Dynamic trafficking of STAT5 depends on an unconventional nuclear localization signal

Multiplexed cellular profiling identifies an organoselenium compound as an inhibitor of CRM1-mediated nuclear export

Chromosomal region maintenance 1 (CRM1 also known as Xpo1 and exportin-1) is the receptor for the nuclear export controlling the intracellular localization and function of many cellular and viral proteins that play a crucial role in viral infections and cancer. The inhibition of CRM1 has emerged as a promising therapeutic approach to interfere with the lifecycle of many viruses, for the treatment of cancer, and to overcome therapy resistance. Recently, selinexor has been approved as the first CRM1 inhibitor for the treatment of multiple myeloma, providing proof of concept for this therapeutic option with a new mode of action. However, selinexor is associated with dose-limiting toxicity and hence, the discovery of alternative small molecule leads that could be developed as less toxic anticancer and antiviral therapeutics will have a significant impact in the clinic. Here, we report a CRM1 inhibitor discovery platform. The development of this platform includes reporter cell lines that monitor CRM1 activity by using red fluorescent protein or green fluorescent protein-labeled HIV-1 Rev protein with a strong heterologous nuclear export signal. Simultaneously, the intracellular localization of other proteins, to be interrogated for their capacity to undergo CRM1-mediated export, can be followed by co-culturing stable cell lines expressing fluorescent fusion proteins. We used this platform to interrogate the mode of nuclear export of several proteins, including PDK1, p110α, STAT5A, FOXO1, 3, 4 and TRIB2, and to screen a compound collection. We show that while p110α partially relies on CRM1-dependent nuclear export, TRIB2 is exported from the nucleus in a CRM1-independent manner. Compound screening revealed the striking activity of an organoselenium compound on the CRM1 nuclear export receptor.

Genomic Mutations of the STAT5 Transcription Factor Are Associated with Human Cancer and Immune Diseases

Signal transducer and activation of transcription 5 (STAT5) is a key transcription factor that regulates various biological processes in mammalian development. Aberrant regulation of STAT5 has also been causally linked to many diseases, including cancers and immune-related diseases. Although persistent activation of STAT5 due to dysregulation of the signaling cascade has been reported to be associated with the progression of solid tumors and leukemia, various genomic mutations of STAT5 have also been found to cause a wide range of diseases. The present review comprehensively summarizes results of recent studies evaluating the intrinsic function of STAT5 and the link between STAT5 mutations and human diseases. This review also describes the types of disease models useful for investigating the mechanism underlying STAT5-driven disease progression. These findings provide basic knowledge for understanding the regulatory mechanisms of STAT5 and the progression of various diseases resulting from aberrant regulation of STAT5. Moreover, this review may provide insights needed to create optimal disease models that reflect human disease associated STAT5 mutations and to design gene therapies to correct STAT5 mutations.

Disruption of STAT5A and NMI signaling axis leads to ISG20-driven metastatic mammary tumors

Molecular dynamics of developmental processes are repurposed by cancer cells to support cancer initiation and progression. Disruption of the delicate balance between cellular differentiation and plasticity during mammary development leads to breast cancer initiation and metastatic progression. STAT5A is essential for differentiation of secretory mammary alveolar epithelium. Active STAT5A characterizes breast cancer patients for favorable prognosis. N-Myc and STAT Interactor protein (NMI) was initially discovered as a protein that interacts with various STATs; however, the relevance of these interactions to normal mammary development and cancer was not known. We observe that NMI protein is expressed in the mammary ductal epithelium at the onset of puberty and is induced in pregnancy. NMI protein is decreased in 70% of patient specimens with metastatic breast cancer compared to primary tumors. Here we present our finding that NMI and STAT5A cooperatively mediate normal mammary development. Loss of NMI in vivo caused a decrease in STAT5A activity in normal mammary epithelial as well as breast cancer cells. Analysis of STAT5A mammary specific controlled genetic program in the context of NMI knockout revealed ISG20 (interferon stimulated exonuclease gene 20, a protein involved in rRNA biogenesis) as an unfailing negatively regulated target. Role of ISG20 has never been described in metastatic process of mammary tumors. We observed that overexpression of ISG20 is increased in metastases compared to matched primary breast tumor tissues. Our observations reveal that NMI-STAT5A mediated signaling keeps a check on ISG20 expression via miR-17–92 cluster. We show that uncontrolled ISG20 expression drives tumor progression and metastasis.

A novel nuclear localization region in SIPA1 determines protein nuclear distribution and epirubicin-sensitivity of breast cancer cells

Signal-induced proliferation-associated protein 1 (SIPA1) is highly expressed and mainly located in the nucleus in some breast cancer cell lines and clinical tumor tissues. Previous study revealed that nuclear localization of SIPA1 is functionally involved in breast cancer metastasis in the lymphatic gland. In the current study, we identified a non-typical region (140-179aa) of SIPA1 as a novel nuclear localization region (NLR) which is crucial for translocating the proteins into the nucleus in HEK293 cells and breast cancer cells. This region contained one basic amino acid, His160, and had no common features of typical nuclear localization signals. In addition, overexpressing SIPA1 without NLR could suppress breast cancer cell proliferation but could not promote cell migration in MCF7 cells. Furthermore, we found that a high expression of SIPA1 upregulated the expression of ABCB1, encoding multi-drug resistance protein MDR1, and promoted the resistance to epirubicin in breast cancer cells, while this effect was largely abolished in the cells with the expression of NLR-deleted SIPA1. This study overall, identified a nuclear localization-dependent region determining the nuclear distribution of SIPA1 and its regulation on epirubicin-sensitivity in breast cancer cells, which could be a potential drug target to facilitate the development of breast cancer chemotherapy.

Human growth disorders associated with impaired GH action: Defects in STAT5B and JAK2

Growth hormone (GH) promotes postnatal human growth primarily by regulating insulin-like growth factor (IGF)-I production through activation of the GH receptor (GHR)-JAK2-signal transducer and activator of transcription (STAT)-5B signaling pathway. Inactivating STAT5B mutations, both autosomal recessive (AR) and dominant-negative (DN), are causal of a spectrum of GH insensitivity (GHI) syndrome, IGF-I deficiency and postnatal growth failure. Only AR STAT5B defects, however, confer additional characteristics of immune dysfunction which can manifest as chronic, potentially fatal, pulmonary disease. Somatic activating STAT5B and JAK2 mutations are associated with a plethora of immune abnormalities but appear not to impact human linear growth. In this review, molecular defects associated with STAT5B deficiency is highlighted and insights towards understanding human growth and immunity is emphasized.

The roles of post-translational modifications and coactivators of STAT6 signaling in tumor growth and progression

Signal transducers and activators of transcription 6 (STAT6) are highly expressed in various tumors and associated with tumorigenesis, immunosuppression, proliferation, metastasis and poor prognosis in human cancers. In response to IL-4/13, STAT6 is phosphorylated, dimerizes and triggers transcriptional regulation after recruitment of coactivators to transcriptosome, such as CBP/p300, SRC-1, PARP-14 and PSF. Post-translational modifications, including phosphorylation, ubiquitination, ADP-ribosylation and acetylation, have been explored for molecular mechanisms of STAT6 in tumor development and management. STAT6 has been developed as a specific biomarker for distinguishing and diagnosing tumor phenotypes, although it is observed to be frequently mutated in metastatic tumors. In this article, we focus mainly on the structural characteristics of STAT6 and its role in tumor growth and progression.

Unphosphorylated STAT3 in heterochromatin formation and tumor suppression in lung cancer

BACKGROUND

Aberrant JAK/STAT activation has been detected in many types of human cancers. The role of JAK/STAT activation in cancer has been mostly attributed to direct transcriptional regulation of target genes by phosphorylated STAT (pSTAT), while the unphosphorylated STAT (uSTAT) is believed to be dormant and reside in the cytoplasm. However, several studies have shown that uSTATs can be found in the nucleus. In addition, it has been shown that tissue-specific loss of STAT3 or STAT5 in mice promotes cancer growth in certain tissues, and thus these STAT proteins can act as tumor suppressors. However, no unifying mechanism has been shown for the tumor suppressor function of STATs to date. We have previously demonstrated a non-canonical mode of JAK/STAT signaling for Drosophila STAT and human STAT5A, where a fraction of uSTAT is in the nucleus and associated with Heterochromatin Protein 1 (HP1); STAT activation (by phosphorylation) causes its dispersal, leading to HP1 delocalization and heterochromatin loss.

METHODS

We used a combination of imaging, cell biological assays, and mouse xenografts to investigate the role of STAT3 in lung cancer development.

RESULTS

We found that uSTAT3 has a function in promoting heterochromatin formation in lung cancer cells, suppressing cell proliferation in vitro, and suppressing tumor growth in mouse xenografts.

CONCLUSIONS

Thus, uSTAT3 possesses noncanonical function in promoting heterochromatin formation, and the tumor suppressor function of STAT3 is likely attributable to the heterochromatin-promoting activity of uSTAT3 in the non-canonical JAK/STAT pathway.

Nucleocytoplasmic Shuttling of STATs. A Target for Intervention?

Signal transducer and activator of transcription (STAT) proteins are transcription factors that in the latent state are located predominantly in the cytoplasm. Activation of STATs through phosphorylation of a single tyrosine residue results in nuclear translocation. The requirement of tyrosine phosphorylation for nuclear accumulation is shared by all STAT family members but mechanisms of nuclear translocation vary between different STATs. These differences offer opportunities for specific intervention. To achieve this, the molecular mechanisms of nucleocytoplasmic shuttling of STATs need to be understood in more detail. In this review we will give an overview on the various aspects of nucleocytoplasmic shuttling of latent and activated STATs with a special focus on STAT3 and STAT5. Potential targets for cancer treatment will be identified and discussed.

Mechanisms Mediating Nuclear Trafficking Involved in Viral Propagation by DNA Viruses

Typical viral propagation involves sequential viral entry, uncoating, replication, gene transcription and protein synthesis, and virion assembly and release. Some viral proteins must be transported into host nucleus to facilitate viral propagation, which is essential for the production of mature virions. During the transport process, nuclear localization signals (NLSs) play an important role in guiding target proteins into nucleus through the nuclear pore. To date, some classical nuclear localization signals (cNLSs) and non-classical NLSs (ncNLSs) have been identified in a number of viral proteins. These proteins are involved in viral replication, expression regulation of viral genes and virion assembly. Moreover, other proteins are transported into nucleus with unknown mechanisms. This review highlights our current knowledge about the nuclear trafficking of cellular proteins associated with viral propagation.

Nonclassical GH Insensitivity: Characterization of Mild Abnormalities of GH Action

GH insensitivity (GHI) presents in childhood with growth failure and in its severe form is associated with extreme short stature and dysmorphic and metabolic abnormalities. In recent years, the clinical, biochemical, and genetic characteristics of GHI and other overlapping short stature syndromes have rapidly expanded. This can be attributed to advancing genetic techniques and a greater awareness of this group of disorders. We review this important spectrum of defects, which present with phenotypes at the milder end of the GHI continuum. We discuss their clinical, biochemical, and genetic characteristics. The objective of this review is to clarify the definition, identification, and investigation of this clinically relevant group of growth defects. We also review the therapeutic challenges of mild GHI.

Oxymatrine Attenuates Tumor Growth and Deactivates STAT5 Signaling in a Lung Cancer Xenograft Model

Oxymatrine (OMT) is a major alkaloid found in radix Sophorae flavescentis extract and has been reported to exhibit various pharmacological activities. We elucidated the detailed molecular mechanism(s) underlying the therapeutic actions of OMT in non-small cell lung cancer (NSCLC) cells and a xenograft mouse model. Because the STAT5 signaling cascade has a significant role in regulating cell proliferation and survival in tumor cells, we hypothesized that OMT may disrupt this signaling cascade to exert its anticancer effects. We found that OMT can inhibit the constitutive activation of STAT5 by suppressing the activation of JAK1/2 and c-Src, nuclear localization, as well as STAT5 binding to DNA in A549 cells and abrogated IL-6-induced STAT5 phosphorylation in H1299 cells. We also report that a sub-optimal concentration of OMT when used in combination with a low dose of paclitaxel produced significant anti-cancer effects by inhibiting cell proliferation and causing substantial apoptosis. In a preclinical lung cancer mouse model, OMT when used in combination with paclitaxel produced a significant reduction in tumor volume. These results suggest that OMT in combination with paclitaxel can cause an attenuation of lung cancer growth both in vitro and in vivo.

Oscillatory IL-2 stimulus reveals pertinent signaling timescales of T cell responsiveness

Cell response to extracellular ligand is affected not only by ligand availability, but also by pre-existing cell-to-cell variability that enables a range of responses within a cell population. We developed a computational model that incorporates cell heterogeneity in order to investigate Jurkat T cell response to time dependent extracellular IL-2 stimulation. Our model predicted preferred timing of IL-2 oscillatory input for maximizing downstream intracellular STAT5 nuclear translocation. The modeled cytokine exposure was replicated experimentally through the use of a microfluidic platform that enabled the parallelized capture of dynamic single cell response to precisely delivered pulses of IL-2 stimulus. The in vitro results demonstrate that single cell response profiles vary with pulsatile IL-2 input at pre-equilibrium levels. These observations confirmed our model predictions that Jurkat cells have a preferred range of extracellular IL-2 fluctuations, in which downstream response is rapidly initiated. Further investigation into this filtering behavior could increase our understanding of how pre-existing cellular states within immune cell populations enable a systems response within a preferred range of ligand fluctuations, and whether the observed cytokine range corresponds to in vivo conditions.

Moderate High Temperature Condition Induces the Lactation Capacity of Mammary Epithelial Cells Through Control of STAT3 and STAT5 Signaling

In lactating mammary glands, alveolar mammary epithelial cells (MECs) synthesize and secrete milk components. MECs also form less permeable tight junctions (TJs) to prevent the leakage of milk components. During lactation, MECs are exposed to temperature changes by metabolic heat production and air ambient temperature. In this study, we investigated whether temperature changes influence milk production ability and TJ barriers in MECs by using two lactating culture models. The results showed that 39 °C treatment activated milk production and enhanced the formation of less-permeable TJs. In contrast, 41 °C treatment caused adverse effects on the TJ barrier and cell viability, although the milk production ability of MECs was temporarily up-regulated. MECs cultured at 37 °C showed relatively low milk production ability and high proliferation activity. Furthermore, we investigated three kinds of transcription factors relating to lactogenesis, signal transducer and activator of transcription 5 (STAT5), STAT3 and glucocorticoid receptor (GR). STAT5 signaling was activated at 39 and 41 °C by an increase in total STAT5. However, long-term treatment led to a decrease in total STAT5. STAT3 signaling was inactivated by high temperature treatment through a decrease in total STAT3 and inhibited phosphorylation of STAT3. GR signaling was continuously activated regardless of temperature. These results indicate that a moderate high temperature condition at 39 °C induces a high lactation capacity of MECs through control of STAT5 and STAT3 signaling. In contrast, long-term exposure at 41 °C leads to a decline in milk production capacity by inactivation of STAT5 and a decrease in the total number of MECs.

Dominant-negative STAT5B mutations cause growth hormone insensitivity with short stature and mild immune dysregulation

Growth hormone (GH) insensitivity syndrome (GHIS) is a rare clinical condition in which production of insulin-like growth factor 1 is blunted and, consequently, postnatal growth impaired. Autosomal-recessive mutations in signal transducer and activator of transcription (STAT5B), the key signal transducer for GH, cause severe GHIS with additional characteristics of immune and, often fatal, pulmonary complications. Here we report dominant-negative, inactivating STAT5B germline mutations in patients with growth failure, eczema, and elevated IgE but without severe immune and pulmonary problems. These STAT5B missense mutants are robustly tyrosine phosphorylated upon stimulation, but are unable to nuclear localize, or fail to bind canonical STAT5B DNA response elements. Importantly, each variant retains the ability to dimerize with wild-type STAT5B, disrupting the normal transcriptional functions of wild-type STAT5B. We conclude that these STAT5B variants exert dominant-negative effects through distinct pathomechanisms, manifesting in milder clinical GHIS with general sparing of the immune system.

Severe growth hormone insensitivity syndrome (GHIS) with immunodeficiency is caused by autosomal recessive mutations in STAT5B. Here the authors report heterozygous STAT5B mutations with dominant-negative effects, causing mild GHIS without immune defects.

A novel cell-penetrating peptide protects against neuron apoptosis after cerebral ischemia by inhibiting the nuclear translocation of annexin A1

Nuclear translocation of annexin A1 (ANXA1) has recently been reported to participate in neuronal apoptosis after cerebral ischemia. Prevention of the nuclear translocation of ANXA1 should therefore inhibit neuronal apoptosis and protect against cerebral stroke. Here, we found that, in the repeat III domain of ANXA1, the amino-acid residues from R228 to F237 function as a unique nuclear translocation signal (NTS) and are required for nuclear translocation of ANXA1. Intriguingly, we synthesized a cell-penetrating peptide derived by conjugating the trans-activator of transcription (Tat) domain to the NTS sequence. This Tat-NTS peptide specifically blocked the interaction of ANXA1 with importin β and, consequently, the nuclear translocation of ANXA1 without affecting the nucleocytoplasmic shuttling of other proteins. The Tat-NTS peptide inhibited the transcriptional activity of p53, decreased Bid expression, suppressed activation of the caspase-3 apoptosis pathway and improved the survival of hippocampal neurons subjected to oxygen-glucose deprivation and reperfusion in vitro. Moreover, using a focal brain ischemia animal model, we showed that the Tat-NTS peptide could be efficiently infused into the ischemic hippocampus and cortex by unilateral intracerebroventricular injection. Injection of the Tat-NTS peptide alleviated neuronal apoptosis in the ischemic zone. Importantly, further work revealed that administration of the Tat-NTS peptide resulted in a dramatic reduction in infarct volume and that this was correlated with a parallel improvement in neurological function after reperfusion. Interestingly, the effects of Tat-NTS were injury specific, with little impact on neuronal apoptosis or cognitive function in sham-treated nonischemic animals. In conclusion, based on its profound neuroprotective and cognitive-preserving effects, it is suggested that the Tat-NTS peptide represents a novel and potentially promising new therapeutic candidate for the treatment of ischemic stroke.

Jak Stat signaling and cancer: Opportunities, benefits and side effects of targeted inhibition

The effects of Jak Stat signaling and the persistent activation of Stat3 and Stat5 on tumor cell survival, proliferation and invasion have made the Jak Stat pathway a favorite target for drug development and cancer therapy. This notion was strengthened when additional biological functions of Stat signaling in cancer and their roles in the regulation of cytokine dependent inflammation and immunity in the tumor microenvironment were discovered. Stats act not only as transcriptional inducers, but affect gene expression via epigenetic modifications, induce epithelial mesenchymal transition, generate a pro-tumorigenic microenvironment, promote cancer stem cell self-renewal and differentiation, and help to establish the pre-metastatic niche formation. The effects of Jak Stat inhibition on the suppression of pro-inflammatory responses appears most promising and could become a strategy in the prevention of tumor progression. The direct and mediated mechanisms of Jak Stat signaling in and on tumors cells, the interactions with other signaling pathways and transcription factors and the targeting of the functionally crucial secondary modifications of Stat molecules suggest novel approaches to the future development of Jak Stat based cancer therapeutics.

The 91-205 amino acid region of AcMNPV ORF34 (Ac34), which comprises a potential C3H zinc finger, is required for its nuclear localization and optimal virus multiplication

During baculovirus infection, most viral proteins must be imported to the nucleus to support virus multiplication. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) orf34 (ac34) is an alphabaculovirus unique gene that is required for optimal virus production. Ac34 distributes in both the cytoplasm and the nuclei of virus-infected Sf9 cells, but contains no conventional nuclear localization signal (NLS). In this study, we investigated the nuclear targeting domains in Ac34. Transient expression assays showed that Ac34 localized in both the cytoplasm and the nuclei of Sf9 cells, indicating that no viral protein is required for Ac34 nuclear localization. Subcellular localization analysis of Ac34 truncations and internal deletions fused with green fluorescent protein in plasmid-transfected Sf9 cells identified that the 91-205 amino acid (aa) region is required for Ac34 nuclear localization. Mutations in a potential C3H zinc finger (aa 116-131) in Ac34 resulted in exclusive cytoplasmic distribution of GFP:Ac34, suggesting that the zinc finger is required for Ac34 nuclear localization. To assess the functional importance of Ac34 in the nucleus during virus replication, recombinant AcMNPV bacmids containing a series of Ac34 truncations, internal deletions, or site mutations fused with HA tags were constructed. Subcellular localization analysis showed that Ac34 with internal deletions in aa 91-205 or site mutations in the potential zinc finger was predominantly distributed in the cytoplasm. Viral plaque assays and virus growth curves indicated that disruption of Ac34 nuclear localization significantly impaired virus replication. Taken together, our findings demonstrated that the nuclear localization of Ac34 requires the 91-205 aa region and its nuclear localization is essential for optimal virus replication.

Lung epithelial tip progenitors integrate glucocorticoid- and STAT3-mediated signals to control progeny fate

Insufficient alveolar gas exchange capacity is a major contributor to lung disease. During lung development, a population of distal epithelial progenitors first produce bronchiolar-fated and subsequently alveolar-fated progeny. The mechanisms controlling this bronchiolar-to-alveolar developmental transition remain largely unknown. We developed a novel grafting assay to test if lung epithelial progenitors are intrinsically programmed or if alveolar cell identity is determined by environmental factors. These experiments revealed that embryonic lung epithelial identity is extrinsically determined. We show that both glucocorticoid and STAT3 signalling can control the timing of alveolar initiation, but that neither pathway is absolutely required for alveolar fate specification; rather, glucocorticoid receptor and STAT3 work in parallel to promote alveolar differentiation. Thus, developmental acquisition of lung alveolar fate is a robust process controlled by at least two independent extrinsic signalling inputs. Further elucidation of these pathways might provide therapeutic opportunities for restoring alveolar capacity.

Nuclear import of the thyroid hormone receptor α1 is mediated by importin 7, importin β1, and adaptor importin α1

The thyroid hormone receptor α1 (TRα1) is a nuclear receptor for thyroid hormone that shuttles rapidly between the nucleus and cytoplasm. Our prior studies showed that nuclear import of TRα1 is directed by two nuclear localization signals, one in the N-terminal A/B domain and the other in the hinge domain. Here, we showed using in vitro nuclear import assays that TRα1 nuclear localization is temperature and energy-dependent and can be reconstituted by the addition of cytosol. In HeLa cells expressing green fluorescent protein (GFP)-tagged TRα1, knockdown of importin 7, importin β1 and importin α1 by RNA interference, or treatment with an importin β1-specific inhibitor, significantly reduced nuclear localization of TRα1, while knockdown of other importins had no effect. Coimmunoprecipitation assays confirmed that TRα1 interacts with importin 7, as well as importin β1 and the adapter importin α1, suggesting that TRα1 trafficking into the nucleus is mediated by two distinct pathways.

Dual-Specificity Phosphatase 4 Regulates STAT5 Protein Stability and Helper T Cell Polarization

Immune responses are critically regulated by the functions of CD4 helper T cells. Based on their secreted cytokines, helper T cells are further categorized into different subsets like Treg or Th17 cells, which suppress or promote inflammatory responses, respectively. Signals from IL-2 activate the transcription factor STAT5 to promote Treg but suppress Th17 cell differentiation. Our previous results found that the deficiency of a dual-specificity phosphatase, DUSP4, induced STAT5 hyper-activation, enhanced IL-2 signaling, and increased T cell proliferation. In this report, we examined the effects of DUSP4 deficiency on helper T cell differentiation and STAT5 regulation. Our in vivo data showed that DUSP4 mice were more resistant to the induction of autoimmune encephalitis, while in vitro differentiations revealed enhanced iTreg and reduced Th17 polarization in DUSP4-deficient T cells. To study the cause of this altered helper T cell polarization, we performed luciferase reporter assays and confirmed that, as predicted by our previous report, DUSP4 over-expression suppressed the transcription factor activity of STAT5. Surprisingly, we also found that DUSP4-deficient T but not B cells exhibited elevated STAT5 protein levels, and over-expressed DUSP4 destabilized STAT5 in vitro; moreover, this destabilization required the phosphatase activity of DUSP4, and was insensitive to MG132 treatment. Finally, domain-mapping results showed that both the substrate-interacting and the phosphatase domains of DUSP4 were required for its optimal interaction with STAT5, while the coiled-coil domain of STAT5 appeared to hinder this interaction. Our data thus provide the first genetic evidence that DUSP4 is important for helper T cell development. In addition, they also help uncover the novel, DUSP4-mediated regulation of STAT5 protein stability.

Diversification of importin-α isoforms in cellular trafficking and disease states

The human genome encodes seven isoforms of importin α which are grouped into three subfamilies known as α1, α2 and α3. All isoforms share a fundamentally conserved architecture that consists of an N-terminal, autoinhibitory, importin-β-binding (IBB) domain and a C-terminal Arm (Armadillo)-core that associates with nuclear localization signal (NLS) cargoes. Despite striking similarity in amino acid sequence and 3D structure, importin-α isoforms display remarkable substrate specificity in vivo. In the present review, we look at key differences among importin-α isoforms and provide a comprehensive inventory of known viral and cellular cargoes that have been shown to associate preferentially with specific isoforms. We illustrate how the diversification of the adaptor importin α into seven isoforms expands the dynamic range and regulatory control of nucleocytoplasmic transport, offering unexpected opportunities for pharmacological intervention. The emerging view of importin α is that of a key signalling molecule, with isoforms that confer preferential nuclear entry and spatiotemporal specificity on viral and cellular cargoes directly linked to human diseases.

Stat5 Exerts Distinct, Vital Functions in the Cytoplasm and Nucleus of Bcr-Abl+ K562 and Jak2(V617F)+ HEL Leukemia Cells

Signal transducers and activators of transcription (Stats) play central roles in the conversion of extracellular signals, e.g., cytokines, hormones and growth factors, into tissue and cell type specific gene expression patterns. In normal cells, their signaling potential is strictly limited in extent and duration. The persistent activation of Stat3 or Stat5 is found in many human tumor cells and contributes to their growth and survival. Stat5 activation plays a pivotal role in nearly all hematological malignancies and occurs downstream of oncogenic kinases, e.g., Bcr-Abl in chronic myeloid leukemias (CML) and Jak2(V617F) in other myeloproliferative diseases (MPD). We defined the mechanisms through which Stat5 affects growth and survival of K562 cells, representative of Bcr-Abl positive CML, and HEL cells, representative for Jak2(V617F) positive acute erythroid leukemia. In our experiments we suppressed the protein expression levels of Stat5a and Stat5b through shRNA mediated downregulation and demonstrated the dependence of cell survival on the presence of Stat5. Alternatively, we interfered with the functional capacities of the Stat5 protein through the interaction with a Stat5 specific peptide ligand. This ligand is a Stat5 specific peptide aptamer construct which comprises a 12mer peptide integrated into a modified thioredoxin scaffold, S5-DBD-PA. The peptide sequence specifically recognizes the DNA binding domain (DBD) of Stat5. Complex formation of S5-DBD-PA with Stat5 causes a strong reduction of P-Stat5 in the nuclear fraction of Bcr-Abl-transformed K562 cells and a suppression of Stat5 target genes. Distinct Stat5 mediated survival mechanisms were detected in K562 and Jak2(V617F)-transformed HEL cells. Stat5 is activated in the nuclear and cytosolic compartments of K562 cells and the S5-DBD-PA inhibitor most likely affects the viability of Bcr-Abl⁺ K562 cells through the inhibition of canonical Stat5 induced target gene transcription. In HEL cells, Stat5 is predominantly present in the cytoplasm and the survival of the Jak2(V617F)⁺ HEL cells is impeded through the inhibition of the cytoplasmic functions of Stat5.

Components and regulation of nuclear transport processes

The spatial separation of DNA replication and gene transcription in the nucleus and protein translation in the cytoplasm is a uniform principle of eukaryotic cells. This compartmentalization imposes a requirement for a transport network of macromolecules to shuttle these components in and out of the nucleus. This nucleo‐cytoplasmic transport of macromolecules is critical for both cell physiology and pathology. Consequently, investigating its regulation and disease‐associated alterations can reveal novel therapeutic approaches to fight human diseases, such as cancer or viral infection. The characterization of the nuclear pore complex, the identification of transport signals and transport receptors, as well as the characterization of the Ran system (providing the energy source for efficient cargo transport) has greatly facilitated our understanding of the components, mechanisms and regulation of the nucleo‐cytoplasmic transport of proteins in our cells. Here we review this knowledge with a specific emphasis on the selection of disease‐relevant molecular targets for potential therapeutic intervention.

STATs get their move on

Understanding the mechanisms that regulate dynamic localization of a protein within a cell can provide critical insight to its functional molecular interactions. Signal transducers and activators of transcription (STATs) play essential roles in development, proliferation, and immune defense. However the consequences of STAT hyperactivity can predispose to diseases including autoimmunity and cancer. To function as transcription factors STATs must gain access to the nucleus, and knowledge of the mechanisms that regulate STAT nuclear trafficking can provide a means to control STAT action. This review presents a synopsis of some of the studies that address the nuclear dynamics of the STAT proteins. Evidence suggests that not all STATs are the same. Nuclear import of STAT1 and STAT4 appears linked to their tyrosine phosphorylation and the formation of parallel dimers via reciprocal phosphotyrosine and Src homology 2 domain interactions. This dimer arrangement generates a conformational nuclear localization signal. STAT2 is imported continually to the nucleus in an unphosphorylated state due to its association with IRF9, but the dominant nuclear export signal of STAT2 shuttles the complex back to the cytoplasm. Following STAT2 tyrosine phosphorylation, it can form dimers with STAT1 to affect nuclear import as the trimeric complex (ISGF3). Distinctly, STAT3, STAT5, and STAT6 are continually imported to the nucleus independent of tyrosine phosphorylation. Mutational studies indicate the nuclear localization signals in these STATs require the conformational structure of their coiled-coil domains. Increases in STAT nuclear accumulation following cytokine stimulation appear coordinate with their ability to bind DNA.