STAT3 is enriched in nuclear bodies

Optogenetic manipulation identifies the roles of ERK and AKT dynamics in controlling mouse embryonic stem cell exit from pluripotency

ERK and AKT signaling control pluripotent cell self-renewal versus differentiation. ERK pathway activity over time (i.e., dynamics) is heterogeneous between individual pluripotent cells, even in response to the same stimuli. To analyze potential functions of ERK and AKT dynamics in controlling mouse embryonic stem cell (ESC) fates, we developed ESC lines and experimental pipelines for the simultaneous long-term manipulation and quantification of ERK or AKT dynamics and cell fates. We show that ERK activity duration or amplitude or the type of ERK dynamics (e.g., transient, sustained, or oscillatory) alone does not influence exit from pluripotency, but the sum of activity over time does. Interestingly, cells retain memory of previous ERK pulses, with duration of memory retention dependent on duration of previous pulse length. FGF receptor/AKT dynamics counteract ERK-induced pluripotency exit. These findings improve our understanding of how cells integrate dynamics from multiple signaling pathways and translate them into cell fate cues.

Embryonic stem cell ERK, AKT, plus STAT3 response dynamics combinatorics are heterogeneous but NANOG state independent

Signaling is central in cell fate regulation, and relevant information is encoded in its activity over time (i.e., dynamics). However, simultaneous dynamics quantification of several pathways in single mammalian stem cells has not yet been accomplished. Here we generate mouse embryonic stem cell (ESC) lines simultaneously expressing fluorescent reporters for ERK, AKT, and STAT3 signaling activity, which all control pluripotency. We quantify their single-cell dynamics combinations in response to different self-renewal stimuli and find striking heterogeneity for all pathways, some dependent on cell cycle but not pluripotency states, even in ESC populations currently assumed to be highly homogeneous. Pathways are mostly independently regulated, but some context-dependent correlations exist. These quantifications reveal surprising single-cell heterogeneity in the important cell fate control layer of signaling dynamics combinations and raise fundamental questions about the role of signaling in (stem) cell fate control.

Phase-Separated Subcellular Compartmentation and Related Human Diseases

In live cells, proteins and nucleic acids can associate together through multivalent interactions, and form relatively isolated phases that undertake designated biological functions and activities. In the past decade, liquid–liquid phase separation (LLPS) has gradually been recognized as a general mechanism for the intracellular organization of biomolecules. LLPS regulates the assembly and composition of dozens of membraneless organelles and condensates in cells. Due to the altered physiological conditions or genetic mutations, phase-separated condensates may undergo aberrant formation, maturation or gelation that contributes to the onset and progression of various diseases, including neurodegenerative disorders and cancers. In this review, we summarize the properties of different membraneless organelles and condensates, and discuss multiple phase separation-regulated biological processes. Based on the dysregulation and mutations of several key regulatory proteins and signaling pathways, we also exemplify how aberrantly regulated LLPS may contribute to human diseases.

Interleukin-6 at the Host-Tumor Interface: STAT3 in Biomolecular Condensates in Cancer Cells

It was recognized over 30 years ago that the polyfunctional cytokine interleukin-6 (IL-6) was an almost invariant presence at the host-tumor interface. The IL-6 in the tumor microenvironment was produced either by the cancer cell or by host stromal cells, or by tumor-infiltrating immune cells, or all of them. IL-6 effects in this context included local changes in tumor cell-cell and cell-substrate adhesion, enhanced motility, epithelial to mesenchymal transformation (EMT), and changes in cell proliferation rates in both solid tumors as well as hematologic dyscrasias. Locally produced IL-6 enhanced cancer-targeting functions of tumor-infiltrating macrophages and immune cells. Additionally, the sex-biased phenotype of certain cancers [e.g., hepatocellular carcinoma (HCC) which is 3-5-fold more common in men] was related to the inhibition of macrophage-derived IL-6 production by estradiol-17β (E2). In many circumstances, locally produced IL-6 reached the peripheral circulation and elicited systemic effects such as cachexia and paraneoplastic syndrome (including fever, increased erythrocyte sedimentation rate, increased levels of C-reactive protein in serum, hypoalbuminemia). This review highlights the EMT produced by IL-6 in cancer cells, as well as mechanisms underlying sex bias in HCC, enhanced IL-6 expression in cancer cells resulting from mutations in p53, consequent alterations in STAT3 transcriptional signaling, and the newer understanding of STAT3 nuclear bodies in the cancer cell as phase-separated biomolecular condensates and membraneless organelles (MLOs). Moreover, the perplexing issue of discrepant measurements of IL-6 in human circulation using different assays, especially in patients undergoing immunotherapy, is discussed. Additionally, the paradoxical chaperone (enhancing) effect of anti-IL-6 “neutralizing” antibodies on IL-6 in vivo and consequent limitations of immunotherapy using anti-IL-6 mAb is considered.

Analyzing signaling activity and function in hematopoietic cells

Cells constantly sense their environment, allowing the adaption of cell behavior to changing needs. Fine-tuned responses to complex inputs are computed by signaling pathways, which are wired in complex connected networks. Their activity is highly context-dependent, dynamic, and heterogeneous even between closely related individual cells. Despite lots of progress, our understanding of the precise implementation, relevance, and possible manipulation of cellular signaling in health and disease therefore remains limited. Here, we discuss the requirements, potential, and limitations of the different current technologies for the analysis of hematopoietic stem and progenitor cell signaling and its effect on cell fates.

Myristoylation-mediated phase separation of EZH2 compartmentalizes STAT3 to promote lung cancer growth

N-myristoylation is a crucial signaling and pathogenic modification process that confers hydrophobicity to cytosolic proteins. Although different large-scale approaches have been applied, a large proportion of myristoylated proteins remain to be identified. EZH2 is overexpressed in lung cancer cells and exerts oncogenic effects via its intrinsic methyltransferase activity. Using a well-established click chemistry approach, we found that EZH2 can be modified by myristoylation at its N-terminal glycine in lung cancer cells. Hydrophobic interaction is one of the main forces driving or stabilizing liquid-liquid phase separation (LLPS), raising the possibility that myristoylation can modulate LLPS by mediating hydrophobic interactions. Indeed, myristoylation facilitates EZH2 to form phase-separated liquid droplets in lung cancer cells and in vitro. Furthermore, we provide evidence that myristoylation-mediated LLPS of EZH2 compartmentalizes its non-canonical substrate, STAT3, and activates STAT3 signaling, ultimately resulting in accelerated lung cancer cell growth. Thus, targeting EZH2 myristoylation may have significant therapeutic efficacy in the treatment of lung cancer. Altogether, these observations not only extend the list of myristoylated proteins, but also indicate that hydrophobic lipidation may serve as a novel incentive to induce or maintain LLPS.

Biomolecular condensates in cell biology and virology: Phase-separated membraneless organelles (MLOs)

Membraneless organelles (MLOs) in the cytoplasm and nucleus in the form of 2D and 3D phase-separated biomolecular condensates are increasingly viewed as critical in regulating diverse cellular functions. These functions include cell signaling, immune synapse function, nuclear transcription, RNA splicing and processing, mRNA storage and translation, virus replication and maturation, antiviral mechanisms, DNA sensing, synaptic transmission, protein turnover and mitosis. Components comprising MLOs often associate with low affinity; thus cell integrity can be critical to the maintenance of the full complement of respective MLO components. Phase-separated condensates are typically metastable (shape-changing) and can undergo dramatic, rapid and reversible assembly and disassembly in response to cell signaling events, cell stress, during mitosis, and after changes in cytoplasmic "crowding" (as observed with condensates of the human myxovirus resistance protein MxA). Increasing evidence suggests that neuron-specific aberrations in phase-separation properties of RNA-binding proteins (e.g. FUS and TDP-43) and others (such as the microtubule-binding protein tau) contribute to the development of degenerative neurological diseases (e.g. amyotrophic lateral sclerosis, frontotemporal lobar degeneration, and Alzheimer's disease). Thus, studies of liquid-like phase separation (LLPS) and the formation, structure and function of MLOs are of considerable importance in understanding basic cell biology and the pathogenesis of human diseases.

Human Antiviral Protein MxA Forms Novel Metastable Membraneless Cytoplasmic Condensates Exhibiting Rapid Reversible Tonicity-Driven Phase Transitions

Phase-separated biomolecular condensates of proteins and nucleic acids form functional membrane-less organelles (e.g., stress granules and P-bodies) in the mammalian cell cytoplasm and nucleus. In contrast to the long-standing belief that interferon (IFN)-inducible human myxovirus resistance protein A (MxA) associated with the endoplasmic reticulum (ER) and Golgi apparatus, we report that MxA formed membraneless metastable (shape-changing) condensates in the cytoplasm. In our studies, we used the same cell lines and methods as those used by previous investigators but concluded that wild-type MxA formed variably sized spherical or irregular bodies, filaments, and even a reticulum distinct from that of ER/Golgi membranes. Moreover, in Huh7 cells, MxA structures associated with a novel cytoplasmic reticular meshwork of intermediate filaments. In live-cell assays, 1,6-hexanediol treatment led to rapid disassembly of green fluorescent protein (GFP)-MxA structures; FRAP revealed a relative stiffness with a mobile fraction of 0.24 ± 0.02 within condensates, consistent with a higher-order MxA network structure. Remarkably, in intact cells, GFP-MxA condensates reversibly disassembled/reassembled within minutes of sequential decrease/increase, respectively, in tonicity of extracellular medium, even in low-salt buffers adjusted only with sucrose. Condensates formed from IFN-α-induced endogenous MxA also displayed tonicity-driven disassembly/reassembly. In vesicular stomatitis virus (VSV)-infected Huh7 cells, the nucleocapsid (N) protein, which participates in forming phase-separated viral structures, associated with spherical GFP-MxA condensates in cells showing an antiviral effect. These observations prompt comparisons with the extensive literature on interactions between viruses and stress granules/P-bodies. Overall, the new data correct a long-standing misinterpretation in the MxA literature and provide evidence for membraneless MxA biomolecular condensates in the uninfected cell cytoplasm.IMPORTANCE There is a long-standing belief that interferon (IFN)-inducible human myxovirus resistance protein A (MxA), which displays antiviral activity against several RNA and DNA viruses, associates with the endoplasmic reticulum (ER) and Golgi apparatus. We provide data to correct this misinterpretation and further report that MxA forms membraneless metastable (shape-changing) condensates in the cytoplasm consisting of variably sized spherical or irregular bodies, filaments, and even a reticulum. Remarkably, MxA condensates showed the unique property of rapid (within 1 to 3 min) reversible disassembly and reassembly in intact cells exposed sequentially to hypotonic and isotonic conditions. Moreover, GFP-MxA condensates included the VSV nucleocapsid (N) protein, a protein previously shown to form liquid-like condensates. Since intracellular edema and ionic changes are hallmarks of cytopathic effects of a viral infection, the tonicity-driven regulation of MxA condensates may reflect a mechanism for modulation of MxA function during viral infection.

Biomolecular condensates in cancer cell biology: interleukin-6-induced cytoplasmic and nuclear STAT3/PY-STAT3 condensates in hepatoma cells

We highlight previous incompletely understood cell biology data in the STAT3 signaling field with respect to interleukin-6 (IL-6)-induced activation of this transcription factor in hepatoma cells to generate cytoplasmic and nuclear STAT3 bodies. We provide a novel re-interpretation of the previous observations. We show that IL-6-induced GFP-STAT3/PY-STAT3 cytoplas-mic and nuclear bodies represent phase-separated biomolecular condensates. These structures represent examples of a cytokine-induced phase transition which occurs within 10-15 min of exposure to the cytokine, and which was Tyr phosphorylation dependent. Evidence that these IL-6-induced cytoplasmic and nuclear GFP-STAT3 bodies in live cells represented phase-separated condensates came from the observation that 1,6-hexanediol caused their disassembly within 30-60 seconds. Moreover, these STAT3 condensates also showed rapid tonicity-driven phase transitions - disassembly under hypotonic conditions and reassembly when cells were returned to isotonic medium. That STAT3 condensates were rapidly disassembled in hypotonic buffer commonly used for cell fractionation points to a limitation of studies of STAT3 biochemistry using hypotonic swelling and mechanical breakage. Overall, the new data help reinterpret IL-6-induced cytoplasmic and nuclear STAT3 bodies as phase-separated biomolecular condensates, and bring the concept of membrane-less organelles to the cytokine-induced STAT transcription factor field and cancer cell biology.

STAT3 Cooperates With Phospholipid Scramblase 2 to Suppress Type I Interferon Response

Type I interferon (IFN-I) is a pluripotent cytokine that modulates innate and adaptive immunity. We have previously shown that STAT3 suppresses IFN-I response in a manner dependent on its N-terminal domain (NTD), but independent of its DNA-binding and transactivation ability. Using the yeast two-hybrid system, we have identified phospholipid scramblase 2 (PLSCR2) as a STAT3 NTD-binding partner and a suppressor of IFN-I response. Overexpression of PLSCR2 attenuates ISRE-driven reporter activity, which is further aggravated by co-expression of STAT3. Moreover, PLSCR2 deficiency enhances IFN-I-induced gene expression and antiviral activity without affecting the activation or nuclear translocation of STAT1 and STAT2 or the assembly of ISGF3 complex. Instead, PLSCR2 impedes promoter occupancy by ISGF3, an effect further intensified by the presence of STAT3. Moreover, palmitoylation of PLSCR2 is required for its binding to STAT3 and for this suppressive activity. In addition to STAT3, PLSCR2 also interacts with STAT2, which facilitates the suppressive effect on ISGF3-mediated transcriptional activity. Together, these results define the role of a novel STAT3–PLSCR2 axis in fine-tuning IFN-I response.

Zinc deficiency affects the STAT1/3 signaling pathways in part through redox-mediated mechanisms

Zinc deficiency affects the development of the central nervous system (CNS) through mechanisms only partially understood. We previously showed that zinc deficiency causes CNS oxidative stress, damaging microtubules and impairing protein nuclear shuttling. STAT1 and STAT3 transcription factors, which require nuclear import for their functions, play major roles in CNS development. Thus, we investigated whether zinc deficiency disrupts STAT1 and STAT3 signaling pathways in the developing fetal CNS, characterizing the involvement of oxidative stress and the cytoskeleton in the adverse effects. Maternal (gestation day 0–19) marginal zinc deficiency (MZD) reduced STAT1 and STAT3 tyrosine phosphorylation and their nuclear translocation in the embryonic day 19 (E19) rat brain. Similar effects were observed in zinc depleted IMR-32 neuroblastoma cells, with an associated decrease in STAT1- and STAT3-dependent gene transactivation. Zinc deficiency caused oxidative stress (increased 4-hydroxynonenal-protein adducts) in E19 brain and IMR-32 cells, which was prevented in cells by supplementation with 0.5 mM α-lipoic acid (LA). In zinc depleted IMR-32 cells, the low tyrosine phosphorylation of STAT1, but not that of STAT3, recovered upon incubation with LA. STAT1 and STAT3 nuclear transports were also restored by LA. Accordingly, chemical disruption of the cytoskeleton partially reduced STAT1 and STAT3 nuclear levels. In summary, the redox-dependent tyrosine phosphorylation, and oxidant-mediated disruption of the cytoskeleton are involved in the deleterious effects of zinc deficit on STAT1 and STAT3 activation and nuclear translocation. Therefore, disruption of the STAT1 and STAT3 signaling pathways may in part explain the deleterious effects of maternal MZD on fetal brain development.

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Zinc deficits impair STAT1/STAT3 signaling in fetal brain and neuroblastoma cells.

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Zinc deficiency inhibits STAT1 and STAT3 tyrosine phosphorylation and nuclear translocation.

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Zinc deficiency causes oxidative stress (high HNE-protein adducts) in fetal brain and cells.

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Lipoic acid reverts zinc deficiency-associated decreased STAT1/STAT3 nuclear shuttling.

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Zinc deficiency-associated oxidative stress impairs STAT1/STAT3 modulation.

HDAC Inhibitor Sodium Butyrate-Mediated Epigenetic Regulation Enhances Neuroprotective Function of Microglia During Ischemic Stroke

Cerebral ischemia leads to neuroinflammation and activation of microglia which further contribute to stroke pathology. Understanding regulation of microglial activation will aid in the development of therapeutic strategies that mitigate microglia-mediated neurotoxicity in neuropathologies, including ischemia. In this study, we investigated the epigenetic regulation of microglial activation by studying histone modification histone 3-lysine 9-acetylation (H3K9ac) and its regulation by histone deacetylase (HDAC) inhibitors. In vitro analysis of activated microglia showed that HDAC inhibitor, sodium butyrate (SB), alters H3K9ac enrichment and transcription at the promoters of pro-inflammatory (Tnf-α, Nos2, Stat1, Il6) and anti-inflammatory (Il10) genes while inducing the expression of genes downstream of the IL10/STAT3 anti-inflammatory pathway. In an experimental mouse (C57BL/6NTac) model of middle cerebral artery occlusion (MCAO), we observed that SB mediates neuroprotection by epigenetically regulating the microglial inflammatory response, via downregulating the expression of pro-inflammatory mediators, TNF-α and NOS2, and upregulating the expression of anti-inflammatory mediator IL10, in activated microglia. Interestingly, H3K9ac levels were found to be upregulated in activated microglia distributed in the cortex, striatum, and hippocampus of MCAO mice. A similar upregulation of H3K9ac was detected in lipopolysaccharide (LPS)-activated microglia in the Wistar rat brain, indicating that H3K9ac upregulation is consistently associated with microglial activation in vivo. Altogether, these results show evidence of HDAC inhibition being a promising molecular switch to epigenetically modify microglial behavior from pro-inflammatory to anti-inflammatory which could mitigate microglia-mediated neuroinflammation.

Role of ND10 nuclear bodies in the chromatin repression of HSV-1

Herpes simplex virus (HSV) is a neurotropic virus that establishes lifelong latent infection in human ganglion sensory neurons. This unique life cycle necessitates an intimate relation between the host defenses and virus counteractions over the long course of infection. Two important aspects of host anti-viral defense, nuclear substructure restriction and epigenetic chromatin regulation, have been intensively studied in the recent years. Upon viral DNA entering the nucleus, components of discrete nuclear bodies termed nuclear domain 10 (ND10), converge at viral DNA and place restrictions on viral gene expression. Meanwhile the infected cell mobilizes its histones and histone-associated repressors to force the viral DNA into nucleosome-like structures and also represses viral transcription. Both anti-viral strategies are negated by various HSV countermeasures. One HSV gene transactivator, infected cell protein 0 (ICP0), is a key player in antagonizing both the ND10 restriction and chromatin repression. On one hand, ICP0 uses its E3 ubiquitin ligase activity to target major ND10 components for proteasome-dependent degradation and thereafter disrupts the ND10 nuclear bodies. On the other hand, ICP0 participates in de-repressing the HSV chromatin by changing histone composition or modification and therefore activates viral transcription. Involvement of a single viral protein in two seemingly different pathways suggests that there is coordination in host anti-viral defense mechanisms and also cooperation in viral counteraction strategies. In this review, we summarize recent advances in understanding the role of chromatin regulation and ND10 dynamics in both lytic and latent HSV infection. We focus on the new observations showing that ND10 nuclear bodies play a critical role in cellular chromatin regulation. We intend to find the connections between the two major anti-viral defense pathways, chromatin remodeling and ND10 structure, in order to achieve a better understanding of how host orchestrates a concerted defense and how HSV adapts with and overcomes the host immunity.

Quantifying the dynamics of the oligomeric transcription factor STAT3 by pair correlation of molecular brightness

Oligomerization of transcription factors controls their translocation into the nucleus and DNA-binding activity. Here we present a fluorescence microscopy analysis termed pCOMB (pair correlation of molecular brightness) that tracks the mobility of different oligomeric species within live cell nuclear architecture. pCOMB amplifies the signal from the brightest species present and filters the dynamics of the extracted oligomeric population based on arrival time between two locations. We use this method to demonstrate a dependence of signal transducer and activator of transcription 3 (STAT3) mobility on oligomeric state. We find that on entering the nucleus STAT3 dimers must first bind DNA to form STAT3 tetramers, which are also DNA-bound but exhibit a different mobility signature. Examining the dimer-to-tetramer transition by a cross-pair correlation analysis (cpCOMB) reveals that chromatin accessibility modulates STAT3 tetramer formation. Thus, the pCOMB approach is suitable for mapping the impact oligomerization on transcription factor dynamics.

Impact of the N-Terminal Domain of STAT3 in STAT3-Dependent Transcriptional Activity

The transcription factor STAT3 is constitutively active in many cancers, where it mediates important biological effects, including cell proliferation, differentiation, survival, and angiogenesis. The N-terminal domain (NTD) of STAT3 performs multiple functions, such as cooperative DNA binding, nuclear translocation, and protein-protein interactions. However, it is unclear which subsets of STAT3 target genes depend on the NTD for transcriptional regulation. To identify such genes, we compared gene expression in STAT3-null mouse embryonic fibroblasts (MEFs) stably expressing wild-type STAT3 or STAT3 from which NTD was deleted. NTD deletion reduced the cytokine-induced expression of specific STAT3 target genes by decreasing STAT3 binding to their regulatory regions. To better understand the potential mechanisms of this effect, we determined the crystal structure of the STAT3 NTD and identified a dimer interface responsible for cooperative DNA binding in vitro. We also observed an Ni(2+)-mediated oligomer with an as yet unknown biological function. Mutations on both dimer and Ni(2+)-mediated interfaces affected the cytokine induction of STAT3 target genes. These studies shed light on the role of the NTD in transcriptional regulation by STAT3 and provide a structural template with which to design STAT3 NTD inhibitors with potential therapeutic value.

The STAT3 HIES mutation is a gain-of-function mutation that activates genes via AGG-element carrying promoters

Cytokine or growth factor activated STAT3 undergoes multiple post-translational modifications, dimerization and translocation into nuclei, where it binds to serum-inducible element (SIE, ‘TTC(N3)GAA’)-bearing promoters to activate transcription. The STAT3 DNA binding domain (DBD, 320–494) mutation in hyper immunoglobulin E syndrome (HIES), called the HIES mutation (R382Q, R382W or V463Δ), which elevates IgE synthesis, inhibits SIE binding activity and sensitizes genes such as TNF-α for expression. However, the mechanism by which the HIES mutation sensitizes STAT3 in gene induction remains elusive. Here, we report that STAT3 binds directly to the AGG-element with the consensus sequence ‘AGG(N3)AGG’. Surprisingly, the helical N-terminal region (1–355), rather than the canonical STAT3 DBD, is responsible for AGG-element binding. The HIES mutation markedly enhances STAT3 AGG-element binding and AGG-promoter activation activity. Thus, STAT3 is a dual specificity transcription factor that promotes gene expression not only via SIE- but also AGG-promoter activity.

Cigarette smoke-induced MMP2 and MMP9 secretion from aortic vascular smooth cells is mediated via the Jak/Stat pathway

It is hypothesized that cigarette smoke may increase MMP2 and MMP9 secretion through Jak/Stat pathway in the aorta, thereby facilitating abdominal aortic aneurysm (AAA) formation/progression in smokers. We observed through zymograms that treatment of male rat aortic vascular smooth muscle cells (RASMC) with an aqueous extract of cigarette smoke (CSE) for 24 hours resulted in a significant increase in pro-MMP9 (P = .005) and a modest increase in pro-MMP2 (P = .055) production. Western blot with protein extracts from CSE-treated RASMC showed up-regulation of pStat3, pJak2, and T-Jak2 and unchanged levels of T-Stat3. Transfection of RASMC with small interfering RNAs for Jak2, Stat3, or both Jak2 and Stat3 significantly reduced pro-MMP9 (P < .005) and pro-MMP2 (P < .05) in medium of CSE-treated RASMC compared with control small interfering RNA-transfected cells. Immunoprecipitation with total Jak2 antibody showed increased pStat3 and T-Stat3 in the cytoplasm and nucleus of CSE-treated RASMC. Immunofluorescence revealed increased presence of pJak2, T-Jak2, pStat3, and T-Stat3 in the cytoplasm and nucleus of the CSE-treated cells. Treatment of control human tissues with CSE resulted in pro-MMP9 secretion and up-regulation of the Jak/Stat proteins. In addition, AAA tissues showed more pJak2 and pStat3 than control human tissues. Therefore, inhibiting the Jak/Stat pathway could be a potential therapeutic approach in the treatment of AAA.

Cellular senescence and protein degradation: breaking down cancer

Autophagy and the ubiquitin-proteasome pathway (UPP) are the major protein degradation systems in eukaryotic cells. Whereas the former mediate a bulk nonspecific degradation, the UPP allows a rapid degradation of specific proteins. Both systems have been shown to play a role in tumorigenesis, and the interest in developing therapeutic agents inhibiting protein degradation is steadily growing. However, emerging data point to a critical role for autophagy in cellular senescence, an established tumor suppressor mechanism. Recently, a selective protein degradation process mediated by the UPP was also shown to contribute to the senescence phenotype. This process is tightly regulated by E3 ubiquitin ligases, deubiquitinases, and several post-translational modifications of target proteins. Illustrating the complexity of UPP, more than 600 human genes have been shown to encode E3 ubiquitin ligases, a number which exceeds that of the protein kinases. Nevertheless, our knowledge of proteasome-dependent protein degradation as a regulated process in cellular contexts such as cancer and senescence remains very limited. Here we discuss the implications of protein degradation in senescence and attempt to relate this function to the protein degradation pattern observed in cancer cells.

Phosphorylated STAT3 physically interacts with NPM and transcriptionally enhances its expression in cancer

The signal transducer and activator of transcription 3 (STAT3) can be activated by the tyrosine kinase domain of the chimeric protein nucleophosmin/anaplastic lymphoma kinase (NPM/ALK), and has a pivotal role in mediating NPM/ALK-related malignant cell transformation. Although the role of STAT3 and wild-type NPM in oncogenesis has been extensively investigated, the relationship between both molecules in cancer remains poorly understood. In the present study, we first demonstrate that STAT3 phosphorylation at tyrosine 705 is accompanied by a concomitant increase in the expression level of NPM. Nuclear co-translocation of phosphorylated STAT3 with NPM can be triggered by interferon-alpha (IFN-α) stimulation of Jurkat cells and phosphorylated STAT3 co-localizes with NPM in cancer cells showing constitutive STAT3 activation. We further demonstrate that STAT3 phosphorylation can transcriptionally mediate NPM upregulation in IFN-α-stimulated Jurkat cells and is responsible for maintaining its expression in cancer cells showing constitutive STAT3 activation. Inhibition of STAT3 phosphorylation or knockdown of NPM expression abrogates their simultaneous transnuclear movements. Finally, we found evidence for a physical interaction between NPM and STAT3 in conditions of STAT3 activation. In conclusion, NPM is a downstream effector of the STAT3 signaling, and can facilitate the nuclear entry of phosphorylated STAT3. These observations might open novel opportunities for targeting the STAT3 pathway in cancer.

Arginine residues within the DNA binding domain of STAT3 promote intracellular shuttling and phosphorylation of STAT3

Acetylation-dependent inactivation of STAT1 can be mimicked by the exchange of its lysine residues K410 and K413 to glutamine residues. STAT3 harbors non-acetylatable arginine moieties at the corresponding sites R414 and R417. It is unclear whether the mutation of these sites to glutamine residues antagonizes STAT3 activation. Here, we show that an arginine-glutamine-exchange at the STAT3 moieties R414 and R417 (R414Q and R417Q) reduces cytokine-dependent tyrosine phosphorylation of STAT3. This inhibitory effect can be partially rescued by phosphatase inhibition. In addition, the R414Q and R417Q mutations enhance the nuclear accumulation of unphosphorylated STAT3. STAT3 R414Q and STAT3 R417Q show a reduced response to cytokine stimulation emanating from the plasma membrane. Moreover, these STAT3 mutants have no direct inhibitory effect on the cytokine-induced activation of STAT1/STAT3-mediated gene expression. Since the mutations R414Q and R417Q reside within the STAT3 DNA binding domain (DBD), the STAT3 R414Q and R417Q mutants also lack intrinsic activity as transcription factors. Furthermore, in contrast to wild-type STAT3 they cannot compensate for a loss of STAT1 and they cannot promote STAT1/STAT3-dependent transcriptional activation. We further analyzed a STAT3 arginine-lysine-exchange mutant (R414K/R417K). This molecule mimics corresponding lysine residues found within the DBD of STAT1. Compared to wild-type STAT3, the STAT3 R414K/R417K mutant shows attenuated tyrosine phosphorylation and it is a less active transcription factor. In addition, STAT3 R414K/R417K is not activated by deacetylase inhibition. On the other hand, C-terminal acetylation of STAT3 is intact in STAT3 R414K/R417K. Our results suggest that the exchange of amino acid residues within the DBDs of STAT1/STAT3 affects their phosphorylation as well as their intracellular shuttling.

STAT3 and MCL-1 associate to cause a mesenchymal epithelial transition

Embryo implantation is effected by a myriad of signaling cascades acting on the embryo-endometrium axis. Here we show, by using MALDI TOF analysis, far-western analysis and colocalization and co-transfection studies, that STAT3 and MCL-1 are interacting partners during embryo implantation. We show in vitro that the interaction between the two endogenous proteins is strongly regulated by estrogen and progesterone. Implantation, pregnancy and embryogenesis are distinct from any other process in the body, with extensive, but controlled, proliferation, cell migration, apoptosis, cell invasion and differentiation. Cellular plasticity is vital during the early stages of development for morphogenesis and organ homeostasis, effecting the epithelial to mesenchymal transition (EMT) and, the reverse process, mesenchymal to epithelial transition (MET). STAT3 functionally associates with MCL-1 in the mammalian breast cancer cell line MCF7 that overexpresses STAT3 and MCL-1, which leads to an increased rate of apoptosis and decreased cellular invasion, disrupting the EMT. Association of MCL-1 with STAT3 modulates the normal, anti-apoptotic, activity of MCL-1, resulting in pro-apoptotic effects. Studying the impact of the association of STAT3 with MCL-1 on MET could lead to an enhanced understanding of pregnancy and infertility, and also metastatic tumors.

Consequences of the disease-related L78R mutation for dimerization and activity of STAT3

STAT3 (signal transducer and activator of transcription 3) is a transcription factor centrally involved in diverse processes including hematopoiesis, immunity and cancer progression. In response to cytokine stimulation STAT3 is activated through phosphorylation of a single tyrosine residue. The phosphorylated STAT3 dimers are stabilized by intermolecular SH2 domain/phosphotyrosine interactions. These activated dimers accumulate in the nucleus and bind to specific DNA sequences resulting in target gene expression. We analysed and compared the structural organizations of the unphosphorylated latent and phosphorylated activated STAT3 dimers using Förster resonance energy transfer (FRET) in fixed and living cells. The latent dimers are stabilized by homotypic interactions between the N-terminal domains. A somatic mutation (L78R) found in inflammatory hepatocellular adenomas (IHCA) which is located in the N-terminal domain of STAT3 disturbs latent dimer formation. Applying intramolecular FRET we verify a functional role of the SH2 domain in latent dimer formation suggesting the parallel orientation of the protomers in the latent STAT3 dimer similar to activated STAT3 dimers but in contrast to the latent dimers of STAT1 and STAT5. Our findings reveal unique structural characteristics of STAT3 within the STAT family and contribute to the understanding of the L78R mutation found in IHCA.

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.

Dual inhibition of Janus and Src family kinases by novel indirubin derivative blocks constitutively-activated Stat3 signaling associated with apoptosis of human pancreatic cancer cells

Constitutively-activated JAK/Stat3 or Src/Stat3 signaling plays a crucial role in tumor cell survival, proliferation, angiogenesis and immune suppression. Activated JAK/Stat3 or Src/Stat3 has been validated as a promising molecular target for cancer therapy. However, prolonged inhibition of Src family kinases (SFKs) leads to reactivation of signal transducer and activator of transcript 3 (Stat3) and tumor cell survival through altered JAK/Stat3 interaction. This compensatory feedback suggests that dual inhibition of Janus kinases (JAKs) and SFKs might be a promising strategy for targeting downstream Stat3 signaling in the clinic. In this study, we identify that the natural product derivative E738 is a novel dual inhibitor of JAKs and SFKs. The IC(50) values of E738 against recombinant JAKs and SFKs in vitro are in the ranges of 0.7-74.1 nM and 10.7-263.9 nM, respectively. We observed that phosphorylation of both Jak2 and Src was substantially inhibited in the submicromolar range by E738 in cultured human pancreatic tumor cells, followed by blockade of downstream Stat3 activation. E738 down-regulated expression of the Stat3 target proteins Mcl-1 and survivin, associated with induction of apoptosis. Computational models and molecular dynamics simulations of E738/Tyk2 or E738/Src in silico suggest that E738 inhibits both tyrosine kinase 2 (Tyk2) and Src as an ATP-competitive ligand. Moreover, the planar E738 molecule demonstrates a strong binding affinity in the compact ATP-binding site of Tyk2. In sum, E738 is the first dual inhibitor of JAKs and SFKs, followed by inhibition of Stat3 signaling. Thus, according to in vitro experiments, E738 is a promising new therapeutic agent for human pancreatic cancer treatment by blocking both oncogenic pathways simultaneously.

Dynamics and non-canonical aspects of JAK/STAT signalling

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway directly links ligand-binding to a membrane-bound receptor with the activation of a transcription factor. This signalling module enables the cell to rapidly initiate a transcriptional response to external stimulation. The main components of this evolutionary conserved module are cytokines that specifically bind to cytokine receptors leading to the activation of receptor-associated Janus tyrosine kinases (JAKs). The receptor-bound JAKs activate STAT transcription factors through phosphorylation of a single tyrosine residue. Activated STAT dimers translocate into the nucleus to induce target gene expression. In this article we will review current opinions on the molecular mechanism and on intracellular dynamics of JAK/STAT signalling with a special focus on the cytokine receptor glycoprotein 130 (gp130) and STAT3. In particular we will concentrate on non-canonical aspects of Jak/STAT signalling including preassembled receptor complexes, preformed STAT dimers, STAT trafficking and non-canonical functions of STATs.

Alkylation of cysteine 468 in Stat3 defines a novel site for therapeutic development

Stat3 is a latent transcription factor that promotes cell survival and proliferation and is often constitutively active in multiple cancers. Inhibition of Stat3 signaling pathways suppresses cell survival signals and leads to apoptosis in cancer cells, suggesting direct inhibition of Stat3 function is a viable therapeutic approach. Herein, we identify a small molecule, C48, as a selective Stat3-family member inhibitor. To determine its mechanism of action, we used site-directed mutagenesis and multiple biochemical techniques to show that C48 alkylates Cys468 in Stat3, a residue at the DNA-binding interface. We further demonstrate that C48 blocks accumulation of activated Stat3 in the nucleus in tumor cell lines that overexpress active Stat3, leading to impressive inhibition of tumor growth in mouse models. Collectively, these findings suggest Cys468 in Stat3 represents a novel site for therapeutic intervention and demonstrates the promise of alkylation as a potentially effective chemical approach for Stat3-dependent cancers.

Dynamics of the STAT3 transcription factor: nuclear import dependent on Ran and importin-β1

The signal transducer and activator of transcription-3 (STAT3) induces transcription of genes that control differentiation, inflammation, proliferation, and tumor cell invasion. Cytokines such as interleukin-6 and interferon stimulate the specific tyrosine phosphorylation of STAT3, which confers its ability to bind consensus DNA targets. In addition, unphosphorylated STAT3 has been demonstrated to induce specific gene expression. STAT3 must gain entrance to the nucleus to impact transcription, however access to the nucleus is a tightly regulated process. Because nuclear trafficking is critical to the function of STAT3, we investigated the molecular mechanisms by which STAT3 is imported to the nucleus. Live cell imaging techniques were used with STAT3 tagged with green fluorescence protein (GFP) or photoactivatable GFP to follow the cellular dynamics of both unphosphorylated and tyrosine phosphorylated forms. Cytokine activation did not alter the rate of STAT3 nuclear import or nuclear export. In addition, Förster resonance energy transfer experiments revealed homomeric interaction of unphosphorylated STAT3 dependent on its amino terminus, but this dimerization is not necessary for its nuclear import. Previous work demonstrated the adapter importin-α3 binds to STAT3 and is required for nuclear import. To determine whether STAT3 nuclear import is mediated by the importin-α/importin-β1 heterodimer, the effects of siRNA to importin-β1 were evaluated. Results indicate STAT3 nuclear import is dependent on the function of importin-β1. Since the Ran GTPase is necessary to bind importin-β1 in the nucleus for release of importin-α-cargo, the effect of a GTPase deficient mutant of Ran was tested. Expression of the Ran interfering mutant inhibited STAT3 nuclear import. This study defines importin-α/importin-β1/Ran as the molecular mechanism by which STAT3 traffics to the nucleus.

Cytokine-induced paracrystals prolong the activity of signal transducers and activators of transcription (STAT) and provide a model for the regulation of protein solubility by small ubiquitin-like modifier (SUMO)

The biological effects of cytokines are mediated by STAT proteins, a family of dimeric transcription factors. In order to elicit transcriptional activity, the STATs require activation by phosphorylation of a single tyrosine residue. Our experiments revealed that fully tyrosine-phosphorylated STAT dimers polymerize via Tyr(P)-Src homology 2 domain interactions and assemble into paracrystalline arrays in the nucleus of cytokine-stimulated cells. Paracrystals are demonstrated to be dynamic reservoirs that protect STATs from dephosphorylation. Activated STAT3 forms such paracrystals in acute phase liver cells. Activated STAT1, in contrast, does not normally form paracrystals. By reversing the abilities of STAT1 and STAT3 to be sumoylated, we show that this is due to the unique ability of STAT1 among the STATs to conjugate to small ubiquitin-like modifier (SUMO). Sumoylation had one direct effect; it obstructed proximal tyrosine phosphorylation, which led to semiphosphorylated STAT dimers. These competed with their fully phosphorylated counterparts and interfered with their polymerization into paracrystals. Consequently, sumoylation, by preventing paracrystal formation, profoundly curtailed signal duration and reporter gene activation in response to cytokine stimulation of cells. The study thus identifies polymerization of activated STAT transcription factors as a positive regulatory mechanism in cytokine signaling. It provides a unifying explanation for the different subnuclear distributions of STAT transcription factors and reconciles the conflicting results as to the role of SUMO modification in STAT1 functioning. We present a generally applicable system in which protein solubility is maintained by a disproportionately small SUMO-modified fraction, whereby modification by SUMO partially prevents formation of polymerization interfaces, thus generating competitive polymerization inhibitors.

Pasteurella multocida Toxin-induced Pim-1 expression disrupts suppressor of cytokine signalling (SOCS)-1 activity

Pasteurella multocida Toxin (PMT) is a mitogenic protein toxin that manipulates signal transduction cascades of mammalian host cells and upregulates Janus kinase (JAK) and signal transducers of transcription (STAT) activity. Here we show that in the presence of PMT, increased levels of suppressors of cytokine signalling-1 (SOCS-1) proteins significantly enhance STAT activity. This occurs via PMT-induced expression of the serine/threonine kinase Pim-1 and subsequent threonine phosphorylation of SOCS-1. The ability of SOCS-1 to act as an E3 ubiquitin ligase is regulated by its phosphorylation status. Thus, the tyrosine kinase JAK2 cannot be marked for proteasomal degradation by threonine phosphorylated SOCS-1. Consequently, the expression levels of JAK2 are increased, eventually leading to hyperactivity of JAK2 and its target, the transcription factor STAT3. Eventually this causes increased anchorage-independent cell growth that correlates with the expression levels of SOCS-1. Interestingly, endogenous SOCS-1 production after Toll-like receptor activation also causes STAT3 hyperactivation. Thus we hypothesize that P. multocida Toxin alters host cell signalling using mechanisms that have so far only been known to be employed by oncogenic viral kinases to avoid host immune defence mechanisms.

Anti-inflammatory effect of insulin in the human hepatoma cell line HepG2 involves decreased transcription of IL-6 target genes and nuclear exclusion of FOXO1

The liver is an important target for interleukin-6 (IL-6) action leading to an increased inflammatory response with impaired insulin signaling and action. The aims of this study are to address if insulin is anti-inflammatory and attenuates IL-6-induced inflammation in the human hepatoma cell line HepG2 and if this involves signal transducer and activator of transcription 3 (STAT3) signal transduction. It was found that insulin significantly reduced IL-6-induced gene transcription of serum amyloid 1 (SAA1), serum amyloid 2 (SAA2), haptoglobin, orosomucoid, and plasmin activator inhibitor-1 (PAI-1). However, the authors did not find any evidence that insulin inhibited IL-6 signal transduction, i.e., no effect of insulin was detected on STAT3 phosphorylation or its translocation to cell nucleus. The potential role of PKCδ was also analyzed but no evidence of its involvement was found. Taken together, these results suggest that the anti-inflammatory effect of insulin on IL-6 action is exerted at the level of the transcriptional activation of the genes. Further analysis revealed that insulin regulates nuclear localization of FOXO1, which is an important co-activator for STAT3 mediated transcription. Insulin induced nuclear exit and Thr24 phosphorylation of FOXO1, thus, inhibiting STAT3-mediated transcription.

Genotoxic stress causes the accumulation of the splicing regulator Sam68 in nuclear foci of transcriptionally active chromatin

DNA-damaging agents cause a multifaceted cellular stress response. Cells set in motion either repair mechanisms or programmed cell death pathways, depending on the extent of the damage and on their ability to withstand it. The RNA-binding protein (RBP) Sam68, which is up-regulated in prostate carcinoma, promotes prostate cancer cell survival to genotoxic stress. Herein, we have investigated the function of Sam68 in this cellular response. Mitoxantrone (MTX), a topoisomerase II inhibitor, induced relocalization of Sam68 from the nucleoplasm to nuclear granules, together with several other RBPs involved in alternative splicing, such as TIA-1, hnRNP A1 and the SR proteins SC35 and ASF/SF2. Sam68 accumulation in nuclear stress granules was independent of signal transduction pathways activated by DNA damage. Using BrU labelling and immunofluorescence, we demonstrate that MTX-induced nuclear stress granules are transcriptionally active foci where Sam68 and the phosphorylated form of RNA polymerase II accumulate. Finally, we show that MTX-induced relocalization of Sam68 correlates with changes in alternative splicing of its mRNA target CD44, and that MTX-induced CD44 splicing depends on Sam68 expression. These results strongly suggest that Sam68 is part of a RNA-mediated stress response of the cell that modulates alternative splicing in response to DNA damage.

The JAK2 inhibitor AZD1480 potently blocks Stat3 signaling and oncogenesis in solid tumors

Persistent activation of Stat3 is oncogenic and is prevalent in a wide variety of human cancers. Chronic cytokine stimulation is associated with Stat3 activation in some tumors, implicating cytokine receptor-associated Jak family kinases. Using Jak2 inhibitors, we demonstrate a central role of Jaks in modulating basal and cytokine-induced Stat3 activation in human solid tumor cell lines. Inhibition of Jak2 activity is associated with abrogation of Stat3 nuclear translocation and tumorigenesis. The Jak2 inhibitor AZD1480 suppresses the growth of human solid tumor xenografts harboring persistent Stat3 activity. We demonstrate the essential role of Stat3 downstream of Jaks by inhibition of tumor growth using short hairpin RNA targeting Stat3. Our data support a key role of Jak kinase activity in Stat3-dependent tumorigenesis.

Fibroblast growth factor inhibits interferon gamma-STAT1 and interleukin 6-STAT3 signaling in chondrocytes

Activation of fibroblast growth factor receptor 3 (FGFR3) leads to attenuation of cartilage growth. The members of the STAT family of transcription factors are believed to participate in FGFR3 signaling in cartilage, however the molecular mechanism of this action is poorly understood. Here, we demonstrate that a chronic FGF stimulus leads to accumulation of STAT1, 3, 5 and 6, evident in both in vitro chondrocyte model and murine limb explant cultures. Despite the accumulation, both endogenous and cytokine-induced activation of STAT1 and STAT3 is impaired by FGF, as demonstrated by imaging of active STAT nuclear translocation and analyses of STAT activatory phosphorylation and transcriptional activation. Further, we demonstrate that FGF induces expression of CIS, SOCS1 and SOCS3 inhibitors of gp130, a common receptor for the IL6-family of cytokines. Since cytokine-gp130 signaling represents an important positive regulator of cartilage, its inhibition may contribute to the growth-inhibitory effect of FGFR3 in cartilage.

Systems biology of JAK/STAT signalling

Signalling in multicellular organisms is mediated by complex networks that integrate extracellular and intracellular signals to generate appropriate responses regulating cell proliferation, differentiation and survival. Downstream of many cytokine and growth hormone receptors, receptor-associated JAKs (Janus kinases) activate transcription factors of the STAT (signal transducer and activator of transcription) protein family and thereby mediate signal transduction from the plasma membrane to the nucleus. The JAK/STAT pathway has been shown to be constitutively activated in a wide array of human malignancies. To elucidate mechanisms contributing to tumour formation and identify system properties of the JAK/STAT signalling pathway, a systems biology approach can be employed. So far the majority of studies available have focused on down-regulation of the signalling pathway based on simulations. However, a data-based model of the core module of the JAK2/STAT5 signalling pathway showed that rapid nucleocytoplasmic cycling of STAT5 is an essential pathway property. In the future, combining assays for quantitative analysis at different levels will be important to gain deeper insight into molecular mechanisms regulating intracellular communication mediated by such complex dynamic systems as signalling pathways and their targets.

An oxazole-based small-molecule Stat3 inhibitor modulates Stat3 stability and processing and induces antitumor cell effects

Stat3 is hyperactivated in many human tumors and represents a valid target for anticancer drug design. We present a novel small-molecule Stat3 inhibitor, S3I-M2001, and describe the dynamics of intracellular processing of activated Stat3 within the context of the biochemical and biological effects of the Stat3 inhibitor. S3I-M2001 is an oxazole-based peptidomimetic of the Stat3 Src homology (SH) 2 domain-binding phosphotyrosine peptide that selectively disrupts active Stat3:Stat3 dimers. Consequently, hyperactivated Stat3, which hitherto occurs as "dotlike" structures of nuclear bodies, undergoes an early aggregation into nonfunctional perinuclear aggresomes and a late-phase proteasome-mediated degradation in malignant cells treated with S3I-M2001. Thus, S3I-M2001 inhibited Stat3-dependent transcriptional regulation of tumor survival genes, such as Bcl-xL. Furthermore, Stat3-dependent malignant transformation, survival, and migration and invasion of mouse and human cancer cells harboring persistently activated Stat3 were inhibited by S3I-M2001. Finally, S3I-M2001 inhibited growth of human breast tumor xenografts. The study identifies a novel Stat3 inhibitor, S3I-M2001, with antitumor cell effects mediated in part through a biphasic loss of functional Stat3. The study represents the first on intracellular Stat3 stability and processing following inhibition by a small molecule that has significant antitumor activity.

Physical and functional interactions between STAT3 and KAP1

Signal transducers and activators of transcription (STATs) mediate cell proliferation, differentiation and survival in immune responses, hematopoiesis, neurogenesis and other biological processes. For example, STAT3 has been reported to be constitutively activated in numerous cancer cells. To clarify the molecular mechanisms underlying the STAT activation, we performed yeast two-hybrid screening and identified KAP1/TIF1beta as a novel STAT-binding partner. KAP1 is a universal corepressor protein for the Kruppel-associated box zinc-finger protein superfamily of transcriptional repressors. We found endogenous KAP1 associated with endogenous STAT3 in vivo. Importantly, small-interfering RNA-mediated reduction of KAP1 expression enhanced interleukin (IL)-6-induced STAT3-dependent transcription and gene expression. Furthermore, reduction of KAP1 expression resulted in the marked accumulation of STAT3 phosphorylated on Ser727 in the nucleus, a modification that regulates its transcriptional activation. These results indicate that KAP1 may serve as a transcriptional regulator of the IL-6/STAT3 signaling pathway.

Nucleocytoplasmic shuttling of persistently activated STAT3

Persistent activation of the transcription factor STAT3 has been detected in many types of cancer and plays an important role in tumor progression, immune evasion and metastasis. To analyze persistent STAT3 activation we coexpressed STAT3 with v-Src. We found that tyrosine phosphorylation of STAT3 by v-Src is independent of Janus kinases (Jaks), the canonical activators of STATs. The STAT3-induced feedback inhibitor, suppressor of cytokine signaling 3 (SOCS3), did not interfere with STAT3 activation by v-Src. However, the protein inhibitor of activated STAT3 (PIAS3) suppressed gene induction by persistently activated STAT3. We measured nucleocytoplasmic shuttling of STAT3 in single cells by bleaching the YFP moiety of double-labelled STAT3-CFP-YFP in the cytoplasm. Analysis of the subcellular distribution of CFP and YFP fluorescence over time by mathematical modeling and computational parameter estimation revealed that activated STAT3 shuttles more rapidly than non-activated STAT3. Inhibition of exportin-1-mediated nuclear export slowed down nucleocytoplasmic shuttling of v-Src-activated STAT3 resulting in reduced tyrosine phosphorylation, decreased induction of STAT3 target genes and increased apoptosis. We propose passage of persistently activated STAT3 through the nuclear pore complex as a new target for intervention in cancer.

Stat3 isoforms, alpha and beta, demonstrate distinct intracellular dynamics with prolonged nuclear retention of Stat3beta mapping to its unique C-terminal end

Two isoforms of Stat3 (signal transducer and activator of transcription 3) are expressed in cells, alpha (p92) and beta (p83), both derived from a single gene by alternative mRNA splicing. The 55-residue C-terminal transactivation domain of Stat3alpha is deleted in Stat3beta and replaced by seven unique C-terminal residues (CT7) whose function remains uncertain. We subcloned the open reading frames of Stat3alpha and Stat3beta into the C terminus of green fluorescent protein (GFP). Fluorescent microscopic analysis of HEK293T cells transiently transfected with GFP-Stat3alpha or GFP-Stat3beta revealed similar kinetics and cytokine concentration dependence of nuclear accumulation; these findings were confirmed by high throughput microscope analysis of murine embryonic fibroblasts that lacked endogenous Stat3 but stably expressed either GFP-Stat3alpha or GFP-Stat3beta. However, although time to half-maximal cytoplasmic reaccumulation after cytokine withdrawal was 15 min for GFP-Stat3alpha, it was >180 min for GFP-Stat3beta. Furthermore, although the intranuclear mobility of GFP-Stat3alpha was rapid and increased with cytokine stimulation, the intranuclear mobility of GFP-Stat3beta in unstimulated cells was slower than that of GFP-Stat3alpha in unstimulated cells and was slowed further following cytokine stimulation. Deletion of the unique CT7 domain from Stat3beta eliminated prolonged nuclear retention but did not alter its intranuclear mobility. Thus, Stat3alpha and Stat3beta have distinct intracellular dynamics, with Stat3beta exhibiting prolonged nuclear retention and reduced intranuclear mobility especially following ligand stimulation. Prolonged nuclear retention, but not reduced intranuclear mobility, mapped to the CT7 domain of Stat3beta.

Preformed STAT3 transducer complexes in human HepG2 cells and rat hepatocytes

Interleukin 6 (IL-6) is a pleiotropic cytokine that mediates a variety of functions, including induction of the acute-phase response in hepatocytes. IL-6 initiates its action by binding to its cell surface receptor, followed by activation of Janus kinases and tyrosine phosphorylation of the signal transducer and transcription factor (STAT) 3. Although it has been suggested that cholesterol- and sphingolipid-enriched membrane domains, called lipid rafts, and caveolin are involved in this process, their roles in the earliest stages of IL-6-mediated signaling are far from being understood. Here we show that pretreatment of HepG2 hepatoma cells with methyl-beta-cyclodextrin (MbetaCD), which removes cholesterol and destroys lipid rafts, inhibited tyrosine phosphorylation of STAT3 in IL-6-activated, but not PV-activated cells. Furthermore, when the cells were lysed under conditions preserving lipid rafts, no IL-6- or PV-induced phosphorylation of STAT3 was observed. Although most of the STAT3 was found in large MbetaCD-resistant assemblies in both non-activated and IL-6-activated cells, its association with lipid rafts was weak or undetectable. The extent of IL-6-induced tyrosine phosphorylation of STAT3 was comparable in cells expressing low or high levels of caveolin. Similar STAT3 transducer complexes were observed in freshly isolated rat hepatocytes. The combined data suggest that STAT3 tyrosine phosphorylation occurs in preformed transducer complexes that can be activated in the absence of intact lipid rafts or caveolin.

Crosstalk between signaling pathways of adrenoreceptors and signal transducers and activators of transcription 3 (STAT3) in heart

Recently, there have been important advancements in our understanding of the signaling mechanisms of adrenoreceptors (AR) and signal transducers and activators of transcription 3 (STAT3). While their crucial roles in the pathological processes of the heart are well established, accumulating evidence suggests there is a complex pattern of crosstalk between these 2 signaling pathways. Moreover, the potential for crosstalk occurs at multiple levels in each signaling cascade and involves receptor transactivation, G proteins, small GTPases, cyclic adenosine 3',5'-monophosphate/protein kinase A, protein kinase C, scaffold/adaptor proteins, protein tyrosine kinases, and mitogen-activated protein kinases. In addition, post-translational modification (eg acetylation) of STAT3 may provide a link between STAT3 and AR signaling. In particular, crosstalk between these 2 systems in the heart would appear to be dependent upon the species/tissue studied, developmental stage, and eliciting stimulus. This at least partly accounts for the epigenetic effects on biological function that is mediated by the 2 signaling pathways. Elucidation of these mechanisms will provide new targets in the development of novel clinical strategies for heart disorders.

Suppression of STAT3 activity by Duplin, which is a negative regulator of the Wnt signal

Duplin was originally isolated as a negative regulator of beta-catenin-dependent T-cell factor (Tcf) transcriptional activity in the Wnt signaling pathway. However, Duplin knockout mice exhibit embryonic lethality at 5.5-em day, suggesting that Duplin has important roles other than as a negative regulator of the Wnt signal. To identify new roles of Duplin, the Duplin-binding proteins were screened. PIAS3, which is a SUMO E3 ligase and acts as an inhibitor of signal transducer and activator of transcription (STAT3), was identified as a Duplin-binding protein. Duplin was sumoylated, but PIAS3 affected neither the sumoylation of Duplin nor its ability to inhibit Tcf-4 activity. Like PIAS3, Duplin suppressed the leukemia-inhibitory factor (LIF)-induced STAT3 transcriptional activity. Duplin did not affect the LIF-dependent tyrosine phosphorylation or nuclear localization of STAT3 but inhibited the formation of complex between STAT3 and DNA. Although STAT3 is not modified with SUMO, PIAS3 inhibited the STAT3 activity in a manner partially depending on its SUMO E3 ligase activity. Duplin suppressed the LIF-dependent STAT3 activity independently of sumoylation. These results demonstrate that Duplin inhibits not only Tcf-4 but also STAT3, suggesting that Duplin may act as a repressor for multiple transcriptional factors.

Conditioning injury-induced spinal axon regeneration requires signal transducer and activator of transcription 3 activation

Sensory axons in the adult spinal cord do not regenerate after injury. This is essentially because of inhibitory components in the damaged CNS, such as myelin-associated inhibitors and the glial scar. However, if the sciatic nerve is axotomized before injury of the dorsal column, injured axons can regenerate a short distance in the spinal cord. Here, we show that sciatic nerve transection results in time-dependent phosphorylation and activation of the transcription factor, signal transducer and activator of transcription 3 (STAT3), in dorsal root ganglion (DRG) neurons. This effect is specific to peripheral injuries and does not occur when the dorsal column is crushed. Sustained perineural infusion of the Janus kinase 2 (JAK2) inhibitor AG490 to the proximal nerve stump can block STAT3 phosphorylation after sciatic nerve transection and results in reduced growth-associated protein 43 upregulation and compromised neurite outgrowth in vitro. Importantly, in vivo perineural infusion of AG490 also significantly attenuates dorsal column axonal regeneration in the adult spinal cord after a preconditioning sciatic nerve transection. We conclude that STAT3 activation is necessary for increased growth ability of DRG neurons and improved axonal regeneration in the spinal cord after a conditioning injury.

Molecular dynamics of STAT3 on IL-6 signaling pathway in living cells

Signal transducer and activator of transcription 3 (STAT3) is a critical signal transducer of interleukin-6 (IL-6) signaling. To investigate the mobility and the dynamics of STAT3 complex on IL-6 signaling in living cells, we generated a chimeric gene consisting of STAT3 fused to enhanced green fluorescence protein, STAT3-GFP. STAT3-GFP was expressed in Hep3B cells and the dynamics of this protein were analyzed by fluorescence correlation spectroscopy. After IL-6 stimulation, STAT3 translocated from the cytoplasm to the nucleus, as shown previously. According to the analysis of STAT3 diffusion in stable transformants, the number of STAT3 molecules at the cytoplasmic membrane and in the cytoplasm decreased after IL-6 stimulation. In the nucleus, the diffusion speed of STAT3 complex strongly decreased after IL-6 stimulation. Furthermore, we found that STAT3 existed as a complex whose molecular weight was less than 400kDa before IL-6 addition. However, IL-6 stimulation induced the formation of STAT3 dimer as a megacomplex form whose molecular weight was more than 1MDa at the cytoplasm and a very slow diffusion complex in the nucleus.

Accelerated and blastic phases of chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) may have a biphasic or triphasic course, whereby patients who were initially diagnosed in the chronic phase (CP) develop more aggressive disease, frequently pass through an intermediate or accelerated phase (AP), and finally evolve into an acute leukemia like blastic phase (BP). A slowing in the rate of development of AP or BP has accompanied successive improvements in therapy for patients who have CP CML. Variable diagnostic criteria for AP and BP are used in the literature, rendering comparisons difficult. The management of patients in AP or BP consistently has been less effective than the management of those inCP for all modalities of therapy. This article reviews the current diagnostic criteria, therapeutic strategies, outcomes, and investigational therapies for AP and BP CML.

Real Time Analysis of STAT3 Nucleocytoplasmic Shuttling

The transcription factor STAT3 is most important for the signal transduction of interleukin-6 and related cytokines. Upon stimulation cytoplasmic STAT3 is phosphorylated at tyrosine 705, translocates into the nucleus, and induces target genes. Notably, STAT proteins are also detectable in the nuclei of unstimulated cells. In this report we introduce a new method for the real time analysis of STAT3 nucleocytoplasmic shuttling in living cells which is based on the recently established fluorescence localization after photobleaching (FLAP) approach. STAT3 was C-terminally fused with the cyan (CFP) and yellow (YFP) variants of the green fluorescent protein. In the resulting STAT3-CFP-YFP (STAT3-CY) fusion protein the YFP can be selectively bleached using the 514-nm laser of a confocal microscope. This setting allows studies on the dynamics of STAT3 nucleocytoplasmic transport by monitoring the subcellular distribution of fluorescently labeled and selectively bleached STAT3-CY. By this means we demonstrate that STAT3-CY shuttles continuously between the cytosol and the nucleus in unstimulated cells. This constitutive shuttling does not depend on the phosphorylation of tyrosine 705 because a STAT3(Y705F)-CY mutant shuttles to the same extent as STAT3-CY. Experiments with deletion mutants reveal that the N-terminal moiety of STAT3 is essential for shuttling. Further studies suggest that a decrease in STAT3 nuclear export contributes to the nuclear accumulation of STAT3 in response to cytokine stimulation. The new approach presented in this study is generally applicable to any protein of interest for analyzing nucleocytoplasmic transport mechanisms in real time.