Nucleophosmin/B23 negatively regulates GCN5-dependent histone acetylation and transactivation

Functions of the native NPM1 protein and its leukemic mutant

The nucleophosmin (NPM1) gene encodes for the most abundant nucleolar protein. Thanks to its property to act as histone chaperone and to shuttle between the nucleus and cytoplasm, the NPM1 protein is involved in multiple cellular function that are here extensively reviewed and include the formation of the nucleolus through liquid-liquid phase separation, regulation of ribosome biogenesis and transport, control of DNA repair and centrosome duplication as well as response to nucleolar stress. NPM1 is mutated in about 30-35% of adult acute myeloid leukemia (AML). Due to its unique biological and clinical features, NPM1-mutated AML is regarded as a distinct leukemia entity in the WHO 5th edition and ICC classifications of myeloid malignancies. The NPM1 mutant undergoes changes at the C-terminus of the protein that leads to its delocalization in the cytoplasm of the leukemic cells. Here, we focus also on its biological functions discussing the murine models of NPM1 mutations and the various mechanisms that occur at cytoplasmic and nuclear levels to promote and maintain NPM1-mutated AML.

Nucleophosmin, a multifunctional nucleolar organizer with a role in DNA repair

Nucleophosmin (NPM1) is a mostly nucleolar protein with crucial functions in cell growth and homeostasis, including regulation of ribosome biogenesis and stress response. Such multiple activities rely on its ability to interact with nucleic acids and with hundreds of proteins, as well as on a dynamic subcellular distribution. NPM1 is thus regulated by a complex interplay between localization and interactions, further modulated by post-translational modifications. NPM1 is a homopentamer, with globular domains connected by long, intrinsically disordered linkers. This configuration allows NPM1 to engage in liquid-liquid phase separation phenomena, which could underlie a key role in nucleolar organization. Here, we will discuss NPM1 conformational and functional versatility, emphasizing its emerging, and still largely unexplored, role in DNA damage repair. Since NPM1 is altered in a subtype of acute myeloid leukaemia (AML), we will also present ongoing research on the molecular mechanisms underlying its pathogenic role and potential NPM1-targeting therapeutic strategies.

Nucleophosmin interaction with APE1: Insights into DNA repair regulation

Nucleophosmin (NPM1), an abundant, nucleolar protein with multiple functions affecting cell homeostasis, has also been recently involved in DNA damage repair. The roles of NPM1 in different repair pathways remain however to be elucidated. NPM1 has been described to interact with APE1 (apurinic apyrimidinic endonuclease 1), a key enzyme of the base excision repair (BER) pathway, which could reflect a direct participation of NPM1 in this route. To gain insight into the possible role(s) of NPM1 in BER, we have explored the interplay between the subnuclear localization of both APE1 and NPM1, the in vitro interaction they establish, the effect of binding to abasic DNA on APE1 conformation, and the modulation by NPM1 of APE1 binding and catalysis on DNA. We have found that, upon oxidative damage, NPM1 is released from nucleoli and locates on patches throughout the chromatin, perhaps co-localizing with APE1, and that this traffic could be mediated by phosphorylation of NPM1 on T199. NPM1 and APE1 form a complex in vitro, involving, apart from the core domain, at least part of the linker region of NPM1, whereas the C-terminal domain is dispensable for binding, which explains that an AML leukemia-related NPM1 mutant with an unfolded C-terminal domain can bind APE1. APE1 interaction with abasic DNA stabilizes APE1 structure, as based on thermal unfolding. Moreover, our data suggest that NPM1, maybe by keeping APE1 in an "open" conformation, favours specific recognition of abasic sites on DNA, competing with off-target associations. Therefore, NPM1 might participate in BER favouring APE1 target selection as well as turnover from incised abasic DNA.

MYC cofactors: molecular switches controlling diverse biological outcomes

The transcription factor MYC has fundamental roles in proliferation, apoptosis, tumorigenesis, and stem cell pluripotency. Over the last 30 years extensive information has been gathered on the numerous cofactors that interact with MYC and the target genes that are regulated by MYC as a means of understanding the molecular mechanisms controlling its diverse roles. Despite significant advances and perhaps because the amount of information learned about MYC is overwhelming, there has been little consensus on the molecular functions of MYC that mediate its critical biological roles. In this perspective, the major MYC cofactors that regulate the various transcriptional activities of MYC, including canonical and noncanonical transactivation and transcriptional repression, will be reviewed and a model of how these transcriptional mechanisms control MYC-mediated proliferation, apoptosis, and tumorigenesis will be presented. The basis of the model is that a variety of cofactors form dynamic MYC transcriptional complexes that can switch the molecular and biological functions of MYC to yield a diverse range of outcomes in a cell-type- and context-dependent fashion.

Aluminum can induce alterations in the cellular localization and expression of three major nucleolar proteins in root tip cells of Allium cepa var. agrogarum L

A 50 μM aluminum (Al) could induce nucleolar materials containing the argyrophilic proteins scattered in the nuclei and extruded from the nuclei into the cytoplasm in the root tip cells of Allium cepa. Unfortunately, what kinds of nucleolar proteins are affected has not been reported till now. In order to go deeper into the understanding of the cytological effects of Al on nucleolus and nucleolar proteins, alterations in the cellular localization and expression of three major nucleolar proteins: nucleophosmin, nucleolin, and fibrillarin were further examined under the treatment with Al in the root tip cells of A. cepa in the present study. Cytological effects of Al on nucleolus were observed by silver-staining method and three major nucleolar proteins: nucleophosmin, nucleolin, and fibrillarin were examined by western blotting. The results indicated that in the presence of 50 μM Al for 48 h the nucleolar proteins were translocated from nucleolus to nucleoplasm and cytoplasm. Western blotting data demonstrated the relatively higher expression of the three major nucleolar proteins when compared with control. Evidence from the present investigation indicated that Al had toxic effects on Ag-NOR proteins, nucleophosmin and nucleolin, and other kinds of nucleolar proteins, fibrillarin.

Knockdown of nucleophosmin induces S-phase arrest in HepG2 cells

Nucleophosmin/B23 (NPM) is a universally expressed nucleolar phosphoprotein that participates in proliferation, apoptosis, ribosome assembly, and centrosome duplication; however, the role of NPM in cell cycle regulation is not well characterized. We investigated the mechanism by which NPM is involved in cell cycle regulation. NPM was knocked down using siRNA in HepG2 hepatoblastoma cells. NPM translocation following actinomycin D (ActD) treatment was investigated using immunofluorescent staining. Expression of NPM and other factors involved in cell cycle regulation was examined by Western blotting. Cell cycle distribution was measured using flow cytometry to detect 5-ethynyl-2'-deoxyuridine (EdU) incorporation. Cell proliferation was quantified by the MTT assay. Knockdown of NPM increased the percentage of HepG2 cells in S phase and led to decreased expression of P53 and P21Cip1/WAF1. S-phase arrest in HepG2 cells was significantly enhanced by ActD treatment. Furthermore, knockdown of NPM abrogated ActD-induced G2/M phase cell cycle arrest. Taken together, these data demonstrate that inhibition of NPM has a significant effect on the cell cycle.

The Multifunctional Nucleolar Protein Nucleophosmin/NPM/B23 and the Nucleoplasmin Family of Proteins

The nucleophosmin (NPM)/nucleoplasmin family of nuclear chaperones has three members: NPM1, NPM2, and NPM3. Nuclear chaperones serve to ensure proper assembly of nucleosomes and proper formation of higher order structures of chromatin. In fact, this family of proteins has such diverse functions in cellular processes such as chromatin remodeling, ribosome biogenesis, genome stability, centrosome replication, cell cycle, transcriptional regulation, apoptosis, and tumor suppression. Of the members of this family, NPM1 is the most studied and is the main focus of this review. NPM2 and NPM3 are less well characterized, and are also discussed wherever appropriate. The structure–function relationship of NPM proteins has largely been worked out. Other than the many processes in which NPM1 takes part, the major interest comes from its involvement in human cancers, particularly acute myeloid leukemia (AML). Its significance stems from the fact that AML with mutated NPM1 accounts for ∼30% of all AML cases and usually has good prognosis. Its clinical importance also comes from its involvement in virus replication, particularly in the era of outbreaks of infectious diseases.

NPM1/B23: A Multifunctional Chaperone in Ribosome Biogenesis and Chromatin Remodeling

At a first glance, ribosome biogenesis and chromatin remodeling are quite different processes, but they share a common problem involving interactions between charged nucleic acids and small basic proteins that may result in unwanted intracellular aggregations. The multifunctional nuclear acidic chaperone NPM1 (B23/nucleophosmin) is active in several stages of ribosome biogenesis, chromatin remodeling, and mitosis as well as in DNA repair, replication and transcription. In addition, NPM1 plays an important role in the Myc-ARF-p53 pathway as well as in SUMO regulation. However, the relative importance of NPM1 in these processes remains unclear. Provided herein is an update on the expanding list of the diverse activities and interacting partners of NPM1. Mechanisms of NPM1 nuclear export functions of NPM1 in the nucleolus and at the mitotic spindle are discussed in relation to tumor development. It is argued that the suggested function of NPM1 as a histone chaperone could explain several, but not all, of the effects observed in cells following changes in NPM1 expression. A future challenge is to understand how NPM1 is activated, recruited, and controlled to carry out its functions.

Regulation of E2F1-induced apoptosis by the nucleolar protein RRP1B

Regulation of the E2F family of transcription factors is important in control of cellular proliferation; dysregulation of the E2Fs is a hallmark of many cancers. One member of the E2F family, E2F1, also has the paradoxical ability to induce apoptosis; however, the mechanisms underlying this selectivity are not fully understood. We now identify a nucleolar protein, RRP1B, as an E2F1-specific transcriptional target. We characterize the RRP1B promoter and demonstrate its selective response to E2F1. Consistent with the activation of E2F1 activity upon DNA damage, RRP1B is induced by several DNA-damaging agents. Importantly, RRP1B is required for the expression of certain E2F1 proapoptotic target genes and the induction of apoptosis by DNA-damaging agents. This activity is mediated in part by complex formation between RRP1B and E2F1 on selective E2F1 target gene promoters. Interaction between RRP1B and E2F1 can be found inside the nucleolus and diffuse nucleoplasmic punctates. Thus, E2F1 makes use of its transcriptional target RRP1B to activate other genes directly involved in apoptosis. Our data also suggest an underappreciated role for nucleolar proteins in transcriptional regulation.

Nucleophosmin (NPM1) mutations in adult and childhood acute myeloid leukaemia: towards definition of a new leukaemia entity

Nucleophosmin (NPM) is a ubiquitously expressed chaperone protein that shuttles rapidly between the nucleus and cytoplasm, but predominantly resides in the nucleolus. It plays key roles in ribosome biogenesis, centrosome duplication, genomic stability, cell cycle progression and apoptosis. Somatic mutations in exon 12 of the NPM gene (NPM1) are the most frequent genetic abnormality in adult acute myeloid leukaemia (AML), found in approximately 35% of all cases and up to 60% of patients with normal karyotype (NK) AML. In children, NPM1 mutations are far less frequent, occurring in 8-10% of all AML cases, and in approximately 25% of those with a NK. NPM1 mutations lead to aberrant localization of the NPM protein into the cytoplasm, thus the designation, NPMc+ AML. NPMc+ AML is seen predominantly in patients with a NK and is essentially mutually exclusive of recurrent chromosomal translocations. Patients with NPM1 mutations are twice as likely as those who lack an NPM1 mutation to also have a FMS-like tyrosine kinase (FLT3) internal tandem duplication (ITD) mutation. NPMc+ AML is also characterized by a unique gene expression signature and microRNA signature. NPMc+ AML has important prognostic significance, as NPMc+ AML, in the absence of a coexisting FLT3-ITD mutation, is associated with a favourable outcome. NPM1 mutations have also shown great stability during disease evolution, and therefore represent a possible marker for minimal residual disease detection. Given its distinctive biologic and clinical features and its clear clinical relevance, NPMc+ AML is included as a provisional entity in the 2008 WHO classifications. There is still much to be learned about this genetic alteration, including its exact role in leukaemogenesis, how it interacts with other mutations and why it confers a more favourable prognosis. Further, it represents a potential therapeutic target warranting research aimed at identifying novel small molecules with activity in NPMc+ AML.

Role of nucleophosmin in acute myeloid leukemia

Nucleophosmin (NPM) is a nucleolar phosphoprotein implicated in the regulation of multiple cellular functions, which possesses both oncogenic and tumor-suppressor properties. Mutations of the NPM1 gene leading to the expression of a cytoplasmic mutant protein, NPMc+, are the most frequent genetic abnormalities found in acute myeloid leukemias. Acute myeloid leukemias with mutated NPM1 have distinct characteristics, including a significant association with a normal karyotype, involvement of different hematopoietic lineages, a specific gene-expression profile and clinically, a better response to induction therapy and a favorable prognosis. NPMc+ maintains the capacity of wild-type NPM to interact with a variety of cellular proteins, and impairs their activity by delocalizing them to the cytoplasm. In this review we summarize recent discoveries concerning NPM function, and discuss their possible impact on the pathogenesis of acute myeloid leukemias with mutated NPM1.

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.