Phosphorylation-dependent migration of retinoblastoma protein into the nucleolus triggered by binding to nucleophosmin/B23

Nucleophosmin: A Nucleolar Phosphoprotein Orchestrating Cellular Stress Responses

Nucleophosmin (NPM1) is a key nucleolar protein released from the nucleolus in response to stress stimuli. NPM1 functions as a stress regulator with nucleic acid and protein chaperone activities, rapidly shuttling between the nucleus and cytoplasm. NPM1 is ubiquitously expressed in tissues and can be found in the nucleolus, nucleoplasm, cytoplasm, and extracellular environment. It plays a central role in various biological processes such as ribosome biogenesis, cell cycle regulation, cell proliferation, DNA damage repair, and apoptosis. In addition, it is highly expressed in cancer cells and solid tumors, and its mutation is a major cause of acute myeloid leukemia (AML). This review focuses on NPM1's structural features, functional diversity, subcellular distribution, and role in stress modulation.

Linker Histone H1.2 Directs Genome-wide Chromatin Association of the Retinoblastoma Tumor Suppressor Protein and Facilitates Its Function

The retinoblastoma tumor suppressor protein pRb is a master regulator of cellular proliferation, principally through interaction with E2F and regulation of E2F target genes. Here, we describe the H1.2 linker histone as a major pRb interaction partner. We establish that H1.2 and pRb are found in a chromatin-bound complex on diverse E2F target genes. Interrogating the global influence of H1.2 on the genome-wide distribution of pRb indicated that the E2F target genes affected by H1.2 are functionally linked to cell-cycle control, consistent with the ability of H1.2 to hinder cell proliferation and the elevated levels of chromatin-bound H1-pRb complex, which occur in growth-arrested cells. Our results define a network of E2F target genes as susceptible to the regulatory influence of H1.2, where H1.2 augments global association of pRb with chromatin, enhances transcriptional repression by pRb, and facilitates pRb-dependent cell-cycle arrest.

Antagonistic roles of Drosophila Tctp and Brahma in chromatin remodelling and stabilizing repeated sequences

Genome stability is essential for all organisms. Translationally controlled tumour protein (TCTP) is a conserved protein associated with cancers. TCTP is involved in multiple intracellular functions, but its role in transcription and genome stability is poorly understood. Here, we demonstrate new functions of Drosophila TCTP (Tctp) in transcription and the stability of repeated sequences (rDNA and pericentromeric heterochromatin). Tctp binds Brahma (Brm) chromatin remodeler to negatively modulate its activity. Tctp mutants show abnormally high levels of transcription in a large set of genes and transposons. These defects are ameliorated by brm mutations. Furthermore, Tctp promotes the stability of repeated sequences by opposing the Brm function. Additional regulation of pericentromeric heterochromatin by Tctp is mediated by su(var)3-9 transcriptional regulation. Altogether, Tctp regulates transcription and the stability of repeated sequences by antagonizing excess Brm activity. This study provides insights into broader nuclear TCTP functions for the maintenance of genome stability.

Genome stability is important for normal cellular function. Here, Hong and Choi show that translationally controlled tumour protein (TCTP) in Drosophila regulates pericentromeric chromatin remodelling and transcription via negatively regulating a chromatin remodeler Brahma.

DNA replication initiator Cdc6 also regulates ribosomal DNA transcription initiation

RNA-polymerase-I-dependent ribosomal DNA (rDNA) transcription is fundamental to rRNA processing, ribosome assembly and protein synthesis. However, how this process is initiated during the cell cycle is not fully understood. By performing a proteomic analysis of transcription factors that bind RNA polymerase I during rDNA transcription initiation, we identified that the DNA replication initiator Cdc6 interacts with RNA polymerase I and its co-factors, and promotes rDNA transcription in G1 phase in an ATPase-activity-dependent manner. We further showed that Cdc6 is targeted to the nucleolus during late mitosis and G1 phase in a manner that is dependent on B23 (also known as nucleophosmin, NPM1), and preferentially binds to the rDNA promoter through its ATP-binding domain. Overexpression of Cdc6 increases rDNA transcription, whereas knockdown of Cdc6 results in a decreased association of both RNA polymerase I and the RNA polymerase I transcription factor RRN3 with rDNA, and a reduction of rDNA transcription. Furthermore, depletion of Cdc6 impairs the interaction between RRN3 and RNA polymerase I. Taken together, our data demonstrate that Cdc6 also serves as a regulator of rDNA transcription initiation, and indicate a mechanism by which initiation of rDNA transcription and DNA replication can be coordinated in cells.

Insights into the regulation of neuronal viability by nucleophosmin/B23

The vastness of the neuronal network that constitutes the human brain proves challenging when trying to understand its complexity. Furthermore, due to the senescent state they enter into upon maturation, neurons lack the ability to regenerate in the face of insult, injury or death. Consequently, their excessive death can be detrimental to the proper functioning of the brain. Therefore, elucidating the mechanisms regulating neuronal survival is, while challenging, of great importance as the incidence of neurological disease is becoming more prevalent in today's society. Nucleophosmin/B23 (NPM) is an abundant and ubiquitously expressed protein that regulates vital cellular processes such as ribosome biogenesis, cell proliferation and genomic stability. As a result, it is necessary for proper embryonic development, but has also been implicated in many cancers. While highly studied in the context of proliferative cells, there is a lack of understanding NPM's role in post-mitotic neurons. By exploring its role in healthy neurons as well as its function in the regulation of cell death and neurodegeneration, there can be a better understanding of how these diseases initiate and progress. Owing to what is thus far known about its function in the cell, NPM could be an attractive therapeutic target in the treatment of neurodegenerative diseases.

Targeting the nucleolus for cancer intervention

The contribution of the nucleolus to cancer is well established with respect to its traditional role in facilitating ribosome biogenesis and proliferative capacity. More contemporary studies however, infer that nucleoli contribute a much broader role in malignant transformation. Specifically, extra-ribosomal functions of the nucleolus position it as a central integrator of cellular proliferation and stress signaling, and are emerging as important mechanisms for modulating how oncogenes and tumor suppressors operate in normal and malignant cells. The dependence of certain tumor cells to co-opt nucleolar processes to maintain their cancer phenotypes has now clearly been demonstrated by the application of small molecule inhibitors of RNA Polymerase I to block ribosomal DNA transcription and disrupt nucleolar function (Bywater et al., 2012 [1]). These drugs, which selectively kill tumor cells in vivo while sparing normal cells, have now progressed to clinical trials. It is likely that we have only just begun to scratch the surface of the potential of the nucleolus as a new target for cancer therapy, with "suppression of nucleolar stress" representing an emerging "hallmark" of cancer. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

Nucleolar Protein B23 Interacts with Japanese Encephalitis Virus Core Protein and Participates in Viral Replication

Japanese encephalitis virus (JEV) core protein is detected not only in the cytoplasm but also in the nucleoli of infected cells. We previously showed that a mutant JEV lacking the nucleolar localization of the core protein impaired viral replication in mammalian cells. In this study, we identified a nucleolar phosphoprotein B23 as a protein binding with the core protein of JEV but not with that of dengue virus. The region binding with JEV core protein was mapped to amino acid residues 38 to 77 of B23. Upon JEV infection, some fraction of B23 was translocated from the nucleoli to the cytoplasm, and cytoplasmic B23 was colocalized with the core protein of wild-type JEV but not with that of the mutant JEV. Furthermore, overexpression of dominant negatives of B23 reduced JEV replication. These results suggest that B23 plays an important role in the intracellular localization of the core protein and replication of JEV.

Cell-cycle-dependent localization of human cytomegalovirus UL83 phosphoprotein in the nucleolus and modulation of viral gene expression in human embryo fibroblasts in vitro

The nucleolus is a multifunctional nuclear compartment widely known to be involved in several cellular processes, including mRNA maturation and shuttling to cytoplasmic sites, control of the cell cycle, cell proliferation, and apoptosis; thus, it is logical that many viruses, including herpesvirus, target the nucleolus in order to exploit at least one of the above-mentioned functions. Recent studies from our group demonstrated the early accumulation of the incoming ppUL83 (pp65), the major tegument protein of human cytomegalovirus (HCMV), in the nucleolus. The obtained results also suggested that a functional relationship might exist between the nucleolar localization of pp65, rRNA synthesis, and the development of the lytic program of viral gene expression. Here we present new data which support the hypothesis of a potentially relevant role of HCMV pp65 and its nucleolar localization for the control of the cell cycle by HCMV (arrest of cell proliferation in G1-G1/S), and for the promotion of viral infection. We demonstrated that, although the incoming pp65 amount in the infected cells appears to be constant irrespective of the cell-cycle phase, its nucleolar accumulation is prominent in G1 and G1/S, but very poor in S or G2/M. This correlates with the observation that only cells in G1 and G1/S support an efficient development of the HCMV lytic cycle. We propose that HCMV pp65 might be involved in regulatory/signaling pathways related to nucleolar functions, such as the cell-cycle control. Co-immunoprecipitation experiments have permitted to identify nucleolin as one of the nucleolar partners of pp65.

Detection of nucleophosmin and FMS-like tyrosine kinase-3 gene mutations in acute myeloid leukemia

BACKGROUND AND OBJECTIVES

Nucleophosmin gene mutations are frequently reported in acute myeloid leukemia (AML) patients with normal karyotype, which is also frequently associated with internal tandem duplication mutations in the FMS-like tyrosine kinase-3 gene. We sought to detect the nucleophosmin and FMS-like tyrosine kinase-3 (FLT3) internal tandem duplication (ITD) mutations among Iranian patients with AML and to assess the relationship between these mutations and the subtypes of the disease.

DESIGN AND SETTING

Cross-sectional study of patients referred during 2007 through 2009.

PATIENTS AND METHODS

Bone marrow and peripheral blood samples of 131 AML patients were randomly collected at the time of diagnosis and prior to treatment and the DNA extracted. After amplifying the nucleophosmin and FLT3 gene regions, positive cases were screened by conformation-sensitive gel electrophoresis and agarose gel electrophoresis techniques.

RESULTS

Of 131 patients, 23 (17.5%) (0.95% CI=0.107-0.244) had nucleophosmin gene mutations. The highest frequency of such mutations was found among the subtypes of M4 (30.4%), M3 (21.7%) and M5 (17.4%). There was a high frequency of these mutations in the M3 subtype as well as a high frequency of allele D in all subtypes. Also, 21 (16.0%) samples (0.95% CI=0.092-0.229) had FLT3/ITD mutation, of which 8 samples had mutant nucleophosmin (8 of 23, 35%), and another 13 samples had wild-type nucleophosmin gene (13 of 108, 12%). There was a high degree of association between the occurrence of nucleophosmin and FLT3/ITD mutations (P=.012).

CONCLUSION

Our data showed a high frequency of NPM1 mutations in the monocytic subtypes of AML, as well as a high degree of association between the occurrence of NPM1 and FLT3/ITD mutations.

Dephosphorylation of nucleophosmin by PP1β facilitates pRB binding and consequent E2F1-dependent DNA repair

We report a new pathway through which PP1β signals to nucleophosmin (NPM) in response to DNA damage. UV induces dephosphorylation of NPM at multiple sites, leading to enhancement of complex formation between NPM and retinoblastoma tumor suppressor protein and the subsequent upregulation of E2F1. Consequently, such signaling pathway potentiates the cellular DNA repair capacity.

Nucleophosmin (NPM) is an important phosphoprotein with pleiotropic functions in various cellular processes. Although phosphorylation has been postulated as an important functional determinant, possible regulatory roles of this modification on NPM are not fully characterized. Here, we find that NPM is dephosphorylated on various threonine residues (Thr199 and Thr234/237) in response to UV-induced DNA damage. Further experiments indicate that the serine/threonine protein phosphatase PP1β is a physiological NPM phosphatase under both the genotoxic stress and growth conditions. As a consequence, NPM in its hypophosphorylated state facilitates DNA repair. Finally, our results suggest that one possible mechanism of this protective response lies in enhanced NPM-retinoblastoma tumor suppressor protein (pRB) interaction, leading to the relief of the repressive pRB–E2F1 circuitry and the consequent transcriptional activation of E2F1 and several downstream DNA repair genes. Thus, this study unveils a key phosphatase of NPM and highlights a novel mechanism by which the PP1β–NPM pathway contributes to cellular DNA damage response.

Identification of target genes for wild type and truncated HMGA2 in mesenchymal stem-like cells

BACKGROUND

The HMGA2 gene, coding for an architectural transcription factor involved in mesenchymal embryogenesis, is frequently deranged by translocation and/or amplification in mesenchymal tumours, generally leading to over-expression of shortened transcripts and a truncated protein.

METHODS

To identify pathways that are affected by sarcoma-associated variants of HMGA2, we have over-expressed wild type and truncated HMGA2 protein in an immortalized mesenchymal stem-like cell (MSC) line, and investigated the localisation of these proteins and their effects on differentiation and gene expression patterns.

RESULTS

Over-expression of both transgenes blocked adipogenic differentiation of these cells, and microarray analysis revealed clear changes in gene expression patterns, more pronounced for the truncated protein. Most of the genes that showed altered expression in the HMGA2-overexpressing cells fell into the group of NF-kappaB-target genes, suggesting a central role for HMGA2 in this pathway. Of particular interest was the pronounced up-regulation of SSX1, already implicated in mesenchymal oncogenesis and stem cell functions, only in cells expressing the truncated protein. Furthermore, over-expression of both HMGA2 forms was associated with a strong repression of the epithelial marker CD24, consistent with the reported low level of CD24 in cancer stem cells.

CONCLUSIONS

We conclude that the c-terminal part of HMGA2 has important functions at least in mesenchymal cells, and the changes in gene expression resulting from overexpressing a protein lacking this domain may add to the malignant potential of sarcomas.

Human papillomavirus type 16 E6/E7 upregulation of nucleophosmin is important for proliferation and inhibition of differentiation

The E6 and E7 oncoproteins of high-risk human papillomaviruses (HPVs) are together sufficient to cause cellular transformation. Nucleophosmin (NPM) was identified as a protein with increased levels in two-dimensional (2-D) gel analysis of human foreskin keratinocytes (HFKs) expressing E7 following methylcellulose-induced differentiation. Analysis of NPM expression in E7-expressing cells and E6- and E7-expressing cells in culture and in organotypic rafts confirmed the increased levels observed in 2-D gel analysis. The elevated expression of NPM was determined to be posttranscriptional and was attributed to increased v-akt murine thymoma viral oncogene (AKT) activity in the E6- and E7-expressing cells. Depletion of NPM caused a reduction in the replicative capacity of E7- and E6/E7-expressing HFKs and an increase in markers of differentiation. Also, the p53 and pRb tumor suppressor levels are increased with the knockdown of NPM in E6/E7-expressing cells, and, interestingly, p14(ARF) is relocalized from the nucleolus to the nucleoplasm and cytoplasm in these cells. The results show for the first time that NPM is required for the proliferation and inhibition of differentiation observed in HPV E6- and E7-expressing primary cells.

NOM1 targets protein phosphatase I to the nucleolus

Protein phosphatase I (PP1) is an essential eukaryotic serine/threonine phosphatase required for many cellular processes, including cell division, signaling, and metabolism. In mammalian cells there are three major isoforms of the PP1 catalytic subunit (PP1alpha, PP1beta, and PP1gamma) that are over 90% identical. Despite this high degree of identity, the PP1 catalytic subunits show distinct localization patterns in interphase cells; PP1alpha is primarily nuclear and largely excluded from nucleoli, whereas PP1gamma and to a lesser extent PP1beta concentrate in the nucleoli. The subcellular localization and the substrate specificity of PP1 catalytic subunits are determined by their interaction with targeting subunits, most of which bind PP1 through a so-called "RVXF" sequence. Although PP1 targeting subunits have been identified that direct PP1 to a number of subcellular locations and/or substrates, no targeting subunit has been identified that localizes PP1 to the nucleolus. Identification of nucleolar PP1 targeting subunit(s) is important because all three PP1 isoforms are included in the nucleolar proteome, enzymatically active PP1 is present in nucleoli, and PP1gamma is highly concentrated in nucleoli of interphase cells. In this study, we identify NOM1 (nucleolar protein with MIF4G domain 1) as a PP1-interacting protein and further identify the NOM1 RVXF motif required for its binding to PP1. We also define the NOM1 nucleolar localization sequence. Finally, we demonstrate that NOM1 can target PP1 to the nucleolus and show that a specific NOM1 RVXF motif and the NOM1 nucleolar localization sequence are required for this targeting activity. We therefore conclude that NOM1 is a PP1 nucleolar targeting subunit, the first identified in eukaryotic cells.

Ebp1 association with nucleophosmin/B23 is essential for regulating cell proliferation and suppressing apoptosis

Ebp1 and NPM/B23 are essential for cell proliferation and survival. Ebp1 possesses p42 and p48 isoforms. Whereas p42 exclusively resides in the cytoplasm, p48 localizes in both the cytoplasm and the nucleolus. Here, we show that Ebp1 forms a complex with B23, and this complex plays a critical role in cell proliferation and survival. p42 specifically associates with B23 upon epidermal growth factor stimulation, while p48 constantly binds B23. Moreover, Ser360 phosphorylation in p42, but not p48, is critical for the interaction. p48 constitutively binds B23 in the nucleolus, for which B23 Lys263 sumoylation is indispensable. By contrast, p42 selectively binds unsumoylated B23 mutants. Interestingly, B23 K263R, an unsumoylated mutant, triggers p42 nuclear translocation and interacts with it in the nucleus even in the absence of epidermal growth factor. In contrast, the nucleolar residency of p48 is abolished in B23 K263R cells. During the cell cycle, p42 selectively colocalizes with B23 in the mitotic cells, correlating with its phosphorylation status in mitosis. Knocking down of B23 or Ebp1 substantially decreases ribosome biogenesis and cell survival. Thus, B23 distinctively binds Ebp1 isoforms and regulates cell proliferation and survival through p42 and p48, respectively.

Anti-citrulline antibodies in the diagnosis and prognosis of rheumatoid arthritis: evolving concepts

Citrulline is a non-standard amino acid that can be incorporated into proteins only by post-translational modification of arginine by peptidylarginine deiminase (PAD) enzymes during a variety of biologic processes, including inflammation. Rheumatoid arthritis (RA) is an inflammatory autoimmune disease, with a prevalence of 0.3 to 1% worldwide, which leads to progressive joint erosion and substantial disability if not treated early. A reliable and specific test for a marker present early in the disease would be useful to identify RA patients prior to the occurrence of joint damage. A new group of autoantibodies, the anti-cyclic citrullinated peptide antibodies (anti-CCP), can be detected in up to 80% of patients with RA, are highly specific for the disease, and may be of value for both the diagnosis and the prognosis of RA. The fact that these antibodies may appear before the onset of the disease suggests a potential role in primary prevention. Interestingly, they may also play a role in the pathophysiology of this disabling disease. The process of citrullination, its physiologic role, and citrullination-related pathologies, as well as the use of anti-citrullinated protein antibody tests (ACPA) for the early diagnosis and prognosis of RA and their potential role in the pathophysiology of the disease, are discussed.

Sumoylation of nucleophosmin/B23 regulates its subcellular localization, mediating cell proliferation and survival

Nucleophosmin/B23 is a major multifunctional nucleolar phosphoprotein that plays a critical role in ribosome biogenesis and cell proliferation. Arf tumor suppressor binds B23 and enhances its sumoylation. However, the biological effects of this event remain unknown. Here we show that B23 is sumoylated on both Lysine 230 and 263 residues, but the latter is the major one. Mutation of K263, but not K230, into R abolishes its centrosomal and nucleolar residency. Moreover, Rb binds to wild-type B23, but fails to interact with K263R. Sumoylation enhances B23 binding to Rb. Consequently, B23 potently stimulates E2F1-mediated transcriptional activity, which is abolished in B23 K263R. Further, K263R mutation makes B23 vulnerable to caspase-3 cleavage and sensitizes cells to apoptosis. Surprisingly, K230R mutant strongly binds to phosphatidylinositol-3,4,5-trisphosphate and suppresses DNA fragmentation. Thus, B23 sumoylation regulates its subcellular localization, cell proliferation, and survival activities.

The functional haplotype of peptidylarginine deiminase IV (S55G, A82V and A112G) associated with susceptibility to rheumatoid arthritis dominates apoptosis of acute T leukemia Jurkat cells

Peptidylarginine deiminase IV (PADI4) posttranslationally converts peptidylarginine to citrulline. It plays an essential role in immune cell differentiation and apoptosis. A haplotype of single-nucleotide polymorphisms (SNPs) in PADI4 is functionally relevant as a rheumatoid arthritis (RA) gene. It could increase enzyme activity leading to raised levels of citrullinated protein and stimulating autoantibody. Previously, our study showed that inducible PADI4 causes haematopoietic cell death. Herein, we further investigate whether RA risk PADI4 haplotype (SNP PADI4; S55G, A82V and A112G) and the increase of its enzymatic activity induce apoptosis. In the tetracycline (Tet)-On Jurkat T cells, ionomycin (Ion) only treatment didn't induce apoptosis however it promoted inducible PADI4-decreased cell viability and -enhanced apoptosis. Through in vitro and in vivo PADI enzyme activity assay, we demonstrated that PADI4 enzyme activity of SNP PADI4 was higher than RA non-risk PADI4 haplotype (WT PADI4). The effect of SNP PADI4-induced apoptosis was superior to WT PADI4. In addition, both Ion and SNP PADI4 synergistically provoked apoptosis were compared with both Ion and WT PADI4. Concurrently, in the conditionally inducible SNP PADI4 cells of Ion treatment-induced apoptosis, not only the expression of Bcl-xL was down-regulated and Bax up-regulated, but also cytochrome c was released from mitochondria to cytoplasm in significant amounts. Western blotting data showed the increase in apoptosomal caspase activation during programmed cell death in the inducible SNP PADI4 cells subsequent to Ion treatment. These data demonstrated that both SNP PADI4 increasing their enzyme activity could enhance apoptosis through the mitochondrial pathway and further provide a conceivable explanation in the pathogenesis of RA following the upregulation of PADI4 activity in its SNPs.

Nucleophosmin is a novel Bax chaperone that regulates apoptotic cell death

The proapoptotic B-cell lymphoma-2 family protein Bax is a key regulatory point in the intrinsic apoptotic pathway. However, the factors controlling the process of Bax activation and translocation to mitochondria have yet to be fully identified and characterized. We performed affinity chromatography using peptides corresponding to the mitochondrial-targeting region of Bax, which is normally sequestered within the inactive structure. The molecular chaperone nucleophosmin was identified as a novel Bax-binding protein by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Reciprocal co-immunoprecipitation and proximity assays confirmed the Bax-nucleophosmin protein-protein interaction and verified that nucleophosmin only bound to activated conformationally altered Bax. Confocal microscopy in a cell-based apoptosis model, demonstrated that nucleophosmin translocation from nucleolus to cytosol preceded Bax movement. Specific knockdown of nucleophosmin expression using RNAi attenuated apoptosis as measured by mitochondrial cytochrome c release and activation of the caspase cascade. In a mouse model of ischaemic stroke, subcellular fractionation studies verified that nucleophosmin translocation occurred within 3 h, at a time before Bax translocation but after Bax conformational changes have occurred. Thus, we have elucidated a novel molecular mechanism whereby Bax becomes activated and translocates to the mitochondria to orchestrate mitochondrial dysfunction and apoptotic cell death, which opens new avenues for therapeutic intervention.

Overexpression of peptidylarginine deiminase IV features in apoptosis of haematopoietic cells

Peptidylarginine deiminases (PADIs) convert peptidylarginine into citrulline via posttranslational modification. One member of the family, PADI4, plays an important role in immune cell differentiation and cell death. To elucidate the participation of PADI4 in haematopoietic cell death, we examine whether inducible overexpression of PADI4 enhances the apoptotic cell death. PADI4 reduced the viability in a dose- and time-dependent manner of human leukemia HL-60 cells and human acute T leukemia Jurkat cells. The apoptosis-inducing activities were determined by nuclear condensation, DNA fragmentation, sub-G1 appearance, loss of mitochondrial membrane potential (delta psi(m)), release of mitochondrial cytochrome c into cytoplasm and proteolytic activation of caspase 9 and 3. Following PADI4 overexpression, cells arrest in G1 phase significantly before their entrance into apoptotic cell death. PADI4 increases tumor suppressor p53 and its downstream p21 to control cell cycle. In the detections of protein expression and kinase activity, all protein levels of cyclin-dependent kinases (CDKs) and cyclins are not reduced except cyclin D, however, CDK2 (G1 entry S phase) and CDK1 (G2 entry M phase) enzyme activities are inhibited by conditionally inducible PADI4. p53 also expands its other downstream Bax to induce cytochrome c release from mitochondria. According to these data, we suggest that PADI4 induces apoptosis mainly through cell cycle arrest and mitochondria-mediated pathway. Furthermore, p53 features in PADI4-induced apoptosis by increasing intracellular p21 to control cell cycle and by Bax accumulation to decline Bcl-2 function, destroy delta psi(m), release cytochrome c to cytoplasm and activate the caspase cascade.

Mapping a nucleolar targeting sequence of an RNA binding nucleolar protein, Nop25

Nop25 is a putative RNA binding nucleolar protein associated with rRNA transcription. The present study was undertaken to determine the mechanism of Nop25 localization in the nucleolus. Deletion experiments of Nop25 amino acid sequence showed Nop25 to contain a nuclear targeting sequence in the N-terminal and a nucleolar targeting sequence in the C-terminal. By expressing derivative peptides from the C-terminal as GFP-fusion proteins in the cells, a lysine and arginine residue-enriched peptide (KRKHPRRAQDSTKKPPSATRTSKTQRRRR) allowed a GFP-fusion protein to be transported and fully retained in the nucleolus. When the peptide was fused with cMyc epitope and expressed in the cells, a cMyc epitope was then detected in the nucleolus. Nop25 did not localize in the nucleolus by deletion of the peptide from Nop25. Furthermore, deletion of a subdomain (KRKHPRRAQ) in the peptide or amino acid substitution of lysine and arginine residues in the subdomain resulted in the loss of Nop25 nucleolar localization. These results suggest that the lysine and arginine residue-enriched peptide is the most prominent nucleolar targeting sequence of Nop25 and that the long stretch of basic residues might play an important role in the nucleolar localization of Nop25. Although Nop25 contained putative SUMOylation, phosphorylation and glycosylation sites, the amino acid substitution in these sites had no effect on the nucleolar localization, thus suggesting that these post-translational modifications did not contribute to the localization of Nop25 in the nucleolus. The treatment of the cells, which expressed a GFP-fusion protein with a nucleolar targeting sequence of Nop25, with RNase A resulted in a complete dislocation of the protein from the nucleolus. These data suggested that the nucleolar targeting sequence might therefore play an important role in the binding of Nop25 to RNA molecules and that the RNA binding of Nop25 might be essential for the nucleolar localization of Nop25.

Prevalence and prognostic impact of NPM1 mutations in 1485 adult patients with acute myeloid leukemia (AML)

Mutations of the nucleophosmin (NPM1) gene have recently been described in patients with acute myeloid leukemia (AML). To clarify the prevalence as well as the clinical impact of this mutation, we investigated 1485 patients with AML for NPM1 exon 12 mutations using fragment analysis. A 4 bp insert was detected in 408 of 1485 patients (27.5%). Sequence analysis revealed known mutations (type A, B, and D) as well as 13 novel alterations in 229 analyzed cases. NPM1 mutations were most prevalent in patients with normal karyotype (NK) (324 of 709; 45.7%) compared with 58 of 686 with karyotype abnormalities (8.5%; P < .001) and were significantly associated with several clinical parameters (high bone marrow [BM] blasts, high white blood cell [WBC] and platelet counts, female sex). NPM1 alterations were associated with FLT3-ITD mutations, even if restricted to patients with NK (NPM1-mut/FLT3-ITD: 43.8%; versus NPM1-wt/FLT3-ITD: 19.9%; P < .001). The analysis of the clinical impact in 4 groups (NPM1 and FLT3-ITD single mutants, double mutants, and wild-type [wt] for both) revealed that patients having only an NPM1 mutation had a significantly better overall and disease-free survival and a lower cumulative incidence of relapse. In conclusion, NPM1 mutations represent a common genetic abnormality in adult AML. If not associated with FLT3-ITD mutations, mutant NPM1 appears to identify patients with improved response toward treatment.

Nucleophosmin/B23 regulates transcriptional activation of E2F1 via modulating the promoter binding of NF-kappaB, E2F1 and pRB

Expression of nucleophosmin/B23 and E2F1 and E2F1-dependent transcription increased in U1 bladder cancer cells upon serum stimulation from quiescence. Nucleophosmin/B23-siRNA treatment abrogated such increase of E2F1-dependent transcriptional activity. In identifying physiologically important factors that may occupy E2F1 promoter and regulate its activity in vivo, we found that the pattern of NF-kappaB, E2F1 and pRB recruitment to E2F1 promoter changed in a strikingly dynamic fashion as cells progressed from quiescence into serum-stimulated growth. E2F1 promoter activity in quiescent cells was associated with recruitment of NF-kappaB. NF-kappaB was replaced largely by E2F1 in concert with gene activation during the early stage (12 h) of serum stimulation. At late stage (24 h) of serum stimulation, pRB was then recruited to the E2F1-promoter complex to counterbalance its activity. Upon siRNA-mediated reduction of intracellular nucleophosmin/B23, E2F1 and pRB were recruited to the promoter with the dissociation of NF-kappaB concomitant with gene inactivation. Based on immunoprecipitation experiments, nucleophosmin/B23 was found to be associated with NF-kappaB in cells grown in serum-supplemented but not in serum-deprived medium. Furthermore, nucleophosmin/B23 could also be co-immunoprecipitated with ppRB at the early stage (12 h) but not at the late stage (24 h) of serum stimulation. The results demonstrate a novel mechanism for transcriptional regulation of E2F1 and identify the functional role of nucleophosmin/B23 in modulating the binding of NF-kappaB, E2F1 and pRB to activate E2F1 promoter.

Hepatitis C virus core protein recruits nucleolar phosphoprotein B23 and coactivator p300 to relieve the repression effect of transcriptional factor YY1 on B23 gene expression

Hepatitis C virus (HCV) core has a pleiotropic effect on various promoters. In this study, we found that the expression of nucleolar phosphoprotein B23 was enhanced in HCV core-expressing cells and, moreover, HCV core interacts directly with the C-terminal end of B23. Using sucrose gradient centrifugation analysis and immunoprecipitation assays, HCV core was found in a large complex containing B23 and its interacting partner transcription factor YY1. Both B23 and HCV core associated with YY1 in the central GA/GK-rich and C-terminal zinc finger domain. These physical interactions between core, B23, and YY1 led to ternary complex formation that was bound to the YY1 response element. In a transient cotransfection experiment, relief of the trans-suppression activity of YY1 on the YY1-response element-driven reporter by core and B23 was found. This is also true when examining the effects of these three constructs on the B23 promoter-driven reporter. Additionally, chromatin immunoprecipitation assays indicated that a transcriptional activation complex consisting of core, together with B23, p300, and YY1, was recruited to the YY1 response element of B23 promoter, and this probably occurred through complex formation between core and these three cellular transcription regulators. This is different from the situation in the absence of core, where YY1 and histone deacetylase 1, but not B23 and p300, were associated on the YY1 element as the transcription repression complex. Together, our results indicate that HCV core can recruit B23 and p300 to relieve the repression effect of YY1 on B23 promoter activity, a property that requires the intrinsic histone acetyltransferase activity of p300. Thus, because these three core-associated cellular transcription regulators have a multitude of cellular interacting proteins and are involved in a versatility of cellular processes, the complex formation described here may partially account for the pleiotropic effects of core protein on gene expression and cellular function in HCV-infected cells.

Screening and identification of interacting proteins with hepatitis B virus core protein in leukocytes and cloning of new gene C1

AIM

To investigate the biological function of HBcAg in pathogenesis of HBV replication in peripheral blood mononuclear cells (PBMCs).

METHODS

HBcAg region was amplified by polymerase chain reaction (PCR) and HBV HBcAg bait plasmid pGBKT7-HBcAg was constructed by routine molecular biological methods. Then the recombinant plasmid DNA was transformed into yeast AH109. After the HBV core protein was expressed in AH109 yeast strains (Western blot analysis), yeast-two hybrid screening was performed by mating AH109 with Y187 containing leukocyte cDNA library plasmid. Diploid yeast cells were plated on synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) (QDO) and synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) (TDO). The second screening was performed with the LacZ report gene ( yeast cells were grown in QDO medium containing X-alpha-gal). The interaction between HBV core protein and the protein obtained from positive colonies was further confirmed by repeating yeast-two hybrid. After plasmid DNA was extracted from blue colonies and sequenced, the results were analyzed by bioinformatic methods.

RESULTS

Eighteen colonies were obtained and sequenced, including hypermethylated in cancer 2 (3 colones), eukaryotic translation elongation factor 2 (2 colones), acetyl-coenzyme A synthetase 3 (1 colone), DNA polymerase gamma (1 colone), putative translation initiation factor (1 colone), chemokine (C-C motif) receptor 5 (1 colone), mitochondrial ribosomal protein L41 (1 colone), kyot binding protein genes (1 colone), RanBPM (1 colone), HBeAg-binding protein 3 (1 colone), programmed cell death 2 (1 colone). Four new genes with unknown function were identified.

CONCLUSION

Successful cloning of genes of HBV core protein interacting proteins in leukocytes may provide some new clues for studying the biological functions of HBV core protein.

The perichromosomal layer

In addition to genetic information, mitotic chromosomes transmit essential components for nuclear assembly and function in a new cell cycle. A specialized chromosome domain, called the perichromosomal layer, perichromosomal sheath, chromosomal coat, or chromosome surface domain, contains proteins required for a variety of cellular processes, including the synthesis of messenger RNA, assembly of ribosomes, repair of DNA double-strand breaks, telomere maintenance, and apoptosis regulation. The layer also contains many proteins of unknown function and is a major target in autoimmune disease. Perichromosomal proteins are found along the entire length of chromosomes, excluding centromeres, where sister chromatids are paired and spindle microtubules attach. Targeting of proteins to the perichromosomal layer occurs primarily during prophase, and they generally remain associated until telophase. During interphase, perichromosomal proteins localize to nucleoli, the nuclear envelope, nucleoplasm, heterochromatin, centromeres, telomeres, and/or the cytoplasm. It has been suggested that the perichromosomal layer may contribute to chromosome structure, as several of the associated proteins have functions in chromatin remodeling during interphase. We review the identified proteins associated with this chromosome domain and briefly discuss their known functions during interphase and mitosis.

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.

Autoantibodies to citrullinated proteins in rheumatoid arthritis: clinical performance and biochemical aspects of an RA-specific marker

Rheumatoid arthritis (RA) is a common, systemic autoimmune disease of which the exact etiology is not known. In the past 10 years, substantial progress has been made in the identification of the antigens specifically recognized by the autoantibodies of RA patients. A central factor in this respect is citrullination, a form of post-translational modification that is strongly associated with autoimmunity in RA. Here, we summarize and discuss our current knowledge on (i) autoantibody systems in RA, (ii) the occurrence of peptidylarginine deiminases and (iii) citrullinated proteins in natural and diseased environments, and (iv) genetic factors involved in RA that may influence the generation and presentation of citrullinated proteins and the resulting antibody production against these modified proteins. Citrullination of proteins may play a key role in the initiation and/or the progression of RA. The onset of citrulline-specific autoimmunity in RA is probably mediated by both environmental and genetic factors, and future studies will learn whether therapeutic intervention at the level of citrullination may provide new possibilities to treat RA.

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 localization and mobility analysis of the NF-κB repressing factor NRF

NF-κB plays a central role in mediating pathogen and cytokine-stimulated gene transcription. NF-κB repressing factor (NRF) has been shown to interact with specific negative regulatory DNA elements (NRE) to mediate transcriptional repression by inhibition of the NF-κB activity at certain promoters. mRNA ablation experiments demonstrated that the trans-acting NRF protein is involved in constitutive but not post-stimulated silencing of IFN-β, IL-8 and iNOS genes by binding to cis-acting NRE elements in their promoters.

We have examined the subcellular localization and mobility of the NRF protein. Since neither tagging nor overexpression perturbs NRF localization the GFP-tagged protein was used for detailed localization and mobility studies. Owing to an N-terminal nuclear localization sequence, all NRF fragments that contain this signal show a constitutive nuclear accumulation. C-terminal NRF fragments also localize to the nucleus although no canonical NLS motifs were detected. Full-length NRF is highly enriched in nucleoli and only a small fraction of NRF is found in the nucleoplasm and cytoplasm. This relationship was found to be independent of the protein expression rate. FRAP analysis proved to be a sensitive method to determine protein mobility and made it possible to differentiate between the NRF protein fragments. Nucleolar localization correlated inversely with mobility. The data demonstrate that a series of neighboring fragments in a large central domain of the protein contribute to the strong nucleolar affinity. These properties were not altered by viral infection or LPS treatment. Several sequence motifs for RNA binding were predicted by computer-mediated databank searches. We found that NRF binds to double stranded RNA (dsRNA). This property mapped to several NRF fragments which correlate with the nucleolar affinity domain. Since treatment with actinomycin D releases NRF from nucleoli the identified RNA binding motifs might act as nucleolar localization signals.

Retinoblastoma protein is frequently absent or phosphorylated in anaplastic large-cell lymphoma

The possible role of retinoblastoma protein (Rb) in the pathogenesis of anaplastic large-cell lymphoma (ALCL) is unknown. We investigated Rb protein expression, both total (phosphorylated and underphosphorylated) and active (underphosphorylated), in four anaplastic lymphoma kinase (ALK)-positive ALCL cell lines (Karpas 299, JB-6, SU-DHL1, and SR-786) by Western blot analysis, and in 67 ALCL tumors (30 ALK-positive, 37 ALK-negative) using immunohistochemical methods. We also used fluorescence in situ hybridization and polymerase chain reaction methods to assess for loss of heterozygosity of the rb gene. The findings were correlated with apoptotic rate assessed by the terminal dUTP nick-end labeling assay. Immunoblots showed high total Rb levels in Karpas 299, SU-DHL1 and SR-786 and relatively lower levels in and JB-6. Underphosphorylated Rb was negative or expressed at low levels in all cell lines. In ALCL tumors, total Rb was detected in 44 (66%) and absent in 23 (34%). The mean apoptotic rate was 3.2% in Rb-negative tumors compared with 2.7%, 2.2%, and 1.2% in tumors with <10%, 10 to 50%, and >50% Rb-positive cells, respectively (P = 0.2, Kruskall-Wallis test). In a subset of 25 total Rb-positive tumors we assessed for underphosphorylated Rb, which was detected in 12 tumors. The detection of only total Rb in the remaining 13 tumors suggests that Rb was phosphorylated. Fluorescence in situ hybridization showed allelic loss of the rb gene in 10 (40%) of 25 tumors analyzed and was significantly associated with absence of Rb expression (P = 0.003). Similar results were obtained for loss of heterozygosity of the 13q14 locus. Five-year progression-free survival for patients with Rb-negative ALCL was 89.4% compared with 47.7% for patients with total Rb-positive ALCL (P = 0.006, log-rank test). Similar trends for progression-free survival held true for patients with ALK-positive and ALK-negative tumors analyzed separately. In conclusion, Rb is absent or phosphorylated in most ALCL cell lines and tumors and absence of Rb expression is associated with better clinical outcome in patients with ALCL.

Retinoblastoma tumor suppressor: analyses of dynamic behavior in living cells reveal multiple modes of regulation

The retinoblastoma tumor suppressor, RB, assembles multiprotein complexes to mediate cell cycle inhibition. Although many RB binding partners have been suggested to underlie these functions, the validity of these interactions on the behavior of RB complexes in living cells has not been investigated. Here, we studied the dynamic behavior of RB by using green fluorescent protein-RB fusion proteins. Although these proteins were universally nuclear, phosphorylation or oncoprotein binding mediated their active exclusion from the nucleolus. In vivo imaging approaches revealed that RB exists in dynamic equilibrium between a highly mobile and a slower diffusing species, and genetic lesions associated with tumorigenesis increased the fraction of RB in a highly mobile state. The RB complexes dictating cell cycle arrest were surprisingly dynamic and harbored a relatively short residence time on chromatin. In contrast, this rapid exchange was attenuated in cells that are hypersensitive to RB, suggesting that responsiveness may inversely correlate with mobility. The stability of RB dynamics within the cell was additionally modified by the presence and function of critical corepressors. Last, the RB-assembled complexes present in living cells were primarily associated with E2F binding sites in chromatin. In contrast to RB, E2F1 consistently maintained a stable association with E2F sites regardless of cell type. Together, these results elucidate the kinetic framework of RB tumor suppressor action in transcriptional repression and cell cycle regulation.

Threonine 308 phosphorylated form of Akt translocates to the nucleus of PC12 cells under nerve growth factor stimulation and associates with the nuclear matrix protein nucleolin

We have examined the issue of whether or not in PC12 cells it may be observed a nerve growth factor (NGF) nuclear translocation of an active (phosphorylated) Akt. Western blot analysis with antibodies to either total or phosphorylated Akt showed a maximal nuclear translocation after 15 min of NGF stimulation. NGF increased rapidly and transiently the enzymatic activity of immunoprecipitable nuclear Akt and after 45 min the values returned to a level close to the basal one. Enzyme translocation was blocked by the selective phosphoinositide 3-kinase inhibitor, LY294002. Confocal microscopy of samples stained with antibody to Akt showed an evident increase in immunostaining intensity in the nuclear interior after NGF treatment. Treatment of cells with inhibitors of protein phosphatase PP2A, calyculin A, or okadaic acid, maintained the phosphorylation levels of nuclear Akt. Immunoprecipitation experiments revealed an association between Akt and PP2A that was maximal when nuclear Akt activity was decreased. Both total and active Akt associated with the nuclear matrix and, in particular, with the protein nucleolin, with which Akt co-immunoprecipitated. These findings strongly suggest that the intranuclear translocation of active Akt is an important step in the signaling pathways elicited by the neurotrophin NGF and that the intranuclear control of Akt is achieved through the action of PP2A.