Nuclear pore proteins and cancer

Reduction of Nup107 attenuates the growth factor signaling in the senescent cells

Hypo-responsiveness to growth factors is a fundamental feature of cellular senescence. In this study, we found markedly decreased level of Nup107, a key scaffold protein in nuclear pore complex assembly, in senescent human diploid fibroblasts as well as in organs of aged mice. Depletion of Nup107 by specific siRNA in young human diploid fibroblasts prevented the effective nuclear translocation of phosphorylated extracellular signal-regulated kinase (ERK) following epidermal growth factor (EGF) stimulation, and decreased the expression of c-Fos in consequence. The disturbances in ERK signaling in Nup107 depleted cells closely mirror the similar changes in senescent cells. Knockdown of Nup107 in anaplastic oligodendroglioma cells caused cell death, rather than growth retardation, indicating a greater sensitivity to Nup107 depletion in cancer cells than in normal cells. These findings support the notion that Nup107 may contribute significantly to the regulation of cell fate in aged and transformed cells by modulating nuclear trafficking of signal molecules.

Molecular characterization of alternative SET-NUP214 fusion transcripts in a case of acute undifferentiated leukemia

Cryptic deletions are occasionally reported in hematologic malignancies. The SET-NUP214 fusion gene has been rarely reported in acute myeloid leukemia, acute undifferentiated leukemia, and recurrently in T-cell acute lymphoblastic leukemia. The fusion product is generated by a submicroscopic deletion in the vicinity of 9q34. Herein we present a novel case of acute undifferentiated leukemia with SET-NUP214 rearrangement due to the cryptic deletion of the 9q34 region producing two different types of fusion transcripts by alternative splicing and molecular characterization of the fusion transcripts by fluorescence in situ hybridization, reverse transcriptase-polymerase chain reaction, and array comparative genomic hybridization analyses.

Role of a novel zebrafish nup98 during embryonic development

OBJECTIVE

The nucleoporin NUP98 is a component of the nuclear pore complex that regulates nucleocytoplasmic trafficking. It has been characterized in acute myeloid leukemia as a fusion partner during chromosomal translocation. In this study, we identified a zebrafish nup98 gene and examined its role in embryonic development.

MATERIALS AND METHODS

Two expressed sequence tags with translated sequences homologous to human NUP98 were identified. The gene was cloned by polymerase chain reaction from complementary DNA of zebrafish embryos. Cellular functions of zebrafish NUP98 were investigated in HeLa cells. nup98 expression and developmental functions in zebrafish embryos were investigated by whole-mount in situ hybridization and morpholino knockdown.

RESULTS

Protein sequence of zebrafish nup98 shared 65% identity with its human homolog. Ectopic expression of zebrafish nup98 rescued the defective messenger RNA export due to human NUP98 knockdown in HeLa cells. In zebrafish embryos, nup98 was expressed diffusely in eyes and the developing brain since 18 hours postfertilization. Knockdown of nup98 with morpholino upregulated pu.1 expression by 39% ± 15% (p = 0.0153) and scl expression by 36% ± 7.6% (p = 0.0017). Expression of genes associated with erythropoiesis was unchanged. The morphants also developed intracranial hemorrhage at 48 hours postfertilization due to defective blood vessel development.

CONCLUSIONS

A novel zebrafish nup98 was identified and it serves a role in nucleocytoplasmic trafficking similar to human NUP98. During development, it modulates hematopoietic stem cell and early myeloid development and maintains the integrity of cranial vasculature in the developing central nervous system.

Covalent histone modifications--miswritten, misinterpreted and mis-erased in human cancers

Post-translational modification of histones provides an important regulatory platform for processes such as gene transcription and DNA damage repair. It has become increasingly apparent that the misregulation of histone modification, which is caused by the deregulation of factors that mediate the modification installation, removal and/or interpretation, actively contributes to human cancer. In this Review, we summarize recent advances in understanding the interpretation of certain histone methylations by plant homeodomain finger-containing proteins, and how misreading, miswriting and mis-erasing of histone methylation marks can be associated with oncogenesis and progression. These observations provide us with a greater mechanistic understanding of epigenetic alterations in human cancers and might also help direct new therapeutic interventions in the future.

Targeting DOT1L action and interactions in leukemia: the role of DOT1L in transformation and development

IMPORTANCE OF THE FIELD

The establishment and maintenance of specialized chromatin is crucial for correct gene expression and chromosome stability in mammalian cells. Therefore, epigenetic insults are frequently observed in cancer. Several chromatin modifying enzymes have been implicated in leukemia, and are attractive candidates for the development of therapeutic agents.

AREAS COVERED IN THIS REVIEW

The histone methyltransferase DOT1L is responsible for methylation of histone H3 at lysine 79 and is involved in the pathobiology of several leukemias, the majority of which are characterized by chromosomal translocations involving the mixed lineage leukemia (MLL) gene. Leukemic translocations yield fusion proteins involving MLL and other proteins that physically interact with DOT1L. These oncogenic fusion proteins recruit DOT1L to ectopic loci (including HOX gene clusters), whose mis-expression contributes to the transformed phenotype. Studies from stem cells and certain leukemias suggest a second mechanism of leukemogenesis, in which reduced or mistargeted DOT1L activity yields altered centromeric chromatin and consequent chromosomal instability. Targeting DOT1L enzymatic activity as well as interactions with leukemogenic fusion proteins is discussed as possible leads in therapeutic interventions.

WHAT THE READER WILL GAIN

In this review, we discuss the normal functions of DOT1L, its mechanistic roles in leukemogenesis, and possible strategies for targeting DOT1L in leukemia. DOT1L is an atypical histone lysine methyltransferase in that it does not contain an enzymatic domain common to all other lysine methyltranferases. This attribute makes DOT1L a unique and specifically targetable enzyme. An emerging role for DOT1L under normal cellular conditions as well as transformed conditions is emerging and shedding light on the biology and mechanisms of some translocation-induced leukemias.

TAKE HOME MESSAGE

DOT1L is critical in development, as shown in studies in mouse embryos and embryonic stem cells. DOT1L enzymatic activity is also required for the leukemic transformation capabilities of a number of oncogenic fusion proteins. In addition, interactions between DOT1L and oncogenic fusion proteins are necessary for the transformation process. Therefore, it may be possible to specifically target DOT1L enzymatic activity or DOT1L interactions with leukemogenic fusion proteins.

Nucleoporin Nup98: a gatekeeper in the eukaryotic kingdoms

The nucleoporin Nup98 is an essential component of the nuclear pore complex. This peripheral nucleoporin with its Gly-Leu-Phe-Gly (GLFG) repeat domain contributes to nuclear-cytoplasmic trafficking, including mRNA export. In addition, accumulating studies indicate that Nup98 plays roles in several important biological events such as gene expression, mitotic checkpoint, and pathogenesis. Nup98 is well conserved among organisms belonging to the fungi and animal kingdoms. These kingdoms belong to the eukaryotic supergroup Opisthokonta. However, there is considerable diversity in the Nup98 orthologs expressed in organisms belonging to other eukaryotic supergroups. Intriguingly, in ciliates, a unicellular organism having two functionally distinct nuclei, GLFG-Nup98 is present in one of the nuclei and a distinct Nup98 ortholog is present in the other nucleus, and these different Nup98s participate in a nucleus-selective transport mechanism. In this review, we focus on Nup98 function and discuss how this nucleoporin has evolved in eukaryotic kingdoms.

Characterization of the role of the tumor marker Nup88 in mitosis

Nuclear pore complexes are massive multiprotein channels responsible for traffic between the nucleus and cytoplasm, and are composed of approximately 30 proteins, termed nucleoporins (Nup). Our recent studies indicated that the nucleoporins Rae1 and Tpr play critical roles in maintaining the spindle bipolarity during cell division. In the present study, we found that another nucleoporin, Nup88, was localized on the spindles together with Nup214 during mitosis. Nup88 expression is linked to the progression of carcinogenesis, Nup88 has been proposed as a tumor marker. Overexpression of Nup88 enhanced multinucleated cell formation. RNAi-mediated knockdown of Nup88 disrupted Nup214 expression and localization and caused multipolar spindle phenotypes. Our data indicate that proper expression of Nup88 is critical for preventing aneuploidy formation and tumorigenesis.

Gene regulation by nucleoporins and links to cancer

Nuclear pore complexes (NPCs) composed of approximately 30 individual nucleoporins form huge macromolecular assemblies in the nuclear envelope, through which bidirectional cargo movement between the nucleus and cytoplasm occurs. Beyond their transport function, NPCs can serve as docking sites for chromatin and thereby contribute to the organization of the overall topology of chromosomes in conjunction with other factors of the nuclear envelope. Recent studies suggest that gene-NPC interactions may promote both transcription and the definition of heterochromatin-euchromatin boundaries. Intriguingly, several nucleoporins were linked to cancer, mostly in the context of chromosomal translocations, which encode nucleoporin chimeras. An emerging concept is that tumor cells exploit specific properties of nucleoporins to deregulate transcription, chromatin boundaries, and essential transport-dependent regulatory circuits. This review outlines new mechanistic links between nucleoporin function and cancer pathogenesis.

The C. elegans homolog of nucleoporin Nup98 is required for the integrity and function of germline P granules

C. elegans P granules are conserved cytoplasmic ribonucleoprotein complexes that are unique to the germline and essential for fertility. During most of germline development, P granules are perinuclear and associate with clusters of nuclear pores. In an RNAi screen against nucleoporins, we have identified a specific nucleoporin essential for P granule integrity and function. The C. elegans homolog of vertebrate Nup98 (CeNup98) is enriched in P granules and associates with the translationally repressed, P granule-enriched mRNA nos-2 (nanos homolog). Loss of CeNup98 causes P granules to disperse in the cytoplasm and to release nos-2 mRNA. Embryos depleted for CeNup98 express a nos-2 3'UTR reporter prematurely. In the mouse, Nup98 immunoprecipitates with the germ granule component MVH. Our findings suggest that, in germ cells, the function of Nup98 extends beyond transport at the nuclear pore to include mRNA regulation in the cytoplasm.

Regulation of differentiation by a PHD domain in the NUP98-PHF23 fusion protein

Acute myeloid leukemia (AML) is frequently associated with chromosomal translocations. These translocations produce specific fusion genes that play crucial roles in leukemogenesis. We recently identified a novel NUP98-PHF23 fusion in AML. In this study, we attempt to determine the role of NUP98-PHF23 protein and its plant homeodomain (PHD) and coiled-coil domain in regulation of cellular differentiation and protein distribution. We provide evidence that NUP98-PHF23, through its PHD domain, impairs TPA-induced differentiation of K562 cells. While the fusion protein localizes to the nucleus, its deletion mutant without the PHD domain resides exclusively in the nucleolus, suggesting a potential link between chromatin-binding PHD domain and nuclear architecture.

Nucleoporin translocated promoter region (Tpr) associates with dynein complex, preventing chromosome lagging formation during mitosis

Gain or loss of whole chromosomes is often observed in cancer cells and is thought to be due to aberrant chromosome segregation during mitosis. Proper chromosome segregation depends on a faithful interaction between spindle microtubules and kinetochores. Several components of the nuclear pore complex/nucleoporins play critical roles in orchestrating the rapid remodeling events that occur during mitosis. Our recent studies revealed that the nucleoporin, Rae1, plays critical roles in maintaining spindle bipolarity. Here, we show association of another nucleoporin, termed Tpr (translocated promoter region), with the molecular motors dynein and dynactin, which both orchestrate with the spindle checkpoints Mad1 and Mad2 during cell division. Overexpression of Tpr enhanced multinucleated cell formation. RNA interference-mediated knockdown of Tpr caused a severe lagging chromosome phenotype and disrupted spindle checkpoint proteins expression and localization. Next, we performed a series of rescue and dominant negative experiments to confirm that Tpr orchestrates proper chromosome segregation through interaction with dynein light chain. Our data indicate that Tpr functions as a spatial and temporal regulator of spindle checkpoints, ensuring the efficient recruitment of checkpoint proteins to the molecular motor dynein to promote proper anaphase formation.

Flexible gates: dynamic topologies and functions for FG nucleoporins in nucleocytoplasmic transport

The nuclear envelope is a physical barrier between the nucleus and cytoplasm and, as such, separates the mechanisms of transcription from translation. This compartmentalization of eukaryotic cells allows spatial regulation of gene expression; however, it also necessitates a mechanism for transport between the nucleus and cytoplasm. Macromolecular trafficking of protein and RNA occurs exclusively through nuclear pore complexes (NPCs), specialized channels spanning the nuclear envelope. A novel family of NPC proteins, the FG-nucleoporins (FG-Nups), coordinates and potentially regulates NPC translocation. The extensive repeats of phenylalanine-glycine (FG) in each FG-Nup directly bind to shuttling transport receptors moving through the NPC. In addition, FG-Nups are essential components of the nuclear permeability barrier. In this review, we discuss the structural features, cellular functions, and evolutionary conservation of the FG-Nups.