Biomolecular interaction analysis of IFN gamma-induced signaling events in whole-cell lysates: prevalence of latent STAT1 in high-molecular weight complexes

The increase of long noncoding RNA Fendrr in hepatocytes contributes to liver fibrosis by promoting IL-6 production

Liver fibrosis/cirrhosis is a pathological state caused by excessive extracellular matrix deposition. Sustained activation of hepatic stellate cells (HSC) is the predominant cause of liver fibrosis, but the detailed mechanism is far from clear. In this study, we found that long noncoding RNA Fendrr is exclusively increased in hepatocytes in the murine model of CCl4- and bile duct ligation-induced liver fibrosis, as well as in the biopsies of liver cirrhosis patients. In vivo, ectopic expression of Fendrr aggravated the severity of CCl4-induced liver fibrosis in mice. In contrast, inhibiting Fendrr blockaded the activation of HSC and ameliorated CCl4-induced liver fibrosis. Our mechanistic study showed that Fendrr binds to STAT2 and enhances its enrichment in the nucleus, which then promote the expression of IL-6, and, ultimately, activates HSC in a paracrine manner. Accordingly, disrupting the interaction between Fendrr and STAT2 by ectopic expression of a STAT2 mutant attenuated the pro-fibrotic response inspired by Fendrr in the CCl4-induced liver fibrosis. Notably, the increase of Fendrr in patient fibrotic liver is positively correlated with the severity of fibrosis and the expression of IL-6. Meanwhile, hepatic IL-6 positively correlates with the extent of liver fibrosis and HSC activation as well, thus suggesting a causative role of Fendrr in HSC activation and liver fibrosis. In conclusion, these observations identify an important regulatory crosstalk between hepatocyte Fendrr and HSC activation in the progression of liver fibrosis, which might represent a potential strategy for therapeutic intervention.

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.

Caveolae Microdomains Mediate STAT5 Signaling Induced by Insulin in MCF-7 Breast Cancer Cells

Caveolae are small plasma membrane invaginations constituted for membrane proteins namely caveolins and cytosolic proteins termed cavins, which can occupy up to 50% of the surface of mammalian cells. The caveolae have been involved with a variety of cellular processes including regulation of cellular signaling. Insulin is a hormone that mediates a variety of physiological processes through activation of insulin receptor (IR), which is a tyrosine kinase receptor expressed in all mammalian tissues. Insulin induces activation of signal transducers and activators of transcription (STAT) family members including STAT5. In this study, we demonstrate, for the first time, that insulin induces phosphorylation of STAT5 at tyrosine-694 (STAT5-Tyr(P)⁶⁹⁴), STAT5 nuclear accumulation and an increase in STAT5-DNA complex formation in MCF-7 breast cancer cells. Insulin also induces nuclear accumulation of STAT5-Tyr(P)⁶⁹⁴, caveolin-1, and IR in MCF-7 cells. STAT5 nuclear accumulation and the increase of STAT5-DNA complex formation require the integrity of caveolae and microtubule network. Moreover, insulin induces an increase and nuclear accumulation of STAT5-Tyr(P)⁶⁹⁴ in MDA-MB-231 breast cancer cells. In conclusion, results demonstrate that caveolae and microtubule network play an important role in STAT5-Tyr(P)⁶⁹⁴, STAT5 nuclear accumulation and STAT5-DNA complex formation induced by insulin in breast cancer cells.

STAT1 and Its Crucial Role in the Control of Viral Infections

The signal transducer and activator of transcription (STAT) 1 protein plays a key role in the immune response against viruses and other pathogens by transducing, in the nucleus, the signal from type I, type II and type III IFNs. STAT1 activates the transcription of hundreds of genes, some of which have been well characterized for their antiviral properties. STAT1 gene deletion in mice and complete STAT1 deficiency in humans both cause rapid death from severe infections. STAT1 plays a key role in the immunoglobulin class-switch recombination through the upregulation of T-bet; it also plays a key role in the production of T-bet+ memory B cells that contribute to tissue-resident humoral memory by mounting an IgG response during re-infection. Considering the key role of STAT1 in the antiviral immune response, many viruses, including dangerous viruses such as Ebola and SARS-CoV-2, have developed different mechanisms to inhibit this transcription factor. The search for drugs capable of targeting the viral proteins implicated in both viral replication and IFN/STAT1 inhibition is important for the treatment of the most dangerous viral infections and for future viral pandemics, as shown by the clinical results obtained with Paxlovid in patients infected with SARS-CoV-2.

Stage-specific IFN-induced and IFN gene expression reveal convergence of type I and type II IFN and highlight their role in both acute and chronic stage of pathogenic SIV infection

Interferons (IFNs) play a major role in controlling viral infections including HIV/SIV infections. Persistent up-regulation of interferon stimulated genes (ISGs) is associated with chronic immune activation and progression in SIV/HIV infections, but the respective contribution of different IFNs is unclear. We analyzed the expression of IFN genes and ISGs in tissues of SIV infected macaques to understand the respective roles of type I and type II IFNs. Both IFN types were induced in lymph nodes during early stage of primary infection and to some extent in rectal biopsies but not in PBMCs. Induction of Type II IFN expression persisted during the chronic phase, in contrast to undetectable induction of type I IFN expression. Global gene expression analysis with a major focus on ISGs revealed that at both acute and chronic infection phases most differentially expressed ISGs were inducible by both type I and type II IFNs and displayed the highest increases, indicating strong convergence and synergy between type I and type II IFNs. The analysis of functional signatures of ISG expression revealed temporal changes in IFN expression patterns identifying phase-specific ISGs. These results suggest that IFN-γ strongly contribute to shape ISG upregulation in addition to type I IFN.

Interferon Regulatory Factor 1 Protects against Chikungunya Virus-Induced Immunopathology by Restricting Infection in Muscle Cells

The innate immune system protects cells against viral pathogens in part through the autocrine and paracrine actions of alpha/beta interferon (IFN-α/β) (type I), IFN-γ (type II), and IFN-λ (type III). The transcription factor interferon regulatory factor 1 (IRF-1) has a demonstrated role in shaping innate and adaptive antiviral immunity by inducing the expression of IFN-stimulated genes (ISGs) and mediating signals downstream of IFN-γ. Although ectopic expression experiments have suggested an inhibitory function of IRF-1 against infection of alphaviruses in cell culture, its role in vivo remains unknown. Here, we infected Irf1 ^-/- mice with two distantly related arthritogenic alphaviruses, chikungunya virus (CHIKV) and Ross River virus (RRV), and assessed the early antiviral functions of IRF-1 prior to induction of adaptive B and T cell responses. IRF-1 expression limited CHIKV-induced foot swelling in joint-associated tissues and prevented dissemination of CHIKV and RRV at early time points. Virological and histological analyses revealed greater infection of muscle tissues in Irf1 ^-/- mice than in wild-type mice. The antiviral actions of IRF-1 appeared to be independent of the induction of type I IFN or the effects of type II and III IFNs but were associated with altered local proinflammatory cytokine and chemokine responses and differential infiltration of myeloid cell subsets. Collectively, our in vivo experiments suggest that IRF-1 restricts CHIKV and RRV infection in stromal cells, especially muscle cells, and that this controls local inflammation and joint-associated swelling.IMPORTANCE Interferon regulatory factor 1 (IRF-1) is a transcription factor that regulates the expression of a broad range of antiviral host defense genes. In this study, using Irf1 ^-/- mice, we investigated the role of IRF-1 in modulating pathogenesis of two related arthritogenic alphaviruses, chikungunya virus and Ross River virus. Our studies show that IRF-1 controlled alphavirus replication and swelling in joint-associated tissues within days of infection. Detailed histopathological and virological analyses revealed that IRF-1 preferentially restricted CHIKV infection in cells of nonhematopoietic lineage, including muscle cells. The antiviral actions of IRF-1 resulted in decreased local inflammatory responses in joint-associated tissues, which prevented immunopathology.

Diptoindonesin G promotes ERK-mediated nuclear translocation of p-STAT1 (Ser727) and cell differentiation in AML cells

Exploration of a new differentiation therapy that extends the range of differentiation for treating acute myeloid leukemia (AML) is attractive to researchers and clinicians. Here we report that diptoindonesin G (Dip G), a natural resveratrol aneuploid, exerts antiproliferative activity by inducing G2/M phase arrest and cell differentiation in AML cell lines and primary AML cells. Gene-profiling experiments showed that treating human leukemia HL-60 cells with Dip G was associated with a remarkable upregulation of STAT1 target gene expression, including IFIT3 and CXCL10. Mechanistically, Dip G activated ERK, which caused phosphorylation of STAT1 at Ser727 and selectively enhanced the interaction of p-STAT1 (Ser727) and p-ERK, further promoting their nuclear translocation. The nuclear translocation of p-STAT1 and p-ERK enhanced the transactivation of STAT1-targeted genes in AML cells. Furthermore, in vivo treatment of HL-60 xenografts demonstrated that Dip G significantly inhibited tumor growth and reduced tumor weight by inducing cell differentiation. Taken together, these results shed light on an essential role for ERK-mediated nuclear translocation of p-STAT1 (Ser727) and its full transcriptional activity in Dip G-induced differentiation of AML cells. Furthermore, these results demonstrate that Dip G could be used as a differentiation-inducing agent for AML therapy, particularly for non-acute promyelocytic leukemia therapy.

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.

Multiple transcription factor families regulate axon growth and regeneration

Understanding axon regenerative failure remains a major goal in neuroscience, and reversing this failure remains a major goal for clinical neurology. Although an inhibitory central nervous system environment clearly plays a role, focus on molecular pathways within neurons has begun to yield fruitful insights. Initial steps forward investigated the receptors and signaling pathways immediately downstream of environmental cues, but recent work has also shed light on transcriptional control mechanisms that regulate intrinsic axon growth ability, presumably through whole cassettes of gene target regulation. Here we will discuss transcription factors that regulate neurite growth in vitro and in vivo, including p53, SnoN, E47, cAMP-responsive element binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cell (NFAT), c-Jun activating transcription factor 3 (ATF3), sex determining region Ybox containing gene 11 (Sox11), nuclear factor κ-light chain enhancer of activated B cells (NFκB), and Krüppel-like factors (KLFs). Revealing the similarities and differences among the functions of these transcription factors may further our understanding of the mechanisms of transcriptional regulation in axon growth and regeneration.

Signal transducers and activators of transcription-from cytokine signalling to cancer biology

Signal transducers and activators of transcription (STATs) are, as the name indicates, both signal transducers and transcription factors. STATs are activated by cytokines and some growth factors and thus control important biological processes. These include cell growth, cell differentiation, apoptosis and immune responses. Dysregulation of STATs, either due to constitutive activation or function impairment, can have, therefore, deleterious biological consequences. This review places particular emphasis on their structural organization, biological activities and regulatory mechanisms most commonly utilized by cells to control STAT-mediated signalling. STATs also play important roles in cancer and immune deficiencies and are thus being exploited as therapeutic targets.

Structural characterization of unphosphorylated STAT5a oligomerization equilibrium in solution by small-angle X-ray scattering

Signal transducer and activator of transcription (STAT) proteins play a crucial role in the activation of gene transcription in response to extracellular stimuli. The regulation and activity of these proteins require a complex rearrangement of the domains. According to the established models, based on crystallographic data, STATs convert from a basal antiparallel inactive dimer into a parallel active one following phosphorylation. The simultaneous analysis of small-angle X-ray scattering data measured at different concentrations of unphosphorylated human STAT5a core domain unambiguously identifies the simultaneous presence of a monomer and a dimer. The dimer is the minor species but could be structurally characterized by SAXS in the presence of the monomer using appropriate computational tools and shown to correspond to the antiparallel assembly. The equilibrium is governed by a moderate dissociation constant of K(d) approximately 90 microM. Integration of these results with previous knowledge of the N-terminal domain structure and dissociation constants allows the modeling of the full-length protein. A complex network of intermolecular interactions of low or medium affinity is suggested. These contacts can be eventually formed or broken to trigger the dramatic modifications in the dimeric arrangement needed for STAT regulation and activity.

Proinflammatory cytokines and HIV-1 synergistically enhance CXCL10 expression in human astrocytes

HIV encephalitis (HIVE), the pathologic correlate of HIV-associated dementia (HAD) is characterized by astrogliosis, cytokine/chemokine dysregulation, and neuronal degeneration. Increasing evidence suggests that inflammation is actively involved in the pathogenesis of HAD. In fact, the severity of HAD/HIVE correlates more closely with the presence of activated glial cells than with the presence and amount of HIV-infected cells in the brain. Astrocytes, the most numerous cell type within the brain, provide an important reservoir for the generation of inflammatory mediators, including interferon-gamma inducible peptide-10 (CXCL10), a neurotoxin and a chemoattractant, implicated in the pathophysiology of HAD. Additionally, the proinflammatory cytokines, IFN-gamma and TNF-alpha, are also markedly increased in CNS tissues during HIV-1 infection. In this study, we hypothesized that the interplay of host cytokines and HIV-1 could lead to enhanced expression of the toxic chemokine, CXCL10. Our findings demonstrate a synergistic induction of CXCL10 mRNA and protein in human astrocytes exposed to HIV-1 and the proinflammatory cytokines. Signaling molecules, including JAK, STATs, MAPK (via activation of Erk1/2, AKT, and p38), and NF-kappaB were identified as instrumental in the synergistic induction of CXCL10. Understanding the mechanisms involved in HIV-1 and cytokine-mediated up-regulation of CXCL10 could aid in the development of therapeutic modalities for HAD.

Down modulation of IFN-gamma signaling in alveolar macrophages isolated from smokers

The master cytokine, IFN-gamma possesses a wide spectrum of biological effects and is crucial for development of the highly activated macrophage phenotype characteristically found during inflammation. However, no data exists regarding the potential influence of cigarette smoke on the status of the expression of the cell surface receptor for IFN-gamma (IFN-gammaR) on alveolar macrophages (AM) of smokers. Here in, we report reduction in the expression of the IFN-gammaR alpha-chain on AM of cigarette smokers, when compared with non-smokers. Ensuing from the loss of receptor expression on the AM of smokers there was a decrease in IFN-gamma-mediated cell signaling. This included a decrease in the phosphorylation of signal transducer and activator of transcription (STAT)-1 and induction of interferon regulatory factor (IRF)-1. Further, diminished activation/induction of transcription factors did not appear to result from induction of known members of the 'suppressors of cytokine signaling (SOCS)' family. Decreased IFN-gamma signal transduction in AM from smokers may have an important implication regarding the use of therapeutic IFN-gamma in the lungs of patients that develop respiratory disorders as a result of tobacco use.

Paradigm shifts in the cell biology of STAT signaling

In recent years several of the key tenets of the original cytokine-STAT-signaling paradigm had to be revised. First, the notion that nonphosphorylated "inactive" STATs are present in the cytoplasm as free monomers which dimerized only subsequent to Tyr-phosphorylation has been replaced by the understanding that nonphosphorylated STATs in the cytoplasm exist largely as dimers and high molecular mass "statosome" complexes. Second, the notion that phosphorylation, either of Tyr or Ser residues or both, in STAT species is required for transcriptional activation has been replaced by the realization that nonphosphorylated STATs can be transcriptionally active albeit with respect to sets of target genes distinct from phosphorylated STATs. Third, the notion that it is the activation by phosphorylation of STATs at the plasma membrane that then leads to their import into the nucleus has been replaced by the recognition that even nonphosphorylated STATs shuttle between the cytoplasm and nucleus at all times in a constitutive manner. Fourth, the notion that the trans-cytoplasmic transit of STATs from the plasma membrane to the nuclear import machinery takes place exclusively as a free cytosolic process has been replaced by the understanding that at least a portion of this trans-cytoplasmic transit is mediated via membrane-associated caveolar and endocytic trafficking (the "signaling endosome" hypothesis). Fifth, the targeting and sequestration of activated STAT3 to long-lived endosomes in the cytoplasm requires consideration of STAT3-mediated "signal transduction" from the plasma membrane to cytoplasmic membrane destinations potentially for function(s) in the cytoplasm. Indeed, in tissue sections many discrete histologic cell types display PY-STAT3 almost exclusively in the cytoplasm with little, if any, in the nucleus. New challenges include determining the structural bases for the recruitment of nonphosphorylated dimeric STAT species to the cytosolic face of membranes including at the cytoplasmic tails of respective receptor complexes, the conformational changes subsequent to phosphorylation and the structural bases for the targeting and functions of STAT proteins within the cytoplasm per se.

Cytoplasmic provenance of STAT3 and PY-STAT3 in the endolysosomal compartments in pulmonary arterial endothelial and smooth muscle cells: implications in pulmonary arterial hypertension

Lung vascular lesions in pulmonary arterial hypertension (PAH) are characterized by enlarged, vacuolated ("megalocytotic") pulmonary arterial endothelial (PAEC) and smooth muscle cells (PASMC). We have recently proposed that dysfunction of vesicle tethers, soluble N-ethylmaleimide-sensitive factor attachment proteins (SNAPs), and SNAP receptors (SNAREs), leading to disruptions of intracellular trafficking in the Golgi to plasma membrane (centrifugal) and the plasma membrane to cell interior (centripetal) directions is a key causal mechanism in this disease. In PAH, there was a reciprocal relationship between loss of caveolin-1 (cav-1) in PAECs and increased expression of "activated" tyrosine-phosphorylated STAT3 (PY-STAT3) associated with a block in centrifugal trafficking to/through the Golgi organelle. In the present study, we investigated 1) whether centripetal trafficking of STAT3 and PY-STAT3 in PAECs and PASMCs was membrane-associated, and 2) whether this might be affected in PAH. Immunofluorescence and live cell imaging studies showed that, in both PAEC and PASMC, STAT3 was associated with cytoplasmic vesicles partially colocalizing with markers of the endolysosomal compartments (clathrin, EEA1, Rab5, Rab11, and LAMP1). Overexpression of cav-1 increased the targeting of STAT3 to lysosomes and inhibited STAT3 transcriptional activity. Exposure of PAECs to monocrotaline (MCT) pyrrole, which causes PAH in the rat, led to a loss of caveolar STAT3 with increased sequestration of STAT3 and PY-STAT3 in endosomes. In vivo, marked cytoplasmic sequestration of activated PY-STAT3 was a common feature in PAEC in the rat/MCT model and in cells in the proliferative arterial and plexiform lesions in PAH in humans. These data highlight the epigenetic regulation of centripetal cytokine and growth-factor signaling pathways and its modulation in PAH.

PDGF synergistically enhances IFN-gamma-induced expression of CXCL10 in blood-derived macrophages: implications for HIV dementia

There is increasing cumulative evidence that activated mononuclear phagocytes (macrophages/microglia) releasing inflammatory mediators in the CNS are a better correlate of HIV-associated dementia (HAD) than the actual viral load in the brain. Earlier studies on simian HIV/rhesus macaque model of NeuroAIDS confirmed that pathological changes in brains of macaques with encephalitis were associated with up-regulation of platelet-derived growth factor (PDGF) and the chemokine, CXCL10. Because the complex interplay of inflammatory mediators released by macrophages often leads to the induction of neurotoxins in HAD, we hypothesized that PDGF could interact with IFN-gamma to modulate the expression of CXCL10 in these primary virus target cells. Although PDGF alone had no effect on the induction of CXCL10 in human macrophages, in conjunction with IFN-gamma, it significantly augmented the expression of CXCL10 RNA & protein through transcriptional and posttranscriptional mechanisms. Signaling molecules, such as JAK and STATs, PI3K, MAPK, and NF-kappaB were found to play a role in the synergistic induction of CXCL10. Furthermore, PDGF via its activation of p38 MAPK was able to increase the stability of IFN-gamma-induced CXCL10 mRNA. Understanding the mechanisms involved in the synergistic up-regulation of CXCL10 could aid in the development of therapeutic modalities for HAD.

STAT3 expression in salivary gland tumours

The aim of the present study was to evaluate the signal transducer and activator of transcription (STAT3) expression, which is constitutively active in different types of malignant tumours, in salivary gland tumours. Fifty biopsies of salivary gland tumours (9 pleomorphic adenomas, 12 adenoid cystic carcinomas, 7 epithelial-myoepithelial carcinomas, 10 polymorphous low-grade adenocarcinomas and 12 mucoepidermoid carcinomas) and 10 normal salivary glands were immunohistochemically labeled for STAT3 and Phospho-STAT3 (STAT3P). The labeled sections were qualitatively and quantitatively evaluated. The results showed that, in normal salivary gland, STAT3 was expressed in cytoplasm and STAT3P in nuclei of all tissue cells, except in large mucous acinar cells for which both antibodies were negative. In pleomorphic adenoma, the expression was the same as in normal glands. In malignant tumours, there were variations in the expression of these antibodies. The most important one was the presence of STAT3 in the nuclei of the malignant tumour cells, most evident in the cribriform-type of adenoid cystic carcinoma. Both loss and variation of STAT3P expression were also observed. The presence of STAT3 in the nuclei of malignant salivary gland tumours may represent an important event in oncogenesis probably contributing to tumour cell proliferation while blocking apoptosis. However, further investigation will be necessary to support this hypothesis.

Live cell imaging of interleukin-6-induced targeting of "transcription factor" STAT3 to sequestering endosomes in the cytoplasm

Signal transducer and activator of transcription (STAT) family transcription factors are classically viewed as transducing cytokine- and growth factor-activated signals from the plasma membrane to the cell nucleus for the purpose of activating transcription. We report live cell imaging studies of fluorescently labeled STAT3 expressed in Hep3B hepatocytes that reveal interleukin (IL)-6-activated targeting of STAT3 and PY-STAT3 to relatively long-lived sequestering endosomes in the cytoplasm. This targeting was rapid but transient, required phosphorylation and integrity of Tyr 705 in STAT3, and was blocked by nocodazole, geldanamycin, and indirubin E804 and by overexpression of wild-type caveolin-1. Strikingly, overexpression of the dominant-negative (DN) mutant K44A of the GTPase dynamin II led to marked constitutive accumulation of STAT3 in the endocytic compartment with depletion of the STAT3 nuclear pool. Subsets of the native and K44A-generated STAT3- and PY-STAT3-sequestering endosomes colocalized with MyD88, an adapter protein that integrates pathways of Toll-like receptor and IL-1 transcriptional signaling and stabilization of mRNAs. These data provide direct evidence for the cytokine-induced "signal transduction" by STAT3 from the plasma membrane to a cytoplasmic membrane destination for yet to be elucidated function(s) in the cytoplasm including prolongation of signaling and/or cross talk.

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

We report experiments that infer a radical reorientation of tyrosine-phosphorylated parallel STAT1 dimers to an antiparallel form. Such a change in structure allows easy access to a phosphatase. With differentially epitope-tagged molecules, we show that the two monomers of a dimer remain together during dephosphorylation although they most likely undergo spatial reorientation. Extensive single amino acid mutagenesis within crystallographically established domains, manipulation of amino acids in an unstructured tether that connects the N-terminal domain (ND) to the core of the protein, and the demonstration that overexpressed ND can facilitate dephosphorylation of a core molecule lacking an ND all support this model: When the tyrosine-phosphorylated STAT1 disengages from DNA, the ND dimerizes and somehow assists in freeing the reciprocal pY-SH2 binding between the monomers of the dimer while ND ND dimerization persists. The core of the monomers rotate allowing reciprocal association of the coiled:coil and DNA-binding domains to present pY at the two ends of an antiparallel dimer for ready dephosphorylation.

Novel STAT1 alleles in otherwise healthy patients with mycobacterial disease

The transcription factor signal transducer and activator of transcription-1 (STAT1) plays a key role in immunity against mycobacterial and viral infections. Here, we characterize three human STAT1 germline alleles from otherwise healthy patients with mycobacterial disease. The previously reported L706S, like the novel Q463H and E320Q alleles, are intrinsically deleterious for both interferon gamma (IFNG)–induced gamma-activating factor–mediated immunity and interferon alpha (IFNA)–induced interferon-stimulated genes factor 3–mediated immunity, as shown in STAT1-deficient cells transfected with the corresponding alleles. Their phenotypic effects are however mediated by different molecular mechanisms, L706S affecting STAT1 phosphorylation and Q463H and E320Q affecting STAT1 DNA-binding activity. Heterozygous patients display specifically impaired IFNG-induced gamma-activating factor–mediated immunity, resulting in susceptibility to mycobacteria. Indeed, IFNA-induced interferon-stimulated genes factor 3–mediated immunity is not affected, and these patients are not particularly susceptible to viral disease, unlike patients homozygous for other, equally deleterious STAT1 mutations recessive for both phenotypes. The three STAT1 alleles are therefore dominant for IFNG-mediated antimycobacterial immunity but recessive for IFNA-mediated antiviral immunity at the cellular and clinical levels. These STAT1 alleles define two forms of dominant STAT1 deficiency, depending on whether the mutations impair STAT1 phosphorylation or DNA binding.

Mendelian susceptibility to mycobacterial disease is a rare syndrome. It is defined by the occurrence of severe disease caused by low virulence mycobacteria in otherwise healthy individuals, in whom antiviral immune response is not affected. Eleven known genetic defects, affecting five genes, have been involved in this type of deficient response to infection, involving immune-mediator molecules IL12 and interferon gamma: IL12B, IL12RB1, IFNGR1, IFNGR2, and STAT1. The signal transducer and activator of transcription-1 (STAT1) amino acid change L706S was previously shown to cause disease by impairing STAT1 phosphorylation. Here, we report two new STAT1 mutations that impair STAT1 DNA-binding activity. We show, by functional analysis of the three STAT1 mutant alleles, that they are intrinsically deleterious for both interferon gamma–induced antimycobacterial immunity, which is mediated through gamma-activated factor and for interferon alpha–induced antiviral immunity, which is mediated through interferon-stimulated genes factor 3. Interestingly, the three alleles are dominant for interferon gamma–induced gamma-activated factor–mediated antimycobacterial immunity, but recessive for interferon alpha–induced interferon-stimulated genes factor 3–mediated antiviral immunity at the cellular and clinical levels. These two new STAT1 alleles, which affect the binding of STAT1 to DNA, define distinct novel genetic causes of Mendelian susceptibility to mycobacterial disease and provide further insight into the molecular mechanism of disease.

A role for the cytoskeleton in STAT5 activation in MCF7 human breast cancer cells stimulated with EGF

A rapid increase in the tyrosine phosphorylation of signal transducers and activators of transcription (STAT) proteins has been extensively documented in cells stimulated with cytokines and growth factors. However, the mechanisms by which these transcription factors translocate to the nucleus have not been studied in detail. Our results demonstrate that stimulation of MCF7 cells with epidermal growth factor (EGF) promoted an increase in the phosphorylation of STAT5 at Tyr-694, as revealed by site-specific antibodies that recognized the phosphorylated state of this residue. In addition, EGF stimulated STAT5 nuclear translocation and an increased in STAT5 DNA binding activity. Prevention of microtubules and microfilaments polymerization induced a partial inhibition of STAT5 nuclear translocation and STAT5 DNA binding activity. However, STAT5 phosphorylation at Tyr-694 was dependent on the integrity of microtubule network and it was independent of the integrity of actin cytoskeleton. Furthermore, EGF induced the formation of the associations STAT5-tubulin and STAT5-kinesin heavy chain in a fashion dependent of cytoskeleton integrity. In summary, our results demonstrate, for the first time, that cytoskeleton plays an important role in STAT5 activation and translocation into the nucleus in MCF7 cells stimulated with EGF.

Membrane-associated STAT3 and PY-STAT3 in the Cytoplasm

Signal transduction from the plasma membrane to the nucleus by STAT proteins is widely represented as exclusively a soluble cytosolic process. Using cell-fractionation methods, we observed that approximately 5% of cytoplasmic STAT3 was constitutively associated with the purified early endosome (EE) fraction in human Hep3B liver cells. By 15-30 min after interleukin-6 (IL-6) treatment, up to two-thirds of cytoplasmic Tyr-phosphorylated STAT3 can be associated with the purified early endosome fraction (Rab-5-, EEA1-, transferrin receptor-, and clathrin-positive fraction). Electron microscopy, immunofluorescence, and detergent dissection approaches confirmed the association of STAT3 and PY-STAT3 with early endosomes. STAT3 was constitutively associated with clathrin heavy chain in membrane and in the 1- to 2-MDa cytosolic complexes. The membrane association was dynamic in that, within 15 min of treatment with the vicinal-thiol cross-linker phenylarsine oxide, there was a dramatic increase in bulk STAT3 association with sedimentable membranes. The functional contribution of PY-STAT3 association with the endocytic pathway was evaluated in transient transfection assays using IL-6-inducible STAT3-reporter-luciferase constructs and selective regulators of this pathway. STAT3-transcriptional activation was inhibited by expression constructs for dominant negative dynamin K44A, epsin 2a, amphiphysin A1, and clathrin light chain but enhanced by that for the active dynamin species MxA. Taken together, these studies emphasize the contribution of the endocytic pathway to productive IL-6/STAT3 signaling.

Innate immune responses to dengue virus

Dengue fever/dengue hemorrhagic fever (DF/DHF) has emerged as the most important mosquito-borne viral diseases in tropical areas. The dengue virus (DV) has become endemic in most tropical urban centers throughout the world, and DHF has appeared concomitantly with this expansion. Given the fact that intensity of DV replication during the early times of infection could determine clinical outcomes, which ranges from febrile illness (DF) to life-threatening disease (DHF), it is important to understand the impact of DV infection on innate immunity. Interstitial dendritic cells (DCs) are believed to constitute the first line of the innate host defense against invading DV at the anatomical sites where it replicates after the initial bite by infected mosquito. Early activation of natural killer (NK) cells and type-I interferon-dependent immunity may be also important in limiting viral replication at the early times of dengue infection. The ability of infecting DV to counter the innate antiviral immunity might account for differences in virulence observed between viral strains.

Composition and assembly of STAT-targeting ubiquitin ligase complexes: paramyxovirus V protein carboxyl terminus is an oligomerization domain

Transcription regulators STAT1 and STAT2 are key components of the interferon signaling system leading to innate antiviral immunity. The related STAT3 protein is a regulator of interleukin-6-type cytokine signals and can contribute to both cell growth and death important for cancer gene regulation and tumor survival. These three STAT proteins are targeted for proteasome-mediated degradation by RNA viruses in the Rubulavirus genus of the Paramyxoviridae. A single viral protein, the V protein, assembles STAT-specific ubiquitin ligase complexes from cellular components. Simian virus 5 (SV5) targets STAT1, human parainfluenza virus 2 targets STAT2, and mumps virus targets both STAT1 and STAT3. Analysis of the V-dependent degradation complex (VDC) composition and assembly revealed several features contributing to targeting specificity. SV5 and mumps V proteins require STAT2 to recruit the STAT1 target, yet mumps V protein binds STAT3 independent of STAT1 and STAT2. All Rubulavirus V proteins tested require cellular DDB1 to target STATs for degradation but differ in the use of Roc1, which is essential for mumps V STAT3 targeting. Protein interaction analysis reveals that paramyxovirus V proteins can homo- and heterooligomerize and that the conserved cysteine-rich zinc-binding C-terminal domain is necessary and sufficient for oligomerization. Purified SV5 V protein spontaneously assembles into spherical macromolecular particles, and similar particles constitute SV5 and mumps VDC preparations.

Crystallization and X-ray crystallographic analysis of human STAT1

Unphosphorylated human STAT1 (1-683) has been crystallized in the presence of a phosphopeptide derived from the alpha-chain of human interferon gamma (IFNgamma) receptor. A complete data set has been collected from a KAu(CN)2-derivatized and dehydrated crystal. The crystal belonged to space group P6(1)22, with unit-cell parameters a = b = 102.6, c = 646.5 A, alpha = beta = 90, gamma = 120 degrees.

Structural bases of unphosphorylated STAT1 association and receptor binding

The crystal structure has been determined at 3.0 A resolution for an unphosphorylated STAT1 (1-683) complexed with a phosphopeptide derived from the alpha chain of interferon gamma (IFNgamma) receptor. Two dimer interfaces are seen, one between the N domains (NDs) (amino acid residues 1-123) and the other between the core fragments (CFs) (residues 132-683). Analyses of the wild-type (wt) and mutant STAT1 proteins by static light scattering, analytical ultracentrifugation, and coimmunoprecipitation suggest that STAT1 is predominantly dimeric prior to activation, and the dimer is mediated by the ND interactions. The connecting region between the ND and the CF is flexible and allows two interconvertable orientations of the CFs, termed "antiparallel" or "parallel," as determined by SH2 domain orientations. Functional implications of these dimer conformations are discussed. Also revealed in this structure is the detailed interaction between STAT1 SH2 domain and its docking site on IFNgamma receptor.

Implications of an antiparallel dimeric structure of nonphosphorylated STAT1 for the activation-inactivation cycle

IFN-gamma treatment of cells leads to tyrosine phosphorylation of signal transducer and activator of transcription (STAT) 1 followed by dimerization through a reciprocal Src homology 2-phosphotyrosine interaction near the -COOH end of each monomer, forming a parallel structure that accumulates in the nucleus to drive transcription. Prompt dephosphorylation and return to the cytoplasm completes the activation-inactivation cycle. Nonphosphorylated STATs dimerize, and a previously described interface between N-terminal domain (ND) dimers has been implicated in this dimerization. A new crystal structure of nonphosphorylated STAT1 containing the ND dimer has two possible configurations for the body of STAT1, one of which is antiparallel. In this antiparallel structure, the Src homology 2 domains are at opposite ends of the dimer, with the coiled:coil domain of one monomer interacting reciprocally with the DNA-binding domain of its partner. Here, we find that mutations in either the coiled:coil/DNA-binding domain interface or the ND dimer interface block dimerization of nonphosphorylated molecules and cause a resistance to dephosphorylation in vivo and resistance to a tyrosine phosphatase in vitro. We conclude that a parallel STAT1 phosphodimer not bound to DNA most likely undergoes a conformational rearrangement (parallel to antiparallel) to present the phosphotyrosine efficiently for dephosphorylation.

A STAT3 dimer formed by inter-chain disulphide bridging during oxidative stress

Signal transducer and activator of transcription (STAT) proteins are activated by cytokines and growth factors to play distinct roles in immune responses and developmental processes. STATs were thought to exist as latent, cytoplasmic monomers and activation to require dimer formation was mediated exclusively by reciprocal phospho-tyrosine/SH2-domain interactions, but recent evidence of cytoplasmic STAT complexes, including dimers, and unphosphorylated STATs in the nucleus has challenged these notions. STAT complexes detected by conventional SDS-PAGE, including a STAT3 dimer, have been reported. We show that such complexes can form during cell lysis and be disrupted with DTT, suggesting inter-chain disulphide bridging. STAT3 also forms a related complex in cells upon oxidative stress. We map the interaction to the amino-terminal domain of STAT3 and use mass spectrometry to implicate cysteine 259 as the reactive residue. The redox sensitivity of STAT3 may be significant, given its activation in cells in response to reactive oxygen species.

Bacterial N-acylhomoserine lactone-induced apoptosis in breast carcinoma cells correlated with down-modulation of STAT3

Cell growth is promoted by mitogens and survival factors, which activate intracellular signalling pathways to control cell cycle progression and cellular integrity. Proliferation signals are transmitted through Ras and Rho family small G-proteins coupled to mitogen-activated protein kinase (MAPK) cascades, while survival signals are propagated by lipid-dependent kinases such as phosphatidylinositide 3-kinases (PI3Ks) and protein kinase B (Akt/PKB). Recently, signal transducer and activator of transcription (STAT) proteins were identified as positive regulators of proliferation in a variety of cell types. Persistent activation of these pathways is associated with tumour cell growth, whereas their inhibition can halt proliferation and precipitate apoptotic cell death. The human pathogen Pseudomonas aeruginosa uses quorum-sensing signal molecules (QSSMs) to regulate virulence gene expression. QSSMs also suppress host immune responses although the mechanism of suppression is unknown. Here, we demonstrate that the QSSM N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL) from P. aeruginosa blocks proliferation and induces apoptosis in human BC cell lines. Analyses of signalling events reveal that OdDHL has little or no effect on MAPK cascades, partially inhibits the Akt/PKB pathway and ablates STAT3 activity. Pharmacological inhibition of each pathway independently indicates that STAT3 activity is critical for BC cell proliferation and survival, while a constitutively active STAT3 confers resistance to OdDHL. These results support the notion of OdDHL as a bioactive molecule in eukaryotic systems and a paradigm for a novel class of antiproliferative compounds.

STAT1 mediates differentiation of chronic lymphocytic leukemia cells in response to Bryostatin 1

Bryostatin 1 is known to exhibit in vitro and in vivo activity against chronic lymphocytic leukemia (CLL) cells by inducing their further maturation into plasma-like cells. Signal transducer and activator of transcription (STAT) proteins play a central role in B-lymphocyte growth and function and are aberrantly phosphorylated on serine residues in CLL cells. To determine whether STAT transcription factors are important in Bryostatin 1-induced differentiation of CLL cells, primary CLL cells were examined for signaling events following exposure to Bryostatin 1 in vitro. Western analysis and electrophoretic mobility shift assays revealed that Bryostatin 1 induced tyrosine phosphorylation and DNA binding of STAT1, yet there was no effect on constitutive serine phosphorylation of STAT1. Bryostatin 1-induced STAT1 activation occurred in a manner that was dependent on protein kinase C (PKC), mitogen-activated protein kinase (MAPK), and Janus tyrosine kinase (JAK) activation. Evidence indicates that Bryostatin 1 induces STAT1 activation through an interferon gamma (IFN gamma) autocrine loop. However, STAT1 activation by IFN gamma stimulation alone was not sufficient to induce differentiation. This insufficiency is due to the broader effect on gene expression caused by Bryostatin 1 compared with IFN gamma, as demonstrated by microarray analysis. Both up-regulation of CD22 expression and immunoglobulin M (IgM) production, markers of CLL differentiation, were inhibited by a decoy oligonucleotide for STAT1, indicating that STAT1 is necessary for Bryostatin 1-induced differentiation of CLL cells. This study implicates STAT transcription factors as important mediators of Bryostatin 1-induced differentiation of CLL cells and could possibly lead to improved therapeutic approaches for the treatment of CLL.

STATs dimerize in the absence of phosphorylation

Upon activation by tyrosine kinases, members of the STAT family of transcription factors form stable dimers that are able to rapidly translocate to the nucleus and bind DNA. Although crystal structures of activated, near full-length, Stat1 and Stat3 illustrate how STATs bind to DNA, they provide little insight into the dynamic regulation of STAT activity. To explore the unique structural changes Stat1 and Stat3 undergo when they become activated, full-length inactive recombinant proteins were prepared. To our surprise, even though these proteins are unable to bind DNA, our studies demonstrate that they exist as stable homodimers. Similarly, the Stat1 and Stat3 found in the cytoplasm of unstimulated cells also exhibit a dimeric structure. These observations indicate that Stat1 and Stat3 exist as stable homodimers prior to activation.

Of JAKs, STATs, blind watchmakers, jeeps and trains

Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling is essential but not sufficient for full responses to the interferons (IFNs), most cytokines and some growth factors. The IFN-gamma and interleukin-6 (IL-6) response pathways have been used as model systems to investigate both the signals involved and their organisation. Activated STAT1 diffuses freely in the cytoplasmic and nuclear compartments of the cell providing a 'random walk' element in the IFN-gamma response. Completely foreign chimeric receptors and, remarkably, in the absence of STAT3, the endogenous IL-6 receptor can efficiently mediate an IFN-gamma-like response. Accordingly all of the signals required for an IFN-gamma response can be generated through physiological levels of a foreign ligand. JAK/STAT signalling, therefore, appears 'soft-wired', modular and highly flexible with substantial overlap between different response pathways. The data are consistent with a generic or 'core' set of signals from JAK/receptor complexes with 'add-on' modulation through specific receptor motifs. The cellular background likely profoundly affects the nature of the response.

Role of the JAK/STAT signal transduction pathway in the regulation of gene expression in CNS

Over the last 20 years the JAK/STAT signal transduction pathway has been extensively studied. An enormous amount of data on different cell signal transduction pathways is now available. The JAK/STAT signal transduction pathway is one of the intracellular signaling pathways activated by cytokines and growth factors that was first studied in the hematopoietic system, but recent data demonstrate that this signal transduction is also greatly utilized by other systems. The JAK/STAT pathway is a signaling cascade that links the activation of specific cell membrane receptors to nuclear gene expression. This review is focused on the role of JAK/STAT signal transduction pathway activation in the central nervous system (CNS).

Interactions of STAT3 with caveolin-1 and heat shock protein 90 in plasma membrane raft and cytosolic complexes. Preservation of cytokine signaling during fever

Interleukin-6 (IL-6) initiates STAT3 signaling in plasma membrane rafts with the subsequent transit of Tyr-phosphorylated STAT3 (PY-STAT3) through the cytoplasmic compartment to the nucleus in association with accessory proteins. We initially identified caveolin-1 (cav-1) as a candidate STAT3-associated accessory protein due to its co-localization with STAT3 and PY-STAT3 in flotation raft fractions, and heat shock protein 90 (HSP90) due to its inclusion in cytosolic STAT3-containing 200-400-kDa complexes. Subsequent immunomagnetic bead pullout assays showed that STAT3, PY-STAT3, cav-1, and HSP90 interacted in plasma membrane and cytoplasmic complexes derived from uninduced and stimulated Hep3B cells. This was a general property of STAT3 in that these interactions were also observed in alveolar epithelial type II-like cells, lung fibroblasts, and pulmonary arterial endothelial cells. Exposure of Hep3B cells to the raft disrupter methyl-beta-cyclodextrin for 1-10 min followed by IL-6 stimulation for 15 min preferentially inhibited the appearance of PY-STAT3 in the cav-1-enriched sedimentable cytoplasmic fraction, suggesting that these complexes may represent a trafficking intermediate immediately downstream from the raft. Because IL-6 is known to function in the body in the context of fever, the possibility that HSP90 may help preserve IL-6-induced STAT3 signaling at elevated temperature was investigated. Geldanamycin, an HSP90 inhibitor, markedly inhibited IL-6-stimulated STAT3 signaling in Hep3B hepatocytes cultured overnight at 39.5 degrees C as evaluated by DNA-shift assays, trafficking of PY-STAT3 to the nucleus, cross-precipitation of HSP90 by anti-STAT3 polyclonal antibody, and reporter/luciferase construct experiments. Taken together, the data show that IL-6/raft/STAT3 signaling is a chaperoned pathway that involves cav-1 and HSP90 as accessory proteins and suggest a mechanism for the preservation of this signaling during fever.

Stats: transcriptional control and biological impact

Extracellular proteins bound to cell-surface receptors can change nuclear gene expression patterns in minutes, with far-reaching consequences for development, cell growth and homeostasis. The signal transducer and activator of transcription (STAT) proteins are among the most well studied of the latent cytoplasmic signal-dependent transcription-factor pathways. In addition to several roles in normal cell decisions, dysregulation of STAT function contributes to human disease, making the study of these proteins an important topic of current research.

Clustered charged amino acids of human adenosine deaminase comprise a functional epitope for binding the adenosine deaminase complexing protein CD26/dipeptidyl peptidase IV

Human adenosine deaminase (ADA) occurs as a 41-kDa soluble monomer in all cells. On epithelia and lymphoid cells of humans, but not mice, ADA also occurs bound to the membrane glycoprotein CD26/dipeptidyl peptidase IV. This "ecto-ADA" has been postulated to regulate extracellular Ado levels, and also the function of CD26 as a co-stimulator of activated T cells. The CD26-binding site of human ADA has been localized by homolog scanning to the peripheral alpha2-helix (amino acids 126-143). Among the 5 non-conserved residues within this segment, Arg-142 in human and Gln-142 in mouse ADA largely determined the capacity for stable binding to CD26 (Richard, E., Arredondo-Vega, F. X., Santisteban, I., Kelly, S. J., Patel, D. D., and Hershfield, M. S. (2000) J. Exp. Med. 192, 1223-1235). We have now mutagenized conserved alpha2-helix residues in human and mouse ADA and used surface plasmon resonance to evaluate binding kinetics to immobilized rabbit CD26. In addition to Arg-142, we found that Glu-139 and Asp-143 of human ADA are also important for CD26 binding. Mutating these residues to alanine increased dissociation rates 6-11-fold and the apparent dissociation constant K(D) for wild type human ADA from 17 to 112-160 nm, changing binding free energy by 1.1-1.3 kcal/mol. This cluster of 3 charged residues appears to be a "functional epitope" that accounts for about half of the difference between human and mouse ADA in free energy of binding to CD26.

Association of the chaperone glucose-regulated protein 58 (GRP58/ER-60/ERp57) with Stat3 in cytosol and plasma membrane complexes

Glucose-regulated protein 58 (GRP58/ER-60/ERp57), best known as a chaperone in the endoplasmic reticulum lumen, was previously identified by us as one of several accessory proteins in the S100 cytosol fraction of human hepatoma Hep3B cells that was differentially coshifted by anti-Stat3 antibody in an antibody-subtracted differential protein display assay. In the present study, the association between GRP58 and Stat3 in different cytoplasmic compartments was evaluated using cross-immunoprecipitation and cell-fractionation techniques. In the S100 cytosol fraction, three different anti-GRP58 polyclonal antibodies (pAb) cross-immunoprecipitated Stat3 (but not Stat1), and, conversely, anti-Stat3 pAb cross-immunoprecipitated GRP58. Both cytosolic Stat3 and GRP58 eluted during Superose-6 gel-filtration chromatography in complexes of size 200-400 kDa (statosome I), and anti-Stat3 pAb cross-immunoprecipitated GRp58 from these FPLC elution fractions. Using differential sedimentation and density equilibrium flotation methods, Stat3 and GRP58 were observed to be coassociated with cytoplasmic membranes enriched for the plasma membrane marker 5' nucleotidase but not with those containing the endoplasmic reticulum marker BiP/GRP78. The Stat3 and GRP58-containing plasma membrane fraction also contained Stat1, Stat5b, and gp130. Stat activation by orthovanadate caused the accumulation of PY-Stat3 in the GRP58-containing plasma membrane fraction. However, this PY-Stat3 was DNA-binding deficient. Likewise, excess exogenous recombinant human GRP58 prepared using a baculovirus expression system preferentially inhibited Stat3 DNA-binding activity in the S100 cytosol, suggesting that GRP58 may sequester activated Stat3. The new data confirm the association between GRP58 and Stat3 in cytosolic 200-400-kDa statosome I complexes and show that both GRP58 and Stat family members coassociate in the plasma membrane compartment. We suggest that the chaperone GRP58 may regulate signaling by sequestering inactive and activated Stat3.

Cytokine signaling: STATS in plasma membrane rafts

STAT transcription factors signal from the plasma membrane to the nucleus in response to growth factors and cytokines. We have investigated whether plasma membrane "rafts" are involved in cytokine-activated STAT signaling. Cytokine-free human hepatoma Hep3B cells or cells treated with interleukin-6 (IL-6) or orthovanadate (a general activator of STATs) were fractionated, and plasma membrane raft fractions were obtained by equilibrium sedimentation or flotation through discontinuous sucrose gradients using either non-detergent or detergent-based (saponin or Triton X-100) methods. By Western blotting the plasma membrane raft fractions obtained using either non-detergent or detergent-based methods contained significant amounts of STAT1 and STAT3 (up to approximately 10% of the total cytoplasmic amount) as well as the integral raft proteins caveolin-1 and flotillin-1, the IL-6-receptor signal transducing chain gp130, the interferon-gamma receptor alpha chain (IFN-gammaRalpha), and the chaperone glucose-regulated protein 58 (GRP58/ER-60/ERp57). Upon activation of signaling by IL-6 or orthovanadate the respective Tyr-phosphorylated STAT species were now also observed in the membrane raft fraction but in a form deficient in DNA binding. The data show pre-association of STATs with plasma membrane rafts in flotation fractions, which also contained caveolin-1 and flotillin-1, and suggest that Tyr phosphorylation may not in itself be sufficient to cause the departure of PY-STATs from plasma membrane rafts. Methyl-beta-cyclodextrin, which sequesters cholesterol and disrupts plasma membrane rafts, markedly inhibited IL-6- and IFN-gamma-induced STAT signaling. Signaling through specialized raft microdomains may be a general mechanism operating at the level of the plasma membrane through which cytokines and growth factors activate STAT species (the "raft-STAT signaling hypothesis").

Stat2 binding to the interferon-alpha receptor 2 subunit is not required for interferon-alpha signaling

The interferon-alpha (IFNalpha) receptor consists of two subunits, the IFNalpha receptor 1 (IFNaR1) and 2 (IFNaR2) chains. Following ligand binding, IFNaR1 is phosphorylated on tyrosine 466, and this site recruits Stat2 via its SH2 domain. In contrast, IFNaR2 binds Stat2 constitutively. In this study we have characterized the Stat2-IFNaR2 interaction and examined its role in IFNalpha signaling. Stat2 binds the major IFNaR2 protein but not a variant containing a shorter cytoplasmic domain. The interaction does not require a STAT SH2 domain. Both tyrosine-phosphorylated and non-phosphorylated Stat2 bind IFNaR2 in vitro; however, relatively little phosphorylated Stat2 associates with IFNaR2 in vivo. In vitro binding assays defined IFNaR2 residues 418-444 as the minimal interaction domain and site-specific mutation of conserved acidic residues within this domain disrupted in vitro and in vivo binding. An IFNaR2 construct carrying these mutations was either (i) overexpressed in 293T cells or (ii) used to complement IFNaR2-deficient U5A cells. Unexpectedly, the activity of an IFNalpha-dependent reporter gene was not reduced but, instead, was enhanced up to 2-fold. This suggests that this particular IFNaR2-Stat2 interaction is not required for IFNalpha signaling, but might act to negatively inhibit signaling. Finally, a doubly truncated recombinant fragment of Stat2, spanning residues 136-702, associated with IFNaR2 in vitro, indicating that the interaction with IFNaR2 is direct and occurs in a central region of Stat2 marked by a hydrophobic core.

STAT1 from the cell membrane to the DNA

The binding of interferons (IFNs) to their receptors leads to the phosphorylation and activation of signal transducers and activators of transcription (STATs), and their translocation from the cytoplasm to the nucleus. The mechanisms by which the STATs move to the nuclear pore are not, however, known. Here it is shown that IFN-alpha and -gamma signalling and STAT1 translocation are independent of the actin cytoskeleton or microtubules. Using fluorescence loss in photobleaching (FLIP) and fluorescence recovery after photobleaching (FRAP) experiments, the mobility of a fusion protein of STAT1 with green fluorescent protein (STAT1-GFP) was compared with that of GFP and protein kinase C-GFP. In IFN-gamma-treated and control cells, cytoplasmic STAT1-GFP shows high, energy-independent, mobility comparable to that of freely diffusible GFP. A random walk model for movement of STAT1 from the plasma membrane to the nuclear pore is, therefore, indicated. Nuclear STAT1-GFP showed similar high mobility, with exclusion from nucleoli, consistent with high rates of association and dissociation of STAT1-DNA and/or STAT1-protein complexes in the nucleoplasm of the cell.

Arginine/lysine-rich structural element is involved in interferon-induced nuclear import of STATs

Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors, which mediate interferon (IFN), interleukin, and some growth factor and peptide hormone signaling in cells. IFN stimulation results in tyrosine phosphorylation, dimerization, and nuclear import of STATs. In response to IFN-gamma stimulation, STAT1 forms homodimers, whereas IFN-alpha induction results in the formation of STAT1.STAT2 heterodimers, which assemble with p48 protein in the nucleus. Phosphorylation as such is not sufficient to target STATs into the nucleus; rather, the dimerization triggered by phosphorylation is essential. Although IFN-induced nuclear import of STATs is mediated by the importin/Ran transport system, no classic nuclear localization signal (NLS) has been found in STATs. In the three-dimensional structure of STAT1, we observed a structural arginine/lysine-rich element within the DNA-binding domain of the molecule. We created a series of point mutations in these elements of STAT1 and STAT2 and showed by transient transfection/IFN stimulation assay that this site is essential for the nuclear import of both STAT1 and STAT2. The results suggest that two arginine/lysine-rich elements, one in each STAT monomer, are required for IFN-induced nuclear import of STAT dimers. Import-defective STAT1 and STAT2 proteins were readily phosphorylated and dimerized, but they functioned as dominant negative molecules inhibiting the nuclear import of heterologous STAT protein.

STAT proteins and transcriptional responses to extracellular signals

Signal transducer and activator of transcription (STAT) transcription factors are implicated in programming gene expression in biological events as diverse as embryonic development, programmed cell death, organogenesis, innate immunity, adaptive immunity and cell growth regulation in organisms ranging from slime molds to insects to man. Rapid progress has unearthed much about the activation of STATs by Janus kinases (JAKs) and other tyrosine kinases and their ability to interface with other signaling systems. Once inside the nucleus, the STATs bind to promoters and join other transcriptional activators in the regulation of gene expression.

STAT-signalling through the cytoplasmic compartment: consideration of a new paradigm

The binding of a large number of cytokines and growth factors to their cognate receptors on the surface of mammalian-cell plasma membrane activates a signalling cascade involving the cytoplasmic STAT-family proteins, which is characterized by the nuclear translocation of a cytokine- or growth factor-specific subset of the cytoplasmic pool of the respective tyrosine- and serine-phosphorylated STAT proteins and the consequent transcriptional activation of specific target genes. In the standard model of cytokine-induced STAT signalling such as that elicited by various interferons and interleukins, it is thought that STAT proteins are recruited to the cytoplasmic side of the cell-surface receptor complex from within a monomeric cytosolic pool, and upon tyrosine-phosphorylation by respective Janus kinase family members, dimerize and translocate to the nucleus. The mechanisms which determine and regulate the recruitment of cytosolic STAT proteins to the plasma membrane-receptor complex, the transit of "activated" STATs through the expanse of the cytoplasmic compartment from the plasma membrane to the nuclear pore region, and the transit of STATs through the nuclear pore complex into the nuclear compartment, remain largely unknown. New data from different laboratories suggests consideration of a model for STAT signalling in which STAT proteins function in the cytoplasm not only as free monomers and dimers but as part of heteromeric complexes ("statosomes"), with accessory proteins which may serve to present specific STATs to the plasma membrane-receptor complex, and to chaperone "activated" STATs through the cytoplasmic compartment toward the nucleus and then into the nuclear compartment.

Complex roles of Stat1 in regulating gene expression

Stat1 is a fascinating and complex protein with multiple, yet contrasting transcriptional functions. Upon activation, it drives the expression of many genes but also suppresses the transcription of others. These opposing characteristics also apply to its role in facilitating crosstalk between signal transduction pathways, as it participates in both synergistic activation and inhibition of gene expression. Stat1 is a functional transcription factor even in the absence of inducer-mediated activation, participating in the constitutive expression of some genes. This review summarizes the well studied involvement of Stat1 in IFN-dependent and growth factor-dependent signaling and then describes the roles of Stat1 in positive, negative and constitutive regulation of gene expression as well as its participation in crosstalk between signal transduction pathways. Oncogene (2000).

Association of STATs with relatives and friends

Members of the STAT family of transcription factors are present in species as diverse as mammals, insects and slime molds. Discovered as mediators of interferon-induced signals, the STATs were later shown to drive many different ligand-induced responses through receptor-induced tyrosine phosphorylation and dimerization. STAT1 also functions as a transcription factor, essential for the efficient constitutive expression of certain genes, without needing tyrosine phosphorylation, and phosphorylated STAT1 dimers mediate suppression - rather than activation - of some genes. STATs are present in the cytoplasm of untreated cells in multiprotein complexes, which might aid in their nuclear translocation and differential binding to DNA, thus contributing to the specificity of STAT action. This review explores the diverse protein-protein interactions that underlie the multiple functions of the STATs.

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.

Cellular physiology of STAT3: Where's the cytoplasmic monomer?

In the standard model of cytokine-induced signal transducer and activator of transcription (STAT) protein family signaling to the cell nucleus, it is assumed that STAT3 is recruited to the cytoplasmic side of the cell surface receptor complex from within a cytosolic monomer pool. By using Superose-6 gel-filtration chromatography, we have discovered that there is little monomeric STAT3 (91 kDa) in the cytosol of liver cells (human hepatoma Hep3B cell line and rat liver). The bulk of STAT3 (and STAT1, STAT5a, and -b) was present in the cytosol as high molecular mass complexes in two broad distributions in the size range 200-400 kDa ("statosome I") and 1-2 MDa ("statosome II"). Upon treatment of Hep3B cells with interleukin-6 (IL-6) for 30 min (i) cytosolic tyrosine-phosphorylated STAT3 was found to be in complexes of size ranging from 200-400 kDa to 1-2 MDa; (ii) a small pool of monomeric STAT3 and tyrosine-phosphorylated STAT3 eluting at 80-100 kDa was observed, and (iii) most of the cytoplasmic DNA-binding competent STAT3 (the so-called SIF-A "homodimer") co-eluted with catalase at 230 kDa. In order to identify the protein components of the 200-400-kDa statosome I cytosolic complexes, we used the novel technique of antibody-subtracted differential protein display using anti-STAT3 antibody. Eight polypeptides in the size range from 20 to 114 kDa co-shifted with STAT3; three of these (p60, p20a, and p20b) were co-shifted in an IL-6-dependent manner. In-gel tryptic fragmentation and mass spectroscopy identified the major IL-6-dependent STAT3-co-shifted p60 protein as the chaperone GRP58/ER-60/ERp57. Taken together, these data (i) emphasize the absence of a detectable STAT3 monomer pool in the cytosol of cytokine-free liver cells as posited by the standard model, and (ii) suggest an alternative model for STAT signaling in which STAT3 proteins function in the cytoplasm as heteromeric complexes with accessory scaffolding proteins, including the chaperone GRP58.