Dephosphorylation of phosphotyrosine on STAT1 dimers requires extensive spatial reorientation of the monomers facilitated by the N-terminal domain

Immunophenotyping and Therapeutic Insights from Chronic Mucocutaneous Candidiasis Cases with STAT1 Gain-of-Function Mutations

PURPOSE

Heterozygous STAT1 Gain-of-Function (GOF) mutations are the most common cause of chronic mucocutaneous candidiasis (CMC) among Inborn Errors of Immunity. Clinically, these mutations manifest as a broad spectrum of immune dysregulation, including autoimmune diseases, vascular disorders, and malignancies. The pathogenic mechanisms of immune dysregulation and its impact on immune cells are not yet fully understood. In treatment, JAK inhibitors have shown therapeutic effectiveness in some patients.

METHODS

We analyzed clinical presentations, cellular phenotypes, and functional impacts in five Taiwanese patients with STAT1 GOF.

RESULTS

We identified two novel GOF mutations in 5 patients from 2 Taiwanese families, presenting with symptoms of CMC, late-onset rosacea, and autoimmunity. The enhanced phosphorylation and delayed dephosphorylation were displayed by the patients' cells. There are alterations in both innate and adaptive immune cells, including expansion of CD38+HLADR +CD8+ T cells, a skewed activated Tfh cells toward Th1, reduction of memory, marginal zone and anergic B cells, all main functional dendritic cell lineages, and a reduction in classical monocyte. Baricitinib showed therapeutic effectiveness without side effects.

CONCLUSION

Our study provides the first comprehensive clinical and molecular characteristics in STAT1 GOF patient in Taiwan and highlights the dysregulated T and B cells subsets which may hinge the autoimmunity in STAT1 GOF patients. It also demonstrated the therapeutic safety and efficacy of baricitinib in pediatric patient. Further research is needed to delineate how the aberrant STAT1 signaling lead to the changes in cellular populations as well as to better link to the clinical manifestations of the disease.

Human STAT1 gain of function with chronic mucocutaneous candidiasis: A comprehensive review for strengthening the connection between bedside observations and laboratory research

Germline human heterozygous STAT1 gain-of-function (GOF) variants were first discovered a common cause of chronic mucocutaneous candidiasis (CMC) in 2011. Since then, numerous STAT1 GOF variants have been identified. A variety of clinical phenotypes, including fungal, viral, and bacterial infections, endocrine disorders, autoimmunity, malignancy, and aneurysms, have recently been revealed for STAT1 GOF variants, which has led to the expansion of the clinical spectrum associated with STAT1 GOF. Among this broad range of complications, it has been determined that invasive infections, aneurysms, and malignancies are poor prognostic factors for STAT1 GOF. The effectiveness of JAK inhibitors as a therapeutic option has been established, although further investigation of their long-term utility and side effects is needed. In contrast to the advancements in treatment options, the precise molecular mechanism underlying STAT1 GOF remains undetermined. Two primary hypotheses for this mechanism involve impaired STAT1 dephosphorylation and increased STAT1 protein levels, both of which are still controversial. A precise understanding of the molecular mechanism is essential for not only advancing diagnostics but also developing therapeutic interventions. Here, we provide a comprehensive review of STAT1 GOF with the aim of establishing a stronger connection between bedside observations and laboratory research.

A novel interface between the N-terminal and coiled-coil domain of STAT1 functions in an auto-inhibitory manner

BACKGROUND

STAT1 is an intracellular signaling molecule that is crucially involved in the regulation of the innate immune system by activation of defense mechanisms against microbial pathogens. Phosphorylation-dependent activation of the STAT1 transcription factor is associated with a conversion from an antiparallel to parallel dimer configuration, which after nuclear import binds to DNA. However, not much is known about the specific intermolecular interactions that stabilize unphosphorylated, antiparallel STAT1 complexes prior to activation.

RESULTS

In this study, we identified a previously unknown interdimeric interaction site, which is involved in the termination of STAT1 signaling. Introduction of the glutamic acid-to-alanine point mutation E169A in the coiled-coil domain (CCD) by site-directed mutagenesis led to increased tyrosine phosphorylation as well as accelerated and prolonged nuclear accumulation in transiently transfected cells. In addition, DNA-binding affinity and transcriptional activity were strongly enhanced in the substitution mutant compared to the wild-type (WT) protein. Furthermore, we have demonstrated that the E169 residue in the CCD mediates the release of the dimer from the DNA in an auto-inhibitory manner.

CONCLUSION

Based on these findings, we propose a novel mechanism for the inactivation of the STAT1 signaling pathway, assigning the interface with the glutamic acid residue 169 in the CCD a crucial role in this process. Video Abstract.

Lysine vitcylation is a novel vitamin C-derived protein modification that enhances STAT1-mediated immune response

Vitamin C (vitC) is a vital nutrient for health and also used as a therapeutic agent in diseases such as cancer. However, the mechanisms underlying vitC's effects remain elusive. Here we report that vitC directly modifies lysine without enzymes to form vitcyl-lysine, termed "vitcylation", in a dose-, pH-, and sequence-dependent manner across diverse proteins in cells. We further discover that vitC vitcylates K298 site of STAT1, which impairs its interaction with the phosphatase PTPN2, preventing STAT1 Y701 dephosphorylation and leading to increased STAT1-mediated IFN pathway activation in tumor cells. As a result, these cells have increased MHC/HLA class-I expression and activate immune cells in co-cultures. Tumors collected from vitC-treated tumor-bearing mice have enhanced vitcylation, STAT1 phosphorylation and antigen presentation. The identification of vitcylation as a novel PTM and the characterization of its effect in tumor cells opens a new avenue for understanding vitC in cellular processes, disease mechanisms, and therapeutics.

JAKs and STATs from a Clinical Perspective: Loss-of-Function Mutations, Gain-of-Function Mutations, and Their Multidimensional Consequences

The JAK/STAT signaling pathway plays a key role in cytokine signaling and is involved in development, immunity, and tumorigenesis for nearly any cell. At first glance, the JAK/STAT signaling pathway appears to be straightforward. However, on closer examination, the factors influencing the JAK/STAT signaling activity, such as cytokine diversity, receptor profile, overlapping JAK and STAT specificity among non-redundant functions of the JAK/STAT complexes, positive regulators (e.g., cooperating transcription factors), and negative regulators (e.g., SOCS, PIAS, PTP), demonstrate the complexity of the pathway's architecture, which can be quickly disturbed by mutations. The JAK/STAT signaling pathway has been, and still is, subject of basic research and offers an enormous potential for the development of new methods of personalized medicine and thus the translation of basic molecular research into clinical practice beyond the use of JAK inhibitors. Gain-of-function and loss-of-function mutations in the three immunologically particularly relevant signal transducers STAT1, STAT3, and STAT6 as well as JAK1 and JAK3 present themselves through individual phenotypic clinical pictures. The established, traditional paradigm of loss-of-function mutations leading to immunodeficiency and gain-of-function mutation leading to autoimmunity breaks down and a more differentiated picture of disease patterns evolve. This review is intended to provide an overview of these specific syndromes from a clinical perspective and to summarize current findings on pathomechanism, symptoms, immunological features, and therapeutic options of STAT1, STAT3, STAT6, JAK1, and JAK3 loss-of-function and gain-of-function diseases.

Inborn errors of immunity with loss- and gain-of-function germline mutations in STAT1

STAT1 dysfunction causes a wide range of immune dysregulation phenotypes, which have been classified into four disease types, namely, (i) autosomal recessive (AR) complete STAT1 deficiency, (ii) AR partial STAT1 deficiency, (iii) autosomal dominant (AD) STAT1 deficiency, and (iv) AD STAT1 gain of function (GOF), based on their mode of inheritance and function. Disease types (i, ii, and iii) are caused by STAT1 loss-of-function (LOF) mutations, whereas disease type (iv) is caused by STAT1 GOF mutations. Therefore, the functional analysis of mutations is necessary for the precise diagnosis.

A family-wide assessment of latent STAT transcription factor interactions reveals divergent dimer repertoires

The conversion of STAT proteins from latent to active transcription factors is central to cytokine signalling. Triggered by their signal-induced tyrosine phosphorylation, it is the assembly of a range of cytokine-specific STAT homo- and heterodimers that marks a key step in the transition of hitherto latent proteins to transcription activators. In contrast, the constitutive self-assembly of latent STATs and how it relates to the functioning of activated STATs, is understood less well. To provide a more complete picture, we developed a co-localization-based assay and tested all 28 possible combinations of the seven unphosphorylated STAT (U-STAT) proteins in living cells. We identified five U-STAT homodimers -STAT1, STAT3, STAT4, STAT5A and STAT5B- and two heterodimers -STAT1:STAT2 and STAT5A:STAT5B- and performed semi-quantitative assessments of the forces and characterizations of binding interfaces that support them. One STAT protein -STAT6- was found to be monomeric. This comprehensive analysis of latent STAT self-assembly lays bare considerable structural and functional diversity in the ways that link STAT dimerization before and after activation.

Network Pharmacology and Molecular Modeling to Elucidate the Potential Mechanism of Neem Oil against Acne vulgaris

Acne vulgaris is a common skin disorder with a complicated etiology. Papules, lesions, comedones, blackheads, and other skin lesions are common physical manifestations of Acne vulgaris, but the individual who has it also regularly has psychological repercussions. Natural oils are being utilized more and more to treat skin conditions since they have fewer negative effects and are expected to provide benefits. Using network pharmacology, this study aims to ascertain if neem oil has any anti-acne benefits and, if so, to speculate on probable mechanisms of action for such effects. The neem leaves (Azadirachta indica) were collected, verified, authenticated, and assigned a voucher number. After steam distillation was used to extract the neem oil, the phytochemical components of the oil were examined using gas chromatography-mass spectrometry (GC-MS). The components of the oil were computationally examined for drug-likeness using Lipinski's criteria. The Pharm Mapper service was used to anticipate the targets. Prior to pathway and protein-protein interaction investigations, molecular docking was performed to predict binding affinity. Neem oil was discovered to be a potential target for STAT1, CSK, CRABP2, and SYK genes in the treatment of Acne vulgaris. In conclusion, it was discovered that the neem oil components with PubChem IDs: ID_610088 (2-(1-adamantyl)-N-methylacetamide), ID_600826 (N-benzyl-2-(2-methyl-5-phenyl-3H-1,3,4-thiadiazol-2-yl)acetamide), and ID_16451547 (N-(3-methoxyphenyl)-2-(1-phenyltetrazol-5-yl)sulfanylpropanamide) have strong affinities for these drug targets and may thus be used as therapeutic agents in the treatment of acne.

IL-2-mTORC1 signaling coordinates the STAT1/T-bet axis to ensure Th1 cell differentiation and anti-bacterial immune response in fish

Utilization of specialized Th1 cells to resist intracellular pathogenic infection represents an important innovation of adaptive immunity. Although transcriptional evidence indicates the potential presence of Th1-like cells in some fish species, the existence of CD3+CD4+IFN-γ+ T cells, their detailed functions, and the mechanism determining their differentiation in these early vertebrates remain unclear. In the present study, we identified a population of CD3+CD4-1+IFN-γ+ (Th1) cells in Nile tilapia upon T-cell activation in vitro or Edwardsiella piscicida infection in vivo. By depleting CD4-1+ T cells or blocking IFN-γ, Th1 cells and their produced IFN-γ were found to be essential for tilapia to activate macrophages and resist the E. piscicida infection. Mechanistically, activated T cells of tilapia produce IL-2, which enhances the STAT5 and mTORC1 signaling that in turn trigger the STAT1/T-bet axis-controlled IFN-γ transcription and Th1 cell development. Additionally, mTORC1 regulates the differentiation of these cells by promoting the proliferation of CD3+CD4-1+ T cells. Moreover, IFN-γ binds to its receptors IFNγR1 and IFNγR2 and further initiates a STAT1/T-bet axis-mediated positive feedback loop to stabilize the Th1 cell polarization in tilapia. These findings demonstrate that, prior to the emergence of tetrapods, the bony fish Nile tilapia had already evolved Th1 cells to fight intracellular bacterial infection, and support the notion that IL-2-mTORC1 signaling coordinates the STAT1/T-bet axis to determine Th1 cell fate, which is an ancient mechanism that has been programmed early during vertebrate evolution. Our study is expected to provide novel perspectives into the evolution of adaptive immunity.

Candidate genes and sequence variants for susceptibility to mycobacterial infection identified by whole-exome sequencing

Inborn errors of immunity are known to influence susceptibility to mycobacterial infections. The aim of this study was to characterize the genetic profile of nine patients with mycobacterial infections (eight with BCGitis and one with disseminated tuberculosis) from the Republic of Moldova using whole-exome sequencing. In total, 12 variants in eight genes known to be associated with Mendelian Susceptibility to Mycobacterial Disease (MSMD) were detected in six out of nine patients examined. In particular, a novel splice site mutation c.373–2A>C in STAT1 gene was found and functionally confirmed in a patient with disseminated tuberculosis. Trio analysis was possible for seven out of nine patients, and resulted in 23 candidate variants in 15 novel genes. Four of these genes - GBP2, HEATR3, PPP1R9B and KDM6A were further prioritized, considering their elevated expression in immune-related tissues. Compound heterozygosity was found in GBP2 in a single patient, comprising a maternally inherited missense variant c.412G>A/p.(Ala138Thr) predicted to be deleterious and a paternally inherited intronic mutation c.1149+14T>C. Functional studies demonstrated that the intronic mutation affects splicing and the level of transcript. Finally, we analyzed pathogenicity of variant combinations in gene pairs and identified five patients with putative oligogenic inheritance. In summary, our study expands the spectrum of genetic variation contributing to susceptibility to mycobacterial infections in children and provides insight into the complex/oligogenic disease-causing mode.

Genetic and Functional Identifying of Novel STAT1 Loss-of-Function Mutations in Patients with Diverse Clinical Phenotypes

PURPOSE

Mutations in signal transducer and activator of transcription 1 (STAT1) cause a broad spectrum of disease phenotypes. Heterozygous STAT1 loss-of-function (LOF) mutations cause Mendelian susceptibility to mycobacterial diseases (MSMD) infection, which is attributable to impaired IFN-γ signaling. The identification of novel mutations may extend the phenotypes associated with autosomal dominant (AD) STAT1 deficiency.

METHODS

Five patients with heterozygous STAT1 variations were recruited and their clinical and immunologic phenotypes were analyzed, with particular reference to JAK-STAT1 signaling pathways.

RESULTS

Four, heterozygous STAT1 deficiency mutations were identified, three of which were novel mutations. Two of the mutations were previously unreported mRNA splicing mutations in AD STAT1-deficient patients. Patients with heterozygous STAT1 deficiency suffered not only mycobacterial infection, but also intracellular non-mycobacterial bacterial infection and congenital multiple malformations. AD-LOF mutation impaired IFN-γ-mediated STAT1 phosphorylation, gamma-activated sequence (GAS), and IFN-stimulated response element (ISRE) transcription activity and IFN-induced gene expression to different extents, which might account for the diverse clinical manifestations observed in these patients.

CONCLUSION

The infectious disease susceptibility and phenotypic spectrum of patients with AD STAT1-LOF are broader than simply MSMD. The susceptibility to infections and immunological deficiency phenotypes, observed in AD-LOF patients, confirms the importance of STAT1 in host-pathogen interaction and immunity. However, variability in the nature and extent of these phenotypes suggests that functional analysis is required to identify accurately novel, heterozygous STAT1 mutations, associated with pathogenicity. Aberrant splice of STAT1 RNA could result in AD-LOF for STAT1 signaling which need more cases for confirmation.

STAT and Janus kinase targeting by human herpesvirus 8 interferon regulatory factor in the suppression of type-I interferon signaling

Human herpesvirus 8 (HHV-8), also known as Kaposi’s sarcoma (KS)-associated herpesvirus, is involved etiologically in AIDS-associated KS, primary effusion lymphoma (PEL), and multicentric Castleman’s disease, in which both viral latent and lytic functions are important. HHV-8 encodes four viral interferon regulatory factors (vIRFs) that are believed to contribute to viral latency (in PEL cells, at least) and/or to productive replication via suppression of cellular antiviral and stress signaling. Here, we identify vIRF-1 interactions with signal transducer and activator of transcription (STAT) factors 1 and 2, interferon (IFN)-stimulated gene factor 3 (ISGF3) cofactor IRF9, and associated signal transducing Janus kinases JAK1 and TYK2. In naturally infected PEL cells and in iSLK epithelial cells infected experimentally with genetically engineered HHV-8, vIRF-1 depletion or ablation, respectively, led to increased levels of active (phosphorylated) STAT1 and STAT2 in IFNβ-treated, and untreated, cells during lytic replication and to associated cellular-gene induction. In transfected 293T cells, used for mechanistic studies, suppression by vIRF-1 of IFNβ-induced phospho-STAT1 (pSTAT1) was found to be highly dependent on STAT2, indicating vIRF-1-mediated inhibition and/or dissociation of ISGF3-complexing, resulting in susceptibility of pSTAT1 to inactivating dephosphorylation. Indeed, coprecipitation experiments involving targeted precipitation of ISGF3 components identified suppression of mutual interactions by vIRF-1. In contrast, suppression of IFNβ-induced pSTAT2 was effected by regulation of STAT2 activation, likely via detected inhibition of TYK2 and its interactions with STAT2 and IFN type-I receptor (IFNAR). Our identified vIRF-1 interactions with IFN-signaling mediators STATs 1 and 2, co-interacting ISGF3 component IRF9, and STAT-activating TYK2 and the suppression of IFN signaling via ISGF3, TYK2-STAT2 and TYK2-IFNAR disruption and TYK2 inhibition represent novel mechanisms of vIRF function and HHV-8 evasion from host-cell defenses.

Viral interferon regulatory factors (vIRFs) encoded by Kaposi’s sarcoma- and lymphoma-associated human herpesvirus 8 (HHV-8) are mediators of protection from cellular antiviral responses and therefore are considered to be pivotal for successful de novo infection, latency establishment and maintenance, and productive (lytic) replication. Identification and characterization of their interactions with cellular proteins, the functional consequences of these interactions, and the operation of these mechanisms in the context of infection has the potential to enable the development of novel antiviral strategies targeted to these interactions and mechanisms. In this report we identify vIRF-1 interactions with transcription factors STAT1 and STAT2, the co-interacting component, IRF9, of the antiviral interferon (IFN)-induced transcription complex ISGF3, and the ability of vIRF-1 to inhibit activation and functional associations of IFN-I receptor- and STAT1/2-kinase TYK2, suppress STAT1/2 activation, and dissociate STAT1 from IFN-induced ISGF3 to blunt IFN signaling and promote STAT1 inactivation. These interactions and activities, which mediate suppression of innate cellular defenses against virus replication, represent novel properties among vIRFs and could potentially be exploited for antiviral and therapeutic purposes.

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.

A chemical probe inhibitor targeting STAT1 restricts cancer stem cell traits and angiogenesis in colorectal cancer

Background

Colorectal cancer (CRC) is a worldwide cancer with rising annual incidence. New medications for patients with CRC are still needed. Recently, fluorescent chemical probes have been developed for cancer imaging and therapy. Signal transducer and activator of transcription 1 (STAT1) has complex functions in tumorigenesis and its role in CRC still needs further investigation.

Methods

RNA sequencing datasets in the NCBI GEO repository were analyzed to investigate the expression of STAT1 in patients with CRC. Xenograft mouse models, tail vein injection mouse models, and azoxymethane/dextran sodium sulfate (AOM/DSS) mouse models were generated to study the roles of STAT1 in CRC. A ligand-based high-throughput virtual screening approach combined with SWEETLEAD chemical database analysis was used to discover new STAT1 inhibitors. A newly designed and synthesized fluorescently labeled 4’,5,7-trihydroxyisoflavone (THIF) probe (BODIPY-THIF) elucidated the mechanistic actions of STAT1 and THIF in vitro and in vivo. Colonosphere formation assay and chick chorioallantoic membrane assay were used to evaluate stemness and angiogenesis, respectively.

Results

Upregulation of STAT1 was observed in patients with CRC and in mouse models of AOM/DSS-induced CRC and metastatic CRC. Knockout of STAT1 in CRC cells reduced tumor growth in vivo. We then combined a high-throughput virtual screening approach and analysis of the SWEETLEAD chemical database and found that THIF, a flavonoid abundant in soybeans, was a novel STAT1 inhibitor. THIF inhibited STAT1 phosphorylation and might bind to the STAT1 SH2 domain, leading to blockade of STAT1-STAT1 dimerization. The results of in vitro and in vivo binding studies of THIF and STAT1 were validated. The pharmacological treatment with BODIPY-THIF or ablation of STAT1 via a CRISPR/Cas9-based strategy abolished stemness and angiogenesis in CRC. Oral administration of BODIPY-THIF attenuated colitis symptoms and tumor growth in the mouse model of AOM/DSS-induced CRC.

Conclusions

This study demonstrates that STAT1 plays an oncogenic role in CRC. BODIPY-THIF is a new chemical probe inhibitor of STAT1 that reduces stemness and angiogenesis in CRC. BODIPY-THIF can be a potential tool for CRC therapy as well as cancer cell imaging.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12929-022-00803-4.

Phenotypic variability, not noise, accounts for most of the cell-to-cell heterogeneity in IFN-γ and oncostatin M signaling responses

Cellular signaling responses show substantial cell-to-cell heterogeneity, which is often ascribed to the inherent randomness of biochemical reactions, termed molecular noise, wherein high noise implies low signaling fidelity. Alternatively, heterogeneity could arise from differences in molecular content between cells, termed molecular phenotypic variability, which does not necessarily imply imprecise signaling. The contribution of these two processes to signaling heterogeneity is unclear. Here, we fused fibroblasts to produce binuclear syncytia to distinguish noise from phenotypic variability in the analysis of cytokine signaling. We reasoned that the responses of the two nuclei within one syncytium could approximate the signaling outcomes of two cells with the same molecular content, thereby disclosing noise contribution, whereas comparison of different syncytia should reveal contribution of phenotypic variability. We found that ~90% of the variance in the primary response (which was the abundance of phosphorylated, nuclear STAT) to stimulation with the cytokines interferon-γ and oncostatin M resulted from differences in the molecular content of individual cells. Thus, our data reveal that cytokine signaling in the system used here operates in a reproducible, high-fidelity manner.

The anti-parallel dimer binding interface in STAT3 transcription factor is required for the inactivation of cytokine-mediated signal transduction

Signal transducer and activator of transcription 3 (STAT3) gain-of-function mutations have been widely reported in patients with tumors and haematological malignancies. However, the molecular mechanisms of these pathogenic mutations remain largely uninvestigated. In this study, we have extensively characterized two STAT3 missense mutations, namely a valine-to-alanine exchange in the amino-terminal region (V77A) and a phenylalanine-to-alanine substitution (F174A) in the coiled-coil domain. The two mutants displayed elevated levels of tyrosine phosphorylation, premature nuclear accumulation, and differential transcriptional responses following stimulation of cells with interleukin-6 and interferon-ɣ. In line with their hyper-phosphorylated status, a greater fraction of V77A and F174A proteins was bound to DNA on high-affinity binding sites termed sis-inducible elements (SIE) as compared to the wild-type (WT) protein. Unexpectedly, these STAT3 variants displayed similar kinetics using in vitro kinase and dephosphorylation assays performed with recombinant Janus kinase 2 (JAK2) and Tc45 phosphatase, respectively. This indicates that the two mutations neither affected the susceptibility of STAT3 to the enzymatic activity of the inactivating tyrosine phosphatase nor to the activating kinase. However, experiments triggering intracellular dephosphorylation by the addition of the tyrosine-kinase inhibitor staurosporine to cytokine-pretreated cells showed that the two mutants partially resisted dephosphorylation. From these data, we propose that the F174A missense mutation hinders the exchange from a parallel to an anti-parallel dimer conformation, thereby increasing the ratio of tyrosine-phosphorylated molecules bound to DNA and enhancing gene-dependent transcription. Our data point to the physiological importance of the anti-parallel dimer conformation in the inactivation of the cytokine-induced STAT3 signalling pathway.

STAT1 N-terminal domain discriminatively controls type I and type II IFN signaling

The seven signal transducers of transcription (STATs) are cytokine-inducible modular transcription factors. They transmit the stimulation of cells with type I interferons (IFN-α/IFN-β) and type II interferon (IFN-ɣ) into altered gene expression patterns. The N-terminal domain (NTD) of STAT1 is a surface for STAT1/STAT1 homodimer and STAT1/STAT2 heterodimer formation and allows the cooperative DNA binding of STAT1. We investigated whether the STAT1 NTD-mediated dimerization affected the IFN-induced tyrosine phosphorylation of STAT1, its nuclear translocation, STAT1-dependent gene expression, and IFN-dependent antiviral defense. We reconstituted human STAT1-negative and STAT2-negative fibrosarcoma cells with STAT1, NTD-mutated STAT1 (STAT1AA), STAT1 with a mutated DNA-binding domain (DBD), or STAT2. We treated these cells with IFN-α and IFN-ɣ to assess differences between IFN-α-induced STAT1 homo- and heterodimers and IFN-ɣ-induced STAT1 homodimers. Our data demonstrate that IFNs induce the phosphorylation of STAT1 and STAT1AA at Y701 and their nuclear accumulation. We further reveal that STAT1AA can be phosphorylated in response to IFN-α in the absence of STAT2 and that IFN-ɣ-induced STAT1AA can activate gene expression directly. However, STAT1AA largely fails to bind STAT2 and to activate IFN-α-induced expression of endogenous antiviral STAT1/STAT2 target proteins. Congruent herewith, both an intact STAT1 NTD and STAT2 are indispensable to establish an antiviral state with IFN-α. These data provide new insights into the biological importance of the STAT1 NTD.

The tyrosine kinase c-Abl potentiates interferon-mediated antiviral immunity by STAT1 phosphorylation

Interferon (IFN)-induced activation of the signal transducer and activator of transcription (STAT) family is an important event in antiviral immunity. Here, we show that the nonreceptor kinases c-Abl and Arg directly interact with STAT1 and potentiate the phosphorylation of STAT1 on Y701. c-Abl/Arg could mediate STAT1 phosphorylation independent of Janus kinases in the absence of IFNγ and potentiate IFNγ-mediated STAT1 phosphorylation. Moreover, STAT1 dimerization, nuclear translocation, and downstream gene transcription are regulated by c-Abl/Arg. c-Abl/Arg (abl1/abl2) deficiency significantly suppresses antiviral responses in vesicular stomatitis virus-infected cells. Compared to vehicle, administration of the c-Abl/Arg selective inhibitor AMN107 resulted in significantly increased mortality in mice infected with human influenza virus. Our study demonstrates that c-Abl plays an essential role in the STAT1 activation signaling pathway and provides an important approach for antiviral immunity regulation.

Transcriptional Profiling of STAT1 Gain-of-Function Reveals Common and Mutation-Specific Fingerprints

STAT1 gain-of-function (GOF) is a primary immunodeficiency typically characterized by chronic mucocutaneous candidiasis (CMC), recurrent respiratory infections, and autoimmunity. Less commonly, also immunodysregulation polyendocrinopathy enteropathy X-linked (IPEX)-like syndromes with CMC, and combined immunodeficiency without CMC have been described. Recently, our group and others have shown that different mutation-specific mechanisms underlie STAT1 GOF in vitro, including faster nuclear accumulation (R274W), and reduced mobility (R321, N574I) to near immobility in the nucleus (T419R) upon IFNγ stimulation. In this work, we evaluated the transcriptomic fingerprint of the aforementioned STAT1 GOF mutants (R274W, R321S, T419R, and N574I) relative to STAT1 wild-type upon IFNγ stimulation in an otherwise isogenic cell model. The majority of genes up-regulated in wild-type STAT1 cells were significantly more up-regulated in cells expressing GOF mutants, except for T419R. In addition to the common interferon regulated genes (IRG), STAT1 GOF mutants up-regulated an additional set of genes, that were in part shared with other GOF mutants or mutation-specific. Overall, R274W and R321S transcriptomes clustered with STAT1 WT, while T419R and N574I had a more distinct fingerprint. We observed reduced frequency of canonical IFNγ activation site (GAS) sequences in promoters of genes up-regulated by all the STAT1 GOF mutants, suggesting loss of DNA binding specificity for the canonical GAS consensus. Interestingly, the T419R mutation, expected to directly increase the affinity for DNA, showed the most pronounced effects on the transcriptome. T419R STAT1 dysregulated more non-IRG than the other GOF mutants and fewer GAS or degenerate GAS promotor sequences could be found in the promoter regions of these genes. In conclusion, our work confirms hyperactivation of common sets of IFNγ-induced genes in STAT1 GOF with additional dysregulation of mutation-specific genes, in line with the earlier observed mutation-specific mechanisms. Binding to more degenerate GAS sequences is proposed as a mechanism toward transcriptional dysregulation in R274W, R321S, and N574I. For T419R, an increased interaction with the DNA is suggested to result in a broader and less GAS-specific response. Our work indicates that multiple routes leading to STAT1 GOF are associated with common and private transcriptomic fingerprints, which may contribute to the phenotypic variation observed in vivo.

The pathogenic T387A missense mutation in the gene encoding signal transducer and activator of transcription 1 exhibits a differential gene expression profile

Heterozygous gain-of-function (GOF) mutations in the interferon-driven transcription factor STAT1 (signal transducer and activator of transcription 1) cause chronic mucocutaneous candidiasis (CMC). In this study, we characterized the molecular basis of a CMC-associated missense mutation by introducing a threonine-to-alanine exchange in the STAT1 DNA-binding domain at position 387. This substitution had previously been described in a CMC patient with suppurative eyelid infection and cutaneous abscesses, which are both unusual symptoms in this immunodeficiency. The STAT1-T387A mutant generated was compared to the wild-type protein and, in addition, to the missense mutant in the neighbouring position 386. Our results showed that the T387A mutant displayed distinct properties different from the wild-type molecule, namely elevated levels of tyrosine phosphorylation in conjunction with increased DNA-binding activity, hyperactive transcriptional regulation, and prolonged nuclear accumulation. The elevated tyrosine phosphorylation of the T387A mutant did not result in an increased mRNA production above the level of the wild-type molecule for all transcripts tested, indicating that the transcriptional activity of this mutant is largely gene-dependent. Nonetheless, these data demonstrate that the pathogenic T387A mutation associated with an atypical CMC symptomatology is biochemically similar to other well-characterized GOF mutants, while the H386A mutant was indistinguishable from the wild-type molecule. Our findings are in line with the assumption that the phenotype of this dominant STAT1 GOF mutation probably results from a disturbed shift in the equilibrium between the parallel and antiparallel dimer conformation, which is required for physiological gene activation.

Stat2 stability regulation: an intersection between immunity and carcinogenesis

Signal transducer and activator of transcription (STAT2) is a member of the STAT family that plays an essential role in immune responses to extracellular and intracellular stimuli, including inflammatory reactions, invasion of foreign materials, and cancer initiation. Although the majority of STAT2 studies in the last few decades have focused on interferon (IFN)-α/β (IFNα/β) signaling pathway-mediated host defense against viral infections, recent studies have revealed that STAT2 also plays an important role in human cancer development. Notably, strategic research on STAT2 function has provided evidence that transient regulatory activity by homo- or heterodimerization induces its nuclear localization where it to forms a ternary IFN-stimulated gene factor 3 (ISGF3) complex, which is composed of STAT1 and/or STAT2 and IFN regulatory factor 9 (IEF9). The molecular mechanisms of ISGF3-mediated ISG gene expression provide the basic foundation for the regulation of STAT2 protein activity but not protein quality control. Recently, previously unknown molecular mechanisms of STAT2-mediated cell proliferation via STAT2 protein quality control were elucidated. In this review, we briefly summarize the role of STAT2 in immune responses and carcinogenesis with respect to the molecular mechanisms of STAT2 stability regulation via the proteasomal degradation pathway.

The activity of STAT2, a protein stimulated by molecular signalling systems to activate selected genes in ways that can lead to cancer, is regulated by factors controlling its rate of degradation. Yong-Yeon Cho and colleagues at The Catholic University of Korea in South Korea review the role of STAT2 in links between molecular signals of the immune response and the onset of cancer. They focus on the significance of factors that regulate the stability of STAT2. One key factor appears to be the molecular mechanisms controlling the degradation of STAT2 by cellular structures called proteasomes. These structures break down proteins as part of routine cell maintenance. Deeper understanding of the stimulation, action and degradation of STAT2 will assist efforts to treat the many cancers in which STAT2 activity is involved.

Human STAT1 Gain-of-Function Heterozygous Mutations: Chronic Mucocutaneous Candidiasis and Type I Interferonopathy

Heterozygous gain-of-function (GOF) mutations in STAT1 in patients with chronic mucocutaneous candidiasis (CMC) and hypothyroidism were discovered in 2011. CMC is the recurrent or persistent mucocutaneous infection by Candida fungi, and hypothyroidism results from autoimmune thyroiditis. Patients with these diseases develop other infectious diseases, including viral, bacterial, and fungal diseases, and other autoimmune manifestations, including enterocolitis, immune cytopenia, endocrinopathies, and systemic lupus erythematosus. STAT1-GOF mutations are highly penetrant with a median age at onset of 1 year and often underlie an autosomal dominant trait. As many as 105 mutations at 72 residues, including 65 recurrent mutations, have already been reported in more than 400 patients worldwide. The GOF mechanism involves impaired dephosphorylation of STAT1 in the nucleus. Patient cells show enhanced STAT1-dependent responses to type I and II interferons (IFNs) and IL-27. This impairs Th17 cell development, which accounts for CMC. The pathogenesis of autoimmunity likely involves enhanced type I IFN responses, as in other type I interferonopathies. The pathogenesis of other infections, especially those caused by intramacrophagic bacteria and fungi, which are otherwise seen in patients with diminished type II IFN immunity, has remained mysterious. The cumulative survival rates of patients with and without severe disease (invasive infection, cancer, and/or symptomatic aneurysm) at 60 years of age are 31% and 87%, respectively. Severe autoimmunity also worsens the prognosis. The treatment of patients with STAT1-GOF mutations who suffer from severe infectious and autoimmune manifestations relies on hematopoietic stem cell transplantation and/or oral JAK inhibitors.

A virus-induced conformational switch of STAT1-STAT2 dimers boosts antiviral defenses

Type I interferons (IFN-I) protect us from viral infections. Signal transducer and activator of transcription 2 (STAT2) is a key component of interferon-stimulated gene factor 3 (ISGF3), which drives gene expression in response to IFN-I. Using electron microscopy, we found that, in naive cells, U-STAT2, lacking the activating tyrosine phosphorylation, forms a heterodimer with U-STAT1 in an inactive, anti-parallel conformation. A novel phosphorylation of STAT2 on T404 promotes IFN-I signaling by disrupting the U-STAT1-U-STAT2 dimer, facilitating the tyrosine phosphorylation of STATs 1 and 2 and enhancing the DNA-binding ability of ISGF3. IKK-ε, activated by virus infection, phosphorylates T404 directly. Mice with a T-A mutation at the corresponding residue (T403) are highly susceptible to virus infections. We conclude that T404 phosphorylation drives a critical conformational switch that, by boosting the response to IFN-I in infected cells, enables a swift and efficient antiviral defense.

Impact of the STAT1 N-terminal domain for fibrosarcoma cell responses to ɣ-irradiation

Type I/II interferons (IFNα,β/IFNɣ) are cytokines that activate signal-transducer-and-activator-of-transcription-1 (STAT1). The STAT1 N-terminal domain (NTD) mediates dimerization and cooperative DNA-binding. The STAT1 DNA-binding domain (DBD) confers sequence-specific DNA-recognition. STAT1 has been connected to growth inhibition, replication stress and DNA-damage. We investigated how STAT1 and NTD/DBD mutants thereof affect fibrosarcoma cells. STAT1 and indicated mutants do not affect proliferation of resting and IFNα-treated cells as well as checkpoint kinase signaling, and phosphorylation of the tumor-suppressive transcription factor p53 ensuing ɣ-irradiation. Of the STAT1 reconstituted U3A cells those with STAT1 NTD mutants accumulate the highest levels of the replication stress/DNA-damage marker S139-phosphorylated histone H2AX (ɣH2AX). This is similarly seen with a STAT1 NTD/DBD double mutant, indicating transcription-independent effects. Furthermore, U3A cells with STAT1 NTD mutants are most susceptible to apoptotic DNA fragmentation and cleavage of the DNA repair protein PARP1. These data provide novel insights into the relevance of the STAT1 NTD.

Live Cell Imaging Demonstrates Multiple Routes Toward a STAT1 Gain-of-Function Phenotype

Signal transducer and activator of transcription 1 (STAT1) gain-of-function (GOF) mutations result in a primary immunodeficiency (PID) characterized typically by chronic mucocutaneous candidiasis (CMC), but a wider phenotypic range is reported and remains unexplained from a pathophysiological point-of-view. We hypothesized that different STAT1 GOF mutations may result in distinct molecular mechanisms, possibly explaining the variable phenotypes observed in patients. We selected STAT1 GOF mutants (R274W, R321S, T419R, and N574I) that are spread over the protein and studied their dynamic behavior in vitro in U3A and HeLa cell lines. All GOF mutants showed increased STAT1 phosphorylation compared to STAT1 WT. Real-time imaging demonstrated three underlying mechanisms for STAT1 GOF: (i) R274W showed a faster nuclear accumulation, (ii) both R321S and N574I showed a reduced nuclear mobility and slower dephosphorylation, whereas (iii) T419R was near-immobile in the nucleus, potentially due to enhanced binding to chromatin.

Phospho-Ser727 triggers a multistep inactivation of STAT3 by rapid dissociation of pY705-SH2 through C-terminal tail modulation

Signal transducer and activator of transcription 3 (STAT3) is involved in many biological processes, including immunity and cancer. STAT3 becomes phosphorylated at Tyr705 and Ser727 on IL-6 stimulation. Phospho-Tyr705 (pY705) stabilizes the STAT3 dimer with reciprocal interactions between pY705 and the SH2 of the other molecule and phospho-Ser727 (pS727) accelerates pY705 dephosphorylation. We study how pS727 regulates STAT3 in both structural and biological perspectives. Using STAT3 reconstituted in HepG2-stat3-knockout cells, we show that pS727, together with a handshake N-terminal domain (NTD) interaction, causes rapid inactivation of STAT3 for pY705 dephosphorylation and a chromosome region maintenance 1 (CRM1)-independent nuclear export, which is critical for faithful STAT3 response to the cellular signals. The various N-terminal tags, GFP-related Ruby and FLAG, rendered the export CRM1-dependent and especially FLAG-tag caused nuclear accumulation of STAT3, indicating the presence of conformational changes in inactivation. Impaired reactivation of STAT3 by S727A or FLAG-tag delayed or inhibited the IL-6-induced saa1 mRNA expression, respectively. The detailed analysis of the pY705-SH2 structure identified the C-terminal tail (CTT) from L706 to P715 as a key regulator of the CTT-CTT intermolecular and the CTT-SH2 intramolecular interactions that support pY705-SH2 association. The functional studies using multiple STAT3 mutants indicated that the degree of the two interactions determines the stability of pY705-SH2 interaction. Importantly, Pro715 was critical for the pS727's destabilizing activity and the known phosphorylation and acetylation at the CTT structurally inhibited the pY705-SH2 interaction. Thus, pS727 triggers pY705-SH2 dissociation by weakening the supportive interactions likely through CTT modulation, inducing rapid cycles of STAT3 activation-inactivation for proper function of STAT3.

Direct Targeting Options for STAT3 and STAT5 in Cancer

Signal transducer and activator of transcription (STAT)3 and STAT5 are important transcription factors that are able to mediate or even drive cancer progression through hyperactivation or gain-of-function mutations. Mutated STAT3 is mainly associated with large granular lymphocytic T-cell leukemia, whereas mutated STAT5B is associated with T-cell prolymphocytic leukemia, T-cell acute lymphoblastic leukemia and γδ T-cell-derived lymphomas. Hyperactive STAT3 and STAT5 are also implicated in various hematopoietic and solid malignancies, such as chronic and acute myeloid leukemia, melanoma or prostate cancer. Classical understanding of STAT functions is linked to their phosphorylated parallel dimer conformation, in which they induce gene transcription. However, the functions of STAT proteins are not limited to their phosphorylated dimerization form. In this review, we discuss the functions and the roles of unphosphorylated STAT3/5 in the context of chromatin remodeling, as well as the impact of STAT5 oligomerization on differential gene expression in hematopoietic neoplasms. The central involvement of STAT3/5 in cancer has made these molecules attractive targets for small-molecule drug development, but currently there are no direct STAT3/5 inhibitors of clinical grade available. We summarize the development of inhibitors against the SH2 domains of STAT3/5 and discuss their applicability as cancer therapeutics.

Identification of a distinct subset of disease-associated gain-of-function missense mutations in the STAT1 coiled-coil domain as system mutants

Heterozygous gain-of-function (GOF) mutations in the cytokine-regulated transcription factor STAT1 (signal transducer and activator of transcription 1) lead to chronic mucocutaneous candidiasis (CMC). However, the molecular basis of these pathogenic missense mutations is largely unknown. In this study, we characterized in more detail the CMC-associated GOF substitution mutation of arginine-to-tryptophan at position 274 (R274W) and, in addition, the adjacent glutamine-to-alanine mutation at position 275 (Q275A). Both mutants displayed elevated tyrosine phosphorylation levels, prolonged nuclear accumulation, and increased transcriptional responses to interferon-γ (IFNγ) stimulation. No difference was observed between wild-type (WT) and mutant STAT1 in DNA sequence-specificity or dissociation kinetics from high-affinity DNA-binding elements known as gamma-activated sites (GAS). Furthermore, all variants exhibited similar cooperative DNA binding. Unexpectedly, in vitro dephosphorylation rates using the recombinant STAT1-inactivating Tc45 phosphatase in both the absence and presence of double-stranded GAS elements were similar in all STAT1 variants. Likewise, the rate of tyrosine phosphorylation by Janus kinase 2 (JAK2) was unaltered as compared to the WT molecule, excluding that the phenotype of these mutants is caused by either defective Tc45-catalyzed dephosphorylation or JAK2-induced hyper-activation. Interestingly, within 10 min of IFNγ exposure, the majority of R274W and Q275A molecules had entered the nucleus, whereas the wild-type protein remained predominantly cytosolic. Thus, the exchange of critical residues located at the binding interface in the antiparallel dimer conformer led to a premature accumulation of phospho-STAT1 in the nuclear compartment. In summary, our data show that the hyper-activity of the GOF mutations results, at least in part, from the premature nuclear import of the tyrosine-phosphorylated molecules and not from alterations in their phosphorylation or dephosphorylation rates.

A molecular switch from STAT2-IRF9 to ISGF3 underlies interferon-induced gene transcription

Cells maintain the balance between homeostasis and inflammation by adapting and integrating the activity of intracellular signaling cascades, including the JAK-STAT pathway. Our understanding of how a tailored switch from homeostasis to a strong receptor-dependent response is coordinated remains limited. Here, we use an integrated transcriptomic and proteomic approach to analyze transcription-factor binding, gene expression and in vivo proximity-dependent labelling of proteins in living cells under homeostatic and interferon (IFN)-induced conditions. We show that interferons (IFN) switch murine macrophages from resting-state to induced gene expression by alternating subunits of transcription factor ISGF3. Whereas preformed STAT2-IRF9 complexes control basal expression of IFN-induced genes (ISG), both type I IFN and IFN-γ cause promoter binding of a complete ISGF3 complex containing STAT1, STAT2 and IRF9. In contrast to the dogmatic view of ISGF3 formation in the cytoplasm, our results suggest a model wherein the assembly of the ISGF3 complex occurs on DNA.

Fine-Tuning Cytokine Signals

Cytokines are secreted or otherwise released polypeptide factors that exert autocrine and/or paracrine actions, with most cytokines acting in the immune and/or hematopoietic system. They are typically pleiotropic, controlling development, cell growth, survival, and/or differentiation. Correspondingly, cytokines are clinically important, and augmenting or attenuating cytokine signals can have deleterious or therapeutic effects. Besides physiological fine-tuning of cytokine signals, altering the nature or potency of the signal can be important in pathophysiological responses and can also provide novel therapeutic approaches. Here, we give an overview of cytokines, their signaling and actions, and the physiological mechanisms and pharmacologic strategies to fine-tune their actions. In particular, the differential utilization of STAT proteins by a single cytokine or by different cytokines and STAT dimerization versus tetramerization are physiological mechanisms of fine-tuning, whereas anticytokine and anticytokine receptor antibodies and cytokines with altered activities, including cytokine superagonists, partial agonists, and antagonists, represent new ways of fine-tuning cytokine signals.

Interferon signature in patients with STAT1 gain-of-function mutation is epigenetically determined

STAT1 gain-of-function (GOF) variants lead to defective Th17 cell development and chronic mucocutaneous candidiasis (CMC), but frequently also to autoimmunity. Stimulation of cells with STAT1 inducing cytokines like interferons (IFN) result in hyperphosphorylation and delayed dephosphorylation of GOF STAT1. However, the mechanism how the delayed dephosphorylation exactly causes the increased expression of STAT1-dependent genes, and how the intracellular signal transduction from cytokine receptors is affected, remains unknown. In this study we show that the circulating levels of IFN-α were not persistently elevated in STAT1 GOF patients. Nevertheless, the expression of interferon signature genes was evident even in the patient with low or undetectable serum IFN-α levels. Chromatin immunoprecipitation (ChIP) experiments revealed that the active chromatin mark trimethylation of lysine 4 of histone 3 (H3K4me3), was significantly enriched in areas associated with interferon-stimulated genes in STAT1 GOF cells in comparison to cells from healthy donors. This suggests that the chromatin binding of GOF STAT1 variant promotes epigenetic changes compatible with higher gene expression and elevated reactivity to type I interferons, and possibly predisposes for interferon-related autoimmunity. The results also suggest that epigenetic rewiring may be responsible for treatment failure of Janus kinase 1/2 (JAK1/2) inhibitors in certain patients.

Regulatory Networks Involving STATs, IRFs, and NFκB in Inflammation

Cells engaging in inflammation undergo drastic changes of their transcriptomes. In order to tailor these alterations in gene expression to the requirements of the inflammatory process, tight and coordinate regulation of gene expression by environmental cues, microbial or danger-associated molecules or cytokines, are mandatory. The transcriptional response is set off by signal-regulated transcription factors (SRTFs) at the receiving end of pathways originating at pattern recognition- and cytokine receptors. These interact with a genome that has been set for an appropriate response by prior activity of pioneer or lineage determining transcription factors (LDTFs). The same types of transcription factors are also critical determinants of the changes in chromatin landscapes and transcriptomes that specify potential consequences of inflammation: tissue repair, training, and tolerance. Here we focus on the role of three families of SRTFs in inflammation and its sequels: signal transducers and activators of transcription (STATs), interferon regulatory factors (IRFs), and nuclear factor κB (NFκB). We describe recent findings about their interactions and about their networking with LDTFs. Our aim is to provide a snapshot of a highly dynamic research area.

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.

Toxoplasma gondii stabilises tetrameric complexes of tyrosine-phosphorylated signal transducer and activator of transcription-1 and leads to its sustained and promiscuous DNA binding

Toxoplasma gondii is an obligate intracellular parasite that infects up to 30% of humans worldwide. It can lead to severe diseases particularly in individuals with immature or defective immune responses. Control of T. gondii relies on the IFN-γ-induced signal transducer and activator of transcription-1 (STAT1) pathway. T. gondii, however, largely inactivates STAT1-mediated gene transcription by T. gondii inhibitor of STAT1-dependent transcription (TgIST), a parasite effector protein binding to STAT1. Here, we have analysed requirements of STAT1 to bind TgIST and characterised downstream effects on STAT1 signalling. TgIST bound to STAT1 dimers but more efficiently assembled with STAT1 tetramers, which are essential for effective IFN-γ responsiveness. Such binding was abrogated in N-terminal, but not C-terminal deletion mutants of STAT1. Furthermore, TgIST did not bind to the STAT1^F77A substitution mutant that cannot form STAT1 tetramers, resulting in a complete unresponsiveness of parasite-infected STAT1^F77A -expressing cells to IFN-γ. Remarkably, binding of TgIST considerably increased the affinity of the aberrant STAT1 tetramers for DNA consensus sequence binding motifs and even enabled binding to nonconsensus sequences. Consistent with the increased DNA binding, STAT1 from parasite-infected cells remained phosphorylated at Tyr⁷⁰¹ and Ser⁷²⁷ and was retained within the nucleus in a DNA-bound state. The sustained and promiscuous binding activity particularly of STAT1 tetramers to unspecific DNA sites lacking a consensus STAT1-binding motif is an as yet unrecognised mechanism contributing to the defective IFN-γ-mediated signalling in T. gondii-infected cells.

"Do We Know Jack" About JAK? A Closer Look at JAK/STAT Signaling Pathway

Janus tyrosine kinase (JAK) family of proteins have been identified as crucial proteins in signal transduction initiated by a wide range of membrane receptors. Among the proteins in this family JAK2 has been associated with important downstream proteins, including signal transducers and activators of transcription (STATs), which in turn regulate the expression of a variety of proteins involved in induction or prevention of apoptosis. Therefore, the JAK/STAT signaling axis plays a major role in the proliferation and survival of different cancer cells, and may even be involved in resistance mechanisms against molecularly targeted drugs. Despite extensive research focused on the protein structure and mechanisms of activation of JAKs, and signal transduction through these proteins, their importance in cancer initiation and progression seem to be underestimated. This manuscript is an attempt to highlight the role of JAK proteins in cancer biology, the most recent developments in targeting JAKs, and the central role they play in intracellular cross-talks with other signaling cascades.

Cucurbitacin I inhibits STAT3, but enhances STAT1 signaling in human cancer cells in vitro through disrupting actin filaments

STAT1 and STAT3 are two important members of the STAT (signal transducers and activators of transcription) protein family and play opposing roles in regulating cancer cell growth. Suppressing STAT3 and/or enhancing STAT1 signaling are considered to be attractive anticancer strategies. Cucurbitacin I (CuI) isolated from the cucurbitacin family was reported to be an inhibitor of STAT3 signaling and a disruptor of actin cytoskeleton. In this study we investigated the function and mechanisms of CuI in regulating STAT signaling in human cancer cells in vitro. CuI (0.1-10 mmol/L) dose-dependently inhibited the phosphorylation of STAT3, and enhanced the phosphorylation of STAT1 in lung adenocarcinoma A549 cells possibly through disrupting actin filaments. We further demonstrated that actin filaments physically associated with JAK2 and STAT3 in A549 cells and regulated their phosphorylation through two signaling complexes, the IL-6 receptor complex and the focal adhesion complex. Actin filaments also interacted with STAT1 in A549 cells and regulated its dephosphorylation. Taken together, this study reveals the molecular mechanisms of CuI in the regulation of STAT signaling and in a possible inhibition of human cancer cell growth. More importantly, this study uncovers a novel role of actin and actin-associated signaling complexes in regulating STAT signaling.

Structural analysis of the STAT1:STAT2 heterodimer revealed the mechanism of Sendai virus C protein-mediated blockade of type 1 interferon signaling

Sendai virus (SeV), which causes respiratory diseases in rodents, possesses the C protein that blocks the signal transduction of interferon (IFN), thereby escaping from host innate immunity. We previously demonstrated by using protein crystallography that two molecules of Y3 (the C-terminal half of the C protein) can bind to the homodimer of the N-terminal domain of STAT1 (STAT1ND), elucidating the mechanism of inhibition of IFN-γ signal transduction. SeV C protein also blocks the signal transduction of IFN-α/β by inhibiting the phosphorylation of STAT1 and STAT2, although the mechanism for the inhibition is unclear. Therefore, we sought to elucidate the mechanism of inhibition of the IFN signal transduction via STAT1 and STAT2. Small angle X-ray scattering analysis indicated that STAT1ND associates with the N-terminal domain of STAT2 (STAT2ND) with the help of a Gly-rich linker. We generated a linker-less recombinant protein possessing a STAT1ND:STAT2ND heterodimeric structure via an artificial disulfide bond. Analytical size-exclusion chromatography and surface plasmon resonance revealed that one molecule of Y3 can associate with a linker-less recombinant protein. We propose that one molecule of C protein associates with the STAT1:STAT2 heterodimer, inducing a conformational change to an antiparallel form, which is easily dephosphorylated. This suggests that association of C protein with the STAT1ND:STAT2ND heterodimer is an important factor to block the IFN-α/β signal transduction.

Deciphering of the Human Interferon-Regulated Proteome by Mass Spectrometry-Based Quantitative Analysis Reveals Extent and Dynamics of Protein Induction and Repression

Interferons (IFNs) are pleotropic cytokines secreted upon encounter of pathogens and tumors. Applying their antipathogenic, antiproliferative, and immune stimulatory capacities, recombinant IFNs are frequently prescribed as drugs to treat different diseases. IFNs act by changing the gene expression profile of cells. Due to characteristics such as rapid gene induction and signaling, IFNs also represent prototypical model systems for various aspects of biomedical research (e.g., signal transduction). In regard to the signaling and activated promoters, IFNs can be subdivided into two groups. Here, alterations of the cellular proteome of human cells treated with IFNα and IFNγ were elucidated in a time-resolved manner by quantitative proteome analysis. The majority of protein regulations were strongly IFN type and time dependent. In addition to the expected upregulation of IFN-responsive proteins, an astonishing number of proteins became profoundly repressed especially by IFNγ. Thus, our comprehensive analysis revealed important insights into the human IFN-regulated proteome and its dynamics of protein induction and repression. Interestingly, the new class of IFN-repressed genes comprises known host factors for highly relevant pathogens such as HIV, dengue virus, and hepatitis C virus.

Molecular mechanism and structural basis of gain-of-function of STAT1 caused by pathogenic R274Q mutation

Gain-of-function (GOF) mutations in the STAT1 gene are critical for the onset of chronic mucocutaneous candidiasis (CMC) disease. However, the molecular basis for the gain of STAT1 function remains largely unclear. Here, we investigated the structural features of STAT1 GOF residues to better understand the impact of these pathogenic mutations. We constructed STAT1 alanine mutants of the α3 helix residues of the coiled-coil domain, which are frequently found in CMC pathogenic mutations, and measured their transcriptional activities. Most of the identified GOF residues were located inside the coiled-coil domain stem structure or at the protein surface of the anti-parallel dimer interface. Unlike those, Arg-274 was adjacent to the DNA-binding domain. In addition, Arg-274 was found to functionally interact with Gln-441 in the DNA-binding domain. Because Gln-441 is located at the anti-parallel dimer contact site, Gln-441 reorientation by Arg-274 mutation probably impedes formation of the dimer. Further, the statistical analysis of RNA-seq data with STAT1-deficient epithelial cells and primary T cells from a CMC patient revealed that the R274Q mutation affected gene expression levels of 66 and 76 non-overlapping RefSeq genes, respectively. Because their transcription levels were only slightly modulated by wild-type STAT1, we concluded that the R274Q mutation increased transcriptional activity but did not change dramatically the repertoire of STAT1 targets. Hence, we provide a novel mechanism of STAT1 GOF triggered by a CMC pathogenic mutation.

Alanine-scanning mutagenesis of human signal transducer and activator of transcription 1 to estimate loss- or gain-of-function variants

BACKGROUND

Germline heterozygous mutations in human signal transducer and activator of transcription 1 (STAT1) can cause loss of function (LOF), as in patients with Mendelian susceptibility to mycobacterial diseases, or gain of function (GOF), as in patients with chronic mucocutaneous candidiasis. LOF and GOF mutations are equally rare and can affect the same domains of STAT1, especially the coiled-coil domain (CCD) and DNA-binding domain (DBD). Moreover, 6% of patients with chronic mucocutaneous candidiasis with a GOF STAT1 mutation have mycobacterial disease, obscuring the functional significance of the identified STAT1 mutations. Current computational approaches, such as combined annotation-dependent depletion, do not distinguish LOF and GOF variants.

OBJECTIVE

We estimated variations in the CCD/DBD of STAT1.

METHODS

We mutagenized 342 individual wild-type amino acids in the CCD/DBD (45.6% of full-length STAT1) to alanine and tested the mutants for STAT1 transcriptional activity.

RESULTS

Of these 342 mutants, 201 were neutral, 30 were LOF, and 111 were GOF mutations in a luciferase assay. This assay system correctly estimated all previously reported LOF mutations (100%) and slightly fewer GOF mutations (78.1%) in the CCD/DBD of STAT1. We found that GOF alanine mutants occurred at the interface of the antiparallel STAT1 dimer, suggesting that they destabilize this dimer. This assay also precisely predicted the effect of 2 hypomorphic and dominant negative mutations, E157K and G250E, in the CCD of STAT1 that we found in 2 unrelated patients with Mendelian susceptibility to mycobacterial diseases.

CONCLUSION

The systematic alanine-scanning assay is a useful tool to estimate the GOF or LOF status and the effect of heterozygous missense mutations in STAT1 identified in patients with severe infectious diseases, including mycobacterial and fungal diseases.

Negative regulation of type I IFN signaling by phosphorylation of STAT2 on T387

The transcription factor ISGF3, comprised of IRF9 and tyrosine-phosphorylated STATs 1 and 2, transmits the signal from the type I interferon receptor to the genome. We have discovered a novel phosphorylation of STAT2 on T387 that negatively regulates this response. In most untreated cell types, the majority of STAT2 is phosphorylated on T387 constitutively. In response to interferon-β, the T387A mutant of STAT2 is much more effective than wild-type STAT2 in mediating the expression of many interferon-stimulated genes, in protecting cells against virus infection, and in inhibiting cell growth. Interferon-β-treated cells expressing wild-type STAT2 contain much less ISGF3 capable of binding to an interferon-stimulated response element than do cells expressing T387A STAT2. T387 lies in a cyclin-dependent kinase (CDK) consensus sequence, and CDK inhibitors decrease T387 phosphorylation. Using CDK inhibitors to reverse the constitutive inhibitory phosphorylation of T387 of U-STAT2 might enhance the efficacy of type I interferons in many different clinical settings.

Novel non-canonical role of STAT1 in Natural Killer cell cytotoxicity

STAT1 is an important regulator of NK cell maturation and cytotoxicity. Although the consequences of Stat1-deficiency have been described in detail the underlying molecular functions of STAT1 in NK cells are only partially understood. Here, we describe a novel non-canonical role of STAT1 that was unmasked in NK cells expressing a Stat1-Y701F mutant. This mutation prevents JAK-dependent phosphorylation, subsequent nuclear translocation and cytokine-induced transcriptional activity as verified by RNA-seq analysis. As expected Stat1-Y701F mice displayed impaired NK cell maturation comparable to Stat1^−/− animals. In contrast Stat1-Y701F NK cells exerted a significantly enhanced cytotoxicity in vitro and in vivo compared to Stat1^−/− NK cells in the absence of detectable transcriptional activity. We thus investigated the STAT1 interactome using primary NK cells derived from Stat1^ind mice that inducibly express a FLAG-tagged STAT1. Mass spectrometry revealed that STAT1 directly binds proteins involved in cell junction formation and proteins associated to membrane or membrane-bound vesicles. In line, immunofluorescence studies uncovered the recruitment of STAT1 to the target-cell interphase during NK cell killing. This led us to propose a novel function for STAT1 at the immunological synapse in NK cells regulating tumor surveillance and cytotoxicity.

Host Intracellular Signaling Events and Pro-inflammatory Cytokine Production in African Trypanosomiasis

Pathogens, such as bacteria, viruses, and parasites, possess specific molecules or proteins that are recognized by several host innate immune receptors, leading to the activation of several intracellular signaling molecules and pathways. The magnitude and quality of these events significantly affect the outcome of infection. African trypanosomes, including Trypanosoma congolense, are capable of manipulating the host immune response, including the activity of macrophages, which are the key immune cells that contribute to the immunopathogenesis of African trypanosomiasis. Although it is known that immune hyperactivation and excessive pro-inflammatory cytokine production are the hallmarks of African trypanosomiasis, the mechanisms through which these events are triggered are poorly defined. However, it is known that macrophages may play a significant role in these processes, because phagocytosis of trypanosomes by macrophages initiates intracellular signal transduction cascades that lead to the release of pro-inflammatory cytokines and alteration in cell function. This review highlights recent progress in our understanding of the innate immune receptors, signaling pathways, and transcription factors involved in T. congolense-induced pro-inflammatory cytokine production in macrophages. It will reveal the existence of complex signaling events through which the parasite modulates the host immune response, thus identifying novel targets that could aid in designing strategies to effectively control the disease.

Common variable immunodeficiency, impaired neurological development and reduced numbers of T regulatory cells in a 10-year-old boy with a STAT1 gain-of-function mutation

Recently, gain-of-function (GOF) mutations in the gene encoding signal transducer and activator of transcription 1 (STAT1) have been associated with chronic mucocutaneous candidiasis (CMC). This case report describes a 10-year-old boy presenting with signs of common variable immunodeficiency (CVID), failure to thrive, impaired neurological development, and a history of recurrent mucocutaneous Candida infections. Sequencing of the STAT1 gene identified a heterozygous missense mutation in exon 7 encoding the STAT1 coiled-coil domain (c.514T>C, p.Phe172Leu). In addition to hypogammaglobulinemia with B-cell deficiency, and a low percentage of Th17 cells, immunological analysis of the patient revealed a marked depletion of forkhead-box P3(+)-expressing regulatory T cells (Tregs). In vitro stimulation of T cells from the patient with interferon-α (IFNα) and/or IFNɣ resulted in a significantly increased expression of STAT1-regulated target genes such as MIG1, IRF1, MX1, MCP1/CCL2, IFI-56K, and CXCL10 as compared to IFN-treated cells from a healthy control, while no IFNα/ɣ-mediated up-regulation of the FOXP3 gene was found. These data demonstrate that the STAT1 GOF mutation F172L, which results in impaired stability of the antiparallel STAT1 dimer conformation, is associated with inhibited Treg cell development and neurological symptoms.

Dissecting functions of the N-terminal domain and GAS-site recognition in STAT3 nuclear trafficking

Signal transducer and activator of transcription 3 (STAT3) is a ubiquitous transcription factor involved in many biological processes, including hematopoiesis, inflammation and cancer progression. Cytokine-induced gene transcription greatly depends on tyrosine phosphorylation of STAT3 on a single tyrosine residue with subsequent nuclear accumulation and specific DNA sequence (GAS) recognition. In this study, we analyzed the roles of the conserved STAT3 N-terminal domain (NTD) and GAS-element binding ability of STAT3 in nucleocytoplasmic trafficking. Our results demonstrate the nonessential role of GAS-element recognition for both cytokine-induced and basal nuclear import of STAT3. Substitution of five key amino acids within the DNA-binding domain rendered STAT3 unable to bind to GAS-elements while still maintaining the ability for nuclear localization. In turn, deletion of the NTD markedly decreased nuclear accumulation upon IL-6 treatment resulting in a prolonged accumulation of phosphorylated dimers in the cytoplasm, at the same time preserving specific DNA recognition ability of the truncation mutant. Observed defect in nuclear localization could not be explained by flawed importin-α binding, since both wild-type and NTD deletion mutant of STAT3 could precipitate both full-length and autoinhibitory domain (∆IBB) deletion mutants of importin-α5, as well as ∆IBB-α3 and ∆IBB-α7 isoforms independently of IL-6 stimulation. Despite its inability to translocate to the nucleus upon IL-6 stimulation, the NTD lacking mutant still showed nuclear accumulation in resting cells similar to wild-type upon inhibition of nuclear export by leptomycin B. At the same time, blocking the nuclear export pathway could not rescue cytoplasmic trapping of phosphorylated STAT3 molecules without NTD. Moreover, STAT3 mutant with dysfunctional SH2 domain (R609Q) also localized in the nucleus of unstimulated cells after nuclear export blocking, while upon cytokine treatment the subcellular localization of this mutant had not changed. Our findings support the concept that basal nucleocytoplasmic shuttling of STAT3 is different from active cytokine-induced nuclear import and does not require conserved N- or SH2-terminal domains, preformed dimer formation and GAS-element-specific DNA recognition.

The unique role of STAT2 in constitutive and IFN-induced transcription and antiviral responses

In the canonical pathway of IFN-I-mediated signaling, phosphorylation of STAT1 and STAT2 leads to heterodimerization and interaction with IRF9. This complex, also known as IFN-stimulated gene factor 3 (ISGF3), then translocates into the nucleus and binds the IFN-I-stimulated response element (ISRE) leading to the activation of transcription of over 300 interferon stimulated genes (ISGs). In addition, STAT1 homodimers [known as γ-activated factor (GAF)] are formed and translocate to the nucleus, where they target genes containing the γ-activated sequence (GAS). The primary function of ISGF3 is to mediate a rapid and robust IFN-I activated response by regulating transient transcription of antiviral ISGs. This requires the quick assembly of ISGF3 from its pre-existing components STAT1, STAT2 and IRF9 and transport to the nucleus to bind ISRE-containing ISGs. The exact events that take place in formation, nuclear translocation and DNA-binding of active ISGF3 are still not clear. Over the years many studies have provided evidence for the existence of a multitude of alternative STAT2-containing (ISRE or GAS-binding) complexes involved in IFN-I signaling, emphasizing the importance of STAT2 in the regulation of specific IFN-I-induced transcriptional programs, independent of its involvement in the classical ISGF3 complex. This review describes the unique role of STAT2 in differential complex formation of unphosphorylated and phosphorylated ISGF3 components that direct constitutive and IFN-I-stimulated transcriptional responses. In addition, we highlight the existence of a STAT1-independent IFN-I signaling pathway, where STAT2/IRF9 can potentially substitute for the role of ISGF3 and offer a back-up response against viral infection.

Induction of dissociated cytokine profiles by ISRAA with selective critical involvement of ERK1/2 in its signaling functions

The immune system-released activating agent (ISRAA) is an immune mediator activated as a result of a nerve stimulus initiated by immune challenge. We have previously demonstrated that ISRAA and tumor necrosis factor (TNF) receptor 1 (TNFR1) share an interspecies-conserved motif (72% homology) that induces the apoptosis and proliferation of human peripheral blood mononuclear cells (hPBMCs) in a dose-dependent manner. In the present study, cytokine profiles were examined in response to the stimulation of hPBMCs with ISRAA. Furthermore, the signaling pathways induced by ISRAA were mapped. The results revealed high measurable levels of TNF-α, interleukin (IL)-6, IL-8, IL-10 and interferon (IFN)-γ, but not IL-4, IL-17 (IL-17A) or transforming growth factor (TGF)-β. The analysis of signaling pathways revealed the activation of extracellular-regulated protein kinase (ERK)1/2 as a downstream signal in the mitogen‑activated protein kinase (MAPK) pathway during TNF‑α and IL-6 production and apoptosis, but not during proliferation following stimulation with ISRAA by triggering the Fas-associated protein with death domain (FADD). STAT3 was found to be unphosphorylated in the ISRAA‑stimulated hPBMCs, and STAT3 was ubiquitously expressed in unstimulated cells, suggesting that ISRAA has a protein inhibitor of activated STAT (PIAS)-like activity, by functioning as a negative regulator of the effects of STAT3 on the Janus kinase (JAK)/STAT pathway. The determination of the nature of cytokine responses together with the signaling pathways of cellular activity induced by ISRAA paves the way for the investigation of a potential target of ISRAA and for the development of novel therapeutic approaches for the treatment of immune-regulated disorders.