18) oncogenes in cervical cancer. Here we find that nucleolin is associated with Rb in intact cells in the G1 phase of the cell cycle, and the complex formation is mediated by the growth-inhibitory domain of Rb. Association with Rb inhibits the DNA binding function of nucleolin and in consequence the interaction of nucleolin with the HPV18 enhancer, resulting in Rb-mediated repression of the HPV18 oncogenes. The intracellular distribution of nucleolin in epithelial cells is Rb-dependent, and an altered nucleolin localization in human cancerous tissues results from a loss of Rb. Our findings suggest that deregulated nucleolin activity due to a loss of Rb contributes to tumor development in malignant diseases, thus providing further insights into the molecular network for the Rb-mediated tumor suppression.

Nucleolar localization of aprataxin is dependent on interaction with nucleolin and on active ribosomal DNA transcription

The APTX gene, mutated in patients with the neurological disorder ataxia with oculomotor apraxia type 1 (AOA1), encodes a novel protein aprataxin. We describe here, the interaction and interdependence between aprataxin and several nucleolar proteins, including nucleolin, nucleophosmin and upstream binding factor-1 (UBF-1), involved in ribosomal RNA (rRNA) synthesis and cellular stress signalling. Interaction between aprataxin and nucleolin occurred through their respective N-terminal regions. In AOA1 cells lacking aprataxin, the stability of nucleolin was significantly reduced. On the other hand, down-regulation of nucleolin by RNA interference did not affect aprataxin protein levels but abolished its nucleolar localization suggesting that the interaction with nucleolin is involved in its nucleolar targeting. GFP-aprataxin fusion protein co-localized with nucleolin, nucleophosmin and UBF-1 in nucleoli and inhibition of ribosomal DNA transcription altered the distribution of aprataxin in the nucleolus, suggesting that the nature of the nucleolar localization of aprataxin is also dependent on ongoing rRNA synthesis. In vivo rRNA synthesis analysis showed only a minor decrease in AOA1 cells when compared with controls cells. These results demonstrate a cross-dependence between aprataxin and nucleolin in the nucleolus and while aprataxin does not appear to be directly involved in rRNA synthesis its nucleolar localization is dependent on this synthesis.

Immunodetection of human telomerase reverse-transcriptase (hTERT) re-appraised: nucleolin and telomerase cross paths

The involvement of telomerase in cellular immortalization and senescence has often been assessed by means of telomerase expression at the RNA level and quantification of telomerase activity by the telomeric repeat amplification protocol assay. However, these methods either neglected the existence of various telomerase splice variants, or ignored the nonconventional functions of telomerase independent of its ability to elongate and maintain telomere length. Immunodetection of telomerase is now being recognized as a necessary approach to precisely elucidate its roles in oncogenesis and senescence. A few antibodies directed against the catalytic subunit of the human telomerase (hTERT) are currently used but their specificity is not always demonstrated. A survey of the literature showed inconsistencies and led us to comparatively re-evaluate the most frequently used antibodies. Surprisingly, mass spectrometry, two-dimensional gel analysis and immunofluorescent experiments revealed that the most frequently used hTERT immunoprobe, a mouse monoclonal antibody that was claimed to be directed against an hTERT protein epitope, in fact recognizes nucleolin rather than telomerase. Our findings have interesting implications regarding the biology of nucleolin and telomerase in the context of pathophysiological investigations recently carried out.

DNA damage repair: anytime, anywhere?

Regulation of the DNA damage response is tightly connected to transcription and replication. These DNA transacting processes share common factors and use similar strategies to exert their function. However, unlike replication and transcription, DNA repair systems may be required anywhere, and at any time, whenever DNA damage occurs in the cell nucleus. This raises questions concerning the spatiotemporal organization of genome caretaking. Currently, quantitative live cell imaging techniques combined with methods to induce local DNA damage in a small region of the nucleus are contributing substantially to unravelling the molecular mechanisms underlying the cellular response to DNA damage.

A novel interaction [corrected] of nucleolin with Rad51

Nucleolin associates with various DNA repair, recombination, and replication proteins, and possesses DNA helicase, strand annealing, and strand pairing activities. Examination of nuclear protein extracts from human somatic cells revealed that nucleolin and Rad51 co-immunoprecipitate. Furthermore, purified recombinant Rad51 associates with in vitro transcribed and translated nucleolin. Electroporation-mediated introduction of anti-nucleolin antibody resulted in a 10- to 20-fold reduction in intra-plasmid homologous recombination activity in human fibrosarcoma cells. Additionally, introduction of anti-nucleolin antibody sensitized cells to death induced by the topoisomerase II inhibitor, amsacrine. Introduction of anti-Rad51 antibody also reduced intra-plasmid homologous recombination activity and induced hypersensitivity to amsacrine-induced cell death. Co-introduction of anti-nucleolin and anti-Rad51 antibodies did not produce additive effects on homologous recombination or on cellular sensitivity to amsacrine. The association of the two proteins raises the intriguing possibility that nucleolin binding to Rad51 may function to regulate homologous recombinational repair of chromosomal DNA.

How the Nucleolar Sequestration of p53 Protein or Its Interplayers Contributes to Its (Re)-Activation

The tumor suppressor p53 is a short-lived protein that under normal conditions is reduced to a barely detectable level. The stability of p53 protein is primarily regulated in normal non-transformed cells by two interplayers: Mdm2 and p14(ARF). Relocation of p53, Mdm2, and p14(ARF) to the nucleolus seems to regulate, at least partially, the steady-state of p53. Moreover, there are alternative pathways of the regulation of p53 stability in unstressed cells. Jun-N(amino)-terminal kinase (JNK) and poly(ADP-ribose) polymerase-1 (PARP-1) are involved in the regulation of the steady-state of wild-type (wt) p53 protein. However, in most human cervical carcinomas, which express the high-risk human papilloma viruses (HPVs) E6 protein, a complete switch from Mdm2 to HPV E6-mediated degradation of p53 occurs. Virally encoded E6 protein utilizes the cellular ubiquitin-protein ligase termed E6-associated protein (E6-AP) to target p53 protein for proteolytic degradation. We recently addressed the question of whether p53 protein can be generally reactivated by chemotherapy in HeLa cells despite the E6 activity. We observed an increase of cellular p53 after cisplatin (CP) treatment. p53 protein accumulated preferentially in the nucleoli. We checked the cellular level of E6 during CP therapy. Six hours after application of CP the expression of E6 protein was markedly reduced. This coincided with the increase of cellular p53 level and preceded the nucleolar accumulation of p53 protein, thereby indicating that repression of virally coded E6 protein by CP contributes to the restoration of p53 expression.

Induction of CYP1A mRNA in Carp (Cyprinus carpio) from the Kalamazoo River polychlorinated biphenyl-contaminated superfund site and in a laboratory study

The Kalamazoo River Superfund site in Michigan is contaminated with polychlorinated biphenyls (PCBs), which were heavily discharged into the river from several paper companies as part of the deinking process in the 1950s through 1970s. We characterized biomarkers of chronic PCB exposure in a resident fish population using real-time reverse transcriptase-polymerase chain reaction to examine mRNA expression levels of multiple genes in carp (Cyprinus carpio) liver from PCB contaminated and reference sites in the Kalamazoo River. We also measured these same genes in juvenile carp exposed to dietary PCBs for 4 months. Kalamazoo River carp had significantly increased levels of cytochrome P450 1A (CYP1A) mRNA as did carp fed PCBs in the laboratory. No significant mRNA upregulation occurred in the specific oxidative stress genes (gamma-glutamylcysteine synthetase and magnesium superoxide dismutase) and metabolic genes (phosphoenolpyruvate carboxykinase and nucleolin) examined. These data are consistent with the idea that carp from the Kalamazoo River Superfund Site are responding to PCB exposure via upregulation of CYP1A independent of activation of the oxidative stress response genes normally thought to be co-regulated with CYP1A.

Regulation of p53 translation and induction after DNA damage by ribosomal protein L26 and nucleolin

Increases in p53 protein levels after DNA damage have largely been attributed to an increase in the half-life of p53 protein. Here we demonstrate that increased translation of p53 mRNA is also a critical step in the induction of p53 protein in irradiated cells. Ribosomal protein L26 (RPL26) and nucleolin were found to bind to the 5' untranslated region (UTR) of p53 mRNA and to control p53 translation and induction after DNA damage. RPL26 preferentially binds to the 5'UTR after DNA damage, and its overexpression enhances association of p53 mRNA with heavier polysomes, increases the rate of p53 translation, induces G1 cell-cycle arrest, and augments irradiation-induced apoptosis. Opposite effects were seen when RPL26 expression was inhibited. In contrast, nucleolin overexpression suppresses p53 translation and induction after DNA damage, whereas nucleolin downregulation promotes p53 expression. These findings demonstrate the importance of increased translation of p53 in DNA-damage responses and suggest critical roles for RPL26 and nucleolin in affecting p53 induction.

A 23-amino acid motif spanning the basic domain targets zebrafish myogenic regulatory factor myf5 into nucleolus

Myf5 is a nuclear protein and one of the basic helix-loop-helix (bHLH) myogenic factors that play an important role in muscle specification and differentiation. The motif responsible for the nuclear translocation of Myf5 was unknown. Using on-line monitoring of EGFP (enhanced green fluorescent protein)-tagged zebrafish Myf5 translocation, we demonstrated that Myf5-EGFP protein resided in the nucleoplasm and nucleolus of zebrafish fibroblast cell lines (ZEM2S and ZF4), mammalian nonmuscle cell line (COS1), and muscle cell lines (RD and C2C12). In contrast, zebrafish MyoD-EGFP was localized in the nucleus but did not condense in the nucleolus. Using indirect immunofluorescent staining, we determined that zebrafish Myf5 was colocalized with nucleophosmin/B23, a nucleolus protein. Deletion analysis revealed that amino acid residues 60 to 82 (60KRKASTVDRRRAATMRERRRLKK82) of Myf5 were sufficient and necessary for nucleolus targeting. A GST pulldown assay followed by Western analysis showed that nucleolin/C23 could be pulled down specifically by GST-Myf5, but not by GST-MyoD. Based on these findings, we propose that the distinct functions of Myf5 and MyoD may result from their differential binding affinity to nucleolin/C23.

Cellular UV damage responses--functions of tumor suppressor p53

DNA damage, provoked by ultraviolet (UV) radiation, evokes a cellular damage response composed of activation of stress signaling and DNA checkpoint functions. These are translated to responses of replicative arrest, damage repair, and apoptosis aimed at cellular recovery from the damage. p53 tumor suppressor is a central stress response protein, activated by multiple endogenous and environmental insults, including UV radiation. The significance of p53 in the DNA damage responses has frequently been reviewed in the context of ionizing radiation or other double strand break (DSB)-inducing agents. Despite partly similar patterns, the molecular events following UV radiation are, however, distinct from the responses induced by DSBs and are profoundly coupled with transcriptional stress. These are illustrated, e.g., by the UV damage-specific translocations of Mdm2, promyelocytic leukemia protein, and nucleophosmin and their interactions with p53. In this review, we discuss UV damage-provoked cellular responses and the functions of p53 in damage recovery and cell death.

Proteomic evidence for roles for nucleolin and poly[ADP-ribosyl] transferase in drug resistance

One-hundred twenty-four proteins have been identified in the soluble nuclear protein mixture from MCF-7 human breast cancer cells, of which more than 90% are classically categorized as nuclear proteins. Proteins were also studied from three drug resistant MDF-7 lines, selected previously from the same parent line by exposure to etoposide, to mitoxantrone, or to adriamycin in the presence of verapamil. Both quantitative gel comparisons and stable isotope labeling were used to identify a total of fourteen proteins whose abundances are altered by more than 2-fold in the three resistant lines. Several cytoskeleton proteins, cytokeratin 8, cytokeratin 19, septin 2, and alpha tropomyosin, are decreased in common across the three resistant cell lines. PARP-l (poly[ADP-ribosyl]transefrase or connexion) is found to be less abundant in all three resistant lines. Nucleolin is more abundant in lines resistant to etoposide and mitoxantrone, while the mitotic checkpoint protein BUB 3 is more abundant in the line resistant to adriamycin/verapamil.

A Chromatin-associated and Transcriptionally Inactive p53-Mdm2 Complex Occurs in mdm2 SNP309 Homozygous Cells

In cancer cells, the function of the tumor suppressor protein p53 is usually blocked. Impairment of the p53 pathway results in tumor cells with endogenous overexpression of Mdm2 via a naturally occurring single nucleotide polymorphism (SNP) in the mdm2 gene at position 309. Here we report that in mdm2 SNP309 cells, inactivation of p53 results in a chromatin-associated Mdm2-p53 complex without clearance of p53 by protein degradation. Nuclear accumulation of p53 protein in mdm2 SNP309 cells results after 6 h of camptothecin, etoposide, or mitomycin C treatment, with the p53 protein phosphorylated at Ser15. Chromatin immunoprecipitation demonstrated p53 and Mdm2 bound to p53 responsive elements. Interestingly, although the p53 protein was able to bind to DNA, quantitative PCR showed compromised transcription of endogenous target genes. Additionally, exogenously introduced p53 was incapable of activating transcription from p53 responsive elements in SNP309 cells, confirming the trans-acting nature of the inhibitor. Inhibition of Mdm2 by siRNA resulted in transcriptional activation of these p53 targets. Our data suggest that overproduction of Mdm2, resulting from a naturally occurring SNP, inhibits chromatin-bound p53 from activating the transcription of its target genes.

The nucleolus as a stress sensor: JNK2 inactivates the transcription factor TIF-IA and down-regulates rRNA synthesis

Cells respond to a variety of extracellular and intracellular forms of stress by down-regulating rRNA synthesis. We have investigated the mechanism underlying stress-dependent inhibition of RNA polymerase I (Pol I) transcription and show that the Pol I-specific transcription factor TIF-IA is inactivated upon stress. Inactivation is due to phosphorylation of TIF-IA by c-Jun N-terminal kinase (JNK) at a single threonine residue (Thr 200). Phosphorylation at Thr 200 impairs the interaction of TIF-IA with Pol I and the TBP-containing factor TIF-IB/SL1, thereby abrogating initiation complex formation. Moreover, TIF-IA is translocated from the nucleolus into the nucleoplasm. Substitution of Thr 200 by valine as well as knock-out of Jnk2 prevent inactivation and translocation of TIF-IA, leading to stress-resistance of Pol I transcription. Our data identify TIF-IA as a downstream target of the JNK pathway and suggest a critical role of JNK2 to protect rRNA synthesis against the harmful consequences of cellular stress.

Novel checkpoint response to genotoxic stress mediated by nucleolin-replication protein a complex formation

Human replication protein A (RPA), the primary single-stranded DNA-binding protein, was previously found to be inhibited after heat shock by complex formation with nucleolin. Here we show that nucleolin-RPA complex formation is stimulated after genotoxic stresses such as treatment with camptothecin or exposure to ionizing radiation. Complex formation in vitro and in vivo requires a 63-residue glycine-arginine-rich (GAR) domain located at the extreme C terminus of nucleolin, with this domain sufficient to inhibit DNA replication in vitro. Fluorescence resonance energy transfer studies demonstrate that the nucleolin-RPA interaction after stress occurs both in the nucleoplasm and in the nucleolus. Expression of the GAR domain or a nucleolin mutant (TM) with a constitutive interaction with RPA is sufficient to inhibit entry into S phase. Increasing cellular RPA levels by overexpression of the RPA2 subunit minimizes the inhibitory effects of nucleolin GAR or TM expression on chromosomal DNA replication. The arrest is independent of p53 activation by ATM or ATR and does not involve heightened expression of p21. Our data reveal a novel cellular mechanism that represses genomic replication in response to genotoxic stress by inhibition of an essential DNA replication factor.

Nucleophosmin (B23) targets ARF to nucleoli and inhibits its function

The ARF tumor suppressor is a nucleolar protein that activates p53-dependent checkpoints by binding Mdm2, a p53 antagonist. Despite persuasive evidence that ARF can bind and inactivate Mdm2 in the nucleoplasm, the prevailing view is that ARF exerts its growth-inhibitory activities from within the nucleolus. We suggest ARF primarily functions outside the nucleolus and provide evidence that it is sequestered and held inactive in that compartment by a nucleolar phosphoprotein, nucleophosmin (NPM). Most cellular ARF is bound to NPM regardless of whether cells are proliferating or growth arrested, indicating that ARF-NPM association does not correlate with growth suppression. Notably, ARF binds NPM through the same domains that mediate nucleolar localization and Mdm2 binding, suggesting that NPM could control ARF localization and compete with Mdm2 for ARF association. Indeed, NPM knockdown markedly enhanced ARF-Mdm2 association and diminished ARF nucleolar localization. Those events correlated with greater ARF-mediated growth suppression and p53 activation. Conversely, NPM overexpression antagonized ARF function while increasing its nucleolar localization. These data suggest that NPM inhibits ARF's p53-dependent activity by targeting it to nucleoli and impairing ARF-Mdm2 association.

Nuclear bodies and compartments: functional roles and cellular signalling in health and disease

There is much interest in recent years in the possible role of different nuclear compartments and subnuclear domains in the regulation of gene expression, signalling, and cellular functions. The nucleus contains inositol phosphates, actin and actin-binding proteins and myosin isoforms, multiple protein kinases and phosphatases targeting Cdk-1 and Cdk-2, MAPK/SAPK, and Src-related kinases and their substrates, suggesting the implication of several signalling pathways in the intranuclear organization and function of nuclear bodies (NBs). NBs include the well-characterized Cajal bodies (CBs; or coiled bodies), the nucleolus, perinucleolar and perichromatin regions, additional NBs best illustrated by the promyelocytic leukemia nuclear bodies [PML-NBs, also named PML oncogenic dots (PODs), ND10, Kr-bodies] and similar intranuclear foci containing multi-molecular complexes with major role in DNA replication, surveillance, and repair, as well as messenger RNA and ribosomal RNA synthesis and assembly. Chromatin modifying proteins, such as the CBP acetyltransferase and type I histone deacetylase, accumulate at PML-NBs. PML-NBs and Cajal bodies are very dynamic and mobile within the nuclear space and are regulated by cellular stress (heat shock, apoptosis, senescence, heavy metal exposure, viral infection, and DNA damage responses). NBs strongly interact, using signalling mechanisms for the directional and ordered traffic of essential molecular components. NBs organize the delivery and storage of essential RNAs and proteins that play a role in transcription, pre-mRNA biosynthesis and splicing, and the sequestration and/or degradation of regulatory proteins, such as heterogenous nuclear ribonuclear proteins (hnRNPs), p53, Rb1, CBP, STAT3, and others. The objective of this review is to summarize some aspects of these nuclear structures/bodies/domains, including their proposed roles in cellular signalling and in human diseases, mainly neurodegenerative disorders and cancer.

Nucleolar protein NPM interacts with HDM2 and protects tumor suppressor protein p53 from HDM2-mediated degradation

Nucleophosmin (NPM, B23) is an abundant nucleolar phosphoprotein involved in ribosome biogenesis, and interacts with tumor suppressor proteins p53 and Rb. Here we show that NPM is a UV damage response protein that undergoes nucleoplasmic redistribution and regulates p53 and HDM2 levels and their interaction. By utilizing RNAi approaches and analyses of endogenous and ectopically expressed proteins, we demonstrate that NPM binds HDM2 and acts as a negative regulator of p53-HDM2 interaction. Viral stress, enforced by expression of Kaposi's sarcoma virus K cyclin, causes NPM redistribution, K cyclin-NPM association, and p53 stabilization by dissociation of HDM2-p53 complexes. The results demonstrate novel associations of HDM2 and K cyclin with NPM and implicate NPM as a crucial controller of p53 through inhibition of HDM2.

Disruption of the nucleolus mediates stabilization of p53 in response to DNA damage and other stresses

p53 protects against cancer through its capacity to induce cell cycle arrest or apoptosis under a large variety of cellular stresses. It is not known how such diversity of signals can be integrated by a single molecule. However, the literature reveals that a common denominator in all p53-inducing stresses is nucleolar disruption. We thus postulated that the impairment of nucleolar function might stabilize p53 by preventing its degradation. Using micropore irradiation, we demonstrate that large amounts of nuclear DNA damage fail to stabilize p53 unless the nucleolus is also disrupted. Forcing nucleolar disruption by anti-upstream binding factor (UBF) microinjection (in the absence of DNA damage) also causes p53 stabilization. We propose that the nucleolus is a stress sensor responsible for maintenance of low levels of p53, which are automatically elevated as soon as nucleolar function is impaired in response to stress. Our model integrates all known p53-inducing agents and also explains cell cycle-related variations in p53 levels which correlate with established phases of nucleolar assembly/disassembly through the cell cycle.

S100C/A11 is a key mediator of Ca(2+)-induced growth inhibition of human epidermal keratinocytes

An increase in extracellular Ca2+ induces growth arrest and differentiation of human keratinocytes in culture. We examined possible involvement of S100C/A11 in this growth regulation. On exposure of the cells to high Ca2+, S100C/A11 was specifically phosphorylated at 10Thr and 94Ser. Phosphorylation facilitated the binding of S100C/A11 to nucleolin, resulting in nuclear translocation of S100C/A11. In nuclei, S100C/A11 liberated Sp1/3 from nucleolin. The resulting free Sp1/3 transcriptionally activated p21CIP1/WAF1, a representative negative regulator of cell growth. Introduction of anti-S100C/A11 antibody into the cells largely abolished the growth inhibition induced by Ca2+ and the induction of p21CIP1/WAF1. In the human epidermis, S100C/A11 was detected in nuclei of differentiating cells in the suprabasal layers, but not in nuclei of proliferating cells in the basal layer. These results indicate that S100C/A11 is a key mediator of the Ca(2+)-induced growth inhibition of human keratinocytes in culture, and that it may be possibly involved in the growth regulation in vivo as well.

Perturbations of chromatin structure in human genetic disease: recent advances

Gene expression studies in mammals and simpler eukaryotes have highlighted the central role that chromatin structure and modifications play in both the activation and repression of transcription. Aberrant chromatin structure can cause human genetic disease. Here we discuss recent progress in understanding the molecular mechanisms that underlie three human genetic diseases linked to perturbations of chromatin structure: ICF syndrome, facioscapulohumeral muscular dystrophy and a case of alpha-thalassaemia.