Tumor suppressor ARF degrades B23, a nucleolar protein involved in ribosome biogenesis and cell proliferation

TTF1 control of LncRNA synthesis delineates a tumor suppressor pathway directly regulating the ribosomal RNA genes

The tumor suppressor p14/19ARF regulates ribosomal RNA (rRNA) synthesis by controlling the nucleolar localization of Transcription Termination Factor 1 (TTF1). However, the role played by TTF1 in regulating the rRNA genes and in potentially controlling growth has remained unclear. We now show that TTF1 expression regulates cell growth by determining the cellular complement of ribosomes. Unexpectedly, it achieves this by acting as a "roadblock" to synthesis of the noncoding LncRNA and pRNA that we show are generated from the "Spacer Promoter" duplications present upstream of the 47S pre-rRNA promoter on the mouse and human ribosomal RNA genes. Unexpectedly, the endogenous generation of these noncoding RNAs does not induce CpG methylation or gene silencing. Rather, it acts in cis to suppress 47S preinitiation complex formation and hence de novo pre-rRNA synthesis by a mechanism reminiscent of promoter interference or occlusion. Taken together, our data delineate a pathway from p19ARF to cell growth suppression via the regulation of ribosome biogenesis by noncoding RNAs and validate a key cellular growth law in mammalian cells.

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.

RNA expression of 6 genes from metastatic mucosal gastric cancer serves as the global prognostic marker for gastric cancer with functional validation

BACKGROUND

Molecular analysis of advanced tumors can increase tumor heterogeneity and selection bias. We developed a robust prognostic signature for gastric cancer by comparing RNA expression between very rare early gastric cancers invading only mucosal layer (mEGCs) with lymph node metastasis (Npos) and those without metastasis (Nneg).

METHODS

Out of 1003 mEGCs, all Npos were matched to Nneg using propensity scores. Machine learning approach comparing Npos and Nneg was used to develop prognostic signature. The function and robustness of prognostic signature was validated using cell lines and external datasets.

RESULTS

Extensive machine learning with cross-validation identified the prognostic classifier consisting of four overexpressed genes (HDAC5, NPM1, DTX3, and PPP3R1) and two downregulated genes (MED12 and TP53), and enabled us to develop the risk score predicting poor prognosis. Cell lines engineered to high-risk score showed increased invasion, migration, and resistance to 5-FU and Oxaliplatin but maintained sensitivity to an HDAC inhibitor. Mouse models after tail vein injection of cell lines with high-risk score revealed increased metastasis. In three external cohorts, our risk score was identified as the independent prognostic factor for overall and recurrence-free survival.

CONCLUSION

The risk score from the 6-gene classifier can successfully predict the prognosis of gastric cancer.

Elongin B promotes breast cancer progression by ubiquitinating tumor suppressor p14/ARF

Elongin B (ELOB), a pivotal element in the ELOB/c-Cullin2/5-SOCS-box E3 ubiquitin-protein ligase complex, plays a significant role in catalyzing the ubiquitination and subsequent degradation of a broad spectrum of target proteins. Notably, it is documented to facilitate these processes. However, the regulatory role of ELOB in breast cancer remains ambiguous. In this study, through bio-informatic analysis of The Cancer Genome Atlas and Fudan University Shanghai Cancer Center database, we demonstrated that ELOB was over-expressed in breast cancer tissues and was related to unfavorable prognosis. Additionally, pathway enrichment analysis illustrated that high expression of ELOB was associated with multiple cancer promoting pathways, like cell cycle, DNA replication, proteasome and PI3K - Akt signaling pathway, indicating ELOB as a potential anticancer target. Then, we confirmed that both in vivo and in vitro, the proliferation of breast cancer cells could be significantly suppressed by the down-regulation of ELOB. Mechanically, immunoprecipitation and in vivo ubiquitination assays prompted that, as the core element of Cullin2-RBX1-ELOB E3 ligase (CRL2) complex, ELOB regulated the ubiquitination and the subsequent degradation of oncoprotein p14/ARF. Moreover, the anticancer efficacy of erasing ELOB could be rescued by simultaneous knockdown of p14/ARF. Finally, through analyzing breast cancer tissue microarrays and western blot of patient samples, we demonstrated that the expression of ELOB in tumor tissues was elevated in compared to adjacent normal tissues. In conclusion, ELOB is identified to be a promising innovative target for the drug development of breast cancer by promoting the ubiquitination and degradation of oncoprotein p14/ARF.

SUMOylation regulation of ribosome biogenesis: Emerging roles for USP36

Ribosome biogenesis is essential for cell growth, proliferation, and animal development. Its deregulation leads to various human disorders such as ribosomopathies and cancer. Thus, tight regulation of ribosome biogenesis is crucial for normal cell homeostasis. Emerging evidence suggests that posttranslational modifications such as ubiquitination and SUMOylation play a crucial role in regulating ribosome biogenesis. Our recent studies reveal that USP36, a nucleolar deubiquitinating enzyme (DUB), acts also as a SUMO ligase to regulate nucleolar protein group SUMOylation, thereby being essential for ribosome biogenesis. Here, we provide an overview of the current understanding of the SUMOylation regulation of ribosome biogenesis and discuss the role of USP36 in nucleolar SUMOylation.

Could Targeting NPM1c+ Misfolding Be a Promising Strategy for Combating Acute Myeloid Leukemia?

Acute myeloid leukemia (AML) is a heterogeneous group of diseases classified into various types on the basis of distinct features concerning the morphology, cytochemistry and cytogenesis of leukemic cells. Among the different subtypes, the group "AML with gene mutations" includes the variations of the gene of the multifunctional protein nucleophosmin 1 (NPM1). These mutations are the most frequent (~30-35% of AML adult patients and less in pediatric ones) and occur predominantly in the C-terminal domain (CTD) of NPM1. The most important mutation is the insertion at W288, which determines the frame shift W288Cfs12/Ffs12/Lfs*12 and leads to the addition of 2-12 amino acids, which hamper the correct folding of NPM1. This mutation leads to the loss of the nuclear localization signal (NoLS) and to aberrant cytoplasmic localization, denoted as NPM1c+. Many investigations demonstrated that interfering with the cellular location and oligomerization status of NPM1 can influence its biological functions, including the proper buildup of the nucleolus, and therapeutic strategies have been proposed to target NPM1c+, particularly the use of drugs able to re-direct NPM1 localization. Our studies unveiled a direct link between AML mutations and the neat amyloidogenic character of the CTDs of NPM1c+. Herein, with the aim of exploiting these conformational features, novel therapeutic strategies are proposed that rely on the induction of the selective self-cytotoxicity of leukemic blasts by focusing on agents such as peptides, peptoids or small molecules able to enhance amyloid aggregation and targeting selectively AML-NPM1c+ mutations.

The stability of NPM1 oligomers regulated by acidic disordered regions controls the quality of liquid droplets

The nucleolus is a membrane-less nuclear body that typically forms through the process of liquid-liquid phase separation (LLPS) involving its components. NPM1 drives LLPS within the nucleolus and its oligomer formation and inter-oligomer interactions play a cooperative role in inducing LLPS. However, the molecular mechanism underlaying the regulation of liquid droplet quality formed by NPM1 remains poorly understood. In this study, we demonstrate that the N-terminal and central acidic residues within the intrinsically disordered regions (IDR) of NPM1 contribute to attenuating oligomer stability, although differences in the oligomer stability were observed only under stringent conditions. Furthermore, the impact of the IDRs is augmented by an increase in net negative charges resulting from phosphorylation within the IDRs. Significantly, we observed an increase in fluidity of liquid droplets formed by NPM1 with decreased oligomer stability. These results indicate that the difference in oligomer stability only observed biochemically under stringent conditions has a significant impact on liquid droplet quality formed by NPM1. Our findings provide new mechanistic insights into the regulation of nucleolar dynamics during the cell cycle.

T cell proliferation requires ribosomal maturation in nucleolar condensates dependent on DCAF13

T cells require rapid proliferation to initiate adaptive immunity to prevent pathogen attacks. The nucleolus, a distinct subnuclear membrane-less compartment for ribosomal biogenesis, is indispensable for cell proliferation. However, specific nucleolar proteins involved in rapid T cell proliferation and their underlying molecular regulatory mechanism remain elusive. Here, we identified an essential nucleolar protein, DCAF13, in T cells and revealed its significant regulation of rapid T cell proliferation. Its depletion drastically impairs T cell proliferation due to severe 18S rRNA maturation failure, consequent abnormal ribosome assembly in nucleoli, and insufficient production of nascent proteins. Mechanistically, we propose that DCAF13 promotes NPM1 phase separation to accelerate pre-RNA enrichment and its endonuclease UTP23 for 18S rRNA maturation during T cell proliferation. Our findings reveal the modulatory effect of nucleolar NPM1/DCAF13 phase separation on ribosomal maturation to ensure rapid T cell proliferation and further pathogen clearance for the first time.

Nucleophosmin 1 cooperates with the methyltransferase DOT1L to preserve peri-nucleolar heterochromatin organization by regulating H3K27me3 levels and DNA repeats expression

BACKGROUND

NPM1 is a phosphoprotein highly abundant in the nucleolus. However, additional nuclear functions have been attributed to NPM1, probably through interaction with other nuclear factors. DOT1L is one interaction partner of NPM1 that catalyzes methylation of histone H3 at lysine 79 (H3K79). DOT1L, playing functional roles in several biological processes, is known for its capability to organize and regulate chromatin. For example, DOT1L modulates DNA repeats expression within peri-nucleolar heterochromatin. NPM1 also affects peri-nucleolar heterochromatin spatial organization. However, it is unclear as of yet whether NPM1 and DOT1L functionally synergize to preserve nucleoli organization and genome stability, and generally, which molecular mechanisms would be involved.

RESULTS

We characterized the nuclear function of NPM1 on peri-nucleolar heterochromatin organization. We show that (i) monomeric NPM1 interacts preferentially with DOT1L in the nucleus; (ii) NPM1 acts in concert with DOT1L to maintain each other's protein homeostasis; (iii) NPM1 depletion results in H3K79me2 upregulation and differential enrichment at chromatin binding genes including Ezh2; (iv) NPM1 and DOT1L modulate DNA repeats expression and peri-nucleolar heterochromatin organization via epigenetic mechanisms dependent on H3K27me3.

CONCLUSIONS

Our findings give insights into molecular mechanisms employed by NPM1 and DOT1L to regulate heterochromatin activity and structural organization around the nucleoli and shed light on one aspect of the complex role of both proteins in chromatin dynamics.

Analysis of cuproptosis-related genes in Ulcerative colitis and immunological characterization based on machine learning

Cuproptosis is a novel form of cell death, mediated by protein lipid acylation and highly associated with mitochondrial metabolism, which is regulated in the cell. Ulcerative colitis (UC) is a chronic inflammatory bowel disease that recurs frequently, and its incidence is increasing worldwide every year. Currently, a growing number of studies have shown that cuproptosis-related genes (CRGs) play a crucial role in the development and progression of a variety of tumors. However, the regulatory role of CRGs in UC has not been fully elucidated. Firstly, we identified differentially expressed genes in UC, Likewise, CRGs expression profiles and immunological profiles were evaluated. Using 75 UC samples, we typed UC based on the expression profiles of CRGs, followed by correlative immune cell infiltration analysis. Using the weighted gene co-expression network analysis (WGCNA) methodology, the cluster's differentially expressed genes (DEGs) were produced. Then, the performances of extreme gradient boosting models (XGB), support vector machine models (SVM), random forest models (RF), and generalized linear models (GLM) were constructed and predicted. Finally, the effectiveness of the best machine learning model was evaluated using five external datasets, receiver operating characteristic curve (ROC), the area under the curve of ROC (AUC), a calibration curve, a nomogram, and a decision curve analysis (DCA). A total of 13 CRGs were identified as significantly different in UC and control samples. Two subtypes were identified in UC based on CRGs expression profiles. Immune cell infiltration analysis of subtypes showed significant differences between immune cells of different subtypes. WGCNA results showed a total of 8 modules with significant differences between subtypes, with the turquoise module being the most specific. The machine learning results showed satisfactory performance of the XGB model (AUC = 0.981). Finally, the construction of the final 5-gene-based XGB model, validated by the calibration curve, nomogram, decision curve analysis, and five external datasets (GSE11223: AUC = 0.987; GSE38713: AUC = 0.815; GSE53306: AUC = 0.946; GSE94648: AUC = 0.809; GSE87466: AUC = 0.981), also proved to predict subtypes of UC with accuracy. Our research presents a trustworthy model that can predict the likelihood of developing UC and methodically outlines the complex relationship between CRGs and UC.

Molecular functions of the double-sided and inverted ubiquitin-interacting motif found in Xenopus tropicalis cryptochrome 6

Cryptochromes (CRYs) are multifunctional molecules that act as a circadian clock oscillating factor, a blue-light sensor, and a light-driven magnetoreceptor. Cry genes are classified into several groups based on the evolutionary relationships. Cryptochrome 6 gene (Cry6) is present in invertebrates and lower vertebrates such as amphibians and fishes. Here we identified a Cry6 ortholog in Xenopus tropicalis (XtCry6). XtCRY6 retains a conserved long N-terminal extension (termed CRY N-terminal extension; CNE) that is not found in any CRY in the other groups. A structural prediction suggested that CNE contained unique structures; a tetrahelical fold structure topologically related to KaiA/RbsU domain, overlapping nuclear- and nucleolar-localizing signals (NLS/NoLS), and a novel motif (termed DI-UIM) overlapping a double-sided ubiquitin-interacting motif (DUIM) and an inverted ubiquitin-interacting motif (IUIM). Potential activities of the NLS/NoLS and DI-UIM were examined to infer the molecular function of XtCRY6. GFP-NLS/NoLS fusion protein exogenously expressed in HEK293 cells was mostly observed in the nucleolus, while GFP-XtCRY6 was observed in the cytoplasm. A glutathione S-transferase (GST) pull-down assay suggested that the DI-UIM physically interacts with polyubiquitin. Consistently, protein docking simulations implied that XtCRY6 DI-UIM binds two ubiquitin molecules in a relationship of a twofold rotational symmetry with the symmetry axis parallel or perpendicular to the DI-UIM helix. These results strongly suggested that XtCRY6 does not function as a circadian transcriptional repressor and that it might have another function such as photoreceptive molecule regulating light-dependent protein degradation or gene expression through a CNE-mediated interaction with ubiquitinated proteins in the cytoplasm and/or nucleolus.

Current research on viral proteins that interact with fibrillarin

The nucleolus is a multifunctional nuclear domain primarily dedicated to ribosome biogenesis. Certain viruses developed strategies to manipulate host nucleolar proteins to facilitate their replication by modulating ribosomal RNA (rRNA) processing. This association interferes with nucleolar functions resulting in overactivation or arrest of ribosome biogenesis, induction or inhibition of apoptosis, and affecting stress response. The nucleolar protein fibrillarin (FBL) is an important target of some plant and animal viruses. FBL is an essential and highly conserved S-adenosyl methionine (SAM) dependent methyltransferase, capable of rRNA degradation by its intrinsically disordered region (IDR), the glycine/arginine-rich (GAR) domain. It forms a ribonucleoprotein complex that directs 2'-O-methylations in more than 100 sites of pre-rRNAs. It is involved in multiple cellular processes, including initiation of transcription, oncogenesis, and apoptosis, among others. The interaction with animal viruses, including human viruses, triggered its redistribution to the nucleoplasm and cytoplasm, interfering with its role in pre-rRNA processing. Viral-encoded proteins with IDRs as nucleocapsids, matrix, Tat protein, and even a viral snoRNA, can associate with FBL, forcing the nucleolar protein to undergo atypical functions. Here we review the molecular mechanisms employed by animal and human viruses to usurp FBL functions and the effect on cellular processes, particularly in ribosome biogenesis.

Targeting and Monitoring Acute Myeloid Leukaemia with Nucleophosmin-1 (NPM1) Mutation

Mutations in NPM1, also known as nucleophosmin-1, B23, NO38, or numatrin, are seen in approximately one-third of patients with acute myeloid leukaemia (AML). A plethora of treatment strategies have been studied to determine the best possible approach to curing NPM1-mutated AML. Here, we introduce the structure and function of NPM1 and describe the application of minimal residual disease (MRD) monitoring using molecular methods by means of quantitative polymerase chain reaction (qPCR), droplet digital PCR (ddPCR), next-generation sequencing (NGS), and cytometry by time of flight (CyTOF) to target NPM1-mutated AML. Current drugs, now regarded as the standard of care for AML, as well as potential drugs still under development, will also be explored. This review will focus on the role of targeting aberrant NPM1 pathways such as BCL-2 and SYK; as well as epigenetic regulators (RNA polymerase), DNA intercalators (topoisomerase II), menin inhibitors, and hypomethylating agents. Aside from medication, the effects of stress on AML presentation have been reported, and some possible mechanisms outlined. Moreover, targeted strategies will be briefly discussed, not only for the prevention of abnormal trafficking and localisation of cytoplasmic NPM1 but also for the elimination of mutant NPM1 proteins. Lastly, the advancement of immunotherapy such as targeting CD33, CD123, and PD-1 will be mentioned.

The role of the nucleolus in regulating the cell cycle and the DNA damage response

The nucleolus has long been perceived as the site for ribosome biogenesis, but numerous studies suggest that the nucleolus carefully sequesters crucial proteins involved in multiple cellular functions. Among these, the role of nucleolus in cell cycle regulation is the most evident. The nucleolus is the first responder of growth-related signals to mediate normal cell cycle progression. The nucleolus also senses different cellular stress insults by activating diverse pathways that arrest the cell cycle, promote DNA repair, or initiate apoptosis. Here, we review the emerging concepts on how the ribosomal and nonribosomal nucleolar proteins mediate such cellular effects.

Ribosomopathies and cancer: pharmacological implications

INTRODUCTION

The ribosome is a ribonucleoprotein organelle responsible for protein synthesis, and its biogenesis is a highly coordinated process that involves many macromolecular components. Any acquired or inherited impairment in ribosome biogenesis or ribosomopathies is associated with the development of different cancers and rare genetic diseases. Interference with multiple steps of protein synthesis has been shown to promote tumor cell death.

AREAS COVERED

We discuss the current insights about impaired ribosome biogenesis and their secondary consequences on protein synthesis, transcriptional and translational responses, proteotoxic stress, and other metabolic pathways associated with cancer and rare diseases. Studies investigating the modulation of different therapeutic chemical entities targeting cancer in in vitro and in vivo models have also been detailed.

EXPERT OPINION

Despite the association between inherited mutations affecting ribosome biogenesis and cancer biology, the development of therapeutics targeting the essential cellular machinery has only started to emerge. New chemical entities should be designed to modulate different checkpoints (translating oncoproteins, dysregulation of specific ribosome-assembly machinery, ribosomal stress, and rewiring ribosomal functions). Although safe and effective therapies are lacking, consideration should also be given to using existing drugs alone or in combination for long-term safety, with known risks for feasibility in clinical trials and synergistic effects.

EAPB0503, an Imidazoquinoxaline Derivative Modulates SENP3/ARF Mediated SUMOylation, and Induces NPM1c Degradation in NPM1 Mutant AML

Nucleophosmin-1 (NPM1) is a pleiotropic protein involved in numerous cellular processes. NPM1 shuttles between the nucleus and the cytoplasm, but exhibits a predominant nucleolar localization, where its fate and functions are exquisitely controlled by dynamic post-translational modifications (PTM). Sentrin/SUMO Specific Peptidase 3 (SENP3) and ARF are two nucleolar proteins involved in NPM1 PTMs. SENP3 antagonizes ARF-mediated NPM1 SUMOylation, to promote ribosomal biogenesis. In Acute Myeloid Leukemia (AML), NPM1 is frequently mutated, and exhibits an aberrant cytoplasmic localization (NPM1c). NPM1c mutations define a separate AML entity with good prognosis in some AML patients, rendering NPM1c as a potential therapeutic target. SENP3-mediated NPM1 de-SUMOylation induces resistance to therapy in NPM1c AML. Here, we demonstrate that the imidazoquinoxaline EAPB0503 prolongs the survival and results in selective reduction in the leukemia burden of NPM1c AML xenograft mice. Indeed, EAPB0503 selectively downregulates HDM2 expression and activates the p53 pathway in NPM1c expressing cells, resulting in apoptosis. Importantly, we unraveled that NPM1c expressing cells exhibit low basal levels of SUMOylation paralleled with high SENP3 and low ARF basal levels. EAPB0503 reverted these molecular players by inducing NPM1c SUMOylation and ubiquitylation, leading to its proteasomal degradation. EAPB0503-induced NPM1c SUMOylation is concurrent with SENP3 downregulation and ARF upregulation in NPM1c expressing cells. Collectively, these results provide a strong rationale for testing therapies modulating NPM1c post-translational modifications in the management of NPM1c AML.

It’s Getting Complicated—A Fresh Look at p53-MDM2-ARF Triangle in Tumorigenesis and Cancer Therapy

Anti-tumorigenic mechanisms mediated by the tumor suppressor p53, upon oncogenic stresses, are our bodies’ greatest weapons to battle against cancer onset and development. Consequently, factors that possess significant p53-regulating activities have been subjects of serious interest from the cancer research community. Among them, MDM2 and ARF are considered the most influential p53 regulators due to their abilities to inhibit and activate p53 functions, respectively. MDM2 inhibits p53 by promoting ubiquitination and proteasome-mediated degradation of p53, while ARF activates p53 by physically interacting with MDM2 to block its access to p53. This conventional understanding of p53-MDM2-ARF functional triangle have guided the direction of p53 research, as well as the development of p53-based therapeutic strategies for the last 30 years. Our increasing knowledge of this triangle during this time, especially through identification of p53-independent functions of MDM2 and ARF, have uncovered many under-appreciated molecular mechanisms connecting these three proteins. Through recognizing both antagonizing and synergizing relationships among them, our consideration for harnessing these relationships to develop effective cancer therapies needs an update accordingly. In this review, we will re-visit the conventional wisdom regarding p53-MDM2-ARF tumor-regulating mechanisms, highlight impactful studies contributing to the modern look of their relationships, and summarize ongoing efforts to target this pathway for effective cancer treatments. A refreshed appreciation of p53-MDM2-ARF network can bring innovative approaches to develop new generations of genetically-informed and clinically-effective cancer therapies.

Differential regulation of p27Kip1 depending on culture conditions and its correlation with status of p14ARF and p53

p27^Kip1 is known as a major cyclin-dependent kinase inhibitor and a tumor suppressor, and often functionally hampered at protein level. p27 protein expression levels are frequently low in various cancers and negatively correlated with malignancy of cancer. However, in our previous study, we discovered that p27 overexpression does not inhibit the proliferation of two cancer cell lines due to a functional suppression of p27 by nucleophosmin isoform 1 (NPM1); that is, a qualitative, not quantitative, suppression of p27 function occurs in these cancer cell lines. To clarify the regulation of p27 in several types of cancer, we investigated p27 function in other cancer cell lines, based on proliferation assays in those cell lines carrying doxycycline-inducible p27, and found that MDAH041 cells which express p14ARF, an antagonist of NPM1, show growth inhibition depending on p27 induction. Moreover, to investigate p27 function under anchorage-independent culture conditions, we performed soft agar colony formation assay and observed that the colony formation of some cell lines carrying wild-type p53, a major tumor suppressor, was inhibited depending on p27 induction. These results suggest that p27 function is regulated differentially among cancer cell types under anchorage-dependent and anchorage-independent culture conditions.

The potential role of nucleophosmin (NPM1) in the development of cancer

Nucleophosmin (NPM1) is a well-known nucleocytoplasmic shuttling protein that performs several cellular functions such as ribosome biogenesis, chromatin remodeling, genomic stability, cell cycle progression, and apoptosis. NPM1 has been identified to be necessary for normal cellular functions, and its altered regulation by overexpression, mutation, translocation, loss of function, or sporadic deletion can lead to cancer and tumorigenesis. In this review, we focus on the gene and protein structure of NPM1 and its physiological roles. Finally, we discuss the association of NPM1 with various types of cancer including solid tumors and leukemia.

TRPV1 Activation Promotes β-arrestin2 Interaction with the Ribosomal Biogenesis Machinery in the Nucleolus:Implications for p53 Regulation and Neurite Outgrowth

Transient receptor potential vanilloids (TRPV1) are non-selective cation channels that sense and transduce inflammatory pain signals. We previously reported that activation of TRPV1 induced the translocation of β-arrestin2 (ARRB2) from the cytoplasm to the nucleus, raising questions about the functional role of ARRB2 in the nucleus. Here, we determined the ARRB2 nuclear signalosome by conducting a quantitative proteomic analysis of the nucleus-sequestered L395Q ARRB2 mutant, compared to the cytosolic wild-type ARRB2 (WT ARRB2), in a heterologous expression system. We identified clusters of proteins that localize to the nucleolus and are involved in ribosomal biogenesis. Accordingly, L395Q ARRB2 or WT ARRB2 after capsaicin treatment were found to co-localize and interact with the nucleolar marker nucleophosmin (NPM1), treacle protein (TCOF1) and RNA polymerase I (POL I). We further investigated the role of nuclear ARRB2 signaling in regulating neuroplasticity. Using neuroblastoma (neuro2a) cells and dorsal root ganglia (DRG) neurons, we found that L395Q ARRB2 mutant increased POL I activity, inhibited the tumor suppressorp53 (p53) level and caused a decrease in the outgrowth of neurites. Together, our results suggest that the activation of TRPV1 promotes the ARRB2-mediated regulation of ribosomal biogenesis in the nucleolus. The ARRB2-TCOF1-p53 checkpoint signaling pathway might be involved in regulating neurite outgrowth associated with pathological pain conditions.

MDMX Recruits UbcH5c to Facilitate MDM2 E3 Ligase Activity and Subsequent p53 Degradation In Vivo

MDM2 regulates p53 degradation by functioning as an E3 ubiquitin ligase. The role of MDMX, an MDM2 homolog that lacks E3 ligase activity, in the regulation of p53 degradation remains incompletely understood and sometime controversial. This confusion is due at least in part to studies of p53 degradation mainly carried out in in vitro settings, as elimination of either MDM2 or MDMX from mice results in p53-dependent embryonic lethality, thus obfuscating in vivo studies of the individual roles of MDM2 and MDMX in p53 degradation. To overcome this problem, we generated mice expressing an inducible p53 allele under various MDM2 and MDMX deletion and mutation statuses and studied in vivo p53 degradation. Degradation of p53 in vivo was largely prevented in mice and mouse embryonic fibroblast retaining MDM2 but lacking MDMX. Although MDM2 and MDMX interacted with p53 in the absence of each other, they bound p53 more efficiently as a heterodimer. MDMX, but not MDM2, interacted with ubiquitin-conjugating enzyme UbcH5c, an interaction that was essential for MDMX to enable MDM2 E3 ligase activity for p53 degradation. Grafting the C-terminal residues of MDMX to the C-terminus of MDM2 allowed MDM2 to interact with UbcH5c and enhanced MDM2-mediated p53 degradation in the absence of MDMX. Together, these data indicate that MDMX plays an essential role for p53 degradation in vivo by recruiting UbcH5c to facilitate MDM2 E3 ligase function. SIGNIFICANCE: This study provides the first in vivo evidence of MDMX facilitating MDM2-mediated p53 degradation, clarifying its role in the regulation of this critical tumor suppressor.

Tumor-associated antigen Prame targets tumor suppressor p14/ARF for degradation as the receptor protein of CRL2Prame complex

Protein Preferentially Expressed Antigen in Melanoma (Prame), a tumor-associated antigen, has been found to frequently overexpress in various cancers, which indicates advanced cancer stages and poor clinical prognosis. Moreover, previous reports noted that Prame functions as a substrate recognizing receptor protein of Cullin RING E3 ligases (CRLs) to mediate potential substrates degradation through Ubiquitin Proteasome System (UPS). However, none of the Prame specific substrate has been identified so far. In this study, proteomic analysis of RBX1-interacting proteins revealed p14/ARF, a well-known tumor suppressor, as a novel ubiquitin target of RBX1. Subsequently, immunoprecipitation and in vivo ubiquitination assay determined Cullin2-RBX1-Transcription Elongation Factor B Subunit 2 (EloB) assembled CRL2 E3 ligase complex to regulate the ubiquitination and subsequent degradation of p14/ARF. Finally, through siRNA screening, Prame was identified as the specific receptor protein responsible for recognizing p14/ARF to be degraded. Additionally, via bioinformatics analysis of TCGA database and clinical samples, Prame was determined to overexpress in tumor tissues vs. paired adjacent tissues and associated with poor prognosis of cancer patients. As such, downregulation of Prame expression significantly restrained cancer cell growth by inducing G2/M phase cell cycle arrest, which could be rescued by simultaneously knocking down of p14/ARF. Altogether, targeting overexpressed Prame in cancer cells inactivated RBX1-Cullin2-EloB-Prame E3 ligase (CRL2^Prame) and halted p14/ARF degradation to restrain tumor growth by inducing G2/M phase cell cycle arrest.

NSC348884 cytotoxicity is not mediated by inhibition of nucleophosmin oligomerization

Nucleophosmin (NPM) mutations causing its export from the nucleoli to the cytoplasm are frequent in acute myeloid leukemia (AML). Due to heterooligomerization of wild type NPM with the AML-related mutant, the wild-type becomes misplaced from the nucleoli and its functions are significantly altered. Dissociation of NPM heterooligomers may thus restore the proper localization and function of wild-type NPM. NSC348884 is supposed to act as a potent inhibitor of NPM oligomerization. The effect of NSC348884 on the NPM oligomerization was thoroughly examined by fluorescence lifetime imaging with utilization of FRET and by a set of immunoprecipitation and electrophoretic methods. Leukemia-derived cell lines and primary AML cells as well as cells transfected with fluorescently labeled NPM forms were investigated. Our results clearly demonstrate that NSC348884 does not inhibit formation of NPM oligomers neither in vivo nor in vitro. Instead, we document that NSC348884 cytotoxicity is rather associated with modified cell adhesion signaling. The cytotoxic mechanism of NSC348884 has therefore to be reconsidered.

Mouse Models of Myeloid Malignancies

Mouse models of human myeloid malignancies support the detailed and focused investigation of selected driver mutations and represent powerful tools in the study of these diseases. Carefully developed murine models can closely recapitulate human myeloid malignancies in vivo, enabling the interrogation of a number of aspects of these diseases including their preclinical course, interactions with the microenvironment, effects of pharmacological agents, and the role of non-cell-autonomous factors, as well as the synergy between co-occurring mutations. Importantly, advances in gene-editing technologies, particularly CRISPR-Cas9, have opened new avenues for the development and study of genetically modified mice and also enable the direct modification of mouse and human hematopoietic cells. In this review we provide a concise overview of some of the important mouse models that have advanced our understanding of myeloid leukemogenesis with an emphasis on models relevant to clonal hematopoiesis, myelodysplastic syndromes, and acute myeloid leukemia with a normal karyotype.

Ribosome biogenesis is a downstream effector of the oncogenic U2AF1-S34F mutation

U2 Small Nuclear RNA Auxiliary Factor 1 (U2AF1) forms a heterodimeric complex with U2AF2 that is primarily responsible for 3' splice site selection. U2AF1 mutations have been identified in most cancers but are prevalent in Myelodysplastic Syndrome (MDS) and Acute Myeloid Leukemia (AML), and the most common mutation is a missense substitution of serine-34 to phenylalanine (S34F). The U2AF heterodimer also has a noncanonical function as a translational regulator. Here, we report that the U2AF1-S34F mutation results in specific misregulation of the translation initiation and ribosome biogenesis machinery. The net result is an increase in mRNA translation at the single-cell level. Among the translationally up-regulated targets of U2AF1-S34F is Nucleophosmin 1 (NPM1), which is a major driver of myeloid malignancy. Depletion of NPM1 impairs the viability of the U2AF1-S34F mutant cells and causes ribosomal RNA (rRNA) processing defects, thus indicating an unanticipated synthetic interaction between U2AF1, NPM1, and ribosome biogenesis. Our results establish a unique molecular phenotype for the U2AF1 mutation that recapitulates translational misregulation in myeloid disease.

Increased Expression of NPM1 Suppresses p27Kip1 Function in Cancer Cells

p27^Kip1, a major cyclin-dependent kinase inhibitor, is frequently expressed at low levels in cancers, which correlates with their malignancy. However, in this study, we found a qualitative suppression of p27 overexpressed in some cancer cells. By proteomic screening for factors interacting with p27, we identified nucleophosmin isoform 1 (NPM1) as a novel p27-interacting factor and observed that NPM1 protein was expressed at high levels in some cancer cells. NPM1 overexpression in normal cells suppressed p27 function, and conversely, NPM1 knockdown in cancer cells restored the function in vitro. Furthermore, the tumors derived from cancer cells carrying the combination of p27 overexpression and NPM1 knockdown constructs showed significant suppression of growth as compared with those carrying other combinations in mouse xenograft models. These results strongly suggest that increased expression of NPM1 qualitatively suppresses p27 function in cancer cells.

The interaction between nucleophosmin/NPM1 and the large ribosomal subunit precursors contribute to maintaining the nucleolar structure

Nucleoli are sites where both the large and small ribosomal subunits mature. Biochemical assays have suggested that a multivalent nucleolar protein, NPM1/nucleophosmin contributes to the formation of the outer layer of the nucleolus. Prior works show that NPM1 depletion disorganizes the nucleolar structure. However, the mechanism of how NPM1 regulates the nucleolar structure has been unknown. We demonstrated that NPM1 directly interacts with the large ribosomal subunits and maintains them in the nucleolus. Ectopically localized NPM1 efficiently recruits only the large ribosomal subunit precursors, while ectopically localized large ribosomal subunit by the ribosomal protein RPL4 efficiently recruits NPM1. These results suggest that the nucleolar localization of NPM1 and the large ribosomal subunit precursors are mutually dependent. Furthermore, proteomic and localization analyses suggest that NPM1 plays a crucial role in the accumulation of the late processing machinery of the large ribosomal subunits in the nucleolus. Our results suggest that NPM1 maintains the pre-ribosomes and assembly machinery in the nucleolus, which in turn determines the nucleolar volume.

The Ins and Outs of HOPS/TMUB1 in biology and pathology

Liver regeneration represents an outstanding tool to study not only proliferation, but also other important processes such as inflammation, regenerative response or stem cell biology. Several novel genes have been identified as being involved in the proliferation of residual hepatocytes. One of them, HOPS/TMUB1, is proving to be a significant player in the control of proliferation, both contributing to genomic stability and as a partner of essential molecules. HOPS is an ubiquitin-like protein, shuttling from nucleus to cytoplasm, and it is engaged in a number of biological and physiopathological functions. HOPS overexpression in tumour cell lines strongly reduces proliferation, arresting cell cycle in G₀ /G₁ . HOPS is involved in centrosome assembly and maintenance, and its knockdown causes genomic instability. Moreover, a direct interaction of HOPS with nucleophosmin (NPM) and p19^Arf has been established, resulting in proper control of p19^Arf stability and localization. These data indicate that HOPS acts as a functional bridge in the interaction between NPM and p19^Arf , providing new mechanistic insight into how NPM and p19^Arf will oppose cell proliferation. HOPS exerts a control in p53 stability, directing p53 mitochondrial apoptosis and cytoplasmic localization. HOPS plays a direct role as novel post-translational modifier of p53, much like SUMO or NEDD. HOPS is overexpressed in a high number of human tumours in patients affected by large intestinal, CNS, liver and oesophageal tumours. This review highlights HOPS involvement in distinct cellular functions, establishing its role as a key player in cell biology and pathology in a broader context.

Nucleophosmin in Its Interaction with Ligands

Nucleophosmin (NPM1) is a mainly nucleolar protein that shuttles between nucleoli, nucleoplasm and cytoplasm to fulfill its many functions. It is a chaperone of both nucleic acids and proteins and plays a role in cell cycle control, centrosome duplication, ribosome maturation and export, as well as the cellular response to a variety of stress stimuli. NPM1 is a hub protein in nucleoli where it contributes to nucleolar organization through heterotypic and homotypic interactions. Furthermore, several alterations, including overexpression, chromosomal translocations and mutations are present in solid and hematological cancers. Recently, novel germline mutations that cause dyskeratosis congenita have also been described. This review focuses on NPM1 interactions and inhibition. Indeed, the list of NPM1 binding partners is ever-growing and, in recent years, many studies contributed to clarifying the structural basis for NPM1 recognition of both nucleic acids and several proteins. Intriguingly, a number of natural and synthetic ligands that interfere with NPM1 interactions have also been reported. The possible role of NPM1 inhibitors in the treatment of multiple cancers and other pathologies is emerging as a new therapeutic strategy.

Nucleophosmin 1 Mutations in Acute Myeloid Leukemia

Nucleophosmin (NPM1) is a ubiquitously expressed nucleolar protein involved in ribosome biogenesis, the maintenance of genomic integrity and the regulation of the ARF-p53 tumor-suppressor pathway among multiple other functions. Mutations in the corresponding gene cause a cytoplasmic dislocation of the NPM1 protein. These mutations are unique to acute myeloid leukemia (AML), a disease characterized by clonal expansion, impaired differentiation and the proliferation of myeloid cells in the bone marrow. Despite our improved understanding of NPM1 mutations and their consequences, the underlying leukemia pathogenesis is still unclear. Recent studies that focused on dysregulated gene expression in AML with mutated NPM1 have shed more light into these mechanisms. In this article, we review the current evidence on normal functions of NPM1 and aberrant functioning in AML, and highlight investigational strategies targeting these mutations.

Genotypic and Phenotypic Characteristics of Acute Promyelocytic Leukemia Translocation Variants

Acute promyelocytic leukemia (APL) is a special disease entity of acute myeloid leukemia (AML). The clinical use of all-trans retinoic acid (ATRA) has transformed APL into the most curable form of AML. The majority of APL cases are characterized by the fusion gene PML-RARA. Although the PML-RARA fusion gene can be detected in almost all APL cases, translocation variants of APL have been reported. To date, this is the most comprehensive review of these translocations, discussing 15 different variants. Reviewed genes involved in APL variants include: ZBTB16, NPM, NuMA, STAT5b, PRKAR1A, FIP1L1, BCOR, NABP1, TBLR1, GTF2I, IRF2BP2, FNDC3B, ADAMDTS17, STAT3, and TFG. The genotypic and phenotypic features of APL translocations are summarized. All reported studies were either case reports or case series indicating the rarity of these entities and limiting the ability to drive conclusions regarding their characteristics. However, reported variants have shown variable clinical and morphological features, with diverse responsiveness to ATRA.

Intracellular partners of fibroblast growth factors 1 and 2 - implications for functions

Fibroblast growth factors 1 and 2 (FGF1 and FGF2) are mainly considered as ligands of surface receptors through which they regulate a broad spectrum of biological processes. They are secreted in non-canonical way and, unlike other growth factors, they are able to translocate from the endosome to the cell interior. These unique features, as well as the role of the intracellular pool of FGF1 and FGF2, are far from being fully understood. An increasing number of reports address this problem, focusing on the intracellular interactions of FGF1 and 2. Here, we summarize the current state of knowledge of the FGF1 and FGF2 binding partners inside the cell and the possible role of these interactions. The partner proteins are grouped according to their function, including proteins involved in secretion, cell signaling, nucleocytoplasmic transport, binding and processing of nucleic acids, ATP binding, and cytoskeleton assembly. An in-depth analysis of the network of these binding partners could indicate novel, non-classical functions of FGF1 and FGF2 and uncover an additional level of a fine control of the well-known FGF-regulated cellular processes.

Heterogeneous nuclear ribonucleoprotein (hnRNPL) in cancer

Heterogeneous nuclear ribonucleoprotein L (hnRNPL) is a type of RNA binding protein that is mainly located in the nucleus. hnRNPL protein, encoded by the gene located at 19q13.2, is an important member of the hnRNP family. In recent years, studies have shown that hnRNPL is highly expressed in a variety of tumors and plays a vital role in tumor progression. hnRNPL promotes various biological processes of tumor cells, including proliferation, migration and invasion. In this review, we discuss the clinical significance of hnRNPL by reviewing the mechanism of hnRNPL in the tumorigenesis of various cancers.

NPM1 upregulates the transcription of PD-L1 and suppresses T cell activity in triple-negative breast cancer

Programmed cell death protein-1 (PD-1)/programmed cell death ligand-1 (PD-L1) interaction plays a crucial role in tumor-associated immune escape. Here, we verify that triple-negative breast cancer (TNBC) has higher PD-L1 expression than other subtypes. We then discover that nucleophosmin (NPM1) binds to PD-L1 promoter specifically in TNBC cells and activates PD-L1 transcription, thus inhibiting T cell activity in vitro and in vivo. Furthermore, we demonstrate that PARP1 suppresses PD-L1 transcription through its interaction with the nucleic acid binding domain of NPM1, which is required for the binding of NPM1 at PD-L1 promoter. Consistently, the PARP1 inhibitor olaparib elevates PD-L1 expression in TNBC and exerts a better effect with anti-PD-L1 therapy. Together, our research has revealed NPM1 as a transcription regulator of PD-L1 in TNBC, which could lead to potential therapeutic strategies to enhance the efficacy of cancer immunotherapy.

PD-L1 is highly expressed in triple-negative breast cancers (TNBC). Here, the authors show that nucleophosmin 1 (NPM1) transcriptionally activates PD-L1 expression and inhibits T cell activity in TNBC.

Increased expression of nucleophosmin is associated with the pathophysiology of chronic rhinosinusitis with nasal polyposis

OBJECTIVE

Nucleophosmin (NPM1) has been suggested to be involved in the pathophysiologic mechanism of inflammatory disorders. We measured the expression level of NPM1 in nasal polyp (NP) tissues of patients with chronic rhinosinusitis with nasal polyposis (CRSwNP). We also assessed the correlation between NPM1 expression and other parameters such as eosinophilic infiltration, inflammatory cytokines, and clinical indicators such as Lund-Mackay computed tomography (CT) score.

METHODS

Thirty patients with CRSwNP were included. We performed pre-operative CT scan to determine Lund-Mackay CT scores. During endoscopic sinus surgery, we harvested NP tissues from patients with CRSwNP. We performed Sirius red staining to evaluate eosinophilia and conducted immunohistochemical staining for NPM1 and real-time PCR for cytokines including interleukin (IL)-5, IL-17A, and IL-32.

RESULTS

The mRNA expression of NPM1 was significantly up-regulated in eosinophilic NP tissues (RQ 0.58 ± 0.06), compared to non-eosinophilic NP tissues (RQ 0.38 ± 0.08, p < 0.05). In the epithelium of NP tissue, a significant positive correlation was observed between eosinophilic infiltration and NPM1 expression. The expression of NPM1 was significantly correlated with that of IL-5 (r = 0.6229, p = 0.0004), IL-17A (r = 0.5971, p = 0.001), and IL-32 (r = -0.5985, p = 0.0068). There was no significant correlation between the mRNA expression of NPM1 and the Lund-Mackay CT score (Spearman r = -0.2563, p = 0.1879).

CONCLUSION

Expression of NPM1 was significantly increased in eosinophilic NP tissues from patients with CRSwNP. We observed an association between NPM1 expression and various pro-inflammatory cytokines such as IL-5, IL-17, and IL-32 and eosinophilic infiltration, which is thought to contribute to the pathophysiology of NP.

Nuclear inclusions of pathogenic ataxin-1 induce oxidative stress and perturb the protein synthesis machinery

Spinocerebellar ataxia type-1 (SCA1) is caused by an abnormally expanded polyglutamine (polyQ) tract in ataxin-1. These expansions are responsible for protein misfolding and self-assembly into intranuclear inclusion bodies (IIBs) that are somehow linked to neuronal death. However, owing to lack of a suitable cellular model, the downstream consequences of IIB formation are yet to be resolved. Here, we describe a nuclear protein aggregation model of pathogenic human ataxin-1 and characterize IIB effects. Using an inducible Sleeping Beauty transposon system, we overexpressed the ATXN1(Q82) gene in human mesenchymal stem cells that are resistant to the early cytotoxic effects caused by the expression of the mutant protein. We characterized the structure and the protein composition of insoluble polyQ IIBs which gradually occupy the nuclei and are responsible for the generation of reactive oxygen species. In response to their formation, our transcriptome analysis reveals a cerebellum-specific perturbed protein interaction network, primarily affecting protein synthesis. We propose that insoluble polyQ IIBs cause oxidative and nucleolar stress and affect the assembly of the ribosome by capturing or down-regulating essential components. The inducible cell system can be utilized to decipher the cellular consequences of polyQ protein aggregation. Our strategy provides a broadly applicable methodology for studying polyQ diseases.

Chromatin-bound CRM1 recruits SET-Nup214 and NPM1c onto HOX clusters causing aberrant HOX expression in leukemia cells

We previously demonstrated that CRM1, a major nuclear export factor, accumulates at Hox cluster regions to recruit nucleoporin-fusion protein Nup98HoxA9, resulting in robust activation of Hox genes (Oka et al., 2016). However, whether this phenomenon is general to other leukemogenic proteins remains unknown. Here, we show that two other leukemogenic proteins, nucleoporin-fusion SET-Nup214 and the NPM1 mutant, NPM1c, which contains a nuclear export signal (NES) at its C-terminus and is one of the most frequent mutations in acute myeloid leukemia, are recruited to the HOX cluster region via chromatin-bound CRM1, leading to HOX gene activation in human leukemia cells. Furthermore, we demonstrate that this mechanism is highly sensitive to a CRM1 inhibitor in leukemia cell line. Together, these findings indicate that CRM1 acts as a key molecule that connects leukemogenic proteins to aberrant HOX gene regulation either via nucleoporin-CRM1 interaction (for SET-Nup214) or NES-CRM1 interaction (for NPM1c).

Oncogenic Ras mutant causes the hyperactivation of NF-κB via acceleration of its transcriptional activation

It is well established that nuclear factor κB (NF-κB) acts as one of the most important transcription factors for tumor initiation and progression, as it both protects cells from apoptotic/necrotic signals and accelerates angiogenesis and tumor metastasis, which is mediated via the expression of target genes. However, it has not yet been clarified how oncogenic signals accelerate the activation of NF-κB. In the current study, we utilized untransformed NIH-3T3 cells stably harboring a κB-driven luciferase gene to show that an oncogenic mutant of Ras GTPase augmented TNFα-induced NF-κB activation. Notably, enforced expression of cyclin-dependent kinase inhibitors, such as p27^Kip1 and p21^Cip1 , effectively canceled the accelerated activation of NF-κB, suggesting that oncogenic Ras-induced cell cycle progression is essential for the hyperactivation of NF-κB. Furthermore, we found that Ras (G12V) augmented the transcriptional activation of NF-κB, and this activation required the p38 MAP kinase. We observed that a downstream kinase of p38 MAP kinase, MSK1, was activated by Ras (G12V) and catalyzed the phosphorylation of p65/RelA at Ser-276, which is critical for its transcriptional activation. Significantly, phosphorylation of the p65/RelA subunit at Ser-276 was elevated in patient samples of colorectal cancer harboring oncogenic mutations of the K-Ras gene, and the expression levels of NF-κB target genes were drastically enhanced in several cancer tissues. These observations strongly suggest that oncogenic signal-induced acceleration of NF-κB activation is caused by activation of the p38 MAP kinase-MSK1 signaling axis and by cell cycle progression in cancer cells.

Formation of adenovirus DNA replication compartments and viral DNA accumulation sites by host chromatin regulatory proteins including NPM1

The adenovirus (Ad) genome is believed to be packaged into the virion by forming a chromatin-like structure. The replicated viral genome is likely to be condensed through binding with viral core proteins before encapsidation. Replicated viral genomes accumulate in the central region of the nucleus, which we termed virus-induced postreplication (ViPR) body. However, the molecular mechanism by which the nuclear structure is reorganized and its functional significance in virus production are currently not understood. In this study, we found that viral packaging protein IVa2, but not capsid proteins, accumulated in the ViPR body. In addition, nucleolar chromatin regulatory proteins, nucleophosmin 1 (NPM1), upstream binding factor, and nucleolin accumulated in the ViPR body in late-stage Ad infection. NPM1 depletion increased the nuclease-resistant viral genome and delayed the ViPR body formation. These results suggested that structural changes in the infected cell nucleus depend on the formation of viral chromatin by host chromatin regulatory proteins. Because NPM1 depletion decreases production of the infectious virion, we propose that host factor-mediated viral chromatin remodeling and concomitant ViPR body formation are prerequisites for efficient encapsidation of Ad chromatin.

The PinX1/NPM interaction associates with hTERT in early-S phase and facilitates telomerase activation

Background

Telomere maintenance is an important factor in tumorigenesis. PinX1 is a potent telomerase regulator which also involves in telomerase loading to telomeres. Nucleophosmin (NPM) can partially attenuate PinX1 inhibition of telomerase activity and NPM loading to hTERT requires PinX1. However, the role of the PinX1/NPM interaction in telomerase activity is not fully understood.

Method

The long-term effects of PinX1 and NPM down-regulation on telomere length were investigated by TRF assay. The localization of the PinX1/NPM association and the NPM/PinX1/hTERT complex formation were examined by immunofluorescence studies.

Results

Concurrent long-term down-regulation of PinX1 and NPM led to a substantial decrease in telomere length. The interaction with PinX1 was crucial in NPM localization in the nucleolus during the S phase. PinX1 and NPM associated throughout S phase and the NPM/PinX1/hTERT complex formation peaked during the early-S phase. The PinX1/NPM interaction was shown to localize away from Cajal Bodies at the start of S phase.

Conclusion

PinX1/NPM interaction is important in telomerase regulation during catalysis. NPM is recruited to hTERT by PinX1 and is required in the proposed telomerase modulating unit to activate telomerase when telomere extension occurs during S phase.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0306-y) contains supplementary material, which is available to authorized users.

Signal Transduction in Ribosome Biogenesis: A Recipe to Avoid Disaster

Energetically speaking, ribosome biogenesis is by far the most costly process of the cell and, therefore, must be highly regulated in order to avoid unnecessary energy expenditure. Not only must ribosomal RNA (rRNA) synthesis, ribosomal protein (RP) transcription, translation, and nuclear import, as well as ribosome assembly, be tightly controlled, these events must be coordinated with other cellular events, such as cell division and differentiation. In addition, ribosome biogenesis must respond rapidly to environmental cues mediated by internal and cell surface receptors, or stress (oxidative stress, DNA damage, amino acid depletion, etc.). This review examines some of the well-studied pathways known to control ribosome biogenesis (PI3K-AKT-mTOR, RB-p53, MYC) and how they may interact with some of the less well studied pathways (eIF2α kinase and RNA editing/splicing) in higher eukaryotes to regulate ribosome biogenesis, assembly, and protein translation in a dynamic manner.

Impaired ribosome biogenesis: mechanisms and relevance to cancer and aging

The biosynthesis of ribosomes is a complex process that requires the coordinated action of many factors and a huge energy investment from the cell. Ribosomes are essential for protein production, and thus for cellular survival, growth and proliferation. Ribosome biogenesis is initiated in the nucleolus and includes: the synthesis and processing of ribosomal RNAs, assembly of ribosomal proteins, transport to the cytoplasm and association of ribosomal subunits. The disruption of ribosome biogenesis at various steps, with either increased or decreased expression of different ribosomal components, can promote cell cycle arrest, senescence or apoptosis. Additionally, interference with ribosomal biogenesis is often associated with cancer, aging and age-related degenerative diseases. Here, we review current knowledge on impaired ribosome biogenesis, discuss the main factors involved in stress responses under such circumstances and focus on examples with clinical relevance.

Nucleophosmin in leukemia: Consequences of anchor loss

Nucleophosmin (NPM), one of the most abundant nucleolar proteins, has crucial functions in ribosome biogenesis, cell cycle control, and DNA-damage repair. In human cells, NPM occurs mainly in oligomers. It functions as a chaperone, undergoes numerous interactions and forms part of many protein complexes. Although NPM role in carcinogenesis is not fully elucidated, a variety of tumor suppressor as well as oncogenic activities were described. NPM is overexpressed, fused with other proteins, or mutated in various tumor types. In the acute myeloid leukemia (AML), characteristic mutations in NPM1 gene, leading to modification of NPM C-terminus, are the most frequent genetic aberration. Although multiple mutation types of NPM are found in AML, they are all characterized by aberrant cytoplasmic localization of the mutated protein. In this review, current knowledge of the structure and function of NPM is presented in relation to its interaction network, in particular to the interaction with other nucleolar proteins and with proteins active in apoptosis. Possible molecular mechanisms of NPM mutation-driven leukemogenesis and NPM therapeutic targeting are discussed. Finally, recent findings concerning the immunogenicity of the mutated NPM and specific immunological features of AML patients with NPM mutation are summarized.

Dual Role of the Alternative Reading Frame ARF Protein in Cancer

The CDKN2a/ARF locus expresses two partially overlapping transcripts that encode two distinct proteins, namely p14ARF (p19Arf in mouse) and p16INK4a, which present no sequence identity. Initial data obtained in mice showed that both proteins are potent tumor suppressors. In line with a tumor-suppressive role, ARF-deficient mice develop lymphomas, sarcomas, and adenocarcinomas, with a median survival rate of one year of age. In humans, the importance of ARF inactivation in cancer is less clear whereas a more obvious role has been documented for p16INK4a. Indeed, many alterations in human tumors result in the elimination of the entire locus, while the majority of point mutations affect p16INK4a. Nevertheless, specific mutations of p14ARF have been described in different types of human cancers such as colorectal and gastric carcinomas, melanoma and glioblastoma. The activity of the tumor suppressor ARF has been shown to rely on both p53-dependent and independent functions. However, novel data collected in the last years has challenged the traditional and established role of this protein as a tumor suppressor. In particular, tumors retaining ARF expression evolve to metastatic and invasive phenotypes and in humans are associated with a poor prognosis. In this review, the recent evidence and the molecular mechanisms of a novel role played by ARF will be presented and discussed, both in pathological and physiological contexts.

B23/Nucleophosmin promotes reconstitution of synaptic path in hippocampus after injury

B23, also known as nucleophosmin (NPM), is multifunctional protein directly implicated in cell proliferation, cell cycle progression, and cell survival. In the current study, in addition to confirming its anti-apoptotic function in neuronal survival, we demonstrated that the spatial-temporal expression profile of B23 during development of hippocampal neurons is high in the embryonic stage, down-regulated after birth, and preferentially localized at the tips of growing neuritis and branching points. Overexpression of B23 promotes axon growth with abundant branching points in growing hippocampal neurons, but depletion of B23 impairs axon growth, leading to neuronal death. Following injury to the trisynaptic path in hippocampal slice, overexpression of B23 remarkably increased the number and length of regenerative fibers in the mossy fiber path. Our study suggests that B23 expression in developing neurons is essential for neuritogenesis and axon growth and that up-regulation of B23 may be a strategy for enhancing the reconstitution of synaptic paths after injury to hippocampal synapses.

On the relationship status for Arf and NPM1 - it's complicated

Arf levels are tightly regulated in cells and correlate with the level of ribosome biogenesis and proliferative status of cells. Through multivalent interactions with NPM1 - a regulator of ribosome biogenesis, and Mdm2 - a regulator of cellular fate, Arf integrates within the nucleolar matrix, altering its structure, dynamics and function and therefore modulates the cell cycle.

The nucleolus, an ally, and an enemy of cancer cells

The rates of ribosome production by a nucleolus and of protein biosynthesis by ribosomes are tightly correlated with the rate of cell growth and proliferation. All these processes must be matched and appropriately regulated to provide optimal cell functioning. Deregulation of certain factors, including oncogenes, controlling these processes, especially ribosome biosynthesis, can lead to cell transformation. Cancer cells are characterized by intense ribosome biosynthesis which is advantageous for their growth and proliferation. On the other hand, this feature can be engaged as an anticancer strategy. Numerous nucleolar factors such as nucleolar and ribosomal proteins as well as different RNAs, in addition to their role in ribosome biosynthesis, have other functions, including those associated with cancer biology. Some of them can contribute to cell transformation and cancer development. Others, under stress evoked by different factors which often hamper function of nucleoli and thus induce nucleolar/ribosomal stress, can participate in combating cancer cells. In this sense, intentional application of therapeutic agents affecting ribosome biosynthesis can cause either release of these molecules from nucleoli or their de novo biosynthesis to mediate the activation of pathways leading to elimination of harmful cells. This review underlines the role of a nucleolus not only as a ribosome constituting apparatus but also as a hub of both positive and negative control of cancer development. The article is mainly based on original papers concerning mechanisms in which the nucleolus is implicated directly or indirectly in processes associated with neoplasia.