Translocation of nucleophosmin from nucleoli to nucleoplasm requires ATP

G protein coupled receptor kinase 5 modifies the nucleolar stress response activated by actinomycin D

G protein coupled receptor kinase 5 (GRK5) is localized within the nucleus and moderates functions such as DNA transcription, in addition to its localization at the plasma membrane. In this report, we show that GRK5 modifies the nucleolar stress response activated by the DNA polymerase inhibitor, actinomycin D (ActD). We show an increased sensitivity to the apoptotic effects of ActD on cervical HeLa cells and the breast cancer cell line MDA MB 231 with reduced protein expression of GRK5. We also tested two types of breast cancer cells (MDA MB 231 and MCF7 cells) and found that the rate of response to ActD varied between them because they have innate differences in the protein expression of GRK5. We also found that GRK5 phosphorylates nucleophosmin (NPM1) at T199 before and during the early stages of ActD treatment. Phosphorylation at T199 increases the ability of NPM1 to interact with p14^ARF in vitro, which may affect the protein expression levels of p14^ARF. We found that the expression levels of p14^ARF were lower in the cells transfected with the control shRNA, but higher in cells transfected with GRK5 shRNA. Collectively, this suggests that GRK5 modifies the nucleolar stress response associated with ActD.

The pathophysiology of neurodegenerative disease: Disturbing the balance between phase separation and irreversible aggregation

Liquid-liquid phase separation (LLPS) brings together functionally related proteins through the intrinsic biophysics of proteins in a process that is driven by reducing free energy and maximizing entropy. The process of LLPS allows proteins to form structures, termed membrane-less organelles. These diverse, dynamic organelles are active in a wide range of processes in the nucleus, cytoplasm, mitochondria and synapse, and ranging from bacteria to plants to eukaryotes. RNA and DNA present long chained charged polymers that promote LLPS. Consequently, many RNA binding proteins (RBPs) and DNA binding proteins form membrane-less organelles. However, the highly concentrated phase separated state creates conditions that also promote formation of irreversible protein aggregates. Mutations in RNA and DNA binding proteins that increase the stability of irreversible aggregates also increase the accumulation of irreversible aggregates directly and from membrane-less organelles. Many of the RBPs that exhibit disease-linked mutations carry out cytoplasmic actions through stress granules, which are a pleiotropic type of RNA granule that regulates the translational response to stress. Phosphorylation and oligomerization of tau facilitates its interactions with RBPs and ribosomal proteins, affecting RNA translation; we propose that this is a major reason that tau becomes phosphorylated with stress. Persistent stress leads to the accumulation of irreversible aggregates composed of RBPs or tau, which then cause toxicity and form many of the hallmark pathologies of major neurodegenerative diseases. This pathophysiology ultimately leads to multiple forms of neurodegenerative diseases, the specific type of which reflects the temporal and spatial accumulation of different aggregating proteins.

Dipeptide repeat proteins inhibit homology-directed DNA double strand break repair in C9ORF72 ALS/FTD

BACKGROUND

The C9ORF72 hexanucleotide repeat expansion is the most common known genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), two fatal age-related neurodegenerative diseases. The C9ORF72 expansion encodes five dipeptide repeat proteins (DPRs) that are produced through a non-canonical translation mechanism. Among the DPRs, proline-arginine (PR), glycine-arginine (GR), and glycine-alanine (GA) are the most neurotoxic and increase the frequency of DNA double strand breaks (DSBs). While the accumulation of these genotoxic lesions is increasingly recognized as a feature of disease, the mechanism(s) of DPR-mediated DNA damage are ill-defined and the effect of DPRs on the efficiency of each DNA DSB repair pathways has not been previously evaluated.

METHODS AND RESULTS

Using DNA DSB repair assays, we evaluated the efficiency of specific repair pathways, and found that PR, GR and GA decrease the efficiency of non-homologous end joining (NHEJ), single strand annealing (SSA), and microhomology-mediated end joining (MMEJ), but not homologous recombination (HR). We found that PR inhibits DNA DSB repair, in part, by binding to the nucleolar protein nucleophosmin (NPM1). Depletion of NPM1 inhibited NHEJ and SSA, suggesting that NPM1 loss-of-function in PR expressing cells leads to impediments of both non-homologous and homology-directed DNA DSB repair pathways. By deleting NPM1 sub-cellular localization signals, we found that PR binds NPM1 regardless of the cellular compartment to which NPM1 was directed. Deletion of the NPM1 acidic loop motif, known to engage other arginine-rich proteins, abrogated PR and NPM1 binding. Using confocal and super-resolution immunofluorescence microscopy, we found that levels of RAD52, a component of the SSA repair machinery, were significantly increased iPSC neurons relative to isogenic controls in which the C9ORF72 expansion had been deleted using CRISPR/Cas9 genome editing. Western analysis of post-mortem brain tissues confirmed that RAD52 immunoreactivity is significantly increased in C9ALS/FTD samples as compared to controls.

CONCLUSIONS

Collectively, we characterized the inhibitory effects of DPRs on key DNA DSB repair pathways, identified NPM1 as a facilitator of DNA repair that is inhibited by PR, and revealed deficits in homology-directed DNA DSB repair pathways as a novel feature of C9ORF72-related disease.

Cucurbitacin B inhibits human breast cancer cell proliferation through disruption of microtubule polymerization and nucleophosmin/B23 translocation

Background

Cucurbitacin B, an oxygenated tetracyclic triterpenoid compound extracted from the Thai medicinal plant Trichosanthes cucumerina L., has been reported to have several biological activities including anti-inflammatory, antimicrobial and anticancer. Cucurbitacin B is great of interest because of its biological activity. This agent inhibits growth of various types of human cancer cells lines.

Methods

In this study, we explored the novel molecular response of cucurbitacin B in human breast cancer cells, MCF-7 and MDA-MB-231. The growth inhibitory effect of cucurbitacin B on breast cancer cells was assessed by MTT assay. The effects of cucurbitacin B on microtubules morphological structure and tubulin polymerization were analyzed using immunofluorescence technique and tubulin polymerization assay kit, respectively. Proteomic analysis was used to identify the target-specific proteins that involved in cucurbitacin B treatment. Some of the differentially expressed genes and protein products were validated by real-time RT-PCR and western blot analysis. Cell cycle distributions and apoptosis were investigated using flow cytometry.

Results

Cucurbitacin B exhibited strong antiproliferative effects against breast cancer cells in a dose-dependent manner. We show that cucurbitacin B prominently alters the cytoskeletal network of breast cancer cells, inducing rapid morphologic changes and improper polymerization of the microtubule network. Moreover, the results of 2D-PAGE, real-time RT-PCR, and western blot analysis revealed that the expression of nucleophosmin/B23 and c-Myc decreased markedly after cucurbitacin B treatment. Immunofluorescence microscopy showed that cucurbitacin B induced translocation of nucleophosmin/B23 from the nucleolus to nucleoplasm. Treatment with cucurbitacin B resulted in cell cycle arrest at G₂/M phase and the enhancement of apoptosis.

Conclusions

Our findings suggest that cucurbitacin B may inhibit the proliferation of human breast cancer cells through disruption of the microtubule network and down-regulation of c-Myc and nucleophosmin/B23 as well as the perturbation in nucleophosmin/B23 trafficking from the nucleolus to nucleoplasm, resulting in G₂/M arrest.

Nucleolar changes and fibrillarin redistribution following apatone treatment of human bladder carcinoma cells

Ascorbate and menadione (Apatone) in a ratio of 100:1 kills tumor cells by autoschizis. In this study, vitamin-induced changes in nucleolar structure were evaluated as markers of autoschizis. Human bladder carcinoma (T24) cells were overlain with vitamins or with culture medium. Supernatants were removed at 1-hr intervals from 1 to 4 hr, and the cells were washed with PBS and prepared for assay. Apatone produced marked alterations in nucleolar structure including redistribution of nucleolar components, formation of ring-shaped nucleoli, condensation and increase of the proportion of perinucleolar chromatin, and the enlargement of nucleolar fibrillar centers. Immunogold labeling of the nucleolar rRNA revealed a granular localization in treated and sham-treated cells, and immunogold labeling of the rDNA revealed a shift from the fibrillar centers to the condensed perinucleolar chromatin. Fibrillarin staining shifted from the fibrillar centers and adjacent regions to a more homogeneous staining of the entire nucleolus and was consistent with the percentage of autoschizic cells detected by flow cytometry. Because autoschizis entails sequential reactivation of DNase I and DNase II, and because the fibrillarin redistribution following DNase I and Apatone treatment is identical, it appears that the nucleolar and fibrillarin changes are markers of autoschizis.

Immunodetection of nucleolar proteins and ultrastructure of nucleoli of soybean root meristematic cells treated with chilling stress and after recovery

The nucleolar proteins, fibrillarin and nucleophosmin, have been identified immunofluorescently in the root meristematic cells of soybean seedlings under varying experimental conditions: at 25 degrees C (control), chilling at 10 degrees C for 3 h and 4 days and recovery from the chilling stress at 25 degrees C. In each experimental variant, the immunofluorescence signals were present solely at the nucleolar territories. Fluorescent staining for both proteins was mainly in the shape of circular domains that are assumed to correspond to the dense fibrillar component of the nucleoli. The fewest fluorescent domains were observed in the nucleoli of chilled plants, and the highest number was observed in the plants recovered after chilling. This difference in the number of circular domains in the nucleoli of each variant may indicate various levels of these proteins in each variant. Both the number of circular domains and the level of these nucleolar proteins changed with changes in the transcriptional activity of the nucleoli, with the more metabolically active cell having higher numbers of active areas in the nucleolus and higher levels of nucleolar proteins, and conversely. Electron microscopic studies revealed differences in the ultrastructure of the nucleoli in all experimental variants and confirmed that the number of fibrillar centres surrounded by dense fibrillar component was the lowest in the nucleoli of chilled plants, and the highest in the nucleoli of recovered seedlings.

The F-actin filament capping protein CapG is a bona fide nucleolar protein

Actin works in concert with myosin I to regulate the transcription of ribosomal genes in the nucleolus. Recently, nucleolar actin has been shown to be active in its polymeric form raising the question how actin dynamics is regulated in the nucleolus. Here, we show that the actin capping protein CapG localizes in the nucleolus of cultured cells. CapG transport to the nucleolus is an active and ATP-dependent process. Association of CapG with the nucleolus requires active RNA Polymerase I transcription. In addition, we show that activated Ran GTPase, an interaction partner of CapG, is also transported to the nucleolus. A constitutively active Ran mutant promotes CapG accumulation in the nucleolus indicating that CapG transport to the nucleolus can be supported by Ran. Our results suggest that filamentous actin in the nucleolus might be regulated by actin binding proteins such as CapG.

Inflammation and endothelial dysfunction during aging: role of NF-kappaB

One of the major conceptual advances in our understanding of the pathogenesis of age-associated cardiovascular diseases has been the insight that age-related oxidative stress may promote vascular inflammation even in the absence of traditional risk factors associated with atherogenesis (e.g., hypertension or metabolic diseases). In the present review we summarize recent experimental data suggesting that mitochondrial production of reactive oxygen species, innate immunity, the local TNF-alpha-converting enzyme (TACE)-TNF-alpha, and the renin-angiotensin system may underlie NF-kappaB induction and endothelial activation in aged arteries. The theme that emerges from this review is that multiple proinflammatory pathways converge on NF-kappaB in the aged arterial wall, and that the transcriptional activity of NF-kappaB is regulated by multiple nuclear factors during aging, including nuclear enzymes poly(ADP-ribose) polymerase (PARP-1) and SIRT-1. We also discuss the possibility that nucleophosmin (NPM or nuclear phosphoprotein B23), a known modulator of the cellular oxidative stress response, may also regulate NF-kappaB activity in endothelial cells.

Lysine 263 residue of NPM/B23 is essential for regulating ATP binding and B23 stability

Here, we show that Nucleophsomin/B23 provides lysine 263 as a critical binding site for ATP. Mutagenesis of lysine 263 to asparagine (K263N) disrupts B23 from ATP binding. While B23 WT exclusively localizes to the nucleolus, the B23-K263N is redistributed from the nucleolus to the nucleoplam. Notably, the K263N mutant is unstable, and displayed rapid degradation. Alteration of K263 induced B23 instability through increased ubiquitination and proteaosomal degradation. Moreover, mutation of K263 impedes the mitogenic effect of B23 in PC12 cells. Thus, K263 is a critical site for ATP binding and required for B23 stability, confining B23 in the nucleolus.

Dynamic sorting of nuclear components into distinct nucleolar caps during transcriptional inhibition

Nucleolar segregation is observed under some physiological conditions of transcriptional arrest. This process can be mimicked by transcriptional arrest after actinomycin D treatment leading to the segregation of nucleolar components and the formation of unique structures termed nucleolar caps surrounding a central body. These nucleolar caps have been proposed to arise from the segregation of nucleolar components. We show that contrary to prevailing notion, a group of nucleoplasmic proteins, mostly RNA binding proteins, relocalized from the nucleoplasm to a specific nucleolar cap during transcriptional inhibition. For instance, an exclusively nucleoplasmic protein, the splicing factor PSF, localized to nucleolar caps under these conditions. This structure also contained pre-rRNA transcripts, but other caps contained either nucleolar proteins, PML, or Cajal body proteins and in addition nucleolar or Cajal body RNAs. In contrast to the capping of the nucleoplasmic components, nucleolar granular component proteins dispersed into the nucleoplasm, although at least two (p14/ARF and MRP RNA) were retained in the central body. The nucleolar caps are dynamic structures as determined using photobleaching and require energy for their formation. These findings demonstrate that the process of nucleolar segregation and capping involves energy-dependent repositioning of nuclear proteins and RNAs and emphasize the dynamic characteristics of nuclear domain formation in response to cellular stress.

Nucleophosmin/B23, a Nuclear PI(3,4,5)P3 Receptor, Mediates the Antiapoptotic Actions of NGF by Inhibiting CAD

Phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P(3)] is an essential second messenger implicated in various cellular processes. Cytoplasmic PI(3,4,5)P(3) has been well characterized, but little is known about the physiological role of nuclear PI(3,4,5)P(3). Here, we describe a nuclear PI(3,4,5)P(3) receptor, nucleophosmin (NPM)/B23, that mediates the antiapoptotic effects of NGF by inhibiting DNA fragmentation activity of caspase-activated DNase (CAD). Employing PI(3,4,5)P(3) column and NGF-treated PC12 nuclear extracts, we identified B23 as a nuclear PI(3,4,5)P(3) binding protein. Purification from nuclear extract demonstrates that B23 contributes to DNA fragmentation inhibitory activity. Depletion of B23 from nuclear extracts or knockdown B23 in PC12 cells abolishes NGF-provoked protective effect, whereas overexpression of B23 in PC12 cells prevents apoptosis. Further, hydrolyzing PI(3,4,5)P(3) with PTEN or SHIP abrogates its antiapoptotic activity. Moreover, B23 mutants that can not associate with PI(3,4,5)P(3) fail to prevent DNA fragmentation. Thus, the nuclear B23-PI(3,4,5)P(3) complex regulates the antiapoptotic activity of NGF in the nucleus.

Dynamic sorting of nuclear components into distinct nucleolar caps during transcriptional inhibition

Nucleolar segregation is observed under some physiological conditions of transcriptional arrest. This process can be mimicked by transcriptional arrest after actinomycin D treatment leading to the segregation of nucleolar components and the formation of unique structures termed nucleolar caps surrounding a central body. These nucleolar caps have been proposed to arise from the segregation of nucleolar components. We show that contrary to prevailing notion, a group of nucleoplasmic proteins, mostly RNA binding proteins, relocalized from the nucleoplasm to a specific nucleolar cap during transcriptional inhibition. For instance, an exclusively nucleoplasmic protein, the splicing factor PSF, localized to nucleolar caps under these conditions. This structure also contained pre-rRNA transcripts, but other caps contained either nucleolar proteins, PML, or Cajal body proteins and in addition nucleolar or Cajal body RNAs. In contrast to the capping of the nucleoplasmic components, nucleolar granular component proteins dispersed into the nucleoplasm, although at least two (p14/ARF and MRP RNA) were retained in the central body. The nucleolar caps are dynamic structures as determined using photobleaching and require energy for their formation. These findings demonstrate that the process of nucleolar segregation and capping involves energy-dependent repositioning of nuclear proteins and RNAs and emphasize the dynamic characteristics of nuclear domain formation in response to cellular stress.

Nucleophosmin/B23 is a proliferate shuttle protein associated with nuclear matrix

It has become obvious that a better understanding and potential elucidation of the nucleolar phosphoprotein B23 involving in functional interrelationship between nuclear organization and gene expression. In present study, protein B23 expression were investigated in the regenerative hepatocytes at different periods (at days 0, 1, 2, 3, 4, 7) during liver regeneration after partial hepatectomy on the rats with immunohistochemistry and Western blot analysis. Another experiment was done with immunolabeling methods and two-dimensional (2-D) gel electrophoresis for identification of B23 in the regenerating hepatocytes and HepG2 cells (hepatoblastoma cell line) after sequential extraction with detergents, nuclease, and salt. The results showed that its expression in the hepatocytes had a locative move and quantitative change during the process of liver regeneration post-operation. Its immunochemical localization in the hepatocytes during the process showed that it moved from nucleoli of the hepatocytes in the stationary stage to nucleoplasm, cytoplasm, mitotic spindles, and mitotic chromosomes of the hepatocytes in the regenerating livers. It was quantitatively increased progressively to peak level at day 3 post-operation and declined gradually to normal level at day 7. It was detected in nuclear matrix protein (NMP) composition extracted from the regenerating hepatocytes and HepG2 cells and identified with isoelectric point (pI) value of 5.1 and molecular weight of 40 kDa. These results indicated that B23 was a proliferate shuttle protein involving in cell cycle and cell proliferation associated with nuclear matrix.

Interaction of protein phosphatase 1 delta with nucleolin in human osteoblastic cells

We examined the expression and cytolocalization of the protein phosphatase type 1 delta (PP1delta) isoform and nucleolin in human osteoblastic MG63 and Saos-2 cells. Cellular fractionation of MG63 cells was done and protein was prepared from each fraction. Anti-nucleolin antibody interacted with the 100- and 95-kD proteins present in the whole-cell lysate. The 100-kD protein was detected in nuclear and nucleolar fractions. The 95-kD protein was detected in cytosolic and nucleoplasmic fractions. PP1delta and nucleolin were co-localized in the nucleolus in MG63 and Saos-2 cells revealed by an immunofluorescence method. PP1delta and nucleolin were also co-immunoprecipitated with anti-nucleolin and anti-PP1delta antibodies. In the actinomycin D-treated cells, the subcellular localization of PP1delta and nucleolin was changed. Expression of PP1delta was upregulated with actinomycin D treatment. The level of 100-kD protein did not change in the actinomycin D-treated cells. However, the level of the 95-kD band increased with actinomycin D treatment. These results indicate that PP1delta was associated with nucleolin in the nucleolus of MG63 and Saos-2 cells and that nucleolin is a possible candidate substrate for PP1delta.

Isolation and characterization of a new nucleolar protein, Nrap, that is conserved from yeast to humans

BACKGROUND

The nucleolus is the site of rRNA synthesis and processing in eukaryotic cells, but its composition remains poorly understood.

RESULTS

We have identified a novel nucleolar RNA-associated protein (Nrap) which is highly conserved from yeast (Saccharomyces cerevisiae) to human, with homologues in mouse, Drosophila melanogaster, Caenorhabditis elegans, Arabidopsis thaliana, Schizosaccharomyces pombe, and other species. In the mouse, we have found that Nrap is ubiquitously expressed and is specifically localized in the nucleolus. We have also identified splice variants in human and mouse, and defined the intron-exon structure of the human Nrap gene. Nrap is inherited into daughter nuclei by associating with the condensed chromosomes during mitosis. RNase treatment of permeabilized cells indicated that the nucleolar localization of Nrap is RNA dependent. The effects of actinomycin D, cycloheximide and 5,6-dichloro-beta-d-ribofuranosyl-benzimidazole on Nrap expression and distribution in cultured cells suggest that Nrap is associated with the pre-rRNA transcript.

CONCLUSIONS

Nrap is a large nucleolar protein (of more than 1000 amino acids), and is a new class of protein with new structural and functional motifs. Nrap appears to be associated with ribosome biogenesis by interacting with pre-rRNA primary transcript.

Involvement of nucleophosmin/B23 in the response of HeLa cells to UV irradiation

The steady-state mRNA level of nucleophosmin/B23 in HeLa cells increased after UV irradiation. Nucleophosmin/B23 antisense transfection potentiated ultraviolet (UV)-induced cell killing. A block in G(2)/M phase, larger peak of apoptotic cells and higher caspase-3 in vitro activity were noted in nucleophosmin/B23 antisense-transfected cells compared with vector-transfected cells after UV treatment. Irradiated cells that received vector plasmid exhibited increased levels of [(3)H]thymidine incorporation due to DNA repair synthesis. In contrast, irradiated cells that received nucleophosmin/B23 antisense plasmid exhibited no such increase of [(3)H]thymidine incorporation, indicating inhibition of DNA repair. Cotransfection of cells with vector allowed repair of the damaged chloramphenicol acetyl transferase (CAT) reporter and rescue of CAT activity by host repair machinery. CAT activity in cells cotransfected with nucleophosmin/B23 antisense was less (<50%) than that of vector-transfected cells, indicating reduction of host nucleotide excision repair activity. Lower protein expressions of nucleophosmin/B23 and proliferating cell nuclear antigen (PCNA) were observed in nucleophosmin/B23 antisense-transfected cells compared with vector-transfected cells with or without UV treatment. Cotransfection of nucleophosmin/B23 antisense-transfected HeLa cells with PCNA construct made the cells less susceptible to UV-induced cell killing. Our results indicate that nucleophosmin/B23 correlates with PCNA and DNA repair capacity in cellular sensitivity to UV.

Over-expression of nucleophosmin/B23 decreases the susceptibility of human leukemia HL-60 cells to retinoic acid-induced differentiation and apoptosis

Stable clones of HL-60 cells in which nucleophosmin/B23 was over-expressed were established. Less percentages (4-20%) of nucleophosmin/B23 over-expressed (pCR3-B23) cells exhibited the morphological characteristic of apoptosis as compared with control vector-transfected (pCR3) cells (6-53%) during the 10 microM RA treatment for 1-4 days. In flow cytometry analysis, a block in the G1 phase was noted in all the pCR3-B23 and pCR3 cells after 2 days of 10 microM RA treatment and continued to be observed at all times measured up to 6 days. Smaller peaks of apoptotic cells with less than G1 DNA content were observed in pCR3-B23 as compared with pCR3 cells after 4-6 days of 10 microM RA treatment. As measured by expressions of differentiation markers and the functional assessment of the ability to reduce nitroblue-tetrazolium, our results further showed that over-expression of nucleophosmin/B23 decreased the response of the cells to RA-induced differentiation. Less cleavage of PARP and in vitro caspase-3 activity were observed in PCR3-B23 cells as compared with pCR3 cells treated with 10 microM RA for 3-4 days. IRF-1 was induced after 6 hr of 10 microM RA treatment in the pCR3-B23 and pCR3 cells. Significantly more nucleophosmin/B23 was co-immunoprecipitated with IRF-1 from pCR3-B23 cells than from pCR3 cells during RA treatment (10 microM; 24 hr, 96 hr). The IRF-1 transcriptional activity was found to be attenuated in pCR3-B23 cells as compared with pCR3 cells during the treatment of cells with RA. Nucleophosmin/B23, through interacting with IRF-1, plays an important role in the control of the susceptibility of cells to RA-induced differentiation and apoptosis.

Different kinases phosphorylate nucleophosmin/B23 at different sites during G(2) and M phases of the cell cycle

The recombinant GST-nucleophosmin/B23 and the truncated mutants were tested for phosphorylation in cell-free extracts of G(2) and M phases or by purified kinases. Our results indicated that a threonine residue at amino acids (a.a.) 185-240 was phosphorylated by cdc2 kinase during the entry of mitosis while the serine phosphorylation site at the middle acidic portion of the molecule (a. a. 83-152) was phosphorylated by casein kinase II during G(2) phase. Our results also showed that there was possibly another serine phosphorylation at site other than the middle portion of nucleophosmin/B23 (a.a. 83-152) during the entry of cells into mitosis. The demonstration of the characteristic changes in phosphorylation of nucleophosmin/B23 during the cell cycle implicates important role of nucleophosmin/B23 in the control of the fate of nucleoli and cell growth.

In vivo interaction of nucleophosmin/B23 and protein C23 during cell cycle progression in HeLa cells

By using the cross-linking reagent, DSP, efforts were made to identify the protein(s) that interact with nucleophosmin/B23. A cross-linked protein complex at molecular weight of about 140 kDa was recognized by both nucleophosmin/B23 and protein C23 MAbs. Both C23 and nucleophosmin/B23 could be detected from the cross-linked complex immunoprecipitated by C23 MAb. The association between nucleophosmin/B23 and protein C23 while being observed at interphase and cytokinesis, was not detected in prometaphase and metaphase cells. Interactions of nucleophosmin/B23 with C23 not only could be found in cells in which nucleophosmin/B23 and C23 were both mainly localized to the nucleolus, but also in cells in which nucleophosmin/B23 and C23 had translocated from the nucleolus to the nucleoplasm during actinomycin D-induced cell growth inhibition. The purified recombinant GST-B23 being phosphorylated by prometaphase cell extracts (nocodazole-arrested cells) or cdc2 kinase could still be co-immunoprecipitated with C23. Consequently, the fact that nucleophosmin/B23 did not interact with C23 during mitosis could not be explained simply by mitotic phosphorylation of nucleophosmin/B23. Our findings suggest some possibilities for further elucidation of the actions of nucleophosmin/B23 and protein C23 in cell cycle progression and cell growth.

Berberine-induced apoptosis of human leukemia HL-60 cells is associated with down-regulation of nucleophosmin/B23 and telomerase activity

The steady-state level of nucleophosmin/B23 mRNA decreased during berberine-induced (25 microg/ml, 24 to 96 hr) apoptosis of human leukemia HL-60 cells. A decline in telomerase activity was also observed in HL-60 cells treated with berberine. A stable clone of nucleophosmin/B23 overexpressed in HL-60 cells was selected and found to be less responsive to berberine-induced apoptosis. About 35% to 63% of control vector-transfected cells (pCR3) exhibited morphological characteristics of apoptosis, while about 8% to 45% of nucleophosmin/B23-over-expressed cells (pCR3-B23) became apoptotic after incubation with 15 microg/ml berberine for 48 to 96 hr. DNA extracted from pCR3 cells contained more fragmented DNA than pCR3-B23 cells during treatment with 15 microg/ml berberine for 24 to 48 hr. Our results indicate that berberine-induced apoptosis is associated with down-regulation of nucleophosmin/B23 and telomerase activity. We also suggest that nucleophosmin/B23 may play an important role in the control of the cellular response to apoptosis induction.

A study of correlation between NPM-translocation and apoptosis in cells induced by daunomycin

Human leukemia K562 and HeLa cells were treated with daunomycin (DA) for 1-4 hr. With the indirect immunofluorescence technique, we observed that the nucleolar protein nucleophosmin/B23 (NPM) shifted its location from the nucleolus to the nucleoplasm (NPM-translocation). The degree of NPM-translocation was determined by the relative immunofluorescent intensity in the nucleoli vs the nucleoplasm (defined as localization index, LI). We found that NPM-translocation, as determined by the decrease of LI, correlates with cytotoxicity. The degrees of NPM-translocation, chromatin condensation, and DNA fragmentation in HeLa cells were determined after treatment with 0.1, 0.5 and 1 microg/mL DA for 1 hr. We found that NPM-translocation (LI < 2.5) was observed in cells during the treatment with 0.5 and 1 but not with 0.1 microg/mL DA. Also, cells treated with 1 microg/mL remained in an NPM-translocated state for a longer time (5-6 hr) than those cells treated with 0.5 microg/mL (1-2 hr). Cells treated with 0.5 and 1 microg/mL DA showed increased levels of chromatin condensation beginning at 5 hr after the drug treatment. The number of cells with condensed chromatin increased with both time and drug concentration. No cells with condensed chromatin were observed in samples treated with 0.1 microg/mL DA, which also showed no significant NPM-translocation. Similar results were observed for induction of DNA fragmentation. We found that the drug concentration required for induction of DNA fragmentation and chromatin condensation coincided with the drug concentration required for NPM-translocation. Taken together, these results indicate that NPM-translocation correlates with apoptosis induced by daunomycin.

Down-regulation of nucleophosmin/B23 mRNA delays the entry of cells into mitosis

In investigating the regulation of nucleophosmin/B23 mRNA expression at the entry of mitosis, the results of Northern gel blot analysis showed that the nucleophosmin/B23 mRNA levels significantly increased in prometaphase (nocodazole-arrested) or metaphase (colchicine-arrested) cells collected by mitotic shake-off. A higher level of nucleophosmin/B23 mRNA was detected in all the collected mitotic cells arrested by treatment with nocodazole for 10-18h as compared to that in G2 cells. An attempt was then made to determine whether the regulation of nucleophosmin/B23 mRNA plays a role in the control of entry into mitosis. Down-regulation of nucleophosmin/B23 mRNA by transfection of its antisense construct resulted in the delay of cells entering mitosis. The demonstration of the characteristic changes in the mRNA level of nucleophosmin/B23 during the entry of cells into mitosis implicates the importance of nucleophosmin/B23 in the control of the mitotic fate of nucleoli and cell growth.

Detection of Normal and Chimeric Nucleophosmin in Human Cells

In anaplastic large-cell lymphoma (ALCL), the (2;5) chromosomal translocation creates a fusion gene encoding the 80-kD NPM-ALK hybrid protein. This report describes three new monoclonal antibodies, two of which recognize, by Western blotting, the N-terminal portion of NPM present in the NPM-ALK fusion protein and also in two other NPM fusion proteins (NPM-RAR and NPM-MLF1). The third antibody recognizes the C-terminal portion (deleted in NPM-ALK) and reacts only with wild-type NPM. The three antibodies immunostain wild-type NPM (in paraffin-embedded normal tissue samples) in cell nuclei and in the cytoplasm of mitotic cells. Cerebral neurones, exceptionally, show diffuse cytoplasmic labeling. In contrast to normal tissues, the two antibodies against the N-terminal portion of NPM labeled the cytoplasm of neoplastic cells, in four ALK-positive ALCL, reflecting their reactivity with NPM-ALK fusion protein, whereas the antibody to the C-terminal NPM epitope labeled only cell nuclei. Immunocytochemical labeling with these antibodies can therefore confirm that an ALK-positive lymphoma expresses NPM-ALK (rather than a variant ALK-fusion protein) and may also provide evidence for chromosomal anomalies involving the NPM gene other than the classical (2;5) translocation.

Detection of Normal and Chimeric Nucleophosmin in Human Cells

In anaplastic large-cell lymphoma (ALCL), the (2;5) chromosomal translocation creates a fusion gene encoding the 80-kD NPM-ALK hybrid protein. This report describes three new monoclonal antibodies, two of which recognize, by Western blotting, the N-terminal portion of NPM present in the NPM-ALK fusion protein and also in two other NPM fusion proteins (NPM-RAR and NPM-MLF1). The third antibody recognizes the C-terminal portion (deleted in NPM-ALK) and reacts only with wild-type NPM. The three antibodies immunostain wild-type NPM (in paraffin-embedded normal tissue samples) in cell nuclei and in the cytoplasm of mitotic cells. Cerebral neurones, exceptionally, show diffuse cytoplasmic labeling. In contrast to normal tissues, the two antibodies against the N-terminal portion of NPM labeled the cytoplasm of neoplastic cells, in four ALK-positive ALCL, reflecting their reactivity with NPM-ALK fusion protein, whereas the antibody to the C-terminal NPM epitope labeled only cell nuclei. Immunocytochemical labeling with these antibodies can therefore confirm that an ALK-positive lymphoma expresses NPM-ALK (rather than a variant ALK-fusion protein) and may also provide evidence for chromosomal anomalies involving the NPM gene other than the classical (2;5) translocation.

Reconstitution of HIV-1 rev nuclear export: independent requirements for nuclear import and export

The Rev protein of HIV-1 actively shuttles between nucleus and cytoplasm and mediates the export of unspliced retroviral RNAs. The localization of shuttling proteins such as Rev is controlled by the relative rates of nuclear import and export. To study nuclear export in isolation, we generated cell lines expressing a green fluorescent protein-labeled chimeric protein consisting of HIV-1 Rev and a hormone-inducible nuclear localization sequence. Steroid removal switches off import thus allowing direct visualization of the Rev export pathway in living cells. After digitonin permeabilization of these cells, we found that a functional nuclear export sequence (NES), ATP, and fractionated cytosol were sufficient for nuclear export in vitro. Nuclear pore-specific lectins and leptomycin B were potent export inhibitors. Nuclear export was not inhibited by antagonists of calcium metabolism that block nuclear import. These data further suggest that nuclear pores do not functionally close when luminal calcium stores are depleted. The distinct requirements for nuclear import and export argue that these competing processes may be regulated independently. This system should have wide applicability for the analysis of nuclear import and export.

Evidence for the ability of nucleophosmin/B23 to bind ATP

By taking advantage of its ability to be retained by ATP-agarose, we have demonstrated that nucleophosmin/B23 is capable of binding ATP. The specificity of the binding was confirmed by the absence of significant binding to AMP-agarose and by its loss when nucleophosmin/B23 in nuclear extracts was preincubated with ATP. Preincubation of the nuclear extracts with other ribonucleotide triphosphates (GTP, CTP, UTP) did not compete for the binding of nucleophosmin/B23 to ATP-agarose. The purified recombinant nucleophosmin/B23 was also able to be retained by ATP-agarose. The Kd for binding of ATP to the purified recombinant nucleophosmin/B23, on the basis of retention on a nitrocellulose membrane, was 86.5+/-8.3 microM; the number of binding sites was 0.68 per nucleophosmin/B23 protein molecule. To determine the possible ATP-binding site of nucleophosmin/B23, various deletion clones including the two mutants in which the putative ATP-binding sequence had been deleted were constructed. Deletion of the portions of the molecule (residues 83-152 and 185-240) had little effect on the ATP binding. The C-terminal deleted mutant (residue 242 to the C-terminus deleted) lost most of its ability to be retained by ATP-agarose and to bind [alpha-32P]ATP on a nitrocellulose membrane. The results indicate that the C-terminal portion (residues 242-294) contains the essential ATP-binding site of nucleophosmin/B23.

Isolation and characterization of the human nucleophosmin/B23 (NPM) gene: identification of the YY1 binding site at the 5' enhancer region

NPM (nucleophosmin/B23) is a major nucleolar protein which is 20 times more abundant in tumor or proliferating cells than in normal resting cells. Recently, it was found that NPM gene is located at the breakpoints of the t(2:5), t(3:5) and t(5:17) chromosome translocation. To understand the human NPM gene's structure and regulation, four genomic clones were isolated from the human chromosome 5 library and their DNA sequences analyzed. The human NPM gene has 12 exons of sizes ranging from 58 to 358 bp. The chromosome breakpoint for t(2:5) and t(5:17) translocation is within intron 4 and the breakpoint for t(3:5) translocation is within intron 6. The initiation site is located 96 bp upstream from the ATG site. A typical TATA box (at -25 nt) and a GC box (at -65 nt) were identified in the promoter region. We identified two gel-shift bands (A and B) with DNA fragment E (-741/-250 nt) by EMSA. A DNA footprint was observed at (-371/-344 nt) with the nuclear extract. A double stranded DNA with the footprint sequence (-371/-344 nt) competed the formation of gel-shift bands A and B in EMSA suggesting that proteins A and B bind to the footprint region. We confirmed that protein A is transcription factor YY1. These results suggest that YY1 may play a role in NPM gene expression. This is the first report on human NPM gene structure and sequence.