Nuclear Export Function Research Articles

The nuclear export protein exportin-1 in solid malignant tumours: From biology to clinical trials.

Exportin-1 (XPO1), a crucial protein regulating nuclear-cytoplasmic transport, is frequently overexpressed in various cancers, driving tumor progression and drug resistance. This makes XPO1 an attractive therapeutic target. Over the past few decades, the number of available nuclear export-selective inhibitors has been increasing. Only KPT-330 (selinexor) has been successfully used for treating haematological malignancies, and KPT-8602 (eltanexor) has been used for treating haematologic tumours in clinical trials. However, the use of nuclear export-selective inhibitors for the inhibition of XPO1 expression has yet to be thoroughly investigated in clinical studies and therapeutic outcomes for solid tumours. We collected numerous literatures to explain the efficacy of XPO1 Inhibitors in preclinical and clinical studies of a wide range of solid tumours. In this review, we focus on the nuclear export function of XPO1 and results from clinical trials of its inhibitors in solid malignant tumours. We summarized the mechanism of action and therapeutic potential of XPO1 inhibitors, as well as adverse effects and response biomarkers. XPO1 inhibition has emerged as a promising therapeutic strategy in the fight against cancer, offering a novel approach to targeting tumorigenic processes and overcoming drug resistance. SINE compounds have demonstrated efficacy in a wide range of solid tumours, and ongoing research is focused on optimizing their use, identifying response biomarkers, and developing effective combination therapies. Exportin-1 (XPO1) plays a critical role in mediating nucleocytoplasmic transport and cell cycle. XPO1 dysfunction promotes tumourigenesis and drug resistance within solid tumours. The therapeutic potential and ongoing researches on XPO1 inhibitors in the treatment of solid tumours. Additional researches are essential to address safety concerns and identify biomarkers for predicting patient response to XPO1 inhibitors.

TREX tetramer disruption alters RNA processing necessary for corticogenesis in THOC6 Intellectual Disability Syndrome

THOC6 variants are the genetic basis of autosomal recessive THOC6 Intellectual Disability Syndrome (TIDS). THOC6 is critical for mammalian Transcription Export complex (TREX) tetramer formation, which is composed of four six-subunit THO monomers. The TREX tetramer facilitates mammalian RNA processing, in addition to the nuclear mRNA export functions of the TREX dimer conserved through yeast. Human and mouse TIDS model systems revealed novel THOC6-dependent, species-specific TREX tetramer functions. Germline biallelic Thoc6 loss-of-function (LOF) variants result in mouse embryonic lethality. Biallelic THOC6 LOF variants reduce the binding affinity of ALYREF to THOC5 without affecting the protein expression of TREX members, implicating impaired TREX tetramer formation. Defects in RNA nuclear export functions were not detected in biallelic THOC6 LOF human neural cells. Instead, mis-splicing was detected in human and mouse neural tissue, revealing novel THOC6-mediated TREX coordination of mRNA processing. We demonstrate that THOC6 is required for key signaling pathways known to regulate the transition from proliferative to neurogenic divisions during human corticogenesis. Together, these findings implicate altered RNA processing in the developmental biology of TIDS neuropathology.

The master energy homeostasis regulator PGC-1α exhibits an mRNA nuclear export function

PGC-1α plays a central role in maintaining mitochondrial and energy metabolism homeostasis, linking external stimuli to transcriptional co-activation of genes involved in adaptive and age-related pathways. The carboxyl-terminus encodes a serine/arginine-rich (RS) region and an RNA recognition motif, however the RNA-processing function(s) were poorly investigated over the past 20 years. Here, we show that the RS domain of human PGC-1α directly interacts with RNA and the nuclear RNA export receptor NXF1. Inducible depletion of PGC-1α and expression of RNAi-resistant RS-deleted PGC-1α further demonstrate that its RNA/NXF1-binding activity is required for the nuclear export of some canonical mitochondrial-related mRNAs and mitochondrial homeostasis. Genome-wide investigations reveal that the nuclear export function is not strictly linked to promoter-binding, identifying in turn novel regulatory targets of PGC-1α in non-homologous end-joining and nucleocytoplasmic transport. These findings provide new directions to further elucidate the roles of PGC-1α in gene expression, metabolic disorders, aging and neurodegeneration.

Abstract B018: Novel therapeutics for targeting the aberrant nuclear export machinery in colorectal cancer

Abstract XPO1 is a nuclear export receptor responsible for exporting many key proteins critical for cell survival from the nucleus to the cytoplasm. Recently, we identified a highly statistically significant hotspot mutation in XPO1 R749Q occurring in patients with colorectal cancer. Out of 96 patients identified to have XPO1 R749Q mutations, 27 (28%) were in colorectal cancer (CRC). 95% of XPO1 R749Q mutant CRC were tumor mutation burden-high (TMB-H; >10 mut/Mb) and 0 were mis-match repair deficient (dMMR)/micro satellite instability-high (MSI-H). Co-mutation analysis showed that 95% of XPO1 R749Q CRC were POLE mutated, which was significantly enriched over XPO1 WT tumors. To investigate whether XPO1 R749Q might play a role in CRC biology, we have generated two different isogenic colorectal cancer cell lines bearing the XPO1 R749Q mutation (HCT116 and LS174T) using CRISPR-CAS9 to better understand the role of this aberrant XPO1 mutation on nuclear export and tumorigenesis. Immunofluorescence (IF) and Stochastic Optical Reconstruction Microscopy (STORM) super-resolution experiments showed that R749Q mutant cells had significantly increased localization of XPO1 in the cytoplasm of the XPO1 R749Q mutant cells when compared to wildtype (XPO1 WT) cells, particularly at the edge of the nuclear pores of the XPO1 R749Q mutant cells suggesting increased nuclear export. Mass spectrometry analysis of nuclear and cytoplasmic fractionated proteins confirmed that XPO1 R749Q mutant cells had increased export of proteins from the nucleus compared to XPO1 WT cells. Structural modeling using published structures of XPO1 bound to Ran-GTP predicted that XPO1 R749Q mutation increased affinity of the regulatory H9-loop of XPO1 to Ran, thus favoring increased shuttling and retention in the cytoplasmic compartment. Therefore, we hypothesize that XPO1 R749Q may represent a novel mechanism of nuclear export alteration involved with kinetic tuning of global transport. Despite the increased nuclear export function, XPO1 R749Q cells did not show a proliferative advantage. We thus set out to test whether XPO1 R749Q might be enriched in cancer as an adaptive response to chemotherapy treatment. Using a chemical compound library of >200 FDA-approved cancer therapies we observed a strong therapeutic resistance of XPO1 R749Q cells relative to XPO1 WT cells, specifically to chemotherapies used in the treatment of colon cancer, such as irinotecan. However, these cells remained sensitive to treatment with the XPO1 inhibitor selinexor. Mice xenografted with XPO1 R749Q mutant HCT116 cells showed moderate tumor response to selinexor or irinotecan monotherapies but prolonged tumor responses to combination therapy. Recent clinical data has shown POLE mutations to be a biomarker for response to PD-1 immune checkpoint blockade, especially in CRC. Since our genetic data identify POLE mutations commonly co-occurring with XPO1 R749Q in colon cancer, we are planning to test the combination of PD-1 inhibitors and selinexor to target both these mutations, as well as CRC tumors without these mutations. Citation Format: Tulasigeri M. Totiger, Sana Chaudhry, Skye Montoya, Monika Chojnacka, Gabriel Gaidosh, Jumana Afaghani, Maurizio Affer, Jenna Zabroski, Jacob Jahn, Vindhya Nawaratne, Ryan Notti, Ramiro Verdun, Daniel Bilbao, Josean Rodriguez, Justin Taylor. Novel therapeutics for targeting the aberrant nuclear export machinery in colorectal cancer [abstract]. In: Proceedings of the AACR Special Conference on Colorectal Cancer; 2022 Oct 1-4; Portland, OR. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_1):Abstract nr B018.

Abstract 5536: Selinexor inhibits lymphoma-leukemia tumor progression directly and through activation of anti-cancer immune response

Abstract Background: Selinexor is a Selective Inhibitor of Nuclear Export (SINE) compound that binds covalently to Exportin 1 (XPO1) and inhibits its nuclear export function, resulting in nuclear accumulation of tumor suppressor proteins (TSPs) including p53, pRB, and IκB-α. Selinexor was evaluated for improved survival in tumor-bearing mice with hematological malignancies using EL-4 (T cell lymphoma/leukemia) and C1498 (Acute myeloid leukemia) models. We hypothesized that selinexor increases survival by directly inducing tumor cell apoptosis and blocks immunosuppressive effects of myeloid derived suppressor cells (MDSCs). Methods: Mice were injected with 2×106 EL-4 or 1×106 C1498 cells intravenously followed by oral treatment with saline, vehicle, or different selinexor dosing schedules (5 mg/kg weekly (5), twice weekly on Wed and Fri (5 d1/3) or Mon and Fri (5 d1/5) or 10 mg/kg weekly (10). Selinexor was given by gavage. To examine the effect of selinexor on immune cells during tumor growth, Tumor bearing mice treated with vehicle or selinexor were euthanized at day 14 after tumor injection. Spleens were removed, single cell suspensions were isolated and the percentage of immune cells including MDSCs, regulatory T cells (Tregs) and the production of interferon gamma (IFN-γ) were analyzed by flow cytometry. Tumor bearing mice were monitored for survival in separate experiments. Results: Selinexor at the weekly 5 d1/3 and the 10 dosing significantly increased survival in mice injected with either tumor line. Whereas weekly 5 or 5 d1/5 dosing did not significant affect survival. Selinexor at weekly 10 dosing significantly reduced the population of polymorphonuclear MDSCs (PMN-MDSCs). Selinexor at 10, 5 d1/3 and 5/1/5 weekly dosing significantly increased IFN-γ production by CD8+ and CD4+ T cells when compared to vehicle. There were no differences in Tregs populations. Conclusions: These data demonstrate that different selinexor dosing and schedules have differential efficacy on survival in murine models of leukemia/lymphoma. Selinexor effects are dose and schedule dependent. We found that in contrast to the split dosing, the single weekly high dose of selinexor reduced splenic PMN-MDSC accumulation. While both dosing schedules increased the percentage of IFN-γ positive T cells in tumor bearing mice suggesting that selinexor’s anti-cancer activity is not limited only to direct cytotoxic or cytostatic tumor cells. These data support the concept that once weekly selinexor, at a higher dose (10 mg/kg (approximately 30 mg/m2) which is comparable to the clinical dose schedule (50-60 mg weekly) can modulate the immune system to target cancer cells and could be used alone or in combination with other immunotherapies to enhance the immune system. Citation Format: Hemn Mohammadpour, Yosef Landesman, Philip McCarthy. Selinexor inhibits lymphoma-leukemia tumor progression directly and through activation of anti-cancer immune response [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5536.

Karyopherin abnormalities in neurodegenerative proteinopathies.

Neurodegenerative proteinopathies are characterized by progressive cell loss that is preceded by the mislocalization and aberrant accumulation of proteins prone to aggregation. Despite their different physiological functions, disease-related proteins like tau, α-synuclein, TAR DNA binding protein-43, fused in sarcoma and mutant huntingtin, all share low complexity regions that can mediate their liquid-liquid phase transitions. The proteins’ phase transitions can range from native monomers to soluble oligomers, liquid droplets and further to irreversible, often-mislocalized aggregates that characterize the stages and severity of neurodegenerative diseases.Recent advances into the underlying pathogenic mechanisms have associated mislocalization and aberrant accumulation of disease-related proteins with defective nucleocytoplasmic transport and its mediators called karyopherins. These studies identify karyopherin abnormalities in amyotrophic lateral sclerosis, frontotemporal dementia, Alzheimer’s disease, and synucleinopathies including Parkinson’s disease and dementia with Lewy bodies, that range from altered expression levels to the subcellular mislocalization and aggregation of karyopherin α and β proteins.The reported findings reveal that in addition to their classical function in nuclear import and export, karyopherins can also act as chaperones by shielding aggregation-prone proteins against misfolding, accumulation and irreversible phase-transition into insoluble aggregates. Karyopherin abnormalities can, therefore, be both the cause and consequence of protein mislocalization and aggregate formation in degenerative proteinopathies. The resulting vicious feedback cycle of karyopherin pathology and proteinopathy identifies karyopherin abnormalities as a common denominator of onset and progression of neurodegenerative disease.Pharmacological targeting of karyopherins, already in clinical trials as therapeutic intervention targeting cancers such as glioblastoma and viral infections like COVID-19, may therefore represent a promising new avenue for disease-modifying treatments in neurodegenerative proteinopathies.

The RNA-Binding Protein SBR (Dm NXF1) Is Required for the Constitution of Medulla Boundaries in Drosophila melanogaster Optic Lobes.

Drosophila melanogaster sbr (small bristles) is an orthologue of the Nxf1 (nuclear export factor 1) genes in different Opisthokonta. The known function of Nxf1 genes is the export of various mRNAs from the nucleus to the cytoplasm. The cytoplasmic localization of the SBR protein indicates that the nuclear export function is not the only function of this gene in Drosophila. RNA-binding protein SBR enriches the nucleus and cytoplasm of specific neurons and glial cells. In sbr12 mutant males, the disturbance of medulla boundaries correlates with the defects of photoreceptor axons pathfinding, axon bundle individualization, and developmental neurodegeneration. RNA-binding protein SBR participates in processes allowing axons to reach and identify their targets.

IκBα Nuclear Export Enables 4-1BB-Induced cRel Activation and IL-2 Production to Promote CD8 T Cell Immunity.

Optimal CD8 T cell immunity is orchestrated by signaling events initiated by TCR recognition of peptide Ag in concert with signals from molecules such as CD28 and 4-1BB. The molecular mechanisms underlying the temporal and spatial signaling dynamics in CD8 T cells remain incompletely understood. In this study, we show that stimulation of naive CD8 T cells with agonistic CD3 and CD28 Abs, mimicking TCR and costimulatory signals, coordinately induces 4-1BB and cRel to enable elevated cytosolic cRel:IκBα complex formation and subsequent 4-1BB-induced IκBα degradation, sustained cRel activation, heightened IL-2 production and T cell expansion. NfkbiaNES/NES CD8 T cells harboring a mutated IκBα nuclear export sequence abnormally accumulate inactive cRel:IκBα complexes in the nucleus following stimulation with agonistic anti-CD3 and anti-CD28 Abs, rendering them resistant to 4-1BB induced signaling and a disrupted chain of events necessary for efficient T cell expansion. Consequently, CD8 T cells in NfkbiaNES/NES mice poorly expand during viral infection, and this can be overcome by exogenous IL-2 administration. Consistent with cell-based data, adoptive transfer experiments demonstrated that the antiviral CD8 T cell defect in NfkbiaNES/NES mice was cell intrinsic. Thus, these results reveal that IκBα, via its unique nuclear export function, enables, rather than inhibits 4-1BB-induced cRel activation and IL-2 production to facilitate optimal CD8 T cell immunity.

Structural basis for the nuclear import and export functions of the biportin Pdr6/Kap122.

Importins ferry proteins into nuclei while exportins carry cargoes to the cytoplasm. In the accompanying paper in this issue (Vera Rodriguez et al. 2019. J. Cell Biol. https://doi.org/10.1083/jcb.201812091), we discovered that Pdr6 is a biportin that imports, e.g., the SUMO E2 ligase Ubc9 while depleting the translation factor eIF5A from the nuclear compartment. In this paper, we report the structures of key transport intermediates, namely, of the Ubc9•Pdr6 import complex, of the RanGTP•Pdr6 heterodimer, and of the trimeric RanGTP•Pdr6•eIF5A export complex. These revealed nonlinear transport signals, chaperone-like interactions, and how the RanGTPase system drives Pdr6 to transport Ubc9 and eIF5A in opposite directions. The structures also provide unexpected insights into the evolution of transport selectivity. Specifically, they show that recognition of Ubc9 by Pdr6 differs fundamentally from that of the human Ubc9-importer Importin 13. Likewise, Pdr6 recognizes eIF5A in a nonhomologous manner compared with the mammalian eIF5A-exporter Exportin 4. This suggests that the import of Ubc9 and active nuclear exclusion of eIF5A evolved in different eukaryotic lineages more than once and independently from each other.

The DEAD-box protein Dbp2p is linked to noncoding RNAs, the helicase Sen1p, and R-loops.

The DEAD-box RNA helicase Dbp2p is highly conserved in eukaryotes and has been implicated in transcription, ribosome biogenesis, mRNP assembly, nuclear export, and long noncoding RNA (lncRNA) function. It is not understood how Dbp2p performs these seemingly unrelated biological roles. An important step toward addressing this question is the determination of cellular RNA binding sites of Dbp2p. Here, we identify transcriptome-wide RNA binding sites of Dbp2p from Saccharomyces cerevisiae using UV-crosslinking, denaturing tandem affinity purification, and next generation sequencing. We find that Dbp2p crosslinks to mRNAs and ribosomal RNAs, and markedly to noncoding RNAs, including snoRNA, snRNAs, and tRNAs. In snoRNAs, Dbp2p preferentially crosslinks at sites near the 3′ ends. These sites coincide with regions where RNA–DNA hybrids (R-loops) form and with binding sites of Sen1p, another RNA helicase that functions in transcription termination and 3′ processing of noncoding RNAs. We show that Dbp2p interacts in an RNA-independent manner with Sen1p in vivo. Dbp2p crosslinks to tRNAs and other RNAs also at sites where R-loops form. Collectively, our data link Dbp2p to noncoding RNAs, Sen1p, and R-loops. The transcriptome-wide connection to R-loops provides a unifying theme for diverse cellular roles of Dbp2p.

Dishevelled has a YAP nuclear export function in a tumor suppressor context-dependent manner

Phosphorylation-dependent YAP translocation is a well-known intracellular mechanism of the Hippo pathway; however, the molecular effectors governing YAP cytoplasmic translocation remains undefined. Recent findings indicate that oncogenic YAP paradoxically suppresses Wnt activity. Here, we show that Wnt scaffolding protein Dishevelled (DVL) is responsible for cytosolic translocation of phosphorylated YAP. Mutational inactivation of the nuclear export signal embedded in DVL leads to nuclear YAP retention, with an increase in TEAD transcriptional activity. DVL is also required for YAP subcellular localization induced by E-cadherin, α-catenin, or AMPK activation. Importantly, the nuclear-cytoplasmic trafficking is dependent on the p53-Lats2 or LKB1-AMPK tumor suppressor axes, which determine YAP phosphorylation status. In vivo and clinical data support that the loss of p53 or LKB1 relieves DVL-linked reciprocal inhibition between the Wnt and nuclear YAP activity. Our observations provide mechanistic insights into controlled proliferation coupled with epithelial polarity during development and human cancer.

Size matters: conserved proteins function in length-dependent nuclear export of circular RNAs.

Circular RNAs (circRNAs) comprise a recently appreciated category of RNAs that are in high abundance and serve important biological functions. Although several discoveries have been made regarding the biogenesis and functions of circRNAs, their subcellular trafficking has remained largely unknown. In this issue of Genes & Development, Huang and colleagues (pp. 639-644) reported the first study of the nuclear export of circRNAs. Drosophila Hel25E and its human homologs, UAP56 and URH49, are required for nuclear export of circRNAs. Nuclear export of circRNAs is surprisingly length-dependent, and the length measurement mechanism was shown to be controlled by motifs in Hel25E and its homologs consisting of four amino acids.

Dishevelled has YAP nuclear export function in a tumour suppressor context dependent manner

The Hippo signaling is an evolutionary conserved pathway that inhibits cell proliferation by contact inhibition, and whose loss leads to organ growth and cancer development. The Yes‐associated protein (YAP) transcription co‐activator is a key regulator of the Hippo pathway. Phosphorylation‐dependent YAP translocation is a well‐known intracellular mechanism of the Hippo pathway, however the responsible molecular effectors by which the YAP leads cytosolic translocation remain undefined. Recent findings indicate that oncogenic YAP paradoxically suppresses canonical Wnt activity.We found that DVL, a scaffolding protein of the Wnt pathway as well as a key regulator of Wnt‐independent epithelial polarity, is a molecular effector for nuclear‐cytoplasmic shuttling of YAP in a YAP‐phosphorylation dependent manner. Mutational inactivation of the nuclear export signal in DVL leads to nuclear YAP retention, with increases TEAD transcriptional activity. DVL is also required for YAP intracellular dynamics induced by E‐cadherin, α‐catenin, or AMPK activation. Importantly, the nuclear‐cytoplasmic trafficking is largely dependent on the p53‐Lats2 or LKB1‐AMPK tumor suppressor axes, which determine YAP phosphorylation status. Conversely, loss of p53 or LKB1 relieves DVL‐linked reciprocal inhibition between the canonical Wnt pathway and nuclear YAP activity. Our observations provide novel mechanistic insights into controlled proliferation coupled with epithelial polarity during development and human cancer.Support or Funding InformationNFR‐2017R1A2B3002241 funded by the Korean government (MSIP) andBK 21 PLUS Project, Yonsei University College of Dentistry, Seoul, Korea.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Effects of exportin 1 on nuclear transport and meiotic resumption in porcine full-grown and growing oocytes.

Exportin 1 (XPO1) is a nuclear transport receptor involved in the nuclear export of majority proteins in somatic cells. In mammalian oocytes, however, only the presence of XPO1 has been reported at mRNA and protein levels, and the definitive functions of XPO1 and its effects on the meiotic maturation of oocytes have never been directly examined. In the present study, the expression state and the nuclear-export function of porcine XPO1 were analyzed in porcine oocytes. In addition, we investigated the effects of the overexpression and inhibition of XPO1 on meiotic regulation in full-grown and growing oocytes by mRNA injection and inhibitor treatment. Endogenous XPO1 was stably expressed in porcine oocytes during the germinal vesicle (GV) stage, and the expression of exogenous XPO1 significantly decreased the nuclear localization of XPO1 cargos, snurportin 1, and WEE1B. Inhibition of XPO1 by a specific inhibitor, leptomycin B, delayed the GV breakdown (GVBD), whereas the overexpression of XPO1 by mRNA injection accelerated the GVBD. XPO1 overexpression overcame the meiotic arrest induced by WEE1B expression in full-grown oocytes. Surprisingly, the GVBD of porcine growing oocytes, which could not resume meiosis by the maturation culture in vitro, was induced by the expression of exogenous XPO1. These results showed the presence of XPO1 and its function as a nuclear export receptor in mammalian oocytes, including growing oocytes, and they suggest that the regulation of nuclear transport has a large influence on the GV maintenance and meiotic resumption of oocytes.

The function of the inner nuclear envelope protein SUN1 in mRNA export is regulated by phosphorylation

SUN1, a component of the LINC (Linker of Nucleoskeleton and Cytoskeleton) complex, functions in mammalian mRNA export through the NXF1-dependent pathway. It associates with mRNP complexes by direct interaction with NXF1. It also binds to the NPC through association with the nuclear pore component Nup153, which is involved in mRNA export. The SUN1-NXF1 association is at least partly regulated by a protein kinase C (PKC) which phosphorylates serine 113 (S113) in the N-terminal domain leading to reduced interaction. The phosphorylation appears to be important for the SUN1 function in nuclear mRNA export since GFP-SUN1 carrying a S113A mutation was less efficient in restoring mRNA export after SUN1 knockdown as compared to the wild type protein. By contrast, GFP-SUN1-S113D resembling the phosphorylated state allowed very efficient export of poly(A)+RNA. Furthermore, probing a possible role of the LINC complex component Nesprin-2 in this process we observed impaired mRNA export in Nesprin-2 knockdown cells. This effect might be independent of SUN1 as expression of a GFP tagged SUN-domain deficient SUN1, which no longer can interact with Nesprin-2, did not affect mRNA export.

Inhibition of CRM1-mediated nuclear export of influenza A nucleoprotein and nuclear export protein as a novel target for antiviral drug development

An anti-influenza compound, DP2392-E10 based on inhibition of the nuclear export function of the viral nucleoprotein-nuclear export signal 3 (NP-NES3) domain was successfully identified by our previous high-throughput screening system. Here, we demonstrated that DP2392-E10 exerts its antiviral effect by inhibiting replication of a broad range of influenza A subtypes. In regard to the molecular mechanism, we revealed that DP2392-E10 inhibits nuclear export of both viral NP and nuclear export protein (NEP). More specifically, in vitro pull-down assays revealed that DP2392-E10 directly binds cellular CRM1, which mediates nuclear export of NP and NEP. In silico docking suggested that DP2392-E10 binds at a region close to the HEAT9 and HEAT10 domains of CRM1. Together, these results indicate that the CRM1-mediated nuclear export function of influenza virus represents a new potential target for antiviral drug development, and also provide a core structure for a novel class of inhibitors that target this function.

The Calreticulin control of human stress erythropoiesis is impaired by JAK2V617F in polycythemia vera.

Calreticulin (CALR) is a Ca2+-binding protein that shuttles among cellular compartments with proteins bound to its N/P domains. The knowledge that activation of the human erythropoietin receptor induces Ca2+ fluxes prompted us to investigate the role of CALR in human erythropoiesis. As shown by Western blot analysis, erythroblasts generated invitro from normal sources and JAK2V617F polycythemia vera (PV) patients expressed robust levels of CALR. However, Ca2+ regulated CALR conformation only in normal cells. Normal erythroblasts expressed mostly the N-terminal domain of CALR (N-CALR) on their cell surface (as shown by flow cytometry) and C-terminal domain (C-CALR) in their cytoplasm (as shown by confocal microscopy) and expression of both epitopes decreased with maturation. In the proerythroblast (proEry) cytoplasm, C-CALR was associated with the glucocorticoid receptor (GR), which initiated the stress response. In these cells, Ca2+ deprivation and inhibition of nuclear export increased GR nuclear localization while decreasing cytoplasmic detection of C-CALR and C-CALR/GR association and proliferation in response to the GR agonist dexamethasone (Dex). C-CALR/GR association and Dex responsiveness were instead increased by Ca2+ and erythropoietin. In contrast, JAK2V617F proErys expressed normal cell-surface levels of N-CALR but barely detectable cytoplasmic levels of C-CALR. These cells contained GR mainly in the nucleus and were Dex unresponsive. Ruxolitinib rescued cytoplasmic detection of C-CALR, C-CALR/GR association, and Dex responsiveness in JAK2V617F proErys and its effects were antagonized by nuclear export and Ca2+ flux inhibitors. These results indicates that Ca2+-induced conformational changes of CALR regulate nuclear export of GR in normal erythroblasts and that JAK2V617F deregulates this function in PV.

Abstract 74: Targeting nuclear export for triple-negative breast cancer therapy

Abstract Triple-negative breast cancers (TNBC) comprise the most aggressive subtype of breast cancer. They rapidly acquire resistance to chemotherapy and are also refractory to endocrine therapy and HER2 inhibitors. In fact, no targeted therapies are currently available against these tumors. Genome sequencing studies have shown that TNBCs are exceedingly heterogeneous in genetic mutations as well as copy number variations. Yet, TNBC almost invariably harbor inactivating mutations in TP53, which seems to be a crucial event in the pathogenesis of these tumors. We recently performed a series of siRNA lethality screens in vitro (N = 11 cell lines) to identify, in an unbiased way, recurrent vulnerabilities that could be harnessed as novel targets for TNBC therapy. Hits were genes whose silencing triggered massive cell death in ≥ 30% of TNBC lines tested. The screens re-discovered mitosis as a frequent Achilles’ heel of these tumors (antimitotic drugs are currently a mainstay of TNBC therapy), supporting the validity of the approach. The nuclear transport gene RAN also scored among the most frequent TNBC selective dependencies. Moreover, TNBC cell lines were exquisitely sensitive to selinexor (KPT-330), a first-in-class inhibitor of exportin 1 (XPO-1) currently in phase 2 trials, both in vitro and in vivo. TNBC cells treated with selinexor died of apoptosis. In contrast, normal human primary breast epithelial progenitors (BPE) underwent cell cycle arrest (but not death) upon selinexor treatment, and were able to resume proliferation upon drug removal. The efficacy of selinexor across different TNBC models varied, suggesting that additional factors, beside the TNBC status, are at play. Preliminary data based on a genome-wide CRISPR screen for genes and networks conferring sensitivity to selinexor using a p53-mutant TNBC cell line (MDA-MB-231) implicated a number of TP53-related genes as key mediators of response. Together, these data support the notion that p53-mutant TNBC coopt nuclear export function to silence TP53-associated networks and that XPO-1 inhibitors could be a viable strategy to tackle TNBC in the clinic. Citation Format: Fabio Petrocca. Targeting nuclear export for triple-negative breast cancer therapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 74.

XPO1 Inhibition Preferentially Disrupts the 3D Nuclear Organization of Telomeres in Tumor Cells.

Previous work has shown that the three‐dimensional (3D) nuclear organization of telomeres is altered in cancer cells and the degree of alterations coincides with aggressiveness of disease. Nuclear pores are essential for spatial genome organization and gene regulation and XPO1 (exportin 1/CRM1) is the key nuclear export protein. The Selective Inhibitor of Nuclear Export (SINE) compounds developed by Karyopharm Therapeutics (KPT‐185, KPT‐330/selinexor, and KPT‐8602) inhibit XPO1 nuclear export function. In this study, we investigated whether XPO1 inhibition has downstream effects on the 3D nuclear organization of the genome. This was assessed by measuring the 3D telomeric architecture of normal and tumor cells in vitro and ex vivo. Our data demonstrate for the first time a rapid and preferential disruption of the 3D nuclear organization of telomeres in tumor cell lines and in primary cells ex vivo derived from treatment‐naïve newly diagnosed multiple myeloma patients. Normal primary cells in culture as well as healthy lymphocyte control cells from the same patients were minimally affected. Using both lymphoid and non‐lymphoid tumor cell lines, we found that the downstream effects on the 3D nuclear telomere structure are independent of tumor type. We conclude that the 3D nuclear organization of telomeres is a sensitive indicator of cellular response when treated with XPO1 inhibitors. J. Cell. Physiol. 231: 2711–2719, 2016. © 2016 The Authors. Journal of Cellular Physiology published by Wiley Periodicals, Inc.

Nuclear Export of Pre-Ribosomal Subunits Requires Dbp5, but Not as an RNA-Helicase as for mRNA Export.

The DEAD-box RNA-helicase Dbp5/Rat8 is known for its function in nuclear mRNA export, where it displaces the export receptor Mex67 from the mRNA at the cytoplasmic side of the nuclear pore complex (NPC). Here we show that Dbp5 is also required for the nuclear export of both pre-ribosomal subunits. Yeast temperature-sensitive dbp5 mutants accumulate both ribosomal particles in their nuclei. Furthermore, Dbp5 genetically and physically interacts with known ribosomal transport factors such as Nmd3. Similar to mRNA export we show that also for ribosomal transport Dbp5 is required at the cytoplasmic side of the NPC. However, unlike its role in mRNA export, Dbp5 does not seem to undergo its ATPase cycle for this function, as ATPase-deficient dbp5 mutants that selectively inhibit mRNA export do not affect ribosomal transport. Furthermore, mutants of GLE1, the ATPase stimulating factor of Dbp5, show no major ribosomal export defects. Consequently, while Dbp5 uses its ATPase cycle to displace the export receptor Mex67 from the translocated mRNAs, Mex67 remains bound to ribosomal subunits upon transit to the cytoplasm, where it is detectable on translating ribosomes. Therefore, we propose a model, in which Dbp5 supports ribosomal transport by capturing ribosomal subunits upon their cytoplasmic appearance at the NPC, possibly by binding export factors such as Mex67. Thus, our findings reveal that although different ribonucleoparticles, mRNAs and pre-ribosomal subunits, use shared export factors, they utilize different transport mechanisms.