DNA damage-dependent translocation of B23 and p19 ARF is regulated by the Jun N-terminal kinase pathway

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.

Mutant NPM1 Directly Regulates Oncogenic Transcription in Acute Myeloid Leukemia

The dysregulation of developmental and stem cell-associated genes is a common phenomenon during cancer development. Around half of patients with acute myeloid leukemia (AML) express high levels of HOXA cluster genes and MEIS1. Most of these AML cases harbor an NPM1 mutation (NPM1c), which encodes for an oncoprotein mislocalized from the nucleolus to the cytoplasm. How NPM1c expression in hematopoietic cells leads to its characteristic gene-expression pattern remains unclear. Here, we show that NPM1c directly binds to specific chromatin targets, which are co-occupied by the histone methyltransferase KMT2A (MLL1). Targeted degradation of NPM1c leads to a rapid decrease in gene expression and loss of RNA polymerase II, as well as activating histone modifications at its targets. We demonstrate that NPM1c directly regulates oncogenic gene expression in collaboration with the MLL1 complex and define the mechanism by which MLL1-Menin small-molecule inhibitors produce clinical responses in patients with NPM1-mutated AML.

SIGNIFICANCE

We uncovered an important functional role of mutant NPM1 as a crucial direct driver of oncogenic gene expression in AML. NPM1c can bind to chromatin and cooperate with the MLL complex, providing the first functional insight into the mechanism of Menin-MLL inhibition in NPM1c leukemias. See related article by Wang et al., p. 724. This article is highlighted in the In This Issue feature, p. 517.

Doxorubicin induces an alarmin-like TLR4-dependent autocrine/paracrine action of Nucleophosmin in human cardiac mesenchymal progenitor cells

Background

Doxorubicin (Dox) is an anti-cancer anthracycline drug that causes double-stranded DNA breaks. It is highly effective against several types of tumours; however, it also has adverse effects on regenerative populations of normal cells, such as human cardiac mesenchymal progenitor cells (hCmPCs), and its clinical use is limited by cardiotoxicity. Another known effect of Dox is nucleolar disruption, which triggers the ubiquitously expressed nucleolar phosphoprotein Nucleophosmin (NPM) to be released from the nucleolus into the cell, where it participates in the orchestration of cellular stress responses. NPM has also been observed in the extracellular space in response to different stress stimuli; however, the mechanism behind this and its functional implications are as yet largely unexplored. The aim of this study was to establish whether Dox could elicit NPM secretion in the extracellular space and to elucidate the mechanism of secretion and the effect of extracellular NPM on hCmPCs.

Results

We found that following the double-strand break formation in hCmPCs caused by Dox, NPM was rapidly secreted in the extracellular space by an active mechanism, in the absence of either apoptosis or necrosis. Extracellular release of NPM was similarly seen in response to ultraviolet radiation (UV). Furthermore, we observed an increase of NPM levels in the plasma of Dox-treated mice; thus, NPM release also occurred in vivo. The treatment of hCmPCs with extracellular recombinant NPM induced a decrease of cell proliferation and a response mediated through the Toll-like receptor (TLR)4. We demonstrated that NPM binds to TLR4, and via TLR4, and nuclear factor kappa B (NFkB) activation/nuclear translocation, exerts proinflammatory functions by inducing IL-6 and COX-2 gene expression. Finally, we found that in hCmPCs, NPM secretion could be driven by an autophagy-dependent unconventional mechanism that requires TLR4, since TLR4 inhibition dramatically reduced Dox-induced secretion.

Conclusions

We hypothesise that the extracellular release of NPM could be a general response to DNA damage since it can be elicited by either a chemical agent such as Dox or a physical genotoxic stressor such as UV radiation. Following genotoxic stress, NPM acts similarly to an alarmin in hCmPCs, being rapidly secreted and promoting cell cycle arrest and a TLR4/NFκB-dependent inflammatory response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01058-5.

Post-Translational Regulation of ARF: Perspective in Cancer

Tumorigenesis can be induced by various stresses that cause aberrant DNA mutations and unhindered cell proliferation. Under such conditions, normal cells autonomously induce defense mechanisms, thereby stimulating tumor suppressor activation. ARF, encoded by the CDKN2a locus, is one of the most frequently mutated or deleted tumor suppressors in human cancer. The safeguard roles of ARF in tumorigenesis are mainly mediated via the MDM2-p53 axis, which plays a prominent role in tumor suppression. Under normal conditions, low p53 expression is stringently regulated by its target gene, MDM2 E3 ligase, which induces p53 degradation in a ubiquitin-proteasome-dependent manner. Oncogenic signals induced by MYC, RAS, and E2Fs trap MDM2 in the inhibited state by inducing ARF expression as a safeguard measure, thereby activating the tumor-suppressive function of p53. In addition to the MDM2-p53 axis, ARF can also interact with diverse proteins and regulate various cellular functions, such as cellular senescence, apoptosis, and anoikis, in a p53-independent manner. As the evidence indicating ARF as a key tumor suppressor has been accumulated, there is growing evidence that ARF is sophisticatedly fine-tuned by the diverse factors through transcriptional and post-translational regulatory mechanisms. In this review, we mainly focused on how cancer cells employ transcriptional and post-translational regulatory mechanisms to manipulate ARF activities to circumvent the tumor-suppressive function of ARF. We further discussed the clinical implications of ARF in human cancer.

To control or to be controlled? Dual roles of CDK2 in DNA damage and DNA damage response

CDK2 (cyclin-dependent kinase 2), a member of the CDK family, has been shown to play a role in many cellular activities including cell cycle progression, apoptosis and senescence. Recently, accumulating evidence indicates that CDK2 is involved in DNA damage and DNA repair response (DDR). When DNA is damaged by internal or external genotoxic stresses, CDK2 activity is required for proper DNA repair in vivo and in vitro, whereas inactivation of CDK2 by siRNA techniques or by inhibitors could result in DNA damage and stimulate DDR. Hence, CDK2 seems to play dual roles in DNA damage and DDR. On one aspect, it is activated and stimulates DDR to repair DNA damage when DNA damage occurs; on the other hand, its inactivation directly leads to DNA damage and evokes DDR. Here, we describe the roles of CDK2 in DNA damage and DDR, and discuss the potential application of CDK2 inhibitors as anti-cancer agents.

Nucleolar Stress: hallmarks, sensing mechanism and diseases

The nucleolus is a prominent subnuclear compartment, where ribosome biosynthesis takes place. Recently, the nucleolus has gained attention for its novel role in the regulation of cellular stress. Nucleolar stress is emerging as a new concept, which is characterized by diverse cellular insult-induced abnormalities in nucleolar structure and function, ultimately leading to activation of p53 or other stress signaling pathways and alterations in cell behavior. Despite a number of comprehensive reviews on this concept, straightforward and clear-cut way criteria for a nucleolar stress state, regarding the factors that elicit this state, the morphological and functional alterations as well as the rationale for p53 activation are still missing. Based on literature of the past two decades, we herein summarize the evolution of the concept and provide hallmarks of nucleolar stress. Along with updated information and thorough discussion of existing confusions in the field, we pay particular attention to the current understanding of the sensing mechanisms, i.e., how stress is integrated by p53. In addition, we propose our own emphasis regarding the role of nucleolar protein NPM1 in the hallmarks of nucleolar stress and sensing mechanisms. Finally, the links of nucleolar stress to human diseases are briefly and selectively introduced.

TMPRSS4 induces invasion and proliferation of prostate cancer cells through induction of Slug and cyclin D1

TMPRSS4 is a novel type II transmembrane serine protease found at the cell surface that is highly expressed in pancreatic, colon, and other cancer tissues. Previously, we demonstrated that TMPRSS4 mediates tumor cell invasion, migration, and metastasis. We also found that TMPRSS4 activates the transcription factor activating protein-1 (AP-1) to induce cancer cell invasion. Here, we explored TMPRSS4-mediated cellular functions and the underlying mechanisms. TMPRSS4 induced Slug, an epithelial-mesenchymal transition (EMT)-inducing transcription factor, and cyclin D1 through activation of AP-1, composed of c-Jun and activating transcription factor (ATF)-2, which resulted in enhanced invasion and proliferation of PC3 prostate cancer cells. In PC3 cells, not only c-Jun but also Slug was required for TMPRSS4-mediated proliferation and invasion. Interestingly, Slug induced phosphorylation of c-Jun and ATF-2 to activate AP-1 through upregulation of Axl, establishing a positive feedback loop between Slug and AP-1, and thus induced cyclin D1, leading to enhanced proliferation. Using data from The Cancer Genome Atlas, we found that Slug expression positively correlated with that of c-Jun and cyclin D1 in human prostate cancers. Expression of Slug was positively correlated with that of cyclin D1 in various cancer cell lines, whereas expression of other EMT-inducing transcription factors was not. This study demonstrates that TMPRSS4 modulates both invasion and proliferation via Slug and cyclin D1, which is a previously unrecognized pathway that may regulate metastasis and cancer progression.

Stabilization of the p53-DNA Complex by the Nuclear Protein Dmp1α

We recently reported the existence of a physical interaction between the Myb-like transcription factor Dmp1 (Dmtf1) and p53 in which Dmp1 antagonized polyubiquitination of p53 by Mdm2 and promoted its nuclear localization. Dmp1 significantly stabilized p53-DNA complexes on promoters that contained p53-consensus sequences, which were either supershifted or disrupted with antibodies to Dmp1. Lysates from mice injected with doxorubicin showed that Dmp1 bound to p21Cip1, Bbc3, and Thbs1 gene regulatory regions in a p53-dependent fashion. Our data suggest that acceleration of DNA-binding of p53 by Dmp1 is a critical process for Dmp1 to increase the p53 function in Arf-deficient cells.

Ras-Erk signaling induces phosphorylation of human TLE1 and downregulates its repressor function

Signaling mediated by the Ras-extracellular signal-regulated kinase (Erk) pathway often leads to the phosphorylation of transcriptional regulators, thereby modulating their activity and causing concerted changes in gene expression. In Drosophila, the induction of multiple Ras-Erk pathway target genes depends on prior phosphorylation of the general co-repressor Groucho, a modification that downregulates its repressive function. Here, we show that TLE1, one of the four human Groucho orthologs, is similarly phosphorylated in response to Ras-Erk pathway activation, and that this modification attenuates its capacity to repress transcription. Specifically, unphosphorylated TLE1 dominantly suppresses the induction of Ras-Erk pathway target genes in cultured human cells, and the expression of an unphosphorylatable TLE1 derivative causes severe phenotypes in a transgenic Drosophila model system, whereas a phosphomimetic variant of TLE1 exerts only negligible effects. We present data indicating that TLE1 is rapidly excluded from the nucleus following epidermal growth factor receptor pathway activation, an effect that likely accounts for its inability to mediate effective repression under such conditions. Significantly, we find that unphosphorylated TLE1 blocks oncogenic phenotypes induced by mutated H-Ras in human mammary cells, both in vitro and following their implantation in mice. Collectively, our data strongly indicate that phosphorylation of TLE family members and the consequent downregulation of their repressor function is a key conserved step in the transcriptional responses to Ras-Erk signaling, and possibly a critical event in the tumorigenic effects caused by excessive Ras-Erk pathway activity.

A redox mechanism underlying nucleolar stress sensing by nucleophosmin

The nucleolus has been recently described as a stress sensor. The nucleoplasmic translocation of nucleolar protein nucleophosmin (NPM1) is a hallmark of nucleolar stress; however, the causes of this translocation and its connection to p53 activation are unclear. Using single live-cell imaging and the redox biosensors, we demonstrate that nucleolar oxidation is a general response to various cellular stresses. During nucleolar oxidation, NPM1 undergoes S-glutathionylation on cysteine 275, which triggers the dissociation of NPM1 from nucleolar nucleic acids. The C275S mutant NPM1, unable to be glutathionylated, remains in the nucleolus under nucleolar stress. Compared with wild-type NPM1 that can disrupt the p53–HDM2 interaction, the C275S mutant greatly compromises the activation of p53, highlighting that nucleoplasmic translocation of NPM1 is a prerequisite for stress-induced activation of p53. This study elucidates a redox mechanism for the nucleolar stress sensing and may help the development of therapeutic strategies.

Nucleoplasmic translocation of NPM1 is integral to nucleolar stress sensing. Here, the authors show that nucleolar oxidation is a general cellular stress response, and that oxidation-related glutathionylation of NPM1 triggers its translocation and facilitates p53 activation.

Nucleolin and nucleophosmin: nucleolar proteins with multiple functions in DNA repair

The nucleolus represents a highly multifunctional intranuclear organelle in which, in addition to the canonical ribosome assembly, numerous processes such as transcription, DNA repair and replication, the cell cycle, and apoptosis are coordinated. The nucleolus is further a key hub in the sensing of cellular stress and undergoes major structural and compositional changes in response to cellular perturbations. Numerous nucleolar proteins have been identified that, upon sensing nucleolar stress, deploy additional, non-ribosomal roles in the regulation of varied cell processes including cell cycle arrest, arrest of DNA replication, induction of DNA repair, and apoptosis, among others. The highly abundant proteins nucleophosmin (NPM1) and nucleolin (NCL) are two such factors that transit to the nucleoplasm in response to stress, and participate directly in the repair of numerous different DNA damages. This review discusses the contributions made by NCL and (or) NPM1 to the different DNA repair pathways employed by mammalian cells to repair DNA insults, and examines the implications of such activities for the regulation, pathogenesis, and therapeutic targeting of NPM1 and NCL.

Destabilisation, aggregation, toxicity and cytosolic mislocalisation of nucleophosmin regions associated with acute myeloid leukemia

Nucleophosmin (NPM1) is a multifunctional protein that is implicated in the pathogenesis of several human malignancies. To gain insight into the role of isolated fragments of NPM1 in its biological activities, we dissected the C-terminal domain (CTD) into its helical fragments. Here we focus the attention on the third helix of the NPM1-CTD in its wild-type (H3 wt) and AML-mutated (H3 mutA and H3 mutE) sequences. Conformational studies, by means of CD and NMR spectroscopies, showed that the H3 wt peptide was partially endowed with an α-helical structure, but the AML-sequences exhibited a lower content of this conformation, particularly the H3 mutA peptide. Thioflavin T assays showed that the H3 mutE and the H3 mutA peptides displayed a significant aggregation propensity that was confirmed by CD and DLS assays. In addition, we found that the H3 mutE and H3 mutA peptides, unlike the H3 wt, were moderately and highly toxic, respectively, when exposed to human neuroblastoma cells. Cellular localization experiments confirmed that the mutated sequences hamper their nucleolar accumulation, and more importantly, that the helical conformation of the H3 region is crucial for such a localization.

Transcriptional repression of p27 is essential for murine embryonic development

The Nczf gene has been identified as one of Ncx target genes and encodes a novel KRAB zinc-finger protein, which functions as a sequence specific transcriptional repressor. In order to elucidate Nczf functions, we generated Nczf knockout (Nczf−/−) mice. Nczf−/− mice died around embryonic day 8.5 (E8.5) with small body size and impairment of axial rotation. Histopathological analysis revealed that the cell number decreased and pyknotic cells were occasionally observed. We examined the expression of cell cycle related genes in Nczf−/− mice. p27 expression was increased in E8.0 Nczf−/− mice compared to that of wild type mice. Nczf knockdown by siRNA resulted in increased expression of p27 in mouse embryonic fibroblasts (MEFs). Furthermore, p27 promoter luciferase reporter gene analysis confirmed the regulation of p27 mRNA expression by Nczf. Nczf−/−; p27−/− double knockout mice survived until E11.5 and the defect of axial rotation was restored. These data suggest that p27 repression by Nczf is essential in the developing embryo.

C-Jun recruits the NSL complex to regulate its target gene expression by modulating H4K16 acetylation and promoting the release of the repressive NuRD complex

The proto-oncogene c-Jun plays essential roles in various cellular processes, including cell proliferation, cell differentiation, and cellular apoptosis. Enormous efforts have been made to understand the mechanisms regulating c-Jun activation. The males absent on the first (MOF)-containing non-specific lethal (NSL) complex has been shown to positively regulate gene expression. However, the biological function of the NSL complex is largely unknown. Here we present evidence showing that c-Jun recruits the NSL complex to c-Jun target genes upon activation. The NSL complex catalyzes H4K16 acetylation at c-Jun target genes, thereby promoting c-Jun target gene transcription. More interestingly, we also found that the NSL complex promotes the release of the repressive NuRD complex from c-Jun target genes, thus activating c-Jun. Our findings not only reveal a new mechanism regulating c-Jun activation, but also identify the NSL complex as a c-Jun co-activator in c-Jun-regulated gene expression, expanding our knowledge of the function of the NSL complex in gene expression regulation.

Novel Molecular Markers for Breast Cancer

The use of molecular biomarkers assures that breast cancer (BC) patients receive optimal treatment. Established biomarkers, such as estrogen receptor, progesterone receptor, HER2, and Ki67, have been playing significant roles in the subcategorization of BC to predict the prognosis and decide the specific therapy to each patient. Antihormonal therapy using 4-hydroxytamoxifen or aromatase inhibitors have been employed in patients whose tumor cells express hormone receptors, while monoclonal antibody to HER2 has been administered to HER2-positive BCs. Although new therapeutic agents have been developed in the past few decades, many patients still die of the disease due to relapse; thus, novel molecular markers that predict therapeutic failure and those that can be targets for specific therapy are expected. We have chosen four of such molecules by reviewing recent publications, which are cyclin E, B-Myb, Twist, and DMP1β. The oncogenicity of these molecules has been demonstrated in vivo and/or in vitro through studies using transgenic mice or siRNAs, and their expressions have been shown to be associated with shortened overall or disease-free survival of BC patients. The former three molecules have been shown to accelerate epithelial-mesenchymal transition that is often associated with cancer stem cell-ness and metastasis; all these four can be novel therapeutic targets as well. Thus, large prospective studies employing immunohistochemistry will be needed to establish the predictive values of these molecules in patients with BC.

PRINS, a primate-specific long non-coding RNA, plays a role in the keratinocyte stress response and psoriasis pathogenesis

In the last few years with the recent emergence of high-throughput technologies, thousands of long non-coding RNAs (lncRNAs) have been identified in the human genome. However, assigning functional annotation and determining cellular contexts for these RNAs are still in its infancy. As information gained about lncRNA structure, interacting partners, and roles in human diseases may be helpful in the characterization of novel lncRNAs, we review our knowledge on a selected group of lncRNAs that were identified serendipitously years ago by large-scale gene expression methods used to study human diseases. In particular, we focus on the Psoriasis-susceptibility-Related RNA Gene Induced by Stress (PRINS) lncRNA, first identified by our research group as a transcript highest expressed in psoriatic non-lesional epidermis. Results gathered for PRINS in the last 10 years indicate that it is conserved in primates and plays a role in keratinocyte stress response. Elevated levels of PRINS expression in psoriatic non-lesional keratinocytes alter the stress response of non-lesional epidermis and contribute to disease pathogenesis. Finally, we propose a categorization for the PRINS lncRNA based on a recently elaborated system for lncRNA classification.

Low-Dose Actinomycin-D Induces Redistribution of Wild-Type and Mutated Nucleophosmin Followed by Cell Death in Leukemic Cells

Specific mutations involving C-terminal part of the nucleolar protein nucleophosmin (NPM) are associated with better outcome of acute myeloid leukemia (AML) therapy, possibly due to aberrant cytoplasmic NPM localization facilitating induction of anti-NPM immune response. Actinomycin D (actD) is known to induce nucleolar stress leading to redistribution of many nucleolar proteins, including NPM. We analyzed the distribution of both wild-type and mutated NPM (NPMmut) in human cell lines, before and after low-dose actD treatment, in living cells expressing exogenous fluorescently labeled proteins as well as using immunofluorescence staining of endogenous proteins in fixed cells. The wild-type NPM form is prevalently nucleolar in intact cells and relocalizes mainly to the nucleoplasm following actD addition. The mutated NPM form is found both in the nucleoli and in the cytoplasm of untreated cells. ActD treatment leads to a marked increase in NPMmut amount in the nucleoplasm while a mild decrease is observed in the cytoplasm. Cell death was induced by low-dose actD in all the studied leukemic cell lines with different p53 and NPM status. In cells expressing the tumor suppresor p53 (CML-T1, OCI-AML3), cell cycle arrest in G1/G0 phase was followed by p53-dependent apoptosis while in p53-null HL60 cells, transient G2/M-phase arrest was followed by cell necrosis. We conclude that although actD does not increase NPM concentration in the cytoplasm, it could improve the effect of standard chemotherapy in leukemias through more general mechanisms.

Insights into the regulation of neuronal viability by nucleophosmin/B23

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

GLTSCR2 is an upstream negative regulator of nucleophosmin in cervical cancer

Nucleophosmin (NPM)/B23, a multifunctional nucleolar phosphoprotein, plays an important role in ribosome biogenesis, cell cycle regulation, apoptosis and cancer pathogenesis. The role of NPM in cells is determined by several factors, including total expression level, oligomerization or phosphorylation status, and subcellular localization. In the nucleolus, NPM participates in rRNA maturation to enhance ribosomal biogenesis. Consistent with this finding, NPM expression is increased in rapidly proliferating cells and many types of human cancers. In response to ribosomal stress, NPM is redistributed to the nucleoplasm, where it inactivates mouse double minute 2 homologue to stabilize p53 and inhibit cell cycle progression. These observations indicate that nucleolus-nucleoplasmic mobilization of NPM is one of the key molecular mechanisms that determine the role of NPM within the cell. However, the regulatory molecule(s) that control(s) NPM stability and subcellular localization, crucial to the pluripotency of intercellular NPM, remain(s) unidentified. In this study, we showed that nucleolar protein GLTSCR2/Pict-1 induced nucleoplasmic translocation and enhanced the degradation of NPM via the proteasomal polyubiquitination pathway. In addition, we showed that GLTSCR2 expression decreased the transforming activity of cells mediated by NPM and that the expression of NPM is reciprocally related to that of GLTSCR2 in cervical cancer tissue. In this study, we demonstrated that GLTSCR2 is an upstream negative regulator of NPM.

EphrinB2 controls vessel pruning through STAT1-JNK3 signalling

Angiogenesis produces primitive vascular networks that need pruning to yield hierarchically organized and functional vessels. Despite the critical importance of vessel pruning to vessel patterning and function, the mechanisms regulating this process are not clear. Here we show that EphrinB2, a well-known player in angiogenesis, is an essential regulator of endothelial cell death and vessel pruning. This regulation depends upon phosphotyrosine-EphrinB2 signaling repressing JNK3 activity via STAT1. JNK3 activation causes endothelial cell death. In the absence of JNK3, hyaloid vessel physiological pruning is impaired, associated with abnormal persistence of hyaloid vessels, defective retinal vasculature and microphthalmia. This syndrome closely resembles human persistent hyperplastic primary vitreus (PHPV), attributed to failed involution of hyaloid vessels. Our results provide evidence that EphrinB2/STAT1/JNK3 signaling is essential for vessel pruning, and that defects in this pathway may contribute to PHPV.

Mutation of Foxo3 causes adult onset auditory neuropathy and alters cochlear synapse architecture in mice

Auditory neuropathy is a form of hearing loss in which cochlear inner hair cells fail to correctly encode or transmit acoustic information to the brain. Few genes have been implicated in the adult-onset form of this disease. Here we show that mice lacking the transcription factor Foxo3 have adult onset hearing loss with the hallmark characteristics of auditory neuropathy, namely, elevated auditory thresholds combined with normal outer hair cell function. Using histological techniques, we demonstrate that Foxo3-dependent hearing loss is not due to a loss of cochlear hair cells or spiral ganglion neurons, both of which normally express Foxo3. Moreover, Foxo3-knock-out (KO) inner hair cells do not display reductions in numbers of synapses. Instead, we find that there are subtle structural changes in and surrounding inner hair cells. Confocal microscopy in conjunction with 3D modeling and quantitative analysis show that synaptic localization is altered in Foxo3-KO mice and Myo7a immunoreactivity is reduced. TEM demonstrates apparent afferent degeneration. Strikingly, acoustic stimulation promotes Foxo3 nuclear localization in vivo, implying a connection between cochlear activity and synaptic function maintenance. Together, these findings support a new role for the canonical damage response factor Foxo3 in contributing to the maintenance of auditory synaptic transmission.

BAG3 is upregulated by c-Jun and stabilizes JunD

BAG3 plays a regulatory role in a number of cellular processes, including cell proliferation, apoptosis, adhesion and migration, epithelial-mesenchymal transition (EMT), autophagy activation, and virus infection. The AP-1 transcription factors are implicated in a variety of important biological processes including cell differentiation, proliferation, apoptosis and oncogenesis. Recently, it has been reported that AP-1 protein c-Jun inhibits autophagy and enhances apoptotic cell death mediated by starvation. However, the molecular mechanisms remain unclear. For the first time, the current study demonstrated that serum starvation downregulated BAG3 at the transcriptional level via c-Jun. In addition, the current study reported that BAG3 stabilized JunD mRNA, which was, at least in part, responsible for the promotion of serum starvation mediated-growth inhibition by BAG3.

TMPRSS4 upregulates uPA gene expression through JNK signaling activation to induce cancer cell invasion

TMPRSS4 is a novel type II transmembrane serine protease that is highly expressed in pancreatic, thyroid, colon, and other cancer tissues. Previously, we demonstrated that TMPRSS4 mediates tumor cell invasion, migration, and metastasis. However, the mechanisms by which TMPRSS4 contributes to invasion are not fully understood. Here, we demonstrated that TMPRSS4 induced the transcription of the urokinase-type plasminogen activator (uPA) gene through activating the transcription factors Sp1, Sp3, and AP-1 in mainly a JNK-dependent manner and that the induction of uPA was required for TMPRSS4-mediated cancer cell invasion and signaling events. In addition, the uPA receptor was involved in TMPRSS4-induced signaling activation and subsequent uPA expression probably through its association with TMPRSS4 on the cell surface. Immunohistochemical analysis showed that uPA expression was significantly correlated with TMPRSS4 expression in human lung and prostate cancers. These observations suggest that TMPRSS4 is an important regulator of uPA gene expression; the upregulation of uPA by TMPRSS4 contributes to invasion and may represent a novel mechanism for the control of invasion.

Nucleolar stress in Drosophila melanogaster: RNAi-mediated depletion of Nopp140

Nucleolar stress results when ribosome biogenesis is disrupted. An excellent example is the human Treacher Collins syndrome in which the loss of the nucleolar chaperone, Treacle, leads to p53-dependent apoptosis in embryonic neural crest cells and ultimately to craniofacial birth defects. Here, we show that depletion of the related nucleolar phosphoprotein, Nopp140, in Drosophila melanogaster led to nucleolar stress and eventual lethality when multiple tissues were depleted of Nopp140. We used TEM, immuno-blot analysis and metabolic protein labeling to show the loss of ribosomes. Targeted loss of Nopp140 in larval wing discs caused Caspase-dependent apoptosis which eventually led to defects in the adult wings. These defects were not rescued by a p53 gene deletion, as the craniofacial defects were in the murine model of TCS, thus suggesting that apoptosis caused by nucleolar stress in Drosophila is induced by a p53-independent mechanism. Loss of Nopp140 in larval polyploid midgut cells induced premature autophagy as marked by the accumulation of mCherry-ATG8a into autophagic vesicles. We also found elevated phenoloxidase A3 levels in whole larval lysates and within the hemolymph of Nopp140-depleted larvae vs. hemolymph from parental genotype larvae. Phenoloxidase A3 enrichment was coincident with the appearance of melanotic tumors in the Nopp140-depleted larvae. The occurrence of apoptosis, autophagy and phenoloxidase A3 release to the hemolymph upon nucleolar stress correlated well with the demonstrated activation of Jun N-terminal kinase (JNK) in Nopp140-depleted larvae. We propose that JNK is a central stress response effector that is activated by nucleolar stress in Drosophila larvae.

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.

Expression and functional studies on the noncoding RNA, PRINS

PRINS, a noncoding RNA identified earlier by our research group, contributes to psoriasis susceptibility and cellular stress response. We have now studied the cellular and histological distribution of PRINS by using in situ hybridization and demonstrated variable expressions in different human tissues and a consistent staining pattern in epidermal keratinocytes and in vitro cultured keratinocytes. To identify the cellular function(s) of PRINS, we searched for a direct interacting partner(s) of this stress-induced molecule. In HaCaT and NHEK cell lysates, the protein proved to be nucleophosmin (NPM) protein as a potential physical interactor with PRINS. Immunohistochemical experiments revealed an elevated expression of NPM in the dividing cells of the basal layers of psoriatic involved skin samples as compared with healthy and psoriatic uninvolved samples. Others have previously shown that NPM is a ubiquitously expressed nucleolar phosphoprotein which shuttles to the nucleoplasm after UV-B irradiation in fibroblasts and cancer cells. We detected a similar translocation of NPM in UV-B-irradiated cultured keratinocytes. The gene-specific silencing of PRINS resulted in the retention of NPM in the nucleolus of UV-B-irradiated keratinocytes; suggesting that PRINS may play a role in the NPM-mediated cellular stress response in the skin.

Nucleolin participates in DNA double-strand break-induced damage response through MDC1-dependent pathway

H2AX is an important factor for chromatin remodeling to facilitate accumulation of DNA damage-related proteins at DNA double-strand break (DSB) sites. In order to further understand the role of H2AX in the DNA damage response (DDR), we attempted to identify H2AX-interacting proteins by proteomics analysis. As a result, we identified nucleolin as one of candidates. Here, we show a novel role of a major nucleolar protein, nucleolin, in DDR. Nucleolin interacted with γ-H2AX and accumulated to laser micro-irradiated DSB damage sites. Chromatin Immunoprecipitation assay also displayed the accumulation of nucleolin around DSB sites. Nucleolin-depleted cells exhibited repression of both ATM-dependent phosphorylation following exposure to γ-ray and subsequent cell cycle checkpoint activation. Furthermore, nucleolin-knockdown reduced HR and NHEJ activity and showed decrease in IR-induced chromatin accumulation of HR/NHEJ factors, agreeing with the delayed kinetics of γ-H2AX focus. Moreover, nucleolin-knockdown decreased MDC1-related events such as focus formation of 53 BP1, RNF168, phosphorylated ATM, and H2A ubiquitination. Nucleolin also showed FACT-like activity for DSB damage-induced histone eviction from chromatin. Taken together, nucleolin could promote both ATM-dependent cell cycle checkpoint and DSB repair by functioning in an MDC1-related pathway through its FACT-like function.

Nucleolar protein CSIG is required for p33ING1 function in UV-induced apoptosis

Cellular senescence-inhibited gene (CSIG) protein, a nucleolar protein with a ribosomal L1 domain in its N-terminus, can exert non-ribosomal functions to regulate biological processes, such as cellular senescence. Here, we describe a previously unknown function for CSIG: promotion of apoptosis in response to ultraviolet (UV) irradiation-induced CSIG upregulation. We identified p33ING1 as a binding partner that interacts with CSIG. After UV irradiation, p33ING1 increases its protein expression, translocates into the nucleolus and binds CSIG. p33ING1 requires its nucleolar targeting sequence region to interact with CSIG and enhance CSIG protein stability, which is essential for activation of downstream effectors, Bcl-2-associated X protein, to promote apoptosis. Thus, our data imply that p33ING1–CSIG axis functions as a novel pro-apoptotic regulator in response to DNA damage.

Rad9B responds to nucleolar stress through ATR and JNK signalling, and delays the G1-S transition

The complex formed by Rad9, Rad1 and Hus1 (9-1-1) protects against genomic instability by activating DNA damage checkpoint and DNA damage repair pathways, mainly in response to replication fork collapse and UV lesions. Here we compare the role of Rad9A (also known as Rad9) with the human paralogue Rad9B. Unlike Rad9A, overexpression of Rad9B delays cells in G1 phase. Moreover, Rad9B migrates to nucleoli after nucleolar stress in an ATR- and JNK-dependent manner, in a newly described nucleolar domain structure containing p21. Analysis of chimeras of Rad9A and Rad9B demonstrate that localisation to nucleoli and the block in G1 phase upon overexpression crucially depend on the Rad9B C-terminal tail. Taken together, data presented here show a relationship between Rad9B and pathways for checkpoints, stress response and nucleolar function.

Quantitative nucleolar proteomics reveals nuclear re-organization during stress- induced senescence in mouse fibroblast

Background

Nucleolus is the most prominent mammalian organelle within the nucleus which is also the site for ribosomal biogenesis. There have been many reports indicating the involvement of nucleolus in the process of aging. Several proteins related to aging have been shown to localize in the nucleolus, which suggests the role of this organelle in senescence.

Results

In this study, we used quantitative mass spectrometry to map the flux of proteins into and out of the nucleolus during the induction of senescence in cultured mammalian cells. Changes in the abundance of 344 nucleolar proteins in sodium butyrate-induced senescence in NIH3T3 cells were studied by SILAC (stable isotope labeling by amino acids in cell culture)-based mass spectrometry. Biochemically, we have validated the proteomic results and confirmed that B23 (nucleophosmin) protein was down-regulated, while poly (ADP-ribose) polymerase (PARP) and nuclear DNA helicase II (NDH II/DHX9/RHA) were up-regulated in the nucleolus upon treatment with sodium butyrate. Accumulation of chromatin in the nucleolus was also observed, by both proteomics and microscopy, in sodium butyrate-treated cells. Similar observations were found in other models of senescence, namely, in mitoxantrone- (MTX) treated cells and primary fibroblasts from the Lamin A knockout mice.

Conclusion

Our data indicate an extensive nuclear organization during senescence and suggest that the redistribution of B23 protein and chromatin can be used as an important marker for senescence.

Metalloprotease type III effectors that specifically cleave JNK and NF-κB

Two major arms of the inflammatory response are the NF-κB and c-Jun N-terminal kinase (JNK) pathways. Here, we show that enteropathogenic Escherichia coli (EPEC) employs the type III secretion system to target these two signalling arms by injecting host cells with two effector proteins, NleC and NleD. We provide evidence that NleC and NleD are Zn-dependent endopeptidases that specifically clip and inactivate RelA (p65) and JNK, respectively, thus blocking NF-κB and AP-1 activation. We show that NleC and NleD co-operate and complement other EPEC effectors in accomplishing maximal inhibition of IL-8 secretion. This is a remarkable example of a pathogen using multiple effectors to manipulate systematically the host inflammatory response signalling network.

Jun proteins are starvation-regulated inhibitors of autophagy

The growing number of biological functions affected by autophagy ascribes a special significance to identification of factors regulating it. The activator protein-1 (AP-1) transcription factors are involved in most aspects of cellular proliferation, death, or survival, yet no information regarding their involvement in autophagy is available. Here, we show that the AP-1 proteins JunB and c-Jun, but not JunD, c-Fos, or Fra-1, inhibit autophagy. JunB inhibits autophagy induced by starvation, overexpression of a short form of ARF (smARF), a potent inducer of autophagy, or even after rapamycin treatment. In agreement, acute repression of JunB expression, by JunB knockdown, potently induces autophagy. As expected from autophagy-inhibiting proteins, Jun B and c-Jun expression is reduced by starvation. Decrease in JunB mRNA expression and posttranscriptional events downregulate JunB protein expression after starvation. The inhibition of autophagy by JunB is not mediated by mammalian target of rapamycin (mTOR) regulation, as it occurs also in the absence of mTOR activity, and autophagy induced by JunB knockdown is not correlated with changes in mTOR activity. Nevertheless, the transcriptional activities of c-Jun and JunB are required for autophagy inhibition, and JunB incapable of heterodimerizing is a less effective inhibitor of autophagy. Most importantly, inhibition of autophagy in starved HeLa cells by JunB enhances apoptotic cell death. We suggest that JunB and c-Jun are regulators of autophagy whose expression responds to autophagy-inducing signals.

AP-1--The Jun proteins: Oncogenes or tumor suppressors in disguise?

Since its discovery more than two decades ago the involvement of the Activating protein 1 (AP-1) in proliferation, inflammation, differentiation, apoptosis, cellular migration and wound healing has been intensively studied. A model based on the early studies suggested antagonistic roles for the Jun proteins in proliferation and transformation. c-Jun was suggested to enhance transformation whereas JunB suggested to inhibit it in an antagonistic manner. Surprisingly, despite accumulation of data obtained from animal models regarding the role of Jun proteins in cancer and identification of oncogenic pathways regulating them, their involvement in human cancer was not demonstrated until recently. Here, we will describe the current knowledge about the roles of Jun proteins in human neoplasia. We will focus on the pathological examples demonstrating that the initial dogma has to be reexamined. For example, like c-Jun, JunB seems to play an oncogenic role in lymphomas, particularly in Hodgkin's lympomas. Furthermore, unlike the antagonistic activities of c-Jun and JunB in the transcription of genes coding for major cell cycle regulators such as CyclinD or p16INK4A, the transcription of other cell cycle regulating genes is modified similarly by c-Jun or JunB. Interestingly, some of these genes such as the ones coding for CyclinA or p19(ARF) are important players in either positive or negative regulation of cellular proliferation and survival. Finally, we will also discuss results posing JNK, known so far as the major activator of c-Jun, as a negative regulator of c-Jun level and activity. These recent findings suggest that the role of each Jun protein in neoplasia as well as in cellular survival should be examined in a context-dependent manner.

B23 interacts with PES1 and is involved in nucleolar localization of PES1

PES1, the human homolog of zebrafish pescadillo, is a nucleolar protein that is essential for cell proliferation. We report herein that a nucleolar marker protein B23 physically interacts with PES1 and is involved in the nucleolar localization of PES1. In vivo interaction between B23 and PES1 was verified by co-immunoprecipitation of endogenous B23 and PES1 proteins, and they showed cellular co-localizations under both normal and actinomycin D-induced stress conditions. Furthermore, we mapped their interaction domains via in vitro pulldown assays. When B23 was knocked down by RNA interference, there appeared an increased nucleoplasmic distribution of PES1. Our results support a previous hypothesis that B23 might be a nucleolar hub protein for protein targeting to the nucleolus, and shed light on the nucleolar localization mechanism of PES1. The physical interaction between B23 and PES1 implies that they may participate in ribosome biogenesis in a protein complex.

The Myc-nucleophosmin-ARF network: a complex web unveiled

The multifunctional nucleolar proteins, nucleophosmin (NPM) and the tumor suppressor ARF, have been assigned numerous roles in diverse cellular processes impacting cellular proliferation, tumorigenesis and apoptosis. In addition, both proteins have been linked to the oncogenic function of c-Myc, a transcription factor that drives the majority of human cancers. Both proteins are induced by oncogenic c-Myc, but have opposing outcomes. Whereas loss of ARF accelerates c-Myc-induced tumorigenesis, NPM overexpression enhances c-Myc transformation. Accordingly, ARF expression is lost in many tumors, while NPM expression is elevated. Previously, we demonstrated that ARF interacts directly with c-Myc, leading to inhibition of its transforming activity while enhancing its apoptotic activity, independently of p53. We have recently shown that NPM also binds directly to c-Myc, but with opposite effects compared to ARF. NPM dramatically enhances the oncogenic activity of c-Myc, independently of ARF and p53. In tumor cells, the ARF-p53 pathway is often inactivated while NPM is elevated. However, when NPM and ARF are both expressed with oncogenic c-Myc the outcome of the interactions becomes more complex, since NPM and ARF also interact directly and NPM controls ARF localization. In this report we demonstrate that in the presence of ARF, NPM overexpression dramatically inhibits c-Myc-induced p53-independent apoptosis, while enhancing proliferation and transformation. We find that NPM sequesters ARF in nucleoli, blocking the relocalization of ARF to the nucleoplasm caused by activation of c-Myc. Therefore, the fate of a cell to undergo apoptosis or become transformed is dependent on this complex interacting network of oncogenic and tumor suppressor proteins.

Nucleophosmin redistribution following heat shock: a role in heat-induced radiosensitization

Cellular survival from radiation-induced DNA damage requires access to sites of damage for the assembly of repair complexes and the subsequent repair, particularly the repair of DNA double strand breaks (DSB). Hyperthermia causes changes in protein-protein/DNA interactions in the nucleus that block access to sites of DNA damage. Studies presented here indicate that the nucleolar protein, nucleophosmin (NPM), redistributes from the nucleolus following hyperthermia, increases its association with DNA, and blocks access to DNA DSBs. Reduction of NPM significantly reduces heat-induced radiosensitization, but reduced NPM level does not alter radiation sensitivity per se. NPM knockdown reduces heat-induced inhibition of DNA DSB repair. Also, these results suggest that NPM associates with nuclear matrix attachment region DNA in heat-shocked cells.

Growth inhibition of human cancer cells by 5-aza-2'-deoxycytidine does not correlate with its effects on INK4a/ARF expression or initial promoter methylation status

The cytotoxicity of 5-aza-2'-deoxycytidine (DAC) has been linked to demethylation of the INK4a/ARF tumor suppressor gene locus in various cell systems, but the causality of this association remains unproven. To test this assumption, we have examined the effects of DAC in two human cancer cell lines of differing INK4a/ARF promoter methylation status: MDA-MB-468 breast cancer cells in which INK4a/ARF is unmethylated and normally expressed, and DLD-1 colorectal cancer cells in which INK4a/ARF is methylated and repressed. In MDA-MB-468 cells, DAC induces cytotoxicity in the absence of any detectable increase of p14 or p16 expression, whereas small interfering RNA knockdown of p16/p14 expression fails to attenuate DAC cytotoxicity. In DLD-1 cells, DAC demethylates INK4a/ARF and restores both p16 and p14 expression at concentrations that fail to cause detectable growth inhibition or apoptosis; moreover, neither ARF nor INK4a transgene expression inhibits DLD-1 cell growth despite normalization of p14 and p16 expression. These data imply that neither of these cell lines depends on up-regulated expression of INK4a/ARF for DAC cytotoxicity. We propose that optimal anticancer use of this drug will await unambiguous identification of those DAC target genes primarily responsible for triggering growth inhibition, followed by clarification as to whether these upstream events are caused by hypomethylation or DNA damage.

Parp1 facilitates alternative NHEJ, whereas Parp2 suppresses IgH/c-myc translocations during immunoglobulin class switch recombination

Immunoglobulin class switch recombination (CSR) is initiated by DNA breaks triggered by activation-induced cytidine deaminase (AID). These breaks activate DNA damage response proteins to promote appropriate repair and long-range recombination. Aberrant processing of these breaks, however, results in decreased CSR and/or increased frequency of illegitimate recombination between the immunoglobulin heavy chain locus and oncogenes like c-myc. Here, we have examined the contribution of the DNA damage sensors Parp1 and Parp2 in the resolution of AID-induced DNA breaks during CSR. We find that although Parp enzymatic activity is induced in an AID-dependent manner during CSR, neither Parp1 nor Parp2 are required for CSR. We find however, that Parp1 favors repair of switch regions through a microhomology-mediated pathway and that Parp2 actively suppresses IgH/c-myc translocations. Thus, we define Parp1 as facilitating alternative end-joining and Parp2 as a novel translocation suppressor during CSR.

Identification of nucleolar effects in JNK-deficient cells

The c-Jun N-terminal kinase (JNK) signalling pathway has an established role in cellular stress signalling, cell survival and tumorigenesis. Here, we demonstrate that inhibition of JNK signalling results in partial delocalization of the RNA helicase DDX21 from the nucleolus to the nucleoplasm, increased nucleolar mobility of DDX21 and inhibition of rRNA processing. Furthermore, our results show that JNK signalling regulates DDX21 phosphorylation and protein expression. In conclusion, the results presented in this study reveal a previously unidentified cellular role for JNK signalling in the regulation of nucleolar functions. Based on these results, we propose that JNK-mediated effects on nucleolar homeostasis and rRNA processing should be considered when interpreting cellular phenotypes observed in JNK-deficient cell and animal models.