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)

Transcriptional control of interferon-stimulated genes

Interferon-induced genes are among the best-studied groups of coregulated genes. Nevertheless, intense research into their regulation, supported by new technologies, is continuing to provide insights into their many layers of transcriptional regulation and to reveal how cellular transcriptomes change with pathogen-induced innate and adaptive immunity. This article gives an overview of recent findings on interferon-induced gene regulation, paying attention to contributions beyond the canonical JAK-STAT pathways.

RanBP2/Nup358 Mediates Sumoylation of STAT1 and Antagonizes Interferon-α-Mediated Antiviral Innate Immunity

Type I interferon (IFN-I)-induced signaling plays a critical role in host antiviral innate immune responses. Despite this, the mechanisms that regulate this signaling pathway have yet to be fully elucidated. The nucleoporin Ran Binding Protein 2 (RanBP2) (also known as Nucleoporin 358 KDa, Nup358) has been implicated in a number of cellular processes, including host innate immune signaling pathways, and is known to influence viral infection. In this study, we documented that RanBP2 mediates the sumoylation of signal transducers and activators of transcription 1 (STAT1) and inhibits IFN-α-induced signaling. Specifically, we found that RanBP2-mediated sumoylation inhibits the interaction of STAT1 and Janus kinase 1 (JAK1), as well as the phosphorylation and nuclear accumulation of STAT1 after IFN-α stimulation, thereby antagonizing the IFN-α-mediated antiviral innate immune signaling pathway and promoting viral infection. Our findings not only provide insights into a novel function of RanBP2 in antiviral innate immunity but may also contribute to the development of new antiviral therapeutic strategies.

N-MYC-interacting protein enhances type II interferon signaling by inhibiting STAT1 sumoylation

Signaling desensitization is key to limiting signal transduction duration and intensity. Signal transducer and activator of transcription 1 (STAT1) can mediate type II interferon (IFNγ)-induced immune responses, which are enhanced and inhibited by STAT1 phosphorylation and sumoylation, respectively. Here, we identified an N-MYC interacting protein, NMI, which can enhance STAT1 phosphorylation and STAT1-mediated IFNγ immune responses by binding and sequestering the E2 SUMO conjugation enzyme, UBC9, and blocking STAT1 sumoylation. NMI facilitates UBC9 nucleus-to-cytoplasm translocation in response to IFNγ, thereby inhibiting STAT1 sumoylation. STAT1 phosphorylation at Y701 and sumoylation at K703 are mutually exclusive modifications that regulate IFNγ-dependent transcriptional responses. NMI could not alter the phosphorylation level of sumoylation-deficient STAT1 after IFNγ treatment. Thus, IFNγ signaling is modulated by NMI through sequestration of UBC9 in the cytoplasm, leading to inhibition of STAT1 sumoylation. Hence, NMI functions as a switch for STAT1 activation/inactivation cycles by modulating an IFNγ-induced desensitization mechanism.

An inhibitory effect on the nuclear accumulation of phospho-STAT1 by its unphosphorylated form

BACKGROUND

Unphosphorylated signal transducer and activator of transcription 1 (U-STAT1) has been reported to elicit a distinct gene expression profile as compared to tyrosine-phosphorylated STAT1 (P-STAT1) homodimers. However, the impact of U-STAT1 on the IFNγ-induced immune response mediated by P-STAT1 is unknown. By generating a double mutant of STAT1 with mutation R602L in the Src-homology 2 (SH2) domain and Y701F in the carboxy-terminal transactivation domain mimicking U-STAT1, we investigated the effects of U-STAT1 on P-STAT1-mediated signal transduction.

RESULTS

In this study, we discovered a novel activity of U-STAT1 that alters the nucleo-cytoplasmic distribution of cytokine-stimulated P-STAT1. While the dimerization-deficient mutant R602L/Y701F was not able to display cytokine-induced nuclear accumulation, it inhibited the nuclear accumulation of co-expressed IFNγ-stimulated wild-type P-STAT1. Disruption of the anti-parallel dimer interface in the R602L/Y701F mutant via additional R274W and T385A mutations did not rescue the impaired nuclear accumulation of co-expressed P-STAT1. The mutant U-STAT1 affected neither the binding of co-expressed P-STAT1 to gamma-activated sites in vitro, nor the transcription of reporter constructs and the activation of STAT1 target genes. However, the nuclear accumulation of P-STAT1 was diminished in the presence of mutant U-STAT1, which was not restored by mutations reducing the DNA affinity of mutant U-STAT1. Whereas single mutations in the amino-terminus of dimerization-deficient U-STAT1 similarly inhibited the nuclear accumulation of co-expressed P-STAT1, a complete deletion of the amino-terminus restored cytokine-stimulated nuclear accumulation of P-STAT1. Likewise, the disruption of a dimer-specific nuclear localization signal also rescued the U-STAT1-mediated inhibition of P-STAT1 nuclear accumulation.

CONCLUSION

Our data demonstrate a novel role of U-STAT1 in affecting nuclear accumulation of P-STAT1, such that a high intracellular concentration of U-STAT1 inhibits the detection of nuclear P-STAT1 in immunofluorescence assays. These observations hint at a possible physiological function of U-STAT1 in buffering the nuclear import of P-STAT1, while preserving IFNγ-induced gene expression. Based on these results, we propose a model of a hypothetical import structure, the assembly of which is impaired under high concentrations of U-STAT1. This mechanism maintains high levels of cytoplasmic STAT1, while simultaneously retaining signal transduction by IFNγ. Video Abstract.

SUMO and Transcriptional Regulation: The Lessons of Large-Scale Proteomic, Modifomic and Genomic Studies

One major role of the eukaryotic peptidic post-translational modifier SUMO in the cell is transcriptional control. This occurs via modification of virtually all classes of transcriptional actors, which include transcription factors, transcriptional coregulators, diverse chromatin components, as well as Pol I-, Pol II- and Pol III transcriptional machineries and their regulators. For many years, the role of SUMOylation has essentially been studied on individual proteins, or small groups of proteins, principally dealing with Pol II-mediated transcription. This provided only a fragmentary view of how SUMOylation controls transcription. The recent advent of large-scale proteomic, modifomic and genomic studies has however considerably refined our perception of the part played by SUMO in gene expression control. We review here these developments and the new concepts they are at the origin of, together with the limitations of our knowledge. How they illuminate the SUMO-dependent transcriptional mechanisms that have been characterized thus far and how they impact our view of SUMO-dependent chromatin organization are also considered.

Bifurcation and sensitivity analysis reveal key drivers of multistability in a model of macrophage polarization

In this paper, we present and analyze a mathematical model for polarization of a single macrophage which, despite its simplicity, exhibits complex dynamics in terms of multistability. In particular, we demonstrate that an asymmetry in the regulatory mechanisms and parameter values is important for observing multiple phenotypes. Bifurcation and sensitivity analyses show that external signaling cues are necessary for macrophage commitment and emergence to a phenotype, but that the intrinsic macrophage pathways are equally important. Based on our numerical results, we formulate hypotheses that could be further investigated by laboratory experiments to deepen our understanding of macrophage polarization.

Interferon Response in Hepatitis C Virus-Infected Hepatocytes: Issues to Consider in the Era of Direct-Acting Antivirals

When interferons (IFNs) bind to their receptors, they upregulate numerous IFN-stimulated genes (ISGs) with antiviral and immune regulatory activities. Hepatitis C virus (HCV) is a single-stranded, positive-sense RNA virus that affects over 71 million people in the global population. Hepatocytes infected with HCV produce types I and III IFNs. These endogenous IFNs upregulate a set of ISGs that negatively impact the outcome of pegylated IFN-α and ribavirin treatments, which were previously used to treat HCV. In addition, the IFNL4 genotype was the primary polymorphism responsible for a suboptimal treatment response to pegylated IFN-α and ribavirin. However, recently developed direct-acting antivirals have demonstrated a high rate of sustained virological response without pegylated IFN-α. Herein, we review recent studies on types I and III IFN responses to in HCV-infected hepatocytes. In particular, we focused on open issues related to IFN responses in the direct-acting antiviral era.

Novel signal transducer and activator of transcription 1 mutation disrupts small ubiquitin-related modifier conjugation causing gain of function

BACKGROUND

Sumoylation is a posttranslational reversible modification of cellular proteins through the conjugation of small ubiquitin-related modifier (SUMO) and comprises an important regulator of protein function.

OBJECTIVE

We sought to characterize the molecular mechanism of a novel mutation at the SUMO motif on signal transducer and activator of transcription 1 (STAT1).

METHODS

STAT1 sequencing and functional characterization were performed in transfection experiments by using immunoblotting and immunoprecipitation in STAT1-deficient cell lines. Transcriptional response and target gene activation were also investigated in PBMCs.

RESULTS

We identified a novel STAT1 mutation (c.2114A>T, p.E705V) within the SUMO motif (702IKTE705) in a patient with disseminated Rhodococcus species infection, Norwegian scabies, chronic mucocutaneous candidiasis, hypothyroidism, and esophageal squamous cell carcinoma. The mutation is located in the tail segment and is predicted to disrupt STAT1 sumoylation. Immunoprecipitation experiments performed in transfected cells confirmed absent STAT1 sumoylation for E705V, whereas it was present in wild-type (WT) STAT1 cells, as well as the loss-of-function mutants L706S and Y701C. Furthermore, stimulation with IFN-γ led to enhanced STAT1 phosphorylation, enhanced transcriptional activity, and target gene expression in the E705V-transfected compared with WT-transfected cells. Computer modeling of WT and mutant STAT1 molecules showed variations in the accessibility of the phosphorylation site Y701, which corresponded to the loss-of-function and gain-of-function variants.

CONCLUSION

This is the first report of a mutation in the STAT1 sumoylation motif associated with clinical disease. These data reinforce sumoylation as a key posttranslational regulatory modification of STAT1 and identify a novel mechanism for gain-of-function STAT1 disease in human subjects.

Interferon Independent Non-Canonical STAT Activation and Virus Induced Inflammation

Interferons (IFNs) are a group of secreted proteins that play critical roles in antiviral immunity, antitumor activity, activation of cytotoxic T cells, and modulation of host immune responses. IFNs are cytokines, and bind receptors on cell surfaces to trigger signal transduction. The major signaling pathway activated by IFNs is the JAK/STAT (Janus kinase/signal transducer and activator of transcription) pathway, a complex pathway involved in both viral and host survival strategies. On the one hand, viruses have evolved strategies to escape from antiviral host defenses evoked by IFN-activated JAK/STAT signaling. On the other hand, viruses have also evolved to exploit the JAK/STAT pathway to evoke activation of certain STATs that somehow promote viral pathogenesis. In this review, recent progress in our understanding of the virus-induced IFN-independent STAT signaling and its potential roles in viral induced inflammation and pathogenesis are summarized in detail, and perspectives are provided.

Responses to Cytokines and Interferons that Depend upon JAKs and STATs

Many cytokines and all interferons activate members of a small family of kinases (the Janus kinases [JAKs]) and a slightly larger family of transcription factors (the signal transducers and activators of transcription [STATs]), which are essential components of pathways that induce the expression of specific sets of genes in susceptible cells. JAK-STAT pathways are required for many innate and acquired immune responses, and the activities of these pathways must be finely regulated to avoid major immune dysfunctions. Regulation is achieved through mechanisms that include the activation or induction of potent negative regulatory proteins, posttranslational modification of the STATs, and other modulatory effects that are cell-type specific. Mutations of JAKs and STATs can result in gains or losses of function and can predispose affected individuals to autoimmune disease, susceptibility to a variety of infections, or cancer. Here we review recent developments in the biochemistry, genetics, and biology of JAKs and STATs.

Interplay between Janus Kinase/Signal Transducer and Activator of Transcription Signaling Activated by Type I Interferons and Viral Antagonism

Interferons (IFNs), which were discovered a half century ago, are a group of secreted proteins that play key roles in innate immunity against viral infection. The major signaling pathway activated by IFNs is the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, which leads to the expression of IFN-stimulated genes (ISGs), including many antiviral effectors. Viruses have evolved various strategies with which to antagonize the JAK/STAT pathway to influence viral virulence and pathogenesis. In recent years, notable progress has been made to better understand the JAK/STAT pathway activated by IFNs and antagonized by viruses. In this review, recent progress in research of the JAK/STAT pathway activated by type I IFNs, non-canonical STAT activation, viral antagonism of the JAK/STAT pathway, removing of the JAK/STAT antagonist from viral genome for attenuation, and the potential pathogenesis roles of tyrosine phosphorylation-independent non-canonical STATs activation during virus infection are discussed in detail. We expect that this review will provide new insight into the understanding the complexity of the interplay between JAK/STAT signaling and viral antagonism.

Regulation of transcription factors by sumoylation

Transcription factors (TFs) are among the most frequently detected targets of sumoylation, and effects of the modification have been studied for about 200 individual TFs to date. TF sumoylation is most often associated with reduced target gene expression, which can be mediated by enhanced interactions with corepressors or by interference with protein modifications that promote transcription. However, recent studies show that sumoylation also regulates gene expression by controlling the levels of TFs that are associated with chromatin. SUMO can mediate this by modulating TF DNA-binding activity, promoting clearance of TFs from chromatin, or indirectly, by influencing TF abundance or localization.

Canonical and Non-Canonical Aspects of JAK-STAT Signaling: Lessons from Interferons for Cytokine Responses

Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signal transduction mediates cytokine responses. Canonical signaling is based on STAT tyrosine phosphorylation by activated JAKs. Downstream of interferon (IFN) receptors, activated JAKs cause the formation of the transcription factors IFN-stimulated gene factor 3 (ISGF3), a heterotrimer of STAT1, STAT2 and interferon regulatory factor 9 (IRF9) subunits, and gamma interferon-activated factor (GAF), a STAT1 homodimer. In recent years, several deviations from this paradigm were reported. These include kinase-independent JAK functions as well as extra- and intranuclear activities of U-STATs without phosphotyrosines. Additionally, transcriptional control by STAT complexes resembling neither GAF nor ISGF3 contributes to transcriptome changes in IFN-treated cells. Our review summarizes the contribution of non-canonical JAK-STAT signaling to the innate antimicrobial immunity imparted by IFN. Moreover, we touch upon functions of IFN pathway proteins beyond the IFN response. These include metabolic functions of IRF9 as well as the regulation of natural killer cell activity by kinase-dead TYK2 and different phosphorylation isoforms of STAT1.

Receptor dimer stabilization by hierarchical plasma membrane microcompartments regulates cytokine signaling

The interaction dynamics of signaling complexes is emerging as a key determinant that regulates the specificity of cellular responses. We present a combined experimental and computational study that quantifies the consequences of plasma membrane microcompartmentalization for the dynamics of type I interferon receptor complexes. By using long-term dual-color quantum dot (QD) tracking, we found that the lifetime of individual ligand-induced receptor heterodimers depends on the integrity of the membrane skeleton (MSK), which also proved important for efficient downstream signaling. By pair correlation tracking and localization microscopy as well as by fast QD tracking, we identified a secondary confinement within ~300-nm-sized zones. A quantitative spatial stochastic diffusion-reaction model, entirely parameterized on the basis of experimental data, predicts that transient receptor confinement by the MSK meshwork allows for rapid reassociation of dissociated receptor dimers. Moreover, the experimentally observed apparent stabilization of receptor dimers in the plasma membrane was reproduced by simulations of a refined, hierarchical compartment model. Our simulations further revealed that the two-dimensional association rate constant is a key parameter for controlling the extent of MSK-mediated stabilization of protein complexes, thus ensuring the specificity of this effect. Together, experimental evidence and simulations support the hypothesis that passive receptor confinement by MSK-based microcompartmentalization promotes maintenance of signaling complexes in the plasma membrane.

S100A6 Regulates Endothelial Cell Cycle Progression by Attenuating Antiproliferative Signal Transducers and Activators of Transcription 1 Signaling

OBJECTIVE

S100A6, a member of the S100 protein family, has been described as relevant for cell cycle entry and progression in endothelial cells. The molecular mechanism conferring S100A6's proliferative actions, however, remained elusive.

APPROACH AND RESULTS

Originating from the clinically relevant observation of enhanced S100A6 protein expression in proliferating endothelial cells in remodeling coronary and carotid arteries, our study unveiled S100A6 as a suppressor of antiproliferative signal transducers and activators of transcription 1 signaling. Discovery of the molecular liaison was enabled by combining gene expression time series analysis with bioinformatic pathway modeling in S100A6-silenced human endothelial cells stimulated with vascular endothelial growth factor A. This unbiased approach led to successful identification and experimental validation of interferon-inducible transmembrane protein 1 and protein inhibitors of activated signal transducers and activators of transcription as key components of the link between S100A6 and signal transducers and activators of transcription 1.

CONCLUSIONS

Given the important role of coordinated endothelial cell cycle activity for integrity and reconstitution of the inner lining of arterial blood vessels in health and disease, signal transducers and activators of transcription 1 suppression by S100A6 may represent a promising therapeutic target to facilitate reendothelialization in damaged vessels.

SUMOylation and SENP3 regulate STAT3 activation in head and neck cancer

Hyperphosphorylation of signal transducer and activator of transcription 3 (STAT3) has been found in various types of human cancers, including head and neck cancer (HNC). Although smoking is critical in the development and progression of HNC, how tobacco components activate STAT3 is unclear. We demonstrated that exposure of HNC cell lines to a tobacco extract induced a rapid Y705 phosphorylation of STAT3 and a rapid increase in the SUMO protease SENP3 that depended on a simultaneous increase in reactive oxygen species. We identified that SUMOylation at the lysine 451 site facilitated STAT3 binding to the phosphatase TC45 through an SUMO-interacting motif of TC45. SENP3 could thus enhance STAT3 phosphorylation by de-conjugating the SUMO2/3 modification of STAT3. Knocking-down of SENP3 greatly impaired basal and induced STAT3 phosphorylation by tobacco extract or interleukin 6. A correlation between SENP3 protein levels and STAT3 Y705 phosphorylation levels in human laryngeal carcinoma specimens was found, which was more significant in the specimens derived from the smoker patients and with poor clinicopathological parameters. Our data identified SUMOylation as a previously undescribed post-translational modification of STAT3 and SENP3 as a critical positive modulator of tobacco- or cytokine-induced STAT3 activation. These findings provide novel insights into the hyperphosphorylation of STAT3 in development of HNC.

STAT3, a Key Parameter of Cytokine-Driven Tissue Protection during Sterile Inflammation - the Case of Experimental Acetaminophen (Paracetamol)-Induced Liver Damage

Acetaminophen (APAP, N-acetyl-p-aminophenol, or paracetamol) overdosing is a prevalent cause of acute liver injury. While clinical disease is initiated by overt parenchymal hepatocyte necrosis in response to the analgetic, course of intoxication is substantially influenced by associated activation of innate immunity. This process is supposed to be set in motion by release of danger-associated molecular patterns (DAMPs) from dying hepatocytes and is accompanied by an inflammatory cytokine response. Murine models of APAP-induced liver injury emphasize the complex role that DAMPs and cytokines play in promoting either hepatic pathogenesis or resolution and recovery from intoxication. Whereas the function of key inflammatory cytokines is controversially discussed, a subclass of specific cytokines capable of efficiently activating the hepatocyte signal transducer and activator of transcription (STAT)-3 pathway stands out as being consistently protective in murine models of APAP intoxication. Those include foremost interleukin (IL)-6, IL-11, IL-13, and IL-22. Above all, activation of STAT3 under the influence of these cytokines has the capability to drive hepatocyte compensatory proliferation, a key principle of the regenerating liver. Herein, the role of these specific cytokines during experimental APAP-induced liver injury is highlighted and discussed in a broader perspective. In hard-to-treat or at-risk patients, standard therapy may fail and APAP intoxication can proceed toward a fatal condition. Focused administration of recombinant STAT3-activating cytokines may evolve as novel therapeutic approach under those ill-fated conditions.

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.

Flaviviral NS4b, chameleon and jack-in-the-box roles in viral replication and pathogenesis, and a molecular target for antiviral intervention

Dengue virus and other flaviviruses such as the yellow fever, West Nile, and Japanese encephalitis viruses are emerging vector-borne human pathogens that affect annually more than 100 million individuals and that may cause debilitating and potentially fatal hemorrhagic and encephalitic diseases. Currently, there are no specific antiviral drugs for the treatment of flavivirus-associated disease. A better understanding of the flavivirus-host interactions during the different events of the flaviviral life cycle may be essential when developing novel antiviral strategies. The flaviviral non-structural protein 4b (NS4b) appears to play an important role in flaviviral replication by facilitating the formation of the viral replication complexes and in counteracting innate immune responses such as the following: (i) type I IFN signaling; (ii) RNA interference; (iii) formation of stress granules; and (iv) the unfolded protein response. Intriguingly, NS4b has recently been shown to constitute an excellent target for the selective inhibition of flavivirus replication. We here review the current knowledge on NS4b.

Resveratrol modulates cytokine-induced Jak/STAT activation more efficiently than 5-aminosalicylic acid: an in vitro approach

Background

Many advances have been recently made focused on the valuable help of dietary polyphenols in chronic inflammatory diseases. On the other hand, current treatment options for intestinal bowel disease patients are unsatisfying and, for this reason, it is estimated that many patients use dietary supplements to achieve extra benefits.

Aim

The aim of this work was to analyze under a mechanistic perspective the anti-inflammatory potential of resveratrol, a natural polyphenolic compound, and to compare it with a pharmaceutical agent, 5-aminosalicylic acid, using the intestinal HT-29 cell line, as a cellular model.

Methodology and Principal Findings

In the present study, HT-29 colon epithelial cells were pre-treated with 25 µM resveratrol and/or 500 µM 5-aminosalicylic acid and then exposed to a combination of cytokines (IL-1α, TNF-α, IFN-γ) for a certain period of time. Our data showed that resveratrol, used in a concentration 20 times lower than 5-aminosalicylic acid, was able to significantly reduce NO and PGE₂ production, iNOS and COX-2 expression and reactive oxidant species formation induced by the cytokine challenge. However, as already verified with 5-aminosalicylic acid, in spite of not exhibiting any effect on IkB-α degradation, resveratrol down-regulated JAK-STAT pathway, decreasing the levels of activated STAT1 in the nucleus. Additionally, resveratrol decreased the cytokine-stimulated activation of SAPK/JNK pathway but did not counteract the cytokine-triggered negative feedback mechanism of STAT1, through p38 MAPK.

Conclusion/Significance

Taken together, our results show that resveratrol may be considered a future nutraceutical approach, promoting remission periods, limiting the inflammatory process and preventing colorectal cancer, which is common in these patients.

Interplay between sumoylation and phosphorylation for protection against α-synuclein inclusions

Parkinson disease is associated with the progressive loss of dopaminergic neurons from the substantia nigra. The pathological hallmark of the disease is the accumulation of intracytoplasmic inclusions known as Lewy bodies that consist mainly of post-translationally modified forms of α-synuclein. Whereas phosphorylation is one of the major modifications of α-synuclein in Lewy bodies, sumoylation has recently been described. The interplay between α-synuclein phosphorylation and sumoylation is poorly understood. Here, we examined the interplay between these modifications as well as their impact on cell growth and inclusion formation in yeast. We found that α-synuclein is sumoylated in vivo at the same sites in yeast as in human cells. Impaired sumoylation resulted in reduced yeast growth combined with an increased number of cells with inclusions, suggesting that this modification plays a protective role. In addition, inhibition of sumoylation prevented autophagy-mediated aggregate clearance. A defect in α-synuclein sumoylation could be suppressed by serine 129 phosphorylation by the human G protein-coupled receptor kinase 5 (GRK5) in yeast. Phosphorylation reduced foci formation, alleviated yeast growth inhibition, and partially rescued autophagic α-synuclein degradation along with the promotion of proteasomal degradation, resulting in aggregate clearance in the absence of a small ubiquitin-like modifier. These findings suggest a complex interplay between sumoylation and phosphorylation in α-synuclein aggregate clearance, which may open new horizons for the development of therapeutic strategies for Parkinson disease.

Regulation and function of signal transducer and activator of transcription 3

Signal transducer and activator of transcription 3 (STAT3), a member of the STAT family, is a key regulator of many physiological and pathological processes. Significant progress has been made in understanding the transcriptional control, posttranslational modification, cellular localization and functional regulation of STAT3. STAT3 can translocate into the nucleus and bind to specific promoter sequences, thereby exerting transcriptional regulation. Recent studies have shown that STAT3 can also translocate into mitochondria, participating in aerobic respiration and apoptosis. In addition, STAT3 plays an important role in inflammation and tumorigenesis by regulating cell proliferation, differentiation and metabolism. Conditional knockout mouse models make it possible to study the physiological function of STAT3 in specific tissues and organs. This review summarizes the latest advances in the understanding of the expression, regulation and function of STAT3 in physiological and tumorigenic processes.

Regulation of type I interferon responses

Type I interferons (IFNs) activate intracellular antimicrobial programmes and influence the development of innate and adaptive immune responses. Canonical type I IFN signalling activates the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway, leading to transcription of IFN-stimulated genes (ISGs). Host, pathogen and environmental factors regulate the responses of cells to this signalling pathway and thus calibrate host defences while limiting tissue damage and preventing autoimmunity. Here, we summarize the signalling and epigenetic mechanisms that regulate type I IFN-induced STAT activation and ISG transcription and translation. These regulatory mechanisms determine the biological outcomes of type I IFN responses and whether pathogens are cleared effectively or chronic infection or autoimmune disease ensues.

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.

An RNAi-based dimorphic genetic screen identified the double bromodomain protein BET-1 as a sumo-dependent attenuator of RAS-mediated signalling

Attenuation of RAS/RAF/MAPK signalling is essential to prevent hyperactivation of this oncogenic pathway. In C. elegans, the sumoylation pathway and a combination of histone tail modifications regulate gene expression to attenuate the LET-60 (RAS) signalling pathway. We hypothesised that a number of chromatin regulators are likely to depend on sumoylation to attenuate the pathway. To reveal these, we designed an RNAi-based dimorphic genetic screen that selects candidates based on their ability to act as enhancers of a sumo mutant phenotype, such interactions would suggest that the candidates may be physically associated with sumoylation. We found 16 enhancers, one of which BET-1, is a conserved double bromodomain containing protein. We further characterised BET-1 and showed that it can physically associate with SMO-1 and UBC-9, and that it can be sumoylated in vitro within the second bromodomain at lysine 252. Previous work has shown that BET-1 can bind acetyl-lysines on histone tails to influence gene expression. In conclusion, our screening approach has identified BET-1 as a Sumo-dependent attenuator of LET-60-mediated signalling and our characterisation suggests that BET-1 can be sumoylated.

Sumoylation: a regulatory protein modification in health and disease

Posttranslational modification with small ubiquitin-related modifier (SUMO) proteins is now established as one of the key regulatory protein modifications in eukaryotic cells. Hundreds of proteins involved in processes such as chromatin organization, transcription, DNA repair, macromolecular assembly, protein homeostasis, trafficking, and signal transduction are subject to reversible sumoylation. Hence, it is not surprising that disease links are beginning to emerge and that interference with sumoylation is being considered for intervention. Here, we summarize basic mechanisms and highlight recent developments in the physiology of sumoylation.

Dominant-negative activity of the STAT3-Y705F mutant depends on the N-terminal domain

Background

STAT3 is a transcription factor of central importance in chronic inflammation and cancer. In response to cytokine stimulation STAT3 is phosphorylated on a single tyrosine residue at position 705, dimerizes and accumulates in the nucleus to induce target gene expression. The substitution of tyrosine 705 to phenylalanine leads to a dominant-negative STAT3 mutant (STAT3-YF) which influences the activation of WT-STAT3 in stimulated cells through a mechanism that is not completely understood. In this study we analyzed the molecular mechanism of STAT3-YF dominant-negative activity in IL-6-induced STAT3 signaling and the relevance of the N-terminal domain.

Results

Expression of STAT3-YF-YFP impairs tyrosine phosphorylation, nuclear translocation and the transcriptional activity of WT-STAT3 in IL-6-stimulated cells. The fluorescently labelled STAT3-YF mutant binds to a phosphorylated gp130 receptor-peptide comparable to WT-STAT3-YFP. STAT3-YF-YFP forms homodimers as well as heterodimers with WT-STAT3 in the presence and absence of IL-6. The preformed heterodimers in unstimulated cells are detectable by colocalization of STAT3-CFP with STAT3-YF-YFP fused to a nuclear localization signal. STAT3/STAT3-YF heterodimers are not able to bind to DNA in stimulated cells, but the presence of the mutant reduces DNA-binding of WT-STAT3 homodimers. STAT3-YF-ΔN-YFP lacking the N-terminal domain forms no dimers and only marginally affects the activity of WT-STAT3.

Conclusion

Our findings demonstrate that dominant-negative STAT3-YF affects the activation of WT-STAT3 at multiple levels. Unexpectedly, the N-terminal domain of STAT3-YF plays an important role for the dominant-negative effect. We show that (i) STAT3-YF competes with WT-STAT3 in binding to activated gp130-receptors, (ii) the formation of WT-STAT3/STAT3-YF heterodimers in IL-6-stimulated cells results in inactive, semiphosphorylated dimers which do not bind to DNA and thus fail to induce target gene expression, (iii) the N-terminal domain-mediated formation of preformed STAT3/STAT3-YF heterodimers in unstimulated cells which affects the IL-6-induced homodimerization of WT-STAT3 contributes to the dominant-negative effect of STAT3-YF. These findings will contribute to our understanding of naturally occuring dominant-negative STAT3 mutants that cause the hyper-IgE syndrome.

Evidence against a role for β-arrestin1 in STAT1 dephosphorylation and the inhibition of interferon-γ signaling

Signal transducer and activator of transcription 1 (STAT1) is activated by tyrosine phosphorylation upon interferon-γ (IFNγ) stimulation, which results in the expression of genes with antiproliferative and immunomodulatory functions. The inactivation of STAT1 occurs through tyrosine dephosphorylation by the tyrosine phosphatase TC45. It was proposed that recruitment of TC45 required the direct interaction of STAT1 with the scaffold protein β-arrestin1, making β-arrestin1 an essential negative regulator of STAT1 and IFNγ signaling (Mo et al., 2008). We tested the relevance of β-arrestin1 for STAT1 activity. Our results do not confirm β-arrestin1 as a STAT1-interacting protein. The STAT1 phosphorylation/dephosphorylation cycle was found to be unaffected by both the overexpression and the genetic deletion of β-arrestin1. Accordingly, β-arrestin1 did not inhibit STAT1 transcriptional activity or the induction of IFNγ target genes in response to IFNγ. Our data indicate that β-arrestin1 is dispensable for STAT1 dephosphorylation and the termination of IFNγ signaling.

Sumoylation in neurodegenerative diseases

The yeast SUMO (small ubiquitin-like modifier) orthologue SMT3 was initially discovered in a genetic suppressors screen for the centromeric protein Mif2 (Meluh and Koshland in Mol Bio Cell 6:793-807, 1). Later, it turned out that the homologous mammalian proteins SUMO1 to SUMO4 are reversible protein modifiers that can form isopeptide bonds with lysine residues of respective target proteins (Mahajan et al. in Cell 88:97-107, 2). This was the discovery of a post-translational modification called sumoylation, which enzymatically resembles ubiquitination. However, very soon it became clear that SUMO attachments served a far more diverse role than ubiquitination. Meanwhile, numerous cellular processes are known to be subject to the impact of SUMO modification, including transcription, protein targeting, protein solubility, apoptosis or activity of various enzymes. In many instances, SUMO proteins create new protein interaction surfaces or block existing interaction domains (Geiss-Friedlander and Melchior in Nat Rev in Mol Cell Biol 8:947-956, 3). For the past few years, sumoylation attracted increasing attention as a versatile regulator of toxic protein properties in neurodegenerative diseases. In this review, we summarize the growing knowledge about the involvement of sumoylation in neurodegeneration, and discuss the underlying molecular principles affected by this multifaceted and intriguing post-translational modification.

Structure-function analysis indicates that sumoylation modulates DNA-binding activity of STAT1

Background

STAT1 is an essential transcription factor for interferon-γ-mediated gene responses. A distinct sumoylation consensus site (ψKxE) ⁷⁰²IKTE⁷⁰⁵ is localized in the C-terminal region of STAT1, where Lys703 is a target for PIAS-induced SUMO modification. Several studies indicate that sumoylation has an inhibitory role on STAT1-mediated gene expression but the molecular mechanisms are not fully understood.

Results

Here, we have performed a structural and functional analysis of sumoylation in STAT1. We show that deconjugation of SUMO by SENP1 enhances the transcriptional activity of STAT1, confirming a negative regulatory effect of sumoylation on STAT1 activity. Inspection of molecular model indicated that consensus site is well exposed to SUMO-conjugation in STAT1 homodimer and that the conjugated SUMO moiety is directed towards DNA, thus able to form a sterical hindrance affecting promoter binding of dimeric STAT1. In addition, oligoprecipitation experiments indicated that sumoylation deficient STAT1 E705Q mutant has higher DNA-binding activity on STAT1 responsive gene promoters than wild-type STAT1. Furthermore, sumoylation deficient STAT1 E705Q mutant displayed enhanced histone H4 acetylation on interferon-γ-responsive promoter compared to wild-type STAT1.

Conclusions

Our results suggest that sumoylation participates in regulation of STAT1 responses by modulating DNA-binding properties of STAT1.

The JAK-STAT pathway at twenty

We look back on the discoveries that the tyrosine kinases TYK2 and JAK1 and the transcription factors STAT1, STAT2, and IRF9 are required for the cellular response to type I interferons. This initial description of the JAK-STAT pathway led quickly to additional discoveries that type II interferons and many other cytokines signal through similar mechanisms. This well-understood pathway now serves as a paradigm showing how information from protein-protein contacts at the cell surface can be conveyed directly to genes in the nucleus. We also review recent work on the STAT proteins showing the importance of several different posttranslational modifications, including serine phosphorylation, acetylation, methylation, and sumoylation. These remarkably proficient proteins also provide noncanonical functions in transcriptional regulation and they also function in mitochondrial respiration and chromatin organization in ways that may not involve transcription at all.

Biology and significance of the JAK/STAT signalling pathways

Since its discovery two decades ago, the activation of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway by numerous cytokines and growth factors has resulted in it becoming one of the most well-studied intracellular signalling networks. The field has progressed from the identification of the individual components to high-resolution crystal structures of both JAK and STAT, and an understanding of the complexities of the molecular activation and deactivation cycle which results in a diverse, yet highly specific and regulated pattern of transcriptional responses. While there is still more to learn, we now appreciate how disruption and deregulation of this pathway can result in clinical disease and look forward to adoption of the next generation of JAK inhibitors in routine clinical treatment.

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.

SUMO conjugation of STAT1 protects cells from hyperresponsiveness to IFNγ

The biologic effects of IFNγ are mediated by the transcription factor STAT1. The activity of STAT1 is inhibited by small ubiquitin-like modifier (SUMO) conjugation. This occurs both directly through decreasing STAT1 tyrosine phosphorylation and indirectly by facilitating STAT1 dephosphorylation consequential to increased STAT1 solubility because of suppressed paracrystal assembly. However, the physiologic implications of SUMO conjugation have remained unclear. Here, we used fibroblasts and bone marrow-derived macrophages (BMMs) from knockin mice expressing SUMO-free STAT1 to explore the consequences of STAT1 sumoylation for IFNγ signaling. Our experiments demonstrated buffer property of paracrystals for activated STAT1, such that SUMO-mediated paracrystal dispersal profoundly reduced phosphorylation of STAT1, which affected both the activating tyrosine 701 and the transcription-enhancing serine 727. Accordingly, the curtailed STAT1 activity in the nucleus caused by SUMO conjugation resulted in diminished transcription of IFNγ-responsive genes; and increased the IFNγ concentration more than 100-fold required to trigger lipopolysaccharide-induced cytotoxicity in bone marrow-derived macrophages. These experiments identify SUMO conjugation of STAT1 as a mechanism to permanently attenuate the IFNγ sensitivity of cells, which prevents hyperresponsiveness to this cytokine and its potentially self-destructive consequences. This sets the mode of SUMO-mediated inhibition apart from the other negative STAT regulators known to date.

STAT1 signaling is not regulated by a phosphorylation-acetylation switch

The treatment of cells with histone deacetylase inhibitors (HDACi) was reported to reveal the acetylation of STAT1 at lysine 410 and lysine 413 (O. H. Krämer et al., Genes Dev. 20:473-485, 2006). STAT1 acetylation was proposed to regulate apoptosis by facilitating binding to NF-κB and to control immune responses by suppressing STAT1 tyrosine phosphorylation, suggesting that STAT1 acetylation is a central mechanism by which histone deacetylase inhibitors ameliorate inflammatory diseases (O. H. Krämer et al., Genes Dev. 23:223-235, 2009). Here, we show that the inhibition of deacetylases had no bearing on STAT1 acetylation and did not diminish STAT1 tyrosine phosphorylation. The glutamine mutation of the alleged acetylation sites, claimed to mimic acetylated STAT1, similarly did not diminish the tyrosine phosphorylation of STAT1 but precluded its DNA binding and nuclear import. The defective transcription activity of this mutant therefore cannot be attributed to STAT1 acetylation but rather to the inactivation of the STAT1 DNA binding domain and its nuclear import signal. Experiments with respective cDNAs provided by the authors of the studies mentioned above confirmed the results reported here, further questioning the validity of the previous data. We conclude that the effects and potential clinical benefits associated with histone deacetylase inhibition cannot be explained by promoting the acetylation of STAT1 at lysines 410 and 413.